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Saturday, 31 August 2013

Catastrophe Country

This is landslide territory -- the western fjords (Vestfirdir) of Iceland -- the north-western peninsula of the country, where I spent a good deal of time in the 1970s.  The map shows the locations of more than a hundred major landslides.

A cluster of landslides beneath the plateau edge near Sudureyri.  M = moraines.  L = landslides.

Landform map of a larger area including that covered in the satellite image above. 

Why are there so many landslides beneath the steep cliffs of this area?  Well, the topography is very suitable, with very steep basalt cliffs around the edges of the plateau country that makes up the interior of the peninsula.  Steep basalt cliffs seem to be very amenable to collapse.  This area is geologically much older than that part of Iceland where most of the volcanic activity now occurs, but it is quite possible that earthquakes have triggered many of the landslides since the end of the last glacial episode -- so most of them have occurred since about 12,000 years ago.

Another mechanism for slope collapse and landsliding on a large scale is pressure release following deglaciation.  When a glacial trough has been filled with ice, and when that ice melts away, the ice which has been pressing against the sides and base of the trough is of course no longer there to "support" rock which may be fractured or unstable -- and collapses become more or less inevitable, especially if there are minor tremors associated with isostatic adjustment following the disappearance of the ice load.

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