D.3. Plate Tectonics

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By, McKenzie and Parker and also Morgan, independently in 1967.
A tectonic plate (also called lithospheric plate) is a massive, irregularly-shaped slab of solid rock, generally composed of both continental and oceanic lithosphere.
Plates move horizontally over the asthenosphere as rigid units.
The lithosphere includes the crust and top mantle with its thickness range varying between 5-100 km in oceanic parts and about 200 km in the continental areas.
A plate may be referred to as the continental plate or oceanic plate depending on which of the two occupy a larger portion of the plate.
Pacific plate is largely an oceanic plate whereas the Eurasian plate may be called a continental plate.
The theory of plate tectonics proposes that the earth’s lithosphere is divided into seven major and some minor plates.
Young Fold Mountain ridges, trenches, and/or faults surround these major plates.

Major plates

Antarctica and the surrounding oceanic plate
North American (with western Atlantic floor separated from the South American plate along the Caribbean islands) plate
South American (with western Atlantic floor separated from the North American plate along the Caribbean islands) plate
Pacific plate
India-Australia-New Zealand plate
Africa with the eastern Atlantic floor plate
Eurasia and the adjacent oceanic plate.

Minor plates

These plates have been constantly moving over the globe throughout the history of the earth.
It is not the continent that moves as believed by Wegener.
Continents are part of a plate and what moves is the plate.

Later discoveries reveal that the continental masses, resting on the plates, have been wandering all through the geological period, and Pangaea was a result of converging of different continental masses that were parts of one or the other plates.
Scientists using the palaeo magnetic data have determined the positions held by each of the present continental landmass in different geological periods.
Position of the Indian sub- continent (mostly Peninsular India) is traced with the help of the rocks analysed from the Nagpur area.

There are three types of plate boundaries:

Divergent Boundaries	Where new crust is generated as the plates pull away from each other. The sites where the plates move away from each other are called spreading sites. The best-known example of divergent boundaries is the Mid-Atlantic Ridge. At this, the American Plate(s) is/are separated from the Eurasian and African Plates.
Convergent Boundaries	Where the crust is destroyed as one plate dived under another. (Nepal quack ) The location where sinking of a plate occurs is called a subduction zone. There are three ways in which convergence can occur. These are: (i) between an oceanic and continental plate; (ii) between two oceanic plates; and (iii) between two continental plates.
Transform Boundaries	Where the crust is neither produced nor destroyed as the plates slide horizontally past each other. Transform faults are the planes of separation generally perpendicular to the mid- oceanic ridges. As the eruptions do not take all along the entire crest at the same time, there is a differential movement of a portion of the plate away from the axis of the earth. Also, the rotation of the earth has its effect on the separated blocks of the plate portions.