Theory of plate tectonics Upsc

Theory of plate tectonics

The theory of plate tectonics is a scientific theory that explains the formation of major landforms due to the earth’s underlying movements.

What is the theory of plate tectonics?

The interest in the distribution of oceans and continents came to life after the introduction of the concept of seafloor spreading.

After that, it was McKenzie and Parker and also Morgan 1967 came out with another theory. This theory is based on a Lithospheric plate also called a Tectonic plate.

The tectonic plate is a massive, irregularly shaped block of solid rock and it is generally made of both continental and oceanic plates.

Their theory is collectively called Plate Tectonic theory.

Plate Tectonic theory

According to the theory of plate tectonics, the plates move horizontally over the Asthenosphere as solid units.

Plate Classification

The lithosphere consists of the crust and top mantle with a thickness of 5 to 100 km in oceanic parts and 200 km in continental areas.

Depending on the occupied area of the plate, the lithosphere is classified as an oceanic or continental plate.

  • If a large portion of the plate occupies the ocean it is called an Oceanic plate
  • If a large portion of the plate occupies the continent it is called a continental plate.
Plate tectonic theory
Plate tectonic theory – Position of continents through geological past Source:Ncert

An example of an Oceanic plate is the Pacific plate and the Continental plate is the Eurasian plate. According to this theory, the earth’s lithosphere is divided into seven major and some minor plates.

These plates are surrounded by Young fold mountain ridges, trenches, and faults. The major leaves are as follows:

Major and minor plates of the world

7 Major plates

The seven major plates of earth are:

  1. Antarctica and surrounding oceanic plates,
  2. North American Plate(with the western Atlantic floor separated from the South American Plate along with the Caribbean Islands),
  3. South American Plate (With the western Atlantic floor separated from the North American Plant along with the Caribbean islands),
  4. Pacific Plate,
  5. India-Australia-New Zealand Plate,
  6. Africa with the eastern Atlantic floor plate,
  7. Eurasia and the adjacent oceanic plate.

Above is the list of major plates. Now let us see the list of some important minor plates below:

  • Cocos Plate – It is found Between Central America and the Pacific plate
  • Nazca plate – It is found between South America and the Pacific plate
  • Arabian Plate – It is found in the Saudi Arabian landmass
  • Philippine Plate – It is found between the Asiatic and Pacific plate
  • Caroline Plate – It is found between the Philippine and Indian Plate that is North of Guinea
  • Fuji Plate – It is found in the North-East of Australia
Tectonic Plates across the World in World map
Tectonic Plates across the World by Source: Ncert

These tectonic plates are dynamic, they keep moving on.

Wegner believed that the continents move by his Continental Drift Theory.

Alfred Wegner

But it is not, Actually, the continents reside over, the tectonic plates. It is the tectonic plates that actually move. And the fact is all the plates have moved in the past and will continue to move in future as well.

In the continental drift theory, Wegner believed that all the continents were once together as a single landmass called Pangaea. But later research revealed that these plates are wandering throughout geological history and Pangea was the result of the converging of different continental masses, which once were present somewhere else.

By using Palaeomagnetic data, the position of the current continental landmass is founded.

As per the data, the Indian subcontinent is traced with the help of rocks found in the Nagpur area.

The plate boundaries are of three types:

  1. Divergent Boundaries
  2. Convergent Boundaries
  3. Transform Boundaries

Divergent Boundaries

Whenever the plates move from each other, a new crust is generated. The spreading sites are sites where the plates move away from each other.

Example: the Mid-Atlantic Ridge divergent boundaries, as a result of the American plate, separated the front of the Eurasian and African Plates.

Convergent Boundaries

One plate dives under another where the crust is destroyed, Subduction zone is the location where the sinking of a plate occurs.

The convergence can occur in three ways, these are 1. Between an oceanic and continental plate; 2. between two oceanic plates; 3. between two continental plates

Transform Boundaries

Plates horizontally slide over each other, the crust is neither created nor destroyed. Transform faults are the planes of separation generally perpendicular to the mid-oceanic ridges.

The eruption does not take all along the entire crest at the same time, there is a differential movement of a portion of the plate away from the axis of the earth and also the earth rotation has effects on the separated block of the plate“.

Plate Movement Rate

The rate of the plate movement is determined by the strips of the normal and reverse magnetic fields that are parallel to the mid-oceanic ridges. Accordingly, the slowest plate movement rate is found in Arctic Ridge at less than 2.5cm/yr.

The fastest plate movement rate is found in the East Pacific rise near Easter Island in the south pacific about, west of Chile about 3400 km.

Plate Movement Force

When Alred Wegner proposed the Continental Drift Theory, most scientists believed that the earth was static and motion solid object but after various studies such as seafloor spreading, Plate Techtonic, etc, scientists found earth was continuously moving and dynamic. Now plate movement is accepted by everyone.

Below the Rigid plate, the mobile rock is believed to moving in a circular manner. The heated material is rise above, spreads, gets cooled and sinks to greater depths and this cycle is repeated continuously and called convection cell or convection flow.

The two main sources of heat within the earth are Radioactive decay and residual heat. This idea is first proposed by Arthur Holmes’s 1930s, later Harry Hess went about Seafloor Spreading.

The slow movement of the hot, softened mantle is responsible for the force behind the plate movement.

Also, Check the Indian Plate Movement

Conclusion

We have discussed Plate tectonic theory, Its types and the evidence that supports it. Also, we have discussed the Tectonic plates boundaries.

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* * All the Notes in this blog, are referred from Tamil Nadu State Board Books and Samacheer Kalvi Books. Kindly check with the original Tamil Nadu state board books and Ncert Books.
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