Continental Shelves, Slopes, and Rises Virtually all continents
are surrounded by a gently sloping submerged plain called the
continental shelf, which is an underwater extension of the
coastal plain. The continental shelves are the regions of the
oceans best known and the most exploited commercially. It is
this region where virtually all of the petroleum, commercial
sand and gravel deposits, and fishery resources are found. It is
also the locus of waste dumping. Changes in sea level have
alternatingly exposed and inundated portions of the continental
shelf. Continental shelves vary in width from almost zero up to
the 1,500-km-wide (930-mi) Siberian shelf in the Arctic Ocean.
They average 78 km (48 mi) in width. The edge of the shelf
occurs at a depth that ranges from 20 to 550 m (66 to 1,800 ft),
averaging 130 m (430 ft). The shelves consist of vast deposits
of sands, muds, and gravels, overlying crystalline rocks or vast
thicknesses of consolidated sedimentary rocks. Although there is
a great variation in sh