Chaiten Volcano

Chaiten volcano (962m) is located in Southern Chile. The 2008 eruption represents the first historical activity at the volcano. Prior to the present eruption, the volcano consisted of a 2.5x4km diameter caldera (or explosion crater) largely filled by a rhyolitic obsidian lava dome. 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.

The 2008 Plinian eruption began on the 2nd of May with emission of ash to an altitude of up to 20km from two vents (which merged on the 6th May) on the N side of the old lava dome. A brief period of seismic unrest was noted on the day preceeding the eruption, although minor tremors may have occured unnoticed much earlier since the volcano was not being monitored. Ash was more or less continuously erupted to an altitude of around 10km on the following days, until on May 6th the intensity of the eruption increased significantly, with ash clouds reaching an altitude of 30km (according to NASA). It is presumably during this phase of the eruption that pyroclastic flows swept down the N flank of the volcano, possibly as a result of a partial column collapse or laterally directed explosive activity. The intensity of the eruption has since waned slightly although ash clouds reaching altitudes of up to 10km were frequently observed until May 21. Ash venting has continued to lesser heights of several km in the following weeks. Significantly, as from May 21, dome extrusion 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 have since opened on the W (8 May), SE (13 May) and S (as from 12 June) sides of the old dome. By July, activity had largely shifted the the S where a new dome had completely covered the S side of the pre-2008 dome. Activity in the S sector is problematic as the town of Chaiten lies S of the volcano. Further, it appears that the drainage from the crater is being disrupted. This poses a serious threat to the town, since if rainwater accumulates in the crater and is suddenly released, this could lead to a massive Lahar.

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 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.

An increase of seismicity on July 23 suggesting movement of magma into the zone under the volcano, together with increased ash venting has led to fears that a second climactic phase of the eruption could lie ahead. It would not be unusual for a dome-building eruption to involve multiple 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.

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 demonstrate that the N flank remains a high risk area whilst dome-building activity continues 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. The images below (taken on 09.07.08) are possibly the first published images from the ground on the W side of the devastated area. 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.

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.

Chaiten Volcano Erupting Steam and Ash, 9th July 2008	Pyroclastic Flow Damage, N Flank	Pyroclastic Flow Damage, N Flank

Pyroclastic Flow Damage, N Flank	Pyroclastic Flow Damage, N Flank	Edge of Pyroclastic Flow Impacted Area

Trees Uprooted by Pyroclastic Flows	Pyroclastic Flow Damage, N Flank	Pyroclastic Flow Damage, N Flank

Healthy Forest, NW Flank. Damaged Forest Behind.	Drainages Redirected by Eruption Caused Erosion of Road N of Volcano.

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, with the banks of the river being overflown for the first time on 12.05.08 and on several subsequent occasions. This has resulted in the burial or destruction of many properties, especially near the mouth of the river, flooding of up to 90% of Chaiten town and the nearby airport, and the formation of a wide sediment delta at the mouth of the river which has filled the harbour with sediments. Fortunately, a small dock remains at the N end of town. Whilst one bridge was swept away, the main bridge has survived so far.

Whilst residents were allowed to return to salvage belongings in early July, the town essentially remains 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.

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.

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 Chaiten Town	Lahar Deposits in Chaiten Town

Preparing to Remove Belongings from Supermarket	Channels Made to Allow Drainage	Inhabitants Bringing Belongings to Dock

Lahar Deposits inside Supermarket	Lahar Deposits inside Supermarket	Lahar Deposits inside Supermarket

New Delta Formed by Lahar Deposits at River Mouth	Damage to Coastal Road (Effect of Tremors/Flooding ?)

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. On May 28, for example, ash carried at low altitudes by winds blowing in a northerly direction led to temporary closure of several airports in Chile.

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

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