There are seven major tectonic plates on the planet that are further subdivided into dozens of smaller plates. Geologists do not always agree on how to subdivide the minor plates. Each plate is in motion relative to the other plates.
The world's volcanoes and mountains, as well as earthquakes, occur where they do owing to the types of interactions at the boundaries of plates.
Plates move, much as icebergs do, slowly over time in what geologists refer to as continental drift. Alfred Wegener initially proposed the theory of continental drift in 1912, contending that at one time all of the earth's plates were joined as one. Evidence discovered in 1952 confirmed his theory.