The Earth's crust is constantly moving, both vertically and horizontally, at rates of up to several inches a year.
Plates have three types of boundary.
One type, like the Mid-Atlantic Ridge which runs along the floor of the Atlantic Ocean, moves apart by seafloor spreading at several inches per year. The third type is a boundary where plates move horizontally past each other such as the San Andreas fault located on the North American Plate. Because earthquakes and often volcanoes can be found along all three types of plate boundary, plate tectonic theory not only is important in scientific study it also has significance for people living near those boundaries. These plates may include both continental and oceanic crust. The North American plate, for example, includes the western Atlantic Ocean and most of the North American continent.