Most of the floor of the Pacific basin is hilly with an average relief of roughly 1,000 feet, but about 10 per cent is smooth. About 4/5 of the smooth area (4×10⁶ square miles) is distributed around groups of existing or drowned ancient islands and has the form of aprons which are here called "archipelagic aprons." In the central and western Pacific, aprons encircle the Caroline, Cook, Ellis, Gilbert, Line, Marquesas, and Marshall islands and the guyots of the Mid-Pacific Mountains, Emperor Seamounts, and the Magellan Seamounts. The apron around the southern part of the Hawaiian Islands is exceptionally thin and has been deformed. In the eastern Pacific the Revillagigedo Islands and isolated Clipperton Island lack extensive aprons, and the Galapagos apron s thin.

Seismic refraction measurements show that volcanic rocks are kilometers thick in the vicinity of all islands where studies have been made. This material forms the bulk of the archipelagic aprons and buries the pre-existing hilly terrane of the sea floor so as to form a relatively smooth plain. The perfection of levelling and smoothness of the aprons indicates deposition of sediment by turbidity currents.