The theory of plate tectonics.

The Earth’s lithosphere (crust and upper mantle) is divided into seven large, and several smaller plates . These plates are constantly moving, and are driven by convection currents in the mantle. Plate boundaries mark the sites of the world’s major land forms, and they are also areas where mountain-building, volcanoes and earthquakes can be found.

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However, in order to account for such activity at the plate boundaries, several points should be noted 

  • Continental crust is less dense than oceanic crust so it does not sink. 
  • Whereas oceanic crust is continuously being created and destroyed, continental crust is permanent, and hosts the oldest rocks on the planet (the shieldlands). 
  • Continental plates may be composed of both continental and oceanic crust (e.g. Eurasia). 
  • Continental crust may extend further than the margins of the land masses (when continental crust is covered by an ocean, it is known as continental shelf). 
  • It is not possible for plates to overlap, so they may either crumple up to form mountain chains, or one plate must sink below the other. 
  • If two plates are moving apart, new crust is formed in the intervening space, as no ‘gaps’ may occur in the Earth’s crust. 
  • The earth is not expanding, so if newer crust is being created in one area, older crust must be being destroyed elsewhere. 
  • Plate movements are geologically fast and continuous. Sudden movements manifest themselves as earthquakes. 
  • Very little structural change takes place in the centre of the plates (the shield lands). Plate margins mark the sites of the most significant landforms, including volcanoes, batholith intrusions, fold mountains, island arcs and deep-sea trenches

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