Glaciers form mainly by the accumulation and compaction of snow, and are deformed by flow under the influence of gravity. When snow falls it is porous, and with time the pore spaces close by precipitation and compaction. When snow first falls, it has a density of about 1/10th that of ice; after a year or more the density is transitional between snow and ice, and it is called firn. After several years the ice reaches a density of 0.9 g/cm3, and it flows under the force of gravity. At this point glaciers are considered to be metamorphic rocks, composed of the mineral ice.
The mass and volume of glaciers constantly change in response to the seasons and to global climate variations. The mass balance of a glacier is determined by the relative amounts of accumulation and ablation (mass loss through melting and evaporation or calving). Some years see a mass gain leading to glacial advance, whereas some periods have a mass loss and a glacial retreat (the glacial front or terminus shows these effects).
Glaciers have two main zones, best observed at the end of the summer ablation period. The zone of accumulation, found in the upper parts of the glacier, remains covered by the remnants of the previous winter's snow. Below this the zone of ablation is characterized by older, dirtier ice from which the previous winter's snow has melted. An equilibrium line, marked by where the amount of new snow exactly equals the amount that melts that year, separates these two zones.
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