As we know, the crust of the earth consists essentially of about 35 kms thick layer of solid rock matter which varies in thickness from about 5 km in the ocenic areas to even such thickness as 70 to 80 kms in the mountainous regions of the Alps and the Himalayas.

That the crust is not fully rigid, but has been repeatedly deformed in the geologic past and is subjected to movements even at the present time is confirmed by earthquakes frequent on the ocean-floors and rarer in continents, volcanism, folding and faulting of large expenses of rock strata and recent elevation and depression of coastal areas.

Evidences derived chiefly from palaeomagnetic studies and those of earthquake waves show that within the upper mantle there is a soft layer which behaves plastically because of increased temperature and pressure. This layer of the upper mantle is known as ‘astheno- sphere’ and is approximately 150 kms thick.

The crust of the earth (oceanic and continental) together with the uppermost portion of the mantle which overlies the asthenosphere constitute the lithosphere. The rigid lithosphere is capable of moving bodily over the asthenosphere and is disjointed into large segments or blocks extensively by faults or thrusts.

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These block are known as ‘lithospheric plates’ which are in motion relative to each other.

The ‘concept of Plate-tectonics’ involves a world-wide net-work of moving lithospheric plates. The concept was formulated by the American scientists Hess and Dietz.

The main ideas underlying the hypothesis of ‘plate-tectonics’ are the out-come of the study of the structure of the ocean-floor and the discovery of zones of the formation of young oceanic-crust on mid-oceanic ridges and zones of the absorp­tion of the crust in trenches. This concept takes into account the sat­isfactory parts of the hypotheses of ‘Continental-drift’ and ‘Ocean-floor spreading’.

The basic idea of plate-tectonics, as has been stated in the above paragraph, involves the movement of lithospheric plates and the geo­logic activities associated with them. Accordingly, the total system of plate motions is commonly referred to as ‘plate-tectonics’.