As shown in Figure Stresses are greater in areas where the ice surface is relatively steep. Like rock, ice behaves in a brittle fashion under low pressure conditions shallow depths in the glacier , and plastically at higher pressures deeper in the glacier. Stress also affects how ice deforms; at high stress ice will either break or deform plastically ductile deformation depending on the pressure conditions.
Under brittle deformation conditions low pressures, shallow depths in the glacier , stress is released when the ice cracks, so does not build up to high values. Within the upper 50 — m of ice above the dashed red line, in Figure Under ductile deformation conditions higher pressures deeper in the glacier , stress can accumulate, and the ice will flow plastically in response to that stress. Ice deforms plastically if deeper than about m in the glacier, and in this region stress levels can accumulate to high values kilopascals or greater, Figure When the lower ice of a glacier flows, it moves the upper ice along with it.
It may seem from the stress patterns red numbers and arrows in Figure The lower ice deforms flows and the upper part is carried along and deforms through brittle deformation if subjected to sufficient stress. The upper part of the glacier moves faster than the base of the glacier because there is friction between the base of the glacier and the surface beneath it that slows the movement of the ice at the base.
The plastic lower ice of a glacier can flow over irregularities in the rocks under the glacier. However, the upper rigid ice cannot flow in this way, and because it is being carried along by the lower ice, it tends to crack in locations when the lower ice flows over changes in the topography below the glacier.
This leads to formation of crevasses in areas where the rate of flow of the deeper, plastic ice is changing. In the area shown in Figure In addition to deformation, another important aspect of glacier flow is basal sliding , which is sliding movement between the base of the glacier and the underlying material. The base of a glacier can be cold below the freezing point of water or warm above the freezing point. If it is warm, a film of water can form between the ice and the material underneath, and the ice will be able to slide over this surface Figure If the base is cold, the ice will be frozen to the material underneath and it will be stuck — unable to slide along its base.
In this case, all the movement of the ice will be by internal flow. There are several factors that can influence warming of the ice and basal flow at the base of an alpine glacier. Friction between the base of the glacier and the surface underneath generates heat and can lead to melting of the ice at the base of the glacier.
Rainwater and meltwater from upper regions of the glacier can percolate down and transfer heat to warm the base of the glacier and enhance basal sliding, particularly in warmer seasons.
Geothermal heat from below also contributes to melting at the base of glaciers in regions with high heat flow due to volcanic activity. Another factor that controls the temperature at the base of a glacier is the thickness of the ice. The force of gravity acting on thicker ice can enhance friction and melting at the base. Ice is also a good insulator so can prevent accumulated heat from escaping. The leading edge of an alpine glacier is typically relatively thin see Figure Since the leading edge of the glacier is frozen to the ground, and the rest of the glacier behind continues to slide forward, this causes the trailing ice to be pushed or thrust over top of the leading edge, forming thrust faults in the ice Figure Just as the base of a glacier moves slower than the surface, the edges, which are more affected by friction along the channel walls, also move slower.
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