· 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.
1. Eurasian plate,
2. African plate,
3. Indo-Australian plate,
4. Pacific plate,
5. North American plate,
6. South American plate and
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 the distribution of earth’s crust, formation of mountain chains and distribtion of earthquakes and volcanism.
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.
· 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
· The convectional current are caused due to radio-activity.
· These currents get diverted on approaching the crust layer.
Divergent boundary :
· Diverging currents produce tension at the contact-zone of crust leading to fracture.
· Magamatic 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 overrides 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”.
Fracture or transform boundary fault :
· 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.
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.