The average thickness of ice in Antarctica is about 1 mile (1.6 km).
An ice sheet is a mass of glacial ice of more than 50,000 square kilometers (19,000 square miles). Ice sheets contain about 99% of the freshwater on Earth and are sometimes called continental glaciers. As ice sheets extend to the coast and over the ocean, they become ice shelves.
A mass of glacial ice covering less area than an ice sheet is called an ice cap. A series of connected ice caps is called an ice field. Making up ice fields, ice caps, and eventually, ice sheets are individual glaciers.
Today, there are only two ice sheets in the world: the Antarctic ice sheet and the Greenland ice sheet. During the last glacial period, however, much of the Earth was covered by ice sheets.
Ice sheets formed like other glaciers. Snow accumulates year after year, then melts. The slightly melted snow gets harder and compresses. It slowly changes texture from fluffy powder to a block of hard, round ice pellets. New snowfalls and buries the grainy snow. The hard snow underneath gets even denser. It is known as firn.
As the years go by, layers of firn build on top of each other. When the ice grows thick enough—about 50 meters (165 feet)—the firn grains fuse into a huge mass of solid ice. At this point, the glacier begins to move under its weight.
Ice sheets tend to be slightly dome-shaped and spread out from their center. They behave plastically, or like a liquid. An ice sheet flows, oozes, and slides over uneven surfaces until it covers everything in its path, including entire valleys, mountains, and plains.
Compression and geothermal energy sometimes cause the bottom of an ice sheet to be slightly warmer than the ice above it. The bottom of the ice sheet melts, causing the ice above it to move at a faster rate than the rest of the ice sheet. These fast-moving glaciers are called ice streams.
Ice streams can move as quickly as 1,000 meters (.6 mile) every year. The slightly warmer, softer ice of the ice stream is where most of the ice sheet's crevasses are located.