Before studying Plate Tectonics Theory, let us first understand what a tectonic plate is.
What is a Tectonic Plate?
A tectonic plate is a hard rigid and elastic segment of lithosphere which is moving and floating over Aesthenosphere.
A Plate contains three parts of lithosphere.
2- Moho Discontinuity
3- Hard and rigid part of mantle above aesthenosphere connected with crust.
To know about internal structure of earth Click Here
What is Plate Tectonics?
The whole mechanism of the evolution, nature and motion of plates and resultant reactions is called Plate Tectonics. In other Words, the process of plate motion is referred to as Plate Tectonics.
How are Plates classified?
On the basis of their size plates have been classified into two categories.
1- Major Plates
2- Minor Plates
The theory of Plate Tectonics proposes that the earth’s lithosphere is divided into Seven Major and Some Minor Plates.
What are Major and Minor Plates?
The major Plates are as Follows:
1- Antarctica and the surrounding Oceanic Plate
2- North American Plate
3- South American Plate
4- Indo- Australian- New Zealand Plate
5- Pacific Plate
6- African and Eastern Atlantic Floor Plate
7- Eurasian and Adjacent Oceanic Floor Plate
Some Imp Minor Plates are listed below:
1- Cocos Plate- Between Central America and Pacific Plate
2- Nazca Plate- Between South America and Pacific Plate
3- Arabian Plate- Mostly the south Arabian landmass
4- Philippine Plate- Between the Asiatic and Pacific Plate
5- Caroline Plate- Between the Philippine and Indian Plate
6- Fuji Plate- North East of Australia
Pacific Plate is the largest Plate which is Oceanic Plate, Eurasian is Purely a continental plate while all other plates are partly oceanic and partly continental.
What are Plate Margins and Plate Boundaries?
Plate Margins are outer parts of plates and Plate boundaries are Zone of interactions of Margins of Plates which are moving in respect of each other.
It may be highlighted that that tectonically plate boundaries and Plate Margins are most important because all tectonic activities occur along the plate margins, For e.g, Seismic activities , mountain building, vulcanicity etc.
On the basis of Nature of interaction, Plate margins/Boundaries have been classified into 3 categories :
1) Convergent / Destructive / Consuming Plate Margins
2) Divergent / Constructive / Accreting Plate Margins
3) Transform / Conservative / Shear plate Margins
What are Convergent Plate Margins?
These are the places where two plates move towards each other or two plates converge along a line and leading edge of one plate ( e.g Plate 1) overrides the other plate ( e.g Plate 2) and the overridden plate ( plate 2) is subducted into mantle of Overriding plate (plate 1) and thus part of crust is lost in the mantle.
When two continental plates converge, they interact to form Mountain Ranges. For e.g Indo- Australian Plate converged with Eurasian Plate to give Rise to Great Himalayas.
What are Divergent Plate Margins?
These represent zones of divergence i.e plates are continuously coming apart from each other.
When a divergent boundary crosses a land, the rift valleys get formed, examples include- Rio Grande rift in new Mexico, East Africa Rift in Kenya and Ethiopia.
Oceanic Plates split apart to form Mid Oceanic Ridges.
What are Transform Plate Margins?
Places where plates slide past each other are called Transform Boundaries. The plates on either side of a transform boundary are merely sliding past each other, thus neither creating nor destroying the Crust.
The most famous Transform boundary in the world is San Andrea’s Fault.
Although transform boundaries are not marked by spectacular surface features, their sliding motion causes lots of earthquakes.
What are the forces behind movement of Plates?
Movement of Plates is under the influence of two forces :
Thermal convergent Current
These are slow and gradually rising currents from Mantle- Core Boundary. These are generated by radiogenic heat produced at Mantle – Core Boundary through radioactive decay.
Rising and laterally spreading thermal convective currents below lithosphere in aesthenosphere are responsible for divergence of Plates.
Cooling of Thermal Convective Currents which are laterally converging is responsible for their subsidence and down-flow Those Plates which are located above laterally converging and subsiding Thermal Convective Current form Convergent Plate Boundaries Above.
2) Magma Plumes
It is a long slender , rising thermal convective current. It rises from mantle- core boundary. Magma plume explains rifting of plates and intra plate volcanoes. (To be Explained later)
These two forces generate initial movement in plates.
Along Convergent Plate Boundaries, Gravity Pull force works along with laterally converging and subsiding Thermal Convective Currents.
Along Divergent Plate Boundaries, Gravity Push works with rising and laterally spreading thermal Convective Currents.