Plate tectonics is a theory that explains the origin of most of the major features of the 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, the 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 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. They move at speeds up to about 4 inches (10 cm) per year.