Chaiten Volcano

Chaiten volcano (962m (pre-2008)) is located in Southern Chile. Until 2008, the volcano consisted of a 2.5x4km diameter caldera (or explosion crater) largely filled by a rhyolitic obsidian lava dome. No historical activity had being recorded until the volcano suddenly burst into life in 2008. The present eruption is of great scientific interest since it is the first rhyolitic eruption since Novarupta in 1912. Rhyolitic lava is extremely viscous since it contains high levels of silicate (69-84%). The last eruption before 2008 can be approximately dated by analysing its deposits. Pebbles of obsidian originating from the dome have been dated at around 5600 years old (Stern et al., 2002, Anales del Inst. de la Patagonia Vol.30, p.167-174). A pyroclastic surge and tephra fall deposit N of Chaiten has been dated at 9370 years old based on charcoal contained therein (Naranjo and Stern, Rev. Geol. de Chile 31(2), p.225-240 (2004)). Tree-trunks under the deposit were dated at 9810 years old. The surge deposit is 3.5m thick 25km N of Chaiten volcano and 1.5m thick at a distance of 40km and is overlain by a tephra layer of 1.6 and 0.3m, respectively. The tephra layer largely consists of rhyolitic pumice and lapilli and is coated by a small deposit of mafic scoria. The different layers probably result from distinct phases of a massive eruption which formed the crater in which the obsidian dome subsequently developed. It is also possible that the large rhyolitic (Mic2) deposits 20km E of Chaiten and previously attributed to Minchinmavida (=Michinmahuida) volcano (Naranjo and Stern, 2004) also derive from Chaiten, since they are compositionally related to other Chaiten eruption products, but dissimilar to other Minchinmavida eruptate deposits (Watt et al. 2009. J. Geophys. Res. 114, doi:10.1029). However, these were dated at only between about 1850 and 3800 years old which seems problematic in view of the over 5000 year dome sitting over the main vent of Chaiten. Even if only one of these deposits is attributable to Chaiten, it shows that the volcano is capable of eruptions of a significantly greater magnitude than the present one.

Chaiten Lava Dome viewed from North Caldera Rim on 6. Dec. 2009 (Stitched Panorama Image). Dome was in phase of degassing and low level extrusion.

The course of the 2008 Plinian eruption has been described in detail (Lara 2009. Andean Geol. 36(1), p.125-129; Carn et al. 2009. Eos Trans. 90(24), p.205-206). The eruption began at 4:00 local time on the 2nd of May with a Plinian phase lasting for about 6 hours during which ash was emitted to an altitude of up to 20km. At the height of the eruption, sheaths of lightning surrounded the eruption column. It is thought that the distribution of electrical discharges around the column in such large eruptions is facilitated by the formation of a rotating volcanic mesocyclone (Chakraborty et al. 2009. Nature 458, p.497-500). By May 3rd, seismicity declined abruptly, possibly due to erosion of a conduit to the surface, yet sub-plinian ash-emissions to altitudes of around 10 km continued unabated. The subplinian activity was punctuated by two further Plinian phases with onsets of May 6 (8:32 local) and May 7 (23:36 local) with column heights reaching around 20 and 13 km, respectively. It is presumably during the Plinian phases of the eruption that first pyroclastic flows swept down the flanks of the volcano, possibly as a result of a partial column collapse or laterally directed explosive activity (however first direct observations of damage to the N flank forests as photographically documented below were on May 12 (BGVN 33:05)).

Activity was initially from two vents (which merged on the 6th May) on the N side of the old lava dome. Following the 3rd Plinian phase, the intensity of the eruption gradually declined although ash clouds reaching altitudes of up to 10km were frequently observed until May 21. Ash venting continued to lesser heights of several km in the following weeks. Significantly, as from May 21, dome extrusion ("Dome 1") was observed on the N flank of the old dome. Eruptions through the dome had transformed it into a large tephra cone by the end of May. Additional vents since opened on the W (8 May), SE (13 May) and S (as from 12 June) sides of the old dome. The USGS has released estimates of the amount of material extruded at Chaiten in the first months of the eruption. In June and July, approximately 37 and 60 cubic meters / sec were extruded on average, respectively. These extrusion rates far surpass those recorded during phases of dome growth at the much studied Soufriere Hills volcano. By July, activity had largely shifted to the S where a new dome had completely covered the S side of the pre-2008 dome. Activity in the S sector is particularly problematic as the town of Chaiten lies S of the volcano. Further, the drainage from the crater has been disrupted. This may pose a serious threat to the town, since if rainwater accumulates in the crater and is suddenly released, this could lead to a massive Lahar. By late September, about 0.5 cubic km of new dome material had been emplaced. A second dome ("Dome 2") was reported developing at its NE side as from November 2008, following a period of slightly heightened activity. On Feb.19 2009, a partial dome collapse largely involving the south flank of Dome 1 resulted in a lateral blast and a PF flow which extended about 5km along the Chaiten River towards the town. Ash plumes rose to about 10 km altitude. About 10 Million cubic meters of dome had collapsed leaving a large 500x500m scar. Dome growth continued near the top of the scar leading to some spectacular spires and the formation of a complex sometimes referred to as Phase / Dome 3. Activity has continued with dome growth and intermittent episodes of steam and / or ash venting over the following months. By December, the spires had largely degraded and the dome displayed a relatively flat-topped morphology.

Images of lava dome from North Caldera Rim (taken on 6. Dec. 2009):

Overview	NW flank of dome	Overview

NE summit of dome	NE summit of dome

	NW part of caldera / dome

	NW summit of dome (audibly extruding during visit)

Remains of extrusions	Remains of extrusions

	Remains of extrusions

Dome, damaged vegetation on caldera rim.	Dome, damaged vegetation on caldera rim.

Dome, damaged vegetation on caldera rim.	Pillars of lava in eastern caldera rim

Images of lava dome taken from Chaiten town (Nov./Dec. 2009):

South side of dome - note: right side not contained by caldera wall	Increased steam emission following period of heavy rainfall

Incandescence visualized by long exposure photography (early morning 28. Nov. 2009)	Locations of weak incandescence (evening 6. Dec. 2009)

A brief period of seismic unrest preceeding the onset of the eruption with first notable quakes occurring about 24 hours beforehand, although retrospective analysis of seismic stations over 300 km further N revealed volcano-tectonic earthquakes in the region of Chaiten starting on 30. April (Basualto et al. 2008. Eos Trans. AGU, 89(53), Abstr. V43D-2179). The rapid onset of the eruption, with the first local signs of seismic unrest being noticed by the inhabitants of Chaiten only hours before the onset of the eruption, appears to be attributable to an extremely rapid ascent of magma (Castro and Dingwell 2009. Nature 461, p.780-783). This was deduced from geochemical studies on pumice samples, which revealed that the magma had been stored at a pressure of over 120 MPa, and had decompressed at a rate of 40MPa/hour. This was correlated with a magma chamber at a depth of about 5km and an ascent rate of about 1 m/sec. Thus it appears that it took 4 hours or less for the magma to ascend. This ascent rate is unprecedented for historically monitored silicic eruptions and raises concerns about the possibility of warning the local population if a similar phenomena occurs elsewhere. To what extent the activity could have been detected with a sensitive seismic monitoring network can only be speculated on. The same study also determined that the magma had been relatively free of carbon dioxide and was probably water-saturated in view of the violence of the eruption.

It should be added that seismic activity about 10 km below Chaiten was recorded in 2005 (Lange et al. 2009. Tectonophysics 471(1-2), p.96-113). It is possible that this reflected precursory magmatic movements below the volcano. Based on surface deformation data acquired by satellite-based radar interferometry, a model has been proposed which can explain the rapid magma ascent (Wicks et al. 2011. Nature 478, p.374-378). Basaltic-andesitic magma influx maintains a 20 km deep rhyolite-containing magma body under nearby Michimahuida (located 15 km ENE of Chaiten) which is responsible for volcanism at Chaiten. A sill-like body which rises at an average angle of about 45 degrees connects the deep body with the zone under Chaiten. It appears that in the past magma has extended further along the sill as it apparently surfaces W of Chaiten where several pre-historic rhyolitic plugs, including Morro Vilcun are found. Under Chaiten, the sill is at a depth of 8-12 km. Here, a small vertically rising conduit is proposed. It is suggested that rapid rise of the magma prior to the eruption occurred via formation of a dyke which extended almost vertically upwards over a significant length of the magma-containing sill. An earthquake about 2 hours prior to the Plinian eruption is attributed to dyke propagation.

Samples of ash from the eruption collected from various locations over the argentinian border (i.e. 100km downwind) in mid-May were analysed by the Servicio Geologico Minero Argentino and found to have silicate levels from 65% (dacite) up to 75% (rhyolite). Most samples had around 75% silicate, making them medium-silica rhyolites. The grain size of ash collected from similar locations has been assessed and it was found that approx. 10% had a grain size below 4microns, another 10% was from 4-10, and a further 7% was from 10-15microns (Inst. Haz. & Risk Res. Univ. Durham, UK). These microscopic ash particles present a health risk, ranging from irritation of the respiratory system to bronchitis or asthma, and following long-term exposure to the smallest particles, also chronic lung disease. The size distribution of the particles is similar to those analysed from Soufriere Hills volcano on Montserrat or Merapi on Java (Horwell, J. Environ. Monitor. Vol.9, p.1107-1115, (2007)). The compositional nature of the ash samples has varied little during the course of the eruption, suggesting that there may be little or no involvement of mafic magma. Indeed, trace element data suggests that the magma is highly evolved due to long-term pre-eruptive storage and significant assimilation of Patagonian batholith granitoids from the rocks surrounding the magma chamber(s) (Watt et al. 2009. J. Geophys. Res. 114, doi:10.1029).

The determination of rhyolitic material is particularly interesting, since rhyolytic volcanic activity is relatively rare. Due to the high viscosity of rhyolitic magma, such activity tends to be explosive in nature. The last significant rhyolitic eruption was that of the Alaskan volcano Novarupta in 1912. This was the largest eruption in the 20th century and involved eruption of 13 cubic km of magma in only 60 hours. The magma was largely drawn from a magma chamber under nearby mount Katmai, the top of which collapsed during the eruption, forming a 2.5km wide caldera.

It is unclear what lies ahead, yet it would not be unusual for a dome-building eruption to involve further eruptive climaxes. A further eruption similar to that of May 6 could directly threaten Chaiten Town if a significant collapse of the eruption column occurs. This however seems unlikely in the short-term as pressure is constantly being relieved from the system by extrusion and degassing. Further, the location of much of the dome within the caldera should make it relatively resistant to gravitational failure which has been a major factor in triggering powerful explosive episodes during the extrusive eruption at Soufriere Hills Volcano. However, the Caldera rim has already been overgrown on the critical southern flank. During 2010 and 2011, Chaiten has been rather quiet and stable and the dome has cooled somewhat. It is possible that the eruption is completely over.

Destruction on N Flank

The N flank of the volcano has been hit by pyroclastic flows during the climax of the eruption on 06.05.08. The damage to the forest on the N flank was first documented during overflights by SERNAGEOMIN personel in the following days. Further small PFs were observed in the area during a May 21 overflight and demonstrated that the N flank remained a high risk area whilst dome-building activity continued in its vicinity. By June 3, SERNAGEOMIN reported that 2500 hectares of forest had been destroyed by pyroclastic flows on the N and NE flanks of Chaiten volcano. I first visited this location on 9. July 2008. Most trees were snapped off a couple of meters above the ground. The 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.

I climbed the north flank on 3. and 6. Dec. 2009 after gaining permission from the Parque Pumalin administration. Some steep areas on the flank had evidently been stripped of vegetation and soil down to the underlying rocks. In most areas the forest had been flattened but not burned. Only near the caldera rim did some tree trunks have small charred areas. Especially near the caldera rim the bark of tree-trunks that had partly remained standing had been stripped completely off the side facing the volcano. Even about half-way up the flank, tree barks had been removed in places by the impact of projectiles embedded in the flows. In particular, pieces of obsidian (presumably mobilized from the old dome) could be found embedded in tree-trunks at several places.

Rough Sketch of Area Around Chaiten Volcano and Impact of Eruption (North at top, area covered approx. 15x15 km). Red arrows indicate areas most affected by Pyroclastic Flows (PFs). Brown areas indicate washing of mud into Rio Blanco during periods of rainfall, generating mudflows (Lahars) which inundated the center of Chaiten Town.

Forest destroyed by pyroclastic flows	Forest destroyed by pyroclastic flows	Tree trunk torn off by pyroclastic flow

Pyroclastic Flow Damage, N Flank	Tree with bark removed on side facing PF	Edge of Pyroclastic Flow Impacted Area

Small rock embedded in tree-trunk by PFs	Tree with bark removed on side facing PF	Slope stripped of forest, upper N Flank

View over forest destroyed by PFs	Upper slopes partially stripped down to bedrock

First plant (bamboo) reestablishing itself in PF zone	Flattened forest at base of N flank

View of singed forest in July 2008 with ash eruption in background	Overview of N flank PF zone

Caleta Gonzalo road, brown singed forest near Chaiten, glaciated Minchinmavida volcano	Eroded road to Caleta Gonzalo (July 2008). Location approx. middle left edge of image above.

Impact of Eruption on Chaiten Town

The Chilean government declared a state of emergency on May 2 and started evacuation of residents of Chaiten and surrounding settlements. All remaining residents in a radius of 50km from the volcano were instructed to leave on May 6, and on 19 May, a Court Order was in place allowing forcible evacuation of those remaining if necessary. A total of about 10,000 people were evacuated from the area around Chaiten. The town of Chaiten was not significantly hit by the eruption until heavy rainfall swept ash deposited in the crater and on the flanks of the volcano into drainages feeding Chaiten River (Rio Blanco). Large Lahars resulted, which gradually filled the original river-bed. The banks of the river were overflown for the first time on 12.05.08. Lahars flowed over wider areas in subsequent days and since its old bed was full the river changed course and flowed through the streets of Chaiten town. By the 15th, the Main Plaza, Harbour and Airport had been inundated. Within less than a week, the partial burial or destruction of many properties had occurred. Properties were particularly hard hit where the river enters town (i.e. Avenida Rio Blanco area) and downstream of the bridge, especially either side of Pillan road where the river took a (new) sharp turn to the left and eroded a new bed. Up to 90% of Chaiten town was inundated by lahars. A wide sediment delta formed at the mouth of the river and filled the main harbour with sediments. Fortunately, the ferry dock at the N end of town remained useable. Although the lahars reached the height of the bridge, it survived relatively unscathed (and is still in use). Using heavy earth-moving equipment, the banks of the Chaiten (or Blanco) River were shored up. Nevertheless further flooding occurred e.g. after heavy rainfall on 5. Nov. 2008. By mid-2009, the banks of the new river bed had been reenforced with large boulders and they withstood a prolonged period of heavy rainfall in early december 2009 which once again led to severe damage to the road linking Chaiten to Caleta Gonzalo further north (personal observation). This road has been frequently unuseable following the eruption, also restricting access to the Parque Pumalin nature reserve. This is the result of the fact that the road passes the base of the north flank of Chaiten volcano which was cleared of forest by pyroclastic flows during the climactic phase of the eruption. The forest-free land is unable to hold rainwater effectively resulting in rapid runoff which repeatedly has washed away the road leaving several meter deep gulleys.

Images from brief visit on 9. July 2008:

Control Post Road Leading N from Chaiten Town	Policeman Controling Papers	Police Car Patroling Streets of Chaiten

Work to Strengthen River Banks to Protect Town from Lahars	Work to Strengthen River Banks to Protect Town from Lahars

Chaiten Bridge, View Towards Town Center	View from Bridge Upstream	Lahar Deposits Downstream from Bridge

Lahar Deposits in Chaiten Town	Lahar Deposits in Chaiten Town

Lahar Deposits in Chaiten Town	Lahar Deposits in Supermarket	Lahar Deposits in Chaiten Town

Detailed images showing damage to town (December 2009):

Original river bed filled with sediments	Control tower with dome behind	Lahars flooded the entire runway of Chaiten airport

Light areas (top): old river bed going round back of town and airport runway (top left)	Lahar deposits by Rio Blanco	New course of the Rio Blanco river through town

View out of Escuela Italia towards volcano over area cleared by redirected river	Severely damaged buildings of Escuela Italia

New river bed cuts Avenida Libertad at height of Av. Pillan	Harbour area full of sediments on right

Riverside property missing section due to impact of lahar	Twisted branch lodged in window-frame by lahars

Partially buried house (Av. Rio Blanco)	Deep lahar deposits along Av. Rio Blanco	Houses tipped by lahars along Av. Rio Blanco

House deposited in sediment delta	House deposited in sediment delta

House deposited in sediment delta	Small fishing vessel stuck in lahar deposits

Buried window survived as not in most energetic lahar flow path	Window broken by debris in main lahar path

Doll partially stuck in lahar deposits	Buried sofa - Living room flooded by lahars

Whilst residents were allowed to return to salvage belongings in early July, the town essentially remained deserted, apart from patrolling police. Many residents have lost almost everything and accuse the government of poor prioritization during the evacuation, lack of warning regarding the lahars and of insufficient levels of compensation. Nevertheless after a while a number of residents returned to the town. However even basic services were missing and one of 70 inhabitants was reported to have died of hypothermia in April 2009. Since then more residents have returned (see section below on "Conflicts regarding the future of Chaiten town).

For comparison, the effect of Lahars on populated areas can also be seen at Soufriere Hills volcano on Montserrat, where the capital city Plymouth was buried.

Conflicts Regarding the Future of Chaiten Town

Whilst the Socialist Chilean government (as of Dec. 2009) is determined to settle the residents of Chaiten elsewhere and to abandon the present site of the settlement, many local residents are resisting these plans. The government plans to resettle many residents at Santa Barbara bay, 12km to the north. A provisional administrative headquarters and new airport have been set up at Santa Barbara but no significant progress has been made in erecting new homes in the swampy terrain.

Many residents have now returned to Chaiten town, against the will of the government. However, these people are being denied even the most basic services such as electricity, running water, fuel, hospital facilities, administrative and postal services. Further, no financial support is apparently given to those living in the town. However, given that Chaiten is an important portal to the surrounding region of Patagonia, ferry services are running nearly daily. Further, a number of shopkeepers have returned, so that by december 2009, 5 grocery stores were open and reasonably well stocked. Two guest houses and a hardware store also appeared to be open. Generators are being used to provide electricity in individual homes and firewood is readily available. However, with little employment and no schools, the future of the town really relies on a change of Government. The region used to have a thriving tourist trade in the summer season and could probably recover quickly if support is provided.

Whilst some houses were lifted from their foundations or buried deeply, the vast majority suffered only minimally from non-energetic lahars leaving sediments less than 50cm deep. Much of the southern third of town was largely unscathed, including the large and modern "Escuela Basica Chaiten Sur" school. Little critical infrastructure was lost (mainly houses and small parts of the "Escuela Italia" school). Locals report that the water system still functioned in parts of town but was shut down by the police. Further, the electricity network could apparently by restored in about 1 week. Some vandalism and looting has occurred, yet the renewed presence of locals and ongoing police controls is keeping this under control.

Whilst 2010 has seen the arrival of a new (conservative) government, the apparent promise that northern Chaiten would be rebuilt has now been superceded by further studies and favouring of the Santa Barbara site (situation July 2010, personal comm. Nicolas La Penna). A final decision is yet to be made, however the port is being dredged to maintain connectivity and river channeling is being improved. Further, the road to Pumalin has been largely repaired. However, water and electricity have not been reestablished. Nevertheless, local people, with the support of the town hall, have installed an alternative water system fed by a 65m deep well near the docks from where water is treated and pumped to storage tanks above the town. Most of the occupied houses have now been connected to these by a rudimentary pipe network.

"The goverment is denying us light and water, but here is our home"	"Chaiten lives. Do not sacrifice it"

"I love Chaiten" / "Welcome to the zone of zero" / "Zero light, zero water, zero support"	Grafiti on wall of hotel that burnt down due to lack of water for fire-fighting. Blaming government for destruction of livelihood

Sign of hope ? The playground dolphin has been dug out so children may play. The dolphin is a symbol of Chaiten.	Locals in front of only supermarket to open again in Chaiten cities southern district.

Safe zones have been designated in case of heightened activity.	Santa Barbara airstrip. Planes take off on a widened section of road.

Wider Impact

During the first days of the eruption, large volumes of ash were carried downwind to the SE of Chaiten. For example, 30cm of ash was deposited in Futaleufu, 65km ESE of Chaiten near the Argentinian border. Ashfall was also reported in the Argentinian town of Escuel and satellite images showed the ash cloud extending as far as the Atlantic in the following days. Ash from Chaiten has also disrupted air traffic. Chaitens small airport, 15km from the volcano, was rendered inoperational due to ashfall and lahars. The larger airport at the Argentinian city of Bariloche, 225km from Chaiten, was closed for many days during May and June 2008, resulting in cancellation of 158 flights (Guffanti et al. 2008. EOS Trans. AGU, 89(53), Fall Meeting Suppl., Abstract V42C-03). Further, during this period, five aircraft encountered ash near Bariloche. Repairs to one turbine cost about 2.5 million USD. Three of the ash encounters were at altitudes of below 2000 meters and may have involved ash remobilized by strong winds. Airolineas also suspended all flights to Esquel Airport, 120km E of Chaiten for 8 months due to the ashfall (BGVN Weekly Reports 7-13 Jan, 2009). The ashfall also caused flashover damage to insulators in several electrical distribution networks (Wilson et al. 2009. Portland Geol. Soc. Amer. Annual Meeting, Paper 164-10). Flashovers at the transformers at the Futaleufu Hydroelectric Power Plant caused interuptions in the supply to the Argentinian grid. Power lines were also damaged by trees falling onto them due to the weight of the ash collecting on them. The water supply of Futaleufu had to be switched from melt-water to ground-water leading to temporary shortages.

Subsequent detailed analysis of the distribution of ashfall in Argentina following the eruption revealed that it had been deposited in essentially 3 distinct lobes, resulting from changing wind conditions which carried ash to the SE, E and then NE as the first week of the eruption progressed (Watt et al. 2009. J. Geophys. Res. 114, doi:10.1029). Ash samples were collected from a multitude of sites in order to draw an isopach map showing what areas were coverd by different ashfall thicknesses. Comparative analysis revealed that MODIS satellite data was able to identify ashfall deposits as thin as 0.1mm, showing that remote monitoring of even minor ashfall is possible. The 1 and 0.1 mm isopach areas covered an area of 30000 and nearly 200000 square km, respectively. Ashfall units attributable to different eruptive phases were recognizable by virtue of their distribution and grain-size characteristics.

The climatic impact of the eruption is likely to be insignificant. The volume of erupted material was not high in historical terms and the level of sulphur dioxide emitted was low with only about 10 kilotons emitted in the three Plinian eruption columns. Rhyolites have low levels of sulphur and taking these into account only 0.1 cubic km of rhyolite would have been emitted during this phase of the eruption (Carn et al. 2009. Eos Trans. 90(24), p.205-212).

Ash on Pavement in Futaleufu near Argentinian Border	Compacted Ash on Road Leading out of Chaiten to East

Access Info for Visitors

Chaiten can be reached by ferry from Puerto Montt in southern Chile. The volcanoes flanks are largely densely forested making access from the south almost impossible. Access is in principle possible from the north after following the road to Caleta Gonzalez until near where the pyroclastic flows reached it. The area belongs to the Parque Pumalin and permission must be obtained before climbing. From the road it is possible to access the northern flank of the volcano where the forest has been destroyed by pyroclastic flows. The volcano is no longer active although sporadically shows signs of seismic unrest (situation Jan 2014) and sudden explosive episodes may occur without warning. Hence, climbing is extremely dangerous and not recommended. Further, climbing is arduous and should definitely not be attempted alone since the flank is covered in tree debris and sharp broken bamboo until near the caldera rim where it becomes steep and slippery due to a hard surface covered with loose fine volcanic debris.

Photovolcanica Full Index

Further Chaiten Images

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