Glaciers: Mountain

Glacier Formation

Glaciers form where snow and ice accumulation exceed snow and ice melt. As the snow and ice thicken, there will be a point where the mass begins to move due to a combination of the slope the snow is lying on and the pressure of the overlying snow and ice. On steeper slopes, this can occur with as little as 20 m (meters) of snow-ice. There are two types of mountain glaciers, based on their temperature: polar and temperate. A polar glacier has cold ice throughout, never reaching the melting point; these glaciers are confined to the Arctic region and very high-altitude locations with dry climates. A temperate glacier is at the melting point throughout. The snow that forms temperate glaciers is subject to repeated freezing and thawing, which quickly changes it into rounded granular ice, or firn. Over a period of years, the firn undergoes further compaction, as well as grain growth via melting and refreezing, and sinters together with neighboring firn grain, creating glacier ice with air bubbles trapped between the grains. This process takes from 3 to 10 years on temperate glaciers, depending mostly on the accumulation rate—the higher the accumulation rate, the faster the process. On a polar glacier, without the melting and refreezing, the process takes much longer. The accumulation rate is lower on polar glaciers as well, further slowing the process of firn and glacier ice formation, which now must occur mainly via compaction.

The smallest alpine glaciers form high on the mountain slopes and are niche, slope, or cirque glaciers. These glaciers occupy locations that favor snow accumulation from drifting or avalanching and/or provide some radiational shading. As a mountain glacier increases in size, it can begin to flow downvalley, creating a valley glacier. Larger alpine glaciers can merge into a large area, covering a portion of a mountain range; these are ice caps or ice fields, such as the Barnes Ice Cap, Baffin Island, or Juneau Icefield, Alaska. In Patagonia and Alaska, large outlet glaciers from ice fields reach the ocean, to become tidewater glaciers. The larger glaciers, like rivers, are formed from tributary glaciers joining together into a larger trunk outlet glacier.

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