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Saturday, 7 April 2018

The island that sprang out of the sea

I came across this splendid photo of a small island in East Greenland which has popped out of the sea since the end of the last glacial maximum. At one time the island wasn't there at all -- and then a rocky shoal appeared, and after that it just grew and grew -- on the photos you can see all the shorelines quite clearly, showing how the island has expanded and changed shape.  It's still growing.....

This is all down to isostatic recovery. When ice melts, the load on the earth's crust reduces, and the land pops up in response. For every 300m or so of ice load that is taken away, the crust will rebound by about 100m.  When I was working in this area (in Kjove Land) in 1962, we found raised beaches up to 134m, and in some locations it's quite feasible that the highest strandline will be well above 150m.

You don't get many strandlines like these in the UK, because in general the rise in global sea-levels has taken place at a faster rate than isostatic recovery.  But close to the centre of the Celtic Ice Sheet, where the ice load was greatest, you do find raise beaches.  One of the best locations is around Malin Head near the northern tip of Ireland.


Gordon said...

It has been reported lately that Willand a village in Devon is rising 20mm per year.What other reasons could there be for this rise? I did hear that abandoned mine workings filling with water can have the same effect.

BRIAN JOHN said...

There are tectonoic and isostatic adjustments going on all the time. Generally the SW is sinking because of the hinge effect, as northern Britain continues its isostatic rebound after the Devensian glaciation. But the N Sea area id sinking fastest, because of the e=weight of water and accumulating sediment, so maybe there is a small compensatory rise in parts of the SW. Then add in the tectonic factors to the isostatic ones, and all sorts of local variations can occur. I was looking not long ago at some of the recent literature on Wales, and it seems that every county -- and indeed every part of every county -- has a unique tectonic / isostatic history.