Antarctica
Antarctica
[‚ant′ärd·ik·ə]Antarctica
Antarctica
Antarctica
the south polar continent, occupying the central part of the south polar region (the antarctic).
General information. The area of Antarctica is 13,975,000 sq km (including the shelf glaciers and the islands and ice caps connected by them to the mainland, which have an area of 1,582,000 sq km). The area of Antarctica including the continental shelf is 16,355,000 sq km. The long, narrow Antarctic Peninsula stretches in the direction of South America; its northern tip, Cape Sifre, extends to 63° 13’ S lat. (the northernmost point of Antarctica). The center of the continent, which is known as the pole of relative inaccessibility, is located 660 km from the south pole, at approximately 84° S lat., 64° E long. The coastline, which is more than 30,000 km long, has few indentations and along most of its length consists of ice shelves (barriers) as high as several dozens of meters.
Antarctica is the earth’s highest continent. The average altitude of the surface of the ice sheet is 2,040 m, which is 2.8 times the average altitude of the surface of the other continents (730 m). The average altitude of the native sub-glacial surface of Antarctica is 410 m.
Antarctica is divided into eastern and western regions on the basis of differences in geological structure and terrain. The surface of the ice sheet of Eastern Antarctica, which rises abruptly from the coast, becomes nearly horizontal in the interior. The central, highest part of the ice sheet—in the vicinity of the Sovetskoe Plateau—reaches an altitude of 4,000 m and constitutes the main ice shed or glacial center of Eastern Antarctica. Western Antarctica has three glacial centers, which are 2,000–2,500 m high. Numerous large, low plains of shelf ice stretch along the coast (usually 30–100 m above sea level). Two of these are enormous—the Ross Ice Shelf, with an area of 538,000 sq km, and the Filch-ner Ice Shelf, with an area of 483,000 sq km.
The terrain of the native (subglacial) surface of Eastern Antarctica consists of high mountain elevations alternating with deep depressions. Eastern Antarctica’s deepest depression lies south of the Knox Coast. The principal elevations are the Gamburtsev and Vernadskii subglacial mountains in the central part of Eastern Antarctica, which reach an altitude of 3,390 m. The Transantarctic Mountains are partially covered with ice (Mt. Kirkpatrick, 4,530 m). Rising above the surface of the ice are also ranges of Queen Maud Land, the Prince Charles Mountains, and others. The relief of Western Antarctica is more complex. The mountains break through the ice sheet more often, especially on the Antarctic Peninsula. The Sentinel Range of the Ellsworth Highland reaches an altitude of 5,140 m (Mt. Vinson massif), which is the highest point in Antarctica. In close proximity to these mountains is the deepest depression of Antarctica’s subglacial terrain—2,555 m. Antarctica’s continental shelf lies lower (at a depth of 400–500 m) than that of other continents.
E. S. KOROTKEVICH
Geological structure and minerals. The pre-Cambrian Antarctic platform (all of Eastern Antarctica, the central part of Western Antarctica, and part of Marie Byrd Land), which is flanked on the coast of the Pacific sector by Mesozoic plicate formations (the coastal region of the Bellingshausen and Amundsen seas and the Antarctic Peninsula), forms the larger part of the continent. The antarctic platform is heterogeneous in structure, and its age varies in different parts. The greater part of the eastern antarctic coastal region is an upper Archean (lower Proterozoic?) crystalline foundation, 15–20 km thick, consisting of diverse gneisses, crystalline schists, magmatites, dark granites, and other rocks. In various sectors of this foundation are layers of upper Proterozoic and lower Paleozoic sedimentary volcanic formations (the lower layers of the outer casing) or, more frequently, middle and upper Paleozoic terrigenous deposits with sills of trap rock (the upper layers of the outer casing). Precambrian and lower Paleozoic intrusions of gabbro-anorthosites and charnock-ites and early Mesozoic intrusions of nepheline syenites, connected with activization processes, occur widely in the crystalline foundation.
On the outskirts of the platform, within the limits of the Transantarctic Mountains and Marie Byrd Land, is the ancient Caledonian plate. Its foundation is formed by a two-tiered folded thickness, the lower part of which is a pre-Riphaeic gneiss-granite complex and the upper part of which consists of Riphaeic and Cambrian (possibly Ordovician) terrigenous and volcanic deposits up to 10 km thick. Intrusions of Lower Paleozoic granitoids are widespread throughout the plate.
The outer casing of the platform is composed here of a layer of deposits of various periods, from Devonian to Cretaceous (the Beacon series), consisting of diverse sandstones, aleurolites, and clay shales up to 3 km in thickness, with numerous strata of high-ash coals and Gondwanaland flora. In the lower half of the level, the series includes til-lites corresponding to the first glaciation of Antarctica (300 million years ago) and is topped by basalt covers with strata of tillites of the second glaciation of Antarctica (150 million years ago). Interspersing the Beacon sandstones are numerous flat masses of Triassic-Jurassic dolerite.
The fold belt of Antarctica is formed by three structural tiers. The bottom tier was raised toward the end of the Permian or during the Triassic period from a thick (10–12 km), geosynclinal Paleozoic carboniferous-terrigenous bed. The middle tier is semiplatformal (1–5 km), formed from the continental, sedimentary-volcanic Jurassic bed and Cretaceous marine terrigenous deposits. The upper tier, Cenozoic platformal (up to 2 km), is composed of andesite and basalt lavas mixed with strata of sandstone and conglomerates. Antarctica’s fold belt contains numerous gabbro-granite intrusions, mostly Cretaceous. Early Mesozoic folded structures composed of a very thick bed (over 10 km) of poorly metamorphosed carboniferousterrigenous deposits, which form the edge flexure of the antarctic platform, have been discovered in the area where the platform and the fold belt of Antarctica join (within the Ellsworth Highlands).
Deposits of anthracite and iron ore, traces of mica, graphite, rock crystal, and beryl—and also of gold, uranium, molybdenum, copper, nickel, lead, zinc, silver, and titanium—have been found in Antarctica. The paucity of mineral deposits is attributable to imperfect geological knowledge of the continent and its thick ice sheet. There is a great potential for mineral wealth in Antarctica; this conclusion is based on the similarity between the geological structure of the antarctic platform and the Gondwanaland platforms of the other continents in the southern hemisphere, as well as the similarity of the Antarctica fold belt to structures in the Andes.
M. G. RAVICH
The ice sheet and types of terrain. The terrain of Antarctica is divided into two sharply different types: glacial and native. Large, elevated glacial plateaus occupy the interior of the continent, passing into a gently sloping and then sloping-undulating declivity of the ice sheet toward the edges of the continent. The relief of the ice of the coastal region is more complex, with poorly dissected sectors of the edge of the ice sheet alternating with glacial tongues which are traversed by cracks and broad plains of shelf ice, above which rise sloping ice caps.
The basic morphological structures of the native terrain were apparently formed toward the end of the Neocene period, but their renewal took place in the Quaternary period as well. Large interior plains, above which rise block and folded-block formations, may be discerned within the limits of the continent. These mountainous formations were dissected by preglacial erosion and then were worn down by glaciers over almost their entire area. Some of the peaks were not covered by the ice sheet and were transformed by local—mostly cirque—glaciers and physical weathering agents—mainly frost.
The formation of volcanic cones (Erebus, 3,794 m; Terror, 3,262 m; and others) and leveled surfaces of native rock (peneplains) and their elevation to considerable altitudes are linked to the late Paleocene-Neocene and Quaternary periods of development of Antarctica. The mountainous areas often have relief of the alpine type. The terrain of the coastal antarctic oases consists of low hills. The presence of ancient coastal lines and terraces containing fossils of marine organisms indicates that the continent was raised in the Holocene period.
The antarctic ice sheet has apparently existed continuously since the Neocene period—sometimes shrinking, sometimes expanding. At present, most of the continent is covered by a thick ice sheet. No more than 0.2–0.3 percent of the continental area is ice-free. The average thickness of the ice is 1,720 m, and the volume is 24 million cu km, which constitutes approximately 90 percent of all the fresh water on the earth’s surface. All types of glaciers are found in Antarctica—from enormous ice sheets to small hanging and cirque glaciers. The antarctic ice sheet descends into the ocean (except in small sections of the coast composed of native rock), forming long stretches of shelf glaciers—flat ice slabs, as much as 700 m thick, which float on the water and rest at some points on elevations of the ocean bottom.
Depressions in the subglacial terrain, from the central regions of the continent toward the coast, are paths by which the ice moves out into the ocean. The ice moves faster in these than in other areas and is split into innumerable blocks by fissure systems. These are glacier tongues, which resemble mountain valley glaciers but usually flow within shores of ice. The largest of these is the Lambert, which is 700 km long and 30–40 km wide. The ice flow in glacier tongues reaches speeds of 1,000–1,500 m per year. Glaciers are fed by atmospheric precipitation, which amounts to about 2,200 cu km of accumulation per year over the entire ice sheet. Ablation of physical material (ice) occurs largely through the calving of icebergs; surface and subglacial melting and water drainage are minimal. The majority of researchers estimate the substance balance in the antarctic ice sheet as close to zero (until more exact data are obtained).
The areas of the surface that are not covered by ice are icebound by many years of permafrost, which penetrates some distance beneath the ice sheet and to the ocean bottom.
Climate. The climate of Antarctica is polar continental (with the exception of the coastal areas). Even though the winter polar night in central Antarctica lasts for several months, the annual total radiation is close to that of the equatorial zone—at Vostok Station, it is 5 gigajoules per sq m per yr [GJ/(m2 yr)] or 120 kilocalories per sq cm per year [kcal(cm2.yr)]; in the summer it is very high, up to 1.25 GJ/(m2.mo) or 30 kcal/(cm2-mo). Nevertheless, up to 90 percent of the incoming heat is reflected back into the atmosphere by the snow, and only 10 percent of it goes to warm the surface. The radiation balance of Antarctica is therefore negative, and the temperature of the air is very low.
The center of cold on our planet is located in central Antarctica. A temperature of -88.3°C was recorded at Vostok Station on Aug. 24, 1960. The average winter temperature ranges from -60° to -70°C, the average summer temperature from -30° to -50°C. Even in the summer the temperature never rises above -20°C. In the coastal regions, especially in the region of the Antarctic Peninsula, the temperature of the air rises to 10–12°C in the summer, but the average temperature of the warmest month (January) is 1°-2°C. The average monthly temperature in the winter (July) ranges from -8°C on the Antarctic Peninsula to -35°C on the edge of the Ross Ice Shelf. The cold air rolls down from the central regions of Antarctica, forming outward-blowing winds that attain great speeds at the coast (an average annual velocity of up to 12 m per sec) and turn into hurricane winds (up to 50–60 and sometimes 90 m/sec) upon coalescence with cyclonic air streams.
As a consequence of the predominance of descending currents, the relative humidity of the air is low (60–80 percent), and at the coast—especially in Antarctic oases—it falls to 20, and even 5, percent. There is also very little cloud cover. Precipitation is almost entirely in the form of snow. In the center of the continent the average annual snowfall is 30–50 mm; on the lower part of the continental slope it increases to 600–700 mm, diminishes slightly at the foot of the slope (400–500 mm), and then rises again on some of the ice shelves and on the northwest coast of the Antarctic Peninsula (700–800 and even 1,000 mm). Blizzards are very common because of the strong winds and abundant snowfall.
Large areas of exposed rock near the coast with distinctive natural conditions are known as antarctic oases. The largest of these are the Bunger, Westfall, Grierson, and Schiermacher oases and the Wright Valley (Dry Valley). They range in area from several dozen to several hundred sq km.
The nunataks—mountain ranges and peaks that break through the ice sheet—sometimes cover much larger areas.
The antarctic lakes, most of which are found in the coastal oases, are distinctive. Many have no outlets and are unusually high in salinity to the point of bitterness. Some remain frozen through the summer. Lagoons found among the coastal cliffs of the oasis and the encircling ice shelf, beneath which they are connected with the sea, are a characteristic feature of the terrain. The lagoons vary in salinity, depending on the influx of fresh melt water and the closeness of their link with the sea. Some of the lakes are situated in the mountains at altitudes as high as 1,000 m (the Taylor oases, the Wohlthat massif on Queen Maud Land, and the Victoria massif on Victoria Land).
Flora and fauna. The plant and animal world of Antarctica is very poor and distinctive, but not even the harshest areas of the continent are biologically sterile.
Natural regions. The whole of Antarctica and the coastal islands is situated in the antarctic desert zone and may be divided into three subzones, visible on extremely limited ice-free patches of dry land. These subzones are the northern, which comprises the northwest coast of the Antarctic Peninsula and the adjoining islands; the middle, comprising the coastal oases, the islands, cliffs, and mountain ranges along the entire coast of Antarctica; and the southern, comprising areas of dry land in the interior of the continent. Division into altitude belts is of much greater significance in Antarctica. The lower belt consists of the coast, including the ice shelves, up to an altitude of several hundred meters. This belt contains the most diverse scenery in Antarctica—ice shelves, the foot of the glacial slope, the lower parts of the glacial tongues, oases, and nunataks. Snow melts here not only near outcroppings of native rock but also on the ice sheet. Because of its proximity to the ocean and the relatively high atmospheric temperatures, practically all of the continent’s organic life is concentrated in the lower belt. The middle belt (up to an altitude of 3,000 m) includes the slope of the ice sheet, the inland glacial plateau of Western Antarctica, and the mountain ranges. The air temperature is below 0°C throughout the entire year. Thawing is observed only in the summer, near outcroppings of native rock. Because of the nearly constant strong outward-blowing winds, large sastrugi form on the snow-covered surface of the glacial slope. The surface of the inland plateau is covered with small sastrugi. Patches of lichens and algae, arthropods, and infrequently birds are found on the cliffs, which are warmed to above 0°C in the summer. The area above 3,000 m is one of perennial frost. The winds there are weak, the snow is grainy, and no large sastrugi form on the surface of the glacial plateau of central Antarctica. Even the surfaces of the native rocks of the mountain peaks in this belt never warm above the freezing point, and hardly any signs of plant or animal life are to be found.
On the basis of the zones, altitude belts, and the position and nature of the shelf ice, mountain ranges, and other orographical features of the territory, Antarctica may be divided into 15 regions (see Figure 1).
E. S. KOROTKEVICH