CONTINENTAL SLOPE

seaward border of the continental shelf. The world's combined continental slope has a total length of approximately 300,000 km (200,000 miles) and descends at an average angle in excess of 4 from the shelf break at the edge of the continental shelf to the beginning of the ocean basins at depths of 100 to 3,200 m (330 to 10,500 feet). The gradient of the slope is lowest off stable coasts without major rivers and highest off coasts with young mountain ranges and narrow continental shelves. Most Pacific slopes are steeper than Atlantic slopes; gradients are flattest in the Indian Ocean. About one-half of all continental slopes descend into deep-sea trenches or shallower depressions; most of the remainder terminate in fans of sediment or continental rises. The transition from continental crust to oceanic crust usually occurs below the continental slope. Continental slopes are indented by numerous submarine canyons and mounds. The Blake Plateau off the southeastern United States and the continental borderland off southern California are examples of continental slopes separated from continental shelves by plateaus of intermediate depth. Slopes off mountainous coastlines and narrow shelves often have outcrops of rock. The predominant sediments of continental slopes are muds; there are smaller amounts of sediments of sand or gravel.