The floor of the Pacific Ocean is flat over vast areas, but there are many submarine cones or seamounts (guyots) besides platforms and ridges, some of which are capped with islands, especially towards the north and west margins of the basin. For convenience of description, the Pacific Ocean can be divided into an eastern and a western portion along the meridian of longitude 150° west.
The eastern Pacific, as thus defined, is nearly devoid of islands except near the margins of the Americas, and the major portion has a fairly uniform depth of nearly 18,000 feet. A broad, ill-defined submarine platform of less depth (about 13,000 feet), called the Albatross Plateau, is located west of South America, and branches of the platform extend north toward Costa Rica, west to the Tuamotu Archipelago and Marquesas Islands, and southward in the direction of Antarctica. Above the platform rise lonely Easter Island and Sala-y-Gomez. Closer to Chile is the Juan Fernandez group, and to the north are the Galapagos Islands, with Cocos Island rising from the branch that approaches Costa Rica. These volcanic islands are located near the margins of the Albatross Plateau and presumably are situated along fracture lines, with the Galapagos and Marquesas groups at the intersection of major fissures at opposite ends of the platform.
The western Pacific is characterized by multitudinous islands, generally arranged along arcs. Typically the islands rise from submarine platforms, some of which are probably constructed by outpourings of lava from rifts on the flat ocean floor and others from deformation of the earth's crust. Many islands represent the summits of mountain ranges, most of which are below sea level. Some volcanic islands have been built up from extreme ocean depths and without apparent help from crustal uplifts. For example, Mauna Kea (13,784 feet) and Mauna Loa (13,679 feet) on the island of Hawaii rise, in a distance of 50 miles, from ocean depths of 18,000 feet, making them higher mountains from the sea bottom than Mount Everest is above sea level. Within the tropical Pacific are thousands of coral islands and reefs, which are sometimes interspersed with those of volcanic origin and sometimes separated by hundreds of miles from any neighbor.