Only about 10 per cent of the world's land surface is occupied by glaciers today; the ice-sheets of Greenland and Antarctica make up the great bulk of this amount. In the northern parts of Asia and North America large areas exist where freeze-thaw action is one of the dominant processes. This is the periglacial region. The study of the behavior of glacial ice is known as glaciology.
Glaciers will only exist and grow if the annual accumulation of snow exceeds the annual ablation, or melting. Ablation occurs by radiation and by conduction when it rains in summer, the bulk of the melted snow being carried off as glacial melt water. The relation between the amounts of accumulation and ablation is known as the glacier mass budget.
The mass budget determines whether the glacier will advance or retreat: there is usually a lag of several years between a change in the annual budget and the response of the glacier snout, and this enables a study of the net annual budget to be used to predict glacier behavior. The lowest point at which snow manages to remain each year marks the summer firn line, which delimits the two spatial parts of a glacier, an accumulation zone and an ablation zone.
Glacial Erosion. Glacier Erosion processes, taking place at the sole of the glacier, are nearly all mechanical effects. Two principal processes are regarded as important: abrasion and plucking. Abrasion, the grinding and crushing of rocks, is not accomplished by the ice itself as it is too soft, but by the rock debris frozen into the lower layers of the ice. The debris load acts rather like a coarse sandpaper, and also abrades itself as it is moved by the glacier. Plucking or quarrying occurs in response to the drag exerted by the moving ice on the bedrock. Plucking is most effective where the rock has already been weakened by being well jointed, or by being subject to pressure-release and freeze-thaw action beneath the glacier.
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