Landforms of Southeast Asia

Southeast Asia is a corner of the continent of Asia which ends in an assemblage of peninsulas, archipelagos, and partially enclosed seas. Towards the northwest, the physical contact of this region with the rest of Asia is via a mountainous region that includes the eastern Tibetan Plateau, the eastern Himalaya Mountains, the hills and plateaux of Assam (India) and of Yunnan (China). From this high region a number of large, elongated river basins run north–south or northwest–southeast. These are the basins of rivers such as the Irrawaddy, Salween, Chao Phraya, Mekong, and Sông Hóng (Red). An east–west traverse across the mainland part of Southeast Asia, therefore, is a repetition of alluvium-filled valleys of large rivers separated by mountain chains or plateaux. To the south and to the east are coastal plains, rocky peninsulas, and a number of deltas. Beyond lies the outer margin of Southeast Asia, the arcuate islands of Indonesia, and the Philippines with steep volcanic slopes, intermontane basins, and flat coastal plains of varying size. This assemblage of landforms has resulted from a combination of plate tectonics, Pleistocene history, Holocene geomorphic processes, and anthropogenic modifications of the landscape. Most of the world has been shaped by such a combination, but unlike the rest of the world, in Southeast Asia all four are important. The conventional wisdom of a primarily climate-driven tropical geomorphology is untenable here. The first two factors, plate tectonics and the Pleistocene history, have been discussed in Chapters 1 and 2 respectively. In the Holocene, Southeast Asia has been affected by the following phenomena: • The sea rose to its present level several thousand years ago. • The present natural vegetation, a major part of which includes a set of rainforest formations, achieved its distribution. • A hot and humid climate became the norm, except in the high altitudes and the extreme northern parts. • The dual monsoon systems blowing from the northeast in the northern hemispheric winter and from the southwest in the summer (and in general producing a large volume of precipitation) became strongly developed.