Canyon

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Noravank Monastery complex and canyon in Armenia.

A canyon or gorge is a deep valley between cliffs, often carved from the landscape by a river. A famous example is the Grand Canyon in Arizona. In the southwestern United States, canyons are important archeologically because of the many cliff-dwellings built there, largely by the earlier inhabitants, Ancient Pueblo Peoples.

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, but many others have no such association. Canyons cutting the continental slopes have been found at depths greater than two kilometers 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.

Terminology

The word canyon derives from the Spanish word (cañón). The word canyon is generally used in the United States, while the word gorge is more common in Europe and Oceania, though it is also used in some parts of the United States and Canada. The rare word defile is occasionally used in England.


Sometimes large rivers run through canyons as the result of gradual geologic uplift. These are called entrenched rivers, because they are unable to easily alter their course. The Colorado River and the Snake River in the northwestern United States are two examples of tectonic uplift.

Canyons often form in areas of limestone rock. Limestone is to a certain extent soluble, so cave systems form in the rock. When these collapse a canyon is left, for example in the Mendip Hills in Somerset and Yorkshire Dales in Yorkshire, England.

Largest canyons

The Grand Canyon in Arizona, United States, and the Copper Canyon in Chihuahua, Mexico are two of the largest canyons in the world. Yet, the definition of "largest canyon" is rather imprecise, because a canyon system can be "large" in terms of its depth, length, or total area of the system. The definition of "deepest canyon" is similarly imprecise, especially if one includes mountain canyons as well as canyons that cut through relatively flat plateaus (which have a somewhat well-defined rim elevation). The inaccessibility of the major canyons in the Himalaya mountain range is one reason why they are usually not regarded as candidates for the biggest canyon.

The Yarlung Tsangpo Canyon, along the Yarlung Tsangpo River in Tibet, is regarded by some as the deepest canyon in the world, and it is even slightly longer than Grand Canyon. Hence it is regarded by many as the world's largest canyon, followed by the Kali Gandaki Gorge[1] in Nepal, Polung Tsangpo Canyon in Tibet, Cotahuasi Canyon (3,535 m deep and the deepest in the Americas), and the Tekezé gorge (2000m+ deep and deepest in Africa).[2]

Slot canyons are very narrow canyons, often with smooth walls.

Submarine canyons

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.

Formation of canyons on land

Most canyons were formed by a process of long-time erosion from a plateau level. The cliffs are formed because harder rock strata that are resistant to erosion and weathering remain exposed on the valley walls. Canyons are much more common in arid areas than in wetter areas because weathering has a lesser effect in arid zones. Canyon walls are often formed of resistant sandstones or granite.

Formation of submarine canyons

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 200 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.


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Other well-known canyons

Other lesser-known canyon systems include:

  • Black Dragon Canyon in Utah, USA
  • Swaan River makes many gorges in Pothohar plateau Pakistan.

The Blue Mountains west of Sydney in New South Wales, Australia contain many gorges carved into the sandstone rock.

Examples of submarine canyons

  • Congo canyon, the largest river canyon, extending from the Congo river, is 800 km (500 miles) long, and 1,200m (4000 ft) deep.
  • Amazon canyon, extending from the Amazon river
  • Hudson canyon[1], 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.

Canyons on other planetary bodies

  • Valles Marineris on Mars is the largest known canyon in the solar system
  • Ithaca Chasma on Saturn's moon Tethys

See also

Notes

  1. If one measures the depth of a canyon by the difference between the river height and the heights of the highest peaks on either side, the Kali Gandaki Gorge is the world's deepest. The portion of the river between the Dhaulagiri and Annapurna massifs is at an elevation of between 1300 meters and 2600 meters, 5500 to 6800 meters lower than the two peaks. See Annapurna (1:100,000 map), Nepal-Kartenwerk der Arbeitgemeinschaft für vergleichende Hochgebirgsforschung Nr. 9, Nelles Verlag, Munich, 1993. Also see Google Earth.
  2. Ethiopia's Water Dilemma. World Rivers Review, August 2006. (pdf) Retrieved October 12, 2007.

References
ISBN links support NWE through referral fees

  • Abbott, Lon, and Terri Cook. 2004. Hiking the Grand Canyon's Geology. Seattle, WA: Mountaineers Books. ISBN 0898868955.
  • Beus, Stanley S., and Michael Morales. 2003. Grand Canyon Geology. 2nd ed. New York: Oxford University Press. ISBN 0195122992.
  • Erickson, Jon. 2001. Rock Formations and Unusual Geologic Structures. Exploring the Earth's Surface. New York: Facts on File. ISBN 0816043280.
  • Plummer, Charles C., David McGeary, and Diane H. Carlson. 2005. Physical Geology. Boston: McGraw-Hill. ISBN 007252815X.
  • Whitney, Stephen. 1996. A Field Guide to the Grand Canyon. Seattle, WA: Mountaineers. ISBN 0898864895.

External links

All links retrieved September 29, 2007.

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