JustAnyone
Tourist
Reged: 02/16/07
Posts: 2
Loc: Mundelein, IL, USA
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Petermann Glacier, a glacier that is rapidly breaking up. It sits mostly on water. Note sizing scale - glacier valley is 12 miles across. When this glacier melts entirely, the ice sheet can flow much more rapidly into the ocean.
Story about this 8/21/2008 at: http://researchnews.osu.edu/archive/boxice.htm
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blt
Cartographer
Reged: 01/25/03
Posts: 638
Loc: Santa Cruz, Ca
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Thanks for the post JustAnyone. After following your links a bit, I found a good closeup of the crack from Byrd Polar Reasearch Center
(Animation is here )
Also attached is a recent Modis overlay of the Glacier from the same site.
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mspelto
Tourist
Reged: 12/11/06
Posts: 203
Loc: Massachusetts
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Excellent point to concentrate on. Petermann Glacier is not rapidly disappearing. It has experienced a rapid breakup of a portion of the floating section of the glacier for the second time in this decade, and the crack that blt indicates is well back from the glacier front indicating that a further large breakup is likely imminent, not this summer I would not think. The key point is that a floating section of a glacier is stabilized by its connection to the valley walls and any pinning points at the base of the glacier. As it thins, which happens during retreat, the degree of connection is reduced, and this allows the breakup. We have seen this pre-conditioning as it is called with respect to the loss of the Larsen and portions of the Wilkins Ice Shelf. Long term, thinning and then sudden breakup.
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mspelto
Tourist
Reged: 12/11/06
Posts: 203
Loc: Massachusetts
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A bit of an update on the crack seen in blt's post. Petermann Glacier is a much different glacier than the amarine terminating outlet glaciers further south like Helheim or Jakobshavn. Its velocity of 2-3 m/day is much lower than 10-30 m/day observed on the above marine terminating outlet glaciers. It is the northwest corner of Greenland and certainly experiences less melting and less snowfall. The lower 80 km in length and in 1300 km2 in area of the glacier is afloat. This makes it by area the largest floating glacier in the Northern Hemisphere. The ice front is not impressive, unlike the faster outlet glaciers. The calving front is a mere 5-10 m high reflecting the fact that the ice at the front is only 60-70 m thick. The ice at the grounding line is 600-700 m thick. The combination of velocity and thickness yield the volume of material calved each year. Petermann Glacier yields 0.6 km3, whereas Jakobshavns yields close to 40 km3. The thinning between the grounding line and the calving front is mainly via melting as the snowline is at 900 m. The low slope leads to very low velocities, giving the low lying floating section plenty of time to melt, and surface melt ponds are common.
The crack well back of the calving front indicates another 150 km2 is in danger. Once again the key to this glacier’s second major ice loss this decade after limited retreat in the last century, is thinning of the floating tongue weakening the glacier. The loss of this ice should then lead to acceleration, developing more crevassing and glacier retreat. The crack seen in the image of Petermann Glacier, Aster image provided by Ian Howat of Ohio State, is more of a rift, like those on Larsen Ice Shelf than a crevasse. This tranverse rift is further connected to longitudinal-marginal rifts. Illustrating the poor connection of the Petermann Glacier to its margin and lack of a stabilizing force this margin has, even 15 km behind the calving front. This is not the only rift of its kind on the glacier. Also note that like on Larsen Ice Shelf the rift crosscuts surface streams.
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