In 1912 Alfred Wegener published a theory to explain why the Earth looked like a huge jigsaw. He believed the continents were once joined forming a supercontinent he called Pangaea. Over 180 million years ago this supercontinent began to “break up” due to continental drift.
During the 20th Century, scientists developed the theory of Plate Tectonics. The theory suggested that the crust of the Earth is split up into seven large plates (see map below) and a few smaller ones, all of which are able to slowly move around on the Earth’s surface. They float on the semi-molten mantle rocks, and are moved around by convection currents within the very hot rock. See why do plates move? for more details.
The Earth’s main tectonic plates
The are two types of tectonic plates – continental plates and oceanic plates. Continental plates are lighter (less dense) than oceanic plates. Oceanic crust is much younger in geologic age than continental crust. Continental crust is on average thicker than oceanic crust.
Cross section of the Earth illustrating oceanic and continental crust.