Ambae (also known as Aoba) is a large 2,500 km³ basaltic shield volcano located in the New Hebrides arc, part of Vanuatu. Historical eruptions have been recorded since the 16^th century, characterized by periodic phreatic and pyroclastic explosions originating from the summit crater lakes. A tuff cone was constructed within Lake Voui (or Vui) during November 2005-February 2006 (BGVN 31:12, figure 30). The most recent eruption occurred from July to September 2018 (BGVN 44:02) consisting of ash plumes, heavy ashfall, and lahars. This report updates activity from June 2019 through December 2021 and describes a new eruption beginning in December 2021 characterized by explosions, a thermal anomaly, and gas-and-steam emissions. Information primarily comes from the Vanuatu Geohazards Observatory of the Vanuatu Meteorology and Geo-Hazards Department (VMGD) and various satellite data.

Activity during June 2019 through November 2021 was relatively low, according to VMGD. On 23 September 2019 the Volcanic Alert Level (VAL) was lowered from 2 to 1 (on a scale of 0-5). Some gas-and-steam emissions were observed in webcam imagery on 4 and 5 May 2021 from a vent in Lake Voui. Intermittent low-strength thermal anomalies were detected by the MIROVA (Middle InfraRed Observation of Volcanic Activity) volcano hotspot detection system based on analysis of MODIS data during May through July 2021 (figure 94). Seven anomalies were detected during May, three during June, four during July, one during early October, and four during December. The December activity was also reflected in Sentinel-2 infrared satellite imagery, which showed a small thermal anomaly in Lake Voui accompanied by a strong gas-and-steam plume that drifted SW (figure 95). On 5 December gas-and-steam emissions rose 250 m high and explosions ejected wet tephra 10 m above the lake surface, based on information from VMGD. As a result, the VAL was raised to 2 on 27 December.

Figure 94. Intermittent low-strength thermal anomalies at Ambae were detected in the MIROVA (Log Radiative Power) graph during May through December 2021. Seven anomalies occurred during May, three during June, four during July, one during early October, and four during December. Courtesy of MIROVA.

Figure 95. Sentinel-2 infrared and natural color satellite imagery showing increased activity at Ambae. On 4 December, a thermal anomaly (bright orange-yellow) was visible and accompanied by a strong gas-and-steam plume that drifted SW (left). On 9 December a notable white gas-and-steam plume drifted W (right). Images using “Atmospheric penetration” (bands 12, 11, 8a) and “Natural Color” rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

Geologic Background. The island of Ambae, also known as Aoba, is a massive 2,500 km3 basaltic shield that is the most voluminous volcano of the New Hebrides archipelago. A pronounced NE-SW-trending rift zone dotted with scoria cones gives the 16 x 38 km island an elongated form. A broad pyroclastic cone containing three crater lakes (Manaro Ngoru, Voui, and Manaro Lakua) is located at the summit within the youngest of at least two nested calderas, the largest of which is 6 km in diameter. That large central edifice is also called Manaro Voui or Lombenben volcano. Post-caldera explosive eruptions formed the summit craters about 360 years ago. A tuff cone was constructed within Lake Voui (or Vui) about 60 years later. The latest known flank eruption, about 300 years ago, destroyed the population of the Nduindui area near the western coast.

Information Contacts: Geo-Hazards Division, Vanuatu Meteorology and Geo-Hazards Department (VMGD), Ministry of Climate Change Adaptation, Meteorology, Geo-Hazards, Energy, Environment and Disaster Management, Private Mail Bag 9054, Lini Highway, Port Vila, Vanuatu (URL: http://www.vmgd.gov.vu/, https://www.facebook.com/VanuatuGeohazardsObservatory/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Saunders Island, located in the remote South Sandwich Volcanic Arc in the South Atlantic, consists of a large central edifice intersected by two seamount chains. The primary volcanic features include Mount Michael, which contains a 500-m-wide summit crater, the northern Blackstone Plain, and the Ashen Hills, which is a group of overlapping craters formed through phreatomagmatic activity. The current eruption period began during November 2014, characterized by elevated crater temperatures and gas-and-steam emissions (BGVN 46:02). Visits are infrequent because of the remote location and often views are obscured by cloudy weather. This report updates information during June 2020 through January 2022 primarily using satellite data.

Activity has been relatively low since June 2020; no thermal activity was detected in satellite data until November 2020, when it occurred intermittently until February 2021, according to MIROVA data (figure 43). Although weather clouds often obscured the views of the summit, gas-and-steam emissions drifting NE were possibly visible in satellite images on 24 October 2020. A clear view of the crater on 6 November showed a strong thermal anomaly on the crater floor in infrared satellite imagery (figure 44). According to Suomi NPP/VIIRs sensor data, thermal anomalies were detected on 3 and 8 November 2020 and 11 January 2021. On clear weather days during late 2020 into 2021, anomalies were strongly observed in the summit crater at Mount Michael, based on Sentinel-2 infrared satellite imagery (figure 44). Small gas-and-steam emissions were detected on 10 January 2021. On 9 February a grayish plume drifted W from the summit crater, according to natural color satellite imagery (figure 44).

Figure 43. Few thermal anomalies (red dots) were detected at Saunders during November 2020 through February 2021, October 2021, and February 2022 as recorded by the Sentinel-2 MODIS Thermal Volcanic Activity data (bands 12, 11, 8A). Courtesy of MIROVA.

Figure 44. Sentinel-2 infrared and natural color images on clear weather days showing a strong thermal anomaly (bright yellow-orange) within the Mount Michael summit crater at Saunders on 6 November (top left), 11 December (top middle) 2020, 10 January (top right), 19 February (bottom middle), and 7 October (bottom right) 2021. Some degassing was visible on 10 January. A grayish plume was detected drifting W on 9 February (bottom left). By 7 October, the strength of the anomaly had declined. Images using “Atmospheric penetration” rendering (bands 12, 11, 8a) and “Natural color” (bands 4, 3, 2) rendering. Courtesy of Sentinel Hub Playground.

Renewed anomalies were visible again during October 2021 in MIROVA data. Although on 7 October the strength of the anomaly had decreased visibly compared to the previous year (figure 44). Small sulfur dioxide emissions were visible on 24 October 2021, as detected by the TROPOMI instrument on the Sentinel-5P satellite (figure 45). Wispy sulfur dioxide emissions were more often detected during January 2022, which drifted generally N and SW.

Figure 45. Small sulfur dioxide plumes rising from Saunders were detected by the TROPOMI instrument on the Sentinel-5P satellite on 24 October (top left) 2021, 13 January (top right), 28 January (bottom left), and 29 January (bottom right) 2022 that drifted SE, N, SW, and NW, respectively. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. Saunders Island is a volcanic structure consisting of a large central edifice intersected by two seamount chains, as shown by bathymetric mapping (Leat et al., 2013). The young constructional Mount Michael stratovolcano dominates the glacier-covered island, while two submarine plateaus, Harpers Bank and Saunders Bank, extend north. The symmetrical Michael has a 500-m-wide summit crater and a remnant of a somma rim to the SE. Tephra layers visible in ice cliffs surrounding the island are evidence of recent eruptions. Ash clouds were reported from the summit crater in 1819, and an effusive eruption was inferred to have occurred from a N-flank fissure around the end of the 19th century and beginning of the 20th century. A low ice-free lava platform, Blackstone Plain, is located on the north coast, surrounding a group of former sea stacks. A cluster of parasitic cones on the SE flank, the Ashen Hills, appear to have been modified since 1820 (LeMasurier and Thomson, 1990). Analysis of satellite imagery available since 1989 (Gray et al., 2019; MODVOLC) suggests frequent eruptive activity (when weatehr conditions allow), volcanic clouds, steam plumes, and thermal anomalies indicative of a persistent, or at least frequently active, lava lake in the summit crater. Due to this observational bias, there has been a presumption when defining eruptive periods that activity has been ongoing unless there is no evidence for at least 10 months.

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/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard MD 20771, USA (URL: https://so2.gsfc.nasa.gov/); NASA Worldview (URL: https://worldview.earthdata.nasa.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Sakurajima rises from Kagoshima Bay, which fills the Aira Caldera near the southern tip of Japan's Kyushu Island. Frequent explosive and occasional effusive activity has been ongoing for centuries. The Minamidake summit cone and crater has been the location of persistent activity since 1955; the Showa crater on its E flank has also been intermittently active since 2006. Explosions and ash-bearing emissions have continued primarily at the Minamidake crater since the latest eruptive episode began in late March 2017. This report covers ongoing activity at Minamidake from July through December 2021; the Japan Meteorological Agency (JMA) provides monthly reports on activity, and the Tokyo VAAC (Volcanic Ash Advisory Center) issues information about the ash emissions.

Activity decreased significantly during the second half of 2021 beginning in May (BGVN 46:08). The number of monthly explosions and ash emissions were both much lower compared with the first half of the year (table 26), as was the amount of ashfall recorded around the volcano. Thermal energy remained at low levels throughout the period (figure 121) although incandescence was visible from the webcams at night when the weather was clear. Very few ash emissions were reported each month, and ash plumes never rose more than 1,300 m above the summit (figure 122). All of the activity originated from the Minamidaki crater; there were no reports of activity from the Showa crater.

Table 26. Number of monthly explosive events, ash emission events, days of ashfall, ashfall amounts, and sulfur dioxide emissions from Sakurajima’s Minamidake crater at Aira during July-December 2021. Note that smaller ash events are not listed. Ashfall days were measured at Kagoshima Local Meteorological Observatory, and ashfall amounts represent material covering all of the Kagoshima Prefecture. Data courtesy of JMA monthly reports.

Figure 121. Thermal energy remained very low at Sakurajima in the Aira caldera throughout the second half of 2021 according to the MIROVA graph of Log Radiative Power from 3 May through December 2021. Courtesy of MIROVA.

Figure 122. Few ash emissions were observed at Aira’s Sakurajima volcano during July-December 2021, and the ash plume heights never exceeded 1,300 m above the crater rim. The ash emission from the Minamidake summit crater on 28 July 2021 (left) rose 1,300 m above the crater rim as seen from the Kaigata surveillance camera. The ash plume on 26 September 2021 (right) rose to 1,100 m above the crater rim. Courtesy of JMA monthly reports.

There were 61 volcanic earthquakes reported during July, up from 18 in June. Less than a minute of volcanic tremors were recorded during the month, a significant drop from the 22 minutes in June. At the Kagoshima Local Meteorological Observatory, located in Higashikorimoto about 11 km WSW of Minamidake, the total amount of ashfall measured during the six days it was recorded was 1 g/m². The total ashfall for Kagoshima Prefecture during July was about 10,000 tons. JMA reported 800-1,000 tons/day of SO₂ emissions in July. An explosion on 28 July produced an ash plume that rose 1,300 m above the crater rim; no large volcanic bombs were reported. The Tokyo VAAC reported the ash emission on 28 July as rising to 2.4 km altitude and drifting NW. Two small ash emissions occurred at Minamidake on 2 and 10 August 2021. The ash plume on 10 August rose 900 m above the crater rim. 143 volcanic earthquakes were recorded during the month with 5 minutes of volcanic tremor. No ashfall was reported at the Observatory and the total ashfall around the Prefecture for August was about 5,000 tons. JMA reported 600-1,500 tons/day SO₂ emissions during August.

During September 2021 four significant ash emissions events occurred at Minamidake crater and a few very small events, but no larger explosions. On 19 September the Tokyo VAAC reported an ash emission that rose to 2.1 km altitude and remained stationary. The tallest ash plume, on 26 September, rose 1,100 m above the crater rim. Seismometers recorded 124 volcanic earthquakes and three minutes of volcanic tremor. JMA reported 700-2,600 tons/day SO₂ emissions, increasing somewhat at the end of the month. Inclinometers and extensometers recorded slight inflation of the volcano beginning on 13 September after a long period with little change. At the Observatory, ashfall was reported on five days with a total amount measured of 3 g/m². The September total ash mass for the Prefecture was estimated at around 10,000 tons.

One explosion occurred from Minamidake crater on 8 October 2021 that sent volcanic bombs 600-900 m from the crater, the largest event of the period. Clouds prevented visual observations during the explosion. Other smaller ash emission events were also recorded. There were 51 volcanic earthquakes and no tremors recorded during the month. Incandescence from the summit crater was visible throughout the month (figure 123). Sulfur dioxide emissions ranged from 400-1,000 tons/day; they increased temporarily during the second half of September. Inflation of the volcano that was measured from the GNSS observations beginning in mid-September slowed in the beginning of October and ceased by mid-month. Ashfall was recorded at the Observatory on five days during the month with a total amount of 0.5 g/m² measured. The total ashfall in the Kagoshima Prefecture for October was estimated at 7,000 tons. No particular changes were observed at the Minamidake and Showa craters during an overflight on 18 October 2021. On 31 October the Tokyo VAAC reported an ash emission that rose to about 2.4 km altitude. Clear Sentinel images from 3 and 28 October showed a moderate thermal anomaly at vent “A” in Minamidake crater.

Figure 123. Incandescence was visible from the Minamidake summit crater at Sakurajima in the Aira caldera on 14 October 2021, and on most clear nights throughout the period. Courtesy of JMA Annual Report for 2021.

Four recorded ash emission events were recorded by JMA at Minamidake crater during November 2021, and a few other much smaller events, but no large explosions took place. Ash emissions rose to a maximum height of 1,300 m above the crater rim. On 2 November the Tokyo VAAC reported an ash emission that rose to 2.4 km altitude and drifted SW. On 18 November two ash emissions also rose to 2.4 km altitude; one drifted SE and the other drifted NE. Observers on an overflight on 20 November saw no notable changes at the craters or surrounding summit area. An ash emission on 25 November rose to 2.1 km altitude and drifted E. Seismometers detected 57 volcanic earthquakes and no tremors during the month. Field measurements of SO₂ indicated 700-1,200 tons/day were released by the volcano. Inclinometers and extensometers showed inflation during November. Ashfall total measured at the Observatory was less than 0.5 g/m² over five days. The estimate of total ashfall in the prefecture for November was 5,000 tons. Clear skies on 2, 7, and 27 November revealed a thermal anomaly inside the Minamidake crater. In addition, a second faint thermal anomaly was visible inside the Minamidake crater on 27 November at vent ‘B’ (figure 124).

Figure 124. Clear skies on 2 (left) and 27 (right) November 2021 revealed thermal anomalies within the Minamidake crater at Aira’s Sakurajima volcano. The brighter anomaly in both images is from vent A, and the fainter anomaly is vent B. Sentinel 2 satellite images use Atmospheric penetration rendering (bands 12, 11, and 8a). Courtesy of Sentinel Hub Playground.

During December 2021 larger ash emission events occurred seven times, an increase from previous months, and a few smaller events were also noted. The tallest ash plume rose 1,300 m above the crater rim on 2 December; the Tokyo VAAC reported the emission at 2.4 km altitude drifting SE. During an overflight on 8 December, no specific changes were visible at the summit crater area (figure 125). On 9 December an ash plume rose to 2.1 km altitude and drifted E. The next day two emissions rose to 2.4 km; one drifted NE. On 20 December an ash emission rose to 2.1 km altitude and drifted E. On 21 and 28 December ash plumes rose to 2.4 km and extended SE and E. Seismometers measured 136 volcanic earthquakes and less than one minute of tremor that was associated with very small ash emission events. Field measurements indicated 600-1,200 tons of SO₂ were released daily. Inflation of the volcano decreased during December. Only slight ashfall (less than 0.5 g/m³) was recorded on two days of the month at the Observatory. A single thermal anomaly was visible inside the crater in Sentinel satellite imagery on 7 December, but an ash emission was visible on 22 December (figure 126).

Figure 125. The Minimidake and Showa craters at the summit of Sakurajima in the Aira caldera seen in this aerial photo. Two active vents, A and B, are within the Minamidake crater. Image taken during an overflight on 8 December 2021. Courtesy of JMA Monthly Report.

Figure 126. An ash emission rose from the Minamidake crater at Aira caldera’s Sakurajima volcano on 22 December 2021 and was recorded by this Natural color rendering (bands 4, 3, 2) Sentinel 2 satellite image. Courtesy of Sentinel Hub Playground.

Geologic Background. The Aira caldera in the northern half of Kagoshima Bay contains the post-caldera Sakurajima volcano, one of Japan's most active. Eruption of the voluminous Ito pyroclastic flow accompanied formation of the 17 x 23 km caldera about 22,000 years ago. The smaller Wakamiko caldera was formed during the early Holocene in the NE corner of the Aira caldera, along with several post-caldera cones. The construction of Sakurajima began about 13,000 years ago on the southern rim of Aira caldera and built an island that was finally joined to the Osumi Peninsula during the major explosive and effusive eruption of 1914. Activity at the Kitadake summit cone ended about 4850 years ago, after which eruptions took place at Minamidake. Frequent historical eruptions, recorded since the 8th century, have deposited ash on Kagoshima, one of Kyushu's largest cities, located across Kagoshima Bay only 8 km from the summit. The largest historical eruption took place during 1471-76.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

The frequently active Merapi is located immediately north of the major city of Yogyakarta in central Java, Indonesia. Volcanism has included lava flows, pyroclastic flows, lahars, explosions with heavy ashfall, incandescent block avalanches, block-and-ash flows, and cycles of dome growth and destruction. The current eruption period began in December 2020 and has recently consisted of explosions, dome growth, incandescent avalanches, and block-and-ash flows (BGVN 46:09). Two new domes appeared in January and February 2021: one inside the summit crater and one at the top of the SW flank. This report updates similar activity from September 2021 through February 2022 using information primarily from Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG), the Center for Research and Development of Geological Disaster Technology, a branch of PVMBG which specifically monitors Merapi. Additional information came from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), MAGMA Indonesia, and the Darwin Volcanic Ash Advisory Centre (VAAC).

BPPTKG reported that the two lava domes, located just below the SW rim and in the summit crater, continued to grow during September; the SW dome had grown 7 m taller and had an estimated volume of 1.6 million cubic meters and the summit lava dome had grown 1 m wider and had an estimated volume of 2.85 million cubic meters. A total of seven pyroclastic flows descended the SW flank as far as 2.5 km, and 67-144 lava avalanches traveled a maximum of 2 km SW. On 1 September ashfall was reported in several areas, including Srumbung, Salam, Ngluwar, Dukun, Mungkid, Sawangan, Muntilan, and the Salaman sub-districts. White gas-and-steam emissions rose 500 m high, based on observations from the Kaliurang Mount Merapi Observation Post. During 9-10 September a continuous ash emission was observed in satellite imagery and rose to 3 km altitude and drifted E and NW.

Both lava domes continued to grow during October; the SW dome had grown about 5 m higher, with an estimated volume of 1.67 million cubic meters, and the summit dome rose another 4 m with an estimated volume of 2.927 million cubic meters. Two pyroclastic flows descended 2.5 km down the SW flank. There were 30-76 lava avalanches reported that traveled a maximum of 2 km SW. White gas-and-steam emissions rose 700 m high. During 15-21 October a thin ash deposit was reported in the Selo area.

Similar activity continued during November, with active lava domes and 106-212 lava avalanches that traveled 2 km SW. Two pyroclastic flows were reported descending 1.5-2 km on the SW flank; one pyroclastic flow traveled 1.8 km down the SW flank on 13 November and another occurred on 20 November (figure 122). The estimated volume of the SW dome was 1.61 million cubic meters, with the summit dome at almost 2.93 million cubic meters.

Figure 122. Webcam image of an incandescent lava avalanche descending the SW flank at Merapi on 21 November 2021, which generates ash plumes. Courtesy of MAGMA Indonesia.

CVGHM reported a discrete ash plume that rose to 3.9 km altitude and drifted E on 1 December, but it was not detected in satellite imagery due to cloud cover. Two pyroclastic flows traveled 3 km down the SW flank on 1 December. According to a VONA issued by PVMBG, around 2104 on 1 December an ash plume rose 1 km above the summit and drifted E. The height of the SW lava dome decreased about 2 m during 26 November and 2 December, but the estimated volumes at both remained stable at 1.63 million cubic meters for the SW dome and just over 3 million cubic meters for the summit dome. Between 112-190 lava avalanches moved as far as 2 km SW down the Bebeng drainage. On 6 December a pyroclastic flow traveled 1.8 km down the Bebeng drainage on the SW flank at 1644, which lasted two minutes and forty seconds, based on seismic data, according to BPPTKG. At 1643 on 18 December a pyroclastic flow advanced 2 km SW, which produced an ash plume that rose 400 m high (figure 123). During 24-30 December the height of the SW lava dome had decreased by 3 m.

Figure 123. Webcam image of a pyroclastic flow at Merapi extending 2 km down the SW flank, which generated a 400-m-high ash plume on 18 December 2021. Courtesy of MAGMA Indonesia.

Observations during early January 2022 showed that the height of the SW lava dome had increased about 2 m between 31 December and 6 January 2022. Seismicity was recorded at higher levels with more intense earthquakes compared to the previous months. The estimated volume of the SW dome was over 1.67 million cubic meters and just over 3 million cubic meters at the summit dome. The weekly number of detected lava avalanches ranged between 30 and 123, which traveled a maximum of 2.2 km SW down the Bebeng drainage (figure 124). A total of seven pyroclastic flows were reported descending 2-2.5 km down the SW flank.

Figure 124. Webcam image of an incandescent lava avalanche moving down the SW flank at Merapi on 28 January 2022. Courtesy of MAGMA Indonesia.

In February, no significant morphological changes were detected at the lava domes, though seismicity remained relatively high. The estimated volumes of the SW and summit domes was 1.58 million cubic meters and 3.23 million cubic meters, respectively. In the Bebeng drainage, 105-173 weekly lava avalanches were reported, moving as far as 2 km SW, and one of which went 300 m NW. Nine pyroclastic flows extended 1.8-2.8 km. On 2 February the Darwin VAAC issued a notice describing resuspended ash that rose to 3.7 km altitude based on satellite data; no eruption was associated with this event. A ground report indicated that on 6 February ash plumes rose 600 m above the summit and drifted E, though it was likely resuspended. Ashfall was reported in multiple areas within about 20 km to the S, SE, and E, including the Cangkringan, Sleman and Musuk districts. On 18 February minor ashfall was reported in the Pakem District.

The MODIS Log Radiative Power graph produced by the MIROVA project showed that thermal activity was stronger and more frequent during September-October 2021 compared to December through February 2022 (figure 125). This pattern is also reflected in the MODVOLC hotspot detection program, which detected a total of 25 thermal anomalies over the days of 3, 10, 17, and 30 September, 5, 14, 21, 28, and 30 October, 29 November, 26 December 2021, 2 and 9 January, and 24 February 2022. This thermal activity was also detected in Sentinel-2 infrared satellite imagery representing incandescent lava and block avalanches dominantly moving down the SW flank (figure 126).

Figure 125. Frequent low- to moderate-power thermal anomalies at Merapi persisted during September 2021 through February 2022, according to the MIROVA graph (Log Radiative Power). The frequency of the anomalies visibly decreased during mid-October and there was a break in activity during January. The overall strength of the anomalies also declined from September through February. Courtesy of MIROVA.

Figure 126. Incandescent block avalanches and block-and-ash flows at Merapi produced significant thermal anomalies, mainly on the SW flank. Examples from Sentinel-2 infrared satellite imagery are shown here for 20 September (top left), 20 October (top right), 4 November (bottom left) 2021, and 27 February (bottom right) 2022. Sentinel-2 satellite images use Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Geologic Background. Merapi, one of Indonesia's most active volcanoes, lies in one of the world's most densely populated areas and dominates the landscape immediately north of the major city of Yogyakarta. It is the youngest and southernmost of a volcanic chain extending NNW to Ungaran volcano. Growth of Old Merapi during the Pleistocene ended with major edifice collapse perhaps about 2,000 years ago, leaving a large arcuate scarp cutting the eroded older Batulawang volcano. Subsequent growth of the steep-sided Young Merapi edifice, its upper part unvegetated due to frequent activity, began SW of the earlier collapse scarp. Pyroclastic flows and lahars accompanying growth and collapse of the steep-sided active summit lava dome have devastated cultivated lands on the western-to-southern flanks and caused many fatalities.

Information Contacts: Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG), Center for Research and Development of Geological Disaster Technology (URL: http://merapi.bgl.esdm.go.id/, Twitter: @BPPTKG); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); 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/); 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

San Cristóbal is a volcanic complex located in Nicaragua that consists of five volcanic edifices; the youngest cone, San Cristóbal (also known as El Viejo), is capped by a 500 x 600 m-wide crater. Historical eruptions date back to 1528, with frequent eruptions occurring over the past 50 years. Recent activity has been characterized by thermal anomalies, occasional explosions, and ash plumes (BGVN 46:07). This report updates activity from July through December 2021 using information from the Instituto Nicaragüense de Estudios Territoriales (INETER) and various satellite data.

Activity during July through December 2021 was relatively calm and primarily consisted of gas-and-steam emissions and seismicity. Monitoring data reported by INETER consistently included the average sulfur dioxide levels (tons/day) and the total number of earthquakes per month, a small number of which were volcano-tectonic events (table 8). Tremor remained within 50 and 350 RSAM units.

Table 8. Monthly sulfur dioxide measurements and seismicity reported at San Cristóbal during July-December 2021.

Temperature measurements using thermal images showed that gas-and-steam emissions in the crater were 40 °C in August, which was lower than the ~56 °C temperatures measured in July. A 40-m-long by 6-m-wide fracture was observed on the NW wall. Fumaroles persisted in the crater throughout the reporting period (figure 42). Little to no thermal activity was detected near the summit through December; a single weak anomaly was visible in MIROVA data during early October (figure 43). On clear weather days, a weak thermal anomaly was observed in the summit crater in Sentinel-2 infrared satellite imagery (figure 44).

A series of five low-to medium-intensity explosions were recorded between 0522 and 0526 on 15 December, according to INETER. Ash plumes rose 2.5 km above the crater rim and drifted W, resulting in ashfall in La Grecia, Rancherías (8 km NW), and El Viejo (18 km SW). Rocks were ejected 500 m from the vent onto the flanks.

Figure 42. Photo of the crater at San Cristóbal taken in October 2021 showing gas-and-steam emissions. Courtesy of INETER (Sismos y Volcanes de Nicaragua. Octubre, 2021).

Figure 43. Low thermal activity was detected at the summit of San Cristóbal during July through December 2021; this MIROVA (Log Radiative Power) plot indicates only a single weak anomaly in early October. Courtesy of MIROVA.

Figure 44. Sentinel-2 thermal satellite imagery showing a persistent, weak thermal anomaly (bright yellow-orange) at the summit crater of San Cristóbal on clear weather days on 10 July (top left), 14 August (top right), 28 September (bottom left), and 22 November (bottom right) 2021. Images use “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

Geologic Background. The San Cristóbal volcanic complex, consisting of five principal volcanic edifices, forms the NW end of the Marrabios Range. The symmetrical 1745-m-high youngest cone, named San Cristóbal (also known as El Viejo), is Nicaragua's highest volcano and is capped by a 500 x 600 m wide crater. El Chonco, with several flank lava domes, is located 4 km W of San Cristóbal; it and the eroded Moyotepe volcano, 4 km NE of San Cristóbal, are of Pleistocene age. Volcán Casita, containing an elongated summit crater, lies immediately east of San Cristóbal and was the site of a catastrophic landslide and lahar in 1998. The Plio-Pleistocene La Pelona caldera is located at the eastern end of the complex. Historical eruptions from San Cristóbal, consisting of small-to-moderate explosive activity, have been reported since the 16th century. Some other 16th-century eruptions attributed to Casita volcano are uncertain and may pertain to other Marrabios Range volcanoes.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); 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).

Tinakula is located 100 km NE of the Solomon Trench at the N end of the Santa Cruz Islands. It is a small 3.5-km-wide island with a breached summit crater that extends to below sea level; landslides enlarged this scarp in 1965, creating an embayment on the NW coast. Its most recent eruption period began in December 2018 and has consisted of weak thermal anomalies and intermittent gas-and-steam plumes. Similar activity continued during this reporting period from July through December 2021 based on satellite data, as ground observations are rarely available.

Infrared MODIS satellite data processed by MIROVA (Middle InfraRed Observation of Volcanic Activity) detected a total of 18 weak thermal anomalies during July through December: once during late July, three times during early August, seven times during October, four times during November, and three times during late December (figure 48).

Figure 48. Weak thermal anomalies at Tinakula were detected once during late July, three times during early August, seven times during October, four times during November, and three times during late December 2021, based on this MIROVA (Log Radiative Power) graph. Courtesy of MIROVA.

Though cloudy weather often obscured clear views of the summit crater, Sentinel-2 infrared satellite imagery showed a possible lava flow descending the NW flank accompanied by gas-and-steam emissions that drifted NW on 28 October (figure 49). By 27 November the flow on the NW flank had cooled, but there were two small anomalies on the W flank that may represent a different lava flow. On 22 December two small possible lava flows moved a short distance from the summit crater to the NW and W; gas-and-steam emissions drifted generally W.

Figure 49. Sentinel-2 infrared satellite imagery showed a thermal anomaly (bright yellow-orange) on clear weather days on 28 October (left), 27 November (middle), and 22 December (right) 2021 accompanied by gas-and-steam emissions that drifted NW, SW, and SW, respectively. A possible lava flow was visible on the NW flank on 28 October. By 27 November the NW lava flow had stopped, but two anomalies were visible on the W flank, possibly representing a different flow. On 22 December two short possible lava flows were detected on the NW and W flanks. Images processed using “Atmospheric penetration” (bands 12, 11, 8a) rendering. Courtesy of Sentinel Hub Playground.

Geologic Background. The small 3.5-km-wide island of Tinakula is the exposed summit of a massive stratovolcano at the NW end of the Santa Cruz islands. Similar to Stromboli, it has a breached summit crater that extends from the summit to below sea level. Landslides enlarged this scarp in 1965, creating an embayment on the NW coast. The satellitic cone of Mendana is located on the SE side. The dominantly andesitic volcano has frequently been observed in eruption since the era of Spanish exploration began in 1595. In about 1840, an explosive eruption apparently produced pyroclastic flows that swept all sides of the island, killing its inhabitants. Frequent historical eruptions have originated from a cone constructed within the large breached crater. These have left the upper flanks and the steep apron of lava flows and volcaniclastic debris within the breach unvegetated.

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).

Kīlauea is the southeastern-most volcano in Hawaii and overlaps the E flank of the Mauna Loa shield volcano. Its East Rift Zone (ERZ) has been intermittently active for at least 2,000 years. An extended eruption period began in January 1983 and was characterized by open lava lakes and lava flows from the summit caldera and the East Rift Zone. During May 2018 lava migrated into the Lower East Rift Zone (LERZ) and opened 24 fissures along a 6-km-long NE-trending fracture zone that produced lava flows traveling in multiple directions. Lava fountaining was reported in these fissures and the lava lake in Halema’uma’u crater drained (BGVN 43:10).

The previous eruption period, during December 2020-May 2021 in Halema’uma’u crater, included a lava lake, lava flows, fountaining, and gas-and-steam emissions (BGVN 46:06). During January 2021 a levee developed around the perimeter of the lake, creating what is known as a “perched” lake with the center elevated above a moat near the crater wall. The E half of the lake became stagnant and crusted over with cooled lava, allowing the active W half of the lake to rise slightly higher. A new eruption period with similar activity that started in September is described here through December using daily reports, volcanic activity notices, and abundant photo, map, and video data from the US Geological Survey's (USGS) Hawaiian Volcano Observatory (HVO).

Activity during June-August 2021. Activity during June through August was relatively low and consisted primarily of low sulfur dioxide emissions and an increase in the number of earthquakes during August. On 12 August the sulfur dioxide emission rate was 50 tons/day (t/d), which was close to levels associated with the non-eruptive period from late 2018 to late 2020 (less than 50 t/d) and significantly lower compared to the 800 t/d measured during mid-February to mid-April. An earthquake swarm was recorded beneath the S part of the caldera at 1630 on 23 August and continued into the early morning of 25 August. The swarm increased in intensity at 0130 on 24 August and was accompanied by an increase in the rate of ground deformation to the W, as recorded by the Sandhill tiltmeter. Over 275 earthquakes were recorded during this swarm, the largest of which was Mw 3.3; the majority were less than Mw 1. Small earthquakes continued at a rate of at least 10 per hour and peaked at 28 events between 1900 and 2000 on 24 August. The rate of earthquakes decreased to 5-12 during 25 August, and by 26 August ground deformation levels had also decreased. HVO reported that ground deformation beneath the S part of the caldera had stopped on 30 August and the earthquake rate decreased during 30-31 August.

Activity during September 2021. No significant activity was reported through most of September until the 29^th, when HVO detected an increase in earthquakes activity and a change in ground deformation. At approximately 1520 that day incandescence appeared in the summit webcam images, in addition to new fissures on the floor of Halema’uma’u crater that generated lava flows (figure 498). A photo taken at 1615 showed a large plume comprised of steam, carbon dioxide, and sulfur dioxide rising from the fissures. Measurements taken just after the eruption began showed that the rate of the sulfur dioxide emissions was high at around 85,000 t/d. At about 1640 another fissure with several vents opened on the inner W wall of the crater and produced low lava fountains and flows that descended to the crater floor (figure 499). Lava from both fissures pooled on the solidified lava lake surface that quickly began to overturn and form a lava lake. Tephra was deposited in areas SW of the crater. The tallest fountain was near the S end of the lava lake and rose 20-25 m during the night of 29-30 September. Scientists on a helicopter overflight at 0730 on 30 September determined that the lake was about 980 m E-W and 710 m N-S, covering an estimated 52 hectares (figure 500). The W wall was visible, and several fountains rose from the fissure in the center of the lake. Sulfur dioxide rates remained high at around 20,000 t/d. Overnight during 30 September-1 October the fountains rose as high as 15 m above the dominant vent on the W wall, based on reports from field crews (figure 501). In total, the surface of the lava lake had risen 24 m by the morning of 1 October since the eruption began. Cooled and crusted parts of the lake’s surface overturned, also referred to as “overfoundered.” Sulfur dioxide emissions remained high, though had decreased slightly to 12,900 t/d.

Figure 498. An aerial photo of Halema’uma’u crater at Kīlauea’s summit crater on 29 September 2021. Webcam imagery showed fissures at the base of the crater (highlighted with the red circle). Aerial image was taken prior to the reactivation of the lake. Cooler temperatures are blue and warmer are shown in red. Aerial imagery copyright 2018 DigitalGlobe Nextview license. Annotated image courtesy of M. Patrick, USGS.

Figure 499. Telephoto image of one of the low lava fountains near the center of Halema’uma’u crater lava lake at the Kīlauea summit crater taken on 29 September 2021. Courtesy of M. Patrick, USGS.

Figure 500. Thermal map of the Halema’uma’u crater at the summit of Kīlauea based on aerial visual and thermal imagery taken from a helicopter at 0730 on 30 September 2021. Cooler temperatures are blue and warmer are shown in red; the red spots indicate where lava fountains had emerged through the lava lake surface, and the active W vent area. The dimensions of the lava lake are 980 m E-W and 710 m N-S, an area of about 52 hectares. Annotated image courtesy of M. Patrick, USGS.

Figure 501. Photo of Halema’uma’u crater at the summit of Kīlauea at 1800 on 30 September 2021 showing vigorous fountaining activity at the new vent on the western crater wall. This view is from the south caldera rim looking to the W. The fountain here is approximately 15 m high, measured over a five-minute interval. Courtesy of K. Lynn, USGS.

Activity during October 2021. The lake had risen another 2 m by the morning of 2 October for a total of 26 m since the start of the eruption, though the rate of the rise in the lava lake surface has slowed (figure 502); fountains were 7 m tall at the main W wall vent and rose 1-2 m at the S vents, based on observations from field crews. A burst of fountain spattering on the morning of 2 October rose 50-60 m high. On 3 October the sulfur dioxide rate was 14,750 t/d and the lava lake had risen 27 m, partially submerging the fountaining fissures and building a cone around the W vent. The W vent remained the most vigorous during 3-4 October; lava fountains rose 10-15 m with occasional bursts to 20 m and a 12-m-high spatter cone had formed. In the S and central parts of the lake, including along a fissure 35-42 m long, fountains rose 5-10 m. The W end of the lava lake was 1-2 m higher than the E part and the S end was approximately 2-3 m higher than the N end. The active lava lake surface was perched 1-2 m above a 20-m-wide ledge that extended outward to the Halema’uma’u crater wall.

Figure 502. Webcam image of Halema’uma’u crater at the summit of Kīlauea taken at 0600 on 2 October 2021 showing the active lava lake that is slowly filling the crater. The W vent in the crater is located to the bottom right. Courtesy of USGS, HVO.

A VONA report on 4 October described vigorous fountaining up to 50-60 m high that produced significant amounts of pumice, Pele’s hair, and fragments of volcanic glass that were deposited downwind along the rim and beyond the crater. Sulfur dioxide emissions decreased from 85,000 to 7,000-9,000 t/d by 4 October. The central island and several of the smaller eastern islets from the 2020 lava lake were also visible on the lake surface; the smaller islets were covered at the beginning of the eruption but have since re-emerged. A dark ring of cooled lava around the central island was from the partial submersion at the beginning of the eruption.

The surface of the lava lake continued to rise each day by about 1 m, and the depth of lava filling in the lake was 256 m with a surface elevation of 774 m by 6 October. The W vent remained the most vigorous source of lava effusion, where fountains were rising 12-16 m high and bursts to 20 m. The cone building around the W vent had an opening to the E that effused lava to the lake (figure 503). The active vent in the S part of the lake sustained fountains that rose 1-5 m but was no longer active by 8 October. Overturning of the lake’s surface was mainly focused on the W part of the lake. The lava fountain height at the W vent was variable; on 11 October it ranged from 4 to 35 m. The total lava volume erupted since the start of the eruption was an estimated 19.9 million cubic meters on 15 October. Fountains during late October were typically less than 12 m high, according to field crews. The spatter cone that built up around the W vent was approximately 10 m wide. On 29 October field crews reported that fountaining had been replaced by low roiling in the lake and spatter from the W vent. The sulfur dioxide emissions were as high as 6,800 t/d on 12 October but continued to fluctuate up to 1,600 t/d on 16 October.

Figure 503. Photo of the W vent at Kīlauea showing lava fountaining rising 12 m high on 6 October 2021. A spatter cone had built up around the vent and has an opening to the E. Photo taken from the NW rim looking S. Courtesy of K. Lynn, USGS.

Activity during November 2021. Lava continued to erupt from a vent in the W wall of Halema’uma’u crater, though it was described as roiling and spatter activity that fed the lake rather than fountaining (figure 504). The W end of the crater had a maximum elevation of 795 m, based on data from HVO’s laser rangefinder on 1 November; the lake had risen roughly 51 m since 29 September and continued to rise by roughly 1 m/day. The lava lake continued to have about 7-8 m of elevation difference between the active W vent and the stagnant E part. On 3 November the spatter reached 6 m high, and lava was frequently thrown over the edge on the vent cone. The total volume of lava erupted as of 3 November was about 28.2 million cubic meters. The short channel from the vent to the lake crusted over on 4 November. HVO reported that the E edge of the lake was level with the lowest down-dropped blocks from the 2018 collapse event. Activity briefly decreased on 8 November; SO₂ emissions decreased to 250 t/d and webcams showed minimal spatter in the W vent. Activity resumed at normal levels the next day; webcam images showed spatter and ponded lava in the W vent and an increase in the active lava surface area. Lava sporadically oozed out along the cooler outer lake margins (figure 505). By 16 November the total erupted volume was roughly 30 million cubic meters (figure 506). A brief increase in effusion on 22 November lasted a few hours. According to HVO, the area of the lava lake decreased greatly in size for about 9 hours during the late afternoon on 24 November but recovered by the early morning on 25 November. Webcams showed spatter and ponded lava within the W vent and an overflow on the S side of the cone. Sulfur dioxide emissions varied, ranging from 1,200 t/d on 29 November to 6,400 t/d on 23 November; the average rate was 3,000 t/d during the month.

Figure 504. Telephoto image of the W vent of Halema’uma’u crater at the summit of Kīlauea taken on 9 November 2021 where spatter bursts in triggered movement on the lava surface. Spattering rose 6-9 m and occasional ejecta fell onto the rim of the cone. Courtesy of USGS, HVO.

Figure 505. Telephoto image of the W lava lake at Halema’uma’u crater at the summit of Kīlauea taken at 1000 on 9 November 2021. The active lava lake (right) is flowing onto an area of cooled solidified crust (left). The inactive part of the lake surface is about 4 m lower than the active one. Courtesy of J. Schmith, USGS.

Figure 506. Aerial and thermal image of Halema’uma’u crater at the summit of Kīlauea based on observations made during a helicopter overflight on 16 November 2021 at 0830. A crater within the W vent cone contained a small spattering lava pond that was notably warmer. The thermal image shows the extent of the new lava extruded into the lake since the start of the eruption. The active lava lake in the W part of the crater is about 300 m in diameter. The E part of the lake is solidified at the surface, though some lava extrusions can be observed along the perimeter of the lake as the surface continues to rise. The new lava flow due to the new breakouts on the down-dropped block N and E of Halema’uma’u is visibly warmer. Cooler temperatures are blue and warmer are shown in red. Courtesy of HVO, USGS.

Activity during December 2021. Eruptive activity continued during December in Halema’uma’u crater, where lava effused from the W wall vent. The E end of the lake continued to advance onto the down-dropped block on the caldera floor while the W end stood at an elevation of 809 m, based on data from HVO’s permanent laser rangefinder; the total increase in elevation was about 65 m since 29 September. Spatter and ponded lava within the W vent persisted (figure 507), in addition to occasional oozes of lava along the outer lake margins, according to webcam data. HVO reported that between 1600 and 1800 on 3 December the rate of lava effusion sharply decreased, and around 1700 a small portion of the vent cone collapsed. Only a few small hotspots were visible around the vent area. Lava re-entered the vent and resumed erupting between 1730 and 1800 on 6 December. The rate sulfur dioxide emissions varied from 3,500 t/d on 9 December to 125 t/d on 13 December.

Figure 507. Photo of lava erupting from the W vent in Halema’uma’u crater at the summit of Kīlauea, including large spatter bursts on 15 December 2021. Courtesy of B. Carr, USGS.

On 20 December the lava effusion rate began to decrease following a strong deflationary event. By 21 December lava had stopped erupting from the W vent, though incandescence was still visible. The surface of the lake had mostly crusted over, except for a section N of the vent that was less than 50 m in diameter, following a sequence of partial overturning on 20 December; the lake had risen 69 m since the start of the eruption. Measurements from a helicopter overflight during the morning of 21 December indicated that the total lava volume erupted was 38 million cubic meters (figure 508).

Figure 508. Aerial photo of the W part of Halema’uma’u crater at the summit of Kīlauea taken during a helicopter overflight on 21 December 2021. Activity at the lake has paused and only a weak gas-and-steam plume was observed from the W vent (center right). Just N of the vent (toward the bottom of the photo) a small lava pond remained weakly active. The general N direction points to the bottom of the image. Courtesy of D. Downs, USGS.

Around 2000 on 22 December new breakouts were observed from the W vent in the lava lake following strong volcanic tremor at 1930; 30 minutes later the summit began to deflate. This renewed activity began to overflow and feed substantial lava flows to the SE onto inactive, crusted-over portions of the lake against the crater wall until midnight. On 24 December the rate of sulfur dioxide emissions rose to 5,300 t/d in the summit region. Lava was detected oozing out from the margins along the crusted-over E part of the lake, including one onto the lowermost down-dropped block from the 2018 caldera collapse. Another strong deflationary event was reported on 25 December, followed by another decrease in lava effusion; the W vent had stopped erupting.

New breakouts were reported around 1910 on 27 December, following a strong volcanic tremor around 1850. Lava began to overflow and feed lava flows that moved N and S onto the inactive, crusted-over parts of the lake surface. The sulfur dioxide rate was approximately 5,000 t/d in the summit region on 28 December. Due to the decreased rate of lava effusion, HVO reported a cooler crust on the outer half of the lake. No overflows from the active lake were observed during 28-29 December, but several lava breakouts occurred along the margins of the N and E part of the lake. There was also occasional minor activity at the vent on the N side of the spatter cone during the morning of 30 December. Activity picked up again at 1445 on 30 December, which included several large overflows onto older lava lake crust in the evening. Strong incandescence was also visible at night. The surface of the lake had risen a total of 70 m since the start of the eruption.

Geologic Background. Kilauea overlaps the E flank of the massive Mauna Loa shield volcano in the island of Hawaii. Eruptions are prominent in Polynesian legends; written documentation since 1820 records frequent summit and flank lava flow eruptions interspersed with periods of long-term lava lake activity at Halemaumau crater in the summit caldera until 1924. The 3 x 5 km caldera was formed in several stages about 1,500 years ago and during the 18th century; eruptions have also originated from the lengthy East and Southwest rift zones, which extend to the ocean in both directions. About 90% of the surface of the basaltic shield volcano is formed of lava flows less than about 1,100 years old; 70% of the surface is younger than 600 years. The long-term eruption from the East rift zone between 1983 and 2018 produced lava flows covering more than 100 km2, destroyed hundreds of houses, and added new coastline.

Information Contacts: Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawai'i National Park, HI 96718, USA (URL: http://hvo.wr.usgs.gov/).

Erebus, located on Ross Island, Antarctica, is one of the few volcanoes in the world with a persistent lava lake in its summit crater. Since 1972, and probably well before, minor explosions, punctuated by occasional larger Strombolian explosions, have been associated with the active lava lake. Since the Mt. Erebus Volcano Observatory suspended their seismic and infrasonic monitoring program in 2016, the volcano has been primarily monitored by satellite. This report documents activity during 2021.

Sentinel-2 satellite images during 2021 showed a single lava lake, except a much smaller second one on 30 October (figure 30). Occasionally, a white gas plume was visible. Based on MODIS infrared detectors aboard the Terra and Aqua satellites and analyzed using the MODVOLC algorithm, thermal activity during 2021 was much greater than the 2020 level (76) but much less than the 2018 and 2019 levels (774 and 624, respectively) (table 7).

Figure 30. Sentinel-2 infrared satellite images showing the active lava lake in the summit crater of Erebus during 2021. The image on 23 January 2021 (left) shows a single lava lake, while the image on 30 October 2021 (right) shows a larger lake at least one smaller thermal area immediately SW. Images use False Color rendering (bands 12, 11, 4). Courtesy of Sentinel Hub Playground.

Table 7. Number of monthly MODIS-MODVOLC thermal alert pixels recorded at Erebus during 2017-2021. See BGVN 42:06 for data from 2000 through 2016. The table was compiled using data provided by the HIGP - MODVOLC Thermal Alerts System.

Geologic Background. Mount Erebus, the world's southernmost historically active volcano, overlooks the McMurdo research station on Ross Island. It is the largest of three major volcanoes forming the crudely triangular Ross Island. The summit of the dominantly phonolitic volcano has been modified by one or two generations of caldera formation. A summit plateau at about 3,200 m elevation marks the rim of the youngest caldera, which formed during the late-Pleistocene and within which the modern cone was constructed. An elliptical 500 x 600 m wide, 110-m-deep crater truncates the summit and contains an active lava lake within a 250-m-wide, 100-m-deep inner crater; other lava lakes are sometimes present. The glacier-covered volcano was erupting when first sighted by Captain James Ross in 1841. Continuous lava-lake activity with minor explosions, punctuated by occasional larger Strombolian explosions that eject bombs onto the crater rim, has been documented since 1972, but has probably been occurring for much of the volcano's recent history.

Information Contacts: 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).

Langila consists of a group of four small overlapping cones and is located at the W tip of Papua New Guinea’s New Britain Island. Frequent mild-to-moderate explosive eruptions with ash emissions have been recorded since the 19th century from three active summit craters. The current eruption period began in October 2015 and has recently been characterized by intermittent low-level thermal activity, ash plumes, and sulfur dioxide plumes (BGVN 46:05). Similar activity continued during this reporting period of May through December 2021 using information primarily from the Darwin Volcanic Ash Advisory Center (VAAC) and satellite images.

Volcanism remained similar to recent activity. MIROVA recorded intermittent thermal activity at the summit during the reporting period, though between June and September the number and frequency of anomalies was low; during October through November, thermal activity increased (figure 25). According to the MODVOLC hotspot system, thermal anomalies were detected on 1, 15, and 20 October, 9 and 25 November, and 2 December. On clear weather days these small anomalies were visible in the summit crater in Sentinel-2 infrared satellite imagery, sometimes accompanied by gas-and-steam emissions (figure 26).

Figure 25. Intermittent thermal activity at Langila was shown in the MIROVA graph (Log Radiative Power) during May through December 2021. Activity was relatively low in both strength and frequency between June and September compared to October through December. Courtesy of MIROVA.

Figure 26. Sentinel-2 infrared satellite imagery showed a thermal anomaly at the summit of Langila on 3 May (top left), 22 June (top right), 12 July (bottom left), and 6 August (bottom right) 2021, sometimes accompanied by gas-and-steam emissions, as shown on 22 June and 12 July. On 6 August two thermal anomalies were observed at the summit. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

The NASA Global Sulfur Dioxide page, which shows data from the TROPOMI instrument on the Sentinel-5P satellite, showed intermittent weak sulfur dioxide plumes during July through December that drifted in different directions (figure 27). Occasional ash plumes were reported by the Darwin VAAC beginning on 3 May when an ash plume rose to 1.5 km altitude and drifted NE, based on HIMAWARI-8 satellite imagery, which was also accompanied by a thermal anomaly (figure 26). On 15 May an ash plume rose to 3 km altitude and drifted in two directions: S and NW. During 13-14 June ash plumes rose to 2.1-3 km altitude and drifted WNW and W, respectively. An ash plume that rose to 2.4 km altitude drifted WNW and W on 20 August and 11-12 September; a hotspot was also observed at the summit in satellite imagery during those days, according to the Darwin VAAC. A strong ash plume on 21 October rose to 4.6 km altitude and drifted NW.

Figure 27. Small sulfur dioxide plumes were visible above Langila based on data from the TROPOMI instrument on the Sentinel-5P satellite. Plumes drifted SW on 3 August (top left), NW on 5 August (top right), SW on 7 October (bottom left), and S on 23 November (bottom right) 2021. Plumes were also intermittently visible from Manam (to the W) and Bagana (to the E). Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. Langila, one of the most active volcanoes of New Britain, consists of a group of four small overlapping composite basaltic-andesitic cones on the lower E flank of the extinct Talawe volcano in the Cape Gloucester area of NW New Britain. A rectangular, 2.5-km-long crater is breached widely to the SE; Langila was constructed NE of the breached crater of Talawe. An extensive lava field reaches the coast on the N and NE sides of Langila. Frequent mild-to-moderate explosive eruptions, sometimes accompanied by lava flows, have been recorded since the 19th century from three active craters at the summit. The youngest and smallest crater (no. 3 crater) was formed in 1960 and has a diameter of 150 m.

Information Contacts: 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).

The remote island of Heard is located in the southern Indian Ocean and contains the Big Ben stratovolcano, which has been intermittently active since 1910. Activity during late November 2020 through January 2021 produced thermal anomalies over the active Mawson Peak and lava flows (BGVN 46:05), which is similar to activity during this reporting period of February 2021 through January 2022. Information primarily comes from satellite data.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows that the thermal activity was relatively low during February through September 2021, with the exception of four low-power anomalies between late May and early June and two stronger anomalies in mid-July (figure 48). Despite the cloudy weather, Sentinel-2 infrared satellite imagery also detected the stronger anomaly at Mawson Peak on 22 July (figure 49). Very weak anomalies were also visible in satellite imagery on 31 August and 22 and 25 September, though they remained stationary and close to the summit (figure 49).

Figure 48. Thermal activity at Heard was relatively low during April-September 2021 and consisted of four low-power anomalies between late May and early June and two stronger anomalies in mid-July. No thermal activity was detected during February and March 2021 (see figure 46 from BGVN 46:05). Activity notably increased in both power and frequency near the end of October 2021, according to the MIROVA system, shown in this Log Radiative Power graph. Courtesy of MIROVA.

Figure 49. Sentinel-2 infrared satellite imagery of Heard Island’s Big Ben showed weak thermal anomalies (bright yellow-orange) on 22 July (top left), 31 August (top right), 22 September (bottom left), and 25 September (bottom right) 2021. The activity on 22 July was also reflected in MIROVA data. The activity remained confined to the summit during these dates. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

Renewed thermal activity of varying intensities was detected in Sentinel-2 infrared satellite imagery over the summit area and on the NW and W flanks on 4, 11, 14, and 21 November, though weather clouds often prevented clear views of the summit. On 11 November four anomalies just NW of Mawson Peak formed a “v” shape open to the NW, which could indicate a branched lava flow; this increase in thermal activity was also noted in MIROVA data (figure 50). According to the MODVOLC Thermal Algorithm, there was a total of twelve thermal alerts on 15, 20, and 25 November and 15, 24, and 28 December. A strong anomaly was visible on the W flank on 21 November, accompanied by possible gas-and-steam emissions that extended E and SE.

Figure 50. Sentinel-2 infrared satellite imagery of Heard Island’s Big Ben showed stronger thermal anomalies (bright yellow-orange) on 11 November (top left), 4 December (top middle), 26 December (top right), 31 December (bottom left) 2021, 3 January (bottom middle), and 5 January (bottom right) 2022. Activity on 11 November marks the start of increased thermal activity according to the MIROVA graph; four distinct anomalies were detected just NW of Mawson Peak, which persisted into December. On 26 and 31 December a possible lava flow was visible traveling W. Between 3 and 5 January a strong lava flow was noted on the SW flank (black streak). Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

Anomalies continued to be detected through January 2022. A possible lava flow was observed on 4 December on the NW flank, accompanied by gas-and-steam emissions to the SE. A strong gas-and-steam plume occurred on 16 December, extending NE and accompanied by a lava flow. A small possible branched lava flow was visible traveling NW and SW on 26 December, accompanied by a small gas-and-steam emission to the E. This activity was also visible on 31 December. Sometime during 3 and 5 January, a strong lava flow event occurred, descending the SW flank, based on satellite images (figure 50). Clouds obscured views of the summit during a majority of the month, though a small anomaly and gas-and-steam emission drifting N were detected on 20 January. A weaker anomaly was visible on the NW flank on 30 January.

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/); 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).

Frequent activity at Ecuador's Sangay has included pyroclastic flows, lava flows, ash plumes, and lahars reported since 1628. The current eruption began in March 2019 and has continued throughout 2021. Ongoing activity from July-December 2021 covered in this report includes explosions with ash plumes, incandescent ejecta, lava flows, and lahars. Information is provided by Ecuador's Instituto Geofísico, Escuela Politécnica Nacional (IG-EPN), and multiple sources of remote data including the Washington Volcanic Ash Advisory Center (VAAC), the Italian MIROVA Volcano HotSpot Detection System, and Sentinel-2 satellite imagery.

Gas and ash emissions rose 500-1,500 m above the summit of Sangay on most days during July-December 2021. Pulses of activity that produced emissions higher than 2 km above the summit occurred at least once almost every month. Ashfall in Chimborazo Province was reported on 14 and 19 July; a plume on the 19th rose to over 10 km altitude. A new vent appeared on the W side of the summit on 24 November, a new lava flow began on the N flank on 2 December, and lava remained intermittently active in the SE flank ravine throughout the period. Steady levels of thermal energy were present in the MIROVA data from July through early December, with a marked increase in frequency at the middle of the month that corresponded with increased observations of incandescence on the N and W flanks (figure 113). A major explosive event on 2 December produced an ash plume that rose to 15.2 km altitude.

Figure 113. MIROVA Log Radiative Power graph of MODIS data for Sangay from 22 March through December 2021 showed relatively low levels of activity into July followed by increased thermal activity through November. After an explosive event in early December, multiple active vents at the summit produced incandescent material, significantly increasing the thermal activity that was recorded. Courtesy of MIROVA.

Multiple daily VAAC reports during July 2021 indicated frequent ash emissions that rose 100-2,400 m above the summit (5.4-7.7 km altitude) and most often drifted W or SW (figure 114). A larger explosion on 19 July produced a plume that rose to 10.7 km altitude and drifted SE while another part of the plume moved NE at 8.5 km altitude. A few hours later a plume was drifting W at 6.4 km altitude. Ashfall was observed on 14 July in the Guamote and Cebadas sectors from one of the 12 explosions reported that day, and on 19 July in the canton of Guamote in the Chimborazo province. Noises were reported to the W and SW after explosions on 19 and 25 July. Incandescence at the crater accompanied ash emissions on 15 July (figure 115). MODVOLC thermal alerts were recorded on 8, 14, 24, and 31 July. Rains were strong enough to produce lahar signals at the seismic stations on 1-3, 9-13, 16, 18-22, 27, and 29-31 July but no damages were reported.

Figure 114. An ash plume drifted W from the summit of Sangay on 5 July 2021 while a significant thermal anomaly was present at the summit. Similar ash emissions were reported almost every day of the month. Sentinel-2 satellite image uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Figure 115. Incandescence at the summit of Sangay was visible from the ECU 911 webcam on 15 July 2021. Courtesy of IG-EPN Daily report.

Gas and ash emissions drifted SW to NW at 500-1,500 m above the summit during most days in August 2021, although activity was more intermittent than during July, with no emissions reported during 15-18 and 26-31 August. The tallest plume reported by IG-EPN rose 2 km above the summit and drifted SW, W, and NW on 21 August, but no ashfall was reported. A pulse of thermal activity on 4 August that was recorded in satellite imagery (figure 116) led IG-EPN to identify a new flow on the SE flank. Incandescence and ash emissions were both visible in webcams on 5 August, confirming that the flow was confined to the upper flank of the volcano. Explosions produced incandescent blocks that rolled down the SE flank ravine overnight on 13-14 and 19-20 August (figure 117). The next night the lava was also visible in the ravine. MODVOLC thermal alerts were issued on 4, 20, 23, and 24 August. Lahar signals were recorded on 2, 11-13, 16-20, 23, and 26 August.

Figure 116. A new lava flow at Sangay produced a strong thermal anomaly on the SE flank on 4 August 2021 (left). The summit anomaly was also very strong on 24 August (right). Sentinel-2 satellite imagery uses Atmospheric penetration rendering (bands 12, 11, 8A). Courtesy of Sentinel Hub Playground.

Figure 117. Explosions at Sangay on 14 (left) and 19 (right) August 2021 sent incandescent blocks down the SE flank ravine. Courtesy of IG-EPN daily reports.

Steam, gas, and ash emissions from the summit continued throughout September 2021, drifting SW, W, or NW on most days. Plumes were usually reported at 600-1,200 m above the summit; they rose as high as 2,000 m on 8 and 20 September. Incandescence was visible at the crater on the morning of 15 September. Overnight on 23-24 September, Strombolian activity at the summit sent incandescent ejecta down the SE flank ravine (figure 118). MODVOLC thermal alerts on 5, 14, 15, 24, 28, and 30 September indicated continued pulses of thermal activity. Heavy rain produced lahar signals at seismic stations on 1-4, 6, 7, 9, and 11-13 September.

Figure 118. Strombolian activity at Sangay overnight on 23-24 September 2021 sent incandescent ejecta down the SE flank ravine that was recorded by the ECU 911 webcam. Courtesy of IG-EPN daily report.

A single MODVOLC thermal alert was issued on 1 October; multiple alerts were issued each day on 9, 14, and 18 October. Webcam images of a lava flow near the summit were seen overnight on 3-4 October. Incandescent blocks were visible on the SE flank overnight on 5-6 October. On 10 October the ECU 911 webcam showed an active flow on the SE flank. Sentinel satellite images showed strong thermal anomalies at the summit and on the SE flank on 8, 13, and 18 October (figure 119). Satellite images from 13 October indicated continued slow growth of the ravine on the SE flank. When last measured in March 2021 it was 600 (± 40 m) wide; as of 13 October is was 650 (± 40 m) wide. Overnight on 16-17 October incandescent blocks were seen rolling down the SE flank ravine. The Washington VAAC issued multiple ash advisories each day during October 2021. Plumes were reported rising 500-1,200 m above the summit on most days; although on 13, 18-19 (figure 120), and 30 October gas and ash plumes rose 1,200-1,500 m high from multiple emission events. A narrow plume of steam and ash drifted tens of kilometers WSW from the summit on 28 October and corresponded to an increase in seismic tremor signals, according to IG-EPN. Small lahars were recorded as high-frequency seismic signals during 29-31 October.

Figure 119. Strong thermal anomalies on the SE flank and at the summit of Sangay on 8 (top) and 13 (bottom) were accompanied by ash or steam emissions drifting NW in Sentinel-2 satellite imagery. Images uses Atmospheric penetration rendering (bands 12, 22, 8A). Courtesy of Sentinel Hub Playground.

Figure 120. The ECU 911 webcam at Sangay captured a dense ash and steam plume rising 1,500 m from the summit and drifting W on 18 October 2021. Courtesy if IG-EPN daily reports.

A pulse of increased emissions activity on 1 and 2 November 2021 produced plumes that rose higher than 2,000 m above the summit and drifted SW, W, and NW; incandescence was also observed in the webcams at the summit overnight. Ash plumes on most days of the month rose 600-1,200 m above the summit and drifted in multiple directions. Incandescent blocks were observed descending the SE flank ravine overnight on 6-7 November (figure 121). On 8 November trace amounts of ashfall were reported in Guamote and Flores, with moderate ashfall in Pungala and Cebadas. Overnight the ash plumes reached as high as 2,400 m above the summit. Additional pulses of ash emissions that rose as high as 2,000 m above the summit were reported on 12, 14-15, 24, and 28-29 November.

Figure 121. Ash emissions or incandescent activity was recorded daily at Sangay throughout November 2021. Strombolian activity and incandescent material descended the SE flank ravine and were recorded in the ECU-911 webcam on 6 November (left). Overnight on 8-9 November (right), ash emissions that rose as high as 2,400 m above the summit resulted in ashfall reported in several parishes in the Chimborazo province. Courtesy of IG-EPN daily reports.

In a special report issued on 24 November IG-EPN noted that there had been an increase in the rate of explosions since the evening of 17 November, reaching an average of two per minute; most of them were small. They also noted that the slight inflation recorded on most flanks since June 2021 had grown more pronounced in recent weeks. Strombolian activity and a lava flow in the SE flank ravine were recorded in the ECU-911 webcam on 24 November. Also, IG-EPN reported a possible new vent on the upper W flank that day. Steam and ash emissions were visible in Sentinel-2 satellite imagery along with a summit thermal anomaly on 27 November. A single MODVOLC thermal alert was recorded on 4 November, and multiple alerts were recorded each day on 15, 17, 22, and 26-28 November. Two lahar signals were measured each day on 12 and 26 November, and one each on 18 and 19 November.

A seismic swarm that began on the afternoon of 1 December 2021 was detected by the SAGA station located 6 km SW of the summit. The frequency of the events increased from 32 per hour to 60 per hour by the following morning. A major explosive event during the early morning of 2 December produced an ash plume that rose to 15.2 km altitude and drifted W according to the Washington VAAC (figure 122). The plume dissipated quickly and was seen 45 km WSW of the summit about an hour later at 6.1 km altitude. The higher part of the plume was moving S and E at 13.7 km altitude and dissipating more slowly. A second emission a few hours later rose to 7 km altitude and drifted NW. There were no reports of ashfall that day. Multiple VAAC reports were issued daily for the remainder of the month; steam, gas, and ash plumes rose 500-1,500 m above the summit and drifted mostly W and SW (figure 123).

Figure 122. A seismic swarm at Sangay began during the afternoon of 1 December 2021 (local time), shown by the red arrow on the left side in the seismograph from the SAGA station located 6 km SW of the summit. The frequency and amplitude of the seismic events increased until the following morning, as indicated by the yellow arrow on the right side. A major explosion shortly after 0900 UTC (0400 local time) is shown in purple identified by the red arrow labeled ‘Primera explosion’ at the bottom left of the image. Courtesy of IG-EPN (Informe Volcánico Especial – Sangay – 2021 – Number 03, Nuevo pulso de actividad en el volcán Sangay, Quito, 3 de diciembre de 2021).

Figure 123. A dense steam plume at Sangay drifted SW adjacent to multiple small pulses of dark ash emerging from the summit, also drifting SW on 7 December 2021. Sentinel-2 satellite image uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

A new lava flow on the N flank was first reported after the explosions of 2 December; it was observed in Sentinel-2 satellite imagery on 12 December (figure 124). During the second week of December IG technicians installed five new infrasound sensors and a new high-resolution seismic station in the Domono area, northwest of Macas, 34 km from the summit. Between 15 and 17 December they also installed new ash sampling meters W of the volcano in the Guamote Canton, including the communities of Piscinas de Atillo, Punto Cero-Atillo, Cashapamba, Palmira, and Guamote. Webcam images from overnight on 20-21 December showed incandescent blocks rolling down the SE flank ravine (figure 125).

Figure 124. After the major explosion on 2 December 2021 at Sangay a new lava flow was reported on the N flank; its thermal anomaly first appeared in a Sentinel-2 satellite image on 12 December. Also apparent in this image is the new vent on the W side of the summit first reported in late November. Image uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Figure 125. Strombolian explosions at Sangay on 21 December 2021 sent incandescent blocks down the SE flank ravine as seen in the ECU 911 webcam. Courtesy of IG-EPN Daily Report.

Members of the volcanic observers network in Macas (in the Morona Santiago Province) reported loud noises from Sangay on 25 December. Seismic data showed a slight increase in the intensity of the explosions. A lava flow was visible in the SE flank ravine overnight on 27-28 December along with incandescence at the crater. Pulses of gas and ash emissions rose to 1,000-1,500 m altitude and drifted NE, NW, S, and SW on both days (figure 126). On 31 December, IG-EPN reported three active emissions from the summit: the lava flow in the SE flank ravine, pyroclastic ejecta at the western vent first observed in late November, and lava flows from a N-flank vent, all resulting in a significant increase in thermal energy. All three were visible in Sentinel-2 satellite imagery taken on 27 December (figure 126). More multiple per day MODVOLC thermal alerts were recorded in December than previous months, confirming the increase in thermal activity witnessed in the webcams. Alerts were issued on 3, 8, 10, 12, 15, 19, 21, 22, 24, 26, 28, and 31 December. Lahar seismic signals were recorded on 14, 16, and 18-19 December. Sulfur dioxide emissions were generally low throughout the period, though increases in activity during December coincided with more frequent plumes of SO₂ as measured with satellite instruments (figure 127).

Figure 126. Activity increased at the summit of Sangay at the end of December 2021. In a Sentinel-2 image from 27 December (left), three emission sites were producing thermal anomalies; the N vent produced a flow on the N flank, the W vent sent pyroclastic ejecta down the W flank, and the SE vent produced lava that descended the ravine on that flank. Pulses of ash emissions (right) were visible through dense meteoric clouds on 28 December. Sentinel image uses Atmospheric penetration rending (bands 12, 11, 8a). Left image courtesy of Sentinel Hub Playground; right image courtesy of IG-EPN Daily Report.

Figure 127. Strong sulfur dioxide plumes originated from Sangay during December 2021, as seen here in images from 1, 6, 12, and 27 December. Sangay is the triangle at the bottom of the image and Columbia’s Nevado del Ruiz it the topmost symbol, also with frequent SO2 emissions. Data is from the TROPOMI instrument on the Sentinel-5P satellite. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. The isolated Sangay volcano, located east of the Andean crest, is the southernmost of Ecuador's volcanoes and its most active. The steep-sided, glacier-covered, dominantly andesitic volcano grew within horseshoe-shaped calderas of two previous edifices, which were destroyed by collapse to the east, producing large debris avalanches that reached the Amazonian lowlands. The modern edifice dates back to at least 14,000 years ago. It towers above the tropical jungle on the east side; on the other sides flat plains of ash have been sculpted by heavy rains into steep-walled canyons up to 600 m deep. The earliest report of a historical eruption was in 1628. More or less continuous eruptions were reported from 1728 until 1916, and again from 1934 to the present. The almost constant activity has caused frequent changes to the morphology of the summit crater complex.

Information Contacts: Instituto Geofísico, Escuela Politécnica Nacional (IG-EPN), Casilla 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); 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); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard MD 20771, USA (URL: https://so2.gsfc.nasa.gov/).

Russia’s Sheveluch (Shiveluch) volcano in the northeastern region of the Kamchatka peninsula, has produced numerous major eruptions documented in both reported observations dating back several hundred years and also with tephrochronology dating back 10,000 years. A major south-flank collapse during a 1964 Plinian explosion produced a scarp within which a “Young Sheveluch” dome began to form in 1980. Repeated episodes of dome formation and destruction since then have produced major and minor ash plumes, pyroclastic, and block-and-ash flows, and two “whaleback domes” of spine-like extrusions in 1993 and 2020. The whaleback domes were subsequently destroyed with further dome growth and the ongoing explosive activity. This report summarizes activity from July-December 2021 with information provided by the Kamchatka Volcanic Eruptions Response Team (KVERT), the Kamchatka Volcano Station (part of the Kamchatka Branch of the Geophysical Survey, Russian Academy of Science (KB GS RAS), the Tokyo Volcanic Ash Advisory Center (VAAC), and satellite information.

The growth of the lava dome inside the 1964 flank collapse scarp continued throughout July-December 2021. Strong fumarolic activity with gas and steam rising from multiple vents around the dome was recorded in either satellite images or was visible whenever the volcano was not obscured by weather. Incandescence at the dome persisted and incandescent block avalanches were reported on most days. A significant increase in thermal and explosive activity during August coincided with the destruction of part of the “Dolphin” whaleback dome that first appeared in September 2020 (BGVN 45:11, figure 55). Ash emissions were recorded daily for much of August. Activity decreased from September through mid-December, with low levels of thermal activity persisting. A new episode of explosions with ash emissions was recorded in late December. The spike in thermal activity during August is clearly seen in the MIROVA Log Radiative Power graph of activity from 6 April through December 2021 (figure 74).

Figure 74. Thermal activity at Sheveluch increased significantly during August 2021 and then decreased steadily through the end of the year according to the MODIS thermal data presented by the MIROVA project. Courtesy of MIROVA.

Lava dome growth continued during July 2021, with observations of strong fumarolic activity and incandescent blocks on the dome. A thermal anomaly was present almost every day, according to KVERT, and multiple anomalies were observed in Sentinel-2 satellite imagery during the month (figure 75). MODVOLC thermal alerts were recorded on 7, 8, 22, 25, 28, and 30 July. During 6-7 July, KVERT satellite data indicated that resuspended ash extended about 90 km E from the volcano at 3.4 km altitude. The Tokyo VAAC reported an ash emission on 17 July that rose to 3.7 km altitude and drifted SE. Activity increased on 26 July with small explosions and collapses of numerous lava dome blocks observed that day and the next. A gas and steam plume containing ash drifted 45 km E. The Tokyo VAAC reported that the ash emission on 26 July rose to 4.3 km altitude and drifted SE. They noted another explosion on 27 July that was estimated at 6.1 km altitude from seismic data. On 28 July two possible ash plumes were observed drifting E and SE at 3.4 km altitude.

Figure 75. Thermal anomalies within the growing dome at Sheveluch were recorded in Sentinel-2 satellite data on 1, 13, 23, and 31 July 2021. Two of those days are shown here. Images use Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel hub Playground.

Thermal and explosive activity increased significantly during August 2021. The Tokyo VAAC reported a series of ash emissions during 2-4 August that rose to 3.7-3.9 km altitude and drifted S and SE. Multiple MODVOLC thermal alerts were issued on several days including 3, 10, 11, 13, 15-22, 24-25, and 28-29 August(figure 76). In addition to the strong fumarolic activity and incandescence observed daily on the dome, avalanches of incandescent blocks began on 5 August and lasted throughout the month (figure 77). KVERT reported ash emissions extending about 90 km SW from the volcano on 7 August; the Tokyo VAAC reported these emissions at 4.3 km altitude. KVERT noted an increase in dome thermal anomaly brightness from 8 August through the rest of the month. Beginning on 9 August, the Tokyo VAAC issued multiple daily ash alerts through 30 August. Emissions rose to altitudes from 3.7 to 5.5 km that initially drifted SW, switched to SE then back to SW at the end of the month. KVERT reported plumes drifting 50-170 km SW during 10-13 August and reaching as far as 350 km SE during 13-24 August. Steam and gas emissions containing ash were also recorded by Kamchatka Volcano Station researchers during 10-13 August. They also reported small explosions on 10 and 11 August that contributed to the destruction of the ‘Dolphin’ whaleback dome, and noted multiple small collapses and incandescent block avalanches on the S and SE slopes of the dome each night in late August.

Figure 76. Sentinel-2 satellite imagery from 5 (left) and 20 (right) August 2021 indicated the significant increase in thermal activity at Sheveluch during the month compared with July. Images use Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Figure 77. Blocks of incandescent ejecta roll down the flanks of the dome at Sheveluch on 10 August 2021. Photo by Alexey Demyanchuk, courtesy of the Kamchatka Volcano Station.

Both thermal and explosive activity decreased significantly during September 2021, although the lava dome continued to grow. Incandescent blocks were visible on the flank of the dome, and a thermal anomaly was present at the dome throughout the month according to KVERT, decreasing slightly in intensity for the second half of the month. MODVOLC thermal alerts decreased in frequency and intensity compared with August; single or double alerts were recorded on 4, 5, 7, 9, 14-15, and 17-18 September, but no more were reported for the remainder of 2021. Researchers at the Kamchatka Volcano Station reported the noticeable drop in overall activity including far fewer incandescent block avalanches than during August, but continued to report incandescence at the SE edge of the crater at night. Visitors to the volcano experienced ashfall on 8 September; inclement weather prevented views of the summit. The next morning on 9 September a clear view of the volcano revealed a small pyroclastic flow on the flank in the direction of the Baydarnaya River (figure 78). Infrared imagery observed the previous nights had shown an increase in incandescence and small block avalanches on the SE flank. Resuspended ash was reported drifting SE at 3.4 km altitude on 10 September. Dense steam and gas emissions drifted SE from multiple spots around the dome on 13 September (figure 79).

Figure 78. A view looking N towards Sheveluch on 9 September 2021 shows steam and gas rising around the active dome within the 1964 flank collapse scarp. Darker deposits of recent pyroclastic and debris flows are visible on the S flank of the dome. Photo by Alexey Demyanchuk, courtesy of the Kamchatka Volcano Station.

Figure 79. Dense steam and gas emissions at Sheveluch drifted SE from multiple spots around the dome on 13 September 2021. Photo by Alexey Demyanchuk, courtesy of the Kamchatka Volcano Station.

The Tokyo VAAC reported volcanic ash extending W from the volcano on 17 September at 6.1 km altitude. A field team from the KF FRC EGS RAS (Kamchatka Branch of Federal Research Center Unified Geophysical Service of the Russian Academy of Sciences) performed maintenance and repair on the radio telemetered seismic stations in the region on 18 September, and with excellent visibility photographed the larger Sheveluch massif as well as fumarolic activity around the growing dome of “Young Sheveluch” (figures 80 and 81). An eruption on 23 September was reported by the Tokyo VAAC sending ash to 5.2 km altitude and extending SE. KVERT reported the ash cloud visible 22 km E.

Figure 80. In this view looking west on 18 September 2021 the larger “Old Sheveluch” massif is on the right, and the “Young Sheveluch” active dome is in the center left with the steep face of the western scarp of the 1964 flank collapse behind. Fumarolic steam and gas plumes drift E from around the active dome. Photo by Alexey Demyanchuk, courtesy of the Kamchatka Volcano Station.

Figure 81. A closeup view of the active dome of Sheveluch on 18 September 2021 shows a partial collapse of the whaleback lava spine that was visible in June 2021 (BGVN 46:07, figure 72), while the dome continues to grow beneath the spine. Gas and steam emerge from multiple vents around the edges of the growing dome. View is looking east. Photo by Alexey Demyanchuk, courtesy of the Kamchatka Volcano Station.

The growth of the lava dome continued during October 2021; strong fumarolic activity was accompanied by visible incandescence and incandescent block avalanches descending the flank of the dome. KVERT reported a thermal anomaly over the volcano on all nights that were not obscured by clouds. A strong wind on 6-7 October resuspended ash from the S flank to altitudes of 4-6 km that was observed in satellite images extending 200 km SE. The same thing happened on 21 October with the ash rising to 4 km altitude and drifting 40 km SE. KVERT reported a gas and steam plume containing ash on 24 October extending 33 km NE. Sentinel-2 satellite imagery revealed dark debris flows surrounded by fresh snow on the SE flank of the dome on 24 October (figure 82). The Tokyo VAAC reported resuspended ash at 3.4 km altitude drifting E on 30 October.

Figure 82. Dark debris is surrounded by fresh snowfall on the SE flank of the active dome at Sheveluch on 24 October 2021 (left) while a plume of steam and gas drifts NNE over a thermal anomaly (right). KVERT reported ash visible in the steam and gas plume that day. Left image uses Natural color rendering (bands 4, 3, 2), and the identical right image uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Strong fumarolic activity, incandescence, and block avalanches during November 2021 indicated continued growth of the dome. Most clear days revealed a thermal anomaly in satellite imagery according to KVERT. The Tokyo VAAC reported two possible ash emissions on 1 November that rose to 3.7-3.9 km altitude and drifted SE. An eruption on 6 November produced an ash plume that rose to 3.7 km altitude and was visible moving S and W in satellite imagery for about 13 hours before dissipating. On 10 November a team from the Kamchatka Volcano Station was able to make a site visit and observed emissions of gas and steam rising from multiple sites around the edges of the active dome (figure 83). The Tokyo VAAC reported an ash emission on 18 November that rose to 3.4 km altitude and drifted N for about ten hours.

Figure 83. A clear day on 10 November 2021 made it possible for the team from the Kamchatka Volcano Station to sample and photograph activity at Sheveluch. Emissions of gas and steam rose from multiple sites around the edges of the active dome. Photo by Alexey Demyanchuk, courtesy of the Kamchatka Volcano Station.

The growth of the lava dome continued throughout December 2021, indicated by strong fumarolic activity, incandescence, block avalanches, and thermal anomalies in satellite data on most days. The Tokyo VAAC reported a possible ash emission on 21 December that rose to 3.7 km altitude and drifted NW. Beginning on 23 December and lasting through the rest of the month, near-daily ash emissions produced ash clouds that rose to 4.5-6 km altitude and drifted up to 100 km NE or NW (figure 84).

Figure 84. A dense ash plume rose from the dome at Sheveluch on 23 December 2021 and drifted NE. Photo by Alexey Demyanchuk, courtesy of the Kamchatka Volcano Station.

Geologic Background. The high, isolated massif of Sheveluch volcano (also spelled Shiveluch) rises above the lowlands NNE of the Kliuchevskaya volcano group. The 1300 km3 volcano is one of Kamchatka's largest and most active volcanic structures. The summit of roughly 65,000-year-old Stary Shiveluch is truncated by a broad 9-km-wide late-Pleistocene caldera breached to the south. Many lava domes dot its outer flanks. The Molodoy Shiveluch lava dome complex was constructed during the Holocene within the large horseshoe-shaped caldera; Holocene lava dome extrusion also took place on the flanks of Stary Shiveluch. At least 60 large eruptions have occurred during the Holocene, making it the most vigorous andesitic volcano of the Kuril-Kamchatka arc. Widespread tephra layers from these eruptions have provided valuable time markers for dating volcanic events in Kamchatka. Frequent collapses of dome complexes, most recently in 1964, have produced debris avalanches whose deposits cover much of the floor of the breached caldera.

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, Kamchatka Branch of Geophysical Survey of the Russian Academy of Science, (KB GS RAS), 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); 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/).

Ambae (also known as Aoba) is a large 2,500 km³ basaltic shield volcano located in the New Hebrides arc, part of Vanuatu. Historical eruptions have been recorded since the 16^th century, characterized by periodic phreatic and pyroclastic explosions originating from the summit crater lakes. A tuff cone was constructed within Lake Voui (or Vui) during November 2005-February 2006 (BGVN 31:12, figure 30). The most recent eruption occurred from July to September 2018 (BGVN 44:02) consisting of ash plumes, heavy ashfall, and lahars. This report updates activity from June 2019 through December 2021 and describes a new eruption beginning in December 2021 characterized by explosions, a thermal anomaly, and gas-and-steam emissions. Information primarily comes from the Vanuatu Geohazards Observatory of the Vanuatu Meteorology and Geo-Hazards Department (VMGD) and various satellite data.

Activity during June 2019 through November 2021 was relatively low, according to VMGD. On 23 September 2019 the Volcanic Alert Level (VAL) was lowered from 2 to 1 (on a scale of 0-5). Some gas-and-steam emissions were observed in webcam imagery on 4 and 5 May 2021 from a vent in Lake Voui. Intermittent low-strength thermal anomalies were detected by the MIROVA (Middle InfraRed Observation of Volcanic Activity) volcano hotspot detection system based on analysis of MODIS data during May through July 2021 (figure 94). Seven anomalies were detected during May, three during June, four during July, one during early October, and four during December. The December activity was also reflected in Sentinel-2 infrared satellite imagery, which showed a small thermal anomaly in Lake Voui accompanied by a strong gas-and-steam plume that drifted SW (figure 95). On 5 December gas-and-steam emissions rose 250 m high and explosions ejected wet tephra 10 m above the lake surface, based on information from VMGD. As a result, the VAL was raised to 2 on 27 December.

Figure 94. Intermittent low-strength thermal anomalies at Ambae were detected in the MIROVA (Log Radiative Power) graph during May through December 2021. Seven anomalies occurred during May, three during June, four during July, one during early October, and four during December. Courtesy of MIROVA.

Figure 95. Sentinel-2 infrared and natural color satellite imagery showing increased activity at Ambae. On 4 December, a thermal anomaly (bright orange-yellow) was visible and accompanied by a strong gas-and-steam plume that drifted SW (left). On 9 December a notable white gas-and-steam plume drifted W (right). Images using “Atmospheric penetration” (bands 12, 11, 8a) and “Natural Color” rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

Geologic Background. The island of Ambae, also known as Aoba, is a massive 2,500 km3 basaltic shield that is the most voluminous volcano of the New Hebrides archipelago. A pronounced NE-SW-trending rift zone dotted with scoria cones gives the 16 x 38 km island an elongated form. A broad pyroclastic cone containing three crater lakes (Manaro Ngoru, Voui, and Manaro Lakua) is located at the summit within the youngest of at least two nested calderas, the largest of which is 6 km in diameter. That large central edifice is also called Manaro Voui or Lombenben volcano. Post-caldera explosive eruptions formed the summit craters about 360 years ago. A tuff cone was constructed within Lake Voui (or Vui) about 60 years later. The latest known flank eruption, about 300 years ago, destroyed the population of the Nduindui area near the western coast.

Information Contacts: Geo-Hazards Division, Vanuatu Meteorology and Geo-Hazards Department (VMGD), Ministry of Climate Change Adaptation, Meteorology, Geo-Hazards, Energy, Environment and Disaster Management, Private Mail Bag 9054, Lini Highway, Port Vila, Vanuatu (URL: http://www.vmgd.gov.vu/, https://www.facebook.com/VanuatuGeohazardsObservatory/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

The frequently active Merapi is located immediately north of the major city of Yogyakarta in central Java, Indonesia. Volcanism has included lava flows, pyroclastic flows, lahars, explosions with heavy ashfall, incandescent block avalanches, block-and-ash flows, and cycles of dome growth and destruction. The current eruption period began in December 2020 and has recently consisted of explosions, dome growth, incandescent avalanches, and block-and-ash flows (BGVN 46:09). Two new domes appeared in January and February 2021: one inside the summit crater and one at the top of the SW flank. This report updates similar activity from September 2021 through February 2022 using information primarily from Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG), the Center for Research and Development of Geological Disaster Technology, a branch of PVMBG which specifically monitors Merapi. Additional information came from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), MAGMA Indonesia, and the Darwin Volcanic Ash Advisory Centre (VAAC).

BPPTKG reported that the two lava domes, located just below the SW rim and in the summit crater, continued to grow during September; the SW dome had grown 7 m taller and had an estimated volume of 1.6 million cubic meters and the summit lava dome had grown 1 m wider and had an estimated volume of 2.85 million cubic meters. A total of seven pyroclastic flows descended the SW flank as far as 2.5 km, and 67-144 lava avalanches traveled a maximum of 2 km SW. On 1 September ashfall was reported in several areas, including Srumbung, Salam, Ngluwar, Dukun, Mungkid, Sawangan, Muntilan, and the Salaman sub-districts. White gas-and-steam emissions rose 500 m high, based on observations from the Kaliurang Mount Merapi Observation Post. During 9-10 September a continuous ash emission was observed in satellite imagery and rose to 3 km altitude and drifted E and NW.

Both lava domes continued to grow during October; the SW dome had grown about 5 m higher, with an estimated volume of 1.67 million cubic meters, and the summit dome rose another 4 m with an estimated volume of 2.927 million cubic meters. Two pyroclastic flows descended 2.5 km down the SW flank. There were 30-76 lava avalanches reported that traveled a maximum of 2 km SW. White gas-and-steam emissions rose 700 m high. During 15-21 October a thin ash deposit was reported in the Selo area.

Similar activity continued during November, with active lava domes and 106-212 lava avalanches that traveled 2 km SW. Two pyroclastic flows were reported descending 1.5-2 km on the SW flank; one pyroclastic flow traveled 1.8 km down the SW flank on 13 November and another occurred on 20 November (figure 122). The estimated volume of the SW dome was 1.61 million cubic meters, with the summit dome at almost 2.93 million cubic meters.

Figure 122. Webcam image of an incandescent lava avalanche descending the SW flank at Merapi on 21 November 2021, which generates ash plumes. Courtesy of MAGMA Indonesia.

CVGHM reported a discrete ash plume that rose to 3.9 km altitude and drifted E on 1 December, but it was not detected in satellite imagery due to cloud cover. Two pyroclastic flows traveled 3 km down the SW flank on 1 December. According to a VONA issued by PVMBG, around 2104 on 1 December an ash plume rose 1 km above the summit and drifted E. The height of the SW lava dome decreased about 2 m during 26 November and 2 December, but the estimated volumes at both remained stable at 1.63 million cubic meters for the SW dome and just over 3 million cubic meters for the summit dome. Between 112-190 lava avalanches moved as far as 2 km SW down the Bebeng drainage. On 6 December a pyroclastic flow traveled 1.8 km down the Bebeng drainage on the SW flank at 1644, which lasted two minutes and forty seconds, based on seismic data, according to BPPTKG. At 1643 on 18 December a pyroclastic flow advanced 2 km SW, which produced an ash plume that rose 400 m high (figure 123). During 24-30 December the height of the SW lava dome had decreased by 3 m.

Figure 123. Webcam image of a pyroclastic flow at Merapi extending 2 km down the SW flank, which generated a 400-m-high ash plume on 18 December 2021. Courtesy of MAGMA Indonesia.

Observations during early January 2022 showed that the height of the SW lava dome had increased about 2 m between 31 December and 6 January 2022. Seismicity was recorded at higher levels with more intense earthquakes compared to the previous months. The estimated volume of the SW dome was over 1.67 million cubic meters and just over 3 million cubic meters at the summit dome. The weekly number of detected lava avalanches ranged between 30 and 123, which traveled a maximum of 2.2 km SW down the Bebeng drainage (figure 124). A total of seven pyroclastic flows were reported descending 2-2.5 km down the SW flank.

Figure 124. Webcam image of an incandescent lava avalanche moving down the SW flank at Merapi on 28 January 2022. Courtesy of MAGMA Indonesia.

In February, no significant morphological changes were detected at the lava domes, though seismicity remained relatively high. The estimated volumes of the SW and summit domes was 1.58 million cubic meters and 3.23 million cubic meters, respectively. In the Bebeng drainage, 105-173 weekly lava avalanches were reported, moving as far as 2 km SW, and one of which went 300 m NW. Nine pyroclastic flows extended 1.8-2.8 km. On 2 February the Darwin VAAC issued a notice describing resuspended ash that rose to 3.7 km altitude based on satellite data; no eruption was associated with this event. A ground report indicated that on 6 February ash plumes rose 600 m above the summit and drifted E, though it was likely resuspended. Ashfall was reported in multiple areas within about 20 km to the S, SE, and E, including the Cangkringan, Sleman and Musuk districts. On 18 February minor ashfall was reported in the Pakem District.

The MODIS Log Radiative Power graph produced by the MIROVA project showed that thermal activity was stronger and more frequent during September-October 2021 compared to December through February 2022 (figure 125). This pattern is also reflected in the MODVOLC hotspot detection program, which detected a total of 25 thermal anomalies over the days of 3, 10, 17, and 30 September, 5, 14, 21, 28, and 30 October, 29 November, 26 December 2021, 2 and 9 January, and 24 February 2022. This thermal activity was also detected in Sentinel-2 infrared satellite imagery representing incandescent lava and block avalanches dominantly moving down the SW flank (figure 126).

Figure 125. Frequent low- to moderate-power thermal anomalies at Merapi persisted during September 2021 through February 2022, according to the MIROVA graph (Log Radiative Power). The frequency of the anomalies visibly decreased during mid-October and there was a break in activity during January. The overall strength of the anomalies also declined from September through February. Courtesy of MIROVA.

Figure 126. Incandescent block avalanches and block-and-ash flows at Merapi produced significant thermal anomalies, mainly on the SW flank. Examples from Sentinel-2 infrared satellite imagery are shown here for 20 September (top left), 20 October (top right), 4 November (bottom left) 2021, and 27 February (bottom right) 2022. Sentinel-2 satellite images use Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Geologic Background. Merapi, one of Indonesia's most active volcanoes, lies in one of the world's most densely populated areas and dominates the landscape immediately north of the major city of Yogyakarta. It is the youngest and southernmost of a volcanic chain extending NNW to Ungaran volcano. Growth of Old Merapi during the Pleistocene ended with major edifice collapse perhaps about 2,000 years ago, leaving a large arcuate scarp cutting the eroded older Batulawang volcano. Subsequent growth of the steep-sided Young Merapi edifice, its upper part unvegetated due to frequent activity, began SW of the earlier collapse scarp. Pyroclastic flows and lahars accompanying growth and collapse of the steep-sided active summit lava dome have devastated cultivated lands on the western-to-southern flanks and caused many fatalities.

Information Contacts: Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG), Center for Research and Development of Geological Disaster Technology (URL: http://merapi.bgl.esdm.go.id/, Twitter: @BPPTKG); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); 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/); 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Saunders Island, located in the remote South Sandwich Volcanic Arc in the South Atlantic, consists of a large central edifice intersected by two seamount chains. The primary volcanic features include Mount Michael, which contains a 500-m-wide summit crater, the northern Blackstone Plain, and the Ashen Hills, which is a group of overlapping craters formed through phreatomagmatic activity. The current eruption period began during November 2014, characterized by elevated crater temperatures and gas-and-steam emissions (BGVN 46:02). Visits are infrequent because of the remote location and often views are obscured by cloudy weather. This report updates information during June 2020 through January 2022 primarily using satellite data.

Activity has been relatively low since June 2020; no thermal activity was detected in satellite data until November 2020, when it occurred intermittently until February 2021, according to MIROVA data (figure 43). Although weather clouds often obscured the views of the summit, gas-and-steam emissions drifting NE were possibly visible in satellite images on 24 October 2020. A clear view of the crater on 6 November showed a strong thermal anomaly on the crater floor in infrared satellite imagery (figure 44). According to Suomi NPP/VIIRs sensor data, thermal anomalies were detected on 3 and 8 November 2020 and 11 January 2021. On clear weather days during late 2020 into 2021, anomalies were strongly observed in the summit crater at Mount Michael, based on Sentinel-2 infrared satellite imagery (figure 44). Small gas-and-steam emissions were detected on 10 January 2021. On 9 February a grayish plume drifted W from the summit crater, according to natural color satellite imagery (figure 44).

Figure 43. Few thermal anomalies (red dots) were detected at Saunders during November 2020 through February 2021, October 2021, and February 2022 as recorded by the Sentinel-2 MODIS Thermal Volcanic Activity data (bands 12, 11, 8A). Courtesy of MIROVA.

Figure 44. Sentinel-2 infrared and natural color images on clear weather days showing a strong thermal anomaly (bright yellow-orange) within the Mount Michael summit crater at Saunders on 6 November (top left), 11 December (top middle) 2020, 10 January (top right), 19 February (bottom middle), and 7 October (bottom right) 2021. Some degassing was visible on 10 January. A grayish plume was detected drifting W on 9 February (bottom left). By 7 October, the strength of the anomaly had declined. Images using “Atmospheric penetration” rendering (bands 12, 11, 8a) and “Natural color” (bands 4, 3, 2) rendering. Courtesy of Sentinel Hub Playground.

Renewed anomalies were visible again during October 2021 in MIROVA data. Although on 7 October the strength of the anomaly had decreased visibly compared to the previous year (figure 44). Small sulfur dioxide emissions were visible on 24 October 2021, as detected by the TROPOMI instrument on the Sentinel-5P satellite (figure 45). Wispy sulfur dioxide emissions were more often detected during January 2022, which drifted generally N and SW.

Figure 45. Small sulfur dioxide plumes rising from Saunders were detected by the TROPOMI instrument on the Sentinel-5P satellite on 24 October (top left) 2021, 13 January (top right), 28 January (bottom left), and 29 January (bottom right) 2022 that drifted SE, N, SW, and NW, respectively. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. Saunders Island is a volcanic structure consisting of a large central edifice intersected by two seamount chains, as shown by bathymetric mapping (Leat et al., 2013). The young constructional Mount Michael stratovolcano dominates the glacier-covered island, while two submarine plateaus, Harpers Bank and Saunders Bank, extend north. The symmetrical Michael has a 500-m-wide summit crater and a remnant of a somma rim to the SE. Tephra layers visible in ice cliffs surrounding the island are evidence of recent eruptions. Ash clouds were reported from the summit crater in 1819, and an effusive eruption was inferred to have occurred from a N-flank fissure around the end of the 19th century and beginning of the 20th century. A low ice-free lava platform, Blackstone Plain, is located on the north coast, surrounding a group of former sea stacks. A cluster of parasitic cones on the SE flank, the Ashen Hills, appear to have been modified since 1820 (LeMasurier and Thomson, 1990). Analysis of satellite imagery available since 1989 (Gray et al., 2019; MODVOLC) suggests frequent eruptive activity (when weatehr conditions allow), volcanic clouds, steam plumes, and thermal anomalies indicative of a persistent, or at least frequently active, lava lake in the summit crater. Due to this observational bias, there has been a presumption when defining eruptive periods that activity has been ongoing unless there is no evidence for at least 10 months.

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/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard MD 20771, USA (URL: https://so2.gsfc.nasa.gov/); NASA Worldview (URL: https://worldview.earthdata.nasa.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).