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).

Gas emissions and lava flows continued at Crater C in July, as well as sporadic Strombolian activity that sent ash columns >500 m above the crater rim. Explosions have decreased in quantity and magnitude compared to June. The lava flow that started in mid-April on the SW flank remained active as its W lobe descended another 50 m to 1,150 m elevation and the SW lobe traveled 150 m down to 1,100 m. A new lobe from this flow that formed in early July descended to the SW. A new lava flow was also observed on 20 July moving WNW. At 1022 on 21 July a rock avalanche traveled NW to 1,300 m elevation. Another avalanche was observed later that day at 1100, and seven more occurred from 1314 and 1355. The lowest elevation reached by the fronts of these nine avalanches was 1,150 m.

Three distance lines continued to show minor contraction in 1993. The last measurement was made on 21 July. The radial leveling line on the W flank indicated deflation of 8.9 µrad between 26 August 1992 and 21 July 1993. Deformation measured at most dry-tilt stations was insignificant. However, three stations on the SW and W flanks exhibited cumulative inflation of ~10 µrad since 15 April. The seismic station 2.7 km NE of the active crater (VACR) registered 651 events in July, with the highest daily total on 21 July. Most of the seismicity was associated with ash eruptions and pyroclastic activity. The level of explosive activity was less in June and July than earlier in the year. Tremor averaged 13 hours/day, with the highest daily total of 23 hours recorded on 8 July. There was more tremor in July than in any previous month this year.

The following report, of activity from 25 July to 4 August, is from W. Melson (SI), and V. Barboza and Jorge Barquero (OVSICORI). "Arenal showed an extremely low level of pyroclastic activity during this period, the lowest recorded during regular periods of observations by Smithsonian Institution volunteers since 1987. There have been 18 such observation periods, each lasting about 10 days for 24-hours/day, from the Arenal Observatory Lodge about 2.8 km S of the summit (figure 59). Periods of high tremor were common. Ongoing 3-component seismic measurements have shown that tremor produces weak vertical signals but strong horizontal signals (figure 60). The dominance of horizontal over vertical motion has been characteristic of Arenal's seismicity over the past 3 years. Local observers reported that a large fragment of the WNW rim collapsed around 19 or 20 July, forming a notch through which lava continued to move as of 4 August. The lava flow was moving slowly (~10-30 m/day) down the WNW slope, and had reached ~1,100 m elevation below the new cone being built to an approximate elevation of 1,650 m above old Crater C."

Figure 59. Summary of pyroclastic-flow activity at Arenal since 1987 during 18 approximately 10-day periods of 24-hours/day observations from the Arenal Observatory Lodge, 2.8 km S of the summit, made by Earthwatch and Smithsonian Institution volunteer expeditions. "Chug" and "whoosh" events are characterized by their sound. Courtesy of W. Melson.

Figure 60. Tremor levels at Arenal during a 2-hour period on 25 July 1993. Values are averaged over 21.5-second intervals and calculated as equal to the sum of the absolute values of signals in decimillivolts divided by number of counts/21.5 seconds at an AD conversion rate of about 100 counts/second. A Mark L-4C 3D seismometer was used with a 16 bit AD board. Courtesy of W. Melson.

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, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, and R. Sáenz, OVSICORI; W. Melson, SI.

A submarine eruption along the Juan de Fuca Ridge (JDFR) was detected in late June 1993 by the NOAA using hydrophones. Plumes and a lava flow were subsequently identified by research ships along the Coaxial Segment (figure 1), located ~435 km W of the Oregon coast.

Figure 1. General plate tectonic map of the NE Pacific Ocean showing the North American, Pacific, Explorer, Juan de Fuca, and Gorda plates. The Coaxial Segment is just N of Axial Volcano along the Juan de Fuca Ridge. Courtesy of Bill Chadwick, NOAA.

On 26 June a burst of low-magnitude and low-frequency (1-30 Hz) T-phase seismicity (~31 events in the first hour) was accompanied by what appeared to be harmonic tremor near the terminus of the N rift zone of Axial Volcano (a hot-spot volcano adjacent to the ridge) near 46.23°N, 129.83°W. Activity migrated ~37 km NNE over the next two days, with the final locus of activity near 46.52°N, 129.60°W. Another swarm of events occurred on 4 July; activity then declined to a few minor events/day. All T-phase activity was probably below the threshold of land-based seismometers, mb < 2.5, though some events may have been larger. Real-time monitoring of the Juan de Fuca Ridge by the NOAA Vents T-Phase Project had begun only four days earlier, on 22 June, using the U.S. Navy Sound Surveillance System (SOSUS). The raw data are transferred to NOAA's Newport, Oregon labs, where time-versus-frequency plots for selected locations are continuously displayed and monitored.

Scientists aboard the Canadian Survey Ship Tully performed Conductivity-Temperature-Depth (CTD)/transmissometer casts on 1-3 July between 46.20°N and 46.55°N, primarily along the W ridge of a large graben where the NE flank of Axial Volcano merges into the JDFR. A transmissometer is a device that measures the amount of particulate matter in the water by shining a light across a short distance and measuring the amount of light received at the other end. Plumes can be mapped in this way independent of temperature anomalies. Plumes were found along the central axis of the graben, extending between approximately 46.33°N and 46.50°N, in close agreement with the T-phase locations. Strong plumes within several hundred meters of the seafloor (bottom depth ~2,400 m) were detected at 46.50°N, 129.59°W, and there was an intense plume at 1,700-1,800 m depth, at least 700 m above the bottom.

On 9 July, the NOAA Ship Discoverer arrived at the site with a full suite of instrumentation, including CTD/transmissometer, chemical scanner, SeaBeam, seafloor volcanic system monitors (rumbleometers), and the Canadian remotely operated vehicle system, ROPOS, equipped with acoustic and differential GPS navigation. The next day, Discoverer's first cast, at 46.50°N, 129.60°W, confirmed the presence of both deep and shallow plumes. Plumes were identified on the basis of thermal and light-attenuation anomalies, as well as large anomalies in dissolved Mn and Fe using a continuously sensing in-situ chemical scanner. Over the next several days, two along-axis, several cross-axis tow-yos, and several vertical CTD casts were completed. The term "tow-yo" describes the process of alternately lowering and raising the CTD instrumentation during the tow, making a saw-toothed survey through the water column. This allows measurement of the upper and lower boundaries of plumes, as well as their lateral extent in two dimensions. The first along-axis CTD tow mapped the large plumes along the segment and above the N swarm area on 12 July.

Lower plumes, extending 39 km along the axis from 46.23°N to 46.57°N, were characterized by temperature anomalies as high as 0.07°C centered ~150-200 m above the seafloor; in some places the temperature anomalies extended within 10 m of the seafloor (generally the closest approach height of the CTD). Upper plumes were well-defined by temperature, light-attenuation, and chemical anomalies, with a maximum rise height of ~800 m and a maximum temperature anomaly of at least 0.18°C. The upper plumes were first found near 46.50°N and later near 46.27°N, and appeared to have a distinct layered structure, perhaps indicative of separate injections of hydrothermal fluid. Although these plumes have rise heights and temperatures similar to the megaplumes discovered over the Cleft Segment of the JDFR (~160 km S at 44.5-45.0°N) in 1986 and 1987, their volume appears much smaller, only several kilometers along and across axis. Preliminary results indicate dissolved Mn and Fe anomalies in the upper plumes exceeding those found in the 1986 and 1987 megaplumes. The well-defined N edge of the plumes approximates the extent of the T-phase swarm area. The S boundary of the plumes is more diffuse, but there is a fairly distinct diminishment at about 46.23°N, approximately the boundary of the S swarm.

Preliminary analysis of radon-222 in plume samples taken on 12 and 19 July shows a distinct decrease in values for the upper plume, supporting the presumption that they were produced by powerful but short-lived hydrothermal discharges and were not resupplied by continuous venting. Radon-222 estimates for the cessation of the hydrothermal event that produced these upper plumes range from 28 June to 1 July, or 2-5 days after the initial T-phase activity. The lower-plume radon-222 values did not decrease between sampling, indicating that they were sustained by continuous venting in mid-July.

The most intense hydrothermal activity was from the center of the axial valley over about 46.52°N. Because of the coincidence between the T-phase swarms and the hydrothermal activity, this area was chosen for detailed study with the remotely operated vehicle ROPOS. The dive traversed the valley floor from E to W and encountered sediment-free, glassy lava on the W-facing slope of a ridge just N of a small, older seamount centered at 46.52°N, 130.59°W. Subsequent traverses over the ENE-oriented lava ridge revealed it to be ~300 m wide and at least 1,500 m long. A 100-m-wide zone centered over the crest was venting warm water through interstices of unfractured pillow and lobate lava flows. Water temperatures were up to 13°C, and the chemical scanner measured high levels of Fe and Mn in the vent fluids. The S end of the lava ridge was within a graben imaged by the ROPOS's Mesotech sonar system. The next ROPOS dive started at the S terminus of the lava ridge and continued S along the trend defined by the ridge crest. An actively venting fracture system was crossed on every subsequent traverse across this trend, extending 6.5 km S to 46.47°N, with water temperatures up to 23°C. The fracture system crosses a lava plateau that bisects the axial valley.

Basalt samples from the recent flow are very fresh-looking lobate to pillowed types. Upper glass selvages are up to 2.5 cm thick and are sparsely plagioclase-phyric. The glass selvages have no sediment cover (except in areas of hydrothermal discharge) and exhibit a shiny iridescent surface. Some of the samples have surficial staining (probably elemental sulfur) as the result of contact with hydrothermal fluids.

The volcanic zone of the Coaxial Segment between 46.10°N and 46.65°N was resurveyed with SeaBeam multibeam sonar. By the time data collection and analysis were complete, a new lava flow had been located with ROPOS at 46.52°N, 129.58°W. The extent of the new lava flow corresponds very closely to an area of depth change identified between SeaBeam surveys. Comparison of the new survey and the 1981 and 1991 SeaBeam surveys showed a depth change of about 30 m. Onshore processing of the data is still in progress, but no other depth changes had been confirmed by 20 Aug; there could be other new lava flows that are below the detection level of SeaBeam. Except for bacterial filaments, the young lava flow was devoid of sessile organisms, whereas adjacent old lavas hosted many non-vent animals.

Geologic Background. The CoAxial segment of the Juan de Fuca Ridge is located about 435 km west of the Oregon coast, NE of Axial volcano. A submarine eruption along the CoAxial segment that produced thermal plumes and a new lava flow was detected by acoustic hydrophones in June 1993, just days after installation of the hydrophone system. This marked the first deep-sea volcanic eruption that was detected as it was happening. The location of the eruption was at a depth of about 2400 m, about 37 km NE of Axial volcano. Bathymetric surveys indicated that one or more additional sea-floor lava extrusions took place immediately to the east of the 1993 lava flow sometime between 1981-82 and 1991.

Information Contacts: C. Fox, W. Chadwick, R. Embley, and R. Dziak, NOAA Vents T-phase Project.

"The visit . . . on 10-11 November 1992 showed that since the last recorded observation, made using Landsat TM data from 9 February 1987, the lava lake in the N crater had disappeared (presumably frozen over and collapsed), but that the lava lake in the S crater . . . was still active. This interpretation is consistent with visual observations made during an aircraft overflight on 21 September 1992 (17:8-9).

"New satellite data further narrow the window of uncertainty about when the N lava lake became inactive, and provide evidence of the continuity of activity at the S lava lake. Data from the Japanese Earth Resources Satellite-1 (JERS-1, also known as Fuyo-1) recorded on 9 June 1992 show the S lava lake to be a strong source of short-wavelength infrared radiation, which is convincing evidence that it was active. However, there is no trace of any short-wavelength infrared thermal radiance from the N lava lake, indicating that the lake had already ceased to exist.

"JERS-1 was launched on 11 February 1992. Its optical sensors record images in three visible channels, and (more importantly for thermal remote sensing of volcanoes) in four short-wavelength infrared channels: 1.55-1.77, 2.01-2.12, 2.13-2.25, and 2.27-2.40 µm. The image pixels are 18 m across. This contrasts with Landsat TM which has 30 m pixels in two short-wavelength infrared channels (1.55-1.75 and 2.08-2.35 µm). This is believed to be the first demonstration that JERS-1 data can play a useful role in volcano monitoring. The thermal anomaly covers an approximately equidimensional area 4-5 pixels across in all four short-wavelength infrared channels, which is roughly consistent with the size of the lava lake seen during fieldwork five months later."

Geologic Background. Erta Ale is an isolated basaltic shield that is the most active volcano in Ethiopia. The broad, 50-km-wide edifice rises more than 600 m from below sea level in the barren Danakil depression. Erta Ale is the namesake and most prominent feature of the Erta Ale Range. The volcano contains a 0.7 x 1.6 km, elliptical summit crater housing steep-sided pit craters. Another larger 1.8 x 3.1 km wide depression elongated parallel to the trend of the Erta Ale range is located SE of the summit and is bounded by curvilinear fault scarps on the SE side. Fresh-looking basaltic lava flows from these fissures have poured into the caldera and locally overflowed its rim. The summit caldera is renowned for one, or sometimes two long-term lava lakes that have been active since at least 1967, or possibly since 1906. Recent fissure eruptions have occurred on the N flank.

Information Contacts: D. Rothery, Open Univ.

Ash explosions at 0600 and 0625 on 6 May produced eruption columns that rose 2,500 and 2,000 m, respectively, above the summit. There was also a small eruption at 1500 on 8 May. The strongest explosion occurred at 1338-1352 on 12 May, preceded by detonation sounds and incandescent ballistic bombs. A black ash column from this eruption rose 3,000 m and could be seen from the observatory. About 141,000 m³ of material was erupted. There was no damage caused by this eruption. An Indonesian radio broadcast also reported an eruption on 21 May (18:05). A pilot report described a smoke plume at 1000 on 17 August rising ~ 480 m above the rim before drifting NNW and dissipating.

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: W. Tjetjep, VSI; J. Lynch, SAB.

Three phreatic eruptions occurred from the center of the crater lake in early July, but caused no damage. The eruptions, at 0646 on 3 July and at 0835 and 1045 on 4 July, were preceded by increasing seismicity and tremor. The color of the lake also changed from whitish-green to brown prior to the activity.

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

Information Contacts: W. Tjetjep, VSI.

The . . . eruption continued in July with lava entering the ocean on the E and W sides of the Kamoamoa delta. The W Kamoamoa . . . flow built small benches below the old sea cliff that were soon broken apart by the high surf. Activity at this new flow declined throughout July. Lava entered the ocean along a 160-m-wide front on the E side of the Kamoamoa delta through most of the month. New land continued to be very unstable in this area. On 3 July, most of the lower bench at the E entry collapsed (an estimated area of 20 x 150 m), leaving behind a sliver of bench still attached to the delta. Lava flows quickly built a new bench extending ~35 m into the ocean, and two new littoral cones formed on the bench surface.

Lava tubes drained on 23 July and lava stopped pouring into the ocean . . . . The eruption resumed the next day as lava reoccupied the E-51 tube. Two small flows broke out of the tube; however, most of the erupted material traveled within the tube to the coastal plain. A large pahoehoe sheet-flow broke out just below a fault scarp and spread out over older Kamoamoa flows (figure 92). One part of the flow descended the W side of the Kamoamoa flow and entered the ocean on 27 July. Another finger advanced down the E side of the Kamoamoa flow and entered the ocean on 28 July. Isolated portions of Chain of Craters Road were covered on both sides of the Kamoamoa flow. Lava entered the ocean along wide fronts and explosive activity was rare. However, one large explosion on 28 July blew lithic blocks inland of the W entry point.

Figure 92. Recent lava flows (November 1992-July 1993) from Kilauea in the Kamoamoa delta area, July 1993. The new lava flow broke out of the tube on 24 July; lava reached the W Kamoamoa entry on 27 July, and the E entry became active on 28 July. Courtesy of T. Mattox.

A team of French and Swiss geologists who spent the night of 10-11 July at the summit of Pu`u `O`o observed the . . . lava pond for a number of hours. Activity was intense throughout the night, with the level of lava fluctuating 1-2 m. Bubbling was localized in the S and SE parts of the pond. At one point, a large collapse of the inner SSE crater wall produced a wave surge across the lava pond. Waves were several meters high, and covered the entire surface for several seconds. Other smaller landslides occurred during the night, showing the instability of the inner crater walls. The lava pond remained active throughout the month, with the surface fluctuating between 74 and 81 m below the crater rim.

Eruption tremor along the East rift zone continued in July at a relatively steady amplitude of 2x background level. Shallow, long-period microearthquake counts were slightly above average in the first half of the month, but were low later in the month, except for 22-23 July. Counts of short-period events were low beneath the summit and about average along the East rift zone.

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 and P. Okubo, HVO; H. Gaudru, SVE, Switzerland.

Steam-and-ash explosions . . . continued in late July and early August. A gas-and-ash plume was rising as high as 3 km above the crater (8 km altitude) and extending to the NW at 1400 on 25 July according to visual observations made from a helicopter by S. Zharinov and O. Braitseva. Lava flows were observed to extend from the summit crater down the W slope of the volcano, entering and melting the glacier at ~3,000 m elev. Seismic activity, including tremor, was stable but at a high level. The direction and height of the plume was about the same the next day as seen from a helicopter, and ashfall took place in Kliuchi [(30 km NNE)]. On 27 July at 2200, the gas-and-ash plume was as high as 5 km above the crater, extending to the S for an unknown distance. A W-flank lava flow was observed at 0030 on 28 July as well as lava fountains 600-700 m above the crater. On July 29 at 1500, the gas-and-ash cloud was 5 km above the crater and extended SW.

Airborne observations in early August indicated that the gas-and-ash plume was generally 1-3 km above the crater, extending to the N or NW for ~50 km. On 3 August, the gas-and-ash plume, as observed from a helicopter by V. Dvigalo and A. Belousov, rose to 6-7 km altitude and extended to the E in the afternoon and to the N that night. Lava continued to flow down the W slope of the volcano, and ash fell again on Kliuchi. Volcanic tremor remained high and steady in late July and early August.

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: S. Zharinov, IVGG.

Anak Krakatau has now been in continuous eruption for nine months . . . . Continued explosions (140-400/day) have built a new cone to ~280-300 m elev, which is 80-100 m above the old peak (figure 7). A seismometer installed by VSI was destroyed by a ballistic bomb on 16 July; lava flows have previously destroyed two other seismometers. Earlier this year, lava flows filled the valley between the cone and the old rim, and overflowed onto the outer slope of the old rim. The April 1993 lava flow descended the N flank to the coast, where it burned trees. The recommended off-limits zone, 3 km from the island, remains in effect.

Figure 7. Sketch of Anak Krakatau showing topographical changes, as of July 1993, since the eruption began in Nov 1992. View is looking SE; scale is approximate. Drawn by Igan S. Sutawidjaja; courtesy of VSI.

Geologic Background. The renowned volcano Krakatau (frequently misstated as Krakatoa) lies in the Sunda Strait between Java and Sumatra. Collapse of the ancestral Krakatau edifice, perhaps in 416 or 535 CE, formed a 7-km-wide caldera. Remnants of this ancestral volcano are preserved in Verlaten and Lang Islands; subsequently Rakata, Danan, and Perbuwatan volcanoes were formed, coalescing to create the pre-1883 Krakatau Island. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan, and left only a remnant of Rakata. This eruption, the 2nd largest in Indonesia during historical time, caused more than 36,000 fatalities, most as a result of devastating tsunamis that swept the adjacent coastlines of Sumatra and Java. Pyroclastic surges traveled 40 km across the Sunda Strait and reached the Sumatra coast. After a quiescence of less than a half century, the post-collapse cone of Anak Krakatau (Child of Krakatau) was constructed within the 1883 caldera at a point between the former cones of Danan and Perbuwatan. Anak Krakatau has been the site of frequent eruptions since 1927.

Information Contacts: W. Tjetjep, VSI.

Activity since mid-June has perhaps been greater than at any other time during the current eruption . . . . Explosive eruptions have produced ash clouds visible from the surrounding region, and deposited some ash on the outer slopes of the cone. Lava fountaining within the crater has thrown volcanic bombs up to 10 cm long across the crater floor. New vents opened in the S part of the crater floor 8-29 June, where none had existed since detailed observations began in 1988. Carbonatitic lava extrusion down the outer, W slopes of the cone blocked the former climbing route.

Burra Gadiye (Sengo Safaris Ltd.) reached the W crater rim on 6 April. There was little change since late February, although T20 had clearly been the source of lava flows that reached the base of the W wall (figure 28). A loud explosion was heard at 0400 on 22 May at Karatu (70 km S) by Gadiye. An explosion near the summit at that time was later reported by residents of Ngare Sero (~10 km N).

Figure 28. Panorama of Ol Doinyo Lengai crater looking E from the W rim, 6 April 1993. Drawn by C. Nyamweru from a photograph taken by Therese Berhault and Dominique Chevallier.

By the time of an 8 June ascent by Gadiye, T20 had grown to the same approximate height as T5/T9, which was emitting steam. A new vent near the N wall (T21), the source of recent lava flows, had no liquid lava visible. There was also a new dome or low mound (T22) near the W wall (figure 29). A large amount of steam was being emitted from a crack between the two buttes near the W rim, between which the path led. An earthquake was reported on 14 June by Gadiye, and dark, dense plumes from the crater could be seen from Ngare Sero, causing local Maasai herdsmen to move their livestock. The next day, Gadiye and some visitors climbed a considerable distance up the W trail before being stopped by flowing lava that apparently originated between the two buttes. Gadiye was able to dip the end of his walking stick into the very liquid lava flow. Dark ash was being erupted from within the summit crater.

Figure 29. Sketch map of the crater of Ol Doinyo Lengai, 29 June 1993. Arrows on the crater floor indicate fresh lava flows. Drawn by C. Nyamweru from field sketches by C. Shrady and T. Peterson. Locations of T21 and T22 on 8 June are based on an interview with Burra Gadiye by Celia Nyamweru.

Gadiye reported that ash fell on the W flank on 18 June, a black ash cloud rose from the summit at 1300-1500 on 21 June, and ash fell on local residents from 0800 to 1400 on 25 June after black ash clouds were blown E. A pilot who overflew the crater the week before 26 June (M. Bornir), reported that about half the crater was a bubbling black mass, two lava fountains were ejecting black material high into the air at short intervals, a new cone had formed, and smoke was abundant along the crater walls. Small light-colored plumes were seen from the surrounding area on the morning of 26 June, but when a group from St. Lawrence Univ arrived in mid-afternoon, they saw only a few small puffs of ash or steam from the summit at 1818-1821.

On 27 June, C. Nyamweru, C. Shrady, T. Coburn, and G. Cornwell (St. Lawrence Univ), Jumanne Juma (Dorobo Safaris Ltd.), and Danieli Kirway were unable to reach the crater rim from the W. No ash clouds were observed, but steam was seen at or near the W rim and a strong smell of sulfur was occasionally detected far down the outer slopes. A light dusting of fine gray ash on the outer slopes, increasing slightly in thickness towards the rim, covered leaves, spider webs, and small stones. Much of the vegetation near the summit was brown and dead. Steep rock slabs a few hundred meters below the rim were covered with several centimeters of loose ash, making climbing very difficult. The gap between the buttes had been blocked by a rockslide, apparently related to the 15 June lava flow. The dimensions of the lava flow were not measured, but the solid, cold surface was composed of broken slabs, some of which had pahoehoe structures.

Shrady, Coburn, and Cornwell, together with Thad Peterson and J. Juma, reached the E crater rim around 0830 on 29 June; they descended into the crater and remained until just after noon. Most of the crater floor (figure 29) was covered by dark, very fresh lava with both blocky and ropy textures. Much of it was still warm, and small pahoehoe flows were moving on the E half of the main crater floor. Lava flows were especially thick to the S, where the crater floor is now higher than cone T14 in the N. A large slab of the S crater wall near T24 had split from the rim and was standing free on the crater floor. Steam and sulfur fumes were escaping from several areas, particularly the E and N walls; only small amounts of steam were escaping along the W wall. The source of the 15 June lava flow was not determined.

Liquid lava was also erupting from a new vent (T23) just S of T20, with lava fragments being thrown 3-7 m above the rim of the vent at a rate of 4-8 bursts/min. The vent consisted of an outer, smooth, regular conical structure (white in color and higher on the W) surrounding smaller, black cones from which liquid lava was bursting. Activity continued throughout the morning, increasing in frequency and strength by noon, when clots of lava were being thrown 10 m high. The activity sounded like ocean surf, and there was a continuous dull rumble from below the vent. No lava flows issued from T23 that morning, but a very recent flow extended N from between T8 and T14. Lava flows from T23 have covered earlier flows NE of T20.

T20 had grown taller than T5/T9, previously the tallest feature, which was unchanged from February, although the top of the cone was steaming. The upper parts of T8 and T14 were visible, but had some sulfur staining and were surrounded by lava flows from other vents. T8 also exhibited some steam emission and occasional gas bursts. The E side of the T14 cone had collapsed, and was steaming weakly. Vent T24, wider than T20 but not as high, had a partly collapsed open top; its NW wall had collapsed, and it was the source of a large, 7-8 m thick flow of blocky lava (figure 29). On 29 June this flow was not moving, but fragments were falling down the flow front and its color was still dark. A sound similar to flowing water emerged from somewhere below T24. Vent T25, a circular fresh-looking black cone on the floor of the former 'Southern Depression,' was the source of a large flow of blocky lava that covered much of the S crater floor, though no liquid lava was present on 29 June. Feature T26 is a large black half-cone close to the base of the S crater wall, below the summit. Not active on the morning of 29 June, it might have been the source of volcanic bombs (up to 10 cm long) found on the crater floor and on the surface of fairly young lava flows, and possibly the source of some of the ash on the outer slopes. Vent T21 and dome T22, first seen on 8 June, were not observed.

On 3 July (1030-1100), Nyamweru and Shrady flew over the summit under clear conditions. No liquid lava could be seen bubbling in any of the vents or flowing on the crater floor, though much of the S and E parts of the crater were very black. Vegetation was brown on the inner crater walls and high on the outer slopes of the cone. Moderate amounts of steam were escaping from the W and N crater rim, from T20, and from the lower slopes of T5/T9. Sulfur odor could be detected from the aircraft. White ash covered much of the summit, extinct S crater, and the SE slopes, overlying the older gray ash.

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

Information Contacts: C. Nyamweru and C. Shrady, St. Lawrence Univ; Thad Peterson and J. Juma, Dorobo Safaris Ltd., Arusha; B. Gadiye, Sengo Safaris Ltd., Arusha; D. Kirway, Monduli, Tanzania; M. Bornir, Nairobi, Kenya; T. Berhault and D. Chevallier, France.

A flurry of seismicity occurred from Loihi for ~5 hours on the evening of 11 July 1993; only of few events were locatable, the largest was M 2.9. [Originally in Kilauea report.]

Geologic Background. Loihi seamount, the youngest volcano of the Hawaiian chain, lies about 35 km off the SE coast of the island of Hawaii. Loihi (which is the Hawaiian word for "long") has an elongated morphology dominated by two curving rift zones extending north and south of the summit. The summit region contains a caldera about 3 x 4 km wide and is dotted with numerous lava cones, the highest of which is about 975 m below the sea surface. The summit platform includes two well-defined pit craters, sediment-free glassy lava, and low-temperature hydrothermal venting. An arcuate chain of small cones on the western edge of the summit extends north and south of the pit craters and merges into the crests prominent rift zones. Deep and shallow seismicity indicate a magmatic plumbing system distinct from that of Kilauea. During 1996 a new pit crater was formed at the summit, and lava flows were erupted. Continued volcanism is expected to eventually build a new island; time estimates for the summit to reach the sea surface range from roughly 10,000 to 100,000 years.

Information Contacts: P. Okubo, HVO.

"Activity reached a high level in mid-July with a paroxysmal eruption on 14 July from Southern Crater. Activity was then low through the end of the month.

"From 1 to 4 July, both Main and Southern craters released occasional small emissions of mostly white vapours with no audible sounds or night glow. Mild, deep-seated eruptive activity began at Southern Crater on 5 July with occasional forceful emissions of white-grey, ash-laden clouds rising several hundred meters above the summit, accompanied by sub-continuous weak-to-loud roaring and rumbling noises. Rapid degassing started on 7 July with incandescent Strombolian projections up to ~100 m above the summit and rocks cascading down the headwall into SW Valley. This level of activity continued until 11 July, and then declined slightly.

"Activity at Southern Crater intensified at 1200 on 14 July as a new eruptive phase began with continuous strong emission of dark ash-laden clouds. The basaltic scoria-and-ash column reportedly rose to about 11 km and was accompanied by continuous loud roaring sounds. Moderate-heavy ash and scoria fall occurred from 1300 to 2000 on the W flank. Heavy tephra fall caused several hours of partial darkness on the W side of the volcano. The maximum measured ash-fall thickness of 80 mm was recorded between the coastal villages of Yacca and Kuluguma (5 km NW), damaging vegetation and food gardens. No casualties were reported. About 2 mm of ashfall occurred on the mainland at Awar Plantation (20 km W) and Watam village (30 km NW). Ashfall was detected as far as 90 km NW of the volcano. Weather satellite imagery indicated that the emission plume was oriented SW from the volcano and was ~170 km long at 1630 on 14 July.

"Small pyroclastic flows, generated at 1300, descended into SW Valley and stopped 3 km from the coast. At about 1820, moderate-to-large pyroclastic flows in SE Valley stripped and burned vegetation from the upper banks of the valley and along its course before reaching the sea, 5 km from the summit. Lava flows followed the pyroclastic flows into SE and SW Valleys. The largest of the lava flows was in SW Valley, and was ~2.5 km long, 40 m wide, and 3-4 m thick, terminating 3 km from the coast. The level of activity at Southern Crater declined on 15 July and stayed at a low level until the end of the month. Activity consisted of the release of white and blue vapour in small amounts with no audible noises or night glow.

"Main Crater activity remained low throughout the month with the emission of mostly white vapour in small-to-moderate volumes on 6, 16, and 31 July, while moderate volumes of white-grey ash and vapours were released between 7 and 15 July. The seismograph was not operational during most of July. Measurements from water-tube tiltmeters during 1993 have fluctuated over a range of a few microradians, although there may be a slight inflationary trend. The 14 July eruption appears to have had little or no effect on the tilt measurements."

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: B. Talai, R. Stewart, and C. McKee, RVO.

In the last week of March 1993 a fissure was observed to open in the bottom of Santiago crater, with no associated seismic activity. Enlargement of the fissure to 10 m in diameter was accompanied by high-pressure gas emissions. In the first week of April, an earthquake swarm occurred approximately 3 km E of Masaya City; the largest event was M 2.7. At the same time, minor microearthquake activity was registered beneath Santiago crater (1-2 events/day). On 16 June a small ash eruption in the late afternoon lasted for about 13 minutes. That same night incandescence was observed in the bottom of the Santiago crater (BGVN 18:06).

A 7-8 m diameter vent with liquid lava splashing at a depth of about 30-40 m was present on 20 June according to a report by Alain Creusot (BGVN 18:06). An additional report from Creusot indicates that an incandescent hole that had opened above the lava lake was 1-2 m larger in late July. A significant increase in gas emission has maintained a plume rising from the crater. As of 28 June, the temperature of gas emissions from the crater had increased. INETER reported that fumarole temperatures have generally been around 50°C since the beginning of the year, but have now increased to almost 250°C. Seismicity has generally remained low, with a slight increase on 4 July.

Geologic Background. Masaya is one of Nicaragua's most unusual and most active volcanoes. It lies within the massive Pleistocene Las Sierras caldera and is itself a broad, 6 x 11 km basaltic caldera with steep-sided walls up to 300 m high. The caldera is filled on its NW end by more than a dozen vents that erupted along a circular, 4-km-diameter fracture system. The Nindirí and Masaya cones, the source of historical eruptions, were constructed at the southern end of the fracture system and contain multiple summit craters, including the currently active Santiago crater. A major basaltic Plinian tephra erupted from Masaya about 6,500 years ago. Historical lava flows cover much of the caldera floor and there is a lake at the far eastern end. A lava flow from the 1670 eruption overtopped the north caldera rim. Masaya has been frequently active since the time of the Spanish Conquistadors, when an active lava lake prompted attempts to extract the volcano's molten "gold." Periods of long-term vigorous gas emission at roughly quarter-century intervals have caused health hazards and crop damage.

Information Contacts: Oscar Leonel Urbina, Departamento de Volcanes, INETER; Alain Creusot, Instituto Nicaraguense de Energía, Managua, Nicaragua.

Heavy ash eruptions rising 900-1,200 m were reported the morning of 31 July by the U.S. Coast Guard. A lava flow was also noted. According to these observers, the volcano was still erupting sporadically on 10 August, with light- to dark-gray ash plumes rising 2,400 m above the summit. Reports from Atka Island (~110 km W) indicate that weather conditions have frequently prevented observations.

Geologic Background. The elliptical, 11.5 x 24 km island of Seguam, lying between Amlia and Amukta Islands in the central Aleutians, contains two calderas with Holocene post-caldera cones. Growth of the basaltic-to-rhyolitic Wilcox volcano on the east side of the island during the late Pleistocene was followed by edifice collapse and an associated ignimbrite eruption about 9000 years ago, leaving a horseshoe-shaped caldera open to the west, inside which a rhyolitic cone was constructed. The 3 x 4 km wide westernmost caldera has a central scoria cone, Pyre Peak, which rises above the caldera rim and is the source of most of the historical eruptions of Seguam volcano. A very young basaltic field surrounds Pyre Peak, and lava flows partially fill the caldera and reach the southern coast. Older Holocene lava flows were erupted from vents within the eastern caldera, and a monogenetic Holocene cone forms Moundhill volcano on the eastern tip of the island.

Information Contacts: AVO.

A gas-and-steam plume was observed on the afternoon of 29 July rising to 3 km above the crater and extending >50 km S. Similar activity, with a gas-and-steam column rising 1-6 km above the rim, has persisted since the 22 April eruption. The volcano has also remained seismically active. Seismicity registered at a station 8 km from the volcano increased 23-24 and 28-29 July, then decreased after 1 August. Weather conditions prevented visual observations of the summit in early August.

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: S. Zharinov, IVGG.

The following ... is based on fieldwork in late March and early May 1993. "Activity in the crater of Ngauruhoe has declined further over the past year and is at an all time low since records were first kept. Fumarole temperatures have decreased to 90°C after remaining stable at 94°C (boiling point for altitude) since 1986. No volcanic deformation was identified by surveys to points on the crater rim and at the base of the mountain."

Geologic Background. Tongariro is a large volcanic massif, located immediately NE of Ruapehu volcano, that is composed of more than a dozen composite cones constructed over a period of 275,000 years. Vents along a NE-trending zone extending from Saddle Cone (below Ruapehu) to Te Maari crater (including vents at the present-day location of Ngauruhoe) were active during several hundred years around 10,000 years ago, producing the largest known eruptions at the Tongariro complex during the Holocene. North Crater stratovolcano is truncated by a broad, shallow crater filled by a solidified lava lake that is cut on the NW side by a small explosion crater. The youngest cone, Ngauruhoe, is also the highest peak.

Information Contacts: B. Christenson, I. Nairn, P. Otway, and B. Scott, IGNS, Wairakei.

Exogenous growth at dome 11 . . . continued through early August (figure 57). Partial collapses generated about 10 pyroclastic flows/day to the E and NE, leaving a relatively stable part of dome 11 over the remnants of dome 4. No damage was caused by pyroclastic flows in July. Ash-laden plumes from the former Jigokuato crater, rising as high as 1 km, have frequently been observed since mid-July. Volcanic tremor at the summit area increased after 22 July, but no surface changes have been observed.

Figure 57. Sketch map of the lava dome complex at Unzen, early August 1993. Note that N is to the right. Courtesy of S. Nakada.

During growth of dome 11, the E part was pushed outward and has collapsed frequently. A ramp structure formed, and lavas were successively overridden by later flows, like dominoes. The thickness of dome 11 decreased toward the front and lengthened, maintaining the ramp structure. This implies that stretching and sliding along the basal plane are the main reasons for the lengthening of the dome. The E arm of the dome was 400 m long, 150 m wide, and 50 m thick by the end of July, and had grown to 500 m in length by 7 August. Geologists estimated a growth rate of ~200,000 m³/day, with the eruption rate close to 250,000 m³/day.

On 19 July, 32 seismically recorded pyroclastic flows moved E along the Mizunashi Valley, the largest daily count in the month. The series began at 0354 and continued until midnight, including larger flows with seismic durations of 130 seconds at 0415 and 0428, 150 seconds at 0405 and 1131, and 170 seconds at 1700. The largest pyroclastic flow of the series occurred at 1821, with a duration of 190 seconds, and traveled 5.3 km E, crossing Route 57 (figure 58). The road was closed in late June after another pyroclastic flow crossed it. The front of the pyroclastic flow extended 100 m beyond the road, which is the farthest distance a pyroclastic flow has traveled from the active crater during the current period of activity. The longest pyroclastic flows, from W of dome 11, traveled 5.5 km on 8 June and 15 September 1991. There were a total of 353 seismically detected pyroclastic flows in July . . . . The number of evacuees from Shimabara and Fukae has remained unchanged at 3,617 since early July.

Figure 58. Map showing distribution of pyroclastic-flow and debris-flow deposits at Unzen from 1991 through early August 1993. Courtesy of S. Nakada.

The frontal part of the 19 July pyroclastic-flow deposit was as narrow as a few tens of meters, and as long as 1 km (figure 58). Deposits were distributed along a narrow gully within older pyroclastic-flow and debris-flow deposits. A video taken of the 19 July pyroclastic flow showed that the front lobes of the growing ash cloud protruded intermittently from the main mass of the ash cloud for the last 1 km. Ground vibrations, usually recorded by seismometers during movement of block-and-ash flows, had stopped before this event. Deposits at the front of this flow were deficient in large lava blocks, which are common 2 km closer to the source, indicating that these may be a type of pyroclastic-surge deposit.

Seismicity at the lava dome complex was unchanged before and during the pyroclastic-flow activity. Low seismicity at the dome complex, about 20 events/day, was followed by a burst of activity that began on 26 July and culminated with 2,604 events on 7 August, the highest daily count in the present period of volcanism. The previous high was 632 on 11 September 1992. Seismicity gradually declined through 11 August, but remained high at about 800 events/day. Aerial inspections on 3, 5, and 8 August showed no unusual changes on the surface of the dome complex, which continued growing at a rate of 2-3 x 10⁵ m³/day. A total of 1,034 microearthquakes was recorded in July, more than double the total of 506 in June.

Heavy rainfall during the rainy season has frequently caused debris flows along the Nakao and Mizunashi rivers. Debris flows on 4-5 July damaged 25 houses, and another 83 houses were damaged on 16-18 July. Debris flows during the current activity have damaged 1,409 houses. During periods of heavy rainfall, mayors of Shimabara and Fukae issued temporary evacuation orders to 5,000-9,000 residents along the rivers.

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: JMA; S. Nakada, Kyushu Univ.

After 6 years of quiet, eruptive activity resumed on 30 July. Observers in Perryville . . . reported seeing black clouds ejected from the summit beginning at 1430 on 30 July and continuing into early August. A white steam cloud was present during the intervals between black emissions. A small eruption plume was observed on satellite imagery by the U.S. National Weather Service on 30 July, but none were seen in the following days. On the morning of 31 July, Perryville observers saw a gray cloud rising from the volcano and extending to the S. Commercial airline pilots on 2 August observed continued intermittent venting of black ash with a low ash cloud rising nearly 300 m.

Pilot reports on 3 August indicated that black ash was erupting from the summit vent of the intracaldera cone at 30-60 second intervals to a height of 2,400-3,000 m above sea level (840 m above the vent). Pilot reports also described a new steam vent that had formed in the icefield adjacent to the active cone, with the melting perhaps caused by a subglacial lava flow. At mid-day on 3 August, pilots of Reeve Aleutian Airways and personnel from the FWS (Alaska Peninsula/Becharof National Wildlife Refuge) observed the eruption site. They described intermittent ash emission and block ejection from the prominent cinder cone (summit elevation 2,160 m) located within the summit caldera. Material was being thrown an estimated 150-300 m above the cone. Steam was emanating from a depression 180-275 m wide in the glacial ice E of the cone's base. Residents of Port Heiden . . . reported a very light ash fall on the night of 3 August. Residents of Perryville, Chignik, and Chignik Lake, S and E of the volcano, also heard a "rumbling noise" accompanied by a slight tremor at about 2200 that night.

On the morning of 6 August, a resident of Port Heiden observed eruptions of ash and steam occurring at 3-4 minute intervals. These small plumes were reaching only several tens of meters above the summit. There were no reports of ashfall at other nearby villages. On 12 August, a pilot reported ash venting 600-900 m above the crater with the ash cloud carried ENE.

Observations indicate that the eruption is Strombolian (characterized by intermittent bursts of molten material several tens of meters above the vent), similar to previous historical eruptions at this volcano. As this activity continues, occasional ash falls are possible at Perryville and other nearby villages. AVO has no seismic monitoring equipment on Veniaminof and for this reason is not using the Level of Concern Color Code. A scientist from the Geophysical Institute at the Univ of Alaska installed a seismometer in Perryville on 4 August and an additional instrument will be installed closer to the volcano. These should aid efforts to identify future continuing activity.

Geologic Background. Veniaminof, on the Alaska Peninsula, is truncated by a steep-walled, 8 x 11 km, glacier-filled caldera that formed around 3,700 years ago. The caldera rim is up to 520 m high on the north, is deeply notched on the west by Cone Glacier, and is covered by an ice sheet on the south. Post-caldera vents are located along a NW-SE zone bisecting the caldera that extends 55 km from near the Bering Sea coast, across the caldera, and down the Pacific flank. Historical eruptions probably all originated from the westernmost and most prominent of two intra-caldera cones, which rises about 300 m above the surrounding icefield. The other cone is larger, and has a summit crater or caldera that may reach 2.5 km in diameter, but is more subdued and barely rises above the glacier surface.

Information Contacts: AVO.