Antarctica - ăntärkˈtĭkə, –ärˈtĭkə, the fifth largest continent, c.5,500,000 sq mi (14,245,000 sq km), asymmetrically centered on the South Pole and almost entirely within the Antarctic Circle.

Geology and Geography

Antarctica consists of two major regions: W Antarctica (c.2,500,000 sq mi/6,475,000 sq km), a mountainous archipelago that includes the Antarctic Peninsula, and E Antarctica (c.3,000,000 sq mi/7,770,000 sq km), geologically a continental shield. They are joined into a single continental mass by an ice sheet thousands of feet thick. At the seaward margins of the ice sheet masses of ice break off and float away as icebergs, leaving ice cliffs. Where the outward creep of the ice is channeled into ice streams (zones of more rapid flow), great floating ice tongues project into the sea; where mountains retard outward movement, the flow is channeled into great valley glaciers.

Less than 5% of Antarctica is free of ice; these areas include mountain peaks, arid "dry valleys," small coastal areas, and islands. Except for mountain ranges (some buried beneath the ice), much of E Antarctica's rock surface is near sea level; however, the continent's domed, snow-covered glacial surface rises to about 13,000 ft (4,000 m). In W Antarctica there is great variation in the subglacial relief, suggesting mountainous islands or submerged ranges separated by deep sounds beneath the ice cover. Since the 1970s lakes of liquid water have been found underneath the continental ice; the largest known of these is Lake Vostok (174 mi by 36 mi/280 km by 60 km; 1600 ft/500 m deep), which lies 2.5 mi (4 km) beneath the Russian Vostok research station in E Antarctica.

The two major coastal indentations are the Ross Sea, facing the Pacific Ocean, and the Weddell Sea, facing the Atlantic Ocean. At the head of each sea are great ice shelves, the Ross ice shelves in the Ross Sea and the Ronne and the Filchner ice shelves in the Weddell Sea. Partly aground but mostly afloat, these nearly level ice shelves are from 600 to 4,000 ft (180–1,220 m) thick. They move steadily toward the sea and are fed by valley glaciers, ice streams, and surface snow accumulations. Smaller ice shelves are found all along the coast.

The Transantarctic Mts (c.3,500–14,300 ft/1,100–4,400 m high), which extend from the east side of the Filchner Ice Shelf to the western portal of the Ross Sea, form the inner margin of E Antarctica. Primarily formed by block faulting (see mountains), the lower slopes have a complex structure of late Precambrian and early Paleozoic metamorphic rocks. These are overlaid by essentially horizontal sedimentary rock, mainly of continental or near-shore origin and ranging in age from the Devonian period to the early Jurassic, which are similar to rocks found in Australia, S Africa, and E South America; coal-bearing Permian strata are also found there. Distinctive plant, insect, fish, and animal fossils in the Triassic and Jurassic strata strongly indicate that the continents of the Southern Hemisphere are parts of an ancient supercontinent, Gondwanaland, which broke up in the late Mesozoic era. The continents have since drifted to their present positions.

The ice-drowned, mountainous archipelago of W Antarctica is related to the Andes Mts. of South America and is structurally connected to them by way of the Antarctic Peninsula and the Scotia Arc (South Georgia and the South Orkney and South Sandwich islands). The complex structure consists of highly folded metasedimentary strata from Paleozoic to Pliocene epochs. There has been much volcanism down to the present. Mountains of the Antarctic Peninsula rise to c.11,000 ft (3,350 m); the mountains of Marie Byrd Land have comparable heights. The Ellsworth Mts., at the head of Ronne Ice Shelf, are the highest in Antarctica; Vinson Massif (16,860 ft/5,140 m) is the continent's highest peak. A variety of mineral deposits have been discovered in Antarctica, but the extent of the deposits is largely unknown and their relative inaccessibility makes their utility doubtful.

Antarctica is surrounded by the world's stormiest seas. A belt of pack ice surrounds the continent; only a few areas are ice-free at the end of most summers. The physical boundary most widely accepted today for the antarctic region is the Antarctic Convergence, a zone c.25 mi (40 km) wide encircling the earth roughly between the 50th and 60th parallels of latitude. Within this zone the colder and denser north-flowing antarctic surface waters sink beneath warmer and saltier subantarctic waters; the difference in temperature and chemical content of the water on the two sides of the zone is reflected in noticeable differences in air temperature and in marine life.

Climate

Antarctic climate is characterized by low temperature, high wind velocities, and frequent blizzards. Rapidly changing weather is typical of coastal locations, where temperatures for the warmest month average around freezing. Winter minimums drop as low as −40 degrees Fahrenheit (−40 degrees Celsius). High altitude and continuous darkness in winter combine to make the interior of Antarctica the coldest place on earth. Summer temperatures are unlikely to be warmer than 0 degrees Fahrenheit (−18 degrees Celsius); winter mean temperatures are −70 degrees Fahrenheit (−57 degrees Celsius) and lower. The lowest temperature ever recorded on earth was −126.9 degrees Fahrenheit (−88.3 degrees Celsius) at Vostok, a Russian station. Precipitation is in the form of snow; the annual water equivalent in the interior is c.2 in. (5 cm) and c.10 in. (25 cm) in coastal areas. In the dry, dust-free air one can see for tens of miles in clear weather; distances are deceptive, and mirages are common. Scattering of light by blowing snow or low clouds causes "whiteouts" in which the sky blends with the snow-covered surface, eliminating the horizon; no condition is more feared by aviators.

Antarctic Life

There is no native human population in Antarctica, nor are there any large land animals. Few species are adapted to the antarctic environment, but individuals of these few species are numberless. Life that depends completely on the land is limited to microscopic life in summer meltwater ponds, tiny wingless insects living in patches of moss and lichens, and two types of flowering plants (both in the Antarctic Peninsula). Birds and seals that spend part of their time on land (e.g., emperor and Adélie penguins and the brown skua—the most southerly bird and a notorious predator—and Weddell, crabeater, and Ross seals) are dependent on the surrounding sea for food. Antarctic waters are rich in plankton, which serves as food for krill, small shrimplike crustaceans that are the principal food of baleen whales, crabeater seals, Adélie penguins, and several kinds of fish.

Fur and elephant seals, which spend the summers on islands north of lat. 65°S were the basis for 19th-century commercial activity in Antarctica. In the 20th cent., commercial interest shifted to baleen whales. Fur seals are recovering from the slaughter of the 19th cent., as are the elephant seals. Whaling has been declining since the peak year of 1930–31. In 1986 the International Whaling Commission imposed a moratorium on commercial whaling; the moratorium, however, has not been adhered to by all nations.

History of Exploration

Early Expeditions

Although there was for centuries a tradition that another land lay south of the known world, attempts to find it were defeated by the ice. Antarctica's frigid nature was revealed by the second voyage (1772–75) of the English explorer Capt. James Cook. He did not see the continent as he circumnavigated the world, but he was the first to cross the Antarctic Circle. British and U.S. seal hunters followed him to South Georgia, an island in the S Atlantic.

In 1819 the British mariner William Smith discovered the South Shetland Islands. Returning in 1820, he and James Bransfield of the British navy explored and roughly mapped the Shetlands and part of the shore of the Antarctic Peninsula. Searching for rookeries, sealers explored the coastal and offshore regions of the Antarctic Peninsula. Most notable were the British captains James Weddell, George Powell, and Robert Fildes and the Americans Nathaniel B. Palmer, Benjamin Pendleton, Robert Johnson, and John Davis. Davis made the first landing on the antarctic continent (Feb. 7, 1821) at Hughes Bay on the Antarctic Peninsula. First to spend the winter in Antarctica, on King George Island in 1821, were 11 men from the wrecked British vessel Lord Mellville.

After 1822 fur sealing declined, but in 1829–30 Palmer and Pendleton led a sealing and exploring expedition that included Dr. James Eights, the first U.S. scientist to visit Antarctica. John Biscoe, a British navigator, circumnavigated Antarctica from 1830 to 1832, sighting Enderby Land in 1831 and exploring the western side of the Antarctic Peninsula in 1832. John Balleny and Peter Kemp were other British sealers who made discoveries in E Antarctica in the 1830s.

Four naval exploring expeditions visited Antarctica in the first half of the 19th cent. Capt. T. T. Bellingshausen was the leader of a Russian expedition that circumnavigated Antarctica (1819–21). He apparently was the first to see (1820) the part of the continent that is now called Queen Maud Land. In W Antarctica he discovered (1821) Peter I Island and Alexander Island. Admiral J. S. C. Dumont d'Urville led a French expedition to the Pacific Ocean that made two visits to Antarctica. He explored in the area of the Antarctic Peninsula in 1838 and in 1840 discovered Clarie Coast and Adélie Coast in E Antarctica. In 1840 Lt. Charles Wilkes, leader of the U.S. Exploring Expedition to the Pacific (1838–42)