In this shot, the eye immediately falls on the deep hole in the middle of the Greenland ice. Masses of water seem to flow into this gorge from all sides and fall like a waterfall into the depths of the glacier.
Ice holes like in this photo are called glacier mills. They arise when melt water flows along the surface of the glacier or in crevasses in summer. The water contains small rock fragments that are moved in circles by flow turbulence in depressions and thus gradually grind holes up to 20 meters in diameter in the ice surface. In this way, the melt water from the ice surface reaches deeper layers of the ice and accumulates there.
How deep the glacier mills get and how much meltwater they can absorb has now been investigated by researchers led by Matt Covington from the University of Arkansas. To do this, they modeled a glacier mill and also traveled to the Greenland ice sheet in October 2018 and 2019. There, the scientists abseiled themselves into two large glacier mills – like the one on this picture – until they almost reached the surface of the meltwater dammed up below.
Their measurements showed that the glacier mills widened in depth and were much larger than previously assumed. Their cross-section therefore reaches an area of ​​up to 500 square meters, so that the researchers also assume that the holes will have an enormous total volume. From this, the team concluded that the glacier holes can collect large amounts of meltwater.
As the researchers explain, the water masses that accumulate under the ice in the glacier mills can increase the pressure inside the glacier and in the subglacial water reservoirs. The pressure accelerates the speed at which the glaciers slide over the ground. Similar to an ice cube sliding on a thin film of water. “The storage of water in these glacier mills during the daily meltwater flows has a major impact on how much the water pressure changes under the ice,” explain Convington and his colleagues.
Should the summer periods in Greenland become even warmer in the future due to climate change, the researchers suspect that even more meltwater will get into the mills and subglacial water reservoirs, thereby accelerating the sliding of the glaciers. As a result, more meltwater gets into the sea and sea levels rise.