The pronounced ridge in the middle of the Atlantic ocean marks the location of the Mid-Atlantic Ridge where new seafloor is being created by magma rising from the mantle. Thus, the Atlantic is getting bigger with time. The Earth's crust is not a fixed, continuous shell. As these plates move and shift against one another they release enormous amounts of energy in the form of earthquakes. The boundary between the Pacific plate and the North American plate is located along the coast of California.
Earthquakes tend to be concentrated at the boundaries of Earth's plates.
Volcanoes are also more frequent along the plate boundaries. As oceanic plates are driven under continental plates, forcing them into the mantle, they begin to melt. The locations of volcanoes which erupted between 1960 and 1995 are shown.