Introduction-Theory of plate tectonics
The Earth's crust is broken into large pieces called tectonic plates. These plates constantly move at a few centimetres each year. Although it isn't very much, over millions of years the movement allows whole continents to shift thousands of kilometres apart. This process is called continental drift. The plates move because of convection currents in the Earth's mantle. Convection currents occur because the molten rock gets heated up by the core and because less dense, while the molten rock closer to the surface cools down becoming more dense. Thus the less dense molten rock goes up towards the surface and the cool more dense rock falls to the core. This process continues round and round, pushing the tectonic plates little by little.