Why do they move?

Muir Glacier in 1941 and 2004
In 1941 (left), Muir Glacier filled this valley in Glacier National Park and Preserve in Alaska. It was a tidewater glacier, meaning that it flowed out onto the ocean. By 2004 (right), Muir Glacier had retreated 12 kilometers (7 miles) and thinned by more than 800 meters (2,600 feet). —Credit: Photographs by William Osgood Field (1941) and Bruce F. Molnia (2004). From the Glacier Photograph Collection. Boulder, Colorado USA: National Snow and Ice Data Center.

The sheer weight of a thick layer of ice, or the force of gravity on the ice mass, causes glaciers to flow very slowly. Ice is a soft material, in comparison to rock, and is much more easily deformed by this relentless pressure of its own weight. Ice may flow down mountain valleys, fan out across plains, or in some locations, spread out onto the sea. Movement along the underside of a glacier is slower than movement at the top due to the friction with the underlying ground's surface. Where the base of the glacier is very cold, the movement at the bottom can be a tiny fraction of the speed of flow at the surface.

Sometimes a glacier slides over a thin water layer at the glacier's base. The water may result from glacial melt driven by pressure of the overlying ice, or from water working its way through glacier cracks to the base. Glaciers can also slide on a soft, watery sediment bed. This basal sliding may account for most of the movement of thin, cold glaciers on steep slopes. Warm, thick glaciers on gentle slopes owe less of their movement to basal sliding.

Glaciers periodically retreat or advance, depending on the amount of snow accumulation or evaporation or melt that occurs. This retreat and advance refers only to the position of the terminus, or snout, of the glacier. Even as it retreats, the glacier still deforms and moves downslope, like a conveyor belt. In other words, a retreating glacier does not flow uphill; it simply melts faster than it flows.

Alternatively, glaciers may surge, racing forward several meters per day for weeks or even months. In 1986, the Hubbard Glacier in Alaska surged at the rate of 10 meters (32 feet) per day across the mouth of Russell Fjord. In only two months, the glacier had dammed water in the fjord and created a lake.

Last updated: 16 March 2020