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Sunday, 30 October 2011

Iceberg transport

Our discussion about the dumping of erratics on distant beaches prompted me to show these two photos:

The top photo shows a big tabular berg typical of those that calve from the biggest and most active glaciers in Greenland.  Normally, when seen from above, those bergs look clean, but if by some freak a big berg like this flips over, the bottom photo shows what it can look like.  There are huge quantities of debris here, embedded into the ice.  In some cases this may be a layer carried within the glacier on a shear-plane -- in other cases this may be the actual glacier bed that we are looking at.

 Zoom in close on the bottom photo if you can -- and you will see many very large boulders melting out of the till and debris-rich ice.

Where floating glacier snouts are crumbling or collapsing into the sea (as in the "Frozen Planet" sequence from Jakobshavns Isbrae) most of the debris from this smashed-up ice is dumped on the sea bed very close to the glacier snout, or maybe within a few kilometres as the floating brash ice is moved away by tides and currents.

But where big slabs of ice are floated away and caught up by ocean currents, we clearly have the potential for tremendous amounts of sediment to be widely dispersed onto the sea bed, hundreds or even thousands of miles from the calving glacier front.  And if an iceberg like this (or even a small remaining fragment of it) washes up onto some distant shore, we have a simple mechanism for the dumping of boulders like the giant erratics of Devon and Cornwall.

Here are a couple of iceberg track maps, to show just how far these icebergs can travel.


Alex Gee said...

In close up,it seems to me that the black surface, is the sheared off surface of the bedrock the glacier was traversing?.

Although impossible to tell for certain, from just a photograph.

BRIAN JOHN said...

No, I don't think so, Alex. I think what we are looking at here is the dirty (debris-filled) basal layers of the glacier, maybe several metres thick. Sometimes basal shearing processes and basal accretion (freezing-on of debris layers at the ice-rock interface) can lead to very thick accumulations. Just look at those pictures of THOMPSON GLACIER on the Glaciers Online web site! The dirty basal layer can be 20m or more thick....