It is universally accepted by earth scientists that parts of the crust are capable of moving horizontally round the globe, causing the continents slowly to change position in relation to each other. This idea was originally synthesized under the term continental drift by A. Wegner in 1915. Wegner postulated an idea that in ancient time about 200 million years ago, there was one land mass pangea represented by Angaraland northward and the Gondwanaland in south. These two have been separated by the sea of Tethys. He envisaged that "differential gravitation at forces acting upon the protruding land masses split the blocks of the present day continents". These land blocks started drifting northward and westward. The equatorial movement had been accounted by low viscosity of Sima then existing, causing Alpine folds, while westward split had caused the buckling up of the Rockies and Andes.
Considerable opposition to this thesis persisted for many years. In the last fifteen years, however, new discoveries have confirmed his general ideas and have led ultimately to a body of theory which we now call plate tectonics. The concepts embodied in this go a long way towards explaining the distribution and origin of many major relief features.
The basic principle of plate tectonics is very simple: the lithosphere is broken into several sections or plates. Each plate is capable of moving over the asthenosphere, carrying oceanic and continental crust alike. Each plate moves as a single independent body.
Each of them varies 100-150 km in depth and many thousands km's in width. Recent volcanic and Seismic activity is concentrated on along the plate Margin.
The Divergence or Constructive Margin: It occurs where the plates are moving from each other with new material being added up in the form of mid-oceanic ridges in the Pacific, Atlantic and Indian oceans, plate boundaries are characterized by the creation of new crust from below, and in the process slowly away from these central ridges. Elsewhere, as around the periphery of the Pacific Ocean, plates move past each other or collide. The convergence or destructive margin occurs where two plates meet each other. At many zones of collision, one plate overrides the other, the lower plate being absorbed into the mantle, thus maintaining total material balance over the globe, making up for the new crust coming out of the ocean ridges.
At the plate boundaries, major tectonic landforms are created. The areas of collision may be made up of three elements: deep trenches on the ocean floor, some up to 11 km deep, marking areas of plate subduction into the mantle; arc-like row of volcanic islands; and mountain ranges, where the plate appears to be crumpling and thickening. All plate margins, including the mid-oceanic ridge systems, are frequently area of considerable volcanism.
There is a conservative margin as in California where the plates slip past one another without any change in area.
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