 Glaciers are large areas of ice that flow slowly over land. They are found in the coldest regions of the world. These regions are the North and South Poles and the high mountain valleys. The low temperatures in these places allow large amounts of snow to build up and turn into ice.
Image to left: Sectional view of a valley glacier. Cracks in its surface are called crevasses. As it moves, the glacier picks up rocks and piles them in ridges called moraines. Credit: World Book illustration by Oxford Illustrations Ltd., adapted from a drawing by Janet Allin
There are two main kinds of glaciers. These are called continental glaciers and valley glaciers. Continental glaciers are wide, very thick ice sheets. They cover huge areas of land on Greenland and Antarctica. These glaciers build up at their center and slope outward. They flow toward the sea in all directions.
Valley glaciers are long, narrow rivers of ice. They move slowly down high mountain valleys. They are found in the European Alps, the Southern Alps of New Zealand, and other mountain ranges nearer the North and South Poles.
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Glaciers form when more snow falls during the winter than melts in summer. The extra snow builds up in layers. Its increasing weight causes the snow under the surface to turn to ice. The ice becomes so thick that it spreads and moves under its own weight.