useful to recall (because some Luna sites wince with every little trembling
[link to earthice.hi.is
Eruption of Grimsvotn volcano, begins 21 May 2011
The subglacial Grimsvotn volcano, Iceland, began erupting on 21 May 2011, around 18-19 GMT. The eruption was preceded by intensive earthquake activity lasting for about 1 hour. An ‘ash-loaded’ eruption plume rose rapidly up to about 17 km height (estimated 55000 feet from ground based radar, overview flights, and pilot reports). Ash from the lower part of the eruption plume was deflected southwards and from a higher level towards the east. Few hours after the onset of the eruption ash fall began over wide area in the nearest populated areas south of the Vatnajokull ice cap, at a distance of more than 50 km from the eruption site.
Grimsvotn is a basaltic volcano that has the highest eruption frequency of all volcanoes in Iceland. It is located near the centre of the Vatnajokull ice cap, the largest ice cap in Europe. The volcano has a caldera complex, the most recent one hosting the Grimsvotn subglacial caldera lake that is sustained by extensive geothermal activity. The volcano is almost fully ice covered and interaction of magma and meltwater from the ice causes phreatomagmatic explosive activity.
Initial overview flight, with limited visibility, and earthquakes locations suggest an eruption site in the southwestern part of the Grimsvotn caldera. Ice cover there is relatively thin (50-200 m) and meltwater is anticipated to accumulate within the Grimsvotn caldera lake. From there, it may eventually drain in a sudden glacial outburst flood, jokulhlaup, along a subglacial channel and issue from the Skeidararjokull outlet glacier.
The height of the initial plume in the present eruption, 17 km, is much higher than in a preceding eruption at Grimsvotn in 2004. Then an eruption produced a plume reaching an estimated height of 6-10 km above vent. The present plume is also higher than recorded in the Eyjafjallajokull eruption in Iceland last year.
A GPS-station on the rim of the Grímsvotn caldera has revealed continuous inflation and expansion of the volcano of few cm per year since the 2004 eruption, interpreted as inflow of magma to a shallow chamber. Another long-term precursor was increase in seismicity over last few months, including some bursts of tremor. Increase in geothermal activity was as well observed in the months leading to the eruption.