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ASIA, by Encyclopaedia Britannica
Largest continent on Earth. It is bounded by the Arctic, Pacific, and Indian oceans. The western boundary, with Europe, runs roughly north-south along the eastern Ural Mountains; the Caspian, Black, Aegean, and Mediterranean seas; the Suez Canal; and the Red Sea. The islands of Sri Lanka and Taiwan and the archipelagoes of Indonesia (excluding New Guinea), the Philippines, and Japan also form part of Asia. Area: 17,226,000 sq mi (44,614,000 sq km). Population (2004 est.): 3,879,659,000. Mountains and plateaus predominate on the continent, with the highest mountains located in Central Asia and north of the Indian subcontinent. Terrain features include Earth’s highest peak, Mount Everest, at 29,035 ft (8,850 m), and the lowest natural point, the Dead Sea, at 1,312 ft (400 m) below sea level. The largest of Asia’s many arid regions are the Thar and Gobi deserts. It has some of the longest rivers in the world, including the Euphrates, Tigris, Indus, Ganges (Ganga), Yangtze (Chang; the longest river in Asia), Huang He (Yellow), Ob, Yenisey, and Lena. The Caspian, Aral, and Dead seas are major saltwater lakes. About one-fifth of Asia’s landmass is arable. Its principal language groups include Sino-Tibetan, Indo-Aryan, Austronesian, Austroasiatic, and Semitic; important singular languages include Japanese and Korean. East Asia contains three main ethnic groups: Chinese, Japanese, and Korean. The Indian subcontinent is home to a vast diversity of peoples, most of whom speak languages from the Indo-Aryan subgroup of the Indo-European family. Because of the influence of China and the former Soviet Union, the Mandarin Chinese dialect and the Russian language are used widely. Asia is the birthplace of all the world’s major religions and hundreds of minor ones. Hinduism is the oldest major religion to have originated in southern Asia; Jainism and Buddhism emerged in the 6th and 5th centuries bc, respectively. Southwest Asia was the cradle of the so-called Abrahamic religions: Judaism, Christianity, and Islam. Daoism and Confucianism, both of which originated in the 6th or 5th century bc, have profoundly influenced Chinese culture and the cultures of surrounding peoples. Asia is marked by great disparities in wealth. A few countries, notably Japan, Singapore, and the oil-rich countries of the Arabian Peninsula, have attained high standards of living; others, such as Bangladesh and Myanmar, are among the poorest. Between these two extremes lie Russia, China, and India. Asia is a land of great cultural diversity, but there are five main cultural influences: Chinese, Indian, Islamic, European, and Central Asian. China has had great influence in East Asia as the source of Confucianism, artistic styles, and the Chinese writing system. Indian influence has been expressed through Hinduism and Buddhism, affecting the Tibet Autonomous Region of China, Indonesia, Cambodia, and Central Asia. Islam spread from its original Arabian home to become important in the Middle East, South Asia, Central Asia, and elsewhere. Members of the earlier human species Homo erectus migrated from Africa to East Asia at least one million years ago. One of the earliest civilizations to use writing developed in the Tigris and Euphrates river valleys c. 3000 bc (see Mesopotamia). Civilization in the Indus River valley and in northern Syria followed c. mid-3rd millennium bc. Chinese urban civilization began with the Shang dynasty (c. 1600–1046 bc) and continued under the Zhou dynasty (1046–256 bc). Indo-European-speaking peoples (Aryans) began to invade India from the west c. 2000–1500 bc and developed the Vedic religion. A succession of empires and charismatic rulers, including the Macedonian Alexander the Great, spread their political control as far as military power could carry them. In the 13th century ad Genghis Khan and his Mongol successors united much of Asia under their rule. In the 14th century the Turkic warlord Timur conquered much of Central Asia. Muslim Turks destroyed the remnants of the Byzantine Empire in the 15th century. In the 19th century European imperialism began to replace Asian imperialism. Tsarist Russia pushed its political control across Asia to the Pacific Ocean, the British gained control of India and Burma (Myanmar), the French dominated eastern Southeast Asia (see French Indochina), the Dutch occupied the East Indies (Indonesia), and the Spanish and later the U.S. ruled the Philippines. After World War II (1939–45), European imperialism steadily disappeared as former colonies gained independence in the second half of the 20th century.
the world’s largest and most diverse continent. It occupies the eastern four-fifths of the giant Eurasian landmass. Asia is more a geographic term than a homogeneous continent, and the use of the term to describe this vast area always carries the potential of obscuring the enormous diversity among the the regions it encompasses. Asia has both the highest and the lowest points on the surface of the Earth, has the longest coastline of any continent, is subject overall to the world’s widest climatic extremes, and, consequently, produces the most varied forms of vegetation and animal life on Earth. In addition, the peoples of Asia have established the broadest variety of human adaptation found on any of the continents.
The name Asia is ancient, and its origin has been variously explained. The Greeks used it to designate the lands situated to the east of their homeland. It is believed that the name may be derived from the Assyrian word asu, meaning “east.” Another possible explanation is that it was originally a local name given to the plains of Ephesus, which ancient Greeks and Romans extended to refer first to Anatolia (contemporary Asia Minor, which is the western extreme of mainland Asia), and then to the known world east of the Mediterranean. When Western explorers reached South and East Asia in early modern times, they extended this label to the whole of this immense landmass.
Asia is bounded by the Arctic Ocean to the north, the Pacific Ocean to the east, the Indian Ocean to the south, the inland seas of the Atlantic Ocean—the Mediterranean and the Black—to the southwest, and Europe to the west. Asia is separated from North America to the northeast by the Bering Strait and from Australia to the southeast by the seas and straits connecting the Indian and Pacific oceans. The Isthmus of Suez unites Asia with Africa, and it is generally agreed that the Suez Canal forms the border between them. Two narrow straits, the Bosporus and the Dardanelles, separate Anatolia from the Balkan Peninsula.
The land boundary between Asia and Europe is a historical and cultural construct that has been defined variously; only as a matter of agreement is it tied to a specific borderline. The most convenient geographic boundary—one that has been adopted by most geographers—is a line that runs south from the Arctic Ocean along the eastern slope of the Ural Mountains and then turns southwest along the Emba River to the northern shore of the Caspian Sea; west of the Caspian, the boundary follows the Kuma-Manych Depression to the Sea of Azov and the Kerch Strait.
The total area of Asia, including the Caucasian isthmus and excluding the island of New Guinea, amounts to roughly one-third of the land surface of the Earth. The islands—including the Japanese islands, Taiwan, the Indonesian islands, Sakhalin and other Asian Russian islands, Sri Lanka, Cyprus, and numerous smaller islands—account for about 7 percent of the total. (Although New Guinea is mentioned occasionally in this article, it generally is not considered a part of Asia.) The farthest terminal points of the Asian mainland are Cape Chelyuskin in north-central Siberia, Russia (77°43′ N), to the north; the tip of the Malay Peninsula, Cape Piai, or Bulus (1°16′ N), to the south; Cape Baba in Turkey (26°4′ E) to the west; and Cape Dezhnev (Dezhnyov), or East Cape (169°40′ W), in northeastern Siberia, overlooking the Bering Strait, to the east.
Asia has the highest average elevation of the continents and contains the greatest relative relief. The tallest peak in the world, Mount Everest, which reaches an elevation of 29,035 feet (8,850 metres; see Researcher’s Note: Height of Mount Everest); the lowest place on the Earth’s land surface, the Dead Sea, which averages about 1,312 feet (400 metres) below sea level; and the world’s deepest continental trough, occupied by Lake Baikal, which is 5,315 feet (1,620 metres) deep and whose bottom lies 3,822 feet (1,165 metres) below sea level, are all located in Asia. These physiographic extremes and the overall predominance of mountain belts and plateaus are the result of the collision of tectonic plates. In geologic terms, Asia comprises several very ancient continental platforms and other blocks of land that merged over the eons. Most of these had coalesced as a continental landmass by about 160 million years ago, when the core of the Indian subcontinent broke off from Africa and began drifting northeastward to collide with the southern flank of Asia about 50 million years ago. The northeastward movement of the subcontinent continues at about 2.4 inches (6 cm) a year. The impact and pressure continue to raise the Plateau of Tibet and the Himalayas.
Asia’s coastline—some 39,000 miles (62,800 km) in length—is, variously, high and mountainous, low and alluvial, terraced as a result of the land’s having been uplifted, or “drowned” where the land has subsided. The specific features of the coastline in some areas—especially in the east and southeast—are the result of active volcanism; thermal abrasion of permafrost (caused by a combination of the action of breaking waves and thawing), as in northeastern Siberia; and coral growth, as in the areas to the south and southeast. Accreting sandy beaches also occur in many areas, such as the Bay of Bengal.
The mountain systems of Central Asia not only have provided the continent’s great rivers with water from their melting snows but also have formed a forbidding natural barrier that has influenced the movement of peoples in the area. Migration across these barriers has been possible only through mountain passes.
A historical movement of population from the arid zones of Central Asia has followed the mountain passes into the Indian subcontinent. More recent migrations have originated in China, with destinations throughout Southeast Asia. The Korean and Japanese peoples and, to a lesser extent, the Chinese have remained ethnically more homogeneous than the populations of other Asian countries.
Asia’s population is unevenly distributed, mainly because of climatic factors. There is a concentration of population in western Asia as well as great concentrations in the Indian subcontinent and the eastern half of China. There are also appreciable concentrations in the Pacific borderlands and on the islands, but vast areas of Central and North Asia—whose forbidding climates limit agricultural productivity—have remained sparsely populated. Nonetheless, Asia, the most populous of the continents, contains almost three-fifths of the world’s people.
Asia is the birthplace of all the world’s major religions—Buddhism, Christianity, Hinduism, Islam, and Judaism—and of many minor ones. Of these, only Christianity developed primarily outside Asia; it exerts little influence on the continent, though many Asian countries have Christian minorities. Buddhism has had a greater impact outside its birthplace in India and is prevalent in various forms in China, Korea, Japan, the Southeast Asian countries, and Sri Lanka. Islam has spread out of Arabia eastward to South and Southeast Asia. Hinduism has been mostly confined to the Indian subcontinent.
Chakravarthi V. NarasimhanYury Konstantinovich Yefremov, Ed.
This article surveys the physical and human geography of Asia. For in-depth treatment of Asia’s major geographic features, see specific articles by name—e.g., Himalayas, Gobi, and Tigris and Euphrates rivers. For discussion of individual countries of the continent, see specific articles by name—e.g., China, India, and Japan. For discussion of major cities of the continent, see specific articles by name—e.g., Bangkok, Jerusalem, Beijing, and Seoul. The principal treatment of Asian historical and cultural development is contained in the articles on Asian countries, regions, and cities and in the articles Palestine, history of and Islamic world. Related topics are discussed in articles on religion (e.g., Buddhism, Hinduism, and Islam) and arts and literature (e.g., Chinese literature, Japanese literature, Central Asian arts, Southeast Asian arts, and South Asian arts). Area (including Asian Russia but excluding the island of New Guinea), 17,226,200 square miles (44,614,000 square km); insular area, 1,240,000 square miles (3,210,000 square km). Pop. (2008 est.) 4,075,742,000.
Asia is not only the Earth’s largest but also its youngest and structurally most complicated continent. Although Asia’s evolution began almost four billion years ago, more than half of the continent remains seismically active, and new continental material is currently being produced in the island arc systems that surround it to the east and southeast. In such places, new land is continuously emerging and is added to the bulk of the continent by episodic collisions of the island arcs with the mainland. Asia also contains the greatest mountain mass on the Earth’s surface: the Plateau of Tibet and the bordering mountains of the Himalayas, Karakoram, Hindu Kush, Pamirs, Kunlun, and Tien Shan. By virtue of its enormous size and relative youth, Asia contains many of the morphological extremes of the Earth’s land surface—such as its highest and lowest points, longest coastline, and largest area of continental shelf. Asia’s immense mountain ranges, varied coastline, and vast continental plains and basins have had a profound effect on the course of human history. The fact that Asia produces about half of the world’s petroleum and coal, in addition to being a significant contributor to the global production of many minerals (e.g., about three-fifths of the world’s tin), heavily underlines the importance of its geology for the welfare of the world’s population.
The morphology of Asia masks an extremely complex geologic history that predates the active deformations largely responsible for the existing landforms. Tectonic units (regions that once formed or now form part of a single tectonic plate and whose structures derive from the formation and motion of that plate) that are defined on the basis of active structures in Asia are not identical to those defined on the basis of its fossil (i.e., now inactive) structures. It is therefore convenient to discuss the tectonic framework of Asia in terms of two separate maps, one showing its paleotectonic (i.e., older tectonic) units and the other displaying its neotectonic (new and presently active) units.
According to the theory of plate tectonics, forces within the Earth propel sections of the Earth’s crust on various courses, with the result that continents are formed and oceans are opened and closed. Oceans commonly open by rifting—by tearing a continent asunder—and close along subduction zones, which are inclined planes along which ocean floors sink beneath an adjacent tectonic plate and are assimilated into the Earth’s mantle. Ocean closure culminates in continental collision and may involve the accretion of vast tectonic collages, including small continental fragments, island arcs, large deposits of sediment, and occasional fragments of ocean-floor material. In defining the units to draw Asia’s paleotectonic map, it is useful to outline such accreted objects and the lines, or sutures, along which they are joined.
Continuing convergence following collision may further disrupt an already assembled tectonic collage along new, secondary lines, especially by faulting. Postcollisional disruption also may reactivate some of the old tectonic lines (sutures). These secondary structures dominate and define the neotectonic units of Asia. It should be mentioned, however, that most former continental collisions also have led to the generation of secondary structures that add to the structural diversity of the continent.
The paleotectonic units of Asia are divided into two first-order classes: continental nuclei and orogenic (mountain-building) zones. The continental nuclei consist of platforms that stabilized mostly in Precambrian time (between roughly 3.8 billion and 540 million years ago) and have been covered largely by little-disturbed sedimentary rocks; included in this designation are the Angaran (or East Siberian), Indian, and Arabian platforms. There are also several smaller platforms that were deformed to a greater extent than the larger units and are called paraplatforms; these include the North China (or Sino-Korean) and Yangtze paraplatforms, the Kontum block (in Southeast Asia), and the North Tarim fragment (also called Serindia; in western China). The orogenic zones consist of large tectonic collages that were accreted around the continental nuclei. Recognized zones are the Altaids, the Tethysides (further subdivided into the Cimmerides and the Alpides), and the circum-Pacific belt. The Alpides and circum-Pacific belt are currently undergoing tectonic deformation—i.e., they are continuing to evolve—and so are the locations of earthquakes and volcanic eruptions.
The Precambrian continental nuclei were formed by essentially the same plate tectonic processes that constructed the later orogenic zones, but it is best to treat them separately for three reasons. First, the nuclei occupy only about one-fourth of the area of Asia, and less than one-third of this area (i.e., less than 10 percent of Asia’s total) consists of exposed Precambrian rocks that enable geologists to study their development. Second, Precambrian rocks are extremely poor in fossils, which makes global or even regional correlations difficult. Finally, during most of Phanerozoic time (i.e., about the past 540 million years), the nuclei have remained stable and have acted as hosts around which the tectonic collages have accumulated in the Phanerozoic orogenic zones.
The paleotectonic evolution of Asia terminated some 50 million years ago as a result of the collision of the Indian subcontinent with Eurasia. Asia’s subsequent neotectonic development has largely disrupted the continent’s preexisting fabric. The first-order neotectonic units of Asia are Stable Asia, the Arabian and Indian cratons, the Alpide plate boundary zone (along which the Arabian and Indian platforms have collided with the Eurasian continental plate), and the island arcs and marginal basins.
The oldest rocks in Asia are found in the continental nuclei. Rocks more than 3 billion years old are in the Precambrian outcrops of the Angaran and Indian platforms and in the North China paraplatform. They consist of primitive island-arc magmatic and sparse sedimentary rocks sandwiched between younger basaltic and ultrabasic rocks, exposed along what are called greenstone belts. The basement of the Angaran platform was largely formed by about 1.5 billion years ago. The final consolidation of the Indian platform, however, lasted until about 600 million years ago and included various mountain-building episodes with peaks of activity between 2.4 and 2.3 billion years ago, at about 2 billion years ago, between 1.7 and 1.6 billion years ago, and between 1.1 billion and 600 million years ago. In the Arabian platform the formation of the present basement commenced by arc and microcontinent accretion some 900 million years ago and ended about 600 million years ago, although some of the accreted microcontinents had basements more than 2.5 billion years old and may be detached fragments of Africa.
In the North China paraplatform, Chinese geologists have identified a period of intense island-arc magmatism (a process by which molten rock, often formed by the melting of subducted oceanic crust, rises and solidifies to form igneous rock) between 3.5 and 3 billion years ago. These arcs then coalesced into protonuclei by collisions until the end of the Archean Eon (2.5 billion years ago). Final consolidation of the North China paraplatform occurred approximately 1.7 billion years ago. The Yangtze paraplatform is younger, the oldest identified orogenic event being 2.5 billion years old. Its final consolidation took place some 800 million years ago. The Kontum block is poorly known. It contains Precambrian metamorphic rocks with minimum ages of about 2.3 billion years, although the oldest well-dated widespread thermal event falls into the middle Cambrian (about 500 million years ago) and indicates the time of its final consolidation. The North Tarim fragment is really a thin sliver caught up in younger orogenic belts. Its Precambrian history is not entirely dissimilar to that of the Yangtze paraplatform, although not all major breaks in their sedimentary and structural evolution or the details in their sedimentary successions correlate. The Tarim fragment was also stabilized some 800 million years ago.
While other Asian continental nuclei were completing their consolidation, orogenic deformation recommenced along the present southeast and southwest margins of the Angaran platform. This renewed activity marked the beginning of a protracted period of subduction, the development of vast sedimentary piles scraped off sinking segments of ocean floor in subduction zones and accumulated in the form of subduction-accretion wedges at the leading edge of overriding plates, and subduction-related magmatism and numerous collisions in what today is known as Altaid Asia (named for the Altai Mountains). Orogenic deformation in the Altaids was essentially continuous from the late Proterozoic (about 850 million years ago) into the early part of the Mesozoic Era (about 220 million years ago), in some regions—such as Mongolia and Siberia—lasting even to the end of the Jurassic Period (about 145 million years ago).
The construction of the Altaid collage was coeval with the late Paleozoic assembly of the Pangaea supercontinent (between about 320 and 250 million years ago). The Altaids lay to the north of the Paleo-Tethys Ocean (also called Paleo-Tethys Sea), a giant triangular eastward-opening embayment of Pangaea. A strip of continental material was torn away from the southern margin of the Paleo-Tethys and migrated northward, rotating around the western apex of the Tethyan triangle much like the action of a windshield wiper. This continental strip, called the Cimmerian continent, was joined during its northward journey by a collage of continental material that had gathered around the Yangtze paraplatform and the Kontum block, and, between about 210 and 180 million years ago, all this material collided with Altaid Asia to create the Cimmeride orogenic belt.
While the Cimmerian continent was drifting northward, a new ocean, the Neo-Tethys, was opening behind it and north of the Gondwanaland supercontinent. This new ocean began closing some 155 million years ago, shortly after the beginning of the major disintegration of Gondwanaland. Two fragments of Gondwanaland, India and Arabia, collided with the rest of Asia during the Eocene (i.e., about 56 to 34 million years ago) and the Miocene (about 23 to 5.3 million years ago) epochs, respectively. The orogenic belts that arose from the destruction of the Neo-Tethys and the resultant continental collisions are called the Alpides and form the present Alpine-Himalayan mountain ranges. Both the Cimmerides and the Alpides resulted from the elimination of the Tethyan oceans, and collectively they are called the Tethysides.
Most of the island arcs fringing Asia to the east came into being by subduction of the Pacific Ocean floor and the opening of marginal basins behind these arcs during the Cenozoic Era (the past 65 million years). This activity continues today and is the major source of tectonism (seismic and volcanic activity often resulting in uplift) in South and Southeast Asia. In the south and in the southwest, India and Arabia are continuing their northward march, moving at an average of about 1.6 to 2.4 inches (4 to 6 cm) per year. These movements have caused the massive distortion of the southern two-thirds of Asia and produced the nearly continuous chain of mountain ranges between Turkey and Myanmar (Burma) that in places widen into high plateaus in Turkey, Iran, and Tibet. Within and north of these plateaus, geologically young mountains such as the Caucasus and the Tien Shan, large strike-slip faults such as the North Anatolian and the Altun (Altyn Tagh), and rift valley basins such as Lake Baikal—all of which are associated with seismic activity—bear witness to the widespread effects of the convergence of Arabia and India with Stable Asia, in which no notable active tectonism is seen.
The Paleozoic Era
The tectonic events in Asia of the Paleozoic Era (about 540 to 250 million years ago) may be summarized under three categories: events in the Altaids, events in the Tethysides, and events in the continental nuclei. The identification of Asian Paleozoic tectonic events with those associated with the Caledonian and Hercynian orogenies of Europe, as was done in the older literature, largely has been abandoned owing to the recognition of the haphazard nature of tectonic events whose temporal limits widely overlap.
Paleozoic events in the Altaids
The Altaids constitute a large and complex tectonic collage that accreted around the Angaran platform from late in the Proterozoic to early in the Mesozoic Era. Its oldest part, the Baikalides, formed between about 850 and 570 million years ago along the southern periphery of the Angaran platform. A number of island arcs and microcontinents were accreted onto Angara along a suture containing ophiolitic remnants of old ocean floor.
After the Baikalian collisions, rifting outboard of the accreted fragments opened a new oceanic area, the floor of which had begun subducting under the enlarged continental nucleus in early Paleozoic time—perhaps during the Ordovician Period (about 488 to 444 million years ago). This subduction accumulated a large accretionary prism (wedge of deformed and partially metamorphosed sediments and rocks scraped from the ocean floor as it subducted) consisting of deep-sea muds (now slates), sandstones (deposited by large submarine turbidity currents), and siliceous sedimentary rocks (cherts) that were all structurally mixed with ophiolites (fragments of oceanic crust). These rocks now form the basement of much of the Altai Mountains. Much subduction-related magmatism was associated with the growth of the Altai accretionary prism. Another accretionary prism was growing at the same time in the ocean, far from the Altai, and this material now forms the basement of much of Kazakhstan. It was consolidated and made into a small continent by repeated deformation and magmatism throughout the early Paleozoic.
The later Paleozoic development of the Altaid tectonic collage included the convergence and final collision of the Kazakhstan continental block with the enlarged Angaran nucleus during the middle of the Carboniferous Period (about 320 million years ago). The collision occurred along the southwestern Altai suture, the northerly continuation of which is now buried under the younger Mesozoic deposits of the West Siberian Plain. To the east it continues into Mongolia and there unites with the circum-Altaid suture zone coming from the west—i.e., from the Tien Shan. Another Carboniferous collision in the Tien Shan welded the North Tarim fragment to the Altaid collage. Shortly afterward, in the early Permian Period (about 290 million years ago), north-plunging subduction along the present-day Kunlun Mountains—which originally lay flush to the south of the North Tarim fragment—rifted open the Junggar (Dzungarian) and Tarim basins. These are analogous in their tectonic setting to the present-day Sea of Japan (East Sea).
The Altaid evolution came to an end in the west when the Russian platform collided with Asia along the Ural Mountains between the Arctic Ocean and the Aral Sea. This collision occurred during the Carboniferous Period (about 360 to 300 million years ago) in the south but later—during the Permian Period (300 to 250 million years ago)—in the north, creating the supercontinent of Laurasia. Later collisions in the south and southeast terminated the Altaid evolution.
Paleozoic events in the Tethysides
Along the northern margin of the Tethysides, there was a continuous transition from the Altaid evolution into the Tethyside or, more strictly speaking, into the Cimmeride evolution. In northern Tibet the Kunlun Mountains (a part of the Cimmerides) may also be considered the southernmost representatives of the Altaid collage that was described above. They are made up of a huge subduction-accretion complex and of arc-related magmatic rocks—such as granites, granodiorites, and andesites, the ages of which range from Cambrian A to Late Triassic (i.e., from about 540 to 200 million years ago)—that had begun accumulating along the southern margin of the North Tarim fragment, from which this subduction-accretion complex was later separated by the opening of the Tarim Basin during the Permian. This accretionary complex continues westward into the Pamir and Hindu Kush ranges in Tajikistan and northern Afghanistan and finally constitutes almost the entire pre-Triassic basement of Turkmenistan. The North China block became a part of Asia during the late Paleozoic, although a small westerly vanishing, wedge-shaped ocean between it and the rest of nuclear Asia remained open along a line roughly following the present course of the Shilka River in southern Siberia.
Orogenic deformation, magmatism, and metamorphism during the Carboniferous and Permian periods have become known in parts of Asia that then either belonged to Gondwanaland or had just separated from it as a result of the rifting of the Paleo-Tethys Ocean behind the separating Cimmerian continent. In northern and eastern Turkey, southwestern Iran, and Oman, folding and thrust faulting were in places accompanied by granitic and andesitic magmatism and high-temperature, low-pressure metamorphism, all collectively suggesting the activity of a subduction zone dipping under Gondwanaland. The same subduction zone may have been responsible for the rifting of the Neo-Tethys in the middle Permian as a back-arc basin similar to the present-day Sea of Japan.
Late Permian andesitic volcanics in the Hoh Xil Mountains in northern Tibet and late Paleozoic granites in western and peninsular Thailand, accompanied by compressional deformation and metamorphism, also suggest that a subduction zone existed along the northern margin of the Cimmerian continent. In these parts of Asia, the separation of parts of the Cimmerian continent from northern Gondwanaland may have already been under way during the Carboniferous, as shown by the deposition of the Phuket Group—a formation of glacially modified clastic sedimentary rocks in western Thailand some 3,600 feet (1,100 metres) thick—and of correlative rocks in adjacent Myanmar (Burma), Malaysia, and the Indonesian island of Sumatra.
The Yangtze paraplatform and the Kontum block are believed to have been parts of Gondwanaland during the early Paleozoic, but they rifted away from it sometime in the Devonian Period (about 416 to 359 million years ago). Two other fragments in southeastern China, the Huan’an and Dongnanya, have basements that had been consolidated mainly in the late Proterozoic and that also may have rifted from Gondwanaland sometime during the middle Paleozoic.
At least two island arcs collided with the Kontum block along its northeastern margin during the Paleozoic to enlarge it to what is called the Annamia block. The earlier island arc docked along a suture that now coincides with the Annamese Cordillera in northern Vietnam in the Devonian or slightly earlier. The later one collided along a suture zone farther to the north, along the present-day Ma River, during the early Carboniferous and caused a major south-directed deformation that included considerable thrusting.
Subduction during Carboniferous to Permian times was active along the present-day western margin of the Annamia block, giving rise to much arc-related magmatism and mineralization. This same magmatic zone extended down into the eastern half of the Malay Peninsula. Subduction was probably also active along the present western margins of the Huan’an and Dongnanya blocks, although late Paleozoic magmatism there was much sparser than in Southeast Asia.
Paleozoic events in the continental nuclei
Only three major nuclei underwent Paleozoic tectonic events not obviously related to their flanking orogenic belts. The Arabian platform underwent a major extensional tectonic event from the late Proterozoic to the middle Cambrian that created large north-south (“Arabian-trend”) and northwest-southeast (“Najd-trend”) rift basins in which clastics and evaporites (Jubaylah and Hormuz) were deposited. These extensional basins were reactivated repeatedly until the early Carboniferous and then again in the late Permian. Active normal faulting in central Saudi Arabia late in the Ordovician (between 450 and 445 million years ago) was coeval with sediment deposition caused by the Saharan glaciation (the RaÊ¾an shales with striated sandstone boulders). A major marine invasion from the east in the late Permian covered more than half of the Arabian platform. The submergence of the platform coincided with the opening of the Neo-Tethys along its eastern margin and with a global rise in sea level following the late Carboniferous–early Permian glaciation in Gondwanaland. Striated pavements and glacial sedimentary deposits in the southern part of the Arabian platform (e.g., the Al-Khlata Formation in Oman) provide evidence of this glaciation.
A prolonged period of emergence up to the late Carboniferous characterized the Paleozoic history of the Indian platform, except for its northern margin, which was involved in the later Himalayan deformation. Late in the Carboniferous, glacially influenced terrestrial sedimentation commenced with the Talcher tillite formation in erosional bedrock depressions. In the early Permian a number of rift valleys oriented east-west and northwest-southeast originated, possibly related to extensions that farther north led to the opening of the Neo-Tethys. Terrestrial deposition continued in these rifts and in the surrounding areas, with local interruptions, until early in the Cretaceous (about 145 million years ago), forming the Gondwanan deposits. Farther north in what later became the Himalayas, there was continuous marine sedimentation, with only local interruptions related to global changes in sea level and gentle oscillations of the platform.
After the early Cambrian deposition of evaporites in extensional basins, the Angaran platform remained geologically calm, and shallow marine clastic and carbonate rocks were deposited on it. In the late Devonian (370 to 360 million years ago), however, the platform’s present northeastern margin was rifted; in addition to creating a major ocean, this activity produced two large rift valleys that now extend into the Angaran platform (the Vilyuy, or Viliui, and Chatanga rifts). Extensive basaltic volcanism accompanied this rifting event, followed by a period of heavy sedimentation along a northeast-facing continental margin.
The Mesozoic Era
The events in Asia of the Mesozoic (about 250 to 65 million years ago) may be summarized as follows: events in the Tethysides, events in the Altaids, events in the continental nuclei, and events in the circum-Pacific orogenic belts.
Mesozoic events in the Tethysides
As the Cimmerian continent was moving across the Tethyan realm—eliminating the Paleo-Tethys Ocean in front of itself while enlarging the Neo-Tethys behind it—it also began falling apart internally. Thus, a northern fragment (consisting of the Farah block in Afghanistan, the central Pamirs, and the western Qiangtang block in Tibet) became separated from a southern fragment (including the Helmand block in Afghanistan, the southern Pamirs, and the Lhasa block in southern Tibet) by an ocean whose ophiolitic remnants are today encountered in the mountain ranges of eastern Iran, along the Farah River in Afghanistan, and in the Tanggula Mountains in Tibet continuing to Mandalay in Myanmar. This ocean opened in the Permian and closed early in the Cretaceous (i.e., earlier than about 125 million years ago).
The northern fragment of the Cimmerian continent, including much of modern-day Iran and the mountains of northern Turkey along the Black Sea, collided with the Altaid collage along a suture zone that passes north of the Elburz Mountains and south of the Kopet-Dag Range in northern Iran, through the Hindu Kush range in Afghanistan, south of the northern Pamirs and the Kunluns in northern Tibet, and then follows the Jinsha (upper Yangtze) River and continues through western Thailand and into the Malay Peninsula. The collision occurred late in the Triassic in Iran and Southeast Asia (about 220 million years ago) and early in the Jurassic (about 200 million years ago) between Iran and Indochina. This collision created a massive wall of mountains along the southern border of Asia, called the Cimmeride Mountains (the name taken from the ancient people the Cimmerians, in whose homeland north of the Black Sea the first pieces of evidence for this chain were found at the beginning of the 20th century). These mountains extended from Turkey well into Southeast Asia. The large, rich tin-bearing granite belt of western Thailand and Malaysia was formed during this collision.
The southern fragment of the Cimmerian continent soon caught up with the northern fragment; and, following the emplacement in the Late Jurassic (about 160 to 145 million years ago) of a part of the floor of the intervening ocean onto the Lhasa block in the form of a giant ophiolite sheet, the southern fragment also collided with Asia, eliminating the entire Paleo-Tethys and its marginal basins. Widespread aridity in much of Central Asia during the Late Jurassic was probably a result of the rain shadow that formed behind the wall of the Cimmeride Mountains to the south.
The interval from the Late Triassic through the Late Jurassic (about 230 to 145 million years ago) was also the time when the Yangtze paraplatform and the Huan’an, Dongnanya, and Annamia blocks collided with one another and also with the eastern end of the Cimmerian continent and the rest of Asia. This created the multibranched Cimmeride mountain ranges of eastern and southeastern Asia, including the Qin (Tsinling) Mountains that separate North China from South China. Some of the metamorphic rocks in the Dabie Mountains were buried to depths reaching 60 miles (100 km) during the collision of the Yangtze and the North China paraplatforms. These collisions formed another vast tin-bearing granite province in southern China.
In the Middle East the rifting of the Cimmerian continent opened the eastern Mediterranean in the Late Triassic (between about 230 and 200 million years ago), with Turkey moving away from Africa. In the Early Jurassic (200 to 180 million years ago) the Turkish part of the Cimmerian continent continued to disintegrate and to open a number of new Tethyan branches.
The fragmentation of the southern supercontinent Gondwanaland accelerated in the middle Mesozoic. This fragmentation led to the opening of the central and the southern Atlantic and Indian oceans that was partially compensated by the beginning closure of the Neo-Tethys. In Asia the main subduction zones consuming the Neo-Tethyan ocean floor began forming in the Late Jurassic along the northern margin of the ocean in Iran and in what later became the Himalayas. A unified subduction zone—extending from northern Turkey, south of the Pontic Mountains, through southwestern Iran (the present northern slope of the Zagros Mountains) and Makran, north of the Salt Range in Pakistan to the present-day Himalayan suture zone along the valleys of the Indus and Brahmaputra rivers, and from there to Myanmar and Sumatra—came into being during the Early Cretaceous (about 120 to 100 million years ago). The vast Late Cretaceous granitic intrusions of the Trans-Himalaya and the Karakoram ranges and the andesitic volcanics that occupy a thin strip from northern Turkey through Iran and Pakistan to the Karakorams and extend beyond the Himalayas into Myanmar, Sumatra, and Borneo are the result of the rapid destruction of the Neo-Tethyan ocean floor.
In the Early Cretaceous other entirely intraoceanic subduction zones also formed just north of the former Gondwanan continental margins in Turkey, Iran, and Oman. The attempted subduction of these margins resulted in the emplacement of vast portions of the Neo-Tethyan ocean floor on top of these margins in the form of giant ophiolite sheets, such as the Semail Nappe in Oman. These ophiolite nappes (i.e., thrust sheets) are major sources of chromite deposits. Also in the Early Cretaceous a small sliver of continental crust that now forms much of southwestern Sumatra rifted from northwestern Australia. This eventually collided with the rest of Sumatra in the Late Cretaceous, resulting in the opening of the northeastern segment of the Indian Ocean.
Mesozoic events in the Altaids
Most of the Mesozoic events in the Altaids were the echoes of the Cimmeride collisions farther south. In places these collisions split the old Altaid edifice at high angles to the collision front, creating extensional basins such as the Torghay Valley, just north of the Aral Sea, and the West Siberian Plain, which contains little-deformed Jurassic and younger shallow-water and continental sedimentary rocks with significant hydrocarbon reserves. In other places closer to the collision front, the basement was uplifted along major thrust faults, creating mountain ridges (e.g., in the Tupqaraghan Peninsula on the east coast of the Caspian Sea and the Kyzylkum Desert of southern Kazakhstan). Between these, large compressional basins formed (e.g., the Turkmenian basins) or older ones became accentuated (the Tarim and Junggar), within which large sedimentary thicknesses and important hydrocarbon reserves accumulated. The compressional structures were connected in places with extensional structures through large strike-slip fault systems, the best-known of which runs through the Fergana Valley in southern Central Asia.
Mesozoic events in the continental nuclei
The Angaran platform was also affected by the Cimmeride collisions but reacted more mildly than the Altaids. The vast Tunguska trap basalts erupted in the transition between the Permian and Triassic periods, and the eruptions lasted well into the Triassic. They were related to the rifting of the West Siberian Plain and were coeval with basaltic eruptions in the Torghay Valley. The old Proterozoic rifts on the Angaran platform were compressed at the end of the Jurassic, probably in response to the ongoing shortening of the Cimmeride continent.
Major Late Jurassic–Early Cretaceous extension and basaltic volcanism affected especially the northern part of the Arabian platform. This extensional event was part of a much wider extensional province in north-central Africa. Yet another such event occurred in the northern and eastern parts of the platform in the Late Cretaceous, creating deep shelf basins.
During the Mesozoic, the Indian subcontinent separated from Gondwanaland. Its eastern margin formed early in the Cretaceous (about 140 million years ago), when India separated from Australia. The Early Cretaceous rifting event that affected the eastern margin of the Indian platform also led to some rejuvenation of the older Gondwanan rifts. India separated from Madagascar some 85 million years ago. Another rifting along this margin, about 65 million years ago, removed the Seychelles and Saya de Malha banks in the present western Indian Ocean from India and also gave rise to the huge Deccan trap basalt eruptions, which involved about 50 distinct flows in probably less than a million years.
Mesozoic events in the circum-Pacific orogenic belts
The subduction of the floor of the Pacific Ocean dominated the evolution of the Pacific margin of Asia, especially during the second half of the Mesozoic Era. Large subduction-accretion complexes formed in Japan and in Borneo, and the Kolyma block—forming present-day northeastern Asia—collided with the Angaran platform during the Late Jurassic–Early Cretaceous interval. This collision produced the 375-mile- (600-km-) wide Verkhoyansk fold-and-thrust belt, in the front of which coal was deposited in postcollisional molasse basins.
A major magmatic arc flanked Asia between Japan and Indochina in the Late Jurassic to Late Cretaceous interval and joined the Neo-Tethyan arc system in Borneo. Late Cretaceous to Paleogene (about 80 to 55 million years ago) extensional tectonics along this arc formed many of the offshore basins along the Chinese continental margin.
The Cenozoic Era
The Cenozoic (i.e., the past 65 million years) was the time when Asia acquired its present appearance.
Cenozoic events in the Alpide plate boundary zone and in the Arabian and Indian cratons
The most important tectonic event in the Cenozoic history of Asia was its collision with India some 50 million years ago. This collision took place about 1,250 miles (2,000 km) south of the present location of the line of collision along the Indus-Brahmaputra suture behind the main range of the Himalayas. Since the collision India has “bulldozed” the southern margin of Asia, crumpling both Asia and its own northern margin. A horizontal shortening of some 500 miles (800 km) has accompanied this action, much of the distance taken up by massive thrust sheets in the Himalayas. The Plateau of Tibet, the largest and thickest concentration of continental crust on Earth, is a consequence of considerable compression of the Asian continental lithosphere. The plateau has a crustal thickness of some 43 miles (69 km), and widespread volcanicity results from the melting of the lower parts of the thickened continental crust. Extensional basins oriented north-south in Tibet indicate that the massive plateau is spreading under its own weight like a piece of Silly Putty. India still moves northward with respect to Asia at a speed of about 2.4 inches (6 cm) per year, maintaining the high elevations of both the Himalayas and the Plateau of Tibet.
The effects of the convergence reach farther north to Lake Baikal. The old Cimmeride compressional basins of Tarim, Dzungaria, and the other smaller ones have been all rejuvenated, as have the intervening mountain ridges such as the Tien Shan. Large strike-slip faults such as the Altun and the Karakoram have redistributed continental material in front of the moving indenter. In the south the collision created the large Ganges basin south of the Himalayas and may have led to a shortening of the southern tip of the Indian subcontinent in the vicinity of Anai Peak.
The Arabian platform, which collided with Asia in the middle Miocene (about 13 million years ago), has continued to converge with it at a rate of some 1.6 inches (4 cm) per year, in the process uplifting both the Zagros Mountains and the entire high-plateau system of Turkey and Iran, which resembles the Plateau of Tibet. A part of eastern Turkey has been pushed out of the way of the indenting Arabian platform along the North Anatolian Fault.
The widespread and complicated deformation caused or influenced by the two major Alpide collisions characterizes the Alpide plate boundary zone, the major neotectonic province in Asia. The vast salt steppes and deserts of Asia are located in this province, behind the rain shadow of the Alpide ranges.
Subduction under Asia continues in the Tethysides and contributes to tectonism in the Alpide plate boundary zone. Subduction has been consuming the floor of the eastern Mediterranean to the south of Asia Minor, the floor of the Arabian Sea off the coast of the Makran, and the floor of the Indian Ocean around Southeast Asia. The Banda arc of mainly volcanic islands in Indonesia collided with Australia in the Pliocene Epoch (i.e., about 5.3 to 2.6 million years ago), and arc-related magmatism has not yet ceased.
Cenozoic events in Stable Asia
North of the Alpide plate boundary zone are the vast expanses of Siberia, where the absence of seismic activity and the subdued relief indicate an absence of active tectonism. The only exception to this is where the Gakkel spreading centre of the Arctic Ocean is propagating into Asia along the Sadko Trough and the Chersky Mountains.
Cenozoic events in the island arcs and the marginal basins
The subduction zone that was active along the eastern margin of Asia late in the Mesozoic started migrating away from the continent in the Late Cretaceous in China. This led to crustal extension that created a number of the present-day offshore basins along the Chinese continental margin. The South China Sea opened as an ocean-floored marginal basin in the Oligocene Epoch (34 to 23 million years ago). Earlier, a midoceanic subduction zone had come into being along the Kyushu-Palau Ridge, and above it the West Mariana Basin opened in the Oligocene-Miocene interval. Some 5 million years ago the East Mariana Basin began opening behind the present Mariana Island arc. Japan moved away from mainland Asia in the Middle Miocene, opening behind it the Sea of Japan. The Kuril Basin behind the Kuril Islands arc has a similar age.
The Cenozoic history of the island arc systems and the marginal basins they delimit against the Pacific Ocean has been dominated by extensional tectonics of the arc massifs concurrent with mainly basaltic and subordinate andesitic volcanism, limited subduction-accretion, and strike-slip faulting (e.g., the Philippine Fault). Some arcs, such as Sengihe and Halmahera, collided with each other, while others have split apart in recent geologic time to create newer marginal basins such as the Okinawa Trough. Some islands, such as eastern Taiwan or those of the Banda arc, have collided with continents. Of the young marginal basins, only the Sea of Japan may have begun closing again. The extraordinarily complex tectonic evolution of the East and Southeast Asian island arcs and marginal basins constitutes an excellent present-day analogue of the processes that may have produced the Altaid collage during the Paleozoic.
A.M. Celal Sengor
The mountain belts
Characteristic of the surface of Asia is the great predominance of mountains and plateaus, constituting about three-fourths of the continent’s total area. The mountains are grouped into two belts: those located on the stable platforms (cratons) and those located in active orogenic zones. The former usually occur on the margins of the platforms and generally are characterized by smooth eroded peaks and steep faulted slopes. Marginal mountain ranges, with average heights of 8,200 to 9,850 feet (2,500 to 3,000 metres), usually enclose the inner tablelands and plateaus; examples of such ranges include the Western and Eastern Ghats in India, the mountains of the Hejaz and Yemeni highlands on the Arabian Peninsula, and the Lebanon and Anti-Lebanon mountains in the Levant. The Aldan Plateau and Stanovoy Mountains lie along the eastern margin of the Angaran (Siberian) platform, where the isolated and uplifted Putoran Mountains are located in central Siberia.
Mountains of the orogenic zones are much higher in elevation and have a more complicated structure. Tectonic movements in these zones have given rise to structures of different age and composition. Mesozoic and Cenozoic foldings (i.e., those of the past 250 million years) created boundaries between basic types of mountains over vast areas of Asia. The largest mountain belt on Mesozoic structures (i.e., from 250 to 65 million years ago) extends from the Chukchi Peninsula at the eastern extremity of Asia through the Kolyma, Dzhugdzhur, and Stanovoy ranges to the mountains of southern Siberia (the Sayans and the Altai) and to the Tien Shan and Gissar-Alay. The Chersky and Verkhoyansk ranges are the western spurs of this belt.
Along the edges of the Central Asian plateaus extend the elongated mountain chains of the Da Hinggan (Greater Khingan), Taihang, and Daxue ranges. The Hinggan-Bureya mountains (Xiao Hinggan [Lesser Khingan] and Bureya ranges) demarcate the Zeya-Bureya Depression; the Manchurian-Korean and Sikhote-Alin mountain ranges separate the plains of the Amur and Sungari (Songhua) rivers, the Lake Khanka lowland, and the Manchurian (Northeast) Plain. The coastal ranges in the southeast consist of the mountains of southern China and the Annamese Cordillera. A generally latitudinal branch springs from the Pamirs region and runs eastward through the Kunlun, Qilian, and Qin (Tsinling) mountains.
The Alpine-Himalayan mountain belt runs in a west-east direction and includes the Taurus Mountains, the Caucasus, the Zagros and Elburz mountains, the Hindu Kush, the Pamirs, the Karakoram Range, the Plateau of Tibet, and the Himalayas; it then turns to the south and southeast, running through the Rakhine (Arakan) Mountains to the islands of the Malay Archipelago. The western part of this belt consists, for a considerable distance, of two series of mountain chains that converge in dense knots in the Armenian Highland, in the Pamirs, and in the southeast of the Plateau of Tibet; the two chains then diverge to encompass the interior plateaus. The average elevation of highlands and marginal ranges increases from west to east from about 2,600 to 3,000 feet (800 to 900 metres) on the Anatolian Plateau to about 13,000 to 16,400 feet (4,000 to 5,000 metres) on the Plateau of Tibet and from about 8,200 to 11,500 feet (2,500 to 3,500 metres) in the Pontic and Taurus mountains to 19,000 feet (5,800 metres) in the Himalayas.
On the northeastern and eastern edges of Asia, a vast belt of Cenozoic (i.e., of the past 65 million years) folding extends from the Koryak Mountains of the Kamchatka-Koryak arc along the Sredinny (Central) range of the Kamchatka Peninsula. The marginal seas of the western Pacific Ocean are bordered by the East Asian islands, which form the line of arcs running from the Kamchatka Peninsula in the north to the Sunda Islands in the south. Volcanic and seismic activity is characteristic of this belt.
The plains and lowlands
Low plains occupy the rest of the Asian mainland, particularly the vast West Siberian and Turan plains of the interior. The remaining lowlands are distributed either in the maritime regions—such as the North Siberian and Yana-Indigirka lowlands and the North China Plain—or in the piedmont depressions of Mesopotamia, the Indo-Gangetic Plain, and mainland Southeast Asia. These plains have monotonously level surfaces with wide valleys, through which the great Asian rivers and their tributaries flow. The topography of the plains in densely populated regions has been greatly modified through the construction of canals, dams, and levees. To the south of the zone of piedmont depressions lie extensive tablelands and plateaus, including the Deccan plateau in India and the Syrian-Arabian Plateau in the west. In addition, there are the intermontane basins of Kashgaria, Junggar, Qaidam (Tsaidam), and Fergana and the plateaus of central Siberia and the Gobi, all of which lie at elevations of 2,600 to 4,900 feet (800 to 1,500 metres). Most of their surfaces are smooth or gently rolling, with isolated hillocks. The plateaus inside Tibet, the Tien Shan, and the Pamirs lie at elevations of some 12,000 feet (3,700 metres) or more.
A large proportion of the islands of Asia are mountainous. The highlands of Sri Lanka rise to 8,281 feet (2,524 metres); Mount Kinabalu in Malaysia reaches 13,455 feet (4,101 metres); Mount Fuji on the Japanese island of Honshu has an elevation of 12,388 feet (3,776 metres); and many volcanoes of Sumatra, Java, and Mindanao reach 10,000 feet (3,000 metres).
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 (4,000 metres). The eastern margin of the highlands, meanwhile, underwent subsidences of up to 2,300 feet (700 metres). 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.
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, which created deep superimposed gorges and canyons.
Vast areas of Middle, Central, and East Asia, particularly in the Huang He (Yellow River) 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 (300 metres). There are broad expanses of badlands, eolian (wind-produced) relief, and karst topography (limestone terrain associated with vertical and underground drainage). Karst terrain is characteristic of the Kopet-Dag, the eastern Pamirs, the Tien Shan, the Gissar and Alay ranges, the Ustyurt Plateau, the western Taurus Mountains, and the Levant. Tropical karst in South China is renowned for its picturesque residual hills.
The mantle of glaciation from the Pleistocene (i.e., about 2,600,000 to 11,700 years ago) embraced northwestern Asia only to latitude 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 elevation of the permanent snow line is relatively high, averaging between 14,800 and 16,400 feet (4,500 and 5,000 metres) and reaching 21,000 feet (6,400 metres) in central Tibet.
An enormous area of permafrost—some 4.25 million square miles (11 million square km)—covers northern Asia and extends to lower latitudes there 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 (300 to 400 metres).
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 the Kamchatka Peninsula, the Philippines, and the Sunda Islands. The highest active volcano in Asia, Klyuchevskaya, rises to 15,584 feet (4,750 metres) on 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 historical times eruptions also occurred in the interior of the continent in the Xiao Hinggan 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, the Junggar and Tarim basins, Inner Mongolia, the Gobi, and the Sino-Tibetan ranges; Middle Asia, including the Turan Plain, the Pamirs, the Gissar and Alay ranges, and the Tien Shan; South Asia, including the Philippine and Malay archipelagoes, Indochina and peninsular India, the Indo-Gangetic Plain, and the Himalayas; and West (or Southwest) Asia, including the West Asian highlands (Anatolia, Armenia, and Iran), the Levant, and the Arabian Peninsula. Sometimes the Philippines, the Malay Archipelago, and the Indochinese Peninsula, instead of being considered part of South Asia, are grouped separately as Southeast Asia. Yet another variation of the basic categories is commonly made to divide Asia into its cultural regions.
Northeastern Siberia comprises faulted and folded mountains of moderate height, such as the Verkhoyansk, Chersky, and Okhotsk-Chaun mountain arcs, all Mesozoic structures that have been rejuvenated by geologically recent tectonic events. The Koryak Mountains are similar but have a Cenozoic origin. 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 Mountains. 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 Prilenskoye and Aldan plateaus—comprising an ancient peneplain resting on the underlying platform that sometimes outcrops on the surface—are 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 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 Central Yakut Lowland, the drainage basin of the lower Lena River, and on the northern periphery is the North Siberian Lowland, covered with its original marine deposits.
The West Siberian Plain is stratified and is composed of Cenozoic sediments deposited over thicknesses of Mesozoic material, in addition to folded bedrock. The northern part was subjected to several periods of glaciation throughout the Quaternary Period (the past 2.6 million years). In the south, glaciofluvial and fluvial deposits predominate.
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 there has been vigorous contemporary glaciation.
The main features in the northern region of East Asia include the Da Hinggan, Xiao Hinggan, and Bureya 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 elevation, 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 uplifted fragments of the Ryukyu-Korean, Honshu-Sakhalin, and Kuril-Kamchatka mountain-island 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.
Kamchatka is a mountainous peninsula formed from fragments of the Kamchatka-Koryak and Kuril-Kamchatka 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 alluviums and volcanic ashes.
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, Sayan, and Stanovoy mountains—are particularly noticeable. They have clearly defined features resulting from ancient glaciation; contemporary glaciers exist in the Altai.
The Central Asian plains and tablelands include the Junggar 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 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. 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. Glaciers are present throughout the region but are most concentrated at the western end of the Himalayas and in the Karakoram Range.
The Plateau of Tibet represents a fractured alpine zone in which Mesozoic and Cenozoic structures that surround an older central mass have experienced more recent uplifting. Some of the highlands are covered with sandy and rocky desert; elsewhere in this region, alpine highlands are dissected by erosion or are covered with glaciers. The Karakoram Range and the Himalayas were uplifted during late Cenozoic times. Their erosion has exposed older rocks that were deformed during earlier tectonic events.
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 (Ganga), 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 subsidence into which thick accumulations of earlier marine sediments and later continental deposits have washed down from the rising mountains. These sediments provide fertile soil in the Ganges and Brahmaputra basins and in irrigated parts of the Indus basin, while the margins of the Indus basin have become sandy deserts. Peninsular India and Sri Lanka are formed of platform plateaus and tablelands, including the vast Deccan plateau, uplifted in the Mesozoic and Cenozoic. The region includes tablelands with uplifted margins, such as the Western and Eastern Ghats, and terraced and dissected plateaus with lava mantles or intrusions.
Southeast Asia is composed of the Indochinese 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 (Burma) is a fold belt of Cenozoic age. Mountains of medium elevation constitute folded blocks that decrease in size and elevation to the south; the valleys are alluvial and broaden out to the south. Central and eastern Thailand and central and southern Vietnam are characterized by mountains of low and moderate height that have been moderately fractured. The region is one of Mesozoic structures surrounding the ancient mass known as the Kontum block, which comprises plateaus and lowlands filled with accumulated alluvial deposits.
Archipelagoes border the southeastern margin of Asia, consisting mainly of island arcs bordered by deep oceanic trenches. The Indian Ocean arcs—Sumatra, Java, and the Lesser Sunda Islands—consist of fragments of Alpine folds that constitute a complex assemblage of rock types of different ages. Vigorous Cenozoic volcanic activity, continuing up to the present, has formed volcanic mountains, and their steady erosion has filled the adjacent alluvial lowlands with sediment.
Borneo and the Malay Peninsula are formed from fractured continental land situated at the junction of the Alpine-Himalayan and East Asiatic downwarp 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, have been built by ongoing tectonic processes, particularly volcanism. Mountain areas of moderate height, volcanic ranges, alluvial lowlands, and coral reef islets are present throughout these regions.
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 Kazakh region, low mountains on the Tupqaraghan and Türkmenbashy (Krasnovodsk) 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 includes the highlands of Anatolia, the Caucasus, and the Armenian and Iranian highlands.
The highlands of Anatolia—the Pontic Mountains that parallel 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 is a region of discontinuous mountains including the Lesser Caucasus and the Kurt mountains. Geologically recent uplifting, in the form of a knot of mountain arcs, took place during a period of vigorous volcanism during the Cenozoic. The region is seismically active and is known for its destructive earthquakes.
The Iranian highlands comprise mountain arcs (the Elburz, the Kopet-Dag, the mountains of Khorasan, the SafÄ«d Range, and the western Hindu Kush in the north; the Zagros, Makran, Soleyman, and KÄ«rthar mountains in the south), together with the plateaus 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, 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. Plateaus 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) all occur. Ancient marine sands and alluvium, 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.
Source: Encyclopaedia Britannica