Chaitén update, 27 May 2008

SERNAGEOMIN issued a new bulletin on the situation at Chaitén late yesterday, 26 May 2008. It reports that ‘The eruptive activity of the Chaitén volcano has declined to subplinian type, but remains continuous’, and that over the weekend, and ‘in particular on Sunday 25 May’, the eruption column reached an average altitude of less than 3.5km above sea level, with occasional large explosions lifting it to around 5km above sea level. South-westerly winds pushed the plume in a north-easterly direction.

An overflight took place on 24 May, during which ‘it was possible to overfly the volcano’s crater and examine the slight growth of the active volcanic dome, which just rises above the summmit of the old dome’. South of the active dome is a 200-metre crater ‘from which there is a continual expulsion of gas and ash’. The new dome ‘generates, especially to the north, flows and blocks of ash, because of the instability of its steep walls’.

A second overflight carried out by a Navy helicopter on 25 May revealed that many of the rivers in the Chaitén region are carrying significant quantities of ash: ‘It was observed that the Amarillo river was carrying a large burden of ash and abundant pumice of various sizes into the Yelcho river, the waters of which already contained ashes and had also received volcanic material, mainly ash, from the Minchinmahuida river’. The Correntoso river running from the north-east, was observed to be carrying ash into Lake Yelcho, and the Futaleufú and Azul rivers running into the southern end of the lake were also carrying smaller loads of ash, but but the other rivers debouching into the lake were clear. Flooding and the deposition of volcanic sediment continue to affect the town of Chaitén.

Seismic activity is currently declining: ‘Over the past four days VT-type [volcano-tectonic] earthquakes have decreased gradually in number and magnitude, indicating a slight but steady decay of seismic activity’. The explosions ‘associated with emissions of gases, ash and other pyroclasts’ are continuing, but are of ‘lesser intensity and volume’.

The apparent decline in activity should not lead to any complacency about the volcano’s future behaviour, concludes the report:

Finally, although the volcanic activity has declined, the occurrence of future major explosions cannot be excluded, caused by obstruction of the conduit, which could produce the partial destruction of the new (and/or the old) dome and generate pyroclastic flows through the collapse of the eruption column, which would fall in a radial form from the volcano over the adjacent valleys. Nor can the possibility of lateral explosions be disregarded.

The latest (0800 GMT today) ash advisory from Buenos Aires VAAC indicates that the plume is reaching 21000 feet, which is an altitude of about 6.4 kilometres. The aviation alert code, which had been orange yesterday, is now red again. A satellite image from the Argentine Servicio Meteorológico Nacional, taken at 17:45 GMT yesterday, shows a significant plume from Chaitén blowing north-east.

UPDATE: Alan Sullivan points out that, regardless of whether the new dome has shown only ‘slight growth’ in height, it’s got a great deal bigger through lateral expansion.

For all our Chaitén coverage: Chaitén << The Volcanism Blog

Information
Global Volcanism Program: Chaitén – summary information for Chaitén (1508-41)
ONEMI, Oficina Nacional de Emergencia – Chilean government emergencies office (Spanish)
SERNAGEOMIN – volcanology information from the Servicio Nacional de Geología y Minería, Chile (Spanish)
Servicio Meteorológico Nacional: Imágenes Satelitales – real-time satellite images from the Argentine National Meteorological Service: the ‘Norte Patagonia’ region covers Chaitén