The uppermost outer solid and rigid layer of the earth is called crust.
Its thickness varies considerably. It is as little as 5 km thick beneath the oceans at some places but under some mountain ranges it extends upto a depth of 70 km.
Below the crust denser rocks are found, known as mantle crust. This upper part of mantle upto an average depth of 100 km from the surface is solid. This solid mantle plus upper crust form a comparatively rigid block termed as lithosphere.
Mantle is partially molten between 100 to 250 km depth. This zone is said to be asthenosphere, also known as Mohr discontinuity, a simplification of Mohorovicic, the name of the seismologist who discovered it.
The lithosphere is broken into several blocks. These blocks are known as plates, which are moving over asthenosphere. There are seven major plates.
- Eurasian plate
- African plate
- Indo-Australian plate
- Pacific plate
- North American plate
- South American plate
- Antarctic plate
Apart from these major plates minor plates are about 20 in number, a few important among them are –
Arabian plate, Philippine plate, Cocos plate, Nazca plate, Caribbean plate, Scotia plate, etc.
The major and minor plates constitute the whole surface of the earth.
Plate tectonics is a method or way of understanding the land-water distribution of the earth. Tectonics is a sort of movement of plates.
Through the movement, internal forces are explained which are responsible for –
- distribution of earth’s crust;
- formation of mountain chains;
- distribution of earthquakes;
Mechanism of Plate Movement
Arthur Holmes, a British geologist, in 1928 – 1929, proposed that convectional currents exist underneath the lithosphere. The centre of convectional current is not exactly known, but it is believed that it has an average depth of about 100 to 250 km below the surface.
The inception of the current is initiated by heat generation due to radio-active minerals. Due to integration and disintegration of atomic minerals heat is produced and hence the melting of surrounding rocks. In this way currents start operating.
These currents are classified into rising and falling with divergence and convergence activities, respectively.
With rising convectional current, transport of hot and viscous matter takes place upwardly. After reaching about 100 kms below the surface that current gets diverged leading to split into the upper part. The molten material penetrates into the split and thus creation of new surface and the draft of the mammoth plate in opposition direction. It happens below the mid-oceanic ridge.
On the other hand two sets of diverging thermal convectional currents brings two plates together and it is called convergent boundary where subduction takes place.
Plates of lithosphere are constantly in motion because of convectional currents. Their relative motion depends upon the force operating over them.
Plate boundaries are very important and significant structural features. Boundaries are very distinct and easy to identify. They are associated with newly formed mountain systems, oceanic ridges and trenches.
Plates are moving continuously and have relative direction of movement. Based on the direction of movement three types of plate boundaries can, easily, be identified.
- (i) Divergent boundary
- (ii) Convergent boundary
- (iii) Fracture or transform boundary fault
The convectional current are caused due to radio-activity. These currents get diverted on approaching the crust layer.
Diverging currents produce tension at the contact-zone of crust leading to fracture. Maganatic material penetrates into the fractroges and gets solidified. This continuous process pushes the blocks in opposite direction and creates a new zone, known as “zone of construction”.
At convergent boundary, two adjacent plates come further and further closer to each other and collide. When both sides are of continental nature, a mountain formation is evident.
When one of the two is continental and the other maritime again mountain comes into being along the boundary. In this case, continental plate overnides the maritime.
When both plates are of maritime, both of them break, subduct and penetrate below and, hence, trenches are formed. Along this boundary earthquakes and volcanic activities are prominent.
In all these three situations, surface area is reduced, therefore, this is also known as “zone of destruction”.
Transform fault is the one when two adjacent plates slide past each other. Direction of movement may be along or against but they move parallel to each other. Therefore, neither there is any construction of fresh area nor it has any destruction. Hence, it is known as “zone of preservation”.
Plates are not a permanent features but they vary in size and shape. Plates can split or get welded with adjoining plate. Almost all tectonic activities occur along the plate boundaries.
Prior to the advent of plate tectonic theory, the continental drift theory which was proposed by Wegener was criticized, particularly about the forces. In fact, it was outrightly rejected inspite of apparent evidences. But further researches about the material of sea floor and paleomagnetism supported the theory but the proposition of plate tectonic theory in 1960’s has solved the problem of the mechanism of movement.
Plate Tectonics Vs Earthquakes and Volcanoes
The distribution of earthquakes and volcanoes over the globe clearly reveals that they are strongly associated with the boundaries of plates.
Plate boundaries are the zones where every sort of tectonic activity does take place. The release of energy created because of the movement of plates is manifested in this zone in the form of earthquakes and volcanic eruption.
Bibliography : NIOS Geography Book