ROSS ICE SHELF

world's largest body of floating ice, lying at the head of Ross Sea (q.v.), itself an enormous indentation in the continent of Antarctica. The ice shelf lies between about 155 W and 160 E longitude and about 78 S and 86 S latitude. Estimates of its area range from about 192,000 to 208,000 square miles (496,000 to 540,000 square km), making it approximately the size of France. The shelf has served as an important gateway for explorations of the Antarctic interior, including those carried out by many of the most famous expeditions. The great white barrier wall of the shelf's front, first seen in 1841 by the British polar explorer James Clark Ross, rises in places to 160 or 200 feet (50 or 60 m) high and stretches about 500 miles (800 km) between fixed anchor points on Ross Island to the west and the jutting Edward VII Peninsula on the east. With its immense, gently undulating surface reaching back nearly 600 miles (950 km) southward into the heart of Antarctica, the Ross Ice Shelf provides the best surface approach into the continental interior. The McMurdo Sound region on the shelf's western edge thus became the headquarters for Robert F. Scott's 191112 epic sledging trip to the South Pole and also served several Antarctic research programs later in the century. The eastern barrier regions of the ice shelf were headquarters for the Norwegian Roald Amundsen's first attainment of the South Pole on Dec. 14, 1911; for Richard E. Byrd's three U.S. expeditions of 192841 at Little America IIII stations; and for several subsequent expeditions and research programs. The Ross Ice Shelf is fed primarily by giant glaciers, or ice streams, that transport ice down to it from the high polar ice sheet of East Antarctica. The ice shelf has been likened to a vast triangular raft because it is relatively thin and flexible and is only loosely attached to adjoining lands. Giant rifts develop behind the ice shelf's barrier wall and occasionally rupture completely to spawn the huge tabular icebergs that are so characteristic of the Antarctic Ocean. Thus, although the barrier's position appears almost stationary, it actually undergoes continual change by calving and melting that accompany northward movement of the shelf ice. The shelf's mean ice thickness is about 1,100 feet (330 m) along a line at about 79 S latitude. In a southward direction along about 168 W longitude, the ice shelf's thickness gradually increases to more than 2,300 feet (700 m). Estimates suggest that at distances of 100 to 200 miles (160 to 320 km) inland from the barrier, 15 to 20 inches (380 to 500 mm) of ice may be added to the shelf each year by bottom freezing. An estimated net of 4 1013 pounds (1.8 1013 kg) per year is added to the whole shelf by bottom freezing.