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Sunday, 30 October 2011

Bulldozing glaciers

The spectacular time-lapse footage in the first "Frozen Planet" programme should leave one in no doubt that glaciers are capable of tremendous things in the process of landscape change -- moving very fast on occasion (for example, when a surge is under way) and shifting large quantities of debris.  However, one might be left with the impression that glaciers act as bulldozers, pushing debris ahead of themselves as they advance.  That is actually quite a rare occurrence.  One place where this does occur is at the snout of the Thompson Glacier on Axel Heiberg Island.  Have a look at these:

Click to enlarge

The top photo shows the push moraine beyond the glacier snout, and the lower one was taken out near the outer edge of the push moraine, looking back at the snout.  That gives a nice impression of the sheer scale of the phenomenon.

There are some rather unique factors here, in that the glacier is (or was, in the 1960-70 period) advancing strongly along a valley thickly infilled with fluvio-glacial sediments frozen by permafrost.  Great slabs of these frozen gravels have in effect been bulldozed, because the base of the glacier is not sitting on top of the gravels but is at a lower level -- so the forward movement of the glacier is strong enough to displace the gravels rather than riding over them.  Wonderfully impressive, but not all that common.

Most glaciers, as they advance, slide on the bed if basal ice temperatures are around the pressure melting point, or -- in truly polar situations where the glacier bed is frozen onto the bedrock interface -- by various processes of internal deformation.  One of these processes is shearing or thrusting -- and there are spectacular examples of thrust planes on the snout of Thompson Glacier as well.

Here is another photo from Thompson Glacier:

The main feature of this photo is the meltwater stream gushing out from a tunnel high on the glacier snout, but the whole of this ice face is controlled by shearing or thrusting, with ice riding up over slower-moving debris-laden ice at the base of the snout.  These thrusts are responsible for the delivery of the masses of debris from within the glacier, or from its bed, up to the ice surface.

For more photos of this remarkable glacier (which I visited in 1963) see the Glaciers Online web site, and go to Axel Heiberg Island and then to Thompson Glacier.

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