The continent consists of two great regions, East Antarctica and West Antarctica. East Antarctica, the larger of the two, lies mainly between the Transantarctic Mountains and the Indian Ocean. It is more than three times the size of West Antarctica and consists of an ice-capped block of geologically old, stable rock, largely above sea level. In contrast, West Antarctica, with much of its land below sea level, is an ice-covered archipelago of geologically more recent origin. Many of the mountain peaks visible above the ice cap were volcanically formed; some are active volcanoes. Probably best known is Mount Erebus, which towers high above McMurdo Station, a United States scientific base on Ross Island.
The Antarctic ice cap covers some 98 per cent of the continent's surface. It rises as an immense, somewhat flattened dome to more than 13,000 feet (3,960 m) above sea level in the interior. No other continent has as high an average elevation as Antarctica. The ice cap averages 7,000 to 8,000 feet (2,130 to 2,440 m) in thickness; its greatest known thickness is 15,670 feet (4,776 m). Across much of the continent high winds have packed the snow into parallel ridges called sastrugi, which form a major obstacle to aircraft landings and surface transportation. In some places the surface of the ice cap is fractured by wide cracks, or crevasses, often hidden by thin "bridges" of snow.
The Transantarctic Mountains extend across the continent in an S-curve from the Theron Mountains of Coats Land through the Queen Maud Mountains to Cape Adare in Victoria Land---a distance of about 2,300 miles (3,700 km). Their highest peaks and ridges rise above the polar ice, reaching heights of more than 14,000 feet (4,270 m) above sea level. The continent's highest known point is in the Ellsworth Mountains, in Ellsworth Land, where the bare top of Vinson Massif lies 16,864 feet (5,140 m) above sea level. There are also lofty mountain ranges near the coasts of Marie Byrd Land and Queen Maud Land, in the American Highland, and on the Antarctic Peninsula. Jutting through the ice in numerous areas are isolated bare peaks called nunataks.
Nearly all of Antarctica's coast is buried by the polar ice cap. In several areas the ice pushes out into the ocean to form enormous floating sheets called ice shelves. The largest are the Ross Ice Shelf, the Ronne Ice Shelf, the Larsen Ice Shelf, and the Filchner Ice Shelf.
Some ice shelves are fed, in part, by huge glaciers moving down mountain valleys to the coasts. There are also glaciers that flow directly into the sea as long floating tongues, known as glacier tongues or iceberg tongues.
Many billion tons of ice break off from the cliff-like outer edges of the ice shelves each summer and, characteristically in the form of flat-topped icebergs, drift northward. Other masses of ice, called sea ice, originate from the freezing of seawater and extend great distances offshore, especially during winter. The breakup of the sea ice during summer gives rise to freely floating ice known as pack ice.