Sangay is one of the most active volcanoes in Ecuador with the current eruptive period continuing since 26 March 2019. Activity at the summit crater has been frequent since August 1934, with short quiet periods between events. Recent activity has included frequent ash plumes, lava flows, pyroclastic flows, and lahars. This report summarizes activity during July through December 2020, based on reports by Ecuador's Instituto Geofísico, Escuela Politécnica Nacional (IG-EPN), ash advisories issued by the Washington Volcanic Ash Advisory Center (VAAC), webcam images taken by Servicio Integrado de Seguridad ECU911, and various satellite data.

Overall activity remained elevated during the report period. Recorded explosions were variable during July through December, ranging from no explosions to 294 reported on 4 December (figure 80), and dispersing mostly to the W and SW. SO₂ was frequently detected using satellite data (figure 81) and was reported several times to be emitting between about 770 and 2,850 tons/day. Elevated temperatures at the crater and down the SE flank were frequently observed in satellite data (figure 82), and less frequently by visual observation of incandescence. Seismic monitoring detected lahars associated with rainfall events remobilizing deposits emplaced on the flanks throughout this period.

Figure 80. A graph showing the daily number of explosions at Sangay recorded during July through December 2020. Several dates had no recorded explosions due to lack of seismic data. Data courtesy of IG-EPN (daily reports).

Figure 81. Examples of stronger SO2 plumes from Sangay detected by the Sentinel 5P/TROPOMI instrument, with plumes from Nevado del Ruiz detected to the north. The image dates from left to right are 31 August 2020, 17 September 2020, 1 October 2020 (top row), 22 November 2020, 3 December 2020, 14 December 2020 (bottom row). Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Figure 82. This log radiative power MIROVA plot shows thermal output at Sangay during February through December 2020. Activity was relatively constant with increases and decreases in both energy output and the frequency of thermal anomalies detected. Courtesy of MIROVA.

Activity during July-August 2020. During July activity continued with frequent ash and gas emission recorded through observations when clouds weren’t obstructing the view of the summit, and Washington VAAC alerts. There were between one and five VAAC alerts issued most days, with ash plumes reaching 570 to 1,770 m above the crater and dispersing mostly W and SE, and NW on two days (figure 83). Lahar seismic signals were recorded on the 1st, 7th, three on the 13th, and one on the 19th.

Figure 83. Gas and ash plumes at Sangay during July 2020, at 0717 on the 17th, at 1754 on the 18th, and at 0612 on the 25th. Bottom picture taken from the Macas ECU 911 webcam. All images courtesy of IG-EPN daily reports.

During August there were between one and five VAAC alerts issued most days, with ash plumes reaching 600 to 2,070 m above the crater and predominantly dispersing W, SW, and occasionally to the NE, S, and SE (figure 84). There were reports of ashfall in the Alausí sector on the 24th. Using seismic data analysis, lahar signals were identified after rainfall on 1, 7, 11-14, and 21 August. A lava flow was seen moving down the eastern flank on the night of the 15th, resulting in a high number of thermal alerts. A pyroclastic flow was reported descending the SE flank at 0631 on the 27th (figure 85).

Figure 84. This 25 August 2020 PlanetScope satellite image of Sangay in Ecuador shows an example of a weak gas and ash plume dispersing to the SW. Courtesy of Planet Labs.

Figure 85. A pyroclastic flow descends the Sangay SE flank at 0631 on 27 August 2020. Webcam by ECU911, courtesy of courtesy of IG-EPN (27 August 2020 report).

Activity during September-October 2020. Elevated activity continued through September with two significant increases on the 20th and 22nd (more information on these events below). Other than these two events, VAAC reports of ash plumes varied between 1 and 5 issued most days, with plume heights reaching between 600 and 1,500 m above the crater. Dominant ash dispersal directions were W, with some plumes traveling SE, S, SE, NE, and NW. Lahar seismic signals were recorded after rainfall on 1, 2, 5, 8-10, 21, 24, 25, 27, and 30 September. Pyroclastic flows were reported on the 19th (figure 86), and incandescent material was seen descending the SE ravine on the 29th. There was a significant increase in thermal alerts reported throughout the month compared to the July-August period, and Sentinel-2 thermal satellite images showed a lava flow down the SE flank (figure 87).

Figure 86. Pyroclastic flows descended the flank of Sangay on 19 (top) and 20 (bottom) September 2020. Webcam images by ECU911 from the city of Macas, courtesy of IG-EPN (14 August 2018 report).

Figure 87. The thermal signature of a lava flow is seen on SW flank of Sangay in this 8 September 2020 Sentinel-2 thermal satellite image, indicated by the white arrow. False color (urban) satellite image (bands 12, 11, 4) courtesy of Sentinel Hub Playground.

Starting at 0420 on the morning of 20 September there was an increase in explosions and emissions recorded through seismicity, much more energetic than the activity of previous months. At 0440 satellite images show an ash plume with an estimated height of around 7 km above the crater. The top part of the plume dispersed to the E and the rest of the plume went W. Pyroclastic flows were observed descending the SE flank around 1822 (figure 88). Ash from remobilization of deposits was reported on the 21st in the Bolívar, Chimborazo, Los Ríos, Guayas and Santa Elena provinces. Ash and gas emission continued, with plumes reaching up to 1 km above the crater. There were seven VAAC reports as well as thermal alerts issued during the day.

Figure 88. An eruption of Sangay on 22 September 2020 produced a pyroclastic flow down the SE flank and an ash plume that dispersed to the SW. PlanetScope satellite image courtesy of Planet Labs.

Ash plumes observed on 22 September reached around 1 km above the crater and dispersed W to NW. Pyroclastic flows were seen descending the SE flank (figure 89) also producing an ash plume. A BBC article reported the government saying 800 km₂ of farmland had experienced ashfall, with Chimborazo and Bolívar being the worst affected areas (figure 90). Locals described the sky going dark, and the Guayaquil was temporarily closed. Ash plume heights during the 20-22 were the highest for the year so far (figure 91). Ash emission continued throughout the rest of the month with another increase in explosions on the 27th, producing observed ash plume heights reaching 1.5 km above the crater. Ashfall was reported in San Nicolas in the Chimborazo Province in the afternoon of the 30th.

Figure 89. A pyroclastic flow descending the flank of Sangay on 22 September 2020. Webcam image by ECU911 from the city of Macas, courtesy of IG-EPN (Sangay Volcano Special Report - 2020 - No 5, 22 September 2020).

Figure 90. Ashfall from an eruption at Sangay on 22 September 2020 affected 800 km2 of farmland and nearby communities. Images courtesy of EPA and the Police of Ecuador via Reuters (top-right), all via the BBC.

Figure 91. Ash plume heights (left graph) at Sangay from January through to late September, with the larger ash plumes during 20-22 September indicated by the red arrow. The dominant ash dispersal direction is to the W (right plot) and the average speed is 10 m/s. Courtesy of IG-EPN (Sangay Volcano Special Report - 2020 - No 5, 22 September 2020).

Thermal alerts increased again through October, with a lava flow and/or incandescent material descending the SE flank sighted throughout the month (figure 92). Pyroclastic flows were seen traveling down the SE flank during an observation flight on the 6th (figure 93). Seismicity indicative of lahars was reported on 1, 12, 17, 19, 21, 23, 24, and 28 October associated with rainfall remobilizing deposits. The Washington VAAC released one to five ash advisories most days, noting plume heights of 570-3,000 m above the crater; prevailing winds dispersed most plumes to the W, with some plumes drifting NW, N, E to SE, and SW. Ashfall was reported in Alausí (Chimborazo Province) on the 1st and in Chunchi canton on the 10th. SO₂ was recorded towards the end of the month using satellite data, varying between about 770 and 2,850 tons on the 24th, 27th, and 29th.

Figure 92. A lava flow descends the SE flank of Sangay on 2 October 2020. Webcam images courtesy of ECU 911.

Figure 93. A pyroclastic flow descends the Sangay SE flank was seen during an IG-EPN overflight on 6 October 2020. Photo courtesy of S. Vallejo, IG-EPN.

Activity during November-December 2020. Frequent ash emission continued through November with between one and five Washington VAAC advisories issued most days (figure 94). Reported ash and gas plume heights varied between 570 and 2,700 m above the crater, with winds dispersing plumes in all directions. Thermal anomalies were detected most days, and incandescent material from explosions was seen on the 26th. Seismicity indicating lahars was registered on nine days between 15 and 30 November, associated with rainfall events.

Figure 94. Examples of gas and ash plumes at Sangay during November 2020. Webcam images were published in IG-EPN daily activity reports.

Lahar signals associated with rain events continued to be detected on ten out of the first 18 days of November. Ash emissions continued through December with one to five VAAC alerts issued most days. Ash plume heights varied from 600 to 1,400 m above the crater, with the prevailing wind direction dispersing most plumes W and SW (figure 95). Thermal anomalies were frequently detected and incandescent material was observed down the SE flank on the 3rd, 14th, and 30th, interpreted as a lava flow and hot material rolling down the flank. A webcam image showed a pyroclastic flow traveling down the SE flank on the 2nd (figure 96). Ashfall was reported on the 10th in Capzol, Palmira, and Cebadas parishes, and in the Chunchi and Guamote cantons.

Figure 95. Examples of ash plumes at Sangay during ongoing persistent activity on 9, 10, and 23 December 2020. Webcam images courtesy of ECU 911.

Figure 96. A nighttime webcam image shows a pyroclastic flow descending the SE flank of Sangay at 2308 on 2 December 2020. Image courtesy of ECU 911.

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

Information Contacts: Instituto Geofísico (IG-EPN), Escuela Politécnica Nacional, Casilla 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec); ECU911, Servicio Integrado de Seguridad ECU911, Calle Julio Endara s / n. Itchimbía Park Sector Quito – Ecuador. (URL: https://www.ecu911.gob.ec/; Twitter URL: https://twitter.com/Ecu911Macas/); 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/); 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/); Planet Labs, Inc. (URL: https://www.planet.com/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); BBC News “In pictures: Ash covers Ecuador farming land” Published 22 September 2020 (URL: https://www.bbc.com/news/world-latin-america-54247797).

Volcanism at Ebeko, located on the N end of the Paramushir Island in the Kuril Islands, has been ongoing since October 2016, characterized by frequent moderate explosions, ash plumes, and ashfall in Severo-Kurilsk (7 km ESE) (BGVN 45:05). Similar activity during this reporting period of June through November 2020 continues, consisting of frequent explosions, dense ash plumes, and occasional ashfall. Information for this report primarily comes from the Kamchatka Volcanic Eruptions Response Team (KVERT) and satellite data.

Activity during June was characterized by frequent, almost daily explosions and ash plumes that rose to 1.6-4.6 km altitude and drifted in various directions, according to KVERT reports and information from the Tokyo VAAC advisories using HIMAWARI-8 satellite imagery and KBGS (Kamchatka Branch of the Geophysical Service) seismic data. Satellite imagery showed persistent thermal anomalies over the summit crater. On 1 June explosions generated an ash plume up to 4.5 km altitude drifting E and S, in addition to several smaller ash plumes that rose to 2.3-3 km altitude drifting E, NW, and NE, according to KVERT VONA notices. Explosions on 11 June generated an ash plume that rose 2.6 km altitude and drifted as far as 85 km N and NW. Explosions continued during 21-30 June, producing ash plumes that rose 2-4 km altitude, drifting up to 5 km in different directions (figure 26); many of these eruptive events were accompanied by thermal anomalies that were observed in satellite imagery.

Figure 26. Photo of a dense gray ash plume rising from Ebeko on 22 June 2020. Photo by L. Kotenko (color corrected), courtesy of IVS FEB RAS, KVERT.

Explosions continued in July, producing ash plumes rising 2-5.2 km altitude and drifting for 3-30 km in different directions. On 3, 6, 15 July explosions generated an ash plume that rose 3-4 km altitude that drifted N, NE, and SE, resulting in ashfall in Severo-Kurilsk. According to a Tokyo VAAC advisory, an eruption on 4 July produced an ash plume that rose up to 5.2 km altitude drifting S. On 22 July explosions produced an ash cloud measuring 11 x 13 km in size and that rose to 3 km altitude drifting 30 km SE. Frequent thermal anomalies were identified in satellite imagery accompanying these explosions.

In August, explosions persisted with ash plumes rising 1.7-4 km altitude drifting for 3-10 km in multiple directions. Intermittent thermal anomalies were detected in satellite imagery, according to KVERT. On 9 and 22 August explosions sent ash up to 2.5-3 km altitude drifting W, S, E, and SE, resulting in ashfall in Severo-Kurilsk. Moderate gas-and-steam activity was reported occasionally during the month.

Almost daily explosions in September generated dense ash plumes that rose 1.5-4.3 km altitude and drifted 3-5 km in different directions. Moderate gas-and-steam emissions were often accompanied by thermal anomalies visible in satellite imagery. During 14-15 September explosions sent ash plumes up to 2.5-3 km altitude drifting SE and NE, resulting in ashfall in Severo-Kurilsk. On 22 September a dense gray ash plume rose to 3 km altitude and drifted S. The ash plume on 26 September was at 3.5 km altitude and drifted SE (figure 27).

Figure 27. Photos of dense ash plumes rising from Ebeko on 22 (left) and 26 (right) September 2020. Photos by S. Lakomov (color corrected), IVS FEB RAS, KVERT.

During October, near-daily ash explosions continued, rising 1.7-4 km altitude drifting in many directions. Intermittent thermal anomalies were identified in satellite imagery. During 7-8, 9-10, and 20-22 October ashfall was reported in Severo-Kurilsk.

Explosions in November produced dense gray ash plumes that rose to 1.5-5.2 km altitude and drifted as far as 5-10 km, mainly NE, SE, E, SW, and ENE. According to KVERT, thermal anomalies were visible in satellite imagery throughout the month. On clear weather days on 8 and 11 November Sentinel-2 satellite imagery showed ashfall deposits SE of the summit crater from recent activity (figure 28). During 15-17 November explosions sent ash up to 3.5 km altitude drifting NE, E, and SE which resulted in ashfall in Severo-Kurilsk on 17 November. Similar ashfall was observed on 22-24 and 28 November due to ash rising to 1.8-3 km altitude (figure 29). Explosions on 29 November sent an ash plume up to 4.5 km altitude drifting E (figure 29). A Tokyo VAAC advisory reported that an ash plume drifting SSE on 30 November reached an altitude of 3-5.2 km.

Figure 28. Sentinel-2 satellite imagery of a gray-white gas-and-ash plume at Ebeko on 8 (left) and 11 (right) November 2020, resulting in ashfall (dark gray) to the SE of the volcano. Images using “Natural Color” rendering (bands 4, 3, 2), courtesy of Sentinel Hub Playground.

Figure 29. Photos of continued ash explosions from Ebeko on 28 October (left) and 29 November (right) 2020. Photos by S. Lakomov (left) and L. Kotenko (right), courtesy of IVS FEB RAS, KVERT.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows a pulse in low-power thermal activity beginning in early June through early August (figure 30). On clear weather days, the thermal anomalies in the summit crater are observed in Sentinel-2 thermal satellite imagery, accompanied by occasional white-gray ash plumes (figure 31). Additionally, the MODVOLC algorithm detected a single thermal anomaly on 26 June.

Figure 30. A small pulse in thermal activity at Ebeko began in early June and continued through early August 2020, according to the MIROVA graph (Log Radiative Power). The detected thermal anomalies were of relatively low power but were persistent during this period. Courtesy of MIROVA.

Figure 31. Sentinel-2 satellite imagery showed gray ash plumes rising from Ebeko on 11 June (top left) and 16 July (bottom left) 2020, accompanied by occasional thermal anomalies (yellow-orange) within the summit crater, as shown on 24 June (top right) and 25 August (bottom right). The ash plume on 11 June drifted N from the summit. Images using “Natural Color” rendering (bands 4, 3, 2) on 11 June (top left) and 16 July (bottom left) and the rest have “Atmospheric penetration” rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Geologic Background. The flat-topped summit of the central cone of Ebeko volcano, one of the most active in the Kuril Islands, occupies the northern end of Paramushir Island. Three summit craters located along a SSW-NNE line form Ebeko volcano proper, at the northern end of a complex of five volcanic cones. Blocky lava flows extend west from Ebeko and SE from the neighboring Nezametnyi cone. The eastern part of the southern crater contains strong solfataras and a large boiling spring. The central crater is filled by a lake about 20 m deep whose shores are lined with steaming solfataras; the northern crater lies across a narrow, low barrier from the central crater and contains a small, cold crescentic lake. Historical activity, recorded since the late-18th century, has been restricted to small-to-moderate explosive eruptions from the summit craters. Intense fumarolic activity occurs in the summit craters, on the outer flanks of the cone, and in lateral explosion craters.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); Kamchatka Branch of the Geophysical Service, Russian Academy of Sciences (KB GS RAS) (URL: http://www.emsd.ru/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Kuchinoerabujima encompasses a group of young stratovolcanoes located in the northern Ryukyu Islands. All historical eruptions have originated from the Shindake cone, with the exception of a lava flow that originated from the S flank of the Furudake cone. The current eruptive period began in January 2020 and has been characterized by small explosions, ash plumes, ashfall, a pyroclastic flow, and gas-and-steam emissions. This report covers activity from May to October 2020, which includes small explosions, ash plumes, crater incandescence, and gas-and-steam emissions. The primary source of information for this report comes from monthly and annual reports from the Japan Meteorological Agency (JMA) and advisories from the Tokyo Volcanic Ash Advisory Center (VAAC).

Volcanism at Kuchinoerabujima remained relatively low during May through October 2020, according to JMA. During this time, SO2 emissions ranged from 40 to 3,400 tons/day; occasional gas-and-steam emissions were reported, rising to a maximum of 900 m above the crater. Sentinel-2 satellite images showed a particularly strong thermal anomaly in the Shindake crater on 1 May (figure 10). The thermal anomaly decreased in power after 1 May and was only visible on clear weather days, which included 19 August and 3 and 13 October. Global Navigation Satellite System (GNSS) observations identified continued slight inflation at the base of the volcano during the entire reporting period.

Figure 10. Sentinel-2 thermal satellite images showed a strong thermal anomaly (bright yellow-orange) in the Shindake crater at Kuchinoerabujima on 1 May 2020 (top left). Weaker thermal anomalies were also seen in the Shindake crater during 19 August (top right) and 3 (bottom left) and 13 (bottom right) October 2020. Sentinel-2 atmospheric penetration (bands 12, 11, 8A) images; courtesy of Sentinel Hub Playground.

Three small eruptions were detected by JMA on 5, 6, and 13 May, which produced an ash plume rising 500 m above the crater on each day, resulting in ashfall on the downwind flanks. Incandescence was observed at night using a high-sensitivity surveillance camera (figure 11). On 5 and 13 May the Tokyo VAAC released a notice that reported ash plumes rising 0.9-1.2 km altitude, drifting NE and S, respectively. On 20 May weak fumaroles were observed on the W side of the Shindake crater. The SO2 emissions ranged from 700-3,400 tons/day.

Figure 11. Webcam images of an eruption at Kuchinoerabujima on 6 May 2020 (top), producing a gray ash plume that rose 500 m above the crater. Crater incandescence was observed from the summit crater at night on 25 May 2020 (bottom). Courtesy of JMA (Monthly bulletin report 509, May 2020).

Activity during June and July decreased compared to May, with gas-and-steam emissions occurring more prominently. On 22 June weak incandescence was observed, accompanied by white gas-and-steam emissions rising 700 m above the crater. Weak crater incandescence was also seen on 25 June. The SO2 emissions measured 400-1,400 tons/day. White gas-and-steam emissions were again observed on 31 July rising to 800 m above the crater. The SO2 emissions had decreased during this time to 300-700 tons/day.

According to JMA, the most recent eruptive event occurred on 29 August at 1746, which ejected bombs and was accompanied by some crater incandescence, though the eruptive column was not visible due to the cloud cover. However, white gas-and-steam emissions could be seen rising 1.3 km above the Shindake crater drifting SW. The SO2 emissions measured 200-500 tons/day. During August, the number of volcanic earthquakes increased significantly to 1,032, compared to the number in July (36).

The monthly bulletin for September reported white gas-and-steam emissions rising 900 m above the crater on 9 September and on 11 October the gas-and-steam emissions rose 600 m above the crater. Seismicity decreased between September and October from 1,920 to 866. The SO2 emissions continued to decrease compared to previous months, totaling 80-400 tons/day in September and 40-300 tons/day in October.

Geologic Background. A group of young stratovolcanoes forms the eastern end of the irregularly shaped island of Kuchinoerabujima in the northern Ryukyu Islands, 15 km W of Yakushima. The Furudake, Shindake, and Noikeyama cones were erupted from south to north, respectively, forming a composite cone with multiple craters. All historical eruptions have occurred from Shindake, although a lava flow from the S flank of Furudake that reached the coast has a very fresh morphology. Frequent explosive eruptions have taken place from Shindake since 1840; the largest of these was in December 1933. Several villages on the 4 x 12 km island are located within a few kilometers of the active crater and have suffered damage from eruptions.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Nyamuragira (also known as Nyamulagira) is a shield volcano in the Democratic Republic of the Congo with a 2 x 2.3 km caldera at the summit. A summit crater lies in the NE part of the caldera. In the recent past, the volcano has been characterized by intra-caldera lava flows, lava emissions from its lava lake, thermal anomalies, gas-and-steam emissions, and moderate seismicity (BGVN 44:12, 45:06). This report reviews activity during June-November 2020, based on satellite data.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed numerous thermal anomalies associated with the volcano during June-November 2020, although some decrease was noted during the last half of August and between mid-October to mid-November (figure 91). Between six and seven thermal hotspots per month were identified by MODVOLC during June-November 2020, with as many as 4 pixels on 11 August. In the MODVOLC system, two main hotspot groupings are visible, the largest being at the summit crater, on the E side of the caldera.

Figure 91. MIROVA graph of thermal activity (log radiative power) at Nyamuragira during March 2020-January 2021. During June-November 2020, most were in the low to moderate range, with a decrease in power during November. Courtesy of MIROVA.

Sentinel-2 satellite images showed several hotspots in the summit crater throughout the reporting period (figure 92). By 26 July and thereafter, hotspots were also visible in the SW portion of the caldera, and perhaps just outside the SW caldera rim. Gas-and-steam emissions from the lava lake were also visible throughout the period.

Figure 92. Sentinel-2 satellite images of Nyamuragira on 26 July (left) and 28 November (right) 2020. Thermal activity is present at several locations within the summit crater (upper right of each image) and in the SW part of the caldera (lower left). SWIR rendering (bands 12, 8A, 4). Courtesy of Sentinel Hub Playground.

Geologic Background. Africa's most active volcano, Nyamuragira, is a massive high-potassium basaltic shield about 25 km N of Lake Kivu. Also known as Nyamulagira, it has generated extensive lava flows that cover 1500 km2 of the western branch of the East African Rift. The broad low-angle shield volcano contrasts dramatically with the adjacent steep-sided Nyiragongo to the SW. The summit is truncated by a small 2 x 2.3 km caldera that has walls up to about 100 m high. Historical eruptions have occurred within the summit caldera, as well as from the numerous fissures and cinder cones on the flanks. A lava lake in the summit crater, active since at least 1921, drained in 1938, at the time of a major flank eruption. Historical lava flows extend down the flanks more than 30 km from the summit, reaching as far as Lake Kivu.

Information Contacts: Observatoire Volcanologique de Goma (OVG), Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo; MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/exp).

A massive stratovolcano in easternmost Java, Raung has over sixty recorded eruptions dating back to the late 16th Century. Explosions with ash plumes, Strombolian activity, and lava flows from a cinder cone within the 2-km-wide summit crater have been the most common activity. Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM) has installed webcams to monitor activity in recent years. An eruption from late 2014 through August 2015 produced a large volume of lava within the summit crater and formed a new pyroclastic cone in the same location as the previous one. The eruption that began in July 2020 is covered in this report with information provided by PVMBG, the Darwin Volcanic Ash Advisory Center (VAAC), and several sources of satellite data.

The 2015 eruption was the largest in several decades; Strombolian activity was reported for many months and fresh lava flows covered the crater floor (BGVN 45:09). Raung was quiet after the eruption ended in August of that year until July of 2020 when seismicity increased on 13 July and brown emissions were first reported on 16 July. Tens of explosions with ash emissions were reported daily during the remainder of July 2020. Explosive activity decreased during August, but thermal activity didn’t decrease until mid-September. The last ash emissions were reported on 3 October and the last thermal anomaly in satellite data was recorded on 7 October 2020.

Eruption during July-October 2020. No further reports of activity were issued after August 2015 until July 2020. Clear Google Earth imagery from October 2017 and April 2018 indicated the extent of the lava from the 2015 eruption, but no sign of further activity (figure 31). By August 2019, many features from the 2015 eruption were still clearly visible from the crater rim (figure 32).

Figure 31. Little change can be seen at the summit of Raung in Google Earth images dated 19 October 2017 (left) and 28 April 2018 (right). The summit crater was full of black lava flows from the 2015 eruption. Courtesy of Google Earth.

Figure 32. A Malaysian hiker celebrated his climbing to the summit of Raung on 30 August 2019. Weak fumarolic activity was visible from the base of the breached crater of the cone near the center of the summit crater, and many features of the lava flow that filled the crater in 2015 were still well preserved. Courtesy of MJ.

PVMBG reported that the number and type of seismic events around the summit of Raung increased beginning on 13 July 2020, and on 16 July the height of the emissions from the crater rose to 100 m and the emission color changed from white to brown. About three hours later the emissions changed to gray and white. The webcams captured emissions rising 50-200 m above the summit that included 60 explosions of gray and reddish ash plumes (figure 33). The Raung Volcano Observatory released a VONA reporting an explosion with an ash plume that drifted N at 1353 local time (0653 UTC). The best estimate of the ash cloud height was 3,432 m based on ground observation. They raised the Aviation Color Code from unassigned to Orange. About 90 minutes later they reported a second seismic event and ash cloud that rose to 3,532 m, again based on ground observation. The Darwin VAAC reported that neither ash plume was visible in satellite imagery. The following day, on 17 July, PVMBG reported 26 explosions between midnight and 0600 that produced brown ash plumes which rose 200 m above the crater. Based on these events, PVMBG raised the Alert Level of Raung from I (Normal) to II (Alert) on a I-II-III-IV scale. By the following day they reported 95 explosive seismic events had occurred. They continued to observe gray ash plumes rising 100-200 m above the summit on clear days and 10-30 daily explosive seismic events through the end of July; plume heights dropped to 50-100 m and the number of explosive events dropped below ten per day during the last few days of the month.

Figure 33. An ash plume rose from the summit of Raung on 16 July 2020 at the beginning of a new eruption. The last previous eruption was in 2015. Courtesy of Volcano Discovery and PVMBG.

After a long period of no activity, MIROVA data showed an abrupt return to thermal activity on 16 July 2020; a strong pulse of heat lasted into early August before diminishing (figure 34). MODVOLC thermal alert data recorded two alerts each on 18 and 20 July, and one each on 21 and 30 July. Satellite images showed no evidence of thermal activity inside the summit crater from September 2015 through early July 2020. Sentinel-2 satellite imagery first indicated a strong thermal anomaly inside the pyroclastic cone within the crater on 19 July 2020; it remained on 24 and 29 July (figure 35). A small SO₂ signature was measured by the TROPOMI instrument on the Sentinel-5P satellite on 25 July.

Figure 34. MIROVA thermal anomaly data indicated renewed activity on 16 July 2020 at Raung as seen in this graph of activity from 13 October 2019 through September 2020. Satellite images indicated that the dark lines at the beginning of the graph are from a large area of fires that burned on the flank of Raung in October 2019. Heat flow remained high through July and began to diminish in mid-August 2020. Courtesy of MIROVA.

Figure 35. Thermal anomalies were distinct inside the crater of the pyroclastic cone within the summit crater of Raung on 19, 24, and 29 July 2020. Data is from the Sentinel-2 satellite shown with Atmospheric penetration rendering (bands 12, 11, 8A). Courtesy of Sentinel Hub Playground.

After an explosion on 1 August 2020 emissions from the crater were not observed again until steam plumes were seen rising 100 m on 7 August. They were reported rising 100-200 m above the summit intermittently until a dense gray ash plume was reported by PVMBG on 11 August rising 200 m. After that, diffuse steam plumes no more than 100 m high were reported for the rest of the month except for white to brown emissions to 100 m on 21 August. Thermal anomalies of a similar brightness to July from the same point within the summit crater were recorded in satellite imagery on 3, 8, 13, 18, and 23 August. Single MODVOLC thermal alerts were reported on 1, 8, 12, and 19 August.

In early September dense steam plumes rose 200 m above the crater a few times but were mostly 50 m high or less. White and gray emissions rose 50-300 m above the summit on 15, 20, 27, and 30 September. Thermal anomalies were still present in the same spot in Sentinel-2 satellite imagery on 2, 7, 12, 17, and 27 September, although the signal was weaker than during July and August (figure 36). PVMBG reported gray emissions rising 100-300 m above the summit on 1 October 2020 and two seismic explosion events. Gray emissions rose 50-200 m the next day and nine explosions were recorded. On 3 October, emissions were still gray but only rose 50 m above the crater and no explosions were reported. No emissions were observed from the summit crater for the remainder of the month. Sentinel-2 satellite imagery showed a hot spot within the summit crater on 2 and 7 October, but clear views of the crater on 12, 17, and 22 October showed no heat source within the crater (figure 37).

Figure 36. The thermal anomaly at Raung recorded in Sentinel-2 satellite data decreased in intensity between August and October 2020. It was relatively strong on 13 August (left) but had decreased significantly by 12 September (middle) and remained at a lower level into early October (right). Data shown with Atmospheric penetration rendering (bands 12, 11, 8A). Courtesy of Sentinel Hub Playground

Figure 37. A small but distinct thermal anomaly was still present within the pyroclastic cone inside the summit crater of Raung on 7 October 2020 (left) but was gone by 12 October (middle) and did not reappear in subsequent clear views of the crater through the end of October. Satellite imagery of 7 and 12 October processed with Atmospheric penetration rendering (bands 12, 11, 8A). Natural color rendering (bands 4, 3, 2) from 17 October (right) shows no clear physical changes to the summit crater during the latest eruption. Courtesy of Sentinel Hub Playground.

Geologic Background. Raung, one of Java's most active volcanoes, is a massive stratovolcano in easternmost Java that was constructed SW of the rim of Ijen caldera. The unvegetated summit is truncated by a dramatic steep-walled, 2-km-wide caldera that has been the site of frequent historical eruptions. A prehistoric collapse of Gunung Gadung on the W flank produced a large debris avalanche that traveled 79 km, reaching nearly to the Indian Ocean. Raung contains several centers constructed along a NE-SW line, with Gunung Suket and Gunung Gadung stratovolcanoes being located to the NE and W, respectively.

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/); 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/); 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/); 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/); Google Earth (URL: https://www.google.com/earth/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Tom Pfeiffer, Volcano Discovery (URL: http://www.volcanodiscovery.com/); MJ (URL: https://twitter.com/MieJamaludin/status/1167613617191043072).

Indonesia’s Sinabung volcano in north Sumatra has been highly active since its first confirmed Holocene eruption during August and September 2010. It remained quiet after the initial eruption until September 2013, when a new eruptive phase began that continued through June 2018. A summit dome emerged in late 2013 and produced a large lava “tongue” during 2014. Multiple explosions produced ash plumes, block avalanches, and deadly pyroclastic flows during the eruptive period. A major explosion in February 2018 destroyed most of the summit dome. After a pause in eruptive activity from September 2018 through April 2019, explosions resumed during May and June 2019. The volcano was quiet again until an explosion on 8 August 2020 began another eruption that included a new dome. This report covers activity from July 2019 through October 2020 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, the Darwin Volcanic Ash Advisory Centre (VAAC), and the Badan Nacional Penanggulangan Bencana (National Disaster Management Authority, BNPB). Additional information comes from satellite instruments and local news reports.

Only steam plumes and infrequent lahars were reported at Sinabung during July 2019-July 2020. A new eruption began on 8 August 2020 with a phreatic explosion and dense ash plumes. Repeated explosions were reported throughout August; ashfall was reported in many nearby communities several times. Explosions decreased significantly during September, but SO₂ emissions persisted. Block avalanches from a new growing dome were first reported in early October; pyroclastic flows accompanied repeated ash emissions during the last week of the month. Thermal data suggested that the summit dome continued growing slowly during October.

Activity during July 2019-October 2020. After a large explosion on 9 June 2019, activity declined significantly, and no further emissions or incandescence was reported after 25 June (BGVN 44:08). For the remainder of 2019 steam plumes rose 50-400 m above the summit on most days, occasionally rising to 500-700 m above the crater. Lahars were recorded by seismic instruments in July, August, September, and December. During January-July 2020 steam plumes were reported usually 50-300 m above the summit, sometimes rising to 500 m. On 21 March 2020 steam plumes rose to 700 m, and a lahar was recorded by seismic instruments. Lahars were reported on 26 and 28 April, 3 and 5 June, and 11 July.

A swarm of deep volcanic earthquakes was reported by PVMBG on 7 August 2020. This was followed by a phreatic explosion with a dense gray to black ash plume on 8 August that rose 2,000 m above the summit and drifted E; a second explosion that day produced a plume that rose 1,000 m above the summit. According to the Jakarta Post, ash reached the community of Berastagi (15 km E) along with the districts of Naman Teran (5-10 km NE), Merdeka (15 km NE), and Dolat Rayat (20 km E). Continuous tremor events were first recorded on 8 August and continued daily until 26 August. Two explosions were recorded on 10 August; the largest produced a dense gray ash plume that rose 5,000 m above the summit and drifted NE and SE (figure 77). The Darwin VAAC reported the eruption clearly visible in satellite imagery at 9.7 km altitude and drifting W. Later they reported a second plume drifting ESE at 4.3 km altitude. After this large explosion the local National Disaster Management Authority (BNPB) reported significant ashfall in three districts: Naman Teran, Berastagi and Merdeka. Emissions on 11 and 12 August were white and gray and rose 100-200 m. Multiple explosions on 13 August produced white and gray ash plumes that rose 1,000-2,000 m above the summit. Explosions on 14 August produced gray and brown ash plumes that rose 1,000-4,200 m above the summit and drifted S and SW (figure 77). The Darwin VAAC reported that the ash plume was partly visible in satellite imagery at 7.6 km altitude moving W; additional plumes were moving SE at 3.7 km altitude and NE at 5.5 km altitude.

Figure 77. Numerous explosions were recorded at Sinabung during August 2020. An ash plume rose to 5,000 m above the summit on 10 August (left) and drifted both NE and SE. On 14 August gray and brown ash plumes rose 1,000-4,200 m above the summit and drifted S, SW, SE and NE (right) while ashfall covered crops SE of the volcano. Courtesy of PVMBG (Sinabung Eruption Notices, 10 and 14 August 2020).

White, gray, and brown emissions rose 800-1,000 m above the summit on 15 and 17 August. The next day white and gray emissions rose 2,000 m above the summit. The Darwin VAAC reported an ash plume visible at 5.2 km altitude drifting SW. A large explosion on 19 August produced a dense gray ash plume that rose 4,000 above the summit and drifted S and SW. Gray and white emissions rose 500 m on 20 August. Two explosions were recorded seismically on 21 August, but rainy and cloudy weather prevented observations. White steam plumes rose 300 m on 22 August, and a lahar was recorded seismically. On 23 August, an explosion produced a gray ash plume that rose 1,500 m above the summit and pyroclastic flows that traveled 1,000 m down the E and SE flanks (figure 78). Continuous tremors were accompanied by ash emissions. White, gray, and brown emissions rose 600 m on 24 August. An explosion on 25 August produced an ash plume that rose 800 m above the peak and drifted W and NW (figure 79). During 26-30 August steam emissions rose 100-400 m above the summit and no explosions were recorded. Dense gray ash emissions rose 1,000 m and drifted E and NE after an explosion on 31 August. Significant SO₂ emissions were associated with many of the explosions during August (figure 80).

Figure 78. On 23 August 2020 an explosion at Sinabung produced a gray ash plume that rose 1,500 m above the summit and produced pyroclastic flows that traveled 1,000 m down the E and SE flanks. Courtesy of PVMBG (Sinabung Eruption Notice, 23 August 2020).

Figure 79. An explosion on 25 August 2020 at Sinabung produced an ash plume that rose 800 m above the peak and drifted W and NW. Courtesy of PVMBG (Sinabung Eruption Notice, 25 August 2020).

Figure 80. Significant sulfur dioxide emissions were measured at Sinabung during August 2020 when near-daily explosions produced abundant ash emissions. A small plume was also recorded from Kerinci on 19 August 2020. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Explosive activity decreased substantially during September 2020. A single explosion reported on 5 September produced a white and brown ash plume that rose 800 m above the summit and drifted NNE. During the rest of the month steam emissions rose 50-500 m above the summit before dissipating. Two lahars were reported on 7 September, and one each on 11 and 30 September. Although only a single explosion was reported, anomalous SO₂ emissions were present in satellite data on several days.

The character of the activity changed during October 2020. Steam plumes rising 50-300 m above the summit were reported during the first week and a lahar was recorded by seismometers on 4 October. The first block avalanches from a new dome growing at the summit were reported on 8 October with material traveling 300 m ESE from the summit (figure 81). During 11-13 October block avalanches traveled 300-700 m E and SE from the summit. They traveled 100-150 m on 14 October. Steam plumes rising 50-500 m above the summit were reported during 15-22 October with two lahars recorded on 21 October. White and gray emissions rose 50-1,000 m on 23 October. The first of a series of pyroclastic flows was reported on 25 October; they were reported daily through the end of the month when the weather permitted, traveling 1,000-2,500 m from the summit (figure 82). In addition, block avalanches from the growing dome were observed moving down the E and SE flanks 500-1,500 m on 25 October and 200-1,000 m each day during 28-31 October (figure 83). Sentinel-2 satellite data indicated a very weak thermal anomaly at the summit in late September; it was slightly larger in late October, corroborating with images of the slow-growing dome (figure 84).

Figure 81. A new lava dome appeared at the summit of Sinabung in late September 2020. Block avalanches from the dome were first reported on 8 October. Satellite imagery indicating a thermal anomaly at the summit was very faint at the end of September and slightly stronger by the end of October. The dome grew slowly between 30 September (top) and 22 October 2020 (bottom). Photos taken by Firdaus Surbakti, courtesy of Rizal.

Figure 82. Pyroclastic flows at Sinabung were accompanied ash emissions multiple times during the last week of October, including the event seen here on 27 October 2020. Courtesy of PVMBG and CultureVolcan.

Figure 83. Block avalanches from the growing summit dome at Sinabung descended the SE flank on 28 October 2020. The dome is visible at the summit. Courtesy of PVMBG and MAGMA.

Figure 84. A very faint thermal anomaly appeared at the summit of Sinabung in Sentinel 2 satellite imagery on 28 September 2020 (left). One month later on 28 October the anomaly was bigger, corroborating photographic evidence of the growing dome. Atmospheric penetration rendering (bands 12, 11, 8A) courtesy of Sentinel Hub Playground.

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/); 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); 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/); The Jakarta Post, 3rd Floor, Gedung, Jl. Palmerah Barat 142-143 Jakarta 10270 (URL: https://www.thejakartapost.com/amp/news/2020/08/08/mount-sinabung-erupts-again-after-year-of-inactivity.html);Rizal (URL: https://twitter.com/Rizal06691023/status/1319452375887740930); CultureVolcan (URL: https://twitter.com/CultureVolcan/status/1321156861173923840).

The remote Heard Island is located in the southern Indian Ocean and contains the Big Ben stratovolcano, which has had intermittent activity since 1910. The island’s activity, characterized by thermal anomalies and occasional lava flows (BGVN 45:05), is primarily monitored by satellite instruments. This report updates activity from May through October 2020 using information from satellite-based instruments.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed frequent thermal activity in early June that continued through July (figure 43). Intermittent, slightly higher-power thermal anomalies were detected in late August through mid-October, the strongest of which occurred in October. Two of these anomalies were also detected in the MODVOLC algorithm on 12 October.

Figure 43. A small pulse in thermal activity at Heard was detected in early June and continued through July 2020, according to the MIROVA system (Log Radiative Power). Thermal anomalies appeared again starting in late August and continued intermittently through mid-October 2020. Courtesy of MIROVA.

Sentinel-2 thermal satellite imagery showed a single thermal anomaly on 3 May. In comparison to the MIROVA graph, satellite imagery showed a small pulse of strong thermal activity at the summit of Big Ben in June (figure 44). Some of these thermal anomalies were accompanied by gas-and-steam emissions. Persistent strong thermal activity continued through July. Starting on 2 July through at least 17 July two hotspots were visible in satellite imagery: one in the summit crater and one slightly to the NW of the summit (figure 45). Some gas-and-steam emissions were seen rising from the S hotspot in the summit crater. In August the thermal anomalies had decreased in strength and frequency but persisted at the summit through October (figure 45).

Figure 44. Thermal satellite images of Heard Island’s Big Ben volcano showed strong thermal signatures (bright yellow-orange) sometimes accompanied by gas-and-steam emissions drifting SE (top left) and NE (bottom right) during June 2020. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Figure 45. Thermal satellite images of Heard Island’s Big Ben volcano showed persistent thermal anomalies (bright yellow-orange) near the summit during July through October 2020. During 14 (top left) and 17 (top right) July a second hotspot was visible NW of the summit. By 22 October (bottom right) the thermal anomaly had significantly decreased in strength in comparison to previous months. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

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

Information Contacts: MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Sabancaya, located in Peru, is a stratovolcano that has been very active since 1986. The current eruptive period began in November 2016 and has recently been characterized by lava dome growth, daily explosions, ash plumes, ashfall, SO₂ plumes, and ongoing thermal anomalies (BGVN 45:06). Similar activity continues into this reporting period of June through September 2020 using information from weekly reports from the Observatorio Vulcanologico INGEMMET (OVI), the Instituto Geofisico del Peru (IGP), and various satellite data. The Buenos Aires Volcanic Ash Advisory Center (VAAC) issued a total of 520 reports of ongoing ash emissions during this time.

Volcanism during this reporting period consisted of daily explosions, nearly constant gas-and-ash plumes, SO₂ plumes, and persistent thermal anomalies in the summit crater. Gas-and-ash plumes rose to 1.5-4 km above the summit crater, drifting up to 35 km from the crater in multiple directions; several communities reported ashfall every month except for August (table 7). Sulfur dioxide emissions were notably high and recorded almost daily with the TROPOMI satellite instrument (figure 83). The satellite measurements of the SO₂ emissions exceeded 2 DU (Dobson Units) at least 20 days each month of the reporting period. These SO₂ plumes sometimes persisted over multiple days and ranged between 1,900-10,700 tons/day. MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows frequent thermal activity through September within 5 km of the summit crater, though the power varied; by late August, the thermal anomalies were stronger compared to the previous months (figure 84). This increase in power is also reflected by the MODVOLC algorithm that detected 11 thermal anomalies over the days of 31 August and 4, 6, 13, 17, 18, 20, and 22 September 2020. Many of these thermal hotspots were visible in Sentinel-2 thermal satellite imagery, occasionally accompanied by gas-and-steam and ash plumes (figure 85).

Table 7. Persistent activity at Sabancaya during June through September included multiple daily explosions that produced ash plumes rising several kilometers above the summit and drifting in multiple directions; this resulted in ashfall in communities within 35 km of the volcano. Satellite instruments recorded daily SO₂ emissions. Data courtesy of OVI-INGEMMET, IGP, and the NASA Global Sulfur Dioxide Monitoring Page.

Figure 83. Sulfur dioxide plumes were captured almost daily from Sabancaya during June through September 2020 by the TROPOMI instrument on the Sentinel-5P satellite. Some of the largest SO2 plumes occurred on 19 June (top left), 5 July (top right), 30 August (bottom left), and 10 September (bottom right) 2020. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Figure 84. Thermal activity at Sabancaya varied in power from 13 October 2019 through September 2020, but was consistent in frequency, according to the MIROVA graph (Log Radiative Power). A pulse in thermal activity is shown in late August 2020. Courtesy of MIROVA.

Figure 85. Sentinel-2 thermal satellite imagery showed frequent gas-and-steam and ash plumes rising from Sabancaya, accompanied by ongoing thermal activity from the summit crater during June through September 2020. On 23 June (top left) a dense gray-white ash plume was visible drifting E from the summit. On 3 July (top right) and 27 August (bottom left) a strong thermal hotspot (bright yellow-orange) was accompanied by some degassing. On 1 September (bottom right) the thermal anomaly persisted with a dense gray-white ash plume drifting SE from the summit. Images using “Natural Color” rendering (bands 4, 3, 2) on 23 June 2020 (top left) and the rest have “Atmospheric penetration” rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

OVI detected slight inflation on the N part of the volcano, which continued to be observed throughout the reporting period. Persistent thermal anomalies caused by the summit crater lava dome were observed in satellite data. The average number of daily explosions during June ranged from 18 during 1-7 June to 9 during 15-21 June, which generated gas-and-ash plumes that rose 1.5-4 km above the crater and drifted 30 km SE, S, SW, NE, W, and E (figure 86). The strongest sulfur dioxide emissions were recorded during 1-7 June measuring 8,400 tons/day. On 20 June drone video showed that the lava dome had been destroyed, leaving blocks on the crater floor, though the crater remained hot, as seen in thermal satellite imagery (figure 85). During 22-28 June there were an average of 13 daily explosions, which produced ash plumes rising to a maximum height of 4 km, drifting NE, E, and SE. As a result, ashfall was reported in the districts of Chivay, Achoma, Ichupampa, Yanque, and Coporaque, and in the area of Sallali. Then, on 27 June ashfall was reported in several areas NE of the volcano, which included the districts of Madrigal, Lari, Achoma, Ichupampa, Yanque, Chivay, and Coporaque.

Figure 86. Multiple daily explosions at Sabancaya produced ash plumes that rose 1.5-4 km above the crater during June 2020. Images are showing 8 (left) and 27 (right) June 2020. Courtesy of OVI (Reporte Semanal de Monitorio de la Actividad de la Volcan Sabancaya, RSSAB-24-2020/INGEMMET Semana del 08 al 14 de junio del 2020 and RSSAB-26-2020/INGEMMET Semana del 22 al 28 de junio del 2020).

Slight inflation continued to be monitored in July, occurring about 4-6 km N of the crater, as well as on the SE flank. Daily explosions continued, producing gas-and-ash plumes that rose 2-2.6 km above the crater and drifting 15-30 km E, NE, NW, SE, SW, S, and W (figure 87). The number of daily explosions increased slightly compared to the previous month, ranging from 20 during 1-5 July to 11 during 13-19 July. SO₂ emissions that were measured each week ranged from 1,900 to 6,000 tons/day, the latter of which occurred during 6-12 July. Thermal anomalies continued to be observed in thermal satellite data over the summit crater throughout the month. During 6-12 July gas-and-ash plumes rose 2.3-2.5 km above the crater, drifting 30 km SE, E, and NE, resulting in ashfall in Achoma and Chivay.

Figure 87. Multiple daily explosions at Sabancaya produced ash plumes that rose 2-3.5 km above the crater during July 2020. Images are showing 7 (left) and 26 (right) July 2020. Courtesy of OVI (Reporte Semanal de Monitorio de la Actividad de la Volcan Sabancaya, RSSAB-28-2020/INGEMMET Semanal: del 06 al 12 de julio del 2020 and RSSAB-30-2020/INGEMMET Semanal: del 20 al 26 de julio del 2020).

OVI reported continued slight inflation on the N and SE flanks during August. Daily explosive activity had slightly declined in the first part of the month, ranging from 18 during the 3-9 August to 9 during 17-23 August. Dense gray gas-and-ash plumes rose 1.7-3.6 km above the crater, drifting 20-30 km in various directions (figure 88), though no ashfall was reported. Thermal anomalies were observed using satellite data throughout the month. During 24-30 August a pulse in activity increased the daily average of explosions to 29, as well as the amount of SO₂ emissions (10,700 tons/day); nighttime incandescence accompanied this activity. During 28-29 August higher levels of seismicity and inflation were reported compared to the previous weeks. The daily average of explosions increased again during 31 August-6 September to 39; nighttime incandescence remained.

Figure 88. Multiple daily explosions at Sabancaya produced ash plumes that rose 1.7-3.6 km above the crater during August 2020. Images are showing 1 (left) and 29 (right) August 2020. Courtesy of OVI (Reporte Semanal de Monitorio de la Actividad de la Volcan Sabancaya, RSSAB-31-2020/INGEMMET Semanal del 27 de julio al 02 de agosto del 2020 and RSSAB-35-2020/INGEMMET Semanal del 24 al 30 de agosto del 2020).

Increased volcanism was reported during September with the daily average of explosions ranging from 33 during 14-20 September to 40 during 28 September-4 October. The resulting gas-and-ash plumes rose 1.8-3.5 km above the crater drifting 25-35 km in various directions (figure 89). SO₂ flux was measured by OVI ranging from 2,600 to 9,700 tons/day, the latter of which was recorded during 31 August to 6 September. During 7-13 September an average of 35 explosions were reported, accompanied by gas-and-ash plumes that rose 2.6-3.5 km above the crater and drifting 30 km SE, SW, W, E, and S. These events resulted in ashfall in Lari, Achoma, and Maca. The following week (14-20 September) ashfall was reported in Achoma and Chivay. During 21-27 September the daily average of explosions was 38, producing ash plumes that resulted in ashfall in Taya, Huambo, Huanca, and Lluta. Slight inflation on the N and SE flanks continued to be monitored by OVI. Strong activity, including SO₂ emissions and thermal anomalies over the summit crater persisted into at least early October.

Figure 89. Multiple daily explosions at Sabancaya produced ash plumes that rose 1.8-2.6 km above the crater during September 2020. Images are showing 4 (left) and 27 (right) September 2020. Courtesy of OVI (Reporte Semanal de Monitorio de la Actividad de la Volcan Sabancaya, RSSAB-36-2020/INGEMMET Semanal del 31 de agosto al 06 de septiembre del 2020 and RSSAB-39-2020/INGEMMET Semanal del 21 al 27 de septiembre del 2020).

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

Information Contacts: Observatorio Volcanologico del INGEMMET (Instituto Geológical Minero y Metalúrgico), Barrio Magisterial Nro. 2 B-16 Umacollo - Yanahuara Arequipa, Peru (URL: http://ovi.ingemmet.gob.pe); Instituto Geofisico del Peru (IGP), Calle Badajoz N° 169 Urb. Mayorazgo IV Etapa, Ate, Lima 15012, Perú (URL: https://www.gob.pe/igp); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Rincón de la Vieja is a remote volcanic complex in Costa Rica that contains an acid lake. Frequent weak phreatic explosions have occurred since 2011 (BGVN 44:08). The most recent eruption period began in January 2020, which consisted of small phreatic explosions, gas-and-steam plumes, pyroclastic flows, and lahars (BGVN 45:04). This reporting period covers April through September 2020, with activity characterized by continued small phreatic explosions, three lahars, frequent gas-and-steam plumes, and ash plumes. The primary source of information for this report is the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA) using weekly bulletins and the Washington Volcanic Ash Advisory Center (VAAC).

Small, frequent, phreatic explosions were common at Rincón de la Vieja during this reporting period. One to several eruptions were reported on at least 16 days in April, 15 days in May, 8 days in June, 10 days in July, 18 days in August, and 13 days in September (table 5). Intermittent ash plumes accompanied these eruptions, rising 100-3,000 m above the crater and drifting W, NW, and SW during May and N during June. Occasional gas-and-steam plumes were also observed rising 50-2,000 m above the crater rim.

Table 5. Monthly summary of activity at Rincón de la Vieja during April through September 2020. Courtesy of OVSICORI-UNA.

During April small explosions were detected almost daily, some of which generated ash plumes that rose 200-1,000 m above the crater and gas-and-steam emissions that rose 50-1,500 m above the crater. On 4 April an eruption at 0824 produced an ash plume that rose 1 km above the crater rim. A small hydrothermal explosion at 0033 on 11 April, recorded by the webcam in Sensoria (4 km N), ejected water and sediment onto the upper flanks. On 15 April a phreatic eruption at 0306 resulted in lahars in the Pénjamo, Azufrada, and Azul rivers, according to local residents. Several small explosions were detected during the morning of 19 April; the largest phreatic eruption ejected water and sediment 300 m above the crater rim and onto the flanks at 1014, generated a lahar, and sent a gas-and-steam plume 1.5 km above the crater (figure 30). On 24 April five events were recorded by the seismic network during the morning, most of which produced gas-and-steam plumes that rose 300-500 m above the crater. The largest event on this day occurred at 1020, ejecting water and solid material 300 m above the crater accompanied by a gas-and-steam plume rising up to 1 km.

Figure 30. Webcam image of small hydrothermal eruptions at Rincón de la Vieja on 19 April 2020. Image taken by the webcam in Dos Ríos de Upala; courtesy of OVSICORI-UNA.

Similar frequent phreatic activity continued in May, with ash plumes rising 200-1,500 m above the crater, drifting W, NW, and SW, and gas-and-steam plumes rising up to 2 km. On 5 May an eruption at 1317 produced a gas-and-steam plume 200 m above the crater and a Washington VAAC advisory reported that an ash plume rose to 2.1 km altitude, drifting W. An event at 1925 on 9 May generated a gas-and-steam plume that rose almost 2 km. An explosion at 1128 on 15 May resulted in a gas-and-steam plume that rose 1 km above the crater rim, accompanied by a gray, sediment-laden plume that rose 400 m. On 21 May a small ash eruption at 0537 sent a plume 1 km above the crater (figure 31). According to a Washington VAAC advisory, an ash plume rose 3 km altitude, drifting NW on 22 May. During the early evening on 25 May an hour-long sequence of more than 70 eruptions and emissions, according to OVSICORI-UNA, produced low gas-and-steam plumes and tephra; at 1738, some ejecta was observed above the crater rim. The next day, on 26 May, up to 52 eruptive events were observed. An eruption at 2005 was not visible due to weather conditions; however, it resulted in a minor amount of ashfall up to 17 km W and NW, which included Los Angeles of Quebrada Grande and Liberia. A phreatic explosion at 1521 produced a gray plume that rose 1.5 km above the crater (figure 31). An eruption at 1524 on 28 May sent an ash plume 3 km above the crater that drifted W, followed by at least three smaller eruptions at 1823, 1841, and 1843. OVSICORI-UNA reported that volcanism began to decrease in frequency on 28-29 May. Sulfur dioxide emissions ranged between 100 and 400 tons per day during 28 May to 15 June.

Figure 31. Webcam images of gray gas-and-steam and ash emissions at Rincón de la Vieja on 21 (left), and 27 (right) May 2020. Both images taken by the webcam in Dos Ríos de Upala and Sensoria; courtesy of OVSICORI-UNA.

There were eight days with eruptions in June, though some days had multiple small events; phreatic eruptions reported on 1-2, 13, 16-17, 19-20, and 23 June generated plumes 1-2 km above the crater (figure 32). During 2-8 June SO2 emissions were 150-350 tons per day; more than 120 eruptions were recorded during the preceding weekend. Ashfall was observed N of the crater on 4 June. During 9-15 June the SO2 emissions increased slightly to 100-400 tons per day. During 16-17 June several small eruptive events were detected, the largest of which occurred at 1635 on 17 June, producing an ash plume that rose 1 km above the crater.

Figure 32. Webcam images of gray gas-and-steam and ash plumes rising from Rincón de la Vieja on 1 (top left), 2 (top right), 7 (bottom left), and 13 (bottom right) June 2020. The ash plume on 1 June rose between 1.5 and 2 km above the crater. The ash plume on 13 June rose 1 km above the crater. Courtesy of OVSICORI-UNA.

Explosive hydrothermal activity was lower in June-September compared to January-May 2020, according to OVSICORI-UNA. Sporadic small phreatic explosions and earthquakes were registered during 22-25 and 29 July-3 August, though no lahars were reported. On 25 July an eruptive event at 0153 produced an ash plume that rose 1 km above the crater. Similar activity continued into August. On 5 and 6 August phreatic explosions were recorded at 0546 and 0035, respectively, the latter of which generated a plume that rose 500 m above the crater. These events continued to occur on 10, 16, 19-20, 22-25, 27-28, and 30-31 August, generating plumes that rose 500 m to 1 km above the crater.

On 3 September geologists observed that the acid lake in the main crater had a low water level and exhibited strong gas emissions; vigorous fumaroles were observed on the inner W wall of the crater, measuring 120°C. Gas-and-steam emissions continued to be detected during September, occasionally accompanied by phreatic eruptions. On 7 September an eruption at 0750 produced an ash plume that rose 50 m above the crater while the accompanying gas-and-steam plume rose 500 m. Several low-energy phreatic explosions occurred during 8-17, 20, and 22-28 September, characterized primarily by gas-and-steam emissions. An eruption on 16 September ejected material from the crater and generated a small lahar. Sulfur dioxide emissions were 100 tons per day during 16-21 September. On 17 September an eruption at 0632 produced an ash plume that rose 700 m above the crater (figure 33). A relatively large eruptive event at 1053 on 22 September ejected material out of the crater and into N-flank drainages.

Figure 33. Webcam image of an eruption plume rising above Rincón de la Vieja on 17 September 2020. Courtesy of OVSICORI-UNA.

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/); 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).

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

Summary of activity during August-November 2020. Eruptive activity continued at Fuego during August-November 2020, very similar to that during the first part of the year (table 22). Ash emissions were reported daily by INSIVUMEH with explosions often in the 6-12 per hour range. Most of the ash plumes rose to 4.5-4.7 km altitude and generally drifted SW, W, or NW, although rarely the wind direction changed and sent ash to the S and SE. Multiple daily advisories were issued throughout the period by the Washington VAAC warning aviators about ash plumes, which were often visible on the observatory webcam (figure 136). Some of the communities located SW of the volcano received ashfall virtually every day during the period. Block avalanches descended the major drainages daily as well. Sounds were heard and vibrations felt from the explosions most days, usually 7-12 km away. The stronger explosions could be felt and heard 20 km or more from the volcano. During late August and early September a lava flow was active on the SW flank, reaching 700 m in length during the second week of September.

Table 22. Eruptive activity was consistently high at Fuego throughout August – November 2020 with multiple explosions every hour, ash plumes, block avalanches, and near-daily ashfall in the communities in certain directions within 10-20 km of the volcano. Courtesy of INSIVUMEH daily reports.

Figure 136. Consistent daily ash emissions produced similar looking ash plumes at Fuego during August-November 2020. Plumes usually rose to 4.5-4.8 km altitude and drifted SW. Courtesy of INSIVUMEH.

The frequent explosions, block avalanches, and lava flows produced a strong thermal signal throughout the period that was recorded in both the MIROVA project Log Radiative Power graph (figure 137) and in numerous Sentinel-2 satellite images (figure 138). MODVOLC data produced thermal alerts 4-6 days each month. At least one lahar was recorded each month; they were most frequent in September and October.

Figure 137. The MIROVA graph of activity at Fuego for the period from 15 January through November 2020 suggested persistent moderate to high-level heat flow for much of the time. Courtesy of MIROVA.

Figure 138. Atmospheric penetration rendering of Sentinel-2 satellite images (bands 12, 11, 8A) of Fuego during August-November 2020 showed continued thermal activity from block avalanches, explosions, and lava flows at the summit and down several different ravines. Courtesy of Sentinel Hub Playground.

Activity during August-November 2020. The number of explosions per hour at Fuego during August 2020 was most often 7-10, with a few days that were higher at 10-15. The ash plumes usually rose to 4.5-4.8 km altitude and drifted SW or W up to 15 km. Incandescence was visible 100-300 m above the summit crater on most nights. All of the major drainages including the Seca, Santa Teresa, Ceniza, Trinidad, Taniluyá, Las Lajas, and Honda were affected by block avalanches virtually every day. In addition, the communities of Panimaché I and II, Morelia, Santa Sofía, Finca Palo Verde, El Porvenir, San Pedro Yepocapa, and Sangre de Cristo reported ashfall almost every day. Sounds and vibrations were reported multiple days every week, often up to 12 km from the volcano, but occasionally as far as 20 km away. Lahars carrying blocks of rocks and debris 1-2 m in diameter descended the SE flank in the Las Lajas and Honda ravines on 6 August. On 27 August a lava flow 150 m long appeared in the Ceniza ravine. It increased in length over the subsequent few days, reaching 550 m long on 30 August, with frequent block avalanches falling off the front of the flow.

The lava flow in the Ceniza ravine was reported at 100 m long on 5 September. It grew to 200 m on 7 September and reached 700 m long on 12 September. It remained 200-350 m long through 19 September, although instruments monitored by INSIVUMEH indicated that effusive activity was decreasing after 16 September (figure 139). A second flow was 200 m long in the Seca ravine on 19 September. By 22 September, active flows were no longer observed. The explosion rate varied from a low of 3-5 on 1 September to a high of 12-16 on 4, 13, 18, and 22-23 September. Ash plumes rose to 4.5-4.9 km altitude nearly every day and drifted W, NW, and SW occasionally as far as 20 km before dissipating. In addition to the active flow in the Ceniza ravine, block avalanches persisted in the other ravines throughout the month. Ashfall continued in the same communities as in August, but was also reported in Yucales on 4 September along with Ojo de Agua and Finca La Conchita on 17 September. The Las Lajas, Honda, and El Jute ravines were the sites of lahars carrying blocks up to 1.5 m in diameter on 8 and 18 September. On 19 and 24 September lahars again descended Las Lajas and El Jute ravines; the Ceniza ravine had a lahar on 19 September.

Figure 139. Avalanche blocks descended the Ceniza ravine (left) and the Las Lajas ravine (right) at Fuego on 17 September 2020. The webcam that captured this image is located at Finca La Reunión on the SE flank. Courtesy of INSIVUMEH (BOLETÍN VULCANOLÓGICO ESPECIAL BEVFGO # 76-2020, 18 de septiembre de 2020, 14:30 horas).

The same activity continued during October 2020 with regard to explosion rates, plume altitudes, distances, and directions of drift. All of the major ravines were affected by block avalanches and the same communities located W and SW of the summit reported ashfall. In addition, ashfall was reported in La Rochela on 2, 3, 7-9 and 30 October, in Ceilán on 3 and 7-9 October, and in Yucales on 5, 14, 18 and 19 October. Multiple strong explosions with abundant ash were reported in a special bulletin on 14 October; high levels of explosive activity were recorded during 16-23 October. Vibrations and sounds were often felt up to 15 km away and heard as far as 25 km from the volcano during that period. Particularly strong block avalanches were present in the Seca and Ceniza ravines on 20, 25, and 30 October. Abundant rain on 9 October resulted in lahars descending all of the major ravines. The lahar in the Las Lajas ravine overflowed and forced the closure of route RN-14 road affecting the community of San Miguel on the SE flank (figure 140). Heavy rains on 15 October produced lahars in the Ceniza, Las Lajas, and Hondas ravines with blocks up to 2 m in diameter. Multiple lahars on 27 October affected Las Lajas, El Jute, and Honda ravines.

Figure 140. Heavy rains on 9 October 2020 at Fuego caused lahars in all the major ravines. Debris from Las Lajas ravine overflowed highway RN-14 near the community of San Miguel on the SE flank, the area devastated by the pyroclastic flow of June 2018. Courtesy of INSIVUMEH (BEFGO #96 VOLCAN DE FUEGO- ZONA CERO RN-14, SAN MIGUEL LOS LOTES y BARRANCA LAS LAJAS, 09 de octubre de 2020).

On 8 November 2020 a lahar descended the Seca ravine, carrying rocks and debris up to 1 meter in diameter. During the second week of November 2020, the wind direction changed towards the SE and E and brought ashfall to San Juan Alotenango, Ciudad Vieja, San Miguel Dueñas, and Antigua Guatemala on 8 November. Especially strong block avalanches were noted in the Seca and Ceniza ravines on 14, 19, 24, and 29 November. During a period of stronger activity in the fourth week of November, vibrations were felt and explosions heard more than 20 km away on 22 November and more than 25 km away on 27 November. In addition to the other communities affected by ashfall during August-November, Quisaché and Santa Emilia reported ashfall on 30 November.

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

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/ ); 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);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).

Kikai is a mostly submarine caldera, 19-km-wide, just S of the Ryukyu Islands of Japan. At the NW rim of the caldera lies the island of Satsuma Iwo Jima (also known as Satsuma-Iojima and Tokara Iojima), and the island’s highest peak, Iodake, a steep stratovolcano. Recent weak ash explosions at Iodake occurred on 2 November 2019 and 29 April 2020 (BGVN 45:02, 45:05). The volcano is monitored by the Japan Meteorological Agency (JMA) and satellite sensors. This report covers the period May-October 2020. During this time, the Alert Level remained at 2 (on a 5-level scale).

Activity at Kikai has been relatively low since the previous eruption on 29 April 2020. During May through October occasional white gas-and-steam emissions rose 0.8-1.3 km above the Iodake crater, the latter of which was recorded in September. Emissions were intermittently accompanied by weak nighttime incandescence, according to JMA (figure 17).

Figure 17. White gas-and-steam emissions rose 1 km above the crater at Satsuma Iwo Jima (Kikai) on 25 May (top) 2020. At night, occasional incandescence could be seen in the Iodake crater, as seen on 29 May (bottom) 2020. Both images taken by the Iwanoue webcam. Courtesy of JMA (An explanation of volcanic activity at Satsuma Iwo Jima, May 2nd year of Reiwa [2020]).

A small eruption at 0757 on 6 October occurred in the NW part of the Iodake crater, which produced a grayish white plume rising 200 m above the crater (figure 18). Faint thermal anomalies were detected in Sentinel-2 thermal satellite imagery in the days just before this eruption (28 September and 3 October) and then after (13 and 23 October), accompanied by gas-and-steam emissions (figures 19 and 20). Nighttime crater incandescence continued to be observed. JMA reported that sulfur dioxide emissions measured 700 tons per day during October, compared to the previous eruption (400-2,000 tons per day in April 2020).

Figure 18. Webcam images of the eruption at Satsuma Iwo Jima (Kikai) on 6 October 2020 that produced an ash plume rising 200 m above the crater (top). Nighttime summit crater incandescence was also observed (bottom). Images were taken by the Iwanoue webcam. Courtesy of JMA (An explanation of volcanic activity at Satsuma Iwo Jima, October 2nd year of Reiwa [2020]).

Figure 19. Weak thermal hotspots (bright yellow-orange) were observed at Satsuma Iwo Jima (Kikai) during late September through October 2020. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Figure 20. Webcam image of a white gas-and-steam plume rising 1.1 km above the crater at Satsuma Iwo Jima (Kikai) on 27 October 2020. Image was taken by the Iwanoue webcam. Courtesy of JMA (An explanation of volcanic activity at Satsuma Iwo Jima, October 2nd year of Reiwa [2020]).

Geologic Background. Kikai is a mostly submerged, 19-km-wide caldera near the northern end of the Ryukyu Islands south of Kyushu. It was the source of one of the world's largest Holocene eruptions about 6,300 years ago when rhyolitic pyroclastic flows traveled across the sea for a total distance of 100 km to southern Kyushu, and ashfall reached the northern Japanese island of Hokkaido. The eruption devastated southern and central Kyushu, which remained uninhabited for several centuries. Post-caldera eruptions formed Iodake lava dome and Inamuradake scoria cone, as well as submarine lava domes. Historical eruptions have occurred at or near Satsuma-Iojima (also known as Tokara-Iojima), a small 3 x 6 km island forming part of the NW caldera rim. Showa-Iojima lava dome (also known as Iojima-Shinto), a small island 2 km E of Tokara-Iojima, was formed during submarine eruptions in 1934 and 1935. Mild-to-moderate explosive eruptions have occurred during the past few decades from Iodake, a rhyolitic lava dome at the eastern end of Tokara-Iojima.

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

Manam, located 13 km off the N coast of Papua New Guinea, is a basaltic-andesitic stratovolcano with historical eruptions dating back 400 years. Volcanism has been characterized by low-level ash plumes, occasional Strombolian activity, lava flows, pyroclastic avalanches, and large ash plumes from Main and South, the two active summit craters. The current eruption period has been ongoing since 2014, typically with minor explosive activity, thermal activity, and SO₂ emissions (BGVN 45:05). This reporting period updates information from April through September 2020, consisting of intermittent ash plumes from late July to mid-September, persistent thermal anomalies, and SO₂ emissions. Information comes from Papua New Guinea's Rabaul Volcano Observatory (RVO), part of the Department of Mineral Policy and Geohazards Management (DMPGM), the Darwin Volcanic Ash Advisory Center (VAAC), and various satellite data.

Explosive activity was relatively low during April through late July; SO₂ emissions and low power, but persistent, thermal anomalies were detected by satellite instruments each month. The TROPOMI instrument on the Sentinel-5P satellite recorded SO₂ emissions, many of which exceeded two Dobson Units, that drifted generally W (figure 76). Distinct SO₂ emissions were detected for 10 days in April, 4 days in May, 10 days in June, 4 days in July, 11 days in August, and 8 days in September.

Thermal anomalies recorded by the MIROVA (Middle InfraRed Observation of Volcanic Activity) system were sparse from early January through June 2020, totaling 11 low-power anomalies within 5 km of the summit (figure 77). From late July through September a pulse in thermal activity produced slightly stronger and more frequent anomalies. Some of this activity could be observed in Sentinel-2 thermal satellite imagery (figure 78). Occasionally, these thermal anomalies were accompanied by gas-and-steam emissions or ash plumes, as shown on 28 July. On 17 August a particularly strong hotspot was detected in the S summit crater. According to the MODVOLC thermal alert data, a total of 10 thermal alerts were detected in the summit crater over four days: 29 July (5), 16 August (1), and 3 (1) and 8 (3) September.

Figure 76. Distinct sulfur dioxide plumes rising from Manam and drifting generally W were detected using data from the TROPOMI instrument on the Sentinel-5P satellite on 28 April (top left), 24 May (top right), 16 July (bottom left), and 12 September (bottom right) 2020. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

Figure 77. Intermittent thermal activity at Manam increased in power and frequency beginning around late July and continuing through September 2020, as shown on the MIROVA Log Radiative Power graph. Courtesy of MIROVA.

Figure 78. Sentinel-2 thermal satellite images showing a persistent thermal anomaly (yellow-orange) at Manam’s summit craters (Main and South) each month during April through August; sometimes they were seen in both summit craters, as shown on 8 June (top right), 28 July (bottom left), and 17 August (bottom right). A particularly strong anomaly was visible on 17 August (bottom right). Occasional gas-and-steam emissions accompanied the thermal activity. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Activity during mid-July slightly increased compared to the previous months. On 16 July seismicity increased, fluctuating between low and moderate RSAM values through the rest of the month. In Sentinel-2 satellite imagery a gray ash plume was visible rising from the S summit crater on 28 July (figure 78). RSAM values gradually increased from a low average of 200 to an average of 1200 on 30 July, accompanied by thermal hotspots around the summit crater; a ground observer reported incandescent material was ejected from the summit. On 31 July into 1 August ash plumes rose to 4.3 km altitude, accompanied by an incandescent lava flow visible at the summit, according to a Darwin VAAC advisory.

Intermittent ash plumes continued to be reported by the Darwin VAAC on 1, 6-7, 16, 20, and 31 August. They rose from 2.1 to 4.6 km altitude, the latter of which occurred on 31 August and drifted W. Typically, these ash plumes extended SW, W, NW, and WSW. On 11 September another ash plume was observed rising 2.4 km altitude and drifting W.

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: Rabaul Volcano Observatory (RVO), Geohazards Management Division, Department of Mineral Policy and Geohazards Management (DMPGM), PO Box 3386, Kokopo, East New Britain Province, Papua New Guinea; 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/); 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).

Explosive volcanism continued through October but caused no damage. There were 31 eruptions . . ., including 14 explosive ones. On 5 October a NOTAM . . . described eruptions at 0136 and 0447 that rose to 3.35 km. On the other hand, JMA reported that at 1628 on 6 October the "highest ash plume of October" rose to 3.3 km, so apparently there was relatively vigorous activity on both days. Volcanic earthquake swarms were detected 130 times, reaching a maximum amplitude of 2 µm. During October, a seismic station 2.3 km NW of Minamidake crater registered 862 distinct events. October ashfall collected at the Kagoshima Meteorological Station, 10 km W, measured 136 g/m².

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

Information Contacts: JMA; [SAB].

Continuing activity in September consisted of Strombolian eruptions and lava output from Crater C and fumarolic activity from Crater D. Two lobes of lava continued to progress toward the Tabacón valley (figure 70). ICE workers suggested that at elevations below 800 m the estimated velocities of lava flows have averaged roughly 2.5 m/day. In some of the steeper upslope reaches flows may have averaged as much as roughly 50 m/day, but velocities were more typically 10-20 m/day. These values are approximate, because field work is hampered by hazards associated with sudden collapse of lava-flow fronts.

Figure 70. Rough field sketch of Arenal, from the ash-sampling locality mentioned in the text (1.8 km W of the summit); view is toward the E. Courtesy of G. Soto, ICE.

In the summit crater, vents active for the past several months had built two small cones. The northernmost cone extruded lava during the past several months. The southerly cone appears to be mainly composed of pyroclastic materials. Toward the crater's center there was a third vent. Summit fumaroles remained vigorous and occasional explosions took place (table 6); at night a red glow still prevailed over the crater area suggesting ponded lava remains molten there. Seismicity reported by ICE appears in table 7; their mid-October sampling found that both pH values and water temperatures remained unchanged.

Table 6. Ash collected downwind at a spot 1.8 km W of Arenal's crater. "Collection Interval" refers to the time period in 1994 when the ash sample accumulated (also shown as "Days," the number of days), but the mass/area value is a computed daily average. Courtesy of G. Soto, ICE.

Table 7. Number of seismic events and tremor duration at Arenal. October values are extrapolated from 20 days of observations. Courtesy of ICE.

OVSICORI-UNA reported that September seismic events often accompanied gas- and ash-bearing eruptions. During September seismic events in the frequency range 1.2-2.5 Hz totaled 657; tremor duration totaled 55 hours.

Geologic Background. Conical Volcán Arenal is the youngest stratovolcano in Costa Rica and one of its most active. The 1670-m-high andesitic volcano towers above the eastern shores of Lake Arenal, which has been enlarged by a hydroelectric project. Arenal lies along a volcanic chain that has migrated to the NW from the late-Pleistocene Los Perdidos lava domes through the Pleistocene-to-Holocene Chato volcano, which contains a 500-m-wide, lake-filled summit crater. The earliest known eruptions of Arenal took place about 7000 years ago, and it was active concurrently with Cerro Chato until the activity of Chato ended about 3500 years ago. Growth of Arenal has been characterized by periodic major explosive eruptions at several-hundred-year intervals and periods of lava effusion that armor the cone. An eruptive period that began with a major explosive eruption in 1968 ended in December 2010; continuous explosive activity accompanied by slow lava effusion and the occasional emission of pyroclastic flows characterized the eruption from vents at the summit and on the upper western flank.

Information Contacts: E. Fernández, J. Barquero, and V. Barboza, OVSICORI; G. Soto and F. Arias, ICE; M. Mora, Univ de Costa Rica.

After ejecting mud and blocks on 12 September, Crater 1 remained restless in October (figure 25). The water-covered crater floor ejected mud intermittently, sometimes accompanied by ash plumes. In one case on 27 October, ejected mud flew more than 100 m above the crater bottom. Tremor amplitude (at Station A, 800 m W of the crater) generally remained less than 1 µm. Some larger tremor episodes exceeded 10 µm and were felt by personnel at the Aso Weather Station.

Figure 25. Seismicity and plume heights at Aso, January-October 1994. Earthquakes and tremor were registered at a station 0.8 km W of Nakadake cone. Plume heights were estimated by personnel at AWS. Courtesy of JMA.

Geologic Background. The 24-km-wide Asosan caldera was formed during four major explosive eruptions from 300,000 to 90,000 years ago. These produced voluminous pyroclastic flows that covered much of Kyushu. The last of these, the Aso-4 eruption, produced more than 600 km3 of airfall tephra and pyroclastic-flow deposits. A group of 17 central cones was constructed in the middle of the caldera, one of which, Nakadake, is one of Japan's most active volcanoes. It was the location of Japan's first documented historical eruption in 553 CE. The Nakadake complex has remained active throughout the Holocene. Several other cones have been active during the Holocene, including the Kometsuka scoria cone as recently as about 210 CE. Historical eruptions have largely consisted of basaltic to basaltic-andesite ash emission with periodic strombolian and phreatomagmatic activity. The summit crater of Nakadake is accessible by toll road and cable car, and is one of Kyushu's most popular tourist destinations.

Information Contacts: JMA.

Cloudy weather prevented observations on most days during the second half of September and October, but seismicity remained at normal levels. A gas-and-steam plume rose to 100 m above the volcano on 16 September, and to 1,000 m the week of 18-24 September. Activity was at normal levels the next two weeks. When conditions permitted, observers in Kozirevsk (~45 km WNW) saw a white steam cloud reaching 500-700 m above the crater on 13 October, 200 m on the 20th and 22nd, and 50 m on the 27th.

Geologic Background. Prior to its noted 1955-56 eruption, Bezymianny had been considered extinct. The modern volcano, much smaller in size than its massive neighbors Kamen and Kliuchevskoi, was formed about 4700 years ago over a late-Pleistocene lava-dome complex and an ancestral edifice built about 11,000-7000 years ago. Three periods of intensified activity have occurred during the past 3000 years. The latest period, which was preceded by a 1000-year quiescence, began with the dramatic 1955-56 eruption. This eruption, similar to that of St. Helens in 1980, produced a large horseshoe-shaped crater that was formed by collapse of the summit and an associated lateral blast. Subsequent episodic but ongoing lava-dome growth, accompanied by intermittent explosive activity and pyroclastic flows, has largely filled the 1956 crater.

Information Contacts: V. Kirianov, IVGG; AVO.

A news report on 3 November noted that gas emissions from the summit are frequent, many minor volcanic earthquakes have been felt during the last two years, and nearby hot springs were also emitting volcanic gases. The official Xinhua News Agency quoted Ruoxin Liu from the State Seismological Bureau, but we have received no direct confirmation.

Charles Dunlap, Susanne Horn, and Hans Schmincke worked on and around the summit with Chinese geologist Tang Deping during 21-25 July 1993, but saw no emissions. One hot spring area was observed by Dunlap in the N-flank valley, which begins at the lake outlet into the Erdobaihe River. These springs were next to the trail to the waterfall and on up to the lake's edge; eggs were boiled in the spring water for sale to tourists. A weak sulfur smell was detected, but it was not as pronounced as at some springs in Yellowstone or Long Valley (USA). No other emissions were noticed from these springs. Another hot spring location W of this valley was not visited, but apparently it is popular as a bath. On the E border of the crater lake (Korean side), water from a hot spring with a temperature of 700°C was being pumped to the crater rim to provide healing potions.

Baitoushan (Korean name P'aektu-san) is a large stratovolcano on the Korea-Manchurian border ~300 km SE of Changchun and 325 km WSW of Vladivostok, Russia. The 60-km-diameter volcano was constructed over the Changbaishan (Laoheidingz) shield volcano and has a 5-km-wide summit caldera. One of the world's largest known Holocene explosive eruptions took place around 1000 A.D., depositing tephra as far away as N Japan and forming in part the 850-m-deep depression filled by Tianchi Lake. The much better exposed pyroclastic deposits on the North Korean side studied by Horn and Schmincke are extremely thick and include major ignimbrites. Four historical eruptions have been recorded since the 15th century (1413, 1597, 1668, and 1702). Chinese geologists spoken to by Dunlap thought that these historical events were probably phreatic explosions, and that there have possibly been occasional gas emissions within approximately the last 50 years.

Geologic Background. Massive Changbaishan stratovolcano (also known as Baitoushan and by the Korean names of Baegdu, Paektu, or P'aektu-san), is located along the China/Korea border. A 5-km-wide, 850-m-deep summit caldera is filled by scenic Lake Tianchi (Sky Lake). A large Korean-speaking population resides near the volcano on both sides of the border. The 60-km-diameter dominantly trachytic and rhyolitic volcano was constructed over the Changbaishan (Laoheidingzi) shield volcano. Satellitic cinder cones are aligned along a NNE trend. One of the world's largest known Holocene explosive eruptions took place here about 946 CE, depositing rhyolitic and trachytic tephra as far away as northern Japan and forming in part the present caldera. Minor historical eruptions have been recorded since the 15th century.

Information Contacts: C. Dunlap, University of California - Santa Cruz; S. Horn and H. Schmincke, GEOMAR; Xinhua News Agency, China; UPI.

The following describes [fieldwork] between 23 September and 14 October 1994.

"There are continuing signs that activity is increasing. At the Chasm (La Voragine), 1-4 very low rumbles/min were heard, but on 14 October six explosions much louder than those heard in June/July (19:07) were heard in 10 minutes. The Bocca Nuova was also producing around one distinct long explosive blast per minute, as opposed to the faint gas puffs heard in the summer. However, no audible explosions were heard when the Chasm was active on 14 October. Northeast and Southeast craters were quiet as in June/July, but temperatures more than 100°C higher were measured at the fumaroles on their outer slopes. Another sign of increasing activity was that during the five days of levelling (25-30 September), 22 earth tremors were detected by the shaking of the instrument. This is > 10 times higher than 1993, and the largest total of tremors noted in this way since September 1991, before the 1991-93 eruption.

"The levelling traverse showed a slight subsidence of the summit since June 1994, the maximum value being just under 3 cm compared to the Piano Provenzana, 6.5 km NNE of the summit. The subsidence is more or less concentric around the summit, with the exception of some stations on the upper E flank and over the 1991-93 dyke, which have subsided nearly a centimetre more than those nearby.

"On 14 October the areas of active fumaroles measured during June were visited. These were measured again using a Minolta/Land 330 hand-held radiometer (8.5-14.5 mm). Temperatures were not corrected for spectral emissivity, so all radiant temperatures are given as brightness temperatures (table 5). At the N, W, and S rim of Northeast Crater, maximum fumarole and rift temperatures were 105-135°C higher than those measured in June. H₂S was also smelled in the vicinity of these high-temperature fumaroles. Higher maximum temperatures were also measured from rifts at the N rim of Southeast Crater, these being up to 170°C higher than those measured in June. It is stressed that these rises in temperature may be the result of different fumaroles being measured on the two dates, though in view of the thorough coverage in June this seems unlikely. Elsewhere, fumarole temperatures were similar to those measured in June. Fumarolic activity only was observed on the floor of Northeast Crater, which was measured from the rim at 40.1°C. The bocca on the floor of the Chasm was measured from the crater rim at 339°C. At the Bocca Nuova, a temperature of 173°C was measured for the SE bocca and of 40.7°C for the NW floor; these were measured from the crater rim. At Southeast Crater, fumaroles decreased in temperature and number around the W and E rims, such that fumaroles were few and cool on the S rim."

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: J. Murray and A. Harris, Open Univ.

During October activity at Galeras remained low. In terms of seismicity, on 20 October sporadic "screw-type" events reappeared. Screw-type events are comparatively monochromatic and with slowly decaying coda (late arriving) waves. They were so-named because their seismograph records look similar to the profile of a finely threaded screw. They are considered significant because they preceded five of the six eruptions between July 1992 and June 1993; on the other hand they have also occurred without being followed by an eruption. During October, seismic stations located 0.9-2.4 km from the active crater detected seven screw-type events. The codas of the screw-type events had durations of 31-63 seconds and a computed damping coefficient of 0.02. The seismic signals detected at all three stations had the same dominant frequency, ~ 2.5 Hz, and the spectra ranged from ~ 2.4 to 10.3 Hz.

Small earthquakes (M<2.4) took place at depths up to 5 km. These earthquakes had epicenters clustered beneath and around the active crater, most plotting within a radius of ~4 km. Butterfly-type events also took place. The SO₂ flux obtained by the mobile COSPEC method showed fairly low values: 38-832 t/d. Degassing continued to be concentrated chiefly on the active cone's W fringe with smaller fumaroles at the interior of the main crater.

Geologic Background. Galeras, a stratovolcano with a large breached caldera located immediately west of the city of Pasto, is one of Colombia's most frequently active volcanoes. The dominantly andesitic complex has been active for more than 1 million years, and two major caldera collapse eruptions took place during the late Pleistocene. Long-term extensive hydrothermal alteration has contributed to large-scale edifice collapse on at least three occasions, producing debris avalanches that swept to the west and left a large horseshoe-shaped caldera inside which the modern cone has been constructed. Major explosive eruptions since the mid-Holocene have produced widespread tephra deposits and pyroclastic flows that swept all but the southern flanks. A central cone slightly lower than the caldera rim has been the site of numerous small-to-moderate historical eruptions since the time of the Spanish conquistadors.

Information Contacts: INGEOMINAS, Pasto.

An eruption late on 15 October sent a plume ~ 300 m above the summit . . ., according to news reports. No casualties or damage were reported, although some ash fell in several villages on the slopes of the volcano and the explosion shook buildings.

Geologic Background. Gamalama is a near-conical stratovolcano that comprises the entire island of Ternate off the western coast of Halmahera, and is one of Indonesia's most active volcanoes. The island was a major regional center in the Portuguese and Dutch spice trade for several centuries, which contributed to the thorough documentation of Gamalama's historical activity. Three cones, progressively younger to the north, form the summit. Several maars and vents define a rift zone, parallel to the Halmahera island arc, that cuts the volcano. Eruptions, recorded frequently since the 16th century, typically originated from the summit craters, although flank eruptions have occurred in 1763, 1770, 1775, and 1962-63.

Information Contacts: Antara News Agency; Reuters.

During October, the crater lake at Irazú remained high, covering the crater floor with yellow-colored water. In addition to active flank fumaroles on the NW, subaqueous fumaroles bubbled consistently in the N, NW, W, SW, and SE parts of the lake, near the crater wall. Rockslides were seen coming down the N, SW, and E crater wall. Seismic events in October totaled 18 earthquakes with S minus P values of 2-3 seconds; some events reached M 2 with epicenters <3 km from the crater and focal depths of 4.0-4.5 km. Geodetic and leveling surveys in September found no significant changes.

Geologic Background. Irazú, one of Costa Rica's most active volcanoes, rises immediately E of the capital city of San José. The massive volcano covers an area of 500 km2 and is vegetated to within a few hundred meters of its broad flat-topped summit crater complex. At least 10 satellitic cones are located on its S flank. No lava flows have been identified since the eruption of the massive Cervantes lava flows from S-flank vents about 14,000 years ago, and all known Holocene eruptions have been explosive. The focus of eruptions at the summit crater complex has migrated to the W towards the historically active crater, which contains a small lake of variable size and color. Although eruptions may have occurred around the time of the Spanish conquest, the first well-documented historical eruption occurred in 1723, and frequent explosive eruptions have occurred since. Ashfall from the last major eruption during 1963-65 caused significant disruption to San José and surrounding areas.

Information Contacts: E. Fernández, J. Barquero, and V. Barboza, OVSICORI-UNA; G. Soto and F. Arias, ICE; Mauricio Mora, Escuela Centroamericana de Geología, Univ de Costa Rica.

"A minor seismic unrest occurred on the morning of 18 October, the second one this year, after 15 years of dormancy at this caldera. The local seismograph recorded a large number of low-frequency events starting at about 0200 on 18 October. Events occurred at a rate of up to 2-4/minute. The activity waned after 0930. Although of short duration, this swarm of events was similar to the unrest recorded between 17 May and mid-June 1994, when the long-term deflation of the caldera floor was interrupted."

Geologic Background. Karkar is a 19 x 25 km wide, forest-covered island that is truncated by two nested summit calderas. The 5.5-km-wide outer caldera was formed during one or more eruptions, the last of which occurred 9000 years ago. The eccentric 3.2-km-wide inner caldera was formed sometime between 1500 and 800 years ago. Parasitic cones are present on the N and S flanks of this basaltic-to-andesitic volcano; a linear array of small cones extends from the northern rim of the outer caldera nearly to the coast. Most historical eruptions, which date back to 1643, have originated from Bagiai cone, a pyroclastic cone constructed within the steep-walled, 300-m-deep inner caldera. The floor of the caldera is covered by young, mostly unvegetated andesitic lava flows.

Information Contacts: C. McKee and P. de Saint-Ours, RVO.

"In September, lava continued to enter the ocean in the Laeapuki area . . . . The W branch of the tube on the bench stopped transporting lava, and flows entering the ocean consolidated in front of the 27 July littoral cone. Littoral explosions increased in size and frequency coincident with the consolidation of the littoral tube system. On 14 September, ~10-15 m of the active bench collapsed into the ocean. The bench built out into the ocean until 1 October, when part of the active bench collapsed again. Flows built a small, thick bench following each collapse. Near the end of September, the flux at this ocean entry appeared to diminish, possibly because of the diversion of lava to a prolific E flow. Lava continued to enter the ocean in this area until 5 October, when the eruption paused for the first time since April.

"The large surface flow that broke out on 20 August at 270 m elevation continued to cover new land on the E side of the Kamoamoa flow-field. Throughout most of September there were active breakouts on this flow from the base of Pulama pali to below Paliuli. All of these breakouts were fluid pahoehoe toes and sheet flows. Sheet flows on the E margin of the flow field frequently ignited methane explosions, which were recorded by the Wahaula seismometer. Breakouts began to close the gap between the Kamoamoa and Kupaianaha flows; <200 m separated the two flow fields. Lava from this E flow entered the ocean on the E side of the Kamoamoa flow field intermittently during 2-9 October.

"Two pauses in October were only the 4th and 5th to occur since E-53 began in February 1993. On 6 October, all surface activity stopped, no lava entered the ocean, and there was no lava in the tube system. By the following morning lava had reoccupied the tube all the way to the Laeapuki ocean entry and fed breakouts close to 270 m elevation. Lava also continued to ooze and dribble into the ocean on the E side of the flow field. Following this pause, a number of breakouts were observed on Pulama pali and on the E flow. Lava entering the ocean in the Laeapuki area began to build a new bench E of the littoral cone formed on 27 July. Lava from the E flow entered the ocean once again on 22 October. On 24 October, the eruption appeared to be sputtering — flows slowed and then surged, entries died and then reactivated. By 25 October, all surface activity had stagnated. The eruption restarted the following day, and this time the tube system was reoccupied to only 550 m elevation. Below this elevation, large channelized aa and pahoehoe flows swept down the flow field. By 31 October, these flows had cascaded over Paliuli and begun to make their way to the ocean.

"Pu`u `O`o pond was a little more dynamic during this interval. From 13 September to 6 October, the pond level slowly dropped from 79 to 88 m below the crater rim. At its lowest level, the entry of lava from the W side of the pond was clearly visible. In October, the pond level rose from 88 to 60 m below the crater rim and activity on the pond surface became more vigorous. There was little change around the active vents, except that the collapse pit on the W flank of Pu`u `O`o doubled in size during September."

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: T. Mattox, HVO.

Activity had decreased by 4 October, and continued to decline the following week. Continuous tremor after 3 October and into early November had a maximum amplitude of 0.23-0.53 µm, registered 11 km from the volcano. On 5 and 7-9 October the volcano was obscured by clouds, but on 6 October the fumarolic plume from the summit crater rose ~600 m above the rim and was directed NE. Observers in Kliuchi [(30 km NNE)] reported decreased activity during 8-15 October. Gas-and-steam columns rising from two apertures at the summit reached 2,500 m above the crater on 10 October and 800 m on 14 October. Once again during clear weather a gas-and-steam column was seen rising 200 m above the summit crater on 17, 22, and 23 October and to 800-1,500 m on 18-20 October. During 27-29 October the column rose 200-800 m above the summit. The volcano was obscured by clouds from 30 October to 2 November.

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

Information Contacts: V. Kirianov, IVGG; AVO.

Eruptive activity in September and October at both craters consisted of moderate and intermittent Vulcanian explosions. Crater 3 was active during the first nine days of the month. It released a moderately thick vapor plume, with occasional dark gray ash clouds, accompanied by explosions and rumbling sounds, and resulting in light ash falls onto the NW flank and coastal villages. For the remainder of September and October, it only emitted very thin wisps of vapor, occasionally accompanied by blue vapor.

At Crater 2, background levels of moderate white and blue vapour emissions continued, and very weak night glow was seen on 7 September. However, activity picked up on the 12th and 13th with occasional dark ash-laden, convoluting Vulcanian explosions. Similar low-level eruptive activity resumed on 15-18, 24, and 28-29 September.

A good correlation could be seen between the level of seismicity and volcanic activity in September. The two local seismographs recorded 2-5 explosive events/day during 1-9 September at Crater 3, and then 2-8 events/day during each of the intermittent phases of activity at Crater 2. Seismicity remained at a low level throughout October.

Emissions from Crater 2 in October consisted of thin white vapour with occasional dark gray, ash-laden convoluting columns rising up to a few hundred meters above the crater. Fine ash fell on downwind coastal areas. Weak night glow accompanied these explosions on 3, 6, 9, 21-22, and 30 October.

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

Information Contacts: C. McKee and P. de Saint-Ours, RVO.

"Following intermittent periods of minor eruptive activity during the previous months, activity at S Crater was low during the first week of September. Weak white-pale grey emissions returned, accompanied by occasional roaring sounds and low-level seismicity (~1,000 small long-period events/day, with a scaled amplitude of 7-10 mm). Periods of stronger activity occurred on 8-11, 14-20, 22, and 29 September.

"Starting at 1845 on 8 September, a loud explosion accompanied a period of incandescent projections to 150 m above the crater, followed by the sounds of blocks tumbling into the radial valleys. For the next three days, grey ash-laden clouds were intermittently ejected above the crater, with weak glow and incandescent projections at night. The eruptive sequence ended with one hour of loud explosions and incandescent projections to 500 m above the crater on the 11th. This was accompanied by a marked rise in seismic amplitude (up to 16 mm), but little change in the event rate (950-1,300/day).

"From 14-20 September, S Crater emitted ash-laden vapour up to 600 m above the crater, and there was light ashfall on the NW flank and on coastal villages. It was accompanied by weak-loud roaring sounds and a moderate level of seismicity (~850-1,200 events/day, with amplitudes of 12-14 mm). When this active phase ended on the 20th, the amplitude of the background seismicity rose markedly to ~15 mm. With the outbreak of the next eruptive phase, the amplitude decreased but the daily event count rose to ~1,500.

"Very thin white and blue vapour is all that was emitted by S Crater on 21 September, but from then onwards, large dark ash clouds were rising at 10-20-minute intervals, to 800-1,000 m above the crater. No sound or night glow was visible for the first few days. On the 26th, the ash column reached 2,000 m above the crater and weak incandescent projections were seen throughout the night, reaching ~200 m above the crater at intervals of 1-2 hours. This level of activity, with a background seismicity of 1,400 events/day of moderate amplitude (11-13 mm), lasted until the 28th. The dark emissions became continuous on the 29th but then died out progressively.

"South Crater was mildly active in early October. Weak to moderate emissions of white and grey vapour were released at intervals of 10-20 minutes, resulting in light ashfall downwind. A weak glow and incandescent projections were visible on the nights of 2-3 and 7 October. Throughout this time the seismicity was at a moderately low eruptive level of 1,300-1,500 events/day of 10-14 mm maximum amplitude. The water-tube tiltmeter at Tabele Observatory showed no trend.

"Starting on 14 October, seismicity increased to 15 mm maximum amplitude and Strombolian explosions occurred at intervals of 2-15 minutes, with roaring and explosion sounds. On the 16th, seismicity rose to 1,640 events of 16 mm maximum amplitude, accompanying Strombolian projections 125-320 m above the crater. Through the 17th, the moderately strong and loud Strombolian activity became sub-continuous. Ballistic blocks cascaded down the headwall of SW Valley and into the upper SE Valley. After 1500, a forceful column of ash was rising 6-10 km above the vent. At nightfall, continuous incandescent projections reached 1,100-2,000 m above the crater. The strength of the eruption seemed to increase after midnight until daybreak, with explosions rattling the walls of the . . . observatory. Seismicity peaked-up simultaneously with innumerable events of relative maximum amplitude of 130 mm. A lava flow poured out at a very high rate through a breach on the E side of S Crater and followed the N wall of the SE valley.

"Activity declined during the 18th. The ash column was still rising 4-6 km, with moderately strong roaring sounds and explosions, and the amplitude of earthquakes was still up to 30 mm. The eruption gradually waned after 1630. In the evening, explosions were 2-4 minutes apart, accompanied by weak incandescent projections. The lava flow entered the sea sometime during the night. On the 19th, S Crater had only weak-to-moderate, less forceful emission and seismicity had dropped to non-eruptive levels (~1,000 events/day of 10 mm maximum amplitude). Interestingly, there was no response of the tiltmeter to this eruption.

"Aerial and field inspections on the 18th (R. Middleton) and 19-20th (B. Talai) revealed an absence of pyroclastic-flow deposits, which is unusual for an eruption of this intensity at Manam. The lava flow was of aa-type, <50 m wide up-slope and bounded by levees. It broadened when reaching the base of the terminal cone, between 800 and 600 m elev. It reached a maximum width of ~300 m at 260 m elev where the main front stopped, and a thickness of 3-5 m. The smaller lobe that progressed to the sea following a dry creek on the N side of the valley had a flow front ~100 m wide and 4-5 m high. It extended the coast out by 10-15 m, but had stopped flowing by the 19th. The only damage was to the forest and a copra dryer.

"In the SW valley, effects were limited to a large build-up of talus at the foot of the rock face, down to ~900 m elevation. On the NW side of the island, downwind ash deposits were limited to ~3 mm of fine grey ash with scattered scoria fragments of <1 cm, in a fan area only ~1 km wide. After a 3-day period of inactivity and through the rest of October, weak white and blue vapour emission and weak glow at night recurred.

"All through September, activity at Main Crater consisted of weak, thin to moderately thick emissions of white vapour, without noise or night glow, as in the previous months. There was, somewhat surprisingly, no significant change in the trend and fluctuations of tilt measurements. Activity in Main Crater also remained undisturbed during October, as it released only occasional thin white vapour."

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: C. McKee and P. de Saint-Ours, RVO.

Collapse of the active summit dome on 22 November produced pyroclastic block-and-ash flows and glowing surges that traveled SSW up to 7.5 km from the summit (figure 13). As of 28 November, 41 people had died and another 43 were at hospitals in serious condition. All of the victims lived in areas near the banks of the Boyong River. That river flows off Merapi's S flanks and, at ~28 km map distance from the summit, passes through the city of Yogyakarta (population ~50,000). The threats to areas on Merapi's S flank were noted in February 1994, when rockfalls were first observed and reported along the Boyong River. Every month since March, the possibility of SW-flank destruction had been mentioned in Berita Merapi (Merapi News) informing local governments, including Sleman Regency (where this disaster took place), of hazards posed by nuées ardentes. Rockfalls from the dome have recently traveled down the Boyong and other rivers for distances of 500-1,500 m.

Figure 13. Deposits of the Merapi eruption of 22 November 1994 shown on a 500-m-contour base map of the SW quadrant with the primary drainages and some towns labeled. Courtesy of Sukhyar, MVO.

The eruption was preceded by low-frequency earthquakes on 20 October. Multiphase seismic events and rockfalls continued to be recorded at normal levels, with occasional low-frequency events, but one tremor episode occurred on 3 November. On 4 November this change in seismic behavior was reported to the Chief of Regencies. During 21-22 November, a team from MVO climbed to the summit to observe dome development and to install an extensometer station to measure the offset along cracks.

The first nuée ardente was recorded instrumentally at 1014 on 22 November, and was observed visually from the Plawangan, Ngepos, Babadan, and Jrakah observation posts. The team at the summit saw a vertical plume that originated from a location somewhere on the S part of the dome.

The intensity of the nuées ardentes increased at 1020, prompting the observer at Plawangan to send a warning to the forestry officer at Kaliurang (figure 13), a well-known tourist resort. The officer then yelled a warning to the local people. Five minutes later (1025) MVO instructed all observation posts and radio stations of the Regional Task Force that the alert status had been raised to the highest level (Level 4), and that evacuations should begin. At 1045 the observer at Plawangan sent a message to the Chief of Pakem District, but he was already in the field, probably because he had heard the previous warning. Another evacuation warning was radioed to regional task forces at 1100. By 1215 the first victim had been discovered. The Plawangan observation post was abandoned at 1508 and the personnel temporarily moved to Kaliurang. The nuées ardentes had diminished by 1720 that evening.

A NOAA/NESDIS volcano hazards alert stated that at 1346 on 22 November a plume rose to ~10 km. At that time winds aloft were toward the W at 18 km/hour. These same points were repeated in an aviation safety alert (NOTAM).

A UNDHA report on 23 November stated that 25 of 40 employees building a water treatment facility were still missing, while 15 were found dead. Evacuees totalled 6,026 from the neighboring villages in the subdistrict of Pakem. Evacuation and emergency response measures had been undertaken by the local authorities and community members. The UNDHA reported that local volcanology officials advised authorities and local people to remain on alert for seven days.

A 23 November Tokyo Kyodo broadcast (in English) reported "Indonesia's team for disaster safety in Yogjakarta said ash rain has reached Temanggung, ~45 km NW of Merapi." A UPI news report stated that, on the morning of 23 November, an official of the natural disasters office in Sleman said that 118 people were in three hospitals suffering from serious burns. The report further stated that "hundreds of homes have collapsed and thousands of cattle were buried by ash." On 26 November UPI reported that >4,700 people remained in evacuation centers.

According to press accounts and other information collected by the U.S. Embassy and issued on 23 and 25 November, most of the casualties occurred when superheated gases swept through two small villages (Desa Purwobinangun and Desa Hargobinangun in the Sleman district). The eruption ignited ~500 hectares of rainforest near Kaliurang, which press reports said had been damaged by ashfall. Embassy reports on 25 November stated that an estimated 34-200 people were still missing (there had been no communication with some affected villages on the slopes of the volcano). Well over 500 injured persons had been treated at local hospitals. The 25 November Embassy report said that "Local authorities are now concerned about an accumulation of volcanic material [on Merapi's flanks]. It is feared that the approaching rainy season could dislodge this material (estimated in the range of 11 million m³) causing dangerous [mudflows] in the villages below. City officials in Yogyakarta . . . are reported to be constructing a third catchment dam to regulate volcanic material entering the Code river, which runs through the city."

A 23 November Reuters press report stated that "The official Antara news agency said that despite warnings, local people were reluctant to leave the area, regarding the volcano as sacred and likely to offer some supernatural signs if it were to cause a major disaster."

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: Sukhyar, MVO; SAB; UNDHA; AP; Reuters; UPI; ANS.

Heavy rains caused the nearly dry crater lake to rise 1.8 m with respect to the level in September, filling it enough so that the diameter reached about 180 m. A pan-like structure on the crater floor became covered by silt and pale-green 60°C lake water. In October, a zone of boiling water was located at a site in the NW quadrant of the crater, outside the lake. The zone produced tiny (1- to 2-m high) phreatic eruptions and modest (<100-m high) gas columns. Fumaroles on the dome appeared unchanged. During October, low-frequency seismic events at Poás totaled 3,630 (see table 6).

Geologic Background. The broad, well-vegetated edifice of Poás, one of the most active volcanoes of Costa Rica, contains three craters along a N-S line. The frequently visited multi-hued summit crater lakes of the basaltic-to-dacitic volcano, which is one of Costa Rica's most prominent natural landmarks, are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the 2708-m-high complex stratovolcano extends to the lower northern flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, is cold and clear and last erupted about 7500 years ago. The more prominent geothermally heated northern lake, Laguna Caliente, is one of the world's most acidic natural lakes, with a pH of near zero. It has been the site of frequent phreatic and phreatomagmatic eruptions since the first historical eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.

Information Contacts: E. Fernández, J. Barquero, and V. Barboza Moreira, OVSICORI-UNA; G. Soto and F. Arias, ICE; M. Mora, UCR.

During late-October, Carlos Valdéz-González and co-workers identified a sudden, prominent (roughly 1.6- to 10-fold) increase in daily earthquakes compared to previous months (figure 4). Station locations and the terms "A-", "B-", and "AB-type" were previously defined (19:1-2). Although Figure 4 shows only B-type events, the other two types remained at 0-1 events/day during September and October. Prior to mid-October, the daily count of B-type events generally remained below 10, but by 28 October they climbed to 26. The B-type events for the first half of 1994 were previously published (19:06). Carlos Valdés-González noted that this was the fastest rate of increase in the last 23 months.

Figure 4. Daily number of B-type seismic events at Popocatépetl, May-October, 1994. Courtesy of Carlos Valdés-González, UNAM.

Ignacio Galindo contributed the following report.

"A new series of ultraviolet absorption correlation spectrometry (COSPEC) measurements was made by scientists from Univ de Colima (A. González, J.C. Gavilanes and C. Navarro), UNAM (H. Hidalgo) and USGS (T. Casadevall) on 5 November from a rented Cessna 310 airplane. The measurements were requested by the Secretaría de Gobernación through the Centro Nacional para la Prevención de Desastres (CENAPRED). Between 1024 and 1148 on 5 November, the plume was traversed 12 times at an altitude between 3,539 and 4,545 m a.s.l. [above sea level] in partially cloudy conditions. The aircraft's global positioning system (GPS) computed the wind speed independently for each traverse. These measurements were each used to make individual SO₂ flux calculations, removing the need to use average wind speed (19:08). This procedure is advantageous when the wind speed varies significantly. SO₂ data were sent to a datalogger, besides the typical COSPEC strip chart. All the recorded data were transferred into a personal computer where evaluation software produced the final SO₂ results together with a statistical analysis of the time series. A manual SO₂ determination using data from strip chart records (as reported in 19:08) was also made by C. Navarro; it reproduced the average values within 2.4% on average.

"The SO₂ flux on 5 November ranged from 924 to 1,877 metric tons/day (t/d), with a standard deviation of 285 t/d and an average value of 1,261 t/d. Table 1 compares our recent measurements with those of 4 May, which were determined with the same methodology (19:04). The SO₂ flux increased substantially between 4 May and 5 November. Although our determinations show absolute values less than those reported by other authors (19:1 & 8), both data sets show increased SO₂ flux."

Table 1. Popocatépetl SO₂ flux measurements on 4 May and 5 November 1994. Courtesy of Ignacio Galindo, Univ de Colima.

Geologic Background. Volcán Popocatépetl, whose name is the Aztec word for smoking mountain, rises 70 km SE of Mexico City to form North America's 2nd-highest volcano. The glacier-clad stratovolcano contains a steep-walled, 400 x 600 m wide crater. The generally symmetrical volcano is modified by the sharp-peaked Ventorrillo on the NW, a remnant of an earlier volcano. At least three previous major cones were destroyed by gravitational failure during the Pleistocene, producing massive debris-avalanche deposits covering broad areas to the south. The modern volcano was constructed south of the late-Pleistocene to Holocene El Fraile cone. Three major Plinian eruptions, the most recent of which took place about 800 CE, have occurred since the mid-Holocene, accompanied by pyroclastic flows and voluminous lahars that swept basins below the volcano. Frequent historical eruptions, first recorded in Aztec codices, have occurred since Pre-Columbian time.

Information Contacts: Guillermo González-Pomposo¹, Carlos Valdés-González, and A. Arciniega-Ceballos, Departamento de Sismología y Volcanología, Instituto de Geofísica, UNAM; Ignacio Galindo, Arturo González, J.C. Gavilanes, Carlos Navarro, CUICT-Univ de Colima; Hugo Delgado, Instituto de Geofísica, UNAM; T. J. Casadevall, USGS; ¹Also at Benmérita Univ Autónoma de Puebla.

"The eruption . . . continued throughout October. However, only one of the two centres initially active, Tavurvur, on the NE part of the caldera, remained in eruption. It displayed moderate Vulcanian-type activity, accompanied by the production of a lava flow. Eruptive activity at the other intra-caldera cone, Vulcan, on the W side of the bay, ended on 2 October. Thereafter, its activity was reduced to weak fumaroles and bubbling pools of water at the bottom of its new NE crater.

"Overall, the level of activity at Tavurvur progressively decreased, in spite of variations in the strength, frequency, ash content, and height of its Vulcanian explosions. Only one crater was active on the E side of the cone; up to four were active in the first few weeks of the eruption. During the first few days of October, explosive phases occurred at intervals of 30-120 seconds. They produced billowing columns rising dynamically, with large ballistic fragments, up to 400-800 m above the crater. In between, ash emission was usually continuous though less forceful. Occasionally, the vent remained free of emissions for a few minutes. A second vent on the W side of the same crater occasionally produced a darker but weaker emission, with apparently unrelated frequency. Depending on wind strength, the emission plume levelled off between 1 and 2 km height, and spread W over the town of Rabaul, the pale yellow to brown mass remaining visible for 20 km.

"Through October, the interval between explosive phases increased, though irregularly, to 1-4 minutes. Explosions were irregular in strength but rose less and less frequently to >600 m, and the ash content of the plume decreased. The visible extension of the plume also decreased to ~15 km. Longer periods of weak activity were commonly followed by larger (and louder) explosions that ejected ballistic material as far as 1.5 km from Tavurvur's summit, onto the lower slopes of the cone or into Greet Harbour. During periods of lesser ash content in the emission, these projections caused incandescent night displays (22-27 October). At times of dense ash emission, lightning occurred under and around the plume. Sound effects of the eruption were variable. Rumbling sounds were the most common and apparently louder during periods of lesser ash content in the emission. At other times, Tavurvur could be silent for a couple of hours, or even days, without noticeable change in activity. The largest explosions (like at 0640 on 14 October or 2125 on the 16th) were heard as impressive, sharp detonations up to 20 km away and their air-waves were felt up to 10 km away.

"Backfall of material around the vent progressively built a cone ~30 m high with a radius of ~80 m. Light ashfall on the town of Rabaul and beyond it on the N coast continued throughout October. The first torrential rainfalls of the pending rainy season contributed to the major destruction within the town area. Most buildings in the S and central parts of Rabaul township collapsed under the weight of 0.3-1.2 m of ash/mud. Subsequent rainfalls also caused large flash-floods of mud that temporarily cut off access roads and flooded several buildings and villages. Earthmoving equipment was used to construct drains and barriers in an attempt to alleviate destruction in the remaining parts of town from expected mudflows at the start of the rainy season in December.

"A viscous lava flow, aa to blocky in texture, began on 30 September from a source SW of the main active vent of Tavurvur. Its flow rate was extremely low and its progression slow. On 5 October, as this lobe was still moving within the lower W part of the crater, a new lobe formed and started to override it. On the 8th, an outbreak of apparently more fluid, darker lava started on the W side of the original lobe source. The two initial lobes merged together on 12 October as they started to spill over the lower side of the crater rim onto the W flank of Tavurvur cone. On the 14th, a new lobe started to form from an outbreak through the flow, near the initial source. This became the main feeder to the combined flow system, although it progressed slower and slower until 25-27 October when the flow-front stopped ~100 m below the rim of the cone, 2/3 of the way to the coast.

"The extensive pumice raft, formed as a result of the early Plinian phases and pyroclastic surges, kept drifting across the bay in response to wind shifts. At times of strong SE winds it occupied the N half of the bay, packing to thicknesses of up to 1.7 m (G. Halls, Hydrographic Surveys, Pty Ltd, pers. communication). A few hours of lull or a reversal in the trade wind, and it decompressed and spread over the SE part of the bay, only to drift back a few hours later.

"Ten of the 14 stations of the RVO seismic network were progressively disabled by volcanic products, lightning, interruption of power supply, or vandalism, within the first week of the eruption. By early October, however, in a prompt response to an RVO and PNG Government invitation, a team from the USGS Volcano Disaster Assistance Program was on-site deploying a network of 10 digitized stations with P-picker, Tom Murray's RSAM, and Willie Lee's data management systems on personal computers.

"Following the end of eruptive activity on the Vulcan side, seismicity was scattered under the whole caldera, including outside the usual annular seismic zone. A high concentration of events at Tavurvur corresponded to explosion earthquakes. The level of seismicity indicated by RSAM and the number of detected events showed a general decline, with some fluctuations, throughout the month (figure 20). Most detected events consisted of low-frequency and explosion earthquakes with delayed air-phases distinctive throughout the network.

Figure 20. Fluctuations in the level of seismicity recorded at Rabaul, October 1994. Courtesy of RVO.

"All real-time ground deformation monitoring (electronic tilts and tide gauges) had progressively been lost over the last few years prior to the eruption by lack of appropriate funding. From the onset of the eruption, ash density in the bay prevented EDM monitoring. For the first week thereafter the only accessible ground deformation data were from two water-tube tiltmeters on the outer caldera rim. They indicated radial deflation of the caldera, which started with the triggering earthquakes (M_L 5.1) on 18 September and amounted to 30 and 37 µrad, respectively, by the end of September. By late September a few other stations had been recovered, including a dry-tilt array near the centre of the caldera at the S end of Matupit Island. In early October two electronic tiltmeters were deployed by the USGS team. Sea shore surveying around the bay resumed on 27 September, and geodetic levelling to Matupit Island on 4 October.

"All collected data revealed a caldera-wide subsidence amounting to ~1 m near the centre and 20-30 cm near the edges. The resulting bowl-shaped subsidence is, however, perturbed by the residuals of a pre-eruption uplift on the night of 18-19 September around the two pending eruptive centres, which amounted to 5-6 m on the E shore of Vulcan and 1-2 m at Tavurvur and Matupit Island. Minor caldera subsidence continued through October, although mainly affecting the central area within 3 km of Tavurvur. The maximum measured subsidence amounted to 20 cm at the Tavurvur tide gauge, near the long-recognized apex of ground deformation, with progressively decreasing rates from ~1.5 to 0.4 cm/day. Simultaneously, the Matupit Island tiltmeter recorded a deflation of >110 µrad, radial to the same centre of deformation, at a slowly decreasing rate (figure 21)."

Figure 21. Changes recorded by the Matupit Island tiltmeter, October 1994. Although an upward trend is seen on the plot, the change reflects a steady deflation of the central part of the caldera. Courtesy of RVO.

Geologic Background. The low-lying Rabaul caldera on the tip of the Gazelle Peninsula at the NE end of New Britain forms a broad sheltered harbor utilized by what was the island's largest city prior to a major eruption in 1994. The outer flanks of the 688-m-high asymmetrical pyroclastic shield volcano are formed by thick pyroclastic-flow deposits. The 8 x 14 km caldera is widely breached on the east, where its floor is flooded by Blanche Bay and was formed about 1400 years ago. An earlier caldera-forming eruption about 7100 years ago is now considered to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the northern and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and western caldera walls. Several of these, including Vulcan cone, which was formed during a large eruption in 1878, have produced major explosive activity during historical time. A powerful explosive eruption in 1994 occurred simultaneously from Vulcan and Tavurvur volcanoes and forced the temporary abandonment of Rabaul city.

Information Contacts: C. McKee and P. de Saint-Ours, with additional contributions fromRVO Staff, RVO; T. Murray, A. Lockhart, and E. Endo, CVO; R. Johnson, AGSO; H. Davies, Univ of Papua New Guinea.

Seismic station RIN (5 km W of the active crater) received 31 events of high-frequency. The events were only detected locally, they had Richter magnitudes of less than 1, and S minus P times of less than 2 seconds. For comparison, during April, the local seismic station received only 13 low-frequency events. In contrast, there were 283 low-frequency events during the previous month, the most reported so far this year.

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: E. Fernández, J. Barquero, and V. Barboza, OVSICORI.

An eruption in June (19:05) sent ash plumes 2,000 m above the summit, resulting in ashfall on nearby villages. Activity of some kind was apparently continuing in late October. A NOTAM from the Bali FIR reported a volcanic ash cloud up to 900 m above the summit, with an average of one eruption per day.

On 3 November, a cold lahar from the summit area traveled down the Kokok Jenggak River. Thirty people from the village of Aikmel who were collecting water from the river were killed; one person remained missing as of 9 November. No damage to the village was reported. Local volcanologists noted that additional lahars could be triggered by heavy rainfall.

Geologic Background. Rinjani volcano on the island of Lombok rises to 3726 m, second in height among Indonesian volcanoes only to Sumatra's Kerinci volcano. Rinjani has a steep-sided conical profile when viewed from the east, but the west side of the compound volcano is truncated by the 6 x 8.5 km, oval-shaped Segara Anak (Samalas) caldera. The caldera formed during one of the largest Holocene eruptions globally in 1257 CE, which truncated Samalas stratovolcano. The western half of the caldera contains a 230-m-deep lake whose crescentic form results from growth of the post-caldera cone Barujari at the east end of the caldera. Historical eruptions dating back to 1847 have been restricted to Barujari cone and consist of moderate explosive activity and occasional lava flows that have entered Segara Anak lake.

Information Contacts: UNDHA; BOM Darwin, Australia.

Several hours of observations were made on 7 August by J. Sesiano from the N rim of Jonggring Seloko crater. Gas-and-ash plumes rose hundreds of meters above the crater. Generally mild explosions occurred at intervals of ~15-20 minutes, each resulting in a white plume that barely rose above the crater rim. The explosions originated from the same vent where very slow lava extrusion was feeding a flow moving SE that exhibited red glow and incandescent cracks at night. Based on the movement of unique morphological features of the lava flow, a velocity of tens of meters/day was estimated. Incandescent boulders were thrown from the flow front by violent explosions that occurred an average of 4-5 times/day. Collapses of the lava flow, located on a 35° slope, sent boulders down into the valley accompanied by small pyroclastic flows. Whistles and roaring noises were heard almost continuously, similar to the noises heard at a busy airport: jets taking off, landing, turning off engines, and disappearing into the distance. Thunder-like claps, rhythmic pulses (~1 Hz frequency, for ~10 minutes), and other sounds could also be heard. Seismicity recorded by VSI during 5-14 August indicated that activity was at normal levels, with 40-100 explosion events/day (19:07).

A NOTAM issued from the Bali Flight Information Region (FIR) on 24 October noted volcanic ash from Semeru, but the cloud top and drift direction were unknown.

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

Information Contacts: J. Sesiano, Univ de Genéve; BOM Darwin, Australia.

Seismicity remained at normal levels (1-4 events/day) through the second half of September and early October. A gas-and-steam plume rose ~800 m above the extrusive dome during 18-24 September. Starting on 4 October, daily seismicity rose to 9 events, followed by 21 events the next day and 14 events on 6 October. By 9 October the gas-and-steam plume was rising up to 1,000 m above the crater rim and was directed NE for ~1 km. Seismicity at or near the active dome remained above normal (5-15 events/day), and weak tremor was recorded for ~30 minutes/day during 8-26 October. A gas-and-steam plume rising 1,000-2,500 m above the crater was observed from Kliuchi (8 km S) on 8-15 October. The plume rose 400 m above the crater on the 23rd and 200 m on the 27th; the volcano was obscured by clouds the remainder of the time through 3 November. Seismic activity in late October-early November remained above normal levels, with 7-19 events/day occurring at or near the active dome, and weak volcanic tremor lasting for 24-84 minutes/day.

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

Information Contacts: V. Kirianov, IVGG; AVO.

Following the slow decrease of tremor energy during June, all seismicity increased in July (figure 36). Tremor energy reached an unusually high peak on 27 July; at the same time, a peak in the number of events was recorded. Although more events were recorded on 19 July (864), that was a period of almost continuous explosive activity. A considerable number of saturating events were recorded after 20 July. Volcano guides observed very strong external activity, with pyroclastic material often reaching the usual tourist zones. A decline in tremor energy was observed after 10 August; a slow increase then followed, reaching a new maximum at the end of the month. The number of recorded events followed a similar trend. Another major decrease in tremor energy characterized the first half of September; later fluctuations remained in a "low-energy" range. Vigorous eruptions seen on 21-22 August occurred during a period of low seismicity compared to late July and late August.

Figure 36. Seismicity recorded at Stromboli, 27 June-29 September 1994. Open bars show the number of recorded events/day, the solid bars those with ground velocities >100 Nm/s (instrument saturation level). The line shows daily tremor energy computed by averaging hourly 60-second samples. The seismic station is located 300 m from the craters at 800 m elevation. Courtesy of R. Carniel.

Observations of crater activity were made by R. Carniel (Univ of Udine) during field work with R. Schick (Univ of Stuttgart) and collaborators at the end of September and early October. Similar observations were made by geologists from Open Univ during 1-13 October, with detailed explosion counts for 3 hours on 1 October, 4 hours on the 5th, and one hour on the 9th. Explosions sent incandescent ejecta, ash, and/or gas to heights of <=300 m, from as many as 10 active vents (figure 37). No active vents were observed in Crater 2, but a hornito (2/1) was visible, and there was minor degassing from an unknown source. Brightness temperatures of fumaroles along the zone E of the Pizzo Sopra la Fossa (39-77°C) were measured by Open Univ geologists with a Minolta/Land Cyclops Compac 3 hand-held radiometer (8-14 mm).

Figure 37. Sketch map of the active craters at Stromboli, 1-13 October 1994. Courtesy of A. Harris [and A. Maciejewski].

Within Crater 1 in late September, Carniel noted three cones ~25 m high that had been built during the very strong activity in July and August (1/5, 1/6, & 1/7; figure 37). Continuous red glow at night could be seen from the top of each. Two other Crater 1 vents were active, the first (1/4) producing short, lateral explosions with large pyroclasts ejected onto the Sciara del Fuoco, and the second closer to Pizzo producing longer and higher explosions (<=200 m). Directed explosions suggested the possibility of a third vent close to the second one. When one of the two W-most cones in Crater 1 erupted (typically with strong degassing and little pyroclastic material) the other exhibited weak degassing. When the second vent erupted, the red glow from the remaining cone strengthened, sometimes with minor degassing.

Crater 1 contained six active vents during visits by Open Univ scientists. Explosions from vents 1/1 (~2/hour), 1/2 (4-9/hour), and 1/3 (0-2/hour) sent incandescent ejecta, occasionally with ash, to heights of 30-250 m. Glow was seen above 1/1 and 1/2 on the night of 5 October. Up to 40% of the ejecta from 1/2 and 1/3 fell outside of the crater area. These explosions were often followed by a gradually fading gas-jet noise of variable length. Explosions seen by the Open Univ team from 1/4 (2/hour) sent incandescent ejecta, including bombs and spatter, 30-150 m E onto the Sciara del Fuoco. On 5 October hornito 1/5 was the source of gas-jet eruptions, and a small amount of incandescent ejecta rose ~50 m; during 10 October more ejecta were seen in 100-m-high gas jets. Hornito 1/7 constantly degassed, and its summit vent was incandescent with a continuous gas flare 1-2 m high. On 10 October this flare increased 1-2 seconds before vent 1/3 erupted. Hornito 1/6 and vents 1/8 and 1/9 vents were only degassing.

The lava pond in Crater 3 had become a small spatter cone (3/2) when observed by Carniel, with a hole through which magma could be seen; activity was limited to degassing. One vent produced high, black, mushroom-shaped columns, and the second (in front towards Pizzo) sent pyroclasts >200 m above the craters. The opening of a new vent was also observed. Explosions from Crater 3 on 28 September were stronger, although less frequent, than from Crater 1. On 5 October the same sequence was observed, with the second vent exploding first and fewer pyroclasts ejected near the end of the explosion by a very small vent to the right of the older one. Guides reported that this vent was first observed on 1 October, when similar explosions from the small vent ejected spatter.

Open Univ geologists noted that only vent 3/2 was active on 1 October, with 3 emissions/hour of brown ash and blocks. By 5 October the quantity of ash emitted had decreased, but the amount of incandescent ejecta had increased, and more frequent explosions (5/hour) were accompanied by loud detonations. Ejecta rose 80-300 m, with some material landing outside of the crater or on the inner crater wall. During night observations on 5 October vent 3/2 would start erupting ~1-3 seconds after 3/1. On 8 October, Crater 3 released gas, sometimes accompanied by minor amounts of ejecta <30 m above the crater rim, and small brown ash clouds 30-100 m high. Similar activity on 9 October was accompanied by an increasing amount of brown ash and incandescent ejecta. During 1 October small lava fountains from vents 3/3 and 3/4 were simultaneous with gas emissions from 3/3. Vent 3/4 was also continuously active with puffs of gas (~1/second). The interior of vent 3/4 was incandescent by day, and glow was observed above 3/1, 3/2, and 3/3 at night. During the night of 5 October the brightness temperature of 3/4 was measured as 873°C, using a Minolta/Land Cyclops 152 hand-held radiometer (0.7-1.1 mm), similar to October 1988 (13:11). Incandescent gas puffs were seen above 3/4 during the night of 10 October. Only minor gas emission was observed from vent 3/5.

Geologic Background. Spectacular incandescent nighttime explosions at this volcano have long attracted visitors to the "Lighthouse of the Mediterranean." Stromboli, the NE-most of the Aeolian Islands, has lent its name to the frequent mild explosive activity that has characterized its eruptions throughout much of historical time. The small island is the emergent summit of a volcano that grew in two main eruptive cycles, the last of which formed the western portion of the island. The Neostromboli eruptive period took place between about 13,000 and 5,000 years ago. The active summit vents are located at the head of the Sciara del Fuoco, a prominent horseshoe-shaped scarp formed about 5,000 years ago due to a series of slope failures that extend to below sea level. The modern volcano has been constructed within this scarp, which funnels pyroclastic ejecta and lava flows to the NW. Essentially continuous mild Strombolian explosions, sometimes accompanied by lava flows, have been recorded for more than a millennium.

Information Contacts: R. Carniel, Univ di Udine; A. Harris and A. Maciejewski, Open Univ.

The 4th anniversary of Unzen's current eruptive episode took place on 17 November. During the first half of November, Unzen's surface activity reached the lowest level seen in the course of 3.5 years of lava dome growth; earthquakes also reached a low level. From mid-October through mid-November the eruption had a low rate of lava extrusion (<10⁴ m³/day) and a low frequency of pyroclastic flows.

During November, only the N slope moved, and the dome's slow endogenous growth produced velocities as low as a few meters in several tens of days. During mid-October to mid-November the top of the endogenous dome occupied an area 400 x 300 m that was covered with oxidized lava fragments and blocks. During this interval the dome's top became flat to partly convex downward. A small spine 20 m across sprouted near the center of the flat dome top in early October. Extrusion during October caused the spine to rise at the rate of 1 m/day, double the November rate. By mid-November the spine had reached ~50 m high.

Small rockfalls originated at the uppermost NE slopes on the endogenous dome. They typically took place episodically, with many falls confined to a few days during intervals of 2-3 weeks. Some of them developed into pyroclastic flows with travel distances <2 km. During mid-October through mid-November pyroclastic flows lacked accompanying pyroclastic surges. On 26 and 27 October, partial collapses of lava blocks from old lobes generated pyroclastic flows, which traveled ~2.5 km SE and ~2.2 km NE. No pyroclastic flows took place in early to mid-November, which probably reflects the low extrusion rate during this period; in contrast to earlier large Merapi-type pyroclastic flows that seemed to result from large collapses driven by high extrusion rates.

COSPEC analysis by the Tokyo Institute of Technology in late September showed that SO₂ flux from the dome had remained at the low value of ~40 t/d since February 1994. Based on air-photograph measurements by the Geographical Survey Institute of Japan, the total volume of magma erupted from May 1991 to September 1994 was 0.20 km³ (dense-rock-equivalent value), twice the volume of the current dome (0.10 km³). The average eruption rate from February until the beginning of September (7 months) was 6 x 10⁴ m³/day (±2 x 10⁴ m³/day).

During October, microearthquakes detected 3.6 km W of the dome (station A) totaled 993; seven pyroclastic flows were caused by dome collapse. The pyroclastic flows were detected remotely using a seismic station 1 km WSW of the dome and four sets of visible and infrared video cameras.

Geologic Background. The massive Unzendake volcanic complex comprises much of the Shimabara Peninsula east of the city of Nagasaki. An E-W graben, 30-40 km long, extends across the peninsula. Three large stratovolcanoes with complex structures, Kinugasa on the north, Fugen-dake at the east-center, and Kusenbu on the south, form topographic highs on the broad peninsula. Fugendake and Mayuyama volcanoes in the east-central portion of the andesitic-to-dacitic volcanic complex have been active during the Holocene. The Mayuyama lava dome complex, located along the eastern coast west of Shimabara City, formed about 4000 years ago and was the source of a devastating 1792 CE debris avalanche and tsunami. Historical eruptive activity has been restricted to the summit and flanks of Fugendake. The latest activity during 1990-95 formed a lava dome at the summit, accompanied by pyroclastic flows that caused fatalities and damaged populated areas near Shimabara City.

Information Contacts: S. Nakada, Kyushu Univ; JMA.

Beginning about 0730 in the morning of 26 September residents of the Centro de Ski Villarrica-Pucón (a ski resort) saw "scrolls of black vapor" emitted about once each minute from the main crater of Villarrica volcano. Vapor rose ~500-750 m above the summit. . . . Four such small explosions took place in the morning, the last, at 1100, coincided with a strong tremor felt at the ski resort.

Figure 3 shows the ash distribution seen by aerial observers in the upper part of the ski area (Piedra Blanca). The distribution was composed of thin ash chiefly visible due to the contrast with the white snow. One part of the ash distribution was bounded by a SE-trending band of heavier deposition. This ash fall deposit extended over 8 km, visible to the east as far as the limit of contrasting background snow.

Figure 3. Ash distribution following the 26 September 1994 Villarrica eruption (mapping by Hugo Moreno on 26 September).

Later on 26 September, between 2030 and 2130, observers saw incandescence above the crater that they attributed to glowing lava in the crater reflected in the fumarolic column. The next day (27 September) was partly cloud-covered, but strong fumarolic activity formed low-lying scrolls directed toward the E. Later, during a clearing in the clouds, observers saw a 500-m-long ash fall layer extending W.

Several seismic stations were installed on 26 September. Although two seismic stations were installed farther from the summit, it was not until 1630 that the station closest to the summit was installed near the Rio Voipir (at the 500-m contour, 13.5 km E of Villarrica). The record there showed continuous harmonic tremor along with other seismic events until about 2110. After that, and until 0600 on 27 September, tremor fell abruptly; however, three long-period volcanic earthquakes occurred in this interval. At 0700 harmonic tremor returned.

Starting at both 0741 and 0800 similar seismic sequences consisted of early events followed by a later event. The same sequence repeated about every 4 hours until the last one ended at 1000 on 28 September. The 4-hour sequence was interpreted as magmatic injections leading to gas-charged explosions. Thus, the main part of the eruptive episode lasted ~3.5 hours (0730-1100 on 26 September). It produced a magmatic eruption with a VEI of 1. The seismic signature associated with frequent gas-charged explosions was not previously seen at this volcano.

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: H. Moreno, G. Fuentealba, and M. Petit-Breuilh, SERNAGEOMIN, Temuco.

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

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

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

Geologic Background. The word volcano is derived from Vulcano stratovolcano in Italy's Aeolian Islands. Vulcano was constructed during six stages during the past 136,000 years. Two overlapping calderas, the 2.5-km-wide Caldera del Piano on the SE and the 4-km-wide Caldera della Fossa on the NW, were formed at about 100,000 and 24,000-15,000 years ago, respectively, and volcanism has migrated to the north over time. La Fossa cone, active throughout the Holocene and the location of most of the historical eruptions, occupies the 3-km-wide Caldera della Fossa at the NW end of the elongated 3 x 7 km island. The Vulcanello lava platform forms a low, roughly circular peninsula on the northern tip of Vulcano that was formed as an island beginning in 183 BCE and was connected to Vulcano in about 1550 CE. Vulcanello is capped by three pyroclastic cones and was active intermittently until the 16th century. The latest eruption from Vulcano consisted of explosive activity from the Fossa cone from 1898 to 1900.

Information Contacts: A. Harris, Open Univ.