How do glaciers move?
The sheer weight of a thick layer of ice and the fact that it deforms as a "plastic" material, combined with gravity's influence, causes glaciers to flow very slowly. Ice may flow down mountain valleys, fan across plains, or in some locations, spread out to the sea. Movement along the underside of a glacier is slower than movement at the top due to the friction created as it slides along the ground's surface.
Glaciers periodically retreat or advance, depending on the amount of snow accumulation or ablation that occurs. This retreat or 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. For most glaciers, retreating and advancing are very slow occurrences, noticeable only over a long time. However, when glaciers retreat rapidly, movement may be visible over a few months or years. For instance, massive glacier retreat has been recorded in Glacier Bay, Alaska (see photos below). Other glaciers have been photographed at intervals showing dramatic recession.
Muir Glacier, located in Glacier Bay, Alaska, photographed by W. Field in Aug. 1941 (left) and B. Molnia in Sep. 1976 (middle) and Aug. 2004 (right). Note how the glacier has retreated and exposed rock in 1976 that has since become lush vegetation in 2004. —Courtesy NSIDC Glacier Photo Collection
Alternatively, glaciers may surge, racing forward several meters per day for weeks or even months. In 1986, the Hubbard Glacier in Alaska began to surge at the rate of 10 meters per day across the mouth of Russell Fjord. In only two months, the glacier had dammed water in the fjord and created a lake. This illustrates how quickly a surging glacier can change its surroundings.
