用英语介绍世界五大洲

Asia
The largest of the world's continents. With its peninsular extension, commonly called the continent of Europe, it is the major portion of the broad east-west extent of the Northern Hemisphere land masses. In many ways Asia is more a cultural concept than a physical entity. There is no logical physical separation between Asia and Europe, and even Africa is separated from Asia merely by the width of the Suez Canal. For convenience, however, the Eurasian land mass is considered to be divided by the Ural Mountains into Europe in the west and Asia in the east. Thus restricted, Asia has an area of about 17,700,000 mi2 (45,800,000 km2), about one-third of the land area of the Earth. In the north, Siberia reaches past the 80th latitude. Southward, India and Sri Lanka (Ceylon) reach nearer than 10°N of the Equator, while the Indonesian islands extend more than 10°S of the Equator. The continental heart of Asia is more than 2000 mi (3200 km) from the nearest ocean. See also Continent; Europe.
Topography
In the topographic framework of Asia, the great mountain systems are the most impressive features. From the central knot of the mighty Pamirs and Kopet Dagh in the heart of the continent originate chains radiating in several directions. In the Peter the First Range there are such heights as Qullai Ismoili Somoni, 24,584 ft (7493 m), and Lenin Peak, 23,377 ft (7125 m), above sea level. Running westward through Afghanistan is the Hindu Kush, reaching elevations over 20,000 ft (6100 m). The mountain trendline continues, after a jog northwestward, in the Elburz of northern Iran and thence in the Armenian highlands and the Caucasus, each with elevations reaching 18,000 ft (5500 m), decreasing thereafter to the Pontus and Taurus ranges of northern and southern Turkey. In western and southern Iran are the massive Zagros and Makran ranges.
Southeastward from the Pamir knot run the three most imposing mountain chains on Earth: the Karakorum, which continues the line of the Hindu Kush eastward in an arc convex to the north; the Himalaya in an arc convex to the south; and the shorter Trans-Himalaya, or Nyen-chen Tangla, north of the Himalaya, with higher average elevations but peaks of lesser height. In all of these, the average elevations exceed 4 mi (6400 m), with several scores of peaks reaching a height in excess of 25,000 ft (7600 m) above sea level. Everest, 29,141 ft (8882 m), and Kinchinjunga, 28,146 ft (8579 m), lie in the Himalaya, while the peak designated as K2, 28,250 ft (8611 m), rises in the Karakorum.
In eastern Tibet the Himalaya and Nyen-chen Tangla bend sharply toward the south, and the former is cut through by the gorge of the Brahmaputra River. From the bend zone, great ridges divided by deep gorges run south to form the Burma-China frontiers and the mountain backbones of the Malay peninsula and Vietnam. The Nan-ling system of south China diverges eastward to divide the Yang-tzu (Yangtze) from the Hsi (Si) drainage.
From the western Himalaya, the 11,000-ft (3400-m) Sulaiman Range runs south and, together with the Kirthar Range, divides West Pakistan from Afghanistan.
Beginning at heights over 20,000 ft (6100 m) and branching off from the Karakorum south of Kashgar, the Kuen-lun Mountains run eastward across western China. Genetically they form the longest mountain system of China. With their eastward extensions in the 12,000 ft (3700 m) Ch'in-ling and the lesser Ta-pieh mountains and Huai-yang hills, they reach almost to the Pacific. Together with the northeastward arc of the Altyn Tagh and the Nan Shan branching from it, the Kuen-lun forms the northern wall of the * plateau. Near the eastern end of the Kuen-lun proper lie the Amne Machin Mountains, with peaks up to 25,000 ft (7600 m) in elevation.
Northeastward of the Pamir knot runs the east-west oriented Tien Shan, over 1000 mi (1600 km) long and maintaining heights of 18,000–20,000 ft (5500–6100 m) over much of its length. Roughly parallel and trending east and west is a series of great ranges to its north, with mutual connections in the west. These include the Altai-Sayan, the Tannu Ola, and the Kentei, which form natural boundaries for Outer *. They continue the systems of young mountains crossing central Asia; farther northeast, they extend further in the Stanovoi Mountains of Eastern Siberia.
The Asian plateaus are in various stages of erosion and thus present a great variety of landscapes. The * plateau is a prime example. The western half, because of little rainfall, exhibits a rolling topography with relatively slight local relief except where mountain chains cross it; it is a land of internal drainage basins. Average elevations are over 16,000 ft (4900 m). The eastern half is humid or subhumid and is cut by numerous rivers, producing deep canyons and great ridges. In contrast to this is the *n plateau. This plateau consists mostly of vast, rather level plains 3000–5000 ft (900–1500 m) high, surmounted in places by mountains, and containing broad, shallow basins divided by land swells of low elevation.
Other major topographic units of Asia are blocs of hill lands. Most of southern China and much of southeastern Asia comprise hills which may be roughly defined as slope lands with local relief under 1000–1500 ft (300–450 m) although in absolute elevation they may rise many thousands of feet above sea level. Hilly lands are found to predominate in the northern part of the Indian peninsula and along both flanks of the Indian plateau, where they are called ghats. In southern India are the Nilgiri and Cardomom hills, rising to mountainous elevations of 8000 ft (2400 m). Many parts of different plateaus have hilly regions where erosion has produced uneven local relief, as in the Shan or North Vietnam plateau. Hills are prominent features of southwestern Asia, including eastern Mediterranean regions, such as Israel, Syria, and Lebanon.
The most significant topographic units of Asia are the great alluvial plains and river deltas. The gross drainage pattern of Asia is radial; the rivers flow from the highlands in the heart of the continent and run outward in all directions. Only in the south, east, and north sectors of the continent do the rivers reach the sea. Flowing into the peripheral seas of the Pacific are such mighty rivers as the Mekong, the Hsi, the Yang-tzu, the Huai, the Yellow, and the Amur, each building large, heavily populated plains and, with the exception of the Amur, densely settled deltas. The Yellow Plain (North China Plain), with some 125,000 mi2 (324,000 km2) of area, and the Yangtzu Plain, with about 75,000 mi2 (194,000 km2), are among the most extensive alluvial plains of the Earth. In the shallow South China, East China, and Yellow seas, the deltas of the first five rivers mentioned above are pushing steadily seaward.
Important sectors of Asia, containing some 200,000,000 people, are completely insular. The most important are the Japanese, Philippine, and Indonesian islands and *. Almost all of Asia's islands lie in great volcanic arcs bounding large seas off the continent's Pacific coast. At least 160 active volcanoes are found here and in Kamchatka. Few islands lie along the Asiatic coasts of the Indian Ocean, although the Sunda chain of Indonesia has perhaps more of a claim to Indian Ocean frontage than to Pacific frontage. Sri Lanka is the only significant island in the northern part of the Indian Ocean west of Sumatra. In the Persian Gulf off the north coast of Arabia lies the small island Bahrein.
Few islands lie off the alluviated coastlands of northern Siberia. Some moderately large ones are included in the barren and rocky Severnaya Zemlya group, the New Siberian Islands, and Wrangel Island. The Commander Islands and Karaginski Island lie in the Bering Sea only a short distance from the Aleutians.
Climates
Five major climatic types may be distinguished in the Asian region: (1) the monsoonal system of eastern Asia, (2) the monsoonal system of southern Asia, (3) the equatorial regions of southeastern Asia and their extension into the Southern Hemisphere as they are influenced by the Australian monsoon, (4) the winter rainfall areas of southwestern Asia, and (5) the cyclonic and convectional storm systems of central and northern Asia.
Fundamental to understanding the climates of Asia are the vastness of the unbroken landmass and the long latitudinal stretch from the polar realm to south of the Equator. These are responsible for the great temperature and humidity extremes that occur. The greatest ranges of temperatures in the world have been recorded in interior Asia. Continentality, therefore, is the outstanding feature of climates of interior Asia. In coastal and insular areas of east Asia, however, winds moving over the warm, northward-flowing Japan Current and the western Pacific waters moderate the coastland and island climates. See also Monsoon meteorology.
The driest portions of Asia include the vast areas of southern *, Hsin-chiang, former Soviet Central Asia, and southwestern Asia. Except for small, favored mountain areas, most of this region from the Gobi to the Red Sea gets less than 10 in. (25 cm) of precipitation per year. With the exception of southern Arabia, which is subtropical desert, these are mid-latitude desert and dry steppe regions. Favored with higher rainfall are the Yemen Mountains and the coastal mountains of Turkey, together with Lebanon, Syria, and northern Israel. The highlands of Armenia and the Elburz of Iran are favored also with more abundant rainfall, which may range from 25 to 50 in. (64 to 127 cm) or more per year.
The northeastern Siberian mountains and the Arctic coastal lands also receive meager rainfall, less than 8 in. (20 cm), but are not dry because evaporation is low and the water table is high. Most of Siberia has permafrost below a few feet of surface soil, so that rainwater does not filter far down into the earth. Between the arid belt of central Asia and the northeast Siberian low-precipitation zone, the annual rainfall ranges between 10 and 18 in. (25 and 45 cm).
In eastern Asia the precipitation increases in a southeasterly direction from interior Asia to the coast. The annual maximum seldom exceeds 80 in. (203 cm) in the wetter southeast coastal regions, whereas this drops to less than 30 in. (76 cm) in the North China Plain and less than 15 in. (38 cm) at the Great Wall. In some mountainous parts of Japan and *, the yearly average may be more than 100 in. (254 cm).
In the Indian subcontinent rainfall is heaviest along the western plateau fringe and in East Bengal, where it may average over 100 in. (254 cm) per year. The interior of the peninsula is relatively dry. Northwestern India and Pakistan share the drought of southwestern Asia. With the exception of the extreme north, Ceylon generally has abundant rainfall.
Southeastern Asia has the heaviest rainfall of the entire Asiatic region. The mainland mountains facing the southwest summer monsoon crossing the Bay of Bengal, and parts of the Vietnamese and Laotian cordilleras facing the humidified northeast winter monsoons of eastern Asia, regularly get average rainfalls of 120–150 in. (305–381 cm) or even more. Equally heavy rainfalls occur in the southwestern half of Sumatra, southwestern Java, the northwestern half of Borneo, and the Pacific fringe of the Philippine Islands. With a few small exceptions, southeastern Asia has no areas that are subject to severe drought.
Vegetation
Asia's vegetation belts and zones follow, in general, the climatic patterns from desert lands through tropical to Arctic margins.
A wide belt of tundra made irregular by topography occupies the entire Arctic lowland of Siberia with widths varying from 250 to 500 mi (400 to 800 km) north and south. It is widest in the extreme northeast and it extends southward and inland with higher elevations. The frozen subsoil permits the growth of little more than mosses, lichens, dwarfed trees, and scrub. See also Permafrost; Tundra.
The largest unbroken expanse of forest in the world is the Siberian taiga, a dominantly coniferous forest of larches, spruce, fir, and pines, with such deciduous trees as birch and aspen occurring intermixed with the conifers or taking over as a secondary growth in burnt-over areas. The width of this belt in Siberia is more than 1000 mi (1600 km) and it stretches about 4000 mi (6400 km) from the Sea of Okhotsk to the Urals.
Various admixtures of coniferous and deciduous trees compose the vegetation of mid-latitude mixed forests. In the west Siberian plain there is a narrow zone of mixed taiga and deciduous forests including oaks, maples, ash, and lindens. This zone, with a width of 50–100 mi (80–160 km), lies somewhat south of the parallel of 60°N and fades into the steppelands that form the great spring-wheat region of Siberia. Mixed midlatitude deciduous and coniferous forest areas of a similar type occupy most of Korea, the northern half of Honshu in Japan, and the hill lands surrounding the Yellow Plain, as well as the Ch'in-ling Mountains. In southern Asia these forests are found chiefly in a narrow belt of mountain land in the outer ranges of the Himalaya. The remaining areas of these mixed forests run from the Elburz Mountains through the Armenian highlands and the Black Sea fringe of Turkey to the Aegean coast, and in southwestern Asia in the Elburz of northern Iran.
From the mixed and deciduous forests of the west Siberian plain southward, an increasingly dry steppeland is encountered. It extends for 400–500 mi (640–800 km) in a belt about 1000 mi (1600 km) long between the Urals and the Altai-Sayan and associated uplands. The northern half of this belt with its higher annual precipitation of 12–16 in. (30–40 cm) is the agricultural heart of the plain. The southern part gradually changes to desert steppe and then to desert along about the 50th parallel. Eastward of Lake Baikal a broadened steppe zone occupies the Trans-Baikal region extending southward to the Gobi Desert of southern * and eastward to the Great Hsing-an Mountains, where the zone, about 200 mi (320 km) wide, runs southward in Inner *. The steppe zone in Inner * widens with the increasing moisture south of the Great Wall to include most of China's loess plateau. Grasses also form the natural vegetation of the Manchurian plain, with tall grass in the eastern portion thinning out to short-grass steppe in the Hsing-an Mountain flanks. The Gobi Desert is flanked by steppelands to its north, east, and south, as well as by mountain steppe zones in the eastern Altai and eastern T'ien Shan.
Mixed evergreen forests appear to be limited mostly to interior southern China and to Japan from the Kwanto Plain southward, South of the Yang-tzu Valley, this forest type extends from the coast at Shanghai to the gorge lands of eastern Tibet. In Asia the characteristic trees of the mixed forest include broad-leafed evergreen trees such as banyans and camphor, and coniferous trees such as pines, cedars, and cypresses, as well as varieties of bamboo.
Tropical and subtropical rainforest is restricted to warm or hot regions of southern and southeastern Asia which get ample rainfall the year round or get so much rain during a large part of the year that a high groundwater table is maintained during the short dry season. The subtropical sectors are found along the southeastern China coast, in *, and in northern Burma; they merge with the tropical rainforest farther south, where rainfall and temperature increase. See also Rainforest.
Monsoon tropical deciduous forests comprise the tropical parts of Asia which have a moderately high rainfall but a long dry season (usually in the low-sun period or winter). These forests consist mostly of mixed species, but sometimes a single species becomes dominant as a result of selection from frequent burnings.
A large region of savanna grassland surrounds the Thar Desert of northwestern India and occupies most of the Indus Valley, the Punjab, and the Kathiawar peninsula. Much of the drier interior peninsular Deccan of India also has this as a natural vegetation. Other Asian regions with similar cover are found in Yemen and the region in southeastern Arabia from Oman as far westward as the Qatar peninsula; and similar vegetation extends over the Korat plateau of Thailand, lower Thailand west of Bangkok, southern Cambodia, and small areas in interior Borneo and the Philippines. See also Savanna.
Immense areas of central and southwestern Asia have little or no vegetative cover, and bare rock alternates with sand veneering. In places shifting sand dunes are formed. Although the deserts are not necessarily lifeless, the vegetation is so widely spaced that much bare ground is exposed. The tropical desert areas generally receive their meager rainfall in torrential downpours on rare occasions. After such rains numerous herbs may spring to life and flower, while the bunch grass here and there may become green for a short season.