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When was Chaitén’s last eruption? 5 November 2008

Posted by admin in Chaitén, Chile, eruptions, volcanology.
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The current eruption of Chaitén volcano in southern Chile began on 2 May 2008. But when was Chaitén’s last eruption? The Global Volcanism Program gives the date as ‘7420 BC ± 75 years’, and describes the dating technique used to give this result as ‘radiocarbon (uncorrected)’. The summary page for the volcano gives more information: ‘A pyroclastic-surge and pumice layer that was considered to originate from the eruption that formed the elliptical 2.5 x 4 km wide summit caldera was dated at about 9400 years ago’. This data in turn comes from a 2004 article: José A. Naranjo & Charles R. Stern, ‘Holocene tephrochronology of the southernmost part (42°30′-45°S) of the Andean Southern Volcanic Zone’, Revista geológica de Chile, vol. 31, no. 2 (December 2004), pp. 225-240 (available online via scielo.cl).

This dated event has been quoted by everyone reporting on the Chaitén eruption, including this blog, as the last time Chaitén erupted. But has there been more recent eruptive activity that we have known nothing about? The idea is not intrinsically unlikely, given the remoteness of the area, the paucity of the historical record (Naranjo & Stern note that of the eight southern Andean volcanoes they study,* only Minchinmávida has any reliable record of historical activity) and our very limited information about Chaitén before this year’s eruption began.

Werner Luis has recently considered the evidence for more recent eruptions at Chaitén on a section of his web site: Durante cuanto tiempo estuvo inactivo el Chaitén? (‘For how long was Chaitén inactive?’). He suggests three indicators that suggest more recent eruptive activity. First, the lack of vegetation on the old, pre-2008, dome, as compared with the thickly forested surrounding terrain. Second, the relative lack of erosion shown by the old dome when compared with nearby slopes cut by deep gullies and ravines. Third, the evidence of ash samples taken from beneath the town of Chaitén. These ash samples have just been analyzed and organic material within the samples has provided dates for the last two possible eruptions:

1.- 210 +/- 40 years (i.e. 1758-1838)
2.- 310 +/- 40 years (i.e. 1658-1738)

(Source: Seminar ‘Volcanismo en Chile y Red Nacional de Vigilancia Volcánica’, 7 October 2008, La Moneda, Santiago de Chile, organized by SERNAGEOMIN.)

This tephrochronology data is very interesting, and it would add a new dimension to our understanding of Chaitén if the occurrence of recent eruptive activity could be established. The potential problem with the ash samples is proving that the ash in question was deposited by Chaitén and not another nearby volcano. The obvious candidate is Minchinmávida, where eruptions took place in 1742, 1834 and 1835, and possibly 1650 ± 50 years (eruption is UNCERTAIN, warns the Global Volcanism Program). Minchinmávida, however, has tended to deposit its ash to the east in accordance with prevailing winds, as Naranjo & Stern show in their 2004 paper, and while lahars from the 1835 eruption reached the coast at Punta Chana, due west of Minchinmávida and about 25km north of Chaitén town, they were able to do so because of a direct path along a river valley, which is not the case between Minchinmávida and Chaitén. I have no information on whether the ash has been fingerprinted as Chaitén’s [see UPDATE below].

At the moment we do not have enough information for definitive conclusions. It will be very interesting to see what evidence emerges to help us fill in the many unknowns of Chaitén’s eruptive history. If the dome itself is younger than previously thought, for example, what will that mean for our understanding of the current dome-building activity at the volcano?

* Naranjo & Stern’s eight volcanoes: Chaitén, Minchinmávida, Corcovado, Yanteles, Melimoyu, Mentolat, Macá, Cay.

UPDATE: since writing the above I have heard from a (very reliable) informant from Chile who confirms that the ash samples from Chaitén town are ‘without doubt from the Chaitén volcano’.

For all our Chaitén coverage: Chaitén « The Volcanism Blog.

Global Volcanism Program: Chaitén – summary information for Chaitén (1508-41)
ONEMI, Oficina Nacional de Emergencia – Chilean government emergencies office (Spanish)
SERNAGEOMIN – Servicio Nacional de Geología y Minería (Spanish)
Erupción del Volcán Chaitén – extensive coverage of the Chaitén eruption

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From the geoblogosphere – dikes and tsunamis 9 October 2008

Posted by admin in blogs, geoblogosphere, geoscience.
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A couple of very interesting new geoblogosphere posts of strong volcanic interest:

Dike swarms and continental barcodes from Highly Allocthonous – the ‘magnetic barcodes’ of dikes in now separate continental fragments can be matched up, showing that at the time the dikes were created the fragments in question were in close enough proximity to be affected by the same episode of igneous intrusion.

Volcanic flank collapse and tsunamis from Dave’s Landslide Blog – a discussion of the Tongatapu boulders, possibly the largest tsunami debris yet identified, the result (it is argued) of a submarine volcanic eruption and flank collapse. 

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Vesuvius, AD79 – something fishy? 29 September 2008

Posted by admin in archaeology, current research, Italy, Vesuvius.
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How does fish sauce enable us to get a more precise fix on the date of the AD79 eruption of Vesuvius? Archaeological researchers at Pompeii have the answer. At the Eruptions blog, Dr Erik Klemetti reports on the latest from the archaeology/volcanology/gastronomy interface.

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