Glaciers form when accumulating snow compresses into firn and eventually turns into ice. In some cases, perennial snow accumulates on the ground and lasts all year. This makes a snowfield and not a glacier since it is a thin accumulation of snow. Snow and glacial ice actually have a fair amount of void space (porosity) that traps air. As the snow settles, compacts, and bonds with underlying snow, the amount of void space diminishes. When the snow gets buried by more snow, it compacts into granular firn (or névé) with less air and it begins to resemble ice more than snow. Continual burial, compression, and recrystallization make the firn denser and more ice-like. Eventually, the accumulated snow turns fully to ice, however, small air pockets remain trapped in the ice and form a record of the past atmosphere.
As the ice accumulates, it begins to flow downward under its own weight. An early study of glacier movement conducted in 1948 on the Jungfraufirn Glacier in the Alps installed hollow vertical rods in the ice and measured the tilt over two years. The study found that the top part was fairly rigid and the bottom part flowed internally. A P-T diagram of ice shows that ice actually melts under pressure (one of the unique properties of water) so ice at the base of a typical glacier is actually melting. About half of the overall glacial movement was from sliding on a film of meltwater along the bedrock surface and a half from internal flow . These studies show that the ice near the surface (about the upper 165 feet [50 meters] depending on location, temperature, and flow rate) is rigid and brittle . This upper zone is the brittle zone, the portion of the ice in which ice breaks when it moves to form large cracks along the top of a glacier called crevasses. These crevasses can be covered and hidden by a snow bridge and thus are a hazard for glacier travelers.
Below the brittle zone, there is so much weight of the overlying ice (typically exceeding 100 kilopascals-approximately 100,000 times atmospheric pressure) that it no longer breaks when force is applied to it but rather it bends or flows. This is the plastic zone and, within this zone, the ice flows. The plastic zone represents the great majority of the ice of a glacier and often contains a fair amount of sediment from as large as boulders and as small as silt and clay which act as grinding agents. The bottom of the plastic zone slides and grinds across the bedrock surface and represents the zone of erosion.
3. Gerrard, J. A. F., Perutz, M. F. & Roch, A. Measurement of the velocity distribution along a vertical line through a glacier. Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences 213, 546–558 (1952).