Physical Environment

The configuration of the Asian mountain system has been determined by tectonic forces, the principal one being the impact of the Gondwana Plate thrusting beneath the Angara Plate. This collision is expressed in the great axes of folding spanning the continent east-west and then sweeping south to north through the island chain along the Pacific Rim. The basement of these Tertiary mountains is of Cenozoic formation, less than 20 million years old. The youthfulness of the mountains is evident from their extreme elevation and high relief. Older Paleozoic and Mesozoic formations in the north-west and eastern Australia also have fold mountains, but these have been long eroded into plateaus and peneplains with some ranges as ancient ribs. As a general pattern, the folded structure of the Cenozoic formations has a rough surface configuration, while those of the Paleozoic and Mesozoic systems are broken or smooth (Table 4).

Apart from structure and relief, other major components that influence physical environment are climate, soils, and vegetation. The mountains of Asia encompass a wide variety of climatic regions as they span a great latitudinal range. This is evident from three extreme regions. The first is one of high altitude at the core of the continent where the climate varies with elevation, latitude, and exposure, tending mostly towards temperate to alpine. The second extreme relates to the subarctic one in eastern Russia with low precipitation and very long winters. The third one relates to the tropical islands of South-East Asia where all months are warm or hot with diurnal rain. Other climatic variations are the semi-arid to arid of the west, wet and dry tropical of the south, and the temperate marine climate of South Australia and New Zealand (Table 4).

© Madhukar Rana

20. A Varied Topography, Kabhrepalanchowk, Nepal

The dominant soils of highland Asia are mostly of a mountainous type that is shallow and that may be grass-covered or barren depending on the climate of the elevation zone. Other prevalent types are the chernozemic of the North-West supporting grasslands, the desertic in the West, and the podzolic of the South and South-East. In terms of vegetation type, most of Central and West Asia are xerophytic with patches of grass. Another extensive type is the mixed forest of deciduous and evergreen species along the southern slopes of the Himalayan Range and much of North-East Asia. South-East Asia is the domain of broad-leaved evergreen forests. The extremes are represented by the boreal taiga of the extreme north and savanna grasslands of Australia.

Asian mountains have great environmental diversity (Plate 20), ranging from cold and hot deserts to tropical rain forests. Yet, whatever the bio-climatic regime, superior elevation and steep slopes are their distinctive features. Elevation exposes them to erosive elements and slope facilitates the gravitational flow of materials downhill. Thus, mountain areas are intensely affected by processes of surface erosion, either of water or wind. This has led to the erroneous notion that the mountains are fragile. In fact, mountains represent a high energy area with much mass wasting without which there would be no depositional material for the adjoining plains. Therefore, it seems more realistic to consider mountains as dynamic landforms.