ERNAGEOMIN 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’.

Moreover, intense and continuous noise has been perceptible since 24 July, when an emission of ash accumulated to the depth of 3cm around Chaitén. On 27 July weather conditions again favoured the fall of fine ash on the town, albeit in a very restricted manner. The preceding describes, regardless, eruptive activity that is weak but sustained.

The foregoing notwithstanding, the latest seismic data clearly shows an elevated seismicity, which has been sustained for several weeks, both in the number of earthquakes and their magnitude (a large number of them being felt by people over several days) and recorded by stations at a distance, with an increase in the number of earthquakes with a magnitude greater than 3.5M. Likewise, it was possible to verify that the greater part of the earthquakes of greater energy were located to the south-east and south-south-east of the Chaitén volcano.

Weather is cloudy at the volcano, but the cloud base was high enough for viewing when the big eruption began soon after dawn. The webcam showed a huge widening of the plume base — the whole dome-top blowing away — then an immense darkness, then the paler ashfall running coastward. The camera may not survive long, if this is as big an event as I suspect. Winds have shifted to W at upper levels, but NE near the surface, so at least some of the plume is coming over the camera location.