Volcán Chaitén began a mighty explosive eruption on 2 May 2008, without any signs recognized as a warning. With a VEI of 4-5, Chaitén delivered a Plinian eruption – the most violent event recorded in Chile since the one of Quizapú volcano in 1932. The collateral damage of the event (mainly the generation of lahars and the subsequent overflow of nearby rivers) almost completely destroyed the city of Chaitén. SERNAGEOMIN consider Chaitén as the fourth most dangerous volcano in the country.

The coastal town of Chaitén lies 10 km SSW of the volcano, at the Gulf of Corcovado; it had a population of 5000 before the eruption.

Chaitén Volcano is situated near the southern end of the Andean Southern Volcanic Zone (SVZ). Volcanism here is controlled by subduction of the Nazca plate under the South American plate. Another important player is the 1,200-km-long Liquiñe-Ofqui fault zone (LOFZ). This active intra-arc fault system runs near or under many of the volcanoes in the central and southern parts of the SVZ. Chaitén lies along a forearc sliver between the subduction zone and the LOFZ. Deformation modeling suggested that the 2008 eruption of Chaitén was fuelled by magma residing along an ENE dipping reverse fault related to the LOFZ.

 

VOLCANO

Before the eruption, Chaitén was a small 3-km-diameter post-glacial caldera or explosion crater. It was filled with a weathered obsidian lava dome of rhyolitic composition. Almost all volcanoes in the SVZ erupt basalt and/or basaltic andesite. Chaitén was (and still is) the only volcano in the southern part of the SVZ known to have erupted a large volume of rhyolite.

Due to the perceived lack of eruptive activity of Chaitén during the Holocene, the volcano was not considered to be a significant threat. It was thought to have erupted last at 9370 (14C) BP. Therefore it was not actively monitored. A study in 2004 did not recognize tephra fall deposits from Chaitén east of the volcano. And yet, deposits of rhyolite pumice dated between 3,820 and 1,840 BP had been found in Argentina, near the city of Esquel – some 100 km to the E. These deposits were geochemically similar to the Chaitén rhyolite pumice. Alas, they were attributed instead to an eruption of nearby Michinmahuida Volcano, even though that volcano has never erupted rhyolite.

Meanwhile, through tephrachronology and radiocarbon dating, more previous eruptions have been identified. In a 2015 study, a research team analyzed the sediments of Lago Teo. They detected the remains of 26 past eruptions, 10 of which were from Chaitén volcano. Five of those again occured in historical time. The diagram below shows the thickness of volcanic sediment layers from Chaitén in Lake Teo:

Wicks et al. (2011) modeled the magma reservoir as a large, dipping, sill-like body residing under Minchinmávida volcano (~20 km E of Chaitén) and rising towards the surface to the W of Chaitén. 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 over pressure draining rhyolitic magma from a mafic reservoir beneath Minchinmávida, or 2) an event like the M 9.5, 1960 Chilean earthquake.

This would have created permeability and a pressure gradient, allowing the overpressured magma to migrate to beneath Chaitén. The volcano is located just east of, and in the middle of, the zone ruptured by the Mw 9.5 great Chilean earthquake of 1960.

Riddles and surprises

Why Rhyolite?
Chaitén’s ash, compared with other southern Andean volcanoes, suggests distinct evolutionary paths for the Chaitén magma. Its 2008 event was the first explosive rhyolitic eruption since Novarupta/Alaska in 1912.

Rhyolites are rare in this part of the southern Andean volcanic arc. Even so, rhyolite is the only magma type Chaitén has ever produced in significant volumes, and no mafic magma has been found in the current eruptive period. This highly evolved nature may be a result of long-term magma storage and melting of the granitoid basement. The question remains, however, why this situation is not found at other regional volcanic centers? The Hudson 1991 ash, the product of the only other historic large explosive eruption in the region, is far less evolved, with a broader compositional range.

Even though Chaitén had been studied inside and out after its great eruption, it still has its surprises in store: In a 2017 study, scientists described a small subsidiary lava dome that had been overlooked before. It is nested within an upper stream valley, less than two km SW of the present volcano. Lava flows of a few kilometers from this dome have been recognised and found to be of different, i. e. less silicic, rock composition. Also, ash deposits of this type had also been found further afield. The researchers concluded that here is another aspect of the possible future behaviour of Chaitén to keep an eye on – different magmas under a volcano can lead to different eruption patterns.

 

THE 2008 ERUPTION

There were only about 24 hours between the first earthquakes felt in the town of Chaitén and the onset of eruption. Yet, in this earthquake prone country, nobody thought of the volcano when the ground rattled. The more so as it – apparently – hadn’t erupted for almost ten thousand years. Also, such short precursory periods had been recorded for basaltic eruptions (e.g. Hekla in Iceland), but not for silicic volcanoes such as Chaitén. Researchers found out later that the magma had risen from depths of at least 5 km in about four hours!

After the initial confusion about where the earthquakes came from, more thorough analysis located them right under Chaitén volcano, which changed the whole scenario: one had to re-consider – the threat from a basaltic, non-explosive volcano versus from one “whose last studied eruption was as big as the one that buried Pompeii”.

Brief Summary of the Eruption
The rhyolitic eruption of Chaitén began on 1 May 2008 and can be described in five main 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 (Oct. 2008-Febr. 2009), and
5. endogenous lava dome growth (Febr. 2009-early 2010).

Following those phases of activity, ash plumes and thermal emissions declined during 2010-2011; intermittent thermal anomalies and vapor emissions dominated activity during 2011-April 2013. Since April 2013, thermal alerts were no longer recorded by the MODVOLC system.

The Alert Level remained at 6 or Red (the highest of Chiles alert level system) from the beginning on 1 May 2008 through April 2010. The Yellow alert was lowered to Green on 3 May 2011 – three years after onset of the eruption.

Some noteworthy points in the time line

Initial days
On April 30, 2008, an earthquake swarm of significant magnitude began near the volcano. The succession of earthquakes was crowned by the onset of a sudden violent eruption shortly before midnight (local time) on May 1, 2008. As it was dark, this was initially thought to come from Michinmahuida volcano. The first Plinian eruption began later that night. Chile’s government declared a state of emergency and the first several hundred people were evacuated from the coastal town of Chaitén.

In the following days 8 to 12 thousand people from settlements around the volcano had to leave their homes. Even the town of Futaleufú, 80 km ESE of Chaitén – who had reported ~30 cm of ash – was evacuated. Extensive, cross-continent ash deposition lead to the disruption of everyday life in the entire region and abroad. As the area around Chaitén town lacked road infrastructure most residents evacuated by boat.

On 6 May, the eruption became more forceful and generated a wider and darker Plinian ash plume. Observers reported that an ash plume rose to >30 km and drifted SSE (satellite images later pinned the hight of this plume at 21 km). Winds widely dispersed the ash eastward across Argentina and out to the South Atlantic Ocean, upsetting airline traffic.

By that time, two explosion craters on the dome’s N flank had united to form one 800m wide crater. All remaining people in Chaitén were ordered to evacuate, as well as anyone within 50 km of the volcano. Lahars and floods inundated the town of Chaitén, causing widespread destruction.

Further on

The sketch on the left shows the impact of the eruption around Chaitén volcano. Red arrows indicate areas most affected by pyroclastic flows. Brown arrows indicate washing of mud into Rio Blanco during periods of rainfall, generating Lahars which inundated the center of Chaitén Town. (R. Roscoe)

During the following months and years three new lava domes were emplaced within the caldera on top of the existing dome. They grew very rapidly extruding 20 to 100 m³/s of lava, pushed up pinnacles and experienced collapses. On Feb.19, 2009, a partial dome collapse ripped the southern flank of the first new dome. It resulted in a lateral blast and a pyroclastic flow which extended about 5km along the Rio Blanco (or Chaitén River) towards the town.

The domes eventually filled the caldera to its rim on the S side, and they exceeded the height of the caldera’s topographic summit of 1112 m by several hundred meters. This was all accompanied by fluctuating seismicity, lateral blasts, steam and ash emissions, density surges, pyroclastic flows and debris avalanches. Rain falling on this ash blanket generated a hyper-concentrated-flow lahar, followed closely by mud flooding down over several days.

 

Present Activity

Since 2011 Chaitén volcano has been degassing, sometimes vigorously, and dome growths continued at a very slow rate. Occasional increases in seismic activity have shown that this eruptive period might not be quite over yet. In many nights one can see the glowing “eyes” of the hot degassing vents on SERNAGEOMIN’s webcam.

(Unfortunately, for non-Spanish speakers it is not possible to read the current Volcanic Activity Reports provided by SERNAGEOMIN. The PDFs are created in a way that they can neither be translated by Google Translate, nor can one copy the text into another device for translation. They are not encrypted, and they seem not to be images. Any suggestion how to translate them? (ex.: Los Lagos No 1. 2018)

 

THE FATE of CHAITÉN TOWN

The Rio Blanco, which played an infamous role in this event, used to drain the waters from Chaitén’s caldera and slopes into the gulf, bypassing the town and it’s harbour. During the eruption, masses of volcanic debris from lahars and avalanches were washed into the coastal plain, silting up the bay – and finally changing the course of the river. Now the river run through the middle of town, burying all and sundry under several feet of mud. This may not have been the first time nor the last time that the bay has been made uninhabitable. So, what became of the town?

C. Farías Vega wrote an excellent summary in the La Tercera on 05/09/2017 (link below). The short and the long of it is this:

– The Chilean government did the right thing, listened to scientists and evacuated the whole area within 2 days of initial eruption.
– Government issued a disaster stipend of $1200 and $2200 per month per family; financial aid to small businesses was granted and a 90 day freeze on payment of existing loans (figures not verified).
– Later, authorities again listened to science and prepared relocation of the town to a safer place, 9 km north along the coast. To that end, they made acceptable offers of help to the former residents of Chaitén.

But then…
– Residents a) didn’t want to leave their home town and b) didn’t trust the government. In fact, they suspected some foul play, the offer seemed too good to be true.
– The first residents returned to Chaitén, mainly better situated people. They started to rebuild their homes and businesses and resisted relocation.
– Elections brought a new government… new disasters had to be looked after… local business pressed government to give up relocation plans altogether…
– Everything petered out and came to nothing. Now there are the businesses back in town, and people necessary to work for them. About a third of the former population. The rest has dispersed to other places.

The infrastructure is being rebuilt and reinforced, and a lively town will develop again with time. Up to the next and perhaps bigger disaster… Chaitén is not a safe location, and it will never be the same as it was.

This disaster could have been turned to the good for the future of the town as a whole. Instead it has become a double-disaster through distrust, power games and politics: many lost their homes and all lost former neighbors and friends – their grown community.

 

EPILOGUE

As bad as the eruption was for the population, it provided huge insights to the science of volcanology; many aspects of it have been studied in detail. Also, disaster management agencies have learnt from it. Most importantly: the event focused the attention of the Chilean government on its volcano hazards and resulted in a new national volcano monitoring program. By now the Red Nacional de Vigilancia Volcánica has grown into an effective network under SERNAGEOMIN.

~~~

Disclaimer: I am not a scientist, all information in this (and any of my other posts) is gleaned from the www and/or from books I have read, so hopefully from people who do get things right! 🙂 If you find something not quite right, or if you can add some more interesting stuff, please leave a comment.

 

Enjoy! – GRANYIA

 

SOURCES & FURTHER READING
– GVP, Chaitén
– Chaitén, SERNAGEOMIN
– Overview of Chaitén V. and its 2008-2009 Eruption (2013)
– EOS, Awakening of Chaitén Volcano (2009)
– Evidence of mid- to late-Holocene explosive rhyolitic eruptions from Chaitén V.
– Acute sedimentation response […] Chaitén (2013, PDF)
– […] Postglacial Activity from Volcán Chaitén (2014, PDF)
– Fallout and distribution of volcanic ash over Argentina following the May 2008 explosive eruption of Chaitén
– Why Eerie Green Lightning […]
– Chaitén, miscommunication and mistrust (La Tercera)
– Destiny Unbound, Chaitén town (2012, blog)
– Photovolcanica, Chaitén
– 18,000 year-long eruptive record from Volcan Chaitén […] (2017)
– The Chaitén rhyolite lava dome: […] (2013)
– A “new” dome in Chaitén volcano (2017, blog)