Glaciers are sometimes described as rivers of ice. This is misleading, because the glaciers associated with ice ages and with the Greenland and Antarctic ice caps are not like rivers, although they flow.
A glacier begins as a fall of winter snow that fails to melt the following summer. This snow is called firn and its edge, beyond which the winter snow does melt in summer, is the firn line. More snow falls the following winter, to become a new layer of firn, and its weight compresses the snow beneath it until the snow turns into solid ice.
Although it does not melt in summer, some snow and ice are lost. When the Sun shines strongly, ice can vaporize without melting first. This is called sublimation. The wind can also remove loose, powdery snow. This type of loss is called ablation. If there is a zone of accumulation, where the winter snowfall exceeds losses by sublimation and ablation, then a glacier will form.
Obviously, it will form first on high ground, where the temperature is lowest. After years or decades of accumulation, the weight of the glacier will start the ice at the base sliding downhill. The upper layer of ice is solid and brittle, but the pressure from above makes the ice near the base slightly plastic, so it will bend and slide over the underlying rock surface. That is when the glacier begins to flow. As the base of the glacier moves, the brittle surface ice breaks up into sections with deep crevasses between them. The surface of a glacier is extremely rough and uneven—and not in the least like the surface of a river.
The glacier may flow between hills by breaking and grinding away the softer rock to form a U-shaped valley, in which case it is known as a valley glacier, or it may spread to cover low-lying ground and become an ice sheet. The glacier ends at a zone of ablation, where the annual loss of ice by sublimation, melting, and ablation balance the quantity of ice being pushed forward.
Rocks, gravel, and other material that is pushed forward by the glacier stop moving when they reach the zone of ablation. That is where the debris accumulates to become a moraine— known as a terminal moraine if it lies ahead of the tip, or snout, of a glacier. Material thrust to the side forms a lateral moraine. Geologists and geomorphologists—scientists who study the way landscapes form—use the presence of moraines to determine the extent of glaciers that disappeared long ago.
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