The Peninsula represents a stable block of the earth’s crust which has remained unaffected by orogenic or mountain building movements since the close of the Precambrian period. It is a shield area composed of some of the oldest rocks of the earth’s crust which have undergone much crushing and metamorpliism.
On this basement of crystalline rocks lie some later sediments and extensive lava flows (Deccan Trap). The region has been exposed from ancient times to the agents of erosion and is at present an extensive plateau approaching peneplanation. The roots of old mountains are discernible, but the present day elevations are erosion remnants left in various stages of denudation.
According to Alfred Wegener’s theory of Continental Drift about 200 million years ago all the continents of the world were grouped together to constitute the super land mass of Pangaea which was surrounded by one large ocean called Panthals. About 135 million years ago Pangaea was broken into two major marts: (i) the Angaraland or Laurasia consisting of North America, Europe, and Asia north of the Himalayas, and (ii) the Gondawanaland comprising South America, Africa, Australia and India. These were separated by a narrow channel of sea called Tethys. Later on the Gondawanaland was also disintegrated and its broken parts were drifted apart giving rise to the Atlantic and the Indian oceans. According to the recent Plate Tectonic theory the northward movement of India started about 120 million years ago and about 50 million years ago, India and Australia became part of a single plate, the Indian plate.
Last to leave the company of India was Madagascar during the Cretaceous period. During this time there was down warping along the western and the eastern coasts of the Peninsula which resulted into the formation of Arabian Sea and the Bay of Bengal. The central part remained as a table-land called the Peninsula.
A Horst-a number of eminent geologists consider the Peninsula as a vast plateau or table-land which has never since the Precambrian times yielded to any compressive forces and all its stratified rocks have, therefore, mostly preserved their original horizontality. Though horizontally disposed they show remarkable signs of having given way or yielded to tensional and vertical forces, resulting in a number of normal faults, some of them being fairly extensive.
One of these faults is believed to run along the entire length of the western coast of the Peninsula and the other along its eastern coast, large blocks of the country having sunk beneath the Arabian Sea on one side and the Bay of Bengal on the other.
The existing peninsular block is, accordingly, an upstanding remnant of a much larger region. A structure of this kind is in geology known as the horst (R.C. Mehdiratta, 1962, pp. 31-32). The regular coastline lacking creeks and inlets, absence of large delta deposits, seaward extension of the Deccan Trap, the Sahyadri as water divide are some of the evidences which very well support this argument that the western coast of the Peninsula is the result of
An extensive normal fault, it’s down throw side being towards the Arabian Sea. In addition to the fault that has given rise to the coast line, the Peninsula has also suffered from a number of faults in its interior.
Deccan Trap-The Deccan Trap has been formed by the outpouring of the basalt over the peninsular surface dirough fissure eruption during Cretaceous period (65 million years ago). The scientists belonging to the National Geophysical Research Institute (NGRI) have proposed a new theory of clockwise volcanism whereby a cyclic phase of volcanism has been observed on global scale on a regular interval of 33 million years. The Deccan lava flow also belongs to one of such phase of volcanism under which basalt was up welled to the surface from the tepid of 50 km (cf. old views of 200 km from die mantle). They have also rejected the Hot Spot diary of W.J. Morgan (Hindustan Times, Dec. 14, 1987).