What are the tectonic plates? How they are formed?

 Before knowing about the plate tectonic theory, we need to know the meaning of the plate and tectonic. So, the plates are the parts of the lithosphere and they are formed due to Exogenetic force and Endogenetic force. These forces were acting on the lithosphere continuously and broke it into 27 parts (7 are major and 20 are minor) and each part of the lithosphere is known as a plate. The major seven plates are as follows-

1)      African plate

2)      Antarctica plate

3)      Indo-Australian plate

4)      North American plate

5)      South American plate

6)      Pacific plate

7)      Eurasian plate

The term tectonic represents the movement of plates. There are three types of plate tectonics or plate movements-

1) 

      Convergent plate tectonic: When two plates move in the same direction towards each other, then the convergent plate tectonic occurs. Due to this kind of movement of plates, one plate eventually slides beneath the other plate through a collision, which leads to the formation of volcanic activities, earthquakes, large landslides, etc. on the Earth’s surface. The area on the earth, at which the convergent plate tectonic or movement occur, such boundary is known as a convergent plate boundary. This boundary is also known as a destructive plate boundary. 

 

2)      

      Divergent plate tectonics: When two plates move in the opposite direction away from each other then the movement of plates is known as divergent plate tectonics. Due to this kind of plate movement, a fault is occurring in the weaker zone on the Earth’s surface, through which magma rises to the Earth’s surface and form a mid-oceanic ridge. The area on the Earth, at which the divergent plate tectonic or movement occur, such boundary is known as a divergent plate boundary. This boundary is also known as a constructive plate boundary.

 

3)     

     Transform plate tectonics: when two plates slide past one another, then the movement of plates is known as transform plate tectonic or transform plate movement. The San Andreas The fault is an example of transform boundaries, which is formed by the pacific plate and North American plate. The motion of plates in transform boundaries is predominantly horizontal. Transform boundaries are found in the ocean basin.

 

 

The plate tectonic theory is a combination theory, which is made up of ideas of two earlier theories-

1)      Continental Drift Theory:

This theory was proposed by Alfred Wegener in 1910. According to this theory, at the beginning of the Mesozoic era (about 200 million years ago), all the continents of the Earth were united together by making a single supercontinent, known as “Pangaea”. A huge ocean surrounded this supercontinent, known as “Panthalassa”. Due to the gravitational force of the Sun, Moon, and the other planets, this supercontinent broke into two parts, the northern part is known as “Laurasia or Angara land” and the southern part is known as “Gondwana land”. Since the gravitational force acting continuously on both the northern part and the southern part, the northern part moved towards the west and the southern part moved towards the equator. The northern part and the southern part further broke down into small pieces, which are known as continents. These continents were drifted to their present position with time and they float over the magma layer just like pieces of woods floating on the water.

 

2)      Sea-floor Spreading Theory: Sea-floor spreading, which was proposed by Harry Hess in the early 1960s.

   

    Before know about the Sea-floor Spreading theory, we need to know about the “Convection Current Theory” because the concept of Sea-floor spreading is taken from this theory.

     

    Convection Current Theory was proposed by British geologist A. Holmes in 1927. According to this theory, Due to the decay of radioactive substances below the Earth’s surface, heat is produced in the Earth’s mantle, which leads to the formation of a molten layer, known as magma. Magma exerts a constant force on the Earth’s surface so that they can come out the surface of the Earth from the Earth’s mantle, which produces convection current in the Earth’s mantle.

 

According to Sea-floor Spreading theory, Due to this convection current, a tensional force occurs, which influence the plates to move away from each other, as a result in a fault is formed at the weaker zone of the earth’s surface, through which magma come out the surface of the earth and formed a mid-oceanic ridge.

 

Modern Plate Tectonic Theory was developed from the 1950s to the 1970s. This theory explains that the top crust of the Earth is a mosaic of several rigid segments called plates, which include not only the solid upper crust but also part of the denser mantle below. They float on the plastic upper mantle known as the “asthenosphere”.

     Plates may diverge, converge or move in parallels.