Plate Tectonics

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Plate Tectonics the theory explains that the earth's crust is composed mainly of seven major and some minor rigid plates that are in motion and are floating on the mobile hot mantle materials below. The plate motions and their mutual interactions are suggested to have controlled the major geological events like earthquakes, volcanism, mountain building etc. Three kinds of mutual plate boundaries are described, ie (i) convergent boundary - when the two plates move towards each other and ultimately converge and collide; in this case one plate plunges beneath the other. This kind of plate collision results in the formation of mountains as well as volcanism near the plate boundary. (ii) Divergent boundary - when two plates move away from each other. This kind of boundary generates a new ocean floor and underwater volcanism. (iii) Transform fault boundary - when the two plates slide past each other. All the three kinds of plate motions generate earthquakes.

The bengal basin, occupied mostly by Bangladesh, is the result of plate collision between the Indian plate and the Asian plate. Before Cretaceous time (125 million years before present), the Indian plate including part of Bangladesh (greater Rangpur and Dinajpur area) was joined together with Antarctica, Africa, Australia and South America, forming a super continent called Gondwanaland. The remaining part of Bangladesh did not exist at that time. It was only after the rifting, northward drifting of the Indian plate and the ultimate collision with the Asian plate that the himalayas were formed and the deltaic plain of Bangladesh created.

The collision between the Indian plate and the Asian plate took place in stages, beginning in the Eocene time (50 to 55 million years before present) when an initial uplift of the Himalayas occurred. By late Eocene (35 to 40 million years before present) the last remnant of intervening the tethys sea between the Indian plate and the Asian plate probably disappeared. During this time the direction of the Indian plate convergence changed from north to northeast with increasing collision with Southeast Asia. Since the Oligocene time (35 million years before present), plate collision continued and major sediments were shed by the rising Himalayas as large river systems started filling up the proto-Bengal basin to the south. From Miocene onward (from 25 million years before present), rapid rise in the Himalayas accompanied by rapid subsidence in the basin resulted in deposition of a huge sedimentary pile with the simultaneous development of a mega-delta. This delta building continues at present as the Ganges-Brahmaputra delta. In the meantime subduction of the Indian plate beneath the Asian plate created a sutured zone in the northern Himalayas while in the east the Indo-Burman range marks the zone of plate collision to the east. The collision of the Indian plate and the Asian plate continues and this is evidenced from the release of stress in the form of earthquakes in the vicinity of the plate boundary from time to time. [Badrul Imam]

See also tectonic framework.