Movement of ice sheets. An ice sheet moves downslope in a number of directions from a central area of high altitude and is not restricted to a channel or valley. The ice sheet must expand because of the constant accumulation of ice and snow. Ice sheets do not move as quickly as alpine glaciers because there is less slope and more mass involved. Ice sheets move mostly by plastic flow. Mountain ranges are completely buried by the ice sheet at the South Pole, which is greater than 3,000 meters thick.
Movement of valley glaciers. Glaciers can move more than 15 meters a day. The larger volumes of ice on steeper slopes move more quickly than the ice on the more gentle slopes farther down the valley. These dynamics allow a glacier to replenish the ice that is lost in the zone of wastage. Glaciers in temperate zones tend to move the most quickly because the ice along the base of the glacier can melt and lubricate the surface. Other factors that affect the velocity of a glacier include the roughness of the rock surface (friction), the amount of meltwater, and the weight of the glacier.
Basal sliding and plastic flow. A valley glacier has various components of flow. First, the entire glacier moves as a single mass over the underlying rock surface. The pressure from the weight of the glacier generates a layer of water that helps the ice glacier move downslope. This process is called basal sliding.
In addition to basal sliding, which slowly moves the glacier downslope as a unit, plastic flow causes glacial ice buried underneath more than about 50 meters to move like a slow‐moving, plastic stream. The central and upper portions of a glacier, as do those portions of a stream, flow more quickly than those near the bottom and sides, where friction between the ice and valley walls slows down the flow. In general, the rate of plastic flow is greater than the rate of basal sliding.
Above a depth of about 50 meters, the weight of the overlying ice is not sufficient to cause plastic flow. This more rigid upper zone, which is called the zone of fracture, is carried along the top of the plastic flow piggyback style. Sometimes the zone of fracture moves faster than the underlying plastic flow. When this happens, especially down a steep slope, the surface breaks into a series of deep fissures called crevasses. Crevasses also result where a valley glacier curves because the ice flows faster toward the outside of the curve than the inside. A steep, rapid descent may result in an icefall, a piled‐up mass of splintered ice blocks from a series of rapidly formed crevasses.