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).

Table 15 lists the ground-based 48-inch lidar measurements at 0.69 µm taken with a ruby laser in Hampton, Virginia (37.1°N, 76.3°W) during 1998. The lowest levels of aerosol loading ever reported in the 24-year lidar record at Hampton were measured during the summer of 1998.

Table 15. Lidar data from Virginia, USA, for April-December 1998 showing altitudes of aerosol layers. Backscattering ratios are for the ruby wavelength of 0.69 µm. The integrated values show total backscatter, expressed in steradians^-1, integrated over 300-m intervals from the tropopause to 30 km. Courtesy of Mary Osborne.

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 thorugh 1989. Lidar data and other atmospheric observations were again published intermittently between 1995 and 2001; those reports are included here.

Information Contacts: Mary Osborn, NASA Langley Research Center (LaRC), Hampton, VA 23681 USA.

A distinct change in seismic activity began on 3 December. About 120 shallow events of very low magnitude were recorded during 3-6 December. The only days during the episode when observation was not obscured by cloud were 1 and 3-6 December, but no plumes were seen those days.

Geologic Background. Avachinsky, one of Kamchatka's most active volcanoes, rises above Petropavlovsk, Kamchatka's largest city. It began to form during the middle or late Pleistocene, and is flanked to the SE by Kozelsky volcano, which has a large crater breached to the NE. A large collapse scarp open to the SW was created when a major debris avalanche about 30,000-40,000 years ago buried an area of about 500 km2 to the south, underlying the city of Petropavlovsk. Reconstruction of the volcano took place in two stages, the first of which began about 18,000 years before present (BP), and the second 7,000 years BP. Most eruptions have been explosive, with pyroclastic flows and hot lahars being directed primarily to the SW by the collapse scarp, although there have also been relatively short lava flows. The frequent historical eruptions have been similar in style and magnitude to previous Holocene eruptions.

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

The eruption at Colima began by 20 November 1998 following 17 days of continuous seismic unrest and deformation of the summit cone. Gabriel Reyes, Juan José Ramírez, and Yuri Taran noted that fumarolic gases monitored during previous years may have also shown precursory variations in chemical composition and temperature. New fractures in the summit region were observed on repeated occasions by Abel Cortes and J.C. Gavilanes during ascents on 27 November 1997, 18 March 1998, and 5 May 1998. Between 1614 and 1800 on 17 November, Carlos Navarro and Cortes visited La Yerbabuena, a town on the SW flank 9 km from the summit crater, where they heard more than 10 episodes of rumbling noise coming from the volcano. Cloudy weather did not allow direct observation of the volcano, but based on previous experience they interpreted the noises to be the result of small rockslides.

During the morning of 18 November the settlement of La Yerbabuena (~180 inhabitants) was evacuated voluntarily and in orderly fashion with the assistance of the Colima Observatory Information Group and the local civil protection and military authorities. During that day residents also evacuated the settlement of Juan Barragan (120 people) 10 km SE of the summit.

The first helicopter overflight took place between 0800 and 1000 on 19 November but cloudy weather obscured large parts of the summit area. Observers did note a vigorous fumarolic plume blowing W. That night the Red Sismológica Telemétrica del Estado de Colima (RESCO) reported strong seismic activity and harmonic tremor over periods lasting for 6 minutes. They also registered increased rockfall signals.

At 0700 on 21 November the new lava dome had almost entirely filled the 1994 crater (BGVN 23:10; figure 25). At 1130 that morning, lava started spilling out of the summit crater area producing block-and-ash flows to rush down the S slopes at 3- to 5-minute intervals. The block-and-ash flows were mostly emplaced within the eastern branch of Barranca El Cordobán. The most voluminous flows reached the 2,400 m contour, a distance of more than 4 km from the crater.

Figure 25. Photograph of Colima taken from the SW at 0930 on 21 November 1998. By this time the new dome has entirely filled the 1994 crater and was about to spill out onto the S face (on the right-central portion of the photo). Courtesy of J.C. Gavilanes, Universidad de Colima.

Figure 26. An oblique, wide-angle aerial photo taken with the camera somewhat tilted (note horizon in upper corner) showing a pyroclastic event at 0830 on 22 November 1998. The event left a series of block-and-ash flows reaching a maximum distance of 4.5 km W of the summit. Courtesy of Abel Cortés, Colima Volcano Observatory, Universidad de Colima.

Another flight on 21 November revealed that the lava flow had advanced ~150 m downslope, and had a width of 100 m and a thickness of ~20 m. The lava flow continued advancing such that on 22 November it was 170 m long; on 23 November, 270 m; and on 24 November, 370 m. Block-and-ash flows emplaced during the morning of 25 November in the central branch of Barranca El Cordobán reached 1,900 m elevation. Observers and photographs revealed two additional lava flows as seen from both Rancho El Jabalí (10 km SW of the summit) during the night of 25 November and from Cofradia de Suchitlan (15 km SW) during the night of 26 November; these flows also descended the SW flank and headed towards two drainages (the W branch of Barranca El Cordobán and the S branch of Barranca La Lumbre).

Figure 27. Time-lapse photograph (30 minutes) of Colima taken from a point near Suchitlan (15 km SW) around 2300 on 26 November 1998. Courtesy of Claus Siebe, UNAM.

On 28 November, S. Rodríguez, J.M. Espíndola, and C. Siebe observed the advance of the westernmost lava flow from a 3,100-m-elevation vantage point. Most of the time the flow's front and margins relinquished large blocks (up to 10 m across) producing strong rumbling noises. Small block-and-ash flows moved quietly compared with the rockfalls from the lava flow. Still, it was possible to collect samples of the lava flow.

Viewed in thin section the new lava contained, in decreasing abundance, phenocrysts of zoned plagioclase, hypersthene, pleochroic resorbed brown hornblende, and subordinate magnetite in a microcrystalline to glassy matrix. Chemical analysis indicated that the new lava is very similar in composition to previous eruptions (table 5).

Table 5. Chemistry of freshly erupted Colima lava sampled on 28 November 1998. Courtesy of S. Rodríguez, J.M. Espíndola, and C. Siebe; analysis made by Rufino Lozano, Laboratorio de Fluorescencia de Rayos X at Instituto de Geología, UNAM.

On 2 December the three lava flows on the SSW flanks had reached these estimated lengths: 1,000 m (more westerly flow), 1,200 m (central flow), and 900 m (SE flow). Good views of these flows were obtained during an overflight the next day (figure 28). Around this time, it seemed most probable that the ongoing eruption would remain mostly effusive and not exceed the magnitude of eruptions witnessed here during past decades. Accordingly, inhabitants of La Yerbabuena were allowed to return to their homes on 1 December.

Figure 28. The state of lava flow advance on Colima at 0850 on 3 December 1998. Photograph taken from the SW by Juan Carlos Gavilanes. Courtesy of J.C. Gavilanes, Universidad de Colima.

Fine ash produced thus far during the eruption consisted mostly of non-juvenile material related to rockfalls and small block-and-ash flows. Two stations, one located at Rancho El Jabalí (10 km SW of the summit) and the other at La Becerrera (12 km SW of the summit), registered maximum ashfalls on 23 and 26 November, respectively; both with daily loads of around 50 g/m². The wind mostly dispersed this ash towards the SW and W.

COSPEC measurements carried out by Gavilanes and Cortes since 30 October 1998 showed a marked increase in SO₂ flux (table 6). The highest discharge, measured on 26 November, yielded an estimate of more than 16,000 metric tons/day.

Table 6. COSPEC measurements for SO₂ fluxes at Colima volcano at stated dates in 1998. Fluxes are in metric tons/day and were rounded to three significant figures. Measurements on 11 February, 14 April, 2 May, and 25 May were below the detection limit. Extrusion began on 20 November. Courtesy of Juan Carlos Gavilanes and Abel Cortes, Universidad de Colima and Colima Volcano Observatory.

Geologic Background. The Colima complex is the most prominent volcanic center of the western Mexican Volcanic Belt. It consists of two southward-younging volcanoes, Nevado de Colima (the high point of the complex) on the north and the historically active Volcán de Colima at the south. A group of late-Pleistocene cinder cones is located on the floor of the Colima graben west and east of the complex. Volcán de Colima (also known as Volcán Fuego) is a youthful stratovolcano constructed within a 5-km-wide scarp, breached to the south, that has been the source of large debris avalanches. Major slope failures have occurred repeatedly from both the Nevado and Colima cones, producing thick debris-avalanche deposits on three sides of the complex. Frequent recorded eruptions date back to the 16th century. Occasional major explosive eruptions have destroyed the summit (most recently in 1913) and left a deep, steep-sided crater that was slowly refilled and then overtopped by lava dome growth.

Information Contacts: Juan Carlos Gavilanes, Carlos Navarro, Abel Cortés, Alicia Cuevas, and Esther Ceballos, Universidad de Colima; Claus Siebe, Juan Manuel Espíndola, Instituto de Geofísica, UNAM; Sergio Rodríguez-Elizarrarás, Instituto de Geología, UNAM.

The following report summarizes activity observed at each of the four summit craters of Etna (figure 70) from June through September 1998. In early June, Northeast Crater was quiet while Bocca Nuova, Southeast Crater and Voragine were displaying the highest level of activity seen in many months. Generally high levels of activity continued until a major explosive eruption from Voragine on 22 July. Strong Southeast Crater explosions on 15 September destroyed the intracrater cone, which was soon replaced.

Figure 70. Map of the summit craters of Mount Etna, 10 July 1998. Courtesy of J.C. Tanguy and G. Patanè.

During early July all four craters were erupting simultaneously, a fact never recorded since their birth; only the Central Voragine has degree of permanence; Northeast Crater (NEC) appeared in 1911, the Bocca Nuova (BN) in 1968 and the Southeast Crater (SEC) in 1971.

Most of the information for this report was compiled by Boris Behncke at the Istituto di Geologia e Geofisica, University of Catania (IGGUC), and published on his internet web site. The compilation was based on personal visits to the summit, telescopic observations from Catania, and other sources. Additional separate reports were provided by Tanguy and Patanè (10-14 July observations) and Murray, Stevens, and Craggs (15 September observations). Aviation notices were issued by the Toulouse (France) Volcanic Ash Advisory Center.

Activity at Southeast Crater (SEC). There were at least three explosive vents on the intracrater cone during 4-5 June. Activity usually alternated between the N and S vents. When both exploded simultaneously, a third NW vent produced weak incandescent projections. Vigorous growth around the vents elevated the summit to 20 m above the SEC rim. Lava flowed towards the NE flank where it spilled down to the base of the SEC cone. During a visit on 11 June, SEC had the usual two vents active, and fresh bombs scattered over the crater floor. Recent flows had built a high mound on the E side of the cone; an active flow issued from the vent area. By the morning of 15 June the lava flow at SEC had reached its southern base and was advancing slowly.

Explosive activity on 22 June occurred from three vents on the intracrater cone, and lava issued from a vent halfway up the S flank. Explosive Strombolian activity occurred in distinct cycles separated by quiet periods of up to one hour, although lava effusion persisted. The beginning of each cycle was marked by a flame of burning gas at the summit. More vigorous bursts would then follow at a larger vent. Explosions would become increasingly frequent and rise higher (up to 150 m above the vent), showering the southern part of SEC with bombs. Activity would then shift back to the SW vent where each Strombolian burst was accompanied by a gas flame. Intermittent explosive and effusive activity continued on 24 and 28 June.

During a summit visit by Behncke and members of L'Association Volcanologique Europeenne (LAVE) of Paris on 4 July, explosive activity at SEC was intense, with bombs falling outside the crater. Activity from the top of the intracrater cone sent jets of bombs and scoria up to 150 m. Lava issued from three vents, one feeding a flow over the SW crater rim. On the evening of 7 July LAVE members reported that the active lava flow on the SW flank of SEC was ~200 m long. The summit visit on 13 July was made by Giovanni Sturiale, Sandro Privitera, and Behncke (IGGUC), and Jürg Alean. At SEC, Strombolian activity was vigorous, bombs fell frequently outside the crater, and lava emission was continuing. Recent lava had filled the SW part of the crater to within 1-2 m of the rim. In all other areas the pre-1997 rim of SEC has been buried by overflowing lava. Jürg Alean visited on 14 July and reported that SEC continued to produce Strombolian activity. From the Torre del Filosofo hut a small lava flow could be seen descending the SE flank of SEC; incandescent blocks frequently detached from the flow front.

Vigorous activity occurred at SEC during the 22 July Voragine episode, and during the days after activity was limited to SEC where vigorous lava fountaining and effusion occurred. On 24 July, Sturiale and Privitera observed vigorous Strombolian activity, with many bombs falling outside the crater. However, SEC activity declined and virtually ceased by the end of July.

As of the night of 17-18 August, there had been no resumption of the SEC eruption. The crater was seen erupting later on 18 August by Privitera. When Monaco and Behncke visited on 20 August, virtually no activity was observed. As of 26 August SEC appeared quiet, although the activity in July had built the intracrater cone to 40-50 m higher than the crater rim. Bombs were scattered all over the crater area and beyond. Two post-22 July lava flows had spilled onto the S and NE flanks. No activity occurred during a visit by Behncke and Sturiale on 9 September.

Explosive activity from SEC was observed by scientists from Open University (Murray and Stevens) and the University of London (Craggs) on the morning of 15 September. Several ash clouds erupting from the summit between 0745 and 0800 were seen from 10 km S. At 0815 bomb-laden ash clouds were observed from near the Piccolo Rifugio (4.5 km S of the summit craters). At 0822 an exceptionally large explosion sent meter-sized bombs ~300-400 m above the crater rim. One more minor explosion was observed before the summit was obscured at 0826. Observation recommenced at the Pizzi Denieri volcano observatory. The summit was usually obscured by clouds, but five explosions during 0928-0936 were audible above gale-force winds and engine noise. Ash clouds were seen from Mt. Nero on the NE rift (6 km from the summit craters) at 1003, and at 1006 an explosion was heard.

Explosions continued all afternoon, causing ashfall in inhabited areas on the E flank. During the afternoon, while conducting fieldwork 50 km S of Etna, Behncke and Sturiale saw black ash fountains piercing weather clouds above the summit. These pyroclastic jets rose several hundred meters above the summit before drifting E. Observations by Behncke on 19 September revealed that the explosions ejected lithics and fresh bombs, which were abundant in the saddle between SEC and the main summit cone. Some of the bombs were up to 5 m across and had flattened upon impact. Bombs tens of centimeters in diameter formed a continuous deposit on the NW side of the crater. Most of the intracrater cone was destroyed, and a crater ~80 m across formed in its place (figure 71).

Figure 71. Sketch drawing showing Southeast Crater of Etna as seen from the NW on 19 September 1998. The former intracrater cone has been replaced by an explosion crater that contains a small new cone with two erupting vents, and a small non-eruptive vent that lies below the major breach in the crater wall in the left center. Lava that had overflowed the SW crater rim until July 1998 is shown in a dark pattern at right. Courtesy of Boris Behncke.

Vigorous activity on 17 and 18 September ejected bombs described as having been "several meters across" by a group of British geologists led by J.B. Murray working in the area. The beginning of a lava flow down the NE flank of SEC is not known, but it was reported by mountain guides to have been moving on 17-18 September. During a 19 September visit by Behncke, Strombolian bursts occurred from two vents in the explosion crater, around which a small cone had begun to grow. Lava emission from a vent high on the SEC cone was feeding a flow that advanced towards Valle del Leone.

Activity at SEC was continuing on 21 and 25 September with intense Strombolian activity; incandescent bombs jetted 150-200 m high. Continued vigorous activity during the last week of September caused rapid growth of the intracrater cone until ti was higher than ever before, having almost entirely covered the remains of its predecessor.

Activity at Bocca Nuova (BN). Eruptive activity during 4-5 June was occurring at both previously active areas. Night incandescence and bomb ejections were seen in a deep pit within the SE eruptive area. Noisy activity occurred at the NW eruptive area, at the bottom of the collapsed cone that had grown in 1997. At least five vents were producing explosions and lava fountains accompanied by bursts of burning gas. Several lava flows extended over the crater floor.

Observations were made for 30 minutes on 11 June from the crater rim. The SE vents had fountains of ash and bombs rising ~50 m. At the NW eruptive area, three vents were active, and the collapse pit was filled with pyroclastics and recent lava flows. Two large (30 and 50 m diameter) vents were in the central part of the filled pit while a smaller vent (~5 m in diameter) lay 50-70 m S; this latter one produced weak lava sputterings, building a low hornito. The two larger vents showed a repetitive eruptive behavior for the first 15 minutes of observation, then erupted simultaneously in a series of ash-free lava fountains. For about ten minutes there were bomb ejections from both major vents. Centimeter-sized scoria and Pele's hair were deposited all over the SE sector and on SEC.

Activity was less intense on 15 June; during a 1-hour stay in the summit area, strong explosions from the large cone ejected ash-rich jets of bombs up to 100 m above the crater rim. Visits to BN are dangerous due to frequent blasts of large quantities of meter-sized bombs. Most blasts observed on 22 June lasted up to 10 minutes. The source vent lay in the partially collapsed 1997 cone at the N eruptive area; it produced almost continuous minor explosions between the large detonations, ejecting large clots of fluid lava. A small vent to the south ejected minor sprays of meter-sized bombs. Continuous lava fountaining occurred from a SE vent. During the 22 June visit the central vent was the site of pulsating gas jets, and vigorous lava fountaining occurred at the larger SW vent. A large asymmetrical cone leaning against the thin wall between the Voragine and BN had grown around the vent. Vigorous activity was continuing on 24 and 28 June.

During a summit visit by Behncke and members of LAVE on 4 July, all four summit craters were active. The summit visit on 13 July by Sturiale, Privitera, Behncke, and Alean showed low levels of activity; a small cone had grown around the main vent. The N eruptive area was the site of Strombolian bursts every 5-10 minutes. A fairly large cone had grown at this vent, the first time that significant cone growth had occurred in BN since late 1997. Lava had covered the S crater floor.

A visit by J.C. Tanguy and G. Patanè during 10-14 July revealed that, with respect to the preceding year, the bottom of BN had raised considerably owing to the tephra deposition, so that the strongest explosions from the NW vent (figure 72) sometimes showered the external slope with bombs. By 12 July the explosions were reduced in strength and frequency. Jürg Alean visited on 14 July and reported that the N cone produced fountains heavily charged with bombs; many fell on the crater rims and in the Voragine.

Figure 72. Sketch of the Bocca Nuova and Voragine craters at Etna, 10 July 1998. Courtesy of J.C. Tanguy and G. Patanè.

Vigorous activity occurred at BN during the 22 July Voragine episode. On the afternoon of the 23rd, Carmelo Monaco (IGGUC) saw bright incandescence in BN even in bright daylight from an airplane approaching Catania. Activity was noted on 25 July and increased the following day according to Claude Grandpey (LAVE); activity at the NW area occurred from a small vent while the SE area had three vents emitting gas and bombs. In late July and early August, numerous vents erupted explosively at the NW area; subsidence of the central crater floor by a few meters occurred on 1 August. The SE vents displayed spectacular lava cascades from one vent into the other, the lower vent filling until an explosion cleared it. Kloster (LAVE) reported a lava lake in this area on 7 and 10 August, but during the following days there was only Strombolian activity.

On 20 August Monaco and Behncke observed moderate eruptions at the N vent area. Besides the summit vent, there were at least four smaller flank vents which had erupted recently. On 26 August frequent ash emissions were occurring. Growth of a small cone above the diaframma (septum between the craters) culminated in the fracturing of this cone and a cascade of lava into BN in late August.

A visit to the summit by Behncke and Sturiale on 9 September revealed that one vent at the summit of the NW cone was the site of Strombolian bursts alternating with bomb and ash emissions. Four smaller vents on the flanks on the cone were weakly degassing. Weak Strombolian activity occurred from two SE vents where a small cone was growing in a collapse depression. Behncke saw activity at similar levels on 19 September. As of 25 September there was low-level activity.

Activity at Voragine. On 3 June, near-continuous cannon-shot like detonations were heard kilometers away, and Marco Fulle (Osservatorio Astronomico, Trieste) observed magma bubbles within the vent burst at the onset of fire-fountaining episodes. When observed during 4-5 June, the vent in the SW crater floor had enlarged notably since 6 April and shifted away from the diaframma, and a low pyroclastic cone had grown around it. On the evening of 4 June, activity at the Voragine was observed for about 4 hours. A sustained fountain jetted from the vent, showering the SW part of the crater floor with bombs; many also fell into BN. This fountain lasted about 75 minutes, followed by pyroclastic material sliding from the inner walls of the vent into its throat. After a few minutes, a small vent opened below the inner SE rim of the vent and emitted jets of incandescent lava. Ejections soon resumed at the main vent, and a flame of burning gas persisted at the subsidiary vent accompanied by weak pyroclastic sprays. A new period of fountaining at the main vent resulted in the continuous fall of bombs into BN. The subsidiary vent was soon buried. At times, portions of the inner walls collapsed, causing ash-rich fountains.

The Voragine was not visited on 11 June, but very strong explosive activity was heard more than 10 km S, and high fountains contained meter-sized bombs. On 15 June the focus of activity had shifted to the central vent, previously active between July and December 1997. This vent ejected continuous lava fountains while a lava flow covered the E half of the crater floor. Fountains played up to 200 m above the vent, with all bombs falling back into the crater. At times, the magma level dropped, and the character of the activity changed to discrete explosions. The SW vent exhibited noisy gas emissions alternating with ash emission and lava fountaining. Vigorous activity was continuing on the evening of 24 June. During a summit visit on 28 June, Monaco observed fountaining from the central vent; the SW vent was less active and mostly ejected ash.

A scoria deposit extending SE, produced by a Voragine lava fountaining episode on 1 July, was examined by Behncke and members of LAVE on 4 July. Both vents in the Voragine were in vigorous, alternating activity. Eruptive cycles at the SW vent produced jets of fragmented pyroclastics. As activity waned at this vent, projections of large bombs would initiate at the central vent, increasing in frequency and height into a pulsating fountain at least 100 m above the crater floor.

Stefano Branca (IGGUC) reported that frequent explosions were audible throughout 6 July at Viagrande, a village at the SE flank of Etna; air concussions associated with the explosions shook windows and rattled doors. The explosions probably originated at the Voragine, the site of recent noisy activity. On 7 July explosions were still audible but less intense. Members of LAVE observed activity that evening from the SW vent that dropped bombs as far as the S rim of NEC.

A visit by J.C. Tanguy and G. Patanè during 10-14 July revealed activity at the large SW cone (figure 72) near the diaframma and from a central cone. By 12 July the two vents hurled large lava lumps and bombs in a fountain-like manner, some of which fell outside the crater. On 13 July this activity was stronger. That afternoon activity decreased, but two flows began from a fissure NE of the central cone. Lava rapidly invaded the northern, lowest part of the Voragine. During the peak effusive activity the two lava flows reached a speed estimated at 3-4 m/s. On the morning of 14 July, only the SW vent showed Strombolian explosions. Lava flows had entirely disappeared under a layer of tephra erupted during the night.

During a visit on 13 July by Sturiale, Privitera, Behncke, and Alean, the most vigorous activity occurred at the Voragine. On 12 July, lava fountains roared up to 200 m above the crater rim for three hours from the SW vent. Powerful jets of bombs mixed with ash were also ejected. The cone around the SW vent was higher in places than the diaframma; the vent was 30-50 m across. Activity varied from isolated powerful explosions to long-lasting lava fountains. At times dense ash plumes with large bombs rose from the vent. Explosions from the central cone blasted lava in all directions. Small lava fountains and ash emissions occurred from two fissures. On at least 20 occasions during 90 minutes of observation the magma surface in the vent domed up, forming a huge bubble that exploded. Explosions later ejected meter-sized bombs to 200 m or higher; many fell into BN, outside the Voragine, or on the E slope of the main summit cone not far from SEC. Jürg Alean visited on 14 July and reported that both vents showed intense activity. The SW vent was filled almost to the rim by lava which was fountaining vigorously. The central vent displayed a similar eruptive behavior as on the previous visit, but no lava bubbles were observed. On 20 July lava fountaining from the Voragine was common.

A major eruptive event began from the Voragine at about 1835 on 22 July. The following is based on preliminary information from scientists of the IGGUC (mainly Giovanni Sturiale and Sandro Privitera) and others who visited after the event as late as 20 August. According to eyewitnesses on the SW side of the main summit cone, huge lava fountains rose from the Voragine, and heavy tephra falls began in the summit area. A large mushroom-shaped tephra column rose up to 10 km above the summit. The plume was then driven S and SE, and widespread ashfalls occurred more than 30 km away. Sand-sized tephra fell in Catania, leaving a deposit about 1 mm thick. For the first time since 24 September 1986 (when NEC had a powerful explosive eruption) the Fontanarossa airport of Catania had to be closed (it was reopened after 15 hours). The Toulouse (France) Volcanic Ash Advisory Center issued 17 aviation notices warning pilots about the ash during 22-29 July. The tephra falls caused traffic problems on roads and highways. Close to the summit, a thick scoria deposit buried the dirt roads leading to the Rifugio Torre del Filosofo and around the western base of the main summit cone. Sturiale and Privitera reported that at Torre del Filosofo the thickness of the scoria deposit was about 50 cm.

It appears that both vent areas produced lava fountains and a tall tephra column. Rapid accumulation of ejecta in the saddle on the NW rim led to a lava flow between the NEC and the main summit cone (figure 73). The flow covered the road connecting the N and S flanks of Etna, and eroded a deep scar into the the S flank of the NEC cone. Continuing pyroclastic activity produced a thick scoria and bomb deposit, with bombs up to 5 m in length. A scoria fan extended 1-1.5 km NW. In the area of the diaframma a lava flow covered the crater floor to several meters depth. On the E flank of the main summit cone a thick pyroclastic deposit formed. In towns on the E and SE flank, the tephra deposit was a few millimeters to a few centimeters thick. Morphological changes within the Voragine consisted mainly of a large amount of filling of the crater followed by subsidence. Parts of the SW crater rim also collapsed.

Figure 73. Sketch map of Etna's summit craters showing features frequently mentioned in the updates, and approximate extent of recent lava flows as of 5 October 1998. Active vents are indicated as gray dots in the craters. Courtesy of Boris Behncke.

Vigorous activity occurred simultaneously at BN and SEC on 22 July Voragine event, indicating that the episode affected much of the central conduit system at some depth, possibly due to the rise of a batch of fresh gas-rich magma. Lava fountaining from the Voragine continued intensely through the night of 22-23 July.

According to Grandpey (LAVE), the Voragine appeared "full of materials" on 25 July with no trace of the former intracrater cones. No further activity occurred until 3 August when Kloster (LAVE) saw explosions ejecting bombs. Two days later, three vents erupted in the center of the Voragine. On 7 August small flows on the crater floor were followed by explosive activity. Powerful Strombolian activity with bomb ejections and ash emission caused light ashfall on the SE flank on 18 August, reaching the outskirts of Catania.

On 19 August explosive ash emissions sent small plumes up to several hundred meters above the summit. When Monaco and Behncke visited on 20 August, vigorous activity occurred from two vents. Very light ashfalls on 21 and 24 August reached Catania; ash emissions were also produced on 26 August. A number of reports indicated continued activity through the end of August.

On 6 September bombs fell on the outer W slope of the Voragine and on 7 September ash emission occurred throughout the day. A visit to the summit by Behncke and Sturiale on 9 September revealed continuous moderately strong Strombolian activity from a SW vent; sporadic explosive activity from the vent next to the diaframma sent bombs over the crater rim. At least three other vents were quietly degassing. Similar activity was continuing as of 19 September. On 30 September strong ash and gas emissions rose hundreds of meters.

Activity at Northeast Crater (NEC). Deep-seated Strombolian activity within the central pit resumed in mid-May according to Vittorio Scribano (Istituto di Scienze della Terra, Catania University). Night glow was observed on the evening of 22 June from 3 km NE. During a summit visit by Behncke and members of LAVE on 4 July, all four craters were active; for the first time since 28 March eruptive activity was observed directly at NEC. The eruption site was a 30-m-diameter vent in the NW part of the central pit while a SW vent (~15-20 m in diameter) emitted dense vapor plumes. Small Strombolian bursts from the larger vent occurred every 2-5 minutes, with most ejecta falling back into the pit.

A visit on 13 July by Sturiale, Privitera, Behncke, and Alean revealed mild Strombolian activity from the central pit that ejected bombs. When Monaco and Behncke visited on 20 August, NEC was degassing quietly. Strong fumarolic activity was occurring on 26 August and 9 September.

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

Information Contacts: Boris Behncke, Istituto di Geologia e Geofisica, Palazzo delle Scienze, Università di Catania, Corso Italia 55, 95129 Catania, Italy; J.C. Tanguy and G. Patanè, University of Catania, Istituto di Geologia e Geofisica, 55 Corso Italia, 95129 Catania, Italy; John Murray and Nicki Stevens, Department of Earth Sciences, Open University, Milton Keynes, United Kingdom; Emma Craggs, Geology Dept, Royal Holloway College, University of London, United Kingdom; Volcanic Ash Advisory Center (VAAC) Toulouse, Météo-France, 42 Avenue Gaspard Coriolis, F-31057 Toulouse cedex, France (URL: http://www.meteo.fr/vaac/)

Since a volcanic crisis in February 1989 (SEAN 14:02-14:05), Observatorio Vulcanológico y Sismológico de Pasto (OVSP) has been constantly monitoring Galeras. The following is from their bi-monthly reports for late 1998.

During September and October 1998, low-level seismic activity continued at Galeras (figure 90). Most of the energy released (1.6 x 10¹⁶ ergs) was due to earthquakes associated with a fracture process. Volcano-tectonic earthquakes registered during these two months totaled 79, ranging from 0.5 to 16 km in depth. One remarkable earthquake occurred at 0209 on 21 September: it was located at 1°15.75'N, 77°19.16'W at a depth of 8 km, released 1.21 x 10¹⁶ ergs of energy, and had a coda magnitude of 3.4. This earthquake was felt in Pasto City and neighboring settlements. It was the most energetic event of 1998 to date.

Figure 90. Location of seismic events at Galeras during September-October 1998. Courtesy OVSP.

Seismic processes related to fluid dynamics (i.e. long-period events and tremor episodes) released a total of 5.18 x 10¹⁴ ergs. Of these events, nine had small amplitudes with long coda and quasi-monochromatic frequencies—so-called "screw type" or "Tornillo" characteristics. Coda values spanned 19-65 s and dominant frequencies ranged 1.82-4.0 Hz. An unusual event occurred 23 October, when harmonic tremor lasted approximately one hour. This episode released 7.09 x 10⁹ ergs.

Galeras, a 4,276 m high andesitic stratovolcano, has a cone that rises 150 m above the floor of the summit caldera. The caldera is open to the west. The active crater is located ~9 km W of Pasto, a city of 350,000 persons. More than 400,000 people live within the volcano's zone of influence. At least six major eruptions have been identified during the past 4,500 years, last in 1886. These eruptions were Vulcanian with inferred low-altitude eruption columns (<10 km) that produced small-volume pyroclastic flows. During the last 500 years eruptions have been characterized by gas-and-ash emissions, small lava flows, and pyroclastic flows that have traveled up to 15 km from the crater.

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

Information Contacts: Patricia Ponce V., and Pablo Chamorro C., Observatorio Vulcanológico y Sismológico de Pasto (OVSP), Carrera 31, 18-07 Parque Infantil, PO Box 1795, Pasto, Colombia (URL: https://www2.sgc.gov.co/volcanes/index.html).

On 18 December an eruption occurred within the caldera of the subglacial Grímsvötn volcano, 10 km S of the 1996 eruption that resulted in a catastrophic flood. Scientists quickly investigated; the information that follows is from the Nordic Volcanological Institute (NVI).

Eruptive activity. The eruption began at 0920 on 18 December. Ten minutes later a plume (figure 4) was observed that eventually rose 10 km above the Vatnajökull glacier and persisted throughout the day. The plume could be seen from Reykjavik, 200 km W. Winds deflected the plume, causing tephra fallout onto the glacier up to 50 km SE. The London Volcanic Ash Advisory Center issued aviation notices later that day and throughout the eruption.

Figure 4. Photo of the eruption plume from Grímsvötn as it appeared from an aircraft on 18 December 1998. Courtesy of NVI; photo by Karl Grönvold.

The eruption was preceded by a mild increase in seismicity for several weeks. A small earthquake swarm began at 2200 on 17 December and a sharp increase in earthquake activity began at 0330 on 18 December. This latter activity was replaced by continuous tremor at 0920, marking the beginning of the eruption. The Icelandic Meteorological Office and the Science Institute monitored seismicity during the eruption.

Vents were located along a 1,300-m-long E-W oriented fissure on the S caldera fault, similar to eruptions in 1934 and 1983, at the foot of Mt. Grímsfjall (which rises ~300 m above the flat ice shelf of the Grímsvötn subglacial lake). The eruption penetrated the caldera lake and its ice shelf, from ice/water depth of ~100 m. Activity was most vigorous at one crater, but several other craters on the short eruptive fissure were also active with less frequent explosions.

The eruption was slightly less vigorous on 19 December. The plume was continuous, but somewhat lower, rising to 7-8 km. Tephra continued to fall SE. A small part of the Grímsvötn ice shelf next to the eruption site had melted without raising the water level of the caldera lake significantly. Activity was mostly limited to one crater.

An overflight on 20 December from 1045 to 1215 revealed variable activity. The eruption plume extended to 7 km altitude. Initially the plume was light-colored, and narrow at its base. Later the ash content of the plume greatly increased, and the plume turned black. It collapsed down to 1-2 km, created a base surge, and Mt. Grímsfjall disappeared into an ash cloud.

Photographs from 27 December showed intermittent eruptive activity between 1124 and 1240. The plume was discontinuous, fed by intermittent crater activity. It rose to a maximum of 4.5 km and distributed ash near the crater; bombs up to 0.5 m in diameter were ejected onto Grímsfjall. The eruption has resulted in the formation of a tephra ring that lies partly on ice, but its inner part is likely to be made completely of ash overlying bedrock.

The eruption ended on 28 December. Continuous tremor recorded at the Grimsfjall seismograph, 3 km from the eruption site, stopped at 1050 on 28 December. Small tremor bursts were recorded for another 3 hours, but activity stopped completely at 1400.

This eruption was located 10 km S of the 1996 eruption in Vatnajökull (Gudmundsson and others, 1997), which caused a catastrophic outburst flood from the glacier. This time no major flood ensued because only a small amount of the Grímsvötn ice shelf near the eruption site melted, and water did not flow towards the Grímsvötn caldera lake.

Chemical analyses of ash. The ash analyzed fell during 1000-1200 on 20 December in Suðursveit, ~60 km SE of Grímsvötn. The ash was well sorted with an average grain size of 0.05 mm and density of ~2.7 g/cm³. The areal density of ash fall was estimated at 93 g/m². The ash was aphyric; the glass composition (table 1) can be compared with Grímsvötn ash samples from earlier this century. The composition is similar to earlier samples; however, the recent sample is slightly less evolved, with higher MgO/FeO, Al₂O₃, and CaO, but lower TiO₂. The composition was markedly different from more evolved samples from the 1996 eruption or most of the samples available from the neighboring Bárðarbunga volcanic system.

Table 1. Microprobe analyses of the glass phase from the 20 December 1998 Grímsvötn eruptions (standard deviation in parentheses) and two Grímsvötn hyaloclastites. The analyses from the 1983, 1934, 1922, and 1903 eruptions are from Grönvold and Johannesson (1984). The analyses of the hyaloclastites are from Heikki Makipaa (1978). All analyses are in weight percent. Courtesy NVI.

The potential chemical pollution of the fallout ash was tested by leaching a batch of ash with 6.7 times its mass of de-ionized water. The pH of the leachate was 5.12; the water-soluble components were as follows (mg leachate / kg ash): SiO₂, 7.2; Na, 315.3; K, 32.7; SO₄, 557.8; F, 346.5; Cl, 366.2.

References. Grönvold, K., and Jóhannesson, H., 1984, Eruption in Grímsvötn 1983, course of events and chemical studies of the tephra: Jökull, 34:1-11.

Gudmunsson, M., Sigmundsson, F., and Björnsson, H., 1997, Ice-volcano interaction of the 1996 Gjálp subglacial eruption, Vatnajökull, Iceland: Nature, v. 389, p. 954-957.

Geologic Background. Grímsvötn, Iceland's most frequently active volcano in recent history, lies largely beneath the vast Vatnajökull icecap. The caldera lake is covered by a 200-m-thick ice shelf, and only the southern rim of the 6 x 8 km caldera is exposed. The geothermal area in the caldera causes frequent jökulhlaups (glacier outburst floods) when melting raises the water level high enough to lift its ice dam. Long NE-SW-trending fissure systems extend from the central volcano. The most prominent of these is the noted Laki (Skaftar) fissure, which extends to the SW and produced the world's largest known historical lava flow in 1783. The 15 km3 basaltic Laki lavas were erupted over 7 months from a 27-km-long fissure system. Extensive crop damage and livestock losses caused a severe famine that resulted in the loss of one-fifth of the population of Iceland.

Information Contacts: Karl Grönvold and Freysteinn Sigmundsson, Nordic Volcanological Institute (NVI), Grensásvegur 50, 108 Reykjavík, Iceland (URL: http://nordvulk.hi.is/); Pall Einarsson, Science Institute, University of Iceland; Icelandic Meteorological Office, Reykjavík, Iceland (URL: http://en.vedur.is/).

Measurements and sampling were made in the acid lake of Kawah-Ijen during two transits with a rubber rowboat on 7 and 10 December 1998. The 7 December measurements occurred just after heavy rain and the lake's color was pale and nearly white. In the middle of the lake, the surface temperature was 23.5°C with a pH of 0.48 as a result of dilution by the rain. The temperature near the solfatara at the S side of the lake ranged between 24.6 and 24.9°C with a pH of 0.45. Near the hot sublacustrine spring the temperature was as high as 61.7°C and the pH was 0.60. In the Banyupahit River, 3 km from the dam that closes the lake, the water had a temperature of 21.1°C and a pH of 0.47.

Three days later, 10 December, the lake was pale green with localized brown coloration; the temperature of the surface was 24.8-25.2°C and the pH 0.36-0.38. The highest measured temperature of the solfatara was 224°C, while the CO₂ content of the atmosphere near the lake surface was normal, ~300 ppm.

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

Information Contacts: Jacques-Marie Bardintzeff, Laboratoire de Petrographie-Volcanologie, bat 504 Universite Paris-Sud, 91405 Orsay, France.

Seismicity remained above background levels during 1 November-7 December. Low-level Strombolian activity, including 100-200 earthquakes and gas explosions each day, continued to characterize activity at the volcano. On 24 November a pilot in the vicinity reported an explosive event that sent an ash column 6 km above the summit. The color-coded hazard status remained at Yellow.

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

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

A significant collapse of the lava bench on the coast SE of Kīlauea occurred in early December. Lava continued to flow into the sea via a tube from the Pu`u `O`o vent, and a pit at the vent continued to grow.

A large part of the active lava delta on the SE coast collapsed into the sea sometime between 1200 on 10 December and 0930 on 11 December. A comparison between the shoreline as mapped on 11 and 24 November (figure 125), and the shoreline on 11 December, showed that ~5.8 hectares (ha) was lost. The missing shoreline included ~3.4 ha of land built since August and ~2.4 ha built W of the current lava-entry area (indicated by the steam cloud at the top of figure 125) between 1992 and 1997. Judging from observations of earlier bench collapses, the collapsed area most likely slid into the sea in several segments over a period of tens of minutes to several hours.

Figure 125. View of the Kīlauea lava entry point area looking S on 24 November 1998. Courtesy HVO; photograph by J. Kauahikaua.

The eruption of Pu`u `O`o continued in November as lava flowed to the sea through a lava tube that developed on the coastal plain after a major pause in magma supply to the vent on 12-14 August (BGVN 23:08). Another brief pause occurred on 7-8 November (pause #21 of the current eruptive episode) leading to several small `a`a and pahoehoe flows on the coastal plain, none of which reached the sea. Scientists measured a slight increase in the discharge of lava from the tube system—from 3/day in late October to just over 400,000 m<sup>3</sup>/day in early December. Dense volcanic fumes continued to obscure various pits within Pu`u `O`o most of the time, but sloshing sounds of lava degassing could be heard from the crater rim.

A new pit that developed high on the S flank of Pu`u `O`o about one year ago enlarged significantly in 1998, and recent measurements of cracks around the edge of the pit showed that its walls were slumping slowly into the pit.

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: Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawaii Volcanoes National Park, HI 96718, USA (URL: https://volcanoes.usgs.gov/observatories/hvo/); K. H. Rubin and Mike Garcia, Hawaii Center for Volcanology, University of Hawaii, Dept. of Geology & Geophysics, 2525 Correa Rd., Honolulu, HI 96822 USA (URL: http://www.soest.hawaii.edu/GG/hcv.html).

Following one month of build-up in seismicity and radial tilt (figure 10), intensive eruptive activity resumed on 5 October 1998—the first since its fatal eruption of November-December 1996 (BGVN 21:12).

Figure 10. Seismicity and radial tilt from a water-tube instrument at Manam, August-October 1998. Courtesy of RVO.

Visible increases in activity started on 23-25 September, with intermittent dark ash emissions and night-time incandescent projections to ~200 m above South Crater. In subsequent days of October, the activity decreased to continuous white vapor emissions, first profuse then very weak, and occasional roaring sounds and fluctuating glow. This corresponded to a slight decrease in seismic amplitude levels, but the radial tilt continued to show inflation.

On the morning of 5 October a rapid build-up of activity took place. At 0800 ash emissions became forceful, rising ~2 km above South Crater. By 0815 the first small pyroclastic flows started down SE Valley at 5-10 minute intervals. At 0850, the now-dark ash column rose ~3 km, surrounded by blue vapor. Pyroclastic flows started at 0913, penetrating down SW Valley, and the island's E side underwent heavy ash and scoria fall. After 1020 this crater produced several loud explosions every 10-15 minutes. Loud roaring and banging starting at 1205 heralded a decline in activity. By 1600, thick, dark clouds still rose intermittently, but by 1800 only weak, thin gray emissions were visible. Roaring and banging sounds were heard through the night. Although short-lived, this phase also fed a lava flow into SE Valley that branched into two lobes below 900 m elevation and stopped at ~450 m. A lava flow also started toward SW Valley but stopped at the headwall.

In the following days, the tiltmeter 4 km from the summit (at Tabele Observatory) recorded a drop of 2 µrad while the seismicity decreased to near background levels. Until 15 October, gray ash clouds and occasional deep roaring sounds were observed. Not even red glow remained. By the evening of the 16th, red glow reappeared and incandescent projections rose 100-200 m above South Crater. On 17 October, dark ash clouds rose forcefully with rumbling sounds, and minor ash fell on the island's N side.

On the 18th, Main Crater occasionally emitted gray-brown plumes to 600-700 m, and the seismic amplitude increased. Activity in South Crater became sub-continuous, with incandescent projections to 1,000-1,100 m. On the morning of the 19th, a lava flow issued by South Crater descended into SW Valley. The strength of the eruption declined after 1415 and again after 1600. Yet, by 2225 there was a fountain of incandescent projections 1,400-1,600 m above the crater accompanied by loud roaring all night.

Emissions on the morning of the 20th comprised a thick, dark, ash-laden column. In the afternoon, small pyroclastic flows at 1415 and 1750 reached only to the head of SW Valley. By that time, the lava flow extended to within ~2 km of the coast. A single large explosion at 1715 ejected ballistic blocks 1,500 m above the crater. That night, on-going Strombolian explosions rarely reached 1,100 m above the crater.

On the 21st, Main Crater produced dark columns, rising to ~1,000 m, while the roaring Strombolian eruption persisted in South Crater. That night, small tongues of lava flowed in the upper SE Valley.

Activity began to decrease on the 23rd, when the Strombolian projections gave way to intermittent dark ash clouds to ~800 m above the crater. After 1000 on the 23rd, rumbling diminished. The next day Main Crater forcefully ejected dark columns with ballistic fragments and South Crater continued to issue subdued white emissions, with occasional ones that were gray and forceful. This activity persisted until the end of the month, without sound except for occasional low roaring and a faint glow.

While the seismicity noticeably reflected the variations in eruptive strength, tilt was not affected by the second eruptive phase and it resumed rising steadily thereafter as late as early November (figure 10).

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: Ben Talai and Patrice de Saint-Ours, RVO.

According to reports from local news sources and USAID's Office of Foreign Disaster Assistance (OFDA), earthquake swarms began on the island of Dominica on 11 September and continued intermittently into October. The Seismic Research Unit (SRU) of the University of the West Indies started monitoring the activity on 28 September and determined that [seismicity] was occurring in the S part of the island beneath Morne Patates volcano. By 23 October the [seismicity] had subsided to about two [earthquake] events per hour, with 10% large enough to be felt.

An earthquake recorded by SRU at 1018 on 24 September had its epicenter at 15.28°N, 61.37°W. It occurred at a depth of 15 km with a body-wave magnitude of 2.9 and a Richter magnitude of 1 to 3. A spasmodic (new events were starting before the previous were finished) sequence of activity started about 1500 on 22 October. These events were less than 6 km deep and had a maximum magnitude of 3.5 Richter and an intensity of MM V. On 23 October, an SRU aerial reconnaissance revealed no surface manifestations of the events (i.e., scarps, vents).

The strong [felt earthquakes] on 22-23 October were described as the longest and most intense in recent times. These [earthquakes] caused landslides and road closures, including the main road from the capital, Roseau, to the communities on the S end of the island. The SRU stated on 22 October that 27 was the maximum number of [events] recorded within a 24-hour period since 28 September, noting that the daily numbers were not as high as during the 1974 sequence.

Morne Patates, at the southern tip of Dominica, is an arcuate structure open to Soufriere Bay on the west. It was constructed within an irregular depression on the SW flank of a larger stratovolcano, Morne Plat Pays, whose summit is only 3 km NE. The latest eruptions occurred at about 450 ± 90 years BP (Roobol and others, 1983) from the Morne Patates lava dome just prior to European settlement. At least ten swarms of small-magnitude earthquakes have occurred since 1765. The most recent swarm, between March and October 1986, consisted of 10-30 recorded A-type volcanic shocks in about two hours. No eruptive activity followed any of these swarms and no systematic shallowing was documented to indicate upward migration of magma.

General References. Roobol, M.J., Wright, J.V., and Smith, A.L., 1983, Calderas or gravity-slide structures in the Lesser Antilles Island Arc?: JVGR, v. 19, p. 121-134.

Geologic Background. The Morne Plat Pays volcanic complex occupies the southern tip of the island of Dominica and has been active throughout the Holocene. An arcuate caldera that formed about 39,000 years ago as a result of a major explosive eruption and flank collapse is open to Soufrière Bay on the west. This depression cuts the SW side of Morne Plat Pays stratovolcano and extends to the southern tip of Dominica. At least a dozen small post-caldera lava domes were emplaced within and outside this depression, including one submarine dome south of Scotts Head. The latest dated eruptions occurred from the Morne Patates lava dome about 1270 CE, although younger deposits have not yet been dated. The complex is the site of extensive fumarolic activity, and at least ten swarms of small-magnitude earthquakes, none associated with eruptive activity, have occurred since 1765 at Morne Patates.

Information Contacts: Tina Neal, OFDA/USAID, 1300 Pennsylvania Ave. NW, Washington, DC 20523-8602 (URL: http://www.info.usaid.gov/ofda/ofda.htm); CaKaFete News, 25-12 Street, Canefield, Dominica (URL: http://www.cakafete.com/).

A change in the typical low-level steam-and-gas emission regime in late November and early December suggested that a new lava body was growing inside the crater. The following has been condensed from CENAPRED bulletins.

Low-level activity continued during the first three weeks of November, and included low-intensity, short-duration exhalations of steam and gas with occasional eruptions of ash. Bad weather obstructed observations on many days. Authorities recommended that no one approach within 5 km of the crater because of the danger of sudden explosions. The volcanic alert level remained at yellow, indicating a state of heightened caution. Several A-type earthquakes occurred (on 6, 14, 15, 16, and 17 November; M 2.1-2.9), generally 3-4 km E or SE from the crater, none of which seemed to affect eruptive activity. One exceptional emission occurred at 0109 on 9 November; its intense phase lasted one minute and was followed by 12 minutes of high-frequency tremor.

At 1753 on 19 November a moderately large eruption was followed by five smaller ones. The series lasted seven minutes and produced an ash column that rose 2-3 km above the summit and dissipated NNW. Light ash fall was reported in the neighboring town of Amecameca. At 2019 a slightly smaller exhalation lasted nine minutes.

At 1302 on 22 November the volcano began a substantial increase in activity, starting with a sequence of small ash emissions; light ashfall was reported at Paso de Cortés and Amecameca. This activity continued into the night with about 40 separate emission events by midnight. Exhalations increased, and at about 0430 on 23 November harmonic tremor episodes were recorded. At 0530 incandescence at the crater could be seen, at 0854 high-frequency tremor started, and at 0922 a moderate ash emission generated a column 3 km above the summit. By noon about 100 exhalations had been recorded. Dense fumarolic clouds of gas and steam were blown NW. Beginning at 1245 activity increased again: high-frequency tremor and emissions occurred at a rate of one per minute. Although the summit was obscured by cloud, it was assumed, based on reports from local towns, that ash emissions were continuous. After 1515 seismicity increased to saturation levels on most of the recording instruments. Later, an emission of steam, gas, and ash could be seen. At 1630 seismicity started to decrease.

Small, low-frequency tremor signals began around 0200 on 24 November, and intensified between 0300 and 0600. The tremor was accompanied by continuous emissions of gas, steam, and some ash, blown to the SW. At 1257 another increase of activity began. Low-frequency tremor of variable amplitude was recorded until 1600. Poor visibility prevented direct observation of the summit during most of the day.

A steam plume that rose 2-2.5 km over the summit persisted until 0803 on 25 November when a moderately large explosion lasting one minute produced an ash plume that rose 3-4 km over the summit (figure 28) and threw rock fragments to a distance of 2 km. The top of the plume moved N, while the lower part moved SW; ashfall warnings were issued to towns in those directions. A low-frequency tremor signal followed the explosion and persisted through the day. Other explosions occurred at 1205 and 1658 on 25 November. Although the explosions were heard in nearby towns, there were no reports of large ash emissions, and it is likely that the ejected rock fragments were dispersed around the crater.

Figure 28. Characteristic explosions and associated plumes from Popocatépetl taken by CENAPRED's video monitor during 25-26 November 1998. Scenes 1-3 (top) are from 0803-0805 on 25 November; scenes 4-5 (bottom) are from 1013-1015 on 26 November. Courtesy of CENAPRED.

An increase in tremor was followed by new explosions at 0654 and 0719 on 26 November. Moderate steam-and-ash plumes rose to a height of 1,500 m above the summit. A stronger exhalation at 0931 produced a moderate plume of steam and ash rising 3-3.5 km above the summit. Other explosions at 1013 (figure 28) and 1104 produced higher ash columns. In all cases warnings were issued to air-traffic controllers. A new warning to the general population recommended approaching no closer than 7 km from the crater. Tremor was followed by volcanic earthquakes at 2113 and 2220; both events produced moderately large explosions and ash plumes, and during the later event incandescent lava fragments were thrown to a distance of ~1.5 km.

Moderate explosions were detected in the crater at 1206, 1333, 1749, and 2345 on 27 November, and at 0242 and 1021 on 28 November. All of them, except the third, expelled incandescent fragments of lava around the crater to a distance of 0.5-2 km, and produced moderately large emissions of ash, rising in most cases up to 4 km over the summit. This activity was detected against a background of low-level exhalation and tremor signals of decreasing amplitude. Light ashfall had been reported in Tlacotitlán at 0130 on 28 November. During 28 November activity increased again following several short harmonic tremor signals at 2130. At 2228 a moderate volcano-tectonic event was followed by small tremor episodes.

At 0002 and 0305 on 29 November two explosions were preceded by low-frequency tremor. The second explosion produced a shock wave clearly heard at Paso de Cortes and San Nicolás de los Ranchos. Large quantities of glowing rocks ejected from the crater could be seen falling in a area of ~3 km radius. There was also a large ash emission. At 0654 a moderately large emission, lasting seven minutes, formed an ash plume 4 km above the summit. At 1118 there were several low-frequency harmonic tremors. A moderately large explosion at 1645 ejected incandescent lava blocks around the cone and produced an ash plume up to 7 km above the summit (according to personnel working close to Paso de Cortes).

Tremor episodes and moderate emissions of steam, ash, and gas with occasional explosions persisted over the next week. One explosion at 0929 on 30 November began with a strong shock wave and blast, ejected fragments over its flanks 2-3 km from the crater, and produced an ash column 4 km above the summit. At 1853 on 3 December an explosion ejected incandescent fragments over the SE flanks and produced a moderately large ash cloud, carried by the wind to the SE. The explosion signal lasted one minute, followed by 15 minutes of tremor. At 1255 on 4 December an explosion threw hot debris on the SE flanks and produced an ash plume that rose 4-5 km above the summit. Another explosive eruption at 1511 on 6 December ejected incandescent rocks over the E and N flanks and produced an ash column 5 km above the summit that dispersed to the NW. This event lasted 1.5 minutes and was followed by high-frequency tremor for four minutes. Three explosions were recorded on 7 December at 0241, 0449, and 0623; glowing fragments fell on the E and N flanks and an ash column rose 4 km. The last of these events lasted 1.5 minutes and was followed by high-frequency tremor for 10 more minutes. During 8 December frequent exhalations with durations of 3-10 minutes each produced steam-and-ash columns 2 km above the summit.

Activity became stable at lower levels during the second week of December, persisting until the time of this report (15 December).

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: Servando De la Cruz-Reyna^1,2, Roberto Quaas^1,2, Carlos Valdés G.², and Alicia Martinez Bringas¹. 1 Centro Nacional de Prevencion de Desastres (CENAPRED) Delfin Madrigal 665, Col. Pedregal de Santo Domingo,Coyoacan, 04360, México D.F. (URL: https://www.gob.mx/cenapred/); 2 Instituto de Geofisica, UNAM, Coyoacán 04510, México D.F., México.

Tilt, leveling, and sea-shore surveys continued to record the slow resurgence of the caldera floor observed since April 1996. During October, continued slow magma supply into Rabaul Caldera kept feeding mild Vulcanian activity at Tavurvur cone. Emissions occurred at irregular intervals, from a few minutes to several hours apart. Longer time intervals usually resulted in more powerful and voluminous explosions.

In the beginning of the month, explosions ejected a grayish ash plume 500-1,000 m above the crater. Following a particularly large explosion at 2138 on October 5 (which littered the cone with incandescent ballistic blocks, and displayed dramatic lightning within the dark rising cloud) emissions were larger for a few days, rising to 1,000-3,000 m, although without sounds. During 10-15 October emissions were again milder, hardly rising over 600 m above the crater. Emissions occurring 16-20 October rose to ~1,000 m and were often accompanied by roaring sounds. After 29 October, emissions were again noiseless, and from the 26th onward they became lower in ash content and energy.

October was the transitional period of wind shift. From the 19th, the NW wind began to dominate and bring welcome relief after seven months of very unpleasant, corrosive, and toxic ashfall to Rabaul and neighboring residents.

The recorded seismicity consisted almost exclusively of low-frequency events accompanying the Vulcanian activity from Tavurvur. However, two types of signals were observed: usual short-duration events, and low-amplitude, long-duration (1-3 minutes) events. Their combined number, with an increase in August and September, averaged 46 per day but increased to 81 and 143 on the last two days of October without any corresponding change in visible eruptive activity. The two types of signals usually occurred in subequal amounts, although on 5-7 October the number of long-lasting events started to dominate, while the shorter events prevailed for a few days after the 8th. The amplitude of both types fluctuated substantially for several multi-day intervals during October. Short-duration harmonic signals were also recorded during 16-18 and 24 October. On 20 October the system registered the month's only significant high-frequency event.

A visit to Rabaul by professional photographer George Casey resulted in several images of Tavurvur during August. Casey appreciated the aid kindly given him by RVO staff and was gracious enough to provide us with photos, including one of a small plume on 4 August (figure 32).

Figure 32. The Tavurvur cone at Rabaul emits a small plume on 4 August in this photograph looking SE from the bay's shore. Courtesy of George Casey.

Figure 33. An E-facing close-up photograph of the Tavurvur cone at Rabaul from the bay's shore; it emphasizes its low conical shape, wide-aperture crater, and ongoing emissions. Courtesy of George Casey.

Figure 34. SE-looking overview photograph taken on 11 August from Rabaul caldera's topographic margin looking out over the city of Rabaul, Simpson harbor, and the cones that bound the harbor's NE side. The now-resurging caldera is breached by the sea on its E side. Tavurvur cone, ~10 km distant, lies in the peninsular lowlands to the right of the highest peak, Mother (Kombiu), and the lower peak, South Daughter. Courtesy of George Casey.

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: Ben Talai and Patrice de Saint-Ours, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.

Seismicity was generally at background levels during 1 November-7 December. Clouds obscured the volcano throughout much of the reporting period. On 1, 2, and 6 November steam-and-gas plumes were seen to rise 300 m above the summit before dispersing. High-frequency tremor increased over six hours on both 13 and 15 November. Periods of high-frequency tremor lasted 0.7 hours on 17 November and 3.5 hours on 22 November. Two hours of high-frequency tremor and 3 hours of low-frequency spasmodic tremor were recorded on 2 December. On 5-6 December a plume rose 150 m above the summit.

Geologic Background. The high, isolated massif of Sheveluch volcano (also spelled Shiveluch) rises above the lowlands NNE of the Kliuchevskaya volcano group. The 1,300 km3 andesitic volcano is one of Kamchatka's largest and most active volcanic structures, with at least 60 large eruptions during the Holocene. The summit of roughly 65,000-year-old Stary Shiveluch is truncated by a broad 9-km-wide late-Pleistocene caldera breached to the south. Many lava domes occur on its outer flanks. The Molodoy Shiveluch lava dome complex was constructed during the Holocene within the large open caldera; Holocene lava dome extrusion also took place on the flanks of Stary Shiveluch. Widespread tephra layers from these eruptions have provided valuable time markers for dating volcanic events in Kamchatka. Frequent collapses of dome complexes, most recently in 1964, have produced debris avalanches whose deposits cover much of the floor of the breached caldera.

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

There was a slight increase in activity in October according to reports from the Montserrat Volcano Observatory (MVO). Five small collapse events occurred on the dome, each producing significant deposits of ash up to 3 km away. Pyroclastic flows occurred along most of the volcano's main drainage. Ash fell predominantly W and NW of the volcano, light ash fell in the N of the island. Dome collapses were commonly followed by periods of volcanic tremor and ash venting, and sometimes swarms of volcano-tectonic earthquakes occurred shortly after the collapse events. The dome gradually eroded, leaving some large fractures in the carapace that could lead to larger collapses in the future.

Visual observations. Intermittent small pyroclastic flows originated from all flanks of the dome. The first significant event, at 0801 on 13 October, produced pyroclastic flows in Tuitt's Ghaut and Tyer's Ghaut. Volcanic tremor after the collapse correlated with ash venting from high on the dome's N flank, the ash cloud rapidly reached 7,500 m. The cloud drifted NW, depositing ash on parts of the island.

At 0916 on 18 October, there was another collapse, the ash cloud rose to around 2,000 m and moved W, although the exact direction was uncertain because a low cloud hampered observation. Subsequent volcanic tremor lasted for several hours.

Another small dome collapse occurred at 2241 on 20 October. The ash cloud from this event rose to an estimated 2,500 m, drifting slowly to the W and NW. Observations the following morning revealed that the pyroclastic flows from this event had traveled towards Plymouth as far as Upper Parsons (2.5 km W of the summit). Fallout included some coarse lithic fragments 4 to 5 mm in diameter.

At 0051 on 26 October, a fourth small collapse occurred. The seismic signal lasted for about 12 minutes followed by an extended period of tremor. Reports were received of thunder from the resultant ash cloud, and there was subsequent wet ashfall as far as 7 km N. Information received from NOAA satellite images indicated that the ash cloud reached to between 6,000 and 7,500 m. Observations during the early hours of the morning suggested that there were two ash cloud lobes, one S of Belham Valley and one over the Salem-Old Towne area. The deepest measured ashfall was 25 mm; 4 mm or more fell in other areas. The ash was fine grained, with common accretionary lapilli. During an observation flight on the 27th, steaming could be seen at the edge of the delta, indicating that the pyroclastic flows had traveled into the sea. The flows also reached NE as the Tar River Estate House (3 km from the summit). On the SW side, down the White River, a thin deposit of ash from the pyroclastic flows could be seen as far as about 700 m from the old coastline at O'Garras; when these deposits were emplaced is unknown.

A fifth small dome collapse occurred at 0418 on 31 October; an ash plume first drifted W, and thenN and NE depositing some ash in occupied areas at the island's N end. An observation flight later that day revealed new deposits: a pyroclastic-flow deposit in the White River reaching Galways Soufriere, and another in the Gages valley that did not extend beyond the top of the Gages fan. The White River deposit had numerous large angular blocks resting on its surface.

A large fissure within the dome extended from its base, where it rests against Chances Peak, to its top in the Galways area (S). At the foot of this crack a triangular-shaped opening had developed and appeared to have been the source of the White River pyroclastic-flow.

Unusual wind directions during the latter part of October directed the plume to the N. As a result, residents in N Montserrat smelled strong sulfurous odors.

On 27 October, probing into the pyroclastic deposits in the area of the Farm River in Trant's yielded these depth-temperature relations: 1.0 m and 86°C; 1.4 m and 146°C; and 2.25 m and 239°C. Unusually clear conditions in the early evening of 27 October enabled observers in Old Towne and Salem to see three small glowing areas on the dome; these areas were thought to reveal the dome's incandescent interior exposed during the recent collapse events.

Seismicity, deformation, and environmental monitoring. Over the reporting period seismicity was generally low; however, small dome collapses triggered volcanic tremor and swarms of volcano-tectonic earthquakes. As in the previous month, tremor correlated with intensified ash-and-steam venting from the N flanks of the dome.

Five small collapses occurred between 13 and 31 October. These were marked by pyroclastic-flow signals that lasted several minutes. The collapse on the 13th was preceded by a swarm of small volcano-tectonic earthquakes. Several much larger volcano-tectonic earthquakes occurred during the collapse, the first approximately 30 seconds after the start of the collapse; hypocenters for these events were tightly clustered directly under the lava dome.

The collapse on the 18th was accompanied by a more intense swarm of earthquakes (table 32). The first earthquake occurred about 40 seconds after the beginning of the collapse and was one of the largest earthquakes recorded since the installation of the broadband network; it was felt in the Woodlands area. This earthquake was much richer in low frequencies than typical volcano-tectonic earthquakes on Montserrat, possibly suggesting a larger source dimension. Hypocenters for the largest earthquakes were located S of the volcano. At the start of the swarm, hypocenters were directly under Roaches Mountain; as the swarm progressed, hypocenters migrated to S of Chances Peak. Preliminary calculations showed that the largest events were consistent with oblique-normal faulting in a NE-SW direction.

Table 32. October 1998 earthquake swarms at Soufriere Hills. Courtesy of MVO.

All GPS sites on the volcano and in the N of the island appear stable and there were no significant changes since last month. The EDM reflector on the northern flank was shot from Windy Hill. The line continues to shorten slowly. The site was later destroyed by a pyroclastic flow.

SO₂ flux, measured using the miniCOSPEC instrument, was (in metric tons/day) 1,300 on 9 October, 340 on 21 October, and 280 on 30 October. These results are similar to those measured in recent months, although an apparent decrease occurred late in the month. Sulfur dioxide was also measured at ground level using diffusion tubes around the island. SO₂ in Plymouth (at Police Headquarters) remained high; elsewhere the average levels were very low.

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

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat, West Indies (URL: http://www.mvo.ms/).

On the basis of waveform features and locations, earthquakes in the vicinity of the volcano during November were identified as constituting a separate group. Since September 1998 more than 20 events with magnitudes ranging from 0.5 to 1.0 occurred at shallow depths (<5 km).

Geologic Background. The Ushkovsky (formerly known as Plosky) complex is a large compound volcanic massif located at the NW end of the Kliuchevskaya volcano group. The summit of Krestovsky (Blizhny Plosky) volcano, about 10 km NW of Kliuchevskoy, is the high point of the complex. Linear zones of cinder cones are found on the SW and NE flanks and on lowlands to the west. The Ushkovsky (Daljny Plosky) edifice SE of Krestkovsky is capped by an ice-filled 4.5 x 5.5 km caldera containing two glacier-clad cinder cones with large summit craters. A younger caldera at the summit of Daljny was formed in association with the eruption of large lava flows and pyroclastic material from the Lavovy Shish cinder cones at the foot of the volcano about 8,600 years ago. An explosive eruption took place from the summit cone in 1890.

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

This report summarizes daily visual observations by members of the Proyecto de Observación Villarrica (POVI), volcano guides, and other sources during February to November 1998. In late February, after two months of subsidence, the magmatic column reached the crater floor with a weak and irregular degassing. By mid-March the lava pond was clearly visible as an intermittent red glow from 12 km away. In April and May, three convective magmatic pushes, gas-poor, filled half of the funnel-shaped crater with pahoehoe lava. On 13, 25, and 30 June, small phreatic emissions rose up to 200 m above the summit. Since mid-October, the activity level in the lava pond has varied, with the low levels of degassing intensity occurring at irregular intervals. On 8 November, the red glow was seen for the only time that month.

It is inferred that the red glow indicates that a small volume of usually gas-enriched magma has reached the crater floor in phases and at irregular intervals. This causes a sudden occurrence of the glow, sometimes with increasing intensity and lasting from a few hours up to 3 days. Subsequently, a distinct reduction of the glow intensity is interpreted to mean that an insufficient supply of convecting magma and gas allows the lava pond to form a crust. During the report period, 16 such magmatic pulses were observed and 10 additional pulses were inferred for periods of non-observation due to weather conditions.

Geologic Background. The glacier-covered Villarrica stratovolcano, in the northern Lakes District of central Chile, is ~15 km south of the city of Pucon. A 2-km-wide caldera that formed about 3,500 years ago is located at the base of the presently active, dominantly basaltic to basaltic andesite cone at the NW margin of a 6-km-wide Pleistocene caldera. More than 30 scoria cones and fissure vents are present on the flanks. Plinian eruptions and pyroclastic flows that have extended up to 20 km from the volcano were produced during the Holocene. Lava flows up to 18 km long have issued from summit and flank vents. Eruptions documented since 1558 CE have consisted largely of mild-to-moderate explosive activity with occasional lava effusion. Glaciers cover 40 km2 of the volcano, and lahars have damaged towns on its flanks.

Information Contacts: Werner Keller U., Proyecto de Observacion Villarrica (P.O.V.I.), Wiesenstrasse 8, 86438 Kissing, Germany (URL: https://www.povi.cl/).

Minor eruptive activity continued at White Island through November and early December. The level of activity varied, but observations during visits and instrumental indicators in early December were sufficient to raise the Alert Level from 1 to 2 on 3 December. The current style of activity was expected to continue for some time.

There was evidence that molten magma was the direct cause of eruptive activity, although only weak volcanic tremor accompanied the ash eruptions. A surveillance visit was made on 1 December to assess the ongoing activity, conduct deformation and magnetic surveys, and collect ash and gas samples.

Observations. The active vent at the base of the NW wall of 1978/90 Crater continued to erupt fine-grained volcanic ash during the 1 December visit. The vent size had not changed since the 2 November visit (BGVN 23:10). During the later visit, an ash-charged, tan-brown convecting plume rose to ~800 m before trailing downwind 10-15 km. The volume of ash in the plume was greater than that observed any time during November. The eruptive activity had deposited up to 45 mm of fine, dark gray and brown ash at the crater rim. Samples of ash that fell on 1 December showed a significant change from ash collected on 23 November and earlier. The 1 December ash samples contained fresh, vesiculated glass, suggesting that magma may have risen in the vent and was contributing directly to the eruption. Previously the ash was derived from solidified lava.

A ground-deformation survey showed a consistent trend of minor inflation across the main crater floor, with continued subsidence near the rim of 1978/90 Crater (figure 34). Large-scale post-1990 inflation was evident at the more distal sites (Pegs C and J), with only minor changes over the last 2-3 months. Collapse about the crater rim, which started in July, was continuing but at a lesser rate (Pegs M and W). Provisional results from the magnetic survey indicated heating at depth and shallow cooling about the crater rim area.

Figure 34. Contour plot from White Island showing the height changes (mm) between measurements made 2 November and 1 December 1998. Courtesy IGNS.

Fumarolic discharge pressures from sites 1, 6a (base of Donald Mound), and 13a were not significantly stronger than those observed on 2 and 16 November, and temperatures remained high at these features: site 1, 124°C; site 6a, 107°C; and site 13a, 120°C. Molten sulfur was found in vents at sites 1 and 13a, which is consistent with the temperatures in excess of 119°C. The sulfur mound at site 1 had grown over the vent during November, suggesting that sulfur was being remobilized from depth in response to elevated temperatures. The discharge at site 6a was mildly superheated, but of high pressure, indicating a relatively high gas content. These observations were consistent with general heating of the hydrothermal system.

The lake, which had reformed in the main crater, was the likely result of recent rains. The lake water was cool (~20°C) and had the brown color of the ash falling into it.

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

Information Contacts: B.J. Scott, Manager of Volcano Surveillance, Institute of Geological and Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand (URL: http://www.gns.cri.nz/).