Klyuchevskoy is part of the Klyuchevskaya volcanic group in northern Kamchatka and is one of the most frequently active volcanoes of the region. Eruptions produce lava flows, ashfall, and lahars originating from summit and flank activity. This report summarizes activity during October 2019 through May 2020, and is based on reports by the Kamchatkan Volcanic Eruption Response Team (KVERT) and satellite data.

There were no activity reports from 1 to 22 October, but gas emissions were visible in satellite images. At 1020 on 24 October (2220 on 23 October UTC) KVERT noted that there was a small ash component in the ash plume from erosion of the conduit, with the plume reaching 130 km ENE. The Aviation Colour Code was raised from Green to Yellow, then to Orange the following day. An ash plume continued on the 25th to 5-7 km altitude and extending 15 km SE and 70 km SW and reached 30 km ESE on the 26th. Similar activity continued through to the end of the month.

Moderate gas emissions continued during 1-19 November, but the summit was obscured by clouds. Strong nighttime incandescence was visible at the crater during the 10-11 November and thermal anomalies were detected on 8 and 10-13 November. Explosions produced ash plumes up to 6 km altitude on the 20-21st and Strombolian activity was reported during 20-22 November. Degassing continued from 23 November through 12 December, and a thermal anomaly was visible on the days when the summit was not covered by clouds. An ash plume was reported moving to the NW on the 13th, and degassing with a thermal anomaly and intermittent Strombolian activity then resumed, continuing through to the end of December with an ash plume reported on the 30th.

Gas-and-steam plumes continued into January 2020 with incandescence noted when the summit was clear (figure 33). Strombolian activity was reported again starting on the 3rd. A weak ash plume produced on the 6th extended 55 km E, and on the 21st an ash plume reached 5-5.5 km altitude and extended 190 km NE (figure 34). Another ash plume the next day rose to the same altitude and extended 388 km NE. During 23-29 Strombolian activity continued, and Vulcanian activity produced ash plumes up to 5.5 altitude, extending to 282 km E on the 30th, and 145 km E on the 31st.

Figure 33. Incandescence and degassing were visible at Klyuchevskoy through January 2020, seen here on the 11th. Courtesy of KVERT.

Figure 34. A low ash plume at Klyuchevskoy on 21 January 2020 extended 190 km NE. Courtesy of KVERT.

Strombolian activity continued throughout February with occasional explosions producing ash plumes up to 5.5 km altitude, as well as gas-and-steam plumes and a persistent thermal anomaly with incandescence visible at night. Starting in late February thermal anomalies were detected much more frequently, and with higher energy output compared to the previous year (figure 35). A lava fountain was reported on 1 March with the material falling back into the summit crater. Strombolian activity continued through early March. Lava fountaining was reported again on the 8th with ejecta landing in the crater and down the flanks (figure 36). A strong persistent gas-and-steam plume containing some ash continued along with Strombolian activity through 25 March (figure 37), with Vulcanian activity noted on the 20th and 25th. Strombolian and Vulcanian activity was reported through the end of March.

Figure 35. This MIROVA thermal energy plot for Klyuchevskoy for the year ending 29 April 2020 (log radiative power) shows intermittent thermal anomalies leading up to more sustained energy detected from February through March, then steadily increasing energy through April 2020. Courtesy of MIROVA.

Figure 36. Strombolian explosions at Klyuchevskoy eject incandescent ash and gas, and blocks and bombs onto the upper flanks on 8 and 10 March 2020. Courtesy of IVS FEB RAS, KVERT.

Figure 37. Weak ash emission from the Klyuchevskoy summit crater are dispersed by wind on 19 and 29 March 2020, with ash depositing on the flanks. Courtesy of IVS FEB RAS, KVERT.

Activity was dominantly Strombolian during 1-5 April and included intermittent Vulcanian explosions from the 6th onwards, with ash plumes reaching 6 km altitude. On 18 April a lava flow began moving down the SE flank (figures 38). A report on the 26th reported explosions from lava-water interactions with avalanches from the active lava flow, which continued to move down the SE flank and into the Apakhonchich chute (figures 39 and 40). This continued throughout April and May with sustained Strombolian and intermittent Vulcanian activity at the summit (figures 41 and 42).

Figure 38. Strombolian activity produced ash plumes and a lava flow down the SE flank of Klyuchevskoy on 18 April 2020. Courtesy of IVS FEB RAS, KVERT.

Figure 39. A lava flow descends the SW flank of Klyuchevskoy and a gas plume is dispersed by winds on 21 April 2020. Courtesy of Yu. Demyanchuk, IVS FEB RAS, KVERT.

Figure 40. Sentinel-2 thermal satellite images show the progression of the Klyuchevskoy lava flow from the summit crater down the SE flank from 19-29 April 2020. Associated gas plumes are dispersed in various directions. Courtesy of Sentinel Hub Playground.

Figure 41. Strombolian activity at Klyuchevskoy ejects incandescent ejecta, gas, and ash above the summit on 27 April 2020. Courtesy of D. Bud'kov, IVS FEB RAS, KVERT.

Figure 42. Sentinel-2 thermal satellite images of Klyuchevskoy show the progression of the SE flank lava flow through May 2020, with associated gas plumes being dispersed in multiple directions. 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/); 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).

Nyamuragira (also known as Nyamulagira) is located in the Virunga Volcanic Province (VVP) in the Democratic Republic of the Congo and consists of a lava lake that reappeared in the summit crater in mid-April 2018. Volcanism has been characterized by lava emissions, thermal anomalies, seismicity, and gas-and-steam emissions. This report summarizes activity during December 2019 through May 2020 using information from monthly reports by the Observatoire Volcanologique de Goma (OVG) and satellite data.

According to OVG, intermittent eruptive activity was detected in the lava lake of the central crater during December 2019 and January-April 2020, which also resulted in few seismic events. MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows thermal anomalies within the summit crater that varied in both frequency and power between August 2019 and mid-March 2020, but very few were recorded afterward through late May (figure 88). Thermal hotspots identified by MODVOLC from 15 December 2019 through March 2020 were mainly located in the active central crater, with only three hotspots just outside the SW crater rim (figure 89). Sentinel-2 thermal satellite imagery also showed activity within the summit crater during January-May 2020, but by mid-March the thermal anomaly had visibly decreased in power (figure 90).

Figure 88. The MIROVA graph of thermal activity (log radiative power) at Nyamuragira during 27 July through May 2020 shows variably strong, intermittent thermal anomalies with a variation in power and frequency from August 2019 to mid-March 2020. Courtesy of MIROVA.

Figure 89. Map showing the number of MODVOLC hotspot pixels at Nyamuragira from 1 December 2019 t0 31 May 2020. 37 pixels were registered within the summit crater while 3 were detected just outside the SW crater rim. Courtesy of HIGP-MODVOLC Thermal Alerts System.

Figure 90. Sentinel-2 thermal satellite imagery (bands 12, 11, 8A) confirmed ongoing thermal activity (bright yellow-orange) at Nyamuragira from February into April 2020. The strength of the thermal anomaly in the summit crater decreased by late March 2020, but was still visible. Courtesy of Sentinel Hub Playground.

Geologic Background. Africa's most active volcano, Nyamuragira, is a massive high-potassium basaltic shield about 25 km N of Lake Kivu. Also known as Nyamulagira, it has generated extensive lava flows that cover 1500 km2 of the western branch of the East African Rift. The broad low-angle shield volcano contrasts dramatically with the adjacent steep-sided Nyiragongo to the SW. The summit is truncated by a small 2 x 2.3 km caldera that has walls up to about 100 m high. Historical eruptions have occurred within the summit caldera, as well as from the numerous fissures and cinder cones on the flanks. A lava lake in the summit crater, active since at least 1921, drained in 1938, at the time of a major flank eruption. Historical lava flows extend down the flanks more than 30 km from the summit, reaching as far as Lake Kivu.

Information Contacts: 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/exp.

Nyiragongo is located in the Virunga Volcanic Province (VVP) in the Democratic Republic of the Congo, part of the western branch of the East African Rift System and contains a 1.2 km-wide summit crater with a lava lake that has been active since at least 1971. Volcanism has been characterized by strong and frequent thermal anomalies, incandescence, gas-and-steam emissions, and seismicity. This report summarizes activity during December 2019 through May 2020 using information from monthly reports by the Observatoire Volcanologique de Goma (OVG) and satellite data.

In the December 2019 monthly report, OVG stated that the level of the lava lake had increased. This level of the lava lake was maintained for the duration of the reporting period, according to later OVG monthly reports. Seismicity increased starting in November 2019 and was detected in the NE part of the crater, but it decreased by mid-April 2020. SO₂ emissions increased in January 2020 to roughly 7,000 tons/day but decreased again near the end of the month. OVG reported that SO₂ emissions rose again in February to roughly 8,500 tons/day before declining to about 6,000 tons/day. Unlike in the previous report (BGVN 44:12), incandescence was visible during the day in the active lava lake and activity at the small eruptive cone within the 1.2-km-wide summit crater has since increased, consisting of incandescence and some lava fountaining (figure 72). A field survey was conducted on 3-4 March where an OVG team observed active lava fountains and ejecta that produced Pele’s hair from the small eruptive cone (figure 73). During this survey, OVG reported that the level of the lava lake had reached the second terrace, which was formed on 17 January 2002 and represents remnants of the lava lake at different eruption stages. There, the open surface lava lake was observed; gas-and-steam emissions accompanied both the active lava lake and the small eruptive cone (figures 72 and 73).

Figure 72. Webcam image of Nyiragongo in February 2020 showing an open lava lake surface and incandescence from the active crater cone within the 1.2 km-wide summit crater visible during the day, accompanied by white gas-and-steam emissions. Courtesy of OVG (Rapport OVG February 2020).

Figure 73. Webcam image of Nyiragongo on 4 March 2020 showing an open lava lake surface and incandescence from the active crater cone within the 1.2 km-wide summit crater visible during the day, accompanied by white gas-and-steam emissions. Courtesy of OVG (Rapport OVG Mars 2020).

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data continued to show frequent strong thermal anomalies within 5 km of the summit crater through May 2020 (figure 74). Similarly, the MODVOLC algorithm reported multiple thermal hotspots almost daily within the summit crater between December 2019 and May 2020. These thermal signatures were also observed in Sentinel-2 thermal satellite imagery within the summit crater (figure 75).

Figure 74. Thermal anomalies at Nyiragongo from 27 July through May 2020 as recorded by the MIROVA system (Log Radiative Power) were frequent and strong. Courtesy of MIROVA.

Figure 75. Sentinel-2 thermal satellite imagery (bands 12, 11, 8A) showed ongoing thermal activity (bright yellow-orange) in the summit crater at Nyiragongo during January through April 2020. Courtesy of Sentinel Hub Playground.

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).

Kavachi is a submarine volcano located in the Solomon Islands south of Gatokae and Vangunu islands. Volcanism is frequently active, but rarely observed. The most recent eruptions took place during 2014, which consisted of an ash eruption, and during 2016, which included phreatomagmatic explosions (BGVN 42:03). This reporting period covers December 2016-April 2020 primarily using satellite data.

Activity at Kavachi is often only observed through satellite images, and frequently consists of discolored submarine plumes for which the cause is uncertain. On 1 January 2018 a slight yellow discoloration in the water is seen extending to the E from a specific point (figure 20). Similar faint plumes were observed on 16 January, 25 February, 2 March, 26 April, 6 May, and 25 June 2018. No similar water discoloration was noted during 2019, though clouds may have obscured views.

Figure 20. Satellite images from Sentinel-2 revealed intermittent faint water discoloration (yellow) at Kavachi during the first half of 2018, as seen here on 1 January (top left), 25 February (top right), 26 April (bottom left), and 25 June (bottom right). Images with “Natural color” rendering (bands 4, 3, 2); courtesy of Sentinel Hub Playground.

Activity resumed in 2020, showing more discolored water in satellite imagery. The first instance occurred on 16 March, where a distinct plume extended from a specific point to the SE. On 25 April a satellite image showed a larger discolored plume in the water that spread over about 30 km2, encompassing the area around Kavachi (figure 21). Another image on 30 April showed a thin ribbon of discolored water extending about 50 km W of the vent.

Figure 21. Sentinel-2 satellite images of a discolored plume (yellow) at Kavachi beginning on 16 March (top left) with a significant large plume on 25 April (right), which remained until 30 April (bottom left). Images with “Natural color” rendering (bands 4, 3, 2); courtesy of Sentinel Hub Playground.

Geologic Background. Named for a sea-god of the Gatokae and Vangunu peoples, Kavachi is one of the most active submarine volcanoes in the SW Pacific, located in the Solomon Islands south of Vangunu Island about 30 km N of the site of subduction of the Indo-Australian plate beneath the Pacific plate. Sometimes referred to as Rejo te Kvachi ("Kavachi's Oven"), this shallow submarine basaltic-to-andesitic volcano has produced ephemeral islands up to 1 km long many times since its first recorded eruption during 1939. Residents of the nearby islands of Vanguna and Nggatokae (Gatokae) reported "fire on the water" prior to 1939, a possible reference to earlier eruptions. The roughly conical edifice rises from water depths of 1.1-1.2 km on the north and greater depths to the SE. Frequent shallow submarine and occasional subaerial eruptions produce phreatomagmatic explosions that eject steam, ash, and incandescent bombs. On a number of occasions lava flows were observed on the ephemeral islands.

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

Kuchinoerabujima encompasses a group of young stratovolcanoes located in the northern Ryukyu Islands. All historical eruptions have originated from the Shindake cone, with the exception of a lava flow that originated from the S flank of the Furudake cone. The most recent previous eruptive period took place during October 2018-February 2019 and primarily consisted of weak explosions, ash plumes, and ashfall. The current eruption began on 11 January 2020 after nearly a year of dominantly gas-and-steam emissions. Volcanism for this reporting period from March 2019 to April 2020 included explosions, ash plumes, SO₂ emissions, and ashfall. The primary source of information for this report comes from monthly and annual reports from the Japan Meteorological Agency (JMA) and advisories from the Tokyo Volcanic Ash Advisory Center (VAAC). Activity has been limited to Kuchinoerabujima's Shindake Crater.

Volcanism at Kuchinoerabujima was relatively low during March through December 2019, according to JMA. During this time, SO₂ emissions ranged from 100 to 1,000 tons/day. Gas-and-steam emissions were frequently observed throughout the entire reporting period, rising to a maximum height of 1.1 km above the crater on 13 December 2019. Satellite imagery from Sentinel-2 showed gas-and-steam and occasional ash emissions rising from the Shindake crater throughout the reporting period (figure 7). Though JMA reported thermal anomalies occurring on 29 January and continuing through late April 2020, Sentinel-2 imagery shows the first thermal signature appearing on 26 April.

Figure 7. Sentinel-2 thermal satellite images showed gas-and-steam and ash emissions rising from Kuchinoerabujima. Some ash deposits can be seen on 6 February 2020 (top right). A thermal anomaly appeared on 26 April 2020 (bottom right). Sentinel-2 atmospheric penetration (bands 12, 11, 8A) images courtesy of Sentinel Hub Playground.

An eruption on 11 January 2020 at 1505 ejected material 300 m from the crater and produced ash plumes that rose 2 km above the crater rim, extending E, according to JMA. The eruption continued through 12 January until 0730. The resulting ash plumes rose 400 m above the crater, drifting SW while the SO₂ emissions measured 1,300 tons/day. Ashfall was reported on Yakushima Island (15 km E). Minor eruptive activity was reported during 17-20 January which produced gray-white plumes that rose 300-500 m above the crater. On 23 January, seismicity increased, and an eruption produced an ash plume that rose 1.2 km altitude, according to a Tokyo VAAC report, resulting in ashfall 2 km NE of the crater. A small explosion was detected on 24 January, followed by an increase in the number of earthquakes during 25-26 January (65-71 earthquakes per day were registered). Another small eruptive event detected on 27 January at 0148 was accompanied by a volcanic tremor and a change in tilt data. During the month of January, some inflation was detected at the base on the volcano and a total of 347 earthquakes were recorded. The SO₂ emissions ranged from 200-1,600 tons/day.

An eruption on 1 February 2020 produced an eruption column that rose less than 1 km altitude and extended SE and SW (figure 8), according to the Tokyo VAAC report. On 3 February, an eruption from the Shindake crater at 0521 produced an ash plume that rose 7 km above the crater and ejected material as far as 600 m away. As a result, a pyroclastic flow formed, traveling 900-1,500 m SW. The previous pyroclastic flow that was recorded occurred on 29 January 2019. Ashfall was confirmed in the N part of Yakushima Island with a large amount in Miyanoura (32 km ESE) and southern Tanegashima. The SO₂ emissions measured 1,700 tons/day during this event.

Figure 8. Webcam images from the Honmura west surveillance camera of an ash plume rising from Kuchinoerabujima on 1 February 2020. Courtesy of JMA (Weekly bulletin report 509, February 2020).

Intermittent small eruptive events occurred during 5-9 February; field observations showed a large amount of ashfall on the SE flank which included lapilli that measured up to 2 cm in diameter. Additionally, thermal images showed 5-km-long pyroclastic flow deposits on the SW flank. An eruption on 9 February produced an ash plume that rose 1.2 km altitude, drifting SE. On 13 February a small eruption was detected in the Shindake crater at 1211, producing gray-white plumes that rose 300 m above the crater, drifting NE. Small eruptive events also occurred during 20-21 February, resulting in gas-and-steam emissions that rose 200 m above the crater. During the month of February, some horizontal extension was observed since January 2020 using GNSS data. The total number of earthquakes during this month drastically increased to 1225 compared to January. The SO₂ emissions ranged from 300-1,700 tons/day.

By 2 March 2020, seismicity decreased, and activity declined. Gas-and-steam emissions continued infrequently for the duration of the reporting period. The SO₂ emissions during March ranged from 700-2,100 tons/day, the latter of which occurred on 15 March. Seismicity increased again on 27 March. During 5-8 April 2020, small eruptive events were detected, generating ash plumes that rose 900 m above the crater (figure 9). The SO₂ emissions on 6 April reached 3,200 tons/day, the maximum measurement for this reporting period. These small eruptive events continued from 13-20 and 23-25 April within the Shindake crater, producing gray-white plumes that rose 300-800 m above the crater.

Figure 9. Webcam images from the Honmura Nishi (top) and Honmura west (bottom) surveillance cameras of ash plumes rising from Kuchinoerabujima on 6 March and 5 April 2020. Courtesy of JMA (Weekly bulletin report 509, March and April 2020).

Geologic Background. A group of young stratovolcanoes forms the eastern end of the irregularly shaped island of Kuchinoerabujima in the northern Ryukyu Islands, 15 km W of Yakushima. The Furudake, Shindake, and Noikeyama cones were erupted from south to north, respectively, forming a composite cone with multiple craters. The youngest cone, centrally-located Shindake, formed after the NW side of Furudake was breached by an explosion. All historical eruptions have occurred from Shindake, although a lava flow from the S flank of Furudake that reached the coast has a very fresh morphology. Frequent explosive eruptions have taken place from Shindake since 1840; the largest of these was in December 1933. Several villages on the 4 x 12 km island are located within a few kilometers of the active crater and have suffered damage from eruptions.

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); 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).

Soputan is a stratovolcano located in the northern arm of Sulawesi Island, Indonesia. Previous eruptive periods were characterized by ash explosions, lava flows, and Strombolian eruptions. The most recent eruption occurred during October-December 2018, which consisted mostly of ash plumes and some summit incandescence (BGVN 44:01). This report updates information for January 2019-April 2020 characterized by two ash plumes and gas-and-steam emissions. The primary source of information come from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG) and the Darwin Volcanic Ash Advisory Center (VAAC).

Activity during January 2019-April 2020 was relatively low; three faint thermal anomalies were observed at the summit at Soputan in satellite imagery for a total of three days on 2 and 4 January, and 1 October 2019 (figure 17). The MIROVA (Middle InfraRed Observation of Volcanic Activity) based on analysis of MODIS data detected 12 distal hotspots and six low-power hotspots within 5 km of the summit during August to early October 2019. A single distal thermal hotspot was detected in early March 2020. In March, activity primarily consisted of white to gray gas-and-steam plumes that rose 20-100 m above the crater, according to PVMBG. The Darwin VAAC issued a notice on 23 March 2020 that reported an ash plume rose to 4.3 km altitude; minor ash emissions had been visible in a webcam image the previous day (figure 18). A second notice was issued on 2 April, where an ash plume was observed rising 2.1 km altitude and drifting W.

Figure 17. Sentinel-2 thermal satellite imagery detected a total of three thermal hotspots (bright yellow-orange) at the summit of Soputan on 2 and 4 January and 1 October 2019. Sentinel-2 atmospheric penetration (bands 12, 11, 8A) images courtesy of Sentinel Hub Playground.

Figure 18. Minor ash emissions were seen rising from Soputan on 22 March 2020. Courtesy of MAGMA Indonesia.

Geologic Background. The Soputan stratovolcano on the southern rim of the Quaternary Tondano caldera on the northern arm of Sulawesi Island is one of Sulawesi's most active volcanoes. The youthful, largely unvegetated volcano is located SW of Riendengan-Sempu, which some workers have included with Soputan and Manimporok (3.5 km ESE) as a volcanic complex. It was constructed at the southern end of a SSW-NNE trending line of vents. During historical time the locus of eruptions has included both the summit crater and Aeseput, a prominent NE-flank vent that formed in 1906 and was the source of intermittent major lava flows until 1924.

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/); 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).

Heard Island is located on the Kerguelen Plateau in the southern Indian Ocean and contains Big Ben, a snow-covered stratovolcano with intermittent volcanism reported since 1910. Due to its remote location, visual observations are rare; therefore, thermal anomalies and hotspots detected by satellite-based instruments are the primary source of information. This report updates activity from October 2019 to April 2020.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed three prominent periods of strong thermal anomaly activity during this reporting period: late October 2019, December 2019, and the end of April 2020 (figure 41). These thermal anomalies were relatively strong and occurred within 5 km of the summit. Similarly, the MODVOLC algorithm reported a total of six thermal hotspots during 28 October, 1 November 2019, and 26 April 2020.

Figure 41. Thermal anomalies at Heard from 29 April 2019 through April 2020 as recorded by the MIROVA system (Log Radiative Power) were strong and frequent in late October, during December 2019, and at the end of April 2020. Courtesy of MIROVA.

Six thermal satellite images ranging from late October 2019 to late March showed evidence of active lava at the summit (figure 42). These images show hot material, possibly a lava flow, extending SW from the summit; a hotspot also remained at the summit. Cloud cover was pervasive during the majority of this reporting period, especially in April 2020, though gas-and-steam emissions were visible on 25 April through the clouds.

Figure 42. Thermal satellite images of Heard Island’s Big Ben showing strong thermal signatures representing a lava flow in the SW direction from 28 October to 17 December 2019. These thermal anomalies are located NE from Mawson Peak. A faint thermal anomaly is also captured on 26 March 2020. Satellite images with atmospheric penetration (bands 12, 11, and 8A), courtesy of Sentinel Hub Playground.

Geologic Background. Heard Island on the Kerguelen Plateau in the southern Indian Ocean consists primarily of the emergent portion of two volcanic structures. The large glacier-covered composite basaltic-to-trachytic cone of Big Ben comprises most of the island, and the smaller Mt. Dixon lies at the NW tip of the island across a narrow isthmus. Little is known about the structure of Big Ben because of its extensive ice cover. The historically active Mawson Peak forms the island's high point and lies within a 5-6 km wide caldera breached to the SW side of Big Ben. Small satellitic scoria cones are mostly located on the northern coast. Several subglacial eruptions have been reported at this isolated volcano, but observations are infrequent and additional activity may have occurred.

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).

The Kikai caldera is located at the N end of Japan’s Ryukyu Islands and has been recently characterized by intermittent ash emissions and limited ashfall in nearby communities. On Satsuma Iwo Jima island, the larger subaerial fragment of the Kikai caldera, there was a single explosion with gas-and-steam and ash emissions on 2 November 2019, accompanied by nighttime incandescence (BGVN 45:02). This report covers volcanism from January 2020 through April 2020 with a single-day eruption occurring on 29 April based on reports from the Japan Meteorological Agency (JMA).

Since the last one-day eruption on 2 November 2019, volcanism at Kikai has been relatively low and primarily consisted of 107-170 earthquakes per month and intermittent white gas-and-steam emissions rising up to 1.3 km above the crater summit. Intermittent weak hotspots were observed at night in the summit in Sentinel-2 thermal satellite imagery and webcams, according to JMA (figures 14 and 15).

Figure 14. Weak thermal hotspots (bright yellow-orange) were observed on 7 January (top) and 6 April 2020 (bottom) at Satsuma Iwo Jima (Kikai). Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Figure 15. Incandescence at night on 10 January 2020 was observed at Satsuma Iwo Jima (Kikai) in the Iodake crater with the Iwanogami webcam. Courtesy of JMA (An explanation of volcanic activity at Satsuma Iwo Jima, January 2nd year of Reiwa [2020]).

Weak incandescence continued in April 2020. JMA reported SO₂ measurements during April were 400-2000 tons/day. A brief eruption in the Iodake crater on 29 April 2020 at 0609 generated a gray-white ash plume that rose 1 km above the crater (figure 16). No ashfall or ejecta was observed after the eruption on 29 April.

Figure 16. The Iwanogami webcam captured a brief gray-white ash and steam plume rising above the Iodake crater rim on Satsuma Iwo Jima (Kikai) on 29 April 2020 at 0609 local time. The plume rose 1 km above the crater summit. Courtesy of JMA (An explanation of volcanic activity at Satsuma Iwo Jima, April 2nd year of Reiwa [2020]).

Geologic Background. Kikai is a mostly submerged, 19-km-wide caldera near the northern end of the Ryukyu Islands south of Kyushu. It was the source of one of the world's largest Holocene eruptions about 6,300 years ago when rhyolitic pyroclastic flows traveled across the sea for a total distance of 100 km to southern Kyushu, and ashfall reached the northern Japanese island of Hokkaido. The eruption devastated southern and central Kyushu, which remained uninhabited for several centuries. Post-caldera eruptions formed Iodake lava dome and Inamuradake scoria cone, as well as submarine lava domes. Historical eruptions have occurred at or near Satsuma-Iojima (also known as Tokara-Iojima), a small 3 x 6 km island forming part of the NW caldera rim. Showa-Iojima lava dome (also known as Iojima-Shinto), a small island 2 km E of Tokara-Iojima, was formed during submarine eruptions in 1934 and 1935. Mild-to-moderate explosive eruptions have occurred during the past few decades from Iodake, a rhyolitic lava dome at the eastern end of Tokara-Iojima.

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Fuego is a stratovolcano in Guatemala that has been erupting since 2002 with historical eruptions that date back to 1531. Volcanism is characterized by major ashfalls, pyroclastic flows, lava flows, and lahars. The previous report (BGVN 44:10) detailed activity that included multiple ash explosions, ash plumes, ashfall, active lava flows, and block avalanches. This report covers this continuing activity from October 2019 through March 2020 and consists of ash plumes, ashfall, incandescent ejecta, block avalanches, and lava flows. The primary source of information comes from the Instituto Nacional de Sismologia, Vulcanología, Meteorología e Hidrologia (INSIVUMEH), the Washington Volcanic Ash Advisory Center (VAAC), and various satellite data.

Summary of activity October 2019-March 2020. Daily activity persisted throughout October 2019-March 2020 (table 20) with multiple ash explosions recorded every hour, ash plumes that rose to a maximum of 4.8 km altitude each month drifting in multiple directions, incandescent ejecta reaching a 500 m above the crater resulting in block avalanches traveling down multiple drainages, and ashfall affecting communities in multiple directions. The highest rate of explosions occurred on 7 November with up to 25 per hour. Dominantly white fumaroles occurred frequently throughout this reporting period, rising to a maximum altitude of 4.5 km and drifting in multiple directions. Intermittent lava flows that reached a maximum length of 1.2 km were observed each month in the Seca (Santa Teresa) and Ceniza drainages (figure 128), but rarely in the Trinidad drainage. Thermal activity increased slightly in frequency and strength in late October and remained relatively consistent through mid-March as seen in the MIROVA analysis of MODIS satellite data (figure 129).

Table 20. Activity summary by month for Fuego with information compiled from INSIVUMEH daily reports.

Figure 128. Sentinel-2 thermal satellite images of Fuego between 21 November 2019 and 20 March 2020 showing lava flows (bright yellow-orange) traveling generally S and W from the crater summit. An ash plume can also be seen on 21 November 2019, accompanying the lava flow. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Figure 129. Thermal activity at Fuego increased in frequency and strength (log radiative power) in late October 2019 and remained relatively consistent through February 2020. In early March, there is a small decrease in thermal power, followed by a short pulse of activity and another decline. Courtesy of MIROVA.

Activity during October-December 2019. Activity in October 2019 consisted of 6-20 ash explosions per hour; ash plumes rose to 4.8 km altitude, drifting up to 25 km in multiple directions, resulting in ashfall in Panimaché I and II (8 km SW), Morelia (9 km SW), San Pedro Yepocapa (8 km NW), Sangre de Cristo (8 km WSW), Santa Sofía (12 km SW), El Porvenir (8 km ENE), Finca Palo Verde, La Rochela and San Andrés Osuna. The Washington VAAC issued multiple aviation advisories for a total of nine days in October. Continuous white gas-and-steam plumes reached 4.1-4.4 km altitude drifting generally W. Weak SO₂ emissions were infrequently observed in satellite imagery during October and January 2020 (figure 130) Incandescent ejecta was frequently observed rising 200-400 m above the summit, which generated block avalanches that traveled down the Seca (W), Taniluyá (SW), Ceniza (SSW), Trinidad (S), El Jute, Honda, and Las Lajas (SE) drainages. During 3-7 October lahars descended the Ceniza, El Mineral, and Seca drainages, carrying tree branches, tree trunks, and blocks 1-3 m in diameter. During 6-8 and 13 October, active lava flows traveled up to 200 m down the Seca drainage.

Figure 130. Weak SO2 emissions were observed rising from Fuego using the TROPOMI instrument on the Sentinel-5P satellite. Top left: 17 October 2019. Top right: 17 November 2019. Bottom left: 20 January 2020. Bottom right: 22 January 2020. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

During November 2019, the rate of explosions increased to 5-25 per hour, the latter of which occurred on 7 November. The explosions resulted in ash plumes that rose 4-4.8 km altitude, drifting 10-20 km in the W direction. Ashfall was observed in Panimaché I and II, Morelia, Santa Sofía, Porvenir, Sangre de Cristo, Finca Palo Verde, and San Pedro Yepocapa. Multiple Washington VAAC notices were issued for 11 days in November. Continuous white gas-and-steam plumes rose up to 4.5 km altitude drifting generally W. Incandescent ejecta rose 100-500 m above the crater, generating block avalanches in Seca, Taniluyá, Trinidad, Las Lajas, Honda, and Ceniza drainages. Lava flows were observed for a majority of the month into early December measuring 100-900 m long in the Seca and Ceniza drainages.

The number of explosions in December 2019 decreased compared to November, recording 8-19 per hour with incandescent ejecta rising 100-400 m above the crater. The explosions generated block avalanches that traveled in the Seca, Taniluya, Ceniza, Trinidad, and Las Lajas drainages throughout the month. Ash plumes continued to rise above the summit crater to 4.8 km drifting up to 25 km in multiple directions. The Washington VAAC issued multiple daily notices almost daily in December. A continuous lava flow observed during 6-15, 21-22, 24, and 26 November through 9 December measured 100-800 m long in the Seca and Ceniza drainages.

Activity during January-March 2020. Incandescent Strombolian explosions continued daily during January 2020, ejecting material up to 100-500 m above the crater. Ash plumes continued to rise to a maximum altitude of 4.8 km, resulting in ashfall in all directions affecting Morelia, Santa Sofía, Sangre de Cristo, San Pedro Yepocapa, Panimaché I and II, El Porvenir, Finca Palo Verde, Rodeo, La Rochela, Alotenango, El Zapote, Trinidad, La Reina, and Ceilán. The Washington VAAC issued multiple notices for a total of 12 days during January. Block avalanches resulting from the Strombolian explosions traveled down the Seca, Ceniza, Taniluyá, Trinidad, Honda, and Las Lajas drainages. An active lava flow in the Ceniza drainage measured 150-600 m long during 6-10 January.

During February 2020, INSIVUMEH reported a range of 4-16 explosions per hour, accompanied by incandescent material that rose 100-500 m above the crater (figure 131). Block avalanches traveled in the Santa Teresa, Seca, Ceniza, Taniluya, Trinidad, Las Lajas, Honda, La Rochela, El Zapote, and San Andrés Osuna drainages. Ash emissions from the explosions continued to rise 4.8 km altitude, drifting in multiple directions as far as 25 km and resulting in ashfall in the communities of Panimache I and II, Morelia, Santa Sofia, Sangre de Cristo, San Pedro Yepocapa, Rodeo, La Reina, Alotenango, Yucales, Siquinalá, Santa Lucia, El Porvenir, Finca Los Tarros, La Soledad, Buena Vista, La Cruz, Pajales, San Miguel Dueñas, Ciudad Vieja, San Miguel Escobar, San Pedro las Huertas, Antigua, La Rochela, and San Andrés Osuna. Washington VAAC notices were issued almost daily during the month. Lava flows were active in the Ceniza drainage during 13-20, 23-24, and 26-27 February measuring as long as 1.2 km.

Figure 131. Incandescent ejecta rose several hundred meters above the crater of Fuego on 6 February 2020, resulting in block avalanches down multiple drainages. Courtesy of Crelosa.

Daily explosions and incandescent ejecta continued through March 2020, with 8-17 explosions per hour that rose up to 500 m above the crater. Block avalanches from the explosions were observed in the Seca, Ceniza, Trinidad, Taniluyá, Las Lajas, Honda, Santa Teresa, La Rochela, El Zapote, San Andrés Osuna, Morelia, Panimache, and Santa Sofia drainages. Accompanying ash plumes rose 4.8 km altitude, drifting in multiple directions mostly to the W as far as 23 km and resulting in ashfall in San Andrés Osuna, La Rochela, El Rodeo, Chuchu, Panimache I and II, Santa Sofia, Morelia, Finca Palo Verde, El Porvenir, Sangre de Cristo, La Cruz, San Pedro Yepocapa, La Conchita, La Soledad, Alotenango, Aldea la Cruz, Acatenango, Ceilan, Taniluyá, Ceniza, Las Lajas, Trinidad, Seca, and Honda. Multiple Washington VAAC notices were issued for a total of 15 days during March. Active lava flows were observed from 16-21 March in the Trinidad and Ceniza drainages measuring 400-1,200 m long and were accompanied by weak to moderate explosions. By 23 March, active lava flows were no longer observed.

Geologic Background. Volcán Fuego, one of Central America's most active volcanoes, is also one of three large stratovolcanoes overlooking Guatemala's former capital, Antigua. The scarp of an older edifice, Meseta, lies between Fuego and Acatenango to the north. Construction of Meseta dates back to about 230,000 years and continued until the late Pleistocene or early Holocene. Collapse of Meseta may have produced the massive Escuintla debris-avalanche deposit, which extends about 50 km onto the Pacific coastal plain. Growth of the modern Fuego volcano followed, continuing the southward migration of volcanism that began at the mostly andesitic Acatenango. Eruptions at Fuego have become more mafic with time, and most historical activity has produced basaltic rocks. Frequent vigorous historical eruptions have been recorded since the onset of the Spanish era in 1524, and have produced major ashfalls, along with occasional pyroclastic flows and lava flows.

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/); 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/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); Crelosa, 3ra. avenida. 8-66, Zona 14. Colonia El Campo, Guatemala Ciudad de Guatemala (URL: http://crelosa.com/, post at https://www.youtube.com/watch?v=1P4kWqxU2m0&feature=youtu.be).

The current moderate explosive eruption of Ebeko has been ongoing since October 2016, with frequent ash explosions that have reached altitudes of 1.3-6 km (BGVN 42:08, 43:03, 43:06, 43:12, 44:12). Ashfall is common in Severo-Kurilsk, a town of about 2,500 residents 7 km ESE, where the Kamchatka Volcanic Eruptions Response Team (KVERT) monitor the volcano. During the reporting period, December 2019-May 2020, the Aviation Color Code remained at Orange (the second highest level on a four-color scale).

During December 2019-May 2020, frequent explosions generated ash plumes that reached altitudes of 1.5-4.6 km (table 9); reports of ashfall in Severo-Kurilsk were common. Ash explosions in late April caused ashfall in Severo-Kurilsk during 25-30 April (figure 24), and the plume drifted 180 km SE on the 29th. There was also a higher level of activity during the second half of May (figure 25), when plumes drifted up to 80 km downwind.

Table 9. Summary of activity at Ebeko, December 2019-May 2020. S-K is Severo-Kurilsk (7 km ESE of the volcano). TA is thermal anomaly in satellite images. In the plume distance column, only plumes that drifted more than 10 km are indicated. Dates based on UTC times. Data courtesy of KVERT.

Figure 24. Photo of ash explosion at Ebeko at 2110 UTC on 28 April 2020, as viewed from Severo-Kurilsk. Courtesy of KVERT (L. Kotenko).

Figure 25. Satellite image of Ebeko from Sentinel-2 on 27 May 2020, showing a plume drifting SE. Image using natural color rendering (bands 4, 3, 2) courtesy of Sentinel Hub Playground.

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

Information Contacts: 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/); Institute of Volcanology and Seismology, Far Eastern Branch, Russian Academy of Sciences (IVS FEB RAS), 9 Piip Blvd., Petropavlovsk-Kamchatsky 683006, Russia (URL: http://www.kscnet.ru/ivs/eng/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Piton de la Fournaise is a massive basaltic shield volcano on the French island of Réunion in the western Indian Ocean. Recent volcanism is characterized by multiple fissure eruptions, lava fountains, and lava flows (BGVN 44:11). The activity during this reporting period of November 2019-April 2020 is consistent with the previous eruption, including lava fountaining and lava flows. Information for this report comes from the Observatoire Volcanologique du Piton de la Fournaise (OVPF) and various satellite data.

Activity during November 2019-January 2020 was relatively low; no eruptive events were detected, according to OVPF. Edifice deformation resumed during the last week in December and continued through January. Seismicity significantly increased in early January, registering 258 shallow earthquakes from 1-16 January. During 17-31 January, the seismicity declined, averaging one earthquake per day.

Two eruptive events took place during February-April 2020. OVPF reported that the first occurred from 10 to 16 February on the E and SE flanks of the Dolomieu Crater. The second took place during 2-6 April. Both eruptive events began with a sharp increase in seismicity accompanied by edifice inflation, followed by a fissure eruption that resulted in lava fountains and lava flows (figure 193). MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed the two eruptive events occurring during February-April 2020 (figure 194). Similarly, the MODVOLC algorithm reported 72 thermal signatures proximal to the summit crater from 12 February to 6 April. Both of these eruptive events were accompanied by SO2 emissions that were detected by the Sentinel-5P/TROPOMI instrument (figures 195 and 196).

Figure 193. Location maps of the lava flows on the E flank at Piton de la Fournaise on 10-16 February 2020 (left) and 2-6 April 2020 (right) as derived from SAR satellite data. Courtesy of OVPF-IPGP, OPGC, LMV (Monthly bulletins of the Piton de la Fournaise Volcanological Observatory, February and April 2020).

Figure 194. Two significant eruptive events at Piton de la Fournaise took place during February-April 2020 as recorded by the MIROVA system (Log Radiative Power). Courtesy of MIROVA.

Figure 195. Images of the SO2 emissions during the February 2020 eruptive event at Piton de la Fournaise detected by the Sentinel-5P/TROPOMI satellite. Top left: 10 February 2020. Top right: 11 February 2020. Bottom left: 13 February 2020. Bottom right: 14 February 2020. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Figure 196. Images of the SO2 emissions during the April 2020 eruptive event at Piton de la Fournaise detected by the Sentinel-5P/TROPOMI satellite. Left: 4 April 2020. Middle: 5 April 2020. Right: 6 April 2020. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

On 10 February 2020 a seismic swarm was detected at 1027, followed by rapid deformation. At 1050, volcanic tremors were recorded, signaling the start of the eruption. Several fissures opened on the E flank of the Dolomieu Crater between the crater rim and at 2,000 m elevation, as observed by an overflight during 1300 and 1330. These fissures were at least 1 km long and produced lava fountains that rose up to 10 m high. Lava flows were also observed traveling E and S to 1,700 m elevation by 1315 (figures 197 and 198). The farthest flow traveled E to an elevation of 1,400 m. Satellite data from HOTVOLC platform (OPGC - University of Auvergne) was used to estimate the peak lava flow rate on 11 February at 10 m3/s. By 13 February only one lava flow that was traveling E below the Marco Crater remained active. OVPF also reported the formation of a cone, measuring 30 m tall, surrounded by three additional vents that produced lava fountains up to 15 m high. On 15 February the volcanic tremors began to decrease at 1400; by 16 February at 1412 the tremors stopped, indicating the end of the eruptive event.

Figure 197. Photo of a lava flow and degassing at Piton de la Fournaise on 10 February 2020. Courtesy of OVPF-IPGP.

Figure 198. Photos of the lava flows at Piton de la Fournaise taken during the February 2020 eruption by Richard Bouchet courtesy of AFP News Service.

Volcanism during the month of March 2020 consisted of low seismicity, including 21 shallow volcanic tremors and near the end of the month, edifice inflation was detected. A second eruptive event began on 2 April 2020, starting with an increase in seismicity during 0815-0851. Much of this seismicity was located on the SE part of the Dolomieu Crater. A fissure opened on the E flank, consistent with the fissures that were active during the February 2020 event. Seismicity continued to increase in intensity through 6 April located dominantly in the SE part of the Dolomieu Crater. An overflight on 5 April at 1030 showed lava fountains rising more than 50 m high accompanied by gas-and-steam plumes rising to 3-3.5 km altitude (figures 199 and 200). A lava flow advanced to an elevation of 360 m, roughly 2 km from the RN2 national road (figure 199). A significant amount of Pele’s hair and clusters of fine volcanic products were produced during the more intense phase of the eruption (5-6 April) and deposited at distances more than 10 km from the eruptive site (figure 201). It was also during this period that the SO2 emissions peaked (figure 196). The eruption stopped at 1330 after a sharp decrease in volcanic tremors.

Figure 199. Photos of a lava flow (left) and lava fountains (right) at Piton de la Fournaise during the April 2020 eruption. Left: photo taken on 2 April 2020 at 1500. Right: photo taken on 5 April 2020 at 1030. Courtesy of OVPF-IPGP (Monthly bulletin of the Piton de la Fournaise Volcanological Observatory, April 2020).

Figure 200. Photo of the lava fountains erupting from Piton de la Fournaise on 4 April 2020. Photo taken by Richard Bouchet courtesy of Geo Magazine via Jeannie Curtis.

Figure 201. Photos of Pele’s hair deposited due to the April 2020 eruption at Piton de la Fournaise. Samples collected near the Gîte du volcan on 7 April 2020 (left) and a cluster of Pele’s hair found near the Foc-Foc car park on 9 April 2020 (right). Courtesy of OVPF-IPGP (Monthly bulletin of the Piton de la Fournaise Volcanological Observatory, April 2020).

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

Information Contacts: Observatoire Volcanologique du Piton de la Fournaise, Institut de Physique du Globe de Paris, 14 route nationale 3, 27 ème km, 97418 La Plaine des Cafres, La Réunion, France (URL: http://www.ipgp.fr/fr); 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/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); GEO Magazine (AFP story at URL: https://www.geo.fr/environnement/la-reunion-fin-deruption-au-piton-de-la-fournaise-200397); AFP (URL: https://twitter.com/AFP/status/1227140765106622464, Twitter: @AFP, https://twitter.com/AFP); Jeannie Curtis (Twitter: @VolcanoJeannie, https://twitter.com/VolcanoJeannie).

Although tephrochronology has dated activity at Sabancaya back several thousand years, renewed activity that began in 1986 was the first recorded in over 200 years. Intermittent activity since then has produced significant ashfall deposits, seismic unrest, and fumarolic emissions. A new period of explosive activity that began in November 2016 has been characterized by pulses of ash emissions with some plumes exceeding 10 km altitude, thermal anomalies, and significant SO₂ plumes. Ash emissions and high levels of SO₂ continued each week during December 2019-May 2020. The Observatorio Vulcanologico INGEMMET (OVI) reports weekly on numbers of daily explosions, ash plume heights and directions of drift, seismicity, and other activity. The Buenos Aires Volcanic Ash Advisory Center (VAAC) issued three or four daily reports of ongoing ash emissions at Sabancaya throughout the period.

The dome inside the summit crater continued to grow throughout this period, along with nearly constant ash, gas, and steam emissions; the average number of daily explosions ranged from 4 to 29. Ash and gas plume heights rose 1,800-3,800 m above the summit crater, and multiple communities around the volcano reported ashfall every month (table 6). Sulfur dioxide emissions were notably high and recorded daily with the TROPOMI satellite instrument (figure 75). Thermal activity declined during December 2019 from levels earlier in the year but remained steady and increased in both frequency and intensity during April and May 2020 (figure 76). Infrared satellite images indicated that the primary heat source throughout the period was from the dome inside the summit crater (figure 77).

Table 6. Persistent activity at Sabancaya during December 2019-May 2020 included multiple daily explosions with ash plumes that rose several kilometers above the summit and drifted in many directions; this resulted in ashfall in communities within 30 km of the volcano. Satellite instruments recorded SO₂ emissions daily. Data courtesy of OVI-INGEMMET.

Figure 75. Sulfur dioxide anomalies were captured daily from Sabancaya during December 2019-May 2020 by the TROPOMI instrument on the Sentinel-5P satellite. Some of the largest SO2 plumes are shown here with dates listed in the information at the top of each image. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Figure 76. Thermal activity at Sabancaya declined during December 2019 from levels earlier in the year but remained steady and increased slightly in frequency and intensity during April and May 2020, according to the MIROVA graph of Log Radiative Power from 23 June 2019 through May 2020. Courtesy of MIROVA.

Figure 77. Sentinel-2 satellite imagery of Sabancaya confirmed the frequent ash emissions and ongoing thermal activity from the dome inside the summit crater during December 2019-May 2020. Top row (left to right): On 6 December 2019 a large plume of steam and ash drifted N from the summit. On 16 December 2019 a thermal anomaly encircled the dome inside the summit caldera while gas and possible ash drifted NW. On 14 April 2020 a very similar pattern persisted inside the crater. Bottom row (left to right): On 19 April an ash plume was clearly visible above dense cloud cover. On 24 May the infrared glow around the dome remained strong; a diffuse plume drifted W. A large plume of ash and steam drifted SE from the summit on 29 May. Infrared images use Atmospheric penetration rendering (bands 12, 11, 8a), other images use Natural Color rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

The average number of daily explosions during December 2019 decreased from a high of 16 the first week of the month to a low of five during the last week. Six pyroclastic flows occurred on 10 December (figure 78). Tremors were associated with gas-and-ash emissions for most of the month. Ashfall was reported in Pinchollo, Madrigal, Lari, Maca, Achoma, Coporaque, Yanque, and Chivay during the first week of the month, and in Huambo and Cabanaconde during the second week (figure 79). Inflation of the volcano was measured throughout the month. SO₂ flux was measured by OVI as ranging from 2,500 to 4,300 tons per day.

Figure 78. Multiple daily explosions at Sabancaya produced ash plumes that rose several kilometers above the summit. Left image is from 5 December and right image is from 11 December 2019. Note pyroclastic flows to the right of the crater on 11 December. Courtesy of OVI (Reporte Semanal de Monitorio de la Actividad de la Volcan Sabancaya, RSSAB-49-2019/INGEMMET Semana del 2 al 8 de diciembre de 2019 and RSSAB-50-2019/INGEMMET Semana del 9 al 15 de diciembre de 2019).

Figure 79. Communities to the N and W of Sabancaya recorded ashfall from the volcano the first week of December and also every month during December 2019-May 2020. The red zone is the area where access is prohibited (about a 12-km radius from the crater). Courtesy of OVI (Reporte Semanal de Monitorio de la Actividad de la Volcan Sabancaya, RSSAB-22-2020/INGEMMET Semana del 25 al 31 de mayo del 2020).

During January and February 2020 the number of daily explosions averaged 4-20. Ash plumes rose as high as 3.4 km above the summit (figure 80) and drifted up to 30 km in multiple directions. Ashfall was reported in Chivay, Yanque, and Achoma on 8 January, and in Huambo on 25 February. Sulfur dioxide flux ranged from a low of 1,200 t/d on 29 February to a high of 8,200 t/d on 28 January. Inflation of the edifice was measured during January; deformation changed to deflation in early February but then returned to inflation by the end of the month.

Figure 80. Ash plumes rose from Sabancaya every day during January and February 2020. Left: 11 January. Right: 28 February. Courtesy of OVI (Reporte Semanal de Monitorio de la Actividad de la Volcan Sabancaya, RSSAB-02-2020/INGEMMET Semana del 06 al 12 de enero del 2020 and RSSAB-09-2020/INGEMMET Semana del 24 de febrero al 01 de marzo del 2020).

Explosions continued during March and April 2020, averaging 8-29 per day. Explosions appeared to come from multiple vents on 11 March (figure 81). Ash plumes rose 3 km above the summit during the first week of March and again the first week of April; they were lower during the other weeks. Ashfall was reported in Madrigal, Lari, and Pinchollo on 27 March and 5 April. On 17 April ashfall was reported in Maca, Ichupampa, Yanque, Chivay, Coporaque, and Achoma. Sulfur dioxide flux ranged from 1,900 t/d on 5 March to 10,700 t/d on 30 March. Inflation at depth continued throughout March and April with 10 +/- 4 mm recorded between 21 and 26 April. Similar activity continued during May 2020; explosions averaged 6-16 per day (figure 82). Ashfall was reported on 6 May in Chivay, Achoma, Maca, Lari, Madrigal, and Pinchollo; heavy ashfall was reported in Achoma on 12 May. Additional ashfall was reported in Achoma, Maca, Madrigal, and Lari on 23 May.

Figure 81. Explosions at Sabancaya on 11 March 2020 appeared to originate simultaneously from two different vents (left). The plume on 12 April was measured at about 2,500 m above the summit. Courtesy of OVI-INGEMMET (Reporte Semanal de Monitorio de la Actividad de la Volcan Sabancaya, RSSAB-11-2020/INGEMMET Semana del 9 al 15 de marzo del 2020 and RSSAB-15-2020/INGEMMET Semana del 6 al 12 de abril del 2020).

Figure 82. Explosions dense with ash continued during May 2020 at Sabancaya. On 11 and 29 May 2020 ash plumes rose from the summit and drifted as far as 30 km before dissipating. Courtesy of OVI-INGEMMET (Reporte Semanal de Monitorio de la Actividad de la Volcan Sabancaya , RSSAB-20-2020/INGEMMET Semana del 11 al 17 de mayo del 2020 and RSSAB-22-2020/INGEMMET Semana del 25 al 31 de mayo del 2020).

Geologic Background. Sabancaya, located in the saddle NE of Ampato and SE of Hualca Hualca volcanoes, is the youngest of these volcanic centers and the only one to have erupted in historical time. The oldest of the three, Nevado Hualca Hualca, is of probable late-Pliocene to early Pleistocene age. The name Sabancaya (meaning "tongue of fire" in the Quechua language) first appeared in records in 1595 CE, suggesting activity prior to that date. Holocene activity has consisted of Plinian eruptions followed by emission of voluminous andesitic and dacitic lava flows, which form an extensive apron around the volcano on all sides but the south. Records of historical eruptions date back to 1750.

Information Contacts: Observatorio Volcanologico del INGEMMET (Instituto Geológical Minero y Metalúrgico), Barrio Magisterial Nro. 2 B-16 Umacollo - Yanahuara Arequipa, Peru (URL: http://ovi.ingemmet.gob.pe); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

During December [1997], lavas emitted beginning in September continued to flow down Arenal's W flank. They reached 1,400 m elevation and mass wasting carried some material to as low as 1,000 m elevation. December eruptive rates and intensities were low; also, the number of earthquakes and hours of tremor were both at or near the minimum values seen during the course of the year. This pattern continued into January 1998. Still, on infrequent occasions the active crater (Crater C) discharged plumes reaching at least 1 km in height above the crater.

Lavas vented in late January continued to flow in February, descending to 1,100 m elevation, and branching near 1,300 m elevation to form a new arm directed to the NW down the Tabacón river valley. During March, this new arm flowed down to reach 1,200 m elevation; the main channel extended to 1,000 m elevation; another arm branched off to the W at 1,400 m elevation and descended about 100 m.

Observers noted two pyroclastic flows during January-February. The first reached 1,100 m elevation on the SE flank. The second followed a similar path and reached 900 m elevation.

The number of low-frequency earthquakes (<4.0 Hz) during January and February, while still low, rose more than 25% over the number during December. The hours of tremor during January-February also remained low; during the latter month the dedicated seismic station (VACR) registered only 58 hours, the lowest monthly record in at least two years. During March, seismicity appeared to rise again, but the seismic system only functioned 18 days of the month. During this time the system recorded 80 hours of tremor.

OVSICORI-UNA scientists noted fumarolic activity in crater D as well as acid rain on the volcano's leeward flanks (towards the NW, W, and SE). In these sectors, some species of plants sustained visible leaf damage.

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: E. Fernandez, V. Barboza, R. Van der Laat, R. Saenz, E. Duarte, E. Malavassi, T. Marino, M. Martinez, and E. Hernandez, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Mauricio Mora Fernandez, Sección de Sismologia, Vulcanologia y Exploración Geofisica, Escuela Centroamericana de Geología, Universidad de Costa Rica, P.O. Box 35-2060, San José, Costa Rica.

Table 13 lists atmospheric lidar data from Hampton, Virginia for 8 April 1997 through 26 February 1998, and from Garmisch-Partenkirchen, Germany for 3 November 1997 to 14 April 1998. The aerosol backscatter measured at Hampton on 26 February 1998 shows a typical winter increase in stratospheric aerosol compared to measurements made the previous summer. The increase from summer to winter is generally a function of the difference in tropopause height between the two seasons. In this case there is a significant decrease in integrated stratospheric aerosol compared to measurements obtained during the winter of 1997 (Bulletin v. 22, nos. 1, 3).

Table 13. Lidar data collected for Virginia (April 1997-February 1998) and Germany (November 1997-April 1998) showing altitudes of aerosol layers. Backscattering rations from Hampton are for the ruby wavelength of 0.69 µm; those from Garmisch-Partenkirchen are for the Nd-YAG wavelength of 0.53 µm, with equivalent ruby values in parentheses. The integrated value shows total backscatter, expressed in steradians^-1, integrated over 300-m intervals from the tropopause to 30 km for both Virginia and Germany. Courtesy of Mary Osborne and Horst Jäger.

A graph of integral stratospheric aerosol backscatter (figure 5) shows how the stratospheric aerosol load had declined by the end of 1997 to pre-Pinatubo values. More observations are needed to decide whether a new background level has been reached or will be reached in the near future.

Figure 5. Graph showing the log of the lidar backscatter versus time at Garmisch-Partenkirchen, Germany for the latter two-thirds of 1991 through end-1997. The plotted data are preliminary 532 nm integral values of stratospheric aerosol backscatter (integrated from the tropopause or cirrus to the top of the aerosol layer) versus time. Labeled arrows indicate the eruptions of Pinatubo and Kliuchevskoi. 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: Mary Osborn, NASA Langley Research Center (LaRC), Hampton, VA 23665 USA; Horst Jäger, Fraunhofer -- Institut für Atmosphärische Umweltforschung, Kreuzeckbahnstrasse 19, D-82467 Garmisch-Partenkirchen, Germany.

No seismicity registered under the volcano during 2 March-5 April. On 5-7, 10, and 12-14 March, fumarolic plumes rose 50-300 m above the volcano. Fumarolic plumes on 16-20 and 22 March rose 50-200 m above the volcano and moved 5-10 km SSE. On 30-31 March and 1-4 April fumarolic plumes rose 100-500 m above the volcano.

Geologic Background. Prior to its noted 1955-56 eruption, Bezymianny had been considered extinct. The modern volcano, much smaller in size than its massive neighbors Kamen and Kliuchevskoi, was formed about 4700 years ago over a late-Pleistocene lava-dome complex and an ancestral edifice built about 11,000-7000 years ago. Three periods of intensified activity have occurred during the past 3000 years. The latest period, which was preceded by a 1000-year quiescence, began with the dramatic 1955-56 eruption. This eruption, similar to that of St. Helens in 1980, produced a large horseshoe-shaped crater that was formed by collapse of the summit and an associated lateral blast. Subsequent episodic but ongoing lava-dome growth, accompanied by intermittent explosive activity and pyroclastic flows, has largely filled the 1956 crater.

Information Contacts: 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.

When scientists from the Alaska Volcano Observatory (AVO) conducted an overflight to Chiginagak on 11 March, the summit was visible but a thin cloud layer at about 1,700-1,900 m altitude obscured the fumarolic areas. Above the fumaroles, however, bulbous gray clouds penetrated through the thin cloud layer and extended to about 2,100 m altitude.

A strong sulfur smell was noticed 16-49 km downwind of the volcano. The gray clouds and sulfur smell supported observations from Pilot Point (60 km NW) that indicated continued vigorous fumarolic activity. Increased fumarolic activity has been reported at the volcano beginning as early as mid-1997 (BGVN 22:11 and 23:01). According to AVO, the increased activity did not imply an imminent eruption.

Geologic Background. The symmetrical, calc-alkaline Chiginagak stratovolcano located about 15 km NW of Chiginagak Bay contains a small summit crater, which is breached to the south, and one or more summit lava domes. Satellitic lava domes occur high on the NW and SE flanks of the glacier-mantled volcano. An unglaciated lava flow and an overlying pyroclastic-flow deposit extending east from the summit are the most recent products of Chiginagak. They most likely originated from a lava dome at 1687 m on the SE flank, 1 km from the summit of the volcano, which has variably been estimated to be from 2075 to 2221 m high. Brief ash eruptions were reported in July 1971 and August 1998. Fumarolic activity occurs at 1600 m elevation on the NE flank of the volcano, and two areas of hot-spring travertine deposition are located at the NW base of the volcano near Volcano Creek.

Information Contacts: 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 following is a summary of observations from scientists at the Observatoire du Piton de la Fournaise and Observatoires Volcanologiques (OVPF), Institut de Physique du Globe de Paris, and the Laboratoire des Sciences de la Terre, Université de la Réunion.

Narrative. An eruption broke out on Piton de la Fournaise (PdF) at 1505 on 9 March 1998, after an unusually long period of 63 months rest. PdF (figure 41) had an average eruption rate of more than one per year in the last several decades. For a time three fissure vents were simultaneously active. The eruption continued at one fissure vent (Piton Kapor) at least as late as 20 April 1998.

Figure 41. Schematic map of Piton de la Fournaise showing the 9 and 11-12 March vents, newly named scoria cones and related features, and the extent of lava flows as of 15 March. Courtesy of Thomas Staudacher, OVPF.

Following escalating seismicity seen over the past two years, a seismic swarm developed at 0338 on 9 March (figures 42, 43, 44, and 45). The swarm was under the edifice, centered slightly W of the small Bory crater, a feature that lies immediately W of the larger Dolomieu crater. In the first observation of its kind at PdF, hypocenters progressed towards the surface prior to the eruption (figure 44).

Figure 42. The number of seismic events accumulated annually at Piton de la Fournaise during 1996, 1997, and early 1998 (three separate curves). The seismic swarm at the end of November 1996 was not followed by an eruption. A significant change in the earthquake rate started in July 1997 and accelerated in early 1998. Courtesy of OVPF.

Figure 43. Located earthquakes at Piton de la Fournaise from 1957 on 6 March through 1857 on 10 March (top) and a vertical, E-W cross section showing hypocenters from 0000 on 8 March through 1200 on 9 March (bottom). Coordinates (labeled tic marks) for horizontal distances on the map and cross section are 5 km apart; this scale differs from the vertical scale on the cross section. Courtesy of OVPF.

Figure 44. During the seismic crisis on PdF hypocenters migrated upward during the pre-eruptive 36-hour period shown (0000 on 6 March-1200 on 9 March). This was the first observation of its kind at PdF; pre-eruptive seismicity had usually remained diffusely distributed within the whole edifice. Courtesy of Jean Battaglia and Nelly Rousseau, OVPF.

Figure 45. Pre-eruptive earthquake counts at Piton de la Fournaise and seismic moments for 8-9 March 1998 (times are GMT). Noteworthy points are labeled as follows: at A, focal depths of the volcano-tectonic events started at ~5 km below sea-level; at B they reached 3 km; at C, 2 km; and at D, 1 km. At E, there occurred the first long-period (near 1 Hz) event since 1993. Venting started at 1505 (1105 GMT). Courtesy of OVPF.

The summit deformed rapidly beginning around 1400. An example of clear and sudden inflation appears in figure 46, documenting changes in radial and tangential inflation at station "Bory." Another multi-component station ("Soufriere"; immediately N of Dolomieu crater) underwent similarly rapid, though larger amplitude, displacement beginning at 1410 and peaking at 1424 to 1429 (undergoing up to 200 µrad of tilt). Inflation at Soufriere station indicated migration of magma towards the N eruptive fissures. Surface venting started there at 1505.

Figure 46. Ground deformation at the summit of Piton de la Fournaise on 9 March during 1200-1700 (0800-1300 GMT). Surface venting began at 1505 (1105 GMT). The Bory two-component inclinometer, ~200 m S of Bory crater, measures tilt aligned radial and tangential to the volcano. The rapid inflation at 1011 GMT was linked to near-surface dike emplacement. Contact the authors for collateral inclinometer and extensometer data at other summit stations. Courtesy of OVPF.

EDM and GPS measurements showed concordant displacements at points around the summit (figures 47 and 48). The time-sequence of EDM data indicated that essentially all deformation occurred at the time of eruption. Consistent with the deformation, eruptive fissures developed between the reflectors to the NE and NW of the summit.

Figure 47. Automated electronic distance meter (EDM) measurements at Piton de la Fournaise taken from an instrument on the NW rim of the Enclos Fouqué caldera (star, labeled 1B10). The EDM computed distances and azimuths to 13 reflectors (triangles) on the flanks of the terminal cone. The numbers indicate centimeters of total displacement between 1000 and 1400 GMT on 9 March. Weather permitting, these measurements were made every hour and telemetered to the observatory in near real-time. Only reflectors E of the fissures underwent measurable relative motion, moving E up to 34 cm. Courtesy of OVPF.

Figure 48. GPS measurements at Piton de la Fournaise showing horizontal displacements in centimeters from GPS positioning in November 1997 and 15 March 1998. Courtesy of OVPF.

At 1505 on 9 March tilt on the northern summit inclinometer reversed and seismic tremor commenced, indicating the final stages of dyke emplacement and the onset of venting. Although at the time, bad weather impaired visual observation, venting was recognized, starting on a 150-m-long N-S fissure around 2,450 m elevation on the N flank of the terminal cone (figure 41). The fissure system quickly developed in an en echelon pattern stretching downslope to approximately 2,100 m elevation. Major venting migrated to the fissure's lower stretches where lava fountaining up to 50 m high fed a flow that descended E (towards an area of the N caldera called the Plaine des Osmondes). Vigorous venting continued through the night of 9 March.

A few discrete seismic events were observed through the tremor during the next two days (10-11 March). The approximate locations of the events were SW of Bory crater. During 10-11 March venting continued in the N along two 100-m-long fissures. At the time, scientists lacked visual observations of the flow front due to cloud cover. Earthquakes at Piton de la Fournaise generally cease after an eruption has broken out, but in this case they continued, hence the impending opening of a new eruption fissure was forecast for the next few hours or days.

In accord with this forecast, during the night of 11 March until 0245 the next morning, a new, isolated eruptive fissure opened WSW of the Bory crater. The vent established itself S of the other erupting fissures, at ~2,200 m elevation (figure 41). Although lava escaped at a much lower rate here than along the northern vents, this southern fissure emitted lava along a zone ~100 m in length. Fountaining lava reached ~10 m high and fed a flow that by 0800 on 11 March had traveled 200-300 m downslope.

During the following days, eruptions continued at both the two northern fissures as well as the southern fissure. Estimated emission rates on the N were 30-50 m³/s and on the S at 5-10 m³/s. Issuing from the northern fissures, E-traveling lava descended to ~1,100 m elevation by 15 March. Here, ~4 km away from the vents, the flow front became stationary. Around the same time, lava issuing at the southern fissure reached an estimated length of 1,500 m. Maximum lava temperatures reached 1,167°C at the northern vents and 1,157°C at the southern vent.

Venting was progressively restricted to limited stretches of the three fissures where scoria cones started to grow. By 19 March the scoria cones were ~40 m high and 120 m long at the upper-elevation northern site, ~35 m high at the lower-elevation northern site, and 15 m high at the southwestern site.

Features at these cones were designated as the Maurice and Katia Krafft crater, Piton Kapor, and the Fred Hudson crater (figure 41). Activity at the three cones continued, but progressively decreased until venting was restricted to Piton Kapor by 31 March. Piton Kapor was still quite active as of 20 April 1998.

Preliminary petrography indicated that the lavas were mostly aphyric basalts carrying a small but variable number of millimeter-sized olivine crystals. Under the assumption that their composition lay close to the so-called "stationary basalts," modeling indicated that they vented at temperatures close to their liquidus.

Premonitory geophysical observations. Clear-cut long-term observations on the various surveillance networks that signaled an impending eruption were, as is customary at PdF, discrete and few. Increasing seismicity late in 1997 and accelerating in early 1998 were signs that an abnormal situation was developing. However, other crises, albeit of smaller intensities, occurred in November 1996 and July 1997 and did not result in an eruption. Small perturbations were seen on the deformation (inclinometry, geodesy, and extensometry) networks months before the present event but were not interpreted as premonitory. These signs most probably corresponded to magma intrusions within the edifice.

Surveillance network observations. It was only a few hours before the 9 March outbreak that short-term signs definitely signaled an impending eruption and civil authorities were warned of a maximum alert. Critical signs included seismic, tilt, and deformation data (summarized on figures 42 to 48). In addition, a total-field magnetometer network provided clear pre- and syn-eruptive signals that remain under interpretation. Measurements on about 50 of the approximately 100 microgravity-benchmark and GPS-array stations were repeated between 18 and 31 March with two Scintrex CG-3M gravimeters. The array was last surveyed in December 1997. A few stations showed variations of relatively small amplitude. Interpretations must await correction of the elevation changes and comparison with the recordings provided by the two permanent monitoring stations installed in December 1997. Radon stations did not show any unusual pattern either before or during the first stages of the outbreak as was hoped from previous behavior during intrusive events (BGVN 21:12).

The Observatoire Volcanologique du Piton de la Fournaise(OVPF) was built in 1979 after the devastation of the 1977 eruption owing to the financial help of the Institut National des Siences de l'Univers, France. The Observatory became operational in 1980; since then, tens of eruption have been closely observed and, most often, forecast sufficiently in advance to alleviate possible personal and material damages.

Besides the information contacts listed below, report contributors also included Kei Aki, Valérie Ferazzini, Louis-Philippe Ricard, Nelly Rousseau, Jean Battaglia, Nicolas Villeneuve, Philippe Kowalski, Philippe Catherine, Denis Wégerlé, Grégory Durand, Nadia Talibart, Jacques Lebreton, Maolidi Assoumani, Massimo Bonfiglio, Bernard Robineau, Jean-Lambert Join, Eric Delcher, Jean-Luc Folio, Jean-Luc Hoareau, Cécile Savin, Hamidou Nassor, Evelyn Maillot, Jean-Claude Lépine, Martine Hirn-Sapin, Christine Deplus, Pierre Briole, Sylvain Bonvalot, Jacques Zlotnicki, Germinal Gabalda, Philippe Labazuy, Alfred Hirn, Jean-Claude Delmond, Guy Aubert, Michel Diament, and Janine Gouin.

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

Information Contacts: Thomas Staudacher, Observatoire Volcanologique du Piton de la Fournaise (OVPF), 14 RN3, le 27Km, 97418 La Plaine des Cafres, La Réunion, France; Patrick Bachèlery, Département des Sciences de la Terre, Université de la Réunion, BP 7151, 15 Avenue Rene Cassin, 97715 Saint Denis Cedex 9, La Réunion, France; Michel P. Semet and Jean-Louis Cheminée, Observatoires Volcanologiques, Institut de Physique du Globe de Paris, 4 Place Jussieu, 75252 Paris Cedex 05, France (URL: http://www.ipgp.jussieu.fr/).

During March-October 1997 a series of phreatic explosions took place within Guagua Pichincha's caldera (figure 5). No precursory signals were detected prior to the activity. The intensity of these explosions peaked in May 1997; the last explosive signal was detected on 18 October 1997. This activity resembled phreatic explosions that occurred in 1981, 1990, and 1993.

Figure 5. Monthly counts of explosion signals at Guagua Pichincha detected by Instituto Geofisico seismic stations during 1997. Courtesy of the Instituto Geofisico.

Larger explosions on 15, 16, 18, 20, and 22 May, 22 and 23 July, and 18 October were detected by four short-period seismic stations located around the volcano. Tremor signals following these explosions had reduced displacements of 2. The largest explosion occurred on 29 May at 0654; its signal was recorded at eight sites, including seismic stations at the volcanoes Cotopaxi (58 km away), Cotacachi (60 km away), and Cayambe (70 km away). The accompanying tremor signal had a reduced displacement of 8.9 cm². An A-type fracture event located just outside the E caldera rim at 3 km depth preceded the explosion.

Following the 20 May explosion, volcanologists observed two new, white, 250-m-tall fumarolic plumes rising from the explosion crater. The crater showed evidence of recent collapses on its interior S and SW sides. Fine pulverized rock deposits covered more than 2 km² in the N part of the caldera bottom. Blocks up to 50 cm across were scattered over the caldera floor as far as 1 km from the crater; impact craters up to 2 m in diameter were formed. No juvenile material was found.

During 1997, the number of events at stations close to the caldera remained at normal values except during September-October, when a large number of events were detected at stations 1.0-1.2 km from the crater. However, at stations over 10 km away, the number of events remained at normal values. Low seismicity preceded phreatic activity in 1990 and 1993. The hypocenter locations of high-frequency events were at depths <5 km beneath the caldera floor (figure 6).

Figure 6. Epicenter map (top) and E-W cross-section (bottom) of high-frequency events at Guagua Pichincha during 1997. Courtesy of Instituto Geofisico.

A swarm of 26 local earthquakes (M <3) lasted less than 1 hour on 16 December 1997. This was the first such swarm detected at Guagua Pichincha since continuous seismic monitoring began in 1981. EDM deformation monitoring of the phreatic crater and outer flanks of the dome revealed no change with regard to the baseline established in 1988.

Thermocouple measurements of fumarole temperatures on the dome showed values of 120-120.7°C, the same as during prior measurements in 1995, but lower than those detected in February 1994 (138-139°C). Prior to 1994, fumarole temperatures were constant at 87°C. Analyses of spring water from the caldera and the surrounding area gave essentially the same results as in 1988.

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: Mario Ruiz Romero, Instituto Geofísico de la Escuela Politécnica Nacional.

During 26-27 December a small seismic swarm at Irazú consisted of 220 earthquakes. At the swarm's peak 109 earthquakes occurred in 15 hours. The epicenters fell 20 km NNW of the summit, originating on a local fault. The largest earthquake, at 0154 on 27 December, was M 2.9. It had a focal depth of 5 km and an epicenter 20 km NW of the summit. For comparison, during the months of January, February, and March 1998, the respective counts consisted of 58, 59, and 70 local earthquakes.

During 20 and 22 February seven earthquakes took place, including one of M 2.3 and another of M 1.8. Both of these events had epicenters within 7 km of the summit; their respective focal depths were at 8 km and 1 km.

During January the lake in the active crater remained greenish yellow and lacked bubbling along its shores. These areas were not mentioned as active again during February-March, although the lake's color was later described as light green. The monthly fluctuations in lake level noted for December to March were under a meter. During early 1998 small landslides continued to occur along the crater's N, E, and W walls. During February, fumaroles remained active on the volcano's NW flanks; their visible outputs remained moderate and their temperatures measured 91°C.

Geologic Background. Irazú, one of Costa Rica's most active volcanoes, rises immediately E of the capital city of San José. The massive volcano covers an area of 500 km2 and is vegetated to within a few hundred meters of its broad flat-topped summit crater complex. At least 10 satellitic cones are located on its S flank. No lava flows have been identified since the eruption of the massive Cervantes lava flows from S-flank vents about 14,000 years ago, and all known Holocene eruptions have been explosive. The focus of eruptions at the summit crater complex has migrated to the W towards the historically active crater, which contains a small lake of variable size and color. Although eruptions may have occurred around the time of the Spanish conquest, the first well-documented historical eruption occurred in 1723, and frequent explosive eruptions have occurred since. Ashfall from the last major eruption during 1963-65 caused significant disruption to San José and surrounding areas.

Information Contacts: E. Fernandez, V. Barboza, R. Van der Laat, R. Saenz, E. Duarte, E. Malavassi, T. Marino, M. Martinez, and E. Hernandez, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Mauricio Mora Fernandez, Sección de Sismologia, Vulcanologia y Exploración Geofisica, Escuela Centroamericana de Geología, Universidad de Costa Rica, P.O. Box 35-2060, San José, Costa Rica.

Seismicity remained above background level during 2 March-5 April and low-level Strombolian activity continued. As many as 70-100 gas-and-ash or gas-and-steam explosions occurred daily. Ash and steam rose 300-400 m above the crater during the first week of March.

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.

The E rift zone eruption at Kilauea remained steady during March. Seismicity was low, little inflation or deflation occurred at the summit, and magma moved through shallow conduits towards the E rift zone without disturbing the ground surface. The eruption has continued in this fashion since a brief surge in January (BGVN 22:12).

On 11 March glowing holes were observed in the Pu`u `O`o crater floor and in the crater vent; however, no lava escaped from the area. Researchers at the University of Hawaii also observed several large fissures and cracks within the cone edifice. Fumes issued from the cracks and surrounding area; during the last two weeks of March, profuse fumes obscured views of the crater vent. Skylights S of Pu`u `O`o cone revealed lava flowing toward the sea.

Although lava continued to travel in tubes from the Pu`u `O`o vent area to the ocean, surface flows have been sparse since early February (BGVN 23:02). Lava broke out of tubes on the Pulama Pali on 2 and 10 March, but both flows lasted less than a day. Small flows issued from weak points in the lava tubes on the coastal plain on 3-7, 10, and 14 March. Most of the breakouts were near the Waha`ula ocean entry.

Kilauea is one of five coalescing volcanoes that comprise the island of Hawaii. Historically its eruptions originated primarily from the summit caldera or along one of the lengthy E and SW rift zones that extend from the summit caldera to the sea. This latest Kilauea eruption began in January 1983 along the E rift zone. The eruption's early phases, or episodes, occurred along a portion of the rift zone that extends from Napau Crater on the uprift end to ~8 km E on the downrift end. Activity eventually centered on what was later named Pu`u `O`o. More than 223 hectares of new land have been added to the island and local communities have suffered more than $100 million in damages since the beginning of the eruption.

Geologic Background. Kilauea, which overlaps the E flank of the massive Mauna Loa shield volcano, has been Hawaii's most active volcano during historical time. Eruptions are prominent in Polynesian legends; written documentation extending back to only 1820 records frequent summit and flank lava flow eruptions that were interspersed with periods of long-term lava lake activity that lasted until 1924 at Halemaumau crater, within the summit caldera. The 3 x 5 km caldera was formed in several stages about 1500 years ago and during the 18th century; eruptions have also originated from the lengthy East and SW rift zones, which extend to the sea on both sides of the volcano. About 90% of the surface of the basaltic shield volcano is formed of lava flows less than about 1100 years old; 70% of the volcano's surface is younger than 600 years. A long-term eruption from the East rift zone that began in 1983 has produced lava flows covering more than 100 km2, destroying nearly 200 houses and adding new coastline to the island.

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

During 2 March-5 April, seismicity under the volcano remained above background level and earthquakes at 25-30 km depth were recorded. Surface earthquakes were detected on 14 March from 0040-0105.

Fumarolic plumes rose 50-100 m above the volcano on 5, 7, 10, 13-15, 16, 18-20, and 22 March. On 30-31 March, and 1, 3, and 5 April the fumarolic plume rose 50-400 m above the volcano and moved 3-10 km SE. A gas-and-steam plume on 12 March rose 200-1,000 m and traveled more than 5 km ESE. On 17 March, a gas-and-steam plume rose 2-3 km above the volcano and drifted 5-10 km SE.

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.

An 8 April 1998 report stated that during the past week Llaima increased its output of small explosions and ash emissions. The amplitude of seismic signals also increased, although the frequency of signals remained fixed at 1.5 Hz. Seismic amplitude (RSAM) values during March averaged about 25% above those of February. Daily RSAM estimates in March jumped to nearly 30 RSAM units on a few days but more frequently only reached about 10 RSAM units. A sample of the seismic record is shown on figure 9.

Figure 10. Sample seismic record at Llaima (Meli station) on 22 April 1998 beginning at 0400. The tic marks are at 1-minute intervals. Courtesy of OVDAS.

Geologic Background. Llaima, one of Chile's largest and most active volcanoes, contains two main historically active craters, one at the summit and the other, Pichillaima, to the SE. The massive, dominantly basaltic-to-andesitic, stratovolcano has a volume of 400 km3. A Holocene edifice built primarily of accumulated lava flows was constructed over an 8-km-wide caldera that formed about 13,200 years ago, following the eruption of the 24 km3 Curacautín Ignimbrite. More than 40 scoria cones dot the volcano's flanks. Following the end of an explosive stage about 7200 years ago, construction of the present edifice began, characterized by Strombolian, Hawaiian, and infrequent subplinian eruptions. Frequent moderate explosive eruptions with occasional lava flows have been recorded since the 17th century.

Information Contacts: Gustavo Fuentealba¹ and Paola Peña S., Observatorio Volcanológico de Los Andes del Sur (OVDAS), Manantial 1710-Carmino del Alba, Temuco, Chile; ¹Universidad de La Frontera (UFRO), Departamento Ciencias Fisicas, Universidad de la Frontera, Avda. Francisco Salazar 01145, Casilla 54-D, Temuco, Chile.

Measurements during a 28 February visit revealed higher-than-normal fumarolic temperatures in the summit area. The high temperatures were associated with a recent period of aridity, during which time fumarolic activity increased. Temperatures ranged from 318-748°C (figure 7).

Figure 7. Sketch of Momotombo's active crater showing fumarole temperatures on 28 February. Areas of fumarolic activity are gray. View is towards the S; the crater is ~150 m wide. Courtesy of A. Creusot.

Geologic Background. Momotombo is a young stratovolcano that rises prominently above the NW shore of Lake Managua, forming one of Nicaragua's most familiar landmarks. Momotombo began growing about 4500 years ago at the SE end of the Marrabios Range and consists of a somma from an older edifice that is surmounted by a symmetrical younger cone with a 150 x 250 m wide summit crater. Young lava flows extend down the NW flank into the 4-km-wide Monte Galán caldera. The youthful cone of Momotombito forms an island offshore in Lake Managua. Momotombo has a long record of Strombolian eruptions, punctuated by occasional stronger explosive activity. The latest eruption, in 1905, produced a lava flow that traveled from the summit to the lower NE base. A small black plume was seen above the crater after a 10 April 1996 earthquake, but later observations noted no significant changes in the crater. A major geothermal field is located on the south flank.

Information Contacts: Alain Creusot, Instituto Nicaraguense de Energía, Managua, Nicaragua.

A 14 February visit to Cerro Negro's crater revealed a general decrease in fumarole temperatures since Alain Creusot last measured temperatures there on 23 December 1996 (BGVN 21:12). The highest temperature found on his latest visit was 340°C. For comparison, in October 1996 fumarole temperatures were as high as 700°C.

Geologic Background. Nicaragua's youngest volcano, Cerro Negro, was created following an eruption that began in April 1850 about 2 km NW of the summit of Las Pilas volcano. It is the largest, southernmost, and most recent of a group of four youthful cinder cones constructed along a NNW-SSE-trending line in the central Marrabios Range. Strombolian-to-subplinian eruptions at intervals of a few years to several decades have constructed a roughly 250-m-high basaltic cone and an associated lava field constrained by topography to extend primarily NE and SW. Cone and crater morphology have varied significantly during its short eruptive history. Although it lies in a relatively unpopulated area, occasional heavy ashfalls have damaged crops and buildings.

Information Contacts: Alain Creusot, Instituto Nicaraguense de Energía, Managua, Nicaragua.

Poás monthly reports from OVSICORI-UNA since November 1997, and as recently as March, have noted that its N crater lake has remained turquoise green and continued to host rafts of suspended sulfur. The lake's surface normally sits at ~2,300 m elevation. Although during 1997 the lake's surface reached a high stand, it descended during early 1998, dropping 3 m due to lack of rain. Mauricio Mora Fernandez provided plots of the lake water's pH, temperature, sulfate, and chlorine for the past several years (figures 67 and 68). Fernandez also reported that during February 1998 fumarolic activity continued in five areas within the active crater (figure 69). During April, he found a sixth fumarolic area on the dome's N slope.

Figure 67. Water pH and temperature measured in the N crater lake at Poás (right- and left-hand scales, respectively), 1993 to early 1998. The time scale is not linear. OVSICORI-UNA staff collected the lake geochemistry data. Courtesy of M. Mora Fernandez.

Figure 68. Aqueous sulfate and chlorine in the N crater lake at Poás, 1993 to early 1998. Time scale is not linear. OVSICORI-UNA staff collected the lake geochemistry data. Courtesy of M. Mora Fernandez.

Figure 69. The active crater at Poás viewed from the S. Numbers 1-5 correspond to areas with fumaroles active during February 1998; a sixth area, located N of the dome (on the side away from the view, not labeled), became active in April 1998. Courtesy of M. Fernandez.

Area 1, the fumarolic field located at the crater's S end, became active in May 1995 and remained comparatively stable thermally until at least March. During 1997-early 1998, the field extended S, SW, and W within the larger crater. During February 1998, the area's average temperature remained constant at ~92°C; during April, it attained 93°C. Steam and high concentrations of SO₂ and Cl gas escaped from the fumaroles; sulfur crystals were deposited around the vents. Mora noted that in the time since the fumaroles appeared, hydrothermal alteration became more rapid and reduced competency of the rock, leading up to two landslides in the area.

In area 2, the field W of the crater lake, a large landslide occurred during February. It took place at a spot where hydrothermal alteration resulted from three fumaroles that sent white gas plumes dominantly toward the SW. More fumaroles sprung up in this field during April.

In area 3, the field at the lake's N end, new fumaroles appeared during roughly the second half of 1997. These continued without important changes through April 1998; their emissions were white and not very vigorous.

In area 4, a field on the dome's E slope, small fumaroles produced white plumes. The emissions were not vigorous but their average February-April temperatures were 92-93°C. Some new fumaroles noted in this area during April had temperatures averaging 93°C.

In areas 5 and 6, fields located respectively on the dome's E and N slopes, vigorous fumaroles gave off mainly white plumes. During April, area 5 plumes had temperatures of 92°C and ascended to tens of meters before dispersing. Area 6, which became active in April, gave off plumes that covered the nearby slope with sulfur deposits.

OVSICORI-UNA reported that the pyroclastic cone in the crater discharged a plume that during January rose 400 m above the crater rim. They also noted that during February the rain collection network located around the active crater yielded samples with increased acidity. During this same month, residents 5.5 km SE of the crater reported occasional sulfur odors.

Seismic data from an OVSICORI-UNA station 2.7 km SW of the active crater revealed a noticeable rise in the duration of tremor during February and March 1998. Tremor generally occurred in discontinuous episodes, although one episode on 21 February carried on for 2.5 hours. Also, an anomalously large number of low-frequency earthquakes took place during February 1998 (figure 70)—a count of this magnitude was last seen in January 1996. In contrast, medium and high frequency earthquakes were not particularly abundant in February or March 1998 (figure 70). Many of the low-frequency earthquakes were attributed to continuous degassing.

Figure 70. Seismicity at Poás during January 1997-March 1998. Number of low-frequency earthquakes and hours of tremor (top); number of high- and medium-frequency earthquakes (bottom). Note that the scales are different. Courtesy of OVSICORI-UNA.

Geologic Background. The broad, well-vegetated edifice of Poás, one of the most active volcanoes of Costa Rica, contains three craters along a N-S line. The frequently visited multi-hued summit crater lakes of the basaltic-to-dacitic volcano, which is one of Costa Rica's most prominent natural landmarks, are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the 2708-m-high complex stratovolcano extends to the lower northern flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, is cold and clear and last erupted about 7500 years ago. The more prominent geothermally heated northern lake, Laguna Caliente, is one of the world's most acidic natural lakes, with a pH of near zero. It has been the site of frequent phreatic and phreatomagmatic eruptions since the first historical eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.

Information Contacts: E. Fernandez, V. Barboza, R. Van der Laat, R. Saenz, E. Duarte, E. Malavassi, T. Marino, M. Martinez, and E. Hernandez, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Mauricio Mora Fernandez, Sección de Sismologia, Vulcanologia y Exploración Geofisica, Escuela Centroamericana de Geología, Universidad de Costa Rica, P.O. Box 35-2060, San José, Costa Rica.

Eruptive activity at Tavurvur persisted during March following the 3 February eruption (BGVN 23:02), producing ash emissions, small pyroclastic flows, and relatively low but fluctuating seismicity. Seismicity peaked around20 March, when eruptions became more energetic, and was probably related to near-surface eruptive activity.

Deformation monitoring indicated steady inflation at Tavurvur. Readings from the Sulphur Creek water tube (3.5 km NW of Tavurvur) revealed a change of ~3 µrad tilt away from the volcano during March. Leveling and real-time GPS also showed continuing inflation.

Tavurvur continued to erupt throughout March and emitted ash at intervals of ~10 minutes to several hours; the rapidly convecting column sometimes rose 2-4 km. After emissions had ceased for more than 10-20 minutes, activity would often recommence with explosions that threw large numbers of blocks from the vent. Blocks up to 1 m in diameter were regularly thrown 1 km S and W of the vent, landing out to sea. Large blocks (~3-4 m across) littered the rim and upper slopes of Tavurvur, probably produced during larger-than-usual explosions on 7 and 8 March.

The 8 March explosion sent red oxide-covered lava blocks and boulders over the S crater rim and down the S flank of Tavurvur, where the flow traveled ~1 km. This mass was described as being "pushed" from the vent immediately prior to the explosion. At other times the ash plume underwent partial column collapse and sent short, billowing flows randomly down the cone's flanks. The flows deposited light gray dust ~50-150 m downslope in well-defined tongues.

During 18-26 March night glow became more evident; occasionally lava fountains sent glowing fragments 200-300 m above the crater rim for up to 5 minutes at a time. During 26-31 March intermittent ash emissions with discrete explosions after longer periods of quiescence resumed.

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: Ben Talai, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.

Beginning at 1428 on 15 February, Rincón de la Vieja volcano discharged phreatic eruptions from the main crater. Ten eruptions took place in the first 15 hours of activity; only two followed in the subsequent 13 hours. During the course of the outburst subsidiary fumarolic activity also became more vigorous; it remained elevated until 18 February.

During 15-17 February numerous steam plumes rose hundreds of meters above the volcano. On 17 February one outburst sent a steam plume to a height of 2 km above the crater. This plume was seen by residents on the N and NE flanks of the volcano. A dozen eruptions around this time were small and lacked associated mudflows. An exception, at 0514 on 16 February, produced a modest mudflow that traveled about 9 km/hour and left a capping deposit of mud 30-cm thick in the upper reaches of the Pénjamo and Azul rivers. Rivers had been low in the region, attributed to the El Niño phenomena, with the result that the mudflow was relatively dry. The mudflow had a large impact on local fish and other stream organisms. Sediment from the mudflow was found 12.3 km from the main crater.

Inspecting the 16 February deposit near the summit on 1 March, scientists inferred from the scorching, burning, and other damage to vegetation on the NE flanks that there must have been several smaller eruptions around that time as well. Mudflows failed to develop due to the paucity of surface water in local drainages.

The 1 March visit also revealed the lake's temperature, 48°C, its color, light gray, the presence of suspended sulfur in the lake, and a haze of condensed gases above the lake. An outgassing fumarole on the SW wall made loud hissing noises (similar to gases exiting a high pressure valve) audible from the crater's rim. Columns of gas rose about 200 m above the crater before being blown E. Those inspecting the scene noted strong sulfurous odors, and experienced irritated skin and eyes. The material erupted was uniformly fine- to medium-grained, lacking either bombs, blocks, or impact craters. This contrasted with deposits left by previous eruptions in 1991 and 1995.

The local seismic station (RIN3) lies 5 km SW of the active crater. The station registered microearthquakes as follows: during January, 18 (including 3 of high frequency and 9 of low frequency); during February, 48 (including 1 of high frequency, 21 of low frequency); during March, 7. In assessing their records of the 48 February microearthquakes, seismologists recognized 20 eruptions including 11 comparatively high-intensity phreatic eruptions mainly registered on 15-18 February. Banded tremor occurred on 15 and 16 February during the main eruptive interval; the tremor prevailed for a total of ~6.5 hours. Low in frequency, the tremor had amplitudes that ranged between 1.0 and 37 mm. The larger amplitude registered during the eruption's initial phase, at 1428 on 15 February. Tremor amplitudes later declined to the 1-4 mm range. As with the 1991 and 1995 eruptions, seismic precursors were absent.

Geologic Background. Rincón de la Vieja, the largest volcano in NW Costa Rica, is a remote volcanic complex in the Guanacaste Range. The volcano consists of an elongated, arcuate NW-SE-trending ridge that was constructed within the 15-km-wide early Pleistocene Guachipelín caldera, whose rim is exposed on the south side. Sometimes known as the "Colossus of Guanacaste," it has an estimated volume of 130 km3 and contains at least nine major eruptive centers. Activity has migrated to the SE, where the youngest-looking craters are located. The twin cone of 1916-m-high Santa María volcano, the highest peak of the complex, is located at the eastern end of a smaller, 5-km-wide caldera and has a 500-m-wide crater. A plinian eruption producing the 0.25 km3 Río Blanca tephra about 3500 years ago was the last major magmatic eruption. All subsequent eruptions, including numerous historical eruptions possibly dating back to the 16th century, have been from the prominent active crater containing a 500-m-wide acid lake located ENE of Von Seebach crater.

Information Contacts: E. Fernandez, V. Barboza, R. Van der Laat, R. Sáenz, E. Duarte, E. Malavassi, T. Marino, M. Martinez, and E. Hernandez, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Mauricio Mora Fernandez, Sección de Sismologia, Vulcanologia y Exploración Geofisica, Escuela Centroamericana de Geología, Universidad de Costa Rica, P.O. Box 35-2060, San José, Costa Rica.

Seismicity was about at background level during 2 March-5 April. Gas-and-steam plumes rose 100 m above the volcano on 7 and 13-15 March. On 16-18, 22, and 30-31 March, and 1 and 3 April, gas-and-steam plumes rose 100-500 m above the volcano. Clouds obscured observations of the volcano on several days in early April.

Geologic Background. The high, isolated massif of Sheveluch volcano (also spelled Shiveluch) rises above the lowlands NNE of the Kliuchevskaya volcano group. The 1300 km3 volcano is one of Kamchatka's largest and most active volcanic structures. The summit of roughly 65,000-year-old Stary Shiveluch is truncated by a broad 9-km-wide late-Pleistocene caldera breached to the south. Many lava domes dot its outer flanks. The Molodoy Shiveluch lava dome complex was constructed during the Holocene within the large horseshoe-shaped caldera; Holocene lava dome extrusion also took place on the flanks of Stary Shiveluch. At least 60 large eruptions have occurred during the Holocene, making it the most vigorous andesitic volcano of the Kuril-Kamchatka arc. Widespread tephra layers from these eruptions have provided valuable time markers for dating volcanic events in Kamchatka. Frequent collapses of dome complexes, most recently in 1964, have produced debris avalanches whose deposits cover much of the floor of the breached caldera.

Information Contacts: 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.

This report condenses Scientific Reports of the Montserrat Volcano Observatory (MVO) covering February. During 1-14 February, seismic activity increased, heavy ashfalls reached the N part of the island, and dome growth continued. Activity during 15-28 February was dominated by rapid dome growth and elevated seismicity.

Visual observations. Low clouds during the first two weeks of February often hampered dome observations. However, on 6 February observers on a police boat reported continued growth in the 26 December collapse scar above the White River. By 10 February the growing dome almost completely filled the 26 December scar, approaching the volume prior to the collapse. In addition, two spines were observed on the dome's S side, and the talus slope below the growth area had grown considerably. Steam-and-ash venting continued and was vigorous during periods of elevated seismicity and rockfall.

Rockfalls and small pyroclastic flows occurred mainly on the Galways side of the dome, but a few small rockfalls were observed in the upper part of Tuitt's Ghaut. Fresh pyroclastic-flow deposits in the upper part of the White River were probably emplaced during the elevated activity of 5-6 February.

On 15 February several rockfalls and small pyroclastic flows traveled down the White River valley. Visibility was poor until 25 February when vigorous ash venting, rockfalls in the White River valley, and several stubby spines atop the dome were observed.

Seismicity. Earthquake activity during 1-14 February mainly consisted of rockfalls and hybrid earthquakes with some tremor. Most swarm events, including 21 locatable volcano-tectonic earthquakes, were concentrated below the dome complex's N sector and had shallow focal depths (2-4 km below the summit). During 15-28 February fewer rockfalls but comparatively more earthquakes and seismic swarms (table 27) occurred than in preceding weeks. The swarms were not followed by surface activity.

Table 27. Number of hybrid, long-period (LP), and volcano-tectonic (VT) events detected during earthquake swarms at Soufriere Hills during February 1998. Courtesy of MVO.

At the beginning of February, seismicity displayed a cyclic pattern with peak amplitudes occurring every 6-8 hours; by 14 February, the cycle had lengthened to 8-12 hours. By 22 February, the cycle was ~14 hours long. Peak amplitudes increased during 1-14 February; these peaks generally coincided with elevated rockfall activity. Towards the end of February, the peaks were dominated by hybrid earthquakes and tremor.

Ground deformation. Two GPS occupations of LEESNET (includes sites at Old Towne, Waterworks, St. Georges Hill, and Lees Yard) were made during 1-14 February. No movement within this network was detected. Meanwhile, GPS surveys at Harris, Hermitage, Lees Yard, Perches, St. Georges Hill, Old Towne, Blakes, and Lookout Yard North confirmed that the Hermitage and Perches sites continued to move NNE. Sites on the volcano's N and NW flanks remained relatively stable.

Electronic tiltmeters were installed at Hermitage and on Gages Mountain to provide data on deformation of the volcano's NE flank. The EDM reflector on the N crater wall (Peak B) was shot from Windy Hill during 15-28 February. During 25 January-late February a 5-cm shortening occurred on this line. Lines between the Lees Yard reflector and sites at MVO south and the Waterworks Estate did not show any movement.

Volume measurements. A 10 February theodolite survey of the dome from Garibaldi Hill and the Delta petrol station revealed that the dome's highest point was 970 m. On 27 February, theodolite measurements from Garibaldi Hill and the old observatory in Old Towne showed that the highest point on the dome had reached 997 m. More theodolite measurements on 1 March from South Soufriere Hills and Perches Mountain gave a height of 1011 m, revealing 14 m of vertical growth in only 2 days.

Environmental monitoring. Sulfur dioxide diffusion tube measurements during 1-14 February showed raised (10-12 ppb) SO₂ levels in Plymouth and at St. Georges Hill and low (0-0.6 ppb) levels at Weekes, MVO south, and Lawyers. During 15-28 February SO₂ levels at Plymouth, MVO south, and Lawyers were higher than earlier in the month, but levels at St. Georges Hill were reduced by half. The site in Plymouth showed very high values (30.2 ppb) because it was surrounded by ~30-cm-thick tephra deposits and redeposited debris from nearby pyroclastic-flow deposits.

The mass of fine ash deposited in N Montserrat during several 28 January-7 February ashfalls was calculated using an array of ash collection trays. The mass totaled more than 1 kg/m²; most of this ash was produced during episodes of ash venting and rockfall activity. At most locations the ash collected during 3-5 February accounted for more than 50% of the local monthly ash accumulation.

Dust Trak monitoring at four fixed sites to measure airborne particles revealed elevated values (0.05-0.38 mg/m³) during ashfalls on 4-5 February. Levels were even higher (0.11-0.43 mg/m³) on 7 February due to resuspension of the ash. Sites in the S part of the island showed higher concentrations than in the N. During 15-28 February, no major ash fall occurred and levels were low (3) at all sites; however, a diffuse volcanic plume was occasionally blown N, causing light ash fall and hazy conditions.

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, P. O. Box 292, Plymouth, Montserrat (URL: http://www.mvo.ms/).

Beginning at about 0900 on 26 March, an unusual cloud or steam plume in the vicinity of Spurr volcano was observed from Anchorage (125 km E). However, seismicity remained at normal levels and nothing unusual was noted in satellite images of the area. The level of concern remained at green, indicating normal seismic and fumarolic activity.

Geologic Background. The summit of Mount Spurr, the highest volcano of the Aleutian arc, is a large lava dome constructed at the center of a roughly 5-km-wide horseshoe-shaped caldera open to the south. The volcano lies 130 km W of Anchorage and NE of Chakachamna Lake. The caldera was formed by a late-Pleistocene or early Holocene debris avalanche and associated pyroclastic flows that destroyed an ancestral edifice. The debris avalanche traveled more than 25 km SE, and the resulting deposit contains blocks as large as 100 m in diameter. Several ice-carved post-caldera cones or lava domes lie in the center of the caldera. The youngest vent, Crater Peak, formed at the breached southern end of the caldera and has been the source of about 40 identified Holocene tephra layers. Eruptions from Crater Peak in 1953 and 1992 deposited ash on the city of Anchorage.

Information Contacts: 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.

Scientists visited Telica's crater on 7 February. They observed a slight increase in fumarolic activity and an active collapse zone on the S crater rim. Light incandescence seen at night had an estimated temperature of 550°C.

Geologic Background. Telica, one of Nicaragua's most active volcanoes, has erupted frequently since the beginning of the Spanish era. This volcano group consists of several interlocking cones and vents with a general NW alignment. Sixteenth-century eruptions were reported at symmetrical Santa Clara volcano at the SW end of the group. However, its eroded and breached crater has been covered by forests throughout historical time, and these eruptions may have originated from Telica, whose upper slopes in contrast are unvegetated. The steep-sided cone of Telica is truncated by a 700-m-wide double crater; the southern crater, the source of recent eruptions, is 120 m deep. El Liston, immediately E, has several nested craters. The fumaroles and boiling mudpots of Hervideros de San Jacinto, SE of Telica, form a prominent geothermal area frequented by tourists, and geothermal exploration has occurred nearby.

Information Contacts: Alain Creusot, Instituto Nicaraguense de Energía, Managua, Nicaragua.

OVSICORI-UNA scientists have taken sporadic samples of the chemistry, pH, and temperature of Turrialba's fumaroles (figures 2 and 3). During January, fumaroles had low emissions but the temperature of one fumarole remained fixed at 90°C (figure 3). Small landslides down the N and S sides of the crater walls covered fumaroles on the crater floor during January; however, during this time new fumaroles also appeared on the crater floor as well.

Figure 2. Chlorine and sulfate in Turrialba fumarolic condensate at [nine] sampling dates during late 1996-early [1997]. For sampling and analytical methods, contact the authors. Courtesy of OVSICORI-UNA.

Figure 3. The pH and temperature of Turrialba fumarolic condensate at four sampling dates during the interval late 1996 to early 1998. Courtesy of OVSICORI-UNA.

The local seismic station ("VTU," located 500 m S of the active crater) was out of service during September-December 1997. After that, the station registered microearthquakes as follows: January, 53; February, 83; and March 96. Two of the February earthquakes, one high- and one low-frequency, also registered on the more distant seismic station IRZ2, ~15 km from the active crater. Besides the 96 microearthquakes registered during March, several more low- and high-frequency earthquakes also took place.

Geologic Background. Turrialba, the easternmost of Costa Rica's Holocene volcanoes, is a large vegetated basaltic-to-dacitic stratovolcano located across a broad saddle NE of Irazú volcano overlooking the city of Cartago. The massive edifice covers an area of 500 km2. Three well-defined craters occur at the upper SW end of a broad 800 x 2200 m summit depression that is breached to the NE. Most activity originated from the summit vent complex, but two pyroclastic cones are located on the SW flank. Five major explosive eruptions have occurred during the past 3500 years. A series of explosive eruptions during the 19th century were sometimes accompanied by pyroclastic flows. Fumarolic activity continues at the central and SW summit craters.

Information Contacts: E. Fernandez, V. Barboza, R. Van der Laat, R. Saenz, E. Duarte, E. Malavassi, T. Marino, M. Martinez, and E. Hernandez, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Mauricio Mora Fernandez, Sección de Sismologia, Vulcanologia y Exploración Geofisica, Escuela Centroamericana de Geología, Universidad de Costa Rica, P.O. Box 35-2060, San José, Costa Rica.

Luis Hernan Ecueñique, a manager in charge of "Las Cavernas," a tourist attraction 8 km from Villarrica's active crater, noted that during late March through at least early April there had been an ascent of magma in the central crater. Erupted material reached ~100 m from the crater's edge. Local tour guides had also informed him that explosions had deposited tephra on the N flanks. Measurements within "Las Cavernas" (which are lava tubes) indicated the air temperature rose by about 2°C.

A digital seismic station 21 km from the crater failed to detect either an increase in the number of seismic events or a shift in their character; the system did register a minor increase in event amplitude.

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

Information Contacts: Gustavo Fuentealba¹ and Paola Peña S., Observatorio Volcanológico de Los Andes del Sur (OVDAS), Manantial 1710-Carmino del Alba, Temuco, Chile; ¹Universidad de La Frontera (UFRO), Departamento Ciencias Fisicas, Universidad de la Frontera, Avda. Francisco Salazar 01145, Casilla 54-D, Temuco, Chile.