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 landforms, and they are also areas where mountain-building, volcanoes, and earthquakes can be found. 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 pl...