What are Tectonic Plates? Definition, Theory & Size

plate tectonics, Major and minor plates

According to tectonic plates theory, the outermost part of the Earth’s interior is made up of two layers:

  • the Lithosphere, comprising the Crust and
  • the solidified uppermost part of the Mantle.

Below the lithosphere lies the asthenosphere, which forms the inner part of the mantle. The asthenosphere behaves like a super heated and extremely viscous liquid.

The lithosphere essentially floats on the asthenosphere and is broken up into what are called tectonic plates. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: convergent, divergent and transform.

Convergent boundaries: where two plates are colliding.

convergent boundary, also known as a destructive plate boundary, is a region of active deformation where two or more tectonic plates or fragments of the lithosphere are near the end of their life cycle.

Subduction is a geological process that takes place at convergent boundaries of tectonic plates where one plate moves under another and is forced or sinks due to gravity into the mantle. Regions where this process occurs are known as subduction zones. The plate being forced under is eventually melted and destroyed.

Divergent boundaries – where two plates are moving apart.

The space created can also fill with new crustal material sourced from molten magma that forms below. Divergent boundaries can form within continents but will eventually open up and become ocean basins.

Transform boundaries – where plates slide passed each other.

The relative motion of the plates is horizontal. They can occur underwater or on land, and crust is neither destroyed nor created.

Because of friction, the plates cannot simply glide past each other. Rather, stress builds up in both plates and when it exceeds the threshold of the rocks, the energy is released – causing earthquakes.

The last occurs where two plates move laterally relative to each other, creating a strike-slip fault. Earthquakes, volcanic activity, mountain building, and oceanic trench formation can occur along these plate boundaries.


Tectonic plates according to their sizes

Major/main tectonic plates

These plates comprise the bulk of the continents and the Pacific Ocean.

Minor tectonic plates

These smaller plates are often not shown on major plate maps, as the majority do not comprise significant land area. A minor plate is any plate with an area less than 20 million km2 but greater than 1 million km2.



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