In broad outline, the physical form of this region is relatively simple. The Indian subcontinent is bounded to the north by the Himalayas, the greatest mountain range on Earth, that stretches for over 2,000 km east-west from Burma to Afghanistan. This great mountain chain contains all 14 of the worldÕs 8,000 m peaks, culminating in the worldÕs highest mountain, Mount Everest (8.848 m) [known as Chomolungma in Tibetan]. To the north of the Himalaya is the Tibetan Plateau, a vast region of mountains and plateaux with an average altitude of c. 5,000 m. The three great rivers of South Asia originate in the Himalayas and the Tibetan Plateau. To the west, The Indus rises north of the Karakoram Mountains, passes through the Himalayan range in a series of deep gorges, and emerges onto the Plains of Pakistan. It meets the coast of the Arabian Sea in a great delta system. Farther east, in India, the Ganges rises in the Garhwal region in the southern Himalayas. Its early course is steep, but most of its course winds across the great Ganges Plain of northern India. The third great river, the Brahmaputra, rises in Tibet, to the north of the Himalayas (where it is known as the Tsangpo), it flows eastward, parallel to the Himalayas, then passes through the range in a deep gorge. Both the Ganges and Brahmaputra reach the sea in the complex Ganges-Brahmaputra Delta that builds out into the Bay of Bengal. The Indus and Brahmaputra are both thought to have been in existence since before the Himalayas were created, and valley erosion by the rivers has kept pace with the mountains as they were uplifted across the riversÕ paths.
The great flood plains of the Indus and Ganges form a wide connected arc of flat land that curves from the Arabian Sea to the Bay of Bengal, known as the Indo-Gangetic Plain. Southward, the subcontinent forms a triangle that stretches southward over 20 degrees of latitude. The centre of this triangle is a block of high ground known as the Deccan Plateau. Much of this area is underlain by basalt lavas, recording one of the world's largest ever volcanic episodes. The lavas cover an area of1.5 million square km, and have an estimated volume of 512,000 cubic km (the 1980 eruption of Mount St. Helens produced 1 cubic km of volcanic material). Most of the basalt was erupted between 65 and 60 million years ago. Gases released by the eruption may have changed the global climate, aiding the general global cooling during the Tertiary Period. The Deccan Plateau has a broad west-east tilt (higher in the west, lower in the east). Thus, much of the drainage of south India is by long, eastward flowing rivers that rise close to the west coast. Westward-flowing rivers tend to be shorter and steeper. The Deccan plateau descends to the sea on either side of India in steep slopes known as the Western and Eastern Ghats. Narrow coastal plains fringe the subcontinent below the Ghats.
To understand the origin of the South Asian landmass, we need to consider the shape of the continents long before our present epoch, during the Permian c. 225 million years ago. At that time, all of the worldÕs continental plates formed a single landmass, known to geologists as Pangaea (from the Greek meaning Ô all landsÕ). The area that is now India was in the southern hemisphere, wedged between Africa, Australia, and Antarctica, whereas the area that is now central Asia was located far to the north. The rocks underlying India are extremely ancient basement rocks or shield, overlain by younger sediments. The Indian and Central Asian components of Pangaea were separated by a wide arm of the sea, known as Tethys. During the Triassic, Pangaea began to break up, and separated into northern and southern masses - Laurasia (consisting of North America and Eurasia) to the north, and Gondwanaland (consisting of South America, Africa, India, Australia, and Antarctica) to the south. India broke away from the other parts of Gondwanaland, and began to drift northward.
As the Indian Plate converged with the Asian Plate, a subduction zone developed between the two, in which the oceanic crust that lay north of the Indian landmass was subducted beneath the Asian Plate, much as the Plates under the Pacific ocean are being subducted beneath Japan at the present. As in modern Japan, the subduction was associated with volcanoes, which formed in what is now Tibet due to the upwelling of material melted during the subduction process. Around 55 - 40 million years ago (Early Tertiary) the Indian and Asian landmasses came into contact, and the conveyor belt began to jam. The speed of the Indian Plate was reduced from 10-20 cm per year to about 5 cm per year (the current rate). As the continents converged, a huge fault developed, plunging downwards from south to north. The crust under the fault plane (The Main Central Thrust) continued to move northward and downward beneath Asia, whereas the material above the fault plane was thrust southward over India. The thrusting was associated with the folding of the overlying rocks, which were buckled and uplifted. About 20-10 million years ago, the thrust became inactive, and a second thrust (The Main Boundary Fault) developed at a deeper level, along which movement continued. The Himalayas were uplifted as the result of this great collision, which continues today.
However, although the Himalayas are still being uplifted at rates varying from 1-10 mm per year, they are not on average getting higher. This is because erosion is very active at such high altitudes. Rock material is loosened from the high peaks by freeze-thaw, seismic shocks, and snow avalanches. Large, catastrophic rock avalanches and flow-slides can be found in most valleys. Rock material is entrained by glaciers, and by steep, rapidly flowing rivers. The incision of the Himalayan landscape by rivers results in very high relative relief (difference between the lowest and highest parts of a catchment). This is typically 3,000 metres, and in some areas, such as the central Indus gorge in Pakistan, it can be over 5,000 metres.
The Himalayas are thought to have a large impact on world climate. On a seasonal time-scale, the presence of the Himalayas strongly influences the monsoon circulation by enhancing the warm, low-pressure cell that forms over South Asia in the summer (Lecture 2). Additionally, heavy snow-cover over the Himalays in winter is thought to delay the monsoon onset the following year. On longer timescales, the uplift of the Himalayas may have helped the global cooling that caused the most recent ice age conditions (Late Tertiary/Quaternary), due to the removal of CO2 from the atmosphere by weathering. The uplift of land exposes easily erodible rocks to intense erosion. In turn, this accelerates chemical weathering of suspended silt particles as they are carried away by rivers. The chemical weathering of silicate minerals removes dissolved CO2, so acts to remove this gas from the atmosphere-hydrosphere system. Such CO2 "draw-down"following the uplift of the Himalayas is thought by some researchers to have initiated global cooling and the onset of the Quaternary Ice Age.
One very important consequence of active tectonism in the Himalayas is the occurrence of earthquakes. Continental collision and uplift of the Himalayas is not a smooth, continuous process, but consists of a series of sudden movements separated by periods of no apparent movement. During the "quiet" periods, stresses gradually build up below the surface, which are released suddenly when a threshold is reached. The sudden movement sends a series of shock waves out from the epicentre, which are felt at the surface as an earthquake. Numerous earthquakes occur in the Himalayas and South Asia every year, most of them minor.
The most recent earthquake in South Asia was very serious. Centred upon the town of Bhuj in Gujurat, India, an earthquake measuring 7.9 on the Richter scale occurred at 8:46 a.m. (local time) on Friday 26th January 2001. At least 30,000 people are estimated to have died in the quake. Another 55,000 have been injured and about half a million made homeless. Only about 18 quakes of this strength occur globally each year, so this was a rare large quake in a region that is frequently shaken by smaller ones. The last comparable quake in the region occurred on 16 June, 1819, killing 2,000 people. That quake was estimated to be about magnitude 7.7, slightly less than the current quake.
An excellent guide to Plate tectonics, including very clear diagrams showing the drift of India and the formation of the Himalayas can be found at this United States Geological Survey site: http://pubs.usgs.gov/publications/text/dynamic.html
(see especially "Understanding Plate Motions" and "Historical Perspective")
Deccan lavas: http://volcano.und.nodak.edu/vwdocs/volc_images/europe_west_asia/india/deccan.html