how and why glaciers move
...the firn line’. Ice moves in three main ways, internal deformation, basal sliding and bed deformation. Ice always moves in the same direction, following gravity, whether it is advancing or retreating. Retreating and advancing refers to the relative position of the glacier terminus (nose). Retreating ice means that ablation exceeds accumulation, and the mass of ice is decreasing. Advancing ice means that accumulation exceeds ablation and the mass of ice is increasing. Both of these affect the rate of movement. Internal deformation is when Ice deforms under its own weight because of gravity. The deformation is because of the sum of tiny movements on the faces of the ice crystals making up the glacier. The thicker the ice, the faster the flow because of internal deformation. The warmer the ice, the faster the movement and the higher the pressure, the faster the movement. Movement of a glacier by internal deformation is very slow, at about tens of meters per year. E.g. Much of the movement of the interior of the Antarctic ice sheet is by internal deformation. Basal Sliding is the second way ice can move. True basal sliding means that the base of the ice sheet is near the pressure melting point, and that some water is present. The pressure melting point is reached because high pressure actually reduces the temperature at which ice will melt. The thicker the ice, the lower the temperature at which it will melt, and the higher the chance there will be water at the base therefore increasing rates of movement. Large parts of the West Antarctic Ice Sheet are at the basal melting point. The reason the ice moves faster when the water is present is because water reduces friction. The water may also come from rain water or surface meltwater that has worked through the cracks in the ice. Or, it may originate from melting upstream in the glacier. This water then flows toward the terminus (nose) of the glacier. Movement by basal sliding is ten time faster than movement by internal deformation. Basal sliding is extremely important in how much a glacier erodes the landscape, and the features that are created by the ice. Deforming Substrate is the third and final way the glaciers can move, water is not the only material that can cause sliding sediment, or the rock debris under the ice sheet, also can increase movement at the base of a glacier. If the glacier is sitting on a soft sediment bed that has some water in it, the sediment can move and carry the ice sheet with it just as if it were riding on a water layer. Valley glaciers of Alaska winding through the mountains, carrying material and sculpting the landscape around them. 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. 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 Fiord. In only two months, the glacier had dammed water in the fjord and created a lake. The speed at which glaciers move depends on a number of factors, including their temperature, the amount of meltwater at the bottom of the ice, the steepness of the slope, and the nature of the rock surface over which they move. As temperatures falls (a climatic deterioration) the ice thickens because of this increase in pressure and mass the ice moves faster, it advances and erodes and transports more vigorously on the other hand as temperatures rise (a climatic amerlioration) the glacier shrinks therefore slowing movement and the ice fronts recedes eroding and transporting less material. If there is an abrupt change in temperature ice movement may cease altogether leaving stag...