Plate Tectonics:

               The plate tectonic theory was developed during the early decade of the 20th century. The term 'plate' was first used by Canadian geophphycist JT Wilson in 1965. McKenzie and Parker discussed in detail the mechanism of plate motions on the basis of Euler's Geometrical theorem in 1967.

            

               Plate Tectonics theory describing the large scale motion of tectonic plates of the Earth's lithosphere over the asthenosphere. The theory has been built on the concept of continental drift and sea floor spreading both of which have been successfully incorporated into the plate Tectonics theory. All tectonic activities occur along the plat margins e.g. seimic events, vulcanicity, mountain building, faulting etc.

Distribution of the Plates:

         Plates are hard and rigid segments of lithosphere including upper part of upper  mantle and oceanic and continental crust that floats on the asthenosphere. There are seven major plates on the Earth's surface. These are the Eurasian, Antarctica, North American, South American, Pacific, African and Indian plates and minor plates are China, Phillipines, Arabian, Iran, Nazca, Coco, Caribian and Scotia plates. There are more than 20 plates with area of 100000 - 10000000 sq km. The Pacific plate is the largest plate covers about 1/5th of the Earth's surface.

Movement of Plates:

             The lithospheric plates move over the plastic asthenosphere under the influence of thermal convective currents. Plate movements are categorised into three division, these are        

                 (i). Divergent plate boundary,

                 (ii). Convergent plate boundary ,

                (iii). Transform plate boundary.

(i). Divergent Plate Boundary:

             These plate margins are formed due to the divergence of lithospheric plates under the influence of lateral outflow of thermal convective currents. There is continuous upwelling of molten lava and therefore new oceanic crust is formed. That is why they are also called constructive plate margins. These type of plate margin are characterized by mid-oceanic ridges where two oceanic plates split apart to form new oceanic crust and move in opposite directions e.g. mid-oceanic ridge of Atlantic Ocean challenger ridge, Socotra and Chagos ridges of Indian Ocean.

 (ii). Convergent Plate Boundary:

                      These are known as destructive plate margins because two plates converge together above laterally inflowing thermal convecting currents. The denser plate is subducted inside the lighter plate and as it reaches the molten part of mantle, it is completely consumed and destroyed. In these plate boundary rise various features such as volcanoes, trenches, fold mountains etc.

                Convergent Plates Boundaries are further classified into three classes- 

(a). Ocean- Ocean Collision: Ocean- Ocean Collision form a strench, an island arc and a marginal sea e.g. Japan strench, Japan island and Japan sea.

(b). Continent- Ocean Collision:  Collision of both the plates causes the sediments of the continental side to compress and fold forming forming fold mountains on the oceanic side, strench is formed.

(c). Continent- Continent Collision:  Continent- Continent collision produce a single large continental mass with internal mountain range. e.g. The Himalaya, the Alps, the Ural, the Atlas mountains.

                 

(iii). Transform/ Conservative Plate Boundary:

                    It is the boundary where the crust is neither produced nor destroyed as the plates slide horizontally pass each other. Transform faults are the planes of separation generally perpendicular to the mid oceanic ridges. There are no eruption along transform faults. But some seismic activity takes place due to interlocking of plates and subsequent release of elastic strain energy in the form of seismic waves. e.g. San Andreas fault in America.