Some of the ideas discussed in this blog are published in my new book called "The Stonehenge Bluestones" -- due for publication on June 1st 2018. After that, it will be available by post and through good bookshops everywhere. Bad bookshops might not have it....
To order, click

Thursday, 29 August 2013

A mega-canyon beneath the Ice Sheet in North Greenland

This is really interesting!  high-definition ice--penetrating radar has revealed this splendid canyon beneath the ice sheet in North Greenland.  With the ice sheet stripped away, the top image shows the canyon winding its way northwards towards the coast.  According to Prof Bamber, it is up to 800m deep and about 10 km wide. (Actually, that is less impressive than you might think -- the trough of Nordvestfjord is over 3,000m deep in places, from plateau surface to fjord bottom.)  But the impression is that this is a very old fluvial system.

The lower image shows the exit of the canyon, which is now currently occupied by the Petermann Glacier.  There is now much speculation that the canyon is very old indeed, and that it acts as a conduit for meltwater beneath the ice sheet, in turn affecting the behaviour of the ice.

In case you wonder what this has to do with Stonehenge, let me enlighten you.  If Salisbury Plain ever was overridden by glacier ice, it would have been by a very large ice mass (part of the British-Irish Ice Sheet) moving cross country from the west.  We are still learning about what happens beneath these large glaciers.  Sometimes they slide and scrape things on their beds, amd sometimes they don't.  Slippage and freezing-on are the two scenarios normally talked about, associated with ice at its pressure melting point (sometimes called "temperate" ice) and ice which id below the PMP (referred to as "polar" ice).  But in recent years it is becoming clear that the situation is much more complex than that, and that there can be sub-glacial lakes and also conduits or canyons which can affect ice behaviour at great depth.    This is where it becomes important for the transport and deposition of the bluestones -- every bit of research like this one tells us a bit more, and gives us more to think about......

Vol. 341 no. 6149 pp. 997-999
DOI: 10.1126/science.1239794

Paleofluvial Mega-Canyon Beneath the Central Greenland Ice Sheet

Subglacial topography plays an important role in modulating the distribution and flow of basal water. Where topography predates ice sheet inception, it can also reveal insights into former tectonic and geomorphological processes. Although such associations are known in Antarctica, little consideration has been given to them in Greenland, partly because much of the ice sheet bed is thought to be relatively flat and smooth. Here, we present evidence from ice-penetrating radar data for a 750-km-long subglacial canyon in northern Greenland that is likely to have influenced basal water flow from the ice sheet interior to the margin. We suggest that the mega-canyon predates ice sheet inception and will have influenced basal hydrology in Greenland over past glacial cycles.

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