Extensive lava flows, bomb-laden Strombolian explosions, and ash plumes from Mackenney crater have characterized the persistent activity at Pacaya since 1961. The latest eruptive period began with intermittent ash plumes and incandescence in June 2015; the growth of a new pyroclastic cone inside the summit crater was confirmed later that year and has continued, producing frequent loud Strombolian explosions rising above the crater rim and ongoing ash emissions. In addition, flank fissures have been the source of lava flows during 2019-2021. A significant increase in both effusive and explosive activity that began in February 2021 continued through mid-May. Activity during March-May 2021 is covered in this report with information provided by Guatemala's Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), multiple sources of satellite data, and photographs from observers on the ground.

Summary of activity during March-May 2021. Incandescent explosions, ash emissions, and subsequent ashfall increased substantially at the beginning of March 2021 from the already increased levels during February. Explosions sent ejecta hundreds of meters high and hundreds of meters from the summit; ash plumes drifted tens to hundreds of kilometers and ashfall occurred almost daily in communities within tens of kilometers of Mackenney crater. The most extensive ash emissions forced closure of the International Airport in Guatemala City on 22 March. Ash emissions decreased during April and were intermittent into the first half of May, after which they tapered off.

Effusive activity also increased significantly during March 2021; by early in the month as many as three lava flows with multiple branches, all about 1 km long, were simultaneously active on multiple flanks. A new fast-moving flow appeared on the SW flank during the second half of March and rapidly reached 1.5 km in length, flowing NW then SW, ultimately extending over 3 km. It had multiple branches that caused vegetation fires, destroyed significant cropland, and crossed roads before stopping in mid-April. A new flow emerged along a similar path at the end of April and grew to over 2 km long in early May before activity at its source fissure ended on 17 May. High temperatures remained at many flow areas around the volcano for the rest of the month.

The high levels of activity are reflected in the MIROVA radiative power data for the period which show the increase in intensity to very high levels through mid-April, followed by a pulse in late April and early May that corresponds to explosions and lava flows. Thermal activity decreased significantly by the third week of May (figure 160). The MODVOLC thermal alert data shows a similar pattern with multiple alerts issued most days in March and for the first half of April, and another pulse of activity from 27 April-13 May. Significant sulfur dioxide emissions were recorded in satellite data several times in March and April and corresponded to periods of increased explosive and effusive activity (figure 161).

Figure 160. The ongoing eruption at Pacaya increased significantly in intensity in December 2020 and continued to increase through March 2021 as seen in this MIROVA log radiative power graph. Abundant ash emissions and extensive lava flows emerged from numerous fissures until activity decreased substantially in mid-May 2021. Courtesy of MIROVA.

Figure 161. Pulses of increased sulfur dioxide emissions at Pacaya were measured by the TROPOMI instrument on the Sentinel-5P satellite multiple times during March and April 2021, including (top row, left to right) on 5, 10, and 21 March, and (bottom row) 6, 8, and 16 April. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Activity during March 2021. A notable increase in seismicity early on 1 March 2021 coincided with increased Strombolian activity. Observatorio Volcán de Pacaya geologists observed explosions sending ejecta 500 m above the rim of Mackenney crater accompanied by plumes of ash and gas that reached 3.5 km altitude and drifted W and SW. For most of March high levels of Strombolian activity sent ejecta 200-400 m high each day, sometimes higher, reaching 800 m on 3 March, 800-1,000 m on 5 March, and 700 m on 10 March (figure 162). Sounds as loud as a train locomotive or plane engine from the explosions were frequently reported, and ejecta was sometimes scattered 500-600 m from the cone. Explosive activity with ejecta and ash emissions were also reported from the fissure feeding the lava flow on the S flank 300 m below Mackenney crater. On 14 March, ejecta from the fissure sent block avalanches 1,300 m down the S flank.

Figure 162. Strombolian activity at Pacaya sent ejecta hundreds of meters above the summit and down the flanks on 4 March 2021 while effusion continued on the SW flank, also producing an intense glow. Image by Reuters photographer Josue Decavele taken from Los Rios. Courtesy of Reuters Pictures.

The increase in explosive activity also included an increase in dense ash emissions and resulting ashfall during March 2021. Ash plume heights ranged from 3 to 5.5 km altitude, and often drifted W, NW, or SW. The Washington VAAC reported an ash plume centered about 75 km WSW of the summit on 1 March. On 3 March a dense ash emission was drifting W from the summit at 3.7 km altitude. The next day ash was detected almost 100 km SW just off the Mexican Pacific coast before dissipating. The altitude of the ash emissions increased to 4.9 km on 5 March; puffs drifting W were visible in satellite images extending over 250 km from the summit the next day. Pulses of activity lasted between 15 minutes and 13 hours, and produced tephra fallout around the volcano, dense ash plumes that drifted 3-5 km, and finer ash plumes that drifted more than 60 km.

Explosions on 7 March caused lava fountains 100-500 m above the crater. The following day ash plumes were drifting 45 km SW at 3 km altitude. On 9 March ash plumes fanned out from the NW to the SW about 30 km from the summit before dissipating. From 11 March onward multiple daily discrete ash emissions extended at least 30-50 km WNW and SW from the summit at altitudes of 3.7-4.3 km altitude, and much farther on some days. The plumes reached 90 km WSW on 12 March, and 140 km W on 14 March. The next day, ash emissions extended over 100 km WSW, with remnants visible in satellite images almost 185 km away by the end of the day. On 16 March they drifted 170 km WNW at 4.3 km altitude and on 18 March the ash emissions were observed drifting SW at 3.4 km altitude extending 185 km from the summit. Dense gray-black emissions were accompanied by white steam emissions on 21 March (figure 163).

Figure 163. Dense dark gray ash emissions rose from the summit of Pacaya on 21 March 2021 causing significant ashfall around the region. In addition, white steam plumes surrounded the summit. Courtesy of CONRED.

Dense ash clouds seen on 22 March 2021 were drifting rapidly SSE at 4.9 km altitude as far as 75 km, SE at 6.1 km altitude, and visible in satellite imagery moving E at 7.6 km altitude up to 25 km from the volcano. The next day they were drifting NE at 3 km altitude up to 90 km away, and SW at 4.6 km altitude. A narrow ash plume was detected in visible satellite imagery on 28 March drifting about 80 km NW of the summit before dissipating. Over the next two days a plume was detected moving SW at 3 km altitude about 130-150 km from the summit. In addition, another plume was drifting NW at 4.3 km altitude on 31 March causing dense ash to cover the summit of Fuego that was visible on webcams. The lower plume was visible over 300 km SW of Pacaya before it dissipated (figure 164).

Figure 164. Haze from ash emissions at Pacaya extends for tens of kilometers across the region in multiple directions after many days of emissions, while a fresh ash plume rises above the volcano in the left foreground on 31 March 2021. Ash drifted NW up to 50 km and was reported in Sacatepéquez and Chimaltenango. In the middle right to the NW is the large Agua volcano, and behind it to the right are Fuego and Acatenango. Ash from Pacaya was visible in Fuego webcams that day. Courtesy of INSIVUMEH.

Communities all around Pacaya were affected by ashfall many times throughout March 2021 (figure 165, table 7). Most of the communities were within 10 km of the summit, but ashfall reached more than 20 km away multiple times. During the bigger ashfall events, blocks more than 6 cm in diameter fell on the flanks of the volcano, while lapilli (2 mm to 6 cm) fell up to 5 km away, and fine ash was observed up to 30 km away (figure 166). The most significant ashfall events occurred during 22-23 March when ash drifted tens of kilometers in multiple directions and caused the closure of La Aurora International Airport in Guatemala City (figure 167).

Figure 165. Communities all around Pacaya were affected by ashfall throughout March 2021. The red oval was the area where INSIVUMEH cautioned residents to be prepared for ashfall and lapilli after explosions on 3 March. All of the communities shown by yellow stars were affected by ashfall at some point during March. Courtesy of INSIVUMEH (Boletin Volcanologico especial BEPAC-41-2021, Eruption, Volcan Pacaya, 3 de marzo 2021, 11:55 horas).

Table 7. Communities reporting ashfall from Pacaya during March 2021. Information courtesy of INSIVUMEH.

Figure 166. Lapilli-size tephra (2 mm to 6 cm) from Pacaya was reported several times during March 2021 in communities as far as 5 km away, including this example on 16 March 2021 from Concepcion El Cedro (4 km NNW). Courtesy of INSIVUMEH (BOLETIN VULCANOLOGICO ESPECIAL BEPAC-56-2021, ACTUALIZACION DE ACTIVIDAD Y CAIDA DE TEFRA, 16 de marzo 2021, 09:05 horas).

Figure 167. A plane at the La Aurora international airport in Guatemala City was dusted with ash from Pacaya on 23 March 2021, forcing closure of the airport for much of the day. Photo by Moises Castillo/AP, courtesy of CNN.

Lava flow activity also increased significantly during March 2021. At the end of February, an active flow on the S flank remained about 1 km long, shedding incandescent blocks hundreds of meters from its advancing front. By 3 March, three flows with multiple branches were active on the SSW flank; they were 800-1,000 m long (figure 168). On 5 and 6 March two flows with many branches extended 300-500 m down the S flank (figure 169). Flows were active on the SW, S, and SE flanks on 7 March. The S-flank-flow with two branches reached 1 km long by 8 March and had incandescent blocks constantly falling of the leading edge. It increased steadily in length, reaching 1.8 km by 16 March (figure 170).

Figure 168. Three flows were active on the S and SW flanks of Pacaya on 3 March 2021, seen here with an infrared camera. Courtesy of INSIVUMEH.

Figure 169. On 5 March 2021 two main flows with multiple branches extended 300-500 m down the S flank of Pacaya causing very bright thermal signatures in satellite imagery. Sentinel-2 image uses Atmospheric penetration rendering (bands 12, 11 and 8a). Courtesy of Sentinel Hub Playground.

Figure 170. Two branches of the S-flank lava flow at Pacaya were each about 1.4 km long on 12 March 2021. Courtesy of INSIVUMEH (FOTOGRAFÍAS RECIENTES DE VOLCANES).

Two new flows emerged from the S and SE flanks on the morning of 18 March (figure 171). The S-flank flow grew to 500 m and part of it overflowed outside the plateau. The SE-flank flow was 400 m long in front of the village of Los Llanos, causing fires in the vegetation which continued for several days (figure 172). On 20 March the SE flank flow caused a strong thermal signature in satellite imagery with incandescent blocks falling downslope far beyond the front (figure 173). During the night of 20-21 March, a new flow appeared on the SW flank and grew to 500 m long; the flow on the SE flank reached 850 m. The following day the rapidly growing SW-flank-flow reached 1,500 m long, causing vegetation fires on ranches in Las Granadillas.

Figure 171. Two new flows emerged from the flanks of Pacaya on 18 March 2021 as seen in this FLIR thermal webcam image. The S-flank flow (center) grew to 500 m with two active branches. The SE flank flow (right) descended 400 m near the village of Los Llanos and burned vegetation. A third fissure higher on the SW flank (upper left) also had a short active flow. Courtesy of INSIVUMEH (Boletin Vulcanologico Especial BEPAC 58-2021, 18 March 2021).

Figure 172. Burning vegetation from a lava flow on Pacaya’s SE flank was controlled by CONRED workers on 20 March 2021. It was burning at the Los Llanos farmhouse, Finca el Muñeco, Villa Canales. Photo by Sergio Girón, courtesy of CONRED.

Figure 173. On 20 March 2021, a flow on the SE flank of Pacaya was about 400 m long with incandescent blocks falling several hundred meters downslope to the SE and causing fires in the vegetation. A strong thermal signature was also present from explosive activity inside Mackenney crater (top). Sentinel-2 image uses Atmospheric penetration rendering (bands 12, 11 and 8a). Courtesy of Sentinel Hub Playground.

Three flows were active on 22 March 2021, with existing flows on the SW (1,500 m) and SE flanks (300 m), and a new flow on the E (500 m) flank. By 25 March activity was focused on the SW-flank flow which had reached 2.5 km in length (figure 174). It was about 400 m wide and 2.5 m high, burning vegetation as it advanced, and causing damage on coffee and avocado plantations. By 31 March the flow exceeded 3 km in length with multiple active fronts. One of the flow fronts near the community of La Breña was still advancing, but the one at the Campo Alegre farm had stopped moving. The flow continued to cause fires, destroy crops and buildings, and block roads (figure 175).

Figure 174. A large flow on Pacaya’s SW flank had reached 2.5 km long by 25 March 2021 (left) and over 3 km long 5 days later on 30 March (right). It flowed W from a fissure on the W flank, then NW around a higher area before continuing SSW. The flow caused fires, destroyed crops and buildings, and blocked roads. Sentinel-2 images use Atmospheric rendering (bands 12, 11 and 8a). Courtesy of Sentinel Hub Playground.

Figure 175. The large lava flow on Pacaya’s SW flank had traveled over 2.5 km by 27 March 2021 when this wide-angle drone image was taken. One of the fronts of the flow was near the community of La Breña and the other was near the Campo Alegre farm. Courtesy of CONRED.

Activity during April 2021. On 1 April 2021 remnant plumes from earlier ash emissions were moving SW over the Pacific about 400 km from the summit at 4.3 km altitude, while newer emissions were drifting S at 3.4 km altitude towards the coast. Continuous ash emissions were reported by the Washington VAAC through April 4 (figure 176) drifting tens of kilometers mostly SW at 3.5-4.5 km altitude. Ash drifted up to 20 km S and SW during the first week and caused frequent ashfall in communities on the SE, S, and SW flanks, with the most affected being Los Pocitos, El Rodeo, and El Patrocinio. A few moderate to strong explosions sent ejecta 100-500 m above the Mackenney crater. By 9 April ash emissions were more sporadic and tended to drift only 5-10 km SW, W, and NW, and no ashfall was reported. The VAAC reported occasional emissions observed in the webcam on 8 and 14 April. An ash plume was detected on 16 April moving NNW at 3.4 km altitude. Strombolian activity diminished and activity changed to primarily steam and gas plumes rising 200 m above the crater after this. A short episode of sporadic explosions during 24-29 April sent ejecta to 250 m above the crater, generated loud noises, and produced ash emissions that rose a few hundred meters and drifted several kilometers.

Figure 176. Daily explosions at Pacaya produced dense ash emissions rising to 3.5-4.5 km altitude during the first part of April, including on 2 April 2021 when the ash drifted S and SE. Multiple branches of the lava flow on the SW flank were also burning vegetation near Las Granadillas and Buena Vista (smoke plumes in the foreground). Courtesy of CONRED.

The SW-flank flow that began during 20-21 March remained active into early April and was 2.8-3 km long during the first week. It continued to advance during the second week and reached 3.7 km long with multiple active branches that were burning vegetation (figure 177). During 7-11 April it was advancing W and N in the area of La Breña and W and S in the area of El Patrocinio and El Rodeo on the Campo Alegre farm (figure 178). By 10 April this flow was 400 m from El Patrocinio and 250 m from San José El Rodeo. By 13 April it was burning avocado and coffee plantations 370 m from houses in El Patrocinio (figure 179). Another active front to the south was 250 m E of El Rodeo and had blocked the road between El Rodeo, El Caracol, and Los Pocitos. The seismic activity associated with the lava effusion decreased significantly beginning on 16 April.

Figure 177. Lava from Pacaya’s SW-flank flow was 300 m wide and extended more than 3 km by 7 April 2021; it was burning vegetation in its path as it advanced at about 5 meters per hour. Courtesy of CONRED.

Figure 178. The SW-flank flow at Pacaya continued to advance during the first half of April 2021 as seen here on 4 (left) and 9 (right) April. The communities of La Breña, El Patrocinio, and El Rodeo were the most affected. Sentinel-2 images use Atmospheric rendering (bands 12, 11 and 8a). Courtesy of Sentinel Hub Playground.

Figure 179. By 14 April 2021 the SW-flank flow at Pacaya was 3.7 km long and several hundred meters wide. It had multiple active branches that came within a few hundred meters of the communities of El Patrocinio and San José El Rodeo and had burned significant acreage on coffee and avocado plantations. It also blocked the road between El Rodeo, El Caracol, and Los Pocitos. Courtesy of CONRED.

During 18-20 April 2021 the branch near La Breña stopped advancing, and by 21 April the branch near El Patrocinio had stopped (figure 180), although temperatures remained high and gas emissions from vents along the flow continued in many places through the end of April. A lava flow appeared on the SE flank on 27 April, following a few days of renewed explosive activity, and grew to 175 m by 29 April. INSIVUMEH reported another new flow on the N flank on 29 April (figure 181); it advanced rapidly to the NW around Cerro Chino, and then turned towards the SW, reaching 1.6 km long by later in the day when the leading edge was located about 100 m from La Breña with several active flow fronts.

Figure 180. The lava flow on the SW flank of Pacaya stopped advancing a few hundred meters before reaching El Patrocinio in San Vicente Pacaya, home to about 350 people, on 21 April 2021. Photo by Moises Castillo/Associated Press, courtesy of KTLA.

Figure 181. A lava flow emerged on the N flank of Pacaya on 29 April 2021 and advanced rapidly NW around Cerro Chino and then SW towards La Breña, reaching 1.6 km long by the end of the day. Courtesy of Colred Los Llanos.

Activity during May 2021. Sporadic emissions of steam and gas with occasional ash were typical from Mackenney crater at the beginning of May 2021. Possible ash emissions were seen in satellite data on 1 May drifting W at 3.4 km altitude. Dense plumes, some with abundant ash, were reported on 8 May drifting W and S. Strombolian activity on 10 May from the NW-flank fissure was feeding the flow which began on 29 April; it sent ejecta 50-150 m high, and loud noises were heard. The Washington VAAC reported minor amounts of ash observed in satellite images moving SW from the summit during 10-13 May, when intermittent pulses of dense ash were reported drifting W and SW from the crater. Intermittent ash emissions rose to 3.7 km altitude on 14 May and were observed about 100 km SW before dissipating. Ash plumes drifted up to 5 km W on 15 and 16 May, causing ashfall during 16 and 17 May in El Patrocinio and El Rodeo (figure 182). During 18-21 May constant steam and gas, and periodic ash, emissions drifted 5-10 km NW and W at about 3 km altitude with ashfall reported in communities such as San Francisco de Sales, Concepción El Cedro, Aldea El Patrocinio, and San Miguel Petapa. For the remainder of May, small quantities of ash accompanied dense steam and gas emissions that rose 200-700 m above the summit and drifted W, SW, and S up to 5 km. El Patrocinio, El Rodeo, and other fincas in that area within 10 km reported ashfall on 26 May.

Figure 182. Pulses of dense ash emissions from the summit of Pacaya were noted on 16 May 2021 by a team of volcanologists from Boise State and Michigan Tech Universities. Steam and gas from still-hot lava flows rose from the flanks. Courtesy of Geo_Sci_Jerry.

The N-flank flow that began on 29 April 2021 continued to advance into early May. It had originally flowed NW, then curved around Cerro Chino and headed W. It was 2 km long and advancing in the vicinity of La Breña on 3 May. On 5 May incandescent ejecta was observed at the fissure feeding the flow, which had advanced to the S of La Breña where incandescent blocks continued to fall off the front of the advancing flow. On 6 May the flow reached 2.3 km in length on the W flank, with only one of the fronts continuing to advance slowly. Small explosions were reported at the fissure. The lava flow continued to advance laterally in places as incandescent material spilled over the edges. Explosions from the fissure on 9 May threw material 15 m away as the flow continued moving slowly W (figure 183). By 11 May the flow was no longer advancing at its front but was still expanding due to overflows along its edges. Explosions from the fissure on 14 May launched ejecta 40 m (figure 184), and the flow front again moved slowly westward; by then it was about 2.3 km long (figure 185). Activity at the fissure ceased by 17 May.

Figure 183. Strombolian explosions at the fissure feeding the W-flank lava flow at Pacaya were visible on the night of 8 May 2021. Although the lava flowed rapidly, it didn’t advance significantly after the first week of May; instead the lava flowed laterally and spread out over the flanks in several places until activity at the fissure ceased on 17 May. Copyrighted photo by David Rojas, used with permission.

Figure 184. The fissure on the NW flank of Pacaya was still active on 14 May 2021. Explosions produced ash and ejecta that rose 40 m above the fissure. Courtesy of CONRED.

Figure 185. The flow on the NW flank of Pacaya was also still active on 14 May 2021. It was over 2 km long and still actively flowing but no longer advancing. Sentinel-2 image uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Geologic Background. Eruptions from Pacaya, one of Guatemala's most active volcanoes, are frequently visible from Guatemala City, the nation's capital. This complex basaltic volcano was constructed just outside the southern topographic rim of the 14 x 16 km Pleistocene Amatitlán caldera. A cluster of dacitic lava domes occupies the southern caldera floor. The post-caldera Pacaya massif includes the ancestral Pacaya Viejo and Cerro Grande stratovolcanoes and the currently active Mackenney stratovolcano. Collapse of Pacaya Viejo between 600 and 1500 years ago produced a debris-avalanche deposit that extends 25 km onto the Pacific coastal plain and left an arcuate somma rim inside which the modern Pacaya volcano (Mackenney cone) grew. A subsidiary crater, Cerro Chino, was constructed on the NW somma rim and was last active in the 19th century. During the past several decades, activity has consisted of frequent strombolian eruptions with intermittent lava flow extrusion that has partially filled in the caldera moat and armored the flanks of Mackenney cone, punctuated by occasional larger explosive eruptions that partially destroy the summit of the growing young stratovolcano.

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/ ); Coordinadora Nacional para la Reducción de Desastres (CONRED), Av. Hincapié 21-72, Zona 13, Guatemala City, Guatemala (URL: http://conred.gob.gt/www/index.php, https://twitter.com/ConredGuatemala/status/1393207685756203011); Colred Los Llanos, Coordinadora local para la reduccion de desastres, Los Llanos, Villa Canales (URL: https://www.facebook.com/Colred-Los-Llanos-102105058094847); 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/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Reuters Pictures (URL: https://twitter.com/reuterspictures/status/1367472450418704387); CNN (URL: https://www.cnn.com/2021/03/23/americas/guatemala-airport-volcano-closure-latam-intl/index.html); KTLA, (URL: https://ktla.com/news/nationworld/lava-from-guatemalas-pacaya-volcano-threatens-small-communities-that-live-nearby/); David Rojas, (URL: https://twitter.com/DavidRojasGt/status/1391592159221063680); Geo_Sci_Jerry (URL: https://twitter.com/SciJerry/status/1394083192773222406).

Etna is located on the island of Sicily, Italy, and has had eruptions that date back 3,500 years. Its most recent eruptive period began in September 2013 and more recently has been characterized by frequent Strombolian explosions, effusive activity, and ash emissions. Activity has commonly originated from the summit areas, including the Northeast Crater (NEC), the Voragine-Bocca Nuova (or Central) complex (VOR-BN), the Southeast Crater (SEC, formed in 1978), and the New Southeast Crater (NSEC, formed in 2011). Another crater, referred to as the "cono della sella" (saddle cone), developed during early 2017 in the area between SEC and NSEC. This report covers activity from December 2020 through March 2021, consisting of frequent Strombolian explosions of variable intensity, effusive activity, ash emissions, and ashfall. Information for this report comes from weekly and special reports by the Osservatorio Etneo (OE), part of the Catania Branch of Italy's Istituo Nazionale di Geofisica e Vulcanologica (INGV).

Summary of activity during December 2020-March 2021. Intra-crater Strombolian explosions that varied in frequency and intensity throughout the reporting period, and the accompanying ash plumes that rose to a maximum altitude of 11 km, primarily originated from the Southeast Crater (SEC), Voragine Crater (VOR), and occasionally the Northeast Crater (NEC) and Bocca Nuova Crater (BN). Beginning in mid-February a series of short lava fountaining events occurred in the SEC that continued through March. These episodes were also characterized by accompanying ash plumes, incandescent ejecta, and lava flows.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows strong and frequent thermal anomalies throughout the reporting period (figure 319). Some of these anomalies were markedly high in mid-December, mid-January, and mid-March. According to the MODVOLC thermal algorithm, a total of 190 alerts were detected in the summit craters during December through March; thermal anomalies were reported for nine days in December, eleven days in January, fifteen days in February, and sixteen days in March. Frequent Strombolian activity contributed to distinct SO₂ plumes that drifted in multiple directions (figure 320).

Figure 319. Strong and frequent thermal anomalies at Etna were detected during December 2020 through March 2021, as reflected in the MIROVA data (Log Radiative Power). Some thermal anomalies were significantly high in mid-December, mid-January, and mid-March. Courtesy of MIROVA.

Figure 320. Distinct SO2 plumes from Etna were detected by the TROPOMI instrument on the Sentinel-5P satellite on multiple days during December 2020 to March 2021 due to frequent Strombolian explosions, including 22 December (top left) 2020, 20 January (top right), 21 February (bottom left), and 7 March (bottom right) 2021. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Activity during December 2020. During December, INGV reported intra-crater Strombolian explosions in the SEC, NEC, and BN. During the more intense SEC explosions material was ejected onto the flanks. Gas-and-steam emissions were reported in the VOR. A field survey on 13-14 December showed that the SEC was an irregular ellipse 150 x 230 m open to the SW. On 13 December Strombolian activity intensified at 2020 and around 2300 evolved to lava fountains which lasted through 2350, though explosions continued. Collapses of the SW part of the SEC at 2315 resulted in pyroclastic flows that traveled up to 2 km, covering the Monte Frumento Supino cone (SSW flank). Around that time two fissures opened on the SW flank of the SEC and produced lava flows until about 2350 (figure 321). A third minor pyroclastic flow went down the SSW flank at 2330. Two lava fountains were seen during 0050-0110 on 14 December.

Figure 321. Photos of Strombolian activity at the Southeast Crater at Etna on the evening of 14 December 2020 (left) seen from Tremestieri Etneo (20 km S) and a thermal image showing the Bocca Nuova, Voragine, and three active vents in the SEC seen from the Montagnola (EMOV) thermal camera at 0949 (UTC) on 15 December (right). Courtesy of INGV (Report 52/2020, ETNA, Bollettino Settimanale, 14/12/2020 – 20/12/2020, data emissione 22/12/2020).

During a field inspection on 14 December scientists noted that the two lava flows on the S and SW flanks were cooling; the S flow had widened near the base of the SEC and formed four main lobes, one of which had stopped just NW of the cones that formed in 2002-2003 (figure 322). The SW flow traveled SSW, branched, curved around the W part of Monte Frumento Supino, and then stopped. An explosion in the easternmost SEC vent generated an ash plume at 1352 that rose to 4 km altitude and drifted S. Additionally, on 14 December sporadic ash explosions resumed in the VOR; incandescent ejecta was visible at night. On 15 December a new lava flow formed on the SW flank of the SEC at 0924 that advanced a few hundred meters. Eruptive activity briefly stopped in the E vent of the SEC during the afternoon of the 15th and during 16-18 December exhibited strong degassing and nighttime incandescence.

Figure 322. Map of the Southeastern Crater (SEC) at Etna showing active lava flows and the cono della sella (red dot). The light green hatch mark represents the location of the eruptive fissure that opened on the SEC flank. The lava flow extended about 2 km SW, and by 14 December had formed four main lobes. The black arrow represents the direction of the pyroclastic flow after the collapse of the SW portion of the SEC cone. This map uses ground observations and thermal image analysis on a PlanetScope satellite image. Courtesy of INGV (Report 51/2020, ETNA, Bollettino Settimanale, 07/12/2020 – 13/12/2020, data emissione 15/12/2020).

Seismic tremor amplitude gradually increased on 20 December, though weather conditions prevented observations. On 21 December at 1008 Strombolian activity increased in the SEC from the central and easternmost vents. Activity evolved to lava fountaining that lasted an hour, as well as an ash plume that rose to 10 km altitude and drifted NE. An active lava flow was still visible on the SW part of the cone which had collapsed on 13 December. A second flow was observed at 1521 on the S slope of the SEC that descended toward the Valle del Bove. Activity continued through the night (figure 323), and on 22 December at 0350 Strombolian activity increased in the central and easternmost vents; around 0415 a lava flow from the SW flank traveled W, overlapping cooling lava from 21 December. Lava fountaining began again at 0519 and fed three lava flows: one from the S flank traveled SW for 2.8 km and was 600 m wide, branching off to the W and E of Monte Frumento Supino, a second that traveled 2.8 km E toward the Valle del Bove, and a third that originated at the E vent of the SEC that traveled 1.3 km ENE toward the Valle del Leone (figure 324). At 0520 a few small phreatic explosions in the Valle del Bove were due to the lava flow interacting with snow. By 0600 the lava fountains gradually subsided and stopped, though Strombolian explosions persisted at varying intensities. On 24 December at 0830 explosive activity in the E vent of the SEC gradually increased, ejecting material above the crater rim and emitting ash that drifted E.

Figure 323. Photo of Etna’s Southeast Crater showing a new episode of lava fountaining during the early morning on 22 December 2020 viewed from Tremestieri Etneo, south of the volcano. Photo by Boris Behncke, INGV.

Figure 324. Thermal webcam images showing (top left) Strombolian explosions and (top right) lava fountaining in the Southeast Crater seen from the Nicolosi (ENT) and Montagnola (EMOT) cameras on 22 December 2020. Lava flows were visible traveling toward the Valle del Bove and Valle del Leone seen from the Monte Cagliato (EMCT) (bottom left) and Schiena della’asino (ESR) (bottom right). Courtesy of INGV (Report 53/2020, ETNA, Bollettino Settimanale, 21/12/2020 – 27/12/2020, data emissione 29/12/2020).

On 29 December at 0750 there was a gradual increase in explosive activity in the E vent of the SEC, producing ash emissions that drifted ENE. Around 0900 Strombolian activity further intensified, ejecting coarse material onto the E flank of SEC (figure 325), but by 1000 the explosions had decreased. Intra-crater Strombolian activity in the NEC, VOR, and BN continued with sporadic ash emissions through the rest of the month; explosions in the VOR intensified, ejecting material above the crater rim.

Figure 325. Photos of the Strombolian activity at the Voragine (left background) and Southeast Crater (right foreground) at Etna on the evening of 28 December (left) and ash emissions rising from the SEC on the morning of 29 December (right) 2020. Photos by Boris Behncke, INGV.

Activity during January 2021. Activity in January continued with intra-crater Strombolian explosions of variable intensity in the SEC, NEC, VOR, and BN with sporadic ash emissions. On 4 and 6 January at least two episodes of intense Strombolian explosions produced continuous ash plumes that drifted E and ejected coarse pyroclastic material. A lava flow on 17 January breached the SEC at 0740 and traveled to the base of the cone toward the Valle del Bove (figure 326); the lava effusion rate increased at 0819, and the flow reached an elevation of 3 km by 1000. Volcanic tremor amplitude and Strombolian activity intensified at 2000 on 18 January, evolving into lava fountains through 2130. A lava flow emerged in the E vent of the SEC at 2015 and moved 2 km ESE toward the Valle del Bove. Lava fountaining produced a plume that drifted SE, resulting in ashfall in Fleri and Acicastello (figure 327). During 2130-2147 a second lava flow on the N side of the SEC reached a length of 1.3 km. By 19 January the explosions decreased in intensity and the lava flows had begun to cool. On 20 January a new lava flow on the N side of the SEC traveled ENE at 0140, overlapping the previous flow on the 18th; by 1830 it was no longer active. The VOR was characterized by almost continuous Strombolian explosions that ejected material above the crater rim. Satellite imagery from 27 January showed that a small lava flow from a vent in the N section of the VOR was pouring into the BN. The BN also produced Strombolian explosions that often ejected material above the crater rim. At night, summit crater incandescence was observed in the NEC.

Figure 326. Map of the summit craters of Etna showing the active vents and lava flow field on 18 January 2021. The base is modified from a 2014 DEM created by Laboratorio di Aerogeofisica-Sezione Roma 2. The hatch marks indicate the crater rims: BN = Bocca Nuova; VOR = Voragine; NEC = North East Crater; SEC = South East Crater. Red circles indicate areas with ash emissions and/or Strombolian activity. Yellow circles indicate steam and/or gas emissions only. The red shape highlights the active lava flow on 18 January and the yellow and orange shape highlights the cooling lava flow from 17 January. Courtesy of INGV (Report 04/2021, ETNA, Bollettino Settimanale, 18/01/2021 – 26/01/2021, data emissione 26/01/2021).

Figure 327. Photos of lava fountains and an ash plume in the SEC at Etna that resulted in ashfall on the SE flank (top) as well as in Fleri (bottom left) and Acicastello (bottom right). Photo a was taken from Tremestieri on the S side of the volcano. Courtesy of INGV (Report 04/2021, ETNA, Bollettino Settimanale, 18/01/2021 – 26/01/2021, data emissione 26/01/2021).

Activity during February 2021. Variable Strombolian activity continued into February at all four summit craters; the last time this occurred was during 1998-1999. The most intense, almost continuous, Strombolian explosions at the SEC originated from two vents in the eastern top of the cone; less intense activity occurred at the S vent. Intra-crater Strombolian activity at the NEC sometimes produced nighttime incandescence. Explosions at the BN sometimes ejected coarse material above the crater rim. A field inspection on 5 February showed that three scoria cones had been built around vents at the bottom of the crater. Another nearby cone occasionally produced dense emissions. Intra-crater lava flows continued to spill into the BN from the VOR, overlapping those formed in late January. On 6 February around 0530 Strombolian activity intensified in the E vent of the SEC and produced an ash plume that drifted E.

During the morning of 15 February explosive activity at the SEC intensified, with activity continuing at the E vents. Sporadic and sometimes violent explosions were also observed at the saddle cone; intra-crater explosive activity continued in the BN, VOR, and NEC. On 16 February at 1700 lava began advancing down the E flank of the SEC for a few kilometers. A partial cone collapse at 1705 produced a pyroclastic flow that traveled 1.5 km along the W wall of the Valle del Bove. The activity changed to lava fountains around 1710, rising 500-600 m high and generating an ash plume that rose to 6-10 km altitude and drifted S (figure 328). Centimeter-sized lapilli and ash was observed in Nicolosi (16 km S), Mascalucia (19 km S), and as far as Catania (29 km SSE) while fine ashfall was reported in Syracuse (60-80 km SSE). Lava flows continued to advance into the Valle del Bove, reaching an elevation of 2 km by 1759. Another lava flow from the SEC traveled N toward the Valle del Leone; smaller lava flows traveled N and S, reaching 2.9 km elevation. Explosive activity decreased and lava fountaining stopped between 1800 and 1838, though ashfall continued; by 2025 the lava flows had stopped. Strombolian activity persisted at the SEC overnight during 16-17 February and stopped by 0715 on 17 February, though sporadic explosions were reported in the VOR at 0420, 0435, 0444.

Figure 328. Photos during 15-16 February of Strombolian activity at the summit craters at Etna on 15 February 2021 (top left); a pyroclastic flow that occurred at the beginning of the eruptive event on 16 February at 1805 (top right); an eruption plume that was a result from the eruptive event on 16 February, seen from the S (bottom left); map of the lava flows on 16 February showing the active vents (red dots), degassing vents (yellow dots), summit craters (black hatch marks), and direction of the lava flows (light green and dark green), as well as the maximum length (4 km) and volume (2.6 million cubic meters) (bottom right). Courtesy of INGV (Report 08/2021, ETNA, Bollettino Settimanale, 15/02/2021 – 21/02/2021, data emissione 23/02/2021).

An eruptive event began at 2330 on 17 February, about 30 hours after the previous one, with a lava flow from the E vents in the SEC, followed by lava fountaining at 0100 on the 18th that rose 600-700 m (figure 329). The lava flow advanced toward the Valle del Bove, the NE, SE, and SW through the saddle vent (“bocca della sella”), covering an area of about 1 km. A second flow on the N flank of the SEC moving toward the Valle del Leone was about 1 km long. Another flow was reported on the S side of the SEC. The resulting ash plume drifted SE, causing ashfall in Zafferana, Etna, and Acireale. The lava fountains ended between 0140-0155 on 18 February, though the lava flows continued to advance.

Figure 329. Activity at Etna during 17-18 February 2021 included lava flows and fountaining. The initial lava flow is seen in a thermal camera image just before midnight from Monte Cagliato on the E side of the volcano (top left). Lava fountains that rose 600-700 m high and lava flows are seen from Milos shortly after midnight (top right). An eruption plume seen from Milos at 0020 on 18 February was accompanied by nighttime incandescence, lava fountains, and lava flows (bottom left). A map of the lava flows on 17-18 February shows the active vents (red dots), degassing vents (yellow dots), summit craters (black hatch marks), and direction of the lava flows (yellow and green), as well as the maximum length (4.1 km) and volume (4 million cubic meters) of the flows (bottom right). Courtesy of INGV (Report 08/2021, ETNA, Bollettino Settimanale, 15/02/2021 – 21/02/2021, data emissione 23/02/2021).

During the morning of 19 February a lava flow effused from the E vents in the SEC at 0855, followed by a rapid increase in explosions and renewed lava fountaining (figure 330). A line of 4-5 vents produced “fan-shaped” lava fountains at 0953. An ash plume rose to 10 km altitude and drifted SE, causing ashfall in some towns. The lava flow that descended toward the Valle del Bove interacted with snow, causing strong explosions, and were accompanied by rockfalls on the flanks of the SEC. By 1110 the explosive activity had stopped.

Figure 330. Thermal images of the lava flow at Etna around 0900 (local) on 19 February 2021 taken with the thermal camera in Monte Cagliato (top left). Later lava fountains reached 600-700 m high, based on the thermal image from Monte Cagliato (top right). A strong ash plume was observed from Pisano (SE) (bottom left). A map of the lava flows on 19 February showing the active vents (red dots), degassing vents (yellow dots), summit craters (black hatch marks), and direction of the lava flows (orange and green), as well as the maximum length (3.8 km) and volume (4 million cubic meters) (bottom right). Courtesy of INGV (Report 08/2021, ETNA, Bollettino Settimanale, 15/02/2021 – 21/02/2021, data emissione 23/02/2021).

Weak Strombolian activity was visible in the late afternoon of 20 February (figure 331). At 2230 a small lava flow from the E vent in the SEC descended 150-200 m into the Valle del Bove. By 2300 the activity had changed to pulsating lava fountains. Beginning at 0100 on 21 February more western vents became active and the E vents ejected lava 600-800 m high. At 0128 lava fountains were ejecting lava up to 1 km high and were sustained for about 10 minutes (figure 331). At the same time, a lava flow from the saddle vent moved a few hundred meters SW. An ash plume rose to 10 km altitude, resulting in ashfall on the SW flank. At 0200 the lava fountains decreased in intensity and by 0220 explosive activity stopped. Periodic ash emissions rose from both the S and E vents later in the evening. A lava flow in the SEC advanced 1 km toward the Valle del Bove. Lava fountains and Strombolian explosions continued at multiple vents. Activity intensified again during 0218-0220 on the 22nd, with lava fountains over 1 km high sending incandescent material onto the flanks. Lava flows in the Valle del Bove reached 3.5-4 km from the crater. During 0430-0515 about 20 strong explosions from SEC vents ejected incandescent bombs that landed at the base of the cone. The NEC was characterized by strong degassing and crater incandescence, often accompanied by Strombolian activity.

Figure 331. Images of weak Strombolian activity in the eastern vents of the SEC at Etna at sunset on 20 February 2021 (top left). Thermal image from the Bronte thermal camera showing strong Strombolian activity at 0131 (local) on 21 February (top right). A strong ash plume at 0205 on 21 February was observed from Tremestieri Etneo (bottom left). A map of the lava flows during 20-21 February showing the active vents (red dots), degassing vents (yellow dots), summit craters (black hatch marks), and direction of the lava flows (red and green), as well as the maximum length (3.2 km) and volume (2.9 million cubic meters) (bottom right). Courtesy of INGV (Report 08/2021, ETNA, Bollettino Settimanale, 15/02/2021 – 21/02/2021, data emissione 23/02/2021).

During the evening on 22 February weak Strombolian explosions were visible in the SEC. The frequency and intensity of the explosions increased and by 2210 material was ejected onto the flanks. Jets of lava were ejected 300 m high at 2305, and by 2327 lava fountains were reported from a second SEC vent. Lava overflowed the crater at 2328 toward the Valle del Bove. Within the first hour of 23 February lava fountains rose more than 1.5 km and an ash plume reached 10 km altitude, causing ashfall to the NW. Lava overflowed the S vent and descended SW. At 0115 the lava fountains decreased. Strombolian activity intensified again at 0450, accompanied by ash emissions. Two lava flows traveled SW and SE, the latter of which reached 1.7-1.8 km elevation. By 1000 the lava flows were no longer active; the flow on the SW flank had traveled a few hundred meters, overlapping the previous flows.

The lava fountaining episodes continued; Strombolian activity at the two vents in the SEC increased during the late afternoon on 24 February that evolved into lava fountains reaching 400 m above the crater. Ash emissions also persisted in the SEC. Lava overflows from the crater headed ESE toward the Valle del Bove as far as 2-4 km and in the S area of the SEC. During 1900-2122 the lava fountains reached 500 m high and a resulting ash plume rose as high as 11 km altitude. A second lava flow traveled SW and at 2100 a pyroclastic flow descended 1 km into the Valle del Bove. The lava fountains in the SEC stopped by 2335, though the lava flow remained active in the SW and E sections.

Weak Strombolian activity on 28 February was visible at 0810 that evolved to lava fountains at 0839, feeding lava flows that traveled E toward the Valle del Bove. The fountains abruptly intensified at 0902 with jets of lava rising 700 m above the crater rim. An ash plume rose as high as 11 km altitude and drifted ESE, resulting in ashfall to the E (figure 332). A small lava flow at the S part of the SEC began at 0909, followed by a pyroclastic flow at 0920. The lava fountains ended abruptly at 0933, though the lava descending E remained active. By 1526 the lava flow in the Valle del Bove was no longer active.

Figure 332. Photo of a strong ash plume rising above Etna’s Southeast Crater on the morning of 28 February 2021 that drifted ESE, with ashfall visible. Taken from Tremestieri Etneo. Photo by Boris Behncke, INGV.

Activity during March 2021. Weak Strombolian activity resumed on 2 March at 1145 in the SEC, which increased in intensity at 1234 with ash emissions. From 1324 to 1550 lava fountains generated an ash plume 9 km above the crater, depositing ash and lapilli in Nicolosi, Aci San Antonio (18 km SE), Pedara (15 km SSE), and Catania (29 km SSE). On 4 March Strombolian explosions increased at 0200 and produced ash emissions that dispersed NE (figure 333). At the same time, Strombolian activity from VOR ejected material above the crater. Degassing persisted in the NEC. Around 0320 the Strombolian explosions in the SEC evolved to lava fountains and at 0515 a lava flow from the E section of the base of the cone was traveling toward the Valle del Bove. Strombolian activity in VOR changed to lava fountains at 0859 that were 300 m high. An ash plume rose 11 km above the crater, depositing ash and lapilli in Fiumefreddo (19 km ENE), Linguaglossa (17 km ESE), and the area of Reggio Calabria. Lava fountains continued.

Figure 333. Photos of the beginning an eruptive episode characterized by an early lava flow originating from Etna’s Southeast Crater (right foreground) and an explosion at the Voragine Crater (left background) on 4 March 2021 (left). Dense gray ash plumes and white degassing plumes were visible from several summit vents on 4 March (right). Taken from Tremestieri Etneo. Photos by Boris Behncke, INGV.

Another eruptive episode on 7 March starting between 0100 and 0200 included Strombolian explosions and minor lava effusions at the E base of the SEC that descended into the Valle del Bove. At 0430 an increase in Strombolian activity generated an ash plume that rose to 5 km altitude and drifted E. The lava reached an elevation of 2.8 km altitude by 0450. Strombolian activity intensified again at 0520 and the lava flow advanced to 2.7 km elevation. Lava fountains at 0720 generated another ash plume that rose to 10 km altitude and drifted E. INGV-OE personnel reported ash and lapilli deposits in Milo (11 km ESE), Fornazzo (10 km ESE), Trepunti (17 km ESE), Giarre (17 km ESE), Macchia di Giarre (16 km ESE), Mascali (18 km E), Riposto (19 km ESE), and Torre Archirafi (20 km ESE). Strombolian activity resumed at 1050 and was over by 1500.

Similar Strombolian activity in the SEC on 10 March changed to lava fountaining and a large eruption plume that rose to at least 9 km altitude and drifted ENE (figure 334). Ash and lapilli were reported in Mascali, Giarre, and Fiumefreddo. A lava flow from the S vent reached an elevation of 1.8 km. By 0430 on 10 March the lava fountaining had stopped, though sporadic ash emissions continued until 0700. On 12 March Strombolian activity in the SEC and accompanying ash emissions began again. As the activity intensified, lava overflowed the E part of the SEC, descending toward the Valle del Bove. Lava fountaining was observed up to 500 m and generating an ash plume that rose to 6 km altitude and drifted E. Within an hour, lava had advanced from an elevation of 2.8 km to 2 km. By 0939 the ash plume had risen to 9-10 km altitude and resulted in ashfall in Fleri, Milo, Fornazzo , Giarre, Santa Venerina (15 km SE), and Torre Archirafi (20 km ESE) (figure 335). Lava fountaining had stopped at 1050, though weak Strombolian activity and ash emissions persisted until 1115. The lava flow advanced as far as 1.7 km elevation while a second lava flow expanded on the W slope of the Valle del Bove for an average length of 3 km and a volume of roughly 1 million cubic meters. Strombolian activity continued in the NEC, BN, and VOR, producing minor ash emissions.

Figure 334. Photo of the nighttime lava fountaining activity at Etna during 9-10 March 2021. Courtesy of INGV Youtube channel.

Figure 335. Photo of an ash plume rising above Etna’s Southeast Crater on the morning of 12 March 2021. Taken from Tremestieri Etneo. Photo by Boris Behncke, INGV.

On 14 March Strombolian activity began at 2110 that evolved into lava fountaining at 0048 on the 15th (figure 336). Lava traveled toward the Valle del Bove as an ash plume drifted E (figure 337). By 0343 lava fountaining had stopped, though weak Strombolian activity and lava flows continued. On 17 March at 0155 weak Strombolian activity was observed, changing into lava fountaining at 0319. An ash plume drifted SE and a lava flow was moving toward the Valle del Bove, the latter of which overlapped the one from 15 March. Due to cloud cover, observations were limited and discontinuous. Fountaining activity stopped at 0717 and was followed by explosive activity. Weather conditions cleared the summit on 18 March at 2142, showing explosions in the SEC and a lava flow in the Valle del Bove. On 19 March at 0734 explosive activity was visible in the SEC, which intensified at 0915, accompanied by ash emissions. Lava fountaining started at 0935 with an accompanying ash plume that drifted ENE. By 1136 lava fountaining had stopped and changed to Strombolian activity, which gradually decreased. Only sporadic explosions were visible with minor ash emissions by 1350; lava flows along the Valle del Bove were reported in the late morning.

Figure 336. Photo of a lava fountain episode at Etna’s Southeast Crater during the night of 14-15 March 2021. Taken from Tremestieri Etneo. Photo by Boris Behncke, INGV.

Figure 337. A map of the lava flows on 15 March 2021 showing the active vents (red dots), degassing vents (yellow dots), summit craters (black hatch marks), and direction of the lava flows (blue), as well as the maximum length (2.7 km) and volume (1.1 million cubic meters) (bottom right). Courtesy of INGV (Report 12/2021, ETNA, Bollettino Settimanale, 15/03/2021 – 21/03/2021, data emissione 23/03/2021).

Though weather conditions often prevented a clear view of the summit, weak Strombolian activity was reported in the SEC at 2005 on 23 March, which had evolved into lava fountaining at 0330 on 24 March (figure 338). At 0335 a lava flow from the SEC was seen branching toward the Valle del Bove and the SE. A pyroclastic flow followed the lava at 0336, descending into the Valle del Bove. The lava fountains generated an ash plume that rose to 6-7 km altitude and drifted SSE, resulting in ashfall on the S slope and in Catania. Lava fountaining gradually decreased at 0700 and by 0945, it had stopped; the lava flows continued to advance. Intra-crater Strombolian activity continued in the NEC, BN, and VOR, accompanied by sporadic weak ash emissions. After the fountains stopped, another ash plume was seen rising to 4.5 km altitude and drifting SE. At night, ashfall was reported in Milia and Trecastagni (16 km SE). The explosions had stopped by 1347. By 25 March the two active lava flows had stopped.

Figure 338. Photos of the lava fountain episode and incandescent Strombolian activity at Etna’s Southeast Crater during 23 (left) and 24 (right) March 2021. Taken from Tremestieri Etneo. Photos by Boris Behncke, INGV.

On 30 March weak Strombolian activity in the SEC resumed around 0607 with a single ash explosion that quickly dispersed near the summit (figure 339). Over the course of the day activity at the SEC gradually changed from degassing to continuous weak Strombolian activity at about 1830 from at least two active vents. This activity increased during the night, throwing lava above the crater rim accompanied by sporadic ash emissions. Several lava flows effused from the S base vent. The main part of the flow traveled toward the Valle del Bove with other smaller flows descending to the S and SW. Two other vents at the S base had opened by the evening, one of which ejected spatter a few tens of meters high. Throughout the night, periods of lava fountaining were detected while the main lava flow descended the W wall of the Valle del Bove. Strombolian activity intensified at 1850 and produced an ash plume that rose to 4 km altitude and drifted SSW. At 0000 there was a gradual transition from Strombolian activity to lava fountaining.

Figure 339. Photo of an ash plume rising from Etna’s Southeast Crater on the morning of 30 March 2021. Photo by Boris Behncke, INGV.

Geologic Background. Mount Etna, towering above Catania, Sicily's second largest city, has one of the world's longest documented records of historical volcanism, dating back to 1500 BCE. Historical lava flows of basaltic composition cover much of the surface of this massive volcano, whose edifice is the highest and most voluminous in Italy. The Mongibello stratovolcano, truncated by several small calderas, was constructed during the late Pleistocene and Holocene over an older shield volcano. The most prominent morphological feature of Etna is the Valle del Bove, a 5 x 10 km horseshoe-shaped caldera open to the east. Two styles of eruptive activity typically occur, sometimes simultaneously. Persistent explosive eruptions, sometimes with minor lava emissions, take place from one or more summit craters. Flank vents, typically with higher effusion rates, are less frequently active and originate from fissures that open progressively downward from near the summit (usually accompanied by Strombolian eruptions at the upper end). Cinder cones are commonly constructed over the vents of lower-flank lava flows. Lava flows extend to the foot of the volcano on all sides and have reached the sea over a broad area on the SE flank.

Information Contacts: Sezione di Catania - Osservatorio Etneo, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/it/ ); Boris Behncke, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy; 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/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/).

Guatemala's Volcán de Fuego has been erupting vigorously since 2002; reported eruptions date back to 1531. These eruptions have resulted in major ashfalls, pyroclastic flows, lava flows, and damaging lahars, including a series of explosions and pyroclastic flows in early June 2018 that caused several hundred fatalities. Activity consisting of explosions with ash emissions, block avalanches, and lava flows has continued since 2018; activity during December 2020-March 2021 is covered in this report. Daily reports are provided by the Instituto Nacional de Sismologia, Vulcanología, Meteorología e Hidrologia (INSIVUMEH); aviation alerts of ash plumes are issued by the Washington Volcanic Ash Advisory Center (VAAC). Satellite data provide valuable information about thermal anomalies and ash emissions.

The many hourly explosions at Fuego throughout December 2020-March 2021 produced vibrations that rattled roofs and windows in the communities around the volcano every day, sometimes heard and felt as far as 20 km away. The explosions produced incandescent block avalanches that descended the flank ravines (barrancas), with a few of the blocks traveling as far as the vegetation near the bottom. The Seca, Ceniza, and Taniluya ravines were most often affected, but blocks were also reported many times in the Trinidad, Santa Teresa, El Jute, Las Lajas, and Honda ravines. Incandescent ejecta could be seen rising 100-300 m above the summit on most nights. Ash plumes rose to 4.4-4.8 km altitude every day and usually drifted W and SW; the Washington VAAC issued 2-5 ash advisories daily. Ashfall was a near-daily occurrence throughout the period. Effusive activity from 13-15 February produced two lava flows; a series of pyroclastic flows on 14 February affected the Ceniza canyon. For several days after the effusive activity, strong explosions caused ashfall in communities up to 50 km away. The MIROVA graph of thermal anomalies showed persistent high heat levels throughout the period with a brief spike to higher levels during mid-February when the lava flows were active (figure 141). MODVOLC thermal alerts were issued on multiple days each month including eight days in December 2020, 11 days in January 2021, 12 days in February, and seven days in March. Sentinel-2 satellite data showed thermal anomalies inside the summit crater five or six times each month, in all available non-cloudy images.

Figure 141. Consistently high levels of thermal anomalies continued at Fuego during July 2020-March 2021. A brief spike in mid-February 2021 corresponded to two lava flows and a pyroclastic flow. Courtesy of MIROVA.

Explosive activity continued at Fuego during December 2020. Seven to eleven explosions per hour were typical; a few days had 10-15 explosions per hour. Gas and ash emissions rose to 4.4-4.8 km every day with ash plumes drifting usually W and SW 10-15 km, occasionally to 20-25 km (figure 142). Plumes drifted over 10 km N and NE on 6 December, 20-25 km S and SW on 13 and 14 December, and 30 km E, SE, and N during 28-31 December. Vibrations were heard and felt up to 15 km away on the W and SW flanks on 7 December. Ashfall was reported almost daily in multiple communities including Panimache I and II, Morelia, Santa Sofia, Los Yucales, Sangre de Cristo, Finca Palo Verde, and San Pedro Yepocapa. In addition, ashfall was reported on 10 December in Ojo de Agua and Santa Isabel, on 14 Dec in Ojo de Agua and Santa Emilia, in Santa Emilia on 20 and 21 December, and in Chimaltenango to the N on 31 December.

Figure 142. An ash plume rose from the summit of Fuego early on 9 December 2020 while blocks descended multiple ravines and resuspended ash on the flanks. Photo by Fredy Arnoldo Esquit Chiquitá, 07:56 am hora local, courtesy of INSIVUMEH.

The ash plume drift direction continued to be N and NW on 1 and 2 January 2021 resulting in ashfall reported in San Pedro Yepocapa, La Soledad, and San Miguel Duenas. According to INSIVUMEH, plumes drifted 20-25 km those days. In addition to ashfall in Panimache I, Morelia, Santa Sofia, and Yucales most days of the month, ashfall was reported in La Rochela on 3 and 6 January and Ceilan on 6 January. Ashfall was reported to the N in Acatenango on 10 January after activity increased; rumbling was heard 20 km away. Explosions produced ejecta which rose 300 m and sent incandescent blocks around the crater rim and onto the upper flanks. High levels of activity continued the next day and produced ashfall in San Pedro Yepocapa, Santa Sophia, Morelia, Panimache II, El Porvenir Yepocapa, Sangre de Cristo, and at finca Palo Verde. Pulses of incandescent ejecta rising 100-300 m were common during the second half of January and ashfall continued on many days in the same communities to the W and SW. Remobilized ash triggered by incandescent blocks descending the ravines was reported in the last week of January. The number of explosions per hour was 6-12 on many days and they produced noises as loud as a train engine that lasted for several minutes at a time.

Explosive activity during February 2021 remained the same as previous months, with 7-15 explosions per hour, train engine noises that lasted for 3-10 minutes, and gas and ash plumes that rose usually to 4.5-4.8 km altitude and drifted W, SW, and S. Rumblings that rattled windows and roofs were heard 15-20 km away on 5 and 10 February; incandescent blocks descended the ravines for hundreds of meters (figure 143). Near-daily reports of ashfall in communities to the W, SW, and S continued; most affected were Panimache I, Morelia, Santa Sofia, Porvenir, Finca Asuncion, Rochela, Santa Sofia, Yucales, Sangre de Cristo, Palo Verde and Yepocapa. In addition Ceilan, El Zapote, and El Rodeo reported ashfall on 5 February when winds carried ash to S and SE.

Figure 143. Incandescent block avalanches could be seen descending a ravine on the NW flank of Fuego in Sentinel-2 satellite imagery on 3 February 2021. A diffuse ash plume drifts S from the summit. Image uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

In a special report issued on 13 February INSIVUMEH noted that the seismic station had registered a change in the eruptive pattern on 12 February. During the night a lava flow emerged from the summit and traveled 1,000 m down the Ceniza ravine on the SW flank (figure 144). It produced incandescent blocks at the leading edge that fell farther, reaching the vegetation. Loud noises similar to a train engine were audible 8 km from the volcano. At 2100 on 13 February a second flow began in the Seca ravine that grew to 500 m long. Incandescent ejecta rose 200 m above the crater and constant loud noises were reported. By this time the Ceniza flow had reached 1,500 m. The following morning both flows remained active; the barranca Ceniza flow was 1,300 m long and the barranca Seca flow was 500 m long. Persistent explosions of ejecta to 200 m above the crater continued along with loud noises. The incandescent blocks spalling off the front of the flows remobilized ash that drifted S, SE, and SW.

Figure 144. Two lava flows were active on the flanks of Fuego on 13 February 2021. A flow in the Ceniza ravine on the SW flank grew to 1,500 m long, while a 500-m-long flow descended the Seca ravine on the NW flank. Sentinel-2 image uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Beginning at 1020 on 14 February 2021 a series of pyroclastic flows were observed in the Ceniza ravine. They lasted for three minutes and traveled several hundred meters. Ashfall was reported in Alotenango, El Porvenir, and Finca La Reunion. By the end of the day the Ceniza lava flow was active for 800 m and the Seca flow reached 200 m. Seismic energy decreased noticeably the next day along with a decrease in the flow rate and thermal energy. Explosions continued with ash plumes drifting E, NE, and N up to 50 km resulting in ashfall in Porvenir and Alotenango. INSIVUMEH considered the effusive eruption over by the evening of 15 February, and noted a decrease in the rate of explosions to 12-14 per hour (figure 145).

Figure 145. Although explosions at Fuego on 15 February 2021 had decreased in frequency, they still produced ash plumes and blocks rolling down the ravines that caused plumes of resuspended ash. Courtesy of INSIVUMEH (BOLETIN VULCANOLOGICO ESPECIAL BEFGO 023-2021, Guatemala, 15 de febrero de 2021, 15:30 horas).

Loud explosions continued 16 February and produced abundant ash that drifted E, NE, and N. The Washington VAAC reported intermittent ash emissions seen in satellite images moving ESE at 4.9 km altitude extending around 110 km from the summit before dissipating. Ashfall was reported in Celian, San Andres Ozuna, Rochela, Zapote, and El Rodeo. On 17 February ash plumes rose to 4.5-4.8 km altitude and drifted N, NE, and E as far as 50 km and caused ashfall in many communities, including as far away as Guatemala City. The wind changed to the E and SE later in the day, and plumes drifted 30-40 km over the departments of Sacatepequez, Escuintla, and Guatemala. Ash plumes from Pacaya were also affecting the same areas that day. The following day ash plumes were drifting 40 km SW. For the remainder of February ashfall affected the same communities to the SW and W as earlier in the month. The incandescent ejecta that rose 350 m above the summit on 28 February produced a strong thermal anomaly in satellite data that also showed incandescent blocks descending all the ravines around the summit (figure 146).

Figure 146. Incandescent ejecta was observed 350 m above the summit of Fuego on 28 February 2021 and produced a strong thermal anomaly shown in this Sentinel-2 satellite image. Also visible is incandescent ejecta around all the ravines near the summit and a small ash plume drifting WNW. Image uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Explosive activity continued throughout March 2021, producing ash plumes that rose to 4.5-4.8 km altitude and drifted mostly W and SW (figure 147). This resulted in ashfall most days in the same communities as before that were located 10-20 km away. The loud rumblings continued daily, lasting for 2-5 minutes at a time and rattling windows and roofs all around the volcano. Incandescent ejecta rose 100-300 m and the blocks traveled down all of the ravines, sometimes reaching the vegetation.

Figure 147. Numerous ash emissions at Fuego during March 2021 were captured in Sentinel-2 satellite images along with the frequent thermal anomalies. Ash plumes drifted W on 5 and 20 March (top row) and NW on 25 and 30 March 2021 (bottom row). Images for 5 and 25 March use Natural color rendering (bands 4, 3, 2). Images for 20 and 30 March show Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Geologic Background. Volcán Fuego, one of Central America's most active volcanoes, is also one of three large stratovolcanoes overlooking Guatemala's former capital, Antigua. The scarp of an older edifice, Meseta, lies between Fuego and Acatenango to the north. Construction of Meseta dates back to about 230,000 years and continued until the late Pleistocene or early Holocene. Collapse of Meseta may have produced the massive Escuintla debris-avalanche deposit, which extends about 50 km onto the Pacific coastal plain. Growth of the modern Fuego volcano followed, continuing the southward migration of volcanism that began at the mostly andesitic Acatenango. Eruptions at Fuego have become more mafic with time, and most historical activity has produced basaltic rocks. Frequent vigorous historical eruptions have been recorded since the onset of the Spanish era in 1524, and have produced major ashfalls, along with occasional pyroclastic flows and lava flows.

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/ ); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); 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).

Kavachi is an active submarine volcano in the SW Pacific, located in the Solomon Islands south of Gatokae and Vangunu islands. Volcanism has been characterized by phreatomagmatic explosions that ejected steam, ash, and incandescent bombs. The previous report described discolored plumes extending from a single point during early September 2020 (BGVN 45:10); similar activity was recorded for this reporting period covering October 2020 through April 2021 using satellite data.

Activity at Kavachi is most frequently observed through Sentinel-2 satellite imagery and has recently been characterized by discolored submarine plumes. On 2 October 2020 a slight yellow-green discoloration in the water was observed extending NE from a specific point (figure 23). Similar faint discolored plumes were intermittently recorded on 27 October, 1 November 2020, and 25 January 2021, which each extended NE, SW, and SW, respectively, from a point source above the summit where previous activity has occurred. Intermittent discolored plumes were also visible during March 2021 (figure 24). The plume discoloration on 1 March extended S from the origin point. On 11 March, the discoloration remained near the origin point. A narrow plume extended several kilometers W on 26 March, followed by a short plume seen towards the NW on 31 March. The only plume seen in April was a broad diffuse area of discoloration extending S on the 10th (figure 24). No discoloration near the volcano was observed in May.

Figure 23. Sentinel-2 satellite images of a discolored plume (light yellow-green) at Kavachi beginning on 2 October 2020 (top left) that extended NE. Additional plumes were visible during clear weather on 27 October (top right) that extended NE, on 1 November (bottom left) 2020 that extended SW, and strongly on 25 January 2021 (bottom right) that extended SW. Images with “Natural color” rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

Figure 24. Sentinel-2 satellite imagery of discolored plumes (light yellow-green) at Kavachi during March-April 2021. On 1 March (top left) the plume was observed extending S with a strongly discolored origin point. On 11 March (top right) the plume remained close to the origin point and did not seem to extend outward. On 26 March (bottom left) the plume was narrow and strongly extended W for several kilometers. On 10 April (bottom right) the plume extended S. Images with “Natural color” rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

Geologic Background. Named for a sea-god of the Gatokae and Vangunu peoples, Kavachi is one of the most active submarine volcanoes in the SW Pacific, located in the Solomon Islands south of Vangunu Island. Sometimes referred to as Rejo te Kvachi ("Kavachi's Oven"), this shallow submarine basaltic-to-andesitic volcano has produced ephemeral islands up to 1 km long many times since its first recorded eruption during 1939. Residents of the nearby islands of Vanguna and Nggatokae (Gatokae) reported "fire on the water" prior to 1939, a possible reference to earlier eruptions. The roughly conical edifice rises from water depths of 1.1-1.2 km on the north and greater depths to the SE. Frequent shallow submarine and occasional subaerial eruptions produce phreatomagmatic explosions that eject steam, ash, and incandescent bombs. On a number of occasions lava flows were observed on the ephemeral islands.

Information Contacts: Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

The volcanic Semisopochnoi Island in the western Aleutian Islands contains a group of cones within a caldera complex (figure 5). The active Cerberus center has three summit craters, with the current activity originating from North Cerberus. Since September 2018, typical activity has produced minor ash deposits within the vicinity. This bulletin summarizes activity that occurred from April 2020 through May 2021 based on information given by the Alaska Volcano Observatory (AVO), supplemented by satellite data.

Figure 5. This satellite image of Semisopochnoi Island, Alaska, shows the major surface features with an 8-km-wide caldera in the center. As of 2021, Mount Cerberus is the most active of three cones within the caldera complex. The North, East, and South Cerberus craters are indicated, with a faint gas plume dispersing NE from the active North crater on 22 August 2020. Base satellite image from Sentinel-2 using Natural color (bands 4, 3, 2) rendering. Courtesy of Sentinel Hub Playground.

Intermittent small explosions occurred at Semisopochnoi during early 2020. An AVO Volcano Activity Notice for Aviation (VONA) issued on 1 April reported no indication of activity over the previous two weeks and seismicity at background levels. Satellite data show detectable sulfur dioxide (SO₂) emission and visible steam plumes. Low-level unrest continued into early June with occasional small earthquakes, including a few small low-frequency events and episodic tremor, occasional steam plumes, and detectable SO₂ emissions. An increase in tremor was detected around 12-13 June, and infrasound and seismicity indicated rapid degassing events on 17 and 19 June, with activity declining again by the 20th. AVO noted that clear satellite images acquired on the 21st showed minor ash deposits near the crater, likely from the elevated activity during the previous week, and vigorous gas and steam emission (figure 6). Steam and gas emission continued through to the end of the month then intermittently through July. A 200-km-long SO₂ plume was detected on 15 July and low-level unrest continued.

Figure 6. Minor ash deposits are visible on the Semisopochnoi North Cerberus Crater and a steam plume is shown dispersing ESE on 21 June 2020. Sentinel-2 satellite image with Natural color (bands 4, 3, 2) rendering. Courtesy of Sentinel Hub Playground.

A gas plume was reported on 7 August and seismicity was above background to the 9th, after that seismicity was at very low levels with no more significant events detected. Infrequent small earthquakes were detected through September and minor steam emissions on the 22nd. Seismicity remained low throughout October. No eruptive activity had detected since mid-June and seismicity had declined to very low levels prior to seismic data transmission failing on 11 November. Due to the lack of data, on 20 November the Aviation Color Code and Alert Level were reduced to Unassigned.

There were no reports of activity during December 2020 or January 2021. A satellite image acquired on 7 February showed several small ash deposits extending at least 3 km from the North Cerberus Crater, likely produced by a small explosion the previous week (figure 7). Steam emission prevented views into the crater and clouds obscured the volcano over the following week.

Figure 7. This Landsat 8 image acquired on 7 February 2021 at Semisopochnoi shows several linear ash deposits from the North Cerberus Crater. This reflects low-level explosive activity. Landsat 9 true Color – pansharpened scene. Courtesy of Sentinel Hub Playground.

On 10 March a satellite image revealed a recently emplaced ash deposit that extended 1.5 km from the crater, with a steam plume being blown to the E (figure 8). Several similar small ash deposits had been noted by AVO in the previous weeks. No activity was observed or detected through 18 March, other than a possible gas plume that day. At 0350 on the 19th a small explosion was detected by infrasound monitoring. Another small explosion was detected at 0230 on the 21st, followed by a series of smaller explosions. During 22-23 March three explosions were detected. Cloud cover prevented visual observation of these events, but possible SO₂ plumes were detected and a confirmed plume on the 23rd indicated further unrest. A probable ash deposit and plume were imaged on the 24th (figure 9). Activity continued with intermittent explosions and SO₂ plumes detected through the 27th.

Figure 8. This 10 March 2021 WorldView-3 satellite image shows ash deposits from low-level explosive activity at the Cerberus North Crater at Semisopochnoi. The ash extends to 1.5 km from the vent and has been partly remobilized by wind. A plume emanating from the crater is being blown to the E. Figure by Hannah Dietterich, courtesy of AVO.

Figure 9. An ash deposit is present between the dashed lines, deposited on snow (red) in this Planet Labs near-IR false color satellite image acquired on 24 March 2021. The deposit extends over 8 km ESE across Semisopochnoi from the North Cerberus Crater and a plume is also visible in the same area. Image courtesy of AVO.

Several small low-altitude ash and gas plumes were detected in satellite images on 30 March and 1 April. Cloud cover prevented satellite views until 12 April, when new ash deposits and low-level ash emissions were observed extending at least to the coastline, accompanied by weak infrasound signals. Low-level activity was also detected the following day. Sustained ash emission that began on the morning of the 15th (figure 10) produced a plume extending more than 350 km E to altitudes of 6 km; activity continued through the next day with a change in direction to the N at around 3 km altitude. Ash emission continued over the following days with a VONA released on the 22nd reporting an ash plume reaching 3 km and extending about 75 km S (figure 11). Through to the end of April ash and SO₂ plumes were either observed or noted as probably occurring under cloudy conditions.

Figure 10. This Sentinel-3 satellite scene acquired on 15 April 2021 shows plumes from Semisopochnoi dispersed over 330 km from the vent. The insert shows a zoomed-in view of the island and the proximal ash plume. Original image by Hannah Dietterich, AVO.

Figure 11. This Planet Labs satellite image acquired on 22 April 2021 shows an ash plume produced by the North Cerberus Crater and dispersing S. Ash deposits are visible on the flanks of the cone. Figure by Hannah Dietterich, AVO.

The volcano was often obscured during the first week of May, with activity possibly continuing at a low level without detection. A gas plume was detected on the 11th, and an ash plume is visible in satellite images acquired on the 17th (figure 12). Small explosions and SO₂ emissions were detected through 21 May. An ash emission reaching 3 km altitude that was seen by an AVO field crew on 29 May was also observed in satellite data moving SW. Elevated temperatures were detected in the North Cerberus Crater. Ash emissions were produced again on the 30th and observed by an AVO field crew (figure 13). Seismic data transmission was restored on 26 May.

Figure 12. Satellite images of Semisopochnoi acquired on 17 and 29 May (top), and a photograph taken on 29 May 2021 (bottom) show weak activity at the North Cerberus Crater, including ash emission, gas emission, and elevated temperature on the crater floor. Sentinel-2 color infrared (vegetation, bands 8, 3, 4) scene at the top left and the false color (urban, bands 12, 11, 4) scene at the top right courtesy of Sentinel Hub Playground. Photo courtesy of Hannah Dietterich, AVO.

Figure 13. Minor ash emissions produced on 30 May 2021 at Semisopochnoi’s North Cerberus Crater around 1320 local time, taken from a helicopter during field work. Both top and bottom-left photos are taken from the SE. Photos courtesy of Hannah Dietterich, AVO.

Geologic Background. Semisopochnoi, the largest subaerial volcano of the western Aleutians, is 20 km wide at sea level and contains an 8-km-wide caldera. It formed as a result of collapse of a low-angle, dominantly basaltic volcano following the eruption of a large volume of dacitic pumice. The high point of the island is Anvil Peak, a double-peaked late-Pleistocene cone that forms much of the island's northern part. The three-peaked Mount Cerberus was constructed within the caldera during the Holocene. Each of the peaks contains a summit crater; lava flows on the N flank of Cerberus appear younger than those on the south side. Other post-caldera volcanoes include the symmetrical Sugarloaf Peak SSE of the caldera and Lakeshore Cone, a small cinder cone at the edge of Fenner Lake in the NE part of the caldera. Most documented eruptions have originated from Cerberus, although Coats (1950) considered that both Sugarloaf and Lakeshore Cone could have been recently active.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA (URL: https://avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://dggs.alaska.gov/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Planet Labs, Inc. (URL: https://www.planet.com/).

Piton de la Fournaise is located on the French island of Réunion in the western Indian Ocean. Its previous most recent eruption occurred during February into April 2020, characterized by fissure eruptions, fountaining, and significant lava flows (BGVN 45:05). This report covers May through December 2020, describing the new eruption in early December that was characterized by lava fountains and flows, using information from the Observatoire Volcanologique du Piton de la Fournaise (OVPF) and various satellite data.

Slight deformation was recorded after the end of the April eruption, but overall activity during May-November 2020 was low, with no eruptive events, according to OVPF. Starting around 16 June seismicity resumed, which included 77 shallow volcano-tectonic earthquakes during the month and occasional rockfall events in the Dolomieu Crater. This increase in seismicity was accompanied by inflation at the base and summit of the volcano. Shallow volcano-tectonic earthquakes continued to be reported under the Dolomieu Crater during July-November accompanied by rockfall events. In late September the number of shallow volcano-tectonic earthquakes increased markedly to 1,648, but then decreased to 129 in October and only four in November.

OVPF reported that during 0510-0554 on 4 December a seismic swarm of about 101 volcano-tectonic earthquakes was accompanied by minor, but rapid, inflation just below the center and N rim of the Dolomieu Crater. Seismicity decreased after 0600, but inflation continued through 6 December. A second seismic crisis began at 0228 on 7 December, accompanied by rapid inflation. Fissures opened on the WSW flank of the Dolomieu Crater at 0440 at elevations ranging from 2.2-2.3 km and spanning a 700-m-long area; lava began to erupt from these fissures during 0455-0500 (figure 202). Scientists on an overflight at 0700-0730 observed lava fountains rising 15 m high from the three active fissures and short lava flows (figure 203). By 1700 the fissure at an elevation of 2.3 km was the most active, with five small vents, while the other two were showing less intense activity. Satellite data via the HOTVOLC platform showed a lava flow rate of 5 and 30 m³/s during 7 December. The eruption period ended at 0715 on 8 December, following a gradual decrease in tremor and a three-hour phase of seismic signals that indicated degassing. Twenty-one volcano-tectonic earthquakes were recorded during that day under the W rim of the Dolomieu Crater. Another six earthquakes were reported during the morning of 9 December through 0900. Surficial activity was no longer visible.

Figure 202. Photo of the active fissure vents on the WSW flank of the Dolomieu Crater and the lava fountains accompanied by degassing at Piton de la Fournaise at 0730 on 7 December 2020. Courtesy of OVPF-IPGP (Bulletin d'activité du lundi 7 décembre 2020).

Figure 203. Photo of the lava fountains up to 15 m high at Piton de la Fournaise during 7-8 December 2020. Courtesy of OVPF-IPGP.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed brief, but significant, thermal activity during early December, reflecting the new eruption. This thermal activity was also visible in Sentinel-2 thermal satellite imagery on 7 December 2020, showing lava flows and possibly lava fountains from the fissures on the SW and W flanks (figure 204). Accompanying this activity were SO₂ emissions that were detected by the Sentinel-5P/TROPOMI instrument (figure 205).

Figure 204. Sentinel-2 infrared satellite image of the thermal activity (bright yellow-orange) on the S and SW flanks of Piton de la Fournaise on 7 December 2020. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

Figure 205. Image of the SO2 emissions that occurred during the eruption at Piton de la Fournaise on 7 December 2020 detected by the Sentinel-5P/TROPOMI satellite. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. The massive Piton de la Fournaise basaltic shield volcano on the French island of Réunion in the western Indian Ocean is one of the world's most active volcanoes. Much of its more than 530,000-year history overlapped with eruptions of the deeply dissected Piton des Neiges shield volcano to the NW. Three calderas formed at about 250,000, 65,000, and less than 5000 years ago by progressive eastward slumping of the volcano. Numerous pyroclastic cones dot the floor of the calderas and their outer flanks. Most historical eruptions have originated from the summit and flanks of Dolomieu, a 400-m-high lava shield that has grown within the youngest caldera, which is 8 km wide and breached to below sea level on the eastern side. More than 150 eruptions, most of which have produced fluid basaltic lava flows, have occurred since the 17th century. Only six eruptions, in 1708, 1774, 1776, 1800, 1977, and 1986, have originated from fissures on the outer flanks of the caldera. The Piton de la Fournaise Volcano Observatory, one of several operated by the Institut de Physique du Globe de Paris, monitors this very active volcano.

Information Contacts: Observatoire Volcanologique du Piton de la Fournaise, Institut de Physique du Globe de Paris, 14 route nationale 3, 27 ème km, 97418 La Plaine des Cafres, La Réunion, France (URL: http://www.ipgp.fr/fr); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); 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).

Heard is a remote island located in the southern Indian Ocean that contains the Big Ben stratovolcano, which has had intermittent activity since 1910. More recent activity since 2012 through October 2020 has been characterized by thermal anomalies in the summit crater and lava flows, primarily identified based on information from satellite data (BGVN 45:11). This report covers similar activity that continued during November 2020 and January 2021.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows a total of three thermal anomalies of varying power during November 2020 (figure 46). Sentinel-2 thermal satellite imagery shows a single thermal anomaly on 9 November 2020 and later, on 11 November two strong thermal anomalies, possibly two lava flows, were observed descending the S and SW flanks (figure 47). These thermal anomalies were also detected by the MIROVA system. Weaker thermal anomalies were observed on 18 and 20 January 2021 in the summit crater. No new thermal activity was detected after November through April 2021 by the MIROVA system.

Figure 46. Only three thermal anomalies at Heard were detected during November 2020, according to the MIROVA system, shown in this Log Radiative Power graph. The strongest thermal anomaly represents the two possible lava flows that were observed in Sentinel-2 infrared satellite data. No thermal anomalies were observed during December through April 2021. Courtesy of MIROVA.

Figure 47. Sentinel-2 infrared satellite imagery of Heard Island’s Big Ben volcano showed a thermal anomaly (bright yellow-orange) on clear weather days on 9 (top left) and 11 (top right) November 2020, along with 18 (bottom left) and 20 (bottom right) January 2021. On 11 November two strong thermal anomalies, possibly representing different lava flows, were observed descending to the S and SW flanks, though much of the activity was covered by clouds. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

Geologic Background. Heard Island on the Kerguelen Plateau in the southern Indian Ocean consists primarily of the emergent portion of two volcanic structures. The large glacier-covered composite basaltic-to-trachytic cone of Big Ben comprises most of the island, and the smaller Mt. Dixon lies at the NW tip of the island across a narrow isthmus. Little is known about the structure of Big Ben because of its extensive ice cover. The historically active Mawson Peak forms the island's high point and lies within a 5-6 km wide caldera breached to the SW side of Big Ben. Small satellitic scoria cones are mostly located on the northern coast. Several subglacial eruptions have been reported at this isolated volcano, but observations are infrequent and additional activity may have occurred.

Information Contacts: 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).

Although tephrochronology has dated activity at Sabancaya back several thousand years, renewed activity that began in 1986 was the first recorded in over 200 years. A new period of explosive activity that began in November 2016 has been characterized by pulses of ash emissions with some plumes exceeding 10 km altitude, frequent thermal anomalies, and significant SO₂ plumes. Daily ash emissions and high levels of SO₂ continued during October 2020-March 2021, the period covered in this report. The Observatorio Vulcanologico INGEMMET (OVI) reported daily on numbers of explosions, ash plume heights and directions of drift, seismicity, ashfall, and other activity; IGP (Instituto Geophysico del Peru) issued weekly reports of activity.

Activity at Sabancaya during October 2020-March 2021 consisted of multiple daily explosions with dense gas-and-ash plumes, SO₂ emissions, and persistent thermal anomalies from the summit crater. OVI and INGEMMET confirmed the presence of a second dome inside the summit crater in mid-November. Ash plumes rose 1-2 km above the summit on most days; higher plumes occasionally reached 2.5-3.0 km above the summit. Ash most commonly drifted W and SW, but there were significant periods where the wind sent ash N and NE. Ashfall was reported in communities around the volcano on 3-8 days each month. The Buenos Aires Volcanic Ash Advisory Center (VAAC) issued three or four daily reports of ongoing ash emissions throughout the period. The plumes generally rose to 7.3-8.2 km altitude and drifted in various directions, often visible in satellite imagery.

Moderate levels of thermal energy were reported by the MIROVA project during July 2020-March 2021. Power values were generally consistent, with a small increase in November and a brief drop from late December through January 2021 (figure 90). The MODVOLC system reported nine thermal alerts in October, 12 in November, one in December, none in January, and four each both February and March. Moderate to strong SO₂ emissions were recorded virtually every day during the period by the TROPOMI instrument on the Sentinel-5P satellite (figure 91).

Figure 90. Thermal anomalies at Sabancaya from July 2020-March 2021 persisted at low to moderate power levels through the period. A small increase in frequency occurred during November 2020 and a decrease in thermal activity was apparent from late December through January 2021. Courtesy of MIROVA.

Figure 91. High levels of SO2 emissions from Sabancaya were recorded by the TROPOMI instrument on the Sentinel-5P satellite during October 2020-March 2021. Examples of the larger plumes each month are shown here; the date is along the top of each image. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

The Buenos Aires VAAC reported intermittent pulses of ash moving SE from the summit at 7.6 km altitude on 1 October 2020 (figure 92). Continuous ash emissions were reported at 7.3-7.9 km altitude throughout the month, resulting in several reports of ashfall. Ash was reported in Lluta on 5-6 October, in Taya on 6 October, in Huanca on 10 and 15 October, in Maca and Lari on 18, 27, and 28 October, in Madrigal on 18 and 27-29 October, and in Chivay on 27 and 29 October. Ash was also visible in satellite imagery on 16 and 26 October. Thermal anomalies were present in satellite images on 1, 6, 11, 16, and 26 October (figure 93).

Figure 92. A dense plume of ash rose to 7.6 km altitude and drifted SE from the summit of Sabancaya on 1 October 2020. Courtesy of OVI-INGEMMET (REPORTE DIARIO DE LA ACTIVIDAD DEL VOLCAN Sabancaya, RDSAB-250-2020 / DGAR-INGEMMET Diario: 01 de octubre del 2020).

Figure 93. Thermal anomalies and ash emissions appeared in Sentinel-2 satellite imagery at Sabancaya multiple times during October 2020. On 11 October a strong thermal anomaly was present along with a dense steam plume drifting W (left). On 26 October both an ash plume and a thermal anomaly were present (right). Images use Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Strong thermal anomalies appeared in satellite images on 5, 10, 15, 20, and 25 November 2020. Two distinct ash plumes rose from the summit crater on 5 November, and thermal anomalies were also present in multiple areas of the crater in satellite images that day (figure 94). OVI reported the existence of a second dome in the summit crater the next day. In a report issued on 17 November INGEMMET named the second dome “Iskay” and identified it in the NE part of the crater using satellite images; it was about 12,000 m² in size (figure 95). On 25 November thermal signals from both domes were visible in satellite imagery along with an ash plume drifting W that rose to 2,900 m above the summit (figure 96). Ashfall was reported in both Madrigal and Lari on 14 and 16 November, in Lluta on 25 November, and in Huambo on 25-28 November.

Figure 94. Thermal and ash activity at Sabancaya on 5 November 2020 both suggested the presence of a second dome inside the summit crater. Two ash plumes rose from the summit (left) and a strong thermal anomaly appeared in the NE quadrant of the crater (right). Left image is from INGEMMET webcam. Right image is Atmospheric penetration rendering (bands 12, 11, 8a) of Sentinel-2 satellite image. Courtesy of OVI-INGEMMET (REPORTE DIARIO DE LA ACTIVIDAD DEL VOLCAN, RDSAB-285 2020 / DGAR INGEMMET Diario: 05 de noviembre del 2020) and Sentinel Hub Playground.

Figure 95. OVI-INGEMMET used webcams and satellite imagery to identify a new dome inside the summit crater of Sabancaya in November 2020. (A) Incandescence over the crater taken by a surveillance camera ~25 km NE on 9 November. (B) Incandescent ejecta rises from two locations within the summit crater taken by a surveillance camera ~4.7 km N on 9 November. (C) Sentinel-2 near-infrared satellite image from 15 November showing hotspots inside the crater; source: MOUNTS. (D) Planet Labs satellite image taken on 16 November showing the location of the growing dome; source: PlanetLabs Inc. Figure prepared by C. Laverde (SGC). Courtesy of OVI-INGEMMET (OVI / 17 noviembre, 2020 / Comentarios desactivados en INGEMMET RATIFICA LA PRESENCIA DEL SEGUNDO DOMO DE LAVA EN EL CRÁTER DEL VOLCÁN SABANCAYA DURANTE SU ACTUAL PROCESO ERUPTIVO / Noticias, Novedades).

Figure 96. The dense ash plume that rose 2.9 km above the summit of Sabancaya on 25 November 2020 (left) could be seen drifting W in satellite imagery that day (right). It rose from the new dome in the NE quadrant of the crater, which appeared as a strong thermal anomaly in the Sentinel-2 satellite image using Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of OVI-INGEMMET (REPORTE DIARIO DE LA ACTIVIDAD DEL VOLCAN Sabancaya, RDSAB-305 2020 / DGAR INGEMMET Diario: 25 de noviembre del 2020) and Sentinel Hub Playground.

Ash emissions continued from both areas of the summit during December 2020 (figure 97); a thermal anomaly was recorded in a satellite image on 15 December. Ashfall was reported in Cabanaconde on 1, 8, and 10 December and was also reported in Pinchollo and Madrigal on 10 December. Two dense ash plumes on 5 January 2021 confirmed that both summit domes were still active (figure 98). Ash emissions appeared in satellite images on 4, 9, and 29 January 2021, along with a thermal anomaly on 29 January (figure 99). Ashfall was reported in Huanca on 15 January, Huambo on 18 January, Madrigal and Pinchollo on 27 January, and in Chivay on 29 January. A lahar descended the Pinchollo ravine near Huayraray around 1610 on 20 January.

Figure 97. Ash emissions continued from two areas within the summit crater of Sabancaya during December 2020. On 9 December the taller plume rose to 2,400 m above the summit (left). On 16 December two dense ash plumes were captured by one of the webcams (right). Courtesy of OVI-INGEMMET (REPORTE DIARIO DE LA ACTIVIDAD DEL VOLCAN Sabancaya, RDSAB –319-2020/ DGAR –INGEMMET Diario: 09 de diciembre del 2020 and RDSAB –324-2020/ DGAR –INGEMMET Diario: 16 de diciembre del 2020).

Figure 98. Two dense ash plumes rising from the summit of Sabancaya on 5 January 2021 confirmed that both domes were still active. Courtesy of OVI-INGEMMET (VOLCAN SABANCAYA Reporte de monitoreo volcanico, 05 enero del 2021, RMVSAB, 002 2021 / DGAR INGEMMET).

Figure 99. Ash emissions from Sabancaya appeared in Sentinel-2 satellite images on 4, 9, and 29 January 2021. The plume on 9 January (left) was visible in the Natural color rendering (bands 4, 3, 2) and the plume on 29 January (right) partially obscured a thermal anomaly at the summit in an Atmospheric penetration rendering (bands 12, 11, 8a) image. Courtesy of Sentinel Hub Playground.

Daily ash emissions continued during February 2021 with plumes that rose as high as 3 km above the summit (figure 100). Ash plumes were visible in satellite images on 13 and 18 February (figure 101). Ashfall was reported in Huanca on 5 February, Pinchollo on 11 February, and in Madrigal on 11, 15, and 16 February. Two small to moderate lahars were reported in the area of Sallalli on 4 February. Ashfall was also reported in Huambo on 8, 9, and 17 March, Cabanaconde on 9 March, and in Huanca on 15 and 25 March. Satellite images indicated ash emissions on 10 March and thermal anomalies on 10 and 25 March (figure 102).

Figure 100. Daily ash emissions rose from Sabancaya during February 2021. On 4 February the plume rose to 2.6 km above the summit and drifted S and SE. Courtesy of OVI-INGEMMET (VOLCAN SABANCAYA, Region Arequipa, Reporte de monitoreo volcanico, RMVSAB-024 2021 / DGAR INGEMMET 04 febrero del 2021).

Figure 101. Ash plumes were a daily occurrence at Sabancaya during February 2021. A large plume covered the volcanic edifice and nearby area on 13 February (left). A smaller plume on 18 February was concentrated over the summit (right). Natural color rendering (bands 4, 3, 2) of Sentinel-2 images. Courtesy of Sentinel Hub Playground.

Figure 102. Ash emissions and thermal activity continued at Sabancaya during March 2021. On 4 March ash emissions rose to 1.8 km above the summit and drifted W and SW (left). On 10 March ash emissions rose to 900 m and drifted W and NW while a strong thermal anomaly was evident inside the summit crater (right). Sentinel-2 satellite image uses Atmospheric penetration rendering (bands 12, 11, 8a) courtesy of Sentinel Hub Playground. Left image courtesy of INGEMMET (VOLCAN SABANCAYA, Region Arequipa, Reporte de monitoreo volcanico, RMVSAB-044 2021/ DGAR INGEMMET 04 marzo del 2021).

Geologic Background. Sabancaya, located in the saddle NE of Ampato and SE of Hualca Hualca volcanoes, is the youngest of these volcanic centers and the only one to have erupted in historical time. The oldest of the three, Nevado Hualca Hualca, is of probable late-Pliocene to early Pleistocene age. The name Sabancaya (meaning "tongue of fire" in the Quechua language) first appeared in records in 1595 CE, suggesting activity prior to that date. Holocene activity has consisted of Plinian eruptions followed by emission of voluminous andesitic and dacitic lava flows, which form an extensive apron around the volcano on all sides but the south. Records of historical eruptions date back to 1750.

Information Contacts: Observatorio Volcanologico del INGEMMET (Instituto Geológical Minero y Metalúrgico), Barrio Magisterial Nro. 2 B-16 Umacollo - Yanahuara Arequipa, Peru (URL: http://ovi.ingemmet.gob.pe); Instituto Geofisico del Peru (IGP), Calle Badajoz N° 169 Urb. Mayorazgo IV Etapa, Ate, Lima 15012, Perú (URLhttps://www.igp.gob.pe/servicios/centro-vulcanologico-nacional/inicio); 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/); 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/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

During January-October 2020, activity at Telica consisted of frequent seismicity, fumarolic emissions, and beginning in July, explosions that generated gas-and-ash plumes (BGVN 45:11). The following report describes activity between November 2020 and February 2021 using monthly and special bulletins published by the Instituto Nicaragüense de Estudios Territoriales (INETER) and satellite images.

Following some small ash explosions in late October, activity decreased in November 2020. INETER scientists visiting the volcano on 24 November observed small pulses of gas and ash originating from the SW walls and flanks, and heard jet sounds inside the crater. The team also noted abundant gas emissions on the N and NW walls and subsidence inside the crater.

A new series of ash explosions began at 0538 on 30 November (figure 52). Similar activity later that morning ejected incandescent ballistics and generated a small gas-and-ash plume (figure 53). Almost continuous explosions for more than three days produced gas-and-ash plumes that rose 50-400 m above the crater rim, depositing most of the ash on the SW slope. The explosive activity on 2 December was visible in satellite imagery (figure 54), and ash plumes the next day reached the ocean (figure 55). After more than three days of almost continuous explosions, explosions became sporadic until 1450 on 8 December. A total of 1,315 small explosions were recorded between 30 November and 8 December. Ashfall during this period was reported in communities to the SW, including Zarandaj (6 km WSW), Cristo Rey (6 km W), Las Colinas (7 km WSW), Garrobo Empinado (8 km SSW), El Panal (9 km SSW), Canta Rana (10 km SW), Ceibo Chachagua (10 km SW), Las Mercedes (10 km SW), Punta Arena (10 km SW), La Virgen (12 km WSW), Filiberto Morales (15 km WSW), Los Cocos (9 km SSW), Verónica Lacayo (10 km SW), and Nuevo Belén.

Figure 52. Webcam photo of an explosion at Telica at 0538 on 30 November 2020. This event was the first in a series of explosions that continued until 8 December. Courtesy of INETER.

Figure 53. Webcam photo of an explosion at Telica at 0921 on 30 November 2020 that generated a dense brown ash plume. Courtesy of INETER.

Figure 54. Sentinel-2 images of Telica on 2 December 2020 in natural color (bands 4, 3, 2) showing a brown gas-and-ash plume streaming SW. Courtesy of Sentinel Hub Playground.

Figure 55. Landsat 8 false-color image (bands B4, B7, B1) of a gas plume (light blue) from Telica extending SW over the Pacific Ocean on 3 December 2020. Courtesy of INETER.

Thermal anomalies seen in Sentinel-2 satellite images were identified again starting on 14 December 2020, for the first time since 10 September. The hotspot in the eastern part of the crater was detected intermittently through 22 February 2021.

SO₂ emissions measured by the Mobile-DOAS technique using 5-7 transects, averaged 99 metric tons/day for November and 821 metric tons/day for December. These values were outside the normal range and represented among the lowest and highest averages, respectively, since at least 2009. SO₂ emissions averaged 129 metric tons/day during January. Small landslides were heard in February, and weak degassing was evident.

According to INETER, about 907,863 seismic events were recorded during 2020, of which 51,275 were long-period (LP) earthquakes; the source of which was 6-10 km deep. In December alone, seismicity was dominated by volcano-tectonic (VT) earthquakes, of which 262,549 were counted. Tremor remained between 20-50 RSAM units. Seismicity continued in early 2021, though INETER did not report any explosive activity. In January 2021 about 140,000 seismic events occurred, of which 24,652 were LP earthquakes. In February about 86,243 seismic events occurred, of which 1,725 were LP earthquakes, 431 were VT earthquakes, and 15,093 were hybrid earthquakes. During both months, tremor remained between 20-50 RSAM units.

During a field visit on 19 January 2021, INETER observed that the crater had increased in diameter to 20 m and in depth to 5 m. The main crater displayed weak gas emissions. During the visit, temperatures of the four fumaroles on the eastern rim were 55-57°C, somewhat lower than the previous three months and significantly lower than during February-July 2020. Temperatures during a visit on 11 February were even lower.

Geologic Background. Telica, one of Nicaragua's most active volcanoes, has erupted frequently since the beginning of the Spanish era. This volcano group consists of several interlocking cones and vents with a general NW alignment. Sixteenth-century eruptions were reported at symmetrical Santa Clara volcano at the SW end of the group. However, its eroded and breached crater has been covered by forests throughout historical time, and these eruptions may have originated from Telica, whose upper slopes in contrast are unvegetated. The steep-sided cone of Telica is truncated by a 700-m-wide double crater; the southern crater, the source of recent eruptions, is 120 m deep. El Liston, immediately E, has several nested craters. The fumaroles and boiling mudpots of Hervideros de San Jacinto, SE of Telica, form a prominent geothermal area frequented by tourists, and geothermal exploration has occurred nearby.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Semeru, located in East Java, Indonesia, contains the active Jonggring-Seloko vent at the Mahameru summit. The current eruptive period began in 2014 and recently has been characterized by ash plumes, pyroclastic flows, incandescent avalanches, and thermal activity. This report covers similar activity consisting of multiple pyroclastic flows, frequent ash plumes, and incandescent avalanches of material during September 2020 through February 2021 using daily and VONA reports from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), the Darwin Volcanic Ash Advisory Centre (VAAC), and various satellite data.

The Darwin VAAC reported intermittent ash plumes during the reporting period that rose between 2.4 and 6.1 km altitude, the latter of which occurred during 1-2 and 5-6 December based on HIMAWARI-8 satellite imagery. Activity during September through late November was dominantly characterized by white-to-gray ash plumes; during 19-20 November crater incandescence was visible and 11 incandescent avalanches of material were reported, traveling 500-1,500 m down the Kembar and Kobokan drainages on the S flank. Crater incandescence continued to be observed through the rest of the month, accompanied by intermittent avalanches of material and ash plumes. On 28 November pyroclastic flows were detected and descended the SE flank, which continued frequently through early February 2021. Explosions, pyroclastic flows, crater incandescence, incandescent avalanches of material, and some lahars were frequently reported during December through early February 2021.

During September intermittent white-and-gray ash plumes rose 200-700 m above the crater and drifted dominantly S and SW, accompanied by occasional thermal anomalies that were visible in satellite images according to PVMBG, VONA advisories, and ground observers. Similar activity continued in October with white-and-gray ash plumes rising 200-1,000 m above the crater and drifting in different directions; the higher plumes were during 16-17 October. On 19 October intermittent rock avalanches were detected.

Gray-and-white ash plumes were recorded rising 100-500 m above the crater, drifting in multiple directions during a majority of November, based on daily reports from PVMBG. During 19-21 November nighttime crater incandescence was accompanied by 11 incandescent avalanches of material that traveled 500-1,500 m down the SE-flank Kembar and Kobokan drainages (figure 45). On 22 November six incandescent avalanches of material extended 100-300 m down the Kembar and Kobokan drainages. During 23-24 November two eruptive events generated gray ash plumes that rose 100-200 m above the crater and drifted SW. On 28 November there was a significant increase in the number of rock avalanches; pyroclastic flows that originated from the ends of the avalanches moved down the SE flank. An accompanying ash plume rose to 4.9 km altitude and drifted SW. As a result, roughly 1,298 people living in five villages and two sub-districts in the Lumajang Regency were affected. Small sulfur dioxide plumes accompanied the more intense eruptive events and were detected in the Sentinel 5P/TROPOMI instrument during late November, late December, and early January (figure 46).

Figure 45. Sentinel-2 thermal satellite images of Semeru on 21 November (left) and 21 December (right) 2020 showing incandescent avalanches of material descending the SE flank. A white gas-and-steam plume accompanies the avalanche on 21 December. Intermittent incandescent avalanches were observed throughout the reporting period on clear weather days. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

Figure 46. Small SO2 plumes from Semeru were detected by the Sentinel 5P/TROPOMI instrument on 25 (top left) and 29 (top right) November, 26 December (bottom left) 2020, and 2 January (bottom right) 2021. The plumes drifted SW on 25 and 29 November and 2 January, and E on 26 December. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Activity continued to increase in December; on 1 December at 0123 pyroclastic flows from the summit lava dome traveled 2-11 km down the Kobokan drainage on the SE flank. According to a VONA issued by PVMBG a resulting ash plume rose 2 km above the crater. Deposits from the pyroclastic flows were as thick as 15 m, destroying some machinery and impacting livestock, agricultural fields, and businesses (figures 47 and 48). BNPB reported that as many as 550 residents had evacuated, though some returned to their homes; one person was reported missing. On 2 December a pyroclastic flow extended 2.5 km down the Kobokan drainage and an accompanying ash plume rose 2.4 km above the crater that drifted E and NE. Ashfall was visible around the summit crater. During 1-9 December near daily avalanches of material, some of which were incandescent, were recorded by the seismic network and traveled 200-2,500 m down the Kobokan drainage on the SE flank (figures 49 and 50). Within that period eruptive events and rockfalls over 5-6 December generated pyroclastic flows that extended 2.5 km down the Kobokan drainage; associated ash plumes rose to 6.1 km altitude that drifted E and NE. During 5-9, 15, and 19-31 December incandescent material was ejected 50-100 m above the crater. Explosions were recorded by the seismic network during 10-12 December but could not be visually confirmed due to weather conditions. On 13 December a pyroclastic flow that originated from the end of an avalanche traveled 1.5 km down the Kembar drainage on the S flank and reached a maximum distance of 3.5 km from the crater. The next day a pyroclastic flow traveled 3.5 km down the Kobokan drainage; rock avalanches traveled 300-1,500 m down both the Kembar and Kobokan drainages during 14-15 December.

Figure 47. Photo of ash deposits on 2 December 2020 in Lumajang, Indonesia, due to the eruptive events at Semeru. Some deposits damaged machinery, agricultural fields, and affected livestock. Photo by Antara Foto/Umarul Faruq; courtesy of Reuters.

Figure 48. Photo of ash deposits on agricultural land in Lumajang, Indonesia, due to the eruptive events at Semeru in early December 2020. Courtesy of AFP.

Figure 49. Photo of summit crater incandescence and an incandescent avalanche of material descending Semeru at night on 3 December 2020. Copyrighted photo courtesy of Øystein Lund Andersen, used with permission.

Figure 50. Photo of a rockfall event and a resulting ash plume at Semeru on 4 December 2020. Copyrighted photo courtesy of Øystein Lund Andersen, used with permission.

On 23, 30, and 31 December pyroclastic flows were reported descending 3.5 km down the Kobokan drainage on the SE flank. During 23-25 December incandescent avalanches of material traveled 100-300 m down the Kobokan drainage. On 30 December observers at the Gunungsawur Observatory (12 km SE) recorded heavy rain that resulted in multiple lahars (figure 51). At 0939 the post’s seismic station detected a lahar in the Kobokan drainage on the SE flank. Another lahar down the SE flank at 0950 was followed by a large volume of water. At 1111 the seismic station detected a lahar and an accompanying pyroclastic flow that traveled 3.5 km down the Kobokan drainage; by 1130 the lahar had damaged bamboo stalls in Sumberwuluh Village (15 km SE).

Figure 51. Photo of a lahar down the Kobokan drainage on the SE flank of Semeru at 0939 on 30 December 2020. A pyroclastic flow was also observed traveling 3.5 km down the Curah Kobokan River drainage. Courtesy of BNPB.

Increased volcanism continued in January 2021; on 1 January a pyroclastic flow was recorded at 1451. Incandescent avalanches were reported during 1-5 and 15 January that traveled 200-1,000 m down the Kobokan drainage, reaching a maximum distance of 1,250 m from the crater. At night incandescent material was ejected 50 m above the crater during 1-6 January. White-and-gray plumes rose 200-300 m above the crater and drifted N. On 16 January incandescent avalanches were detected at 1724 and extended as far as 1 km down the Kobokan drainage; a pyroclastic flow traveled 4-4.5 km down the same drainage during that day. A large ash plume was observed along the length of the pyroclastic flow, rising 2 km above the crater and drifted NE and N (figures 52 and 53). As a result, ashfall was reported in areas to the N. Incandescent material was ejected 30 m above the crater and incandescent avalanches of material descended 100-200 m down the Kobokan drainage (figure 54).

Figure 52. Photo of a dense gray ash plume rising up to 2 km above the summit crater at Semeru on 16 January 2021. Courtesy of BNPB.

Figure 53. Webcam image of dense gray ash clouds due to a pyroclastic flow from Semeru drifting up to 4 km to the SE and S on 16 January 2020. Courtesy of MAGMA Indonesia.

Figure 54. Photo of an incandescent avalanche of material descending the SE flank at Semeru on 17 January 2021. Incandescence and a gray ash plume are also visible in the summit crater. Photo by Agus Harianto; courtesy of AFP/Getty Images.

Gray-and-white plumes continued to be observed during 18-26 January rising 200-500 m above the crater and drifting generally N. Daily incandescent avalanches of material from the Jonggring-Seloko vent during 19-26 January extended 200-1,000 m down the SE flank, accompanied by frequent incandescent ejecta up to 50 m above the crater that continued to be reported through 28 January (figure 55). On 2 February a pyroclastic flow from the end of an incandescent avalanche traveled 2 km down the Kobokan drainage. Activity decreased after the pyroclastic flow on 2 February. White-and-gray ash plumes were reported during 17-18 and 24-25 February, rising 100-500 m above the crater and drifting N, NE, E, and SE.

Figure 55. Sentinel-2 thermal satellite images showing an incandescent summit crater at Semeru on 20 January (left) and 19 February (right) 2021. On 20 January a faint incandescent avalanche of material was visible traveling down the SE flank. Clouds prevented a clear view of the SE flank on 19 February 2021. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

Sentinel-2 thermal satellite imagery shows this thermal activity originating from the summit crater on clear weather days throughout the reporting period. Summit crater incandescence was visible intermittently on 2, 7, 12, 17, and 27 September, and 2, 7, 17, and 22 October; on 17 and 22 October faint incandescence on the SE flank was visible, possibly representing a small avalanche of hot material. On 16 and 21 November the thermal anomaly increased in strength and was accompanied by an incandescent avalanche down the SE flank (figure 45). Further SE-flank Incandescent avalanches of varying lengths were seen on 11, 16, 21, and 31 December. The avalanche on 21 December was accompanied by a white gas-and-steam plume (figure 45). Though clouds prevented a clear view of the various avalanches during January and February, thermal activity was still visible in the summit crater, with a small incandescent avalanche traveling down the SE flank on 20 January (figure 55). Crater incandescence continued through late February.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed intermittent low-power thermal anomalies during September through mid-November (figure 56). A pulse in thermal activity was detected from mid-November through early December with more frequent and moderate-power anomalies, coincident with more eruptive events. In late December another period of stronger thermal activity began that continued through early February 2021. During February the frequency of the anomalies decreased and were variable in power. The MODVOLC algorithm detected thermal anomalies on 11 different days: 24 October; 18, 19, 21, and 23 November; 23, 25, and 28 December 2020; 10 and 24 January; and 13 February 2021. The elevated thermal activity in December is also reflected in the MODVOLC algorithm on 23 December with a high hotspot count of six.

Figure 56. Thermal anomalies at Semeru during September through mid-November 2020 were of variable low intensities and occurred intermittently. In mid-November thermal anomalies increased in frequency and power, a period that continued through early December. In late December another pulse of thermal activity was recorded which continued through early February 2021. During February the thermal anomalies decreased in frequency but were variable in power. Courtesy of MIROVA.

Geologic Background. Semeru, the highest volcano on Java, and one of its most active, lies at the southern end of a volcanic massif extending north to the Tengger caldera. The steep-sided volcano, also referred to as Mahameru (Great Mountain), rises above coastal plains to the south. Gunung Semeru was constructed south of the overlapping Ajek-ajek and Jambangan calderas. A line of lake-filled maars was constructed along a N-S trend cutting through the summit, and cinder cones and lava domes occupy the eastern and NE flanks. Summit topography is complicated by the shifting of craters from NW to SE. Frequent 19th and 20th century eruptions were dominated by small-to-moderate explosions from the summit crater, with occasional lava flows and larger explosive eruptions accompanied by pyroclastic flows that have reached the lower flanks of the volcano.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Badan Nasional Penanggulangan Bencana (BNPB), National Disaster Management Agency, Graha BNPB - Jl. Scout Kav.38, East Jakarta 13120, Indonesia (URL: http://www.bnpb.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); 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/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Øystein Lund Andersen (Twitter: @OysteinLAnderse, https://twitter.com/OysteinLAnderse, URL: http://www.oysteinlundandersen.com); Agence France-Presse (URL: http://www.afp.com/); Reuters (http://www.reuters.com/).

Klyuchevskoy, Kamchatka’s tallest volcano, has had over 100 recorded historical eruptions during the last 300 years characterized by major explosive and effusive events from summit and flank craters. Activity since April 2019 has consisted of ash plumes, Strombolian activity, and active lava flows on multiple flanks. Renewed Strombolian activity and lava flows began in October 2020 and continued into 2021, ending in late March. January-March 2021 is covered in this report with information provided by the Kamchatkan Volcanic Eruption Response Team (KVERT), the Kamchatka Volcanological Station, the Tokyo Volcanic Ash Advisory (VAAC), and satellite data.

Renewed explosive and effusive activity that began in October 2020 (BGVN 46:01) continued into 2021. A lava flow was active in the Kozyrevsky chute on the SW flank through mid-January; a flow again entered the SE-flank Apakhonchi chute in late January. Flow activity and Strombolian explosions from the summit ended in early February. A new vent near the base of the NW flank opened up on 17 February, sending a lava flow N and NW until the third week of March. Steam and gas emissions continued at the summit through the end of March. Explosive activity was very high during January 2021 with one or more ash explosions almost every day; significant ashfall occurred in mid-January. After a significant explosion in early February, subsequent ash plumes were all caused by resuspended ash from earlier events.

The MIROVA thermal anomaly graph for Klyuchevskoy closely follows the activity witnessed on the ground and seen in satellite images. The reappearance of very high levels of Radiative Power in early October 2020 coincided with the effusion of lava flows that remained active until early February 2021. A pulse of thermal energy that began in mid-February corresponded to the new vent on the NW flank about 5 km from the summit (figure 55). Activity stopped at the new vent in mid-March, but low levels of thermal energy remained from the cooling flows and from steam and gas emissions at the summit. MODVOLC thermal alert data included near-daily alerts from 1 January through 3 February. A second pulse of alerts was recorded from 20 February-19 March 2021.

Figure 55. The MIROVA thermal anomaly graph for Klyuchevskoy during April 2020-March 2021 reflects observed surface activity. High levels of Radiative Power in early October 2020 coincided with lava flows on the SE and SW flanks through early February 2021. Thermal anomalies more than 5 km from the summit that began later in February corresponded to effusion at the NW-flank vent. Courtesy of MIROVA.

The Tokyo VAAC reported an ash plume that rose to 7.6 km altitude and drifted SE on 1 January 2021. Two explosions the next day produced ash plumes that drifted SE at 6.1 km altitude. The Kamchatka Volcanological Station reported a landslide in the Apakhonchi chute on the SE flank on 2 January that generated an ash cloud which rose 1 km above the slope and drifted SE (figure 56). One or two daily explosions were reported through 7 January, generating ash plumes that rose to 6.1-7 km altitude and drifted in multiple different directions. KVERT reported ash plumes drifting as far as 70 km the first week and 90 km the second week of the month along with Strombolian and some Vulcanian activity (figure 57). Multiple explosions during 12-17 January produced ash that drifted NE or E at 6.1-7 km altitude, sometimes reaching 200 km from the volcano.

Figure 56. Ash rose from the flank of Klyuchevskoy on 2 January 2021 from a landslide in the Apakhonchi ravine on the SE flank while dense steam plumes rose from the summit crater. Photo by Y. Demyanchuk, courtesy of Kamchatka Volcanological Station.

Figure 57. Strombolian activity sent ejecta tens of meters above the summit of Klyuchevskoy on 14 January 2021. Photo by Y. Demyanchuk, courtesy of Kamchatka Volcanological Station.

A strong explosion on 18 January 2021 sent ash to 7.5 km altitude that drifted 500 km W. Two hours after the explosion ashfall began in Kozyrevsk village 50 km W. After a large snowstorm the next day 1 cm of ash was measured under 50 cm of fresh snow (figure 58). During 20-23 January ash plumes rose to 6.1-7.6 km altitude and drifted SW and NE. A large explosion on 24 January produced ash that initially rose to 8.5 km altitude and drifted W. Ash was later reported extending W at 10.4 km altitude and remained visible in satellite imagery into the next day, drifting 60 km N then NE. The explosion resulted in the collapse of areas along the edge of the Apakhonchi chute and produced multiple ash avalanches that descended 1,700 m from the summit in a few minutes (figure 59). During 28-31 January ash emissions rose to 3-6.1 km altitude and drifted NE and NW; KVERT reported an ash plume extending 92 km E on 29 January.

Figure 58. A layer of ash about 1 cm thick fell near Kozyrevsk village, 50 km W of Klyuchevskoy, after explosions on 18 January 2021; it was sampled two days later following a large snowstorm. Photo by Y. Demyanchuk, courtesy of Kamchatka Volcanological Station.

Figure 59. Ash plumes descended the SE flank of Klyuchevskoy on 24 January 2021 from a collapse of the summit crater rim. Photo by Y. Demyanchuk, courtesy of Kamchatka Volcanological Station.

At the beginning of January 2021, the lava flow in the Kozyrevsky chute on the SW flank that first appeared in images on 10 December 2020 remained active; it was visible in Sentinel 2 satellite imagery on 2, 4, and 7 January (figure 60). In the 14 and 17 January images it was about half as long as it had been previously. In a 22 January image a second flow in the Apakhonchi chute on the SE flank was partly visible under clouds, with a short flow remaining in the SW Kozyrevsky chute. KVERT first reported activity in the Apakhonchi chute on 21 January; satellite imagery on 24, 27, and 29 January and a photo from 28 January confirmed that it remained active while the flow in the Kozyrevsky chute had cooled (figure 61).

Figure 60. Lava flows were active on the SW and SE flanks of Klyuchevskoy during January 2021. An active flow descended the SW-flank Kozyrevsky chute on 4 January (left) and the SE-flank Apakhonchi chute on 29 January. Images use Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Figure 61. Lava continued to flow down the Apakhonchi chute on the SE flank of Klyuchevskoy on 28 January 2021. The image is misdated as 2020. To the left of Klyuchevskoy is Kamen and then the smaller Bezymianny lies farther to the left (summits are about 10 km apart). Photo by Y. Demyanchuk, courtesy of Kamchatka Volcanological Station.

There were far fewer ash emissions during February than January 2021. The Tokyo VAAC reported ash plumes on 2 and 3 February that rose to 5-6.1 km altitude and drifted E, NE, and W. An explosion on 4 February sent an ash plume ESE at 7 km altitude. Two large explosions on 5 February sent ash plumes that drifted ENE to 8.2 and 9.1 km altitude (figure 62). Continuous emissions that drifted W at 6.1 km altitude were noted on 7 February; KVERT reported satellite imagery of ash drifting 200 km W that day. KVERT issued a VONA reporting a significant decrease in seismicity overnight on 7-8 February. According to the seismological service, the volcanic tremor signal dropped to values less than 1 μm/s. The Kamchatka Volcanological Station reported that effusive activity stopped overnight during 8-9 February, and KVERT considered the summit eruption over. A small cinder cone had grown in the SE part of the summit crater during the eruption. During 12-14 February the Tokyo VAAC reported resuspended ash from the flanks at 4.6-5.2 km altitude. Sulfur dioxide emissions are difficult to detect in satellite instruments at high latitudes in winter. In spite of this, distinct SO₂ plumes were recorded on 4 and 15 February drifting away from the volcano (figure 63).

Figure 62. A dense ash emission from Klyuchevskoy on 5 February 2021 rose to over 8 km altitude drifted ENE. Continuous emissions during the next two days resulted in a plume visible in satellite imagery over 200 km W. Photo by Y. Demyanchuk, courtesy of Kamchatka Volcanological Station.

Figure 63. SO2 plumes were measured by the TROPOMI instrument on the Sentinel-5P satellite drifting NW and N from Klyuchevskoy on 4 and 15 February 2021. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

The lava flow on the SE flank was active in the Apakhonchi chute during the first week of February 2021, visible in satellite imagery on 1, 3, and 6 February. After that there was no sign of thermal activity at the summit or either the SW or SE chutes on 13 or 16 February. KVERT reported in a VONA on 17 February that a lateral vent on the NW flank began to open around 1130 UTC at an elevation of about 2,800 m near the Erman glacier. Snowfall during 17-19 February prevented video observation, but bright incandescence that appeared on 20 February suggested lava was flowing from the vent. Satellite imagery on 21 February confirmed incandescence and lava flows at two vents on the NW flank; they continued to grow and were both visible in images on 23 February (figure 64).

Figure 64. Sentinel-2 satellite infrared images with Atmospheric penetration rendering (bands 12, 11, 8a) showed the changes of effusive activity at Klyuchevskoy during February 2021. On 1 February (top left) a flow was active in the Apakhonchi chute on the SE flank. A steam plume rises from the head of the flow at the edge of the summit crater. By 16 February only steam emissions were present at the summit (top right). The first satellite image indicating a new vent on the NW flank was 21 February (bottom left). A closeup of the vent from 23 February showed two lava flows extending NNW (bottom right). Courtesy of Sentinel Hub Playground.

The lava flows on the NW flank melted parts of the Erman glacier and produced a lahar that was reported on 23 February 2021 traveling down the Krutenkaya River about 7 km from the village of Klyuchi. Strombolian ejecta sent bombs 50 m from the vents on 25 February and a small cinder cone had formed at the upper vent (figure 65). Satellite images showed two flows on 26 February but only one was active on 28 February (figure 66).

Figure 65. The new vent and lava flow on the NW flank of Klyuchevskoy produced Strombolian activity and phreatic explosions where the lava met the Erman glacier on 25 February 2021. A small pyroclastic cone is outlined in the glow from the Strombolian activity in the enlarged inset. Photos by Y. Demyanchuk, courtesy of Kamchatka Volcanological Station.

Figure 66. Two flows were active from the vent on the NW flank of Klyuchevskoy on 26 February 2021 (left) but only one was growing in length on 28 February (right). Sentinel-2 images use Atmospheric penetration rendering (bands 12, 11, 8A). Courtesy of Sentinel Hub Playground.

An ash plume was reported by the Tokyo VAAC drifting E at 4.9 km altitude on 5 March 2021. KVERT noted that the ash was resuspended from material on the E flank and extended 375 km E of the volcano. KVERT reported that the lava flow on the NW flank remained active for the first two weeks of March 2021 with Strombolian activity at the cinder cone at the head of the flow. On 2 March a group of volcanologists from the IVS FEB RAS visited the site of the cinder cone and flow on the NW flank. The cone measured about 54 m high and 101 m wide at the base (figure 67). The lava flowed NW, penetrating into the Erman glacier, causing melting and mudflows in the Krutenkaya River (figure 68).

Figure 67. A team of volcanologists from IVS FEB RAS visited the site of the eruption on the NW flank of Klyuchevskoy on 2 March 2021. Ejecta from the growing pyroclastic cone at the vent rose tens of meters high and landed nearby. Photo by A. Elistratov, courtesy of Kamchatka Volcanological Station.

Figure 68. The lava flow on the NW flank of Klyuchevskoy persisted for the first two weeks of March 2021. Multiple branches of lava flowed NW from the new vent on 5 March 2021 (top left), and by 10 March it was focused into a single flow with steam plumes rising from multiple locations (top right). The path of the flow on 13 March was visible in both Atmospheric penetration rendering (bands 12, 11, and 8A) (bottom left) that showed the thermal anomaly and in the Natural color rendering (bands 4,3,2) (bottom right) that showed the dark area of the flow surrounded by ice and snow. Courtesy of Sentinel Hub Playground.

The Kamchatka Volcano Station reported on 14 March 2021 that the flow rate had decreased, and the lava was flowing N (figure 69). A field survey from the Station noted a decrease in activity at the NW-flank vent, now referred to as the Gorshkov vent, on 16 March (figure 70). They reported that only a small flow emerged from the W foot of the cone and flowed N (figure 71). Incandescent ejecta rose about 100 m above the cinder cone every few seconds. KVERT reported that the NW flank activity had stopped by 22 March. Weak incandescence was still observed over the cinder cone and lava flows in video images, but the intensity had decreased significantly. During the last week of March strong winds were responsible for plumes of resuspended ash from loose material on the SE flank near the Apakhonchi chute. On 25 March ash was visible in satellite imagery drifting E at 5.2-5.5 km altitude. KVERT reported it visible 72 km E from the volcano. Gas and steam emissions from the summit crater continued through the end of March (figure 72).

Figure 69. The NW flank flow at Klyuchevskoy shifted more towards the N on 14 March 2021 and was visible in a Sentinel-2 satellite image the next day (left). By 20 March the flow rate had diminished significantly, producing a much smaller thermal anomaly (right). Images use Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Figure 70. A team from the Kamchatka Volcanological Station visited the new cone and vent on the NW flank of Klyuchevskoy on 16 March 2021, noting that activity had decreased since a visit earlier in the month. The active flow is on the left below the cone. In this clear view to the N, beyond the cone Ushkovsky is on the left about 10 km away with the smaller Sredny cone in front of it, and Sheveluch is on the right about 75 km NE. Photo by Y. Demyanchuk, courtesy of Kamchatka Volcanological Station.

Figure 71. The lava flow from the Gorshkov vent on the NW flank of Klyuchevskoy was slowly moving N from the W base of the cone when surveyed on 16 March 2021 by scientists from the Kamchatka Volcanological Station. Video by Y. Demyanchuk, courtesy of Kamchatka Volcanological Station.

Figure 72. Gas and steam emissions from the summit crater of Klyuchevskoy are seen here on 24 March 2021. Photo by Y. Demyanchuk, courtesy of Kamchatka Volcanological Station.

A large plume of SO₂ drifted SW from Klyuchevskoy on 19 March 2021 and was still present the next day drifting E (figure 73). Multiple days of smaller but distinct SO₂ emissions were detected during February and March 2021.

Figure 73. A large plume of SO2 drifted SW from Klyuchevskoy on 19 and 20 March 2021. A smaller plume drifted S on 28 March. Steam and gas emissions from the summit were the only noted activity after the lava flow from the vent on the NW flank had stopped advancing by 22 March. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. Klyuchevskoy (also spelled Kliuchevskoi) is Kamchatka's highest and most active volcano. Since its origin about 6000 years ago, the beautifully symmetrical, 4835-m-high basaltic stratovolcano has produced frequent moderate-volume explosive and effusive eruptions without major periods of inactivity. It rises above a saddle NE of sharp-peaked Kamen volcano and lies SE of the broad Ushkovsky massif. More than 100 flank eruptions have occurred during the past roughly 3000 years, with most lateral craters and cones occurring along radial fissures between the unconfined NE-to-SE flanks of the conical volcano between 500 m and 3600 m elevation. The morphology of the 700-m-wide summit crater has been frequently modified by historical eruptions, which have been recorded since the late-17th century. Historical eruptions have originated primarily from the summit crater, but have also included numerous major explosive and effusive eruptions from flank craters.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); Kamchatka Volcanological Station, Klyuchi, Kamchatka Krai, Russia (URL: http://volkstat.ru/); 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/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/).

Sarychev Peak forms the surface of Matua Island in the Kurile Islands with reported activity dating back to around 1765. Recent activity that started in May 2019 included ash and gas emission and elevated temperatures within the summit crater detected by satellite sensors, with the last reported activity being an ash plume reaching 2.7 km altitude on 10 August and thermal anomalies present until 7 October 2019 (BGVN 44:11). This bulletin summarizes activity during November 2019-April 2021 using reports by the Sakhalin Volcanic Eruption Response Team (SVERT) and the Kamchatka Volcanic Eruptions Response Team (KVERT), along with satellite data.

No cloud-free satellite images were found of the summit in November 2019, but Sentinel-2 satellite images showed weak gas-and-steam emissions on 2 and 20 December. Cloud-free Sentinel-2 images showed gas-and-steam emission through January 2020, and a thermal anomaly was detected in the crater on the 29th (figure 30). No clear satellite images of the summit area were found, but there is evidence of gas emission in February. Evidence of a new eruption is seen in satellite imagery of thin linear ash deposits across the snow on 1, 19, and 30 March 2020, all extending SE from the crater (figure 31). The crater was obscured by gas emissions on the 19th and a clear view of the crater floor showed no thermal anomaly on the 31st.

Figure 30. These thermal satellite images show the Sarychev Peak summit area in December 2019 and January 2020. The images from 2 December 2019, 6 January, and 19 January 2020 show gas emissions (solid arrows). The 29 January image shows a small area with an elevated temperature on the crater floor (dashed arrow). Sentinel-2 thermal satellite images with false color (urban) (bands 12, 11, 4) rendering. Courtesy of Sentinel Hub Playground.

Figure 31. Three ashfall deposits are visible SW of the Sarychev Peak summit through March 2020. Based on satellite images, the deposit at the top was emplaced during an event that occurred during 28 February (ash-free image) and 1 March, the middle during 17 (ash-free image) and 19, and the bottom during 26 (ash-free image) and 29 March. Gas-and-steam emissions are obscuring the view into the crater. All images are at the same scale. Sentinel-2 satellite image with natural (bands 4, 3, 2) rendering. Courtesy of Planet Labs.

The MIROVA system began detecting elevated temperatures in early April 2020, which corresponded to the Sentinel-2 thermal sensor detecting high temperatures on the crater floor (figures 32 and 33). Satellite images also showed continued gas emissions, some days obscuring the view of the crater floor.

Figure 32. This plot shows thermal energy detected at Sarychev Peak by the MIROVA system during March 2020-March 2021. there was an increase in energy detected in April 2020, which was intermittent through to October. After a few months the system detected thermal energy again in mid-January through to early February with a higher output. Courtesy of MIROVA.

Figure 33. Satellite images showing the Sarychev Peak summit crater on 4, 5, 18, and 20 April 2020. The first (top left) PlanetScope image shows the snow-covered summit area with a darker snow-free area on the crater floor. The other three images are Sentinel-2 thermal satellite images with the yellow to red colors indicating high temperatures on the crater floor. There is gas and steam in the crater on the 18th. The high temperature areas correlate to the darker snow-free area in NW part of the crater in the first image; blue colors in the thermal images are snow. Sentinel-2 thermal satellite images have false color (urban) (bands 12, 11, 4) rendering. Courtesy of Planet Labs and Sentinel Hub Playground.

The thermal anomaly on the crater floor continued through May and June, with cloud-free images showing the same area of elevated temperature as the previous months. By 20 May 2019 data from Sentinel-1 Synthetic Aperture Radar (SAR) showed morphological change in the crater associated with the area of high temperature, and this change continued through June. The TROPOspheric Monitoring Instrument (TROPOMI) detected sulfur dioxide (SO2) content within the plume on 27 May (figure 34). Gas-and-steam emission also continued in June, with more substantial plumes visible on 22 and 27 June (figure 35). TROPOMI again detected SO2 on 24 and 25 June; the plume on 24 June was also visible in Sentinel-2 imagery (figure 36).

Figure 34. This image shows a weak gas plume from Sarychev Peak dispersing to the SE on 27 May 2020, as well as other volcanoes in Kamchatka. TROPOspheric Monitoring Instrument (TROPOMI) data showing sulfur dioxide (SO2) in Dobson Units (DU). Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Figure 35. These Planet Scope satellite scenes show gas-and-steam plumes emanating from the Sarychev Peak summit crater and dispersing SSW (left) and NW (right) on 22 and 27 June 2020, respectively. Courtesy of Planet Labs.

Figure 36. Weak gas emission at Sarychev Peak detected by satellite sensors on 25 and 26 June 2020. The top image and the bottom-left images were acquired on the 25th and show the plume being redirected by a meteorological vortex northward before curving to the W and N. Top: Sentinel-2 satellite image with natural color (bands 4, 3, 2) rendering. Courtesy of Planet Labs. Bottom: TROPOspheric Monitoring Instrument (TROPOMI) data showing sulfur dioxide (SO2) in Dobson Units (DU). Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Throughout July satellite data show thermal emission and gas-and-steam emission, mostly within plumes dispersing from the summit crater in different directions and sometimes restricted to within the crater (figure 37). On 18 July a PlanetScope image showed lava extrusion in the crater, at the location of the elevated temperature. Sentinel-2 thermal satellite images showed weak thermal energy detected in the same location during August, and degassing continued (figure 38). By 12 August the deformation on the crater floor was clear in SAR data (figure 39), matching the PlanetScope and Sentinel-2 data. From 21 August through to 12 October there was a reduction in thermal energy detected in Sentinel-2 TIR data, with many days not having clear views of the crater floor. Plume emission continued throughout this time. There were no images showing elevated temperatures during November and December when clouds frequently covered the crater area, and there were also no anomalies detected by the MIROVA system.

Figure 37. The PlanetScope natural color (top) and Sentinel-2 thermal (bottom) satellite images indicate lava in the crater during July 2020. Gas emission is also visible in the images. Sentinel-2 thermal satellite images have false color (urban) (bands 12, 11, 4) rendering. Courtesy of Planet Labs and Sentinel Hub Playground.

Figure 38. PlanetScope and Sentinel-2 satellite images acquired during August 2020 show lava in the crater and gas-and-steam plumes being dispersed in different directions by winds. Sentinel-2 satellite image with natural color (bands 4, 3, 2) rendering. Courtesy of Planet Labs and Sentinel Hub Playground.

Figure 39. These satellite images show the morphological change in the Sarychev Peak summit crater between 10 November 2019 and 12 August 2020. The three gray-scale images use Sentinel-1 Synthetic Aperture Radar (SAR) data acquired on 10 November 2019, 20 May, and 12 August 2020. The color image in the lower left is a Sentinel-2 thermal image acquired on 22 June 2020. The SAR images show morphological changes in the crater in the same location as the elevated temperatures in the thermal images, indicating lava extrusion. Sentinel-1 SAR images are VV, decibel gamma0, and orthorectified. Sentinel-2 thermal satellite images have false color (urban) (bands 12, 11, 4) rendering. Courtesy of Sentinel Hub Playground.

On 11 January 2021 KVERT released a Volcano Observatory Notice for Aviation (VONA) with an elevation of the Aviation Color Code from Green to Yellow. The temperature within the crater had increased above background levels by 79.8°?, indicating that renewed lava extrusion had begun in the crater on the 10th. A gas-and-steam plume extended 36 km NE on the 12th. On 15 January KVERT reported that moderate activity continued, including a gas-and-steam plume that extended 40 km NE. SAR data through January shows the lava volume increasing before flowing over the NW rim and down a preexisting channel on the flank (figure 40). KVERT reported that a lava flow on the northern flank had reached 400 m by the 20th. Lava extrusion with associated moderate gas and steam emission continued throughout the month.

Figure 40. These SAR images of Sarychev Peak during 3 January to 20 February 2021 show lava extrusion filling the summit crater and descending a channel on the NW flank. Note that the 6 January image has a different look angle to the other images, and this alters how the surface appears. Sentinel-1 SAR images are VV, decibel gamma0, and orthorectified. Courtesy of Sentinel Hub Playground.

A 3 February satellite image of the NW flank showed that the lava flow front had reached approximately 1.9 km from the crater rim where it had overflowed (figure 41). The Aviation Color Code was lowered to Green on the 18th with KVERT reporting that the eruption had ended, though thermal anomalies and gas-and-steam emission continued.

Figure 41. Satellite image scenes show the lava flow at Sarychev Peak on 3 and 14 February 2021. Top: PlanetScope image from 3 February showing the lobate lava flow front approximately 1.9 km from the NW crater rim. Bottom: Sentinel-2 satellite scenes from 14 February (thermal infrared to the left and natural color to the right) showing the summit crater area with lava extrusion and the lava flow overtopping the NW rim. Sentinel-2 satellite images have natural color (bands 4, 3, 2) rendering, and thermal false color (urban) (bands 12, 11, 4) rendering. Courtesy of Planet Labs and Sentinel Hub Playground.

Satellite images of the lava flow acquired during March and April show the narrow lava lobe with pressure ridges and levees (figure 42). A comparison between a September 2019 satellite image and a clear 29 April 2021 image shows the change to the crater after the lava emplacement. The last Sentinel-2 image acquired within this period showing elevated temperatures within the crater was on 19 March and there was no more thermal energy detected by the MIROVA system by early February.

Figure 42. The PlanetScope satellite images across the top of this figure show the lava flow on the NW flank of Sarychev Peak during March-April 2021. The different degrees of snow cover show different surface morphological aspects like pressure ridges and levees. The bottom images show the crater on 7 September 2019 for comparison (left) and the lava within the summit crater on 29 April 2021 (right). Fumaroles are also visible around the crater walls in the 2019 image. The top images and bottom right image are PlanetScope satellite images and the lower left image is by CNES/Airbus through Google Earth. Courtesy of Planet Labs and U.S. Dept. of State Geographer Data via Google Earth, ©2019 Google.

Geologic Background. Sarychev Peak, one of the most active volcanoes of the Kuril Islands, occupies the NW end of Matua Island in the central Kuriles. The andesitic central cone was constructed within a 3-3.5-km-wide caldera, whose rim is exposed only on the SW side. A dramatic 250-m-wide, very steep-walled crater with a jagged rim caps the volcano. The substantially higher SE rim forms the 1496 m high point of the island. Fresh-looking lava flows, prior to activity in 2009, had descended in all directions, often forming capes along the coast. Much of the lower-angle outer flanks of the volcano are overlain by pyroclastic-flow deposits. Eruptions have been recorded since the 1760s and include both quiet lava effusion and violent explosions. Large eruptions in 1946 and 2009 produced pyroclastic flows that reached the sea.

Information Contacts: Sakhalin Volcanic Eruption Response Team (SVERT), Institute of Marine Geology and Geophysics, Far Eastern Branch, Russian Academy of Science, Nauki st., 1B, Yuzhno-Sakhalinsk, Russia, 693022 (URL: http://www.imgg.ru/en/, http://www.imgg.ru/ru/svert/reports); Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); 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); Planet Labs, Inc. (URL: https://www.planet.com/); Google Earth (URL: https://www.google.com/earth/).

Kavachi is an active submarine volcano in the SW Pacific, located in the Solomon Islands south of Gatokae and Vangunu islands. Volcanism has been characterized by phreatomagmatic explosions that ejected steam, ash, and incandescent bombs. The previous report described discolored plumes extending from a single point during early September 2020 (BGVN 45:10); similar activity was recorded for this reporting period covering October 2020 through April 2021 using satellite data.

Activity at Kavachi is most frequently observed through Sentinel-2 satellite imagery and has recently been characterized by discolored submarine plumes. On 2 October 2020 a slight yellow-green discoloration in the water was observed extending NE from a specific point (figure 23). Similar faint discolored plumes were intermittently recorded on 27 October, 1 November 2020, and 25 January 2021, which each extended NE, SW, and SW, respectively, from a point source above the summit where previous activity has occurred. Intermittent discolored plumes were also visible during March 2021 (figure 24). The plume discoloration on 1 March extended S from the origin point. On 11 March, the discoloration remained near the origin point. A narrow plume extended several kilometers W on 26 March, followed by a short plume seen towards the NW on 31 March. The only plume seen in April was a broad diffuse area of discoloration extending S on the 10th (figure 24). No discoloration near the volcano was observed in May.

Figure 23. Sentinel-2 satellite images of a discolored plume (light yellow-green) at Kavachi beginning on 2 October 2020 (top left) that extended NE. Additional plumes were visible during clear weather on 27 October (top right) that extended NE, on 1 November (bottom left) 2020 that extended SW, and strongly on 25 January 2021 (bottom right) that extended SW. Images with “Natural color” rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

Figure 24. Sentinel-2 satellite imagery of discolored plumes (light yellow-green) at Kavachi during March-April 2021. On 1 March (top left) the plume was observed extending S with a strongly discolored origin point. On 11 March (top right) the plume remained close to the origin point and did not seem to extend outward. On 26 March (bottom left) the plume was narrow and strongly extended W for several kilometers. On 10 April (bottom right) the plume extended S. Images with “Natural color” rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

Geologic Background. Named for a sea-god of the Gatokae and Vangunu peoples, Kavachi is one of the most active submarine volcanoes in the SW Pacific, located in the Solomon Islands south of Vangunu Island. Sometimes referred to as Rejo te Kvachi ("Kavachi's Oven"), this shallow submarine basaltic-to-andesitic volcano has produced ephemeral islands up to 1 km long many times since its first recorded eruption during 1939. Residents of the nearby islands of Vanguna and Nggatokae (Gatokae) reported "fire on the water" prior to 1939, a possible reference to earlier eruptions. The roughly conical edifice rises from water depths of 1.1-1.2 km on the north and greater depths to the SE. Frequent shallow submarine and occasional subaerial eruptions produce phreatomagmatic explosions that eject steam, ash, and incandescent bombs. On a number of occasions lava flows were observed on the ephemeral islands.

Information Contacts: Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Extensive lava flows, bomb-laden Strombolian explosions, and ash plumes from Mackenney crater have characterized the persistent activity at Pacaya since 1961. The latest eruptive period began with intermittent ash plumes and incandescence in June 2015; the growth of a new pyroclastic cone inside the summit crater was confirmed later that year and has continued, producing frequent loud Strombolian explosions rising above the crater rim and ongoing ash emissions. In addition, flank fissures have been the source of lava flows during 2019-2021. A significant increase in both effusive and explosive activity that began in February 2021 continued through mid-May. Activity during March-May 2021 is covered in this report with information provided by Guatemala's Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), multiple sources of satellite data, and photographs from observers on the ground.

Summary of activity during March-May 2021. Incandescent explosions, ash emissions, and subsequent ashfall increased substantially at the beginning of March 2021 from the already increased levels during February. Explosions sent ejecta hundreds of meters high and hundreds of meters from the summit; ash plumes drifted tens to hundreds of kilometers and ashfall occurred almost daily in communities within tens of kilometers of Mackenney crater. The most extensive ash emissions forced closure of the International Airport in Guatemala City on 22 March. Ash emissions decreased during April and were intermittent into the first half of May, after which they tapered off.

Effusive activity also increased significantly during March 2021; by early in the month as many as three lava flows with multiple branches, all about 1 km long, were simultaneously active on multiple flanks. A new fast-moving flow appeared on the SW flank during the second half of March and rapidly reached 1.5 km in length, flowing NW then SW, ultimately extending over 3 km. It had multiple branches that caused vegetation fires, destroyed significant cropland, and crossed roads before stopping in mid-April. A new flow emerged along a similar path at the end of April and grew to over 2 km long in early May before activity at its source fissure ended on 17 May. High temperatures remained at many flow areas around the volcano for the rest of the month.

The high levels of activity are reflected in the MIROVA radiative power data for the period which show the increase in intensity to very high levels through mid-April, followed by a pulse in late April and early May that corresponds to explosions and lava flows. Thermal activity decreased significantly by the third week of May (figure 160). The MODVOLC thermal alert data shows a similar pattern with multiple alerts issued most days in March and for the first half of April, and another pulse of activity from 27 April-13 May. Significant sulfur dioxide emissions were recorded in satellite data several times in March and April and corresponded to periods of increased explosive and effusive activity (figure 161).

Figure 160. The ongoing eruption at Pacaya increased significantly in intensity in December 2020 and continued to increase through March 2021 as seen in this MIROVA log radiative power graph. Abundant ash emissions and extensive lava flows emerged from numerous fissures until activity decreased substantially in mid-May 2021. Courtesy of MIROVA.

Figure 161. Pulses of increased sulfur dioxide emissions at Pacaya were measured by the TROPOMI instrument on the Sentinel-5P satellite multiple times during March and April 2021, including (top row, left to right) on 5, 10, and 21 March, and (bottom row) 6, 8, and 16 April. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Activity during March 2021. A notable increase in seismicity early on 1 March 2021 coincided with increased Strombolian activity. Observatorio Volcán de Pacaya geologists observed explosions sending ejecta 500 m above the rim of Mackenney crater accompanied by plumes of ash and gas that reached 3.5 km altitude and drifted W and SW. For most of March high levels of Strombolian activity sent ejecta 200-400 m high each day, sometimes higher, reaching 800 m on 3 March, 800-1,000 m on 5 March, and 700 m on 10 March (figure 162). Sounds as loud as a train locomotive or plane engine from the explosions were frequently reported, and ejecta was sometimes scattered 500-600 m from the cone. Explosive activity with ejecta and ash emissions were also reported from the fissure feeding the lava flow on the S flank 300 m below Mackenney crater. On 14 March, ejecta from the fissure sent block avalanches 1,300 m down the S flank.

Figure 162. Strombolian activity at Pacaya sent ejecta hundreds of meters above the summit and down the flanks on 4 March 2021 while effusion continued on the SW flank, also producing an intense glow. Image by Reuters photographer Josue Decavele taken from Los Rios. Courtesy of Reuters Pictures.

The increase in explosive activity also included an increase in dense ash emissions and resulting ashfall during March 2021. Ash plume heights ranged from 3 to 5.5 km altitude, and often drifted W, NW, or SW. The Washington VAAC reported an ash plume centered about 75 km WSW of the summit on 1 March. On 3 March a dense ash emission was drifting W from the summit at 3.7 km altitude. The next day ash was detected almost 100 km SW just off the Mexican Pacific coast before dissipating. The altitude of the ash emissions increased to 4.9 km on 5 March; puffs drifting W were visible in satellite images extending over 250 km from the summit the next day. Pulses of activity lasted between 15 minutes and 13 hours, and produced tephra fallout around the volcano, dense ash plumes that drifted 3-5 km, and finer ash plumes that drifted more than 60 km.

Explosions on 7 March caused lava fountains 100-500 m above the crater. The following day ash plumes were drifting 45 km SW at 3 km altitude. On 9 March ash plumes fanned out from the NW to the SW about 30 km from the summit before dissipating. From 11 March onward multiple daily discrete ash emissions extended at least 30-50 km WNW and SW from the summit at altitudes of 3.7-4.3 km altitude, and much farther on some days. The plumes reached 90 km WSW on 12 March, and 140 km W on 14 March. The next day, ash emissions extended over 100 km WSW, with remnants visible in satellite images almost 185 km away by the end of the day. On 16 March they drifted 170 km WNW at 4.3 km altitude and on 18 March the ash emissions were observed drifting SW at 3.4 km altitude extending 185 km from the summit. Dense gray-black emissions were accompanied by white steam emissions on 21 March (figure 163).

Figure 163. Dense dark gray ash emissions rose from the summit of Pacaya on 21 March 2021 causing significant ashfall around the region. In addition, white steam plumes surrounded the summit. Courtesy of CONRED.

Dense ash clouds seen on 22 March 2021 were drifting rapidly SSE at 4.9 km altitude as far as 75 km, SE at 6.1 km altitude, and visible in satellite imagery moving E at 7.6 km altitude up to 25 km from the volcano. The next day they were drifting NE at 3 km altitude up to 90 km away, and SW at 4.6 km altitude. A narrow ash plume was detected in visible satellite imagery on 28 March drifting about 80 km NW of the summit before dissipating. Over the next two days a plume was detected moving SW at 3 km altitude about 130-150 km from the summit. In addition, another plume was drifting NW at 4.3 km altitude on 31 March causing dense ash to cover the summit of Fuego that was visible on webcams. The lower plume was visible over 300 km SW of Pacaya before it dissipated (figure 164).

Figure 164. Haze from ash emissions at Pacaya extends for tens of kilometers across the region in multiple directions after many days of emissions, while a fresh ash plume rises above the volcano in the left foreground on 31 March 2021. Ash drifted NW up to 50 km and was reported in Sacatepéquez and Chimaltenango. In the middle right to the NW is the large Agua volcano, and behind it to the right are Fuego and Acatenango. Ash from Pacaya was visible in Fuego webcams that day. Courtesy of INSIVUMEH.

Communities all around Pacaya were affected by ashfall many times throughout March 2021 (figure 165, table 7). Most of the communities were within 10 km of the summit, but ashfall reached more than 20 km away multiple times. During the bigger ashfall events, blocks more than 6 cm in diameter fell on the flanks of the volcano, while lapilli (2 mm to 6 cm) fell up to 5 km away, and fine ash was observed up to 30 km away (figure 166). The most significant ashfall events occurred during 22-23 March when ash drifted tens of kilometers in multiple directions and caused the closure of La Aurora International Airport in Guatemala City (figure 167).

Figure 165. Communities all around Pacaya were affected by ashfall throughout March 2021. The red oval was the area where INSIVUMEH cautioned residents to be prepared for ashfall and lapilli after explosions on 3 March. All of the communities shown by yellow stars were affected by ashfall at some point during March. Courtesy of INSIVUMEH (Boletin Volcanologico especial BEPAC-41-2021, Eruption, Volcan Pacaya, 3 de marzo 2021, 11:55 horas).

Table 7. Communities reporting ashfall from Pacaya during March 2021. Information courtesy of INSIVUMEH.

Figure 166. Lapilli-size tephra (2 mm to 6 cm) from Pacaya was reported several times during March 2021 in communities as far as 5 km away, including this example on 16 March 2021 from Concepcion El Cedro (4 km NNW). Courtesy of INSIVUMEH (BOLETIN VULCANOLOGICO ESPECIAL BEPAC-56-2021, ACTUALIZACION DE ACTIVIDAD Y CAIDA DE TEFRA, 16 de marzo 2021, 09:05 horas).

Figure 167. A plane at the La Aurora international airport in Guatemala City was dusted with ash from Pacaya on 23 March 2021, forcing closure of the airport for much of the day. Photo by Moises Castillo/AP, courtesy of CNN.

Lava flow activity also increased significantly during March 2021. At the end of February, an active flow on the S flank remained about 1 km long, shedding incandescent blocks hundreds of meters from its advancing front. By 3 March, three flows with multiple branches were active on the SSW flank; they were 800-1,000 m long (figure 168). On 5 and 6 March two flows with many branches extended 300-500 m down the S flank (figure 169). Flows were active on the SW, S, and SE flanks on 7 March. The S-flank-flow with two branches reached 1 km long by 8 March and had incandescent blocks constantly falling of the leading edge. It increased steadily in length, reaching 1.8 km by 16 March (figure 170).

Figure 168. Three flows were active on the S and SW flanks of Pacaya on 3 March 2021, seen here with an infrared camera. Courtesy of INSIVUMEH.

Figure 169. On 5 March 2021 two main flows with multiple branches extended 300-500 m down the S flank of Pacaya causing very bright thermal signatures in satellite imagery. Sentinel-2 image uses Atmospheric penetration rendering (bands 12, 11 and 8a). Courtesy of Sentinel Hub Playground.

Figure 170. Two branches of the S-flank lava flow at Pacaya were each about 1.4 km long on 12 March 2021. Courtesy of INSIVUMEH (FOTOGRAFÍAS RECIENTES DE VOLCANES).

Two new flows emerged from the S and SE flanks on the morning of 18 March (figure 171). The S-flank flow grew to 500 m and part of it overflowed outside the plateau. The SE-flank flow was 400 m long in front of the village of Los Llanos, causing fires in the vegetation which continued for several days (figure 172). On 20 March the SE flank flow caused a strong thermal signature in satellite imagery with incandescent blocks falling downslope far beyond the front (figure 173). During the night of 20-21 March, a new flow appeared on the SW flank and grew to 500 m long; the flow on the SE flank reached 850 m. The following day the rapidly growing SW-flank-flow reached 1,500 m long, causing vegetation fires on ranches in Las Granadillas.

Figure 171. Two new flows emerged from the flanks of Pacaya on 18 March 2021 as seen in this FLIR thermal webcam image. The S-flank flow (center) grew to 500 m with two active branches. The SE flank flow (right) descended 400 m near the village of Los Llanos and burned vegetation. A third fissure higher on the SW flank (upper left) also had a short active flow. Courtesy of INSIVUMEH (Boletin Vulcanologico Especial BEPAC 58-2021, 18 March 2021).

Figure 172. Burning vegetation from a lava flow on Pacaya’s SE flank was controlled by CONRED workers on 20 March 2021. It was burning at the Los Llanos farmhouse, Finca el Muñeco, Villa Canales. Photo by Sergio Girón, courtesy of CONRED.

Figure 173. On 20 March 2021, a flow on the SE flank of Pacaya was about 400 m long with incandescent blocks falling several hundred meters downslope to the SE and causing fires in the vegetation. A strong thermal signature was also present from explosive activity inside Mackenney crater (top). Sentinel-2 image uses Atmospheric penetration rendering (bands 12, 11 and 8a). Courtesy of Sentinel Hub Playground.

Three flows were active on 22 March 2021, with existing flows on the SW (1,500 m) and SE flanks (300 m), and a new flow on the E (500 m) flank. By 25 March activity was focused on the SW-flank flow which had reached 2.5 km in length (figure 174). It was about 400 m wide and 2.5 m high, burning vegetation as it advanced, and causing damage on coffee and avocado plantations. By 31 March the flow exceeded 3 km in length with multiple active fronts. One of the flow fronts near the community of La Breña was still advancing, but the one at the Campo Alegre farm had stopped moving. The flow continued to cause fires, destroy crops and buildings, and block roads (figure 175).

Figure 174. A large flow on Pacaya’s SW flank had reached 2.5 km long by 25 March 2021 (left) and over 3 km long 5 days later on 30 March (right). It flowed W from a fissure on the W flank, then NW around a higher area before continuing SSW. The flow caused fires, destroyed crops and buildings, and blocked roads. Sentinel-2 images use Atmospheric rendering (bands 12, 11 and 8a). Courtesy of Sentinel Hub Playground.

Figure 175. The large lava flow on Pacaya’s SW flank had traveled over 2.5 km by 27 March 2021 when this wide-angle drone image was taken. One of the fronts of the flow was near the community of La Breña and the other was near the Campo Alegre farm. Courtesy of CONRED.

Activity during April 2021. On 1 April 2021 remnant plumes from earlier ash emissions were moving SW over the Pacific about 400 km from the summit at 4.3 km altitude, while newer emissions were drifting S at 3.4 km altitude towards the coast. Continuous ash emissions were reported by the Washington VAAC through April 4 (figure 176) drifting tens of kilometers mostly SW at 3.5-4.5 km altitude. Ash drifted up to 20 km S and SW during the first week and caused frequent ashfall in communities on the SE, S, and SW flanks, with the most affected being Los Pocitos, El Rodeo, and El Patrocinio. A few moderate to strong explosions sent ejecta 100-500 m above the Mackenney crater. By 9 April ash emissions were more sporadic and tended to drift only 5-10 km SW, W, and NW, and no ashfall was reported. The VAAC reported occasional emissions observed in the webcam on 8 and 14 April. An ash plume was detected on 16 April moving NNW at 3.4 km altitude. Strombolian activity diminished and activity changed to primarily steam and gas plumes rising 200 m above the crater after this. A short episode of sporadic explosions during 24-29 April sent ejecta to 250 m above the crater, generated loud noises, and produced ash emissions that rose a few hundred meters and drifted several kilometers.

Figure 176. Daily explosions at Pacaya produced dense ash emissions rising to 3.5-4.5 km altitude during the first part of April, including on 2 April 2021 when the ash drifted S and SE. Multiple branches of the lava flow on the SW flank were also burning vegetation near Las Granadillas and Buena Vista (smoke plumes in the foreground). Courtesy of CONRED.

The SW-flank flow that began during 20-21 March remained active into early April and was 2.8-3 km long during the first week. It continued to advance during the second week and reached 3.7 km long with multiple active branches that were burning vegetation (figure 177). During 7-11 April it was advancing W and N in the area of La Breña and W and S in the area of El Patrocinio and El Rodeo on the Campo Alegre farm (figure 178). By 10 April this flow was 400 m from El Patrocinio and 250 m from San José El Rodeo. By 13 April it was burning avocado and coffee plantations 370 m from houses in El Patrocinio (figure 179). Another active front to the south was 250 m E of El Rodeo and had blocked the road between El Rodeo, El Caracol, and Los Pocitos. The seismic activity associated with the lava effusion decreased significantly beginning on 16 April.

Figure 177. Lava from Pacaya’s SW-flank flow was 300 m wide and extended more than 3 km by 7 April 2021; it was burning vegetation in its path as it advanced at about 5 meters per hour. Courtesy of CONRED.

Figure 178. The SW-flank flow at Pacaya continued to advance during the first half of April 2021 as seen here on 4 (left) and 9 (right) April. The communities of La Breña, El Patrocinio, and El Rodeo were the most affected. Sentinel-2 images use Atmospheric rendering (bands 12, 11 and 8a). Courtesy of Sentinel Hub Playground.

Figure 179. By 14 April 2021 the SW-flank flow at Pacaya was 3.7 km long and several hundred meters wide. It had multiple active branches that came within a few hundred meters of the communities of El Patrocinio and San José El Rodeo and had burned significant acreage on coffee and avocado plantations. It also blocked the road between El Rodeo, El Caracol, and Los Pocitos. Courtesy of CONRED.

During 18-20 April 2021 the branch near La Breña stopped advancing, and by 21 April the branch near El Patrocinio had stopped (figure 180), although temperatures remained high and gas emissions from vents along the flow continued in many places through the end of April. A lava flow appeared on the SE flank on 27 April, following a few days of renewed explosive activity, and grew to 175 m by 29 April. INSIVUMEH reported another new flow on the N flank on 29 April (figure 181); it advanced rapidly to the NW around Cerro Chino, and then turned towards the SW, reaching 1.6 km long by later in the day when the leading edge was located about 100 m from La Breña with several active flow fronts.

Figure 180. The lava flow on the SW flank of Pacaya stopped advancing a few hundred meters before reaching El Patrocinio in San Vicente Pacaya, home to about 350 people, on 21 April 2021. Photo by Moises Castillo/Associated Press, courtesy of KTLA.

Figure 181. A lava flow emerged on the N flank of Pacaya on 29 April 2021 and advanced rapidly NW around Cerro Chino and then SW towards La Breña, reaching 1.6 km long by the end of the day. Courtesy of Colred Los Llanos.

Activity during May 2021. Sporadic emissions of steam and gas with occasional ash were typical from Mackenney crater at the beginning of May 2021. Possible ash emissions were seen in satellite data on 1 May drifting W at 3.4 km altitude. Dense plumes, some with abundant ash, were reported on 8 May drifting W and S. Strombolian activity on 10 May from the NW-flank fissure was feeding the flow which began on 29 April; it sent ejecta 50-150 m high, and loud noises were heard. The Washington VAAC reported minor amounts of ash observed in satellite images moving SW from the summit during 10-13 May, when intermittent pulses of dense ash were reported drifting W and SW from the crater. Intermittent ash emissions rose to 3.7 km altitude on 14 May and were observed about 100 km SW before dissipating. Ash plumes drifted up to 5 km W on 15 and 16 May, causing ashfall during 16 and 17 May in El Patrocinio and El Rodeo (figure 182). During 18-21 May constant steam and gas, and periodic ash, emissions drifted 5-10 km NW and W at about 3 km altitude with ashfall reported in communities such as San Francisco de Sales, Concepción El Cedro, Aldea El Patrocinio, and San Miguel Petapa. For the remainder of May, small quantities of ash accompanied dense steam and gas emissions that rose 200-700 m above the summit and drifted W, SW, and S up to 5 km. El Patrocinio, El Rodeo, and other fincas in that area within 10 km reported ashfall on 26 May.

Figure 182. Pulses of dense ash emissions from the summit of Pacaya were noted on 16 May 2021 by a team of volcanologists from Boise State and Michigan Tech Universities. Steam and gas from still-hot lava flows rose from the flanks. Courtesy of Geo_Sci_Jerry.

The N-flank flow that began on 29 April 2021 continued to advance into early May. It had originally flowed NW, then curved around Cerro Chino and headed W. It was 2 km long and advancing in the vicinity of La Breña on 3 May. On 5 May incandescent ejecta was observed at the fissure feeding the flow, which had advanced to the S of La Breña where incandescent blocks continued to fall off the front of the advancing flow. On 6 May the flow reached 2.3 km in length on the W flank, with only one of the fronts continuing to advance slowly. Small explosions were reported at the fissure. The lava flow continued to advance laterally in places as incandescent material spilled over the edges. Explosions from the fissure on 9 May threw material 15 m away as the flow continued moving slowly W (figure 183). By 11 May the flow was no longer advancing at its front but was still expanding due to overflows along its edges. Explosions from the fissure on 14 May launched ejecta 40 m (figure 184), and the flow front again moved slowly westward; by then it was about 2.3 km long (figure 185). Activity at the fissure ceased by 17 May.

Figure 183. Strombolian explosions at the fissure feeding the W-flank lava flow at Pacaya were visible on the night of 8 May 2021. Although the lava flowed rapidly, it didn’t advance significantly after the first week of May; instead the lava flowed laterally and spread out over the flanks in several places until activity at the fissure ceased on 17 May. Copyrighted photo by David Rojas, used with permission.

Figure 184. The fissure on the NW flank of Pacaya was still active on 14 May 2021. Explosions produced ash and ejecta that rose 40 m above the fissure. Courtesy of CONRED.

Figure 185. The flow on the NW flank of Pacaya was also still active on 14 May 2021. It was over 2 km long and still actively flowing but no longer advancing. Sentinel-2 image uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Geologic Background. Eruptions from Pacaya, one of Guatemala's most active volcanoes, are frequently visible from Guatemala City, the nation's capital. This complex basaltic volcano was constructed just outside the southern topographic rim of the 14 x 16 km Pleistocene Amatitlán caldera. A cluster of dacitic lava domes occupies the southern caldera floor. The post-caldera Pacaya massif includes the ancestral Pacaya Viejo and Cerro Grande stratovolcanoes and the currently active Mackenney stratovolcano. Collapse of Pacaya Viejo between 600 and 1500 years ago produced a debris-avalanche deposit that extends 25 km onto the Pacific coastal plain and left an arcuate somma rim inside which the modern Pacaya volcano (Mackenney cone) grew. A subsidiary crater, Cerro Chino, was constructed on the NW somma rim and was last active in the 19th century. During the past several decades, activity has consisted of frequent strombolian eruptions with intermittent lava flow extrusion that has partially filled in the caldera moat and armored the flanks of Mackenney cone, punctuated by occasional larger explosive eruptions that partially destroy the summit of the growing young stratovolcano.

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/ ); Coordinadora Nacional para la Reducción de Desastres (CONRED), Av. Hincapié 21-72, Zona 13, Guatemala City, Guatemala (URL: http://conred.gob.gt/www/index.php, https://twitter.com/ConredGuatemala/status/1393207685756203011); Colred Los Llanos, Coordinadora local para la reduccion de desastres, Los Llanos, Villa Canales (URL: https://www.facebook.com/Colred-Los-Llanos-102105058094847); 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/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Reuters Pictures (URL: https://twitter.com/reuterspictures/status/1367472450418704387); CNN (URL: https://www.cnn.com/2021/03/23/americas/guatemala-airport-volcano-closure-latam-intl/index.html); KTLA, (URL: https://ktla.com/news/nationworld/lava-from-guatemalas-pacaya-volcano-threatens-small-communities-that-live-nearby/); David Rojas, (URL: https://twitter.com/DavidRojasGt/status/1391592159221063680); Geo_Sci_Jerry (URL: https://twitter.com/SciJerry/status/1394083192773222406).

The volcanic Semisopochnoi Island in the western Aleutian Islands contains a group of cones within a caldera complex (figure 5). The active Cerberus center has three summit craters, with the current activity originating from North Cerberus. Since September 2018, typical activity has produced minor ash deposits within the vicinity. This bulletin summarizes activity that occurred from April 2020 through May 2021 based on information given by the Alaska Volcano Observatory (AVO), supplemented by satellite data.

Figure 5. This satellite image of Semisopochnoi Island, Alaska, shows the major surface features with an 8-km-wide caldera in the center. As of 2021, Mount Cerberus is the most active of three cones within the caldera complex. The North, East, and South Cerberus craters are indicated, with a faint gas plume dispersing NE from the active North crater on 22 August 2020. Base satellite image from Sentinel-2 using Natural color (bands 4, 3, 2) rendering. Courtesy of Sentinel Hub Playground.

Intermittent small explosions occurred at Semisopochnoi during early 2020. An AVO Volcano Activity Notice for Aviation (VONA) issued on 1 April reported no indication of activity over the previous two weeks and seismicity at background levels. Satellite data show detectable sulfur dioxide (SO₂) emission and visible steam plumes. Low-level unrest continued into early June with occasional small earthquakes, including a few small low-frequency events and episodic tremor, occasional steam plumes, and detectable SO₂ emissions. An increase in tremor was detected around 12-13 June, and infrasound and seismicity indicated rapid degassing events on 17 and 19 June, with activity declining again by the 20th. AVO noted that clear satellite images acquired on the 21st showed minor ash deposits near the crater, likely from the elevated activity during the previous week, and vigorous gas and steam emission (figure 6). Steam and gas emission continued through to the end of the month then intermittently through July. A 200-km-long SO₂ plume was detected on 15 July and low-level unrest continued.

Figure 6. Minor ash deposits are visible on the Semisopochnoi North Cerberus Crater and a steam plume is shown dispersing ESE on 21 June 2020. Sentinel-2 satellite image with Natural color (bands 4, 3, 2) rendering. Courtesy of Sentinel Hub Playground.

A gas plume was reported on 7 August and seismicity was above background to the 9th, after that seismicity was at very low levels with no more significant events detected. Infrequent small earthquakes were detected through September and minor steam emissions on the 22nd. Seismicity remained low throughout October. No eruptive activity had detected since mid-June and seismicity had declined to very low levels prior to seismic data transmission failing on 11 November. Due to the lack of data, on 20 November the Aviation Color Code and Alert Level were reduced to Unassigned.

There were no reports of activity during December 2020 or January 2021. A satellite image acquired on 7 February showed several small ash deposits extending at least 3 km from the North Cerberus Crater, likely produced by a small explosion the previous week (figure 7). Steam emission prevented views into the crater and clouds obscured the volcano over the following week.

Figure 7. This Landsat 8 image acquired on 7 February 2021 at Semisopochnoi shows several linear ash deposits from the North Cerberus Crater. This reflects low-level explosive activity. Landsat 9 true Color – pansharpened scene. Courtesy of Sentinel Hub Playground.

On 10 March a satellite image revealed a recently emplaced ash deposit that extended 1.5 km from the crater, with a steam plume being blown to the E (figure 8). Several similar small ash deposits had been noted by AVO in the previous weeks. No activity was observed or detected through 18 March, other than a possible gas plume that day. At 0350 on the 19th a small explosion was detected by infrasound monitoring. Another small explosion was detected at 0230 on the 21st, followed by a series of smaller explosions. During 22-23 March three explosions were detected. Cloud cover prevented visual observation of these events, but possible SO₂ plumes were detected and a confirmed plume on the 23rd indicated further unrest. A probable ash deposit and plume were imaged on the 24th (figure 9). Activity continued with intermittent explosions and SO₂ plumes detected through the 27th.

Figure 8. This 10 March 2021 WorldView-3 satellite image shows ash deposits from low-level explosive activity at the Cerberus North Crater at Semisopochnoi. The ash extends to 1.5 km from the vent and has been partly remobilized by wind. A plume emanating from the crater is being blown to the E. Figure by Hannah Dietterich, courtesy of AVO.

Figure 9. An ash deposit is present between the dashed lines, deposited on snow (red) in this Planet Labs near-IR false color satellite image acquired on 24 March 2021. The deposit extends over 8 km ESE across Semisopochnoi from the North Cerberus Crater and a plume is also visible in the same area. Image courtesy of AVO.

Several small low-altitude ash and gas plumes were detected in satellite images on 30 March and 1 April. Cloud cover prevented satellite views until 12 April, when new ash deposits and low-level ash emissions were observed extending at least to the coastline, accompanied by weak infrasound signals. Low-level activity was also detected the following day. Sustained ash emission that began on the morning of the 15th (figure 10) produced a plume extending more than 350 km E to altitudes of 6 km; activity continued through the next day with a change in direction to the N at around 3 km altitude. Ash emission continued over the following days with a VONA released on the 22nd reporting an ash plume reaching 3 km and extending about 75 km S (figure 11). Through to the end of April ash and SO₂ plumes were either observed or noted as probably occurring under cloudy conditions.

Figure 10. This Sentinel-3 satellite scene acquired on 15 April 2021 shows plumes from Semisopochnoi dispersed over 330 km from the vent. The insert shows a zoomed-in view of the island and the proximal ash plume. Original image by Hannah Dietterich, AVO.

Figure 11. This Planet Labs satellite image acquired on 22 April 2021 shows an ash plume produced by the North Cerberus Crater and dispersing S. Ash deposits are visible on the flanks of the cone. Figure by Hannah Dietterich, AVO.

The volcano was often obscured during the first week of May, with activity possibly continuing at a low level without detection. A gas plume was detected on the 11th, and an ash plume is visible in satellite images acquired on the 17th (figure 12). Small explosions and SO₂ emissions were detected through 21 May. An ash emission reaching 3 km altitude that was seen by an AVO field crew on 29 May was also observed in satellite data moving SW. Elevated temperatures were detected in the North Cerberus Crater. Ash emissions were produced again on the 30th and observed by an AVO field crew (figure 13). Seismic data transmission was restored on 26 May.

Figure 12. Satellite images of Semisopochnoi acquired on 17 and 29 May (top), and a photograph taken on 29 May 2021 (bottom) show weak activity at the North Cerberus Crater, including ash emission, gas emission, and elevated temperature on the crater floor. Sentinel-2 color infrared (vegetation, bands 8, 3, 4) scene at the top left and the false color (urban, bands 12, 11, 4) scene at the top right courtesy of Sentinel Hub Playground. Photo courtesy of Hannah Dietterich, AVO.

Figure 13. Minor ash emissions produced on 30 May 2021 at Semisopochnoi’s North Cerberus Crater around 1320 local time, taken from a helicopter during field work. Both top and bottom-left photos are taken from the SE. Photos courtesy of Hannah Dietterich, AVO.

Geologic Background. Semisopochnoi, the largest subaerial volcano of the western Aleutians, is 20 km wide at sea level and contains an 8-km-wide caldera. It formed as a result of collapse of a low-angle, dominantly basaltic volcano following the eruption of a large volume of dacitic pumice. The high point of the island is Anvil Peak, a double-peaked late-Pleistocene cone that forms much of the island's northern part. The three-peaked Mount Cerberus was constructed within the caldera during the Holocene. Each of the peaks contains a summit crater; lava flows on the N flank of Cerberus appear younger than those on the south side. Other post-caldera volcanoes include the symmetrical Sugarloaf Peak SSE of the caldera and Lakeshore Cone, a small cinder cone at the edge of Fenner Lake in the NE part of the caldera. Most documented eruptions have originated from Cerberus, although Coats (1950) considered that both Sugarloaf and Lakeshore Cone could have been recently active.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA (URL: https://avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://dggs.alaska.gov/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Planet Labs, Inc. (URL: https://www.planet.com/).