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glaciers in which the ice flows from ice divides located in the interior to the periphery; the ice flows in the direction of the surface slope, without a direct dependence on subglacial relief.
Ice sheets form in areas where the snow line descends to the level of the lowlands. Small ice sheets (particularly those having steeply sloped surfaces, the domes) are also encountered on high plateaus. The position of the ice divides and the shape of the surface are caused by the distribution of accumulation and the drainage conditions; that is, both are indirectly dependent on subglacial relief. If subglacial features are small in comparison with ice thickness, then the radial profile of the surface approaches a semi-oval, and convexity of the semi-oval increases as the glacier becomes smaller. To a small degree, the convexity of the profile also increases with a drop in ice temperature and with an acceleration in accumulation. The loss of ice in Antarctica occurs predominantly by the discharge of ice into ice shelves and the breaking off of icebergs, and in warmer regions by melting in the marginal zone.
Of the total area of ice sheets (14.4 million sq km), 85.3 percent is accounted for by the continental ice sheets of Antarctica, which consist of five large contiguous sheets and a number of peripheral small sheets and domes. The ice sheets of Greenland constitute 12.1 percent, and 2.6 percent is constituted by the small ice sheets of the Canadian Arctic Archipelago, Iceland, Spitsbergen, Franz Josef Land, Novaia Zemlia, Severnaia Zemlia, and other polar islands, as well as by the mountain regions of Patagonia and the Scandinavian Peninsula. Ice sheets attained even greater development during periods of the culmination of Pleistocene glaciation.
Continental ice sheets are the world poles of cold: in the center of Antarctica the mean annual temperature reaches — 61°C, and in the center of Greenland it reaches — 32°C. The temperature of the ice is negative to the bottom (in the center of Antarctica the ice temperature on the bottom is as low as — 30°C), but in the narrow marginal zone, in deep basins of the sea floor (to — 2.6 km below sea level), and under outlet glaciers as well as in southern Greenland and the north of the Antarctic Peninsula, the temperature reaches the melting point.
The rate of movement increases from the ice divides to the periphery, where there is a differentiation into inactive areas (above rises in the floor) and outlet glaciers in which the speed reaches several kilometers per year.
P. A. SHUMSKII