The Pacific Plate is located in the Pacific Ocean and is mainly composed of oceanic crust. Its eastern boundary is the San Andreas transform fault, which separates it from the North American Plate. Its western boundary is the Mariana Trench, which separates it from the Philippine Sea Plate. Extension at the mid-ocean ridge in the Pacific Ocean is on-going and associated with extensive neutral-basic volcanic eruptions. Studies of the magnetic stripes on the oceanic crust and abundant seismic data indicate that the Pacific Plate is still expanding and subsiding towards the eastern and western margins, creating the most active volcanic and seismic belt in the world.