These cracks, called hydrofractures, drain the entire lake within an hour or two. Water flows out of the lake bottom faster than the water goes over Niagara Falls. The influx of water at the base of the ice sheet causes the ice sheet to slide faster toward the coast. That accelerates the outflow of ice from land to sea and causes sea levels to rise faster.EndFragment     EndFragment EndFragment     

The process starts when the returning sun begins to melt snow and ice into water that pools in depressions in the ice sheet to form lakes. Depressions can form at the ice surface as the ice sheet flows slowly forward over depressions in the bedrock. Moulins are conduits that connect the surface to the base of the ice sheet thousands of feet below.

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The depression directs the flow of ice downward, compressing ice on either side together at the bottom of the depression. As melting increases, more streams feed the expanding lake.EndFragment     

An outflow stream from the lake spills water into the moulin, which transports it to the base of the ice sheet. EndFragment     

The tensional stress builds up until it is relieved by a sudden large crack in the ice that extends below the lake. The huge volume of water in the lake surges into the opening, widening and extending it, and keeping it filled with water all the way to base of the ice sheet. EndFragment EndFragment     

Water slowly accumulates between the bedrock and the base of the ice sheet. It eventually creates a bulge that jacks up the entire ice sheet. That creates tension that overcomes the compressional forces. The ice begins to stretch apart at the surface. The water also lubricates the interface between ice and rock, allowing the ice sheet to slide faster toward the coast. GPS receivers record the ice moving upward and forward.

Thousands of supraglacial lakes form each spring and summer on top of the Greenland Ice Sheet as sunlight returns to the region. Many lakes disappear within hours when large cracks form below them, draining the lakes and sending torrents of water to the base of the ice sheet. To learn what triggers this phenomenon, a team of scientists installed a network of GPS units around a lake. EndFragment

Triggering Hydrofractures