Ambrym contains a 12-km-wide caldera and is part of the New Hebrides Arc, located in the Vanuatu archipelago. The two currently active craters within the caldera are Benbow and Marum, both of which have produced lava lakes, explosions, lava flows, and gas-and-ash emissions. The previous eruption occurred during late January 2022 and was characterized by ash plumes, sulfur dioxide plumes, and crater incandescence (BGVN 47:05). This report covers a new, short eruption during January 2024, which consisted of a lava effusion and an explosion. Information comes from the Vanuatu Meteorology and Geohazards Department (VMGD) and satellite data.

VMGD reported that at 2217 on 13 January an eruption began at Benbow Crater, based on webcam and seismic data. The eruption was characterized by a loud explosion, intense crater incandescence (figure 55), and gas-and-steam emissions. As a result, the Volcano Alert Level (VAL) was raised from 1 to 3 (on a scale of 0-5). A lava flow was reported in Benbow Crater, which lasted for four days. Satellite data showed that 1,116 tons of sulfur dioxide per day (t/d) were emitted on 14 January (figure 56). During the morning of 15 January, ground reports noted loud explosions and minor earthquakes. The sulfur dioxide flux on 15 January was 764 t/d. During 15-17 January activity decreased according to webcam images, seismic data, and field observations. No sulfur dioxide emissions were reported after 15 January. Gas-and-ash emissions also decreased, although they continued to be observed through 31 January, and crater incandescence was less intense (figure 57). The VAL was lowered to 2 on 17 January.

Figure 55. Webcam image showing strong nighttime incandescence coming from Benbow Crater at Ambrym at 2030 on 14 January 2024. Courtesy of VMGD.

Figure 56. A sulfur dioxide plume with a volume of 1,116 t/d was detected on 14 January 2024 drifting W from Ambrym. Courtesy of MOUNTS via VMGD.

Figure 57. Thermal activity was visible in a clear infrared (bands B12, B11, B4) satellite image at Benbow Crater on 23 January 2024. Courtesy of Copernicus Browser.

Geologic Background. Ambrym is a large basaltic volcano with a 12-km-wide caldera formed during a major Plinian eruption with dacitic pyroclastic flows about 1,900 years ago. A thick, almost exclusively pyroclastic sequence, initially dacitic then basaltic, overlies lava flows of a pre-caldera shield volcano. 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 been frequently reported since 1774, though mostly limited to extra-caldera eruptions that would have affected local populations. Since 1950 observations of eruptive activity from cones within the caldera or from flank vents have occurred almost yearly.

Information Contacts: Geo-Hazards Division, Vanuatu Meteorology and Geo-Hazards Department (VMGD), Ministry of Climate Change Adaptation, Meteorology, Geo-Hazards, Energy, Environment and Disaster Management, Private Mail Bag 9054, Lini Highway, Port Vila, Vanuatu (URL: http://www.vmgd.gov.vu/, https://www.facebook.com/VanuatuGeohazardsObservatory/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Popocatépetl, located 70 km SE of Mexica City, Mexico, contains a 400 x 600 m-wide summit crater. Records of activity date back to the 14th century; three Plinian eruptions, the most recent of which took place about 800 CE, have occurred since the mid-Holocene, accompanied by pyroclastic flows and voluminous lahars that swept basins below the volcano. The current eruption period began in January 2005, characterized by numerous episodes of lava dome growth and destruction within the summit crater. Recent activity has been characterized by daily gas-and-ash emissions, ashfall, and explosions (BGVN 48:09). This report covers similar activity during August through November 2023, according to daily reports from Mexico's Centro Nacional de Prevención de Desastres (CENAPRED) and various satellite data.

Daily gas-and-steam emissions, containing some amount of ash, continued during August through November 2023. CENAPRED reported the number of low-intensity gas-and-ash emissions or “exhalations” and the minutes of tremor, which sometimes included harmonic tremor in their daily reports (figure 220). A total of 21 volcano-tectonic (VT) tremors were detected throughout the reporting period. The average number of exhalations was 117 per day, with a maximum number of 640 on 25 September. Frequent sulfur dioxide plumes that exceeded two Dobson Units (DU) and drifted in multiple directions were visible in satellite data from the TROPOMI instrument on the Sentinel-5P satellite (figure 221).

Figure 220. Graphs showing the number of daily “exhalations” (in blue, top), and the number of minutes of tremor (in gold, bottom) at Popocatépetl each day during August through November 2023. The maximum number of daily exhalations was 640 on 25 September 2023; the maximum duration of 1,323 minutes of tremor was detected on 14 November 2023. Data from CENAPRED daily reports.

Figure 221. Strong sulfur dioxide plumes were detected at Popocatépetl and drifted in different directions on 26 August 2023 (top left), 5 September 2023 (top right), 9 October 2023 (bottom left), and 21 November 2023 (bottom right). Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Activity during August was relatively low and mainly consisted of occasional explosions, ash emissions, and light ashfall. There were 30 explosions (25 minor explosions and four moderate explosions), and nine VT-type events detected. An average number of 60 exhalations occurred each day, which mostly consisted of water vapor, volcanic gases, and a small amount of ash. On 2 August the National Center for Communications and Civil Protection Operations (CENACOM) reported light ashfall in Ocuituco (22 km SW), Yecapixtla (31 km SW), Cuautla (43 km SW), and Villa de Ayala (47 km SW). On 7 August light ashfall was observed in Atlautla (16 km W). A minor explosion at 0305 on 11 August was accompanied by crater incandescence. Explosions at 0618 on 13 August produced a gas-and-ash plume that rose above the summit, and at 0736 another explosion produced a puff of gas-and-ash (figure 222). Two minor explosions were detected at 0223 and 0230 on 16 August that generated eruptive columns with low ash content rising 800 m and 700 m above the crater, respectively. On 24 August an eruptive event lasted 185 minutes and consisted of light ash emissions that did not exceed 300 m above the crater. According to the Washington VAAC, ash plumes identified in daily satellite images rose to 4.6-7.6 km altitude and drifted in multiple directions, the highest of which occurred on 29 August.

Figure 222. Webcam image of an ash plume rising above Popocatépetl at 0738 on 13 August 2023. Courtesy of CENAPRED daily report.

There was an average of 156 exhalations each day during September, a monthly total of seven VT-type events, and 29 explosions, 14 of which were minor and nine of which were moderate. A gas-and-ash plume rose to 2 km above the summit and drifted WSW at 1216 on 1 September. CENACOM reported at 1510 observations of ashfall in Ozumba (18 km W), Atlautla, Tepetlixpa (20 km W), and Ecatzingo (15 km SW), as well as in Morelos in Cuernavaca (65 km WSW), Temixco (67 km WSW), Huitzilac (67 km W), Tepoztlán (49 km W), and Jiutepec (59 km SW). The next day, gas-and-ash plumes rose to 2 km above the summit (figure 223). At 1100 ashfall was reported in Amecameca (15 km NW), Ayapango (24 km WNW), Ozumba, Juchitepec, Tenango del Aire (29 km WNW), Atlautla, and Tlalmanalco (27 km NW). A gas-and-ash plume rose to 1 km above the summit and drifted WNW at 1810. During 5-6, 8-9, 12, 14, 19, and 24-25 September ashfall was reported in Amecameca, Atlautla, Ozumba, Tenango del Aire, Tepetlixpa, Juchitepec, Cuernavaca, Ayala, Valle de Chalco (44 km NW), Ixtapaluca (42 km NW), La Paz (50 km NW), Chimalhuacán, Ecatepec, Nezahualcóyotl (60 km NW), Xochimilco (53 km SE), Huayapan, Tetela del Volcano (20 km SW), Yautepec (50 km WSW), Cuautla (43 km SW), Yecapixtla (30 km SW) and possibly Tlaltizapán (65 km SW), Tlaquiltenango, and Tepalcingo. According to the Washington VAAC, ash plumes identified in daily satellite images rose to 5.8-9.1 km altitude and drifted in multiple directions, the highest of which was identified during 1-2 August.

Figure 223. Webcam image of a strong ash plume rising 2 km above Popocatépetl around 0342 on 2 September 2023. Courtesy of CENAPRED daily report.

Activity during October and November was relatively low. An average of 179 exhalations consisting of gas-and-steam and ash emissions were reported during October and 73 during November. Only one VT-type event and two explosions were detected during October and four VT-type events and one explosion during November. A satellite image from 0101 on 14 October showed ash fanning out to the NW at 6.7 km altitude and an image from 0717 showed a continuously emitted ash plume drifting WNW and NW at the same altitude. Ash emissions at 1831 on 14 October were ongoing and visible in webcam images slowly drifting W at an altitude of 6.4 km altitude (figure 224). On 24 October a tremor sequence began at 0310 that generated a gas-and-ash plume that rose 800 m above the summit and drifted W. Another tremor sequence occurred during 1305-1900 on 25 October that consisted of continuous ash emissions. Ash plumes identified in daily satellite images rose to 5.5-8.5 km altitude and drifted in different directions during October, according to the Washington VAAC. The highest ash plume was detected on 23 October. During 10-13 November ash plumes rose to 6.7 km altitude and drifted N, NNW, NE, and NW. On 13 November a M 1.5 VT-type event was detected at 0339 and light ashfall was reported in Amecameca, Cocotitlán (34 km NW), and Tenango del Aire, and Ocuituco. On 14 November ash plumes rose to 6 km altitude and drifted N, NE, and SE and light ashfall was reported in Cuernavaca (64 km W). The Washington VAAC reported frequent ash plumes that rose to 5.8-7.9 km altitude and drifted in several directions; the highest ash plume was recorded on 28 November.

Figure 224. A strong ash plume rising above Popocatépetl at 0553 on 14 October 2023. Image has been color corrected. Courtesy of CENAPRED daily report.

Satellite data. MODIS thermal anomaly data provided through MIROVA (Middle InfraRed Observation of Volcanic Activity) showed frequent low-to-moderate thermal anomalies during the reporting period (figure 225). The intensity of the anomalies was lower compared to previous months. According to data from MODVOLC thermal alerts, a total of ten hotspots were detected at the summit crater on 2 August and 2, 4, 9, 19, and 26 September. Thermal activity in the summit crater was visible in infrared satellite data and was sometimes accompanied by ash plumes, as shown on 17 November (figure 226).

Figure 225. Frequent low-to-moderate power thermal anomalies were detected at Popocatépetl during July through November 2023. During October through November the intensity of the anomalies was lower compared to previous months. Courtesy of MIROVA.

Figure 226. Infrared (bands B12, B11, B4) satellite images show a persistent, yet variably strong, thermal anomaly (bright yellow-orange) in the summit crater of Popocatépetl on 9 August 2023 (top left), 19 August 2023 (top right), 28 October 2023 (bottom left), and 17 November 2023 (bottom right). A strong ash plume drifted S on 17 November. Courtesy of Copernicus Browser.

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

Information Contacts: Centro Nacional de Prevención de Desastres (CENAPRED), Av. Delfín Madrigal No.665. Coyoacan, México D.F. 04360, México (URL: http://www.cenapred.unam.mx/, Daily Report Archive https://www.gob.mx/cenapred/archivo/articulos); 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 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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Volcán El Reventador, located in Ecuador, is a stratovolcano with a 4-km-wide avalanche scarp open to the E that was formed by edifice collapse. The largest recorded eruption took place in 2002 producing a 17-km-high eruption column, pyroclastic flows that traveled as far as 8 km, and lava flows from summit and flank vents. Recorded eruptions date back to the 16th century and have been characterized by explosive events, lava flows, ash plumes, and lahars. Frequent lahars have built deposits on the scarp slope. The current eruption period began in July 2008 and has recently been characterized daily explosions, gas-and-ash emissions, and block avalanches (BGVN 48:08). This report covers similar activity during August through November 2023 using daily reports from Ecuador's Instituto Geofisico (IG-EPN) and satellite data.

During August through November 2023, IG-EPN reported daily explosions, gas-and-ash plumes that rose as high as 1.3 km above the crater, and frequent crater incandescence, often accompanied by incandescent block avalanches that affected one or multiple flanks. More daily explosions were detected during November, with an average total of 46 per day.

Table 19. Monthly summary of explosions and plume heights recorded at Reventador from August through November 2023. Data could not be collected for 29-30 September 2023 and 6-23 October 2023. Data courtesy of IG-EPN (August-November 2023 daily reports).

Activity during August consisted of 6-75 daily explosions, nighttime crater incandescence, and incandescent avalanches of material. Frequent seismicity was mainly characterized by long-period (LP) events, harmonic tremor (TRARM), tremor-type (TRE), and volcano tectonic (VT)-type events. Daily gas-and-ash emissions rose 200-1,300 m above the summit and drifted W, SW, NW, NE, N, and E, based on webcam and satellite images. The Washington VAAC also reported occasional ash plumes that rose 400-1,600 m above the crater and drifted NW. Avalanches of incandescent material were reported during 1-2, 6-7, 9-14, 16-17, 18-21, and 26-29 August, which traveled 500-900 m below the crater and affected multiple flanks (figure 180). During 24-25 August incandescent material was ejected 300 m above the crater.

Figure 180. Infrared webcam image of incandescent avalanches descending the flanks of Reventador at 2158 (local time) on 21 August 2023. A gas-and-ash plume accompanied this activity more than 700 m above the crater as indicated by the black dotted lines. The white dotted line indicates the direction of the avalanches. The southern flank is located on the left of the photo. Courtesy of IG-EPN (INFORME DIARIO DEL VOLCAN EL REVENTADOR No. 2023-233, 21 de agosto de 2023).

Gas-and-ash emissions and seismicity characterized by LP, VT, TRARM, and TRE-type events continued during September; data were not available for 29-30 September. Daily gas-and-ash emissions rose 200-1,000 m above the crater and generally drifted W, NW, and SW (figure 181). Near-daily explosions ranged from 16-53 per day, often accompanied by incandescent avalanches, which affected one or multiple flanks and traveled 100-800 m below the crater. During 2-3 September incandescent material was ejected 200 m above the crater and was accompanied by blocks rolling down the flanks. During 16-17 September incandescent material was ejected 100-200 m above the crater and avalanches descended 600 m below the crater. During 21-22 and 24-26 September incandescent material was ejected 100-300 m above the crater. According to the Washington VAAC, ash plumes rose 700 m above the crater and drifted SW, W, and NW on 3, 16, and 20 September, respectfully.

Figure 181. Webcam image of a gas-and-ash plume rising above Reventador on 13 September 2023. Courtesy of IG-EPN (INFORME DIARIO DEL VOLCAN EL REVENTADOR No. 2023-257, 14 de septiembre de 2023).

During October, daily explosions, gas-and-ash plumes, and crater incandescence continued, with 16-40 explosions recorded each day (figure 182); data was not available for 6-23 October. Seismicity consisted of LP, TRE, and TRARM-type events. Gas-and-ash emissions rose 200-1,000 m above the crater and drifted W, SW, NW, SSW, NNW, and NE. The Washington VAAC reported that ash plumes rose 1-1.3 km above the crater and drifted W, SW, and NW during 1-5 October. During 30 September-1 October incandescent avalanches descended 700 m below the crater. Ejected material rose 200 m above the crater during 2-5 October and was accompanied by avalanches of material that traveled 250-600 m below the crater rim; incandescent avalanches were also reported during 23-29 October.

Figure 182. Photo showing nighttime crater incandescence and an explosion at Reventador on 25 October 2023. Courtesy of IG-EPN (INFORME DIARIO DEL VOLCAN EL REVENTADOR No. 2023-299, 26 de octubre de 2023).

Daily explosions, LP, TRARM, VT, and TRE-type events, crater incandescence, and avalanches of material continued during November. There were 26-62 daily explosions detected throughout the month. Gas-and-ash emissions rose 300-1,200 m above the crater and drifted in different directions (figure 183). The Washington VAAC reported that ash plumes rose 700-1,620 m above the crater and drifted NW, W, WNW, SW, E, SE, and ESE. Frequent incandescent avalanches descended 500-1,000 m below the crater. Explosions ejected material 100-300 m above the crater during 4-7, 11-12, and 19-23 November.

Figure 183. Webcam image showing an ash plume rising several hundred meters above Reventador on 21 November 2023. Courtesy of IG-EPN (INFORME DIARIO DEL VOLCAN EL REVENTADOR No. 2023-325, 21 de noviembre de 2023).

Satellite data. MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed intermittent thermal anomalies of low-to-moderate power (figure 184). Thermal activity mainly consisted of incandescent avalanches descending the flanks due to the frequently detected explosions. The MODVOLC hotspot system identified a total of ten hotspots on 3 August, 7, 18, 12, 22, and 28 September, and 7, 9, and 19 November.

Figure 184. Intermittent low-to-moderate intensity thermal activity was detected at Reventador during August through November 2023, based on this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Geologic Background. Volcán El 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 stratovolcano has 4-km-wide avalanche scarp open to the E formed by edifice collapse. A young, unvegetated, cone rises from the amphitheater floor to a height comparable to the rim. It has been the source of numerous lava flows as well as explosive eruptions visible from Quito, about 90 km ESE. Frequent lahars in this region of heavy rainfall have left extensive deposits on the scarp slope. The largest recorded 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/); 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/).

Erta Ale in Ethiopia has a 50-km-wide edifice that rises more than 600 m from below sea level in the Danakil depression. The summit caldera is 0.7 x 1.6 km and contains at least two pit craters (North and South). Another larger 1.8 x 3.1-km-wide depression is located SE of the summit and is bounded by curvilinear fault scarps on the SE side. Lava flows from fissures have traveled into the caldera and locally overflowed the crater rim. The current eruption has been ongoing since 1967, with at least one long-term active lava lake present in the summit caldera. Recent fissure eruptions from 2017 have occurred on the SE flank (BGVN 42:07). Recent activity has been characterized by minor thermal activity at the S crater and an active lava lake at the N crater (BGVN 48:06). This report covers strong lava lake activity primarily at the N pit crater during June through November 2023 using information from satellite infrared data.

Infrared satellite images generally showed an active lava lake as the N pit crater and variable thermal activity at the S pit crater during the reporting period. On 7 June two strong thermal anomalies were detected at the S pit crater and two weaker anomalies were visible at the N pit crater. Those anomalies persisted throughout the month, although the intensity at each declined. On 2 July a possible lava lake was identified at the S pit crater, filling much of the crater. On 7 July both pit craters contained active lava lakes (figure 120). By 12 July the thermal activity decreased; two smaller anomalies were visible through the rest of the month at the S pit crater while the N pit crater showed evidence of cooling.

Figure 120. Infrared satellite images (bands B12, B11, B4) showed strong thermal anomalies at both the N and S pit craters at Erta Ale on 7 July 2023 (top left). On 25 September 2023 (top right) thermal activity intensified at the N pit crater, which overflowed and traveled SE for several hundred meters, as shown on 15 October 2023 (bottom left) and 29 November 2023 (bottom right). Courtesy of Copernicus Browser.

Renewed lava lake activity was identified at the N pit crater, based on a satellite image from 11 August, with two smaller anomalies visible at the S pit crater. By 16 August the lava lake in the N pit had begun to cool and only a small thermal anomaly was identified. Activity restarted on 21 August, filling much of the E and SE part of the N pit crater. The thermal activity at the N pit crater intensified on 31 August, particularly in the NW part of the crater. On 5 September lava filled much of the N pit crater, overflowing to the W and SW. During at least 10-20 September thermal activity at both craters were relatively low.

According to a satellite image on 25 September, strong thermal activity resumed when lava overflowed the N pit crater to the S, SW, and NE (figure 120). A satellite image taken on 5 October showed lava flows from the N had spilled into the S and begun to cool, accompanied by two weak thermal anomalies at the S pit crater. On 15 October lava flows again traveled SE and appeared to originate from the S pit crater (figure 120). Following these events, smaller thermal anomalies were visible on the SE rim of the N pit crater and within the S pit crater.

Lava was visible in the NW part of the N pit crater according to a satellite image taken on 4 November. By 9 November the intensity had decreased, and the lava appeared to cool through the rest of the month; young lava flows were visible along the W side of the S pit crater on 24 and 29 November. Lava flows occurred at the N pit crater trending NE-SW and along the E side on 29 November (figure 120).

During the reporting period, the MIROVA (Middle InfraRed Observation of Volcanic Activity) thermal detection system recorded consistent activity during the first half of 2023 (figure 121). Beginning in June 2023, thermal activity increased and remained variable in intensity through the end of the year indicating the presence of an active lava lake and lava flows. The MODVOLC thermal detection system registered a total of 63 anomalies during 7, 8, and 23 July, 10 and 18 August, 3, 5, 16, 23, 24, and 25 September, 15 and 20 October, and 21, 24, 26, 28, and 30 November. Some of these stronger thermal anomalies were also detected in Sentinel-2 infrared satellite images that showed an active lava lake at the N pit crater and subsequent lava overflows from both pit craters (figure 120).

Figure 121. Graph of Landsat 8 and 9 OLI (red dots) and MODIS (blue bars) thermal anomalies at Erta Ale during 2022-2023. Thermal activity was relatively consistent during much of this time and during June through November activity became more variable due to lava flows and a strong active lava lake. Courtesy of MIROVA.

Geologic Background. The Erta Ale basaltic shield volcano in Ethiopia has a 50-km-wide edifice that rises more than 600 m from below sea level in the Danakil depression. The volcano includes a 0.7 x 1.6 km summit crater hosting steep-sided pit craters. Another larger 1.8 x 3.1 km wide depression elongated parallel to the trend of the Erta Ale range is located SE of the summit and is bounded by curvilinear fault scarps on the SE side. Basaltic lava flows from these fissures have poured into the caldera and locally overflowed its rim. The summit caldera usually also holds at least one long-term lava lake that has been active since at least 1967, and possibly since 1906. Recent fissure eruptions have occurred on the N flank.

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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Ubinas, located in Peru, has had 24 eruptions since 1550, which more recently have been characterized by explosions, ash plumes, and lahars (BGVN 45:03). This report covers a new eruption during June through December 2023 based on reports from Instituto Geofisico del Peru (IGP), Instituto Geológico Minero y Metalúrgico (INGEMMET), and satellite data.

IGP reported that seismic unrest began on 17 May, followed by an increase in seismicity during the second half of the month. There were 168 volcano-tectonic (VT) earthquakes detected, which are associated with rock fracturing processes, and 171 long-period (LP) earthquakes recorded during 16-24 May, which are associated with the movement of volcanic fluid.

Seismicity and fumarolic activity at the crater level continued to increase during June. During 1-18 June there was an average of 631 VT-type earthquakes and 829 LP earthquakes recorded. Webcams showed gas-and-steam emissions rising 500 m above the summit and drifting SE. In addition, the maximum value of emitted sulfur dioxide during this period was 337 tons/day. During 19-22 June an average of 315 VT-type events and 281 LP-type events and tremor were reported. On 20 June the Gobierno Regional de Moquegua raised the Volcano Alert Level (VAL) to Yellow (the second level on a four-color scale), based on recommendations from IGP. Webcam images showed ash emissions rising 1 km above the summit and drifting E at 0011 on 22 June, which IGP reported marked the start of a new eruption. Sporadic and diffuse gas-and-ash emissions continued to rise 800-1,500 m above the summit through the rest of the month and drifted mainly E, N, NW, W, SW, and NE. During 23-25 June there was an average of 402 VT-type earthquakes and 865 LP-type events detected. During 26-28 June the earthquakes associated with ash emissions, which have been observed since 22 June, decreased, indicating the end of the phreatic phase of the eruption, according to IGP. A thermal anomaly was detected in the crater for the first time on 26 June and was periodically visible through 4 July (figure 61). During 29-30 June there was an average of 173 VT-type earthquakes and 351 LP-type events recorded, and sulfur dioxide values ranged between 600 t/d and 1,150 t/d. During this same time, seismicity significantly increased, with 173 VT-type earthquakes, 351 LP-type events, and harmonic tremor which signified rising magma. The Gobierno Regional de Moquegua raised the Alert Level to Orange (the third level on a four-color scale) on 30 June based on the recommendation from IGP and INGEMMET.

Figure 61. A strong thermal anomaly (bright yellow-orange) at Ubinas was visible in an infrared (bands B12, B11, B4) satellite image on 28 June 2023 (left). Natural color images showed an ash plume rising above the summit on 3 July 2023 (middle) and 12 August 2023 (right). Courtesy of Copernicus Browser.

Activity during July consisted of continued seismicity and gas-and-ash emissions. Gas-and-ash emissions rose as high as 5 km above the summit and drifted as far as 40 km in different directions during 1, 4-6, 16, 20-23, 26, and 29 July, based on webcam and satellite images. During 1-2 July an average of 72 VT-type earthquakes and 114 LP-type events were detected. In addition, during that time, ashfall was reported in Ubinas (6.5 km SSE) and Querapi (4.5 km SE). During 2-3 July INGEMMET reported gas-and-ash plumes rose 400 m above the summit and drifted SW, causing ashfall in downwind areas as far as 5 km. During 3-4 July there was an average of 69 VT-type earthquakes and 96 LP-type events reported. On 4 July starting around 0316 there were 16 seismic signals associated with explosive activity and ash emissions detected (figure 62). According to INGEMMET an explosion ejected ballistics and generated a gas-and-ash plume that rose 5.5 km above the summit and drifted SW and S. Ashfall was recorded in Querapi, Ubinas, Sacohaya (7 km SSE), Anascapa (11 km SE), San Miguel (10 km SE), Tonohaya (7 km SSE), Huatahua, Huarina, Escacha (9 km SE), and Matalaque (17 km SSE), and was most significant within 5 km of the volcano. IGP noted that ash fell within a radius of 20 km and deposits were 1 mm thick in towns in the district of Ubinas.

Figure 62. Webcam image showing an ash plume rising 2.5 km above the summit of Ubinas on 4 July 2023. Courtesy of INGEMMET.

During 5-9 July an average of 67 VT-type events and 47 LP-type events were reported. A period of continuous gas-and-ash emissions occurred on 5 July, with plumes drifting more than 10 km SE and E. A total of 11 seismic signals associated with explosions also detected on 6, 16, 17, and 22 July. On 6 July explosions recorded at 0747 and 2330 produced gas-and-ash plumes that rose as high as 3.5 km above the summit and drifted as far as 30 km NW, NE, SE, and S. According to the Washington VAAC the explosion at 0747 produced a gas-and-ash plume that rose to 9.1 km altitude and drifted SW, which gradually dissipated, while a lower-altitude plume rose to 7.6 km altitude and drifted NE. Gobierno Regional de Moquegua declared a state of emergency for districts in the Moquegua region, along with Coalaque Chojata, Icuña, Lloque, Matalaque, Ubinas, and Yunga of the General Sánchez Cerro province, to be in effect for 60 days. On 7 July an ash plume rose to 7.3 km altitude and drifted E at 0320. At 0900 and 1520 gas-and-steam plumes with diffuse ash rose to 6.7 km altitude and drifted SE. Small ash emissions were visible in satellite and webcam images at 0920 and 1520 on 8 July and rose as high as 6.4 km altitude and drifted SE. During 10-16 July there was an average of 80 VT-type earthquakes and 93 LP-type events reported. INGEMMET reported that during 9-11 July sulfur dioxide emissions were low and remained around 300 t/d.

During 17-23 July an average of 46 VT-type events and 122 LP-type events were detected. On 20 July at 0530 an explosion generated an ash plume that rose 3-4.5 km above the crater and drifted 65 km toward Arequipa. An explosion on 21 July at 0922 produced a gas-and-ash plume that rose 5 km above the summit (figure 63). Ashfall was reported in Querapi, Ubinas, Tonohaya, Anascapa, Sacohaya, San Miguel, Escacha, Huatagua (14 km SE), Huarina, Escacha (9 km SE), Matalaque, Logén, Santa Lucía de Salinas, and Salinas de Moche. An explosion on 22 July at 1323 generated an ash plume that rose 5.5 km above the summit and drifted NE, E, and SE. During 24-30 July there were five volcanic explosions detected and an average of 60 VT-type events and 117 LP-type events. An explosion on 29 July at 0957 produced an ash plume that rose 2.5 km above the summit and drifted as far as 40 km NE, E, and SE. As a result, significant ashfall was reported in Ubinas and Matalaque.

Figure 63. Webcam image of Ubinas showing an ash plume rising as high as 5 km above the summit at 0930 on 21 July 2023. Courtesy of INGEMMET.

During August, explosions, gas-and-ash emissions, and seismic earthquakes persisted. During 31 July to 6 August there was an average of 115 VT-type events and 124 LP-type events reported. Gas-and-ash emissions were observed during 1, 6, 10, 13-14, 17-18, 21, and 23 August and they drifted as far as 20 km in different directions; on 14 and 18 August continuous ash emissions extended as far as 40 km S, SE, and NE. An explosion was detected at 2110 on 1 August, which generated a gas-and-ash plume that rose 5.4 km above the summit and drifted SE and E. The explosion ejected blocks and incandescent material as far as 3 km from the crater onto the SW, S, and SE flanks. Ashfall was reported in Ubinas and Chojata (19 km ESE). Gas-and-ash emissions rose as high as 2 km above the summit and drifted in different directions through 5 August, sometimes causing ashfall within a 15-km-radius. An explosion at 0009 on 6 August ejected blocks and produced a gas-and-ash plume that rose 1.4 km above the summit and drifted SE and E, which caused ashfall in Ubinas and Chojata and other areas within a 30-km radius. During 7-13 August there was an average of 102 VT-type events and 60 LP-type events detected. INGEMMET reported that sulfur dioxide emissions were low on 7 August and averaged 400 t/d.

One volcanic explosion that was recorded on 10 August, producing gas-and-ash emissions that rose 2.4 km above the summit and drifted as far as 25 km SE and E. Ashfall was observed in Ubinas, Matalaque, and Chojata. During 10-11 and 13-14 August sulfur dioxide values increased slightly to moderate levels of 2,400-3,700 t/d. The average number of VT-type events was 104 and the number of LP-type events was 71 during 14-21 August. Two explosions were detected at 0141 and 0918 on 21 August, which produced gas-and-ash emissions that rose 3.5 km above the summit and drifted 50 km N, NE, W, and NW (figure 64). The explosion at 0918 generated an ash plume that caused ashfall in different areas of San Juan de Tarucani. During 22-27 August the average number of VT-type events was 229 and the average number of LP-type events was 54. An explosion was reported at 1757 on 25 August, which generated a gas-and-ash plume that rose 4.2 km above the summit and drifted in multiple directions as far as 25 km. During 28 August through 3 September gas-and-ash emissions rose 600 m above the summit and drifted as far as 5 km E and SE. During this time, there was an average of 78 VT-type events and 42 LP-type events.

Figure 64. Webcam image showing an ash plume rising 3 km above the summit of Ubinas on 21 August 2023 at 0932. Courtesy of INGEMMET.

Gas-and-steam emissions rose 600-2,600 m above the summit and drifted as far as 15 km in multiple directions during September. During 4-10 and 11-17 September there was an average of 183 VT-type events and 27 LP-type events, and 114 VT-type events and 86 LP-type events occurred, respectively. On 14 September an explosion at 1049 generated a gas-and-ash plume that rose 2.6 km above the summit and drifted as far as 15 km E, NE, SE, and S (figure 65). During 14-16 September an average of three hours of seismic tremor related to ash emissions was recorded each day. During 18-24 September the average number of VT-type events was 187 and the average number of LP-type events was 45. During 25 September and 1 October, there was an average number of 129 VT-type events and 52 LP-type events detected.

Figure 65. Webcam image showing an ash plume rising 2.6 km above the summit of Ubinas on 14 September 2023. Courtesy of INGEMMET.

Relatively low activity was reported during October; during 2-9 October there was an average number of 155 VT-type events and 27 LP-type events recorded. On 1 October at 1656 seismic signals associated with ash emissions were recorded for an hour and thirty minutes; the ash plumes rose as high as 1 km above the summit and drifted more than 10 km E, S, and SW. On 4 October IGP reported that an ash plume drifted more than 15 km SW and S. Sulfur dioxide emissions were 1,250 t/d on that day. On 7 October a gas-and-ash plume rose 1.9 km above the summit and drifted NE, E, and SE. On 4 October the amount of sulfur dioxide emissions was 1,250 t/d. During 10-15 October there was an average number of 225 VT-type events and 34 LP-type events recorded. On 11 October at 1555 a single seismic signal associated with an ash pulse was recorded; the gas-and-ash emissions rose 700 m above the summit and drifted SW and W. There was an average of 204 VT-type events and 25 LP-type events detected during 16-22 October and 175 VT-type events and 17 LP-type events during 23-29 October. On 27 October at 0043 a gas-and-ash emission rose 500 m above the summit and drifted SE and E. A minor thermal anomaly was visible on the crater floor. During 30 October to 5 November there was an average of 95 VT-type events and 24 LP-type events detected.

Activity remained relatively low during November and December and consisted mainly of gas-and-steam emissions and seismicity. Gas-and-steam emissions rose 900-1,100 m above the summit and drifted mainly E, SE, N, and NE. IGP detected an average of 166 VT-type events and 38 LP-type events during 6-15 November, 151 VT-type events and 17 LP-type events during 16-30 November, 143 VT-type events and 23 LP-type events during 1-15 December, and 129 VT-type events and 21 LP-type events during 16-31 December. No explosions or ash emissions were recorded during November. The VAL was lowered to Yellow (the second level on a four-color scale) during the first week of November. According to the Washington VAAC an ash emission was identified in a satellite image at 0040 on 11 December that rose to 5.5 km altitude and drifted NW. Webcam images at 0620 and 1220 showed continuous gas-and-steam emissions possibly containing some ash rising as high as 7 km altitude. Webcam images during 10-31 December showed continuous gas-and-ash emissions that rose as high as 2.5 km above the summit and drifted up to 5 km NW, W, and SW. On 12 December continuous ash emissions drifted more than 10 km N and NW.

Geologic Background. The truncated appearance of Ubinas, Perú's most active volcano, is a result of a 1.4-km-wide crater at the summit. It is the northernmost of three young volcanoes located along a regional structural lineament about 50 km behind the main volcanic front. The growth and destruction of Ubinas I was followed by construction of Ubinas II beginning in the mid-Pleistocene. The upper slopes of the andesitic-to-rhyolitic Ubinas II stratovolcano are composed primarily of andesitic and trachyandesitic lava flows and steepen to nearly 45°. The steep-walled, 150-m-deep summit crater contains an ash cone with a 500-m-wide funnel-shaped vent that is 200 m deep. Debris-avalanche deposits from the collapse of the SE flank about 3,700 years ago extend 10 km from the volcano. Widespread Plinian pumice-fall deposits include one from about 1,000 years ago. Holocene lava flows are visible on the flanks, but activity documented since the 16th century has consisted of intermittent minor-to-moderate explosive eruptions.

Information Contacts: Instituto Geofisico del Peru (IGP), Calle Badajoz N° 169 Urb. Mayorazgo IV Etapa, Ate, Lima 15012, Perú (URL: https://www.gob.pe/igp); Observatorio Volcanologico del INGEMMET (Instituto Geológical Minero y Metalúrgico), Barrio Magisterial Nro. 2 B-16 Umacollo - Yanahuara Arequipa, Peru (URL: http://ovi.ingemmet.gob.pe); Gobierno Regional Moquegua, Sede Principal De Moquegua, R377+5RR, Los Chirimoyos, Moquegua 18001, Peru (URL: https://www.gob.pe/regionmoquegua); 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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Kanaga lies within the Kanaton caldera at the northern tip of Kanaga Island. The caldera rim forms a 760-m-high arcuate ridge south and east of Kanaga; a lake occupies part of the SE caldera floor. Most of its previous recorded eruptions are poorly documented, although they date back to 1763. Fumarolic activity at Kanaga occurs in a circular, 200-m-wide, 60-m-deep summit crater and produces vapor plumes sometimes seen on clear days from Adak, 50 km to the east. Its most recent eruption occurred in February 2012, which consisted of numerous small earthquakes, a possible weak ash cloud, and gas-and-steam emissions (BGVN 38:03). This report covers a new eruption during December 2023, based on information from the Alaska Volcano Observatory (AVO).

A small explosion was detected in local infrasound and seismic data at 2231 on 18 December, followed by elevated seismicity. No ash emissions were visible in partly cloudy satellite images. On 19 December the Volcano Alert Level (VAL) was raised to Advisory (the second level on a four-level scale) and the Aviation Color Code (ACC) was raised to Yellow (the second color on a four-color scale). The rate of seismicity significantly declined after the 18th, although it remained elevated through 30 December. Small, daily earthquakes occurred during 19-28 December. Satellite observations following the event showed a debris flow extending 1.5 km down the NW flank. Possible minor gas-and-steam emissions were visible in a webcam image on 20 December. Weakly elevated surface temperatures were identified in satellite data during 23-26 December. A series of cracks extending from the inner crater to the upper SE flank and debris deposits on the upper flanks were observed in satellite images on 27 December. AVO reported that these were likely formed during the 18 December event. Local webcam and seismic data were temporarily offline due to a power failure during 4-28 January.

On 28 January connection to the seismic stations and webcams was restored and webcam images showed gas-and-steam emissions at the summit. Occasional earthquakes were also detected each day. A period of weak seismic tremor was observed on 31 January. During February, the number of earthquakes declined. On 27 February AVO lowered the VAL to Normal (the lowest level on a four-level scale) and the ACC to Green (the lowest color on a four-color scale) due to decreased levels of seismicity and no new surface changes or elevated temperatures based on satellite and webcam data.

Geologic Background. Symmetrical Kanaga stratovolcano is situated within the Kanaton caldera at the northern tip of Kanaga Island. The caldera rim forms a 760-m-high arcuate ridge south and east of Kanaga; a lake occupies part of the SE caldera floor. The volume of subaerial dacitic tuff is smaller than would typically be associated with caldera collapse, and deposits of a massive submarine debris avalanche associated with edifice collapse extend nearly 30 km to the NNW. Several fresh lava flows from historical or late prehistorical time descend the flanks of Kanaga, in some cases to the sea. Historical eruptions, most of which are poorly documented, have been recorded since 1763. Kanaga is also noted petrologically for ultramafic inclusions within an outcrop of alkaline basalt SW of the volcano. Fumarolic activity occurs in a circular, 200-m-wide, 60-m-deep summit crater and produces vapor plumes sometimes seen on clear days from Adak, 50 km to the east.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA (URL: https://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 (URL: http://dggs.alaska.gov/).

Klyuchevskoy, located on the Kamchatka Peninsula, has produced frequent moderate-volume explosive and effusive eruptions and more than 100 flank eruptions have occurred during the past 3,000 years. Eruptions recorded since the late 17th century have resulted in frequent changes to the morphology of the 700-m-wide summit crater. Eruptions over the past 400 years have primarily originated from the summit crater, although numerous major explosive and effusive eruptions have also occurred from flank craters. The previous eruption ended in November 2022 and consisted of Strombolian activity (BGVN 47:12). This report covers a new eruption during June through December 2023, characterized by Strombolian explosions, lava flows, and ash plumes. Information primarily comes from weekly and daily reports from the Kamchatkan Volcanic Eruption Response Team (KVERT) and various satellite data.

KVERT reported that a Strombolian eruption began at 2323 on 22 June. A thermal anomaly was visible in satellite data starting on 22 June (figure 75). As a result, the Aviation Color Code (ACC) was raised to Yellow (the second lowest level on a four-color scale). During 4-6 and 13 July small ash clouds were occasionally observed over the crater. On 19 July a new lava flow began to effuse along the Apakhonchich drainage on the SE flank, which continued through 19 August. Lava fountaining was reported on 21 July in addition to the active lava flow, which continued through 23 August and during 27-30 August. During 22-23 and 27-30 August the lava flow was active along the Apakhonchich drainage on the SE flank.

Figure 75. Photo of Strombolian activity at the summit crater of Klyuchevskoy on 5 July 2023. Photo has been color corrected. Courtesy of Yu Demyanchuk via Volkstat.

Similar activity was observed during September. Lava fountaining resumed on 2 September and continued through 31 October. In addition, on 2 September a lava flow began to effuse along the Kozyrevsky drainage on the SW flank. During 3-5 September resuspended ash plumes rose to 3-3.5 km altitude and extended as far as 170 km E by 1940 on 4 September. The ACC was raised to Orange (the third level on a four-color scale) at 1240 on 4 September. The ACC was briefly lowered back to Yellow at 1954 that same day before returning to Orange during 1532-1808 on 5 September due to resuspended ash plumes that rose to 3 km altitude and drifted 120 km E at 1500. KVERT reported that Strombolian activity continued, feeding the lava flows advancing down the Apakhonchichsky and Kozyrevsky drainages through most of the month. During 25 September through 16 October the lava flow was only active in the Apakhonchichisky drainage (figure 76). During 9-12 September resuspended ash plumes rose to 1.5-4 km altitude and extended 550 km E and SE. On 22 September resuspended ash plumes rose to 2-2.5 km altitude and drifted 50-90 km E, which prompted KVERT to raise the ACC to Orange; the ACC was lowered back to Yellow on 24 September. On 29 September phreatic explosions generated ash plumes that rose to 5.2-5.3 km altitude.

Figure 76. Photo of Strombolian explosions at the summit of Klyuchevskoy accompanied by ash plumes and a lava flow descending the Apakhonchichsky on the SE flank on 28 September 2023. Photo has been color corrected. Courtesy of Yu Demyanchuk, IVS FEB RAS, KVERT.

Activity during October continued with lava fountains, lava flows, and ash plumes. Strombolian activity with lava fountains continued at the crater and active lava flows alternately descended the Apakhonchichisky and Kozyrevsky drainages on the SE and S flanks (figure 77). During 11-12 October gas-and-steam plumes containing some ash rose to 5.5-6 km altitude and extended as far as 65 km NE and SE. The ACC was raised to Orange on 11 October. According to observers at the Kamchatka Volcanological Station, lava effusion was almost continuous, and incandescent material was ejected as high as 300 m above the crater rim. On 13 October at 1420 an ash plume rose to 5-5.5 km altitude and drifted 90-100 km SE. During 14-16 October gas-and-steam plumes containing some ash rose to 4-6 km altitude and drifted 40-145 km ESE and E. On 16 October lava on the SE flank melted the snow and ice, causing phreatic explosions and large collapses of material from the margins of the flow. At 1500 an ash plume rose to 6.5-7 km altitude and drifted 70 km ENE. On 17 October an ash plume was reported extending 360 km NE. Gray-red ashfall was observed in Klyuchi at 0700; this ash was resuspended from older material.

Figure 77. Photo of Strombolian activity at the summit crater of Klyuchevskoy on 23 October 2023. Photo has been color corrected. Courtesy of Yu Demyanchuk, IVS FEB RAS, KVERT.

During 22-31 October phreatic explosions generated ash plumes mainly containing ash from collapses of previously deposited pyroclastic material that rose to 7 km altitude and extended as far as 280 km NE, E, SW, and S on 23 and 29 October the ash plumes rose to 8 km altitude. Ash plumes during 27-29 October rose to 8 km altitude and drifted as far as 300 km SE, ESE, and E. Lava fountains rose up to 500 m above the crater during 27-31 October. Scientists from the Kamchatka Volcanological Station visited the volcano on 28 October and reported that the cinder cone at the summit had grown. They also observed advancing lava on the E flank that extended about 2 km from the summit to 2,700 m elevation, incandescent ejecta 500 m above the crater, and avalanches in the Apakhonchichsky drainage. On 31 October activity intensified, and lava flows were reported moving in the Kretovsky, Kozyrevsky, and Apakhonchichisky drainages on the NW, SW, and SE flanks. At 0930 an ash plume rose to 7 km altitude and at first drifted 169 km SW and then 646 km SE. KVERT reported ash plumes rose to 14 km altitude and extended as far as 1,500 km SSE. The ACC was raised to Red (the highest level on a four-color scale). During 31 October to 1 November ash plumes rose as high as 14 km altitude and drifted as far as 2,255 km ESE.

Activity on 1 November intensified. The lava fountains rose as high as 1 km above the summit (figure 78) and fed the lava flows that were active on the Kretovsky, Kozyrevsky, and Apakhonchichsky drainages on the NW, SW, and SE flanks. Ash plumes rose to 10-14 km altitude and drifted as far as 1,500 km SSE (figure 79). According to the Kamchatka Volcanological Station, observers reported pyroclastic flows descending the flanks. Lahars descended the Studenoy River, blocking the Kozyrevsky-Petropavlovsk federal highway and descended the Krutenkaya River, blocking the road E of Klyuchi. According to news articles the ash plumes caused some flight cancellations and disruptions in the Aleutians, British Columbia (Canada), and along flight paths connecting the Unites States to Japan and South Korea. Ash plumes containing old ash from collapses in the Apakhonchichsky drainage due to phreatic explosions rose to 9.5-9.8 km altitude and drifted 192 km SW at 1400 and to 8.7 km altitude and drifted 192 km SW at 1710 on 1 November.

Figure 78. Photo of the Strombolian activity at Klyuchevskoy accompanied by strong ash plumes taken on 1 November 2023. Photo has been color corrected. Courtesy of Yu Demyanchuk via Volkstat.

Figure 79. Webcam image of an explosive eruption at Klyuchevskoy accompanied by strong ash plumes on 1 November 2023. Courtesy of KB GS RAS, KVERT.

On 2 November ash plumes rose to 6-14 km altitude; the ash plume that rose to 14 km altitude decreased to 6.5 km altitude and drifted NNE by 2000 and continued to drift more than 3,000 km ESE and E. The ACC was lowered to Orange. On 3 November ash plumes rose to 5-8.2 km altitude and drifted 72-538 km ENE, NNE, and ESE; at 0850 an ash plume rose to 6-6.5 km altitude and drifted more than 3,000 km ESE throughout the day. During 4-6 and 8-10 November resuspended ash plumes associated with collapses of old pyroclastic material from the sides of the Apakhonchichsky drainage due to phreatic explosions rose to 4.5-5.5 km altitude and extended 114-258 km NE, ENE, and E. KVERT reported that the eruption stopped on 5 November and the lava flows had begun to cool. Resuspended ash plumes rose to 5-6 km altitude and drifted 60 km E at 0820 on 13 November and to 5 km and 4.5 km altitude at 1110 and 1430 and drifted 140 km E and 150 km ESE, respectively. On 15 November the ACC was lowered to Green.

Activity was relatively low during most of December. On 27 December Strombolian activity resumed based on a thermal anomaly visible in satellite data. On 30 December an ash plume rose to 6 km altitude and extended 195 km NW. The ACC was raised to Orange. On 31 December video and satellite data showed explosions that generated ash plumes that rose to 5-6.5 km altitude and drifted 50-230 km WNW and NW. Though a thermal anomaly persisted through 1 January 2024, no explosions were detected, so the ACC was lowered to Yellow.

Satellite data. Thermal activity was strong throughout the reporting period due to frequent lava fountaining and lava flows. MODIS thermal anomaly data provided through MIROVA (Middle InfraRed Observation of Volcanic Activity) showed strong activity during the entire reporting period, resulting from lava fountaining and lava flows (figure 80). According to data from MODVOLC thermal alerts, a total of 336 hotspots were detected in June (3), July (30), August (11), September (52), October (217), and November (23). Thermal activity was also visible in infrared satellite images, often showing a strong thermal anomaly at the summit crater and a lava flow affecting primarily the SE and SW flanks (figure 81).

Figure 80. Strong thermal activity was detected at Klyuchevskoy during the end of June through early November 2023, according to this MIROVA graph (Log Radiative Power). High levels of activity coincided with lava flows on the SE and SW flanks and Strombolian activity. Courtesy of MIROVA.

Figure 81. Infrared (bands B12, B11, B4) satellite images show a strong thermal anomaly (bright yellow-orange) in the summit crater of Klyuchevskoy, which over time became a lava flow that primarily affected the SE and SW flanks. Lava flows shown here occurred on 31 July 2023 (top right), 27 August 2023 (left middle), 29 September 2023 (right middle), 24 October 2023 (bottom left), and 29 October 2023 (bottom right). Courtesy of Copernicus Browser.

Geologic Background. Klyuchevskoy is the highest and most active volcano on the Kamchatka Peninsula. Since its origin about 6,000 years ago, this symmetrical, basaltic stratovolcano has produced frequent moderate-volume explosive and effusive eruptions without major periods of inactivity. It rises above a saddle NE of Kamen volcano and lies SE of the broad Ushkovsky massif. More than 100 flank eruptions have occurred during approximately the past 3,000 years, with most lateral craters and cones occurring along radial fissures between the unconfined NE-to-SE flanks of the conical volcano between 500 and 3,600 m elevation. Eruptions recorded since the late 17th century have resulted in frequent changes to the morphology of the 700-m-wide summit crater. These eruptions over the past 400 years have originated primarily from the summit crater, but have also included numerous major explosive and effusive eruptions from flank craters.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); Kamchatka Volcanological Station, Kamchatka Branch of Geophysical Survey, (KB GS RAS), Klyuchi, Kamchatka Krai, Russia (URL: http://volkstat.ru/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Mount Agung, located on the E end of the island of Bali, Indonesia, rises above the SE rim of the Batur caldera. The summit area extends 1.5 km E-W, with the highest point on the W and a steep-walled 800-m-wide crater on the E. Recorded eruptions date back to the early 19th century. A large and deadly explosive and effusive eruption occurred during 1963-64, which was characterized by voluminous ashfall, pyroclastic flows, and lahars that caused extensive damage and many fatalities. More recent activity was documented during November 2017-June 2019 that consisted of multiple explosions, significant ash plumes, lava flows at the summit crater, and incandescent ejecta. This report covers activity reported during April-May 2022 and December 2022 based on data from the Darwin Volcanic Ash Advisory Center (VAAC).

Activity during 2022 was relatively low and mainly consisted of a few ash plumes during April-May and December. An ash plume on 3 April rising to 3.7 km altitude (700 m above the summit) and drifting N was reported in a Darwin VAAC notice based on a ground report, with ash seen in HIMAWARI-8 visible imagery. Another ash plume was reported at 1120 on 27 May that rose to 5.5 km altitude (2.5 m above the summit); the plume was not visible in satellite or webcam images due to weather clouds. An eruption was reported based on seismic data at 0840 on 13 December, with an estimated plume altitude of 3.7 km; however, no ash was seen using satellite imagery in clear conditions before weather clouds obscured the summit.

Geologic Background. Symmetrical Agung stratovolcano, Bali's highest and most sacred mountain, towers over the eastern end of the island. The volcano, whose name means "Paramount," rises above the SE rim of the Batur caldera, and the northern and southern flanks extend to the coast. The summit area extends 1.5 km E-W, with the high point on the W and a steep-walled 800-m-wide crater on the E. The Pawon cone is located low on the SE flank. Only a few eruptions dating back to the early 19th century have been recorded in historical time. The 1963-64 eruption, one of the largest in the 20th century, produced voluminous ashfall along with devastating pyroclastic flows and lahars that caused extensive damage and many fatalities.

Information Contacts: Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/).

Saunders is one of eleven islands that comprise the South Sandwich Islands in the South Atlantic. The active Mount Michael volcano has been in almost continuous eruption since November 2014 (BGVN 48:02). Recent activity has resulted in intermittent thermal anomalies and gas-and-steam emissions (BGVN 47:03, 48:02). Visits are infrequent due to its remote location, and cloud cover often prevents satellite observations. Satellite thermal imagery and visual observation of incandescence during a research expedition in 2019 (BGVN 28:02 and 44:08) and a finding confirmed by a National Geographic Society research team that summited Michael in November 2022 reported the presence of a lava lake.

Although nearly constant cloud cover during February 2023 through January 2024 greatly limited satellite observations, thermal anomalies from the lava lake in the summit crater were detected on clear days, especially around 20-23 August 2023. Anomalies similar to previous years (eg. BGVN 48:02) were seen in both MIROVA (Middle InfraRed Observation of Volcanic Activity) data from MODIS instruments and in Sentinel 2 infrared imagery. The only notable sulfur dioxide plume detected near Saunders was on 25 September 2023, with the TROPOMI instrument aboard the Sentinel-5P satellite.

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

Information Contacts: MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard MD 20771, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser (URL: https://dataspace.copernicus.eu/browser).

Tengger Caldera, located at the N end of a volcanic massif in Indonesia’s East Java, consists of five overlapping stratovolcanoes. The youngest and only active cone in the 16-km-wide caldera is Bromo, which typically produces gas-and-steam plumes, occasional ash plumes and explosions, and weak thermal signals (BGVN 44:05, 47:01). This report covers activity during January 2022-December 2023, consisting of mostly white gas-and-steam emissions and persistent weak thermal anomalies. Information was provided by the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM) and satellite imagery. The Alert Level remained at 2 (on a scale of 1-4), and visitors were warned to stay at least 1 km from the crater.

Activity was generally low during the reporting period, similar to that in 2021. According to almost daily images from MAGMA Indonesia (a platform developed by PVMBG), white emissions and plumes rose from 50 to 900 m above the main crater during this period (figure 24). During several days in March and June 2022, white plumes reached heights of 1-1.2 km above the crater.

Figure 24. Webcam image showing a gas-and-steam plume from the Bromo cone in the Tengger Caldera on 2 April 2023. Courtesy of MAGMA Indonesia.

After an increase in activity at 2114 on 3 February 2023, a PVMBG team that was sent to observe white emissions rising as high as 300 m during 9-12 February and heard rumbling noises. A sulfur dioxide odor was also strong near the crater and measurements indicated that levels were above the healthy (non-hazardous) threshold of 5 parts per million; differential optical absorption spectroscopy (DOAS) measurements indicated an average flux of 190 metric tons per day on 11 February. Incandescence originating from a large fumarole in the NNW part of the crater was visible at night. The team observed that vegetation on the E caldera wall was yellow and withered. The seismic network recorded continuous tremor and deep and shallow volcanic earthquakes.

According to a PVMBG press release, activity increased on 13 December 2023 with white, gray, and brown emissions rising as high as 900 m above Bromo’s crater rim and drifting in multiple directions (figure 25). The report noted that tremor was continuous and was accompanied in December by three volcanic earthquakes. Deformation data indicated inflation in December. There was no observable difference in the persistent thermal anomaly in the crater between 11 and 16 December 2023.

Figure 25. Webcam image showing a dark plume that rose 900 m above the summit of the Bromo cone in the Tengger Caldera on 13 December 2023. Courtesy of MAGMA Indonesia.

All clear views of the Bromo crater throughout this time, using Sentinel-2 infrared satellite images, showed a weak persistent thermal anomaly; none of the anomalies were strong enough to cause MODVOLC Thermal Alerts. A fire in the SE part of the caldera in early September 2023 resulted in a brief period of strong thermal anomalies.

Geologic Background. The 16-km-wide Tengger caldera is located at the northern end of a volcanic massif extending from Semeru volcano. The massive volcanic complex dates back to about 820,000 years ago and consists of five overlapping stratovolcanoes, each truncated by a caldera. Lava domes, pyroclastic cones, and a maar occupy the flanks of the massif. The Ngadisari caldera at the NE end of the complex formed about 150,000 years ago and is now drained through the Sapikerep valley. The most recent of the calderas is the 9 x 10 km wide Sandsea caldera at the SW end of the complex, which formed incrementally during the late Pleistocene and early Holocene. An overlapping cluster of post-caldera cones was constructed on the floor of the Sandsea caldera within the past several thousand years. The youngest of these is Bromo, one of Java's most active and most frequently visited volcanoes.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); 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/).

Shishaldin is located on the eastern half of Unimak Island, one of the Aleutian Islands. Frequent explosive activity, primarily consisting of Strombolian ash eruptions from the small summit crater, but sometimes producing lava flows, has been recorded since the 18th century. The previous eruption ended in May 2020 and was characterized by intermittent thermal activity, increased seismicity and surface temperatures, ash plumes, and ash deposits (BGVN 45:06). This report covers a new eruption during July through November 2023, which consisted of significant explosions, ash plumes, ashfall, and lava fountaining. Information comes from daily, weekly, and special reports from the Alaska Volcano Observatory (AVO) and various satellite data. AVO monitors the volcano using local seismic and infrasound sensors, satellite data, web cameras, and remote infrasound and lightning networks.

AVO reported that intermittent tremor and low-frequency earthquakes had gradually become more regular and consistent during 10-13 July. Strongly elevated surface temperatures at the summit were identified in satellite images during 10-13 July. On 11 July AVO raised the Aviation Color Code (ACC) to Yellow (the second color on a four-color scale) and Volcano Alert Level (VAL) to Advisory (the second level on a four-level scale) at 1439. Later in the day on 11 July summit crater incandescence was observed in webcam images. Observations of the summit suggested that lava was likely present at the crater, which prompted AVO to raise the ACC to Orange (the second highest color on a four-color scale) and the VAL to Watch (the second highest level on a four-level scale). The US Coast Guard conducted an overflight on 12 July and confirmed that lava was erupting from the summit. That same day, sulfur dioxide emissions were detected in satellite images.

A significant explosion began at 0109 on 14 July that produced an ash plume that rose to 9-12 km altitude and drifted S over the Pacific Ocean (figure 43). Webcam images and photos taken around 0700 from a ship SW off Unimak Island showed small lahar deposits, which were the result of the interaction of hot pyroclastic material and snow and ice on the flanks. There was also ashfall on the SW and N flanks. A smaller explosion at 0710 generated an ash plume that rose to 4.5 km altitude. Webcam images and pilot reports showed continued low-level ash emissions during the morning, rising to less than 4.6 km altitude; those emissions included a small ash plume near the summit around 1030 resulting from a small explosion.

Figure 43. Photo of a strong ash plume that rose to 9-12 km altitude on the morning of 14 July 2023. Lahar deposits were visible on the SW flank (white arrows). Photo has been color corrected. Courtesy of Christopher Waythomas, AVO.

Seismic tremor amplitude began increasing at around 1700 on 15 July; strongly elevated surface temperatures were also reported. An ash plume rose to 4.6 km altitude and drifted SSE at 2100, based on a satellite image. A continuous ash plume during 2150 through 2330 rose to 5 km altitude and extended 125 km S. At 2357 AVO raised the ACC to Red (the highest color on a four-color scale) and the VAL to Warning (the highest level on a four-level scale), noting that seismicity remained elevated for more than six hours and explosion signals were frequently detected by regional infrasound (pressure sensor) networks. Explosions generated an ash plume that rose to 4.9 km altitude and drifted as far as 500 km SE. Activity throughout the night declined and by 0735 the ACC was lowered to Orange and the VAL to Watch. High-resolution satellite images taken on 16 July showed pyroclastic deposits extending as far as 3 km from the vent; these deposits generated lahars that extended further down the drainages on the flanks. Ash deposits were mainly observed on the SSE flank and extended to the shore of Unimak Island. During 16-17 July lava continued to erupt at the summit, which caused strongly elevated surface temperatures that were visible in satellite imagery.

Lava effusion increased at 0100 on 18 July, as noted in elevated surface temperatures identified in satellite data, increasing seismic tremor, and activity detected on regional infrasound arrays. A significant ash plume at 0700 rose to 7 km altitude and continued until 0830, eventually reaching 9.1 km altitude and drifting SSE (figure 44). As a result, the ACC was raised to Red and the VAL to Warning. By 0930 the main plume detached, but residual low-level ash emissions continued for several hours, remaining below 3 km altitude and drifting S. The eruption gradually declined and by 1208 the ACC was lowered to Orange and the VAL was lowered to Watch. High-resolution satellite images showed ash deposits on the SW flank and pyroclastic deposits on the N, E, and S flanks, extending as far as 3 km from the vent; lahars triggered by the eruption extended farther down the flanks (figure 45). Lava continued to erupt from the summit crater on 19 July.

Figure 44. Photo of an ash-rich plume rising above Shishaldin to 9.1 km altitude on 18 July 2023 that drifted SE. View is from the N of the volcano and Isanotski volcano is visible on the left-hand side of the image. Photo has been color corrected. Courtesy of Chris Barnes, AVO.

Figure 45. Near-infrared false-color satellite image of Shishaldin taken on 18 July 2023 showing ash deposits on the N, E, and S flanks extending as far as 3 km from the vent due to recent eruption events. Courtesy of Matthew Loewen, AVO.

Elevated surface temperatures were detected in satellite images during 19-25 July, despite occasional weather cloud cover, which was consistent with increased lava effusion. During 22-23 July satellite observations acquired after the eruption from 18 July showed pyroclastic flow and lahar deposits extending as far as 3 km down the N, NW, and NE flanks and as far as 1.5 km down the S and SE flanks. Ash deposits covered the SW and NE flanks. No lava flows were observed outside the crater. On 22 July a sulfur dioxide plume was detected in satellite data midday that had an estimated mass of 10 kt. In a special notice issued at 1653 on 22 July AVO noted that eruptive activity had intensified over the previous six hours, which was characterized by an hours-long steady increase in seismic tremor, intermittent infrasound signals consistent with small explosions, and an increase in surface temperatures that were visible in satellite data. Pilots first reported low-level ash plumes at around 1900. At 2320 an ash plume had risen to 9 km altitude based on additional pilot reports and satellite images. The ACC was increased to Red and the VAL to Warning at 2343. Satellite images indicated growth of a significantly higher ash plume that rose to 11 km altitude continued until 0030 and drifted NE. During the early morning hours of 23 July ash plumes had declined to 4.6 k altitude. Seismic tremor peaked at 0030 on 23 July and began to rapidly decline at 0109; active ash emissions were no longer visible in satellite data by 0130. The ACC was lowered to Orange and the VAL to Watch at 0418; bursts of increased seismicity were recorded throughout the morning, but seismicity generally remained at low levels. Elevated surface temperatures were visible in satellite data until about 0600. On 24 July pilots reported seeing vigorous gas-and-steam plumes rising to about 3 km altitude; the plumes may have contained minor amounts of ash.

During 24-25 July low level seismicity and volcanic tremor were detected at low levels following the previous explosion on 23 July. Strongly elevated surface temperatures were observed at the summit crater in satellite data. Around 2200 on 25 July seismicity began to increase, followed by infrasound signals of explosions after 0200 on 26 July. An ash plume rose to 3 km altitude at 0500 and drifted ENE, along with an associated sulfur dioxide plume that drifted NE and had an estimated mass of 22 kt. Diffuse ash emissions were visible in satellite data and rose to 6.1-7.6 km altitude and extended 125 km from the volcano starting around 1130. These ash events were preceded by about seven hours of seismic tremor, infrasound detections of explosions, and five hours of increased surface temperatures visible in satellite data. Activity began to decline around 1327, which included low-frequency earthquakes and decreased volcanic tremor, and infrasound data no longer detected significant explosions. Surface temperatures remained elevated through the end of the month.

Seismicity, volcanic tremor, and ash emissions remained at low levels during early August. Satellite images on 1 August showed that some slumping had occurred on the E crater wall due to the recent explosive activity. Elevated surface temperatures continued, which was consistent with cooling lava. On 2 August small explosive events were detected, consistent with low-level Strombolian activity. Some episodes of volcanic tremor were reported, which reflected low-level ash emissions. Those ash emissions rose to less than 3 km altitude and drifted as far as 92.6 km N. Pilots that were located N of the volcano observed an ash plume that rose to 2.7 km altitude. Seismicity began to increase in intensity around 0900 on 3 August. Seismicity continued to increase throughout the day and through the night with strongly elevated surface temperatures, which suggested that lava was active at the surface.

An ash cloud that rose to 7.6-7.9 km altitude and drifted 60-75 km NE was visible in a satellite image at 0520 on 4 August. Pilots saw and reported the plume at 0836 (figure 46). By 0900 the plume had risen to 9.1 km altitude and extended over 100 km NE. AVO raised the ACC to Red and the VAL to Warning as a result. Seismic tremor levels peaked at 1400 and then sharply declined at 1500 to slightly elevated levels; the plume was sustained during the period of high tremor and drifted N and NE. The ACC was lowered to Orange and the VAL to Watch at 2055. During 5-14 August seismicity remained low and surface temperatures were elevated based on satellite data due to cooling lava. On 9 August a small lava flow was observed that extended from the crater rim to the upper NE flank. It had advanced to 55 m in length and appeared in satellite imagery on 11 August. Occasional gas-and-steam plumes were noted in webcam images. At 1827 AVO noted that seismic tremor had steadily increased during the afternoon and erupting lava was visible at the summit in satellite images.

Figure 46. Photo showing an ash plume rising above Shishaldin during the morning of 4 August 2023 taken by a passing aircraft. The view is from the N showing a higher gas-rich plume and a lower gray ash-rich plume and dark tephra deposits on the volcano’s flank. Photo has been color corrected. Courtesy of Chris Barnes, AVO.

Strong explosion signals were detected at 0200 on 15 August. An ash cloud that was visible in satellite data extended 100 km NE and may have risen as high as 11 km altitude around 0240. By 0335 satellite images showed the ash cloud rising to 7.6 km altitude and drifting NE. Significant seismicity and explosions were detected by the local AVO seismic and infrasound networks, and volcanic lightning was detected by the World Wide Lightning Location Network (WWLLN). A sulfur dioxide plume associated with the eruption drifted over the S Bering Sea and parts of Alaska and western Canada. Seismicity was significantly elevated during the eruption but had declined by 1322. A pilot reported that ash emissions continued, rising as high as 4.9 km altitude. Elevated surface temperatures detected in satellite data were caused by hot, eruptive material (pyroclastic debris and lava) that accumulated around the summit. Eruptive activity declined by 16 August and the associated sulfur dioxide plume had mostly dissipated; remnants continued to be identified in satellite images at least through 18 August. Surface temperatures remained elevated based on satellite images, indicating hot material on the upper parts of the volcano. Small explosions were detected in infrasound data on the morning of 19 August and were consistent with pilot reports of small, short-lived ash plumes that rose to about 4.3 km altitude. Low-level explosive activity was reported during 20-24 August, according to seismic and infrasound data, and weather clouds sometimes prevented views. Elevated surface temperatures were observed in satellite images, which indicated continued hot material on the upper parts of the volcano.

Seismic tremor began to increase at around 0300 on 25 August and was followed by elevated surface temperatures identified in satellite images, consistent with erupting lava. Small explosions were recorded in infrasound data. The ACC was raised to Red and the VAL to Warning at 1204 after a pilot reported an ash plume that rose to 9.1 km altitude. Seismicity peaked at 1630 and began to rapidly decline at around 1730. Ash plumes rose as high as 10 km altitude and drifted as far as 400 km NE. By 2020 the ash plumes had declined to 6.4 km altitude and continued to drift NE. Ash emissions were visible in satellite data until 0000 on 26 August and seismicity was at low levels. AVO lowered the ACC to Orange and the VAL to Watch at 0030. Minor explosive activity within the summit crater was detected during 26-28 August and strongly elevated surface temperatures were still visible in satellite imagery through the rest of the month. An AVO field crew working on Unimak Island observed a mass flow that descended the upper flanks beginning around 1720 on 27 August. The flow produced a short-lived ash cloud that rose to 4.5 km altitude and rapidly dissipated. The mass flow was likely caused by the collapse of spatter that accumulated on the summit crater rim.

Similar variable explosive activity was reported in September, although weather observations sometimes prevented observations. A moderate resolution satellite image from the afternoon of 1 September showed gas-and-steam emissions filling the summit crater and obscuring views of the vent. In addition, hot deposits from the previous 25-26 August explosive event were visible on the NE flank near the summit, based on a 1 September satellite image. On 2 and 4 September seismic and infrasound data showed signals of small, repetitive explosions. Variable gas-and-steam emissions from the summit were visible but there was no evidence of ash. Possible summit crater incandescence was visible in nighttime webcam images during 3-4 September.

Seismicity began to gradually increase at around 0300 on 5 September and activity escalated at around 0830. A pilot reported an ash plume that rose to 7.6 km altitude at 0842 and continued to rise as high as possibly 9.7 km altitude and drifted SSE based on satellite images (figure 47). The ACC was raised to Red and the VAL to Warning at 0900. In addition to strong tremor and sustained explosions, the eruption produced volcanic lightning that was detected by the WWLLN. Around 1100 seismicity decreased and satellite data confirmed that the altitude of the ash emissions had declined to 7.6 km altitude. By 1200 the lower-altitude portion of the ash plume had drifted 125 km E. Significant ash emissions ended by 1330 based on webcam images. The ACC was lowered to Orange and the VAL to Watch at 1440. Satellite images showed extensive pyroclastic debris flows on most of the flanks that extended 1.2-3.3 km from the crater rim.

Figure 47. Webcam image taken from the S of Shishaldin showing a vertical ash plume on 5 September 2023. Courtesy of AVO.

During 6-13 September elevated surface temperatures continued to be observed in satellite data, seismicity remained elevated with weak but steady tremor, and small, low-frequency earthquakes and small explosions were reported, except on 12 September. On 6 September a low-level ash plume rose to 1.5-1.8 km altitude and drifted SSE. Occasional small and diffuse gas-and-steam emissions at the summit were visible in webcam images. Around 1800 on 13 September seismic tremor amplitudes began to increase, and small explosions were detected in seismic and infrasound data. Incandescent lava at the summit was seen in a webcam image taken at 0134 on 14 September during a period of elevated tremor. No ash emissions were reported during the period of elevated seismicity. Lava fountaining began around 0200, based on webcam images. Satellite-based radar observations showed that the lava fountaining activity led to the growth of a cone in the summit crater, which refilled most of the crater. By 0730 seismicity significantly declined and remained at low levels.

Seismic tremor began to increase around 0900 on 15 September and rapidly intensified. An explosive eruption began at around 1710, which prompted AVO to raise the ACC to Red and the VAL to Warning. Within about 30 minutes ash plumes drifted E below a weather cloud at 8.2 km altitude. The National Weather Service estimated that an ash-rich plume rose as high as 12.8 km altitude and produced volcanic lightning. The upper part of the ash plume detached from the vent around 1830 and drifted E, and was observed over the Gulf of Alaska. Around the same time, seismicity dramatically decreased. Trace ashfall was reported in the community of False Pass (38 km ENE) between 1800-2030 and also in King Cove and nearby marine waters. Activity declined at around 1830 although seismicity remained elevated, ash emissions, and ashfall continued until 2100. Lightning was again detected beginning around 1930, which suggested that ash emissions continued. Ongoing explosions were detected in infrasound data, at a lower level than during the most energetic phase of this event. Lightning was last detected at 2048. By 2124 the intensity of the eruption had decreased, and ash emissions were likely rising to less than 6.7 km altitude. Seismicity returned to pre-eruption levels. On 16 September the ACC was lowered to Orange and the VAL to Watch at 1244; the sulfur dioxide plume that was emitted from the previous eruption event was still visible over the northern Pacific Ocean. Elevated surface temperatures, gas-and-steam emissions from the vent, and new, small lahars were reported on the upper flanks based on satellite and webcam images. Minor deposits were reported on the flanks which were likely the result of collapse of previously accumulated lava near the summit crater.

Elevated seismicity with tremor, small earthquakes, and elevated surface temperatures were detected during 17-23 September. Minor gas-and-steam emissions were visible in webcam images. On 20 September small volcanic debris flows were reported on the upper flanks. On 21 September a small ash deposit was observed on the upper flanks extending to the NE based on webcam images. Seismic tremor increased significantly during 22-23 September. Regional infrasound sensors suggested that low-level eruptive activity was occurring within the summit crater by around 1800 on 23 September. Even though seismicity was at high levels, strongly elevated surface temperatures indicating lava at the surface were absent and no ash emissions were detected; weather clouds at 0.6-4.6 km altitude obscured views. At 0025 on 24 September AVO noted that seismicity continued at high levels and nearly continuous small infrasound signals began, likely from low-level eruptive activity. Strongly elevated surface temperatures were identified in satellite images by 0900 and persisted throughout the day; the higher temperatures along with infrasound and seismic data were consistent with lava erupting at the summit. Around 1700 similarly elevated surface temperatures were detected from the summit in satellite data, which suggested that more vigorous lava fountaining had started. Starting around 1800 low-level ash emissions rose to altitudes less than 4.6 km altitude and quickly dissipated.

Beginning at midnight on 25 September, a series of seismic signals consistent with volcanic flows were recorded on the N side of the volcano. A change in seismicity and infrasound signals occurred around 0535 and at 0540 a significant ash cloud formed and quickly reached 14 km altitude and drifted E along the Alaska Peninsula. The cloud generated at least 150 lightning strokes with thunder that could be heard by people in False Pass. Seismicity rapidly declined to near background levels around 0600. AVO increased the ACC to Red and the VAL to Warning at 0602. The ash cloud detached from the volcano at around 0700, rose to 11.6 km altitude, and drifted ESE. Trace to minor amounts of ashfall were reported by the communities of False Pass, King Cove, Cold Bay, and Sand Point around 0700. Ash emissions continued at lower altitudes of 6-7.6 km altitude at 0820. Small explosions at the vent area continued to be detected in infrasound data and likely represented low-level eruptive activity near the vent. Due to the significant decrease in seismicity and ash emissions the ACC was lowered to Orange and the VAL to Watch at 1234. Radar data showed significant collapses of the crater that occurred on 25 September. Satellite data also showed significant hot, degassing pyroclastic and lahar deposits on all flanks, including more extensive flows on the ENE and WSW sections below two new collapse scarps. Following the significant activity during 24-25 September, only low-level activity was observed. Seismicity decreased notably near the end of the strong activity on 25 September and continued to decrease through the end of the month, though tremor and small earthquakes were still reported. No explosive activity was detected in infrasound data through 2 October. Gas-and-steam emissions rose to 3.7 km altitude, as reported by pilots and seen in satellite images. Satellite data from 26 September showed that significant collapses had occurred at the summit crater and hot, steaming deposits from pyroclastic flows and lahars were present on all the flanks, particularly to the ENE and WSW. A small ash cloud was visible in webcam images on 27 September, likely from a collapse at the summit cone. High elevated surface temperatures were observed in satellite imagery during 27-28 September, which were likely the result of hot deposits on the flanks erupted on 25 September. Minor steaming at the summit crater and from an area on the upper flanks was visible in webcam images on 28 September.

During October, explosion events continued between periods of low activity. Seismicity significantly increased starting at around 2100 on 2 October; around the same time satellite images showed an increase in surface temperatures consistent with lava fountaining. Small, hot avalanches of rock and lava descended an unspecified flank. In addition, a distinct increase in infrasound, seismicity, and lightning detections was followed by an ash plume that rose to 12.2 km altitude and drifted S and E at 0520 on 3 October, based on satellite images. Nighttime webcam images showed incandescence due to lava fountaining at the summit and pyroclastic flows descending the NE flank. AVO reported that a notable explosive eruption started at 0547 and lasted until 0900 on 3 October, which prompted a rise in the ACC to Red and the VAL to Warning. Subsequent ash plumes rose to 6-7.6 km altitude by 0931. At 1036 the ACC was lowered back to Orange and the VAL to Watch since both seismic and infrasound data quieted substantially and were slightly above background levels. Gas-and-steam emissions were observed at the summit, based on webcam images. Trace amounts of ashfall were observed in Cold Bay. Resuspended ash was present at several kilometers altitude near the volcano. During the afternoon, low-level ash plumes were visible at the flanks, which appeared to be largely generated by rock avalanches off the summit crater following the explosive activity. These ash plumes rose to 3 km altitude and drifted W. Trace amounts of ashfall were reported by observers in Cold Bay and Unalaska and flights to these communities were disrupted by the ash cloud. Satellite images taken after the eruption showed evidence of pyroclastic flows and lahar deposits in drainages 2 km down the SW flank and about 3.2 km down the NE flank, and continued erosion of the crater rim. Small explosion craters at the end of the pyroclastic flows on the NE flank were noted for the first time, which may have resulted from gas-and-steam explosions when hot deposits interact with underlying ice.

During 4 October seismicity, including frequent small earthquakes, remained elevated, but was gradually declining. Ash plumes were produced for over eight hours until around 1400 that rose to below 3.7 km altitude. These ash plumes were primarily generated off the sides of the volcano where hot rock avalanches from the crater rim had entered drainages to the SW and NE. Two explosion craters were observed at the base of the NE deposits about 3.2 km from the crater rim. Webcam images showed the explosion craters were a source of persistent ash emissions; occasional collapse events also generated ash. Seismicity remained elevated with sulfur dioxide emissions that had a daily average of more than 1,000 tons per day, and frequent small earthquakes through the end of the month. Frequent elevated surface temperatures were identified in satellite images and gas-and-steam plumes were observed in webcam images, although weather conditions occasionally prevented clear views of the summit. Emissions were robust during 14-16 October and were likely generated by the interaction of hot material and snow and ice. During the afternoon of 21 October a strong gas-and-steam plume rose to 3-4.6 km altitude and extended 40 km WSW, based on satellite images and reports from pilots. On 31 October the ACC was lowered to Yellow and the VAL was lowered to Advisory.

Activity in November was characterized by elevated seismicity with ongoing seismic tremor and small, low-frequency earthquakes, elevated surface temperatures, and gas-and-steam emissions. There was an increase in seismic and infrasound tremor amplitudes starting at 1940 on 2 November. As a result, the ACC was again raised to Orange and the VAL was increased to Watch, although ash was not identified in satellite data. An ash cloud rose to 6.1 km altitude and drifted W according to satellite data at 2000. By 0831 on 3 November ash emissions were no longer visible in satellite images. On 6 and 9 November air pressure sensors detected signals consistent with small explosions. Small explosions were detected in infrasound data consistent with weak Strombolian activity on 19 and 21 November. Seismicity started to decrease on 21 November. On 25 November gas-and-steam emissions were emitted from the vent as well as from a scarp on the NE side of the volcano near the summit. A gas-and-steam plume extended about 50 km SSE and was observed in satellite and webcam images on 26 November. On 28 November small explosions were observed in seismic and local infrasound data and gas-and-steam emissions were visible from the summit and from the upper NE collapse scarp based on webcam images. Possible small explosions were observed in infrasound data on 30 November. Weakly elevated surface temperatures and a persistent gas-and-steam plume from the summit and collapse scarps on the upper flanks. A passing aircraft reported the gas-and-steam plume rose to 3-3.4 km altitude on 30 November, but no significant ash emissions were detected.

Satellite data. MODIS thermal anomaly data provided through MIROVA (Middle InfraRed Observation of Volcanic Activity) showed a strong pulse of thermal activity beginning in July 2023 that continued through November 2023 (figure 48). This strong activity was due to Strombolian explosions and lava fountaining events at the summit crater. According to data from MODVOLC thermal alerts, a total of 101 hotspots were detected near the summit crater in July (11-14, 16-19, 23-24 and 26), August (4, 25-26, and 29), September (5, 12, and 17), and October (3, 4, and 8). Infrared satellite data showed large lava flows descending primarily the northern and SE flanks during the reporting period (figure 49). Sulfur dioxide plumes often exceeded two Dobson Units (DUs) and drifted in different directions throughout the reporting period, based on satellite data from the TROPOMI instrument on the Sentinel-5P satellite (figure 50).

Figure 48. Graph of Landsat 8 and 9 OLI thermal data from 1 June 2024 showing a strong surge in thermal activity during July through November 2023. During mid-October, the intensity of the hotspots gradually declined. Courtesy of MIROVA.

Figure 49. Infrared (bands B12, B11, B4) satellite images show several strong lava flows (bright yellow-orange) affecting the northern and SE flanks of Shishaldin on 18 July 2023 (top left), 4 June 2023 (top right), 26 September 2023 (bottom left), and 3 October 2023 (bottom right). Courtesy of Copernicus Browser.

Figure 50. Strong sulfur dioxide plumes were detected at Shishaldin and drifted in different directions on 15 August 2023 (top left), 5 September 2023 (top right), 25 September 2023 (bottom left), and 6 October 2023 (bottom right). Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. The symmetrical glacier-covered Shishaldin in the Aleutian Islands is the westernmost of three large stratovolcanoes in the eastern half of Unimak Island. The Aleuts named the volcano Sisquk, meaning "mountain which points the way when I am lost." Constructed atop an older glacially dissected edifice, it is largely basaltic in composition. Remnants of an older edifice are exposed on the W and NE sides at 1,500-1,800 m elevation. There are over two dozen pyroclastic cones on its NW flank, which is covered by massive aa lava flows. Frequent explosive activity, primarily consisting of Strombolian ash eruptions from the small summit crater, but sometimes producing lava flows, has been recorded since the 18th century. A steam plume often rises from the summit crater.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA (URL: https://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 (URL: http://dggs.alaska.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/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Ioto (Iwo-jima), located about 1,200 km S of Tokyo, lies within a 9-km-wide submarine caldera along the Izu-Bonin-Mariana volcanic arc. Previous eruptions date back to 1889 and have consisted of dominantly phreatic explosions, pumice deposits during 2001, and discolored water. A submarine eruption during July through December 2022 was characterized by discolored water, pumice deposits, and gas emissions (BGVN 48:01). This report covers a new eruption during October through December 2023, which consisted of explosions, black ejecta, discolored water, and floating pumice, based on information from the Japan Meteorological Association (JMA), the Japan Coast Guard (JCG), and satellite data.

JMA reported that an eruption had been occurring offshore of Okinahama on the SE side of the island since 21 October, which was characterized by volcanic tremor, according to the Japan Maritime Self-Defense Force (JMSDF) Iwo Jima Air Base (figure 22). According to an 18 October satellite image a plume of discolored water at the site of this new eruption extended NE (figure 23). During an overflight conducted on 30 October, a vent was identified about 1 km off the coast of Okinahama. Observers recorded explosions every few minutes that ejected dark material about 20 m above the ocean and as high as 150 m. Ejecta from the vent formed a black-colored island about 100 m in diameter, according to observations conducted from the air by the Earthquake Research Institute of the University of Tokyo in cooperation with the Mainichi newspaper (figure 24). Occasionally, large boulders measuring more than several meters in size were also ejected. Observations from the Advanced Land Observing Satellite Daichi-2 and Sentinel-2 satellite images also confirmed the formation of this island (figure 23). Brown discolored water and floating pumice were present surrounding the island.

Figure 22. Map of Ioto showing the locations of recorded eruptions from 1889 through December 2023. The most recent eruption occurred during October through December 2023 and is highlighted in red just off the SE coast of the island and E of the 2001 eruption site. A single eruption highlighted in green was detected just off the NE coast of the island on 18 November 2023. From Ukawa et al. (2002), modified by JMA.

Figure 23. Satellite images showing the formation of the new island formation (white arrow) off the SE (Okinahama) coast of Ioto on 18 October 2023 (top left), 27 November 2023 (top right), 2 December 2023 (bottom left), and 12 December 2023 (bottom right). Discolored water was visible surrounding the new island. By December, much of the island had been eroded. Courtesy of Copernicus Browser.

Figure 24. Photo showing an eruption off the SE (Okinahama) coast of Ioto around 1230 on 30 October 2023. A column of water containing black ejecta is shown, which forms a new island. Occasionally, huge boulders more than several meters in size were ejected with the jet. Dark brown discolored water surrounded the new island. Photo has been color corrected and was taken from the S by the Earthquake Research Institute, University of Tokyo in cooperation of Mainichi newspaper. Courtesy of JMA.

The eruption continued during November. During an overflight on 3 November observers photographed the island and noted that material was ejected 169 m high, according to a news source. Explosions gradually became shorter, and, by the 3rd, they occurred every few seconds; dark and incandescent material were ejected about 800 m above the vent. On 4 November eruptions were accompanied by explosive sounds. Floating, brown-colored pumice was present in the water surrounding the island. There was a brief increase in the number of volcanic earthquakes during 8-14 November and 24-25 November. The eruption temporarily paused during 9-11 November and by 12 November eruptions resumed to the W of the island. On 10 November dark brown-to-dark yellow-green discolored water and a small amount of black floating material was observed (figure 25). A small eruption was reported on 18 November off the NE coast of the island, accompanied by white gas-and-steam plumes (figure 23). Another pause was recorded during 17-19 November, which then resumed on 20 November and continued erupting intermittently. According to a field survey conducted by the National Institute for Disaster Prevention Science and Technology on 19 November, a 30-m diameter crater was visible on the NE coast where landslides, hot water, and gray volcanic ash containing clay have occurred and been distributed previously. Erupted blocks about 10 cm in diameter were distributed about 90-120 m from the crater. JCG made observations during an overflight on 23 November and reported a phreatomagmatic eruption. Explosions at the main vent generated dark gas-and-ash plumes that rose to 200 m altitude and ejected large blocks that landed on the island and in the ocean (figure 26). Discolored water also surrounded the island. The size of the new island had grown to 450 m N-S x 200 m E-W by 23 November, according to JCG.

Figure 25. Photo of the new land formed off the SE (Okinahama) coast of Ioto on 10 November showing discolored water and a small amount of black floating material were visible surrounding the island. Photo has been color corrected. Photographed by JCG courtesy of JMA.

Figure 26. Photo of the new land formed off the SE (Okinahama) coast of Ioto on 23 November showing a phreatomagmatic eruption that ejected intermittent pulses of ash and dark material that rose to 200 m altitude. Photo has been color corrected. Photographed by JCG courtesy of JMA.

The eruption continued through 11 December, followed by a brief pause in activity, which then resumed on 31 December, according to JMA. Intermittent explosions produced 100-m-high black plumes at intervals of several minutes to 30 minutes during 1-10 December. Overflights were conducted on 4 and 15 December and reported that the water surrounding the new island was discolored to dark brown-to-dark yellow-green (figure 27). No floating material was reported during this time. In comparison to the observations made on 23 November, the new land had extended N and part of it had eroded away. In addition, analysis by the Geospatial Information Authority of Japan using SAR data from Daichi-2 also confirmed that the area of the new island continued to decrease between 4 and 15 December. Ejected material combined with wave erosion transformed the island into a “J” shape, 500-m-long and with the curved part about 200 m offshore of Ioto. The island was covered with brown ash and blocks, and the surrounding water was discolored to greenish-brown and contained an area of floating pumice. JCG reported from an overflight on 4 December that volcanic ash-like material found around the S vent on the NE part of the island was newly deposited since 10 November (figure 28). By 15 December the N part of the “J” shaped island had separated and migrated N, connecting to the Okinahama coast and the curved part of the “J” had eroded into two smaller islands (figure 27).

Figure 27. Photos of the new island formed off the SE (Okinahama) coast of Ioto on 4 December 2023 (left) and 15 December 2023 (right). No gas-and-ash emissions or lava flows were observed on the new land. Additionally, dark brown-to-dark yellow-green discolored water was observed surrounding the new land. During 4 and 15 December, the island had eroded to where the N part of the “J” shape had separated and migrated N, connecting to the Okinahama coast and the curved part of the “J” had eroded into two smaller islands. Courtesy of Copernicus Browser.

Figure 28. Photo of new volcanic ash-deposits (yellow dashed lines) near the S vent on the NE coast of Ioto taken by JCG on 4 December 2023. White gas-and-steam emissions were also visible (white arrow). Photo has been color corrected. Courtesy of JMA.

References. Ukawa, M., Fujita, E., Kobayashi, T., 2002, Recent volcanic activity of Iwo Jima and the 2001 eruption, Monthly Chikyu, Extra No. 39, 157-164.

Geologic Background. Ioto, in the Volcano Islands of Japan, lies within a 9-km-wide submarine caldera. The volcano is also known as Ogasawara-Iojima to distinguish it from several other "Sulfur Island" volcanoes in Japan. The triangular, low-elevation, 8-km-long island narrows toward its SW tip and has produced trachyandesitic and trachytic rocks that are more alkalic than those of other volcanoes in this arc. The island has undergone uplift for at least the past 700 years, accompanying resurgent doming of the caldera; a shoreline landed upon by Captain Cook's surveying crew in 1779 is now 40 m above sea level. The Motoyama plateau on the NE half of the island consists of submarine tuffs overlain by coral deposits and forms the island's high point. Many fumaroles are oriented along a NE-SW zone cutting through Motoyama. Numerous recorded phreatic eruptions, many from vents on the W and NW sides of the island, have accompanied the uplift.

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); Japan Coast Guard (JCG) Volcano Database, Hydrographic and Oceanographic Department, 3-1-1, Kasumigaseki, Chiyoda-ku, Tokyo 100-8932, Japan (URL: https://www1.kaiho.mlit.go.jp/GIJUTSUKOKUSAI/kaiikiDB/kaiyo22-2.htm); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Asahi, 5-3-2, Tsukiji, Chuo Ward, Tokyo, 104-8011, Japan (URL: https://www.asahi.com/ajw/articles/15048458).

Sixteen explosions were recorded in October. No explosions had been recorded for about a month after a large explosion on 24 September, but the level of activity increased toward the end of October. Ash plumes rose to 3,000 m above the crater of Minami-dake on 29 and 31 October. Ten air shocks were felt and seven explosion sounds were heard during the month. October ash accumulation was 619 g/m² at [KLMO].

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 caldera, along with several post-caldera cones. The construction of Sakurajima began about 13,000 years ago on the southern rim and built an island that was joined to the Osumi Peninsula during the major explosive and effusive eruption of 1914. Activity at the Kitadake summit cone ended about 4,850 years ago, after which eruptions took place at Minamidake. Frequent eruptions since the 8th century have deposited ash on the city of Kagoshima, located across Kagoshima Bay only 8 km from the summit. The largest recorded eruption took place during 1471-76.

Information Contacts: JMA.

Vigorous explosions were reported by the beginning of October, continuing to average ~5/day through the month.

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: I. Boschini and G. Alvarado, ICE.

Stratospheric aerosol concentrations measured by lidar from Hawaii, Virginia, and Germany in October remained stable or declined slightly from the previous month (figure 48). Integrated backscatter over Mauna Loa, Hawaii on 27 October was the lowest since the 1982 eruption of El Chichón.

Figure 48. Lidar data from Virginia, Hawaii, and Germany, showing altitudes of aerosol layers during October-November 1987. Note that some layers have multiple peaks. Backscattering ratios are for the ruby wavelength of 0.69 µm. Integrated values show total backscatter, expressed in steradians-1, integrated over 300-m intervals from 16-33 km at Mauna Loa and from the tropopause to 30 km at Hampton, Virginia. Altitudes of maximum backscattering ratios and coefficients are shown for each layer at Mauna Loa.

Geologic Background. The enormous aerosol cloud from the March-April 1982 eruption of Mexico's El Chichón persisted for years in the stratosphere, and led to the Atmospheric Effects section becoming a regular feature of the Bulletin. Descriptions of the initial dispersal of major eruption clouds remain with the individual eruption reports, but observations of long-term stratospheric aerosol loading will be found here.

Information Contacts: Thomas DeFoor, Mauna Loa Observatory, P.O. Box 275, Hilo, HI 96720 USA; William Fuller, NASA Langley Research Center, Hampton, VA 23665 USA; Horst Jäger, Fraunhofer-Institut für Atmosphärische Umweltforschung, Kreuzeckbahnstrasse 19, D-8100 Garmisch-Partenkirchen, West Germany.

Following the September pyroclastic avalanches, Bagana emitted continuous moderate-strong white vapour for most of October. Weak glow from the summit was visible during the first 2 weeks of the month. Seismicity was at a low level, except from 15-19 October when a total of 75 events were recorded.

Geologic Background. Bagana volcano, in a remote portion of central Bougainville Island, is frequently active. This massive symmetrical cone was largely constructed by an accumulation of viscous andesitic lava flows. The entire edifice could have been constructed in about 300 years at its present rate of lava production. Eruptive activity is characterized by non-explosive effusion of viscous lava that maintains a small lava dome in the summit crater, although occasional explosive activity produces pyroclastic flows. Lava flows with tongue-shaped lobes up to 50 m thick and prominent levees descend the flanks on all sides.

Information Contacts: J. Mori, D. Lolok, and P. Lowenstein, RVO.

Rockslides and fumarolic activity were observed 11-12 October and gas plumes with some ash reached 1 km height. A NOAA 9 satellite image returned at 0518 on 13 October showed a very thin, weak plume drifting 60-80 km NE.

Geologic Background. The modern Bezymianny, much smaller than its massive neighbors Kamen and Kliuchevskoi on the Kamchatka Peninsula, was formed about 4,700 years ago over a late-Pleistocene lava-dome complex and an edifice built about 11,000-7,000 years ago. Three periods of intensified activity have occurred during the past 3,000 years. The latest period, which was preceded by a 1,000-year quiescence, began with the dramatic 1955-56 eruption. This eruption, similar to that of St. Helens in 1980, produced a large open crater that was formed by collapse of the summit and an associated lateral blast. Subsequent episodic but ongoing lava-dome growth, accompanied by intermittent explosive activity and pyroclastic flows, has largely filled the 1956 crater.

Information Contacts: S. Fedotov and I. Menyailov, IV; W. Gould, NOAA/NESDIS.

Although a NOAA 10 weather satellite image on 14 July showed a cloud that extended ~200 km from the vicinity of Una Una, no reports were received of an eruption from the island, uninhabited since residents were evacuated before the paroxysmal explosion of 23 July 1983.

Yosihiro Sawada inspected 14-15 July imagery from the GMS weather satellite. By 0800 on 14 July, less than an hour after the NOAA 10 image, the cloud retained a plume-like structure, but its origin was several tens of kilometers W of Una Una. An infrared image 24 hours later showed a chain of weather clouds extending SW from several sources just W of Una Una. On the image returned three hours later, these clouds had combined to form a plume-like feature similar to the one seen the previous day. This evidence, and the absence of a reported eruption, suggested that weather clouds may have produced the 14 July plume.

Geologic Background. Colo volcano forms the isolated small island of Una Una in the middle of the Gulf of Tomini in northern Sulawesi. The broad, low volcano contains a 2-km-wide caldera with a small central cone. Three eruptions have been recorded since the late 1800's, but two of those caused widespread damage over the island. The last eruption, in 1983, produced pyroclastic flows that swept over most of the island shortly after all residents had been evacuated.

Information Contacts: Y. Sawada, JMA.

Beginning 14 October, two vents on a sheer wall of the N crater began ejecting fine ash and steam to 300 m above the summit. Pulsations of activity occurred 3 or 4 times a day. Ejections reached 1 km height, with a plume that extended horizontally 10-15 km NE. Fissures that formed on the outer E flank of the crater emitted steam. No seismicity preceded or accompanied the ash-steam ejections. The press reported that ash and gas began to be emitted from the main crater before 2100 on 19 October, and the crater lake was emptied of water. A 1-km-high ash column formed on 27 October and the next day a small amount of ash fell on the town of Severo-Kurilsk (a few kilometers away). No casualties resulted from the eruptions. Additional equipment was installed to monitor the volcano. Variations in gas composition of near-crater fumaroles were observed 4 years ago, and an increase in the temperatures of fumarole emissions has been noted since the October eruption. The press reported that scientists did not rule out the possiblilty of a lahar which could pose a danger to the S part of Severo-Kurilsk.

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

Information Contacts: S. Fedotov and I. Menyailov, IV; G. Steinberg, Yuzhno-Sakhalinsk; Moscow Domestic Service; Soviet Sakhalin newspaper.

JMSA observed pale green discolored water above the volcano on 17 September.

Geologic Background. Fukutoku-Oka-no-ba is a submarine volcano located 5 km NE of the island of Minami-Ioto. Water discoloration is frequently observed, and several ephemeral islands have formed in the 20th century. The first of these formed Shin-Ioto ("New Sulfur Island") in 1904, and the most recent island was formed in 1986. The volcano is part of an elongated edifice with two major topographic highs trending NNW-SSE, and is a trachyandesitic volcano geochemically similar to Ioto.

Information Contacts: Y. Sawada, JMA; D. Shackelford, Fullerton, CA.

A large explosion on 2 October at 2252 deposited a light dusting of ash in about a 15 km² area around the volcano. During the week of 9-15 October, white fume was continuously present above the crater, rising to heights of 300 m. During that week, about 13 tectonic and 11 volcanic events were recorded/day.

Geologic Background. Iliboleng stratovolcano was constructed at the SE end of Adonara Island across a narrow strait from Lomblen Island. The volcano is capped by multiple, partially overlapping summit craters. Lava flows modify its profile, and a cone low on the SE flank, Balile, has also produced lava flows. Historical eruptions, first recorded in 1885, have consisted of moderate explosive activity, with lava flows accompanying only the 1888 eruption.

Information Contacts: VSI.

On 16 November at 1047 an eruption began with two explosions that sent a black ash and gas column to at least 2,500 m above the summit. Small eruptions continued every 5-10 minutes. Roads leading to the mountain were closed and about 150 tourists and others in the summit area were evacuated to a hotel on the N side of the crater at 1100. Heavy ashfall occurred on the S part of the island and more than 30 volcanic clasts fell near a shrine on the crater rim causing several small fires. A window was broken at a tea house near the crater but there were no reports of casualties or major damage. JMA recorded >1,000 small earthquakes before the eruption, and weak earthquakes continued afterwards. During an earthquake swarm in early October [12:9] the maximum daily number of recorded earthquakes reached twice the previous maximum for 1987.

The JMSA and the Japan Maritime Self-Defense Force (JMSDF) mobilized patrol boats and escort vessels off the island.

Geologic Background. Izu-Oshima volcano in Sagami Bay, east of the Izu Peninsula, is the northernmost of the Izu Islands. The broad, low stratovolcano forms an 11 x 13 km island constructed over the remnants of three older dissected stratovolcanoes. It is capped by a 4-km-wide caldera with a central cone, Miharayama, that has been the site of numerous recorded eruptions datining back to the 7th century CE. More than 40 cones are located within the caldera and along two parallel rift zones trending NNW-SSE. Although it is a dominantly basaltic volcano, strong explosive activity has occurred at intervals of 100-150 years throughout the past few thousand years. A major eruption in 1986 produced spectacular lava fountains up to 1,600 m high and a 16-km-high eruption column; more than 12,000 people were evacuated from the island.

Information Contacts: A. Izumo, Yokohama Science Center; Japan Times, Tokyo; UPI.

Activity . . . continued through October with three flows moving toward the coast. The westernmost lobe, up to 5 m thick in places, broadened but mostly stayed on top of September flows. The flow intermittently entered Royal Gardens subdivision, and the footpath from the road into the housing area was covered in late October, leaving the few remaining residents with no way of reaching their homes. The middle lobe was entering the ocean inside the national park. The new ocean front was ~700 m wide W of Kupapau point, moving W along the seacliffs toward the national park visitor center (at Wahaula; figure 53). The E lobe has completely buried the 1984 aa flow, and was intermittently entering the ocean 600 m E of Kupapau point along a front ~50 m wide. The level of the lava pond . . . was fairly constant, from nearly full to 6 m below the rim. There have been no overflows from the pond and all lava production has been carried away in the tube system.

Figure 53. Sketch map of part of the SE coast of Hawaii with flows from the new eruptive vent in the lava shield, November 1986-January 1988. The dotted line shows the location of the new shoreline.

Harmonic tremor continued at low and constant amplitude . . . . The number of shallow microearthquakes was about average in the summit region and East rift zone. Most of the earthquakes . . . in October were centered along the S flank of Kīlauea and SE flank of Mauna Loa at intermediate depths of 5-10 km. Twelve events with preliminary magnitudes of 2.5-4.1 were the strongest during the month.

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

Information Contacts: C. Heliker and R. Koyanagi, HVO.

Many small volcanic earthquakes were recorded 14-19 October by the seismograph 1 km NE of Yugama, the largest of the three crater lakes. The monthly total of volcanic earthquakes reached 114, compared to the monthly average of less than 20. Seismic activity declined to a low level after 22 October. Epicenters were estimated to be several hundred meters E of Yugama. JMA's Mobile Volcanological Observation Team established two additional seismographs and began 10 days of observation on 18 October. No significant changes in fumarolic activity were observed during field surveys on 23 and 26 October.

A phreatic explosion from Yugama crater ejected 20-cm blocks to 600-700 m from the crater lake on 13 November 1983. Small phreatic explosions occurred from Yugama and Karegama (the E crater lake) on 26 July and 21 December 1983.

Geologic Background. The Kusatsu-Shiranesan complex, located immediately north of Asama volcano, consists of a series of overlapping pyroclastic cones and three crater lakes. The andesitic-to-dacitic volcano was formed in three eruptive stages beginning in the early to mid-Pleistocene. The Pleistocene Oshi pyroclastic flow produced extensive welded tuffs and non-welded pumice that covers much of the E, S, and SW flanks. The latest eruptive stage began about 14,000 years ago. Historical eruptions have consisted of phreatic explosions from the acidic crater lakes or their margins. Fumaroles and hot springs that dot the flanks have strongly acidified many rivers draining from the volcano. The crater was the site of active sulfur mining for many years during the 19th and 20th centuries.

Information Contacts: JMA.

Activity . . . continued at a low level in October except for a single explosion on the 9th, which produced a dark brown ash column ~300 m high. Weak-moderate white vapour was emitted continually from Crater 2 during the month, while Crater 3 was quiet.

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: J. Mori, D. Lolok, and P. Lowenstein, RVO.

The following report on August and September activity . . . is from sources (in parentheses) who have climbed the volcano several times in the past few years.

Between 2 and 4 August a group visited the floor of the N crater and noted major morphologic changes since they last saw it in December 1985 and May 1986. They estimated that new lava flows had raised the crater floor ~40 m. The saddle that separates the larger, active part of the N crater from its small S corner or shelf (figure 3) was only ~7-8 m high . . . . Highly liquid, very hot, jet-black lava splashed out from the single active vent in the S central crater floor (1/3 of the way across the crater from the S edge), and formed small thin flows. The group estimated that ~25% of the crater floor had been covered by liquid lava during the 24 hours preceding their visit. Two inactive vents were observed closer to the N side of the crater floor in addition to many fumaroles. However, the party did not see any large holes in the crater floor or hear any of the rumbling noises that they noted in December 1985. (Bill Waldron)

During a visit 9-10 September 1987, lava bubbled up and flowed from one vent on the N side of the crater. A number of small inactive vents were noted on other parts of the crater floor. An observer who paced the crater floor estimated the dimensions to be 275 x 260 m and confirmed that the slope from the S saddle down into the N crater floor was on the order of 7 m high. (Thad Peterson)

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

Information Contacts: C. Nyamweru, Kenyatta Univ.

Harmon Craig provided additional observations of the 11-12 October eruption. When the RV Melville first arrived . . . on 11 October, bright green discolored water had spread in a crescent-shaped zone ~2.5 km wide that extended 1.5 km from a point source. The water remained discolored during the 2 days that the ship was in the area. Episodes of erupting gas bubbles were most intense during the afternoon, with bubbles rising sporadically at 10-minute to 1-hour intervals. Following eruptions of bubbles, an area of chocolate-brown water, perhaps 100 m across, appeared in the center of the green discolored water. Craig saw a total of ~30-40 basaltic rocks brought to the surface by gas bubbles. The depth to the summit was 30-100 m; the summit had a flat area at 100 m depth and pinnacles rose as much as 70 m above this surface.

The RSP reported that a seismic crisis began again on 14 October at [1200]. Continuous intense noise was recorded until the onset of explosive phenomena at about [1730], persisting until [0320 the next morning] when it was followed by weak but continuous activity that was ongoing as of 16 October.

Geologic Background. Discovered by the detection of teleseismic waves in 1967, Macdonald seamount (also known as Tamarii seamount) rises from a depth of about 1,800 m to within 27 m of the ocean surface at the eastern end of the Austral Islands. The basaltic submarine volcano marks the site of a hotspot that was the source of the Austral-Cook island chain. The summit, named after volcanologist Gordon Macdonald, consists of a flat plateau about 100 x 150 m wide with an average depth of about 40 m. The summit plateau is capped with spatter cones that form steep-sided pinnacles. Most eruptions have been seismically detected, but in 1987 and 1989 pumice emission was observed from research vessels. Pumice rafts observed in the South Pacific in 1928 and 1936 may also have originated here.

Information Contacts: H. Craig, Scripps Institute of Oceanography; J. Talandier, LDG Tahiti.

There was a marked, though moderate increase in the level of activity at the beginning of the month. Seismic amplitudes began a progressive increase during the first week of October, when visible changes were first noticed. A weak fluctuating night glow at Southern Crater was accompanied by low rumbling and an increase in the rate of vapor emission (including blue vapour). Main Crater showed no significant change in the intensity of night glow but the amount of blue and white-gray vapor increased. Seismic amplitudes remained steady and elevated through the end of the month.

Radial tilt, which had shown a steady rise since August, showed a rapid deflation of ~3 µrad after 15 October. Deflation was continuing at the end of the month.

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: J. Mori, D. Lolok, and P. Lowenstein, RVO.

"Mauna Loa's latest eruption was in March-April 1984 on the Northeast rift zone. The eruption was associated with a large collapse (figure 10) and seismicity that peaked during and following the eruption (figure 11). Since 1984 the volcano has begun to reinflate, as shown by outward tilt and horizontal extension of Mauna Loa's summit area.

Figure 10. EDM data along three lines across Mauna Loa's summit caldera, January 1983-4 November 1987. HVO 93 is on the NW rim of the caldera, and the three lines cross the caldera in ESE, SE, and S directions, respectively, to the SE rim. Courtesy of HVO.

Figure 11. Daily number of recorded short-period (top) and long-period (middle) summit microearthquakes, and NE rift events (bottom) at Mauna Loa, January 1984-4 November 1987. Courtesy of HVO.

Seismicity. "Following the increase and peak in seismicity of the last eruption in Mar-Apr 1984, the number of shallow microearthquakes had slowly decreased (figure 11). Most of the post-eruption events were attributed to the gradual structural adjustments from the major deflation at the summit (resulting from the voluminous magma withdrawal) and the principal eruptive vent near Pu'u Ulaula on the Northeast rift zone. The post-eruption pattern of decreasing seismicity is indicated by the daily number of summit microearthquakes and Northeast rift events.

"There has been no significant seismic activity beneath the summit and rift zones of Mauna Loa since the 1984 eruption, and the present level of shallow seismicity is relatively low. There has been some increase in intermediate-depth events beneath the volcano noticed over the past year. Most of the events are very small, recorded only on a few summit and rift stations, and essentially < 0.5 in magnitude.

Ground deformation. "Deformation studies show that Mauna Loa reinflation has been steady; at this writing the summit has recovered over 1/3 of the amount of subsidence that took place during the eruption, as measured by both dry tilt and cross-caldera EDM (figure 10).

"A forecast of the next Mauna Loa eruption will depend on two things: an increase in shallow earthquakes and tilt recovery. The last two eruptions showed a precursory period of 1 year (July 1975 eruption) and 4 years (March 1984 eruption) respectively from the time of increased seismicity to the onset of eruption. There is no absolute level of tilt recovery at which we can specify that Mauna Loa will erupt. However, we would consider roughly 90% recovery from the 1984 deflation would indicate a state of readiness to erupt. On the basis of the data shown here we would not expect a Mauna Loa eruption for at least five years. We will update this estimate as we continue to monitor tilt and seismicity . . . ."

Geologic Background. Massive Mauna Loa is a basaltic shield volcano that rises almost 9 km from the ocean floor to form the world's largest Holocene volcano. Flank eruptions typically occur from the lengthy NE and SW rift zones, and from the Moku'aweoweo summit is caldera, which is within an older and larger 6 x 8 km caldera. Two of the youngest large debris avalanches documented in Hawaii traveled nearly 100 km from Mauna Loa; the second of the Alika avalanches was emplaced about 105,000 years ago (Moore et al., 1989). Almost 90% of the surface of the volcano is covered by lavas less than 4,000 years old (Lockwood and Lipman, 1987). Beginning about 1,500 years ago, a series of voluminous overflows from a summit lava lake covered about 25% of the volcano's surface. Over the last 750 years, from shortly after the formation of Moku'aweoweo caldera until the present, an additional 25% of the volcano has been covered with lava flows, mainly from summit and NW rift zone vents.

Information Contacts: T. Wright, R. Koyanagi, and A. Okamura, HVO.

Discolored water within a 100-m radius of Myojin-sho, the central cone of the submarine volcano, was recognized by observers on a fishing boat at 1040 on 21 October. No discolored water had been seen at 0603 that day and none was seen the next day during a JMSA overflight.

Geologic Background. Beyonesu Rocks are part of the barely exposed rim of the largely submarine Myojinsho caldera. Formation of the 8-9 km wide caldera was followed by construction of a large (2.6 km3) lava dome and/or lava flow complex on the caldera floor, originally located at a depth of 1,000-1,100 m. Most eruptions recorded since the late-19th century have occurred from the dacitic post-caldera Myojinsho lava dome on the NE caldera rim. Deposits from submarine pyroclastic flows associated with growth of the dome cover it and extend both into the NE part of the caldera and down its outer slopes. An explosive submarine eruption from Myojinsho in 1952 destroyed a Japanese research vessel, killing all 31 on board. Submarine eruptions have also been observed from other points on the caldera rim and outside of the caldera. The Beyonesu Rocks were named after the French warship the Bayonnaise, which was surveying volcanic islands south of Tokyo Bay in 1850.

Information Contacts: JMA.

Observations from the N foot of the volcano (8 km from the crater) on 7 September showed that the fuming from the upper NE flank was continuing. Summit fuming had visibly increased since 7 August observations, but fuming from the upper NE flank had decreased in volume. On 14 and 24 September, white vapors were seen rising from the E flank.

Geologic Background. Niigata-Yakeyama, one of several Japanese volcanoes named Yakeyama ("Burning Mountain"), is an andesitic-to-dacitic lava dome in Niigata prefecture in west-central Honshu, about 20 km from the coast. The volcano was constructed on a base of Tertiary mountains beginning about 3,100 years ago. Three major eruptions in the past 1,000 years have produced pyroclastic flows and lava flows that traveled mainly down the Hayakawa river valley to the N and NW. The first of these eruptions, in 887 CE (and possibly 989 CE), produced the Hayakawa pyroclastic flow, which reached the coast, and the massive Mae-yama lava flow, which traveled about 6.5 km down the Hayakawa river valley. The summit lava dome was emplaced during the 1361 CE eruption, and the last magmatic eruption took place in 1773 CE. Eruptive activity since 1773 has consisted of relatively minor phreatic explosions from several radial fissures and explosion craters that cut the summit and flanks of the dome.

Information Contacts: Y. Sawada, JMA; D. Shackelford, Fullerton, CA.

Activity increased 17 October when ash-rich explosions from the NW summit vent occurred every 30 seconds to several minutes. Ash plumes, reported through 9 November attained a maximum altitude of 4500-5800 m on 30 October (table 4). Plumes stretched E as much as 200 km and tephra fell at Canoe Bay, 45 km ENE, on 19 October. Blocks and spatter ejected to 30 m from the near-summit vent formed a short spatter flow. Night glow from the summit was visible on several evenings in mid-October. Incandescent rocks and spatter were reportedly continuing in early November. Ash deposits were last reported on 4 September (SEAN 12:09).

Table 4. Reports of activity at Pavlof, 16 October-27 November 1987, compiled by John Reeder and M.E. Yount from the following observers: Steve Hakala (Canoe Bay); Dan Coy (Sand Point Air); Scotty Gibbens and John Sarvis (USFWS); Deedee and Tom O'Malia (Cold Bay); Marsha Brown, Theresa Dubber, Chuck Taylor, Jim Yakal (FAA Flight Service, Cold Bay); Steve Shivers (USGS, Anchorage); James Gibson, George Wooliver, Chuck Nickerson, Gary Lintner, Harding (Reeve Aleutian Airways); Guy Morgan, Bryan Carricaburu, Doug Ruberg (Peninsula Airways); Harold Johnson Sr. (Nelson Lagoon); Will Gould (NOAA/NESDIS); John Sarvis (USFWS); Sand Point Air pilot; James Fredenhagen (Reeve Aleutian Airways);Coast Guard pilot. Unattributed reports were compiled by M.E. Yount.

Geologic Background. The most active volcano of the Aleutian arc, Pavlof is a Holocene stratovolcano that was constructed along a line of vents extending NE from the Emmons Lake caldera. Pavlof and Pavlof Sister to the NE form a dramatic pair of symmetrical, glacier-covered stratovolcanoes that overlook Pavlof and Volcano bays. Little Pavlof is a smaller cone on the SW flank of Pavlof volcano, near the rim of Emmons Lake caldera. Unlike Pavlof Sister, eruptions have frequently been reported from Pavlof, typically 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 recorded 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: J. Reeder, ADGGS; M.E. Yount, USGS Anchorage.

Seismicity remained at a very low level during October with only 33 events recorded. The 10 events large enough to be located occurred in the Greet Harbour and Vulcan areas . . . . Tilt data, EDM, levelling, and strand line measurements indicated that no ground deformation had occurred, apart from a very slight subsidence at the Vulcan Headland.

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 asymmetrical 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 1,400 years ago. An earlier caldera-forming eruption about 7,100 years ago is thought to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the N and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and W 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: J. Mori, D. Lolok, and P. Lowenstein, RVO.

. . . Crater Lake temperature fell. . . . to 28.5°C on 16 October. The lake was steaming and dark yellow-green (sulfur) slicks were periodically observed near the lake center. Deformation measurements indicated a minor 8 mm (inflationary) increase in crater diameter. Seismicity since 8 September has been dominated by low-amplitude 2 Hz volcanic tremor. No eruptive activity has been reported. During a visit on 28 September a geologist observed a seiche with amplitude of 50 mm and period of ~35 seconds. A similar seiche was observed on 17 October 1986.

Geologic Background. Ruapehu, one of New Zealand's most active volcanoes, is a complex stratovolcano constructed during at least four cone-building episodes dating back to about 200,000 years ago. The dominantly andesitic 110 km3 volcanic massif is elongated in a NNE-SSW direction and surrounded by another 100 km3 ring plain of volcaniclastic debris, including the NW-flank Murimoto debris-avalanche deposit. A series of subplinian eruptions took place between about 22,600 and 10,000 years ago, but pyroclastic flows have been infrequent. The broad summait area and flank contain at least six vents active during the Holocene. Frequent mild-to-moderate explosive eruptions have been recorded from the Te Wai a-Moe (Crater Lake) vent, and tephra characteristics suggest that the crater lake may have formed as recently as 3,000 years ago. Lahars resulting from phreatic eruptions at the summit crater lake are a hazard to a ski area on the upper flanks and lower river valleys.

Information Contacts: P. Otway, NZGS Wairakei; B. Scott, NZGS Rotorua.

A small ash emission occurred on 25 October at 0946. Seismicity had begun to migrate toward the crater in mid-October, and the activity was accompanied by a sudden increase in harmonic tremor that lasted for ~15 minutes. Tremor was otherwise very weak during October. During the month, 301 high-frequency, 593 low-frequency, and 125 shallow (explosion) events were recorded, up from 154, 533, and 15 in September. Changes in deformation remained small to moderate. Rates of SO₂ emission were higher than in any previous month of 1987, with median values of close to 2,000 t/d.

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: E. Parra, INGEOMINAS, Manizales.

Eruptive activity continued in October at a rate of ~18 explosions/day, with plumes reaching ~600 m above the crater rim. The number of daily explosions has declined steadily from 50-70/day in early 1987, and the October rate is the lowest since the eruption started in July 1985.

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 eruptions have been recorded since 1512, most of them during in the 20th century.

Information Contacts: VSI.

. . . Vulcanian activity in October.

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: VSI.

"Recorded earthquakes began increasing 30 October. A maximum of 19 events/day was recorded on 31 October; 5 of the events were felt by residents of Agoncillo, 10.7 km SW of the crater on the W shore of Taal Lake (figure 3). The events were recorded but not felt on the volcano island, the site of historic eruptions in the Taal Caldera. Some residents of Agoncillo also reported rumbling sounds accompanying the earthquakes. Isolated reports of sulfurous stench were unconfirmed. Many residents of Agoncillo and nearby villages described the quakes as having prominent vertical movements, similar to those prior to the 1965 eruption, prompting a few families to immediately evacuate to adjacent villages. These areas were among those devastated during the 1965 phreatic explosion.

Figure 3. Map of crater lake and vicinity, after Land Use of Taal Volcano and Environs, PHIVOLCS, 1:50,000.

"Events on 6 November had epicenters on Manalao Point on the W shore of Taal Lake and across the lake from the 1965-77 eruption site (Mt. Tabaro; figure 3). The earthquakes had depths of 6.4 km and suggested ground fracturing beneath the caldera.

"Thermal conditions and steam emissions of the main crater lake and the 1965-77 eruption site remained within normal levels. However, the water-tube tiltmeter on the island indicated a slight inflation-deflation episode of the main crater 20 April-28 October, with the pre-inflation position now being occupied. Likewise, thermal areas near the epicenter have remained unchanged. Lake-level monitoring stations around Taal Lake, some installed only last July, indicate some warping along the lakeshore, a condition that cannot yet be related to volcanic activity.

"PHIVOLCS had installed additional seismic and ground deformation stations to monitor the events. Local authorities were informed and were prepared for any contingencies."

Geologic Background. Taal is one of the most active volcanoes in the Philippines and has produced some powerful eruptions. The 15 x 20 km Talisay (Taal) caldera is largely filled by Lake Taal, whose 267 km2 surface lies only 3 m above sea level. The maximum depth of the lake is 160 m, with several submerged eruptive centers. The 5-km-wide Volcano Island in north-central Lake Taal is the location of all observed eruptions. The island is composed of coalescing small stratovolcanoes, tuff rings, and scoria cones. Powerful pyroclastic flows and surges have caused many fatalities.

Information Contacts: PHIVOLCS.

Intermittent ash eruption, with occasional larger explosive events, has continued since the last inspection on 7 September. An overflight on [15] October showed that the main crater floor was covered with dark ash. Congress Crater appeared to have deepened and the active vent emerged some distance above the crater floor.

Seismicity has remained at moderate levels since 7 September, dominated by medium- to high-frequency volcanic tremor. NZGS personnel thought it likely that episodes recorded 17-22 and 25-28 September accompanied significant ash emission. Several eruption-type earthquake sequences have been recorded since 7 September. These eruptive episodes, probably of similar size to the observed 7 September episode, have deposited block ejecta and left impact craters on the W part of the main crater floor. Volcanic earthquake events (E-type) that occurred between 7 September and 16 October were strongest on 12 and 13 September. Each event began with a sequence of C-type events followed 50-70 seconds later by a symmetric envelope of higher frequency (4-10 Hz) signal, interpreted as the time of the explosive eruption of ash and blocks from the vent (figure 9).

Figure 9. Typical example of an E-type volcanic earthquake at White Island, beginning with a sequence of C-type events. Courtesy of NZGS.

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: A. Cody, I. Nairn, and B. Scott, NZGS Rotorua.