Antarctica, fifth in size among the world’s continents. Its landmass is almost wholly covered by a vastice sheet.
Lying almost concentrically around theSouth Pole, Antarctica—the name of which means “opposite to the Arctic”—is the southernmost continent, a circumstance that has had momentous consequences for all aspects of its character. It covers about 5.5 million square miles (14.2 million square km), and would be essentially circular except for the outflaringAntarctic Peninsula, which reaches toward the southern tip ofSouth America (some 600 miles [970 km] away), and for two principal embayments, theRoss Sea and theWeddell Sea. These deep embayments of the southernmost Pacific and Atlantic oceans make the continent somewhat pear-shaped, dividing it into two unequal-sized parts. The larger is generally known as East Antarctica because most of it lies in east longitudes. The smaller, wholly in west longitudes, is generally called West Antarctica. East and West Antarctica are separated by the 1,900-mile-longTransantarctic Mountains. Whereas East Antarctica consists largely of a high, ice-covered plateau, West Antarctica consists of an archipelago of mountainous islands covered and bonded together by ice.
The continental ice sheet contains approximately 7 million cubic miles (29 million cubic km) of ice, representing about 90 percent of the world’s total. The average thickness is about 1.5 miles (2.45 km). Many parts of the Ross and Weddell seas are covered by ice shelves, or ice sheets floating on the sea. These shelves—theRoss Ice Shelf and the Filchner-Ronne Ice Shelf—together with other shelves around the continental margins, constitute about 10 percent of the area of Antarctic ice. Around the Antarctic coast, shelves, glaciers, and ice sheets continually “calve,” or discharge, icebergs into the seas.
Because of this vast ice, the continent supports only a primitive indigenous population of cold-adapted land plants and animals. The surrounding sea is as rich in life as the land is barren. With the decline of whaling and sealing, the only economic base in the past, Antarctica now principally exports the results of scientific investigations that lead to a better understanding of the total world environment. The present scale of scientific investigation of Antarctica began with the International Geophysical Year (IGY) in 1957–58. Although early explorations were nationalistic, leading to territorial claims, modern ones have come under the international aegis of theAntarctic Treaty. This treaty, which was an unprecedented landmark in diplomacy when it was signed in 1959 by 12 nations, preserves the continent for nonmilitary scientific pursuits.
Antarctica, the most remote and inaccessible continent, is no longer as unknown as it was at the start of IGY. All its mountain regions have been mapped and visited by geologists, geophysicists, glaciologists, and biologists. Some mapping data are now obtained by satellite rather than by observers on the surface. Many hidden ranges and peaks are known from geophysical soundings of the Antarctic ice sheets. By using radio-echo sounding instruments, systematic aerial surveys of the ice-buried terrains can be made.
The ice-choked and stormy seas around Antarctica long hindered exploration by wooden-hulled ships. No lands break the relentless force of the prevailing west winds as they race clockwise around the continent, dragging westerly ocean currents along beneath. The southernmost parts of the Atlantic, Pacific, and Indian oceans converge into a cold, oceanic water mass with singularly unique biologic and physical characteristics. Early penetration of this Southern (or Antarctic) Ocean, as it has been called, in the search for fur seals led in 1820 to the discovery of the continent. Icebreakers and aircraft now make access relatively easy, although still not without hazard in stormy conditions. Many tourists have visited Antarctica, and it seems likely that, at least in the short run, scenic resources have greater potential for economic development than do mineral and biological resources.
The termAntarctic regions refers to all areas—oceanic, island, and continental—lying in the cold Antarctic climatic zone south of theAntarctic Convergence, an important boundary with little seasonal variability, where warm subtropical waters meet and mix with cold polar waters. For legal purposes of theAntarctic Treaty, the arbitrary boundary of latitude 60° S is used. The familiar map boundaries of the continent known as Antarctica, defined as the South Polar landmass and all its nonfloating grounded ice, are subject to change with future changes of climate. The continent was ice-free during most of its lengthy geologic history, and there is no reason to believe it will not become so again in the probably distant future.
The geologic evolution of Antarctica has followed a course similar to that of the other southern continents. The earliest chapters in Antarctica’s rather fragmentary record extend far back, perhaps as much as 3 billion years, into earlyPrecambrian time. Similarity in patterns of crustal and biological evolution in the southern continents can be traced back some 150 million years, and evolutionary courses began to diverge conspicuously by about 70 million years ago, in the lateMesozoic Era. Plant and animal migration routes that apparently had interconnected all the southern continents were largely cut off by the outset of the Cenozoic Era (i.e., about 65.5 million years ago). Antarctica became isolated at a time when land mammals diversified and flourished elsewhere, populating all the other continents of the world. Antarctica had long been thought to be a migratory path for marsupials moving between southern continents in early Cenozoic time. But documentation for the theory was not discovered until 1982, when the first mammal remains, amarsupial fossil, were found onSeymour Island in the Weddell Sea. The subsequent growth of Antarctica’s ice sheets cut off any further migrations by land animals.
Now bathed by polar ice, Antarctica has abundant fossil evidence that its climate and terrain at one time supported far more populous flora and fauna than today’s few seedless plants and primitive insects. Much of Antarctica was denselyforested inMesozoic times (about 251 million to 65.5 million years ago), dominated by southern conifers of podocarps and araucarias, with undergrowth of rainforest-type ferns. Angiosperm trees, particularly the southern beech,Nothofagus, appeared during theCretaceous Period (about 146 to 65.5 million years ago) and lingered in places until about 2 million years ago as Antarctica drifted poleward, cooled, and became glaciated. Remains of luxuriant extinct floras, as well as fossils of Mesozoic reptiles, dinosaurs, and amphibians, have been discovered, and these compare so closely to those of other southern continents that many geologists have postulated former contiguity of these lands in a single giant continent calledGondwana. Continental stratigraphic evidence and the dating of seafloors seem to indicate that the supercontinent broke apart along Jurassic rift faults 180 to 160 million years ago, and fragments such asAfrica andAustralia separated from Antarctica in Jurassic to Cretaceous time (about 200 to 65.5 million years ago) and in the early Cenozoic Era. Early stages of rifting were marked by immense outpourings of plateau lavas (Kirkpatrick Basalt, on Mount Kirkpatrick) and by related sill intrusions (Ferrar Dolerites) across Antarctica, including one of the world’s largest layered gabbroic igneous complexes, the Dufek intrusion, in the Pensacola Mountains.
Modern theory ties mobile zones to the interaction and jostling of immense crustalplates (seeplate tectonics). Modern plate boundaries may be far different from ancient ones presumably marked by old fold belts. Ancient Antarctic mobile belts, such as are followed by today’sTransantarctic Mountains, terminate at continental margins abruptly, as if sliced off, and seemingly reappear in other lands across young ocean basins. Much research has been concentrated on attempting to match intercontinentally the detailed structure of opposed coasts, such as between Antarctica and Australia, in an effort to learn whether they had been actually connected before the latest cycle of crustal spreading from intervening mid-oceanic ridges. Similarities between ancient mobile belts now suggest to some geologists that Antarctica may even have been connected to southwesternNorth America more than 600 million years ago, in late Precambrian time.
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