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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); Institute of Volcanology and Seismology, Far Eastern Branch, Russian Academy of Sciences (IVS FEB RAS), 9 Piip Blvd., Petropavlovsk-Kamchatsky 683006, Russia (URL: http://www.kscnet.ru/ivs/eng/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/).

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

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

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

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

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

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

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Information Contacts: Sezione di Catania - Osservatorio Etneo, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/it/); Toulouse Volcanic Ash Advisory Center (VAAC), Météo-France, 42 Avenue Gaspard Coriolis, F-31057 Toulouse cedex, France (URL: http://www.meteo.fr/aeroweb/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Boris Behncke, Sonia Calvari, and Marco Neri, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: https://twitter.com/etnaboris, Image at https://twitter.com/etnaboris/status/1183640328760414209/photo/1).

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

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

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

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

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

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

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

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

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

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

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

Information Contacts: Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG), Center for Research and Development of Geological Disaster Technology (URL: http://merapi.bgl.esdm.go.id/, Twitter: @BPPTKG); Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Badan Nasional Penanggulangan Bencana (BNPB), National Disaster Management Agency, Graha BNPB - Jl. Scout Kav.38, East Jakarta 13120, Indonesia (URL: http://www.bnpb.go.id/, Twitter: https://twitter.com/BNPB_Indonesia); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Jamie S. Sincioco, Phillipines (Twitter: @jaimessincioco, Image at https://twitter.com/jaimessincioco/status/1227966075519635456/photo/1).

Erta Ale is a shield volcano located in Ethiopia and contains multiple active pit craters in the summit and southeastern caldera. Volcanism has been characterized by lava flows and large lava flow fields since 2017. Surficial lava flow activity continued within the southeastern caldera during November 2019 until early April 2020; source information was primarily from various satellite data.

The number of days that thermal anomalies were detected using MODIS data in MODVOLC and NASA VIIRS satellite data was notably higher in November and December 2019 (figure 96); the number of thermal anomalies in the Sentinel-2 thermal imagery was substantially lower due to the presence of cloud cover. Across all satellite data, thermal anomalies were identified for 29 days in November, followed by 30 days in December. After December 2019, the number of days thermal anomalies were detected decreased; hotspots were detected for 17 days in January 2020 and 20 days in February. By March, these thermal anomalies became rare until activity ceased. Thermal anomalies were identified during 1-4 March, with weak anomalies seen again during 26 March-8 April 2020.

Figure 96. Graph comparing the number of thermal alerts using calendar dates using MODVOLC, NASA VIIRS, and Sentinel-2 satellite data for Erta Ale during November 2019-March 2020. Data courtesy of HIGP - MODVOLC Thermal Alerts System, NASA Worldview using the “Fire and Thermal Anomalies” layer, and Sentinel Hub Playground.

MIROVA (Middle Infrared Observation of Volcanic Activity) analysis of MODIS satellite data showed frequent strong thermal anomalies from 18 April through December 2019 (figure 97). Between early August 2019 and March 2020, these thermal signatures were detected at distances less than 5 km from the summit. In late December the thermal intensity dropped slightly before again increasing, while at the same time moving slightly closer to the summit. Thermal anomalies then became more intermittent and steadily decreased in power over the next two months.

Figure 97. Two time-series plots of thermal anomalies from Erta Ale from 18 April 2019 through 18 April 2020 as recorded by the MIROVA system. The top plot (A) shows that the thermal anomalies were consistently strong (measured in log radiative power) and occurred frequently until early January 2020 when both the power and frequency visibly declined. The lower plot (B) shows these anomalies as a function of distance from the summit, including a sudden decrease in distance (measured in kilometers) in early August 2019, reflecting a change in the location of the lava flow outbreak. A smaller distance change can be identified at the end of December 2019. Courtesy of MIROVA.

Unlike the obvious distal breakouts to the NE seen previously (BGVN 44:04 and 44:11), infrared satellite imagery during November-December 2019 showed only a small area with a thermal anomaly near the NE edge of the Southeast Caldera (figure 98). A thermal alert was seen at that location using the MODVOLC system on 28 December, but the next day it had been replaced by an anomaly about 1.5 km WSW near the N edge of the Southeast Caldera where the recent flank eruption episode had been centered between January 2017 and January 2018 (BGVN 43:04). The thermal anomaly that was detected in the summit caldera was no longer visible after 9 January 2020, based on Sentinel-2 imagery. The exact location of lava flows shifted within the same general area during January and February 2020 and was last detected by Sentinel-2 on 4 March. After about two weeks without detectable thermal activity, weak unlocated anomalies were seen in VIIRS data on 26 March and in MODIS data on the MIROVA system four times between 26 March and 8 April. No further anomalies were noted through the rest of April 2020.

Figure 98. Sentinel-2 thermal satellite imagery of Erta Ale volcanism between November 2019 and March 2020 showing small lava flow outbreaks (bright yellow-orange) just NE of the southeastern calderas. A thermal anomaly can be seen in the summit crater on 15 November and very faintly on 20 December 2019. Imagery on 19 January 2020 showed a small thermal anomaly near the N edge of the Southeast Caldera where the recent flank eruption episode had been centered between January 2017 and January 2018. The last weak thermal hotspot was detected on 4 March (bottom right). Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

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

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

Rincón de la Vieja is a remote volcanic complex in Costa Rica containing an acid lake that has regularly generated weak phreatic explosions since 2011 (BGVN 44:08). The most recent eruptive period occurred during late March-early June 2019, primarily consisting of small phreatic explosions, minor deposits on the N crater rim, and gas-and-steam emissions. The report period of August 2019-March 2020 was characterized by similar activity, including small phreatic explosions, gas-and-steam plumes, ash and lake sediment ejecta, and volcanic tremors. The most significant activity during this time occurred on 30 January, where a phreatic explosion ejected ash and lake sediment above the crater rim, resulting in a pyroclastic flow which gradually turned into a lahar. Information for this reporting period of August 2019-March 2020 comes from the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA) using weekly bulletins.

According to OVSICORI-UNA, a small hydrothermal eruption was recorded on 1 August 2019. The seismicity was low with a few long period (LP) earthquakes around 1 August and intermittent background tremor. No explosions or emissions were reported through 11 September; seismicity remained low with an occasional LP earthquake and discontinuous tremor. The summit’s extension that has been recorded since the beginning of June stopped, and no significant deformation was observed in August.

Starting again in September 2019 and continuing intermittently through the reporting period, some deformation was observed at the base of the volcano as well as near the summit, according to OVSICORI-UNA. On 12 September an eruption occurred that was followed by volcanic tremors that continued through 15 September. In addition to these tremors, vigorous sustained gas-and-steam plumes were observed. The 16 September weekly bulletin did not describe any ejecta produced as a result of this event.

During 1-3 October small phreatic eruptions were accompanied by volcanic tremors that had decreased by 5 October. In November, volcanism and seismicity were relatively low and stable; few LP earthquakes were reported. This period of low activity remained through December. At the end of November, horizontal extension was observed at the summit, which continued through the first half of January.

Small phreatic eruptions were recorded on 2, 28, and 29 January 2020, with an increase in seismicity occurring on 27 January. On 30 January at 1213 a phreatic explosion produced a gas column that rose 1,500-2,000 m above the crater, with ash and lake sediment ejected up to 100 m above the crater. A news article posted by the Universidad de Costa Rica (UCR) noted that this explosion generated pyroclastic flows that traveled down the N flank for more than 2 km from the crater. As the pyroclastic flows moved through tributary channels, lahars were generated in the Pénjamo river, Zanjonuda gorge, and Azufrosa, traveling N for 4-10 km and passing through Buenos Aires de Upala (figure 29). Seismicity after this event decreased, though there were still some intermittent tremors.

Figure 29. Photo of a lahar generated from the 30 January 2020 eruption at Rincon de la Vieja. Photo taken by Mauricio Gutiérrez, courtesy of UCR.

On 17, 24, and 25 February and 11, 17, 19, 21, and 23 March, small phreatic eruptions were detected, according to OVSICORI-UNA. Geodetic measurements observed deformation consisting of horizontal extension and inflation near the summit in February-March. By the week of 30 March, the weekly bulletin reported 2-3 small eruptions accompanied by volcanic tremors occurred daily during most days of the week. None of these eruptions produced solid ejecta, pyroclastic flows, or lahars, according to the weekly OVSICORI-UNA bulletins during February-March 2020.

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/); Luis Enrique Brenes Portuguéz, University of Costa Rica, Ciudad Universitaria Rodrigo Facio Brenes, San José, San Pedro, Costa Rica (URL: https://www.ucr.ac.cr/noticias/2020/01/30/actividad-del-volcan-rincon-de-la-vieja-es-normal-segun-experto.html).

Manam is a basaltic-andesitic stratovolcano that lies 13 km off the northern coast of mainland Papua New Guinea; it has a 400-year history of recorded evidence for recurring low-level ash plumes, occasional Strombolian activity, lava flows, pyroclastic avalanches, and large ash plumes from Main and South, the two active summit craters. The current eruption, ongoing since June 2014, produced multiple large explosive eruptions during January-September 2019, including two 15-km-high ash plumes in January, repeated SO₂ plumes each month, and another 15.2 km-high ash plume in June that resulted in ashfall and evacuations of several thousand people (BGVN 44:10).

This report covers continued activity during October 2019 through March 2020. Information about Manam is primarily provided by Papua New Guinea's Rabaul Volcano Observatory (RVO), part of the Department of Mineral Policy and Geohazards Management (DMPGM). This information is supplemented with aviation alerts from the Darwin Volcanic Ash Advisory Center (VAAC). MODIS thermal anomaly satellite data is recorded by the University of Hawai'i's MODVOLC thermal alert recording system, and the Italian MIROVA project; sulfur dioxide monitoring is done by instruments on satellites managed by NASA's Goddard Space Flight Center. Satellite imagery provided by the Sentinel Hub Playground is also a valuable resource for information about this remote location.

A few modest explosions with ash emissions were reported in early October and early November 2019, and then not again until late March 2020. Although there was little explosive activity during the period, thermal anomalies were recorded intermittently, with low to moderate activity almost every month, as seen in the MODIS data from MIROVA (figure 71) and also in satellite imagery. Sulfur dioxide emissions persisted throughout the period producing emissions greater than 2.0 Dobson Units that were recorded in satellite data 3-13 days each month.

Figure 71. MIROVA thermal anomaly data for Manam from 17 June 2019 through March 2020 indicate continued low and moderate level thermal activity each month from August 2019 through February 2020, after a period of increased activity in June and early July 2019. Courtesy of MIROVA.

The Darwin VAAC reported an ash plume in visible satellite imagery moving NW at 3.1 km altitude on 2 October 2019. Weak ash emissions were observed drifting N for the next two days along with an IR anomaly at the summit. RVO reported incandescence at night during the first week of October. Visitors to the summit on 18 October 2019 recorded steam and fumarolic activity at both of the summit craters (figure 72) and recent avalanche debris on the steep slopes (figure 73).

Figure 72. Steam and fumarolic activity rose from Main crater at Manam on 18 October 2019 in this view to the south from a ridge north of the crater. Google Earth inset of summit shows location of photograph. Courtesy of Vulkanologische Gesellschaft and Claudio Jung, used with permission.

Figure 73. Volcanic debris covered an avalanche chute on the NE flank of Manam when visited by hikers on 18 October 2019. Courtesy of Vulkanologische Gesellschaft and Claudio Jung, used with permission.

On 2 November, a single large explosion at 1330 local time produced a thick, dark ash plume that rose about 1,000 m above the summit and drifted NW. A shockwave from the explosion was felt at the Bogia Government station located 40 km SE on the mainland about 1 minute later. RVO reported an increase in seismicity on 6 November about 90 minutes before the start of a new eruption from the Main Crater which occurred between 1600 and 1630; it produced light to dark gray ash clouds that rose about 1,000 m above the summit and drifted NW. Incandescent ejecta was visible at the start of the explosion and continued with intermittent strong pulses after dark, reaching peak intensity around 1900. Activity ended by 2200 that evening. The Darwin VAAC reported a discrete emission observed in satellite imagery on 8 November that rose to 4.6 km altitude and drifted WNW, although ground observers confirmed that no eruption took place; emissions were only steam and gas. There were no further reports of explosive activity until the Darwin VAAC reported an ash emission in visible satellite imagery on 20 March 2020 that rose to 3.1 km altitude and drifted E for a few hours before dissipating.

Although explosive activity was minimal during the period, SO₂ emissions, and evidence for continued thermal activity were recorded by satellite instruments each month. The TROPOMI instrument on the Sentinel-5P satellite captured evidence each month of SO₂ emissions exceeding two Dobson Units (figure 74). The most SO₂ activity occurred during October 2019, with 13 days of signatures over 2.0 DU. There were six days of elevated SO₂ each month in November and December, and five days in January 2020. During February and March, activity was less, with smaller SO₂ plumes recording more than 2.0 DU on three days each month. Sentinel-2 satellite imagery recorded thermal anomalies at least once from one or both of the summit craters each month between October 2019 and March 2020 (figure 75).

Figure 74. SO2 emissions at Manam exceeded 2 Dobson Units multiple days each month between October 2019 and March 2020. On 3 October 2019 (top left) emissions were also measured from Ulawun located 700 km E on New Britain island. On 30 November 2019 (top middle), in addition to a plume drifting N from Manam, a small SO2 plume was detected at Bagana on Bougainville Island, 1150 km E. The plume from Manam on 2 December 2019 drifted ESE (top right). On 26 January 2020 the plume drifted over 300 km E (bottom left). The plumes measured on 29 February and 4 March 2020 (bottom middle and right) only drifted a few tens of kilometers before dissipating. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Figure 75. Sentinel-2 satellite imagery with Atmospheric penetration rendering (bands 12, 11, and 8a) showed thermal anomalies at one or both of Manam’s summit craters each month during October 2019-March 2020. On 17 October 2019 (top left) a bright anomaly and weak gas plume drifted NW from South crater, while a dense steam plume and weak anomaly were present at Main crater. On 25 January 2020 (top right) the gas and steam from the two craters were drifting E; the weaker Main crater thermal anomaly is just visible at the edge of the clouds. A clear image on 5 March 2020 (bottom left) shows weak plumes and distinct thermal anomalies from both craters; on 20 March (bottom right) the anomalies are still visible through dense cloud cover that may include steam from the crater vents as well. Courtesy of Sentinel Hub Playground.

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: Rabaul Volcano Observatory (RVO), Geohazards Management Division, Department of Mineral Policy and Geohazards Management (DMPGM), PO Box 3386, Kokopo, East New Britain Province, Papua New Guinea; MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Vulkanologische Gesellschaft (URL: https://twitter.com/vulkanologen/status/1194228532219727874, https://twitter.com/vulkanologen/status/1193788836679225344); Claudio Jung, (URL: https://www.facebook.com/claudio.jung.1/posts/10220075272173895, https://www.instagram.com/jung.claudio/).

Near-constant fountains of lava at Stromboli have served as a natural beacon in the Tyrrhenian Sea for at least 2,000 years. Eruptive activity at the summit consistently occurs from multiple vents at both a north crater area (N area) and a southern crater group (CS area) on the Terrazza Craterica at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the volcano-island (figure 168). Periodic lava flows emerge from the vents and flow down the scarp, sometimes reaching the sea; occasional large explosions produce ash plumes and pyroclastic flows. Thermal and visual cameras that monitor activity at the vents are located on the nearby Pizzo Sopra La Fossa, above the Terrazza Craterica, and at multiple locations on the flanks of the volcano. Detailed information for Stromboli is provided by Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) as well as other satellite sources of data; September-December 2019 is covered in this report.

Figure 168. This shaded relief map of Stromboli’s crater area was created from images acquired by drone on 9 July 2019 (In collaboration with GEOMAR drone group, Helmholtz Center for Ocean Research, Kiel, Germany). Inset shows Stromboli Island, the black rectangle indicates the area of the larger image, the black curved and the red hatched lines indicate, respectively, the morphological escarpment and the crater edges. Courtesy of INGV (Rep. No. 50/2019, Stromboli, Bollettino Settimanale, 02/12/2019 - 08/12/2019, data emissione 10/12/2019).

Activity was very consistent throughout the period of September-December 2019. Explosion rates ranged from 2-36 per hour and were of low to medium-high intensity, producing material that rose from less than 80 to over 150 m above the vents on occasion (table 7). The Strombolian activity in both crater areas often sent ejecta outside the crater rim onto the Terrazza Craterica, and also down the Sciara del Fuoco towards the coast. After the explosions of early July and late August, thermal activity decreased to more moderate levels that persisted throughout the period as seen in the MIROVA Log Radiative Power data (figure 169). Sentinel-2 satellite imagery supported descriptions of the constant glow at the summit, revealing incandescence at both summit areas, each showing repeating bursts of activity throughout the period (figure 170).

Table 7. Monthly summary of activity levels at Stromboli, September-December 2019. Low-intensity activity indicates ejecta rising less than 80 m, medium-intensity is ejecta rising less than 150 m, and high-intensity is ejecta rising over 200 m above the vent. Data courtesy of INGV.

Figure 169. Thermal activity at Stromboli was high during July-August 2019, when two major explosions occurred. Activity continued at more moderate levels through December 2019 as seen in the MIROVA graph of Log Radiative Power from 8 June through December 2019. Courtesy of MIROVA.

Figure 170. Stromboli reliably produced strong thermal signals from both of the summit vents throughout September-December 2019 and has done so since long before Sentinel-2 satellite imagery was able to detect it. Image dates are (top, l to r) 5 September, 15 October, 20 October, (bottom l to r) 14 November, 14 December 2019, and 3 January 2020. Sentinel-2 imagery uses Atmospheric penetration rendering with bands 12, 11, and 8A, courtesy of Sentinel Hub Playground.

After a major explosion with a pyroclastic flow on 28 August 2019, followed by lava flows that reached the ocean in the following days (BGVN 44:09), activity diminished in early September to levels more typically seen in recent times. This included Strombolian activity from vents in both the N and CS areas that sent ejecta typically 80-150 m high. Ejecta from the N area generally consisted of lapilli and bombs, while the material from the CS area was often finer grained with significant amounts of lapilli and ash. The number of explosive events remained high in September, frequently reaching 25-30 events per hour. The ejecta periodically landed outside the craters on the Terrazza Craterica and even traveled partway down the Sciara del Fuoco. An inspection on 7 September by INGV revealed four eruptive vents in the N crater area and five in the S crater area (figure 171). The most active vents in the N area were N1 with mostly ash emissions and N2 with Strombolian explosions rich in incandescent coarse material that sometimes rose well above 150 m in height. In the S area, S1 and S2 produced jets of lava that often reached 100 m high. A small cone was observed around N2, having grown after the 28 August explosion. Between 11 and 13 September aerial surveys with drones produced detailed visual and thermal imagery of the summit (figure 172).

Figure 171. Video of the Stromboli summit taken with a thermal camera on 7 September 2019 from the Pizzo sopra la Fossa revealed four active vents in the N area and five active vents in the S area. Images prepared by Piergiorgio Scarlato, courtesy of INGV (Rep. No. 37.2/2019, Stromboli, Bollettino Giornaliero del 10/09/2019).

Figure 172. An aerial drone survey on 11 September 2019 at Stromboli produced a detailed view of the N and CS vent areas (left) and thermal images taken by a drone survey on 13 September (right) showed elevated temperatures down the Sciara del Fuoco in addition to the vents in the N and CS areas. Images by E. De Beni and M. Cantarero, courtesy of INGV (Rep. No. 37.5/2019, Stromboli, Bollettino Giornaliero del 13/09/2019).

Strombolian activity from the N crater on 28 September and 1 October 2019 produced blocks and debris that rolled down the Sciara del Fuoco and reached the ocean (figure 173). Explosive activity from the CS crater area sometimes produced ejecta over 150 m high (figure 174). A survey on 26 November revealed that a layer of ash 5-10 cm thick had covered the bombs and blocks that were deposited on the Pizzo Sopra la Fossa during the explosions of 3 July and 28 August (figure 175). On the morning of 27 December a lava flow emerged from the CS area and traveled a few hundred meters down the Sciara del Fuoco. The frequency of explosive events remained relatively constant from September through December 2019 after decreasing from higher levels during July and August (figure 176).

Figure 173. Strombolian activity from vents in the N crater area of Stromboli produced ejecta that traveled all the way to the bottom of the Sciara del Fuoco and entered the ocean. Top images taken 28 September 2019 from the 290 m elevation viewpoint by Rosanna Corsaro. Bottom images captured on 1 October from the webcam at 400 m elevation. Courtesy of INGV (Rep. No. 39.0/2019 and Rep. No. 40.3, Stromboli, Bollettino Giornaliero del 29/09/2019 and 02/10/2019).

Figure 174. Ejecta from Strombolian activity at the CS crater area of Stromboli rose over 150 m on multiple occasions. The webcam located at the 400 m elevation site captured this view of activity on 8 November 2019. Courtesy of INGV (Rep. No. 45.5/2019, Stromboli, Bollettino Giornaliero del 08/11/2019).

Figure 175. The Pizzo Sopra la Fossa area at Stromboli was covered with large blocks and pyroclastic debris on 6 September 2019, a week after the major explosion of 28 August (top). By 26 November, 5-10 cm of finer ash covered the surface; the restored webcam can be seen at the far right edge of the Pizzo (bottom). Courtesy of INGV (Rep. No. 49/2019, Stromboli, Bollettino Settimanale, 25/11/2019 - 01/12/2019, data emissione 03/12/2019).

Figure 176. The average hourly frequency of explosive events at Stromboli captured by surveillance cameras from 1 June 2019 through 5 January 2020 remained generally constant after the high levels seen during July and August. The Total value (blue) is the sum of the average daily hourly frequency of all explosive events produced by active vents.

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

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy, (URL: http://www.ct.ingv.it/en/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Semeru is a stratovolcano located in East Java, Indonesia containing an active Jonggring-Seloko vent at the Mahameru summit. Common activity has consisted of ash plumes, pyroclastic flows and avalanches, and lava flows that travel down the SE flank. This report updates volcanism from September 2019 to February 2020 using primary information from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM) and the Darwin Volcanic Ash Advisory Centre (VAAC).

The dominant activity at Semeru for this reporting period consists of ash plumes, which were frequently reported by the Darwin VAAC. An eruption on 10 September 2019 produced an ash plume rising 4 km altitude drifting WNW, as seen in HIMAWARI-8 satellite imagery. Ash plumes continued to rise during 13-14 September. During the month of October the Darwin VAAC reported at least six ash plumes on 13, 14, 17-18, and 29-30 October rising to a maximum altitude of 4.6 km and moving primarily S and SW. Activity in November and December was relatively low, dominated mostly by strong and frequent thermal anomalies.

Volcanism increased in January 2020 starting with an eruption on 17 and 18 January that sent a gray ash plume up to 4.6 km altitude (figure 38). Eruptions continued from 20 to 26 January, producing ash plumes that rose up to 500 m above the crater that drifted in different directions. For the duration of the month and into February, ash plumes occurred intermittently. On 26 February, incandescent ejecta was ejected up to 50 m and traveled as far as 1000 m. Small sulfur dioxide emissions were detected in the Sentinel 5P/TROPOMI instrument during 25-27 February (figure 39). Lava flows during 27-29 February extended 200-1,000 m down the SE flank; gas-and-steam and SO₂ emissions accompanied the flows. There were 15 shallow volcanic earthquakes detected on 29 February in addition to ash emissions rising 4.3 km altitude drifting ESE.

Figure 38. Ash plumes rising from the summit of Semeru on 17 (left) and 18 (right) January 2020. Courtesy of MAGMA Indonesia and via Ø.L. Andersen's Twitter feed (left).

Figure 39. Small SO2 plumes from Semeru were detected by the Sentinel 5P/TROPOMI instrument during 25 (left) and 26 (right) February 2020. Courtesy of NASA Goddard Space Flight Center.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed relatively weak and intermittent thermal anomalies occurring during May to August 2019 (figure 40). The frequency and power of these thermal anomalies significantly increased during September to mid-December 2019 with a few hotspots occurring at distances greater than 5 km from the summit. These farther thermal anomalies to the N and NE of the volcano do not appear to be caused by volcanic activity. There was a brief break in activity during mid-December to mid-January 2020 before renewed activity was detected in early February 2020.

Figure 40. Thermal anomalies were relatively weak at Semeru during 30 April 2019-August 2019, but significantly increased in power and frequency during September to early December 2019. There was a break in activity from mid-December through mid-January 2020 with renewed thermal anomalies around February 2020. Courtesy of MIROVA.

The MODVOLC algorithm detected 25 thermal hotspots during this reporting period, which took place during 25 September, 18 and 21 October 2019, 29 January, and 11, 14, 16, and 23 February 2020. Sentinel-2 thermal satellite imagery shows intermittent hotspots dominantly in the summit crater throughout this reporting period (figure 41).

Figure 41. Sentinel-2 thermal satellite imagery detected intermittent thermal anomalies (bright yellow-orange) at the summit of Semeru, which included some lava flows in late January to early February 2020. Sentinel-2 atmospheric penetration (bands 12, 11, 8A) images courtesy of Sentinel Hub Playground.

Geologic Background. Semeru, the highest volcano on Java, and one of its most active, lies at the southern end of a volcanic massif extending north to the Tengger caldera. The steep-sided volcano, also referred to as Mahameru (Great Mountain), rises above coastal plains to the south. Gunung Semeru was constructed south of the overlapping Ajek-ajek and Jambangan calderas. A line of lake-filled maars was constructed along a N-S trend cutting through the summit, and cinder cones and lava domes occupy the eastern and NE flanks. Summit topography is complicated by the shifting of craters from NW to SE. Frequent 19th and 20th century eruptions were dominated by small-to-moderate explosions from the summit crater, with occasional lava flows and larger explosive eruptions accompanied by pyroclastic flows that have reached the lower flanks of the volcano.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); 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); 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/); Øystein Lund Andersen (Twitter: @OysteinLAnderse, https://twitter.com/OysteinLAnderse, URL: http://www.oysteinlundandersen.com).

Frequent historical eruptions have been reported from Mexico's Popocatépetl going back to the 14th century. Activity increased in the mid-1990s after about 50 years of quiescence, and the current eruption, ongoing since January 2005, has included numerous episodes of lava-dome growth and destruction within the 500-m-wide summit caldera. Multiple emissions of steam and gas occur daily, rising generally 1-3 km above the summit at about 5,400 m elevation; many contain small amounts of ash. Larger, more explosive events with ash plumes and incandescent ejecta landing on the flanks occur frequently. Activity through August 2019 was typical of the ongoing eruption with near-constant emissions of water vapor, gas, and minor ash, as well as multiple explosions with ash plumes and incandescent blocks scattered on the flanks (BGVN 44:09). This report covers similar activity from September 2019 through February 2020. Information comes from daily reports provided by México's Centro Nacional de Prevención de Desastres (CENAPRED); ash plumes are reported by the Washington Volcanic Ash Advisory Center (VAAC). Satellite visible and thermal imagery and SO₂ data also provide helpful observations of activity.

Activity summary. Activity at Popocatépetl during September 2019-February 2020 continued at the high levels that have been ongoing for many years, characterized by hundreds of daily low-intensity emissions that included steam, gas, and small amounts of ash, and periods with multiple daily minor and moderate explosions that produce kilometer-plus-high ash plumes (figure 140). The Washington VAAC issued multiple daily volcanic ash advisories with plume altitudes around 6 km for many, although some were reported as high as 8.2 km. Hundreds of minutes of daily tremor activity often produced ash emissions as well. Incandescent ejecta landed 500-1,000 m from the summit frequently. The MIROVA thermal anomaly data showed near-constant moderate to high levels of thermal energy throughout the period (figure 141).

Figure 140. Emissions continued at a high rate from Popocatépetl throughout September 2019-February 2020. Daily low-intensity emissions numbered usually in the hundreds (blue, left axis), while less frequent minor (orange) and moderate (green) explosions, plotted on the right axis, occurred intermittently through November 2019, and increased again during February 2020. Data was compiled from CENAPRED daily reports.

Figure 141. MIROVA log radiative power thermal data for Popocatépetl from 1 May 2019 through February 2020 showed a constant output of moderate energy the entire time. Courtesy of MIROVA.

Sulfur dioxide emissions were measured with satellite instruments many days of each month from September 2019 thru February 2020. The intensity and drift directions varied significantly; some plumes remained detectable hundreds of kilometers from the volcano (figure 142). Plumes were detected almost daily in September, and on most days in October. They were measured at lower levels but often during November, and after pulses in early and late December only small plumes were visible during January 2020. Intermittent larger pulses returned in February. Dome growth and destruction in the summit crater continued throughout the period. A small dome was observed inside the summit crater in late September. Dome 85, 210-m-wide, was observed inside the summit crater in early November. Satellite imagery captured evidence of dome growth and ash emissions throughout the period (figure 143).

Figure 142. Sulfur dioxide emissions from Popocatépetl were frequent from September 2019 through February 2020. Plumes drifted SW on 7 September (top left), 30 October (top middle), and 21 February (bottom right). SO2 drifted N and NW on 26 November (top right). On 2 December (bottom left) a long plume of sulfur dioxide hundreds of kilometers long drifted SW over the Pacific Ocean while the drift direction changed to NW closer to the volcano. The SO2 plumes measured in January (bottom center) were generally smaller than during the other months covered in this report. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Figure 143. Sentinel-2 satellite imagery of Popocatépetl during November 2019-February 2020 provided evidence for ongoing dome growth and explosions with ash emissions. Top left: a ring of incandescence inside the summit crater on 8 November 2019 was indicative of the growth of dome 85 observed by CENAPRED. Top middle: incandescence on 8 December inside the summit crater was typical of that observed many times during the period. Top right: a dense, narrow ash plume drifted N from the summit on 17 January 2020. Bottom left: Snow cover made ashfall on 6 February easily visible on the E flank. On 11 February, the summit crater was incandescent and nearly all the snow was covered with ash. Bottom right: a strong thermal anomaly and ash emission were captured on 21 February. Bottom left and top right images use Natural color rendering (bands 4, 3, 2); other images use Atmospheric penetration rendering to show infrared signal (bands 12, 11, 8A). Courtesy of Sentinel Hub Playground.

Activity during September-November 2019. On 1 September 2019 minor ashfall was reported in the communities of Atlautla, Ozumba, Juchitepec, and Tenango del Aire in the State of Mexico. The ash plumes rose less than 2 km above the summit and incandescent ejecta traveled less than 100 m from the summit crater. Twenty-two minor and three moderate explosions were recorded on 4-5 September along with minor ashfall in Juchitepec, Tenango del Aire, Tepetlixpa, and Atlautla. During a flyover on 5 September, officials did not observe a dome within the crater, and the dimensions remained the same as during the previous visit (350 m in diameter and 150 m deep) (figure 144). Ashfall was reported in Tlalmanalco and Amecameca on 6 September. The following day incandescent ejecta was visible on the flanks near the summit and ashfall was reported in Amecameca, Ayapango, and Tenango del Aire. The five moderate explosions on 8 September produced ash plumes that rose as high as 2 km above the summit, and incandescent ejecta on the flanks. Explosions on 10 September sent ejecta 500 m from the crater. Eight explosions during 20-21 September produced ejecta that traveled up to 1.5 km down the flanks (figure 145). During an overflight on 27 September specialists from the National Center for Disaster Prevention (CENAPRED ) of the National Coordination of Civil Protection and researchers from the Institute of Geophysics of UNAM observed a new dome 30 m in diameter; the overall crater had not changed size since the overflight in early September.

Figure 144. CENAPRED carried out overflights of Popocatépetl on 5 (left) and 27 September (right) 2019; the crater did not change in size, but a new dome 30 m in diameter was visible on 27 September. Courtesy of CENAPRED (Sobrevuelo al volcán Popocatépetl, 05 y 27 de septiembre).

Figure 145. Ash plumes at Popocatépetl on 19 (left) and 20 (right) September 2019 rose over a kilometer above the summit before dissipating. Courtesy of CENAPRED (Reporte del monitoreo de CENAPRED al volcán Popocatépetl 19 y 20 de septiembre).

Fourteen explosions were reported on 2 October 2019. The last one produced an ash plume that rose 2 km above the summit and sent incandescent ejecta down the E slope (figure 146). Ashfall was reported in the municipalities of Atlautla Ozumba, Ayapango and Ecatzingo in the State of Mexico. Explosions on 3 and 4 October also produced ash plumes that rose between 1 and 2 km above the summit and sent ejecta onto the flanks. Additional incandescent ejecta was reported on 6, 7, 15, and 19 October. The communities of Amecameca, Tenango del Aire, Tlalmanalco, Cocotitlán, Temamatla, and Tláhuac reported ashfall on 10 October; Amecameca reported more ashfall on 12 October. On 22 October slight ashfall appeared in Amecameca, Tenango del Aire, Tlalmanalco, Ayapango, Temamatla, and Atlautla.

Figure 146. Incandescent ejecta at Popocatépetl traveled down the E slope on 2 October 2019 (left); an ash plume two days later rose 2 km above the summit (right). Courtesy of CENAPRED (Reporte del monitoreo de CENAPRED al volcán Popocatépetl 2 y 4 de octubre).

During 2-3 November 2019 there was 780 minutes of tremor reported in four different episodes. The seismicity was accompanied by ash emissions that drifted W and NW and produced ashfall in numerous communities, including Amecameca, Juchitepec, Ozumba, Tepetlixpa, and Atlautla in the State of México, in Ayapango and Cuautla in the State of Morelos, and in the municipalities of Tlahuac, Tlalpan, and Xochimilco in Mexico City. A moderate explosion on 4 November sent incandescent ejecta 2 km down the slopes and produced an ash plume that rose 1.5 km and drifted NW. Minor ashfall was reported in Tlalmanalco, Amecameca, and Tenango del Aire, State of Mexico. Similar ash plumes from explosions occurred the following day. Scientists from CENAPRED and the Institute of Geophysics of UNAM observed dome number 85 during an overflight on 5 November 2019. It had a diameter of 210 m and was 80 m thick, with an irregular surface (figure 147). Multiple explosions on 6 and 7 November produced incandescent ejecta; a moderate explosion late on 11 November produced ejecta that traveled 1.5 km from the summit and produced an ash plume 2 km high (figure 148). A lengthy period of constant ash emission that drifted E was reported on 18 November. A moderate explosion on 28 November sent incandescent fragments 1.5 km down the slopes and ash one km above the summit.

Figure 147. A new dome was visible inside the summit crater at Popocatépetl during an overflight on 5 November 2019. It had a diameter of 210 m and was 80 m thick. Courtesy of CENAPRED (Sobrevuelo al volcán Popocatépetl, 05 de noviembre).

Figure 148. Ash emissions and explosions with incandescent ejecta continued at Popocatépetl during November 2019. The ash plume on 1 November changed drift direction sharply a few hundred meters above the summit (left). Incandescent ejecta traveled 1.5 km down the flanks on 11 November (right). Courtesy of CENAPRED (Reporte del monitoreo de CENAPRED al volcán Popocatépetl 1 y 12 de noviembre).

Activity during December 2019-February 2020. Throughout December 2019 weak emissions of steam and gas were reported daily, sometimes with minor amounts of ash, and minor explosions were only reported on 21 and 27 December. On 21 December two new high-resolution webcams were installed around Popocatépetl, one 5 km from the crater at the Tlamacas station, and the second in San Juan Tianguismanalco, 20 km away. Ash emissions and incandescent ejecta 800 m from the summit were observed on 25 December (figure 149). Incandescence at night was reported during 27-29 December.

Figure 149. Incandescent ejecta moved 800 m down the flanks of Popocatépetl during explosions on 25 December 2019 (left); weak emissions of steam, gas, and minor ash were visible on 27 December and throughout the month. Courtesy of CENAPRED (Reporte del monitoreo de CENAPRED al volcán Popocatépetl 25 y 27 de diciembre).

Continuous emissions of water vapor and gas with low ash content were typical daily during January 2020. A moderate explosion on 9 January produced an ash plume that rose 3 km from the summit and drifted NE. In addition, incandescent ejecta traveled 1 km from the crater rim. A minor explosion on 21 January produced a 1.5-km-high plume with low ash content and incandescent ejecta that fell near the crater (figure 150). The first of two explosions late on 27 January produced ejecta that traveled 500 m and a 1-km-high ash plume. Constant incandescence was observed overnight on 29-30 January.

Figure 150. Although fewer explosions were recorded at Popocatépetl during January 2020, activity continued. An ash plume on 19 January rose over a kilometer above the summit (top left). A minor explosion on 21 January produced a 1.5-km-high plume with low ash content and incandescent ejecta that fell near the crater (top right). Smaller emissions with steam, gas, and ash were typical many days, including on 22 (bottom left) and 31 (bottom right) January 2019. Courtesy of CENAPRED (Reporte del monitoreo de CENAPRED al volcán Popocatépetl 19, 21, 22 y 31 de enero).

A moderate explosion on 5 February 2020 produced an ash plume that rose 1.5 km and drifted NNE. Explosions on 10 and 13 February sent ejecta 500 m down the flanks (figure 151). During an overflight on 18 February scientists noted that the internal crater maintained a diameter of 350 m and its approximate depth was 100-150 m; the crater was covered by tephra. For most of the second half of February the volcano had a continuous emission of gases with minor amounts of ash. In addition, multiple explosions produced ash plumes that rose 400-1,200 m above the crater and drifted in several different directions.

Figure 151. Ash emissions and explosions continued at Popocatépetl during February 2020. Dense ash drifted near the snow-covered summit on 6 February (top left). Incandescent ejecta traveled 500 m down the flanks on 13 February (top right). Ash plumes billowed from the summit on 18 and 22 February (bottom row). Courtesy of CENAPRED (Reporte del monitoreo de CENAPRED al volcán Popocatépetl, 6, 15, 18 y 22 de febrero).

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

Information Contacts: Centro Nacional de Prevención de Desastres (CENAPRED), Av. Delfín Madrigal No.665. Coyoacan, México D.F. 04360, México (URL: http://www.cenapred.unam.mx/), Daily Report Archive http://www.cenapred.unam.mx:8080/reportesVolcanGobMX/BuscarReportesVolcan); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

The dacitic Santiaguito lava-dome complex on the W flank of Guatemala's Santa María volcano has been growing and actively erupting since 1922. Ash explosions, pyroclastic, and lava flows have emerged from Caliente, the youngest of the four vents in the complex, for more than 40 years. A lava dome that appeared within the summit crater of Caliente in October 2016 has continued to grow, producing frequent block avalanches down the flanks. Daily explosions with ash plumes and block avalanches continued during September 2019-February 2020, the period covered in this report, with information primarily from Guatemala's INSIVUMEH (Instituto Nacional de Sismologia, Vulcanologia, Meterologia e Hidrologia) and the Washington VAAC (Volcanic Ash Advisory Center).

Constant fumarolic activity with steam and gas persisted from the Caliente dome throughout September 2019-February 2020. Explosions occurred multiple times per day, producing ash plumes that rose to altitudes of 3.1-3.5 km and usually drifted a few kilometers before dissipating. Several lahars during September and October carried volcanic blocks, ash, and debris down major drainages. Periodic ashfall was reported in communities within 10 km of the volcano. An increase in thermal activity beginning in November (figure 101) resulted in an increased number of observations of incandescence visible at night from the summit of Caliente through February 2020. Block avalanches occurred daily on the flanks of the dome, often reaching the base, stirring up small clouds of ash that drifted downwind.

Figure 101. The MIROVA project graph of thermal activity at Santa María from 12 May 2019 through February 2020 shows a gradual increase in thermal energy beginning in November 2019. This corresponds to an increase in the number of daily observations of incandescence at the summit of the Caliente dome during this period. Courtesy of MIROVA.

Constant steam and gas fumarolic activity rose from the Caliente dome, drifting W, usually rising to 2.8-3.0 km altitude during September 2019. Multiple daily explosions with ash plumes rising to 2.9-3.4 km altitude drifted W or SW over the communities of San Marcos, Loma Linda Palajunoj, and Monte Claro (figure 102). Constant block avalanches fell to the base of the cone on the NE and SE flanks. The Washington VAAC reported an ash plume visible in satellite imagery on 10 September at 3.1 km altitude drifting W. On 14 September another plume was spotted moving WSW at 4.6 km altitude which dissipated quickly; the webcam captured another plume on 16 September. Ashfall on 27 September reached about 1 km from the volcano; it reached 1.5 km on 29 September. Lahars descended the Rio Cabello de Ángel on 2 and 24 September (figure 102). They were about 15 m wide, and 1-3 m deep, carrying blocks 1-2 m in diameter.

Figure 102. A lahar descended the Rio Cabello de Ángel at Santa Maria and flowed into the Rio Nima 1 on 24 September 2019. Courtesy of INSIVUMEH (Reporte Semanal de Monitoreo: Volcán Santiaguito (1402-03), Semana del 21 al 27 de septiembre de 2019).

Througout October 2019, degassing of steam with minor gases occurred from the Caliente summit, rising to 2.9-3.0 km altitude and generally drifting SW. Weak explosions took place 1-5 times per hour, producing ash plumes that rose to 3.2-3.5 km altitude. Ashfall was reported in Monte Claro on 2 October. Nearly constant block avalanches descended the SE and S flanks, disturbing recent layers of fine ash and producing local ash clouds. Moderate explosions on 11 October produced ash plumes that rose to 3.5 km altitude and drifted W and SW about 1.5 km towards Río San Isidro (figure 103). The following day additional plumes drifted a similar distance to the SE. The Washington VAAC reported an ash emission visible in satellite imagery at 4.9 km altitude on 13 October drifting NNW. Ashfall was reported in Parcelamiento Monte Claro on 14 October. Some of the block avalanches observed on 14 October on the SE, S, and SW flanks were incandescent. Ash drifted 1.5 km W and SW on 17 October. Ashfall was reported near la finca Monte Claro on 25 and 28 October. A lahar descended the Río San Isidro, a tributary of the Río El Tambor on 7 October carrying blocks 1-2 m in diameter, tree trunks, and branches. It was about 16 m wide and 1-2 m deep. Additional lahars descended the rio Cabello de Angel on 23 and 24 October. They were about 15 m wide and 2 m deep, and carried ash and blocks 1-2 m in diameter, tree trunks, and branches.

Figure 103. Daily ash plumes were reported from the Caliente cone at Santa María during October 2019, similar to these from 30 September (left) and 11 October 2019 (right). Courtesy of INSIVUMEH (Reporte Semanal de Monitoreo: Volcán Santiaguito (1402-03), Semana del 28 de septiembre al 04 de octubre de 2019; Reporte Semanal de Monitoreo: Volcán Santiaguito (1402-03), Semana del 05 al 11 de octubre de 2019).

During November 2019, steam plumes rose to 2.9-3.0 km altitude and generally drifted E. There were 1-3 explosions per hour; the ash plumes produced rose to altitudes of 3.1-3.5 km and often drifted SW, resulting in ashfall around the volcanic complex. Block avalanches descended the S and SW flanks every day. On 4 November ashfall was reported in the fincas (ranches) of El Faro, Santa Marta, El Viejo Palmar, and Las Marías, and the odor of sulfur was reported 10 km S. Incandescence was observed at the Caliente dome during the night of 5-6 November. Ash fell again in El Viejo Palmar, fincas La Florida, El Faro, and Santa Marta (5-6 km SW) on 7 November. Sulfur odor was also reported 8-10 km S on 16, 19, and 22 November. Fine-grained ash fell on 18 November in Loma Linda and San Marcos Palajunoj. On 29 November strong block avalanches descended in the SW flank, stirring up reddish ash that had fallen on the flanks (figure 104). The ash drifted up to 20 km SW.

Figure 104. Ash plumes rose from explosions multiple times per day at Santa Maria’s Santiaguito complex during November 2019, and block avalanches stirred up reddish clouds of ash that drifted for many kilometers. Courtesy of INSIVUMEH. Left, 11 November 2019, from Reporte Semanal de Monitoreo: Volcán Santiaguito (1402-03), Semana del 09 al 15 de noviembre de 2019. Right, 29 November 2019 from BOLETÍN VULCANOLÓGICO ESPECIAL BESTG# 106-2019, Guatemala 29 de noviembre de 2019, 10:50 horas (Hora Local).

White steam plumes rising to 2.9-3.0 km altitude drifted SE most days during December 2019. One to three explosions per hour produced ash plumes that rose to 3.1-3.5 km altitude and drifted W and SW producing ashfall on the flanks. Several strong block avalanches sent material down the SW flank. Ash from the explosions drifted about 1.5 km SW on 3 and 7 December. The Washington VAAC reported a small ash emission that rose to 4.9 km altitude and drifted WSW on 8 December, and another on 13 December that rose to 4.3 km altitude. Ashfall was reported up to 10 km S on 24 December. Incandescence was reported at the dome by INSIVUMEH eight times during the month, significantly more than during the recent previous months (figure 105).

Figure 105. Strong thermal anomalies were visible in Sentinel-2 imagery at the summit of the Caliente cone at Santa María’s Santiaguito’s complex on 19 December 2019. Image uses Atmospheric Penetration rendering (bands 12, 11, 8A). Courtesy of Sentinel Hub Playground.

Activity during January 2020 was similar to that during previous months. White plumes of steam rose from the Caliente dome to altitudes of 2.7-3.0 km and drifted SE; one to three explosions per hour produced ash plumes that rose to 3.2-3.4 km altitude and generally drifted about 1.5 km SW before dissipating. Frequent block avalanches on the SE flank caused smaller plumes that drifted SSW often over the ranches of San Marcos and Loma Linda Palajunoj. On 28 January ash plumes drifted W and SW over the communities of Calaguache, El Nuevo Palmar, and Las Marías. In addition to incandescence observed at the crater of Caliente dome at least nine times, thermal anomalies in satellite imagery were detected multiple times from the block avalanches on the S flank (figure 106).

Figure 106. Incandescence at the summit and in the block avalanches on the S flank of the Caliente cone at Santa María’s Santiaguito’s complex was visible in Sentinel-2 satellite imagery on 8 and 13 January 2020. Atmospheric penetration rendering images (bands 12, 11, 8A) courtesy of Sentinel Hub Playground.

The Washington VAAC reported an ash plume visible in satellite imagery at 4.6 km altitude drifting W on 3 February 2020. INSIVUMEH reported constant steam degassing that rose to 2.9-3.0 km altitude and drifted SW. In addition, 1-3 weak to moderate explosions per hour produced ash plumes to 3.1-3.5 km altitude that drifted about 1 km SW. Small amounts of ashfall around the volcano’s perimeter was common. The ash plumes on 5 February drifted NE over Santa María de Jesús. On 8 February the ash plumes drifted E and SE over the communities of Calaguache, El Nuevo Palmar, and Las Marías. Block avalanches on the S and SE flanks of Caliente dome continued, creating small ash clouds on the flank. Incandescence continued frequently at the crater and was also observed on the S flank in satellite imagery (figure 107).

Figure 107. Incandescence at the summit and on the S flank of the Caliente cone at Santa María’s Santiaguito’s complex was frequent during February 2020, including on 2 (left) and 17 (right) February 2020 as seen in Sentinel-2 imagery. Atmostpheric Penetration rendering imagery (bands 12, 11, 8A) courtesy of Sentinel Hub Playground.

Geologic Background. Symmetrical, forest-covered Santa María volcano is part of a chain of large stratovolcanoes that rise above the Pacific coastal plain of Guatemala. The sharp-topped, conical profile is cut on the SW flank by a 1.5-km-wide crater. The oval-shaped crater extends from just below the summit to the lower flank, and was formed during a catastrophic eruption in 1902. The renowned Plinian eruption of 1902 that devastated much of SW Guatemala followed a long repose period after construction of the large basaltic-andesite stratovolcano. The massive dacitic Santiaguito lava-dome complex has been growing at the base of the 1902 crater since 1922. Compound dome growth at Santiaguito has occurred episodically from four vents, with activity progressing W towards the most recent, Caliente. Dome growth has been accompanied by almost continuous minor explosions, with periodic lava extrusion, larger explosions, pyroclastic flows, and lahars.

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/ ); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); 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/).

On 9 November the Japanese Meteorological Agency (JMA) issued two "Volcanic Advisories" and a "Volcano Observation Report" following a small-scale eruption at Me-Akan volcano ~225 km E of Sapporo. New ash deposits were observed on trees in the nearby town of Akan, located E of the volcano near Lake Akan, and trace amounts of ash were distributed up to ~10 km E from the summit crater. JMA and Hokkaido University seismometers detected 4 minutes of tremor beginning at 1441 on 9 November. No additional earthquake or tremor events followed.

According to the local news agency, Asahi Shinbun, one of their aircraft flew near the snow-covered summit of the volcano at approximately 0900 on 10 November. White-colored "smoke" was seen to rise 700 m above the Ponmachineshiri crater (figure 7). Observers also noted that snow fields up to 1 km S and E of the crater were gray in color. There were no reports of injuries or damage.

Figure 7. Summit view of Ponmachineshiri, part of the Me-Akan volcano group, [in 1996]. The water-filled Aonuma crater is in the foreground, First crater is center, and the smoking Fourth crater is on the right. Courtesy of JMA; photo by Keiji Wada, Hokkaido University of Education, Asahikawa.

Researchers from Hokkaido University, the Geological Survey of Japan (Hokkaido Branch), Geological Survey of Hokkaido, and JMA (Sapporo and Kushiro) surveyed ash deposits from the 9 November eruption, and examined the ash under a petrological microscope. They estimated the total mass of the deposits as ~1,000 metric tons (t), smaller than the ~2,000 t eruption in 1996 (BGVN 21:10). The ash consisted of older, altered rock-fragments (andesite), minerals and clay. They found trace amounts of angular, fresh basalt fragments containing gray glass. They considered it likely that new magma reacted with water in a hydrothermal system, resulting in a phreatomagmatic eruption in which chips of solidified new magma were issued together with larger amounts of fragments of older rocks altered hydrothermally beneath the crater.

Geologic Background. Akan is a 13 x 24 km caldera located immediately SW of Kussharo caldera. The elongated, irregular outline of the caldera rim reflects its incremental formation during major explosive eruptions from the early to mid-Pleistocene. Growth of four post-caldera stratovolcanoes, three at the SW end of the caldera and the other at the NE side, has restricted the size of the caldera lake. Conical Oakandake was frequently active during the Holocene. The 1-km-wide Nakamachineshiri crater of Meakandake was formed during a major pumice-and-scoria eruption about 13,500 years ago. Within the Akan volcanic complex, only the Meakandake group, east of Lake Akan, has been historically active, producing mild phreatic eruptions since the beginning of the 19th century. Meakandake is composed of nine overlapping cones. The main cone of Meakandake proper has a triple crater at its summit. Historical eruptions at Meakandake have consisted of minor phreatic explosions, but four major magmatic eruptions including pyroclastic flows have occurred during the Holocene.

Information Contacts: J. Miyamura, Japan Meteorological Agency, Kishocho-881, 3-4 Ote-machi, Chiyoda-ku, Tokyo 100-0004, Japan; Mitsuhiro Nakagawa, Department of Earth and Planetary Material Sciences, Graduate School of Science, Hokkaido University, N-10 W-8 Kita-ku, Sapporo 060, Japan; Asahi Shimbun News, Tokyo, Japan (URL: http://www.asahi.com/); Keiji Wada, Hokkaido University of Education at Asahikawa, Hokumoncho 9-chome, Asahikawa 070,Japan (URL: http://www.asa.hokkyodai.ac.jp/research/staff/wada/EV/E-Welcome.html); Volcano Research Center, Earthquake Research Institute (ERI), University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html).

Following the 1995 phreatic explosion at Lake Voui (BGVN 20:02 and 20:08) a bathymetric survey of the crater lake was carried out. The 1996 survey confirmed the location of activity that had first been observed in 1992 on a SPOT satellite image. Monitoring of Lake Voui has continued through November 1998.

The average temperature over the whole 1 x 2 km surface of the lake (figures 7 and 8) stayed at ~30°C during November 1996-November 1998, due in part to constant streams of gas that issued from the main vent. As a comparison, in June 1995, three months after the phreatic explosion, the surface temperature was 45°C.

Figure 7. Schematic map of the summit area of Aoba volcano. Monitoring equipment includes: (1) a hydrophone at a depth of 10 m; (2) temperature sensors at a depth of 7 m; (3) power supply, electronics, and ARGOS satellite transmitter station; and, (4) a terrestrial data station measuring seismicity, heat flow, and rainfall. Courtesy Centre ORSTOM, Vanuatu.

Figure 8. Photograph of Aoba showing Lake Voui. Water discoloration marks the zone of activity. The power and transmitter station is located on the islet at the center. Lake Lakua is in the right background. Courtesy Centre ORSTOM, Vanuatu.

The ten major compounds dissolved in the lake's water have changed in concentration with time (table 1), but the samples, taken at the surface and at depths of 15-50 m, were consistent throughout the lake at any one time.

Table 1. Synopsis of the physical and chemical analysis of the waters of Voui lake derived from samples taken during 1995-98. Chemical constituents and ratios are given in mg/L. Courtesy Centre ORSTOM, Vanuatu.

The average volume of the lake was estimated at 50 x 10⁶ m³, but the level varied significantly. A drop of 275 cm in surface elevation was observed between June 1997 and October 1998. Rainfall varied between 500 and 600 cm/year in the summit area.

Monitoring was conducted twice per year, complemented by seismic recordings taken from a station set up in the dry lake bed of Ngoro. This system is similar to that used on Tanna Island, Vanuatu (BGVN 21:08). The range of monitoring equipment in place on Aoba since 1996 was extended in October 1998 by the installation of an acoustic recording station (0.1-150 KHz) and a device for continuous measurement of lake-water temperature. The data are relayed through an ARGOS satellite transmitter. Identical stations have been set up on Kelut in Indonesia and at Lake Taal in the Philippines.

Geologic Background. The island of Ambae, also known as Aoba, is a massive 2500 km3 basaltic shield that is the most voluminous volcano of the New Hebrides archipelago. A pronounced NE-SW-trending rift zone dotted with scoria cones gives the 16 x 38 km island an elongated form. A broad pyroclastic cone containing three crater lakes (Manaro Ngoru, Voui, and Manaro Lakua) is located at the summit within the youngest of at least two nested calderas, the largest of which is 6 km in diameter. That large central edifice is also called Manaro Voui or Lombenben volcano. Post-caldera explosive eruptions formed the summit craters about 360 years ago. A tuff cone was constructed within Lake Voui (or Vui) about 60 years later. The latest known flank eruption, about 300 years ago, destroyed the population of the Nduindui area near the western coast.

Information Contacts: Michel Lardy, Inès Rodriguez, Douglas Charley, and Pascal Gineste, Centre ORSTOM, P.O.Box 76, Port-Vila, Vanuatu; Michel Halbwachs, and Jacques Grangeon, Université de Savoie, Campus Scientifique, F3376, Le Bourget du Lac, Cédex France; Janette Tabbagh, Centre de Téléobservation Informatisée des volcans, CNRS-CRG, Garchy, France.

Rapid lava effusion began from Colima's summit lava dome in late November. The 1998 lava extrusion, the first since 1991, followed months of seismic unrest and a subsequent explosion at the summit on 6 July, leading to local evacuations.

The night of 19 November was marked by strong seismicity and a large number of rockfalls (lasting 2-4 minutes) down the summit's W, SW, and S sectors. Although a previous helicopter flight could not confirm the prescence of new lava, at 0730 on 20 November geologists saw that the crater formed by explosions in 1994 contained a fresh, nearly black circular lava dome with a rough, wrinkled surface. At that time, based on the 1994 crater's dimensions (135 m in diameter and 50 m deep), the dome was approximately 30 x 50 x 15 m in size. Fumaroles were noted along the dome's margins. Other fumaroles in the area of the N-NW summit continued to emit a high output of gases. By 1800 on 20 November both seismicity and rockfalls had dropped to low levels.

Surprisingly rapid dome growth took place that night, and a 0730 flight on 21 November disclosed that the 1994 crater (~3.8 x 10⁵ m³ in volume) was then full and new lava spilled out the S side. Up to this point Colima's eruption appeared quite similar to the 1991 lava extrusion episode, but the new lava erupted at a considerably higher rate. In 1991 it took about 16 days to form a dome of comparable size.

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

Information Contacts: Carlos Navarro Ochoa, Colima Volcano Observatory, Universidad de Colima, Ave. 25 de Julio 965, Colima 28045, Colima, México.

The following report summarizes activity observed at each of the four summit craters of Etna from 15 January through May 1998. Southeast Crater was active throughout this period, with explosions and lava flows both within the crater and on the flanks of the cone. Activity at Bocca Nuova alternated between ash emissions from collapses and vigorous magmatic eruptions until early April. Voragine exhibited intermittent low-level activity. Northeast Crater had a lava fountaining episode in late March, its first significant activity since August 1996. Additional summit crater eruptive episodes after May 1998 will be described in future issues.

Information for this report was compiled by Boris Behncke at the University of Catania and published on his internet web site. The compilation was based on personal visits to the summit, telescopic observations from Catania, and other sources.

Seismicity on the W flank. Seismic activity resumed on 15 January with weak tremors ~6 km below the W flank (Monte Palestra area) and several shallow shocks on the SW slope. Seismicity was low but a tremor occurred on the W flank, and another directly below the summit craters, on 19 January. After about two weeks of relative seismic quiet, earthquakes occurred again below the W flank on 31 January and below the summit craters on 1 February. Mild seismic activity was occurring again on 9 February in the Monte Palestra area (W flank at around 2,000 m), in the same area that has been affected repeatedly by seismic activity since late December.

Activity at Southeast Crater. On 16 January, explosive and effusive activity resumed at Southeast Crater (SEC). On 18 January there were three active lava flows on the southern slopes of SEC. A lava flow which moved towards the W rim of Valle del Bove stopped shortly on 20 January. After two days of weak or absent eruptive activity, SEC resumed Strombolian activity on 22 January. On 28 January a lava tongue extended to the W rim of Valle del Bove; at dusk there was vigorous explosive activity and two small lava flows were visible. During the evening of 29 January, Strombolian activity occurred from the intracrater cone while a lava flow was overflowing down the SE flank.

Clear weather on 4 February revealed fresh lava flows on the S and ESE flanks of SEC. Explosive activity continued on 9 February while small lava flows moved down its SE flank. On 10 February, SEC was the site of continuous powerful Strombolian explosions that dropped bombs and scoriae beyond the crater rims. Activity alternated between two vents, only one erupting at any given time. The S vent produced fountains that showered the whole southern sector of SEC with bombs. The N vent sent vertical fountains of bombs up to 200 m high. Some bombs that fell on the W crater rim were up to 30 cm long. Smaller projectiles even fell at the lower slope of the main cone, 100 m from the erupting vent. Lava flowed from a vent on the SE side of the intracrater cone. A lava tongue spilled over the crater rim on its ENE side. Other recent lava tongues had extended just beyond the base of the cone; the longest flow to the ESE (produced in mid-December 1997) had advanced to within ~50 m of the W rim of Valle del Bove. The only significant remainder of SEC's former rim is on the W and NW side where it stands 15 m above the lava field surrounding the central cone. In all other areas the crater is filled and has overflowed in many places. The appearance of the crater's interior is that of a low lava shield topped by a cone that is 30-40 m high.

By 11 February, growth on the NW side of the intracrater cone had raised its summit by at least 1 m since the day before. Two vents were active in its summit crater, and for the first time these were seen to erupt simultaneously. The vigor of the activity increased notably after 1930, when jets of bombs frequently rose up to 250 m above the vent. Lava from the vent on the SE base of the intracrater cone rapidly covered the SE sector of the crater floor and began to spill down the upper outer flank of SEC. By 2000, it had extended some 50-100 m downslope. Activity continued at similar levels through 15 February.

Strombolian activity was intermittent on 17 February, and degassing alternated with bomb ejections while a lava flow slowly moved down the SSE flank of the SEC cone. New lava flows from the intracrater cone covered ~25% of the crater floor, and a new lava lobe began spilling down the outer flank of SEC adjacent to the still-active SSE flow. A lava flow on the SW flank of SEC during 20-25 February appeared to be flowing on the NW side of the January flow. Strombolian activity occurred on the night of 25 February, and a very minor lava lobe spilled over the SE crater rim.

The eruption continued on 5 March with lava effusion on the flanks of SEC. As of 11 March lava continued to spill down the SE flank of SEC. Around 16-19 March, SEC appeared to be the only center of eruptive activity with weak Strombolian activity accompanied by minor overflows of lava. Lava flows began moving down the SSW flank of SEC on 20 and 21 March, but explosive activity was weak. During the Northeast Crater episode of 27-28 March, SEC was intensely active, with vigorous and continuous Strombolian bursts, and a lava flow spilling down the SW flank of the SEC cone. Moderate Strombolian activity continued, but effusive activity on the SW flank ceased sometime during 29 March.

Significant morphologic changes were noted on 6 April that had occurred since the previous visit on 17 February. The summit of the intracrater conelet had collapsed or been destroyed in late March. A depression on the lower E flank of the conelet was the site of a new effusive vent. The effusive vent area that had been active for many months in the S and SE sectors of the conelet's flank was inactive. Lava had buried the old rim of SEC on all sides except the W and NW where the old rim stood a few meters above the lava field. Lava had overflowed onto the northern outer flank of SEC, forming a short lobe. On the SW flank of SEC a lava flow active from mid-February until early March had extended to near the base of the 1971 "Observatory Cone".

The new effusive vent on the eastern base of the conelet had apparently formed only shortly before the visit because the depression around it had not yet been filled. Extrusion at this site had been preceded by subsidence at the base of the conelet. Meter-sized slabs of older lava had been uplifted and tilted, and fresh lava was being squeezed through the cracks, accompanied by high-pressure gas venting. A more vigorous flow issued from a U-shaped vent, similar to ephemeral vents seen on other occasions. Yet another flow began to issue from below an upheaved slab of older lava with spectacular lava stalagtites on its bottom. These two flows spilled 150 m down the NE flank of SEC.

Explosive activity on 6 April occurred from two vents within the crater of the central conelet, but they never erupted simultaneously; one vent was very noisy while the other erupted silently. SEC continued to erupt on 27 April, with small Strombolian explosions and lava effusion. Scientists who visited the crater on 14 May reported that lava was overflowing onto the flanks, and Strombolian activity was occurring from the summit of the conelet.

Vigorous explosive and continuous effusive activity as well as morphological changes were observed at SEC during a visit on 21 May with students from North Dakota State University. The central conelet was observed at close range, and the main effusive vent could be approached amidst a rain of light scoriae. Strombolian activity occurred from a single vent in the NW summit area of the conelet. Explosions occurred incessantly, and many ejected bombs 200 m above the vent. As on many other occasions, a distinct periodicity could be noted in the activity, each cycle culminating in a series of powerful Strombolian blasts heavily charged with meter-sized bombs. Overlapping lobes on the E side of the conelet had built a low shield, and the depression which had formed at the E base of the conelet was completely filled.

Vigorous explosive activity occurred on 24 May from the central conelet of SEC, and two flows were descending the SE cone. Some explosions ejected incandescent bombs at least 200 m high. Giovanni Sturiale and Boris Behncke, both of Catania University, visited SEC on 28 May; the central conelet was somewhat higher in the vent area than on 20 May. The main vent at the E base of the conelet was issuing lava that spilled over the E rim of SEC (buried under at least 30 m of lava since July 1997). Most flows stop at the base of the cone and are followed by the formation of new flows. Vigorous explosive activity dropped bombs on the N side of the central conelet. The current activity is known as Etna's "persistent summit activity" which became famous from descriptions of Northeast Crater which in the 1950's to 1970's produced similar activity.

Activity at Bocca Nuova. Very dense gas emissions were occurring from Bocca Nuova (BN) on 19 January; some contained ash. Explosions from BN were audible 8 km from the summit on 20 January, but magmatic activity alternated with collapses, generating dense ash plumes. Bright glow was visible on 22 January. BN was emitting white steam with some dark ash plumes derived from crater wall collapse on 28 January. On 28-29 January periods of intense incandescence indicated vigorous but intermittent activity at both the SE and the N eruptive centers.

Intense glow was again visible at BN on 4 February, indicating vigorous intracrater activity. Activity on 8 February continued without significant changes; there were emissions of dark ash indicating collapse of the crater walls. Magma again withdrew from BN (as indicated by internal collapse) on 9 February. Later that day collapse in BN ended; at nightfall, bright incandescence was visible.

The overall appearance of BN on 10 February was similar to before the collapses that accompanied the seismic crises on the W flank. The collapse had affected only the summit areas of the two large cones, and the N cone had subsided several meters. Activity had resumed at both cones. Jets of bombs, at times mixed with ash, rose tens of meters above the vents, and occasional explosions ejected bombs. Eruptive activity from the northern cone had resumed at a new vent close to the center of BN. A vent in the deepest part of the ~150-m-wide crater of the cone was vigorously degassing. A third vent rarely produced spectacular ash emissions. The main eruptive vent (on the S rim of the cone) was in constant eruption, with powerful bomb ejections about every 2 seconds. Many ejections rose above the W rim of BN, which stands 70-80 m above the vent. Every 5-10 minutes, this vent would produce larger eruptions, ejecting continuous fountains mixed with ash.

Activity in BN increased notably when seen on 11 February. Activity was continuous at both cones. During the afternoon, periods of near-continuous ash emissions were accompanied by powerful explosions. At night, both eruptive areas produced intense continuous glow. Occasional larger explosions ejected bombs up to 150 m above the SE rim of Bocca Nuova. The eruption in BN continued on 15 February without significant modifications. There were vigorous bomb ejections, many of which dropped bombs on the outer slopes of the main summit cone.

During another visit on 17 February, both eruptive centers of BN were active. One vent, 30-35 m in diamater, was ejecting continuous lava fountains and occasional large jets to above the crater rim. The northern eruptive center was the site of continuous very narrow incandescent fountains, and a small lava flow. Occasional violent explosions occurred from the vent on the southern rim of the collapse structure which had been the most active vent in this area one week earlier. Activity in BN during 20-23 February was characterized by low-level bomb ejections with occasional larger jets of bombs. Virtually continuous ash emissions began at BN on the afternoon of 24 February. The ash emissions were followed that evening by vigorous magmatic activity, probably from the SE vents, that caused a bright fluctuating glow until daylight.

BN continued to erupt in early March, although the activity appeared to decrease. On 5 March there was weak activity at BN. As of 11 March sporadic night glow was visible at BN. This crater was completely inactive during a 6 April visit. Wholesale collapse had occurred at the N and SE eruptive areas. A vast collapse depression had formed at the former, leaving only the N part of the large cone that had grown there until the end of 1997. Explosion sounds heard on 27 April possibly came from BN. The local mountain guides reported on 21 May that there had been no recent activity at BN. Activity resumed from BN at the end of May after several months of little activity.

Activity at Voragine. Eruptive activity reportedly included the Voragine on 20 January, but it was inactive during a summit visit on 10 February. During a 6 April visit, the first to this crater since 10 February, a few minor morphologic changes were noted. The most significant was the formation of a new crater <10 m in diameter on the central conelet. Some growth had occurred, and the crater floor was covered with finer-grained tephra. The SW vent at the base of the septum between Voragine and BN had enlarged to ~40 m in diameter. This vent was the only site of eruptive activity within the crater during the visit. Large explosions every 3-5 minutes ejected bombs tens of meters high, some of which flew into BN. Scientists at the summit on 14 May reported vigorous activity from the vent in the SW part of the Voragine and numerous fresh bombs. Loud detonations on 24 May indicated explosive activity; some were accompanied by dense vapor and gas plumes.

Activity at Northeast Crater. In one of the most spectacular eruptive events of the past few years, Northeast Crater (NEC) produced a 2-hour episode of lava fountaining during the night of 27-28 March. The event marks a resumption of more vigorous activity at NEC, which has displayed only weak activity since August 1996.

Volcanic tremor was registered by seismic stations in the summit area early on 27 March. At about 1000, Northeast Crater began to emit ash plumes that continued until shortly after 1600. By nightfall, sporadic ejections of incandescent bombs sometimes rose several hundred meters above the crater. The Strombolian ejections gradually increased in intensity and became virtually continuous by 2200. Shortly before midnight, the ejections merged into a continuous pulsating fountain rising 300-350 m above the rim of the active vent within the collapse pit in the S-central part of the crater. Large bombs fell onto the lava platform and into the adjacent Voragine and BN craters, some fell 1 km S and SW of the vent. Loud detonations were heard on the E and SE flanks where hundreds of thousands of people watched the display at a safe distance. By about 0130, the activity began to decline and was virtually over after 0200. This eruption appears to be another episode of lava fountaining similar to those at the same crater between November 1995 and June 1996, and many times during the late 1970's and early 1980's. The next day, NEC emitted a few ash plumes several hundred meters above the summit, but there was no evidence of renewed Strombolian activity.

When the crater was visited on 6 April, centimeter-sized, highly inflated scoriae were abundant a few hundred meters S of the 1971 "Observatory Cone," and the deposit was nearly continuous on the W side of that cone, with maximum clast sizes exceeding 5 cm. Closer to SEC the deposit was no longer continuous, but clasts up to 10 cm long were found. Close to NEC, little fallout was found. A few impact craters were seen in the N part of the Voragine floor while on its N wall bombs had formed a nearly continuous cover. On the S and SE rim of NEC the deposit was at most a few meters thick. The inner terrace surrounding the central pit, previously 5-10 m below the outer terrace, had subsided at least 10 m, exposing huge caverns in the vertical scarp along which subsidence took place; these were formed during the summer of 1996 when the crater was filled with lava which crusted over and later drained. The dimensions of the central pit had changed little, but its floor had risen to within ~50-60 m of the lowest point on the rim. There was no evidence of fresh ejecta around these vents indicating that no significant eruptive activity had taken place there since the 27-28 March eruption.

Local mountain guides reported on 21 May that there had been no recent activity at NEC. However, on the morning of 1 June there was a series of ash emissions.

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

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

The sequence of phreatic explosions initiated on 7 August (BGVN 23:09) continued from 28 October through 17 November (table 1). A substantial number of days were marked by one phreatic explosion. Visible explosions rose at most a few kilometers above the summit. Many explosions were accompanied by tremor; they were seismically characterized with reduced displacements.

Table 1. Some details of Guagua Pichincha's phreatic explosions, their size (as reduced displacements), and associated tremor, 27 October through 17 November 1998. A "--" signifies the data is either inapplicable or not reported. Extracted from the daily reports posted on the website of IG-EPN.

As illustrated in the previous report (BGVN 23:09), volcano-tectonic, long-period, and multiphase earthquakes all escalated prominently during mid-September. During the current reporting interval, these remained elevated but did not increase, and the numbers of the various events, particularly volcano-tectonic and multiphase earthquakes, may have moderated or diminished slightly.

The number of explosions in a single day reached a new high for this crisis: four occurred on 7 November. The previous one-day record, three, had occurred only on two days in mid-October. Yet, the 7 November blasts were followed by four consecutive days with no explosions and, during 8-20 November no day had more than one explosion. As an indication of the pace of the venting, during 7 August-3 November the daily reports noted 59 explosions.

The highest plume seen during the reporting interval came from an explosion at 0715 on 3 November. It rose to ~3 km above the summit. Clear atmospheric conditions enabled residents to see it from the city of Quito. Although atmospheric conditions frequently blocked visibility, local observers saw fumarolic plumes rising from 100 to 1000 m. Thus, on 28 October a plume rose 100 m; on 9, 11, and 14 November, respectively, plumes rose 300, 600, and 1,000 m high. A plume on 4 November was of ambiguous origin, but it rose 1,000 m.

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

Information Contacts: Instituto Geofísico, Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador; El Comercio newspaper, Quito, Ecuador (URL: http://www.elcomercio.com); El Universo newspaper, Quito, Ecuador (URL: http://www.eluniverso.com); La Hora newspaper, Quito, Ecuador (URL: http://www.lahora.com); Volcanic Disaster Assistance Program, U.S. Geological Survey, 5400 MacArthur Blvd., Vancouver, Washington 98661 USA (URL: https://volcanoes.usgs.gov/observatories/cvo/); ORSTOM, A.P. 17-11-6596, Quito, Ecuador (URL: http://www.ird.fr/).

Subsequent to the 3 September earthquake (BGVN 23:09), seismicity was low. Except for a few days, the number of tremors during October was <10/day, about the same level as in February-March 1998. The last tremor was observed on 3 November. This implies that the volcanic seismicity crisis (BGVN 23:09) has ended.

Geologic Background. Viewed from the east, Iwatesan volcano has a symmetrical profile that invites comparison with Fuji, but on the west an older cone is visible containing an oval-shaped, 1.8 x 3 km caldera. After the growth of Nishi-Iwate volcano beginning about 700,000 years ago, activity migrated eastward to form Higashi-Iwate volcano. Iwate has collapsed seven times during the past 230,000 years, most recently between 739 and 1615 CE. The dominantly basaltic summit cone of Higashi-Iwate volcano, Yakushidake, is truncated by a 500-m-wide crater. It rises well above and buries the eastern rim of the caldera, which is breached by a narrow gorge on the NW. A central cone containing a 500-m-wide crater partially filled by a lake is located in the center of the oval-shaped caldera. A young lava flow from Yakushidake descended into the caldera, and a fresh-looking lava flow from the 1732 eruption traveled down the NE flank.

Information Contacts: Yukio Hayakawa, Faculty of Education, Gunma University, Aramaki, Maebashi 371, Japan.

On 5 October, the Kamchatka Volcanic Eruptions Response Team reported that seismicity remained above background level. The low-level Strombolian eruptive activity that has characterized the volcano for more than two years continued. About 100-200 earthquakes and gas explosions occurred every day.

On 24 October Tass reported that a Russian-Japanese expedition of volcanologists had finished their work on Karymsky. The participants had spent two weeks at a location 3 km from the mountain studying seismic, acoustic, and other phenomena related to the eruption.

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

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

Increasing activity culminated in an eruption on 3 November. In the early afternoon the volcano rumbled three times and ash covered the nearby village of Palempok. Residents also noticed a strong sulfur smell. Rumbling was heard twice on 6 November by residents of Tangkil and Palempok. Unfortunately, the seismograph used to monitor the volcano had been inoperative since 3 November.

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: R. Sukhyar, Volcanological Survey of Indonesia (VSI), Bandung, Indonesia (URL: http://www.vsi.esdm.go.id/).

The eruption of Pu`u `O`o continued in October as lava moved 11 km to the sea through both small, intermittent surface flows and through a lava tube that developed after a pause on 12-14 August (BGVN 23:08).

By 19 October, a 300-m-wide lava bench had grown W of the prominent littoral cone at a new ocean entry, extending 60 m beyond the old shoreline. Surface flows obscured the old sea cliff that once marked the relatively safe visitor viewing areas (figure 124).

Figure 124. An aerial view of the Kamokuna lava bench on the SE coast of Kilauea, 24 September 1998. Note location of the former sea cliff. The bench was ~ 150 m wide at the W entry area, near the larger white plume. Photograph by J. Kauahikaua; courtesy HVO.

Dense volcanic fumes from Pu`u `O`o obscured its crater for several weeks, and no lava has been seen in the crater for many months, although there have been reports of glow at night near the summit. In late October, Pu`u `O`o was releasing ~2,000 tons/day of SO₂. This discharge is equivalent to the gas contained in ~400,000 m³ of lava, in concurrence with measurements of lava discharge above the lava tube ~5 km from the vent.

A new skylight formed above the lava tube at 635 m elevation showed lava moving 7-9 m below the surface. This part of the tube formed in August 1997, and since then flowing lava eroded the underlying flows to form a tube that is taller than it is wide.

Pu`u `O`o is the only active vent at Kilauea. The vent area is complex and slowly forms new pits, cracks, and collapse areas. Since the current eruption began in January 1983, a mosaic of flows has buried 16 km of the coastal highway to a depth of 23 m and created nearly 2.6 km² of new land. Recently, lava has flowed into the sea at three entry points near Kamokuna, 4.8 km E of the end of the "Chain of Craters Road" in Hawaii Volcanoes National Park. The easternmost entry has been active since August 1997, but is slowly dying as ruptures in the main tube divert lava elsewhere. Other entry points evolved in September and October 1998. The deltas or benches formed at sea entry points are unstable, collapsing without warning. The largest such collapse occurred a few years ago and involved 10 ha of bench material (105 m²).

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

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

During October seismicity under the volcano was generally above background levels. Hypocenters of earthquakes recorded through the period were concentrated at two levels: near the summit crater and at depths of 25-30 km. On 1, 14, 15, 18, and 19 October a fumarolic plume was observed during the daylight hours rising 50 m above the summit. On 9 October the plume rose to 100 m above the summit. No fumarolic plumes were seen on 30 September, 2, 3, 6, 11, or 16 October. Clouds prevented direct observation of the summit during the remainder of the month. The alert status remained "green" indicating normal activity through October.

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

Crater 2 emitted thin to thick white vapor throughout September, with an occasional ash component. Weak roaring noises were reported on 1 September. One large explosion on 21 September sent ash to an altitude of 2-3 km and resulted in ashfalls to the SW. Crater 3 was quiet, emitting only thin white vapor.

The activity at Crater 2 during October was moderate and uneventful. Pale gray ash clouds rose intermittently to ~500 m, without sound. On 21 October, however, weak roaring and rumbling sounds accompanied emissions to 1,000-1,500 m and a bright fluctuating night glow.

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: Patrice de Saint-Ours, Steve Saunders, and Ben Talai, RVO.

An inflation of ~10 µrad for September was recorded at Tabele Observatory, ~3 km SW of the summit. This deformation, together with increased seismicity, audible rumblings, and night glow evident in the middle of the month, was thought to indicate the onset of renewed activity.

Intense eruptive activity resumed at Manam in late September for the first time since its fatal eruption of November-December 1996. A visible increase in activity started during 23-26 September, with intermittent dark ash emissions and incandescent projections at night to ~200 m above South Crater. On subsequent days activity decreased to continuous white vapor emissions, first profuse then very weak, and occasional roaring sounds and fluctuating red glow. This corresponded to a slight decrease in seismic amplitude levels, but the radial tilt kept showing inflation.

Significant eruptive activity throughout October, including ash emissions, pyroclastic flows, and lava flows, will be described in the next issue.

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: Patrice de Saint-Ours, Steve Saunders, and Ben Talai, RVO.

Eruptive activity occurred at Nyamuragira volcano beginning on 17 October. During the following week several Strombolian explosions and effusive activity were reported. Lava "gently gushed" from the cone and through a fissure in its side, according to an official at the National Scientific Research Centre (CNRS) quoted in a Reuters news report. On 19 October the central crater opened and the lava flowed into the surrounding forest. Glow was visible at night from the city of Goma, ~30 km SE of the volcano. The flows were still active but diminishing at the time of the last report on 25 October. Scientists are not able to visit the site because of the threat of civil unrest. Virunga National Park has been closed for months.

An SO₂ plume was first detected by the Earth Probe Total Ozone Mapping Spectrometer (TOMS) on 18 October. Although the image resolution is not sufficient to differentiate between Nyamuragira and Nyiragongo as a plume source, the former has previously emitted large amounts of sulfur dioxide. Imagery the next day (figure 16) showed that the plume extended ~700 km SW from the volcano and covered an area of 300,000 km². Scientists at the Goddard Space Flight Center calculated that this plume contained 115 kilotons (kt) of SO₂. An SO₂ plume was detected on each day from 18 through 29 October. On 29 October the plume was directed to the N and contained 10 kt of SO₂. No SO₂ was detected in images taken from 30 October through 4 November. Visible satellite imagery acquired by the Toulouse Volcanic Ash Advisory Center on 20 October did not show any evidence of an ash plume, but convective clouds were obscuring the area.

Figure 16a. Detail of Total Ozone Mapping Spectrometer (TOMS) satellite image of the SO2 plume over Nyamuragira on 19 October 1998. Darker areas represent higher concentrations; those areas contained within black represent higher concentrations than the black areas. Courtesy of George Stephens and Robert Farquhar, NOAA/NESDIS.

Figure 16b. Color Total Ozone Mapping Spectrometer (TOMS) satellite image of the SO2 plume over Nyamuragira on 19 October 1998. Red areas represent higher concentrations. Courtesy of George Stephens and Robert Farquhar, NOAA/NESDIS.

Historical eruptions at Nyamuragira have occurred within the summit caldera and from numerous flank fissures and cinder cones. Twentieth-century flank lava flows extend 30 km from the summit. This eruption was the first from Nyamuragira since December 1996 (BGVN 21:10). Nyamuragira is one of two frequently active volcanoes in that part of Virunga National Park; the other is Nyiragongo, which sits closer to Goma.

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

Information Contacts: C. Akumbi, Goma Volcano Observatory, Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo; Stephen J. Schaefer, Joint Center for Earth System Technology (NASA-UMBC), Mail Code 921, NASA Goddard Space Flight Center, Greenbelt, MD 20771 USA; George Stephens, NOAA/NESDIS, E/SP22, 5200 Auth Road, Camp Springs, MD 20746-4304, USA; Robert D. Farquhar, NOAA/NESDIS, FB-4, Suitland, MD 20233-9909 USA; Volcanic Ash Advisory Center (VAAC) Toulouse, Météo-France, 42 Avenue Gaspard Coriolis, F-31057 Toulouse cedex, France; Reuters Limited.

There were a few instances of moderate disturbance during October, and a relatively large emission occurred on 17 October; otherwise, Popocatépetl remained generally stable at low levels of eruptive activity, including almost daily emissions of steam and gas. Since the possibility of explosions remained, authorities recommended that no one approach within 4 km of the crater. The caution light remained "yellow" throughout the month.

Steam-and-gas fumaroles rose up to 500 m above the summit several times during the first week of October. The emissions usually blew SE. Two slightly larger exhalations lasting 5 minutes each at 0218 and 1409 on 4 October may have also released ash, but this was unconfirmed owing to bad weather obstructing views of the volcano. At 2312 on 5 October an explosive event began. An intense two minute phase was followed by 30 minutes of steam, gas, and ash emission that formed a plume 4 km above the crater. Glow was also seen at this time. Activity quickly diminished to previous low levels.

At 1715 on 17 October a larger exhalation began: its intense phase lasted about 16 minutes and produced an ash column (figure 27). The plume rose 2 km above the summit and blew NW (towards Mexico City).

Figure 27. Basal portion of an ash column from Popocatepetl on the afternoon of 17 October as seen from a video monitor. Courtesy of CENAPRED.

The ash column was initially detected by Doppler radar located at CENAPRED headquarters in Mexico City, and staff there immediately informed air-traffic controllers. The ash emission persisted for 20 minutes, after which the volcano returned to its previous low-level activity (steam and gas emissions only). One hour after the beginning of the event, reports were received of ashfall at Amecameca, Tenango del Aire, and other towns NW of the volcano.

At 2040 another smaller exhalation took place with a duration of only 1 minute. At about 2100 light ash from the earlier eruption fell at CENAPRED headquarters, UNAM, and at other places in SW Mexico City. Activity soon dropped to characteristic low-intensity exhalations. A similar moderate emission lasted 1 minute at 1859 on 24 October; the event was followed by low-amplitude, high-frequency tremor for about 20 minutes, producing a 2,500-m-high column of gas, water vapor, and ash.

A-type earthquakes were recorded at 0956 on 16 October (M 2.6, at a point 6.6 km below the summit), at 2227 on 22 October (M 2.0, at a point 7 km below the crater), at 1751 (M 2.1) and 1919 (M 1.8) on 29 October, and at 0942 (M 2.4) on 30 October. Two minutes of low-amplitude, low-frequency tremor began at 1355 on 29 October. None of these events seemed to affect activity at the volcano.

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

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

The activity at Tavurvur continued as in previous months, with regular Vulcanian eruptions mainly emitting dust with few blocks. These events occurred at intervals of ten minutes to one hour; the longer the preceding interval, the more powerful the eruption.

The overall trend of seismic activity remained low, although short periods of increased activity were observed. During the first two weeks, on 5, 6, 8, and 10 September, bands of discontinuous non-harmonic low-amplitude tremor lasted from a few minutes to about an hour. This activity was coupled with a daily average of 10 discrete low-frequency earthquakes. From 13 September, an increase in low-frequency events became more apparent, with the highest number of 128 recorded on the 18th. This increase continued until 23 September, after which the activity declined to previous levels. Event counts recorded at the KPT seismic station, ~1.5 km W from Tavurvur crater, showed an increase during the month. The total number of events was about 675 compared to about 154 in August. RSAM values also showed a general increase. A few high-frequency earthquakes on 3 September were too small to be located, only seismic stations to the N of the Rabaul Harbor Network recorded them.

A water-tube tiltmeter at Sulphur Creek (3.5 km from Tavurvur) showed a 3.5-mm inflation of Tavurvur for the month. This inflation has been continuing ever since a 20-µrad deflation associated with an eruption on 14 March 1997. In other words, eruptions after 14 March 1997 have lacked significant deflation, and since then cumulative inflation has totaled ~30 µrad.

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: Patrice de Saint-Ours, Steve Saunders, and Ben Talai, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.

Activity was monitored during 1-9 September using detailed field observations combined with satellite and aerial remote sensing data. Activity was generally similar to that reported in August. On 6 September a large eruption began. In the preceding days activity had fluctuated. On 1 September, the only activity observed was a small white gas cloud at 0944. Gas clouds were emitted from 0748 until 0942 on 2 September. These predominantly white and gray clouds rose only 200 m above the crater before dissipating. The only exception was a period of ten minutes when brown and dark gray clouds issued from the crater. The sole emission the following day was a small white gas cloud at 1506. On 4 and 5 September small gas emissions were observed from the fumarole on the S side of the cone.

Activity on 6 September was first noted at 0702 when large white and gray gas clouds rose from the whole crater. At 0704 part of the gas column began to sink and move down the upper flanks, obscuring the E-flank ice walls. The gray and brown gas cloud was densest on the S side of the crater and appeared to be expanding as it rose. At 0711, the whiter part of the cloud rose upward while the dark gray portion dropped ash on the N side of the cone. Wind speeds at the summit appeared to increase, and the 400-m-high column began to be pushed N. At 0716 more gas descended the flanks. At 0735 observers on the edge of the easternmost lava flow could smell sulfur.

The main gas emission continued to be from the S side of the crater and at 0740 another cloud descended over halfway down the flanks. At 0743 a large white and dark gray gas cloud emerged from the crater. Ash fell from it onto the upper and mid-slopes. Another large gray, white, and brown plume filled the whole crater at 0746 and billowing to 400 m. At 0749 the plume color changed to brown, yellow, and dark gray. Ash was blown N. New gas clouds emerged from the crater on average every 30 seconds. At 0824 the cloud color returned to white and light gray for a few minutes before it once again became brown, gray, and yellow. The brown portion seemed to contain the ash. Gas once again descended the upper slopes at 0846. Winds at the summit began to pull the top of the plumes apart and by 0854 they were almost flat across the crater.

There was a reduction in gas emission at 1143. Gas continued to periodically descend the upper slopes and ashfall appeared to be mainly on the N slopes. At 1155 a gas cloud descended to mid-slope. The interval between gas emissions grew during the afternoon. After three hours of white- and gray-colored gas clouds, yellow, white, and brown clouds emerged again at 1604. This marked renewal of activity was similar to that in the early morning. Gas originated mainly from the southern fumarole and occasionally descended the upper slopes. Gas clouds rose 500 m and formed a cumulo-like mass. At 1737 there was a big gas release, part of which descended the cone slope while the main cloud rose and curled N over the crater. After this the intensity of the activity from the cone diminished and gas clouds became light gray.

On 7 September a faint brown haze was noted over Sabancaya at 0630. Dust in the atmosphere obscured viewing. Gas clouds were observed at 0643, 0704, 0719, and 1210. Visibility improved around mid-day, and ashfall was observed on the S side of the cone at 1243. At 1652 a small gas cloud descended the upper slopes. From 1740 until dark, gas emissions were continuous, but none were seen the following day. On 9 September observers on a morning flight around the volcano observed light emissions from fumaroles on the N and S crater rims. Fresh sulfur deposits existed on the crater walls. The crater itself was deeper than the year before and the floor could not be seen. Recent ash eruptions had covered the ice walls on the E side.

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

Information Contacts: Mark Bulmer, Frederick Engle, and Andrew Johnston, Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Washington DC 20560-0315 USA; Guido Salas, Departamento Academico de Geoloia y Geofisica, Universidad Nacional de San Augustin, Arequipa, Perú; Elian Perea, Universidad Nacional de San Augustin, Arequipa, Perú.

On 30 October 1998 a disastrous event (called a "mudflow" in newspapers) occurred on the S flank of Casita volcano. According to official reports, the incident killed between 1,560 and 1,680 people, displaced hundreds more, destroyed several towns and settlements, and disrupted the Pan American Highway at numerous bridges. On 11 and 12 November the first scientific team visited the volcano to investigate the disaster. The team examined the summit area on the first day and made a complete traverse of the devastated zone as far S as the Pan American Highway on the second day. This report presents the team's conclusions and provides some recommendations regarding future risks.

Background. Casita is within the Cordillera Maribios, a 70-km-long volcanic chain that extends from the N shore of Lake Managua to the vicinity of Chinandega. Casita is part of the San Cristóbal volcanic complex, which consists of five principal volcanic edifices. The largest volcano in Nicaragua, San Cristóbal lies 4 km WNW of Casita and has exhibited frequent episodes of historical activity; at present it is emitting a vigorous fumarolic plume. For these reasons San Cristóbal has been studied in greater detail.

Casita is a composite volcano with deeply dissected morphology. The top of the volcano consists of a cluster of dacite domes. At its summit is a 1-km diameter crater that could be reached by a road - now impassable - to service telecommunication towers. A set of prominent NE-trending normal faults cut the summit area bounding each side of the crater. Explosion craters on the southern plain are aligned along a conjugate set of fractures trending NW-SE. No historical volcanic activity has been reported at Casita; however, the domes of the summit area are autobrecciated and exhibit strong hydrothermal alteration, which is consistent with low-temperature fumarolic activity.

Meteorological conditions. Hurricane Mitch was a major factor in the disaster. Abnormal rainfall related to Mitch began on 25 October. By 27 October the precipitation reached 100 mm/day and increased continuously to a maximum of ~500 mm/day on 30 October, the day of the avalanche. The total rainfall in October was 1,984 mm. Within three days, precipitation dropped to normal levels. For comparison, the average rainfall for October is 328 mm; thus the rainfall associated with the disaster was more than 6 times the average.

Source zone. The main source of the avalanche was 200 m SW of the volcano summit, and 60 to 80 m below the telecommunication towers. A secondary source was located at the same elevation but 100 m SE of the summit. The rock in this area is a hydrothermally altered and brecciated dacite dome. The principal rupture occurred along a ~500-m-long segment of a NE-trending fault that intersects the summit. A slab measuring ~20 m thick, 60 m high, and 150 m long detached slid down the fault plane that was inclined about 45 degrees SE. The volume of source block for the first rockslide was ~200,000 m³.

Avalanche event. Inhabitants of the lower plains described the sound of the avalanche as similar to a helicopter. Multiple witnesses gave the time as between 1030 and 1100 on 30 October. The main slide mass immediately shattered into its original breccia blocks coated by vein precipitates. The initial SE movement of the avalanche blocks was deflected to the SW along a deep gully oriented parallel to the fault. A smaller part of the avalanche surmounted a small ridge and continued SE towards the village of Argelia.

For the first 2 km the main avalanche remained confined to a narrow valley. The top of the flow was 150 to 250 m wide; its depth, 30 to 60 m. A typical cross section of the peak flow was 7,500 to 9,000 m². The flow swashed back and forth on its downward course. Super-elevation calculations at locations of overbank flow gave a velocity of ~15 m/s in the upper reaches. Deposits high on the volcano consisted of altered dacite blocks up to meter-size. They contained essentially no matrix, with the finest particles centimeter-sized. The margin of the avalanche was sharp and flying rocks scarred the adjacent trees at 2-3 m height. A few trees were decapitated at heights of several meters.

At a prominent break in slope 2-3 km from the source, large ramps of avalanche materials formed imbricate ridges. Here the deposits, 4-6 m thick, still lacked matrix. The avalanche materials were essentially clast supported. The avalanche scoured blocks of lava from the walls, and up to 10 m deep into clay-rich soil in the base of the valley where it passed.

Lahar runout flow. Soon after the onset of the avalanche, a lahar runout flow, as defined in Scott (1988), initiated from the major accumulation zone of the primary avalanche. In other words, the source of the lahar runout flow formed in the thickest accumulation of debris at the mouth of the avalanche valley, 3 km from the summit and 3 km above the towns of El Porvenir (formerly Augusto Cesar Sandino) and Rolando Rodriguez. The populations of these two towns were respectively 600 and 1,250 according to the last census. The location of the sites of El Porvenir and Rolando Rodriguez could only be found by GPS data; there remained almost no evidence of former human habitation.

Apparently the lahar runout flow resulted from rapid dewatering of the saturated avalanche. The flood surge moved as a hyperconcentrated flow, depositing a thin (~40 cm thick) layer of gravel with some clay matrix on the overbank zones, and transporting meter-size blocks within the incised channels. The peak height of the flood surge was 3 m as it entered El Porvenir, as evidenced by stripped bark from the few standing trees. Nearly all vegetation and soil was removed by the leading edge of the wave. However, a few islands of vegetation were spared on some hills. The width of the flood surge in its upper reaches was ~1,500 m. Assuming an average peak depth of about 3 m, this yields a cross sectional area of flood surge at 4,500 m².

Casualties and damage. Based on observations in the field, the towns of El Porvenir and Rolando Rodriguez were destroyed beyond recognition. It is unknow how many people survived. Visible cadavers and dead livestock on the overbank had been burned for sanitary reasons. Many other small hamlets, residences, and farms were destroyed.

Future hazard potential. The disaster of 30 October, was produced by the coincidence of two discrete events: extraordinarily heavy rains and an avalanche. Neither of these alone would have produced such extensive damage to the surrounding area. In this respect note that the towns of El Porvenir and Rolando Rodriguez were established only a few decades ago in this area of high geologic risk. To reduce threats for new settlements, comprehensive geologic hazard studies can help identify regions with elevated risk.

In the absence of another episode of heavy rainfall, the new deposits seem to be stable. In fact, there is little mud or silt within the deposits at higher elevations to facilitate remobilization. However, the conditions near the summit that favored the rockslide avalanche still exist. Altered and fractured dacite occurs on steep slopes at a high elevation. Destabilizing events, such as an earthquake or torrential rains, could produce another avalanche in an adjacent area. The probability of such an extreme avalanche seems remote. However, an assessment of the associated hazards and risks should be undertaken.

Reference. Scott, Kevin M., 1988, Origins, behavior, and sedimentology of lahars and lahar-runout flows in the Toutle-Cowlitz River system: U.S.G.S. Professional Paper 1447-A, 74 p.

Geologic Background. The San Cristóbal volcanic complex, consisting of five principal volcanic edifices, forms the NW end of the Marrabios Range. The symmetrical 1745-m-high youngest cone, named San Cristóbal (also known as El Viejo), is Nicaragua's highest volcano and is capped by a 500 x 600 m wide crater. El Chonco, with several flank lava domes, is located 4 km W of San Cristóbal; it and the eroded Moyotepe volcano, 4 km NE of San Cristóbal, are of Pleistocene age. Volcán Casita, containing an elongated summit crater, lies immediately east of San Cristóbal and was the site of a catastrophic landslide and lahar in 1998. The Plio-Pleistocene La Pelona caldera is located at the eastern end of the complex. Historical eruptions from San Cristóbal, consisting of small-to-moderate explosive activity, have been reported since the 16th century. Some other 16th-century eruptions attributed to Casita volcano are uncertain and may pertain to other Marrabios Range volcanoes.

Information Contacts: Michael F. Sheridan, SUNY, Buffalo, New York; Claus Siebe, UNAM, Mexico; Christophe Bonnard, EPFL Lausanne, Switzerland; Wilfried Strauch; Martha Navarro, Jorge Cruz Calero, and Nelson Buitrago Trujillo, INETER, Nicaragua.

Seismicity remained generally at background levels during October. During 1, 16, and 23 October plumes were seen rising 200 m above the volcano. On 19 and 24 October, gas-and-steam plumes rose 100 m above the volcano. No plumes were seen on 2, 3, and 9 October. During other days the summit was obscured by cloud. The level-of-concern color code remained green.

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

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

Moderate activity prevailed at Stromboli from January to May 1997 (BGVN 22:03). During this period there was a slight decrease in tremor intensity and a slight increase in the number of recorded events (figure 56). Events exceeding the saturation level of the summit seismic station numbered fewer than 10% of the total recorded.

Figure 56. Seismicity detected at the summit of Stromboli from January 1997 through August 1998. Gray bars show the number of recorded events/day, and the black bars those saturating the instrument (ground velocity exceeding 100 µm/s). The line shows daily tremor intensity computed by averaging hourly 60-second samples. The seismic station is located 300 m from the craters at 800 m elevation. Courtesy of Roberto Carniel.

There was a marked increase in the total number of events during June-July 1997, sometimes in excess of 300 per day. Following a month-long lapse, an even larger long-term increase began in September that continued until November 1997. There were several days in this interval when triggering of the seismic station was almost continuous and tremor intensity reached high values, behavior that usually coincided with continuous spattering at the vents. No seismic data were recorded between 24 November 1997 and 9 January 1998. Activity had returned to moderate levels by the time seismic data acquisition resumed on 10 January 1998 (figure 56). The number of daily events rapidly decreased, as did tremor intensity.

At 1130 on 16 January 1998, a strong explosion in the crater area was similar to others at Stromboli during the last few years; one comparable event occurred on 4 September 1996 (BGVN 22:03). Such explosions are not a danger to the villages of Stromboli and Ginostra (figure 57), but they may be dangerous for tourists visiting the summit because bombs easily reach the usual observation points. Another risk is that fires, started by incandescent bombs, may spread in the vegetation. In the case of the 16 January eruption, bad weather prevented tourists from climbing the volcano and rain extinguished any wildfires.

Figure 57. Sketch map of Stromboli Island, showing locations referred to in the text. Courtesy of Roberto Carniel.

A new rise in seismicity began a few days after the explosion. A peak was reached during 16-20 February; on 19 February, 405 events were recorded, and on 20 February tremor intensity was high and 43 saturating events were noted. After this increase, activity decreased steadily with only a few fluctuations until the end of April. The total number of events recorded during the decrease was sometimes

During May-June seismic activity increased. During July two sharp drops in the level of activity were observed: the number of events did not exceed 80 per day during 1-3 July, and went below 50 per day during 22-24 July. Tremor intensity reached the minimum of the year on 22 July. There was a slight upturn in August.

At 1726 on 23 August, another powerful explosion occurred at the craters. The strong blast was heard throughout the island, and a column of ash and lapilli shot over the craters. Incandescent bombs fell over a vast area towards Vallonazzo, Labronzo, and Forgia Vecchia. At least one other explosion followed. Several fires started in vegetation on the upper slopes; the largest one, near Forgia Vecchia, was not extinguished until the next day. Fortunately, although a high number of tourists were on the island, no one was hurt. A dark ash column was eventually replaced by a large, light ash cloud. Small lapilli fell in Ginostra. Bombs were found on the tourist path down to 750 m elevation, and in other directions bombs fell to 500 m. Authorities immediately blocked public access to the upper part of the volcano. The explosion also caused significant morphological changes to the rim of Crater 1 towards Semaforo Labronzo.

Another strong explosion, perhaps more energetic than that of 23 August, happened at 1914 on 8 September. A considerable atmospheric shock wave was reported at the village of Stromboli, and broken windows were reported near San Bartolo. Ash and small lapilli fell near Ginostra and several bush fires were started by bombs on the volcano's slopes. Unfortunately, the seismic station was not operational at the time due to a technical problem.

Stromboli, a small island N of Sicily, has been in almost continuous eruption for over 2,000 years. It is the namesake for small Strombolian explosions, which hurl incandescent scoriae above the crater rim several times a day, with infrequent larger eruptions.

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: Roberto Carniel, Dipartimento di Georisorse e Territorio, Universitá di Udine, Via Cotonificio, 114 I-33100 Udine; Jürg Alean, Kantonsschule Zürcher Unterland, CH-8180 Bülach, Switzerland.

A white vapor plume was present throughout September; it appeared to vary in thickness, probably as a result of atmospheric conditions. Observed seismicity was low to moderate. An aerial inspection on 1 October, as part of the Ulawun Decade Volcano workshop, showed the summit crater to be open, ~150-200 m in diameter, with vertical sides descending at least 50 m before being lost in thick white fume.

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: Patrice de Saint-Ours, Steve Saunders, and Ben Talai, RVO.

A minor eruption at White Island in August (BGVN 23:08), which was investigated by volcanologists from the Institute of Geological and Nuclear Sciences (IGNS), persisted until late in September. Analysis of samples collected during the visits continued through September. Eruptive activity recommenced in late October, prompting another investigative visit on 2 November. The following reports is summarized from IGNS Science Alert Bulletins.

A new active vent in the NW corner of the 1978-1990 Crater Complex produced intermittent weak ash emissions during late August and early September that rose 100-1,500 m above the island. September ash contained more fresh volcanic glass than previous samples, but this failed to give clear indication of new magma being the source because the eruptions came from a crusted-over magma body.

Weak volcanic tremor on 10-11 September appeared on seismic records and impacted estimates of the Real-Time Seismic Amplitude (RSAM). The RSAM outputs a number of 'counts' over set time intervals. The higher the counts the stronger the volcanic tremor signal and the stronger the volcanic activity. The RSAM count level in mid-September was about 12-13, on a scale of several thousand, having risen from the typical background of 2-3 counts. There were no reports of ash after 18 September and seismicity was reduced to background levels. The Alert Level was reduced from 2 to 1 on 29 September.

Minor eruptive activity recommenced on 24 October. Small amounts of ash were emitted on 24-25 October, and on 31 October a steam-and-ash column rose in calm conditions to 1,500-1,600 m above the volcano. Weak volcanic tremor reappeared at about the same time as the ash eruptions recommenced; however seismicity remained low.

A surveillance visit was made on 2 November to assess the activity, conduct a deformation survey, and collect ash and gas samples. The level of activity varied during this visit, but the most energetic activity observed was not sufficient to raise the Alert Level. The active vent at the base of the NW wall of the 1978-1990 crater had grown slightly since August. A very weak ash-charged reddish-gray convecting plume was emitted. Occasional yellowish hues were present in the plume, consistent with the periodic eruption of hydrothermal sulfur from the vent. The maximum temperature measured in the ash column was 451°C.

Eruptive activity over previous days had deposited 15 mm of fine dark gray ash at the crater rim. Examination of the ash indicated no change in character from that of the July-August eruptions. Ground-deformation surveys showed a consistent trend of minor deflation across the main crater floor, with the largest changes (20-30 mm) near the crater rim. However, fumarole temperatures had increased nominally since August 31. Fumarole ##1 was at 113°C (up from 101°C), was moderately dry, and had molten sulfur in the orifice (indicating temperatures in excess of 119°C in the vent). Donald Mound continued to discharge only low-pressure steam from diffuse areas of steaming ground, and the cracks around Peg M continued to discharge steam close to the boiling point. Maximum temperature at Noisy Nellie was 140°C (up from 126°C), whereas pressures were similar to those observed in August. Fumarole 13a was 111°C, a slight increase from August (105°C). The plume from the island appeared to carry a heavier SO₂ burden than observed in August.

The uninhabited 2 x 2.4 km White Island is the emergent summit of a 16 x 18 km submarine volcano. The island consists of two overlapping stratovolcanoes; the summit crater appears to be breached to the SE because the shoreline corresponds to the level of several notches in the SE crater wall. Intermittent steam and tephra eruptions have occurred throughout the short historical period, but its activity also forms a prominent part of Maori legends.

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

Information Contacts: B.J. Scott, Institute of Geological and Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand.