The lithosphere of the Earth is not a single piece but is made up of continental plates containing land mass and oceanic plates are under water.
Plate tectonic theory
The lithosphere of the Earth is not a single piece but is made up of continental plates containing land mass and oceanic plates are under water. They fix together like jigsaw puzzle. The plates float over magma in many directions with different speeds (see Fig. 16.2).
Convection currents develop in the viscous mantle because of the prevailing high temperature and pressure gradients between the crust and the core. The convective flow of mantle causes the crust and same portion of the mantle to slide in the hot molten outer core. This sliding of the Earth’s mass takes place in pieces called ‘Tectonic plates’. The surface of the Earth consists of seven major tectonic plates and many smaller ones (Fig. 16.2). Sometimes, the plate in the front is slower than the plate behind it and they collide (forming mountains) and sometimes two plates move away from one another (creating rifts). In another case, two plates move side by side in opposite directions. These three types of inter-plate interactions are the convergent, divergent and transform boundaries. The relative movement of these plate boundaries varies across the Earth; on average it is of the order of couple of tens of centimetres/year.
When the crust is subjected to tectonic forces, it bends slightly. Because the crust is rigid and stress exceeds the strength of the rock, the crust breaks and snaps into a new position. Vibrations called seismic waves are generated and travel both through the Earth and along the surface. The seismic wave causes the movement which we call earthquakes.