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Tuesday, 20 May 2014

Outlet glacier troughs -- now and then


The spectacular image at the top is one which was recently published, showing part of the Antarctic Peninsula.  It's a bit confusing because the black area in the middle, with an irregular outline, is not water as you might think, but the ice cap that runs along the spine of the peninsula.  So here we have the highest part of the landscape, with large glaciers draining away from it towards the sea on both sides of the peninsula.  You can see the calving snouts quite clearly, at the coastline. 

The most interesting thing about the glacial troughs, filled with streaming ice, is that they are very wide, with short stumpy "feeder troughs" carrying ice down from the ice cap, but with very few tributary glaciers feeding into them  in their middle and lower sections.  This means that the main troughs have been carrying so much ice that they have effectively chopped off or truncated any tributary valleys that might have existed in this landscape prior to the arrival of the ice.

As soon as I saw this image I thought of Iceland, where there are very similar troughs, especially in the area to the west of Akureyri.  That's the area shown in the lower photo, flipped through 90 degrees so that the comparison is a bit easier to appreciate.  Now of course these valleys are all green and ice-free, although some snowfields and small glaciers still exist on the plateau that once supported an independent ice cap.  Parts of this area are also quite mountainous.

These photos illustrate what happens when you get very heavy ice discharge from the highest source areas but not many supplements afterwards -- so that the erosive capacity of the long glacier is enormous, enabling it to chop off spurs and divides as it grinds its way towards the sea.

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