Glaciers are formed in high mountains or around the polar region. During the winter the snow piles up and the glacier grows. But during the summer some of the snow melts and evaporates. Year after year the snow piles up in layers. The increasing weight causes the snow crystals under the surface to become compact and turn into grainlike pellets called firn. At depths of 50 feet the firn is compressed further into dense crystals of glacial ice.
As years pass the slab of ice gets bigger and grows thicker and becomes too big to stand still. The ice starts to move down the hill. When the ice moves it is called a glacier.
During the summer some of the snow and ice melts but in some areas of the glacier the temperature doesn’t rise high enough to melt the snow and ice. Glaciers may also increase and decrease in size because of the changes in the climate around the glacier. For example, the ice sheet on Greenland is shrinking because of the rise in the temperature in the area.
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