Why are some supraglacial lakes able to drain in merely hours?

The Greenland Ice Sheet is the second largest body of ice in the world, covering roughly 80 percent of the island. As the melt season begins, surface meltwater reaches the bed underneath the glacier via conduits on the interior of the ice sheet. This process is often accompanied by the formation and sometimes sudden drainage of supraglacial lakes, bodies of water that form on top of glaciers.

While most supraglacial lakes drain relatively slowly, over many days or months, roughly 13 percent of lakes drain in less than one day, sometimes in a matter of hours. This less common type of lake drainage occurs when a crack forms at the base of the ice sheet and propagates to the surface, resulting in a crack that runs the entire depth of the glacier. These cracks are then opened through hydro-fractures that form directly beneath the lake basin and typically close up after the water has drained, though they can stay open for the melt season if there is continued stream flow.

Currently, scientists do not know what triggers the formation of kilometer length hydro-fractures, though they can only form when the supraglacial lake contains sufficient water volume to continuously fill the fracture as it is propagating from the surface to the bed. However, many lakes that contain the same volumes of water do not exhibit draining, even over multiple summers.

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