The continental shelf is a portion of the continent to which it is adjacent, and not actually part of the ocean floor. As a result of continual earth movement, the shelf is continuously exposed and covered by water. Even when covered by water, as it is today, it shows signs of once having been dry land. Fossil river beds, for example, are characteristic of some slopes. Remnants of glacial action can also be found in some regions of the continental shelf.
The continental shelf tends to be quite flat, with an average slope of less than 6.5 ft (2 m) for each mile (km) of distance. It varies in width from a few miles (kms) to more than 932 mi (1,500 km) with a worldwide average of about 43 mi (70 km). Some of the widest continental slopes are to be found along the northern coastline of Russia and along the western edge of the Pacific Ocean, from Alaska to Australia. Very narrow continental slopes are to be found along the western coastline of South America and the comparable coasts of west Africa. The average depth at which the continental shelf begins to fall off toward the ocean floor (the beginning of the continental slope) is about 440 ft (135 m).
Materials washed off the continents by rivers and streams gradually work their way across the continental shelf to the edge of the continental slope. In some instances, the flow of materials can be dramatically abrupt as, for example, following an earthquake. At the outer edge of the continental shelf, eroded materials are dumped, as it were, over the edge of the shelf onto the sea floor below.
The continental shelf is one of the best studied portions of the ocean bottom. One reason for this fact, of course, is that it is more accessible to researchers than are other parts of the sea floor. More than that, however, the waters above the continental shelf are the richest fishing grounds in the world. A number of nations have declared that their national sovereignty extends to the end of the continental shelf around their territory—often a distance of 120 mi (200 km)—to protect their marine resources.
Included among those resources are extensive mineral deposits. Many nations now have offshore wells with which they extract oil and natural gas from beneath the continental shelf.