Strombolian explosions and lava flows continue during January-April 2023

Stromboli, located in Italy, has exhibited nearly constant lava fountains for the past 2,000 years; recorded eruptions date back to 350 BCE. Eruptive activity occurs at the summit from multiple vents, which include a north crater area (N area) and a central-southern crater (CS area) on a terrace known as the ‘terrazza craterica’ at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the volcano-island. Activity typically consists of Strombolian explosions, incandescent ejecta, lava flows, and pyroclastic flows. Thermal and visual monitoring cameras are located on the nearby Pizzo Sopra La Fossa, above the terrazza craterica, and at multiple flank locations. The current eruption period has been ongoing since 1934 and recent activity has consisted of frequent Strombolian explosions and lava flows (BGVN 48:02). This report updates activity during January through April 2023 primarily characterized by Strombolian explosions and lava flows based on reports from Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) and various satellite data.

Frequent explosive activity continued throughout the reporting period, generally in the low-to-medium range, based on the number of hourly explosions in the summit crater (figure 253, table 16). Intermittent thermal activity was recorded by the MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data (figure 254). According to data collected by the MODVOLC thermal algorithm, a total of 9 thermal alerts were detected: one on 2 January 2023, one on 1 February, five on 24 March, and two on 26 March. The stronger pulses of thermal activity likely reflected lava flow events. Infrared satellite imagery captured relatively strong thermal hotspots at the two active summit craters on clear weather days, showing an especially strong event on 8 March (figure 255).

Figure 253. Explosive activity persisted at Stromboli during January through April 2023, with low to medium numbers of daily explosions at the summit crater. The average number of daily explosions (y-axis) during January through April (x-axis) are broken out by area and as a total, with red for the N area, blue for the CS area, and black for the combined total. The data are smoothed as daily (thin lines) and weekly (thick lines) averages. The black squares along the top represent days with no observations due to poor visibility (Visib. Scarsa). The right axis indicates the qualitative activity levels from low (basso) to highest (altissimo) with the green highlighted band indicating the most common level. Courtesy of INGV (Report 17/2023, Stromboli, Bollettino Settimanale, 18/04/2023 - 24/04/2023).

Table 16. Summary of type, frequency, and intensity of explosive activity at Stromboli by month during January-April 2023; information from webcam observations. Courtesy of INGV weekly reports.

Figure 254. Intermittent thermal activity at Stromboli was detected during January through April 2023 and varied in strength, as shown in this MIROVA graph (Log Radiative Power). A pulse of activity was captured during late March. Courtesy of MIROVA.

Figure 255. Infrared (bands B12, B11, B4) satellite images showing persistent thermal anomalies at both summit crater on 1 February 2023 (top left), 23 March 2023 (top right), 8 March 2023 (bottom left), and 27 April 2023. A particularly strong thermal anomaly was visible on 8 March. Courtesy of Copernicus Browser.

Activity during January-February 2023. Strombolian explosions were reported in the N crater area, as well as lava effusion. Explosive activity in the N crater area ejected coarse material (bombs and lapilli). Intense spattering was observed in both the N1 and N2 craters. In the CS crater area, explosions generally ejected fine material (ash), sometimes to heights greater than 250 m. The intensity of the explosions was characterized as low-to-medium in the N crater and medium-to-high in the CS crater. After intense spattering activity from the N crater area, a lava overflow began at 2136 on 2 January that flowed part way down the Sciara del Fuoco, possibly moving down the drainage that formed in October, out of view from webcams. The flow remained active for a couple of hours before stopping and beginning to cool. A second lava flow was reported at 0224 on 4 January that similarly remained active for a few hours before stopping and cooling. Intense spattering was observed on 11 and 13 January from the N1 crater. After intense spattering activity at the N2 crater at 1052 on 17 January another lava flow started to flow into the upper part of the Sciara del Fuoco (figure 256), dividing into two: one that traveled in the direction of the drainage formed in October, and the other one moving parallel to the point of emission. By the afternoon, the rate of the flow began to decrease, and at 1900 it started to cool. A lava flow was reported at 1519 on 24 January following intense spattering in the N2 area, which began to flow into the upper part of the Sciara del Fuoco. By the morning of 25 January, the lava flow had begun to cool. During 27 January the frequency of eruption in the CS crater area increased to 6-7 events/hour compared to the typical 1-7 events/hour; the following two days showed a decrease in frequency to less than 1 event/hour. Starting at 1007 on 30 January a high-energy explosive sequence was produced by vents in the CS crater area. The sequence began with an initial energetic pulse that lasted 45 seconds, ejecting predominantly coarse products 300 m above the crater that fell in an ESE direction. Subsequent and less intense explosions ejected material 100 m above the crater. The total duration of this event lasted approximately two minutes. During 31 January through 6, 13, and 24 February spattering activity was particularly intense for short periods in the N2 crater.

Figure 256. Webcam images of the lava flow development at Stromboli during 17 January 2023 taken by the SCT infrared camera. The lava flow appears light yellow-green in the infrared images. Courtesy of INGV (Report 04/2023, Stromboli, Bollettino Settimanale, 16/01/2023 - 22/01/2023).

An explosive sequence was reported on 16 February that was characterized by a major explosion in the CS crater area (figure 257). The sequence began at 1817 near the S2 crater that ejected material radially. A few seconds later, lava fountains were observed in the central part of the crater. Three explosions of medium intensity (material was ejected less than 150 m high) were recorded at the S2 crater. The first part of this sequence lasted approximately one minute, according to INGV, and material rose 300 m above the crater and then was deposited along the Sciara del Fuoco. The second phase began at 1818 at the S1 crater; it lasted seven seconds and material was ejected 150 m above the crater. Another event 20 seconds later lasted 12 seconds, also ejecting material 150 m above the crater. The sequence ended with at least three explosions of mostly fine material from the S1 crater. The total duration of this sequence was about two minutes.

Figure 257. Webcam images of the explosive sequence at Stromboli on 16 February 2023 taken by the SCT and SCV infrared and visible cameras. The lava appears light yellow-green in the infrared images. Courtesy of INGV (Report 08/2023, Stromboli, Bollettino Settimanale, 13/02/2023 - 19/02/2023).

Short, intense spattering activity was noted above the N1 crater on 27 and 28 February. A lava overflow was first reported at 0657 from the N2 crater on 27 February that flowed into the October 2022 drainage. By 1900 the flow had stopped. A second lava overflow also in the N crater area occurred at 2149, which overlapped the first flow and then stopped by 0150 on 28 February. Material detached from both the lava overflows rolled down the Sciara del Fuoco, some of which was visible in webcam images.

Activity during March-April 2023. Strombolian activity continued with spattering activity and lava overflows in the N crater area during March. Explosive activity at the N crater area varied from low (less than 80 m high) to medium (less than 150 m high) and ejected coarse material, such as bombs and lapilli. Spattering was observed above the N1 crater, while explosive activity at the CS crater area varied from medium to high (greater than 150 m high) and ejected coarse material. Intense spattering activity was observed for short periods on 6 March above the N1 crater. At approximately 0610 a lava overflow was reported around the N2 crater on 8 March, which then flowed into the October 2022 drainage. By 1700 the flow started to cool. A second overflow began at 1712 on 9 March and overlapped the previous flow. It had stopped by 2100. Material from both flows was deposited along the Sciara del Fuoco, though much of the activity was not visible in webcam images. On 11 March a lava overflow was observed at 0215 that overlapped the two previous flows in the October 2022 drainage. By late afternoon on 12 March, it had stopped.

During a field excursion on 16 March, scientists noted that a vent in the central crater area was degassing. Another vent showed occasional Strombolian activity that emitted ash and lapilli. During 1200-1430 low-to-medium intense activity was reported; the N1 crater emitted ash emissions and the N2 crater emitted both ash and coarse material. Some explosions also occurred in the CS crater area that ejected coarse material. The C crater in the CS crater area occasionally showed gas jetting and low intensity explosions on 17 and 22 March; no activity was observed at the S1 crater. Intense, longer periods of spattering were reported in the N1 crater on 19, 24, and 25 March. Around 2242 on 23 March a lava overflow began from the N1 crater that, after about an hour, began moving down the October 2022 drainage and flow along the Sciara del Fuoco (figure 258). Between 0200 and 0400 on 26 March the flow rate increased, which generated avalanches of material from collapses at the advancing flow front. By early afternoon, the flow began to cool. On 25 March at 1548 an explosive sequence began from one of the vents at S2 in the CS crater area (figure 258). Fine ash mixed with coarse material was ejected 300 m above the crater rim and drifted SSE. Some modest explosions around Vent C were detected at 1549 on 25 March, which included an explosion at 1551 that ejected coarse material. The entire explosive sequence lasted approximately three minutes.

Figure 258. Webcam images of the lava overflow in the N1 crater area of Stromboli on 23 March 2023 taken by the SCT infrared camera. The lava appears light yellow-green in the infrared images. The start of the explosive sequence was also captured on 25 March 2023 accompanied by an eruption plume (e) captured by the SCT and SPT infrared webcams. Courtesy of INGV (Report 13/2023, Stromboli, Bollettino Settimanale, 20/03/2023 - 26/03/2023).

During April explosions persisted in both the N and CS crater areas. Fine material was ejected less than 80 m above the N crater rim until 6 April, followed by ejection of coarser material. Fine material was also ejected less than 80 m above the CS crater rim. The C and S2 crater did not show significant eruptive activity. On 7 April an explosive sequence was detected in the CS crater area at 1203 (figure 259). The first explosion lasted approximately 18 seconds and ejected material 400 m above the crater rim, depositing pyroclastic material in the upper part of the Sciara del Fuoco. At 1204 a second, less intense explosion lasted approximately four seconds and deposited pyroclastic products outside the crater area and near Pizzo Sopra La Fossa. A third explosion at 1205 was mainly composed of ash that rose about 150 m above the crater and lasted roughly 20 seconds. A fourth explosion occurred at 1205 about 28 seconds after the third explosion and ejected a mixture of coarse and fine material about 200 m above the crater; the explosion lasted roughly seven seconds. Overall, the entire explosive sequence lasted about two minutes and 20 seconds. After the explosive sequence on 7 April, explosions in both the N and CS crater areas ejected material as high as 150 m above the crater.

Figure 259. Webcam images of the explosive sequence at Stromboli during 1203-1205 (local time) on 7 April 2023 taken by the SCT infrared camera. Strong eruption plumes are visible, accompanied by deposits on the nearby flanks. Courtesy of INGV (Report 15/2023, Stromboli, Bollettino Settimanale, 03/04/2023 - 09/04/2023).

On 21 April research scientists from INGV made field observations in the summit area of Stromboli, and some lapilli samples were collected. In the N crater area near the N1 crater, a small cone was observed with at least two active vents, one of which was characterized by Strombolian explosions. The other vent produced explosions that ejected ash and chunks of cooled lava. At the N2 crater at least one vent was active and frequently emitted ash. In the CS crater area, a small cone contained 2-3 degassing vents and a smaller, possible fissure area also showed signs of degassing close to the Pizzo Sopra La Fossa. In the S part of the crater, three vents were active: a small hornito was characterized by modest and rare explosions, a vent that intermittently produced weak Strombolian explosions, and a vent at the end of the terrace that produced frequent ash emissions. Near the S1 crater there was a hornito that generally emitted weak gas-and-steam emissions, sometimes associated with “gas rings”. On 22 April another field inspection was carried out that reported two large sliding surfaces on the Sciara del Fuoco that showed where blocks frequently descended toward the sea. A thermal anomaly was detected at 0150 on 29 April.

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy, (URL: http://www.ct.ingv.it/en/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Information is preliminary and subject to change. All times are local (unless otherwise noted)

April 1971 (CSLP 34-71)

Lava flow on 3 April reaches the sea

Card 1168 (06 April 1971) Lava flow on 3 April reaches the sea

"A lava flow on the Sciara del Fuoco occurred on 3 April 1971. The flow occurred on the active slope between the crater and the sea on the NW side of the volcano. The crater is at an altitude of 800 m. Lava reached the sea and the activity has momentarily stopped."

Information Contacts: Dr. Glot, International Laboratoire de Volcanologie, Paris, France; H. Tazieff, Paris, France.

May 1971 (CSLP 34-71)

Two explosions and a small lava flow on 1 May

Card 1196 (06 May 1971) Two explosions and a small lava flow on 1 May

"Catania, 1 May 1971: On the first of May at 1345 two explosions with emission of ashes have been observed at the crater of Stromboli. They opened the crater which had been blocked since the explosion of 31 March 1971 by collapsed loose materials. After the two explosions on 1 May a small amount of lava was emitted and flowed down the Sciara del Fuoco. The explosive activity continues in the form of small emissions of vapor and ashes which occur at irregular intervals of at least 15 minutes. No precursory nor contemporaneous.....have been registered on the seismograph of the Geophysical Observatory at Lipari, a distance of about 15 km from Stromboli.

"Catania, 4 May 1971: The lava flow of Stromboli....the activity is reduced to its usual state."

Information Contacts: Lo Giudice and Alfredo Rittman, Instituto Internazionale di Vulcanologia, Catania, Sicily, Italy.

November 1975 (NSEB 01:02) Cite this Report

Effusive activity began on 5 November; lava reaches the northern coast

For the first time since the spring of 1971, a new effusive phase began at 2030 on 5 November. Three summit craters were active, and lava flowing down the Sciara del Fuoco reached the N coast. By mid November effusive activity was decreasing below the 700 m level on the Sciara. The eruption had no adverse effects on the island's population or agriculture.

Further References. Villari, L., ed., 1980, The Aeolian Islands, an active volcanic arc in the Mediterranean Sea: Rendiconti Soc. Ital. Mineral. Petrol., v. 36, 185 p. + refs.

Capaldi, G., and others, 1978, Stromboli and its 1975 eruption: BV, v. 41, p. 259-285.

Information Contacts: G. Nappi, IIV, Catania.

January 1986 (SEAN 11:01) Cite this Report

Explosions and nuée ardente, then lava flow to sea

"On 6 December at 0745 an eruptive crisis with lava flow emission started at Stromboli, interrupting a 10-year period of persistent mild explosive activity during which liquid lava was constantly in the summit craters. Inception of the eruption was accompanied by a violent explosive phase and by the opening of a trough a few tens of meters wide on the NE margin of the crater platform. The newly formed eruptive trough, ~100 m long, developed from the easternmost vent of the main crater platform (825 m elevation) roughly NE down to an altitude of 680 m (figure 1). During the initial explosive phase, which lasted ~1.5 hours, ash clouds rich in steam rose to an altitude of 2,000-3,000 m. At about 0815, a small nuée ardente descended the NW flank's Sciara del Fuoco (near the Fili del Fuoco side), reaching the sea. The explosive activity was mainly confined to the newly formed trough, which was probably widened by ejection of abundant old and fumarolically altered material. The first emission of a lava flow was from a temporary vent a few tens of meters higher than the eventual main eruptive vent, preceding or accompanying the explosive phase. The first lava flow descended toward the easternmost edge of the Sciara, forming a small lava field (at the foot of Bastimento rock). Later that morning, the lava vent was displaced N to an elevation of ~680 m because of further collapse of the eruptive trough. From this point, a second lava flow started to descend the 40° steep slope of the Sciara del Fuoco, reaching the sea at about 1100.

Figure 1. Map of the summit and NW flank of Stromboli showing the newly formed eruptive gorge (1), the first lavas on the morning of 6 December 1985 (2), the lava flows of 6 December 1985-1 January 1986 (3), and the path of a nuée ardente on 6 December 1985 (4). Courtesy of Mauro Rosi and Alessandro Sbrana.

"During the following days, the lava front advanced a few tens of meters into the sea, forming a small peninsula at the NE end of the Sciara del Fuoco. Advance of the lava flow into the sea was frequently accompanied by secondary phreatic explosions and strong steaming. The lateral drainage of the degassed lava was followed by a drop of the lava level in the upper craters, changing their activity from lava fountains (5 December) to rhythmic puffs of ash clouds and continuous steaming.

"A visit to the summit on 8 December revealed a thin ashfall cover containing millimeter-sized accretionary lapilli deposited during the initial phreatomagmatic explosive phase. Outpouring of lava continued through December and January, with an average decline in lava output and a tendency to accumulate materials on the E side of the Sciara del Fuoco. Lava was still emerging on 27 January.

"Preliminary seismic investigations (Biviano and others, 1985) indicated that no relevant variations occurred in the 3 months preceding the eruption. An isolated event (M >2.5) located a few kilometers below the volcano was recorded on 18 November, ~2 weeks before the eruption. Microseismicity related to the very shallow explosivity of the volcano revealed a weak tendency to increase as the eruption approached. Preliminary analysis of the lava emitted in the first few days of the eruption indicated that the rock is a silica undersaturated shoshonitic basalt (SiO₂ 49.5%, Ne 1.1), containing phenocrysts of olivine, clinopyroxene, and plagioclase very similar to those erupted in historic time at Stromboli.

"The 6 December phenomena and their precise timing were reconstructed from eyewitness observations and photographs. Field investigations were performed 7-14 December and 29 December-4 January by Rosi and Sbrana (Univ of Pisa) and Giovanni Frazzetta (IIV Catania). Preliminary information on seismicity was provided by G. Neri (IIV Catania) and extracted from Biviano and others, 1985. XRF analysis of major elements was performed at the Univ of Pisa.

References. Biviano, A., Di Prima, S., Falsaperla, S., Marturano, M., Neri, G., Pellegrino, A., and Velardita, S., 1985, Relazione informativa sull'attività sismica a Stromboli: internal report to the National Group of Volcanology, CNR.

Capaldi, G., Guerra, I., Lo Bascio, A., Luongo, G., Rapolla, A., Scarpa, R., del Pezzo, E., Martini, M., Ghiara, M. R., Lirer, L., Munno, R., and La Volpe, L., 1978, Stromboli and its 1975 eruption: BV, v. 41, p.259-285.

Further References. De Fino, M., La Volpe, L., Falsaperla, S., Frazzetta, G., Neri, G., Francalanci, L., Rosi, M., and Sbrana, A., 1988, The Stromboli eruption of December 6, 1985-April 25, 1986: volcanological, petrological and seismological data: Rendiconti della Società Italiana di Mineralogia e Petrologia, v. 43, p. 1,021-1,038.

Falsaperla, S., and Neri, G., 1986, Seismic monitoring of volcanoes: Stromboli (southern Italy): Periodico di Mineralogia, v. 55, p. 153-163.

Information Contacts: M. Rosi and A. Sbrana, Univ di Pisa.

June 1986 (SEAN 11:06) Cite this Report

Strombolian activity; small lava flows

. . . the Société de Volcanologie Genève (SVG) visited Stromboli in early April and observed continued lava production. On 3 April, two vents were active in the summit terrace. Strombolian activity at the main vent ejected tephra to 100-150 m height in explosions that occurred every 20-25 minutes. Explosions at 30-35-minute intervals from the second vent ejected only old material. An effusive vent in the upper NNE part of the Sciara del Fuoco (below and N of the main vent) quietly emitted two small lava flows. The main flow, ~1.5 m wide, moved N at ~0.5 m/s; its front was about halfway down the Sciara del Fuoco. The other flow was only 0.5 m wide and was moving more slowly, to the NNE. Activity the next day was similar, but the smaller lava flow had stopped advancing. On 5 April, Strombolian activity had declined; explosions were less frequent and tephra did not rise as far. One lava flow, ~2 m wide, was active, moving NNE at 1-1.5 m/s.

Information Contacts: P. Vetsch, SVG, Switzerland.

September 1986 (SEAN 11:09) Cite this Report

Intermittent lava fountains from summit lava lake after lava flow production ends; tourist killed near summit vent

"The effusive eruption [ended] on 25 April 1986. During the eruption, the effusive vent, at 680 m elevation on the NE side of the Sciara del Fuoco, showed a gradual decline in lava output rate. The decrease in lava output was accompanied by the development of many small flows that advanced tens to hundreds of meters downslope, often covering previously erupted materials. In the very final stage, the progressive accumulation of lava around the vent led to the growth of a small (10-m-high) exogenous dome. At the end of the eruption, the new aa lava field covered ~1/3 of the Sciara del Fuoco; despite its 37° slope, virtually all of the erupted material accumulated on the subaerial part of the edifice. At the end of the effusive phase, Stromboli returned to its normal state, with a persistent lava lake producing intermittent lava fountains in the summit craters.

"On 24 July, Dr. Perez Bastardas Albert, a 33-year-old biologist from Barcelona, Spain, was killed near the edge of the W crater while climbing the mountain with his brother (but without local authorized guides). According to ... local police ... and volcano guides, the two brothers were at the edge of the crater at about 1100 when an explosion took place. They ran away, but Dr. Perez was hit in the head by a falling block and died instantly ~15 m from the crater rim. Recovery of the body was performed the same day, but with some difficulty because of the risk of falling material."

Information Contacts: M. Rosi, Univ di Pisa; G. Frazzetta, IIV.

November 1988 (SEAN 13:11) Cite this Report

Incandescent tephra from several vents

Geologists visited Stromboli in October and November. Surface temperatures associated with Strombolian and fumarolic activity were studied for comparison with remotely sensed data. Temperatures were recorded using a Minolta Cyclops 33 radiometer operating at [8-14 mm] wavelength.

Craters 1, 3, and 4 were estimated to be 45 m deep; the floor of Crater 2 was not seen. Crater 1 contained three active vents (figure 2). On 26 and 27 October, a circular pit ([vent 1-1]) 1 m in diameter contained lava that glowed bright orange in daylight. Infrequent incandescent scoria and gas eruptions from vent [1-1] closely followed activity in [vent 1-2], a rubbly surface ~3 m away. At 10-30-minute intervals, [1-2] was ruptured by violent emission of blue incandescent gases (visible only after dark) followed by glowing scoria ejection. Ejecta from [1-1] and [1-2] usually fell within the crater, but on one occasion fell several meters beyond the S rim. A 2-m-diameter glowing vent ([1-3]) almost continuously ejected spatter to 50 m height. The maximum brightness temperature for the lava surface in vent [1-3] was 835°C, although readings sometimes fell as low as 240°C (when taken through dense fumes that frequently accumulated within the crater).

Figure 2. Sketch of active craters at Stromboli, October-November 1988. Courtesy of C. Oppenheimer and D. Rothery.

On 28 October, vent [1-1] had increased in area four-fold but was no longer glowing visibly in daylight. Incandescent scoria, ejected at 5-30-minute intervals, usually fell inside the crater although medium-coarse ash often reached the crater rim. Tephra ejections were usually immediately preceded by brief, rapid, bluish-gray steam emission and a roaring sound. Vent [1-3] was faintly glowing in daylight. Separate activity at former site [1-2] was no longer evident.

When geologists returned 3-6 November, a large volume of dark scoria and agglutinate had accumulated within the crater. Two new incandescent cracks had developed near site [1-1] and sharp cracking noises, like pistol shots, were often heard. Activity was dominated by ~7 explosions/hour at site [1-1]. Two distinct eruption types (ejecting tephra well above the crater rim) were recognized: very brief, violent detonations expelling gases and comminuted scoria; and longer duration (5-10-second) ejections of incandescent, highly vesiculated bombs. Site [1-3] glowed brightly but erupted infrequently.

Crater 2 was inaccessible and could not be observed directly. The crater continuously emitted high volumes of thick white fume. An eruption on 28 October that ejected a brown ash plume may have originated from this crater. A very low inner wall separated craters 3 and 4, and it was difficult to determine the origin of some eruptions. Dirty-brown plumes, attributed to Crater 3, frequently reached a height of 200-300 m above the crater rim. Brief (3-8-second) powerful incandescent bomb ejections at 5-40-minute intervals deposited tephra onto the crater's outer flanks. Infrequent explosions, clearly from Crater 4, obliquely ejected incandescent scoria over the crater's E rim, accompanied by small mushrooming ash columns. On 5 November, two small glowing vents, a few meters apart, were observed on the crater floor.

Information Contacts: C. Oppenheimer and D. Rothery, Open Univ.

March 1989 (SEAN 14:03) Cite this Report

Brief stronger explosions; one tourist injured

A series of explosions occurred in Stromboli's summit craters [26] March at [0927]. The explosions lasted a few minutes and were significantly more intense than the volcano's regular intermittent activity. Ejecta consisted of fluid bombs, blocks, and ash that were scattered within a few hundred meters of the vents. Tourists observing the volcano nearby were hit by the shower of tephra. No one was directly injured by the fallout, but a girl fell and broke her arm when panic caused a rush towards the village.

According to numerous eyewitnesses, the explosions began without premonitory activity. Seismic records from the Ginostra station (1.8 km from the crater) recorded no significant variation before or after the event. However, a slight increase in volcanic tremor amplitude and energy of seismic shocks was observed a few days before the explosions by a portable seismograph operating on the N flank (1.5 km from the vents). The rate of gas emission and frequency of explosions gradually returned to a normal level in the following days.

Further Reference. Falsaperla, S., Montalto, A., and Spampinato, S., 1989, Analysis of seismic data concerning explosive sequences on Stromboli volcano in 1989: Bolletino del Gruppo Nazionale per la Vulcanologia, 1989-1, p. 249-258.

Information Contacts: S. Falsaperla, G. Frazzetta, and E. Privitera, IIV; M. Rosi, Univ di Pisa.

September 1989 (SEAN 14:09) Cite this Report

Explosions eject bombs and spatter

Geologists from Ruhr Univ . . . visited Stromboli 15-18 September. On 15 September between 1900 and 2100, 10 eruptions occurred from at least 3 vents, ejecting glowing spatter to 50-100 m above the crater terrace. No ash plumes were observed, and only two of the eruptions were audible from . . . Punta Labronzo, at 125 m elevation, ~2 km NNE of the craters near the N tip of the island.

Activity seemed comparatively weak and irregular during observations of the crater terrace from about 1730 on 16 September until 1000 the next day (from Pizzo sopra la Fossa, 918 m elevation). Small Strombolian explosions from at least four vents ejected bombs and spatter to <125 m above the crater rims; no ejecta reached the observation site, roughly 200 m from the vents. Individual bursts were separated by quiet intervals of 2-45 minutes. The SE walls of the active vents seemed to have grown unusually high. The southernmost vent ([1-1] or [1-2] on figure 2) was most active with about 3 explosions/hour, but bombs were rarely ejected significantly higher than the crater rim and were mostly directed obliquely S or SW, preventing close access to the crater terrace. Next to the most active vent was a second ([1-3] on figure 2) which apparently contained a lava lake. Although not visible, the lava lake's presence was suggested by a continuous glow and dull surf-like sounds. Geologists suggested that the two vents probably coalesced sometime after midnight on 17 September, as explosive activity from the first vent was much reduced and glow was also visible above it. Between 0900 and 1000, two brown ash plumes rose from the center of Crater 1 to perhaps 200 m above the vents, dropping some ash on the observation platform. A small ash plume was visible during the evening when the volcano was frequently observed from the E coast.

Craters 3 and 4 were less active, but one or two explosions/hour from Crater 4 produced the highest lava fountains (up to 100 m) and the longest eruptive episodes. Crater 3 erupted only three times during the observation period, but often made loud crashing noises without visible ejections. Two vents erupted simultaneously only once during the observation period ([1-1/1-2] and 4). No earthquakes were felt.

Information Contacts: B. Behncke, Ruhr Univ, Germany.

October 1989 (SEAN 14:10) Cite this Report

Frequent incandescent tephra ejection similar to 1988

During 5 October fieldwork, Open Univ geologists noted that morphology of the active craters appeared similar to that of late 1988. Crater 1 (see figure 2) was the most active, with two open vents at about the same positions as [1-1] and [1-3], both containing magma at or near the surface. Vent [1-1], which had developed a deeper intracrater, erupted at a mean interval of ~8 minutes [see also 15:08], often ejecting incandescent tephra over the crater's NE rim, and occasionally over the SE rim. Bombs rose as much as 150 m above the rim. Explosions from vent [1-3] were less frequent and less powerful. Crater 3 was also active, with eruptions at an average of once every 45 minutes, ejecting glowing bombs obliquely to the W, into adjoining Crater 2.

Information Contacts: C. Oppenheimer, Open Univ.

April 1990 (BGVN 15:04) Cite this Report

Lava fountaining and ash emission from several vents

Stromboli was visited on 29 March and 1-2 April. Poor weather on the 29th obscured the active summit vents until late afternoon, when six eruptive episodes were observed between 1745 and 1830. Five were accompanied by emission of dark gray to black ash plumes that rose 150-200 m above the two active vents. The activity produced lava fountains ~100 m high, often falling onto the Sciara del Fuoco. On one occasion, a large glowing block rolled down the NNW side of the Sciara to ~100 m below the vents.

Observations from the summit between 1730 on 1 April and 0200 the next morning revealed morphologic changes that had occurred in the vent area since September 1989 (14:09). Crater A [termed Crater 1 in previous reports; compare figure 2 and figure 3 below] included at least five vents, four of which were active. The vent that had been most active in September had collapsed, but a new deep vent that had formed 10-20 m to the NW produced an average of 2-3 eruptions/hour before midnight. Glowing spatter from the eruptions rarely escaped the vent, but black ash plumes rose ~100 m above the rim. Eruptions became more frequent and intense after midnight, with lava fountains rising to ~100 m above the rim, larger ash plumes, and heavy falls of bombs and spatter onto Crater A's NE rim (figure 4). Each of the eruptions was accompanied by a few seconds of deep but not very loud rumbling. In the NE part of Crater A, a cluster of open vents (several very small and three larger) contained active magma and glowed intensely at night. They emitted burning gases but no spatter.

Figure 3. Sketch map of the summit area of Stromboli showing vent configurations observed 1-2 April 1990. Courtesy of B. Behncke.

Figure 4. Oblique sketch of the summit area of Stromboli, looking roughly W. Courtesy of B. Behncke.

Crater B [coalesced from craters formerly termed 2, 3, and 4; compare figures 2 and 3 below] included at least 4 vents, and others were probably hidden from view by intense gas emission and topographic obstacles. In its center was a symmetrical spatter cone 10-15 m high with a glowing summit vent and a small steaming hornito near its SW base. The cone was covered with yellowish green sublimates and was not ejecting tephra. Vent 5, frequently active in September, erupted only once every 1-2 hours. Its eruptions lasted up to 30 seconds, sometimes consisting of several pulses of lava fountains that reached 100 m above the rim, accompanied by loud rumbling. Vigorous emission of gas (smelling strongly of H₂S and [SO₂]) from the E wall of Crater B obscured vent 5 from direct observation. Vent 4a, at most 2 m in diameter, was near the base of a steep pinnacle, possibly a hornito, ~10 m high. The vent produced very loud high-pressure gas emissions, sometimes lasting 20 seconds, every 20-30 minutes, feeding a very faint bluish gas column ~20-30 m high. A few were accompanied by ejection of several glowing blocks, probably from the conduit walls. A slight continuous tremor was felt from ~50 m away during one of the gas emission episodes. Vent 3, between craters A and B, remained inactive during the observation period.

During the afternoon of 3 April, ash emission reportedly became stronger and could be seen from villages on the SE and E sides of the island.

Information Contacts: B. Behncke, Ruhr Univ, Germany.

August 1990 (BGVN 15:08) Cite this Report

Strong explosions; one crater filled by tephra

An automatic telemetering short-period seismic station was installed near the active craters in October 1989 to monitor explosive activity and volcanic tremor. During that month, "normal Strombolian" activity from Craters 1 and 3 produced ~160 explosive events/day (figure 5). During the second half of November, several events with peculiar waveforms were recorded over a 4-day period. Volcano guides (who cooperate with volcanologists by noting visible activity, as reported below) observed a new cone inside Crater 1 in December 1989, and another cone that showed explosive activity between craters 1 and 2 in February 1990. In March, small cones produced bluish vapor inside Crater 3; emissions were accompanied by dull rumbling.

Figure 5. Average number of seismic events/hour recorded at Stromboli, October-November 1989 and April-July 1990. Arrow marks 18 June explosions. Courtesy of M. Riuscetti.

During the end of May and the first half of June (28 and 30 May, and 3, 4, 11, 13, and 15 June) "normal" activity was observed at craters 1 and 3 with continuous explosions and ejection of incandescent material to 10-50 m height. Crater 2 was not active during this period. Morphologic changes to the new cones in Crater 1 were not evident.

At least four large explosions occurred on 18 June between 1700 and 1710. Ejecta fell onto the NW flank's Sciara del Fuoco and ash emission could be seen from S. Bartolo village on the NE side of the island. The wall between craters 2 and 3 collapsed. After this episode, explosive activity with ejection of small glowing blocks was observed at Crater 2 (on 19, 21, 24, 25, and 28 June, and 2 July) with almost continuous noisy gas emission (on 25 June, and 4, 7, and 9 July). Craters 1 and 3 were active, with ejection of glowing material to ~100-150 m height (on 24 and 26 June, and 1, 3, 4, 5, 7, and 9 July).

An increase in the number of low-energy explosion earthquakes occurred 9-16 July, while tremor amplitude decreased slightly. The number of events saturating the seismometer then increased sharply, while low-energy shocks dropped to near the long-term mean (figure 6). Ash and lapilli emissions were continuous from the three craters, with increases in ejecta height and emission frequency after 19 July. The strong eruptive activity declined after 26 July. Crater 2 had been completely filled by tephra, but included two active vents characterized by synchronous noisy explosions. Activity at Crater 3 was dominated by prolonged silent ash emissions. Continuous strong explosions from Crater 1 have been observed since 1 August.

Figure 6. Top: detail of figure 5, showing daily average number of seismic events/hour recorded at Stromboli, 21 June-31 July, 1990. Bottom: number of events with amplitude ³ full scale (solid line) and average relative tremor amplitude (dashed line), 9 July-4 August, 1990. Mean values for the period are shown. Courtesy of M. Riuscetti.

Information Contacts: M. Riuscetti, Univ di Udine; volcano guides:Prospero Cultrera, Nino Aquilone, and Antonio Zerilli, Stromboli, Italy.

September 1990 (BGVN 15:09) Cite this Report

Continued Strombolian activity; new vents

The number of explosions and seismometer-saturating events fluctuated during August, and were apparently inversely correlated (figure 7). The highest peak in saturating events, 86, occurred on 13 August. Average tremor amplitude increased during August, to 12% higher than in July.

Figure 7. Average number of seismic events/hour recorded at Stromboli, August 1990. The mean value for the period is shown. Courtesy of Marcello Riuscetti.

Explosive activity persisted in Crater 3, throwing cinders and ash to >100 m. Crater 2 resumed visible explosive activity with continuous rumbling 5-10 August, while Crater 1 showed fumarolic activity 6-13 August and explosive activity following the 13th. New vents opened in Crater 2's NW wall on 25 August, and subsequently in Craters 1 and 3 (figure 8). Lapilli and cinders continued to fill the craters, completely covering the two cones in Crater 1 (15:08).

Figure 8. Sketch from "Pizzo sopra la Fossa" looking NW at the summit of Stromboli, August 1990. Courtesy of Marcello Riuscetti.

Information Contacts: M. Riuscetti, Univ di Udine.

October 1990 (BGVN 15:10) Cite this Report

Strong tephra ejection; increased seismicity

At the end of August, explosive activity in Crater 1 became nearly continuous and tremor amplitude increased. The monthly average tremor amplitude was twice as high in September as in August. The daily number of events that saturated seismometers oscillated around a mean of 30 until 20 September before rapidly decreasing (figure 9). Saturating events and tremor amplitude reached a minimum during the first week in October, then remained at levels similar to those preceding the strong activity in the second half of July.

Figure 9. Number of seismometer-saturating events/day (solid line); and average tremor amplitude (dashed line) at Stromboli, mid-June to November 1990. Courtesy of M. Riuscetti.

Volcano guides reported the following activity. 1-6 September: Ejection of hot lapilli was continuous from vents 1 and 2 in crater C1 (figure 10). Violent explosions with ash emission (150-200 m high) occurred from C3. 7-12 September: Activity was similar from C1 and C3. Block ejection and gas emission took place from C2. 13-20 September: Ejection of hot lapilli and noisy gas emission occurred from C1, while continuous minor explosions ejected small blocks from C2. Tephra was filling C3, where 4 new vents were forming on 15 September. 21 September-4 October: Most activity was concentrated in C1 and C3, with frequent explosions ejecting hot lapilli to as high as 200 m.

Figure 10. Sketch from "Pizzo sopra la Fossa" looking NW at the summit of Stromboli, October 1990. Courtesy of M. Riuscetti.

Geologists visited the summit area 4-10 October. The activity was more vigorous than had been seen during 25 years of study at Stromboli. Strong explosions at 15-20-minute intervals fed powerful brown ash emissions that reached about 300 m height (from vent 4 of C3). Nearly continuous bomb ejection from vent 1 of C1 was evident at night and gases were red. Stronger explosions were synchronous from many of the vents in the 3 craters. Lava spilled out every few tenths of a second from one small cone (1) in C3. One vent (3) in C1 ejected gas nearly every second. Fumarolic activity was very intense, especially from the W rim of C3. At least 3 new vents had formed (3 in C1 and 2 & 3 in C3) with continuous whistling and rare explosions.

11-16 October: Activity continued, but with an apparent slight decline. 17-28 October: Observations from the summit area were not available, but seismicity and reports from a village at the foot of the volcano suggested decreasing activity.

Information Contacts: M. Riuscetti, Univ di Udine.

November 1990 (BGVN 15:11) Cite this Report

Vigorous tephra ejection and lava fountaining from summit vents

Although the eruption was apparently somewhat less vigorous in late October, activity observed by Boris Behncke on 7-8 November was stronger than during his previous visits in September 1989 and March-April 1990. Vigorous gas emission fed a dense plume that obscured the vent area during the day, but visibility improved after sunset and a clear view of the craters was possible after 2000 on 7 November. Although vent morphology had changed somewhat, vent configuration was much the same as in April.

The main focus of activity was a cluster of at least four vents in C1 (at the NE end of the summit crater group) that were almost continuously erupting. Intense bomb and spatter ejection started at 1717 on 7 November and continued for at least 2 hours, with loud roaring like a jet aircraft. The strongest eruptions occurred from C1's easternmost vent (1), site of nearly continuous bomb ejections in early October. On 7 November, vent 1 ejected lava fountains to 100 m height, often followed within seconds by eruptions from vent 3, the southernmost vent in C1. Individual bursts of spatter, mostly from vent 3, were accompanied by loud explosions; gas had been emitted from this vent nearly once a second in early October. Between stronger bursts, very small lava fountains were continuously active within vents 2 and 3. Spatter was ejected to ~20 m height every 10-20 seconds from vent 3 and another vent to the NE. At 0100 on 8 November, fountains rose 40-50 m from the latter vent, and loud roaring was continuing. None of the vents produced ash plumes after 1800 on 7 November. The former vent at the NE end of C1 had apparently ceased erupting and may have been buried by the growing cone at vent 1, a prominent feature that had been too small to be visible from the summit in April.

Eruptive episodes from C3 (at the SW end of the summit crater group) occurred about twice an hour, producing lava fountains that rose as much as 100 m, and sometimes diffuse brown ash plumes and light tephra falls onto the summit platform. Most episodes consisted of several pulses of fountaining over a period of ~30 seconds. A strong eruptive episode at 1710 on 7 November was followed by bursts of spatter at intervals of 10-20 seconds until 1717. Another particularly violent burst at 2000 covered most of the crater area with glowing bombs and spatter. An area of 3 pits in C3 that had contained actively degassing lava in April was occupied by two small (<1 m diameter) vents that emitted low fountains of spatter. Much of C3 had been filled with recent pyroclastics.

Information Contacts: B. Behncke, Ruhr Univ, Germany.

December 1990 (BGVN 15:12) Cite this Report

Activity drops to occasional explosions; seismicity declines

Strombolian activity, abnormally vigorous during early October, began to decline in late October, and reached "normal" levels by the end of November. On 23 November, weak fumarolic activity was observed on the W rim of Crater 3, and continuous rumbling punctuated by rare explosions were reported from vent 3 in Crater 1. No activity was observed in Crater 2. The average tremor amplitude and the number of major shocks decreased to levels lower than in June when the paroxysmal phase began (figure 9).

Information Contacts: M. Riuscetti, Univ di Udine.

April 1991 (BGVN 16:04) Cite this Report

Explosive activity from a single crater; strong seismicity

Explosive activity was at low levels from January through March, seldom exceeding the long-term average of six recorded explosions/hour (figure 11). Visits to the summit on 30 March and 9 April revealed that activity was restricted to Crater 1, and that the small cone 1 in Crater 3 had collapsed, forming a glowing red vent. The number of earthquakes exceeding instrument saturation level was quite high from the end of January to the beginning of February (~30/day), and 11-17 March (~19/day; figure 12). Average tremor amplitude returned to normal following a low in December.

Figure 11. Daily average number of seismically recorded explosion events/hour at Stromboli, January-March 1991. The mean value for the period is shown. Courtesy of M. Riuscetti.

Figure 12. Number of seismometer-saturating events/day (upper curve); and average tremor amplitude (lower curve) at Stromboli, January-March 1991. Courtesy of M. Riuscetti.

Information Contacts: M. Riuscetti, Univ di Udine.

May 1991 (BGVN 16:05) Cite this Report

More frequent explosions

The number of seismically recorded explosions increased briefly in late March and persistently from mid-April (figure 13). After mid-April, the number of earthquakes exceeding instrument saturation level decreased from an average of ~20/day since January to

Figure 13. Average number of seismically recorded explosion events/hour at Stromboli, 15 March-15 May 1991. The mean value for the period is shown. Courtesy of M. Riuscetti.

Figure 14. Daily number of seismometer-saturating events (lower curve); and average tremor amplitude (upper curve) at Stromboli, 15 March-15 May 1991. Courtesy of M. Riuscetti.

Volcano guides reported infrequent small explosive activity at the 3 craters during visits to the summit on 10, 15, and 16 April, and 1 May. In early May, the first complete gas sampling at Stromboli was made during an inter-explosive phase at fumaroles (410°C) on the NW rim of the active crater complex (table 1).

Table 1. Chemical composition (in volume %) of fumarolic gases from Stromboli, early May 1991. Courtesy of M. Martini.

Local residents reported a significant increase in the number of explosions on 19 May, after several weeks of weak activity. During a visit to the summit on the evening of 21 May, frequent strong explosions were observed at craters 1 and 3, with large ejections of incandescent material. Thirty explosions were counted between 2000 on the 21st and 0600 the next day. Many ejecta fell onto the N flank's Sciara del Fuoco. Crater 2 and the small cones continuously emitted gas and vapor.

Information Contacts: M. Riuscetti, Univ di Udine; M. Martini, Univ di Firenze; H. Gaudru, Société Volcanologique Européenne (SVE), Switzerland.

June 1991 (BGVN 16:06) Cite this Report

Explosions eject glowing fragments and gas columns

The number of recorded explosion shocks remained elevated through late June (figure 15), with mean rates relatively stable near the long-term "normal value" of 6/hour. Average tremor amplitude declined slightly at the beginning of June while the number of saturating earthquakes rose sharply (figure 16). Volcano guides reported that the activity was concentrated at Crater 3, where explosions ejected glowing fragments and white gas columns that rose 100-200 m in late June. Explosions were rare from other craters, but tephra built small cones in Crater 2. White gas emission was continuous.

Figure 15. Average number of explosion events/hour at Stromboli, 18 May-21 June 1991. The mean value for the period is shown. At the end of May, the Mark L4 seismometer was replaced by a Willmore MK III set to a natural undamped frequency of 2 seconds, with the gain adjusted to keep the same overall amplitude response at 1 Hz. Courtesy of M. Riuscetti.

Figure 16. Average number of seismometer-saturating events (lower curve) and average tremor amplitude (upper curve) at Stromboli, 18 May-21 June 1991. Courtesy of M. Riuscetti.

Information Contacts: M. Riuscetti, Univ di Udine.

July 1991 (BGVN 16:07) Cite this Report

Continued explosions from two craters

The number and intensity of explosions has continued to fluctuate in recent months, with the average rate remaining slightly higher since mid-March. During a summit visit on the night of 31 July-1 August, >50 explosions were observed between 2100 and 0600. The strongest ejected incandescent material toward the edge of the summit area. Most of the explosions were from Crater 1, the rest from Crater 3, with only gas emission evident from Crater 2 and from a small cone. On this occasion and during other visits over the past several years, durations of precursory noises appeared linked to explosive vigor, with stronger explosions following noises lasting 3-5 seconds, whereas 1-2-second noises preceded weak explosions [see also 16:08].

Information Contacts: H. Gaudru, SVE, Switzerland; T. De St. Cyr, Fontaines St. Martin, France.

August 1991 (BGVN 16:08) Cite this Report

Continued moderate explosive activity; gas data

Explosive activity was restricted to crater C1 (NE part of the summit area; figure 17) during 9 August fieldwork (by F. Iacop, Institute of Earth Sciences, Univ of Udine). C1's central cone ejected hot tephra at ~20-minute intervals, and as a result, it had grown more rapidly than the crater's other two active cones. Glow from two small radial fissures in crater C2 was clearly visible at night. Sustained noisy gas emissions occurred about once an hour. Volcano guides had reported that activity was concentrated in crater C3 (SW part of the summit area), but at its cone 1 only hot vapor emission was occurring, from two vents, on 9 August. Rare explosions, mostly ejecting tephra, took place at bocca 4. The average number of recorded earthquakes remained near the normal value of 6/hour in July, declining below that level in the month's last week (figure 18). Average tremor amplitude also remained relatively constant through the end of July, while large shocks nearly disappeared after a peak on 29 June (figure 19). [see 16:09 for 28-29 August observations].

Figure 17. Active craters at Stromboli as seen from the somma, 6 September 1991. Crosses mark small vents active during the 6 September fieldwork. Courtesy of the Société Volcanologique Européenne.

Figure 18. Average number of explosion events/hour at Stromboli, 22 June-31 July 1991. The mean value for the period is shown. Courtesy of M. Riuscetti.

Figure 19. Number of seismometer-saturating events/day (lower curve) and average daily tremor amplitude (upper curve) at Stromboli, 22 June-31 July 1991. Courtesy of M. Riuscetti.

Moderate activity was observed in early September, with explosive episodes about every 15 minutes at crater C3 and roughly hourly at C1. Activity increased in the 3 hours of observations after 2300 on 6 September, with many moderate to strong explosions from the SW part of C3. Ejections of incandescent bombs and scoria sometimes lasted several minutes. Thick white vapor plumes rose from C2 and a small cone in its center, while blue SO₂-rich plumes emerged from several other vents. Explosions from C1 were vigorous, ejecting glowing fragments and dark brown columns that rose 200 m above the crater. C3's smaller explosive bursts, consisting of tephra-poor incandescent gas jets, were usually preceded by comparatively brief periods of increasing, noisy gas puffs; larger explosions that ejected a higher proportion of tephra followed longer intervals, with fewer or no precursory gas puffs. Geologists attributed this pattern to intermittent closure (by cooling) of the lava-filled conduits to gas-bubble rise from the underlying magma body, allowing higher pressure to build at depth.

Airborne COSPEC measurements by an Italian-French cooperative program in May-July indicated a total SO₂ flux somewhat lower than that measured by the same means in 1980 and 1984 (1,000 ± 200 t/d average; Allard and others, in press), consistent with the current moderate activity. Geologists concluded that combined with microprobe determination of the initial and residual sulfur content of Stromboli's lava, the SO₂ flux data require the degassing of 0.1 km³/year (average) of magma, three orders of magnitude more than the co-erupted volume. Thus, gas output is essentially derived from magma stored within the volcano. To assess the amount of diffuse magmatic degassing through the volcanic pile, other than from the craters, infrared mass spectrometric profiling of CO₂ concentrations in the ground began on 11 September. High CO₂ levels (80-90%), associated with subsurface thermal anomalies, were found to characterize the Pizzo sopra La Fossa crater terrace (at the summit rim, SE of the active craters). Concentrations gradually decreased toward the rim of this former crater, and no CO₂ anomaly was detected in outer areas to the S (down to the Vancori rampart).

Reference. Allard, P., Carbonelle, J., Le Bronec, J., Metrich, N., and Zetwoog, P., Volatile flux and magma degassing budget at Stromboli volcano: Geophysical Research Letters, in review.

Information Contacts: M. Riuscetti, Univ di Udine; Patrick Allard, CNRS-CEA, France; J.C. Baubron, BRGM, France; H. Gaudru and Rolf Haubrichs, SVE, Switzerland; Yvonne Miller, Univ de Genève, Switzerland.

September 1991 (BGVN 16:09) Cite this Report

Continued tephra ejection from several vents

Activity was moderate from August through mid-September, with occasional explosions from craters C1 and C3 (about 1-4 hourly/crater). During 1 August to 25 September, the average number of explosion shocks/hour gradually increased to the "normal" value of 6; (figure 20). Relatively low levels of activity lasted from mid-July to mid-August. The number of instrument-saturating events was low and concentrated on a few days during the second week in August, while average tremor amplitude remained relatively stable until it declined in late August (figure 21).

Figure 20. Average number of explosion shocks/hour at Stromboli, 1 August-25 September 1991. The mean value for the period is shown by the dotted line. Courtesy of M. Riuscetti.

Figure 21. Number of seismometer-saturating events/day (lower curve) and average daily tremor amplitude in volts (upper curve) at Stromboli, 1 August-25 September 1991. Courtesy of M. Riuscetti.

The following report from Jonathan Dehn and Boris Behncke, on 28-29 August activity, supplements information in 16:8. "During a visit to Stromboli's summit and craters on 28-29 August, the volcano, much less active than during visits in March-April 1990 and November 1990, was in a state similar to that observed in mid-September 1989. Eruptions came from only two vents, one at the NE end of C1 (figure 22), and the other in the center of C3. The former erupted frequently during the evening of 28 August producing fountains up to 150 m high. Such eruptions lasted at least 10 seconds and began without any noticeable premonitory signs. Bombs and spatter fell over much of the C1 area and into C2. After midnight on 29 August, explosions from this vent became much less frequent, averaging 1-2/hour. The other active vent produced ~2-4 explosions/hour but much lower fountains, and ejecta mostly fell back into C3. These explosions were often preceded by an increasingly bright glow from the interior of C3.

Figure 22. Sketch of Stromboli's crater terrace as seen from Pizzo sopra la Fossa, 28-29 August 1991. Features (such as cone at vent 3 in C1) are not to scale. Courtesy of J. Dehn and B. Behncke.

"During the night, four glowing but not erupting vents (1 and 2 in C2, and 1 and 2 in C3) could be observed, at the same positions as in late 1990. Two more vents (2 and 3) were visible within or near C1. Vent 2 appeared to be a larger pit created by collapse of the former vents 1 and 2. A symmetrical cone ~10 m high had been built around C1's vent 3, feeding a dense gas plume.

"C3 could be approached around daybreak (0530-0630) on 29 August, and observations of the crater's interior and its eruptive activity were made during a 45-minute period. The SW half of the crater was occupied by a pit ~50 m deep. This pit contained at least three active vents, of which two were open and displaying continuous glow and gas emission. Minor spattering from these vents occurred at times, often heralding eruptions from a third vent, in the center of the pit's floor, that was buried under a cover of bombs and scoriae. Eruptions from this vent fragmented the old overlying material, forming diffuse ash plumes followed by the ejection of fresh glowing spatter. All of the ejecta from C3 fell back into the pit and onto its walls, and slumping of material from the pit's walls commonly covered the vent within seconds after each eruption.

"Vents 1 and 2 (in C3) were also observed at close range. They contained small active lava ponds, or molten sulfur, and displayed pulsating gas emission. The ponds were located within a cavern, but no obvious vertical conduit was seen. The rim of C3 was covered with a thin coating of fine tephra and fresh Pele's hair, but no ejecta fell in this area during the stay on the crater rim. From 0630 until about 1100 (29 August), the end of the summit visit, eruptions were separated by intervals ranging from 10 to 40 minutes."

Information Contacts: J. Dehn and B. Behncke, GEOMAR, Kiel; M. Riuscetti, Univ di Udine.

October 1991 (BGVN 16:10) Cite this Report

More frequent explosions; new zone of fumaroles

The number of recorded explosion shocks increased irregularly through October (figure 23), a continuation of the generally increasing trend that followed the low activity of mid-July to mid-August. Tremor amplitude also increased (figure 24). The number of seismometer-saturating events was quite low and concentrated during the last week of October, when tremor was stronger and explosions were more frequent.

Figure 23. Average number of explosion shocks/hour at Stromboli, 25 September-31 October, 1991. The mean value for the period is shown by the dotted line. Courtesy of M. Riuscetti.

Figure 24. Number of seismometer-saturating events/day (lower curve) and average daily tremor amplitude in volts (upper curve) at Stromboli, 25 September-31 October, 1991. Courtesy of M. Riuscetti.

A team from the Univ di Udine climbed the volcano during the first week in October. Two vents were active in Crater 1, the first near the E rim, the second a 30-m-high cone on its NW flank (facing the Sciara del Fuoco). Explosions produced plumes 100-150 m high, and ejected lapilli and sand-sized tephra. Crater 2 was still marked by two radial fissures with clearly visible night glow, but its rim was no longer well-defined. Cone 1 in Crater 3 continued to produce white vapor from two vents. The westernmost vent of Crater 3 appeared to be the most active, and was at the center of a rapidly enlarging chasm. It ejected clouds of black, sand-sized tephra to a maximum height of 300 m.

Along the ridge extending E from Pizzo sopra la Fossa (the observation point SE of the active craters), a zone that once consisted of a few isolated hot spots had evolved to a thermal anomaly in September (16:08) and a continuous line of fumaroles in October.

Information Contacts: M. Riuscetti, Univ di Udine.

March 1992 (BGVN 17:03) Cite this Report

Less-frequent eruptive episodes

Visitors ... in mid-March reported that activity was at a low level, with only a few eruptive episodes an hour. Despite frequent heavy rains, no dark, ash-laden phreatomagmatic eruption plumes were observed. When viewed from a ferryboat for about an hour during the morning of 18 March, the volcano erupted only once, shortly after 0730, otherwise emitting only a gas plume.

Information Contacts: B. Behncke, GEOMAR, Kiel.

May 1992 (BGVN 17:05) Cite this Report

Frequent explosions; increased seismicity

Seismic activity remained at a low level (around 100 explosions/day) from the beginning of 1992 through 8 April, when the seismic station was shut down for maintenance and conversion to a 3-component system. When operations resumed on 17 May, seismicity was unusually high, and the number of recorded events on 19 May was the largest since the station was installed in October 1989 (figure 25). Tremor amplitude briefly remained at November 1991 levels, but decreased rapidly beginning 20 May.

Figure 25. Seismicity recorded at Stromboli, January-May 1992. Open bars show the total number of seismic events/day, while solid bars tally those with ground velocities exceeding 100 mm/s. The line represents tremor energy computed using 60-second samples taken every hour, then averaged daily. Courtesy of M. Riuscetti.

Daily summit observations 10-19 May revealed that activity was concentrated in craters C1 (vent 1) and C3 (vent 4) with glowing tephra ejected to 100-150 m height. Noisy vapor emissions lasting 15-20 seconds, accompanied by modest spatter ejection, occurred from a fissure in C2, on the W rim. Very modest activity continued from the small spatter cone in C3.

During the night of 16-17 May, Beat Gasser saw activity from several vents. Loud explosions occurred ~4 times an hour from C1, ejecting lava to as much as 300 m height for 5-10 seconds. Several explosions typically occurred at intervals of 5-10 minutes, followed by ~30 minutes of repose. Between explosions, a steady red glow and lava spattering were visible inside the crater, with spatter seldom reaching the crater's outer walls. Spattering declined before explosions. Crater C2 produced noisy 10-15-second gas emissions about once an hour. Ejections of a few red tephra fragments from C2 were seen during the night. East of C2, a steady red glow was visible at night within a small vent that was the source of pulsing gas emissions at 3-second intervals. Eruptions occurred about twice an hour from C3, but like those from C1 were not evenly spaced. Two eruptions typically occurred roughly 10 minutes apart, followed by nearly an hour of quiet. The three active craters never erupted simultaneously, and their eruptions were separated by intervals of at least 5 minutes.

Information Contacts: M. Riuscetti, Univ di Udine; B. Gasser, Kloten, Switzerland.

June 1992 (BGVN 17:06) Cite this Report

Small explosions and seismicity continue

Fieldwork during the first week in June revealed that eruptive activity was mainly concentrated in craters C1 (vent 1) and C3 (vent 4), which fed black plumes no more than 100 m high. Seismicity remained high in June (figure 26), near the 180 events/day reached in the last third of May. A minimum of 108 events was recorded on 24 June. After declining rapidly about 20 May, tremor energy returned to levels characteristic of the period since November 1991.

Figure 26. Seismicity at Stromboli, June 1992. Open bars show the number of recorded events per day, black bars those with ground velocities exceeding 100 mm/s. The curve represents the each day's average of tremor energies on hourly 60-second samples. Courtesy of M. Riuscetti.

Information Contacts: M. Riuscetti, Univ di Udine.

August 1992 (BGVN 17:08) Cite this Report

Frequent weak explosions; tremor

Daily summit observations revealed frequent weak explosions and vigorous fumarolic activity in July and August. Tremor amplitude was constant, and seismic activity was slightly above average. Tremor amplitude, measured with hourly 60-second samples, generally averaged 0.4-0.6 v, with one day >0.7 v (3 July). There were >100 recorded seismic events per day throughout the period; 16 days had >200 events (4 August and 15-29 August); 16 and 26 August had >250. The last two weeks in August also had more strong shocks (ground velocities >100 mm/s), but no more than 50/day.

Information Contacts: M. Riuscetti, Univ di Udine.

October 1992 (BGVN 17:10) Cite this Report

Continuous spatter ejection; occasional vigorous explosions; seismicity increases

Fieldwork during the third week in October revealed that eruptive activity was mainly concentrated in crater 1, in the NE part of the summit area. Spatter ejection was nearly continuous and explosions were frequent from C1's NE vent. Rare strong explosions from vent 5 of C3 (SW summit area) fed black plumes about 200 m high. Strong noisy degassing occurred from C2, with some spatter emission from vents that had developed from pre-existing fissures. C1 had been partially filled with debris and the spatter cone in C3 destroyed.

Tremor energy and the number of earthquakes increased steadily in September and October (figure 27). The strongest activity was recorded 14-16 October with > 20 events/hour (20% with ground velocities exceeding 100 mm/s).

Figure 27. Seismicity recorded at Stromboli, September-October 1992. Open bars show the number of recorded events per day, the solid bars those with ground velocities exceeding 100 Nm/s. The curve represents tremor energy computed using 60-second samples taken every hour, then averaged daily. Courtesy of M. Riuscetti.

Information Contacts: M. Riuscetti, Univ di Udine.

January 1993 (BGVN 18:01) Cite this Report

Short series of violent explosions ejects tephra column

A short series of violent explosions occurred from the summit craters on 10 February at 1610 GMT, ejecting a large tephra column. Lithic blocks and lava fragments fell to 1 km from the summit, and heavy ashfall occurred at the village of Ginostra, ~2 km SW of the summit. Only weak degassing from the summit craters was visible during the next two days.

A sequence of three explosion earthquakes that occurred within <2 minutes of one another was recorded by the Ginostra station of the Aeolian Island Seismic Network, operated by the IIV. The last earthquake was followed by high-amplitude tremor that lasted for 8 minutes, then gradually declined. No other anomalous seismic activity was recorded during the succeeding hours, although spectral amplitude of tremor was remarkably low. No seismicity associated with the explosive activity was detected by any other stations in the IIV network. Tilt data from a shallow borehole station on the lower N flank (at Punta Labronzo) did not show any deformation suggesting significant magma storage in the volcanic edifice.

Geologists noted that the activity appears to be comparable to similar episodes in 1988 and 1989, thought to be caused by shallow gas accumulation building pressure in a feeder pipe.

Information Contacts: S. Falsaperla and L. Velardita, IIV.

February 1993 (BGVN 18:02) Cite this Report

Seismicity and tremor resume after 10 February explosions

Seismicity between 1 November 1992 and 28 February 1993 was stable except for two episodes of heightened activity (figure 28). High tremor energy and >25 explosion-earthquakes/hour were recorded on 6 December. On 10 February, three strong explosions occurred followed by a sudden decrease of tremor to about one-sixth the energy and one-third the number of events. In the week preceding the 10 February explosions, a large number of strong shocks occurred, with a maximum of 76 on 7 February. After a few days of low seismicity following the explosions, tremor amplitude and the number of events gradually increased, reaching an isolated maximum on 22 February.

Figure 28. Seismicity recorded at Stromboli, November 1992-February 1993. Open bars show the number of recorded events/day, the solid bars those with ground velocities >100 mm/s. The lines show daily tremor energy computed by averaging daily 60-second samples. Courtesy of M. Riuscetti.

Information Contacts: M. Riuscetti, Univ di Udine.

April 1993 (BGVN 18:04) Cite this Report

Explosive activity increases; detailed description of crater

Steve Matthews and Abigail Church observed vigorous Strombolian activity on 22 April at two 20-m-high hornitos in crater C1 (figure 29). Incandescent gas explosions occurred at 3-10 second intervals, followed ~0.5 seconds later by ejection of spatter. Semi-liquid bombs up to 2 m across reached up to 100 m above the vents. Stronger activity from all three craters every 10-20 minutes consisted of gas emissions lasting as long as 15 seconds that ejected spatter as high as 250 m. These stronger events appeared to occur in pairs from craters C3 and C2. An explosion from C3's vent 4 was often followed a few minutes later by an explosion from vent 2 in C2. Within C1, these larger explosions were only produced from the NE hornito (vent 1). Many incandescent fumaroles were visible on the hornitos and the floors of all three craters. Small amounts of spatter were also ejected from the fumaroles during the strongest explosive episodes. At about 2030, shortly after sunset, lava was observed flowing slowly from a breach or bocca in the SW hornito (vent 2) in C1. When observations ended at 2200, the lava flow had divided and was beginning to form a moat around the hornitos.

Figure 29. Sketch of the summit craters and vents at Stromboli, 22 April (top), and 3 May 1993 (bottom). Crater walls could not be distinguished on 3 May due to the abundant gas and steam. Vent numbers are in parentheses. Field of view is ~200 m across. Courtesy of S. Matthews, A. Church, and S. O'Meara.

A high level of eruptive activity was reported by Steve O'Meara on 2-6 May. Roaring noises could be heard in San Vincenzo (~2.5 km NE) the afternoon of 2 May, which became periodic by late afternoon, occurring about once every 20-30 minutes. A gray fountain was observed from the lower NE slopes around 1830 that rose several hundred meters above the NE-most vent. Several strong explosions later that evening sent incandescent boulders rolling down the steep slope of the Sciara del Fuoco (figure 29). There were at least 8 vents active for several hours during summit observations the night of 3-4 May. Eruptive activity increased dramatically after the nearly full moon rose, peaked when the moon culminated in the southern sky, and waned before moonset. Lunar perigee (when the moon is closest to the Earth) occurred that night around 0100.

Vent 1 in crater C1 (figure 29) was a large dome-shaped mound with a summit crater and shallow floor filled with incandescent bombs from other vent explosions. Approximately every 30 minutes a powerful explosive blast, which sounded like a large cannon firing, violently blew the debris from the vent to heights of 200-300 m. Increased crater glow preceded these eruptions and most others. C1's vent 2 is a small cone with a peanut-shaped throat adjacent to and W of vent 1. This vent was continuously active with jetting sounds, blue flames, and spatter ejection. Thin streams of lava were erupted about every 5 minutes, with larger 100-m sprays of lava about every 15 minutes. Vent 3 in C1 (E of and adjacent to vent 1) exhibited continuous glow and erupted synchronously with either vent 1 or vent 2, ejecting material to a height of ~100 m. Occasionally, vents 1-3 would erupt together. Ejecta from vent 3 was directed slightly NE, while blasts from vents 1 and 2 were directed vertically. These explosions only lasted for a few seconds. Another less-active vent in C1, S of and adjacent to vent 1, also appeared to erupt synchronously with vents 1-3 to heights of tens of meters.

In crater C2, vent 1 had three glowing components, though only the western-most one produced sporadic minor eruptions, spraying lava ~10-30 m above its steep, narrow cone. Eruptions from vent 2 in C2 occurred every 30-45 minutes and lasted 20-40 seconds each. Eruptions began with a strong jetting sound, after which a thin spray of lava would shoot out, followed by more vigorous jetting and extensive lava production. Lava fountains reached heights of up to 150 m. Lava was visible in the vent for about a minute after each eruption, with the surface continually being fractured by escaping gases. The lava would then slowly sink into the vent until it was no longer visible, although glow remained.

Two small adjacent vents in crater C3 were each surrounded by wide, shallow cinder rims. Eruptions were more frequent at the W vent, where explosions sent material 300-400 m high about every 10 minutes during the most active periods. These eruptions occurred without warning and were accompanied by a loud roaring noise. The largest eruptions from this vent produced very broad, expansive plumes shaped like large evergreen trees, which reached 30 m above the summit of the volcano. Another vent farther S may also have erupted, but that area was obscured by fumes and steam.

The frequency of eruptions from each vent changed with time, but not the sounds, making it possible to know which vents were erupting. By the morning of 3 May, activity had declined to one large explosion and a couple of smaller ones approximately every 20 minutes. During the most active periods of the night, >20 strong eruptions occurred every hour. Activity increased again after moonrise on 4 May and remained strong into the early morning. Orange glow reflected by the clouds was observed that night in San Vincenzo. The next day, powerful eruptions continued from crater C3 and vent 1 in C1, but with less frequency. Vents 2 and 3 in C1 glowed but did not have any strong eruptions. Observations ended about 2400 on 5 May. Seven eruptions were seen from the ferry 2100-2200 on 6 May.

Marcello Riuscetti reports that Stromboli guides observed a new cone in crater C1 and renewed activity at the C3 spatter cone in mid-May. On 16 May a small lava emission occurred from the base of a cone in C3. During the night the flow traveled 30 m down the slope, reaching the feeding fissure of the 1985 eruption before stopping. The flow resumed 18 May, covering ~60 m of 1985 lava NE towards the Sciara del Fuoco. Strong tremor and frequent explosions accompanied the lava flow.

Seismicity (number and energy of shocks, tremor energy) increased in March and April after the low of 11 February (18:02). The level of seismicity was very high in April (figure 30), with nearly continuous explosions in the second and third weeks.

Figure 30. Seismicity recorded at Stromboli, March-April 1993. Open bars show the number of recorded events/day, the solid bars those with ground velocities >100 mm/s. The lines show daily tremor energy computed by averaging hourly 60-second samples. The number of daily events are off the scale for the 2nd and 3rd weeks of April due to the nearly continuous explosions during that period. Courtesy of M. Riuscetti.

Information Contacts: S. Matthews, Univ College London, London; A. Church, Natural History Museum, London; S. O'Meara, Sky & Telescope; M. Riuscetti, Univ di Udine.

May 1993 (BGVN 18:05) Cite this Report

Strombolian activity decreases

The strong Strombolian activity that accompanied the 18 May eruptive phase had stopped by 28 May. Observations on 20 May revealed the end of lava emissions, intense Strombolian activity from the active vents of Crater 1 (E crater), and degassing from Crater 3 (W crater). Two cones were built by Strombolian activity inside Crater 1. The smaller one, only a few meters high, is located near the E rim. The second, located in the W part of Crater 1, was about 10 m high. Strombolian activity was continuous in the small E cone, with ejection of bombs to a few meters, but was discontinuous and stronger in the W cone. Frequent explosions sent incandescent juvenile scoriae to 100 m above the crater rim; they fell up to tens of meters away from the rim. Significant ejections occurred at intervals of about 8-10 minutes. The funnel-shaped bottom of Crater 3 was obstructed by material, resulting in strong bursts of gas at intervals of 20-30 minutes that formed a column up to 100 m high. The column consisted of brown, non-juvenile dust and blocks that fell back into the crater.

The intensity of Strombolian activity at Crater 1 was lower on 27 May than during the previous visit, with fewer explosions and a decreased volume of ejected material. However, gas explosions at Crater 3 were continuing with the same frequency and intensity. No sounds indicative of Strombolian activity were heard during about one hour of observation 28 May. Only weak degassing was observed from the vents in Crater 1. The degassing was interrupted at intervals of 15-20 minutes by noisy gas explosions that formed a plume up to 100 m high composed of brown, non-juvenile dust and a few scoria blocks. At Crater 3, activity was similar to previous visits, with gas explosions and brown dust emissions at intervals of 20-30 minutes.

Information Contacts: S. Calvari, IIV.

September 1993 (BGVN 18:09) Cite this Report

Eruptive activity and seismicity decline from high levels in May; two strong explosions in October

After the eruption of a small lava flow in mid-May accompanied by high seismicity, there was an abrupt decay of seismic activity back to "normal levels" in early June (figure 31). Stromboli guides reported very low activity at the craters, with rare ejection of black ash from crater C3 and spatter from C1 during May to August. Two strong explosions felt at 0210 on 16 October destroyed the small spatter cone in C3 that was built during the October 1990 eruption. Large blocks and spatter up to 2 m in diameter were ejected as far as 500 m from the crater, and reddish ash fell on the NW slope of the volcano along the Sciara del Fuoco. One woman was injured by hot ashes while sleeping near the crater area, and some bushes caught fire along the slopes. Tremor had begun to increase around 1100 of the previous day and then fell below the detection limits of the instruments one hour after the explosions.

Figure 31. Seismicity recorded at Stromboli, May-August 1993. Open bars show the number of recorded events/day, the solid bars those with ground velocities >100 microns/second. The lines show daily tremor energy computed by averaging hourly 60-second samples. Courtesy of M. Riuscetti.

Information Contacts: M. Riuscetti, Univ di Udine.

October 1993 (BGVN 18:10) Cite this Report

Explosive activity ejects lithic fragments and large bombs

An eruptive episode in the early hours of 23 October produced some strong explosions at both Crater 1 and Crater 2, ejecting spatter and lithic clasts. Two people who spent the night near the summit were injured by incandescent material during the explosions. Crater 1 (the easternmost of the summit craters) ejected bombs that measured up to 2 m across. Many bombs fell as far as 500 m from the craters and formed a deposit that covered the area near the vents. The eruption also destroyed an E-W line of small Strombolian cones in the crater, formed by activity in May. At least two explosions in Crater 2, with minor magmatic contributions as inferred by abundant lithics, produced a wide chasm and a small pit crater. Quiet gas emissions, with no Strombolian activity, continued through October.

Explosions from Crater 3 a week earlier, on 16 October, ejected large blocks and spatter up to 500 m from the crater. Ashfall from the Crater 3 explosions injured one woman sleeping near the crater area. Activity from mid-May through August was very low, with rare ejection of black ash from Crater 3 and spatter from Crater 1. Guides reported small explosions and negligible fumarolic activity in September. Seismicity dropped abruptly in early June, and had declined to a level of <150 events/day throughout the second half of September.

Information Contacts: S. Calvari, IIV; M. Riuscetti, Univ di Udine.

January 1994 (BGVN 19:01) Cite this Report

Seismicity continues to rise following October explosions

A sudden decrease in seismicity followed the explosive episodes of mid-October 1993. Both number of recorded events and tremor level declined to below "normal" levels (figure 32). This pattern is similar to that observed after other explosive episodes in February and May 1993. Although problems with solar panel efficiency resulted in data loss during December, an increase in seismicity was still evident through December 1993 and January 1994.

Figure 32. Seismicity recorded at Stromboli, October 1993-January 1994. Open bars show the number of recorded events/day, the solid bars those with ground velocities >100 microns/second. The lines show daily tremor energy computed by averaging hourly 60-second samples. Courtesy of R. Carniel.

Information Contacts: R. Carniel, Univ di Udine.

March 1994 (BGVN 19:03) Cite this Report

Normal Strombolian activity; crater descriptions

"On two of three visits during 9-12 March, very detailed observations of crater morphology and eruptive activity were made. The volcano continues its millennia-long eruption; the intensity of the current activity is considered normal and characteristic of Stromboli's persistent activity. A brief visit to the Pizzo Sopra la Fossa (figure 33) was carried out on the afternoon of 9 March, but due to dense weather clouds few visual observations were possible. The noise of explosions was audible every 10-15 minutes, and continuous lava splashing could be heard. Breaks in the cloud cover revealed vigorous degassing in the entire crater area.

Figure 33. Sketch map of the crater area at Stromboli. Bold numbers indicate craters, smaller numbers are vents. Courtesy of B. Behncke.

"The second summit climb and overnight stay was undertaken during much improved weather conditions, from about 1700 on 10 March until 0700 the next morning. The active craters were observed from the beginning of the visit until 0200 on 11 March. Observations were made at close range from the rim of crater 3 (the SW-most active crater) from 2130 until 2300. Eruptions from at least 3 vents all produced largely ash-free lava fountains that rose <=150 m. Vent 4 in Crater 3 (figure 34) ejected low lava fountains about every 10 minutes between 1700 and 2000, but then remained inactive for several hours. The eruptions made little noise, similar to eruptions from the same vent during visits in September 1989, March and November 1990, and August 1991. Another vent (1 & 2) was present in the NE part of Crater 3, at the location where several small incandescent pits and conelets existed in 1990-91. However, there is now a larger and deeper pit with much more vigorous activity. The pit is roughly circular and has a diameter of about 30-50 m; its bottom (and active bocca) is not visible from any accessible place on the crater rim. Nonetheless, it appears probable that there is an active, vigorously spattering lava pond in the pit.

Figure 34. Sketch of Stromboli's crater 3 seen from the SE rim of crater 1, 12 March 1994. Made from a composite photograph. View is to the SW. Courtesy of B. Behncke.

"During the 90-minute observation from the crater rim, remarkable fluctuations in pit activity were seen. There would be a period of very low-level activity (up to 5 minutes long) when little or no spatter was thrown above the pit lip. Then bombs and spatter would be obliquely projected against the S wall of the pit for several minutes. This was followed by more vigorous vertical fountains of gradually increasing height. For ~ 10-20 minutes there would be a stupendous display of such fountains until a sequence of very large fountains (up to 100 m high) marked the end of increased activity. The heat of the large fountains could be felt on the crater rim; fortunately, no bombs fell closer than 25 m to the vantage point. Three such large fountains, or fountaining sequences, were observed during the stay on the crater rim.

"Crater 2 was inactive and not visible, but vent 4 at the SW end of Crater 1 had very violent and loud eruptions every 20-30 minutes, sometimes at shorter intervals. These eruptions began instantaneously with crashing sounds and ejection of a very thin, tall, vertical incandescent column. Within ~1 second, another fountain would shoot obliquely from a second vent a few meters away and jet right through the first column; these eruptions lasted <5 seconds. Several of them were followed within the next few minutes by a series of up to four more eruptions of gradually decreasing intensity. Many bombs from the oblique fountains fell into the adjacent pit with continuous spattering. Similar activity continued after our departure to make observations from Pizzo Sopra la Fossa. Loud crashing noises from vent 4 of Crater 1 were frequently heard during attempts to sleep below the observation platform and the next morning when descending towards the village of Stromboli.

"The summit was climbed a third time during daylight on 12 March, and a visit was made to the craters from 0900 until 1100. All of the craters are significantly deeper than during visits in March 1990 and August 1991. The pit (vent 1 & 2) in Crater 3 (figure 34) was still continuously spattering and ejecting small lava fountains, but there were fewer large fountains. Vent 4 in Crater 3 ejected low lava fountains ~ 3 times, but was hidden by dense gas-and-steam clouds most of the time. Striking changes have occurred in Crater 1, probably during the violent explosions of October 1993. All cinder cones observed within this crater in 1990-91 have vanished; now there is an elongate chasm up to 60 m deep that appears to have a large but inactive fissure on its floor. An irregularly shaped vent in the NE portion of the crater, not active 10-11 March, erupted several times. These eruptions had durations of up to 30 seconds and produced low (~50 m) fountains mixed with very dense steam-and-gas plumes and accompanied by relatively loud rumblings. The gas plumes made the stay on the crater rim inconvenient but did not cause other problems.

"The most impressive eruptions came from vents 3 & 4 at the SW end of Crater 1. These vents lie within a larger depression of highly irregular shape; one bocca continuously emitted a bluish gas column at high pressure from a mouth maybe 2 m in diameter. Most eruptions came without any warning, especially when gas plumes caused poor visibility. However, several were preceded by brief roaring noises. The eruptions themselves began with immense crashing noises that were heart-rending at a distance of <= 50 m. Initially a diffuse ash plume would boil up from vent 3 and turbulently shoot to ~ 50 m, then large but continuously fragmenting incandescent lava lumps would be ejected at extremely high velocity. Great turbulence within the rising fountain violently tossed and turned the bombs, which therefore did not travel along the parabolic trajectories commonly observed during Strombolian eruptions. At times there were very loud but brief gas emissions from this vent that did not develop into eruptions; one particularly violent eruption was followed by several minutes of powerful degassing.

"After the end of the 12 March summit visit, ash plumes from vent 4 in Crater 1 became more common. During departure from the island on the morning of 14 March, a dense brown ash plume rose several hundred meters above the weather clouds that covered the summit."

Information Contacts: B. Behncke, Geomar, Kiel, Germany.

June 1994 (BGVN 19:06) Cite this Report

Variable seismicity, but generally low; moderate-low activity in late May

Visual observations made by volcano guides in April indicated moderate activity, in terms of both the number and vigor of the eruptions; pyroclastic material rarely reached the crater rims. Field work was carried out by R. Carniel, F. Iacop, and G. Salemi (Univ of Udine) during 21-26 May to study the correlation of external activity with seismicity. Activity at the middle crater (C2) during this period consisted of fumarolic emission, without explosions or strong degassing. The lava pond in the pit of the SW crater, C3 (19:03), displayed almost continuous spattering. This activity could be distinguished by short intervals of noise during the day, and by the red light reflected by gas above the vent at night. Neither the surface of the pond nor the fumarolic vents could be seen. Two other vents closer to the SW rim of C3 were also active. The first exhibited moderate explosions. The other produced a few small explosions on 22 May, but much more vigorous activity on 24 and 25 May during the peak of tremor (figure 35), with high, black, mushroom-shaped columns; pyroclastic material fell well beyond the SW crater rim. Within the NE crater (C1), two active vents were observed. The first, in the front towards the Sciara del Fuoco, was characterized by very high and long-lasting explosions (up to 40 seconds). The second, near the boundary with C2, produced smaller explosions; during the night a pale reddish light could be seen that intensified a few seconds before the explosions began.

Figure 35. Seismicity recorded at Stromboli, May-June 1994. Open bars show the number of recorded events/day, the solid bars those with ground velocities >100 microns/second (instrument saturation level). The line shows daily tremor energy computed by averaging hourly 60-second samples. Courtesy of Roberto Carniel.

Following increased seismic activity in December 1993 and January 1994 (19:01), seismicity was fairly constant through February and early March (<150 events/day), except for a brief increase during the last week of February (150-200 events/day). On 10 March, the number of seismic events/day increased into the 200-250 range, and remained at that level through the end of the month. Starting on 12 March, tremor energy showed a general decrease, and the number of stronger seismic events increased. This pattern has been observed several times in the past. The summit visits by Boris Behncke on 9-12 March (19:03) took place during a period of higher seismicity and tremor energy. Seismicity decreased again on 28 March to around 200 events/day, and by 10 April the rate had declined to ~ 150 events/day; it remained stable at that level for two weeks. Tremor amplitude during this time fluctuated, but generally seemed to decrease. Tremor energy increased abruptly on 25 April, just two days before the number of daily events again decreased.

During May, tremor energy decreased and the daily number of events increased compared to April, peaking at 173 events on 8 May (figure 35). Tremor energy increased from 14 to 24 May before steadily decreasing in June. The number of recorded events increased in June, with a maximum of 175 on 10 June, and there were a considerable number of larger events. Larger events are defined as those with ground velocities >100 mm/second, saturating the seismic station located very close to the crater area.

Information Contacts: R. Carniel, Univ di Udine.

September 1994 (BGVN 19:09) Cite this Report

Intense activity from ten vent locations

Extraordinarily intense activity was observed 21-22 August during an ascent and 8 hours on the summit (Pizzo sopra la Fossa). Significant morphologic changes had taken place in the crater area since March 1994 (19:03). Due to the vigorous activity, the craters could not be approached; however, the position and shape of eruptive vents were visible due to the filling of the craters. During the observation period, 10 boccas produced eruptions (compared with 4 in March), most of which were generally clustered and showed sympathetic to simultaneous activity. There were rarely any 10-minute intervals without eruptions, and for periods of up to several hours there was continuous lava fountaining from up to 3 vents at the same time. There was no regularity in the succession, size, or timing of the eruptions. Crater 2 was inactive.

Crater 1, the NE-most active crater, had 6 active boccas, most of which had formed spatter cones. None of these cones had been present during the crater visits in March; during the present visit, however, Crater 1 was filled almost to its rim with cones and erupted pyroclastics. Growth of these spatter cones since March had been much more vigorous than the formation of the earlier cones (1986-93), which were destroyed by explosions in October 1993. Only 5 months before this visit, Crater 1 had been a deep (>60 m) chasm, with no indication of incipient cones. The new cones were, after only 5 months of growth, larger than the pre-October 1993 cones.

The northernmost two vents, 1A and 1B, formed a broad, flat cone ~5 m high that displayed continuous incandescence. Vent 1A formed a crater 5-10 m wide on top of the cone and was the site of frequent brief lava fountains, but also had periods of quasi-continuous lava jetting and spraying. The focus of the explosions was apparently very close to the surface judging from the broad angle of the jets that sprayed large clumps of lava over a wide area, thus contributing to the broad, flat shape of the cone. The largest fountains from 1A rose higher than Pizzo sopra la Fossa, maybe to heights of 250 m. Vent 1B on the NE flank only became active towards the closing stages of the largest eruptions of 1A, ejecting a narrow fountain obliquely NE.

A cluster of vents was present in the central part of Crater 1, the most active among them (2A) was located on top of a tall, steep, spatter cone about 20-25 m high. Vent 2A (diameter <=3 m) was the site of activity ranging from continuous spattering to vigorous, long-lasting fountains that reached heights >250 m. There were at least four periods of continuous and vigorous fountaining, at 1930-2000, 2300-2400 (21 August), 0100-0200, and 0700-0800 (22 August), spraying rapid successions of lava 100 m above the vent and producing a continuous loud roaring sound. All fountains from 2A were vertical and relatively narrow. Frequently the entire cone was covered by cascading spatter forming small, rootless flows. Towards the morning of 22 August, the upper ~3 m of the cone was destroyed by vigorous gas emissions and explosive fountaining. Vent 2B, on the SE flank of cone 2A, was somewhat wider (<=5 m) and had formed a low, flat conelet. Its activity was restricted to minor oblique ejections of spatter towards the E that always preceded major activity from cone 2A. A very small incandescent vent (2C) was present on the S flank of 2A; it did not eject any solid material.

In the SW sector of Crater 1, two similarly shaped spatter cones (3A & 3B) were each ~10 m high. They were at the site of the twin boccas of March (labeled ##4 at that time). The activity of these boccas was stupendously symmetrical, producing a pair of equally shaped narrow, tall (> 100 m) vertical fountains of equal height, initially of bluish burning gas followed by the ejection of lava fragments. Magmatic eruptions lasted up to 15 seconds and were accompanied by very loud crashing noises.

Crater 3, largely filled with new pyroclastic material, had two principal eruptive sites that had not developed into cones due to the wide dispersal of ejecta beyond the crater. Vent 1 lay in the NE part of Crater 3, at the site of the pit containing the active lava pond 5 months earlier. The vent was very small (<=3 m diameter) and had built a low mound of very large agglutinated bombs to above the almost level surface of pyroclastics filling the crater. Activity from this bocca was highly irregular, with repose periods of >30 minutes, and continuous fountaining episodes up to 60 minutes long. Larger fountains every 10-45 minutes sprayed incandescent tephra up to 150 m high. During periods of continuous fountaining, the focus of the explosions migrated towards the surface, as evidenced by the increasingly wide angle of the fountains. The vent area was covered by a continuous sheet of incandescent spatter, but no lava outflow took place.

The most impressive eruptions took place from a cluster of three closely spaced, continuously incandescent vents (2) at the SW end of Crater 3, probably corresponding to vents 3 and 4 in March (19:03). Eruptions began instantaneously and sent very broad jets to heights of up to 300 m, covering an area far beyond the crater rim. During daylight, some of these eruptions produced spectacular plumes that rose up to 500 m above the vents (350 m above the summit). The eruptions made little noise, but sometimes produced heat waves that could be intensely felt on Pizzo sopra la Fossa. At times, two eruptions occurred within a 5-minute period, whereas others were separated by up to 60 minutes.

During the week preceding and 10 days after the visit, occasional large ash puffs (up to 350-400 m above the summit) were seen from neighboring islands, and frequent lava fountains were seen at night from N Lipari Island (26 August) and Alicudi Island (30-31 August), indicating that Stromboli was in a state of increased activity at least from mid-August until the end of the month.

Information Contacts: G. Giuntoli and B. Behncke, GEOMAR, Kiel, Germany.

October 1994 (BGVN 19:10) Cite this Report

High seismicity during July-September; eruptive activity described

Following the slow decrease of tremor energy during June, all seismicity increased in July (figure 36). Tremor energy reached an unusually high peak on 27 July; at the same time, a peak in the number of events was recorded. Although more events were recorded on 19 July (864), that was a period of almost continuous explosive activity. A considerable number of saturating events were recorded after 20 July. Volcano guides observed very strong external activity, with pyroclastic material often reaching the usual tourist zones. A decline in tremor energy was observed after 10 August; a slow increase then followed, reaching a new maximum at the end of the month. The number of recorded events followed a similar trend. Another major decrease in tremor energy characterized the first half of September; later fluctuations remained in a "low-energy" range. Vigorous eruptions seen on 21-22 August occurred during a period of low seismicity compared to late July and late August.

Figure 36. Seismicity recorded at Stromboli, 27 June-29 September 1994. Open bars show the number of recorded events/day, the solid bars those with ground velocities >100 Nm/s (instrument saturation level). The line shows daily tremor energy computed by averaging hourly 60-second samples. The seismic station is located 300 m from the craters at 800 m elevation. Courtesy of R. Carniel.

Observations of crater activity were made by R. Carniel (Univ of Udine) during field work with R. Schick (Univ of Stuttgart) and collaborators at the end of September and early October. Similar observations were made by geologists from Open Univ during 1-13 October, with detailed explosion counts for 3 hours on 1 October, 4 hours on the 5th, and one hour on the 9th. Explosions sent incandescent ejecta, ash, and/or gas to heights of <=300 m, from as many as 10 active vents (figure 37). No active vents were observed in Crater 2, but a hornito (2/1) was visible, and there was minor degassing from an unknown source. Brightness temperatures of fumaroles along the zone E of the Pizzo Sopra la Fossa (39-77°C) were measured by Open Univ geologists with a Minolta/Land Cyclops Compac 3 hand-held radiometer (8-14 mm).

Figure 37. Sketch map of the active craters at Stromboli, 1-13 October 1994. Courtesy of A. Harris [and A. Maciejewski].

Within Crater 1 in late September, Carniel noted three cones ~25 m high that had been built during the very strong activity in July and August (1/5, 1/6, & 1/7; figure 37). Continuous red glow at night could be seen from the top of each. Two other Crater 1 vents were active, the first (1/4) producing short, lateral explosions with large pyroclasts ejected onto the Sciara del Fuoco, and the second closer to Pizzo producing longer and higher explosions (<=200 m). Directed explosions suggested the possibility of a third vent close to the second one. When one of the two W-most cones in Crater 1 erupted (typically with strong degassing and little pyroclastic material) the other exhibited weak degassing. When the second vent erupted, the red glow from the remaining cone strengthened, sometimes with minor degassing.

Crater 1 contained six active vents during visits by Open Univ scientists. Explosions from vents 1/1 (~2/hour), 1/2 (4-9/hour), and 1/3 (0-2/hour) sent incandescent ejecta, occasionally with ash, to heights of 30-250 m. Glow was seen above 1/1 and 1/2 on the night of 5 October. Up to 40% of the ejecta from 1/2 and 1/3 fell outside of the crater area. These explosions were often followed by a gradually fading gas-jet noise of variable length. Explosions seen by the Open Univ team from 1/4 (2/hour) sent incandescent ejecta, including bombs and spatter, 30-150 m E onto the Sciara del Fuoco. On 5 October hornito 1/5 was the source of gas-jet eruptions, and a small amount of incandescent ejecta rose ~50 m; during 10 October more ejecta were seen in 100-m-high gas jets. Hornito 1/7 constantly degassed, and its summit vent was incandescent with a continuous gas flare 1-2 m high. On 10 October this flare increased 1-2 seconds before vent 1/3 erupted. Hornito 1/6 and vents 1/8 and 1/9 vents were only degassing.

The lava pond in Crater 3 had become a small spatter cone (3/2) when observed by Carniel, with a hole through which magma could be seen; activity was limited to degassing. One vent produced high, black, mushroom-shaped columns, and the second (in front towards Pizzo) sent pyroclasts >200 m above the craters. The opening of a new vent was also observed. Explosions from Crater 3 on 28 September were stronger, although less frequent, than from Crater 1. On 5 October the same sequence was observed, with the second vent exploding first and fewer pyroclasts ejected near the end of the explosion by a very small vent to the right of the older one. Guides reported that this vent was first observed on 1 October, when similar explosions from the small vent ejected spatter.

Open Univ geologists noted that only vent 3/2 was active on 1 October, with 3 emissions/hour of brown ash and blocks. By 5 October the quantity of ash emitted had decreased, but the amount of incandescent ejecta had increased, and more frequent explosions (5/hour) were accompanied by loud detonations. Ejecta rose 80-300 m, with some material landing outside of the crater or on the inner crater wall. During night observations on 5 October vent 3/2 would start erupting ~1-3 seconds after 3/1. On 8 October, Crater 3 released gas, sometimes accompanied by minor amounts of ejecta <30 m above the crater rim, and small brown ash clouds 30-100 m high. Similar activity on 9 October was accompanied by an increasing amount of brown ash and incandescent ejecta. During 1 October small lava fountains from vents 3/3 and 3/4 were simultaneous with gas emissions from 3/3. Vent 3/4 was also continuously active with puffs of gas (~1/second). The interior of vent 3/4 was incandescent by day, and glow was observed above 3/1, 3/2, and 3/3 at night. During the night of 5 October the brightness temperature of 3/4 was measured as 873°C, using a Minolta/Land Cyclops 152 hand-held radiometer (0.7-1.1 mm), similar to October 1988 (13:11). Incandescent gas puffs were seen above 3/4 during the night of 10 October. Only minor gas emission was observed from vent 3/5.

Information Contacts: R. Carniel, Univ di Udine; A. Harris and A. Maciejewski, Open Univ.

January 1995 (BGVN 20:01) Cite this Report

Seismicity low and stable in late 1994

After the decrease in tremor intensity that characterized the first half of September, seismic activity generally remained stable from the end of September to mid-December (figure 38). Tremor intensity remained fairly constant, and the number of recorded events was consistently ~300/day. In contrast, the number of major events (ground speed >100 µm/s) started increasing in October and reached a maximum of 141 shocks on 14 November, which represents 38% of the total number of recorded events. This could suggest a greater source depth for the explosions, although visual observations are needed to support this hypothesis. The number of major shocks then decreased to "normal" values during late November and early December.

Figure 38. Seismicity recorded at Stromboli, 1 October-9 December 1994. Open bars show the number of recorded events/day, the solid bars those with ground velocities >100 µm/s (instrument saturation level). The line shows daily tremor energy computed by averaging hourly 60-second samples. The seismic station is located 300 m from the craters at 800 m elevation. Courtesy of Roberto Carniel.

Information Contacts: R. Carniel, Univ di Udine.

April 1995 (BGVN 20:04) Cite this Report

Explosion on 5 March and tremor; crater observations

Due to funding problems, the power supply to the 3-component summit seismic station maintained by the University of Udine was interrupted from 10 December 1994 until 13 January 1995. The previous report (BGVN 20:01) described seismic activity through 9 December. This station has been operating since 1989, but may be permanently shut down in June if funding is not continued.

Stromboli island was visited by Giada Giuntoli and Boris Behncke on 19-24 April. Generally, the volcano showed much less activity than during a previous visit in August 1994, but an increase was evident on 23 April, resulting in the resumption of eruptions from Crater 1, which had been inactive for several weeks. Behncke also provided a review of crater morphology changes since 1989.

Seismicity, early 1995. Throughout 13 January-4 April the daily number of shocks remained roughly constant at 200-400 (figure 39). On 26 February tremor intensity began to decrease, and for a few days its average value remained stable below 3 Volts x seconds (Vs). However, the number of major shocks remained high. On 5 March a large explosion accompanied the return of tremor intensity to more usual values of around 5 Vs. The explosion threw pyroclastic material towards Forgia Vecchia and Fossetta, a depression SW of the crater area. The ejecta rose high enough to be clearly seen from the village of Stromboli, where the explosion was strongly felt. Tremor level continued to increase following the explosion; after a short decrease it quickly increased again to a peak of 10.8 Vs on 30 March. The number of major shocks decreased after 13 March. The increase in tremor intensity after the 5 March event did not match the behavior recorded after the explosions of 10 February 1993 and 16 October 1993 (BGVN 18:01, 18:02, and 18:09). On those occasions a remarkable decrease of all seismicity, and of the tremor level in particular, was noted immediately afterwards.

Figure 39. Seismicity recorded at Stromboli, 13 January-4 April 1995. Open bars show the number of recorded events/day, the solid bars those with ground velocities >100 micron/s (instrument saturation level). The line shows daily tremor energy computed by averaging hourly 60-second samples. The seismic station is located 300 m from the craters at 800 m elevation. Courtesy of Roberto Carniel.

Activity on 20 April 1995. During a summit visit on 20 April between 0000 and 1500, activity was low compared to previous visits (September 1989, March and November 1990, August 1991, and March and August 1994); only three vents were erupting, in contrast to 10 in August. A detailed record of the eruptions was made for ~4 hours (table 2). The most notable change was the almost complete inactivity of Crater 1 (figures 40 and 41), which had contained at least six erupting vents in August 1994. Only vent 1/3 displayed some brief weak explosions, mostly of burning gas carrying a few incandescent fragments from the conduit walls. Crater 2 was not erupting, as in March and August 1994, but was the site of loud gas emissions.

Table 2. Eruptive activity at Stromboli observed between 0800 and 1210 on 20 April 1995, from Pizzo sopra la Fossa. Courtesy of Boris Behncke.

The most active vents were in Crater 3. Vent 3/1 activity consisted of almost continuous low spattering from a small lava pond with occasional bursts to ~60 m above the vent; similar activity was seen in March 1994 (BGVN 19:03). Rare bursts of large incandescent lava clots (up to 5 m in diameter) were accompanied by faint "whooshing" noises. Only twice were bombs ejected beyond the pit of 3/1, onto the NE wall of Crater 3. Eruptions from vent 3/2 occurred at intervals ranging from 2 minutes to >1 hour (see table 1), with periods of more frequent eruptions alternating with periods of very low activity. For example, six eruptions occurred during a 25-minute period (0845-0910), while from 0910 until 1210 there were only five more. Some of these eruptions consisted of loud gas emissions with very low spatter fountains, but most produced incandescent fountains 80-100 m high. Between sunrise on 20 April (at about 0700) and noon, the eruptions produced ash plumes up to 250 m high. Most of the ejected material fell back into the pit, but sometimes the entire NW rim of Crater 3 was covered with pyroclastics, and bombs rolled down the Sciara del Fuoco.

Activity on 21 and 23 April 1995. When observed from Punta Labronzo, on the N side of the island, on the evening of 21 April activity consisted of frequent low lava fountains from vent 3/2 and fluctuating incandescence over vent 3/1. Small ash plumes produced by eruptions from 3/2 were driven down the Sciara del Fuoco by strong winds. A dramatic change was evident late on 23 April, when the volcano was again observed from Punta Labronzo. Crater glow was much more intense, though still intermittent, and a persistent glow was visible at a small spot in the gap on the NE rim of Crater 1 (formed by the 5 March explosion). Vent 3/2 erupted as during the preceding days with somewhat larger ash plumes. However, a vent in the N part of Crater 1 ended the period of unusual inactivity of this crater, erupting spectacularly at intervals of 10-25 minutes. These eruptions were very brief (< 5 seconds) and produced cannon-shot-like bangs. Narrow incandescent columns rose obliquely to at least 150 m above the vent before falling onto the Sciara del Fuoco, depositing abundant incandescent material on the steep slope. For 3-5 minutes, incandescent material would cascade down to about half of the Sciara's extension, with a few large blocks tumbling farther. None appeared to reach the sea during the 1-hour observation period.

Figure 40. Sketch map of the summit area of Stromboli, April 1995, showing the three craters and locations of vents. Courtesy of Boris Behncke.

Morphologic changes occur almost continuously, with alternating constructive and destructive processes. Periods of spatter-cone growth and crater filling usually last from a few months to several years and are followed by either crater-floor subsidence or explosive disruption of the cones. Cone growth was continuous from at least 1989 (maybe 1986) until October 1993, interrupted only by small-scale cone collapse and minor explosions. At the same time, the craters were filled to their rims with tephra and minor lava flows (as in May 1993; BGVN 18:04). Two large explosions in October 1993 blew out all of the material from the craters, leaving deep (>60 m) and wide chasms with near-vertical walls, still present in March 1994 (BGVN 19:03). New spatter cones grew rapidly during unusually vigorous activity in the summer and autumn of 1994, reaching much larger dimensions than the 1989-93 cones. In March 1995, parts of these cones were again removed by powerful explosions similar to, but smaller than, the October 1993 explosions. Also during early 1995, subsidence in Crater 3 created two pits at least 50 m deep.

Crater 1 has been the site of the most pronounced spatter-cone growth during 1989-95. Very small cones rarely formed at vent 3/1 and within the one vent of Crater 2. Most of the filling of craters 2 and 3 was due to the accumulation of pyroclastics. Three large, steep-sided cones and several smaller ones grew within Crater 1 between March and August 1994, the largest at vent 1/2 in the central portion of the crater, reaching ~30 m above its base. A powerful explosion in March 1995 blew out a pit 60-70 m in diameter and some 40 m deep with vertical walls, removing half of the cone (figure 41), and exposing the now-inactive conduit. Some of the smaller 1994 cones were also destroyed during the March explosion. The "twin cones" above vents 1/4 and 1/5 had grown much larger since August 1994, reaching ~25 m above their bases. Crater 2 had changed little since the summer of 1994. The small (~5 m high) hornito in its center, first observed in October 1994 (BGVN 19:10) was still present.

Figure 41. View of the crater terrace from Pizzo Sopra la Fossa, 20 April 1995. Courtesy of Boris Behncke.

Crater 3, which had been filled with pyroclastics in August 1994, had two major depressions at the sites of vents 3/1 and 3/2. These depressions differ from the explosion pit in Crater 1, lacking its vertical walls and sharp rim, and may have formed in response to the lowering of the magmatic column sometime during November 1994 when the period of high-level activity ended. Another major change since 1989 is the significant upward growth of the entire crater terrace, most notable on the NW side facing the Sciara del Fuoco. This change is also evident on the profile views of Crater 1 taken from an observation point ~400 m NW (figure 42). Since the early and mid-20th century, the crater terrace has grown upwards by 50-100 m, completely burying the formerly conspicuous Filo di Baraona (figure 40), a frequently cited reference point in older literature at the SW end of the crater terrace. The highest point of the crater terrace is the SW rim of Crater 3, lying at ~780-800 m elevation (some 40 m above its NE rim), at the site of the former Filo di Baraona. This is significantly higher than the ~725 m estimated by Hornig-Kjarsgaard and others (1993).

Figure 42. Comparative profile views of Crater 1 from the NE, illustrating the repeated growth and destruction of spatter cones between September 1989 and April 1995. The June 1993 sketch is based on photographs taken by Jon Dehn (Geological Survey of Japan, Hokkaido) and shows two lava lobes (arrows) from the vigorous May 1993 activity extending downslope. Courtesy of Boris Behncke.

Reference. Hornig-Kjarsgaard, I., Keller, J., Koberski, K., Stadlbauer, E., Francalanci, L., and Lenhart, R., 1993, Geology, stratigraphy and volcanological evolution of the island of Stromboli, Aeolian arc, Italy: Acta Vulcanologica, v. 3, p. 21-68.

Information Contacts: Roberto Carniel, Dipartimento di Georisorse e Territorio, via Cotonificio 114, I-33100 Udine, Italy; Giada Giuntoli and Boris Behncke, GEOMAR Research Center, Dept. of Volcanology and Petrology, Christian-Albrechts-Universitat zu Kiel, Wischhofstr. 1-3, 24148 Kiel, Germany.

May 1995 (BGVN 20:05) Cite this Report

Slight late-May increase in seismicity; crater observations

During the 5 April-10 June reporting interval, the most significant low-intensity period occurred between 27 April and 6 May (figure 43), with an average of

Figure 43. Seismicity reported at Stromboli, 5 April-10 June. Open bars show the number of recorded events/day, the solid bars those with ground velocities >100 µ/s (instrument saturation level). The line shows daily tremor energy computed by averaging hourly 60-second samples. The seismic station is located 300 m from the craters at 800 m elevation. Courtesy of Roberto Carniel.

The second half of May was characterized by a reprise in the total number of recorded events and in the number of events that saturated the acquisition system with ground velocities >100 µ/s. Access limitations imposed by local authorities were also eased once normal activity resumed. A slight decrease in seismicity characterized early June, with ~100-150 events/day, only a few major shocks, and continued low tremor intensity.

During 30 April-1 May Richard Pichl observed powerful explosions every 20-25 minutes, throwing incandescent ejecta from a vent in the northernmost part of Crater 1 onto the Sciara del Fuoco, each accompanied by a "cannon-shot" bang. There was also a 1-2 hour period of continuous, violent degassing. A second vent in the central part of the crater erupted once during the night. Black ash plumes rose from Crater 3; no incandescent ejecta were reported. There was unusually strong fumarolic activity from numerous sources within Crater 1.

On the night of 23-24 May, SVE members observed explosive Strombolian activity from the Pizzo Sopra after several weeks of weak activity. Vent 3/1 (figure 44) was the most active, with intense explosions at ~15-25 minute intervals sending incandescent ejecta ~100 m above Crater 3. Vent 3/2, inside a small depression, displayed continuous incandescence with occasional weak jets and sprays of lava. Vent 1/1 ejected incandescent material every 30-40 minutes SW onto the outer slope of Crater 1 and the rim of Crater 3. Two hornitos near the rim of Crater 1 exhibited weak vapor emissions. Yellow sulfur deposits surrounded several fumaroles on the external slope of Crater 1. Intense vapor emissions from several areas in Craters 1 and 3 were observed.

Figure 44. Sketch of the active craters at Stromboli, 24 May 1995. View is from the Pizzo Sopra looking NW. Courtesy of H. Gaudru.

Information Contacts: Roberto Carniel, Dipartimentto di Georisorse e Territorio, via Cotonificio 114, I-33100 Udine, Italy; Henry Gaudru, Societe Volcanologique Europeenne (SVE), C.P. 1 - 1211 Geneva 17, Switzerland; Richard Pichl, Institute of Hydrogeology, Engineering Geology & Applied Geophysics, Charles University, Prague, Czech Republic.

August 1995 (BGVN 20:08) Cite this Report

Seismicity generally low from mid-June to mid-September

The only significant tremor variations for the period 11 June-15 September 1995 were between 12 and 27 August, when intensity slowly increased (figure 45). The following four days were characterized by a rapid return of the intensity to the range observed throughout the first half of September. Noteworthy is the lack of a great number of saturating events compared to March-May 1995, including the 5 March explosion (BGVN 20:04 and 20:05). This means that explosive activity was either less energetic or shallower, with more energy released towards the air than into seismic waves. The total number of recorded events showed a rise and subsequent decrease from mid-June to mid-July, with two minima of 50-60 events/day and a maximum of almost 200 events/day recorded on 26 June. Another rise starting in mid-July reached values of 260-280 events on 28 August and 3 September, corresponding to the period of decreased tremor.

Figure 45. Seismicity detected at Stromboli, 11 June-15 September 1995. Open bars show the number of recorded events/day, the solid bars those with ground velocities >100 µ/s (instrument saturation level). The line shows daily tremor energy computed by averaging hourly 60-second samples. The seismic station is located 300 m from the craters at 800 m elevation. Courtesy of Roberto Carniel.

Information Contacts: Roberto Carniel, Dipartimento di Georisorse e Territorio, via Cotonificio 114, I-33100 Udine, Italy.

December 1995 (BGVN 20:11) Cite this Report

Low-level ash plumes and lava fountains during September-October

In contrast to very intense activity seen in summer-autumn 1994, Boris Behncke noted that activity remained low from early 1995 through October. The low level of activity, also shown by seismic data acquired by the University of Udine (see recent Bulletins), was interpreted by some researchers as a possible precursor of a more powerful eruption in the near future, resulting in a warning and access restrictions in April-May.

Eruptions during August-October produced low lava fountains and ash plumes. Activity from vent 3/1 (figure 46) consisted of night glow and spatter ejections, at times throwing bombs outside the crater. Vent 1/1 had periods of vigorous lava fountaining, often dropping incandescent bombs on the Sciara del Fuoco, particularly in early September. During dry weather, a dense gas plume often formed a hazy layer at 850-900 m altitude that extended for tens of kilometers.

Figure 46. Map of the crater terrace at Stromboli, 19-20 September 1995, showing active vents. The map was produced using EDM and triangulation measurements. Vent numbering is consistent with sketch maps from April 1995 (BGVN 20:04). Courtesy of Andy Harris and Nicki Stevens.

During a 19-20 September visit by Andy Harris and Nicki Stevens, activity was observed from five vents (figure 47). A 4-m-diameter vent in the side of a hornito (1/4), had incandescent walls and an internal temperature of 940°C, as measured with a Minolta/Land Cyclops 152 infrared (0.8-1.1 µm) thermometer. Gas-jet eruptions from this vent sent incandescent gas and minor ejecta ~50 m high. Regular explosions from vents 1/2 and 3/2 ejected bombs and brown ash clouds up to ~100 m. Seven eruptions during a 90-minute period from vent 2/1 sent bombs to a height of ~50 m. No explosions were seen from vent 3/1, but it exhibited continuous night glow and apparently quietly ejected a few bombs to no more than 10 m above the crater rim.

Observations by Behncke on 28-29 September showed that craters 2 and 3 had not changed significantly since a visit on 20 April (BGVN 20:04). Vent 3/1 showed fluctuating glow at night but had no ejections. Vent 3/2 had very weak emissions of reddish ash every 5-20 minutes. Crater 1 had been largely filled with small spatter cones during the summer of 1994, but their destruction began with a powerful phreatic explosion on 5 March 1995 (BGVN 20:04). However, the twin cones (1/4 & 5) in vent area 1/3 remained. Neither of them had erupted after September/October 1994, but an incandescent vent (~10 m wide) at the SE base of the SW cone (1/4) had brief noisy gas explosions that emitted a diffuse incandescent gas cloud.

Vigorous eruptions observed by Behncke from vent 1/1 ejected black ash plumes that occasionally rose >100 m. After dark, incandescent ejections were seen, and loud roaring noises were audible. Reports by other observers in early October disclosed continuing low-level eruptions from vents 1/1 and 3/2 and incandescence from vents 1/3 and 3/1. In addition to the vents active in September, a vent behind the twin cones in Crater 1 and a vent in the NW part of Crater 3 were active when observed by Open University geologists on 15 and 30 October.

Information Contacts: Boris Behncke and Giada Giuntoli, Department of Volcanology and Petrology, GEOMAR, Wischhofstr. 1-3, 24148 Kiel, Germany; Andy Harris, Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, United Kingdom; Nicki Stevens, ESSC, University of Reading, P.O. Box 227, Reading RG2 2AB, United Kingdom.

February 1996 (BGVN 21:02) Cite this Report

Intense eruptive phase followed by a drop in seismicity

The following presents previously unreported observations of October 1995 activity made by Roberto Carniel (University of Udine), and seismicity recorded near the summit since mid-September 1995. A contribution from the Istituto Internazionale di Vulcanologia (IIV) provides information about a significant explosive event on 16 February.

October 1995 activity. Abundant light fumarolic activity was seen in the crater area on 13 October 1995 by Carniel. A shallow lava pond in vent 3/1 (see map in BGVN 20:11/12) was inferred by continuous night glow and ejection of small spatters that sometimes reached 3-4 m above the crater rim and only rarely fell outside the vent area. The other active vent in the SW crater (vent 3/2) produced regular explosions, with ejecta reaching considerable heights. Crater 2 was quiet, exhibiting neither explosions nor the gas-jet activity that often characterizes this crater. In Crater 1, the only activity occurred at a hole in cone 1/4; it consisted of continuous gas puffing, strong glow visible during the day, and very short blasts of air and smoke. Vent 1/1 was also active, although its eruptions were not as spectacular as those from 3/2, with occasional emission of a black cloud and ejection of sufficient material to trigger noteworthy movement of pyroclasts down the Sciara del Fuoco.

Mauro Coltelli (IIV) noted that the low lava fountains reported during August-October (BGVN 20:11/12) were typical of the Strombolian activity at the volcano, which was relatively low during that period.

Seismicity recorded at the summit, September-December 1995.Seismic activity recorded by the University of Udine summit station during the last three months of 1995 showed little variation in volcanic tremor intensity (figure 47). The daily number of recorded events was low (<100) in mid-September, reached a maximum of 337 on 18 October, and then decreased again until November. This period was interesting because of rapid transitions between days of very quiet activity (3-5 and 8-9 November) and days with a greater number of events (6-7 November). The minimum of the period was reached on 8 November, with only 53 events recorded during a full day of operation. A greater number of stronger events (either more energetic or less shallow) was recorded at the end of November and in early December (high of 46 events on 4 December).

Figure 47. Seismicity detected at the summit of Stromboli, 16 September 1995-29 February 1996. Open bars show the number of recorded events/day, and the solid bars those saturating the instrument (ground velocity exceeding 100 µm/s). The line shows daily tremor intensity computed by averaging hourly 60-second samples. The seismic station is located 300 m from the craters at 800 m elevation. No data were collected during the gaps on the plot, intervals when solar panel efficiency was insufficient to provide power for continuous acquisition. In cases of partial operation, the number of recorded events and the tremor intensity were normalized to the period of acquisition. However, because stronger events (those saturating the instrument) can easily be clustered in a short period of time, they were not normalized and the plotted values show the actual numbers recorded. Courtesy of Roberto Carniel.

January-February 1996 activity. January and the first half of February showed increased seismicity, with an average of 200-300 events/day and higher tremor intensity recorded at the summit. IIV reported that explosive activity during the first half of February remained low, ranging from days with an explosion almost every hour to days with a very few explosions. The main activity consisted of Crater 3 explosions that ejected minor spatter and ash puffs. Crater 2 exhibited continuous degassing, rarely interrupted by short periods of low-level spattering. Crater 1 produced daily strong gas explosions, sometimes with minor spatter.

At 2258 GMT on 16 February the seismic stations of the IIV permanent network on Stromboli recorded a sequence of explosion events, some of which were characterized by remarkable amplitudes. The events occurred in a very short time and were followed by increased tremor amplitude lasting ~12 minutes. Thereafter, the increment of tremor amplitude gradually vanished. The seismicity marked an intense eruptive phase from the summit craters. Eyewitnesses in Stromboli village reported a strong blast followed in the next few minutes by some incandescent bombs and glow on the summit; a dark column rose 200-300 m above the craters. No significant activity was observed by local residents for the next several hours. An IIV field survey on 20 February revealed that the bombs fell on an area 200-300 m wide. Both black scoriaceous bombs, covered by Pele's hairs, and reddish fumarolized blocks were observed; the vent that produced these materials was probably in Crater 2 or 3, but no relevant morphological variation of the shape of these craters was observed.

The University of Udine summit seismic station showed a general drop in activity after the event (figure 47). This repeats the pattern already observed after the explosions of 10 February and 16 October 1993 (BGVN 18:01 and 18:09) and after the small lava flow of May 1993 (BGVN 18:09), when similar abrupt decays were observed. The following days show increasing seismicity in terms of both tremor intensity and number of events.

Information Contacts: Roberto Carniel, Dipartimento di Georisorse e Territorio, Univ. di Udine, via Cotonificio 114, I- 33100 Udine; Mauro Coltelli, IIV (URL: http://www.ingv.it/).

April 1996 (BGVN 21:04) Cite this Report

Increased seismicity and Crater 1 activity after mid-April

Following the strong crater explosion on 16 February (BGVN 21:02), the summit seismic station of the University of Udine showed a general drop in seismic activity, as seen previously after paroxysmal phases at Stromboli (BGVN 18:01, 18:04, and 18:09). A new increase in seismic activity continued through early March with elevated average tremor intensity, while the numbers of recorded events and saturating events decreased (figure 48). After a fall in the tremor intensity on 9 and 10 March, all parameters started showing a general increasing trend that continued through the first week of April.

Figure 48. Seismicity detected at the summit of Stromboli, 1 March-15 May 1996. Open bars show the number of recorded events/day, and the solid bars those saturating the instrument (ground velocity exceeding 100 µm/s). The line shows daily tremor intensity computed by averaging hourly 60-second samples. The seismic station is located 300 m from the craters at 800 m elevation. Courtesy of Roberto Carniel.

After a brief drop in the tremor intensity (6-8 April) and in the number of events (10-11 April), a new period of increasing seismicity began. The number of recorded events suddenly increased between 15 April (286 events) and 16 April (540 events); in the following days numbers become even greater, as the seismic station was triggered almost every minute. Stromboli volcano guide N. Zerilli confirmed that on those days Crater 1 (figure 49) erupted almost continuously, with fountaining to 40-50 m above the crater. This was the source of a rapid succession of moderate explosion-quakes that caused the high number of events recorded by the station. The number of more energetic events was also high. Strong explosive activity was continuing as of 15 May.

Figure 49. Sketches of the Stromboli craters based on ground-based stereo photographs: a) October 1993 and b) April 1996. View is towards the NW. Question marks (?) on the 1993 sketch indicate areas impossible to see because of fumaroles. Vents 3/2? and 3/3? on the 1996 sketch are indirectly inferred from the trajectory of ejecta. Courtesy of Jürg Alean.

Field observations were made by J. and P. Alean, R. Carniel and F. Iacop during 21-28 April, although poor weather allowed only three summit visits. The most striking activity during the period was almost continuous spattering at vent 1/2 (figure 49b). Ejecta up to (and occasionally in excess of) 2 m in diameter were thrown at least 50 m high about every 30 minutes. For very short time periods (a few minutes) spattering activity at vent 1/2 would almost disappear, then increase and sometimes reach an intensity similar to the more normal, larger Strombolian eruptions. The latter occurred in Crater 1 from three distinct vents. On 21 April there were 20 eruptions at Crater 1 in 3.5 hours, some of them reaching heights of ~200 m. Cone 1/4 produced smoke rings on 22 April. On 28 April, Crater 1 produced 45 "normal" Strombolian explosions between 1300 and 1900 GMT, apart from the continuous spattering described above. Intense red glow from Crater 1 illuminated steam and clouds above it. This glow was one of the most intense ever seen by these scientists during their visits to the volcano; it could often be seen even from S. Vincenzo village.

Apart from fumarolic emissions, Crater 2 remained inactive. Crater 3 eruptions on 21 April were relatively small, albeit very noisy. Scoria did not reach heights of more than 100 m. It appeared as if the material was ejected from several individual vents or a fissure within the crater. A lot of ash was ejected, occasionally producing black mushroom-shaped clouds. Vent 3/1 had grown to an impressive size since 1994. They saw only one eruption from it, on 21 April at 1815 GMT; all the others were generated at vents 3/2 or 3/3. On 28 April activity at Crater 3 increased (55 eruptions between 1300 and 1900 GMT). By about 2100 GMT eruptions were occurring at intervals of 1-5 minutes, most of them exceeding 200 m in altitude. The ash content was clearly less than on 21 April, and brown ash clouds had become rare.

Figure 49 illustrates the change in crater morphology between October 1993 (figure 49a) and April 1996 (figure 49b). These sketches were drawn based on stereo photographs taken from the ground with a 35 mm single-lens reflex camera. The most striking morphological changes are probably in Crater 1, which saw the construction of a series of cones and their subsequent destruction, which led to the present configuration near vents 1/3 and 1/4. The zone around vent 1/2 in the foreground now appeared more open towards the Sciara del Fuoco, thus allowing better visibility of the crater from Punta Labronzo. Changes were also observed at vent 3/1, now better separated from Crater 2 on the rear and more connected to the rest of Crater 3 due to slumping of blocks between vents 3/1 and 3/3.

Information Contacts: Jürg Alean, Kantonsschule Zürcher Unterland, CH-8180 Bülach, Switzerland; Roberto Carniel, Dipartimento di Georisorse e Territorio, via Cotonificio 114, I-33100 Udine.

May 1996 (BGVN 21:05) Cite this Report

Continued high levels of activity through mid-June; two larger explosions

Seismicity began slowly increasing in mid-March before a sudden jump in tremor intensity on 15-16 April (BGVN 21:04). Observations made by Marco Fulle confirmed that the elevated seismicity corresponded to increased eruptive activity. During the night of 15-16 April about 100 explosions occurred. Continuous fountains from the N part of vent 1/2 (see sketch in BGVN 21:04) rose 50 m and lasted 1-2 hours. The S part of vent 1/2 produced large explosions to heights of 150-200 m that deposited bombs on the terrace beyond vent 3/2. Activity from vent 3/1 consisted of continuous pulsing of incandescent gas and explosions every 2-3 hours. Vent 3/2 produced simultaneous explosions every 10-30 minutes from two vents. Similar activity and ~50 explosions were seen the night of 20-21 April. Additional observations included glowing ex-hornitos in vent 1/3 with regular steam pulses. Vent 3/2 explosions covered the terrace S of Crater 3 with bombs.

Observations of summit activity made during 21-28 April by Alean, Carniel, and Iacop revealed similar activity consisting of continuous spattering and intermittent explosions from Crater 1 (BGVN 21:04). Seismicity remained at high levels through mid-May (BGVN 21:04).

IIV report of 1 and 6 June explosions. At 2147 on 1 June, local seismic stations maintained by the Istituto Internazionale di Vulcanologia (IIV) recorded a powerful event lasting ~3 minutes. Eyewitnesses at Stromboli village reported a single strong blast followed by the fallout of red bombs on the upper N slope. Incandescent bombs fell on vegetation, causing a fire that was extinguished by Civil Protection aircraft in the late morning of 2 June. More than twenty tourists were visiting the summit at the time of the explosion. Some of them reported light burns caused by hot lapilli fallout and minor injuries made while escaping on the steep slope.

A field survey early on 2 June revealed that the explosion occurred at Crater 1. The chain of hornitos inside Crater 1 was blown out, leaving a large deep depression in the N side of the crater floor. The ejected material completely covered the summit, falling more than 500 m to the S and E, and reaching ~1,000 m on the N sector, where it fell on the vegetation. The deposit was made of black scoriaceous bombs, covered by Pele's hair, reddish blocks, and a small amount of fine material. On the Pizzo area, where people usually stay to observe the activity (250 m SE from the vent), the falling bombs ranged between 10 and 50 cm in size, and they covered the area with a density of 3-4/m².

Strombolian activity after this event shortly returned to a medium intensity and a normal frequency (3-4 events/hour). In the days after there were several hours without any activity alternating to mild Strombolian activity and after 5 June spattering activity lasting several minutes was occasionally observed.

At 0452 on 6 June another strong seismic event from Crater 1 was smaller than the 1 June event and lasted ~1 minute. The eruption was recorded by the surveillance video camera on the Pizzo Sopra La Fossa, 120 m above the vent and 250 m away; the camera had been restored two days earlier. A few people observed the explosion and reported an ash column to a few hundred meters high and bomb fallout on the Sciara del Fuoco. The video showed a very fast gray-brown jet that ascended at ~30 m/second at the upper limit of the camera view; most of the bomb and block fallout was behind the camera. The ash emission lasted ~2 minutes, but at the end only overpressured steam was emitted.

After the explosion, Strombolian activity continued at Crater 1. During fieldwork that afternoon, activity was characterized by low-intensity explosions with emission of bombs and brown ash, interrupted by sporadic strong explosions that produced a larger amount of bombs followed by an almost continuous spattering for 5-15 minutes. All pyroclastic materials fell close to the craters but during the larger explosion some bombs were thrown a few hundred meters from the vents. The Strombolian activity continued through at least 10 June, showing periods of mild explosions interrupted by strong explosions and short periods of continuous spattering.

Observations on 8-9 and 11-12 June. Marco Fulle made observations from Pizzo sopra la Fossa for six hours on the night of 8-9 June. Vent 1/2 exhibited continuous fountaining 50 m high with larger pulses every 5-10 minutes and ejection of meter-sized lava clots. The vent also produced 35 explosions 100-200 m high, with bombs over the Sciara del Fuoco and the terrace up to Crater 2, and meter-sized lava clots inside Crater 1. Vent 3/1 was inactive, but vent 3/2 produced 20 explosions 50 m high with a lot of ash and bombs ejected inside the crater.

Observations from Pizzo sopra la Fossa were again made for six hours on the night of 11-12 June. Vent 1/2 again produced continuous fountaining and 46 explosions. Vent 3/1 remained inactive. Vent 3/2 generated 37 explosions 100-250 m high with minor ash. Fountaining occurred during the explosions and near-vertical jets of bombs fell S of the crater rim and over vent 3/1.

Information Contacts: Mauro Coltelli, CNR Istituto Internazionale di Vulcanologia (IIV), Piazza Roma 2, Catania, Italy (URL: http://www.ingv.it/en/); Marco Fulle, Osservatorio Astronomico, Via Tiepolo 11, I-34131 Trieste, Italy.

March 1997 (BGVN 22:03) Cite this Report

Summary of seismic and volcanic activity during May 1996-January 1997

The following summarizes the interval from 15 May 1996 to 31 January 1997. Eruptive activity increased in mid-April 1996 (BGVN 21:04) and continued through mid-June with significant explosions (BGVN 21:05). A flight on 16 July 1996 documented a plume rising from Stromboli using a Wide Angle Optoelectronic Stereo Scanner (figure 50). This image of the entire island also shows older lava flows, the Sciara del Fuoco, and the Pizzo Sopra La Fossa (120 m above and 250 m SE of the vent), where people often make observations from.

Figure 50. Wide Angle Optoelectronic Stereo Scanner (WAOSS) image of Stromboli taken on 16 July 1996. Courtesy of Martin Scheele, DLR.

Figure 51 illustrates the change in crater morphology between April 1996 and September 1996. Unfortunately the morphological changes in Crater 1 (typically a site of ongoing changes) were obscured on the September stereo photographs; however, the other two craters underwent noteworthy changes. In Crater 3, the front vent (3/3), which was essentially a pit in April, had become a small cone by September as a tephra apron enclosed the vent. In the westernmost part of Crater 3, vents 3/1 and 3/2 merged into a single chasm distinct from the rest of the crater.

Figure 51. Sketches made from terrestrial stereo photographs shot on 25 April 1996 and 19 September 1996. View is towards the NW. The sketches allow comparison of morphological changes in the crater area between these two dates; vents and craters labeled 3/2? and 3/3? were inferred indirectly from ejecta trajectories; areas labeled "?" were obscured by particulate and gases. Courtesy of Jürg Alean (original photos by Alean and Carniel).

During the first half of August the activity increased at Crater 3, in particular at vent 3/1, where the magma rose to increasingly shallow levels. During 16--17 August 1996, amid vigorous seismicity (figure 52), local volcano guide Nino Zerilli saw a small lava flow discharged from vent 3/1. The lava flow proceeded for only a few meters inside the crater and stopped by the evening of 18 August. Around this same time interval magma also reached high levels in Crater 1.

On 22 August 1996 at 0230 a tourist was hit by volcanic ejecta while in a sleeping bag ~80 m from the crater. He had to be transported by helicopter to Messina for head surgery. Zerilli reported that during the previous days ejecta from Crater 3 had been thrown as far as observation sites between the Pizzo and Crater 3. Thus, this injury appeared to have been more a case of camping too close to the crater rather than an especially violent outburst that particular night.

By 26 August, vent 3/1 was completely inactive, whereas vents 3/2 and 3/3 had almost joined, and their activity consisted of very powerful Strombolian explosions sending small fragments onto the Pizzo. At Crater 1, vent 1/4 only rarely exploded but did so with strong gas jets. In contrast, vent 1/3 ceased the continuous activity that it had begun on 16 April 1996, a change interpreted as either another pause there or the end of an unusually long period of continuous spattering.

On 4 September 1996 at 1545 a blast threw incandescent pyroclastic material onto slope vegetation. This started several fires and the explosion was clearly heard and watched from the village of Stromboli. According to the local newspaper "Il Piccolo" some tourists were caught by the explosion in the crater area and six of them were slightly injured. After this, the Mayor of Lipari ordered the closure of the path to the craters. After the blast, however, Strombolian activity continued to decrease. In terms of seismicity, both the number of major events and the total number of events decreased to very low values, a situation which prevailed through the end of January 1997 (figure 52).

Figure 52. Seismicity recorded at Stromboli 15 May 1996-31 January 1997. Open bars show the number of recorded events/day, and the solid bars those saturating the instrument (ground velocity exceeding 100 µm/s). The line shows daily tremor intensity computed by averaging hourly 60-second samples. The seismic station is 300 m from the craters at 800 m elevation. Arrows highlight the two powerful explosions of 1 June and 4 September 1996. Courtesy of R. Carniel.

A visit by Carniel at the end of September revealed extremely low activity as judged by the few audible eruptions seen or heard; however, smoke often covered most of the crater area. Remarkable were some very long rumbles, which could be heard even from the village of Stromboli. These rumbles were probably associated with strong degassing from Crater 2, although this interpretation remained unconfirmed by direct observation.

Matthias Hort and Ralf Seyfried visited and photographed portions of the volcano during 30 September-2 October (figures 53 and 51). They witnessed small eruptions from vent 2 at Crater 3. These eruptions took place while Craters 1 and 2 were inactive. Rumbling noises heard on 30 September from Crater 1 disappeared within the next two days. Overall, activity gradually declined from 30 September to 2 October. According to Hort, the most spectacular event during the observation period was an ash emission that produced a plume to 100 m above the craters. Periods of up to 2 hours passed without eruption.

Figure 53. Ash and bomb emission from Crater 3 seen from Pizzo on 30 September 1996. Courtesy of Matthias Hort, GEOMAR.

Figure 54. View of vent 1 in Crater 3, 30 September 1996. Bright incandescence is seen during daylight, indicating active magma at shallow depth. Courtesy of Matthias Hort, GEOMAR.

On the evening of 10 October Boris Behncke saw two lava fountains shooting up from Crater 3 during a 5- minute interval, reaching 80-100 m above the vent (to the height of Pizzo sopra la Fossa). Each fountain lasted ~20 seconds. About 10 minutes after the second fountain, an eruption apparently occurred at Crater 1.

Unusually low seismic activity appeared on 10 November, when the seismic station recorded only 17 events within 24 hours (none of them saturating the acquisition system). Exceptionally long intervals without a single event occurred on several other days: 7 hours on 25 October, 8 hours on 10 November, 10 hours during the night between 12 and 13 November. Although the tremor intensity slowly increased from September 1996 (1-2 V.s) to January 1997 (3-5 V.s), there was no significant increase in the number of recorded events or saturating events. Thus, activity at the end of January was still considered "low to moderate."

Information Contacts: Roberto Carniel, Dipartimento di Georisorse e Territorio, via Cotonificio 114, I-33100 Udine; Jürg Alean, Kantonsschule Zürcher Unterland, CH-8180 Bülach, Switzerland; Matthias Hort, Ralf Seyfried, and Boris Behncke, Geomar Research Center for Marine Geosciences, Wischhofstrasse 1-3, 24148 Kiel, Germany; Martin Scheele, Institut für Weltraumsensorik (Institute for Space Sensor Technology), Deutsche Forschungsanstalt für Luft- und Raumfahrt (German Aerospace Research Establishment), Forschungszentrum Berlin-Adlershof, Rudower Chaussee 5, 12489 Berlin (URL: http://www.dlr.de/).

May 1997 (BGVN 22:05) Cite this Report

New map of the crater terrace

A 7-hour visit on 23 May led to the construction of a crater terrace map (figure 55). The Crater 2 pit mapped in 1994 and 1995 had filled in and was occupied by an inactive lava flow fed from a small truncated cone. From information by Jürg Alean (BGVN 22:03) it was inferred that this flow was emplaced between 25 April and 19 September 1996. A new subsidence bowl was forming to the SE of the former Crater 2 pit. This was occupied by a pit crater and three vents (2/1, 2/2, and 2/3), none of which exhibited explosive activity. Vent 2/1, ~4 m in diameter, was the source of regular (~1/s) gas puffs and occasional gas release, and also showed night incandescence.

Figure 55. Sketch map of Stromboli's crater terrace drawn on 23 May 1997 and fitted to the map from the September 1995 EDM survey (BGVN 20:11/12). Label A indicates the pit in Crater 2 mapped in both 1994 (BGVN 19:10) and 1995. Label B designates a new subsidence bowl. Label C indicates the location of a vent and small lava flow erupted sometime between 25 April and 19 September 1996. Label D indicates the spot where hornitos existed in 1994 and 1995. Courtesy of Andy Harris.

Activity during the 23 May visit was at lower levels than seen in either October 1994 (BGVN 19:10) or September 1995 (BGVN 20:11/12). However, excellent viewing conditions revealed that major changes had occurred since the 1994 and 1995 surveys (BGVN 22:03).

During the intervals 1100-1400 and 1900-2200, no eruptions were observed from Crater 3. However, at about1400 on 24 May, an explosion from Crater 3 fed a brown ash cloud that rose ~200 m above the crater rim. This was observed from the sea.

Night-time temperature measurements obtained from Pizzo Sopra la Fossa using a Minolta/Land 152 infrared thermometer (corrected for an emissivity of 0.956) gave 2/1 vent temperatures of 670-699°C. Vents 2/2 and 2/3 (~3 x 1 m and <1 m wide, respectively) were actively degassing without incandescence. The site of hornitos in 1994 and 1995 was occupied by a pit, with a wall on the NE side.

Crater 1 was occupied by two active vents (1/1 and 1/2). Between 1055 and 1155 on 23 May eight explosions occurred from these two vents. The first two explosions sent bombs 100-200 m above the crater rim with ~20% of the ejecta landing on the upper Sciara. The following six explosions sent ejecta up to 50 m above the vent.

For the next two hours, Crater 1 exploded ~1-2 times/hour, but additional sloshing and gas release sounds were occasionally audible from Pizzo Sopra la Fossa. By the evening of 23 May, Crater 1 activity had escalated to levels similar to the 1055-1155 period. As darkness fell, an intense pulsating glow visible over Crater 1 could have been due to a small, active lava pond on the crater floor. This may have accounted for the sloshing sounds heard earlier in the day.

Stromboli, a small island N of Sicily, has been in almost continuous eruption for over 2,000 years. Its small Strombolian explosions, which hurl incandescent scoriae above the crater rim, occur several times a day, but larger eruptions are less frequent.

Information Contacts: Andy Harris, HIGP/SOEST, University of Hawaii, 2525 Correa Road, Honolulu, HI 96822 USA.

October 1998 (BGVN 23:10) Cite this Report

Larger explosions in January, August, and September 1998

Moderate activity prevailed at Stromboli from January to May 1997 (BGVN 22:03). During this period there was a slight decrease in tremor intensity and a slight increase in the number of recorded events (figure 56). Events exceeding the saturation level of the summit seismic station numbered fewer than 10% of the total recorded.

Figure 56. Seismicity detected at the summit of Stromboli from January 1997 through August 1998. Gray bars show the number of recorded events/day, and the black bars those saturating the instrument (ground velocity exceeding 100 µm/s). The line shows daily tremor intensity computed by averaging hourly 60-second samples. The seismic station is located 300 m from the craters at 800 m elevation. Courtesy of Roberto Carniel.

There was a marked increase in the total number of events during June-July 1997, sometimes in excess of 300 per day. Following a month-long lapse, an even larger long-term increase began in September that continued until November 1997. There were several days in this interval when triggering of the seismic station was almost continuous and tremor intensity reached high values, behavior that usually coincided with continuous spattering at the vents. No seismic data were recorded between 24 November 1997 and 9 January 1998. Activity had returned to moderate levels by the time seismic data acquisition resumed on 10 January 1998 (figure 56). The number of daily events rapidly decreased, as did tremor intensity.

At 1130 on 16 January 1998, a strong explosion in the crater area was similar to others at Stromboli during the last few years; one comparable event occurred on 4 September 1996 (BGVN 22:03). Such explosions are not a danger to the villages of Stromboli and Ginostra (figure 57), but they may be dangerous for tourists visiting the summit because bombs easily reach the usual observation points. Another risk is that fires, started by incandescent bombs, may spread in the vegetation. In the case of the 16 January eruption, bad weather prevented tourists from climbing the volcano and rain extinguished any wildfires.

Figure 57. Sketch map of Stromboli Island, showing locations referred to in the text. Courtesy of Roberto Carniel.

A new rise in seismicity began a few days after the explosion. A peak was reached during 16-20 February; on 19 February, 405 events were recorded, and on 20 February tremor intensity was high and 43 saturating events were noted. After this increase, activity decreased steadily with only a few fluctuations until the end of April. The total number of events recorded during the decrease was sometimes

During May-June seismic activity increased. During July two sharp drops in the level of activity were observed: the number of events did not exceed 80 per day during 1-3 July, and went below 50 per day during 22-24 July. Tremor intensity reached the minimum of the year on 22 July. There was a slight upturn in August.

At 1726 on 23 August, another powerful explosion occurred at the craters. The strong blast was heard throughout the island, and a column of ash and lapilli shot over the craters. Incandescent bombs fell over a vast area towards Vallonazzo, Labronzo, and Forgia Vecchia. At least one other explosion followed. Several fires started in vegetation on the upper slopes; the largest one, near Forgia Vecchia, was not extinguished until the next day. Fortunately, although a high number of tourists were on the island, no one was hurt. A dark ash column was eventually replaced by a large, light ash cloud. Small lapilli fell in Ginostra. Bombs were found on the tourist path down to 750 m elevation, and in other directions bombs fell to 500 m. Authorities immediately blocked public access to the upper part of the volcano. The explosion also caused significant morphological changes to the rim of Crater 1 towards Semaforo Labronzo.

Another strong explosion, perhaps more energetic than that of 23 August, happened at 1914 on 8 September. A considerable atmospheric shock wave was reported at the village of Stromboli, and broken windows were reported near San Bartolo. Ash and small lapilli fell near Ginostra and several bush fires were started by bombs on the volcano's slopes. Unfortunately, the seismic station was not operational at the time due to a technical problem.

Stromboli, a small island N of Sicily, has been in almost continuous eruption for over 2,000 years. It is the namesake for small Strombolian explosions, which hurl incandescent scoriae above the crater rim several times a day, with infrequent larger eruptions.

Information Contacts: Roberto Carniel, Dipartimento di Georisorse e Territorio, Universitá di Udine, Via Cotonificio, 114 I-33100 Udine; Jürg Alean, Kantonsschule Zürcher Unterland, CH-8180 Bülach, Switzerland.

June 1999 (BGVN 24:06) Cite this Report

Vents in summit craters still active; variable seismicity

After the strong summit explosions of 23 August and 8 September 1998 (BGVN 23:10), at least two others followed during 1998: the first at 1805 on 24 November, the second at 0245 on 28 December. Both were reported by Pierre Cottens from the village of Stromboli, with pyroclasts clearly seen from the village, reaching estimated heights of at least 700 m over the craters in November and 500 m in December, but with little ashfall over the village. Technical problems kept the seismic station maintained by the University of Udine out of service through the end of January 1999.

Seismicity during February-June 1999. The seismic station was restored on 1 February 1999, and in the first part of the month the daily number of events declined from 130 to 50-80/day (figure 58). Saturating events showed a similar trend. The second half of February was characterized by an increasing number of events, reaching a maximum of 275 events on 2 March. During this period the tremor intensity showed fluctuations around an average of 3.6 V s, with an isolated peak on 23 February.

Figure 58. Seismicity detected at the summit of Stromboli from February through June 1999. The gray bars show the number of recorded events/day, and the black bars those saturating the instrument (ground velocity exceeding 100 µm/s). The line shows daily tremor intensity computed by averaging hourly 60-second samples. The seismic station is located 300 m from the craters at 800 m elevation. Courtesy of Roberto Carniel.

During 12-20 March there was a general decrease in activity, with a minimum number of 52 seismic events recorded on 16 March, and a minimum tremor intensity of 1.8 V s on 15 March. The gain in activity was observed first in the tremor intensity, with a local maximum of 4.7 V s on 29 March, then in the number of events, which reached a high of 208 events on 30 March.

A sharp decline was observed on 7 April both in the tremor intensity (from 3.0 to 1.2) and in the number of events (from 180 to 76). Saturating events also stopped, after an average of five saturated events during the three preceding days. Seismic activity increased slightly during the following two days, and on 9 April Cottens reported two strong blasts at about 0300, separated by a few minutes. Bad weather did not allow observation of the summit area, but the noise was similar to that produced by the strong eruptions of 1998. The seismic station recorded an event with greater than usual energy, which may have been from one of the explosions. Another decrease of activity was observed the day after the explosion, with the number of daily events falling to 83 and the tremor intensity to 1.9. The following days were characterized by a slow increase in seismicity, but activity remained low to moderate for the rest of the month.

After another gap in the seismic acquisition, the activity was still low after mid-May 1999, with <100 events/day between 20-24 May. The last week of May showed a rise in the number of events, with a maximum of 183 events on 26 May. A relatively high number of saturating events was also observed, starting on 23 May and peaking on 27 May (21 saturating events) and 30 May (23 saturating events). Another minimum in the number of events (66), number of saturating events (0) and tremor intensity (1.6) was recorded on 5-6 June 1999. During several days on the island in the second half of June, seismic activity remained at low to moderate levels, with a short duration increase in the tremor intensity on 21-22 June.

Observations during 17-20 June 1999. Observations during 17-20 June allowed mapping of the crater terrace (figure 59). Observations were made over two 3-4 hour periods: 2109-0100 (18-19 June), and 2030-2320 (19 June).

Figure 59. Sketch map of Stromboli's Crater Terrace drawn on 18-19 June 1999 from Pizzo sopra la Fossa and fitted to the map produced from a September 1995 EDM survey of the Crater Terrace (BGVN 20:11/12). Courtesy of Andy Harris and Roberto Carniel

Crater 1 contained four active vents (figure 59). During the first observation period, the vicinity of Vent 1/2 was the source of persistent widespread glow; but no ejecta was observed. Vent 1/1, however, was the source of glow and near-persistent low-energy activity, characterized by repeated phases of ejecta emission separated by periods of little or no emissions. Each phase consisted of numerous pulses of ejecta. During each pulse a few (typically 1-10) bombs were ejected <10 m above the crater rim, with pulses every few seconds. Around 11 periods of this persistent, pulsing emission were observed. Each period lasted 2-29 minutes and was separated by intervals of 3-28 minutes with occasional bomb emissions. This pattern at Vent 1/1 was broken by 16 high-energy events during which explosions sent ejecta ~100 m high.

Vent 1/3 produced high-energy events only, with 18 observed during the first period. High energy events from vents 1/1, 1/3, and 1/4 were synchronized. It was difficult to distinguish eruptions from 1/4 and 1/1 from the viewing angle, so eruptions from these two vents may have been occasionally assigned to the wrong vent; the eruption counts for 1/1 and 1/4 together were therefore combined. Vents 1/3, 1/1, and/or 1/4 erupted together on 11 occasions. On these occasions ejection from each vent was either synchronous or closely linked. During such synchronized events there appeared to be no set order in terms of which of the three vents started erupting and which followed. Vents 1/1 (and/or 1/4) and 1/3 erupted on their own on 5 and 7 occasions, respectively.

As in the first observation period, glow persisted above Vent 1/2 throughout the second period. Unlike the previous evening, however, ejecta was observed from Vent 1/2, where activity followed the style observed at Vent 1/1 during the previous evening. Around eight periods of low-energy but persistent pulsing activity were observed. These lasted 3-33 minutes and were separated by 2-23-minute-long periods of apparently no emission. This pattern of activity from Vent 1/2 was interrupted by five high-energy events lasting 4-6 seconds.

Vent 1/1 issued regular gas puffs (typically one puff every 1-2 seconds), but the frequency of ejecta emission had declined, with the periods of persistent, pulsing activity observed the previous evening replaced by discrete emissions. Ejections were observed from 1/1 on nine occasions, where only one event was of the high-energy type, and the remainder were short (<40-second-long) periods during which 1-10 bombs were ejected <10 m above the rim in pulses. As in the previous period, Vent 1/3 was characterized by high-energy events only. Seven occurred during the second period, of which three were synchronous with high energy events from 1/2 or 1/1.

Although no activity or glow was observed from Crater 2, a hornito (figure 59) had grown in the vicinity of the low cone observed during May 1997 (BGVN 22:05). The area surrounding Crater 2 no longer seemed to be marked by a crater-like feature. The subsidence bowl observed SE of Crater 2 in May 1997 had developed into a deep, funnel-shaped pit, which was the source of faint glow. On a previous map (BGVN 22:05), the vent numbering indicated that this area was part of Crater 2; this interpretation is questionable and this feature could be considered a crater by itself now. On figure 59 we denote this vent as 3/3 (therefore part of Crater 3) in order to allow for easy comparison with older maps of the crater terrace (e.g., BGVN 22:03).

Crater 3 was the location of two additional active vents (figure 59). Glow was observed from vent 3/1 only after an eruption at 2317 on 18 June. Eruptions from Crater 3 were high-energy only, and typically larger (in terms of volume and height, attaining heights of 150 ± 50 m) and longer (lasting 8-23 seconds) than those from Crater 1. Nine and eleven eruptions, respectively, occurred from Crater 3 during the two observation periods.

During our descent in the early hours of 19 June there was a rock fall/slide at about 0200 lasting 1-2 minutes. Boulders from the cliffs on the N edge of the Rina Grande rolled and bounced down the Rina Grande. Owing to the steepness of this flank, once in motion the rocks probably did not stop until they reached the sea in the direction of Forgia Vecchia, crossing the route that descends the Rina Grande ash slope within seconds. Such events pose a very serious hazard to anyone using this route, especially at night.

Information Contacts: Andy Harris and Dawn Pirie, Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, United Kingdom; Sarah Sherman, 41-485D Kalanianaole Hwy., Waimanolo, HI 96795 USA; Roberto Carniel, Dipartimento di Georisorse e Territorio, Università di Udine, Via Cotonificio, 114, I-33100 Udine (URL: http://www.swisseduc.ch/stromboli/).

January 2000 (BGVN 25:01) Cite this Report

1999 seismic summary and some stronger-than-usual eruptions

Reports of a strong eruption on the morning of 26 August were submitted to Stromboli On-line (maintained by Jürg Alean and Roberto Carniel) by a variety of sources, including Gianfranco Cincotta, Franco Iacop, Danielle Cottens, Klaus Jäger, and others. Another strong eruption was noted on 10 October. Throughout 1999 activity apparently continued at low levels, with between 50 and 250 daily seismic events registered at the summit (figure 60).

Figure 60. Seismicity detected at the summit of Stromboli during 1999. The gray bars show the number of recorded events/day, and the black bars those saturating the instrument (ground velocity greater than 100 µm/s). The line shows daily tremor intensity computed by averaging hourly 60-second samples. The seismic station is located 300 m from the craters at 800 m elevation. Courtesy of Roberto Carniel.

The first report of the 26 August explosion came from Gianfranco Cincotta, who reported that a normal explosion from Crater 3 at 0058 was followed by a more intense one at Crater 1. Then, after a tremor, the large explosion came from the vent in front of Crater 2, which is considered part of Crater 3 for "historical" reasons but has almost reached the status of an independent crater. The event happened as the evening tourist group descended the volcano and the night group was still climbing. Pyroclastic material was clearly visible from the village and fell over a wide area, including the path section with the observation points around the 700-m level. Fortunately this time few pyroclasts reached the reed vegetation on the upper slopes. Nevertheless, Danielle Cottens saw some small fires from Scari. According to Cincotta, 10 tourists asked for medical assistance. All were slightly injured, mostly because of the sudden run from the craters. A man inside his sleeping bag on the Pizzo was slightly burned by pyroclasts.

Henrik Berger also reported via Stromboli On-line that he and friends were camping on the Pizzo the night of 25-26 August; there were a total of seven people on the summit. A normal eruption of Crater 3 produced the usual amount of ash and little glowing lava. Perhaps one minute later, at about 0057, a significant tremor occurred, immediately followed by an eruption at Crater 1. Less than one second afterwards the small incandescent middle vent exploded with a bang. Lava shot from this vent to about twice the usual height. Most pyroclasts were ejected by Crater 1. The group fled in panic as incandescent volcanic bombs up to 30 x 50 cm impacted nearby. A lot of non-glowing material was also ejected. One woman's hand was hit by a glowing piece of scoria. A girl who had been sleeping barefoot in her sleeping bag suffered minor burns on her feet and hands. On the other side of Pizzo another person suffered burns on his belly.

On 15 September Boris Behncke was at the summit and found numerous scoriaceous bombs on and around the Pizzo sopra la Fossa that had flattened upon impact; many had diameters of 20 cm or more. In one case, a bomb had fallen on the rim of one of the primitive shelters made by tourists in the summit area. Eruptive activity was at relatively low levels, and observations into Crater 3 were made for about 10 minutes. The crater floor was covered with a mixture of previously ejected scoriae and partially altered blocks fallen off the crater walls. Explosions pierced this mixture in at least two locations, and incandescent scoriae sprayed 15-20 m above the crater floor, which lay about 30 m below observation points on the S crater rim. During three hours of observation explosions occurred from up to three vents in Craters 1 and 3, and a persistent fluctuating glow was visible after nightfall in the northernmost part of Crater 1. This glow had been visible to Behncke previously during August while passing Stromboli island twice on the ferry connecting the Aeolian Islands with Naples.

Danielle Cottens reported via Stromboli On-line that while she was on the Pizzo around 1830 on 10 October the volcano was showing intense activity, with bombs often reaching the base of the Pizzo. Around 1930 she decided to go back to the village. After 30 minutes, at about 2000, a significant explosion occurred that ejected ash and scoria onto Pizzo and in the Fossetta. Other tourist groups climbing the volcano decided to go back.

Information Contacts: Stromboli On-line, maintained by Jürg Alean and Roberto Carniel (URL: http://www.swisseduc.ch/stromboli/); Roberto Carniel, Dipartimento di Georisorse e Territorio, Universitá di Udine, Via Cotonificio, 114 I33100 Udine; Jürg Alean, Kantonsschule Zürcher Unterland, CH8180 Bülach, Switzerland; Boris Behncke, Dipartimento di Scienze Geologiche, Palazzo delle Scienze, Università di Catania (DSGUC), Corso Italia 55, 95129 Catania, Italy.

August 2000 (BGVN 25:08) Cite this Report

Low-to-moderate eruptive activity January-September 2000

Activity at Stromboli during January-September 2000 was low-to-moderate, as it was throughout 1999 (BGVN 25:01). During a field campaign on 10-19 May 2000 researchers from various universities closely observed activity at the volcano, which resulted in detailed accounts of activity at Stromboli's three main craters. During January-September several small Strombolian eruptions occurred at the volcano, with the largest reported eruptions on 4 July and 6 September 2000.

Activity during January-May 2000. During January and February 2000, low-to-moderate seismicity was recorded by the University of Udine's summit station (figure 61). During the two months there were brief periods with very few seismic events; for example on 1-7 January there were [less than 50] events/day. On 17 February, after a short gap in acquisition, the tremor intensity increased greatly from the average value of [~4 to a peak of ~8] volt seconds (V-s), but then returned to average values the following day.

Figure 61. Seismicity detected at the summit of Stromboli during January-[early October] 2000. [Data collection continued through October and will be reported in a future issue.] The yellow bars show the number of recorded events/day, and the red bars those saturating the instrument (ground velocity greater than 100 µm/s). The line shows daily tremor intensity computed by averaging hourly 60-second samples. The seismic station is located 300 m from the craters at 800 m elevation. Courtesy of Roberto Carniel.

During the first half of March seismicity was even lower than in the previous months, often with [less than 50] events/day and occasionally (1, 5-9, 11 March) as few as [less than 20] events/day (figure 61). During the second half of March, the number of events returned to normal values (100-120 events/day) and during April increased to 140-160 events/day. Despite the increase in the number of events from March to April, there was no significant variation in tremor intensity.

A technical problem prohibited the acquisition of data at the beginning of May. During 10-19 May the Universities of Udine, Florence, Hawaii, and Open University carried out a field campaign at Stromboli. During the 10 days, scientists collected near-continuous seismic, thermal, and infrasonic measurements. They also constructed an activity log that totaled ~71 hours and was divided into nine 2- to 15-hour long segments that covered all days in the observation period except 13 May. It is important to note that the number of events recorded by the seismic station are not directly comparable to the number of eruptions logged by visual observation because some eruptions may not have produced a seismic signal that was detectable by a short-period seismometer. For this survey the naming convention was changed from previous reports. The craters that were previously named 1,2, and 3, were renamed NE, Central and SW Craters, respectively.

Several active vents were observed in the NE and [SW] craters; up to five were in the NE Crater (figure 62). The western cluster (1/1) consisted of vents that emitted ejecta in three different directions: obliquely to the SW, obliquely to the NE, and symmetrically. This zone was the source of strong nighttime glow, and near-continuous sloshing/puffing sounds that were accompanied by regular emissions of single bombs between Strombolian events. The eastern cluster (1/2) consisted of two vents that would often erupt together. During the night this zone was the source of weak, intermittent glow. Vent clusters 1/1 and 1/2 were located on the floors of two coalesced depressions within the main NE Crater. The SW Crater was the location of at least two active vents (3/1) ~10 m apart on the SE portion of the crater. No night glow was observed from either vent. A zone of high-temperature fumaroles was aligned along crater-bounding fractures at the E rim of the SW Crater.

Figure 62. Sketch map of Stromboli's Crater Terrace drawn on 10-19 May 2000 from Pizzo sopra la Fossa and fitted to the map produced from a September 1995 EDM survey of the Crater Terrace (BGVN 20:11/12). Note: For this survey the naming convention was changed, craters that were previously named 1,2, and 3, were renamed NE, Central, and SW Craters, respectively. Courtesy of Andy Harris.

During 10-14 May, activity at 1/1 was characterized by emissions of molten ejecta in heavily loaded, single-shot fountains that reached heights of 50-350 m and landed as far as ~120 m away to the eastern edge of the SW Crater. Fountain heights and ejected volumes appeared to increase from 10-11 May, when maximum ejecta heights were 100-200 m, to 14 May, when ejecta reached up to 230-350 m. Bombs rarely landed within 50 m of Pizzo sopra la Fossa, which was ~140 m away from the 1/1 vent cluster. The 1/2 vent cluster was the source of pulsing, 20- to 30-second emissions of incandescent ejecta, ash, and gas. Ejecta were emitted in a diffuse spray, and ash-and-gas emissions reached <100 m above the vent. Two vents within 10 m of each other often erupted together 2-4 times per hour, with the westernmost vent usually erupting after the eastern vent began to erupt. Activity at 3/1 was characterized by 10- to 40-second emissions of gas and brown ash, containing minor ejecta. The ash clouds from 3/1 rose up to 300 m above the vent before detaching from it.

Overnight on 14-15 May the style of activity changed when all vents switched to gas-dominated emissions that reached lower heights (<75 m) than previously. Events also became increasingly quiet, such that by the evening of 15 May some emissions were completely silent. Ejections from 1/1 continued to be short, 1- to 2-second, single-shot events, but they contained low volumes of molten ejecta with minor amounts of ash. The ejections rose <75 m above the vent with gas being the dominant component. At 1/2 the emissions were gas dominated. Long-lasting emissions at 3/1 consisted almost entirely of gas and rose <100 m above the vent. This gas-dominated style of activity continued until the final day of observations on 19 May.

During the activity change the frequency of events remained relatively stable, with 11-17 events/hour. During 10-14 May the frequency decreased at 1/1 from 6-7 events/hour to 3-6 events/hour, and during 15-19 May increased at 3/1 from 2-5 events/hour to 5-8 events/hour.

The Central Crater was not active during the observation period. The hornito initially observed during July 1999 (BGVN 24:06) was the source of low-temperature fumarolic activity. The Central Crater was entirely filled, resembling a saddle between the SW and NE Craters, rather than a crater. The deep, funnel-shaped pit SE of the Central Crater that developed during 1997-99 (BGVN 24:06) appeared to be a well-established feature. Its morphology suggested that it was an eastern extension of the SW Crater with a low septum separating it from the SW Crater.

An ~5-m-wide, persistently degassing active vent (3A) was observed on the floor of the funnel-shaped pit. Another actively degassing vent (3B), which was established on the NW rim of the 3A pit during July 1999, was ~2 m wide and was not surrounded by any constructive features (e.g. hornito, spatter rampart, or crater features), suggesting that eruptions were rare or did not occur from it. A maximum temperature of 639°C was measured from the gas-heated walls of vent 3B. However, the temperature of vent 3B was variable and during low-activity periods the vent temperature fell below 550°C (the lowest detection limit of the thermal infrared thermometer). The variation in temperature was obvious during the day from increases and decreases in the vigor of degassing: vigorous degassing occurred at high temperatures, and weak degassing occurred at low temperatures. Similar variations in glow, temperature, and degassing were observed at vent 3A. The activity at 3A and 3B vents appeared to be inversely correlated; a decrease in temperature and degassing at 3B would often synchronize with an increase at 3A, and vice versa.

Overall, during the observation period a total of 916 Strombolian events were recorded, with 383, 198, 335 events occurring at vent clusters 1/1, 1/2, and 3/1, respectively. The overall eruption rate was 13 events/hour, with a range of 6-20 events/hour and a typical repose of 5 minutes (the maximum repose was 37 minutes). Breaking these statistics down by vent, 1/1, 1/2, and 3/1 were respectively active with 5, 3, and 5 events/hour, with ranges of 0-12, 0-6, and 0-9 events/hour, and mean reposes of 12, 22, and 12 minutes (the maximum repose times were 194, 136, and 117 minutes).

Activity during June to mid-August. According to Sistema Poseidon, during mid-to-late June there was frequent Strombolian activity of medium intensity, with tephra reaching ~100-150 m above the crater's edge. This activity was regular, with about one event every 20 minutes and less regular ejections of ash and secondary debris coming from a second active vent in the same crater. By the end of June activity at the other summit craters was limited to abundant steam emissions. In early August, explosive eruptions occurred primarily in the northern vent and more sporadic eruptions occurred in the southern vent.

Small eruptions on 4 July and 6 September. Stromboli On-Line reported that at about 0935 on 4 July two large explosions were separated by 1-2 seconds. Only a small amount of ash was observed from the eruptions. They also reported that at 0554 on 6 September a violent eruption woke up the population. The ash cloud from the eruption rose to at least 1 km in altitude. The timing of the eruptions is shown on the plots of event frequency and intensity during January-September (figure 61).

Information Contacts: Andy Harris, Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, U.K.; Roberto Carniel, Dipartimento di Georisorse e Territorio, Università di Udine, via Cotonificio 114, I-33100 Udine, Italy (URL: http://www.swisseduc.ch/stromboli/); Maurizio Ripepe, Dipartimento di Scienze della Terra, Università di Firenze, via G. La Pira 4, I-50121 Firenze, Italy; John Bailey, Department of Geology and Geophysics, SOEST, University of Hawai' i, 2525 Correa Road, Honolulu, HI 96822, USA; Stromboli On-line, maintained by Jürg Alean and Roberto Carniel (URL: http://www.swisseduc.ch/stromboli/); Jürg Alean, Kantonsschule Zürcher Unterland, CH8180 Bülach, Switzerland; Sistema Poseidon, a cooperative project supported by both the Italian Government and the Sicilian Regional Government, and operated by several scientific institiutions (URL: http://www.ct.ingv.it/en/chi-siamo/la-sezione.html).

April 2001 (BGVN 26:04) Cite this Report

Variable seismicity during late 2000 and early 2001; spatter ejected above crater rims

This report covers the period of October 2000-February 2001, describing variable activity since the last report (BGVN 25:08).

During the last three months of 2000 the seismic activity recorded by the University of Udine summit station (figure 63) remained in the low to moderate range, comparable to the previous months. Short periods were again characterized by a very low number of events. In particular, under 50 events were recorded on several days during the period 29 October-16 November. The period between late December 2000 and early January 2001 was characterized by slightly increased activity, with more than 250 events/day and a tremor intensity occasionally exceeding the value of 5 volt·seconds (V·s). A drop in seismicity at the end of January was followed by an increase in all parameters during February, with the number of seismic events again exceeding 250 per day on 20 and 21 February. "Major" seismic events, which caused instrument saturations, were concentrated in the period 17-22 February, with a peak of 44 and 48 saturating events on 20 and 21 February, respectively (figure 63). The tremor intensity showed a slower but steady growth, with values slightly over 2 V·s at the beginning of February, and near 6 V·s at the end of February.

Figure 63. Seismicity detected at the summit of Stromboli during the period October 2000 - February 2001. Open bars signify the number of recorded events per day; solid bars, saturating events (i.e. with ground velocity exceeding 100 µm/s). The line shows the daily average of tremor intensity derived from hourly 60-second samples. The seismic station is located 300 m from the craters at 800 m elevation. Courtesy of Roberto Carniel.

Jürg Alean visited the volcano several times during mid-October 2000. On 10 October 2000, he observed about 6 eruptions/hour at the Southwest Crater (formerly called Crater 3), expelling large amounts of ash, sometimes almost hiding the glowing spatters that were being ejected to moderate heights. Vent 3B glowed intensely but never ejected lava. The Northeast Crater (formerly called Crater 1) had grown substantially over the last 7 years (figure 64) and was clearly separated into two pits, both of which were glowing at night with vent 1/1 glowing more strongly (vent clusters 1/1 and 1/2 are located on the floors of two coalesced depressions within the main Northeast Crater).

Figure 64. Different sketch maps of Stromboli's Crater Terrace drawn on the basis of ground stereo photos from 1993, 1996, and 2000. These highlight the changes in the morphology of the crater area during these seven years. Other maps and sketches can be found on the Stromboli On-line website. Courtesy of Jürg Alean.

Alean reported that on 13 October Southwest Crater had more eruptions, ~10 per hour. Further, the kind of activity was different with a total absence of ash and a much longer duration of individual eruptions, some over 30 sec; spatters reached ~100 m above the crater rim. Activity had also increased at Northeast Crater: vent 1/1 delivered several eruptions/hour, mostly at an oblique angle towards Pizzo, while vent 1/2 erupted only weakly about once/hour. On 15 October, the sudden occurrence of very noisy degassing events heard several times at Stromboli village (e.g. 1106, 1125, 1145 and 1245) confirmed the variability of the eruptive activity during this week. No similar noises were heard later that day.

Roberto Carniel visited the summit crater area on 29 November 2000 and noted that there were significant changes since Alean's earlier visit in October. The hornito, located where the Central Crater (formerly called Crater 2) was located, was showing more intense degassing, and a new vent had opened between the hornito itself and vent 3B (figure 64), thus suggesting a renewal of the activity in this sector. This new vent, which Carniel labeled 3C, showed continuously puffing degassing, similar to vent 3B. At night, both were incandescent and continuously moderate spattering was observed at vent 3C, with small-sized spatters sometimes reaching the vent rim, landing on the crater terrace. Vent 3C also showed Strombolian eruptions, with vertically ejected, molten material reaching over 50 m above the rim.

Information Contacts: Roberto Carniel, Dipartimento di Georisorse e Territorio, Universitá di Udine, via Cotonificio 114, I-33100 Udine, Italy; Jürg Alean, Rheinstrasse 6, CH-8193 Eglisau, Switzerland; Stromboli On-line website, maintained by Alean and Carniel (URL: http://www.swisseduc.ch/stromboli/).

July 2001 (BGVN 26:07) Cite this Report

Continued Strombolian activity during March-May 2001; crater morphology changes

This report discusses the period March-May 2001, which included a 10-day interval of field observations. Observers and instruments documented variable volcanism and seismicity ongoing since the last report (BGVN 26:04).

Seismic activity increased in early March, as recorded by the University of Udine summit station, which is located 300 m from the craters at 800 m elevation. On 13 March the number of events per day reached 200. The latter half of March was characterized by a decrease of tremor intensity and by a noteworthy number of saturating events (peaking at 63 on 20 March). In April a reprise of the tremor intensity occurred but the number of triggering seismic events decreased. May was characterized by an increased number of seismic events.

During 14 days (10-24 May) of continuous seismic, thermal, and infrasonic measurements, the authors recorded a detailed 16-hour-long log of activity, and they updated the crater terrace map (figures 65 and 66). During the period, 1,050 seismically determined Strombolian events were recorded overall. These came from the NE, Central, and SW craters, with respective craters discharging 463, 42, and 545 events, respectively. These data provide an average daily rate (over the 62-hour period) that ranged from 5 to 31 events/hour and averaged 17 events/hour. Thus, the average repose time between eruptions was ~3.5 min; the largest measured repose time for the three vents was 22 minutes.

Figure 65. A sketch map showing Stromboli's Crater. The Terrace was drawn during 10-24 May 2001 from Pizzo Sopra la Fossa and fitted to the map produced from the September 1995 EDM survey of the Crater. Note that, the prefix "2" has been used to denote Central Crater vents as opposed to the "3" prefix used BGVN 25:08. The map does not use contours, instead the long lines show the steepest gradient of the slope. Courtesy of Andy Harris.

Figure 66. A panoramic view of Stromboli's crater terrace area taken on 10 May from Pizzo Sopra la Fossa and highlighting plumes from synchronous activity at the two SW Crater vents (3/3, 3/2). Brown ash rose from 3/2 and gray ash rose from 3/3. Courtesy of Dave Rothery.

Breaking these statistics down by individual crater, the NE, Central, and SW craters had respective daily averages that ranged as follows: 2-21, 0-3, and 1-19 events/hour. The crater's average event rates were 8, 1, and 9 events/hour, respectively. This gives average repose times for the craters of 8, 69, and 7 min, respectively. For comparison, the maximum repose times at NE, Central, and SW craters were 46, 420 and 105 minutes.

As in May 2000 (BGVN 25:08), the NE Crater consisted of two smaller pits separated by a low septum, the two pits being the location of one and two active vents, respectively. Of these, the western-most vent (1/1) and eastern-most vent (1/2) were most active, with average rates of 4 and 3 events/hour, respectively, compared with ~1 event/hour for vent 1/3. The SW crater contained three active vents that often showed paired or synchronous activity. However, the exact combination of paired eruptions varied daily. For example, on 16 May, an eruption from 3/1 would often be followed by one from 3/2 within a few seconds; but, on 19 May, an eruption from 3/1 would be followed by one from 3/3. As in previous years, eruptions from the SW crater had longer durations and were richer in ash than those from the NE crater.

The frequency and style of activity at the NE crater showed significant variations. During 10-11 May, the NE crater erupted up to 10 times/hour. Events at vent 1/1 were characterized by single-shot, ejecta-loaded Strombolian eruptions, while those at vent 1/2 were long duration (typically 10-20 s), gas-rich eruptions with diffuse ejecta sprays. During 14-15 May, the eruption rate increased to 12-17 events/hour, as eruptions at 1/1 switched to longer duration (~10 s), gas-rich ejections mixed with ash and small bombs. At the same time, events from vent 1/2 contained more bombs that reached ~300 m above the crater. On 16 May, maximum eruption rates declined to 8 events/hour, and ejections from 1/1 and 1/2 were characterized by diffuse sprays of small incandescent bombs mixed with ash to ~200 m. During 17-20 May, activity from both vents was characterized by strong eruptions, often occurring in multiple pulses, with heavy bomb loads to 200-300 m above the crater, and maximum eruption rates of 21 events/hour. Activity declined by 21 May and, by 23 May, activity consisted of gas-rich eruptions with rare-to-no bombs and maximum eruption rates of 5-6 events/hour.

During 11-20 May, the eruption rate at the SW crater increased from 1-12 events/hour (11-16 May) to 6-19 events/hour (18-20 May). Events were typified by 20- to 40-second-long emissions of gas, ash, and bombs. During this period, the ash component appeared to decline and the bomb component appeared to increase. The area inundated by bombs gradually increased, reaching the outer flank of the NE crater by 17 May. Activity peaked on 22 May when strong eruptions with heavy bomb loads were observed. At this time bombs hit the cliff below the Pizzo Sopra la Fossa and cleared the lower section of the pizzo ridge where the lowest tourist shelters are located. Bombs ~0.5 m in diameter fell within 20 m of that location and the path was littered with fresh scoria tens of centimeters in diameter. By 23 May, activity had changed entirely with the eruption rate down to 5-6 events/hour and activity characterized by gas- and ash-rich ejections with few or no bombs.

The Central Crater had evolved significantly since May 2000, when the a funnel-shaped pit that had developed during 1997-99 in the SE sector of the crater (BGVN 24:06) was active with a single degassing vent only (BGVN 25:08). Over the intervening period this pit has filled and now has an inactive hornito. Since May 2000, a new hornito (2B) has developed on the rim of this pit, with a 5-10 m wide vent (2A) at its base. The 2A vent was incandescent by night and radiometer-measured temperatures were in the range 726-577°C.

The summit of the 2B hornito was occupied by an open vent that was the source of continuous gas emission with weakly formed puffs, but no eruptions during the observation period. Vent 2A was the source of vigorous degassing with well-formed puffs. Frequent vigorous phases here often sent one or two pieces of scoria to a height of 10 m above the vent rim. This vent was also the location of rare Strombolian explosions, with just 11 observed during the entire 62-hour observation period.

A new ~2 m wide vent (2C) had also opened towards the center of the Central Crater, and appeared to be the source of a small lava flow that was not observed during May 2000. The surface shows a pahoehoe form, and the flow extends around the base of the inactive hornito 2E and laps up against the back wall of the Crater Terrace (figure 65). Vent 2C was also the source of rare (24 over the entire observation period) Strombolian eruptions, characterized by loud, emissions that created well-formed column-shaped ejecta-bearing plumes.

Explanation of seismic events. In the discussion above, the number of seismic "events" is not directly comparable to the number of "eruptions" for two reasons. First, not all eruptions produce a seismic signal in the frequency range recorded by the short-period seismometer installed by University of Udine. Second, the seismic acquisition at Udine employs a trigger algorithm, which, although not perfectly efficient, has been kept constant since the installation of the 3-component station in 1992 to guarantee coherency between the graphs presented in the Bulletin.

Information Contacts: Andy Harris, Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, U.K.; Roberto Carniel, Dipartimento di Georisorse e Territorio, Università di Udine, via Cotonificio 114, I-33100 Udine, Italy (URL: http://www.swisseduc.ch/stromboli/); Maurizio Ripepe, Dipartimento di Scienze della Terra, Università di Firenze, via G. La Pira 4, I-50121 Firenze, Italy; Emanuele Marchetti, Dipartimento di Scienze della Terra, Università di Firenze, via G. La Pira 4, I-50121 Firenze, Italy; John Bailey, Department of Geology and Geophysics, SOEST, University of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA; Scott Rowland, Department of Geology and Geophysics, SOEST, University of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA; Jürg Alean, Kantonsschule Zürcher Unterland, CH8180 Bülach, Switzerland; Dave Rothery, Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, U.K.; Jonathan Dehn, University of Alaska Fairbanks, 903 Koyukuk Drive, Fairbanks, AK 99775, USA; Stromboli On-line, maintained by Jürg Alean and Roberto Carniel (URL: http://www.swisseduc.ch/stromboli/).

October 2001 (BGVN 26:10) Cite this Report

Major explosion at Stromboli kills a tourist on 20 October 2001

Continuing Strombolian activity resulted in crater-morphology changes during March-May 2001 (BGVN 26:07). Similar activity has continued at Stromboli (figure 67), with a major explosion occurring on 20 October.

On 20 October 2001 at 0237 ejecta from a major explosion at the active vents reached a group of tourists a few hundred meters away near Pizzo sopra La Fossa, severely injuring a woman who died a few days later at Messina hospital. On behalf of the Italian Civil Protection, Franco Barberi and others visited the crater on 21 October. The following is their report, including a summary of field observations, information provided by a Stromboli guide, and a brief examination of images recorded by a video camera at Pizzo sopra La Fossa managed by the Section of Catania of the Istituto Nazionale di Geofisica e Vulcanologia.

Figure 67. Seismicity detected near the summit of Stromboli during May-early November 2001. The gray-shaded bars signify the number of recorded events per day; the much smaller solid-black bars signify saturating events (i.e. with ground velocity exceeding 100 µm/s). The line shows the daily average of tremor intensity derived from hourly 60-second samples. The seismic station is located 300 m from the craters at 800 m elevation. Courtesy of Jürg Alean and Roberto Carniel.

Major explosion on 20 October 2001. In the days preceding the explosion, since at least the afternoon of 16 October, Stromboli's activity was intense at crater 3 and less intense at craters 1 and 2. A strong explosion from crater 2 occurred at about 0100 on 17 October and was followed by a lingering prolonged incandescence within the crater indicating that magma was very near to the surface. The situation remained almost the same, with fluctuations, until the morning of 19 October, when activity decreased. It again gained vigor in the afternoon and during the following night, mostly from crater 1 and subordinately from crater 2. Activity at crater 3 was less frequent compared with that of the preceding days.

The major explosion at 0237 of 20 October is clearly visible on the recorded images. The ejected material was mostly made of large blocks of strongly altered old lava. At Pizzo sopra La Fossa, where the tourists were located, several blocks of these old lavas were observed, with a maximum size of 40 x 40 x 20 cm, and clear impact pits in the loose soil (a dozen over a surface of 40 m²). Fresh scoriaceous material was rare, making it difficult to assess whether any juvenile clasts were associated with emission of the solid blocks. Fresh scoriae were abundant descending the path that approaches crater 3. The guide confirmed that in the days preceding the accident, glowing scoriae from crater 3 landed on the path to the crater rim.

Examination of the freshly ejected material revealed two previously recognized pumice and scoria suites. Some fresh material showed the intermingling of black scoria and golden pumice characteristic of type "b" of the major explosions of Stromboli described in the background section below. The nature of emitted products indicates that the explosion of 20 October was of type "a." Both types of major explosions appear to have occurred within a couple of days.

The 20 October explosion produced a new crater hole, a few ten's of meters across, between craters 1 and 3 near a small pre-existing hornito. For a few hours after the explosion, activity remained high at crater 1 and some glowing scoria set fire to vegetation in the upper part of a neighboring small valley. On the morning of 21 October, the activity had returned to normal levels, with Strombolian explosions at about 20-minute intervals, mostly from crater 1.

Civil protection implications. Every year thousands of tourists climb Stromboli, attracted by its persistent activity. The Pizzo sopra La Fossa is an ideal site for observation, as it dominates the crater terrace hosting the active craters a few hundred meters below (see maps in BGVN 15:04). The sudden occurrence of major explosive bursts, with ejecta landing on Pizzo sopra La Fossa, represents a threat for these tourists. Accordingly, access to the crater zone is prohibited and it is forbidden to spend the night there. However, there is no apparent enforcement of the restrictions, and there are only a few warning notices at the beginning of the path. For many years Italian volcanologists have, in vain, suggested that local authorities build appropriate shelters at Pizzo sopra La Fossa, to allow tourists to observe the volcanic activity in reasonably safe conditions. The 20 October incident could have been avoided. For example, the tourists could have heeded the suggestion to immediately leave the zone, given by the guide, who found them at that dangerous site 4 hours before the explosion.

The search for possible geophysical and geochemical precursors of these major explosions continues. It remains the main objective of the volcanological research on Stromboli (Carapezza and Federico, 2000).

Background on petrology and eruption dynamics. Activity occurs mostly from three craters, conventionally named 1 to 3 from NE to SW. The craters are located at ~750 m elevation within the "crater terrace," a flat area in the upper part of Sciara del Fuoco, a depression cutting the NW flank of the volcano.

The ordinary persistent activity of the volcano (Strombolian), is characterized by continuous emission of steam and gas, frequent explosions of moderate energy, and a magma column at persistently high levels in the conduit that feeds the eruptive vents. During the normal Strombolian activity, explosions occur on average every 10-20 minutes and produce jets of gas, fragments of black, crystal-rich (~50% of phenocrysts), shoshonitic scoriae, and solid blocks of pre-existing lavas, usually strongly altered, ripped from the conduit walls. The jet height is around 200-300 m and ejecta fall mostly within the crater terrace without affecting the Pizzo sopra La Fossa zone and the access paths (Barberi and others, 1993).

This ordinary activity is periodically interrupted, on average twice a year, by more violent explosions that may be associated with lava-flow emission. Jets reach a height up to 500 m, and molten as well as solid fragments fall over an area with a radius of several hundred meters. The summit zone is frequently visited by tourists. Much rarer are the so-called "eruptive paroxysms" that occur on average every 5-15 years and may also affect the inhabited zones of the island (Stromboli and Ginostra villages) with showers of bombs and blocks, ash falls, glowing avalanches, and even small tsunamis (Barberi and others, 1993).

From the nature of the erupted products, two different types of major explosions have been identified at Stromboli (Barberi and others, 1993): a) events characterized by the emission of mainly solid old lava blocks, with subordinate fragments of new magma. They are probably produced by the sudden expansion of over-pressured gas pockets accumulated in proximity to the magma column in zones with obstructed conduits; b) sudden and violent lava fountains with high emission rates of vesiculated glowing magma fragments and minor solid blocks. During these events, in addition to the usual black scoriae of the ordinary Strombolian activity, and closely intermingled with them, are crystal poor (~10% of phenocrysts) golden pumices. The chemical compositions of the two types of lapilli are nearly identical, and the black scoria is interpreted as the degassed and largely crystallized equivalent of the golden pumice (Bertagnini and others, 1999; Metrich and others, 2001). Therefore, the major type "b" explosions seem generated by the injection of a discrete volume of deep, gas-rich, and largely liquid magma, into the cooler, crystal-rich, more viscous and degassed magma permanently present in the upper part of the eruptive system.

References. Barberi, F., Rosi, M., and Sodi, A., 1993, Volcanic hazard assessment at Stromboli based on review of historical data: Acta Vulcanol. 3, p.173-187.

Bertagnini, A., Coltelli, M., Landi, P., Pompilio, M., and Rosi, M., 1999, Violent explosions yield new insight into dynamics of Stromboli volcano: Eos, Transactions of the American Geophysical Union, v. 80, p. 633, 636.

Carapezza, M.L., and Federico, C., 2000, The contribution of fluid geochemistry to the volcano monitoring of Stromboli: J. Volcanol. Geoth. Res., v. 95, p. 227-245.

Metrich, N., Bertagnini, A., Landi, P., and Rosi, M., 2001, Crystallization driven by decompression and water loss at Stromboli volcano (Aeolian Islands, Italy): J. Petrol., v. 42, no. 8, p. 1471-1490.

Information Contacts: Franco Barberi, Dipartimento di Scienze Geologiche, Università di Roma Tre, Largo San L. Murialdo 1, 00146 Roma; Maria Luisa Carapezza, Gruppo Nazionale per la Vulcanologia, INGV, Via Nizza 128, 00198 Roma; Jürg Alean, Kantonsschule Zürcher Unterland, CH8180 Bülach, Switzerland; Roberto Carniel, Dipartimento di Georisorse e Territorio, Università di Udine, via Cotonificio 114, I-33100 Udine, Italy (URL: http://www.swisseduc.ch/stromboli/).

January 2002 (BGVN 27:01) Cite this Report

Fallout from 23 January explosion carpets popular tourist area

On 23 January at 2054 a large explosion occurred at Stromboli. The explosion was accompanied by a loud noise that was heard at all of the villages on the island and ashfall that lasted for several minutes.

On 24 January, staff from Istituto Nazionale di Geofisica e Vulcanologia - Sezione di Catania (INGV-CT) visited the area SE of the summit craters near Il Pizzo Sopra la Fossa between the Bastimento and La Fossetta. They found the area covered with ash and blocks, mostly comprised of lithic material, with some clasts up to 60 cm in diameter, and with minor amounts of spatter up to 1.7 m long. No golden-colored pumice was found, which typically characterizes the most energetic events of Stromboli (Bertagnini and others, 1999). The greatest density of lithics on the ground was found in a ~200-m-wide belt between the craters and Il Pizzo. Spatter was more frequent NE of Il Pizzo. Fine-grained material covered the crater zone and the volcano's NE flank to the village of Stromboli, ~2 km to the NE. A continuous carpet of fallout material covered the zone of Il Pizzo, a spot where many tourists visit. The explosion would have posed a serious threat to tourists had it occurred during a visit. Fallout from the eruption also damaged equipment located near the summit.

During the 2.5 hours of the survey observers recorded only five weak explosions from Crater 1 and none from Craters 2 and 3. This activity was much weaker than that observed after the major explosion of 20 October 2001 (BGVN 26:10), when 15 explosions were recorded from Crater 1 and 8 from Crater 3 during a 1-hour period.

Thermal images on 24 January showed that Crater 2 had a higher temperature than the other active craters. Maximum temperatures recorded at this crater were 320°C averaged over a pixel area of 40 cm, much higher than the 200°C recorded on 20 October 2001. The high temperatures were due to spatter coating the crater's inner walls following the 23 January explosion. Measurements also revealed that the diameter of Crater 2 had grown from an estimated 10 m in October to ~26 m after the January explosion.

From the type and distribution of erupted products and the morphological changes observed at the craters, observers suggested that the eruptive event of 23 January could be related to the obstruction of the conduit of one of the craters. Gas pressure within the conduit probably built up until a major explosion occurred, ejecting mostly lithics. Conduit opening was followed by intense magmatic explosions and spatter fallout. During the present phase, observers were concerned by the lack of explosive activity at Crater 3. This may suggest an obstruction of this crater, which might be followed by a new violent episode similar to the one on 23 January.

Reference. Bertagnini A., Coltelli M., Landi P., Pompilio M., and Rosi M., 1999, Violent explosions yield new insights into dynamics of Stromboli volcano: EOS Transactions, AGU, v. 80, n. 52, p. 633-636.

Information Contacts: Sonia Calvari, Massimo Pompilio and Daniele Andronico, Istituto Nazionale di Geofisica e Vulcanologia - Sezione di Catania (INGV-CT) Piazza Roma 2, 95123 Catania, Italy.

July 2002 (BGVN 27:07) Cite this Report

Increased activity beginning in June 2002 culminates in strong explosion on 24 July

Except for the large explosion on 23 January 2002 (BGVN 27:01), activity at Stromboli was subdued until late June. However, activity fluctuated during 5-23 July (figure 68) and culminated with an explosion on the 24th. The explosion was observed (and photographed) from the harbor of the village of Stromboli by Boris Behncke and a group of French excursionists. Thermal camera and infrared spectrometer measurements were performed on 16 May and 22 June.

Figure 68. Seismic activity at Stromboli during January through mid-August 2002. Major events (those saturating the seismic station) became prominent starting in mid-June. Courtesy of Stromboli On-Line.

On 9 July, most activity was concentrated at several vents within the NE crater; the most vigorous vent was located farthest NE. Explosions sent bursts of incandescent bombs up to 150 m above this vent, with many directed obliquely toward the NE. The SW crater produced only a few weak explosions, while the central crater was inactive and remained so through 23 July.

On 17 July, the activity was much weaker; explosions occurred only at the most northeastern vent of the NE crater and produced jets of bombs that rose only a few tens of meters high. On 23 July the same vent produced incandescent jets up to 150 m high, some directed toward the Sciara del Fuoco, with spectacular cascades of bombs rolling toward the sea. Two smaller vents in the SW part of the NE crater were the site of smaller simultaneous explosions. Rather long-lived small explosions came from the SW crater and were often accompanied by ash plumes.

The explosion at about0745 on 24 July produced a brown, mushroom-shaped plume that rose ~500 m above the summit. The explosion occurred as a single burst followed by weaker ash emissions. While a loud detonation was heard in the village of Stromboli, no noise was noted at the village harbor. As a consequence of the explosion, light ash fell over the harbor area and on a number of nearby vessels. It is not known which crater produced the explosion and whether there were any persons present at the summit. Viewed from the neighboring island of Panarea during the evening of 24 July, the volcano continued its normal explosive activity with incandescent bursts at intervals of several tens of minutes.

Thermal camera and infrared spectrometer measurements by Instituto Nazionale di Geofisica e Vulcanologia (INGV). On 16 May and 22 June, a Fourier transform infrared (FTIR) spectrometer (figure 69) and FLIR thermal camera (figure 70) measurements of activity were performed on Crater 1 [NE Crater] and Crater 3 [SW Crater] from Pizzo Sopra la Fossa. Time, location, and estimates of intensity of periodic explosions were recorded.

Figure 69. Photo showing the FTIR spectrometer instrument located at Pizzo Sopra la Fossa during measurements of Crater 1 in May and June 2002. Courtesy INGV.

Figure 70. Photo showing the FLIR thermal camera located at Pizzo Sopra la Fossa during measurements of Crater 1 in May and June 2002. Courtesy INGV.

On 16 May, activity at the summit was primarily located at Crater 1 and consisted of relatively weak explosions containing smaller amounts of pyroclasts than had been seen in the preceding months. Examples of some explosions recorded with the FLIR are shown in figure 71.

Figure 71. FLIR images of Stromboli taken on 16 May 2002. Crater 1a at 1314 (top) shows pyroclastics estimated to be ~ 40 m above the source. Crater 1b at 1319 (bottom) has only a gas cavity visible. Temperature ranges vary with each image. Courtesy INGV.

On 22 June, FTIR and FLIR measurements were made from Pizzo Sopra la Fossa, observing all three craters. Measurements began at 1142 and concluded at 1350. Activity at the summit was primarily located at Crater 1 and consisted of explosions that propelled scoria to a maximum height of ~250 m above the craters. Occasional ash emissions were observed from Craters 1a and 3. The activity was of significantly greater intensity compared with that observed in May. Figure 72 shows the resulting images from measurements taken with the FLIR camera.

Figure 72. FLIR images of Stromboli taken on 22 June 2002. Crater 1a at 1150 (top) and Crater 3 at 1220 (bottom). Temperature ranges vary with each image. Courtesy INGV.

Information Contacts: Boris Behncke, Dipartimento di Scienze Geologiche, Universita di Catania, Corso Italia 55, 95129 Catania, Italy (URL: http://www.swisseduc.ch/stromboli/boris); Instituto Nazionale di Geofisica e Vulcanologia (INGV); Sezione di Catania (URL: http://www.ct.ingv.it/); Stromboli On-Line (URL: http://www.stromboli.net/).

December 2002 (BGVN 27:12) Cite this Report

Landslides on 30 December cause two tsunamis; damage in nearby villages

Following heightened seismicity during June-July 2002 that culminated in an explosion on 24 July (BGVN 27:07), major activity lessened until late December.

On 28 December, an effusive eruption started at the base of Crater 1 of the NE Crater in the summit area. This eruption ended on 29 December and a helicopter-borne thermal camera survey that day revealed three lava flows that had spread in the eastern Sciara del Fuoco and had reached the sea. Along the coast, the joined flows were ~300 m wide, but were no longer being fed.

Visibility improved on 30 December, when a new survey found an eruptive fissure running NE. The fissure started from the base of Crater 1 at ~700 m elevation and spread down to ~600 m elevation, along a length of ~200 m. On 30 December observers saw a ~200-m-long lava flow emitted from the base of the fissure, spreading in the upper Sciara del Fuoco into a small depression.

Landslides and tsunami. On 30 December at 1315 and 1322 two landslides formed along the Sciara del Fuoco. They reached the sea accompanied by fine (0.1 mm grain-size) wet dust falling on the SE flank of the island (from rock collisions during the landslides). The volume of the first landslide was estimated at ~6 x 10⁶ m³ of rock while the second was smaller at ~5 x 10⁶ m³ of rock. These landslides detached the lava from the 28 December eruption along the slope together with a large portion of the ground below.

The large volume of rock crashing into the sea caused two tsunamis, each with waves several meters high. The waves spread onto the villages of Stromboli and Ginostra damaging buildings and boats and injuring several people (according to news reports, six people were evacuated by helicopter and taken to two hospitals on Sicily). Large waves were reported on the northern coast of Sicily, 60 km S of Stromboli. The two separate landslides were formed from two distinct bodies of rock, and left a ridge on the Sciara del Fuoco wall between them. This ridge may collapse in the future; its volume is estimated to be similar to that of the first landslide.

As of 6 January 2003, the effusive eruption and thin lava flows continued along the Sciara del Fuoco. Two vents located at ~500 m and ~300 m elevation in the middle of the Sciara del Fuoco were feeding two narrow flows that merged and reached the sea. Occasional small landslides from the unstable walls of the Sciara covered the lava flows with a thin talus. Concern over another major landslide had diminished due to several small-volume rockfalls from the walls of the depression. The summit craters had not shown any explosive activity since the start of the eruption on 28 December, and no earthquakes were recorded by the indigenous seismic network. Two shocks recorded by INGV seismic stations were directly related to the spreading of the two landslides on the Sciara del Fuoco.

Previous tsunamis at Stromboli occurred in 1930, 1944, and 1954. These were related either to paroxysmal eruptive activity, to landslides along the Sciara del Fuoco, or to pyroclastic flows, but not associated with lava flow venting.

Information Contacts: Sonia Calvari, Instituto Nazionale di Geofisica e Vulcanologia (INGV); Sezione di Catania (URL: http://www.ct.ingv.it/); Stromboli On-Line (URL: http://www.stromboli.net/).

January 2003 (BGVN 28:01) Cite this Report

Lava emissions continue into January; crater morphology changes

The effusive eruption at Stromboli, which began 28 December 2002, continued into January 2003. Effusion of lava occurred at a main vent located at 500 m elevation in the middle of the Sciara del Fuoco, within the scar remaining after the 30 December 2002 landslide. The position of this vent has been rather stable since its opening, also on 30 December. Another vent, located at 600 m elevation at the NE base of Crater 1, has been active several times during the eruption, forming low-effusion rate, short lava flows lasting from a few hours to a few days. Effusion rates along the Sciara del Fuoco from the 500 m vent were very variable. During peaks in effusion rate, aa lava flows were reaching the sea causing phreatic explosions at the front. A decrease in effusion rate formed a fan of thin, narrow lava flows spreading on the upper flow field without reaching the sea.

Activation of the 600 m vent occurred each time the 500 m vent showed a marked decrease in effusion rate, suggesting a temporary magma level rise within the feeder conduit of the volcano. This observation was confirmed by an approximately 50°C increase in temperature at the bottom of the craters during activation of the 600 m vent, recorded during daily thermal mapping from a helicopter.

Lava flow emission along the Sciara del Fuoco formed a very thick flow field within the landslide scar of 30 December. Occasional small landslides from the unstable walls of the Sciara cover the lava flows with talus, increasing the thickness and instability of the flow field.

During a helicopter-borne thermal survey carried out on 12 January, arcuate cracks were detected around the southern base of the summit craters of the volcano. Other fractures, oriented NE-SW, cut through the craters. These probably result from drainage of magma in the upper part of the conduit. Collapse of the crater floor in early January significantly changed the morphology of the upper part of the volcano. Crater 2 (the middle crater) has disappeared, and Crater 1 (NE) and Crater 3 (SW) were joined together to form a unique, elongate depression. No explosive activity has been detected at the summit craters of the volcano since the start of the effusion within the Sciara del Fuoco.

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania Piazza Roma 2, 95123 Catania (URL: http://www.ct.ingv.it/).

April 2003 (BGVN 28:04) Cite this Report

Strong explosion on 5 April covers much of the summit in pyroclastic deposits

On the morning of 5 April, scientists from INGV-CT were conducting a daily helicopter flight with a portable thermal camera, surveying the active lava flow field on the upper sector of the Sciara del Fuoco, above a flat zone at the base of the 28 December 2002 eruptive fissure. Three vents along this surface were feeding small lava flows, and the summit craters were producing a very diluted gas cloud. A few minutes after the start of the survey, which began about 0900, the gas plume from the craters being blown W was suddenly crossed by a reddish ash emission, which was interpreted as resulting from further collapses within the craters. However, the red ash was soon replaced by darker juvenile material coming from Crater 1 (the NE crater) that formed a hot jet with a cauliflower shape rapidly growing above the crater. A few seconds later, Crater 3 also produced a hot jet of juvenile material. Data from the seismic network confirmed that the explosion began at 0912.

The eruptive process then evolved very rapidly, with jets from craters 1 and 3 joining together. A very powerful explosion pushed the helicopter away from the crater. A mushroom-shaped dark cloud rose from the craters, expanding vertically to an altitude of ~2 km, 1 km above the volcano's summit (figure 73). The eruptive cloud was surrounded at its base by a dark-gray cloud, while it was still expanding vertically and assuming the mushroom shape. Bombs, ash, and blocks fell on the NE flank above 400 m elevation, burning vegetation. Most of the ejecta drifted W, falling on Ginostra (~1.5 km from the summit) and destroying two houses; no people were injured.

Figure 73. Photograph of the expanding eruption plume at Stromboli on 5 April 2003. Courtesy of INGV.

Continuing the helicopter survey after the eruption, observers saw that the lava-flow field on the upper Sciara del Fuoco was completely covered by a brown carpet of debris ejected from Crater 1 during the initial phase of the event. A thick steam cloud rose above the debris due to vaporization from the wet material by the underlying lava flows. Meanwhile, several alternating black and reddish pulses occurred, mainly from Crater 3. Several fingers of light-brown debris were expanding from the NW flank of Crater 1 along the mid-section of the Sciara del Fuoco. The upper part of the volcano above 700 m elevation was completely covered by pyroclastic products. Within a few minutes after the start of the eruption, the upper Sciara del Fuoco had active flows emerging from the layer of debris covering the lava-flow field. The explosive event caused abundant emission of pumice mixed with small brown scoria. The pumice contained small crystals and was very vesiculated. Lithic fragments of lava with light-gray groundmass and centimeter-sized crystals of pyroxene were common in the pumice.

A helicopter survey on 8 April showed four active vents pouring lava onto the upper Sciara del Fuoco at 590 m elevation. Two of the flows were expanding along the middle Sciara del Fuoco, causing detachment of blocks from the flow front and small rockfalls that reached the sea. Within the summit craters a thick layer of debris had accumulated following the event of 5 April.

Information Contacts: Sonia Calvari, Istituto Nazionale di Geofisica e Vulcanologia, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/).

May 2003 (BGVN 28:05) Cite this Report

Lava effusion continues through mid-June; infrared satellite observations

The latest eruptive episode from Stromboli began on 28 December 2002 (BGVN 28:01) and included a significant explosion on 5 April (BGVN 28:04). This report includes field observations provided by the Istituto Nazionale di Geofisica e Vulcanologia (INGV) through mid-June 2003. Thermal alerts based on infrared satellite imagery over the course of this eruption have been compiled and summarized by scientists at The Open University.

Activity during 17 April-16 June 2003. Effusion of lava from vents located at ~600 m elevation, on the upper eastern corner of the Sciara del Fuoco, continued until 16 June with a generally decreasing effusion rate. This caused a significant increase in the thickness of the lava field formed since 15 February to over 50 m. Since the 5 April eruption, the summit craters of the volcano have been blocked by fallout material obstructing the conduit. Small, occasional, short-lived explosions of hot juvenile material were observed on 17 April during a helicopter survey with a hand-held thermal camera, and on 3 May from the SAR fixed camera located at 400 m elevation on the E rim of the Sciara del Fuoco.

The effusion rate from the 600-m-elevation vents on the Sciara del Fuoco showed a significant decline between 1 and 4 May, when inflation of the upper lava flow field was detected through daily helicopter-borne thermal surveys. Inflation stopped on 6 May, when two new vents opened on the inflated crust of the flow field, causing drainage and spreading new lava flows along the Sciara del Fuoco. Between the end of May and early June, gas-rich magma was extruded from the 600 m vents on the upper Sciara del Fuoco. Spattering built up two hornitos, which in a few days reached an estimated height of over 6 m. This activity was accompanied by lava flow effusion along the upper Sciara del Fuoco, with lava descending to 150 m elevation.

On 1 June, Strombolian activity resumed at Crater 1 (NE crater). It was revealed first through helicopter-borne thermal surveys, and then by direct observations from the eastern Sciara del Fuoco rim. Most of the ejecta fell within the crater, and from the lower slopes of the volcano only pulsating dark ash emissions were observed. Strombolian activity stopped around 6 June, and occasional lava flows occurred at the hornitos at 600 m elevation on 11 June. The summit craters showed discontinuous ash emission until mid-June, and the SAR fixed camera at 400 m elevation showed a Strombolian explosion with abundant ash emission on the night of 15 June.

MODVOLC Thermal Alerts. MODIS thermal anomalies for Stromboli covering the period from the start of MODIS data acquisition over Europe in May 2000 until the present were compiled using data available at http://modis.higp.hawaii.edu/.

With the exception of single-pixel alerts on 8 July and 19 September 2000 (with alert ratios of -0.798 and -0.794, both barely above the -0.800 automatic detection threshold of the thermal alerts algorithm), activity at Stromboli remained below the automatic detection threshold until November 2002 (figure 74). In that month there were two single-pixel alerts, barely above detection threshold (-0.790 on 12 November and -0.792 on 28 November). Thermal infrared radiance was higher than ever before at the time of the MODIS overpass on 20 December 2002, when there was a two-pixel alert, with alert ratios of -0.667 and -0.749.

Figure 74. Alert-ratio, number of alert pixels, and summed 4 µm (MODIS band 21) spectral radiance for MODIS thermal alerts on Stromboli between 1 November 2002 and 13 May 2003. MODIS data courtesy of the HIGP MODIS Thermal Alert Team.

These five dates were the only MODIS thermal alerts prior to the start of effusive activity on 28 December 2002 (BGVN 27:12 and 28:01). The source of the radiance to trigger these alerts was evidently incandescence at one or more of the active vents. In the case of a volcano such as Stromboli, prior to December 2002, isolated thermal alerts are more likely to represent the chance coincidence of a short-lived peak of incandescence with the time of MODIS overpass, rather than a sustained emission of infrared radiation. However the November-December 2002 thermal alerts can with hindsight be seen to have been indicators of enhanced activity in the lead-up to the 28 December effusive eruption.

On 28 December 2002 MODIS recorded its highest ever alert ratio at Stromboli (+0.419) and highest summed radiance at 4.0 µm (MODIS band 21) in a seven-pixel alert, corresponding to the daily MODIS overpass at 2115 UTC. This is a record of radiance from 300-m-wide lava flows from the NE crater (BGVN 27:12). Subsequent to that date, thermal alerts have occurred persistently at Stromboli, and evidently reflect ongoing lava effusion. The general trend of the highest alert ratio on each date, the number of alert pixels, and the summed 4.0 µm radiance for all alert pixels on each date shows an exponential decline.

There are no thermal alerts for 3-7 April 2003 inclusive, which could be because of cloud cover. There is thus no direct record of the explosion on the morning of 5 April that completely covered the upper 200 m of the volcano with bombs. However, the mild intensification of subsequent thermal-alerts indicates slight re-invigoration of the on-going lava effusion.

Information Contacts: Sonia Calvari, Istituto Nazionale di Geofisica e Vulcanologia, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/); David A Rothery and Diego Coppola, Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, United Kingdom. MODIS data courtesy of the HIGP MODIS Thermal Alert Team.

July 2003 (BGVN 28:07) Cite this Report

Flank eruption finished as of 22 July; activity resumed at summit craters on 17 April

Effusion of lava from vents located at about 600 m elevation on the upper eastern corner of the Sciara del Fuoco decreased in early June and completely stopped between 21 and 22 July. The decreasing effusion rate caused shorter lava flows, which during July did not spread below 600 m elevation. The upper part of the lava flow field, formed since 15 February on the upper Sciara del Fuoco, reached an estimated thickness of more than 50 m as a result of the slower rate.

[After] the 5 April eruption (BGVN 28:04), the summit craters of the volcano [were] blocked by fallout debris obstructing the conduit. [By 17 April the blockage was apparently cleared because] small, occasional, and short-lived explosions of juvenile, hot material were observed at Crater 3 (the SW crater) [that day] during a helicopter survey with a hand-held thermal camera, and at Crater 1 (the NE crater) on 3 May from the SAR fixed camera located at 400 m on the eastern rim of the Sciara del Fuoco.

Strombolian activity from Crater 1 (NE crater) became more frequent and intense in June, and almost continuous in July, with spatter often falling outside the crater. In July, Crater 3 (SW crater) activity consisted mainly of degassing and sporadic ash emissions, with Strombolian explosions becoming more common in the second half of July.

Erosion of the N flank of Crater 1 by landslides in the upper Sciara del Fuoco increased in July, with the 30 December 2002 landslide scar extending backward and upslope, cutting the flank of the cone 50 m below the crater rim.

Information Contacts: Sonia Calvari, Istituto Nazionale di Geofisica e Vulcanologia, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/).

August 2003 (BGVN 28:08) Cite this Report

Explosive activity in the summit craters and thermal signatures in the lava-flow field

The latest effusive eruption at Stromboli ended between 21 and 22 July (BGVN 28:07), when active lava flows on the upper Sciara del Fuoco were no longer visible. Since then explosive Strombolian activity became more common at both summit craters. Four active vents were observed within Crater 1 (the NE crater), and there was one funnel-shaped incandescent depression within Crater 3 (the SW crater). Strombolian activity during the first half of August was very intense at Crater 1, causing a spatter cone to form on the crater floor and incandescent bombs to fall on the outer flanks. Explosive activity at Crater 3 was apparently deeper, and was often accompanied by ash emission.

During the first half of August, thermal images of the apparently inactive lava flow field revealed thermal signatures within cracks on the upper flow field, and within skylights along two lava tubes in the upper Sciara del Fuoco, at ~550 m elevation. Temperatures of over 300°C, and incandescence of these hot spots, suggest endogenous growth. Incandescence and thermal signatures at these sites were not observed between 22 and 31 July.

Information Contacts: Sonia Calvari, Istituto Nazionale di Geofisica e Vulcanologia, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/).

February 2004 (BGVN 29:02) Cite this Report

After 10 February 2004, explosions at upper limit of that typically seen

According to aviation reports from the U.S. Air Force, the web camera at Stromboli captured shots of light ash emissions on 7 and 11 November 2003. In both cases plumes rose to ~ 2.5 km elevation. The Stromboli Web video camera showed a small explosion on 10 December 2003 that produced a plume to a height of ~ 1 km above the volcano. No ash was visible on satellite imagery.

The Istituto Nazionale di Geofisica e Vulcanologia (INGV) in Catania reported that explosive activity at Stromboli's three summit craters increased after 10 February 2004, leading to significant growth of the cinder cones inside the craters. Several powerful explosions, especially from Crater 1 (the NE crater) and Crater 3 (the SW crater), sent scoriae 200 m above the rims. These powerful explosions led to fallout of fresh bombs and lapilli on Il Pizzo Sopra la Fossa (an area atop the volcano about 100 m above the crater terrace) in early March. As of 8 March, Strombolian activity was occurring at the volcano, with variations in the number and frequency of explosions within normally observed limits, and the intensity of explosions at the higher limit of commonly observed activity.

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/); Toulouse VAAC, Météo-France, 42 Avenue G. Coriolis, 31057 Toulouse, France (URL: http://www.meteo.fr/vaac/); AGI Online news service, Italy (URL: http://www.agi.it/).

March 2004 (BGVN 29:03) Cite this Report

Webcams at various wavelengths document increased explosions in February 2004

Explosive activity at the summit craters of Stromboli volcano resumed in early June 2003, before the end of the effusive eruption that finished between 21 and 22 July 2003. Eruptive activity at this volcano is monitored by Istituto Nazionale di Geofisica e Vulcanologia (INGV-CT). They have installed two web cameras at an elevation of 920 m on Il Pizzo Sopra la Fossa and at an elevation of 400 m along the E margin of the Sciara del Fuoco, the depression on the N flank of the volcano. Additionally, a web thermal camera is located at the 400-m elevation site noted above, and a web infrared camera is positioned at Il Pizzo Sopra la Fossa. The 2 cameras (thermal and video) at the 400-m elevation site give important insights when visibility is insufficient at the more distant cameras. The infrared camera at Il Pizzo provides both a continuous view of the activity at the summit craters and a quantification of the energy released by the explosions at the three summit craters through an automated system called VAMOS (Cristaldi and others, 2004).

According to aviation reports from the U.S. Air Force, the web camera at Stromboli captured shots of light ash emissions on 7 and 11 November 2003. In both cases plumes rose to ~ 2.5 km altitude. According to the Toulouse VAAC the Stromboli Web video camera showed a small explosion on 10 December that produced a plume to a height of ~ 1 km above the volcano. No ash was visible on satellite imagery.

According to the INGV-CT, explosive activity at the three summit craters increased after 10 February 2004, leading to a significant growth of the cinder cones inside the craters. Several powerful explosions, especially from crater 1 (the NE-crater) and crater 3 (the SW-crater) carried scoriae 200 m above the craters. These explosions led to fallout of fresh bombs and lapilli on Il Pizzo Sopra la Fossa (the top of the volcano, ~ 100 m above the crater terrace) in early March. Samples of lapilli and scoriae collected on Stromboli by local guides have been analyzed with the scanning electron microscope and microanalysis instruments of INGV-CT (Corsaro and others, 2004). Measurements of glass compositions indicated that products erupted until 25 February 2004 are related to the black scoriaceous volcanics normally erupted during Strombolian activity. Golden basaltic pumices were absent from available samples; such pumices at this volcano have been generally associated with paroxysmal explosive events (Bertagnini and others, 1999) such as that of 5 April 2003. Analysis of components carried out on several ash samples allowed scientists at INGV-CT to recognize sideromelane and tachylite as the main components, making up ~ 80% of the erupted ash (Andronico and others, 2004). The activity of this volcano as of 8 March 2004 can be described, fittingly, as Strombolian with variations in the number and frequency of explosions within normally observed limits, and intensity of explosions at the higher limit of commonly observed activity.

References. Andronico, D., Caruso, S., Cristaldi, A., and Del Carlo, P., 2004, Caratterizzazione delle ceneri emesse dallo Stromboli nel Gennaio-Febbraio 2004: INGV-CT Internal Report, Prot. int. n° UFVG2004/034.

Corsaro, R.A., Miraglia, L., and Zanon, V., 2004, Caratterizzazione dei vetri presenti nei prodotti emessi dallo Stromboli durante il mese di febbraio: 2004 INGV-CT Internal Report, Prot. int. UFVG2004/033.

Cristaldi, A., Contelli, M., and Mangiagli, S., 2004, Rapporto settimanale sull'attivit eruttiva dello Stromboli: 22-29 Febbraio 2004. INGV-CT Internal Report, Prot. int. n° UFVG2004/031 [download at http://www.ct.ingv.it/].

Bertagnini, A., Coltelli, M., Landi, P., Pompilio, M., and Rosi, M., 1999, Violent explosions yield new insights into dynamics of Stromboli volcano. Eos, American Geophysical Union Transactions, 80, 52: 633-636.

Information Contacts: Sonia Calvari, Istituto Nazionale di Geofisica e Vulcanologia, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/); Charles Holliday, Air Force Weather Agency (AFWA), Satellite Applications Branch, Offutt AFB, NE 68113-4039, USA.

April 2007 (BGVN 32:04) Cite this Report

Flank eruption begins on 27 February 2007

According to Sonia Calvari of Istituto Nazionale di Geofisica e Vulcanologia (INGV-CT), a flank eruption started on Stromboli volcano on 27 February 2007 and continued to at least 15 March. Compared to the previous flank eruption during 2002-2003, lava effusion was about an order of magnitude greater. Initially, a NE fissure opened on the NE flank of the NE-crater, and lava emitted from the fissure formed three branches and rapidly reached the sea (figure 75).

Figure 75. Lava from Stromboli reaching the sea on 27 February 2007. Courtesy of the INGV-CT 2007 Stromboli eruption web site.

Late on the eruption's first day, the three initial flows stopped and a new vent opened at the E Margin of the Sciara del Fuoco at about 400 m elevation. In a few days, this vent emitted sufficient lava to build a lava bench several tens of meters wide, which significantly modified the coastline. These lava emissions stopped for a few hours on 9 March, after which another vent opened at about 550 m elevation on the N flank of the NE-crater, almost in the same position as one of the vents of the 2002-2003 eruption. The 550-m vent was active for less than 24 hours and, when it ceased emitting lava, the 400-m vent reopened, again feeding lava to the sea.

On 15 March 2007, while the effusion from the 400-m vent continued, a major explosion occurred at 2137 (2037 UTC). This event, similar to that on 5 April 2003 (BGVN 28:04), was recorded by all the INGV-CT monitoring web cams. As in 2003, the 2007 event occurred during a flank effusive eruption, when the summit craters were obstructed by debris fallen from the crater rims. Still images and videos can be downloaded from the INGV-CT webpage dedicated to the 2007 Stromboli eruption.

Satellite imagery. Satellite imagery revealed an ash plume fanning SSE from the eruption site beginning at 1215 UTC on 27 February 2007. Another eruption was observed on MET-8 split-window IR (infrared) imagery on the same day at 1830 UTC. Ash then blew SSE at 46-56 km/hour.

Information Contacts: Sonia Calvari, Istituto Nazionale di Geofisica e Vulcanologia Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/); INGV-CT 2007 Stromboli eruption website (URL: http://www.ct.ingv.it/stromboli2007/main.htm); U.S. Air Force Weather Agency (AFWA)/XOGM, Offutt Air Force Base, NE 68113, USA.

March 2010 (BGVN 35:03) Cite this Report

Explosions and lava flows in 2009; recent reports on 2007 eruption

Sonia Calvari of the Istituto Nazionale di Geofisica e Vulcanologia (INGV) reported that the 2007 eruptive episode at Stromboli started on 27 February and finished on 2 April (BGVN 32:04) Additional details about this eruption can be found in Barberi and others (2009) and Calvari and others (2010). Eruptions later in 2007 and during 2008 will be reported in a later issue; summaries of activity in 2009 and January 2010 are included below.

Activity during 2009. The summit activity in 2009 was very unusual, producing four or five intracrater lava flows. Lava within the crater depression was extruded on 22-25 April, 3 May, and 30 August 2009. On 8 November a major explosion from the vents in the central crater fragmented and destroyed part of the E flank of the cinder cone there. The explosion produced an eruptive column over 350 m high that drifted SE and was soon followed by a lava flow from the widened central vent. The lava flow spread within the crater depression for a few minutes and reached a maximum distance of ~ 60 m. After the 8 November explosion, activity returned to background levels.

Strong seismic activity was recorded on 24 November 2009. Observers saw an explosive eruption cloud and the emission of a lava flow. Ejecta fallout affected the summit area, particularly the Pizzo sopra la Fossa, where numerous volcanic bombs landed. Also affected was the eastern downwind flank, where a layer of pumice was deposited on the beach. The fallout of incandescent material caused some vegetation fires on the E flank. After this explosive activity, seismicity returned to the level previously observed.

Activity during January 2010. According to the INGV website, at 1912 UTC on 4 January 2010, the network of surveillance cameras recorded an explosion that affected the central vent area. During a first phase, coarse pink pyroclastic materials (bombs and possibly lithic particles) were erupted from the entire crater terrace. A second phase followed with the emission of a small ash plume. Beginning at 0757 UTC on 7 January, the IR camera located on the Pizzo sopra la Fossa showed spattering lava in the central portion of the crater, leading to a series of lava flows; the lava stopped around 0100 UTC on 8 January. At 1448 UTC on 10 January, the INGV network of surveillance cameras recorded a strong explosion that affected the N portion of the crater, causing a major fallout of volcanic bombs at Pizzo sopra la Fossa and high on the NE part of the volcano.

References. Barberi, F., Rosi, M., and Scendone, R. (eds), 2009, The 2007 eruption of Stromboli: Journal of Volcanology and Geothermal Research, v. 182, no. 3-4, p. 123-280.

Calvari, S., Lodato, L., Steffke, A., Cristaldi, A., Harris, A.J.L., Spampinato, L., and Boschi, E., 2010, The 2007 Stromboli eruption: Event chronology and effusion rates using thermal infrared data: Journal Geophysical Research, Solid Earth, 115, B4, B04201, doi:10.1029/2009JB006478.

Information Contacts: Sonia Calvari, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/).

September 2011 (BGVN 36:09) Cite this Report

Recent activity; plumbing insights; new water vapor flux technique; hydrogeology

Activity at Stromboli (figure 76), since February 2010 (BGVN 35:03) through 11 October 2011 was generally of medium to low intensity, with minor fluctuations typical for Stromboli. Istituto Nazionale di Geofisica e Vulcanologia (INGV) reported occasional episodes of increased activity; note that the dates provided below are illustrative and by no means all-inclusive. This generally occurred as either more intense explosions or increased spattering (figure 77). More intense explosions often generated coarse pyroclastics and/or expelled erupted products farther than during typical activity, sometimes to the summit platform (Pizzo sopra la Fossa) overlooking the active craters, and beyond. For example, some specific pronounced events took place on 19 December 2010, 5 August, and 5 September 2011.

Figure 76. Index map (inset) and aerial photograph of Stromboli, showing the active vents and Sciara del Fuoco, a Pleistocene landslide scarp. Underlined names indicate coastal towns. Index map courtesy of Ginkgo Maps; aerial photograph courtesy of the Italian Air Force.

Figure 77. Photograph of an eruption at Stromboli at approximately 1930 on 15 July 2011 showing spattering behavior. Courtesy of Tom Pfeiffer, Volcano Discovery.

On 30 June 2010, incandescent material thrown from the vent caused small fires on the upper flanks of the volcano. Increased spattering activity often resulted in intra- and, less frequently, extra-crater lava flows (figure 78a and b, respectively) such as those seen during 18-23 October 2010, 1-2 August, and 7 September 2011. The extra-crater lava flows of 1-2 August from the northernmost vent were the first observed since December 2010; they extended only a few hundred meters downslope (figure 78a). Extra-crater lava flows were often accompanied by land- or rock-slides down Sciara del Fuoco, fed by material that broke free from lava flow fronts and rolled down slope (e.g. 1 August 2011).

Figure 78. Intra-crater (a) and extra-crater (b) lava flows at Stromboli. (a) Spatter fed lava flow emitted from a small cone on the SW rim of the crater terrace; a small group of glowing vents can be seen at bottom left (circled). Photographed during the night of 11-12 August 2011 by Gijs de Reijke. (b) Aerial view of the summit area of Stromboli, highlighting extra-crater lava flows onto Sciara del Fuoco. The northern (N) and southern (S) vent areas are labeled in red; lava flows were emitted during 11-12 December 2010 (1, yellow), 1-2 August 2011 (2, pink), and on 18 August 2011 (3, red). Photo by Mauro Coltelli; courtesy of Istituto Nazionale di Geofisica e Vulcanologia (INGV).

Some of the parameters reported by INGV during February 2010 to October 2011 included seismic, deformation, visual observation, and gas flux. The flux of CO₂ and SO₂, and particularly the CO₂/SO₂ ratio measured in the plumes, provided insight into the provenance of the generally discreet gas discharges ("gas slugs") that regularly burst upon reaching Stromboli's vent. INGV reported that a coupled increase in the CO₂/SO₂ ratio and decrease of SO₂ flux (seen, for example, during the weeks of 5-12 and 19-26 September 2011) indicated an increased contribution of volatiles from deeper portions of the magmatic system.

Recent insight into magma generation. A study of erupted ash textures and compositions by D'Oriano and others (2011) yielded results comparable with the implications of the CO₂/SO₂ ratio reported by INGV. The authors use the commonly accepted terms for two separate populations of magma at Stromboli; these terms are presented and used herein.

"LP magmas" are those with low porphyricity (similar sized phenocrysts) that are considered to have relatively deeper origins and simple cooling histories. "HP magmas" are those with high porphyricity (multiple size populations of phenocrysts) that are considered to reside in a shallow reservoir in the crust and have more complex, multiple stage cooling histories.

D'Oriano and others (2011) found that there is a coupled, possibly persistent ascent of deep-derived CO₂ and small amounts of LP magma. They concluded that the coupled ascent of LP magma and CO₂ is transient and does not disturb the HP magma that resides in the shallow reservoir. See figure 79 for a schematic of the plumbing and magma storage zones of Stromboli.

Figure 79. Schematic cross-section of Stromboli showing the crustal plumbing system and highlighting HP and LP magma storage zones. From Aiuppa and others (2010), based upon earlier works.

New technique for measuring plume water vapor concentration. LIDAR (Light Detection and Ranging) has been used for the first time to measure water vapor flux of a volcanic plume. Fiorani and others (2011) used a CO₂ laser-based LIDAR system called ATLAS (Agile Tuned Lidar for Atmospheric Sensing) at Stromboli to measure both wind speed and water vapor concentration of the erupted plume. When combined, the measurements yielded water vapor flux of the plume. Their measurements agreed with measurements obtained from traditional methods, and were the first such measurements at an active volcano.

Hydrogeology of Stromboli. A detailed geophysical survey of Stromboli revealed some hydrogeological features of the volcano (figure 80). Revil and others (2011) conducted a survey measuring electrical resistivity, soil CO₂ concentrations, soil temperature, and self-potential along two profiles (a and b, figures 80 and 81). Their survey focused on the Pizzo crater (near the active vents), and the Rina Grande sector collapse on the E side of the island. They published a detailed schematic interpretation of fluid-flow pathways along the two profiles (figure 81).

Figure 80. Geologic map of Stromboli. Red dots denote geophysical profiles of Revil and others (2011); (a) profile across Pizzo crater, near the active vents, and (b) profile down the length of the Rino Grande sector collapse, on the E flank of Stromboli. The two profiles are coincident to the W of Neo-Stromboli crater. Modified from Revil and others (2011).

Figure 81. Interpretive hydrogeology diagram of Revil and others (2011) at Stromboli highlighting fluid flow pathways along two profiles; (a) profile across Pizzo crater, and (b) profile down the length of the Rino Grande sector collapse. See legend at right; areas in gray indicate the inferred extent of the hydrothermal system; orange lines and arrows indicate hot water flow; gray arrows indicate gas discharge; black solid and dashed lines indicate faults. See Revil and others (2011) for further details.

The authors identified an unconfined aquifer above the villages of Scari and San Vincenzo. They stated that the Rina Grande sector collapse is the "most important structural control for magmatic and hydrothermal fluids" in the upper part of the Stromboli edifice, and that it hosts the two main diffuse degassing areas of the edifice. They further concluded that the hydrothermal system reaches shallow levels in the lower part of the Rina Grande collapse (profile b, figures 80 and 81). This fact, they wrote, "raises questions about the mechanical stability of this [E] flank of the edifice".

References. Aiuppa, A., Bertagnini, A., Métrich, N., Moretti, R., Di Muro, A., Liuzzo, M., Tamburello, G., 2010, A model of degassing for Stromboli volcano, Earth and Planetary Science Letters, v. 295, no. 1-2, p. 195-204 (DOI: 10.1016/j.epsl.2010.03.040).

D'Oriano, C., Bertagnini, A., and Pompilio, M., 2011, Ash erupted during normal activity at Stromboli (Aeolian Islands, Italy) raises questions on how the feeding system works, Bulletin of Volcanology, v. 73, no. 5, p. 471-477 (DOI:10.1007/s00445-010-0425-0).

Fiorani, L., Colao, F., Palucci, A., Poreh, D., Aiuppa, A., and Giudice, G., 2011, First-time lidar measurement of water vapor flux in a volcanic plume, Optics Communications, v. 284, no. 5, p. 1295-1298 (DOI:10.1016/j.optcom.2010.10.082).

Revil, A., Finizola, A., Ricci, T., Delcher, E., Peltier, A., Barde-Cabusson, S., Avard, G., Bailly, T., Bennati, L., Byrdina, S., Colonge, J., Di Gangi, F., Douillet, G., Lupi, M., Letort, J., and Tsang Hin Sun, E., 2011, Hydrogeology of Stromboli volcano, Aeolian Islands (Italy) from the interpretation of resistivity tomograms, self-potential, soil temperature and soil CO₂ concentration measurements, Geophysical Journal International, v. 186, no. 3, p. 1078-1094 (DOI:10.1111/j.1365-246X.2011.05112.x).

Information Contacts: Boris Behncke and Mauro Coltelli, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, 95125 Catania (URL: http://www.ct.ingv.it/); Ginkgo Maps (URL: http://www.ginkgomaps.com/); Italian Air Force (URL: http://www.aeronautica.difesa.it/); Tom Pfeiffer, Volcano Discovery (URL: http://www.volcanodiscovery.com/); Gijs de Reijke, Arnhem High School, Nijmegen, Netherlands.

December 2013 (BGVN 38:12) Cite this Report

Small-to-moderate eruptions continue through February 2013

Our last report, (BGVN 36:09), covered activity at Stromboli through 11 October 2011, characterized by explosions, spattering, rockslides, and occasional lava flows. Similar activity persisted through February 2013. Stromboli (volcano and island) sits N of Sicily in the Tyrrhenian sea along the N side of the Aeolian archipelago.

Activity during 2011. The activity documented in October 2011 continued into November, December, and January 2012, and was concentrated at the two active vent areas in the northern and southern portions of the crater terrace. The wide array of activity noted above fluctuated. There were, along with frequent episodes of spattering, in particular on 10 December 2011. The spattering episodes occurred in the southern area and did not lead to the formation of any lava flows.

Activity during 2012. Stromboli exhibited two periods of isolated activity early in 2012, discussed next. On 16 February there were two sequences of up to 6 explosion earthquakes of medium-high amplitude, and ~9 Very Long Period (VLP) events per hour.

On 6 March, instruments detected both tremor coupled with a strong explosion and at least three major explosions. The first event, at 0643 UTC, presented a VLP component with amplitude ~10-times higher than the daily average. The last event, at 0645, was also of very high amplitude. The VLP events occurred at a rate of ~12 per hour, at a low level with a single event of high amplitude, corresponding to the first event in the sequence at 0643. The VLP sequence was followed by an increase in tremor lasting ~30 minutes. The tremor amplitude was medium-low with its peak corresponding to the earlier events seen around 0643.

On 22 November 2012, the persistent explosive activity at Stromboli showed a clear increase, with episodes of spattering and low lava fountains from two vents in the northern and central portions of the crater terrace.

Beginning on 23 December 2012, repeated lava overflows from the crater terrace generated small lava flows down the northern and northwestern sectors of the Sciara del Fuoco (see images below), and were accompanied by numerous landslides.

Major lava flows occurred on the evening of 23 December 2012 (to the N), during 25-27 December 2012 (to the NW), and on the morning of 7 January 2013 (to the NW). Lava vented from points just below the rim of the northernmost explosive vent on the crater terrace. During the intervals between the main effusive episodes, lava vented at extremely low, releasing numerous incandescent blocks down the Sciara del Fuoco (the area within the sector collapse). At times, small lava flows advanced for a few tens of meters before disintegrating into blocks, such as on the morning of 10 January 2013 (see the last photo in the sequence below in figure 82).

Figure 82. Frames extracted from video recorded by the visible surveillance camera at 400 m elevation on Stromboli (SQV) during the effusive episodes between 23 December 2012 and 10 January 2013. (Note date format: DD-MM-YY.) The first frame (upper left) shows the sliding of material caused by the emplacement of a lava flow onto unstable material on the slope of the Sciara del Fuoco. Courtesy INGV.

Around this time, in all cases, the effusion of lava was preceded, and often accompanied, by intense explosive activity on the crater terrace.

More insight into behavior at Stromboli during this December 2012-March 2013 eruptive phase can be found in Di Traglia and others (2014). They applied the ground-based InSAR monitoring system at Stromboli volcano, linking changes in displacement and other field-monitored observations to volcanism.

Activity during January-February 2013. A new phase of intermittent effusive activity at Stromboli consisting of small overflows of lava from the crater terrace began on 8 February 2013 and continued with significant fluctuations until the morning of 17 February. During this interval, several episodes of effusive activity occurred, which produced lava flows reaching several tens to a few hundred meters in length in the northern and northwestern sectors of the Sciara del Fuoco.

Spattering from vent N2, which lies at the top of a hornito perched on the NW rim of the crater terrace, continued for a few hours, and then diminished during the late afternoon of 14 February. See Figure 83. Subsequently, effusive activity diminished considerably, producing only very small lava overflows that extended a few tens of meters downslope to the NW. On the morning of 17 February, all effusive activity ceased and mild Strombolian activity resumed.

Figure 83. Frames extracted from video recorded by the thermal monitoring camera at 400 m elevation on Stromboli (SQT), showing effusive activity on 11 February (top) and 14 February 2013 (bottom).

After an interval of 10 days of normal Strombolian activity, Stromboli again produced small lava overflows from the crater terrace from the afternoon of 27 February 2013 through the following night. A second episode of lava overflow started on the evening of 1 March and ceased the next afternoon. Both overflows were fed by continuous spattering from vent N2.

Reference. Di Traglia, F., Intrieri, E., Nolesini, T., Bardi, F., Del Ventisette, C., Ferrigno, F., Frangioni, S., Frodella, W., Gigli, G., Lotti, A., Tacconi Stefanelli, C., Tanteri, L., Leva, D., Casagli, N., 2014, The ground-based InSAR monitoring system at Stromboli volcano: linking changes in displacement rate and intensity of persistent volcanic activity, Bulletin of Volcanology 76:786DOI 10.1007/s00445-013-0786-2.

Information Contacts: Boris Behncke and Mauro Coltelli, Istituto Nazionale di Geofisica e Vulcanologia (INGV) Osservatorio Etneo (Catania), 95125 Catania (URL: http://www.ct.ingv.it/).

November 2015 (BGVN 40:11) Cite this Report

Lava flows and ash plumes complement classic Strombolian activity of spattering; lapilli and bombs through December 2013

Italy's Stromboli volcano, located on the N side of the Aeolian archipelago in the Tyrrhenian Sea, is best known for its iconic lava fountains that have graced its summit essentially continuously for at least the last 400 years. Confirmed historical observations of its eruptions go back 2,000 years. In addition, lava flows, tephra explosions, and pyroclastic flows have all contributed to its geologic history.

Stromboli continued its frequent, mild explosive activity through the end of 2013, with a few additional noteworthy events which are well documented in weekly reports by the Instituto Nazionale de Geofisica e Vulcanologia (INGV) Sezione de Catania. These reports are briefly summarized below. INGV monitors the gas geochemistry (CO₂ and SO₂), deformation, and seismology, as well as the surficial activity at Stromboli. The thermal camera located on the Pizzo sopra la Fossa near the summit was restored in July 2013. This allowed renewed visual monitoring of the eruptive activity which consistently occurs from two well defined crater areas at the head of the Sciara del Fuoco (figure 84), a large scarp that runs from the summit down the NW side of the island (see BGVN 36:09 for a detailed geologic map).

Figure 84. The crater terrace view from the thermal camera placed on the Pizzo sopra la Fossa of Stromboli. The two areas outlined in dashed lines indicate the limits of the regions of primary eruptive activity (AREA N, AREA S) and each is approximately 100-200 m wide. The abbreviations and the arrows indicate the names and locations of the active vents. Image courtesy INGV (Stromboli Weekly Bulletin, 30 July 2013).

Strombolian activity at low-to-moderate intensity occurred throughout the year; periodic, brief, high-intensity explosions also occurred. Additionally, several small lava flows in March, April, and May from the North Crater area (Area N) sent material down the Sciara del Fuoco to the sea. A large lava flow during 17-18 April created a large MODVOLC thermal anomaly, and caused abundant steam plumes from material entering the sea. From September through the end of the year, explosive activity increased significantly at the South Crater (Area S) with a marked increase in the frequency of eruptive activity, as well as two significant high-intensity explosive events on 4 and 25 December.

Vents in both the northern and southern areas consistently produced predominantly coarse material (lapilli and bombs) at low-to-medium intensity throughout 2013. The ejecta generally rose to heights of 80-150 m during explosions that occurred with a low-to-moderate frequency rate of 6-12 per hour. The ejecta tended to be deposited on the upper part of the Sciara del Fuoco and caused minor landslides consisting of small bombs and lava fragments.

During 27-28 February a lava flow came from the N2 vent where intense spattering caused incandescent material rolling down the slope to transform into a continuous lava flow down the N side of the scarp. The lava flow eventually reached the coast and created dense clouds of steam on contact with the seawater. A second flow also traveled down the same area of the flank during 1-2 March.

On 13 and 14 March intense spattering from the N Area evolved into a lava fountain and caused a small flow down the N side of the scarp (Sciara del Fuoco). SO₂ flux values increased from average levels of 260 t/d to as high as 800 t/d during this event. Additional small lava flows from the N Area were observed on 3, 12, and 13 April.

During 17-19 April, two large lava flows emerged from the N Area and flowed about 400 m down the Sciara del Fuoco. The cooling lava front produced large lava blocks that rolled down the slope to the sea, creating large steam plumes which lifted abundant lithic particles (figure 85). The heat from this flow was recorded in the MODVOLC thermal anomaly data as 15 pixels during 17-19 April.

Figure 85. The lava flow of 17 April 2013 produced from both of the vents in the N Area of Stromboli. Courtesy of INGV, photo by F. Murè (Stromboli Weekly Bulletin, 23 April 2013).

Additional smaller lava flows during 26-28 April created numerous minor landslides and were accompanied by seismic tremors. During May the N Area remained active, with vigorous lava spattering from N2 on 1 May, and an ash cloud from N1 on 3 May. The lava spattering sent large fragments down the scarp and created a flow that reached several hundred meters before breaking into blocks that rolled down to the sea. Average-to-high-intensity explosions of coarse material occurred on 5 May from the S Area vents, and another lava flow from N2 was recorded on 7 May.

Activity decreased significantly at the N Area vents after early May, and intensity and frequency of events increased at the S Area beginning in June. In late July when the thermal camera on the Pizzo sopra la Fossa was restored, observations of the volcanic activity at the S Area were significantly improved. INGV also began plotting the average number of hourly explosions at the end of July (figure 86). Values for the second half of the year averaged less than 10 events per hour, except for increased activity in the S Area in mid October and November.

Figure 86. Trend of hourly average daily frequency of explosive events from 25 July through the end of 2013 for Stromboli. Values within gray band are estimated. Courtesy of INGV (Stromboli Weekly Bulletin, 30 December 2013).

In August, the frequency and intensity of explosions briefly increased at vent S4, with periods of vigorous spattering, strong incandescence, and some projectiles exceeding 150 m in height from the crater terrace. A short episode of medium-intensity (80-150 m high) explosions was recorded on 27 August from vent N1 that included ash mixed with coarse material. In September and October vent S3 exhibited brief periods of intense spattering on several occasions, with increasing frequency during October. A VT (volcano-tectonic) seismic event was recorded on 14 October at a depth of 4.5 km below the area of Labronzo, on the N side of the island. In Late October into November the activity shifted to vent S4 with frequent low intensity (less than 80 m high) explosions. The frequency of events peaked at 22 per hour on 2 November from the S Area vents (figure 86). Explosions of varying intensity continued at vent S4 throughout November, slowly decreasing in frequency to 5-7 per hour by the end of the month.

A series of high intensity explosions took place at the South Crater (Area S) on 4 December. Vents S2 and S4 sent ejecta to 200 m high, and a plume of ash and lapilli exceeded 250 m above the crater terrace. Fallout from these explosions covered the S part of the crater terrace, with finer particles drifting S and depositing on the slope of the scarp. The high-intensity explosions were followed by medium-intensity explosions and a lava fountain 30 to 50 m high that lasted for several minutes. The final event of the year was a series of six medium-to-high-intensity explosions late in the day on 25 December that occurred at Area S in rapid succession over two and a half minutes (figure 87). Large bombs were ejected SE from the vents. A small column of ash and lapilli from one explosion rose and rapidly dispersed to the NE, and was followed by lava fountaining and additional plumes of ash with bombs and lapilli.

Figure 87. Significant frames of the explosive sequence of 25 December 2013 from Stromboli, recovered from surveillance camera of the Pizzo sopra la Fossa. The top two frames show the strongest events from the S2 and S4 vents, and the lower two show two explosions from the S1 vent. Courtesy of INGV (Stromboli Weekly Bulletin, 30 December 2013).

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/en/); Hawai'i Institute of Geophysics and Planetology (HIGP), MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/).

January 2017 (BGVN 42:01) Cite this Report

Ongoing explosive activity during 2014, followed by large lava flows into the sea in August

Italy's Stromboli volcano, best known for lava fountain eruptions, has been essentially continuously active for at least the last 400 years. Confirmed historical observations of its eruptions go back 2,000 years. Frequent, mild explosive activity in 2013 was accompanied by lava flows and ash plumes (BGVN 40:11). Activity increased significantly during 2014 as reported by the Instituto Nazionale de Geofisica e Vulcanologia (INGV), Sezione de Catania, who monitors the gas geochemistry, deformation, and seismology, as well as the surficial activity. The Toulouse Volcanic Ash Advisory Center (VAAC) reports on ash plumes potentially affecting air travel. The activity at the summit consistently occurs from vents within two well defined north and south crater areas (figure 88) at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the island (see BGVN 36:09 for geologic map).

Figure 88. The crater terrace view of Stromboli from the SE (compare with figure 84, BGVN 40:11) showing active vents at the North and South Areas. Thermal image created 29 July 2014 by Luigi Lodato, INGV-OE, during an overflight by a Catania Coast Guard helicopter. Courtesy of INGV (Bollettino Stromboli 2014, 5 August).

A gradual increase of Strombolian activity from January through May 2014 was followed by small lava flows in June onto the Sciara del Fuoco. Several more lava flows in early July contributed to landslides that sent debris down the scarp into the ocean. In mid-July, four additional flows emerged and traveled down the scarp, with the flow on 19 July reaching the coastline. A strong surge in explosion frequency and intensity in early August caused 300-m-high fountains, followed by lava flows into the ocean for several days between 6 and 13 August. Minor ash emissions in early October sent plumes as high as 3 km altitude. Lava flows continued intermittently into October, but they no longer made it to the coast. Activity diminished significantly at the end of October 2014.

Activity during January-March 2014. After an explosive sequence on 25 December 2013 sent lapilli, bombs, and an ash plume above the summit craters, activity was quieter for several months. INGV reported that a medium-intensity explosive sequence of four events occurred from the S crater area on 4 January 2014. Lava fountaining with lapilli and bombs landing on the S part of the crater terrace and the S and E edges of the Sciara del Fuoco were reported, along with a minor landslide along the scarp. For the remainder of January, fountain explosion heights ranged from low (less than 80 m) to medium (less than 120 m) from the North (N) Area vents. Explosions of lapilli and bombs mixed with ash averaged 2-3 per hour. The South (S) Area vents sometimes exhibited medium-high-intensity (over 150 m) activity with discontinuous spattering, averaging 3-7 explosions per hour.

Only vent N1 in the North Area was active in February 2014. It was characterized by low- to medium-intensity explosive activity, emitting lapilli and bombs mixed with ash at a rate of 2-3 explosions per hour. In the South Area, vents S1, S3, and S4 were active at weak levels with emissions of fine ash mixed with some coarse material, at a rate averaging less than 6 explosions per hour.

Typical Strombolian activity during March included low- to medium-intensity explosive activity from both vent areas. A sequence of three explosions on 7 March from the South Vents led to the fallout of bombs on the upper side of the Sciara del Fuoco. At vent S1 vigorous activity on 14 March produced the rapid accumulation of lava fragments around the vent that flowed downward inside the terrace crater before subsiding. On 17 March small explosions at vent S3 briefly formed a new nearby vent with a persistent thermal anomaly. An increase in SO₂ flux was observed in mid-March by INGV. The average frequency of explosions increased in the last week of March to 10-13 per hour, and the seismic amplitudes were also slightly higher in the second half of the month.

Activity during April-June 2014. Lapilli and bombs mixed with fine ash were typical from all vents during April 2014. The South Area had greater activity, with 3 or 4 vents active during the month, although the activity level was generally low- to medium-intensity in both areas. Frequency of events was generally average, ranging from 9 to 15 per hour. Activity in May 2014 was much the same as April in the N Area until the very end of the month when vent N2 began low-intensity explosive activity. All four vents in the S Area were active throughout May. Two intervals of high intensity spattering were reported on 13 and 19 May from the S Area vents. Explosion rates increased slightly during the month to averages of 11 to 18 per hour.

Activity continued to increase in both vent areas during June 2014. Explosions increased to a rate of more than 20 per hour several times during the month, accompanied by longer periods of spattering. Seismic tremor amplitudes also increased beginning at the end of May. Two periods of vigorous spattering led to lava flows. On 17 June, 70 minutes of vigorous spattering from vent S1 fed a lava overflow within the crater that flowed NE for a few tens of meters before cooling. On the morning of 22 June, vent N2 showed a marked increase in both frequency and intensity of activity. It was characterized by vigorous spattering and discrete bursts of high-intensity (over 200 m high) lava jets. The lava flowed from a crack at the edge of the vent and spread to the upper part of the Sciara del Fuoco. It flowed down the scarp for a few hundred meters before stopping early on 23 June. The South Area vents also had explosions over 200 m high beginning on 23 June. A lava flow emerged from vent S1 on 27 June; on 29 June, vent N2 produced two lava flows, the first remained within the crater, and the second, starting in the afternoon, continued flowing into 30 June, reaching the upper Sciara del Fuoco before stopping.

The first anomalies from the MODVOLC thermal alert system using MODIS satellite thermal data in 2014 appeared in early June and increased during the lava flow emissions that occurred at the end of the month.

Activity during July-October 2014. Three lava flows emerged from Vent N2 on 1, 4, and 7 July 2014. The first flowed E for two hours over the 29 June flow within the crater, and was followed later in the day by a second flow that moved towards the Sciara del Fuoco as did the flows on 4 and 7 July. Modest slumping of material around the western portion of the small pyroclastic cone that formed around Vent N2 led to a collapse and landslide that spread rapidly down the Sciara del Fuoco on 7 July. This led to a lava overflow on the upper part of the scarp for several hours during 7 July. Additional lava flows from Vent N2 occurred on 9 and 10 July as large blocks rolling down the scarp coalesced into a lava flow that continued until the evening of 10 July. Small landslides were triggered on the steep flanks of the scarp, and fine debris was carried downslope, almost to the coastline (figure 89).

Figure 89. An INGV thermal infrared camera located at 400 m elevation recorded effusive activity on 9 and 10 July 2014 at Stromboli. a) July 10, b) July 9, c) the arrows indicate small landslide events on the Sciara del Fuoco observed from the sea and also d) from an elevation of 190 m near the Observatory, during an inspection carried out by INGV-OE personnel on 14 July. Courtesy of INGV (Bollettino Stromboli 2014, 15 July).

Four lava flows emerged from vent N2 on 15, 16, 17, and 19 July, while activity at vent N1 continued as low- to high-intensity (up to 200 m high) explosions with lapilli and bomb ejections. The new flows were emplaced just north of the earlier flows. The flow on 19 July made it to the shoreline. Meanwhile, constant spattering and low-intensity explosions continued in the South area at all four vents. The locus of activity shifted during 21 and 22 July from the North Area to the South Area.

During 3 and 4 August, there was a strong surge in explosion frequency to averages of over 30 per hour with peaks of around 100 per hour. This resulted in high-intensity explosions (to over 300 m in height above the vents) from both the North and South Areas. A new lava overflow from the crater terrace began in the early afternoon of 6 August, following the same path down the center of the Sciara del Fuoco as other recent flows. Landslides of hot material quickly reached the coastline, raising large plumes of steam. Pulsating flows of lava later reached the coast and continued flowing into the early hours of 7 August. A new lava overflow from the N Area vents in the early morning of 7 August quickly formed a broad lava field at 600 m elevation and flowed onto the Sciara del Fuoco. Several arms of the lava flowed toward the coast and entered the sea (figure 90).

Figure 90. Three lava flows entering the sea at Stromboli, while two others, in the foreground, are about to reach the coastline. Taken from the SCT camera at 06:23 GMT on 7 August 2014. Courtesy of INGV (Stromboli Update, 7 August 2014, 0745 GMT).

Lava emissions continued from the N Area vents, reaching the coastline intermittently for several days, fanning out and covering large areas of the scarp, and generating steam jets and explosions with blocks of lava sent tens of meters high as the lava entered the ocean (figure 91). During this time, explosive activity decreased noticeably at the vents, while strong degassing continued. The lava continued to flow along the eastern edge of the Sciara del Fuoco with new flows covering earlier cooling flows as they traveled down the scarp to the coastline until 13 August. Lava effusion continued until mid-October but flows gradually retreated up the scarp, no longer reaching the sea.

Figure 91. Lava flows entering the sea at Stromboli during effusive activity on 10 August 2014. Courtesy of INGV (Stromboli Update, 10 August 2014, 1400 GMT).

Sporadic ash emissions in early October 2014 led to several reports from the Toulouse VAAC. Ash was reported in the vicinity of Stromboli at a low levels on 30 September, but it was not identifiable on satellite data. It was reported below 1.8 km altitude on 8 October, below 2.4 km on 9 October and below 3 km on 11 October.

A few intermittent MODVOLC thermal anomalies were recorded in July and then substantial anomalies appeared in August, with multiple-per-day continuously during 7-29 August. Almost daily multi-pixel anomalies continued in September and October, but ended abruptly on 28 October. Only one more anomaly was recorded on 8 November 2014. No additional reports on Stromboli were issued by INGV after the 16 October 2014 update.

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/en/); Toulouse Volcanic Ash Advisory Center (VAAC), Météo-France, 42 Avenue Gaspard Coriolis, F-31057 Toulouse cedex, France (URL: http://www.meteo.fr/vaac/); Hawai'i Institute of Geophysics and Planetology (HIGP), MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/).

July 2017 (BGVN 42:07) Cite this Report

Persistent low- and moderate-level explosive activity during 2015 and 2016

Confirmed historical eruptions at Italy's Stromboli volcano go back 2,000 years as this island volcano in the Tyrrhenian Sea has been a natural beacon for eons with its near-constant fountains of lava. Explosive activity during 2014 generated numerous lava flows that traveled down the flanks, including several that reached the ocean during August (BGVN 42:01). The volcano was quieter during 2015 and 2016 as reported by the Instituto Nazionale de Geofisica e Vulcanologia (INGV), Sezione de Catania, who monitors the gas geochemistry, deformation, and seismology, as well as the surficial activity at Stromboli. Their weekly reports are summarized briefly below. Eruptive activity at the summit consistently occurs from multiple vents at both a north crater area (N Area) and a southern crater group (S Area) on the Terrazza Craterica at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the island. Thermal and visual cameras placed on the nearby Pizzo Sopra La Fossa monitor activity at the Terrazza Craterica.

No reports were issued by INGV after a report of 16 October 2014. The last activity at Stromboli in 2014 captured remotely was a MODVOLC thermal alert on 8 November 2014. Low- to medium-intensity explosions from the active vent areas at the summit characterized activity throughout 2015 and 2016. Occasional bursts of higher-intensity activity sent ash, lapilli, and bombs across the Terrazza Craterica and onto the head of the Sciara del Fuoco.

Activity during 2015. While no thermal anomalies were identified in MODIS data during 2015 or 2016, the eruptive activity continued at low-to-moderate levels. Strombolian explosions were frequent from both crater areas during January 2015. Six explosions accompanied by abundant ash emissions erupted from the N Area on 12 and 13 January. In the S Area, vents also produced tephra containing lapilli and small bombs. A high-intensity burst from the S Area on 23 January contained ash and a few lapilli and bombs.

Intermittent explosive activity continued at both crater areas during February 2015. Medium-to-low intensity explosive activity during the first week characterized the N Area, with the ejection of bombs mixed with ash. Strombolian activity increased on 7 February. In the S Area, explosions were characterized by the ejection of fine ash with lesser coarse material (lapilli and bombs). An energetic explosive event took place at the S Area on 15 February (figure 92). It was the strongest event since the activity of August 2014, and was followed by several explosions over the following 12 hours that contained abundant tephra.

Figure 92. The explosive sequence of 15 February 2015 at the S Area crater of Stromboli. Images captured by the thermal and visual cameras located on the Pizzo Sopra La Fossa span a two-minute interval that starts at 1109:08 on 15 February (A) and goes through 1110:52 (D). Image E is from the same moment as image D. Courtesy of INGV (Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 17/2/2015).

Low-intensity explosions characterized both the N and S Areas during March and early April 2015. Beginning on the afternoon of 15 April, the intensity and number of explosions increased significantly in the N Area for about 48 hours. Low- to medium-intensity explosions continued at both crater areas during May. On the evening of 11 May, and again during 13-15 May, a continuous glow was observed from the S Area caused by significant spattering activity. Strombolian activity was also noted from both crater areas on 20 and 21 May, and was more frequently observed during June 2015 (figure 93).

Figure 93. Images from the Pizzo Sopra La Fossa visual camera show the increased Strombolian activity of June 2015 at Stromboli. On 11 June a double explosion of medium intensity from the two vents located in the S Area occurred just 10 seconds apart (top). The morphology of the terrace is visible in the lower left image, and the only explosion observed from the N Area was simultaneous with an explosion from the S Area (bottom right). Courtesy of INGV (Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 16/6/2015).

Members of an expedition to the summit on 1 July 2015 observed explosive activity from the S Area vents (figure 94). Activity continued at low-to-moderate levels during July. On 16 July, a strongly intense explosive sequence occurred at both crater areas (figure 95). The first explosion occurred in the S Area at 0103. A larger explosion a few seconds later produced a jet of bombs and lapilli that lasted for about 15 seconds and rose about 300 m into the air, depositing material across the Terrazza Craterica area and the upper part of the Sciara del Fuoco. A third explosion, this time from the N Area, occurred about one minute later, ejecting bombs and lapilli 200 m into the air. Intense spattering continued from both crater areas for the next hour, after which activity resumed at lower levels.

Figure 94. Explosive activity from the S Area photographed from the Pizzo Sopra La Fossa at Stromboli on 1 July 2015. Photo by B. Behncke, courtesy of INGV (Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 7/7/2015).

Figure 95. The explosive sequence of 16 July 2015 at Stromboli. A) the first explosion from the S Area; B) the second and strongest explosion from another S Area vent; C) bombs ejected across the Terrazza Craterica; D) maximum height of the eruptive column; E) the third explosion rises from the N Area; F) glowing bombs and lapilli are ejected during the third explosion. Courtesy of INGV (Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 21/7/2015).

Low-intensity explosions accompanied by weak and discontinuous spattering with ash, lapilli, and bombs characterized activity from both areas for most of August except for a short-lived (2-hour) vigorous explosive event at the S Area beginning around 2300 on 8 August 2015. Activity was more vigorous at the N Area from 23 August through the end of the month. Low- and medium-low intensity explosions were typical during September with only a few days of medium- to medium-high intensity explosive events. Activity during October was difficult for INGV to monitor due to difficulty with their equipment, but it appeared to continue at low-to-moderate levels.

A series of medium- and medium-high intensity events occurred during 7-9 November 2015 from the N Area and were captured by the thermal camera on the Pizzo Sopra La Fossa (figure 96). Two vents in the N Area produced strong explosions at the same time generating plumes with fine ash and lapilli that likely reached over 200 m above the Terrazza Craterica. Another strong explosion from the N Area occurred on 19 November, sending coarse ejecta onto the top of the Sciara del Fuoco.

Figure 96. A strong explosion on 8 November 2015 from the N Area from 2053:26 to 2053:40 (14 seconds). Courtesy of INGV (Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 10/11/2015).

Explosions during 12 and 14 December ejected bombs and lapilli onto the Sciara del Fuoco. During an overflight on 16 December thermal imagery showed hot explosive material from the N Area deposited on the Sciara del Fuoco, and freshly erupted material surrounding the S Area as well.

Activity during 2016. During January 2016 windy and cloudy weather conditions and technical equipment problems made observations difficult for INGV, but activity was generally low to moderate at both crater areas. A strong Strombolian explosion from the N Area on 14 January was one of the larger events of the month, sending lapilli and bombs to the top of the Sciara del Fuoco. Numerous explosions from the N Area of medium-to-medium-high intensity were typical during February. Explosions at the S Area were generally low intensity. Clear weather on 15 February provided an excellent view of the two crater areas on the Terrazza Craterica (figure 97). A brief event on 21 February at the S Area caused weak spattering around the vent.

Figure 97. The Terrazza Craterica at Stromboli as seen on a clear day from the Pizzo Sopra La Fossa on 15 February 2016, showing the two crater areas (AREA N, AREA S). Photo by F. Ciancitto, courtesy of INGV (Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 16/2/2016).

During March 2016, two vents were active in the S Area, and one in the N Area until 16 March, when a second vent began activity (figure 98). The typical frequency of events during low-level activity is 0-1 explosions per hour. Rarely, higher energy events will deposit material on the Sciara del Fuoco. After a month of low-intensity activity at both crater areas, there was a rapid intensification of explosive activity at the S Area at around 2130 on 28 April, which continued through 1 May (figure 99).

Figure 98. The Terrazza Craterica as viewed from the thermal camera on the Pizzo Sopra La Fossa at Stromboli during 16 March 2016. In (A) and (B), the vents of the S Area (Area S) (1, 4) are active with occasional spattering from vent 3. In C), vent 2 of the N Area (Area N) is active; the arrow marks the new vent triggered on 16 March, in conjunction with an explosion from vent 2. Courtesy of INGV (Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 22/3/2016).

Figure 99. The Terrazza Craterica as viewed from the visible camera on the Pizzo Sopra La Fossa at Stromboli during 29-30 April 2016. On 29 April (A), simultaneous explosive activity was observed at two vents in the S Area (yellow and white arrows) and one the N Area (red arrow). On 30 April (B), daylight illuminates the profile of the Terrazza Craterica and the position of the three vents shown in (A). Courtesy of INGV (Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 3/5/2016).

Generally low-level activity during most of May was interrupted during 6-11 May with persistent incandescence and pulsating spattering at the S Area vents. Occasional medium-to-high-intensity explosions from the S Area produced significant ash emissions during the second half of May, and often sent lapilli and bombs on to the Terrazza Craterica, and occasionally onto the Sciara del Fuoco.

Events with medium-to-high intensity levels continued at the S Area during June 2016, which resulted in ash emissions covering much of the Terrazza Craterica. Intensity increased in the N area by the third week of June. Two site inspections on 6 and 7 July by INGV provided details of the ongoing changes in morphology at the Terrazza Craterica (figure 100). At the N Area, they noted two distinct vents, while in the S area they observed a large crater depression with subvertical walls that had many deep vents on the S side. Episodic explosive activity from the N Area was accompanied by small ash plumes. In the S Area, landslides occurred along the southernmost wall, lasting for tens of seconds and producing small ash clouds.

Figure 100. The Terrazza Craterica at Stromboli on the morning of 6 July 2016. The S Area (on the left) is a large crater depression with subvertical walls that has many deep vents on the S side; two distinct vents are visible from the N Area on the right. Photo by D. Andronico, courtesy of INGV (Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 12/7/2016).

During late July, persistent incandescence was visible at night from the Pizzo Sopra La Fossa coming from the northernmost vent of the S Area, which continued until 19 August. Activity diminished from this vent and appeared at the southernmost vent of the S Area on 20 August. Explosions of incandescent lava were observed about ten meters above the crater rim. Occasional high-intensity explosions from the N Area resulted in coarse ash emissions during August, especially during 27 and 28 August when two vents were active at the N area, sending bombs, lapilli, and ash onto the Sciara del Fuoco.

By the end of August activity was concentrated mostly in the N Area where two active vents ejected lapilli, bombs, and abundant ash in explosions that occurred at a rate of 2-3 per hour. On 29 September, a nighttime pulsating glow was observed from the Pizzo Sopra La Fossa visible camera emanating from the southernmost vent of the S area. Observations of the glow persisted until 6 October. INVG inferred the glow was related to deep explosive activity. Typical low-to-moderate activity during October included Strombolian activity several tens of meters above the crater rim and frequent ash emissions, primarily from the S Area.

During November and December 2016, low- and moderate-level activity continued, with persistent incandescence observed at northern vent of the S Area for most of December, and rare low-and-medium-intensity explosions observed at the N Area (figure 101).

Figure 101. Typical activity during November and December 2016 at Stromboli is represented in images captured by the visible camera located on the Pizzo Sopra La Fossa on 6 and 7 November. A) the main vents active on the Terrazza Craterica. The yellow and white arrows point respectively to the southern and northern vents of the S Area; the green and red arrows point respectively to the southern and northern vents of the N Area. B) bomb-laden explosion from the southern vent (yellow arrow) of the S Area. C) an explosion from the southern vent (green arrow) of the N Area. D) the northern vent (red arrow) of the N Area explodes and sends a bomb outside the crater. Courtesy of INGV (Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 8/11/2016).

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/en/).

February 2018 (BGVN 43:02) Cite this Report

Moderate increase in thermal energy and explosion rate, April-August 2017

Confirmed historical eruptions at Italy's Stromboli volcano go back 2,000 years as this island volcano in the Tyrrhenian Sea has been a natural beacon for eons with its near-constant fountains of lava. Eruptive activity at the summit consistently occurs from multiple vents at both a north crater area (N Area) and a southern crater group (S or CS Area) on the Terrazza Craterica at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the island (figures 102 and 103). Thermal and visual cameras placed on the nearby Pizzo Sopra La Fossa monitor activity at the Terrazza Craterica. Eruptive activity continued at low to moderate levels during 2015 and 2016, with intermittent periods of frequent explosions from both crater areas that sent ash, lapilli, and bombs across the Terrazza Craterica and onto the head of the Sciara del Fuoco (BGVN 42:07).

Figure 102. A view of Stromboli looking SW with the Sciara del Fuoco on the NW flank on the right. Image taken during 10-12 June 2017. Copyrighted photo by Martin Rietze, used with permission.

Figure 103. A view to the NW of the Terrazza Craterica from the summit of Stromboli shows the CS Area (left) and N Area (right) vents during 10-12 June 2017. Copyrighted photo by Martin Rietze, used with permission.

This report covers activity from January-October 2017. Activity similar to 2016 continued through March 2017 when an increase began in explosion rates. The increase peaked during June and then declined through August, returning to background levels in September (figures 104). Thermal energy increased beginning in early May and lasted through mid-August (figure 105). Multiple MODVOLC thermal alerts were issued for Stromboli between 4 May and 25 August 2017. Weekly reports of activity were provided by the Instituto Nazionale de Geofisica e Vulcanologia (INGV), Sezione de Catania, which monitors the gas geochemistry, deformation, and seismology, as well as the surficial activity.

Figure 104. Increased rates of explosive activity at Stromboli were recorded between early April and late August 2017, peaking during mid-June. Rates declined to background levels by early September. The green line represents the number of daily explosions from the S Area, the red line is the number of daily explosions from the N Area, and the blue line is the cumulative of the two areas. Graph includes activity from 28 March-30 October 2017. Courtesy of INGV (Rep. 44/2017, Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 31/10/2017).

Figure 105. After a lengthy period of low to intermittent thermal activity during 2015 and 2016, a distinct increase in thermal energy was recorded in satellite thermal imagery and is shown in the MIROVA system data for the year ending on 25 August 2017. Courtesy of MIROVA.

Activity during January 2017 consisted of low to moderate intensity explosions from the southern crater area (S Area), and low intensity explosions at the northern crater area (N Area). Two vents in the S Area generated explosive activity. Modest explosions with ash and lapilli occurred regularly from the southernmost vent, and rare explosions were observed from the northernmost vent (figure 106). At the northern crater area (N Area) the southern vent was active, generating ash and lapilli that was ejected a few tens of meters from the vent. There were no explosions from the northern vent in the N Area.

Figure 106. Typical activity at Stromboli's Terrazza Craterica during January 2017 photographed from visible cameras on the Pizzo sopra la Fossa. Left: Explosions at the S Area on 23 January 2017 included moderate activity at the southern vent (yellow arrow) and low activity at the northern vent (white arrow). Right: The southern vent (green arrow) of the N Area showed moderate explosive activity on 17 January 2017. Courtesy of INGV (Rep. 04/2017, Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 24/01/2017).

There were no notable changes in activity until the second week of February 2017 when explosive activity returned to the northern vent of the N Area. During the third week of February, a gradual increase in the rate and intensity of the explosions at both areas was observed which lasted throughout the rest of the month (figure 107). Coarse pyroclastic material was ejected onto the Terrazza Craterica and occasionally onto the Sciara del Fuoco. The stronger explosions generated modest plumes of dilute ash that quickly dissipated.

Figure 107. Explosive activity at Stromboli during the third week of February 2017: A) The colored arrows indicate the active vents in the S and N Areas as seen by the visible camera of the Pizzo. B) Explosion at the northern vent (blue arrow) of the N area (visible camera). C) Explosion at the southern vent (yellow arrow) of the S area (visible camera). D-F) explosions from the N and S Areas taken by the 400 level Thermal camera. Courtesy of INGV (Rep. 08/2017, Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 21/02/2017).

During the first week of March 2017, the most active vents were the southernmost vent of the S Area and the northernmost vent of the N Area. The strongest explosions from the northern vent of the N Area produced dilute ash emissions and pyroclastic ejecta that landed on the upper part of the Sciara del Fuoco. By the third week of March, and through the end of the month, most of the activity had shifted to the vents in the N Area and diminished in the S Area. On 28 March, Etna Observatory personnel restored operations at both the infrared and visible cameras on the Pizzo sopra la Fossa which allowed for more detailed observations of the activity at the summit (figure 108).

Figure 108. The Terrazza Craterica at Stromboli seen from the thermal camera on the Pizzo sopra la Fossa on 31 March 2017, showing active vents in the two crater areas (AREA N, AREA CS). The abbreviations and arrows indicate the names and locations of the active vents. Courtesy of INGV (Rep. 14/2017, Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del, vulcano Stromboli del 04/04/2017).

Throughout April 2017, the N1 vent produced low (less than 80 m high) to medium (80-150 m) intensity explosions containing ash, lapilli, and bombs. The N2 vent showed sporadic low intensity explosive activity with occasional ash emissions until 20 April when more coarse (lapilli and bombs) material was ejected. Vent C showed continuous degassing throughout the month, and low intensity explosions began there during the third week of April, causing intense spattering on 29 April. The S1 vent showed sporadic and weak explosive activity of low intensity with the ejection of coarse material until the third week when activity ceased. Vent S2 showed explosive activity of medium-low intensity (less than 120 m high) of coarse material sometimes mixed with ash. Explosion rates were around 2-10 events per hour during the first half of the month, rising to 10-15 per hour for the second half of April.

In the N Area, the N1 and N2 vents continued with a similar level of activity throughout May 2017 (figure 109). Explosions of low to medium intensity sent coarse ejecta of lapilli and bombs up to 150 m high at N1 and 120 m high at N2. The rate of explosions in the N Area ranged from 4-12 per hour.

Figure 109. The Terrazza Craterica at Stromboli seen from the thermal camera located on the Pizzo sopra la Fossa on 18 May 2017, showing active vents in the two crater areas (AREA N, AREA CS). The abbreviations and arrows indicate the names and locations of the active vents. The vents in the N Area exhibited similar levels of activity throughout the month. Courtesy of INGV (Rep. 21/2017, Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 23/05/2017).

In the S Area, activity was more variable during May, and the rate of explosions ranged from 2-10 per hour. Vent C also continued with intense degassing and low-intensity explosions and spattering. On 13 May, two emission points were observed at vent C, one a few meters S of the other. Vent S1 showed no activity until late in the second week of May when low to moderate intensity explosions rose up to 150 m with coarse ejecta. During 14-15 May, a second vent opened a few meters north of S1, and simultaneous explosions from both S1 vents sent jets of gas and incandescent material into the air. Activity decreased to low intensity explosions (less than 80 m high) with ejecta during the third week, but then increased significantly during the last week of the month. Ejecta reached 200 m high from the S1 vents (figure 110). The southern S1 vent built a surrounding hornito and produced high and narrow jets of incandescent material, while the northern emission point produced more modest jets of gas and material. Vent S2 was quiet for most of May, producing only low-intensity explosions of coarse material sometimes mixed with ash for a few days near the beginning of the month.

Figure 110. The Terrazza Craterica at Stromboli seen from the thermal camera on the Pizzo sopra la Fossa on 29 May 2017, showing active vents in the two crater areas (AREA N, AREA CS). The abbreviations and arrows indicate the names and locations of the active vents. The S1 vent in the CS Area produced high intensity jets of incandescent material that rose 200 m during the last week of the month. Courtesy of INGV (Rep. 22/2017, Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 30/05/2017).

An increase in activity during June 2017 was apparent at both the N and S Areas (figure 111). Video taken by drone and from the summit during 10-12 June shows periodic explosions with ash, lapilli, and bombs ejected around the Terrazza Craterica (See Information Contacts for link). Vent N1 was characterized by low to medium-high intensity explosive activity that ejected lapilli and bombs to 200 m and was sometimes accompanied by ash that drifted S over the island. N2 also showed variable activity which ranged from low to high intensity (ejecta rising over 200 m high) during the first week, and low to medium-high (ejecta rose to 150 m) for the rest of the month (figure 112). Numerous bombs and lapilli were deposited both inside and outside the crater rim. Intense spattering was reported at N2 on 11, 12, 18, 19, and 26 June. The explosion rate in the N Area was 9-18 per hour.

Figure 111. Thermal activity increased during June 2017 at Stromboli. Simultaneous explosions from both the S (left) and N (right) Areas during 10-12 June 2017 were photographed from the summit. Copyrighted photo by Martin Rietze, used with permission.

Figure 112. Increased thermal activity was apparent in the N Area of the Terrazza Craterica at Stromboli as seen from the thermal camera located on the Pizzo sopra la Fossa on 5 June 2017. Courtesy of INGV (Rep. 23/2017, Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 06/06/2017).

In the CS Area, sporadic low-intensity explosions (less than 80 m high) characterized vent C, with modest spattering reported on 11, 12, 13, 26, 30 June 2017. Activity at S1 continued from two vents simultaneously with low to medium intensity explosive activity (figure 113 and 114). The vent at S2 reactivated briefly on 3 June after about a month of quiet with weak spattering activity but was not active again during the month. The CS Area was characterized by an explosion frequency of 1-10 per hour.

Figure 113. Explosions of incandescent ejecta from the CS Area at Stromboli during 10-12 June 2017. Copyrighted photo by Martin Rietze, used with permission.

Figure 114. Increased activity at the CS Area of Stromboli on 26 June 2017 was recorded by the thermal camera located on the Pizzo sopra la Fossa. Activity at S1 continued from two vents simultaneously with low to medium intensity explosive activity for most of the month. Courtesy of INGV (Rep. 26/2017, Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 27/06/2017).

During July 2017, thermal activity at the vents remained moderate to high; explosions at the N1 vent sent lapilli and bombs, sometimes mixed with ash, to 200 m above the vent. At vent N2, lapilli and bombs were ejected outside the crater rim, sometimes rolling down the Sciara del Fuoco to the ocean. The hourly frequency of explosions ranged from 5-18. At S1, both vents exploded simultaneously with lapilli, bombs and occasional ash rising to 150 m numerous times.

Beginning in the afternoon of 26 July, an explosive sequence at the CS Area lasting about 90 seconds was recorded with the thermal and visible image cameras on the Pizzo sopra la Fossa (figure 115). It began with explosions from vents C and S1, followed by a second explosion at S2. More explosions from C and S1 sent debris to the SE and were followed by fountaining to about 50 m from the vents for about a minute. INGV personnel witnessed 10-cm-diameter bombs on the SW side of the Pizzo at about 850 m elevation during a 30 July site visit.

Figure 115. The explosive sequence of 26 July 2017 at Stromboli was recorded by the thermal and visible cameras located on the Pizzo sopra la Fossa. Details of the 90-second-long event are described in the text. Courtesy of INGV (Rep. 31/2017, Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 01/08/2017).

A return to background activity during August consisted of explosions of varying intensity from low (less than 80 m) to medium-low (ejecta sometimes reached 120 m in height) at both the N and CS Area vents. Explosion frequency ranged from 2-11 per hour, decreasing significantly by the end of the month. Activity continued to diminish during September. Periodic spattering from vent C occurred. Only one vent was active in the CS Area during the month. A brief increase in intensity at vent N1 during 8-9 September sent ejecta over 150 m high. By the end of September, few explosions reached over 80 m in height. A brief episode of intense spattering at vent C on 24 September sent bombs and lapilli to 40 m above the vent. Explosion frequency averaged only 2-6 per hour by the end of September.

Continuous spattering, occasionally intense, from vent C continued during October. The vents in the N Area produced low to moderate intensity explosions, and one vent in the CS Area produced low intensity explosions. A strong explosive sequence in the CS Area lasted for about five minutes on 23 October 2017 (figure 116). The first explosion of the sequence came from vent C and lasted 30 seconds. It destroyed the hornito formed around the vent. About a minute later, two explosions occurred at the S1 vent, reaching about 120 m in height and dispersing to the SE. Another explosion at vent C about 3 minutes later sent ejecta 100 m high. The event ended with a series of small ash emissions that rose a few tens of meters. Low intensity activity continued from both areas through the end of October, with low explosion rates of around 2-6 per hour.

Figure 116. An explosive sequence from the CS Area at Stromboli on 23 October 2017 lasted about five minutes. Ejecta from vents C and S1 rose 100-150 m above the vents and dispersed SE. Courtesy of INGV (Rep. 43/2017, Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 24/10/2017).

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy, (URL: http://www.ct.ingv.it/en/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP), MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Martin Rietze, Taubenstr. 1, D-82223 Eichenau, Germany (URL: https://mrietze.com/, https://www.youtube.com/channel/UC5LzAA_nyNWEUfpcUFOCpJw/videos, http://mrietze.com/web16/Stromb_Vesuv17.htm).

April 2018 (BGVN 43:04) Cite this Report

Intermittent explosions and 100-m-long lava flow, November 2017-February 2018

Confirmed historical eruptions at Stromboli go back 2,000 years; this island volcano in the Tyrrhenian Sea has been a natural beacon with its near-constant fountains of lava for eons. Eruptive activity at the summit consistently occurs from multiple vents at both a north crater area (N Area) and a southern crater group (S or CS Area) on the Terrazza Craterica at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the island. Thermal and visual cameras that monitor activity at the vents are located on the nearby Pizzo Sopra La Fossa, above the Terrazza Craterica, and at a location closer to the summit craters.

Eruptive activity during January-October 2017 peaked during June and then declined through August, returning to background levels in September; it included intermittent periods of frequent explosions from both crater areas that sent ash, lapilli, and bombs across the Terrazza Craterica and onto the head of the Sciara del Fuoco (BGVN 43:02). This report covers similar activity from November 2017-February 2018. Weekly reports of activity were provided by Italy's Instituto Nazionale de Geofisica e Vulcanologia (INGV), Sezione de Catania, which monitors the gas geochemistry, deformation, and seismicity, as well as surficial activity.

An explosive sequence on 1 November 2017 followed less than two weeks after a similar event on 23 October (BGVN 43:02) in the CS Area, creating Strombolian activity that sent ejecta 300 m high. Intermittent explosions and spattering continued until the next large explosion on 1 December, also in the CS Area. A general increase in seismicity was recorded during December 2017; intense spattering in the N Area on 15 December formed a lava flow that traveled 100 m N from the rim of the vent before stopping. The number of explosive events remained high (more than 20 per hour) through December, when both the intensity and rate of activity declined significantly, reaching levels below 10 events per hour in early February, and remaining there for the rest of the month. The general levels of intensity in the N and CS Areas, apart from the larger explosive events, were variable throughout November 2017-February 2018, generally increasing during December and decreasing during January (table 3). This pattern of activity is also reflected in the variation of the thermal activity that was recorded in the MIROVA thermal data during that time (figure 117), and the MODVOLC thermal alert data which recorded two alerts in November, and 14 in December, but none after that through February 2018.

Table 3. General intensity and activity levels at the summit vents in the N Area and CS Area at Stromboli, November 2017-February 2018. Intensity values correspond to the height of the ejecta above the vent: Low = less than 80 m high, Med-Low = less than 120 m High, Medium = less than 150 m high, Med-High = sometimes to 200 m, High = over 200 m. Coarse ejecta consisted of lapilli and bombs, and fine ejecta was primarily ash and smaller lava fragments.

Figure 117. MIROVA thermal data for Stromboli for the year ending on 2 May 2018 showed a gradual increase in thermal energy during mid-November 2017, peaking in mid-December, and then decreasing rapidly in early January to low levels by the end of the month that persisted through February 2018. Courtesy of MIROVA.

On 1 November 2017 at 0829 UTC a strong explosive sequence that lasted about 2 minutes was observed in the CS Area of the Terrazza Craterica (figure 118). The first explosion sent bombs and lapilli around the slopes of the terrace and the ejecta exceeded 300 m in height. Two more explosions followed soon after, sending material about 150 m into the air.

Figure 118. The explosive sequence of 1 November 2017 at Stromboli, taken from the INGV thermal and visual cameras at the 400 m level sent ash, bombs, and lapilli as high as 300 m. The time period covered by the explosions is about two minutes. Courtesy of INGV (Report 45/2017, Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 07/11/2017).

A survey by INGV scientists during 3-5 November 2017 evaluated the effects of this and the previous explosion on 23 October on the Terrazza Craterica. They noted that a large depression with a vent at the base, formed in the CS Area after the 23 October explosions, had been significantly enlarged during the 1 November explosions. Continuous spattering and strong incandescence were observed during the survey at the 4-m-wide C vent. They also observed that the explosive activity at S2 was produced by three emission points. They noted that the N1 site consisted of a single hornito and a secondary vent on the side flank (figure 119).

Figure 119. Several changes to the Terrazza Craterica at Stromboli were visible after the two strong explosive sequences of 23 October and 1 November 2017. a) The Terrazza Craterica on 5 November 2017; b) vent C on 30 September 2017 and c) on 5 November 2017 after the two major explosions; d) vent N1 on 30 September and e) on 5 November 2017. Photograph by D. Andronicus, courtesy of INGV (Report 45/2017, Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 07/11/2017).

The 23 October 2017 explosions ejected light brown scoriaceous material S and SE, almost reaching the Pizzo Sopra La Fossa 300 m to the E. A wide band of lithic blocks was also observed on the N flank of the W part of the Valle della Luna, an open area located S of the Terrazza, over a ridge at a higher elevation. During the 1 November explosions abundant black scoriaceous material formed spatter that covered the entire Terrazza Craterica and reached the W wall of the Pizzo facing the craters. Some of this material additionally landed on the NW ridge of the Valle della Luna and on its N flank. Blocks as large as 2 m were ejected during the 1 November event, along with reddish debris that dispersed in a wide area of the Terrazza Craterica and onto the SE flank at the S end of the Pizzo.

Vent C exhibited continuous degassing activity interrupted by short spattering episodes observed mainly on 15 and 21 November 2017. During 20-24 November, a new vent opened between vents S2 and C, which was sporadically active with incandescence and small explosions of fine-grained material. Three emission points were active from the C vent area at the end of November (figure 120).

Figure 120. The two crater areas on the Terrazza Craterica at Stromboli are visible from the thermal camera on the Pizzo Sopra La Fossa, seen here on 27 November 2017. The abbreviations and arrows indicate the names and locations of the active vents. Three emission points were active at vent C at the end of November 2017. Courtesy of INGV (Report 48/2017, Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 28/11/2017).

On 1 December 2017 at 1242 UTC a strong new explosive sequence in the CS crater area was recorded by the seismic network, although weather conditions permitted only observations of incandescence during the event. A large crater was noted a few days later in the area where the three emission points had been active at vent C. A general increase in seismic activity was observed beginning on 4 December that included increases in tremor amplitude, frequency and amplitude of VLP quakes, and the amplitude of explosion earthquakes. On 9 December, numerous explosions from vent S2 combined with strong winds and sent debris as far as the Pizzo Sopra La Fossa located 300 m E (figure 121).

Figure 121. The infrared camera on the Pizzo Sopra La Fossa captured an explosion produced by vent S2 in the CS Area at Stromboli on 9 December 2017; ejecta reached the Pizzo area. Courtesy of INGV (Report 50/2017, Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 12/12/2017).

The general increase in seismicity continued into the second week of December 2017. On 15 December 2017 intense spattering began at vent N1 at 1019 UTC. At 1330 the lava overflowed the crater rim and flowed N towards the Pianoro area, the N facing slope of the Terrazza Craterica, reaching about 100 m from the rim of N1 before stopping by 1530 that afternoon (figure 122).

Figure 122. Images taken by the infrared camera at the 400 m level of the lava overflow on 15 December 2017 from the N1 vent at Stromboli. Courtesy of INGV (Report 51/2017, Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 19/12/2017).

A survey by INGV scientists on 15 December 2017 revealed that the biggest change caused by the 1 December explosion was the formation of a new cone at vent S2 (figure 123a) with an inner crater that was almost 40 m wide. Emissions of dark ash 2-3 times per hour were observed along with spattering and ejected blocks of lava. Vent C, which had been a small pit crater prior to the explosion (figure 123b), had become a small cone that was degassing from the crater, with two smaller lateral vents exhibiting weak but continuous spattering activity (figure 123c). Vent N2 was characterized by infrequent Strombolian activity (1-2 explosions per hour). Most of the activity on 15 December was at vent N1 (figure 123a, e), where INGV scientists observed a new vent with continuous and increasing spattering that soon formed a lava flow. The flow traveled quickly across the crater area. Between 1300 and 1420, 3-4 violent and prolonged explosions at N1 ejected lava fragments tens of meters from at least four emission points. The area was covered with abundant scoriaceous material with average dimensions of 5-6 cm, and numerous fragments of black scoriaceous spatter ranging in size from 20 to 40 cm long; a few were as large as 100 cm.

Figure 123. INGV scientists recorded the changes at Stromboli's summit on 15 December 2017 that resulted from the explosions of 1 December, as well as events that day that generated a short lava flow. a) the Terrazza Craterica on 15 December 2017; b) vent C on 5 November and c) on 15 December after the explosions of 1 December created a cone; d) vent N1 on 5 November and e) on 15 December; the lava flows were produced by vents N1a and N1d. Photo by D. Andronicus, courtesy of INGV (Report 51/2017, Bollettino settimanale sul monitoraggio vulcanico, geochimico, delle deformazioni del suolo e sismico del vulcano Stromboli del 19/12/2017.

Activity diminished during January 2018; low- to medium-intensity explosions were typical in the N Area and degassing continued with intermittent explosive activity at the CS Area. During February 2018 activity decreased further with the overall explosion rate averaging generally less than 10 events per hour, a significant decline after the increases in activity that began in early November 2017 (figure 124).

Figure 124. The hourly frequency of the explosive events at Stromboli as recorded by the surveillance cameras from 1 July 2017-5 March 2018, averaged by day. The information is grouped by explosions at the N Area and the CS Area, and also shown as the total average. The Total value is the sum of the average hourly frequency by day of all the explosive events produced by the active vents. Courtesy of INGV (Repprt 10/2018, Stromboli, Bollettino Settimanale, 26/02/2018 - 04/03/2018, issue date 06/03/2018).

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy, (URL: http://www.ct.ingv.it/en/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/).

August 2018 (BGVN 43:08) Cite this Report

Continued Strombolian activity from five active summit vents through March-June 2018

Stromboli is a persistently active volcano in the Aeolian Islands, Italy, with confirmed historical eruptions going back over about 2,000 years. The active summit craters on the crater terrace are situated above the Sciara del Fuoco, a steep talus slope on the NW side of the island that leads to the Tyrrhenian Sea below. The NE crater (Area N) includes the active N1 and N2 vents, while the Central and SW craters (Area CS) contains the C, S1, and S2 vents (figures 125 and 126).

Figure 125. False color thermal Sentinel-2 satellite image of Stromboli volcano with the locations of the Sciara del Fuoco and the active craters and vents. Four of the active vents are visible in this image as bright yellow-orange areas. Image acquired on 27 June 2018 and processed using bands 12, 11, 4. Courtesy of Sentinel Hub Playground.

Figure 126. Thermal image of the Stromboli crater terrace area showing the N (area N), and the central and S (area CS) craters with the active vents. Image taken by the Pizzo webcam, courtesy of INGV (report number 11/2018 for the period 5 to 11 March, released on 13 March 2018).

Typical activity comprises degassing and multiple explosions per hour that range from tens of seconds to a few minutes, known as Strombolian activity, which is named after this particular volcano (figure 127). The activity usually consists of low-intensity explosions that eject material (ash, lapilli, and blocks) up to 80 m above the crater and medium-low intensity explosions that eject material up to 120 m above the crater. This report describes the activity at Stromboli through March to June 2018 and summarizes reports published by the Istituto Nazionale di Geofisica e Vulcanologia (INGV).

Figure 127. The daily frequency of explosions per hour produced by all the active vents at Stromboli during the period 1 January to 2 July 2018. Red indicates explosions within the N crater, green indicates activity at the central-S craters, and blue indicates the number of total events. Courtesy of INGV (report number 27/2018 for the period 25 June to 7 July, released on 3 July 2018).

Characteristic Strombolian activity occurred throughout March, typically consisting of 5-11 events per hour that ejected material up to 120 m above the craters. High-energy explosive events occurred on 7 and 18 March, both lasting around 40 seconds and ejecting material to a height of 400 m (figures 128 and 129).

Figure 128. A high-energy explosive event on 7 March 2018 at the N2 vent of Stromboli. Top images (frames a to c) are thermal images, with the corresponding visible images across the bottom (frames d to f). Images were taken by the Pizzo webcams, courtesy of INGV (report number 11/2018 for the period 5 to 11 March, released on 13 March 2018).

Figure 129. Thermal infrared images of the high-energy explosive event on 18 March 2018 at Stromboli. The images show approximately 40 seconds of the explosive sequence recorded by the Pizzo webcam, courtesy of INGV (report number 12/2018 for the period 12 to 18 March, released on 20 March 2018).

Typical Strombolian activity continued through April with 6-12 explosive events per hour, with two high-energy explosive events on 24 and 26 April that lasted nine and three minutes, respectively. Both events ejected material across the Sciara del Fuoco, producing ash plumes and lava fountaining (figure 130). Low to medium-low intensity activity continued through May and June, with explosions per hour in the range of 3-15 and 6-13, respectively.

Figure 130. INGV noted an intense explosive sequence on 26 April 2018 at Stromboli. Top images (frames A to C) show the thermal signature of the explosion; bottom images (frames G to I) are the corresponding visible images. The sequence produced abundant ash, incandescent material, lava fountaining, and ejected large blocks to a height of 250 m above the vent that then fell around the crater and on the Sciara del Fuoco. Courtesy of the INGV (Blog INGVvulcani entry for 16 July 2018).

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/en/); Blog INGVvulcani, Istituto Nazionale di Geofisica e Vulcanologia (INGV) (URL: https://ingvvulcani.wordpress.com/2018/07/16/stromboli-e-le-sue-esplosioni/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

December 2018 (BGVN 43:12) Cite this Report

Explosive activity produced ash, lapilli, and bombs, with occasional spattering during July-October 2018

Stromboli is a persistently-active volcano that currently has five active vents in the crater terrace area that sits above the steep slope of the Sciara del Fuoco. For several decades, activity has been focused at three main craters, the North crater (N area) and the Central and South craters (CS area), each with multiple frequently-active vents.

This report summarizes activity for July-October 2018 (table 4) and is based on reports by Istituto Nazionale di Geofisica e Vulcanologia (INGV) and satellite data. Intensities associated with explosions are based on the following heights of material ejected from the crater and are as follows. Very low is less than 40 m; low is 40-80 m; medium is 80-150 m; and high is greater than 150 m (figure 131). Overall, the intensity of all vents ranged from very low to medium, with variations in the eruption of ash and lapilli to bomb-sized material (less than 2 mm, 2-64 mm, and over 64 mm, respectively). The variations in activity of the five active vents during July-October is seen in Sentinel-2 thermal satellite data (figure 132).

Table 4. Activity at Stromboli during July-October 2018 summarized by vent areas: N Area (North) with vents N1 and N2; CS Area (central-south) with vents C, S1, and S2. Maximum reported heights for each month are given as meters above the crater. Data courtesy of INGV weekly reports.

Figure 131. A thermal image of the active craters of Stromboli showing the Central-South crater (Area CS) and northern crater (Area N), with the active vents S1, S1, C, N1, N2. The white horizontal lines show the heights attributed to explosion intensity: low (bassa), medium (media), and alta (high). Image taken on 29 October 2018, courtesy of INGV (Report No. 44/2018, released on 30 October 2018).

Figure 132. Infrared Sentinel-2 satellite images showing thermal variations at vents on Stromboli during July-October 2018. The active vents are shown in bright yellow-orange and gas plumes appear as light blue-white areas emanating from the vents. Courtesy of Sentinel Hub Playground.

During July Strombolian activity continued with explosions of low to medium-low intensity in the N Area; variable explosions ejected mainly lapilli and bombs along with some ash at the N1 vent, and mainly ash with lapilli and bombs at the N2 vent. Explosive activity was absent or sporadic at the N2 vent during 4-5 July. There was a rapid increase in explosion frequency at the N1 vent on the 14th, and on the 16th lapilli and bombs were ejected. During 16-29 July explosive activity in the N Area was focused at the N2 vent. The average frequency of explosions in the N area ranged from 1-19 per hour. Explosion intensity in the CS Area ranged from low to medium at both the S1 and S2 vents. The C vent produced continuous degassing that was interrupted by intense spattering on the 26th. Activity at the S1 vent was characterized as jets with some ash, lapilli, and blocks, and explosions with ash, lapilli, and blocks occurred at the S2 vent. The average frequency of explosions in the CS area ranged from 1 to 11 per hour during July. The total number of explosions at Stromboli increased in mid-July and remained elevated compared to previous months through the end of October (figure 133).

Figure 133. Graph showing the average number of explosions at Stromboli per day from 1 January to 20 October 2018. The red data are for the N crater area, green are for the CS crater area, and dark blue are the total explosions per day for all active vents. There was a period from mid-January to mid-July when there was a reduction in the frequency of explosions, which then increased to around a total of 15-30 per day. Courtesy of INGV (Report No. 44/2018 released on 30 October 2018).

Similar activity continued through August with the exception of a strong explosion at the C vent that lasted approximately one minute at 1508 on 18 August (figure 134). The explosion ejected an ash plume that rapidly dissipated. Coarse pyroclastic material fell on the crater terrace area and the upper part of the Sciara del Fuoco, and rolled down to the ocean. Occasional intense spattering at the C vent was also observed on the 27th, interrupting the continuous degassing from two vents. Medium to low, and occasionally high-intensity gas jets that incorporated incandescent material were frequent at the S1 vent through August. Low- to medium-intensity explosive activity occurred at the S2 event throughout the month. Explosions averaged 1-11 per hour for the entire CS area during August. The N area produced variable explosions that ejected lapilli and bombs with some ash at the N1 vent. During 8-12 August most of the activity in the N area continued to be focused at the N2 vent, and during this time it produced intense spattering activity. During the rest of the month activity at the N2 vent was characterized by variable explosive activity that produced lapilli and bombs with occasional spattering. The average frequency of explosions for the month was 2-20 per hour.

Figure 134. The major explosion at the Stromboli C vent on 18 August 2018 as seen in thermal and photograph images. The brief (less than one minute) explosion produced an ash plume that deposited material around the vent and on the Sciara del Fuoco. Courtesy of INGV (Report No. 34/2018 released on 21 August 2018).

The typical activity persisted through September with explosions producing ash, lapilli, and blocks (figures 135 and 136), gas jets with incandescent material (figures 137 and 138), and degassing. Over the month there was an average of 2-12 explosive events per hour at the N area, and an average of 4-20 events per hour at the CS area. Variable explosions that ejected lapilli and bombs with some ash characterized activity at the N2 vent, and mainly ash with some lapilli and bombs were typically ejected at the N1 vent. Continuous emissions originated from two points within the C vent and was occasionally interrupted by explosions and spattering. Jets of gas and incandescent material continued at the S1 vent and explosive activity with some ash and lapilli occurred at the S2 vent. The S2 vent had two active points from the 10 September onwards.

Figure 135. Ash plumes and degassing on 10 September. Courtesy of Benjamin Simons, The University of Auckland.

Figure 136. Thermal infrared video screenshots showing multiple active vents in the Stromboli central-south crater area on 10 September 2018. Vents are actively degassing and explosions eject hot lapilli and blocks at two craters. Courtesy of Benjamin Simons, The University of Auckland.

Figure 137. A gas jet with incandescent lapilli and bombs from the Stromboli central-south crater area on 10 September 2018. White gas plumes are visible emanating from other vents in the central-south and north craters. Courtesy of Benjamin Simons, The University of Auckland.

Figure 138. Screenshots of a video showing a gas jet with incandescent lapilli, bombs, and ash from the Stromboli Central-South crater area on 10 September 2018. A large bomb can be seen ejecting from the vent in the top photo. White plumes are a result of degassing of the surrounding vents. Courtesy of Benjamin Simons, The University of Auckland.

During October variable explosions continued to produce low-to medium-intensity explosions that ejected lapilli and bombs, and sometimes ash, at the N1 vent, and very low- to low-intensity explosions that produced mostly ash with some lapilli and bombs at the N2 vent. Explosions averaged 1-13 events per hour through the month. The CS area produced a higher average of 6-20 explosions per hour for October. Sustained degassing continued at two points in the C vent. Low- to medium-low-intensity jets on incandescent material occurred at the S1 vent, and the same intensity of explosive activity was reported at the S2 vent.

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/en/); Benjamin Simons, The University of Auckland, 23 Symonds Street, Auckland, 1010, New Zealand (URL: https://unidirectory.auckland.ac.nz/people/profile/bsim836); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

March 2019 (BGVN 44:03) Cite this Report

Constant explosions from both crater areas during November 2018-February 2019

Nearly constant fountains of lava at Stromboli have served as a natural beacon in the Tyrrhenian Sea for at least 2,000 years. Eruptive activity at the summit consistently occurs from multiple vents at both a north crater area (N Area) and a southern crater group (CS Area) on the Terrazza Craterica at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the island. Thermal and visual cameras that monitor activity at the vents are located on the nearby Pizzo Sopra La Fossa, above the Terrazza Craterica, and at a location closer to the summit craters.

Eruptive activity from November 2018 to February 2019 was consistent in terms of explosion intensities and rates from both crater areas at the summit, and similar to activity of the past few years (table 5). In the North Crater area, both vents N1 and N2 emitted a mixture of coarse (lapilli and bombs) and fine (ash) ejecta; most explosions rose less than 80 m above the vents, some reached 150 m. Average explosion rates ranged from 4 to 21 per hour. In the CS crater area continuous degassing and occasional intense spattering were typical at vent C, vent S1 was a low-intensity incandescent jet throughout the period. Explosions from vent S2 produced 80-150 m high ejecta of ash, lapilli and bombs at average rates of 3-16 per hour. Thermal activity at Stromboli was actually higher during November 2018-February 2019 than it had been in previous months as recorded in the MIROVA Log Radiative Power data from MODIS infrared satellite information (figure 139).

Table 5. Summary of activity levels at Stromboli, November 2018-February 2019. Low intensity activity indicates ejecta rising less than 80 m and medium intensity is ejecta rising less than 150 m. Data courtesy of INGV.

Figure 139.Thermal activity at Stromboli increased during November 2018-February 2019 compared with the preceding several months as recorded in the MIROVA project log radiative power data taken from MODIS thermal satellite information. Courtesy of MIROVA.

Activity at the N area was very consistent during November 2018 (figure 140). Explosions of low-intensity (less than 80 m high) to medium-intensity (less than 150 m high) occurred at both the N1 and N2 vents and produced coarse material (lapilli and bombs) mixed with ash, at rates averaging 6-16 explosions per hour. In the SC area continuous degassing was reported from vent C with a brief period of intense spattering on 26 November. At vent S1 low- to medium-intensity incandescent jetting was reported. At vent S2, low- and medium-intensity explosive activity produced a mixture of coarse and fine (ash) material at a frequency of 3-18 events per hour.

Figure 140. The Terrazza Craterica at Stromboli on 12 November 2018 as viewed by the thermal camera placed on the Pizzo sopra la Fossa, showing the two main crater areas and the active vents within each area that are discussed in the text. Heights above the crater terrace, as indicators of intensity of the explosions, are shown divided into three intervals of low (basso), medium (media), and high (alta). Courtesy of INGV (Report 46/2018, Stromboli, Bollettino Settimanale 05/11/2018 - 11/11/2018, data emissione 13/11/2018).

Similar activity continued during December at both crater areas, although there were brief periods of more intense activity. Low- to medium-intensity explosions at both N area vents produced a mixture of coarse and fine-grained material at rates averaging 4-21 per hour. During 6-7 December ejecta from the N vents fell onto the upper part of the Sciara del Fuoco and rolled down the gullies to the coast, producing tongues of debris (figure 141). An explosion at N1 on 12 December produced a change in the structure of the crater area. During 10-16 December the ejecta from the N area landed outside the crater on the Sciara del Fuoco. Intense spattering was observed from N2 on 18, 22, and 31 December. In the CS area, continuous degassing took place at vent C, along with a brief period of intense spattering on 1-2 December. Low to medium intensity incandescent jets persisted at S1 along with low-and medium-intensity explosions of coarse and fine-grained material at vent S2. Rates of explosion at the CS area were higher at the beginning of December (10-18 per hour) and lower during the last week of the month (3-4 per hour).

Figure 141. Images from the Q 400 thermal camera at Stromboli taken on 6 December 2018 showed the accumulation of pyroclastic material in several gullies on the upper part of the Sciara del Fuoco following an explosion at vent N2 at 1520 UTC. The images illustrate the rapid cooling of the pyroclastic material in the subsequent two hours. Courtesy of INGV (Report 50/2018, Stromboli, Bollettino Settimanale, 03/12/2018 - 09/12/2018, data emissione 11/12/2018).

Explosive intensity was low (ejecta less than 80 m high) at vent N1 at the beginning of January 2019 and increased to medium (ejecta less than 150 m high) during the second half of the month, producing coarse ejecta of lapilli and bombs. Intensity at vent N2 was low to medium throughout the month with both coarse- and fine-grained material ejected. Explosions from N2 sent large blocks onto the Sciara del Fuoco several times throughout the month and usually was accompanied by intense spattering. Explosion rates varied, with averages of 9 to 16 per hour, throughout the month in the N area. In the CS area continuous degassing occurred at vent C, and low-intensity explosions of coarse-grained material were reported during the second half of the month. Low-intensity incandescent jets at S1 along with low- and medium-intensity explosions of coarse and fine-grained material at S2 persisted throughout the month.

A helicopter overflight of Stromboli on 8 January 2019 allowed for detailed visual and thermal observations of activity and of the morphology of the vents at the summit (figure 142). Vent C had two small hornitos, and a small scoria cone was present in vent S1, while a larger crater was apparent at S2. In the N crater area vent N2 had a large scoria cone that faced the Sciara del Fuoco to the north; three narrow gullies were visible at the base of the cone (figure 143). Vent S1 was a large crater containing three small vents aligned in a NW-SE trend; INGV scientists concluded the vents formed as a result of the 12 December 2018 explosion. Thermal images showed relatively low temperatures at all fumaroles compared with earlier visits.

Figure 142. Thermal images from Stromboli taken during the overflight of 8 January 2019 showed the morphological structure of the individual vents of the N and CS crater areas. Courtesy of INGV (Report 03/2019, Stromboli, Bollettino Settimanale, 07/01/2019 - 13/01/2019, (data emissione 15/01/2019).

Figure 143. An image taken at Stromboli during the overflight of 8 January 2019 shows the morphological structure of the summit Terrazza Craterica with three gullies at the base of the scoria cone of vent N2. The top thermal image (inset a) shows that the fumaroles in the upper part of the Sciara del Fuoco have low temperatures. Courtesy of INGV (Report 03/2019, Stromboli, Bollettino Settimanale, 07/01/2019 - 13/01/2019, data emissione 15/01/2019).

Activity during February 2019 declined slightly from the previous few months. Explosions at vent N1 were of medium-intensity and produced coarse material (lapilli and bombs). At N2, low-intensity explosions produced fine ash. Average explosion rates in the N area ranged from 4-11 per hour. At the CS area, continuous degassing and low-intensity explosions produced coarse and fine-grained material from vents C and S2 while low-intensity incandescent jets were active at S1. The explosion rates at the CS area averaged 2-13 per hour.

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy, (URL: http://www.ct.ingv.it/en/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/).

September 2019 (BGVN 44:09) Cite this Report

Major explosions on 3 July and 28 August 2019; hiker killed by ejecta

Near-constant fountains of lava at Stromboli have served as a natural beacon in the Tyrrhenian Sea for at least 2,000 years. Eruptive activity at the summit consistently occurs from multiple vents at both a north crater area (N area) and a southern crater group (CS area) on the Terrazza Craterica at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the volcano-island. Periodic lava flows emerge from the vents and flow down the scarp, sometimes reaching the sea; occasional large explosions produce ash plumes and pyroclastic flows. Thermal and visual cameras that monitor activity at the vents are located on the nearby Pizzo Sopra La Fossa, above the Terrazza Craterica, and at multiple locations on the flanks of the volcano. Detailed information for Stromboli is provided by Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) as well as other satellite sources of data; March-August 2019 is covered in this report.

Typical eruptive activity recorded at Stromboli by INGV during March-June 2019 was similar to activity of the past few years (table 6); two major explosions occurred in July and August with a fatality during the 3 July event. In the north crater area, both vents N1 and N2 emitted fine (ash) ejecta, occasionally mixed with coarser lapilli and bombs; most explosions rose less than 80 m above the vents, some reached 150 m. Average explosion rates ranged from 1 to 12 per hour. In the CS crater area continuous degassing and occasional intense spattering were typical at vent C, vent S1 was a low-intensity incandescent jet throughout the period. Explosions from vent S2 produced 80-150 m high ejecta of ash, lapilli, and bombs at average rates of 2-17 per hour.

After a high-energy explosion and lava flow on 25 June, a major explosion with an ash plume and pyroclastic flow occurred on 3 July 2019; ejecta was responsible for the death of a hiker lower down on the flank and destroyed monitoring equipment near the summit. After the explosion on 3 July, coarse ejecta and multiple lava flows and spatter cones emerged from the N area, and explosion rates increased to 4-19 per hour. At the CS area, lava flows emerged from all the vents and spatter cones formed. Explosion intensity ranged from low to very high with the finer ash ejecta rising over 250 m from the vents and causing ashfall in multiple places on the island. This was followed by about 7 weeks of heightened unrest and lava flows from multiple vents. A second major explosion with an ash plume and pyroclastic flow on 28 August reshaped the summit area yet again and scattered pyroclastic debris over the communities on the SW flank near the ocean.

Table 6. Summary of activity levels at Stromboli, March-August 2019. Low-intensity activity indicates ejecta rising less than 80 m, medium-intensity is ejecta rising less than 150 m, and high-intensity is ejecta rising over 200 m above the vent. Data courtesy of INGV.

Thermal activity was low from March through early June 2019 as recorded in the MIROVA Log Radiative Power data from MODIS infrared satellite information. A sharp increase in thermal energy coincided with a large explosion and the emergence of numerous lava flows from the summit beginning in late June (figure 144). High heat-flow continued through the end of August and dropped back down at the beginning of September 2019 after the major 28 August explosion.

Figure 144. Thermal activity at Stromboli was low and intermittent from 12 November 2018 through early June 2019, based on this MIROVA plot of thermal activity through August 2019. A spike in thermal energy in late June coincided with a major explosion on 3 July and the emergence of lava from the summit area. Heightened activity continued from 3 July through 28 August with multiple lava flows emerging from both crater areas. Courtesy of MIROVA.

Activity during March-June 2019. Activity was low during March 2019. Low- to medium-intensity explosions occurred at both vents N1 and N2 in the north area. Ejecta was mostly coarse grained (lapilli and bombs) from N1 and fine-grained ash mixed with some coarse material from N2. Intense spattering activity was reported from N2 on 29 March. Explosion rates were reported at 5-12 per hour. At the CS area, medium-intensity explosions from both south area vents produced lapilli and bombs mixed with ash at a rate of 2-9 explosions per hour.

During a visit to the Terrazza Craterica on 2 April 2019, degassing was visible from vents N1, N2, C, and S2; activity continued at similar levels to March throughout the month. Low- and medium-intensity explosions with coarse ejecta, averaging 3-12 per hour, were typical at vent N1 while low-intensity explosions with fine-grained (ash) ejecta occurred at a similar rate from N2. Continuous degassing was observed at the C vent, and low-intensity incandescent jets were present at S1 throughout the month. Multiple emission points from S2 (as many as 4) produced low- to medium-intensity explosions at rates of 4-14 explosions per hour; the ejecta was mostly fine-grained mixed with some coarse material. Frequent explosions on 19 April produced abundant pyroclastic material in the summit area.

Low to medium levels of explosive activity at all of the vents continued during May 2019. Emissions consisted mostly of ash occasionally mixed with coarser material (lapilli and bombs). Rates of explosion were 2-8 per hour in the north area, and 5-16 per hour in the CS Area. Explosions of low-intensity continued from all the vents during the first part of June at rates averaging 2-12 per hour, although brief periods of high-frequency explosions (more than 21 events per hour) were reported during the week of 10 June. Strong degassing was observed from crater C during an inspection on 12 June (figure 145); by the third week, continuous degassing was interrupted at C by sporadic short-duration spattering events.

Figure 145. The Terrazza Craterica as seen from the Pizzo sopra la Fossa (above, near the summit) at Stromboli on 12 June 2019. In red are the two craters (N1 and N2) of the N crater area, in green is the CS crater area with 2 vents (C1 and C2) in the central crater and S2, the largest and deepest crater in the CS area, also with at least two vents. S1 is hidden by the degassing of crater C. Photograph by Giuseppe Salerno, courtesy of INGV (Report 25/2019, Stromboli, Bollettino Settimanale, 10/06/2019-16/06/2019).

Late on 25 June 2019, a high-energy explosion that lasted for 28 seconds affected vent C in the CS area. The ejecta covered a large part of the Terrazza Craterica, with abundant material landing in the Valle della Luna. An ash plume rose over 250 m after the explosion and drifted S. After that, explosion frequency varied from medium-high (17/hour) on 25 June to high (25/hour) on 28 June. On 29 June researchers inspected the summit and noted changes from the explosive events. Thermal imagery indicated that the magma level at N1 was almost at the crater rim. The magma level at N2 was lower and explosive activity was less intense. At vent C, near-constant Strombolian activity with sporadic, more intense explosions produced black ash around the enlarged vent. At vent S2, a pyroclastic cone at the center of the crater produced vertical jets of gas, lapilli, and bombs that exceeded 100 m in height (figure 146).

Figure 146. A high-energy explosion at Stromboli late on 25 June 2019 affected vent C in the CS Area (top row). The ejecta covered a large part of the Terrazza Craterica. An ash plume rose over 250 m after the explosion and drifted S. On 29 June (bottom row) thermal imagery indicated that the magma level at N1 was almost at the crater rim. At vent C, near-constant Strombolian activity was interrupted with sporadic, more intense explosions. At vent S2, a pyroclastic cone at the center of the crater produced vertical jets of gas, lapilli, and bombs that exceeded 100 m in height. Photo 2f by L. Lodato, courtesy of INGV (Rep 27/2019, Stromboli, Bollettino Settimanale, 24/06/2019-30/06/2019).

Activity during July 2019. A large explosion accompanied by lava and pyroclastic flows affected the summit and western flank of Stromboli on 3 July 2019. Around 1400 local time an explosion from the CS area generated a lava flow that spilled onto the upper part of the Sciara del Fuoco. Just under an hour later several events took place: lava flows emerged from the C vent and headed E, from the N1 and N2 vents and flowed N towards Bastimento, and from vent S2 (figure 147). The emergence of the flows was followed a minute later by two lateral blasts from the CS area, and a major explosion that involved the entire Terrazza Craterica lasted for about one minute (figure 148). Within seconds, the pyroclastic debris had engulfed and destroyed the thermal camera located above the Terrazza Craterica on the Pizzo Sopra La Fossa and sent a plume of debris across the W flank of the island (figure 149). Two seismic stations were also destroyed in the event. The Toulouse VAAC reported a plume composed mostly of SO₂ at 9.1 km altitude shortly after the explosion. They noted that ash was present in the vicinity of the volcano, but no significant ashfall was expected. INGV scientists observed the ash plume at 4 km above the summit.

Figure 147. A major eruptive event at Stromboli on 3 July 2019 began with an explosion from the CS area that generated a lava flow at 1359 (left). About 45 minutes later (at 1443:40), lava flows emerged from all of the summit vents (right), followed closely by a major explosion. Courtesy of INGV (Eruzione Stromboli. Comunicato straordinario del 4 luglio 2019).

Figure 148. A major explosion at Stromboli beginning at 1445 on 3 July 2019 was preceded by lava flows from all the summit vents in the previous 60 seconds (top row). This thermal camera (SPT) and other monitoring equipment on the Pizzo Sopra La Fossa above the vents were destroyed in the explosion (bottom row). Courtesy of INGV (Il parossismo dello Stromboli del 3 luglio 2019 e l'attività nei giorni successivi: il punto della situazione al 13 luglio 2019).

Figure 149. The monitoring equipment at Stromboli on the Pizzo Sopra La Fossa above the summit was destroyed in the major explosion of 3 July 2019 (left, photo by F. Ciancitto). Most of the W half of the island was affected by pyroclastic debris after the explosion, including the town of Ginostra (right). Courtesy of INGV (Report 28/2019, Stromboli, Bollettino Settimanale, 01/07/2019 - 07/07/2019).

Two pyroclastic flows were produced as a result of the explosions; they traveled down the Sciara and across the water for about 1 km before collapsing into the sea (figure 150). A hiker from Sicily was killed in the eruption and a Brazilian friend who was with him was badly injured, according to a Sicilian news source, ANSA, and the New York Post. They were hit by flying ejecta while hiking in the Punta dei Corvi area, due W of the summit and slightly N of Ginostra, about 100 m above sea level according to INGV. Most of the ejecta from the explosion dispersed to the WSW of the summit. Fallout also ignited vegetation on the slopes which narrowly missed destroying structures in the town. Ejecta blocks and bombs tens of centimeters to meters in diameter were scattered over a large area around the Pizzo Sopra La Fossa and the Valle della Luna in the direction of Ginostra. Smaller material landed in Ginostra and was composed largely of blonde pumice, that floated in the bay (figure 151). The breccia front of the lava flows produced incandescent blocks that reached the coastline. High on the SE flank, the abundant spatter of hot pyroclastic ejecta coalesced into a flow that moved 200-300 m down the flank before cooling, crossing the path normally used by visitors to the summit (figure 152).

Figure 150. At the time of the major explosion of Stromboli on 3 July 2019 people on a German ship located about 2 km off the northern coast captured several images of the event. (a) Two pyroclastic flows traveled down the Sciara del Fuoco and spread over the sea up to about 1 km from the coast. (b) The eruption column was observed rising several kilometers above the summit as debris descended the Sciara del Fuoco. (c) Fires on the NW flank were started by incandescent pyroclastic debris. The photos were taken by Egon Karcher and used with permission of the author by INGV. Courtesy of INGV (Il parossismo dello Stromboli del 3 luglio 2019 e l'attività nei giorni successivi: il punto della situazione al 13 luglio 2019).

Figure 151. Pumice filled the harbor on 4 July 2019 (left) and was still on roofs (right) on 7 July 2019 in the small port of Ginostra on the SW flank of Stromboli after the large explosion on 3 July 2019. Photos by Gianfilippo De Astis, courtesy of INGV (Il parossismo dello Stromboli del 3 luglio 2019 e l'attività nei giorni successivi: il punto della situazione al 13 luglio 2019).

Figure 152. A small lava flow high on the SE flank of Stromboli formed during the 3 July 2019 event from abundant spatter of hot pyroclastic ejecta that coalesced into a flow and moved 200-300 m down the flank before cooling, crossing the path normally used by visitors to the summit. Photo by Boris Behncke taken on 9 July 2019, courtesy of INGV (Il parossismo dello Stromboli del 3 luglio 2019 e l'attività nei giorni successivi: il punto della situazione al 13 luglio 2019).

INGV scientists inspected the summit on 4 and 5 July 2019 and noted that the rim of the Terrazza Craterica facing the Sciara del Fuoco in both the S and N areas had been destroyed, but the crater edge near the central area was not affected. In addition, the N area appeared significantly enlarged and deepened, forming a single crater where the former N1 and N2 vents had been located; an incandescent jet was active in the CS area (figure 153). Explosive activity declined significantly after the major explosions, although moderate overflows of lava continued from multiple vents, especially the CS area where the flows traveled about halfway down the southern part of the Sciara del Fuoco; lava also flowed E towards Rina Grande (about 0.5 km E of the summit). The main lava flows active between 3 and 4 July produced a small lava field along the Sciara del Fuoco which flowed down to an elevation of 210 m in four flows along the S edge of the scarp (figure 154). Additional block avalanches rolled to the coastline.

Figure 153. The summit craters of Stromboli were significantly altered during the explosive event of 3 July 2019. The rim of the Terrazza Craterica facing the Sciara del Fuoco in both the CS and N areas was destroyed, but the crater edge near the CS area was not affected. In addition, the N area was significantly enlarged and deepened, forming a single crater where the former N1 and N2 vents had been located; an incandescent jet was active in the CS area. Courtesy of INGV (Report 28/2019, Stromboli, Bollettino Settimanale, 01/07/2019 - 07/07/2019).

Figure 154. The main lava flows active between 3 and 4 July at Stromboli after the major explosion on 3 July 2019 produced a small lava field along the Sciara del Fuoco. Left: Aerial photo taken by Stefano Branca (INGV-OE) on 5 July; the yellow arrow shows a small overflow from the N crater area, the red arrow shows the largest overflow from the CS crater area. Right: Flows from the CS area traveled down to an elevation of 210 m in four flows along the S edge of the scarp. Additional block avalanches rolled to the coastline. Right photo by Francesco Ciancitto taken on 5 July 2019. Courtesy of INGV (Il parossismo dello Stromboli del 3 luglio 2019 e l'attività nei giorni successivi: il punto della situazione al 13 luglio 2019).

During the second week of July lava flows continued; on 8 July volcanologists reported two small lava flows from the CS area flowing towards the Sciara del Fuoco. A third flow was noted the following day. The farthest flow front was at about 500 m elevation on 10 July, and the flow at the center of the Sciara del Fuoco was at about 680 m. An overflow from the N area during the evening of 12 July produced two small flows that remained high on the N side of the scarp; lava continued flowing from the CS area into the next day. A new flow from the N area late on 14 July traveled down the N part of the scarp (figure 155).

Figure 155. During the second week of July 2019 lava flows at Stromboli continued from both crater areas. Top left: Lava flows from the CS area flowed down the Sciara on 9 July while Strombolian activity continued at the summit, photo by P. Anghemo, mountain guide. Bottom left: A lava flow from the CS area at Stromboli is viewed from Punta dei Corvi during the night of 12-13 July 2019. Photo by Francesco Ciancitto. Right: The active flows on 10 July (in red) were much closer to the summit crater than they had been during 3-4 July (in yellow). Courtesy of INGV, top left and right photos published in Report 29/2019, Stromboli, Bollettino Settimanale, 08/07/2019 - 14/07/2019; bottom left photo published in 'Il parossismo dello Stromboli del 3 luglio 2019 e l'attività nei giorni successivi: il punto della situazione al 13 luglio 2019'.

A new video station with a thermal camera was installed at Punta dei Corvi, a short distance N of Ginostra on the SW coast, during 17-20 July 2019. During the third week of July lava continued to flow from the CS crater area onto the southern part of the Sciara del Fuoco, but the active flow area remained on the upper part of the scarp; block avalanches continuously rolled down to the coastline (figure 156). During visits to the summit area on 26 July and 1 August activity at the Terrazza Craterica was observed by INGV scientists. There were at least six active vents in the N area, including a scoria cone and an intensely spattering hornito; the other vents were ejecting coarse material in jets of Strombolian activity. In the CS area, a large scoria cone was clearly visible from the Pizzo, with two active vents generating medium- to high-intensity explosions rich in volcanic ash mixed with coarse ejecta (figures 157 and 158). Some of the finer-grained material in the jets reached 200 m above the vents. A second smaller cone in the CS area faced the southernmost part of the Sciara del Fuoco and produced sporadic low-intensity "bubble explosions." Effusive activity decreased during the last week of July; the active lava front was located at about 600 m elevation. Blocks continued to roll down the scarp, mostly from the explosive activity, and were visible from Punta dei Corvi.

Figure 156. Lava continued to flow from the CS area at Stromboli during the third week of July 2019, although the active flow area remained near the top of the scarp. Block avalanches continued to travel down the scarp. Image taken by di Francesco Ciancitto from Punta dei Corvi on 19 July 2019. Courtesy of INGV (Report 30/2019, Stromboli, Bollettino Settimanale, 15/07/2019 - 21/07/2019).

Figure 157. Thermal and visible images of Terrazza Craterica at the summit of Stromboli from the Pizzo Sopra La Fossa on 1 August 2019 showed significant changes since the major explosion on 3 July 2019. A large scoria cone was present in the CS area (left) and at least six vents from multiple cones were active in the N area (right). The active lava flow 'Trabocco Lavico' emerged from the southernmost part of the CS area (far left). Courtesy of INGV (Report 32/2019, Stromboli, Bollettino Settimanale, 29/07/2019 - 04/08/2019.

Figure 158. At the summit of Stromboli on 1 August 2019 two active vents inside a large cone in the CS area generated medium- to high-intensity explosions rich in volcanic ash mixed with coarse ejecta (left). There were at least six active vents in the N area (right), including a scoria cone and an intensely spattering hornito; the other vents were ejecting coarse material in jets of Strombolian activity. Courtesy of INGV (Report 32/2019, Stromboli, Bollettino Settimanale, 29/07/2019 - 04/08/2019).

Activity during August 2019. A small overflow of lava on 4 August 2019 from the N area lasted for about 20 minutes and formed a flow that went a few hundred meters down the Sciara del Fuoco. Observations made at the summit on 7 and 8 August 2019 indicated that nine vents were active in the N crater area, three of which had scoria cones built around them (figure 159). They all produced low- to medium-intensity Strombolian activity. In the CS area, a large scoria cone was visible from the summit that generated medium- to high-intensity explosions rich in volcanic ash, which sometimes rose more than 200 m above the vent. Lava overflowing from the CS area on 8 August was confined to the upper part of the Sciara del Fuoco, at an elevation between 500 and 600 m (figure 160). Occasional block avalanches from the active lava fronts traveled down the scarp. Ashfall was reported in the S. Bartolo area, Scari, and Piscità during the first week of August.

Figure 159. Nine vents were active in the N crater area of Stromboli on 7 August 2019, three of which had scoria cones built around them. They all produced low- to medium-intensity Strombolian activity (top). In the CS area (bottom), a large scoria cone was visible from the summit that generated medium- to high-intensity explosions rich in volcanic ash, which sometimes rose more than 200 m above the vent. Visible images taken by S. Consoli, thermal images taken by S. Branca. Courtesy of INGV (Report 33/2019, Stromboli, Bollettino Settimanale, 05/08/2019 - 11/08/2019).

Figure 160. Multiple Lava flows were still active on the Sciara del Fuoco at Stromboli on 7 August 2019. Top images by INGV personnel S Branca and S. Consoli, lower images by A. Di Pietro volcanological guide. Courtesy of INGV (Report 33/2019, Stromboli, Bollettino Settimanale, 05/08/2019 - 11/08/2019).

Drone surveys on 13 and 14 August 2019 confirmed that sustained Strombolian activity continued both in the N area and the CS area. Lava flows continued from two vents in the CS area; they ceased briefly on 16 and 17 August but resumed on the 18th, with the lava fronts reaching 500-600 m elevation (figure 161). A fracture field located in the southern part of the Sciara del Fuoco was first identified in drone imagery on 9 July. Repeated surveys through mid-August indicated that about ten fractures were identifiable trending approximately N-S and ranged in length from 2.5 to 21 m; they did not change significantly during the period. An overflight on 23 August identified the main areas of activity at the summit. A NE-SW alignment of 13 vents within the N area was located along the crater edge that overlooks the Sciara del Fuoco. At the CS area, the large scoria cone had two active vents, there was a pit crater, and two smaller scoria cones. A 50-m-long lava tube emerged from one of the smaller lava cones and fed two small flows that emerged at the top of the Sciara del Fuoco (figure 162).

Figure 161. Detail of a vent at Stromboli on 14 August 2019 located in the SW part of the Sciara del Fuoco at an elevation of 730 m. Flow is tens of meters long. Courtesy of INGV (COMUNICATO DI DETTAGLIO STROMBOLI del 20190816 ORE 17:05 LT).

Figure 162. Thermal and visual imagery of the summit of Stromboli on 23 August 2019 revealed a NE-SW alignment of 13 vents within the N area located along the crater edge that overlooks the Sciara del Fuoco. At the CS area, the large scoria cone had two active vents (1 and 2), there was a pit crater (3), and two smaller scoria cones (4). A 50-m-long lava tube formed from one of the smaller lava cones (5) and fed two small flows that emerged at the top of the Sciara del Fuoco. Photos by L. Lodato and S. Branca, courtesy of INGV (Report 35/2019, Stromboli, Bollettino Settimanale, 19/08/2019 - 25/08/2019).

INGV reported a strong explosion from the CS area at 1217 (local time) on 28 August 2019. Ejecta covered the Terrazza Craterica and sent debris rolling down the Sciara del Fuoco to the coastline. A strong seismic signal was recorded, and a large ash plume rose more than 2 km above the summit (figure 163). The Toulouse VAAC reported the ash plume at 3.7-4.6 km altitude, moving E and rapidly dissipating, shortly after the event. Once again, a pyroclastic flow traveled down the Sciara and several hundred meters out to sea (figures 164). The entire summit was covered with debris. The complex of small scoria cones within the N area that had formed since the 3 July explosion was destroyed; part of the N area crater rim was also destroyed allowing lava to flow down the Sciara where it reached the coastline by early evening.

Figure 163. A major explosion at Stromboli on 28 August 2019 produced a high ash plume and a pyroclastic flow. The seismic trace from the STR4 station (top left) indicated a major event. The ash plume from the explosion was reported to be more than 2 km high (right). The thermal camera located at Stromboli's Punta dei Corvi on the southern edge of the Sciara del Fuoco captured both the pyroclastic flow and the ash plume produced in the explosion (bottom left). Seismogram and thermal image courtesy of INGV (INGVvulcani blog, 30 AGOSTO 2019INGVVULCANI, Nuovo parossismo a Stromboli, 28 agosto 2019). Photo by Teresa Grillo (University of Rome) Courtesy of AIV - Associazione Italiana di Vulcanologia.

Figure 164. A pyroclastic flow at Stromboli traveled across the sea off the W flank for several hundred meters on 28 August 2019 after a major explosion at the summit. Photo by Alberto Lunardi, courtesy of INGV (5 SETTEMBRE 2019INGVVULCANI, Quando un flusso piroclastico scorre sul mare: esempi a Stromboli e altri vulcani).

At 1923 UTC on 29 August a lava flow was reported emerging from the N area onto the upper part of the Sciara del Fuoco; it stopped at mid-elevation on the slope. About 90 minutes later, an explosive sequence from the CS area resulted in the fallout of pyroclastic debris around Ginostra. Shortly after midnight, a lava flow from the CS area traveled down the scarp and reached the coast by dawn, but the lava entry into the sea only lasted for a short time (figure 165).

Figure 165. Lava flows continued for a few days after the major explosion of 28 August 2019 at Stromboli. Left: A lava flow emerged from the N crater area on 29 August 2019 and traveled a short distance down the Sciara del Fuoco. Incandescent blocks from the flow front reached the ocean. Photo by A. DiPietro. Right: A lava flow that emerged from the CS crater area around midnight on 30 August 2019 made it to the ocean around dawn, as seen from the N ridge of the Sciara del Fuoco at an altitude of 400 m. Photo by Alessandro La Spina. Both courtesy of INGV. Left image from 'COMUNICATO DI ATTIVITA' VULCANICA del 2019-08-29 22:20:06(UTC) – STROMBOLI', right image from INGVvulcani blog, 30 AGOSTO 2019 INGVVULCANI, 'Nuovo parossismo a Stromboli, 28 agosto 2019'.

An overflight on 30 August 2019 revealed that after the explosions of 28-29 August the N area had collapsed and now contained an explosive vent producing Strombolian activity and two smaller vents with low-intensity explosive activity. In the CS area, Strombolian activity occurred at a single large crater (figure 166). INGV reported an explosion frequency of about 32 events per hour during 31 August-1 September. The TROPOMI instrument on the Sentinel-5P satellite captured small but distinct SO₂ plumes from Stromboli during 28 August-1 September, even though they were challenging to distinguish from the larger signal originating at Etna (figure 167).

Figure 166. A 30 August 2019 overflight of Stromboli revealed that after the explosions of 28-29 August the N area had collapsed and now contained a single explosive vent producing Strombolian activity and two smaller vents with low intensity explosive activity. In the CS area, a single large crater remained with moderate Strombolian activity. No new lava flows appeared on the Sciara del Fuoco, only cooling from the existing flows was evident. Courtesy of INGV (Report 35.6/2019, Stromboli, Daily Bulletin of 08/31/2019).

Figure 167. Small but distinct SO2 signals were recorded from Stromboli during 28 August through 1 September 2019; they were sometimes difficult to discern from the larger signal originating at nearby Etna. Courtesy of NASA Goddard Space Flight Center.

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy, (URL: http://www.ct.ingv.it/en/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Toulouse Volcanic Ash Advisory Center (VAAC), Météo-France, 42 Avenue Gaspard Coriolis, F-31057 Toulouse cedex, France (URL: http://www.meteo.fr/aeroweb/info/vaac/); AIV, Associazione Italiana di Vulcanologia (URL: https://www.facebook.com/aivulc/photos/a.459897477519939/1267357436773935; ANSA.it, (URL: http://www.ansa.it/sicilia/notizie/2019/07/03/-stromboli-esplosioni-da-cratere-turisti-in-mare); The New York Post, (URL: https://nypost.com/2019/07/03/dozens-of-people-dive-into-sea-to-escape-stromboli-volcano-eruption-in-italy/).

April 2020 (BGVN 45:04) Cite this Report

Strombolian activity continues at both summit crater areas, September-December 2019

Near-constant fountains of lava at Stromboli have served as a natural beacon in the Tyrrhenian Sea for at least 2,000 years. Eruptive activity at the summit consistently occurs from multiple vents at both a north crater area (N area) and a southern crater group (CS area) on the Terrazza Craterica at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the volcano-island (figure 168). Periodic lava flows emerge from the vents and flow down the scarp, sometimes reaching the sea; occasional large explosions produce ash plumes and pyroclastic flows. Thermal and visual cameras that monitor activity at the vents are located on the nearby Pizzo Sopra La Fossa, above the Terrazza Craterica, and at multiple locations on the flanks of the volcano. Detailed information for Stromboli is provided by Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) as well as other satellite sources of data; September-December 2019 is covered in this report.

Figure 168. This shaded relief map of Stromboli’s crater area was created from images acquired by drone on 9 July 2019 (In collaboration with GEOMAR drone group, Helmholtz Center for Ocean Research, Kiel, Germany). Inset shows Stromboli Island, the black rectangle indicates the area of the larger image, the black curved and the red hatched lines indicate, respectively, the morphological escarpment and the crater edges. Courtesy of INGV (Rep. No. 50/2019, Stromboli, Bollettino Settimanale, 02/12/2019 - 08/12/2019, data emissione 10/12/2019).

Activity was very consistent throughout the period of September-December 2019. Explosion rates ranged from 2-36 per hour and were of low to medium-high intensity, producing material that rose from less than 80 to over 150 m above the vents on occasion (table 7). The Strombolian activity in both crater areas often sent ejecta outside the crater rim onto the Terrazza Craterica, and also down the Sciara del Fuoco towards the coast. After the explosions of early July and late August, thermal activity decreased to more moderate levels that persisted throughout the period as seen in the MIROVA Log Radiative Power data (figure 169). Sentinel-2 satellite imagery supported descriptions of the constant glow at the summit, revealing incandescence at both summit areas, each showing repeating bursts of activity throughout the period (figure 170).

Table 7. Monthly summary of activity levels at Stromboli, September-December 2019. Low-intensity activity indicates ejecta rising less than 80 m, medium-intensity is ejecta rising less than 150 m, and high-intensity is ejecta rising over 200 m above the vent. Data courtesy of INGV.

Figure 169. Thermal activity at Stromboli was high during July-August 2019, when two major explosions occurred. Activity continued at more moderate levels through December 2019 as seen in the MIROVA graph of Log Radiative Power from 8 June through December 2019. Courtesy of MIROVA.

Figure 170. Stromboli reliably produced strong thermal signals from both of the summit vents throughout September-December 2019 and has done so since long before Sentinel-2 satellite imagery was able to detect it. Image dates are (top, l to r) 5 September, 15 October, 20 October, (bottom l to r) 14 November, 14 December 2019, and 3 January 2020. Sentinel-2 imagery uses Atmospheric penetration rendering with bands 12, 11, and 8A, courtesy of Sentinel Hub Playground.

After a major explosion with a pyroclastic flow on 28 August 2019, followed by lava flows that reached the ocean in the following days (BGVN 44:09), activity diminished in early September to levels more typically seen in recent times. This included Strombolian activity from vents in both the N and CS areas that sent ejecta typically 80-150 m high. Ejecta from the N area generally consisted of lapilli and bombs, while the material from the CS area was often finer grained with significant amounts of lapilli and ash. The number of explosive events remained high in September, frequently reaching 25-30 events per hour. The ejecta periodically landed outside the craters on the Terrazza Craterica and even traveled partway down the Sciara del Fuoco. An inspection on 7 September by INGV revealed four eruptive vents in the N crater area and five in the S crater area (figure 171). The most active vents in the N area were N1 with mostly ash emissions and N2 with Strombolian explosions rich in incandescent coarse material that sometimes rose well above 150 m in height. In the S area, S1 and S2 produced jets of lava that often reached 100 m high. A small cone was observed around N2, having grown after the 28 August explosion. Between 11 and 13 September aerial surveys with drones produced detailed visual and thermal imagery of the summit (figure 172).

Figure 171. Video of the Stromboli summit taken with a thermal camera on 7 September 2019 from the Pizzo sopra la Fossa revealed four active vents in the N area and five active vents in the S area. Images prepared by Piergiorgio Scarlato, courtesy of INGV (Rep. No. 37.2/2019, Stromboli, Bollettino Giornaliero del 10/09/2019).

Figure 172. An aerial drone survey on 11 September 2019 at Stromboli produced a detailed view of the N and CS vent areas (left) and thermal images taken by a drone survey on 13 September (right) showed elevated temperatures down the Sciara del Fuoco in addition to the vents in the N and CS areas. Images by E. De Beni and M. Cantarero, courtesy of INGV (Rep. No. 37.5/2019, Stromboli, Bollettino Giornaliero del 13/09/2019).

Strombolian activity from the N crater on 28 September and 1 October 2019 produced blocks and debris that rolled down the Sciara del Fuoco and reached the ocean (figure 173). Explosive activity from the CS crater area sometimes produced ejecta over 150 m high (figure 174). A survey on 26 November revealed that a layer of ash 5-10 cm thick had covered the bombs and blocks that were deposited on the Pizzo Sopra la Fossa during the explosions of 3 July and 28 August (figure 175). On the morning of 27 December a lava flow emerged from the CS area and traveled a few hundred meters down the Sciara del Fuoco. The frequency of explosive events remained relatively constant from September through December 2019 after decreasing from higher levels during July and August (figure 176).

Figure 173. Strombolian activity from vents in the N crater area of Stromboli produced ejecta that traveled all the way to the bottom of the Sciara del Fuoco and entered the ocean. Top images taken 28 September 2019 from the 290 m elevation viewpoint by Rosanna Corsaro. Bottom images captured on 1 October from the webcam at 400 m elevation. Courtesy of INGV (Rep. No. 39.0/2019 and Rep. No. 40.3, Stromboli, Bollettino Giornaliero del 29/09/2019 and 02/10/2019).

Figure 174. Ejecta from Strombolian activity at the CS crater area of Stromboli rose over 150 m on multiple occasions. The webcam located at the 400 m elevation site captured this view of activity on 8 November 2019. Courtesy of INGV (Rep. No. 45.5/2019, Stromboli, Bollettino Giornaliero del 08/11/2019).

Figure 175. The Pizzo Sopra la Fossa area at Stromboli was covered with large blocks and pyroclastic debris on 6 September 2019, a week after the major explosion of 28 August (top). By 26 November, 5-10 cm of finer ash covered the surface; the restored webcam can be seen at the far right edge of the Pizzo (bottom). Courtesy of INGV (Rep. No. 49/2019, Stromboli, Bollettino Settimanale, 25/11/2019 - 01/12/2019, data emissione 03/12/2019).

Figure 176. The average hourly frequency of explosive events at Stromboli captured by surveillance cameras from 1 June 2019 through 5 January 2020 remained generally constant after the high levels seen during July and August. The Total value (blue) is the sum of the average daily hourly frequency of all explosive events produced by active vents.

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy, (URL: http://www.ct.ingv.it/en/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

August 2020 (BGVN 45:08) Cite this Report

Strombolian explosions persist at both summit craters during January-April 2020

Stromboli is a stratovolcano located in the northeastern-most part of the Aeolian Islands composed of two active summit vents: the Northern (N) Crater and the Central-South (CS) Crater that are situated at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the volcano. The ongoing eruption began in 1934 and has been characterized by regular Strombolian explosions in both summit craters, ash plumes, and occasional lava flows (BGVN 45:08). This report updates activity from January to April 2020 with information primarily from daily and weekly reports by Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) and various satellite data.

Activity was consistent during this reporting period. Explosion rates ranged from 1-20 per hour and were of variable intensity, producing material that rose from less than 80 to over 250 m above the vents (table 8). Strombolian explosions were often accompanied by gas-and-steam emissions, spattering, and lava flows which has resulted in fallout deposited on the Sciara del Fuoco and incandescent blocks rolling toward the coast up to a few hundred meters down the slopes of the volcano. According to INGV, the average SO₂ emissions measured 300-650 tons/day.

Table 8. Summary of activity at Stromboli during January-April 2020. Low-intensity activity indicates ejecta rising less than 80 m, medium-intensity is ejecta rising less than 150 m, and high-intensity is ejecta rising over 200 m above the vent. Data courtesy of INGV.

During January 2020, explosive activity mainly originated from three vents in the N crater and at least three vents in the CS crater. Ejecta from numerous Strombolian explosions covered the slopes on the upper Sciara del Fuoco, some of which rolled hundreds of meters down toward the coast. Explosion rates varied from 2-12 per hour in the N crater and 9-14 per hour in the CS crater; ejected material rose 80-200 m above the craters. According to INGV, a small cone growing in the S1 crater produced some explosions that ejected coarse material mixed with fine ash. On 18 and 19 January a lava flow was observed, both of which originated in the N crater. In addition, two explosions were detected in the N crater that was associated with two landslide events.

Explosive activity in February primarily originated from 2-3 eruptive vents in the N crater and at least three vents in the CS crater (figure 177). The Strombolian explosions ejected material 80-250 m above the craters, some of which fell onto the upper part of the Sciara. Explosion rates varied from 3-12 per hour in the N crater and 2-14 per hour in the CS crater (figure 178). On 3 February a short-lived lava flow was reported in the N crater.

Figure 177. A drone image showed spattering accompanied by gas-and-steam emissions at Stromboli rising above the N crater on 15 February 2020. Courtesy of INGV (Rep. No. 08/2020, Stromboli, Bollettino Settimanale, 10/02/2020 - 16/02/2020, data emissione 18/02/2020).

Figure 178. a) Strombolian explosions during the week of 17-23 February 2020 in the N1 crater of Stromboli were seen from Pizzo Sopra La Fossa. b) Spattering at Stromboli accompanied by white gas-and-steam emissions was detected in the N1 and S2 craters during the week of 17-23 February 2020. c) Spattering at Stromboli accompanied by a dense ash plume was seen in the N1 and S2 craters during the week of 17-23 February 2020. All photos by F. Ciancitto, courtesy of INGV (Rep. No. 09/2020, Stromboli, Bollettino Settimanale, 17/02/2020 - 23/02/2020, data emissione 25/02/2020).

Ongoing explosive activity continued into March, originating from three eruptive vents in the N crater and at least three vents in the CS crater. Ejected lapilli and bombs rose 80-250 m above the craters resulting in fallout covering the slopes in the upper Sciara del Fuoco with blocks rolling down the slopes toward the coast and explosions varied from 4-13 per hour in the N crater and 1-16 per hour in the CS crater. Discontinuous spattering was observed during 9-19 March. On 19 March, intense spattering was observed in the N crater, which produced a lava flow that stretched along the upper part of the Sciara for a few hundred meters. Another lava flow was detected in the N crater on 28 March for about 4 hours into 29 March, which resulted in incandescent blocks breaking off the front of the flow and rolling down the slope of the volcano. On 30 March a lava flow originated from the N crater and remained active until the next day on 31 March. Landslides accompanied by incandescent blocks rolling down the Sciara del Fuoco were also observed.

Strombolian activity accompanied by gas-and-steam emissions continued into April, primarily produced in 3-4 eruptive vents in the N crater and 2-3 vents in the CS crater. Ejected material from these explosions rose 80-250 m above the craters, resulting in fallout products covering the slopes on the Sciara and blocks rolling down the slopes. Explosions varied from 4-15 per hour in the N crater and 1-10 per hour in the CS crater. On 1 April a thermal anomaly was detected in satellite imagery accompanied by gas-and-steam and ash emissions downstream of the Sciara del Fuoco. A lava flow was observed on 15 April in the N crater accompanied by gas-and-steam and ash emissions; at the front of the flow incandescent blocks detached and rolled down the Sciara (figure 179). This flow continued until 16 April, ending by 0956; a thermal anomaly persisted downslope from the lava flow. Spatter was ejected tens of meters from the vent. Another lava flow was detected on 19 April in the N crater, followed by detached blocks from the front of the flow rolling down the slopes. Spattering continued during 20-21 April.

Figure 179. A webcam image of an ash plume accompanied by blocks ejected from Stromboli on 15 April 2020 rolling down the Sciara del Fuoco. Courtesy of INGV via Facebook posted on 15 April 2020.

Moderate thermal activity occurred frequently during 16 October to April 2020 as recorded in the MIROVA Log Radiative Power graph using MODIS infrared satellite information (figure 180). The MODVOLC thermal alerts recorded a total of 14 thermal signatures over the course of nine different days between late February and mid-April. Many of these thermal signatures were captured as hotspots in Sentinel-2 thermal satellite imagery in both summit craters (figure 181).

Figure 180. Low to moderate thermal activity at Stromboli occurred frequently during 16 October-April 2020 as shown in the MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 181. Thermal anomalies (bright yellow-orange) at Stromboli were observed in thermal satellite imagery from both of the summit vents throughout January-April 2020. Images with Atmospheric Penetration rendering (bands 12, 11, 8A); courtesy of Sentinel Hub Playground.

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/en/, Facebook: https://www.facebook.com/ingvvulcani/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

September 2020 (BGVN 45:09) Cite this Report

Strombolian activity continues at both summit craters during May-August 2020

Stromboli, located in northeastern-most part of the Aeolian Islands, is composed of two active summit vents: the Northern (N) Crater and the Central-South (CS) Crater that are situated at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the volcano. The current eruption period began in 1934, continuing to the present with volcanism characterized by consistent Strombolian explosions in both summit craters, ash plumes, pyroclastic flows, and occasional lava flows (BGVN 45:08). This report updates activity consisting of dominantly Strombolian explosions and ash plumes from May to August 2020 with information primarily from daily and weekly reports by Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) and various satellite data.

Activity was consistent during this reporting period. Explosion rates ranged from 1-23 events per hour and were of variable intensity, producing material that typically rose from less than 80 to over 300 m above the crater. One ash plume on 19 July rose 1 km above the crater and high energy ballistics were ejected 500 m above the crater during the week of 20-26 July (table 9). Strombolian explosions were often accompanied by gas-and-steam emissions and spattering that has occasionally resulted in material deposited on the slopes of the Sciara del Fuoco. According to INGV, the average SO₂ emissions measured 250-300 tons/day.

Table 9. Summary of activity at Stromboli during May-August 2020. Low-intensity activity indicates ejecta rising less than 80 m, medium-intensity is ejecta rising less than 150 m, and high-intensity is ejecta rising over 200 m above the vent. Data courtesy of INGV.

Explosive activity was relatively consistent during May 2020 and was mainly produced in 3-4 eruptive vents in the N crater and at least two eruptive vents in the CS crater. As a result of some explosions fallout covered the slopes of the Sciara del Fuoco. Explosion rates varied from 1-17 per hour in the N crater and 1-8 per hour in the CS crater; ejected material rose 80-250 m above the craters.

During June, explosions originated from 2-3 eruptive vents in the N crater and at least 2-3 localized vents in the CS crater. The Strombolian explosions ejected material 80-200 m above the craters, some of which fell back onto the Sciara (figure 182). Explosion rates varied from 5-14 per hour in the N crater and 2-9 per hour in the CS crater. Spattering was typically observed in the CS crater.

Figure 182. An explosion at Stromboli produced gas-and-steam and ash emissions on 18 June 2020 was observed in the CS crater in the Sciara del Fuoco. Courtesy of INGV (Rep. No. 26/2020, Stromboli, Bollettino Settimanale, 15/06/2020 - 21/06/2020, data emissione 23/06/2020).

Ongoing explosive activity continued into July, originating from 2-3 eruptive vents in the N crater and 3-4 eruptive vents in the CS crater. Explosions varied from 3-12 per hour in the N crater and 1-11 per hour in the CS crater; ejected lapilli and bombs rose 80-1,000 m above the craters (figure 183). On 19 July a high-energy explosion between 0500 and 0504 produced an ash plume containing ejecta more than 50 cm that rose to a maximum of 1 km above the crater, with fallout reaching the Pizzo sopra la Fossa and resulting in ashfall on the Sciara and the towns of Liscione and Roccette. During the week of 20-26 July explosions in the E portion of the volcano ejected ballistics 500 m above the crater; the size and shape of these varied between slag bombs to clasts greater than 50 cm.

Figure 183. Webcam (left column) and thermal (right column) images of explosive activity at Stromboli on 29 July (top row) and 2 August (bottom row) 2020 originated from the N and CS craters, producing spatter and ash plumes. Courtesy of INGV (Rep. No. 32/2020, Stromboli, Bollettino Settimanale, 27/07/2020 - 02/08/2020, data emissione 04/08/2020).

Strombolian activity accompanied by discontinuous spattering continued during August. Total daily explosions varied from 3-23 per hour ejecting material that up to 200-250 m above the craters. During the first half of the month the explosions were low-intensity and consisted of fine material. On 13 August the intensity of the explosions increased, producing an ash plume that rose 300 m above the crater drifting SE and resulting in a significant amount of ashfall on the Sciara. During the week of 17-23, explosions in the N1 crater ejected material 200 m above the crater while explosions in the CS crater ejected material 250 m above the crater, predominantly during 22 August in the S2 crater (figure 184).

Figure 184. Images of gas-and-steam and ash plumes rising from the N2 (left), S2 (middle), and CS craters (right) at Stromboli on 22 August 2020. Courtesy of INGV (Rep. No. 35/2020, Stromboli, Bollettino Settimanale, 17/08/2020 - 23/08/2020, data emissione 25/08/2020).

Moderate thermal activity was relatively consistent from October 2019 through mid-April 2020; during May-August thermal activity became less frequent and anomalies were lower in power based on the MIROVA Log Radiative Power graph using MODIS infrared satellite information (figure 185). Though there were no detected MODVOLC thermal alerts during this reporting period, many thermal hotspots were observed in Sentinel-2 thermal satellite imagery in both summit craters (figure 186).

Figure 185. Low to moderate thermal activity at Stromboli occurred frequently from 16 September to mid-April 2020 as shown in the MIROVA graph (Log Radiative Power). During May-August thermal activity decreased and was less frequent compared to the previous months. Courtesy of MIROVA.

Figure 186. Weak thermal anomalies (bright yellow-orange) at Stromboli were observed in thermal satellite imagery from both of the summit vents throughout May-August 2020. Images with atmospheric penetration (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy, (URL: http://www.ct.ingv.it/en/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

February 2021 (BGVN 46:02) Cite this Report

Explosions, incandescent ejecta, lava flows, and pyroclastic flows during September-December 2020

Stromboli, located in the northeastern Aeolian Islands, is composed of two active summit craters: the Northern (N) crater and the Central-South (CS) crater that are situated at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the volcano (figure 187). The current eruption period began in February 1934 and has been recently characterized by Strombolian explosions at both summit craters, ash plumes, and SO₂ plumes (BGVN 45:09). This report covers activity consisting of dominantly Strombolian explosions, incandescent ejecta, and ash plumes from September to December 2020, with information primarily from daily and weekly reports by Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) and various satellite data.

Figure 187. Photo of the summit craters at Stromboli showing the North and Central-South crater areas with the location of each active vent: N1 and N2 in the N crater and S1, S2, and C in the CS crater. Photo was taken from the Pizzo sopa la Fossa during an expedition on 22 August by INGV-OE personnel. Courtesy of INGV (Rep. No. 37/2020, Stromboli, Bollettino Settimanale, 31/08/2020 - 06/09/2020, data emissione 08/09/2020).

Activity was consistent during this reporting period. Explosion rates typically ranged from 1-14 events per hour and varied in intensity that ejected material 80-250 m above the N crater and 150-250 m above the CS crater (table 10). An ash plume on 16 November rose 1 km above the crater, accompanied by a pyroclastic flow descending the Sciara del Fuoco to the NW as far as 200 m. As a result, some ash and lapilli fell in the town of Stromboli (2 km NE). Strombolian explosions were often accompanied by gas-and-steam emissions, occasional spattering that deposited material on the Sciara del Fuoco, small lava flows, and small pyroclastic flows. According to INGV, the daily SO₂ emissions measured 250-300 tons/day.

Table 10. Summary of activity at Stromboli during September-December 2020. Low-intensity activity indicates ejecta rising less than 80 m, medium-intensity is ejecta rising less than 150 m, and high-intensity is ejecta rising over 200 m above the vent. Data courtesy of INGV.

During September the frequency of the Strombolian explosions in the N crater typically ranged from 2-14 per hour; in the CS crater there were 1-10 explosions per hour. N1 consisted of three points of emissions that produced low- to high-intensity explosions, launching lapilli and bombs, sometimes mixed with fine ash, 80-200 m above the N crater and were distributed radially (figure 188); N2 typically showed low-intensity explosions (less than 80 m above the crater). Medium- to high-intensity explosions ejected mostly fine material mixed with some coarse tephra 250 m above the CS crater. On 28 September the number of explosive events reached a high of 22 per hour.

Figure 188. Webcam images of Strombolian activity at Stromboli in the N1 crater on 29 September (left) and in the CS crater on 4 October (right) 2020. Images captured by the SCV surveillance cameras. Courtesy of INGV (Rep. No. 41/2020, Stromboli, Bollettino Settimanale, 28/09/2020 - 04/10/2020, data emissione 06/10/2020).

Explosions with occasional spatter continued in October at a rate of 2-13 per hour in the N crater and 1-4 per hour in the CS crater. In the N crater, N1 consisted of 2-4 eruptive vents that produced explosions of variable intensity while N2 contained two vents that primarily produced low-intensity explosions. Lapilli and bombs, sometimes mixed with fine ash, were ejected 80-250 m above the N crater. Fine ash sometimes mixed with coarse-to-medium tephra rose 150-250 m above the CS crater. Spatter was reported from two hornitos that formed in the N1 crater (figure 189). On 11 October sporadic ash emissions and coarse ejecta were observed above the S2 crater, episodic ash emissions rose above the S1 crater, and occasional degassing with modest spattering were visible in the C crater.

Figure 189. Drone images showing gas-and-steam emissions and Strombolian activity at Stromboli during 8-9 October 2020. The white annotations label the craters and the red show the active hornitos (H). The N2H2 label shows a small explosion (right). Images from the HPHT Lab from INGV-Roma 1. Courtesy of INGV (Rep. No. 42/2020, Stromboli, Bollettino Settimanale, 05/10/2020 - 11/10/2020, data emissione 13/10/2020).

Strombolian explosions persisted into November. The N1 crater consisted of 2-3 vents, producing explosions of variable intensity; the N2 crater also consisted of 2-3 active vents that produced low- to medium-intensity explosions. The frequency of explosions ranged from 2-10 per hour in the N crater and 1-4 per hour in the CS crater. Lapilli and bombs, sometimes mixed with fine ash, rose 80-250 m above the N crater and fine material was ejected 150 m above the CS crater. On 10 November an explosion was detected at 2104 in the S2 crater of the CS area, producing pyroclastic material that was distributed radially along the Sciara del Fuoco, followed by an ash plume (figure 190). Within 30 seconds, another pulse of activity from the C crater in the northern part of the CS area produced intense lava fountaining that ejected coarse incandescent material 300 m above the crater, lasting about two minutes. At 2106 a small explosion was detected in the N2 crater, ending the explosive sequence.

Figure 190. Thermal (rows 1 and 3) and webcam (rows 2 and 4) images showing the evolution of the explosion at Stromboli on the evening of 10 November 2020 accompanied by an ash plume and incandescent ejecta. Images captured by the SCT and SQV surveillance cameras. Courtesy of INGV (Rep. No. 47/2020, Stromboli, Bollettino Settimanale, 09/11/2020 - 15/11/2020, data emissione 17/11/2020).

During an overflight by the 2nd Air Unit of the Coast Guard of Catania on 11 November, scientists identified degassing in the entire summit crater area; a small lava flow was observed in the S1 crater, originating from an intra-crater vent. Additional thermal anomalies were noted at the bottom of the C, N1, and N2 craters. Strong fumaroles were visible originating from a hornito located outside the S1 crater on the Sciara del Fuoco. A second hornito was visible on the slope of the Sciara del Fuoco near the N2 crater. On 16 November a major explosion was detected at 1017 in the N crater area and on the edge of the N2 crater. Thermal and visible images captured the resulting dense, gray ash plume that rose 1 km above the crater and the accompanying pyroclastic flow that descended the Sciara del Fuoco as far as 200 m (figure 191). Some ash and lapilli fell over the town of Stromboli, about 2 km away on the NE coast of the island. A sequence of explosive events at 0133 on 21 November was detected in three different craters: the first two events occurred in the N1 and N2 craters, and the third occurred in the C crater. Coarse material was ejected 300 m above the crater and was distributed radially, affecting the upper part of the Sciara del Fuoco. A small ash plume was also visible.

Figure 191. Thermal (top row) and webcam (bottom row) images showing the evolution of the explosion at Stromboli on the morning of 16 November 2020 accompanied by a significant gray ash plume. Images captured by the SCT and SCV surveillance cameras. Courtesy of INGV (Rep. No. 48/2020, Stromboli, Bollettino Settimanale, 16/11/2020 - 22/11/2020, data emissione 24/11/2020).

During December, similar Strombolian explosions were reported. There were two eruptive vents in the N1 crater and 2-4 in the N2 crater that produced explosions of low intensity and low-to-medium intensity, respectively. The frequency of explosions ranged from 1-13 per hour in the N crater and 1-5 per hour in the CS crater. Fine ash mixed with some coarse material (lapilli and bombs) was ejected 80-150 m above the N crater and mostly fine material rose 150 m above the CS crater. Some spattering activity was reported in the N2 crater, which contributed to the formation of hornitos that produced incandescent material. On 6 December an explosive sequence of events was detected in the CS crater area at 0612. An explosion ejected material 300 m above the crater that were distributed radially, depositing on the upper Sciara del Fuoco. In addition, two small lava flows formed (figure 192). A second explosion was recorded at 0613, characterized by lava fountaining in the CS crater that reached a height of 200 m. Similar activity in the N and CS craters were also captured by webcam images on 21 and 27 December, which showed lava fountaining, accompanied by a small pyroclastic flow (figure 193).

Figure 192. Thermal images of the explosion at Stromboli in the CS crater on 6 December 2020, accompanied by incandescent ejecta and two small lava flows. Some lava fountaining was visible in the bottom center image at 0513:47. Images captured by the SCT surveillance camera. Courtesy of INGV (Rep. No. 50/2020, Stromboli, Bollettino Settimanale, 30/11/2020 - 06/12/2020, data emissione 08/12/2020).

Figure 193. Webcam (top row) and thermal (bottom row) images of Strombolian activity in the N (left column) and CS (right column) crater areas at Stromboli on 21 December (top right) and 27 December (top left and bottom row) 2020. This activity included a small pyroclastic flow and lava fountaining. Images captured by the SCV and SCT surveillance cameras. Courtesy of INGV (Rep. No. 53/2020, Stromboli, Bollettino Settimanale, 21/12/2020 - 27/12/2020, data emissione 29/12/2020).

Intermittent and low-power thermal activity was detected during September through December, according to the MIROVA Log Radiative Power graph using MODIS infrared satellite information (figure 194). Though there were no detected MODVOLC thermal alerts during this reporting period, many thermal hotspots were visible in one or both summit craters on clear weather days using Sentinel-2 thermal satellite imagery, which is due to Strombolian activity (figure 195).

Figure 194. Intermittent, low thermal activity at Stromboli was recorded by the MIROVA graph (Log Radiative Power) during September through December 2020. The frequency of the thermal anomalies had decreased compared to the previous months of May through August; a total of eleven thermal anomalies were detected during this reporting period. Courtesy of MIROVA.

Figure 195. Weak thermal anomalies (bright yellow-orange) at Stromboli were visible in Sentinel-2 thermal satellite imagery from typically both summit craters during September through December 2020. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy, (URL: http://www.ct.ingv.it/en/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

May 2021 (BGVN 46:05) Cite this Report

Strombolian explosions, lava flows, fountains, and spatter during January-April 2021

Stromboli, in the northeastern Aeolian Islands, includes the active Northern (N) and Central-South (CS) craters in the summit area at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW flank. The current eruption period began February 1934 and has been recently characterized by Strombolian explosions, incandescent ejecta, lava flows, and pyroclastic flows (BGVN 46:02). This report updates activity consisting of similar eruptive events during January through April 2021 using information from daily and weekly reported by Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) and various satellite data.

Activity was consistent during this reporting period. The average explosion rates ranged from 2-28 per hour in the N crater and 1-8 per hour in the CS crater, though individual explosions varied in intensity (table 11). Ejected material rose up to 250 m above both the N and CS craters. Strombolian explosions were often accompanied by gas-and-steam emissions and frequent intense spattering in the N crater, depositing material on the Sciara del Fuoco. Lava fountains were also reported. On 18, 22, and 24 January four lava flows were detected in the N crater area. According to INGV, the average SO₂ emissions measured 250-300 tons/day.

Table 11. Summary of activity at Stromboli during January-April 2021. Data courtesy of INGV.

Activity during January consisted of Strombolian explosions in the N and CS craters ranging from 7-18 per hour and 1-4 per hour, respectively. The frequency of explosive events was relatively low during 1-6 January (less than 10 events per hour), increased on 7 and 8 January (up to 18 events per hour), then dropped again during 9-10 January (12-14 events per hour). The N1 vent contained two points of emission, producing low-intensity explosions that ejected fine ash, coarse lapilli, and bombs. The N2 vent contained four points of emission that generated explosions of more variable intensity and ejected coarse lapilli. Spattering at this vent also constructed some hornitos, which in turn generated jets of incandescent material. Spattering was more energetic during 7-8 January and then started to decline to weak and less frequent intervals.

During 18, 22, and 24 January INGV reported four lava flows in the N crater beyond the edge of the N2 vent, extending onto the upper Sciara del Fuoco (figure 196). Two points of emission between vents N1 and N2 have been designated P1 and P2 (figure 197), the latter of which has been observed since October. The first lava flow originated from P1 and was reported on 18 January at 1115; incandescent blocks of material traveled a few tens of meters in the upper part of the Sciara before stopping (reaching an elevation of 700 m). A second lava flow, originating from P2 occurred between 1600 and 2100 on 18 January, stopped in the central part of the Sciara. The third lava flow also originated from P2 on 22 January between 1222 and 2100, extending for a few hundred meters in the upper and central part of the Sciara (figure 198); this flow had a greater volume compared to the first two and accumulated at the coastline. The fourth lava flow was detected on 24 January at 1956 from P2 and continued until the morning of 25 January; its volume was less than the third flow. In the CS crater at least two vents were active, one of which emitted only ash, while the other ejected coarse incandescent material 250 m above the crater.

Figure 196. Thermal webcam images showing the evolution of the lava flows (bright red, yellow, and green) at Stromboli during 18 (a-d), 22 (e-f), and 24 (g-h) January 2021 accompanied by some gas-and-steam emissions. Images captured by the SCT surveillance camera. Courtesy of INGV (Rep. No. 04/2021, Stromboli, Bollettino Settimanale, 18/01/2021 - 24/01/2021, data emissione 26/01/2021).

Figure 197. Thermal webcam image indicating the relative locations of the crater areas (N1, N2, and CS) and lava emission points (P1 and P2) on Stromboli. Image taken on 18 January 2021 captured by the SCT surveillance camera. Courtesy of INGV (Rep. No. 04/2021, Stromboli, Bollettino Settimanale, 18/01/2021 - 24/01/2021, data emissione 26/01/2021).

Figure 198. Webcam image of the effusive activity at Stromboli at 1920 (local time) on 22 January 2021. Image captured by the SCV surveillance camera. Courtesy of INGV.

Similar explosive activity continued in February. The average explosion rates ranged from 5-28 per hour in the N crater and 1-6 per hour in the CS crater. The N1 vent continued to produce low-intensity explosions that ejected material less than 80 m high, and sometimes up to 250 m, that consisted of fine ash mixed with coarse lapilli and bombs. On the evening of 4 February the lava fountains exceeded 100 m above the vent. The N2 vent had stronger explosions that ejected coarse material 250 m high; intense and frequent spattering was also observed in this vent, particularly on 8, 10, and 13 February. Weak spattering was observed through 17 February. During 24-25 February observations were made by the HPHT Lab from INGV-Roma 1 and FlyEye Team of OE as part of the UNO Departmental Project. Drone images were taken to document the active vents and craters (figure 199). In addition, a Digital Surface Model (DSM) was created to map the summit craters (figure 200). Crater measurements showed that CS1 was greater than 90 m wide, CS2 was about 70 m, and CS3 has a diameter of 43 m. In the N crater, N1 and N2 measured 47 and 67 m in diameter, respectively.

Figure 199. Drone images of the summit crater at Stromboli documenting the craters (yellow outline) and the location of the active vents (red dots). White gas-and-steam emissions were also recorded in these images. Courtesy of INGV (Rep. No. 09/2021, Stromboli, Bollettino Settimanale, 22/02/2021 - 28/02/2021, data emissione 02/03/2021).

Figure 200. Digital Surface Model (DSM) of Stromboli’s summit crater area with a resolution of 10 cm. Each crater is outlined with a different color: CS1 (yellow), CS2 (orange), CS3 (dark red), N1 (light green), and N2 (dark green). The red dots represent active locations at the time of the survey. Courtesy of INGV (Rep. No. 11/2021, Stromboli, Bollettino Settimanale, 08/03/2021 - 14/03/2021, data emissione 16/03/2021).

During the week of 22-28 February two active vents in the N crater produced explosions that ejected lapilli and bombs up to 100-120 m high. Four vents were visible in N2, one of which was a hornito near the edge of the N1 crater that was characterized by degassing. Only two of these four vents were active, and the resulting explosions were small and contained coarse material. INGV also reported an extensive sulfur deposit on the hornito and on the N1 vent. In the CS crater ash sometimes mixed with coarse material was ejected 250 m high. Three vents were identified in the CS1 crater: one was inactive, one was degassing, and the largest vent was characterized by spattering and ejecta. In the SW section (CS2), there were at least four active vents: a hornito that produced some explosions and ejected coarse material, two vents behind the hornito that produced simultaneous explosions and ash emissions, and a deep crater further N with a vent on the floor that was characterized by small explosions and gas emissions.

Strombolian explosions persisted in March, though at an overall lower rate compared to February: 2-14 per hour in the N crater and 1-8 per hour in the CS crater. The N1 vent continued to generate low-intensity explosions that ejected material 80-250 m high, accompanied by minor ash emissions, while the N2 vent had similar explosions that ejected mixed coarse material. In the CS crater coarse material mixed with ash was ejected 250 m above the crater. An episode of stronger explosive activity lasting 3 minutes and 30 seconds early on 1 March consisted of about 10 explosions in the summit area (figure 201). Three strong explosive pulses were detected starting at 0232. At 0233 a strong explosion ejected material 350 m above the crater that fell along the upper part of the Sciara in the W and toward the Pizzo in the E. Two simultaneous explosions at 0235 from the CS crater and N1 lasted 22 seconds, ending the sequence. Resulting ejecta was deposited along the Sciara del Fuoco and toward Pizzo.

Figure 201. Thermal webcam images showing the explosive sequence at Stromboli on 1 March 2021 from 0232 (a) through 0236 (h) local time (the webcam timestampes are reported in UTC). Images captured by the SCT surveillance camera. Courtesy of INGV (Rep. No. 10/2021, Stromboli, Bollettino Settimanale, 01/03/2021 - 07/03/2021, data emissione 09/03/2021).

Field observations were conducted on 31 March and 2-3 April by INGV staff to measure any changes in the summit crater and describe the activity. According to INGV, N1 showed two vents with low-to-medium explosions that ejected mainly coarse material mixed with ash. Two vents in one cone at N2 had little activity, but a hornito produced gas-and-steam emissions. CS1 has two vents adjacent to each other; the northernmost exhibited gas-and-steam emissions. The southernmost vent was occasionally explosive and ejected coarse material. CS2 contained three emission points with a SW-NE orientation, showing explosive activity of varying intensities and ejecting material of different sizes. CS3 had little explosive activity; sporadic modest explosions generated reddish ash. The average rate of explosions in the N crater was 2-18 per hour, and in the CS crater it was 1-7 per hour. N1 continued to eject coarse material (lapilli and bombs) up to 250 m high while N2 ejected fine ash mixed with coarse material. After 12 April both the intensity and volume of ejecta decreased in N1. The N2 vent showed low-intensity explosions and fine ejecta through 15 April; coarse material and weak spattering characterized 17 April. In the CS crater, at least three emission points were observed that generated explosions and ejected coarse material was mixed with medium-sized ash up to 250 m high.

Intermittent, low-power thermal activity was detected during January through April, according to the MIROVA Log Radiative Power graph using MODIS infrared satellite information (figure 202). During late January through early February, a cluster of stronger thermal anomalies were detected. Two thermal alerts were reported by the MODVOLC system on 21 January, around when the cluster of anomalies were detected by MIROVA. Sentinel-2 infrared satellite imagery showed a consistent thermal hotspot in both summit craters on clear days (figure 203).

Figure 202. Intermittent low thermal activity at Stromboli was recorded by the MIROVA system (Log Radiative Power) from MODIS satellite data during January through April 2021; a cluster of stronger anomalies occurred during late January through early February. A single low-power anomaly was detected in early March, with four in April. Courtesy of MIROVA.

Figure 203. Persistent thermal anomalies (bright yellow-orange) at Stromboli were visible in Sentinel-2 infrared satellite imagery from both summit craters during January through April 2021. On 7 January (top left) and 16 February (top right) 2021 three thermal anomalies were visible in the summit area. By 8 March (bottom left) and 27 April (bottom right) only two anomalies were observed. Images with “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy, (URL: http://www.ct.ingv.it/en/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

October 2021 (BGVN 46:10) Cite this Report

Pyroclastic flow extends 1 km into the sea on 19 May 2021

Nearly constant fountains of lava at Italy’s Stromboli have served as a defining character of this SE Tyrrhenian Sea volcano for at least 2,000 years. Eruptive activity at the summit consistently occurs from multiple vents at both a north crater area (N area) and a central-southern crater area (CS area) on a cratered terrace known as the ‘terrazza craterica’ at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the volcano-island. Periodic lava flows emerge from the vents and flow down the scarp, sometimes reaching the sea; occasional large explosions produce ash plumes and pyroclastic flows. Thermal and visual cameras that monitor activity at the vents are on the nearby Pizzo Sopra La Fossa, above the terrazza craterica, and at multiple flank locations. Detailed information for Stromboli is provided by Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) as well as several satellite sources of data; May-August 2021 is covered in this report.

Summary of activity during May-August 2021. Stromboli maintained typical levels of explosive activity throughout most of May-August 2021 with the exception of a significant pyroclastic explosion on 19 May, and short periods of increased activity in late June and late July (figure 204, table 12). The thermal activity for the period was high during late May and June, but low during the rest of the period according to the MIROVA graph of radiative power (figure 205). Sentinel-2 satellite imagery also showed fluctuations in the number of active vents and in the amount of thermal intensity at the vents throughout the period, with a strong thermal anomaly from the 19 May pyroclastic flow (figure 206).

Figure 204. The average number of hourly explosions (left axis) at Stromboli during January-August 2021 are broken out by area and as a total with blue for the CS area, red for the N area and black for the combined total. The data are smoothed as daily (thin lines) and weekly (thick lines) averages. The right axis indicates the qualitative descriptors of the activity levels from low (basso) to highest (altissimo) with the green highlighted band indicating the most common level of activity. Courtesy of INGV (Report 36/2021, Stromboli, Bollettino Settimanale, 30/08/2021 - 05/09/2021).

Table 12. Summary of type, frequency, and intensity of explosive activity at Stromboli by month. Courtesy of INGV weekly reports.

Figure 205. MIROVA thermal anomaly data for Stromboli from 8 February through August 2021 indicated a spike in late May and June that corresponded to a pyroclastic flow event on 19 May followed by several lava flows in May and June. Courtesy of MIROVA.

Figure 206. Variations in the size, shape, and intensity of thermal anomalies at Stromboli during May-August 2021, seen in Sentinel-2 satellite imagery, correlate with fluctuating levels of explosive activity at various vents within the two major crater areas (N and CS). Dates are shown on each image. The large pyroclastic flow followed by a lava flow on the Sciara del Fuoco on the NW flank appears as a strong linear thermal anomaly in the 22 May image (top center). Images use Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Activity during May 2021. During a visit on 26 April 2021 INGV scientists noted that the N area had three emission sites that included one hornito producing a mixture of ash, lapilli, and bombs; the CS area had one hornito and a deep crater area producing lapilli and bombs (figure 207). A Digital Surface Model (DSM) of the crater area compiled from data on 8 May compared with the 31 March model (figure 208) revealed that the N1 crater had widened by 8 m in the SW direction and had three active vents. Inside the N2 crater, two scoria cones with a diameter of about 20 m each had formed and two others had coalesced to form a 25 m wide feature where previously there had been crater depressions. There were four active vents. Craters CS 1 and 2 were coalescing and contained ten vents, seven of which were active. The southwestern edge had widened by a few meters towards the SW. The CS 3 crater was unchanged with one inactive vent.

Figure 207. A view of the terrazza craterica on 26 April 2021, taken from the Pizzo Sopra la Fossa, with subcraters in the N and CS areas labeled. a) overview of the terrazza craterica. b) detail photo of the CS area. Courtesy of INGV (Report 18/2021, Stromboli, Bollettino Settimanale, 26/04/2021 - 02/05/2021).

Figure 208. An orthophoto (left) and shaded Digital Surface Model (DSM, right) of the summit of Stromboli from 8 May 2021 highlight the different crater areas with different colors: CS1 in yellow, CS2 in orange, CS3 in dark red; N1 light green, and N2 in dark green. Red circles indicate active vents, yellow circles are degassing vents, and green circles are inactive vents. Courtesy of INGV (Report 19/2021, Stromboli, Bollettino Settimanale, 03/05/2021 - 09/05/2021).

During field observations over 10-16 May 2021, INGV scientists noted that intense spattering had built a large cone in the N2 crater area, and frequent explosions in the CS area were interrupted by intense spattering from the two vents of the cone in that area (figure 209). The vent in the S part of the CS area also produced explosions of coarse tephra that, at times, exceeded 200 m in height. Intense degassing was first noted on the flank of the N area beginning on 11 May with weak and discontinuous explosive activity observed by field personnel on 14 May (figure 210).

Figure 209. Infrared images of the terrazza craterica at Stromboli taken during 10-16 May 2021 show the CS area and the N area with a) an explosion in the CS area and the two cones, N-cono1 and CS-cono2 and b) an explosion at the N-cono1 cone in the N2 area. Courtesy of INGV (Report 20/2021, Stromboli, Bollettino Settimanale, 10/05/2021 - 16/05/2021).

Figure 210. An area on the flank of N area cone of Stromboli (red circle) showed intense degassing with mild and discontinuous explosive ejections of lapilli and bombs during 10-16 May 2021. Courtesy of INGV (Report 20/2021 Stromboli, Bollettino Settimanale, 10/05/2021 - 16/05/2021).

On 19 May at 1251 UTC, after an episode of intense spattering activity from the N2 crater in the N area, the edge of the N2 crater overlooking the Sciara del Fuoco collapsed. The collapse caused the formation of a pyroclastic flow which reached the coastline at a speed of about 50 m/s and traveled more than 1 km across the ocean surface. The Toulouse VAAC reported a narrow ash plume that rose to about 2 km altitude and drifted E (figure 211). Trace amounts of ashfall were recorded in San Vincenzo on the E flank. Additional pyroclastic flows of lesser intensity continued for about 30 minutes. A lava flow began from the collapse site about 20 minutes after the explosion and flowed down the Sciara to the coast (figure 212). Webcams indicated that on 20 May the lava flow was still being fed.

Figure 211. An ash plume from a partial collapse of the N2 crater at Stromboli on 19 May 2021 drifted E over houses in San Vincenzo on the E flank of the island. Photograph by Carlo Lanza, courtesy of INGV (Stromboli activity of 19 May 2021).

Figure 212. The lava flow descending the Sciara del Fuoco entered the sea in a plume of steam on 19 May 2021 at Stromboli. It was preceded by a pyroclastic flow in the same location that extended more than a kilometer over the water before dissipating. Courtesy of INGV (Report 21/2021, Stromboli, Bollettino Settimanale, 17/05/2021 - 23/05/2021).

The volume of pyroclastic material released as of 24 May 2021 was estimated at 450,000 ± 150,000 m³. Observations by INGV personnel on 26-27 May confirmed that the lava flow was no longer active, as suggested by satellite data two days earlier. Six vents were actively spattering at the N2 crater, with four of them aligned and erupting simultaneously (figure 213). They also noted that the pyroclastic cone that had occupied the center of the N2 crater until 16 May had been partially destroyed; no fresh ejecta was found on the Pizzo above the vents. Ash emissions and gas jets resumed at the N area without noticeable spattering on 27 May. A new DSM model with data from 27 May allowed comparison with the 7 May data to determine the volume and location of material involved in the 19 May event (figure 213); the collapse involved both the N2 crater area and the upper part of the Sciara del Fuoco near the N area. INGV determined that the minimum volume of material involved was likely around 70,000 m³, although they concluded that significant material had been added to the crater between 7 May and 19 May, indicating that the volume of material removed was likely greater. A similar analysis for the lava flow that was active on the Sciara del Fuoco between 19 and 24 May indicated that the volume of material added to the slope was around 300,000 m³ (± 100,000). Again, this value was considered an underestimate because it didn’t consider the material that flowed into the N2 crater or was deposited below sea level (figure 214).

Figure 213. The effects of the collapsed area of the N2 crater at Stromboli could be seen in both photographic images (left) and DSM data (right). On the left, a photo taken on 26-27 May 2021 showed multiple active vents (white arrows) at the edge of the collapsed rim (blue line). The right image is a shaded model of the crater area comparing 7 and 27 May where the difference between the DSMs is expressed in meters and shown in a color scale from red to blue indicating the thicknesses of the accumulated material in red and of the lost material in blue; SdF = Sciara del Fuoco. Courtesy of INGV (Report 22/2021, Stromboli, Bollettino Settimanale, 24/05/2021 - 30/05/2021).

Figure 214. The material added from the lava flow of 19-20 May 2021 is shown as the difference of the shaded model of the Sciara del Fuoco area at Stromboli between 7 and 27 May 2021 in meters of thickness in shades of red. The dotted line indicates the area under analysis. Threshold value of resolvable height differences is 2 m. Courtesy of INGV (Report 23/2021, Stromboli, Bollettino Settimanale, 31/05/2021 - 06/06/2021).

Explosive activity was observed in the webcams throughout May 2021. The N Area included explosions from three vents at the N1 crater and four vents at the N2 crater, occurring at a rate of 8-15/hour of varying intensity from low to medium-high (sometimes the ejecta reached 250 m high); short intervals of intense spattering occurred during 27-30 April, 2, 6, 12, and 15 May. Ejecta was primarily lapilli and bombs, with some ash. Prior to the events of 19 May, the explosion frequency was around 12-15 events/hour, and dropped to 8-10 events/hour afterwards. The CS area had at least three vents with medium and high intensity explosions (over 250 m high) of coarse and fine ejecta. The explosion frequency of the CS Area was 5-7/hour before 19 May and dropped to low or nothing afterwards.

Activity during June-August 2021. Activity recorded in the webcams during June 2021 indicated that in the N area, crater N1 had two vents and produced explosions varying from low to medium intensity with the ejecta consisting of lapilli and bombs mixed with ash reaching 150 m high. The N2 crater, with four vents, showed variable explosive activity from low to medium-high (sometimes the ejecta reached 250 m) of mainly lapilli and bombs, with continuous spattering that intensified during 3-5, 7, 12, 13, 15, 17 and 20 June; intense spattering caused short lava overflows on 21, 23, and 25 June. The average frequency of the explosions fluctuated between 6 and 16 events/hour. In the CS area there were at least two vents with explosions of primarily ash, of medium and sometimes high intensity. The explosive activity showed average hourly frequency values ranging from less than 1 to 6 events/hour. During the last week of the month there were as many as eight active vents in the CS Area. Two were at the S side and produced medium-high intensity explosions up to about 200 m of primarily ash, and one was more in the center and produced discontinuous low-intensity explosions with more lapilli and bombs.

Two lava flows were recorded from the N2 crater on 11 and 13 June. The first flow on 11 June emerged from a vent on the S edge of the cone inside the N2 crater (figure 215) and traveled only a few tens of meters in the upper area of the Sciara before it stopped but then produced blocks that rolled down the slope for about eight hours. The second flow on 13 June began after an episode of intense spattering; the flow lasted about one hour and traveled a short distance down the upper part of the Sciara.

Figure 215. Two small lava flows on 11 (top) and 13 (bottom) June 2021 at Stromboli were seen in one of the flank thermal cameras. Courtesy of INGV (Report 24/2021, Stromboli, Bollettino Settimanale, 07/06/2021 - 13/06/2021).

INGV carried out a site visit during 15-17 June 2021 at the summit and along the Sciara del Fuoco at an elevation of about 290 m. They observed on 15 June that explosive activity was concentrated in the N area, which included three active vents (figure 216). A single vent was active at the N1 crater producing episodic and medium intensity Strombolian explosions to 100 m, and there were alternating phases of strong spattering with ejecta exceeding 200 m, and intense degassing from two small cones at the head of the Sciara del Fuoco. Two main vents were active in the CS area; the southernmost was characterized by sporadic and intense ash explosions that included some lapilli and bombs rising to 200 m, and the more central vent had discontinuous low intensity explosions with more incandescent ejecta. A small vent with incandescence was active on the N side of the cone in the center of the terrazza craterica. A landslide on the Sciara del Fuoco during the afternoon of 16 June produced multiple blocks on the slope between 700 and 300 m elevation. This was followed on the evening of 17 June by the opening of a 30-m-long E-W fissure which produced a flow that descended the Sciara del Fuoco to 335 m elevation adjacent to the flow of 19 May (figure 217). A subsequent overflow on 19 June from the same fissure only traveled a few tens of meters before stopping. Additional small overflows were noted on 21, 23, and 25 June that remained at or above 700 m elevation on the Sciara del Fuoco.

Figure 216. A view of the terrazza craterica from the Pizzo de la Fossa on 15 June 2021 at Stromboli showed degassing from numerous active vents in both the N and CS areas. Details provided in the text. Photo by D. Andronico, courtesy of INGV (Report 25/2021, Stromboli, Bollettino Settimanale, 14/06/2021 - 20/06/2021).

Figure 217. Thermal and orthophoto images of the Sciara del Fuoco at Stromboli show the outlines of the flows of 19 May and 17 and 19 June 2021. The orthophoto image (right) is inverted relative to the thermal image (left) and shows the 17 June flow outlined in red. Courtesy of INGV (Report 25/2021, Stromboli, Bollettino Settimanale, 14/06/2021 - 20/06/2021).

Three vents were active in the N area during July 2021; crater N1 produced low to medium-high intensity explosions of lapilli and bombs sometimes mixed with ash (figure 218). Crater N2, with two vents, showed low intensity explosive activity (less than 80 m high) of lapilli and bombs with intense spattering at times. The average frequency of the explosions ranged between 1 and 10 events/hour. In the CS area, five vents were active; vent S1, on the cone overlooking the Sciara, produced low intensity explosions of ash, sometimes mixed with lapilli and bombs. Most explosive activity was observed at the S2 crater, with two vents that produced medium-high intensity explosions of lapilli, bombs, and ash where numerous explosions had ejecta that exceeded 150 m in height. A few low intensity explosions of lapilli and bombs occurred at crater C when it was not degassing. The explosive activity had average hourly frequency values ranging from 2 to 9 events/hour. The intensity of the activity decreased during the second half of July.

Figure 218. The crater terrace ‘terrazza craterica’ of Stromboli as seen by the restored thermal camera placed on Pizzo sopra del Fossa showing the Central-South (CS) area and North (N) area. The abbreviations and arrows indicate the names and locations of the craters and active vents. The area above the crater terrace is divided into three height ranges relating to the intensity of the explosions. Courtesy of INGV (Report 28/2021, Stromboli, Bollettino, Settimanale, 05/07/2021 - 11/07/2021).

During 14 July, the terrazza craterica inside the CS area was impacted by a medium-high energy explosive sequence. The first explosion produced pyroclastic ejecta from both of the C vents. A few seconds later a smaller explosion from S1 was followed by two explosions of lapilli and bombs from the S2 crater. The explosions enlarged the crater around the C vents. Observations by INGV scientists on 23 and 25 July provided details of activity not observed in the webcams (figure 219). In the N area, the N1 crater had two active vents with simultaneous explosions of ash at the southern vent and spatter at the northern vent. The N2 crater had four active vents, including one with gas emissions and bombs and three with only degassing. In the CS area, and in particular at crater CS1, five active vents were observed, two of which produced abundant ash emission, sometimes simultaneously, and three were primarily degassing vents at the eastern edge. The active vent at CS2 was only degassing with no ejecta, and the active vent at CS3 had sporadic explosions and produced mostly reworked ash.

Figure 219. Details of the craters and active vents on the terrazza craterica of Stromboli were observed on 23 and 25 July 2021 by INGV scientists and are described in the text. Crater areas are shown in different colors and letters. Vents with ejecta are shown in red, vents with only degassing are shown in yellow, and a fumarole is circled in orange. ‘Cenere’ is ash. Courtesy of INGV (Report 30/2021, Stromboli, Bollettino Settimanale, 19/07/2021 - 25/07/2021).

The typical eruptive activity of July was interrupted by a large explosion on 28 July 2021 followed by another on 1 August. The 28 July explosion occurred in the CS area near the C and S2 crater vents. The explosion reached about 200 m above the terrace; ejecta landed on the Pizzo and also sent blocks down the Sciara del Fuoco (figure 220). A second explosion on the night of 1 August also originated from the C vents of the CS Area and sent ejecta in all directions, including up to the Pizzo and down the Sciara del Fuoco. The explosion was followed by continuous and intense spattering activity for about two minutes from crater S2.

Figure 220. The large explosion of 28 July 2021 at Stromboli was recorded through the INGV webcams on the Pizzo, and at 400 and 190 m above sea level. The explosion came from the C crater in the CS area and expanded also into the S2 crater, sending ejecta around a large area beyond the crater terrace and onto the Sciara del Fuoco. Courtesy of INGV (Report 31/2021, Stromboli, Bollettino Settimanale, 26/07/2021 - 01/08/2021).

Eruptive activity during August was produced by three vents in the N area and six vents in the CS area. Crater N1 with two vents, produced medium-low intensity explosions of lapilli and bombs which caused the growth of hornitos and sent ejecta in all directions around the vents during the first half of the month. Later in the month, the activity was more low intensity explosions of ash with fewer lapilli and bombs. The N2 crater exhibited low intensity explosive activity with ash to less than 80 m high sometimes mixed with lapilli and bombs. The average frequency of the explosions fluctuated between 3 and 6 events/hour. In the CS area, vents at the S1 crater, located on the cone facing the Sciara, produced low intensity explosions of ash mixed with lapilli and bombs while the three vents at the S2 crater produced simultaneous explosions of low to medium-high intensity of ash mixed with lapilli and bombs. Crater C produced occasionally intense degassing. The frequency of the explosions fluctuated between 4 and 9 events/hour.

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy, (URL: http://www.ct.ingv.it/en/; MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Toulouse Volcanic Ash Advisory Center (VAAC), Météo-France, 42 Avenue Gaspard Coriolis, F-31057 Toulouse cedex, France (URL: http://www.meteo.fr/aeroweb/info/vaac/).

February 2022 (BGVN 47:02) Cite this Report

Strombolian explosions and minor flow activity, September-December 2021

Nearly constant fountains of lava at Italy’s Stromboli have served as a defining character of this SE Tyrrhenian Sea volcano for at least 2,000 years; recorded eruptions go back to 300 BCE. Eruptive activity at the summit consistently occurs from multiple vents at both a north crater area (N area) and a central-southern crater area (CS area) on a terrace known as the ‘terrazza craterica’ at the head of the Sciara del fuoco, a large scarp that runs from the summit down the NW side of the volcano-island. Periodic lava flows emerge from the vents and flow down the scarp, sometimes reaching the sea; occasional large explosions produce ash plumes and pyroclastic flows. Thermal and visual monitoring cameras are on the nearby Pizzo Sopra La Fossa, above the terrazza craterica, and at multiple flank locations. Detailed information for Stromboli is provided by Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) as well as several satellite sources of data; September-December 2021 is covered in this report.

Stromboli maintained typical levels of explosive activity throughout September-December 2021 with a modest increase in activity in the north crater area during November and December (figure 221, table 13). Thermal activity for the period remained low with a slight increase at the end of the year according to the MIROVA graph of radiative power (figure 222). Sentinel-2 satellite imagery also showed minor fluctuations in the number of active vents and in the amount of thermal intensity at the vents throughout the period (figure 223).

Figure 221. Typical levels of activity continued at Stromboli during September-December 2021 with a modest increase in the number of hourly explosions in the north crater area during November and December. The average number of hourly explosions (left axis) during June-December are broken out by area and as a total with blue for the CS area, red for the N area, and black for the combined total. The data are smoothed as daily (thin lines) and weekly (thick lines) averages. The black circles along the top represent days with no observation from poor visibility (Visib. Scarsa). The right axis indicates the qualitative descriptors of the activity levels from low (basso) to highest (altissimo) with the green highlighted band indicating the most common level of activity. Courtesy of INGV (Report 01/2022, Stromboli, Bollettino Settimanale, 27/12/2021 - 02/01/2022).

Table 13. Summary of type, frequency, and intensity of explosive activity at Stromboli by month from September-December 2021; information from webcam observations. Courtesy of INGV weekly reports.

Figure 222. Thermal activity at Stromboli decreased during September-December 2021 after a spike in activity caused by lava flows during May and June as seen in the MIROVA project graph of Log Radiative Power for 21 March through December 2021. Courtesy of MIROVA.

Figure 223. Minor variations in the size, shape, and intensity of thermal anomalies at Stromboli during September-December 2021, seen in Sentinel-2 satellite imagery, correlate with fluctuating levels of activity at various vents within the two major crater areas (N and CS). Dates are shown on each image. Images use Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Activity during September-October 2021. INGV personnel made a site visit to the summit on 2 September 2021 to examine recent changes. In the N crater area, crater N1 had a cone with two vents and an inactive hornito, as well as three additional vents near the head of the Sciara del fuoco (figure 224). The observed explosions contained mostly ash and were of medium-low frequency and intensity. Crater N2 had at least two active vents that produced explosions of lapilli and bombs mixed with ash, but with very low frequency (less than 1 event/hour) and medium intensity. In the CS area, Crater C had three vents, two of which were degassing with no explosive activity. Crater S1 was an inactive cone. Crater S2 had two vents with pulsing emissions, a partly destroyed hornito, and a vent in a depression producing Strombolian explosions of lapilli and bombs mixed with ash. The event frequency was medium-low (5-10 events/hour) and the intensity was medium (the ejecta did not exceed 150 m in height). An additional cone facing the Sciara del fuoco was observed. At 1909 UTC on 11 September webcams recorded a high-energy explosive event that affected the N crater area. Cloud cover prevented detailed observations, but abundant fallout of pyroclastic material on the Sciara del fuoco was observed (figure 225).

Figure 224. Five active vents (indicated by arrows) were observed in the N crater area of Stromboli on 2 September 2021. View is towards the SE. Courtesy of INGV (Report 36/2021, Stromboli, Bollettino Settimanale, 30/08/2021 - 05/09/2021).

Figure 225. Webcams on Stromboli at 190 m elevation captured the high-energy explosive event that occurred in the N crater area on 11 September 2021 and sent pyroclastic material down the Sciara del fuoco. Courtesy of INGV (Report 37/2021, Stromboli, Bollettino Settimanale, 06/09/2021 - 12/09/2021).

A drone image from 23 September captured a view of the terrazza craterica showing both crater areas and the active vents (figure 226). During their site visit that day INGV noted that four vents were active at N1, with low frequency, medium intensity explosions of mostly ash. The active vent at N2 also had a similar frequency, intensity, and composition. At crater C in the CS area two vents issued pulsating gas emissions, and S1 had no explosive activity. Crater S2 had four vents, one with intermittent spattering activity. The explosions were low frequency with variable intensity of mostly lapilli and bombs.

Figure 226. Both N and CS crater areas and the active vents were visible in a drone image taken over the terrazza craterica of Stromboli on 23 September 2021. Red circles indicate the two main craters of the N crater area, and green circles delineate the main features of the CS area. Courtesy of INGV (Report 38/2021, Stromboli, Bollettino Settimanale, 20/09/2021 - 26/09/2021).

Observations from the webcams (figure 227) throughout September 2021 indicated that crater N1 had two vents with low intensity explosions (less than 80 m high) of ash mixed with lapilli and bombs through 23 September. After this, the intensity of the explosions increased to medium and high values with numerous explosions exceeding 150 m in height. The N2 crater vent produced low intensity explosions of ash, lapilli, and bombs. Explosion frequency in the N area was 2-7 events/hour. In the CS area, crater S1 had no activity during the first week, but produced moderate to constant explosive activity of ash during 6-15 September before returning to sporadic explosive activity for the second half of the month. Three vents were active in crater S2 producing variable intensity explosions of mostly coarse material reaching and occasionally exceeding 150 m in height. Crater C had mostly intense degassing interrupted with modest explosions of lapilli and bombs during the second week of the month and on 25-26 September. The explosion frequency of the CS area fluctuated between 3 and 11 events/hour.

Figure 227. A view of Stromboli’s terrazza craterica from the ‘Pizzo’ webcam above the terrace showing the CS and N areas on 26 September 2021. Active vents are shown by arrows. The 80 and 150 m lines are used to measure ejecta heights above the terrazza craterica. Courtesy of INGV (Report 39/2021, Stromboli, Bollettino Settimanale, 20/09/2021 - 26/09/2021).

During October 2021 webcam images indicated that crater N1 had two active vents with variable intensity explosions of ash mixed with lapilli and bombs. A third vent with similar ejecta became active after 11 October. A number of the explosions exceeded 150 m in height during the last week of the month. Crater N2 also had two vents producing low intensity explosions of ash, lapilli, and bombs interrupted by a major explosion on 6 October. The northernmost vent in the N2 crater had continuous explosions on 2 October. A third vent was active in the N2 crater during 25-31 October, and intense spattering from N2 was observed during 27, 30, and 31 October. The explosion frequency at the N crater area ranged from 2-14 events/hour. At the CS area, vent S1 produced sporadic low intensity explosions of ash until 18 October. Sporadic explosions of lapilli and bombs recurred there during the last week of the month. The S2 area produced variable intensity explosions of lapilli and bombs from three vents. Crater C produced intense degassing interrupted by modest explosions of lapilli and bombs until 11 October when activity diminished. Explosions of lapilli and bombs at crater C occurred again on 23 and 26 October. The explosion rate in the CS area ranged from 4-12 events/hour.

A major explosion from a vent in the N2 crater began in the early afternoon of 6 October. The ejecta was sent in all directions out from the terrazza craterica, and also rose as high as the Pizzo Sopra La Fossa above the terrace. Numerous blocks rolled down the upper part of the Sciara del fuoco, creating a thick ash blanket on the slope. The explosion lasted for about ten seconds, and the subsequent ash plume was visible for about 20 seconds (figure 228). During a site visit on 9-12 October, INGV personnel carried out drone overflights to collect data for a new digital terrane model. Light and dark-colored meter-size ejecta appeared around the N1 crater from the 6 October explosion. A new crater with approximate dimensions of 37 x 47 m, was located between areas N1 and N2 (figure 229).

Figure 228. The webcam on Stromboli at 190 m elevation captured visible and infrared images of an explosion from the N2 summit crater on 6 October 2021. Courtesy of INGV (Report 41/2021, Stromboli, Bollettino Settimanale, 04/10/2021 - 10/10/2021).

Figure 229. INGV scientists carried out a drone overflight of the summit of Stromboli during 9-12 October 2021 collecting data for a new digital terrane model (top) and observing the changes that resulted from the 6 October explosion at the N crater area (bottom). N area craters are shown in blue with the new crater highlighted in red and shown in closeup in the lower images; CS area craters are outlined in green. Courtesy of INGV (Report 42/2021, Stromboli, Bollettino Settimanale, 11/10/2021 - 17/10/2021).

Activity during November-December 2021. Activity recorded in webcam data during November 2021 indicated that at the N1 crater had two active vents with medium-low intensity explosions of lapilli and bombs. Short periods of intense spattering were observed at the vent overlooking the Sciara del fuoco on 8 and 12 November. At the N2 crater, three vents produced variable intensity explosions of lapilli and bombs. Occasional intense spattering was noted on 3, 5-9, and 14 November, and became persistent after that. On 21 November a lava flow emerged from the crater in the N area that had formed during the 6 October explosion (figure 230). Additional flows during 25-27 November emerged from the hornito in the N2 crater. The average explosion frequency was 6-17 events/hour. In the CS area, during the first week of the month craters S1 and C had sporadic, low-intensity explosions of lapilli and bombs and then activity ceased for the rest of the month. Two active vents at crater S2 produced low intensity explosions of lapilli and bombs along with short periods of intense spattering. The explosion frequency in the CS area ranged from 4-10 events/hour.

Figure 230. The crater terrace of Stromboli as viewed on 21 November 2021 by the thermal camera on the Pizzo sopra la Fossa showing both the CS crater area and the N crater area. The intense thermal signature in the N area was from the lava flow that emerged from the N area that morning. The abbreviations and arrows indicate the names and locations of the active vents, the area above the crater terrace is divided into three height ranges relating to the intensity of the explosions. Courtesy of INGV (Report 47/2021, Stromboli, Bollettino Settimanale, 15/11/2021 - 21/11/2021).

A field inspection on 20 November 2021 by INGV personnel included drone imagery taken 400 m above the summit. Visible and thermal images captured the explosive activity at both crater areas. At least two degassing vents with minor ash emissions were observed at N1, and six vents with one spattering were noted at N2. At the CS area, two vents were spattering, and one was degassing. In addition, fumarolic activity was observed on the upper portion of the Sciara del fuoco (figure 231) aligned NNW with a length of about 60 m at the edges of both crater areas. On the morning of 21 November 2021 intense spattering from the vent between craters N1 and N2 produced a small blocky flow. Blocks rolled down the upper part of the Sciara del fuoco for approximately one hour. Beginning in the morning of 25 November, intense activity produced blocks of lava from a hornito in crater N2 that traveled down the Sciara. Intermittent flow activity continued through 27 November with pulses that sent blocks to the coastline multiple times (figure 232).

Figure 231. Thermal and visible images of the summit of Stromboli on 20 November 2021 taken by a drone indicate high levels of thermal activity at both the CS area and the N area (white ovals), and numerous degassing vents along the top of the Sciara del fuoco. Red circles indicate vents with emissions of lapilli and bombs, and black circles indicate vents with gas or ash emissions. Courtesy of INGV (Report 47/2021, Stromboli, Bollettino Settimanale, 15/11/2021 - 21/11/2021).

Figure 232. Thermal imagery from the Stromboli webcam at 190 m elevation during 25-27 November 2021 showed intermittent pulses of flow activity originating in the N crater area that sent lava blocks as far as the coastline multiple times. Courtesy of INGV (Report 48/2021, Stromboli, Bollettino Settimanale, 22/11/2021 - 28/11/2021).

During December 2021 webcam images recorded explosions from crater N1 that were low to medium intensity from two vents with lapilli and bombs mixed with some ash. At crater N2 three vents produced variable intensity explosions of lapilli and bombs with weak and occasional spattering which increased in intensity on 12 December. Two more vents were active at N2 beginning on 13 December and intense spattering continued through 16 December. Some explosions in the N area reached 170 m high during 20-26 December and sent abundant ejecta onto the Sciara del fuoco (figure 233). Episodes of significant spattering during that week produced blocky flows that reached the upper part of the Sciara del fuoco. The average explosion frequency was 2-18 events/hour.

Figure 233. High levels of explosive activity at Stromboli during 21-26 December 2021 produced ejecta that rose as high as 170 m in the N crater area (a) and up to 190 m in the CS area and sent blocks of lava down the upper part of the Sciara del fuoco (b). Courtesy of INGV (Report 52/2021, Stromboli, Bollettino Settimanale, 20/12/2021 - 26/12/2021).

In the CS area, crater S1 had no significant activity for the month. Crater C had modest explosions of lapilli and bombs during 12-19 December but was otherwise quiet. Two vents were active at crater S2 with medium-low intensity explosions of lapilli and bombs at the beginning of the month. A third vent became active during the week of 13 December. The explosion intensity increased to high, with a number of explosions exceeding 150 m in height on 12 December. High levels of explosive activity at S2 during 20-26 December produced ejecta that rose as high as 190 m and consisted mostly of ash with lesser amounts of lapilli and bombs. The explosion rate was 0-7 events/hour.

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy, (URL: http://www.ct.ingv.it/en/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

July 2022 (BGVN 47:07) Cite this Report

Strombolian explosions and incandescent ejecta during January-June 2022

Stromboli is a SE Tyrrhenian Sea stratovolcano in Italy that has exhibited nearly constant lava fountains for the past 2,000 years; recorded eruptions date back to 300 BCE. Eruptive activity occurs at the summit from multiple vents, which include a north crater area (N area) and a central-southern crater (CS area) on a terrace known as the ‘terrazza craterica’ at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the volcano-island. Activity typically consists of Strombolian explosions, incandescent ejecta, lava flows, and pyroclastic flows. Thermal and visual monitoring cameras are located on the nearby Pizzo Sopra La Fossa, above the terrazza craterica, and at multiple flank locations. This report covers activity during January through June 2022 using Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) and various satellite data.

Stromboli continued to exhibit a range of explosive activity during January through June 2022 with a gradual decrease in the number of hourly explosions at the summit crater during April through June (figure 234, table 14). Thermal activity for this period remained relatively low, with a short spike in activity toward the end of May, based on the MIROVA graph of radiative power (figure 235). Sentinel-2 infrared satellite imagery showed minor thermal intensity fluctuations at the active summit vents (figure 236).

Figure 234. The range of activity continued at Stromboli during January through June 2022, showed a general decline in the number of hourly explosions at the summit crater during April through June. The average number of hourly explosions (left axis) during January-June are broken out by area and as a total with blue for the CS area, red for the N area, and black for the combined total. The data are smoothed as daily (thin lines) and weekly (thick lines) averages. The black squares along the top represent days with no observation from poor visibility (Visib. Scarsa). The right axis indicates the qualitative descriptors of the activity levels from low (basso) to highest (altissimo) with the green highlighted band indicating the most common level of activity. Courtesy of INGV (Report 27/2022, Stromboli, Bollettino Settimanale, 27/06/2022 - 03/07/2022).

Table 14. Summary of type, frequency, and intensity of explosive activity at Stromboli by month from January-June 2022; information from webcam observations. Courtesy of INGV weekly reports.

Figure 235. Thermal activity at Stromboli was relatively low during January through June 2022, though there was a notable spike of three strong anomalies toward the end of May, as shown in this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 236. Minor variations in the size, shape, and intensity of thermal anomalies at the two major crater areas (N and CS) from the active vents of Stromboli on 22 January 2022 (top left), 21 February 2022 (top middle), 28 March 2022 (top right), 12 April 2022 (bottom left), 12 May 2022 (bottom middle), and 26 June 2022 (bottom right), as shown in Sentinel-2 infrared satellite images. Some gas-and-steam emissions were also visible accompanying the thermal activity. Images use “Atmospheric penetration” rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Activity during January-February 2022. Variable low to medium explosive activity occurred in the N crater area during January, ejecting coarse lapilli and bombs mixed with ash as high as 130 m above the crater. Spattering was localized to the N2 crater in the upper part of the Sciara del Fuoco and continued during 1-2 January. Lower frequency explosions in the CS area, mostly from the S2 crater, ejected mainly fine to some coarse products 100-120 m above the terrazza craterica. Gas-and-steam emissions dominated the S1 crater; explosions rarely occurred at this crater. Based on webcam images, there was an increase in explosive spattering in the N2 crater on 16 January and as a result, a continuous accumulation of pyroclastic material in the upper part of the Sciara del Fuoco. Around 1930 a small lava flow was observed in the upper part of the Sciara del Fuoco, the front of which consisted of large, incandescent blocks that rolled down the flanks (figure 237); by 2326 the blocks had stopped rolling along the scarp.

Figure 237. Webcam images showing spattering activity in the N summit crater area of Stromboli from the N2 crater on 16 January 2022 (infrared image shown on the right). Incandescent material accompanied this activity, descending the flanks toward the coastline. Courtesy of INGV (Report 03/2022, Stromboli, Bollettino Settimanale, 10/01/2022 - 16/01/2022).

Strombolian explosions that varied from low-to-high intensity were observed with spattering in the N crater area during February. The explosions ejected fine ash mixed with coarse lapilli and bombs 80-150 m high. The CS area did not show significant explosive activity. During 2-4 and 9-12 February discontinuous spattering activity was notably stronger. On 11 February incandescent gases rose from the S1 crater, accompanied by coarse material; at the same time, two vents in the S2 crater generated low-to-medium explosions that ejected coarse and fine material higher than 80 m high.

Activity during March-April 2022. Similar explosive Strombolian events continued during March with spattering in the N crater area. Explosions in the N crater area varied in intensity, with fine to coarse material ejected 80-150 m high. Short, intense spattering activity occurred on 3 March. The CS area did not show significant explosive activity; low intensity explosions that ejected products less than 80 m high from two emission points in the S2 crater. Intense degassing was observed from the N2 crater, which included gas-and-steam rings (figure 238). During 10-12, 16-17, 25, 27, and 30 March low-to-medium explosive activity originated from the N2 crater that contained four emission points, ejecting coarse material less than 120 m high; spattering activity was intense for short intervals.

Figure 238. Photos from Pizzo at Stromboli during early March 2022 showing strong gas-and-steam emissions (left) and degassing rings (right) rising from the N2 crater. Courtesy of INGV (Report 10/2022, Stromboli, Bollettino Settimanale, 28/02/2022 - 06/03/2022).

During April the N1 crater generated explosions of various intensity that ejected material 80-150 m high. During 8-9, 12, and 16 April four emission points in the N2 crater showed low-to-medium explosive activity, accompanied by spattering, that ejected material less than 120 m high. There was not significant explosive activity in the CS area, but low-to-medium explosions ejected products at least 80 m high. A vent near the CS produced episodic but prolonged and dense ash emissions associated with Strombolian activity in the southernmost vent. In the N1 crater, an asymmetrical hornito was open toward the Sciara del Fuoco and contained a vent that produced continuous spattering and discontinuous explosions that ejected coarse material.

Activity during May-June 2022. Activity persisted through May, with low intensity Strombolian explosions in the N and CS area. According to INGV, the S2 crater, which consisted of two emissions points, showed explosions varying from low to high, where some coarse products reached 250 m high. Two field inspections were made on 5 and 7 May. On 5 May mild explosions characterized the S1 and N1 craters, ejecting fine to coarse material from the vents. The N2 crater dominantly showed gas-and-steam emissions. On 7 May cloud cover mostly obscured views of the summit, but INGV reported that activity was less frequent and intense.

On 13 May at 1643 a sequence of six main explosions was detected in the CS area that began from a southern vent in the S2 crater that rapidly evolved to the S1 crater (figure 239). This explosion lasted about 8 seconds, ejecting material 300 m above the crater, and falling to the E and SE, as well as in Pizzo. The resulting ash cloud drifted S. A second explosion at 1643 occurred at the S1 crater, but was of a lower intensity than the initial one, and lasted about 10 seconds. The explosions during 1643-1644 were also detected from the S1 crater, though the intensity was lower, and deposited material on Pizzo and Sciara del Fuoco. Finer material was dispersed to the SE. After this explosive sequence, INGV reported that activity stopped through at least the 15th.

Figure 239. Infrared webcam images showing the explosive sequence that occurred at Stromboli during 1643-1647 (local time) on 13 May 2022. Material was ejected 190 m high according to webcam data. Courtesy of INGV (Report 20/2022, Stromboli, Bollettino Settimanale, 09/05/2022 - 15/05/2022).

A small collapse detected on 20 May caused a landslide along the Sciara del Fuoco. A thermal camera mounted on a drone detected a major thermal anomaly was detected in the N1 crater; activity in the N1 crater has been characterized by explosions and reddish-brown ash emissions. In the CS area, anomalies were also recorded. On 25 May two strong explosions were reported by INGV in the CS area (figure 240): the first of which occurred at 1611 from the N vent in the S2 crater, ejecting material 250 m high and falling in the main crater area; the second event ended at 1612 and occurred in the CS area, ejecting coarse material 80 m high. The resulting ash cloud drifted S.

Figure 240. Infrared webcam images showing two explosions at Stromboli during 1611-1612 (local time) on 25 May 2022. Material was ejected as high as 250 m according to webcam data. Courtesy of INGV (Report 22/2022, Stromboli, Bollettino Settimanale, 23/05/2022 - 29/05/2022).

During June, surveillance cameras showed that explosive activity at the N1 crater consisted of low-to-high intensity explosions where material was ejected 80-180 m high. Three emission points in the N2 crater showed gas-and-steam emissions accompanied by occasional weak spattering activity. The S2 crater showed low intensity explosions that ejected fine material 80-150 m high. The most active crater was N1, which contained two main vents, the first of which faced Pizzo and produced ash emissions on 29 and 30 June and the second, that overlooked the Sciara, emitted ash mixed with coarse incandescent materials on 29 June (figure 241). Crater N2 and S2 showed continuous diffuse degassing. The vents in the CS area did not show significant explosive behavior, though two emission points from the S2 crater were characterized by low intensity explosions.

Figure 241. Photos of the summit crater of Stromboli, labeling each of the active craters (S1, S2, N1, and N2) (top). Ash emissions were visible rising from the N1 crater on the Pizzo side on 29 and 30 June 2022 (bottom). Courtesy of INGV (Report 27/2022, Stromboli, Bollettino Settimanale, 27/06/2022 - 03/07/2022).

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy, (URL: http://www.ct.ingv.it/en/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

February 2023 (BGVN 48:02) Cite this Report

Frequent Strombolian explosions persist with lava flows during July-December 2022

Stromboli, located in Italy, has exhibited nearly constant lava fountains for the past 2,000 years; recorded eruptions date back to 300 BCE. Eruptive activity occurs at the summit from multiple vents, which include a north crater area (N area) and a central-southern crater (CS area) on a terrace known as the ‘terrazza craterica’ at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the volcano-island. Activity typically consists of Strombolian explosions, incandescent ejecta, lava flows, and pyroclastic flows. Thermal and visual monitoring cameras are located on the nearby Pizzo Sopra La Fossa, above the terrazza craterica, and at multiple flank locations. The current eruption period has been ongoing since 1934 and recent activity has consisted of Strombolian explosions and incandescent ejecta (BGVN 47:07). Activity described here includes daily explosions, incandescent ejecta, spatter activity, and lava flows during July through December 2022 using reports from Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) and various satellite data.

Frequent explosive activity continued throughout the reporting period, generally in the low-to-medium range, based on the number of hourly explosions in the summit crater (figure 242, table 15). Intermittent thermal activity was recorded by the MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data (figure 243). There was a gradual increase in thermal power and frequency from September through October and again during early-to-mid November. The strongest thermal activity was detected during early December and continued at a high level through the end of the month. These thermal activity increases likely reflected the period in which active lava flows were reported in the Sciara del Fuoco that preceded intense spattering activity. Sentinel-2 infrared satellite imagery captured relatively strong thermal hotspots at the two active summit craters on clear weather days, showing an especially strong event on 6 October, where a lava flow covered much of the NW flank (figure 244).

Figure 242. Explosive activity persisted at Stromboli during July through December 2022, with low to medium numbers of daily explosions at the summit crater. The average number of daily explosions (y-axis) during July-December (x-axis) are broken out by area and as a total, with red for the N area, blue for the CS area, and black for the combined total. The data are smoothed as daily (thin lines) and weekly (thick lines) averages. The black squares along the top represent days with no observations due to poor visibility (Visib. Scarsa). The right axis indicates the qualitative activity levels from low (basso) to highest (altissimo) with the green highlighted band indicating the most common level. Courtesy of INGV (Report 01/2023, Stromboli, Bollettino Settimanale, 26/12/2022 - 01/01/2023).

Table 15. Summary of type, frequency, and intensity of explosive activity at Stromboli by month during July-December 2022; information from webcam observations. Courtesy of INGV weekly reports.

Figure 243. Intermittent thermal activity at Stromboli occurred during July through December 2022 with increases in power and frequency during early October, mid-November, and December, as shown in this MIROVA graph (Log Radiative Power). The strongest thermal activity was detected during early and late December. Courtesy of MIROVA.

Figure 244. A persistent thermal anomaly was detected at the two major crater areas (N and CS) from the active vents of Stromboli during clear weather days on 31 July 2022 (top left), 25 August 2022 (top right), 24 September 2022 (middle left), 4 October 2022 (middle right), 9 October 2022 (bottom left), and 13 November 2022 (bottom right). During 4 and 9 October the thermal anomaly intensified and by 9 October there was a strong lava flow field that covered much of the NW flank. On 13 November degassing rings were also visible above the summit. Images use “Atmospheric penetration” rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Activity during July-August 2022. Variable low-to-medium Strombolian explosions were reported in both the N and CS crater areas also accompanied by spattering activity (figure 245). The N1 and S2 craters, both of which have two emission points, generated low to medium intensity explosions that ejected fine to coarse material 30-150 m above the crater. Explosions at the N1 crater ejected coarse, partially reworked material mixed with fine ash, which remained suspended a few tens of meters above the crater and dispersed as a light-colored plume, eventually falling in the innermost vent area. Explosive activity at the S2 crater was sporadic and mainly consisted of mainly reworked fine ash and light brown ash plumes. Craters N2 (in the N area), and C and S1 (in the CS area) exhibited frequent degassing and were less explosive. INGV reported that crater incandescence from the degassing craters was visible during the day and night (figure 246). Explosions on 18 July were higher energy and consisted of mainly juvenile material. An explosive sequence at N2 on 25 July was characterized by continuous, diffuse degassing. Explosions affected both the N and CS crater areas; at 0256 an explosion of coarse mixed with fine material was detected at the S2 crater; at 0256 a more intense explosion was detected in the N2 crater, ejecting material 300 m above the crater, and falling along the Sciara del Fuoco. This stronger explosion lasted about two minutes and was accompanied by a lava fountain that gradually decreased in intensity and became spattering after about six minutes. At 0257 a pyroclastic flow originating from the deposits of the previous explosion extended toward the coastline for a few hundred meters. A low intensity explosion with fine material was detected at 0258. The overall duration of the explosive sequence lasted about eight minutes.

Figure 245. Shaded terrain model showing the location of the active craters at Stromboli updated on 17 July 2022. The red circles indicate explosive activity, and the yellow circles indicate intense continuous degassing. Courtesy of INGV (Report 29/2022, Stromboli, Bollettino Settimanale, 11/07/2022 - 17/07/2022).

Figure 246. Aerial images of the N2 crater at Stromboli taken on 15 July 2022 showing crater incandescence. Intense degassing was observed on the outer flank of the N2 crater on the Sciara side. Courtesy of INGV (Report 29/2022, Stromboli, Bollettino Settimanale, 11/07/2022 - 17/07/2022).

On 27 July a lava overflow was reported in the N2 crater area. After the explosive sequence on 25 July, spatter activity was reported in the N2 crater area which, by 1708, had turned into a lava flow (figure 247). Two branches reached the central part of the Sciara at an elevation of 600 m. By 2000 the flow had stopped. During 8-14 August the N2 crater showed sporadic explosions consisting of low intensity coarse material and weak spattering. Some observational data was lost from webcams at 400 m elevation during the latter half of August due to bad weather damaging an optical fiber link on 12 August.

Figure 247. Webcam images of the lava flow development at Stromboli during the evening of 27 July 2022 taken by the SCT infrared camera. The lava flow appears light yellow-green in the infrared images. Courtesy of INGV (Report 31/2022, Stromboli, Bollettino Settimanale, 25/07/2022 - 31/07/2022).

Activity during September-October 2022. Hourly Strombolian explosions were variably low-to-moderate in intensity. Ejected material rose between 80 to more than 150 m above the crater and generally consisted of coarse material mixed with fine ash. In the northern crater area, N1 crater generally showed little degassing activity, but there were episodic ash emissions observed on 9 September. The N2 crater showed intense gas-and-steam emissions and few explosions consisting of lava fragments. Gas-and-steam emissions were relatively low in the CS area. Weak infrequent ash emissions consisting of some coarse material mixed with fine ash were sometimes observed from at least two vents near the S2 crater. Near the central crater (C) gas-and-steam emissions were almost absent.

On 25 September a strong explosion from a vent in the N2 crater was accompanied by intense spattering. Shortly afterwards a lava overflow was reported in the N crater area along the central and summit of the Sciara del Fuoco, lasting around four hours (figure 248). A strong explosion was observed in the N2 crater area on 29 September (figure 249). The first pulse was the most energetic, lasting about eight seconds and ejecting fine material 300 m above the crater that fell along the Sciara del Fuoco. This activity was followed by at least four low-intensity explosive events that ejected material less than 100 m above the crater. The N2 crater showed continuous spattering after the 29 September explosion.

Figure 248. Webcam images of the effusive event at the N crater area of Stromboli on 25 September 2022. A strong explosion was captured at 0229 UTC accompanied by strong spattering activity. By 0247 a lava flow had developed. Courtesy of INGV (Report 39/2022, Stromboli, Bollettino Settimanale, 19/09/2022 - 25/09/2022).

Figure 249. Infrared webcam images showing the explosive event at Stromboli that occurred at 1324 UTC on 29 September 2022. The explosion ejected material as high as 300 m above the crater that was deposited along the Sciara del Fuoco. Courtesy of INGV (Report 40/2022, Stromboli, Bollettino Settimanale, 26/09/2022 - 02/10/2022).

On 3 and 4 October two lava flows were observed at the N crater area. The first flow originated from a fissure on the external cone in the N2 crater area in the Sciara del Fuoco on 3 October. As the lava flow grew in volume, a lava channel formed along the Sciara del Fuoco up to the coast. By 1600 UTC the lava flow had begun to cool. On 4 October a new lava flow from the N crater area overlapped the previous flow and loose material moved along the Sciara del Fuoco. Another lava flow was observed from the N2 crater area beginning on 0721 UTC on 9 October. At 0722 UTC a pyroclastic flow began due to a collapse on the edge of the N2 crater that faced the Sciara del Fuoco (figure 250). The pyroclastic flow moved rapidly and reached the coastline by 0723 UTC, spreading out over the sea a few hundred meters. The pyroclastic flow was then followed by a strong lava flow that developed into two branches and reached the sea in a few minutes.

Figure 250. A sequence of webcam photos showing the evolution of the pyroclastic flow that originated from the N2 crater area at 0721 on 9 October 2022 (a-h). In addition, a lava flow was also seen in an infrared webcam image and during field observations (i and l). Courtesy of INGV (Report 41/2022, Stromboli, Bollettino Settimanale, 03/10/2022 - 09/10/2022).

The effusive activity that began on 9 October developed a lava flow field that consisted of numerous overlapping minor flows. Frequent collapses of material occurred from the lava channel that was formed during 3-4 October, rolling along the Sciara del Fuoco, and reaching the coast, forming a delta. As the hot material interacted with the sea, lobes formed, which generated gas-and-ash emissions that rose to 1 km altitude. Low-level intense spattering activity was variable and contained within the N2 crater with occasional phases of lava fountaining. By 14 October the eruptive activity markedly decreased. INGV reported that by 15 October, according to surveillance cameras and field observations, the flow along the Sciara del Fuoco had stopped. On 16 October the spattering activity in the N2 crater had stopped. After the spattering ended there were numerous series of low-intensity ash events in the N2 crater. Observations from a drone overflight on 16 October revealed new fractures with thermal anomalies between N1 and N2. During 21-22 October two emission points showed low-intensity explosive activity with coarse material and intense spattering activity in the N2 crater area.

Activity during November-December 2022. Explosive activity persisted in both the N and CS crater areas consisting of coarse-to-fine material; spattering activity mainly occurred in the N crater area. The N1 crater contained two emission points that produced explosions of low-to-medium intensity that ejected coarse material up to 120 m above the crater. The N2 crater also contained two emission points that generated low-to-high intense explosions where material sometimes reached more than 150 m above the crater. The S2 crater generated explosions that ejected fine material mixed with coarse material 150-250 m above the crater. No explosive activity was noted in the S1 and C craters. Continuous spattering was reported on 2 November and became more intense during 6 and 8-9 November. A lava flow was observed on 16 November beginning at 0705 UTC, based on seismic data. Due to bad weather conditions during the latter half of the month, visual observations of the summit craters could not be made.

During December, only one explosion was noted in the CS crater area; explosions and spattering mostly occurred in the N crater area. On 4 December at 1410 UTC a lava flow was observed during the afternoon in the upper part of the Sciara del Fuoco, followed by a small flow starting around 1428 UTC that traveled along the central part of the Sciara del Fuoco (figure 251). The two lava flows were 50 m and 80 m below the crater rim of the N crater, respectively, and reached the coast around 1600 UTC. Pyroclastic flows were detected at 1431 and 1518 UTC, the latter of which originated from a fracture on the NW flank of the N crater area within the Sciara del Fuoco and reached the coast in a few seconds. They traveled along the Sciara del Fuoco and then several hundreds of meters into the sea. Accompanying gas-and-ash plumes rose to 1 km altitude. Intense spattering activity was reported hours after the lava and pyroclastic flows began.

Figure 251. Map of the active (red) and cooling (yellow) lava flow moving NW from the N crater area at Stromboli on 7 December 2022. The background image is the shaded relief model of the Sciara del Fuoco and crater area obtained by processing images acquired during several drone overflights. Courtesy of INGV (Report 50/2022, Stromboli, Bollettino Settimanale, 05/12/2022 - 11/12/2022).

The effusive activity from 4 December was observed at least through 8 December, based on webcam images. Both explosive and effusive activity was observed in the N crater area from three vents within the Sciara del Fuoco after 4 December. Additionally, intense spattering activity accompanied the lava flow. No explosive activity was observed in the CS area. Activity that occurred during 4-9 December generated a collapse of material on the N area of the crater terrace, and to a lesser extent, in the CS terrace.

On 16 December at 1208 UTC, after intense spattering activity in the N crater area near N2, a lava flow began to flow in the central area of the Sciara del Fuoco. The flow gradually decreased in volume after a few hours before stopping. Similarly, at 0842 on 19 and 1447 on 21 December (UTC), two lava flows were observed from the vents in the N crater area that spread along the central area of the Sciara del Fuoco after intense spattering activity near the N2 vent. The only explosive activity that was detected in the CS crater area occurred on 22 December, when fine material mixed with coarse material was ejected higher than 150 m above the crater. A lava flow during 26-28 December traveled to 400 m elevation and, similar to previous lava flow events, was preceded by intense spattering activity from the N1 vent that gradually intensified (figure 252). The lava flow began at 2013 UTC on 26 December and had ended by 0830 on 28 December. The flow was mainly fed by the N1 vent and to a lesser extent, by the N2 vent during 0505-0712 UTC.

Figure 252. Webcam images showing the evolution of the lava flow activity (bright yellow-red) from the N1 vent in the N crater area of Stromboli during 26-28 December 2022 taken by the INGE-OE surveillance camera. In images E and F, the N2 vent also partly fed the flow. Courtesy of INGV (Report 01/2023, Stromboli, Bollettino Settimanale, 26/12/2022 - 01/01/2023).

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy, (URL: http://www.ct.ingv.it/en/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

July 2023 (BGVN 48:07) Cite this Report

Strombolian explosions and lava flows continue during January-April 2023

Stromboli, located in Italy, has exhibited nearly constant lava fountains for the past 2,000 years; recorded eruptions date back to 350 BCE. Eruptive activity occurs at the summit from multiple vents, which include a north crater area (N area) and a central-southern crater (CS area) on a terrace known as the ‘terrazza craterica’ at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the volcano-island. Activity typically consists of Strombolian explosions, incandescent ejecta, lava flows, and pyroclastic flows. Thermal and visual monitoring cameras are located on the nearby Pizzo Sopra La Fossa, above the terrazza craterica, and at multiple flank locations. The current eruption period has been ongoing since 1934 and recent activity has consisted of frequent Strombolian explosions and lava flows (BGVN 48:02). This report updates activity during January through April 2023 primarily characterized by Strombolian explosions and lava flows based on reports from Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) and various satellite data.

Frequent explosive activity continued throughout the reporting period, generally in the low-to-medium range, based on the number of hourly explosions in the summit crater (figure 253, table 16). Intermittent thermal activity was recorded by the MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data (figure 254). According to data collected by the MODVOLC thermal algorithm, a total of 9 thermal alerts were detected: one on 2 January 2023, one on 1 February, five on 24 March, and two on 26 March. The stronger pulses of thermal activity likely reflected lava flow events. Infrared satellite imagery captured relatively strong thermal hotspots at the two active summit craters on clear weather days, showing an especially strong event on 8 March (figure 255).

Figure 253. Explosive activity persisted at Stromboli during January through April 2023, with low to medium numbers of daily explosions at the summit crater. The average number of daily explosions (y-axis) during January through April (x-axis) are broken out by area and as a total, with red for the N area, blue for the CS area, and black for the combined total. The data are smoothed as daily (thin lines) and weekly (thick lines) averages. The black squares along the top represent days with no observations due to poor visibility (Visib. Scarsa). The right axis indicates the qualitative activity levels from low (basso) to highest (altissimo) with the green highlighted band indicating the most common level. Courtesy of INGV (Report 17/2023, Stromboli, Bollettino Settimanale, 18/04/2023 - 24/04/2023).

Table 16. Summary of type, frequency, and intensity of explosive activity at Stromboli by month during January-April 2023; information from webcam observations. Courtesy of INGV weekly reports.

Figure 254. Intermittent thermal activity at Stromboli was detected during January through April 2023 and varied in strength, as shown in this MIROVA graph (Log Radiative Power). A pulse of activity was captured during late March. Courtesy of MIROVA.

Figure 255. Infrared (bands B12, B11, B4) satellite images showing persistent thermal anomalies at both summit crater on 1 February 2023 (top left), 23 March 2023 (top right), 8 March 2023 (bottom left), and 27 April 2023. A particularly strong thermal anomaly was visible on 8 March. Courtesy of Copernicus Browser.

Activity during January-February 2023. Strombolian explosions were reported in the N crater area, as well as lava effusion. Explosive activity in the N crater area ejected coarse material (bombs and lapilli). Intense spattering was observed in both the N1 and N2 craters. In the CS crater area, explosions generally ejected fine material (ash), sometimes to heights greater than 250 m. The intensity of the explosions was characterized as low-to-medium in the N crater and medium-to-high in the CS crater. After intense spattering activity from the N crater area, a lava overflow began at 2136 on 2 January that flowed part way down the Sciara del Fuoco, possibly moving down the drainage that formed in October, out of view from webcams. The flow remained active for a couple of hours before stopping and beginning to cool. A second lava flow was reported at 0224 on 4 January that similarly remained active for a few hours before stopping and cooling. Intense spattering was observed on 11 and 13 January from the N1 crater. After intense spattering activity at the N2 crater at 1052 on 17 January another lava flow started to flow into the upper part of the Sciara del Fuoco (figure 256), dividing into two: one that traveled in the direction of the drainage formed in October, and the other one moving parallel to the point of emission. By the afternoon, the rate of the flow began to decrease, and at 1900 it started to cool. A lava flow was reported at 1519 on 24 January following intense spattering in the N2 area, which began to flow into the upper part of the Sciara del Fuoco. By the morning of 25 January, the lava flow had begun to cool. During 27 January the frequency of eruption in the CS crater area increased to 6-7 events/hour compared to the typical 1-7 events/hour; the following two days showed a decrease in frequency to less than 1 event/hour. Starting at 1007 on 30 January a high-energy explosive sequence was produced by vents in the CS crater area. The sequence began with an initial energetic pulse that lasted 45 seconds, ejecting predominantly coarse products 300 m above the crater that fell in an ESE direction. Subsequent and less intense explosions ejected material 100 m above the crater. The total duration of this event lasted approximately two minutes. During 31 January through 6, 13, and 24 February spattering activity was particularly intense for short periods in the N2 crater.

Figure 256. Webcam images of the lava flow development at Stromboli during 17 January 2023 taken by the SCT infrared camera. The lava flow appears light yellow-green in the infrared images. Courtesy of INGV (Report 04/2023, Stromboli, Bollettino Settimanale, 16/01/2023 - 22/01/2023).

An explosive sequence was reported on 16 February that was characterized by a major explosion in the CS crater area (figure 257). The sequence began at 1817 near the S2 crater that ejected material radially. A few seconds later, lava fountains were observed in the central part of the crater. Three explosions of medium intensity (material was ejected less than 150 m high) were recorded at the S2 crater. The first part of this sequence lasted approximately one minute, according to INGV, and material rose 300 m above the crater and then was deposited along the Sciara del Fuoco. The second phase began at 1818 at the S1 crater; it lasted seven seconds and material was ejected 150 m above the crater. Another event 20 seconds later lasted 12 seconds, also ejecting material 150 m above the crater. The sequence ended with at least three explosions of mostly fine material from the S1 crater. The total duration of this sequence was about two minutes.

Figure 257. Webcam images of the explosive sequence at Stromboli on 16 February 2023 taken by the SCT and SCV infrared and visible cameras. The lava appears light yellow-green in the infrared images. Courtesy of INGV (Report 08/2023, Stromboli, Bollettino Settimanale, 13/02/2023 - 19/02/2023).

Short, intense spattering activity was noted above the N1 crater on 27 and 28 February. A lava overflow was first reported at 0657 from the N2 crater on 27 February that flowed into the October 2022 drainage. By 1900 the flow had stopped. A second lava overflow also in the N crater area occurred at 2149, which overlapped the first flow and then stopped by 0150 on 28 February. Material detached from both the lava overflows rolled down the Sciara del Fuoco, some of which was visible in webcam images.

Activity during March-April 2023. Strombolian activity continued with spattering activity and lava overflows in the N crater area during March. Explosive activity at the N crater area varied from low (less than 80 m high) to medium (less than 150 m high) and ejected coarse material, such as bombs and lapilli. Spattering was observed above the N1 crater, while explosive activity at the CS crater area varied from medium to high (greater than 150 m high) and ejected coarse material. Intense spattering activity was observed for short periods on 6 March above the N1 crater. At approximately 0610 a lava overflow was reported around the N2 crater on 8 March, which then flowed into the October 2022 drainage. By 1700 the flow started to cool. A second overflow began at 1712 on 9 March and overlapped the previous flow. It had stopped by 2100. Material from both flows was deposited along the Sciara del Fuoco, though much of the activity was not visible in webcam images. On 11 March a lava overflow was observed at 0215 that overlapped the two previous flows in the October 2022 drainage. By late afternoon on 12 March, it had stopped.

During a field excursion on 16 March, scientists noted that a vent in the central crater area was degassing. Another vent showed occasional Strombolian activity that emitted ash and lapilli. During 1200-1430 low-to-medium intense activity was reported; the N1 crater emitted ash emissions and the N2 crater emitted both ash and coarse material. Some explosions also occurred in the CS crater area that ejected coarse material. The C crater in the CS crater area occasionally showed gas jetting and low intensity explosions on 17 and 22 March; no activity was observed at the S1 crater. Intense, longer periods of spattering were reported in the N1 crater on 19, 24, and 25 March. Around 2242 on 23 March a lava overflow began from the N1 crater that, after about an hour, began moving down the October 2022 drainage and flow along the Sciara del Fuoco (figure 258). Between 0200 and 0400 on 26 March the flow rate increased, which generated avalanches of material from collapses at the advancing flow front. By early afternoon, the flow began to cool. On 25 March at 1548 an explosive sequence began from one of the vents at S2 in the CS crater area (figure 258). Fine ash mixed with coarse material was ejected 300 m above the crater rim and drifted SSE. Some modest explosions around Vent C were detected at 1549 on 25 March, which included an explosion at 1551 that ejected coarse material. The entire explosive sequence lasted approximately three minutes.

Figure 258. Webcam images of the lava overflow in the N1 crater area of Stromboli on 23 March 2023 taken by the SCT infrared camera. The lava appears light yellow-green in the infrared images. The start of the explosive sequence was also captured on 25 March 2023 accompanied by an eruption plume (e) captured by the SCT and SPT infrared webcams. Courtesy of INGV (Report 13/2023, Stromboli, Bollettino Settimanale, 20/03/2023 - 26/03/2023).

During April explosions persisted in both the N and CS crater areas. Fine material was ejected less than 80 m above the N crater rim until 6 April, followed by ejection of coarser material. Fine material was also ejected less than 80 m above the CS crater rim. The C and S2 crater did not show significant eruptive activity. On 7 April an explosive sequence was detected in the CS crater area at 1203 (figure 259). The first explosion lasted approximately 18 seconds and ejected material 400 m above the crater rim, depositing pyroclastic material in the upper part of the Sciara del Fuoco. At 1204 a second, less intense explosion lasted approximately four seconds and deposited pyroclastic products outside the crater area and near Pizzo Sopra La Fossa. A third explosion at 1205 was mainly composed of ash that rose about 150 m above the crater and lasted roughly 20 seconds. A fourth explosion occurred at 1205 about 28 seconds after the third explosion and ejected a mixture of coarse and fine material about 200 m above the crater; the explosion lasted roughly seven seconds. Overall, the entire explosive sequence lasted about two minutes and 20 seconds. After the explosive sequence on 7 April, explosions in both the N and CS crater areas ejected material as high as 150 m above the crater.

Figure 259. Webcam images of the explosive sequence at Stromboli during 1203-1205 (local time) on 7 April 2023 taken by the SCT infrared camera. Strong eruption plumes are visible, accompanied by deposits on the nearby flanks. Courtesy of INGV (Report 15/2023, Stromboli, Bollettino Settimanale, 03/04/2023 - 09/04/2023).

On 21 April research scientists from INGV made field observations in the summit area of Stromboli, and some lapilli samples were collected. In the N crater area near the N1 crater, a small cone was observed with at least two active vents, one of which was characterized by Strombolian explosions. The other vent produced explosions that ejected ash and chunks of cooled lava. At the N2 crater at least one vent was active and frequently emitted ash. In the CS crater area, a small cone contained 2-3 degassing vents and a smaller, possible fissure area also showed signs of degassing close to the Pizzo Sopra La Fossa. In the S part of the crater, three vents were active: a small hornito was characterized by modest and rare explosions, a vent that intermittently produced weak Strombolian explosions, and a vent at the end of the terrace that produced frequent ash emissions. Near the S1 crater there was a hornito that generally emitted weak gas-and-steam emissions, sometimes associated with “gas rings”. On 22 April another field inspection was carried out that reported two large sliding surfaces on the Sciara del Fuoco that showed where blocks frequently descended toward the sea. A thermal anomaly was detected at 0150 on 29 April.

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy, (URL: http://www.ct.ingv.it/en/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).