As Greenland’s glaciers melt, gigantic chunks of ice are breaking off. They are so large that they are causing powerful earthquakes as they tumble into the ocean.

A team of researchers from Swansea University in the UK, the Lamont-Doherty Earth Observatory at Columbia University, and a number of other institutions, studied GPS data from Greenland’s fast-moving Helheim Glacier, and the glacier’s calving front, where icebergs break off into the ocean, and correlated this with seismic data for earthquake timings. They found that large earthquakes, in the 4.6 to 5.2 range, are generated when billion-ton ice sheets break off from the glacier’s forward face.

As the ice sheet breaks off, it tumbles forward in the water, with the bottom of the piece pushing back violently against the glacier, driving the ice back inland. “The horizontal and vertical motion then rebound rapidly,” says Meredith Nettles of the Lamont-Doherty Earth Observatory at Columbia University, a study co-author. “That calving iceberg is pushing the remaining part of the glacier backwards hard enough that we can actually measure that, hard enough that it reverses the whole flow of the front of the glacier temporarily. And it’s that force pushing on the remaining glacier and the rocks beneath it that gives us the seismic [activity].”

“These are all around magnitude 4.6 to 5.2, they’re all pretty close to magnitude 5, which is a pretty big earthquake,” continues Nettles. He also explains the sheer size of these gigaton-sized chunks of ice: “If you took the whole National Mall, and covered it up with ice, to a height about four times as high as the [Washington] monument, all the way down from the Capitol steps to the Lincoln Memorial.”

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