SUBMARINE CANYON

any of a class of narrow, steep-sided valleys cut into continental slopes. Submarine canyons are so called because they resemble river canyons on land. Undersea canyons are found along the slopes of most continental margins. They also occur along the slopes of the Hawaiian Islands and possibly certain other ocean islands. The majority of these V-shaped depressions have steep, rocky walls thousands of metres high. Those of the Grand Bahama Canyon, which are thought to be the highest, rise nearly 5 km (3 miles) from the canyon floor. The walls of the Grand Canyon of the Colorado River, by comparison, measure about 1.6 km (1 mile) high. Most submarine canyons extend only about 48 km (30 miles) or less in length, but a few are more than 320 km (200 miles) long. They usually are many kilometres in width, as, for instance, the Grand Bahama Canyon, which measures 37 km (23 miles) at its widest point. A relatively large number of submarine canyons are located directly off river canyons of adjacent land areas and may have once been connected to the latter. Yet, in most cases, the characteristics of the submarine variety and those of the nearby land canyons are quite distinct. The submarine canyons, for example, tend to have steeper side slopes, much higher gradients, and considerably narrower floors. Moreover, the drainage pattern of submarine canyons differs from that of their terrestrial counterparts. The submarine canyons have a substantial number of tributaries at their heads but generally do not have as many tributaries in their lower courses as do the land canyons. There exist at the mouths of many submarine canyons enormous fanlike sediment deposits similar to the alluvial fans associated with many river canyons. The sediments of such a marine fan appear to have been channeled down the canyon by large-scale turbidity currentsi.e., gravity-induced underflows of denser water, known as density currents, in which the density difference is caused by suspended debris. For years the origin of submarine canyons has been the subject of much debate among investigators. Various ideas have been proposed, but prevailing theory favours subaerial erosion as the starting point for a good number of undersea canyons. Such erosion is thought to have begun with the lowering of sea level during the glaciations of the Pleistocene Epoch (about 1,600,000 to 10,000 years ago). It is perceived, however, that subaerial erosion alone could scarcely have excavated deep canyons that extend down to the seafloor. Evidence seems to suggest that the principal agents responsible for the formation of submarine canyons are marine processes, most notably the erosion and transportation of sediments by turbidity currents activated by the slumping of unconsolidated rock material near the heads of the canyons.