Klyuchevskoy, located in northern Kamchatka, has had historical eruptions dating back 3,000 years characterized by major explosive and effusive eruptions from the flank craters. The current eruption began in April 2019 and has recently consisted of Strombolian activity, ash plumes, and an active lava flow descending the SE flank (BGVN 45:09). This report covers September-December 2020 and describes similar activity of Strombolian explosions, ash plumes, and active lava flows beginning in early October. Information primarily comes from weekly and daily reports from the Kamchatkan Volcanic Eruption Response Team (KVERT), the Tokyo Volcanic Ash Advisory (VAAC), and satellite data.

Activity from July through September was relatively low, with no thermal activity detected during August-September. On 2 October renewed Strombolian explosions began at 1003, ejecting ash 300-400 m above the summit and producing gas-and-steam plumes with some ash that drifted down the E flank (figure 48). That night, crater incandescence was visible. On 5 October KVERT reported that a lava flow began to effuse along the Apakhonchich chute at 0100. During 7-8 October activity intensified and was characterized by strong explosions, collapses of the sides of the drainage, strong thermal anomalies, and ash plumes that extended over 200 km SE from the crater; the lava flow remained active and continued to descend the SE flank. A Tokyo VAAC advisory issued on 7 October reported that an ash plume rose to 8.8 km altitude and drifted E and SE; during 8-9 October ash plumes rose to 5.5 km altitude and drifted as far as 270 km SE. A strong, bright, thermal anomaly was observed daily in satellite imagery, which represented the new lava flow. Strombolian explosions continued throughout the month, accompanied by gas-and-steam plumes containing some ash and an active lava flow advancing down the Apakhonchich chute on the SE flank (figure 49).

Figure 48. Photos of a gray ash plume (left) and the beginning of the lava flow (right), represented as summit crater incandescence at Klyuchevskoy on 2 October 2020 at 1030 and 2100, respectively. Photos by Y. Demyanchuk; courtesy of Volkstat.

Figure 49. Photo of Strombolian explosions at the summit of Klyuchevskoy accompanied by ash emissions and a lava flow advancing down the SE-flank Apakhonchich chute on 25 October 2020. Photo by Y. Demyanchuk (color corrected); courtesy of Volkstat.

Similar activity continued to be reported in November, consisting of Strombolian explosions, ash plumes, and a lava flow advancing down the SE flank. A bright thermal anomaly was observed in thermal satellite imagery each day during the month. During 16-19 November explosions recorded in satellite and video data showed ash plumes rising to 7.5 km altitude and drifting as far as 108 km to the NE, E, SE, and S (figure 50). On 19 November an ash cloud 65 x 70 km in size drifted 50 km SE, according to a KVERT VONA (Volcano Observatory Notice for Aviation). During 26-30 November video and satellite data showed that gas-and-steam plumes containing some ash rose to 7 km altitude and extended as far as 300 km NW and E, accompanied by persistent moderate explosive-effusive activity (figure 51).

Figure 50. Photo of the Strombolian and Vulcanian explosions at Klyuchevskoy on 18 November 2020 which produced a dense gray ash plume. Photo by Yu. Demyanchuk, IVS FEB RAS, KVERT

Figure 51. Photo of the summit of Klyuchevskoy (right foreground) showing incandescent Strombolian explosions, the lava flow descending the Apakhonchich chute on the SE flank, and a gray ash plume on 29 November 2020. Kamen volcano is the cone at back left. Photo by Y. Demyanchuk (color corrected); courtesy of Volkstat.

Moderate explosive-effusive activity continued through December; a strong daily thermal anomaly was visible in satellite images. During 2-3 December gas-and-steam plumes containing some ash rose to 7 km altitude and extended 300 km NW and E. Intermittent gas-and-ash plumes continued through the month. On 7 December KVERT reported that a new lava flow began to advance down the Kozyrevsky chute on the S flank, while the flow on the SE flank continued. Strombolian explosions in the crater ejected incandescent material up to 300 m above the crater on 8 December while hot material was deposited and traveled 350 m below the crater. A cinder cone was observed growing in the summit crater and measured 75 m tall.

Strombolian and Vulcanian activity continued during 11-25 December, accompanied by the lava flow on the S flank; according to Sentinel-2 thermal satellite images, the effusion on the SE flank had stopped around 13 December and had begun to cool. The lava flow in the Kozyrevsky chute spalled off incandescent material that continued to travel an additional 350 m. Gas-and-steam plumes that contained some ash rose to 6 km altitude and drifted up to 350 km generally E. On 24 December the Kamchatka Volcanological Station field team visited Klyuchevskoy to do work on the field stations. The scientists observed explosions that ejected incandescent material 300 m above the crater and the S-flank lava flow (figure 52). On 28 December KVERT reported that the moderate explosive-effusive eruption continued, but the intensity of the explosions had significantly decreased. The lava flow on the S flank continued to effuse, but its flow rate had already decreased.

Figure 52. Photos of a dense ash plume (left) and a color corrected photo of the lava flow advancing on the S flank (right) of Klyuchevskoy on 24 December 2020, accompanied by incandescent Strombolian explosions and a gray ash plume. Photos by Y. Demyanchuk; courtesy of Volkstat.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows frequent and strong thermal activity beginning in early October and continuing through December 2020, which is represented by the active lava flows reported in the summit crater (figure 53). According to the MODVOLC thermal algorithm, a total of 615 thermal alerts were detected at or near the summit crater from 1 October to 31 December; none were reported in September. Sentinel-2 thermal satellite imagery frequently showed the progression of the active lava flows as a strong thermal anomaly descending the SE flank during October through late November and the SW flank during December, sometimes even through weather clouds (figure 54). The thermal anomalies were commonly accompanied by a gas-and-steam plume that drifted mainly E and NE. A total of 164 VAAC advisories were issued from 2 October through 31 December.

Figure 53. Strong and frequent thermal anomalies were detected in early October at Klyuchevskoy and continued through December 2020, as recorded by the MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 54. Sentinel-2 thermal satellite images showing the progression of two lava flows (bright yellow-orange) originating from the summit crater at Klyuchevskoy from 4 October through December 2020. Crater incandescence was visible on 4 October (top left), which marked the beginning of the lava flow. By 31 October (top right) the active flow had traveled down the Apakhonchich chute on the SE flank, accompanied by a gas-and-steam plume that drifted NE. On 10 November (bottom left) the lava flow continued down the SE flank; the darker black color represents parts of the lava flow that began to cool. The gas-and-steam plume drifted E from the summit. On 25 December (bottom right) a new lava flow was observed descending the SW flank, also accompanied by a strong gas-and-steam plume. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Geologic Background. Klyuchevskoy (also spelled Kliuchevskoi) is Kamchatka's highest and most active volcano. Since its origin about 6000 years ago, the beautifully symmetrical, 4835-m-high basaltic stratovolcano has produced frequent moderate-volume explosive and effusive eruptions without major periods of inactivity. It rises above a saddle NE of sharp-peaked Kamen volcano and lies SE of the broad Ushkovsky massif. More than 100 flank eruptions have occurred during the past roughly 3000 years, with most lateral craters and cones occurring along radial fissures between the unconfined NE-to-SE flanks of the conical volcano between 500 m and 3600 m elevation. The morphology of the 700-m-wide summit crater has been frequently modified by historical eruptions, which have been recorded since the late-17th century. Historical eruptions have originated primarily from the summit crater, but have also included numerous major explosive and effusive eruptions from flank craters.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); Kamchatka Volcanological Station, Klyuchi, Kamchatka Krai, Russia (URL: http://volkstat.ru/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

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).

Tinakula is located 100 km NE of the Solomon Trench at the N end of the Santa Cruz. The current eruption began in December 2018 and has recently been characterized by intermittent small thermal anomalies and gas-and-steam plumes (BGVN 45:07), which continued into the current reporting period of July-December 2020. Information primarily comes from various satellite data, as ground observations are rarely available.

Infrared MODIS satellite data processed by MIROVA (Middle InfraRed Observation of Volcanic Activity) showed a total of ten low-power thermal anomalies during July through December; one anomaly was detected in early July, two in late August, three in November, and four in December (figure 44). A single MODVOLC alert was issued on 16 December, which was visible in Sentinel-2 thermal satellite imagery on 17 December (figure 45). Though clouds often obscured the view of the summit crater, Sentinel-2 satellite imagery showed intermittent dense gas-and-steam plumes rising from the summit that drifted in different directions (figure 45).

Figure 44. Low-power thermal anomalies at Tinakula were detected intermittently during April-December 2020 by the MIROVA system (Log Radiative Power). Courtesy of MIROVA.

Figure 45. Sentinel-2 satellite imagery shows ongoing gas-and-steam plumes rising from Tinakula during July-December 2020. A small thermal anomaly (bright yellow-orange) is visible on 17 December (bottom right) using “Atmospheric penetration” (bands 12, 11, 8a) rendering. All other images using “Natural color” rendering (bands 4, 3, 2); courtesy of Sentinel Hub Playground.

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

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

Erebus, located on Ross Island, Antarctica, and overlooking the McMurdo research station, is the southernmost active volcano in the world. The stratovolcano, which frequently has active lava lakes in its 250-m wide summit crater, is primarily monitored by satellite.

Thermal activity during 2020 was at lower levels than in recent years. The total number of thermal pixels, as recorded by MODIS thermal emission instruments aboard NASA’s Aqua and Terra satellites, was 76 (table 6), similar to low totals recorded in 2000 and 2015.

Table 6. Number of monthly MODIS-MODVOLC thermal alert pixels recorded at Erebus during 2017-2020. See BGVN 42:06 for data from 2000 through 2016. The table was compiled using data provided by the Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System.

Sentinel-2 satellite images showed two lava lakes, with one diminishing in size during the year (figure 29). Occasionally a gas plume could be observed. The volcano was frequently covered by atmospheric clouds on days when the satellite passed over.

Figure 29. Infrared Sentinel-2 thermal images of the summit crater area of Erebus in 2020. Left: Image on 28 February 2020 showing two lava lakes in the summit crater. Right: Image on 4 October 2020 showing a single primary lake, with a much diminished second lake immediately SW. The main crater is 500 x 600 m wide. Both images are using the Atmospheric Penetration filter (bands 12, 11, 8A). Courtesy of Sentinel Hub Playground.

Geologic Background. Mount Erebus, the world's southernmost historically active volcano, overlooks the McMurdo research station on Ross Island. It is the largest of three major volcanoes forming the crudely triangular Ross Island. The summit of the dominantly phonolitic volcano has been modified by one or two generations of caldera formation. A summit plateau at about 3,200 m elevation marks the rim of the youngest caldera, which formed during the late-Pleistocene and within which the modern cone was constructed. An elliptical 500 x 600 m wide, 110-m-deep crater truncates the summit and contains an active lava lake within a 250-m-wide, 100-m-deep inner crater; other lava lakes are sometimes present. The glacier-covered volcano was erupting when first sighted by Captain James Ross in 1841. Continuous lava-lake activity with minor explosions, punctuated by occasional larger Strombolian explosions that eject bombs onto the crater rim, has been documented since 1972, but has probably been occurring for much of the volcano's recent history.

Information Contacts: 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).

Sakurajima is the active volcano within the Aira Caldera in Kyushu, Japan. With several craters historically active, the current activity is concentrated in the Minamidake summit crater. Activity usually consists of small explosions producing ashfall and ballistic ejecta, with occasional pyroclastic flows and lahars. The current eruption has been ongoing since 25 March 2017, but activity has been frequent over the past few hundred years. This bulletin summarizes activity that occurred during July through December 2020 and is largely based on reports by the Japan Meteorological Agency (JMA) and satellite data. The Alert Level remains at 3 on a 5-level scale. There was no activity at the Showa crater in 2020.

The number of recorded explosive and ash eruptions for 2020 at the Minamidake crater were 221 and 432, respectively (228 and 393 the previous year). Activity declined in July and remained low through the end of December. There was ash reported on 79 days of the year, most frequently in January, and only 26 of those days during August-December (table 24 and figure 104). The largest ash plumes during this time reached 5 km at 0538 on 9 August, 3 km at 1959 on 17 December, and 3.5 km at 1614 on 29 December. The decline in events was reflected in thermal data, with a decline in energy detected during June through October (figure 105). Recorded SO₂ was generally high in the first half of the year then began to decrease from April to around 1,000 tons/day until around late May. Emissions increased after August and were extremely high in October. There were no notable changes in the geothermal areas around the craters.

Table 24. Number of monthly total eruptions, explosive eruptions, days of ashfall, and ashfall amounts from Sakurajima's Minamidake crater at Aira during 2020. Note that smaller events that did not reach the threshold of explosions or eruptions also occurred. Ashfall was measured at Kagoshima Local Meteorological Observatory; ash weights are rounded down to the nearest 0.5 g/m² and zero values indicate that less than this amount was recorded. Data courtesy of JMA.

Figure 104. The total calculated observed ash erupted from Aira's Sakurajima volcano. Top: Annual values from January 1980 to November 2020. Bottom: the monthly values during January 2009 through November 2020. Courtesy of JMA (January 2021 Sakurajima monthly report).

Figure 105. Thermal data detected at Aira's Sakurajima volcano during February through December 2020 by the MIROVA thermal detection system that uses MODIS satellite middle infrared data. There was a decline in activity during June-September, with energy emitted in November-December remaining lower than earlier in the year. Courtesy of MIROVA.

During July "very small" explosions were observed on the 1st, 2nd, and 8th, with the last explosion producing a plume up to 600 m above the crater. These events didn't generate enough of an ash plume to be counted as either a quiet or explosive eruption, leaving no eruptions reported during July. No incandescence was observed at the crater since 3 June. Field surveys on 2, 13, and 21 July detected 600 to 1,300 tons of SO₂ per day.

An explosion occurred at 0538 on 9 August, producing an ash plume to 5 km above the crater, dispersing NE (figure 106). This was the largest explosion observed through the Sakurajima surveillance camera since 8 November 2019. Ashfall was reported in Kagoshima City, Aira City, Kirishima City, Yusui Town, and parts of Miyazaki and Kumamoto Prefectures. Ashfall measured to be 300 g/m² in Shirahama on Sakurajima island (figure 106). No ballistic ejecta were observed due to clouds at the summit, but very small explosions were occasionally observed afterwards.

Figure 106. An explosion at Aira's Sakurajima volcano at 0538 on 9 August 2020 (top, taken from the Ushine surveillance camera in Kagoshima) produced ashfall in Shirahama on Sakurajima (bottom). The plume contains a white steam-rich portion on the left, and a darker relatively ash-rich portion on the right. Images courtesy of JMA (Sakurajima August 2020 monthly report).

A small lake or pond in the eastern Minamidake crater was first observed in PlanetScope satellite imagery on 1 August (through light cloud cover) and intermittently observed when the summit was clear through to the 22nd (figure 107). The summit is obscured by cloud cover in many images before this date. An observation flight on 14 August confirmed weak gas emission from the inner southern wall of the Showa crater, and a 200-m-high gas plume rose from the Minamidake crater, dispersing SE (figure 108). Thermal imaging showed elevated temperatures within the crater. SO₂ measurements were conducted during field surveys on the 3rd, 13th, 24th and 31st, with amounts similar to July at 600 to 1,400 tons per day.

Figure 107. A crater lake is visible in the eastern part of the Minamidake summit crater at Aira's Sakurajima volcano on 5, 18, and 22 August 2020. Four-band PlanetScope satellite images courtesy of Planet Labs.

Figure 108. Gas emissions from the Minamidake and Showa craters at Sakurajima in the Aira caldera on 14 August 2020. Photos taken from the from Kagoshima Prefecture disaster prevention helicopter at 1510-1513. Courtesy of JMA (Sakurajima August monthly report).

Activity continued at Minamidake crater throughout September with seven observed eruptions sending plumes up to 1.7 km above the crater, and additional smaller events (figure 109). An ash plume reached 1 km at 0810 on the 15th. Ashfall was reported on four days through the month with a total of 2 g/m² measured. Incandescence was observed in nighttime surveillance cameras from the 9-10th for the first time since 2 June, then continued through the month. There was an increase in detected SO₂, with measurements on the 11th and 25th ranging from 1,300 to 2,000 tons per day.

Figure 109. Examples of activity at Aira's Sakurajima volcano on 4, 10, and 14 September 2020. The images show an ash plume reaching 1.7 km above the crater (top left), a gas-and-steam plume (bottom left), and incandescence at night visible in a gas-and steam plume (right). Images courtesy of JMA (September 2020 Sakurajima monthly report).

During October two eruptions and occasional smaller events occurred at the Minamidake crater and there were six days where ashfall occurred at the Kagoshima Local Meteorology Observatory (including remobilized ash). An ash plume rose to 1.7 km above the crater at 1635 on the 3rd and 1 km on the 30th. Incandescence was observed at night through the month (figure 110). Gas surveys on the 20th, 21st, 23rd, and 26th recorded 2,200-6,600 tons of SO₂ per day, which are high to very high levels and a large increase compared to previous months. An observation flight on the 13th confirmed lava in the bottom of the Minamidake crater (figure 111). Gas emissions were rising to 300 m above the Minamidake crater, but no emissions were observed at the Showa crater (figure 112).

Figure 110. Gas emissions and incandescence seen above the Sakurajima Minamidake crater at Aira on 10 and 23 October 2020. Courtesy of JMA (Sakurajima October 2020 monthly report).

Figure 111. Lava was observed on the floor of the Minamidake summit crater at Aira's Sakurajima volcano on 13 October 2020, indicated by the yellow dashed line. Courtesy of JMA (Sakurajima October 2020 monthly report).

Figure 112. An observation flight on 13 October 2020 noted gas emissions up to 300 m above the Minamidake crater at Sakurajima, but no emissions from the Showa crater. Courtesy of JMA (Sakurajima October 2020 monthly report).

Eight ash eruptions and six explosive eruptions occurred during November as well as additional very small events. At 1551 on the 3rd an ash plume reached 1.8 km above the crater and an event at 1335 on the 10th produced large ballistic ejecta out to 600-900 m from the crater (figure 113). Ashfall was reported on 11 days this month (including remobilized ash). Incandescence was observed at night and elevated temperatures in the Minamidake crater were detected by satellites (figure 114). Detected SO₂ was lower this month, with amounts ranging between 1,300 and 2,200 on the 9th, 18th and 24th.

Figure 113. Ash plumes at Aira's Sakurajima volcano rise from the Minamidake crater in November 2020. Left: an ash plume rose to 1.8 km above the crater at 1551 on the 3rd and drifted SE. on 3 (left) and 10 (right) November 2020. Right: An explosion at 1335 on the 10th produced an ash plume to 1.6 km above the crater and ballistic ejecta out to 600-900 m, with one projectile indicated by the red arrow. Courtesy of JMA (Sakurajima November 2020 monthly report).

Figure 114. An ash plume drifts SE from the Minamidake crater at Aira's Sakurajima volcano on 8 November 2020. This thermal image also shows elevated temperatures in the crater. Sentinel-2 False color (urban) satellite image (bands 12, 11, 4) courtesy of Sentinel Hub Playground.

During December there were 25 ash eruptions and 18 explosive eruptions recorded, with large ballistic ejecta reaching 1.3-1.7 km from the crater (figure 115). An explosion on the 2nd sent an ash plume up to 1 km above the crater and ballistic ejecta out to 1-1.3 km, and an event at 0404 on the 12th produced incandescent ballistic ejecta reached out to 1.3-1.7 km from the crater. At 1959 on 17 December an explosion generated an ash plume up to 3 km above the crater and ejecta out to 1.3-1.7 km. A photograph that day showed an ash plume with volcanic lightning and incandescent ejecta impacting around the crater (figure 116). On the 18th an ash plume reached 1.8 km and ejecta impacted out to 1-1.3 km. An event at 1614 on the 29th produced an ash plume reaching 3.5 km above the crater. Elevated temperatures within the Minamidake crater and plumes were observed intermittently in satellite data through the month (figure 117). This month there were four days where ashfall was recorded with a total of 14 g/m². Incandescence continued to be observed at night through the month. High levels of gas emission continued, with field surveys on 2nd, 7th, 16th and 21st recording values ranging from 1,500 to 2,900 tons per day at the Observatory located 11 km SW.

Figure 115. Explosions at Aira's Sakurajima volcano from the Minamidake summit crater in December 2020. Top: An explosion recorded at 0404 on the 12th produced incandescent ballistic ejecta out to 1.3-1.7 km from the crater, with an example indicated in the red circle. Bottom: An explosion at 1614 on the 29th produced an ash plume up to 3.5 km above the crater, and ballistic ejecta out to 1.3-1.7 km. Courtesy of JMA (top, from Sakurajima December 2020 monthly report) and Volcano Time Lapse (bottom).

Figure 116. An explosion from Sakurajima's Minamidake crater at Aira produced an ash plume with volcanic lightning on 17 December 2020. Photograph taken from Tarumizu city, courtesy of Kyodo/via Reuters.

Figure 117. Activity at Aira's Sakurajima volcano during December 2020. Top: Sentinel-2 thermal satellite image showing a diffuse gas-and-steam plume dispersing to the SE with elevated temperatures within the Minamidake summit crater on the 22nd. PlanetScope satellite image showing an ash plume dispersing between the N and E on the 26th. Sentinel-2 False color (urban) satellite image (bands 12, 11, 4) courtesy of Sentinel Hub Playground. PlanetScope satellite image courtesy of Planet Labs.

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

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); 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/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Planet Labs, Inc. (URL: https://www.planet.com/); Kyodo/via REUTERS, "Photos of the Week" (URL: https://www.reuters.com/news/picture/photos-of-the-week-idUSRTX8HYLR); Volcano Time-Lapse, YouTube (URL: https://www.youtube.com/watch?v=jTgd152oGVo).

Japan’s Nishinoshima volcano, located about 1,000 km S of Tokyo in the Ogasawara Arc, erupted above sea level in November 2013 after 40 years of dormancy. Activity lasted for two years followed by two brief eruptions in 2017 and 2018. The next eruption, from early December 2019 through August 2020, included ash plumes, incandescent ejecta, and lava flows; it produced a large pyroclastic cone with a wide summit crater and extensive lava flows that significantly enlarged the island. This report covers the end of the eruption and cooling during September 2020-January 2021. Information is provided primarily from Japan Meteorological Agency (JMA) monthly reports and the Japan Coast Guard (JCG), which makes regular observation overflights.

Ash emissions were last reported on 27 August 2020. The very high levels of thermal energy from numerous lava flows, ash, and incandescent tephra that peaked during early July decreased significantly during August and September. Continued cooling of the fresh lava and the summit crater lasted into early January 2021 (figure 107). Monthly overflights and observations by scientists confirmed areas of steam emissions at the summit and on the flanks and discolored water around the island, but no eruptive activity.

Figure 107. High levels of thermal activity at Nishinoshima during June and July 2020 resulted from extensive lava flows and explosions of incandescent tephra. Although the last ash emission was reported on 27 August 2020, cooling of new material lasted into early January 2021. The MIROVA log radiative power graph of thermal activity covers the year ending on 3 February 2021. Courtesy of MIROVA.

Thermal activity declined significantly at Nishinoshima during August 2020 (BGVN 45:09). Only two days had two MODVOLC alerts (11 and 30), and four other days (18, 20, 21, 29) had single alerts. During JCG overflights on 19 and 23 August there were no ash emissions or lava flows observed, although steam plumes rose over 2 km above the summit crater during both visits. The last ash emission was reported by the Tokyo VAAC on 27 August 2020. No eruptive activity was observed by JMA during an overflight on 5 September, but steam plumes were rising from the summit crater (figure 108). No significant changes were observed in the shape of the pyroclastic cone or the coastline. Yellowish brown discolored water appeared around the western half of the island, and high temperature was still measured on the inner wall of the crater. Faint traces of SO₂ plumes were present in satellite images in early September; the last plume identified was on 18 September. Six days with single MODVOLC alerts were recorded during 3-19 September, and the final thermal alert appeared on 1 October 2020.

Figure 108. No eruptive activity was observed during a JMA overflight of Nishinoshima on 5 September 2020, but steam rose from numerous places within the enlarged summit crater (inset). Courtesy of JMA and JCG (Monthly report of activity at Nishinoshima, September 2020).

Steam plumes and high temperatures were noted at the summit crater on 28 October, and brown discolored water was present around the S coast of the island (figure 109), but there were no other signs of volcanic activity. Observations from the sea conducted on 2 November 2020 by researchers aboard the Maritime Meteorological Observatory marine weather observation ship "Ryofu Maru" confirmed there was no ongoing eruptive activity. In addition to steam plumes at the summit, they also noted steam rising from multiple cracks on the cooling surface of the lava flow area on the N side of the pyroclastic cone (figure 110). Only steam plumes from inside the summit crater were observed during an overflight on 24 November.

Figure 109. On a JCG overflight above Nishinoshima on 28 October 2020 there were no signs of eruptive activity; steam plumes were present in the summit crater and brown discolored water was visible around the S coast of the island. Courtesy of JMA and JCG (Monthly report of activity at Nishinoshima, October 2020).

Figure 110. Observations of Nishinoshima by staff aboard the Maritime Meteorological Observatory ship "Ryofu Maru" on 2 November 2020 showed a steam plume rising from the lava flow area on the N side of the pyroclastic cone (arrow) and minor steam above the cone. Courtesy of JMA (Monthly report of activity at Nishinoshima, November 2020).

JMA reduced the warning area around the crater on 18 December 2020 from 2.5 to 1.5 km due to decreased activity. On 7 December a steam plume rose from the inner wall of the summit crater and thermal imaging indicated the area was still hot. Brown discolored water was observed on the SE and SW coasts. Researchers aboard a ship from the Earthquake Research Institute at the University of Tokyo and the Marine Research and Development Organization reported continued steam plumes in the summit crater, around the lava flows on the N flank, and along the S coast during 15-29 December (figure 111). Steam plumes and elevated temperatures were still measured inside the summit crater during an overflight by the Japan Coast Guard on 25 January 2021, and discolored water persisted on the SE and SW coasts; there was no evidence of eruptive activity.

Figure 111. Observations of Nishinoshima from the sea by researchers from the Earthquake Research Institute (University of Tokyo) and the Marine Research and Development Organization, which took place from 15-29 December 2020, showed fumarolic acitivity not only inside the summit crater, but also in the lava flow area on the N side of the pyroclastic cone (left, 20 December) and in places along the southern coast (right, 23 December). (Monthly report of activity at Nishinoshima, December 2020).

Geologic Background. The small island of Nishinoshima was enlarged when several new islands coalesced during an eruption in 1973-74. Another eruption that began offshore in 2013 completely covered the previous exposed surface and enlarged the island again. Water discoloration has been observed on several occasions since. The island is the summit of a massive submarine volcano that has prominent satellitic peaks to the S, W, and NE. The summit of the southern cone rises to within 214 m of the sea surface 9 km SSE.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Japan Coast Guard (JCG) Volcano Database, Hydrographic and Oceanographic Department, 3-1-1, Kasumigaseki, Chiyoda-ku, Tokyo 100-8932, Japan (URL: http://www.kaiho.mlit.go.jp/info/kouhou/h29/index.html); Volcano Research Center (VRC-ERI), Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan (URL: http://www.eri.u-tokyo.ac.jp/topics/ASAMA2004/index-e.html); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); 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/).

Nyiragongo is a stratovolcano in the DR Congo with a deep summit crater containing a lava lake and a small active cone. During June 2018-May 2020, the volcano exhibited strong thermal signals primarily due to the lava lake, along with incandescence, seismicity, and gas-and-steam plumes (BGVN 44:05, 44:12, 45:06). The volcano is monitored by the Observatoire Volcanologique de Goma (OVG). This report summarizes activity during June-November 2020, based on satellite data.

Infrared MODIS satellite data showed almost daily strong thermal activity during June-November 2020 from MIROVA (Middle InfraRed Observation of Volcanic Activity), consistent with a large lava lake. Numerous hotspots were also identified every month by MODVOLC. Although clouds frequently obscured the view from space, a clear Sentinel-2 image in early June showed a gas-and-steam plume as well as a strong thermal anomaly (figure 76).

Figure 76. Sentinel-2 satellite imagery of Nyiragongo on 1 June 2020. A gas-and-steam is visible in the natural color image (bands 4, 3, 2) rising from a pit in the center of the crater (left), while the false color image (bands 12, 11, 4) reveals a strong thermal signal from a lava lake (right). Courtesy of Sentinel Hub Playground.

During the first half of June 2020, OVG reported that SO₂ levels had decreased compared to levels in May (7,000 tons/day); during the second half of June the SO₂ flux began to increase again. High levels of sulfur dioxide were recorded almost every day in the region above or near the volcano by the TROPOspheric Monitoring Instrument (TROPOMI) aboard the Copernicus Sentinel-5 Precursor satellite (figure 77). According to OVG, SO₂ flux ranged from 819-5,819 tons/day during June. The number of days with a high SO₂ flux decreased somewhat in July and August, with high levels recorded during about half of the days. The volume of SO₂ emissions slightly increased in early July, based on data from the DOAS station in Rusayo, measuring 6,787 tons/day on 8 July (the highest value reported during this reporting period), and then declined to 509 tons/day by 20 July. The SO₂ flux continued to gradually decline, with high values of 5,153 tons/day in August and 4,468 tons/day in September. The number of days with high SO₂ decreased further in September and October but returned to about half of the days in November.

Figure 77. TROPOMI image of SO2 plume on 27 June 2020 in the Nyiragongo-Nyamulagira area. The plume drifted SSE. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

During 12-13 July a multidisciplinary team of OVG scientists visited the volcano to take measurements of the crater using a TCRM1102 Plus2 laser. They noted that the crater had expanded by 47.3 mm in the SW area, due to the rise in the lava lake level since early 2020. The OVG team took photos of the small cone in the lava lake that has been active since 2014, recently characterized by white gas-and-steam emissions (figure 78). OVG noted that the active lava lake had subsided roughly 20 m (figure78).

Figure 78. Photos (color corrected) of the crater at Nyiragongo showing the small active cone generating gas-and-steam emissions (left) and the active lava lake also characterized by white gas-and-steam emissions on 12 July 2020 (right). Courtesy of OVG (Rapport OVG Juillet 2020).

Geologic Background. One of Africa's most notable volcanoes, Nyiragongo contained a lava lake in its deep summit crater that was active for half a century before draining catastrophically through its outer flanks in 1977. The steep slopes of a stratovolcano contrast to the low profile of its neighboring shield volcano, Nyamuragira. Benches in the steep-walled, 1.2-km-wide summit crater mark levels of former lava lakes, which have been observed since the late-19th century. Two older stratovolcanoes, Baruta and Shaheru, are partially overlapped by Nyiragongo on the north and south. About 100 parasitic cones are located primarily along radial fissures south of Shaheru, east of the summit, and along a NE-SW zone extending as far as Lake Kivu. Many cones are buried by voluminous lava flows that extend long distances down the flanks, which is characterized by the eruption of foiditic rocks. The extremely fluid 1977 lava flows caused many fatalities, as did lava flows that inundated portions of the major city of Goma in January 2002.

Information Contacts: Observatoire Volcanologique de Goma (OVG), Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo; 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 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/).

Whakaari/White Island, located in the Bay of Plenty 50 km offshore of North Island, has been New Zealand’s most active volcano since 1976. Activity has been previously characterized by phreatic activity, explosions, and ash emissions (BGVN 42:05). The most recent eruption occurred on 9 December 2019, which consisted of an explosion that generated an ash plume and pyroclastic surge that affected the entire crater area, resulting in 21 fatalities and many injuries (BGVN 45:02). This report updates information from February through November 2020, which includes dominantly gas-and-steam emissions along with elevated surface temperatures, using reports from the New Zealand GeoNet Project, the Wellington Volcanic Ash Advisory Centre (VAAC), and satellite data.

Activity at Whakaari/White Island has declined and has been dominated by white gas-and-steam emissions during the reporting period; no explosive eruptive activity has been detected since 9 December 2019. During February through 22 June, the Volcanic Activity Level (VAL) remained at a 2 (moderate to heightened volcanic unrest) and the Aviation Color Code was Yellow. GeoNet reported that satellite data showed some subsidence along the W wall of the Main Crater and near the 1914 landslide scarp, though the rate had reduced compared to previous months. Thermal infrared data indicated that the fumarolic gases and five lobes of lava that were first observed in early January 2020 in the Main Crater were 550-570°C on 4 February and 660°C on 19 February. A small pond of water had begun to form in the vent area and exhibited small-scale gas-and-steam-driven water jetting, similar to the activity during September-December 2019. Gas data showed a steady decline in SO₂ and CO₂ levels, though overall they were still slightly elevated.

Similar activity was reported in March and April; the temperatures of the fumaroles and lava in the Main Crater were 746°C on 10 March, the highest recorded temperature to date. SO₂ and CO₂ gas emissions remained elevated, though had overall decreased since December 2019. Small-scale water jetting continued to be observed in the vent area. During April, public reports mentioned heightened gas-and-steam activity, but no eruptions were detected. A GeoNet report issued on 16 April stated that high temperatures were apparent in the vent area at night.

Whakaari remained at an elevated state of unrest during May, consisting of dominantly gas-and-steam emissions. Monitoring flights noted that SO₂ and CO₂ emissions had increased briefly during 20-27 May. On 20 May, the lava lobes remained hot, with temperatures around 500°C; a nighttime glow from the gas emissions surrounding the lava was visible in webcam images. Tremor levels remained low with occasional slightly elevated episodes, which included some shallow-source volcanic earthquakes. Satellite-based measurements recorded several centimeters of subsidence in the ground around the active vent area since December 2019. During a gas observation flight on 28 May there was a short-lived gas pulse, accompanied by an increase in SO₂ and CO₂ emissions, and minor inflation in the vent area (figure 96).

Figure 96. Photo of a strong gas-and-steam plume rising above Whakaari/White Island on 28 May 2020. Courtesy of GeoNet.

An observation flight made on 3 June reported a decline in gas flux compared to the measurements made on 28 May. Thermal infrared images taken during the flight showed that the lava lobes were still hot, at 450°C, and continued to generate incandescence that was visible at night in webcams. On 16 June the VAL was lowered to 1 (minor volcanic unrest) and on 22 June the Aviation Color Code had decreased to Green.

Minor volcanic unrest continued in July; the level of volcanic tremors has remained generally low, with the exception of two short bursts of moderate volcanic tremors in at the beginning of the month. Temperatures in the active vents remained high (540°C) and volcanic gases persisted at moderate rate, similar to those measured since May, according to an observation flight made during the week of 30 July. Subsidence continued to be observed in the active vent area, as well as along the main crater wall, S and W of the active vents. Recent rainfall has created small ponds of water on the crater floor, though they did not infiltrate the vent areas.

Gas-and-steam emissions persisted during August through October at relatively high rates (figures 97 and 98). A short episode of moderate volcanic tremor was detected in early August, but otherwise seismicity remained low. Updated temperatures of the active vent area were 440°C on 15 September, which had decreased 100°C since July. Rain continued to collect at the crater floor, forming a small lake; minor areas of gas-and-steam emissions can be seen in this lake. Ongoing subsidence was observed on the Main Crater wall and S and W of the 2019 active vents.

Figure 97. Photo of an observation flight over Whakaari/White Island on 8 September 2020 showing white gas-and-steam emissions from the vent area. Photo courtesy of Brad Scott, GeoNet.

Figure 98. Image of Whakaari/White Island from Whakatane in the North Island of New Zealand showing a white gas-and-steam plume on 26 October 2020. Courtesy of GeoNet.

Activity during November was primarily characterized by persistent, moderate-to-large gas-and-steam plumes that drifted downwind for several kilometers but did not reach the mainland. The SO₂ flux was 618 tons/day and the CO₂ flux was 2,390 tons/day. New observations on 11 November noted some occasional ash deposits on the webcams in conjunction with mainland reports of a darker than usual plume (figure 99). Satellite images provided by MetService, courtesy of the Japan Meteorological Agency, confirmed the ash emission, but later images showed little to no apparent ash; GNS confirmed that no eruptive activity had occurred. Initial analyses indicated that the ash originated from loose material around the vent was being entrained into the gas-and-steam plumes. Observations from an overflight on 12 November showed that there was no substantial change in the location and size of the active vents; rainfall continued to collect on the floor of the 1978/90 Crater, reforming the shallow lake. A small sequence of earthquakes was detected close to the volcano with several episodes of slightly increased volcanic tremors.

During 12-14 November the Wellington VAAC issued multiple advisories noting gas, steam, and ash plumes that rose to 1.5-1.8 km altitude and drifted E and SE, based on satellite data, reports from pilots, and reports from GeoNet. As a result, the VAL was increased to 2 and the Aviation Color Code was raised to Yellow. Scientists on another observation flight on 16 November reported that small amounts of ash continued to be present in gas-and-steam emissions, though laboratory analyses showed that this ash was resuspended material and not from new eruptive or magmatic activity. The SO₂ and CO₂ flux remained above background levels but were slightly lower than the previous week’s measurements: 710 tons/day and 1,937 tons/day. Seismicity was similar to the previous week, characterized by a sequence of small earthquakes, a larger than normal volcanic earthquake located near the volcano, and ongoing low-level volcanic tremors. During 16-17 November plumes with resuspended ash were observed rising to 460 m altitude, drifting E and NE, according to a VAAC advisory (figure 99). During 20-24 November gas-and-steam emissions that contained a minor amount of resuspended ash rose to 1.2 km altitude and drifted in multiple directions, based on webcam and satellite images and information from GeoNet.

Figure 99. Left: Photo of a gas observation flight over Whakaari/White Island on 11 November 2020 showing some dark particles in the gas-and-steam plumes, which were deposited on some webcams. Photo has been color corrected and straightened. Courtesy of GeoNet. Right: Photo showing gas, steam, and ash emissions rising above the 2019 Main Crater area on 16 November 2020. Courtesy of GNS Science (17 November 2020 report).

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows a total of eleven low-power thermal anomalies during January to late March 2020; a single weak thermal anomaly was detected in early July (figure 100). The elevated surface temperatures during February-May 2020 were detected in Sentinel-2 thermal satellite images in the Main Crater area, occasionally accompanied by gas-and-steam emissions (figure 101). Persistent white gas-and-steam emissions rising above the Main Crater area were observed in satellite imagery on clear weather days and drifting in multiple directions (figure 102). The small lake that had formed due to rainfall was also visible to the E of the active vents.

Figure 100. Low-power, infrequent thermal activity at Whakaari/White Island was detected during January through late March 2020, as reflected in the MIROVA data (Log Radiative Power). A single thermal anomaly was shown in early July. Courtesy of MIROVA.

Figure 101. Sentinel-2 thermal satellite images in the Main Crater area of Whakaari/White Island show residual elevated temperatures from the December 2019 eruption, accompanied by gas-and-steam emissions and drifting in different directions during February-May 2020. Images using “Atmospheric penetration” rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Figure 102. Sentinel-2 images showing persistent white gas-and-steam plumes rising from Main Crater area of Whakaari/White Island during March-November 2020 and drifting in multiple directions. A small pond of water (light blue-green) is visible in the vent area to the E of the plumes. On 11 November (bottom right), the color of the plume is gray and contains a small amount of ash. Images using “Natural color” rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

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

Information Contacts: New Zealand GeoNet Project, a collaboration between the Earthquake Commission and GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3352, New Zealand (URL: http://www.geonet.org.nz/); GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3352, New Zealand (URL: http://www.gns.cri.nz/); Wellington Volcanic Ash Advisory Centre (VAAC), Meteorological Service of New Zealand Ltd (MetService), PO Box 722, Wellington, New Zealand (URL: http://www.metservice.com/vaac/, http://www.ssd.noaa.gov/VAAC/OTH/NZ/messages.html); 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); Brad Scott, GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3352, New Zealand (URL: https://twitter.com/Eruptn).

Kerinci, located in Sumatra, Indonesia, has had numerous explosive eruptions since 1838, with more recent activity characterized by gas-and-steam and ash plumes. The current eruptive episode began in April 2018 and has recently consisted of intermittent brown ash emissions and white gas-and-steam emissions (BGVN 45:07); similar activity continued from June through November 2020. Information primarily comes from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM, or the Center of Volcanology and Geological Hazard Mitigation), MAGMA Indonesia, the Darwin Volcanic Ash Advisory Centre (VAAC), and satellite data.

Activity has been characterized by dominantly white and brown gas-and-steam emissions and occasional ash plumes, according to PVMBG. Near daily gas-and-steam emissions were observed rising 50-6,400 m above the crater throughout the reporting period: beginning in late July and continuing intermittently though November. Sentinel-2 satellite imagery showed frequent brown emissions rising above the summit crater at varying intensities and drifting in different directions from July to November (figure 21).

Figure 21. Sentinel-2 satellite imagery of brown emissions at Kerinci from July through November 2020 drifting in multiple directions. On 27 July (top left) the brown emissions drifted SW. On 31 August (top right) the brown emissions drifted W. On 2 September (bottom left) slightly weaker brown emissions drifting W. On 4 November (bottom right) weak brown emissions mostly remained within the crater, some of which drifted E. Images using “Natural Color” rendering (bands 4, 3, 2), courtesy of Sentinel Hub Playground.

During June through July the only activity reported by PVMBG consisted of white gas-and-steam emissions and brown emissions. On 4 June white gas-and-steam emissions rose to a maximum height of 6.4 km above the crater. White-and-brown emissions rose to a maximum height of 700 m above the crater on 2 June and 28 July.

Continuous white-and-brown gas-and-steam emissions were reported in August that rose 50-1,000 m above the crater. The number of ash plumes reported during this month increased compared to the previous months. In a Volcano Observatory Notice for Aviation (VONA) issued on 7 August at 1024, PVMBG reported an ash plume that rose 600 m above the crater and drifted E, SE, and NE. In addition, the Darwin VAAC released two notices that described continuous minor ash emissions rising to 4.3 km altitude and drifting E and NE. On 9 August an ash plume rose 600 m above the crater and drifted ENE at 1140. An ash plume was observed rising to a maximum of 1 km above the crater, drifting E, SE, and NE on 12 August at 1602, according to a PVMBG VONA and Darwin VAAC advisory. The following day, brown emissions rose to a maximum of 1 km above the crater and were accompanied by a 600-m-high ash plume that drifted ENE at 1225. Ground observers on 15 August reported an eruption column that rose to 4.6 km altitude; PVMBG described brown ash emissions up to 800 m above the crater drifting NW at 0731 (figure 22). During 20-21 August pilots reported an ash plume rising 150-770 m above the crater drifting NE and SW, respectively.

Figure 22. Webcam image of an ash plume rising above Kerinci on 15 August 2020. Courtesy of MAGMA Indonesia.

Activity in September had decreased slightly compared to the previous month, characterized by only white-and-brown gas-and-steam emissions that rose 50-300 m above the crater; solely brown emissions were observed on 30 September and rose 50-100 m above the crater. This low level of activity persisted into October, with white gas-and-steam emissions to 50-200 m above the crater and brown emissions rising 50-300 m above the crater. On 16 October PVMBG released a VONA at 0340 that reported an ash plume rising 687 m above the crater and drifting NE. On 17 October white, brown, and black ash plumes that rose 100-800 m above the crater drifted NE according to both PVMBG and a Darwin VAAC advisory (figure 23). During 18-19 October white, brown, and black ash emissions rose up to 400 m above the crater and drifted NE and E.

Figure 23. Webcam image of a brown ash emission from Kerinci on 17 October 2020. Courtesy of MAGMA Indonesia.

Geologic Background. Gunung Kerinci in central Sumatra forms Indonesia's highest volcano and is one of the most active in Sumatra. It is capped by an unvegetated young summit cone that was constructed NE of an older crater remnant. There is a deep 600-m-wide summit crater often partially filled by a small crater lake that lies on the NE crater floor, opposite the SW-rim summit. The massive 13 x 25 km wide volcano towers 2400-3300 m above surrounding plains and is elongated in a N-S direction. Frequently active, Kerinci has been the source of numerous moderate explosive eruptions since its first recorded eruption in 1838.

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

Suwanosejima, an andesitic stratovolcano in Japan's northern Ryukyu Islands, was intermittently active for much of the 20th century, producing ash plumes, Strombolian explosions, and ashfall. Continuous activity since October 2004 has included intermittent explosions which generate ash plumes that rise hundreds of meters above the summit to altitudes between 1 and 3 km. Incandescence is often observed at night and ejecta periodically reaches over a kilometer from the summit. Ashfall is usually noted several times each month in the nearby community on the SW flank of the island. Ongoing activity for the second half of 2020, which includes significantly increased activity in December, is covered in this report with information provided by the Japan Meteorological Agency (JMA), the Tokyo Volcanic Ash Advisory Center (VAAC), and several sources of satellite data.

A steady increase in activity was reported during July-December 2020. The number of explosions recorded increased each month from only six during July to 460 during December. The energy of the explosions increased as well; ejecta was reported 600 m from the crater during August, but a large bomb reached 1.3 km from the crater at the end of December. After an increased period of explosions late in December, JMA raised the Alert Level from 2 to 3 on a 5-level scale. The MIROVA graph of thermal activity indicated intermittent anomalies from July through December 2020, with a pulse of activity in the second half of December (figure 48).

Figure 48. MIROVA thermal activity for Suwanosejima for the period from 3 February through December 2020 shows pulses of activity in February and April, with intermittent anomalies until another period of frequent stronger activity in December. Courtesy of MIROVA.

Six explosions were recorded during July 2020, compared with only one during June. According to JMA, the tallest plume rose 2,000 m above the crater rim. Incandescent ejecta was occasionally observed at night. The Tokyo VAAC reported a number of ash plumes that rose to 1.2-2.7 km altitude and drifted NW and W during the second half of the month (figure 49). Activity increased during August 2020 when thirteen explosions were reported. The Tokyo VAAC reported a few ash plumes during 1-6 August that rose to 1.8-2.4 km altitude and drifted NW; a larger pulse of activity during 18-22 August produced plumes that rose to altitudes ranging from 1.8 to over 2.7 km. Ashfall was reported on 19 and 20 August in the village located 4 km SSW of the crater; incandescence was visible at the summit and ash plumes drifted SW in satellite imagery on 19 August (figure 50). A MODVOLC thermal alert was issued on 19 August. On 21 August a large bomb was ejected 600 m from the Otake crater in an explosion early in the day; later that afternoon, an ash plume rose to more than 2,000 m above the crater rim. During 19-22 August, SO₂ emissions were recorded each day by the TROPOMI instrument on the Sentinel-5P satellite (figure 51).

Figure 49. An ash emission at Suwanosejima rose to 2.7 km altitude and drifted NW on 27 July 2020. Courtesy of JMA (Volcanic activity commentary material on Suwanosejima, July 2020).

Figure 50. Ash drifted SW from the summit crater of Suwanosejima on 19 August 2020 and a bright thermal anomaly was present at the summit. Residents of the village 4 km SW reported ashfall that day and the next. Courtesy of Sentinel Hub Playground.

Figure 51. A period of increased activity at Suwanosejima during 19-22 August 2020 produced SO2 emissions that were measured by the TROPOMI instrument on the Sentinel-5P satellite. Nishinoshima, was also producing significant SO2 at the same time. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Thirteen explosions were recorded during September 2020, with the highest ash plumes reaching 2,000 m above the crater rim, and bombs falling 400 m from the crater. Ashfall was recorded on 20 September in the community located 4 km SSW. The Tokyo VAAC reported intermittent ash plumes during the month that rose to 1.2-2.1 km altitude and drifted in several directions. Incandescence was frequently observed at night (figure 52). Explosive activity increased during October with 22 explosions recorded. Ash plumes rose over 2,000 m above the crater rim, and bombs reached 700 m from the crater. Steam plumes rose 2,300 m above the crater rim. Ashfall and loud noises were confirmed several times between 2 and 14 October in the nearby village. A MODVOLC thermal alert was issued on 6 October. The Tokyo VAAC reported multiple ash plumes throughout the month; they usually rose to 1.5-2.1 km altitude and drifted in many directions. The plume on 28 October rose to over 2.7 km altitude and was stationary.

Figure 52. Incandescence at night and ash emissions were observed multiple times at Suwanosejima during September and October 2020 including on 21 and 26 September (top) and 29 October 2020. Courtesy of JMA (Volcanic activity commentary material on Suwanosejima, September and October 2020).

Frequent explosions occurred during November 2020, with a sharp increase in the number of explosions to 105 events compared with October. Ash plumes rose to 1,800 m above the crater rim and bombs were ejected 700 m. Occasional ashfall and loud noises were reported from the nearby community throughout the month. Scientists measured no specific changes to the surface temperature around the volcano during an overflight early on 5 November compared with the previous year. At 0818 on 5 November a small ash explosion at the summit crater was photographed by the crew during an observation flight (figure 53). On 12 and 13 November, incandescent ejecta fell 600 m from the crater and ash emissions rose 1,500 m above the crater rim (figure 54).

Figure 53. A minor explosion produced a small ash plume at Suwanosejima during an overflight by JMA on the morning of 5 November 2020. The thermal activity was concentrated at the base of the explosion (inset). Image taken from off the E coast. Courtesy of JMA (Volcanic activity commentary material on Suwanosejima, November 2020).

Figure 54. On 12 and 13 November 2020 incandescent ejecta from Suwanosejima reached 600 m from the crater (top) and ash emissions rose 1,500 m above the crater rim (bottom). Courtesy of JMA (Volcanic activity commentary material on Suwanosejima, November 2020).

During December 2020 there were 460 explosions reported, a significant increase from the previous months. Ash plumes reached 1,800 m above the summit. Three MODVOLC thermal alerts were issued on 25 December and two were issued the next day. The number of explosions increased substantially at the Otake crater between 21 and 29 December, and early on 28 December a large bomb was ejected to 1.3 km SE of the crater (figure 55). A second explosion a few hours later ejected another bomb 1.1 km SE. An overflight later that day confirmed the explosion, and ash emissions were still visible (figure 56), although cloudy weather prevented views of the crater. Ashfall was noted and loud sounds heard in the nearby village. A summary graph of observations throughout 2020 indicated that activity was high from January through May, quieter during June, and then increased again from July through the end of the year (figure 57).

Figure 55. Early on 28 December 2020 a large explosion at Suwanosejima sent a volcanic bomb 1.3 km SE from the summit (bright spot on left flank in large photo). Thermal imaging taken the same day showed the heat at the eruption site and multiple fragments of warm ejecta scattered around the crater area (inset). Courtesy of JMA (Volcanic activity commentary material on Suwanosejima, December 2020).

Figure 56. Ash emissions were still visible midday on 28 December 2020 at Suwanosejima during a helicopter overflight by the 10th Regional Coast Guard. Image taken from the SW flank of the volcano. Two large explosions earlier in the day had sent ejecta more than a kilometer from the crater. Courtesy of JMA (Volcanic activity commentary material on Suwanosejima, December 2020).

Figure 57. Activity summary for Suwanosejima for January-December 2020 when 764 explosions were recorded. Black bars represent the height of steam, gas, or ash plumes in meters above the crater rim, gray volcano icons represent explosions, usually accompanied by an ash plume, red icons represent large explosions with ash plumes, orange diamonds indicate incandescence observed in webcams. Courtesy of JMA (Suwanosejima volcanic activity annual report, 2020).

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

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

Karangetang (also known as Api Siau) is located on the island of Siau in the Sitaro Regency, North Sulawesi, Indonesia and consists of two active summit craters: a N crater (Kawah Dua) and a S crater (Kawah Utama, also referred to as the “Main Crater”). More than 50 eruptions have been observed since 1675. The current eruption began in November 2018 and has recently been characterized by frequent incandescent block avalanches, thermal anomalies in the crater, and gas-and-steam plumes (BGVN 45:06). This report covers activity from June through November 2020, which includes dominantly crater anomalies, few ash plumes, and gas-and-steam emissions. Information primarily comes 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 various satellite data.

Activity decreased significantly after mid-January 2020 and has been characterized by dominantly gas-and-steam emissions and occasional ash plumes, according to PVMBG. Daily gas-and-steam emissions were observed rising 25-600 m above the Main Crater (S crater) during the reporting period and intermittent emissions rising 25-300 m above Kawah Dua (N crater).

The only activity reported by PVMBG in June, August, and October was daily gas-and-steam emissions above the Main Crater and Kawah Dua (figure 47). MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows intermittent low-power thermal anomalies during June through late July, which includes a slight increase in power during late July (figure 48). During 14-15 July strong rumbling from Kawah Dua was accompanied by white-gray emissions that rose 150-200 m above the crater. Crater incandescence was observed up to 10 m above the crater. According to webcam imagery from MAGMA Indonesia, intermittent incandescence was observed at night from both craters through 25 July. In a Volcano Observatory Notice for Aviation (VONA) issued on 5 September, PVMBG reported an ash plume that rose 800 m above the crater.

Figure 47. Webcam image of gas-and-steam plumes rising above the two summit craters at Karangetang on 16 June 2020. Courtesy of MAGMA Indonesia.

Figure 48. Intermittent low-power thermal anomalies at Karangetang were reported during June through July 2020 with a slight increase in power in late July, according to the MIROVA graph (Log Radiative Power). No thermal activity was detected during August to late October; in mid-November a short episode of increased activity occurred. Courtesy of MIROVA.

Thermal activity increased briefly during mid-November when hot material was reported extending 500-1,000 m NW of the Main Crater, accompanied by gas-and-steam emissions rising 200 m above the crater. Corresponding detection of MODIS thermal anomalies was seen in MIROVA graphs (see figure 48), and the MODVOLC system showed alerts on 13 and 15 November. On 16 November blue emissions were observed above the Main Crater drifting W. Sentinel-2 thermal images showed elevated temperatures in both summit craters throughout the reporting period, accompanied by gas-and-steam emissions and movement of hot material on the NW flank on 19 November (figure 49). White gas-and-steam emissions rose to a maximum height of 300 m above Kawah Dua on 22 November and 600 m above the Main Crater on 28 November.

Figure 49. Persistent thermal anomalies (bright yellow-orange) at Karangetang were detected in both summit craters using Sentinel-2 thermal satellite imagery during June through November 2020. Gas-and-steam emissions were also occasionally detected in both craters as seen on 17 June (top left) and 20 September (bottom left) 2020. On 19 November (bottom right) the Main Crater (S) showed a hot thermal signature extending NW. Images using “Atmospheric penetration” rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Geologic Background. Karangetang (Api Siau) volcano lies at the northern end of the island of Siau, about 125 km NNE of the NE-most point of Sulawesi island. The stratovolcano contains five summit craters along a N-S line. It is one of Indonesia's most active volcanoes, with more than 40 eruptions recorded since 1675 and many additional small eruptions that were not documented in the historical record (Catalog of Active Volcanoes of the World: Neumann van Padang, 1951). Twentieth-century eruptions have included frequent explosive activity sometimes accompanied by pyroclastic flows and lahars. Lava dome growth has occurred in the summit craters; collapse of lava flow fronts have produced pyroclastic flows.

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

Colombia’s broad, glacier-capped Nevado del Ruiz has an eruption history documented back 8,600 years, including documented observations since 1570. Ruiz remained quiet for 20 years after the deadly September 1985-July 1991 eruption until a period of explosive activity from February 2012 into 2013. Renewed activity beginning in November 2014 included ash and gas-and-steam plumes, ashfall, and the appearance of a slowly growing lava dome inside the Arenas crater in August 2015. Additional information has caused a revision to earlier reporting that eruptive activity ended in May 2017 and began again that December (BGVN 44:12); activity appears to have continued throughout 2017 with intermittent ash emissions and thermal evidence of dome growth. Periods of increased thermal activity alternated with periods of increased explosive activity during 2018-2019 and into 2020; SO₂ emissions persisted at significant levels. The lava dome has continued to grow through 2020. This report covers ongoing activity from July-December 2020 using information from reports by the Servicio Geologico Colombiano (SGC) and the Observatorio Vulcanológico y Sismológico de Manizales, the Washington Volcanic Ash Advisory Center (VAAC) notices, and various sources of satellite data.

Gas and ash emissions continued throughout July-December 2020; they generally rose to 5.8-6.1 km altitude with the highest reported plume at 6.7 km altitude on 7 December. SGC interpreted repeated episodes of “drumbeat seismicity” as an indication of continued dome growth throughout the period. Satellite thermal anomalies also suggested that dome growth continued. The MIROVA graph of thermal activity suggests that the dome was quiet in July and early August, but small pulses of thermal energy were recorded every few weeks for the remainder of 2020 (figure 115). Plots of the cumulative number and magnitude of seismic events at Nevado del Ruiz between January 2010 and November 2020 show a stable trend with periodic sharp increases in activity or magnitude throughout that time. SGC has adjusted the warning levels over time according to changes in the slope of the curves (figure 116).

Figure 115. Thermal energy shown in the MIROVA graph of log radiative power at Nevado del Ruiz from 3 February 2020 through the end of the year indicates that higher levels of thermal energy lasted through April 2020; a quieter period from late May-early August was followed by low-level persistent anomalies through the end of the year. Courtesy of MIROVA.

Figure 116. Changes in seismic frequency and energy at Nevado del Ruiz have been monitored by SGC for many years. Left: the cumulative number of daily VT, LP-VLP, TR, and HB seismic events, recorded between 1 January 2010 and 30 November 2020. The arrows highlight the days with the highest number of seismic events; the number and type of event is shown under the date. Right: The cumulative VT and HB seismic energy recorded between 1 January 2010 and 30 November 2020. The arrows highlight the days with the highest energy; the local magnitude of the event is shown below the date. SGC has adjusted the warning levels over time (bar across the bottom of each graph) according to changes in the slope of the curves. Courtesy of SGC (INFORME TÉCNICO – OPERATIVO DE LA ACTIVIDAD VOLCÁNICA, SEGMENTO VOLCÁNICO NORTE DE COLOMBIA – NOVIEMBRE DE 2020).

Activity during July-December 2020. Seismic energy increased during July compared to June 2020 with events localized around the Arenas crater. The depth of the seismicity varied from 0.3-7.8 km. Some of these signals were associated with small emissions of gas and ash, which were confirmed through webcams and by reports from officials of the Los Nevados National Natural Park (NNNP). The Washington VAAC reported a possible ash emission on 8 July that rose to 6.1 km altitude and drifted NW. On 21 July a webcam image showed an ash emission that rose to the same altitude and drifted W; it was seen in satellite imagery possibly extending 35 km from the summit but was difficult to confirm due to weather clouds. Short- to moderate-duration (less than 40 minutes) episodes of drumbeat seismicity were recorded on 5, 13, 17, and 21 July. SCG interprets this type of seismic activity as related to the growth of the Arenas crater lava dome. Primarily WNW drifting plumes of steam and SO₂ were observed in the webcams daily. The gas was occasionally incandescent at night. The tallest plume of gas and ash reached 1,000 m above the crater rim on 30 July and was associated with a low-energy tremor pulse; it produced ashfall in parts of Manizales and nearby communities (figure 117).

Figure 117. Images captured by a traditional camera (top) and a thermal camera (bottom) at Nevado del Ruiz showed a small ash emission in the early morning of 30 July 2020. Ashfall was reported in Manizales. The cameras are located 3.7 km W of the Arenas crater. Courtesy of SGC (Emisión de ceniza Volcan Nevado del Ruiz Julio 30 de 2020).

Seismicity increased in August 2020 with respect to July. Some of the LP and TR (tremor) seismicity was associated with small emissions of gas and ash, confirmed by web cameras, park personnel, and the Washington VAAC. The Washington VAAC received a report from the Bogota MWO of an ash emission on 1 August that rose to 6.1 km altitude and drifted NW; it was not visible in satellite imagery. Various episodes of short duration drumbeat seismicity were recorded during the month. The tallest steam and gas plume reached 1,800 m above the rim on 31 August. Despite the fact that in August the meteorological conditions made it difficult to monitor the surface activity of the volcano, three ash emissions were confirmed by SGC.

Seismicity decreased during September 2020 with respect to August. Some of the LP and TR (tremor) seismicity was associated with small emissions of gas and ash, confirmed by web cameras, park personnel and the Washington VAAC. The Washington VAAC reported an ash emission on 16 September that rose to 6.1 km altitude and drifted NW. A minor ash emission on 20 September drifted W from the summit at 5.8 km altitude. A possible emission on 23 September drifted NW at 6.1 km altitude for a brief period before dissipating. Two emissions were reported drifting WNW of the summit on 26 September at 5.8 and 5.5 km altitude. Continuous volcanic tremors were registered throughout September, with the higher energy activity during the second half of the month. One episode of drumbeat seismicity on 15 September lasted for 38 minutes and consisted of 25 very low energy earthquakes. Steam and gas plumes reached 1,800 m above the crater rim during 17-28 September (figure 118). Five emissions of ash were confirmed by the webcams and park officials during the month, in spite of difficult meteorological conditions; three of them occurred between 15 and 20 September.

Figure 118. A dense plume of steam rose from Nevado del Ruiz in the morning of 17 September 2020. Courtesy of Gonzalo.

Seismicity increased during October with respect to September. A few of the LP and tremor seismic events were associated with small emissions of gas and ash, confirmed by web cameras, park personnel, and the Washington VAAC. The Washington VAAC issued advisories of possible ash emissions on 2, 6, 9, 11, 15, 17, 18, and 21 October. The plumes rose to 5.6-6.4 km altitude and drifted primarily W and NW. Steam plumes were visible most days of the month (figure 119). Only a few were visible in satellite data, but most were visible in the webcams. Several episodes of drumbeat seismicity were recorded on 13, 22-25, and 27 October, which were characterized by being of short duration and consisting of very low energy earthquakes. The tallest plume during the month rose about 2 km above the crater rim on 18 October. Ash emissions were recorded eight times during the month by SGC.

Figure 119. A steam plume mixed with possible ash drifted SE from Nevado del Ruiz on 7 October 2020. Courtesy of vlucho666.

During November 2020, the number of seismic events decreased relative to October, but the amount of energy released increased. Some of the seismicity was associated with small emissions of gas and ash, confirmed by webcams around the volcano. The Washington VAAC reported ash emissions on 22 and 30 November; the 22 November event was faintly visible in satellite images and was also associated with an LP seismic event. They rose to 5.8-6.1 km altitude and drifted W. Various episodes of drumbeat seismicity registered during November were short- to moderate-duration, very low energy, and consisted of seismicity associated with rock fracturing (VT). Multiple steam plumes were visible from communities tens of kilometers away (figure 120).

Figure 120. Multiple dense steam plumes were photographed from communities around Nevado del Ruiz during November 2020, including on 18 (top) and 20 (bottom) November. Top image courtesy of Jose Fdo Cuartas, bottom image courtesy of Efigas Oficial.

Seismic activity increased in December 2020 relative to November. It was characterized by continuous volcanic tremor, tremor pulses, long-period (LP) and very long-period (VLP) earthquakes. Some of these signals were associated with gas and ash emissions, one confirmed through the webcams. The Washington VAAC reported ash emissions on 5 and 7 December. The first rose to 5.8 km altitude and drifted NW. The second rose to 6.7 km altitude and drifted W. A single discrete cloud was observed 35 km W of the summit; it dissipated within six hours. Drumbeat seismic activity increased as well in December; the episode on 3 December was the most significant. Steam and gas emissions continued throughout the month; a plume of gas and ash reached 1,700 m above the summit on 20 December, and drifted NW.

Sentinel-2 satellite data showed at least one thermal anomaly inside the Arenas crater each month during August-December 2020, corroborating the seismic evidence that the dome continued to grow throughout the period (figure 121). Sulfur dioxide emissions were persistent, with many days every month recording DU values greater than two with the TROPOMI instrument on the Sentinel 5-P satellite (figure 122).

Figure 121. Thermal anomalies at Nevado del Ruiz were recorded at least once each month during August-December 2020 suggesting continued growth of the dome within the Arenas crater at the summit. Courtesy of Sentinel Hub Playground.

Figure 122. Sulfur dioxide emissions were persistent at Nevado del Ruiz during August-December 2020, with many days every month recording DU values greater than two with the TROPOMI instrument on the Sentinel 5-P satellite. Ecuador’s Sangay had even larger SO2 emissions throughout the period. Dates are at the top of each image. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Additional reports of activity during 2017. Activity appears to have continued during June-December 2017. Ash emissions were reported by the Bogota Meteorological Weather Office (MWO) on 13 May, and by SGC on 28 May. During June, some of the recorded seismic events were associated with minor emissions of ash; these were confirmed by webcams and by field reports from both the staff of SGC and the Los Nevados National Natural Park (PNNN). Ash emissions were confirmed in webcams by park officials on 3, 16, and 17 June. Gas emissions from the Arenas crater during July 2017 averaged 426 m above the crater rim, generally lower than during June. The emissions were mostly steam with small amounts of SO₂. Emissions were similar during August, with most steam and gas plumes drifting NW. No ash emissions were reported during July or August.

SGC reported steam and gas plumes during September that rose as high as 1,650 m above the crater rim and drifted NW. On 21 September the Washington VAAC received a report of an ash plume that rose to 6.4 km altitude and drifted NNW, although it was not visible in satellite imagery. Another ash emission rising to 6.7 km altitude was reported on 7 October; weather clouds prevented satellite observation. An episode of drumbeat seismicity was recorded on 9 October, the first since April 2017. While SGC did not explicitly mention ash emissions during October, several of the webcam images included in their report show plumes described as containing ash and gas (figure 123).

Figure 123. Plumes of steam, gas, and ash rose from Arenas crater at Nevado del Ruiz most days during October 2017. Photographs were captured by the webcams installed in the Azufrado Canyon and Cerro Gualí areas. Courtesy of SGC (INFORME DE ACTIVIDAD VOLCANICA SEGMENTO NORTE DE COLOMBIA, OCTUBRE DE 2017).

The Washington VAAC received a report from the Bogota MWO of an ash emission that rose to 6.1 km altitude and drifted NE on 8 November 2017. A faint plume was visible in satellite imagery extending 15 km NE from the summit. SGC reported that plumes rose as high as 2,150 m above the rim of Arenas crater during November. The plumes were mostly steam, with minor amounts of SO₂. A diffuse plume of ash was photographed in a webcam on 24 November. SGC did not report any ash emissions during December 2017, but the Washington VAAC reported “a thin veil of volcanic ash and gases” visible in satellite imagery and webcams on 18 December that dissipated within a few hours. In addition to the multiple reports of ash emissions between May and December 2017, Sentinel-2 thermal satellite imagery recorded at least one image each month during June-December showing a thermal anomaly at the summit consistent with the slowly growing dome first reported in August 2015 (figure 124).

Figure 124. Thermal anomalies from the growing dome inside Arenas crater at the summit of Nevado del Ruiz appeared at least once each month from June-December 2017. A strong anomaly was slightly obscured by clouds on 3 June (top left). On 2 August, a steam plume obscured most of the crater, but a small thermal anomaly is visible in its SE quadrant (top right). Strong anomalies on 30 November and 20 December (bottom) have a ring-like form suggestive of a growing dome. Atmospheric penetration rendering (bands 12, 11, 8A), courtesy of Sentinel Hub Playground.

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

Information Contacts: 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); 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/); Gonzalo (URL: https://twitter.com/chaloc22/status/1306581929651843076); Jose Fdo Cuartas (URL: https://twitter.com/JoseFCuartas/status/1329212975434096640); Vlucho666 (URL: https://twitter.com/vlucho666/status/1313791959954268161); Efigas Oficial (URL: https://twitter.com/efigas_oficial/status/1329780287920873472).

A seismic instrument at Arenal has registered declining seismicity since late 1996 (figure 87). The instrument resides at Arenal Observatory Lodge, 2.8 km S of the summit. This three-component instrument (a Marks Products L-4-3D seismometer) interfaced to an automatic data acquisition system has been in nearly continuous operation for several years. The automated system looked at the output during 19-second-long intervals. The plots show seismicity in terms of the monthly percentage of these intervals with maximum amplitudes over 0.1 mV. Each plot corresponds to one of the instrument's three orthogonal components. The vertical component had the lowest amplitudes with only a few percent of the intervals over the threshold, and those occurred mainly in 1995-96. The horizontal components behaved with broad-scale similarity; but the N-S component had more vigorous response, in late 1996 and 1997 over 20% of the intervals extended over the stated threshold.

Figure 87. Arenal's monthly seismicity during early 1995-late 1998 as measured 2.8 km S of the summit. The vertical axis, scaled identically on all three components, shows the percent of intervals registered over the threshold of 100 microvolts. The missing interval on the N-S (radial) component reflects instrument malfunction. From late 1996 until the last reported data point in late 1998, all three components showed declining seismicity. Courtesy of William Melson and Sara James.

At Arenal, seismic levels have typically correlated positively with the intensity of pyroclastic outbursts. Decreased seismic intensity has often correlated with the escape of lava flows without pyroclastic outbursts. In harmony with the seismic data, observers noted both escaping lavas and fewer pyroclastic outbursts during much of 1998.

Geologic Background. Conical Volcán Arenal is the youngest stratovolcano in Costa Rica and one of its most active. The 1670-m-high andesitic volcano towers above the eastern shores of Lake Arenal, which has been enlarged by a hydroelectric project. Arenal lies along a volcanic chain that has migrated to the NW from the late-Pleistocene Los Perdidos lava domes through the Pleistocene-to-Holocene Chato volcano, which contains a 500-m-wide, lake-filled summit crater. The earliest known eruptions of Arenal took place about 7000 years ago, and it was active concurrently with Cerro Chato until the activity of Chato ended about 3500 years ago. Growth of Arenal has been characterized by periodic major explosive eruptions at several-hundred-year intervals and periods of lava effusion that armor the cone. An eruptive period that began with a major explosive eruption in 1968 ended in December 2010; continuous explosive activity accompanied by slow lava effusion and the occasional emission of pyroclastic flows characterized the eruption from vents at the summit and on the upper western flank.

Information Contacts: Jorge Barquero, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; William Melson and Sara James, Department of Mineral Sciences, Smithsonian Institution, Washington, DC 20560-0119 USA.

Atmospheric lidar measurements from Germany (table 16) from July through December 1998 showed no significant change compared to levels recorded earlier in 1998 (Bulletin v. 23, no. 6). Layer altitudes were in the 12-29 km range, with peaks at 14.0-21.9 km.

Table 16. Lidar data from Germany (July-December 1998) showing altitudes of aerosol layers. Backscattering ratios are for the Nd-YAG wavelength of 532 nm, with the equivalent ruby values (690 nm) in parentheses. Courtesy of Horst Jäger.

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: Horst Jäger, Fraunhofer-Institut fuer Atmosphaerische, Umweltforschung, IFU, Kreuzeckbahnstr., 19 D-82467, Garmisch-Partenkirchen, Germany.

This report is based primarily on official releases mainly covering December 1998. On 1 December three lava lobes existed, and the longest (central) about 800 m long and 350 m wide. Loose material traveled downslope 4.5 km. Monitored parameters indicated that the volcano was relatively stable, suggesting that neighboring communities were not at risk. On 1 December, the evacuation order was rescinded for the SW- flank community of Yerbabuena, 9 km from the summit. However, scientists reported increased activity on 9 December, when the longest flow reached 1.2 km from the summit. Summit winds changed at that time and began blowing ash away from villages.

By 10 December the central lobe had extended to ~1.7 km in length, and the E lobe reached a distance of ~1.3 km from the summit. In accord with these advances, the number of incandescent blocks escaping both near the summit and at the lava fronts increased. Figure 29 shows the three lava lobes on 11 December.

A press release on 14 December stated that during the previous 72 hours the volcano had generally remained at low intensity. The width of the central lava flow width remained at 350 m but in the following days the front crept forward to reach the following lengths: on 14 December; ~1.8 km; on 16 December, ~1.9 km; and on 4 January, ~2.9 km. The shorter, E lobe on 14-15 December had remained at ~1.35 km from the summit; on 16 December it reached 1.40 km. Blocks continued to break off and feed small avalanches but they remained within a 4.5-km radius.

Figure 29. Fresh block lava flowing down Colima's upper slopes as seen from the air off the SW side on 11 December 1998. Volcan de Colima appears in the right foreground; the peak of Nevado de Colima can be seen in the left-central part of the photo obscured by haze. Lava flows followed incisions of the Cordoban drainage (barrancas). From left to right these headwaters comprise the Western, Central, and Eastern Cordoban. Photograph by Juan Carlos Gavilanes, Colima Volcano Observatory.

Figure 30. A block-and-ash flow seen at Colima on 11 December 1998 descending from the dome on the W side of Barranca Cordoban and beside one of the new lava flows. View is from a point at 2,150 m, 4.25 km S of the dome. Photograph by Juan Carlos Gavilanes, Colima Volcano Observatory.

SO₂ monitoring. A COSPEC flight on 31 December 1998 made five transects below the plume that resulted in an SO₂ flux estimate of 4,930 ± 1,040 metric tons/day. An important component of a flux estimate comes from the wind velocity measurement, in this case computed by GPS. The average value was 4.7 m/s.

Satellite views. Peter Mouginis-Mark and over 10 other collaborators, colleagues, and co-workers have created a website displaying processed GOES 8 and GOES 10 satellite images of potential hot spots. Colima is one of eleven selected sites; each site gets imaged by a GOES satellite an average of once every 15 minutes.

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: Mauricio Breton Gonzalez; Carlos Navarro Ochoa, and Juan Carlos Gavilanes, Colima Volcano Observatory, Universidad de Colima, Ave. 25 de Julio 965, Colima 28045, Colima, México; Peter Mouginis-Mark, GOES Hotspot Monitoring System, Hawaii Institute of Geophysics and Planetology, University of Hawaii, 2525 Correa Road, Honolulu, Hawaii 96822 (URL: http://modis.higp.hawaii.edu/).

The following report summarizes activity observed at each of the four summit craters of Etna from October through December 1998. Bocca Nuova and Voragine exhibited some explosive activity during this period, but Northeast Crater was quiet. Southeast Crater had 15 distinct eruptive episodes. Most of the information for this report was compiled by Boris Behncke at the Istituto di Geologia e Geofisica, University of Catania (IGGUC), and posted on his internet website. The compilation was based on personal visits to the summit, telescopic observations from Catania, and other sources.

Activity at Southeast Crater (SEC). Poor visibility precluded observations after 30 September, when intense activity was rapidly building the intracrater cone and a new lava flow was spilling down the SW flank of the SEC cone (figure 74). On the evening of 1 October, no incandescence was visible at the crater, and brief glimpses of the summit during the following days revealed that no further growth of the intracrater cone had occurred. A group encamped ~800 m S of SEC reported that on 4 October there were no eruptions, but that activity renewed during the night.

Figure 74. Sketch map of Etna's summit craters showing recent eruption products as of 1 November 1998. The approximate extent of recent lava flows from Southeast Crater has been added. This map is not completely accurate regarding the distribution of the new flows, but locates some frequently mentioned features. Courtesy of Boris Behncke.

The summit was visited on 5 October by a group including Boris Behncke and Giovanni Sturiale (IGGUC), Marco Fulle (Trieste Astronomical Observatory), and Jürg Alean (Stromboli On-line). Strombolian bursts hurled incandescent bombs up to 200 m above the vent. A small conelet a few meters high that had grown around this vent was destroyed by explosions around 1300, and the active vent widened to 8-10 m diameter, with a low pyroclastic mound around it. Meter-sized lava blobs jetted continuously from the vent, and bombs showered SEC and its N flank. Many explosions were caused by the bursting of magma bubbles. Although the intracrater cone had grown significantly, it did not entirely fill the 15 September explosion crater. Alean stayed on the summit until the late evening of 5 October and returned the next afternoon. Strombolian activity culminated in a paroxysmal eruptive episode (the sixth since the crater resumed its activity on 15 September) on the evening of 5 October when fluid lava moved ~1 km down its E flank within a few hours. Strombolian activity at the intracrater cone was weaker, but increased slightly during the evening.

Vigorous eruptive activity resumed on the evening of 11 October at SEC, which had shown only low levels of activity during the preceding days. The renewed activity (the eighth eruptive episode) consisted of lava fountains and a lava flow that extended ~700 m downslope adjacent to the 5 October flow. Strombolian activity at the intracrater cone continued the next morning. Fulle reported "zero activity" at SEC on 13 October.

The episode of lava fountaining and lava emission during the night of 11-12 October was the seventh at SEC since the reawakening of the crater on 15 September. The activity had established a repetitive pattern of periods of relative calm or very low-level activity that lasted up to several days, followed by episodes of very intense Strombolian activity that culminated in lava fountains and short-lived, rapid effusion of lava flows for 1-2 days. The same crater displayed a series of episodes in September 1989 that marked the uprise of a voluminous batch of fresh, gas-rich magma, and culminated in a flank eruption in Valle del Leone. That eruption was accompanied by the formation of a non-eruptive fracture system down the SW flank to about 1,600 m elevation, close to one of the most densely populated areas on Etna. The new eruptive episodes at SEC, however, were of much smaller magnitude and occurred at greater intervals, and there was no geophysical evidence that magma was intruding at shallow levels into the fracture systems that radiate from the central conduit system.

Strombolian activity at SEC resumed on the evening of 16 October, after three days of no eruptive activity. According to Fulle, who witnessed the resumption of activity, there were first some high-pressure gas emissions during the late afternoon, without the ejection of pyroclastics. Strombolian activity had initiated sometime before 1900. Remote observation with binoculars from 5 km N of Catania by Sturiale during the night of 17-18 October revealed that the intracrater cone fractured on its southern side and issued lava. While Strombolian activity from the summit vent of the intracrater cone culminated in about nine hours of paroxysmal activity with lava fountains several hundred meters high, the new lava flow advanced in up to five lobes a few hundred meters downslope, slowing at the base of the SEC cone. Further flows spilled down the E and W sides of the cone. According to Sturiale, the most intense activity occurred around 0300; the episode ended at around 0630. The intracrater cone had merged with the N outer flank of the pre-1997 SEC cone; the summit of the cone was conservatively estimated to stand at 3,220 m, 30-40 m higher than the highest pre-1997 rim of SEC.

On 24 October SEC produced its ninth eruptive episode since 15 September. Activity began to intensify at around 1700 and was at its climax between 1900 and 2100 when Strombolian bursts jetted hundreds of meters above the cone. At times several vents appeared to be active. The main lava flow advanced to the base of the intracrater cone where it bifurcated into at least five lobes that spilled down the S flank. As of 2100, these active lobes had reached the base of the cone, and movement appeared to be slowing. Another lava flow spilled down the SW side of the SEC cone. Like the previous episode, the 24 October eruption was preceded by about 24 hours of weak Strombolian activity on the evening of 23 October. The paroxysmal event itself lasted only a few hours but was very intense, with about 2 hours of near-continuous lava fountaining. The lava flows on the S flank came close to the tourist lookout ~500 m N of the Torre del Filosofo hut, and then turned SE towards Valle del Bove, reaching ~1 km in maximum length. The SW lava flow did not extend beyond the base of the SEC cone. By 2300 all activity was over, but a brief revival of Strombolian activity occurred at around 0200 the next morning.

The tenth eruptive episode from SEC in seven weeks took place on the early morning of 1 November. In a characteristic pattern established during the recent episodes, the 1 November event was preceded on 31 October by the resumption of very mild Strombolian activity, and an increase in seismicity. While no effusive or explosive activity was evident until shortly after midnight (observation by Sturiale), lava began to spill down the S flank of the SEC cone before 0030. Low fountains began to play in the summit vent by 0130, and continued through at least 0430. The culminating phase began at around 0500 and lasted two or three hours; during this phase lava fountains continuously jetted hundreds of meters above the erupting vent, and numerous lava lobes spilled down the S flank of the SEC. Two lobes stopped about 100-150 m short of the tourist outlook, but other lobes turned SE at the base of the cone and reached ~600 m from the crater. Loud explosion noises were audible in towns on the lower flanks of Etna. It appears that initially the magma rose within the conduit and overflowed quietly without being accompanied by vigorous degassing, and this relatively quiet phase lasted a few hours. The 24 October episode was also reported to have initiated with the quiet overflow of lava prior to vigorous fountaining.

Behncke and Carmelo Monaco (IGGUC) visited the summit craters starting on 1300 on 1 November, roughly six hours after the end of that morning's eruptive episode and cessation of all lava outflow. Mild Strombolian activity continued through 1700, but there was no active or incandescent lava and Behncke was able to approach the spillover point on the S side of the intracrater cone, walking on still-hot but stagnant lava emplaced that morning. The spill-over area was a narrow channel, ~10 m deep, whose upper sides were plastered with large spatter; this channel extended to the base of the intracrater cone where it divided into two major channels that fed the lava flows on the outer S flank of the SEC cone. About 20-25 m farther W a similar spillover channel partly filled with 1 November ejecta was probably active during the 24 October episode.

The most striking effect of the five eruptive episodes since 5 October was the growth of the intracrater cone, which had become an imposing structure occupying almost all of the former SEC depression. A crater ~25-30 m wide occupied the summit of the intracrater cone.

Weak and infrequent Strombolian activity began again on the evening of 6 November; the next morning, SEC produced eruptive episode 11. Strombolian activity gradually increased through the night of 7 November and early morning, and the culminating phase of the episode began around 0830 on 8 November. By 1100, vigorous fountaining from the summit was accompanied by lava outflow onto the S flank. Shortly after 1330 the main phase of the episode was over, and no active lava was visible.

As of 16 November there had been no significant activity since 7 November, as revealed by seismic data (information from G. Patanè of the Osservatorio Sismologico di Acireale and IGGUC) and the lack of morphological changes to the summit cone. Sandro Privitera (IGGUC) reached the Torre del Filosofo hut on 15 November and witnessed a single ash emission from the cone before clouds hampered observations.

After 11 days of silence SEC produced its 12th eruptive episode in nine weeks on 18 November. After several days of weak seismicity, earthquakes began to increase in frequency during the late afternoon of 17 November (information from Patanè), and weak Strombolian activity began sometime around 2000 (information from J.C. Tanguy). This activity continued throughout the night, gradually increasing in vigor. The most intense activity occurred around 1030-1130 with high lava fountains, frequent ash emissions, and lava overflow onto the S flank. By 1230, most pyroclastic activity had ceased, and lava movement apparently stopped, although vigorous steaming from the new lava continued, and intense seismicity persisted for some time.

The 13th episode occurred on 29 November, again after a quiet interval of 11 days. Due to bad weather conditions, the activity could not be observed, but loud detonations were audible 25-30 km from the summit. The effects of this episode were studied during a visit on 3 December by Behncke. Lava had spilled through the breach in the S crater rim and reached the base of the cone. The summit of the newly formed cone at SEC was climbed to observe the vent that had produced all the recent activity; there was no eruptive activity, and only weak gas emissions occurred. The summit crater was ~50-80 m wide, its rim being highest on the SE side. The crater floor was relatively flat and had a central pit ~15 m wide in its center. From the crater rim it was possible to see that the summit of SEC was only about 20 m lower than the rim of the former summit crater (elevation 3,260 m), and thus SEC has grown at least 60-80 m since mid-1997.

It was SEC more than 14 days later that SEC began its fourteenth eruptive episode in three months, on 13-14 December. As usual, Strombolian activity began some 24 hours or so earlier. Carmelo Monaco (IGGCT) heard explosion sounds at Montagnola from the direction of SEC, but clouds prevented observations. However, at about 1930 the summit became visible from Catania, when Strombolian bursts occurred every few seconds. Between 2000 and 2030 a growing incandescent spot became visible below the fountain. During the next two hours, lava spilled down the S flank, and pyroclastic ejections became gradually stronger. The culminating phase began at about 0430, marked by strong seismic activity (information from Patanè). Tephra was carried S, leaving a dark streak on the snow. On 14 December, when viewed from Catania, the cone of SEC was covered with new pyroclastics and appeared to have grown; activity had returned to low levels.

The fifteenth eruptive episode from SEC occurred on 29 December, after the longest quiet interval between two episodes observed so far, and was essentially similar to the preceding episodes, with vigorous lava fountaining, tephra emission, and small lava flows.

Activity at Bocca Nuova (BN), Voragine, and Northeast Crater (NEC). The summit craters were visited on 5 October by Behncke, Sturiale, Fulle, and Alean. NEC was limited to forceful gas emission from a 30-m-wide vent on the floor of its about 80-m-deep central pit. Activity in the Voragine occurred in one vent in its SW part, which was ~100 m wide, tens of meters deep, and ejected bombs in near-continuous bursts; four other vents in the Voragine were degassing quietly. Within BN, both the NW and SE vent areas produced Strombolian activity. At the former, two vents in the W part of the cone were the sites of continuous minor bomb ejections culminating in fountains ~100 m high every 5-15 minutes. Only very few bombs fell outside the crater, but abundant fresh-looking bombs indicated that stronger activity had occurred within the preceding two days. Extensive fracturing of the lava flow that had entered the Bocca Nuova on 22 July indicated that minor subsidence had also affected a wider area. At the SE vents continuous Strombolian activity occurred from two vents in the collapse depression formed in early 1998.

Alean reported that activity in the Voragine and the SE vents in BN was stronger on 6 October. Fulle indicated that low-level activity persisted through 10 October and that during his observations on 12 and 13 October there was ongoing eruptive activity in BN and the Voragine. Eruptions from the SW vent in the Voragine ejected bombs into BN. An increase in the vigor of the ejections of the NW vent in BN was noted by Fulle on 15-16 October; explosions from that site ejected large (up to 1.5 m) black bombs onto the NW and N crater rims. On the morning of 25 October there was a dense gas plume issuing from BN.

Behncke and Monaco observed activity at BN and the Voragine on 1 November. In the former, the NW cone did not produce visible eruptions although explosion sounds could occasionally be heard. In the SE eruptive area three vents were the site of Strombolian activity. For the first time since the 22 July eruption it was possible to enter the Voragine, which was much shallower than before that event. Only the large SW vent was erupting, but that activity was very deep-seated, and only on one occasion did bombs rise above the lip of the vent. Very little degassing occurred from the large central vent, and the general impression was that the Voragine was quieter than at any time during the past six months. A weak gas plume was seen rising from the NEC central pit. There was continued weak activity in BN and Voragine through at least 10 November.

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: Boris Behncke, Istituto di Geologia e Geofisica, Palazzo delle Scienze, Universitá di Catania, Corso Italia 55, 95129 Catania, Italy.

As late as 16 January, the volcanic crisis near Ecuador's capital, Quito, continued as visually observed activity and seismic indices showed little sign of either halting or escalating. Relevant histograms showing the seismic indices were posted by the Instituto Geofísico, Escuela Politécnica Nacional (IG-EPN) on their website. In addition to photos and regular updates, the site discusses such topics as civil defense and hazard planning. The current eruptive crisis began when a series of modest phreatic eruptions followed the large 4 August earthquake that struck near the coast ~175 km SW (BGVN 23:08 and 23:09). This summary covers the interval 28 October 1998-16 January 1999.

Summit observations on 27 October disclosed fumaroles off-gassing at both the 1981 crater and the adjacent 1998 crater, and steam wafting to 300 m. The last few days of October were marked by comparative quiet, with few phreatic eruptions, and associated tremor under 30 minutes in duration.

On 9 November seismically detected explosions reached a new high of 4/day; the previous maxima occurred when 3/day took place on two days in mid-October. During 22 November through 22 December instruments detected very few explosions. A visit to the crater area on 20 November disclosed abundant fresh debris on the rim, and numerous impact craters scattered about the area. Poor weather prohibited systematic determination of ash plume heights, but on 23 December one plume rising to 3 km altitude was seen from Quito. Explosions then resumed, with three explosions on 10 January. Patterns in the number of daily multiphase, long-period, and volcano-tectonic earthquakes were somewhat similar, with lows in the weeks surrounding 28 November and significant upswings thereafter. Many seismic events were shallow, at depths of several kilometers. Intervals of spasmodic tremor up to two or three hours also occurred during the reporting interval.

Scientists computed reduced displacements of the seismically detected explosions. For the following intervals the maxima can be summarized as follows: November, 14.8 cm²; December, 15 cm²; and 1-16 January, 13.3 cm². Two so-called "tornillo" (screw-type) seismic events were noted on 3 January. The term arises from the seismic record of these events that looks like the profile of a screw. The broad, higher amplitude portion of the screw occurs early in the arrival sequence; the signal's amplitude decays slowly, finally reaching background at the point of the screw.

On 13 November field workers observed deposits from mud and debris flows that had come down the Cinto and Cristal rivers on 4 November. Triggered by a small rockslide on the SW flank, these flows were also seen by WSW-flank inhabitants who reside in the village of La Playa. The flows could be observed up to 15 km from the crater. The flow material also mixed with thin ash produced during phreatic explosions.

On 19 November a guard at a local refuge reported an absence of activity at both the crater and fumaroles; however, at 1049 that day, the fumarole known as 'La Locomotora,' located on the caldera's S wall, expelled vapor reaching 400 m high. This fumarole remained active throughout the reporting period. Another fumarole, known as Las Alineadas, escaped from the S dome area and was briefly mentioned on several days during mid-December as the source of sulfurous gases and loud noises; on 26 December Alineadas discharged an 800-m-tall vapor plume. On 13, 14, and 16 January crater fumaroles gave off vapor plumes that reached 1 km in altitude.

Press reports. An unusually clear, though undated photo of the two intracaldera craters can be found at the El Comercio website. The same site has over 70 articles (in Spanish) devoted to Guagua; pieces that are direct, practical, informative, and—given the circumstances—surprisingly upbeat. They convey a sense of the human side of a volcanic crisis without undue sensationalism. Topics include: broccoli growth in the volcano's soil, the vulnerability of a local marketplace in the event of an eruption, "Geophysicists—the volcano doctors," "Gasoline: there is a distribution plan," and the merging of art and science in an attempt to glean past eruptive behavior.

One article, titled "Guagua: mud and ash could effect 31,000," discusses the poor state of roofs in 18 separate zones in N upland areas along the E half of Quito's urban margins, an area described as high-risk. An impressive figure illustrates the locations and names of the zones, their populations, and shows how each ranks in terms of relative risk from mud flows and ash fall. Finally, for each of these zones, the figure indicates the average estimated risk of roofs to weight-bearing loads.

Geologic Background. Guagua Pichincha and the older Pleistocene Rucu Pichincha stratovolcanoes form a broad volcanic massif that rises immediately to the W of Ecuador's capital city, Quito. A lava dome is located at the head of a 6-km-wide breached caldera that formed during a late-Pleistocene slope failure ~50,000 years ago. Subsequent late-Pleistocene and Holocene eruptions from the central vent in the breached caldera consisted of explosive activity with pyroclastic flows accompanied by periodic growth and destruction of the central lava dome. One of Ecuador's most active volcanoes, it is the site of many minor eruptions since the beginning of the Spanish era. The largest historical eruption took place in 1660, when ash fell over a 1000 km radius, accumulating to 30 cm depth in Quito. Pyroclastic flows and surges also occurred, primarily to then W, and affected agricultural activity, causing great economic losses.

Information Contacts: Instituto Geofísico, Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador; El Comercio newspaper, Quito, Ecuador (URL: http://www.elcomercio.com); El Universo newspaper, Quito, Ecuador (URL: http://www.eluniverso.com).

Seismicity remains elevated. The low-level Strombolian eruptive activity that has characterized the volcano for the past two years continued during December. About 300-400 earthquakes and gas explosions occur every day. Satellite imagery on 16 December showed an ash-poor plume extending 200 km E. No change in seismicity was noticed. The level of concern color code remained yellow.

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

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

During 7-27 December seismicity under the volcano was generally at background. Hypocenters concentrated both at shallow depths near the summit crater and at depths of 25-30 km. On 7 December a fumarolic plume rose 500 m above the crater and extended >10 km E. During 8-11 December a plume rose 50 m above the crater before moving 2-3 km SE and E. On 21 December the plume rose 100 m above the crater, extending 10 km NW. On most other days during December, the volcano was obscured by clouds.

Beginning at 2352 on 23 December a series of shallow earthquakes with magnitudes smaller than M 2 began to be recorded beneath the volcano and at distances of >100 km. At 0400 on 24 December the activity abruptly decreased, although remaining still slightly above background until 1000 that day. Satellite images obtained during and after this anomaly did not show large areas of airborne ash. The level of concern color code was increased to yellow.

Geologic Background. Klyuchevskoy (also spelled Kliuchevskoi) is Kamchatka's highest and most active volcano. Since its origin about 6000 years ago, the beautifully symmetrical, 4835-m-high basaltic stratovolcano has produced frequent moderate-volume explosive and effusive eruptions without major periods of inactivity. It rises above a saddle NE of sharp-peaked Kamen volcano and lies SE of the broad Ushkovsky massif. More than 100 flank eruptions have occurred during the past roughly 3000 years, with most lateral craters and cones occurring along radial fissures between the unconfined NE-to-SE flanks of the conical volcano between 500 m and 3600 m elevation. The morphology of the 700-m-wide summit crater has been frequently modified by historical eruptions, which have been recorded since the late-17th century. Historical eruptions have originated primarily from the summit crater, but have also included numerous major explosive and effusive eruptions from flank craters.

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

The ongoing Vulcanian eruption at Crater 2 continued throughout November and December. Emissions consisted chiefly of gray ash clouds that drifted SW, resulting in fine ashfall. On 2 November a significant ash column was ejected forcefully up to ~2 km above the crater. Emissions during November were sometimes accompanied by roaring and rumbling sounds. No night glow was reported.

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

Information Contacts: Herman Patia, RVO.

Volcanologists observed pyroclastic flows and lava flows at Manam in mid-November and mild Strombolian eruptions during the last week of December.

November activity. In early November, Main Crater emitted pale gray ash clouds at irregular intervals, accompanied by roaring and rumbling, while South Crater released both white vapor and ash clouds that rose 500-800 m. On 6 November, following deep roaring and rumbling sounds, both craters emitted thick, dark, convoluting clouds that rose 600 m above the summit. Beginning at 0047 on 7 November, very loud explosions blasted out of South Crater at 5-20 minute intervals. The explosions led to ash columns that sent pyroclastic flows ~1 km down the SW valley. Later emissions produced a steadier and more forceful dark gray cloud accompanied by large explosions 20-60 minutes apart. At 1938 a large explosion sent pyroclastic flows ~2 km down the SW valley. Similar explosions were heard during the next few hours and incandescent projections were seen. A strong explosion at 2334 produced pyroclastic flows that ran ~2 km down SW Valley and 1 km down the SE valley. Similar activity continued until 10 November.

During the morning of 13 November frequent explosions led to small pyroclastic flows; later activity was irregular and accompanied by roaring noises. Activity increased at 1637 and sometime before 1900 lava began fountaining 180-350 m above the crater rim. Lava then spilled over the rim and flowed ~1-2 km down the SE and SW valleys. Ash emissions and loud noise were continuous while the lava was flowing. At 2100 the ash column abated, sending pyroclastic flows into the NW valley.

At 0221 on 14 November a voluminous ash column was produced and lava fragments ejected ~400-500 m above the summit. The incandescent projections within this column lit the mountain spectacularly. Later a very thick, dark gray ash cloud rose ~2 km above the summit and lava flowed ~2 km down the SE and SW valleys. Most ashfall was toward the SE, but a shower of ash with grain size of 1.5-2.0 mm fell at Tabele Observatory 4 km to the SW. Activity ended at about 2100 and afterward there were no noises heard or glows seen. From then until the end of the month activity at South Crater was restricted to mostly thin, white vapor emissions.

December activity. Emission from South Crater from 30 November to 20 December consisted mostly of white vapor with an occasional ash cloud rising 500 m above the summit. The ash clouds drifted SE and left a fine ashfall. Weak roaring noises were heard during 1-2 December and weak but steady glow was visible on the 1st.

A brief episode of Strombolian activity occurred on 24 December. At 1217 deep, weak explosions were followed by discontinuous, forceful emissions of dark-gray, convoluting clouds that rose 1,000-1,200 m above the summit. The emissions occurred at 1-2 minute intervals accompanied by roaring sounds. The activity continued until 1330 before declining to occasional emissions of thin, gray ash clouds. At night projections of incandescent lava fragments reached 200 m above the summit. Fine scoria and ashfall were reported on the SE of the island. During the next two days, moderate emissions were sometimes accompanied by forceful ejections of thick, dark gray ash clouds rising 800 m above the summit. A fluctuating glow and weak projections of glowing lava fragments were visible. From the 28th until the month's end South Crater released only white vapor while Main Crater released white vapor with occasional pale gray ash.

Geophysics. A steady accumulation of tilt took place before mid-November; it measured ~2 µrad at the radial water-tube tiltmeter of Tabele Observatory heralding renewed eruptive activity. Although during the mid-November eruption tilt appeared unaffected, seismicity reflected the changes in eruptive intensity; after 20 November a deflation of ~1.5 µrad was recorded. During 1-21 December, the tiltmeter recorded an inflation of ~1 µrad.

Geologic Background. The 10-km-wide island of Manam, lying 13 km off the northern coast of mainland Papua New Guinea, is one of the country's most active volcanoes. Four large radial valleys extend from the unvegetated summit of the conical basaltic-andesitic stratovolcano to its lower flanks. These valleys channel lava flows and pyroclastic avalanches that have sometimes reached the coast. Five small satellitic centers are located near the island's shoreline on the northern, southern, and western sides. Two summit craters are present; both are active, although most observed eruptions have originated from the southern crater, concentrating eruptive products during much of the past century into the SE valley. Frequent eruptions, typically of mild-to-moderate scale, have been recorded since 1616. Occasional larger eruptions have produced pyroclastic flows and lava flows that reached flat-lying coastal areas and entered the sea, sometimes impacting populated areas.

Information Contacts: Herman Patia, RVO.

Only three lakes in the world are known to contain high concentrations of dissolved gas in their bottom waters: Lakes Nyos and Monoun in Cameroon and Lake Kivu in East Africa. The release of large quantities of gas from lakes is very rare; however, massive carbon dioxide gas (CO₂) releases from Lake Monoun in 1984 (SEAN 09:08) and Lake Nyos in 1986 (SEAN 11:08) resulted in the loss of nearly 1,800 lives.

A joint team comprising U.S., Cameroonian, and Japanese scientists continues to investigate the cause of these lethal CO₂ releases, the potential for future events, and hazard remediation. The following is a summary of the team's preliminary findings as contained in a report for April-May 1998 (Kling and others, 1998).

The report stated that the total gas content in the two lakes was very high and continued to build from supplies of CO₂ that discharge from underground springs in the bottom of the lakes. Likelihood of a gas release can be estimated from the degree of gas saturation (the ratio of gas pressure to hydrostatic pressure) in the bottom waters. Measurements established that the subsurface gas pressure is mainly due to dissolved CO₂.

Figure 3 shows gas pressure plotted against depth in the lake from measurements made in Lake Nyos starting in 1989. Pressure has increased at all depths below 170 m since then; the largest increases occurred in the bottom 20 m of the lake. Gas pressure at lake bottom exceeds 13 bars, more than 60% of the saturation value based on an ambient hydrostatic pressure at that depth (21 bars).

Figure 3. Gas pressure as a function of depth in Lake Nyos for four years (1989, 1990, 1992, and 1998). The 210 m depth represents the lake bottom. From Kling and others (1998).

In Lake Monoun gas pressure also increased, reaching about 83% of the saturation pressure at 60-m depth. While the current gas saturation for both lakes remains below 100%, any large disturbance of the water column could trigger a violent release of the residual gas in these lakes. A frequently cited analogy to this process is the removal of the cap from a bottle of soda and the consequent drop in confining pressure enabling the gas dissolved in the soda to form bubbles. Once bubbles are formed in the lake they rise rapidly and drag the deep water toward the surface, drawing additional water upward in a chain reaction that can violently liberate enormous amounts of gas. Thus, the threat of a future lethal gas release is increasing.

A brief summary of the approximate water temperature and significant chemical parameters (alkalinity, pH, dissolved oxygen, and conductivity) in Lake Nyos as a function of depth is presented in table 1; the trends in Lake Monoun are similar. Comprehensive details are available in the complete report. Both lakes are warmer at shallow depths and near the bottom than at intermediate depths; for Nyos the coolest temperatures (~22.5°C) appear at near 40-m depth. Nyos lake water has become increasingly similar to that seen in 1985 prior to the massive CO₂ release.

Table 1. Some physicochemical parameters of water in Lake Nyos. The temperature values are estimates taken from plots in the original report. Conductivity refers to specific conductivity in microSiemens per centimeter. Data from Kling and others (1998).

A simple remedy to eliminate future lethal gas buildup involves removal of gas from the gas-rich bottom water of the lakes by pumping it through pipes to a suitable disposal area on the surface (BGVN 15:11). The scheme has been submitted by the Cameroonian government to the Japanese Embassy and Ministry of Foreign Affairs for funding. The energy released due to bubble formation (degassing) as the fluid rises is sufficient to drive the pumping operation without any external power source. This approach was validated by two demonstration projects, one in Lake Monoun in 1992 and one in Lake Nyos in 1995. In addition, future gas buildups can be prevented by continually flushing the bottom water out of the lakes through a pipe.

Lake Nyos has a weak natural dam at the outlet whose failure would cause a devastating flood that could affect up to 10,000 people in the downstream flood plain. However, pumping the gas-rich bottom water as noted above would also lower the lake water level, eliminating the flooding threat.

The people living close to these lakes need to be made aware of the risks from gas release and potential flooding. Toward this end, several Cameroonian Ministries have cooperated to produce a national plan for prevention and management of natural hazards.

In 1997 an international committee was established to help coordinate and advise the degassing efforts. This committee, named the NMDP Advisory Committee (Nyos-Monoun Degassing Project), includes members from six countries.

References. Kling, G., Evans, W., Tanyileke, G., and Kusakabe, M., 1998, Scientific investigation of Lakes Nyos and Monoun, Cameroon: Preliminary report, April-May 1998, for NMDP Advisory Committee. http://www.biology.lsa.umich.edu/~gwk/research/nm98rept.html.

Geologic Background. Numerous maars and basaltic cinder cones lie on or near the deeply dissected rhyolitic and trachytic Mount Oku massif along the Cameroon volcanic line. The Mount Oku stratovolcano is cut by a large caldera. The Oku volcanic field is noted for two crater lakes, Lake Nyos to the N and Lake Monoun to the S, that have produced catastrophic carbon-dioxide gas release events. The 15 August 1984, gas release at Lake Monoun was attributed to overturn of stratified lake water, triggered by an earthquake and landslide. The Lake Nyos event on 21 August 1986, caused at least 1,700 fatalities. The emission of ~1 km3 of magmatic carbon dioxide has been attributed either to overturn of stratified lake waters as a result of a non-volcanic process, or to phreatic explosions or injection of hot gas into the lake.

Information Contacts: George Kling, University of Michigan, Ann Arbor MI 48109, USA; William Evans, U.S. Geological Survey, Menlo Park CA 94025, USA; Gregory Tanyileke, IRGM, BP 4110, MINREST, Yaounde, Cameroon; Minoru Kusakabe, Okayama University, Misasa, Tottori-ken 682-0192, Japan.

The following explanation of the recent seismicity on Dominica (BGVN 23:11) was provided by John Shepherd of the Seismic Research Unit.

"There has indeed been a series of earthquakes in Dominica, West Indies, over the past few months. The sequence reached an apparent climax on 22-23 October 1998 when a total of about 370 earthquakes occurred, of which over 100 were felt. Since then earthquake numbers have declined irregularly with smaller maxima in numbers on 6 and 30 December 1998. At the present time (5 February 1999) the rate of activity has declined to a few earthquakes per week, which is about the background level which we have observed for the past 30 years.

"The earthquakes are part of a pattern which has continued for at least 250 years. The present earthquakes are NOT directly associated with Morne Patates or any other volcano in Dominica and there have been absolutely no other signs of volcanic activity. Dominica is in the center of the tectonically-active Lesser Antilles, and non-volcanic earthquakes are frequent. Dominicans refer to felt earthquakes as 'tremors.' This has no scientific significance; the earthquakes are conventional local earthquakes and no volcanic tremor has been recorded.

"There is undoubtedly a continuous volcanic hazard and many Dominicans have become concerned because this particular set of earthquakes follows closely on the recent events in the nearby island of Montserrat. For this reason the government of the Commonwealth of Dominica has conducted an intensive program of public awareness in which we have participated. We are also assisting in the preparation of an updated volcanic hazards map for Dominica."

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

Information Contacts: John B. Shepherd, Head of Seismic Research, The University of the West Indies, St. Augustine, Trinidad.

Weather clouds obstructed visibility during much of December. Scientists and civil authorities continued to recommend that no one get closer than 7 km from the crater. The hazard status remained Yellow.

At 0140 on 9 December a five-minute series of explosions ejected incandescent fragments over the flanks. These explosions were preceded by tremor and an A-type earthquake of low magnitude. At 0929 on 10 December another explosion ejected rocks onto the E flank (figure 29) and produced an ash column ~4 km high. High-frequency tremor was associated with the emission of gas, steam, and some ash plumes rising several thousand meters. An M 2.7 earthquake, 2 km SW and 11 km beneath the crater, occurred at 1839 on 13 December but did not affect eruptive activity.

Figure 29. Series of images showing the N flank of Popocatépetl from a monitoring camera taken at 0931-0932 on 10 December. Courtesy CENAPRED.

On 15 December the instrument station at Canario, on the N flank, went out of operation due to the intense eruptions of recent weeks. At 1750 on 15 December an explosion lasting one minute ejected incandescent fragments over a radius of 2-3 km. The explosion also produced an ash plume 3-4 km above the summit.

After an explosion at 1847 on 17 December activity fell immediately, with only isolated low-intensity exhalations and periods of high-frequency, low-amplitude tremor. According to field and aeronautical reports, the ash column reached 4-5 km above the crater and slowly dispersed ENE. Glowing fragments that fell on forested and grassy areas produced fires that persisted into the night but did not present hazards for nearby towns.

Following several A-type events, a moderate exhalation occurred at 2010 on 20 December. This event produced ash carried NW before falling over the airport in México City. Flight operations were closed between 2330 and 0115. Several hours of high-frequency, medium-intensity tremor were recorded during 24 December. At 0044 on 31 December an A-type earthquake took place with a magnitude of 3.5 and depth of 12 km under the summit.

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

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

Eruptive activity was continuous during November and December, dominated by intermittent emissions of small, pale-gray ash clouds from the Tavurvur cone. Some larger ash-laden explosions reached 2.5 km high.

Visual observations. The first 13 days of November were a continuation of the style of activity observed at the end of October (BGVN 23:11) when emissions occurred minutes or hours apart. Ash clouds were usually released quietly, but occasional dark, ash-laden explosions rose 600-1,500 m above the summit. At 1334 and 2019 on 6 November two such explosions occurred: the latter produced an ash column 3 km high and sent pyroclastic flows down the sides of the cone. Moderate explosions were also heard occasionally during 13-23 November, a period of otherwise low activity. On 27 November emissions of dark gray ash at 1712 and 1909 rose ~1,000 m. Emissions subsequently became more frequent, thicker, and darker. Another significant explosion occurred at 1500 on 29 November. Light ashfalls fell mainly to the SE (over the sea), although occasional shifts of wind resulted in ashfalls over populated areas.

Emissions during December also occurred at irregular intervals; however, emissions became more frequent during 5-19 December and at the end of the month. Six large explosions during the month produced dark, ash-laden plumes that rose more than 1,000-1,500 m. Three of these explosions (on the 8, 27, and 29 December) produced ash columns that rose 2,500 m and showered the flanks with lava fragments. The ash was blown mainly to the SE, but some wind changes resulted in fine ashfall over Rabaul. Field observations suggested that the fragments were accidental materials from the vent area. Occasional roaring noises were heard during the month.

Ground deformation. Ground deformations were very slow during both months, though still indicating an uplifting trend that has continued since April 1997. However, sea shore survey measurements showed a reversal, which might be an artifact of the large flow of ocean water westward due to the reversal of El Niño, although this phenomenon is not well understood.

Seismicity. Only 633 low-frequency events were recorded during November; 2,843 were recorded in December. During 10-12 and 29-30 November low-amplitude harmonic signals were recorded. Between 12 and 28 November, the pattern of seismicity was characterized by low-frequency events of low amplitude and long duration. A noticeable emergence of moderate and large explosions (1-3 per day between 14 and 20 November) occurred during this period. A sequence of high-frequency events having an average S-P interval of 3.5 s occurred NE of the caldera during 20-25 November. It was not discovered whether these events were from the usual NE focus of earthquakes (which have had an S-P interval of 1-2.5 s). After 29 November the number and amplitude of the events increased.

A marked increase to a daily average of 100 low-frequency events during 5-19 and 29-31 December was associated with more frequent ash emissions. During 20-28 December ~60 events were recorded daily. Short bands of harmonic tremor were recorded during the second week of December and again on the 18th, 22nd, and 27th. Two high-frequency events were located NE of the caldera.

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

Information Contacts: Herman Patia, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.

Activity during November was dominated by small-volume pyroclastic flows down the Gages, White River, and Tar River valleys. The pyroclastic flows reached the sea and left a narrow, deep cleft in the dome. Ash was deposited over the whole island, but heavy rains cleared the dust from inhabited areas. Seismicity was dominated by rockfalls and volcano-tectonic earthquakes, the latter occasionally occurring in swarms. Some of the larger seismic events were felt throughout the island.

Visual observations. As in October (BGVN 23:10), volcanic activity during November was dominated by intermittent, small pyroclastic flows from all of the dome flanks. On 2 November several small rockfall events were recorded, some followed by low-amplitude tremor.

At 0821 on 3 November a larger dome collapse sent pyroclastic flows down the Tar River as far as the sea and down the White River valley as far as Galway's Soufriere. The ash cloud from this event reached >3,100 m and drifted W. Most of the ash fell S of the Belham valley.

A major dome collapse occurred at 2117 on 5 November. The pyroclastic flows from this collapse traveled down the White River valley to the sea, depositing two blocky lobes on the White River delta. The surge cloud climbed halfway up the N slope of Fergus Mountain. A small, fresh, and predominantly fine-grained pyroclastic-flow deposit was also observed in Ginkgoes Ghaut near Reids Estate. The ash cloud from this event drifted W and reached a height of ~6,200 m. The pyroclastic flows originated from a deep gully between Chances Peak and the dome above Galway's.

Two small pyroclastic flows occurred at 0920 on 8 November and at 0847 on 9 November. These traveled down the White River and the associated ash clouds reached heights of ~1,800 and 3,100 m.

At 0607 on 12 November, the largest dome collapse in the current series occurred, followed by vigorous ash venting. Pyroclastic flows traveled down Gages, Tar River, and White River valleys. The ash cloud reached a height of ~7,700 m; ashfall covered the island but mainly affected the Richmond Hill area. The pyroclastic flows that traveled down Gages valley almost reached the sea at Plymouth; some burning was observed near the port buildings. For the first time, pyroclastic flows reached the War Memorial and the Post Office. Lobes of material reached into the Amersham area and a large water tower was transported into the upper parts of Parsons. Pyroclastic flows also reached the sea at the Tar River delta and the old coastline at the bottom of the White River valley. In the weeks following this collapse there were a few small pyroclastic flows and periods of low-amplitude seismic tremor coupled with ash venting.

Activity during November cut a deep channel into the dome. The channel is ~150 m deep and 30 m wide and bisects the dome between the head of the Tar River and the top of Gages valley. The channel sides are extremely steep and overhanging in places. Several large cracks formed in various sectors of the dome, including in the area above White River and Tyer's Ghaut.

On 16 November, deposits near the War Memorial showed a temperature of 386°C at a depth of 1 m. During 28-29 November, heavy rain caused mudflows down all flanks. New material was deposited on the Belham Bridge (1 m depth), in Plymouth, and on the airport runway.

Seismicity, deformation, and environmental monitoring. A swarm of volcano-tectonic (VT) earthquakes occurred on 1 November (42 events within about 3 minutes); the largest was felt throughout the island. The hypocenters were located SW of the volcano under Chances Peak. Rockfall signals and pyroclastic flows dominated seismicity (70% of recorded events). VT earthquakes (28% of recorded events) beneath the dome often followed rapidly after the larger collapse events. There was a second swarm of VT earthquakes on 25 November with 42 events within about 5 minutes; a pyroclastic flow occurred shortly after the swarm started.

GPS measurements made during the latter part of the month in collaboration with University of Puerto Rico staff determined that Long Ground has moved ~4 cm E since March 1998.

The miniCOSPEC measured an SO₂ flux of 740 metric tons per day on 2 November, similar to the flux measured the previous 2 months. Sulfur dioxide also was measured at ground level using diffusion tubes around the island. SO₂ levels varied depending on the prevailing winds, but overall were lower during November than in previous months.

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

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat, West Indies (URL: http://www. geo.mtu.edu/volcanoes/west.indies/soufriere/govt).

A multiple-vent crater, named Metra, took form on the floor of main 1978/90 crater during 7-11 January. The other active vent (PeeJay) in the main crater formed in August 1998 and continued to emit dense volcanic gas and steam with some volcanic ash. Volcanic tremor levels decreased with the decline in activity at Metra, but remain slightly above typical background for White Island.

A visit was made on 12 January to assess the ongoing activity, conduct a deformation survey, collect ash and gas samples, and service the seismic installation. Results from that visit are reported below. An Alert Level 2 remained; explosive eruptions producing ballistic ejecta are considered possible, particularly if Metra Crater reactivates.

PeeJay vent. The size of PeeJay vent, located at the base of the NW wall of 1978/90 crater (figure 35), had not changed appreciably since a visit in November 1998 (BGVN 23:10 and 23:11). During the January 1999 visit the only generally active vent was PeeJay. It emitted considerable volumes of gas and steam under high pressure, and carried a minor amount of dark gray ash. The plume rose to ~300-350 m before trailing off downwind 10-15 km. The volume of ash contained in the plume was less than that observed over the past month.

Figure 35. Sketch map of the crater area showing the position of the vents and ground deformation contours (heavy black lines; deformations in mm). Courtesy IGNS.

Measurements made near Peg Z, 140 m ENE of PeeJay, show that about 120 mm of fine ash had accumulated between 1 December 1998 and 12 January 1999, in at least 12 episodes of ashfall, mainly from PeeJay. Near Peg M, 280 m SE of PeeJay, only half that thickness was recorded.

Metra Crater. A considerable portion of the floor of 1978/90 Crater had collapsed, forming a multiple-vent, collapse-crater feature subsequently named Metra Crater (figure 35). The margins of this feature were characterized by scalloped areas that had subsided 5-10 m. On the N side were very fresh cracks in the ground; more collapse was deemed likely in this area. During the visit, Metra was essentially inactive but vivid white steam was emitted (figure 36). The deeper vents in Metra (15-20 m deep) contained pools of muddy, dark gray water and are likely the site of previously observed hydrothermal eruptions. Eruptions from Metra during the period from 7 January, when it was first seen, through 11 January produced a surrounding apron of closely spaced ballistic blocks on the 1978/90 crater floor. Scattered impact craters containing blocks up to 40 cm were seen on the 1978/90 crater rim in areas frequented by visitors, and isolated impacts were noted up to 350 m from Metra near Noisy Nellie. Observations on 17 January confirmed that no eruptions were occurring at Metra Crater; overnight rains had flooded the crater floor.

Figure 36. Close-up view of the new Metra crater at White Island, which appeared during January 1999. Courtesy IGNS.

Other observations. The ground deformation survey showed a consistent trend of minor deflation across the main crater floor, with continued subsidence near the rim of 1978/90 Crater. Data from two selected pegs (figure 37) show the large-scale post-1990 inflation and minor deflation over the last 2-3 months.

Figure 37. Plot showing temporal height changes of two selected pegs at White Island. Heights are in meters. Courtesy IGNS.

Volcanic tremor declined following the high that accompanied the formation of Metra (figure 38). Afterwards, tremor remained slightly higher than before the formation of the new vents. Tremor levels were low during 11-13 January before dramatically rising to a peak overnight on 14-15 January—the highest levels since those that accompanied the formation of the Metra Crater on 6-7 January. A further peak occurred on the evening of 15 January. Between 1030 and 1500 on 16 January explosive activity at Metra Crater tossed blocks up to 400 m from the crater. Observations from a helicopter operator, who was over the island during 1200-1220, suggested that multiple vents were active, each one erupting differently. Volcanic tremor levels reached a low on the morning of the 17th.

Figure 38. Plot showing volcanic tremor at White Island, 5-17 January 1999. Courtesy IGNS.

Discharge temperatures and characteristics for fumaroles on the main crater floor were little changed from previous measurements made on 1 December. Fumarole ##1 measured 111°C, but tubes removed from the vent were coated in molten sulfur, indicating temperatures in the conduit of at least 119°C. Elemental sulfur continued to accumulate near this and neighboring vents on the S crater wall at high rates. Discharges on Donald Mound and Gully were very weak. Noisy Nellie and ##13a discharge pressures were strong, with temperatures of 134°C and 115°C, respectively.

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

Information Contacts: Brad Scott, Wairakei Research Centre, Institute of Geological and Nuclear Sciences (IGNS) Limited, Private Bag 2000, Wairakei, New Zealand (URL: https://www.gns.cri.nz/).