Plate Tectonic Theory is a comprehensive theory which offers explanations for various relief features and tectonic events viz. mountain building, folding and faulting, continental drift, vulcanicity, seismic events (earthquakes) etc.
Base of the Plate Tectonic Theory:
The rigid lithospheric slabs or rigid and solid landmasses having a thickness of about 100 km composed of Earth’s crust and some portion of upper mantle are technically called ‘Plates’. The term ‘plate’ was first used by Canadian Geologist John Tuzo Wilson in 1965. The whole mechanism of the evolution, nature and motion and resultant reactions of plates is called ‘Plate Tectonics’.
Plate tectonic theory is a great scientific achievement of the decade of 1960s is based on two major scientific evidences e.g.:
(i) Evidence of Palaeo magnetism and
(ii) Evidences of sea floor spreading six major and 20 minor plates have been identified so far.
Mckenzie and Parker discussed in detail the mechanism of plate motions on the basis of Euler’s Geometrical Theorem in 1967. Prof. Hary Hess (1960) elaborated the mechanism of Plate movement on the basis of the evidences of sea floor spreading. W.J. Morgan and Le Pichon elaborated the various aspects of plate tectonics in 1968.
Three types of Plate Boundaries on the origin of continents and ocean basins have been identified e.g:
(i) Constructive plate boundaries
(ii) Destructive plate boundaries
(iii) Conservative plate boundaries.
(i) Constructive Plate Boundaries are also called as divergent plate boundary’, spreading boundary, or accreting plate boundary, represent zones of divergence along the mid oceanic ridges and are characterised by continuous addition (accretion) of materials as there is constant upwelling of motion materials (basaltic lava) from below the mid-oceanic ridges.
These basalt lavas are cooled and solidified and are added to the trailing margins of the divergent plates and thus new oceanic crust is continuously formed. In fact, oceanic plates split apart along the mid-oceanic ridges and moves in opposite directions and thus transform faults are formed.
(ii) Destructive Plate Boundaries are also known as ‘Consuming Plate Boundaries’ or ‘Convergent Plate Boundary’ are those where two plates collide against each other and the leading edge of one plate having relatively lighter material overrides the other plate and the overridden plate boundary of relative denser material is sub ducted or thrust into the upper mantle and thus a part of the crust in lost in the mental. This is why convergent plate margins are called destructive margins, Collision zone, subduction zone and ‘Benioff Zone’. See Fig.
(iii) Conservative Plate Boundaries are also known as Sheer Plate Boundaries, and Transform Boundaries. Transform faults are those where two plates slip past each other without any collision along the transform fault and thus crust is neither created nor destroyed.
Mechanism of the Plate Tectonic Theory:
According to Plate Tectonic Theory, mountains are formed due to collision of two convergent plates. Mountains are always formed along the destructive plate boundaries. It is obvious that the process of mountain building is associated with destructive plate boundaries of two convergent plates.
The Plate Tectonic Theory envisages the formation of mountains due to compression of sediments caused by the collision of two convergent plate boundaries. Two plates moving together under the impact of thermal convective currents collide against each other and the plate boundary having relatively denser material is sub-ducted under the other plate boundary of relatively lighter materials.
This subduction zone is also called Benioff Zone. The subduction of plate boundaries causes lateral compressive force which ultimately squeezes and folds the sediments and material of the margins at the plates and thus mountains are formed. Rockie Mountains find their origin in collision of American and Pacific Plates while Andes again are result of same collision. Himalayas rose out of collision of Indo-Australian plate against that of Eurasian. Indian plate is still in process of insertion under Eurasian plate and Himalayas are rising.