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).

The eruption at Sheveluch has continued for more than 20 years, with strong explosions that have produced ash plumes, lava dome growth, hot avalanches, numerous thermal anomalies, and strong fumarolic activity (BGVN 44:05). During this time, there have been periods of greater or lesser activity. The most recent period of increased activity began in December 2018 and continued through October 2019 (BGVN 44:11). This report covers activity between November 2019 to April 2020, a period during which activity waned. The volcano is monitored by the Kamchatka Volcanic Eruptions Response Team (KVERT) and Tokyo Volcanic Ash Advisory Center (VAAC).

During the reporting period, KVERT noted that lava dome growth continued, accompanied by incandescence of the dome blocks and hot avalanches. Strong fumarolic activity was also present (figure 53). However, the overall eruption intensity waned. Ash plumes sometimes rose to 10 km altitude and drifted downwind over 600 km (table 14). The Aviation Color Code (ACC) remained at Orange (the second highest level on a four-color scale), except for 3 November when it was raised briefly to Red (the highest level).

Figure 53. Fumarolic activity of Sheveluch’s lava dome on 24 January 2020. Photo by Y. Demyanchuk; courtesy of KVERT.

Table 14. Explosions and ash plumes at Sheveluch during November 2019-April 2020. Dates and times are UTC, not local. Data courtesy of KVERT and the Tokyo VAAC.

KVERT reported thermal anomalies over the volcano every day, except for 25-26 January, when clouds obscured observations. During the reporting period, thermal anomalies, based on MODIS satellite instruments analyzed using the MODVOLC algorithm recorded hotspots on 10 days in November, 13 days in December, nine days in January, eight days in both February and March, and five days in April. The MIROVA (Middle InfraRed Observation of Volcanic Activity) volcano hotspot detection system, also based on analysis of MODIS data, detected numerous hotspots every month, almost all of which were of moderate radiative power (figure 54).

Figure 54. Thermal anomalies at Sheveluch continued at elevated levels during November 2019-April 2020, as seen on this MIROVA Log Radiative Power graph for July 2019-April 2020. Courtesy of MIROVA.

High sulfur dioxide levels were occasionally recorded just above or in the close vicinity of Sheveluch by the TROPOspheric Monitoring Instrument (TROPOMI) aboard the Copernicus Sentinel-5 Precursor satellite, but very little drift was observed.

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: 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/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); 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/).

The ongoing eruption at Dukono is characterized by frequent explosions that send ash plumes to about 1.5-3 km altitude (0.3-1.8 km above the summit), although a few have risen higher. This type of typical activity (figure 13) continued through at least March 2020. The ash plume data below (table 21) were primarily provided by the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG) and the Darwin Volcanic Ash Advisory Centre (VAAC). During the reporting period of October 2019-March 2020, the Alert Level remained at 2 (on a scale of 1-4) and the public was warned to remain outside of the 2-km exclusion zone.

Table 21. Monthly summary of reported ash plumes from Dukono for October 2019-March 2020. The direction of drift for the ash plume through each month was highly variable; notable plume drift each month was only indicated in the table if at least two weekly reports were consistent. Data courtesy of the Darwin VAAC and PVMBG.

Figure 13.Satellite image of Dukono from Sentinel-2 on 12 November 2019, showing an ash plume drifting E. Image uses natural color rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

During the reporting period, high levels of sulfur dioxide were only recorded above or near the volcano during 30-31 October and 4 November 2019. High levels were recorded by the Ozone Mapping and Profiler Suite (OMPS) instrument aboard the Suomi National Polar-orbiting Partnership (NPP) satellite on 30 October 2019, in a plume drifting E. The next day high levels were also recorded by the TROPOspheric Monitoring Instrument (TROPOMI) aboard the Copernicus Sentinel-5 Precursor satellite on 31 October (figure 14) and 4 November 2019, in plumes drifting SE and NE, respectively.

Figure 14. Sulfur dioxide emission on 31 October 2019 drifting E, probably from Dukono, as recorded by the TROPOMI instrument aboard the Sentinel-5P satellite. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. Reports from this remote volcano in northernmost Halmahera are rare, but Dukono has been one of Indonesia's most active volcanoes. More-or-less continuous explosive eruptions, sometimes accompanied by lava flows, occurred from 1933 until at least the mid-1990s, when routine observations were curtailed. During a major eruption in 1550, a lava flow filled in the strait between Halmahera and the north-flank cone of Gunung Mamuya. This complex volcano presents a broad, low profile with multiple summit peaks and overlapping craters. Malupang Wariang, 1 km SW of the summit crater complex, contains a 700 x 570 m crater that has also been active during historical time.

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

Mount Etna is a stratovolcano located on the island of Sicily, Italy, with historical eruptions that date back 3,500 years. The most recent eruptive period began in September 2013 and has continued through March 2020. Activity is characterized by Strombolian explosions, lava flows, and ash plumes that commonly occur from the summit area, including the Northeast Crater (NEC), the Voragine-Bocca Nuova (or Central) complex (VOR-BN), the Southeast Crater (SEC, formed in 1978), and the New Southeast Crater (NSEC, formed in 2011). The newest crater, referred to as the "cono della sella" (saddle cone), emerged during early 2017 in the area between SEC and NSEC. This reporting period covers information from October 2019 through March 2020 and includes frequent explosions and ash plumes. The primary source of information comes from the Osservatorio Etneo (OE), part of the Catania Branch of Italy's Istituo Nazionale di Geofisica e Vulcanologica (INGV).

Summary of activity during October 2019-March 2020. Strombolian activity and gas-and-steam and ash emissions were frequently observed at Etna throughout the entire reporting period, according to INGV and Toulouse VAAC notices. Activity was largely located within the main cone (Voragine-Bocca Nuova complex), the Northeast Crater (NEC), and the New Southeast Crater (NSEC). On 1, 17, and 19 October, ash plumes rose to a maximum altitude of 5 km. Due to constant Strombolian explosions, ground observations showed that a scoria cone located on the floor of the VOR Crater had begun to grow in late November and again in late January 2020. A lava flow was first detected on 6 December at the base of the scoria cone in the VOR Crater, which traveled toward the adjacent BN Crater. Additional lava flows were observed intermittently throughout the reporting period in the same crater. On 13 March, another small scoria cone had formed in the main VOR-BN complex due to Strombolian explosions.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows multiple episodes of thermal activity varying in power from 22 June 2019 to March 2020 (figure 286). The power and frequency of these thermal anomalies significantly decreased between August to mid-September. The pulse of activity in mid-September reflected a lava flow from the VOR Crater (BGVN 44:10). By late October through November, thermal anomalies were relatively weaker and less frequent. The next pulse in thermal activity reflected in the MIROVA graph occurred in early December, followed by another shortly after in early January, both of which were due to new lava flows from the VOR Crater. After 9 January the thermal anomalies remained frequent and strong; active lava flows continued through March accompanied by Strombolian explosions, gas-and-steam, SO2, and ash emissions. The most recent distinct pulse in thermal activity was seen in mid-March; on 13 March, another lava flow formed, accompanied by an increase in seismicity. This lava flow, like the previous ones, also originated in the VOR Crater and traveled W toward the BN Crater.

Figure 286. Multiple episodes of varying activity at Etna from 22 June 2019 through March 2020 were reflected in the MIROVA thermal energy data (Log Radiative Power). Courtesy of MIROVA.

Activity during October-December 2019. During October 2019, VONA (Volcano Observatory Notice for Aviation) notices issued by INGV reported ash plumes rose to a maximum altitude of 5 km on 1, 17, and 19 October. Strombolian explosions occurred frequently. Explosions were detected primarily in the VOR-BN Craters, ejecting coarse pyroclastic material that fell back into the crater area and occasionally rising above the crater rim. Ash emissions rose from the VOR-BN and NEC while intense gas-and-steam emissions were observed in the NSEC (figure 287). Between 10-12 and 14-20 October fine ashfall was observed in Pedara, Mascalucia, Nicolosi, San Giovanni La Punta, and Catania. In addition to these ash emissions, the explosive Strombolian activity contributed to significant SO2 plumes that drifted in different directions (figure 288).

Figure 287. Webcam images of ash emissions from the NE Crater at Etna from the a) CUAD (Catania) webcam on 10 October 2019; b) Milo webcam on 11 October 2019; c) Milo webcam on 12 October 2019; d) M.te Cagliato webcam on 13 October 2019. Courtesy of INGV (Report 42/2019, ETNA, Bollettino Settimanale, 07/10/2019 - 13/10/2019, data emissione 15/10/2019).

Figure 288. Strombolian activity at Etna contributed to significant SO2 plumes that drifted in multiple directions during the intermittent explosions in October 2019. Top left: 1 October 2019. Top right: 2 October 2019. Middle left: 15 October 2019. Middle right: 18 October 2019. Bottom left: 13 November 2019. Bottom right: 1 December 2019. Captured by the TROPOMI instrument on the Sentinel 5P satellite, courtesy of NASA Global Sulfur Dioxide Monitoring Page.

The INGV weekly bulletin covering activity between 25 October and 1 November 2019 reported that Strombolian explosions occurred at intervals of 5-10 minutes from within the VOR-BN and NEC, ejecting incandescent material above the crater rim, accompanied by modest ash emissions. In addition, gas-and-steam emissions were observed from all the summit craters. Field observations showed the cone in the crater floor of VOR that began to grow in mid-September 2019 had continued to grow throughout the month. During the week of 4-10 November, Strombolian activity within the Bocca Nuova Crater was accompanied by gas-and-steam emissions. The explosions in the VOR Crater occasionally ejected incandescent ejecta above the crater rim (figures 289 and 290). For the remainder of the month Strombolian explosions continued in the VOR-BN and NEC, producing sporadic ash emissions. Isolated and discontinuous explosions in the New Southeast Crater (NSEC) also produced fine ash, though gas-and-steam emissions still dominated the activity at this crater. Additionally, the explosions from these summit craters were frequently accompanied by strong SO2 emissions that drifted in different directions as discrete plumes.

Figure 289. Photo of Strombolian activity and crater incandescence in the Voragine Crater at Etna on 15 November 2019. Photo by B. Behncke, taken by Tremestieri Etneo. Courtesy of INGV (Report 47/2019, ETNA, Bollettino Settimanale, 11/11/2019 - 17/11/2019, data emissione 19/11/2019).

Figure 290. Webcam images of summit crater activity during 26-29 November and 1 December 2019 at Etna. a) image recorded by the high-resolution camera on Montagnola (EMOV); b) and c) webcam images taken from Tremestieri Etneo on the southern slope of Etna showing summit incandescence; d) image recorded by the thermal camera on Montagnola (EMOT) showing summit incandescence at the NSEC. Courtesy of INGV (Report 49/2019, ETNA, Bollettino Settimanale, 25/11/2019 - 01/12/2019, data emissione 03/12/2019).

Frequent Strombolian explosions continued through December 2019 within the VOR-BN, NEC, and NSEC Craters with sporadic ash emissions observed in the VOR-BN and NEC. On 6 December, Strombolian explosions increased in the NSEC; webcam images showed incandescent pyroclastic material ejected above the crater rim. On the morning of 6 December a lava flow was observed from the base of the scoria cone in the VOR Crater that traveled toward the adjacent Bocca Nuova Crater. INGV reported that a new vent opened on the side of the saddle cone (NSEC) on 11 December and produced explosions until 14 December.

Activity during January-March 2020. On 9 January 2020 an aerial flight organized by RAI Linea Bianca and the state police showed the VOR Crater continuing to produce lava that was flowing over the crater rim into the BN Crater with some explosive activity in the scoria cone. Explosive Strombolian activity produced strong and distinct SO2 plumes (figure 291) and ash emissions through March, according to the weekly INGV reports, VONA notices, and satellite imagery. Several ash emissions during 21-22 January rose from the vent that opened on 11 December. According to INGV’s weekly bulletin for 21-26 January, the scoria cone in the VOR crater produced Strombolian explosions that increased in frequency and contributed to rapid cone growth, particularly the N part of the cone. Lava traveled down the S flank of the cone and into the adjacent Bocca Nuova Crater, filling the E crater (BN-2) (figure 292). The NEC had discontinuous Strombolian activity and periodic, diffuse ash emissions.

Figure 291. Distinct SO2 plumes drifting in multiple directions from Etna were visible in satellite imagery as Strombolian activity continued through March 2020. Top left: 21 January 2020. Top right: 2 February 2020. Bottom left: 10 March 2020. Bottom right: 19 March 2020. Captured by the TROPOMI instrument on the Sentinel 5P satellite, courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Figure 292. a) A map of the lava field at Etna showing cooled flows (yellow) and active flows (red). The base of the scoria cone is outlined in black while the crater rim is outlined in red. b) Thermal image of the Bocca Nuova and Voragine Craters. The bright orange is the warmest temperature measure in the flow. Courtesy of INGV, photos by Laboratorio di Cartografia FlyeEye Team (Report 10/2020, ETNA, Bollettino Settimanale, 24/02/2020 - 01/03/2020, data emissione 03/03/2020).

Strombolian explosions continued into February 2020, accompanied by ash emissions and lava flows from the previous months (figure 293). During 17-23 February, INGV reported that some subsidence was observed in the central portion of the Bocca Nuova Crater. During 24 February to 1 March, the Strombolian explosions ejected lava from the VOR Crater up to 150-200 m above the vent as bombs fell on the W edge of the VOR crater rim (figure 294). Lava flows continued to move into the W part of the Bocca Nuova Crater.

Figure 293. Webcam images of A) Strombolian activity and B) effusive activity fed by the scoria cone grown inside the VOR Crater at Etna taken on 1 February 2020. C) Thermal image of the lava field produced by the VOR Crater taken by L. Lodato on 3 February (bottom left). Image of BN-1 taken by F. Ciancitto on 3 February in the summit area (bottom right). Courtesy of INGV; Report 06/2020, ETNA, Bollettino Settimanale, 27/01/2020 - 02/02/2020, data emissione 04/02/2020 (top) and Report 07/2020, ETNA, Bollettino Settimanale, 03/02/2020 - 09/02/2020, data emissione 11/02/2020 (bottom).

Figure 294. Photos of the VOR intra-crater scoria cone at Etna: a) Strombolian activity resumed on 25 February 2020 from the SW edge of BN taken by B. Behncke; b) weak Strombolian activity from the vent at the base N of the cone on 29 February 2020 from the W edge of VOR taken by V. Greco; c) old vent present at the base N of the cone, taken on 17 February 2020 from the E edge of VOR taken by B. Behncke; d) view of the flank of the cone, taken on 24 February 2020 from the W edge of VOR taken by F. Ciancitto. Courtesy of INGV (Report 10/2020, ETNA, Bollettino Settimanale, 24/02/2020 - 01/03/2020, data emissione 03/03/2020).

During 9-15 March 2020 Strombolian activity was detected in the VOR Crater while discontinuous ash emissions rose from the NEC and NSEC. Bombs were found in the N saddle between the VOR and NSEC craters. On 9 March, a small scoria cone that had formed in the Bocca Nuova Crater and was ejecting bombs and lava tens of meters above the S crater rim. The lava flow from the VOR Crater was no longer advancing. A third scoria cone had formed on 13 March NE in the main VOR-BN complex due to the Strombolian explosions on 29 February. Another lava flow formed on 13 March, accompanied by an increase in seismicity. The weekly report for 16-22 March reported Strombolian activity detected in the VOR Crater and gas-and-steam and rare ash emissions observed in the NEC and NSEC (figure 295). Explosions in the Bocca Nuova Crater ejected spatter and bombs 100 m high.

Figure 295. Map of the summit crater area of Etna showing the active vents and lava flows during 16-22 March 2020. Black hatch marks indicate the crater rims: BN = Bocca Nuova, with NW BN-1 and SE BN-2; VOR = Voragine; NEC = North East Crater; SEC = South East Crater; NSEC = New South East Crater. Red circles indicate areas with ash emissions and/or Strombolian activity, yellow circles indicate steam and/or gas emissions only. The base is modified from a 2014 DEM created by Laboratorio di Aerogeofisica-Sezione Roma 2. Courtesy of INGV (Report 13/2020, ETNA, Bollettino Settimanale, 16/03/2020 - 22/03/2020, data emissione 24/03/2020).

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

Information Contacts: Sezione di Catania - Osservatorio Etneo, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/it/); Toulouse Volcanic Ash Advisory Center (VAAC), Météo-France, 42 Avenue Gaspard Coriolis, F-31057 Toulouse cedex, France (URL: http://www.meteo.fr/aeroweb/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/); 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); Boris Behncke, Sonia Calvari, and Marco Neri, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: https://twitter.com/etnaboris, Image at https://twitter.com/etnaboris/status/1183640328760414209/photo/1).

Merapi is a highly active stratovolcano located in Indonesia, just north of the city of Yogyakarta. The current eruption episode began in May 2018 and was characterized by phreatic explosions, ash plumes, block avalanches, and a newly active lava dome at the summit. This reporting period updates information from October 2019-March 2020 that includes explosions, pyroclastic flows, ash plumes, and ashfall. The primary reporting source of activity comes from Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG, the Center for Research and Development of Geological Disaster Technology, a branch of PVMBG) and Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM).

Some ongoing lava dome growth continued in October 2019 in the NE-SW direction measuring 100 m in length, 30 m in width, and 20 m in depth. Gas-and-steam emissions were frequent, reaching a maximum height of 700 m above the crater on 31 October. An explosion at 1631 on 14 October removed the NE-SW trending section of the lava dome and produced an ash plume that rose 3 km above the crater and extended SW for about 2 km (figures 90 and 91). The plume resulted in ashfall as far as 25 km to the SW. According to a Darwin VAAC notice, a thermal hotspot was detected in HIMAWARI-8 satellite imagery. A pyroclastic flow associated with the eruption traveled down the SW flank in the Gendol drainage. During 14-20 October lava flows from the crater generated block-and-ash flows that traveled 1 km SW, according to BPPTKG.

Figure 90. An ash plume rising 3 km above Merapi on 14 October 2019.

Figure 91. Webcam image of an ash plume rising above Merapi at 1733 on 14 October 2019. Courtesy of BPPTKG via Jaime S. Sincioco.

At 0621 on 9 November 2019, an eruption produced an ash plume that rose 1.5 km above the crater and drifted W. Ashfall was observed in the W region as far as 15 km from the summit in Wonolelo and Sawangan in Magelang Regency, as well as Tlogolele and Selo in Boyolali Regency. An associated pyroclastic flow traveled 2 km down the Gendol drainage on the SE flank. On 12 November aerial drone photographs were used to measure the volume of the lava dome, which was 407,000 m3. On 17 November, an eruption produced an ash plume that rose 1 km above the crater, resulting in ashfall as far as 15 km W from the summit in the Dukun District, Magelang Regency (figure 92). A pyroclastic flow accompanying the eruption traveled 1 km down the SE flank in the Gendol drainage. By 30 November low-frequency earthquakes and CO2 gas emissions had increased.

Figure 92. An ash plume rising 1 km above Merapi on 17 November 2019. Courtesy of BPPTKG.

Volcanism was relatively low from 18 November 2019 through 12 February 2020, characterized primarily by gas-and-steam emissions and intermittent volcanic earthquakes. On 4 January a pyroclastic flow was recorded by the seismic network at 2036, but it wasn’t observed due to weather conditions. On 13 February an explosion was detected at 0516, which ejected incandescent material within a 1-km radius from the summit (figure 93). Ash plumes rose 2 km above the crater and drifted NW, resulting in ashfall within 10 km, primarily S of the summit; lightning was also seen in the plume. Ash was observed in Hargobinangun, Glagaharjo, and Kepuharjo. On 19 February aerial drone photographs were used to measure the change in the lava dome after the eruption; the volume of the lava had decreased, measuring 291,000 m3.

Figure 93. Webcam image of an ash plume rising from Merapi at 0516 on 13 February 2020. Courtesy of MAGMA Indonesia and PVMBG.

An explosion on 3 March at 0522 produced an ash plume that rose 6 km above the crater (figure 94), resulting in ashfall within 10 km of the summit, primarily to the NE in the Musuk and Cepogo Boyolali sub-districts and Mriyan Village, Boyolali (3 km from the summit). A pyroclastic flow accompanied this eruption, traveling down the SSE flank less than 2 km. Explosions continued to be detected on 25 and 27-28 March, resulting in ash plumes. The eruption on 27 March at 0530 produced an ash plume that rose 5 km above the crater, causing ashfall as far as 20 km to the W in the Mungkid subdistrict, Magelang Regency, and Banyubiru Village, Dukun District, Magelang Regency. An associated pyroclastic flow descended the SSE flank, traveling as far as 2 km. The ash plume from the 28 March eruption rose 2 km above the crater, causing ashfall within 5 km from the summit in the Krinjing subdistrict primarily to the W (figure 94).

Figure 94. Images of ash plumes rising from Merapi during 3 March (left) and 28 March 2020 (right). Images courtesy of BPPTKG (left) and PVMBG (right).

Geologic Background. Merapi, one of Indonesia's most active volcanoes, lies in one of the world's most densely populated areas and dominates the landscape immediately north of the major city of Yogyakarta. It is the youngest and southernmost of a volcanic chain extending NNW to Ungaran volcano. Growth of Old Merapi during the Pleistocene ended with major edifice collapse perhaps about 2000 years ago, leaving a large arcuate scarp cutting the eroded older Batulawang volcano. Subsequently growth of the steep-sided Young Merapi edifice, its upper part unvegetated due to frequent eruptive activity, began SW of the earlier collapse scarp. Pyroclastic flows and lahars accompanying growth and collapse of the steep-sided active summit lava dome have devastated cultivated lands on the western-to-southern flanks and caused many fatalities during historical time.

Information Contacts: Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG), Center for Research and Development of Geological Disaster Technology (URL: http://merapi.bgl.esdm.go.id/, Twitter: @BPPTKG); Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Badan Nasional Penanggulangan Bencana (BNPB), National Disaster Management Agency, Graha BNPB - Jl. Scout Kav.38, East Jakarta 13120, Indonesia (URL: http://www.bnpb.go.id/, Twitter: https://twitter.com/BNPB_Indonesia); 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/); Jamie S. Sincioco, Phillipines (Twitter: @jaimessincioco, Image at https://twitter.com/jaimessincioco/status/1227966075519635456/photo/1).

In November 1998-May 1999, Arenal's explosive activity continued, although at a reduced pace compared with past years. Crater C continued to emit gases, lava flows, and sporadic Strombolian eruptions. Fumarolic activity persisted at Crater D.

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, E. Duarte, R. Saenz, E. Malavassi, M. Martinez, and R. Van der Laat, T. Marino, and E. Hernandez, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Wendy Perez Fernandez, Seccion de Seismologia, Vulcanologia y Exploracion Geofisica, Escuela Centroamericana de Geologia, Universidad de Costa Rica, POB 35-2060, San José, Costa Rica.

During the week of 9-15 March, white ash plumes rose 10-100 m above the crater. Booming noises were heard six times, and volcanic earthquakes increased drastically compared to the previous week (table 1). It was later determined that an ash eruption had begun on 15 March, sending bluish-white plumes 10-50 m high. On 17 March, two relatively recent craters merged when a connecting ridge collapsed as a result of earthquakes and small eruptions.

Table 1. Weekly seismicity recorded at Batur, March-July 1999. Types of events include volcanic A-type, volcanic B-type, tectonic, explosion earthquakes, and ash emissions (small explosion earthquakes). No data were available for the period 30 March-19 April. Maximum explosion amplitudes were reported as 2-26 mm during May; more typical, smaller amplitudes were 1.5-24 mm during May-July. Courtesy of VSI.

Volcanic activity was dominated by emission events (small explosions) during the week of 23-29 March; the eruption plume was white-blue in color, rising 10-100 m above crater rim. Booming noises were heard three times on 22 March, but no glow was observed. Reports are not available for most of April, but during late April through the middle of May the seismic record was dominated by explosion events. Ash plumes, described as "white" or "white-bluish" were observed rising 50-100 m above the crater. Neither explosion sounds nor glow was observed.

Six explosions on 17 May ejected materials that fell around the crater. Another explosion on 25 May was accompanied by incandescent ejections that fell around the crater. Explosion sounds were heard on 17 occasions during the week of 25-31 May. "White ash emissions" rose only to 25 m during 1-14 June, but varied between 10 and 100 m the rest of the month. Similar activity continued through mid-July, and a variable rumbling noise was heard the week of 13-19 July.

Geologic Background. The historically active Batur is located at the center of two concentric calderas NW of Agung volcano. The outer 10 x 13.5 km wide caldera was formed during eruption of the Bali (or Ubud) Ignimbrite about 29,300 years ago and now contains a caldera lake on its SE side, opposite the satellitic Gunung Abang cone, the topographic high of the complex. The inner 6.4 x 9.4 km wide caldera was formed about 20,150 years ago during eruption of the Gunungkawi Ignimbrite. The SE wall of the inner caldera lies beneath Lake Batur; Batur cone has been constructed within the inner caldera to a height above the outer caldera rim. The Batur stratovolcano has produced vents over much of the inner caldera, but a NE-SW fissure system has localized the Batur I, II, and III craters along the summit ridge. Historical eruptions have been characterized by mild-to-moderate explosive activity sometimes accompanied by lava emission. Basaltic lava flows from both summit and flank vents have reached the caldera floor and the shores of Lake Batur in historical time.

Information Contacts: Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

Details about another large explosion on 17 July that sent a plume to over 10 km altitude will be reported in a future issue. Prior to that, during the month of June, degassing and explosions continued. The highest level of activity in 13 days was recorded on 1 June, but by 4 June activity had dropped to the lowest level in a month and it remained low through the end of June. Accordingly, during June explosions and degassing events became less frequent, shorter in duration, and of lower intensity. On 5 July one of two explosions sent an ash column to ~2,500 m and produced ashfall 10-20 km W of the summit.

A 3 June aerial inspection verified that no new summit crater has been formed, but did record a recent increase in the size of the crater formed during the 10 February and 10 May explosions. The crater diameter is now 180-200 m, with the deepest sector measuring 70 m.

Early in June authorities continued the evacuation of La Yerbabuena, in the state of Colima, and Juan Barragán, Los Machos, El Borbollón, and Durazno, in the state of Jalisco. The Observatory recommended controlled access to areas within 8.5 km of the summit and alert to persons within 11.5 km. People in other high-risk areas were told to be prepared to evacuate. By 10 June, however, reduced activity enabled authorities to relax restrictions. They trimmed the evacuated zone to within 6.5 km of the summit and maintained immediate response capacity to 8.5 km (including the villages of La Yerbabuena, Juan Barragán, El Agostadero, Los Machos, Borbollón, and El Durazno). Radio alert was maintained to a 11.5 km radius of the summit (encompassing the settlements of Causentla, Cofradía de Tonila, Atenguillo, Saucillo, El Fresnal, El Embudo). Authorities allowed residents to return to evacuated communities in both Colima and Jalisco. On 2 July, due to recent rain and the potential for lahars, authorities recommended avoiding the bed of the Cordobán river.

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

Information Contacts: Colima Volcano Observatory, University of Colima, Ave. 25 de Julio 965, Colima 28045 México (URL: https://portal.ucol.mx/cueiv/).

During the winter of 1998-99, Southeast Crater underwent a series of short eruptions. On 4 February 1999 a similar outbreak led to fracturing of the SE cone with subsequent lava flows to the NNE and SSE (observation by Giuseppe Scarpinati). The SE flow developed a lava field between 2,900 and 2,800 m elevation on the rim of the Valle del Bove (BGVN 24:05), with numerous branches that nearly reached its bottom at about 2,000 m elevation. This mild effusive activity was characterized by very small and slow degassed lava flows coming from ephemeral vents through the superficially congealed crust of the field.

Temperature measurements were carried out on lavas from several of these effusive vents during the first days of April. Special thanks are due to Antonio and Orazio Nicoloso and the Etnean Guides for assistance in the field. Consumable "Temtip" Pt / Pt-10%Rh thermocouples were used following a procedure described in Archambault and Tanguy (1976). The maximum lava temperature recorded at depths of 40-60 cm within the fluid lava was 1,085 ± 5°C (figure 78), slightly higher than that measured during the last two major flank eruptions of 1983 and 1991-93 (1,070-1,080°C). As already shown in Archambault and Tanguy (1976), such small lava flows showed a strong thermal gradients, so that measurements made at 5-15 cm depth give results 10-15°C lower than the true internal temperature (figure 79). This means that devices reaching only the superficial parts of the flows are unsuitable for such temperature measurements.

Figure 78. Histogram showing the number of Etna lava measurements at each temperature; the 16 measurements were taken at depth on fluid lava.

Figure 79. Lava flow temperatures measured at Etna during early April plotted versus depth. Different symbols (circles, squares, and triangles) represent different sets of measurements.

Although relatively low for a trachybasalt (or basaltic hawaiite) lava, the temperature of 1,085°C is consistent with their high crystal content of ~40% phenocrysts. As usual in recent Etna lavas, these phenocrysts consist of plagioclase, clinopyroxene, olivine and titanomagnetite lying in a glassy alkaline groundmass.

Reference. Archambault, C., and Tanguy, J.C., 1976, Comparative temperature measurements on Mount Etna lavas: Journal of Volcanology and Geothermal Research, 1, 113-125.

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

Information Contacts: Roberto Clocchiatti, Lab. Pierre Sue, CEA Saclay, France; Charles Rivière, CGE, France; Santo La Delfa, Giuseppe Patanè, and Jean-Claude Tanguy, University of Catania, Italy.

The "yellow alert" status was uninterrupted as Guagua Pichincha expelled steam and ash throughout June. Explosions occurred on 31 May and on 1, 5, 6, 7, 8, 10, 11, 12, 13, 17, 24, 28, and 30 June. Explosions were more frequent in early-to mid-June, but the 28 June explosion was the largest in three months. A large explosion on 11 June sent a steam-and-ash column to ~5 km that lasted for about 5 minutes before dispersing to the south. Explosions were usually accompanied by long periods of tremor. A sulfur smell persisted throughout June and loud noises were also common. Steam frequently escaped to heights between 15 and 1,200 m from vents known as Alineadas, 1981 Crater, and Locomotora, along with those in the NW area of the summit. Alineadas discharged the highest plumes. Phreatic explosions as well as volcano-tectonic (VT), long-period (LP), and hybrid earthquakes occurred almost daily throughout June at levels similar to the last few months (figures 14 and 15). In addition to activity on the volcano, the seismic swarm N of Quito has altered. This may reflect changes in the regional stress field.

Figure 14. Monthly totals at Guagua Pichincha for phreatic explosions from August 1998 through June 1999. Courtesy of Instituto Geofisico.

Figure 15. Monthly totals at Guagua Pichincha for seismic events (LP, VT, and hybrid) from August 1998 through June 1999. Courtesy of Instituto Geofisico.

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

Information Contacts: Instituto Geofísico, Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador.

In late 1998 and early 1999, seismographic station IRZ2 continued to register a few microseisms and occasional earthquakes (figure 13). During February and March, the color of the lake was clear yellow; in May, green. As is typical, the lake contained zones of constant bubbling. Weak fumarolic activity continued on the NE flank.

Figure 13. Monthly seismicity at Irazú, January 1998-April 1999. Courtesy of OVSICORI-UNA.

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, E. Duarte, R. Saenz, E. Malavassi, M. Martinez, and R. Van der Laat, T. Marino, and E. Hernandez, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Wendy Perez Fernandez, Seccion de Seismologia, Vulcanologia y Exploracion Geofisica, Escuela Centroamericana de Geologia, Universidad de Costa Rica, POB 35-2060, San Jose, Costa Rica.

Crater 2 exhibited mild, continuous volcanic activity during May and very low activity during June. The May activity primarily consisted of the escape of moderately thick gray to brown ash clouds. Weak rumbling and roaring noises occasionally accompanied the emissions and fairly significant ash columns were forcefully ejected to 2 km height on 4, 9, 11, and 30 May. The ash clouds drifted NW, resulting in downwind ashfall. The June activity was summarized as the escape of mostly small to moderate amounts of vapor. Occasional ash-bearing (gray-brown) ash clouds were seen. In both months, there was no visible night glow, Crater 3 remained quiet and only occasionally released thin white vapor. The seismograph remained inoperative.

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

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

Eruptions from the summit crater continued at a steady state. The activity level was low, but new observations during the rainy season conflicted with previous assumptions of color and age relationships among the natrocarbonatite flows. On 3 April two fresh flows, each having volumes of ~30 m³, had apparently issued from two separate hornitos in the summit crater. The pahoehoe flows were extruded over older lavas saturated with water from recent rainfall and earlier that day had generated a prominent steam plume many hundreds of meters high and visible from Engare Sero, 10 km N of the volcano. Large areas of the highest part of the N crater floor remained unusually hot (>150°C) throughout the day and sounds of sloshing magma suggest that substantial collapse of this region might be imminent. Small amounts of lapilli surrounding a hornito indicated a recent small scale (~10 m) lava fountain.

Cross-sections cut through the strongly porphyritic ~0.5 m flows (gregoryite and neyereite) revealed compact non-vesicular interiors and a near-glassy crystal-supported matrix. During a rain squall individual rain drops were observed whitening the flow surface (perhaps 50-150°C) in seconds. The margins of the recent flows also had turned white during cooling (figure 60). Clearly the timing between eruption and direct precipitation can change the age/color relationship previously established for Lengai, and is unreliable during the rainy season, an important consideration for photo-reconnaissance interpretations.

Figure 60. View of lava flows at Ol Doinyo Lengai on 3 April 1999 showing alteration of the flow margins from dark gray to white due to hydration during cooling. Courtesy of Matthew Genge.

Intense fumarole activity from at least five of the larger hornitos, and from crater-rim fissures and cooling cracks on the lava flows, continued throughout 3-4 April (figure 61). Hornitos produced continuous steam and/or H₂S gas, although one emanated blue and then black smoke. Another large hornito produced substantial heat haze but no visible gas. Fumaroles from cooling cracks in the fresh lava were associated with delicate salt deposits (figure 62).

Figure 61. Fumarolic activity from hornitos in the crater of Ol Doinyo Lengai on 3 April 1999. Courtesy of Matthew Genge.

Figure 62. Salt deposits lining cooling cracks on recent lava flows at Ol Doinyo Lengai, 3 April 1999. Courtesy of Matthew Genge.

At the time of the visit lava that flowed over the crater rim in the N and E (see BGVN 24:02) extended many hundreds of meters down the flanks of the volcano. The state of weathering of these flows suggested they were 1-2 weeks old, consistent with reports that glowing lava could be observed on the flanks of the volcano from Engare Sero at night during this period. Lava was also close to the NW rim of the crater. During this rainy season (as in the last one) vigorous flowing streams of water delivered both volcanic solutes (soda-rich) and volcanic detritus directly to Lake Natron, which was wet from shore to shore.

Geologic Background. The symmetrical Ol Doinyo Lengai is the only volcano known to have erupted carbonatite tephras and lavas in historical time. The prominent stratovolcano, known to the Maasai as "The Mountain of God," rises abruptly above the broad plain south of Lake Natron in the Gregory Rift Valley. The cone-building stage ended about 15,000 years ago and was followed by periodic ejection of natrocarbonatitic and nephelinite tephra during the Holocene. Historical eruptions have consisted of smaller tephra ejections and emission of numerous natrocarbonatitic lava flows on the floor of the summit crater and occasionally down the upper flanks. The depth and morphology of the northern crater have changed dramatically during the course of historical eruptions, ranging from steep crater walls about 200 m deep in the mid-20th century to shallow platforms mostly filling the crater. Long-term lava effusion in the summit crater beginning in 1983 had by the turn of the century mostly filled the northern crater; by late 1998 lava had begun overflowing the crater rim.

Information Contacts: Matthew J. Genge, Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom; Matt Balme and Adrian P. Jones, Department of Geological Sciences, University College, London, United Kingdom.

The following summarizes activity at Long Valley during the second half of 1997 and all of 1998 (Hill, 1998). A summary of activity during 1996 and the first half of 1997 can be found in BGVN 22:11 and 22:12.

Summary of activity during July-December, 1997.After nearly a year of relative seismic quiescence within the caldera, earthquake swarm activity returned in early July. The deeper focus (>10 km) , long-period (LP) earthquakes centered beneath the SW flank of Mammoth Mountain and the Devil's Postpile area, which had increased in early 1997, continued at an elevated rate through the end of the year. Altogether over 200 of these events were detected during 1997, all M <2.0 (figure 21). This exceeded the total number of deep LP events detected from their onset in 1989 through 1996. Earthquake activity in the shallow crust beneath Mammoth Mountain (depths <10 km) at the SW margin of the caldera remained low throughout 1997.

Figure 21. Earthquake epicenters in the Long Valley region during 1997. Courtesy of USGS.

The gradual increase in the inflation rate of the resurgent dome begun in late May 1997 marked the onset of an episode of unrest in the caldera that intensified at an accelerating rate through the summer and fall, culminating in a series of strong earthquake swarms from mid-November through early January 1998. The extension rate of an 8-km baseline spanning the resurgent dome peaked at over 20 cm/year during the second week of November. Following a strong earthquake swarm on 22 November that included three M >4.5 earthquakes, the deformation pattern briefly changed to one dominated by right-oblique slip along the WNW-striking "S-moat fault zone." In early December and through the rest of 1997, the deformation resumed the earlier pattern of dome inflation with a relatively steady 12-15 cm/year extension rate. By the end of 1997, the 8-km baseline was 7 cm longer than in late May.

The earthquake activity associated with the swarms begun in early July developed over a broad 15-km zone spanning the S-moat and southern margin of the resurgent dome. Typically, several areas within this zone were active simultaneously. This swarm activity, which included more than 12,000 M >1.2 and 120 M >3.0 earthquakes over a 7-month period through mid-January 1998, had a cumulative seismic moment of 3.3 x 10²4 dyne-cm, equivalent to a single earthquake of M 5.4. The peak in seismicity from mid-November through early January included eight M >4.0 earthquakes. Focal mechanism data for the larger earthquakes indicated a dominantly right-lateral slip along a WNW-trending fault zone within the S-moat, although broadband seismograms admit the possibility of a dilatational component (volume increase) in the source mechanism for some of the M >4 earthquakes. The great majority of earthquakes had the broadband character of brittle, double-couple events (tectonic or volcano-tectonic earthquakes). A few shallow (<3 km) events beneath the southern half of the resurgent dome had energy concentrated in the 1-3 Hz band typical of shallow LP earthquakes. Whether the unusual appearance of the seismograms can be attributed to the earthquake source or wave propagation effects remains to be determined.

Monitoring of the gases around Mammoth Mountain showed two noteworthy changes in 1997, both more likely related to the increased deep LP earthquake activity beneath the SW flank of the mountain than to the much stronger activity within the caldera. The helium isotope ratio, ³He/4He, in samples collected in May and October from the MMF fumarole on the NE flank of Mammoth Mountain showed an increase with respect to the gradually declining values measured in late 1995 through early 1997. Continuous CO₂ monitors in tree-kill areas on opposite sides of Mammoth Mountain detected an abrupt increase in CO₂ soil-gas concentrations beginning in late September and ending in early December before significant snow accumulations.

The episode of persistent unrest during the second half of 1997 is the third most energetic activity in the caldera since the intense earthquake swarms of May 1980 (which included four M 6 earthquakes) and January 1983 (which included two M 5.3 earthquakes). The strongest seismic activity in the 1980 and 1983 episodes occurred within the first few days of the swarm sequences. The 1997 activity level accelerated gradually over a 4-month period prior to the strongest seismicity, which then spread out over nearly an additional 3 months. This activity provided the first test of the color-code notification system for ranking activity levels within the caldera. "Condition Green" (no immediate risk) remained throughout the year but the activity peaks on 22 and 30 November came close to meeting the guidelines for "Condition Yellow" (watch).

Summary of activity during 1998. Three events dominated activity during 1998. First, a decline in the acceleration of the resurgent dome uplift and the persistent earthquake swarm activity in the S-moat, which began in June-July 1997 and peaked in November-December 1998. Second and third, M 5.1 earthquakes on 8 June and 14 July 1998, centered W of the Hilton Creek fault and 2-4 km S of the caldera, together with their aftershock sequences.

In the S-moat of the caldera, the final phase of the strong earthquake swarms that dominated activity through the second half of 1997 extended into January 1998. An earthquake (M 4.8) on 31 December marked a shift in the center of the most intense activity from beneath the western part to the eastern-central section. A strong swarm burst occurred during 1-5 January in which >2,000 earthquakes were detected and located, including more than a dozen with M >3. On 6 January, a M 4.1 earthquake was the last of nine earthquakes with magnitudes of 4.0-4.9 in the S-moat since 13 November 1997.

Activity in the S-moat area declined to background levels by midsummer, interrupted by occasional M >3 earthquakes and brief swarms, particularly in February and March. The latter period included several days with 80-180 small swarm events. A M 3.2 earthquake occurred on 13 February, and swarm events on 2, 6-7, and 15-16 March included earthquakes of M >2.5. The last six months of 1998 included five M >3.0 caldera earthquakes. The two largest, on 14 July and 7 December, had magnitudes of 3.7 (the 14 July event occurred 2 hours after the M 5.1 earthquake).

Geodimeter measurements confirmed that the inflation rate of the resurgent dome gradually slowed from a peak of 30 cm/year in November 1997 to ~15 cm/year in early 1998 and by mid-May had dropped to 1-2 cm/year, a rate that persisted through the end of 1998.

Long-period earthquake activity 10-25 km beneath Devil's Postpile and the SW flank of Mammoth Mountain continued through 1998 (figure 22). Some 140 of the LP events were detected in 1998, just over half the 1997 number (250 events). That rate is still higher than any time since the mid-1989 onset of LP activity; during 1989-96 a total of only 165 LP events were detected. In 1998, many of the LP earthquakes were preceded by several tens of seconds of a tremor-like signal with a dominant frequency around 1 Hz. This was a change from the character of the LP activity in earlier years, when the tremor-like signals were generally of shorter duration and the dominant frequency was 2-3 Hz.

Figure 22. Earthquake epicenters in the Long Valley region during 1998. Courtesy of USGS.

An event of M 1.8 at a depth of 12 km on 11 November was the largest LP event recorded since the initial activity in 1989; it was followed by a week of ~25 smaller events. Occasional LP earthquakes continued to occur through 1998 at depths between 15 and 30 km centered beneath an area roughly 7 km W of Mono Craters. One of the largest in this area, M ~2.4, occurred at a depth of 24 km on 26 September.

Over the summer months, field studies around the flanks of Mammoth Mountain suggested that the CO₂ flux rate had been slowly decreasing over the past 3 years. Airborne measurements in September and November detected a CO₂ plume downwind, consistent with the multiple sources around the flanks of the mountain.

The first of the two M 5.1 earthquakes occurred on 8 June just 1.5 km S of the caldera at a depth of 6.7 km. The second occurred on 14 July and was centered 3 km to the SSE at 6.2 km depth. Both earthquakes were located within the footwall of the E-dipping Hilton Creek fault; neither appeared to have involved slip on the fault itself. Both earthquakes were followed by rich aftershock sequences that tailed off though the end of the year and, together, included nine earthquakes with magnitudes between 4.0 and 4.5. The aftershock epicenters defined a nearly orthogonal pattern in map view. Those of the 8 June event were confined to a WNW lineation through the mainshock epicenter, whereas those of the 14 July event were confined to a more diffuse SSW lineation through the mainshock epicenter. Both lineations cut across the Hilton Creek fault and intersect just E of the 8 June epicenter.

Earthquake activity elsewhere in the region showed no significant variation from background activity over the past several years.

References. Hill, David P., 1997, Long Valley Caldera monitoring report (July-December 1997): U.S. Geological Survey, Volcano Hazards Program.

Hill, David P., 1998, Long Valley Caldera monitoring report (October- December 1998): U.S. Geological Survey, Volcano Hazards Program.

Geologic Background. The large 17 x 32 km Long Valley caldera east of the central Sierra Nevada Range formed as a result of the voluminous Bishop Tuff eruption about 760,000 years ago. Resurgent doming in the central part of the caldera occurred shortly afterwards, followed by rhyolitic eruptions from the caldera moat and the eruption of rhyodacite from outer ring fracture vents, ending about 50,000 years ago. During early resurgent doming the caldera was filled with a large lake that left strandlines on the caldera walls and the resurgent dome island; the lake eventually drained through the Owens River Gorge. The caldera remains thermally active, with many hot springs and fumaroles, and has had significant deformation, seismicity, and other unrest in recent years. The late-Pleistocene to Holocene Inyo Craters cut the NW topographic rim of the caldera, and along with Mammoth Mountain on the SW topographic rim, are west of the structural caldera and are chemically and tectonically distinct from the Long Valley magmatic system.

Information Contacts: David Hill, U.S. Geological Survey, MS 977, 345 Middlefield Rd., Menlo Park, CA 94025 USA (URL: https://volcanoes.usgs.gov/observatories/calvo/).

Mild to weak eruptive activity from Main Crater continued in May and June. During May there were small to moderate pale gray ash emissions. Ash clouds then rose 500-600 m above the summit before being blown NW with resulting light ashfall on 1, 22, 26-27, and 31 May. A slight wind change on 15 and 25 May caused the ash clouds to drift SW and the ashfall to move downwind. There were no noises or night glow observed in May. A weak, steady glow was visible between 2 and 5 May.

Although Southern Crater was generally quiet with only thin white emissions, a small explosion took place on 10 June. A weak discharge of lava fragments accompanied loud noises that lasted only a short time. Later, on 26 June, Main Crater vented gray-brown ash clouds resulting in ash fall over some parts of the island.

During May-June, seismicity remained low. Evidence for deformation was absent at the water-tube tiltmeter located at Tabele Observatory, 4 km from the summit on the SW flank.

Inhabitants of the 10-km-wide island of Manam reside on one of Papua New Guinea's most active volcanoes. Four large radial valleys extend from the unvegetated summit of the conical stratovolcano to its lower flanks. These "avalanche valleys," regularly spaced 90 degrees apart, channel lava flows and pyroclastic avalanches that have sometimes reached the coast. Five satellitic centers are located near the island's shoreline. Two summit craters are present and both are active, although most historical eruptions have originated from the southern crater and drained into the SE avalanche valley. Frequent historical eruptions have been recorded since 1616.

More recent activity began in December 1956 and lasted through January 1966. Lava flows and a nuee ardente from the South Crater occurred in June and December 1974, and intermittent moderate explosive activity has continued into 1993, with peaks of activity in 1982 and 1984.

Geologic Background. The 10-km-wide island of Manam, lying 13 km off the northern coast of mainland Papua New Guinea, is one of the country's most active volcanoes. Four large radial valleys extend from the unvegetated summit of the conical 1807-m-high basaltic-andesitic stratovolcano to its lower flanks. These "avalanche valleys" channel lava flows and pyroclastic avalanches that have sometimes reached the coast. Five small satellitic centers are located near the island's shoreline on the northern, southern, and western sides. Two summit craters are present; both are active, although most historical eruptions have originated from the southern crater, concentrating eruptive products during much of the past century into the SE valley. Frequent historical eruptions, typically of mild-to-moderate scale, have been recorded since 1616. Occasional larger eruptions have produced pyroclastic flows and lava flows that reached flat-lying coastal areas and entered the sea, sometimes impacting populated areas.

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

At 1658 on 22 June Mayon emitted an ash column that rose 7-10 km above the vent (figure 8). The emission was recorded by the seismic network of the Philippines Institute of Volcanology and Seismology (PHIVOLCS) as an explosion that lasted for 10 minutes. No volcanic earthquakes nor other visible signs of abnormal activity were observed before the explosion. During May, however, low-frequency volcanic earthquakes had been recorded intermittently, accompanied by faint crater glow.

Figure 8. A column of steam and ash rising from Mayon's crater and a pyroclastic flow descending its SE flank during its sudden isolated explosion on 22 June. Photograph courtesy of PHILVOCS.

The explosion represented an isolated event as activity immediately declined to typical incidents of weak steaming without measurable seismicity. Faint glow was seen the next day at the summit crater. An aerial survey noted a new explosion pit at the summit; the small diameter pit was later described as a deep hole lined with sulfur deposits (figure 9). The presence of sulfur suggested that lava had not yet ascended to the surface.

Figure 9. The crater of Mayon as it appeared after the 22 June explosion. A new, circular explosion pit developed on the crater floor; the shadow formed along the rim of this pit can be seen in this NW-looking photo shot through the breech. Courtesy of PHILVOCS.

Beginning at 0700 on 25 June there was a slight increase in seismicity and SO₂ emission. The COSPEC measured an SO₂ flux of 4,800 tons/day, compared with 4,200 tons/day the previous day. SO₂ fluxes normally average 500 tons/day. A short interval of high-frequency tremor was also recorded.

Tremor, light steaming, low-frequency volcanic earthquakes, and elevated SO₂ fluxes continued for several days. Also, deformation surveys conducted with laser-ranging EDM equipment indicated sustained inflation on the SE slope.

PHIVOLCS maintained an alert status of "Level 1," advising the public not to venture within 6 km of the summit area (figure 10). In particular, residents were advised to avoid the Bonga pyroclastic fan, an area on the SE side of the volcano that contains a deep canyon and lies directly below the crater rim notch. This fan was the site of most of the fatalities in the 1993 eruption and is considered the area most vulnerable to future pyroclastic flows.

Figure 10. Details on Mayon and vicinity taken from a volcanic hazards map (PHILVOCS, 1999). The legend describes some effects of the 1993 eruption. The solid black circle represents the 6-km-radius safety zone currently in effect. An additional 1-km-wide precautionary zone lies to the SE of the volcano below the Bonga Pyroclastic Fan. Some local cities and river drainage are also shown. Courtesy of PHILVOCS.

Reference. Philippine Institute of Volcanology and Seismology (PHIVOLCS), 1999, 1999 Mayon permanent danger & high susceptibility areas (map), URL: http://www.philonline.com/~seismo/Volcanoes/Mayon/MayonHazMaps.htm.

Geologic Background. Beautifully symmetrical Mayon, which rises above the Albay Gulf NW of Legazpi City, is the Philippines' most active volcano. The structurally simple edifice has steep upper slopes averaging 35-40 degrees that are capped by a small summit crater. Historical eruptions date back to 1616 and range from Strombolian to basaltic Plinian, with cyclical activity beginning with basaltic eruptions, followed by longer term andesitic lava flows. Eruptions occur predominately from the central conduit and have also produced lava flows that travel far down the flanks. Pyroclastic flows and mudflows have commonly swept down many of the approximately 40 ravines that radiate from the summit and have often devastated populated lowland areas. A violent eruption in 1814 killed more than 1,200 people and devastated several towns.

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

Fumarole temperatures have been rising on Momotombo since 1996. An April 1996 summit visit reported that temperatures were unchanged over the past year (BGVN 21:04), but by November of that year the temperatures had begun to fluctuate (BGVN 21:11). Fumarole temperatures in November 1996 registered between 130 and 677°C, higher than those recorded in October but lower than those in April. The flux was attributed to seasonal change; an intense rainy season and associated erosion accounted for the October lows. Temperatures had increased again by July 1997 to 232-773°C (BGVN 22:07).

In October 1997 higher than normal fumarole temperatures were attributed to arid conditions. During a 1 September visit to the summit area, fumarole temperatures were measured at their usual points and found escalated. The maximum temperature was 740°C. Because the temperature increase coincided with a dry spell, the heightened fumarolic activity was not deemed alarming.

Elevated temperatures were also reported in March 1998 (BGVN 23:03). Measurements during a 28 February visit revealed higher-than-normal fumarolic temperatures in the summit area. The high temperatures were again associated with a recent period of aridity, during which time fumarolic activity increased. Temperatures ranged from 318 to 748°C.

On 24 April 1999 the summit area was visited and temperatures again found inflated in the five areas of fumarolic activity. The maximum temperatures were, in the south area of the crater, 725°C and, in the north, 550°C. Significant changes in the crater morphology were noted and attributed to the strong erosion induced by Hurricane Mitch in October 1998.

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.

The French Polynesian Seismic Network recorded a strong acoustic crisis originating from Kermadec, very near Monowai Seamount, during 5-10 June. The acoustic crisis started at 0300 on 6 June (1500 on 5 June GMT) and ended at 0100 on 11 June (1300 on 10 June GMT) with no precursory activity (figure 7). Two distinct episodes were separated by a 4-hour period of inactivity. The network recorded more than 394 strong acoustic T-waves; the strongest took place at 1115 on 6 June (2315 on 5 June GMT). The first signal received on 6 June began with an impulse and lasted more than 12 minutes. Many of them had explosive characteristics, short and strong. The activity stopped with a small explosive T-wave. No tremor was recorded.

Figure 7. Acoustic wave amplitudes recorded by the French Polynesian Seismic Network during 5-10 June. The source of these waves was near the Monowai Seamount. Courtesy of Olivier Hyvernaud.

Monowai Seamount lies midway between the Kermadec and Tonga Islands, ~1,400 km NE of New Zealand. The adjacent trench is much shallower (~4 km) compared with the Tonga and Kermadec trenches (9-11 km deep). There was a T-wave swarm in November 1995 (BGVN 20:11/12). Other noteworthy recent activity at Monowai included a possible eruption in 1944, and about seven documented eruptions during 1977-90 (BGVN 16:03).

Geologic Background. Monowai, also known as Orion seamount, rises to within 100 m of the sea surface about halfway between the Kermadec and Tonga island groups. The volcano lies at the southern end of the Tonga Ridge and is slightly offset from the Kermadec volcanoes. Small parasitic cones occur on the N and W flanks of the basaltic submarine volcano, which rises from a depth of about 1500 m and was named for one of the New Zealand Navy bathymetric survey ships that documented its morphology. A large 8.5 x 11 km wide submarine caldera with a depth of more than 1500 m lies to the NNE. Numerous eruptions from Monowai have been detected from submarine acoustic signals since it was first recognized as a volcano in 1977. A shoal that had been reported in 1944 may have been a pumice raft or water disturbance due to degassing. Surface observations have included water discoloration, vigorous gas bubbling, and areas of upwelling water, sometimes accompanied by rumbling noises.

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

A team from Southwest Research Institute and Arizona State University visited Cerro Negro during 24 February-2 March. Their objectives were to map geomorphologic changes to the cone resulting from the 1995 eruption, estimate the respirable fraction of ash suspended above the 1992 and 1995 tephra blankets, and perform stratigraphic studies of tephra deposits.

A topographic map was prepared from 8,700 GPS measurements collected on and around the cinder cone (figures 12 and 13). Precision of individual measurements was within 1 cm and the survey had gross precision within 0.5 m.

Figure 12. Topographic map of Cerro Negro volcano, prepared during 24 February-2 March. The contour interval is 10 m. Shaded triangles indicate the 1995 eruption vents, arrows show the direction of 1995 lava flows from the crater, shaded circles show positions of active fumarole areas, and the shaded square indicates the position of the INETER seismic station. Courtesy of Chuck Conner.

Figure 13. Color-shaded topographic map of Cerro Negro volcano, prepared during 24 February-2 March. The contour interval is 10 m. The black square indicates the position of the INETER seismic station. Courtesy of Chuck Conner.

The cinder cone's morphology was changed significantly by the 1992 and 1995 eruptions. During the 1992 eruption the crater widened and now has an average diameter of 340 m. It was widest, 418 m, along its NE-SW axis because of a broad shoulder of tephra deposited on the SW side of the cone (during the 1992 and 1995 eruptions prevailing wind directions were from the NE). The diameter of the base of the cone was 945 m, measured between Cerro La Mula Ridge to the N of the cone and the Cristo Rey vent on the S. Data derived from air photos taken in 1986 revealed that this basal diameter had not changed substantially since then despite the 1992 and 1995 eruptions. The high point on Cerro Negro was measured at 728 m elevation, giving the cone a height of 258 m when measured relative to a topographic break in slope at the cone base, just west of Cristo Rey vent.

A small pyroclastic cone within the 1992 crater was created during the 1995 eruption. This new cone had a rim diameter of 124-130 m. Its crater depth was 94 m from the W rim (corresponding to the high point on Cerro Negro) and 44 m from the low point on the E side. Lava flows, emitted from at least two boccas on the 1992 crater floor during the 1995 activity, breached the N side of the 1992 cone. Because of this breach the low point on the 1992 crater rim was 9 m below the average rim elevation of 625 m.

No substantial degassing or other signs of volcanic unrest were observed during the seven days that the team was on-site and the area seemed unchanged since the cessation of the 1995 eruption. No thermal activity was observed on the 1995 lava flows, even on flow levees thicker than 3 m. These lava flows appeared to have cooled substantially as a result of the more than 3 m rainfall the region received during Hurricane Mitch in the Fall of 1998. Two fumarole areas were active; one fumarole area on the E of the crater has likely persisted since before the 1992 eruption. These fumaroles were easily accessible due to the breaching of the N crater rim by the 1992 lava flows. A new fumarole area had been established high on the W rim of the 1995 pyroclastic cone, just below the high point on Cerro Negro. Sulfur, anhydrite, and halite were deposited in these areas. Low-temperature fumaroles (<100°C) were found along arcuate fractures that paralleled the 1992 cone rim. These fractures were most prominent on the S side of the cone where they were faulted with 0.1-1.0 m displacements, down toward the outer cone flank and up toward the crater, because of slumping on the outer flank. They were radial on the SW of the cone, where the outer cone slope was buttressed by 1992 and 1995 tephra accumulations. Similar fractures and small faults also occurred on the NW rim and around the 1995 pyroclastic cone. Low-temperature fumaroles were also found on the southernmost 1995 bocca and a small mound within the 1992 crater. Fumaroles and steaming ground persisted on Cerro La Mula Ridge.

Crater fumarole measurements. The temperature of crater fumaroles has been measured during visits by Alain Creusot since 1995 (BGVN 21:11, 21:12, and 23:03). From November 1995, when a significant eruption took place, to October 1996, fumarole temperatures were as high as 700°C. On 27 November 1996, a visit to the crater found that fumarole temperatures had generally decreased by 80-100°C and the maximum temperature was only 630°C. On 23 December 1996 the maximum fumarole temperature was only 606°C. An additional decrease of fumarole temperatures was noted on a 5 September 1997 visit; the maximum temperature measured was only 405°C (previously unreported). The highest temperature found on 14 February 1998 was 340°C.

References. Hill and others, 1998, Eruptions of Cerro Negro volcano, Nicaragua, and risk assessment for future eruptions, Geological Society of America, Bulletin, v. 110, p. 1231-1241.

Connor and others, 1996, Soil ²²²Rn pulse during the initial phase of the June-August 1995 eruption of Cerro Negro, Nicaragua, Journal of Volcanology and Geothermal Research, vol. 73, p.119-127.

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: Chuck Connor, Peter La Femina, Brittain Hill, James Weldy, Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 6220 Culebra Rd, San Antonio, TX, 78238-5166; Kurt Roggensack and Berry Cameron, Department of Geological Sciences, Arizona State University, Tempe, AZ; Alain Creusot, Instituto Nicaraguense de Energía, Managua, Nicaragua.

Seismicity registered at station POA2, located 2.8 km SW of the active crater, has declined since early 1998 (figures 71 and 72). Since then, gas columns continued to reach altitudes between 500 and 600 m above the floor of the crater as they had during the interval of greater seismicity. The pyroclastic cone remained the focus of fumarolic activity. The crater's W, E, and SE walls continued to slip into the lake. The lake maintained a constant bubbling on its S and SE edges. The active lake's color varied considerably; for example, at various times during April 1999, the ~32°C lake water appeared green, turquoise, or light blue. In November 1998, the lake appeared greenish turquoise and had a temperature of 29°C.

Figure 71. Low-frequency earthquakes at Poás each month during January 1998-May 1999. Courtesy of OVSICORI-UNA.

Figure 72. Monthly earthquakes of medium- and high-frequency (number of events on y-axis scale) at Poás and the tremor duration (in hours on y-axis scale). The plot covers the interval January 1998-February 1999. 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, E. Duarte, R. Saenz, E. Malavassi, M. Martinez, and R. Van der Laat, T. Marino, and E. Hernandez, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Wendy Perez Fernandez, Seccion de Seismologia, Vulcanologia y Exploracion Geofisica, Escuela Centroamericana de Geologia, Universidad de Costa Rica, POB 35-2060, San Jose, Costa Rica.

The low level of activity displayed in May continued, with small exhalations and minor fumarolic emissions until 12 June. However, seismic activity increased on 12 June and continued for the next 10 or 11 days. At 1209 and 1600 on 12 June, two M 2.2 volcano-tectonic events occurred under the crater and SW of the volcano. At 1542 on 15 June, a large earthquake (M 6.7) centered between the states of Puebla and Oaxaca did not affect the volcano. Bad weather had obstructed visibility earlier, but that afternoon observers saw small fumarolic emissions of steam and gas.

Seismicity increased on 16 June as several volcano-tectonic events were recorded in the morning, most with magnitudes between 2.5 and 3 and two larger ones with M >3. These events were located 4-7 km below the summit crater. The last event occurred at 0206 on 17 June. This seismicity did not produce any important external manifestations except a small exhalation on the morning of 17 June accompanied by a light ash puff blown to the W.

On 21 June two earthquakes in Guerrero did not effect the volcano. No other events were reported for the month and by 30 June the radius of restricted access was reduced to 5 km from the 7 km previously recommended.

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

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

The mild Vulcanian activity continuing since November 1998 continued through June 1999. With time, the eruption's from Rabaul's Tavurvur cone appeared to be progressively waning in intensity. Still, during May and June several moderate explosions occurred.

Some May and June explosions sent ash clouds 1 km above the summit. The ash clouds drifted NW, some resulted in light ashfall over Rabaul Town. The mild ash-bearing outbursts in June occurred with very long intervals (sometimes 24 hours) between them. Notable outbursts took place on 9 days during the month (3, 5, 6, 9, 13, 15, 16, 17, 19, 23 and 25 June); although many only lasted 2-10 minutes, the last one of the set prevailed for 25 minutes. Typical plumes rose to 500 m high; SE winds typically blew these plumes and fine ash fell, including some on Rabaul Town.

In accord with these visual observations, both deformation and caldera seismicity remained low. Although during May, April, and March there had been 150, 142, and 120 low-frequency earthquakes, respectively, during June there occurred only 38 such earthquakes. The two located earthquakes appeared to the NE of the main ring fault. Anomalously, during the past 2 years there has been an absence of recorded high-frequency earthquakes on the ring fault. Instead, located earthquakes have consistently struck NE of the ring-fault system.

On the 16th a regional earthquake directly E of Wide Bay triggered a 31-cm tsunami. Since then, more than 20 regional events have occurred within a 20 km radius of the initial quake.

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.

Seismicity during 17 months through April 1999 (figure 14) showed pronounced peaks at over 100 events/month in one parameter, microseisms, during September- October 1998. Otherwise, relative quiet prevailed; microseisms, high-frequency, and low-frequency events all generally took place fewer than 20 times/month. Tremor was nearly absent during roughly half the months of 1998 and in 1999 during January, February, and April. Months with 2-10 hours of tremor included February, August, September 1998 and March and May 1999.

Figure 14. Selected seismic parameters at Rincón de la Vieja during January 1998-May 1999. The arrows indicate months with detected eruptions (mainly in February 1998, a month with ~10; the other indicated months with fewer than 2). Courtesy of OVSICORI-UNA.

In March, fumarolic activity continued on the NE, S, and SW walls of the main crater. The lake had a gray color and contained suspended particles of sulfur. The temperature of the lake was 35.5°C.

During May, the main crater's N-flank fumarolic activity fluctuated in temperature between 68°C and 92°C. The lake in the crater was light blue with particles of sulfur, and a temperature of 37°C. On the S and N walls, there were columns of gases that irritated eyes and skin.

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, E. Duarte, R. Sáenz, E. Malavassi, M. Martinez, and R. Van der Laat, T. Marino, and E. Hernandez, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Wendy Perez Fernandez, Seccion de Seismologia, Vulcanologia y Exploracion Geofisica, Escuela Centroamericana de Geologia, Universidad de Costa Rica, POB 35-2060, San José, Costa Rica.

After the strong summit explosions of 23 August and 8 September 1998 (BGVN 23:10), at least two others followed during 1998: the first at 1805 on 24 November, the second at 0245 on 28 December. Both were reported by Pierre Cottens from the village of Stromboli, with pyroclasts clearly seen from the village, reaching estimated heights of at least 700 m over the craters in November and 500 m in December, but with little ashfall over the village. Technical problems kept the seismic station maintained by the University of Udine out of service through the end of January 1999.

Seismicity during February-June 1999. The seismic station was restored on 1 February 1999, and in the first part of the month the daily number of events declined from 130 to 50-80/day (figure 58). Saturating events showed a similar trend. The second half of February was characterized by an increasing number of events, reaching a maximum of 275 events on 2 March. During this period the tremor intensity showed fluctuations around an average of 3.6 V s, with an isolated peak on 23 February.

Figure 58. Seismicity detected at the summit of Stromboli from February through June 1999. The gray bars show the number of recorded events/day, and the black bars those saturating the instrument (ground velocity exceeding 100 µm/s). The line shows daily tremor intensity computed by averaging hourly 60-second samples. The seismic station is located 300 m from the craters at 800 m elevation. Courtesy of Roberto Carniel.

During 12-20 March there was a general decrease in activity, with a minimum number of 52 seismic events recorded on 16 March, and a minimum tremor intensity of 1.8 V s on 15 March. The gain in activity was observed first in the tremor intensity, with a local maximum of 4.7 V s on 29 March, then in the number of events, which reached a high of 208 events on 30 March.

A sharp decline was observed on 7 April both in the tremor intensity (from 3.0 to 1.2) and in the number of events (from 180 to 76). Saturating events also stopped, after an average of five saturated events during the three preceding days. Seismic activity increased slightly during the following two days, and on 9 April Cottens reported two strong blasts at about 0300, separated by a few minutes. Bad weather did not allow observation of the summit area, but the noise was similar to that produced by the strong eruptions of 1998. The seismic station recorded an event with greater than usual energy, which may have been from one of the explosions. Another decrease of activity was observed the day after the explosion, with the number of daily events falling to 83 and the tremor intensity to 1.9. The following days were characterized by a slow increase in seismicity, but activity remained low to moderate for the rest of the month.

After another gap in the seismic acquisition, the activity was still low after mid-May 1999, with <100 events/day between 20-24 May. The last week of May showed a rise in the number of events, with a maximum of 183 events on 26 May. A relatively high number of saturating events was also observed, starting on 23 May and peaking on 27 May (21 saturating events) and 30 May (23 saturating events). Another minimum in the number of events (66), number of saturating events (0) and tremor intensity (1.6) was recorded on 5-6 June 1999. During several days on the island in the second half of June, seismic activity remained at low to moderate levels, with a short duration increase in the tremor intensity on 21-22 June.

Observations during 17-20 June 1999. Observations during 17-20 June allowed mapping of the crater terrace (figure 59). Observations were made over two 3-4 hour periods: 2109-0100 (18-19 June), and 2030-2320 (19 June).

Figure 59. Sketch map of Stromboli's Crater Terrace drawn on 18-19 June 1999 from Pizzo sopra la Fossa and fitted to the map produced from a September 1995 EDM survey of the Crater Terrace (BGVN 20:11/12). Courtesy of Andy Harris and Roberto Carniel

Crater 1 contained four active vents (figure 59). During the first observation period, the vicinity of Vent 1/2 was the source of persistent widespread glow; but no ejecta was observed. Vent 1/1, however, was the source of glow and near-persistent low-energy activity, characterized by repeated phases of ejecta emission separated by periods of little or no emissions. Each phase consisted of numerous pulses of ejecta. During each pulse a few (typically 1-10) bombs were ejected <10 m above the crater rim, with pulses every few seconds. Around 11 periods of this persistent, pulsing emission were observed. Each period lasted 2-29 minutes and was separated by intervals of 3-28 minutes with occasional bomb emissions. This pattern at Vent 1/1 was broken by 16 high-energy events during which explosions sent ejecta ~100 m high.

Vent 1/3 produced high-energy events only, with 18 observed during the first period. High energy events from vents 1/1, 1/3, and 1/4 were synchronized. It was difficult to distinguish eruptions from 1/4 and 1/1 from the viewing angle, so eruptions from these two vents may have been occasionally assigned to the wrong vent; the eruption counts for 1/1 and 1/4 together were therefore combined. Vents 1/3, 1/1, and/or 1/4 erupted together on 11 occasions. On these occasions ejection from each vent was either synchronous or closely linked. During such synchronized events there appeared to be no set order in terms of which of the three vents started erupting and which followed. Vents 1/1 (and/or 1/4) and 1/3 erupted on their own on 5 and 7 occasions, respectively.

As in the first observation period, glow persisted above Vent 1/2 throughout the second period. Unlike the previous evening, however, ejecta was observed from Vent 1/2, where activity followed the style observed at Vent 1/1 during the previous evening. Around eight periods of low-energy but persistent pulsing activity were observed. These lasted 3-33 minutes and were separated by 2-23-minute-long periods of apparently no emission. This pattern of activity from Vent 1/2 was interrupted by five high-energy events lasting 4-6 seconds.

Vent 1/1 issued regular gas puffs (typically one puff every 1-2 seconds), but the frequency of ejecta emission had declined, with the periods of persistent, pulsing activity observed the previous evening replaced by discrete emissions. Ejections were observed from 1/1 on nine occasions, where only one event was of the high-energy type, and the remainder were short (<40-second-long) periods during which 1-10 bombs were ejected <10 m above the rim in pulses. As in the previous period, Vent 1/3 was characterized by high-energy events only. Seven occurred during the second period, of which three were synchronous with high energy events from 1/2 or 1/1.

Although no activity or glow was observed from Crater 2, a hornito (figure 59) had grown in the vicinity of the low cone observed during May 1997 (BGVN 22:05). The area surrounding Crater 2 no longer seemed to be marked by a crater-like feature. The subsidence bowl observed SE of Crater 2 in May 1997 had developed into a deep, funnel-shaped pit, which was the source of faint glow. On a previous map (BGVN 22:05), the vent numbering indicated that this area was part of Crater 2; this interpretation is questionable and this feature could be considered a crater by itself now. On figure 59 we denote this vent as 3/3 (therefore part of Crater 3) in order to allow for easy comparison with older maps of the crater terrace (e.g., BGVN 22:03).

Crater 3 was the location of two additional active vents (figure 59). Glow was observed from vent 3/1 only after an eruption at 2317 on 18 June. Eruptions from Crater 3 were high-energy only, and typically larger (in terms of volume and height, attaining heights of 150 ± 50 m) and longer (lasting 8-23 seconds) than those from Crater 1. Nine and eleven eruptions, respectively, occurred from Crater 3 during the two observation periods.

During our descent in the early hours of 19 June there was a rock fall/slide at about 0200 lasting 1-2 minutes. Boulders from the cliffs on the N edge of the Rina Grande rolled and bounced down the Rina Grande. Owing to the steepness of this flank, once in motion the rocks probably did not stop until they reached the sea in the direction of Forgia Vecchia, crossing the route that descends the Rina Grande ash slope within seconds. Such events pose a very serious hazard to anyone using this route, especially at night.

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

Information Contacts: Andy Harris and Dawn Pirie, Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, United Kingdom; Sarah Sherman, 41-485D Kalanianaole Hwy., Waimanolo, HI 96795 USA; Roberto Carniel, Dipartimento di Georisorse e Territorio, Università di Udine, Via Cotonificio, 114, I-33100 Udine (URL: http://www.swisseduc.ch/stromboli/).

During the 17 months ending in May 1999, microseisms varied from ~30 to ~180 a month (figure 5). A 4-fold progressive increase began after December 1998.

Figure 5. A histogram showing Turrialba's monthly microseisms during January 1998- April 1998. Courtesy of OVSICORI-UNA.

In March 1999, the main crater's fumaroles were visible on the NE, N, NW, E and SW walls. Escaping gases appeared constant and had a temperature of 89°C. During March, the seismographic station VTU, located 0.5 km NE of the active crater registered a total of 252 earthquakes. Of those 81 had high frequency, with S-P duration of less than 1.5 seconds and frequencies greater than 3.0 Hz. The 166 microseisms registered had amplitudes under 10 mm, short durations, and frequencies between 2.1 and 3.0 Hz. An earthquake registered at 1846 on 7 March, with a Richter magnitude of 2.3, a depth of 7 km, and an epicenter 4 km NE of the main crater.

During April, the station VTU registered 287 earthquakes. Of those, 105 were of high frequency (with S-P of less than 1.5 seconds and frequencies above 3.0 Hz), and 4 were of low frequency. The 178 microseisms registered were of short duration; their dominant frequencies were between 2.1 and 3.0 Hz.

During May, a total of 309 events were recorded, of which 120 were type AB with S-P less than 1.5 seconds and frequencies less than 3.0 Hz. There were 3 low-frequency events. The 186 microseisms registered had amplitudes under 10 mm.

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, E. Duarte, R. Saenz, E. Malavassi, M. Martinez, and R. Van der Laat, T. Marino, and E. Hernandez, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Wendy Perez Fernandez, Seccion de Seismologia, Vulcanologia y Exploracion Geofisica, Escuela Centroamericana de Geologia, Universidad de Costa Rica, POB 35-2060, San Jose, Costa Rica.

White Island was visited on 30 June by IGNS scientists who reported that no significant eruptive activity had occurred since the explosive activity in April (BGVN 24:04). PeeJay vent was inactive and the only significant emissions of steam and gas came from the vent that formed in early May (at a spot E of PeeJay vent).

As the group arrived on 30 June a weak white steam-and-gas plume was rising 500-700 m above the volcano and moving ENE (figure 43). The plume was being fed by emissions from within the 1978/90 Crater Complex; the new vent E of PeeJay was the plume's strongest contributing source. Most of the gas and steam emitted (at high velocity) from this vent was from an inclined orifice near the S side of the vent. Other prominent sources included fumaroles near the lakeshore and in the valley wall W of the lake. PeeJay vent was inactive and had been filled by sediment that formed a flat floor about 3 m below the vent's rim. Activity had stopped in late May or early June.

Figure 43. View of the 1978/90 Crater complex at White Island looking toward the NW on 30 June. Photograph courtesy of IGNS.

The light-green colored lake within Metra Crater had enlarged and flooded into the NNE embayments of this crater. There was no evidence of further explosive activity from Metra Crater. No strong ebullition or convection was observed in the lake.

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

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