Lakes and Reservoirs of Asia Formation Diversity Distribution

Asia, the world's largest continent (44.39 million km2), lies entirely in the Northern Hemisphere extending from the Equator to the Arctic Circle. The Ural and Caucasus mountains, and the Caspian Sea are accepted by some as the border with Europe, others extending the border to include the Black Sea, the Aegean Sea, and the Bosphorus and Dardanelles straits. The continent is separated from Africa by the Suez Canal and the Red Sea, and from North America by the Bering Strait.

Geologically, Asia consists of Precambrian landmasses, the Arabian and Indian peninsulas in the south and the central Siberian plateau in the north, as well as areas of very recent origin. The Asian continent however owes its present geomorphological features to the geological events of the past 50 million years. After the breaking apart of the Gondwanaland, the Arabian and Indian plates drifted north and northeastwards. The thrust of the Indian plate under the Asian plate obliterated the Tethys Sea and resulted in the rise of Himalayan ranges and the Tibetan Plateau. The collision of the Arabian plate with west Asia pushed up the Caucasus mountains and reduced the Tethys sea to the brackish Sarmantian Lake. The subsequent epeiro-genic processes (large scale upliftment and subsidence of land) and volcanic and glacial activities have resulted in a highly complex pattern of landforms.

Physiographically, Asia comprises of (a) the vast central zone of high plateaus, rising to more than 4500 m and the surrounding high mountain ranges,

(b) the northern lowlands of Central Asia and Siberia,

(c) the eastern lowlands of eastern China and southeast Asia drained by mighty rivers such as Yangtse, Hwangho, and Mekong, (d) the southern peninsular plateaus of India and Arabia, and (e) vast plains of major rivers (Tigris-Euphrates, Indus, Ganga, and Brahmaputra) that are separated by various spurs of mountain ranges (Figure 1). Most important of the highland plateaus is the Pamir Plateau (often referred to as the Pamir Knot) from which high mountain ranges radiate both eastwards and westwards. The Hindukush, Koh-i-Baba, and Suleiman mountains radiate westwards, enclose the high plateaus of Afghanistan, Baluchistan, and Iran, and later coalesce in the Armenian Knot. Eastwards, Kunlun, Karakorum, and the Himalaya, which are the youngest (Tertiary period), enclose between them the large

Tibetan Plateau. The most northerly range, Tien Shan is the oldest and arches over a depression, the Tarim basin, which is bordered on the south by the Kunlun range.

The Himalaya, after a sharp southward turn, extends eastwards in a large arc across north India, and includes the world's highest peaks. The Himalayan range again turns southwards to continue as Arakan Yoma through Myanmar, and can be traced further south in Andaman-Nicobar islands and into Sumatra and Java Islands.

The eastern lowlands of China and southeast Asia are intersected by a series of relatively low ranges running in the north-south direction. The Deccan Plateau of the Indian subcontinent is separated from the Ganga-Brahmaputra plain by the Vindhyan and Satpura ranges and is also rimmed on the west and the east by a series of low mountain ranges (the Western Ghats and Eastern Ghats) that coalesce in the south.

The climate in Asia varies between extremes of both temperature and precipitation. It is governed by the monsoon winds and influenced by the high mountain ranges. After the temperatures start soaring during the summer (April onwards), the southwest monsoon brings very large amounts of precipitation over the south and southeast Asia. The large mountain ranges influence the spatial distribution of rainfall, resulting in sharp gradients. During winter, monsoons from the north move into East Asia and cause cold dry weather. Bitter cold polar winds keep part of northern Siberia frozen throughout the year. More than one-third of the Asian continent is therefore arid (annual precipitation <200 mm), semi-arid (200-500mm), or even hyper-arid (<25 mm), and is either too hot or too cold. This includes most of the Middle East, Central Asia, Mongolia, western China (much of Qinghai, Xinjiang, and Xizang (=Tibet) provinces), and parts of the Indian subcontinent (most of Afghanistan and Pakistan, western India and parts of peninsular India, and even Sri Lanka). Interestingly, these dry regions have the largest concentration of natural, though small, lakes.

Thus, the continent is remarkably unique for its great contrasts and extremes because it includes the highest mountain peak (Mt Everest, 8850 m) and the lowest place on land (Dead Sea, 394 m below sea level), most rainy and most arid areas, as well as the hottest and coldest places on the Earth. Not only did innumerable plants and animals originate in Asia, but

Ecosystems Map Asia"/>
Figure 1 Map of Asia (from

the human species and human civilization also evolved here. Today, Asia is the home to three-fifths of the world's human population, which exhibits as high diversity of human cultures as the continent's biological diversity.

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