Glaciers

Ice masses take on a variety of characteristics as they flow and retreat. Glaciers that pour down a valley from mountainous ground, for example, usually follow paths originally formed by rivers of snowmelt in the spring and summer. These glaciers, termed alpine or mountain glaciers, end either in valleys or in the ocean and tend to increase the sharpness and steepness of the mountains surrounding them by eroding them. They are thus, partially responsible for carving the high-relief mountain peaks of the Himalayas, Andes, and alpine regions of the Cascades and Northern Rocky Mountains.

Piedmont glaciers are large, gently sloping ice mounds. Piedmont glaciers are common in Alaska, Greenland, Iceland, and Antarctica.

Glaciers often form in small bowl-like valleys called cirques on the sides of mountains. Found in Norway, Iceland, Greenland, and Antarctica, glaciers within cirques usually do not move out of their basinlike areas.

The largest form of glacier is called an ice sheet or continental glacier, a huge ocean of ice that spreads slowly outward from its center. Ice sheets may cover millions of square miles and are so heavy that they cause the continental crust beneath to float lower on the Earth's mantle, like a heavy-laden barge. The largest ice sheet is found on Antarctica, where the ice is more than 2.5 mi (4 km) thick at its center, hiding entire mountain ranges (mapped using seismic waves and radar). The Antarctic ice sheet covers more than 5 million sq mi (12.9 million sq km), which exceeds the combined areas of the United States, Mexico, and Central America. It contains about 90% of all the world's ice and 70% of its fresh water. The Greenland ice sheet is 670,000 sq mi (1,735,000 sq km) in area, covering virtually the entire island. Smaller ice sheets are found in Iceland, northern Canada, and Alaska.