The andesitic Volcán El Reventador lies almost 100 km E of the main axis of active volcanoes in Ecuador and has historical eruptions with numerous lava flows and explosive events going back to the 16th century. An eruption in November 2002 generated a 17-km-high eruption cloud, pyroclastic flows that traveled 8 km, and multiple lava flows. Eruptive activity has been continuous since 2008. Daily explosions with ash emissions and ejecta of incandescent blocks rolling hundreds of meters down the flanks have been typical for many years. Similar activity continued during August 2020-January 2021, the period covered in this report, with information provided by Ecuador's Instituto Geofisico (IG-EPN), the Washington Volcano Ash Advisory Center (VAAC), and infrared satellite data.

Near-daily emissions of gas and ash often rose 500-1,000 m above the summit and drifted mostly in a westerly direction throughout August 2020-January 2021. Incandescence at night was produced by explosions of ejecta that sent blocks rolling hundreds of meters down the flanks of the pyroclastic cone inside the summit caldera. IG-EPN reported the presence of a new dome inside the crater in early August. A small lava flow about 400 m long persisted on the NE flank through at least the end of 2020; another flow was observed on the N flank in January. Small pyroclastic flows were reported a few times, and ashfall occurred in the San Rafael region (10 km SSE) at the end of October. After a relatively quiet June 2020, thermal activity increased to moderate levels and remained there throughout the period (figure 132).

Figure 132. Thermal activity at Reventador was consistent at moderate to high levels from late June 2020 through January 2021, according to this MIROVA project graph of log radiative power at the volcano. Courtesy of MIROVA.

Gas and ash emissions rose 500-1,000 m above the summit almost every day during August 2020 (figure 133). Incandescence and explosions at the summit crater, visible at night, were accompanied many nights by incandescent blocks that rolled 500-700 m down various flanks. The Washington VAAC issued 1-4 alerts most days, reporting ash observed in satellite data that rose 700-1,400 m above the summit. Drift directions were generally NW, W, or SW. IG reported a pyroclastic flow on the NE flank on 4 August, and a new 200-m-long lava flow near the summit on the NE flank was seen on 10 August (figure 134). By 19 August the lava flow had reached 350 m long; it remained active for the rest of the month but didn’t increase in length. Based on the analysis of webcam photographs and infrared images, they confirmed the growth of a new dome on 17 August (figure 135). MODVOLC thermal alerts were recorded on 3 and 11 August.

Figure 133. Gas and ash rose 500-1,000 m above the summit of Reventador most days during August 2020, as seen here on 17 August. Courtesy of IG-EPN (INFORME DIARIO DEL ESTADO DEL VOLCÁN REVENTADOR No. 2020-231, LUNES, 17 AGOSTO 2020).

Figure 134. IG-EPN reported a new lava flow on the NE flank of Reventador on 10 August 2020. It was about 400 m long and persisted through the end of 2020. Courtesy of IG-EPN (INFORME DIARIO DEL ESTADO DEL VOLCÁN REVENTADOR No. 2020-224, LUNES, 10 AGOSTO 2020).

Figure 135. Infrared images show volcanic activity at Reventador during August 2020, including a pyroclastic flow on 4 August (top right), a lava flow on 6 August (middle left), and a lava dome on 17 August (middle right and bottom row). Courtesy of IG-EPN (Prepared by Cámar IR, S Vallejo; Informe Especial del Volcán El Reventador No. 2-2020).

Incandescence from summit explosions was visible most nights in September 2020; explosions sent glowing blocks 500-800 m down multiple flanks on many nights. The lava flow on the NE flank remained active, growing slightly from 350 to 400 m in length. Three or four VAAC alerts were issued each day for ash plumes that rose usually 700-1,400 m above the summit and drifted NW. IG webcams captured images of ash emissions rising 600-900 m above the summit on most days; a few exceeded 1,000 m in height. IG reported pyroclastic flows on the N flank on 3 and 4 September, and on the W flank on 6 September. Pyroclastic deposits were observed on the E flank of the cone on 26 September, and the webcams captured a pyroclastic flow in the early morning of 29 September along the WSW flank that reached 600 m from the summit (figure 136). All of the pyroclastic flows remained inside the summit caldera. MODVOLC thermal alerts were recorded on 11, 12, and 20 September.

Figure 136. A pyroclastic flow was visible on the WSW flank of Reventador on 29 September 2020 along with an ash plume that rose hundreds of meters above the summit. Courtesy of IG-EPN (IGAlInstante Informativo VOLCÁN REVENTADOR No. 005, MARTES, 29 SEPTIEMBRE 2020).

The 400- to 450-m-long lava flow that first emerged on the NE flank in early August remained active, as seen in thermal imagery, throughout October 2020 (figure 137). Emissions of gas and ash continued rising daily 500-1,000 m above the summit and drifting in multiple different directions. Multiple VAAC reports were issued on most days; the plumes increased in height and frequency during the second half of the month, reaching 1,400 m above the summit. Incandescent blocks rolled 500-800 m down the flanks on most nights. MODVOLC thermal alerts were issued on five days during the month, on 2, 11, 14, 25, and 27 October; five alerts were issued on 25 October. Occasional pyroclastic flows were recorded on the N flank on 21 October. Fine-grained ashfall was reported in the San Rafael region (on the border between the Napo and Sucumbios provinces, 10 km ESE) on 28 and 30 October (figure 138).

Figure 137. The lava flow on the NE flank of Reventador was about 450 m long and active throughout October 2020. In this 6 October 2020 infrared image incandescent ejecta rose from the summit and the lava flow was visible on the NE flank. Courtesy of IG-EPN (INFORME DIARIO DEL ESTADO DEL VOLCÁN REVENTADOR No. 2020-281, MARTES, 6 OCTUBRE 2020).

Figure 138. IGEPN official S. Vallejo reported ashfall on a vehicle in the San Rafael region on the border between the Napo and Sucumbios provinces, 10 km ESE of Reventador on 28 and 30 October. US penny for scale. Photo by S. Vallejo, courtesy of IG-EPN (IGAlInstante Informativo VOLCÁN REVENTADOR No. 007, VIERNES, 30 OCTUBRE 2020).

Steam, gas, and ash emissions continued throughout November 2020, with many plumes rising 800-1,000 m above the summit and drifting NW (figure 139). Multiple daily VAAC reports indicated plumes visible in satellite imagery 1,000-1,400 m above the summit on most days. The lava flow remained active on the NE flank with thermal imagery indicating a strong heat signal 400-450 m from the summit. The explosions that produced the incandescent blocks were strongest during 5-7 November when the blocks rolled as far as 1,000 m from the summit. Cloudy weather and rain obscured views of activity at the end of the month, and a lahar was measured by seismic instruments on 27 November, but no damage was reported. MODVOLC alerts were issued on 3, 10, 26, and 30 November. Cloudy weather during the first week of December prevented many observations, but clearer skies later in the month indicated ongoing activity that included gas and ash emissions rising about 1,000 m and drifting NW; incandescent blocks rolled 500 m down the flanks following explosions inside the crater. Only a single MODVOLC alert was issued on 25 December. The 450-m-long lava flow on the NE flank remained active.

Figure 139. Many ash plumes at Reventador rose 800-1,000 m above the summit during November 2020. They were visible on some days when the mountain was not; clear days revealed blocks rolling down the NE flank and raising ash clouds as they rolled (bottom left). Courtesy of IG-EPN (INFORME DIARIO DEL ESTADO DEL VOLCÁN REVENTADOR Nos. 2020-312, 2020-315, 2020-323, and 2020-327).

A new pulse of lava was first reported from a vent on the N flank on 10 January 2021 and remained active for the rest of the month. That same day incandescent blocks traveled 700 m down the NE flank. Pyroclastic flows were observed on the night of 14 January on the N flank. Satellite imagery on 16 January showed multiple areas of thermal activity at the summit and on the NNE flank (figure 140). On 21 January the ejecta from the explosions rose a hundred meters or more into the air over the pyroclastic cone in addition to traveling several hundred meters down the NE flank (figure 141). MODVOLC thermal alerts were issued on 4, 13, and 31 January.

Figure 140. Sentinel-2 satellite imagery of Reventador on 16 January 2021 indicated strong thermal anomalies at the summit and on the NE flank, even through the frequently dense cloud cover. Courtesy of Sentinel Hub Playground.

Figure 141. On 21 January 2021 the ejecta from explosions at Reventador could be seen rising a hundred meters or more over the pyroclastic cone in addition to traveling several hundred meters down the NE flank. Courtesy of IG-EPN (INFORME DIARIO DEL VOLCAN REVENTADOR No. 2021-021, Quito, jueves 21 de enero de 2021).

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/); 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 Popocatépetl is an active stratovolcano near Mexico City that has had frequent historical eruptions dating back to the 14th century. The current eruption has been ongoing since January 2005 and has more recently consisted of lava dome growth and destruction, frequent explosions, and emissions of ash plumes and incandescent ejecta. Activity through July 2020 was characterized by hundreds of daily low-intensity emissions that included gas-and-steam and small amounts of ash, and multiple daily minor and moderate explosions that sent ash plumes more than 1 km above the crater (BGVN 45:08). This report covers somewhat decreased activity from August 2020 through January 2021 using information from México's Centro Nacional de Prevención de Desastres (CENAPRED), the Washington Volcanic Ash Advisory Center (VAAC), and various satellite data.

Popocatépetl had ongoing water vapor, gas, and ash emissions throughout August 2020-January 2021, but far fewer minor and moderate explosions than during the period of the previous report. Ash emissions generally rose to 5.8-7.1 km altitude and drifted in many different directions. Ashfall was reported in multiple communities during August, October, and numerous times in January 2021. Thermal anomalies were recorded in satellite images inside the summit crater a few times each month. The MIROVA thermal anomaly data indicated persistent, low levels of activity throughout the reporting period (figure 162). CENAPRED reported the number of low-intensity emissions or ‘exhalations’ and the number of minutes of tremor in their daily reports (figure 163). Tremor activity was very high at the beginning of August, and then again during January 2021. The daily number of exhalations was highest during late October and November 2020.

Figure 162. MIROVA thermal anomaly data for Popocatépetl for the year ending on 3 February 2021 showed persistent low levels of activity from August 2020 through January 2021, the period covered in this report. Courtesy of MIROVA.

Figure 163. CENAPRED reported the number of exhalations (low-intensity emissions) and the number of minutes of tremor at Popocatépetl in their daily reports. Tremor activity was very high at the beginning of August, and then again during January 2021 (yellow columns). The daily number of exhalations was highest during late October and November 2020 (blue columns). Data courtesy of CENAPRED daily monitoring reports.

During August 2020 daily water vapor and gas emissions often contained small quantities of ash. In addition, low-intensity emissions or exhalations with larger quantities of ash occurred tens of times per day. The daily number of minutes of tremor was over 1,000 at the beginning of the month but dropped back to lower levels of a few tens or hundreds of minutes later in the month. Slight amounts of ashfall were reported in Amecameca and Ozumba in the State of Mexico on 1 August. On 2 August the 1159 minutes of tremor were sometimes accompanied by incandescent ejecta that fell into and a short distance from the summit crater. The Washington VAAC observed an ash emission drifting NE at 6.1 km altitude on 2 August that later rose to 7.6 km altitude. It fanned out from the summit to the N and E for about 15 km. Similar observations were made virtually every day of the month; ash or gas-and-ash emissions generally rose to 5.8-7.6 km altitude and drifted a few tens of kilometers in different directions before dissipating. Constant gas emissions and incandescence were reported at night during 10-23 August; an ash emission that rose to 600 m above the crater rim and drifted W on 14 August was captured in the webcam (figure 164). The largest SO₂ emissions during the period were captured by the TROPOMI instrument on the Sentinel-5P satellite during 2-5 August (figure 165).

Figure 164. An ash emission at Popocatépetl rose to 600 m above the crater rim and drifted W on 14 August 2020. Dense steam emissions also drifted just above the summit. Courtesy of CENAPRED (Reporte del monitoreo de CENAPRED al volcán Popocatépetl hoy 14 de Agosto).

Figure 165. The largest SO2 emissions at Popocatépetl during the period were captured by the TROPOMI instrument on the Sentinel-5P satellite during 2-5 August 2020. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Gas and occasional weak ash emissions accompanied the tens of daily low-intensity emissions during September 2020; thermal activity was very low with weak anomalies inside the summit present in satellite images on 3, 8, and 13 September. Ash emissions were visible from a webcam on 18 September and in satellite imagery on 23 September (figure 166). Weak incandescence above the crater was only reported by CENAPRED during 26 and 27 September. The Washington VAAC reported intermittent ash emissions throughout the month that commonly rose to 6-7 km altitude and drifted over 50 km downwind before dissipating.

Figure 166. Ash emissions were visible from a webcam at Popocatépetl on 18 September (left) and in satellite imagery on 23 September 2020 (right). Right image is from Sentinel-2 with natural color rendering (bands 4, 3, 2). Left image courtesy of CENAPRED (Reporte del monitoreo de CENAPRED al volcán Popocatépetl hoy 18 de septiembre). Right image courtesy of Sentinel Hub Playground.

Water-vapor and gas emissions with small quantities of ash similar to those seen in September were also typical activity during October 2020. Tens or a few hundred daily low-intensity emissions often produced ash plumes visible in the webcams (figure 167). Ashfall was reported in Tetela del Volcano (20 km SW), in the state of Morelos, and in Amecameca (20 km NW), Atlautla (17 km W), Ayapango (22 km NW) and Ecatzingo (15 km SW), in the State of Mexico on 7 October; a small amount of ashfall was also reported in Amecameca on 13 October. The Washington VAAC issued multiple daily ash advisories throughout the month; many ash plumes were visible in satellite imagery. Incandescence appeared over the summit crater at night during 10-16 October, and was noted in satellite imagery on 3, 8, 18, 23, and 28 October. Incandescence and ash emissions were both captured in satellite imagery on 8 and 18 October (figure 168). Personnel from the Institute of Geophysics of the National Autonomous University of Mexico (UNAM) and the National Center for Disaster Prevention (CENAPRED) conducted an overflight on 16 October and verified that the inner crater at the summit was covered in tephra and about 360-390 m in diameter and 120-170 m deep (figure 169).

Figure 167. Ash plumes and steam rose hundreds of meters above Popocatépetl on 5 (left) and 10 (right) October 2020. Courtesy of CENAPRED (Reporte del monitoreo de CENAPRED al volcán Popocatépetl hoy 5 de octubre y 10 de octubre de 2020).

Figure 168. Thermal anomalies at the summit of Popocatépetl and ash plumes drifting SW were both present in satellite imagery on 8 (left) and 18 (right) October 2020. Images are using Atmospheric penetration rendering (bands 12, 11, 8A). Courtesy of Sentinel Hub Playground.

Figure 169. Personnel from the Institute of Geophysics of the National Autonomous University of Mexico (UNAM) and the National Center for Disaster Prevention (CENAPRED) conducted an overflight of Popocatépetl on 16 October 2020 and verified that the inner crater at the summit was covered in tephra, about 360-390 m in diameter, and 120-170 m deep. Courtesy of CENAPRED (Sobrevuelo al volcán Popocatépetl, 16 de octubre de 2020).

Activity during November 2020 consisted primarily of weak emissions of steam and gas with occasional small quantities of ash that rose a short distance above the summit crater (figure 170). The Washington VAAC reported ash emissions on 19 days during the month, most rising to 5.8-6.7 km altitude and drifting for a few tens of kilometers before dissipating. CENAPRED reported a few hundred low-intensity emissions daily, but only a few tens of minutes of tremor each day, significantly lower than previous months. Satellite imagery showed weak thermal anomalies inside the summit crater on 2, 7, 12, 22, and 27 November.

Figure 170. Activity during November 2020 at Popocatépetl consisted primarily of weak emissions of steam and gas with occasional small quantities of ash that rose a short distance above the summit crater such as this one on 2 November. Courtesy of CENAPRED (Reporte del monitoreo de CENAPRED al volcán Popocatépetl hoy 02 de noviembre).

Emissions of steam and gas with occasional low quantities of ash continued during December 2020. Six explosions on 5 December produced small ash plumes that rose 500-1,000 m above the crater. The next day two explosions produced plumes that rose less than 1,500 m above the crater and drifted NE. Incandescent ejecta was captured in the webcam on 14 December (figure 171). The Washington VAAC issued multiple aviation alerts nearly every day of the month; ash plumes generally rose to 6-7 km altitude and drifted 30-50 km before dissipating. Activity increased during the second half of the month (figure 172). Visible ejecta was seen in webcams during low-energy emissions on 24 December, accompanied by an ash plume that rose 1,000 m above the crater. The next day an ash emission rose 300 m. Ejecta was noted on the SE flank after an explosion on 27 December, and ash plumes rose to 500-1,400 m above the crater each day through the end of December and into January 2021. Thermal anomalies appeared in satellite data inside the summit crater on 2, 17, 22, and 27 December.

Figure 171. Explosions at Popocatépetl produced dense ash emissions and incandescent ejecta. On 6 December the ash plume rose to 1,500 m above the crater and drifted NE (left). On 14 December 2020 incandescent ejecta rose a few hundred meters above the summit crater (right). Courtesy of CENAPRED (Reporte del monitoreo de CENAPRED al volcán Popocatépetl, 7 de diciembre y 15 de Diciembre de 2020).

Figure 172. Ash emissions occurred daily at Popocatépetl during December 2020. On 20 December the dense plume rose about one kilometer above the summit (left). On 31 December a thermal inversion was the likely reason that the ash from the summit flowed down the flank towards the webcam (right). Courtesy of CENAPRED (Reporte del monitoreo de CENAPRED al volcán Popocatépetl, 20 de diciembre y 31 de Diciembre de 2020).

Daily ash emissions were reported by the Washington VAAC during January 2021, rising to 5.8-7.0 km altitude and drifting tens or hundreds of kilometers before dissipating (figure 173). Ash plumes rose 500-600 m above the crater on 1 and 2 January; at least one explosion each of those days produced incandescent ejecta in and around the crater. The Washington VAAC reported the ash plume from 1 January as visible in the webcam and satellite imagery over 200 km NE from the summit before dissipating, and one on 6 January visible about 100 km E of the volcano (figure 174). Ashfall was reported each day during 4-6 January in Puebla to the NW. On 8 January ashfall occurred in Atlixco (23 km SE), San Andrés Cholula (35 km E), San Nicolás de los Ranchos (15 km ENE) and Domingo Arenas (22 km NE), all in the state of Puebla. The following day ashfall was reported in San Salvador el Verde (30 km NNE) and San Nicolás de los Ranchos. Multiple explosions with ash plumes rising 500-700 m were reported on 14 and 15 January followed the next day by ashfall in San Nicolás de los Ranchos. Trace amounts of ash were reported in Tetela del Volcán (18 km SW) in the State of Morelos on 22 January. An explosion on 26 January ejected ash 700 m high and sent incandescent fragments a short distance from the crater rim. Ashfall on 28 January was reported in Ixtlacuixtla de Mariano, Nativitas and part of the center of Tlaxcala (50 km NE). The circular inner crater rim at the summit was sharply defined in a satellite image taken on 31 January 2021; a thermal anomaly was also present inside the crater (figure 175).

Figure 173. Ash plumes were reported daily at Popocatépetl during January 2021, including on 19 (left) and 21 (right) January, some rising over a kilometer above summit and drifting for tens of kilometers before dissipating. Courtesy of CENAPRED (Reporte del monitoreo de CENAPRED al volcán Popocatépetl, 20 y 21 de Enero de 2021).

Figure 174. The Washington VAAC reported an ash plume at Popocatépetl from 1 January 2020 as visible over 200 km NE from the summit before dissipating (left), and one on 6 January as visible about 100 km E of the volcano (right). Sentinel-2 satellite images are with Natural color (bands 4, 3, 2) and Atmospheric penetration (bands 12, 11, 8a) rendering. Courtesy of Sentinel Hub Playground.

Figure 175. A thermal anomaly inside the summit crater of Popocatépetl seen in this Sentinel-2 image was surrounded by a distinct gray circle that was the rim of the inner crater on a clear 31 January 2021. Image uses Atmospheric penetration rendering (bands 12, 11, 8A). Courtesy of Sentinel Hub Playground.

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

Extensive lava flows, bomb-laden Strombolian explosions, and ash plumes emerging from Mackenney crater have characterized the persistent activity at Pacaya since 1961. The latest eruptive episode began with intermittent ash plumes and incandescence in June 2015; the growth of a new pyroclastic cone inside the summit crater was confirmed later that year. The pyroclastic cone has continued to grow, producing Strombolian explosions rising above the crater rim and frequent loud explosions. In addition, fissures on the flanks of the summit crater have produced an increasing number of lava flows traveling distances of over one kilometer down multiple flanks during 2019 and 2020 (figure 129). Increasing explosive and effusive activity during August-November 2020 is covered in this report with information provided by Guatemala's Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), multiple sources of satellite data, and numerous photographs from observers on the ground.

Figure 129. Lava flows traveled down the flank of Pacaya during July 2019 while ash emissions and incandescent ejecta marked the summit of Fuego located 30 km NW. The large edifice on the right is Agua, and the one between it and Fuego is Acatenango, which last erupted in the early 20th century. Photo courtesy David Rojas, used with permission.

After a brief pause in effusive activity at the end of July 2020, two lava flows appeared on the NW flank on 12 August. Another flow began on the NE flank ten days later, and multiple flows were active for the remainder of the month, some reaching 650 m long. Multiple lava flows issued from fissures on the N flank and elsewhere throughout September. A flow on the NE flank was reported as 1,200 m long and was visible from Guatemala City on 8 September. A new flow on the S flank was very active later in the month. Flows were persistent on most of the flanks throughout October; a flow appeared from a fissure on the W flank on 20 October and reached 1 km in length by 24 October. Block avalanches spalled off the front of the flows and generated small ash plumes. Multi-branched flows on the W and SW flanks from the W flank fissure remained active throughout November. The slowdown in effusive activity in late July and early August 2020 is apparent in the MIROVA thermal anomaly data, as is the significant increase in activity during September that persisted into November 2020 (figure 130).

Figure 130. Thermal activity at Pacaya decreased in late July and early August 2020 but then increased significantly in early September and remained high through November 2020; numerous lava flows were reported during the periods of increased thermal activity. Thermal data is shown from 3 February through November 2020. Courtesy of MIROVA.

The break in the lava flow activity that began on 25 July 2020 (BGVN 45:08) lasted until 12 August. During that time, steam plumes were reported rising 25-75 m above the summit and drifting generally S or SW as far as 6 km before dissipating. Strombolian explosions rose 25-150 m above the rim of Mackenney crater and ejecta reached 50 m from the rim; noises as loud as a train engine were heard in nearby communities. Incandescence was observed nearly constantly along with persistent seismic tremor activity. On 12 August two lava flows emerged on the NW flank, each reaching about 150 m long. Incandescence from the flows was visible each day through 21 August on the NW flank in the area just above Cerro Chino (figure 131). The active flows were 100-200 m long during this period. A new lava flow appeared on the NE flank and grew to 300 m in length on 22 August.

Figure 131. A thermal anomaly from a lava flow on the NNW flank of Pacaya was present in Sentinel-2 satellite imagery on 17 August 2020 in addition to a thermal anomaly at the center of the pyroclastic cone inside the summit crater. Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Multiple lava flows were active on the NW, N, and NE flanks for the rest of the August. Incandescence on 24 August from the NW-flank flow near Cerro Chino indicated it was 250-300 m long. During 27 and 28 August flows were reported on the N and NNE flanks, 600 and 300 m long, respectively (figure 132). Incandescent pulses were reported from the crater overnight on 28-29 August; the NW flank flow remained active and was 300 m long. MODVOLC reported three thermal alerts on 29 August. The next day, 30 August, incandescence from the 650-m-long N flank flow and 300-m-long NE flank flow continued. Constant crater incandescence accompanied dense gray ash emissions on 31 August; the lava flow on the N flank remained incandescent for 350-400 m, but there was no incandescence or degassing from the NE-flank flow on the last day of the month.

Figure 132. A 600-m-long lava flow was visible on the N flank of Pacaya as seen from Villa Nueva, part of Guatemala City, late on 27 August 2020. Courtesy of Sh!ft.

White and blue steam and gas plumes were present daily throughout September 2020. They drifted in multiple directions as far as 8 km from the summit before dissipating. Strombolian activity was constant, building up the pyroclastic cone inside of Mackenney crater and sending ejecta as far as 50 m from the rim. Ejecta rose 50-150 m on most days; it was reported at 200 m high on 3, 9, and 14 September and was heard loudly and rattled windows nearby on 17 and 27 September. Constant crater incandescence with prolonged degassing of dense gray ash plumes was reported on 5, 10, 15, 17, and 21 September.

Multiple lava flows issued from fissures on the N flank and elsewhere throughout the month. Two lava flows on 1 September on the N flank were 50 and 350 m long. The next day three flows on the same flank were 300, 350, and 650 m long. On 3 September a new flow appeared on the E flank and extended 600 m from its source in addition to two flows on the N flank. For the next several days multiple flows were active on the N and NE flanks, reaching 450 m on the NE flank on 7 September. The next day the flow on the NE flank reached 1,200 m in length and was visible from Guatemala City. Activity continued with multiple flows 150-300 m long through 12 September (figure 133).

Figure 133. Lava flows at Pacaya were active on multiple flanks on 11 September 2020, including one that reached over a kilometer in length on the NE flank. Atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

On 13 September 2020 the flows on the N and NE flanks reached 600 and 300 m long, while a third flow reached 150 m down the S flank. The flow on the S flank was the most active during 14-23 September, extending 550 m from its source and producing numerous block avalanches from the flow front (figure 134). During the last week of the month the focus of the flow activity returned to the NE, N, and NW flanks where multiple flows were reported, some up to 550 m long, along with constant Strombolian activity (figures 135). Increased thermal activity resulted in MODVOLC thermal alerts reported on seven days during the second half of the month.

Figure 134. A large lava flow on the S flank of Pacaya during 14-23 September 2020 produced block avalanches from the flow front. It was seen here in Sentinel-2 satellite imagery on 21 September 2020 using atmospheric penetration rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Figure 135. Strombolian explosions sent ejecta 40-70 m above the crater at Pacaya on 26 September 2020. In addition, a lava flow 200 m long descended the N flank. Courtesy of CONRED.

Gas and steam plumes persisted throughout October 2020. They generally rose a few hundred meters above the summit and usually drifted S or W up to 10 km. Strombolian explosions continued daily, reported at 75-150 m high for most of the month. In a special report on 8 October INSIVUMEH noted increased Strombolian activity that sent bombs and fine ash 200-300 m above the crater, with ash emissions drifting 12 km W. During the last week of the month the ejecta reached 250 m high on several days. Loud noises and shock waves were periodically reported; vibrations were felt in San Francisco de Sales on 23 October and in areas to the S of Guatemala City on 27 October. INSIVUMEH reported ash emissions that drifted 8-10 km S and W from the summit on 23 October. The Washington VAAC reported ash emissions seen in satellite imagery drifting 15 km NE at 3.7 km altitude on 28 October. Weak sulfur dioxide emissions were recorded by the TROPOMI instrument on 6, 20, and 26 October (figure 136).

Figure 136. Weak SO2 emissions from Pacaya were recorded by the TROPOMI instrument on the Sentinel 5P satellite on 6, 20, and 26 October 2020. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Numerous lava flows were active throughout the month of October 2020 on multiple flanks (figure 137). During 1-4 October INSIVUMEH reported one or two flows active on the N and NE flanks that were 100-500 m long (figure 138). On 4 October there was a 200-m-long flow on the S flank, and another flow on the W flank. The S-flank flow grew to 250 m long by 8 October, had block avalanches spalling off the front, and fine ash that was stirred up by the wind. The next day three flows were active; they were 400 m long on the NE flank, 300 m on the N flank, and 200 m on the W flank. The N-flank flow was the most active during 11-15 October, reaching 650 m long. The W-flank flow was very active from 20 October through the end of the month, issuing from a fissure at mid-flank. It reached 1 km in length by 24 October and burned vegetation at the flow front (figures 139). A flow on the NE flank was 350 m long on 26 October (figure 140). MODVOLC issued thermal alerts on 7 days of the month, including seven alerts on 5 October.

Figure 137. Numerous lava flows were active throughout the month of October 2020 on multiple flanks of Pacaya. On 1 October the flows were concentrated on the N flank (left), and on 31 October a long flow was active on the W flank in addition to strong thermal activity at the summit crater (right). Atmospheric penetration rendering (bands 12, 11, and 8a) of Sentinel-2 satellite data. Courtesy of Sentinel Hub Playground.

Figure 138. A lava flow 125 m long on the N flank of Pacaya was active on 1 October 2020. Courtesy of CONRED.

Figure 139. A flow on the W flank of Pacaya was over 1 km long by 24 October 2020 when it was burning vegetation as it traveled downslope. Courtesy of Noti7.

Figure 140. An active flow on the SW flank of Pacaya issuing from a fissure on the W flank was over 1 km long on 26 October 2020 and had multiple branches flowing down the slope. Numerous people were camped on the slope below the flow. Photo by Mariana Lemus.

Although the weather was cloudy for much of November 2020, white steam and blue gas plumes were visible drifting S or W from the summit on many days, some reaching 10 km from the volcano before dissipating. Sporadic Strombolian explosions rose 100-200 m above the pyroclastic cone inside Mackenney crater; the explosions were often accompanied by small ash plumes that rose a few hundred meters and drifted downwind 8-10 km before dissipating. A small SO2 plume was recorded in the TROPOMI satellite data on 8 November, the same day that INSIVUMEH and the Washington VAAC reported an ash emission drifting NE at 3.4 km altitude over the village of Los Llanos and others in the area (figure 141). An increase in activity reported by INSIVUMEH on 15 November consisted of Strombolian explosions sending material up to 300 m above the summit and ejecting bombs up to 100 m outside the crater.

Figure 141. Ash and steam emissions were observed at Pacaya on 8 November 2020. Courtesy of CONRED.

Lava flows were still very active on the SW flank throughout November, emerging from a fissure a few hundred meters down from the summit that initially opened on 20 October. The main flow was 600 m long on 1 November and grew to 1,200 m long by 11 November (figure 142). On 5 November there were four separate branches of the SW-flank flow that were active. Block avalanches were common at the flow front. On 14 November a second flow was observed emerging from a fissure higher up on the SW flank from the earlier flow; they both were active for several days. INSIVUMEH issued a special report indicating increased effusion on 15 November from the SW-flank fissure. Block avalanches were occurring from the front of the 1-km-long flow, which had several branches. The blocks were 1-3 m in diameter and created small plumes of ash when moving as far as 500 m down the slope. An explosion during the night of 14-15 November at the SW-flank fissure created incandescent ejecta and ash emissions for several hours (figure 143). The flow remained active throughout the rest of November; on 26 November two flows were active from the main fissure, 500 and 400 m long (figure 144). On 30 November the main flow on the SW flank had three branches and extended 600 m from the mid-flank fissure.

Figure 142. A fissure on the W flank of Pacaya that opened on 20 October 2020 sent multiple flows down the W and SW flanks during November. The flow extended more than a kilometer on 10 November (left). It had moved in a SW direction by 20 November (center) and had three major branches active on 25 November (right). Atmospheric penetration rendering (bands 12, 11, and 8a) of Sentinel-2 satellite data. Courtesy of Sentinel Hub Playground.

Figure 143. An explosion at the fissure on the W flank of Pacaya during the night of 14-15 November 2020 produced incandescent ejecta almost as bright as that coming from the Strombolian activity inside the summit crater. For several hours dense ash emissions were visible at the fissure vent (inset). Large copyrighted photo courtesy of David Rojas, used with permission; inset courtesy of Prensa Objetiva.

Figure 144. Two flows with multiple branches were active on the W and SW flanks of Pacaya on 26 November 2020. Both copyrighted photos courtesy of David Rojas, used with permission.

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

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/ ); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Coordinadora Nacional para la Reducción de Desastres (CONRED), Av. Hincapié 21-72, Zona 13, Guatemala City, Guatemala (URL: http://conred.gob.gt/www/index.php) (URL: https://twitter.com/ConredGuatemala/status/1310057080162844673, https://conred.gob.gt/monitoreo-a-flujo-de-lava-en-el-volcan-pacaya/) ; 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/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); David Rojas, Guatemala (URL: https://www.instagram.com/davidrojasgtfoto/, https://twitter.com/DavidRojasGt/); Mariana Lemus, Guatemala (URL: https://www.instagram.com/marianalemusgt/); Noti7 (URL: https://twitter.com/Noti7Guatemala/status/1320169410833883136); Sh!ft (URL: https://twitter.com/kevingt_/status/1299204020662304768); Prensa Objetiva (URL: https://twitter.com/noticiasprensa/status/1328102695832612865).

Stromboli, located in the northeastern Aeolian Islands, is composed of two active summit craters: the Northern (N) crater and the Central-South (CS) crater that are situated at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the volcano (figure 187). The current eruption period began in February 1934 and has been recently characterized by Strombolian explosions at both summit craters, ash plumes, and SO₂ plumes (BGVN 45:09). This report covers activity consisting of dominantly Strombolian explosions, incandescent ejecta, and ash plumes from September to December 2020, with information primarily from daily and weekly reports by Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) and various satellite data.

Figure 187. Photo of the summit craters at Stromboli showing the North and Central-South crater areas with the location of each active vent: N1 and N2 in the N crater and S1, S2, and C in the CS crater. Photo was taken from the Pizzo sopa la Fossa during an expedition on 22 August by INGV-OE personnel. Courtesy of INGV (Rep. No. 37/2020, Stromboli, Bollettino Settimanale, 31/08/2020 - 06/09/2020, data emissione 08/09/2020).

Activity was consistent during this reporting period. Explosion rates typically ranged from 1-14 events per hour and varied in intensity that ejected material 80-250 m above the N crater and 150-250 m above the CS crater (table 10). An ash plume on 16 November rose 1 km above the crater, accompanied by a pyroclastic flow descending the Sciara del Fuoco to the NW as far as 200 m. As a result, some ash and lapilli fell in the town of Stromboli (2 km NE). Strombolian explosions were often accompanied by gas-and-steam emissions, occasional spattering that deposited material on the Sciara del Fuoco, small lava flows, and small pyroclastic flows. According to INGV, the daily SO₂ emissions measured 250-300 tons/day.

Table 10. Summary of activity at Stromboli during September-December 2020. Low-intensity activity indicates ejecta rising less than 80 m, medium-intensity is ejecta rising less than 150 m, and high-intensity is ejecta rising over 200 m above the vent. Data courtesy of INGV.

During September the frequency of the Strombolian explosions in the N crater typically ranged from 2-14 per hour; in the CS crater there were 1-10 explosions per hour. N1 consisted of three points of emissions that produced low- to high-intensity explosions, launching lapilli and bombs, sometimes mixed with fine ash, 80-200 m above the N crater and were distributed radially (figure 188); N2 typically showed low-intensity explosions (less than 80 m above the crater). Medium- to high-intensity explosions ejected mostly fine material mixed with some coarse tephra 250 m above the CS crater. On 28 September the number of explosive events reached a high of 22 per hour.

Figure 188. Webcam images of Strombolian activity at Stromboli in the N1 crater on 29 September (left) and in the CS crater on 4 October (right) 2020. Images captured by the SCV surveillance cameras. Courtesy of INGV (Rep. No. 41/2020, Stromboli, Bollettino Settimanale, 28/09/2020 - 04/10/2020, data emissione 06/10/2020).

Explosions with occasional spatter continued in October at a rate of 2-13 per hour in the N crater and 1-4 per hour in the CS crater. In the N crater, N1 consisted of 2-4 eruptive vents that produced explosions of variable intensity while N2 contained two vents that primarily produced low-intensity explosions. Lapilli and bombs, sometimes mixed with fine ash, were ejected 80-250 m above the N crater. Fine ash sometimes mixed with coarse-to-medium tephra rose 150-250 m above the CS crater. Spatter was reported from two hornitos that formed in the N1 crater (figure 189). On 11 October sporadic ash emissions and coarse ejecta were observed above the S2 crater, episodic ash emissions rose above the S1 crater, and occasional degassing with modest spattering were visible in the C crater.

Figure 189. Drone images showing gas-and-steam emissions and Strombolian activity at Stromboli during 8-9 October 2020. The white annotations label the craters and the red show the active hornitos (H). The N2H2 label shows a small explosion (right). Images from the HPHT Lab from INGV-Roma 1. Courtesy of INGV (Rep. No. 42/2020, Stromboli, Bollettino Settimanale, 05/10/2020 - 11/10/2020, data emissione 13/10/2020).

Strombolian explosions persisted into November. The N1 crater consisted of 2-3 vents, producing explosions of variable intensity; the N2 crater also consisted of 2-3 active vents that produced low- to medium-intensity explosions. The frequency of explosions ranged from 2-10 per hour in the N crater and 1-4 per hour in the CS crater. Lapilli and bombs, sometimes mixed with fine ash, rose 80-250 m above the N crater and fine material was ejected 150 m above the CS crater. On 10 November an explosion was detected at 2104 in the S2 crater of the CS area, producing pyroclastic material that was distributed radially along the Sciara del Fuoco, followed by an ash plume (figure 190). Within 30 seconds, another pulse of activity from the C crater in the northern part of the CS area produced intense lava fountaining that ejected coarse incandescent material 300 m above the crater, lasting about two minutes. At 2106 a small explosion was detected in the N2 crater, ending the explosive sequence.

Figure 190. Thermal (rows 1 and 3) and webcam (rows 2 and 4) images showing the evolution of the explosion at Stromboli on the evening of 10 November 2020 accompanied by an ash plume and incandescent ejecta. Images captured by the SCT and SQV surveillance cameras. Courtesy of INGV (Rep. No. 47/2020, Stromboli, Bollettino Settimanale, 09/11/2020 - 15/11/2020, data emissione 17/11/2020).

During an overflight by the 2nd Air Unit of the Coast Guard of Catania on 11 November, scientists identified degassing in the entire summit crater area; a small lava flow was observed in the S1 crater, originating from an intra-crater vent. Additional thermal anomalies were noted at the bottom of the C, N1, and N2 craters. Strong fumaroles were visible originating from a hornito located outside the S1 crater on the Sciara del Fuoco. A second hornito was visible on the slope of the Sciara del Fuoco near the N2 crater. On 16 November a major explosion was detected at 1017 in the N crater area and on the edge of the N2 crater. Thermal and visible images captured the resulting dense, gray ash plume that rose 1 km above the crater and the accompanying pyroclastic flow that descended the Sciara del Fuoco as far as 200 m (figure 191). Some ash and lapilli fell over the town of Stromboli, about 2 km away on the NE coast of the island. A sequence of explosive events at 0133 on 21 November was detected in three different craters: the first two events occurred in the N1 and N2 craters, and the third occurred in the C crater. Coarse material was ejected 300 m above the crater and was distributed radially, affecting the upper part of the Sciara del Fuoco. A small ash plume was also visible.

Figure 191. Thermal (top row) and webcam (bottom row) images showing the evolution of the explosion at Stromboli on the morning of 16 November 2020 accompanied by a significant gray ash plume. Images captured by the SCT and SCV surveillance cameras. Courtesy of INGV (Rep. No. 48/2020, Stromboli, Bollettino Settimanale, 16/11/2020 - 22/11/2020, data emissione 24/11/2020).

During December, similar Strombolian explosions were reported. There were two eruptive vents in the N1 crater and 2-4 in the N2 crater that produced explosions of low intensity and low-to-medium intensity, respectively. The frequency of explosions ranged from 1-13 per hour in the N crater and 1-5 per hour in the CS crater. Fine ash mixed with some coarse material (lapilli and bombs) was ejected 80-150 m above the N crater and mostly fine material rose 150 m above the CS crater. Some spattering activity was reported in the N2 crater, which contributed to the formation of hornitos that produced incandescent material. On 6 December an explosive sequence of events was detected in the CS crater area at 0612. An explosion ejected material 300 m above the crater that were distributed radially, depositing on the upper Sciara del Fuoco. In addition, two small lava flows formed (figure 192). A second explosion was recorded at 0613, characterized by lava fountaining in the CS crater that reached a height of 200 m. Similar activity in the N and CS craters were also captured by webcam images on 21 and 27 December, which showed lava fountaining, accompanied by a small pyroclastic flow (figure 193).

Figure 192. Thermal images of the explosion at Stromboli in the CS crater on 6 December 2020, accompanied by incandescent ejecta and two small lava flows. Some lava fountaining was visible in the bottom center image at 0513:47. Images captured by the SCT surveillance camera. Courtesy of INGV (Rep. No. 50/2020, Stromboli, Bollettino Settimanale, 30/11/2020 - 06/12/2020, data emissione 08/12/2020).

Figure 193. Webcam (top row) and thermal (bottom row) images of Strombolian activity in the N (left column) and CS (right column) crater areas at Stromboli on 21 December (top right) and 27 December (top left and bottom row) 2020. This activity included a small pyroclastic flow and lava fountaining. Images captured by the SCV and SCT surveillance cameras. Courtesy of INGV (Rep. No. 53/2020, Stromboli, Bollettino Settimanale, 21/12/2020 - 27/12/2020, data emissione 29/12/2020).

Intermittent and low-power thermal activity was detected during September through December, according to the MIROVA Log Radiative Power graph using MODIS infrared satellite information (figure 194). Though there were no detected MODVOLC thermal alerts during this reporting period, many thermal hotspots were visible in one or both summit craters on clear weather days using Sentinel-2 thermal satellite imagery, which is due to Strombolian activity (figure 195).

Figure 194. Intermittent, low thermal activity at Stromboli was recorded by the MIROVA graph (Log Radiative Power) during September through December 2020. The frequency of the thermal anomalies had decreased compared to the previous months of May through August; a total of eleven thermal anomalies were detected during this reporting period. Courtesy of MIROVA.

Figure 195. Weak thermal anomalies (bright yellow-orange) at Stromboli were visible in Sentinel-2 thermal satellite imagery from typically both summit craters during September through December 2020. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

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

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy, (URL: http://www.ct.ingv.it/en/); 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 glaciated Saunders Island is located in the remote South Sandwich Volcanic Arc in the South Atlantic between Candlemas (to the north) and Montagu (to the south) islands. The main volcanic features are Mount Michael, lava flows on the northern Blackstone Plain, and the Ashen hills complex near the eastern Nattriss Point (figure 31). The Ashen Hills complex is a group of overlapping craters formed through phreatomagmatic activity, with the largest crater opening towards the NW (figure 32). Gas emissions have been remotely observed from the ice-filled Old crater to the SE, with reports of gas plumes extending back to 1820 (LeMasurier et al., 1990; Patrick and Smellie, 2013; Liu et al., 2021). The current eruption period, centered at the 500-m-diameter Mount Michael summit crater, has been ongoing since at least 12 November 2014, based on remote sensing analysis (Gray et al., 2019). Activity consists of a lava lake, persistent degassing, and intermittent explosions producing ash plumes (Patrick and Smellie 2013; Gray et al. 2019). Visits are infrequent due to the remote location, and cloud and plume cover often prevents satellite observations. This report summarizes activity during June 2019 through May 2020 primarily using satellite data, as well as observations from visiting scientists.

Figure 31. This 24 December 2019 satellite image (PlanetScope 3-Band scene) of Saunders Island shows the locations of the active Mount Michael summit crater and other features on the island. Courtesy of Planet Labs.

Figure 32. Images of the southeastern area of Saunders Island taken in January 2020. The top left image shows Nattriss Point with Ashen Hills in the background. The other photos show the crater and flanks of the Ashen Hills complex with rill and gully features from fluvial erosion. White and black speckled features in the images are penguins. Photos courtesy of Emma Liu and the 2020 Pelagic Australis expedition group.

Activity during June-December 2019. Ashfall deposits on the flanks were sometimes visible on the snow and ice (figure 33). MIROVA thermal anomaly data during June 2019 through June 2020 showed few days where high temperatures were detected by this sensor, but the active summit crater floor is often obscured by cloud cover or condensed gas-and-steam plumes. The TROPOspheric Monitoring Instrument (TROPOMI) detected frequent sulfur dioxide (SO₂) plumes of varying concentrations that are dispersed in different directions by wind (figure 34). Small condensed gas-and-steam plumes are often visible in satellite imagery within the crater, and some larger plumes are also imaged (figure 35). All satellite images where the summit crater was not obscured by either cloud cover or gas-and-steam plumes showed elevated temperatures within the summit crater, with three distinct areas visible possibly indicating multiple active vents (figure 36).

Figure 33. This satellite image of Saunders Island acquired on 15 September 2019 shows the snow and ice-covered island and a recent ashfall deposit on the NE flank towards Cordelia Bay, with a green sediment plume in the water. Sentinel-2 image with Natural color (bands 4, 3, 2) rendering. Courtesy of Planet Labs.

Figure 34. These images show data acquired by the TROPOspheric Monitoring Instrument (TROPOMI) that demonstrate detected SO2 (sulfur dioxide) from Mount Michael on Saunders Island on 2, 3, 25, and 29 September 2019. These are examples of gas plumes through the month with wind dispersing the plumes in different directions. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Figure 35. This 10 October 2019 satellite image shows Saunders Island and the surrounding area with light cloud cover, and a condensed gas-and-steam plume from the summit crater drifting towards the E to SE. Sentinel-2 image with Natural color (bands 4, 3, 2) rendering. Courtesy of Planet Labs.

Figure 36. These two Sentinel-2 thermal satellite images of Saunders Island acquired on 2 and 24 December 2019 show three distinct areas of elevated temperature within the Mount Michael summit crater (yellow to red). While the locations of the thermal anomalies look different in these images, the angle of the view into the crater is not specified. Blue is Ice, black is ocean water. Sentinel-2 image with False color (Urban) (bands 12, 11, 4) rendering. Courtesy of Sentinel Hub Playground.

Activity during January-May 2020. During January through May 2020 various remote sensing data showed the same activity as the previous seven months, with abundant cloud cover over the island. The Sentinel-2 satellite imaged a vertical plume on 13 March rising then being dispersed NE (figure 37). Intermittent observations of SO₂ plumes continued through TROPOMI data analysis (figure 38). A clear view of the summit area on 29 May showed the ice-free active summit crater producing a weak gas-and-steam plume, and ash deposition on the NE to SE upper flanks (figure 39).

Figure 37. This Sentinel-2 satellite image of the Mount Michael summit area on Saunders Island with a gas-and-steam plume rising from the summit crater above the cloud cover, and dispersing NE. The plume and clouds are casting dark shadows below them. Sentinel-2 image with False color (Urban) (bands 12, 11, 4) rendering. Courtesy of Sentinel Hub Playground.

Figure 38. Examples of SO2 gas plumes originating from Saunders detected by the TROPOMI instrument on 14 and 18 March 2020. The plumes are dispersing N to NNE. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Figure 39. This 29 May 2020 Planet Scope satellite image shows the summit area of Mount Michael above cloud cover with the active summit crater and the old crater to the SE. There is a weak gas plume rising from the crater and ashfall on the upper E flank. Courtesy of Planet Labs.

Research expedition in January 2020. The team of the 2020 Pelagic Australis expedition visited the island on 5-8 January 2020, with shore landings on the last three days, to quantify gas emissions from the island. The following information is from the published expedition results (Liu et al., 2021), with photos supplied by volcanologist Emma Liu.

Across the South Sandwich islands they used a combination of a ground-based and drone-mounted gas detectors (Multi-GAS), a UV imaging camera, sample collection, and NDIR spectrometer analyses to quantify gas output. They confirmed that the summit crater is a persistent source of gas emissions with 145 ± 59 tons per day of SO₂ and a CO₂ flux of 179 ± 76 tons per day. On the 5th they observed a vertical plume and on the 7th they observed the plume drifting down the E flank before rising (figure 40). They noted that the surface was steaming and was warm to the touch, suggesting widespread geothermal activity. The non-glaciated surfaces of the island contain tephra deposits, with units exposed by erosion and preserved within snow and ice (figure 41). Explosions have emplaced tephra layers across the island as well as ballistic blocks and bombs on the E flank (figure 42; Liu et al., 2021).

Figure 40. These images show the gas emissions from Mount Michael on Saunders Island in January 2020. The top right image is a vertical gas plume rising from the summit crater on the evening of the 5th. The two photos on the right are looking towards the E on the 7th. The bottom left image is a low-lying condensed gas plume on the 8th travelling down the E flank before rising. Courtesy of Emma Liu, and Liu et al. (2021).

Figure 41. Tephra layers are preserved within the stratigraphy of snow and ice on Saunders Island. Scale shown by penguins (top) and volcanologist Kieran Wood (right). Photos courtesy of Emma Liu.

Figure 42. Dense volcanic blocks up to a meter in size are widespread on Saunders Island. The block in the foreground has a height of approximately 35 cm; the Chinstrap penguin in the foreground is around 50 cm tall. Courtesy of Emma Liu and Liu et al. (2021).

References: Liu E J, Wood K, Aiuppa A, Giudice G, Bitetto M, Fischer T P, McCormick Kilbride B T, Plank T, Hart T, 2021. Volcanic activity and gas emissions along the South Sandwich Arc. Bull Volcanol 83. https://doi.org/10.1007/s00445-020-01415-2

LeMasurier W E, Thomson J W, Baker P E, Kyle P R, Rowley P D, Smellie J L, Verwoerd W J, 1990. Volcanoes of the Antarctic Plate and Southern Ocean. American Geophysical Union, Washington, D.C.

Lachlan-Cope T, Smellie J L, Ladkin R, 2001. Discovery of a recurrent lava lake on Saunders Island (South Sandwich Islands) using AVHRR imagery. J. Volcanol. Geotherm. Res. 112: 105-116.

Gray D M, Burton-Johnson A, Fretwell P T, 2019. Evidence for a lava lake on Mt. Michael volcano, Saunders Island (South Sandwich Islands) from Landsat, Sentinel-2 and ASTER satellite imagery. J. Volcanol. Geotherm. Res. 379:60-71. https://doi.org/10.1016/j.volgeores.2019.05.002

Derrien A, Richter N, Meschede M, Walter T, 2019. Optical DSLR camera- and UAV footage of the remote Mount Michael Volcano, Saunders Island (South Sandwich Islands), acquired in May 2019. GFZ Data Services. http://doi.org/10.5880/GFZ.2.1.2019.003

Patrick M R, Smellie J L, 2013. Synthesis A spaceborne inventory of volcanic activity in Antarctica and southern oceans, 2000–10. Antarct Sci 25:475–500. https://doi.org/10.1017/S0954102013000436

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

Information Contacts: Emma Liu, University College London, Kathleen Lonsdale Building, 5 Gower Place, London, WC1E 6BS, United Kingdom; Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Planet Labs, Inc. (URL: https://www.planet.com/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Santa Maria is one of the most active volcanoes in Guatemala. Major features are the Santa Maria edifice with the large crater that formed in the 1902 eruption, and the Santiaguito dome complex about 2.5 km down the SW flank that includes the currently active Caliente dome (figure 113). Activity typically includes ash plumes, gas emissions, lava extrusion, and avalanches. This report summarizes activity during August 2020 through January 2021 and is based on reports by Instituto Nacional de Sismología, Vulcanología, Meteorología e Hidrología (INSIVUMEH), Coordinadora Nacional para la Reducción de Desastres (CONRED), and satellite data.

Figure 113. Main features of the Santa Maria complex are shown in this March 2021 Planet Labs satellite image monthly mosaic. The large scarp is the wall of the crater produced during the 1902 eruption. Within that the El Brujo, El Monje, La Mitad domes, and the currently active Caliente dome, are from W to E. Courtesy of Planet Labs.

Throughout August weak to moderate explosions were reported most days, some days occurring 2-4 times per hour. These produced ash plumes to an altitude of 3.5 km, typically reaching 3.4 km. The plumes were dispersed mostly W and SW, sometimes S, SE, and NW. Degassing was reported throughout the month, with plumes reaching 3.5 km, but most often 3-3.1 km altitude. On the 3rd, ashfall was reported in San Marcos Palajuno (8 km SW), Loma Linda (6 km WSW) and others in that direction, and again on the 29th. It was also reported in Monte Claro (S of the summit) on the 12th and light ashfall occurred on the flanks through the month. Explosions on the 23rd produced weak pyroclastic flows that traveled down the SW flank of the dome. The activity produced frequent avalanches on the S, SW, and SE flanks of the dome, some reaching the base of the dome and some depositing fine ash onto the flanks. The sound of explosions and degassing were reported most days and incandescence was frequently seen at the crater at night.

This activity continued through September, maintaining the same eruptive pattern of weak and moderate explosions, gas emission, lava extrusion, and avalanches. Incandescence continued to be visible at the crater. There was ashfall reported in Monte Claro, Aldea San Marcos Palajunoj and other surrounding communities on the 7th, Monte Claro on the 11th, and across the Palajunoj area on the 28th. On the afternoon of 25 September lahars occurred in the Cabello de Ángel and Nimá I drainages. Lava extrusion was reported on the morning of the 29th along with resulting block-and-ash flows.

Throughout October explosions, gas emission, avalanches, and elevated crater temperatures producing nighttime incandescence (figure 114) continued in the same manner as the previous months. From the 9th the extrusion of lava was observed over the dome, generating block-and-ash flows mainly down the W flank. Ashfall was reported in of Loma Linda and El Rosario Palajunoj and others in the area on the 13th, 7 km SW on the 18th, and in San Marcos Palajunoj and nearby areas on the 23rd. Lava extrusion generated constant avalanches down multiple flanks from the 23rd, with some producing small ash plumes as they descended.

Figure 114. This Shortwave Infrared (SWIR) image of Santa Maria acquired on 19 October by the Landsat 8 satellite shows elevated temperatures at the Caliente dome. The contour intervals are 30 m. Courtesy of USGS and INSIVUMEH.

Throughout November gas emissions and explosions continued to produce gas-and-steam and ash plumes that rose up to 3.4 km altitude. Lava extrusion also continued down the W flank, producing incandescence and frequent avalanches down the SE, S, SW, and W flanks, as well as less frequent block-and-ash flows (figure 115). An increase in thermal energy detected towards the end of the month resulted from this extrusion (figure 116). Ashfall occurred around the volcano from explosions and avalanches. Ashfall was reported SE within the villages of Las Marías, Calaguache and others nearby on the 12th and 22nd, and SSW over the village of San Marcos Palajunoj, Loma Linda and Fincas in the Palajunoj area on the 27th. Degassing and explosions were intermittently heard in nearby communities with reports of sounds similar to an airplane turbine. An explosion on the 16th produced an ash plume up to 3.6 km altitude and pyroclastic flows down the flanks (figure 117).

Figure 115. This nighttime Landsat 8 Shortwave Infrared (SWIR) satellite image of Santa Maria with the contours of the Caliente dome overlain was acquired on 20 November 2020. There are elevated temperatures within the summit crater and lava is flowing down a channel on the western flank. The contour intervals are 20 m. Courtesy of USGS and INSIVUMEH.

Figure 116. This MIROVA log radiative power plot shows the thermal energy released at Santa Maria between April 2020 to February 2021. There was a decrease in energy emitted from May to November, followed by an increase in the frequency and the energy released on some days. The black vertical lines like the two in January-February are more than 5 km from the summit and are likely not a result of volcanic activity. Courtesy of MIROVA.

Figure 117. An explosion from the Caliente dome of Santa Maria is seen here at 0715 on 16 November 2020. The photo shows the ash plume that rose to 3.6 km altitude and pyroclastic flows descending the flanks. The seismogram shows the explosion in the center of the bottom line (the times on the left are given in UTC). Courtesy of INSIVUMEH.

Gas emissions and weak to moderate explosions continued throughout December, producing plumes reaching 3.4 km altitude along with ongoing lava extrusion producing avalanches (figures 118 and 119). Ash from explosions and avalanches was intermittently emplaced onto the flanks, and ashfall was reported in the villages of San Marcos and Loma Linda Palajunoj on the 7th, and in Loma Linda and Finca Montebello on the 11th. Activity increased from 0430 on 11 December 2020 with the generation of moderate to powerful avalanches as well as block-and-ash flows from lava extrusion and accumulation, with 13 events recorded between that time and when a report was released at 0900. The intensity continued with block-and-ash flows and pyroclastic flows moving down the W and SW flanks that generated ash plumes which extended 20 km downwind.

Figure 118. Plumes rise from the Caliente dome at Santa Maria on 9 (top left) and 15 (top right) December 2020. A faint plume rises from the summit of the Caliente dome and another plume rises from a possible avalanche down the SW flank (bottom). Courtesy of INSIVUMEH (Fotografías Recientes de Volcanes).

Figure 119. A gas-and-steam plume rises from the degassing Caliente dome at Santa Maria on 30 December 2020. Around this time weak and moderate explosions produced ash plumes up to 3-3.4 km altitude, resulting in ashfall on the flanks. Courtesy of CONRED.

The high level of background activity associated with lava extrusion continued through January. Satellite images show the lava flow advancing down the W-flank channel (figure 120), reaching approximately 250 m by the 11th. Avalanches also continued, producing ash that was emplaced nearby (figure 121). On the 22nd the collapse of dome material produced a pyroclastic flow to the E and SE. Explosions ejected ash to 3.4 km altitude, with ashfall that was reported in the Aldeas de San Marcos and Loma Linda Palajunoj on the 1st, Aldeas de San Marcos and Loma Linda Palajunoj on the 11th, Aldeas de San Marcos y Loma Linda Palajunoj, Fca. El Patrocinio during the 20-21st. Ashfall was again reported on the 31st to the west on farms, in Aldeas de San Marcos, and in Loma Linda Palajunoj. Sounds generated by explosions were sometimes heard around 10 km away.

Figure 120. PlanetScope satellite images of Santa Maria acquired on 20 December 2020 and 10 and 11 January 2021 show the development of a lava flow down a channel on the W flank (white arrows). In the latest image the flow is approximately 250 m long. Courtesy of Planet Labs.

Figure 121. Thermal infrared satellite images of Santa Maria acquired on 12 and 22 January 2021 show higher temperatures on the Caliente dome. Top: Elevated thermal areas are detected at the summit and hot material is emplaced down the W-flank channel. Bottom: Elevated temperatures at the summit of the lava dome, with a possible avalanche on the E flank. Sentinel-2 thermal satellite images with false color (urban) (bands 12, 11, 4) rendering. 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/); Coordinadora Nacional para la Reducción de Desastres (CONRED), Av. Hincapié 21-72, Zona 13, Guatemala City, Guatemala (URL: http://conred.gob.gt/www/index.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/); Planet Labs, Inc. (URL: https://www.planet.com/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Activity at Bromo, the youngest and only active cone within the 16-km-wide Tengger caldera in East Java, is characterized by occasional explosions with ash plumes followed by periods of relative quiet with only gas-and-steam emissions (BGVN 44:05). There have been more than 30 eruptive periods since 1900. During the first seven months of 2019, ash explosions occurred on 18 February 2019 and became especially numerous in March and April, with more explosive activity in July 2019 (BGVN 44:05, 44:08). The volcano is monitored by the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM) and by the Darwin Volcanic Ash Advisory Centre (VAAC).

Following the ash explosion on 28 July 2019, satellite observations frequently showed a white gas-and-steam plume in the Bromo crater (figure 19). No additional eruptive activity was reported until 26-27 December 2020 when PVMBG reported white-and-gray plumes rose 50-700 m above the summit of Bromo’s cone. The next day, at 0550 on 28 December, an observer spotted a gas-and-ash emission rising at least 500 m above the summit. The Darwin VAAC was unable to confirm if there was ash in the plume based on satellite data, but ashfall was reported in the Ngadirejo area, about 5 km NE. The Alert Level remained at 2 (on a scale of 1-4), and visitors were warned to stay outside a 1-km radius of the crater.

Figure 19. Satellite image of the Tengger Caldera on 12 September 2020, with a typical white plume visible in the Bromo crater. Sentinel-2 image with natural color rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

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

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Lewotolok (also known as Lewotolo) is located on the eastern end of a peninsula connected to Lembata (formerly Lomblen) that extends north into the Flores Sea. Eruptions date back to 1660, characterized by explosive activity in the summit crater. Typical activity has consisted of seismicity and thermal anomalies near the summit crater (BGVN 36:12 and 41:09). A new eruption that began in late November 2020 was characterized by increased seismicity, dense, gray ash plumes, nighttime crater incandescence, and ashfall. This report covers activity through January 2021 using information primarily from the 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.

Summary of activity during February 2012-October 2020. Activity from February 2012 to November 2020 was relatively low and consisted primarily of a persistent thermal anomaly in the summit crater since at least March 2016 and occasional white gas-and-steam emissions. During January 2012 intermittent white gas-and-steam plumes rose 15-500 m above the crater, accompanied by crater incandescence; no thermal anomalies were reported during 16-24 January. On 6 January there were 500 people in the Lembata district evacuated due to reports of ash plumes that were observed by local residents, the smell of sulfur, and the sound of rumbling (BGVN 36:12).

Thermal activity dates back to 13 October 2014 using MODIS data in MODVOLC satellite data (BGVN 41:09; figure 3). According to the MODVOLC algorithm, a total of seven thermal alerts were detected on 13 October 2014 (1), 27 September 2015 (1), 2, 3, and 4 (2) October 2015, and 5 November 2017 (1). The number of thermal alerts in both MODVOLC and Sentinel-2 satellite data had increased slightly in 2020 compared to 2018 and 2019, though cloud cover often prevented visual confirmation for the latter (figure 3). Sentinel-2 thermal satellite imagery captured occasional thermal anomalies in the summit crater during 2016-2019 (figure 4). White gas-and-steam plumes were intermittently reported from September 2017 through 2 March 2018 that rose as high as 500 m above the crater and drifted dominantly E and W, according to PVMBG.

Figure 3. Graph comparing the number of thermal anomalies using MODVOLC alerts and Sentinel-2 satellite data for Lewotolok during January 2014-January 2021 for MODVOLC and 20 March 2016-January 2021 for Sentinel-2 thermal satellite data. Data courtesy of HIGP - MODVOLC Thermal Alerts System and Sentinel Hub Playground.

Brief seismicity, which included shallow and deep volcanic earthquakes was detected during October 2017. On 9 October 2017 PVMBG issued a VONA (Volcano Observatory Notice for Aviation) reporting that white gas-and-steam emissions rose 500 m above the crater. On 10 October BNPB (Badan Nacional Penanggulangan Bencana) reported that five earthquakes 10-30 km below Lewotolok and ranging in magnitude of 3.9-4.9 as recorded by Badan Meteorologi, Klimatologi, dan Geofisika (BMKG). These seismic events were felt by local populations and resulted in an evacuation of 723 people. The only activity reported between January 2018 and October 2020 was white gas-and-steam plumes that rose 5-100 m above the crater drifting primarily E and W and an occasional thermal anomaly in the summit crater (figure 4).

Figure 4. Sentinel-2 thermal satellite imagery shows a thermal anomaly in the summit crater of Lewotolok during 20 March 2016 (top left), 8 July 2017 (top right), 13 July 2018 (bottom left), and 12 August 2019 (bottom right). Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

New eruption starting in November 2020. On 26 November 2020 a continuous tremor began at 1943, followed by a series of volcanic earthquakes at 1947 and deep volcanic earthquakes at 1951, 1952, 1953, and 2255; white gas-and-steam emissions rose 20 m above the crater. Deep volcanic earthquakes were again recorded at 0242, 0537, 0556 on 27 November. At 0557 an explosion produced a gray ash plume that rose 500 m above the crater and drifted W; by 0630 the plume turned white, according to PVMBG (figure 5). Seismicity decreased slightly after the explosion, but tremor continued. During 27-28 November dense white gas-and-steam plumes rose as high as 500 m above the crater and nighttime crater incandescence was observed.

Figure 5. Webcam image of a dense gray ash plume rising 500 m above the crater of Lewotolok on 27 November 2020. Courtesy of MAGMA Indonesia.

During the morning of 29 November seismicity increased again and consisted of six deep volcanic earthquakes, continuous tremor occurred around 0930. A second explosion was recorded at 0945 that produced an ash plume 4 km above the crater, accompanied by incandescent material that was ejected above the crater (figure 6). The ash plume consisted of two levels: the lower-level drifted W and NW and the upper-level drifted E and SE. The large, gray ash plume was captured in a satellite image as it spread generally E and W (figure 7). Ashfall and a sulfur odor was reported in several surrounding villages; videos from social media showed tephra falling onto the roofs of residential areas. BPBD evacuated residents in 28 villages in two sub-districts; by 29 November at 1300 about 900 people had been evacuated. At 1900 Strombolian activity was observed and during the night, crater incandescence was visible.

Figure 6. Photos of the eruption at Lewotolok on 29 November 2020 that produced a dense, gray ash plume 4 km above the crater. Courtesy of Devy Kamil Syahbana, PVMBG (left) and MAGMA Indonesia (right).

Figure 7. Satellite image showing a strong gray ash plume above Lewotolok on 29 November 2020, expanding roughly E and W. Courtesy of Sentinel Hub Playground and the European Space Agency, Copernicus.

The eruption continued from 29 November into 1 December, where the white-and-gray ash plumes rose 700-2,000 m above the crater and drifted SE and W, accompanied by incandescent material that was ejected above the crater and the smell of sulfur, according to PVMBG (figure 8). A large sulfur dioxide plume was reported drifting SE and extending over the N half of Australia by 30 November (figure 9). By 1300 that day, 4,628 people had been evacuated. Incandescent lava flows near the summit were visible and incandescent material traveled down the flanks during 30 November and 1 December.

Figure 8. Webcam image of the continuous eruption at Lewotolok showing a dense gray ash plume rising above the cloud-covered summit on 30 November 2020. Courtesy of MAGMA Indonesia.

Figure 9. SO2 plume from Lewotolok captured by the Sentinel-5P/TROPOMI instrument on 30 November 2020 drifting SE and along the N part of Australia. Courtesy of Simon Carn and the NASA Global Sulfur Dioxide Monitoring Page.

White-and-gray plumes continued frequently through January 2021, rising 100-1,500 m above the crater, drifting in multiple directions, accompanied by nighttime crater incandescence and occasional incandescent ejecta (figure 10). During 1-8 December gray plumes rose 100-1,000 m above the crater and drifted E, W, and SW accompanied by nightly crater incandescence and incandescent material ejected as high as 20 m above the crater. By 5 December at 2200 about 9,028 residents had been evacuated to 11 evacuation centers, according to BNPB. Black, gray, and brown ash plumes were visible daily during 9-15 December, rising 1 km above the crater, accompanied by nightly Strombolian explosions that ejected material above the crater. More Strombolian explosions on most nights over 16-29 December ejected material 100-300 m above the crater; in addition, the sounds of rumbling and banging could be heard. The material was deposited as far as 1 km from the crater E and SE during 24-25 and 27-31 December and 4-7 January 2021. Strombolian activity continued into January, accompanied by frequent gray-and-white ash plumes, rumbling and banging sounds, and incandescent ejecta up to 600 above the crater that extended as far as 500 m E, SE, and W. Crater incandescence was visible up to 600 m above the crater.

Figure 10. Webcam images showing continuing dense gray ash plumes from Lewotolok on 1 December 2020 (top) and 8 January 2021 (bottom). Courtesy of MAGMA Indonesia.

A consistent level of thermal activity was recorded in the Sentinel-2 MODIS Thermal Volcanic Activity from February 2019 through October 2020; in early December 2020 a slight increase in thermal anomalies were detected (figure 11). This data reflects the start of the new eruption in late November 2020. According to the MODVOLC thermal algorithm, five thermal hotspots were detected between January 2020 and January 2021 on 3 September (1), 29 November (2), 24 December (1), and 5 January 2021 (1). Some of this thermal activity was also observed in Sentinel-2 thermal satellite imagery in the summit crater (figure 12).

Figure 11. Sentinel-2 MODIS Thermal Volcanic Activity data (bands 12, 11, 8A) shows consistent thermal activity (red dots) at Lewotolok during February 2020 through December 2020. Stronger thermal anomalies in early December is likely due to the new eruption that began in late November 2020. Courtesy of MIROVA.

Figure 12. Sentinel-2 thermal satellite imagery showing a thermal anomaly in the summit crater of Lewotolok on 25 October (top left), 9 November (top right), and 3 January 2021 (bottom right). On 14 December (bottom left) a Natural Color image showed a gray ash emission above the clouds and drifted E. On 3 January 2021 (bottom right) two thermal anomalies were visible in the summit crater accompanied by gas-and-steam emissions drifting NE. Sentinel-2 satellite images with “Natural Color” rendering (bands 4, 3, 2) on 14 December 2020, all other images use “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

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/); Badan Nasional Penanggulangan Bencana (BNPB), National Disaster Management Agency, Graha BNPB - Jl. Scout Kav.38, East Jakarta 13120, Indonesia (URL: http://www.bnpb.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.vsi.esdm.go.id/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); European Space Agency (ESA), Copernicus (URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus); 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/); Simon Carn, Dept of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Dr., Houghton, MI 49931, USA (URL: https://so2.gsfc.nasa.gov/).

Soufrière St. Vincent is the northernmost stratovolcano on St. Vincent Island in the southern part of the Lesser Antilles. The NE rim of the 1.6-km-wide summit crater is cut by a crater (500 m wide and 60 m depth) that formed in 1812. Recorded eruptions date back to 1718, with notable eruptions occurring in 1812, 1902, and 1979. The eruption of 1979 was characterized by ashfall, pyroclastic flows, and lahars, in addition to a series of Vulcanian explosions during 13-26 April 1979 that destroyed the lava dome in the summit crater, which had formed during a 1971 effusive eruption (SEAN 04:04). As a result, more than 20,000 people were evacuated. Beginning around 3 May 1979 another lava dome began to form in the main crater (SEAN 04:05; Shepherd et al., 1979) that continued to grow until the end of October 1979, expanding to 850 m in diameter and 120 m high (SEAN 04:11; Cole et al., 2019).

No further eruptive activity took place until December 2020, when a new lava dome began to grow SW of the pre-existing 1979 lava dome, accompanied by increased seismicity, crater incandescence, and gas-and-steam emissions. This report reviews information through February 2021 using bulletins from the University of the West Indies Seismic Research Centre (UWI-SRC), the National Emergency Management Organisation (NEMO), and various satellite data. Soufrière St. Vincent is monitored by the SRC assisted by the Soufrière Monitoring Unit (SMU) from the Ministry of Agriculture in Kingstown. As of 2004, the monitoring network had consisted of five seismic stations, eight GPS stations, and several dry tilt sites. Seismic data are transmitted from field sites to the Belmont Observatory (9 km SSW), which is operated by the SMU (figure 4). On 1 January 2021 a new seismic station was installed at Georgetown, on 10 January one was installed in Owia, followed on 15 January by another on the upper S flank, station SSVA at the summit on 18 January, and in Fancy on 21 January. In February 2021 the USGS-USAID (US Geological Survey-US Agency for International Development), through the Volcano Disaster Assistance Program (VDAP), donated equipment to build four more seismic stations.

Figure 4. Location map of the Belmont Observatory (yellow star) located in Rosehall, St. Vincent, 9 km SSW from the Soufrière St. Vincent summit crater (red triangle). Base map satellite imagery courtesy of Google Earth.

A spike in seismicity was recorded during June-July 2019 (figure 5), though no cause was reported. The number of events sharply declined after July but continued intermittently through November 2020. Seismicity began to increase in early November through 23 December 2020, which included 126 earthquakes described as volcano-tectonic events and rockfall signals that were captured on one reliable seismic station (SVB) located 9 km from the volcano. The maximum daily count was 11 events on 16 November. After 23 December a total of eight events were detected before seismicity briefly subsided.

Figure 5. Daily count of volcanic earthquakes recorded at Soufrière St. Vincent during 1 January 2019 through February 2021. Increased seismicity was detected during June-July 2019 and mid-October 2020 through February 2021. An installation of station SVV on 6 January 2021 at Wallibou is annotated on this graph. Data courtesy of UWI-SRC.

Activity during December 2020. Staff members of the Soufrière Monitoring Unit (SMU) made visual observations of the crater on 16 December and reported minor changes in fumarolic activity and a small lake on the E side of the crater floor. On 27 December UWI-SRC and NEMO reported that an effusive eruption had begun, which was characterized by a new lava dome in the main crater on the SW perimeter of the 1979 dome (figures 6 and 7). A thermal hotspot in the crater was also detected that day using satellite data by NASA FIRMS. As a result, the Volcanic Alert Level (VAL) was raised to Orange (the second highest level on a four-color scale) on 29 December (figure 8). The Volcano Ready Communities Project, a collaboration between NEMO SVG and UWI Seismic Research Centre, distributed their volcano hazard map for the surrounding communities, in preparation for a potential evacuation (figure 9).

Figure 6. Photo of the first documented observation of the new lava dome at Soufrière St. Vincent on 27 December 2020 taken from the E side of the summit. Courtesy of Melanie Grant, IG, UWI-SRC.

Figure 7. Photo of an early observation of the new lava dome at Soufrière St. Vincent on 29 December 2020 growing WSW of the 1979 lava dome on the SW edge of the summit crater, accompanied by gas-and-steam emissions. The dome was estimated to be 60 m high on 30 December. Courtesy of Kemron Alexander (color corrected), SMU, UWI-SRC.

Figure 8. Volcanic Hazard Alert Level System for Soufriere St. Vincent. Courtesy of UWI-SRC.

Figure 9. Volcanic hazard map for Soufrière St. Vincent, showing different areas that are likely to experience hazardous volcanic events which would require evacuations. The hazard map is divided into four zones: Zone 1 (Red), which is a very high hazard location; Zone 2 (Orange), which is a high hazard location; Zone 3 (Yellow), which is a moderate hazard location; and Zone 4 (Green), which is a low hazard location. This poster was created prior to the current eruption as part of the Volcano Ready Communities Project, a collaboration between NEMO SVG and UWI Seismic Research Centre. Courtesy of UWI-SRC and NEMO.

Activity during January-February 2021. Observations made during a field visit on 5 January, during a helicopter overflight on 6 January, and based on 9 January drone video noted that the new dome was expanding to the W on the WSW edge of the 1979 lava dome and continued to gradually grow through February 2021 (figure 10). Growth of the 2020/21 lava dome produced small, hot rockfalls and gas-and-steam emissions that were visible from the Belmont Observatory. The gas emissions were most notable from a small depression at the top of the dome. Two seismic stations were installed on the flank of the volcano at Wallibou (SVV) and at the summit (SSVA) on 6 and 18 January, respectively.

Figure 10. Map showing the growth of the new 2020/21 lava dome at Soufrière St. Vincent from 27 December 2020 to 12 February 2021. The dome is located on the SW edge of the crater rim and WSW of the 1979 lava dome that is covered in vegetation. Courtesy of UWI-SRC.

Seismic stations recorded 573 events through 0730 on 30 January; this number continued to grow into February (up to 703 events by 0830 on 4 February) (figure 5). Observations on 14 January showed that the dome was growing taller and expanding to the E and W. An overflight on 15 January showed extensive vegetation damage on the E, S, and W inner crater walls; damage previously noted on the upper SW crater rim had expanded downslope (figure 11). Scientists visited on 16 January and recorded temperatures of 590°C at the dome surface (figure 12). During 15-17 January residents to the W of the volcano reported nighttime crater incandescence. Persistent gas-and-steam emissions were observed rising above the dome, as well as from the contact between the 2020/21 and 1979 domes during the rest of the month and through February.

Figure 11. Oblique aerial view of the lava dome at Soufrière St. Vincent between the 1979 dome and the SW crater rim on 15 January 2021, accompanied by gas-and-steam emissions. On this day, the dome was 340 m long, 160 m wide, and 80 m high. Courtesy of Adam Stinton, MVO, UWI-SRC.

Figure 12. Thermal measurements were taken at the base of the freshly extruded lava dome at Soufrière St. Vincent on 16 January 2021. Top: Photo (color corrected) of the base of the new lava dome. Bottom: Thermal FLIR (Forward-Looking InfraRed) image of the base of the new lava dome showing a maximum temperature of 590.8°C. Courtesy of Adam Stinton, MVO, UWI-SRC.

Sulfur dioxide emissions were first detected on 1 February using a Multi-Gas Instrument and a filter pack; the dome had reached an estimated volume of 5.93 million cubic meters. Vegetation on the NW part of the crater (N of the dome) was damaged, likely due to fire. The dome continued to expand laterally to the N and S, according to reports issued on 6 and 8 February. After that it grew about 15 m to the NW and SE, according to 11 and 15 February reports (figure 13). NEMO reported that the growth rate of the lava dome ranged from 1.9 to 2.13 m³/s (figure 14). Active gas-and-steam emissions originated dominantly at contact areas between the pre-existing 1979 dome and the 2020/21 dome, as well as at the top of the new dome.

Figure 13. Photo of the 2020/21 lava dome (dark mass at left) at Soufrière St. Vincent on 12 February 2021 showing continuous gas-and-steam emissions and damaged vegetation on the 1979 lava dome (right). On this day, the dome was 618 m long, 232 m wide, 90 m high, and an estimated volume of 6.83 million cubic meters. Courtesy of Kemron Alexander, SMU, UWI-SRC.

Figure 14. Estimated lava extrusion rates and added volume of material at Soufrière St. Vincent’s 2020/21 lava dome during 27 December 2020 through 3 February 2021. Calculations were based on UAV photography and photogrammetry. Data courtesy of UWI-SRC.

Thermal satellite data. MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows the beginning of thermal activity in late December 2020 and continuing at a lower power into early February (figure 15). A single MODVOLC thermal alert was detected on 29 December. This activity marks the beginning of the effusive eruption and the formation of the new lava dome. Sentinel-2 thermal satellite imagery detected a thermal anomaly on the SW side of the main crater during clear weather days in January 2021, which represents the active 2020/21 lava dome (figure 16). Fresh, hot material is also visible surrounding the thermal anomaly, which demonstrates the growth of the lava dome over time.

Figure 15. Thermal activity at Soufrière St. Vincent was detected beginning in late December 2020 and continued through early February 2021, as reflected in the MIROVA data (Log Radiative Power). The power of the thermal anomalies had slightly decreased after December. Courtesy of MIROVA.

Figure 16. Sentinel-2 thermal satellite imagery showing a persistent thermal anomaly (bright yellow-orange) in Soufrière St. Vincent’s growing lava dome on the WSW edge of the main crater during 3 January through 28 January 2021. The dark black color is the freshly cooled material from the effusive activity, which also demonstrates the increasing size of the lava dome. Images using “Atmospheric penetration” rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Field work during mid-January 2021. SRC collected rock samples from the new lava dome and sent them to scientists from the University of East Anglia, University of Plymouth, and University of Oxford on 16 January 2021 as a collaborative project to analyze their composition and compare them with the composition of rocks erupted in 1902, 1971, and 1979. Analyses showed that the new 2020/21 lava dome was basaltic andesite, similar in composition to the earlier domes (figure 17).

Figure 17. Backscattered electron image of a sample from the 2020/21 lava dome showing groundmass texture. Low-contrast dark gray crystals are feldspar microlites in glass (darkest gray). Some of the larger feldspar crystals have Ca-rich cores (paler gray). Clinopyroxenes also make up the groundmass (brighter gray) and some are breaking down to Fe-oxides (small oxides at edges of clinopyroxene bottom center and bottom right). In some areas dark glass is devitrifying (paler gray irregular shapes within dark gray glassy patches). Fe-Ti oxides are also common (bright white crystals). Total image width is about 0.3 mm. Image and description courtesy of Bridie Davies, UEA.

References: Cole P D, Robertson R E A, Fedele L, Scarpati C, 2019. Explosive activity of the last 1000 years at La Soufrière, St Vincent, Lesser Antilles. J. Volcanol. Geotherm. Res., 371:86-100.

Shepherd, J. B., Aspinall, W. P., Rowley, K. C., Pereira, J., Sigurdsson, H., Fiske, R. S., Tomblin, J. F., 1979. The eruption of Soufrière volcano, St Vincent April–June 1979. Nature, 282 (5734), 24–28. doi:10.1038/282024a0.

Geologic Background. Soufrière St. Vincent is the northernmost and youngest volcano on St. Vincent Island. The NE rim of the 1.6-km wide summit crater is cut by a crater formed in 1812. The crater itself lies on the SW margin of a larger 2.2-km-wide caldera, which is breached widely to the SW as a result of slope failure. Frequent explosive eruptions after about 4,300 years ago produced pyroclastic deposits of the Yellow Tephra Formation, which cover much of the island. The first historical eruption took place in 1718; it and the 1812 eruption produced major explosions. Much of the northern end of the island was devastated by a major eruption in 1902 that coincided with the catastrophic Mont Pelée eruption on Martinique. A lava dome was emplaced in the summit crater in 1971 during a strictly effusive eruption, forming an island within a lake that filled the crater. A series of explosive eruptions in 1979 destroyed the 1971 dome and ejected the lake; a new dome was then built.

Information Contacts: University of the West Indies Seismic Research Centre (UWI-SRC), University of the West Indies, St. Augustine, Trinidad & Tobago, West Indies (URL: http://www.uwiseismic.com/); National Emergency Management Organisation (NEMO), Government of Saint Vincent and the Grenadines, Biseé, PO. Box 1517, Castries, Saint Lucia, West Indies (URL: http://nemo.gov.lc/); 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); Google Earth (URL: https://www.google.com/earth/); Bridie Davies, University of East Anglia, Norwich Research Park, Norwich, Norfolk, NR4 7TJ, UK (URL: https://people.uea.ac.uk/bridie_davies).

Erta Ale, located in Ethiopia, is a highly active volcano that contains a 0.7 x 1.6 km, elliptical summit caldera with multiple pit craters that frequently host active lava lakes. Another larger 1.8 x 3.1 km wide depression SE of the summit is bounded by curvilinear fault scarps on the SE side. Recent activity has been characterized by lava flow outbreaks (BGVN 45:05) and thermal anomalies detected from pit craters in the summit caldera (BGVN 45:05 and 45:10). This report covers activity from October 2020 through February 2021 and is characterized by a brief period of strong thermal anomalies in late November, which sharply declined in December. Information primarily comes from satellite data.

Activity at Erta Ale had gradually decreased compared to previous months; thermal activity during this reporting period remained primarily in the N summit caldera. MIROVA (Middle Infrared Observation of Volcanic Activity) analysis of MODIS satellite data shows a total of four low-power thermal anomalies from October through most of November. At the end of November, a brief surge of strong thermal activity was detected in the S pit crater of the summit caldera, followed by a sharp decrease the following days (figure 102). Similarly, the MODVOLC system detected a total of eight thermal alerts; two were detected on 29 November and six were detected on 30 November, primarily focused in the summit caldera. Only two thermal anomalies were recorded in the MIROVA graph after this surge of activity; one in mid-December and one in early January. Thermal data from NASA VIIRS detected hotspots on 28-30 November, 1-3 December, and 8 December.

Figure 102. A total of four low-power thermal anomalies were recorded at Erta Ale during October through most of November 2020. Beginning in late November into early December a strong but brief surge of thermal activity was detected according to the MIROVA system (Log Radiative Power). Only two low-power thermal anomalies were recorded after the activity in early December; one in mid-December and one in early January 2021. Courtesy of MIROVA.

According to Sentinel-2 thermal satellite images, a weak thermal anomaly was first visible on 20 October in the summit caldera. Intermittent, weak anomalies were also detected in the summit caldera on 25 and 30 October and 4, 9, 19, and 24 November. On 29 November the thermal activity increased significantly, detected as a strong hotspot in the S pit crater of the summit caldera (figure 103). This brief increase in power was also recorded in the MIROVA graph and by the MODVOLC thermal algorithm. By 4 December the size and power of this thermal activity decreased significantly, though it was still visible in the summit caldera. Thermal activity was no longer observed after 4 December until clear weather days on 2 and 12 February when a faint anomaly was detected.

Figure 103. Sentinel-2 thermal satellite images of Erta Ale during 30 October 2020 to 12 February 2021 showing a single thermal anomaly (bright yellow-orange) in the S pit crater of the summit caldera that varies in strength. Top left: 30 October 2020 shows a faint thermal anomaly in the S pit crater. Top right: 29 November 2020 shows the strongest thermal anomaly in the S pit crater during the reporting period and is also reflected in the MIROVA graph and detected by the MODVOLC system. Bottom left: 4 December 2020 shows that the thermal anomaly from activity in late November remains hot but begins to decrease in strength. Bottom right: 12 February 2021 again shows thermal activity from the S pit but weaker than the previous November and December. Sentinel-2 images with “Atmospheric penetration” (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

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

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

Bagana is a remote volcano located in central Bougainville Island in Papua New Guinea with eruptions dating back to 1842. The current eruption period began in February 2000, with more recent activity characterized by thermal anomalies along with gas-and-steam and ash plumes (BGVN 44:12 and 45:07). Typical activity consists of episodes of lava flows and intermittent strong passive degassing, especially sulfur dioxide. This report covers activity from May-December 2020 using primarily thermal data and satellite imagery.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed a cluster of intermittent low-power thermal anomalies during June through early August, followed by a period of quiescence during August to mid-October, with the exception of two anomalies detected in early September (figure 44). Thermal activity slightly increased again by mid-October and continued infrequently through December at low levels. This pattern of thermal activity is also reflected in three Sentinel-2 thermal satellite images that showed faint, roughly linear, thermal anomalies, indicative of lava flows trending NE and NW on 21 June, NE on 1 July, and W on 23 November (figure 45). On clear weather days, gas-and-steam emissions could be seen in satellite imagery on 30 August, 4 October, and 23 November, each of which drifted W (figure 45). Gas-and-steam emissions on 13 December drifted E.

Figure 44. Intermittent low-power thermal anomalies were detected at Bagana during late May-December 2020 as recorded by the MIROVA system (Log Radiative Power). Relatively higher power and frequency anomalies were detected during June-early August. Thermal activity declined after early August into mid-October, with the exception of two thermal anomalies in early September. Activity increased again slightly by mid-October and continued through December, but at a lower power and frequency. Courtesy of MIROVA.

Figure 45. Sentinel-2 thermal satellite imagery showing weak thermal anomalies at Bagana during June through December 2020. Top left: Faint, linear thermal anomalies on 21 June 2020 on the NE and NW flanks, which could represent lava effusion, though clouds covered much of the area. Top right: Hot material traveling down the NE flank on 1 July 2020. Middle left and right: Gas-and-steam emissions rising from the summit crater and drifting W on 30 August and 4 October 2020; very faint thermal anomalies can be observed in the crater. Bottom left: Gas-and-steam emissions in the summit crater drifted W on 23 November 2020, and a probable lava flow is visible extending down the NW flank. Bottom right: Gas-and-steam emissions rose above the summit crater on 13 December 2020 and drifted E. Sentinel-2 images with "Atmospheric penetration" (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Geologic Background. Bagana volcano, occupying a remote portion of central Bougainville Island, is one of Melanesia's youngest and most active volcanoes. This massive symmetrical cone was largely constructed by an accumulation of viscous andesitic lava flows. The entire edifice could have been constructed in about 300 years at its present rate of lava production. Eruptive activity is frequent and characterized by non-explosive effusion of viscous lava that maintains a small lava dome in the summit crater, although explosive activity occasionally producing pyroclastic flows also occurs. Lava flows form dramatic, freshly preserved tongue-shaped lobes up to 50 m thick with prominent levees that descend the flanks on all sides.

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).

Kadovar is located in the Bismark Sea offshore from the mainland of Papua New Guinea about 25 km NNE from the mouth of the Sepik River. Its first confirmed eruption began in early January 2018, characterized by ash plumes and a lava extrusion that resulted in the evacuation of around 600 residents from the N side of the island (BGVN 43:03). Activity has recently consisted of intermittent ash plumes, gas-and-steam plumes, and thermal anomalies (BGVN 45:07). Similar activity continued during this reporting period of July-December 2020 using information from the Rabaul Volcano Observatory (RVO), the Darwin Volcanic Ash Advisory Center (VAAC), and various satellite data.

RVO issued an information bulletin on 15 July reporting minor eruptive activity during 1-5 July with moderate light-gray ash emissions rising a few hundred meters above the Main Crater. On 5 July activity intensified; explosions recorded at 1652 and 1815 generated a dense dark gray ash plume that rose 1 km above the crater and drifted W. Activity subsided that day, though fluctuating summit crater incandescence was visible at night. Activity increased again during 8-10 July, characterized by explosions detected on 8 July at 2045, on 9 July at 1145 and 1400, and on 10 July at 0950 and 1125, each of which produced a dark gray ash plume that rose 1 km above the crater. According to Darwin VAAC advisories issued on 10, 16, and 30 July ash plumes were observed rising to 1.5-1.8 km altitude and drifting NW.

Gas-and-steam emissions and occasional ash plumes were observed in Sentinel-2 satellite imagery on clear weather days during August through December (figure 56). Ash plumes rose to 1.2 and 1.5 km altitude on 3 and 16 August, respectively, and drifted NW, according to Darwin VAAC advisories. On 26 August an ash plume rose to 2.1 km altitude and drifted WNW before dissipating within 1-2 hours. Similar activity was reported during September-November, according to several Darwin VAAC reports; ash plumes rose to 0.9-2.1 km altitude and drifted mainly NW. VAAC notices were issued on 12 and 22 September, 4, 7-8, and 18 October, and 18 November. A single MODVOLC alert was issued on 27 November.

Figure 56. Sentinel-2 satellite data showing a consistent gas-and-steam plume originating from the summit of Kadovar during August-December 2020 and drifting NW. On 21 September (top right) a gray plume was seen drifting several kilometers from the island to the NW. Images with “Natural color” (bands 4, 3, 2) rendering; courtesy of Sentinel Hub Playground.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows intermittent low-power anomalies during July through December 2020 (figure 57). Some of this thermal activity in the summit crater was observed in Sentinel-2 thermal satellite imagery, accompanied by gas-and-steam emissions that drifted primarily NW (figure 58).

Figure 57. Intermittent low-power thermal anomalies at Kadovar were detected in the MIROVA graph (Log Radiative Power) during July through December 2020. The island location is mislocated in the MIROVA system by about 5.5 km SE due to older mis-registered imagery; the anomalies are all on the island. Courtesy of MIROVA.

Figure 58. Sentinel-2 satellite data showing thermal anomalies at the summit of Kadovar on 23 July (top left), 7 August (top right), 1 September (bottom left), and 21 September (bottom right) 2020, occasionally accompanied by a gas-and-steam plume drifting dominantly NW. Two thermal anomalies were visible on the E rim of the summit crater on 23 July (top left) and 7 August (top right). Images with “Atmospheric penetration” (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Geologic Background. The 2-km-wide island of Kadovar is the emergent summit of a Bismarck Sea stratovolcano of Holocene age. It is part of the Schouten Islands, and lies off the coast of New Guinea, about 25 km N of the mouth of the Sepik River. Prior to an eruption that began in 2018, a lava dome formed the high point of the andesitic volcano, filling an arcuate landslide scarp open to the south; submarine debris-avalanche deposits occur in that direction. Thick lava flows with columnar jointing forms low cliffs along the coast. The youthful island lacks fringing or offshore reefs. A period of heightened thermal phenomena took place in 1976. An eruption began in January 2018 that included lava effusion from vents at the summit and at the E coast.

Information Contacts: Rabaul Volcano Observatory (RVO), Geohazards Management Division, Department of Mineral Policy and Geohazards Management (DMPGM), PO Box 3386, Kokopo, East New Britain Province, Papua New Guinea; Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); 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, one of Antarctica's most well known volcanoes, contains a 7-km-wide caldera flooded by the sea. Deception Island is located at the SW end of the Shetland Islands, NE of Graham Land Peninsula, and was constructed along the axis of the Bransfield Rift spreading center. A narrow passageway named Neptunes Bellows provides entrance to a natural harbor that was utilized as an Antarctic whaling station. Numerous vents located along ring fractures circling the low, 14-km-wide island have been active during historical time. Maars line the shores of 190-m-deep Port Foster, the caldera bay. Among the largest of these maars is 1-km-wide Whalers Bay, at the entrance to the harbor. Eruptions from Deception Island during the past 8700 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, Sicily's second largest city, has one of the world's longest documented records of historical 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 horseshoe-shaped 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. Beautifully symmetrical Mayon, which rises above the Albay Gulf NW of Legazpi City, is the Philippines' most active volcano. The structurally simple edifice has steep upper slopes averaging 35-40 degrees that are capped by a small summit crater. Historical eruptions date back to 1616 and 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 devastated 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. Niuafo'ou is an isolated volcanic island in the north central Lau Basin about 170 km west 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 caldera lake, named Big Lake (or Vai Lahi), which contains several small islands and pyroclastic cones on its NE shore. Historical eruptions, mostly from circumferential fissures on the west-to-south side of the island, have been recorded since 1814 and have often damaged villages on this small ring-shaped island. A major eruption at Niuafo'ou in 1946 forced evacuation of most of its 1200 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.