Glaciers are built up from layers of fused snow that spread out under their own weight as they accumulate, leaving a frozen record of their life history behind. Studying ice-penetrating radar images to piece together the ages of different layers in the Greenland sheet, researchers found that only significant melting could explain the thickness of an area resting above a lengthy ice flow. The old crust beneath this 600-square-kilometer region should melt about a few millimeters of ice each year; instead, up to 10 centimeters liquify. Although its true nature and duration are not clear, the hotspot is comparable in size and strength to the Yellowstone caldera, a place where the crust collapsed after a volcanic eruption. "The rates of melting are a surprise," lead author Mark Fahnestock says, "and the fact that it's localized in the right place [to cause the ice flow]."
Ice sheets and flows are important stores and sources of water, he adds, so understanding where they come from should improve glaciologists' understanding of glacier behavior and corresponding changes in sea level. In an accompanying article, geologist Christina Hulbe of Portland State University notes that water melting from the base of a glacier can alter the way it moves and stores energy. This unexpected new finding, she writes, "reminds us that basal melt water matters and that it may be important in places we never suspected."