H₂S, SO₂, HF, HCl, and other gases tending to increase during 1998-99

Periodic inspections of Vulcano have indicated that the total output from the fumarolic field at the crater "La Fossa" started to decrease in 1995. However, an analysis of the chemical compositions of the gaseous phases conducted as a part of an overall evaluation of the volcanic system provides important volcanological details. Different trends observed between the fumaroles located on the rim and those inside the crater (see BGVN 22:11) indicate that the Fossa cone is affected by deformation, possibly the result of increased vapor pressure at depth. The chemical data from samples collected at the same locations during 1998 and 1999 (table 6) indicate similar trends of increasing magmatic gases. This could imply widespread opening of the system, which could affect the stability of the edifice.

Table 6. Chemical data measured at Vulcano, 1998-99. Courtesy of Marino Martini.

In addition to the data reported in table 6, temperature measurements taken along the N rim of Fossa crater some time during the period 2-16 July 1999 were reported by Claude Grandpey. The temperature in July was 340°C compared to ~410°C the previous April. Grandpey also reported stable temperatures (i.e., 95-100°C) at the fumaroles on the isthmus between Vulcano and Vulcanello.

Vulcano is located at the southern boundary of the Aeolian Islands, about 25 km from northern Sicily. It last erupted in 1888-90 when numerous meter-sized bombs and blocks fell in the area now occupied by the village of Vulcano Porto, which hosts thousands of tourists daily during the summer season. Vulcanello, the youngest part of Vulcano Island, began to form only ~2,100 years ago as an isolated island that later became connected with the main island. The latest activity at Vulcanello occurred in the 16th century when lava flows, now covered by large hotel complexes, were extruded.

Information Contacts: Marino Martini, Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, 50125, Firenze, Italy; Claude Grandpey, L'Association Volcanologique Europénne (LAVE), 7 rue de la Guadelopue, 75018 Paris, France.

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

June 1985 (SEAN 10:06) Cite this Report

Microearthquake swarm and slight uplift

"Daily earthquake frequency in the Vulcano area showed, from 24 April, a significant increase (figure 1). Seismic energy may also be considered unusual even though magnitudes have not exceeded 2.5. Epicenters lay predominantly in the Gran Cratere area or very close to it (figure 2); focal depths were generally <1 km. The timing of event and energy distribution reveals the swarm character of the seismicity. Waveforms lead us to hypothesize that both degassing and fracturing phenomena occurred. A leveling survey carried out in May showed a slight uplift (1 cm) of the epicentral area with respect to the S part of the island."

Figure 1. Daily frequency of local earthquakes recorded 1 January-30 May 1985 by the Vulcano Cratere seismic station on the N flank. Courtesy of IIV.

Figure 2. Epicenters of the seven highest energy earthquakes that occurred in the Vulcano area 24 April-30 May 1985. Courtesy of IIV.

Further References. Frazzetta, G., Gillot, P.Y., La Volpe, L., and Sheridan, M.F., 1984, Volcanic hazards at fossa of Vulcano: Data from the last 6000 years: BV, v. 47, p. 105-125.

Falsaperla, S., and Neri, G., 1986, Seismic monitoring of volcanoes: Vulcano (southern Italy): Periodico di Mineralogia, v. 55, p. 143-152.

Information Contacts: S. Falsaperla and G. Neri, IIV.

February 1988 (SEAN 13:02) Cite this Report

New fracture system

"After the last explosive event, in 1888-90, activity has been mainly fumarolic emissions of varying intensity at the crater. A new fracture system, transverse to the main alignment along which the more recent activity had originated, opened during the last few months.

"No important seismic activity was detected at the same time, and the progressive spreading trend beginning in 1985 was not substantially modified. The rate of longitudinal opening (some meters/month), however, along with the well-defined magmatic character of the gaseous species emitted through these fractures, appear as a clear indication of an increasing pressure from beneath."

Information Contacts: M. Martini, Univ di Firenze.

May 1988 (SEAN 13:05) Cite this Report

Higher fumarole temperatures and changing gas chemistry

OV has regularly collected data on fumarole temperatures, gas chemistry, and radon emission at Vulcano. Since September 1987, the temperature of a fumarole (F5) on the crater rim has increased from 310 to 335°C. In December, a new vent near F5 had a temperature of 370°C. Measurements since then (through May 1988) do not show continued temperature increases, although values reached 415° near the bottom of the crater on 1 April. Chemical analyses of fumarolic fluids show clear variations in several gases with time. H₂S, H₂, H₂O vapor, and the S/C ratio strongly increased (figure 3), while Cl and CO₂ decreased significantly. Equilibrium temperatures calculated from the gas/water shift reaction (CO + H₂O <-> CO₂ + H₂) do not show any variation with an average temperature of ~ 400°C. Measurements of water wells also show an increase in radon activity (figure 4).

Figure 3. Percentages of H2S, H2, and H2O; and S/C ratio in gases from fumarole F5 on Vulcano's crater rim, July 1987-April 1988.

Figure 4. Temperature (top), and radon tracks/cm2/day (bottom), at a well (Pozzo Bambara) near Vulcano, 8 June 1986-11 April 1988.

OV geologists believe that the variations could be explained by two main hypotheses: 1) An increase in permeability of the shallow system because of more fracturing (13:02); 2) An increase in heat migration from deeper to shallower reservoirs, buffered by water as indicated by the higher water vapor content of the collected gases.

On 21 April, a landslide occurred on the NE flank of the island. Visual estimates yield a total avalanche volume of <50,000 m³. OV geologists note that a very limited phreatic explosion cannot be excluded as a cause for the landslide.

Information Contacts: D. Tedesco, J.-P. Toutain, L. Bottiglieri, R. Pece, and G. Luongo, OV.

November 1988 (SEAN 13:11) Cite this Report

Fumarole temperature profiles

Geologists visited Vulcano on 25 and 30 October and 1, 7, and 8 November to obtain spatial and temporal surface temperature distributions (figure 5). Fumaroles typically formed linear zones of acid steam discharge. Transects of a zone extending 40 m somewhat radially along the N rim of Gran Cratere were surveyed.

Figure 5. Apparent surface temperature profiles transverse to an elongated fumarolic zone on the N rim of Vulcano's Gran Cratere measured at 0930 (filled circles) and 1230 (open circles) on 8 November. Temperatures were recorded using a Minolta Cyclops 33 radiometer operating at [8-14 µm] wavelength. Each datum is the integrated temperature for the whole field of view of the radiometer, between 2.5 and 4.5 cm in diameter. The later survey shows warming of the downwind side of the zone by hot clouds of condensates blowing across the area. Courtesy of C. Oppenheimer and D. Rothery.

Gas temperatures (recorded by a thermocouple) were stable during the visits. The highest temperature measured was 396°C, although surface temperatures above 300°C were uncommon and highly localized (<0.01 m²) near vents. Because sublimates were unstable above about 150°C, there was a gray sublimate-free belt within 5-40 cm of the vents, flanked on either side first by white ammonium chloride, then yellow sulfur sublimates.

Further Reference. Oppenheimer, C., and Rothery, D., 1991, Infrared monitoring of volcanoes by satellite: J. of the Geological Society, London, v. 148, p. 563-569.

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

January 1989 (SEAN 14:01) Cite this Report

CO₂ in soil increases

CO₂ concentrations in soil near Vulcano's summit crater increased substantially between measurements in October 1987 and November 1988 (table 1), correlated with increased fumarolic activity. Changes in concentrations and flow rates of CO₂ gas have been correlated to volcanic activity by Elskens, Tazieff, and Tonai (1969).

Table 1. CO₂ soil concentrations near Vulcano's summit crater, in mole %. Equipment, sites, and the person making the measurements were all identical for both data sets.

Reference. Elskens, I., Tazieff, H., and Tonai, F., 1969, investigations nouvelles sur les gaz volcaniques: BV, v. 32, no. 3, p. 523-575.

Information Contacts: R.X. Faivre Pierret, DPT/SPIN/LESI CEA CEN, Grenoble, France; F. Le Guern, CNRS, CFR, CEN, Saclay, France.

October 1989 (SEAN 14:10) Cite this Report

Fumaroles deposit sulfur

Geologists from Ruhr Univ visited Vulcano on 20 September and observed it from nearby Lipari Island on 19 and 21 September. During the afternoon of 19 September, strong gas emission occurred from the N part of Fossa Grande Crater, site of Vulcano's last eruption in 1888-1890. Gases were generally driven W by strong winds, but a white plume occasionally rose 300 m above the crater. Strong fumarolic activity occurred from numerous vents and fissures during the 20 September visit. Most of the activity was concentrated in the crater's N sector, but some occurred from the outer N flank and the inner S crater rim. Activity was most intense from a fissure that cut the crater rim along a roughly N-S trend. Just inside the crater, the fissure was up to 0.5 m wide and 1 m deep, with sharp edges coated with sulfur sublimates. Gases escaped with a hissing noise at the most active points. The volume of visible steam seemed to decrease during the afternoon, probably because of a decrease in humidity. Steam emission was still somewhat reduced when viewed from Lipari Island the next day.

During field studies 27-28 September and 3-4 October, Open Univ geologists noted that fumaroles seemed little changed from their previous visit in October-November 1988 (13:11). The fumarole fissure, ~ 40 m long, that crossed the N rim of the crater, was substantially deeper in places, perhaps from the loss of rock particles ejected by the pressurized gas flow. Gas temperatures along the fissure were generally about 275°C, with the highest value, 407°, at a vent on the crater's NE rim. Solid sulfur was abundant near fumaroles, as loose masses of yellow crystals, thin-walled tubules and cups containing drops of acid solution, and sulfur stalactites a few centimeters long within recessed vents. Liquid sulfur was also present, commonly as yellow, orange, or red droplets and dribbles, although one vent had produced a molten sulfur flow ~2 m long, and another contained a pool of liquid sulfur ~ 10 cm in diameter, with a temperature of 115.2°C.

Information Contacts: B. Behncke, Ruhr Univ, Germany; C. Oppenheimer, Open Univ.

March 1990 (BGVN 15:03) Cite this Report

High-temperature fumaroles; gas chemistry; small seismic swarms

Fumarolic activity at Vulcano remained at a very high level in 1989. The temperature of a fumarole (F5) on the crater rim (figure 6) has remained stable at 310 ± 5°C; more than 90 samples have been collected since July 1987. In contrast, a fumarole (FF) inside the crater showed very high temperatures, reaching a maximum of 550°C in August-September 1989, 100° hotter than in 1988. February 1990 temperatures were 515° and 312° at FF and F5 respectively.

Figure 6. Map of Vulcano, showing locations of F5 and FF fumaroles.

Major chemical species (H₂O, CO₂, H₂S, and SO₂) showed large variations in concentration (figure 7). ³He/4He ratios were very high for all crater fumaroles (~60% mantle-derived He), remaining stable during 1989 at ~ 7.5-8.0 x 10^-6. The 13C/¹²C ratio followed a similar trend to that of CO₂, with very wide oscillations from about d13C 0.00 to -2.20+. Geologists noted that the chemical and isotopic trends suggest mixing of different sources.

Figure 7. Variations in concentrations of H2O (top), CO2, (center) and SO2 and H2S (bottom) at Vulcano's fumarole F5, 1987-90. Courtesy of OV.

Seismic activity was monitored by a permanent network installed by IIV, and a digital mobile seismic network operated by OV since 1987. Seismicity was at a low level and characterized by low-energy earthquakes occurring in swarm sequences. On the basis of their wave shapes and spectral characteristics, the earthquakes were divided into "Volcano-tectonic" and "Volcanic" events (figure 8) using the classification of Latter (1981). Volcano-tectonic earthquakes outside the Fossa cone and around the island showed clear P and S phases, high frequency contents, and represented the most energetic events (M < 1.6). Volcanic-type events showed very regular wave trains that were sometimes sharply monochromatic, and were characterized by low dominant frequencies and an absence of clearly identifiable phases. Their energy reached 1011-1012 ergs and their magnitudes were negative. Particle motion analysis revealed the presence of Rayleigh and Rayleigh-like waves with a prograde rotation; the arrivals of these two phases followed one another during such earthquakes. Geologists interpreted these events, centered in the Fossa crater, as being related to fumarolic gas flow at shallow depth.

Figure 8. Seismograms showing events classified as "Volcano-tectonic" (top) and "Volcanic" (bottom) at Vulcano.

Reference. Latter, J.H., 1981, Volcanic earthquakes and their relationship to eruptions at Ruapehu and Ngāuruhoe volcanoes: JVGR, v. 9, p. 293-310.

Information Contacts: D. Tedesco, S. Vulcano, and G. Luongo, OV.

April 1990 (BGVN 15:04) Cite this Report

Continued fumarolic activity

A summit climb on 31 March revealed only minor changes since September 1989 (14:10). Gas emission continued from the fissure on the N rim, at high pressure from its 10-15-cm-wide central portion. Rocks up to 5 mm in diameter were re-ejected when thrown into the fissure's central section. The resulting gas plume rose 300-400 m during rainy weather on 3 April, but was considerably smaller at other times. Weak fumarolic activity was also occurring on the outer SE crater wall, and a new fumarole had formed on the NW flank.

Information Contacts: B. Behncke, Ruhr Univ.

August 1990 (BGVN 15:08) Cite this Report

High fumarole temperatures and geochemical changes; seismicity suggests complex fumarolic system

"OV geologists visited Vulcano island in recent months. Temperatures of the sampled crater fumaroles F5, F5AT, and FA (figure 9) were 300°, 420°, and 537°C respectively on 18 August. During two night inspections inside the crater, bright glow was discovered at all fumaroles up to 530°C and blue flames were discovered at some points in the fumarolic field, probably revealing burning of molten sulfur.

Figure 9. Map of Vulcano showing the locations of fumaroles (F5, F5AT, and FA), and the seismic stations (CNW and CNE) used during the May 1990 microseismicity study. Courtesy of the Osservatorio Vesuviano.

Geochemistry. "Several chemical variations have been observed since April 1990 in fluids sampled at F5 fumaroles. A sharp decrease in H₂O content similar to that recorded in 1988 (see figure 7) has occurred. Consequently, CO₂, SO₂, N₂, HCl, and HF increased in content. At the same time, the S/C ratio significantly decreased. Chemical variations seemed to follow the trend recorded in 1988. These data agree with an unpublished model by Tedesco et al. of possible mixing between shallow and deep fluids, continuously occurring in different proportions before gas escapes from fumarolic vents.

Geophysics. "A microseismicity study of Vulcano crater by the OV in the summer of 1988 revealed the presence of Rayleigh and Rayleigh-like waves with a prograde rotation (15:03). The analyzed earthquakes were low-frequency events, with energy up to 1012 ergs, showing phases not clearly identifiable on seismograms. Most scientists believe them to be related to gas flow in fumarolic conduits (Blot, 1971; Latter, 1971). Particle motion analysis revealed retrograde and prograde elliptical orbit phases that followed one another during such earthquakes (figures 10 and 11). This physical phenomenology was interpreted as due to propagation and reflection of tube waves in a fluid-filled conduit (White, 1983; Toksoz and Stewart, 1984; Hardage, 1985). According to such a model, the successive rotation inversions of particle motion would be generated from alternating downgoing and upgoing tube waves. The non-correlativity of phase arrivals among the seismic network stations suggested complex circulations discriminated by tube heights, because of the presence of several reflecting points (in fact seismographs operated at different altitudes on Vulcano island).

Figure 10. Particle motion on the vertical-radial plane (with respect to crater axes) derived from Vulcano seismogram in figure 11. Numbered frames correspond to seismogram segments and represent several seconds; arrows mark rotation inversions relating to phase arrivals. Courtesy of the OV.

Figure 11. Filtered three-component signal of the Vulcano microearthquake analyzed in figure 10. Dotted lines discriminate temporal intervals (numbered frames in figure 10) for particle motion, and arrows mark arriving of retrograde and prograde phases. Courtesy of OV.

"In May 1990, a survey was carried out to verify the possible presence of correlativity and synchrony of phase arrivals at two seismic stations placed at the same altitude on the top of the crater. Stations were installed at ~90° from each other with respect to the crater axes (figure 9). Notwithstanding the low activity level during the 2-week recording period, the few events analyzed show the same phenomenology observed on 1988 records. Unfortunately, the expected correlativity was absent. The negative result, not invalidating the proposed model, suggested a complex geometry of the tube-like source structure, such as non-vertical orientation."

References. Blot, C., 1971, Etude sismologique de Vulcano: Cahiers ORSTOM serie Géophysique, no. 11.

Hardage, B.A., 1985, Vertical seismic profiling, Part A: Principles, in Helbig, K., and Treitel, S., eds., Handbook of Geophysical Exploration: Geophysical Press, p. 71-95.

Latter, J.K., 1971, Near Surface seismicity of Vulcano, Aeolian Islands, Sicily: BV, v. 35, p. 117-126.

Toksoz, M.N., and Stewart, R.R., eds., 1984, Vertical seismic profiling, Part B: Advanced Concepts, in Helbig, K., and Treitel S., eds., Handbook of Geophysical Exploration: Geophysical Press, p. 256-313.

White, J.E., 1983, Underground sound: application of seismic waves: Elsevier, New York, p. 139-191.

Information Contacts: D. Tedesco, S. Vulcano, and G. Luongo, OV.

April 1991 (BGVN 16:04) Cite this Report

Fumarole temperatures increase

Observations at "La Fossa" crater in recent years have included changes in fumarole temperatures and chemical compositions, ground deformation, and opening of new fractures. Data collected since a systematic surveillance program began in 1977 have allowed geologists to identify different stages during which changing contributions of magmatic gases and water caused fluctuating fumarole outputs. The interaction of heat rising from depth with shallow aquifers has produced changes in water vaporization and pressure as the heat/water ratio varied.

Only minor crater activity occurred until 1987, probably because of the constraints imposed by a limited fracture system on the thermal input. Since then, a sharp change has been observed, with ground inflation and significant increases in the maximum temperature and water concentration of emitted fluids.

In 1990, a further increase in the maximum temperature (to 620°C) and decrease in water contents of fumarole fluids were interpreted as a consequence of increased heat flow, causing significant aquifer depletion (15:08).

The most recent (April 1991) observations indicate that fumarole temperatures are again increasing, and significant vaporization as well as new inflation can be expected. Geologists noted that the long-lasting instability of La Fossa's NW sector could result in some form of collapse that could create problems for the local community.

Further References. Falsaperla, S., Frazzetta, G., Neri, G., Nunnari, G., Velardita, R., and Villari, L., 1989, Volcano monitoring in the Aeolian Islands (southern Tyrrhenian Sea): the Lipari-Vulcano eruptive complex, in Latter, J.H., ed., Volcanic Hazards: Assessment and Monitoring: Springer-Verlag, p. 339-356.

Martini, M., 1989, The forecasting significance of chemical indicators in areas of quiescent volcanism: examples from Vulcano and Phlegrean Fields (Italy), in Latter, J.H., ed., Volcanic Hazards: Assessment and Monitoring: Springer-Verlag, p. 372-383.

Martini, M., Giannini, L., Buccianti, A., Prati, F., Legittimo, P.C., Iozelli, P., and Capaccioni, B., 1991, 1980-1990: Ten years of geochemical investigation at Phlegrean Fields (Italy): Journal of Volcanology and Geothermal Research, v. 48, p. 161-171.

Martini, M., Giannini, L., and Capaccioni, B., 1991, Geochemical and seismic precursors of volcanic activity: Acta Vulcanologia, v. 1, p. 7-11.

Martini, M., Giannini, L., and Capaccioni, B., 1991, The influence of water on chemical changes of fumarolic gases: different characters and their implications in forecasting volcanic activity: Acta Vulcanologia, v. 1, p. 13-16.

Information Contacts: M. Martini, Univ di Firenze.

March 1992 (BGVN 17:03) Cite this Report

Vigorous fumarolic activity

Vigorous fumarolic activity was continuing from the N rim of the historically active crater (Fossa Grande) and from thermal areas on the upper N flank during a visit on the afternoon of 18 March. Most of the fumaroles were concentrated along the N crater rim, inside the N part of the crater, and on the N flank of the wall of tephra built during Vulcano's last eruption, in 1888-90. The main fumarole field appears to have extended a short distance to the E along the N crater rim, where new vents had formed since Behncke's last visit in November 1990. A new linear zone of high-pressure gas emission has developed roughly parallel to the large fissure that formed after 1988 on the N crater wall. Fumarolic activity from scattered vents on the upper N flank seemed to have increased since November 1990, and a less-prominent thermal area on the outer SE flank included at least 7 weak fumarolic vents.

Intense hydrothermal alteration and erosional undercutting have occurred on the upper N flank, around the S rim of the 18th-century Forgia Vecchia craters. Extension cracks have appeared within a few meters of the steep N slope, and deep gullies extend toward the coastal town of Porto di Levante.

Information Contacts: B. Behncke, GEOMAR, Kiel.

October 1994 (BGVN 19:10) Cite this Report

Fumarole observations and temperatures from Gran Cratere

"Gran Cratere was visited on 7 and 11 October 1994 by Open Univ geologists and observations were made of the fumarole zone, which extends from the floor of the lower crater to the rim of the upper crater, and onto the NE outer crater flanks. On 7 October, temperatures of >500 fumaroles were measured (table 2) with a Minolta/Land Cyclops Compac 3 hand-held radiometer (8-14 mm). The only area within the fumarole zone not sampled was that extending from the rim of the lower crater to its floor. Because radiant temperatures have not been corrected for spectral emissivity, all are given as brightness temperatures.

Table 2. Summary of fumarole and fissure temperatures measured at Gran Cratere, Vulcano, 7 October 1994. The upper temperature range of the Compac 3 is given as 500°C by the manufacturer. Courtesy of A. Harris, Open Univ.

"Fumaroles along the crater rim are located in a sinuous 1-3 m wide fissure that runs along the NE crater rim for ~200 m. Within this zone, low-temperature (54-148°C) and medium-temperature (164-286°C) fumaroles dominate and sublimates are common. Maximum temperatures (305-449°C) came from fumaroles within gray rubble-filled depressions, which occurred less commonly along this fissure line. The crater rim fumaroles were bounded at the N end by a rubble-filled fissure, ~60 m long, which cuts the rim obliquely with a N-S trend and extends onto the outer and inner slopes of the crater. This fissure contains fumaroles at temperatures between 134 and 345°C (table 2). The upper slopes of the inner NE flank of the upper crater and S edge of the fumarole zone were dominated by low- to medium-temperature fumaroles, with less common high-temperature fumaroles in rubble-filled depressions and fissures. However, the lower slopes of the inner NE flank of the upper crater were dominated by an area (~70 x 15 m) of gray rubble and high-temperature fumaroles (211-507°C), with lower temperature fumaroles (60-191°C) and sublimates far less common. High temperatures were found in the middle and towards the N side of this area. During measurements there was constant discharge of gases from the fumaroles."

Information Contacts: A. Harris, Open Univ.

April 1995 (BGVN 20:04) Cite this Report

Fumaroles at Fossa Grande and Forgia Vecchia craters

During an 18 Apri visit by Boris Behncke to the Fossa Grande crater the most vigorous fumaroles were present on the N inner crater rim and near its bottom. The main focus of fumarolic activity had shifted notably from the crater rim towards its center since his March 1992 visit (BGVN 17:03). Some of the spectacular fissures on the outer N crater wall were inactive, but several large fumaroles had formed near the crater floor. Molten sulfur was present in many fumaroles on the crater rim. Fumarolic activity on the oversteepened S part of the 18th century Forgia Vecchia craters and on the upper SE slope of the cone has changed little since 1992. Fumaroles were also active at Gran Cratere in October 1994.

Information Contacts: Giada Giuntoli and Boris Behncke, GEOMAR Research Center, Dept. of Volcanology and Petrology, Christian-Albrechts-Universitat zu Kiel, Wischhofstr. 1-3, 24148 Kiel, Germany.

June 1995 (BGVN 20:06) Cite this Report

Fumarole observations and measurements

SVE members who visited Gran Cratere on the Fossa Cone on 21 May observed the fumarole zone that extends from the floor of the lower crater to the rim of the upper crater and onto the NE flanks of the outer crater. Fumarolic activity has remained steady for several months with maximum temperatures of 500-600°C. Although the E-W fissure inside the crater (near the fumarole area) still appeared to be moving, scientists at Palermo University reported no increased seismicity or inflation.

Periodic fumarole surveys made by Marino Martini within the "La Fossa" crater between April 1993 and April 1995 showed a significant decrease in temperatures. Fumarole emissions during this period exhibited increased H₂O and CO₂ gas with a corresponding decrease in volcanic gases (table 3). Marino suggested that the changes were caused by increased permeability, allowing additional shallow groundwater to dilute the fluids eventually emitted at the surface. Increased vapor pressure could affect the precarious stability of the NW slopes of the crater, a serious potential hazard.

Table 3. Fumarole temperatures and gas compositions at Vulcano, April 1993 and April 1995. Courtesy of Marino Martini.

Information Contacts: Henry Gaudru, Societe Volcanologique Europeenne (SVE), C.P. 1 - 1211 Geneva 17, Switzerland; Marino Martini, Department of Earth Sciences, University of Florence, Via G. La Pira 4, 50121 Florence, Italy.

October 1995 (BGVN 20:10) Cite this Report

Fumarolic activity notably diminished from previous years

Fumarolic activity, vigorous in the late 1980s and through 1994, notably diminished in 1995 (BGVN 20:04 and 20:06). During observations in September, the steam and gas output of the most conspicuous fumaroles, at the N rim of the Fossa Grande crater, was back to pre-1985 levels, and no longer formed sizeable gas plumes. Some of the formerly most vigorous fumaroles and steaming cracks were no longer active. Strong gas emission still occurred from fumaroles in the oversteepened and unstable Forgia Vecchia area, below the N rim of the Fossa Grande, and hydrothermal alteration continued to weaken the rock. Several blocks of strongly altered rock with volumes of ~100-500 m³ each had already detached and subsided by 10-20 cm, and may fall. However, it was uncertain whether they would reach the S margin of the village below the Fossa cone. Fumarolic activity also continued from numerous places on the beach N of the "Faraglione" and on the low isthmus connecting Vulcanello to the main body of Vulcano island. During a visit to the western-most (and most recent) crater of Vulcanello on 13 September, no evidence of recent fumarolic activity was found in its NE part where intense fumarolic activity took place until the mid-19th century.

Information Contacts: Boris Behncke and Giada Giuntoli, Department of Volcanology and Petrology, GEOMAR, Wischhofstr. 1-3, 24148 Kiel, Germany.

April 1996 (BGVN 21:04) Cite this Report

Decrease in fumarole temperatures

The "La Fossa" crater was visited during 9-11 May by a group from the Federal Institute of Technology in Zurich. Fumarolic emissions were observed on the SW inner crater wall, on the outer N slope ~100 m below the crater rim, and on the NE outside flank about half way down from the rim towards the sea. During the night of 9-10 May, several new fissures, 2-3 m long and 2-5 cm wide, opened on the inner crater slopes. They formed as an extension of a major fissure reaching W from fumarole FF, concentric to the crater rim. Temperatures of gases emitted from these fissures ranged from 160 to 220°C. During the same night, pre-existing fissures widened by a few centimeters (

Fumarole temperatures were measured on the NE crater rim and on the inner crater flanks, but those from radial fractures in the inner crater were not measured. Maximum temperature observed was 507°C on an extension fissure of fumarole FF on the inner crater slopes (table 4). This compares to the maximum temperature of 552°C in the same period last year at the same location. Temperatures on the crater rim peaked at 326°C at fumarole F5 compared to 512°C last year. Temperatures of outlets situated at the edge of the slope from the inner crater to its floor reach a maximum of 435°C. Fumarole temperatures therefore showed decreasing trends, but maximum temperatures remained high. The decrease was strongest at the rim fumaroles.

Table 4. Measured temperatures at La Fossa Crater, Vulcano, in May 1995 and 1996. Fumaroles F0/F1 and F5 are located at the crater rim; FF, FA and the extension fissure occur in the inner crater. Courtesy of C. Wahrenberger.

Temperature measurements were done using a Cr-Al Type K thermocouple at ~5 cm below the surface. All 1996 measurements were taken at the same locations as those made in 1995. Temperatures at each point were also taken on three successive days; deviations in 1996 were

Information Contacts: Christoph Wahrenberger, Terry M. Seward, and Volker Dietrich, Institute for Mineralogy and Petrography, Federal Institute of Technology, Sonneggstrasse 5, 8092 Zurich, Switzerland.

July 1997 (BGVN 22:07) Cite this Report

Fumarolic emissions during April from Fossa Grande

Fumarolic emissions observed by Boris Behncke during 24-30 April from the Fossa Grande crater appeared more voluminous and denser than during 1995-96. The main focus of the fumarolic activity was in the N-central part of the crater, but fumaroles also appeared more vigorous on the N crater rim.

Information Contacts: Boris Behncke, Istituto di Geologia e Geofisica, Palazzo delle Scienze, Corso Italia 55, 95129 Catania, Italy.

November 1997 (BGVN 22:11) Cite this Report

Trends in fumarolic gas composition during 1996-97

Periodic observations of the chemical composition of fumarolic gases have been made at Vulcano since 1977. Several fumaroles with different temperatures but similar chemical compositions were observed. Differing trends in fumarolic gas composition at different locations on Vulcano have been observed during 1996-97.

Table 5 shows the trend in chemical composition of gases emitted by fumaroles on the rim and inside the crater during 1996-97. For fumaroles on the rim, percentages of typical magmatic species such as SO₂, H2, and CO increased during 1996-97; percentages decreased for fumaroles inside the crater. Scientists estimated that the magmatic system was opening on the rim and closing inside the crater. This evolution pattern revealed that the stability of the system was affected by deformation of the Fossa cone produced by increased vapor pressures at depth.

Table 5. Fumarolic gas composition (percentages) on the rim (A) and inside the crater (B) of Vulcano, 1996-97. Courtesy of M. Martini.

Correction: Boris Behncke (Istituto di Geologia e Geofisica at Università di Catania) noted that during a visit in late April 1997 (BGVN 22:07) steam emissions from the Fossa Grande crater appeared slightly more voluminous than during visits in 1995 and 1996. This statement may have created a false impression of renewed increase in fumarolic activity when it was actually due to the relative humidity of the air. Fieldwork by other scientists during spring 1997 revealed low fumarole temperatures and less abundant emissions. This was confirmed by Behncke during June-July and 10-12 October when fumarolic emissions were the lowest since 1989.

Information Contacts: Marino Martini, Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, 50125, Firenze, Italy; Boris Behncke, Istituto di Geologia e Geofisica, Universitá di Catania, Corso Italia 55, 95129 Catania, Italy.

October 1999 (BGVN 24:10) Cite this Report

H₂S, SO₂, HF, HCl, and other gases tending to increase during 1998-99

Periodic inspections of Vulcano have indicated that the total output from the fumarolic field at the crater "La Fossa" started to decrease in 1995. However, an analysis of the chemical compositions of the gaseous phases conducted as a part of an overall evaluation of the volcanic system provides important volcanological details. Different trends observed between the fumaroles located on the rim and those inside the crater (see BGVN 22:11) indicate that the Fossa cone is affected by deformation, possibly the result of increased vapor pressure at depth. The chemical data from samples collected at the same locations during 1998 and 1999 (table 6) indicate similar trends of increasing magmatic gases. This could imply widespread opening of the system, which could affect the stability of the edifice.

Table 6. Chemical data measured at Vulcano, 1998-99. Courtesy of Marino Martini.

In addition to the data reported in table 6, temperature measurements taken along the N rim of Fossa crater some time during the period 2-16 July 1999 were reported by Claude Grandpey. The temperature in July was 340°C compared to ~410°C the previous April. Grandpey also reported stable temperatures (i.e., 95-100°C) at the fumaroles on the isthmus between Vulcano and Vulcanello.

Vulcano is located at the southern boundary of the Aeolian Islands, about 25 km from northern Sicily. It last erupted in 1888-90 when numerous meter-sized bombs and blocks fell in the area now occupied by the village of Vulcano Porto, which hosts thousands of tourists daily during the summer season. Vulcanello, the youngest part of Vulcano Island, began to form only ~2,100 years ago as an isolated island that later became connected with the main island. The latest activity at Vulcanello occurred in the 16th century when lava flows, now covered by large hotel complexes, were extruded.

Information Contacts: Marino Martini, Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, 50125, Firenze, Italy; Claude Grandpey, L'Association Volcanologique Europénne (LAVE), 7 rue de la Guadelopue, 75018 Paris, France.