Antarctica

I

Introduction

Antarctica, fifth largest of Earth’s seven continents. Antarctica surrounds the South Pole and is a place of extremes. It is the southernmost, coldest, iciest, driest, windiest, most remote, and most recently discovered continent. Nearly the entire landmass lies within the Antarctic Circle. Air temperatures of the high inland regions fall below -80°C (-110°F) in winter and rise only to -30°C (-20°F) in summer. Massive ice sheets built up from snow over millions of years cover almost all of the continent and float in huge ice shelves on coastal waters. In winter frozen sea water (sea ice) more than doubles the size of the Antarctic ice cap. Antarctica's vast areas of ice on land and on sea play a major role in Earth’s climate and could be strongly affected by global warming. The melting of Antarctic ice could dramatically raise global sea level.

Antarctica means “opposite to the Arctic,” Earth’s northernmost region. Antarctica is completely encircled by the Southern Ocean. The entire area south of the Antarctic Convergence zone where cold Antarctic waters sink below warmer waters on the northern boundary of the Southern Ocean is referred to as the Antarctic region.

The small human presence on Antarctica is made up of visiting scientists, support staff, and tourists. The last continent to be discovered, Antarctica remained hidden behind barriers of fog, storm, and sea ice until it was first sighted in the early 19th century (see Polar Exploration). Because of the extreme cold and the lack of native peoples, forests, land animals, and obvious natural resources, the continent remained largely neglected for decades after discovery. Scientific expeditions and seal hunters had explored only fragments of its coasts by the end of the 19th century, while the interior remained unknown. Explorers first reached the South Pole in 1911, and the first permanent settlements—scientific stations—were established in the early 1940s. From that time the pace of exploration and scientific research has accelerated rapidly. In the mid- to late 20th century, the region’s majestic scenery and wildlife began to attract increasing numbers of tourists.

Seven nations—Argentina, Australia, the United Kingdom, Chile, France, New Zealand, and Norway—claim territory in Antarctica. Other nations, including the United States and Russia, do not acknowledge these claims and make no claims of their own, but reserve rights to claim territory in the future. Since 1961 the continent has been administered under the Antarctic Treaty, an international agreement to preserve the continent for peaceful scientific study.

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II

Land and Climate of Antarctica

The continent of Antarctica is shaped somewhat like a comma, with a round body surrounding the pole and a tail curving toward South America. The round portion, lying mainly in the Eastern Hemisphere, makes up East Antarctica. The tail and its thickened base, located entirely in the Western Hemisphere, form West Antarctica. East Antarctica includes the regions of Queen Maud Land, Enderby Land, and Wilkes Land; West Antarctica includes Palmer Land, Ellsworth Land, and Marie Byrd Land. Antarctica lies 1,000 km (600 mi) from South America, its nearest neighbor; 4,000 km (2,500 mi) from Africa; and 2,500 km (1,600 mi) from Australia.

With an area of 14 million sq km (5.4 million sq mi), Antarctica is larger than either Europe or Australia. Its average elevation of more than 2,000 m (6,500 ft) is over twice that of Asia, the next highest continent. However, much of this mass is ice. Below this ice, East Antarctica is a landmass about the size of Australia, and West Antarctica is a collection of islands. Only 2.4 percent of the total continental area is exposed rock. Exposed areas include the peaks of several mountain ranges and other smaller scattered outcrops, both of which poke through the ice cover, as well as dry valleys, glacier-carved areas that are kept clear of snow by gusty winds. Only about 2 percent of the coast is exposed cliffs or beaches; the rest is made up of ice cliffs that extend beyond the end of the continental rock.

The Ross and Weddell seas indent the thickened base of West Antarctica where it meets East Antarctica, while the Amundsen and Bellingshausen seas lie at West Antarctica’s outer edge. Numerous bays also indent the outer edge of East Antarctica, creating a jagged coastline.

The geographic South Pole lies near the center of the continent in East Antarctica. This point is where all map lines of longitude converge at the southern end of Earth’s axis of rotation. Distinct from the geographic south pole is the south magnetic pole, where the lines of force of Earth’s magnetic field emerge vertically, arching upward over the planet to enter Earth again at the north magnetic pole. The south-seeking end of a compass needle points to the south magnetic pole. The south magnetic pole is currently located off the Adélie Coast of East Antarctica, but was on land when it was first recorded in 1909. The south magnetic pole has migrated gradually out to sea with changes in the fields, a phenomenon known as polar wandering. The south geomagnetic pole is the hypothetical location of the magnetic pole if Earth’s magnetic field were a perfect bar magnet. The south geomagnetic pole is located near Vostok station in East Antarctica.

A

Land of Ice

The total volume of the ice sheet covering Antarctica is estimated at 29 million cu km (7 million cu mi), or about 90 percent of the world’s ice. (Greenland contains about 8 percent of the global ice mass, with most of the rest found as glaciers in mountainous regions around the world.) The Antarctic ice sheet holds about 70 percent of all the freshwater on Earth. If the entire ice sheet melted, the oceans of the world would rise by 60 m (200 ft). Some 11 percent of the ice sheet consists of ice shelves—massive floating slabs of permanent ice fringing the continent—that are anchored to the rock and extend into the surrounding ocean. The largest, Ross Ice Shelf, is about the size of France. The Antarctic ice sheet has an average thickness of 2,160 m (7,090 ft); its greatest recorded depth is more than 4,700 m (15,400 ft).

Antarctica’s ice sheet formed over millions of years. As new snow falls, it compresses the layers of older snow beneath it into ice. The physical characteristics of the ice sheet are constantly changing as new ice forms and slides outward toward the coasts. Large masses of moving ice known as glaciers move down the continent’s five major drainage systems in two ways. If there is a layer of water between the glacier and the bedrock beneath it, the whole glacier can slide under the force of gravity. Alternatively, the weight of the accumulated snow and ice can cause the ice crystals of the glacier to form into layers, which glide over one another. Glaciers flow either into ice shelves or directly out to the edges of the continent, where portions break off and form floating masses called icebergs (see Ice: Icebergs). Carried by circumpolar currents and prevailing winds, these icebergs drift westward around the continent and then northward to the Antarctic Convergence before gradually breaking up and melting upon contact with warmer waters.

Icebergs that break away from Antarctica and melt raise sea level by adding water that was locked up as land ice. Increased snow fall that adds to the Antarctic ice sheet could offset the melting of icebergs and glaciers, however. The flow of glaciers to the sea has been accelerating in parts of West Antarctica. Large portions of ice shelves have also broken off. Since ice shelves float on water, they would not raise sea level when they melt. However, rising sea levels in the future would lift ice shelves and lead to more break ups and melting of ice that was once on land.

In addition to icebergs and ice shelves, ocean waters close to the continent contain floating sea ice. In contrast to icebergs and ice shelves, which are frozen fresh water, sea ice is frozen sea water. As sea water freezes, it expels salt, which collects as concentrated brine between the ice crystals. The briny water eventually drains from the underside of the sea ice, creating a downward flow of heavier salty water. As a result, sea ice that survives for a number of months can be virtually fresh. Some sea ice is attached to the land (fast ice) and some drifts with wind and currents (pack ice). Sea ice melts and freezes seasonally, covering up to 21 million sq km (8 million sq mi) in late winter and only 5 million sq km (2 million sq mi) in summer. Unlike melting glaciers and ice sheets, melting sea ice does not directly raise sea level. However, loss of sea ice exposes darker ocean water to sunlight and heats the ocean.

The maximum area of sea ice surrounding Antarctica each winter varies from year to year. A marked decline during the 1970s appears to have reversed in more recent decades, except in the Antarctic Peninsula area. This area has lost almost 40 percent of its sea ice since the start of the 1980s. Sea ice is important to marine life. Krill feed on algae that live under the sea ice and are released when the ice melts in spring and summer. In turn, many marine animals feed on krill. Emperor penguins breed on flat expanses of sea ice close to land during the winter. Seals give birth on sea ice in early spring.

Ice on land and on sea reflects sunlight back into space, which has a cooling effect on Earth’s climate. Shrinking ice shelves or reduced sea ice in summer allows the exposed dark ocean to absorb more heat, raising temperatures.

B

Land Regions of Antarctica

During the early part of the Mesozoic Era (which lasted from 251 million to 65 million years ago) Antarctica was a part of Pangaea, the giant supercontinent that included nearly all land on Earth. About 240 million years ago Pangaea began to split into two giant landmasses called Laurasia and Gondwanaland (also called Gondwana). Gondwanaland consisted of the present continent of Antarctica along with Australia, South America, Africa, and the Indian subcontinent. Gondwanaland itself began to break up about 185 million years ago. Antarctica gradually moved towards the South Pole. About 80 million years ago, Antarctica split from Australia and became an isolated land mass that drifted southward into its polar position. Climatic cooling began about 34 million years ago during the Oligocene Epoch. Coupled with mountain building, this cooling initiated the formation of permanent ice on land, resulting in the Antarctic ice sheet. The giant ice sheet in East Antarctica has been a stable feature for about 15 million years, beginning in the Miocene Epoch. The ice sheet on West Antarctica has collapsed and reformed a number of times during the same period.

East Antarctica makes up about two-thirds of the continent’s area. The land beneath the ice consists of a basement complex of ancient gneisses, schists, and other metamorphic rocks overlaid by sediments from the Cambrian and Permian periods. Containing evidence of tropical forests and deserts prior to the more recent glacial conditions, these layers reflect the region’s complex climatic history. Covering the land is a huge, complex ice dome rising from coastal plains to a high plateau more than 4,000 m (13,000 ft) in elevation. The Transantarctic Mountains stretch 3,500 km (2,200 mi) along the entire western flank of East Antarctica, separating it from West Antarctica. This range holds back the plateau ice of East Antarctica like a massive dam, penetrated by glaciers that flow into the Ross and Filchner-Ronne ice shelves to the west. High peaks—some rising to more than 4,300 m (14,000 ft)—poke through the ice sheet, and other portions of the range are dry valleys that are free of ice. Toward the Indian and Pacific ocean coasts lie several minor ice domes and lower plateaus, some penetrated by other mountain ranges.

The remaining third of the continent, West Antarctica, consists of a much lower, undulating ice sheet covering a complex of mountainous islands. The Antarctic Peninsula, the “tail” of land reaching toward South America, was formed by the same geologic processes that formed the Andes Mountains on that continent. Many islands, including the South Shetland Islands, lie off the Antarctic Peninsula. Deep trenches and basins beneath the ice separate the islands of West Antarctica. Vinson Massif, the highest point in Antarctica, has an elevation of 4,897 m (16,066 ft) and lies in the Sentinel Range near the Filchner-Ronne Ice Shelf. Several active volcanoes, including Mount Erebus on Ross Island, dot coastal and island areas. The multilayered land formations of West Antarctica, folded and transformed by geologic structural deformations, reveal a distinct, more turbulent geological history than that of East Antarctica. These formations consist of a Precambrian basement covered by volcanic sediments from the Carboniferous Period, Mesozoic Era, and Cenozoic Era. These volcanic sediments are in turn covered by plant-bearing Jurassic and later Cenozoic sediments.

Soils are extremely rare in Antarctica, occurring only in ice-free areas where fine rock debris can accumulate. In summer, the soils provide a damp substrate for plants such as lichens and moss, and for microorganisms. The continent’s two species of flowering plants may gain a foothold in favorable conditions found in West Antarctica. There is no Antarctic equivalent of Arctic tundra.

Unusual geological features on Antarctica include dry valleys and lakes buried deep beneath the ice sheet. Dry valleys are ice-free valleys carved by glaciers that have retreated during the past few thousand years. The valleys receive little or no snow and are mainly surfaced by rock or gravel, with summer streams and almost permanently frozen lakes. Unusual plants, animals, and microbes survive in the simple ecosystems. Much larger bodies of water called subglacial lakes have been found under the ice sheet. The largest known is Lake Vostok, which covers about 11,655 square km (5,400 square mi) in area, similar in size to Lake Ontario. Such lakes may contain interesting microbes that are unknown elsewhere, but scientists have not yet found ways of drilling down to to the lakes without severely contaminating them. A complex system of subglacial lakes may also affect how fast ice sheets and glaciers move to the coast and break up as icebergs.

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