Large mass of perennial ice that forms on land through the recrystallization of snow and that moves forward under its own weight. The term ice sheet is commonly applied to a glacier that occupies an extensive tract of relatively level land and that flows from the centre outward. Glaciers occur where snowfall in winter exceeds melting in summer, conditions that prevail only in high mountain areas and polar regions. Glaciers occupy about 11% of the Earth's land surface but hold roughly three-fourths of its fresh water; 99% of glacier ice lies in Antarctica and Greenland.

This entry comes from Encyclopædia Britannica Concise.
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