The theory of plate tectonics states that the Earth's crust is rigid and is divided into about 12 main slabs or plates that float on an underlying plastic zone.
Deformation occurs along the edges of the plates as they move and grind together.
New crust is created along spreading centers, another type of plate boundary where subsea mountain chains that rise above the ocean floor in many of world's oceans. Iceland is one place where a spreading center is above sea level. Earthquakes, volcanoes, and mountain belts are generally located near the edges of the plates. Plate tectonics implies that the Earth's crust is constantly moving and changing through time.