Why do they move?
In 1941, Muir Glacier extended far down this valley in Glacier National Park and Preserve in Alaska. It was a tidewater glacer, meaning that it flowed out onto the ocean, and had a terminus 60 meters (200 feet) high.
—Credit: Photograph by W. O. Field. 1941. Muir Glacier: From the Glacier Photograph Collection. Boulder, Colorado USA: National Snow and Ice Data Center/World Data Center for Glaciology. Digital media.
By 1976, Muir Glacier had started retreating back up the valley.
—Credit: Photograph by Bruce F. Molnia. 1976. Muir Glacier: From the Glacier Photograph Collection. Boulder, Colorado USA: National Snow and Ice Data Center/World Data Center for Glaciology. Digital media.
Between 1941 and 2004, when this photograph was taken, Muir Glacier retreated twelve kilometers (seven miles) and thinned by more than 800 meters (2625 feet). Ocean water now fills the valley the glacier once occupied.
—Credit: Photograph by Bruce F. Molnia. 2004. Muir Glacier: From the Glacier Photograph Collection. Boulder, Colorado USA: National Snow and Ice Data Center/World Data Center for Glaciology. Digital media.
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. Other glaciers have been photographed at intervals showing dramatic recession.
This Landsat satellite image shows Bering Glacier in Alaska, the largest glacier in North America. Although the glacier has been retreating and thinning recently, it still surges about every 20 years or so.
—Credit: NASA image by Robert Simmon, based on data provided by the Landsat 7 Science Team
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 (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. This illustrates how quickly a surging glacier can change its surroundings.