The theory of plate tectonics developed by geoscientists during the early 1960s describing the large-scale motion of seven large plates and the movements of a larger number of smaller plates of the Earth’s lithosphere. It is now widely accepted that most revolutionary concept in the history of GEOLOGY as a science. According to this concept, supported of course by convincing data and observations, the earth’s outer brittle layer extending down to a depth of 150-200 Km and called lithosphere, is actually divided into several blocks or slabs or plates. These lithospheric plates have been and are even now in process of gradual shifting or drifting with respect to each other.
Facts which led to the theory of plate tectonics
(i) Mapping of the oceanic floor.
(ii) Study of seismic data.
(iii) The pattern of anomalies of the earth’s magnetic field.
According to this concept the earth’s surface is comprised of several rigid but relatively thin plates, from the surface to a depth of 150-200 km consisting of both continental and oceanic crust with them, These plates are supported from below on a hot plastic flexible part of upper mantle zone known as Asthenosphere on which these are capable of shifting or drifting with respect to each other . This plate movement is very slow, these plates vary in size.
There are about 20 crustal plates on the earth’s surface out of which 12 major are:-
- The Pacific Plate
- The North American Plate
- The South American Plate
- The African Plate
- The Antarctic Plate
- The Indian Plate
- The European Plate
- The Arabian Plate
- The Nazea Plate
- The Caribbean Plate
- The Scotia Plate
- The Philippine Plate
MOVEMENT OF PLATES
Almost all seismic, volcanic and tectonic activities are located around the plate margins. There is three principle type of plate boundaries depending on the relative motion of adjacent plate.
(1) Diverging Boundaries
(2) Converging Boundaries
(3) Transform Boundaries
(1) Diverging Boundaries:–
These are zones where two adjoining plates are in the process of moving away from each other. It is best seen in the case of the North American plate versus Eurasian plate. In the process of plate separation, the magma rises up from the asthenosphere and fills the gap thus created. By this process, a new crust is formed. This phenomenon is called “seafloor spreading”.
(2) Converging Boundaries:-
These are narrow zones where two adjoining plates are closing on each other. These boundaries are also called “destructive boundaries” or “subduction zones” These occur at deep trenches. The dipping plates sink into the asthenosphere where lithosphere melts to produce magma. Thus these areas are where plates edges are destroyed.
(3) Transform Boundaries:-
These are zones where the adjoining boundaries just slide past each other. The margins at which the plates neither gain nor lose surface area are called “conservative boundaries”.