When Does a Glacier Move?
A glacier moves because the huge pressure of the ice and snow lowers the freezing point of its lower layers, causing them to melt. Consequently, the river of ice tends to flow like tar, although very slowly. The glacier must be quite old before enough material accumulates to exert the pressure needed to move it—usually when it is about 60 feet thick.
The movement of glaciers is too slow to be noticed by the eye, but measurements have shown they may travel as much as 150 feet a day. The bodies of mountaineers buried by avalanches have been carried several miles in a few years. Glaciers cover 10 per cent of the earth’s land surface.
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 created as it slides along the ground’s surface, and in some cases 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.
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. For most glaciers, retreating and advancing are very slow occurrences, requiring years or decades to have a significant effect.
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. Glaciers that once terminated in the ocean have now receded onto land, retreating far up valleys. Over the past several decades, scientists and researchers have begun to capture data and photographic evidence of this recession over time.
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.