Reventador is a stratovolcano located in the Cordillera Real, Ecuador with historical eruptions dating back to the 16th century. The most recent eruptive period began in 2008 and has continued through July 2020 with activity characterized by frequent explosions, ash emissions, and incandescent block avalanches (BGVN 45:02). This report covers volcanism from February through July 2020 using information primarily from the Instituto Geofísico (IG-EPN), the Washington Volcano Ash Advisory Center (VAAC), and various infrared satellite data.

During February to July 2020, IG-EPN reported almost daily explosions, gas-and-steam and ash emissions, and frequent crater incandescence. The highest average number of explosions per day was 26 in March, followed by an average of 25 explosions per day in June (table 12). Ash plumes rose to a maximum height of 2.5 km above the crater during this reporting period with the highest plume height recorded on 5 May 2020. Frequently at night, crater incandescence was observed, occasionally accompanied by incandescent block avalanches traveling as far as 900 m downslope from the summit of the volcano.

Table 12. Monthly summary of eruptive events recorded at Reventador from February through July 2020. Data courtesy of IG-EPN (February to July 2020 daily reports).

During February 2020 there were between 2 and 32 explosions recorded each day, accompanied by gas-and-steam and ash emissions that rose about 700-1,300 m above the crater. At night and early morning crater incandescence was observed frequently throughout the reporting period from 1 February and onward. Incandescent block avalanches were also detected intermittently beginning on 5 February when incandescent blocks rolled 300-800 m downslope from the summit on all sides of the volcano. On 6 and 21 February, gas-and-steam and ash emissions rose to a high of 1.3 km above the crater, according to Washington VAAC notices (figure 125).

Figure 125. Webcam images of ash plumes rising from Reventador on 6 February 2020. Courtesy of IG-EPN (Informe diario del estado del Volcán Reventador No. 2020-37).

Between 7 and 47 daily explosions were detected during March. On 17 March, rainfall generated two small lahars, accompanied by ash emissions that rose 1 km above the crater drifting NW. That same day, ashfall was reported in San Rafael (8 km ESE) SE of the volcano (figure 126). On 19 March ashfall was also reported in El Chaco (30 km SW), according to SNGR-Umeva-Orellana. At night, crater incandescence was observed and was occasionally accompanied by block avalanches traveling as far as 900 m downslope of the summit. Gas-and-steam and ash emissions continued, rising to 2.2 km above the crater on 28 March, according to the Washington VAAC notices.

Figure 126. Photo of ashfall SE of Reventador on 17 March 2020. Courtesy of IG-EPN (IG al Instante Informativo Volcán Reventador No. 001).

Activity persisted in April, characterized by almost daily gas-and-steam and ash emissions that rose to 1.4 km above the crater (figure 127) and intermittent crater incandescence observed at night and in the morning. The number of daily explosions detected per day ranged between 2 and 40, many of which were accompanied by block avalanches that traveled as far as 800 m downslope from the summit.

Figure 127. Webcam images showing gas-and-steam and ash plumes rising from Reventador on 7 (top) and 14 (bottom) April 2020. Courtesy of IG-EPN (Informe diario del estado del Volcán Reventador No. 2020-99 and 2020-105).

During May, volcanism remained consistent, characterized by intermittent crater incandescence, gas-and-steam and ash emissions that rose 2.5 km above the crater, and daily explosions that ranged between 6 and 56 per day. On 14 May a Washington VAAC advisory stated there were three ash emissions that rose to a maximum of about 2.5 km above the crater and drifted W. At night, crater incandescence was observed accompanied by incandescent blocks that traveled 300 m below the summit on the SE flank; the furthest blocks traveled during this month was 800 m downslope.

The average number of daily explosions increased from 22 in May to 25 in June, and ranged between 0 and 51 per day, accompanied by gas-and-steam and ash emissions that rose 1.3 km above the crater (figure 128). At night, crater incandescence continued to be observed with occasional blocks rolling down the flanks up to 800 m downslope from the summit.

Figure 128. Webcam images showing gas-and-steam and ash plumes rising from Reventador on 7 (top) and 18 (bottom) June 2020. Courtesy of IG-EPN (Informe diario del estado del Volcán Reventador No. 2020-160 and 2020-171).

By July, the average number of daily explosions decreased to 15 and gas-and-steam and ash emissions continued (figure 129). The maximum ash plume height during this month reached 1.4 km above the crater, according to a Washington VAAC advisory. Explosions still continued, ranging between 2 and 38 per day; explosions were not recorded in every daily report during this month. At night, crater incandescence was commonly observed and was sometimes accompanied by incandescent block avalanches that traveled as far as 800 m downslope from the summit. On 1 July a webcam image showed an explosion that produced an ash plume that rose 1 km above the crater drifting W and small pyroclastic flows near the cone. Another explosion on 5 July resulted in an ash plume that rose up to 1 km above the crater drifting W and NW accompanied by crater incandescence, a block avalanche that moved up to 800 m downslope, and a pyroclastic flow. On 22 and 24 July explosions ejected blocks that traveled downslope from the summit and were accompanied by pyroclastic flows that traveled down the N flank for 600 m.

Figure 129. Image of an explosion at Reventador on 16 July 2020 that produced an ash column rising 500 m above the crater drifting N. Photo by Darwin Yánez (SNGRE, Servicio Nacional de Gestión de Riesgos y Emergencias del Ecuador), courtesy of IG-EPN (Informe Especial Reve N1 2020).

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed intermittent thermal anomalies within 5 km of the summit during 5 October 2019 and July 2020 (figure 130). There was a small decline in power from late April to late May, followed by a brief break in thermal anomalies from late May to mid-June 2020. In comparison, the MODVOLC algorithm identified nine thermal alerts between February and July 2020 near the crater summit on 5 February (2), 7 February (2), 12 February (1), 22 March (1), 27 April (1), 10 June (1), and 7 July (1). Some thermal anomalies can be seen in Sentinel-2 thermal satellite imagery on days with little cloud cover (figure 131).

Figure 130. Thermal anomalies at Reventador persisted intermittently during 5 October 2019 through July 2020 as recorded by the MIROVA system (Log Radiative Power). Courtesy of MIROVA.

Figure 131. Sentinel-2 thermal satellite images of Reventador on 6 (top left) and 11 (top right) February, 17 March (bottom left), and 10 June (bottom right) showing a thermal hotspot in the central summit crater. Images with “Atmospheric penetration” (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Geologic Background. Reventador is the most frequently active of a chain of Ecuadorian volcanoes in the Cordillera Real, well east of the principal volcanic axis. The forested, dominantly andesitic Volcán El Reventador stratovolcano rises to 3562 m above the jungles of the western Amazon basin. A 4-km-wide caldera widely breached to the east was formed by edifice collapse and is partially filled by a young, unvegetated stratovolcano that rises about 1300 m above the caldera floor to a height comparable to the caldera rim. It has been the source of numerous lava flows as well as explosive eruptions that were visible from Quito in historical time. Frequent lahars in this region of heavy rainfall have constructed a debris plain on the eastern floor of the caldera. The largest historical eruption took place in 2002, producing a 17-km-high eruption column, pyroclastic flows that traveled up to 8 km, and lava flows from summit and flank vents.

Information Contacts: Instituto Geofísico, Escuela Politécnica Nacional (IG-EPN), Casilla 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Barren Island is a remote island east of India in the Andaman Islands. Its most recent eruptive period began in September 2018 with volcanism characterized by thermal anomalies and small ash plumes (BGVN 45:02). This report updates information from February through July 2020 using various satellite data as a primary source of information.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed intermittent low-power thermal anomalies within 5 km of the summit from early September 2019 through July 2020 (figure 44). The frequency of the thermal anomalies decreased during mid-May 2020 with only six detected between June and July 2020. Sentinel-2 thermal satellite imagery showed weak thermal hotspots in the summit crater on 5 and 10 February, and 5 April (figure 45). In comparison, Suomi NPP/VIIRS sensor data registered elevated temperatures during 13-14 May and 18 July. Intermittent gas-and-steam emissions were also observed in Sentinel-2 satellite imagery on days with little to no cloud coverage. A small ash plume was observed in Sentinel-2 satellite imagery drifting NW on 24 June; there was no thermal anomaly detected that day (figure 46).

Figure 44. Intermittent thermal anomalies at Barren Island for 10 August 2019 through July 2020 were detected by the MIROVA graph (Log Radiative Power). The frequency of the anomalies decreased after mid-May. Courtesy of MIROVA.

Figure 45. Sentinel-2 thermal images show weak thermal anomalies (bright yellow-orange) at Barren Island on 5 February (left) and 5 April (right) 2020. Images with False color (bands 12, 11, 4) rendering courtesy of Sentinel Hub Playground.

Figure 46. Sentinel-2 satellite image showing a small ash plume rising from Barren Island and drifting NW on 24 June 2020. Image with Natural color (bands 4, 3, 2) rendering courtesy of Sentinel Hub Playground.

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

Information Contacts: 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); NASA Worldview (URL: https://worldview.earthdata.nasa.gov/).

Pacaya, located in Guatemala, is a highly active volcano that has previously produced continuous Strombolian explosions, multiple lava flows, and the formation of a small cone within the crater due to the constant deposition of ejected material (BGVN 45:02). This reporting period updates information from February through July 2020 consisting of similar activity that dominantly originates from the Mackenney crater. Information primarily comes from reports by the Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH) in Guatemala and various satellite data.

Strombolian explosions were recorded consistently throughout this reporting period. During February 2020, explosions ejected incandescent material 100 m above the Mackenney crater. At night and during the early morning the explosions were accompanied by incandescence from lava flows. Multiple lava flows were active during most of February, traveling primarily down the SW and NW flanks and reaching 500 m on 25 February. On 5 February the lava flow on the SW flank divided into three flows measuring 200, 150, and 100 m. White and occasionally blue gas-and-steam emissions rose up to 2.7 km altitude on 11 and 14 February and drifted in multiple directions. On 16 February Matthew Watson utilized UAVs (Unmanned Aerial Vehicle) to take detailed, close up photos of Pacaya and report that there were five active vents at the summit exhibiting lava flows from the summit, gas-and-steam emissions, and small Strombolian explosions (figure 122).

Figure 122. Drone image of active summit vents at Pacaya on 16 February 2020 with incandescence and white gas-and-steam emissions. Courtesy of Matthew Watson, University of Bristol, posted on 17 February 2020.

Activity remained consistent during March with Strombolian explosions ejecting material 100 m above the crater accompanied by occasional incandescence and white and occasionally blue gas-and-steam emissions drifting in multiple directions. Multiple lava flows were detected on the NW and W flanks reaching as far as 400 m on 9-10 March.

In April, frequent Strombolian explosions were accompanied by active lava flows moving dominantly down the SW flank and white gas-and-steam emissions. These repeated explosions ejected material up to 100 m above the crater and then deposited it within the Mackenney crater, forming a small cone. On 27 April seismicity increased at 2140 due to a lava flow moving SW as far as 400 m (figure 123); there were also six strong explosions and a fissure opened on the NW flank in front of the Los Llanos Village, allowing gas-and-steam to rise.

Figure 123. Infrared image of Pacaya on 28 April 2020, showing a lava flow approximately 500 m long and moving down the S flank on the day after seismicity increased and six strong explosions were detected. Courtesy of ISIVUMEH (Reporte Volcán de Pacaya July 2020).

During May, Strombolian explosions continued to eject incandescent material up to 100 m above the Mackenney crater, accompanied by active lava flows on 1-2, 17-18, 22, 25-26, and 29-30 May down the SE, SW, NW, and NE flanks up to 700 m on 30 May. White gas-and-steam emissions continued to be observed up to 100 m above the crater drifting in multiple directions. Between the end of May and mid-June, the plateau between the Mackenney cone and the Cerro Chiquito had become inundated with lava flows (figure 124).

Figure 124. Aerial views of the lava flows at Pacaya to the NW during a) 18 September 2019 and b) 16 June 2020 showing the lava flow advancement toward the Cerro Chiquito. Both images have been color corrected. Courtesy of INSIVUMEH (Reporte Volcán de Pacaya July 2020).

Lava flows extended 700 m on 8 June down multiple flanks. On 9 June, a lava flow traveled N and NW 500 m and originating from a vent on the N flank about 100 m below the Mackenney crater. Active lava flows continued to originate from this vent through at least 19 June while white gas-and-steam emissions were observed rising 300 m above the crater. At night and during the early mornings of 24 and 29 June Strombolian explosions were observed ejecting incandescent material up to 200 m above the crater (figure 125). These explosions continued to destroy and then rebuild the small cone within the Mackenney crater with fresh ejecta. Active lava flows on the SW flank were mostly 100-600 m long but had advanced to 2 km by 30 June.

On 10 July a 1.2 km lava flow divided in two which moved on the NE and N flanks. On 11 July, another 800 m lava flow divided in two, on the N and NE flanks (figure 126). On 14 and 19 July, INSIVUMEH registered constant seismic tremors and stated they were associated with the lava flows. No active lava flows were observed on 18-19 July, though some may have continued to advance on the SW, NW, N, and NE flanks. On 20 July, lava emerged from a vent at the NW base of the Mackenney cone near Cerro Chino, extending SE. Strombolian explosions ejected incandescent material up to 200 m above the crater on 22 July, accompanied by active incandescent lava flows on the SW, N, NW, NE, and W flanks. Three lava flows on the NW flank were observed on 22-24 July originating from the base of the Mackenney cone. Explosive activity during 22 July vibrated the windows and roofs of the houses in the villages of San Francisco de Sales, El Patrocinio, El Rodeo, and others located 4 km from the volcano. The lava flow activity had decreased by 25 July, but remnants of the lava flow on the NW flank persisted with weak incandescence observed at night, which was no longer observed by 26 July. Strombolian explosions continued to be detected through the rest of the month, accompanied by frequent white gas-and-steam emissions that extended up to 2 km from the volcano; no active lava flows were observed.

Figure 125. Photos of Pacaya on 11 July 2020 showing Strombolian explosions and lava flows moving down the N and NE flanks. Courtesy of William Chigna, CONRED, posted on 12 July 2020.

Figure 126. Infrared image of Pacaya on 20 July 2020 showing a hot lava flow accompanied by gas-and-steam emissions. Courtesy of INSIVUMEH (BEPAC 47 Julio 2020-22).

During February through July 2020, multiple lava flows and thermal anomalies within the Mackenney crater were detected in Sentinel-2 thermal satellite imagery (figure 127). These lava flows were observed moving down multiple flanks and were occasionally accompanied by white gas-and-steam emissions. Thermal anomalies were also recorded by the MIROVA (Middle InfraRed Observation of Volcanic Activity) system during 10 August through July 2020 within 5 km of the crater summit (figure 128). There were a few breaks in thermal activity from early to mid-March, late April, early May, and early June; however, each of these gaps were followed by a pulse of strong and frequent thermal anomalies. According to the MODVOLC algorithm, 77 thermal alerts were recorded within the summit crater during February through July 2020.

Figure 127. Sentinel-2 thermal satellite images of Pacaya showing thermal activity (bright yellow-orange) primarily as lava flows originating from the summit crater during February to July 2020 frequently accompanied by white gas-and-steam emissions. All images with "Atmospheric penetration" (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Figure 128. The MIROVA thermal activity graph (Log Radiative Power) at Pacaya during 10 August to July 2020 shows strong, frequent thermal anomalies through late July with brief gaps in activity during early to mid-March, late April, early May, and early June. Courtesy of MIROVA.

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

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Matthew Watson, School of Earth Sciences at the University of Bristol (Twitter: @Matthew__Watson, https://twitter.com/Matthew__Watson); William Chigna, CONRED (URL: https://twitter.com/william_chigna).

Sangeang Api is a 13-km-wide island located off the NE coast of Sumbawa Island, part of Indonesia's Lesser Sunda Islands. Documentation of historical eruptions date back to 1512. The most recent eruptive episode began in July 2017 and included frequent Strombolian explosions, ash plumes, and block avalanches. The previous report (BGVN 45:02) described activity consisting of a new lava flow originating from the active Doro Api summit crater, short-lived explosions, and ash-and-gas emissions. This report updates information during February through July 2020 using information from the Darwin Volcanic Ash Advisory Center (VAAC) reports, Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, or CVGHM) reports, and various satellite data.

Volcanism during this reporting period was relatively low compared to the previous reports (BGVN 44:05 and BGVN 45:02). A Darwin VAAC notice reported an ash plume rose 2.1 km altitude and drifted E on 10 May 2020. Another ash plume rose to a maximum of 3 km altitude drifting NE on 10 June, as seen in HIMAWARI-8 satellite imagery.

The MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data detected a total of 12 low power thermal anomalies within 5 km from the summit during February through May 2020 (figure 42). No thermal anomalies were recorded during June and July according to the MIROVA graph. Though the MODVOLC algorithm did not detect any thermal signatures between February to July, many small thermal hotspots within the summit crater could be seen in Sentinel-2 thermal satellite imagery (figure 43).

Figure 42. Thermal anomalies at Sangeang Api from 10 August 2019 through July 2020 recorded by the MIROVA system (Log Radiative Power) were infrequent and low power during February through May 2020. No thermal anomalies were detected during June and July. Courtesy of MIROVA.

Figure 43. Sentinel-2 thermal satellite imagery using “Atmospheric penetration” (bands 12, 11, 8A) rendering showed small thermal hotspots (orange-yellow) at the summit of Sangeang Api during February through June 2020. Courtesy of Sentinel Hub Playground.

Geologic Background. Sangeang Api volcano, one of the most active in the Lesser Sunda Islands, forms a small 13-km-wide island off the NE coast of Sumbawa Island. Two large trachybasaltic-to-tranchyandesitic volcanic cones, Doro Api and Doro Mantoi, were constructed in the center and on the eastern rim, respectively, of an older, largely obscured caldera. Flank vents occur on the south side of Doro Mantoi and near the northern coast. Intermittent historical eruptions have been recorded since 1512, most of them during in the 20th century.

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/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Stromboli is a stratovolcano located in the northeastern-most part of the Aeolian Islands composed of two active summit vents: the Northern (N) Crater and the Central-South (CS) Crater that are situated at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the volcano. The ongoing eruption began in 1934 and has been characterized by regular Strombolian explosions in both summit craters, ash plumes, and occasional lava flows (BGVN 45:08). This report updates activity from January to April 2020 with information primarily from daily and weekly reports by Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) and various satellite data.

Activity was consistent during this reporting period. Explosion rates ranged from 1-20 per hour and were of variable intensity, producing material that rose from less than 80 to over 250 m above the vents (table 8). Strombolian explosions were often accompanied by gas-and-steam emissions, spattering, and lava flows which has resulted in fallout deposited on the Sciara del Fuoco and incandescent blocks rolling toward the coast up to a few hundred meters down the slopes of the volcano. According to INGV, the average SO2 emissions measured 300-650 tons/day.

Table 8. Summary of activity at Stromboli during January-April 2020. Low-intensity activity indicates ejecta rising less than 80 m, medium-intensity is ejecta rising less than 150 m, and high-intensity is ejecta rising over 200 m above the vent. Data courtesy of INGV.

During January 2020, explosive activity mainly originated from three vents in the N crater and at least three vents in the CS crater. Ejecta from numerous Strombolian explosions covered the slopes on the upper Sciara del Fuoco, some of which rolled hundreds of meters down toward the coast. Explosion rates varied from 2-12 per hour in the N crater and 9-14 per hour in the CS crater; ejected material rose 80-200 m above the craters. According to INGV, a small cone growing in the S1 crater produced some explosions that ejected coarse material mixed with fine ash. On 18 and 19 January a lava flow was observed, both of which originated in the N crater. In addition, two explosions were detected in the N crater that was associated with two landslide events.

Explosive activity in February primarily originated from 2-3 eruptive vents in the N crater and at least three vents in the CS crater (figure 177). The Strombolian explosions ejected material 80-250 m above the craters, some of which fell onto the upper part of the Sciara. Explosion rates varied from 3-12 per hour in the N crater and 2-14 per hour in the CS crater (figure 178). On 3 February a short-lived lava flow was reported in the N crater.

Figure 177. A drone image showed spattering accompanied by gas-and-steam emissions at Stromboli rising above the N crater on 15 February 2020. Courtesy of INGV (Rep. No. 08/2020, Stromboli, Bollettino Settimanale, 10/02/2020 - 16/02/2020, data emissione 18/02/2020).

Figure 178. a) Strombolian explosions during the week of 17-23 February 2020 in the N1 crater of Stromboli were seen from Pizzo Sopra La Fossa. b) Spattering at Stromboli accompanied by white gas-and-steam emissions was detected in the N1 and S2 craters during the week of 17-23 February 2020. c) Spattering at Stromboli accompanied by a dense ash plume was seen in the N1 and S2 craters during the week of 17-23 February 2020. All photos by F. Ciancitto, courtesy of INGV (Rep. No. 09/2020, Stromboli, Bollettino Settimanale, 17/02/2020 - 23/02/2020, data emissione 25/02/2020).

Ongoing explosive activity continued into March, originating from three eruptive vents in the N crater and at least three vents in the CS crater. Ejected lapilli and bombs rose 80-250 m above the craters resulting in fallout covering the slopes in the upper Sciara del Fuoco with blocks rolling down the slopes toward the coast and explosions varied from 4-13 per hour in the N crater and 1-16 per hour in the CS crater. Discontinuous spattering was observed during 9-19 March. On 19 March, intense spattering was observed in the N crater, which produced a lava flow that stretched along the upper part of the Sciara for a few hundred meters. Another lava flow was detected in the N crater on 28 March for about 4 hours into 29 March, which resulted in incandescent blocks breaking off the front of the flow and rolling down the slope of the volcano. On 30 March a lava flow originated from the N crater and remained active until the next day on 31 March. Landslides accompanied by incandescent blocks rolling down the Sciara del Fuoco were also observed.

Strombolian activity accompanied by gas-and-steam emissions continued into April, primarily produced in 3-4 eruptive vents in the N crater and 2-3 vents in the CS crater. Ejected material from these explosions rose 80-250 m above the craters, resulting in fallout products covering the slopes on the Sciara and blocks rolling down the slopes. Explosions varied from 4-15 per hour in the N crater and 1-10 per hour in the CS crater. On 1 April a thermal anomaly was detected in satellite imagery accompanied by gas-and-steam and ash emissions downstream of the Sciara del Fuoco. A lava flow was observed on 15 April in the N crater accompanied by gas-and-steam and ash emissions; at the front of the flow incandescent blocks detached and rolled down the Sciara (figure 179). This flow continued until 16 April, ending by 0956; a thermal anomaly persisted downslope from the lava flow. Spatter was ejected tens of meters from the vent. Another lava flow was detected on 19 April in the N crater, followed by detached blocks from the front of the flow rolling down the slopes. Spattering continued during 20-21 April.

Figure 179. A webcam image of an ash plume accompanied by blocks ejected from Stromboli on 15 April 2020 rolling down the Sciara del Fuoco. Courtesy of INGV via Facebook posted on 15 April 2020.

Moderate thermal activity occurred frequently during 16 October to April 2020 as recorded in the MIROVA Log Radiative Power graph using MODIS infrared satellite information (figure 180). The MODVOLC thermal alerts recorded a total of 14 thermal signatures over the course of nine different days between late February and mid-April. Many of these thermal signatures were captured as hotspots in Sentinel-2 thermal satellite imagery in both summit craters (figure 181).

Figure 180. Low to moderate thermal activity at Stromboli occurred frequently during 16 October-April 2020 as shown in the MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 181. Thermal anomalies (bright yellow-orange) at Stromboli were observed in thermal satellite imagery from both of the summit vents throughout January-April 2020. Images with Atmospheric Penetration rendering (bands 12, 11, 8A); courtesy of Sentinel Hub Playground.

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

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/en/, Facebook: https://www.facebook.com/ingvvulcani/); 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).

Columbia’s broad, glacier-capped Nevado del Ruiz has an eruption history documented back 8,600 years, and historical observations since 1570. It’s profound notoriety stems from an eruption on 13 November 1985 that produced an ash plume and pyroclastic flows onto the glacier, triggering large lahars that washed down 11 valleys, inundating most severely the towns of Armero (46 km W) and Chinchiná (34 km E) where approximately 25,000 residents were killed. It remains the second deadliest volcanic eruption of the 20th century after Mt. Pelee killed 28,000 in 1902. Ruiz remained quiet for 20 years after the September 1985-July 1991 eruption until a new explosive event occurred in February 2012; a series of explosive events lasted into 2013. Renewed activity beginning in November 2014 included ash and gas-and-steam plumes, ashfall, and the appearance of a lava dome inside the Arenas crater in August 2015 which has regularly displayed thermal anomalies through 2019. This report covers ongoing activity from January-June 2020 using information primarily from reports by the Servicio Geologico Colombiano (SGC) and the Observatorio Vulcanológico y Sismológico de Manizales, the Washington Volcanic Ash Advisory Center (VAAC) notices, and various sources of satellite data.

Gas and ash emissions continued at Nevado del Ruiz throughout January-June 2020; they generally rose to 5.8-6.1 km altitude with the highest reported plume at 7 km altitude during early March. SGC confirmed the presence of the growing lava dome inside Arenas crater during an overflight in January; infrared satellite imagery indicated a continued heat source from the dome through April. SGC interpreted repeated episodes of ‘drumbeat seismicity’ as an indication of continued dome growth throughout the period. Small- to moderate-density sulfur dioxide emissions were measured daily with satellite instruments. The MIROVA graph of thermal activity indicated a heat source consistent with a growing dome from January through April (figure 102).

Figure 102. The MIROVA graph of thermal activity at Nevado del Ruiz from 2 July 2019 through June 2020 indicated persistent thermal anomalies from mid-November 2019-April 2020. Courtesy of MIROVA.

Activity during January-March 2020. During January 2020 some of the frequent tremor seismic events were associated with gas and ash emissions, and several episodes of “drumbeat” seismicity were recorded; they have been related by SGC to the growth of the lava dome on the floor of the Arenas crater. An overflight on 10 January, with the support of the Columbian Air Force, confirmed the presence of the dome which was first proposed in August 2015 (BGVN 42:06) (figure 103). The Arenas crater had dimensions of 900 x 980 m elongate to the SW-NE and was about 300 m deep (figure 104). The dome inside the crater was estimated to be 173 m in diameter and 60 m high with an approximate volume of 1,500,000 m³ (figures 105 and 106). In addition to the dome, the scientists also noted ash deposits on the summit ice cap (figure 107). The Washington VAAC reported an ash plume on 19 January that rose to 5.5 km altitude and drifted SW, dissipating quickly. On 30 January they reported an ash plume visible in satellite imagery extending 15 km NW from the summit at 5.8 km altitude. A single MODVOLC alert was issued on 15 January and data from the VIIRS satellite instrument reported thermal anomalies inside the summit crater on 14 days of the month. Sulfur dioxide plumes with DU values greater than 2 were recorded by the TROPOMI satellite instrument daily during the month.

Figure 103. SGC confirmed the presence of a lava dome inside the Arenas crater at Nevado del Ruiz on 10 January 2020. The dome is shown in brown, and zones of fumarolic activity are labelled around the dome. Courtesy of SGC (El Nuevo Domo de Lava del Volcán Nevado del Ruiz y la Geomorfología Actual del Cráter Arenas 2020).

Figure 104. A view of the Arenas crater at the summit of Nevado del Ruiz on 10 January 2020 (left) is compared with a view from 2010 (right). They were both taken during overflights supported by the Colombian Air Force (FAC). Ash deposits on the ice fields are visible in both images. Fumarolic activity rises from the inner walls of the crater in January 2020. Courtesy of SGC (El Nuevo Domo de Lava del Volcán Nevado del Ruiz y la Geomorfología Actual del Cráter Arenas 2020).

Figure 105. The dome inside the Arenas crater at Nevado del Ruiz appeared dark against the crater rim and ash-covered ice field on 10 January 2020. Features observed include (A) the edge of the Arenas crater, (B) a secondary crater 150 m in diameter located to the west, (C) interior cornices, (D) the lava dome, (E) a depression in the center of the dome caused by possible subsidence and cooling of the lava, (F) a source of gas and ash emission with a diameter of approximately 15 m (secondary crater), and (G, H, and I) several sources of gas emission located around the crater. Courtesy of SGC (El Nuevo Domo de Lava del Volcán Nevado del Ruiz y la Geomorfología Actual del Cráter Arenas 2020).

Figure 106. Images of the summit of Nevado del Ruiz captured by the PlanetScope satellite system on 14 March 2018 (A) and 10 January 2020 (B) show the lava dome at the bottom of Arenas crater. Courtesy Planet Lab Inc. and SGC (El Nuevo Domo de Lava del Volcán Nevado del Ruiz y la Geomorfología Actual del Cráter Arenas 2020).

Figure 107. Ash covered the snow and ice field around the Arenas crater at the summit of Nevado del Ruiz on 10 January 2020. The lava dome is the dark area on the right. Courtesy of SGC (posted on Twitter @sgcol).

The Washington VAAC reported multiple ash plumes during February 2020. On 4 February an ash plume was observed in satellite imagery drifting 35 km W from the summit at 5.8 km altitude. The following day a plume rose to 6.1 km altitude and extended 37 km W from the summit before dissipating by the end of the day (figure 108). On 6 February an ash cloud was observed in satellite imagery centered 45 km W of the summit at 5.8 km altitude. Although it had dissipated by midday, a hotspot remained in shortwave imagery until the evening. Late in the day another plume rose to 6.7 km altitude and drifted W. Diffuse ash was seen in satellite imagery on 13 February fanning towards the W at 5.8 km altitude. On 18 February at 1720 UTC the Bogota Meteorological Weather Office (MWO) reported an ash emission drifting NW at 5.8 km altitude; a second plume was reported a few hours later at the same altitude. Intermittent emissions continued the next day at 5.8-6.1 km altitude that reached as far as 50 km NW before dissipating. A plume on 21 February rose to 6.7 km altitude and drifted W (figure 109). Occasional emissions on 25 February at the same altitude reached 25 km SW of the summit before dissipating. A discrete ash emission around 1550 UTC on 26 February rose to 6.1 km altitude and drifted W. Two similar plumes were reported the next day. On 28 and 29 February plumes rose to 5.8 km altitude and drifted W.

Figure 108. Emissions rose from the Arenas crater at Nevado del Ruiz on 5 February 2020. The Washington VAAC reported an ash plume that day that rose to 6.1 km altitude and drifted 37 km W before dissipating. Courtesy of Camilo Cupitre.

Figure 109. Emissions rose from the Arenas crater at Nevado del Ruiz around 0600 on 21 February 2020. The Washington VAAC reported ash emissions that day that rose to 6.7 km altitude and drifted W. Courtesy of Manuel MR.

SGC reported several episodes of drumbeat type seismicity on 2, 8, 9, and 27 February which they attributed to effusion related to the growing lava dome in the summit crater. Sentinel-2 satellite imagery showed ring-shaped thermal anomalies characteristic of dome growth within Arenas crater several times during January and February (figure 110). The VIIRS satellite instrument recorded thermal anomalies on twelve days during February.

Figure 110. Persistent thermal anomalies from Sentinel-2 satellite imagery during January and February 2020 suggested that the lava dome inside Nevado del Ruiz’s Arenas crater was still actively growing. Atmospheric penetration rendering (bands 12, 11, 8A) courtesy of Sentinel Hub Playground.

On 4, 14, and 19 March 2020 thermal anomalies were visible in Sentinel-2 satellite data from within the Arenas crater. Thermal anomalies were recorded by the VIIRS satellite instrument on eight days during the month. Several episodes of drumbeat seismicity were recorded during the first half of the month and on 30-31 March. Distinct SO₂ plumes with DU values greater than 2 were recorded by the TROPOMI satellite instrument daily throughout February and March (figure 111). The Washington VAAC reported an ash emission on 1 March that rose to 5.8 km altitude and drifted NW; it was centered 15 km from the summit when detected in satellite imagery. The next day a plume was seen in satellite imagery moving SW at 7.0 km altitude, extending nearly 40 km from the summit. Additional ash emissions were reported on 4, 14, 15, 21, 28, 29, and 31 March; the plumes rose to 5.8-6.7 km altitude and drifted generally W, some reaching 45 km from the summit before dissipating.

Figure 111. Distinct SO2 plumes with Dobson values (DU) greater than 2 were recorded by the TROPOMI satellite instrument daily during February and March 2020. Ecuador’s Sangay produced smaller but distinct plumes most of the time as well. Dates are shown at the top of each image. Courtesy of NASA’s Sulfur Dioxide Monitoring Page.

Activity during April-June 2020. The Washington VAAC reported an ash emission that rose to 6.7 km altitude and drifted W on 1 April 2020. On 2 April, emission plumes were visible from the community of Tena in the Cundinamarca municipality which is located 100 km ESE (figure 112). The unusually clear skies were attributed to the reduction in air pollution in nearby Bogota resulting from the COVID-19 Pandemic quarantine. On 4 April the Bogota MWO reported an emission drifting SW at 5.8 km altitude. An ash plume on 8 April rose to 6.7 km altitude and drifted W. On 25 April the last reported ash plume from the Washington VAAC for the period rose to 6.1 km altitude and was observed in satellite imagery moving W at 30 km from the summit; after that, only steam and gas emissions were observed.

Figure 112. On the evening of 2 April 2020, emission plumes from Nevado del Ruiz were visible from Santa Bárbara village in Tena, Cundinamarca municipality which is located 100 km ESE. The unusually clear skies were attributed to the reduction in air pollution in the nearby city of Bogota resulting from the COVID-19 Pandemic quarantine. Photo by Williama Garcia, courtesy of Semana Sostenible (3 April 2020).

Distinct SO₂ plumes with DU values greater than 2 were recorded by the TROPOMI satellite instrument daily throughout the month. On 13 April, a Sentinel-2 thermal image showed a hot spot inside the Arenas crater largely obscured by steam and clouds. Cloudy images through May and June prevented observation of additional thermal anomalies in satellite imagery, but the VIIRS thermal data indicated anomalies on 3, 4, and 26 April. SGC reported low-energy episodes of drumbeat seismicity on 4, 9, 10, 12, 15, 16, 20, and 23 April which they interpreted as related to growth of the lava dome inside the Arenas crater. The seismic events were located 1.5-2.0 km below the floor of the crater.

Small emissions of ash and gas were reported by SGC during May 2020 and the first half of June, with the primary drift direction being NW. Gas and steam plumes rose 560-1,400 m above the summit during May and June (figure 113). Drumbeat seismicity was reported a few times each month. Sulfur dioxide emissions continued daily; increased SO₂ activity was recorded during 10-13 June (figure 114).

Figure 113. Gas and steam plumes rose 560-1,400 m above the summit of Nevado del Ruiz during May and June 2020, including in the early morning of 11 June. Courtesy of Carlos-Enrique Ruiz.

Figure 114. Increased SO2 activity during 10-13 June 2020 at Nevado del Ruiz was recorded by the TROPOMI instrument on the Sentinel-5P satellite. Sangay also emitted SO2 on those days. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. Nevado del Ruiz is a broad, glacier-covered volcano in central Colombia that covers more than 200 km2. Three major edifices, composed of andesitic and dacitic lavas and andesitic pyroclastics, have been constructed since the beginning of the Pleistocene. The modern cone consists of a broad cluster of lava domes built within the caldera of an older edifice. The 1-km-wide, 240-m-deep Arenas crater occupies the summit. The prominent La Olleta pyroclastic cone located on the SW flank may also have been active in historical time. Steep headwalls of massive landslides cut the flanks. Melting of its summit icecap during historical eruptions, which date back to the 16th century, has resulted in devastating lahars, including one in 1985 that was South America's deadliest eruption.

Information Contacts: El Servicio Geológico Colombiano (SGC), Diagonal 53 No. 34-53 - Bogotá D.C., Colombia (URL: https://www.sgc.gov.co/volcanes, https://twitter.com/sgcol); 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/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); NASA Worldview (URL: https://worldview.earthdata.nasa.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Camilo Cupitre (URL: https://twitter.com/Ccupitre/status/1225207439701704709); Manuel MR (URL: https://twitter.com/ElPlanetaManuel/status/1230837262088384512); Semana Sostenible (URL: https://sostenibilidad.semana.com/actualidad/articulo/fumarola-del-nevado-del-ruiz-fue-captada-desde-tena-cundinamarca/49597); Carlos-Enrique Ruiz (URL: https://twitter.com/Aleph43/status/1271800027841794049).

Japan's 24-km-wide Asosan caldera on the island of Kyushu has been active throughout the Holocene. Nakadake has been the most active of 17 central cones for 2,000 years; all historical activity is from Nakadake Crater 1. The largest ash plume in 20 years occurred on 8 October 2016. Asosan remained quiet until renewed activity from Crater 1 began in mid-April 2019; explosions with ash plumes continued through the first half of 2020 and are covered in this report. The Japan Meteorological Agency (JMA) provides monthly reports of activity; the Tokyo Volcanic Ash Advisory Center (VAAC) issues aviation alerts reporting on possible ash plumes, and Sentinel-2 satellite images provide data on ash emissions and thermal activity.

The Tokyo VAAC issued multiple daily reports of ash plumes from Nakadake Crater 1 from 1 January-14 June 2020. They were commonly at 1.8-2.1 km altitude, and often drifted E or S. JMA reported that ashfall continued downwind from the ash plumes until mid-June; seismic activity was relatively high during January and February and decreased steadily after that time. The measured SO₂ emissions ranged from 1,000-4,900 tons per day through mid-June and dropped to 500 tons per day during the second half of June. Intermittent thermal activity was recorded at the crater through mid-May.

Explosive activity during January-June 2020. Ash plumes rose up to 1.1 km above the crater rim at Nakadake Crater 1 during January 2020 (figure 70). Ashfall was confirmed downwind of an explosion on 7 January. During February, ash plumes rose up to 1.7 km above the crater, and ashfall was again reported downwind. The crater camera provided by the Aso Volcano Museum occasionally observed incandescence at the floor of the crater during both months. Incandescence was also occasionally observed with the Kusasenri webcam (3 km W) and was seen on 20 February from a webcam in Minamiaso village (8 km SW).

Figure 70. Ash plumes rose up to 1.1 km above the Nakadake Crater 1 at Asosan during January 2020 (left) and up to 1.7 km above the crater during February 2020 (right) as seen in these images from the Kusasenri webcam. Ashfall was reported downwind multiple times. Courtesy of JMA (Volcanic activity commentary material for Mt. Aso, January and February 2020).

During March 2020, ash plumes rose as high as 1.3 km. Ashfall was reported on 9 March in Ichinomiyamachi, Aso City (figure 71). In field surveys conducted on the 18th and 25th, there was no visible water inside the crater, and high-temperature grayish-white plumes were observed. The temperature at the base of the plume was measured at 300°C (figure 72).

Figure 71. Ashfall from Asosan appeared on 9 March 2020 in Ichinomiyamachi, Aso City around 10 km N. Courtesy of JMA (Volcanic activity commentary material for Mt. Aso, March 2020).

Figure 72. During a field survey of Nakadake Crater 1 at Asosan on 25 March 2020, JMA staff observed a gray ash plume rising from the crater floor (left). The maximum temperature of the ash plume was measured at about 300°C with an infrared thermal imaging device (right). Courtesy of JMA (Volcanic activity commentary material for Mt. Aso, March 2020).

Occasional incandescence was observed at the bottom of the crater during April and May 2020; ash plumes rose 1.1 km above the crater on most days in April and were slightly higher, rising to 1.8 km during May, although activity was more intermittent (figure 73). A brief increase in SO₂ activity was reported by JMA during field surveys on 7 and 8 May; satellite data captured small plumes of SO₂ on 1 and 6 May (figure 74). A brief increase in tremor amplitude was reported by JMA on 16 May.

Figure 73. Although activity at Asosan’s Nakadake Crater 1 was more intermittent during April and May 2020 than earlier in the year, ash plumes were still reported most days and incandescence was seen at the bottom of the crater multiple times until 15 May. Left image taken 11 May 2020 from the Kusachiri webcam; right image taken from the crater webcam on 10 May provided by the Aso Volcano Museum. Courtesy of JMA (Volcanic activity commentary material for Mt. Aso, May 2020).

Figure 74. The TROPOMI instrument on the Sentinel-5P satellite detected small but distinct SO2 plumes from Asosan on 1 and 6 May 2020. Additional small plumes are visible from Aira caldera’s Sakurajima volcano. Courtesy of NASA’s Global Sulfur Dioxide Monitoring Page.

The last report of ash emissions at Nakadake Crater 1 from the Tokyo VAAC was on 14 June 2020. JMA also reported that no eruption was observed after mid-June. On 8 June they reported an ash plume that rose 1.4 km above the crater. During a field survey on 16 June, only steam was observed at the crater; the plume rose about 100 m (figure 75). In addition, a small plume of steam rose from a fumarole on the S crater wall.

Figure 75. A steam plume rose about 100 m from the floor of Nakadake Crater 1 on 16 June 2020. A small steam plume was also observed by the S crater wall. Courtesy of JMA (Volcanic activity commentary material for Mt. Aso, June 2020).

Thermal activity during January-June 2020. Sentinel-2 satellite data indicated thermal anomalies present at Nakadake Crater 1 on 2 January, 6 and 21 February, 16 April, and 11 May (figure 76). In addition, thermal anomalies from agricultural fires appeared in satellite images on 11 February, 7 and 17 March (figure 77). The fires were around 5 km from the crater, thus they appear on the MIROVA thermal anomaly graph in black, but are likely unrelated to volcanic activity (figure 78). No thermal anomalies were recorded in satellite data from the Nakadake Crater 1 after 11 May, and none appeared in the MIROVA data as well.

Figure 76. Thermal anomalies appeared at Asosan’s Nakadake Crater 1 on 2 January, 6 and 21 February, 16 April and 11 May 2020. On 2 January a small ash plume drifted SSE from the crater (left). On 6 February a dense ash plume drifted S from the crater (center). Only a small steam plume was visible above the crater on 21 February (right). Images use Atmospheric penetration rendering (bands 12, 11, 8A), courtesy of Sentinel Hub Playground.

Figure 77. Thermal anomalies from agricultural fires located about 5 km from the crater appeared in satellite images on 11 February, and 7 and 17 March 2020. Although a dense ash plume drifted SSE from the crater on 11 February (left), no thermal anomalies appear at the crater on these dates. Images use Atmospheric penetration rendering (bands 12, 11, 8A), courtesy of Sentinel Hub Playground.

Figure 78. The MIROVA project plot of Log Radiative Power at Asosan from 29 June 2019 through June 2020 shows only a few small thermal alerts within 5 km of the summit crater during January-June 2020, and a spike in activity during February and March located around 5 km away. These data correlate well with the Sentinel-2 satellite data that show intermittent thermal anomalies at the summit throughout January-May and agricultural fires located several kilometers from the crater during February and March. Courtesy of MIROVA.

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

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Sakurajima rises from Kagoshima Bay, which fills the Aira Caldera near the southern tip of Japan's Kyushu Island. Frequent explosive and occasional effusive activity has been ongoing for centuries. The Minamidake summit cone has been the location of persistent activity since 1955; the adjacent Showa crater on its E flank has also been intermittently active since 2006. Numerous explosions and ash-bearing emissions have been occurring each month at either Minamidake or Showa crater since the latest eruptive episode began in late March 2017. This report covers ongoing activity at Minamidake from January through June 2020; the Japan Meteorological Agency (JMA) provides regular reports on activity, and the Tokyo VAAC (Volcanic Ash Advisory Center) issues tens of reports each month about the frequent ash plumes.

Activity continued during January-June 2020 at Minamidake crater with tens of explosions each month. The Tokyo VAAC issued multiple daily reports of ash emissions during January and February. Less activity occurred during the first half of March but picked up again with multiple daily reports from mid-March through mid-April. Emissions were more intermittent but continued through early June, when activity decreased significantly. JMA reported explosions with ash plumes rising 2.5-4.2 km above the summit, and ejecta traveling generally up to 1,700 m from the crater, although a big explosion in early June send a large block of tephra 3 km from the crater (table 23). Thermal anomalies were visible in satellite imagery on a few days most months and were persistent in the MIROVA thermal anomaly data from November 2019 through early June 2020 (figure 94). Incandescence was often visible at night in the webcams through early June; the Showa crater remained quiet throughout the period.

Table 23. Monthly summary of eruptive events recorded at Sakurajima's Minamidake crater within the Aira Caldera, January through June 2020. The number of events that were explosive in nature are in parentheses. No events were recorded at the Showa crater during this time. Ashfall is measured at the Kagoshima Local Meteorological Observatory (KLMO), 10 km W of Showa crater. Data courtesy of JMA (January to June 2020 monthly reports).

Figure 94. Persistent thermal anomalies were recorded in the MIROVA thermal energy data for the period from 2 July 2019 through June 2020. Thermal activity increased in October 2019 and remained steady through May 2020, decreasing abruptly at the beginning of June. Courtesy of MIROVA.

Explosions continued at Minamidake crater during January 2020 with 65 ash plumes reported. The highest ash plume rose 2.5 km above the crater on 30 January, and incandescent ejecta reached up to 1,700 m from the Minamidake crater on 22 and 29 January (figure 95). Slight inflation of the volcano since September 2019 continued to be measured with inclinometers and extensometers on Sakurajima Island. Field surveys conducted on 15, 20, and 31 January measured the amount of sulfur dioxide gas released as very high at 3,400-4,700 tons per day, as compared with 1,000-3,000 tons in December 2019.

Figure 95. An explosion at the Minamidake summit crater of Aira’s Sakurajima volcano on 29 January 2020 produced an ash plume that rose 2.5 km above the crater rim and drifted SE (left). On 22 January incandescent ejecta reached 1,700 m from the summit during explosive events. Courtesy of JMA (Sakurajima Volcanic Activity Commentary, January 2020).

About the same number of explosions produced ash plumes during February 2020 (67) as in January (65) (figure 96). On 10 February a large block was ejected 1,800 m from the crater, the first to reach that far since 5 February 2016. The tallest plume, on 26 February rose 2.6 km above the crater. Sentinel-2 satellite imagery indicated two distinct thermal anomalies within the Minamidake crater on both 1 and 6 February (figure 97). Activity diminished during March 2020 with only 10 explosions out of 26 eruptive events. On 21 March a large bomb reached 1,700 m from the crater. The tallest ash plume rose 3 km above the crater on 17 March. Scientists noted during an overflight on 16 March that a small steam plume was rising from the inner wall on the south side of the Showa crater; a larger steam plume rose to 300 m above the Minamidake crater and drifted S (figure 98). Sulfur dioxide emissions were similar in February (1,900 to 3,100 tons) and March (1,300 to 3,400 tons per day).

Figure 96. An ash plume rose from the Minamidake crater at the summit of Aira’s Sakurajima volcano on 6 February 2020 at 1752 local time, as seen looking S from the Kitadake crater. Courtesy of JMA (Sakurajima Volcanic Activity Commentary, February 2020).

Figure 97. Sentinel-2 satellite imagery revealed two distinct thermal anomalies within the Minamidake crater at Aira’s Sakurajima volcano on 1 and 6 February 2020. Images use Atmospheric penetration rendering (bands 12, 11, 8A). Courtesy of Sentinel Hub playground.

Figure 98. During an overflight of Aira’s Sakurajima volcano on 16 March 2020, JMA captured this view to the SW of the Kitadake crater on the right, the steam-covered Minamidake crater in the center, and the smaller Showa crater on the left adjacent to Minamidake. Courtesy of JMA and the Maritime Self-Defense Force 1st Air Group P-1 (Sakurajima Volcanic Activity Commentary, March 2020).

During April 2020, ejecta again reached as far as 1,700 m from the crater; 14 explosions were identified from the 51 reported eruptive events, an increase from March. The tallest plume, on 4 April, rose 3.8 km above the crater (figure 99). The same number of eruptive events occurred during May 2020, but 24 were explosive in nature. A large plume on 9 May rose to 4.2 km above the rim of Minamidake crater, the tallest of the period (figure 100). On 20 May, incandescent ejecta reached 1,300 m from the summit. Sulfur dioxide emissions during April (1,700-2,100 tons per day) and May (1,200-2,700 tons per day) were slightly lower than previous months.

Figure 99. A large ash plume at Aira’s Sakurajima volcano rose from Minamidake crater at 1621 on 4 April 2020. The plume rose to 3.8 km above the crater and drifted SE. Courtesy of JMA (Sakurajima Volcanic Activity Commentary, April 2020).

Figure 100. Activity continued at Aira’s Sakurajima volcano during May 2020. A large plume rose to 4.2 km above the summit and drifted N in the early morning of 9 May (left). The Kaigata webcam located at the Osumi River National Highway Office captured abundant incandescent ejecta reaching 1,300 m from the crater during the evening of 20 May. Courtesy of JMA (Sakurajima Volcanic Activity Commentary, May 2020).

A major explosion on 4 June 2020 produced 137 Pa of air vibration at the Seto 2 observation point on Sakurajima Island. It was the first time that air vibrations exceeding 100 Pa have been observed at the Seto 2 station since the 21 May 2015 explosion at the Showa crater. The ash plume associated with the explosion rose 1.5 km above the crater rim. During an 8 June field survey conducted in Higashisakurajima-cho, Kagoshima City, a large impact crater believed to be associated with this explosion was located near the coast 3 km SSW from Minamidake. The crater formed by the ejected block was about 6 m in diameter and 2 m deep (figure 101); fragments found nearby were 10-20 cm in diameter (figure 102). A nearby roof was also damaged by the blocks. Smaller bombs were found in Kurokami-cho, Kagoshima City, around 4- 5 km E of Minamidake on 5 June; the largest fragment was 5 cm in diameter. Multiple ash plumes rose to 3 km or more above the summit during the first ten days of June; explosions on 4 and 5 June reached 3.7 km above the crater (figure 103). Larger than normal inflation and deflation before and after the explosions was recorded during early June in the inclinometers and extensometers located on the island. Incandescence at the summit was observed at night through the first half of June. The Tokyo VAAC issued multiple daily ash advisories during 1-10 June after which activity declined abruptly. Two brief explosions on 23 June and one on 28 June were the only two additional ash explosions reported in June.

Figure 101. A large crater measuring 6 m wide and 2 m deep was discovered 3 km from the Minamidake crater in Higashisakurajima, part of Kagoshima City, on 8 June 2020. It was believed to be from the impact of a large block ejected during the 4 June explosion at Aira’s Sakurajima volcano. Photo courtesy of Kagoshima City and JMA (Sakurajima Volcanic Activity Commentary, June 2020).

Figure 102. Fragments 10-30 cm in diameter from a large bomb that traveled 3 km from Minamidake crater on Sakurajima were found a few days after the 4 June 2020 explosion at Aira. Courtesy of JMA, photo courtesy of Kagoshima City (Sakurajima Volcanic Activity Commentary, June 2020).

Figure 103. An ash plume rose 3.7 km above the Minamidake crater at Aira’s Sakurajima volcano on 5 June 2020 and was recorded in Sentinel-2 satellite imagery. Image uses Atmospheric penetration rendering (bands 12, 11, 8A). Courtesy of Sentinel Hub playground.

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

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); 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).

Nevados de Chillán is a complex of late-Pleistocene to Holocene stratovolcanoes in the Chilean Central Andes. An eruption started with a phreatic explosion and ash emission on 8 January 2016 from a new crater (Nicanor) on the E flank of the Nuevo crater, itself on the NW flank of the large Volcán Viejo stratovolcano. Strombolian explosions and ash emissions continued throughout 2016 and 2017; a lava dome within the Nicanor crater was confirmed in early January 2018. Explosions and pyroclastic flows continued during 2018 and 2019, with several lava flows appearing in late 2019. This report covers continuing activity from January-June 2020 when ongoing explosive events produced ash plumes, pyroclastic flows, and the growth of new dome inside the crater. Information for this report is provided primarily by Chile's Servicio Nacional de Geología y Minería (SERNAGEOMIN)-Observatorio Volcanológico de Los Andes del Sur (OVDAS), and by the Buenos Aires Volcanic Ash Advisory Center (VAAC).

Explosions with ash plumes rising up to three kilometers above the summit area were intermittent from late January through early June 2020. Some of the larger explosions produced pyroclastic flows that traveled down multiple flanks. Thermal anomalies within the Nicanor crater were recorded in satellite data several times each month from February through June. A reduction in overall activity led SERNAGEOMIN to lower the Alert Level from Orange to Yellow (on a 4-level, Green-Yellow-Orange-Red scale) during the first week of March, although tens of explosions with ash plumes were still recorded during March and April. Explosive activity diminished in early June and SERNAGEOMIN reported the growth of a new dome inside the Nicanor crater. By the end of June, a new flow had extended about 100 m down the N flank. Thermal activity recorded by the MIROVA project showed a drop in thermal energy in mid-December 2019 after the lava flows of September-November stopped advancing. A decrease in activity in January and February 2020 was followed by an increase in thermal and explosive activity in March and April. Renewed thermal activity from the growth of a new dome inside the Nicanor crater was recorded beginning in mid-June (figure 52).

Figure 52. MIROVA thermal anomaly data for Nevados de Chillan from 8 September 2019 through June 2020 showed a drop in thermal activity in mid-December 2019 after the lava flows of September-November stopped advancing. A decrease in activity in January and February 2020 was followed by an increase in explosive activity in March and April. Renewed thermal activity from the growth of a new dome inside the Nicanor crater was recorded beginning in mid-June. Courtesy of MIROVA.

Weak gas emissions were reported daily during January 2020 until a series of explosions began on the 21st. The first explosion rose 100 m above the active crater; the following day, the highest explosion rose 1.6 km above the crater. The Buenos Aires VAAC reported pulse emissions visible in satellite imagery on 21 and 24 January that rose to 3.9-4.3 km altitude and drifted SE and NE, respectively. Intermittent explosions continued through 26 January. Incandescent ejecta was observed during the night of 28-29 January. The VAAC reported an isolated emission on 29 January that rose to 5.2 km altitude and drifted E. A larger explosion on 30 January produced an ash plume that SERNAGEOMIN reported at 3.4 km above the crater (figure 53). It produced pyroclastic flows that traveled down ravines on the NNE and SE flanks. The Washington VAAC reported on behalf of the Buenos Aires VAAC that an emission was observed in satellite imagery on 30 January that rose to 4.9 km altitude and was moving rapidly E, reaching 15 km from the summit at midday. The altitude of the ash plume was revised two hours later to 7.3 km, drifting NNE and rapidly dissipating. Satellite images identified two areas of thermal anomalies within the Nicanor crater that day. One was the same emission center (CE4) identified in November 2019, and the second was a new emission center (CE5) located 60 m NW.

Figure 53. A significant explosion and ash plume from the Nicanor crater at Nevados de Chillan on 30 January 2020 produced an ash plume reported at 7.3 km altitude. The left image was taken within one minute of the initial explosion. Images posted by Twitter accounts #EmergenciasÑuble (left) and T13 (right); original photographers unknown.

When the weather permitted, low-altitude mostly white degassing was seen during February 2020, often with traces of fine-grained particulate material. Incandescence at the crater was observed overnight during 4-5 February. The Buenos Aires VAAC reported an emission on 14 February visible in the webcam. The next day, an emission was visible in satellite imagery at 3.9 km altitude that drifted E. Episodes of pulsating white and gray plumes were first observed by SERNAGEOMIN beginning on 18 February and continued through 25 February (figure 54). The Buenos Aires VAAC reported pulses of ash emissions moving SE on 18 February at 4.3 km altitude. Ash drifted E the next day at 3.9 km altitude and a faint plume was briefly observed on 20 February drifting N at 3.7 km altitude before dissipating. Sporadic pulses of ash moved SE from the volcano on 22 February at 4.3 km altitude, briefly observed in satellite imagery before dissipating. Thermal anomalies were visible from the Nicanor crater in Sentinel-2 satellite imagery on 23 and 28 February.

Figure 54. An ash emission at Nevados de Chillan on 18 February 2020 was captured in Sentinel-2 satellite imagery drifting SE (left). Thermal anomalies within the Nicanor crater were measured on 23 (right) and 28 February. Images use Atmospheric penetration rendering (bands 12, 11, 8a); courtesy of Sentinel Hub Playground.

Only low-altitude degassing of mostly steam was reported for the first half of March 2020. When SERNAGEOMIN lowered the Alert Level from Orange to Yellow on 5 March, they reduced the affected area from 5 km NE and 3 km SW of the crater to a radius of 2 km around the active crater. Thermal anomalies were recorded at the Nicanor crater in Sentinel-2 imagery on 4, 9, 11, 16, and 19 March (figure 55). A new series of explosions began on 19 March; 44 events were recorded during the second half of the month (figure 56). Webcams captured multiple explosions with dense ash plumes; on 25 and 30 March the plumes rose more than 2 km above the crater. Fine-grained ashfall occurred in Las Trancas (10 km SW) on 25 March. Pyroclastic flows on 25 and 30 March traveled 300 m NE, SE, and SW from the crater. Incandescence was observed at night multiple times after 20 March. The Buenos Aires VAAC reported several discrete pulses of ash that rose to 4.3 km altitude and drifted SE on 20 and 21 March, SW on 25 March, and SE on 29 and 30 March. Another ash emission rose to 5.5 km altitude later on 30 March and drifted SE.

Figure 55. Sentinel-2 Satellite imagery of Nevados de Chillan during March 2020 showed thermal anomalies on five different dates at the Nicanor crater, including on 9, 11, and 16 March. A second thermal anomaly of unknown origin was also visible on 11 March about 2 km SW of the crater (center). Images use Atmospheric penetration rendering (bands 12, 11, 8a); courtesy of Sentinel Hub Playground.

Figure 56. Forty-four explosive events were recorded at Nevados de Chillan during the second half of March 2020 including on 19 March. Courtesy of SERNAGEOMIN webcams and chillanonlinenoticia.

In their semi-monthly reports for April 2020, SERNAGEOMIN reported 94 explosive events during the first half of the month and 49 during the second half; many produced dense ash plumes. The Buenos Aires VAAC reported frequent intermittent ash emissions during 1-13 April reaching altitudes of 3.7-4.3 km (figure 57). They reported the plume on 8 April visible in satellite imagery at 7.3 km altitude drifting SE. An emission on 13 April was also visible in satellite imagery at 6.1 km altitude drifting NE.

Figure 57. Sentinel-2 satellite imagery captured a strong thermal anomaly and an ash plume drifting SE from Nevados de Chillan on 10 April 2020. Image uses Atmospheric penetration rendering (bands 12, 11, 8a); courtesy of Sentinel Hub Playground.

During the second half of April 2020, SERNAGEOMIN reported that only one plume exceeded 2 km in height; on 21 April, it rose to 2.4 km above the crater (figure 58). The Buenos Aires VAAC reported isolated pulses of ash on 18, 26, 28, and 30 April. During the second half of April SERNAGEOMIN also reported that a pyroclastic flow traveled about 1,200 m from the crater rim down the SE flank. The ash from the pyroclastic flow drifted SE and S as far as 3.5 km. Satellite images showed continued activity from multiple emission centers around the crater. Pronounced scarps were noted on the internal walls of the crater, attributed to the deepening of the crater from explosive activity.

Figure 58. Tens of explosions were reported at Nevados de Chillan during the second half of April 2020 that produced dense ash plumes. The plume on 21 April rose 2.4 km above the Nicanor crater. Photo by Josefa Carrasco Acuña from San Fabián de Alico; posted by Noticias Valpo Express.

Intermittent explosive activity continued during May 2020. The plumes contained abundant particulate material and were accompanied by periodic pyroclastic flows and incandescent ejecta around the active crater, especially visible at night. The Buenos Aires VAAC reported several sporadic weak ash emissions during the first week of May that rose to 3.7-5.2 km altitude and drifted NE. SERNAGEOMIN reported that only one explosion produced an ash emission that rose more than two km above the crater during the first two weeks of the month; on 6 May it rose to 2.5 km above the crater and drifted NE. They also observed pyroclastic flows on the E and SE flanks that day. Additional pyroclastic flows traveled 450 m down the S flank during the first half of the month, and similar deposits were observed to the N and NE. Satellite observations showed various emission points along the NW-trending lineament at the summit and multiple erosion scarps. Major erosion was noted at the NE rim of the crater along with an increase in degassing around the rim.

During the second half of May 2020 most of the ash plumes rose less than 2 km above the crater; a plume from one explosion on 22 May rose 2.2 km above the crater; the Buenos Aires VAAC reported the plume at 5.5 km altitude drifting NW (figure 59). Continuing pyroclastic emissions deposited material as far as 1.5 km from the crater rim on the NNW flank. There were also multiple pyroclastic deposits up to 500 m from the crater directed N and NE during the period. SERNAGEOMIN reported an increase in steam degassing between Nuevo-Nicanor and Nicanor-Arrau craters.

Figure 59. Explosions produced dense ash plumes and pyroclastic flows at Nevados de Chillan multiple times during May 2020 including on 22 May. Courtesy of SERNAGEOMIN.

Webcam images during the first two weeks of June 2020 indicated multiple incandescent explosions. On 3 and 4 June plumes from explosions reached heights of over 1.25 km above the crater; the Buenos Aires VAAC reported them drifting NW at 3.9 km altitude. Incandescent ejecta on 6 June rose 760 m above the vent and drifted NE. In addition, pyroclastic flows were distributed on the N, NW, E and SE flanks. Significant daytime and nighttime incandescence was reported on 6, 9, and 10 June (figure 60). The VAAC reported emission pulses on 6 and 9 June drifting E and SE at 4.3 km altitude.

Figure 60. Multiple ash plumes with incandescence were reported at Nevados de Chillan during the first ten days of June 2020 including on 6 June, after which explosive activity decreased significantly. Courtesy of SERNAGEOMIIN and Sismo Alerta Mexicana.

SERNAGEOMIN reported that beginning on the afternoon of 9 June 2020 a tremor-type seismic signal was first recorded, associated with continuous emission of gas and dark gray ash that drifted SE (figure 61). A little over an hour later another tremor signal began that lasted for about four hours, followed by smaller discrete explosions. A hybrid-type earthquake in the early morning of 10 June was followed by a series of explosions that ejected gas and particulate matter from the active crater. The vent where the emissions occurred was located within the Nicanor crater close to the Arrau crater; it had been degassing since 30 May.

Figure 61. A tremor-type seismic signal was first recorded on the afternoon of 9 June 2020 at Nevados de Chillan. It was associated with the continuous emission of gas and dark gray ash that drifted SE, and incandescent ejecta visible after dark. View is to the S, courtesy of SERNAGEOMIN webcam, posted by Volcanology Chile.

After the explosions on the afternoon of 9 June, a number of other nearby vents became active. In particular, the vent located between the Nuevo and Nicanor craters began emitting material for the first time during this eruptive cycle. The explosion also generated pyroclastic flows that traveled less than 50 m in multiple directions away from the vent. Abundant incandescent material was reported during the explosion early on 10 June. Deformation measurements showed inflation over the previous 12 days.

SERNAGEOMIN identified a surface feature in satellite imagery on 11 June 2020 that they interpreted as a new effusive lava dome. It was elliptical with dimensions of about 85 x 120 m. In addition to a thermal anomaly attributed to the dome, they noted three other thermal anomalies between the Nuevo, Arrau, and Nicanor craters. They reported that within four days the base of the active crater was filled with effusive material. Seismometers recorded tremor activity after 11 June that was interpreted as associated with lava effusion. Incandescent emissions were visible at night around the active crater. Sentinel-2 satellite imagery recorded a bright thermal anomaly inside the Nicanor crater on 14 June (figure 62).

Figure 62. A bright thermal anomaly was recorded inside the Nicanor crater at Nevados de Chillan on 14 June 2020. SERNAGEOMIN scientists attributed it to the growth of a new lava dome within the crater. Image uses Atmospheric penetration rendering (bands 12, 11, 8a); courtesy of Sentinel Hub Playground.

A special report from SERNAGEOMIN on 24 June 2020 noted that vertical inflation had increased during the previous few weeks. After 20 June the inflation rate reached 2.49 cm/month, which was considered high. The accumulated inflation measured since July 2019 was 22.5 cm. Satellite imagery continued to show the growth of the dome, and SERNAGEOMIN scientists estimated that it reached the E edge of the Nicanor crater on 23 June. Based on these images, they estimated an eruptive rate of 0.1-0.3 m³/s, about two orders of magnitude faster than the Gil-Cruz dome that emerged between December 2018 and early 2019.

Webcams revealed continued low-level explosive activity and incandescence visible both during the day and at night. By the end of June, webcams recorded a lava flow that extended 94 m down the N flank from the Nicanor crater and continued to advance. Small explosions with abundant pyroclastic debris produced recurring incandescence at night. Satellite infrared imagery indicated thermal radiance from effusive material that covered an area of 37,000 m², largely filling the crater. DEM analysis suggested that the size of the crater had tripled in volume since December 2019 due largely to erosion from explosive activity since May 2020. Sentinel-2 satellite imagery showed a bright thermal anomaly inside the crater on 27 June.

Geologic Background. The compound volcano of Nevados de Chillán is one of the most active of the Central Andes. Three late-Pleistocene to Holocene stratovolcanoes were constructed along a NNW-SSE line within three nested Pleistocene calderas, which produced ignimbrite sheets extending more than 100 km into the Central Depression of Chile. The largest stratovolcano, dominantly andesitic, Cerro Blanco (Volcán Nevado), is located at the NW end of the group. Volcán Viejo (Volcán Chillán), which was the main active vent during the 17th-19th centuries, occupies the SE end. The new Volcán Nuevo lava-dome complex formed between 1906 and 1945 between the two volcanoes and grew to exceed Volcán Viejo in elevation. The Volcán Arrau dome complex was constructed SE of Volcán Nuevo between 1973 and 1986 and eventually exceeded its height.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/, https://twitter.com/Sernageomin); 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/); 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); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); #EmergenciasÑuble (URL: https://twitter.com/urgenciasnuble/status/1222943399185207296); T13, Channel 13 Press Department (URL: https://twitter.com/T13/status/1222951071443771394); Chillanonlinenoticia (URL: https://twitter.com/ChillanOnline/status/1240754211932995595); Noticias Valpo Express (URL: https://twitter.com/NoticiasValpoEx/status/1252715033131388928); Sismo Alerta Mexicana (URL: https://twitter.com/Sismoalertamex/status/1269351579095691265); Volcanology Chile (URL: https://twitter.com/volcanologiachl/status/1270548008191643651).

Tinakula is a remote stratovolcano located 100 km NE of the Solomon Trench at the N end of the Santa Cruz. In 1971, an eruption with lava flows and ash explosions caused the small population to evacuate the island. Volcanism has previously been characterized by an ash explosion in October 2017 and the most recent eruptive period that began in December 2018 with renewed thermal activity. Activity since then has consisted of intermittent thermal activity and dense gas-and-steam plumes (BGVN 45:01), which continues into the current reporting period. This report updates information from January-June 2020 using primary source information from various satellite data, as ground observations are rarely available.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed weak, intermittent, but ongoing thermal activity during January-June 2020 (figure 41). A small cluster of slightly stronger thermal signatures was detected in late February to early March, which is correlated to MODVOLC thermal alert data; four thermal hotspots were recorded on 20, 27, and 29 February and 1 March. However, observations using Sentinel-2 satellite imagery were often obscured by clouds. In addition to the weak thermal signatures, dense gas-and-steam plumes were observed in Sentinel-2 satellite imagery rising from the summit during this reporting period (figure 42).

Figure 41. Weak thermal anomalies at Tinakula from 26 June 2019 through June 2020 as recorded by the MIROVA system (Log Radiative Power) were intermittent and clustered more strongly in late February to early March.

Figure 42. Sentinel-2 satellite imagery shows ongoing gas-and-steam plumes rising from Tinakula during January through May 2020. Images with atmospheric penetration (bands 12, 11, 8a) rendering; courtesy of Sentinel Hub Playground.

Three distinct thermal anomalies were observed in Sentinel-2 thermal satellite imagery on 22 January, 11 April, and 6 May 2020, accompanied by some gas-and-steam emissions (figure 43). The hotspot on 22 January was slightly weaker than the other two days, and was seen on the W flank, compared to the other two that were observed in the summit crater. According to MODVOLC thermal alerts, a hotspot was recorded on 6 May, which corresponded to a Sentinel-2 thermal satellite image with a notable anomaly in the summit crater (figure 43). On 10 June no thermal anomaly was seen in Sentinel-2 satellite imagery due to the presence of clouds; however, what appeared to be a dense gas-and-steam plume was extending W from the summit.

Figure 43. Sentinel-2 thermal satellite images showing a weak thermal activity (bright yellow-orange) on 22 January 2020 on the W flank of Tinakula (top) and slightly stronger thermal hotspots on 11 April (middle) and 6 May (bottom) in at the summit, which are accompanied by gas-and-steam emissions. Images with atmospheric penetration (bands 12, 11, 8a) rendering; courtesy of Sentinel Hub Playground.

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

Information Contacts: MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); 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).

Ibu is an active stratovolcano located along the NW coast of Halmahera Island in Indonesia. Volcanism has recently been characterized by frequent ash explosions, ash plumes, and small lava flows within the crater throughout 2019 (BGVN 45:01). Activity continues, consisting of frequent white-and-gray emissions, ash explosions, ash plumes, and lava flows. This report updates activity through June 2020, using data from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Darwin Volcanic Ash Advisory Centre (VAAC), and various satellites.

Volcanism during the entire reporting period dominantly consisted of white-and-gray emissions that rose 200-800 m above the summit drifting in multiple directions. The ash plume with the maximum altitude of 13.7 km altitude occurred on 16 May 2020. Sentinel-2 thermal satellite imagery detected multiple smaller hotspots within the crater throughout the reporting period.

Continuous ash emissions were reported on 6 February rising to 2.1 km altitude drifting E, accompanied by a hotspot visible in infrared satellite imagery. On 16 February, a ground observer reported an eruption that produced an ash plume rising 800 m above the summit drifting W, according to a Darwin VAAC notice. Ash plumes continued through the month, drifting in multiple directions and rising up to 2.1 km altitude. During 8-10 March, video footage captured multiple Strombolian explosions that ejected incandescent material and produced ash plumes from the summit (figures 21 and 22). Occasionally volcanic lightning was observed within the ash column, as recorded in video footage by Martin Rietze. This event was also documented by a Darwin VAAC notice, which stated that multiple ash emissions rose 2.1 km altitude drifting SE. PVMBG published a VONA notice on 10 March at 1044 reporting ash plumes rising 400 m above the summit. PVMBG and Darwin VAAC notices described intermittent eruptions on 26, 28, and 29 March, all of which produced ash plumes rising 300-800 m above the summit.

Figure 21. Strombolian explosions recorded at the crater summit of Ibu during 8-10 March 2020 ejected incandescent ejecta and a dense ash plume. Video footage copyright by Martin Rietze, used with permission.

Figure 22. Strombolian explosions recorded at the crater summit of Ibu during 8-10 March 2020 ejected incandescent ejecta and ash. Frequent volcanic lightning was also observed. Video footage copyright by Martin Rietze, used with permission.

A majority of days in April included white-and-gray emissions rising up to 800 m above the summit. A ground observer reported an eruption on 9 April, according to a Darwin VAAC report, and a hotspot was observed in HIMAWARI-8 satellite imagery. Minor eruptions were reported intermittently during mid-April and early to mid-May. On 12 May at 1052 a VONA from PVMBG reported an ash plume 800-1,100 m above the summit. A large short-lived eruption on 16 May produced an ash plume that rose to a maximum of 13.7 km altitude and drifted S, according to the Darwin VAAC report. By June, volcanism consisted predominantly of white-and-gray emissions rising 800 m above the summit, with an ash eruption on 15 June. This eruptive event resulted in an ash plume that rose 1.8 km altitude drifting WNW and was accompanied by a hotspot detected in HIMAWARI-8 satellite imagery, according to a Darwin VAAC notice.

The MIROVA (Middle InfraRed Observation of Volcanic Activity) system detected frequent hotspots during July 2019 through June 2020 (figure 23). In comparison, the MODVOLC thermal alerts recorded a total of 24 thermal signatures over the course of 19 different days between January and June. Many thermal signatures were captured as small thermal hotspots in Sentinel-2 thermal satellite imagery within the crater (figure 24).

Figure 23. Thermal anomalies recorded at Ibu from 2 July 2019 through June 2020 as recorded by the MIROVA system (Log Radiative Power) were frequent and consistent in power. Courtesy of MIROVA.

Figure 24. Sentinel-2 thermal satellite imagery (bands 12, 11, 8A) showed occasional thermal hotspots (bright orange) in the Ibu summit crater during January through June 2020. Courtesy of Sentinel Hub Playground.

Geologic Background. The truncated summit of Gunung Ibu stratovolcano along the NW coast of Halmahera Island has large nested summit craters. The inner crater, 1 km wide and 400 m deep, contained several small crater lakes through much of historical time. The outer crater, 1.2 km wide, is breached on the north side, creating a steep-walled valley. A large parasitic cone is located ENE of the summit. A smaller one to the WSW has fed a lava flow down the W flank. A group of maars is located below the N and W flanks. Only a few eruptions have been recorded in historical time, the first a small explosive eruption from the summit crater in 1911. An eruption producing a lava dome that eventually covered much of the floor of the inner summit crater began in December 1998.

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/); 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); Martin Rietze, Taubenstr. 1, D-82223 Eichenau, Germany (URL: https://mrietze.com/, https://www.youtube.com/channel/UC5LzAA_nyNWEUfpcUFOCpJw/videos, video at https://www.youtube.com/watch?v=qMkfT1e4HQQ).

Suwanosejima is an active stratovolcano located in the northern Ryukyu Islands. Volcanism has previously been characterized by Strombolian explosions, ash plumes, and summit incandescence (BGVN 45:01), which continues to occur intermittently. A majority of this activity originates from vents within the large Otake summit crater. This report updates information during January through June 2020 using monthly reports from the Japan Meteorological Agency (JMA), the Tokyo Volcanic Ash Advisory Center (VAAC), and various satellite data.

During 3-10 January 2020, 13 explosions were detected from the Otake crater rising to 1.4 km altitude; material was ejected as far as 600 m away and ashfall was reported in areas 4 km SSW, according to JMA. Occasional small eruptive events continued during 12-17 January, which resulted in ash plumes that rose 1 km above the crater rim and ashfall was again reported 4 km SSW. Crater incandescence was visible nightly during 17-24 January, while white plumes rose as high as 700 m above the crater rim.

Nightly incandescence during 7-29 February, and 1-6 March, was accompanied by intermittent explosions that produced ash plumes rising up to 1.2 km above the crater rim (figure 44); activity during early February resulted in ashfall 4 km SSW. On 19 February an eruption produced a gray-white ash plume that rose 1.6 km above the crater (figure 45), resulting in ashfall in Toshima village (4 km SSW), according to JMA. Explosive events during 23-24 February ejected blocks onto the flanks. Two explosions were recorded during 1-6 March, which sent ash plumes as high as 900-1,000 m above the crater rim and ejected large blocks 300 m from the crater.

Figure 44. Surveillance camera images of summit incandescence at Suwanosejima on 29 January (top left), 21 (middle left) and 23 (top right) February, and 25 March (bottom left and right) 2020. Courtesy of JMA (Monthly bulletin reports 511, January, February, and March 2020).

Figure 45. Surveillance camera images of which and white-and-gray gas-and-steam emissions rising from Suwanosejima on 5 January (top), 19 February (middle), and 24 March 2020 (bottom). Courtesy of JMA (Monthly bulletin reports 511, January, February, and March 2020).

Nightly incandescence continued to be visible during 13-31 March, 1-10 and 17-24 April, 1-8, 15-31 May, 1-5 and 12-30 June 2020; activity during the latter part of March was relatively low and consisted of few explosive events. In contrast, incandescence was frequently accompanied by explosions in April and May. On 28 April at 0432 an eruption produced an ash plume that rose 1.6 km above the crater rim and drifted SE and E, and ejected blocks as far as 800 m from the crater. The MODVOLC thermal alerts algorithm also detected four thermal signatures during this eruption within the summit crater. An explosion at 1214 on 29 April caused glass in windows to vibrate up to 4 km SSW away while ash emissions continued to be observed following the explosion the previous day, according to the Tokyo VAAC.

During 1-8 May explosions occurred twice a day, producing ash plumes that rose as high as 1 km above the crater rim and ejecting material 400 m from the crater. An explosion on 29 May at 0210 produced an off-white plume that rose as high as 500 m above the crater rim and ejected large blocks up to 200 m above the rim. On 5 June an explosion produced gray-white plumes rising 1 km above the crater. Small eruptive events continued in late June, producing ash plumes that rose as high as 900 m above the crater rim.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed relatively stronger thermal anomalies in late February and late April 2020 with an additional six weaker thermal anomalies detected in early January (2), early February (1), mid-April (2), and mid-May (1) (figure 46). Sentinel-2 thermal satellite imagery in late January through mid-April showed two distinct thermal hotspots within the summit crater (figure 47).

Figure 46. Prominent thermal anomalies at Suwanosejima during July-June 2020 as recorded by the MIROVA system (Log Radiative Power) occurred in late February and late April. Courtesy of MIROVA.

Figure 47. Sentinel-2 thermal satellite images showing small thermal anomalies (bright yellow-orange) from two locations within the Otake summit crater at Suwanosejima. Images with “Atmospheric penetration” (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Geologic Background. The 8-km-long, spindle-shaped island of Suwanosejima in the northern Ryukyu Islands consists of an andesitic stratovolcano with two historically active summit craters. The summit is truncated by a large breached crater extending to the sea on the east flank that was formed by edifice collapse. Suwanosejima, one of Japan's most frequently active volcanoes, was in a state of intermittent strombolian activity from Otake, the NE summit crater, that began in 1949 and lasted until 1996, after which periods of inactivity lengthened. The largest historical eruption took place in 1813-14, when thick scoria deposits blanketed residential areas, and the SW crater produced two lava flows that reached the western coast. At the end of the eruption the summit of Otake collapsed forming a large debris avalanche and creating the horseshoe-shaped Sakuchi caldera, which extends to the eastern coast. The island remained uninhabited for about 70 years after the 1813-1814 eruption. Lava flows reached the eastern coast of the island in 1884. Only about 50 people live on the island.

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/); 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).

A visit to the summit caldera on 8-9 July did not permit an approach to the lava lakes in the Benbow and Marum craters due to poor weather. An overflight on the night of 20 July permitted observations of surface bubbling in Marum's lava lake. Two other overflights, on 21 and 22 July, allowed observation of activity in both lakes for several minutes. During these observations, the surface of the Benbow lake was fairly calm. However, Marum's lava lake, ~100 m in diameter, exhibited occasional explosions that threw glowing magma fragments some meters above the surface; bubbling was clearly visible from the airplane.

Geologic Background. Ambrym, a large basaltic volcano with a 12-km-wide caldera, is one of the most active volcanoes of the New Hebrides Arc. A thick, almost exclusively pyroclastic sequence, initially dacitic then basaltic, overlies lava flows of a pre-caldera shield volcano. The caldera was formed during a major Plinian eruption with dacitic pyroclastic flows about 1,900 years ago. Post-caldera eruptions, primarily from Marum and Benbow cones, have partially filled the caldera floor and produced lava flows that ponded on the floor or overflowed through gaps in the caldera rim. Post-caldera eruptions have also formed a series of scoria cones and maars along a fissure system oriented ENE-WSW. Eruptions have apparently occurred almost yearly during historical time from cones within the caldera or from flank vents. However, from 1850 to 1950, reporting was mostly limited to extra-caldera eruptions that would have affected local populations.

Information Contacts: Henry Gaudru, C. Pittet, C. Bopp, and G. Borel, Société Volcanologique Européenne, C.P. 1, 1211 Genève 17, Switzerland (URL: http://www.sveurop.org/); Michel Lardy, Centre ORSTOM, B.P. 76, Port Vila, Vanuatu.

On 18 September AVO received a pilot report of a small ash plume above Amukta. An Alaska Airlines pilot noted black and gray ash clouds rising ~300 m above the summit crater during overflights on 17 and 18 September. The ash plumes extended ~16 km S over the Pacific Ocean before dissipating. No plume was visible on satellite imagery.

Geologic Background. The symmetrical Amukta stratovolcano lies in the central Aleutians SW of Chagulak Island and is the westernmost of the Islands of the Four Mountains group. Amukta was constructed at the northern side of an arcuate caldera-like feature that is open to the sea along the southern coast of the 8-km-wide Amukta Island. The 1066-m-high stratovolcano overlies a broad shield volcano and is topped by a 400-m-wide crater. A cinder cone is located near the NE coast. Amukta has had several eruptions in historical time from both summit and flank vents.

Information Contacts: Alaska Volcano Observatory (AVO); NOAA/NESDIS Satellite Analysis Branch, Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA.

Some small pyroclastic flows took place in September but eruptions were milder than the previous month. Eruptions were often separated by 10-60 minute intervals, and plumes seldom rose much over 1 km. During September, a new lava flow began moving toward the crater's SW side.

Noteworthy eruptions took place several times during September. An eruption at 0926 on the 11th generated a pyroclastic flow that traveled SW; the associated plume reached 1,230 m altitude. At 1700 on the 29th eyewitnesses saw a rockslide off a lava flow that led to a small avalanche (figure 80). Also, at 1720 that same day, an ash-column collapse produced a small pyroclastic flow (figure 80). At 1634 on the 30th a pyroclastic flow swept NW; the associated plume reached 1,000 m altitude.

Figure 80. Arenal seen from the NNW looking towards the active flow field (shaded). The sketch shows events visible at 1720 on 29 September 1996: (A) the avalanche deposit laid down ~20 minutes earlier, and (B) the ash-laden column collapsing to create a small pyroclastic flow. Courtesy of G.J. Soto, ICE.

During September, OVSICORI-UNA reported about average monthly seismic activity: 875 events and 300 hours of tremor (station VACR, 2.7 km NE of Crater C). ICE reported above-average seismic activity during September: 86 events and 4.78 hours of tremor (Fortuna Station, 3.5 km E of Crater C). OVSICORI-UNA noted that many of the seismic events were associated with Strombolian eruptions.

Although the volcano's distance network has generally shown a cumulative contraction since the initial measurements in 1991, a small pulse of inflation (reaching 5 ppm) took place in April 1996. Due to accumulating lava and pyroclastic materials, the summit of the active crater (C) grew 1.65 m between April and September 1996. This growth rate was consistent with the average rate of 4.13 m/year seen thus far in 1996 and close to the overall average of 5.33 m/year.

Arenal's post-1968 Strombolian-type eruptions have produced basaltic-andesite tephra and lavas. The volcano lies directly adjacent to Lake Arenal, a dammed reservoir for generating hydroelectric power.

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, E. Duarte, V. Barboza, R. Van der Laat, E. Hernandez, M. Martinez, and R. Sáenz, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; G.J. Soto and J.F. Arias, Oficina de Sismología y Vulcanología del Arenal y Miravalles (OSIVAM), Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica.

On the morning of 12 August, the ~250,000 residents of Puerto Montt (35 km SW) and Puerto Varas (36 km SW) were alarmed by strong fumarolic emissions from the 1.5-km-diameter main crater of Calbuco. In May 1995 a weak fumarole was noticed and filmed from a helicopter. Prior to that, Calbuco had showed no signs of activity since a 1972 eruption that lasted for ~4 hours.

Calbuco is a very explosive late Pleistocene to Holocene andesitic volcano S of Lake Llanquihue that underwent edifice collapse in the late Pleistocene, producing a volcanic debris avalanche that reached the lake. One of the largest historical eruptions in southern Chile took place from Calbuco in 1893-1894. Violent eruptions ejected 30-cm bombs to distances of 8 km from the crater, accompanied by voluminous hot lahars. Several days of darkness occurred in San Carlos de Bariloche, Argentina (>100 km SE). Strong explosions occurred in April 1917, and a lava dome formed in the crater accompanied by hot lahars. Another short explosive eruption in January 1929 also included an apparent pyroclastic flow and a lava flow. The last major eruption of Calbuco, in 1961, sent ash columns 12-15 km high and produced plumes that dispersed mainly to the SE as far as Bariloche; two lava flows were also emitted.

Geologic Background. Calbuco is one of the most active volcanoes of the southern Chilean Andes, along with its neighbor, Osorno. The late-Pleistocene to Holocene andesitic volcano is immediately SE of Lake Llanquihué in the Chilean lake district. Guanahuca, Guenauca, Huanauca, and Huanaque, all listed as synonyms of Calbuco (Catalog of Active Volcanoes of the World), are actually synonyms of nearby Osorno volcano (Moreno 1985, pers. comm.). The edifice is elongated in a SW-NE direction and is capped by a 400-500 m wide summit crater. The complex evolution included collapse of an intermediate edifice during the late Pleistocene that produced a 3-km3 debris avalanche that reached the lake. It has erupted frequently during the Holocene, and one of the largest historical eruptions in southern Chile took place from Calbuco in 1893-1894 that concluded with lava dome emplacement. Subsequent eruptions have enlarged the lava-dome complex in the summit crater.

Information Contacts: Hugo Moreno, Observatorio Volcanologico de los Andes del Sur (OVDAS), Universidad de la Frontera, Casilla 54-D, Temuco, Chile.

Activity observed during 14-15 July consisted of a large steam-and-gas plume with a strong SO₂ odor. Numerous fumarolic zones covered with yellow sulfur deposits dotted the interior wall of the crater. Fairly strong degassing was taking place from the NW part of the depression. An active fumarole rose from the high interior N part of the crater (T = 119 ± 5°C). The dominant vent sent a plume W from the caldera. The highest temperature of the hot sub-lacustrine fumaroles in the NE part of the lake, in the vicinity of the seismic station, varied between 34 and 65°C. The northernmost attained a temperature of 62°C.

The cone that dominates the NW part of the caldera is composed of five principal craters. The bottom of the northernmost crater is occupied in part by a small shallow pool of greenish water. The active crater is situated on the SE flank of the cone (Mt. Garat).

Geologic Background. The roughly 20-km-diameter Gaua Island, also known as Santa Maria, consists of a basaltic-to-andesitic stratovolcano with an 6 x 9 km wide summit caldera. Small parasitic vents near the caldera rim fed Pleistocene lava flows that reached the coast on several sides of the island; several littoral cones were formed where these lava flows reached the sea. Quiet collapse that formed the roughly 700-m-deep caldera was followed by extensive ash eruptions. Construction of the historically active cone of Mount Garat (Gharat) and other small cinder cones in the SW part of the caldera has left a crescent-shaped caldera lake. The symmetrical, flat-topped Mount Garat cone is topped by three pit craters. The onset of eruptive activity from a vent high on the SE flank in 1962 ended a long period of dormancy.

Information Contacts: Henry Gaudru, C. Pittet, C. Bopp, and G. Borel, Société Volcanologique Européenne, C.P. 1, 1211 Genève 17, Switzerland (URL: http://www.sveurop.org/); Michel Lardy, Centre ORSTOM, B.P. 76, Port Vila, Vanuatu.

On 19 September seismic activity increased and more than 20 earthquakes (M <= 1.8) were recorded beneath Gorely. However, no sign of eruptive activity was observed around the crater on 20 September. During 23-30 September seismicity returned to background levels.

Geologic Background. Gorely volcano consists of five small overlapping stratovolcanoes constructed along a WNW-ESE line within a large 9 x 13.5 km caldera. The caldera formed about 38,000-40,000 years ago accompanied by the eruption of about 100 km3 of tephra. The massive complex includes 11 summit and 30 flank craters, some of which contain acid or freshwater crater lakes; three major rift zones cut the complex. Another Holocene stratovolcano is located on the SW flank. Activity during the Holocene was characterized by frequent mild-to-moderate explosive eruptions along with a half dozen episodes of major lava extrusion. Early Holocene explosive activity, along with lava flows filled in much of the caldera. Quiescent periods became longer between 6000 and 2000 years ago, after which the activity was mainly explosive. About 600-650 years ago intermittent strong explosions and lava flow effusion accompanied frequent mild eruptions. Historical eruptions have consisted of moderate Vulcanian and phreatic explosions.

Information Contacts: Tom Miller, Alaska Volcano Observatory (AVO), 4200 University Drive, Anchorage, AK 99508-4667, USA; Vladimir Kirianov, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia.

The Nordic Volcanical Institute reported that from late in the evening of 30 September until 13 October a subglacial eruption occurred along part of the East Rift Zone that traverses beneath the NW side of Vatnajökull, Europe's largest continental glacier (Björnsson and Einarsson, 1991; Björnsson and Gudmundsson, 1993). This part of the Rift Zone includes both Bardarbunga and Grímsvötn fissure systems and their respective central volcanoes, each containing a substantial caldera (figure 1).

Figure 1. Area map showing the erupting fissure and recent seismicity along the East Rift Zone in the Grímsvötn-Bardarbunga region. Shaded regions indicate exposed land surface, unshaded regions indicate glaciers; ice-surface contour values are undisclosed. The solid sub-circular lines depict the larger extents of the named central volcanoes; hachured lines indicate the respective caldera topographic margins. Dots show earthquake epicenters for 29 September-2 October. Balloons depict available earthquake fault plane solutions for some events over M 4. Courtesy of the Icelandic Meteorological Office.

The eruption was preceded by an unusual sequence of earthquakes. One, at 1048 on 29 September, was Ms 5.4 and centered near Bardarbunga caldera's N rim (figure 1). Similar earthquakes have occurred beneath Bardarbunga many times during the last 22 years. Unlike this event, however, none of the previous large earthquakes had either significant aftershocks or preceded magmatic activity.

In the two hours following the M 5.4 event there were numerous earthquakes, including five larger than M 3. These were recorded at the two analog seismic stations just NW of Bardarbunga and at the S rim of the Grímsvötn caldera. Shortly after 1300 on 30 September, Science Institute seismologists informed Civil Defense authorities and the scientific community about this unusual seismicity and the possibility of impending eruptive activity.

The seismic swarm continued throughout 30 September, with increasing intensity. Hundreds of earthquakes were recorded each day, including over 10 events larger than M 3. The earthquakes were located in the N part of Bardarbunga and migrated towards Grímsvötn. They were accompanied by high-frequency (>3 Hz) continuous tremor of the same type as was frequently observed during intrusive activity within the Krafla volcanic system during 1975-84.

The Civil Defense Council issued a warning of a possible eruption at 1900 on 30 September. Later that evening earthquake activity near Grímsvötn decreased markedly, while that near Bardarbunga continued. At about 2200 the seismograph at Grímsvötn began recording continuous small-amplitude eruption tremor. The sudden decrease in earthquake activity and the onset of tremor may be taken as evidence that an eruption began between 2200 and 2300 on September 30. Tremor amplitude increased very slowly during the next hours, reaching a maximum at about 0600 on 1 October.

The eruption site was spotted from aircraft in the early morning of 1 October. By that time two elongate, 1-2 km wide and N23E-trending subsidence bowls or cauldrons had developed in the ice surface. These bowls were located to Bardarbunga's SSE, along a fissure on Grímsvötn's N flank (figure 1). The bowls (one of which is shown in figures 2 and 3) appeared in the glacial ice above a 4-6-km-long NNE-trending fissure; ice in this location had been considered 400-600 m thick, though some later estimates put the ice thickness more precisely at 450 m. The eruption was most powerful under the northernmost bowl, causing it to subside 50 m over 4 hours.

Figure 2. A subsidence bowl developed in glacial ice on Grímsvötn's N flank., 1 October 1996. Courtesy of R. Axelsson.

Figure 3. A detail from 1 October showing inward stepping crevasses of the subsidence bowl with a fixed-wing airplane and its shadow for scale. Courtesy of R. Axelsson.

The resulting meltwater drained into Grímsvötn caldera (figure 1) raising the ice shelf above the caldera lake. The lake was covered by 250 m of ice and held in place by an ice dam. Widening and deepening of the bowls during the day added an estimated 0.3 km³ of water to the Grímsvötn lake in less than 24 hours. On 1 October a shallow linear subsidence structure extended from the eruption site to the subglacial Grímsvötn caldera lake, the surface manifestation of the subglacial pathway for water draining into Grímsvötn.

By 1 October the lake's surface had risen 10-15 m (to 1,410 m). During the first week of the eruption meltwater production was thought to be ~5,000 m³/second, but it later slowed. Glacier bursts (jökulhlaups) were thought to be likely, if not imminent. Water from Grímsvötn crater lake was expected to emerge at an outlet at the edge of the glacier ~50 km S. N-directed floods were also expected if the eruptive fissure continued to propagate N.

Helgi Torfason noted that although a previous glacier burst took place last summer (with 3,000 m³/second flow rates), the affected bridges were designed to withstand surges with meltwater fluxes 3x that size. On the other hand, a 1938 eruption, in almost exactly the same place (Gudmundsson and Björnsson, 1991) caused glacier bursts with fluxes ~5 or 6 times as large.

At 0447 on the morning of 2 October a vent on the floor of one bowl broke through the ice and the eruption began a subaerial phase. At 0800 vigorous explosive activity was observed in the crater with the eruption column rising to 4-5 km altitude. One account noted that rhythmic explosions resulted in black ash clouds rising 500 m while the buoyant eruption column rose to 3 km. In the afternoon the opening in the ice was several hundred meters wide. The eruptive fissure apparently extended 3 km farther N, because on the ice surface observers saw a new, elongated, N-trending ice cauldron. Some 2 October reports noted a steam column that rose to ~10 km altitude.

On 3 October the ice bowl over the northernmost part of the fissure had grown ~2 km since the previous day. By this time the glacier had subsided over an area 8-9 km long and 2-3 km wide. Subaerial eruptions pulsated, alternating between quiet periods and explosive activity. Ash mainly dispersed N but also SSW. The opening at the eruption site grew larger. Eruptive intensity began to decline on this day but tremor continued. A TV photographer captured footage of two lightning strikes traveling along the ash cloud that was widely shown on news reports. The water level in the vent was ~50-200 m below the original ice surface. The surface of Grímsvötn lake was at 1,460 m. Ash samples collected on this day had water-soluble fluorine contents of ~130 ppm, ~10% the amount found in Hekla ash, reducing concerns about the immediate danger to grazing animals. Initial electron microprobe analysis of the ash indicated that it was basaltic andesite in composition.

The eruption continued on 4 October. It was noted that the caldera lake was higher than at any point in this century. Poor weather intervened for the next few days, but on 7 and 9 October the eruption continued from the 9-km-long fissure; thin ash covered about half of the 8,100 km² Vatnajökull glacier. On 9 October J-M. Bardintzeff and a visiting French team saw a 4-km-high plume as well as violent phreatic ash emissions between 1230 and 1415.

On 10 October eruptive intensity appeared similar to the low levels seen since 3 October. Occasional eruptions carried black ash clouds to ~3 km and vapor with finer ash to 4 km. Minor ashfall was limited to the Vatnajökull glacier. An 11 October flight confirmed that emissions continued, but lacked rooster-tail-shaped explosions seen previously and may have declined in intensity. The eruptive crater was still water covered. Grímsvötn ice cover had bulged upward but signs of escaping water were absent. The caldera lake's total volume was estimated at >2 km³.

A Canadian Space Agency satellite radar image from 17 October was processed by Troms Satellite Station. In this image they found increased backscatter compared to earlier in the month; they suggested that this may have been due to cooler ice caused by a return to stability around the crater. In accord with this observation, on 18 October NVI announced that the eruption had apparently stopped on 13 October.

The eruption left material piled up to form a subglacial ridge; the highest part of this ridge supported an eruptive crater that reached a few to tens of meters out of meltwater at the eruptive site. Cooling eruptive materials continued to melt significant volumes of ice.

Increased CO₂ and H₂S in N-flowing river water suggested some flow of meltwater from the eruptive site. As of 18 October most of the meltwater was still directed towards the Grímsvötn caldera lake, with no signs of the awaited glacier burst. GPS measurements in October documented the lake's rise on the 12th (1,500 m), 15th (1,504 m), and 17th (1,505 m). Glacier bursts from the crater lake have typically occurred at the much lower lake level of ~1,450 m.

The recent eruption was a continuation of geophysical events in the Vatnajökull area that began in 1995 and possibly earlier. In July 1995 and August 1996 there were glacial floods from subglacial geothermal areas NW of Grímsvötn. In both cases, after the water reservoir drained, distinct tremor episodes occurred. Presumably, these pressure releases triggered small eruptions. In February 1996 there was an intense, week-long earthquake swarm centered on Hamarinn volcano (figure 1).

Besides the prospect of glacier bursts, the eruption was watched closely because the 1783-84 Laki (Skaftár Fires) and 1783-85 Grímsvötn eruptions vented on the Rift Zone within ~70 km of the current eruption. The 27-km-long Laki fissures active in 1783-84 start ~40 km SW of Grímsvötn's center. The Laki eruption produced 14.7 ± 0.1 km³ of basaltic lavas (Thordarson and Self, 1993) making it the largest known lava eruption in history. Sulfur and other gases released produced an acid haze (aerosol) that perturbed the weather in Western Eurasia, the North Atlantic, and the Arctic. An estimated 9,350 Icelanders died in the "haze famine" from 1783-86, an interval that included two severe winters, crop failures, livestock and fish deaths, and various illnesses, including fluorine poisoning (Stothers, 1996).

References. Björnsson, H., and Gudmundsson, M.T., 1993, Variations in the thermal output of the subglacial Grímsvötn caldera, Iceland: Geophysical Research Letters, v. 20, p. 2127-2130.

Björnsson, H., and Einarsson, P., 1991, Volcanoes beneath Vatnajökull, Iceland: evidence from radio-echo sounding, earthquakes and jökulhlaups: Jökull, v. 40, p. 147-168.

Gudmundsson, M.T., and Björnsson, H., 1991, Eruptions in Grímsvötn, Vatnajökull, Iceland, 1934-1991: Jökull, v. 41, p. 21-45.

Stothers, R.B., 1996, The great dry fog of 1783: Climatic Change, Kluwer Academic Publishers, v. 32, p.79-89.

Thordarson, T., and Self, S., 1993, The Laki (Skaftár Fires) and Grímsvötn eruptions in 1783-1785: Bulletin of Volcanology, Springer-Verlag, v. 55, p. 233-263.

Further Reference. Worsley, P., 1997, The 1996 volcanically induced glacial mega-flood in Iceland - cause and consequence: Geology Today, Blackwell Science, Ltd., v. 13., no. 6, p. 222-227.

Geologic Background. Grímsvötn, Iceland's most frequently active volcano in historical time, lies largely beneath the vast Vatnajökull icecap. The caldera lake is covered by a 200-m-thick ice shelf, and only the southern rim of the 6 x 8 km caldera is exposed. The geothermal area in the caldera causes frequent jökulhlaups (glacier outburst floods) when melting raises the water level high enough to lift its ice dam. Long NE-SW-trending fissure systems extend from the central volcano. The most prominent of these is the noted Laki (Skaftar) fissure, which extends to the SW and produced the world's largest known historical lava flow during an eruption in 1783. The 15-cu-km basaltic Laki lavas were erupted over a 7-month period from a 27-km-long fissure system. Extensive crop damage and livestock losses caused a severe famine that resulted in the loss of one-fifth of the population of Iceland.

Information Contacts: Nordic Volcanological Institute (NVI), Grensásvegur 50, 108 Reykjavík, Iceland (URL: http://nordvulk.hi.is/); Páll Einarsson, Bryndís Brandsdóttir, Magnús Tumi Gudmundsson, and Helgi Björnsson, Science Institute, Dunhagi 3, 107 Reykjavík, Iceland (URL: https://www.hi.is/); Icelandic Meteorological Office, Geophysics Department, Reykjavík, Iceland (URL: http://en.vedur.is/); J-M. Bardintzeff, Lab. Petrographi-Volcanologie, bat 504, Universite Paris-Sud, 91305 Orsay, France; Helgi Torfason, National Energy Authority, Grensasvegur 9, 108 Reykjavík, Iceland; Tromsø Satellite Station, N-9005, Tromsø, Norway; R. Axelsson, Morgunbladid News (photographer), Reykjavík, Iceland.

A small shallow earthquake swarm occurred beneath Iliamna during mid-May. After two months of ensuing quiescence, seismic activity increased on 1 August (BGVN 21:08). During September and the first half of October, 6 to 27 events were recorded each day at depths within the edifice to 9 km below sea level. Most of them were less than M 1.0 and the largest was M 3.2. All events seemed to be volcano-tectonic, and no long-period earthquakes or tremors that usually precede eruptions were detected. This seismicity was likely related to an intrusion of magma, but doest not mean that an eruption is imminent.

Geologic Background. Iliamna is a prominentglacier-covered stratovolcano in Lake Clark National Park on the western side of Cook Inlet, about 225 km SW of Anchorage. Its flat-topped summit is flanked on the south, along a 5-km-long ridge, by the prominent North and South Twin Peaks, satellitic lava dome complexes. The Johnson Glacier dome complex lies on the NE flank. Steep headwalls on the S and E flanks expose an inaccessible cross-section of the volcano. Major glaciers radiate from the summit, and valleys below the summit contain debris-avalanche and lahar deposits. Only a few major Holocene explosive eruptions have occurred from the deeply dissected volcano, which lacks a distinct crater. Most of the reports of historical eruptions may represent plumes from vigorous fumaroles E and SE of the summit, which are often mistaken for eruption columns (Miller et al., 1998). Eruptions producing pyroclastic flows have been dated at as recent as about 300 and 140 years ago, and elevated seismicity accompanying dike emplacement beneath the volcano was recorded in 1996.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA; NOAA/NESDIS Satellite Analysis Branch (SAB), Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA.

During September and the first half of October, seismicity remained above background and was indicative of continued low-level Strombolian eruptive activity. Gas-and-ash explosions occurred every 3-25 minutes, commonly generating ash-and-steam plumes 300-700 m high. However, the eruptive activity increased on 13 October. Volcanic bombs were ejected to 500 m above the crater; eruptive plumes from separate explosions rose to 3-5 km above Karymsky and extended >200 km NE and E. AVO analysis of satellite imagery confirmed a hot spot at the volcano.

Geologic Background. Karymsky, the most active volcano of Kamchatka's eastern volcanic zone, is a symmetrical stratovolcano constructed within a 5-km-wide caldera that formed during the early Holocene. The caldera cuts the south side of the Pleistocene Dvor volcano and is located outside the north margin of the large mid-Pleistocene Polovinka caldera, which contains the smaller Akademia Nauk and Odnoboky calderas. Most seismicity preceding Karymsky eruptions originated beneath Akademia Nauk caldera, located immediately south. The caldera enclosing Karymsky formed about 7600-7700 radiocarbon years ago; construction of the stratovolcano began about 2000 years later. The latest eruptive period began about 500 years ago, following a 2300-year quiescence. Much of the cone is mantled by lava flows less than 200 years old. Historical eruptions have been vulcanian or vulcanian-strombolian with moderate explosive activity and occasional lava flows from the summit crater.

Information Contacts: Tom Miller, Alaska Volcano Observatory; Vladimir Kirianov, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry.

During August and September, the eruption along the east rift zone continued without significant change and flows entered the ocean only at Lae`apuki in Hawaii Volcanoes National Park (figure 101). During the first ten days of August, the lava pond within Pu`u `O`o was sluggish and ~100 m below the lowest part of the rim. Glows from the pond reflecting off the fume cloud over the cone were often seen at night. After a short eruptive pause on 21 August, most of the lava was confined to tubes all the way to the sea, with only a few small surface flows from breakouts. Shortly after midnight on 29 August, a large collapse removed two-thirds of the active lava bench at Lae`apuki. During the early morning of 19 September, a large block of the Lae`apuki bench slid into the ocean. Sufficient energy was transferred to the ground for the HVO seismic network to detect the event, which lasted for eight minutes.

Figure 101. Map of recent lava flows from Kilauea's east rift zone, June-September 1996. Contours are in feet. Courtesy of the Hawaiian Volcano Observatory, USGS.

The lava flow field from this eruption that began in 1983 covers 23,475 acres, and ~820 acres of the flow field have been resurfaced by new lava since the beginning of June, when the eruption restarted after a five-day pause (BGVN 21:05). A total of 540 acres of new land has been added to the island since lava began entering the ocean in late 1986. As has been the case with other long-lived ocean entries, bench collapses at Lae`apuki have increased in frequency and are occurring about every two weeks. After each collapse, a severed lava tube or incandescent fault scarp is exposed and violent explosions follow. Types of explosive events observed at Lae`apuki after mid-August included sudden rock blasts, sustained and powerful steam jets, lava fountains, and "bubble-bursts" from holes in the tube above the entry.

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

Information Contacts: Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawaii National Park, HI 96718, USA (URL: http://www.soest.hawaii.edu/hvo/).

Seismicity was at or a little above normal background levels in late July and August. Historical activity at Koryaksky has been largely fumarolic, although a weak explosive eruption took place in 1956-57 from the summit crater and a radial fissure on the upper NW flank.

Geologic Background. The large symmetrical Koryaksky stratovolcano is the most prominent landmark of the NW-trending Avachinskaya volcano group, which towers above Kamchatka's largest city, Petropavlovsk. Erosion has produced a ribbed surface on the eastern flanks of the 3430-m-high volcano; the youngest lava flows are found on the upper W flank and below SE-flank cinder cones. Extensive Holocene lava fields on the western flank were primarily fed by summit vents; those on the SW flank originated from flank vents. Lahars associated with a period of lava effusion from south- and SW-flank fissure vents about 3900-3500 years ago reached Avacha Bay. Only a few moderate explosive eruptions have occurred during historical time, but no strong explosive eruptions have been documented during the Holocene. Koryaksky's first historical eruption, in 1895, also produced a lava flow.

Information Contacts: Tom Miller, Alaska Volcano Observatory (AVO), 4200 University Drive, Anchorage, AK 99508-4667, USA; Vladimir Kirianov, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia.

At about 1140 on 29 September, a Qantas Airlines pilot reported a thick plume near Krakatau that rose to an altitude of 3,700 m and drifted NW at low levels and E at high levels. There was no definite signature on GMS satellite images.

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: Bureau of Meteorology, Northern Territory Regional Office, P.O. Box 735, Darwin NT 0801, Australia; NOAA/NESDIS Satellite Analysis Branch (SAB), Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA.

Crater 3 remained quiet during September. Moderate Vulcanian activity at Crater 2 continued until 14 September; after then the activity declined to weak emissions of thin, white vapor. Emissions from Crater 2 produced thin white to thick gray vapor-and-ash clouds, which rose to a few hundred meters above the crater rim. Ash-laden emissions were commonly accompanied by low rumbling sounds. On 4-6, 10, and 13-14 September, strong explosions resulted in light ashfall on populated areas to the NW. Weak, steady crater glows were observed on most nights before 14 September. The Langila seismographs were inoperative during September.

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

Information Contacts: Chris McKee and Ben Talai, RVO.

The following report summarizes morphological changes in the summit crater seen during visits on 16 July, 17 August, and 24 September (figures 42-46). The crater was estimated to be ~400 m in diameter. Emissions of carbonatitic lava have been observed on many visits since July 1995 (BGVN 20:10, 20:11/12, 21:04, and 21:06).

On 16 July Celia Nyamweru and Mark Alvin reported that cone T39 was bubbling and splashing clots of molten lava every 30-60 seconds. The largest splashes reached 1-2 m above the vent. There was a recently formed pahoehoe flow ~50 m long and 2-3 m wide coming from the E side of cone T37. The continuous noise of gas escaping at high pressure was heard from a new vent, T38, between T5T9 and T20. Another new vent, T40, had formed by the N wall of the crater; it had produced a pahoehoe flow that covered a large portion of N and NE crater floor. At the time of the visit the sound of bubbling lava was coming from within this vent. Considerable volumes of steam were escaping from a longitudinal crack trending NW-SE on the W part of the crater floor, and sulfur fumes were escaping from a deep open crack on the E rim.

Figure 42. Sketch of the Ol Doinyo Lengai crater looking W from the E rim, 16 July 1996. Courtesy of C. Nyamweru.

Figure 43. Sketch of the Ol Doinyo Lengai crater looking NNW from the SE Rim, 16 July 1996. Courtesy of C. Nyamweru, from a photo by B.A. Gadiye.

T24 was partially filled with lava from T37S; there was some sulfur staining and steaming emissions on it. T5T9 was also emitting small amounts of steam (figure 44). T37S, now a broad cone with several peaks, was taller than T5T9. It had emitted several pahoehoe flows toward E and between T5T9 and the crater wall, totally covering F35. T37N showed an open pit below an overhanging wall, and T36 had a spine recently formed on its top. T20 appeared white-to-pale brown, with a rounded top and some steam emission. Near its base T35 had almost completely crumbled and collapsed. A small open circular vent (not numbered) at the base of the E wall had covered some of the vegetation on the crater wall with spatters of lava. It was surrounded by an overhang with small lava stalactites. Slight warmth was perceived from the vent but the lava stalactites were white. T15, T8, and T14, buried under recent flows from T40 and T37S, were no longer visible. The crater walls had several vertical cracks on the NW side, the lowest wall, facing E, was ~8 m high.

Figure 44. Photo of T5T9 in the Ol Doinyo Lengai crater, 16 July 1996. The estimated height of the cone is ~10 m. Estimated crater diameter (left to right) is ~400 m. Courtesy of C. Nyamweru.

Christoph Weber reported that on 17 August the crater floor had been covered with new black aa and pahoehoe lava flows. Weber had met another traveler, however, who had observed no eruptice activity about 14 days earlier. When Weber visited, he estimated the thickness of the fresh flows as typically ~20-30 cm. Fresh flows were easy to distinguish because they change from black to grayish white as they cool. They were often stacked, particularly on flow field F37, the one most active at that time, forming a composite of new flow material perhaps a meter thick overall. The area covered by these new flows was ~30,000 m². Thus, in the first half of August, the volume of erupted lava was on the order of 30,000 m³. Because of the rough irregular surfaces on some flows, their contacts with successive flows often contained considerable void space. Many of the flows were tube-fed, the tubes typically being 10- to 150-m long. When Weber left on 17 August lavas still poured out. He also observed a lava fountain ~3-m-high on T37. On 24 September some students from St. Lawrence University observed continuous bubbling and spattering of lavas from several vents.

Figure 45. Sketch of the Ol Doinyo Lengai crater looking S from the N rim, 17 August 1996. Courtesy of Christoph Weber, revised by C. Nyamweru.

Figure 46. Sketch map of the Ol Doinyo Lengai crater, 17 August 1996. Courtesy of Christoph Weber, revised by C. Nyamweru.

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: Celia Nyamweru, Department of Anthropology, St. Lawrence University, Canton, NY 13617 USA; Christoph Weber, Kruppstrasse 171, 42113 Wuppertal, Germany.

The onset of an intense earthquake swarm, which began in mid-July, prompted a rapid-response cruise and submersible dives during early August (BGVN 21:07). Scientists from the University of Hawaii once again used the research vessel Ka'imikai O Kanaloa (R/V KOK) and PISCES V manned submersible to carry out two follow-up research cruises over Loihi during 26-28 September and 2-10 October, respectively. The following summarized observations are from reports of the Hawaii Center for Volcanology.

Observations on 26-28 September. During 26 September the divers found hydrothermal venting on the bottom of the newly formed Pele's Pit (figure 9). In the summit area N of East Pit, no volcanic activity was observed, but a number of broken-up pillows were discovered. There was no activity at West Pit, however, the divers saw columnar basalt that appeared to be teetering due to collisions from debris slides. Some noise was heard with sonobuoys the next day. In East Pit on 27 September, divers saw a mudslide but no venting. Visibility was poor due to particles coming from Pele's Pit via a channel between the two pits. In Pele's Pit, active venting was observed on the upper W wall below Pele's Lookout. The divers encountered vents early during the dive on 28 September. The dive was aborted after the submersible brushed an unseen wall and damaged a thruster.

Figure 9. Sketch map of the Loihi Seamount. View is from the SSE. After Carlowicz (1996); original image by J.R. Smith, Jr., University of Hawaii.

Observations on 2-3 October. The dive on 2 October began in the "sand channel" between the pre-existing East Pit and the new Pele's Pit. The bottom of the channel was covered with a thick layer of fine-grained sediments. A miniature temperature recorder (MTR) was deployed, and a maximum vent-fluid temperature of >18°C was measured. At the W end of the vent field at Pele's Pit (1,175-m depth), numerous vents were seen; most were covered with white, streaming mats. This area, dubbed the rubble zone, extended perhaps 50-60 m in diameter, and was marked with several locations of recent slides and a few relatively stable benches. At night a tow-yo survey of nearly 18-km length was run up the W side of the main N-S axis of the seamount. A nephelometer detected a large number of plumes over the N half of the survey concentrated at ~1,350 and 1,050 m depth beside a large summit plume at a depth of ~1,150 m.

Vents were found the next day with a maximum vent-fluid temperature of 77°C, a much higher temperature than any previously measured at Loihi. A hydrocast into Pele's Pit showed that water-temperature anomalies had greatly decreased after the rapid-response cruise in August (a few tenths of a degree vs. three degrees). However, a distinct turbidity maximum remained in the bottom waters.

Observations on 4-6 October. A submersible dive up the S rift was conducted to investigate the origin of a hydrothermal plume at 1,350-m depth detected on 2 October. A new hydrothermal vent field was found on the rift axis at 1,325-m depth, and was named "Naha Vents". This extensive vent field contained many fresh fractures, including a fissure (1-3 m wide) that vented large volumes of water. A smaller vent had a measured temperature of 11.2°C. The dive concluded farther up the rift at the site of the previously active Kapo's Vents (1,250-m depth); no hydrothermal activity was observed there. At night a ship-based water sampling program included a ~13 km long SW-NE tow-yo survey across the summit (the tow was run parallel to the predominantly NE current). A hydrothermal plume was first detected 6.5 km downcurrent from the summit.

Observations on 5 October showed that the Naha vent field was ~20 x 30 m, and was heavily covered with nontronite deposits and tan bacterial mats. The field contained many small vents, as well as diffuse flows through fractured pillows and large fissures. The highest vent-fluid temperature was 22.7°C. Night water sampling (vertical hydrocast) 1.4 km downcurrent (NE) from the summit revealed six major turbidity maxima at depths of 1,050-1,330 m. The strongest signal, at 1,080 m, was associated with a significant temperature anomaly. This suggested that there might be an undiscovered major source of venting at the summit (all of the vents discovered thus far are below 1,180 m).

Water sampling the night of 6 October better located the sources of the large shallow (1,000-1,105 m depth) turbidity and temperature-anomaly maxima observed on 5 October. Hydrocasts and tow-yos across the seamount suggested that a major venting site should be just S of Pele's Pit near the top of the S rift.

Observations on 7-10 October. An MTR showed a slow increase in temperature from 48 to 53°C over its deployment during 4-7 October, with some daily variations. The dive on 7 October explored a site covered with nontronite-coated gravels where diffuse venting was observed at a depth of 1,099 m. This field was likely an early stage of the "finger vent"-type hydrothermal fields seen previously on Loihi, and was named "Ula Vents". The dive concluded on the steep W flank of the summit at a site of previously observed intermittent venting (Maximilian Vents) at 1,249-m depth. A night water sampling program ran two perpendicular 5-km-long tow-yo sections near the summit. In the both runs, plume maxima were in the vicinity of Kapo's Vents. A hydrocast at West Pit indicated a substantial particle plume above the pit with no associated temperature anomaly.

The 8 October dive began just W of the site of Kapo's Vents, a small field that was active in the late 1980s. As on the section of the S rift already explored, large volumes of clay- to gravel-sized sediments covered much of the area. Pele's hair and flat sheets of glass that formed as walls of large lava bubbles were common. One interesting feature was ~5-cm-diameter holes at several sites in the sand layer that appeared to be locations of recently terminated venting. An area of modest venting through a mound of small nontronite-covered boulders was found at a depth of 1,196 m. A maximum vent-fluid temperature of 17.2°C was measured. At night a S-to-N tow 3 km W of the seamount axis showed that the bulk of the hydrothermal plume above Loihi had shifted from the WSW to the NE over the previous few days.

Dive operations the next day focused on completing work at Lohiau Vents. The dive finished at the E end of the vent field and collected rocks bearing several high-temperature sulfide minerals; these suggested that vent-fluid temperatures during the July-August seismic event might have been much higher. The hydrothermal site sampled on 8 October at a depth of 1,196 m on the S rift was confirmed to be a new field. It was named "Pohaku Vents".

On 10 October, a repeat of the tow-yo section made on 8 October revealed that the plume had shifted to nearly due N. This shift during only a few days indicated the speed at which the ocean currents carrying the Loihi plumes could change their orientation. During the whole cruise, 71 km of tow-yos were conducted, making Loihi one of the most intensively studied submarine hydrothermal systems.

Reference. Carlowicz, M., 1996, Earthquake swarm heats up Loihi: EOS, v. 77, no. 42, p. 405-406.

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: Hawaii Center for Volcanology, Department of Geology & Geophysics, University of Hawaii at Manoa, 2525 Correa Road, Honolulu, HI 96822 USA (URL: http://www.soest.hawaii.edu/GG/hcv.html); Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawaii National Park, HI 96718, USA (URL: http://www.soest.hawaii.edu/hvo/).

An overflight on 21 and 22 July allowed observation of the summit for a few minutes. Activity at the two summit craters consisted of fumarolic emissions from the S interior wall of the principal crater, which is also the highest. A few yellow sulfur deposits carpet the interior walls of the cone, principally on the S and SW.

Geologic Background. The small 7-km-wide conical island of Lopevi, known locally as Vanei Vollohulu, is one of Vanuatu's most active volcanoes. A small summit crater containing a cinder cone is breached to the NW and tops an older cone that is rimmed by the remnant of a larger crater. The basaltic-to-andesitic volcano has been active during historical time at both summit and flank vents, primarily along a NW-SE-trending fissure that cuts across the island, producing moderate explosive eruptions and lava flows that reached the coast. Historical eruptions at the 1413-m-high volcano date back to the mid-19th century. The island was evacuated following major eruptions in 1939 and 1960. The latter eruption, from a NW-flank fissure vent, produced a pyroclastic flow that swept to the sea and a lava flow that formed a new peninsula on the western coast.

Information Contacts: Henry Gaudru, C. Pittet, C. Bopp, and G. Borel, Société Volcanologique Européenne, C.P. 1, 1211 Genève 17, Switzerland (URL: http://www.sveurop.org/); Michel Lardy, Centre ORSTOM, B.P. 76, Port Vila, Vanuatu.

During the night of 27 September, a lahar triggered by unusually heavy rainfalls occurred on the E flank of Maderas and destroyed the village of El Corozal (~3 km from the volcano) and other settlements.

Five children and an adult were killed, and several more people injured. The full extent of the damage became evident only after a few days: rocks, mud, and water had destroyed 36 houses and heavily damaged crops; some areas were covered with 2 m of mud and water. About 250 people were affected by the lahar and evacuated to a local school.

Two policemen, who climbed the volcano two days after the lahar, observed a small crater at the starting point of the lahar. They presumed that a minor volcanic explosion could have triggered the event, but this has not been confirmed by Nicaraguan volcanologists. A local farmer reported a strange thunder sound minutes before the lahar came down.

Geologic Background. Volcán Maderas is a roughly conical stratovolcano that forms the SE end of the dumbbell-shaped Ometepe island in Lake Nicaragua. The basaltic-to-trachydacitic edifice is cut by numerous faults and grabens, the largest of which is a NW-SE-oriented graben that cuts the summit and has at least 140 m of vertical displacement. The small Laguna de Maderas lake occupies the bottom of the 800-m-wide summit crater, which is located at the western side of the central graben. The SW side of the edifice has been affected by large-scale slumping. Several pyroclastic cones, some of which may have originated from littoral explosions produced by lava flow entry into Lake Nicaragua, are situated on the lower NE flank down to the level of Lake Nicaragua. The latest period of major growth was considered to have taken place more than 3000 years ago, but later detailed mapping has shown that the most recent dated eruptive activity took place about 70,000 years ago and that it has likely been inactive for tens of thousands of years (Kapelanczyk et al., 2012). A lahar in September 1996 killed six people in an E-flank village, but associated volcanic activity was not confirmed.

Information Contacts: Wilfried Strauch, Instituto Nicaraguense de Estudios Territoriales (INETER), Dept. of Geophysics, Managua, Nicaragua.

During early September, both Main and South Craters emitted weak to moderate white vapor. Main Crater started to produce occasional puffs of gray vapor and ash on 13 September, and became more forceful and frequent (at a-few-minute intervals) the next day. This increased eruptive activity during mid-September resulted in very light ashfall over villages and garden areas on the NW side of the island. This is the first time that Main Crater has been active since mid-December 1992. The activity began to decline on 20 September. Occasional roaring or rumbling sounds were heard, but neither glow nor incandescent projection was seen at night. By 26 September emissions were weak and took place every 30 minutes.

During 16-29 September, activity at South Crater also slightly increased with occasional blue and gray emissions. Mild Vulcanian explosions took place every 5-10 minutes on 22-27 September. However, neither night glow nor incandescent projection was observed over the crater.

There was no seismic monitoring at Manam during September. Measurements from the water-tube tiltmeters at Tabele Observatory (4 km SW of the summit) have shown no tilt change since April 1996.

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: Chris McKee and Ben Talai, RVO.

Pacaya erupted more forcefully than usual beginning late on 10 October. Based on an INSIVUMEH report, between about 2300 on 10 October and 0200 on 11 October Pacaya produced a moderate Strombolian eruption with sustained fountaining of incandescent materials up to 500 m high.

The plume's maximum height reached ~3.7 km altitude; within that plume the ash column rose to ~700 m. During the eruption winds blew from the NNE at 35 km/hour with gusts to 45 km/hour; they carried fine ash toward the town of Esquintla. A report from Puerto San Jose, a city on the Pacific coast ~60 km SW, indicated that the earlier dark ash cloud had thinned during the day.

The explosive eruption was followed by significant lava effusion from the crater. The longest lava flow traveled SW for 1.5 km over the surface of an older flow field. At 0300 the flow front's velocity was 100 m/hour; it came within 300 m of the relatively flat area reached by the 1991 lava flow. Lava ceased venting at dawn; however, the SW flow remained incandescent and slowly moving. Although eruptive strength diminished, some tremor persisted on 11 October. On that day satellite images (Band 2 on GOES-8) showed a small hot spot. An INSIVUMEH report on 14 October noted that ongoing eruptions continued into the morning of the 12th. After that the eruptive vigor and amount of tremor both dropped and no new lava vented from the crater.

On 16 October INSIVUMEH reported that Pacaya continued to expel abundant white steam. At that time there were no audible explosions, underground booming noises, or newly vented lava flows. Tremor was present, presumably related to the degassing seen at the surface. Eddy Sanchez noted that 38 people were evacuated from neighboring villages during the height of the eruption.

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

Information Contacts: Eddy Sanchez and Otoniel Matías, Seccion Vulcanologia, INSIVUMEH (Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia of the Ministerio de Communicaciones, Transporte y Obras Publicas), 7A Avenida 14-57, Zona 13, Guatemala City, Guatemala; NOAA/NESDIS Satellite Analysis Branch, Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA.

Residents of the Alaska Peninsula observed small glowing plumes from Pavlof on 15 September. During the next week, seismicity was vigorous and eruptions were intermittent (BGVN 21:08). At 1328 on 24 September seismicity began to increase, suggesting stronger eruptive activity. This increased level of seismicity persisted through the first half of October. Visual observations and satellite imagery verified that increased seismicity correlated with eruptions of ash and bombs up to 1,200 m above the summit.

On 26 September satellite imagery showed a small steam-and-ash plume extending ~45 km SE. A pilot subsequently reported a steam plume to an estimated altitude of 3,700 m. AVO staff doing airborne observations during 27-30 September reported low-level fountaining and occasional small explosions of incandescent material in the summit crater. The small explosions produced sporadic steam-and-ash plumes to 610 m above the vent. The largest plume drifted S for ~110 km and appeared faintly on satellite images. Incandescent spatter was deposited on the NW summit slope or moved down a deep gully on the NW side of the volcano.

During 4-11 October lava fountaining from two vents continued to heights of a few hundred meters above the summit. Incandescent spatter-fed lava flows moved down the steep, snow- and ice-covered slope, widening at the base and extending NW. Occasional water-rich slurries of ash and mud descended the N flank. Diffuse plumes of steam, gas, and ash rose to as high as 6 km above sea level and drifted 160 km downwind. On 15 October eruptive activity increased and seismicity reached the highest levels yet observed. Satellite imagery and pilot reports showed ongoing lava fountaining from two vents near the summit. Pilot reports indicated that diffuse ash layers reached 7,300-m altitude and extended perhaps as far as 50 km SE.

Geologic Background. The most active volcano of the Aleutian arc, Pavlof is a 2519-m-high Holocene stratovolcano that was constructed along a line of vents extending NE from the Emmons Lake caldera. Pavlof and its twin volcano to the NE, 2142-m-high Pavlof Sister, form a dramatic pair of symmetrical, glacier-covered stratovolcanoes that tower above Pavlof and Volcano bays. A third cone, Little Pavlof, is a smaller volcano on the SW flank of Pavlof volcano, near the rim of Emmons Lake caldera. Unlike Pavlof Sister, Pavlof has been frequently active in historical time, typically producing Strombolian to Vulcanian explosive eruptions from the summit vents and occasional lava flows. The active vents lie near the summit on the north and east sides. The largest historical eruption took place in 1911, at the end of a 5-year-long eruptive episode, when a fissure opened on the N flank, ejecting large blocks and issuing lava flows.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

Mild eruptions continued at Tavurvur during September. Weak, white to pale-gray vapor-and-ash emissions took place at short irregular intervals, and plumes rose ~1,000 m above the crater. These emissions were occasionally accompanied by roaring sounds. On 2, 7, and 9-12 September, strong explosions sent ash clouds up to 4 km above the crater, resulting in light ashfall on Matupit Island and Rabaul town.

After the explosions on 26 August (BGVN 21:08), the release of SO₂ was at a low level of ~200 metric tons/day (t/d). However, the flux rate gradually increased and reached ~1,500 t/d on the night of the 11 September explosions. Seismicity showed variations similar to the SO₂ flux. The background seismicity level was 5-20 low-frequency events/hour and 30-100 RSAM (Real-time Seismic Amplitude Measurement) units. From 8 to 10 September, seismicity increased to ~40 low-frequency events/hour and 100-200 RSAM units. After the eruption on 11 September, seismicity returned to a normal level (3-15 events/hour and 25-100 RSAM units). Ground deformation was not evident around the mid-September eruptions.

After 18 September, seismic activity increased to medium levels (30-40 events/hour and 50-150 RSAM units). Likewise, the flux rates of SO₂ changed from 200-400 t/d to 1,000-1,500 t/d by the end of September. Beginning on 22 September, tiltmeters recorded deflation of the central caldera reservoir at a rate of up to 1 µrad/day. Following these anomalies, strong eruptions took place in early October, sending ash clouds to an altitude of 5.5 km.

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

Information Contacts: C. McKee and B. Talai, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea; NOAA/NESDIS Satellite Analysis Branch (SAB), Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA.

During May-July, seismic activity at Ruiz remained quite low. Significant volcano-tectonic earthquake swarms occurred on 8, 10, 11, 16, and 23 May, and 7, 15, and 18 June (figure 48). Most were located at depths of <7 km and within 3 km of Arenas Crater. The strongest volcano-tectonic earthquake (M 2.2) was recorded at 1636 on 10 May. Swarms of long-period events were registered on 9, 20, 23, and 25 May. Scientists working in the field reported that an isolated long-period event at 1153 on 29 May was correlated with an explosion-like sound possibly caused by the fall of solid material. The analog recorders detected this event, but the digital systems did not.

Figure 48. Released energy and number of volcano-tectonic and long-period events at Ruiz during May-July 1996. Scales are approximate. Courtesy of INGEOMINAS.

Visual monitoring indicated that normal white gas plumes occurred over the Ruiz summit and reached an altitude of <2 km. The FARALLONES electronic tiltmeter did not record any significant deformations during May-July.

A new fumarolic field and a hot spring, both called "El Calvario," were found 1.7 km NE of Arenas Crater at an elevation of 4,628 m. The fumarole had a temperature of 84°C and pH of 3.8. Emissions consisted of: H₂O vapor, 95.5%; CO₂, 4.3%; total S, 0.18%; and HCl, 0.001%. The water from the hot spring had the following features: temperature, 66.4°C; pH, 2.7; Cl, 10 ppm; and SO₄, 1,545 ppm.

Geologic Background. Nevado del Ruiz is a broad, glacier-covered volcano in central Colombia that covers more than 200 km2. Three major edifices, composed of andesitic and dacitic lavas and andesitic pyroclastics, have been constructed since the beginning of the Pleistocene. The modern cone consists of a broad cluster of lava domes built within the caldera of an older edifice. The 1-km-wide, 240-m-deep Arenas crater occupies the summit. The prominent La Olleta pyroclastic cone located on the SW flank may also have been active in historical time. Steep headwalls of massive landslides cut the flanks. Melting of its summit icecap during historical eruptions, which date back to the 16th century, has resulted in devastating lahars, including one in 1985 that was South America's deadliest eruption.

Information Contacts: John Jairo Sánchez, Alvaro Pablo Acevedo, Fernando Gil Cruz, John Makario Londoño, Jairo Patiño Cifuentes, Claudia Alfaro Valero, Hector Mora Páez, Cesar A. Carvajal, Luis Fernando Guarnizo, and Jair Ramirez, INGEOMINAS Observatorio Vulcanológico y Sismológico de Manizales (OVSM), A.A. 1296, Manizales, Caldas, Colombia.

The main crater (Caliente) of Santa María's active dome, Santiaguito, issued a 300-m-high explosion at 0631 on 14 October. Ash from the explosion blew E and small avalanches traveled down the E and S flanks. Brief explosions from the Caliente vent at Santiaguito were last reported in November 1993. However, it is likely that there has been near-continuous low-level activity since that time.

Geologic Background. Symmetrical, forest-covered Santa María volcano is part of a chain of large stratovolcanoes that rise above the Pacific coastal plain of Guatemala. The sharp-topped, conical profile is cut on the SW flank by a 1.5-km-wide crater. The oval-shaped crater extends from just below the summit to the lower flank, and was formed during a catastrophic eruption in 1902. The renowned Plinian eruption of 1902 that devastated much of SW Guatemala followed a long repose period after construction of the large basaltic-andesite stratovolcano. The massive dacitic Santiaguito lava-dome complex has been growing at the base of the 1902 crater since 1922. Compound dome growth at Santiaguito has occurred episodically from four vents, with activity progressing W towards the most recent, Caliente. Dome growth has been accompanied by almost continuous minor explosions, with periodic lava extrusion, larger explosions, pyroclastic flows, and lahars.

Information Contacts: Eddie Sánchez and Otoniel Matías, INSIVUMEH.

A pilot report from Qantas Airlines on 1 August noted an ash cloud at an altitude of 4,000 m. Animated visible and infrared GMS satellite data through 0832 on 2 August did not reveal any discernible ash plume.

Another Qantas pilot report indicated that Semeru erupted at 1625 and 1637 on 12 September with ash reaching 4,600-m altitude and drifting NW; no plume was seen on satellite imagery. At approximately 0640 the next day a localized plume was evident on satellite imagery drifting SSW to ~35 km away. Eruptive activity was again observed by Qantas pilots who reported at 1154 on 29 September thick black "smoke" at 6 km altitude. Another aircraft report at 2110 later that day indicated ash to 6 km moving N and NW. Satellite data showed local high cloud cover throughout the day, but no apparent ash plume.

On 6 October an eruption was reported by Qantas pilots at 1418. The dense plume was rising to ~4.6 km altitude with no significant drift.

Semeru is the highest and one of the most active volcanoes of Java. It lies at the S end of a volcanic massif extending N to the Tengger Caldera and has been in almost continuous eruption since 1967.

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

Information Contacts: Bureau of Meteorology, Northern Territory Regional Office, P.O. Box 735, Darwin, NT 0801, Australia; NOAA/NESDIS Satellite Analysis Branch (SAB), Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA; Tom Fox, International Civil Aviation Organization (ICAO), 999 University Street, Montreal, Quebec H3C 5H7, Canada.

The following condenses the weekly Scientific Reports of the Montserrat Volcano Observatory (MVO) and stated sources for the period 1 September-1 October.

Observations during 1-14 September. The early days of the month were characterized by several periods of intense rockfalls and pyroclastic flows from the E flank of the lava dome. The steepening of the dome's active flank caused a partial gravitational collapse on 2 and 3 September. The resulting pyroclastic flows were generally confined to the S part of the Tar River valley although they came from N of Castle Peak (figure 10). The pyroclastic flows caused significant erosion in the middle part of the valley and deposition in the lower part and at the mouth of the Tar River, on the pyroclastic-flow delta built up since late July. Excavation of a deep (>10 m) channel from the base of the new dome through the upper part of the talus fan confined the flows giving them greater run-out potential. The scar left on the E flank was soon refilled by continuous rockfall activity and new dome growth. Samples of the pyroclastic-flow deposits on the delta contained less vesicular material than other deposits since late July, and were typically ash-rich, very poorly sorted, and contained juvenile lava blocks to at least 50 cm diameter.

Figure 10. Map of Montserrat showing selected towns and features.

The pyroclastic flows of 2 and 3 September produced ash clouds that rose 6 km, but there was no evidence of vertical columns from the summit of the dome. The ash clouds deposited 1-2 cm of ash in the Cork Hill area, and >5 mm farther N in the Old Towne area. MVO estimated the volume of ash deposited on 2 and 3 September to be equivalent to a rock volume of 7 x 10⁴ m³. In addition to this description from MVO, a local newspaper, The Montserrat Reporter, said these events caused ash to fall on nearly every part of the island from St. Patrick's in the SW, to St. John's in the N, and from Plymouth in the W to Long Ground in the NE, including Bramble Airport. For the remainder of the period, rockfall and associated pyroclastic-flow activity was confined almost exclusively to the E flank. After the major ash falls of 2 and 3 September more moderate amounts were deposited W of the volcano.

Signals from rockfalls and pyroclastic flows dominated the seismic records during this observation period. Long-period and hybrid events remained at background levels and tremor was generally low. Volcano-tectonic earthquakes occurred exclusively in short swarms lasting 1-6 hours. The volcano-tectonic earthquakes were all located <2 km below sea level beneath the crater.

The passage of a hurricane caused several days of strong winds and heavy rain making visual observation of the dome difficult, and causing flash floods that deposited ~60 cm of sediment in Fort Ghaut's lower reaches.

Observations during 15-21 September. Several small pyroclastic flows occurred on 15 September, the largest reaching beyond the Tar River Soufriere. Ash clouds from rockfalls and flows were generally blown NW. Intense ash and steam venting during 1250-1320 on 15 September came from the highest part of the dome W of the active area.

Near-continuous rockfalls started late on the morning of 16 September and by mid-afternoon, numerous pyroclastic flows were being produced by gravitational collapse from the lava dome. Many of these pyroclastic flows reached the sea, extending considerably the depositional fan at the mouth of the Tar River valley. Continuous ash production from the flows fed into a convective column that reached heights of 2-3 km and deposited ash on areas W of the volcano. Activity slowed somewhat in the middle of the evening as pyroclastic flow generation stopped.

Activity restarted at 2342 on 17 September with a small explosive eruption. A laterally directed explosion projected ballistic clasts toward the E (over the Hermitage area and into Long Ground village) and an eruption column was briefly sustained. More than half of the houses in Long Ground were damaged by blocks falling through roofs, doors, and windows. Eight buildings, including the Pentecostal Church, were burnt in Long Ground, all from extremely hot rocks falling on them. The Tar River Estate House was partially demolished by a pyroclastic surge. Gravel-sized material of both pumiceous and dense nature was deposited at Cork Hill, Richmond Hill, and Fox's Bay from the eruption column. The Montserrat Reporter noted that many vehicles had lost their windscreens from "falling pebble rocks". On the other hand, MVO data suggested that the number of windscreen breakages was actually quite low and that ash loading contributed substantially to breakages. All ash erupted during the night was blown W over Plymouth and Richmond Hill and both of these areas received heavy ashfall.

In an electronic forum, Douglas Darby, an eyewitness, reported: "From Iles Bay, you could hear something coming from the direction of the volcano, at about [2345 on 17 September]. It sounded like a low roar, the first time ever in Iles Bay that you could hear any noise from the volcano. Immediately after, thunder and lightning began and it was obvious that this was not anything experienced before . . . And then the rain of stones began . . . Visually you could not really see much at that time but we thought we could see a low level of glowing all across the area where we know is Tar River, from the direction of the pyroclastic flows."

Reports from the NOAA Satellite Analysis Branch indicated that the ash column attained a height of at least 12 km and caused the closure of the airport in Guadeloupe on the morning of 18 September. Pilot and NOAA reports and personal communication with Tom Casadevall indicated that an Air Canada flight inadvertently entered the ash plume on 17 September. Dave Schneider of MTU collected and processed two AVHRR scenes of the ash plume from 18 September: at 0544 the plume was 175 km long E-W and 75 km wide N-S, at 1018 the cloud became very diffuse as it extended 550 km E and 85 km N-S (figure 11).

Figure 11. AVHRR images of the 18 September ash cloud from Soufriere Hills. Courtesy of Dave Schneider, MTU.

A major collapse scar cut deeply into the new dome's E flank. Some material was eroded from Castle Peak and a large volume was deposited in the Tar River Valley. The delta at the mouth of the Tar River Valley was enlarged and the vegetation was completely destroyed. MVO estimates stated that perhaps 25-30% of the new dome was removed.

Several small rockfalls from the inner steep-sided walls of the scar, particularly on the N and NW, generated small ash clouds and deposited new debris at the base of the valley. On 19 September field workers found pumice clasts of up to 95 g at 3 km and clasts up to 3.5 g at 6 km. On 22 September a sampling expedition to the Tar River area obtained a temperature of 373°C at a depth of 45 cm in the pyroclastic-flow deposits close to the Tar River Estate House.

Seismicity during this period was characterized by brief swarms of volcano-tectonic earthquakes from a shallow source. These swarms occurred immediately before the most intense rockfalls and increased in frequency and duration preceding the 17-18 September explosion. After 18 September the frequency of volcano-tectonic earthquakes decreased from 2-3 swarms/day to single isolated events at the end of the observation period. Long-period and hybrid events remained low, averaging <11 events/day; low-amplitude tremor was recorded on the Gages seismometer.

Observations during 24-30 September. Activity kept decreasing in intensity during the last part of the month. On 24 September visual observations of the scar's interior showed no signs of new material apart from debris derived from rockfalls off the side walls. Abundant steaming and sulfur deposits were observed at the base of the scar. Rockfalls were very small, mainly concentrated within the scar and associated with continued stabilization of the inner walls of the scar. The lack of large rockfalls suggests that any new dome growth was limited to the interior of the dome, probably at the base of the scar feature caused by the 17 September explosion. On 26 September some red-hot rock and high-temperature gases were seen in the bottom of the scar, suggesting that fresh magma was getting close to the surface again; however material falling from the scar walls covered any new dome growth. Light ashfall, possibly associated with small rockfalls into the scar, was observed by a field team near Chances Peak on 28 September.

On 30 September some areas to the SW and along the base of the scar showed light swelling. This may be due to new dome growth beneath the blocky deposits that line the base of the scar. The N part of the scar had a vertical cliff face with a nearly horizontal, bowl-shaped base, grading downward and outward to the Tar River Valley. Several unstable blocks were observed on the top inner parts of the NE sides of the scar.

Small rockfalls were the most dominant type of seismic signal recorded during this period, but hybrid and volcano-tectonic activity became more prominent during the latter part of the week. Volcano-tectonic earthquakes reappeared from 26 September onwards. They were transitional to hybrid events with a short high-frequency onset and low-frequency coda. The levels of long-period and hybrid events remained comparatively low throughout this period, averaging <11 events/day. Hybrid activity increased somewhat during the latter part of the week in tandem with the increase in volcano-tectonic activity. Tremor levels were high during the earlier parts of the week due to heavy rains. In Fort Ghaut, mudflows resulted from remobilization of thick ash deposits from the 17-18 September explosion.

EDM measurements. Measurements taken on 11 September from White's Yard to Castle Peak showed a 1 cm/day shortening trend, slightly higher than the trend established since mid-July. The Galway's to Chances Peak line was measured on 13 September, but it continued to show inconsistent changes, although shortening was predominant.

On 16 September a shortening of 2.8 cm on the St. George Hill-Farrell's line (N triangle) was measured since 22 August, whereas the two other lines in this triangle -- Windy Hill-Farrell's and St. George's Hill-Windy Hill -- did not change. Between 16 and 21 September the lines St. George's Hill-Farrell's and Windy Hill-Farrell's lengthened by 4 and 9 mm, respectively. These changes, however, are not considered to be related to the 17-18 September explosion. On 25 September the N triangle showed shortening on the St. George Hill-Farrell's and Windy Hill-Farrell's lines of 4 and 11 mm, respectively. Although little consistency is found in the changes of this triangle, a slight overall trend of shortening is observed.

Line lengths between Lower-Upper Amersham and Lower Amersham-Chances Peak showed changes of +48 mm and -1 mm, respectively, during 20-26 September. On 30 September the Galloways-Chances Peak line was found to have lengthened 13 mm during the previous 16 days.

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

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.mvo.ms/); NOAA/NESDIS Satellite Analysis Branch (SAB), Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA; Bennette Roach, The Montserrat Reporter, v. XII nos. 33 and 35, Tom Casadevall, U.S. Geological Survey, Menlo Park, CA 90210 USA; Dave Schneider, Michigan Technological University, Houghton MI 49931, USA; Doug Darby, 6 Satinwood Road, Rocky Point, NY 11778 USA.

Above-background seismicity started on 7 September (BGVN 21:08); a follow-up report indicated that Villarrica's microseismicity again increased starting on 26 September and was continuing as late as 3 October. The events seen were of short-duration with dominant frequencies of 1.75 Hz and they appeared in swarms (figure 6). Some isolated events occurred in the 0.7-1 Hz range. In this same time interval the crater was the scene of abundant to occasionally intense degassing.

Figure 6. One of Villarrica's ongoing swarms of long-period seismic events (station VVN), 0900 to 0927 (GMT) on 26 September 1996. Reference marks are at one minute intervals. Courtesy of Gustavo Fuentealba and Paola Peña.

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

Information Contacts: Gustavo Fuentealba C.¹ and Paola Peña, Programa Riesgo Volcánico de Chile (PRV), OVDAS; 1-also at Depto. Ciencias Fisicas, Universidad de La Frontera, Temuco, Chile.

Observations in April, May, and July indicated continued increases in heat flow and inflation of the Main Crater floor. Low-level volcanic tremor that began in late July continued through August. Since the tremor commenced it appears that heat-flow has decreased, as has the deformation. Measurements in late August indicated that the crater-wide deformation and heating of the last 2-3 years appears to have peaked without eruptive activity. Since the last report (BGVN 21:04), monitoring visits were made on 18 April, 16 May, 24 July, and 28 August 1996.

Crater observations. On 18 April, the lake occupied Royce, Wade, Princess, and TV1 craters, with the S part of the divide between Princess and Wade craters 2-3 m above the lake. The lake was light turquoise, with a few brown surface slicks. A fumarole in the N wall of Wade Crater was audible from the edge of the 1978/90 Crater Complex; it was the only significant steam source in the complex.

Donald Mound was steaming vigorously, with that part exposed in the wall of the 1978/90 Crater Complex and the SE slopes the dominant features. Sulfur deposits were obvious on Donald Mound and the 1978/90 wall. The area of mud pots at the base of Donald Mound was also steaming vigorously. The whole area was wet and some mud pots included areas of significant sulfur deposition. Collapse was actively occurring between the 1978/90 Crater Complex and Donald Duck, causing brown slicks on the lake surface.

An ejecta apron with material up to 12 m from the vent was observed by charter pilot J. Tait on 4 June. Calm and clear conditions on 9 June allowed a tall steam plume to develop above the island; it was mistaken as an eruption plume by several coastal observers and the media. However, pilots R. Fleming and J. Tait, on the island at the time, observed no unusual activity. On 11 June R. Fleming reported a dramatic rise in lake level (>5 m) in three weeks. Strong convection in the lake caused fountaining up to 3-4 m high in the embayment below the May '91 vent.

Fumarolic discharge continued to increase on the crater floor when measured on 28 August, although temperatures had moderated somewhat since May. Springs, consisting largely of steam condensate, continued to discharge, and two new such features had developed along the boundary between the E and central sub-craters. Maximum temperatures on Donald Mound were 311°C, down ~100°C from May. A large fumarole discharging a bright yellow, sulfur-laden plume had developed ~5 m below the inner crater rim that intersects Donald Mound. The crater lake was mostly obscured by steam, but it appeared gray in color; maximum temperature as recorded by pyrometer was 69°C.

Magnetic survey. A comprehensive survey of the magnetic network was conducted on 16 May with the exception of a few sites at Donald Mound that were inaccessible due to hydrothermal activity. Contouring the changes since the partial survey on 23 January 1996 showed that the decreases at Donald Mound with corresponding increases to the S were continuing. These results suggested continued shallow (50-100 m deep) heating. A weaker negative anomaly W of Noisy Nellie, presumably resulting from heating on the N side of the complex, continued the trend observed during 6 July-12 December 1995.

A positive anomaly E of Donald Mound (site D10b) showed a change of +518 nT, although the site is near a new mud hole, so the effect may be local. Positive changes at Site G (+126 nT) and nearby sites are unusual because decreases are usually recorded when there is heating at Donald Mound. This anomaly may suggest cooling, perhaps around 100-200 m deep, at the E edge of the area of hydrothermal activity, possibly related to the rising water table.

Deformation. Levelling surveys on 18 April and 16 May were conducted over the entire network except over Donald Mound due to intense steam and hot, soft ground. Both surveys revealed broadly similar patterns and rates of continuing uplift centered on Donald Mound and extending SE. Relative subsidence continued NW of Donald Duck Crater, although part of that may be due to slumping induced by encroachment from the 1978/90 Crater Complex. The inflation pattern during the previous five months remained similar to that since Donald Mound began rising in late 1993.

A partial levelling survey was done on 28 August; three pegs near Donald Mound could not be accessed, two were lost due to crater wall collapses, and one was buried under a landslide. Since about 1992-93, levelling surveys have shown a systematic crater-wide uplift. However, this survey showed a dramatic reversal of the uplift trend, with minor subsidence observed over much of the Main Crater floor. The larger subsidences were focused about the Donald Mound area and the margins of the 1978/90 Crater Complex. These changes are consistent with the thermal changes observed on 28 August and may indicate that the present inflationary-heating episode is over or declining.

Geologic Background. The uninhabited Whakaari/White Island is the 2 x 2.4 km emergent summit of a 16 x 18 km submarine volcano in the Bay of Plenty about 50 km offshore of North Island. The island consists of two overlapping andesitic-to-dacitic stratovolcanoes. The SE side of the crater is open at sea level, with the recent activity centered about 1 km from the shore close to the rear crater wall. Volckner Rocks, sea stacks that are remnants of a lava dome, lie 5 km NW. Descriptions of volcanism since 1826 have included intermittent moderate phreatic, phreatomagmatic, and Strombolian eruptions; activity there also forms a prominent part of Maori legends. The formation of many new vents during the 19th and 20th centuries caused rapid changes in crater floor topography. Collapse of the crater wall in 1914 produced a debris avalanche that buried buildings and workers at a sulfur-mining project. Explosive activity in December 2019 took place while tourists were present, resulting in many fatalities. The official government name Whakaari/White Island is a combination of the full Maori name of Te Puia o Whakaari ("The Dramatic Volcano") and White Island (referencing the constant steam plume) given by Captain James Cook in 1769.

Information Contacts: B.J. Scott, Institute of Geological and Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand.

Although very intense activity was recorded during 1994, volcanism decreased in 1995 and was at normal levels (explosions, lava fountaining, and ash emissions) in November 1995. After a period of significant increase in the number and intensity of explosions during June 1996, activity returned to a quieter, but sustained, level (figure 6).

Figure 6. Seismicity at Yasur recorded every 4 hours by the seismometer 2 km from the summit, 24 May-23 July 1996. The upper line shows all events with a seismograph displacement greater than 12 µm. The vertical bars on the bottom of the graph indicate the number of larger events, those with a displacement greater than 60 µm. Thick lines are an 8-measurement (32-hour) running mean. Note that the scale is logarithmic. Courtesy of ORSTOM.

Observations made during 3-5 July showed that explosive Strombolian activity was fairly significant. Heavy ash-and-steam plumes, visible from surrounding villages, frequently rose several hundreds of meters above the volcano, accompanied by loud rumbling/roaring noises. The summit crater is ~250 m deep, and is occupied by three smaller active craters (figure 7). During observation the explosive activity and intense degassing came from six vents (one in Crater A; three in Crater B; two in Crater C).

Figure 7. Sketch map showing the summit craters at Yasur, 3-5 July 1996. Observation points are indicated by an "X". Courtesy of Henry Gaudru, SVE.

Crater A was a pit with a S vertical wall ~100 m high. On the morning of 3 July between 1130 and 1330 the activity was principally characterized by frequent and intermittent explosions that generated ejections of magma fragments to several dozens of meters above the vent, sometimes surpassing the upper rim of the crater. A steam-and-ash plume regularly followed the explosive activity.

Crater B, smaller than A and separated from it by a small wall, had more sustained explosive activity from several vents, of which two (B1-B2) were particularly active with strong degassing. Bombs were regularly ejected >300 m vertically, often surpassing the highest point on the crater rim. The most active vent (B1) showed activity phases of continuous, very violent jets that lasted between 1 and 5 minutes, notably between 1930 and 2230 on 3 July. Pressurized gas intermittently generated a blue-orange flame. Good-sized magma fragments projected several meters above this vent were accompanied by strong detonations and intense degassing. Based on calculations made following several hours of observations, the ejection speed was estimated at 230-250 m/second. A third vent (B3) near the E rim was also very active but in a less violent and frequent manner. Two other vents, more westward, visible for an instant, showed mainly intense degassing sometimes accompanied by magma ejections to some meters above the red glow.

Crater C is a large depression with a lava lake in its center, usually agitated by surface movements. Violent explosions sent heavy gray-black ash plumes several hundreds of meters above the crater. Weak magma ejections also occurred from a glowing zone SW of the main lava lake. On the night of 3-4 July an intermittent flame came from the interior of this pit. Several times during the night, Strombolian explosions occurred simultaneously in these two areas.

A count of magma-ejecting explosions made over three 1-hour periods showed that Crater B was consistently more active. On 3 July between 1800 and 1900 a total of 63 explosions were distributed as follows: Crater A, 10; Crater B, 33; Crater C, 20. On 3 July between 2030 and 2130 a total of 51 explosions were distributed as follows: Crater A, 8; Crater B, 26; Crater C, 17. On 4 July between 1000 and 1100 a total of 54 explosions were distributed as follows: Crater A, 10; Crater B, 28; Crater C, 16.

On 5 July between 1430 and 1600, activity was much less frequent than the previous days, with explosions followed by long minutes of silence. The lava lake was quite visible in Crater C. During this period craters A and C were more active than B. At 1545 a larger explosion from Crater B generated some bomb falls at the extreme edge of the crater.

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

Information Contacts: Henry Gaudru, C. Pittet, C. Bopp, and G. Borel, Société Volcanologique Européenne, C.P. 1, 1211 Genève 17, Switzerland (URL: http://www.sveurop.org/); Michel Lardy, Centre ORSTOM, B.P. 76, Port Vila, Vanuatu.