The theory of plate tectonic presented in early 1960’s, explains that the lithosphere is broken into seven large segments (and several smaller) called plates separated by boundaries.
The uppermost part of the earth has two layers with different deformation properties.
- The upper rigid layer called lithosphere is about 100 km thick below the continents and about 50 km thick under the oceans, consisting of crust and upper mantle rocks.
- The lower layer called as the asthenosphere is extends down to about 70 km depth.
The lithosphere plates are not stationary, they float in a complex pattern with a velocity 2 to 10km per year on the soft rocks of the underlying asthenosphere like raft on a lake.
The major continental plates are:
- African plate
- South American plate
- North American plate
- Eurasian plate
- Indo-Australian plate
- Antarctic plate
- Pacific plate
The great forces thus generated at plate boundaries build mountain ranges, cause volcanic eruptions and earthquake. The earthquake that occur at plate boundaries is inter-plate earthquake, and the earthquake that occur far from the plate boundaries are called intra-plate earthquake.
The types of plate tectonic boundaries are:
1. Divergent boundary
Divergent boundaries or spreading ridge are areas along the edges of plate that move away from each other. This is the location where the less dense molten rock from the mantle rises upwards and becomes part of the crust after cooling. This occurs in rifts and valleys formation.
2. Convergent boundary
It is also known as subduct boundaries. It is formed when either oceanic lithosphere subducts beneath oceanic lithosphere (ocean-ocean convergence) or when oceanic lithosphere subducts beneath continental lithosphere (ocean-continental convergence).
An oceanic trench or mountains forms at the junction of two plates where they meet.
3. Transform boundary
Transform boundaries occur along the plate margins where two plate move pass each other without destroying or creating new crust. Here the two plates may move horizontally across each other or they may shift vertically with respect to each other.