Submarine canyon

Shaded relief image of seven submarine canyons imaged on the continental slope off New York, using multibeam echosounder data, the Hudson Canyon is the furthest to the left
Perspective view shaded relief image of the San Gabriel and Newport submarine canyons off Los Angeles
The Congo Canyon off southwestern Africa, about 300 km visible in this view
Heavily canyoned northern margin to the Biscay abyssal plain, with the Whittard Canyon highlighted
Bering Sea showing the larger of the submarine canyons that cut the margin

A submarine canyon is a steep-sided valley cut into the sea floor of the continental slope, sometimes extending well onto the continental shelf, having nearly vertical walls, and occasionally having canyon wall heights of up to 5 km, from canyon floor to canyon rim, as with the Great Bahama Canyon.[1] Just as above-sea-level canyons serve as channels for the flow of water across land, so it is commonly held that submarine canyons serve as channels for the flow of turbidity currents across the ocean floor. Turbidity currents are flows of the relatively heavy sediment laden waters that are generated by rivers, streams, submarine landslides, earthquakes, and other soil disturbances. These heavier waters tend to travel downwards across sloped ground, in a fashion similar to an above-sea-level river, as they naturally seek the calmer waters of the abyssal plain, where the particles of turbidity can finally settle and sedimentate out.[2]

The one thing that nearly all submarine canyons have in common is their termination point. Nearly all submarine canyons terminate at the bottom of the deep intercontinental seabed floor (abyssal plain), approximately 2 km deep or more. About 3% of submarine canyons include valleys that have cut transversally, fully across continental shelves, and which begin with their upstream ends in alignment with and quite close to the mouths of large rivers, such as the Hudson Canyon. A larger 28.5% of submarine canyons have only managed to cut back partially across their continental shelves, and a full 68.5% of submarine canyons have not managed at all to cut significantly across their continental shelves, having their upstream beginnings or "heads" only at the outermost edges of their continental shelves. [3]

The formation of submarine canyons is believed to occur as the result of a certain process of underwater erosion known as turbidity current erosion. While at first glance, the erosion patterns of submarine canyons may appear to mimic those of above sea level river-canyons, due to the markedly different erosion processes that have been found to take place at the underwater soil/ water interface, several notably different erosion patterns have been observed in the formation of typical submarine canyons.[2][4]

Many canyons have been found at depths greater than 2 km 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.[5] Turbidites are often formed at the downstream mouths or ends of submarine canyons.

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.[6] 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.[3] 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.[7]

Examples

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. Slumping is generally used for rotational movement of masses on a hillside. Landslides, or slides, generally comprise the detachment and displacement of sediment masses.

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.

In addition to the processes described above, submarine canyons that are especially deep may form by another method. In certain cases, a sea with a bed significantly below sea level is cut off from the larger ocean to which it is usually connected. The sea which is normally repleted by contact and inflow from the ocean is now no longer replenished and hence dries up over a period of time, which can be very short if the local climate is arid. In this scenario, rivers that previously flowed into the sea at a sea level elevation now can cut far deeper into the bottom of the bed now exposed. The Messinian Salinity Crisis is an example of this phenomenon; between five and six million years ago, the Mediterranean Sea became isolated from the Atlantic Ocean and evaporated away in roughly a thousand years. During this time, the Nile River delta, among other rivers, extended far beyond its present location, both in depth and length. In a cataclysmic event, the Mediterranean sea basin was flooded. One relevant consequence is that the submarine canyons eroded are now far below the present sea level.

See also

References

  1. ↑ Shepard, F.P., 1963. Submarine Geology. Harper & Row, New York
  2. 1 2 Continental Margin Sedimentation: From Sediment Transport to Sequence Stratigraphy (Special Publication 37 of the IAS) March 2009, by Charles Nittroeur, pg 372.
  3. 1 2 Harris, P.T., Whiteway, T., 2011. Global distribution of large submarine canyons: geomorphic differences between active and passive continental margins. Marine Geology 285, 69–86.
  4. ↑ Submarine Canyon by Richard Strickland, 2004
  5. ↑ Giddings, J.A.; Wallace M.W.; Haines P.W.; Mornane K. (2010). "Submarine origin for the Neoproterozoic Wonoka canyons, South Australia". Sedimentary Geology (Elsevier) 223 (1-2): 35–50. Bibcode:2010SedG..223...35G. doi:10.1016/j.sedgeo.2009.10.001. Retrieved 26 January 2012.
  6. ↑ Harris, P.T. (2011). "Seafloor Geomorphology–Coast, Shelf, and Abyss". In Harris P.T. & Baker E.K. Seafloor Geomorphology as Benthic Habitat: GeoHAB Atlas of Seafloor Geomorphic Features and Benthic Habitats. Elsevier. pp. 125–127. ISBN 978-0-12-385141-3. Retrieved 26 January 2012.
  7. ↑ Harris, P.T., MacMillan-Lawler, M., Rupp, J., Baker, E.K., 2014. Geomorphology of the oceans. Marine Geology 352, 4-24.
  8. ↑ Arthur Newell Strahler, Physical Geography. New York: John Wiley & Sons, Inc., 1960, Second Edition, p. 290
  9. ↑ New World Encyclopedia, Canyon. http://www.newworldencyclopedia.org/entry/Canyon

External links

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