At the poles and on high mountains, vast areas are covered in ice—in rivers of ice called glaciers, and in layers of ice called ice sheets or ice caps. Ice is a major force of erosion on land, where glaciers gouge deep valleys in the landscape.
Glaciers form when more snow falls each winter than melts each summer. As the snow builds up, the top layers press down on the layers below and turn them to ice. When enough ice has formed, the glacier’s great weight and gravity set it moving slowly downhill, at a rate of about 3.3 ft (1 m) a day. The glacier continues to advance as long as more snow is falling at its top than ice is melting at its tip.
Ice caps are vast, domed sheets of ice that cover the land in the polar regions. They are formed as snow builds up year after year, creating a thick layer of ice. Three-quarters of the world’s fresh water is locked up in the polar ice caps. The ice cap covering Antarctica at the South Pole is over 2.5 miles (4 km) deep.
Icebergs form where glaciers and ice caps meet the ocean. Huge chunks of ice “calve,” or break off, and fall into the water. Ocean currents may then carry the floating ice to warmer waters, where it can endanger shipping. In 1912, the luxury liner Titanic was sunk by an iceberg, and more than 1,500 people drowned.