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/).

Jenifer Piatt, a meteorologist with the Air Force Weather Agency in the Satellite Applications Branch, notified Bulletin staff on 17 June 2005 that haze had appeared near Ambrym on MODIS imagery over the past few days. Over the past several months, this volcano had been emitting SO₂ and sometimes light ash. She informed us of several recent news articles that addressed this event and provided several satellite images (figure 14).

Figure 14. Images for 0250 UTC 15 June 2005 (top), and 0240 UTC 17 June 2005 (bottom) disclosing the area around Vanuatu including Ambryn. The images came from NASA's AQUA MODIS satellite with a resolution of 500 m. SO2 plumes from Ambrym are labeled. NASA image courtesy of USAF Weather Agency.

Tony Ligo wrote on 1 June 2005 in the Port Villa Presse that acid rain continued to fall in W Ambrym Island in Vanuatu, even after ash from the volcano had stopped falling. This prompted the provincial secretary general to discuss the need for new water sources. The Vanuatu government, through the department of Rural Water Supply, agreed to provide a drilling rig to the Malampa provincial government to drill on W Ambrym as soon as possible.

The government also recognized the value of scientific and technical data; in order to effectively respond to such environmental problems the government needs to get more young people studying in this area. The article noted that Vanuatu only has one volcanologist, Charley Douglas, with enough background to give accurate data on current activity.

Aid and food have been sent to affected areas on the western coast of the island, and a contingency evacuation plan is required for resettling people should this be necessary in the future. Health issues have been raised regarding hygiene, respiratory problems, asthma, and malnutrition over the past couple of months. Of great concern are health problems particular to children, including exposure to excess fluoride and the consequent risk of bone disease.

The National Aeronautics and Space Administration (NASA) Earth Observatory web site reported that Ambrym volcano was the strongest point source of SO₂ on the planet for the first months of 2005; it had been steadily emitting SO₂ for at least 6 months, and satellite images produced using data collected by the Ozone Monitoring Instrument (OMI) on NASA's Aura satellite during the first 10 days of March 2005 show high concentrations of SO₂ drifting NW.

The web site article noted that "Ambrym is not erupting in the traditional sense with thick ash plumes and explosive bursts of lava, rather it is leaking SO₂ gas from active lava lakes in what scientists call 'passive' or 'non-eruptive' emissions. Despite these gentle names, the volcano still threatens the local population. SO₂ has a strong smell and can irritate the eyes and nose and make breathing difficult. Higher in the atmosphere, SO₂ combines with water to create rain laced with sulfuric acid. On Ambrym, acid rain has destroyed staple crops and contaminated the water supply, leaving communities in need of food aid." In the past, satellites have been able to monitor SO₂ emissions only from large eruptions or the most powerful passive degassing. All other SO₂ emissions remain at low altitudes and have low SO₂ concentrations that were hard to see from space.

On 15 July 2004, NASA launched its Aura satellite carrying the OMI, which is part of a collaboration between the Netherlands' Agency for Aerospace Programs, the Finnish Meteorological Institute, and NASA. With greater spatial resolution (the ability to "zoom-in" to see greater detail) and higher sensitivity to SO₂ than any previous space-borne sensor, OMI allows scientists to study passive volcanic degassing on a daily basis for the first time.

The image in figure 15 is an example of the instrument's preliminary, uncalibrated, and unvalidated data. This new view of passive volcanic emissions could lead to significant advances in understanding both volcanic eruptions and the impact of SO₂ on climate. Changes in passive emissions can be a precursor to explosive eruptions, and thus provide a warning signal that activity may be changing.

Figure 15. A zone of elevated atmospheric SO2 from Ambrym during the interval 1-10 March 2005. The units on the scale bar reflect SO2 in terms of Dobson Units (DU). (A Dobson Unit represents the physical thickness of the SO2 gas if a 1 cm2 column of the atmosphere were brought to 0EC and 1 atmosphere pressure. A value of 300 Dobson Units equals three millimeters.) To process the OMI spectrometer data, two different pairs of measured UV wavelengths are averaged. The mean of pairs 1 and 2 is written as "P1-P2 mean" on the scale bar. Courtesy of Simon Carn.

Geologic Background. Ambrym, a large basaltic volcano with a 12-km-wide caldera, is one of the most active volcanoes of the New Hebrides Arc. A thick, almost exclusively pyroclastic sequence, initially dacitic then basaltic, overlies lava flows of a pre-caldera shield volcano. The caldera was formed during a major Plinian eruption with dacitic pyroclastic flows about 1,900 years ago. Post-caldera eruptions, primarily from Marum and Benbow cones, have partially filled the caldera floor and produced lava flows that ponded on the floor or overflowed through gaps in the caldera rim. Post-caldera eruptions have also formed a series of scoria cones and maars along a fissure system oriented ENE-WSW. Eruptions have apparently occurred almost yearly during historical time from cones within the caldera or from flank vents. However, from 1850 to 1950, reporting was mostly limited to extra-caldera eruptions that would have affected local populations.

Information Contacts: Jenifer E. Piatt, HQ Air Force Weather Agency Satellite Applications Branch; Simon Carn, TOMS Volcanic Emissions Group, University of Maryland, 1000 Hilltop Circle, Baltimore, MD 21250, USA (URL: https://so2.gsfc.nasa.gov/); NASA Earth Observatory Natural Hazards web page (URL: http://earthobservatory.nasa.gov/NaturalHazards/).

Members of the Indian Coast Guard observed a new eruption on the morning of 28 May 2005. An ash plume originated from a vent on the W side of the summit of the central cone; fresh black lava flows did not reach the sea (figure 10). The eruption continued through at least 6 June. Fresh lava emissions had been noted by Indian Coast Guard personnel who patrol the area regularly. A large amount of steam was emitted due to heavy rainfall onto the hot lava surfaces. Heavy monsoon rains prevented access to the island. However, the Geological Survey of India (GSI) was planning a monitoring program and field expedition to the island.

Figure 10. Photograph of Barren Island erupting on 28 May 2005 taken from a helicopter. The black lava in the foreground is of 1994-95 eruption. A lava flow that did not reach the sea issues from a steaming flank vent. View is towards the ESE. Courtesy of the Indian Coast Guard.

Dornadula Chandrasekharam (Indian Institute of Technology) noted on 6 July that by that date the eruption had ceased, with only steam emissions continuing after three weeks of heavy monsoon rains. The Indian Coast Guard also confirmed to Chandrasekharam that the eruption was first noticed on 28 May, contrary to some press reports indicating that activity was seen on the 27th. Patrol helicopters saw no activity on 25 and 26 May, and did not observe the island on the 27th.

Press reports. A report in the 31 May edition of The Hindu stated that defense forces witnessed intermittent billowing smoke and "flame" from the volcano. The same article referenced a Press Trust of India (PTI) report that military forces that landed on the island "experienced a hot breeze and found themselves stepping on fresh lava" where earlier patrol teams had been able to reach the crater. Another article from The Hindu reported that on 2 June teams of the Indian Coast Guard vessel CG Sagar landed on the island in an inflatable raft while a helicopter hovered overhead. The report described eruptive activity consisting of lava and "fireballs" from the crater every few seconds. The purpose of the expedition was to "collect samples of the lava flowing into the rough sea" that would be given to scientists. Coast Guard members and various other government officials made an aerial survey of the island on 3 June according to a PTI report published in The Hindu the next day. The Lt. Governor of Andaman, Ram Kapse, saw "smoke and lava rising from the crater." Coast Guard sources stated that the volume of "smoke" had increased and lava was still flowing out of the crater.

A report in The Daily Telegrams on 17 February 2005 quoted K.N. Mathur, Director General of the GSI, regarding a scientific visit to Barren Island on 16 February. At that time, Mathur noted, the team observed "no serious volcanic activities on the island." A similar report in the 18 February edition of the Trinity Mirror carried a quote from Mathur that "There is no activity in the crater and it remained as it was found during GSI's last visit in 2003." These media reports were reproduced on the GSI website.

Geologic Background. Barren Island, a possession of India in the Andaman Sea about 135 km NE of Port Blair in the Andaman Islands, is the only historically active volcano along the N-S volcanic arc extending between Sumatra and Burma (Myanmar). It is the emergent summit of a volcano that rises from a depth of about 2250 m. The small, uninhabited 3-km-wide island contains a roughly 2-km-wide caldera with walls 250-350 m high. The caldera, which is open to the sea on the west, was created during a major explosive eruption in the late Pleistocene that produced pyroclastic-flow and -surge deposits. Historical eruptions have changed the morphology of the pyroclastic cone in the center of the caldera, and lava flows that fill much of the caldera floor have reached the sea along the western coast.

Information Contacts: Dornadula Chandrasekharam, Department of Earth Sciences, Centre of Studies in Resources Engineering, Indian Institute of Technology, Bombay 400076, India (URL: http://www.geos.iitb.ac.in/index.php/dc); Geological Survey of India, 27 Jawaharlal Nehru road, Kolkata 700016, India (URL: https://www.gsi.gov.in/); The Daily Telegrams, India; Trinity Mirror, Chennai, India; The Hindu, 859 and 860 Anna Salai, Chennai 600002, Tamil Nadu, India (URL: http://www.hinduonnet.com/); Press Trust of India, PTI Building, 4, Parliament Street, New Delhi 110001, India (URL: http://www.ptinews.com/); Indian Coast Guard, National Stadium Complex, New Delhi 110 001, India.

Table 2 below tabulates the seismic activity by date of the volcano prior to and subsequent to its eruption on 6 February 2005, but little was reported concerning that event. The volcano erupted again on 7 February. That eruption was accompanied by a strong smell of SO2 or H₂S in the villages of Hebing and Hale and apparently rendered a villager unconscious.

Table 2. A summary of counts for different earthquake types (type B volcanic, type A volcanic, emission, low frequency, and tectonic), tremor, amplitude, and Alert Level at Egon volcano. Unreported data indicated by "--". Courtesy of the Directorate of Volcanology and Geological Hazard Mitigation (DVGHM) DVGHM.

On 8 February 2005 a fissure about 1 km long appeared along the southern slope. Vegetation along the fissure's margins had died, indicating that a gas blow out had occurred there. On 14 February 2005 at 1830 another explosion occurred. It was accompanied by significant seismic activity (see table 2). This latest eruption ejected ash and glowing material as high as 50 m above the summit. Volcanic earthquakes were frequent.

Distances increased for electronic distance measurements (EDM) during April, July, and October 2004 and during February 2005 (the last four measurements). During 25-27 February 2005 ash plumes rose to 50 m high. Volcano status remained at alert level 4 (the highest hazard status).

Geologic Background. Gunung Egon, also known as Namang, sits within the narrow section of eastern Flores Island. The barren, sparsely vegetated summit region has a 350-m-wide, 200-m-deep crater that sometimes contains a lake. Other small crater lakes occur on the flanks. A lava dome forms the southern summit. Solfataric activity occurs on the crater wall and rim and on the upper S flank. Reports of eruptive activity prior to explosive eruptions beginning in 2004 are unconfirmed. Emissions were often observed above the summit during 1888-1892. Strong emissions in 1907 reported by Sapper (1917) was considered by the Catalog of Active Volcanoes of the World (Neumann van Padang, 1951) to be an historical eruption, but Kemmerling (1929) noted that this was likely confused with an eruption on the same date and time from Lewotobi Lakilaki.

Information Contacts: Dali Ahmad, Hetty Triastuty, Nia Haerani, and Sri Kisyati, Directorate of Volcanology and Geological Hazard Mitigation (DVGHM), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

Ongoing seismicity continued at Karangetang during January-February 2005. Lava avalanches were noted on 3 January and during the week of 17-23 January. The volcano was last discussed in a report on thermal alerts and a pilot's report of an ash plume to 7.5 km altitude (BGVN 29:03, which updated through May 2004). Table 11 presents a summary of the reported seismic and other data during January and February 2005.

Table 11. A summary of observations made at Karangatang during 3 January-February 2005. Courtesy of DVGHM.

Geologic Background. Karangetang (Api Siau) volcano lies at the northern end of the island of Siau, about 125 km NNE of the NE-most point of Sulawesi island. The stratovolcano contains five summit craters along a N-S line. It is one of Indonesia's most active volcanoes, with more than 40 eruptions recorded since 1675 and many additional small eruptions that were not documented in the historical record (Catalog of Active Volcanoes of the World: Neumann van Padang, 1951). Twentieth-century eruptions have included frequent explosive activity sometimes accompanied by pyroclastic flows and lahars. Lava dome growth has occurred in the summit craters; collapse of lava flow fronts have produced pyroclastic flows.

Information Contacts: Directorate of Volcanology and Geological Hazard Mitigation (DVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

Langila was last reported on in BGVN 29:06, as part of a MODIS data summary, although the last prominent event there was on 18 January 2003, when a large explosion produced a thick dark ash column that penetrated the weather clouds over the summit area (BGVN 28:03).

A plume from Langila was visible on satellite imagery on 17 December 2004 according to the Darwin VAAC. The plume reached an unknown height and extended NW.

Between 28 April 2005 and 4 May 2005 the Rabaul Volcano Observatory (RVO) received reports of activity at Langila characterized by forceful emissions of thick white to gray ash-laden clouds rising ~ 700-800 m above the summit crater. Occasional continuous rumbling and explosive noises were heard and incandescence was visible at night. During early May, incandescent lava fragments were ejected. Activity increased at about 1300 on 4 May 2005, when white-to-gray ash emissions changed to dark ash clouds. Explosions became frequent, with incandescent lava fragments ejected again, and very bright glow was visible during the night. Around 1200 on 5 May 2005 the color of the ash emissions changed from dark gray to white-to-gray. A lava flow was produced but no further detail is available. Based on information from RVO, the Darwin VAAC reported that ash emissions from Langila rose to ~ 2.1 km altitude on 3 May. A very small plume and a hot spot were visible on satellite imagery. Ash clouds from the eruption were blown generally NW towards Kilenge ~ 100 km away, where light to moderate ashfall was reported.

According to the Darwin VAAC, low-level ash plumes emitted from Langila were visible on satellite imagery during 8-13 June 2005. RVO reported to the Darwin VAAC that moderate eruptive activity was expected to continue.

The International Federation of Red Cross and Red Crescent Societies (IFRC) reported that eruptive activity occurred at Langila on 2 June with more ash than normal being emitted from the volcano. Prevailing winds carried most of the initial ashfall to the sea, but lower-level winds redirected the ash back onto the island. About 10,000 people live near the volcano, and there were reports of increased cases of respiratory problems and eye irritation. During an aerial inspection of the area on 6 June 2005, IFRC determined that ~ 3,490 people had been affected by the eruption, mainly in the villages of Aitavala, Masele, Kilenge, Ongaea, Potne, and Sumel, but also to a lesser extent in Vem, Galegale, Tauale, and Laut. Ashfall damaged small food gardens and contaminated some water sources. The provincial government encouraged voluntary evacuation of affected areas.

During 16-17 June 2005, ash plumes from Langila were visible on satellite imagery (figure 4). The heights of the plumes were not reported.

Figure 4. On 21 June 2005 the Moderate Resolution Imaging Spectroradiometer (MODIS), flying on NASA's Aqua satellite, captured this image of Langila, Ulawun, and Rabaul. At the time MODIS captured this image, Langila showed the biggest plume of volcanic ash, followed by Ulawun. In all cases, winds pushed the ash clouds NW over the ocean. NASA image courtesy Jesse Allen, based on data from the MODIS Rapid Response Team at NASA.

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: Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, Northern Territory 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea; International Federation of Red Cross And Red Crescent Societies (IFRC), Langila Volcano Information Bulletin No. 1 (URL: https://reliefweb.int/).

The 4 May 2005 early morning eruption of Lascar was described in BGVN 30:04. Note that the time conversion in that issue was in error by 1 hour. The following information is based on a report prepared for Bulletin staff by Jose Viramonte of the Universidad Nacional de Salta, and Lizzette Rodriguez of Michigan Technological University.

Viramonte and Rodriguez estimated that the 4 May 2005 eruption column rose to a height of ~ 10-11 km, based on numerical models of temperature and wind measurements from the Servicio Metereológico Nacional, Argentina at different altitudes at the time of the eruption. The column traveled rapidly to the SE under the influence of the strong tropospheric winds with predominant direction from the NW to the SE.

Residents of the towns of Talabre (located 15 km W of the volcano) and Jama (located 60 km ENE of the volcano) did not report earthquakes or explosions. The Instituto GEONORTE of the Universidad Nacional de Salta reported very fine ashfall at 0545 in the city of Salta, located ~ 285 km SSE of the volcano. Ash sample collection, carried out by GEONORTE personnel for 2.5 hours, measured a rate of 0.4 g/ (m² h). Grain size analyses of the ash showed a strong mode at diameters of 4-8 phi (0.062-0.003 mm) (figure 29); the ash was composed predominantly of andesitic lithic fragments and broken crystals of two pyroxenes (hyperstene and augite) and plagioclase, with very scarce glass shards.

Figure 29. Histogram of the grain size of ash deposited at the city of Salta by the 4 May 2005 Lascar eruption. Courtesy of Jose Viramonte and Lizzette Rodriguez.

The Buenos Aires VAAC and the Comisión Nacional de Actividades Espaciales (CONAE) processed different bands from MODIS data: b29-b32 for SO₂, b31-b32 for ash, and b30-b32 for SO₄. The first two band combinations showed the Lascar plume in coincidence with the b5-b4 band combination from NOAA-17 (figure 30).

Figure 30. NOAA-17 image of a SE-directed plume from Lascar at 1440 UTC (1040 local time), obtained with the difference of channels 4 and 5 from the AVHRR sensor. The plume can be better identified withing the ellipse on higher resolution reproductions. Courtesy of Jose Viramonte and Lizzette Rodriguez.

The grain size and shape of the ash, its composition, and the interpretation of the satellite data, suggest that Lascar volcano had a short phreato-vulcanian eruption.

On May 25, Felipe Aguilera of the Universidad Católica del Norte, Antofagasta, Chile, climbed up to the crater of Lascar volcano (figure 31). He reported three new strong fumaroles a few meters from the S border of the crater, and sampled the sulfur sublimates (figure 32). No new bombs or blocks were seen around the crater area.

Figure 31. View of Lascar's NE crater, looking NE (see arrow, upper left) with fumaroles present along a number of fractures to the N and E sides. The active crater is just out of view in the image foreground. Picture taken by Felipe Aguilera on 25 May 2005. Courtesy of Jose Viramonte and Lizzette Rodriguez.

Figure 32. Schematic diagram showing the position of fumaroles on Lascar after the eruption on 4 May 2005. Also indicated are several new post-eruption fumaroles that developed on the S crater margin. Courtesy of Jose Viramonte and Lizzette Rodriguez.

Recent and future work. A team of scientists from Michigan Technological University, the University of Hawaii, the Universidad Nacional de Salta, the Universidad de Chile, and the Universidad Nacional de Córdoba, conducted a field campaign at Lascar from 29 November to 8 December 2004. During this period, SO₂ emissions were measured using two mini-UV spectrometers; aerosols were measured using two Microtops II sun photometers, and temperatures of the vent fumaroles were measured using a Forward Looking IR Radiometer (FLIR). Preliminary processing of the gas data showed a decrease since 2003 in the emissions, with SO₂ fluxes around 500 tons/day (Rodríguez et al., 2005). This contrasts with the fluxes determined by Mather et al. (2004) on January 2003, which were on the order of 2,300 tons/day. Observations of the SO₂ index, using ASTER TIR images, have shown a decrease in the size of the SO₂ anomaly from 2000 to the first half of 2004 (Castro Godoy and Viramonte, 2004).

Temperature measurements made at the crater on 2 December 2004 by University of Hawaii scientists using a FLIR indicated low temperatures for the fumarole field, which represented a decrease when compared with the results of direct measurements conducted in October 2002 by Franco Tassi and others (Tassi et al., 2004; BGVN 28:03). Similar observations have been made using ASTER SWIR and TIR images (Silvia Castro, GEOSAR-AR program), which have shown a decrease in the absolute temperatures and the size of the thermal anomaly since October 2002 (Castro Godoy and Viramonte, 2004). Images during the month of April 2005 showed a slight increase in the area and maximum temperature of the anomaly at the beginning of the month, followed by a decrease at the end of April, prior to the eruption. Decreases in the thermal activity have been observed in previous eruptive cycles, prior to explosive events (Oppenheimer et al., 1993; Matthews et al., 1997).

The data collected during the 2004 field campaign will help in the understanding of the pre-eruptive conditions at Lascar. SO₂ emission rates on 7 December 2004 will be used to ground truth the satellite data from an ASTER overpass at 1436 UTC (1036 local time), and recently acquired ASTER data will be used to investigate SO₂ emissions during the period close to the 4 May 2005 eruption. Scientists from Università degli studi di Firenze (Italy), Universidad Católica del Norte (Chile), and Universidad Nacional de Salta (Argentina) are conducting a systematic gas sample campaign at Lascar and other active volcanoes on the Central Volcanic Zone. Finally, scientists from the Universidad Católica del Norte and the Universidad Nacional de Salta are processing data from Landsat TM and ETM+ images, with the objective of understanding the behavior of Lascar volcano during the 1998-2004 period.

References. Castro Godoy, S. and Viramonte, J.G., 2004, Micro FTIR field measurement for volcanic mapping, SO2 and temperature monitoring using ASTER images in Lascar Volcano, southern central Andes: IAVCEI General Assembly, Book of Abstracts, Pucón, Chile, 14-20 November.

Mather, T.A., Tsanev, V.I., Pyle, D.M., McGonigle, A.J.S., Oppenheimer, C., and Allen, A.G., 2004, Characterization and evolution of tropospheric plumes from Lascar and Villarrica volcanoes, Chile: Journal of Geophysical Research, v. 109.

Matthews, S.J., Gardeweg, M.C., and Sparks, R.S.J., 1997, The 1984 to 1996 cyclic activity of Lascar volcano, northern Chile: cycles of dome growth, dome subsidence, degassing and explosive eruptions: Bulletin of Volcanology, v. 59, p.72-82.

Oppenheimer, C., Francis, P., Rothery, D., Carlton, D., and Glaze, L., 1993, Interpretation and comparison of volcanic thermal anomalies in Landsat Thematic Mapper infrared data: Volcán Lascar, Chile, 1984-1991: Journal of Geophysical Research, 98, p. 4269-4286.

Rodríguez, L.A., Watson, I.M., Viramonte, J., Hards, V., Edmonds, M., Cabrera, A., Oppenheimer, C., Rose, W.I., and Bluth, G.J.S., 2005, SO₂ conversion rates at Lascar and Soufriere Hills volcanoes: 9th Gas Workshop, Palermo, Italy, May 1-10.

Tassi, F., Viramonte, J., Vaselli, O., Poodts, M., Aguilera, F., Martínez, C., Rodríguez, L.A., and Watson, I.M., 2004, First geochemical data from fumarolic gases at Lascar volcano, Chile: 32nd International Geological Congress, Florence, August 20-28, 2004.

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

Information Contacts: Raúl Becchio and José G. Viramonte, Instituto GEONORTE and CONICET, Universidad Nacional de Salta, Buenos Aires 177, Salta 4400, Argentina (URL: http://www.unsa.edu.ar/); Lizzette A. Rodríguez and Matthew Watson, Michigan Technological University, Houghton, MI 49931, USA (URL: http://www.geo.mtu.edu/volcanoes/); Felipe Aguilera, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile (URL: http://www.ucn.cl/en/carrera/geology/); Silvia Castro Godoy, GEOSAT-AR Project, SEGEMAR, Buenos Aires, Argentina (URL: http://www.segemar.gov.ar/); Matt Patrick and Rob Wright, HIGP-University of Hawaii, Honolulu, HI 96822, USA (URL: http://www.higp.hawaii.edu/volcanology.html); Sergio Haspert and Ricardo Valenti, VAAC Buenos Aires - Div. VMSR, Servicio Meteorologico Nacional, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/productos.php).

The relative quiescence in Long Valley caldera that began in early 1999 persisted through 2004 according to the U.S. Geological Survey's weekly reports and the 2004 annual summary of the Long Valley Observatory. Those manuscripts provide the basis for this synopsis. Seismicity in the adjacent Sierra Nevada block S of the caldera gradually died away over the same period, although background levels remained somewhat higher than within the caldera.

The resurgent dome continued to undergo minor fluctuations in deformation as reflected in changes in the lengths of baselines onto the dome. Over the past 6 years, the center of the resurgent dome has sustained the roughly 75-cm uplift that accumulated during the recurring unrest from 1979 through 1999.

Seismicity within both the caldera and the Sierra Nevada block to the S remained low through 2004. The two most notable earthquake sequences within the caldera were a minor swarm at the end of January and the first few days of February in the S moat, and a M 3.0 earthquake on 20 September located at the S margin of the caldera just N of Convict Lake. The latter was the first earthquake greater than M 3.0 within the caldera since the cluster of earthquakes on 4 November 2002, events centered beneath the S moat just S of the Highway 395-203 junction. The swarm in early February 2004 was located in the same general area of the S moat, but the epicenters fell along a SW trend in contrast to the WNW trend shown by most earthquake sequences in that area.

Seismicity within the adjacent Sierra Nevada block continued to be somewhat elevated compared to that in the caldera through 2004. The Sierra Nevada activity included about seven earthquakes over M 3, the largest of which was an M 3.7 earthquake on 12 January 2004 located 2 km E of Red Slate Mountain (19 km S of the caldera and 15 km WSW of Tom's Place). Most of the activity remained concentrated in the NNE-trending aftershock zone associated with the three earthquakes over M 5 during June and July 1998 and May 1999.

The most noteworthy seismic activity in the general vicinity of Long Valley caldera during 2004 was the prolonged earthquake swarm in the Adobe Hills centered roughly 20 km E of Mono Lake and 20 km NNE of Long Valley caldera (figure 30). Its onset was marked by a M 2.3 earthquake at 0002 on 18 September, followed by M 3.2 and 4.1 earthquakes at 0007 and 0008, respectively. Activity intensified through mid-afternoon of 18 September, with M 5.5 and M 5.4 earthquakes at 1602 and 1643, respectively. These produced widely felt shaking in the area from Bridgeport to Bishop. Seismicity declined gradually through the remainder of the year and into early 2005. By the end of December 2004, this Adobe Hill swarm had produced well over 1,000 detectable earthquakes including ~ 48 over M 3 and 6 equal or over M 4.

Figure 30. All earthquake epicenters detected in the Long Valley region for 2004. Courtesy of U.S. Geological Survey, Long Valley Observatory (2005).

The mid-crustal long-period (LP) volcanic earthquakes, which began beneath the SW flank of Mammoth Mountain during the 1989 Mammoth Mountain earthquake swarm, continued through 2004 but at a much reduced rate compared with the peak in LP activity from early 1997 through mid-1998.

In early 2005 seismicity was generally minor (up to M 2.5) in and around the caldera. An M 4.2 earthquake occurred S of Long Valley caldera on 13 March 2005 at 1409. The event, which produced light shaking in Mammoth Lakes and Bishop was located in the Sierra Nevada ~ 12 miles SW of Toms Place near Grinnell Lake. It was followed by a series of 18 aftershocks, the largest which were M 2.8 and M 2.3. The last earthquake of similar magnitude in this area occurred in 1999 on 17 May. In addition to the M 4.2 main shock/aftershock sequence, two other significant earthquakes occurred in the Adobe Hills area E of Mono Lake, and a third occurred on 13 March in the Sierra Nevada S of the caldera, near Mount Baldwin. All three had magnitudes under M 2.0. From that time to mid-June 2005, seismicity was generally in the range of M 1-2, with a very few occurring to M 3.

Carbon dioxide (CO₂) concentrations measured in the Horseshoe Lake tree-kill area on the S flank of Mammoth Mountain showed no significant changes for 2004 with respect to the past several years. A survey of scattered areas of vegetation die-off and diffuse CO₂ flux on the resurgent dome completed in 2004 indicated anomalous CO₂ emissions from the kill areas were ~9 metric tons/day (compared with ~ 300 tons/day from Mammoth Mountain). The d13C-CO₂ values of the diffuse emissions were similar to values previously reported for CO₂ from hot springs and thermal wells around Long Valley, indicating a common source. The areas of elevated CO₂ flux tend to be associated with locally elevated soil temperatures. Some of the older areas near the Casa Diablo power plant are likely related to geothermal power production, but development of new areas may reflect a delayed response of the hydrothermal system to the 1997 unrest episode (including an additional 10-cm uplift of the resurgent dome accompanied by intense earthquake swarm activity in the S moat).

Thermal spring discharge in Hot Creek Gorge, which had dropped by about 20% in the last half of 2003, followed by a recovery beginning in January 2004, reached normal discharge values by June 2004. Fluid levels in key monitoring wells continued to decline, with some wells reaching their lowest values since records began in 1985.

Reference. U.S. Geological Survey—Long Valley Observatory, 2005, Long Valley Observatory Quarterly Report, October-December 2004 and Annual Summary for 2004 (URL: http://lvo.wr.usgs.gov/).

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

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

Manam erupted several times during October to December 2004 and January 2005. A strong eruption on 24 October 2004, preceded by a buildup in seismicity and a felt earthquake, was described in BGVN 29:10. This eruption generated pyroclastic flows, and its plume was imaged from space. The eruption sent ash and condensed water in the form of ice to a maximum height of ~ 15 km altitude. On 10-11 November 2004, a Strombolian eruption occurred; the ash column was estimated to have risen ~ 5-6 km above the crater. On 23-24 November 2004 Manam's main crater ejected glowing lava and discharged an ash cloud that rose ~ 10 km high. A lava flow was also reported to be heading for two villages on the island. Details and reports of eruptions in November and December 2004 were included in BGVN 29:11.

The eruption at Manam on the evening of 27 January 2005 (BGVN 30:02) was more severe than the previous ones during the current eruptive period. During 27-28 January 2005 there were 14 people injured and one person killed at Warisi village. The reports of the Rabaul Volcano Observatory (RVO) and the Darwin VAAC, and an analysis of the Manam eruption clouds by Andrew Tupper of the Darwin VAAC, were summarized in BGVN 30:02. In late January, five commercial flights were cancelled from Rabaul, East New Britain, delaying about 100 passengers.

Documented occurrence of olfactory fatigue. A report received from Andrew Tupper discussed an encounter of an aircraft with an airborne gas plume that took place about 2300 UTC on 29 January (0800 on the 30th, East Timor time) reported to him by a pilot. The encounter took place at a considerable distance from Manam, and a map is helpful to visualize the region's geography (figure 21). The incident involved entry into a visibly anomalous, hazy-blue cloud that turned out to contain sulfurous odor (figure 22). Although Tupper and the pilot discussed other possibilities for the cloud's origin, Tupper came to the conclusion that the cloud was volcanic fog (vog) erupted from Manam.

Figure 21. The airport at Dili, East Timor (Indonesia), located about 2,200 km WSW of Manam.

Key portions of the pilot's message conveyed to us by Tupper follow.

"On descent into Dili, approaching 10,000 feet at 12 nautical miles [~ 3 km altitude and ~ 22 km from the airport] aircraft control levers were pulled back to flight idle just prior to entering a thin layer of smooth stratus cloud [figure 22].

Figure 22. The hazy blue cloud that produced a sulfur smell in the cockpit of the plane approaching Dili. The photo was taken by the air crew (names not given).

"Shortly after passing into the cloud, a strange smell was soon noticed in the cockpit; once the accusations of responsibility had passed, it quickly became apparent that the smell was not the result of a bodily function. The smell became very strong, with high sulfur content. As a precaution the Captain directed the First Officer to don his oxygen mask. The smell persisted but began to weaken on descent, and landing was accomplished without incident. After landing, First Officer removed the oxygen mask and noted the smell had remained. The captain had by this time become desensitized to the smell. Upon shutdown, unloading was halted, until such time as the cargo hold could be examined for a source of the smell. No smell remained."

Tupper and the pilot discussed possible sources for the smell. The cloud displayed a distinct blue haze (Tupper commented that "it's difficult to tell from the attached photo whether the blue is all that out-of-the-ordinary, but obviously they thought it interesting enough to take a photo!"). The cloud sat on the hills and appeared to have fog-like characteristics. The pilot described the odor as sharper and more metallic than the smell of H₂S (a description consistent with SO₂, the odor of which is sometimes described as metallic or akin to a struck-match.

What caused the sulfurous-smelling stratus cloud? The sulfur content may have come from either nearby volcanoes, none of which have been reported as active, or from industrial production (possibly Kupang). Due to a serious dengue outbreak in East Timor, it may have been the result of chemical mosquito control. Many chemical methods of mosquito control are based on sulfur products. Malathion is one such product; it contains mercaptan, which has a strong noxious odor. (Organic compounds with HS bound to carbon are called mercaptans or thiols and those of low molecular weight have strong smells. Small doses of mercaptan are often used to give natural gas a distinctive odor.) One possible way to explain the sulfurous gases was morning fog moving up the hills of Dili in response to anabatic (upslope-blowing) winds, which also carried residual insecticide.

Tupper spoke to or emailed the pilot several more times to get the following other details. The aircraft was an Embraer E120, a 30 seat turbo prop, with 20-25 people on board. The cabin attendant also noticed the smell, but no passengers commented. Despite the speculation about chemicals above, this was the only trip on which the smells had been noticed by the pilot.

According to Claire Witham, human perception of SO₂ odor varies depending on the individual's sensitivity, but SO₂ is generally perceived between 0.3-1.4 ppm and is easily noticeable at 3 ppm. This is generally below the level where health effects (e.g. respiratory response) might be noted. In general an exposure limit of 1-5 ppm is the threshold for respiratory response in healthy individuals upon exercise or deep breathing, whilst at 3-5 ppm the gas is easily noticeable and may cause a fall in lung function in persons at rest, and increased airway resistance. Asthmatic individuals may respond at much lower concentrations, and prolonged exposure to low concentrations carries increased risk for those with pre-existing heart and lung diseases. A more detailed review of gas hazards and guidelines has just gone online on the International Volcanic Health Hazard Network.

Significant in this event is that the flight crew thought that the smell had dissipated. The First Officer, who was wearing an oxygen mask, remained able to detect that the smell persisted. This indicates that the others in the crew lost their ability recognize that the sulfurous odors remained, a well-know effect of sulfurous gases called olfactory fatigue ('bombarded nerve receptors'), a potentially confusing situation for pilots focused on escaping from a volcanic plume (Wunderman, 2004).

Tupper conducted dispersion modeling of the 27 January 2005 Manam eruption (figure 23). The results suggested that the SO₂ cloud from the volcano probably passed over East Timor on the night before the incident and at higher altitudes. This is supported to a limited extent by the preliminary ozone and SO₂ monitoring results (figure 24), which suggest that the bulk of the cloud went N, but that part of the cloud traveled over the Banda Sea and passed over East Timor. The low level winds are highly unlikely to have carried the SO₂ to East Timor, but there was significant storm activity on the night when the cloud would have passed over. Excluding other explanations on the grounds that the eruption / encounter timing are unlikely to be mere coincidence, the most likely explanation for the flight crew's experience is that some eruption products from Manam were rained out over East Timor on the night of 29 January 2005. If SO₂ had been incorporated into ice particles, which then rained out, the particles would have melted and released SO₂ at about the level of the encounter, where the temperature was a bit above freezing. According to this scenario, the plane then flew through the resultant vog/stratus the next morning.

Figure 23. An ash dispersion model for the eruption cloud associated with the eruption of Manam on 27 January 2005. The model takes into account wind at various altitudes and other meteorological data, and predicts the movement of material injected in the atmosphere. The model used, NOAA hysplit, adopted the boundary condition that material was above the volcano between 10 and 24 km altitude starting at 1400 on 27 January. The results shown predict the dispersal for the interval 1200-1400 on 29 January. The model indicates that some material from Manam's 27 January eruption traveled WSW to where the aircraft-gas plume encounter took place. The model is a product of the NOAA Air Resources Lab with this particular run provided by Andrew Tupper.

Figure 24. A satellite image of atmospheric SO2 burden from Manam made about 12 hours after the 27-28 January 2005 eruption. The image resulted from the NASA Ozone Monitoring Instrument (OMI), which flew over the region on NASA's new Aura satellite. This image was produced from preliminary, uncalibrated data provided by the OMI. The OMI detected a large cloud of SO2 drifting W over the island of New Guinea. The gas is measured in Dobson Units (DU), a reflection of the number of molecules in a square centimeter of the atmosphere. Darker pixels cover the areas of highest concentration, while the lowest concentrations are represented by lighter ones (red and pink, respectively, on the colored electronic version of the Bulletin). If you were to compress all of the SO2 in a column of the atmosphere into a flat layer at standard temperature and pressure, one Dobson Unit would be 0.01 mm (millimeters) thick and would contain 0.0285 grams of SO2 per m2. On January 28, the atmosphere over New Guinea contained up to 50 Dobson Units (red regions), or 1.425 grams of SO2 per square meter. NASA image and caption courtesy Simon Carn, Joint Center for Earth Systems Technology.

Infrasound reports. The Comprehensive Nuclear Test Ban Treaty Organisation (CTBTO) is installing a world-wide network of 60 infrasound stations as part of the International Monitoring System (IMS) for detection of nuclear tests. The stations, some of which are already functioning, use microbarographs (acoustic pressure sensors) to detect very low-frequency (0.01-10 Hz) sound waves in the atmosphere produced by natural and anthropogenic events.

The eruption at Manam on 27 January at about 1400 UTC was detected at several infrasound stations around the Pacific (table 3). In one case a signal was received at a distance exceeding 10,000 km. The sound of the explosion took more than ten hours to reach that most distant station, located in Washington state (USA). The difference in the calculated and measured signal azimuths is likely caused by high atmosphere winds, and is reasonable given the great distances that the signal traveled.

Table 3. Arrival times and great circle paths for infrasound signal from Manam eruption on 27 January 2005 received at CTBTO infrasound stations. Azimuth and Distance data are for the calculated great circle path from the station to the volcano. Courtesy of Robert North.

Subsequent RVO observations. Although it remained active, Manam calmed considerably during February-May 2005. During the first two weeks of February 2005, emissions from Manam continued. On 15 February 2005, the alert level was reduced from 3 to 2. Mild eruptive activity was observed from Manam's Southern crater during the third week of February. Weak-to-moderate ash explosions rose a few hundred meters above the crater and drifted E and SE, depositing fine ash in areas downwind. Throughout February, seismicity was at low levels, with small low-frequency earthquakes occurring and no volcanic tremor. Throughout March, weak-to-moderate emissions from both the Main and Southern craters continued to produce occasional ash clouds during most days. On 15 March, a thin plume from Manam was visible on satellite imagery. On 24 March, emissions from Main crater rose to ~ 1 km above the summit. On 28 March, a moderate explosion produced an ash plume to a height of ~ 1.2 km above the summit. Ash plumes drifted N, depositing ash on the island. Seismic activity fluctuated between low and moderate, with low-frequency earthquakes recorded.

During April and May 2005, mild eruptive activity continued at the volcano. Manam remained at alert level 2 from February 2005 through at least late May. A thin plume extending 55 km NW on 4 May was seen on satellite imagery by the Darwin VAAC. The ash cloud remained below 3 km altitude.

Reference. Wunderman, R., 2004, Sulfurous odors: A signal of entry into an ash plume—perhaps less reliable for escape, Second International Conference on Volcanic Ash and Aviation Safety (Alexandria, Virginia, USA), 21-24 June 2004 (Plenary Session 1: Encounters, Damage, and Socioeconomic Consequences, poster P 1.2, Socioeconomic consequences) (http://www.ofcm.gov/ICVAAS/Proceedings2004/ICVAAS2004-Proceedings.htm).

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: Andrew Tupper, Darwin Volcanic Ash Advisory Centre, Australian Bureau of Meteorology (URL: http://www.bom.gov.au/info/vaac); Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea; David Innes, Flight Safety Office, Air Niugini, PO Box 7186, Boroko, Port Moresby, National Capital District, Papua New Guinea (URL: http://www.airniugini.com.pg/); International Volcanic Health Hazard Network (URL: http://www.ivhhn.org/); Simon Carn, TOMS Volcanic Emissions Group, Univ. of Maryland, 1000 Hilltop Circle, Baltimore, MD 21250, USA (URL: https://so2.gsfc.nasa.gov/); Claire Witham, Meteorology Office, FitzRoy Road, Exeter, EX1 3PB, UK; Robert North, SAIC Monitoring Systems Division, 1953 Gallows Rd., Vienna, VA 22182, USA; NOAA Air Resources Lab (ARL), Room 3316, 1315 East-West Highway, Silver Spring, MD 20910, USA (URL: http://www.arl.noaa.gov/ready/).

Crisis escalates. Instituto Geofísico (IG) members noted that eruptions at Reventador in Ecuador's eastern cordillera continued into at least early July 2005. Observers documented thick blocky lava flows, occasional Vulcanian explosions, new fumarolic activity on the N flank of the cone, and venting of vapor, gases, and fine ash. This followed a spate of increased seismicity during April to early June 2005. Lava flows had extended 4 km from the summit vent toward the SE, in the direction of the main highway across this region, a route that links the important oilfields in the Amazon basin with Quito, the capital. The lava flows were sequentially numbered (Lava ##3, ##4, etc.).

Lava ##3, a flow that began in November 2004 (BGVN 29:11), advanced slowly and ceased movement by early January 2005. Following relatively low seismic activity in late 2004 and early 2005, the IG monitoring network began to register bands of harmonic tremor starting 1 April (figure 18). Through 8 April 2005, instruments recorded 45 tremor episodes, each lasting 10 to 60 minutes. Dominant frequency peaks were between 1 and 1.5 Hz. Given that strong incandescence was observed by a guard of PetroEcuador from 14 km away, the tremor was interpreted to signal the rise of magma into the upper part of the cone through an open conduit.

Figure 18. Seismic events registered at Reventador since August 2004. Courtesy of IG.

Lava ##4 erupted coincident with this strong tremor and was the most important surface manifestation. It was first observed in an overflight on 12 April, escaping from a summit crater conduit that had formed a carapace. It was seen flowing down the SW crater notch onto the cone's flanks and then onto the SW and SE caldera floor. The flow partially covered Lava ##3 (figure 19), resulting in layers of recent lava in some places reaching more than 50 m thick. This emplacement was observed during several days of work on the seismic instrumentation and sampling within the caldera carried out by IG personnel during 19-22 April. During the same overflights, a new fumarole field was observed on the lower S flank of the cone, a spot very close to the upper Reventador River, in the same place where thermal anomalies were observed on 11 March 2005.

Figure 19. Location of lava flows related to eruptive activity within the Reventador caldera since 2002. Photo taken looking at the SE flank on 6 May 2005 by P. Ramón. Provided courtesy of IG.

Starting on 15 May there was an important increase in the intensity of harmonic tremor, often preceded by low frequency (< 1 Hz) long-period events, a conspicuous aspect of behavior that was absent in April. Many of the long-period events, particularly those occurring during 17-21 May, were of such magnitude that they registered at seismic stations on other volcanoes (e.g., Cerro Negro and Guagua Pichincha) more than 100 km distant.After this elevated activity in mid May, there was a decrease in the number of events, dropping to an average of 88 per day. During this period Lava ##4 continued to flow, moving at the rate of about 20 m/day, advancing particularly strongly along the caldera's S wall in a stream channel (Rió Marker) cut through the 2002 pyroclastic deposits. Lava reached 25 meters thick when seen during a 22-23 May visit, during which time strong roars and the sounds of 'many jet planes' blared from the vent. These sounds indicated a strong gas flux, although little vapor was observed. At this time, there was an absence of both explosions and incandescence in the summit crater.

An overflight on 25 May confirmed the emergence of a new flow (Lava ##5). It followed the same route as ##4, but was comprised of three principal lobes. The middle lobe, which represented the most conspicuous and largest volume, advanced down the Río Marker's channel (figure 20).

Figure 20. Lavas 4 and 5 flowing down the Marker's stream channel along the SE margin of Reventador's caldera. Photo taken on 17 June 2005 by P. Ramón. Provided courtesy of IG.

Reventador's activity in June 2005 began with an important swarm of volcano-tectonic and hybrid seismic events—starting on the 2nd and continuing through the 3rd. Of particular note, tremor continued for more than 10 hours, and provided background to the discrete volcano-tectonic and hybrid events Hybrid events had not been registered since November 2004. Following these important swarms, instruments registered strong, full-amplitude bands of spasmodic tremor, comprised to some extent by packages of long-period events lasting for hours to days on end.

During these early days of June, there was an intensification of incandescence in the crater and later, the emission of gases and slight ash. On 8 June, a 100 km long vapor/ash column extended from the volcano into the S part of Quito at ~ 7 km altitude and caused a very slight powdering of ash, which was brought down by a gentle rain and left cars dappled with circular spots.

A trip by IG volcanologists into the caldera on 11-12 June disclosed strong Strombolian fountaining in the summit crater. Lava ##5 continued to flow atop the stalled Lava ##4. Measurements of SO₂ flux with a mini-DOAS (differential optical absorption spectroscopy) resulted in an estimate of ~ 2,500 metric tons/day.

Three other seismic stations were installed around the caldera with the helicopter help of the petroleum company OCP during 16-19 June. One broad-band seismograph and infrasound system was also installed, thanks to collaboration with Jeff Johnson of the University of New Hampshire. During this period no Strombolian activity was observed, but Vulcanian explosions (figure 21) occurred with little warning. A 24-hour period during 18-19 June included at least seven discrete explosions, producing strong infrasound and seismic responses. Many of these explosions discharged columns that rose 2-3 km above the summit (and some, up to as high as ~ 6 km above the summit) and were clearly heard within the caldera. Large incandescent blocks could be seen thrown several hundreds of meters into the air, falling on the cone's upper slopes. Ash content in the columns was moderate. Explosions were discrete and often terminated within 4 minutes. Thermal alerts were identified by the Hawaii Institute of Geophysics and Planetology (HIGP). Observations on 30 June and 1 July noted recent lava flows in the upper Marker river valley (figure 22).

Figure 21. One of Reventador's discrete Vulcanian explosions observed during a 19 June 2005 helicopter flight. The view is from the E of Reventador caldera looking toward the W. Photo taken on by P. Ramón; provided courtesy of IG.

Figure 22. A photo of Reventador's Lava ##4 flow front (which had reddish hues) overtopped by Lava ##5 (more nearly white). The shot was taken in the Río Marker at 1100 on 30 June 2005. By 1 July, Lava ##5 had still not advanced beyond the terminus of Lava ##4. Photo by P. Ramón, provided courtesy of IG.

The 4-6 discrete explosive degassing events/day observed in June led the IG authors to surmise that there were a series of temporary plugs in the upper part of the conduit. This behavior was thought to reflect magma becoming more crystal rich.

As of 6 July, harmonic tremor, occasional explosions, and long-period and volcano-tectonic signals all continued to register at Reventador on the IG's telemetered monitoring network. Strong Strombolian fountaining was observed from distances of 6.5 and 14 km during the evening and one of the lobes of Lava ##5 was advancing down the caldera wall (following the Río Marker), but abruptly slowed to perhaps only ~ 20 m/day. In comparison, this flow-front velocity had earlier attained ~ 70 m/day (during 19-23 June) and ~ 50 m/day (during 23-30 June). The diminished rate of advance and continuing high-amplitude tremor suggested that perhaps a new lava flow (Lava ##6) had broken out high on the flanks, a conjecture yet to be confirmed by press time. Lava ##5 was still 1.2 km from the steep incline, a point where it could begin rapid descent to the alluvial fan where the highway and petroleum pipeline are located.

Geologic Background. Reventador is the most frequently active of a chain of Ecuadorian volcanoes in the Cordillera Real, well east of the principal volcanic axis. The forested, dominantly andesitic Volcán El Reventador stratovolcano rises to 3562 m above the jungles of the western Amazon basin. A 4-km-wide caldera widely breached to the east was formed by edifice collapse and is partially filled by a young, unvegetated stratovolcano that rises about 1300 m above the caldera floor to a height comparable to the caldera rim. It has been the source of numerous lava flows as well as explosive eruptions that were visible from Quito in historical time. Frequent lahars in this region of heavy rainfall have constructed a debris plain on the eastern floor of the caldera. The largest historical eruption took place in 2002, producing a 17-km-high eruption column, pyroclastic flows that traveled up to 8 km, and lava flows from summit and flank vents.

Information Contacts: Patricia Mothes, Patricio Ramón, Pete Hall, Daniel Andrade, and Liliana Troncoso, Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Jeffrey B. Johnson, Dept. of Earth Sciences, James Hall University of New Hampshire, Durham, NH 03824, USA.

BGVN 26:03 reported hydrothermal activity at Rotorua on 26 January 2001 involving the ejection of mud and ballistic blocks. BGVN 28:12 reported that the New Zealand Institute of Geological and Nuclear Sciences reported two subsequent hydrothermal eruptions in Rotorua caldera at Kuirau Park around 1100 on 6 November 2003 (figure 5). The eruptions occurred just meters from the site of the large blowout in 2001. The area is known for this kind of geothermal activity. The following information is primarily from Ashley Cody.

Figure 5. Rotorua is the NW-most caldera of the Taupo volcanic zone, in the Bay of Plenty region of New Zealand's North Island. Courtesy of UNAVCO.

In late May 2004 a geothermal well ~ 40 m deep at Tokaanu on Lake Taupo (~ 100 km S of Rotorua) blew out suddenly, erupting mud and scalding waters to ~ 15 m high and flooding surrounding properties for several days until it could be quenched and a new headworks fitted. This well may have been standing open and just suddenly began boiling, since its casing seemed to be intact.

About 0100 on Saturday 29 June 2004 the blowout of a geothermal well in Rotorua blew muddy water and rubbly debris to ~ 15 m high and showered muck over houses and cars to a radius of ~ 100 m accompanied by noise "like a jet aircraft." It went on until about 0400 on 30 June 2004 when it was quenched with a pumped cold water supply. It was cement-grouted shut a few days later. The well was 100 m deep and cased to 47.5 m.

Starting 18 July 2004 in the early afternoon, many earthquakes were strongly felt by many people in the area ~30 km N of Rotorua and ~20 km NW from Kawerau, in the northern North Island, or central Bay of Plenty. By 23 July more than 200 earthquakes were recorded in this area, most at less than 10 km depth.

In Lake Rotoehu, about 20 km N of Rotorua city, eyewitnesses reported a water column 100 m high that occurred at the same time as a strongly-felt ML 5.4 earthquake at about 1600 on 18 July 2004 at ~5 km depth. Shortly afterward a big series of waves occurred on the lake, and swept up beaches much higher than ever seen before.

Ground rupturing was reported at several sites along southern shores of Lake Rotoehu. Many houses were evacuated due to damage such as walls breaking apart and houses shifting off their foundations. The main road was blocked in many places and more than 200 houses were evacuated due to their becoming unsafe to live in. Several people were killed by trees falling down banks onto cars and houses during the earthquakes.

On Thursday 17 March 2005 at about 1435, a blowout was observed from the northern end of Ruapeka Bay on Lake Rotorua, at Ohinemutu. It shot dark grey muddy waters and steam to ~ 6 m for 3-4 minutes. An eyewitness called the council safety inspector, Peter Brownbridge. On 18 March at about 1500, the safety inspector saw two more shots each ~ 1 m high from the same spot in the lake. This previously unknown vent is ~ 25 m NW from a clear flowing hot spring known simply as S1233, in the bed of the lake just 10 m W of the tip of Muruika Point. This is where a prehistoric account relates of a sunken village, where a sudden disturbance occurred one night and many people were killed. From verbal genealogy records, this event may have occurred about early 1700s-1720s. Today rows of timber posts are still standing below water level in the lake here.

According to a report in The Dominion Post by Mike Watson on 21 April 2005, one of the largest hydrothermal eruptions in the Rotorua area since 1948 took place about 1030 on 19 April 2005 and was witnessed by two farmers (figure 6).

Figure 6. The geothermal eruption roughly midway between Rotorua and Taupo on 19 April 2005 left a 50-m wide crater. Courtesy of Ashley Cody.

A huge column of hot steam, mud and rocks was thrown 200 m in the air. The eruption happened in an inaccessible area at Ngatamariki scenic reserve, close to the Waikato River, and about 8 km from Orakei Korako geothermal springs, roughly halfway between Taupo and Rotorua. The column was visible 10 km away and left a 50 m-wide crater and two hectares of debris. With the energy now taken out of the vent, no further eruption was expected.

The major part of the eruption lasted about two hours but it was still spewing steam up to 10 m high five hours later. The eruption sent out 7,000-10,000 m³ of material. Mud and 50 cm-diameter rocks covered a 70-100 m radius from the crater site, which had previously been covered by 2 m-high blackberry bushes and fallen trees (figure 7). The ground may take months to cool. According to Ashley Cody, the site had been heating up in the past year, with three new hot springs forming.

Figure 7. The hydrothermal eruption roughly halfway between Taupo and Rotorua left ash and mud covering the surrounding area to a depth of 4 m (light-colored material on ground surface, coating some trees, and choking the stream). Courtesy of Ashley Cody.

Geologic Background. The 22-km-wide Rotorua caldera is the NW-most caldera of the Taupo volcanic zone. It is the only single-event caldera in the Taupo Volcanic Zone and was formed about 220,000 years ago following eruption of the more than 340 km3 rhyolitic Mamaku Ignimbrite. Although caldera collapse occurred in a single event, the process was complex and involved multiple collapse blocks. The major city of Rotorua lies at the south end of the lake that fills much of the caldera. Post-collapse eruptive activity, which ceased during the Pleistocene, was restricted to lava dome extrusion without major explosive activity. The youngest activity consisted of the eruption of three lava domes less than 25,000 years ago. The major thermal areas of Takeke, Tikitere, Lake Rotokawa, and Rotorua-Whakarewarewa are located within the caldera or outside its rim, and the city of Rotorua lies within and adjacent to active geothermal fields.

Information Contacts: Ashley Cody, Consulting Geologist, 10 McDowell Street, Rotorua, New Zealand; Ron Keam, Physics Department, The University of Auckland, Private Bag 92-019, Auckland, New Zealand; Mike Watson, The Dominion Post.

White Island was last reported on in BGVN 29:03, covering the period to March 2004. At that time, approximately two years had passed since any significant eruption, but the New Zealand Institute of Geological and Nuclear Sciences (GNS) continues to monitor White Island. This report is a summary of their brief reports.

From April 2004 until June 2005, seismicity and hydrothermal activity at White Island remained at low levels, with some brief periods of weak to moderate volcanic tremor recorded during September to November of 2004. The level of the crater lake has risen significantly over this period, from 12-13 m below the overflow level in April 2004 to only 3-4 m below overflow level in June 2005 (figure 46). Some of this increase was caused by landslides in July 2004 and by heavy rains in May 2005. Steam and gas emissions have been minor, with the exception of a large plume visible from the mainland on 15 October 2004. The alert level remained at 1 (on a scale of 0-5), indicating some degree of unrest but no threat of eruption.

Figure 46. The crater lake on White Island, taken 9 January 2005, when the lake level was about 5 m below the overflow level and rising. Courtesy of Franz Jeker.

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

Information Contacts: Institute of Geological and Nuclear Sciences (GNS), Private Bag 2000, Wairakwi, New Zealand (URL: http://www.gns/cri.nz); GeoNet, a project sponsored by the New Zealand Government through these agencies:Earthquake Commission (E.C.), Geological and Nuclear Sciences (GNS), and Foundation for Research, Science and Technology (FAST). Geonet can be contacted at the above GNS address (URL: http://www.geonet.org.nz/contact.htm); Franz Jeker, Rigistrasse 10, 8173 Neerach, Switzerland.