The crust, or the solid shell of the earth is about 25 miles thick over the continents, and about 4 miles thick under the ocean floors. The movement of the plates is considered as the Plate Tectonic Theory. Diagram of the earth's major tectonic plates, showing locations of volcanoes. Tectonic forces, volcanoes and earthquakes all occur together and are interrelated. Spreading Where two plates spread apart, cracks or rifts form in the earth's crust.
Volcanoes make new land.
Rifts on continents can create valleys, as is seen in the East African Rift Valley. But it means that in your lifetime, North American will move westward the length of a man's body. When two plates move toward each other, the denser one is forced under the other in a process called subduction. Deep trenches are formed when subduction happens in the ocean, also it gives rise to earthquake activity and volcanic islands. Japan is such a place, in the Pacific Ring of Fire.