Plate tectonics is a theory that explains the origin of most of the major features of Earth's surface. For example, the theory tells us why most volcanoes occur where they do, why there are high ridges and deep trenches in the oceans, and how mountains form.
According to this theory, Earth has an outer shell made up of about 30 rigid pieces called tectonic plates. Some of these plates are gigantic. For instance, most of the Pacific Ocean covers a single plate.
The plates move about on a layer of rock that is so hot it flows, even though it remains solid. The plates are moving very slowly relative to one another. They move at speeds up to about 4 inches (10 centimeters) per year.
Plates have been moving about for hundreds of millions of years. So, in spite of their very low speeds, some of them have moved vast distances. In fact, over the past several hundred million years, plate movement has changed the map of Earth drastically. Earth scientists have determined that before about 200 million years ago, all the continents were part of a supercontinent called Pangaea.
