Evolution and Geomorphology of the Indian Subcontinent
About 140 million years ago the main landmass on Earth was concentrated together in a super continent called Gondwana which started to break up in four tectonic plates; African, Antarctic, Australian and Indian plates. The rifting is thought to be caused by the rising of a mantle plume which caused the Indian plate to drift northwards and resulted in the opening of the Indian Ocean. The velocity of the drifting of the Indian plate northwards was surprisingly high, 18 to 20 cm per year prior to the collision with the Eurasian plate. During the same period of time the adjacent African and Australian plates moved much slower, 2 to 4 cm per year. It is speculated that the Indian plate had such a high drifting velocity because of its low lithospheric thickness which extends to about 100 km where the other plates that formed Gondwanaland have lithospheric thicknesses of above 180 km which increased the drag and decreased the drift velocity.
As the Indian plate is moving northward relative to the Eurasian plate and collides with it, a convergent boundary is created. On the opposite side, the Indo-African boundary is divergent. The western Indo-Arabian boundary is lateral relative to each other giving rise to a transform boundary. It was previously thought that the Indian and the Australian plates formed one single plate as there is no clear type of boundary but recent seismologic evidence suggests that the two plates will have a transform boundary as the drift velocities of these two plates are different even if the general direction of motion of the two plates is similar.
The collision of the Indian plate into the Eurasian plate about 50 million years ago resulted in the erection of the Himalayan mountain chain which contains the highest peaks on the Earth today.
The Eurasian plate was partly crumpled and buckled up above the Indian plate but due to their low density/high buoyancy neither continental plate could be subducted. This caused the continental crust to thicken due to folding and faulting by compressional forces pushing up the Himalaya and the Tibetan Plateau. The continental crust here is twice the average thickness at around 75 km. The thickening of the continental crust marked the end of volcanic activity in the region as any magma moving upwards would solidify before it could reach the surface.
The Himalayas are still rising by more than 1 cm per year as India continues to move northwards into Asia, which explains the occurrence of shallow focus earthquakes in the region today. However the forces of weathering and erosion are lowering the Himalayas at about the same rate. The Himalayas and Tibetan plateau trend east-west and extend for 2,900 km, reaching the maximum elevation of 8,848 metres (Mount Everest – the highest point on Earth).