Explosions, ash plumes, pyroclastic flows, and thermal activity during June-October 2022

Nevados de Chillán, located in the Chilean Central Andes, has had multiple recorded eruptions dating back to the seventeenth century. The current eruption began in January 2016 with a phreatic explosion and ash emissions from the new Nicanor crater on the E flank of Nuevo crater. Recently, activity has consisted of pyroclastic flows, gas-and-ash plumes, and a new lava dome (Dome 4) that was detected during early March 2022 (BGVN 47:06). This report updates information during June through October 2022 that describes continued explosions, ash plumes, pyroclastic flows, and thermal activity, based on information from Chile's Servicio Nacional de Geología y Minería (SERNAGEOMIN)-Observatorio Volcanológico de Los Andes del Sur (OVDAS) and satellite data.

During June through 17 October 2022 there was continuing eruptive activity in the Nicanor crater, which initially consisted of seismic events, sulfur dioxide emissions, and thermal anomalies (table 3). During August through mid-October, explosion events became more frequent and generated ash plumes that rose as high as 2.7 km above the crater. Explosions were also accompanied by pyroclastic flows that traveled no farther than 800 m down multiple flanks. Incandescent ejecta was visible above the crater rim in surveillance cameras, with some material falling back onto the proximal flanks.

Table 3. Summary of seismic events, maximum sulfur dioxide values, and number of thermal anomalies at Nevados de Chillán during June-October 2022. Data from SERNAGEOMIN bi-weekly reports.

Low thermal activity was occasionally detected by satellite data during July through October, as shown in the MIROVA (Middle InfraRed Observation of Volcanic Activity) Log Radiative Power graph (figure 99). There was a small cluster of thermal activity that occurred during mid-September through early October, which occurred around the same time as strong explosive eruptions. A single thermal hotspot was detected on 13 October according to the MODVOLC thermal algorithm. Some of this activity was also reflected in Sentinel-2 infrared satellite imagery as a small thermal hotspot in the active Nicanor crater (figure 100).

Figure 99. Occasional low-power thermal anomalies were detected at Nevados de Chillán during mid-July through early October 2022, according to this MIROVA graph (Log Radiative Power). No thermal activity was detected in June and three anomalies were detected during each month in July and August. There was a small cluster of thermal activity during mid-September through early October. Courtesy of MIROVA.

Figure 100. Sentinel-2 infrared satellite imagery of Nevados de Chillán showing a weak but persistent thermal anomaly at the active Nicanor crater on 19 July 2022 (top left), 11 August 2022 (top right), 5 September 2022 (bottom left), and 12 October 2022 (bottom right). Thermal activity was confined to the crater. Images use “Atmospheric penetration” rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Activity during June consisted of explosions that generated eruptive plumes with little to no pyroclastic content and crater incandescence that was visible up to 150 m above the crater; the highest plumes reached 940 m above the crater on 19 June. Seismicity was characterized by 56 volcano-tectonic (VT) events, 1,231 long-period (LP) events, and 472 tremor-type (TR) events. There were 320 LP-type events that were linked to surface-level explosions. Sulfur dioxide emission data was obtained by Differential Absorption Optical Spectroscopy (DOAS) equipment, which were installed on the SSE and ESE flanks of the active Nicanor crater. The average emission value ranged from 445 ± 95 to 453 ± 85 t/d, with a maximum daily value of 767 t/d recorded on 8 June. These sulfur dioxide levels are above base levels but are consistent with the presence of the new lava dome (Dome 4) that formed during early March 2022.

Explosive activity from the active crater continued during July and typically generated white gas-and-steam plumes less than 500 m high; due to weather clouds visibility was often obstructed. Seismicity included 67 VT-type events, 1,416 LP-type events, and 454 TR-type events. Of the 1,416 LP-type events, 212 were linked to surface-level explosions. On 13 July an explosion generated an avalanche mixed with deposits on the NE flank of the Nuevo crater that reached 470 m long. Ejecta traveled less than 200 m from the vent and were deposited on the E slope. On 29 July an eruptive episode produced a plume that rose 1,290 m above the crater. Explosions toward the end of the month also ejected a moderate amount of pyroclastic material less than 200 m from the vent onto the nearby flanks. DOAS data showed that the average sulfur dioxide emission value during the month ranged from 554 ± 29 to 601 ± 273 t/d. The maximum daily value was 1,056 t/d on 21 July.

During August, continuous explosions produced eruption plumes with little pyroclastic content that generally rose less than 500 m above the crater; on 2 and 20 August plumes exceeded 1 km above the crater. Intermittent night incandescence was visible to less than 50 m high and did not go beyond the crater rim. A total of 92 VT-type events, 1,382 LP-type events, and 532 TR-type events were detected throughout the month; of the LP-type events, 364 were linked to surface-level explosions. On 11 August surveillance cameras showed an extrusion of material inside the active crater, which SERNAGEOMIN reported was preceded by an increase in sulfur dioxide rates, plume heights associated with explosions, and increased explosion frequency. An explosion on 29 August generated a plume that rose 2 km above the crater and drifted S (figure 101). In addition, pyroclastic flows were reported, traveling less than 500 m on the E and NE flanks. The average sulfur dioxide emission value throughout the month ranged from 413 ± 69 to 507 ± 83 t/d, according to DOAS data. The maximum daily value was 1,017 t/d on 9 August. There was a slight decrease in sulfur dioxide emission rates toward the end of the month.

Figure 101. Photo of an ash plume that rose 2 km above the crater of Nevados de Chillán on 29 August 2022 and drifted S. The explosion was accompanied by pyroclastic flows that traveled less than 500 m on the E and NE flanks. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV) Region De Nuble, Complejo Volcanico Nevados de Chillan, 29 de agosto de 2022, 1108 Hora local (Chile continental)).

Three surveillance cameras continued to detect explosions and eruption plumes composed of mostly gas-and-steam that rose between 500-1,000 m above the crater during September. Seismicity consisted of 180 LP-type events, 1,200 LP-type events, and 332 TR-type events; of the LP-type events, 237 events were linked to surface-level explosions. On 12 September a strong explosion generated an eruption plume that rose 1.4 km above the crater. Toward the end of the month, the explosive events contained a higher volume of pyroclastic content, according to SERNAGEOMIN; ejecta was dispersed SE and block avalanches traveled as far as 500 from the crater rim. On 16, 19, and 30 September explosions produced plumes that carried a higher volume of pyroclastic material more than 360 m above the crater. Two explosions were reported on 19 September: the first rose 1.1 km above the crater and drifted SE (figure 102). A block avalanche occurred on the W flank that traveled 500 m from the crater rim. The second explosion produced an incandescent eruptive column that rose 1.7 km above the crater and drifted SE; ashfall was visible to the SE and ejecta was reported on the W flank. According to SERNAGEOMIN, these explosions also partially destroyed Dome 4. DOAS data recorded that sulfur dioxide emission rates during the month ranged from 315 ± 85 to 323 ± 76 t/d; the maximum daily value was 870 t/d on 12 September.

Figure 102. Photo of an ash plume rising 1.1 km above the crater of Nevados de Chillán that drifted SE on 19 September 2022. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV) Region De Nuble, Complejo Volcanico Nevados de Chillan, 19 de septiembre de 2022, 0813 Hora local (Chile continental)).

More frequent explosions and incandescent material were reported during early October, based on surveillance camera data. Six explosions generated plumes that rose more than 1 km above the crater and three exceeded 2 km. Seismicity has been characterized by 131 VT-type events, 710 LP-type events, and 122 TR-type events. There were 84 LP-type events that were linked to surface-level explosions. Explosions on 2 and 13 October were accompanied by 300-m-long pyroclastic flows and ejecta that traveled more than 500 m from the crater. A strong explosion on 9 October generated an eruption plume that rose 2.4 km above the crater and drifted NNW (figure 103); ashfall was reported in the same direction. In addition, pyroclastic flows were observed on the N, NNE, NE, and E flanks and traveled as far as 766 m. The flows mixed with snow on the volcano, which resulted in avalanche-type flows that moved 1.3 km down the N and NE flanks. Incandescent ejecta was visible at least as high as 800 m above the crater on 15 October. On 16 and 17 October there were three high-energy explosions; on 17 October the eruption column rose 2.7 km above the crater. During this explosion, pyroclastic flows were generated along the N, NW, W, NE, and E flanks and reached as far as 780 m on the NE flank. Some of these flows interacted with snow on the volcano and produced an avalanche-type flow that reached up to 2 km on the NE flank. DOAS measurements showed that the sulfur dioxide emission rates ranged from 47 ± 18 to 284 ± 83 t/d, with a maximum daily value of 911 t/d on 12 October. After the event on 17 October, there was an abrupt drop in surface activity with no eruptive columns or crater incandescence observed.

Figure 103. Photo of an ash plume rising 2.4 km above the crater of Nevados de Chillán that drifted NNW on 9 October 2022. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV) Region De Nuble, Complejo Volcanico Nevados de Chillan, 9 de octubre de 2022, 1930 Hora local (Chile continental)).

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/, https://twitter.com/Sernageomin); 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).

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

July 1979 (SEAN 04:07) Cite this Report

Eruption continuing since 1973

When visited by Oscar González-Ferrán on 21 February 1979, the eruption of Nevados de Chillán that began in July 1973 was continuing. An explosion lasting 1.5 hours produced a cloud, containing bombs, [blocks], and ash, that rose almost 2 km before reaching a windy layer that prevented further upward movement. The new cone had grown to about the same height as the adjacent 1906 cone [Volcán Nuevo], where fumarolic activity persisted (figure 1).

Figure 1. Sketch of the Nevados de Chillán complex; the upper sketch shows details of the area active on 21 February 1979. Courtesy of Oscar González-Ferrán.

[Hugo Moreno reports that by 1983 the phreatomagmatic eruption had almost ended. From 1983 to 1987, only a few explosions have been reported (about one every 2 months). These generated small pyroclastic flows, by column collapse, over the snow cover. By late 1987, the dome extruded earlier in the eruption had been covered by tephra that built a new cone (named Tata) about 30 m higher than neighboring Volcán Nuevo.]

Further Reference. Deruelle, B., 1977, New activity of the Nevados de Chillán: C.R. Acad. Sci. Paris, serie D, v. 284, p. 1651-1654.

Information Contacts: O. González-Ferrán, Univ. de Chile, Santiago.

March 2004 (BGVN 29:03) Cite this Report

A small eruption, the first since 1986, during August-September 2003

Nevados de Chillán was active from 1973 through 1983; after that, phreatomagmatic eruptions were reported to have almost ended. A small (VEI 1) eruption, the first since 1986, was noted by local inhabitants and tourists in August-September 2003. Low magnitude explosive events occurred over the week ending 27 August 2003, sending brown-gray to white gas-and-ash columns up to heights of 500 m for periods of up to 25 minutes. Resulting deposits were ~ 1 cm deep over a sharply defined 2.2 km wide zone to the SSE. Prevailing winds were strong around the time of the eruption (figure 2). Explosions then became more sporadic, occurring at 2-3 day intervals, until ceasing in mid-September.

Figure 2. Strong prevailing winds blowing over the Nevados de Chillán complex caused the resulting plume to remain at low altitude. This photo was taken in early September 2003. The plume blew towards the SSE. Courtesy Servicio Nacional de Geoligica y Mineria.

An inspection of the eruption site on 22 January 2004 by Servicio Nacional de Geoligica y Mineria scientists revealed a new compound, fissure-like, double crater in the saddle between the cones Nuevo (which erupted during 1906-1945) and Arrau (which erupted during 1973-1986) (figure 3). This new ~ 64 m long double crater consisted of a NW situated, 25 x 14 m crater and a SE situated, 39 x 28 m crater. These craters lie to the NW of Arrau cone and become surrounded by an area of intense fumaroles towards Nuevo cone. The fumaroles are water-vapor rich but give off a weak sulfur odor. On Nuevo's E side they had temperatures of up to 88°C (table 1). While no previous measurements were available, this area showed more intense fumarolic activity than seen during a January 1994 visit and 1998 air photographs. During the recent visit the local heat-flow appeared concentrated adjacent to Nuevo cone, rather than Arrau cone. This, and the fissure-like form of the 2003 crater, were taken as evidence for possible future eruptions closer to Nuevo cone.

Figure 3. Aerial view and cross section of the Nevados de Chillán complex, showing the new crater in relation to Nuevo and Arrau cones, and indicating SSE-oriented ash dispersal. Courtesy Servicio Nacional de Geoligica y Mineria.

Table 1. Site names, locations (as UTM coordinates), and fumarole temperatures describing conditions at Nevados de Chillán on 22 January 2004. The fumaroles were located near the 2003 vent. Courtesy of J.A. Naranjo and L.E. Lara, SERNAGEOMIN.

In addition to dispersal and deposition of loose ash, the January inspection noted agglutinates forming a series of 2 m long ridges or 'dunes' (figure 4). The agglutinates consisted of wet black clusters of ash spheres with 0.5- to 1-cm diameters. A large number of dead insects in the agglutinated ash suggested extreme conditions such as the presence of toxic gasses. When dry, the ash was dark gray with a lithic-rich polymodal composition. Particle sizes ranged from dust to 4-5 mm, of which 5-10% was coarse-grained, lithic-rich lapilli composed of black, gray, and red aphyric andesites and ~ 60% was fine- to medium-grained lapilli composed of lithic clasts, quartz, and plagioclase crystals. Below the 1 mm size range, black glassy shards appeared with cleaved vesicle surfaces and blocky or plate-like shapes. The remnant fraction was light-gray fine ash.

Figure 4. January 2004 view of dried ash deposits from Nevados de Chillán's 2003 eruption. The darker deposits lay atop remnant snow pack. Courtesy Servicio Nacional de Geoligica y Mineria.

Reference. Naranjo, J.A., and Lara, L.E., 2004, August-September 2003 small vulcanian eruption at the Nevados de Chillán Volcanic Complex (36°50'S), Southern Andes (Chile). Revista Geológica de Chile, Vol. 31, No. 2, p. 359-366. DOI: 10.4067/S0716-02082004000200011.

Information Contacts: Jose A. Naranjo and Luis E. Lara, Servicio Nacional de Geoligica y Mineria (SERNAGEOMIN), Av. Santa Maria 0104, Santiago, Chile.

June 2016 (BGVN 41:06) Cite this Report

Eruptive episode begins in January 2016 with phreatic explosions

A small eruption from Nevados de Chillán during August-September 2003 was the first since 1986 (BGVN 29:03). Subsequent activity included ash plumes in 2009, and since December 2015, heightened seismic activity, phreatic explosions, and gas and steam emissions from fumaroles. Nevados de Chillán is a group of stratovolcanoes with Cerro Blanco (Volcán Nevado) at the NW end, Volcán Viejo (Volcán Chillán) at the SE end, with a group of lava domes known as Volcán Nuevo in the middle; the 1973-1986 Volcán Arrau dome complex is SE of Volcán Nuevo.

This report covers activity from 2009 through May 2016. With the exception of the Buenos Aires Volcanic Ash Advisory Center (VAAC) report in January 2009, all of the information has been provided by Chile's Servicio Nacional de Geología y Minería (SERNAGEOMIN) - Observatorio Volcanológico de Los Andes del Sur (OVDAS).

Based on Significant Meteorological Information (SIGMET) and analysis of satellite imagery, the Buenos Aires VAAC reported that during 21-22 January 2009, ash plumes rose to altitudes of 3.7-6.1 km and drifted 50-80 km SE. No other information or ground confirmation of ash emission is available about this activity, and no other activity was reported over the next seven years.

The webcam recorded a small white plume on 9 December 2015 rising from the crater. On 31 December 2015, SERNAGEOMIN-OVDAS reported that both seismicity and hydrothermal system activity had increased, prompting an elevation of the Alert Level to Yellow (on a 3-color scale). They also noted the installation of two new seismic stations as well as three GPS stations, with real-time transmission. During December 2015 the seismic network recorded 1,259 seismic events, including 186 volcano-tectonic (VT) earthquakes, 1,030 long-period (LP) earthquakes, 40 short episodes of spasmodic tremor, and three tornillo-type events. The majority of these events were concentrated around the southern cone of the complex at a depth of less than 5 km.

A phreatic explosion at 1755 on 8 January 2016, associated with an LP earthquake, generated a short-duration ash emission. At least one additional phreatic ash explosion occurred on 9 January. During an overflight that same day, volcanologists observed a new crater on the E flank of the Volcán Nuevo lava-dome complex, about 40 m from the edge of the crater. On 29 January a phreatic explosion at 1425 from the Arrau dome complex generated an ash emission that was associated with a seismic tremor signal. During an overflight on 30 January, volcanologists observed that the series of recent phreatic explosions had formed a new crater about 50 m from Arrau Crater, on the E flank. The new crater was 25-30 m wide and at a similar elevation as the crater formed on 8 January.

Monitoring stations registered a VT earthquake at 0635 on 11 February 2016, located 5.3 km SE of Chillán at a depth of 1 km. During 16-29 February 2016, the seismic network recorded 579 seismic events, including 65 VT earthquakes, 510 LP earthquakes, and four discreet tremors. Webcams showed steam emissions from a small fumarole rising to a maximum height of 300 m above the crater.

On 18 March 2016 the number of LP earthquakes increased to 3-4 events per minute starting at 0844. During 1-15 April steam rose from the Arrau dome complex; a small amount of ash was present in the emissions during 7-9 April, rising at most 400 m. On 9 May at 1303, seismic stations recorded an explosion that generated a plume that rose 1.7 km above the crater rim.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN) (URL: http://www.sernageomin.cl/volcanes.php); Observatorio Volcanológico de los Andes del Sur (OVDAS), Temuco (URL: http://www.sernageomin.cl/volcan-observatorio.php); Oficina Nacional de Emergencia Ministerio del Interior (ONEMI) (URL: http://www.onemi.cl/); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php?lang=es).

October 2017 (BGVN 42:10) Cite this Report

Intermittent ash emissions from new craters along the E flanks of Volcáns Nuevo and Arrau persist through September 2017

Nevados de Chillán, in the Chilean Central Andes, is a complex of late-Pleistocene to Holocene stratovolcanoes constructed along a NNW-SSE trend (figure 5). The Nuevo and Arrau craters, active during 1906-1945 and 1973-1986, respectively, are adjacent vents on the NW cone of a large stratovolcano complex 5 km SE of Cerro Blanco; the summit 1 km SE of Arrau is named Volcán Viejo (figure 6). A short eruption during August-September 2003 created a new fissure vent between the Nuevo and Arrau craters (BGVN 29:03, figure 3). Increased seismicity and fumarolic activity were recorded during December 2015, and a new eruption started with a phreatic explosion and ash emission on 8 January 2016 from a new crater on the E flank of Nuevo cones (BGVN 41:06). This report adds information about the beginning of the event and continues with activity through September 2017. Information for this report is provided by Chile's Servicio Nacional de Geología y Minería (SERNAGEOMIN) - Observatorio Volcanológico de Los Andes del Sur (OVDAS), Oficina Nacional de Emergencia - Ministerio del Interior (ONEMI), Corporación Ciudadana Red Nacional de Emergencia (RNE), and by the Buenos Aires Volcanic Ash Advisory Center (VAAC).

Figure 5. This photograph, taken by an astronaut aboard the International Space Station on 11 June 2013, shows three of the largest features of the Nevados de Chillán volcanic complex: Cerro Blanco, Volcán Nuevo, and Volcán Viejo. North is to the lower right. New eruptive activity began in January 2016 from craters located in between Volcán Nuevo and Volcán Viejo.

Figure 6. Detailed location map identifying features of the Nevados de Chillán complex, and the warning zones around the volcano. The colors represent High (maroon), Medium (orange-red), and Low (gold) probabilities of pyroclastic material accumulation of more than one centimeter during a VEI 6 event. Circles with hatch marks inside represent craters. Stars are "Centro de emission,", blue ovals are hot springs. Diagonal cross-hatch is the area most susceptible to pyroclastic material greater than 6.4 cm in diameter in a radius of about 4 km around the active vents. The blue grid lines are spaced four km apart. Courtesy of SERNAGEOMIN, excerpted from Orozco et al. (2016).

Ash emissions at Nevados de Chillán began on 8 January 2016, and were intermittent through September 2017. Four new craters emerged in a NNE trend along the flanks of Volcán Nuevo and Volcán Arrau; two eventually merged into a single 100-m-diameter crater. Most plumes were brief pulses of steam and ash that rose 200-300 m above the craters. Larger events sent a few plumes as high as 2.2 km above the summit (to 5.4 km altitude). Strong prevailing winds quickly dissipated most ash plumes. Periods of multiple small explosions lasted for 1-2 weeks, separated by periods of relative quiet characterized by only steam-and-gas emissions from the active craters and nearby fumarolic centers. The first observable incandescence at the craters was noted in early March 2016. Incandescent bombs were thrown 300 m above the craters during July and September 2016, and 500 m high during March-May 2017 when blocks also fell with 500 m of the craters.

Activity during 2016. After the first explosion with ash emissions on 8 January 2016, nine more pulses of ash were emitted the next day, and small sporadic emissions were reported in the following days (figure 7). OVDAS researchers flew over the volcano on 9 January and concluded that the explosions came from a new crater on the E slope of Volcán Nuevo, about 40 m from the edge of the crater. Researchers from the University of Cambridge who visited the site on 13 January observed continuous degassing at the new 20-m-wide crater. The Buenos Aires VAAC noted puffs of steam and gas dissipating a few hundred meters above the summit (at 3.7 km altitude) in satellite imagery on 16 January 2016. ONEMI reported an ash emission on 29 January that originated from the Arrau crater (see figure 6). During an overflight on 30 January, OVDAS researchers saw occasional explosions from the new crater at Nuevo, as well as activity at a new 30-m-diameter crater about 50 m from the Arrau crater on its NE flank (figure 8). Several fumaroles were also identified on the E flank of Arrau crater.

Figure 7. Ash emission at Nevados de Chillán on 9 January 2016 from the edifice that contains the Nuevo and Arrau craters. The peak to the right is Volcán Viejo. Courtesy of Corporación Ciudadana Red Nacional de Emergencia (RNE) (Advierten nuevo pulso volcánico en el Nevados de Chillán, 9 January 2016).

Figure 8. Photograph showing the Arrau crater at Nevados de Chillán observed during a flyover on 30 January 2016. Ash emissions from a new crater on the NE flank (at right) were reported on 29 January. Courtesy of Corporación Ciudadana Red Nacional de Emergencia (RNE) (Otro cráter más se formó en Nevados de Chillán, 31 January 2016).

During the first two weeks of February 2016, there were 175 episodes of discrete tremor; webcams recorded explosions that ejected material from both craters. The Buenos Aires VAAC reported a brief ash emission on 3 February that dissipated quickly near the summit. During an overflight on 11 February coordinated with ONEMI, scientists identified a third crater, which created a 150-m-long NNE trend with the other two active craters identified during January. During the second half of February, emissions consisted mostly of steam plumes rising no more than 300 m above the crater.

Activity during March 2016 was characterized by steam plumes rising from the active craters; on 3 March, however, a small ash emission was observed. Incandescence was observed in the crater area on the night of 9 March. SERNAGEOMIN reported the beginning of an episode of long-period (LP) seismicity on 18 March, with a pulsating pattern of 3-4 events per minute. During the second half of March, LP and tremor activity was associated with ash emissions. Notably, a low-energy tremor on 30 March lasted for several hours, and concurrently a dense ash plume rose 200 m.

Ash emissions were observed on 7, 8, 9, 18, and 19 April 2016. Plumes were reported rising 400 m on 8 April, and 200 m on 18 and 19 April. Incandescence was observed along with the ash on 18 April. A significant explosion on 9 May 2016 generated an ash plume that rose 1,700 m above the summit (figure 9). The Buenos Aires VAAC reported the ash plume at 3.9 km altitude (700 m above the summit) drifting SE. An overflight by OVDAS on 9 May confirmed the presence of three active craters on the active summit, with the central one having enlarged by 50% since the previous overflight on 11 February. Only pulsating steam emissions were observed in the webcam during the remainder of May and June 2016.

Figure 9. An ash plume rises 1,700 m above the active crater area at Nevados de Chillán after an explosion in the early morning of 9 May 2016. Courtesy of SERNAGEOMIN.

Only steam emissions were reported during the first half of July 2016, but on 21 July an ash-laden emission sent incandescent bombs 300 m above the crater. The Buenos Aires VAAC reported that the webcam showed an ash emission to 3.4 km altitude (200 m above the craters) that day. Webcam Images obtained on 25 July showed debris from an explosion scattered 300 m down the NE flank. During the next few days, ash emissions were inferred from the seismic tremors, but weather conditions prevented direct observations.

During the first two weeks of August 2016, 14 explosions were recorded from the new craters on the E flanks of Nuevo and Arrau. The largest explosion, on 8 August, sent an ash plume 2 km above the crater, according to SERNAGEOMIN (figure 10). The Buenos Aires VAAC reported brief ash emissions on 1, 4, 8, and 9 August at altitudes of 3.7, 3.4, 4.3, and 3.7 km altitude, respectively. Fresh ashfall was visible on the flanks during a flyover on 12 August (figure 11). On the few days when the weather permitted observation of the summit during the remainder of the month, only steam plumes were observed rising no more than 400 m above the crater.

Figure 10. An ash emission at Nevados de Chillán rises 2 km above the active craters on 8 August 2016. Courtesy of Corporación Ciudadana Red Nacional de Emergencia (RNE) (Volcán Nevados de Chillán presenta nuevo pulso eruptive, 8 August 2016).

Figure 11. Fresh ashfall coats the flanks of the active summit at Nevados de Chillán on 12 August 2016, after a large explosion on 8 August. Courtesy of Corporación Ciudadana Red Nacional de Emergencia (RNE) (Registro aéreo muestra actual pulso eruptivo de volcán Nevados de Chillán, 12 August 2016).

Pulsating steam plumes, interrupted by periodic ash emissions, were typical during September 2016. During the first two weeks of the month, 37 recorded explosions were characterized by a high concentration of particulate material. The largest explosion, during the evening of 1 September, generated incandescent bombs for 20 minutes. Incandescence was observed during nighttime explosions a number of times. The Buenos Aires VAAC noted a pilot report of an ash cloud moving SW at 5.2 km altitude on 2 September. They also reported a weak emission of steam and gas with possible diffuse ash visible in the webcam that day. Another pilot report on 6 September indicated an ash cloud moving NE at 6.4 km altitude from a brief but intense emission event around 1420 UTC (figure 12). SERNAGEOMIN noted in their late September report that there had been six explosive episodes since January 2016, with the latest one that occurred during 1-10 September being the strongest.

Figure 12. An ash plume rises from Nevados de Chillán on 6 September 2016. Courtesy of Corporación Ciudadana Red Nacional de Emergencia (RNE) (Volcán Nevados de Chillán registró nuevo pulso eruptive, 6 September 2016).

Explosive activity was recorded on 3, 7, and 8 October 2016 by SERNAGEOMIN; The events were low-energy episodes that emitted small quantities of ash. The Buenos Aires VAAC noted a pilot report on 3 October of an ash cloud moving SE near the summit. It was visible in the webcam but not in satellite imagery, and dissipated quickly. The tallest emission of those days rose to 300 m above the crater on 7 October. During an overflight on 22 October, the continued presence of the three craters along the E flanks of Nueva and Arrau reported previously was confirmed. In addition, the existence of a fourth crater was noted along the same trend as the others. The Buenos Aires VAAC noted ash emissions on 26 and 28 October rising to between 3.7 and 4.3 km altitude and dissipating quickly near the summit.

Seismic activity during the first half of November 2016 included 17 explosions from the active craters. An explosion on 18 November generated an ash plume that rose 1.2 km (figure 13). The Buenos Aires VAAC noted a pilot report of possible ash emissions between 4.6 and 6.1 km altitude on 17 and 27 November, although neither were identified in satellite data.

Figure 13. An ash emission rises 1.2 km above the active crater area at Nevados de Chillán on 18 November 2016. Courtesy of Corporación Ciudadana Red Nacional de Emergencia (RNE) (Sernageomin emite reporte especial por actividad volcánica del complejo Nevados de Chillán, 18 November 2016).

Explosions associated with LP and tremor seismicity continued into December 2016. There were 14 explosive seismic events during the second half of the month, reported by SERNAGEOMIN. The largest occurred on 28 December. The Buenos Aires VAAC noted pilot reports of ash emissions that dissipated near the summit on 13, 28, and 29 December.

Activity during January-September 2017. Explosions related to LP and tremor seismicity increased again on 5 January 2017. The Buenos Aires VAAC reported a dark fumarolic plume drifting E at 4.5 km altitude on 6 January that was observed by a pilot and in the webcam. On 11 and 13 January, the webcam showed sporadic puffs of ash that dissipated very quickly. The largest event occurred on 15 January; the Buenos Aires VAAC reported a narrow plume of ash in satellite imagery at 3.9 km altitude moving W. The webcam also showed sporadic and small puffs that dissipated quickly. An event on 16 January produced an emission that rose 700 m above the crater according to SERNAGEOMIN. This was the last LP-associated explosion of the month. Scientists on a 20 January overflight noted low-intensity steam plumes from the Nuevo and Arrau craters, and from the Chudcún crater which formed in 2003 between them (see figure 6). Yellow and ocher-colored areas, indicating the presence of precipitated sulfur, were visible around the fumaroles and craters.

Low-level degassing rising less than 200 m above the crater was the only surface activity observed during February 2017. A new stage of explosive activity began on 7 March 2017 with emissions that rose as high as 300 m above the crater. The Buenos Aires VAAC noted a pilot report of an ash plume at 3.7 km altitude, and a short-lived puff of ash seen in the webcam. On 11 March, eight explosions sent incandescent blocks up to 0.5 km from the active craters, and emissions rose to 500 m above the crater. Another series of eight explosions on 14 March produced incandescent material and sent an ash plume 1.5 km above the craters. The Buenos Aires VAAC reported intermittent emissions rising up to 4.9 km altitude that day, followed by continuing steam emissions. The following day they noted a small plume near the volcano at 3.9 km altitude visible in satellite data.

During a flyover on 15 March, OVDAS scientists noted that two of the craters (craters 3 and 4) had merged into a single crater 100 m in diameter (figure 14). They also observed five explosions within the space of an hour, the highest resulting plume rose 900 m above the active crater. Webcam images during 16-17 March showed ash emissions rising to 2 km above the crater. The Buenos Aires VAAC reported an ash emission visible in satellite imagery at 5.5 km altitude moving SW on 16 March. For the remainder of the month, only weak degassing under 200 m above the crater was observed. Beginning on 24 March, low-level incandescence at night was reported for the rest of the month.

Figure 14. OVDAS scientists photographed two merged craters (3+4) at Nevados de Chillán on 15 March 2017. They also witnessed five explosions from one crater within an hour (yellow arrow). Courtesy of Corporación Ciudadana Red Nacional de Emergencia (RNE) (Complejo Volcánico Nevados de Chillán tiene cráter de 100 metros de diámetro, 24 March 2017).

Between 1 and 12 April 2017, there were 56 intermittent explosions marking a new phase of activity according to SERNAGEOMIN. The webcams around the complex imaged emissions up to 3 km above the crater throughout the month. The Buenos Aires VAAC reported sporadic emissions of ash visible in the webcam on 3 and 6-8 April. A faint emission at 3.7 km altitude was spotted in satellite imagery on 10 April. From 16 to 30 April, there were 79 intermittent explosions recorded. During dusk and dawn, incandescent material was observed traveling 600 m down the flanks, with some episodes lasting for 60 minutes. The Buenos Aires VAAC reported a brief ash emission and incandescent material visible in the webcam on 17 April, and sporadic ash emissions that rose to 3.9 km altitude on 21, 29, and 30 April (figure 15).

Figure 15. An ash emission on 30 April 2017 at Nevados de Chillán rose to 3.9 km altitude (700 m above the craters), and was photographed by a twitter user near the volcano. Courtesy of Corporación Ciudadana Red Nacional de Emergencia (RNE) (Nuevo pulso eruptivo de volcán Nevados de Chillán preocupa en la región del Bío Bío, 30 April 2017).

Nine intermittent explosions occurred between 1 and 11 May 2017. The webcams showed emissions from the explosions rising generally 300 m above the craters according to SERNAGEOMIN. Intermittent explosions increased again during 27-31 May. Emissions rose to 1.5 km above the craters and incandescent blocks could be seen traveling 600 m down the flank. Periods of constant incandescence lasted for 30 minutes.

This explosive episode continued into June 2017, with 23 intermittent explosions between 1 and 5 June. The largest emission event on 5 June sent a plume 2.2 km above the craters (figure 16). The Buenos Aires VAAC observed the ash plume at 4.6 km altitude in satellite imagery. During 6-15 June, only steam emissions rising to 300 m were reported. Intermittent explosions on 20, 22, 25, and 26 June produced plumes that rose only 200 m above the craters; cloudy weather prevented observation from the webcams during these events.

Figure 16. Twitter users in Chile shared this image of an ash plume rising from the active craters at Nevados de Chillán with regional authorities on 5 June 2017. The Buenos Aires VAAC reported the plume rising to 4.6 km altitude. Courtesy of Corporación Ciudadana Red Nacional de Emergencia (RNE) (Volcán Nevados de Chillán emite nuevo pulso eruptive, 5 June 2017).

No explosive events were observed in the webcams during the first half of July 2017; only steam plumes rising 200 m were reported. A single low-energy explosion was recorded on 31 July; the emission rose to only 100 m above the crater. During August 2017, there were 83 intermittent explosions associated with ash emissions recorded by SERNAGEOMIN. The emissions rose to about 300 m above the active craters; a few larger emissions rose 1,000 m. The Buenos Aires VAAC noted a pilot report of ash emissions on 17 August; the webcam captured a brief emission that dissipated rapidly.

About 150 intermittent explosions were reported during September 2017. The highest plumes, generally composed of steam and ash, rose 2,000 m above the craters. The Buenos Aires VAAC observed a narrow plume of ash in satellite imagery moving N at 3.9 km altitude and dissipating rapidly on 15 September, and a similar plume moving SE near the summit on 26 September 2017.

Reference: Orozco, G.; Jara, G.; Bertin, D. 2016. Peligros del Complejo Volcánico Nevados de Chillán, Región del Biobío. Servicio Nacional de Geología y Minería, Carta Geológica de Chile, Serie Geología Ambiental 28: 34 p., 1 mapa escala 1:75.000. Santiago.

Information Contacts: Servicio Nacional de Geología y Minería, (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile ( URL: http://www.sernageomin.cl/); Oficina Nacional de Emergencia - Ministerio del Interior (ONEMI), Beaucheff 1637/1671, Santiago, Chile (URL: http://www.onemi.cl/); Corporación Ciudadana Red Nacional de Emergencia (RNE, Citizen Corporation National Emergency Network), Avda. Vicuña Mackenna Nº3125, San Joaquín, Santiago de Chile, Chile (URL: http://www.reddeemergencia.cl/); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php?lang=es).

June 2018 (BGVN 43:06) Cite this Report

Hundreds of ash-bearing explosions; dome appears in crater in mid-December 2017

Nevados de Chillán is a complex of late-Pleistocene to Holocene stratovolcanoes constructed in the Chilean Central Andes. The Nuevo and Arrau craters are adjacent vents on the NW flank of the cone of the large stratovolcano referred to as Volcán Viejo. An eruption started with a phreatic explosion and ash emission on 8 January 2016 from a new crater on the E flank of Nuevo. Explosions continued through September 2017 with ash plumes rising several kilometers and Strombolian activity sending ejecta hundreds of meters (BGVN 42:10). This report covers continuing activity from September 2017-May 2018. Information for this report is provided by Chile's Servicio Nacional de Geología y Minería (SERNAGEOMIN)-Observatorio Volcanológico de Los Andes del Sur (OVDAS), Oficina Nacional de Emergencia-Ministerio del Interior (ONEMI), and by the Buenos Aires Volcanic Ash Advisory Center (VAAC).

About 150 ash-bearing explosions were recorded during September and October 2017, with plumes rising almost 2 km above the summit. Activity decreased during the second half of October, and no ash plumes were recorded during November. A significant increase in activity in early December led to over 200 explosions with ash emissions. An overflight on 21 December 2017 produced images of a fissure at the bottom of the new crater. The presence of a growing lava dome in the crater was confirmed in early January 2018. Frequent Strombolian explosions produced nighttime incandescence at the summit and down the flanks. Hundreds of ash-bearing explosions occurred during February 2018; the largest plume rose 2.5 km above the summit, and many smaller pulses produced ash and steam that rose 1.5 km. Sporadic incandescence at night and continued explosions of magmatic gases were typical during March 2018. A large explosion on 31 March coincided with the first appearance of a low-level MODIS thermal anomaly in the MIROVA data, and incandescence from explosions at night indicated that the dome continued to grow during April and May. SERNAGEOMIN reported that the top of the lava dome was visible from the E flank for the first time at the end of May 2018.

Activity during September-December 2017. SERNAGEOMIN reported 117 ash-bearing explosions between 16 and 30 September 2017 (figure 17). The one that released the most energy occurred on 19 September. The plumes of steam and ash rose up to 1,800 m above the crater. The Buenos Aires VAAC observed a narrow plume of ash in satellite imagery moving N at 3.9 km altitude and dissipating rapidly on 15 September, and a similar plume moving SE near the summit on 26 September 2017.

Figure 17. Over 100 ash-bearing explosions were reported at Nevados de Chillán during late September 2017, including ones on 15 September (upper left), 20 September (upper right), 23 September (lower left) and 24 September (lower right). Courtesy of SERNAGEOMIN.

During the first two weeks of October 2017 there were 30 ash-bearing explosions recorded. The Buenos Aires VAAC reported small sporadic puffs of ash on 6 October 2017 that were visible in the webcam (figure 18), but not in satellite data, and a similar dense but short-lived plume on 14 October. SERNAGEOMIN reported a series of pulsating low-energy explosions visible in the webcam that drifted SW on 11 and 12 October 2017, and rose no more than 1 km above the summit.. Only two ash-bearing explosions were recorded during the second half of the month. The volcano was much quieter during November; plumes of steam were observed rising only 100 m above the summit throughout the month, with no ash-bearing plumes reported.

Figure 18. Ash plumes at Nevados de Chillán on 6 (left) and 11 (right) October 2017 were two of the 30 plumes recorded during the first half of October. Courtesy of SERNAGEOMIN.

A significant increase in activity in early December 2017 resulted in 245 explosions associated with ash emissions during the first two weeks, some rising as high as 3,000 m above the summit. The Buenos Aires VAAC reported a puff of ash on 1 December that rose to 3.7 km altitude and drifted S, dissipating rapidly. The next day another plume rose slightly higher, to 4.3 km. A dense emission on 4 December rose to 4.9 km and drifted SE before dissipating in a few hours and was not visible in satellite data. On 11 and 14 December, short-lived emissions rose to 4.3 km (figure 19). A yellow cloud of sulfur formed on 11 December within 300 m of the active crater. The webcams also recorded sporadic nighttime incandescence during increased explosions in the early morning of 14 December. Continuous steam emissions with pulses of minor ash were first noted on 16 December; they were visible in satellite imagery the next day at 3.9-4.3 km altitude drifting NE, and by 18 December, consisted only of water vapor.

Figure 19. An increase in explosive activity at Nevados de Chillán in December 2017 resulted in numerous explosions with ash plumes including on 1 December (upper left), 2 December (upper right), 4 December (lower left), and 11 December (lower right). Courtesy of SERNAGEOMIN.

In a special report released on 19 December, OVDAS-SERNAGEOMIN reported an increase in surface activity over the previous three days, recording minor explosions averaging four per hour, and seismic pulses lasting 5-10 minutes; they also noted harmonic tremor with the increase in explosion frequency. A detailed review of images taken during an overflight on 21 December revealed a fissure 30-40 m long trending NW at the bottom of the crater. Incandescence at night was regularly observed after 20 December (figure 20), and ash emissions rose to 3,000 m above the summit during the second half of the month.

Figure 20. Phreatic explosions with steam and minor ash were common at Nevados de Chillán during the last two weeks of December 2017. Ash emissions and pyroclastic flows (top image) were noted during 12-19 December, and numerous incandescent blocks accompanied the explosions on 28 December (bottom image). Courtesy of SERNAGEOMIN.

Activity during January-April 2018. SERNAGEOMIN volcanologists identified a growing lava dome within the new crater during two overflights on 9 and 12 January 2018 (figures 21); it was emerging from the fissure first identified on 21 December. During the first two weeks of January SERNAGEOMIN reported 1,027 pulsating explosions associated primarily with magmatic gases, and very little ash that rose up to 1,000 m above the summit. Confirmed ash emissions were reported on 11 January at 4.3 km altitude faintly visible moving SE in satellite imagery, according to the Buenos Aires VAAC. Nighttime incandescence from the growing dome was periodically observed (figure 22). Based on the overflight data and satellite imagery, they calculated a growth rate for the dome of 1,360 m³ per day. They estimated the size at 37,000 m³ by mid-month.

Figure 21. During an overflight at Nevados de Chillán on 9 January 2018, SERNAGEOMIN scientists observed the growing dome within the crater. Courtesy of SERNAGEOMIN.

Figure 22. Incandescence at night increased from the growing dome at Nevados de Chillán on 13 January 2018. Courtesy of SERNAGEOMIN.

Overflights on 23 and 31 January measured temperatures of 305-480°C over the surface of the dome, with the highest values at the fissure. The growth rate calculated after these overflights was 2,540 m³ per day. The webcam revealed emissions of ash and water vapor during the second half of the month that rose less than 1,000 m above the summit crater.

An explosion on 2 February 2018 sent an ash plume to 2,500 m above the summit (figure 23). Vibrations from the explosion were reported in Las Trancas (10 km) and at the Gran Hotel Termas de Chillan (5 km). SERNAGEOMIN began referring to the active crater as Nicanor, and the dome was named Gil-Cruz. During the first two weeks of February, 840 explosions associated with plumes of magmatic gases were reported. The plumes generally rose as high as 1,500 m above the summit and were often accompanied by incandescence at night. Two overflights on 7 and 14 February recorded temperatures of 500 and 550°C. SERNAGEOMIN determined a dome growth rate of 1,389 m³ per day, and a total volume of 82,500 m³ by mid-month. At least four explosions on 14 February were characterized by two simultaneous plumes, one of white steam and the other darker with a higher ash content according to SERNAGEOMIN. The highest plume that day reached 1,200 m above the summit crater. The Buenos Aires VAAC also reported a small pulse of ash on 14 February that rose to 4.6 km altitude and drifted SE. The dome continued to grow slowly during the rest of February, with a small increase in size noted during a 22 February flyover. Plumes of mostly water vapor with minor ash rose a maximum of 1,080 m above the summit during the hundreds of small explosions that took place.

Figure 23. A substantial explosion on 2 February 2018 at Nevados de Chillán sent an ash plume 2,500 m above the summit and generated vibrations that were felt 10 km from the summit. Courtesy of SERNAGEOMIN.

Sporadic incandescence at night and continued explosions of magmatic gases were typical during March 2018, with plume heights reaching 2,000 m over the Nicanor crater. During an overflight on 11 March, a temperature of 330°C was measured around the Gil-Cruz dome, which had grown to a volume of about 100,000 m³ but still remained below the crater rim. Morphological changes in the still-slowly growing dome included fracture lines and unstable large vertical blocks. A significant decrease in seismic energy was noted beginning on 24 March that ended when two larger explosions occurred on 30 and 31 March (figure 24).

Figure 24. A substantial explosion on 31 March 2018 at Nevados de Chillán generated distinct ash and steam plumes (top) and sent several large blocks down the flanks (bottom). Courtesy of SERNAGEOMIN.

During an overflight on 3 April 2018, scientists observed energetic pulses of steam and minor ash from the central NW-SE trending fissure inside the crater. They noted that lapilli from explosions had been ejected as far as 1 km from the fissure, and that the Gil-Cruz dome had increased in volume since 11 March; they also observed an area of subsidence on the top of the growing dome (figure 25). The dome was expanding toward the E side of the crater, and the top of the dome rose above the crater rim. They measured a maximum temperature of 670°C on the surface of the dome. The decrease in daily seismicity, the larger explosions of the previous days, and the increased size of the dome with greater risk of collapse, pyroclastic flows, and lahars, all led SERNAGEOMIN to raise the alert level at Chillan to Orange on 5 April 2018.

Figure 25. The growing lava dome at Nevados de Chillán, referred to as Gil-Cruz, had an active steam plume at the center when photographed by SERNAGEOMIN during an overflight on 3 April 2018. Courtesy of SERNAGEOMIN.

The Buenos Aires VAAC reported continuous emissions of steam and gas with minor ash along with a small pulse of ash on 2 April 2018. Low-altitude plumes of mostly water vapor were common throughout April 2018. Incandescence from explosions was visible on clear nights during the month, and ejecta rose as high as 250 m above the crater and was scattered around the crater rim. Seismicity remained constant at moderate levels related to the repeated explosions and the growth of the dome. A faint ash plume could be seen in visible satellite imagery on 18 April at 3.7 km altitude drifting E.

Observations reported on 1 May 2018 from the previous flyover indicated that the rate of growth of the dome had slowed to about 690 m³ per day, and the estimated volume had grown to about 150,000 m³. Activity remained at similar levels throughout May 2018. Seismic instruments recorded long-period seismicity and tremor episodes similar to previous months that corresponded with surface explosions and the extrusion of the lava dome. Seismic energy levels were moderate but fluctuated at times. Plumes of predominantly water vapor with minor gas rose a few hundred meters above the summit drifting generally S or SE before dissipating. Incandescence was often observed on clear nights, accompanied by ejection of incandescent blocks that were observed generally 100 to 150 m above the active crater. A larger explosive event took place on 7 May. Occasional plumes with minor ash were reported on 11 May. SERNAGEOMIN reported on 24 May 2018 that the top of the lava dome was visible from the E flank.

Information Contacts: Servicio Nacional de Geología y Minería, (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); Oficina Nacional de Emergencia - Ministerio del Interior (ONEMI), Beaucheff 1637/1671, Santiago, Chile (URL: http://www.onemi.cl/); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php); 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/).

December 2018 (BGVN 43:12) Cite this Report

Dome growth and destruction with several explosive events, June-November 2018

Nevados de Chillán is a complex of late-Pleistocene to Holocene stratovolcanoes in the Chilean Central Andes. An eruption started with a phreatic explosion and ash emission on 8 January 2016 from a new crater (Nicanor) on the E flank of the Nuevo crater, which lies on the NW flank of the cone of the large stratovolcano referred to as Volcán Viejo. Strombolian explosions and ash emissions continued throughout 2016 and 2017. The presence of a lava dome within the Nicanor crater was confirmed in early January 2018; it continued to grow through May 2018. This report covers continuing activity from June-November 2018 when growth and destruction of the dome alternated in a series of explosive events. Information for this report is provided primarily by Chile's Servicio Nacional de Geología y Minería (SERNAGEOMIN)-Observatorio Volcanológico de Los Andes del Sur (OVDAS), and by the Buenos Aires Volcanic Ash Advisory Center (VAAC).

Activity at the Nevados de Chillán volcanic complex from June-November 2018 consisted of continued steam-and-gas emissions and periodic explosions with ash plumes and incandescent ejecta; these caused frequent changes to the size and shape of the Gil-Cruz dome within the Nicanor crater. Incandescent material as far as 300 m down the flank was seen in nighttime and thermal webcam images on multiple occasions. Larger explosive events during 13-15 July, 7-8 August, 11-12 September, 13 October, and 7 November produced significant ash plumes that rose a few kilometers above the summit, covered much of the area around the crater with fresh ash and blocks as large as a meter in diameter, and caused noticeable changes to the size and shape of the dome. A 400-m-long pyroclastic flow traveled down the E flank on 12 September 2018. The highest ash plume, on 7 November, rose almost 4 km above the summit and drifted SE.

Intermittent seismic and effusive activity continued during June 2018. Seismicity consisted of long-period earthquakes (LP) and tremor episodes (TR) related to the growth of the viscous lava dome located in the Nicanor crater, and occasional volcano-tectonic (VT) seismic events. Gray emissions and dark ash covering the snow were reported several times during the month. The dome was visible on clear days from the webcam located in Portezuelo (70 km NW); the thermal camera there showed intermittent evidence of emissions as well, usually as nighttime incandescence and ejecta scattered around the crater. Incandescent material traveled 300 m down the slope on 22 June. The Buenos Aires VAAC reported a brief emission on 23 June that rose to 4.6 km altitude and drifted NE before dissipating. It was accompanied briefly by a hotspot detected in thermal imagery.

Low-altitude steam and gas plumes were visible throughout July 2018 with periodic nighttime incandescence and ejecta blocks occasionally visible around the crater. Three explosions on 13, 14, and 15 July produced seismic events and significant ejecta, and resulted in partial destruction of the dome (figure 26). The event on 13 July was recorded as a M 3.7 located 430 m below the summit. During the night of 13-14 July images showed incandescence and ejecta on the NE flank near the crater ranging from centimeter to meter in size. The thermal webcam measured temperatures around 300?C. The second explosion on 14 July was recorded as a M 3.9 event located 1.4 km below the summit; webcam images in clear weather the following afternoon showed the extent of the new material on the NNE flank (figure 27). The third explosion in the early morning of 15 July was measured as a M 3.8 event and produced an incandescent column 340 m high. Additional ejecta on the NNE slope was visible in the webcam that afternoon. The Buenos Aires VAAC reported a pulse of ash moving ESE on 15 July at 6.4 km altitude. A video taken by SERNAGEOMIN during an overflight on 16 July showed ejecta around the flanks and steam rising from the partly destroyed dome. Intermittent, low-altitude steam-and-gas emissions continued for the rest of the month; light gray emissions were reported from 26 July through the end of the month.

Figure 26. Images of the Gil-Cruz dome inside the Nicanor crater at Nevados de Chillán show changes in the character of the dome between 4 April and 16 July 2018 after a series of explosions on 13, 14, and 15 July 2018. The arrows show the main area covered by incandescent ejecta during the explosions. Left image courtesy of Nicolás Luengo V. and used with permission, right image taken during a SERNAGEOMIN overflight and copyright by Carabineros de Chile.

Figure 27. Images from a SERNAGEOMIN webcam showing the NE slope of Nevados de Chillán on several dates in July 2018. Significant ejecta from an explosion during the night of 13-14 July covered the rim of the crater and traveled down the NNE slope over the snow (top). Additional new ejecta appeared on the NNE slope on 15 July 2018 after a third explosion in three days (bottom left). Steam and gas plumes rose from the crater on 24 July 2018 (bottom right) and for the remainder of the month after the explosions during 13-15 July. Courtesy of SERNAGEOMIN.

An explosion midday on 7 August 2018 produced abundant high-temperature ejecta around the crater and a 1.5 km high ash plume, according to SERNAGEOMIN. Intermittent gray plumes were reported the next day and for the remainder of August, along with incandescence at night from high-temperature degassing and smaller explosive events (figure 28). The Buenos Aires VAAC reported sporadic and small puffs of ash visible in the webcam on 27 August.

Figure 28. Activity during August 2018 included a number of ash plumes and incandescent explosions at Nevados de Chillán. Skier Birgit Erti captured this image of an ash plume rising after an explosion on 8 August (left); courtesy of Jaime S. Sincioco. Incandescence from explosions at Nevados de Chillán on 16 August (right) was typical of activity throughout the month; courtesy of SERNAGEOMIN.

Intermittent gray emissions and minor incandescence at night were typical of the activity during September 2018, except for a series of explosive events during 11-13 September (figures 29). An explosion on 11 September produced ejecta that traveled 300 m down the slope. The largest event, on 12 September, produced a 2.5-km-high dense ash plume and a pyroclastic flow that went 400 m down the E slope. Communities within 1 km of the crater reported ashfall. Drone video footage from 13 September posted by Nicolas Luengo V. showed the path of a block-and-ash flow down the flank and dense steam emissions with ash rising from the partially destroyed dome (figure 30) (Luengo and Palma, 2018). The Buenos Aires VAAC reported a small ash plume at 4.3-4.9 km altitude drifting SSW on 14 September. Satellite images from 16 September again showed partial destruction of the growing dome at the summit from the explosive events.

Figure 29. Incandescent explosions on 12 September 2018 (left) generated significant ash and ejecta, including a pyroclastic flow, that spread down the flank of Nevados de Chillán. Fresh deposits from the explosions were visible on 14 September (right) from the webcam. Courtesy of SERNAGEOMIN.

Figure 30. Dense steam-and-ash rose from the dome inside the Nicanor crater at Nevados de Chillán on 13 September 2018 in multiple explosive events. Courtesy of Nicolás Luengo, used with permission.

The Buenos Aires VAAC reported an ash emission to 6.1 km altitude on 13 October 2018 seen in multispectral imagery under mostly clear skies moving SSE, and another isolated emission at the same altitude moving SE on 31 October. SERNAGEOMIN reported abundant ejecta scattered around the crater after the 13 October event. Another explosive event on 7-8 November produced incandescent ejecta and ash plumes that were the highest of the reporting period, rising to 7 km altitude and moving SE as reported by the Buenos Aires VAAC (figure 31).

Figure 31. Explosive events at Nevados de Chillán on 7 and 8 November 2018 were recorded by the SERNAGEOMIN webcam on the NE flank (left, 8 November), and by Samuel Opazo T (right, 7 November), likely taken from a community about 40 km NW. Courtesy of SERNAGEOMIN and Samuel Opazo T.

For most of November 2018, pulsating emissions from the crater were accompanied by nighttime incandescence with small explosions and short-range ejecta. The SERNAGEOMIN webcam captured images of explosions on 23, 27, and 29 November. The Buenos Aires VAAC observed weak pulses of ash in satellite imagery at 3.9 km altitude on 23 and 27 November. The intermittent explosions with incandescent blocks and ash from June through November 2018 produced occasional low to moderate thermal anomalies that were captured by the MIROVA project (figure 32).

Figure 32. Low to moderate power thermal anomalies at Nevados de Chillán were intermittent between June and November 2018, increasing slightly in both intensity and frequency towards the end of the period. Courtesy of MIROVA.

Reference: Luengo, Nicolas and Palma, Jose Luis, 2018, Morfometría y tasas de extrusión del domo de lava del Complejo Volcánico Nevados de Chillán mediante el uso de drones eimágenes satelitales, Concepción, Chile, XV Congreso Geológico Chileno, University of Concepción, DOI:10.13140/RG.2.2.35386.64966/1.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/), 16 July 2018 overflight video on YouTube (https://www.youtube.com/watch?v=SVFklfEnWXI); 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/); Nicolas Luengo, University of Concepcion (Twitter: @nluengov), 13 September drone video footage on YouTube (https://www.youtube.com/watch?v=BZt5X3rWoFM); Jaime S. Sincioco (Twitter: @jaimessincioco); Samuel Opazo T (Twitter: @OpazoSamuel).

June 2019 (BGVN 44:06) Cite this Report

Small ash explosions and dome growth during December 2018-May 2019; ballistic ejecta deposited around the crater, with a pyroclastic flow in May

The current Nevados de Chillán eruption period began on 8 January 2018 with a phreatic explosion from the new Nicanor crater, within the Nuevo crater; a new dome was observed within this crater the next day. Dome growth continues with explosions that eject ash plumes and incandescent ejecta. This bulletin summarizes activity from December 2018 through May 2019 and is based on reports by Chile's Servicio Nacional de Geología y Minería (SERNAGEOMIN)-Observatorio Volcanológico de Los Andes del Sur (OVDAS) and satellite imagery.

Throughout December 2018 pulsating emissions from the Nicanor crater produced white plumes predominantly composed of water vapor, with occasional ash ejections giving the plume a gray appearance. Incandescence was frequently observed during the night due to the ejection of hot ballistic ejecta emplaced around the crater during explosions. After 11 months of observations, the dacite dome in the crater maintained a semi-stable extrusion rate of around 345 m³/day. Explosions were reported on 7, 17, 28, and 29 December.

Similar background activity continued through January with pulsating gas-and-steam plumes occasionally including ash, and incandescence observed during the nights due to hot ejecta around the crater. Explosions were recorded at 0500 and 1545 on 11 January, and on 13, 21, and 31 January (figures 33 and 34). During the night explosions and incandescent ejecta were observed impacting the area around the crater.

Figure 33. An explosion at Nevados de Chillán on 11 January 2019. The explosion ejected incandescent blocks that impacted the flanks. The timestamp is at the top left of each image; screenshots are of footage courtesy of SERNAGEOMIN.

Figure 34. An explosion at Nevados de Chillán on 31 January 2019 produced an ash plume from the Nicanor crater. Courtesy of SERNAGEOMIN.

Activity continued through February similar to previous months. The dome in the crater maintained a low extrusion, and activity alternated between dome growth and partial destruction during explosions. Steam-and-gas plumes with occasional ash content continued, with plumes reaching 1 km and drifting in multiple directions. Incandescence was observed during the night. Explosions were reported on 15 February.

During March through May, typical activity consisting of pulsating emission of steam plumes with occasional ash content, and incandescence at night, continued. Intermittent explosions associated with the partial destruction of the dome continued, with events reported on 1 March at 2323, and on 4, 7, and 8 March. Several explosions were reported during 8-9 and 23-30 April. Three explosions were reported on 3 May with one of them producing a 2-km-high ash plume and a pyroclastic flow on 10 May (figure 35). Additional explosions occurred on the 12 and 18 May.

Figure 35. An explosion at Nevados de Chillán on 10 May 2019 produced an ash plume that rose to 2 km above the crater and a pyroclastic flow. The white plume in the bottom two images is steam from the interaction of the hot pyroclastic material and the snow. Screenshots are of a video courtesy of SERNAGEOMIN with timestamps indicated in the top left of each image.

Satellite data from December 2018 through May 2019 recorded intermittent thermal energy, with an increase after February 2019 (figure 36). Thermal anomalies from MODIS instruments were detected by the MODVOLC system on 29 March and 17 May 2019 (two anomalies). A thermal anomaly in the Nicanor crater was persistent in Sentinel-2 data throughout this period.

Figure 36. Thermal anomalies at the active Nicanor crater of the Nevados de Chillán complex. Top: Sentinel-2 thermal image of showing the location of the thermal anomaly (orange). Bottom: MIROVA log radiative power plot of MODIS thermal infrared data from September 2018 through May 2019. Thermal signatures are intermittent and increase after February 2019. Note that the black lines are not from the crater and are unlikely to be related to volcanic activity. Courtesy of Sentinel Hub Playground and MIROVA.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); 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).

January 2020 (BGVN 45:01) Cite this Report

Many explosions, ash plumes, lava and pyroclastic flows June-December 2019

Nevados de Chillán is a complex of late-Pleistocene to Holocene stratovolcanoes in the Chilean Central Andes. An eruption started with a phreatic explosion and ash emission on 8 January 2016 from a new crater (Nicanor) on the E flank of the Nuevo crater, which lies on the NW flank of the cone of the large stratovolcano referred to as Volcán Viejo. Strombolian explosions and ash emissions continued throughout 2016 and 2017; a lava dome within the Nicanor crater was confirmed in early January 2018. Explosions and pyroclastic flows continued during 2018 and the first half of 2019. This report covers continuing activity from June-December 2019 when ongoing explosive events produced ash plumes, lava, and pyroclastic flows. Information for this report is provided primarily by Chile's Servicio Nacional de Geología y Minería (SERNAGEOMIN)-Observatorio Volcanológico de Los Andes del Sur (OVDAS), and by the Buenos Aires Volcanic Ash Advisory Center (VAAC).

Nevados de Chillán was relatively quiet during June 2019, generating only a small number of explosions with ash plumes. This activity continued during July; some events produced incandescent ejecta around the crater. By August a distinct increase in activity was noticeable; ash plumes were larger and more frequent, and incandescent ejecta rose hundreds of meters above the summit a number of times. Frequent explosions were typical during September; the first of several blocky lava flows emerged from the crater mid-month. Inflation that began in mid-July continued with several centimeters of both horizontal and vertical displacement. By October, pyroclastic flows often accompanied the explosive events in addition to the ash plumes, and multiple vents opened within the crater. Three more lava flows had appeared by mid-November; explosions continued at a high rate. Activity remained high at the beginning of December but dropped abruptly mid-month. MODVOLC measured three thermal alerts in September, two in October, seven in November, and six in December. This period of increased thermal activity closely matches the thermal anomaly data reported by the MIROVA project (figure 37), which included an increase at the end of August 2019 that lasted through mid-December before stopping abruptly. Several lava flows and frequent explosions with incandescent ejecta and pyroclastic flows were reported throughout the period of increased thermal activity.

Figure 37. MIROVA thermal anomaly data for Nevados de Chillán from 3 February through December 2019 show low activity during June-August and increasing activity from August through mid-December. This correlates with ground and satellite observations of lava flows, incandescent explosions, ash plumes, and pyroclastic flows during the period of increased thermal activity. Courtesy of MIROVA.

Activity during June-August 2019. Nevados de Chillán remained relatively quiet during June 2019 with a few explosions of ash. At the active Nicanor crater, located on the E flank of the Volcán Nuevo dome, predominantly white steam plumes were observed daily in the nearby webcams. The growth rate of the dome inside the crater was reported by SERNAGEOMIN as continuing at about 260 m³/day. They noted an explosion on 3 June; the Buenos Aires VAAC reported a puff of ash seen from the webcam drifting SE at 3.7 km altitude (figure 38). The webcam indicated sporadic weak emissions continuing that day and the next. Minor explosions were also reported on 7-8 June and included incandescence observed at night and ejecta deposited around the crater rim. The Buenos Aires VAAC reported a narrow ash plume drifting ENE in multispectral imagery under clear skies late on 7 June. The webcams showed sporadic emissions of ash at 3.4 km altitude on 19 June that dissipated rapidly.

Figure 38. Explosions at Nevados de Chillán on 3 (left) and 20 (right) June 2019 produced ash plumes that quickly dissipated in the strong winds. Courtesy of the SERNAGEOMIN Portezuelo webcam, Pehuenia Online (left) and Eco Bio Bio La Red Informativa (right).

Minor pulsating explosive activity continued during July 2019 with multiple occurrences of ash emissions. Ash emissions rose to 3.7 km altitude on 4 July and were seen in the SERNAGEOMIN webcam; the VAAC reported an emission on 8 July that rose to 4.3 km altitude and drifted SE. Monitoring stations near the complex recorded an explosive event early on 9 July; incandescence with gases and ejecta were deposited around the crater and an ash plume rose to 3.9 km and drifted SE. Small ash plumes from sporadic puffs on 12 July rose to 4.6 km altitude. An explosive event on 14 July also produced incandescent ejecta around the crater along with weak sporadic ash emissions. Single ash emissions on 18 (figure 39) and 22 July at 3.7 km altitude drifted ESE from summit before dissipating; another emission on 26 July was reported at 4.3 km altitude.

Figure 39. Local news sources reported ash emissions at Nevados de Chillán on 18 July 2019. Courtesy of INF0SCHILE (left) and Radio Ñuble (right).

A distinct increase in the intensity and frequency of explosive activity was recorded during August 2019. SERNAGEOMIN noted ash emissions and explosions during 3-4 August in addition to the persistent steam plumes above the Nicanor crater (figure 40). The Buenos Aires VAAC reported a single puff on 3 August that was seen in the webcam rising to 3.9 km altitude and dissipating quickly. The next day a pilot reported an ash plume estimated at 5.5 km altitude drifting E. It was later detected in satellite imagery; the webcam revealed continuous emission of steam and gas with intermittent puffs of ash. SERNAGEOMIN issued a special report (REAV) on 6 August noting the increase in size and frequency of explosions, some of which produced dense ash plumes that rose 1.6 km above the crater along with incandescent ejecta. They also reported that satellite imagery indicated a 1.5-km-long lahar that traveled down the NNE flank as a result of the interaction of the explosive ejecta with the snowfall near the summit.

Figure 40. Climbers captured video of a significant explosion at Nevados de Chillán on 4 August 2019. Courtesy of CHV Noticias.

Beginning on 9-10 August 2019, and continuing throughout the month, SERNAGEOMIN observed explosive nighttime activity with incandescent ejecta scattered around the crater rim along with moderate levels of seismicity each day. A diffuse ash plume was detected in satellite imagery by the VAAC on 9 August drifting NW at 4.9 km altitude. SERNAGEOMIN issued a new warning on 12-13 August that the recent increase in activity since the end of July suggested the injection of a new magmatic body that could lead to larger explosive events with pyroclastic and lava flows. They reported pyroclastic ejecta from multiple explosions on 13 August rising 765 and 735 m above the crater. Drone images taken between 4 and 12 August showed the destruction of the summit dome from multiple explosions with the Nicanor Crater (figure 41). The VAAC reported sporadic pulses of volcanic ash drifting N during 12-14 August, visible in satellite imagery estimated at 4.3 km altitude. By 17-18 August, they noted constant steam emissions interspersed with gray plumes during explosive activity.

Figure 41. Drone images taken at Nevados de Chillán between 4 and 12 August 2019 showed destruction of the dome caused by multiple explosions at the summit crater. Courtesy of Movisis.org Internacional.

An increase in seismicity, especially VT events, during 21-22 August 2019 resulted in multiple special REAV reports from SERNAGEOMIN. They noted on 21 August that an explosion produced gas emissions and pyroclastic material that rose 1,400 m above the crater; the next day material rose 450 m. That night, in addition to incandescent ejecta around the crater, they reported small high-temperature flows on the N flank which extended to the NNE flank a few days later. The VAAC reported pulses of ash plumes moving SE on 22 August at 4.3 km altitude. A faint ash cloud was visible in satellite imagery on 29 August drifting E at 3.7 km altitude (figure 42). The cloud was dissipating rapidly as it moved away from the summit. Sporadic ash emissions from intermittent explosions continued moving ESE then N and NE; they were reported daily through 5 September. They continued to rise in altitude to 3.9 km on 30 August, 4.3 km on 1 September, and 4.6 km on 3 September.

Figure 42. Incandescence at the summit of Nevados de Chillán and ashfall covering snow to the E was captured in Sentinel-2 satellite imagery on 29 August 2019. Courtesy of Copernicus EMS.

Activity during September-October 2019. Frequent explosions from Nicanor crater continued during September 2019, producing numerous ash plumes and small high-temperature flows along the NNE flank. A webcam detected a small lateral vent on the NNE flank about 50 m from the crater rim emitting gas and particulates on 2-3 September. Multiple explosions during 3-5 September were associated with gas and ash emissions and incandescent ejecta deposited around the crater rim (figure 43). The network of GNSS stations recording deformation of the volcanic complex confirmed on 3-4 September that inflation, which had been recorded since mid-July 2019, was continuing at a rate of about 1 cm/month. Blocks of incandescent ejecta from numerous explosions were observed rolling down the N flank on 6-7 September and the E flank the following night.

Figure 43. Activity at Nevados de Chillán on 3 September 2019 included ash and steam explosions (left) and incandescent ejecta at the summit (right). Courtesy of Carlos Bustos and SERNAGEOMIN webcams.

SERNAGEOMIN reported on 9-10 September that satellite imagery revealed a new surface deposit about 130 m long trending NNE from the center of crater. They reported an increase in the level of seismicity from moderate to high on 10-11 September and observed incandescent ejecta at the summit during several explosions (figure 44). During a flyover on 12 September scientists confirmed the presence of a new blocky lava flow emerging from Nicanor Crater and moving down the NNE flank of Nuevo volcano. The flow was about 600 m long, 100 m wide, and 5 m thick with a blocky surface and incandescent lava at the base within the active crater. Measurements with a thermal camera indicated a temperature around 800°C within the active crater, and greater than 100°C on the surface of the flow. Frequent high-energy explosions that day produced incandescent ejecta that could be seen from Las Trancas and Shangri-La (figure 45). Ashfall 0.5 cm thick was reported 2 km from the volcano to the SW. The flow was visible from the webcam located N of Nicanor on 16-17 September. Satellite imagery indicated the flow was about 550 m long and moving at a rate of about 21 m/day.

Figure 44. A blocky lava flow moved down the NNE flank of Nevados de Chillán on 11 September (left); incandescent ejecta covered the summit area the next night (right). Courtesy of EarthQuakesTime (left), Red Geocientifica de Chile (right) and SERNAGEOMIN Webcams.

Figure 45. The SERNAGEOMIN Portezuelo webcam revealed the blocky lava flow, incandescent ejecta and ash emissions at Nevados de Chillán on 12 September 2019. Courtesy of American Earthquakes (left), PatoArias (right), and SERNAGEOMIN.

During 18-22 September 2019 multiple special reports of seismicity were released each day with incandescent ejecta, gas, and particulate emissions often observed at the summit crater; the lava flow remained active. On 24 September ashfall was reported about 15 km NW in communities including Las Trancas; small pyroclastic flows were observed the following day. Horizontal inflation of 2.4 cm was reported on 25 September, and vertical inflation was measured at 3.4 cm since mid-July. SERNAGEOMIN noted that while the frequency of explosions had increased, the energy released had decreased. Morphological changes in Nicanor crater suggested that it was growing at its SW edge and eroding the adjacent Arrau crater; the NE edge of the crater was unstable.

Plumes of steam and ash continued along with the explosions for the remainder of the month. During the night, incandescent ejecta was observed, and the low-velocity lava flow continued to move. Multiple VAAC reports were issued virtually every day of September. Pulses of ash were moving SE at 4.3 km altitude on 7-8 September. For most of the rest of the month sporadic emissions with minor amounts of ash were observed in either the webcam or satellite images at an altitude of 3.7 km, occasionally rising to 4.3 km. They drifted generally SE but varied somewhat with the changing winds. Continuous ash emissions were observed during 24-25 September that rose as high as 4.9 km altitude and drifted E, clearly visible in satellite imagery. After that, the altitude dropped back to 3.7 km and the plume was only faintly and intermittently visible in satellite imagery.

Low-altitude gray ash plumes were observed rising from Nicanor crater almost every day that weather permitted during October 2019. Incandescent ejecta was frequently observed at night. Beginning on 6-7 October, SERNAGEOM reported pyroclastic flows traveling short distances from the crater most days. They traveled 1.13 km down the NNE flank, 0.42 km down the NNW flank and 0.88 km down the SW flank. The blocky lava flow on the NNE flank was no longer active (figure 46). During 9-12 October, multiple special reports of increased seismic activity (REAVs) were issued each day. Inflation continued throughout the month. On 10 October the total horizontal deformation (since mid-July) was 3 cm, with a rate of movement a little over 1 cm/month; the total vertical displacement was 4.5 cm, with a rate of 1.93 cm/month during the previous 30 days.

In a special report issued on 11 October, SERNAGEOMIN mentioned that analysis of satellite imagery indicated a new emission center within the Nicanor crater adjacent to the dome vent active since December 2017 and to the lava flow of September. The new center was oval shaped with an E-W dimension of 60 m and a N-S dimension of 55 m, located about 90 m SE of the old, still active center, and was the site of the explosive activity reported since 30 September.

Figure 46. Drone footage posted 10 October 2019 from Nevados de Chillán shows steam emissions from the Nicanor crater and a blocky lava flow down NNE flank. The snow-covered cone in background is Volcan Baños. Courtesy of Volcanologia Chile and copyright by Nicolas Luengo V.

On 16 October a new blocky flow was observed on the NE flank of the Nicanor Crater; it was about 70 m long, moving about 30 m/day. By 21 October it had reached 130 m in length, and its rate of advance had slowed significantly. Beginning on 25 October seismicity decreased noticeably and much less surface activity was observed at the crater. Explosions at the end of the month produced steam plumes, gas emissions and minor pulsating ash emissions.

The Buenos Aires VAAC reported a puff of ash at 4.9 km altitude on 1 October moving SE. Continuous emission of steam and gas with sporadic puffs of ash that rose to around 3.7-4.3 km altitude were typical every day after that until 25 October usually drifting S or E; they were most often visible in the webcams, and occasionally visible in satellite imagery when weather conditions permitted. A diffuse plume of ash was detected on 16 October drifting SE at 4.6 km altitude. The VAAC reported incandescence visible at the summit in webcam images on 22 October; a significant daytime explosion on 24 October produced a large incandescent ash cloud (figure 47). The next day the VAAC detected weak pulses of ash plumes in satellite images extending E from the summit for 130 km. Intermittent ash emissions were reported drifting SE at 3.7-4.3 km each day from 29-31 October.

Figure 47. A large incandescent ash plume at Nevados de Chillán on 24 October 2019 sent ejecta around the summit (left); a dense ash plume was produced during an explosion on 30 October 2019 (right). Courtesy of Cristian Farian (left) and SERNAGEOMIN (right); both images taken from the SERNAGEOMIN webcams.

Activity during November-December 2019. Moderate seismicity continued during November 2019 with recurrent episodes of pulsating gas and ash emissions. Incandescent ejecta was visible many nights that the weather conditions were favorable (figure 48). In the Daily Report (RAV) issued on 6 November, SERNAGEOMIN noted that the original 700-m-long blocky lava flow on the NNE flank active during September had been partly covered by another flow, about 350 m long. They also reported that pyroclastic density currents were observed in the area immediately around the crater extending in several directions. They extended 850 m down the SW flank, 670 m down the NW flank, 1,680 m down the N flank, and 440 m to the NNE.

Changes in the crater area indicated a growth of the SW edge of the Nicanor Crater, continuing to erode the Arrau crater, with the constant emission of gas, ash, and incandescent ejecta that produced plumes up to 1.8 km high. SERNAGEOMIN also observed activity from a vent at the NE edge of the crater that included gas emission and ejecta, but no lava flow. The fourth lava flow observed in recent months (L4) was identified on the NNE slope on 13 November adjacent to the earlier flows; it was about 70 m long and slowly advancing. By 19 November L4 consisted of two lobes and extended about 90 m from the edge of the Nicanor crater advancing at an average rate of 0.4 m/hour. The vent producing L4 was located about 60 m SSE of the vent that produced the earlier flows (L1, L2, and L3). By 28 November the flow had reached a length of 165 m and was no longer advancing. A series of explosions reported on 25-27 and 30 November produced ejecta that rose 800, 1,000, 1,300, and 700 m above the crater.

Figure 48. Incandescent ejecta at Nevados de Chillán was clearly visible at night on 3 November 2019. Courtesy of Claudio Kanisius.

Ash emissions were reported by the Buenos Aires VAAC during most of November, usually visible from the webcams, but often also seen in satellite imagery. The plumes generally reached 3.7-4.6 km altitude and drifted SSE. They usually occurred as continuous emission of steam and gas accompanied by sporadic pulses of ash but were sometimes continuous ash for several hours. They were visible about 100 km E of the summit on 2 November, and over 200 km SE the following day. A narrow plume of ash was seen in visual satellite imagery extending 50 km E of the summit on 9 November. Intermittent incandescence at the summit was seen from the webcam on 18 November. Pulses of ash were detected in satellite imagery extending 125 km SE on 22 November. Strong puffs of ash briefly rose to 4.9 km altitude and drifted NE on 26 November (figure 49); incandescence during the nighttime was visible in the webcam on 28 November.

Figure 49. An explosion on 26 November 2019 at Nevados de Chillán produced a dense ash plume and small pyroclastic flows down the flank. Courtesy of Volcanes de Chile and the SERNAGEOMIN Portezuelo webcam.

Pulsating emissions of gas and ejecta continued into December 2019. Five explosions were reported on 1 December that produced gas plumes which rose 300-800 m above the crater. Three more explosions occurred on 3 December sending gas plumes 500-1,000 m high. SERNAGEOMIN reported on 4 December that explosive activity was observed from four vents within the Nicanor crater. This activity triggered new pyroclastic flows that extended 1,100 m E and 400 m S. By 5 December the total vertical inflation reported since July was 8 cm. A large explosion on 5 December sent material 1.6 km above the summit and pyroclastic flows down the flanks (figure 50). The webcams at Andarivel and Portezuelo showed a pyroclastic flow moving 400 m W, a direction not previously observed; this was followed by additional pyroclastic flows to the N and E.

Figure 50. A large explosion at Nevados de Chillán on 5 December 2019 produced an ash plume that rose 1.6 km above the summit and sent pyroclastic flows down the flanks. Courtesy of SERNAGEOMIN.

On 9 December SERNAGEOMIN noted that the increase to four active vents was causing erosion on the S and SE edges of the crater making the most affected areas to the SW, S, SE and E of the crater. Major explosions reported that day produced pyroclastic flows that descended down the E and ESE flanks and particulate emissions that rose 1 km. The SW flank near the crater was also affected by ejecta and pyroclastic debris carried by the wind. The most extensive pyroclastic flows travelled down the E flank for the next several days; explosions on 10 December sent material 1.2 km high. Three explosions were noted on 11 December; the first sent incandescence close to 200 m high, and the second produced a column of particulate material 1.2 km high. The first of two explosions on 12 December sent material 1.8 km above the crater and pyroclastic flows down the flanks (figure 51). Although explosions were reported on 13 and 14 December, cloudy skies prevented observations of the summit.

Figure 51. A large explosion at Nevados de Chillán on 12 December 2019 produced an ash plume that rose 1.8 km above the summit and sent pyroclastic flows down the flanks. Courtesy of Volcanes de Chile and SERNAGEOMIN.

Intermittent ash emissions were reported by the Buenos Aires VAAC during 1-13 December 2019. They rose to 3.7-4.3 km and drifted generally E. Pulses of ash were detected at 4.9 km altitude moving S in satellite imagery on 9 December. The last reported ash emission for December was on the afternoon of 12 December; puffs of ash could be seen in satellite imagery moving E at 4.6 km altitude. A decrease in particulate emissions and explosions was reported beginning on 14 December, and no further explosions were recorded by infrasound devices after 15 December. The deposits from the earlier pyroclastic flows had reached 600 m E and 300 m W of the crater. Seismic activity was recorded as low instead of moderate beginning on 25 December. A total horizontal inflation of about 6 cm since July was measured at the end of December.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/), Twitter: @Sernageomin; 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/); Cristian Farias Vega, Departamento de Obras Civiles y Geología, Universidad Católica de Temuco, Vilcún, Región de La Araucanía, Chile (URL: https://twitter.com/cfariasvega/status/1187471827255226370); Copernicus Emergency Management Service (Copernicus EMS), Joint Research Centre, European Union (URL: https://emergency.copernicus.eu/, https://twitter.com/CopernicusEMS/status/1168156474817818624); Volcanes de Chile, Proyectos de la Fundación Volcanes de Chile, Chile (URL: https://www.volcanesdechile.net/, https://twitter.com/volcanesdechile/status/1199496839491395585); Pehuenia Online, Pehuenia, Argentina (URL: http://pehueniaonline.com.ar/, https://twitter.com/PehueniaOnline/status/1135703309824745472); Eco Bio Bio La Red Informativa, Bio Bio Region, Chile (URL: http://emergenciasbiobio.blogspot.com/, https://twitter.com/Eco_BioBio_II/status/1141734238590574593); INF0SCHILE (URL: https://twitter.com/INF0SCHILE/status/1151849611482599425); Radio Ñuble AM y FM, Chillán, Chile (URL: http://radionuble.cl/linea/, lhttps://twitter.com/RadioNuble/status/1151858189299781632); CHV Noticias, Santiago, Chile (URL: https://www.chvnoticias.cl/, https://twitter.com/CHVNoticias/status/1159263718015819777); Movisis.org Internacional, Manabi, Ecuador (URL: https://movisis.org/, https://twitter.com/MOVISISEC/status/1160778823031558144); Carlos Bustos (URL: https://twitter.com/cbusca1970/status/1168932243873644548); EarthQuakesTime (URL: https://twitter.com/EarthQuakesTime/status/1171654504841908229); Red Geocientifica de Chile (URL: https://twitter.com/RedGeoChile/status/1171972482875703296); American Earthquakes (URL: https://twitter.com/earthquakevt/status/1172271139760091136); PatoArias, Talca, Chile (URL: https://twitter.com/patoarias/status/1172287142191665153); Volcanologia Chile, (URL: http://www.volcanochile.com/joomla30/, https://twitter.com/volcanologiachl/status/1182707451554078720); Claudio Kanisius (URL: https://twitter.com/ClaudioKanisius/status/1191182878346031104).

July 2020 (BGVN 45:07) Cite this Report

Explosions and pyroclastic flows continue; new dome emerges from Nicanor crater in June 2020

Nevados de Chillán is a complex of late-Pleistocene to Holocene stratovolcanoes in the Chilean Central Andes. An eruption started with a phreatic explosion and ash emission on 8 January 2016 from a new crater (Nicanor) on the E flank of the Nuevo crater, itself on the NW flank of the large Volcán Viejo stratovolcano. Strombolian explosions and ash emissions continued throughout 2016 and 2017; a lava dome within the Nicanor crater was confirmed in early January 2018. Explosions and pyroclastic flows continued during 2018 and 2019, with several lava flows appearing in late 2019. This report covers continuing activity from January-June 2020 when ongoing explosive events produced ash plumes, pyroclastic flows, and the growth of new dome inside the crater. Information for this report is provided primarily by Chile's Servicio Nacional de Geología y Minería (SERNAGEOMIN)-Observatorio Volcanológico de Los Andes del Sur (OVDAS), and by the Buenos Aires Volcanic Ash Advisory Center (VAAC).

Explosions with ash plumes rising up to three kilometers above the summit area were intermittent from late January through early June 2020. Some of the larger explosions produced pyroclastic flows that traveled down multiple flanks. Thermal anomalies within the Nicanor crater were recorded in satellite data several times each month from February through June. A reduction in overall activity led SERNAGEOMIN to lower the Alert Level from Orange to Yellow (on a 4-level, Green-Yellow-Orange-Red scale) during the first week of March, although tens of explosions with ash plumes were still recorded during March and April. Explosive activity diminished in early June and SERNAGEOMIN reported the growth of a new dome inside the Nicanor crater. By the end of June, a new flow had extended about 100 m down the N flank. Thermal activity recorded by the MIROVA project showed a drop in thermal energy in mid-December 2019 after the lava flows of September-November stopped advancing. A decrease in activity in January and February 2020 was followed by an increase in thermal and explosive activity in March and April. Renewed thermal activity from the growth of a new dome inside the Nicanor crater was recorded beginning in mid-June (figure 52).

Figure 52. MIROVA thermal anomaly data for Nevados de Chillan from 8 September 2019 through June 2020 showed a drop in thermal activity in mid-December 2019 after the lava flows of September-November stopped advancing. A decrease in activity in January and February 2020 was followed by an increase in explosive activity in March and April. Renewed thermal activity from the growth of a new dome inside the Nicanor crater was recorded beginning in mid-June. Courtesy of MIROVA.

Weak gas emissions were reported daily during January 2020 until a series of explosions began on the 21st. The first explosion rose 100 m above the active crater; the following day, the highest explosion rose 1.6 km above the crater. The Buenos Aires VAAC reported pulse emissions visible in satellite imagery on 21 and 24 January that rose to 3.9-4.3 km altitude and drifted SE and NE, respectively. Intermittent explosions continued through 26 January. Incandescent ejecta was observed during the night of 28-29 January. The VAAC reported an isolated emission on 29 January that rose to 5.2 km altitude and drifted E. A larger explosion on 30 January produced an ash plume that SERNAGEOMIN reported at 3.4 km above the crater (figure 53). It produced pyroclastic flows that traveled down ravines on the NNE and SE flanks. The Washington VAAC reported on behalf of the Buenos Aires VAAC that an emission was observed in satellite imagery on 30 January that rose to 4.9 km altitude and was moving rapidly E, reaching 15 km from the summit at midday. The altitude of the ash plume was revised two hours later to 7.3 km, drifting NNE and rapidly dissipating. Satellite images identified two areas of thermal anomalies within the Nicanor crater that day. One was the same emission center (CE4) identified in November 2019, and the second was a new emission center (CE5) located 60 m NW.

Figure 53. A significant explosion and ash plume from the Nicanor crater at Nevados de Chillan on 30 January 2020 produced an ash plume reported at 7.3 km altitude. The left image was taken within one minute of the initial explosion. Images posted by Twitter accounts #EmergenciasÑuble (left) and T13 (right); original photographers unknown.

When the weather permitted, low-altitude mostly white degassing was seen during February 2020, often with traces of fine-grained particulate material. Incandescence at the crater was observed overnight during 4-5 February. The Buenos Aires VAAC reported an emission on 14 February visible in the webcam. The next day, an emission was visible in satellite imagery at 3.9 km altitude that drifted E. Episodes of pulsating white and gray plumes were first observed by SERNAGEOMIN beginning on 18 February and continued through 25 February (figure 54). The Buenos Aires VAAC reported pulses of ash emissions moving SE on 18 February at 4.3 km altitude. Ash drifted E the next day at 3.9 km altitude and a faint plume was briefly observed on 20 February drifting N at 3.7 km altitude before dissipating. Sporadic pulses of ash moved SE from the volcano on 22 February at 4.3 km altitude, briefly observed in satellite imagery before dissipating. Thermal anomalies were visible from the Nicanor crater in Sentinel-2 satellite imagery on 23 and 28 February.

Figure 54. An ash emission at Nevados de Chillan on 18 February 2020 was captured in Sentinel-2 satellite imagery drifting SE (left). Thermal anomalies within the Nicanor crater were measured on 23 (right) and 28 February. Images use Atmospheric penetration rendering (bands 12, 11, 8a); courtesy of Sentinel Hub Playground.

Only low-altitude degassing of mostly steam was reported for the first half of March 2020. When SERNAGEOMIN lowered the Alert Level from Orange to Yellow on 5 March, they reduced the affected area from 5 km NE and 3 km SW of the crater to a radius of 2 km around the active crater. Thermal anomalies were recorded at the Nicanor crater in Sentinel-2 imagery on 4, 9, 11, 16, and 19 March (figure 55). A new series of explosions began on 19 March; 44 events were recorded during the second half of the month (figure 56). Webcams captured multiple explosions with dense ash plumes; on 25 and 30 March the plumes rose more than 2 km above the crater. Fine-grained ashfall occurred in Las Trancas (10 km SW) on 25 March. Pyroclastic flows on 25 and 30 March traveled 300 m NE, SE, and SW from the crater. Incandescence was observed at night multiple times after 20 March. The Buenos Aires VAAC reported several discrete pulses of ash that rose to 4.3 km altitude and drifted SE on 20 and 21 March, SW on 25 March, and SE on 29 and 30 March. Another ash emission rose to 5.5 km altitude later on 30 March and drifted SE.

Figure 55. Sentinel-2 Satellite imagery of Nevados de Chillan during March 2020 showed thermal anomalies on five different dates at the Nicanor crater, including on 9, 11, and 16 March. A second thermal anomaly of unknown origin was also visible on 11 March about 2 km SW of the crater (center). Images use Atmospheric penetration rendering (bands 12, 11, 8a); courtesy of Sentinel Hub Playground.

Figure 56. Forty-four explosive events were recorded at Nevados de Chillan during the second half of March 2020 including on 19 March. Courtesy of SERNAGEOMIN webcams and chillanonlinenoticia.

In their semi-monthly reports for April 2020, SERNAGEOMIN reported 94 explosive events during the first half of the month and 49 during the second half; many produced dense ash plumes. The Buenos Aires VAAC reported frequent intermittent ash emissions during 1-13 April reaching altitudes of 3.7-4.3 km (figure 57). They reported the plume on 8 April visible in satellite imagery at 7.3 km altitude drifting SE. An emission on 13 April was also visible in satellite imagery at 6.1 km altitude drifting NE.

Figure 57. Sentinel-2 satellite imagery captured a strong thermal anomaly and an ash plume drifting SE from Nevados de Chillan on 10 April 2020. Image uses Atmospheric penetration rendering (bands 12, 11, 8a); courtesy of Sentinel Hub Playground.

During the second half of April 2020, SERNAGEOMIN reported that only one plume exceeded 2 km in height; on 21 April, it rose to 2.4 km above the crater (figure 58). The Buenos Aires VAAC reported isolated pulses of ash on 18, 26, 28, and 30 April. During the second half of April SERNAGEOMIN also reported that a pyroclastic flow traveled about 1,200 m from the crater rim down the SE flank. The ash from the pyroclastic flow drifted SE and S as far as 3.5 km. Satellite images showed continued activity from multiple emission centers around the crater. Pronounced scarps were noted on the internal walls of the crater, attributed to the deepening of the crater from explosive activity.

Figure 58. Tens of explosions were reported at Nevados de Chillan during the second half of April 2020 that produced dense ash plumes. The plume on 21 April rose 2.4 km above the Nicanor crater. Photo by Josefa Carrasco Acuña from San Fabián de Alico; posted by Noticias Valpo Express.

Intermittent explosive activity continued during May 2020. The plumes contained abundant particulate material and were accompanied by periodic pyroclastic flows and incandescent ejecta around the active crater, especially visible at night. The Buenos Aires VAAC reported several sporadic weak ash emissions during the first week of May that rose to 3.7-5.2 km altitude and drifted NE. SERNAGEOMIN reported that only one explosion produced an ash emission that rose more than two km above the crater during the first two weeks of the month; on 6 May it rose to 2.5 km above the crater and drifted NE. They also observed pyroclastic flows on the E and SE flanks that day. Additional pyroclastic flows traveled 450 m down the S flank during the first half of the month, and similar deposits were observed to the N and NE. Satellite observations showed various emission points along the NW-trending lineament at the summit and multiple erosion scarps. Major erosion was noted at the NE rim of the crater along with an increase in degassing around the rim.

During the second half of May 2020 most of the ash plumes rose less than 2 km above the crater; a plume from one explosion on 22 May rose 2.2 km above the crater; the Buenos Aires VAAC reported the plume at 5.5 km altitude drifting NW (figure 59). Continuing pyroclastic emissions deposited material as far as 1.5 km from the crater rim on the NNW flank. There were also multiple pyroclastic deposits up to 500 m from the crater directed N and NE during the period. SERNAGEOMIN reported an increase in steam degassing between Nuevo-Nicanor and Nicanor-Arrau craters.

Figure 59. Explosions produced dense ash plumes and pyroclastic flows at Nevados de Chillan multiple times during May 2020 including on 22 May. Courtesy of SERNAGEOMIN.

Webcam images during the first two weeks of June 2020 indicated multiple incandescent explosions. On 3 and 4 June plumes from explosions reached heights of over 1.25 km above the crater; the Buenos Aires VAAC reported them drifting NW at 3.9 km altitude. Incandescent ejecta on 6 June rose 760 m above the vent and drifted NE. In addition, pyroclastic flows were distributed on the N, NW, E and SE flanks. Significant daytime and nighttime incandescence was reported on 6, 9, and 10 June (figure 60). The VAAC reported emission pulses on 6 and 9 June drifting E and SE at 4.3 km altitude.

Figure 60. Multiple ash plumes with incandescence were reported at Nevados de Chillan during the first ten days of June 2020 including on 6 June, after which explosive activity decreased significantly. Courtesy of SERNAGEOMIIN and Sismo Alerta Mexicana.

SERNAGEOMIN reported that beginning on the afternoon of 9 June 2020 a tremor-type seismic signal was first recorded, associated with continuous emission of gas and dark gray ash that drifted SE (figure 61). A little over an hour later another tremor signal began that lasted for about four hours, followed by smaller discrete explosions. A hybrid-type earthquake in the early morning of 10 June was followed by a series of explosions that ejected gas and particulate matter from the active crater. The vent where the emissions occurred was located within the Nicanor crater close to the Arrau crater; it had been degassing since 30 May.

Figure 61. A tremor-type seismic signal was first recorded on the afternoon of 9 June 2020 at Nevados de Chillan. It was associated with the continuous emission of gas and dark gray ash that drifted SE, and incandescent ejecta visible after dark. View is to the S, courtesy of SERNAGEOMIN webcam, posted by Volcanology Chile.

After the explosions on the afternoon of 9 June, a number of other nearby vents became active. In particular, the vent located between the Nuevo and Nicanor craters began emitting material for the first time during this eruptive cycle. The explosion also generated pyroclastic flows that traveled less than 50 m in multiple directions away from the vent. Abundant incandescent material was reported during the explosion early on 10 June. Deformation measurements showed inflation over the previous 12 days.

SERNAGEOMIN identified a surface feature in satellite imagery on 11 June 2020 that they interpreted as a new effusive lava dome. It was elliptical with dimensions of about 85 x 120 m. In addition to a thermal anomaly attributed to the dome, they noted three other thermal anomalies between the Nuevo, Arrau, and Nicanor craters. They reported that within four days the base of the active crater was filled with effusive material. Seismometers recorded tremor activity after 11 June that was interpreted as associated with lava effusion. Incandescent emissions were visible at night around the active crater. Sentinel-2 satellite imagery recorded a bright thermal anomaly inside the Nicanor crater on 14 June (figure 62).

Figure 62. A bright thermal anomaly was recorded inside the Nicanor crater at Nevados de Chillan on 14 June 2020. SERNAGEOMIN scientists attributed it to the growth of a new lava dome within the crater. Image uses Atmospheric penetration rendering (bands 12, 11, 8a); courtesy of Sentinel Hub Playground.

A special report from SERNAGEOMIN on 24 June 2020 noted that vertical inflation had increased during the previous few weeks. After 20 June the inflation rate reached 2.49 cm/month, which was considered high. The accumulated inflation measured since July 2019 was 22.5 cm. Satellite imagery continued to show the growth of the dome, and SERNAGEOMIN scientists estimated that it reached the E edge of the Nicanor crater on 23 June. Based on these images, they estimated an eruptive rate of 0.1-0.3 m³/s, about two orders of magnitude faster than the Gil-Cruz dome that emerged between December 2018 and early 2019.

Webcams revealed continued low-level explosive activity and incandescence visible both during the day and at night. By the end of June, webcams recorded a lava flow that extended 94 m down the N flank from the Nicanor crater and continued to advance. Small explosions with abundant pyroclastic debris produced recurring incandescence at night. Satellite infrared imagery indicated thermal radiance from effusive material that covered an area of 37,000 m², largely filling the crater. DEM analysis suggested that the size of the crater had tripled in volume since December 2019 due largely to erosion from explosive activity since May 2020. Sentinel-2 satellite imagery showed a bright thermal anomaly inside the crater on 27 June.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/, https://twitter.com/Sernageomin); 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/); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); #EmergenciasÑuble (URL: https://twitter.com/urgenciasnuble/status/1222943399185207296); T13, Channel 13 Press Department (URL: https://twitter.com/T13/status/1222951071443771394); Chillanonlinenoticia (URL: https://twitter.com/ChillanOnline/status/1240754211932995595); Noticias Valpo Express (URL: https://twitter.com/NoticiasValpoEx/status/1252715033131388928); Sismo Alerta Mexicana (URL: https://twitter.com/Sismoalertamex/status/1269351579095691265); Volcanology Chile (URL: https://twitter.com/volcanologiachl/status/1270548008191643651).

November 2020 (BGVN 45:11) Cite this Report

Frequent explosions, a lava flow on the N flank, and lava dome growth during July-October 2020

Nevados de Chillán, located in the Chilean Central Andes, is a volcanic complex composed of late-Pleistocene to Holocene stratovolcanoes. On 8 January 2016 an explosion created the Nicanor Crater on the NW flank of Volcán Viejo. Recent activity consists of explosions, ash plumes, pyroclastic flows, and a new lava dome in the Nicanor Crater (BGVN 45:07). This report covers July through October 2020; activity is characterized by frequent explosions, ash plumes, a lava flow on the N flank, and continued lava dome growth. The primary source of information comes from the Servicio Nacional de Geología y Minería (SERNAGEOMIN)-Observatorio Volcanológico de Los Andes del Sur (OVDAS), the Buenos Aires Volcanic Ash Advisory Center (VAAC), and satellite data.

Since 27 June webcams have showed an active lava flow that originated from the Nicanor Crater and descended the N flank. Activity during July consisted of 210-473 volcano-tectonic seismic events and 565-614 explosive events. Ash plumes rising 1.1-1.2 km above the crater and were accompanied by day and nighttime incandescence on the E edge of the Nicanor Crater. Due to these explosions, SERNAGEOMIN reported that tephra and other pyroclastic deposits were deposited within 400 m to the E of the crater. On 1 July a Buenos Aires VAAC advisory reported that a webcam showed ash emissions rising to 4.3 km altitude. Continuous explosions the next day produced ash plumes that rose 500 m above the crater. During 1-2 July the active lava flow had reached 40 m long and descended at a rate of 0.2 m³ per second. On 6 July an explosion at 0837 generated a gas-and-ash plume that rose 1.2 km above the crater and drifted SE; sporadic ash emissions were also observed on 7 July, according to a VAAC advisory. SERNAGEOMIN webcams showed that the lava flow that began on 27 June continued down the N flank, while a new lobe 55-194 m long moved toward the NE flank of Nicanor Crater. Gas plumes were also observed rising above the active crater, as noted on 20 July (figure 63). On 29 July weak ash emissions rose 3.9 km altitude and drifted SE, according to a VAAC report. During that day, the volume of the lava dome measured 400,000 m³ and grew at a rate of 0.1 m³ per second. Throughout the month, the lava flow continued to descend the N flank of the Nicanor Crater, reaching 520 m at a rate of 0.7-0.6 m per hour. Some unconsolidated blocks up to a meter in size detached from the front of the flow and moved up to 240 m. Sulfur dioxide emissions during the month averaged 823 tons/day with a high value of 1,815 tons/day reported on 29 July.

Figure 63. A white gas-and-steam plume was observed at Nevados de Chillán on 20 July 2020. Courtesy of SERNAGEOMIN webcam, posted by Volcanology Chile.

During August SERNAGEOMIN reported 68-75 volcano-tectonic seismic events and 497-578 explosive events, the latter of which ejected material as far as 300 m E and NE from Nicanor Crater. Associated ash plumes rose 800-980 m above the crater and were accompanied by day and nighttime crater incandescence. The lava dome continued to grow during the month, reaching a thickness of 41 m, according to SERNAGEOMIN. SO₂ emissions were an average value of 134-205 tons/day with a high value of 245 tons/day reported on 3 August. On 15 August a VAAC advisory reported weak and sporadic gas-and-ash emissions at the summit; on 20 August a hotspot was detected in satellite imagery, though an ash plume was not observed. The active lava flow on the N flank extended 490-495 m and moved at a rate of 0.07-0.06 m per hour. On 31 August a webcam showed an ash plume rising above the volcano, accompanied by the advancing lava flow on the N flank (figure 64).

Figure 64. An explosion at Nevados de Chillán produced an ash plume on 31 August 2020. A lava flow accompanies the ash plume on the N flank. Courtesy of SERNAGEOMIN.

Similar activity continued into September, with 45-48 volcano-tectonic and 591-621 explosive events. Ash plumes rose to 1.5 km above the crater and were accompanied by day and nighttime incandescence on the E edge of Nicanor Crater. During 1-15 September explosions at the lava dome produced ash plumes that rose to less than 1.5 km altitude, resulting in ashfall within 300 m E and NE of the crater; ejecta from larger explosions was also observed to the ESE. Satellite images showed partial destruction of the lava dome as well as loss of some material due to successive explosions at the beginning of the month. Overall, the dome continued to increase in size, reaching a volume of 180,000 m³ and a thickness of 45 m since August (41 m). The lava dome measured 93 m NW-SE and 104 m SW-NE. By 15 September the 500-m-long lava flow had descended the NNE flank and continued to advance at a rate of 1.7 m per hour. The W levee of the flow channel had ruptured, which caused the toe of the lava flow to thicken. On 20 September ash emissions rose to 3.7 km altitude and drifted NE and ENE, according to a VAAC advisory. On 22 September gas emissions, weak and sporadic ash emissions, and occasional explosions accompanied the lava flow. Through the remainder of the month, the lava flow persisted, measuring 615 m, and advancing at a rate of 0.4 m per hour; its volume was 487,000 m³ (figure 65). SO₂ emissions were an average value of 111-358 tons/day with a high value of 503 tons/day reported on 22 September.

Figure 65. Photo (color corrected) of the incandescent lava flow at night descending the NNE flank of Nevados de Chillán on 21 September 2020. Photo by Jose Fauna, courtesy of Volcanology Chile.

During October there were 34-61 volcano-tectonic seismic events reported, as well as 607-644 explosive events, seven of which generated ash plumes that rose 1-1.5 km above the crater. Day and nighttime incandescence in the E edge of Nicanor Crater remained. Ash deposits associated with the explosive activity were distributed to the E and NE as far as 300 m from the crater; denser pyroclastic deposits from stronger explosions were located to the N and NE. The lava flow on the N slope persisted, extending 614-683 m from the crater rim at a rate of 0.1-0.82 m per hour with a width of 80.2 m near the crater rim and up to 112.8 m near the toe. The lava dome also continued to grow since it was last measured in September; it was 115 m wide at the base by 107 m high. SO₂ emissions were an average value of 167-355 tons/day with a high value of 588 tons/day reported on 26 October. On 29 October an ash plume was detected in satellite imagery and rose to 3.7 km altitude and drifted W, according to a VAAC advisory (figure 66). SERNAGEOMIN reported that a 25-m-diameter subcrater had formed on the E inner edge of Nicanor Crater at the top of the lava dome. On 30 October, intermittent gas-and-ash emissions were visible at the summit in satellite imagery, rising to 3.9 km altitude and drifting SE.

Figure 66. Webcam image of an explosion at Nevados de Chillán on 29 October 2020 that produced an ash plume that rose 360 m above the crater and drifted SW. Courtesy of SERNAGEOMIN.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows frequent low-power thermal activity beginning in early June and continuing through October 2020 due to frequent explosions, the continued lava dome growth in Nicanor Crater, and the lava flow that descended the N flank (figure 67). On clear weather days, two thermal anomalies in the summit craters are observed in Sentinel-2 thermal satellite imagery; one represents the growing lava dome and the other is the lava flow on the N flank (figure 68). On 25 September an ash plume was observed drifting S.

Figure 67. Frequent low-power thermal activity at Nevados de Chillán continued during July through October 2020, according to the MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 68. Sentinel-2 satellite imagery showed a persistent thermal anomaly (bright yellow-orange) in the summit crater of Nevados de Chillán during July through October 2020. On 29 July (top left), a third faint thermal anomaly was detected on the N flank, indicating a lava flow. On 25 September (bottom left) an ash plume was visible drifting S. Images using “Atmospheric penetration” rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php); 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); Volcanology Chile (URL: https://twitter.com/volcanologiachl); Jose Fauna, Caracol sector, San Fabián de Alicom, Chile (URL: https://twitter.com/josefauna).

May 2021 (BGVN 46:05) Cite this Report

N-flank flow grows 200 m while dome growth and explosions continue during November 2020-April 2021

Nevados de Chillán is a large complex of late-Pleistocene to Holocene stratovolcanoes in the Chilean Central Andes that has had multiple historical eruptions dating back to the seventeenth century. The most recent eruption started with a phreatic explosion and ash emission on 8 January 2016 from a new crater (Nicanor) on the E flank of the Nuevo crater, itself on the NW flank of the large Volcán Viejo stratovolcano. Strombolian explosions and ash emissions continued throughout 2016 and 2017; a lava dome within the Nicanor crater was confirmed in early 2018. Explosions and pyroclastic flows continued into 2020; several lava flows appeared in late 2019. New dome growth began in late June 2020, accompanied by a new flow descending the N flank from the crater rim. This report covers continuing activity from November 2020-April 2021 when ongoing explosive events produced ash plumes, and growth of the dome inside the crater continued along with the lava flow extending farther down the N flank. Information for this report is provided primarily by Chile's Servicio Nacional de Geología y Minería (SERNAGEOMIN)-Observatorio Volcanológico de Los Andes del Sur (OVDAS), and from satellite data.

Activity at Nevados de Chillán during November 2020-April 2021 was similar to the previous few months. The flow that appeared on the N flank at the end of June lengthened by more than 200 m and increased significantly in volume. A second thermal anomaly on the edge of Nicanor crater at the head of the flow first appeared in satellite imagery on 14 July 2020 and persisted in all subsequent clear images through April 2021 along with the anomaly from the growing dome inside the crater. Tens of seismic explosive events were measured daily; many produced plumes of gas and ash. The dome inside the crater continued to grow even though explosive events intermittently destroyed parts of the dome. An increase in the flow rate was observed at the very end of April, and a new lava flow appeared on the NE flank at the beginning of May. Thermal activity shown in the MIROVA graph indicated a constant level of heat energy from July 2020 through the end of April 2021 (figure 69).

Figure 69. The MIROVA graph of Log Radiative Power at Nevados de Chillan from 8 July 2020 through April 2021 showed a very consistent pattern of thermal energy from a lava flow and growing dome until the end of April when an increase in the flow rate created a new lava flow. Courtesy of MIROVA.

During the first half of November 2020 there were 61 volcano-tectonic (VT) and 575 explosive events recorded. Plume heights did not exceed 1,140 m above the Nicanor crater and only a few of them contained identifiable ash (figure 70). Incandescence was observed at the E edge of crater on clear nights. SERNAGEOMIN reported that a small crater continued to grow on the inner E rim of the Nicanor crater and was identified in satellite images; during some explosions, two sources of emissions were apparent. During the second half of the month 70 VT and 573 explosive seismic events were recorded. Webcam images indicated that the plumes from the explosions remained at low altitude (less than 1,000 m above the crater rim), and little to no tephra was noted; the emissions were primarily gas. Most of the emissions originated from the central area of the dome inside the crater, but the second emission site, located at the E edge of the crater remained active.

Figure 70. A small ash emission rose to 1,280 m above the summit of Nevados de Chillan on 9 November 2020 and was typical of the explosive activity that occurred regularly throughout the reporting period. Courtesy of SERNAGEOMIN.

The lava flow (L5) on the N flank of Volcan Nuevo continued to advance during November 2020, reaching about 720 m from the rim of Nicanor crater by mid-month with an estimated speed of 1.4 m/day the flow only advanced about 10 m during the second half of November. Its size was estimated at about 580,000 m³ by the middle of the month. Dome growth inside the crater continued as well; it was measured in mid-November as 45 m high with a volume a little over 200,000 m³. During an overflight in mid-November SERNAGEOMIN scientists noted a fracture about 50 m long on the N side of the dome that extended NE-SW, was 10 m deep, and connected the dome to the central channel of the L5 lava flow. Inflation rates of 3-5 mm/month were recorded at GNSS stations around the active crater at mid-month. The vertical inflation increased to a rate of 12 mm/month during the second half of the month while the horizontal displacement remained low at 2 mm/month.

VT seismic events increased in frequency during December 2020 to 192 during the first half of the month and 236 during the second half. The numbers of explosions remained similar with 551 during the first two weeks and 582 during the second. Explosion plumes usually remained less than 1,200 m above the summit and contained moderate amounts of particulates (figure 71). Most of the explosions appeared to come from the dome inside the crater; later in the month a few explosions appeared to cause partial destruction of the NE corner of the dome and produced limited pyroclastic flows adjacent to the crater.

Figure 71. Plumes of gas and ash from Nevados de Chillan, along with thermal anomalies, were frequently detected by satellite instruments, as seen here on 9 (top) and 29 (bottom) December 2020. The plumes are drifting SE, and two thermal anomalies were present at the summit; the brighter one is from the center of the dome inside Nicanor crater and the dimmer one slightly to the NW is at the head of flow L5 that descends the N flank. Image uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

The L5 lava flow was 750 m long and 20 m wide by 15 December, and it had grown 15 m by the end of the month. The rate of dome growth appeared to increase during the second half of December, with the dome growing over the crater rim on the NNE edge, causing block avalanches to descend the NNE flank. Deformation data indicated a high level of inflation, with a horizontal displacement rate of 5 mm/month and a vertical rate of 11 mm/month. The accumulated volume of new material from both the dome and the flow since late June 2020 was estimated at a little over 1,400,000 m³ at the end of December.

The number of seismic events continued to increase in January 2021 with 386 VT events recorded for 1-15 January and 244 events 16-31 January. For explosions, 659 events occurred during the first half of the month, and 676 were reported for the second half. The explosions from the dome rose up to 1,200 m above the crater during the first half of January and 1,400 m high during the second half. Dome growth continued with activity focused in the NNE part of the crater; fragments traveled up to 100 m down that flank. Increased flow from the fissure at the head of the L5 flow led to an increase in the flow rate at the head of the flow to about 1.7 m/day by mid-month when it was 788 m long. The inflation rate remained constant during the first half of the month. Intense continuous degassing was observed from the front of the L5 flow during 23-25 January (figure 72). This coincided with a rise in thermal radiance in the same area. By the end of the month the flow was 808 m long, and the central channel had enlarged to 40 m wide near the head of the flow. An increased flow rate observed in the webcams was accompanied by the development of lateral lobes at the head of the flow. A slight decrease in the rate of inflation was recorded near the end of the month.

Figure 72. Intense degassing and a rise in thermal radiance were observed at the head of the L5 flow at Nevados de Chillan during 23-25 January 2021. The left Sentinel-2 image from 25 January 2021 shows the thermal anomaly at the front of the L5 flow (top), the anomaly at the head of the flow, and the anomaly from the dome inside the Nicanor crater (bottom). The right image shows strong degassing at the L5 flow front at the base of the channel of the active flow. Courtesy of SERNAGEOMIN.

A bright thermal anomaly near the front of the L5 flow on 4 February 2021 was attributed by SERNAGEOMIN to the rupture of the crust and partial collapse of that area due to an increased flow rate (figure 73). Dome growth continued in February with ejecta from explosions reaching 70-100 m from the crater rim. The central channel of the L5 flow continued to widen from the inferred increase in flow rate (figure 74); it was 710 m long by the end of the month. The total flow length was estimated at 860 m by 15 February and 900 m long by the end of the month. Its volume measured on 15 February was about 1,700,000 m³. Inflation continued at a rate of 5-8 mm/month near the active crater, but a small deflation was recorded 11 km E. Eruption plumes rose no higher than 1,100 m above the crater during February 2021 and contained various amounts of particulate matter (figure 75). The number of VT seismic events decreased to 165 during the first half of the month and to 106 during the second half, but the number of explosive events was relatively constant at 598 during 1-15 February and 471 during 16-28 February.

Figure 73. A bright thermal anomaly appeared at the head of the L5 flow at Nevados de Chillan on 4 February 2021. SERNAGEOMIN attributed it to the rupture of the crust and partial collapse of that area due to increased flow. Sentinel-2 image uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Figure 74. The growth of the L5 lava flow on the N flank of Nicanor crater and dome inside the crater at Nevados de Chillan was evident in these images comparing them from 1 December 2020 and 8 February 2021. Courtesy of SERNAGEOMIN.

Figure 75. A plume of steam and ash rose from the summit of Nevados de Chillan on 22 February 2021 after a VT seismic event that was located 3.3 km deep with a magnitude of 3.0. Courtesy of Volcanologia Chile.

Incandescence at night was often visible from the E side of the crater during March 2021. The explosion heights were up to 1,200 m during the beginning of the month, and below 1,000 m during the latter half when most of the plumes rose less than 500 m. There were 232 VT events and 534 explosions during 1-15 March and 214 VT events and 556 explosions during 13-31 March. The estimated volume of the dome on 6 March was a little over 300,000 m³. Dome growth continued with ejecta traveling up to 160 m from the dome; the L5 flow front was 80 m wide by the middle of the month and measured 950 m long in satellite imagery; detachment blocks fell from the front and sides of the flow. The rate of inflation decreased during the month. During an overflight on 24 March SERNAGEOMIN observers noted a significant increase in the quantity of pyroclastic debris inside Nicanor crater compared with the previous overflight on 2 December 2020. Blocks as large as 50 cm were scattered around the dome, and a few reached the adjacent Nuevo and Arrau craters. Ejecta was scattered up to 140 m from the crater rim. A collapse on the N rim had sent debris down that flank. The main fissure feeding the flow had grown to 20 m wide and almost 70 m long by the end of the month (figure 76). Measurements of the volume of effusive material on 24 March from a DEM indicated that the extruded volume since June 2020 of the flow was just over 1,900,000 m³ and for the dome was almost 412,000 m³.

Figure 76. Scientists doing fieldwork at Nevados de Chillan on 26 March 2021 collected samples of lava from the L5 flow on the N flank of Nicanor crater. The lava emerged from a fissure at the edge of the crater. The brown material on the left is the part of the dome that had grown over the NE rim of the crater and sent debris down the flank. Courtesy of SERNAGEOMIN.

Both explosive and effusive activity increased during April 2021, although the height of the plumes remained below 800 m. Continued dome growth beyond the NE edge of the crater had created an area of unconsolidated deposits on the NE flank. The dome extended 140 m beyond the crater by mid-month. The fissure connecting the dome and the N-flank L5 flow measured 60 m long, 25 m wide, and 8 m high by 15 April, and had grown to 80 m long and 32 m wide by the end of the month (figure 77). The total volume of extruded material from both the dome and the flow on 13 April was about 2,760,000 m³. Lava spines were reported during the second half of April. The lava dome continued to grow and reached 66 m high by mid-month. Most of the growth during the first half of the month was concentrated on the edges, with a depression in the center of the dome. During the second half of April the dome growth was concentrated on the W edge of the crater. The lava flow was about the same length at the end of April as it had been at the end of March. Horizontal inflation increased to 12.5 mm/month during the first half of April but was stable during the second half; the vertical component indicated inflation from all the stations, with the maximum deformation of 21 mm at station FRSC. An increase in the flow rate was observed beginning on 28 April (figure 78), and a new lava flow on the NE flank was observed on 4 May 2021.

Figure 77. An annotated image of the N flank of Nevados de Chillan from 6 April 2021 identified the central channel of the N-flank L5 flow and its front (dashed yellow line), the overall active flow area (red dashed line), detachment blocks falling off the active flow channel (derrumba de rocas), the lava dome (yellow circle), pyroclastic debris on the NE flank from the dome (Depositos piroclasticos provenientes del domo), and an eruptive column of gas and ash emerging from the dome inside Nicanor Crater. Courtesy of Volcanologia Chile.

Figure 78. Steam and ash emissions and thermal anomalies continued at Nevados de Chillan during April 2021, seen here in Sentinel-2 satellite images. On 15 April (left) a plume of steam and ash drifted SE from the Nicanor crater. On 28 April (right) a surge in flow activity produced a strong thermal anomaly at the front of the L5 flow on the N flank of the crater in addition to the anomalies from the head of the flow and the growing dome. Left image uses Natural color rendering (bands 4,3,2) and right image uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/, https://twitter.com/Sernageomin); 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/); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Volcanología Chile (URL: https://www.volcanochile.com/joomla30/, Twitter: @volcanologiachl).

December 2021 (BGVN 46:12) Cite this Report

New lava flows and domes accompany heightened explosive activity, May-November 2021

Nevados de Chillán is a large complex of late-Pleistocene to Holocene stratovolcanoes in the Chilean Central Andes that has had multiple recorded eruptions dating back to the seventeenth century. The most recent started with a phreatic explosion and ash emission on 8 January 2016 from a new crater (Nicanor) on the E flank of Nuevo crater, itself on the NW flank of the large Volcán Viejo stratovolcano. Strombolian explosions and ash emissions continued throughout 2016 and 2017; the growth of a lava dome within Nicanor was confirmed in early 2018. Multiple lava flows emerged from the crater beginning in late 2019, while explosions, block-and-ash pyroclastic flows, and frequent ash plumes continued into 2021. This report covers activity from May-November 2021 when ongoing eruptive activity produced ash plumes, the growth of domes inside the crater, and multiple lava flows extending down the N and NE flanks. Information for this report is provided primarily by Chile's Servicio Nacional de Geología y Minería (SERNAGEOMIN)-Observatorio Volcanológico de Los Andes del Sur (OVDAS), the Buenos Aires Volcanic Ash Advisory Center (VAAC) and from satellite data.

During May-November 2021 there was continuing activity on the existing L5 flow and the development of three new flows (L6, L7, and L8) that traveled hundreds of meters down the N and NE flanks. Ash plumes rose generally 1-2 km above the crater, generated by the hundreds to thousands of explosions reported each month. Multiple daily reports of ash emissions were issued by the Buenos Aires VAAC during May through mid-August, followed by more intermittent emissions through September, before returning to multiple daily reports for most of October and November. Block and ash flows descended several hundred meters down the flanks, mostly N and NE, multiple times each month. Incandescent ejecta rose several hundred meters above the summit during many of the explosions, with ejecta falling back into the crater and also onto the flanks. The growth of two new domes was reported in September and October, followed by surges of lava flow activity in November. Thermal activity, as reflected in the MIROVA data (figure 79), closely followed the activity observed at the various flows.

Figure 79. An increase in the thermal energy at Nevados de Chillan appeared at the beginning of May 2021 in conjunction with the onset of a new lava flow (L6) from the NE crater rim. The lava flows remained active through mid-August but were quiet in September. Renewed activity in October and November again increased the thermal output. Courtesy of MIROVA.

Activity during May-June 2021. At the beginning of May 2021, the L5 lava flow, which first appeared on the N flank of Nicanor crater in June 2020, was reported as extending 938 m from the edge of the crater, about 50 m thick at the front, and moving at a rate of about 10 cm/hour. An increase in the eruptive rate had been noted after 28 April. The character of the eruption changed on 2 May with higher, denser, ash plumes, and greater intensity of incandescence. Explosions generated block and ash pyroclastic flows that extended less than 400 m down the NE flank and resulted in partial destruction of the dome inside the crater. Shorter block-and-ash flows also affected the SW flank, and ash was deposited around the crater rim.

Satellite imagery on 4 May indicated a new thermal anomaly at the NE crater rim. This was confirmed the next day as a new lava flow (L6) reached 100 m from the rim (figure 80). Thermal camera measurements indicated a maximum temperature at L6 of 278°C and a new area of high temperature at the edge of L5 that registered as 307°C. Flow L5 continued to advance on the N flank with a maximum temperature of 208°C (figure 81).

Figure 80. A view looking SW at the summit of Nevados de Chillan shows lava flows L5 and L6 and pyroclastic flow deposits on the NE flank at 1505 UTC on 5 May 2021. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanic (REAV), Region de Nuble, Complejo Volcanico Nevados de Chillan, 5 de mayo de 2021, 1840 Hora local (Chile continental)).

Figure 81. A thermal camera at Nevados de Chillan measured temperatures at the L6 flow of 278°C, at a high-temperature area on the edge of L5 at 307°C, and near the front of the L5 flow at 208°C, on 5 May 2021. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanic (REAV), Region de Nuble, Complejo Volcanico Nevados de Chillan, 5 de mayo de 2021, 1840 Hora local (Chile continental)).

During 6-15 May there were 127 VT seismic events and 1,652 discrete explosions reported. Explosions produced ash and steam plumes that reached up to 1,300 m above the crater (figure 82). Increased explosive activity resulted in a greater volume of ash in the emissions (figure 83). Additionally, the intensity, frequency, and extent of distribution of incandescent ejecta from the dome also increased, affecting the E, SE, and NE flanks. Pyroclastic flows extended 1.1 km NE and 230 m SE from the crater, and ash deposits were reported between 3.5 and 5 km away on the SW, S, and SE flanks.

Figure 82. Ash and steam emissions from Nevados de Chillan rose 500 m (scale on left) and drifted N on 9 May 2021. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De Nuble, Complejo Volcanico Nevados de Chillan, 09 de mayo de 2021, 12:10 Hora local (Chile continental)).

Figure 83. A dense, dark gray, ash plume rose 280 m from the crater and drifted S on 15 May 2021 at Nevados de Chillan. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De Nuble, Complejo Volcanico Nevados de Chillan, 15 de mayo de 2021, 11:45 Hora local (Chile continental)).

The fissure feeding flow L5 grew to 40 m wide and 80 m long during the first half of May. Its growth resulted in the partial collapse of the NE rim, producing a deposit 160 m long. Activity at the dome was characterized by destruction of its E portion, which resulted in the source of the new L6 flow. By 15 May, L5 had reached 966 m from the rim, moving at a rate of 20 cm/hour and the secondary flow from the NE rim near L5 had reached 220 m from the crater rim. Flow L6 was moving rapidly, advancing at a rate of 3.6 m/hour and grew to 640 m in length. Sentinel-2 satellite imagery showed the advance of the L6 flow down the NE flank during May (figure 84)

Figure 84. Sentinel-2 satellite imagery of Nevados de Chillan showed the advance of the new L6 lava flow on the NE flank during May. Flow L5 remained stable with little advance at the front. Images uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Explosive activity increased significantly during the second half of May with 3,668 recorded events. Ash plumes rose as high as 1,400 m above the rim and nighttime incandescence was concentrated in the E part of the crater. Ejecta affected the E, SE, and NE flanks, and block-and-ash pyroclastic flows reached 1.1 km NE of the volcano. Flow L5 did not advance, its front remaining at 966 m from the rim, but new material flowed over the existing channel and widened the lower part of the flow. L6 continued to advance at a velocity of 1.16 m/hour and reached 894 m from the rim. The secondary lobe off L5 also continued to advance and recorded a temperature similar to L6. Data from the GNSS network showed a vertical component deflation of 51 mm during 2-14 May.

Explosions continued at a high rate during the first half of June 2021 with 2,853 discrete events. Ash plumes rose as high as 1,140 m above the summit; nighttime incandescence was concentrated on the E part of the crater, and ejecta affected the E and SE flanks. Pyroclastic flows reached 600 m down the E flank and 400 m on the S flank. A series of LP seismic events on 14 June produced incandescent ejecta that rose 200 m and sent multiple blocks down the flanks. During the second half of the month plumes, frequently with ash, reached 1,400 m above the summit; 1,901 explosive events were recorded (figure 85). Incandescent ejecta rose as high as 500 m and affected the N, E, and S flanks (figure 86). Pyroclastic flows on 19, 27, 28, and 29 June traveled up to 500 m down the NE flank.

Figure 85. An explosion on 28 June 2021 at Nevados de Chillan produced an ash plume that rose 740 m above the crater rim and drifted N. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De Nuble, Complejo Volcanico Nevados de Chillan, 28 de junio de 2021, 12:40 Hora local (Chile continental)).

Figure 86. Pulsating emissions of ash and ejecta caused partial destruction of the dome at Nevados de Chillan on 29 June 2021. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De Nuble, Complejo Volcanico Nevados de Chillan, 29 de junio de 2021, 18:39 Hora local (Chile continental)).

Flows L5 and L6 remained active during the first half of June. Flow L5 reached 1 km in length with a very slow advance rate as most of the activity produced flows overtop of the existing channel that spread out laterally, widening the flow on the distal slope and causing frequent block avalanches. Flow L6 remained at 894 m by mid-June; activity was largely widening the flow in the middle section. A new pulse of lava on 6 June produced levees along the L6 channel and created a zone of accumulation near the toe of the flow. By the end of June, flow L5 had reached 1,022 m from the crater rim; activity on L6 continued, but it expanded laterally and thickened without extending in length. The flow rate increased during 17-19 and 27-29 June, resulting in new lobes advanced over existing flows with temperatures considerably higher than observed previously (figure 87). There was also increased nighttime incandescence, additional block-and-ash pyroclastic flows, and higher plumes. Fresh lava at flow L5 extended the front by at least 100 m. Two new emission centers with explosive activity and gas emissions appeared at the crater rim between flows L5 and L6.

Figure 87. Activity at both flows L5 and L6 at Nevados de Chillan increased during 17-19 June 2021 producing strong thermal anomaly signals in Sentinel-2 satellite data. Emissions from the crater were often visible. Image uses Atmospheric penetration rending (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Activity during July-August 2021. During the first half of July 2021 there were 2,208 explosive events identified either acoustically or visually; ash plumes reached up to 1,140 m above the crater. The high levels of explosive activity produced frequent nighttime incandescence and incandescent blocks descended the N, E, and S flanks, reaching as far as 500 m down the E flank. SERNAGEOMIN issued numerous special reports describing explosions that produced ash plumes rising several hundred meters and drifting in multiple directions, including multiple events of pulsating ash and ejecta on 10 July. No pyroclastic flows were observed. Flow L5 was measured at 1,064 m in length from the crater rim, advancing slowly at 50 cm/hour by mid-month. The overall length of L6 remained at 894 m, but a surge of activity from the L6 vent during 5-10 July produced a thermal anomaly 420 m down the flow. Both flows showed significant instability, with collapses and block avalanches from the flow fronts on the NE slope.

High levels of explosive activity continued during the second half of July with 2,964 events either seen in webcams or detected acoustically. Ash-rich plumes rose up to 1,900 m above the crater. A new explosive center located SE of the L5 flow vent was responsible for explosions with incandescent blocks that fell up to 500 m down all of the flanks. Pyroclastic flows occurred on 27 and 28 July on the NE flank, both N and S of L6, and to the NNW (figure 88). The farthest-reaching pyroclastic flow traveled 555 m down the NE flank on 28 July. Frequent collapses continued from the edges of both lava flows. Flow L5 continued advancing slowly at 10 cm/hour, reaching 1,101 m in length; flow L6 did not advance until the last four days of the month. High thermal radiance was observed along the entire length of L5, while a significant decrease in the temperature of L6 was observed until a new pulse of lava was detected on 27 July (figure 89); it advanced 100 m in four days.

Figure 88. Block-and-ash flows descend the NE flank of Nevados de Chillan during heightened explosive activity on 27 July 2021. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De Nuble, Complejo Volcanico Nevados de Chillan, 27 de julio de 2021, 16:30, Hora local (Chile continental)).

Figure 89. A new pulse of lava along flow L6 at Nevados de Chillan on 27 July 2021 extended down the NNE flank. Image uses Atmospheric penetration rendering (bans 12, 11, 8a). Courtesy of Sentinel Hub playground.

The increase in explosive activity continued into August 2021 with over 3,000 events recorded from either acoustic measurements or emissions seen in webcams during the first half of the month. Dense ash plumes rose up to 1,480 m above the summit. Two adjacent emissions centers were active inside the Nicanor crater. The high levels of explosive activity produced incandescent blocks and ash that dispersed up to 500 m down all the flanks. During 8-12 August the explosion rate increased, and pyroclastic flows affected the NE flank, extending 570 m E, 450 m N, and 270 m NE (figure 90). Four closely spaced explosions on 13 August produced a series of plumes that rose to 1,240 m and were captured by the Arauco webcam to the N (figure 91). Collapses were common along the flow fronts of both L5 and L6, sometimes triggered by explosions.

Figure 90. A 760-m-high ash plume drifted NE from Nevados de Chillan at sunset on 10 August 2021. Multiple pyroclastic flows also descended up to 500 m down the NE and E flanks. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De Nuble, Complejo Volcanico Nevados de Chillan, 10 de agosto de 2021, 18:30, Hora local (Chile continental)).

Figure 91. A sequence of four explosions at Nevados de Chillan on 13 August 2021 within about 40 minutes produced a series of ash clouds that rose 1,240 m high and drifted E. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De Nuble, Complejo Volcanico Nevados de Chillan, 13 de agosto de 2021, 15:50, Hora local (Chile continental)).

Flow L5 continued advancing during the first half of August at a much higher rate of 1.2 m/hour compared to previous months, and reached 1,277 m from the crater rim, while L6 remained constant at 850 m. Webcams showed a constant supply of fresh material in the central channel of L5; the flow changed course to the NW after encountering a topographic barrier. Sentinel-2 images showed a thermal anomaly in the center channel of L6 for about the first week, after which there was only a strong anomaly at the emission point inside the central crater. A site visit to the front of L5 during the first two weeks of August corroborated the instability and recurring collapses of the flow as seen in webcam images. The thickness of the flow front was estimated at 20 m and maximum temperatures there were 440-460°C; most temperatures in the medial part of the L5 flow were around 265°C. Scientists observed levees and secondary flows along the length of the flow, and a high degree of alteration and instability at the front. Rock and ash samples of the L5 flow were taken.

A sharp decline in activity was reported for the second half of August 2021. Poor weather reduced visibility; only 476 explosions were observed. Most of the plumes, which reached up to 1,080 m, consisted of gas and steam, but they were more frequently interspersed with ash-bearing plumes at the end of the month. The drainages on the NE flank were affected the most by pyroclastic flows and blocks from the flow-front collapses, especially from flow L5, with the longest pyroclastic flow reaching 560 m from the crater rim. Although flow L5 grew to 1,380 m long with a flow rate of 1 m/hour, flow L6 remained static through the end of August, and the overall thermal anomalies decreased significantly. Data recorded in the GNSS instruments suggested an inflation of the volcano of 5 mm between 27 July and the end of August.

Activity during September-November 2021. Slightly over 300 explosions were either observed in webcams or measured with acoustic monitors during the first half of September 2021. New harmonic tremors were detected on 7 September. During 10 September there was a sudden increase in the size of the LP seismicity which continued through 15 September along with another pulse of harmonic tremor on 14 September. The plumes from explosive events reached 1,060 m above the crater rim, most with high concentrations of ash during the first week, changing to mostly steam and gas by the end of the second week. A few of the explosions contained very dense concentrations of maroon-colored tephra originating from the center of the crater. A pyroclastic flow on 3 September reached 550 m down the NE flank. Little nighttime incandescence was reported except for 14 September when ejecta rose less than 200 m high and remained within the crater.

Flows L5 and L6 showed very little movement; during the first week, the central channel of L5 was active but diminished by the end of the second week. Satellite images showed no thermal anomalies in the channels of either flow throughout the month. The growth of a new lava dome (Dome 3) inside the crater was confirmed by satellite imagery on 15 September and was measured to be 27 x 40 by SERNAGEOMIN (figure 92). The fissure feeding the dome was located slightly NW of the fissure that fed flow L6. Inflation continued to be measured with a vertical rate of about 10 mm/month and a horizontal radial inflation pattern of about 5 mm/month.

Figure 92. A Skysat image of the summit crater of Nevados de Chillan from 15 September 2021 shows the growth of a new dome with dimensions of about 27 x 40 m. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De Nuble, Complejo Volcanico Nevados de Chillan, 15 de septiembre de 2021, 19:45, Hora local (Chile continental)).

Although the number of explosive events increased in the second half of the month, the overall energy of the events decreased significantly. Most plumes rose only 300 m; the highest reached 1 km on 27 September. They were white and gray with low quantities of ash. No pyroclastic flows were observed, but permanent incandescence was visible from inside Nicanor crater. A fourth dome was identified adjacent to Dome 3 in satellite images on 18 September with elliptical dimensions of 36 x 43 m. Dome 3 continued to enlarge and was estimated to be 39 x 64 m on 24 September. Evidence from the webcams on 29 September suggested that a new flow (L7) issued from Dome 4, between flows L5 and L6, and measured about 50 m from the crater rim, moving at a rate of about 40 cm/hour. Also apparent was a reactivation from the vent that fed flow L5, which grew to 50 m in length, with the new material descending adjacent to the previous flow. Vertical inflation continued at a rate of about 50 mm/month, and the horizontal component showed a slight increase as well.

Explosions during the first half of October 2021 produced white and gray plumes with a moderate content of ash and ejecta. Most plumes rose less than 400 m, but the tallest reached 1,620 m on 9 October. Incandescence was visible during both day and night, and ejecta that rose as high as 350 m landed around the crater and on the NW rim in the area of the new lava flow. Flow L7 remained active with the main channel in the saddle between L5 and L6; a secondary lobe (L7b) traveled over the previous L6 channel. By 18 October the central channel of L7 was estimated to be 930 m long from the emission point, moving at a speed of 1.5 m/hour. Flow L7b had reached 350 m by 14 October when movement ceased. The new L8 flow (figure 93), located W of L5, reached 280 from the emission point, moving at a speed of 20 cm/hour. Flow L5 remained inactive.

Figure 93. New lava effusion at Nevados de Chillan during the first two weeks of October 2021 produced flows L7, L7b, and L8, shown here in Sentinel 2 satellite imagery on 7 October. Image uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Satellite imagery from 6 and 7 October showed the area of Dome 3 to be about 4,000 m². Vertical inflation continued to be measured at a rate of 50 mm/m with a total accumulation since early August of 1.4 cm. A significant increase in the temperature around the active crater was measured in thermal imagery, associated with the growth of flows L7 and L8. During explosive activity on 3 and 7-10 October temperatures exceeded the range of the thermal camera measurements (over 360°C). The average temperature of L7 was 152°C on 4 October and a maximum temperature of 280°C was observed during collapses associated with an increase in the velocity of the flow. Flow L8 was initially measured at 106°C on 7 October, and the maximum temperature of 240°C was measured the next day when the flow rate increased.

Many of the 956 recorded explosive events that occurred during the second half of October produced plumes of gas and ash. The plumes rose from the Nicanor crater, and alternated white, vapor-dense plumes and gray plumes with ash. The highest plume reached 1,580 m above the crater on 18 October. Occasional stronger explosive events sent incandescent ejecta onto the N slope and pyroclastic flows onto the NE slope as far as 500 m. The height of the ejecta didn’t exceed 350 m from its source at the L8 flow vent on the NE rim of the crater. Surface explosions increased in energy on 18 and 23 October. The flow rate for flows L7 and L8 decreased during the second half of October. By 29 October L7 had reached 943 m in length, and L8 had reached 382 m. The lobe L7b did not advance after 14 October. The geodetic network indicated deformation stability with little or no continued inflation observed.

Explosions from the Nicanor crater during the first half of November continued to alternate between steam-dense white plumes and ash-laden gray plumes. Discrete explosions produced emissions that rose as high as 2 km on 8 November. The stronger explosions caused incandescent blocks on the N slope as well as pyroclastic flows on the NE flank. The largest events, on 8 and 10 November sent material as far as 700 m down the flank and ash emissions to 1,120 m above the summit that drifted SE.

Neither flows L7 or L8 extended in length during the period, but both remained active with lava moving in their main channels. Satellite images also identified a new effusive body around the emission center of L7 estimated at 80 m long and 55 m wide. SERNAGEOMIN inferred that the extrusion began around 28 October, and that most of the structure was destroyed during explosions on 8 November. Following this, a new pulse of lava from L7 was observed in webcams on 9 and 14 November, with growth of the flow to the SE and E. An increase in the vertical inflation rate of the volcano to 10 mm/month was noted, but the horizontal component remained stable.

Both gray and white explosions continued for the rest of November 2021 with the tallest plume reaching 1,700 m above the summit on 22 November. Pulsating explosions released multiple plumes on several different days (figure 94). Incandescent ejecta was frequently reported on the N flank, and the number of daily explosions increased towards the end of the month. Occasional pyroclastic flows were observed on the NE flank, reaching up to 500 m from the crater rim. There were multiple new pulses of lava feeding the L7 and L8 flows. A new pulse on 19 November at flow L7 sent lava 477 m to the N and 465 m down the L7b channel to the NE. A pulse in the flow L8 channel reached 198 m the same day, moving with a velocity of 30 cm/hour. Vertical inflation continued at an estimated rate of about 10 mm/month; both vertical and horizontal inflation increased the most during 20-23 November.

Figure 94. Pulsating ash emissions from Nevados de Chillan were captured in satellite imagery multiple times during the second half of November 2021. Note different scale bar in each image. Two widely spaced plumes drifted SE on 19 November (top left). Five separate pulses could be identified on 21 November, three NE of the crater, one over the crater, and one S of the crater (top right). A single event with a dense plume projected SE from the crater on 26 November (bottom left), and two widely spaced pulses of emissions drifted E on 29 November (bottom right). Thermal anomalies were present inside the summit crater in all of the images. Images all use Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/, https://twitter.com/Sernageomin); 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/); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

June 2022 (BGVN 47:06) Cite this Report

Pyroclastic flows, gas-and-ash plumes, and a new lava dome during December 2021-May 2022

Nevados de Chillán, located in the Chilean Central Andes, has had multiple recorded eruptions dating back to the seventeenth century. The most recent eruption began in January 2016 with a phreatic explosion and ash emission from the new Nicanor crater on the E flank of Nuevo crater. More recently, lava flows and domes, along with multiple daily reports of ash emissions and block-and-ash flows, have been reported (BGVN 46:12). Two new lava domes were described in September and October 2021, followed by a surge of lava flow activity in November. This report covers activity from December 2021 through May 2022 and describes seismic and thermal activity, gas-and-ash plumes, and pyroclastic flows. Information comes primarily from Chile's Servicio Nacional de Geología y Minería (SERNAGEOMIN)-Observatorio Volcanológico de Los Andes del Sur (OVDAS), the Buenos Aires Volcanic Ash Advisory Center (VAAC), and satellite data.

According to an analysis of digital elevation models, about 1.5 million cubic meters of effusive material was emitted during 17 September-2 December 2021. During December 2021-May 2022 there was continuing eruptive activity in the Nicanor crater, which consisted dominantly of seismic events, sulfur dioxide emissions, and thermal anomalies (table 2). By 13 December the L7 and L8 flows were no longer advancing, based on Sentinel-2 L2A imagery. Gas-and-ash plumes rose generally 1-2.2 km above the crater, generated by the several hundreds of explosions detected each month. Intermittent pyroclastic flows accompanied these eruptive events, traveling as far as 580 m down multiple flanks. Incandescent ejecta rose several hundred meter above the crater rim during many of the explosions, with some material falling back on the proximal flanks. A new dome (Dome 4) was first detected in early March.

Table 2. Summary of seismic events, maximum sulfur dioxide values, and number of thermal anomalies at Nevados de Chillán during December 2021-May 2022. Data from SERNAGEOMIN bi-weekly reports.

During mid-November 2021 there were multiple surges along the L7 and L8 flows; lava from the L7 flow reached 477 m to the N and 465 m down the L7b channel to the NE and 198 m in the L8 channel (BGVN 46:12). Thermal activity gradually decreased in energy from mid-November through early December 2021, due to the cessation of the L7 and L8 lava flows (figure 95). After December, the thermal anomalies became intermittent and relatively low in power compared to November. Four anomalies were detected by the MODVOLC thermal system during this time, on 1 December 2021, 14 January, and 28 January 2022. Sentinel-2 infrared satellite imagery showed a persistent thermal anomaly in the active Nicanor crater, sometimes accompanied by a gas-and-ash plume that drifted NE and E (figure 96).

Figure 95. During mid-November 2021, there was a surge of multiple new lava flow pulses from Nevados de Chillán that was reflected in this MIROVA graph (Log Radiative Power). Following this spike in thermal activity, there was a gradual decrease in thermal energy that continued through early December 2021. Thermal activity became less frequent through May 2022. Courtesy of MIROVA.

Figure 96. Sentinel-2 infrared satellite imagery of Nevados de Chillán showing a persistent thermal anomaly in the Nicanor crater on 24 December 2021 (top left), 7 February 2022 (top right), 1 March 2022 (bottom left), and 10 May 2022 (bottom right). Incandescent material was visible descending the NE flank on 7 February. A gas-and-ash plume drifted NE and E on 1 March and 10 May, respectively. Images use “Atmospheric penetration” rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Activity during December 2021 consisted of frequent seismic events and daily explosive events that originated from the Nicanor crater. Surveillance cameras showed that explosions often ejected gas-and-ash plumes 300 m high; the highest was 1.6 km on 4 December. Strong nighttime incandescence was visible on 7 and 11 December, accompanied by material that was ejected less than 600 m away. Based on high-resolution SkySat satellite images, the L7 lava flow advanced 560 m with a speed of 0.5 m/hour along the central lobe and 596 m with a speed of 1.6 m/hour along the secondary lobe. According to analysis of Sentinel-2 L2A images, the L7 and L8 flows no longer advanced after 13 December. On 24-25 December a gas-and-ash plume rose 1.8 km above the crater. Some new effusive material was reported in the Nicanor crater, based on thermal camera images and Sentinel-2 L2A images during 16-31 December. The average sulfur dioxide values, recorded by Differential Absorption Optical Spectroscopy (DOAS) equipment, were 236 ± 51 t/d during 1-15 December and 196 ± 42 t/d during 16-31 December.

Seismicity increased in January 2022 and intermittent incandescence was observed throughout the month. Around 1944 on 6 January an explosion generated a pyroclastic flow which traveled less than 400 m NE from the crater rim, according to SERNAGEOMIN. A gas-and-ash plume from the event rose 2.2 km above the crater (figure 97), accompanied by nighttime incandescence visible less than 250 m high. A small cone was growing in the active crater due to the constant accumulation of pyroclasts. A gas-and-ash plume rose 1.6 km high on 19 January. Explosions generated pyroclastic flows on 19, 25, and 26 January that traveled less than 500 m down the NE, N, and NW flanks. During 25-26 January incandescent material was ejected 500 m toward the N flank. The average sulfur dioxide value was 269 ± 67 t/d during 1-15 January and 285 ± 60 t/d during 16-31 January.

Figure 97. Webcam image of the gas-and-ash plume that rose 2.2 km above Nevados de Chillán on 6 January 2022 as a result of an explosion. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV) Region De Nuble, Complejo Volcanico Nevados de Chillan, 6 de enero de 2022, 2145 Hora local (Chile continental)).

Similar explosive activity and seismicity persisted in February with intermittent nighttime incandescence. Pyroclastic flows due to continued explosions were detected on 5, 7, 9, 12, 13, and 19-20 February which descended the NE and E flanks as far as 580 from the crater rim. An explosion on 13 February generated a gas-and-ash plume that rose 1.9 km above the crater and produced a pyroclastic flow. Gas-and-ash plumes rose 1.6 km high on 16 February, 1.2 km high on 19 February, and 1.7 km high on 20 February. Material was ejected less than 500 m from the crater, accompanied by gas-and-steam emissions during 25-28 February. The event on 20 February also ejected incandescent material radially less than 500 m from the crater onto the N and W flanks. The average sulfur dioxide value was 137 ± 29 t/d during 1-15 February and 221 ± 70 t/d during 16-28 February.

On 1 March new effusive material was observed in the Nicanor crater using Planet satellite images, which was also reflected as a thermal anomaly in Sentinel-2 L2A images. Explosions continued, generating gas-and-ash plumes as high as 1.6 km above the crater. Plumes on 10, 11, 13, and 15 March rose 1 km, 1.2 km, 1 km, and 1.6 km high, respectively. Some explosions generated pyroclastic flows down the SE and E flanks as far as 460 m from the crater rim. Intermittent incandescence of varying power persisted in the active crater. On 13 March incandescence rose as high as 230 m. According to a SkySat satellite image from 15 March the effusive mass in the active crater (now called “Dome 4”) was 57 m long, 33 m wide, growing to the NE and SE, and covered an area of 1.7 m² (figure 98). On 25 March a pyroclastic flow descended the NW flank of the active crater as far as 500 m. Incandescent material was ejected 500 m to the NE and N on 23, 25, and 31 March. The average sulfur dioxide value was 454 ± 24 t/d during 1-15 March and 537 ± 140 t/d during 16-31 March. The increase in sulfur dioxide rates was due to a new effusion in the Nicanor crater, according to SERNAGEOMIN.

Figure 98. SkySat image showing the Nicanor crater at Nevados de Chillán. New effusive material (labeled “Cuerpo Efusivo”) was observed on 15 March 2022. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcánica (REAV), Región De Ñuble, Complejo Volcánico Nevados de Chillán, 17 de marzo de 2022, 19:30 Hora local (Chile continental)).

The new lava effusion was still visible in the Nicanor crater during April. Gas-and-ash plumes rose as high as 1.6 km on 15 April. As a result, a pyroclastic flow descended 500 m down the N and NE slope. Crater incandescence remained at low to moderate levels. The average sulfur dioxide value was 595 ± 80 t/d during 1-15 April and 551 ± 68 t/d during 16-30 April.

Explosions persisted during May, producing gas-and-ash plumes as high as 900 m on 16 May, which generated pyroclastic flows down the SE, NE, and W flanks as far as 400 m from the crater rim. Low to moderate incandescence was also visible up to 200 m high. Based on a SkySat image, numerous fissures and emission centers were noted in a E-W trend in the S and SW part of the growing lava dome, characterized by passive degassing. On 20 and 21 May gas-and-ash plumes rose 920 m high and generated proximal pyroclastic flows down the E flank as far as 400 m from the crater rim; material was ejected 350 m down the W slope, which included ashfall deposits. Incandescence was visible up to 200 m high on 31 May. The average sulfur dioxide value decreased compared to the previous month to 466 ± 54 t/d during 1-15 May and 363 ± 58 t/d during 16-31 May.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/, https://twitter.com/Sernageomin); 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).

December 2022 (BGVN 47:12) Cite this Report

Explosions, ash plumes, pyroclastic flows, and thermal activity during June-October 2022

Nevados de Chillán, located in the Chilean Central Andes, has had multiple recorded eruptions dating back to the seventeenth century. The current eruption began in January 2016 with a phreatic explosion and ash emissions from the new Nicanor crater on the E flank of Nuevo crater. Recently, activity has consisted of pyroclastic flows, gas-and-ash plumes, and a new lava dome (Dome 4) that was detected during early March 2022 (BGVN 47:06). This report updates information during June through October 2022 that describes continued explosions, ash plumes, pyroclastic flows, and thermal activity, based on information from Chile's Servicio Nacional de Geología y Minería (SERNAGEOMIN)-Observatorio Volcanológico de Los Andes del Sur (OVDAS) and satellite data.

During June through 17 October 2022 there was continuing eruptive activity in the Nicanor crater, which initially consisted of seismic events, sulfur dioxide emissions, and thermal anomalies (table 3). During August through mid-October, explosion events became more frequent and generated ash plumes that rose as high as 2.7 km above the crater. Explosions were also accompanied by pyroclastic flows that traveled no farther than 800 m down multiple flanks. Incandescent ejecta was visible above the crater rim in surveillance cameras, with some material falling back onto the proximal flanks.

Table 3. Summary of seismic events, maximum sulfur dioxide values, and number of thermal anomalies at Nevados de Chillán during June-October 2022. Data from SERNAGEOMIN bi-weekly reports.

Low thermal activity was occasionally detected by satellite data during July through October, as shown in the MIROVA (Middle InfraRed Observation of Volcanic Activity) Log Radiative Power graph (figure 99). There was a small cluster of thermal activity that occurred during mid-September through early October, which occurred around the same time as strong explosive eruptions. A single thermal hotspot was detected on 13 October according to the MODVOLC thermal algorithm. Some of this activity was also reflected in Sentinel-2 infrared satellite imagery as a small thermal hotspot in the active Nicanor crater (figure 100).

Figure 99. Occasional low-power thermal anomalies were detected at Nevados de Chillán during mid-July through early October 2022, according to this MIROVA graph (Log Radiative Power). No thermal activity was detected in June and three anomalies were detected during each month in July and August. There was a small cluster of thermal activity during mid-September through early October. Courtesy of MIROVA.

Figure 100. Sentinel-2 infrared satellite imagery of Nevados de Chillán showing a weak but persistent thermal anomaly at the active Nicanor crater on 19 July 2022 (top left), 11 August 2022 (top right), 5 September 2022 (bottom left), and 12 October 2022 (bottom right). Thermal activity was confined to the crater. Images use “Atmospheric penetration” rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Activity during June consisted of explosions that generated eruptive plumes with little to no pyroclastic content and crater incandescence that was visible up to 150 m above the crater; the highest plumes reached 940 m above the crater on 19 June. Seismicity was characterized by 56 volcano-tectonic (VT) events, 1,231 long-period (LP) events, and 472 tremor-type (TR) events. There were 320 LP-type events that were linked to surface-level explosions. Sulfur dioxide emission data was obtained by Differential Absorption Optical Spectroscopy (DOAS) equipment, which were installed on the SSE and ESE flanks of the active Nicanor crater. The average emission value ranged from 445 ± 95 to 453 ± 85 t/d, with a maximum daily value of 767 t/d recorded on 8 June. These sulfur dioxide levels are above base levels but are consistent with the presence of the new lava dome (Dome 4) that formed during early March 2022.

Explosive activity from the active crater continued during July and typically generated white gas-and-steam plumes less than 500 m high; due to weather clouds visibility was often obstructed. Seismicity included 67 VT-type events, 1,416 LP-type events, and 454 TR-type events. Of the 1,416 LP-type events, 212 were linked to surface-level explosions. On 13 July an explosion generated an avalanche mixed with deposits on the NE flank of the Nuevo crater that reached 470 m long. Ejecta traveled less than 200 m from the vent and were deposited on the E slope. On 29 July an eruptive episode produced a plume that rose 1,290 m above the crater. Explosions toward the end of the month also ejected a moderate amount of pyroclastic material less than 200 m from the vent onto the nearby flanks. DOAS data showed that the average sulfur dioxide emission value during the month ranged from 554 ± 29 to 601 ± 273 t/d. The maximum daily value was 1,056 t/d on 21 July.

During August, continuous explosions produced eruption plumes with little pyroclastic content that generally rose less than 500 m above the crater; on 2 and 20 August plumes exceeded 1 km above the crater. Intermittent night incandescence was visible to less than 50 m high and did not go beyond the crater rim. A total of 92 VT-type events, 1,382 LP-type events, and 532 TR-type events were detected throughout the month; of the LP-type events, 364 were linked to surface-level explosions. On 11 August surveillance cameras showed an extrusion of material inside the active crater, which SERNAGEOMIN reported was preceded by an increase in sulfur dioxide rates, plume heights associated with explosions, and increased explosion frequency. An explosion on 29 August generated a plume that rose 2 km above the crater and drifted S (figure 101). In addition, pyroclastic flows were reported, traveling less than 500 m on the E and NE flanks. The average sulfur dioxide emission value throughout the month ranged from 413 ± 69 to 507 ± 83 t/d, according to DOAS data. The maximum daily value was 1,017 t/d on 9 August. There was a slight decrease in sulfur dioxide emission rates toward the end of the month.

Figure 101. Photo of an ash plume that rose 2 km above the crater of Nevados de Chillán on 29 August 2022 and drifted S. The explosion was accompanied by pyroclastic flows that traveled less than 500 m on the E and NE flanks. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV) Region De Nuble, Complejo Volcanico Nevados de Chillan, 29 de agosto de 2022, 1108 Hora local (Chile continental)).

Three surveillance cameras continued to detect explosions and eruption plumes composed of mostly gas-and-steam that rose between 500-1,000 m above the crater during September. Seismicity consisted of 180 LP-type events, 1,200 LP-type events, and 332 TR-type events; of the LP-type events, 237 events were linked to surface-level explosions. On 12 September a strong explosion generated an eruption plume that rose 1.4 km above the crater. Toward the end of the month, the explosive events contained a higher volume of pyroclastic content, according to SERNAGEOMIN; ejecta was dispersed SE and block avalanches traveled as far as 500 from the crater rim. On 16, 19, and 30 September explosions produced plumes that carried a higher volume of pyroclastic material more than 360 m above the crater. Two explosions were reported on 19 September: the first rose 1.1 km above the crater and drifted SE (figure 102). A block avalanche occurred on the W flank that traveled 500 m from the crater rim. The second explosion produced an incandescent eruptive column that rose 1.7 km above the crater and drifted SE; ashfall was visible to the SE and ejecta was reported on the W flank. According to SERNAGEOMIN, these explosions also partially destroyed Dome 4. DOAS data recorded that sulfur dioxide emission rates during the month ranged from 315 ± 85 to 323 ± 76 t/d; the maximum daily value was 870 t/d on 12 September.

Figure 102. Photo of an ash plume rising 1.1 km above the crater of Nevados de Chillán that drifted SE on 19 September 2022. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV) Region De Nuble, Complejo Volcanico Nevados de Chillan, 19 de septiembre de 2022, 0813 Hora local (Chile continental)).

More frequent explosions and incandescent material were reported during early October, based on surveillance camera data. Six explosions generated plumes that rose more than 1 km above the crater and three exceeded 2 km. Seismicity has been characterized by 131 VT-type events, 710 LP-type events, and 122 TR-type events. There were 84 LP-type events that were linked to surface-level explosions. Explosions on 2 and 13 October were accompanied by 300-m-long pyroclastic flows and ejecta that traveled more than 500 m from the crater. A strong explosion on 9 October generated an eruption plume that rose 2.4 km above the crater and drifted NNW (figure 103); ashfall was reported in the same direction. In addition, pyroclastic flows were observed on the N, NNE, NE, and E flanks and traveled as far as 766 m. The flows mixed with snow on the volcano, which resulted in avalanche-type flows that moved 1.3 km down the N and NE flanks. Incandescent ejecta was visible at least as high as 800 m above the crater on 15 October. On 16 and 17 October there were three high-energy explosions; on 17 October the eruption column rose 2.7 km above the crater. During this explosion, pyroclastic flows were generated along the N, NW, W, NE, and E flanks and reached as far as 780 m on the NE flank. Some of these flows interacted with snow on the volcano and produced an avalanche-type flow that reached up to 2 km on the NE flank. DOAS measurements showed that the sulfur dioxide emission rates ranged from 47 ± 18 to 284 ± 83 t/d, with a maximum daily value of 911 t/d on 12 October. After the event on 17 October, there was an abrupt drop in surface activity with no eruptive columns or crater incandescence observed.

Figure 103. Photo of an ash plume rising 2.4 km above the crater of Nevados de Chillán that drifted NNW on 9 October 2022. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV) Region De Nuble, Complejo Volcanico Nevados de Chillan, 9 de octubre de 2022, 1930 Hora local (Chile continental)).

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/, https://twitter.com/Sernageomin); 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).