Archive for April 2012

Askja volcano, Iceland: the little mystery of the ice-free crater lake   Leave a comment

Recent reports from Iceland over the past week have mentioned the fact that at the Askja volcano deep in the uninhibited interior of Iceland there is an ice-free lake. But why is this newsworthy? Bit of background before you get my view/opinion.

View of the water-filled 4-5 km diameter depression (caldera) created in the aftermath of the 1875 AD eruption. Taken from the eastern rim, looking west.

The lake in question occupies an oval 4-5 km diameter depression formed in the aftermath of the 1875 eruption at Askja. The lake is Iceland’s deepest lake at c.220 metres deep, and probably took about 30 years to fill to the brim after the eruption, judging by surveys taken afterwards, notably by the Danish surveyor Caroc. But after the formation of the depression (i.e. caldera) which became filled by water, there was a series of eruptions in the 1920s, some of which were around the lake margins and clearly controlled by the caldera fault system that created the c.220 m deep depression. Even now, on a still and humid day, steam rises along a fair bit of the lake shore indicating that the caldera fault system produced in 1875 still acts as a conduit for uprising heat from the underlying plexus of cooling magma storage pods.

Back to the present ‘issue’. At this time of year the lake should be covered by ice, with thick rafts on the Eastern shore where wind had moved the early formed but isolated sheets. I have seen these personally, as they can persist till June. But the images show most of the lake free of ice, apart from the eastern shore which is where some of the most vigorous geothermal areas are. However this may simply be that there is thicker ice here plus the heat is escaping without melting completely though the ice (i.e. laterally).

So what can be doing this? An obvious answer is that the water is either too warm to freeze or that the water is being moved too quickly for ice to form easily. But the Met Office records do not support water being moved purely by wind. So an additional heat input into the lake seems likely, but does this imply that an eruption is imminent? Definitely not! Though it is possible that this activity may the precursor to an eruption (after all, there is at least one island in the lake formed by a 1920s eruption). More likely is an additional but short-term heat input from the geothermal system. This would do more to stir the lake waters than to heat them up, so aerial surveys looking for higher water temperatures are likely to find nothing definitive unless the thermal input is high enough to overcome the effective buffering of such a large water mass and enable warm water to reach the lake surface.

I rather hope an eruption does not happen, or if it does then it does not disturb the lava flow I intend to work on in late August 2012. Rather quirkily it is called ‘Batshraun’ which means ‘boat lava’ and was named because some country folk left a boat by the lakeside in the 1920s and when they came back the next summer they found where they left the boat covered by a new lava flow. Finally, it is very difficult to get to Askja at this time of year. Helicopter is the only swift way. During the coming weeks Askja will be looked at carefully and we will know more about the significance of the ice-free lake. There could of course be an innocuous and innocent explanation unrelated to anything volcanic and/or geothermal… such as a sudden thaw pouring a thin sheet of water onto the ice.

There are plans for a scientific team from Iceland to visit the lake, and if they manage to do this soon then this little mystery might be solved.

Eastern shore of the lake in Askja in August 2010. Steam rises from one of the vigorous geothermal areas along this margin of the 1875 AD caldera.


Posted April 11, 2012 by davemcgarvie in Uncategorized

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