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, which have produced lava lakes, explosions, lava flows, ash, and gas emissions. The previous eruption period, which began in May 2008, ended in December 2018 with lava fountains, lava flows, a submarine fissure eruption, and drainage of the lava lake (BGVN 45:03). After those events no eruptive activity was reported until January 2022, which consisted of ash plumes and crater incandescence. This report updates information from April 2020 through March 2022 and describes a short eruption period during 25 January to the beginning of February 2022. Information comes from the Vanuatu Meteorology and Geohazards Department (VMGD) and satellite data.

Activity during April 2020 through December 2021 was relatively low and consisted mainly of gas-and-steam emissions from both Benbow and Marum craters. On 25 January 2022 the Volcanic Alert Level (VAL) was raised from Level 1 to 2 (on a scale of 0-5) due to increased activity beginning around 0400. Significant gas-and-steam emissions were observed rising from Marum, and gas-and-ash emissions rose from Benbow at 0515 (figure 52). A sulfur dioxide plume that exceeded 2 DUs (Dobson Units) was detected on 25 January and drifted SE, following the ash plume, based on data from the Sentinel-5 instrument (figure 53). At night on 25 January, crater incandescence from Benbow was visible in webcam images, which represented a lava flow that had effused from a new vent on the NW part of the crater floor. Incandescence persisted through 27 January, according to VMGD (figure 54).

Figure 52. Webcam image of Ambrym at 0645 on 25 January 2022 showing an ash plume rising above the Benbow crater. Courtesy of VMGD.

Figure 53. A sulfur dioxide plume was detected on 25 January 2022 based on Sentinel-5P data after increased activity, including an ash plume, occurred at Ambrym. The plume drifted SE. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Figure 54. Webcam image showing visible nighttime incandescence coming from the Benbow crater at Ambrym at 2000 on 25 January 2022. Activity was visible through the early morning of 26 January, accompanied by gas-and-steam emissions. Courtesy of VMGD.

By early February, crater incandescence was no longer visible. On 2 February sulfur dioxide emissions were still detected in satellite images and drifted E. According to VMGD, no gas-and-steam emissions were visible from Benbow during early February through April and seismicity had decreased. As a result, the VAL was lowered to Level 1 on 28 April.

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

Information Contacts: 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/); 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/).

Popocatépetl is located 70 km SE of Mexico City, Mexico, and contains a 400 x 600 m wide crater. Records of activity date back to the 14th century and the current eruption period has been ongoing since January 2005, which has included numerous episodes of lava-dome growth and destruction within the summit caldera. Recently, activity has consisted of intermittent crater incandescence, frequent ash explosions, and ash emissions. This report covers the period from August 2021 through January 2022, characterized by daily low-intensity gas-and-ash emissions, volcano-tectonic tremors, and crater incandescence, based on information from México's Centro Nacional de Prevención de Desastres (CENAPRED) and various satellite data.

Gas-and-steam emissions, some of which contained ash, remained ongoing throughout August 2021-January 2022. Some minor ashfall was reported during mid-September. CENAPRED reported the number of low-intensity gas-and-ash emissions or “exhalations” and the number of minutes of tremor in their daily reports (figure 185). Tremor activity reached an average of 261 minutes per day from August to January 2022, but dropped off significantly after September. A total of about 72 volcano-tectonic (VT) tremors were detected throughout the reporting period. The average number of gas-and-ash emissions was 54 per day, with a maximum number of 260 on 25 September. Occasional small to moderate sulfur dioxide plumes were detected with satellite instruments each month (figure 186).

Figure 185. CENAPRED reported the number of daily “exhalations” (in blue, left scale), and the number of minutes of tremor (in gold, right scale) at Popocatépetl each day during August 2021-January 2022. The number of daily exhalations fluctuated throughout the period, reaching as high as 260 on 25 September 2021, followed by a gradual decline through the rest of the reporting period. Similarly, there was some fluctuation in the duration of tremors during August through September; on 15 August 2021 a maximum duration of 1,192 minutes of tremor was detected. After September, the minutes of tremor decreased significantly and often were not detected. Data from CENAPRED daily reports.

Figure 186. Moderate sulfur dioxide plumes were detected at Popocatépetl intermittently from August 2021 through January 2022, as seen on 26 October (top left), 14 November (top right), 3 December (bottom left) 2021, and 2 January (bottom right) 2022. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

MODIS thermal anomaly data provided through MIROVA showed low-level, intermittent activity during August through early October 2021, followed by a brief break in activity (figure 187). Thermal activity resumed in early November at a higher level and frequency compared to previous months. According to data from MODVOLC thermal alerts, a total of nine hotspots were detected on 10 and 16 September, 14 and 28 November, 6 December, and 1, 2, and 15 January 2022. Sentinel-2 infrared satellite imagery also showed persistent thermal anomalies in the summit crater multiple times each month from August 2021 through January 2022 (figure 188).

Figure 187. The MIROVA graph of thermal anomalies at Popocatépetl from 11 April 2021 through January 2022 shows low-level, intermittent activity during May through early October, followed by a short break. Activity increased in both power and frequency in November and remained elevated through January. Courtesy of MIROVA.

Figure 188. Sentinel-2 infrared satellite images showing thermal anomalies in the summit crater of Popocatépetl on 9 August (top left), 3 October (top right), 12 November (middle left), 27 November (middle right), 7 December (bottom left) 2021, and 31 January (bottom right) 2022. Images use Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Activity during August and September was relatively high compared to the rest of the reporting period. There were an average number of 92 exhalations per day, which consisted mostly of water vapor, volcanic gases, and a small amount of ash, the most of which occurred on 25 September with 260. On clear nights, crater incandescence was observed, though cloudy weather was common. About 36 VT earthquakes were also noted throughout the two months. A total of 46 minor to moderate explosions were detected. A moderate explosion was reported on 6 August at 2137, accompanied by incandescent material that was ejected a short distance from the crater.

Explosions were detected at 2135, 2254, and 2345 on 1 September, followed by a VT earthquake. Two explosions on 11 September (at 1222 and 1323) generated ash plumes that rose 1.8-2 km above the crater, accompanied by incandescent material ejected from the crater. A gas-and-ash plume at 1015 on 15 September rose 2.2 km above the crater; a minor explosion at 1441 that day produced an ash plume 1.8 km above the crater and drifted NW. According to information from the National Center for Communication and Civil Protection Operations (CENACOM), light ashfall was reported in the municipalities of Valle de Chalco, Chiautla, Ixtapaluca, Nezahualcóyotl, La Paz, Ecatepec, Ayapango, Temamatla, Tenango del Aire, Tlalmanalco, Amecameca, Tepetlixpa, Tlalnepantla, and Acolman in the México State. Three minor explosions on 17 September resulted in light ashfall in the municipalities of Valle de Chalco, Ixtapaluca, Chalco, Tlalmanalco, Amecameca, Ayapango, Tenango del Aire, Temamatla, and Ecatzingo, as well as in Iztapalapa, Xochimilco, and Tlahuac in México City. Some ashfall was reported on 18 September in Cuernavaca.

The number of daily exhalations and duration of VT earthquakes declined after September, though they continued to be detected during October and November, with an average of 43 exhalations per day; 129 exhalations occurred on 2 October. A total of 45 VT earthquakes was detected during these two months and 11 minor to moderate explosions were recorded. On 2 October an explosion at 0619 generated an ash plume that rose 1.2 km above the crater (figure 189) and ejected incandescent material onto the slopes. A low-intensity explosion detected at 0057 on 5 October produced an ash plume that rose 800 m above the crater and drifted W.

Figure 189. Webcam image of a gas-and-ash plume that rose 1.2 km above the crater of Popocatépetl on 2 October 2021. Courtesy of CENAPRED daily report.

The Institute of Geology at the National Autonomous University of Mexico (UNAM), with support from the National Guard and CENAPRED, conducted an overflight on 5 November to make visual observations; the internal crater had a diameter of 390-400 m and a depth of 160-200 m (figure 190), and no new lava was seen. On 19 November an explosion at 1714 generated an ash plume that rose 2 km above the crater and drifted NE. A minor explosion on at 0230 on 21 November produced an ash plume that rose 600 m above the crater and drifted NW, and at 0136 the next day another small explosion produced an ash plume that rose 800 m above the crater and drifted NE.

Figure 190. Aerial photo of the summit crater and inner crater of Popocatépetl taken on 5 November 2021. The internal crater had an approximate diameter of 400 m and a depth of 200 m. Courtesy of CENAPRED daily report.

Activity remained relatively low during December and January 2022, consisting of an average number of 27 exhalations reported per day, a total of 47 VT earthquakes, and only six minor to moderate explosions, most of which occurred during January. A minor explosion was detected on 13 January at 0420, producing an ash plume that rose 600 m above the crater and drifted SE. The next day on 14 January a minor explosion at 2255 generated a plume containing a small amount of ash rose 1 km above the crater and drifted NE. Some material was also ejected a short distance from the crater (figure 191).

Figure 191. Webcam image of an explosion at Popocatépetl on 13 January 2022 that generated an ash plume 1 km above the crater that drifted NE. Incandescent material was ejected a short distance from the crater. Courtesy of CENAPRED daily report.

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); 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Ibu is located along the NW coast of Halmahera Island in Indonesia and contains an inner crater measuring 1-km-wide and 400 m deep, and an outer crater that measures 1.2-km-wide breached on the N side. The first observed and recorded eruption occurred in 1911 and consisted of a small explosion in the summit crater. The current eruption period began in April 2008 and has recently been characterized by intermittent low-level ash plumes, light ashfall, and thermal anomalies (BGVN 46:09). This report covers similar activity of low-level ash plumes, minor ashfall, and thermal activity during September 2021 through February 2022 using information from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Darwin Volcanic Ash Advisory Centre (VAAC), and various satellite data. The Volcano Alert Level remained at 2 (on a scale of 1-4), and the public was warned to stay at least 2 km away from the active crater and 3.5 km away on the N side.

Activity during this reporting period was relatively low, primarily consisting of occasional ash plumes rising to 2.4 km altitude, minor ashfall, and thermal activity. Daily explosions ranged from 55-102; the greatest number of eruption events was detected on 8 September 2021. Frequent white-and-gray emissions rose 200-1,000 m above the summit and drifted in different directions, according to PVMBG. Data from the MIROVA plot (Log Radiative Power) showed intermittent low-to-moderate thermal anomalies during September 2021 through mid-February 2022, including a short break in activity during late November to mid-December 2021 (figure 36). According to MODIS data from MODVOLC, a total of 10 thermal alerts were detected on 28 September, 15 and 19 October, 13, 16, and 27 November 2021, 28 January 2022, and 4 February. Thermal anomalies were also captured in Sentinel-2 infrared satellite images on 2 September and 12 October 2021, and again on 5 January and 14 February 2022 (figure 37).

Figure 36. Persistent low- to- moderate-power thermal anomalies were detected at the summit crater of Ibu during September 2021 through mid-February 2022, though a short break in activity occurred during late November to mid-December 2021, as shown by the MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 37. Thermal anomalies of varying intensity were visible in the summit crater of Ibu on 2 September 2021 (top left), 12 October 2021 (top right), 5 January 2022 (bottom left), and 14 February 2022 (bottom right). Two anomalies, one in the summit crater and a smaller one to the NW of it, appeared on 12 October, 5 January, and 14 February. Images using “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

Intermittent ash plumes were reported during September 2021 while white-and-gray emissions rose 200-800 m above the summit each day. Daily eruption events ranged from 55-102, the higher number occurring on 8 September, according to PVMBG daily reports. On 6 September an ash plume rose 400 m above the summit at 0916 and drifted N. Another ash plume on 18 September rose 1 km above the summit at 1135 and drifted W (figure 38). Two avalanches were reported on 30 September traveling 1 km to the W.

Figure 38. Images of various ash plumes rising above Ibu on 18 September (top left), 11 November (top right), 18 December (bottom left) 2021, and 16 February (bottom right) 2022 and drifting in different directions. The ash plume on 18 September rose 1 km above the summit and drifted W. The ash plume on 16 February rose to 2.1 km altitude and drifted S. Courtesy of MAGMA Indonesia.

White-and-gray emissions persisted every day during October, rising 200-800 m above the summit. Daily eruption events ranged from 0-81, the most of which occurred on 28 October. Some light ashfall was reported in the towns to the W of the volcano on 1 October. On 7 and 19 October an explosion generated an ash plume that rose 800 m above the summit. According to a VONA issued on 9 October, an ash plume rose to 1.7 km altitude at 1651 and drifted NE. Another VONA published on 16 October described an ash plume that rose 600 m above the summit at 0838 and drifted N. On 31 October, an ash plume rose 500 m above the summit at 0818.

Similar low activity continued in November; white-and-gray emissions rose 200-800 m above the summit and intermittent ash plumes rose as high as 2.3 km altitude. Daily eruption events ranged from 1-87, the most of which occurred on 26 November. On 6 November an ash plume rose to 2.3 km altitude at 0637 and drifted W and S. Two days later, on 8 November, an ash plume rose 1 km above the summit at 0957 and drifted S. During 10-11 November, ash plumes drifted W, causing some light ashfall in communities to the W of the volcano. VONAs issued on 16 and 19 November described ash plumes rising 1-8-2.3 km altitude that drifted N and S.

During December, the primary form of activity was white-and-gray emissions rising 200-1,000 m above the summit. Daily eruption events ranged from 56-87, the most of which occurred on 18 December. On 20 December, an avalanche traveled 100-400 m to the NW.

Low activity during January 2022 consisted of daily white-and-gray emissions that rose 200-1,000 m above the summit. During 1-4 January, avalanches were detected each day, but they were not visually observed. Daily eruption events ranged from 66-94, the most of which occurred on 17 January. During 8-9 January, strong rumbling sounds were heard and minor ashfall was reported to the W of the volcano.

During February, white-and-gray emissions persisted, rising 200-1,000 m above the summit, while intermittent ash plumes rose to 2.3 km altitude. Daily eruption events ranged from 46-82, the most of which occurred on 2 and 4 February. Ash plumes were reported on 14, 16, 18, and 23 February, according to VONA notices, rising to 2.1-2.3 km altitude and drifting W, S, and SW (figure 38).

Geologic Background. The truncated summit of Gunung Ibu stratovolcano along the NW coast of Halmahera Island has large nested summit craters. The inner crater, 1 km wide and 400 m deep, has contained several small crater lakes. The 1.2-km-wide outer crater is breached on the N, creating a steep-walled valley. A large cone grew ENE of the summit, and a smaller one to the WSW has fed a lava flow down the W flank. A group of maars is located below the N and W flanks. The first observed and recorded eruption was a small explosion from the summit crater in 1911. Eruptive activity began again in December 1998, producing a lava dome that eventually covered much of the floor of the inner summit crater along with ongoing explosive ash emissions.

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

Villarrica has had documented eruptions dating back to 1558, which have consisted largely of mild-to-moderate explosive activity and occasional lava effusions. Its currently active cone has a 2-km-wide caldera at its base. The current eruption period has been ongoing since December 2014 and more recently has been characterized by low-level activity of thermal anomalies, gas-and-steam emissions, and sulfur dioxide emissions. This reporting period of September 2021 through February 2022 continues this pattern of low-level activity. Information for this report primarily comes from the Southern Andes Volcano Observatory (Observatorio Volcanológico de Los Andes del Sur, OVDAS), part of Chile's National Service of Geology and Mining (Servicio Nacional de Geología y Minería, SERNAGEOMIN) and satellite data.

Seismicity and thermal activity were relatively low in September. Continuous tremor, as many as 162 long period (LP), and 4 volcano-tectonic (VT) seismic events were recorded. Gas-and-steam emissions rose less than 500 m above the crater rim. A single thermal anomaly was visible above the volcano on 20 September, according to MIROVA data, and three anomalies were detected in Sentinel-2L2A satellite imagery on 7, 10, and 20 September (figure 116).

Figure 116. Intermittent thermal anomalies of varying intensity were visible in Sentinel-2 infrared satellite images of Villarrica during September 2021 through February 2022 on clear weather days. Images use Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

The number of LP-type events increased during October; as many as 2,142 were detected, in addition to 5 VT-type events. White gas-and-steam emissions of varying intensities throughout the month rose 1 km above the crater rim. MIROVA reported only one thermal anomaly, on 26 October, and Sentinel-2L2A imagery showed six anomalies on 7, 10, 12, 17, 27, and 30 October, according to SERNAGEOMIN (figure 116).

LP, tremor (TR), and VT-type seismic events were detected during November; about 504 LP and 88 VT-type earthquakes were recorded. The highest magnitude earthquake was 3.4, detected on 28 November about 10.8 km ESE of the crater. Throughout the month, gas-and-steam emissions rose less than 500 m above the crater rim. According to a notice from the Buenos Aires VAAC, an ash plume rose to 3.4 km altitude and drifted SE on 6 November. Analysis of Sentinel-2L2A satellite images showed nine thermal anomalies that occurred on 1, 6, 11, 16,19, 21, 24, 26, and 29 November (figure 116).

The number of LP earthquakes increased during December. About 2,888 LP earthquakes were recorded over the month, in addition to as many as 614 TR-type events. Gas-and-steam emissions persisted, rising less than 500 m above the crater rim. According to data from Sentinel-2L2A satellite imagery, ten thermal anomalies were visible in the crater on 4, 9, 14, 16, 19, 21, 24, 26, 29, and 31 December. SERNAGEOMIN used equipment for Differential Absorption Optical Spectroscopy (DOAS) to measure an average sulfur dioxide value of 412 and 608 t/d on 28 and 30 December, respectively.

During January 2022, LP and TR-type seismicity persisted, the latter of which presented an energy value (RSAM) between 0.1 and 0.4 units. About 3,766 earthquakes were detected, most of which were LP type, 53 were TR-type events, and five were VT-type events. Consistent gas-and-steam emissions rose to a maximum height of 540 m on 6 January. Based on DOAS measurements of sulfur dioxide emissions, an average value of 595 t/d was recorded, with a daily maximum value of 1,763 t/d on 19 January. Sentinel-2L2A satellite images showed a total of 8 thermal hotspots within the crater on 5, 8, 10, 13, 15, 25, 28, and 30 January (figure 116).

Seismicity in February was considered low, with the RSAM units measuring between 0.2 and 0.5. About 2,811 LP-type, 4 TR-type, and 1 VT-type seismic events were detected. The average value of sulfur dioxide emissions was 451 t/d, with a daily maximum value of 774 t/d on 15 February. On 2 February an ash plume rose to 2.7-4.6 km altitude and drifted E at 1050, based on data from two webcam images and information from SERNAGEOMIN. By 1130 the plume was barely visible in satellite images. Webcam images showed continuous gas-and-steam emissions with sporadic puffs of ash rising as high as 4.9 km altitude, although by 2330, the ash was no longer visible. There were seven thermal anomalies detected during the month on 2, 4, 7, 9, 12, 14, and 17 February (figure 116). Low-intensity incandescence was visible from 12 to 15 February. Gas-and-steam emissions persisted, with a maximum height of 520 m above the crater rim on 21 February.

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

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

The Santiaguito lava-dome complex of Guatemala's Santa María volcano has been actively erupting since 1922. It formed within a large crater on the SW flank which was created during the 1902 eruption. Ash explosions, pyroclastic flows, and lava flows have emerged from Caliente, the youngest of the four vents in the complex, for more than 40 years. The Caliente vent has an elevation of about 2.5 km, and the summit of Santa Maria is about 3.7 km elevation. A lava dome that appeared within the summit crater of Caliente in October 2016 has continued to grow, producing frequent block avalanches down the flanks. Daily explosions with ash plumes and block avalanches continued during this report period of August 2021-January 2022, with information primarily from Guatemala's INSIVUMEH (Instituto Nacional de Sismologia, Vulcanologia, Meterologia e Hidrologia).

Activity during August consisted of gas-and-steam emissions rising to 3-3.3 km altitude and drifting SW, weak to moderate explosions that produced gray ash plumes to 2.8-3.5 km altitude that drifted mainly W and SW, and constant weak to moderate block-and-ash avalanches down the W, SW, and S flanks, some of which reached the base of the lava dome. As a result of the avalanches, some fine ash particles were occasionally detected around the volcano. Crater incandescence was frequently observed. On 4 August an explosion generated an ash plume that rose to 2.8 km altitude and drifted W and SW for 6 km, followed by fine ashfall in the villages of Loma Linda Palajunoj, San Marcos Palajunoj, and farms in the surrounding area. Incandescent block avalanches traveled 600 m down the W flank on 8, 18, and 23 August. Explosions on 14 and 24 August produced ash plumes that rose to 2.8 km altitude and drifted as far as 8 km S and SW, resulting in ashfall in the villages of Las Marías and Calaguache on the 14th and in San Marcos and Loma Linda Palajunoj, and farms in the area on the 24th. Due to heavy rains on 23 August, lahars were detected in the Cabello de Ángel, located S of the volcano, according to CONRED.

Similar explosions and ash plumes continued in September. Gas-and-steam plumes rose to 2.7-3.5 km altitude and drifted generally W and SW from the Caliente vent. Weak to moderate explosions produced ash plumes that rose to 3-3.4 km altitude and drifted primarily to the W and SW. Incandescent block-and-ash avalanches descended mainly the W, SW, and S flanks, which sometimes reached the base of the dome and continued to generate fine ashfall surrounding the volcano. Heavy rainfall on 1 September generated lahars in the Cabello de Ángel river south of the volcano. The river carried blocks up to 1 m in diameter, fine sediments, and tree trunks and branches. On 17 September the smell of sulfur was reported from several communities up to 7 km S of the lava dome as block avalanches continued down the W flank of the Caliente dome. An explosion on 18 September produced an ash plume that rose to 3 km altitude that dispersed W as far as 10 km, and resulted in ashfall in San Marcos Palajunoj, Loma Linda Palajunoj, and farms in the area. Strong rainfall resulted in lahars descending the San Isidro and Cabello de Ángel rivers, moving blocks 30 cm to 1 m in diameter and material that was deposited on the upper part of the volcano.

Explosions and ash plumes persisted in October. Gas-and-steam emissions rose 3-3.5 km altitude and gray and white eruption columns rose to 3-3.4 km altitude that drifted generally S and SW. Block avalanches descended the W, SW, and S flanks of the Caliente dome, commonly generating fine ashfall around the volcano. On 3 October an explosion generated an ash plume that rose to 2.8-3 km altitude and drifted S and SW as far as 10 km, resulting in ashfall in San Marcos Palajunoj, Loma Linda Palajunoj, and farms in the area. Lahars were detected in the El Tambor river on 19 October, located SSW of the volcano that carried blocks up to 3 m in diameter, fine sediment, and tree trunks and branches.

During November, weak to moderate block avalanches mainly affected the SW, W, NW, and S flanks of the Caliente dome. Weak to moderate explosions produced white and gray ash plumes that rose to 3.5 km altitude and drifted generally SW and W, sometimes resulting in ashfall near the volcano. Constant incandescence was observed above the lava dome in addition to gas-and-steam emissions rising to 3 km altitude (figure 129). On 2 November an explosion produced an ash plume that rose to 3.4 km altitude and drifted SW, causing ashfall in the Monte Claro area. On 20 November an explosion generated an ash plume that rose to 3.4 km altitude and drifted W and SW and ashfall was reported in Monte Claro. Explosions during 28 and 29 November ejected ash plumes to 3.3-3.4 km altitude that extended 10 km to the W, NW, SW, and S, causing ashfall in the villages of San Marcos Palajunoj and Loma Linda Palajunoj.

Figure 129. Photo of white gas-and-steam emissions rising above the Caliente dome of Santa María at 0845 on 24 November 2021. Courtesy of CONRED (Informative Bulletin No. 532-2021).

Activity consisting of weak degassing over the Caliente dome rising 150-800 m above the vent and weak to moderate avalanches persisted during December. Some of the avalanches were preceded by explosions; effusive activity continued to build up the W flank and the lava dome. Intermittent ash plumes rose to altitudes of 3-3.4 km and extended generally to the S, SW, and W. On 3 December an ash plume rose to 3 km altitude and drifted NW and W as far as 10 km and, as a result, ashfall was reported in San Marcos Palajunoj and Loma Linda. An explosion on 13 December produced an ash plume that dispersed to the W and SW as far as 15 km and ashfall was reported in the villages of San Marcos and Loma Linda Palajunoj. Ash was also reported in these villages on 17, 18, 28, and 31 December. During the night and early morning of 16-17 December intense incandescence was observed on the W and SW edge of the Caliente dome, accompanied by block avalanches descending the W, SW, and S flanks. On 29 December an explosion expelled an ash plume to 3.2 km altitude that drifted for 10 km to the W and NW, followed by fine ashfall in Loma Linda Palajunoj, San Marcos Palajunoj, and Llanos del Pinal.

White and gray gas-and-steam emissions rose 100-600 m above the crater that generally expanded to the W and SW and weak-to-moderate explosions ejecting ash plumes to 2.8-3.4 km altitude persisted during January 2022. At night and during the early morning, persistent and strong incandescence was observed from the Caliente dome. Frequent moderate-to-strong block avalanches continued descend mostly the W, SW, and S flanks, and weaker ones down the NE, SE, and E flanks, some of which reached the base of the dome. During 2-3, 8, 12-14, 17-18, 22-23 January ash plumes rose to 3.1-3.3 km altitude that drifted W and NW for 10-15 km, causing ashfall in the villages of Loma Linda Palajunoj, San Marcos Palajunoj, and Llanos del Pinal. On 7, 17, 22 January two lava flows were reported on the W and SW flanks with lengths of 500 and 700 m, respectively. In addition, explosions generated an ash plume that rose to 3.2 km altitude and drifted W and NW for 10 km, causing ashfall in the villages of Loma Linda Palajunoj, San Marcos Palajunoj, and Llanos del Pinal. On 29 January moderate to strong ashfall was reported in Loma Linda Palajunoj, San Marcos Palajunoj, El Palmar, and Quetzaltenango. From the early morning on 30 January through 1800 on 31 January, 10 pyroclastic flows were detected by seismic sensors and surveillance cameras. Gas-and-steam emissions persisted over the Caliente dome as high as 600 m.

Intermittent low-level thermal activity was recorded during August through October 2021, as reported by MIROVA (figure 130). Beginning in November, the frequency and power of the anomalies gradually increased through January 2022. According to MODVOLC infrared satellite data from NASA’s MODIS instrument, a total of 21 hotspots were detected over the Caliente vent on 17, 19, 23, 26, and 30 December and 2, 12, 18, 19, 20, 24, and 29 January. On clear weather days, Sentinel-2 infrared satellite imagery showed a small thermal anomaly over the Caliente vent; on 2 January 2022 a linear anomaly was visible on the W flank, which was likely the result of incandescent blocks or a pyroclastic flow (figure 131).

Figure 130. Low-level intermittent thermal activity was detected at Santa María during August through October 2021, based on data from MIROVA (Log Radiative Power). Anomalies were followed by an increase in both frequency and power beginning in November and continuing through January 2022. Courtesy of MIROVA.

Figure 131. Sentinel-2 infrared satellite imagery showed a small thermal anomaly over the Caliente vent of Santa María on 24 October (top left), 13 November (top right), 3 December (bottom left) 2021, and 2 January (bottom right) 2022. A linear thermal anomaly was visible on the W flank originating from the Caliente vent on 2 January, which was likely due to either incandescent blocks or a pyroclastic flow. Images use Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

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

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

Grímsvötn is a volcano located under the Vatnajökull glacier in the southeastern quadrant of Iceland (figure 19). Its most recent eruption in 2011 included explosions with multiple 15-20 km altitude ash plumes that produced ashfall tens of kilometers away (BGVN 36:06). Periodic jökulhlaups (glacial outburst floods) from Vatnajökull have been recognized for centuries (Einarsson, 2009), and have occurred regularly since the end of the last ice age when a lake fed by glacial meltwater breaches its dam and drains. The best known jökulhlaups from Vatnajökull occur from three separate places: Grímsvötn volcano, the Skaftá cauldrons, and glacial lake Grænalón. The most dangerous jökulhlaups in Iceland have been associated with subglacial volcanic activity due to the greater volume of meltwater produced (Einarsson, 2009). This report summarizes past eruptions, jökulhlaups, and possible co-occurrences of the two with respect to Grímsvötn, and then describes in more detail events since the last eruption in 2011. Since then, jökulhlaups have been reported on the Skeiðarársandur outwash plain in 2014, 2015, 2016, and most recently during November-December 2021. Information is primarily from the Icelandic Met Office (IMO).

Figure 19. Grímsvötn volcano is located under the western half of the Vatnajökull glacier in southeastern Iceland. Jökulhlaups originating from the caldera lake travel 50 km S, emerge from the S tip of the Skeiðarárjökull lobe of the Vatnajökull glacier, enter the Gígjukvísl river, and travel S across the Skeiðarársandur outwash plain. The most recent jökulhlaup from Grímsvötn occurred during November-December 2021. Base map courtesy of Google Earth.

Jökulhlaups from Grímsvötn have been known since at least the fourteenth century (Þórarinsson, 1939 and 1974, in Einarsson, 2009), but the relationship between eruptions and floods from the subglacial crater lake is not well understood. In 1953, Sigurður Þórarinsson suggested a correlation between jökulhlaups and Grímsvötn eruptions. He proposed that if a large volume of water was stored in the lake, that the pressure release following the sudden removal of water during a flood could facilitate magma movement and trigger an eruption. This type of scenario was proposed to explain coincident eruptions and jökulhlaups in 1922 and 1938. Records preserved since 1922 have indicated a few eruptions that were coincident with jökulhlaups, several eruptions with no evidence of jökulhlaups, and a larger number of jökulhlaups with no prior or subsequent documented eruption (table 2). The only recent confirmed event where a jökulhlaup was followed by an eruption took place in 2004.

Table 2. Jökulhlaups and eruptions at Grímsvötn from 1922-2021. For pre-1972 events, only those with possible co-occurring jökulhlaups and eruptions are listed. For 1972 and later both jökulhlaups and eruptions as reported by GVP or IMO are listed. Sources are listed in the table.

Until 2009, jökulhlaups from Grímsvötn had emerged from under the S side of the Vatnajökull glacier on the S and E margins of the Skeiðarárjökull lobe of the glacier, draining into the Skeiðará river (figure 20). The Skeiðará river had previously deposited large amounts of sediment on the eastern part of the Skeiðarársandur plains over the centuries. In addition, the glacier had carved a trench during times of advance. In the summer of 2009, water ceased to enter the channel of the Skeiðará; the retreat of the glacier over the previous 15 years led to a shift in the direction of meltwater flow. Beginning in 2010, floodwater that emerged from beneath the eastern part of the Skeiðarárjökull glacier went westwards along the margin and then entered the Gígjukvísl river (figure 21).

Figure 20. Jökulhlaups from Grímsvötn currently emerge from the S margin of the Skeiðarárjökull lobe of Vatnajökull glacier and drain into the Gígjukvísl River. Until 2009 they drained into the Skeiðará river. Courtesy of IMO.

Figure 21. Floodwater flows along the terminus of Skeiðarárjökull glacier on 3 November 2010. IMO scientists are measuring water flow and chemistry from the glacier edge (left). The flow traveled west along the margin of the lobe into the Gígjukvísl river (right). Photos taken during scientific flight with the Icelandic Coast Guard (TF-SIF) by Matthew J. Roberts and Egill Axelsson, courtesy of Icelandic Met Office.

Two subglacial geothermal areas 10-15 km NW of Grímsvötn cause surface depressions to form due to melting of the glacier at its base; they are known as the Eastern and Western Skaftá cauldrons (figure 22), named for the river where the floods discharge. The eastern cauldron is a little less than 3 km in diameter while the western one is about 2 km in diameter. The lows in the glacier surface lead to local minima in the fluid potential at the base of the glacier and therefore lakes are formed under both cauldrons, sealed by the ice overburden pressure at the rim (Björnsson, 2002). For the western cauldron, analyses suggest that the water formed by geothermal melting of ice is the largest (71%) component of inflow to the lake, followed by geothermal fluid (19%) and surface meltwater (10%). Jökulhlaups from underneath the Skaftá cauldrons occur almost every year and 45 jökulhlaups were recorded in the Skaftá river between 1955 and 2009 (Einarsson, 2009). Although the Skaftá cauldrons are located less than 20 km from Grímsvötn, the rivers where their respective jökulhlaups occur are located about 100 km apart; the Skaftá river is off the SW edge of the Vatnajökull glacier, while the Grímsvötn jökulhlaups have only been observed on the Skeidarársandur outwash plain located 50 km S of the volcano. There has not been any coincidence of activity recorded between these areas.

Figure 22. The Eastern and Western Skaftá cauldrons discharge water as jökulhlaups almost yearly; the water travels 40 km subglacially and emerges at the SW edge of the Vatnajökull glacier into the Skaftá river. There is no documented connection with Grímsvötn caldera and its associated jökulhlaups which discharge 50 km S of the volcano at the S edge of the Skeiðarárjökull lobe of the Vatnajökull glacier (bottom right of figure) into the Gígjukvísl river. Until 2009 Grímsvötn jökulhlaups drained into the Skeiðará river located farther E. Figure from Einarsson (2009).

Jökulhlaups and eruptions during 1922-2011. A Skeiðará river jökulhlaup began in late September 1922 and an eruption at Grímsvötn was first reported on 29 September and last observed on 23 October of that year. There were no reports of jökulhlaups during eruptions in 1933 and 1934. Jökulhlaups that occurred on the Skeiðará and Sula rivers in May 1938 have been associated with an eruption that occurred a few kilometers N of Grímsvötn, though very little eruptive activity above the glacier was recorded. Jökulhlaups from Grímsvötn were reported in June 1939, April and May 1941, September 1945, and February 1948, but there were no confirmed reports of eruptions around those times. A Skeiðará river jökulhlaup during 7-21 July 1954 was attributed to a subglacial eruption, but this is uncertain. Jökulhlaups reported in March 1972 (CSLP 20-72), September 1976, and January-February 1982 (SEAN 07:02) had no accompanying reports of eruptions. The May-June 1983 eruption of Grímsvötn (SEAN 08:05) melted the overlying ice and revealed an oval-shaped lake, 300 m in diameter, initially covered by raft ice. Multiple explosions were observed from the lake; no jökulhlaups were reported in the Skeiðará river.

The 30 September-13 October 1996 Gjálp eruption was located on a N-trending fissure between Bardarbunga and Grímsvötn (BGVN 21:09; Björnsson, 2002) about 8 km NW of Grímsvötn. For five weeks the meltwater generated by the eruption went into the Grímsvötn caldera, raising the lake level to the highest ever recorded (Gudmundsson and others, 1997). By 4 November 1996 the lake had risen to 1,510 m, the level needed to float the ice dam, and ice-quakes marked the onset of drainage. About 10.5 hours later water emerged from the margin of Skeiðarárjökull as a flood wave, in the most rapid jökulhlaup ever recorded from Grímsvötn (Björnsson, 2002).

The December 1998 eruptive vents at Grímsvötn were at the foot of Mt. Grímsfjall, which rises about 300 m above the flat ice shelf over the subglacial lake (BGVN 23:11). No major flood occurred in conjunction with this event; only a small amount of the ice shelf near the eruption site melted.

High-frequency tremor on 26 October 2004 indicated the beginning of a flood from the subglacial caldera lake. An eruption began on 1 November 2004, melting through 150-200 m of ice to the surface in about an hour. Observations on 2 November revealed that the eruption was occurring from a circular vent about 1 km in diameter in the SE part of the crater where a surface ice cauldron had been mapped a year earlier (figure 5, BGVN 29:10). The jökulhlaup from the draining caldera reached a maximum discharge rate of 3,000-4,000 m³/s on the afternoon of 2 November in the affected rivers on the coastal plain. Discharge declined quickly after the peak with no damage reported to roads or bridges. The total volume of the jökulhlaup was about 0.5 km³. On 3 November pulses of activity produced ash plumes rising from 8-14 km above the volcano.

On 31 October 2010, a jökulhlaup originating at subglacial lake Grímsvötn emerged from beneath the Skeiðarárjökull glacier, according to IMO. Observations on the bridge over the Gígjukvísl river confirmed a steady water level increase that indicated the onset of a jökulhlaup, with initial discharge rates of about 145 m³/s. Flow reached a maximum of 2,600 m³/s on the Gígja river on 3 November before declining. Seismicity recorded during the event was all attributed to the jökulhlaup, and no evidence was detected either seismically or visually of an eruption (figure 23).

Figure 23. An aerial view of the eastern edge of Mt. Grímsfjall on 3 November 2010 revealed fresh crevasses at its base in the ice-cover over the adjacent subglacial lake Grímsvötn. The elongated trench in the ice gives an indication of the lake surface and the extent of the subsidence after the jökulhlaup. Water presumably drained from this region around the eastern edge of Grímsfjall. Photo taken during scientific flight with the Icelandic Coast Guard (TF-SIF) by Matthew J. Roberts and Egill Axelsson, courtesy of Icelandic Met Office (IMO).

An eruption from Grímsvötn began on 21 May 2011 following about an hour of tremor (BGVN 36:06). Ash plumes from the eruption rose to 15-20 km altitude, disrupted airline traffic, and drifted toward northern Europe. Ashfall was reported from the Reykjavík area in the SW to Tröllaskagi Peninsula in the N. Explosions lasted for approximately a week; on 28 May tremor rapidly decreased then disappeared. No changes in the water levels were recorded in either the Gígja (Gígjukvísl) or Núpsvötn rivers. IMO noted that the eruption occurred from the same site in the SW part of the Grímsvötn caldera as the 2004 eruption, and ice-melt was not expected to be great. Visual observation on 26 May indicated that little ice had melted during the eruption, and no jökulhlaup occurred.

Jökulhlaups during 2012-2021. The water level rose slightly in the Gígjukvísl river on 22 November 2012 and was attributed by IMO to a small flood releasing from Grímsvötn's subglacial lake; no eruption was reported. Another small jökulhlaup with a maximum discharge rate of about 1,000 m³/s was reported on 27 March 2014, causing a rise in the water level in the Gígjukvísl River (figure 24). Electrical conductivity measurements indicated a considerable increase of a geothermal contribution to the river water, confirming its source at Grímsvötn's subglacial lake. Seismic tremor had increased due to the flood and not volcanic activity.

Figure 24. Water at the Skeiðará river outlet at the Skeiðarárjökull ice margin on 27 March 2014 indicated a small jökulhlaup from Grímsvötn's subglacial lake. The photo is taken at the E margin of Skeiðarárjökull and the mountains in the background are Krossgilstindur (near) and Færnes (distant). Photo copyright by Njáll Fannar Reynisson, courtesy of IMO.

A small jökulhlaup from Grímsvötn's subglacial lake occurred on 6 May 2015, increasing the water level in the Gígjukvísl River. Electrical conductivity measurements indicated a considerable increase of a geothermal contribution to the river water. Based on information from the Institute of Earth Sciences, the water available for drainage was estimated at 0.2-0.3 km³, similar to floods in November 2012 and March 2014. Seismic tremor had increased due to the flood and not volcanic activity, and no eruption was reported. The IMO reported another small jökulhlaup originating from Grímsvötn in the Gígjukvísl river on the Skeiðarársandur outwash plain on 23 August 2016. Seismicity had been first observed at Mt. Grímsfjall on 19 August and lasted for about 24 hours (figure 25). On 22 August, data indicated that the ice shelf over the subglacial lake had subsided by about 5 m since 18 August. It was estimated that only 0.1-0.15 km³ of water was present in the lake. No eruption occurred.

Figure 25. Mt. Grímsfjall was visible from the Grímsvötn's ice shelf during an IMO expedition 5-7 August 2016. The ice shelf subsided by about 5 m during the August 2016 jökulhlaup. The buildings of the Glacier Research Society at Eystri Svíahnúk are visible as two small dots within the snowfield along the top of Grímsfjall. Courtesy of IMO.

Between previous eruptions of Grímsvötn, deformation data was interpreted by IMO as indicating gradual accumulation of new magma at depth. In early June 2020 IMO scientists measured high levels of SO₂ in the SW part of the caldera, close to the 2004 and 2011 eruption sites (figure 26); they interpreted this as magma degassing at a shallow level. In addition, the area where geothermal activity could be detected at the surface had notably increased. Although seismic activity had been increasing for the previous year, it was still below levels reached prior to the eruptions of 2004 and 2011.

Figure 26. A specialist from the Icelandic Meteorological Office performed gas measurements at Grímsvötn during the first half of June 2020, as increasing SO2 levels suggested magma degassing. The location of the photo is near the site of the 2011 eruption. Photograph by Melissa Anne Pfeffer, courtesy of IMO.

Elevation measurements of the ice shelf over Grímsvötn made by IES showed that it rose 10 m during the first half of 2020 to a level it had not exceeded since October 2010. In response to the increased possibility for a flood or an eruption, IMO installed a continuous GPS measurement station on the ice cap in early June 2020. Webcams were also installed at Hamarinn and Skeiðarársandur looking towards Grímsvötn.

In August 2020 IMO added a webcam to Mt. Grímsfjall, overlooking the Grímsvötn ice field above the lake. The GPS devices showed a potential drop in the ice sheet in early August, but the electrical conductivity and water level in the Gígjukvísl river were both normal. IMO discovered that the instruments on the ice shelf had tilted (figure 27) as a result of high rates of melting of the ice sheet from unusually hot weather. The IMO raised the Aviation Color Code to Yellow on 30 September 2020, noting that seismicity had increased over the past month, ice cauldrons had deepened in several places over the caldera signifying increased geothermal activity, surface deformation surpassed the level prior to the 2011 eruption, and magmatic gases were present in emissions over the summer. Additionally, water levels in the subglacial lake were comparable to levels prior to floods in 2004 and 2010.

Figure 27. IMO scientists repaired tilted GPS monitoring equipment on the Grímsvötn ice sheet over the subglacial lake in mid-August 2020 after unusually hot weather increased the melting rate at the surface and disrupted the equipment. The GPS meter is on the left, and on the right is a wind turbine that powers the measuring device (right). Photograph by Björn Ólafsson, courtesy of IMO.

On 24 November 2021 IMO reported that the ice sheet over Grímsvötn's caldera had subsided 60 cm in the previous few days and the rate of subsidence had accelerated during the previous 24 hours. These measurements indicated that it was likely that water had started to leave the lake. The flow rate in the river on 29 November was measured at 240 m³/s; water levels in the Gígjukvísl drainage rose overnight during 30 November-1 December (figure 28) and by 2 December the flow rate had reached 930 m³/s, ten times the normal seasonal rate. Water from the lake drained from the E side of Skeiðarárjökull lobe and from a channel in the middle of the lobe into the Gígjukvísl River. Daily measurements showed that the flow rate continued to rise; it peaked at about 2,800 m³/s during the morning of 5 December, and then declined rapidly. While the water was draining from Grímsvötn lake, subsidence of the overlying ice sheet continued; by 2 December it had reached 17 m and continued to fall rapidly. The maximum subsidence of 77 m was measured by IMO during 5-6 December.

Figure 28. The water level rose substantially in the Gígjukvísl river between 28 November (left) and 1 December 2021 (right) from the jökulhlaup that began on 24 November draining the subglacial lake at Grímsvötn. View is to the S from the IMO webcam. Courtesy of IMO.

Early on 6 December 2021 two earthquakes, M 2.3 and 3.6 were recorded, followed by several M 1 aftershocks. This resulted in the IMO briefly raising the Aviation Color Code to Orange, based on concerns of a possible relationship between the draining lake and increased eruption probability. With no further significant seismic activity, the alert level was lowered to Yellow the next day. IMO lowered the Aviation Color Code further to Green on 12 January 2022, noting that seismicity had returned to normal levels with a few earthquakes detected over the previous few weeks.

References: Björnsson H, 2002, Subglacial lakes and jökulhlaups in Iceland. Global and Planetary Change, 35: 255-271.

Brandsdottir B, 1984, Seismic activity in Vatnajökull in 1900-1982 with special reference to Skeidararhlaups, Skaftarhlaups and Vatnajökull eruptions. Jokull, 34: 141-150.

Einarsson B, 2009, Jökulhlaups in Skaftá: A study of jökulhlaup from the Western Skaftá cauldron in the Vatnajökull ice cap. Iceland, Icelandic Meteorological Office, VÍ 2009-006, ISSN 1670-8261.

Gudmundsson M T, Bjornsson H, 1991, Eruptions in Grímsvötn, Vatnajökull, Iceland, 1934-1991, Jokull, 41: 21-45.

Gudmundsson M T, Sigmundsson F, Björnsson H, 1997, Ice-volcano interaction of the 1996 Gjálp subglacial eruption, Vatnajökull, Iceland. Nature, 389:6654, 954-957. DOI: 10.1038/40122.

Þórarinsson S, 1939, The ice dammed lakes of Iceland with particular reference to their values as indicators of glacier oscillations. Geografiska Annaler, 21 (3-4), 216-242.

Þórarinsson S, 1953, The crater groups in Iceland. Bulletin of Volcanology, 14: 3-44. https://doi.org/10.1007/BF02596003

Þórarinsson S, 1974. Vötnin stríð. Saga Skeiðarárhlaupa og Grímsvatnagosa. Bókaútgáfa, Menningarsjóðs. Reykjavík. [In Icelandic].

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

Information Contacts: Icelandic Met Office (IMO), Reykjavík, Iceland (URL: http://en.vedur.is/); Google Earth (URL: https://www.google.com/earth/).

Ethiopia's Erta Ale basaltic shield volcano has had at least one active lava lake since the mid-1960s, and possibly much earlier. Two active craters (north and south pits) within the larger oval-shaped summit caldera have exhibited periodic lava fountaining and lava lake overflows for many years. During January 2017-March 2020 a vent at the southeast caldera, a few kilometers SE of the summit caldera, produced lava flows that extended many kilometers from the main vents. Since March 2020 pulses of activity from both pit craters in the summit caldera have been recorded in numerous satellite thermal images. This report covers activity from September 2021-April 2022 when both the north and south pits were intermittently active. Information comes primarily from satellite imagery and thermal data, although reports and photographs from ground-based expeditions that periodically visit the site are occasionally available.

Multiple breakouts from persistent lava flows originating at the southeast caldera tapered off in March 2020; after this, intermittent low levels of thermal activity were focused at the N and S pit craters of the summit caldera, with occasional pulses of stronger activity in late November 2020 at the S pit crater and in late March and late May 2021 at the N pit crater (BGVN 46:02, 46:10). A return to more vigorous thermal activity in September 2021 was indicated by MODIS satellite data as an increased number of MODVOLC thermal alerts, increases in the MIROVA log radiative power frequency and intensity (figure 107), and numerous Sentinel-2 satellite images of thermal anomalies. Visitors to the site in December 2021, and January and February 2022, confirmed the active lava lake at the S pit crater.

Figure 107. The Log Radiative Power for thermal activity at Erta Ale increased during September 2021 and remained elevated through April 2022, as seen in the MIROVA graph of activity from 6 June 2021 through April 2022 (top). The MIROVA distance graph (bottom) confirms that all of the activity was located at the N and S pit craters, compared with the earlier flow activity located many kilometers from the summit and southeast calderas (figure 97, BGVN 45:05). Courtesy of MIROVA.

Only a few of the Sentinel-2 satellite images of Erta Ale are cloudy, providing a consistent record of the changes in thermal activity in the summit caldera during the period. The S pit crater had thermal anomalies of varying intensity throughout, but the N pit crater was more intermittent. A very weak anomaly appeared at the N pit on 31 August 2021, but then not again until 10 October. It was larger on 15 October and bright on 30 October. At the S pit, the anomaly was present in all September and October images, and brightest on 20 September. Double anomalies appeared at the S pit for most of October (figure 108). MODVOLC thermal alerts were recorded on four days in September and nine days of October.

Figure 108. A very weak anomaly appeared at the N pit of Erta Ale on 31 August 2021 (top left), but then not again until 10 October. It was larger on 15 October (bottom 2nd from left) and very bright on 30 October (bottom right). At the S pit, the anomaly was present in all September and October images, and brightest on 20 September (top, 2nd from right). Double anomalies appeared at the S pit for most of October (bottom). Sentinel-2 images use Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

The thermal anomaly was weak at the N pit on 4 November 2021, brighter on 19 November, brightest on 24 November, and very weak or absent on 29 November. It was very bright on 14 December, but absent the other days of December. At the S pit, two anomalies were present in most images, one much brighter than the other, with a similar size and intensity throughout (figure 109). MODVOLC thermal alerts were issued on eleven days during November and six days during December.

Figure 109. The thermal anomaly was weak at the N pit of Erta Ale on 4 November 2021 (top left), brighter on 19 November (top, 2nd from left), brightest on 24 November (top, 2nd from right) and very weak or absent on 29 November (right). It was very bright on 14 December (bottom 2nd from left), but absent the other days of December. At the S pit, two anomalies were present in most images from November and December, one much brighter than the other, with a similar size and intensity throughout the period. Sentinel-2 images use Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

No thermal anomaly was present at the N pit for all of January or February 2022. At the S pit, a single weak anomaly was present during January, and a brighter single or double anomaly was present throughout February and early March (figure 110). A single MODVOLC thermal alert was issued on 24 January, and alerts were reported on four days during February. A weak thermal anomaly was present at the N pit in the 29 March 2022 image and persisted in all of the April images. Consistent size and intensity double anomalies, one bright and one dim, were present at the S pit in all images for March and April (figure 111). MODVOLC thermal alerts were issued for five days during March and six days during April 2022.

Figure 110. No thermal anomaly was present in satellite images at the N pit of Erta Ale for all of January and February 2022. At the S pit, a single weak anomaly was present during January, and a brighter single or double anomaly was present throughout February and early March. Sentinel-2 images use Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Figure 111. A weak thermal anomaly was present at the N pit of Erta Ale in the 29 March 2022 image (top right), and persisted in all of the April images (bottom). Double anomalies of consistent size and intensity, one bright and one dim, were present at the S pit in all images for March and April. Sentinel-2 images use Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Visitors to the S pit crater in late December 2021 and early January 2022 reported that the lake measured about 200 m in diameter and the level was about 30 m below the edge of the crater with fresh lava overhanging the crater rim (figure 112). Two collapses of the crater rim, widening the lake, were observed on 31 December-1 January 2022. In a second visit during 28-29 January 2022 the lake was still about 200 m in diameter with periodic small lava fountains (figure 113). On a 24 February 2022 visit the lava lake showed signs of constant activity with slabs of solidified crust moving around on the surface, bright orange lava in the cracks and spattering of lava a few tens of meters above the lake surface (figure 114).

Figure 112. Visitors to Erta Ale on 31 December-1 January 2022 reported that the crater at the S pit was about 200 m in diameter and the lava lake was about 30 m below the rim, with fresh lava hanging off the crater rim. Photo by Enku Mulugeta, courtesy of Volcano Discovery.

Figure 113. The lava lake at Erta Ale’s S pit crater was still about 200 m wide on 28 January 2022 and active with periodic lava fountains. Photo by Enku Mulugeta, courtesy of Volcano Discovery.

Figure 114. On a 24 February 2022 visit, Erta Ale’s lava lake at the S pit crater showed signs of constant activity with slabs of solidified crust moving around on the surface with bright orange lava in the cracks and spattering of lava a few tens of meters above the lake surface. Photo by Enku Mulugeta, courtesy of Volcano Discovery.

Geologic Background. The Erta Ale basaltic shield volcano is the most active in Ethiopia, with a 50-km-wide edifice that rises more than 600 m from below sea level in the barren Danakil depression. It is the namesake and most prominent feature of the Erta Ale Range. The volcano includes a 0.7 x 1.6 km elliptical 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. Fresh-looking 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, or 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Tom Pfeiffer, Volcano Discovery (URL: http://www.volcanodiscovery.com/).

Suwanosejima is an 8-km-long island located in the northern Ryukyu Islands, Japan, consisting of a stratovolcano and two historically active summit craters. Volcanism was intermittent for much of the 20th century, characterized by Strombolian explosions, ash plumes, and ashfall. The current eruption began in October 2004 and has recently consisted of intermittent explosions, ash emissions, and incandescent ejecta (BGVN 46:09). Incandescence is often observed at night and ejecta periodically reaches as far as a kilometer from the summit. Ashfall is usually noted several times each month in the nearby community on the SW flank of the island. This report overs activity from July 2021 through March 2022 using information from the Japan Meteorological Agency (JMA), the Tokyo Volcanic Ash Advisory Center (VAAC), and several sources of satellite data.

Intermittent explosions were detected in the Otake crater, generating ash plumes that rose 5.4 km above the crater rim. Larger volcanic bombs were ejected as far as 1.9 km from the crater. Crater incandescence was frequently visible at night. The MIROVA Log Radiative Power graph of the MODIS thermal anomaly data through March 2022 indicates intermittent pulses of increased thermal activity in in late April to early May 2021, July 2021, September 2021, two small pulses in early and late November 2021, and early January 2022 that appear to correspond to increased periods of explosive activity (figure 67). According to data from the MODVOLC thermal algorithm, a total of eight hotspots were detected: one on 29 August, one on 10 September, two on 21 September, one on 23 September 2021, one on 4 January 2022, and two on 6 January. Reports of eruption sounds were heard in Toshima village (4 km SSW), in addition to occasional ashfall events.

Figure 67. The MIROVA Log Radiative Power graph of the MODIS thermal anomaly data from 4 April 2021 through March 2022 indicates intermittent pulses of increased thermal activity in late April to early May 2021, July 2021, September 2021, two small pulses in early and late November 2021, and early January 2022 that appear to correspond to increased periods of explosive activity. Courtesy of MIROVA.

Ongoing explosions during July generated ash plumes that rose 1.6-3.6 km high (figure 68). Bombs were ejected as far as 1 km from the crater, accompanied by occasional crater incandescence that was visible at night. An explosion on 8 July at 0439 ejected bombs 800 m NW. An explosion at 1319 on 12 July generated an ash plume that rose 3 km high and later at 2330 a second explosion produced an ash plume 3.6 km high. Eruption sounds were heard in Toshima village; ashfall deposits were reported in the village during 12-19 July. On 29 July JMA lowered the Volcano Alert Level to 2 (on a 5-level scale) at 1100 and warned the public to stay 1 km from the crater.

Figure 68. Photo of an eruption plume rising 3.8 km high above Suwanosejima at 1814 on 31 July 2021. Courtesy of JMA (Volcanic activity commentary for Suwanosejima, July 2021).

During August, explosions continued, producing ash plumes that rose 2.2-4.8 km high. Large blocks were ejected as far as 500 m from the crater, accompanied by nightly crater incandescence. Ashfall was occasionally reported in Toshima village. On 19 August an explosion at 0137 and 1613 produced an ash plume that rose 3 km and 2.2 km NE and N above the crater, respectively. A third explosion at 2059 generated an ash plume that rose 2.5 km above the crater and drifted N. A small amount of ashfall was reported in Yakushima, Nishinoomote, and Nakatane, as well as Toshima village. Explosions at 0628 and 0713 on 20 August produced ash plumes that rose 2.5-3 km above the crater and drifted N, resulting in ashfall in Toshima village, with smaller amounts in Yakushima, Mishima, Ibusuki, Minamikyushu, and Makurazaki. An explosion at 0617 on 21 August resulted in an ash plume 3.2 km above the crater that drifted N; a large amount of ash (over 1 mm) was detected in Toshima village and a smaller amount (less than 0.1 mm) was detected in Makurazaki, Minamisatsuma, Minamikyushu, Kagoshima, Ibusuki, and Hioki. A second explosion that day at 0906 produced an ash plume up to 3.2 km high that drifted N. On 28 August an explosion occurred at 1231 that produced an ash plume 4.8 km above the crater, though weather clouds prevented clear views of the summit.

Explosions persisted during September, with ash plumes rising 2.4-5.4 km high. Some crater incandescence was visible at night, which was also detected in Sentinel-2 infrared satellite imagery on clear weather days (figure 69). On 17 September explosions were detected at 0212 and 0218, which ejected material 1 km SE from the crater. JMA raised the Alert Level to 3 at 0235. On 20 September an explosion ejected material as far as 1.2 km SE from the crater. An eruptive event at 0711 on 26 September produced an ash plume that rose 5.4 km high, though weather clouds prevented a clear view of the summit. As a result, ejecta traveled 800 m from the crater and a large amount of ash was reported in Toshima village.

Figure 69. A thermal anomaly (bright yellow-orange) and occasional gray ash plumes were recorded in Sentinel-2 satellite imagery at Suwanosejima on 23 September (top left), 28 October (top right) 2021, 26 January (bottom left), and 12 March (bottom right) 2022. The size of the hotspot visibly decreased during 2022. Image uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

During October, frequent explosions continued, generating ash plumes up to 3.3 km high and ejecting bombs as far as 1.1 km from the crater. Occasional crater incandescence was noted at night, as well as ashfall in Toshima village. On 26 October an explosion at 1317 produced an ash plume that rose 3.3 km above the crater and ejected material was reported up to 1.9 km from the crater (figure 70). Similar activity continued in November with explosions that produced ash plumes 1.2-2.7 km above the crater. Large volcanic bombs were ejected 800 m from the crater and occasional ashfall was reported in Toshima village. During late November crater incandescence was visible nightly.

Figure 70. Photo of an eruption plume rising 3.3 km above Suwanosejima at 1317 on 26 October 2021. Courtesy of JMA (Volcanic activity commentary for Suwanosejima, October 2021).

Explosions during December generated ash plumes 1.5-3.1 km high and ejected large blocks as far as 800 m from the crater. Some ash plumes rose 1-3.4 km high in early December but were not associated with explosive events. Occasional crater incandescence was visible at night. Hundreds of explosions persisted during January 2022 with ash plumes rising 1.4-3 km high and nighttime crater incandescence. Material was ejected 1.1 km from the crater. During the latter half of the month, rumbling sounds and ashfall were reported in Toshima village.

Figure 71. Photo of crater incandescence from Suwanosejima at 0106 on 9 January 2022. Courtesy of JMA (Volcanic activity commentary for Suwanosejima, January 2022).

Similar activity continued during February and March with intermittent explosions and ash plumes rising 1.3-2.8 km high. Bombs were ejected 900 m from the crater and crater incandescence persisted. Ashfall also was also intermittently reported in Toshima village.

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

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Lewotolok (also known as Lewotolo) is located on the island of Lembata (Lomblen) in the Lesser Sunda Islands of Indonesia. The current eruption period began in late November 2020 and has been recently characterized by explosions with ash emissions rising up to 1.5 km above the summit and incandescent ejecta. A cone had formed in the summit crater over the SE rim, which contains a smaller crater. Intermittent thermal activity was also visible in satellite imagery according to MIROVA data, MODVOLC, and Sentinel-2 infrared data. This report covers similar explosive activity during August 2021 through January 2022 with information provided by Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM, or the Center of Volcanology and Geological Hazard Mitigation), MAGMA Indonesia, and satellite data.

Intermittent ash plumes were reported rising to a maximum of 2.1 km above the summit; incandescent ejecta was also observed rising hundreds of meters above the summit and extending a similar distance from the crater. Thermal anomalies in MIROVA data indicated pulses of higher activity during late June to early July, and again in October through November (figure 27). Sentinel-2 infrared satellite imagery provided thermal and visual evidence of thermal activity inside the summit crater on multiple occasions (figure 28). According to data from the MODVOLC thermal algorithm, a total of eight hotspots were detected on 8, 13, 15, 17, 20, and 22 October, and 9 November.

Figure 27. Periods of increased thermal activity at Lewotolok were shown on the MIROVA graph (Log Radiative Power) during late June-early July and October-November 2021. Few low-power anomalies were also detected during January 2022. Courtesy of MIROVA.

Figure 28. Occasional thermal anomalies were detected at the summit of Lewotolok on 31 August (top left), 5 October (top right), 24 November (bottom left) 2021, and 3 January (bottom right) 2022. Two distinct anomalies were present on 24 November and 3 January. Sentinel-2 satellite imagery uses Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Daily white, gray, and black emissions rose 50-2,000 m above the summit during August. Occasional banging noises were also reported. On 10 August an eruption at 0729 produced an ash plume that rose 1.5 km above the summit and drifted S (figure 29). Incandescent material was ejected 200-350 m radially from the summit and was accompanied by banging noises. Ash plumes on 11 and 13 August rose 2 km and 1 km above the summit, respectively, both of which drifted W. During 15-16 August incandescence was observed from the SW part of the crater. On 18 August an ash plume at 1820 rose 1 km above the summit and drifted W and material was ejected 500 m SE. Ash plumes rose 1.2-1.5 km above the summit and drifted generally W on 22, 24, and 26 August.

Figure 29. An ash plume rose 1.5 km above the summit of Lewotolok and drifted S on 10 August 2021. Courtesy of MAGMA Indonesia and PVMBG.

Similar activity during September was characterized by daily white, gray, and black emissions rising 50-700 m above the summit. Ash plumes rose 200-600 m above the summit. Incandescent material was ejected as high as 300 m above the summit during 1-2 September. Some incandescent material traveled as far as 1 km to the SE on 5 September (figure 30). On 15 September an ash plume rose 600 m above the summit and during 15-16 September incandescent material was ejected 100-300 m above the summit to the E (figure 31). Crater incandescence was visible on 17 September, followed by incandescent ejecta as high as 300 m above the summit that was reported on 18, 19, and 21 September.

Figure 30. Incandescent material from Lewotolok traveled as far as 1 km to the SE on 5 September 2021. Courtesy of MAGMA Indonesia and PVMBG.

Figure 31. Incandescent material from Lewotolok was ejected 100-300 m above the summit to the E on 15 September 2021. Courtesy of MAGMA Indonesia and PVMBG.

During October white, gray, and black emissions rose 50-1,000 m above the summit. Ash plumes were reported rising as high as 1 km above the summit. Rumbling and banging sounds were reported daily. On 1 October an ash plume rose 800 m above the summit and drifted W and incandescent ejecta during 1-2 October rose 300 m above the summit (figure 32). During 3-5 October incandescent material was ejected as far as 700 m from the vent in multiple directions. On 6 October material was ejected as far as 1 km to the SE and 300 m to the SW; an associated ash plume at 1723 rose 600 m above the summit. A VONA stated that on 7 October an ash plume rose 1.9 km above the summit and drifted W. Daily incandescent ejecta were detected during 8-16 October rising up to 300 m above the crater, one event of which moved to the SE. On 11 October there were 24 eruption events reported with ash emissions rising 200 m above the summit, and during 13-15 October there were a total of 77 eruption events with similar ash emissions up to 1 km above the summit. An ash plume at 0548 on 25 October rose 2.1 km above the summit. Incandescent pulses and weak rumbling sounds were reported on 30 October.

Figure 32. Incandescent ejecta rose as high as 300 m above the summit of Lewotolok on 1 October 2021. Courtesy of MAGMA Indonesia and PVMBG.

White, gray, and black emissions persisted in November, rising 50-2,000 m above the summit. During 1-3 November incandescent material was ejected 100-200 m above the summit, and frequent rumbling and banging sounds were reported. On 6 October there were 16 eruption events that produced white and gray emissions 200-300 m above the summit; rock avalanches and crater incandescence were also observed (figure 33). Incandescent material was frequently ejected as far as 400 m away from the vent in multiple directions. During 19-20 November incandescent material was ejected 200 m E from the vent and crater incandescence remained visible during 22-30 November. On 24-25 and 30 November incandescent material was ejected 200-500 m above the summit to the E and SE.

Figure 33. Incandescent material and rock avalanches were visible above the Lewotolok summit on 6 October 2021. Courtesy of MAGMA Indonesia and PVMBG.

Activity continued during December, with white, gray, and black emissions rising 50-800 m above the summit. During 7-8 December incandescent material was ejected 300 m above the summit; faint rumbling accompanied this activity. On 13, 14, and 15 December ash plumes rose to 500 m, 700 m, and 600 m above the summit, respectively (figure 34). Incandescent material was ejected on multiple days during 15-29 December as high as 300 m in different directions. Ash plumes on 25 and 27 December rose 500 m and 400 m above the summit, respectively.

Figure 34. An ash plume rose 500 m above the summit of Lewotolok on 13 December 2021. Courtesy of MAGMA Indonesia and PVMBG.

During January 2022, white, gray, and black emissions rose as high as 600 m above the summit and drifted E and SE and incandescent material was ejected 300 m from the vent, accompanied by rumbling. At 0848 on 11 January an ash plume rose 700 m above the summit and drifted E (figure 35). During 11-14 January ash plumes also rose as high as 700 m above the summit, drifting E, SE, and W.

Figure 35. An ash plume rose 700 m above the summit and drifted E from Lewotolok on 11 January 2022. Courtesy of MAGMA Indonesia and PVMBG.

Geologic Background. The Lewotolok (or Lewotolo) stratovolcano occupies the eastern end of an elongated peninsula extending north into the Flores Sea, connected to Lembata (formerly Lomblen) Island by a narrow isthmus. It is symmetrical when viewed from the north and east. A small cone with a 130-m-wide crater constructed at the SE side of a larger crater forms the volcano's high point. Many lava flows have reached the coastline. Eruptions recorded since 1660 have consisted of explosive activity from the summit crater.

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

Volcán El Reventador is located 100 km E of the main axis of active volcanoes in Ecuador and has had eruptions recorded since the 16th century, characterized by explosive events and lava flows. The most recent eruption began in 2008 and has recently consisted of ash explosions, lava flows, and block avalanches (BGVN 46:08). This report describes daily explosions, ash plumes, incandescent block avalanches, lava flows, and occasional pyroclastic flows and lahars during August 2021 through January 2022, based on daily reports from Ecuador's Instituto Geofisico (IG-EPN), the Washington Volcano Ash Advisory Center (VAAC), and infrared satellite data.

During August 2021 to January 2022, IG-EPN reported daily explosions, gas-and-steam and ash plumes, and frequent crater incandescence, often accompanied by incandescent block avalanches and lava flows. The highest average number of explosions per day was 74 in August, followed by 68 in October (table 14). During January 2022, the average number of daily explosions had declined to 32. Ash plumes rose to a maximum height of 1.7 km above the crater during 2-3 September and 10 October. At night and early morning, frequent crater incandescence was visible, occasionally accompanied by lava flows generally on the S and NE flanks and incandescent block avalanches traveling as far as 800 m from the summit.

Table 14. Monthly summary of explosions and plume heights recorded at Reventador from August 2021 through January 2022. Data courtesy of IG-EPN (August 2021 to January 2022 daily reports).

Summit crater activity was relatively similar during August and September 2021. There were 34-100 daily explosive events during these two months, which generated gas-and-steam and ash plumes that rose 300-1,700 m above the crater that drifted in multiple directions. At night, crater incandescence was frequently visible and incandescent block avalanches intermittently traveled 300-800 m down all the flanks, but primarily the NE and SE flanks. A lava flow was observed descending the S and NE flank during 2-3 August. On 12 August an ash plume rose as high as 1.6 km above the crater (figure 149). An active lava flow was detected on the NE flank during the afternoon of 12 August through the late morning of 13 August. During 2-3 September ash plumes varied from 1.4-1.7 km above the crater and drifted SW and W (figure 149). Incandescent blocks during 29-30 November were observed rolling down the E flank for about 800 m and accompanied crater incandescence.

Figure 149. Webcam images of gas-and-ash plumes rising as high as 1.6 km above the crater on 12 August 2021 (left) and 1.7 km above the crater and drifting W and SW on 3 September 2021 (right). Courtesy of IG-EPN (INFORME DIARIO DEL VOLCAN REVENTADOR No. 2021-225, 12 de agosto de 2021 and INFORME DIARIO DEL VOLCAN REVENTADOR No. 2021-247, 03 de septiembre de 2021).

During October and November, daily explosions continued, with 8-105 per day and resulting gas-and-ash plumes that rose 100-1,700 m above the crater and drifted in different directions. Nighttime crater incandescence was frequently visible, accompanied by occasional block avalanches that descended 300-800 m down all the flanks, especially the S and SE flanks. According to Washington VAAC reports, ash emissions rose as high as 1.7 km above the summit and drifted NW and W on 2 and 8-10 October 2021 (figure 150). A pyroclastic flow was captured descending the S flank at 0617 on 25 October 2021 (figure 151). Ashfall was reported in Gonzalo Pizarro on 27 October. On 7 November an ash plume rose 1.2 km above the summit and drifted W and SW (figure 150). During 8-9 November ash emissions rose as high as 1.4 km above the summit and drifted SW and N, according to a Washington VAAC notice. Some light ashfall was reported during the morning of 12 and 14 November in San Luis del El Chaco. A lava flow was detected on the NE flank advancing slowly during 14-16 November.

Figure 150. Webcam image of a gas-and-ash plume that rose 1.7 km above the summit on 10 October 2021 (left) and 1.2 km above the summit on 7 November 2021 (right). Courtesy of IG-EPN (INFORME DIARIO DEL VOLCAN REVENTADOR No. 2021-277, 10 de octubre de 2021 and INFORME DIARIO DEL VOLCAN REVENTADOR No. 2021-312, 07 de noviembre de 2021).

Figure 151. Webcam image of a pyroclastic flow descending the S flank of Reventador at 0617 on 25 October 2021. Courtesy of IG-EPN (INFORME DIARIO DEL VOLCAN REVENTADOR No. 2021-299, 25 de octubre de 2021).

Activity persisted during December and January 2022, characterized by 7-103 daily explosions, frequent ash plumes, and crater incandescence; however, weather and clouds in January often prevented clear views of the summit. Ash plumes rose 500-1,400 m above the summit and drifted in different directions, especially W, SW, and NW. Nighttime crater incandescence was occasionally accompanied by incandescent blocks that descended 300-800 m on all sides of the volcano, including the S, SE, and NE flanks. On 1, 2, and 15 December ash plumes rose 1.3 km above the summit and drifted W and SW, respectively. An active lava flow was reported traveling down the N flank during 1-2 and 25-26 December. During the night of 13 December and during 16-19 and 22-27 December, a lava flow descended the NE flank, based on data from surveillance cameras (figure 152). A lava flow on the E flank was reported during 14-15 and 23-24 January. On 4, 29, and 30 January ash plumes rose 1.4 km above the summit and drifted S and SE.

Figure 152. Thermal image of an active lava flow descending the NE flank of Reventador at 0720 on 25 January. Courtesy of IG-EPN (INFORME DIARIO DEL VOLCAN REVENTADOR No. 2021-025, 25 de enero de 2022).

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed intermittent thermal anomalies of varying intensity during April 2021 through January 2021, which reflected the active lava flows and incandescent block avalanches occurring throughout that time (figure 153). In comparison, the MODVOLC thermal algorithm 37 thermal anomalies were detected by the MODVOLC thermal algorithm on 4 and 20 August, 5, 14, and 28 September, 1, 2, and 3 October, 4, 17, 22, 24, and 29 November, 31 December 2021, and 2, 7, and 9 January 2022. Some thermal anomalies and incandescent avalanches on the NE flank were visible in Sentinel-2 infrared satellite imagery, though clouds often obscured the view of the summit (figure 154).

Figure 153. Frequent thermal activity was detected at Reventador at varying levels during August 2021 through January 2022, based on this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 154. Sentinel-2 infrared satellite images of Reventador on 4 August (top left), 2 December (top right), 12 December (bottom left), and 27 December (bottom right) 2021 showing a strong thermal anomaly at the summit crater. On 12 December an incandescent block avalanche could be seen through the clouds descending the NE flank. Images with “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

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

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

The Hunga Tonga-Hunga Ha’apai volcano includes the small islands of Hunga Tonga and Hunga Ha’apai along with shallow reefs along the caldera rim of a much larger submarine edifice in the western South Pacific Ocean (see figure 25; BGVN 47:02), west of the main inhabited islands in the Kingdom of Tonga. It is one of 12 confirmed submarine volcanoes along the Tofua Arc, a segment of the larger Tonga-Kermadec volcanic arc. The Tonga-Kermadec arc formed as a result of subduction of the Pacific Plate beneath the Indo-Australian Plate. The capital city of Tonga, Nuku’alofa, is located 65 km S on the island of Tongatapu. New Zealand lies 2,000 km S, and Australia is over 3,000 km SW.

The first recorded eruption at this site occurred in 1912, followed by an eruption in 1937, and then S of the islands in 1988. In 2009 a short eruption increased the land area at Hunga Ha’apai; around this time the two islands were each about 2 km long (BGVN 34:03). During December 2014 through January 2015 eruptive activity added land between the islands, creating the merged Hunga Tonga-Hunga Ha’apai island (BGVN 40:01). Major Surtseyan explosions and eruption plumes were detected beginning in late December 2021, which initially reshaped the central part of the combined island (BGVN 47:02) before stronger activity on 14 January 2022 removed most of the 2014-2015 material. The next day, 15 January, an even larger eruption generated a plume that reached at least 20 km altitude, caused a tsunami across the Pacific Ocean, and triggered shock waves through the atmosphere; only small remnant of the islands remained visible above the ocean surface. This report will update details of the large 14-15 January 2022 events using information primarily from the Tonga Geological Services (TGS), Tongan and New Zealand news outlets, the Wellington and Darwin Volcanic Ash Advisory Centers (VAAC), and various satellite data.

Activity during 1-13 January 2022. According to observations of satellite images, little eruptive activity was detected during 1-2 January. On 2 January a Sentinel-2 natural color image showed green-yellow discolored water in the area west of the volcano, accompanied by white gas-and-steam emissions drifting NE on the SW end of the westernmost island (figure 44). Pumice rafts roughly 10 m wide were also visible in Sentinel-2 satellite imagery, drifting about 100 km W of the volcano on 2 January (see figure 41; BGVN 47:02). The surface area of the island had grown from approximately 2 km² on 10 December 2021 to 3.6 km² on 2 January 2022. The vent was estimated to be at 90 m elevation and the base of the active cone had grown from 500 to 1,700 m wide.

Figure 44. A Sentinel-2 satellite image showing green-yellow discolored water surrounding the area to the W of Hunga Tonga-Hunga Ha’apai captured on 2 January 2022. Some white gas-and-steam emissions are visible rising from the SW end of the island. Image uses “Natural color” rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

A small ash plume was detected during 2220-2230 on 3 January that rose to 6-7 km altitude and drifted 5-10 km NE, according to TGS (figure 45). At 2230 the plume decreased in altitude to 5 km. During 3-4 January a cyclone passed through the area, obscuring views of the volcano. A minor ash plume was again detected between 0000 and 0010 on 5 January, which rose to 8 km altitude and drifted 15 km NE; by 0010 the plume had drifted 18 km ENE. Some pumice had washed up on the northwestern beaches of Tongatapu Island on 5 January (see figure 42; BGVN 47:02) and a sulfurous odor was reported on the Nuku’alofa waterfront on 6 January.

Figure 45. HIMAWARI-8 AHI satellite images captured a small ash plume rising 5-10 km NE from Hunga Tonga-Hunga Ha’apai (purple box) at 2220 (left) and 2230 (right) on 3 January 2022. At 2220 the plume rose to 6-7 km altitude and by 2230, it had decreased to 5 km, based on the color scale on the bottom right of each image. Courtesy of Tonga Geological Services.

Activity during 14 January 2022. A sub-aerial eruption that began at 0420 on 14 January generated a mushroom-shaped eruption plume consisting of gas-and-steam and ash which was 5 km wide at the base and rose to the stratosphere, 18-20 km altitude, according to TGS (figure 46). The plume expanded radially to 240-260 km diameter and extended over Tongatapu (70 km S), ‘Eua (106 km SSE), Ha’apai (130 km NE), and Vava’u (270 km NNE). Geologists on a boat observed Surtseyan pulses ejecting dark, dense material into the air and pyroclastic flows expanding over the ocean around 1700-1830 (figure 47). The activity removed approximately the middle third of the island that had been enlarged over the previous weeks, according to analysis of a Planet Labs satellite image acquired at 1525 on 15 January (figure 48).

Figure 46. A massive eruption plume rising above Hunga Tonga-Hunga Ha’apai was captured by the GOES-17 satellite (NOAA) at 0640 on 14 January 2022. The plume rose above 16 km altitude and expanded radially at the top to 240-260 km in diameter. Tongan islands are outlined in blue. Courtesy of CIMSS and SSEC.

Figure 47. Successive photos of the explosion at Hunga Tonga-Hunga Ha’apai taken at 1712 (top), 1727 (middle), and 1816 (bottom) on 14 January 2022. The view at 1712 is towards the NE with the volcano in the foreground. The photo at 1727 looks E with Hunga Ha’apai in the foreground and Hunga Tonga on the far right. In the 1816 image the view is to the N with Hunga Ha’apai to the left of the plume. The plume measures 5 km wide at the base and rises to 18-20 km altitude in each of these photos. Courtesy of Tonga Geological Services.

Figure 48. The center section of Hunga Tonga-Hunga Ha’apai became submerged as a result of the large explosion on 14 January 2022. It is surrounded by green-yellow discolored water and gray-brown pumice rafts. Image courtesy of Tanya Harrison, Planet Labs.

A national tsunami warning was issued at 1112 due to abnormal swirling water off the coast of Nuku’alofa, according to a Matangitonga news article. The Tonga Meteorological Services (TMS) also issued tsunami warnings for areas including ‘otu Mu’omu’a in Ha’apai (Nomuka, Mango, Fonoifua), ‘Atataa, ‘Eueiki, and Tongatapu mo ‘Eua. Fluctuating tsunami waves were recorded off the coast of Tongatapu throughout the day; waves reached 200 m inland and rose as high as 15 m in elevation on the Good Samaritan Beach, according to TGS. The highest tsunami wave was recorded around 1800, measuring 30 cm, according to the Nuku’alofa tidal gauge.

The eruption on 14 January continued for more than 12 hours, with the plume reaching 20 km altitude, based on data from HIMAWARI-8 AHI satellite imagery captured between 1410 and 2330. Ashfall was reported on the Mango and Fonoi (75 km ENE) islands of Ha’apai. The eruption plume also contained an estimated sulfur dioxide mass of 0.05 Tg (50 kilotons), based on satellite data. As a result, the smell of sulfur was reported over Tongatapu, Ha’apai, and ‘Eua. Ashfall was reported on many islands, including Fonoi and Mango. All domestic flights in Tonga were canceled on 15 January. By 2230 the altitude of the plume had decreased to 18 km. The Global Lightning Detection Network (GLD360) ground-based network detected 191,309 lightning events from 0334 on 14 January through 0134 on 15 January, or up to 30,000 events per hour.

Activity during 15 January 2022. The eruption was intermittent during 0043 through 0604, according to the Wellington VAAC; plumes rose to 14 km altitude. The Nuku’alofa tidal gauge recorded waves less than 10 cm high through 1000 on 15 January, causing the National Tsunami Warning Centre to cancel the tsunami marine warning for Tongatapu, Ha’apai, and southern Tonga. Based on observations from satellite images, an ash plume rising to 14 km altitude was detected at 0720 on 15 January that drifted downwind to the E from the volcano due to an eruption that lasted 10-15 minutes.

A larger submarine eruption began at 1700 on 15 January from vents just below the surface of the ocean, generating an eruption plume of gas, steam, and some ash. A report from the Wellington VAAC stated that the plume had risen to 15.2 km altitude by 1918; the top of the plume was at least 600 km in diameter by 1903 as seen in satellite images (figure 49). According to news reports and social media posts, residents in Nuku’alofa heard multiple loud booms and saw a large expanding eruption plume that eventually covered all the Tongan islands. The eruptive activity lasted eight minutes.

Figure 49. HIMAWARI-8 satellite image showing the significant eruption plume that rose from Hunga Tonga Hunga Ha’apai taken at 1800 (local time) on 15 January 2022. The top of the plume measured at least 600 km in diameter and rose to at least 20 km altitude, possibly higher. Tongan islands are outlined in blue. Courtesy of CIMSS.

Ashfall eventually measuring several centimeters thick started around 1800 on 15 January and lasted about 10 hours (figure 50), which also affected water tanks and some residents reported having difficulty from breathing the ash in the air. The eruption caused a communications black out across the country, which included damaging the underwater fiber-optic cable providing access to the internet and damage to satellite terminals due to being covered in ash.

Figure 50. Drone image of the tsunami aftermath from the Hunga Tonga-Hunga Ha’apai eruption taken on 16 January 2022. Image shows ashfall in Nuku’alofa. Courtesy of Tonga Geological Services.

During 1719-2300 on 15 January there were almost 400,000 lightning events recorded in the plume by the GLD360 network, with 200,000 of those occurring during 1800-1900. The plume continued to rise to at least 20 km altitude, though other satellite datasets suggested that some of it may have risen as high as 30 km. At 0343 on 16 January the plume was at 19.2 km altitude. Based on volcanic ash advisories issued by the Wellington VAAC and then by the Darwin VAAC, the horizontal extent of the plume grew from 18,000 km² at 1739 on the 15th to 12 million km² by 1300 on 19 January. The plume narrowed and lengthened along an E-W axis, moving W over Australia. According to data from the TROPOMI instrument on the Sentinel-5P satellite, a sulfur dioxide plume was detected with a mass of roughly 50 kilotons on 14 January, 420 kilotons that drifted W on 16 January, and 400 kilotons that continued to drift W on 17 January (figure 51).

Figure 51. A large sulfur dioxide plume was released from Hunga Tonga-Hunga Ha’apai on 14 January 2022 (top) that measured roughly 50 kilotons. Significant plumes with a mass of roughly 400-420 kilotons continued to be visible on 16 January (middle) and 17 January (bottom) drifted W. Courtesy of NASA Global Sulfur Dioxide Monitoring Page via Simon Carn.

The explosions produced multiple pressure (shock) waves that rippled through surrounding weather clouds at a rate of 300 m/s, though the pressure wave from the largest explosion propagated across the planet. The sonic boom from this wave was heard at great distances, including within about two hours in New Zealand (2,300 km SW), Samoa, Fiji (500 km NW), Vanuatu, Cook Islands, and Alaska (9,370 km NE km). The pressure wave was also recorded by infrasound and weather instruments worldwide as it circled the Earth, with instruments detecting it again when it arrived from the opposite direction.

Warnings were issued for the N and E coasts of New Zealand’s North Island and the Chatham Islands; multiple boats were destroyed overnight in Tutukaka Marina in Far North (figure 52). According to an RNZ article, more than 120 people had been evacuated from the Far North between 2300 and 0000. Tsunami waves reaching 1.2 m high hit Nuku’alofa during the evening, breaching the shoreline and flooding coastal roads and properties. At Mango Island (75 km ENE), the tsunami waves reached 500 m inland and rose as high as 12 m high, destroying buildings and trees (figure 53). According to the Tongan Navy, tsunami waves that hit Mango Island reached 5-10 m high. According to an official update from the Tongan government on 20 January, tsunami waves reached up to 15 m high, which affected the W coasts of Tongatapu, 'Eua, and Ha'apai Islands and the water depth in the village of Kanokupolu was 1.5 m.

Figure 52. Photos of boats that were damaged by strong tsunami waves during the night of 15 January 2022 as a result of the Hunga Tonga-Hunga Ha’apai eruption and possibly Cyclone Cody at Tutukaka Marina in Far North. Photo by Sam Olley. Courtesy of Radio New Zealand.

Figure 53. Video footage of Mango Island (75 km NE) showing the damage caused by the tsunami waves generated by Hunga Tonga-Hunga Ha’apai from the eruption on 15 January 2022 coupled with an acoustic wave that permeated the atmosphere. The resulting wave measured 12 m high and moved as far as 500 m inland. Traces of the waves can be observed based on the dark brown rings surrounding the coast on the sand. Ashfall lasting 10 hours was also reported after 1800 on the 15th. Footage was captured on 20 January 2022. Video captured by Nikolasi Heni. Courtesy of Tonga Geological Services.

In Japan, around 230,000 people were advised to evacuate across eight prefectures due to the tsunami risk. The resulting waves were recorded in the Kominato district of Amami-Oshima Island in Kagoshima Prefecture at 2355. The waves reached 80 cm in Japan, disrupting train services, flights, and damaging harbors and boats. In Anchorage, Alaska, the US National Weather Service reported maximum waves heights of 20-100 cm on Alaskan coastlines and along the British Columbia coast, waves were 16-29 cm. By the morning of 16 January, the tsunami warning had been extended to the W coast of South Island. Reports indicated that the villages of Sopu, Kolomotua, Kolofo'ou, Ma'ufanga, Patangata were also affected by the tsunami, especially areas near Vuna Road.

Activity during 16-31 January 2022. No new eruptive events were detected after the large explosive eruption on 15 January. A satellite image taken on 18 January showed that most of the previously combined island had been destroyed, leaving only a small part of the NE island of Hunga Tonga (200 m long) and the SW island of Hunga Ha’apai (700 m long) visible above the ocean surface (figure 54). Pumice rafts were visible to the SE of Hunga Ha’apai. The Aviation Color Code was lowered to Green on 19 January. According to the Darwin VAAC, the plume continued to drift W at altitudes between 12.8 and 19.2 km during 19-22 January; the ash was diffuse and difficult to distinguish from meteorological clouds, though the sulfur dioxide signal was stronger. The leading edge of the plume reached the E coast of Africa on 22 January, but by 2150 (local time) the Darwin VAAC noted that ash was no longer detectable.

Figure 54. Following the major eruption on 15 January 2022, this image taken on 18 January shows that most of the previous combined island had been destroyed, leaving only small remnants of the NE island of Hunga Tonga (200 m long) and the SW island of Hunga Ha’apai (700 m long) visible above the ocean surface. A few days after the eruptive activity, pumice rafts were still visible in the area. Images courtesy of Tanya Harrison, Planet Labs.

The Tongan Ministry of Foreign Affairs and Trade confirmed three deaths and multiple people injured following the explosion and tsunami on 15 January. Aerial surveillance images showed that buildings and villages had washed away or been badly damaged, roofs were covered with ash, trees were uprooted, vegetation was brown and damaged, and modified coastlines contained sediment-laden waters. All the houses on Mango Island were destroyed due to the tsunami waves; a few temporary tarpaulin shelters were still visible. Only two houses remained on Fonoifua, and extensive damage was also reported on Nomuka Island (figure 55). Evacuations began on 19 January, which included people on the islands of Mango, Atata, and Fonoifua; about 150 people were evacuated to other islands. About 89 people remained in evacuation shelters on the island of ‘Eua. According to the Tongan government, parts of the W side of Tongatapu, including Kanokupolu, were evacuated after dozens of houses were damaged (figure 55), and in the central district many houses were damaged in Kolomotu'a and on the island of 'Eua. Numerous buildings were reported gone on Atata Island. Roughly 62 survivors were evacuated from Mango Island to Tongatapu on 22 January via a Navy patrol boat. According to a news article from Radio New Zealand, two deaths were reported from a beach in Peru (over 10,000 km) due to high tsunami waves.

Figure 55. Drone images of Nomuka Island (top) and Kanokupolu Village (bottom) showing the damage caused by the Hunga Tonga-Hunga Ha’apai eruption taken on 16 January 2022. Image on the bottom shows the remnants of the Vakaloa Resort. Footage of Nomuka Island was recorded by Nikolasi Heni. Courtesy of Tonga Geological Services.

According to a media release from the Government of Tonga on 21 January, all the islands were affected by ashfall and tsunami waves. A relief flight from New Zealand brought telecommunication equipment and a repair vessel was sent to repair the damaged seafloor fiber-optic cable; domestic flights remained suspended. Dozens of Mw 4.5-5 earthquakes were detected in the vicinity of the volcano after the eruption, at least through 24 January, though the type of earthquake signal was unknown.

Geologic Background. The small andesitic islands of Hunga Tonga and Hunga Ha'apai are part of the western and northern remnants of the rim (~6 km diameter) of a largely submarine caldera located about 30 km SSE of Falcon Island. The topmost sequence of welded and unwelded ignimbrite units from a caldera-forming eruption was 14C dated to 1040-1180 CE (Cronin et al., 2017; Brenna et al. 2022). At least two additional welded pumice-rich ignimbrite units and nonwelded pyroclastic flow deposits, below paleosols and other volcaniclastic deposits, indicated more very large previous eruptions (Cronin et al., 2017; Brenna et al. 2022). Several submarine eruptions have occurred at this caldera system since the first recorded eruption in 1912, including 1937 and S of the islands in 1988. A short eruption in 2009 added land to to Hunga Ha'apai. At that time the two islands were each about 2 km long, displaying inward-facing sea cliffs with lava and tephra layers dipping gently away from the caldera. An eruption during December 2014-January 2015 was centered between the islands, and combined them into one larger structure. Major explosive eruptions in late 2021 initially reshaped the central part of the combined island before stronger activity in mid-January 2022 removed most of the 2014-15 material; an even larger eruption the next day sent an eruption plume high into the stratosphere, triggered shock waves through the atmosphere and tsunami across the Pacific Ocean, and left only small remnants of the islands above the ocean surface.

Information Contacts: Tonga Geological Services, 51 Vaha'akolo Road, Nuku’alofa, Tonga (URL: https://www.facebook.com/tongageologicalservice); Matangi Tonga Online, PO Box 958, Nuku‘alofa, Tonga (URL: https://matangitonga.to/); RNZ (Radio New Zealand) Pacific, PO Box 123, Wellington 6140, New Zealand (URL: https://www.rnz.co.nz); 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/); Wellington Volcanic Ash Advisory Centre (VAAC), Meteorological Service of New Zealand Ltd (MetService), PO Box 722, Wellington, New Zealand (URL: http://www.metservice.com/vaac/, http://www.ssd.noaa.gov/VAAC/OTH/NZ/messages.html); Planet Labs, Inc. (URL: https://www.planet.com/); 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/); Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin-Madison, Space Science and Engineering Center, 1225 West Dayton Street, Madison, WI 53706, USA (URL: http://tropic.ssec.wisc.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); 1 News, New Zealand (URL: https://www.1news.co.nz/); Tanya Harrison, Planet Labs, Inc. (URL: https://twitter.com/tanyaofmars); Chris Vagasky, National Lightning Safety Council (URL: https://twitter.com/COweatherman); Simon Carn, Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA (URL: http://www.volcarno.com/, Twitter: @simoncarn).

Sangeang Api is a complex volcano on the small island of Sangeang in the Lesser Sunda Islands, Indonesia, consisting of two volcanic cones. After a major explosion in May 2014, activity continued until November 2015, with thermal anomalies indicating possible lava dome growth or lava flows (BGVN 39:02 and 41:10). Another eruptive period during July 2017 into June 2020 included occasional weak ash explosions with ash plumes and emissions, hot material discharged from the summit crater, periods of numerous thermal anomalies, summit incandescence, and infrequent Strombolian activity (BGVN 42:09, 43:11, 44:05, 45:02, and 45:08). The volcano is monitored by Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, or CVGHM), the Darwin Volcanic Ash Advisory Center (VAAC), and by various satellites.

The only documented activity during August 2020-February 2022 was an ash plume on 17 February 2022 reported by the Darwin VAAC that rose to an altitude of 4-4.9 km (~3 km above the summit) and drifted SW and WSW. During August 2020-February 2022, Sentinel-2 satellite images during this time were usually obscured by weather clouds, but no thermal signals or volcanic activity were observed when there were clear views. During May 2021 through February 2022, the MIROVA (Middle InfraRed Observation of Volcanic Activity) volcano hotspot detection system recorded ten scattered hotspots within 5 km of the summit; the cause of the weak anomalies is unknown.

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

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

A short episode of explosive submarine volcanism was recorded 24 April 2001 by the Laboratoire de Géophysique's (LDG) Pomariorio (PMO) seismic station on Rangiroa Atoll, Tuamotu Archipelago. This episode began at 1110 UTC, and ended at 1900 UTC, with more than 40 explosive T-waves at a fairly uniform rate. The wave forms were similar to those of December 1989 (from a source NW of Supply Reef, SEAN 14:12), and suggested a source in the Mariana Islands. LDG scientists identified these explosive events on records from some other IRIS and Freesia stations, and computed a well-constrained location at 20.34°N, 145.02°E with an error of 15 km (figure 1).

Figure 1. Map showing Ahyi and other volcanic edifices along part of the Mariana Arc just north of 20°N, 145°E. The location of the April 2001 activity is indicated, as well as activity reported between Farallon de Pajaros and Supply Reef in 1967, 1969, 1979, 1985, and 1989. Contour interval is 200 m; bathymetry is based on US Navy narrow-beam SASS data. Thick black bars show 1985 dredge locations. Scale and volcanic activity locations are approximate. Base map modified from Bloomer and others (1989).

The summit of Ahyi lies within this location uncertainty, approximately 10 km N. Ahyi seamount is a large conical submarine volcano that rises to within about 140 m of the sea surface about 18 km SE of Farallon de Pajaros. Water discoloration has been observed over the volcano, and in 1979 the crew of a fishing boat felt shocks over the summit area followed by upwelling of sulfur-bearing water (SEAN 04:11).

Regional volcanic activity. Most of the recent historical activity in this area is based on acoustic detection methods from great distances, making exact location determinations difficult. The following presents background information about other volcanoes close to the April 2001 event, with a description of recent volcanism.

The small 2-km-wide island of Farallon de Pajaros (also known as Uracas) is the northernmost and most active volcano of the Mariana Islands. Its relatively frequent historical eruptions dating back to the mid-19th century have caused it to be referred to as the "lighthouse of the western Pacific." Flank fissures have fed historical lava flows that form platforms along the coast. Summit vents have also been active during historical time, and eruptions have been observed from nearby submarine vents. Aerial observations of fuming were reported in July 1981 (with discolored water), August 1990, and May 1992. Makhahnas seamount, which rises to within 640 m of the sea surface, lies about 10 km SW. A possible eruption during March-April 1967 on the SW flank of this seamount was identified on the basis of T-phase recordings by Norris and Johnson (1969).

Supply Reef is a conical submarine volcano that rises to within 8 m of the sea surface. The seamount lies about 10 km NW of the Maug Islands, the emergent summit of a submarine volcano that is joined to Supply Reef by a low saddle at a depth of about 1,800 m. Several submarine eruptions have been detected by sonar signals originating from points very approximately located at distances of 15-25 km NW of Supply Reef. An event in March 1969 was detected using T-phase recordings and located by the crew of a fishing boat who heard explosion sounds and saw water discoloration (CSLP Cards 528 and 534). Activity in August-September 1985 (SEAN 10:09 and 10:11) and September and December 1989 (SEAN 14:10 and 14:12) were in the same approximate location, 30 km S of Farallon de Pajaros, about midway between Makhahnas and Supply Reef. Both of these events were identified and located using T-phase data, but discolored water was also observed during the 1985 event by an airline pilot.

References. Bloomer, S.H., Stern, R.J., and Smoot, N.C., 1989, Physical volcanology of the submarine Mariana and Volcano arcs: Bulletin of Volcanology, v. 51, p. 210-224.

Norris, R.A., and Johnson, R.H., 1969, Submarine volcanic eruptions recently located in the Pacific by Sofar hydrophones: Journal of Geophysical Research, v. 74, no. 2, p. 650-664.

Geologic Background. Ahyi seamount is a large conical submarine volcano that rises to within 75 m of the sea surface about 18 km SE of the island of Farallon de Pajaros (Uracas) in the northern Marianas. Water discoloration has been observed there, and in 1979 the crew of a fishing boat felt shocks over the summit area of the seamount, followed by upwelling of sulfur-bearing water. On 24-25 April 2001 an explosive eruption was detected seismically by a station on Rangiroa Atoll, Tuamotu Archipelago. The event was well constrained (+/- 15 km) at a location near the southern base of Ahyi. An eruption in April-May 2014 was detected by NOAA divers, hydroacoustic sensors, and seismic stations.

Information Contacts: Olivier Hyvernaud, Laboratoire de Géophysique, PO Box 640, Pamatai, Tahiti, French Polynesia.

The following report, discussing volcanic aerosol optical thicknesses since 1960 as derived from lunar eclipse observations, was provided by Richard Keen. About once per year, on average, the moon is eclipsed as it passes into the Earth's shadow; at these times the moon can be used as a remote sensor of the global average optical depth of stratospheric aerosols of volcanic origin. Volcanic aerosols and lunar eclipses can be linked because the moon is visible during total lunar eclipses due to sunlight refracted into the shadow (umbra) by the Earth's atmosphere (primarily by the stratosphere), stratospheric aerosols reduce the transmission of sunlight into the umbra, and the path length of sunlight through a stratospheric aerosol layer is about 40 times the vertical thickness of the layer. Therefore, the brightness of the eclipsed moon is extremely sensitive to the amount of aerosols in the stratosphere.

Methodology and data reduction. Aerosol optical thicknesses can be calculated for the date of an eclipse from the difference between the observed brightness of the eclipse and a modeled brightness computed for an aerosol-free standard atmosphere, modified by assumed distributions of ozone and cloud. Details of this technique, applied to observations during 1960 through 1982, appear in Keen (1983); updates following the eruption of Pinatubo appeared in February 1993 (Bulletin v. 18, no. 2) and November 1997 (Bulletin v. 22, no. 11). This report updates the time series through the lunar eclipse of 9 January 2001, the last total lunar eclipse until May 2003.

Figure 12 plots the global optical thicknesses derived from 38 total or near-total lunar eclipses during 1960-2001. Results from eight eclipses during 1880-1888 have been added to figure 12 to allow comparison with the effects of Krakatau in 1883. The plotted values are actual derived optical depths, modified as follows: Due to the higher concentration of aerosols from Agung and El Chichón in the Southern and Northern Hemispheres, respectively, a sampling bias due to the moon's passing though the southern or northern portion of the umbra was removed by using an empirical adjustment factor of 0.8 (thus, if the moon passed S of the Earth's shadow axis during an eclipse following an Agung eruption, the derived optical thickness was multiplied by 0.8, while the derived value was divided by 0.8 if the moon passed N of the axis). Furthermore, no lunar eclipses occurred until 18 months following the Pinatubo eruption in June 1991, while results from Agung and El Chichón indicate that peak optical depths occurred about 9 months after those eruptions. Therefore, for plotting purposes on figure 12, the time series of optical thicknesses following Pinatubo was extrapolated backwards to a date 9 months after the eruption using a composite decay curve (with a time constant of 1.92 years) derived from the Agung and El Chichón eclipse data. Finally, the global optical depths were set to zero on the dates of the eruptions of Krakatau, Agung, Fuego, and Pinatubo; observed values were near zero for eclipses close to the dates of the eruptions of Fernandina and El Chichón.

Figure 12. Global optical thicknesses derived from 38 total or near-total lunar eclipses during 1880-1888 and 1960-2001. Details about the methodology and data reduction used to construct this figure are in the report text. Courtesy of Richard Keen.

The time series. The volcanic eruptions probably responsible for the major peaks in the times series are identified, although the identification of Fernandina with the 1968 peak is highly uncertain. Comparative maximum global optical thicknesses are: Pinatubo (1991), 0.15; Krakatau (1883), 0.13; Agung (1963), 0.10; El Chichón (1982), 0.09; Fernandina (1968), 0.06; Fuego (1974), 0.04.

The results indicate that the volcanic aerosol veil from Pinatubo disappeared between the eclipses of November 1993, and April 1996, with optical depth probably reaching zero sometime in 1995. Since 1995, optical depths have stayed near zero ( ± 0.01), indicating no further major injections of volcanic aerosols into the stratosphere. However, slight increases to observed values slightly above 0.01 in 1979 and in late 1997 are close to the noise level due to the uncertainty in the brightness observations; if real, they could indicate aerosols from the eruptions of Soufriere St. Vincent (1979) and Soufriere Hills on Montserrat (1997).

Acknowledgments. Thanks are due to the following observers who supplied observations of the three eclipses in the 2000-2001 series: C. Drescher, F. Farrell, M. Matiazzo, A. Pearce, and D. Seargent (Australia), W. de Souza and J. Aguiar (Brazil), J. Finn (Canada), K. Hornoch (Czech Republic), A. Shahin (Dubai, United Arab Emirates), G. Glitscher (Germany), N. Abanda, S. Abdo, W. Abu Alia, E. Al-Ashi, H. Al-Dalee', A. Al-Niamat,K. Al-Tell, and M. Odeh (Jordan), R. Bouma (Netherlands), B. Granslo and O. Skilbrei (Norway), A. Pereira and C. Vitorino (Portugal), J. Atanackov and J. Kac (Slovenia), T. Cooper (South Africa), T. Karhula and P. Schlyter (Sweden), R. Eberst and A. Pickup (UK), R. Keen, T. Mallama, and J. Marcus (USA).

References. Keen, R., 1983, Volcanic aerosols and lunar eclipses: Science, v. 222, p. 1011-1013.

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: Richard A. Keen, Program for Atmospheric and Oceanic Sciences (PAOS) , 311 UCB, University of Colorado, Boulder, CO 80309 USA.

This report describes two visits to the rim of Colima's main crater (17 March and 26 May 2001) and summarizes collateral data collected around that time. On the earlier visit, observers found an enlarged main crater, they noted the disappearance of an older (1994) crater, and they photographed a recent crater with a sulfur-encrusted, warped, and fractured floor. By the time of the later visit, an unusual new dome had appeared, composed of more fragmentary and lighter color clasts than typical for Colima's lava domes. Effusive activity was previously seen during November 1998-February 1999.

Crater rim observations. On 17 March 2001, Nick Varley and Juan Carlos Gavilanes ascended to Colima's crater rim (figures 40 and 41). It was the first visit there since January 1999. Circumnavigating the main crater, they prepared a map of the current crater and environs (figure 40). The main crater was 230-260 m in diameter, 15-40 m deep, and ~1.4 x 10⁶ m³ in volume. Its diameter had grown two-fold larger than it was before the 1998-99 eruption, reaching its largest size since the early 1960s.

Figure 40. A sketch map of Colima's crater zone showing the main summit crater geometry after the 1998-99 eruption, and the dome seen on 26 May 2001. The small triangles on the crater rim indicate GPS-surveyed points (way points obtained using various receivers on 17 March and 26 May 2001); values at the map margins are UTM coordinates. The photograph shown in figure 41 was shot from the vantage point indicated by the bold rectangle on the main crater's eastern rim. Historical lava flows traveled down the volcano along routes indicated by small arrows. Fumaroles Fa and Fb indicate areas with temperatures over 850°C and over 800°C during December 1995 and May 1998, respectively. The locations of the craters formed during the 1994 and 1987 explosions were based on an August 1996 survey by A. Cortés, J.C. Gavilanes, and J. Ramos. The current map was prepared by J.C. Gavilanes, N. Varley, A. Rivera, and J. Heredia.

Figure 41. Pre-extrusion views of Colima's up-warped crater floor as seen from the point on the main crater rim indicated on the map (figure 40) on 17 March 2001. The upper photo provides an overview shot of the 22 February 2001 crater; the lower photo is zoomed in on the deformed crater floor. The crater floor displays both fractures and buckling of sufficient intensity to create a visibly undulatory surface. The color version of the photos shows bright yellow sulfur incrustations over extensive portions of the up-warped crater floor. Photo and caption provided courtesy of J.C. Gavilanes.

On their 17 March visit Varley and Gavilanes found a smaller crater located inside the main crater's N sector (figure 40). This inner crater was assumed to be formed by the 22 February 2001 explosion. The inner crater was then estimated to be 127 m in diameter, 15 m deep, and ~0.2 x 10⁶ m³ in volume. In the NE sector of the inner crater they observed an inflated, buckled, and fractured surface (figure 41). They inferred that this inflated surface stemmed from an intrusion initiated sometime after the 22 February explosion.

Figure 42 records the scene Varley and Gavilanes found when they ascended to the crater rim on 26 May 2001. Close to the inflated surface observed on 17 March they found a new lava dome. It stood ~115 m across its base, ~57 m across its top, ~30 m high, and was ~0.15 x 10⁶ m³ in volume. The two observers also noted that in comparison to conditions witnessed during the previous crater ascent, new and stronger fumarolic zones surrounded the new dome, mainly to its N, NE, and E (figure 40).

Figure 42. A photo of the new dome shot from the Colima's E crater rim on 26 May 2001. The photo of the new dome was taken from the vantage point indicated by the rectangle on figure 40, ~ 135 m from the center of the dome. Courtesy of J.C. Gavilanes.

Collateral observations. Later review of seismic, deformation, and GOES radiation data (figure 43) showed that dome extrusion may have started on 8 May, a day with distinct increases in both thermal radiation and tilt. No increase in seismic activity was observed; the proposed explanation for this is that the lava was plastic enough to avoid the shear fracturing of surrounding structures. Assuming that the extrusion started on 8 May 2001, the resulting growth rate (for 8-26 May, 19 days) was ~0.1 m³ s-1. Fieldwork in the crater's vicinity took place over a 3-hour interval and included gas sampling. Only a small rockfall was heard.

Figure 43. Plots of four monitored parameters at Colima acquired during April-May 2001. The common time axis allows the comparison of seismic (RSAM) data (A), remotely sensed radiance (B), and tilt (C and D). The tilt data (C and D) were recorded at a station 1.02 km E of the dome. The arrow indicates the inferred date when the dome began extruding. Seismic data represent the cumulative amplitude of reduced seismic energy (RSAM) measured at station EZV4, 1.7 km from the crater. Seismicity remained relatively quiet (see text). The radiance plot (B) was made using mid-infrared (3.9 mm) data. This plot presents infrared volcanic radiance acquired by NOAA's geostationary GOES-8 satellite. The radiance values shown depict the hottest pixel within the 500 x 500 pixel box that lies centered on Colima. These data were made available by the Institute of Geophysics & Planetology of the University of Hawaii. The figure was compiled by V. M. Zobin using data processed by the University of Hawaii, and data collected and processed by T. Dominguez, C. Navarro, and H. Santiago.

The new dome appeared anomalous in certain ways. It was not composed of large dark-colored blocks (as observed for the effusive events that occurred during the last 40 years), but instead consisted mainly of smaller-sized blocks with a light-gray color. The new dome could be an example of endogenous dome growth, where no new molten material reaches the surface.

On 1 May 2001 the measured SO₂ flux was 200 t/d, and on March 16 it was 145 t/d. These are only slightly higher than mean values recorded during the calm period of 1997, which were less than 100 t/d.

Geologic Background. The Colima volcanic 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 caldera, 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 historical 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: Observatorio Vulcanológico de la Universidad de Colima, Colima, Col., 28045, México; Facultad de Ciencias de la Universidad de Colima, Colima, Col., 28045, México (URL: http://www.ucol.mx/).

During the most recent austral summer, December 2000-March 2001, the Spanish Antarctic Programme (SAP) carried out its yearly survey of Deception Island. Researchers from Spain, Italy, and México took part in the seismological, magnetic, and geochemical study of the entire island.

The seismic network's stations were deployed in a variety of configurations (figure 15). The instruments used were as follows: two dense seismic antennas each with 16 short-period seismometers, two small antennas each with four seismometers, three short-period seismometers, two broadband seismic stations, and four autonomous three-component short-period seismic stations.

Figure 15. Seismic instruments deployed in the December 2000-March 2001 field survey of Deception Island. Seismic arrays are detailed in large squares. Courtesy of SAP.

Seismicity is summarized in figure 16. Registered seismic events featured volcano-tectonic earthquakes (VT), a few episodes of volcanic tremor, long-period events (LP), and hybrid events (VT + LP). More than 75 VT, 500 LP, and 20 hybrid events were recorded; this constituted moderate activity compared to previous surveys. Hybrid events, which were difficult to detect in previous studies, peaked at the end of January 2001. Volcanic tremor episodes occurred with durations between hours and a few days; workers interpreted these events, together with the LP events, as a consequence of hydrothermal activity.

Figure 16. Histogram of the volcano-tectonic (VT), long period (LP), and hybrid events recorded during 20 December 2000-15 February 2001. Courtesy of SAP.

The magnetic field in the area was monitored using a proton magnetometer deployed near the Argentinean base, which is the position used in previous surveys (figure 17). The recorded values of the magnetic field are being processed and corrected according to external variations in order to observe whether volcano-magnetic effects produced variation in the local magnetic field.

Figure 17. Map showing morphological features, bases, and the sites selected to measure CO2 flux. Courtesy of the SAP.

Geochemical investigations consisted of recording gas composition and temperature of the fumaroles in Fumarole Bay and measuring CO₂ flux at 26 points around the island (figure 16). The chemical analyses of the fumarolic samples are being processed. Fumarole temperatures averaged ~100°C, similar to values of previous years. The majority of points, including those bordering Fumarole Bay, had a very low flux of CO₂. Two of them, however, Murature Point and Cerro Caliente hill (figure 17), had high fluxes. Future studies will conduct similar surveys in order to establish a CO₂ flux map for the entire island.

Geologic Background. Ring-shaped Deception Island, at the SW end of the South Shetland Islands, NE of Graham Land Peninsula, was constructed along the axis of the Bransfield Rift spreading center. A narrow passageway named Neptunes Bellows provides an entrance to a natural harbor within the 8.5 x 10 km caldera open to the ocean that was utilized as an Antarctic whaling station. Numerous vents along ring fractures circling the low 14-km-wide island have been reported active for more than 200 years. Maars line the shores of 190-m-deep Port Foster caldera bay. Among the largest of these maars is 1-km-wide Whalers Bay, at the entrance to the harbor. Eruptions during the past 8,700 years have been dated from ash layers in lake sediments on the Antarctic Peninsula and neighboring islands.

Information Contacts: Alicia García and Ramón Ortiz, Dpto. Volcanología, Museo Nacional de Ciencias Naturales, CSIC, José Gutierrez Abascal 2, 28006, Madrid, Spain; Jesús M. Ibáñez, Enrique Carmona, José Benito Martín, and Carmen Martínez, Instituto Andaluz de Geofísica, Apartado 2145, University of Granada, 18071 Granada, Spain; José Luis Pérez-Cuadrado, Universidad de Cartagena, 30202 Murcia, Spain; Mauricio Bretón, Universidad de Colima, Colima, Col., 28045, México; Mario La Rocca, Osservatorio Vesuviano, Via Diocleziano 328, 80124 Napoli, Italy.

As reported by Sistema Poseidon, activity at Etna (figure 85) during December 2000-8 April 2001 was characterized by episodic Strombolian blasts, steam and ash emissions, and lava flows.

Figure 85. Aerial photograph of Etna looking E towards the Bocca Nuova vent within the central crater on 6 December 2000. Northeast Crater is also partially visible (in the left background), as well as Southeast Crater (right). Courtesy of Sistema Poseidon.

Minor activity during December 2000 through mid-January 2001. Low-intensity gas emissions dominated activity during this period. Observations on 6 December revealed three distinct cavities in the interior of the Bocca Nuova (BN) vent. The two near the center of the crater trended NW, were deep and full of material, and were delineated by pit-craters. The smaller cavity to the SE was encircled by a high wall of scoria; it weakly emitted light brown ash, possibly due to internal collapse. White steam emissions from BN in early January were visible during the early morning hours, and became more evident as each day progressed due to increased humidity. Sporadic ash ejections also occurred.

At the end of December, adverse atmospheric conditions prevented detailed observations, but during rare periods of visibility observers saw snow covering the W flanks of the central crater and Southeast Crater (SEC). A weak intermittent fumarolic emission emerged from the base of the fracture that runs from the SEC to the lava cairn at its base. The SEC also produced weak fumarolic emissions in early January from the W edge of the crater's summit. On the evening of 14 January a weak, diffused illumination was observed at SEC, likely coming from the E edge of the crater, where during recent months there was visible night incandescence.

Increased activity during mid-late January 2001. The BN vent produced abundant steam during the middle of January. Brown ash was weakly emitted on 16 and 19 January; darker ash ejections occurred on the 18th and 21st. Ash fell on the E flank of the volcano for five hours during the morning of the 18th, and weak illumination was visible for 30 minutes that night coming from BN. Ash-and-gas emissions increased toward the end of January. Isolated night glow suggested weak explosive Strombolian activity confined to inside the central crater. Activity alternated between visible degassing and intense phases of ash emission; one particularly acute phase occurred on 31 January.

New activity initiated from SEC on the evening of 15 January. Low-energy Strombolian eruptions were seen at night by distant observers. Activity increased in frequency during 16-17 January, reaching a maximum on 18 January when explosions occurred every 3-4 minutes, interspersed with high-energy episodes that repeated at variable intervals of ~1-2 hours. Ejected material from these events reached ~50 m high on the edge of the SEC, falling back into the crater. Strombolian activity continued through 19 January. Lava began to flow from the radial fracture cutting the N flank of the SEC beginning during the day on 21 January and persisting discontinuously until the end of the month. Intermittent flows formed several finger-like fronts. The flow reached down to ~2,800 m elevation, and remained confined to the Valle del Leone.

Strombolian explosions at Bocca Nuova during February-April 2001. During the nights of 1 and 4 February, frequent illumination was observed in the BN vent. Strombolian activity continued from BN throughout February. As during January, strong degassing and dark gray ash emissions were sporadic. High ambient humidity during morning hours made gas plumes distinct, especially on 10 February; activity was particularly consistent during 20-22 February. The fixed Montagnola camera captured images of frequent flashes from the crater interior, but activity did not extend beyond the crater area.

The BN vent produced increased explosive activity during March from two vents (W and E) inside the depression. The W vent exhibited Strombolian explosions; during some periods these were continuous and sent incandescent material just above the crater rim. A small number of lava fragments fell outside of the crater and rolled down its flanks. Explosive activity at the E vent did not eject material above the crater rim. Alternating degassing and dark gray ash emission continued as in February. Fine-grained material blown by wind fell as far as 2 km from the summit. Activity was more intense on 6 and 28 March when BN emitted copious amounts of ash from the NW and SE sectors of the crater. The Montagnola camera detected almost continuous night illumination of the crater, suggesting Strombolian activity from multiple vents. Strombolian activity also occurred from Northeast Crater, although it was rarely visible.

Strombolian activity and ash emission from BN continued throughout April. On the evening of 4 April an intense phase at the S zone of the central crater included ejection of some incandescent material above the crater rim. During 7-8 April, a slight increase in the frequency of ash emissions was observed, while night-time incandescence was sporadic.

Lava flows from Southeast Crater during February-April 2001. Early in February lava emission from the N-flank of SEC diminished; it produced modest regular lava flows for the rest of the month. On 4 February observers saw intense flashes that indicated explosive lava ejection from the fracture. Flashes and illumination visible in camera footage evidenced erratic SEC effusive activity throughout February. One early February lava flow from a vent at 3,100 m continued for several days. Bubbles frequently burst from the lava, indicating high gas content within the magma. The lava flow was ~2 m wide near the source, grew to 5 m wide toward the base, and reached an elevation of 2,900 m. During mid-February a vent at 3,150 m elevation produced a flow down a 2-m-wide canal. The flow ran N initially, but ~100 m downslope it headed E and formed a lava tube about 20-25 m wide. The flow moved toward the Valle del Bove, in the direction of Monte Sinome; it continued through the end of the month and reached 2,600 m elevation.

Through mid-March lava continued to flow from the fracture at 3,080 m elevation on the SEC's N flank. Near the vent the flow was ~1 m wide and ~80 cm deep. After having flowed less than 2 m it divided into two forks that ran roughly parallel to each other. The principal flow retained a width of ~1 m and headed N for ~100 m before deviating toward the NE and reaching an elevation of ~2,800 m. The secondary flow was about half a meter in width; it traveled at ~4 m/s near the fork and ~2 m/hour near the flow front where it spread to ~5 m across at an elevation of about 2,970 m. Effusive activity appeared to diminish on 23 March. The vent observed three days before was no longer active. A single flow was fed by a new vent about 5 m below the previous vent. A steep slope at the vent's mouth produced flow velocities of ~6 m/minute. This flow reached down to an elevation of 2,950 m, where it traveled at 1 m/hour over the flows of three days before. The flow front measured 5 m wide and 1 m high. On 30 March conspicuous white vapor issued from the SEC.

A 4 April survey of the flows revealed a moderate flow from the N flank of SEC. The vent had built up a small cone ~6 m tall at 3,095 m elevation. Two flows, each ~1 m wide and 1-2 m deep, traveled away from the cone and joined together 20-25 m away, flowing E. The flows in the two channels moved at a speed of ~0.1 m/s and an estimated 0.2-0.4 m³ of molten material emerged each second. The maximum length of the overall flow was ~350 m. During the evening of 8 April strong, persistent illumination from the E base of SEC probably indicated a new lava flow. The incandescence was distinctly visible as it reflected off of a steam plume from the summit crater.

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: Sistema Poseidon, a cooperative project supported by both the Italian and the Sicilian regional governments, and operated by several scientific institutions (URL: http://www.ct.ingv.it/en/chi-siamo/la-sezione.html).

In 1998, after 5.5 years of calm, Piton de la Fournaise erupted twice. Two eruptions occurred in 1999, while in 2000, three eruptions took place (BGVN 25:12). Only 4.5 months after the last eruption in October 2000, Piton de la Fournaise erupted once more on 27 March 2001 at 1320. As described below, precursor extensometer and tiltmeter measurements, in conjunction with historical data, provided an accurate forecast of an eruption sometime near the end of March. The March eruption was followed by another at 1350 on 11 June.

Geodetic measurements. After 1 January 2001, the Château Fort extensometer showed a significant, regular increase (figure 61), and, beginning 21 January, the Magne extensometer showed the same tendency. Plots of the measurements from these two stations show remarkably constant slopes of 0.0038 mm/day at Château Fort and 0.005 mm/day at Magne. In 1999 and 2000, such variations were observed 2-3 months before the eruptions of 19 July 1999, 23 June 2000, and 23 October 2000 on the E and SE flanks of the volcano. Using these historical data and the fact that the maximal variation of spread for all these eruptions was 0.25 to 0.35 mm for the Château Fort station and 0.3 to 0.5 mm for the Magne station, extrapolations of the deformation were used to forecast a late March eruption.

Figure 61. Extensometer measurements from the Château Fort station at Piton de la Fournaise during mid-December 2000-early April 2001. Courtesy of T. Staudacher, OVPF.

Almost simultaneous with the extensometer-measured tilt increases, important variations were registered by the Dolomieu Sud and La Soufrière tiltmeters. The Dolomieu Sud radial tiltmeter measurements increased considerably after 6 January 2001 compared to those for the previous two years; similar variations were observed before the 12 October 2000 and 28 September 1999 eruptions (figure 62). The measured increase of ~110 µrad of radial tilt as observed at Dolomieu Sud between January and March 2001 could not be explained by temperature changes. Rather, it indicated a significant inflation of the summit prior to the eruption.

Figure 62. Tilt variation from the Dolomieu Sud station at Piton de la Fournaise compared between 1999, 2000, and 2001. Courtesy of T. Staudacher, OVPF.

Seismicity. Intense seismicity on Piton de la Fournaise increased early in 2001. During 20 January-10 February, 133 tremors were registered (generally M < 0.5). Then, after 13 days of calm, a new series of tremors began on 25 February that included 315 events. These events were weak (M < 1.5), but increased in intensity with respect to the events earlier in the year. On 3 March, 40 summit tremors occurred within one hour, and a total of 126 tremors were observed that day. All of these tremors took place beneath the Dolomieu crater at ~0.5 km below sea level.

The number of tremors increased again starting on 12 March and continuing until the eruption on 27 March. Tremor hypocenters measured on 23 March occurred 1.5 km below sea level, but rose the next day to 0.5 km below sea level. Seismometers recorded 145 tremors on 25 March. Tremor intensity increased gradually during the period with numerous events of M 1.0-1.9. In addition, precursory seismicity and deformation measurements were correlated as shown in figure 63. Figure 63 indicates that, in January, summit inflation preceded the first period of seismicity by about 10 days, while the second increase in inflation, which began on 24 January, occurred simultaneously with the second period of strong seismicity. The latter continued essentially until the eruption. On 27 March, 120 tremors were detected, including one at 1255 of M 2.0. At 1320, an eruption began on the SE flank. Tremor that began with the eruption on 27 March diminished regularly until 2 April; after eight days of activity, the eruption ended on 4 April at about 0700.

Figure 63. Total number of earthquakes at Piton de la Fournaise compared with tilt variation during 1 January - mid-April 2001. Note that the total number of earthquakes exceeds the scale of the figure during and after the 27 March eruption. Courtesy of T. Staudacher, OVPF.

Ground observations. Ground observations were undertaken several hours after the eruption began. Five major fissures were active; their exact positions were determined later using GPS measurements. The first fissure, ~250 m long, began 100 m below the edge of Dolomieu Sud while the last ended between Piton Morgabim and the Signal de L'Enclos. The general trend of the fissures was ESE.

Three significant aa flows were observed. The first was fed by the highest fissure and descended along the S flank ending at about 1,800 m elevation. A second flow, which began at a lower altitude, wound around the Piton Morgabim toward the S and along the path of the previous flows from the June and October 2000 eruptions. The most significant flow was fed by the lowest fissure, which went N along the path of the June and October 2000 flows and came down the Grandes Pentes. By 27 March at 1700, this flow reached an elevation of 700 m, descending to 500 m on 28 March and continuing down to 350 m elevation on 29 March. These fissures were active for only several hours, and on 28 March the eruption became concentrated on the last fissure where the cone Piton Tourkal formed during the next few days. The cone was located midway between the Signal de l'Enclos and the Piton Morgabim (figure 64).

Figure 64. Photograph showing lava flows and the future location of the soon-to-be-formed Piton Tourkal cone, between the Signal de l'Enclos (bottom left) and the Piton Morgabim (middle left). Courtesy of T. Staudacher, OVPF.

Between 27 March and 3 April, a total of nine samples were gathered for chemical analysis. On 3 April, the lava temperature was measured to be 1,150°C. No significant variation in the rates of radon emission was measured during 27 March - 3 April.

Continuous extensometer and tiltmeter variations occurred, and increased seismic activity was recorded beginning in late May. A short seismic crisis with 126 recorded events started on 11 June at 1327 and, at 1350, extensometer variations indicated that a new eruption had started on the SE flank in the same area as the 27 March eruption. En echelon fissures formed on the S flank at ~2,500 m elevation, 200 m below the Dolomieu summit crater. More fissures were located between 2,000 and 1,800 m elevation on the E flank at the southern base of crater Signal de l'Enclos and N of the Ducrot crater. Several lava flows descended the Grand Brûlé but progressed very slowly; at 1700 the front of the lava flow reached an elevation of 1,450 m. On the morning of 12 June, only the lower fissure at 1,800 m elevation was still active. It measured ~200 m long, with several lava fountains that sent material 20-30 m high. The lava flow followed the N border of the 27 March lava flow and reached about 400 m elevation on the Grand Brûlé.

Geologic Background. The massive Piton de la Fournaise basaltic shield volcano on the French island of Réunion in the western Indian Ocean is one of the world's most active volcanoes. Much of its more than 530,000-year history overlapped with eruptions of the deeply dissected Piton des Neiges shield volcano to the NW. Three calderas formed at about 250,000, 65,000, and less than 5000 years ago by progressive eastward slumping of the volcano. Numerous pyroclastic cones dot the floor of the calderas and their outer flanks. Most historical eruptions have originated from the summit and flanks of Dolomieu, a 400-m-high lava shield that has grown within the youngest caldera, which is 8 km wide and breached to below sea level on the eastern side. More than 150 eruptions, most of which have produced fluid basaltic lava flows, have occurred since the 17th century. Only six eruptions, in 1708, 1774, 1776, 1800, 1977, and 1986, have originated from fissures on the outer flanks of the caldera. The Piton de la Fournaise Volcano Observatory, one of several operated by the Institut de Physique du Globe de Paris, monitors this very active volcano.

Information Contacts: Thomas Staudacher and Jean Louis Cheminée, Observatoire Volcanologique du Piton de la Fournaise, Institut de Physique du Globe de Paris, Institut National des Sciences de l'Univers, 14 RN3 - Km 27, 97418 La Plaine des Cafres, Réunion, France (URL: http://www.ipgp.fr/fr/ovpf/observatoire-volcanologique-piton-de-fournaise).

According to reports by the Observatorio Vulcanológico y Sismológico de Pasto (OVSP), volcanic unrest at Galeras continued during 16 April 2000-March 2001. However, OVSP reports for November-December 2000 were not available when this report went to press.

Two small eruptive episodes occurred on 22 April and 18 May 2000. The associated seismic records included long-period (LP) events and spasmodic tremor similar to those registered during eruptive episodes on 21 March and 5 April 2000 (BGVN 25:03). Elevated seismicity continued with two volcano-tectonic (VT) events on 30 July and 17 September 2001. These events were focused ENE of the active cone; previous activity initiating within this source region was sporadic. During January-March 2001 activity continued at low levels. VT events occurred during mid- to late-January, and were followed by similar events during late March.

New crater formation during April 2000. Spasmodic tremor starting on 22 April at 1558 lasted for 175 seconds, followed by three smaller tremor episodes with durations of 90, 320, and 170 seconds, respectively. Five small LP events also occurred; the final LP event was recorded at 1634. Peak frequency for the main event was ~5.0 Hz (figure 91), but at the nearest station to the active crater other frequencies ranging from 1 to 13 Hz were observed.

Figure 91. Main event seismic signal from 22 April 2000 at 1558 and its spectrum recorded at Anganoy station, 0.9 km E of Galeras's crater. Courtesy of OVSP.

Field inspections on 27 April revealed that within the Chavas fumarole area, on the WSW edge of the main crater, a new crater approximately 8 x 4 m in area and 1.5 m deep had formed. Several gas-emitting fissures were observed along the crater wall. Temperatures recorded at the border of the new crater on 27 April and 1 May were 408°C and 393°C, respectively, which are not anomalously higher than those observed previously.

During 16 April-30 June 2000, radon-222 emissions from soil monitored at several stations around Galeras showed values of 78-2,966 picocuries/liter (pCi/l). These levels are similar to those found in previous months. The highest value corresponded to the Sismo 2 station, located 5 km NE of the summit.

Activity during May-October 2000. An eruptive event at 1411 on 18 May was seismically characterized by an initial LP event with a dominant frequency of ~2.1 Hz figure 92), followed by five spasmodic tremor episodes and nine more LP events. The last LP event was recorded at 1806 later that day.

Figure 92. Main event seismic signal from 18 May 2000 at 1411 and its spectrum recorded at Anganoy station, 0.9 km E of Galeras's crater. Courtesy of OVSP.

On 30 July at 0935 an earthquake swarm occurred 9 km ENE of the active cone, in the suburban area adjacent to the city of Pasto. The main event (M 4.5) was distinctly felt inside the city and in other neighboring communities. Aftershocks of lesser magnitude (M 2.3-3.4) continued through 4 August.

On 17 September 2000 at 2246 residents of Pasto and neighboring communities felt a M 3.9 event. Seismographs also detected aftershocks of M 2.6. Figure 93 shows a map view of volcano-tectonic earthquakes that occurred during July-October 2000. According to a report, movement of fluids within volcanic conduits remained at low levels.

Figure 93. Map view showing volcano-tectonic earthquakes registered at Galeras during July-October 2000. Courtesy of OVSP.

During 1 July-30 October 2000, radon-222 emission from soil monitored around Galeras showed average values lower than 3,000 pCi/l. Peak values at the Zanjón station, located 16 km NW of the summit, reached 9,620 pCi/l on 8 September. The highest values at the San Antonio 2 station, 14 km W of the summit, occurred on 13 July and 1 September with recorded values of 15,119 pCi/l and 11,587 pCi/l, respectively.

Activity during January-March 2001. A VT earthquake swarm located near the active crater occurred during 15-17 January. The swarm was composed of 17 quakes with depths less than 3.5 and M < 1.3. A single event on 24 January and two more on 26 January (M 2.3-2.7, depths of 6-8 km) followed. Seismometers recorded three further events (M 2.5-2.7, depths of 8-9 km) on 20, 21, and 23 March. The majority of the January-March 2001 earthquakes occurred NE of the summit and were felt in the neighboring communities of Pasto and Puyito. During the first quarter of 2001, instruments detected 52 events located within the active cone area (figures 94 and 95).

Figure 94. Map view showing volcano-tectonic earthquakes registered at Galeras during January-March 2001. Courtesy of OVSP.

Figure 95. Cross-sectional view (N-S) showing earthquakes registered at Galeras during January-March 2001. Courtesy of OVSP.

The occurrence of four tornillo ("screw-type") events with dominant frequencies of 3.2, 8.7, 12.8, and 18.7 Hz suggested that flow of volcanic material within interior conduits continued at low levels. Tremor episodes of short duration were also recorded. Spectral analysis of the registered tremor showed dominant frequencies of 2.3-3.5 Hz.

Field workers at Galeras near the Chavas fumarole (W of the active crater) reported hearing a sound similar to the rushing current of a river, which correlated with increased rates of gas emission.

During 2000 the temperature of the Deformes fumarole (S of the active crater) measured an average of 111°C and showed a slight cooling over time. The fumarole temperature averaged 100°C during the first three months of 2001.

During 1 January-31 March 2001, radon-222 emission from soil measured up to 4,000 pCi/l at most stations. The San Juan 1 station (10 km NE of the active cone) and Sismo 5 station (7 km N of the active cone) detected higher values of 6,754 pCi/l and 5,455 pCi/l, res

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

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

Since the last report (BGVN 25:04), activity was variable at Mayon. The following report covers activity during April 2000-May 2001, but does not include the event that began on 24 June 2001; details of that eruption will appear in a subsequent issue. This report was compiled from reports posted on the Philippine Institute of Volcanology and Seismology (PHIVOLCS) website.

April-June 2000. Mayon's hazard status remained at 2 (on a scale of 0-5) as of 2 April. At that time, no entry was allowed within the 6-km-radius Permanent Danger Zone (PDZ) and the 7-km-radius Extended Danger Zone (EDZ) in the SE sector. Low-frequency (LF) and high-frequency (HF) earthquakes, and short-duration HF tremors, were recorded. Around this time, SO₂ flux increased from 3,600 metric tons/day (t/d) to 6,210 t/d. The summit crater emitted a weak to moderate steam plume which drifted WSW. Faint crater glow was observed during the evening. Similar activity continued through the end of April, although the SO₂ emission rate had decreased to 4,061 t/d as of 26 April.

Seismicity during 2-3 May included seven LF earthquakes with relative amplitudes of 55-56 mm, but there was no other variation in activity. On May 3 PHIVOLCS raised the Alert Level from 2 to 3. The next Mayon volcano bulletin, issued on 1 June, noted that SO₂ flux on 21 May was 680 t/d, slightly above the baseline of 500 t/d.

By 1 June the hazard status had been decreased to Alert Level 0. Seismicity had also decreased markedly; only two HF events and two short- duration HF tremors were reported on 1 June. Crater illumination resumed the same day. SO₂ flux readings were not available for the month.

July 2000. On 16 July at 0629 a phreatic explosion occurred that was visible only from the E due to thick clouds on the other sides. The explosion produced a small volume of gray ash as well as steam clouds that rose ~1 km above the summit before drifting NNE. Mayon Volcano Observatory at Ligñon Hill (MVO) seismographs recorded an explosion-type seismic signal that lasted for 1.5 minutes. Tiltmeters at Buang and Mayon Resthouse stations did not, however, detect significant ground movement, which suggested that the explosion was caused by shallow activity.

On 30 July at 1315, Mayon produced a mild ash ejection. MVO reported a small ash plume that rose 1 km. Seismicity associated with the event lasted for about 1 minute. As with the 16 July event, other monitoring, including SO₂ flux readings, did not indicate further activity. Mayon's Alert Level was undisclosed for the month.

August-December 2000. A mild ash ejection at 1432 on 31 August sent a small gray ash cloud ~1 km above the summit. An activity update on 1 September noted that small explosions similar to those in July had occurred in the previous weeks. PHIVOLCS suggested that these shallow explosions were probably due to rainwater seepage into the February-March 2000 lava deposits (BGVN 25:04). No further reports were issued in 2000.

January 2001. A resurgence of activity was observed as of [8] January. MVO reported an apparently growing lava dome which emitted voluminous gases from its summit. During the previous week there had been increases in both the number of earthquakes and in tilt, presumably due to magma ascent. [These] events led PHIVOLCS to set the Alert Level to 2.

On 10 January aerial observers noted that the dome appeared to have a spiny, blocky surface, which resulted from the crater floor being pushed upward by rising magma. Slight incandescence was also emanating from the crater. Correlation spectrometer (COSPEC) measurements detected an elevated SO₂ emission rate of 2,300 t/d. Seismicity also remained elevated. Ground deformation measured on the N flank continued to indicate tilting. Over the next week, activity remained high. Crater glow, however, was weak, and only visible from a distance with a telescope.

Activity escalated further after 19 January. Sixty seismic events occurred on 20 January, and a high number of earthquakes continued to occur. SO₂ flux spiked up to ~8,070 t/d. A brown steam puff rose from the lava dome at 0932 on 22 January. This brief emission of ash-laden steam coincided with a volcanic earthquake. A second ash emission occurred later the same day. Alert Level 3 became effective as of 25 January. Five ash emissions rose from Mayon's summit on 28 January followed by two more the next day. Plumes rose ~500 m and generally drifted WNW or NW. The earthquakes associated with these late January events were noticeably larger than those in previous weeks. Inflation of the edifice was also detected.

February-May 2001. The Alert Level remained at 3 for the entire period; high seismicity and moderate steaming prevailed. Inflationary trends were shown by tiltmeter readings through the end of March, when uplift tapered off slightly. On 24 February a small ash-and-steam plume rose 250 m and was blown ENE. SO₂ flux decreased through February with a reading of 2,889 t/d on the 28th. Crater glow was observed rarely during February, and not at all during March.

On 2 April the SO₂ flux rose to 7,205 t/d, but then dropped to 444 t/d two days later. SO₂ emission rates ranged from ~2,000 to 4,000 t/d during the rest of April. Low-intensity crater glow was observed sporadically during the month. On 7 May more intense crater glow was observed. A small ash emission occurred at 1752 on 11 May and sent material 50 m above the summit.

On 12 May a series of explosions were detected by a seismometer S of the summit. Ash ejection occurred, and late in the day the SE portion of the dome partially collapsed, causing a small lava avalanche that reached ~300 m down into Bonga Gully. Following the avalanche, MVO workers noted incandescence at the dome and continuing rockfalls into the gully. Workers speculated that active magma transport upward toward the crater was increasing.

Rockfalls due to molten lava fragments rolling down from the dome dominated activity during 13-14 May. When conditions cleared briefly on 14 May observers saw that the partial dome collapse had produced a V-shaped gash; this breach was the source of the outpouring lava. Avalanches had reached 500 m downslope as of this date.

Rockfalls and lava emissions ceased on 15 May but resumed the following day. Fresh lava began to refill the previously formed gash. SO₂ flux remained high, and tiltmeters detected consistent inflation through 31 May. Similar activity, accompanied by elevated seismicity that included rockfall-induced signals, continued through the month.

Geologic Background. Symmetrical Mayon, which rises above the Albay Gulf NW of Legazpi City, is the most active volcano of the Philippines. The steep upper slopes are capped by a small summit crater. Recorded eruptions since 1616 CE range from Strombolian to basaltic Plinian, with cyclical activity beginning with basaltic eruptions, followed by longer term andesitic lava flows. Eruptions occur predominately from the central conduit and have also produced lava flows that travel far down the flanks. Pyroclastic flows and mudflows have commonly swept down many of the approximately 40 ravines that radiate from the summit and have often damaged populated lowland areas. A violent eruption in 1814 killed more than 1,200 people and devastated several towns.

Information Contacts: Raymundo S. Punongbayan and Ernesto Corpuz, Philippine Institute of Volcanology and Seismology (PHIVOLCS), C.P. Garcia Avenue, U.P. Diliman, 1101 Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/).

On 8 May 1999 a group of natives were traveling around the E shore of Vai Si'i, the smaller of the two lakes that occupy the caldera in the center of the island. The water level in the lake was reported to be noticeably higher (about 0.5 m) than usual. At a locality on the E shore of the lake, below the caldera wall (figure 3) a new hot spring had formed. At the time of this observation it was below the level of the lake. Bubbles were being produced from the site and the water was noticeably warmer than usual.

Figure 3. Map showing the location of the new hot spring adjacent to the Vai Si'i crater lake in the caldera of Niuafo'ou that was reported in May 1999 and observed in June 1999. Courtesy of Paul Taylor.

This report of the new hot spring was communicated to Paul Taylor, a volcanic geologist who was conducting a workshop on the island during the first week of June 1999. When Taylor visited the lake on 1 June the water level had returned to its normal level, but the hot spring was clearly present in a small embankment on the side of the track that followed the edge of the lake. A small amount of steam and a quantity of hot water were still being produced by the spring at that time. The temperature of the water was estimated to be about 70-80°C. A small stream of the warm water was flowing across the track and into Vai Si'i. A strong smell of sulfur was present in the immediate area of the spring. A large deposit of dark, sulfur-rich mud was present along the shore within Vai Si'i near the new hot spring. Vegetation had withered noticeably and a large number of dead fish were present along the shoreline. The new hot spring represents the first reported activity in the NE part of the central caldera, and the first activity reported on the island in more than a decade.

Geologic Background. Niuafo'ou ("Tin Can Island") is a low 8-km-wide island that forms the summit of a largely submerged basaltic shield volcano in the north central Lau Basin about 170 km W of the northern end of the Tofua volcanic arc. The circular island encloses a 5-km-wide caldera that is mostly filled by a lake whose bottom extends to below sea level. The inner walls of the caldera drop sharply to the lake, named Big Lake (or Vai Lahi), which contains several small islands and pyroclastic cones on its NE shore. Eruptions recorded since 1814, mostly from circumferential fissures on the west-to-south side of the island, have often damaged villages. A major eruption in 1946 forced evacuation of most of its 1,200 inhabitants.

Information Contacts: Paul W. Taylor, Australian Volcanological Investigations, PO Box 291, Pymble, NSW 2073, Australia.

Beginning on 11 May 2001 volcanic activity increased above normal levels, with small eruptions producing gas-and-ash clouds that deposited small amounts of ash on a neighboring town. The previous report of anomalous volcanic activity at San Cristóbal was in May 2000 when a series of lahars occurred as a result of the remobilization of ash that had been deposited on the volcano from the 20 November eruption (BGVN 25:02 and 25:05).

On 22 July 2000, ten months prior to the May 2001 eruption, Alain Creusot visited the summit of the volcano. He reported that seismic activity during 18-19 July caused two lakes to empty that were observed during a previous trip. He also found that active fissures inside the crater were partially sealed, which caused the intensity of degassing to decrease.

INETER reported that on 11 May 2001 tremor began to rise at a seismic station on San Cristóbal (figure 9). The tremor reached a maximum level at noon and then slightly diminished, but stayed at relatively high levels for several days. Seismic activity during this period exceeded the maximum level of seismicity throughout the entire December 1999-February 2000 eruption (BGVN 25:02). Beginning on 11 May INETER personnel stationed near the summit of the volcano occasionally observed small plumes of volcanic gas with small amounts of ash emanating from the volcano. In contrast, on 10 May very low levels of gas were emitted from the crater. On 14 May observers noted that gas emissions with small amounts of ash continued. On 17 May the level of seismic activity significantly increased, and pulses of gas and ash rose ~100 m above the crater rim. Small amounts of ash fell in the town of Santa Barbara, 14 km SW of the volcano.

Figure 9. Seismic amplitude recorded at CRIN seismic station on San Cristóbal during 7-17 May 2001. Courtesy of INETER.

INETER noted that rain could mix with ash deposited on the flanks of the volcano and generate dangerous lahars. This occurred after the 1999-early 2000 eruption when rainfall in May 2000 mixed with ash that accumulated on the flanks of the volcano. The lahars were especially strong in the S part of the volcano.

According to news reports, on 21 June an explosion at San Cristóbal sent an ash cloud to a maximum height of 800 m. The cloud extended approximately 25 km downwind of the crater, and ash fell in the town of Chinandega, ~15 km SW of the volcano.

Geologic Background. The San Cristóbal volcanic complex, consisting of five principal volcanic edifices, forms the NW end of the Marrabios Range. The symmetrical 1745-m-high youngest cone, named San Cristóbal (also known as El Viejo), is Nicaragua's highest volcano and is capped by a 500 x 600 m wide crater. El Chonco, with several flank lava domes, is located 4 km W of San Cristóbal; it and the eroded Moyotepe volcano, 4 km NE of San Cristóbal, are of Pleistocene age. Volcán Casita, containing an elongated summit crater, lies immediately east of San Cristóbal and was the site of a catastrophic landslide and lahar in 1998. The Plio-Pleistocene La Pelona caldera is located at the eastern end of the complex. Historical eruptions from San Cristóbal, consisting of small-to-moderate explosive activity, have been reported since the 16th century. Some other 16th-century eruptions attributed to Casita volcano are uncertain and may pertain to other Marrabios Range volcanoes.

Information Contacts: Wilfried Strauch and Virginia Tenorio, Department of Geophysics, Instituto Nicaragüense de Estudios Territoriales (INETER), P.O. Box 1761, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); Alain Creusot, Instituto Nicaraguense de Energía, Managua, Nicaragua (URL: http://www.ine.gob.ni/); La Noticia (URL: http://www.lanoticia.com.ni/); El Nuevo Diario (URL: http://www.elnuevodiario.com.ni/); La Prensa (URL: http://www.laprensa.com.ni/).

An unusual cloud formation was spotted on 12 June satellite imagery from the Balleny Islands region by Petty Officer Eugenia Dowling, of the U.S. National Ice Center, while performing a weekly analysis of Ross Sea imagery. In addition to AVHRR (Advanced Very High Resolution Radiometer), the National Ice Center uses OLS (Optical Line Scan) Imagery from a Defense Meteorological Satellite (visible/IR, 0.55 km resolution). The cloud was seen in OLS imagery and brought to the attention of Paul Seymour, who then forwarded it for further evaluation to Ralph Meiggs, Applied Technology Branch Chief and part of the NOAA Operational Significant Event Imagery team. From there it came to the attention of the Washington Volcanic Ash Advisory Center (VAAC), who consulted with volcanologists and other international meteorologists familiar with identifying volcanic plumes from satellite data.

Preliminary interpretations based on satellite data were made by analysts in the United States (NOAA/Washington VAAC), Australia (Bureau of Meteorology/Darwin VAAC), and New Zealand (MetService NZ/Wellington VAAC). More detailed research and analysis was provided by Fred Prata of Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO), Atmospheric Research Division. Thoughtful comments were also provided by Steve Pendelbury and Neil Adams of the Australian Bureau of Meteorology.

The feature was first seen on AVHRR imagery from 1352 UTC on 12 June 2001. It appeared to be almost detached from the island on AVHRR images at 1631 and 1652, but was still visible emanating from the island on MODIS imagery at 2245.

Preliminary interpretations from Volcanic Ash Advisory Centers. Based on analysis of NOAA-14, -15, and -16 AVHRR imagery by the Washington VAAC, the plume reached a size of ~20 x 200 km and an altitude of ~1,300 m (later analysis, below, showed the plume to be much higher); temperatures were estimated to be about -53°C (220 K). Channel differencing revealed no ash content, which suggests that the cloud was mainly steam. A short video was created from satellite imagery showing the progression of the plume.

During a discussion with Andrew Tupper (Darwin VAAC), Lance Cowled, a senior meteorologist in the Tasmania/Antarctic office of the Australian Bureau of Meteorology, noted that at first sight it looked like a banner cloud shed by the island that developed with the onset of cirrus overcast thickening, but that it may have been caused by an interaction between the moisture field and any gas being emitted. The summit of Sturge Island has a lower elevation (1,167 m) than both nearby Young Island (1,340 m) and Buckle Island (1,239 m). With this in mind, Tupper stated that the chance of a banner cloud forming only on Sturge without some volcanic influence was less likely, but difficult to know without more topographical knowledge of the islands.

James Travers, Operations Manager for the Aviation Services Division of the MetService NZ and Wellington VAAC, stated that, based on his experience, the feature was more likely to be associated with volcanic activity rather than with an orographically induced cloud.

Analysis by Australian CSIRO Atmospheric Research. Fred Prata (CSIRO Atmospheric Research) obtained MODIS (Moderate Resolution Imaging Spectroradiometer), ATSR-2 (Along Track Scanning Radiometer), and AVHRR-2 LAC (Local Area Coverage) data for this mysterious plume seen on AVHRR GAC (Global Area Coverage) data. His analysis and interpretation follows. "My first impression was that it was volcanic in origin. However, the AVHRR LAC, MODIS and ATSR-2 data do not show an ash signature when processed using a technique that usually discriminates ash (figure 1). So, either there was no ash or it's not volcanic. The case for it being volcanic with no ash is sustainable as the MODIS 7.3 µm channel does give an indication of SO₂, but this signal is weaker than normal (figure 2). It is also possible that the ash is there but the signal is concealed by ice coating the ash. We have seen a few instances of this in the past. The plume could also be mostly steam (and then ice or liquid water drops once in the atmosphere). The case for it not being volcanic relies on the observation that there were winds streaming over these islands which spawned a cloud (looking like a banner cloud) in the lee of Sturge Island. You can easily convince yourself that this is possible when looking at the NOAA animation. I have examined MODIS 250-m data (at different times of year) and found that when Sturge forms these clouds the other islands also form clouds (Buckle and Young) and more often the clouds are lee waves rather than banner clouds.

Figure 1. Satellite image of the Sturge Island plume from AVHRR LAC data acquired on 12 June 2001 at 1652 UTC showing the extent of the plume. The temperature difference image of the 11 µm channel - 12 µm channel (T4-T5) is usually negative for 'ash' plumes. This positive difference suggests that there is no ash content, or an undetectable amount. These data are at the edge of the satellite reception capability, resulting in many missing or bad lines. Courtesy of F. Prata, CSIRO.

Figure 2. Satellite image of the Sturge Island plume showing MODIS 1-km data acquired on 12 June 2001 at 2245 UTC. This image of temperature difference between the 6.7 and 7.3 µm channels is an SO2 sensitive combination, giving some indication of SO2, but the interpretation is not clear in this case. Young and Buckle islands, to the NW, exhibit no plume. Courtesy of F. Prata, CSIRO.

"Looking at AVHRR temperatures I find that the thickest part of the plume (near the island) is at around 213 K (12 µm) and the surrounding scene temperatures are 250 K or higher. This puts the cloud top at around 6 km assuming a lapse rate of 6.5 K per km and the cloud is opaque (which it isn't quite). The cloud also extends a long way downwind (I calculate that it is visible for 300 km from Sturge) and there is no such cloud coming off Young or Buckle. Finally, looking at the AVHRR LAC it is apparent that there are regions in the plume that are more opaque - as if there were discrete pulses, possibly from several eruptions (figure 3). So my conclusion is that it is more likely to be an eruption cloud than a banner cloud, but there is a degree of doubt."

Figure 3. Satellite image of the Sturge Island plume showing AVHRR LAC data acquired on 12 June 2001 at 1352 UTC. The image is an 11 µm brightness temperature (K) image with black as cold and white as warm, annotated to show the possible "puffs" or pulses of volcanic activity. Courtesy of F. Prata, CSIRO.

Further comments by Australian Bureau of Meteorology. Steve Pendelbury, a Supervisory Meteorologist in the Bureau of Meteorology and his colleague Neil Adams (Senior Meteorologist) identified the plume as a banner cloud, and noted that the "pulses" seen in AVHRR imagery seemed like lee wave activity. The plume was similar to one recorded on AVHRR imagery over Heard Island where orographic banner was suspected. Orographic influence is also suggested because the upwind part of the plume mirrors the breadth of the island. A reason for the plume only being off this island is the differences in island height and perhaps variations in the static stability with height. They noted that the estimated height of the plume top (6 km by Fred Prata's estimation) would mean that ejected volcanic material, albeit even steam, would have had to rise approximately 5 km; this might be difficult in the intrinsically stable atmosphere of high southern latitude waters, but orographic clouds can form that high via vertically propagating waves. Another possibility, assuming that the moisture could have risen to 6 km, is that volcanic venting provided moisture needed to produce a cloud in otherwise invisible lee waves that may be present downwind of all three islands. They agreed that the data are inconclusive.

AVHRR band 4 mosaics from the Casey HRPT ground station, reduced to 4 km resolution, showed a good banner cloud along with a wake cloud evident off Young Island, the northern island in the Balleny Island chain, at 0830 UTC on 5 July image. Another image at 2130 UTC still has evidence of a wake cloud but the banner cloud is no longer visible.

Seismicity. No earthquakes recorded within 100 km of the Balleny Islands during 6-20 June 2001 were present in the USGS National Earthquake Information Center's database as of 20 June.

Summary of interpretations. Basic observations about this cloud/plume are as follows: It is unlikely that this plume contained ash, but there may have been some SO₂ content. This plume clearly originated above Sturge Island, but not above the two other Balleny Islands with higher elevations. The cloud was not consistent throughout the period it was observed, exhibiting variable opacity. Explanations can be constructed to explain all of these features that are based on orographic influences, volcanic emissions, or some combination of the two. Local static stability might have assisted cloud formation above this lower-elevation island, but not above the nearby higher islands. Water vapor provided by volcanic emissions may also have resulted in cloud formation, either directly or orographically. Likewise, the variable opacity of the cloud could be caused by pulses of emissions or orographic lee waves. Without independent evidence of volcanism, the satellite imagery is not conclusive.

Background. A 160-km-long chain of volcanic islands forms the Balleny Islands just off the coast of Antarctica's Victoria Land. The islands are located at the southern end of a submarine ridge system that extends north to New Zealand, but is offset by the Indian-Antarctic ridge system. No detailed geologic studies have been conducted in the inaccessible Balleny Islands.

Sturge is the largest and southernmost of the Balleny Islands. The 44-km-long island is completely mantled by an icecap and has a prominent summit, Russel Peak, at the northern end. "Volcanic activity" was reported on a U.S. Navy chart, but no indications of present or past activity were noted in 1959 (Catalog of Active Volcanoes of the World).

Buckle Island is in the center of the Balleny Islands. The elongated, 21-km-long island is capped by a gently sloping icecap that descends steeply to the sea between rocky cliffs. Dark eruption columns were reported during 1839 and 1899.

Young Island is the northernmost and second largest of the Balleny Islands. Captain Balleny, the discoverer of the islands, reported "smoke" issuing from Freeman Peak on Young Island on 12 February 1839. The island has a broad plateau-like summit reaching 1,340 m and is almost completely mantled by ice.

Geologic Background. Sturge is the largest and southernmost of the Balleny Islands, which are located just off the coast of Antarctica's Victoria Land. The 44-km-long island is completely mantled by an icecap and has a prominent summit, Russel Peak, at the northern end. "Volcanic activity" was reported on a U.S. Navy chart, but no indications of present or past activity were noted in 1959 (Catalog of Active Volcanoes of the World). No detailed geologic studies have been conducted in the inaccessible Balleny Islands.

Information Contacts: Grace Swanson, Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch, NOAA/NESDIS/E/SP23, NOAA Science Center Room 401, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/); Fred Prata, Senior Principal Research Scientist, CSIRO Atmospheric Research, PB 1 Aspendale, Victoria 3195, Australia (URL: https://www.cmar.csiro.au/); Steve Pendelbury and Lance Cowled, Weather Services, Bureau of Meteorology, GPO Box 727G, Hobart, Tasmania 7001, Australia; Neil Adams, Antarctic Co-operative Research Centre and Bureau of Meteorology, PO Box 421, Kent Town, SA 5071, Australia; Andrew Tupper, Darwin VAAC, Northern Territory Regional Office, Bureau of Meteorology, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); National Ice Center, Federal Building 4, 4251 Suitland Road, Washington, DC 20395 USA (URL: http://www.natice.noaa.gov/); National Earthquake Information Center (NEIC), US Geological Survey, Mail Stop 967, Federal Center Box 25046, Denver, CO 80225, USA (URL: http://earthquakes.usgs.gov/).

United States Geological Survey (USGS) scientists detected a slight uplift of the ground surface over a broad region centered 5 km W of South Sister volcano in the Three Sisters region (figure 1). The area is located within the central Oregon Cascade range, 35 km W of Bend, and 100 km E of Eugene, Oregon. The measured uplift, which occurred during 1996-2000, covered an area ~15-20 km in diameter; the maximum amount of uplift at the region's center was ~10 cm. Several close aerial inspections of the area revealed no unusual surface features.

Figure 1. Radar interferogram showing ground uplift pattern centered ~ 5 km W of South Sister. Each shaded region represents ~ 2.8 cm of ground movement in the direction of the satellite. In this case, four concentric shaded bands show that the surface moved toward the satellite (close to vertical) by as much as 10 cm between August 1996 and October 2000. Data gaps occur where forest vegetation or other factors hinder the acquisition of useful radar data. A numerical model places the source of the uplift ~ 7 km beneath the ground surface. After a color version by Wicks and others (2001), which uses radar images from the European Space Agency's ERS satellites.

The uplift was detected by using satellite radar interferometry (InSAR), which uses satellite data to make radar images of the ground surface (figure 1). InSAR can detect even minor (down to a few centimeters) changes in ground elevation over time. Images from 1996 and 2000 were compared and revealed the rise in ground level. The exact timing of uplift between the two dates, or whether it will continue, is unknown, but is being studied further.

The specific cause of the uplift was also uncertain. Uplift in the Three Sisters region may reflect intrusion of a relatively small volume of magma at a possible depth of 7 km. If this is the result of intrusion, it indicates that the region remains active, but does not suggest eruptive activity without additional precursors. In the Three Sisters area, earthquake activity appeared to be at or near background levels and gas emissions were low as of May 2001. The USGS plans to enhance the existing monitoring network in the region to more accurately detect possible precursors and to better understand the uplift phenomenon. Installation of one or more additional seismometers, a global positioning system (GPS) receiver, a resurvey of existing benchmarks and installation of new ones, and periodic airborne and ground-based sampling of gases are all being considered.

References. Wicks, C., Jr., Dzurisin, D., Ingebritsen, S.E., Thatcher, W., and Lu, Z., 2001, Ground uplift near the Three Sisters volcanic center, central Oregon Cascade Range, detected by satellite radar interferometry: in prep.

Geologic Background. The north-south-trending Three Sisters volcano group dominates the landscape of the Central Oregon Cascades. All Three Sisters stratovolcanoes ceased activity during the late Pleistocene, but basaltic-to-rhyolitic flank vents erupted during the Holocene, producing both blocky lava flows north of North Sister and rhyolitic lava domes and flows south of South Sister volcano. Glaciers have deeply eroded the Pleistocene andesitic-dacitic North Sister stratovolcano, exposing the volcano's central plug. Construction of the main edifice ceased at about 55,000 yrs ago, but north-flank vents produced blocky lava flows in the McKenzie Pass area as recently as about 1600 years ago. Middle Sister volcano is located only 2 km to the SW and was active largely contemporaneously with South Sister until about 14,000 years ago. South Sister is the highest of the Three Sisters. It was constructed beginning about 50,000 years ago and was capped by a symmetrical summit cinder cone formed about 22,000 years ago. The late Pleistocene or early Holocene Cayuse Crater on the SW flank of Broken Top volcano and other flank vents such as Le Conte Crater on the SW flank of South Sister mark mafic vents that have erupted at considerable distances from South Sister itself, and a chain of dike-fed rhyolitic lava domes and flows at Rock Mesa and Devils Chain south of South Sister erupted about 2000 years ago.

Information Contacts: Cascades Volcano Observatory (CVO), U.S. Geological Survey (USGS), 5400 MacArthur Blvd., Vancouver, WA 98661 USA (URL: https://volcanoes.usgs.gov/observatories/cvo/); Volcano Hazards Team, USGS, 345 Middlefield Road, Menlo Park, CA 94025-3591 USA (URL: http://volcanoes.usgs.gov/); Pacific Northwest Seismograph Network, University of Washington Geophysics Program, Box 351650, Seattle, WA 98195-1650 USA (URL: http://www.geophys.washington.edu/SEIS/PNSN/); Oregon Department of Geology and Mineral Industries, 800 NE Oregon St., Suite 965, Portland, OR 97232 USA (URL: http://www.oregongeology.org/sub/default.htm).

On 30 April 2001 a moderate-sized ash cloud from an eruption at Ulawun was visible on Geostationary Meteorological Satellite (GMS), U.S. National Oceanic and Atmospheric Administration (NOAA) weather satellite, and Total Ozone Mapping Spectrometer (TOMS) imagery. There had been no reports of anomalous volcanic activity at Ulawun since the 28 September-2 October 2000 eruption sent an ash cloud 12-15 km above the volcano (BGVN 25:11).

The Darwin VAAC received a pilot report that a "smoke" cloud had been emitted from Ulawun on 30 April at 0730. The Rabaul Volcano Observatory (RVO) confirmed the report. The cloud reached an altitude of ~9 km and drifted NW and SW, expanding to 80-113 km in radius. GMS and NOAA weather satellite imagery indicated that the cloud may have reached a maximum height of ~13.7 km and that the eruption ceased by approximately 1530. By 3 May volcanic activity had decreased, but, because further ash emissions could occur, RVO placed the volcano at Stage 2 Alert. RVO reported that limited evacuations occurred. Ash was not observed on satellite imagery after the 30 April eruption, although ash clouds may have been obscured by meteorological clouds near the volcano.

On 30 April around noon, a few hours after reports of an eruption at Ulawun, the Earth Probe TOMS detected a SO₂ cloud over SW New Britain,. A gap between successive TOMS swaths over the volcano unfortunately precluded measurement of the full extent of this cloud. Elevated levels of SO₂ were recorded in a region bounded approximately by longitudes 147°E and 150°E (swath edge) and by latitudes 5°S and 7°S, at a maximum distance of ~400 km WSW from Ulawun. The highest SO₂ concentrations (38 milli atm cm) were recorded in a NNW-SSE trending region ~300 km WSW of the volcano. Preliminary analysis indicates that the portion of the cloud visible in TOMS imagery contained ~5 kilotons of SO₂.

Geologic Background. The symmetrical basaltic-to-andesitic Ulawun stratovolcano is the highest volcano of the Bismarck arc, and one of Papua New Guinea's most frequently active. The volcano, also known as the Father, rises above the N coast of the island of New Britain across a low saddle NE of Bamus volcano, the South Son. The upper 1,000 m is unvegetated. A prominent E-W escarpment on the south may be the result of large-scale slumping. Satellitic cones occupy the NW and E flanks. A steep-walled valley cuts the NW side, and a flank lava-flow complex lies to the south of this valley. Historical eruptions date back to the beginning of the 18th century. Twentieth-century eruptions were mildly explosive until 1967, but after 1970 several larger eruptions produced lava flows and basaltic pyroclastic flows, greatly modifying the summit crater.

Information Contacts: Darwin VAAC, Regional Director, Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, Northern Territory 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Simon Carn, Joint Center for Earth System Technology (NASA/UMBC), University of Maryland Baltimore County, 1000 Hilltop Circle Baltimore, MD 21250.