Antarctica is the fifth largest and southernmost continent. Its position at the South Pole, together with its elevation and ice-and-snow cover, generates the coldest climate on Earth. Its enormous ice sheet covers all but 2 to 3 percent of Antarctica and extends over the encircling ocean. If released by melting, this amount of water would cause the sea level to rise more than 60 m (200 ft) worldwide. The summer population is several thousand, but only a few hundred scientists and support personnel stay during the winter. They live in semipermanent bases, the largest of which is the U.S. base at McMurdo, Ross Island. The continent is more than half again as large as the United States and covers about 14,245,000 sq km (5,500,000 sq mi), including 1,640,000 sq km (633,000 sq mi) of floating ice shelves. At least a third of the coastline (about 30,000 km/18,600 mi) is hidden beneath perennial ice. Most of the continent lies within the Antarctic Circle (lat. 66 deg30' S). It is completely isolated by the Antarctic Ocean. The South Shetland Islands adjoin the \TAntarctic Peninsula\t. Other islands in the Antarctic region, but removed from the continent itself, include South Georgia, South Sandwich, South Orkney, Bouvet, Heard, Balleny, Scott, and Peter. These lie within the Antarctic Convergence, the fluctuating zone between about 48 deg and 60 deg south latitude where cold Antarctic surface waters descend below the warmer waters of the South Pacific, South Atlantic, and Indian oceans. Islands such as the Kerguelen Islands that lie north of the Antarctic Convergence contain glaciers and are considered loosely within the Antarctic region. Because seasonal or perennial sea ice is found along all coasts, protected harbors are not available to ships much of the year. The continent may be subdivided into two major parts on the basis of topography and geology: west Antarctica, which is south of South America, and east Antarctica, in the Eastern Hemisphere. The boundary between them roughly follows the 0 deg-180 deg meridian, from the east side of the Ronne-Filchner Ice Shelf in the \TWeddell Sea\t to the west side of the \TRoss Ice Shelf\t in the Ross Sea. These enormous floating masses of \Tpack ice\t are derived mostly from glaciers discharging from the continent; the Ronne-Filchner Ice Shelf encompasses about 500,000 sq km (193,050 sq mi), and the Ross Ice Shelf about 538,720 sq km (208,000 sq mi). The Ross Ice Shelf alone is about equal to the area of Texas. Probably an additional 600,880 sq km (232,000 sq mi) of ice shelves fringe the coast of the continent. In striking contrast to the ice cover are the "dry valley" areas of the continent, especially in Victoria Land, which receive little precipitation and do not have a continuous snow cover. The Transantarctic Mountains (2,000 to 4,572 m/6,562 to 15,000 ft) extend from Victoria Land southward along the west side of the Ross Sea to the Weddell Sea on the opposite side of the continent. Coal, uranium, and a variety of metallic minerals have been found on land but are not considered economically exploitable. Offshore petroleum and natural gas have more economic potential. Fishing boats take whales, seal, and krill (small protein-rich crustaceans) from the surrounding ocean. Growing controversy surrounding the issues of environmental protection and resource development in Antarctica was intensified by oil spills there in 1989. TOPOGRAPHY Antarctica has two topographies--one of perennial snow and ice (about 2 km/1.24 mi thick) and one of land. The snow-and-ice surface in east Antarctica is an irregularly shaped elliptical dome that rises to a maximum elevation of more than 4,267 m (14,000 ft). A deep bay in the margin of the continent at the Amery Ice Shelf and Lambert Glacier produces a topographic sag in the ice almost to the center of the dome. Broad topographic highs rise above both sides of it. In Queen Maud Land rugged mountains retain a high ice surface inland. The ice surface of the dome rises more gently from the coast south of New Zealand and Australia. The surface of the ice sheet in west Antarctica is much more irregular. A broad ice high (2,000 to 3,000 m/6,562 to 9,842 ft) extends from the center of the ice dome in east Antarctica past the South Pole and into the center of west Antarctica at the Ellsworth Mountains. Major ice flow is diverted to the Ronne-Filchner and Ross ice shelves on opposite sides of the ridge and is reflected in long shallow troughs following that drainage. In west Antarctica the only other large area of ice surface above 2,000 m (6,562 ft) is that in the Executive Committee Range near the coast between the Amundsen Sea and the Ross Sea. A broad saddle with ice elevations between 1,800 and 2,000 m (5,906 and 6,562 ft) connects the Executive Committee Range and the ridge near the Ellsworth Mountains. A smaller area of ice slightly higher than 2,000 m (6,562 ft) lies at the base of the Antarctic Peninsula. Most of Antarctica's bedrock topography is known only indirectly. The first drill hole to penetrate rock--that at Byrd Station (elevation 1,530 m/5,020 ft), which was successful on Jan. 29, 1968, at 2,164 m (7,106 ft). The first measurements (1951-52) of ice thickness, in Queen Maud Land, were by G. de Q. Robin, who used seismic reflection and refraction methods. Today, airborne radio-echo soundings can penetrate the ice and produce continuous records of the rock surface. The data collected thus far indicate an elongate basin, named the Wilkes Subglacial Basin, several hundred meters below sea level and parallel to and west of the Transantarctic Mountains of Victoria Land, in east Antarctica. The Polar Basin extends from the South Pole toward Queen Maud Land. In Wilkes Land, deep embayments with small rises penetrate from the coast at longitude 145 deg and 115 deg. In those embayments preglacial shorelines would have extended many hundreds of kilometers into the present continent. A bedrock high above 3,000 m (9,842 ft) is centered at the Gamburtsev Mountains, near the Pole of Inaccessibility. In the rest of east Antarctica the bedrock topography ranges from about 1,000 m (3,280 ft) above to slightly below sea level. In contrast, much of the bedrock surface of west Antarctica is between sea level and -1,000 m (-3,280 ft), and depressions almost 3,000 m (9,840 ft) below sea level are known. In ranges where rock-surface elevations are between 1,000 and 3,000 m (3,281 and 9,842 ft), mountain peaks protrude above the ice surface. Most notable are the ranges in Marie Byrd Land and Eights Coast and the irregular ridge that extends from the Horlick Mountains through the Ellsworth Mountains to the Antarctic Peninsula. The highest point in Antarctica is Vinson Massif (5,140 m/16,863 ft) in the Sentinel Range. The weight of ice has depressed the land by an average of 560 m (1,838 ft), roughly 0.28 times the average thickness of ice (2,000 m/6,562 ft). If the ice were removed, the land would rise until most of east Antarctica was above sea level, but west Antarctica would consist largely of a shallow sea. By the time (1957-58) of the \TInternational Geophysical Year\t (\TIGY\t) many regions were still unknown. Now all major areas of rock outcrop have been seen or photographed from the air, if not actually studied on the ground. GEOLOGY A vast Precambrian \Tcontinental shield\t makes up east Antarctica. High-grade \Tmetamorphic\t \Lrock\ls dominate the coast. The Ross mobile belt, a major tectonic unit, includes the Transantarctic Mountains. An inner belt of sedimentary and volcanic rocks of the Gondwana System are Devonian to Jurassic in age; an outer belt is Precambrian to Cambrian in age. Cenozoic volcanic rocks, mainly basaltic, are present on the west side of the Ross Sea. In west Antarctica the Antarctic Peninsula and the coastal area to Marie Byrd Land make up the Andean mobile belt, which consists mostly of upper Paleozoic to Mesozoic rocks. The Ellsworth mobile belt includes the Ellsworth and Whitmore mountains and a broad arc that swings out to the Filchner Ice Shelf. It contains rocks of mostly late Precambrian to late Paleozoic age. Most plant and animal fossils are found in the Gondwana System of sedimentary beds in the Transantarctic Mountains or in the low-grade metamorphic rocks of the Antarctic Peninsula. Fossils (including the first land mammal fossil, a marsupial, found in 1982) and rock types, ages, structure, and metamorphism permit correlation of Antarctica with the other continents believed to have once been united in the ancestral continent of \TGondwanaland\t. Other fossil evidence (including dinosaur remains, first found in 1986) indicates that Antarctica once had a climate milder than its present one. The Kukri \Tpeneplain\t, an erosional surface on the Precambrian and lower Paleozoic basement rocks, has been identified throughout the Transantarctic Mountains. A Tertiary peneplain has been interpreted at several places in the Antarctic Peninsula and Pensacola, Shackleton, and Prince Charles mountains. Active volcanoes are confined to the Transantarctic Mountains of Victoria Land, Marie Byrd Land, and the South Shetland Islands. Block-faulted mountains are mostly in the Transantarctic Mountains, Queen Maud Land, and Antarctic Peninsula. Soils in Antarctica are limited to the ice-free areas, only 2% to 3% of the continent, and are patchy even there. They are thin, commonly alkaline, and have little humus, although they have many soil-forming bacteria. A "desert pavement" of rock fragments is common. Under patches of lichens and mosses or penguin rookeries, organic acids play a discernible role in soil formation. Polygonal patterned ground, produced by growth of ice or sand wedges, develops on most soils. CLIMATE The Antarctic \Tpolar-climate\t boundary (10 deg C/50 deg F isotherm for the warmest month) encompasses about 12% of the surface of the globe, an area twice as large as that of the Arctic. It includes all of the Antarctic continent except the extreme northern tip of the Antarctic Peninsula. The vast area of snow and ice reflects about three-fourths of the incoming radiation. Vostok at 78 deg28' south latitude and 3,505 m (11,500 ft) elevation has an average temperature for the warmest month of only -33 deg C (-27 deg F ), an absolute minimum of -88.3 deg C (-126.9 deg F), and an absolute maximum of -21 deg C (-5.8 deg F). The absolute minimum figure is the lowest temperature ever recorded on Earth. Coastal areas are much warmer, with annual mean temperatures of -15 deg to -10 deg C (5 deg to 14 deg F). Most precipitation falls as snow. This amounts to only 50 mm (2 in) in the interior but is highly variable, from 500 to more than 1,000 mm (20 to 40 in), on the coast. An important aspect of the atmosphere is the low-level temperature \Tinversion\t. Marine air subsides over the radiationally cooled surface, generating minute ice spicules most of the time. However, the bulk of the precipitation comes from the cyclonic storms that diverge into the interior from the ocean. The continent has the strongest sustained westerly winds on Earth. In the mid-1980s, atmospheric studies showed that an ozone "hole"--a dramatic drop in ozone concentration in an area of the ozone layer above the continent--was appearing and then disappearing each Antarctic spring. Studies of past records revealed that this had been occurring for several years. Whether this "hole" is a product of human activities or a natural process is not yet known. Scientists are also measuring the Antarctic ice in an attempt to determine whether the \Tgreenhouse effect\t, a global warming trend caused by rising levels of atmospheric carbon dioxide, is melting the Antarctic ice cap (which could raise global sea levels) or whether the warming air, because it can hold more moisture, is increasing snowfall over Antarctica, thus increasing the size of the continent. \TRobert\t F. \TBlack\t ANTARCTIC LIFE Most of the life on and around the Antarctic continent is supported by the sea, because the continent itself is barren. The life consists primarily of a few species of lichens and mosses; some small floating plants called phytoplankton, which occur in certain freshwater lakes in ice-free desert areas; and a few arthropods, the most numerous of which are several groups of mites. The sea, on the other hand, is extremely rich because in many areas water movement is vertical and brings nutrients from the bottom to the surface. These areas of upwelling supply nutrients for phytoplankton, which multiply quickly during the summer (October through February) and become food for small floating animals called zooplankton. Several species of zooplankton are important to the Antarctic marine ecosystem; however, the krill is usually recognized as the key. These shrimplike organisms feed directly on various small plants and are in turn an important food source for many fish, birds, and mammals. Due to the exploitation of certain Antarctic species, several agreements to conserve Antarctica's living resources have been signed. A fairly large number of fish species are found in the Antarctic, but they are small in size or few in number. At present knowledge about the population dynamics and life history of the Antarctic fish is limited, and, hence, experts are uncertain about their general role in the ecosystem. Certain species are known to be primary users of krill; many others use food sources such as other species of zooplankton. Some are predatory on species of fish and squid. The cephalapods, which include the squid and the octopus, are relatively abundant in Antarctica and are known to be important in the diets of certain whales, seals, penguins, flying birds, and fish. The birds of the Antarctic ecosystem include mostly sea birds, particularly the penguins, albatrosses, and petrels. Their distribution is primarily limited by the Antarctic continent to the south and to the northern edge of the pack ice. Seven species of penguins are confined to the pack ice region, and they constitute more than 80% of the birds in the Antarctic region. The most common penguin is the Adelie, a primary consumer of krill. This species nests in ice-free areas mostly on offshore islands around the Antarctic continent. Colonies of more than 100,000 Adelies are fairly common from early October through January, when they congregate to lay eggs and hatch young. The emperor penguin, another species, is considered typically Antarctic. The emperors are well known and nest much of their life on sea ice close to the continent but lay eggs and raise their young on land. The female emperor lays one egg. The male emperor incubates the egg by placing it on top of his feet and covering the upper surface of the egg with his body, which has a special indentation called a brood pouch. The egg is incubated through the early spring season, when severe storms can occur. Other species of penguin common in the Antarctic pack ice region are gentoo, chin strap, and king penguin. The flying birds, characterized primarily by the petrels (a diverse group of 24 species), nest on various offshore islands and on rock outcroppings. Antarctic marine mammals include the seal and the whale. The whales can be conveniently classified into two groups, the baleen and tooth whales. Four species of large baleen whales--the blue whale, fin whale, sei whale, and humpback whale--inhabit Antarctic waters regularly; occasional inhabitants are the minke whale and southern right whale, two other species of baleen whales. Ten species of toothed whales can be found in Antarctic waters. The two species considered most important are the large sperm whale, the only large-toothed whale to occupy the region, and the killer whale. The following four species of seals are found in the pack ice region: the crabeater, the Weddell, the leopard seal, and Ross's seal. The crabeater seal is by far the most abundant and has been estimated to number around 20 million. The crabeater has lobed teeth, presumably an adaptation for sieving krill from the water. The Weddell seal occupies regions of pack ice close to the Antarctic continent. It climbs out of the water through cracks in the ice to give birth to pups and breed from early September through early November, depending upon the region. The leopard seal is a predatory animal that is known to feed upon penguins, fish, other seals, and occasionally krill. Little is known about Ross's seal, the numbers of which constitute less than 0.5% of the seals of the region. Typically, the southern fur seal and the southern elephant seal are found farther north than the four species of true Antarctic seal, and they occupy sub-Antarctic islands during the pupping and breeding season, which occurs from September through December. Donald B. Siniff EXPLORATION AND TERRITORIAL CLAIMS Although recent cartographic studies suggest that the broad outlines of the continent may have been known before the mid-16th century, it is generally thought that James \TCook\t, on Jan. 16, 1773, was the first to cross the Antarctic Circle, and Adm. Fabian von \TBellingshausen\t and M. P. Lazarev, on Jan. 28, 1820, were probably the first to sight land in the Antarctic. John Davis, captain of the American sealer Huron, was the first to land, at Hughes Bay on the Antarctic Peninsula, on Feb. 7, 1821. On Jan. 23, 1895, Leonard Kristensen and Carsten \TBorchgrevink\t landed near Cape Adare, and they were thus the first to set foot on the main part of the continent. In 1898, Borchgrevink led the first expedition to spend a winter on the continent, also at Cape Adare. Their stay ushered in the "heroic phase" of exploration by parties that wintered over for scientific purposes. Several such expeditions were launched during the next decade. Otto \TNordenskjold\t explored the east coast of the Antarctic Peninsula (1901-04). Robert F. \TScott\t and Ernest H. \TShackleton\t led three scientific parties (1901-04, 1907-09, and 1910-13) from bases on Ross Island and made major advances in geology, meteorology, and oceanography. In 1908, T. W. E. David of Scott's party ascended Mount \TErebus\t (3,795 m/12,450 ft), and later reached the South Magnetic Pole (lat. 72 deg25' S, long. 155 deg16' E) at an altitude of more than 2,130 m (7,000 ft). The pole has since moved to the Adelie Coast. In 1909, Shackleton traversed the Ross Ice Shelf, ascended the Beardmore Glacier, and reached 88 deg23' south latitude on the polar ice sheet. On Oct. 20, 1911, Roald \TAmundsen\t of Norway left from the Bay of Whales, on the east side of the Ross Ice Shelf, with four companions on skis and with dogsleds. They reached the South Pole (altitude 2,804 m/9,200 ft) on Dec. 14, 1911, via the Axel Heiberg Glacier. Scott started from Ross Island on Oct. 24, 1911, and followed Shackleton's route of 1909. Scott, Wilson, Bowers, Oates, and Evans man-hauled one sledge to the South Pole on Jan. 17, 1912; all perished on the return. In 1911-14, Sir Douglas \TMawson\t explored the Adelie Coast and Queen Mary Land. On Nov. 16, 1928, C. B. Eielson and Sir George Hubert \TWilkins\t made the first airplane flight in Antarctica. Between 1926 and 1937, Lars Christensen made aerial surveys and mapped the coastal area from the Weddell Sea to the Shackleton Ice Shelf. Adm. Richard E. \TByrd\t used aircraft extensively during four expeditions: 1928-30, 1933-35, 1939-41, and 1946-47. From a base at Little America, Byrd, Bernt Balchen, A. C. McKinley, and Harold June flew to the South Pole and back on Nov. 28-29, 1929. Between 1933 and 1939, Lincoln \TEllsworth\t made four expeditions to Antarctica. In 1935 he flew from the tip of the Antarctic Peninsula to the Bay of Whales in the Ross Sea, landing four times during twelve days. In 1934-37 a British expedition used planes, boats, and dogs to study much of the Antarctic Peninsula, and in 1939 a German group photographed a large area in Queen Maud Land. In 1946-48, Comdr. Finn Ronne and a group, his wife among them, mapped a large area from the Antarctic Peninsula to the Filchner Ice Shelf. The Norwegian-British-Swedish expedition of 1949-52 conducted major scientific traverses into the interior and laid the foundation for the \TIGY\t. During the \TIGY\t, 12 nations manned over 50 wintering-over stations in Antarctica. The Pole of Inaccessibility (lat. 82 deg06' S, long. 54 deg58' E) was reached on Dec. 14, 1958, by a Soviet \TIGY\t tractor train. Between Nov. 24, 1957, and Mar. 2, 1958, Sir Vivian \TFuch\t's party crossed the continent from the Weddell Sea to the Ross Sea. In 1982, British explorers Sir Ranulph Fiennes and Charles Burton completed the first crossing of both poles in a single circumnavigation of the earth. China launched its first major Antarctic expedition in 1984. The Special Committee on Antarctic Research (\Tscar\t), set up in 1957, now coordinates all phases of Antarctic research. Emphasis today is on long-term investigations of such large-scale phenomena as ocean currents, underice topography, and the relationship of the Antarctic ice sheet to global climate. The United States and the \TUSSR\t recognize no territorial claims in Antarctica and make none, although reserving the right to do so. The Antarctic Treaty was ratified on June 23, 1961, by the 12 \TIGY\t nations referred to above--Argentina, Australia, Chile, France, New Zealand, Norway, and the United Kingdom (all of which claim parts of the continent), plus Belgium, Japan, South Africa, the \TUSSR\t, and the United States. Brazil, China, India, Poland, Uruguay, Italy, and Germany later became voting signatories; other nations not conducting major scientific activity in Antarctica have signed the document but do not have voting rights. The Antarctic Treaty states specifically that Antarctica shall be used for peaceful purposes only and supports the freedom of scientific investigation and cooperation. As part of the treaty, all territorial claims were frozen until a 1991 review. In 1988 an agreement to regulate the future exploitation of Antarctica's mineral resources was negotiated, but was widely criticized by environmentalists and by developing nations demanding that the continent's resources be shared under United Nations supervision. This agreement fell apart in 1989 when Australia and France refused to sign it. In 1991 delegates drafted a new agreement to ban Antarctic mining for 50 years. Robert F. Black Bibliography: Adie, R. ed., Antarctic Geology and Geophysics (1973); Auburn, F. M., Antarctic Law and Politics (1982); Bonner, N., and Walton, D., eds., Key Environments: Antarctica (1985); Byrd, R. E., Little America (1930); Charney, J. I., ed., The New Nationalism and the Use of Common Spaces (1982); Deacon, G., The Antarctic Circumpolar Ocean (1984); Dolgin, I. M., Climate of Antarctica (1986); Laws, R. M., ed., Antarctic Ecology (1984); Llano, G., ed., Adaptations within Antarctic Ecosystems (1977); Mickleburgh, E., Beyond the Frozen Sea (1988); Orrego-Vicuna, F., ed., Antarctic Resources Policy (1984); Parfit, M., South Light (1986); Pyne, S. J., The Ice (1987); Quigg, P. W., A Pole Apart: The Emerging Issues of Antarctica (1982) and Antarctica: The Continuing Experiment (1985); Ray, G. C., and McCormick-Ray, M. G., Wildlife of the Polar Regions (1981); Suter, K. D., Antarctica: Private Property or Public Heritage (1990); Walton, David, Antarctic Science (1987).