Sedimentary rocks constitute about 10 percent of the earth’s crust by volume but cover 75 percent of the earth’s surface. Most of the rocks are concentrated on continents (500 million cu km) and continental slopes (190 million cu km); the ocean floor accounts for 250 million cu km. On continents about 75 percent by volume of all sedimentary rock is confined to geosynclinal regions, and about 25 percent is associated with platforms. More than 75 percent of the useful minerals extracted from the earth’s interior are contained in sedimentary rock.