Once the ice reaches a thickness of about 20 meters (66 ft) it will begin to move under the pressure of its weight. Glaciers move across the surface by internal deformation and basal slip. Under the weight of accumulating ice,the ice is deformed and begins to move by pseudo-plastic flow. Glaciers slip over the surface lubricated by meltwater at their base. Generally speaking, flow velocity in a glacier is greatest near the surface of the ice and decreases towards the bottom. The surface moves faster than the base does due to internal deformation and basal slipping. The actual forward movement of a particle of ice in the glacier is about 1,000 feet per year. A typical glacier will move at about 10 inches a day, though some move more rapidly like Greenland's Jakobshavn glacier. [ View "Fastest Glacier" from Nova scienceNow] Variations in the speed of the ice caused by surface irregularities results in differential expansion and compression of the ice and the development of crevasses. A deadly situation for hikers, crevasses can open and close with little warning.
A glacial surge occurs as an abrupt movement that can cover tens of meters per day. The exact cause is not well-known, but may result from water pressure building at the base which "floats" the glacier. In 1986 the Hubbard Glacier surged across the mouth of the Russell fjord in Alaska cutting it off from Yakut Bay. Glacier movement exceeded 112 feet per day, compared to a normal rate of 10 inches per day! The Black Rapids Glacier, AK in Figure19.7 is a surging glacier. Tributary glaciers enter the main valley from the left. Massive amounts of glacial sediment have been deposited along the sides of the glacier and up against the valley walls. The tell-tale sign of a surging glacier are the looped moraines that snake their way across the ice.
Figure19.7 Black Rapids Glacier. Alaska