Antarctica

Fifth in size among the world's continents, Antarctica's landmass is almost wholly covered by a vast ice sheet. It lies almost concentrically around the South 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.

Antarctica covers about 14.2 million square km (5.5 million square miles), and would be essentially circular except for the outflaring Antarctic Peninsula, which reaches toward the southern tip of South America (some 970 km [600 miles] away), and for two principal embay-ments, the Ross Sea and the Weddell 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 3,060-km-long (1,900-mile-long) Transantarctic 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 29 million cubic km (7 million cubic miles) of ice, representing about 90 percent of the world's total. The average thickness is about 2.45 km (1.5 miles). Many parts of the Ross and Weddell seas are covered by ice shelves, or ice sheets floating on the sea. These shelves—the Ross 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.

Filchner Ronne Ice Shelf Antarctica
A 2009 NASA satellite image of Antarctica, showing a vast expanse of the continent's coastline that is occupied by the Ross and Filchner-Ronne ice shelves. MODIS Rapid Response Team, NASAs Goddard Space Flight Center

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 the Antarctic Treat). 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 term Antarctic regions refers to all areas—oceanic, island, and continental—lying in the cold Antarctic climatic zone south of the Antarctic Convergence, an important boundary with little seasonal variability, where warm subtropical waters meet and mix with cold polar waters. For legal purposes of the Antarctic Treaty, the arbitrary boundary of latitude 6o° 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.

Physical Features

There are two faces of the present-day continent of Antarctica. One, seen visually, consists of the exposed rock and ice-surface terrain. The other, seen only indirectly by seismic or other remote-sensing techniques, consists of the ice-buried bedrock surface. Both evolved through long and slow geologic processes.

Effects of glacial erosion and deposition dominate everywhere in Antarctica, and erosional effects of running water are relatively minor. Yet, on warm summer days, rare and short-lived streams of glacial meltwater do locally exist. The evanescent Onyx River, for example, flows from Lower Wright Glacier terminus to empty into the nond-rained basin of Lake Vanda near McMurdo Sound.

Glacially sculptured landforms now predominate, as they must have some 300 million years ago, in an earlier period of continental glaciation of all of Gondwana.

With an average elevation of about 2,200 metres (7,200 feet) above sea level, Antarctica is the world's highest continent. (Asia, the next, averages about 3,000 feet.) The vast ice sheets of East Antarctica reach heights of 11,500 feet or more in four main centres: Dome A (Argus) at 81° S, 77° E; Dome C at 75° S, 125° E; Dome Fuji at 77° S, 40° E; and Vostok station at 77° S, 104° E. Without its ice, however, Antarctica would probably average little more than about 1,500 feet. It would then consist of a far smaller continent (East Antarctica) and a nearby island archipelago. A vast lowland plain between 90° E and 150° E (today's Polar and Wilkes subglacial basins) would be fringed by the ranges of the Transantarctic Mountains and of the Gamburtsev Mountains, 6,500 to 13,000 feet high. The rest might be a hilly to mountainous terrain. Relief in general would be great, with elevations ranging from 4,897 metres (16,066 feet) at Vinson Massif in the Sentinel Range, the highest point in Antarctica, to more than 8,200 feet below sea level in an adjoining marine trough to the west (Bentley Subglacial Trench). Areas that are now called "lands," including most of Ellsworth Land and Marie Byrd Land, would be beneath the sea.

Ice-scarred volcanoes, many still active, dot western Ellsworth Land, Marie Byrd Land, and sections of the coasts of the Antarctic Peninsula and Victoria Land, but principal activity is concentrated in the volcanic Scotia Arc. Only one volcano, Gaussberg (90° E), occurs along the entire coast of East Antarctica. Long dormant, Mount Erebus, on Ross Island, showed increased activity from the mid-1970s. Lava lakes have occasionally filled, but not overspilled, its crater, but the volcano's activity has been closely monitored because Antarctica's largest station

(McMurdo Station, U.S.) lies on its lower flank. One of several violent eruptions of Deception Island, a volcanic caldera, in 1967-70 destroyed nearby British and Chilean stations. Whereas volcanoes of the Antarctic Peninsula and Scotia Arc are mineralogically similar to the volcanoes typical of the Pacific Ocean rim, the others in Antarctica are chemically like those of volcanoes along the East African Rift Valley

Antarctic Glaciation

Antarctica provides the best available picture of the probable appearance 20,000 years ago of northern North America under the great Laurentide Ice Sheet. Some scientists contend that the initial glacier that thickened over time to become the vast East Antarctic Ice Sheet originated in the Gamburtsev Mountains more than 14 million years ago. Other glaciers, such as those forming in the Sentinel Range perhaps as early as 50 million years ago, advanced down valleys to calve into the sea in West Antarctica. Fringing ice shelves were built and later became grounded as glaciation intensified. Local ice caps developed, covering West Antarctic island groups as well as the mountain ranges of East Antarctica. The ice caps eventually coalesced into great ice sheets that tied together West and East Antarctica into the single continent that is known today. Except for a possible major deglaciation as recently as 3 million years ago, the continent has been largely covered by ice since the first glaciers appeared.

Causal factors leading to the birth and development of these continental ice sheets and then to their decay and death are, nevertheless, still poorly understood. The factors are complexly interrelated. Moreover, once developed, ice sheets tend to form independent climatic patterns and thus to be self-perpetuating and eventually perhaps even self-destructing. Cold air masses draining off Antarctic lands, for example, cool and freeze surrounding oceans in winter to form an ice pack, which reduces solar energy input by increasing reflectivity and makes interior continental regions even more remote from sources of open oceanic heat and moisture. The East Antarctic Ice Sheet has grown to such great elevation and extent that little atmospheric moisture now nourishes its central part.

The volume of South Polar ice must have fluctuated greatly at times since the birth of the ice sheets. Glacial erratics and glacially striated rocks on mountain summits now high above current ice-sheet levels testify to an overriding by ice at much higher levels. General lowering of levels caused some former glaciers flowing from the polar region through the Transantarctic Mountains to recede and nearly vanish, producing such spectacular "dry valleys" as the Wright, Taylor, and Victoria valleys near McMurdo Sound.

Doubt has been shed on the common belief that Antarctic ice has continuously persisted since its origin by the discovery reported in 1983 of Cenozoic marine diatoms—believed to date from the Pliocene Epoch (about 5.3 to 2.6 million years ago)—in glacial till of the Beardmore Glacier area. The diatoms are believed to have been scoured from young sedimentary deposits of basins in East Antarctica and incorporated into deposits of glaciers moving through the Transantarctic Mountains. If so, Antarctica may have been free or nearly free of ice as recently as about 3 million years ago, when the diatom-bearing beds were deposited in a marine seaway. Additionally, the Antarctic Ice Sheet may have undergone deglaciations perhaps similar to those that occurred later during interglacial stages in the Northern Hemisphere. Evidence of former higher sea levels found in many areas of the Earth seems to support the hypothesis that such deglaciation occurred. If Antarctica's ice were to melt today, for example, global sea levels would probably rise about i50 to 200 feet.

The Antarctic Ice Sheet seems to be approximately in a state of equilibrium, neither increasing nor decreasing significantly according to the best estimates. Snow precipitation is offset mainly by continental ice moving seaward by three mechanisms—ice-shelf flow, ice-stream flow, and sheet flow. The greatest volume loss is by calving from shelves, particularly the Ross, Ronne, Filchner, and Amery ice shelves. Much loss also occurs by bottom melting, but this is partly compensated by a gain in mass by accretion of frozen seawater. The quantitative pattern and the balance between gain and loss are known to be different at different ice shelves, but melting probably predominates. The smaller ice shelves in the Antarctic Peninsula are currently retreating, breaking up into vast fields of icebergs, likely due to rising temperature and surface melting.

The West Antarctic Ice Sheet (WAIS) has been the subject of much research because it may be unstable. The Ross Ice Shelf is largely fed by huge ice streams descending from the WAIS along the Siple Coast. These ice streams have shown major changes — acceleration, deceleration, thickening, and thinning—in the last century or so. These alterations have affected the grounding line, where grounded glaciers lift off their beds to form ice shelves or floating glacier tongues. Changes to the grounding line may eventually transform the WAIS proper, potentially leading to removal of this ice sheet and causing a major rise in global sea level. Although the possibility of all this happening in the next i00 years is remote, major modifications in the WAIS in the 21st century are not impossible and could have worldwide effects.

These ice sheets also provide unique records of past climates from atmospheric, volcanic, and cosmic fallout; precipitation amounts and chemistry; temperatures; and even samples of past atmospheres. Thus ice-core drilling, and the subsequent analysis of these cores, has provided new information on the processes that cause climate to change. A deep coring hole at the Russian station Vostok brought up a climate and fallout history extending back more than 400,000 years. Although near the bottom, drilling has stopped because a huge freshwater lake lies between the ice and the bed at this location. Lake Vostok has probably been isolated from the atmosphere for tens of millions of years, leading to speculation of what sort of life may have evolved in this unusual setting. Research is being conducted on how to answer this question without contaminating the water body. Lake Vostok has also attracted the attention of the planetary science community, because it is a possible test site for future study of Jupiter's moon Ganymede. Ganymede possesses a layer of liquid water beneath a thick ice cover and thus has a potential for harbouring life.

Thousands of meteorites have been discovered on "blue ice" areas of the ice sheets. Only five fragments had been found by 1969, but since then more than 9,800 have been recovered, mainly by Japanese and American scientists. Most specimens appear to have landed on Antarctic ice sheets between about 700,000 and 10,000 years ago. They were carried to blue ice areas near mountains where the ancient ice ablated and meteorites became concentrated on the surface. Most meteorites are believed to be from asteroids and a few from comets, but some are now known to be of lunar origin. Other meteorites of a rare class called shergottites had a similar origin from Mars. One of these Martian shergottites has minute structures and a chemical composition that some workers have suggested is evidence for life, though this claim is very controversial.

The SurrouNDiNg Seas

The seas around Antarctica have often been likened to the moat around a fortress. The turbulent "Roaring Forties" and "Furious Fifties" lie in a circumpolar storm track and a westerly oceanic current zone commonly called the West Wind Drift, or Circumpolar Current. Warm, subtropical surface currents in the Atlantic, Pacific, and Indian oceans move southward in the western parts of these waters and then turn eastward upon meeting the Circumpolar Current. The warm water meets and partly mixes with cold Antarctic water, called the Antarctic Surface Water, to form a mass with intermediate characteristics called Subantarctic Surface Water. Mixing occurs in a shallow but broad zone of approximately io° latitude lying south of the Subtropical Convergence (at about 40° S) and north of the Antarctic Convergence (between about 50° and 60° S). The Subtropical Convergence generally defines the northern limits of a water mass having so many unique physical and biological characteristics that it is often given a separate name, the Antarctic, or sometimes the Southern, Ocean; it contains about 10 percent of the global ocean volume.

The two convergences are well defined and important oceanic boundary zones that profoundly affect climates, marine life, bottom sedimentation, and ice-pack and iceberg drift. They are easily identified by rapid changes in temperature and salinity. Antarctic waters are less saline than tropical waters because of their lower temperatures and lesser evaporational concentration of dissolved salts. When surface waters move southward from the Subtropical Convergence zone into the subantarctic climatic belt, their temperatures drop by as much as about 9 to i6°F (5 to 9°C). Across the Antarctic Convergence, from the subantarctic into the Antarctic climatic zone, surface-water temperature drops further.

Whereas the pattern of surface currents, controlled largely by Earth's rotation, winds, water-density differences, and the geometry of basins, is relatively well understood, that of deeper water masses is more complex and less well known. North-flowing Antarctic Surface Water sinks to about 900 metres (3,000 feet) beneath warmer Subantarctic Surface Water along the Antarctic Convergence to become the Subantarctic Intermediate Water. This water mass, as well as the cold Antarctic Bottom Water, spreads far north beyond the Equator to exchange with waters of the Northern Hemisphere. The movement of the Antarctic Bottom Water is identifiable in the Atlantic as far north as the Bermuda Rise. Currents near the continent result in a circumferential belt of surface-water divergence accompanied by upwell-ing of deeper water masses.

Two forms of floating ice masses build out around the continent: glacier-fed semipermanent ice shelves, some of enormous size, such as the Ross Ice Shelf, and an annually frozen and melted ice pack that in winter reaches to about 56° S in the Atlantic and 64° S in the Pacific. Antarctica has been called the pulsating continent because of the annual buildup and retreat of its secondary icefronted coastline. Pushed by winds and currents, the ice pack is in continual motion. This movement is westward in the coastal belt of the East Wind Drift at the continent edge and eastward (farther north) at the belt of the West Wind Drift. Icebergs—calved fragments of glaciers and ice shelves—reach a northern limit at about the Subtropical Convergence. With an annual areal variation about six times as great as that for the Arctic ice pack, the Antarctic pack doubtless plays a far greater role in varying heat exchange between ocean and atmosphere and thus probably in altering global weather patterns. Long-term synoptic studies, now aided by satellite imagery, show long-period thinning in the Antarctic ice-pack regimen possibly related to global climate changes.

As part of the Deep Sea Drilling Project conducted from 1968 to 1983 by the U.S. government, the drilling ship Glomar Challenger undertook several cruises of Antarctic and subantarctic waters to gather and study materials on and below the ocean floor. Included expeditions were between Australia and the Ross Sea (1972-73), in the area south of New Zealand (1973), from southern Chile to the Bellingshausen Sea (1974), and two in the Drake Passage and Falkland Islands area (1974 and 1979-80). Among the ship's most significant findings were hydrocarbons discovered in sediments of Neogene and Paleogene age (some 2.6 million to 65 million years old) in the Ross Sea and rocks carried by icebergs from Antarctica found in late Oligocene sediments (those roughly 23 to 28 million years old) at numerous locations. Researchers inferred from these ice-borne debris that Antarctica was glaciated at least 25 million years ago.

Internationally funded drilling operations began in 1985 with the Ocean Drilling Program, using the new drilling vessel JOIDES Resolution to expand earlier Glomar Challenger studies. Studies in the Weddell Sea (1986-87) suggested that surface waters were warm during Late Cretaceous to early Cenozoic time and that the West Antarctic Ice Sheet did not form until about 10 million to 5 million years ago, which is much later than inferred from evidence on the continent itself. Drilling of the Kerguelen Plateau near the Amery Ice Shelf (1987-88) entailed the study of the rifting history of the Indian-Australian Plate from East Antarctica and revealed that this submerged plateau—the world's largest such feature—is of oceanic origin and not a continental fragment, as had been previously thought.

CLiMATe

The unique weather and climate of Antarctica provide the basis for its familiar appellations—Home of the Blizzard and White Desert. By far the coldest continent, Antarctica has winter temperatures that range from -i28.6°F (-89.2°C), the world's lowest recorded temperature, measured at Vostok Station (Russia) onJuly 2i, i983, on the high inland ice sheet to -76T (-60°C) near sea level. Temperatures vary greatly from place to place, but direct measurements in most places are generally available only for summertime. Only at fixed stations operated since the IGY have year-round measurements been made. Winter temperatures rarely reach as high as 52T (ii°C) on the northern Antarctic Peninsula, which, because of its maritime influences, is the warmest part of the continent. Mean temperatures of the coldest months are -4 to -22°F (-20 to -30°C) on the coast and -40 to -94T (-40 to -70°C) in the interior, the coldest period on the polar plateau being usually in late August just before the return of the sun. Whereas midsummer temperatures may reach as high as 59T (i5°C) on the Antarctic Peninsula, those elsewhere are usually much lower, ranging from a mean of about 32T (0°C) on the coast to between -4 and -3i°F (-20 and -35°C) in the interior. These temperatures are far lower than those of the Arctic, where monthly means range only from about 32T in summer to -3i°F in winter.

Wind chill—the cooling power of wind on exposed surfaces—is the major debilitating weather factor of Antarctic expeditions. Fierce winds characterize most coastal regions, particularly of East Antarctica, where cold, dense air flows down the steep slopes off interior highlands. Known as katabatic winds, they are a surface flow that may be smooth if of low velocity but that may also become greatly turbulent, sweeping high any loose snow, if a critical velocity is surpassed. This turbulent air may appear suddenly and is responsible for the brief and localized Antarctic "blizzards" during which no snow actually falls and skies above are clear. During one winter at Mirnyy Station, gusts reached more than 110 miles per hour on seven occasions. At Commonwealth Bay on the Adelie Coast the wind speed averaged 45 miles per hour (20 metres per second). Gusts estimated at between 140 and 155 miles per hour on Dec. 9, 1960, destroyed a Beaver aircraft at Mawson Station on the Mac. Robertson Land coast. Winds on the polar plateau are usually light, with monthly mean velocities at the South Pole ranging from about 9 miles per hour (4 metres per second) in December (summer) to 17 miles per hour (8 metres per second) in June and July (winter).

The Antarctic atmosphere, because of its low temperature, contains only about one-tenth of the water-vapour concentration found in temperate latitudes. This atmospheric water largely comes from ice-free regions of the southern oceans and is transported in the troposphere into Antarctica mostly in the 140° sector (8o° E to 140° W) from Wilkes Land to Marie Byrd Land. Most of this water precipitates as snow along the continental margin. Rainfalls are almost unknown. Despite the tremendous volume of potential water stored as ice, Antarctica must be considered one of the world's great deserts; the average precipitation (water equivalent) is only about 2 inches (50 mm) per year over the polar plateau, though considerably more, perhaps 10 times as much, falls in the coastal belt.

Lacking a heavy and protective water-vapour-rich atmospheric layer, which in other areas absorbs and reradiates to Earth long-wave radiation, the Antarctic surface readily loses heat energy into space.

Many factors determine Antarctica's climate, but the primary one is the geometry of the Sun-Earth relationship. The 23.5° axial tilt of the Earth to its annual plane of orbit, or ecliptic, around the Sun results in long winter nights and long summer days alternating between both polar regions and causing seasonal variations in climate. On a midwinter day, about June 21, the Sun's rays reach to only 23.5° (not exact, because of refraction) from the South Pole along the latitude of 66.5° S, a line familiarly known as the Antarctic Circle. Although "night" theoretically is six months long at the geographic pole, one month of this actually is a twilight period. Only a few coastal fringes lie north of the Antarctic Circle. The amount of incoming solar radiation, and thus heat, depends additionally on the incident angle of the rays and therefore decreases inversely with latitude to reach a minimum at the geographic poles.

These and other factors are essentially the same for both polar regions. The reason for their great climatic difference primarily lies in their reverse distributions of land and sea. The Arctic is an ocean surrounded by land, while Antarctica is a continent surrounded by ocean. The Arctic Ocean, a climate-ameliorating heat source, has no counterpart at the South Pole, the great elevation and perpetually reflective snow cover of which instead intensify its polar climate. Moreover, during Antarctic winters, freezing of the surrounding sea effectively more than doubles the size of the continent and removes the oceanic heat source to nearly 3,000 km (1,800 miles) from the central polar plateau.

Outgoing terrestrial radiation greatly exceeds absorbed incoming solar radiation. This loss results in strong surface cooling, giving rise to the characteristic Antarctic temperature inversions in which temperature increases from the surface upward to about i,000 feet above the surface. About 90 percent of the loss is replaced by atmospheric heat from lower latitudes, and the remainder by latent heat of water-vapour condensation.

Great cyclonic storms circle Antarctica in endless west-to-east procession, exchanging atmospheric heat to the continent from sources in the southern Atlantic, Pacific, and Indian oceans. Moist maritime air interacting with cold polar air makes the Antarctic Ocean in the vicinity of the Polar Front one of the world's stormiest. Few storms bring snowfalls to interior regions. With few reporting stations, weather prediction has been exceedingly difficult but is now greatly aided by satellite imagery

Antarctica, and particularly the South Pole, attracts much interest in astronomical and astrophysical studies as well as research on the interactions between the Sun and the upper atmosphere of Earth. The South Pole is a unique astronomical location (a station from which the Sun can be viewed continuously in summer) sitting at a high geomagnetic latitude with unequaled atmospheric clarity. It possesses a thick section of pure material (ice) that can be used as a cosmic particle detector. Automatic geophysical observatories on the high polar plateau now record information on the polar ionosphere and magnetosphere, providing data that are critical to an understanding of Earth's response to solar activity.

A major focus of upper atmospheric research in Antarctica is to understand the processes leading to the annual springtime depletion in stratospheric ozone—the "ozone hole"—which has been steadily increasing since it was first detected in 1977. Ozone is destroyed as the result of chemical reactions on the surfaces of particles in polar stratospheric clouds (PSCs). These clouds are isolated within an atmospheric circulation pattern known as the "polar vortex," which develops during the long, cold Antarctic winter. The chemical reactions take place with the arrival of sunlight in spring and are facilitated by the presence of halogens (chlorine and fluorine), which are mostly products of human activity. This process of ozone destruction, which also occurs to a lesser extent in the Arctic, increases the amount of ultraviolet-B radiation reaching Earth's surface, a type of radiation shown to impair photosynthesis in plants, cause an increase in skin cancer in humans, and damage DNA molecules in living things.

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