submarine canyon

Narrow, steep-sided underwater valley cut into a continental slope. Submarine canyons resemble river canyons on land, usually having steep, rocky walls. They are found along most continental slopes. Those of the Grand Bahama Canyon, which are thought to be the deepest, cut nearly 3 mi (5 km) deep into the continental slope. Most submarine canyons extend only about 30 mi (50 km) or less, but a few are more than 200 mi (300 km) long.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

Submarine canyon

A submarine canyon is a steep-sided valley on the sea floor of the continental slope. Many submarine canyons are found as extensions to large rivers; however there are many that have no such association. Canyons cutting the continental slopes have been found at depths greater than 2 km below sea level. They are formed by powerful turbidity currents, volcanic and earthquake activity. Many submarine canyons continue as submarine channels across continental rise areas and may extend for hundreds of kilometers.

Characteristics

Submarine canyons are more common on steep slopes than on gentle slopes. They show erosion through all substrates, from unlithified sediment to crystalline rock. They are more densely spaced on steep slopes while being rare on gentle slopes. The walls are generally very steep and can be near vertical. The walls are subject to erosion by turbidity currents, bioerosion, or slumping.

Examples

Congo canyon, the largest river canyon, extending from the Congo river, is 800 km (500 miles) long, and 1200 metres (3900 ft) deep.
Amazon canyon, extending from the Amazon river
Hudson Canyon, extending from the Hudson river
Ganges canyon, extending from the Ganges river
Indus canyon, extending from the Indus river
Monterey Canyon, off the coast of central California
La Jolla and Scripps canyon, off the coast of La Jolla, southern California
Whittard Canyon, Atlantic Ocean off southwest Ireland
Bering Canyon, in the Bering sea
Zhemchug Canyon the largest submarine canyon in the world, also in the Bering sea

Formation

Many mechanisms have been proposed for the formation of submarine canyons, and during the 1940s and 1950s the primary causes of submarine canyons were subject to active debate.

An early and obvious theory was that the canyons present today were carved during glacial times, when sea level was about 125 meters below present sea level, and rivers flowed to the edge of the continental shelf. However, while many (but not all) canyons are found offshore from major rivers, subaerial river erosion cannot have been active to the water depths as great as 3000 meters where canyons have been mapped, as it is well established (by many lines of evidence) that sea levels did not fall to those depths.

The major mechanism of canyon erosion is now thought to be turbidity currents and underwater landslides. Turbidity currents are dense, sediment-laden currents which flow downslope when an unstable mass of sediment that has been rapidly deposited on the upper slope fails, perhaps triggered by earthquakes. There is a spectrum of turbidity- or density-current types ranging from "muddy water" to massive mudflow, and evidence of both these end members can be observed in deposits associated with the deeper parts of submarine canyons and channels, such as lobate deposits (mudflow) and levees along channels.

Mass wasting, slumping, and submarine landslides are forms of slope failures (the effect of gravity on a hillslope) observed in submarine canyons. Mass wasting is the term used for the slower and smaller action of material moving downhill; and would commonly include the effects of bioerosion: the burrowing, ingestion and defecation of sediment performed by organisms. Slumping is generally used for rotational movement of masses on a hillside. Landslides, or slides, generally comprise the detachment and displacement of sediment masses. All are observed; all are contributory processes.

It is now understood that many mechanisms of submarine canyon creation have had effect to greater or lesser degree in different places, even within the same canyon, or at different times during a canyon's development. However, if a primary mechanism must be selected, the downslope lineal morphology of canyons and channels and the transportation of excavated or loose materials of the continental slope over extensive distances require that various kinds of turbidity or density currents act as major participants.