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Thursday, September 30, 2010

Doug MacAyeal explains glaciological mosh pits to Graham Cogley (like the disintegration of the Larsen B Ice Shelf in 2002 and the Wilkins Ice Shelf in 2008)

Glaciological mosh pits

by Graham Cogley, environmentalresearchweb, September 27, 2010
Last month I found out what a mosh pit is. Doug MacAyeal told me. I gather that anybody more than about 15 years younger than me already knows this, but for the rest of us a mosh pit is the place in front of the stage at a rock concert, where extreme violence is likely to break out. (But apparently it is good-natured violence.)
Like me, Doug MacAyeal was attending a symposium of the International Glaciological Society to mark the 50th anniversary of the Byrd Polar Research Center in Columbus, Ohio. He is a glaciologist who is unusually gifted in the understanding of forces. In fact, one of the reasons he was at the symposium was to accept the Byrd Polar’s Goldthwait Polar Medal, an award made only intermittently to the most distinguished of glaciologists. Doug’s talk entitled “The glaciological mosh pit” made clear why he is a Goldthwait medallist.
The glaciological mosh pit, just before the violent release of gravitational potential energy that justifies the name, is a collection of icebergs, detached from each other but in close physical contact. They are the descendants of blocks of ice in an ice shelf that were formerly separated along crevasses, but the crevasses have now propagated through the whole thickness of the shelf. The bergs are typically much longer than they are wide, but the crucial point is that they are in a gravitationally unstable state, being several timestaller than they are wide.
Although their weight is supported by the water, they would topple over if they were not propping each other up. Do they topple as soon as the crevasses break through to the base of the shelf? If so, why do all the crevasses apparently make the breakthrough at the same time? If not, what keeps them from toppling, and what triggers the eventual catastrophe?
I have trouble sorting out the ways in which this adds up to an exciting mechanical and glaciological problem. First and foremost, perhaps, the mosh pit is already a horrible mess and is about to get very much messier, but there is the prospect of reducing the chaos to intellectual order.
Then there is the question of how it got that way in the first place. There is a link here to global warming. The crevasses probably would not penetrate to the base of the ice shelf if they were not strengthened by an influx of surface meltwater. The ice shelves have been around for a long time, and that they are disintegrating now suggests that surface meltwater is now more abundant than formerly.
A related question is why some floating slabs of ice disintegrate mosh-pit fashion but some others break up along just a few cracks, or even a single crack, to form ice islands.
And then there is the really big question: what happens to the released energy when the berg finally switches from being a vertically extended slab to being a more civilized, horizontally extended slab? It seems that, apart from a little bit of heat and a little bit of noise (waves in the air), nearly all of the gravitational energy becomes kinetic energy (waves in the water). This wave energy has to go somewhere in turn, and it can do an astonishing amount of damage when the waves break.
Doug MacAyeal and his students are grappling with this question along several lines of attack. Doug’s talk was mainly about the theory of the balance of forces on a collection of gravitationally unstable icebergs, and about how tricky it is to write this balance down algebraically. Justin Burton told us about the research group’s efforts to simulate the mosh pit with fake plastic icebergs in a large tank of water, showing fascinating movies of the collapse and “seaward” advance of the collection and the subsequent sloshing about of the water. Nicholas Guttenberg described early work on computational simulation of the mosh pit, with an equally fascinating movie showing “virtual collapse.”
So far we have a sample of only two observed glaciological mosh pits, the disintegration of most of Larsen B Ice Shelf in 2002 and of part of Wilkins Ice Shelf beginning in March 2008. But two examples are more than twice as interesting as one, as well as being infinitely more interesting than none at all. Two mosh pits suggest a pattern, and the possibility of more to come and perhaps to guard against.

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