AS IA
Geologic and climatic influences.
The contemporary relief of Asia was molded primarily under the influences of (1) ancient processes of planation (leveling), (2) larger vertical movements of the surface during the Cenozoic Era, and (3) severe erosive dissection of the edges of the uplifted highlands with the accompanying accumulation of alluvium in low-lying troughs, which were either settling downward or being uplifted more slowly than the adjoining heights.
The interior portions of the uplifted highlands, and the plateaus and tablelands of peninsular India, Arabia, Syria, and eastern Siberia--all of which are relatively low-lying but composed of resistant rock--largely have preserved their ancient peneplaned (i.e., leveled) surfaces. Particularly spectacular uplifting occurred in Central Asia, where the amplitude of this uplift of the mountain ranges of Tibet and of the Pamirs and the Himalayas has exceeded 13,000 feet. The eastern margin of the highlands, meanwhile, underwent subsidences of up to 2,300 feet. Uplifting as a result of fractures at great depths, of which the Kopet-Dag and Fergana ranges provide typical examples, and of folding over a large radius, examples of which may be seen in the Tien Shan and Gissar and Alay ranges, played a significant role. (see also Index: Kopet-Dag Range, Gissar Range) Erosional dissection transformed many ancient plateaus into mountainous regions. Majestic gorges were carved into the highlands of the western Pamirs and southeastern Tibet; the Himalayas, the Kunlun and Sayan mountains, the Stanovoy and Chersky ranges, and the marginal ranges of the West Asian highlands were deeply cut by the rivers, creating deep superimposed gorges and canyons. Vast areas of Middle, Central, and East Asia, particularly in the Huang Ho basin, are covered with loess (a loamy
unstratified deposit formed by wind or by glacial meltwater deposition); the thickness of these deposits on the Loess Plateau of China sometimes exceeds 1,000 feet. There are broad expanses of badlands, eolian (windproduced) relief, and karst topography (limestone terrain associated with vertical and underground drainage). Karst terrain is characteristic of the Kopet-Dag, eastern Pamirs, Tien Shan, Gissar and Alay ranges, Ustyurt Plateau, western Taurus Mountains, and the Levant. Tropical karst in South China is renowned for its picturesque residual hills. The mantle of Pleistocene glaciation embraced northwestern Asia only to 60° N. East of the Khatanga River, which flows from Siberia into the Arctic Ocean, only isolated glaciation of the mantle debris and of the mountains occurred because of the extremely dry climate that existed in northeastern Asia even at that time. The high mountain regions experienced primarily mountain glaciation. There are traces of several periods during which the glaciers advanced--periods separated by warmer interglacial epochs. Glaciation continues in many of the mountainous areas and on the Severnaya Zemlya archipelago. The Karakoram Range, the Pamirs, the Tien Shan, the Himalayas, and the eastern Hindu Kush are noted for the immensity of their contemporary glaciers. Most of the glaciers are retreating. The altitude of the permanent snow line is relatively high, averaging between 14,800 and 16,400 feet and reaching 21,000 feet in central Tibet. An enormous area of permafrost--some 4.25 million square miles (11 million square kilometres)--covers
northern Asia and extends to lower latitudes than anywhere else in the world. Little snowfall occurs because of the aridity, and deep freezing of the soil takes place. The depth of the permafrost in continental northern and eastern Siberia exceeds 1,000 to 1,300 feet. Volcanism has added broad lava plateaus and chains of young volcanic cones to the relief of Asia. Ancient lavas and intrusions of magma, exposed by later erosion, cover the terraced plateaus of peninsular India and central Siberia. Extensive zones of young volcanic relief and contemporary volcanism, however, are confined to the unstable arcs of the East Asian islands, together with Kamchatka, the Philippines, and the Sunda Islands. The highest active volcano in Asia, Klyuchevskaya, rises to 15,584 feet (4,750 metres) on the Kamchatka. Geologically recent volcanism is also characteristic of the West Asian highlands, the Caucasus, Mongolia, the Manchurian-Korean mountains, and the Syrian-Arabian Plateau. In historic times eruptions have also occurred in the interior of the continent in the Lesser Khingan Range and the Anyuy highlands. The regions of Asia. It is common practice in geographic literature to divide Asia into large regions, each grouping together a number of countries. These physiographic divisions usually consist of North Asia, including the bulk of Siberia and the northeastern edges of the continent; East Asia, including the continental part of the Far East region of Siberia, the East Asian islands, Korea, and eastern and northeastern China; Central Asia, including the Plateau of Tibet, and the Dzungarian and Tarim basins, Inner Mongolia, the Gobi, and the Sino-Tibetan ranges; Middle Asia, including the Turan Plain, the Pamirs, the Gissar-Alay Mountains, and the Tien Shan; South Asia, including the Philippine and Malay archipelagoes, Indochina and peninsular India, the IndoGangetic Plain, and the Himalayas; and West Asia, including the West Asian highlands (Asia Minor, Armenia, and Iran), the Levant, and the Arabian Peninsula. On occasion, the Philippines, the Malay Archipelago, and the Indochina peninsula, instead of being considered as part of South Asia, are grouped separately as Southeast Asia; the Arabian Peninsula and the Levant are also sometimes grouped together
separately as Southwest Asia. Yet another variation of the basic categories is commonly made to divide Asia into its cultural regions. North Asia. Frost weathering and permafrost have influenced the relief of North Asia. In northeastern Siberia are found faulted and folded mountains of moderate height, such as the Verkhoyansk, Chersky, and Okhotsk-Chaun mountain arcs, formed of Mesozoic structures rejuvenated by neotectonic uplifting; the Koryak Range, formed of Cenozoic structures, are also in this region. Volcanic activity took place in these areas during the Cenozoic. Some plateaus are found in the areas of the ancient massifs, such as the Kolyma Highlands. Traces of several former centres of mountain glaciers remain, as well as traces of lowland originally covered by the sea, such as the New Siberian Islands. The Lena-Aldan Plateau--an ancient peneplain resting on the underlying platform that sometimes outcrops on the surface--is located in the region. Traces of ancient glaciation also can be distinguished.
The dominant feature of north-central Siberia is the Central Siberian Plateau, a series of platform plateaus and stratified plains that were uplifted in the Cenozoic. They are composed of terraced and dissected mesas with exposed horizontal volcanic intrusions; plains formed from uplifted Precambrian blocks; and a young uplifted mesa, dissected at the edges and partly covered with traprock (Putoran Mountains). On the eastern periphery is the Yakut Lowland, the drainage basin of the lower Lena River, and on the northern periphery the North Siberian Lowland, covered with its original marine deposits. The West Siberian Plain is stratified and is composed of early Cenozoic sediments deposited over thicknesses of Mesozoic material, in addition to folded bedrock. The northern part was earlier subjected to several periods of glaciation; in the south the predominant deposits are those laid down by glacial streams, as well as alluvial deposits. In the northern part of the region are the mountains and islands of the Asian Arctic. The archipelago of Severnaya Zemlya is formed of fragments of fractured Paleozoic
folded structures. Throughout the region vigorous contemporary glaciation has occurred. East Asia. The main features in the northern region of East Asia include the Khingan and Burein ranges, the Zeya-Bureya Depression and the Sikhote-Alin ranges, the lowlands of the Amur and Sungari rivers and Lake Khanka, the Manchurian-Korean highlands running along North Korea's border with China, the ranges extending along the eastern side of the Korean peninsula, the Manchurian Plain, the lowlands of the Liao River basin, and the North China Plain. Most of these features were formed by folding, faulting, or broad zonal subsidence. The mountains are separated by alluvial lowlands in areas where recent subsidence has occurred.
The mountains of southeastern China were formed from Precambrian and Paleozoic remnants of the Yangtze paraplatform by folding and faulting that occurred during the Mesozoic and Cenozoic eras. The mountain ranges are numerous, are of low or moderate altitude, and occupy most of the surface area, leaving only small, irregularly shaped plains. The islands off the coast of East Asia and the Kamchatka Peninsula are related formations. The Ryukyu Islands, Japan, Sakhalin, and the Kuril Islands are fragments, uplifted in varying degrees, of the Ryukyu-Korean, Honshu-Sakhalin, and Kuril-Kamchatka mountainisland arcs. Dating from the Mesozoic and Cenozoic eras, these arcs have complex knots at their junctions, represented by the topography of the Japanese islands of Kyushu and Hokkaido. The mountains are of low or moderate height and are formed of folded and faulted blocks; some volcanic mountains and small alluvial lowlands also are to be found. (see also Index: Sakhalin Island) Kamchatka is a mountainous peninsula formed from fragments of the Kamchatka-Koryak and KurilKamchatka arcs, which occur in parallel ranges. The
geologically young folds enclose rigid ancient structures. Cenozoic (including contemporary) volcanism is pronounced, and the peninsula has numerous geysers and hot springs. Vast plains exist that are composed of alluvia with volcanic ashes. (see also Index: Kamchatka Peninsula) Central Asia and South Siberia. Central Asia consists of mountains, plateaus, and tablelands formed from fragments of the ancient platforms and surrounded by a folded area formed in the Paleozoic and Mesozoic eras.
The mountains of southern Siberia and Mongolia were formed by renewed uplift of old faulted and folded blocks; ranges are separated by intermontane troughs. The Alpine mountains--the Altai, the Mongolian Altai, and the Sayano-Tuvan and Stanovoy highlands--are particularly noticeable. They have clearly defined features resulting from ancient glaciation; contemporary glaciation is active in the Altai. The Central Asian plains and tablelands include the Dzungarian Basin, the Takla Makan Desert, the Gobi, and the Ordos Desert. Relief features vary from surfaces leveled by erosion in the Mesozoic and Cenozoic to stratified plateaus with low mountains, eroded plateaus on which loess had accumulated, and vast sandy deserts covered with wind-borne alluvium and lacustrine deposits. Alpine Asia--sometimes known as High Asia--includes the Pamirs and the eastern Hindu Kush, the Kunlun Mountains, the Tien Shan, the Gissar and Alay ranges, the Plateau of Tibet, the Karakoram Range, and the Himalayas. The Pamirs and the eastern Hindu Kush are sharply uplifted mountains dissected into ridges and
gorges in the west. There is thick glacial cover, and Alpine deserts occur on the plateaus. The Kunlun Mountains, the Tien Shan, and the Gissar and Alay ranges belong to an Alpine region that was formed from folded structures of Paleozoic age. There are glaciers of immense size centred in this region. The Plateau of Tibet represents a fractured Alpine zone in which Mesozoic and Cenozoic structures that surround an older mass in the centre have experienced more recent uplifting. Some of the highlands are covered with detrital desert; elsewhere in this region, Alpine highlands are dissected by erosion or are covered with glaciers. The Karakoram Range and the Himalayas were formed by uplifting that took place in a zone of Cenozoic folds and Mesozoic partially folded areas containing outcrops of the ancient bedrock. Contemporary glaciation in the region is vigorous. South Asia. South Asia, in the limited sense of the term, consists of the Indo-Gangetic Plain, peninsular India, and Sri Lanka. The Indo-Gangetic Plain is formed from the combined alluvial plains of the Indus, Ganges, and Brahmaputra rivers, which lie in a deep marginal depression running north of and parallel to the main range of the Himalayas. It is an area of immature subsidence in which thick accumulations of earlier marine sediments and later continental deposits that washed down from the mountains have been transformed into sandy deserts in the western arid region. Peninsular India and Sri Lanka are formed of platform plateaus and tablelands uplifted in the Mesozoic and Cenozoic and subjected to humid-climate erosion ever since. Tablelands with uplifted margins and terraced and dissected plateaus with lava mantles or intrusions may be distinguished. Southeast Asia. Southeast Asia is composed of the Indochina peninsula and the islands and peninsulas to the southeast of the Asian continent.
The mainland consists of the western mountain area and the central and eastern mountains and plains. The western mountain area of Myanmar is a zone of Cenozoic folding. Mountains of medium altitude are formed of
folded blocks that decrease in size and altitude to the south; the valleys are alluvial and broaden out to the south. The central and eastern region of Thailand and of Vietnam is characterized by mountains of low and moderate height that have been moderately fractured. The region is one of Mesozoic structures surrounding an ancient mass known as the Kontum block, with which are associated plateaus and lowlands filled with accumulated alluvial deposits. Archipelagoes border the southeastern margin of Asia, consisting mainly of island arcs with which peninsulas are associated. The island arcs are bordered by deep oceanic trenches. The Indian Ocean arcs-- Sumatra and the Lesser Sunda Islands--consist of fragments of Alpine folds formed from materials of different ages. Cenozoic and contemporary volcanic activity is manifest, and volcanic mountains as well as alluvial lowlands may be distinguished. Borneo and the Malay Peninsula are formed from fractured continental land situated at the junction of the Alpine-Himalayan and East Asiatic geosynclinal regions. The mountains are composed of folded and faulted blocks; the lowlands are alluvial. The Pacific Ocean island arcs, including Celebes (Sulawesi), the Moluccas, the Philippine Islands, and Taiwan, are fragments of folded Alpine structures that were built up by volcanic products during the Cenozoic. Volcanic activity and the building of coral reefs continue. Mountain areas of moderate height, volcanic ranges, alluvial lowlands, and coral reef islets may all be distinguished in the region.
Middle Asia. Middle Asia includes the plains and hills lying between the Caspian Sea to the west and Lake Balkhash to the east. This area is composed of flat plains on continental platforms of folded Paleozoic and Mesozoic bedrock. Individual uplifted portions form low, rounded hills in the Kazak region, low mountains on the Tupqaraghan and Krasnovodsk (TĂźrkmenbashy) peninsulas of the Caspian Sea, and mesas (isolated hills with level summits and steeply sloping sides) in areas of earlier marine sedimentation, such as the Ustyurt Plateau and the Karakum Desert. Thick accumulations of alluvium have been transported by the wind, forming sandy deserts in the south. Original marine and lacustrine sediments adjoin the shores of the Caspian and Aral seas and Lake Balkhash. West Asia. West Asia includes the highlands of Anatolia, the Greater Caucasus Mountains, and the Armenian and Iranian highlands.
The highlands of Anatolia--the Pontic mountain system that parallels the Black Sea, and the Taurus and Anatolian tablelands--are areas of severe fragmentation, heightened erosional dissection, and isolated occurrences of volcanism. The Greater Caucasus Mountains are a series of upfolded ranges generally running northwest to southeast between the Black and Caspian seas. The Armenian Highland, which includes the Lesser Caucasus and the Kurt mountains, is severely fragmented. Recent uplifting, in the form of a knot of mountain arcs, took place during a period of vigorous volcanism that occurred in the Cenozoic. The region is seismically active and is known for its destructive earthquakes. The Iranian Highlands represent a combination of mountain arcs (the Elburz and Turkmen-Khorasan mountains, the Safid Kuh, and the western Hindu Kush in the north; the Zagros, Makran, Soleyman, and Kirthar mountains in the south), together with the tablelands of the interior, and the central Iranian, eastern Iranian, and central Afghanistan mountains. There are isolated volcanoes of Cenozoic origin, a predominance of
accumulated remnants resulting from ancient erosion, and saline and sandy deserts in the depressions and stony deserts (hammadas) on the tablelands. Southwest Asia. Southwest Asia, like much of southern Asia, is made up of an ancient platform--the northern fragments of Gondwanaland--in which sloping plains occur in the marginal downwarps. Its principal components are the Arabian Peninsula and Mesopotamia.
The Arabian Peninsula is a tilted platform, highest along the Red Sea, on which the stratified plains have undergone erosion under arid conditions. Block tablelands with uplifted margins, Cenozoic lava plateaus, stratified plains, and cuestas (long, low ridges with a steep face on one side and a long gentle slope on the other) may all be distinguished. Ancient marine sands and alluvia, resulting from previous subsidence and sedimentation, now take the form of vast, sandy deserts. Mesopotamia consists of the Tigris and Euphrates floodplains and of the deltas from Baghdad to the Persian Gulf. The original lowland is covered with late Cenozoic sedimentation; the elevated plain, on the other hand, has been dissected by erosion and denudation under the continental conditions prevailing in the late Cenozoic. Geology. The Altai were formed during the great orogenic (mountain-building) upthrusts occurring between 500 and 300 million years ago and were worn down, over geologic time, into a peneplain, a plateau with generally accordant summit heights. Beginning in the Quaternary Period (the past 1.6 million years), new upheavals thrust up magnificent peaks of considerable size. Earthquakes are still common in the region along a fault zone in the Earth's crust; among the most recent quakes are those that occurred near Bayanbulag in 1957 and 1958 and in the vicinity of Lake Zaysan in 1990. Quaternary glaciation scoured the mountains, carving them into rugged shapes, and changed valleys from a V- to a U-shaped cross section; river erosion was also intensive and left its marks on the landscape.
As a result of these differential geologic forces, the highest ridges in the contemporary Altai--notably the Katun, North (Severo) Chu, and the South (Yuzhno) Chu--tower more than 13,000 feet, running latitudinally in the central and eastern portions of the sector of the system within the Gorno-Altay. The Tabyn-Bogdo-Ola (Mongolian: Tavan Bogd Uul), the Mรถnh Hayrhan Uul, and other western ridges of the Mongolian Altai are somewhat lower. The highest peaks are much steeper and rockier than their Alpine equivalents, but the ranges and massifs of the middle Altai, to the north and west, have ridges of about 8,200 feet, whose softer outlines betray their origins in presenting ancient, smoothed surfaces. Valleys are nevertheless jagged and gorgelike. The ridges are separated by structural hollows (notably the Chu, Kuray, Uymon, and Kansk), which are filled with crumbly deposits forming steppe landscapes. Elevations range from 1,600 to 6,600 feet above sea level. The extreme dislocations suffered by the Altai over the course of geologic time have occasioned a variety of rock types, many of them altered by magmatic and volcanic activity. There are large accumulations of geologically young, crumbly sediments in numerous intermontane depressions. The tectonic structures bear commercially exploitable deposits of iron, of such nonferrous and rare metals as mercury, gold, manganese, and tungsten, and of marble. The Altai were formed during the great orogenic (mountain-building) upthrusts occurring between 500 and 300 million years ago and were worn down, over geologic time, into a peneplain, a plateau with generally accordant summit heights. Beginning in the Quaternary Period (the past 1.6 million years), new upheavals thrust up magnificent peaks of considerable size. Earthquakes are still common in the region along a fault zone in the Earth's crust; among the most recent quakes are those that
occurred near Bayanbulag in 1957 and 1958 and in the vicinity of Lake Zaysan in 1990. Quaternary glaciation scoured the mountains, carving them into rugged shapes, and changed valleys from a V- to a U-shaped cross section; river erosion was also intensive and left its marks on the landscape.
As a result of these differential geologic forces, the highest ridges in the contemporary Altai--notably the Katun, North (Severo) Chu, and the South (Yuzhno) Chu--tower more than 13,000 feet, running latitudinally in the central and eastern portions of the sector of the system within the Gorno-Altay. The Tabyn-Bogdo-Ola (Mongolian: Tavan Bogd Uul), the Mรถnh Hayrhan Uul, and other western ridges of the Mongolian Altai are somewhat lower. The highest peaks are much steeper and rockier than their Alpine equivalents, but the ranges and massifs of the middle Altai, to the north and west, have ridges of about 8,200 feet, whose softer outlines betray their origins in presenting ancient, smoothed surfaces. Valleys are nevertheless jagged and gorgelike. The ridges are separated by structural hollows (notably the Chu, Kuray, Uymon, and Kansk), which are filled with crumbly deposits forming steppe landscapes. Elevations range from 1,600 to 6,600 feet above sea level. The extreme dislocations suffered by the Altai over the course of geologic time have occasioned a variety of rock types, many of them altered by magmatic and volcanic activity. There are large accumulations of geologically young, crumbly sediments in numerous intermontane depressions. The tectonic structures bear commercially exploitable deposits of iron, of such nonferrous and rare metals as mercury, gold, manganese, and tungsten, and of marble. Geology.
The Altai were formed during the great orogenic (mountain-building) upthrusts occurring between 500 and 300 million years ago and were worn down, over geologic time, into a peneplain, a plateau with generally accordant summit heights. Beginning in the Quaternary Period (the past 1.6 million years), new upheavals thrust up magnificent peaks of considerable size. Earthquakes are still common in the region along a fault zone in the Earth's crust; among the most recent quakes are those that occurred near Bayanbulag in 1957 and 1958 and in the vicinity of Lake Zaysan in 1990. Quaternary glaciation scoured the mountains, carving them into rugged shapes, and changed valleys from a V- to a U-shaped cross section; river erosion was also intensive and left its marks on the landscape.
As a result of these differential geologic forces, the highest ridges in the contemporary Altai--notably the Katun, North (Severo) Chu, and the South (Yuzhno) Chu--tower more than 13,000 feet, running latitudinally in the central and eastern portions of the sector of the system within the Gorno-Altay. The Tabyn-Bogdo-Ola (Mongolian: Tavan Bogd Uul), the Mรถnh Hayrhan Uul, and other western ridges of the Mongolian Altai are somewhat lower. The highest peaks are much steeper and rockier than their Alpine equivalents, but the ranges and massifs of the middle Altai, to the north and west, have ridges of about 8,200 feet, whose softer outlines betray their origins in presenting ancient, smoothed surfaces. Valleys are nevertheless jagged and gorgelike. The ridges are separated by structural hollows (notably the Chu, Kuray, Uymon, and Kansk), which are filled with crumbly deposits forming steppe landscapes. Elevations range from 1,600 to 6,600 feet above sea level. The extreme dislocations suffered by the Altai over the course of geologic time have occasioned a variety of rock types, many of them altered by magmatic and volcanic activity. There are large accumulations of geologically young, crumbly sediments in numerous intermontane depressions. The tectonic structures bear commercially exploitable deposits of iron, of such nonferrous and rare metals as mercury, gold, manganese, and tungsten, and of marble.