A submarine canyon is a steep-sided valley cut into the seabed of the continental slope, sometimes extending well onto the continental shelf, having nearly vertical walls, and occasionally having canyon wall heights of up to , from canyon floor to canyon rim, as with the Great Bahama Canyon. Just as above-sea-level canyons serve as channels for the flow of water across land, submarine canyons serve as channels for the flow of turbidity currents across the seafloor. Turbidity currents are flows of dense, sediment laden waters that are supplied by rivers, or generated on the seabed by storms, submarine landslides, earthquakes, and other soil disturbances. Turbidity currents travel down slope at great speed (as much as ), eroding the continental slope and finally depositing sediment onto the abyssal plain, where the particles settle out.
About 3% of submarine canyons include shelf valleys that have cut transversely across continental shelves, and which begin with their upstream ends in alignment with and sometimes within the mouths of large rivers, such as the Congo River and the Hudson Canyon. About 28.5% of submarine canyons cut back into the edge of the continental shelf, whereas the majority (about 68.5%) of submarine canyons have not managed at all to cut significantly across their continental shelves, having their upstream beginnings or "heads" on the continental slope, below the edge of continental shelves.
The formation of submarine canyons is believed to occur as the result of at least two main process: 1) erosion by turbidity current erosion; and 2) slumping and mass wasting of the continental slope. While at first glance the erosion patterns of submarine canyons may appear to mimic those of river-canyons on land, several markedly different processes have been found to take place at the soil/water interface.
Many canyons have been found at depths greater than below sea level. Some may extend seawards across continental shelves for hundreds of kilometres before reaching the abyssal plain. Ancient examples have been found in rocks dating back to the Neoproterozoic. Turbidites are deposited at the downstream mouths or ends of canyons, building an abyssal fan.
Characteristics
Submarine canyons are more common on the steep slopes found on active margins compared to those on the gentler slopes found on passive margins. They show erosion through all substrates, from unlithified sediment to crystalline rock. Canyons are steeper, shorter, more dendritic and more closely spaced on active than on passive continental margins. The walls are generally very steep and can be near vertical. The walls are subject to erosion by bioerosion, or slumping. There are an estimated 9,477 submarine canyons on Earth, covering about 11% of the continental slope.
Examples
Avilés Canyon, off the coast of Asturias, Spain, reaches depth of about . Amazon Canyon, extending from the Amazon River Baltimore and Wilmington Canyons, East Coast of Maryland and Delaware States Bering Canyon, in the Bering Sea Congo Canyon, the largest river canyon, extending from the Congo River, is long, and deep Hatteras Canyon, off the coast of North Carolina Hudson Canyon, extending from the Hudson River Ganges Canyon, extending from the Ganges Indus Canyon, extending from the Indus River Kaikoura Canyon, extending offshore from the Kaikoura Peninsula, New Zealand La Jolla and Scripps Canyon, off the coast of La Jolla, Southern California Mona Canyon, off the coast of western Puerto Rico Monterey Canyon, off the coast of central California Nazaré Canyon, off the coast of Portugal, reaches abyssal depths of Perth Canyon, off the coast of Perth, Western Australia, reaches a depth of and contains the world's largest plunge pool Pribilof Canyon, in the Bering Sea Whittard Canyon, Atlantic Ocean off southwest Ireland Zhemchug Canyon the deepest and joint-widest submarine canyon in the world, in the Bering Sea
Formation
Different mechanisms have been proposed for the formation of submarine canyons. Their primary causes have been subject to debate since the early 1930s.
An early and obvious theory was that the canyons present today were carved during glacial times, when sea level was about 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 where canyons have been mapped, as it is well established (by many lines of evidence) that sea levels did not fall to those depths.