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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Information Contacts: Sakhalin Volcanic Eruption Response Team (SVERT), Institute of Marine Geology and Geophysics, Far Eastern Branch, Russian Academy of Science, Nauki st., 1B, Yuzhno-Sakhalinsk, Russia, 693022 (URL: http://www.imgg.ru/en/, http://www.imgg.ru/ru/svert/reports); Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Planet Labs, Inc. (URL: https://www.planet.com/); Google Earth (URL: https://www.google.com/earth/).

Ol Doinyo Lengai is located near the southern end of the East African Rift in Tanzania. It is known for its unique low-temperature carbonatitic lava. Activity primarily occurs in the crater offset to the N about 100 m below the summit where hornitos (small cones) and pit craters produce lava flows and spattering. Eruptions have been recorded since the late 19th century; the current eruptive period began in April 2017 and has recently been characterized by small lava flows in the crater (BGVN 45:09). This report covers similar activity during September 2020 through February 2021 using information primarily from satellite data.

During September 2020 to February 2021 both thermal and natural color satellite imagery showed small lava flows in the summit crater. A total of six weak thermal anomalies were identified in MIROVA data during September (2), October (3), and November (1) 2020 (figure 211). No thermal anomalies were detected after late November, according to the MIROVA graph. Sentinel-2 satellite imagery showed small lava flows within the summit crater throughout the reporting period. On clear weather days, infrequent and faint thermal anomalies were observed in thermal satellite imagery within the crater; new lava flows were identified due to the change in shape, volume, and location of the thermal anomaly (figure 212). On 31 August a faint thermal anomaly was visible in the NW side of the summit crater. On 15 September fresh black lava was observed in the center of the summit crater spreading to the NW and E. Two small thermal anomalies were present on the W and E side of the crater on 20 September. On 24 December both thermal and Natural Color images showed the location of a lava flow as a thermal anomaly and as fresh lava in the center and W side of the crater. On 7 February a gas-and-steam plume was observed drifting E from the crater.

Figure 211. Intermittent low-level thermal anomalies were recorded at Ol Doinyo Lengai, based on the MIROVA thermal data graph (Log Radiative Power) during late August through late November 2020; a total of six weak thermal anomalies were detected between September through November 2020. The black lines are distant anomalies (more than 5 km from the summit) not related to volcanism. Courtesy of MIROVA.

Figure 212. Sentinel-2 thermal and natural color imagery of Ol Doinyo Lengai from 31 August 2020 to 7 February 2021. On clear weather days, thermal anomalies (bright yellow-orange) were faintly visible in the summit crater on 31 August (top left) on the NW side. On 15 September (top right) fresh black lava, which quickly cools to a whitish-brown color, was seen in the crater, reflecting the position of the anomalies visible in the thermal image. Two anomalies were visible on 20 September (middle left) on the W and E side. Two black dots which represent cooled lava and thermal anomalies on the W side of the crater were visible in both 24 December (bottom left) thermal and Natural Color images. A small lava flow was observed in the center of the crater on 7 February (bottom right) 2021. Images are marked with “Atmospheric penetration” rendering (bands 12, 11, 8A) and “Natural Color” rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

Geologic Background. The symmetrical Ol Doinyo Lengai is the only volcano known to have erupted carbonatite tephras and lavas in historical time. The prominent stratovolcano, known to the Maasai as "The Mountain of God," rises abruptly above the broad plain south of Lake Natron in the Gregory Rift Valley. The cone-building stage ended about 15,000 years ago and was followed by periodic ejection of natrocarbonatitic and nephelinite tephra during the Holocene. Historical eruptions have consisted of smaller tephra ejections and emission of numerous natrocarbonatitic lava flows on the floor of the summit crater and occasionally down the upper flanks. The depth and morphology of the northern crater have changed dramatically during the course of historical eruptions, ranging from steep crater walls about 200 m deep in the mid-20th century to shallow platforms mostly filling the crater. Long-term lava effusion in the summit crater beginning in 1983 had by the turn of the century mostly filled the northern crater; by late 1998 lava had begun overflowing the crater rim.

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).

Manam is located 13 km off the N coast of mainland Papua New Guinea and has had eruptions documented since 1616. It contains two active summit craters, Main and South, which have been characterized by occasional Strombolian activity, lava flows, pyroclastic avalanches, and ash plumes. The current eruption period has been ongoing since 2014 with more recent activity consisting of intermittent ash plumes, thermal anomalies, and sulfur dioxide emissions (BGVN 45:10). This report describes similar activity and covers October 2020 through March 2021 using information primarily from the Darwin Volcanic Ash Advisory Center (VAAC) and various satellite data.

Explosive and thermal activity was relatively low during this reporting period. Three ash plumes were reported by the Darwin VAAC based on imagery from the HIMIWARI-8 satellite. On 6 December 2020 an ash plume rose to 2.4 km altitude and drifted SW. The next VAAC notice was for ash detected on 23 January 2021 rising to 4.9 km and drifting SE and N. Then on 21 February an ash plume rose to a maximum altitude of 6 km and drifted W. Intermittent sulfur dioxide plumes were detected using the TROPOMI instrument on the Sentinel-5P satellite, some of which reached at least two Dobson Units (DU) and drifted in multiple directions (figure 79). On 6 December and 23 January, the ash plume that was described in the Darwin VAAC advisory was accompanied by an SO₂ plume. SO₂ plumes that reached a minimum of two DU were recorded for at least 12 days during October, 13 days during November, 15 days during December, 10 days during January, 3 days during February, and 6 days during March.

Figure 79. Distinct sulfur dioxide plumes rising from Manam and drifting in different directions were detected using data from the TROPOMI instrument on the Sentinel-5P satellite on 9 October (top left), 3 November (top right), 6 December (middle left) 2020, 13 January (middle right), 18 February (bottom left), and 18 March (bottom right) 2021. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

Thermal activity during October 2020 through March 2021 was relatively low in power and frequency compared to August and September, as recorded by the MIROVA (Middle InfraRed Observation of Volcanic Activity) system. Two brief pulses of activity were detected during mid-November and late December to mid-January (figure 80). A total of 14 low-power anomalies were recorded: one in early October, three in mid-November, two in December, a maximum number of six in January, one in late February, and one in late March. Some of this activity was captured in Sentinel-2 thermal satellite imagery on clear weather days in both the Main and South summit craters (figure 81).

Figure 80. Thermal activity at Manam was low to moderate in power during October 2020 through March 2021, with notable brief pulses during mid-November and late December through mid-January, as shown on this MIROVA Log Radiative Power graph. One anomaly was detected in early October, three in mid-November, two in December, six in January, one in late February, and one in late March. Courtesy of MIROVA.

Figure 81. Sentinel-2 thermal satellite images show a persistent thermal anomaly (bright yellow-orange) at both of Manam’s summit craters (Main and South) on clear weather days during November 2020 through March 2021. Occasional gas-and-steam emissions accompanied the thermal anomalies as seen on 25 November 2020 (top left), 29 January (top right), 8 February (bottom left), and 20 March (bottom right) 2021. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

Geologic Background. The 10-km-wide island of Manam, lying 13 km off the northern coast of mainland Papua New Guinea, is one of the country's most active volcanoes. Four large radial valleys extend from the unvegetated summit of the conical basaltic-andesitic stratovolcano to its lower flanks. These valleys channel lava flows and pyroclastic avalanches that have sometimes reached the coast. Five small satellitic centers are located near the island's shoreline on the northern, southern, and western sides. Two summit craters are present; both are active, although most observed eruptions have originated from the southern crater, concentrating eruptive products during much of the past century into the SE valley. Frequent eruptions, typically of mild-to-moderate scale, have been recorded since 1616. Occasional larger eruptions have produced pyroclastic flows and lava flows that reached flat-lying coastal areas and entered the sea, sometimes impacting populated areas.

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/); 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).

Dukono, located in northernmost Halmahera, Indonesia, has been erupting continuously since 1933. Volcanism has recently been characterized by frequent ash explosions, ash plumes, and sulfur dioxide plumes (BGVN 45:10). This report updates activity consisting of white-and-gray plumes and sulfur dioxide plumes during October 2020-March 2021 using information primarily from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), the Darwin Volcanic Ash Advisory Centre (VAAC), and satellite data.

Volcanism at Dukono has been characterized by dominantly white-and-gray plumes, accompanied by intermittent ash plumes that drifted in multiple directions. On clear weather days, the ash plumes rose to 1.5-2.4 km altitude, or about 270-1,200 m above the crater, according to PVMBG and the Darwin VAAC advisories (table 23).

Table 23. Monthly summary of reported ash plumes from Dukono during October 2020-March 2021. The direction of drift for the ash plumes was highly variable; notable plume drifts each month were only indicated in the table if at least two weekly reports were consistent. Data courtesy of PVMBG and the Darwin VAAC.

Activity during October 2020 primarily consisted of near daily white-and-gray plumes that rose 100-700 m above the crater and drifted in multiple directions (figure 19). Ash plumes during this month rose between 1.8 and 2.4 km altitude and drifted W, N, NE, E, SW, according to PVMBG VONA notices and the Darwin VAAC advisories. Frequent white gas-and-steam emissions were also observed in webcam images. Similar activity continued in November, with almost daily white-and-gray plumes rising 100-800 m above the crater and drifting in multiple directions. On clear weather days ash plumes were observed up to 2.1 km altitude; on 12 November the ash plume rose up to 2.1 km altitude and drifted SW (figure 19).

Figure 19. Webcam images of white-and-gray plumes rising from Dukono on 8 October (left) and an ash plume on 12 November (right) 2020. Courtesy of MAGMA Indonesia.

In December and January 2021, white-and-gray plumes were 100-700 m above the crater and drifted in multiple directions, dominantly E and W in December and SW in January. According to Darwin VAAC advisories during these two months, ash plumes were seen rising to 2.4 km altitude and drifted notably SE, E, and SW.

Activity in February persisted with white-and gray plumes rising 100-600 m above the crater and drifting dominantly SW and E (figure 20). Intermittent ash plumes rose to 2.1 km altitude during February and 2.4 km altitude during March, drifting in multiple directions. Gas-and-steam plumes were also frequent. During March, almost daily white-and-gray plumes rose 100-800 m above the crater and drifted in multiple directions (figure 20).

Figure 20. Webcam images of white-and-gray plumes rising from Dukono on 25 February (left) and 22 March (right) 2021. Courtesy of MAGMA Indonesia.

The NASA Global Sulfur Dioxide page, using data from the TROPOMI instrument on the Sentinel-5P satellite, showed strong SO₂ plumes rising from Dukono and drifting in various directions (figure 21). In addition to SO₂ plumes, Sentinel-2 thermal satellite imagery showed thermal anomalies of variable intensities on clear weather days (figure 22). Intermittent thermal anomalies recorded by the MIROVA (Middle InfraRed Observation of Volcanic Activity) system during early December 2020 through mid-March 2021 were low in power (figure 23). A brief break in thermal activity occurred during mid- to late-February.

Figure 21. Strong sulfur dioxide emissions rose from Dukono and drifted in multiple directions were detected using the TROPOMI instrument on the Sentinel-5P satellite. SO2 plumes drifted N on 10 October (top left), generally E on 28 November (top right), 13 December (middle left) 2020, and 11 February 2021 (bottom left), SE on 9 January (middle right) 2021, and W on 4 March (bottom right) 2021. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

Figure 22. Sentinel-2 thermal satellite imagery showing a thermal anomaly in the summit crater on 1 November (top left) 2020, 10 January (middle right), 1 March (bottom left), and 16 March (bottom right) 2021, frequently accompanied by gas-and-steam and ash plumes. On 11 November (top right) and 6 December (middle left) 2020 a Natural Color image showed a grayish white ash plume drifting SW and SE, respectively. Sentinel-2 satellite images with “Natural Color” rendering (bands 4, 3, 2) on 11 November and 6 December 2020, all other images use “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

Figure 23. MIROVA (Log Radiative Power) thermal data for Dukono from 3 June 2020 through March 2021 showed intermittent low power thermal activity during early December 2020 through mid-March 2020. A brief break in activity occurred during mid- to late-February. Courtesy of MIROVA.

Geologic Background. Reports from this remote volcano in northernmost Halmahera are rare, but Dukono has been one of Indonesia's most active volcanoes. More-or-less continuous explosive eruptions, sometimes accompanied by lava flows, occurred from 1933 until at least the mid-1990s, when routine observations were curtailed. During a major eruption in 1550, a lava flow filled in the strait between Halmahera and the north-flank cone of Gunung Mamuya. This complex volcano presents a broad, low profile with multiple summit peaks and overlapping craters. Malupang Wariang, 1 km SW of the summit crater complex, contains a 700 x 570 m crater that has also been active during historical time.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); 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).

Indonesia’s Sinabung volcano in north Sumatra had its first confirmed Holocene eruption during August and September 2010. It remained quiet until September 2013 when a new eruptive phase began that continued through mid-2018. Dome growth and destruction resulted in block avalanches, multiple explosions with ash plumes, and deadly pyroclastic flows during the period. After a pause in activity from September 2018 through April 2019, explosions resumed during May and June 2019. Dome growth began again with an explosion on 8 August 2020, and similar activity continued through October 2020. This report covers ongoing activity from November 2020 through February 2021 with information provided by Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), referred to by some agencies as CVGHM or the Indonesian Center of Volcanology and Geological Hazard Mitigation, and the Darwin Volcanic Ash Advisory Centre (VAAC). Additional information comes from satellite instruments and local news reports.

Activity at Sinabung during November 2020-February 2021 was characterized by tens of daily rock avalanches, periodic pyroclastic flows, and ash-bearing explosions. The rock avalanches traveled up to 1,000 m down the E and SE flanks. The pyroclastic flows also traveled down the E and SE flanks, and the largest reached 2.5 km from the summit. Periodic explosions produced ash plumes that rose up to 2 km above the summit and drifted in multiple directions. Although cloudy much of the time, intermittent satellite images showing two thermal anomalies at the summit suggested that the dome remained active (figure 85).

Figure 85. Two thermal anomalies were present at the summit of Sinabung several times during the report period from November 2020-February 2021, including on 2 December 2020 and 10 February 2021, suggesting ongoing dome activity. In addition, frequent pyroclastic flows produced incandescent anomalies on the E flank multiple times including on 10 February 2021. Sentinel-2 images use Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

White steam emissions rose 50-500 m above the summit of Sinabung during most days in November 2020. Block avalanches were frequent during the first half of the month, traveling 200-1,000 m down the S and SE flanks. The Darwin VAAC reported small ash plumes from block avalanches on 1 and 2 November that rose to 3 km altitude and quickly dissipated. Clouds prevented observations during the last week of the month, but tens of seismic events interpreted by PVMBG as block avalanches were detected. Pyroclastic flows were either observed visually or measured seismically on 2-7, 10, 12, 16, 18 and 19 November (figure 86). They most often occurred on the E or SE flanks and traveled 1,500-2,500 m. Seismic signals indicating lahars were recorded on 26, 27, and 30 November.

Figure 86. A pyroclastic flow descended the S flank of Sinabung on 7 November 2020. Courtesy of Rizal.

Nine explosions with ash plumes were reported during November 2020. On 2 November a gray ash plume rose 1,500 m above the summit, to about 3.9 km altitude, and drifted E. The next day the Darwin VAAC reported an explosion to 3.7 km altitude that drifted E. An ash explosion on 4 November was recorded seismically for 117 seconds but was not seen due to fog. An explosion on 10 November produced an ash plume that rose 2 km above the summit and drifted E, along with pyroclastic flows that traveled 1,500-2,500 m down the E and SE flanks. On 18 November an explosion created an ash plume that rose to 3.7 km altitude and drifted SW; it was measured seismically as a continuous volcanic tremor that lasted for 160 seconds. Seismic activity confirmed an explosion on 21 November, but meteoric clouds obscured observations of ash. An ash plume drifting SW at 3 km altitude, about 500 m above the summit, was reported on 25 November. On 29 November an explosion produced an ash plume to the same altitude that drifted E (figure 87). The next day seismic activity indicated another explosion, but it was not observed due to cloudy weather.

Figure 87. An ash plume at Sinabung rose to 3 km altitude and drifted E on 29 November 2020. Courtesy of PVMBG and MAGMA Indonesia.

Explosive activity decreased during December 2020. Steam plumes rose 50-500 m and tens of rock avalanches were recorded seismically every day. On 6 December block avalanches rolled 300-500 m down the E and SE flanks; they traveled 500-1,000 m down the SE flank on 8 December. During 12-14 December they traveled 1,000-1,500 m down the E and S flanks. On 30 and 31 December they were seen moving 500-1,000 m down the same flanks. Lahars were measured seismically on 4 and 5 December with no reports of damage.

An explosion on 2 December produced an ash plume that rose about 500 m above the summit and drifted ESE. Clouds and rain prevented views of the summit on 5 December, but the seismogram recorded an explosive event that lasted for 168 seconds (figure 88). The Darwin VAAC reported an ash plume moving ESE at 3 km altitude on 13 December. Sentinel-2 satellite imagery captured a thermal anomaly on the E flank on 17 December that was likely from a pyroclastic flow (figure 89). Two explosions were recorded each day on 28 and 29 December. On the first day the ash plume from the first explosion rose to 500 m and drifted S. The second explosion was not observed due to weather, but a thermal anomaly was intermittently visible. The explosions on 29 December were only recorded seismically, as was one explosion on 30 December.

Figure 88. The KESDM seismogram at Sinabung recorded an explosive event on 5 December 2020 that lasted for 168 seconds. Courtesy of PVMBG and MAGMA Indonesia.

Figure 89. A thermal anomaly on the E flank of Sinabung on 17 December 2020 was likely from a pyroclastic flow. The summit is obscured by clouds. Sentinel-2 image with Atmospheric penetration rendering (bands 12, 11, and 8a). Courtesy of Sentinel Hub Playground.

Tens of daily rock avalanches continued to be recorded during January 2021, although most were not observed. During 2-5 January they traveled 500-1,200 m down the E and SE flanks, and on 14 January they fell 700-1,000 m down the SE flank. The number of explosions with ash plumes increased significantly from December. On 3 January two explosions were recorded seismically; an ash plume from the first rose 1,000 m above the summit and drifted NW in the morning. A few hours later a second explosion was recorded but not observed due to clouds. Three explosions were recorded each day on 4 and 5 January. The first on 4 January produced a 700-m-high ash plume, the second and third sent ash 1,000 m above the summit to the W and NW (figure 90). The next day, the first explosion sent an ash plume 800 m above the summit that drifted E and SE; the other two were recorded seismically but not observed due to weather. One or two explosions were recorded daily during 6-10 January; most were obscured by clouds. One of the explosions on 8 January produced an ash plume that rose to 700 m and drifted N, and the explosion on 9 January rose to 1,000 m and drifted N and NE. Two explosions were recorded on 12 January, and two or three explosions were reported daily during 16-18 January. Explosions were also recorded on 20-21, 23, 25-27, and 29 January. The three ash plumes on 17 January all rose 500 m above the summit and drifted E, NE, or SE; the plumes on 21 and 27 January rose 500 m and drifted E and SE.

Figure 90. An explosion at Sinabung on 4 January 2021 produced an ash emission that rose 1,000 m above the summit and drifted W and NW. Courtesy of PVMBG and MAGMA Indonesia.

Steam emissions rose 50-700 m above the summit throughout February 2021. Over 100 seismic events from rock avalanches were reported daily; on 6 February a maximum of 231 events were recorded. Numerous explosions, many with pyroclastic flows, were only detected seismically on 5-12, 14, 17, 22, 25, and 28 February. On 6 February the Darwin VAAC reported a continuous ash eruption identified in satellite imagery at 3.1 km altitude drifting NW. PVMBG also reported a pyroclastic flow that traveled 2,500 m down the S flank that day. The Antara News Agency reported an ash plume rising 1,000 m above the summit from a pyroclastic flow and drifting E, SE, and S on 7 February, and another pyroclastic flow on 9 February that traveled 1,000 m down the SE flank (figure 91). Cloudy weather obscured views on most days, but during 12-14 February blocks traveled 500-1,500 m down the S, SE, and E flanks.

Figure 91. A pyroclastic flow traveled 1,000 m down the SE flank of Sinabung on 9 February 2021. Courtesy of Anadolu Agency.

The Darwin VAAC received a report on 10 February of an ash plume at 4.6 km altitude moving E; it was not identifiable in satellite imagery due to meteoric clouds. Two pyroclastic flows on 12 February moved as far as 2,000 m down the E and SE flanks. On 17 February an ash plume rose 1,000 m above the summit and drifted S and W and a pyroclastic flow was reported. A lahar was reported on 21 February. A pyroclastic flow on 22 February traveled 2,000 m down the E and SE flanks. The ash plume from the 25 February event rose to 1,500 m above the summit to about 3.9 km altitude and drifted E and SE (figure 92) and was accompanied by four pyroclastic flows that traveled 500-1,000 m down the E and SE flanks. A discrete ash plume was reported by the Darwin VAAC on 28 February that rose to 3.1 km altitude and drifted SW, dissipating withing six hours. Pyroclastic flow were observed that day moving 1,000-1,250 m down the S, SE, and E flanks.

Figure 92. The ash plume at Sinabung from a 25 February 2021 explosion rose to 1,500 m above the summit and drifted E and SE. Courtesy of PVMBG and MAGMA Indonesia.

Geologic Background. Gunung Sinabung is a Pleistocene-to-Holocene stratovolcano with many lava flows on its flanks. The migration of summit vents along a N-S line gives the summit crater complex an elongated form. The youngest crater of this conical andesitic-to-dacitic edifice is at the southern end of the four overlapping summit craters. The youngest deposit is a SE-flank pyroclastic flow 14C dated by Hendrasto et al. (2012) at 740-880 CE. An unconfirmed eruption was noted in 1881, and solfataric activity was seen at the summit and upper flanks in 1912. No confirmed historical eruptions were recorded prior to explosive eruptions during August-September 2010 that produced ash plumes to 5 km above the summit.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Rizal (URL: https://twitter.com/Rizal06691023/status/1324972883634917376); Antara News Agency (URL: https://www.antaranews.com/berita/1986704/guguran-abu-gunung-sinabung-teramati-setinggi-1000-meter); Anadolu Agency (URL: https://www.aa.com.tr/ba/svijet/indonezija-u-vulkanu-sinabung-odjeknula-eksplozija/2138389).

Barren Island, an uninhabited possession of India in the Andaman Sea, had numerous historical eruptions observed during 1787-1832. No further evidence of activity was found until 1991 when ash plumes, Strombolian explosions, and lava flows that reached the ocean were observed. Intermittent similar eruptions since 2005 have lasted for months to years. Its remoteness makes ground observations rare, but satellite data and reports from the Darwin VAAC (Volcanic Ash Advisory Center) suggest that the most recent eruption which began in September 2018 with lava fountaining, lava flows, and ash emissions has continued with intermittent thermal anomalies at the summit and minor ash emissions since early 2019. This report covers activity from July 2020-February 2021.

The MIROVA thermal anomaly data from April 2020 through February 2021 indicate low levels of thermal activity from April through October 2020. Pulses of activity in early November and late January-early February 2021 correspond to increased thermal activity seen in satellite images during that time (figure 47). Ash emissions were reported by the Darwin VAAC in early November and early December 2020. A strong thermal anomaly was present in satellite imagery on 11 November, and moderate anomalies appeared during February 2021. In addition, during November-February faint thermal anomalies and/or small ash emissions were present in one or more satellite images each month.

Figure 47. The MIROVA thermal anomaly data from April 2020 through February 2021 indicate low levels of thermal activity from April through October 2020. Pulses of activity in early November and late January-early February 2021 corresponded to increased thermal activity seen in satellite images. Courtesy of MIROVA.

After a small ash plume was observed on 24 June 2020 in Sentinel-2 satellite imagery (BGVN 45:08), the only evidence of further activity was a very weak thermal anomaly present inside the summit crater of the pyroclastic cone on 19 July 2020. Satellite images were mostly cloudy during August-October 2020, although the few clear images each month showed no sign of thermal anomalies or ash emissions. Single MODVOLC thermal alerts were issued for Barren Island on 2 and 4 November 2020. The Darwin VAAC reported continuous ash emissions drifting SW at 1.5 km altitude on 5 November. A very faint thermal anomaly was present inside the summit of the pyroclastic cone the next day. A large thermal anomaly and small ash plume were captured in satellite images on 11 November (figure 48). The bright anomaly at the center of the cone was surrounded by a weaker anomaly suggesting incandescent ejecta on the flanks of the cone. A smaller thermal anomaly and similar ash plume were visible in the 16 November 2020 Sentinel-2 satellite images (figure 49).

Figure 48. A large thermal anomaly and small ash plume at Barren Island were captured in Sentinel-2 satellite images on 11 November 2020. In the left image the bright anomaly at the center of the cone was surrounded by a weaker anomaly suggesting incandescent ejecta on the flanks of the cone. Image uses Atmospheric penetration rendering (bands 12, 11, 8a). The ash emission immediately W of the summit crater is more visible in the Natural color rendering (right, bands 4,3,2). Courtesy of Sentinel Hub Playground.

Figure 49. A thermal anomaly at the summit and a discrete ash emission slightly W of the summit of Barren Island were captured in Sentinel-2 satellite imagery on 16 November 2020. Left image uses Atmospheric penetration rendering (bands 12, 11, 8a) and right image shows a closeup of the summit and ash plume in Natural color rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

The Darwin VAAC issued an ash advisory on 8 December 2020 of an ash plume drifting W at 1.8 km altitude. It was only visible in satellite imagery for about two hours before dissipating. A small thermal anomaly appeared at the summit on 21 December. During January 2021 faint thermal anomalies were visible on 5, 20, and 25 January, and ash plumes could be seen on 15 and 25 January in Sentinel-2 images (figure 50). The strength of the thermal activity increased during February 2021, with satellite evidence recorded on 4, 9, 19, and 24 February; an ash emission was visible on 9 February (figure 51).

Figure 50. Ash plumes and thermal anomalies at Barren Island were present in Sentinel-2 satellite images several times during January 2021. The left image from 15 January shows an ash plume drifting W from the summit using Natural color rendering (bands 4, 3, 2). The right image shows a weak thermal anomaly at the summit on 25 January with an ash plume drifting S using Atmospheric penetration rendering (bands 12, 11, and 8A). Courtesy of Sentinel Hub Playground.

Figure 51. Sentinel-2 satellite images showed thermal anomalies at Barren Island several times during February 2020 including on 4 (left) and 9 (right) February. An ash emission drifted S from the summit on 9 February. Images use Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Geologic Background. Barren Island, a possession of India in the Andaman Sea about 135 km NE of Port Blair in the Andaman Islands, is the only historically active volcano along the N-S volcanic arc extending between Sumatra and Burma (Myanmar). It is the emergent summit of a volcano that rises from a depth of about 2250 m. The small, uninhabited 3-km-wide island contains a roughly 2-km-wide caldera with walls 250-350 m high. The caldera, which is open to the sea on the west, was created during a major explosive eruption in the late Pleistocene that produced pyroclastic-flow and -surge deposits. Historical eruptions have changed the morphology of the pyroclastic cone in the center of the caldera, and lava flows that fill much of the caldera floor have reached the sea along the western coast.

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

Merapi volcano in central Java, Indonesia, has a lengthy history of major eruptive episodes. Activity has included lava flows, pyroclastic flows, lahars, Plinian explosions with heavy ashfall, incandescent block avalanches, block-and-ash flows, and dome growth and destruction. Fatalities from these events were reported in 1994, 2006, and in 2010 when hundreds of thousands of people were evacuated. Renewed phreatic explosions in May 2018 cancelled airline fights and generated significant SO2 plumes. A new lava dome appeared in early August 2018; gradual dome growth and then destruction was accompanied by rockfalls, block-and-ash flows, periodic explosions, and pyroclastic flows through June 2020. The period from October 2020 through February 2021 is covered in this report and includes the growth of two new domes in early 2021. Information is provided primarily by Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG), the Center for Research and Development of Geological Disaster Technology, a branch of PVMBG, which monitors activity specifically at Merapi, the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), and the Darwin Volcanic Ash Advisory Centre (VAAC).

Measurements in late July 2020 showed no change in the dome (BGVN 45:10), though satellite evidence for weak thermal activity near the NW crater rim persisted during August-October 2020 (figure 98). A significant increase in the deformation rate and the appearance of numerous rock avalanches at the end of October led PVMBG to raise the Alert Level from II to III and evacuate hundreds of local residents. During November and December 2020 the deformation rate continued to increase and numerous rock avalanches were reported. Incandescent block avalanches were first reported on 4 January 2021. Block-and-ash flows began on 7 January and increased in frequency throughout the month; a new dome was confirmed that day. The deformation rate decreased significantly as the dome grew in size during January. Hundreds of incandescent block avalanches were recorded through the end of the month. A large explosion on 27 January produced a 12.2-km-high ash plume and a large pyroclastic flow; ashfall was reported in numerous communities. Incandescent block avalanches and block-and-ash flows continued frequently during February 2021; a second dome was reported growing near the center of the summit crater on 17 February.

Figure 98. A very small thermal anomaly was recorded in Sentinel-2 satellite data near the NW crater rim at the summit of Merapi during August-October 2020, along with gas emissions. Images are from 21 August 2020 (top left), 15 September 2020 (top right), 20 October 2020 (bottom left), and 13 January 2021 (bottom right). The January anomaly was much larger, noticeable even through cloud cover, six days after PBBTKG scientists confirmed the presence of a new dome growing near the SW crater rim. Courtesy of Sentinel Hub Playground.

The deformation rate at the summit, shortening determined by Electronic Distance Measurements (EDM) interpreted by PBBTK as inflation related to magma moving towards the surface, remained between 1-2 cm per week during August through early -October with just steam-and-gas plumes rising 150-250 m. During the week of 9-15 October PBBTKG reported a deformation rate of 1 cm/day. Drone photographs confirmed no change in the size or shape of the dome on 18 October 2020. The shortening rate increased to 2 cm/day during 16-22 October and the steam-and-gas plumes rose up to 500 m above the summit; the shortening rate increased to 4 cm/day during 23-29 October. PVMBG reported on 28 October that rock avalanches were heard twice in Babadan and Jrakah over the previous 24 hours, but fog prevented observations.

PVMBG raised the Alert Level from II to III on 5 November 2020 based on an increase in both seismicity and the deformation rate. Rock avalanches were heard that day from Babadan. Analyses of the crater area based on photographs from 30 October and 3 November did not show any morphological changes at the dome. The shortening rate, however, increased to 9-10 cm/day during the first three weeks of the month. Rock avalanches were observed on 8 November on the W flank moving as far as 3 km downslope and moving 2 km on 14 November. Photos comparing the SE flank on 11 and 19 November showed that part of the 2018 lava dome had collapsed. Drone images on 16 November also showed a collapse of part of the crater wall. On 22 November rock avalanches from the crater rim moved 1 km down the W flank. Steam and gas emissions were observed from the Babadan Observation Post rising 200-750 m above the summit during the second half of November (figure 99. A photo analysis on 26 November indicated that part of the 1954 lava dome had collapsed since 19 November. The deformation rate had increased to 11 cm/day by the last week of the month. During overflights on 26 and 27 November BNPB and BPPTKG observers noted many new avalanche deposits on the NW, W, and SW flanks. As of 27 November, there were 2,318 people who had been evacuated from the area around the volcano.

Figure 99. Steam and gas emissions at Merapi were observed from the Babadan Observation Post rising 200-750 m above the summit during the second half of November, including on 25 November 2020 shown here. Courtesy of MAGMA Indonesia Volcano Photo Gallery.

Steam and gas plumes rose 150-400 m above the summit throughout December 2020. Rock avalanches were heard but not seen due to foggy weather during the first few days of the month. On 8 December they were seen falling 200 m upstream of Kali Lamat on the W flank and on 14 December they were observed moving downslope 1.5 km on the NW flank upstream of the Senowo River. Rock avalanches were also observed on 23 December moving 1.5 km down the W flank above Kali Sat ravine and on 31 December moving the same distance above the Senowo River. The deformation rate remained high during December, ranging from 9-11 cm/day through 24 December; it rose to 14 cm/day during the last week. Minor changes were seen in photographs of the summit area, but drone data on 5 and 14 December showed no new lava dome. No lava dome was visible in a clear view of the upper part of the SW flank on 20 December (figure 100); the head of BPPTKG-PVMBG noted that the first observed incandescence in that area was on 31 December.

Figure 100. No lava dome was visible in a clear view of the upper part of the SW flank of Merapi on 20 December 2020, although rock avalanches had occurred a number of times during the month; the head of BPPTKG-PVMBG noted that the first observed incandescence in that area was on 31 December. Courtesy of BPPTKG and MAGMA Indonesia Volcano Photo Gallery.

The deformation rate remained very high at 15 cm/day during the first week of January 2021. Rock avalanches were observed on 1 and 3 January that moved 1.5 km from the summit towards Kali Lamat and Kali Senowo on the W and NW flanks. On 4 January incandescent material was observed with a thermal webcam, and rock avalanches were heard at the Babadan Observation Post (figure 101). Incandescent block avalanches were observed 19 times during 4-7 January, traveling 800 m to the upper reaches of Kali Krasak (figure 102). Four block-and-ash flows occurred on 7 January, moving less than 1 km downslope. Comparison of images between 24 December and 7 January revealed a new lava dome. Hanik Humaida, the head of BPPTKG-PVMBG concluded that incandescent lava had appeared at the bottom of the 1997 dome and noted that incandescence had first been observed late on 31 December. PVMBG issued VONAs on 7 and 9 January reporting block-and-ash flows that produced ash plumes which rose to 3.2 km altitude and drifted SW and NW.

Figure 101. Incandescence from the growth of a new dome at Merapi on the SW flank appeared in a thermal webcam image on 4 January 20201. Courtesy of BPPTKG (Terjadi Peningkatan Aktivitas Vulkanik, Teramati Guguran Lava Pijar di Gunung Merapi, 5 January 2021).

Figure 102. Numerous incandescent blocks fell down the SW flank of Merapi from the new lava dome, seen here on 6 January 2021. Courtesy of BPPTKG and MAGMA Indonesia Volcano Photo Gallery.

Incandescent block avalanches were observed 128 times during the second week of January moving as far as 900 m down the SW flank to the upper reaches of Kali Krasak. Two block-and-ash flows were also reported. On 14 January 2021, the measured volume of the new dome was 46,766 m³ with a growth rate of about 8,500 m³/day. Deformation decreased significantly to a shortening rate of 6 cm/day during the second week of the month. Incandescent avalanches continued at a high rate and were reported 282 times during the third week of January (figure 103); they traveled as far as 1,000 m to the upper reaches of the Kali Krasak and Kali Boyong. Block-and-ash flows were recorded 19 times during 15-21 January moving 1,800 m downslope to the SW (figure 104). Compared to the previous week, as measured on 21 January, the new dome had more than doubled in size to 104,000 m³ with an average growth rate of 8,600 m³/day.

Figure 103. There were 20 incandescent block avalanches that fell up to 1,000 m down the SW flank of Merapi from the new dome on 16 January 2021. Courtesy of BPPTKG.

Figure 104. PVMBG reported a block-and-ash flow (referred to as Awan Panas Guguran or APG) at Merapi that traveled approximately 1,000 m down the SW flank towards Kali Krasak on 18 January 2021. Courtesy of BPPTKG and BNPB (Gunung Merapi Kembali Keluarkan Awan Panas Guguran Sejauh 1.000 Meter, 18 January 2021).

The deformation rate decreased further to less than 1 cm/day by the end of the third week of January. A substantial block-and-ash flow on 19 January that moved 1,800 m down the Krasak and Boyong rivers produced a 500-m-high ash plume that drifted E. According to detikNews, ash fell on 19 January in several villages in Musuk and Tamansari Districts in the Boyolali Regency, and in the Kemalang District in the Klaten Regency (figure 105). The Darwin VAAC reported ash visible in the webcam on 20 and 26 January that drifted downwind close to the summit. Over 200 incandescent block avalanches were observed during the last week of January; the maximum distance traveled was 1,500 m down the SW flank. Block-and-ash flow activity increased significantly during 25-27 January with four flows on 25 January and 13 flows on 26 January which produced ash plumes that rose 300-400 m above the summit and traveled 600-1,500 m down the SW flank. PVMBG reported 31 block-and-ash flows on 27 January that traveled as far as 3 km down the SW flank (figure 106).

Figure 105. Ash from Merapi covered plants in Tegalmulyo Village, in the Klaten Regency on 19 January 2021. Photo by Achmad Syauqi, courtesy of detik.com.

Figure 106. A block-and-ash flow at Merapi with it’s associated ash plume seen here on 27 January 2021 was one of 36 such events reported by BPPTKG that day; they traveled up to 3 km from the summit down the SW flank. Courtesy of BNPB (Gunung Merapi Erupsi Besar, Begini Penjelasan BPPTKG, 27 January 2021).

The volume of the 2021 lava dome on 25 January 2021 was 157,000 m³, but by 28 January it was only 62,000 m³ as a result of block-and-ash flows, explosions, and pyroclastic flows that occurred on 26-27 January. An explosion on 27 January was reported by the Darwin VAAC, based on multiple ground reports of a significant eruption, although meteoric clouds obscured most ground observations. The ash plume rose to 12.2 km altitude, drifted NW, and was visible in satellite images. Ash emissions from a superheated pyroclastic flow rose to 6.1 km altitude and drifted NE (figure 107). Satellite imagery and pilot reports indicated that the 12.2 km ash plume dissipated after about five hours, while the plumes generated by the pyroclastic flow continued moving E at 3.7 km altitude for several more hours. Sand-sized ash was reported in several villages in the Tamansari District in Boyolali Regency on the E flank including the Dukuh Beling area, Sudimoro (Sangup Village), Lanjaran Village, Mriyan and in Boyolali City, Central Java on 27 January. Dense ash was also reported in Tegalmulyo Village; Sruni Village and Cluntang in the Musuk District also reported ashfall.

Figure 107. A significant explosion at Merapi on 27 January 2021 produced an ash plume to 12.2 km altitude that drifted NW and a pyroclastic flow that sent ash to 6.1 km altitude and drifted NE. The pyroclastic flow is seen here from Ngrangkah, Umbulharjo, Cangkringan, Sleman Regency. Photo by Jauh Hari Wawan S, courtesy of detik.com.

Multiple incandescent rock avalanches were observed during the first week of February 2021. They traveled 500-1,200 m down the SW flank. On 4 February the volume of the 2021 lava dome on the SW flank was measured at 117,400 m³; the growth rate since 28 January was 12,600 m³/day. On 8 February, 23 incandescent block avalanches were reported that traveled as far as 1,500 m from the summit down the SW flank upstream of Kali Krasak and Kali Boyong. Six incandescent avalanches were reported on 9 February; webcams indicated multiple daily incandescent block avalanches for the rest of the month. When measured on 11 February, the dome had grown significantly to 295,000 m³ at a growth rate of 48,900 m³/day (figure 108).

Figure 108. The 2021 lava dome at Merapi was located at the head of the SW flank, and was almost 300,000 m3 in size on 11 February, two days before this image taken on 13 February 2021. Courtesy of PVMBG and Rizal.

A drone observation on 17 February noted two lava domes at the summit. The first (the 2021 lava dome) was located on the SW flank and was attached to the 1997 lava dome, and a second new dome had appeared more in the center of the summit crater. Based on calculations from aerial photographs, the dome on the SW flank was 258 m long, 133 m wide, and 30 m high, with a volume of 397,500 m³ and growth rate of 25,200 m³/day. The lava dome in the center of the summit crater was 160 m long, 120 m wide, and 50 m high, with a volume of 426,000 m³ and an average growth rate of 10,000 m³/day. Deformation data showed no changes during February. During 24-27 February one or two block-and-ash flows occurred each day, the largest travelled 1,900 m SW (figure 109). The block-and-ash flow on 25 February 2021 at 1652 local time (WIB) produced traces of ashfall in Kali Tengah Lor, Kali Tengah Kidul, Deles, and Tlukan. The volume of the lava dome on the SW flank on 25 February was 618,700 m³ with a growth rate of 13,600 m³/day.

Figure 109. A block-and-ash flow at Merapi on 27 February 2021 descended hundreds of meters down the SW flank and sent ash drifting E mostly below the level of the summit. Courtesy of BPPTKG and MAGMA Indonesia Volcano Photo Gallery.

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: 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/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1, https://magma.esdm.go.id/v1/gunung-api/gallery); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG), Center for Research and Development of Geological Disaster Technology (URL: http://merapi.bgl.esdm.go.id/, https://twitter.com/BPPTKG/status/1350508928740675584); Badan Nasional Penanggulangan Bencana (BNPB), National Disaster Management Agency, Graha BNPB - Jl. Scout Kav.38, East Jakarta 13120, Indonesia (URL: http://www.bnpb.go.id/); 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/); Detik news (URL: https://news.detik.com/, https://news.detik.com/berita-jawa-tengah/d-5339832/hujan-abu-gunung-merapi-jangkau-desa-di-wilayah-krb-ii-klaten, https://news.detik.com/berita-jawa-tengah/d-5350542/gunung-merapi-erupsi-sirene-bahaya-meraung-warga-turun-ke-tempat-aman, https://news.detik.com/berita-jawa-tengah/d-5350625/gunung-merapi-erupsi-besar-boyolali-diguyur-hujan-abu-campur-pasir?_ga=2.230047007.2076450499.1612195171-14950811.1611700211); Rizal (URL: https://twitter.com/Rizal06691023/status/1360488059649757191).

Yasur, located at the SE tip of Tanna Island, contains a 400-m-wide summit crater within the small Yenkahe caldera. Its current eruption has been ongoing since at least 1774 and has consisted of Strombolian and Vulcanian activity. More recently, Strombolian activity and gas-and-ash explosions have been reported (BGVN 45:03 and 45:09). This report covers activity from September 2020 through February 2021 that is characterized by ongoing explosions, gas-and-ash emissions, SO₂ plumes, and thermal anomalies. Information primarily comes from monthly bulletins of the Vanuatu Meteorology and Geo-Hazards Department (VMGD) and various satellite data.

VMGD reported that ongoing explosions and gas-and-ash emissions continued at an elevated level throughout this reporting period, based on ground observations and seismic data. On clear weather days these emissions were captured by Sentinel-2 satellite imagery (figure 75). Some of the more intense explosions may result in larger ejecta falling in or around the summit crater. On 18 January 2021 a webcam image captured a gas-and-ash emission rising above the crater rim at 1500 (figure 76).

Figure 75. Sentinel-2 satellite images showing gas-and-ash emissions rising from the summit crater of Yasur on clear weather days. Ash is visible during 17 October (left) and 21 December 2020 (middle), while white gas-and-steam emissions are observed on 14 February 2021 (right). Sentinel-2 satellite images with “Natural Color” (bands 4, 3, 2) rendering. Courtesy of Sentinel Hub Playground.

Figure 76. Webcam photo of a gas-and-ash emission rising from Yasur on 18 January 2021 taken at 1500. Courtesy of VMGD.

Sulfur dioxide emissions were measured using the Sentinel-5P/TROPOMI satellite instrument for multiple days each month from September through February 2021 (figure 77). The density and drift direction of these SO₂ plumes varied. During 17-19 January relatively dense SO₂ plumes were detected consecutively, and drifted SE (figure 78).

Figure 77. Occasional SO2 plumes of varying densities were observed from Yasur during each month of September 2020 through February 2021. Plumes drifted generally W on 28 September (top left), 29 October (top right), 6 December (middle right), 25 December 2020 (bottom left), slightly N on 14 November (middle left), and SW on 19 February 2021 (bottom right). Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Figure 78. Relatively high-density SO2 plumes from Yasur during 17 (left), 18 (middle), and 19 (right) January 2021 were observed consecutively using the TROPOMI imaging spectrometer on the Sentinel-5P satellite. The plumes drifted SE on each of the days. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Intermittent thermal anomalies recorded by the MIROVA (Middle InfraRed Observation of Volcanic Activity) system during September 2020 through February 2021 were low to moderate in power (figure 79). Brief noticeable break in activity occurred during early December 2020 and for much of January 2021. The MODVOLC thermal alert data recorded 41 thermal signatures primarily within the summit crater over a total of 25 different days during September 2020-February 2021. Some of these thermal anomalies were also captured in Sentinel-2 thermal satellite imagery; thermal anomalies were visible in the N and S vents in the summit crater (figure 80).

Figure 79. MIROVA (Log Radiative Power) thermal data for Yasur from 26 May 2020 through February 2021 showed persistent low to moderate thermal activity. A brief but noticeable break in activity occurred during early December, early January, and late January. Courtesy of MIROVA.

Figure 80. Sentinel-2 thermal satellite images showing strong thermal anomalies (yellow-orange) in the N and S vents of the summit crater at Yasur each month from September 2020 through February 2021. During 22 September (top left), 17 October (top right), and 26 November (middle left), the two thermal anomalies in the crater were roughly the same intensity. On 21 December (middle right) the anomaly was accompanied by a small, gray ash plume. On 15 January (bottom left) and 24 February (bottom right) the intensity of the anomaly in the N vent and then the S vent had decreased slightly. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

Geologic Background. Yasur, the best-known and most frequently visited of the Vanuatu volcanoes, has been in more-or-less continuous Strombolian and Vulcanian activity since Captain Cook observed ash eruptions in 1774. This style of activity may have continued for the past 800 years. Located at the SE tip of Tanna Island, this mostly unvegetated pyroclastic cone has a nearly circular, 400-m-wide summit crater. The active cone is largely contained within the small Yenkahe caldera, and is the youngest of a group of Holocene volcanic centers constructed over the down-dropped NE flank of the Pleistocene Tukosmeru volcano. The Yenkahe horst is located within the Siwi ring fracture, a 4-km-wide, horseshoe-shaped caldera associated with eruption of the andesitic Siwi pyroclastic sequence. Active tectonism along the Yenkahe horst accompanying eruptions has raised Port Resolution harbor more than 20 m during the past century.

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/); 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).

Recent activity at Rincón de la Vieja has been dominated by frequent weak phreatic explosions, with an occasional ash plume, along with gas-and-steam emissions. Sporadic lahars have also been recently reported (BGVN 45:10). The volcano has a hot, churning, acid lake in its main crater. The current report describes activity during October 2020-February 2021, a continuation of the most recent eruptive period that began in January 2020. The primary information source for this report is weekly bulletin from the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA).

According to OVSICORI-UNA, small but frequent hydrothermal explosions continued in October through mid-December 2020, although less energetic than during previous months (figure 34). During the first half of October there were 1-2 daily small explosions. Plumes often rose 500-800 m above the crater rim, but on 1 and 6 October they rose 1 km. Then the number briefly increased to 5-7 small daily explosions before decreasing during the latter part of October; one explosion seen in webcam images on 24 October sent a plume to 1 km above the crater (figure 35).

Figure 34. Graph showing the number of daily eruptions at Rincón de la Vieja during 2020. Following frequent phreatic explosions during April-June, weak intermittent explosions were detected again starting in late July and continuing through December 2020. Courtesy of OVSICORI-UNA.

Figure 35. Webcam photo of Rincón de la Vieja taken on 24 October 2020 at 0808 local time. According to OVSICORI-UNA, the explosion lasted for about a minute and the resulting plume rose to 1 km above the crater. Courtesy of OVSICORI-UNA, as reported by The Nacion.

OVSICORI-UNA reported that in November small-to-moderate hydrothermal explosions increased in amplitude, but became more sporadic and by the end of the month had decreased to only one per day. An explosion at 0835 on 3 November produced a plume that rose 800 m above the crater rim. According to OVSICORI’s weekly bulletin for 23 November, there had been 1,437 explosions since the beginning of 2020. A large explosion on 13 December was the last through at least February 2021. During the week of 18 January OVSICORI changed the Alert Level from 3 to 2 due to the low level of activity.

Geodesic monitoring at the summit by GPS indicated no deformation trend in October, significant contraction in November, some extension in December, but then no significant changes through at least February 2021. Aerial observations on 13 February indicated that the crater lake was at a low water level and had sustained convection. The lake level had dropped 15-20 m since February 2020, and 5-10 m since May 2020. Gas monitoring during October 2020-February 2021 was carried out at the Ojo de Agua Santuarium (4 km N of the active crater); sulfur dioxide in the plume was not detected in significant quantities.

Geologic Background. Rincón de la Vieja, the largest volcano in NW Costa Rica, is a remote volcanic complex in the Guanacaste Range. The volcano consists of an elongated, arcuate NW-SE-trending ridge constructed within the 15-km-wide early Pleistocene Guachipelín caldera, whose rim is exposed on the south side. Sometimes known as the "Colossus of Guanacaste," it has an estimated volume of 130 km3 and contains at least nine major eruptive centers. Activity has migrated to the SE, where the youngest-looking craters are located. The twin cone of Santa María volcano, the highest peak of the complex, is located at the eastern end of a smaller, 5-km-wide caldera and has a 500-m-wide crater. A Plinian eruption producing the 0.25 km3 Río Blanca tephra about 3,500 years ago was the last major magmatic eruption. All subsequent eruptions, including numerous historical eruptions possibly dating back to the 16th century, have been from the prominent active crater containing a 500-m-wide acid lake located ENE of Von Seebach crater.

Information Contacts: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/, https://www.facebook.com/OVSICORI/); The Nacion (URL: https://www.nacion.com/).

Kilauea, which overlaps the E flank of the Mauna Loa shield volcano, is the southeastern-most volcano in Hawaii. It’s East Rift Zone (ERZ) has been intermittently active for at least 2,000 years; the most recent 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 the Halema’uma’u crater drained (BGVN 43:10).

September 2018 marked the end of the previous eruption period after 36 years of continuous activity. A new eruption began during December 2020 in the Halema’uma’u crater, characterized by a new lava lake, lava flows, lava fountaining, and gas-and-steam emissions. This report covers the activity from December 2020 through January 2021 using information provided from the US Geological Survey's (USGS) Hawaiian Volcano Observatory (HVO) in the form of daily reports, volcanic activity notices, and abundant photo, map, and video data.

Monitoring through mid-December 2020. Monitoring data from HVO since the end of the previous eruption in September 2018 included variable rates of seismicity and ground deformation, low rates of sulfur dioxide emissions, and minor morphological changes. Areas of elevated ground temperatures and minor gas emissions persisted in the vicinity of the 2018 LERZ fissures. Since March 2019, GPS stations and tiltmeters at the summit had detected deformation consistent with slow magma accumulation approximately 1-2 km below ground level. In addition, GPS stations in the upper ERZ recorded increased rates of uplift beginning in September. The HVO seismic network recorded 1,450 earthquakes in September, a significant increase over previous months, followed by another increase to 2,100 events in October. The pond at the bottom of the Halema’uma’u crater, which appeared on 25 July 2019, continued to collect water over time, slowly expanding and deepening from 23 m in early January 2020 to 48 m by 3 November 2020 (figure 467).

Figure 467. Photos comparing the growth of the water lake in the Halema’uma’u crater at Kilauea on 18 December 2019 (left) and 23 September 2020 (right). During this time, the lake had risen approximately 25 m and had a surface area of 0.033 km2, compared to December 2019 (0.011 km2). Photos taken from the E rim of Halema’uma’u by K. Mulliken and M. Patrick; courtesy of USGS HVO.

The number of earthquakes detected in November was 1,350, less than what was recorded in October. By late November seismic stations recorded an average of at least 480 shallow, small-magnitude, earthquakes per week underneath the summit and upper ERZ; during 29-30 November HVO recorded over 80 earthquakes beneath the summit, beginning at 2300 on 29 November and continuing for 11 hours. On 2 December, spikes in seismicity were reported, consistent with a small dike intrusion under the S part of the caldera; tiltmeters at the summit detected about 8 cm of caldera floor uplift. At 1745 earthquakes intensified and another spike occurred after 0000 to an average rate of 10-12 earthquakes per hour. Within 24 hours, up to 220 earthquakes were recorded, occurring in clusters under the caldera and upper ERZ, according to HVO. By the afternoon of 3 December, seismicity and ground deformation rates at the summit had decreased and returned to near background levels. On 17 December, the number and duration of long-period seismic signals increased.

Eruptive activity during 20-21 December 2020. On the evening of 20 December at 2030 an earthquake swarm was recorded, accompanied by ground deformation detected by tiltmeters. Shortly after 2130 HVO reported an orange glow within the Halema’uma’u crater at Kilauea’s summit caldera, observed on an infrared monitoring camera, as well as a vigorous gas-and-steam plume, which marked the beginning of the eruption. At 2236 an M 4.4 earthquake was detected below the S flank. The Volcano Alert Level (VAL) was raised to Warning and the Aviation Color Code was raised to Red.

An HVO Volcanic Activity Notice issued on 21 December at 1014 stated that the water lake in the summit crater had boiled away due to new effusive activity, producing a large gas-and-steam emission (figure 468). Three vents in the N, NW, and W walls of the Halema’uma’u crater generated lava flows that fed a growing lava lake at the base of the crater (figure 469). Minor lava fountaining at these vents rose 25 m high; the highest fountain reached 50 m high in the N fissure. The lava lake began rising several meters per hour since the start of the eruption and exhibited a circulating perimeter, but a stagnant center (figure 470). Occasional blasts originated from the ponded lava in the crater. The eruption was confined to the Halema’uma’u crater. On 21 December the VAL was lowered to Warning and the Aviation Color Code decreased to Orange. Sulfur dioxide emission rates remained high at around 30,000 tons/day. In comparison, the emission rates from the pre-2018 lava lake ranged between 3,000-6,500 tons/day.

Figure 468. Webcam image of the summit of Kilauea at 0630 on 21 December 2020. The water lake had been replaced by a lava lake as fissure vents in the wall of Halema’uma’u effused lava into the crater. Strong gas-and-steam emissions were visible. Courtesy of HVO.

Figure 469. Map of the Halema’uma’u crater at Kilauea showing the location of volcanic activity shortly after 2130 on 20 December 2020. The red spots are the approximate locations of the three initial fissure vents effusing lava into the bottom of the Halema’uma’u crater. The water lake at the base of the crater had been replaced with a growing lava lake. The lava is deeper by at least 10 m compared to the water lake in this base map. The base map is from imagery collected on 23 September 2020. The eastern-most vent was characterized by lava fountains up to 50 m high with minor fountaining on the W side. Courtesy of HVO.

Figure 470. Aerial view of the summit of Kilauea during an overflight at 1120 on 21 December 2020 showing two active fissure vents that effused lava into the growing lava lake in the Halema’uma’u crater. The N fissure (right-most) is the dominant stream of lava. The fresh cooling lava appears black, surrounding the center of the lake, which was described as stagnant. Courtesy of HVO.

Activity during 22-25 December 2020. The effusive eruption continued on 22 December from at least two vents on the N and W sides of Halema’uma’u; the third vent between the N and W vents paused between 0730 and 0800. The middle and W vents became inundated by the growing lava lake, while the northern-most vent remained vigorous. As of 1151 the crater lake had grown to 487 m below the crater rim, which suggests that the lake had filled 134 m from the crater floor; the rate at which the lake rose was more than 1 m per hour. Measurements made on 22 December showed that approximately 10-12 million cubic meters of lava had been erupted to that point, with a surface area of about 0.13-0.22km² (figure 471). Another measurement made during the afternoon showed that the volume of the lava lake grew an additional two million cubic meters. The dimensions of the lake were 690 m E-W and 410 m N-S. Overflights were made on 21 and 22 December to obtain natural color and thermal infrared images of the growing lava lake (figure 472).

Figure 471. Location map showing the activity from the new eruption at the summit of Kilauea in the Halema’uma’u crater updated on 22 December 2020 at 1400. Two active fissure vents (orange dots) on the N and W side of the crater fed lava into the growing lava lake (red). The blue dashed line represents the extent of the former water lake (July 2019 to December 2020) that was present in the crater before the eruption and the black dashed line represents the extent of the lava lake that was present during 2008-2018. The current lava lake is larger than both the previous lakes and has formed slightly more N compared to the former lava lake. Map created by M. Zoeller; courtesy of USGS HVO.

Figure 472. Comparison of thermal images taken on 21 December at 1120 (top) and 22 December 2020 at 1130 (bottom) showing the rise and infilling of the lava lake from wall vents in the Halema’uma’u crater at Kilauea’s summit. Images by M. Patrick; courtesy of HVO.

By 23 December the lava lake had deepened to 155 m (figure 473). Two fissure vents on the N and W walls remained active; the W vent fed two narrow channels into the lake and the N vent remained the most vigorous. An island of cooler, solidified, lava within the lava lake that measured 115 x 260 m was drifting slowly eastward, based on a thermal map. During an overflight made later in the day, the approximate surface area was 0.25 km², with dimensions of 460 x 715 m. High SO₂ emissions were an estimated 30,000-40,000 tons/day, based on measurements made on 21 and 23 December.

Figure 473. Plot showing the increasing depth in Kilauea’s summit lava lake since the beginning of the eruption on 20 December 2020 at 2130. A laser rangefinder was used to take measurements of the lava lake surface about 2-3 times per day. The depth of the lake was about 155 m on 23 December at 0630 (top right) compared to 87 m on 21 December at 0630 (bottom right). In comparison, the water lake that was observed in Halema’uma’u before the start of the eruption was 51 m at its deepest. Plot by H. Dietterich; courtesy of HVO.

Measurements taken on 24 and 25 December showed a continuously growing lava lake that was 169 and 176 m deep, respectively, and the volume of the lake had reached 21 million cubic meters. By 25 December the vigorously erupting N fissure vent was starting to become inundated and the W vent displayed intermittent spattering (figure 474). Around 1400 the lake level had dropped by 2 m to reveal a narrow black ledge around the N edge of the crater. The rate of SO₂ emissions decreased to 16,000-20,000 tons/day during 25 December.

Figure 474. Photo of the Halema’uma’u crater at the summit of Kilauea at 0230 on 25 December 2020 showing lava flows and lava fountaining feeding the lake. The main N vent started to become inundated by the growing lava lake. Intermittent activity continued at the W vent. Photo taken from the S rim of the crater by J. Schmith and C. Parcheta; courtesy of HVO.

Activity during 26-31 December 2020. During the morning of 26 December, at 0240, the N vent continued to erupt lava into the lake while the W vent began to effuse more vigorously with up to three narrow lava flows feeding the lake (figure 475). The depth and volume of the lake remained the same as on 25 December: 176 m deep and 21 million cubic meters. Lava fountaining was visible up to 10 m high above the W vent. After 0300, the N vent declined in activity and started to drain lava from the lake. Summit tiltmeters continued to record some deformation. Effusive activity remained confined to Halema’uma’u; the lava lake was 177 m deep as of 0700 m on 27 December. The SO₂ emissions continued to decrease to about 3,300-5,500 tons/day during 27-28 December. Summit tiltmeters continued to record weak inflation.

Figure 475. Photo of the W vent in Halema’uma’u at Kilauea’s summit shows the effusive activity increased on 26 December 2020. Some lava fountaining in this vent was visible while lava flows continued to feed the lake from the N vent. The lava fountaining in the W vent rose at least 10 m high. Photo was taken at 0515 by H. Dietterich; courtesy of HVO.

On 28 December the volume of the lava lake had grown to 21.5 million cubic meters and a thermal map updated on 26 December showed the new dimensions of the lava lake were 520 x 790 m, covering a surface area of 0.29 km². The narrow black ledge visible above the N edge of the crater was about 1-2 m above the lake surface. During 27-28 December the main central island of cooler, solidified, lava drifted slowly W and measured about 110 x 225 m. The island surface was about 6 m above the lake surface and was covered in tephra, possibly remnants of explosive activity generated when lava first reached the water lake. Reduced, but still elevated, SO₂ emissions were 3,300 tons/day; the emission plume carried Pele’s Hair and Pele’s Tears SW, depositing the tephra in areas downwind.

Effusive activity continued, with the lava lake measuring 179-180 m deep with a narrow black ledge around it as of 0400 on 29 December. Multiple narrow lava channels from the W vent fed into the crater. The lava lake volume was slightly more than 22 million cubic meters. The central 135 x 250 m island of solidified lava had drifted slowly W until 2200 on 28 December, then during the morning of 29 December it stalled and began rotating. There were about 10 smaller islands to the E.

On the morning of 30 December, at 0345, the lava lake was 181 m deep with the narrow black ledge around it; the lava lake was an estimated volume of 23 million cubic meters. A spatter cone built around the W vent, while lava effused through crusted-over channels. The main central island was about 6-8 m above the surface of the lake. The rate of SO₂ emissions were 3,800 tons/day.

Similar observations were made during 31 December; the lava lake continued to grow, with the depth of the lake measuring 181-186 m and dimensions of 530 x 800 m, based on thermal mapping. The total surface area was 0.33 km². Spattering continued in the W vent while lava flowed through crusted-over channels into the lake (figure 476). The main island in the lake continued to drift slowly W while roughly 10 smaller islands were observed around the E end of the crater (figure 477). The SO₂ emission rate increased to 4,500-6,300 tons/day, compared to the previous day.

Figure 476. Photo of the active W vent in Halema’uma’u at the summit of Kilauea, viewed from the W crater rim on 31 December 2020 with incandescence, spattering, and a prominent spatter cone; the lava lake is visible in the right background. Photo by B. Carr; courtesy of HVO.

Figure 477. Annotated photo taken from the S rim of Halema’uma’u at the summit of Kilauea at 1700 on 30 December 2020 showing the location of the main central island and the smaller islands located on the eastern part of the crater. The W vent continued to effuse lava, as well as some spattering, while the N vent was inactive. Photo by K. Lynn; courtesy of HVO.

Activity during January 2021. Effusive activity continued within Halema’uma’u during January 2021. Lava originated from the NW side of the crater, with the W vents exhibiting spattering and lava effusions through crusted-over channels into the lava lake. A levee had also begun to develop around the perimeter of the lake (figure 478), creating what is known as a “perched” lake. According to HVO, this is common in lava lakes at Kilauea, and is due to repeated small overflows and the rafting and piling of surface crust that fuses together to form a barrier. During 31 December and 1 January the main island of solidified lava (135 x 250 m) had moved W while the other 10 smaller islands remained near the E side of the lake. Summit tiltmeters recorded weak deflation during 1-2 January. Both SO₂ emission rates and seismicity remained elevated; the SO₂ emission rate was 4,400 tons/day on 1 January.

Figure 478. Photo of the lava lake in Halema’uma’u at Kilauea on 1 January 2021 that has developed a levee (darker black) around the perimeter, allowing the lake to be slightly perched above its base. Photo by M. Patrick; courtesy of HVO.

During 2-3 January the depth of the lake had grown to 189-190 m, had a volume of 26 million cubic meters, and still maintained the narrow black ledge around its perimeter. Measurements on 3 January showed that the lake was perched about a meter above its E and W edges, and discontinuously on the N edge. A thermal webcam showed spatter originating from two places in the W vents and a small dome fountain above the lake crust in front of the W vents (figure 479). The dome fountain had formed where lava was entering the lake from a submerged inlet at the base of the W vent. The height of the dome fountain reached 5 m and the width was an estimated 10 m. The main island, about 6 m above the lake surface, continued to drift W in front of the W vents while the 10 smaller islands remained relatively stationary near the E end of the lake.

Figure 479. Video data showed the lava at Kilauea’s summit crater formed a dome fountain at the inlet to the lava lake in Halema’uma’u during 2-3 January 2021. The fountain is located near the base of the W vents where the inlet had become partially submerged. The 5-m-high dome fountain was about 10 m wide. Video by H. Dietterich; courtesy of HVO.

Lava effusion continued during 4-5 January from vents on the NW side of the crater. The lava lake was perched 1-2 m above its edge and had deepened to 191-192 m (figure 480). A thermal map from 5 January showed the perched lake dimensions had slightly decreased in size to 520 x 760 m, with a volume of about 27 million cubic meters. Summit tiltmeters continued to record weak deflation. Spatter in the W vents was visible from the top of a small cone on the NW wall of Halema’uma’u; the dome fountain persisted in front of the W vents (figure 481). The main island was rotating counterclockwise in front of the W vent while the now 11 smaller islands had generally stayed in the E side of the crater. Measurements on 4 January showed that the island was 7-8 m above the lake surface.

Figure 480. A comparison of the Digital Elevation Model (DEM) and topographic profiles of the Halema’uma’u crater at Kilauea created from aerial imagery collected during helicopter overflights, showing the change in depth and elevation of the lava lake between 26 December 2020 (left) and 5 January 2021 (right). The N vent remained inactive as it became inundated by the rising lava. The central island had migrated W and rotated by 5 January. The depth of the lava lake was 192 m on 5 January. DEMs created by B. Carr, graphic created by K. Mulliken; courtesy of USGS HVO.

Figure 481. Photo of the Halema’uma’u crater at Kilauea at 0545 on 5 January 2021 showing ongoing activity at the W vent, generating a lava flow that feeds both the lake and the dome fountain. Photo by K. Lynn; courtesy of USGS HVO.

HVO continued to monitor the changes in the active lava lake on 6 January, which was 194 m deep and remained perched 1-2 m above its edge. At 1500 rapid deflationary tilt was recorded overnight into 7 January. Lava from the W vents continued to feed the dome fountain through crusted-over channels on the W side of the crater. During the morning of 7 January the dome fountain weakened giving way to spattering at the top of the vent and the formation of a second cone. A thermal map on 7 January showed that the lake size had decreased to 470 x 760 m, covering 0.28 km²; more of the E part appeared to be stagnant while solidified lava was being progressively pulled beneath the molten surface (figure 482). SO₂ emissions were still elevated at 3,400 tons/day on 6 January, but had decreased to 2,700 tons/day the next day. During 7-8 January incandescence was visible from two small cones on the NW wall of Halema’uma’u while lava flowed into the lake through a crusted channel. The main island remained 135 x 250 m; it had moved slightly E while the 11 smaller islands remained stationary.

Figure 482. Thermal image (top) and photo (bottom) of the lava lake at Kilauea showing the larger central island on the W side of the Halema’uma’u crater and 11 smaller islands on the E side of the crater, taken on 7 and 9 January 2021, respectively. The lake is slightly perched and surrounded by a lower ledge of cooler lava along the perimeter (appears pink-purple in the thermal image along the perimeter). The lava effusion at the W vent has become less intense and much of the E half of the lake has stagnated completely, likely because the lake level has not changed significantly in the last three days. Image by M. Patrick (top) and photo by H. Dietterich (bottom); courtesy of HVO.

Incandescence and spatter continued on 9 January at the two W vents as lava descended through a crusted channel into the lake. Summit tiltmeters recorded weak deflation since 1 January, but on the evening of 9 January weak inflation was detected. A newly installed instrument during 9-10 January showed that the lake had risen about a meter since the switch to inflationary tilt. The depth of the lake slightly increased to 196 m below the W vents on the morning of 10 January. The W vents exhibited strong lava flows during the afternoon with spattering and spatter-fed lava flows from the top of the small cones on the NW wall of Halema’uma’u; lava also flowed through crusted-over channels into the lake. Low lava fountaining was also visible during 10-11 January. The SO₂ emission rates were 2,300 tons/day and 2,500 tons/day on 10 and 11 January, respectively.

During the morning of 12 January the lava lake remained at a depth of 196 m below the W vents; the stagnant E half of the lake was about 4 m shallower and had subsided below its perched rims. Low lava fountaining and flows through channels from the top of the small cones were visible. Measurements of the main island on 12 January showed that it was 8 m above the surface, with the highest point at 23 m. By 13 January, the depth of the lake had increased to 198 m. On 13 January a small portion of the active cone had collapsed, causing a second vent to open adjacent to the main vent and effuse lava for less than 20 minutes.

Activity continued in Halema’uma’u with low fountaining, lava flows, and spattering from the W vent through 22 January (figure 483). The depth of the lake continued to increase slowly to 204 m on 22 January. The entire lake was perched 1-2 m above the crust between the levees along the perimeter and the crater wall. All of the islands of solidified lava within the lake were stagnant; the dimensions of the main island were unchanged since 10 January. On 14 January the SO₂ emissions increased to 4,700 tons/day, then decreased to 2,500 tons/day on 16 January. On 19 January at 1746 field crews observed a minor collapse event from the spatter cone on its N rim and open channel margins at the W vent (figure 484). Summit tiltmeters began to detect some deflation on 20 January; the rate of which began to slow by 21 January. Measurements on 22 January showed that the S end of the main island was 12 m above the lava lake surface, with the highest point still around 23 m.

Figure 483. Photo of low fountaining and an accompanying lava flow at the W vent of Halema’uma’u at Kilauea on 15 January 2021. The vent formed a spatter cone around the fountaining as the flow moved through an open channel into the lake. Photo by M. Patrick; courtesy of HVO.

Figure 484. Series of photos showing the W vent at Kilauea (seen from the S rim looking NW) that continued to feed the growing lava lake in Halema’uma’u through an open channel. At 1746 on 19 January field crews observed a minor collapse on the N rim of the spatter cone and channel margins. The photo at 1731 (top left) shows the vent just before the collapse; the photo at 1746 (top right) shows just after the collapse; the photos at 1749 (bottom left) and at 1811 (bottom right) show the destabilization and movement of the portion of the remaining cone flank surrounded by incandescence. Photos by H. Dietterich; courtesy of HVO.

During the morning of 23-25 January the lava lake was about 205 m deep; the W half remained active with low fountaining and a lava flow while the E half was stagnant (figure 485). The E side of the lake was elevated about 1-2 m and the W half was elevated about 4 m above the solidified lava adjacent to the crater wall. HVO reported that summit tiltmeters continued to record variable inflation and deflation. On 23 January SO₂ emission rates were 2,200 tons/day.

Figure 485. Map of the Halema’uma’u crater at the summit of Kilauea on 25 January 2021 showing the locations of the active lava lake (red), the extent of the lava lake (light red), the major islands of solidified lava (yellow), the active W vent (orange), and the inactive N vent (maroon). The depth of the lake is 205 m, the size of the lake is 0.1 km2, and the total lake volume is 31 million cubic meters. In comparison, the dashed blue line represents the final extent of the water lake that evaporated on 20 December 2020 and the dashed black line represents the extent of the 2008-2018 lava lake. Courtesy of USGS HVO.

The depth of the lava lake continued to deepen, and by the evening of 27 January it was 209 m, while the stagnant E half remained up to 5 m lower. The active lake surface no longer extended around the E side of the central island; surface circulation was limited to the W, N, and S sides of the island. Activity in the W vents consisted of slow surface movements at the base of the lava flow and overturning of the crust near its margins. The E side of the lake was elevated approximately 1 m while the W was 3 m above the solidified lava adjacent to the crater wall. All the islands within the lake were stationary. By 28 January only the W part of the lava lake was active. On 29 January, measurements made on the main island showed its edges were 7-8 m above the lake surface.

On the morning of 30 and 31 January, the active W part of the lava lake was 211 and 212 m deep, respectively; the W vent had crusted over except for a single (possibly two) openings that were mostly obscured by degassing, though several incandescent areas on the cone were visible. Surface lava continued to effuse into the central part of Halema’uma’u from the base of the cone (figure 486). A series of surface cracks separated the active and stagnant parts of the lake. During 30-31 January tiltmeters recorded inflation at the summit.

Figure 486. Photo showing the leading edge of an active lava lobe moving S into the central part of Halema’uma’u at Kilauea on 31 January 2021. Photo by M. Patrick; courtesy of HVO.

Geologic Background. Kilauea, which overlaps the E flank of the massive Mauna Loa shield volcano, has been Hawaii's most active volcano during historical time. Eruptions are prominent in Polynesian legends; written documentation extending back to only 1820 records frequent summit and flank lava flow eruptions that were interspersed with periods of long-term lava lake activity that lasted until 1924 at Halemaumau crater, within the summit caldera. The 3 x 5 km caldera was formed in several stages about 1500 years ago and during the 18th century; eruptions have also originated from the lengthy East and SW rift zones, which extend to the sea on both sides of the volcano. About 90% of the surface of the basaltic shield volcano is formed of lava flows less than about 1100 years old; 70% of the volcano's surface is younger than 600 years. A long-term eruption from the East rift zone that began in 1983 has produced lava flows covering more than 100 km2, destroying nearly 200 houses and adding new coastline to the island.

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/).

Extensive lava flows, bomb-laden Strombolian explosions, and ash plumes emerging from Mackenney crater have characterized the persistent activity at Pacaya since 1961. The latest eruptive episode began with intermittent ash plumes and incandescence in June 2015; the growth of a new pyroclastic cone inside the summit crater was confirmed later that year. The cone has continued to grow, producing frequent loud Strombolian explosions rising above the crater rim and ongoing ash emissions. In addition, fissures on the flanks of the summit crater have been the source of an increasing number of lava flows traveling distances of over one kilometer down multiple flanks during 2019 and into 2021. Increasing explosive and effusive activity during December 2020-February 2021 is covered in this report with information provided by Guatemala's Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), multiple sources of satellite data, and numerous photographs from observers on the ground.

Eruptive activity increased substantially during December 2020-February 2021. During December, ash emissions were reported fewer than half the days of the month; by February, dense ash emissions drifted many kilometers most days, and ashfall was reported numerous times in the surrounding communities. Strombolian explosions in December generally rose 50-125 m above the summit of the pyroclastic cone; by February they were commonly rising 300 m or more and sending ejecta 500 m from the summit. Numerous lava flows were reported on the NW, W, and S flanks during the period; a flow that emerged on the SSW flank on 7 January 2021 persisted through the end of February and was 800-1,200 m long. Strombolian activity also occurred at the fissure where the flow emerged, and incandescent blocks rolled hundreds of meters beyond the front of the flow. A steady increase in thermal activity was recorded with the MIROVA Log Radiative Power graph during December 2020 – February 2021 (figure 145). This corresponded to the persistent lava flows on multiple flanks and constant Strombolian activity. Multiple MODVOLC thermal alerts were issued many days each month during the period.

Figure 145. The MIROVA graph of thermal anomalies at Pacaya from 13 May 2020 through February 2021 shows activity increasing in frequency and intensity beginning in late August 2020. Multiple lava flows from fissures on the flanks and Strombolian activity from the pyroclastic cone inside Mackenney crater were reported throughout the period. Courtesy of MIROVA.

The Washington VAAC reported an ash emission at Pacaya that rose to 3.0 km altitude and drifted WSW on 3 December 2020; it dissipated within a few hours. INSIVUMEH reported daily gas and steam plumes that rose a few hundred meters and sometimes drifted as far as 10 km. They also reported ash emissions along with the gas and steam on 10, 12-14, 16-18, 24-25, and 28 December. The ash plumes usually rose 300-400 m and drifted a few kilometers with the wind. On the evening of 28 December ash reached populated places including San José El Rodeo. Strombolian explosions at the summit occurred daily and rose 50-125 m above the Mackenney crater rim (figure 146). Ejecta was reported to heights of 250 m on 13 December and 200 m on 21 December.

Figure 146. Explosions sent ejecta up to 125 meters above the Mackenney cone crater at Pacaya on 29 December 2020. In addition, lava flows with multiple branches were active on the W flank. Courtesy of CONRED (LAVA FLOWS IN PACAYA VOLCANO CONTINUE ACTIVE, 29 December 2020, Informative Bulletin No. 582-2020).

Lava flow activity continued on the SW flank throughout December 2020 and high winds remobilized ash on the flanks a number of times during the month. On 1 December the flow was about 675 m long and moving to the SW. Two branches were active the next day and three were reported on 6 December. A second flow appeared on the NW flank on 9 December on the plateau near Cerro Chino and grew to 250 m long (figure 147). Both flows had incandescent block avalanches spalling off their fronts and rolling at least 100 m. The SW-flank flow remained 450-550 m long through 11 December, and then grew to around 700 m the next day. Branches from both flows extended 700-1,000 m by 15 December and were moving NW, W, and SW. The NW-flank flow was growing through 16 December. Three 600-m-long branches were active on the SW-flank flow on 21 December. In a special bulletin released on 23 December, INSIVUMEH noted that the SW-flank flow was still active from the same mid-flank fissure where it originated on 20 October 2020, and consisted of 5-7 branches with lengths varying from 600-750 m (figure 148). For the remainder of December, multiple branches of the active SW-flank flow were between 525 and 650 m long, with block avalanches falling off the front that generated ash clouds.

Figure 147. Sentinel 2 satellite imagery of Pacaya from 10 December 2020 revealed a thermal anomaly at the summit (lower right of center image), a multi-branch flow 550 meters long on the W flank (left of center image), and a small anomaly from the beginning of a new flow on the NW flank. Courtesy of INSIVUMEH (BOLETÍN VULCANOLÓGICO ESPECIAL BEPAC # 119-2020, Guatemala, 10 de diciembre de 2020, 19:30 horas (Hora Local), “ACTUALIZACIÓN DE LA ACTIVIDAD VOLCÁNICA”).

Figure 148. Sentinel 2 satellite imagery of Pacaya on 20 (left) and 30 (right) December 2020 indicated thermal activity at the summit and on the NW and W flanks. The NW-flank lava flow was active from 9-16 December, and still cooling in the 20 December image. The WSW-flank lava flow had multiple branches between 525 and 650 m long for the last half of the month. Images use Atmospheric Penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

In a special bulletin issued on 1 January 2021 INSIVUMEH reported an increase in eruptive activity that produced Strombolian explosions which sent ejecta 300 m high and up to 100 m from the summit. Constant rumblings like a train and shock waves were heard and felt in nearby communities. Strombolian explosions continued to rise 75-200 m above the rim throughout the month, and numerous gas emissions rose 100-300 m and drifted as far as 10 km (figure 149). Ash emissions were noted on 1, 6, 7, 11, 13, 18, 19, 21-23, 25, 27, and 31 January. On 7 January ash drifted SW at 3 km altitude and ejecta was reported 300 m from the summit. INSIVUMEH noted that the columns of ash reached 300-500 m above the crater that day, generating loud rumbling and shock waves that vibrated roofs and windows in nearby villages. On 12 January explosions sent material 300 m high. A VAAC report on 22 January noted an ash plume drifting NW from the summit at 3.4 km altitude. INSIVUMEH remarked in a special report that day that ash fell in San Vicente Pacaya and in San Francisco de Sales. The ash emissions on 25 January were brown to gray, sporadic overnight and more continuous in the early morning, drifting 1-4 km W. On 27 and 31 January ash drifted 10 km W.

Figure 149. Strombolian explosions rose 75-200 m above the summit of the pyroclastic cone inside Pacaya’s Mackenney crater on 7 January 2020 and throughout the month. On the NW flank, multiple branches of lava appeared as red to white areas in this thermal image. Thermal image courtesy of INSIVUMEH (BOLETÍN VULCANOLÓGICO ESPECIAL BEPAC002-2021, 12:00 horas (Hora Local), EXPLOSIONES CON CENIZA).

Multiple lava flows emerged from the flanks of Pacaya during January 2021. The lava flow that began on 20 October 2020 on the W flank continued to be active through about 8 January with branches flowing 400-600 m W and SW. A flow on the SSW flank began on 2 January from a vent 200 m below the rim of Mackenney crater. By 6 January it was feeding 3-4 flows from the same point, each 400 m long with block avalanches falling off the fronts and moving W, SW, and S down the flanks (figure 150). In the morning of 7 January two flows were seen on the N flank, 200 and 50 m long. Later that night another flow appeared on the SSW flank that lengthened rapidly, reaching 425 m the next day, and was 1,200 m long on 9 January (figure 151). High temperatures were still present on the W and SW flanks from the earlier flows. The SSW flow reached 1,500 m in length on 10 January and fluctuated between 1,200 and 1,600 m through 17 January when Strombolian activity ejecting material 5-10 m high was reported from the fissure. More Strombolian activity at the fissure was noted on 22 January, and the flow remained 800-1,150 m long through the 23rd. The flow reached 1,700 m in length on 25 January; for the rest of the month, it was reported as 800-1,000 m long, with block avalanches traveling an additional 200-400 m from the flow front. Strombolian activity reached 65 m high from the fissure at the head of the flow on 28 January. On 30 January multiple branches of the SSW flow were visible from a vantage point south of the volcano (figure 152).

Figure 150. Multiple flows emerged from a single vent at Pacaya on 5 January 2021. The fissure was located about 300 m below the rim of Mackenney crater on the SSW flank. Incandescent debris falls from the front of the flow generated an ash plume seen at the bottom center of the image. Copyrighted photo by Deybin Fotografia, used with permission.

Figure 151. A lava flow at Pacaya that first emerged on 7 January 2021 on the SSW flank grew quickly to over a kilometer long by 9 January and remained 800-1,000 m long for the rest of the month, often with incandescent blocks falling several hundred meters beyond the front of the flow. A thermal anomaly persisted at the summit of the pyroclastic cone inside Mackenney crater as well from constant Strombolian activity. A weak anomaly was also visible on the NW flank from earlier activity. Atmospheric penetration rendering (bands 12, 11, 8a) of Sentinel 2 images. Courtesy of Sentinel Hub Playground.

Figure 152. Multiple branches of Pacaya’s SSW-flank-flow that began on 7 January 2021 were visible from a vantage point S of the volcano on 30 January. The branches were at least 700 m long with incandescent blocks falling several hundred meters farther down the flanks. The white lights below the flow are from people approaching the flow. Courtesy of David Rojas, used with permission.

Increased Strombolian activity during February 2021 was accompanied by frequent ash emissions that rose to 3.0-3.5 km altitude. The explosions often reached 225 m above the crater rim, and higher during pulses of increased activity. On 5 February ash drifted W, NW, and SW about 4 km and ashfall was reported in San Francisco de Sales, Concepcion el Cedro, and Calderas. A pulse of increased Strombolian activity on 6 February sent ejecta 400-500 m around the pyroclastic cone and columns of ash drifted 6 km NW and N. Ashfall was reported in the same areas as the day before, plus in El Bejucal, Mesías Altas and other communities in that region. Abundant ash emissions were reported by INSIVUMEH overnight on 7-8 February; variable winds dispersed the ash 30 km to the NW and W and 10 km N (figure 153). The ash emissions were accompanied by ejecta that landed 300 m from the summit. By the next day, ash had drifted as far as 66 km W and NW and ashfall was reported in El Patrocinio, El Rodeo, and El Caracol. Prolonged rumbling as loud as an airplane engine was reported from strong degassing. The Washington VAAC reported ash emissions in satellite imagery on 9 February at 3.8 km altitude drifting NW about 65 km from the summit.

Figure 153. Dense ash emissions increased in frequency at Pacaya during February 2021. Ash emissions on 6 (left) and 8 (right) February resulted in ashfall in multiple communities around the volcano and were accompanied by incandescent ejecta falling hundreds of meters from the summit. Courtesy of INSIVUMEH (BOLETÍN VULCANOLÓGICO ESPECIAL BEPAC 006-2021, 009-2021).

High levels of similar activity continued through 10 February when 500-m-high ejecta was observed inside Mackenney crater. An increase in the seismic amplitude on 11 February was accompanied by ash plumes rising to 3.0-3.2 km altitude and drifting 15-20 km W and SW. Ashfall was reported in Patrocinio and El Rodeo. The next day ashfall was reported in San Francisco de Sales, San Jose Calderas, and Concepción el Cedro. On 13 February the Washington VAAC reported ash plumes visible in satellite imagery at 4.3 km altitude moving ENE, and ash fell in Santa Elena Barillas, Mesillas Bajas, and Mesillas Altas as the wind carried ash 6 km W, N, and NE; ash on 14 February drifted 5 km E. A new pulse of activity late on 16 February, the third in a week, produced incandescent material 400 m high; high-pressure gas also created plane engine noises, with roofs and windows rattled in nearby communities. Ashfall from the event was reported in Los Llanos, Los Pocitos, El Cedro, and other communities within 4 km. Another pulse on 18 February sent ejecta 200 m high, variable winds sent ash primarily NE and S. Two more pulses of activity on the morning of 19 February were recorded as increases in seismic amplitude by the PCG5 seismic station (figure 154). The first pulse was accompanied by a new lava flow appearing on the NW flank. The second pulse coincided with ash emissions that rose 500 m above the crater and drifted 8 km S, producing ashfall in Los Pocitos and plantations in that vicinity.

Figure 154. Two increases in seismic amplitude at Pacaya were recorded during the morning of 19 February 2021 at seismic station PCG5. The first corresponded to the effusion of a new lava flow on the NW flank (left), and the second coincided with a pulse of ash plumes that drifted S (right). Courtesy of INSIVUMEH (BOLETÍN VULCANOLÓGICO ESPECIAL BEPAC 31-2021, Incremento de actividad por emission de ceniza y surgimiento de nuevo flujo de lava).

Ash emissions from explosions on 20 February drifted 10-25 km S and SW, resulting in ashfall in El Rodeo and El Patrocinio. That evening incandescent material rose 300-400 m above the summit and ejecta reached 500 m down the flanks of the cone (figure 155). The next day ash plumes rose to 2.8-3.2 km altitude and drifted SW with ashfall reported in San Francisco de Sales, El Cedro, and other plantations in the area (figure 156). During 22-24 February ash emissions rose as high as 800 m above the summit and drifted 3-5 km W, SW, and S. Ashfall drifted over 30 km S and SW on 24 February with ashfall reported in the villages of Los Pocitos, Pacaya, El Rodeo, and El Patrocinio. Pulses of increased activity on 26 February produced an ash plume 2.5 km above the summit. With variable wind directions at different altitudes, the ash drifted both N and S. The Washington VAAC reported the plume drifting N at 3.9 km altitude. This activity was accompanied by incandescent explosions that rose 500 m above the Mackenney crater, and noises as loud as an airplane engine. Similar pulses of activity continued through the end of the month, producing ash plumes that rose to 3.5 km altitude and drifted W and SW; ashfall was reported in El Patrocinio on 28 February.

Figure 155. During the weekend of 20-21 February 2021 when this photo was taken, Strombolian explosions at Pacaya sent ejecta 400 m above the summit of the cone and 500 m down the flanks, while a lava flow remained active on the SSW flank. Copyrighted photo by David Rojas, used with permission.

Figure 156. On 21 February 2021, ash plumes at Pacaya rose to 2.8-3.2 km altitude and drifted SW with ashfall reported in San Francisco de Sales, El Cedro, and other plantations in the area. Courtesy of Luis Figueroa.

The lava flow on the SSW flank was about 900 m long at the beginning of February with block avalanches falling about 100 m from the front of the flow, and Strombolian explosions active at the fissure at the head of the flow. Two distinct branches of the flow were visible on 6 February, one 1,200 and one 800 m long; multiple branches were active throughout the month (figure 157). High levels of activity continued; during 10-12 February the flow was 1,200-1,300 m long and loose blocks were descending an additional 200 m. During 13-18 February high temperature zones were still present on the N and NW flanks from earlier flows. From 14-18 February the S-flank flow was 900-1,100 m long with multiple branches and Strombolian activity at the vent (figure 158). A new flow appeared briefly on the NW flank during 19-20 February. High-temperature zones remained on the NW flank during 22-24 February. The S-flank flow remained active throughout the rest of February and was 800-1,100 m long, with incandescent blocks traveling up to 600 m beyond the flow fronts (figure 159).

Figure 157. Multiple branches of the S-flank lava flow at Pacaya were active throughout February 2021. Strombolian activity was observed at the fissure where the flow emerged, and incandescent blocks rolled hundreds of meters beyond the flow front. The fissure was located about 300 m below the crater rim. The thermal anomaly from the Strombolian activity at the summit of the pyroclastic cone inside Mackenney crater was also visible in most satellite images. Atmospheric penetration rendering of Sentinel 2 image uses bands 12, 11, 8a. Courtesy of Sentinel Hub Playground.

Figure 158. A lava flow about 1 km long on the S flank of Pacaya was active throughout the month; on 16 February 2021 Strombolian activity at the summit and at the head of the flow were visible. Multiple branches of the flow sent incandescent blocks hundreds of meters beyond the flow front. Copyrighted image by Berner Villela, used with permission.

Figure 159. The lava flow on the S flank of Pacaya had several active branches as seen in this thermal image on 21 February 2021. The source fissure vent was about 300 m below the rim of Mackenney crater. Incandescent blocks fell hundreds of meters beyond the fronts of the flows. Courtesy of INSIVUMEH and Roberto Iboy.

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

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/ ); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Deybin Fotografía (URL: https://www.facebook.com/Deybin-fotografía-2316704905277353, https://twitter.com/UniversoNews1/status/1347037016324792327); David Rojas (URL: https://twitter.com/DavidRojasGt/status/1360789438545149957); Luis Figueroa (URL: https://twitter.com/luisficarpediem/status/1363664541318598657); Berner Villela (URL: https://bernervillela.com/galerias/naturaleza, https://twitter.com/soy_502/status/1362846917743366146); Roberto Iboy (URL: https://twitter.com/IboyRoberto/status/1363688900401709057).

Villarrica, located in Chile, has had historical eruptions dating back to 1558. The current eruption period began in December 2014 and more recently has been characterized by summit crater incandescence, Strombolian explosions, and ash emissions (BGVN 45:09). This report covers activity during September 2020 through February 2021, which consists of an active lava lake, explosions, ash plumes, and nighttime crater incandescence. Information is provided by the Southern Andes Volcano Observatory (Observatorio Volcanológico de Los Andes del Sur, OVDAS), part of Chile's National Service of Geology and Mining (Servicio Nacional de Geología y Minería, SERNAGEOMIN), the Projecto Observación Villarrica Internet (POVI), part of the Fundacion Volcanes de Chile, a private research group that studies volcanoes across Chile, the Buenos Aires Volcanic Ash Advisory Center (VAAC), and various satellite data.

Activity during September 2020 was characterized by an active lava lake, white gas-and-steam plumes that rose 500 m above the crater, nighttime crater incandescence that could be observed on clear days, and sporadic ash emissions produced by minor explosions. During 5 and 7 September tephra deposits extended up to 36 m on the E and SE flanks, according to satellite data. On 25 September the seismic network recorded a long-period earthquake associated with a moderate explosion at 1350, which produced an ash plume that rose 800 m above the crater and drifted ENE (figure 104); blocks of ejecta were deposited around the crater. A second explosion was recorded at 1829 in conjunction with another long-period event, which generated an ash plume that rose 450 m above the crater (figure 104). Sentinel L2 A satellite images were used to determine that ashfall extended 3.8 km SSE, 865 m SE, and 275 m N as a result of the explosions during the day. The POVI webcam captured incandescent ejecta at night on 27 September (figure 105).

Figure 104. Explosions at Villarrica on 25 September 2020 at 1350 (top) and 1829 (bottom) produced a long-period seismic signal and ash plumes that rose 800 m and 450 m above the crater, respectively and drifted ENE. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De La Araucania y Los Rios, Volcan Villarrica, 25 de septiembre de 2020, 14:35 Hora local y 25 de septiembre de 2020, 19:20 Hora local).

Figure 105. Incandescent ejecta up to 100 m above the summit of Villarrica was captured in the POVI webcam at night on 27 September 2020. Courtesy of POVI.

Intermittent white gas-and-steam plumes, ash explosions, and nighttime crater incandescence continued during October. On 4 October SERNAGEOMIN reported a long-period event accompanied by a moderate explosion at 1130, generating an ash plume that rose 450 m above the crater and drifted NE. The next day on 5 October two long-period events were recorded at 1343 and 1347 associated with explosions, resulting in ash plumes that rose to 400 m above the crater and drifted SE (figure 106). On 12 October a satellite image showed an ash plume drifting 2.5 km NE and a strip of tephra deposits measuring 200 m wide and 3 km long on the NE flank, as a result of two eruptive events on 9 October, according to POVI and Sentinel-2 satellite imagery.

Figure 106. Explosions at Villarrica on 5 October 2020 produced a long-period seismic signal and an ash plume that rose 400 m above the crater and drifted SE. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De La Araucania y Los Rios, Volcan Villarrica, 5 de octubre de 2020, 14:20 Hora local).

Moderate explosions were detected at 0534 and 0804 on 15 October, associated with two long-period earthquakes. As a result, ash plumes rose as high as 900 m above the crater and gas-and-steam plumes rose to 450 m, according to SERNAGEOMIN. The explosion at 0534 was accompanied by crater incandescence and incandescent ejecta that were deposited on the E flank as far as 3 km. An analysis of Planet Scope and Sentinel-2 satellite images showed that ash deposits extended 4.4 km NE. On 20 October an explosion and long-period event were recorded at 1722 that resulted in an ash plume 240 m above the crater that drifted S (figure 107). Explosions recorded during 22-23 October produced ash plumes that rose 780 m and 180 m above the crater, respectively, according to a Buenos Aires VAAC report and SERNAGEOMIN. The event on 22 October deposited tephra up to 3.8 km on the E flank.

Figure 107. An explosion at Villarrica on 20 October 2020 at 1722 was characterized by a long-period earthquake and a dense, gray ash plume that rose 240 m above the crater and drifted S. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De La Araucania y Los Rios, Volcan Villarrica, 20 de octubre de 2020, 18:00 Hora local).

Ash explosions continued in November, accompanied by intermittent nighttime crater incandescence and white gas-and-steam plumes. On 5 November a pulse of ash was observed at 1442 that rose 350 m above the crater and drifted NW. Similar activity was noted on 6 November at 0757 and 0808 when ash rose 350 m above the crater and at 1412 when ash rose 250 m above the crater, both of which drifted NW (figure 108). According to a Buenos Aires VAAC report on 7 November, an isolated ash plume was detected in satellite images up to 4.3 km altitude, drifted ESE. A Differential Absorption Optical Spectroscopy Unit (DOAS) showed average values of SO₂ totaling 140 tons/day during 7-8 and 15 November with a maximum daily value of 168 tons/day on 7 November. An explosive event at 0051 on 8 November ejected incandescent material and produced an ash plume that rose 220 m above the crater (figure 108). On 10 November OVDAS reported an ash plume rose 320 m above the crater and drifted SSW, accompanied by continuous seismic tremor at 1514. Ash continued to be reported during 16-17 November rising 160 m above the crater and to 3.7 km altitude, respectively. Data from the DOAS showed that SO₂ emissions had slightly increased to an average of 166 tons/day during 16-30 November, with a maximum daily value of 549 tons/day on 22 November.

Figure 108. Explosions that generated ash and incandescent ejecta at the summit of Villarrica were captured by the POVI webcam during 6-8 November 2020 (left to right). Courtesy of POVI.

The number of ash events decreased in December compared to the previous months, though similar activity persisted. On clear nights, crater incandescence was visible, accompanied by white gas-and-steam emissions. SERNAGEOMIN reported a single long-period earthquake associated with a moderate explosion at 1844 on 5 December with a resulting ash plume that rose 160 m above the crater and drifted SSE; some ashfall was detected within 500 m of the crater, based on Sentinel-2, Pleiades, and SkySat data, and incandescent material was deposited on the SSE flanks (figure 109). According to POVI, during an overflight on 9 December scientists observed a lava lake 10-15 m in diameter that was partially covered by solidified floating black lava. Small pulses of gas and ash were observed in the lava lake. Additionally, ballistic blocks and pyroclasts that measured a maximum of 20 cm in diameter had been ejected up to 800 m from the crater during previous eruptive events. The average SO₂ value was 178 tons/day with a maximum daily value of 353 tons/day on 7 December 2020, according to DOAS data.

Figure 109. An explosion at Villarrica on 5 December 2020 at 1844 produced a long-period seismic signal along with an ash plume that rose 160 m crater and drifted SSE. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De La Araucania y Los Rios, Volcan Villarrica, 5 de diciembre de 2020, 19:50 Hora local).

On 16 December at both 1146 and 1156 SERNAGEOMIN reported two ash pulses associated with long-period events. The first ash emission rose 160 m above the crater and drifted NW; the second rose 280 m above the crater and drifted 500 m NE. On 17 December at 1716 another ash plume associated with a long-period event rose 720 m above the crater and drifted ESE (figure 110). Pyroclastic deposits were reported up to 1.3 km N, 3.3 km E, 5 km SE, and 1.8 km SW from the crater, according to data obtained from Sentinel-2 and SkySat. During 18-19 December seismicity increased, intense crater incandescence was visible, and a notable sulfur odor was noted, according to POVI reports. Minor ash emissions rose to low heights on 22 December.

Figure 110. An explosion at Villarrica on 17 December 2020 at 1716 produced an ash plume that rose 720 m above the crater and drifted ESE. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De La Araucania y Los Rios, Volcan Villarrica, 17 de diciembre de 2020, 17:50 Hora local).

During January 2021, the number of explosions with ash plumes continued to decrease compared to the previous months. On clear weather days, occasional nighttime crater incandescence was observed, as well as white gas-and-steam emissions of variable intensities. During an overflight on 2 January scientists observed an incandescent vent at the bottom of the crater that had a solidified lava bridge connecting across a partially crusted-over top (figure 111). DOAS data showed that the average mass of SO₂ plumes had increased compared to November and December to 318 tons/day with a maximum daily value of 789 tons/day on 12 January. During 1-15 January, the highest ash plume reported rose 700 m above the crater, though it was mostly composed of gas-and-steam emissions. During 16-31 January gas-and-steam emissions continued, rising to 1.3 km above the crater on 20 January. The average value of SO₂ plumes increased again to 430 tons/day with a maximum daily value of 789 tons/day on 22 January.

Figure 111. Webcam image of two incandescent vents at Villarrica on 2 January 2021. A bridge of solidified lava separates the two sections and extends across the active lava lake. Courtesy of POVI.

Activity during February continued to decrease compared to the previous months and consisted of primarily white gas-and-steam plumes, nighttime crater incandescence, and SO₂ plumes. On 10 February dense, white gas-and-steam plumes rose 700 m above the crater. During 1-15 February, the average value of SO₂ plumes was 181 tons/day with a maximum daily value of 369 tons/day on 2 February. Long-period earthquakes were recorded by the seismic network at 1146 and 1156 on 16 February with an associated explosion that generated ash plumes 160 m above the crater that drifted NW and 280 m that drifted NE, respectively. During 16-28 February white gas-and-steam plumes rose to a high of 780 m above the crater; SO₂ plumes were an average value of 402 tons/day with a maximum daily value of 1,026 tons/day on 21 February.

Low-power thermal activity was detected during September 2020 through January 2021, according to the MIROVA Log Radiative Power graph using MODIS infrared satellite information (figure 112). Three thermal anomalies were recorded in September, one in October, and four in November; a single stronger anomaly was observed in early November. The number of anomalies increased in late December through late January 2021, though they remained low in power. On clear weather days, a strong thermal anomaly in the summit crater was visible in Sentinel-2 thermal satellite imagery during each month of the reporting period; in February, the strength of the anomaly had slightly decreased compared to previous months (figure 113).

Figure 112. Low-power thermal anomalies were detected in the MIROVA graph (Log Radiative Power) at Villarrica during September 2020 through late January 2021. A pulse of thermal anomalies was recorded during late December 2020 through late January 2021 compared to the previous month but remained low in power. Courtesy of MIROVA.

Figure 113. Sentinel-2 thermal satellite images showing strong thermal anomalies on clear weather days in the summit crater of Villarrica each month from September 2020 through February 2021. The strength of the thermal anomaly in February decreased slightly compared to previous months. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

Geologic Background. Glacier-clad Villarrica, one of Chile's most active volcanoes, rises above the lake and town of the same name. It is the westernmost of three large stratovolcanoes that trend perpendicular to the Andean chain. A 6-km-wide caldera formed during the late Pleistocene. A 2-km-wide caldera that formed about 3500 years ago is located at the base of the presently active, dominantly basaltic to basaltic-andesitic cone at the NW margin of the Pleistocene caldera. More than 30 scoria cones and fissure vents dot the flanks. Plinian eruptions and pyroclastic flows that have extended up to 20 km from the volcano were produced during the Holocene. Lava flows up to 18 km long have issued from summit and flank vents. Historical eruptions, documented since 1558, have consisted largely of mild-to-moderate explosive activity with occasional lava effusion. Glaciers cover 40 km2 of the volcano, and lahars have damaged towns on its flanks.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); Proyecto Observación Villarrica Internet (POVI) (URL: http://www.povi.cl/); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Eruptive activity at Chikurachki began on 25 January 2002. Ash plumes were observed, and a small new crater formed on the SSE part of the summit crater. By mid-February, volcanism decreased, but the Kamchatkan Eruptions Response Team (KVERT) stated that ash explosions could still occur (BGVN 27:01).

During 23-27 February, reports from the town of Severo-Kurilsk revealed renewed activity. On 25, 26, and 27 February ash plumes occasionally rose above the crater and ash fell in the vicinity of Tukharka River. In addition, snow melted very quickly near the volcano. On 8 February an ash plume rose a short distance and drifted NNE. Several clouds were visible on AVHRR satellite imagery that may have been composed of gas and steam from the volcano.

KVERT reported a continuation of eruptive activity through at least 16 March. On that day, beginning at 0700 and lasting until late evening, ash fell in Podgorny settlement, ~20 km SE of the volcano. On a reconnaissance helicopter flight during 1100-1300, observers saw constant gas emissions and sustained ash explosions that rose 200 m above the volcano and extended more than 100 km SE.

Geologic Background. Chikurachki, the highest volcano on Paramushir Island in the northern Kuriles, is actually a relatively small cone constructed on a high Pleistocene volcanic edifice. Oxidized basaltic-to-andesitic scoria deposits covering the upper part of the young cone give it a distinctive red color. Frequent basaltic plinian eruptions have occurred during the Holocene. Lava flows from 1781-m-high Chikurachki reached the sea and form capes on the NW coast; several young lava flows also emerge from beneath the scoria blanket on the eastern flank. The Tatarinov group of six volcanic centers is located immediately to the south of Chikurachki, and the Lomonosov cinder cone group, the source of an early Holocene lava flow that reached the saddle between it and Fuss Peak to the west, lies at the southern end of the N-S-trending Chikurachki-Tatarinov complex. In contrast to the frequently active Chikurachki, the Tatarinov volcanoes are extensively modified by erosion and have a more complex structure. Tephrochronology gives evidence of only one eruption in historical time from Tatarinov, although its southern cone contains a sulfur-encrusted crater with fumaroles that were active along the margin of a crater lake until 1959.

Information Contacts: Olga Chubarova, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia; Tom Miller, Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

This report discusses Etna following the July-August 2001 eruption and through 25 April 2002. According to Boris Behncke, the chief source for this report, this 9-month interval was an unusually quiet one and marked the longest quiet interval since 1995.

A visit to the summit craters on 30 January 2002 revealed low levels of activity and no evidence of energetic outbursts. Loud explosions occurred at intervals of 5-30 minutes within the NW pit of Bocca Nuova, but no solid material was ejected. The rims of the pit were covered with brown lithic ash (which had been emitted in December-January) but there were no blocks or fresh scoriae indicating recent ejections. The pit appeared much the same as in September 2001, with a crescent-shaped flat terrace surrounding a deep, degassing vent in the SE part of the pit.

Most of the present degassing at the summit craters is occurring from a vent in the SW part of the Voragine, which had been much less active during the past 1.5 years. Northeast Crater emitted a fairly dilute plume, and at Southeast Crater, fumarolic activity was concentrated at its W rim where numerous degassing vents lie in a fracture. Mechanized access remained limited after the demise of the cable car and the ski lifts on the S flank during the July-August 2001 eruption (BGVN 26:08 and 27:03). In order to access the summit area one has to hike from ~1,900 m elevation, a trip that takes several hours and leads across the July-August 2001 lava fields.

Numerous small earthquakes, some of which were felt by the local population, were recorded on the S flank (in the area of the largest of the July-August 2001 lava flows), and were interpreted to result from the cooling of the lava. Near-continuous, pulsating emissions of reddish-brown lithic ash began around 9 March at the NW vent of Bocca Nuova, generating a plume that trailed for dozens of kilometers downwind. The same source vent has been the site of deep-seated explosions during the past six months. The emissions may have been caused by collapse within the conduit, which occurred repeatedly after the end of the July-August 2001 eruption, and does not necessarily indicate an intensification of eruptive activity or uprise of fresh magma. On the other hand, the volcano had been quiet for some 8 months at this time, and renewed magmatic activity at the summit was to be expected in the near future.

During the third week of March, emissions of lithic, pink-colored ash continued at Bocca Nuova. These were accompanied by voluminous degassing from Northeast Crater and minor fumarolic activity from Voragine and Southeast Crater. During days without strong wind, these emissions rose vertically to form a spectacular plume that might easily create the impression of true eruptive activity at the summit. However, there is no evidence that fresh magma has risen to near the surface, because no incandescence can be seen at night.

A mid-March summit visit by Giovanni Tomarchio, a cameraman of the Italian television RAI (who is responsible for much of the television footage of Etna in recent years), revealed frequent loud explosions at the SW vent of Bocca Nuova. Although the floor of this vent was not visible, it seemed that the explosions originated somewhere immediately below the visible part of the pit. All recent ejecta were fine lithic ash, which accumulated to form a thick, soft deposit in the summit area. Similar emissions occurred for months at Bocca Nuova during the spring and summer of 1999, prior to the vigorous eruptions at Voragine and Bocca Nuova during September-November of that year.

In late March, after nearly three weeks of ash emissions from Bocca Nuova, Northeast Crater began to emit dark brown to gray ash. The emissions appeared to follow a series of small SE-flank earthquakes during 24-25 March. At least three of the shocks were felt by the local population. On 27 and 28 March the ash emissions from both Bocca Nuova and Northeast Crater rose as distinct puffs to several hundred meters above the summit and seemed more energetic, denser, and darker than during the previous weeks. To a passing airplane pilot they appeared so spectacular that he sent out a warning of an eruption. On 28 March, light ash fell over the S flank as far as Catania (~25 km SSE).

Whether Etna is back in magmatic eruption is the subject of debate. The ash that came from the two craters consisted of fine-grained fragments of rock and was derived from the conduit walls and thus contained no new magmatic material. The ash that fell in Catania on 28 March was distinctly darker than the ash that fell in the summit area during the previous weeks and may contain a certain proportion of juvenile magmatic material, although microscopic examination has not been conducted to confirm this. No glow has been seen so far at the summit during night observations, so it seems unlikely that magma has reached the surface. On 29 March two impressive columns bearing dark ash rose nearly continuously from the two craters to several hundreds of meters (~800 m at one point) above the summit. Shifting winds carried the plume E, S, and W.

During late March through 2 April ash emission continued without interruption from Bocca Nuova, while at Northeast Crater it had apparently stopped. Light ashfalls occurred in downwind areas, at times extending as far as Catania. The emissions took the form of billowing brown plumes, which at times rose several hundred meters above the summit. No incandescence was seen at night. Weather prevented observations after the afternoon of 2 April.

The summit became visible again on 6 April. Bocca Nuova continued to produce weak expulsions of brown-colored (probably lithic) ash, while Northeast Crater emitted only white vapor. Two small (M ~3) earthquakes occurred under the SE flank on 4 April. On 13 April two earthquakes (M 2.7-3) were felt by residents on the SE flank (between the towns of Zafferana and Santa Venerina), their epicenters lying in an area named "Salto della Giumenta," located ~5 km NW of Zafferana. Press sources citing scientists of the Istituto Nazionale di Geofisica e Vulcanologia of Catania gave focal depths of ~4 km below the surface. Numerous earthquakes had occurred within the past few weeks in this area, although their correlation with magma movement within the volcano remained unclear.

Ash emissions continued almost constantly at Bocca Nuova. On 14 April these appeared to be dark gray, and at times were emitted forcefully enough to form plumes several hundred meters high. No incandescence was seen during night observations. A dense plume of brownish-gray ash drifted from Etna's summit across the E sky of Catania as Bocca Nuova emitted pulverized rock from its SE vent. Voragine and Northeast craters gave off dense steamy plumes.

In late April heavy snow fell on Etna; snow-cover reached down to ~1,400 m elevation and access to the summit area was reduced. The snow provided a good opportunity to observe the hot areas at the summit and to confirm that no recent lava outflows have taken place. Snow was melting rapidly on the cones of the summit craters and along the fracture that extends NNE from Southeast Crater. Since 23 April, Bocca Nuova's ash emissions, which had been nearly continuous since early March, decreased markedly. The only visible summit activity during 24-25 April consisted of apparently ash-free gas emissions, mostly from Bocca Nuova and Northeast Crater. Nine months after the climax of its most recent flank eruption, Etna continues its unquiet slumber.

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

Information Contacts: Boris Behncke, Dipartimento di Scienze Geologiche (Sezione di Geologia e Geofisica), Palazzo delle Scienze, Corso Italia 55, 95129 Catania, Italy.

During 7 January through at least 19 May 2002 at Ijen, seismicity was higher than normal. Shallow volcanic and tectonic earthquakes were recorded (table 3). One small explosion earthquake was recorded during the week of 28 January-3 February. A total of three deep volcanic (A-type) earthquakes were registered during early May. Continuous tremor occurred with a maximum amplitude of 0.5-4 mm until mid-March, when it decreased to 0.5-2 mm. During 8-14 April, a white, thin, medium-pressure plume rose 50 m above the summit crater. The following week, the tremor increased to 0.5-6 mm maximum amplitude and remained at similar levels through at least 19 May. The Alert Level remained at 2 during the report period.

Table 3. Earthquakes recorded at Ijen during 7 January through 19 May 2002. Courtesy VSI.

Geologic Background. The Ijen volcano complex at the eastern end of Java consists of a group of small stratovolcanoes constructed within the large 20-km-wide Ijen (Kendeng) caldera. The north caldera wall forms a prominent arcuate ridge, but elsewhere the caldera rim is buried by post-caldera volcanoes, including Gunung Merapi, which forms the high point of the complex. Immediately west of the Gunung Merapi stratovolcano is the historically active Kawah Ijen crater, which contains a nearly 1-km-wide, turquoise-colored, acid lake. Picturesque Kawah Ijen is the world's largest highly acidic lake and is the site of a labor-intensive sulfur mining operation in which sulfur-laden baskets are hand-carried from the crater floor. Many other post-caldera cones and craters are located within the caldera or along its rim. The largest concentration of cones forms an E-W zone across the southern side of the caldera. Coffee plantations cover much of the caldera floor, and tourists are drawn to its waterfalls, hot springs, and volcanic scenery.

Information Contacts: Dali Ahmad, Volcanological Survey of Indonesia (VSI) (URL: http://www.vsi.esdm.go.id/).

During January-May 2002, seismic activity at Kerinci was dominated by small explosion earthquakes. Plumes reached up to 600 m above the summit (table 2). An explosion during 0950-1030 on 4 May produced ash that rose 400 m above the summit. The Alert Level remained at 2 throughout the report period.

Table 2. Seismicity and plume observations at Kerinci during 7 January through 19 May 2002. Courtesy VSI.

Geologic Background. Gunung Kerinci in central Sumatra forms Indonesia's highest volcano and is one of the most active in Sumatra. It is capped by an unvegetated young summit cone that was constructed NE of an older crater remnant. There is a deep 600-m-wide summit crater often partially filled by a small crater lake that lies on the NE crater floor, opposite the SW-rim summit. The massive 13 x 25 km wide volcano towers 2400-3300 m above surrounding plains and is elongated in a N-S direction. Frequently active, Kerinci has been the source of numerous moderate explosive eruptions since its first recorded eruption in 1838.

Information Contacts: Dali Ahmad, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

An eruption at Lokon on 9 February, triggered by extensive rainfall, sent ash plumes to 1 km and deposited ash in surrounding villages. Activity then decreased significantly and remained low through February 2002 (BGVN 27:02). During February through at least April, Tompaluan crater emitted plumes 50-350 m above the crater rim.

During early April deep and shallow volcanic earthquakes increased (table 2). Eruptions on 10 and 12 April ejected glowing material from the crater. A thick white-gray ash plume rose 1 km above the crater rim. During 13-14 April gas/ash explosions occurred nearly continuously, with eight explosions on 13 April and five on the 14th. Ash explosions rose 50-75 m above the crater rim. Tremor amplitude increased from 0.5-2 mm on 11 April to 4-48 mm by 14 April. The Volcanological Survey of Indonesia (VSI) raised the Alert Level to 3 on 12 April. A total of 25 and 68 small explosions per week were registered during 22-28 April and 29 April-5 May, respectively. During the following weeks the number of small explosions dropped to only 6 per week. As of 26 May, tremor fluctuated (0.5-30 mm amplitude) and gas explosions continued.

Table 2. Earthquakes recorded at Lokon during 11 February through 26 May 2002. Courtesy VSI.

Geologic Background. The twin volcanoes Lokon and Empung, rising about 800 m above the plain of Tondano, are among the most active volcanoes of Sulawesi. Lokon, the higher of the two peaks (whose summits are only 2 km apart), has a flat, craterless top. The morphologically younger Empung volcano to the NE has a 400-m-wide, 150-m-deep crater that erupted last in the 18th century, but all subsequent eruptions have originated from Tompaluan, a 150 x 250 m wide double crater situated in the saddle between the two peaks. Historical eruptions have primarily produced small-to-moderate ash plumes that have occasionally damaged croplands and houses, but lava-dome growth and pyroclastic flows have also occurred. A ridge extending WNW from Lokon includes Tatawiran and Tetempangan peak, 3 km away.

Information Contacts: Dali Ahmad, Volcanological Survey of Indonesia (VSI) (URL: http://www.vsi.esdm.go.id/).

Eruptions at Mayon in June and July 2001 were followed by a decrease in seismic activity beginning on 10 August. Low-frequency volcanic earthquakes and SO₂ fluxes were still high and were probably related to shallow magma degassing. While various monitoring parameters continued to reflect significant unrest, the general trend was one of declining activity (BGVN 26:08).

Volcanic activity remained low during August. There was relatively little seismicity, slight inflation, occasional observations of incandescence at the summit, and a moderate amount of steam emission. SO₂ flux remained well above the baseline of 500 metric tons per day (t/d) (table 7). SO₂ emission rates reflected continued degassing of cooling magma, and ground-deformation data continued to indicate the absence of magma intrusion. On 21 August the Alert Level was lowered to 3 and, following a continued decrease in activity, on 19 October it was lowered to 1.

Table 7. Earthquakes, tremor, and SO₂ flux at Mayon during 13-30 August. Differences in reported daily and weekly data during 20-26 August could not be resolved by press time. Courtesy PHIVOLCS.

News reports on 21 November stated that lahars were generated after several days of heavy rainfall mixed with unconsolidated material on the volcano's slopes. According to the civil defense, flooding caused more than 4,800 families to be evacuated from their homes.

The Philippine Institute of Volcanology and Seismology (PHIVOLCS) reported that since 19 October 2001, when the Alert Level was lowered to 1, all measured parameters had continued to decrease to near-baseline levels. Ground deformation data from electronic tiltmeters continued to indicate the volcano's deflated condition, and SO₂ emission rates yielded relatively low values of 450-900 t/d. The observations implied that no active magma intrusion was occurring beneath the active cone. Although incandescence was still visible at night, PHIVOLCS suggested that it was likely due to still-hot magma beneath the crater. As a result of the low activity, on 5 February PHIVOLCS lowered the Alert Level to 0, but reminded the public to avoid the 6 km Permanent Danger Zone, and residents near major river channels emanating from the volcano were advised to be on alert during heavy rainfall because loose pyroclastic deposits could be remobilized as life-threatening stream flows and lahars.

Geologic Background. Beautifully symmetrical Mayon, which rises above the Albay Gulf NW of Legazpi City, is the Philippines' most active volcano. The structurally simple edifice has steep upper slopes averaging 35-40 degrees that are capped by a small summit crater. Historical eruptions date back to 1616 and range from Strombolian to basaltic Plinian, with cyclical activity beginning with basaltic eruptions, followed by longer term andesitic lava flows. Eruptions occur predominately from the central conduit and have also produced lava flows that travel far down the flanks. Pyroclastic flows and mudflows have commonly swept down many of the approximately 40 ravines that radiate from the summit and have often devastated populated lowland areas. A violent eruption in 1814 killed more than 1,200 people and devastated several towns.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), C.P. Garcia Ave., Univ. Philippines Campus, U.P. Diliman, 1101 Quezon City; Associated Press.

The following was extracted from the 8 March final report of the French-British Scientific Team on the January 2002 Nyiragongo eruption (Allard and others, 2002). On 22 January the team, comprised of Patrick Allard, Peter Baxter, Michel Halbwachs, and Jean-Christophe Komorowski, joined local scientists of the Goma Volcano Observatory (GVO), and the UN-OCHA team (Jacques Durieux, Paolo Papale, Dario Tedesco, and Orlando Vaselli) in Goma.

Precursory signals. The January 2002 eruption of Nyiragongo volcano was heralded by precursory phenomena detected since March 2001 by volcanologists of the GVO. Anomalous seismicity occurred. It included both type-C long-period (LP) events and tremor, which persisted after the February-March 2001 eruption of Nyamuragira (BGVN 26:03), 15 km NW of Nyiragongo, and had increased gently over the rest of the year. LP events and volcanic tremor were mainly registered at the Bulengo seismic station (15 km W of Goma) and were minimal, or absent, at the more remote (40 km) Katale station, located closer to Nyamuragira (Akumbi Mbiligi, GVO, pers. comm.). This observation supported the idea of seismo-magmatic processes occurring at, or closer to, Nyiragongo. This was later confirmed by the registration of earthquake swarms (presumed fracturing events) in the Nyiragongo area: first in October 2001 and then on 4 January 2002, 13 days prior to the eruption's onset. The 4 January earthquakes were accompanied by a darkened plume and rumbling sounds at the summit of Nyiragongo (Akumbi and Kasareka, GVO, pers. comm.).

A fracture from the 1977 eruption runs above Shaheru crater (2,700 m elevation and ~2 km S of the summit, figure 15). A fumarolic vent formed at ~2,800 m elevation along this fracture in October 2001. New cracks and increased fumarolic activity were also detected on the southern inner wall of the summit crater, upslope of Shaheru crater. In November 2001, new fumaroles appeared on the N floor of Shaheru crater itself.

Figure 15. Map showing the eruptive chronology, the lava flow field, and phenomena associated with the 17-18 January 2002 eruption of Nyiragongo (geologic base map taken from Thonnard and others, 1965). The map compiles observations of the French-British scientific team, together with the UN volcano surveillance team, the Goma Volcano Observatory, Minerena (Rwanda), UN-OCHA mapping, and includes contributions by D. Garcin and collaborators from the UN, and observers in Goma (subsidence and eyewitness data). Information was preliminary as of 9 February 2002 and subject to change. Courtesy of the French-British team.

An increase in seismicity during 4-17 January included several felt earthquakes and volcanic tremor. On 16 January, a few hours before the eruption onset, an abnormally strong smell of sulfur dioxide was also noticed by the pilot of a small private aircraft flying N of Nyiragongo (Ted Hoaru, pers. comm.).

Chronology of the eruption. According to GVO, Nyiragongo started erupting at 0825 on 17 January. Earthquakes caused the 1977 fracture system running from 2,800 m elevation into Shaheru crater to open and drain the lava stored in the summit crater. The height and energy of the discharging lava during this initial phase is demonstrated by lava boulders that were perched 6-8 m high in trees at distances up to 30 m from the eruptive fracture above Shaheru. Very fluid lava flows, only 10-15 cm thick at their source, moved across the forested SE slopes of Nyiragongo and rapidly cut the road going N from Goma. The outpouring lava left high-stand marks on trees up to a height of 1.5 m upslope of, and within, Shaheru. The 800-m-wide Shaheru crater was filled with a 3-m-thick lava pond.

Two sets of parallel eruptive fractures, ~300 m apart, further propagated through the S flank of Shaheru cone and extended downslope forming a series of grabens (~5-10 m wide) cutting across banana groves, villages, and older volcanic cones. Between 1000 and 1100, lava flows issued from a series of eruptive vents at ~2,300-1,800 m elevation along this system (figure 16), devastating several villages. Between 1400 and 1620 fractures approached the outskirts of Goma and began to form a line of vents SE of Monigi village only 1.5 km NE of Goma airport (see figure 15, including points labeled 1610 and 1620). These lowest fractures produced intense spattering. This led to the voluminous lava flow that ran through the airport and the heart of Goma, finally entering Lake Kivu during the night. Other eruptive vents that opened higher on the volcano produced voluminous lava flows that also reached Goma. Most of these flows were of aa-type, less fluid, black, and 1-3 m thick. Visible fracturing occurred simultaneously with the onset of lava effusions.

Figure 16. Aerial view at Nyiragongo after the January 2002 eruption, showing part of the lava flow field S of Lemera hill and N of Mugara hill. Notice the system of parallel fractures that runs from N (bottom of photo) to S (top of photo), a fissure-vent system that in this instance produced very fluid, pahoehoe lava flows (under 1 m thick). Photo by Jean-Christophe Komorowski. Courtesy of the French-British team.

Another eruptive fissure opened at 1530 at 2,250 m elevation (figure 15) (2 km W of Kibati). Eyewitnesses reported that this western fissure initially produced passively effusive activity feeding pahoehoe lava flows. However, the presence of a scoria-fall deposit extending over 500 m around the vent indicated at least momentary lava fountaining. Lava flows there were voluminous, aa-type, and 1-2 m thick, that cascaded down a significant sector of the volcano (figure 17). These fed a flow advancing towards Monigi and formed the second main flow that reached Goma on the W, stopping a few kilometers from Lake Kivu.

Figure 17. Detail of the large pahoehoe and aa lava flows emitted by Nyiragongo during January 2002 from the W vent, which fed a large flow that reached Goma but not Lake Kivu. The two types of lavas were emitted simultaneously and did not exceed 2 m thick. Photo by Jean-Christophe Komorowski. Courtesy of the French-British team.

From helicopter and ground-based studies of the lava flows the team estimated a total erupted volume of between 20 and 30 x 10⁶ m³, including the lava that flowed into Lake Kivu. First analyses of bulk rock samples (table 3) revealed that lavas erupted from the highest and lowest fractures had very similar compositions, implying their derivation from a single magma batch. These otherwise degassed lavas still contained very high bulk amounts of S, F, and Cl, with slightly higher contents in the products of spattering activity along the Monigi fracture zone. Moreover, the 2002 Nyiragongo lavas are similar to the leucite-bearing nephelinite lavas produced during the 1977 eruption.

Table 3. Chemical analyses of lavas from the January 2002 and January 1977 Nyiragongo eruptions. Analysis at CRPG, CNRS, Vandoeuvre-Les-Nancy, France. All values in wt % (P. Allard, unpublished data, 2002). Courtesy of the French-British team.

The UN reported 147 deaths (of whom 60-100 died in an explosion of the Goma central petrol station on 21 January), 30,000 people displaced, and 14,000 homes destroyed by the eruption. Around 470 injured people reportedly suffered burns, fractures, and gas intoxication. However, Peter Baxter reviewed health aspects of the eruption during a visit to Goma in March, and found no evidence for a large number of people injured or killed. He places the number of deaths at about 70, of which 20 occurred as a result of the petrol station explosion; only a few burn injuries needed hospital treatment, and none of those were serious.

As many as 350,000 people fled from the advancing lava, principally towards nearby Rwanda to the E. After two days the majority returned to Goma, despite hazards from hot lava and burning materials. Despite the lack of observers on the scene at the time, it seems that lava emission stopped during the early morning of 18 January, indicating that the entire flank eruption lasted ~24 hours. However, molten lava continued to flow in tunnels and tubes along the main flow that had reached Lake Kivu and spilled into it for a few more days. This created a new fan-shaped lava delta ~800 m across at its widest point along the previous shoreline. Lava flows destroyed part of the airport and Goma's business and commercial center.

Crater collapse and explosive activity. According to Jacques Durieux (UN-OCHA), the solidified lava floor of Nyiragongo summit crater, lying at 320 m below the rim since 1996, was still in place on 21 January, three days after the end of the eruption, but was cut by a N-S steaming graben. It is most likely that this chilled crater floor, although thick enough to initially resist falling, had been weakened by the lava drainage during 17-18 January. Its broad-scale collapse occurred during the night of 22-23 January. A detailed report by eyewitnesses in Rusaya (8 km SW of the summit) indicated that collapse started at 2051 on 22 January, coinciding with a series of felt earthquakes. It was accompanied by roaring sounds and glow above the crater and followed soon after by hot ashfall over Rusaya, that reportedly formed a layer 10 cm thick. GVO registered intense and continuous seismic tremor over the next four hours. Light ashfall also took place over Goma and Gisenyi that night. A helicopter flight on 24 January allowed the team to observe the ash cover on the forested SW flank. They found Nyiragongo's new crater floor ~700 m (instead of 320 m) below the rim, with a blocky and fuming narrow bottom partly covered by remnants of the former crater floor.

Changes in crater morphology correspond to an estimated bulk volume of ~30 x 10⁶ m³ removed during previous lava drainage and subsequent (unquantified but likely secondary) ash emission. This figure compares well with the bulk volume of lava flows, suggesting that these mainly derived from the lava stored in the crater and upper conduit of the volcano. This conclusion is consistent with the identical composition of bulk lava flow samples from the upper and lower fissure vents (table 3).

Intermittent phreatomagmatic explosive activity inside Nyiragongo crater persisted after the collapse. At 0910 on 24 January a dense cloud was visible above the volcano. On 27 January fresh impacts and fresh tree-destructions were discovered in the forest on the upper N flank. Phreatomagmatic activity in the crater was observed directly on 3 February by GVO volcanologist M. Kasareka who had climbed to the summit.

Fracture system. A large fracture system cut the volcano over an elevation range of 1,100 m and extendeed 20 km from N to S, reaching to within 1 km of Goma (figure 15). In some places along the fractures eruptive vents and phreatic (explosion-caused) craters formed. Field observations, combined with eyewitness accounts confirmed the opening of fractures and emission of lava flows either simultaneously or in close succession during the eruption. The overall fissure propagation velocity averaged 2 km/hour. However, massive post-eruptive fracturing was also observed in some places, correlated with intense post-eruptive seismicity. Two weeks after the eruption strong steaming persisted in several sections of the fracture system.

The system of fractures was spectacularly developed in the Monigi area (1,700 m elevation) where it consists of a down-dropped zone ~25-50 m wide with up to 20 m of vertical downward displacement along vertical walls that extend across the topography for ~2 km (figures 18-20). Several fractures opened 1-3 m and ran parallel on either side of the main fault system; they extended out to a distance of 100-300 m from the axis. The fault system passed through several villages (Kasenyi, Buganra). Continuous steaming (60-80°C) was occurring along the faults. Locally, steam vents formed craters 10-15 m deep.

Figure 18. Fracture and fault system developed at Nyiragongo on 17 January 2002 in Monigi village. Continuing, strongly felt, post-eruptive seismicity further opened the fractures. Some openings in fissures reached up to 2 m wide and 5-10 m deep. Photo by Jean-Christophe Komorowski. Courtesy French-British team.

Figure 19. Fracture system N of Monigi village at Nyiragongo following the January 2002 eruption. The area between the fractures had dropped by ~ 2 m to form a graben (note leaning trees). Steam vented locally from deep pits (5-10 m). Earth cracks parallel to the main depression extend out to ~ 20-30 m. Photo by Jean-Christophe Komorowski. Courtesy French-British team.

Figure 20. A displaced mud-brick house located on the main fracture in Monigi village following the January 2002 eruption of Nyiragongo. The fracture here behaved as a normal fault, with vertical displacement of ~ 0.5 m. Photo by Jean-Christophe Komorowski. Courtesy French-British team.

A 50- to 80-cm-wide fracture at Monigi village also channeled lava to the surface where it formed a thin chilled margin (figure 21). Withdrawal of magma during the fracture's southward propagation, as confirmed by eyewitness accounts, left a drained lava tube. In a few locations lava spatter was ejected up to 15 m away from the fracture indicating short-lived gas-rich lava venting.

Figure 21. The 17-18 January 2002 eruption of Nyiragongo produced this fissure or dike, found near the village of Monigi (figure 15). The dike is ~ 0.6-0.8 m wide and contains a glassy outer envelope of chilled lava (a shell somewhat like a small lava tube). The still-fluid portion of lava drained away southward through the dike conduit towards Goma. Photo by Jean-Christophe Komorowski. Courtesy of the French-British team.

Fracturing occurred over a short time between 1000 and 1300 from N to S, cutting through thick scoria-cone deposits (figure 22) as well as lava flows several meters thick. Fractures left openings 5-10 m deep. The system transected the W flanks of the Mubara cinder cone, where fractures spread over an area of 100-200 m forming several sub-parallel strands with 0.2-3 m of vertical displacement. This area could present future slope stability problems.

Figure 22. Photo following the January 2002 Nyiragongo eruption of the central depression in the Monigi fracture-graben system through old scoria fall deposits from Mugara cinder cone located just N. Width is about 25 m and depth 10-15 m. Local steaming indicated that a dike was near the surface and was involved in the formation of this feature. Photo by Jean-Christophe Komorowski. Courtesy French-British team.

Seismicity. Intense felt seismic activity occurred during but mainly after the eruption, including tectonic earthquakes M 3.5 or larger. The number of earthquakes gradually declined with time but has remained abnormally high. As of early March 2002, earthquakes were still felt intermittently.

The seismic network that operated during the eruption and up until 30 January did not allow an accurate assessment of the location and depth of earthquakes. However, the short time intervals between the arrival of P and S waves as measured on seismograms indicated local sources. The persistence of numerous LP events and sequences of tremor after the eruption raised concerns about the possibility of continuing magma intrusions and phreato-magmatic eruptions inside the summit crater. This intense post-eruptive seismicity, combined with widespread ground subsidence in the Kivu rift (BGVN 27:03), as well as the synchronism of the eruption with 20-km-long fractures and the broadly consistent volumes of bulk lava flows and summit crater collapse, led the team to propose that the 2002 Nyiragongo eruption was most likely triggered by tectonic spreading of the Kivu rift.

Gas emanations. During and after the eruption people in Goma confronted a variety of gas emissions. Abnormal odors of hydrocarbons were reported in many parts of the city, prompting the use of a portable infrared spectrometer allowing in-situ gas analysis. The team found that the smells were due to hydrocarbon-bearing methane- and CO₂-rich gas emanations from the ground, which occurred in areas separated by 300-800 m from the lava flows and which, therefore, had no relationship with organic matter fired or heated by the flows. These emanations, with methane concentrations of a few percent and sometimes approaching the 5% flammability threshold in air were found both outdoors diffusing up through pavements along streets, in gardens, and in buildings. At a school, methane measured under 1%. Near a drain system for rainwater ~200 m from a lava flow's edge methane was found in the air along the ground but at less than 1%. However, at a nearby concrete roof over a drain the methane content was 2%, together with 2% CO₂.

A long fissure passed under a church in the center of Goma. CO₂ emissions caused two women cleaning the church to faint. According to GVO, similar fractures are scattered throughout the area. Heat from engulfing lava flows led to the combustion of both plants and a wide variety of dissimilar materials (houses, cars, petrol tanks, etc.).

Flames of burning gas and vegetation were observed and analyzed in different parts of the flows, both inside and outside the city. On 23 January the team measured a temperature of 500°C for blue flames burning on a still-hot lava flow. The air in cracks near the flames contained about 2% methane, the smell of which was readily detectable in the area. According to witnesses, on the previous day these flames had been orange in color and 1.5 m high, suggesting that the fire was originally caused by the burning of organic matter inside the flow and that the flames resulted from the combustion of distillates of vegetation. Slow combustion of vegetation and organic matter was widespread after the eruption in all the areas affected by Nyiragongo lava flows.

Numerous gas bursts were reported to have occurred during-but mostly after-the eruption, principally during 20-22 January when the most intense seismicity occurred. No one was injured by the explosions. Eyewitnesses to these events saw that the gas bursts shortly followed strongly felt earthquakes and were accompanied by strong smells of hydrocarbon gas. In several places 300-400 m distant from lava flows, these gas bursts ripped through cement and stone pavement in Goma's houses and streets. Gas concentrations stood at 5% CO₂ and 3-4% methane in one case, and at 1% CO₂ and 2.6% methane in an office. Not far away in a garage a 21 January explosion had blown apart a concrete floor 10 cm thick. But when visited 4 days later, a measurable gas anomaly was absent.

Most of the gas bursts occurred at places or in areas that are broadly aligned with the N-S fracture system cutting the volcano and where ground gas emanations were persisting. Although these explosions occurred at the time of felt earthquakes, the associated seismically induced ground movement was not severe enough to have been responsible for the observed localized type of damages. The strong gas smells and the elevated methane concentrations were taken as evidence of a methane-driven origin for the explosions. Sub-surface methane concentrations must have been locally high enough to allow spontaneous ignition of the methane upon contact with oxygen during and following seismic loading. Further study will be necessary to elucidate the origin of that methane. The team emphasized that methane is weakly abundant in permanent gas vents (locally called "mazukus") that occur in the area, emanating through old lava flows, such as those to the W of Goma (CO₂: 93.2 %; methane, CH4: 0.07% by volume).

The team witnessed a small methane burst on 27 January while inspecting ground fractures in Monigi that displayed persistent incandescence and very high temperatures (970°C on 24 January). These sites are located in the middle of a small village and constitute a major attraction for cooking and for children who play nearby. The fracture, through which no lava had erupted, was formed parallel to the main eruptive fractures but there crosses through thick old lava flows. The team inferred that incandescence was caused by the presence at depth of relict heat from the magma body (dike) that fed the nearby lava flows that covered Goma (within 1 km). The gas burst occurred at ~2 m from the site where maximum incandescence had persisted for a week and where scientists were measuring temperatures and collecting gases. Most likely, the scientific fieldwork brought air in contact with a pocket of methane, which then spontaneously burst. A few fist-sized blocks of old lava were popped up to a distance under 1 m, without causing any injuries to the numerous bystanders. At another site, minor bursts occured every few minutes as wind blew through the fractures.

In contrast, minor explosions of phreatic origin also occasionally occurred in different places. For example, at the lava delta, when molten lava entered Lake Kivu, and at Goma when bulldozing the lava flows suddenly depressurized steam produced by the high temperature of lava flows along the ground.

Gas hazard of Lake Kivu. Lake Kivu (485 m deep) is known to contain an immense amount of both CO₂ (1,000 times that in Lake Nyos, Cameroon) and methane stored in solution in its waters. In the case of a major disturbance of the density stratification of gas-charged water in this lake, a huge gas release could occur. Concerns about such a hazard were raised when the lava flowed into the lake, together with the opening of new fractures, strong seismicity, and the poorly understood possibility of an underwater extension of the eruption.

A variety of manifestations were observed at the surface of the lake after the eruption. During 20-21 January, coincident with felt earthquakes, the lake water was seen uprising along the shore 9 km to the W of Goma and, in three separate areas, the water became dark and warm, with gas bubbles and an associated odor (hydrogen sulfide). Many dead fish were seen in and around these areas. Similar phenomena were reported along other sectors of the lake's shore. Additionally, yellow flames were reported to have been seen on occasion at the surface of Lake Kivu well away from the lava flows, suggesting methane burning. Unpleasant odors and experiences were reported by swimmers in Lake Kivu before the eruption, again ascribed to gas emissions. These reports need to be followed up by a survey of gas concentrations at the lake surface.

The hazard of lava flows entering and disturbing the lake waters has not been extensively studied previously. The hot lava could disturb the lake stability by starting lake-water convection. This might trigger a gas burst resulting in a lethal cloud of CO₂ and methane flowing over an unknown area around the lake. In order to assess the problem, Halbwachs organized underwater investigations, first with the help of scuba divers from UN-OCHA and, in a second stage (7-10 February), using a submersible sent from France with the support of EC-ECHO.

Local divers reported the presence of hot water (40-60°C) surrounding the lava delta. Gas bubbling could be observed locally, but its limited extent suggested that neither the gases, nor the solidified lava presented a risk for the local water supplies. In contrast, potential hazard from submarine lava tubes required investigations at greater depth. Despite the poor visibility due to abundant particles in suspension, the surveys with the submersible revealed that the lava flow and tubes had descended to ~80 m depth in the lake by the shore at Goma. Fortunately, such a depth is much smaller than the critical depths of 200-300 m at which Lake Kivu's waters contain more abundant dissolved carbon dioxide, closer to the saturation limit.

In order to evaluate the influence of the hot mass of lava that entered into Lake Kivu on its physico-chemical stratification, during early February a series of samples were collected at varying depths, and 40 vertical lake soundings were undertaken. In collaboration with Halbwachs, these measurements were performed by two limnologists: Klaus Tietze (PDT GmbH, Celle, Germany) and Andreas Lorke (EAWAG Laboratory, Lucerne, Switzerland). They measured depth, temperature, pH, electrical conductivity, turbidity (transparency to white light), and dissolved-oxygen content. Preliminary results suggested a change in the water stratification since the last measurements by Klaus Tietze 20 years ago. A new homogenous water layer was found at depths between 200 and 250 m. Near the lava delta the temperature and turbidity profiles showed some perturbations between 50 and 120 m depth. Away from the delta, a thin (3 m) layer of slightly warmer water lay at ~80 m depth. The turbidity was rather low close to the lava flows but increased rapidly away from it. More synthesis and analytical work continues in order to assess fundamental questions on the stability of Lake Kivu stratification.

References. Allard, P., Baxter, P., Halbwachs, M., and Komorowski, J-C, 2002, Final report of the French-British scientific team: submitted to the Ministry for Foreign Affairs, Paris, France, Foreign Office, London, United Kingdom and respective Embassies in Democratic Republic of Congo and Republic of Rwanda, 24 p.

Thonnard, R.L.G., Denaeyer, M-E., and Antun, P., 1965, Carte volcanologique des Virunga (1/50000), Afrique Central, Feuile No. 1: Centre National de Volcanologie (Belgique), Missions Gèologiques et Gèophysiques aux Virunga, Ministère de L'Education et de la Culture, Bruxelles, Belgium.

Geologic Background. One of Africa's most notable volcanoes, Nyiragongo contained a lava lake in its deep summit crater that was active for half a century before draining catastrophically through its outer flanks in 1977. The steep slopes of a stratovolcano contrast to the low profile of its neighboring shield volcano, Nyamuragira. Benches in the steep-walled, 1.2-km-wide summit crater mark levels of former lava lakes, which have been observed since the late-19th century. Two older stratovolcanoes, Baruta and Shaheru, are partially overlapped by Nyiragongo on the north and south. About 100 parasitic cones are located primarily along radial fissures south of Shaheru, east of the summit, and along a NE-SW zone extending as far as Lake Kivu. Many cones are buried by voluminous lava flows that extend long distances down the flanks, which is characterized by the eruption of foiditic rocks. The extremely fluid 1977 lava flows caused many fatalities, as did lava flows that inundated portions of the major city of Goma in January 2002.

Information Contacts: Patrick Allard, Laboratoire Pierre Süe, CNRS-CEA, Saclay, France; Peter Baxter, University of Cambridge, United Kingdom; Michel Halbwachs, Université de Savoie, Chambéry, France; Jean-Christophe Komorowski, Institut de Physique du Globe de Paris, France.

Instituto Nicaragüense de Estudios Territoriales (INETER) reported that during December 2001 through May 2002 San Cristóbal maintained generally constant levels of seismicity and moderate tremor levels. Thousands of earthquakes per month were recorded, most with frequencies of 4.0 to over 10 Hz. Very few events registered with frequencies less than 1.0 Hz.

The third eruptive stage in 2001 was during 7-25 November, when strong ash-and-gas emissions and rumblings occurred and small amounts of ash fell in surrounding areas. After visiting the crater on 11 and 25 November, and 9 December, Vicente Perez (INETER) reported rockfalls and strong emissions of gas and ash. Fumarolic temperatures on 25 November were ~40-100°C, and were similar during December. On 15 January Pedro Perez observed only sporadic gas emanations during a crater visit.

During November through 23 February seismic tremor generally remained between 20 and 60 RSAM units, with the maximum tremor occurring during 8-14 November, when ash-and-gas emissions were strongest. Tremor frequency was 4.0-6.0 Hz.

Observations on 6 February revealed an overall lack of visible changes at the volcano with the exception of gas emanations in the new crater. On 24 February seismic tremor began to increase until it reached 40 RSAM units. While the tremor increased, the number of earthquakes diminished. Strong rumblings on 22 and 26 February, coinciding with the increase of tremor on 24 February, were accompanied by gas emissions.

Another increase in tremor began on the afternoon of 6 March. Strong seismicity occurred in 2- to 3-hour periods that were generally separated by less than 1 hour of less intense activity. INETER reported that seismic tremor reached more than 50 RSAM units on 7 March. Scientists visiting the volcano found that the amount and temperature of degassing had increased. Reportedly, incandescent material in the crater was reflected on the clouds above it. On 22 March at 2219 an earthquake was felt by most of the population near the volcano. Following this event, more than twelve earthquakes with magnitudes of 2.0-3.2 occurred. According to INETER, associated activity was not strong enough to warrant raising the Alert Level.

During April an average of 30 earthquakes occurred per hour (figure 10), most associated with degassing. Very few events were volcano-tectonic or explosion earthquakes. Seismic tremor remained between 40 and 45 RSAM units.

Figure 10. Seismic amplitude RSAM (top) and number of earthquakes per hour (bottom) at San Cristóbal during April 2002. Courtesy INETER.

On 23 May a strong gas column was observed at San Cristóbal. The Washington Volcanic Ash Advisory Center (VAAC) stated that a surface report had indicated strong activity near the summit. A plume was visible on satellite imagery drifting SW from the summit (figure 11). A video camera near the summit indicated that the altitude of the plume was relatively low, near ~3 km. The Washingon VAAC issued a second notice stating that according to INETER, the emissions consisted solely of gas. The VAAC noted that no plume was detected in satellite imagery later that day. INETER reported that the column was the result of rain in the crater that generated steam. No other phenomena were observed that could indicate an increase in the eruptive activity of the volcano.

Figure 11. Sketch based on satellite imagery depicting a plume drifting SW from San Cristóbal on 23 May 2002. Courtesy NOAA.

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: Virginia Tenorio, Department of Geophysics, Instituto Nicaragüense de Estudios Territoriales (INETER), P.O. Box 1761, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); La Noticia (URL: http://www.lanoticia.com.ni/); El Nuevo Diario (URL: http://www.elnuevodiario.com.ni/); La Prensa (URL: http://www.laprensa.com.ni/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch, NOAA/NESDIS/E/SP23, NOAA Science Center Room 401, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/).

During mid-August 2001 through February 2002 at a new lava dome continued to grow at Soufriere Hills. Small-scale dome collapses generated pyroclastic flows almost continuously, with some reaching and entering the sea on several occasions. Dense ash plumes associated with sea entry and ash venting from the summit generally drifted W and reached up to 3 km altitude. Mudflows occurred in the Belham Valley on several days during periods of torrential rainfall (BGVN 27:01). The lava dome continued to grow during February through at least mid-May 2002. Minor episodes of ash venting occurred from the summit of the dome, and at times incandescence was visible. The dome produced numerous rockfalls and small pyroclastic flows in the upper reaches of the Tar River Valley. SO₂ flux rates reached up to 1,200 metric tons per day (table 40).

Table 40. Seismic and SO2-flux data from Soufriere Hills during 1 February-10 May 2002. Courtesy of MVO.

During flights on 4, 5, and 6 February new pyroclastic-flow deposits were observed in the Tar River to the E (with some flows reaching the sea) and in the White River to the S, derived from the collapse of remnant talus material from the pre-29 July 2001 dome (BGVN 26:07). An observation flight on 14 February revealed minor rockfalls of old, inactive dome material in the upper part of the Gages region. Near-continuous rockfalls and minor pyroclastic flows occurred on the E flank. Minor rockfalls on the N flank of the active dome cascaded between the NE and central buttresses of the older inactive dome.

Activity increased beginning on the evening of 8 March. Small ash clouds (reaching ~2.1 km) arising from small collapses drifted to the W over the Plymouth and Richmond Hill area, although most of the ash fallout occurred over the sea. For a couple days during late March weak winds dispersed the ash towards the NW and N, depositing it over the main populated areas. Large spines on the dome during mid-March periodically collapsed, producing pyroclastic flows down the E flank, some of which reached the Tar River Fan. By late March minor amounts of rockfall debris from the NE flank of the dome had begun to spill into the head of Tuit's Ghaut. Ash venting appeared to have been from a pit-like depression on the summit of the dome.

Increased rockfall and pyroclastic-flow activity over the E flank of the dome coincided with periods of tremor during late April. Small, low-level ash clouds were occasionally visible on satellite imagery. Rockfalls traveled down the SE flank of the dome almost continuously. By early May rockfall talus had begun to spill over the rim of the 29 July 2001 collapse-scar in the extreme SE at the foot of Roches Mountain. Pyroclastic flows on the mornings of 1 and 2 May were the most energetic seismic events recorded for over a month. Activity increased beginning on 8 May, and rockfalls and pyroclastic flows were concentrated on the dome's NE flank.

MVO reported that weather permitting, the daytime entry zone (DTEZ) would remain open. The observatory warned that activity could increase quite suddenly, with a dangerous situation developing in the DTEZ very quickly, and that ash masks should be worn in ashy conditions. The Belham Valley was to be avoided during and after heavy rainfall due to the possibility of mudflows. Access to Plymouth, Bramble airport, and beyond was prohibited. In addition, a maritime exclusion zone around the S part of the island extends two miles beyond the coastline from Trant's Bay in the E to Garibaldi Hill on the W coast.

Seismicity and SO₂ flux. Since 4 February SO₂ measurements were carried out using a remote, telemetered Differential Optical Absorption Spectrometer (DOAS) that scans through the plume, yielding over 600 measurements of SO₂ emission rates per day. The highest SO₂ fluxes were measured after pyroclastic flows. SO₂ emission rates decreased dramatically during early March (table 40).

A swarm of hybrid earthquakes on 22 April was followed by increased numbers of long-period events and a surge in the number of rockfalls over the next four days. Banded tremor also followed the swarm. Weak periods of tremor occurred approximately every 20 hours during 26 April-3 May, and each lasted a few hours. Fluctuations in SO₂ emission rates in late April appeared to reflect variations in the intensity of rockfall activity.

Morphology of the lava dome. During early February the lava dome continued to grow primarily on the E and NE sides, and by late February growth was focused on the E side. The summit of the dome was blocky and massive, in contrast to the spines of previous weeks. On 19 February the dome was crowned by a large spine inclined steeply up towards the SE. The spine changed in size and shape, as it periodically collapsed or disintegrated and grew again as fresh material was extruded. On 26 February the spine had a height of 90 m above the general level of the summit area. At this stage the top of the spine had an elevation of 1,080 m, the highest point measured during the eruption to date.

Observations in early March revealed that the summit of the dome had a generally spiny appearance and on several occasions was crowned by a large spine directed upwards at a high angle towards the E. During mid-March the summit of the dome was dominated by fast-growing large spines (50-70 m high). Theodolite measurements of the dome taken on 20 March yielded a dome height of 1,039 m.

During mid-April, dome growth shifted to the SE area of the dome complex, although small rockfalls occurred in other areas. The summit area had evolved from a large striated lobe to a series of small spines. By late April the lobe on the SE portion of the dome had reached 1,041 m elevation and the NE lobe, which had been highly active during the previous two weeks, stagnated at a height of 1,020 m elevation. Lava dome growth continued on the E side of the dome complex during early May.

The closest GPS station to the dome showed sustained outward movement of ~0.5 cm per month. During periods of dome building, slow subsidence took place at the closest sites at Hermitage, Whites, and Harris. Since January, the EDM reflector on the N flank showed a 5-cm movement away from the lava dome.

Hazard assessment. On 11 March 2002 the Montserrat Volcano Observatory (MVO) issued the following preliminary statement concerning the history and hazard assessment of the current eruption: "The Soufrière Hills Volcano continues its second phase of sustained dome growth, which began in November 1999. Since September 2001, the dome has grown at an average rate of about 2 m³/s (or 400,000 metric tons per day). The summit region of the dome has now reached an altitude of ~990 m, having filled most of the depression formed by the large dome collapse of 29 July 2001. The dome has mainly grown towards the E, although there was a period during late November and early December 2001 when growth was directed W.

"During [September 2001 to March 2002] there have been fluctuations in activity as recorded in seismicity and gas emissions. Pyroclastic flows and almost continuous rockfalls have occurred, mostly directed down the Tar River Valley. For prolonged periods in the last six months, there have been cyclical patterns of enhanced seismicity lasting for a few hours to about a day, during which rockfall and pyroclastic-flow activity has been more intense.

"Continued growth of the dome over this period has meant that hazard levels close to the volcano have increased slightly compared with . . . September 2001. Risk levels will fluctuate as the configuration of the dome changes. In an extreme scenario, a switch in the direction of growth to the N or NW could result in more hazardous conditions along the margins of the Exclusion Zone. Consequently, increased levels of risk might develop in the populated areas bordering the Belham River. Across the remainder of the island, however, it is considered that the general level of risk to the population from volcanic activity is unchanged.

"The main hazards remain pyroclastic flows, explosions, falls of ash and small stones, and volcanic mudflows. The increasing knowledge of the volcano acquired by the experienced observatory staff allows patterns of eruption behavior to be recognized and some forms of activity to be anticipated. During a large dome collapse or explosion, heavy ashfall and the fall of small rock fragments can be expected in the populated areas if the wind is in an unfavorable direction. However, a detailed study of the hazard due to fall of rock fragments has recently been completed, and this indicates that outside the Exclusion Zone significant falls of rock fragments large enough to cause serious injury are unlikely.

"At the moment there is no sign of the volcanic activity diminishing. It is most likely that the eruption will continue for a number of years, although the volcano may be evolving into a persistently active state with the eruption continuing for even longer periods, either continuously or intermittently."

General References. Baker, P.E., 1985, Volcanic hazards on St. Kitts and Montserrat, West Indies: Journal of the Geological Society, London, v. 142, p. 279-295.

Shepherd, J.B, Tomblin, J.F., and Woo, D.A., 1971, Volcano-seismic crisis in Montserrat, West Indies, 1966-67: Bulletin of Volcanology, v. 35, p. 143-163.

Wadge, G., and Isaacs, M.C., 1988, Mapping the volcanic hazards from Soufriere Hills volcano, Montserrat, West Indies using an image processor: Journal of the Geological Society, London, v. 145, p. 541-551.

Geologic Background. The complex, dominantly andesitic Soufrière Hills volcano occupies the southern half of the island of Montserrat. The summit area consists primarily of a series of lava domes emplaced along an ESE-trending zone. The volcano is flanked by Pleistocene complexes to the north and south. English's Crater, a 1-km-wide crater breached widely to the east by edifice collapse, was formed about 2000 years ago as a result of the youngest of several collapse events producing submarine debris-avalanche deposits. Block-and-ash flow and surge deposits associated with dome growth predominate in flank deposits, including those from an eruption that likely preceded the 1632 CE settlement of the island, allowing cultivation on recently devegetated land to near the summit. Non-eruptive seismic swarms occurred at 30-year intervals in the 20th century, but no historical eruptions were recorded until 1995. Long-term small-to-moderate ash eruptions beginning in that year were later accompanied by lava-dome growth and pyroclastic flows that forced evacuation of the southern half of the island and ultimately destroyed the capital city of Plymouth, causing major social and economic disruption.

Information Contacts: Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.mvo.ms/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/).