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In 2008, scientists documented for the first time how the icy bottoms of lakes atop the Greenland Ice Sheet can crack open suddenly—draining the lakes completely within hours and sending torrents of water to the base of the ice sheet thousands of feet below. Now they have found a surprising mechanism that triggers the cracks.
Their study showed that in the six to 12 hours before the lake cracked and drained, the ice around the lake moved upward and slipped horizontally. The scientists say that meltwater had begun to drain through a nearby system of moulins, which connected the surface to the base of the ice sheet 3,215 feet below. The accumulating water creates a bulge that floats the entire ice sheet, creating tension at the surface underneath the lake. The stress builds up until it is relieved by a sudden large crack in the ice below the lake. Once the tension initiates the crack, the volume of water surges into the opening, widening and extending it and keeping it filled with water, all the way to base of the thick ice sheet. These are called hydrofractures, and the scientists have documented how they can drain more than 11 billion gallons of water out of North Lake in about 90 minutes. At times, water flowed out of the lake bottom faster than the water goes over Niagara Falls, the scientists estimated.Image credit: Photo by Laura Stevens, Woods Hole Oceanographic Institution
