Surprising workings that trigger cracks draining lakes laying over the Greenland Ice Sheet has been found.
The discovery was made by the same scientists scientists from Woods Hole Oceanographic Institution (WHOI) and the University of Washington, who in 2008 first reported how the icy bottoms of the lakes can crack open suddenly. The results entirely drained the lakes within hours, sending deluges of water thousands of feet below to the base of the ice sheet base.
Laura Stevens, Massachusetts Institute of Technology-Woods Hole Oceanographic Institution (MIT/WHOI) graduate student, said:
“Our discovery will help us predict more accurately how supraglacial lakes will affect ice sheet flow and sea level rise as the region warms in the future.”
The scientists’ study discovered that 6 to 12 hours prior to the lake cracking and draining, ice surrounding the lake moved upward and slipped horizontally.
Meltwater had started to drain through an adjoining system of vertical conduits through the ice, refered to as moulins, which connected the surface to the base of the ice sheet 3,215 feet below. The gathering water produces a bulge which floats the whole ice sheet, creating tension at the surface underneath the lake.
The stress tension builds until it is alleviated by a precipitous large crack in the ice below the lake.
“In some ways, ice behaves like Silly Putty—if you push up on it slowly, it will stretch; if you do it with enough force, it will crack,” said Stevens. “Ordinarily, pressure at the ice sheet surface is directed into the lake basin, compressing the ice together. But, essentially, if you push up on the ice sheet and create a dome instead of a bowl, you get tension that stretches the ice surface apart. You change the stress state of the surface ice from compressional to tensional, which promotes crack formation.”
Thousands of these supraglacial lakes are formed every spring and summer atop the Greenland Ice Sheet as sunlight comes back to the region. The resulting heat melts snow and ice into water that gathers in depressions in the ice sheet, forming large lakes.
As the are becomes still warmer, more lakes tend to form, thus leading to a predictor of more hydrofractures, more lubrication, more ice sheet slippage, and faster-rising sea levels.
Photo Credit: Laura Stevens, Woods Hole Oceanographic Institution
Greenland supraglacial lake drainages triggered by hydrologically induced basal slip
Nature 522, 73–76 (04 June 2015) doi:10.1038/nature14480