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 11 May, rapid movement of an older landslide was followed by a steam explosion that triggered mud flows and a small tephra emission. The event occurred at Sumikawa-Onsen (a hot spring resort) at the foot of Akita-Yakeyama, ~4 km NE of the summit. The following is based on a report by Shintaro Hayashi.

Although the landslide began moving a few days before 11 May, the sliding accelerated 20 minutes before the explosion. A field party saw the fast-moving landslide and took refuge prior to 0800 on 11 May. The explosion was witnessed at 0800 by a pilot flying over the area; he saw a water-and-steam column rising like a geyser, followed within seconds by black smoke emissions.

The explosion, heard as far as 1.4 km away, triggered a mudflow along the Akagawa River and eventually developed into a debris flow downstream. The field party noticed a thin coat of ash covering the mudflow deposits; they concluded that the tephra had issued from the explosion site.

Hayashi suggested that the explosion was triggered by sudden depressurization of a hot water reservoir under the hot spring due to removal of the overlying debris. The depressurization led to sudden boiling, generating sufficient steam pressure to explode. The volume of erupted material was estimated to be between 1,000 and 10,000 m³.

According to Hiroyuri Hamaguchi the precursory vibration and tremor were recorded by a short-period seismometer 1 km NNE of the hot spring. The landslide was as large as 500 m wide, 150 m long, and 500 m deep. After 2000 on 10 May, tremors of increasing amplitude built up. They declined by midnight and then returned at 0400 on 11 May. A maximum amplitude was reached at 0732, followed by a hiatus during 0753-0757. Short- and long-period events took place at 0757 and 0758, respectively.

Hayakawa reported that two hotels at the foot of Akita Yakeyama were completely destroyed by the landslide and lahar; however, there were no casualties because the staff and guests had evacuated. Air photos taken on 12 May by Asia Air Survey Co. can be seen on the internet.

Geologic Background. One of several Japanese volcanoes named Yakeyama ("Burning Mountain"), Akita-Yakeyama is the most recently active of a group of coalescing volcanoes in NW Honshu immediately west of Hachimantai volcano. The main volcano, Yakeyama, contains a small lava dome in its 600-m-wide summit crater. Tsugamori volcano to the east is a stratovolcano of roughly the same height and has a 2-km-wide crater breached to the NE. The flat-topped parasitic lava dome of Kuroshimori lies 4 km south of Yakedake. Tamagawa Spa at the western foot, one of several thermal areas, is strongly radioactive. The last magmatic eruption formed the Onigajo lava dome in the summit crater about 5000 years ago. The only known historical activity has consisted of somewhat uncertain 19th-century eruptions and mild phreatic eruptions in the 20th century.

Information Contacts: Shintaro Hayashi, Faculty of Education, Akita University, 1-1 Tegata-Gakuen-Cho, Akita 010, Japan; Hiroyuki Hamaguchi, Faculty of Science, Tohoku University, Sendai 980-77, Japan; Yukio Hayakawa, Faculty of Education, Gunma University, 4-2 Aramaki-machi, Mae-bashi-chi, Gunma 371, Japan (URL: http://www.hayakawayukio.jp/); Tatsuro Chiba, Dept of Disaster Prevention, Asia Air Survey Co., 4-2-18 Shinjuku, Shinjuku-ku, Tokyo 160, Japan (URL: http://www.ajiko.co.jp/en/).

During April OVSICORI-UNA scientists noted a decrease in eruptive vigor and seismicity at Arenal compared to the previous month. Lavas erupted beginning in January 1997 advanced during April to reach an elevation of ~800 m on the N flank; lavas erupted in March advanced during April to reach an elevation of 1,150 m. During mid-May some advancing lava fronts had reached an elevation of about 1000 m. As has been typical during recent years, Crater D showed only fumarolic activity. Crater C erupted about 288 times during May.

Tremor occurred for as much as 15 hours/day during April, but both tremor and earthquake counts dropped by about a third compared to March, and further still during May. Nonetheless, on 16 May there were 8 hours of continuous tremor (amplitude, 18 mm; dominant frequency, 2.0-3.9 Hz). This tremor accompanied venting of new lava that traveled down the NNW flank. At least through April, the OVSICORI-UNA distance network continued to undergo radial contraction of ~22 ppm/year.

During 10-14 May, Gerardo Soto saw mild Strombolian eruptions tens of minutes apart, with ash columns up to 500 m above the crater. Although this seemed comparatively quiet, the usual vigorous summit fumarolic outgassing prevailed. A new pyroclastic cone was noted in Crater C (figure 82); it stood tens of meters high. Although the volcano lacked pyroclastic flows while he was watching, well developed pyroclastic-flow fans existed on the N and W flanks and summit.

Figure 82. A rough sketch of Arenal as seen from the NW during 10-14 May 1997. Courtesy of Gerardo Soto, ICE.

Arenal's first chronicled eruption, in 1968, began an unbroken sequence of Strombolian explosions and basaltic andesite discharges from multiple vents. The volcano can be seen from a lodge 2.8 km S of the vent that enables visitors to hear, to see, and occasionally to smell its dynamism.

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

Information Contacts: E. Fernandez, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, W. Jimenez, R. Saenz, E. Duarte, M. Martinez, E. Hernandez, and F. Vega, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; G.J. Soto, Oficina de Sismologia y Vulcanologia del Arenal y Miravalles (OSIVAM), Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica.

The Pinatubo aerosol layer at Garmisch-Partenkirchen declined to a minimum in the summer of 1996 (figure 3 and table 11). Since then no further decay was observed. The January-June 1997 average value of the integrated backscatter represents ~70% of 1991 pre-Pinatubo value. It is too early, however, to establish the aerosol load observed since mid-1996 as a new stratospheric background.

Figure 3. Graph showing the log of the lidar backscatter versus time at Garmisch-Partenkirchen, Germany for the latter two-thirds of 1991 through mid-1997. The plotted data are preliminary 532 nm integral values of stratospheric aerosol backscatter (integrated from the tropopause or cirrus to the top of the aerosol layer) versus time. Labeled arrows indicate the eruptions of Pinatubo and Kliuchevskoi. Courtesy of Horst Jager.

Table 11. Lidar data from Germany (October 1996-June 1997) and Hawaii (July-December 1996) showing altitudes of aerosol layers. Backscattering rations are for the Nd-YAG wavelength of 0.53 um, with equivalent ruby values in parentheses for data from Germany; those from Mauna Loa are for the ruby wavelength of 0.69 um. The integrated value shows total backscatter, expressed in steradians^-1, integrated over 300-m intervals from the tropopause to 30 km at Garmisch-Partenkirchen and 15.8-33 km at Hawaii. The "ci" stands for cirrus clouds; their presence in the tropopause region usually obscures the lower boundary of the aerosol layer. Courtesy of Horst Jager and John Barnes.

Correction: Lidar data from Mauna Loa, Hawaii, for July-December 1996 (Bulletin v. 22, no. 3) was incorrect by a factor of 1,000. Corrected data is presented in this issue (table 11).

Geologic Background. The enormous aerosol cloud from the March-April 1982 eruption of Mexico''s El Chichón persisted for years in the stratosphere, and led to the Atmospheric Effects section becoming a regular feature of the Bulletin thorugh 1989. Lidar data and other atmospheric observations were again published intermittently between 1995 and 2001; those reports are included here.

Information Contacts: Horst Jager, Fraunhofer-Institut fur Atmospharische Umweltforschung, Kreuzeckbahnstrasse 19, D-8100 Garmisch-Partenkirchen, Germany.

A map prepared from observations carried out on 11 April of Voragine and Bocca Nuova craters is presented in figure 66. The position and altitude of the points shown by stars were measured with ranging binoculars (model 1500 DAES; on loan courtesy Leica-France) from two observation points (circles) on the rim of the craters. Photos of the crater interior were also used to draw the map.

Figure 66. Map of the Etna craters prepared using LEICA binoculars. Courtesy of P. Briole, O. Consoli, C. Deplus, and J-L. Froger, IPGP.

Bocca Nuova crater measured ~170 m deep and had two active cones on the crater floor. The N cone, 25 m above the crater floor, was the most active. Its Strombolian activity threw ejecta close to Monumento, a spot on the crater's N rim. The S cone, 35-40 m above the crater floor, appeared composed of two coalescent cones, and was less active then the N one.

The depth of the Voragine crater measured ~150 m. Quiet steam emission was observed escaping from the large hole on the lower part of the crater floor.

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: Pierre Briole, Orazio Consoli, Christine Deplus, and Jean-Luc Froger, Institut de Physique du Globe de Paris, Case 89, 4 place Jussieu, 75252 Paris Cedex 05, France.

Although mid-May speculations suggested that a new volcano might be developing in the SE part of the State of Zacatecas, the incident has been attributed to methane combustion unrelated to volcanism. The event took place near the town of Jerez, ~50 km SW of Zacatecas city. Hugo Delgado received a video made by local residents, asked officials about the event, and provided the following report.

"The place where this phenomena is happening is a flat area (a square area [~20 m on each side]) where the ground is smoking (combustion-like blue smoke). There are several cracks on the ground and inside the cracks the earth looks reddish and hot. The people who sent me the video show how a piece of wood burns [when] they put it inside the crack. The area was isolated from the curious people (crowds of families who want to see what is happening visit the place) [by] digging a furrow around the hot site and posting policemen in order to [prevent] children [from falling] into the hot cracks.

"People from the University of Zacatecas and from the SEMARNAP (Ministry of the Environment, Natural Resources, and Fisheries) have visited the area and concluded that microbial activity on concentrated organic material in the area has produced methane and this started to burn since the beginning of May. Burning of methane has [caused] the ground to glow. According to their report, no deformation of the ground has been detected, nor [were] ashes or sulfuric odors detected during their visit. Samples taken from the ground were chemically analyzed [revealing] mainly organic material in them. This kind of [incident] has occurred before in other [parts] of Zacatecas, according to SEMARNAP.

"[It] seems that somebody (unidentified, but according to the local people, it was a retired scientist [transporting] equipment) came to the region to see the phenomena, and commented that it was the birth of a volcano. Thus, the inhabitants became alarmed. Local newspapers have also published that methane is burning there according to the researchers of the University of Zacatecas.

"Officials from the National Center for Disaster Prevention (CENAPRED) knew about this event, and have received the reports from SEMARNAP and the University of Zacatecas. This has been treated not as a volcanic problem but [an] environmental [one].

"A year ago, there was a similar event in the region. Carlos Gutierrez from CENAPRED visited the zone in order to deploy seismic equipment to observe this event. It was determined that organic material (sedimentary carbon[aceous] deposit[s] in a lacustrine environment during the Pleistocene) was burning underground after the local people incinerated dry grass (a common practice in Mexico to fertilize the land before the rainy season)."

Luca Ferrari provided geological insight into the area. It is on the E flank of the Sierra Madre Occidental, a huge mid-Tertiary volcanic pile related to subduction of the Farallon Plate. Volcanic rocks in the area include extensive silicic ashflow tuffs of late Oligocene to early Miocene age; these are sometimes capped by small volumes of andesitic and basaltic lavas ~20 Ma old. The incident took place more than 200 km N of the active volcanic arc (the Mexican Volcanic Belt, related to the ongoing subduction of the Rivera and Cocos plates). Quaternary intra-plate basalts are absent within a 200 km radius of the site of the incident. From a tectonic point of view, the village of Jerez lies at the N end of the Tlaltenango graben, which formed during Basin and Range extension in the early Miocene. Tectonic activity appears to have slowed since then and no Quaternary faulting is reported in the region.

Geologic Background. False or otherwise incorrect reports of volcanic activity.

Information Contacts: Hugo Delgado, Instituto de Geofisica, U.N.A.M.Circuito Cientifico, C.U. 04510, Mexico D.F., Mexico; Luca Ferrari, Instituto de Geologia, UNAM, Apdo. Postal 376, 36000 Guanajuato, Gto., Mexico.

In April and May there were 107 and 136 earthquakes, respectively, a higher number than is typical. These were mainly detected locally. One M 1.9 earthquake centered 4 km SW of the active crater at a depth of 3.8 km took place at 0054 on 6 April. A M 2.2 earthquake struck at 0437 the same day at 5.2 km depth centered 11 km NW of the crater. On 19 May the station 5 km SW (IRZ2) registed a swarm consisting of 23 high-frequency earthquakes.

Irazú lies along a fault zone shaken by repeated earthquakes in the past 6 years. It was estimated that ~40 of May's 136 earthquakes were associated with faulting. Earlier summaries of monthly earthquakes for December 1996, and January and March 1997 reported 51, 82, and 92 events, respectively.

Irazú's ashfalls frequently reached San Jose, 25 km to the E, during its historically most active period in 1963- 65. That period of intermittent, weak-to-moderate explosive activity severely affected agricultural areas over much of central Costa Rica, causing major economic problems. During the same interval, 46 secondary mudflows swept down the Rio Reventado valley.

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

Information Contacts: E. Fernandez, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, W. Jimenez, R. Saenz, E. Duarte, M. Martinez, E. Hernandez, and F. Vega, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

Vulcanian explosions resumed in late May. During the first 3 weeks of the month Crater 2 released moderate volumes of steam. Then, an explosion on the 22nd at 1510 produced dark gray ash clouds that rose to about 4.5 km above the crater rim. Explosions on the following days of May generated ash clouds to heights of between 2 and 3.5 km. Low rumbling sounds on the 27th presumably accompanied other explosions. Only weak vapor vented at Crater 3 during May. Seismographs remained inoperative.

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

Information Contacts: B. Talai, D. Lolok, P. de Saint-Ours, and C. McKee, RVO.

During the first week of May, Main Crater gently emitted small to moderate ash clouds, similar to those in late April. On 9 May, activity increased slightly and ash clouds were ejected to 500-1,000 m above the summit resulting in light ashfall downwind. Forceful emissions and light ashfalls at Main Crater occurred on the 13th; there were also two loud explosions during 1500-1600. After that, there were weak-moderate ash emissions accompanied by roaring noises and infrequent rumblings. Rumblings on the 6th and 28th were attributed to rocks cascading into Southwest Valley. Activity increased again on the 29th. South Crater weakly emitted steam during May.

Seismicity showed an irregular rise during May (growing from 800 to 1,700 low-frequency events/day). Wave amplitudes, although low, doubled. Water-tube tiltmeters at Manam Volcano Observatory (4 km SW of the summit) showed a very small inflationary change (0.5 µrad), which may be significant because it continues the inflationary pattern evident since early March.

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: B. Talai, D. Lolok, P. de Saint-Ours, and C. McKee, RVO.

Between 17 and 20 April the seismic network of the French Laboratoire de Geophysique in Tahiti recorded an acoustic swarm from Monowai seamount (figure 3). The swarm ended on 20 April at 2058 GMT. It was very similar to the swarm of September 1996, with similar amplitudes and overall duration.

Figure 3. The 17-20 April 1997 acoustic swarm at Monawai seamount shown as a plot of wave amplitude versus time. Courtesy of Olivier Hyvernaud.

The signals for the 17 April acoustic swarm that started at 2007 GMT consisted of strong acoustic waves with a maximum peak-to-peak amplitude of 1.01 m/s. These and later signals were interpreted as an indication of explosive phenomena. The subsequent acoustic waves were weaker, with amplitudes between 50 and 450 millimicrons/second. Overall, the laboratory recorded 136 acoustic waves. Most of the signals clustered into two episodes. The first took place on 18 April during 0013-1911 GMT and included 46 acoustic waves. The second occurred on 19 April during 0308-0810 GMT and included 81 acoustic waves. In addition to including more waves in a shorter time interval, the second episode was stronger.

The above-cited coordinates (25.89°S, 177.19°W) are for the summit of the volcano. A bubbling area was discovered on 17 October 1977 at 25.917°S, 177.233°W. The exact coordinates of the acoustic source discussed here are not well known, and can not be located precisely using currently available T-wave selections.

Proceeding NNE from the Rumble (I, II, III, and IV) seamounts (New Zealand), the next known active volcanoes lie in the Southern Kermadec Islands. From S to N, these consist of Curtis (submarine), Brimstone Island (submarine), Macauley Island (a sub-aerial caldera), Raoul Island (a vigorously active stratovolcano), an unnamed center (submarine), and then Monowai (submarine). Monowai was the source of over six inferred eruptions; in some cases these eruption reports were also based on collateral visual observations such as discolored water and bubbles.

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

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

During April, fumarolic degassing and weak bubbling continued in the 29°C, green-turquoise-colored crater lake. On the N crater floor there appeared a new 80-m-long fracture with fumaroles depositing sulfur; weakly escaping gases there had temperatures of 94°C. The same temperature was measured at the accessible part of the pyroclastic cone, and other fumaroles reached temperatures of 92-93°C. A steam plume rose 300 m above the crater floor.

April seismicity increased to 2,532 events (2,192 low-frequency and 339 medium-frequency). Only one month in the previous year had more events: during January 1996 there were 4,045 events. The high seismicity was not sustained, May 1997 earthquakes only numbered 1,020. In conjunction with medium-frequency earthquakes, people watching the volcano noticed new fumaroles. The distance net showed no significant changes during 1997.

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

Information Contacts: E. Fernandez, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, W. Jimenez, R. Saenz, E. Duarte, M. Martinez, E. Hernandez, and F. Vega, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA).

Inflation recorded since late April culminated in a Strombolian eruption on 1 June. Unlike most of the earlier Strombolian eruptions, this one escalated slowly and sustained moderate-to-high intensities briefly before declining.

Activity during May. The lead-up to June's Strombolian eruption, like that of the April eruption, was characterized by relatively low-pressure, but voluminous gas-rich emissions. Occasionally, these very pale gray emissions produced light ashfalls from columns with heights of 0.6-1 km altitude. Roaring sounds were heard throughout May; loud explosions occurred on the 2nd (at 0105), 12th (1526), 14th (0759), 15th (0052), and 31st (2201). Weak night glows were seen above the crater on the 1, 2, 16, and 25 May.

Twenty-one low-frequency earthquakes (mostly associated with explosions) were recorded during May; most in the first two weeks. The highest numbers of daily earthquakes reached four on the 7th and three on the 8th. There were six high-frequency earthquakes on the 18th. Two of these were located immediately NE of the caldera and one was just outside the SE part of the caldera. Background seismicity remained low at ~20 RSAM units.

The Sulphur Creek water-tube tiltmeters registered N-down tilt in early May, continuing the inflationary pattern seen since late April. About 5 µrad of inflationary tilt had accumulated by mid-May, a time when the tilting seemed to cease or perhaps reverse slightly. At Sulphur Creek, the total inflation since the March eruption was ~10 µrad.

A new electronic tiltmeter was installed at Matupit Island on 8 May and soon indicated WNW-down tilt, suggesting inflation of the magma reservoir. About 18 µrad of this tilting had accumulated by 15 May when tilt changed to WSW-down, a direction radial to Tavurvur. This change was consistent with the behavior of the tiltmeters at Sulphur Creek in mid May. The WSW-down tilt continued through the remainder of the month, amounting to ~20 µrad. The pattern of tilting registered by the electronic tiltmeter on the S part of the Vulcan headland (Vulcan Island) was complicated during the first half of May, but during the second half of the month ~10 µrad of SW-down tilt took place, consistent with the inflation in the central or eastern part of the caldera.

During May the water-tube tiltmeters at Tavuiliu (on Rabaul caldera's SW rim) continued to shift in a SW- down direction. Since the April eruption this tilting had amounted to about 6 µrad. The only late-stage precursor to the 1 June eruption was an unusual E-down tilt of a few microradians recorded at the Matupit Island electronic tiltmeter beginning about midday on 31 May.

Activity during June. Apart from the tilt, through the early morning of 1 June there was little indication of the impending Strombolian eruption. The eruption's early phase began when low-pressure, hazy, white-and-blue emissions rose a few hundred meters above Tavurvur.

Starting about 0700 at Matupit Island, a N-down shift in tilt began at ~1 µrad/hour accompanied by discontinuous tremor (recorded at the nearest seismic station, KPTH, ~1 km away on Matupit Island). By about 0830 on 1 June, the seismicity had climbed to ~50 RSAM units from a normal background of about 20. At 0837, a moderate explosion sent a low-density ash cloud ~1.3 km above the vent. Seismicity briefly reached 200 RSAM units and then declined to ~90 RSAM units by 0900. The column remained at ~1.3 km and seismicity fluctuated between about 60 and 150 RSAM units until 1030 when activity intensified.

Later, at about 1030, the column rose to ~2.5 km and seismicity increased to 250 RSAM units. The column was a pale gray-brown color, with moderate ash content. A strong S wind blew the plume over the E side of Rabaul Town. Then, between 1100 and 1145, eruptive vigor declined and seismicity fell (to 120 RSAM units).

From 1130 until 1930, the eruption was observed at comparatively close range, at 0.5-1.5 km distances. Although the explosions were initially, around 1130, almost continuous, the column only rose to ~0.5 km above the vent. There were multiple active vents within Tavurvur's summit crater, but the principal one was near the crater's S rim. The explosions produced broad, dense, and moderately dark gray emission clouds that assumed the shape of cock's tails. The activity began increasing again at 1145 and lightning was seen in the column at 1200. The N-down tilt that had been in progress since 0700 reversed at 1200 after accumulating ~5 µrad.

A change in the column was noticed at about 1240 as the emissions became distinctly depleted in ash and the sounds grew louder and sharper. By this time seismicity had increased to about 250 RSAM units, where it stayed until 1400. Tavurur's principal vent (on the S side of the crater) started ejecting incandescent lava fragments, including some very large ones. For brief intervals, other vents in the crater issued dark, dense clouds.

The noisy explosions from the principal vent carried brightly incandescent lava fragments with little ash. In contrast, the dense ash-rich explosions from other vents escaped were accompanied by little or no sound.

During a brief lull between about 1400 and 1425 seismicity fell to ~200 RSAM units. Then, at about 1425, distinctly louder explosions began. It appeared that fluid lava had almost reached the crater rim and the explosions were akin to bubbles bursting. The explosions usually involved sustained jetting for periods of over 10 seconds. Intervals between events were typically only a few seconds.

Beginning about 1440, visible shock waves were observed. At about 1451 the explosions were very loud and the eruption column was about 0.5 km high. At 1450, seismicity peaked at 645 RSAM units.

A prolonged period of dense, dark ash emission commenced at about 1500 and seismicity fell sharply. While dark ash clouds billowed upwards from vents in the W part of the crater, the principal vent continued producing nearly ash-free explosions bearing larger incandescent fragments. The dense, dark ash emission had ceased by 1519, and by then seismicity had dropped to 300 RSAM units. Seismicity during 1520-1600 increased to ~500 RSAM units; after that it declined slowly so that by 1800 it reached 300 RSAM units. After 1830 seismicity declined more quickly, so that by 2030 it reached only 90 RSAM units.

For most of the remainder of the eruption the only vent to emit much solid material was the principal vent, which continued to eject nearly ash-free, incandescent lava fragments. Although the bulk of the column remained only ~0.5 km above the vent, beginning in the mid-afternoon some fragments rose ~100 m higher. Explosions throughout the afternoon tended to sustain stronger jets of gas and lava fragments. By about 1800 some of the explosions were more than 1-minute long. In one 5-minute period at about 1800 there were eight explosions.

Witnesses on a boat sailing past Tavurvur's S and W flanks at about 1910-1930 noticed considerably more ejecta landing N of the vent. Some ejecta blew in the strong prevailing S wind as far as Tavurvur's N flank. Between 1200 and 2000 the tiltmeter at Matupit Island had accumulated ~5 µrad of predominantly S-down tilt. Then, at 2000, the tilt shifted to SW down, changing by 0.3 µrad/hour.

During the night of 1 June there were episodes of rhythmic degassing; in addition some very loud detonations shook buildings. Background seismicity fell slowly after 2030 on June 1, descending by midnight to 30 RSAM units. Sustained increases in seismicity returned on 2 June during two intervals: the first, 0100-0145, and the second, 0300-0500. During these intervals, seismicity reached 300 and 250 RSAM units, respectively. In addition, a brief (10 minute) peak in seismicity occurred around 0330 on 2 June; it reached 750 RSAM units.

Overview. Unlike some previous eruptions, no lava flows were generated by the 1-2 June event. Ejected lava fragments showed textural evidence of moderate expansion but lacked evidence of post-emplacement flow. Additionally, the bombs were considerably smaller.

The volume of material erupted on 1 June was very small, possibly only 1 x 10⁵ m³. There was no off- set in tilt as had been seen with the earlier, larger eruptions. Thus, after the 1 June eruption, Tavurvur remained inflated 10 µrad over the tilt encountered after the March eruption (BGVN 22:03). Accordingly, scientists believe that Tavurvur could erupt with similar intensity again in coming weeks.

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: B. Talai, D. Lolok, P. de Saint-Ours, and C. McKee, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.

During April, fumarolic activity remained in the E and S parts of the main crater. In the latter location, escaping gases hissed like a pressure cooker and were audible from the crater rim. Gas columns rose up to 200 m high. Adjacent to the crater, visitors smelled sulfur gases and their throats, eyes, and skin became irritated. Some of the plants damaged during November 1995 showed new signs of recovery. Although the seismic station (RIN3, located 5 km SW of the active crater) remained out of service during May, earthquake counts numbered five events in December 1996 and 24 in January 1997.

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

Information Contacts: E. Fernandez, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, W. Jimenez, R. Sáenz, E. Duarte, M. Martinez, E. Hernandez, and F. Vega, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

According to press reports quoting an official from the Instituto Nicaraguense de Estudios Territoriales (INETER), an eruptive phase began at San Cristóbal during the night of 19-20 May. Because of the possibility of ashfall, authorities declared a state of alert in the city of Chinandega, one of the largest cities in Nicaragua, situated 20 km WSW, the main downwind direction. A report about precursory seismicity was provided by INETER on 18 May. Observations after the start of eruptive activity were sent by Benjamin van Wyk de Vries (Open University), and include information from INETER scientists.

Precursory activity as of 18 May. Strong seismic activity was detected starting in May, by far the strongest seismicity observed since the new seismic station (CRIN, ~3 km NW of the crater at the base of the volcano), was installed at the end of 1992. As of 18 May CRIN was recording 500 volcano-seismic events/day, with frequencies of 1-6 Hz and durations of ~1 minute or more. Sometimes roughly mono-frequency wavetrains of ~3 Hz were observed. RSAM values climbed from2 odor. INETER volcanologists visited the volcano on 17 May, but could not climb to the crater because of high gas concentrations.

On 18 May INETER was preparing an observation point at Casita volcano, ~4 km SE of San Cristóbal. At the seismic station, CRIN, additional channels with low amplification were switched to be transmitted to the Managua data center. The installation of additional seismic stations is planned. INETER volcanologists took gas and water samples for analysis, and local residents were being interviewed to obtain information about recent changes at the volcano. INETER informed the Nicaraguan Government and the Civil Defense Organization about the situation.

Activity during 21-26 May. B. van Wyk de Vries visited the volcano during 21-23 May, setting up the base for a deformation network, and checking the general state of the cone and crater. RSAM levels stayed fairly constant during this period, but began a slow decline at the end of May. Eruptions through 26 May produced light gray clouds of ash that normally rose 50-200 m above the crater. There were a few notable large eruptions, including one at about 0245 on 22 May that reached 800 m. The height differences were partly due to how quickly the plume was pulled over by the wind. One pulse made a noise that was heard 3 km away at the Casita observation post. Pedro Perez (contracted to The Open University), who was at the crater edge on the morning of the 22nd, noted dull noises before each ejection of ash. Sounds were not heard the next day, when the ejections were less powerful. There were periods of up to a few hours with very weak gas release, then equally long periods of eruptions every 5-10 minutes, or a constant plume. If the wind was strong the plume was dragged down the cone to ~900 m, after which it rose to 1,000-1,200 m; during calmer periods the plume rose to 2,500 m.

Summit visit on 23 May. There was no activity when the crater was reached at 1200 on 23 May. Volcanologists stayed for an hour installing a GPS station on the edge of the outer crater. They made a very quick descent into the crater and saw a new vent. Before the start of activity in early May, San Cristóbal was generally the same as when van Wyk de Vries first saw it in 1986. The main change has been the slow progressive growth of the inner pit crater (figure 2). There is an old outer crater ledge on the S side, then the main crater (10-50 m deep); a small inner cone has been gradually hollowed out by a pit crater. The cone has now mostly fallen in, and only a small part to the S and W still exists. The pit is ~80 m deep and Pedro Perez reported that in early May it had a flat bottom. At that time there was little degassing: a few fumaroles on the SW side and a few fumarole mounds on the SE part of the main crater floor. There had been a vigorous fumarole on the edge of the pit crater, with a temperature of ~600°C, but this part had fallen in. On 23 May there was a 4-m-wide vent in the SW part of the pit crater floor; looking in at 45° no bottom could be seen, and in shadow it was not glowing. On the walls of the pit were dark patches of wet rock, where water seepage or fumarolic emanations had discolored the new ash. The whole of the crater was covered by ash. On the crater floor it was 10 cm deep and around the edges 1-2 cm. The upwind crater rim (SE-N) had a slight dusting. On the downwind rim ash had been stripped off the top of rocks but was accumulating in fine layers on the vertical upwind surfaces. The ash was being continually blown around and formed some beautiful ripples.

Figure 2. Summit maps showing crater features at San Cristóbal in 1957 and May 1997. The 1957 map is based on an aerial photo from the INETER collections. Courtesy of B. van Wyk de Vries, The Open University.

Ash covered the W side of the mountain, giving it a light-gray appearance down to ~800 m. Although vegetation hid the ash layer from distant viewers, it continued down to ~600 m where a 1-3 mm thick layer coated the ground and dusted the leaves. Gas masks were worn during the descent from the summit because the ash billowed up. Down at the farm of Las Rojas (500 m elevation) there was a slight dusting of ash; other farms reported heavier falls. Vegetation damage was not observed, possibly because everything is so dry in the area. Very light ashfall was occurring 10-15 km downwind.

No tremor was felt at the crater, though there were ~600 events/day being recorded. Campesinos living on the pass between San Cristóbal and Casita felt shocks on the night of 21-22 May. The only significant deformation features noted on 23 May were several fissures around the edge of the inner pit crater, which indicate that it is still enlarging. The fractures on the main crater floor were covered in ash, which was not fractured, indicating that there was no movement over the previous few days. Pedro Perez noted some rockfalls during explosive activity on the morning of the 22nd, from the N side of the pit crater.

Deformation network.The new GPS network consists of two triangles. The outer triangle is centered on a point on the pass between Casita and San Cristóbal, with one position on Casita, one 2 km SE of Chinandega, and one 10 km NE of Chinandega. The lines are ~10 km long. The center point should form one corner of an inner triangle, the other points being at Las Rojas farm to the W and one as yet undecided on the N flank. These lines are <5 km long. Within the inner triangle, points were placed at ~1,500 m elevation on the SE flank and on the W side of the crater edge at ~1,700 m; both are 1-2 km from the center point. Simultaneous measurements at all four of the outer triangle points were taken with the Open University GPS and the INETER GPS. Two points on the volcano were fixed, and will be re-occupied by INETER personnel. Although the whole of the network is not in place, at least three points on the volcano have been fixed and will indicate any major movement.

Volcanic history. San Cristóbal is a stratovolcano 100 km NW of Managua that has erupted about nine times since the Spanish conquest. Its previous most recent confirmed eruption was a 45-minute ash emission in October 1977 (SEAN 02:10), but a small ash emission may have occurred in November 1987 (SEAN 13:01). The San Cristóbal complex comprises San Cristóbal cone, El Chonco cone, Cerro Montoso, Casita volcano, and La Pelona Caldera (Hazlet, 1987; Van Wyk de Vries and Borgia, 1996). San Cristóbal proper is the youngest feature of the complex. Casita was probably active in the 16th century, and has several active fumarole fields. La Pelona looks as though it was once a large stratocone that underwent a caldera-forming eruption in the Quaternary. El Chonco is an 800-m-high andesite-dacite cone. Cerro Montoso is a 600-m-high andesitic scoria cone cut by large faults.

The complex as a whole has a tendency to produce significant amounts of dacitic magma. Examples include the Chonco cone and Loma La Teta (a dacite dome associated with El Chonco), recent pyroclastic-flow and tephra deposits on San Cristóbal and Casita, and the La Pelona Caldera ignimbrite. This type of magma contrasts with the predominant basalt-basaltic andesite of the other nearby volcanoes, such as Telica, Rota, and El Hoyo/Cerro Negro. Martha Navarro (INETER), who has done geological and hazard mapping at San Cristóbal, noted that the dacitic pyroclastic-flow deposits of San Cristóbal are similar in composition to a thick tephra-fall deposit that the forms subsoil over much of the west side of the cone. On the cone itself, these deposits are covered by more recent andesitic scoriae and bombs, some or all of which come from the historical eruptions in the 17th century and the 1970's. There have not been any historical lava flows, but several lava flows are still only partially vegetated.

References. Hazlett, R.W., 1987, Geology of the San Cristóbal volcanic complex, Nicaragua, in Williams, S.N. and Carr, M.J. (eds.), Richard E. Stoiber 75th Birthday Volume: J. Volcanol. Geotherm. Res., v. 33, p. 223-230.

Van Wyk de Vries, B., and Borgia, A., 1996, The role of basement in volcano deformation, in McGuire, W.J., Jones, A.P., and Neuberg, J. (eds.), Volcano Instability on the Earth and Other Planets: Geological Society Special Publication 110, London, p. 95-110.

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: Wilfried Strauch and Pedro Perez, Department of Geophysics, Instituto Nicaraguense de Estudios Territoriales (INETER), P.O. Box 1761, Managua, Nicaragua; Benjamin van Wyk de Vries, Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, United Kingdom; Reuters.

On 25 June, unusually large pyroclastic flows swept down drainages on the volcano's NNE side reaching almost as far as the airport. Settlements along their path sustained serious damage. Amid rescue efforts on 27 June, MVO reported at least nine people dead, six injured, and 14 missing. Additional information will be provided in next month's Bulletin.

The following summarizes weekly Scientific Reports of the Montserrat Volcano Observatory for the period 11 May-7 June 1997 and NOAA/NESDIS satellite observations during 12 May-6 June. Many of the places mentioned in this report appear on available maps (e.g. BGVN 22:02; Williams, 1997).

A new risk map was released on 6 June (figure 22). Zone A was expanded from the crater to the N as far as Harris, Bramble, and Bethel villages. Areas designated as Zone B included Tuitt's and Spanish Point on the E and Streatham and Farrell's on the W. Bramble Airport, ~5 km NE of the volcano, was moved into zone C.

Figure 22. Map showing the volcanic risk zones for Soufriere Hills Volcano, updated on 6 June 1997.

At the beginning of this reporting period, dome growth (estimated at 2 m³/s) was concentrated on the crater's S side, above the Galway's area. Rockfalls and a few small pyroclastic flows were shed into both the White River and down the S side of the Tar River valley. After 12 May loud roaring sounds caused by vigorous venting of ash and gas from the dome were heard at Whites, Harris, and Farrell's. These were taken to indicate increased gas pressures within the dome. Furthermore, on 12 May an airplane pilot reported ash between 1,830 and 2,440 m altitude.

On 13 May at 0755 a moderate-size pyroclastic flow from the summit region eroded a narrow channel on the E flank of the dome, a spot underlain by the ancestral Castle Peak. The flow went down the Tar River valley, splitting into two branches that traveled down either side of the upper break in slope, and eventually reached the delta at the coast. The ash cloud from this flow reached 3.3 km altitude and later formed a plume conspicuous in visible satellite imagery for 220 km WNW of the summit.

On 14 and 15 May, small, nearly continuous rockfalls and some small pyroclastic flows occurred on the NE and SE flanks of the dome; these traveled either towards the E (Tar River valley) or S (down Galway's side). Beginning at about 2040 on 15 May a 70-minute-long outburst generated moderate-size pyroclastic flows down the E side, creating a small scar ~40 m N of the one formed on 13 May.

Events on 16 May included small-to-moderate pyroclastic flows from the dome's summit. These traveled down the dome's N and NW sides, towards Farrell's wall, which deflected them E toward the Tar River valley. In addition erosion occurred on the dome's N face: talus continued piling up against the N and NNE crater rim.

During the following days activity was concentrated on the N and E flanks of the dome, with three major rockfall chutes developed on the dome's E, NE, and N sides. At the base of one of these chutes rockfall material piled up against the crater's N wall (Farrell's). Several small rockfalls were also heard on the crater's S side (Galway's wall), where new, relatively fine-grained rockfall deposits had blanketed the entire talus slopes.

On 18 May at 0820 the largest pyroclastic flow of this reporting period occurred. First, a large long-period earthquake took place; observers at Whites reported that the entire dome was being shaken just before the flow started. This pyroclastic flow traveled from the summit down both the 15 May chute and the NE chute. Then it passed down the N side of the Tar River valley to stop a few hundred meters from the delta. During that night intense glows were observed over the dome's entire NW face. There were also some incandescent rockfalls and small pyroclastic flows.

Clear visibility on 19 May revealed a new dark extrusion at the top of the NW dome. This area discharged ash and steam and ejected 5- to 20-cm diameter fragments up to ~60 m above the dome. The source of continuous rockfalls with small pyroclastic flows extended from the 15 May chute on the E to the margin of the September 1996 scar on the NW. The remnant wall of the scar prevented material from reaching the rim of Gages wall. The N side of the crater had filled up, with small amounts of dome material falling into the top of Tuitt's Ghaut, the N- flank drainage.

On 21 and 22 May a few small spines toppled, sending rockfalls down the E, NE, and NW flanks. On 22 May at 1300, after a rockfall on the N flank, some blocks reached ~100 m down the N flank (Tuitt's Ghaut). At 1430 a pyroclastic flow on the same flank produced an almost continuous ash plume; lapilli up to 4 mm in diameter were collected at Dyers and ash fragments ~1 mm in size were reported at Farrell's and from Salem up to St. John's.

Observations on 23 May from Chances Peak revealed several small spines and large blocks in the summit area with vigorous venting and gas emissions in the growth area; there was also a cleft in the middle separating the S lobe from the new extrusion in the N.

On 27 May, a large pyroclastic flow generated high on the E side of the dome traveled down the Tar River valley at a speed of 230 km/h , the fastest flow yet documented during the eruption. In the lower part of the valley the flow slowed considerably, and it stopped ~150 m from the sea. That same day, for the first time, moderate-size pyroclastic flows reached Tuitt's Ghaut; later on 29 May discernible material was deposited 400 m down this drainage.

By 31 May, talus slopes over the dome's E and NE flanks had covered the chutes formed by mid-May pyroclastic flows. The upper part of the dome's E face looked more blocky and relatively inactive. When visibility was good, the presence of ash below ~1,600 m was reported almost daily in satellite imagery.

Small pyroclastic flows down Tuitt's Ghaut on 2 June left fresh deposits ~1 km from the crater rim. By 3 June they reached 1.4 km, and by 4 June, 1.8 km. At 1207 on 5 June a pyroclastic flow extended ~2.9 km from the crater rim; a shorter flow followed to ~2 km. All of these pyroclastic flows were confined to the narrow valley and comparatively slow moving, taking about three minutes to descend it. In the first 500 m of the upland portion of the valley all vegetation was stripped from the valley walls. Farther down, some trees were left standing within the deposits. In the upper 1 km of the deposits there was evidence of several small, lobate flows. In general the thermal effects remained confined to 10 m from the deposit's edge, but on bends it rode up the banks of the ghaut (the so- called "bobsled" effect). The front of the flow was marked by a pile of burned logs and coarse debris, and a finer-grained surge had traveled ~100-200 m farther down the ghaut. A pyroclastic flow at 1845 on 6 June traveled ~2 km down from the crater rim; its front carried particularly large boulders. The flow significantly widened the notch in the crater wall through which it traveled; by this time the domes talus created a smooth slope down the ghaut.

NOAA reported ash clouds on 3, 4, and 5 June in visible satellite imagery up to 2,150 m altitude and crossing over the Virgin Islands, 400 km NW.

Seismicity. The shifting focus of dome growth and rising vigor of emission were reflected in a general decline in the number of long-period earthquakes and an increase in the number of hybrid earthquake swarms. Each swarm lasted for a few hours; some intense swarms during 19-21 May reached up to 35 events/hour. Rockfalls remained common and were concentrated during periods of minor dome collapse. The ratio of maximum rockfall amplitudes measured at Galway's Estate Station and Long Ground station served to differentiate between Tar River and White River pyroclastic flows.

Toward the end of May there was a significant reduction in the number of hybrid and long-period earthquakes, and rockfalls. The hybrid earthquake swarms continued until 27 May; although less frequent, they lasted longer.

The number of long-period earthquakes dropped to the normal background (0-4 events/day), the lowest levels since mid-March. The number of rockfalls increased from 1 June, and for the rest of the period were concentrated on the N and E sides of the dome. Periods of enhanced rockfall and pyroclastic-flow activity occurred every 16-20 hours and lasted ~4 hours. In the lulls, rockfalls continued at greatly reduced levels.

After 4 June the number of both long-period and hybrid earthquakes increased again. Over 50% of these shocks triggered rockfalls.

Ground deformation. GPS measurements at station FT3 (Farrell's wall) on 12 May showed continued movement to the NW, consistent with the total 20 cm of displacement noted since January 1997. Data were somewhat equivocal on 17 and 21 May. A GPS occupation at Chances Peak on 23 May suggested that it had moved 3.5 cm WNW since 28 April. Prior to that date, the movement was toward the NW. The change in direction was thought to reflect the dome's northward shift in activity.

Telemetered electronic tiltmeters installed at Chances Peak on 18 and 21 May (stations CP2 and CP3, W and E of the summit, respectively) registered cycles of inflation and deflation, each lasting ~12-18 hours. Progressive intervals and magnitudes of inflation were greater than those of deflation. Inflation occurred with hybrid earthquake swarms, and deflation correlated with peak rockfall/pyroclastic-flow events. RSAM patterns showed a strong correlation with tilt, with the higher spikes reflecting rockfalls, and the lower intensity patterns reflecting the sum of hybrid events and lesser rockfall activity. Thus tilt and RSAM combined provided a predictive capability. Accordingly, when it was possible, missions to close-in areas were scheduled during early inflation, when the likelihood of pyroclastic flows was considered minimal.

Crack 2, which developed into a zone of broad fracture on Chances Peak, was measured on 23 May, and on 4 and 8 June. The shear along the crack was dextral (E block moving S relative to W block) and reached 6 cm. The shear during 23 May-4 June was 2.5 cm. On 23 May a telemetered extensometer installed across part of Crack 2 that day showed almost 5 mm of diurnal change.

Dome volume, COSPEC, and other measurements. Using a combination of theodolite, GPS, and ranging binoculars, scientists on 19 May estimated the summit at 991 m elevation. One major change since the previous survey (15 April) was the inflation of the highest part of the dome above Galway's wall. Another change was the growth of the new extrusion in the N summit area and the talus accumulation in a 300-m-wide zone against the back of Farrell's wall, due to the activity on the N and NE faces. The volume of the dome from this survey was estimated at 60.1 x 10⁶ m³; this established an average extrusion rate during 15 April-19 May of 2.7 x 10⁵ m³/day (3.1 m³/s).

Later dome-volume surveys were severely hampered by poor visibility; however, brief clear windows allowed photos to be taken for both 31 May and 1 June, documenting continued growth of the dome's N side and summit. On the basis of these photos, the dome's volume was 64.6 x 10⁶ m³, a mean growth rate of 3.5 m³/s during 19 May-1 June. As with the last survey this represented a rate considerably above the mean extrusion rate of 2.1 m³/s.

Mini-COSPEC runs that were completed daily, often both in the morning and afternoon, gave results substantially higher than the usual background flux of 200-300 t/d. May and June SO₂ fluxes were as follows: 24 May, 950 metric tons per day (t/d); 26 May, 940 t/d; 27 May, 971 t/d; 28 May, 616 t/d; 29 May, 770 t/d; 30 May, 510 t/d; 2 and 3 June, 475 t/d; 4 June, 2,129 t/d; 5 June, 2,242 t/d; 6 June, 642 t/d; and 7 June, 505 t/d. The high values on 4-5 June correlated with increased pyroclastic flow activity during 4-6 June. Sulfur diffusion tubes collected on 20 April and 4 May mainly showed values similar to those of previous weeks (table 19). The results from Upper Amersham on 17 May presumably increased because of the increase in the level of eruptive activity.

Table 19. SO₂ concentrations in part per billion (ppb) from diffusion tubes at sites around the volcano. Recommended action level is 100 ppb. Courtesy of MVO.

Rainwater collected W and NW of the volcano on 17 May was more acidic than samples from the previous week and chlorides and sulfates were present at substantially higher levels (table 20). After heavy rainfall and continued winds from the S and SE, a rainwater sample collected on 28 May from Lawyers, 2 km north of Salem, had a pH of 3.3. On those same days, new sites to the N of the volcano were also monitored and showed very low pH values. During this period the fluoride content of the rainwater was also elevated. The pH and fluoride returned to normal values when the wind direction changed to WNW at the end of May. Piped ground water had remained unaffected by the low pH of the rainwater.

Table 20. Rainwater geochemistry from 17 May to 1 June. For comparison, WHO guideline values are as follows: pH, 6.5- 8.5; TDS, 1.0 g/l; fluorides, 1.5 mg/l; chlorides, 250 mg/l; sulfates, 250 mg/l. Courtesy of MVO.

Ash was collected on 17 May following several days of increased volcanic activity. The ash was at least 6 mm thick at Upper Amersham, and 4 mm at Lower Amersham, the Plymouth Police Headquarters, and Dagenham. Ash collected on 1 June was noticeably fine and widely distributed from Brodericks to Dyers with the thickest ash fall (2 mm) at Upper Amersham, Dagenham, and Plymouth Police HQ.

Reference. Williams, A.R., 1997, Montserrat, under the Volcano: National Geographic, v. 192, no. 1 (July 1997), p. 58-77.

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

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.mvo.ms/); NOAA/NESDIS Satellite Analysis Branch (SAB), Room 401, 5200 Auth Road, Camp Spring, MD 20746, USA.

Tens of commercial jet aircraft, which are not designed to fly through particulate and corrosive gases, have suffered damage from inadvertently encountering ash clouds that had drifted tens to hundreds of kilometers from erupting volcanoes; in one case, a plane descended more than 6 km before the engines could be restarted (Casadevall, 1994). As a result of this vulnerability, there have been new and evolving strategies for alerting aviators as to the presence, location, and movement of eruption plumes. Conversely, pilots often see aspects of volcanism that merit preservation in the Bulletin. In order to solicit and register these observations, a form for pilots relates a series of key questions (plate 1, back page).

Plate 1. A form developed to help pilots record and submit their observations related to volcanism. Courtesy of Ed Miller, ALPA.

The form, called the "Volcanic Activity Reporting Form," is now included in the US Aeronautical Information Manual (FAA, 1995), a reference used by all large US carriers. A similar form is in use by members of the International Civil Aviation Organization (ICAO).

The form is divided into two parts. The critical upper part (numbers 1-8) gets radioed to air traffic control immediately. Most of the form's lower part provides stated choices on topics such as ash density and color, continuousness of the eruption, as well as the effects on the aircraft and atmosphere (numbers 9-15). The last block (number 16) allows pilots to provide further written information.

The forms are ultimately to be sent (via mail or fax) to GVN for archiving. Expenses for postage or connections by fax can be reimbursed by the GVN.

In addition to the form itself, we wish to receive other aviation observations. These may include eyewitness accounts or photos made by passengers or crew, descriptions of damage, or ash collected by mechanics, as well as relevant weather details from meteorologists. These can (and already do) complement volcanological and atmospheric studies of eruptive activity. Ideally, such multiple perspectives can build a much more comprehensive picture of volcanic processes than can result from any one vantage point.

Every day thousands of people fly across potentially ash-contaminated airspace--to some degree, the people in these planes are just as vulnerable as villages perched on a volcano's flanks. Conventional planes still lack on- board instruments to warn pilots if hazardous atmospheric ash lies ahead. Such plumes are relatively rare, but to consistently avoid them requires interdisciplinary communication and cooperation between both aviators and scientists.

References. Casadevall, T.J. (ed.), 1994, Volcanic ash and aviation safety, Proceeding of the First International Symposium on Volcanic Ash and Aviation Safety (Seattle, Washington, July 1991): U.S. Geological Survey Bulletin 2047, 450 p.

Federal Aviation Administration, 1995, Volcanic Activity Reporting Form: US Aeronautical Information Manual (AIM), 1995 (June), Appendix 2 (1 May 1997), p. A2-1, Superintendent of Documents, US Government Printing Office, Washington, D.C.

Further Reference. Casadevall, T.J., and Thompson, T.B., 1995, Volcanoes and principal aeronautical features, Geophysical Investigation Map GP-1011: U.S. Geological Survey, prepared in cooperation with Jeppesen Sanderson, Inc.

Geologic Background. Special announcements or information of general interest not linked to any specific volcano.

Information Contacts: Captain Ed Miller (Retired), Air Line Pilots Association, 535 Herndon Parkway, P.O. Box 1169, Herndon, VA 20172-1169 USA; Tom Fox, Air Navigation Bureau, International Civil Aviation Organization (ICAO), 999 University St., Montreal H3C 5H7, Canada.

A 7-hour visit on 23 May led to the construction of a crater terrace map (figure 55). The Crater 2 pit mapped in 1994 and 1995 had filled in and was occupied by an inactive lava flow fed from a small truncated cone. From information by Jürg Alean (BGVN 22:03) it was inferred that this flow was emplaced between 25 April and 19 September 1996. A new subsidence bowl was forming to the SE of the former Crater 2 pit. This was occupied by a pit crater and three vents (2/1, 2/2, and 2/3), none of which exhibited explosive activity. Vent 2/1, ~4 m in diameter, was the source of regular (~1/s) gas puffs and occasional gas release, and also showed night incandescence.

Figure 55. Sketch map of Stromboli's crater terrace drawn on 23 May 1997 and fitted to the map from the September 1995 EDM survey (BGVN 20:11/12). Label A indicates the pit in Crater 2 mapped in both 1994 (BGVN 19:10) and 1995. Label B designates a new subsidence bowl. Label C indicates the location of a vent and small lava flow erupted sometime between 25 April and 19 September 1996. Label D indicates the spot where hornitos existed in 1994 and 1995. Courtesy of Andy Harris.

Activity during the 23 May visit was at lower levels than seen in either October 1994 (BGVN 19:10) or September 1995 (BGVN 20:11/12). However, excellent viewing conditions revealed that major changes had occurred since the 1994 and 1995 surveys (BGVN 22:03).

During the intervals 1100-1400 and 1900-2200, no eruptions were observed from Crater 3. However, at about1400 on 24 May, an explosion from Crater 3 fed a brown ash cloud that rose ~200 m above the crater rim. This was observed from the sea.

Night-time temperature measurements obtained from Pizzo Sopra la Fossa using a Minolta/Land 152 infrared thermometer (corrected for an emissivity of 0.956) gave 2/1 vent temperatures of 670-699°C. Vents 2/2 and 2/3 (~3 x 1 m and <1 m wide, respectively) were actively degassing without incandescence. The site of hornitos in 1994 and 1995 was occupied by a pit, with a wall on the NE side.

Crater 1 was occupied by two active vents (1/1 and 1/2). Between 1055 and 1155 on 23 May eight explosions occurred from these two vents. The first two explosions sent bombs 100-200 m above the crater rim with ~20% of the ejecta landing on the upper Sciara. The following six explosions sent ejecta up to 50 m above the vent.

For the next two hours, Crater 1 exploded ~1-2 times/hour, but additional sloshing and gas release sounds were occasionally audible from Pizzo Sopra la Fossa. By the evening of 23 May, Crater 1 activity had escalated to levels similar to the 1055-1155 period. As darkness fell, an intense pulsating glow visible over Crater 1 could have been due to a small, active lava pond on the crater floor. This may have accounted for the sloshing sounds heard earlier in the day.

Stromboli, a small island N of Sicily, has been in almost continuous eruption for over 2,000 years. Its small Strombolian explosions, which hurl incandescent scoriae above the crater rim, occur several times a day, but larger eruptions are less frequent.

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

Information Contacts: Andy Harris, HIGP/SOEST, University of Hawaii, 2525 Correa Road, Honolulu, HI 96822 USA.

Seismicity as of mid-May remained at a high level, similar to recent months. There have been ~160 daily volcano-seismic events detected, with little variation. During March there were ~150 seismic signals/day recorded; in December 1996 there were <100 signals/day (BGVN 22:03).

An eruption on 31 July 1994 produced a gas-and-ash column and detectable ash fell as far as 17 km from the summit (BGVN 19:07). Phreatic explosions continued until 12 August 1994 when seismicity began decreasing (BGVN 19:09).

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

Information Contacts: Wilfried Strauch, Department of Geophysics, Instituto Nicaraguense de Estudios Territoriales (INETER), P.O. Box 1761, Managua, Nicaragua.

Fumaroles emitted comparatively little gas but remained active in the main crater's NE, N, NW, and W parts, with temperatures in the range 89-90°C. In the N and S parts of the crater, small areas of mass-wasting covered some fumaroles. Seismicity at a station 500 m E of the active crater (station VTU) has been measured consistently since May 1996; reported local earthquake counts included 72 in December 1996, 146 in January 1997, 194 in February, 182 in March, and 137 in April. During May, seismic station VTU registered a total of 72 earthquakes. On 10-11 May, four of these were located at 5-6 km depths at 8-9 km distances NE of the crater, with magnitudes of 2.1-2.6. Their origin was possibly related to a local fault.

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

Information Contacts: E. Fernandez, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, W. Jimenez, R. Saenz, E. Duarte, M. Martinez, E. Hernandez, and F. Vega, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.