The Earth's crust is made up of seven principal tectonic plates and numerous other smaller plates. The plates are sections of the crust that "float" on the mantle, which is made up of molten rock. Where the plate's meet, huge forces mean that they can form features such as volcanoes, fold mountains, deep-sea trenches and earthquakes.
There are two main types of tectonic plate. Oceanic crust is often only about 5km thick, but is very dense. Continental crust is considerably thicker, often being approximately 30km deep, but is less dense.
The Earth's Tectonic Plates all move very slowly on the mantle, meeting along the four main boundaries that can be found in the next section. The plates move due to convection currents in the mantle. These are hot currents of molten rock that slowly move within the mantle and cause the plates above them to move, usually by as little as one or two centimetres each year.