A glacier is a huge mass of ice that flows slowly over land. They form in the cold polar regions and in high mountains. The low temperatures in these places enable large amounts of snow to build up and turn inti ice. Most glaciers range in thickness from about 91 to 3,000 meters. Glaciers are formed when more snow falls during the winter than melts and evaporates in summer. The extra snow gradually builds up in layers. Its increasing weight causes the snow crystals under the surface to become compact, grainlike pellets. At depths of 15 meters or more, the pellets are further compressed into thick crystals of ice. These crystals combine to form glacial ice. The ice eventually becomes so thick that it begins to move under the pressure of its own great weight.
Huge moving sheets of ice are wearing down parts of the earth's surface. In Antarctica and Greenland, ice covers all but the tips of the highest mountains. These ice sheets are located in Greenland, Antarctica and other frozen parts of North and South Pole. The ice sheet that covers much of Greenland is growing smaller because of a gradual rise in temperature in the area since the early 1900s.