Canyon

A canyon (from Spanish: cañón; archaic British English spelling: cañon), gorge or chasm, is a deep cleft between escarpments or cliffs resulting from weathering and the erosive activity of a river over geologic time scales. Rivers have a natural tendency to cut through underlying surfaces, eventually wearing away rock layers as sediments are removed downstream. A river bed will gradually reach a baseline elevation, which is the same elevation as the body of water into which the river drains. The processes of weathering and erosion will form canyons when the river's headwaters and estuary are at significantly different elevations, particularly through regions where softer rock layers are intermingled with harder layers more resistant to weathering.
A canyon may also refer to a rift between two mountain peaks, such as those in ranges including the Rocky Mountains, the Alps, the Himalayas or the Andes. Usually, a river or stream carves out such splits between mountains. Examples of mountain-type canyons are Provo Canyon in Utah or Yosemite Valley in California's Sierra Nevada. Canyons within mountains, or gorges that have an opening on only one side, are called box canyons. Slot canyons are very narrow canyons that often have smooth walls.
Steep-sided valleys in the seabed of the continental slope are referred to as submarine canyons. Unlike canyons on land, submarine canyons are thought to be formed by turbidity currents and landslides.
The word canyon is Spanish in origin (cañón, pronounced [kaˈɲon]), with the same meaning. The word canyon is generally used in North America, while the words gorge and ravine (French in origin) are used in Europe and Oceania, though gorge and ravine are also used in some parts of North America. In the United States, place names generally use canyon in the southwest (due to their proximity to Spanish-speaking Mexico) and gorge in the northeast (which is closer to French Canada), with the rest of the country graduating between these two according to geography. In Canada, a gorge is usually narrow while a ravine is more open and often wooded. The military-derived word defile is occasionally used in the United Kingdom. In South Africa, kloof (in Krantzkloof Nature Reserve) is used along with canyon (as in Blyde River Canyon) and gorge (in Oribi Gorge).
Most canyons were formed by a process of long-time erosion from a plateau or table-land level. The cliffs form 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 wet areas because physical weathering has a more localized effect in arid zones. The wind and water from the river combine to erode and cut away less resistant materials such as shales. The freezing and expansion of water also serves to help form canyons. Water seeps into cracks between the rocks and freezes, pushing the rocks apart and eventually causing large chunks to break off the canyon walls, in a process known as frost wedging. Canyon walls are often formed of resistant sandstones or granite.
Sometimes large rivers run through canyons as the result of gradual geological uplift. These are called entrenched rivers, because they are unable to easily alter their course. In the United States, the Colorado River in the Southwest and the Snake River in the Northwest are two examples of tectonic uplift.
Canyons often form in areas of limestone rock. As limestone is soluble to a certain extent, cave systems form in the rock. When a cave system collapses, a canyon is left, as in the Mendip Hills in Somerset and Yorkshire Dales in Yorkshire, England.
A box canyon is a small canyon that is generally shorter and narrower than a river canyon, with steep walls on three sides, allowing access and egress only through the mouth of the canyon. Box canyons were frequently used in the western United States as convenient corrals, with their entrances fenced.
The definition of "largest canyon" is imprecise, because a canyon can be large by its depth, its length, or the total area of the canyon system. Also, the inaccessibility of the major canyons in the Himalaya contributes to their not being regarded as candidates for the biggest canyon. The definition of "deepest canyon" is similarly imprecise, especially if one includes mountain canyons, as well as canyons cut through relatively flat plateaus (which have a somewhat well-defined rim elevation).
The Yarlung Tsangpo Grand Canyon (or Tsangpo Canyon), along the Yarlung Tsangpo River in Tibet, China, is regarded by some as the deepest canyon on Earth at 5,500 metres (18,000 ft). It is slightly longer than Grand Canyon in the United States. Others consider the Kali Gandaki Gorge in midwest Nepal to be the deepest canyon, with a 6,400-metre (21,000 ft) difference between the level of the river and the peaks surrounding it.
Vying for the deepest canyon in the Americas is the Cotahuasi Canyon and Colca Canyon, in southern Peru. Both have been measured at over 3,500 metres (11,500 ft) deep.
Grand Canyon of northern Arizona in the United States, with an average depth of 1,600 metres (5,200 ft) and a volume of 4.17 trillion cubic metres (147 trillion cubic feet), is one of the world's largest canyons. It was among the 28 finalists of the New 7 Wonders of Nature worldwide poll. (Some referred to it as one of the seven natural wonders of the world.)
The largest canyon in Europe is Tara River Canyon.
The largest canyon in Africa is the Fish River Canyon in Namibia.
In August 2013, the discovery of Greenland's Grand Canyon was reported, based on the analysis of data from Operation IceBridge. It is located under an ice sheet. At 750 kilometres (470 mi) long, it is believed to be the longest canyon in the world.
Despite not being quite as deep or long as Grand Canyon, the Capertee Valley in Australia is actually 1 km wider than Grand Canyon, making it the widest canyon in the world.
Some canyons have notable cultural significance. Evidence of archaic humans has been discovered in Africa's Olduvai Gorge. In the southwestern United States, canyons are important archeologically because of the many cliff-dwellings built in such areas, largely by the ancient Pueblo people who were their first inhabitants.
The following list contains only the most notable canyons of the world, grouped by region.
Venus has many craters and canyons on its surface. The troughs on the planet are part of a system of canyons that is more than 6,400 km long.