Declining seismicity, thermal emissions, and plumes during 2013

In this report, we cover declining activity observed at Chaitén during 2012-2013. The Servicio Nacional de Geología y Minería (SERNAGEOMIN) had reduced the Alert Level to Green on 2 May 2011 (BGVN 36:11) and continued to monitor activity and release online bulletins. These activity bulletins were available during February 2012-October 2013. We also note the thermal alerts from MODVOLC during January 2010-April 2013.

The recent rhyolitic eruption of Chaitén began in May 2008 (BGVN 33:04) and, according to Pallister and others (2013), comprised five major phases: (1) an explosive sequence (1-11 May 2008), (2) a transitional time that included ash plumes and lava extrusion (11-31 May 2008), (3) lava flows and dome growth (June-September 2008), (4) spine extrusion and dome growth (October 2008-February 2009), and (5) endogenous lava dome growth (February 2009-early 2010). Following those phases of activity, ash plumes and thermal emissions declined during 2010-2011 (figure 31); intermittent thermal anomalies and vapor emissions dominated activity during 2011-April 2013. Since April 2013, thermal alerts were no longer reported by the MODVOLC system.

Figure 31. On 25 October 2011, this Landsat 5 image captured a small plume rising from Chaitén's active lava dome. A) This wideview includes the caldera and surrounding region of Chaitén. Meandering river systems link the volcanic highlands with the Pacific coast. A) and B) The town of Chaitén is located at the red circle, on the delta formed by runoff from the highlands that include Chaitén ~11 km NE. Note that the scale bars are approximate. Courtesy of USGS/NASA.

SERNAGEOMIN reported decreasing seismicity during 2012-2013 (table 3). Volcano-tectonic (VT) events dominated these datasets while hybrid events were very rare. Local magnitudes (ML) and magnitudes calculated from signal duration (MD) were reported as well as the reduced displacement (calculated for long-period earthquakes).

Table 3. Seismicity during February 2012-September 2013 is reported for total earthquake counts (EQ), volcano-tectonic earthquakes (VT), long-period earthquakes (LP), and hybrid (HYB). Note that "**" indicates reporting of M_L values where M_D values were not reported. Reduced displacement (RD) is reported for LP events. Courtesy of SERNAGEOMIN.

When clear viewing conditions permitted, nighttime incandescence and vapor plumes were captured by the SERNAGEOMIN webcamera during February 2012-September 2013. Observations included plume heights ranging 300-600 m above the crater.

Thermal alerts. Elevated temperatures were detected from Chaitén during much of the eruption that began in May 2008. Thermal anomalies were detected by MODIS sensors on the Terra and Aqua satellites every month until September 2010 (table 4). As the eruption waned, elevated temperatures were observed less often and were absent during the months of July 2011, May 2012, August-October 2012, December 2012, March 2013, and after 2 April 2013. The MODVOLC alerts chiefly occurred in the immediate area of the crater (figure 32).

Table 4. The MODVOLC system generated alerts during January 2010-April 2013. The number of thermal alerts detected per year decreased from 2010 through early 2013. Courtesy of HIGP.

Figure 32. As seen here, from April 2012 to April 2013, MODVOLC generated alerts (red and orange point) on 15 days for Chaitén. Those alerts indicated elevated temperatures within 2 km of the summit area. Courtesy of HIGP.

References. NASA Landsat Program, 2011, Landsat ETM scene LT52320902011298COA00, SLC-Off, USGS, Sioux Falls, October 25, 2011.

Pallister, J.S., Diefenbach, A.K., Burton, W.C., Muñoz, J., Griswold, J.P., Lara, L.E., Lowenstern, J.B., and Valenzuela, C., 2013, The Chaitén rhyolite lava dome: Eruption sequence, lava dome volumes, rapid effusion rates and source of the rhyolite magma, Andean Geology, 40, (2): 277-294.

Information Contacts: Observatorio Volcanológico de los Andes del Sur-Servicio Nacional de Geologia y Mineria (OVDAS-SERNAGEOMIN), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); and 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/).

Information is preliminary and subject to change. All times are local (unless otherwise noted)

April 2008 (BGVN 33:04) Cite this Report

First recorded eruption generates large ash plume; thousands evacuated

The first historical eruption at Chaitén began on the morning of 2 May 2008, following increased seismicity in the region the day before. Chaitén, located W of the larger Minchinmávida (or Michinmahuida) stratovolcano, is a small 3-km-diameter post-glacial caldera or explosion crater (figure 1) which probably was formed ~ 9.4 ka BP, based on dating of scoria-rich surge deposits (Naranjo and Stern, 2004). Within the explosion crater lies an obsidian lava dome of rhyolite composition.

Figure 1. Orthorectified 15-m ASTER infrared (VNIR) satellite image from 1 April 2006 showing Chaitén volcano (upper left), ice-covered Minchinmávida volcano (right), and the town of Chaitén (lower left). Courtesy of Rick Wessels, Alaska Volcano Observatory.

Servicio Nacional de Geología y Minería (SERNAGEOMIN) reported that a pulsating white-to-gray ash plume on 2 May rose to an estimated altitude greater than 21 km and drifted SSE. Based on observations of satellite imagery and pilot reports, the Buenos Aires VAAC reported an ash plume at altitudes of 13.7-16.8 km that drifted NE. According to news articles, Chile's government declared a state of emergency on 2 May and several hundred people were evacuated from the coastal town of Chaitén (10 km SE).

According to news sources, ashfall was reported during 2-6 May both locally and up to hundreds of kilometers away, affecting water supplies and roads. Based on observations of satellite imagery and pilot reports, the Buenos Aires VAAC reported that during 3-6 May ash plumes rose as high as 10.7 km altitude and drifted variably to the SE (figure 2), E, W, and NE. News sources indicated that about 4,000-5,000 people were evacuated from the town of Chaitén and surrounding areas as the eruption continued. On 5 May, ONEMI (Oficina Nacional de Emergencia - Ministerio del Interior) reported that evacuations also took place in Futaleufú, about 65 km ESE of Chaitén, where ~ 30 cm of ash accumulated. One elderly person died during the evacuation efforts. On 6 May, ONEMI and SERNAGEOMIN reported that the eruption became more forceful and generated a wider and darker gray ash plume rising to an estimated altitude of 30 km. All remaining people in Chaitén were ordered to evacuate, as well as anyone within 50 km of the volcano.

Figure 2. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite captured this image of a long, cloud-like plume extending about 700 km SE from Chaitén on 3 May at 1035 local time. The plume rises high over the Andes mountains, drifts across Argentina, and thins over the Atlantic Ocean. Courtesy of NASA Earth Observatory and the MODIS Rapid Response System.

Activity continued, and a lava dome began growing from a vent on the upper flank of the old dome. Lahars and floods also inundated the town of Chaitén, causing widespread destruction. Additional details will be provided in future reports.

References. Naranjo, J.A., and Stern, C.R., 2004, Holocene tephrochronology of the southernmost part (42°30'-45°S) of the Andean Southern Volcanic Zone: Revista Geológica de Chile, v. 31, no. 2, p. 225-240.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Avda Sta María No 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); Oficina Nacional de Emergencia - Ministerio del Interior (ONEMI), Beaucheff 1637 / 1671, Santiago, Chile (URL: http://www.onemi.cl/); José Antonio Naranjo, Departamento de Geología Aplicada, SERNAGEOMIN; Buenos Aires Volcanic Ash Advisory Center (VAAC), Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/productos.php); Rick Wessels, Alaska Volcano Observatory, U.S. Geological Survey, Anchorage, AK, USA (URL: http://www.avo.alaska.edu/); NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/); Associated Press (URL: http://www.ap.org/); Agence France-Presse (URL: http://www.afp.com/).

May 2008 (BGVN 33:05) Cite this Report

Widespread rhyolitic ash; a dome then a tephra cone; destructive lahars

[Chaitén began erupting] on the morning of 2 May 2008 (BGVN 33:04). This report discusses events through 30 May, in particular, summarizing reports ("Noticias") issued by Servicio Nacional de Geología y Minería (SERNAGEOMIN). News and other reports have variously stated 8,000-12,000 people evacuated.

Impacts of ashfall in Argentina also spurred a local government report (Anonymous, 2008) noting that the Argentine Atomic Energy Commission analyzed tephra (pumice and ash) from ashfalls in Argentina. Results established the tephra as a low-silica rhyolite (table 1).

Table 1. Major element analyses (ranges for four samples) from Chaitén's ash. The samples were all from Argentina, at or near the settlements of Corovado (120 km SE of the volcano), Trevelin and Esquel (~100 km E), and Epuyén (~120 km NE). The values presented are weight percent (with total Fe shown as Fe₂O₃). In general, low silica rhyolites are typically about 69-74% SiO₂; high-silica rhyolites, about 75-84%SiO₂. Values here were measured by Laboratorio de Geoquímica de la Comisión Nacional de Energía Atómica, Regional Cuyo (Anonymous, 2008).

News reports during May (and later) stated that ash in and over Argentina closed airports. Many flights were also cancelled.

Chaitén volcano is in southern Chile, at the S end of Patagonia's Lakes district (figure 3). The evacuated town of Chaitén (figure 4) served as the provincial capital of Palena. The town was home to about 4,000 people, but lahars have buried at least portions of it. That town was also the main jumping-off point for Pumalin Park, a new nature sanctuary funded by philanthropist Douglas Tompkins.

Figure 3. Map showing Chaitén volcano and other Southern Volcanic Zone volcanoes in Chile. The large island to the west is Chiloé Island. The passage at the N end of Chiloé island is called the Chacao strait; and the body of water it leads to is the Ancud gulf; farther S it becomes the Corcovado gulf. The city of Puerto Montt and major roads emphasize that the town of Chaitén lacks road access to the N. The town has an airport, but most residents evacuated by boat. Courtesy of Google Earth.

Figure 4. A pre-eruption photograph taken in 2003 looking downward and approximately SW from the International Space Station. Chaitén volcano sits in the lower right, with the larger snow-covered Minchinmavida at lower left. The Blanco river leads from the 3-km-diameter caldera towards the sea and passes through the town of Chaitén (top right), 10 km from the volcano. Photo ISS006E42130; courtesy of NASA.

To the W of Chaitén town lies both the large (190 km long) Chiloé island and the much smaller Talcán island. Talcán island sits ~ 30 km from the town of Chaitén; it served as a staging area for monitoring efforts. Many of the larger rivers along the coast in the region reach the sea at fjords, and Chaitén town sits at the head of a fjord of the same name.

Synopsis of key events. Events during May included the month-long persistence of ash plumes; impressive electrical discharges coincident with some ash plumes; an ash blanket spanning cross-continent; some variable (and difficult to forecast) plume-dispersal patterns; small pyroclastic flows on multiple days; and lahars that progressively engulfed the town of Chaitén.

Some other highlights include the following. By 6 May, two explosion craters on the dome's N flank had united to form one large crater. By 21 May, aerial viewers saw a new dome had extruded in the N-central sector of the older rhyolite dome (figure 5). That new dome continued to grow through May, sprouting on the older dome, and later in the month, forming a large tephra cone. On 24 May, observers saw a vigorous eruption venting from an explosion crater on the old dome. They also noted that the new dome had grown taller than the old one. On 26 May, reports declared that the eruption had entered a less energetic phase (subplinean) with ash plumes rising 3-5 km in altitude.

Figure 5. The dome-filled caldera of Chaitén volcano is seen in an aerial view from the S taken prior to the 2008 eruption. The elliptical 2.5 x 4 km wide summit caldera was formed during an eruption about 9,400 years ago. A rhyolitic, 962-m-high obsidian lava dome occupies much of the caldera floor. Photo by Eric Manríquez T. (Instituto Geográfico Militar).

SERNAGEOMIN began to author reports on Chaitén starting the day the eruption began (2 May 2008, BGVN 32:04). Ten reports discussed May events. In addition, although little discussed in this report, the advent of digital and internet technology enabled eruption observers to share unprecedented numbers of photographs and videos. Satellite data on the eruption was also impressive (eg., NASA's Earth Observatory website featured 14 reports on Chaitén's May impacts).

Activity during 4-31 May. Amid reports of tall plumes on 3-6 May (BGVN 33:04), Andes del Sur (OVDAS) of SERNAGEOMIN installed three non-telemetered seismic stations around the volcano. These stations were later moved to more accessible places to enable more frequent data inspection. Improved seismic stations were installed later in the month (see below).

The 5 May eruptive vigor is partly revealed by an astronaut photo (NASA-ISS, 2008). Taken from a height of 344 km, it showed a plume punching through weather clouds and manifesting powerful vertical transport. It also highlights how weather clouds then would have thwarted ground observers from seeing, and thus assessing, the height of the plume top in those conditions.

Satellite imagery acquired on 5 May and discussed by NASA's Earth Observatory website that day revealed the Chaitén plume and a fresh blanket of ash. The ash blanket stretched from the high Andes to the Atlantic coast, and the ash plume continued E beyond it. Areas of the land surface along the Andes and to the Pacific were obscured by weather clouds.

The 6 May report noted that the eruption intensified at 0820 that day, leading to vigorous explosions of rhythmic character and high sustained energy. An ash column rose to ~ 30 km altitude. At this point in time, the column was taller and wider than those seen in the earlier, initial eruptive phase.

A helicopter flight at 1000 on the 6th indicated that two explosion craters on the dome's N flank had united to form one crater ~ 800 m in diameter. The column height had decreased. Consistent with mobile ash on the ground, the amount of ash in rivers in the region had increased.

After large explosions on 6 and 7 May, earthquakes occurred that were thought to denote moving fluid associated with a magma chamber beneath the volcano. Hypocenter calculations suggested the magma chamber was at less than 5 km depth. The ambient seismicity near the volcano around that time was ~ 35 volcano-tectonic earthquakes per day.

During 8 May, despite frequent low-hanging clouds, viewers glimpsed areas E of Chaitén. Along a N-trending valley there, thin gray spirals of cloud descended into the Rayas river, and ultimately the Rayas itself also began to emit clouds. SERNAGEOMIN's 9 May report explained these phenomena as the result of small pyroclastic flows inferred to have heated the river waters, thus yielding vapor that subsequently condensed to form the spirals of cloud.

The atmosphere, which on 8 May was cloudy during the hours 0715 to 1515, cleared somewhat during 1500-1630. At 1600 viewers saw both the volcano and a NE-blowing mushroom-shaped cloud that reached 14 km altitude. Photographic evidence (not included) showed that to the W side of the column there appeared a smaller cloud that looked denser and medium to dark gray. That smaller cloud was thought to have been associated with a new vent located at the foot of the dome's W side.

At 1300 on 12 May observers on Talcán island saw the upper portion of an ash column, which rose to 8.0 km altitude. Helicopter flights found strong SW winds aloft, blowing 80-100 km/hour to the NE. Despite the wind, during the flight about four explosions rose to similar (8 km) altitudes, thus sustaining the plume.

Aerial inspection of the caldera and dome at 1430 on 12 May revealed that small pyroclastic flows had burned multiple hectares of native forest in the headwaters of the Rayas river on the caldera's N flank and as far as the Austral highway. Similar processes had also devastated vegetation both everywhere within the caldera and on parts of the outer NE flanks. A wide, vertically oriented ash column originated from a vent extending from a crater on the older dome's N flank to its summit.

A photo of the caldera provided in the 13 May report showed a powerful billowing eruption. Incandescent areas spread along the dome's upper SE side, and a blanket of fresh fragmental deposits covered much of the upper dome.

On 12 May a helicopter crew found Chaitén town flooded by lahars traveling down the Blanco river. Based on the two photos in the report, the lahars at that point had covered roughly the lower half of single-story structures closest to the river and as far back as perhaps 3-5 buildings from those closest to the river's former margin. Some buildings closest the river were dislodged, a few had only their uppermost walls and roofs exposed. Significant portions of the town farther from the river still stood above the level of inundation.

Subsequent to 12 May, the river rose yet farther, and lahars took out a bridge. The lahars stretched ~ 200 m farther into the town reaching ~ 40 homes and numerous vehicles. Scenes of the town of Chaitén became the subject of many news reports and some videos posted on the web.

NASA's Earth Observatory posted an image acquired on 12 May (figure 6). Ash was again visible across the entire continent, spreading in a band trending ENE of the volcano. The ash is more visible in this image as the plume is blowing well S of the ash blanket.

Figure 6. Chaitén's plume crosses the bottom of this Terra satellite image from 12 May 2008, becoming more diffuse before reaching the Atlantic coast. The ashfall blanket in an E-W cross-continent band. Courtesy of NASA Earth Observatory by Jeff Schmaltz and Michon Scott (their 12 May report).

The 16 May report noted that the eruption was clearly plinean in nature and the source of continuous plumes. But, in the past two days, the plumes had not risen above 5 km altitude. Seismicity during 14-16 May included swarms of hybrid earthquakes.

At 0730 on 15 May observers saw the upper part of an ash plume reach 4 km altitude. The plume was swept NE in 140 km/hr winds. That day, a helicopter took observers over the town of Chaitén and the Amarillo and Michinmahuida rivers. Wide areas of the region were covered by white tephra. Lahars continued, apparent both to Chaitén's S and along the Blanco river to the coast. Lahars covered the Chaitén airfield and invaded the dock areas, as well as its main plaza, swamping government buildings. The lahars continued rising as the river bed and flood plains filled with sediments. Discolored water was seen widely (including N of Chaitén town in Pumalín bay,). Some elongate pumice rafts were floating in the Corcovado gulf.

In response to the crisis at Chaitén, during mid-May, the United States gave Chile several radio-telemetered seismic stations. Three members of the US Geological Survey (USGS) also joined SERNAGEOMIN and other agencies in Chile to install two stations. The visiting team, there during mid-May to early June, also discussed instrument operations, maintenance, and data interpretation.

The diagram in figure 7 indicates the key components of the portable seismic station initially used (without telemetry) and the new seismic stations installed (with digital instrumentation, telemetry, and linkage to the Internet). Both of the new seismic stations were installed on the mainland, broadcasting to a site on Chiloé island. Photographs of the area were taken during fieldwork (figures 8-10).

Figure 7. Diagrams comparing seismic stations and related components. At Chaitén, the portable stations were replaced by telemetered seismic stations during mid-May 2008. The A/D converter changes continuouos (Analog) signals to a stream of discrete (Digital) numbers. A "wideband" (broadband) seismic sensor can detect ground motions over wide frequency ranges compared to the narrower ranges typical of older instruments. These instruments also have a more uniform response across these varied frequencies, easing data interpretation. Courtesy of SERNAGEOMIN (from their 20 May report).

Figure 8. Chaitén eruption plume seen looking E across the gulf on 19 May 2008. The snow-covered flanks of Michimahuida volcano appear in the background. Courtesy of J.N. Marso (USGS).

Figure 9. Native forest destroyed by pyroclastic surges on the flanks of Chaitén during mid-May to early June 2008. Note the singed band located between the zone of total destruction (bottom) and unaffected forest (top). A road is visible across much of the photo. Photo by A.B. Lockhart, USGS.

Figure 10. (top) Town of Chaitén overrun by lahars during mid-May to early June 2008. Lahars had began to accumulate as a delta at the river mouth. Owing to sedimentation, the river (seen in background) had changed course and was then flowing through the town. The airport is on photo's right side between the town and the steep hill in background. (bottom) This closer view illustrates variable amounts of lahar damage affecting Chaitén town. This town was completely evacuated within several days of the eruption's onset. Photos by A.B. Lockhart, USGS.

At midday on 15 May, the authors described a cold wind that carried fine ash W. Ash fell on the ship Aquiles and on Talcán island, reaching 0.5-1.0 mm thick. In the same time frame, ash fell at the Blanco and Rayas rivers at least as far W as the Chaitén fjord's mouth.

Favorable weather, including strong wind, enabled scientists to assess the caldera on 21 May (figure 11). They found a new dome had emerged, already of significant size. It extruded from an area in the old dome. The eruption at the time was vigorous, though marked by sporadic explosions.

Figure 11. (top) An annotated photo from a helicopter on 21 May 2008 looking S into the erupting Chaitén caldera documenting the emergence of a new dome on the N side of the old one. The old dome (dark color, lower right) emitted a white plume; the new one (pale rose in colored versions of this photo, center) emitted a pale rose plume. Small block-and-ash flows descended the new dome's N side. The dark material in the background is the far (S) caldera wall. The venting ash column delivered considerable ashfall downwind. (bottom) A schematic (plan view) of the caldera seen that day (composed 23 May 2008 by H. Moreno). Both courtesy of SERNAGEOMIN.

An overflight on 24 May revealed the new dome had slightly higher elevation than the old dome. Airborne observers saw the cone's 200-m-diameter crater vigorously expelling gas and ash. This vent was on the higher parts of the old dome in an area just to the S of the active dome.

The eruption, although unceasing, was described in the 26 May report as having decreased to subplinean. It remained in this lower energy state through at least month's end. On 25 May, ash columns reached ~ 3.5 km in altitude, with occasional explosions prompting plumes up to 5 km. Plumes often blew NE.

The 22 May report mentioned a swarm of hybrid earthquakes, in this case with considerable 3 Hz content. Volcano-tectonic earthquakes diminished progressively during 22-26 May, both in number and magnitude. These accompanied a reduction in volcanism.

A substantial tephra cone had developed on top of the new and old domes by 26 May (figure 12). The new dome, pink in color, was still present but lay directly behind the collar of tephra composing the new tephra cone. A summit crater vented plumes of different color. Although a dome had emerged, vigorous ash plume emission continued.

Figure 12. Chaitén seen from a helicopter on 26 May with the camera aimed NE. A tephra cone stood atop the new and old dome complex. The cone's steep upper walls discharged a broad plume from an unusually ample summit crater. The plume was two-toned, with distinctively shaded material on its left and right sides. Lumpy areas on the middle to lower cone correspond to the obsidian on the now buried older dome. Some burned vegetation exists in the bottom center of the photo along the outflowing Blanco river. Photo by J.N. Marso (USGS).

A 26 May Terra image showed some areas of ashfall, but also several unusual features attributed to the eruption. These were described in the 26 May Earth Observatory report. First, rivers and lakes around the volcano were a distinct blue-green color, and this discoloration persisted into the Corcovado gulf, presumably from the waters' high suspended loads.

Second, although views of ridges (topographic highs) were clear and unobstructed in the image, a dendritic pattern of clouds, fog or mist hugged the valleys (topographic lows) for at least 200-300 km N and NE of the volcano. These white, opaque clouds originated from Chaitén.

On 28 May the ash column rose to 3.5-4 km altitude, blowing N to NW. It affected localities hundreds of kilometers away. Chilean airports closed in Puerto Montt, Osorno, Valdivia, and as far as 300 km N in Temuco. Lower altitude winds blew ash farther W, affecting coastal areas between the town of Chaitén and Chumildén, including Talcán island. In these areas, suspended ash appeared as a dense mist, grounding aircraft, including those used for vol;cano inspections.

During the last few days May, the number and magnitude of volcano-tectonic earthquakes diminished, and both low-frequency and hybrid earthquakes were absent. These changes coincided with a drop in the altitudes of eruption columns over the course of about a week.

Reference. Anonymous, 2008, Análisis químicos realizados en la contingencia del Volcán Chaitén: Municpalidad de Lago Puelo, 4 p. Accessed July 2008 (URL: http://www.lagopuelo.gov.ar/extras/riesgos/Analisis_quimicos_realizados_contingencia_Volcan_Chaiten[1].pdf).

Naranjo, J.A., and Stern, C.R., 2004, Holocene tephrochronology of the southernmost part (42°30'-45°S) of the Andean Southern Volcanic Zone: Revista Geológica de Chile, v. 31, no. 2, p. 225-240.

NASA-ISS, 2008, Astronaut photo from the International Space Station taken at 2027 on 5 May 2008 UTC. (Frame 6214, Mission ISS 017; file name, ISS017-E-6214.JPG). Image Science and Analysis Laboratory, NASA-Johnson Space Center (URL: http://eol.jsc.nasa.gov/scripts/sseop/photo.pl?mission=ISS017&roll=E&frame=6214>

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Avenida Santa María 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); Oficina Nacional de Emergencia - Ministerio del Interior (ONEMI), Beaucheff 1637 / 1671, Santiago, Chile (URL: http://www.onemi.cl/); José Antonio Naranjo, Departamento de Geología Aplicada, SERNAGEOMIN, Avenida Santa María 0104, Santiago, Chile; NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/); John Pallaster, Andy B. Lockhart, Jeff N. Marso, and John Ewert, US Geological Survey (USGS), Volcano Disaster Assistance Program (VDAP), 1300 SE Cardinal Court, Bldg. 10, Suite 100, Vancouver, WA 98683, USA; Image Science and Analysis Laboratory, NASA-Johnson Space Center, The Gateway to Astronaut Photography of Earth (URL: http://eol.jsc.nasa.gov/).

June 2008 (BGVN 33:06) Cite this Report

Events of June-July include diminished plumes, substantial seismicity, and lateral blast

Follow previous reports of May 2008 activity (BGVN 33:04, 33:05), this report summarizes Chaitén's behavior from 31 May through 25 July 2008. The bulk of this report came from SERNAGEOMIN (Servicio Nacional de Geología y Minería) and to some extent ONEMI (Oficina Nacional de Emergencia - Ministerio del Interior). A web camera located on a tower in Chaitén town and aimed upstream along the Blanco (Chaitén) river has helped authorities assess both the state of the volcano's plumes and the river (see URL in Information Contacts). In a later section are included some descriptions and photos by Richard Roscoe taken on 9 July.

On 3 June it was reported that lateral blasts or surges (or related processes) had devastated ~ 25 km² of native forest. Other behavior during this interval included consistent ash plumes, which were generally present when the volcano was visible, and continued growth of the intracrater dome and tephra cone. Vent areas and the dome and tephra cone's morphology changed as the dome grew more elongate.

The late May to early June behavior included a short-term seismic decrease, and a weakened eruptive column. During the reporting interval, the column was often noticeably weaker than in early May, but the seismicity was still relatively high. The two main seismic instruments monitoring the volcano (figure 13) registered numerous sustained events through late July, which began to cluster NNE of Chaitén. Some of the earthquakes were up to M 2.6.

Figure 13. Monitoring instrumentation includes two telemetered seismic stations, PUMA (short for Pumalín) and STAB (short for Santa Barbara), which sit adjacent the coast and monitor Chaitén volcano (Cv). On 12 July the stations detected two earthquakes centered NE of the volcano along a major fault trace there (the Liquiñe-Ofqui fault system). The colored versions of the map distinguish second-order faults, which mostly have left-lateral kinematics (red lines), and eroded scarps (yellow lines). Snow-covered Michinmahuida stratovolcano is also a prominent feature (M, along the E margin of map), as is the town of Chaitén (Ct). Courtesy of Luis E. Lara.

SERNAGEOMIN repeatedly interpreted the earthquakes to signify magma ascending from depth. If this magma reached the surface, they noted, vigorous eruptions might return. The high-viscosity of rhyolitic magmas seen here increases potential explosivity. This rhyolitic eruption at Chaitén is the first historically at a monitored volcano. The last significant rhyolitic eruption was at Novarupta volcano in Alaska in 1912.

Chaitén town has largely survived the lahars thus far. A deeper concern is that the growing dome and tephra cone sent bouncing rocks and smaller debris into the caldera's moat. In an early July SERNAGEOMIN report, the authors noted that the caldera's breach, located on the S, appeared blocked by recently eroded products. Small lakes were also then seen on the crater floor. Since the moat area drains to the S through this breach and feeds into the Blanco river, temporary dams in the moat area might seal the caldera's outflow, only to suddenly fail and release large volumes of debris towards the town. Despite this concern, as of 25 July such an event had been absent; however, on 12 July a sudden flood struck Chaitén town (see below).

Activity during June 2008. On 1 June, Chaitén's plume blew W, affecting Chiloé island (including the towns of Queilen, Lebjn, Chonchi, Dalcahue, and Castro, the island's capital). These conditions thwarted work on the seismic network. On 2 June dense fog affected the Gulf of Corcovado, especially adjacent Chiloé island, an atmosphere attributed to remobilization of air-fall ash by wind. That day, a helicopter managed to take off and the view enabled scientists to see an eruptive column to no higher than 3.0 km altitude dispersing SSE.

Seismicity on 2 July was higher than the previous days. Abundant were VT earthquakes, followed by long- period (LP) earthquakes. Between 1 and 2 July, seismic stations registered an average of 5 VT earthquakes per hour (below M 2). At some stations, some of the LP signals were sporadic, lasting less than a minute.

A 5 June SERNAGEOMIN report noted that explosions diminished gradually. Although ash was present, vapor dominated the emissions. A 3 June aerial inspection revealed that the dome's volume and footprint had increased, although it still had not reached the caldera's N wall.

The effects of N and NE flank blasts (or surges, pyroclastic flows, or related processes) were noted during aerial observations from the 3 June flight. The surges had scorched and burned an area of native forest. On this day observers computed an estimate of the damaged area, ~ 2,500 hectares (~ 25 km²). An undated photo looking down on part of the destruction appeared in BGVN 33:05 and more photos appear below. Several SERNAGEOMIN reports mentioned small pyroclastic flows during early and mid-May (12 May in particular, BGVN 33:05). Bulletin editors take the 3 June estimate as reflecting the sum of all devastation to that point in time.

On 3 and 4 June the plume's top stood below 3 km altitude. A 10 June SERNAGEOMIN report noted the continued lowered eruptive and seismic intensity through that time. Plumes continued to remain under 3 km altitude and they still affected air travel.

On 12 June observers at Chaitén town noticed tephra-bearing emissions. Noises had emanated from the volcano that day and the previous one. The SERNAGEOMIN report associated these emissions with two new vents seen on the S flank of the old dome, where craters had developed. Vapor-rich plumes had emerged from these areas and the observers inferred that the vents were possibly due to magma-water interactions. In addition, sudden floods swept into Chaitén town in the afternoon on 12 June, despite a lack of evidence for greater rains across the region. They were inferred as related to the emissions the same day.

Seismicity beneath the volcano on 12 June increased in the morning both in terms of the number of earthquakes and their magnitudes. Most of these events were less than M 2. Two prominent earthquakes struck ~ 5 km farther NE of the volcano, along the Liquiñe-Ofqui fault zone.

The 22 June report noted that observers looking at the contact between the old and new domes had seen two craters there that emitted ash plumes. The observers also noted near-source falls of both blocks and ash.

The same report said that a 17 June aerial inspection documented an ash plume to over 2 km over the volcano's summit that blew N and NW. Roars and associated noise from the eruption included the sound of an explosion at 1430 on 17 June. The resulting column rose to a height above the summit of over 3 km but later dropped to 2 km. Emissions continued from a crater S of the contact between the old and new domes. Immediately to the W of this crater, a new and growing crater issued increasingly large emissions of ash and gas. Numerous smaller vents were also apparent, chiefly emitting steam. Loose material covered parts of the old dome, forming a ring-shaped structure (a tephra cone). That structure's steep sides and inner and outer walls occasionally underwent mass wasting. Poor weather during 19-25 June halted aerial inspections then, but ash fell in Chaitén town and to the W and SE, as well as Queilen and other portions of E Chiloé island.

Following 20 June, seismicity remained stable with ~ 40-45 earthquakes per day. Sporadic numbers of VT earthquakes took place; there was no change in the number of LP earthquakes. Investigators inferred a lack of pressure increase in the volcanic system. During bad weather on 23-25 June some earthquakes again occurred on the Liquiñe-Ofqui fault zone, with epicenters in an area 2-3 km E of the volcano. A power outage struck midday on 25 June. A back-up power supply (UPS) worked for a while, but ultimately the outage caused several hours of lost seismic data at the Queilen processing center. Available data suggested a small increase in both the number and amplitudes of earthquakes during 24-25 June. During 0000-1200 on 25 June, instruments recorded 35 VT earthquakes, and four of those were M 2.2; LP earthquakes were absent.

Seismicity during the days leading up the SERNAGEOMIN report issued on 27 June reflected VT earthquakes generally below M 2, reaching 50 per day. An exception was on the 25th when four earthquakes exceeded M 2.0.

July 2008. On 1 July an ash column rose ~ 3 km above the top of the new dome. It blew N and NE. An aerial observation at close hand discerned two roughly vertical, sub-parallel eruption plumes issuing from vents in the crater. One plume, most active in recent weeks, came from a sector S of the new dome. The second plume came from a sector more to the W of the new dome. A photo of the scene in the 3 July SERNAGEOMIN report also depicted the area of eruption largely engulfed in white clouds from numerous fumaroles on the dome. On 3 July SERNAGEOMIN began a series of reports on unrest at Llaima stratovolcano (which went to Red alert on 10 July). Around 16 July a weather front also moved in across the Chiloé island region. Consecutive SERNAGEOMIN reports discussing Chaitén were only issued on 3 and 21 July, with a lack of much discussion on that volcano for the interval 3-15 July.

During 15-20 July seismicity stood relatively high, with an average of 350-400 VT earthquakes per day. On 20 July more than 20 earthquakes surpassed M 2.6. The next reports noted that on 21 and 22 July VT earthquakes occurred 330 times per day; 60 of those were near M 2.6, and that the number of earthquakes decreased on 24 July. Still, some of the minor earthquakes reached M 2.6 and were detected up to 300 km away. Seismic data around this time were interpreted to reflect magma at depth moving towards the surface, possibly implying a reactivation of the system, although the earthquake's depth was poorly constrained.

Chaitén's plume blew E at ~ 2 km altitude above the summit and appeared weaker than usual when seen as the weather cleared after 1500 on 23 July. During 22-24 July, earthquakes had increased both in number and magnitude, with the largest M ~ 2.6.

A new area of epicenters appeared during 22 and 23 July at a location 6 km ENE of the volcano. Seismic stations located 176 and 296 km from Chaitén, respectively monitoring the volcanoes Calbuco and Puyehue-Cordón Caulle, recorded these events, the first such occurrence since the eruption began. Previously, conspicuous epicenters had mainly occurred to the S and SE. Preliminary hypocenter calculations suggested the larger earthquakes in this NNE area were deeper, at 10-15 km depth.

Arrival times of S- and P-waves at stations Pumalín and Santa Bárbara indicated that the smaller magnitude earthquakes still occurred S and SE of Chaitén, whereas the larger magnitude earthquakes struck in the area 6 km ENE. An inspection flight carried viewers to the N and NE of the volcano on 24 July where they saw that the single active central vent sat to the S of the new dome. The emissions then were intermittent, white, and ash poor. When strongest, a thin plume rose to under 2 km altitude, with strong winds causing dispersion to the S and SE. When viewed on 24 July, the new dome also contained a significant depression in the S sector, at a point immediately N of the main active vent mentioned above. This depression emitted steam and gases. The new dome seemed to have decreased its growth rate, at least in the N sector. Strong steaming emerged from base of the dome's E sector. The observers looked around the new dome on the NW, N and NE sides, and they saw neither ponded areas nor lakes. During 24-27 July, the ash column rose to 2.5 km and occasionally 3.0 km altitude. The most active vent was the previously mentioned one located S of the new dome. The plume blew N and NW where it affected various localities along the coast.

Floating pumice. By early June, the white pumice from the eruption accumulated at river mouths to the volcano's W. Some fragments of pumice were as large as 40 cm in diameter. In addition to the Blanco river, those carrying the pumice included the Yelcho and Negro (respectively entering the sea 2 km and 5 km S of Chaitén town). Pumice rafts in the Gulf were seen in May (BGVN 33:05). During June and at least early July, along beaches of Chiloé (and particularly at Lelbjn, 12 km N of Queilen, a town almost directly W of Chaitén town) floating pumice continued to arrive. This area lies 60-100 km across Corcorvado gulf from the mouth of the Blanco river at Chaitén town. The pumice deposits, which included tree trunks and other debris, covered a thin zone along the shoreline stretching ~ 20 m from the sea's edge when photographed the afternoon of 1 July.

Roscoe's July 2008 photos. One of the subjects Roscoe presented on his PhotoVolcanica website was Chaitén's N devastated area, and some of those photos appear here (figures 14 and 15). The captions were brief and omitted the direction the camera was aimed. He visited the devastated area on 9 July 2008.

Figure 14. One of the parts of the devastation zone containing large lithic blocks (~ 1 m across), the most conspicuous being the one at left, which may have been perched above fallen timber. Trees here fell away from the viewer. Courtesy of Richard Roscoe, PhotoVolcanica.com.

Figure 15. Drainages redirected by Chaitén's eruption caused erosion of this road to the volcano's N. Courtesy of Richard Roscoe, PhotoVolcanica.com.

Roscoe noted that in the area he photographed, "Most trees were snapped off a couple of meters above the ground. The [pyroclastic] flow does not appear to have been hot enough to burn the leaves off the trees at the point we visited at the base of the volcano. Many branches with brown leaves were lying around. Very little pumice was found in the area although much of it may have been swept away during subsequent heavy rainfall."

In Chaitén town, Roscoe documented damage-mitigation and salvaging efforts (figure 16). Two of Roscoe's photos showed heavy equipment (a large backhoe and a bulldozer) reshaping the lahar deposits in an attempt to control encroaching lahars. Other scenes included people retrieving belongings, excavating lahar deposits covering the floor and lower shelves of a grocery store, and improving drainage from and access to their homes.

Figure 16. Work in Chaitén town to strengthen river banks to protect town from lahars. Although laden with tree trunks, the lahars appear quite uniform in color and character, devoid of coarse lithics or large rafted pumices. Courtesy of Richard Roscoe, PhotoVolcanica.com.

Information Contacts: Servicio Nacional de Geología y Minería(SERNAGEOMIN), Avda Sta María No 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); Oficina Nacional de Emergencia - Ministerio del Interior (ONEMI), Beaucheff 1637 / 1671, Santiago, Chile (URL: http://www.onemi.cl/); Luis E. Lara, Departamento de Geología Aplicada, SERNAGEOMIN; Richard Roscoe, Photovolcanica.com (URL: http://www.photovolcanica.com/).

December 2010 (BGVN 35:12) Cite this Report

During June-November2008 dome continues to enlarge despite a lateral blast

What follows summarizes official agency reporting on Chaitén's behavior from 26 July 2008 through early November 2008. Noteworthy events included a large scar that developed on the E dome area by 31 July 2008 from an inferred lateral blast, observance of a new lava dome (or dome lobe), ongoing seismicity, the emergence of the new dome as the highest point in the complex, dome growth to a point where the dome crossed the moat to impinge on the caldera wall, ongoing destruction to the town of Chaitén (10 km SW) by debris-laden floodwaters, and changes in the river system due to lahars. The hazard status remained at Volcanic Alert Red, Level 6—the highest level—throughout the reporting period.

The bulk of this report came from those issued by the Servicio Nacional de Geología y Minería (SERNAGEOMIN) with collaboration from the Oficina Nacional de Emergencia—Ministerio del Interior (ONEMI). Collectively, they and partner organizations listed below under Information Contacts issued over 20 reports we used in discussions here. Below, we reference the four reports from which we extracted figures (ONEMI-SERNAGEOMIN, 2008a, b, c, and d). Respectively, these four reports covered these respective date ranges in 2008: (a, no. 45) 31 July-1 August; (b, no. 55) 23-30 September; (c, no. 58) 14-20 September; and (d, no. 60) 31 October-7 November.

As previously reported, activity at Chaitén during June-July 2008 included diminished plumes compared with earlier activity, substantial seismicity, and lateral blasts that devastated ~25 km² of native forest (BGVN 33:06).

Seismicity, dominated by volcano-tectonic earthquakes, increased in July 2008, with some higher-magnitude earthquakes felt by the residents of the town of Chaitén. According to Carn and others (2009), "During July [2008], ash and steam emissions subsided while lava extrusion continued, accompanied daily by up to 300 hybrid earthquakes, which have characteristics transitional between low- and high-frequency events. Earthquake magnitudes increased from less than 2.5 to 4 by the end of July, raising concerns about renewed explosive eruptions, but seismicity declined in August. By late September 2008, the new lava dome volume was about half a cubic kilometer."

Clouds often obscured the view of Chaitén during 23 July-25 November. However, when visible, gas-and-ash plumes generally rose to an altitude of 1.4-4 km. In one of many examples, a modest (2 km high) plume dropped minor ash on 15 October, leaving plant leaves with a thin sprinkling of ash. Plumes reached over 4 km altitude on 13 August (up to 6-7 km), 27-28 September (5 km), and 26-28 October (7 km). Ash fell in the town of Chaitén at times. Noteworthy cases of ash accumulation including those on 24 July (3 cm thick); 27 July (thickness unstated); and 13 August (1.5 cm thick).

An overflight on 24 July revealed a 2-km-altitude plume that escaped from an area S of a previous vent, a zone on the S flank of the new lava dome. That plume contained emissions from various areas on the dome and included ash-bearing plumes generated by explosions. A gas-and-steam plume was emitted from a depression, also on the S flank, that was possibly formed by an explosion or partial collapse the previous day or two.

The Buenos Aires Volcanic Ash Advisory Center saw thermal anomalies over the lava dome in satellite imagery. Detections occurred during 22-23 August, 20-22 and 25-27 September, 27 and 30 October, and 2, 10, and 12 November 2008.

Scar in the dome. A Twin Otter aircraft flew past the volcano on 31 July 2008, and observers first saw a large scar on the E side of the dome. At some point in the flight they also saw a 2-km-altitude, languid, light-colored plume, which contained a small proportion of ash and blew NE. The plume vented most strongly from the growing dome's upper area. The scar was horseshoe-shaped, ~500 m wide, and ~200 m high, and had formed on an unknown date near the end of July 2008 (figure 17). The scar was inferred to have formed by a lateral explosion rather than collapse, because hummocky morphology typical of landslide deposits was absent on the caldera's floor (the moat) below the scar (OVDAS-SERNAGEOMIN, 2008a). Recent pyroclastic flow deposits associated with the scar also were not conspicuous. The location of the scar on the E dome was judged fortunate; if the same volume of material had blasted from the S dome, it would have traveled into the Chaitén (Blanco) River, and accordingly, to the town of Chaitén. Material from the explosion was transported into drainages that eventually flowed N into the Rayas River, and also entered small lakes at the S base of the dome (figure 18). However a significant volume of the material broken loose from the dome by the directed blast was visible in and adjacent to the SE moat. In the E moat, there were both a lake and a steep erosional scarp (figure 18). Thus, after the blast, the valley's SE drainage became obstructed, whereas the SW drainage remained open. Debris-charged water was visible in many parts of the moat.

Figure 17. Scar of the lateral explosion taken at Chaitén on 31 July 2008 looking at the dome's E side. This view also contains multiple labels detailing the scene. Taken from SERNAGEOMIN (2008a).

Figure 18. Vista of Chaitén's S moat as seen 31 July 2008 from the NE. Taken from OVDASSERNAGEOMIN (2008a).

Observations during August-November 2008. On 1 August, a N-facing camera situated ~1 km SE of town at the Chaitén airfield showed an active eruption plume in the morning, but weather obstructed the plume in the afternoon. This is one of two such web cameras at that airfield in a national system supported by the Chilean civil aviation agency DGAC. The other web camera shows the town of Chaitén (for more information see IFIS, 2011, in References below).

Seismicity decreased beginning in August and remained relatively low and variable through mid-September 2008. Hybrid earthquakes dominated, and, after 20 September, they increased in number and magnitude. SERNAGEOMIN interpreted seismicity during this time within the system as associated with movements of magmatic fluids and rock rupture. In late September, a seismic station was reinstalled in the volcano's N sector (previously sited at Auchemó, on an island 8 km SSW of the town of Chaitén). The new location was chosen in hopes that it would enable seismologists to better assess the volcano's seismic signals.

Although visual observations revealed comparative quiet during 23-25 September with weak plumes of mainly steam to 1.5-km altitude, the intensity of activity rose on the 27 and 28 September, when a series of columns emitted in separate puffs reached to 3- and sometimes 4-km altitude. The plumes were conspicuous from the settlements of Chaitén and Queilen and contained steam and fine ash.

On 30 September, the new dome had grown, but it lacked both any new scars (like the one seen on 31 July) and signs of further collapse at the 31 July scar. Small lakes remained on the N moat (arrow at left, figure 19). Portions of the older dome were still exposed (arrow at right, figure 19).

Figure 19. Aerial photo of the W portion of Chaitén's dome taken 30 September 2008 (from the E; arrows discussed in text). The upper dome gave off steam over a wide area. Taken from OVDAS-SERNAGEOMIN (2008b).

An overflight occurred on 30 September, thanks to a Presidential Delegation, during which the emissions remained similar to those previously described. Observers found it hard to define a single vent area for the emissions, although it appeared that abundant vapor escaped in the N dome area (figure 19).

Observers also commented that the scar on the E dome had changed morphology due to erosion. A photo documented that, by 30 September, new dome growth was sufficient in the NE sector to have reached the NE wall. By this time, the report authors noted, the growing dome's height had exceeded that of the caldera's topographic margin (the summit, 1,112 m elevation, a point on the S rim).

The number of hybrid earthquakes increased beginning on 20 September through early October. Beginning on 4 October, the number of earthquakes over M 3.0 decreased, a decline interpreted as related to slowed dome growth. Seismicity continued to decrease through mid-October, and was concentrated mainly in the volcano's S sector. Volcano-tectonic earthquakes did not exceed 3 per day, with magnitudes less than 1.5. Epicenters plotted on a map for 14-16 November scattered along a broad ~3-km-long line trending tangent to the ESE rim of the caldera, with several on the SE dome. Their focal depths were 0.5-3 km.

A miniDoas (a portable spectrometer based largely on differential optical absorption, Nadeau and Williams-Jones, 2008) was used for ground-based transects of the plume on 15 October 2008 to assess SO₂ fluxes. After several transects along a N65°W path and wind estimates of 5 m/s (from the Chilean Meteorological Directorate), the resulting flux estimate was 190 metric tons per day.

On 29 October, activity at Chaitén increased, characterized by several explosions that darkened the plume and caused it to rise from ~1.6 km to ~3.1 km. SERNAGEOMIN reported that variations in seismicity remained similar to patterns detected during the previous weeks.

On a 30 October flight, scientists observed a landslide that had originated from the active lava dome. The next day, observers described a plume emitted from multiple areas that rose to an altitude of 2.1 km. The white component of the plume (steam and gas) emitted mainly from the center and S portions of the dome. Vents on the dome's N and NE areas produced a gray plume.

An ONEMI-SERNAGEOMIN report (2008d) covered the date range 31 October-7 November 2008. Owing to bad weather, only on 2 November did observers or cameras in the town of Chatén manage to obtain a reasonable view of the volcano (figure 20). That day, they also saw an appreciable white column that appeared to be predominantly steam, similar to many of the plumes of recent months. The plume blew E, and its top reached ~2 km above the dome.

Figure 20. Chaitén as seen on 2 November 2008 from the town of the same name (~10 km SW). The photo on the right zooms in on dome in the framed area. Authors described the dome as "constantly growing." Taken from ONEMI-SERNAGEOMIN (2008d).

On 4 November, ONEMI-SERNAGEOMIN reported that a recent overflight confirmed the presence of a second new lava dome. The new dome grew in the NE part of the first dome that started to form in May 2008 and had a diameter of ~300 m and a height of ~150 m. Spines protruded from the top. Seismicity was concentrated underneath the dome.

Seismicity during 31 October-7 November 2008. Elevated seismicity prevailed during the first week of November. Hybrid earthquakes dominated, averaging 2-5 events per hour. Those over M 3 occurred 1-2 times per hour. They typically had long, low-frequency (1 Hz) codas and focal depths of 0.5-2 km at points below and alongside the caldera.

Volcano-tectonic earthquakes were predominantly of low magnitude (under M 1). A larger event, M 2.2, occurred on 5 November; this was the first event of this type and magnitude in over a month. It was located 2 km W of the caldera margin at 7 km below the surface. The focus was close to, and possibly related to, a major structural lineament that trends NE across the Chaitén substructure at depths below 5 km.

Regarding the town of Chaitén. Heavy rains struck the region on 3 November, and, by 5 November, flooding had inundated parts of the town on the S banks of the Chaitén (Blanco) River (figure 21). One of the drainages with headwaters in the caldera feed into this river, and the moving water carried great quantities of volcanic material downstream. Lahars followed the river, leaving abundant sediments along their path, and they reached the sea at the river's mouth.

Figure 21. The sediment-charged Chaitén river seen in flood on 5 November 2008 looking towards the S side of the town of Chaitén. At distance appear intact portions of the town on higher ground. The river passes E and S of the town and then enters the sea at Chaitén bay on the Corcorvado gulf (see figure 13 in BGVN 33:06 and maps in various issues). Extensive tidal areas lie SE of the town. Taken from ONEMISERNAGEOMIN (2008d).

According to ONEMI-SERNAGEOMIN, during dry conditions, both wind and vehicular traffic in Chaitén kicked up fine ash, which remained suspended in the atmosphere. This ash had obliged the town's few remaining inhabitants to don dust masks. The requisite masks needed to filter out material down to 5 microns in diameter.

References. IFIS (Internet Flight Information Service), 2011, Location map showing camera at Chaitén airfield (URL: http://www.aipchile.cl/aerodromo/ubicacion/designador/SCTN; Images of the volcano, http://www.aipchile.cl/camara/show/id/14); Images of the river and town, http://www.aipchile.cl/camara/show/id/15).

Carn, S. A., Pallister, J. S., Lara, L., Ewert, J. W., Watt, S., Prata, A. J., Thomas, R. J., and Villarosa, G., 2009, The Unexpected Awakening of Chaitén Volcano, Chile, Eos, Transactions, American Geophysical Union, Vol. 90, No. 24, 16 JUNE 2009, pp. 205-206.

Nadeau, P. A. and Williams-Jones, G., 2008, Beyond COSPECRecent advances in SO₂ monitoring technology. In: Williams-Jones, G., Stix, J. & Hickson, C. (eds.) The COSPEC Cookbook: Making SO₂ Measurements at Active Volcanoes. IAVCEI, Methods in Volcanology, 1, 219-233.

OVDAS-SERNAGEOMIN, 2008a, Erupción del volcán Chaitén, Cuadragésimo Quinto Informe Técnico, 31 de julio al 01 de agosto de 2008, 4 pp., Issued 1 August 2008 (in Spanish) [file name: Informe_45_Chaitén_al_01.08.08[1].pdf].

OVDAS-SERNAGEOMIN, 2008b, Erupción del volcán Chaitén, Informe Técnico No. 55, 23-30 de septiembre de 2008, 4 pp., (Issued 30 September 2008 (in Spanish) [file name: Informe_55_Chaitén_al_30.09.08[1].pdf].

OVDAS-SERNAGEOMIN, 2008c, Erupción del volcán Chaitén, Informe Técnico No. 58, 14-20 de Septiembre de 2008, 3 pp. Issued 20 October 2008 (in Spanish), [file name: Informe_58_Chaitén_al_20.10.08.pdf].

OVDAS-SERNAGEOMIN, 2008d, Erupción del volcán Chaitén, Informe Técnico No. 60, 31 de octubre-07 de noviembre de 2008; 3 pp., Issued 8 November 2008 (in Spanish), [file name: Informe_60_Chaitén_al_07.11.08.pdf].

Information Contacts: Oficina Nacional de Emergencia - Ministerio del Interior (ONEMI), Beaucheff 1637 / 1671, Santiago, Chile (URL: http://www.onemi.cl/); Observatorio Volcanológico de los Andes del Sur-Servico Nacional de Geologia y Mineria (OVDAS-SERNAGEOMIN), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); 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/productos.php).

November 2011 (BGVN 36:11) Cite this Report

Declining vigor; magma ascent rate; disaster recovery in the town of Chaitén

The previous report on Chaitén volcano, Chile (BGVN 35:12) noted that a second new lava dome was reported on 4 November 2008. The Alert Level remained at Red (the highest of the alert level system) from the onset of the eruption on 2 May 2008 through April 2010. This report will chronologically summarize the growth of the new lava domes and major eruptive events. In general, a gradual decline in activity occurred during November 2008-September 2011. The Alert Level stood at Yellow during May 2010-May 2011, and was then lowered to Green, where it remained through the end of 2011. Most of the information in this report comes from weekly to monthly reports from the Observatorio Volcanológico de los Andes del Sur-Servicio Nacional de Geología y Minería (OVDAS-SERNAGEOMIN) with collaboration from the Oficina Nacional de Emergencia - Ministerio del Interior (ONEMI). Finally, the current status of the town of Chaitén (which was abandoned by all but a handful of residents who continued to live there without electricity or running water) will be discussed. The potential hazards from dome-collapse-generated block-and-ash flows and lahars from remobilized volcanic deposits were persistent throughout the reporting period.

Portions of this report were initially synthesized and edited by Nick Legg (covering November 2008-March 2009) and Eduardo Guerrero (covering April 2009-July 2009), as part of a graduate student writing assignment in a volcanology class at Oregon State University under the guidance of professor Shan de Silva.

New lava domes observed. Initially occupying the caldera of Chaitén was the lava dome ('old dome') that existed prior to the 2 May 2008 eruption (and for the ~ 9,400 years since the previous eruption). A lava dome ('Dome 1') first observed on 21 May 2008 and a new lava dome ('Dome 2') confirmed on 4 November 2008 during an overflight (BGVN 35:12) were extruded on top of the old dome (figures 22 and 23). A third phase of dome extrusion ('Dome 3') was observed on 29 September 2009.

Figure 22. December 2008 aerial photograph of the Chaitén dome complex taken looking approximately SE into the caldera. The domes were emplaced on top of the old dome, emplaced ~9,400 years ago. Courtesy of OVDAS-SERNAGEOMIN.

Figure 23. Satellite photograph of Chaitén and surroundings, acquired on 30 September 2009. (a) Chaitén volcano (top right corner), Chaitén (Blanco) River, and their proximity to the town of Chaitén (bottom left). Note the significant lahar deposits at the mouth of the Chaitén river and cutting through the town of Chaitén. Inset index map shows Chaitén's location in Southern Chile. (b) Enlarged, annotated view of Chaitén volcano. Dome 1 is in the W side of the caldera and Dome 2 is in the NNE part. Dome 3 is not visible in this photograph. Satellite photograph courtesy of NASA Earth Observatory; index map modified from MapsOf.net.

November 2008-April 2010 (Red Alert). Following the confirmation of the existence of Dome 2 on 4 November 2008, a lateral explosion occurred on 17 November, directed WSW. The explosion was not constrained to either Dome 1 or 2 alone, but was associated with a collapse resulting from continued dome extrusion. By 6 December 2008, both active domes had exceed the height of the caldera rim (Dome 1 by ~ 250 m and Dome 2 by ~ 350 m). SERNAGEOMIN reported an increase in the growth of Dome 2, corresponding to a temporally slight increase in both hybrid (HB) and volcano-tectonic (VT) earthquakes.

Plumes continued to vary in color, indicating varying contributions of steam and ash from both new domes. By 9 January 2009, an observational overflight revealed that the inner caldera had been filled by Domes 1 and 2, and growth of sharp spines or pinnacles was reported.

On 19 January, a major collapse of the Dome 2 summit spines occurred, producing block-and-ash flows that traveled down the SE and E flanks of Chaitén. After the collapse, observers noted a decrease in seismicity and slowed dome extrusion.

During a flyby facilitated by ONEMI and the Chilean Air Force on 21 January 2009, researchers from the University at Buffalo-State University of New York (UB-SUNY) and the University of Chile acquired thermal images of the dome complex (figure 24). The thermal images indicated that, while Dome 1 still had areas of elevated temperature, the highest temperatures (greater than 150°C) corresponded to the pinnacles at the summit of Dome 2 (figure 24a). Images of pyroclastic flow and rockfall deposits resting within the E part of the caldera disclosed elevated temperatures there as well (higher than 80°C in places, figure 24b). Their research on thermal imaging of Chaitén will be showcased in an upcoming publication.

Figure 24. Thermal images of the dome complex of Chaitén acquired on 21 January 2009. (a) The summit of Dome 2, where the highest temperatures were recorded; the same area corresponded to the pinnacles and the area of the most rapid growth of Dome 2. (b) Pyroclastic flow and rockfall deposits within the E part of the caldera, emplaced just prior to 21 January, that were still elevated in temperature. The colors are scaled uniquely for both images with the temperature scales shown at right. The flyby was facilitated by ONEMI and the Chilean Air Force, the images were acquired by Patrick Whelley (UB) and Andrés Paves (University of Chile), and processing was performed by Marc Bernstein (UB); use of the thermal camera was courtesy of Eliza Calder under the UB-SUNY Research Foundation.

By 9 February 2009, a new high-standing pinnacle atop Dome 2 indicated the return of rapid dome growth. This did not coincide with significant increases in seismicity.

On 19 February, a large partial collapse generated pyroclastic flows that traveled down the Chaitén River towards the town of Chaitén (see annotated photo below). This event produced a plume reaching 9.1 km altitude according to the Buenos Aires Volcanic Ash Advisory Center (VAAC). The plume was white (indicating high water vapor content) at the top, and contained abundant ash at the base. The collapse created a scarp measuring approximately 500 m by 500 m. Increased seismicity occurred on the same day; background volcanic tremor occurred from 1028 until 1346, and swarms of earthquakes ranging from M 3.6-4.2 originated at depths of 3-5 km. Due to the amount of material deposited from the collapse (~10 x 10⁶ m³), SERNAGEOMIN reported a significantly higher than normal danger of lahars.

On 25 February 2009, dome growth focused on Dome 1, although seismicity had gradually decreased in frequency (excluding the outstanding events of mid-February discussed above) compared to November-December 2008 values. Despite the decrease, on the afternoon of 3 March, dome collapses occurred every ~ 40 minutes. Within the next few days, SERNAGEOMIN reported that "VT's almost disappeared." A gradual decrease in seismicity continued until 24 March, when the frequency of HB earthquakes increased briefly, but they decreased again by early April.

On 6 April, a prominent spine of lava was observed on the S part of Dome 1 (figure 25). It indicated that dome growth was still concentrated in the W part of the caldera (Dome 1). The next week, the same spine was reported to have a wider base, and the crater reportedly glowed at night. In early May, an aerial photograph captured a close view of a very fractured central lava spine of the dome complex (figure 26).

Figure 25. Images taken by Dirección General de Aeronáutica Civil (DGAC) and a Channel 13 cameraman on 6 April 2009 showing a prominent spine of lava that had grown in the S area of Dome 1. Courtesy of OVDAS-SERNAGEOMIN.

Figure 26. Aerial view of the dome complex in early May 2009 showing a very fractured central pinnacle. Courtesy of Javier Romero.

In the later parts of May 2009, dome growth and seismicity continued to be focused on the W part of the caldera, associated with Dome 1. Although there was no significant increase in seismicity until a slight increase in June, witnesses in the town of Chaitén reporting feeling tremors in May. Otherwise, activity through August consisted of continuous ash-and-steam emissions, small collapses of unstable portions of the domes, and resulting block-and-ash flows. Seasonal precipitation remobilized previously erupted material and lahars reached the town in July.

Seismicity remained slightly elevated (relative to April and May) through September. On 14 September, a surveillance camera captured a plume as wide as the caldera reaching ~ 1.5 km high. This plume was significantly wider than plumes in the previous months. On 29 September, witness reports prompted SERNAGEOMIN to fly past the caldera, and obervers saw evidence of a significant recent collapse.

SERNAGEOMIN's 29 September observation flight provided stunning views of the dome complex leading to the detection of a new dome, Dome 3, that had filled the 19 February 2009 collapse scar (figures 27 and 28). Dome 3 had a depression in its N sector and small central pinnacles (figure 28). The central pinnacle of the whole complex had disappeared, and a large active depression, elongated to the NNW, had formed E of Dome 3. This depression reportedly resulted from either a lateral explosion or a relatively slow and structurally controlled internal collapse.

Figure 27. Aerial views of Chaitén’s dome complex taken following the partial collapses of (a) 19 February and (b) 29 September 2009. (a) The dome complex following the 19 February partial collapse. Dimensions of the central pinnacle (*) and the collapse scarp (**) are given at the lower left. (b) The dome complex following the 29 September partial collapse, showing the first observation of Dome 3. Labels in all capital letters indicate structures or deposits, and labels in all lowercase indicate relative amounts of water vapor and ash in emitted plumes. Courtesy of OVDAS-SERNAGEOMIN; (a) photo by Paul Duhart, (b) photo and interpretation by Jorge Muñoz.

Figure 28. Aerial view of the center of Chaitén's dome complex on 29 September 2009. The then-newly ob served central depression is the most active area in the photograph, and a small new pinnacle, Proto-pinnacle 2, is seen near this area. Labels in all capital letters indicate structures or de pos its, and labels in all low er case indicate relative amounts of water vapor and ash in emitted plumes. Courtesy of OVDAS-SERNAGEOMIN; photo and interpretation by Jorge Muñoz.

Following the events of September, no major outstanding activity was reported through the rest of 2009. In late January 2010, two new telemetered seismic stations were added to the monitoring network, increasing the number of seismic stations around Chaitén to ten. A new observation camera was also installed ~ 800 m from the dome complex.

During the end of 2009 and January 2010, the growth rate of the dome complex slowed, and seismicity declined significantly, with larger earthquakes (stronger than M 3.5) being absent through at least March (figure 29). Following a few months of relatively calmer activity at Chaitén, and after at least a month without emissions of ash, the Alert Level was lowered to Yellow on 1 May 2010.

Figure 29. Daily earthquakes between M 3.5 and M 4.5 at Chaitén, from the onset of the eruption on 2 May 2008 to 1 April 2010 (a month prior to the lowering of the Alert Level to Yellow). Courtesy of OVDAS-SERNAGEOMIN.

1 May 2010-2 May 2011 (Yellow Alert). During the period of Yellow Alert, reported plume heights remained below 2.1 km altitude (Buenos Aires VAAC); compared to the prior, more active period when plumes regularly reached 3-4 km in height, this was a significant decline. Over this period, the Buenos Aires VAAC reported occasional emissions that included ash (table 2). OVDAS-SERNAGEOMIN began recording mainly VT events (interpreted as relating to rock fracturing) and long period (LP) events (interpreted as related to fluid dynamics in and beneath the volcanic edifice). Both types of seismicity remained low throughout the remainder of 2010 and into 2011. However, incandescence of the lava dome surface was observed at night in late January 2011.

Table 2. Emissions from Chaitén's lava dome complex during the period of Yellow Alert (1 May 2010-2 May 2011); '--' indicates information that was not reported. Courtesy of the Buenos Aires VAAC.

3 May 2011-September 2011 (Green Alert). On 2 May 2011, the Alert Level was lowered to Green due to lower levels of activity since January 2011, including (1) seismicity remaining at low levels of occurrence and magnitude; (2) no significant emissions of ash; (3) a lack of dome growth and associated partial collapses; and (4) a lack of visual observations suggesting restlessness.

Since 3 May 2011, ash-free plumes rose no higher than 0.5 km; the exception was one plume in late May or early June. Seismicity remained low, with daily counts averaging fewer than 2 for LP events, less than 10 for VT events, and less than 1 for HB events.

Magma ascent prior to 2 May 2008 eruption. Wicks and others (2011) stated that "Because of the historically rare and explosive nature of rhyolite eruptions and because of the surprisingly short warning before the eruption of the Chaitén volcano, any information about the workings of the magmatic system at Chaitén, and rhyolitic systems in general, is important from both the scientific and hazard perspectives." There were only about 24 hours between the first felt seismicity in the town of Chaitén and the onset of the eruption. Such a short precursory period has been recorded for basaltic eruptions (e.g. Hekla volcano, Iceland; Soosalu and Einarsson, 2002), but not for silicic eruptions such as Chaitén (Castro and Dingwell, 2009).

Castro and Dingwell (2009) used petrological experiments to constrain the temperatures and decompression rates of the magma erupted explosively at Chaitén on 2 May 2008. Their results suggested that the magma rose from depths of at least 5 km in about four hours, shorter than the roughly 1-day period of seismicity that was felt in the town of Chaitén.

Wicks and others (2011) interpreted radar interferometry observations to indicate that the rapid ascent (as reported by Castro and Dingwell, 2009) of the Chaitén magma was controlled by pre-existing faults in the crust beneath Chile (figure 30). Specifically, they modeled a large, dipping, sill-like body (their "reservoir") residing under Minchinmávida volcano (~ 20 km E of Chaitén, "M" on figure 30) and rising towards the surface to the W of Chaitén (Morro Vilcún, "MV" on figure 30). The magma followed a path that intersected an inferred vertical conduit feeding Chaitén. They interpreted the rhyolitic reservoir as originating from either 1) a combination of tectonic stresses and magmatic overpressure draining rhyolitic magma from a mafic reservoir beneath Minchinmávida, or 2) an event similar to the M 9.5, 1960 Chilean earthquake creating permeability and a pressure gradient, allowing the overpressured magma to migrate to beneath Chaitén.

Figure 30. Cross-sectional model of the magmatic system that ultimately erupted at Chaitén (Wicks and others, 2011). The profile A-A' is approximately W-E; Morro Vilcún (MV), Chaitén (Ch), and Minchinmávida (M) are plotted at their relative positions on the surface. Stages a, b, and c are illustrated by a series of model cartoons. (a) The sill-like body (Reservoir) extends to the W, towards the surface, from the magma chamber beneath Minchinmávida; the dike (red body) begins propagation upwards, but at this stage has not intersected Chaitén's conduit. (b) Diking leads to an injection-caused earthquake (inflational, shown by the moment tensor solution "beachball diagram" where black indicates compression and white indicates tension) which occurred 2 hours before the 2 May 2008 eruption began. (c) As the eruption progressed and drained the shallow storage beneath Chaitén, the sill-like reservoir collapsed (shown by the moment tensor solution diagram). In all frames, the Liquiñe-Ofqui Fault Zone (LOFZ) is indicated beneath Minchinmávida volcano. CMT 1 refers to the centroid mean solution (CMT) of a moment magnitude 5.2 earthquake that occurred 2 hours prior to the main eruption on 2 May 2008. CMT 2 refers to an earthquake of magnitude 5.0 that occurred 19 hours after the onset of eruption. From Wicks and others (2011).

Restoring the town and damaged infrastructure. On 9 April 2011, the Chilean government reported that President Sebastián Piñera had announced the "North Chaitén Solution" plan. After restoring basic services (e.g. electricity) to the town of Chaitén in the first months of 2011, President Piñera stated that "everything will be definitively restored, electricity and light has already returned and ... all of the electricity poles are new." He also announced plans to dredge the harbor a second time (necessary due to the amount of remobilized volcanic material deposited in the bay) to allow boats to dock, a plan to install a floating dock, ongoing surveying for new paved roads to other surrounding cities, and plans for the town's school and aerodome.

References. Castro, J.M., and Dingwell, D.B., 2009, Rapid ascent of rhyolitic magma at Chaitén volcano, Chile: Nature, v. 461, p. 780-784 (DOI:10.1038/nature08458).

Soosalu, H., and Einarsson, P., 2002, Earthquake activity related to the 1991 eruption of the Hekla volcano, Iceland: Bulletin of Volcanology, v. 63, p. 536-544.

Wicks, C., de la Llera, J.C., Lara., L.E., and Lowenstern, J., 2011, The role of dyking and fault control in the rapid onset of eruption at Chaitén volcano, Chile, Nature, v. 478, pp. 374-377 (DOI:10.1038/nature10541).

Information Contacts: Observatorio Volcanológico de los Andes del Sur-Servico Nacional de Geologia y Mineria (OVDAS-SERNAGEOMIN), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); Oficina Nacional de Emergencia - Ministerio del Interior (ONEMI), Beaucheff 1637 / 1671, Santiago, Chile (URL: http://www.onemi.cl/); NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/); MapsOf.net (URL: http://mapsof.net/); Marc Bernstien, Eliza Calder, and Patrick Whelley, University at Buffalo, State University of New York, 411 Cooke Hall, Buffalo, NY 14260; Andrés Paves, University of Chile, 2002 Blanco Encalada, Santiago, Chile; 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/productos.php); Javier Romero, Dirección de Vialidad - Ministerio de Obras Públicas, Puerto Montt; Gobierno de Chile, Santiago, Chile (URL: http://www.gob.cl/ or http://www.gob.cl/english/).

August 2013 (BGVN 38:08) Cite this Report

Declining seismicity, thermal emissions, and plumes during 2013

In this report, we cover declining activity observed at Chaitén during 2012-2013. The Servicio Nacional de Geología y Minería (SERNAGEOMIN) had reduced the Alert Level to Green on 2 May 2011 (BGVN 36:11) and continued to monitor activity and release online bulletins. These activity bulletins were available during February 2012-October 2013. We also note the thermal alerts from MODVOLC during January 2010-April 2013.

The recent rhyolitic eruption of Chaitén began in May 2008 (BGVN 33:04) and, according to Pallister and others (2013), comprised five major phases: (1) an explosive sequence (1-11 May 2008), (2) a transitional time that included ash plumes and lava extrusion (11-31 May 2008), (3) lava flows and dome growth (June-September 2008), (4) spine extrusion and dome growth (October 2008-February 2009), and (5) endogenous lava dome growth (February 2009-early 2010). Following those phases of activity, ash plumes and thermal emissions declined during 2010-2011 (figure 31); intermittent thermal anomalies and vapor emissions dominated activity during 2011-April 2013. Since April 2013, thermal alerts were no longer reported by the MODVOLC system.

Figure 31. On 25 October 2011, this Landsat 5 image captured a small plume rising from Chaitén's active lava dome. A) This wideview includes the caldera and surrounding region of Chaitén. Meandering river systems link the volcanic highlands with the Pacific coast. A) and B) The town of Chaitén is located at the red circle, on the delta formed by runoff from the highlands that include Chaitén ~11 km NE. Note that the scale bars are approximate. Courtesy of USGS/NASA.

SERNAGEOMIN reported decreasing seismicity during 2012-2013 (table 3). Volcano-tectonic (VT) events dominated these datasets while hybrid events were very rare. Local magnitudes (ML) and magnitudes calculated from signal duration (MD) were reported as well as the reduced displacement (calculated for long-period earthquakes).

Table 3. Seismicity during February 2012-September 2013 is reported for total earthquake counts (EQ), volcano-tectonic earthquakes (VT), long-period earthquakes (LP), and hybrid (HYB). Note that "**" indicates reporting of M_L values where M_D values were not reported. Reduced displacement (RD) is reported for LP events. Courtesy of SERNAGEOMIN.

When clear viewing conditions permitted, nighttime incandescence and vapor plumes were captured by the SERNAGEOMIN webcamera during February 2012-September 2013. Observations included plume heights ranging 300-600 m above the crater.

Thermal alerts. Elevated temperatures were detected from Chaitén during much of the eruption that began in May 2008. Thermal anomalies were detected by MODIS sensors on the Terra and Aqua satellites every month until September 2010 (table 4). As the eruption waned, elevated temperatures were observed less often and were absent during the months of July 2011, May 2012, August-October 2012, December 2012, March 2013, and after 2 April 2013. The MODVOLC alerts chiefly occurred in the immediate area of the crater (figure 32).

Table 4. The MODVOLC system generated alerts during January 2010-April 2013. The number of thermal alerts detected per year decreased from 2010 through early 2013. Courtesy of HIGP.

Figure 32. As seen here, from April 2012 to April 2013, MODVOLC generated alerts (red and orange point) on 15 days for Chaitén. Those alerts indicated elevated temperatures within 2 km of the summit area. Courtesy of HIGP.

References. NASA Landsat Program, 2011, Landsat ETM scene LT52320902011298COA00, SLC-Off, USGS, Sioux Falls, October 25, 2011.

Pallister, J.S., Diefenbach, A.K., Burton, W.C., Muñoz, J., Griswold, J.P., Lara, L.E., Lowenstern, J.B., and Valenzuela, C., 2013, The Chaitén rhyolite lava dome: Eruption sequence, lava dome volumes, rapid effusion rates and source of the rhyolite magma, Andean Geology, 40, (2): 277-294.

Information Contacts: Observatorio Volcanológico de los Andes del Sur-Servicio Nacional de Geologia y Mineria (OVDAS-SERNAGEOMIN), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); and 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/).