4. Plate Tectonics

Since the advent of the concept of sea floor spreading, the interest in the problem of distribution of oceans and continents was revived. The hypothesis of plate tectonics is an extended and more comprehensive version of the theory of sea-floor spreading. This is a great unifying concept which “draws sea-floor spreading, continental drift, crustal structures and world pattern of seismic and volcanic activity together as aspects of one coherent picture.”

The term plate was first used by Tuzo Wilson in his definition of transform faults in 1965, but the hypothesis of plate tectonics was first outlined by W.J. Morgan in 1967. More or less concurrently but independently D.P. Mackenzie and Parker had arrived at similar conclusions. It first came to be known by the name of New Global Tectonics but after sometime the term Plate Tectonics gained currency. Basic assumptions of plate tectonics are as follows:

1. There is spreading of sea floor and new oceanic crust is being continually created at the active mid-oceanic ridges and destroyed at trenches.

2. The area of the earth’s surface is fixed. It means, the amount of crust consumed almost equals the amount of new crust created.

3. The new crust that is formed becomes part and parcel of a plate.