Pacaya is one of the most active volcanoes in Guatemala, with activity largely consisting of frequent lava flows and Strombolian activity at the Mackenney crater. This report summarizes continued activity during February through July 2019 based on reports by Guatemala's Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH) and Sistema de la Coordinadora Nacional para la Reducción de Desastres (CONRED), visiting scientists, and satellite data.

At the beginning of February activity included Strombolian explosions ejecting material up to 5 to 30 m above the Mackenney crater and a degassing plume up to 300 m. Multiple lava flows were observed throughout the month on the N, NW, and W flanks, reaching 350 m from the crater and resulting in avalanches from the flow fronts. Strombolian activity continued with sporadic to continuous explosions ejecting material 5-75 m above the Mackenney crater. Degassing produced plumes up to 300 m above the crater, and incandescence from the crater and lava flows were seen at night. Daniel Sturgess of Bristol University observed activity on the 24th, noting a 70-m-long lava flow with individual blocks from the front of the flow rolling down the flanks (figure 108). He reported that mild Strombolian explosions occurred every 10-20 minutes and ejected blocks, up to approximately 4 m in diameter, as high as 5-30 m above the crater and towards the northern flank.

Figure 108. An active lava flow on the NW flank of Pacaya on 24 February 2019 with incandescence visible in lower light conditions. Courtesy of Daniel Sturgess, University of Bristol.

Similar activity continued through March with multiple lava flows reaching a maximum of 200 m N and NW, and avalanches descending from the flow fronts. Ongoing Strombolian explosions expelled material up to 75 m above the Mackenney crater. Degassing produced a white-blue plume to a maximum of 900 m above the crater (figure 109) and incandescence was noted some nights.

Figure 109. A degassing plume at Pacaya reaching 350 m above the crater and dispersing to the S on 19 March 2019. Courtesy of CONRED.

During April lava flows continued on the N and NW flanks, reaching a maximum length of 300 m, with avalanches forming from the flow fronts. Degassing formed plumes up to 600 m above the crater that dispersed with various wind directions. Strombolian activity continued with explosions ejecting material up to 40 m above the crater. On the 2nd and 3rd weak rumbles were heard at distances of 4-5 km. Similar activity continued through May with lava flows reaching 300 m to the N, degassing producing plumes up to 600 m above the crater, and Strombolian explosions ejecting material up to 15 m above the crater.

Lava flows continued out to 300 m in length to the N and NW during June (figures 110 and 111). Strombolian activity ejected material up to 30 m above the crater and degassing resulted in plumes that reached 300 m. During July there were multiple active lava flows that reached a maximum of 300 m in length on the N and NW flanks (figure 112). Avalanches generated by the collapse of material at the front of the lava flows were accompanied by explosions ejecting material up to 30 m above the crater.

Figure 110. An active lava flow on Pacaya on 9 June 2019 with incandescent blocks rolling down the flank from the flow front. Courtesy of Paul Wallace, University of Liverpool.

Figure 111. Activity at Pacaya on 22 June 2019 with a degassing plume dispersed to the W and a 300-m-long lava flow. Photos by Miguel Morales, courtesy of CONRED.

Figure 112. Two lava flows were active to the N and NW at Pacaya on 20 July 2019. Photos courtesy of CONRED.

During February through July multiple lava flows and crater activity were detected in Sentinel-2 satellite thermal images (figures 113 and 114) and relatively constant thermal energy was detected by the MIROVA system with a slight decrease in the energy and frequency of anomalies during June (figure 115). The thermal anomalies detected by the MODVOLC system for each month from February through July spanned 6-30, with six during June and 30 during April.

Figure 113. Sentinel-2 thermal satellite images of Pacaya show lava flows to the N and NW during February through April 2019. There was a reduction in visible activity in early March. False color (urban) satellite images (bands 12, 11, 4) courtesy of Sentinel Hub Playground.

Figure 114. Sentinel-2 thermal satellite images of Pacaya showing lava flow and hot avalanche activity during June and July 2019. False color (urban) satellite images (bands 12, 11, 4) courtesy of Sentinel Hub Playground.

Figure 115. MIROVA log radiative power plot of MODIS thermal infrared at Pacaya during October 2018 through July 2019. Detected thermal energy is relatively stable with a decrease through June and subsequent increase during July. Courtesy of MIROVA.

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

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/); Coordinadora Nacional para la Reducción de Desastres (CONRED), Av. Hincapié 21-72, Zona 13, Guatemala City, Guatemala (URL: http://conred.gob.gt/www/index.php); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); 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/); Daniel Sturgess, School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, United Kingdom (URL: http://www.bristol.ac.uk/earthsciences/); Paul Wallace, Department of Earth, Ocean and Ecological Sciences, University of Liverpool, 4 Brownlow Street, Liverpool L69 3GP, United Kingdom (URL: https://www.liverpool.ac.uk/environmental-sciences/staff/paul-wallace/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Frequent historical eruptions at Volcán de Colima date back to the 16th century and include explosive activity, lava flows, and large debris avalanches. The most recent eruptive episode began in January 2013 and continued through March 2017. Previous reports have covered activity involving ash plumes with extensive ashfall, lava flows, lahars, and pyroclastic flows (BGVN 41:01 and 42:08). In late April 2019, increased seismicity related to volcanic activity began again. This report covers activity through July 2019. The primary source of information was the Centro Universitario de Estudios e Investigaciones de Vulcanologia, Universidad de Colima (CUEIV-UdC).

On 11 May 2019, CUEIV-UdC reported an explosion that was recorded by several monitoring stations. A thermal camera located south of Colima captured thermal anomalies associated with the explosion as well as intermittent degassing, which mainly consisted of water vapor (figure 131). A report from the Unidad Estatal de Protección Civil de Colima (UEPCC), and seismic and infrasound network data from CUEIV-UdC, recorded about 60 high-frequency events, 16 landslides, and 14 low-magnitude explosions occurring on the NE side of the crater during 11-24 May. Drone imagery showed fumarolic activity occurring on the inner wall of this crater on 22 May (figure 132).

Figure 131. Gas emissions from Colima during the 11 May 2019 eruption as seen from the Naranjal station. Courtesy of CUEIV-UdC (Boletin Seminal de la Actividad del Volcan de Colima 17 mayo 2019 no 121).

Figure 132. A drone photo showing fumarolic activity occurring within the NE wall of the crater at Colima on 22 May 2019. Courtesy of CUEIV-UdC (Boletin Seminal de la Actividad del Volcan de Colima 24 mayo 2019 no 122).

Small explosions and gas-and-steam emissions continued intermittently through mid-July 2019 concentrated on the NE side of the crater. An overflight on 9 July 2019 revealed that subsidence from the consistent activity slightly increased the diameter of the vent; other areas within the crater also showed evidence of subsidence and some collapsed material on the outer W wall (figure 133). During the weeks of 19 and 26 July 2019, monitoring cameras and seismic data recorded eight lahars.

Figure 133. A drone photo of the crater at Colima on 8 July 2019 shows continuing fumarolic activity and evidence of a collapsed wall on the W exterior side. Courtesy of CUEIV-UdC (Boletin Seminal de la Actividad del Volcan de Colima 12 julio 2019 no 129).

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 4320 m high point of the complex) on the north and the 3850-m-high historically active Volcán de Colima at the south. A group of cinder cones of late-Pleistocene age is located on the floor of the Colima graben west and east of the Colima 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, and have produced a thick apron of debris-avalanche deposits on three sides of the complex. Frequent historical eruptions date back to the 16th century. Occasional major explosive eruptions (most recently in 1913) have destroyed the summit and left a deep, steep-sided crater that was slowly refilled and then overtopped by lava dome growth.

Information Contacts: Centro Universitario de Estudios e Investigaciones de Vulcanologia, Universidad de Colima (CUEIV-UdC), Colima, Col. 28045, Mexico; Centro Universitario de Estudios Vulcanologicos y Facultad de Ciencias de la Universidad de Colima, Avenida Universidad 333, Colima, Col. 28045, Mexico (URL: http://portal.ucol.mx/cueiv/); Unidad Estatal de Protección Civil, Colima, Roberto Esperón No. 1170 Col. de los Trabajadores, C.P. 28020, Mexico (URL: http://www.proteccioncivil.col.gob.mx/).

Masaya, in Nicaragua, contains a lava lake found in the Santiago Crater which has remained active since its return in December 2015 (BGVN 41:08). In addition to this lava lake, previous volcanism included explosive eruptions, lava flows, and gas emissions. Activity generally decreased during March-July 2019, including the number and frequency of thermal anomalies, lava lake levels, and gas emissions. The primary source of information for this report comes from the Instituto Nicareguense de Estudios Territoriales (INETER).

On 21 July 2019 a small explosion in the Santiago Crater resulted in some gas emissions and an ash cloud drifting WNW. In addition to the active lava lake (figure 77), monthly reports from INETER noted that thermal activity and gas emissions (figure 78) were decreasing.

Figure 77. Active lava lake visible in the Santiago Crater at Masaya on 27 June 2019. Photo by Sheila DeForest (Creative Commons BY-SA license).

Figure 78. Gas emissions coming from the Santiago Crater at Masaya on 29 June 2019. Photo by Sheila DeForest (Creative Commons BY-SA license).

On 15 May and 22 July 2019, INETER scientists used a FLIR SC620 thermal infrared camera to measure temperatures of fumaroles on the Santiago Crater. In May 2019 the temperature of fumaroles had decreased by 48°C since the previous month. Between May and July 2019 fumarole temperatures continued to decline; temperatures ranged from 90° to 136°C (figure 79). Compared to May 2019 these temperatures are 3°C lower. INETER reports that the level of the lava lake has been slowly dropping during this reporting period.

Figure 79. FLIR (forward-looking infrared) and visible images of the Santiago Crater at Masaya showing fumarole temperatures ranging from 90° to 136°C. The scale in the center shows the range of temperatures in the FLIR image. Courtesy of INETER (March 2019 report).

According to MIROVA (Middle InfraRed Observation of Volcanic Activity) data from MODIS satellite instruments, frequent thermal anomalies were recorded from mid-March through early May 2019, with little to no activity from mid-May to July 2019 (figure 80). Sentinel-2 thermal images show high temperatures in the active lava lake on 10 March 2019 (figure 81). Thermal energy detected by the MODVOLC algorithm showed 14 hotspot pixels with the most number of hotspots (7) occurring in March 2019.

Figure 80. Thermal anomalies were relatively constant at Masaya from early September 2018 through early May 2019 and then abruptly decreased until mid-June 2019 as recorded by MIROVA. Courtesy of MIROVA.

Figure 81. Sentinel-2 thermal satellite image showing a detected heat signature from the active lava lake at Masaya on 10 March 2019. The lava lake is visible (bright yellow-orange). Approximate diameter of the crater containing the lava lake is 500 m. Thermal (urban) satellite image (bands 12, 11, 4) courtesy of Sentinel Hub Playground.

Geologic Background. Masaya is one of Nicaragua's most unusual and most active volcanoes. It lies within the massive Pleistocene Las Sierras pyroclastic shield volcano and is a broad, 6 x 11 km basaltic caldera with steep-sided walls up to 300 m high. The caldera is filled on its NW end by more than a dozen vents that erupted along a circular, 4-km-diameter fracture system. The twin volcanoes of Nindirí and Masaya, the source of historical eruptions, were constructed at the southern end of the fracture system and contain multiple summit craters, including the currently active Santiago crater. A major basaltic Plinian tephra erupted from Masaya about 6500 years ago. Historical lava flows cover much of the caldera floor and have confined a lake to the far eastern end of the caldera. A lava flow from the 1670 eruption overtopped the north caldera rim. Masaya has been frequently active since the time of the Spanish Conquistadors, when an active lava lake prompted attempts to extract the volcano's molten "gold." Periods of long-term vigorous gas emission at roughly quarter-century intervals cause health hazards and crop damage.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); 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); Sheila DeForest (URL: https://www.facebook.com/sheila.deforest).

The acid lake of Rincón de la Vieja's active crater has generated intermittent weak phreatic explosions regularly since 2011, continuing during the past year through at least August 2019. The volcano is monitored by the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), and the information below comes from its weekly bulletins between 4 March and 2 September 2019. Clouds often prevented webcam and satellite views. The current report describes activity from March through July 2019.

OVSICORI-UNA reported that weak events occurred on 19 March at 1851 and on 29 March 2019 at 2043. A two-minute-long phreatic explosion on 1 April at 0802 produced a plume that rose 600 m above the crater rim. Continuous emissions were visible during 3-4 April, rising 200 m above the crater rim. On 3 April, at 1437, a small explosion was detected. An explosion on 10 April at 0617 produced a gas-and-steam plume that rose 1 km above the crater rim and drifted SE. On 12 April at 0643, a plume rose 500 m. Another event took place at 0700 on 13 April, although poor weather conditions prevented visual observations. On 14 April, OVSICORI-UNA noted that aerial photographs showed a milky-gray acid lake at a relatively low water level with convection cells of several tens meters of diameter in the center and eastern parts of the lake.

According to an OVSICORI-UNA bulletin, a small phreatic explosion occurred on 1 May. Another explosion on 11 May at 0720 produced a white gas-and-steam plume that rose 600 m above the crater rim. Phreatic explosions were recorded on 14 May at 1703 and on 17 May at 0357, though dense fog prevented visual confirmation of both events with webcams. On 15 May a local observer noted a diffuse plume of steam and gas, material rising from the crater, and photographed milky-gray deposits on the N part of the crater rim ejected from the event the day before. A major explosion occurred on 24 May.

OVSICORI-UNA recorded a significant phreatic 10-minute-long explosion that began on 11 June at 0343, but plumes were not visible due to weather conditions. No further phreatic events were reported in July.

Seismic activity was very low during the reporting period, and there was no significant deformation. Short tremors were frequent toward the end of April, but were only periodic in May and June; tremor almost disappeared in July. A few long-period earthquakes were recorded, and volcano-tectonic earthquakes were even less frequent.

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: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/, https://www.facebook.com/OVSICORI/).

Sakurajima rises from Kagoshima Bay, which fills the Aira Caldera near the southern tip of Japan's Kyushu Island. Frequent explosive and occasional effusive activity has been ongoing for centuries. The Minamidake summit cone has been the location of persistent activity since 1955; the Showa crater on its E flank has also been intermittently active since 2006. Numerous explosions and ash-bearing emissions have been occurring each month at either Minamidake or Showa crater since the latest eruptive episode began in late March 2017. This report covers ongoing activity from January through June 2019; the Japan Meteorological Agency (JMA) provides regular reports on activity, and the Tokyo VAAC (Volcanic Ash Advisory Center) issues tens of reports each month about the frequent ash plumes.

From January to June 2019, ash plumes and explosions were usually reported multiple times each week. The quietest month was June with only five eruptive events; the most active was March with 29 (table 21). Ash plumes rose from a few hundred meters to 3,500 m above the summit during the period. Large blocks of incandescent ejecta traveled as far as 1,700 m from the Minamidake crater during explosions in February and April. All the activity originated in the Minamidake crater; the adjacent Showa crater only had a mild thermal anomaly and fumarole throughout the period. Satellite imagery identified thermal anomalies inside the Minamidake crater several times each month.

Table 21. Monthly summary of eruptive events recorded at Sakurajima's Minamidake crater in Aira caldera, January-June 2019. The number of events that were explosive in nature are in parentheses. No events were recorded at the Showa crater during this time. Data courtesy of JMA (January to June 2019 monthly reports).

There were eight eruptive events reported by JMA during January 2019 at the Minamidake summit crater of Sakurajima. They occurred on 3, 6, 7, 9, 17, and 19 January (figure 76). Ash plume heights ranged from 600 to 2,100 m above the summit. The largest explosion, on 9 January, generated an ash plume that rose 2,100 m above the summit crater and drifted E. In addition, incandescent ejecta was sent 800-1,100 m from the summit. Incandescence was visible at the summit on most clear nights. During an overflight on 18 January no significant changes were noted at the crater (figure 77). Infrared thermal imaging done on 29 January indicated a weak thermal anomaly in the vicinity of the Showa crater on the SE side of Minamidake crater. The Kagoshima Regional Meteorological Observatory (KRMO) (11 km WSW) recorded ashfall there during four days of the month. Satellite imagery indicated thermal anomalies inside Minamidake on 7 and 27 January (figure 77).

Figure 76. Incandescent ejecta and ash emissions characterized activity from Sakurajima volcano at Aira during January 2019. Left: A webcam image showed incandescent ejecta on the flanks on 9 January 2019, courtesy of JMA (Explanation of volcanic activity in Sakurajima, January 2019). Right: An ash plume rose hundreds of meters above the summit, likely also on 9 January, posted on 10 January 2019, courtesy of Mike Day.

Figure 77. The summit of Sakurajima consists of the larger Minamidake crater and the smaller Showa crater on the E flank. Left: The Minamidake crater at the summit of Sakurajima volcano at Aira on 18 January 2019 seen in an overflight courtesy of JMA (Explanation of volcanic activity in Sakurajima, March 2019). Right: Two areas of thermal anomaly were visible in Sentinel-2 satellite imagery on 27 January 2019. "Geology" rendering (bands 12, 4, and 2) courtesy of Sentinel Hub Playground.

Activity increased during February 2019, with 15 eruptive events reported on days 1, 3, 7, 8, 10, 13, 14, 17, 22, 24, and 27. Ash plume heights ranged from 600-2,300 m above the summit, and ejecta was reported 300 to 1,700 m from the crater in various events (figure 78). KRMO reported two days of ashfall during February. Satellite imagery identified thermal anomalies at the crater on 6 and 26 February, and ash plumes on 21 and 26 February (figure 79).

Figure 78. An explosion from Sakurajima at Aira on 7 February 2019 sent ejecta up to 1,700 m from the Minamidake summit crater. Courtesy of JMA (Explanation of volcanic activity in Sakurajima, February 2019).

Figure 79. Thermal anomalies and ash emissions were captured in Sentinel-2 satellite imagery on 6, 21, and 26 February 2019 originating from Sakurajima volcano at Aira. Top: Thermal anomalies within the summit crater were visible underneath steam and ash plumes on 6 and 26 February (closeup of bottom right photo). Bottom: Ash emissions on 21 and 26 February drifted SE from the volcano. "Geology" rendering (bands 12, 4, and 2) courtesy of Sentinel Hub Playground.

The number of eruptive events continued to increase during March 2019; there were 29 events reported on numerous days (figures 80 and 81). An explosion on 14 March produced an ash plume that rose 3,500 m above the summit and drifted E. It also produced ejecta that landed 800-1,100 m from the crater. During an overflight on 26 March a fumarole was the only activity in Showa crater. KRMO reported 14 days of ashfall during the month. Satellite imagery identified an ash plume on 13 March and a thermal anomaly on 18 March (figure 82).

Figure 80. A large ash emission from Sakurajima volcano at Aira was photographed by a tourist on the W flank and posted on 1 March 2019. Courtesy of Kratü.

Figure 81. An ash plume from Sakurajima volcano at Aira on 18 March 2019 produced enough ashfall to disrupt the trains in the nearby city of Kagoshima according to the photographer. Image taken from about 20 km away. Courtesy of Tim Board.

Figure 82. An ash plume drifted SE from the summit of Sakurajima volcano at Aira on 13 March (left) and a thermal anomaly was visible inside the Minamidake crater on 18 March 2019 (right). "Geology" rendering (bands 12, 4, and 2) courtesy of Sentinel Hub Playground.

A decline in activity to only ten eruptive events on days 7, 13, 17, 22, and 25 was reported by JMA for April 2019. An explosion on 7 April sent ejecta up to 1,700 m from the crater. Another explosion on 13 April produced an ash plume that rose 2,200 m above the summit. Most of the eruptive events at Sakurajima last for less than 30 minutes; on 22 April two events lasted for almost an hour each producing ash plumes that rose 1,400 m above the summit. Ashfall at KRMO was reported during seven days in April. Two distinct thermal anomalies were visible inside the Minamidake crater on both 12 and 27 April (figure 83).

Figure 83. Two thermal anomalies were present inside Minamidake crater at the summit of Sakurajima volcano at Aira on 12 (left) and 27 (right) April 2019. "Geology" rendering (bands 12, 4, and 2) courtesy of Sentinel Hub Playground.

There were 15 eruptive events during May 2019. An event that lasted for two hours on 12 May produced an ash plume that rose 2,900 m from the summit and drifted NE (figure 84). The Meteorological Observatory reported 14 days with ashfall during the month. Two thermal anomalies were present in satellite imagery in the Minamidake crater on both 17 and 22 May.

Figure 84. An ash plume rose 2,900 m above the summit of Sakurajima at Aira on 12 May 2019 (left); incandescent ejecta went 1,300 m from the summit crater on 13 May. Courtesy of JMA (Explanation of volcanic activity in Sakurajima, May 2019).

During June 2019 five eruptive events were reported, on 11, 13, and 24 June; the event on 11 June lasted for almost two hours, sent ash 2,200 m above the summit, and produced ejecta that landed up to 1,100 m from the crater (figure 85). Five days of ashfall were reported by KRMO.

Figure 85. A large ash plume on 11 June 2019 rose 2,200 m above the summit of Sakurajima volcano at Aira. Courtesy of Aone Wakatsuki.

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

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, 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, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Mike Day, Minnesota, Twitter (URL: https://twitter.com/MikeDaySMM, photo at https://twitter.com/MikeDaySMM/status/1083489400451989505/photo/1); Kratü, Twitter (URL: https://twitter.com/TalesOfKratue, photo at https://twitter.com/TalesOfKratue/status/1101469595414589441/photo/1); Tim Board, Japan, Twitter (URL: https://twitter.com/Hawkworld_, photo at https://twitter.com/Hawkworld_/status/1107789108754038789); Aone Wakatsuke, Twitter (URL: https://twitter.com/AoneWakatsuki, photo at https://twitter.com/AoneWakatsuki/status/1138420031258210305/photo/3).

After a large, deadly explosive and effusive eruption during 1963-64, Indonesia's Mount Agung on Bali remained quiet until a new eruption began in November 2017 (BGVN 43:01). Lava emerged into the summit crater at the end of November and intermittent ash plumes rose as high as 3 km above the summit through the end of the year. Activity continued throughout 2018 with explosions that produced ash plumes rising multiple kilometers above the summit, and the slow effusion of the lava within the summit crater (BGVN 43:08, 44:02). Information about the ongoing eruptive episode comes from Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), also known as the Indonesian Center for Volcanology and Geological Hazard Mitigation (CVGHM), the Darwin Volcanic Ash Advisory Center (VAAC), and multiple sources of satellite data. This report covers the ongoing eruption from February through May 2019.

Intermittent but increasingly frequent and intense explosions with ash emissions and incandescent ejecta characterized activity at Agung during February through May 2019. During February, explosions were reported three times; events on seven days in March were documented with ash plumes and ashfall in surrounding villages. Five significant events occurred during April; two involved incandescent ejecta that traveled several kilometers from the summit, and ashfall tens of kilometers from the volcano. Most of the five significant events reported in May involved incandescent ejecta and ashfall in adjacent villages; air traffic was disrupted during the 24 May event. Ash plumes in May reached altitudes over 7 km multiple times. Thermal activity increased steadily during the period, according to both the MIROVA project (figure 44) and MODVOLC thermal alert data. MAGMA Indonesia reported at the end of May 2019 that the volume of lava within the summit crater remained at about 25 million m³; satellite information indicated continued thermal activity within the crater. Alert Level III (of four levels) remained in effect throughout the period with a 4 km exclusion radius around the volcano.

Figure 44. Thermal activity at Agung from 4 September 2018 through May 2019 was variable. The increasing frequency and intensity of thermal events was apparent from February-May. Courtesy of MIROVA.

Steam plumes rose 30-300 m high daily during February 2019. The Agung Volcano Observatory (AVO) and PVMBG issued a VONA on 7 February (UTC) reporting an ash plume, although it was not visible due to meteoric cloud cover. Incandescence, however, was observed at the summit from webcams in both Rendang and Karangasem City (16 km SE). The seismic event associated with the explosion lasted for 97 seconds. A similar event on 13 February was also obscured by clouds but produced a seismic event that lasted for 3 minutes and 40 seconds, and ashfall was reported in the village of Bugbug, about 20 km SE. On 22 February a gray ash plume rose 700 m from the summit during a seismic event that lasted for 6 minutes and 20 seconds (figure 45). The Darwin VAAC reported the plume visible in satellite imagery moving W at 4.3 km altitude. It dissipated after a few hours, but a hotspot remained visible about 10 hours later.

Figure 45. An ash plume rose from the summit of Agung on 22 February 2019, viewed from the Besakih temple, 7 km SW of the summit. Courtesy of PunapiBali.

Persistent steam plumes rose 50-500 m from the summit during March 2019. An explosion on 4 March was recorded for just under three minutes and produced ashfall in Besakih (7 km SW); no ash plume was observed due to fog. A short-lived ash plume rose to 3.7 km altitude and drifted SE on 8 March (UTC) 2019. The seismic event lasted for just under 4 minutes. Ash emissions were reported on 15 and 17 March to 4.3 and 3.7 km altitude, respectively, drifting W (figure 46). Ashfall from the 15 March event spread NNW and was reported in the villages of Kubu (6 km N), Tianyar (14 km NNW), Ban, Kadundung, and Sukadana. MAGMA Indonesia noted that two explosions on the morning of 17 March (local time) produced gray plumes; the first sent a plume to 500 m above the summit drifting E and lasted for about 40 seconds, while the second plume a few hours later rose 600 m above the crater and lasted for 1 minute and 16 seconds. On 18 March an ash plume rose 1 km and drifted W and NW. An event on 20 March was measured only seismically by PVMBG because fog prevented observations. An eruption on 28 March produced an ash plume 2 km high that drifted W and NW. The seismic signal for this event lasted for about two and a half minutes. The Darwin VAAC reported the ash plume at 5.5 km altitude, dissipating quickly to the NW. No ash was visible four hours later, but a thermal anomaly remained at the summit (figure 47). Ashfall was reported in nearby villages.

Figure 46. Ash plumes from Agung on 15 (left) and 17 (right) March 2019 resulted in ashfall in communities 10-20 km from the volcano. Courtesy of PVMBG and MAGMA Indonesia (Information on G. Agung Eruption, 15 March 2019 and Gunung Agung Eruption Press Release March 17, 2019).

Figure 47. A thermal anomaly was visible through thick cloud cover at the summit of Agung on 29 March 2019 less than 24 hours after a gray ash plume was reported 2,000 m above the summit. "Atmospheric Penetration" rendering (bands 12, 11, and 8A) courtesy of Sentinel Hub Playground.

The first explosion of April 2019 occurred on the 3rd (UTC); PVMBG reported the dense gray ash plume 2 km above the summit drifting W. A few hours later the Darwin VAAC raised the altitude to 6.1 km based on infrared temperatures in satellite imagery. The seismic signal lasted for three and a half minutes and the explosion was heard at the PGA Post in Rendang (12 km SW). Incandescent material fell within a radius of 2-3 km, mainly on the S flank (figure 48). Ashfall was reported in the villages of Telungbuana, Badeg, Besakih, Pempatan, Teges, and Puregai on the W and S flanks (figure 49). An explosion on 11 April also produced a dense gray ash plume that rose 2 km above the summit and drifted W. A hotspot remained about six hours later after the ash dissipated.

Figure 48. Incandescent ejecta appeared on the flanks of Agung after an eruption on 4 April 2019 (local time) as viewed from the observation post in Rendang (8 km SW). Courtesy of Jamie Sincioco.

Figure 49. Ashfall in a nearby town dusted mustard plants on 4 April 2019 from an explosion at Agung the previous day. Courtesy of Pantau.com (Photo: Antara / Nyoman Hendra).

PVMBG reported an eruption visible in the webcam early on 21 April (local time) that rose to 5.5 km altitude and drifted SW. The ash spread W and S and ash fell around Besakih (7 km SW), Rendang (8 km SW), Klungkung (25 km S), Gianyar (20 km WSW), Bangli (17 km WNW), Tabanan (50 km WSW), and at the Ngurah Rai-Denpasar Airport (60 km SW). About 15 hours later a new explosion produced a dense gray ash plume that rose to 3 km above the summit and produced incandescent ejecta in all directions as far as 3 km away (figure 50). The ash spread to the S and ashfall was reported in Besakih, Rendang, Sebudi (6 km SW), and Selat (12 km SSW). Both of the explosions were heard in Rendang and Batulompeh. The incandescent ejecta from the explosions remained within the 4-km exclusion zone. A satellite image on 23 April showed multiple thermal anomalies within the summit crater (figure 51). A dense gray plume drifted E from Agung on 29 April (30 April local time) at 4.6 km altitude. It was initially reported by ground observers, but was also visible in multispectral satellite imagery for about six hours before dissipating.

Figure 50. An explosion at Agung on 21 April 2019 sent incandescent eject 3,000 m from the summit. Courtesy of MAGMA Indonesia (Gunung Agung Eruption Press Release April 21, 2019).

Figure 51. Multiple thermal anomalies were still present within the summit crater of Agung on 23 April 2019 after two substantial explosions produced ash and incandescent ejecta around the summit two days earlier. "Atmospheric Penetration" rendering (bands 12, 11, and 8A) courtesy of Sentinel Hub Playground.

PVMBG reported an eruption on 3 May 2019 that was recorded on a seismogram with a signal that lasted for about a minute. Satellite imagery reported by the Darwin VAAC showed a growing hotspot and possible ash near the summit at 4.3 km altitude moving NE. A few days later, on 6 May, a gray ash plume rose to 5.2 km altitude and drifted slowly W before dissipating; it was accompanied by a seismic signal that lasted for about two minutes. Explosions on 12 and 18 May produced significant amounts of incandescent ejecta (figure 52). The seismic signal for the 12 May event lasted for about two minutes; no plume was observed due to fog, but incandescent ejecta was visible on the flanks and the explosion was heard at Rendang. The Darwin VAAC reported an ash plume from the explosion on 17 May (18 May local time) at 6.1 km altitude in satellite imagery moving E. They revised the altitude a short while later to 7.6 km based on IR temperature and movement; the plume drifted N, NE, and E in light and variable winds. A few hours after that it was moving NE at 7.6 km altitude and SE at 5.5 km altitude; this lasted for about 12 hours until it dissipated. Ashfall was reported in villages downwind including Cutcut, Tongtongan, Bonyoh (20 km WNW), and Temakung.

Figure 52. Explosions on 12 (left) and 18 (right) May (local time) 2019 produced substantial ejecta on the flanks of Agung visible from a distance of 10 km or more in PVMBG webcams. The ash plume from the 18 May event resulted in ashfall in numerous communities downwind. Courtesy of PVMBG (Information Eruption G. Agung, May 13, 2019, Information Eruption G. Agung, May 18, 2019).

The initial explosion on 18 May was captured by a webcam at a nearby resort and sent incandescent ejecta hundreds of meters down the NE flank within 20 seconds (figure 53). Satellite imagery on 3, 8, 13, and 18 May indicated multiple thermal anomalies growing stronger at the summit. All of the images were captured within 24 hours of an explosive event reported by PVMBG (figure 54).

Figure 53. The 18 May 2019 explosion at Agung produced an ash plume that rose to over 7 km altitude and large bombs of incandescent material that traveled hundreds of meters down the NE flank within the first 20 seconds of the explosion. Images taken from a private webcam located 12 km NE. Courtesy of Volcanoverse, used with permission.

Figure 54. Satellite images from 3, 8, 13, and 18 May 2019 at Agung showed persistent and increasing thermal anomalies within the summit crater. All images were captured within 24 hours of explosions reported by PVMBG. "Atmospheric Penetration" rendering (bands 12, 11, and 8A) courtesy of Sentinel Hub Playground.

PVMBG issued a VONA on 24 May 2019 reporting a new ash emission. They indicated that incandescent fragments were ejected 2.5-3 km in all directions from the summit, and the seismic signal lasted for four and a half minutes (figure 55). A dense gray ash plume was observed from Tulamben on the NE flank rising 2 km above the summit. Satellite imagery indicated that the plume drifted SW and ashfall was reported in the villages of Besakih, Pempatan, Menanga, Sebudi, Muncan, Amerta Bhuana, Nongan, Rendang, and at the Ngurah Rai Airport in Denpassar. Additionally, ashfall was reported in the districts of Tembuku, Bangli, and Susut (20 km SW). The Darwin VAAC reported an ash plume visible in satellite imagery at 4.6 km altitude along with a thermal anomaly and incandescent lava visible in webcam imagery. The remains of the ash plume were about 170 km S of the airport in Denpasar (60 km SW) and had nearly dissipated 18 hours after the event. According to a news article several flights to and from Australia were cancelled or diverted, though the International Gusti Ngurah Rai (IGNR) airport was not closed. On 31 May another large explosion produced the largest ash plume of the report period, rising more than 2 km above the summit (figure 56). The Darwin VAAC reported its altitude as 8.2 km drifting ESE visible in satellite data. It split into two plumes, one drifted E at 8.2 km and the other ESE at 6.1 km altitude, dissipating after about 20 hours.

Figure 55. A large explosion at Agung on 24 May 2019 produced incandescent ejecta that covered all the flanks and dispersed ash to many communities to the SW. Courtesy of PVMBG (Gunung Agung Eruption Press Release 24 May 2019 20:38 WIB, Kasbani, Ir., M.Sc.).

Figure 56. An explosion at Agung on 31 May 2019 sent an ash plume to 8.2 km altitude, the highest for the report period. Courtesy of Sutopo Purwo Nugroho, BNPB.

Geologic Background. Symmetrical Agung stratovolcano, Bali's highest and most sacred mountain, towers over the eastern end of the island. The volcano, whose name means "Paramount," rises above the SE caldera rim of neighboring Batur volcano, and the northern and southern flanks extend to the coast. The summit area extends 1.5 km E-W, with the high point on the W and a steep-walled 800-m-wide crater on the E. The Pawon cone is located low on the SE flank. Only a few eruptions dating back to the early 19th century have been recorded in historical time. The 1963-64 eruption, one of the largest in the 20th century, produced voluminous ashfall along with devastating pyroclastic flows and lahars that caused extensive damage and many fatalities.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.vsi.esdm.go.id/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); The Jakarta Post, Mount Agung eruption disrupts Australian flights, (URL: https://www.thejakartapost.com/news/2019/05/25/mount-agung-eruption-disrupts-australian-flights.html); PunapiBali (URL: http://punapibali.com/, Twitter: https://twitter.com/punapibali, image at https://twitter.com/punapibali/status/1098869352588288000/photo/1); Jamie S. Sincioco, Phillipines (URL: Twitter: https://twitter.com/jaimessincioco. Image at https://twitter.com/jaimessincioco/status/1113765842557104130/photo/1); Pantau.com (URL: https://www.pantau.com/berita/erupsi-gunung-agung-sebagian-wilayah-bali-terpapar-hujan-abu?utm_source=dlvr.it&utm_medium=twitter); Volcanoverse (URL: https://www.youtube.com/channel/UCi3T_esus8Sr9I-3W5teVQQ); Sutopo Purwo Nugroho, BNPB (Twitter: @Sutopo_PN, URL: https://twitter.com/Sutopo_PN ).

Frequently active, Indonesia's Mount Kerinci on Sumatra has been the source of numerous moderate explosive eruptions since its first recorded eruption in 1838. Intermittent explosions with ash plumes, usually multiple times per month, have characterized activity since April 2018. Similar activity continued during February-May 2019, the period covered in this report with information provided primarily by the Indonesian volcano monitoring agency, Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), MAGMA Indonesia, notices from the Darwin Volcano Ash Advisory Center (Darwin VAAC), and satellite data. PVMBG has maintained an Alert Level II (of 4) at Kerinci for several years.

On 13 February 2019 the Kerinci Volcano Observatory (KVO), part of PVMBG, noted a brownish-white ash emission that was drifting NE about 400 m above the summit. The seismicity during the event was dominated by continuous volcanic tremor. A brown ash emission was reported on 7 March 2019 that rose to 3.9 km altitude and drifted NE. Ash also drifted 1,300 m down the SE flank. Another ash plume the next morning drifted W at 4.5 km altitude, according to KVO. On 10, 11, and 13 March KVO reported brown ash plumes drifting NE from the summit at about 4.0-4.3 km altitude. The Darwin VAAC observed continuous ash emissions in satellite imagery on 15 March drifting W at 4.3 m altitude that dissipated after about 3 hours (figure 10). A gray ash emission was reported on 19 March about 600 m above the summit drifting NE; local news media noted that residents of Kayo Aro reported emissions on both 18 and 19 March (figure 11). An ash emission appeared in satellite imagery on 25 March (figure 10). On 30 March the observatory reported two ash plumes; a brown emission at 0351 UTC and a gray emission at 0746 UTC that both drifted NE at about 4.4 km altitude and dissipated within a few hours. PVMBG reported another gray ash plume the following day at a similar altitude.

Figure 10. Sentinel-2 satellite imagery of Kerinci from 15 (left) and 25 (right) March 2019 showed evidence of ash plumes rising from the summit. Kerinci's summit crater is about 500 m wide. "Geology" rendering (bands 12, 4, 2), courtesy of Sentinel Hub Playground.

Figure 11. Dense ash plumes from Kerinci were reported by local news media on 18 and 19 March 2019. Courtesy of Nusana Jambi.

Activity continued during April with a brown ash emission reported on 3 April by several different agencies; the Darwin VAAC and PVMBG daily reports noted that the plume was about 500 m above the summit (4.3 km altitude) drifting NE. KVO observed two brown ash emissions on 13 April (UTC) that rose to 4.2 km altitude and drifted NE. Satellite imagery showed minor ash emissions from the summit on 14 April; steam plumes 100-500 m above the summit characterized activity for the remainder of April (figure 12).

Figure 12. A dilute ash emission rose from the summit of Kerinci on 14 April 2019 (left); only steam emissions were present on a clear 29 April in Sentinel-2 imagery (right). "Geology" rendering (bands 12, 4, 2), courtesy of Sentinel Hub Playground.

Ashfall on the NE and S flanks within 7 km of the volcano was reported on 2 May 2019. According to a news article, at least five villages were affected late on 2 May, including Tanjung Bungo, Sangir, Sangir Tengah, Sungai Rumpun, and Bendung Air (figures 13 and 14). The smell of sulfur was apparent in the villages. Brown ash emissions were observed on 3 and 4 May that rose to 4.6 and 4.1 km altitude and drifted SE. The Darwin VAAC reported an emission on 5 May, based on a pilot report, that rose to 6.7 km altitude and drifted NE for about an hour before dissipating. A brown ash emission on 10 May rose 700 m above the summit and drifted SE. Satellite imagery captured ash emissions from the summit on 14 and 24 May (figure 15). For the remainder of the month, 300-700-m-high dense steam plumes were noted daily until PVMBG reported white and brown plumes on 26 and 27 May rising 500-1,000 m above the summit. Although thermal anomalies were not reported during the period, persistent weak SO₂ emissions were identified in TROPOMI instrument satellite data multiple times per month (figure 16).

Figure 13. Ashfall was reported from five villages on the flanks of Kerinci on 2 May 2019. Courtesy of Uzone.

Figure 14. An ash plume at Kerinci rose hundreds of meters on 2 May 2019; ashfall was reported in several nearby villages. Courtesy of Kerinci Time.

Figure 15. Ash emissions from Kerinci were captured in Sentinel-2 satellite imagery on 14 (left) and 24 (right) May 2019. The summit crater is about 500 m wide. "Geology" rendering (bands 12, 4, 2), courtesy of Sentinel Hub Playground.

Figure 16. Weak SO2 anomalies from Kerinci emissions were captured by the TROPOMI instrument on the Sentinel-5P satellite multiple times each month from February to May 2019. Courtesy of NASA Goddard Space Flight Center.

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

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.vsi.esdm.go.id/); 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/); Nuansa Jambi, Informasi Utama Jambi: (URL: https://nuansajambi.com/2019/03/20/gunung-kerinci-semburkan-asap-tebal/); Kerinci Time (URL: https://kerincitime.co.id/gunung-kerinci-semburkan-abu-vulkanik.html); Uzone.id (URL: https://news.uzone.id/gunung-kerinci-erupsi-5-desa-tertutup-abu-tebal).

Suwanosejima is an active volcanic island south of Japan in the Ryuku islands with recent activity centered at Otake crater. The current eruption began in October 2004 and activity has mostly consisted of small ash plumes, ballistic ejecta, and visible incandescence at night. This report summarizes activity during January through June 2019 and is based on reports by the Japan Meteorological Agency (JMA), and various satellite data.

Thermal activity recorded by the MIROVA system was low through January and February after a decline in November (figure 36), shown in Sentined-2 thermal infrared imagery as originating at a vent in the Otake crater (figure 37). During January an explosive event was observed at 1727 on the 3rd, producing a gray plume that rose 600 m above the crater. A white gas-and-steam plume rose to 1.5 km above the crater and nighttime incandescence was observed throughout the month. Reduced activity continued through February with no reported explosive eruptions and light gray plumes up to 900 m above the crater. Incandescence continued to be recorded at night using a sensitive surveillance camera.

Figure 36. MIROVA log radiative power plot of MODIS thermal infrared data at Suwanosejima during September 2018 through June 2019. There was reduced activity in 2019 with periods of more frequent anomalies during March and June. Courtesy of MIROVA.

Figure 37. A Sentinel-2 thermal satellite image shows Suwanosejima with the active Otake crater in the center with elevated temperatures shown as bright orange/yellow. There is a light area next to the vent that may be a gas plume. False color (urban) satellite image (bands 12, 11, 4) courtesy of Sentinel Hub Playground.

There was an increase in thermal energy detected by the MIROVA system in mid-March and there was a MODVOLC thermal alert on the 15th. Occasional small explosions occurred but no larger explosive events were recorded. A white plume was noted on the 27th rising to 900 m above the crater and an event at 1048 on the 30th produced a light-gray plume that rose to 800 m. Incandescence was only observed using a sensitive camera at night (figure 38).

Figure 38. Incandescence from the Suwanosejima Otake crater reflecting in clouds above the volcano. Courtesy of JMA (Volcanic activity of Suwanosejima March 2019).

No explosive events were observed through April. A white gas-and-steam plume rose to 1,200 m above the crater on the 19th and incandescence continued intermittently. Minor explosions were recorded on 5, 30, and 31 May, but no larger explosive events were observed during the month. The event on the 30th produced ash plume that reached 1.1 km above the crater. Similar activity continued through June with one explosive event occurring on the 2nd. Overall, there was a reduction in the number of ash plumes erupted during this period compared to previous months (figure 39).

Figure 39. Observed activity at Suwanosejima for the year ending in July 2019. The black vertical bars represent steam, gas, or ash plume heights (scale in meters on the left axis), yellow diamonds represent incandescence observed in webcams, gray volcano symbols along the top are explosions accompanied by ash plumes, red volcano symbols represent large explosions with ash plumes. Courtesy of JMA (Volcanic activity of Suwanosejima June 2019).

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

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

The Great Sitkin volcano is located about 40 km NE of Adak Island in the Aleutian Islands and has had a few short-lived eruptions over the past 100 years. Prior to the latest activity in early June 2019 described below, small phreatic explosions occurred in June and August 2018 (BGVN 43:09). An eruption in 1974 produced a lava dome in the center of the crater. The Alaska Volcano Observatory (AVO) is the primary source of information for this September 2018-June 2019 reporting period.

Low-level unrest occurred from September 2018 through February 2019 with slightly elevated seismic activity (figure 6). Small explosions were seismically detected by AVO on 30 October, 5 and 16 November, and 11 December 2018, but they were not seen in regional infrasound data and satellite data did not show an ash cloud.

On 1, 7, and 9 June 2019, AVO reported small steam explosions as well as slightly elevated seismic activity. Steam plumes and surficial evidence of an explosion were not observed during these events. On 18 June 2019 weakly elevated surface temperatures were recorded, field crews working on Adak observed some steam emissions, and a gas flight was conducted. Elevated concentrations of carbon dioxide detected above the lava dome were likely associated with the steam explosions earlier in the month (figures 7 and 8). From 23 June through the end of the month seismicity began to decline back to background levels.

Figure 6. A steam plume was seen at the summit of Great Sitkin on 7 December 2018. Photo by Andy Lewis and Bob Boyd; courtesy of AVO/USGS.

Figure 7. Some degassing was observed on the southern flank of the Great Sitkin during an overflight on 18 June 2019. Photo by Laura Clor; image courtesy of AVO/USGS.

Figure 8. View of Great Sitkin with white plumes rising from the summit on 20 June 2019. Photo by Laura Clor, courtesy of AVO/USGS.

Geologic Background. The Great Sitkin volcano forms much of the northern side of Great Sitkin Island. A younger parasitic volcano capped by a small, 0.8 x 1.2 km ice-filled summit caldera was constructed within a large late-Pleistocene or early Holocene scarp formed by massive edifice failure that truncated an ancestral volcano and produced a submarine debris avalanche. Deposits from this and an older debris avalanche from a source to the south cover a broad area of the ocean floor north of the volcano. The summit lies along the eastern rim of the younger collapse scarp. Deposits from an earlier caldera-forming eruption of unknown age cover the flanks of the island to a depth up to 6 m. The small younger caldera was partially filled by lava domes emplaced in 1945 and 1974, and five small older flank lava domes, two of which lie on the coastline, were constructed along northwest- and NNW-trending lines. Hot springs, mud pots, and fumaroles occur near the head of Big Fox Creek, south of the volcano. Historical eruptions have been recorded since the late-19th century.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA (URL: https://avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://dggs.alaska.gov/).

Ibu volcano on Halmahera island in Indonesia began the current eruption episode on 5 April 2008. Since then, activity has largely consisted of small ash plumes with less frequent lava flows, lava dome growth, avalanches, and larger ash plumes up to 5.5 km above the crater. This report summarizes activity during December 2018 through June 2019 and is based on Volcano Observatory Notice for Aviation (VONA) reports by MAGMA Indonesia, reports by Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG) and Badan Nasional Penanggulangan Bencana (BNPB), and various satellite data.

During December PVMBG reported ash plumes ranging from 200 to 800 m above the crater. There were 11 MODVOLC thermal alerts that registered during 1-12 December. An explosion on 12 January 2019 produced an ash plume that reached 800 m above the crater and dispersed to the S (figure 15). A report released for this event by Sutopo at BNPB said that Ibu had erupted almost every day over the past three months; an example given was of activity on 10 January consisting of 80 explosions. There were four MODVOLC thermal alerts through the month.

Figure 15. An eruption at Ibu at 1712 on 21 January 2019 produced an ash plume that rose to 800 m above the crater. Courtesy of BNPB (color adjusted).

Throughout February explosions frequently produced ash plumes as high as 800 m above the crater, and nine MODVOLC thermal alerts were issued. Daily reports showed variable plume heights of 200-800 m most days throughout the month. Wind directions varied and dispersed the plumes in all directions. A VONA released at 1850 on 6 February reported an ash plume that rose to 1,925 m altitude (around 600 m above the summit) and dispersed S. Activity continued through March with the Darwin VAAC and PVMBG reporting explosions producing ash plumes to heights of 200-800 m above the crater and dispersing in various directions. There were ten MODVOLC alerts through the month.

Similar activity continued through April, May, and June, with ash plumes reaching 200-800 m above the crater. There were 12, 6, and 15 MODVOLC Alerts in April, May, and June, respectively.

Planet Scope satellite images show activity at a two vents near the center of the crater that were producing small lava flows from February through June (figure 16). Thermal anomalies were frequent during December 2018 through June 2019 across MODVOLC, MIROVA, and Sentinel-2 infrared data (figures 17 and 18). Sentinel-2 data showed minor variation in the location of thermal anomalies within the crater, possibly indicating lava flow activity, and MIROVA data showed relatively constant activity with a few reductions in thermal activity during January and February.

Figure 16. Planet Scope natural color satellite images showing activity in the Ibu crater during January through June 2019, with white arrows indicating sites of activity. One vent is visible in the 21 February image, and a 330-m-long (from the far side of the vent) lava flow with flow ridges had developed by 24 March. A second vent was active by 12 May with a new lava flow reaching a maximum length of 520 m. Activity was centered back at the previous vent by 23-27 June. Natural color Planet Scope Imagery, copyright 2019 Planet Labs, Inc.

Figure 17. Examples of thermal activity in the Ibu crater during January through May 2019. These Sentinel-2 satellite images show variations in hot areas in the crater due to a vent producing a small lava flow. Sentinel-2 false color (urban) images (bands 12, 11, 4) courtesy of Sentinel Hub Playground.

Figure 18. MIROVA log radiative power plot of MODIS thermal infrared at Ibu from September 2018 through June 2019. The registered energy was relatively stable through December, with breaks in January and February. Regular thermal anomalies continued with slight variation through to the end of June. Courtesy of MIROVA.

Geologic Background. The truncated summit of Gunung Ibu stratovolcano along the NW coast of Halmahera Island has large nested summit craters. The inner crater, 1 km wide and 400 m deep, contained several small crater lakes through much of historical time. The outer crater, 1.2 km wide, is breached on the north side, creating a steep-walled valley. A large parasitic cone is located ENE of the summit. A smaller one to the WSW has fed a lava flow down the W flank. A group of maars is located below the N and W flanks. Only a few eruptions have been recorded in historical time, the first a small explosive eruption from the summit crater in 1911. An eruption producing a lava dome that eventually covered much of the floor of the inner summit crater began in December 1998.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Badan Nasional Penanggulangan Bencana (BNPB), National Disaster Management Agency, Graha BNPB - Jl. Scout Kav.38, East Jakarta 13120, Indonesia (URL: http://www.bnpb.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.vsi.esdm.go.id/); 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/); 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); Planet Labs, Inc. (URL: https://www.planet.com/).

The Ebeko volcano, located on the northern end of the Paramushir Island in the Kuril Islands, consists of many craters, lakes, and thermal features and has been frequently erupting since late February 2017. Typical activity includes ash plumes, explosive eruptions, and gas-and-steam activity. The previous report through November 2018 (BGVN 43:12) described frequent ash explosions that sometimes caused ashfall in Severo-Kurilsk (7 km E). The primary source of information is the Kamchatka Volcanic Eruptions Response Team (KVERT). This report updates the volcanic activity at Ebeko for December 2018 through May 2019.

Frequent moderate explosive activity continued after November 2018. Volcanologists in Severo-Kurilsk observed explosions sending up ash, which drifted N, NE, and E, resulting in ash falls on Severo-Kurilsk on 28 different days between December 2018 and March 2019. On 25 December 2018 an explosion sent ash up to a maximum altitude of 4.5 km and then drifted N for about 5 km. Explosions occurring on 8-10 March 2019 sent ash up to an altitude of 4 km, resulting in ashfall on Severo-Kurilsk on 9-10 March 2019. An ash plume from these explosions rose to a height of 2.5 km and drifted to a maximum distance of 30 km ENE.

Satellite data analyzed by KVERT registered 12 thermal anomalies from December 2018 through May 2019. According to satellite data analyzed by MIROVA (Middle InfraRed Observation of Volcanic Activity), only one thermal anomaly was recorded from December 2018-May 2019, and no hotspot pixels were recognized using satellite thermal data from the MODVOLC algorithm.

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/); 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/).

Klyuchevskoy has had alternating eruptive and less active periods since August 2015. Activity has included lava flows, a growing cinder cone, thermal anomalies, gas-and-steam plumes, and ash explosions. Though some eruptions occur near the summit crater, major explosive and effusive eruptions have also occurred from flank craters (BGVN 42:04 and 43:05). Intermittent moderate gas-and-steam and ash emissions were previously reported from mid-February to mid-August 2018. The Kamchatka Volcanic Eruptions Response Team (KVERT) is the primary source of information for this September 2018-June 2019 reporting period.

KVERT reported that moderate gas-and-steam activity, some of which contained a small amount of ash, and weak thermal anomalies occurred intermittently from the beginning of September 2018 through mid-April 2019. On 21-22 April 2019 webcam data showed a gas-and-steam plume extending about 160 km SE (figure 31). Moderate Strombolian-type volcanism began late April 2019 and continued intermittently through June 2019. On 11-12 June webcam data showed explosions that sent ash up to a maximum altitude of 6 km, with the resulting ash plume extending about 200 km WNW.

Figure 31. Gas-and-steam plume containing some amount of ash rising from the summit of Klyuchevskoy on 22 April 2019. Photo by A. Klimova, courtesy of Institute of Volcanology and Seismology (IVS FEB RAS).

Thermal anomalies were noted by KVERT during two days in September 2018, six days in April 2019, eleven days in May 2019, and six days in June 2019. MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed infrequent weak thermal anomalies December 2018 through early May 2019.

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

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/).

At the request of the Civil Defense office of the CNMI, a team from the USGS and the Hawaii Institute of Geophysics monitored seismicity, deformation, and thermal activity, and investigated the geologic and eruptive history of the island during fieldwork 19-27 April. The following is from their preliminary report.

Geologic overview. "Anatahan, ~9 km long by 3 km wide and elongate along an E-W axis, is topped by a compound caldera of both collapse and explosive origin. The caldera's shape roughly parallels the outline of the island. It can generally be divided into E and W craters; the floor of the E bay is ~250 m below the floor of the main caldera and contains a lake. The W crater essentially comprises the caldera floor and is made up of at least five phreatomagmatic explosion craters. The two highest points on the volcano are peaks at the E and W ends of the caldera (540 and 790 m above sea level, respectively). Slopes dip steeply to the sea from these high points as well as from the caldera walls, at angles averaging about 25°. A number of extra-caldera explosion craters have been identified, especially at the E end of the island.

"The oldest units are interbedded phreatomagmatic ashes and andesite flows exposed at the coast. Most of the flows are plagioclase-phyric and 5-15 m thick, and a number are glassy or partly glassy with well-developed flow lineations. The interbedded ashes and flows dip 20-25° in a generally radial direction from the center of the volcano. At numerous locations along the coast, oxidized cinder layers are exposed, usually accompanied by co-magmatic flows. Along both the N and S coasts, slopes are significantly steeper near the ocean, and ocean-induced mass wasting is undoubtedly the cause. Additionally, however, an E-W-trending set of faults on the S flank has probably contributed to both the steepness of the slopes and the straightness of the coastline.

"The youngest volcanic unit is a light brown to gray phreatomagmatic surge deposit. It is very young, in places plastered on the wave-cut cliffs just above the coastline, or even banked against large talus boulders. This ash blankets all surfaces on the slopes and within the caldera, and has only been eroded from gullies. Its thickness is extremely variable, and isopachs do not give an indication of source location. This deposit appears to be no more than a few hundred years old. Human cultural remains were found under 4 m of base surge deposits, and will be radiocarbon dated to give the deposit's maximum age.

Present activity. "An acid lake and a number of acid pools were present within the E crater. The water in the pools was boiling, turbid due to very fine suspended sediment, and had a pH ranging between 0.7 and 1.2. Water within the larger lake was clear except for areas where upwelling stirred up sediment. Water collected at the edge of one of the upwelling areas was warm and had a pH ranging between 1.2 and 1.9. A large area of vegetation had been killed near the lake and ponds, most likely due to overflows of the hot acid water. The air around the lake was breathable, smelled slightly of sulfur, and probably contained an elevated amount of CO2. Gas had accumulated in one of the water samples that had been collected gas-free. A small explosion crater at the W end of the W bay was observed to be steaming.

"Seismicity was monitored from the village and periodically from the center of the W bay during the 9-day investigation. From 19 until 25 April, daily seismicity consisted of 3-4 distant earthquakes, possibly aftershocks of the 6 April M 7.5 earthquake in the Mariana Trench (centered at 15.27°N, 147.53°E, 32 km depth). Additionally, 3-4 local events of M 3-4 were recorded each day, usually in pairs (figure 1 and table 2). Only one was felt.

Figure 1. Portion of seismogram from the temporary station on Anatahan, 22-23 April 1990. Courtesy of Robert Koyanagi, USGS.

Table 2. Earthquakes recorded on Anatahan by the USGS, 19-27 April 1990, using a revolving drum portable seismograph with 1.0 Hz geophone. Magnitude threshold was greater than 0.5 for distances less than 10 km. Events with distances less than 40 km may be volcanically related and located within ~10 km of Anatahan at varying depths beneath the island (the 10 km distance also includes Sarigan Island, N of Anatahan). Seismic recording did not begin until late 19 April. Data courtesy of R. Koyanagi.

"An EDM network and two radial tilt stations were installed and monitored during the investigation. The tilt stations showed no changes. Because of limited helicopter time, every line of the EDM network was not reoccupied every day, but extensions of 6-91 mm were recorded between measurements on 25 and 26 April."

Geologic Background. The elongate, 9-km-long island of Anatahan in the central Mariana Islands consists of a large stratovolcano with a 2.3 x 5 km compound summit caldera. The larger western portion of the caldera is 2.3 x 3 km wide, and its western rim forms the island's high point. Ponded lava flows overlain by pyroclastic deposits fill the floor of the western caldera, whose SW side is cut by a fresh-looking smaller crater. The 2-km-wide eastern portion of the caldera contained a steep-walled inner crater whose floor prior to the 2003 eruption was only 68 m above sea level. A submarine cone, named NE Anatahan, rises to within 460 m of the sea surface on the NE flank, and numerous other submarine vents are found on the NE-to-SE flanks. Sparseness of vegetation on the most recent lava flows had indicated that they were of Holocene age, but the first historical eruption did not occur until May 2003, when a large explosive eruption took place forming a new crater inside the eastern caldera.

Information Contacts: F. Sasamoto, Office of Civil Defense, Saipan; J. Lockwood, M. Sako, R. Koyanagi, and G. Kojima, HVO; S. Rowland, Univ of Hawaii.

A significant increase in Strombolian activity during March started with a corresponding increase in volcanic earthquakes at the beginning of the month. Between 20 and 24 March, substantially enhanced tremor accompanied lava production from Crater C. A small flow descended the NW flank toward the Río Tabacón, reaching ~700 m elevation. Lava continued to flow onto the NW flank in April, and a second flow began to advance down the S flank. Strombolian eruptions were larger and more frequent than in previous months and gas emission was continuous. Blocks and bombs fell 800-900 m from Crater C. Ash columns rose as much as 1 km and were carried by the wind toward the NW, W, and SW, the area most affected by acid rain. The activity occasionally produced small nuées ardentes.

Arenal remained highly active 1-22 April during 24 hour/day observations by Smithsonian Research Expeditions. Activity consisted of frequent pyroclastic events as well as production of small block lava flows that advanced down the S and WNW flanks. Flow movement down the NNW slope, the dominant direction during the past 3 years, had ceased. A pronounced leveed ridge had been produced by the most active (WNW) flow. Smaller flows were extruded from a small fissure ~100 m S of the summit crater rim, and a few hundred meters S from a spillover along the SSE part of the crater rim. Frequent explosions continued to eject blocks and bombs around the crater. Some fell more than a kilometer away, creating extremely hazardous conditions for ground observation of the crater area. Because the active flow fronts were all within the impact zone, there were no direct observations of flow rates.

Over the 23-day period, 807 pyroclastic events were recorded (figure 25), including sharp explosions [Type 1 in 14:06]; intense gas, block and bomb fountains [Type 2 in 14:06]; and rhythmic, less intense gas emissions, commonly accompanied by blocks and bombs [Type 3 in 14:06]. A "flashing arc", as described by Perret (1912) from eruptions at Vesuvius and Mont. Pelée, was observed from an explosion on 4 April at 1047 (figure 26). Two blocky pyroclastic flows moved down the S flank, each reaching ~800 m from the crater.

Figure 25. Number of pyroclastic events/day at Arenal, 2-24 April 1990. Poor weather conditions frequently prevented viewing of the summit, so different activity types were identified by sound: type 1 ('Explosions') are sharp explosions; type 2 ('Whooshes') corresponds to intense gas, block, and bomb fountains; type 3 ('Chugs') are rhythmic, less intense gas emissions commonly accompanied by blocks and bombs.

Figure 26. Airwave train of an explosion from Arenal that produced a "flashing arc" on 4 April at 1047. The tape recording from which this was derived was made 2.7 km S of the crater, where the event's maximum sound intensity was ~90 dB.

The level of pyroclastic activity (figure 27), sampled at various times over the past 1,000 days at 10-23-day intervals, as well as lava flow directions and rates, continued to show wide variations on both day-to-day and longer time scales. However, the volume of individual lava and pyroclastic flows appears to have gradually declined over the 1,000-day period, with few reaching more than ~800 m from the summit crater. The crater appeared to contain at least two vents, with the S vent the source of much of the pyroclastic activity, while the lava flows emerged from the vent to its N and W.

Figure 27. Comparisons of the number of pyroclastic events heard and (sometimes) seen at the Arenal Observatory, 2.7 km S of the summit crater, by Earthwatch and Smithsonian Research Expeditions volunteers since 27 April 1987. Activity types are as in figure 25. Sampling was over 10-22-day intervals during the periods shown.

Chemical compositions of the highly phyric basaltic andesitic blocks and bombs (table 4) have remained essentially constant over the past 3 years, reflecting both a constant source composition and high magma viscosity. However, petrography has varied, particularly the glass/crystal ratio, high in bombs and near-vent lava flows and much lower in blocks.

Table 4. Compositions of 1987-89 eruption products from Arenal; XRF analyses by Joseph Nelen, SI. The August 1989 lava flow sample is from J. Barquero and E. Fernández. pf = bomb from pyroclastic flow; b = ejected block; B = ejected scoriaceous bomb; L = lava flow.

Reference. Perret, F.A., 1912, The flashing arcs: a volcanic phenomenon: American Journal of Science, ser. 4, v. 34, p. 329-333.

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: W. Melson, SI; E. Fernández and J. Barquero, OVSICORI; Red Sismológica Nacional, ICE; Escuela Centroamericana de Geología, Univ de Costa Rica.

"In the current period of social unrest on Bougainville Island Island, no instrumental data is being recorded, and the only information on Bagana's activity is from visual observations from a site 15 km SSW of the volcano.

"When observations resumed on 3 April, Bagana was in a fairly high level of activity. Thick, white, ash-laden vapour was being forcefully emitted from the summit area. [An explosion in the summit crater] on the 3rd produced a black column, and loud rumbling noises were heard until the 4th.

"Numerous rockfalls (including daytime glowing avalanches) were observed in early April on the SE and E flanks of the cone, where a slowly progressing blocky lava flow has been active since 1987. This activity together with the reportedly stronger vapour and ash emission may suggest that a new pulse of viscous lava extrusion took place in the summit crater in the first few days of April.

"The mountain was often covered by atmospheric clouds or rainstorms, but a weak, night, summit glow was intermittently observed until the 21st, with occasional (night-glowing) rockfalls occurring until the 24th."

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

Information Contacts: P. de Saint-Ours and C. McKee, RVO.

"Seismic activity . . . decreased markedly in April. Following a period of intense activity in early March, the frequency of occurrence and magnitude of earthquakes decreased gradually, with only 27 events of M_L >=3 recorded in April (from a total of 200 events picked up by the Ulawun station, 25 km away). Event frequency ranged between 2 and 7/day. Two isolated earthquakes of M_L 5.6 and 4.2 occurred on the 26th."

Geologic Background. Symmetrical 2248-m-high Bamus volcano, also referred to locally as the South Son, is located SW of Ulawun volcano, known as the Father. These two volcanoes are the highest in the 1000-km-long Bismarck volcanic arc. The andesitic stratovolcano is draped by rainforest and contains a breached summit crater filled with a lava dome. A satellitic cone is located on the southern flank, and a prominent 1.5-km-wide crater with two small adjacent cones is situated halfway up the SE flank. Young pyroclastic-flow deposits are found on the volcano's flanks, and villagers describe an eruption that took place during the late 19th century.

Information Contacts: P. de Saint-Ours, and C. McKee, RVO.

After ~15 days of increased seismicity, an eruption began on 18 April at 1252. Lava production occurred from the SE part of the Enclos Fouqué caldera, with vigorous fountains (~30-50 m high) that built the "Catherine N" eruptive crater, and extrusion of flows that advanced down the Grand Brûlé area. Poor weather during the eruption severely hampered observations.

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: J-P. Toutain and P. Taochy, OVPDLF; P. Bachelery, Univ de la Réunion; J-L. Cheminée, IPGP.

Small phreatic ash emissions were frequent through March, but the last episode was recorded on the 29th (table 2), and none were detected in April. Incandescence continued in April, mainly from the SE part of the crater, which showed changes in morphology, fumarolic activity, and temperature.

The number of seismic events increased 21% in April (to 1,343 low-frequency and 154 high-frequency earthquakes) from March values, but energy release declined 17% in April, to 1.48 x 108 and 7.49 x 107 ergs for high- and low-frequency shocks respectively. The month's largest earthquake reached M 2. Long-period events and tremor bursts decreased in both number and maximum reduced displacement. Continuous tremor remained at very low levels and showed no changes in characteristics. Most high-frequency earthquakes were centered in one of two main source regions, W of the active crater at 2-6 km depth, or SSE of the crater at 2-4 km depth. Two peaks in seismicity were noted. On 4 April, 47 high-frequency earthquakes were recorded, and on the 24th there were 115 low-frequency shocks, 83 long-period events, and 16 bursts of spasmodic tremor.

Deformation data showed no significant changes. Twelve COSPEC measurements of SO₂ emission yielded rates of 961-4,078 t/d, with a mean of 2,146 t/d (calculated using measured wind speeds). Maximum and minimum values occurred on 14 and 16 April respectively.

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

Information Contacts: INGEOMINAS-OVP.

Press reports indicated that an eruption began at 1847 on 25 April, ejecting a thick reddish column ~ 1.5 km high. Authorities inspected areas believed to be at risk from lava flows, but did not immediately order evacuations.

Geologic Background. Gamalama is a near-conical stratovolcano that comprises the entire island of Ternate off the western coast of Halmahera, and is one of Indonesia's most active volcanoes. The island was a major regional center in the Portuguese and Dutch spice trade for several centuries, which contributed to the thorough documentation of Gamalama's historical activity. Three cones, progressively younger to the north, form the summit. Several maars and vents define a rift zone, parallel to the Halmahera island arc, that cuts the volcano. Eruptions, recorded frequently since the 16th century, typically originated from the summit craters, although flank eruptions have occurred in 1763, 1770, 1775, and 1962-63.

Information Contacts: Jakarta Domestic Service.

Fumarolic activity continued on the NE flank, with a mean temperature of 89°C. The lake in the main crater had disappeared. Small landslides persisted on the W and N crater walls.

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: J. Barquero, OVSICORI.

. . . lava production continued from Kupaianaha vent, feeding flows that overran dozens of houses in a community near the coast. Lava production halted briefly in early April and early May but promptly resumed each time, and activity remained vigorous in mid-May. Shallow tremor . . . continued at a low level.

Lava that had advanced to just above [Hwy 130] by the end of March crossed the highway on 2 April (100 m E of its intersection with Highway 137). A small N-facing fault scarp (the Hakuma horst) blocked the flow's direct path to the sea, turning it ESE toward Kalapana Gardens subdivision, in a low-lying area below the scarp (figure 68). Lava entered the subdivision's W edge on 3 April, destroying two houses the next morning, but lava production stopped late that day. Tremor amplitude near Kupaianaha decreased appreciably 4-6 April while lava production was stopped. As in previous instances, USGS geologists believe that the pause in lava production was due to blockage in the upper East rift zone. The summit responded with a sequence of deflation and shallow tremor followed by inflation and an increased number of shallow microearthquakes. Eruptive activity resumed during the night of 6-7 April. By the time of an early morning overflight, lava had reoccupied the tube system along the E side of the flow field, and numerous breakouts were occurring along much of its length, between ~600 and 120 m (1,950 and 400 ft) altitude. The lava formed three flows, the largest advancing along roughly the same path as previous flows toward Hwy 130.

Figure 68. Kalapana Gardens and the E side of Royal Gardens subdivision on Kilauea's S flank, April 1990.

The primary flow moved down the main flow field to ~120 m (400 ft) elevation, then turned E, following approximately the same path as the previous Kalapana Gardens flow. The lava advanced slowly, but by 13 April two separate lobes had crossed Highway 130, one on top of the early April flow along the Hakuma horst, the other farther E. The W lobe entered Kalapana Gardens on 17 April. The two lobes merged by the 20th. Two homes just outside Kalapana Gardens were destroyed on 13 and 15 April, and destruction of houses within the subdivision began on 18 April. By the end of the month, the April flows had overrun nearly 4 dozen houses, had cut off the main access road into the subdivision (Highway 137) and were moving along both Highway 137 and the fault scarp below it. By 3 May, the flows reached Kalapana village, an older settlement to the E. As of 5 May, the Hawaii County Civil Defense Agency reported that the eruption had destroyed 63 houses since 4 April.

A smaller flow, originating from the breakout at ~600 m elevation, moved diagonally toward the E side of the lava field . . . into the "woodchip area," then onto the December 1986 flow. Lava in this area advanced slowly through April, reaching 30 m (100 ft.) elevation by the end of the month. Another small flow originated from a breakout high in the tube system at slightly above 600 m (2,050 ft) elevation, cutting across lava from Kupaianaha vent and advancing W onto earlier aa lava from Pu`u `O`o . . . . This flow entered the extreme NE corner of Royal Gardens subdivision by mid-April, but moved down the E edge of the subdivision without threatening any homes, reaching ~250 m (800 ft) elevation by the end of the month.

When lava production halted briefly on 7 May [see also 15:3], two lobes had reached the water. One partially filled a spring-fed brackish water inlet behind the fault scarp, and the second (W) lobe reached the open ocean (at Harry K. Brown Park) E of Kalapana Gardens. Lava production resumed during the evening of 9 May. Onset of the renewed activity was gradual, but the eruption was again vigorous by the 11th. On 14 May, lava traveling over roughly the same route as previous flows had reached 120 m (400 ft) elevation and was again turning E toward Kalapana Gardens.

Kupaianaha vent . . . generally remained covered with frozen lava at ~18 m below the rim throughout April, having effectively become part of the tube system. After lava production resumed on 9 May, lava in the pond was at times again actively overturning. Lava also returned to the bottom of Pu`u `O`o after 9 May, but dimensions were difficult to estimate at the base of the 180-m-deep crater.

April seismicity generally conformed to the pattern of crustal earthquakes that has persisted beneath Kilauea and the SE part of Mauna Loa. Small (M <2.0) shallow (<5 km) events were mainly centered beneath Kilauea's summit and East rift zone. Of the hundreds of earthquakes processed in April, eight ranged from magnitude 3.0 to 4.4.

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

Information Contacts: C. Heliker, P. Okubo, and R. Koyanagi, HVO; AP.

"Activity was limited to weak or moderate emissions of white vapour from Crater 2, with a weak, steady, red glow at night from 25 March until 6 April, and 9-11 and 27-28 April. Occasional weak, deep rumbling noises were heard on 3 consecutive days 12-14 April. Crater 3 remained inactive, apart from thin white vapour released by fumaroles in the crater."

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: P. de Saint-Ours and C. McKee, RVO.

Field observations suggest that the dome extrusion . . . has stopped since at least November and that the dome has continued to collapse above a withdrawing, degassing, magma column, accompanied by small, mainly phreatomagmatic eruptions.

During a summit climb by C. Oppenheimer on 4 April, little activity was seen on the collapsed dome region during daylight. Almost all of the visibly fuming vents were located beyond its margins, particularly on the E side where several powerful fumaroles were active (figure 6). After dark, very few if any of those vents were seen to be incandescent. The collapsed dome, however, showed numerous glowing red patches, presumed to be high-temperature fumarolic vents concentrated along ring fractures (figure 7). Individual vents were probably <0.5-1 m across; the majority appeared to be only a few centimeters across but formed clusters along roughly arcuate trends close to the edge of the collapsed dome. A broad area in the dome's center had no incandescent sites. There were a few groups of incandescent fumarolic vents beyond the collapsed dome, at locations that seemed to correspond spatially to distinct hot pixels on Landsat TM images of October-December 1989 (processed at Open Univ).

Figure 6. Lascar's active crater in daylight, 4 April 1990, showing locations of strong fumaroles. Sketch by C. Oppenheimer.

Figure 7. Lascar's active crater at night, 4 April 1990. Dark spots on the collapsed dome and crater walls represent incandescent areas. Note that north is down, the opposite orientation to figure 6. Sketch by C. Oppenheimer.

The highest brightness temperature, measured over a vent close to the E margin of the collapsed dome (by an 0.8-1.1 mm infrared thermometer) was 787°C. The glowing region filled about 1/6 of the instrument's field of view; the temperature measured around the incandescent vent was ~540°C. Oppenheimer noted that use of the Planck function suggests an actual temperature of the glowing vent, and therefore the gas, of ~940°C.

A seismometer (Portable Kinemetrics MEQ-800) installed 17 km W of the volcano (in the village of Talabre) began recording local seismic activity on 4 April. The seismic station was established by Juan Thomas (Antofagasta Branch, Dept de Geofísica, Univ de Chile) who also trained the village teacher, Manuel Castillo, in its operation. A second seismometer, installed the next day 7.5 km from the volcano (at Tumbre), had to be retired 2 days later because of logistics and operation problems. Installation was supported by Nelson Allendes and data interpreted by Sergio Barrientos (both with the Dept de Geología y Geofísica, Univ de Chile, Santiago).

Seismograms 4-19 April indicated that Lascar's seismicity was limited to tremor every 2-3 minutes, interpreted as magma movements in a chamber of unknown depth. Geologists suggested that the absence of discrete earthquakes could indicate that there was no rupturing of material adjusting to pressure from ascending magma. Installation of the Talabre seismometer is scheduled to end in late May. However, strong recommendations were made to local authorities that permanent monitoring of Lascar be established with telemetrically controlled seismometers, given its distance from any research center or large city (270 km from Antofagasta and 1,200 km from Santiago).

A small eruptive episode was observed on 6 April at 0840 from Talabre and by MINSAL geologists in Toconao. A pale grayish cloud rose to ~1,000 m above the volcano in 1-2 minutes. No sounds were audible during the activity, which appeared to be phreatomagmatic. The seismometers at Talabre and Tumbre recorded no seismicity at the time of the eruptive episode. During the following 20 minutes, the plume was dispersed to the SE, rapidly turning white. Some ash could be seen falling from its base. By 0910, the plume was indistinguishable from weather clouds and the normal vapor plume had reappeared, rising to ~300 m above the crater rim. The vapor plume was weaker than normal 7-8 April, reaching <100 m above the rim, but had strengthened to the usual 900 m height by the 9th.

An ascent of the volcano's S side by Steve Matthews on 12 April showed the dome to be essentially unchanged, with continuing strong fumarolic activity. Fresh tension cracks just outside the N margin of the dome, produced by further collapse, were photographed. Geologists interpreted the eruptive episode as the result of a dome collapse event, given the tension cracking and lack of associated seismicity.

Geologic Background. Láscar is the most active volcano of the northern Chilean Andes. The andesitic-to-dacitic stratovolcano contains six overlapping summit craters. Prominent lava flows descend its NW flanks. An older, higher stratovolcano 5 km E, Volcán Aguas Calientes, displays a well-developed summit crater and a probable Holocene lava flow near its summit (de Silva and Francis, 1991). Láscar consists of two major edifices; activity began at the eastern volcano and then shifted to the western cone. The largest eruption took place about 26,500 years ago, and following the eruption of the Tumbres scoria flow about 9000 years ago, activity shifted back to the eastern edifice, where three overlapping craters were formed. Frequent small-to-moderate explosive eruptions have been recorded since the mid-19th century, along with periodic larger eruptions that produced ashfall hundreds of kilometers away. The largest historical eruption took place in 1993, producing pyroclastic flows to 8.5 km NW of the summit and ashfall in Buenos Aires.

Information Contacts: M. Gardeweg, SERNAGEOMIN, Santiago; S. Barrientos, Univ de Chile, Santiago; J. Thomas, Proyecto Sismológico Antofagasta; S. Matthews, Univ College London; C. Oppenheimer, Open Univ.

Earthquake swarms continued in the S moat through early May, with bursts of activity (magnitude greater than or equal to 2.5) on 28 and 30 March, 18-20 April, and 6-7 May. The 6-7 May swarm was the most intense activity within the caldera since 1983-84. It began with an earthquake of about M 2.7 on 6 May at 2,238 and produced more than 300 of M >0.5 over the next 24 hours. The earthquakes included nearly 20 of magnitude greater than or equal to 2.5 and three of magnitude greater than or equal to 3, the largest of which was about M 3.5 at 0241. This swarm, as with most of the others that have occurred since the beginning of the year, was centered in the S moat, ~4 km E of the town of Mammoth Lakes (figure 12). Focal depths ranged from <3 km to as deep as 10 km, with most concentrated between 5 and 8 km (figure 13). Earthquakes in the S moat area as small as about M 2.5 are felt in Mammoth Lakes, and residents reported feeling some 15-20 events during a 5 1/2-hour period starting 6 May at 2238.

Figure 12. Epicenters of earthquakes in the Long Valley Caldera region, 1 January-10 May 1990. Courtesy of D. Hill.

Figure 13. E-W cross-section from U to U' (figure 12), showing focal depths of Long Valley area earthquakes, 1 January-10 May 1990. Courtesy of David Hill.

An approximately 5-fold increase in extensional strain across the S moat and resurgent dome began to be recorded by the caldera's 2-color geodimeter network in September 1989. The extension rate continued to average 4-5 ppm/year through early May, although recent measurements indicated that the rate may be slowing somewhat. Small strain changes, apparently associated with the 6-7 May swarm, were detected by borehole dilatometers at distances of 6 and 10 km. The changes were generally ~0.03-0.05 microstrain.

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: D. Hill, USGS Menlo Park.

"A final cumulative ashfall isopach map was made after the Navidad cone eruption, which ended by 23-25 January 1990. The 3 February 1989 preliminary map (see figure 6) was based on 67 data points. After the eruption ended, 40 were checked, but only 19 sites were considered valid. All were located in open spaces within the forest, on flat surfaces and with some grass; therefore, wind or water action has been minimized, although some variation could have been caused by minor ash removal from leaves through wind action. Values increased by 3-19 times the 3 February 1989 measurements.

"Ash thicknesses at six sites previously sampled in February had increased 3- to 4-fold when measured again by 25 September 1989. Consequently rejected were all new measurements <3x those of February, and those that were meaningless in the context of the 19 chosen values. Among them, nine had <70% of the February value x 3, and were located in open windy areas (in the Colorado and Lonquimay valleys). The other 12 sites had more than the February value x 3 and were located in the Naranjo River valley and the E slope of the Cordillera Las Raíces (figure 17). Undoubtedly, the main ash removal agent has been the wind. Water action was observed on slopes less than 20°, and big volumes of ash had been carried down the Cautín and Naranjo headwaters, burying trees, bushes, and wire fences."

Figure 17. Final ash isopach map (in centimeters) after the full 13 months of the Lonquimay eruption. Courtesy of H. Moreno and J. Naranjo.

Geologic Background. Lonquimay is a small, flat-topped, symmetrical stratovolcano of late-Pleistocene to dominantly Holocene age immediately SE of Tolguaca volcano. A glacier fills its summit crater and flows down the S flank. It is dominantly andesitic, but basalt and dacite are also found. The prominent NE-SW Cordón Fissural Oriental fissure zone cuts across the entire volcano. A series of NE-flank vents and scoria cones were built along an E-W fissure, some of which have been the source of voluminous lava flows, including those during 1887-90 and 1988-90, that extended out to 10 km.

Information Contacts: H. Moreno, Univ de Chile; J. Naranjo, SERNAGEOMIN, Santiago.

"Activity remained at a low level. The summit craters were often obscured but when clear were seen to release white vapour in weak amounts. Occasionally, Southern Crater emissions were greyish, containing a little ash. These emissions were accompanied by weak, deep, rumbling sounds. Seismicity was low throughout the month, while the water tube tiltmeter accumulated an unusual radial deflation of 4.5 µrad."

Geologic Background. The 10-km-wide island of Manam, lying 13 km off the northern coast of mainland Papua New Guinea, is one of the country's most active volcanoes. Four large radial valleys extend from the unvegetated summit of the conical 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: P. de Saint-Ours and C. McKee, RVO.

During fieldwork on 17 and 25 April, gas emission in Santiago Crater was limited to a few patches of weakly fuming ground within the inner crater, below the level of the frozen 1965 lava lake. The highest temperature measured on the fuming ground (using an 8-14 µm infrared thermometer from the crater rim) was 50.7°C. Small rockfalls from the inner crater walls were frequently audible. Much of the floor of the innermost crater was covered by debris and the "cannon" vent (first reported in February 1989; 14:02) was no longer visible. However, an opening had formed at the site of a former incandescent vent N of the February-March 1989 lava lake. No incandescence was evident in the crater after dusk on 25 April. Tangential fissures crossing the S rim parking area and nearby had widened in recent weeks.

Geologic Background. Masaya is one of Nicaragua's most unusual and most active volcanoes. It lies within the massive Pleistocene Las Sierras pyroclastic shield volcano and is a broad, 6 x 11 km basaltic caldera with steep-sided walls up to 300 m high. The caldera is filled on its NW end by more than a dozen vents that erupted along a circular, 4-km-diameter fracture system. The twin volcanoes of Nindirí and Masaya, the source of historical eruptions, were constructed at the southern end of the fracture system and contain multiple summit craters, including the currently active Santiago crater. A major basaltic Plinian tephra erupted from Masaya about 6500 years ago. Historical lava flows cover much of the caldera floor and have confined a lake to the far eastern end of the caldera. A lava flow from the 1670 eruption overtopped the north caldera rim. Masaya has been frequently active since the time of the Spanish Conquistadors, when an active lava lake prompted attempts to extract the volcano's molten "gold." Periods of long-term vigorous gas emission at roughly quarter-century intervals cause health hazards and crop damage.

Information Contacts: C. Oppenheimer, Open Univ; B. van Wyk de Vries, INETER.

When visited 21 and 27 April 1990, locations of fumarolic vents had changed little since a year earlier but gas temperatures had generally declined by 50-150°C. The highest recorded at vent F9 was 772°C (down from 880° on 15 April 1989) and temperatures at neighboring vents were 634° (779° in April 1989), 662° (716° in 1989), and 742° (844° in 1989). Other temperatures measured on 27 April 1990 were 575°C (F7), 702° (F8), and 439° (F12). Small dribbles, drops, or pools of amber-colored liquid sulfur were common at most fumaroles. Some very small multiple flows had smooth crusts of solid orange-brown sulfur, which turned bright yellow within minutes of removal from the hot surface. At F8, a tiny pool of liquid sulfur, a few mm deep, had a temperature of 137°C. Evidence for older, larger, sulfur flows included eroded remnants as much as 15 cm thick near F12, where up to 5 m of a pahoehoe type flow was preserved, including its front. The upper portion had apparently been eroded by water.

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: C. Oppenheimer, Open Univ; Benjamin van Wyk de Vries, INETER.

An 8-14 µm infrared thermometer was used on 23 April to measure temperatures of the inner wall of the prominent 20-m-deep chasm formed in the [1952] eruption. Weak fumarolic activity was occurring there, and the maximum recorded temperature was 96°C, probably corresponding closely with the gas temperature.

Geologic Background. Las Pilas volcanic complex, overlooking Cerro Negro volcano to the NW, includes a diverse cluster of cones around the central vent, Las Pilas (El Hoyo). A N-S-trending fracture system cutting across the edifice is marked by numerous well-preserved flank vents, including maars, that are part of a 30-km-long volcanic massif. The Cerro Negro chain of cinder cones is listed separately in this compilation because of its extensive historical eruptions. The lake-filled Asososca maar is located adjacent to the Cerro Asososca cone on the southern side of the fissure system, south of the axis of the Marrabios Range. Two small maars west of Lake Managua are located at the southern end of the fissure. Aside from a possible eruption in the 16th century, the only historical eruptions of Las Pilas took place in the 1950s from a fissure that cuts the eastern side of the 700-m-wide summit crater and extends down the N flank.

Information Contacts: C. Oppenheimer, Open Univ; B. van Wyk de Vries, INETER.

March activity. Continuous gas emission persisted in March. Gases were carried W and SW by prevailing winds, with major impact on the vegetation, infrastructure, and health of the inhabitants of that area. Winds sometimes changed, carrying gas toward the S and SE where other residents were affected.

Water level in the crater lake had dropped, leaving isolated pools around a central remnant lake from which most of the activity occurred. Activity included continuous bubbling, small geyser-like eruptions, and intense gas emission (primarily water vapor). Two shallow ponds to the SE were 5-10 m in diameter with a mean temperature of 80°C. On the NE side there were three shallow molten sulfur ponds with a mean temperature of 160°C. In the N part of the crater the most vigorous fumarole emitted orange gas, probably produced by combustion of sulfur, and fine sulfur particles. The gas temperature was about 400°C and a reddish flame was sometimes observed at the base of the gas column.

A permanent seismic station (POA2) continued to record B-type events, with a mean of 337/day in March (figure 26). An A-type earthquake was felt at MM II, 7 km WNW of the summit (in Bajos del Toro) on 8 March at 0239. A series of A-type shocks began to be recorded on 25 March (figure 27). Inhabitants of flank towns, including Poásito (5.5 km SE of the summit), Fraijanes (7 km SSE), and San Pedro de Poás (13.5 km S) felt a M 2.5 earthquake on 1 April at 0204 that was centered 5 km NE of the active crater at 5 km depth. A second felt event, on 2 April at 0215, was centered 6 km SW of the crater at 15 km depth, and had a magnitude of 3.1.

Figure 26. Number of low-frequency earthquakes recorded at Poás, March-April 1990. March data courtesy of J. Barquero; April data courtesy of G. Soto.

Figure 27. Number of A-type earthquakes recorded at Poás, 25 March-24 April 1990. Courtesy of G. Soto.

Hazel Rymer noted that the March 1990 crater lake was comparable to the lake in April 1989, having apparently evaporated to a low level more rapidly during the 1990 dry season. The base of the mud volcanoes, <1 m above lake level, was 2,282.650 m above sea level on 18 March 1989, while the 30 March 1990 lake level was at 2,278.853 m (elevations tied to a flank benchmark). The location of the hot sulfur lakes, observed last year prior to the ash eruption, was occupied by a vigorous vent, continuously jetting a mixture of sulfurous gases. In early April 1990, boiling mud pools occupied the remainder of the former lake area.

April activity. By the beginning of April, lake level had dropped 4 m since December 1989, shrinking to a small pool in the center of the former lake, where minor phreatic eruptions occurred. Numerous fissures continually emitted gases; at some sites, combustion of sulfur produced flames. By the 18th, the lake's cumulative descent had reached 7 m and it was nearly dry, leaving areas of mud with occasional bubbling caused by the discharge of fumaroles in the bottom of the crater. In addition to activity on the SE part of the crater floor, decsribed in detail below, hot fumaroles were also found in the NE part of the former lake bed (figure 28). The principal fumarole in this area had a vent 2-3 m in diameter. Sprays of very pure yellow sulfur were emitted, as were bluish SO₂ and water vapor. The gases were expelled under high pressure with a jet aircraft sound, and burned with yellow-orange flames. At the beginning of the month, temperature (190°C with an infrared thermometer), sound level, and pressure were less than on the 18th, when activity was stronger with temperatures to 793°C. Since 17 April, the vigorous discharge has caused vibrations felt inside the crater and registered by the summit seismic station (VPS-2). The fumarole ejected evaporitic sediments to 75 m height, and similar activity continued through the end of the month. On the 1953-55 dome, fumaroles emitted gas dominated by water, and precipitated sulfur and sulfates. Maximum fumarole temperature (measured by thermocouple) was 90.3°C.

Figure 28. Sketch map of the active crater at Poás, 18 April 1990. Courtesy of G. Soto.

Gases, carrying sediments because of the dryer lake bottom, were carried mainly W and SW, affecting coffee farms, pastures, and especially forests. The inhabitants of various towns, including San Luis (about 13 km SSE of the summit), Cajón, and San Miguel de Grecia (~11 km SW of the summit), suffered health problems, principally with the respiratory tract, vision, and skin allergies. From the night of 26 April through the following day, a wind change concentrated gases strongly in the Parque Nacional del Poás area and toward the SE flank, affecting strawberry crops.

Seismic activity changed substantially from the previous month. Recorded events dropped from 9,460 in March to 9,190 in April. Of these, 9,026 were of low frequency and 159 were volcano-tectonic or A-type. The latter averaged 5/day, a substantial increase. Volcanic tremor also occurred during April, manifested as prolonged vibration of the edifice caused by gases emerging from fumaroles at high pressure.

12 April fieldwork. G. Soto and Clive Oppenheimer climbed into the crater on 12 April at about noon. Bubbling mud pools occupied positions very similar to those of sulfur ponds seen on the SE part of the crater floor in April 1989. Nearly continuous geysering of mud and clumps of solid sulfur was depositing a ring around one of the mud pools. Columns of material ejected in the SE part of the crater reached 10-15 m height, and temperatures measured by an infrared thermometer were between 70 and 100°C. A vent at the NE fumarole site produced a gentle roar and a blue-tinged turbulent plume of gases with pink-orange flames, just visible in daylight, at its base. The plume intermittently stopped burning, turned a thick bright yellow color for a few minutes, then re-ignited and returned to its previous color.

By 0900 the next day, a small dull-yellow sulfur cone had grown at the site of the previous day's geyser-like eruptions. The cone was 2.2 m high with a basal diameter of about 6 m and a crater 2 m across. Small bursts of coagulated, rather plastic sulfur mixed with some silicate occurred at about 1-second intervals, sending ejecta to about 3 m above the rim. A warm, white, acid, vapor cloud was continuously emitted from the crater. The maximum temperature measured by a thermocouple pushed into the substrate at the summit was 97.6°C; when thrown over the rim, the highest measured temperature was 96°C. The crater was filled to within about 0.75 m of the rim with various-sized pellets of somewhat malleable sulfur mixed with silicate, appearing fluidized from agitation in the upward gas stream. Some of the crudely spherical pellets, collected from the sides and base of the cone, were up to 4 cm in diameter. In cross section, they revealed accretionary shells of aggregated sulfur crystals and minor clay alternating with gray clay-rich layers. Shortly before 1130, the vent appeared to become more confined, and the crater was quickly filled by a growing sub-cone. Within minutes, it grew above the old crater rim, forming a perfect cone 3.5-4 m high. A constant spray of sulfur up to 10 m high issued from a narrow vent at the cone's apex for about 30 minutes, dispersing a fine layer to about 15 m downwind. The conduit was often momentarily blocked before being cleared by a slightly stronger gas burst. The continuous ejection of material also built other cones, a few meters high and rich in pyroclastic sulfur, which periodically collapsed and recycled their contents.

29 April fieldwork. When geologists returned to the volcano on 29 April at about 1245, a convecting grayish-white plume, combined from numerous individual vents, was rising to more than 300 m, accompanied by a continuous roar, like a distant jet aircraft, from the center of the crater floor. Moist drops of acid mud fell from the plume, and appeared to form a thin veneer on the inner crater. Three recent cones, presumably of sulfur, on the SE part of the crater floor were also coated with mud and only weakly fuming, with bright yellow sulfur deposited around their conduits. One was irregularly erupting yellowish sulfur. To the NW, small gas eruptions ejected dark clouds of lake sediment, forming a line or cluster of several wide cones. Fumarolic vents varied in color from bright yellow, to different shades of gray, to white. Some had a pronounced tinge of bluish haze. The NE vent, which had been burning on 12 April, was considerably quieter, but a nearby vent was producing a strong plume with pink-orange flames clearly visible at its base. The plume changed color to yellow when combustion ceased; re-ignition was accompanied by a roaring sound. Bright flames also licked the steep inner walls and rim of the remnants of a small cone nearby. A peak thermocouple temperature of 662°C was recorded with the probe in the flames. The origin of the cone was uncertain, although samples of pink-gray ash were collected nearby. At about 1445, a brief, more powerful eruption occurred from the center of the former lake floor. The plume, presumably of non-juvenile material (lake sediments) rose roughly 50 m and produced a hail of blocks that fell noisily to the muddy crater floor.

The next day at 0700, nearly constant eruptions of gas, yellow-tinged apparently dry ash, and blocks, continued from several vents around the center of the crater. Some fresh sulfur had been erupted over one of the old sulfur cones at the SE site. The activity was similar to that observed in the same region in April 1989. At roughly hourly intervals, considerably more powerful activity ejected thick cauliflowering columns of dark ash, accompanied by blocks trailing white vapor above the level of the 1953-55 dome. The episodes lasted 30-50 seconds. There was no evidence that the ash was juvenile, although no samples were obtained. Two gas eruptions were observed at previously inactive sites. The first, at about 0800, occurred very near one of the active phreatic cones. Gas bubbles burst noisily through mud, hurling expanding shells of mud spatter. A similar but much briefer episode occurred 40 m away at about 0810, near the first set of lake sediment terraces. It left a crudely horseshoe-shaped scar in the mud, and a gray fluidized mudflow moved toward the center of the crater. Another strong eruption was seen at 0944, shortly before geologists left the crater, producing a plume that rose to an estimated height of 100 m.

Gravity data. The following is from Hazel Rymer. "Five years of continuous gravity increases at crater-bottom stations, from March 1985 to March 1989, have been recorded by Open University geophysicists. Since 1987, we have also had good elevation control at these stations and have recorded minor deflation. The maximum changes, >200 microGal increases at stations on the 1953 dome, were accompanied by 30 cm deflation (March 1987-March 1989). Gravity variations were similar elsewhere on the crater floor, but elevation changes were less than 6 cm. These data are interpreted in terms of small dendritic magma intrusions and loss of magmatic gas from beneath the lake area (Rymer and Brown, 1987). Evidence to support this model comes from detailed analysis of the energy budget of the crater lake. While gravity increased gradually from 1985 to 1989, the power output through the lake area jumped in 1986 from a fairly steady 190 MW to about 300 MW, maintained to the present. Thus, steady gravity increase was associated with a sustained power output since the stepped increase in 1986.

"Microgravity and elevation data collected by Open Univ geophysicists and Earthwatch volunteers between 19 and 30 March 1990 revealed gravity decreases of about 50 microGal at crater bottom stations with elevations unchanged from 1989 to within 2 cm."

Reference. Rymer, H. and Brown, G., 1987, Gravity changes as a precursor to volcanic eruption of Poás volcano: Nature, v. 342, p. 902-905.

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: J. Barquero, E. Fernández, and V. Barboza, OVSICORI; Hazel Rymer and C.M.M. Oppenheimer, Open Univ; G. Soto and R. Barquero, ICE; Mario Fernández, UCR.

"Activity was at very low level throughout April, with only 69 recorded earthquakes. There were several days without any recorded events, and the highest daily total was 9 events. Only three earthquakes could be located - one in each of the E, S, and NW parts of the caldera seismic zone. Levelling measurements carried out on 25 April indicated no significant changes from the previous measurements (26 March)."

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: P. de Saint-Ours, and C. McKee, RVO.

This report, from the AVO staff, covers the period 15 April-15 May, 1990. Information about the 15 April explosive episode supplements the initial material in 15:3.

"Lava dome growth punctuated by small explosive events and partial dome collapse continued to occur. In general, seismic and explosive activity were at relatively low levels (figure 11). Intense steaming in the crater area coupled with poor weather have limited viewing of the dome and crater area to 5 observations during the report period. The pattern of explosive eruptions every 3-9 days that had characterized Redoubt's activity from 15 February to 21 April has changed; as of mid-May, the last significant explosive event had occurred on 21 April, 3.5 weeks earlier.

Figure 11. Epicenter map (top) and depth vs. time plot (bottom) of earthquakes recorded near Redoubt by AVO, 15 April-15 May 1990. Squares on the epicenter map mark the positions of seismic stations. Near-summit stations are obscured by numerous seismic signals.

Explosive episode, 15 April. "A moderate explosive event occurred at 1440 and was recorded for 8 minutes at the Spurr station. The explosive event triggered a pyroclastic flow down the N face of the volcano, and a mudflow that carried hot blocks of dense dome rock 1-2 m in diameter 4 km downvalley (to the E end of the Dumbbell Hills; figure 12). A small flood reached the Drift River oil facility about 3 hours after the onset of the episode (about 40% occupied the Drift River channel, the rest the Rust Slough and Cannery Creek channels). Winds carried the tephra N-NW from Redoubt; flat discs of pumice up to 4 cm in diameter were noted about 10 km NW of the vent area. Three cloud-to-ground lightning strikes were detected NW of the volcano.

Figure 12. Sketch map of the Drift River valley and related drainages on the NE flank of Redoubt. The Drift River oil facility is between the mouth of the Drift River and Rust Slough. Courtesy of AVO.

Explosive episode, 21 April. "A small to moderate explosive event occurred at 0611 and was recorded for 4 minutes at the Spurr station. An ash-laden plume was reported to 7.5-9 km and a disc-shaped 'collar' was observed at about half that altitude by personnel at the Drift River oil facility and residents of the Kenai Peninsula. Light ashfalls occurred N-NW of the volcano to about 75 km distance. Heavy steaming prevented good observations of the crater area, but it was apparent that at least some of the lava dome remained in the crater. The episode triggered a small pyroclastic flow and surge. No significant flooding was associated with this episode.

Minor explosive events. "A small seismic event that occurred on 26 April at 1017 was recorded for 2 minutes at Spurr station. An AVO field crew reported an ash plume rising above the volcano and heading SE. Light ashfall reached 100 km SE of the mountain. The episode included a small pyroclastic flow that terminated at about 1,000 m on the N flank. The summit area was partially obscured by clouds, but it appeared that about 80% of the dome was intact and was still oversteepened to the N. An overhanging slab extended from the dome's NE rim; collapse of a similar slab may have triggered the pyroclastic flow. No flooding in the Drift River valley accompanied the event.

"A seismic event on 7 May at 1846 was too small to be recorded at Spurr, but pilots reported a plume to 9.5 km, consisting mostly of steam with a little ash. No pyroclastic flows were reported to have been associated with the event, and no hot blocks were noted by an AVO crew in the field the next day; however, poor weather prevented observations above 750 m altitude. A dusting of ash (presumably from the 7 May event) was seen on the upper flanks on 10 May.

Dome observations. "The dome was only observed five times during the report period, on 21, 25, 26, and 28 April, and 5 May. Thus, no views of the dome were obtained between the explosive episodes on 15 and 21 April, nor since the small explosive event on 7 May. The crater area was exceptionally clear on 25 and 28 April, and in both viewings the dome had a rough, blocky surface with transverse tension cracks exhibiting a smooth, massive interior. On 25 April, the dome was oval in plan, but by the 28th it was elongate N-S and oversteepened on its N side. Blocky talus that covered the canyon floor for a couple of hundred meters N of the dome on 28 April was not observed on the 25th. When the dome was last viewed on 5 May, it was partially obscured by steam and low clouds, but the surface was definitely smooth and massive, not rough and blocky."

Geologic Background. Redoubt is a glacier-covered stratovolcano with a breached summit crater in Lake Clark National Park about 170 km SW of Anchorage. Next to Mount Spurr, Redoubt has been the most active Holocene volcano in the upper Cook Inlet. The volcano was constructed beginning about 890,000 years ago over Mesozoic granitic rocks of the Alaska-Aleutian Range batholith. Collapse of the summit 13,000-10,500 years ago produced a major debris avalanche that reached Cook Inlet. Holocene activity has included the emplacement of a large debris avalanche and clay-rich lahars that dammed Lake Crescent on the south side and reached Cook Inlet about 3,500 years ago. Eruptions during the past few centuries have affected only the Drift River drainage on the north. Historical eruptions have originated from a vent at the north end of the 1.8-km-wide breached summit crater. The 1989-90 eruption had severe economic impact on the Cook Inlet region and affected air traffic far beyond the volcano.

Information Contacts: AVO Staff.

Fieldwork on 12 April revealed that the temperature of Crater Lake had dropped to 29.5°C, continuing a decline from 34°C on 19 March and an 8-year peak of 46.5° on 6 February. Upwelling occurred from three vents in the lake's N vent area, from which yellow sulfur slicks were drifting SE. Chemistry of lake water indicated that HCl-bearing steam was the lake's dominant thermal input. Deformation measurements revealed only minor changes.

Tremor amplitude declined in late March, was increasing again by the beginning of April, then declined toward mid-month. Low-frequency tremor remained uncommon, with 1-Hz signals recorded only on 7 and 8 April.

Geologic Background. Ruapehu, one of New Zealand's most active volcanoes, is a complex stratovolcano constructed during at least four cone-building episodes dating back to about 200,000 years ago. The 110 km3 dominantly andesitic volcanic massif is elongated in a NNE-SSW direction and surrounded by another 100 km3 ring plain of volcaniclastic debris, including the Murimoto debris-avalanche deposit on the NW flank. A series of subplinian eruptions took place between about 22,600 and 10,000 years ago, but pyroclastic flows have been infrequent. A single historically active vent, Crater Lake, is located in the broad summit region, but at least five other vents on the summit and flank have been active during the Holocene. Frequent mild-to-moderate explosive eruptions have occurred in historical time from the Crater Lake vent, and tephra characteristics suggest that the crater lake may have formed as early as 3000 years ago. Lahars produced by phreatic eruptions from the summit crater lake are a hazard to a ski area on the upper flanks and to lower river valleys.

Information Contacts: P. Otway, DSIR Wairakei.

Seismic energy release and the number of earthquakes were at low to moderate levels in April. Seismicity peaked on 11 April with 162 low-frequency events. Earthquakes were dispersed around the crater, with focal depths of 0.5-6.5 km. Pulses of low-energy tremor began 26 April and persisted until the 29th, when an episode of continuous low-energy tremor was associated with a small ash emission. Dry and electronic tilt showed no substantial changes. Measurements of glacial behavior showed significant ablation, reaching a rate of the order of 240 m³/day. The average rate of SO₂ emission, measured by COSPEC, was 1,467 t/d.

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

Information Contacts: C. Carvajal, INGEOMINAS, Manizales.

The Mt. St. Helens seismic network recorded an explosion-type seismic signal on 25 April at 0126. The seismicity appeared similar to that associated with minor tephra emissions on [6] December and 6 January (SEAN 14:11 and 14:12), but both amplitude and duration were much smaller on 25 April. No eruption plume was reported and fresh snowfall shortly after the episode obscured any material that may have been deposited. The number of tiny earthquakes (detected only by crater stations) remained elevated for ~18 hours after the activity, then returned to background levels. Periods of increased local seismicity have continued since late 1987 (figure 43 and SEAN 14:08 and 14:10).

Figure 43. Time vs. depth plot of seismicity at Mt. St. Helens, 1 January 1989-17 May 1990. Arrows mark episodes of explosion-type seismicity. Courtesy of the Geophysics Program, University of Washington.

Geologic Background. Prior to 1980, Mount St. Helens formed a conical, youthful volcano sometimes known as the Fuji-san of America. During the 1980 eruption the upper 400 m of the summit was removed by slope failure, leaving a 2 x 3.5 km horseshoe-shaped crater now partially filled by a lava dome. Mount St. Helens was formed during nine eruptive periods beginning about 40-50,000 years ago and has been the most active volcano in the Cascade Range during the Holocene. Prior to 2200 years ago, tephra, lava domes, and pyroclastic flows were erupted, forming the older St. Helens edifice, but few lava flows extended beyond the base of the volcano. The modern edifice was constructed during the last 2200 years, when the volcano produced basaltic as well as andesitic and dacitic products from summit and flank vents. Historical eruptions in the 19th century originated from the Goat Rocks area on the north flank, and were witnessed by early settlers.

Information Contacts: C. Jonientz-Trisler, University of Washington.

Stromboli was visited on 29 March and 1-2 April. Poor weather on the 29th obscured the active summit vents until late afternoon, when six eruptive episodes were observed between 1745 and 1830. Five were accompanied by emission of dark gray to black ash plumes that rose 150-200 m above the two active vents. The activity produced lava fountains ~100 m high, often falling onto the Sciara del Fuoco. On one occasion, a large glowing block rolled down the NNW side of the Sciara to ~100 m below the vents.

Observations from the summit between 1730 on 1 April and 0200 the next morning revealed morphologic changes that had occurred in the vent area since September 1989 (14:09). Crater A [termed Crater 1 in previous reports; compare figure 2 and figure 3 below] included at least five vents, four of which were active. The vent that had been most active in September had collapsed, but a new deep vent that had formed 10-20 m to the NW produced an average of 2-3 eruptions/hour before midnight. Glowing spatter from the eruptions rarely escaped the vent, but black ash plumes rose ~100 m above the rim. Eruptions became more frequent and intense after midnight, with lava fountains rising to ~100 m above the rim, larger ash plumes, and heavy falls of bombs and spatter onto Crater A's NE rim (figure 4). Each of the eruptions was accompanied by a few seconds of deep but not very loud rumbling. In the NE part of Crater A, a cluster of open vents (several very small and three larger) contained active magma and glowed intensely at night. They emitted burning gases but no spatter.

Figure 3. Sketch map of the summit area of Stromboli showing vent configurations observed 1-2 April 1990. Courtesy of B. Behncke.

Figure 4. Oblique sketch of the summit area of Stromboli, looking roughly W. Courtesy of B. Behncke.

Crater B [coalesced from craters formerly termed 2, 3, and 4; compare figures 2 and 3 below] included at least 4 vents, and others were probably hidden from view by intense gas emission and topographic obstacles. In its center was a symmetrical spatter cone 10-15 m high with a glowing summit vent and a small steaming hornito near its SW base. The cone was covered with yellowish green sublimates and was not ejecting tephra. Vent 5, frequently active in September, erupted only once every 1-2 hours. Its eruptions lasted up to 30 seconds, sometimes consisting of several pulses of lava fountains that reached 100 m above the rim, accompanied by loud rumbling. Vigorous emission of gas (smelling strongly of H₂S and [SO₂]) from the E wall of Crater B obscured vent 5 from direct observation. Vent 4a, at most 2 m in diameter, was near the base of a steep pinnacle, possibly a hornito, ~10 m high. The vent produced very loud high-pressure gas emissions, sometimes lasting 20 seconds, every 20-30 minutes, feeding a very faint bluish gas column ~20-30 m high. A few were accompanied by ejection of several glowing blocks, probably from the conduit walls. A slight continuous tremor was felt from ~50 m away during one of the gas emission episodes. Vent 3, between craters A and B, remained inactive during the observation period.

During the afternoon of 3 April, ash emission reportedly became stronger and could be seen from villages on the SE and E sides of the island.

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: B. Behncke, Ruhr Univ, Germany.

Fumarolic activity continued from the main crater, with temperatures of 90°C, and from the N wall of the central crater.

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: J. Barquero, OVSICORI.

"Activity remained at a very low level, with the summit crater releasing white vapour in small to moderate amounts. Seismicity was limited to a few (<=10) very small low-frequency events/day."

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

Information Contacts: P. de Saint-Ours, and C. McKee, RVO.

A summit climb on 31 March revealed only minor changes since September 1989 (14:10). Gas emission continued from the fissure on the N rim, at high pressure from its 10-15-cm-wide central portion. Rocks up to 5 mm in diameter were re-ejected when thrown into the fissure's central section. The resulting gas plume rose 300-400 m during rainy weather on 3 April, but was considerably smaller at other times. Weak fumarolic activity was also occurring on the outer SE crater wall, and a new fumarole had formed on the NW flank.

Geologic Background. The word volcano is derived from Vulcano stratovolcano in Italy's Aeolian Islands. Vulcano was constructed during six stages during the past 136,000 years. Two overlapping calderas, the 2.5-km-wide Caldera del Piano on the SE and the 4-km-wide Caldera della Fossa on the NW, were formed at about 100,000 and 24,000-15,000 years ago, respectively, and volcanism has migrated to the north over time. La Fossa cone, active throughout the Holocene and the location of most of the historical eruptions, occupies the 3-km-wide Caldera della Fossa at the NW end of the elongated 3 x 7 km island. The Vulcanello lava platform forms a low, roughly circular peninsula on the northern tip of Vulcano that was formed as an island beginning in 183 BCE and was connected to Vulcano in about 1550 CE. Vulcanello is capped by three pyroclastic cones and was active intermittently until the 16th century. The latest eruption from Vulcano consisted of explosive activity from the Fossa cone from 1898 to 1900.

Information Contacts: B. Behncke, Ruhr Univ.

In discussing the week ending on 12 September, "Earthweek" (Newman, 1997) incorrectly claimed that a volcano named "Mount Pinukis" had erupted. Widely read in the US, the dramatic Earthweek report described terrified farmers and a black mushroom cloud that resembled a nuclear explosion. The mountain's location was given as "200 km E of Zamboanga City," a spot well into the sea. The purported eruption had received mention in a Manila Bulletin newspaper report nine days earlier, on 4 September. Their comparatively understated report said that a local police director had disclosed that residents had seen a dormant volcano showing signs of activity.

In response to these news reports Emmanuel Ramos of the Philippine Institute of Volcanology and Seismology (PHIVOLCS) sent a reply on 17 September. PHIVOLCS staff had initially heard that there were some 12 alleged families who fled the mountain and sought shelter in the lowlands. A PHIVOLCS investigation team later found that the reported "families" were actually individuals seeking respite from some politically motivated harassment. The story seems to have stemmed from a local gold rush and an influential politician who wanted to use volcanism as a ploy to exclude residents. PHIVOLCS concluded that no volcanic activity had occurred. They also added that this finding disappointed local politicians but was much welcomed by the residents.

PHIVOLCS spelled the mountain's name as "Pinokis" and from their report it seems that it might be an inactive volcano. There is no known Holocene volcano with a similar name (Simkin and Siebert, 1994). No similar names (Pinokis, Pinukis, Pinakis, etc.) were found listed in the National Imagery and Mapping Agency GEOnet Names Server (http://geonames.nga.mil/gns/html/index.html), a searchable database of 3.3 million non-US geographic-feature names.

The Manila Bulletin report suggested that Pinokis resides on the Zamboanga Peninsula. The Peninsula lies on Mindanao Island's extreme W side where it bounds the Moro Gulf, an arm of the Celebes Sea. The mountainous Peninsula trends NNE-SSW and contains peaks with summit elevations near 1,300 m. Zamboanga City sits at the extreme end of the Peninsula and operates both a major seaport and an international airport.

[Later investigation found that Mt. Pinokis is located in the Lison Valley on the Zamboanga Peninsula, about 170 km NE of Zamboanga City and 30 km NW of Pagadian City. It is adjacent to the two peaks of the Susong Dalaga (Maiden's Breast) and near Mt. Sugarloaf.]

References. Newman, S., 1997, Earthweek, a diary of the planet (week ending 12 September): syndicated newspaper column (URL: http://www.earthweek.com/).

Manila Bulletin, 4 Sept. 1997, Dante's Peak (URL: http://www.mb.com.ph/).

Simkin, T., and Siebert, L., 1994, Volcanoes of the world, 2nd edition: Geoscience Press in association with the Smithsonian Institution Global Volcanism Program, Tucson AZ, 368 p.

Information Contacts: Emmanuel G. Ramos, Deputy Director, Philippine Institute of Volcanology and Seismology, Department of Science and Technology, PHIVOLCS Building, C. P. Garcia Ave., University of the Philippines, Diliman campus, Quezon City, Philippines.

Xinhua News Agency filed a news report on 27 February under the headline "Volcano erupts in Somalia" but the veracity of the story now appears doubtful. The report disclosed the volcano's location as on the W side of the Gedo region, an area along the Ethiopian border just NE of Kenya. The report had relied on the commissioner of the town of Bohol Garas (a settlement described as 40 km NE of the main Al-Itihad headquarters of Luq town) and some or all of the information was relayed by journalists through VHF radio. The report claimed the disaster "wounded six herdsmen" and "claimed the lives of 290 goats grazing near the mountain when the incident took place." Further descriptions included such statements as "the volcano which erupted two days ago [25 February] has melted down the rocks and sand and spread . . . ."

Giday WoldeGabriel returned from three weeks of geological fieldwork in SW Ethiopia, near the Kenyan border, on 25 August. During his time there he inquired of many people, including geologists, if they had heard of a Somalian eruption in the Gedo area; no one had heard of the event. WoldeGabriel stated that he felt the news report could have described an old mine or bomb exploding. Heavy fighting took place in the Gedo region during the Ethio-Somalian war of 1977. Somalia lacks an embassy in Washington DC; when asked during late August, Ayalaw Yiman, an Ethiopian embassy staff member in Washington DC also lacked any knowledge of a Somalian eruption.

A Somalian eruption would be significant since the closest known Holocene volcanoes occur in the central Ethiopian segment of the East African rift system S of Addis Ababa, ~500 km NW of the Gedo area. These Ethiopian rift volcanoes include volcanic fields, shield volcanoes, cinder cones, and stratovolcanoes.

Information Contacts: Xinhua News Agency, 5 Sharp Street West, Wanchai, Hong Kong; Giday WoldeGabriel, EES-1/MS D462, Geology-Geochemistry Group, Los Alamos National Laboratory, Los Alamos, NM 87545; Ayalaw Yiman, Ethiopian Embassy, 2134 Kalorama Rd. NW, Washington DC 20008.

Following the Ms 7.8 earthquake in Turkey on 17 August (BGVN 24:08) an Email message originating in Turkey was circulated, claiming that volcanic activity was observed coincident with the earthquake and suggesting a new (magmatic) volcano in the Sea of Marmara. For reasons outlined below, and in the absence of further evidence, editors of the Bulletin consider this a false report.

The report stated that fishermen near the village of Cinarcik, at the E end of the Sea of Marmara "saw the sea turned red with fireballs" shortly after the onset of the earthquake. They later found dead fish that appeared "fried." Their nets were "burned" while under water and contained samples of rocks alleged to look "magmatic."

No samples of the fish were preserved. A tectonic scientist in Istanbul speculated that hot water released by the earthquake from the many hot springs along the coast in that area may have killed some fish (although they would be boiled rather than fried).

The phenomenon called earthquake lights could explain the "fireballs" reportedly seen by the fishermen. Such effects have been reasonably established associated with large earthquakes, although their origin remains poorly understood. In addition to deformation-triggered piezoelectric effects, earthquake lights have sometimes been explained as due to the release of methane gas in areas of mass wasting (even under water). Omlin and others (1999), for example, found gas hydrate and methane releases associated with mud volcanoes in coastal submarine environments.

The astronomer and author Thomas Gold (Gold, 1998) has a website (Gold, 2000) where he presents a series of alleged quotes from witnesses of earthquakes. We include three such quotes here (along with Gold's dates, attributions, and other comments):

(A) Lima, 30 March 1828. "Water in the bay 'hissed as if hot iron was immersed in it,' bubbles and dead fish rose to the surface, and the anchor chain of HMS Volage was partially fused while lying in the mud on the bottom." (Attributed to Bagnold, 1829; the anchor chain is reported to be on display in the London Navy Museum.)

(B) Romania, 10 November 1940. ". . . a thick layer like a translucid gas above the surface of the soil . . . irregular gas fires . . . flames in rhythm with the movements of the soil . . . flashes like lightning from the floor to the summit of Mt Tampa . . . flames issuing from rocks, which crumbled, with flashes also issuing from non-wooded mountainsides." (Phrases used in eyewitness accounts collected by Demetrescu and Petrescu, 1941).

(C) Sungpan-Pingwu (China), 16, 22, and 23 August 1976. "From March of 1976, various large anomalies were observed over a broad region. . . . At the Wanchia commune of Chungching County, outbursts of natural gas from rock fissures ignited and were difficult to extinguish even by dumping dirt over the fissures. . . . Chu Chieh Cho, of the Provincial Seismological Bureau, related personally seeing a fireball 75 km from the epicenter on the night of 21 July while in the company of three professional seismologists."

Yalciner and others (1999) made a study of coastal areas along the Sea of Marmara after the Izmet earthquake. They found evidence for one or more tsunamis with maximum runups of 2.0-2.5 m. Preliminary modeling of the earthquake's response failed to reproduce the observed runups; the areas of maximum runup instead appeared to correspond most closely with several local mass-failure events. This observation together with the magnitude of the earthquake, and bottom soundings from marine geophysical teams, suggested mass wasting may have been fairly common on the floor of the Sea of Marmara.

Despite a wide range of poorly understood, dramatic processes associated with earthquakes (Izmet 1999 apparently included), there remains little evidence for volcanism around the time of the earthquake. The nearest Holocene volcano lies ~200 km SW of the report location. Neither Turkish geologists nor scientists from other countries in Turkey to study the 17 August earthquake reported any volcanism. The report said the fisherman found "magmatic" rocks; it is unlikely they would be familiar with this term.

The motivation and credibility of the report's originator, Erol Erkmen, are unknown. Certainly, the difficulty in translating from Turkish to English may have caused some problems in understanding. Erkmen is associated with a website devoted to reporting UFO activity in Turkey. Photographs of a "magmatic rock" sample were sent to the Bulletin, but they only showed dark rocks photographed devoid of a scale on a featureless background. The rocks shown did not appear to be vesicular or glassy. What was most significant to Bulletin editors was the report author's progressive reluctance to provide samples or encourage follow-up investigation with local scientists. Without the collaboration of trained scientists on the scene this report cannot be validated.

References. Omlin, A, Damm, E., Mienert, J., and Lukas, D., 1999, In-situ detection of methane releases adjacent to gas hydrate fields on the Norwegian margin: (Abstract) Fall AGU meeting 1999, Eos, American Geophysical Union.

Yalciner, A.C., Borrero, J., Kukano, U., Watts, P., Synolakis, C. E., and Imamura, F., 1999, Field survey of 1999 Izmit tsunami and modeling effort of new tsunami generation mechanism: (Abstract) Fall AGU meeting 1999, Eos, American Geophysical Union.

Gold, T., 1998, The deep hot biosphere: Springer Verlag, 256 p., ISBN: 0387985468.

Gold, T., 2000, Eye-witness accounts of several major earthquakes (URL: http://www.people.cornell.edu/ pages/tg21/eyewit.html).

Information Contacts: Erol Erkmen, Tuvpo Project Alp.

In December 2002 information appeared in Mongolian and Russian newspapers and on national TV that a volcano in Central Mongolia, the Har-Togoo volcano, was producing white vapors and constant acoustic noise. Because of the potential hazard posed to two nearby settlements, mainly with regard to potential blocking of rivers, the Director of the Research Center of Astronomy and Geophysics of the Mongolian Academy of Sciences, Dr. Bekhtur, organized a scientific expedition to the volcano on 19-20 March 2003. The scientific team also included M. Ulziibat, seismologist from the same Research Center, M. Ganzorig, the Director of the Institute of Informatics, and A. Ivanov from the Institute of the Earth's Crust, Siberian Branch of the Russian Academy of Sciences.

Geological setting. The Miocene Har-Togoo shield volcano is situated on top of a vast volcanic plateau (figure 1). The 5,000-year-old Khorog (Horog) cone in the Taryatu-Chulutu volcanic field is located 135 km SW and the Quaternary Urun-Dush cone in the Khanuy Gol (Hanuy Gol) volcanic field is 95 km ENE. Pliocene and Quaternary volcanic rocks are also abundant in the vicinity of the Holocene volcanoes (Devyatkin and Smelov, 1979; Logatchev and others, 1982). Analysis of seismic activity recorded by a network of seismic stations across Mongolia shows that earthquakes of magnitude 2-3.5 are scattered around the Har-Togoo volcano at a distance of 10-15 km.

Figure 1. Photograph of the Har-Togoo volcano viewed from west, March 2003. Courtesy of Alexei Ivanov.

Observations during March 2003. The name of the volcano in the Mongolian language means "black-pot" and through questioning of the local inhabitants, it was learned that there is a local myth that a dragon lived in the volcano. The local inhabitants also mentioned that marmots, previously abundant in the area, began to migrate westwards five years ago; they are now practically absent from the area.

Acoustic noise and venting of colorless warm gas from a small hole near the summit were noticed in October 2002 by local residents. In December 2002, while snow lay on the ground, the hole was clearly visible to local visitors, and a second hole could be seen a few meters away; it is unclear whether or not white vapors were noticed on this occasion. During the inspection in March 2003 a third hole was seen. The second hole is located within a 3 x 3 m outcrop of cinder and pumice (figure 2) whereas the first and the third holes are located within massive basalts. When close to the holes, constant noise resembled a rapid river heard from afar. The second hole was covered with plastic sheeting fixed at the margins, but the plastic was blown off within 2-3 seconds. Gas from the second hole was sampled in a mechanically pumped glass sampler. Analysis by gas chromatography, performed a week later at the Institute of the Earth's Crust, showed that nitrogen and atmospheric air were the major constituents.

Figure 2. Photograph of the second hole sampled at Har-Togoo, with hammer for scale, March 2003. Courtesy of Alexei Ivanov.

The temperature of the gas at the first, second, and third holes was +1.1, +1.4, and +2.7°C, respectively, while air temperature was -4.6 to -4.7°C (measured on 19 March 2003). Repeated measurements of the temperatures on the next day gave values of +1.1, +0.8, and -6.0°C at the first, second, and third holes, respectively. Air temperature was -9.4°C. To avoid bias due to direct heating from sunlight the measurements were performed under shadow. All measurements were done with Chechtemp2 digital thermometer with precision of ± 0.1°C and accuracy ± 0.3°C.

Inside the mouth of the first hole was 4-10-cm-thick ice with suspended gas bubbles (figure 5). The ice and snow were sampled in plastic bottles, melted, and tested for pH and Eh with digital meters. The pH-meter was calibrated by Horiba Ltd (Kyoto, Japan) standard solutions 4 and 7. Water from melted ice appeared to be slightly acidic (pH 6.52) in comparison to water of melted snow (pH 7.04). Both pH values were within neutral solution values. No prominent difference in Eh (108 and 117 for ice and snow, respectively) was revealed.

Two digital short-period three-component stations were installed on top of Har-Togoo, one 50 m from the degassing holes and one in a remote area on basement rocks, for monitoring during 19-20 March 2003. Every hour 1-3 microseismic events with magnitude <2 were recorded. All seismic events were virtually identical and resembled A-type volcano-tectonic earthquakes (figure 6). Arrival difference between S and P waves were around 0.06-0.3 seconds for the Har-Togoo station and 0.1-1.5 seconds for the remote station. Assuming that the Har-Togoo station was located in the epicentral zone, the events were located at ~1-3 km depth. Seismic episodes similar to volcanic tremors were also recorded (figure 3).

Figure 3. Examples of an A-type volcano-tectonic earthquake and volcanic tremor episodes recorded at the Har-Togoo station on 19 March 2003. Courtesy of Alexei Ivanov.

Conclusions. The abnormal thermal and seismic activities could be the result of either hydrothermal or volcanic processes. This activity could have started in the fall of 2002 when they were directly observed for the first time, or possibly up to five years earlier when marmots started migrating from the area. Further studies are planned to investigate the cause of the fumarolic and seismic activities.

At the end of a second visit in early July, gas venting had stopped, but seismicity was continuing. In August there will be a workshop on Russian-Mongolian cooperation between Institutions of the Russian and Mongolian Academies of Sciences (held in Ulan-Bator, Mongolia), where the work being done on this volcano will be presented.

References. Devyatkin, E.V. and Smelov, S.B., 1979, Position of basalts in sequence of Cenozoic sediments of Mongolia: Izvestiya USSR Academy of Sciences, geological series, no. 1, p. 16-29. (In Russian).

Logatchev, N.A., Devyatkin, E.V., Malaeva, E.M., and others, 1982, Cenozoic deposits of Taryat basin and Chulutu river valley (Central Hangai): Izvestiya USSR Academy of Sciences, geological series, no. 8, p. 76-86. (In Russian).

Geologic Background. The Miocene Har-Togoo shield volcano, also known as Togoo Tologoy, is situated on top of a vast volcanic plateau. The 5,000-year-old Khorog (Horog) cone in the Taryatu-Chulutu volcanic field is located 135 km SW and the Quaternary Urun-Dush cone in the Khanuy Gol (Hanuy Gol) volcanic field is 95 km ENE. Analysis of seismic activity recorded by a network of seismic stations across Mongolia shows that earthquakes of magnitude 2-3.5 are scattered around the Har-Togoo volcano at a distance of 10-15 km.

Information Contacts: Alexei V. Ivanov, Institute of the Earth Crust SB, Russian Academy of Sciences, Irkutsk, Russia; Bekhtur andM. Ulziibat, Research Center of Astronomy and Geophysics, Mongolian Academy of Sciences, Ulan-Bator, Mongolia; M. Ganzorig, Institute of Informatics MAS, Ulan-Bator, Mongolia.

An eruption at Mount Elgon was mistakenly inferred when fumes escaped from this otherwise quiet volcano. The fumes were eventually traced to dung burning in a lava-tube cave. The cave is home to, or visited by, wildlife ranging from bats to elephants. Mt. Elgon (Ol Doinyo Ilgoon) is a stratovolcano on the SW margin of a 13 x 16 km caldera that straddles the Uganda-Kenya border 140 km NE of the N shore of Lake Victoria. No eruptions are known in the historical record or in the Holocene.

On 7 September 2004 the web site of the Kenyan newspaper The Daily Nation reported that villagers sighted and smelled noxious fumes from a cave on the flank of Mt. Elgon during August 2005. The villagers' concerns were taken quite seriously by both nations, to the extent that evacuation of nearby villages was considered.

The Daily Nation article added that shortly after the villagers' reports, Moses Masibo, Kenya's Western Province geology officer visited the cave, confirmed the villagers observations, and added that the temperature in the cave was 170°C. He recommended that nearby villagers move to safer locations. Masibo and Silas Simiyu of KenGens geothermal department collected ashes from the cave for testing.

Gerald Ernst reported on 19 September 2004 that he spoke with two local geologists involved with the Elgon crisis from the Geology Department of the University of Nairobi (Jiromo campus): Professor Nyambok and Zacharia Kuria (the former is a senior scientist who was unable to go in the field; the latter is a junior scientist who visited the site). According to Ernst their interpretation is that somebody set fire to bat guano in one of the caves. The fire was intense and probably explains the vigorous fuming, high temperatures, and suffocated animals. The event was also accompanied by emissions of gases with an ammonia odor. Ernst noted that this was not surprising considering the high nitrogen content of guano—ammonia is highly toxic and can also explain the animal deaths. The intense fumes initially caused substantial panic in the area.

It was Ernst's understanding that the authorities ordered evacuations while awaiting a report from local scientists, but that people returned before the report reached the authorities. The fire presumably prompted the response of local authorities who then urged the University geologists to analyze the situation. By the time geologists arrived, the fuming had ceased, or nearly so. The residue left by the fire and other observations led them to conclude that nothing remotely related to a volcanic eruption had occurred.

However, the incident emphasized the problem due to lack of a seismic station to monitor tectonic activity related to a local triple junction associated with the rift valley or volcanic seismicity. In response, one seismic station was moved from S Kenya to the area of Mt. Elgon so that local seismicity can be monitored in the future.

Information Contacts: Gerald Ernst, Univ. of Ghent, Krijgslaan 281/S8, B-9000, Belgium; Chris Newhall, USGS, Univ. of Washington, Dept. of Earth & Space Sciences, Box 351310, Seattle, WA 98195-1310, USA; The Daily Nation (URL: http://www.nationmedia.com/dailynation/); Uganda Tourist Board (URL: http://www.visituganda.com/).