Closely connected with the conception of the continental drift is the more recent concept of Plate-tectonics. A plate is a large, mobile slab of rock and is part of the earth’s surface.

The idea underlying the hy­pothesis of plate-tectonics are associated with the discover/ of zones of formation of young oceanic crust (mid-oceanic ridges) and zones of the absorption of the crust (Trenches).

The plates constitute the rigid outer shell of the earth called the ‘lithosphere’ which include the crust of the earth(both oceanic and continental) and the upper portion of the mantle, which overlies the asthenosphere’, a zone located at 100 to 150 kms depth and behaves plastically. It is a low-seismic velocity zone.

These plates have thickness of 0 to 10 Kms at the ridges to 100 to 150 Kms elsewhere. They move with velocities of 1-6 cm/year.

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According to the available data the lithosphere consists of six major plates bounded by zones of spreading, subduction and folding. These are the Pacific, Eurasian, American, Indian, African, and Antarctic.

A number of factors like-oceanic topograhy, gravity, temperature difference between the ridge-crest and the ocean-floor, convection- current etc. are believed to play major roles in the movement of plates.

Thus the passive continents move on the spreading oceanic-crust, which supports the theory of Continental-drift