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5.2: Classification of Glaciers

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  • One good way of classifying glaciers is by the extent to which their shape and movement are affected by the underlying bedrock topography.

    Glaciers not strongly constrained by underlying topography

    Ice sheet: Superimposed on underlying topography; largely or entirely submerges that topography; ice flow reflects largely the size and shape of the glacier, less the shape of the ground. Ice sheets smaller than about 50,000 km2 in area are called ice caps.

    Ice dome: About symmetrically over land area involved; the top of the dome may be over bedrock highs or bedrock lows. Highest: 4300 m, Antarctica. The convex profile is a reflection mainly of flow mechanics, including the nature of the bottom roughness.

    Outlet glacier: A stream of ice that extends beyond an ice dome and drains it. Outlet glaciers are often very large: several hundred kilometers long and tens of kilometers wide. Commonly they are commonly associated both with large ice sheets and with smaller ice caps. Most of the Greenland ice sheet and about three-quarters of the Antarctic ice sheet is drained in this way.

    Ice shelf: A floating ice sheet that deforms under its own weight. The slab of ice is squeezed between the atmosphere and the ocean. The ice shelf has to be anchored at several points. In Antarctica, ice shelves constitute about 7% of ice area. The ice shelf may be fed partly or not at all by land glaciers. Accumulation: snow on the surface; land glaciers; bottom freezing. Ablation: melting (at the top or the bottom); calving.

    Glaciers strongly constrained by topography

    Ice field: An approximately level area of ice, not an ice cap because not domelike; the flow reflects the underlying bedrock topography (ice fields are usually nestled in among mountains). Size: a few square kilometers to very large. Ice fields grade over into small ice caps. Requirement for existence of an ice field: high and overall gentle topography.

    Valley glacier: A glacier flowing in a rock valley and surrounded by rock walls (therefore long and linear). Fed from an ice field or a cirque (see below). Usually 10–30 km long, but up to 100 km. The terminus can be on land or in the sea (by calving). A valley glacier can also disgorge from mountains and spread out as a flat mass in piedmont areas to form a what is called a piedmont glacier. A good example of a piedmont glacier: the Malaspina Glacier, in Alaska, 40 km across. Valley glaciers are usually vigorous glaciers. And because they are typically present at low as well as high latitudes, they are among the most accessible of glaciers.

    Cirque glacier: A small ice mass, fairly wide relative to its length, occupying a bedrock hollow or basin, usually on a mountain slope.