Which is Earth's deepest canyon depends on how you measure it
Ask for the world's deepest canyon and you get an argument. Some point to Tibet's Yarlung Tsangpo at about 5,500 metres. Others pick Nepal's Kali Gandaki Gorge, where the river runs 6,400 metres below surrounding peaks. Mountain canyons have no clear rim, so even measuring depth becomes a judgment call.
A canyon is a deep cleft between cliffs, carved by a river and by weathering over geological time. Rivers naturally cut downward, carrying sediment away, until their beds approach the level of the water body they drain into. Canyons form where a river's source sits far higher than its mouth, especially where soft rock alternates with harder layers. Most were cut down from a high plateau, and the cliffs stand because resistant strata, often sandstone or granite, survive on the walls while shale and other weak rock wear away.
They are far more common in dry regions, where physical weathering acts in a more concentrated way. Wind and river water attack softer beds, and frost wedging helps too: water seeps into cracks, freezes, expands and levers off slabs of wall. Some big rivers, including the Colorado and the Snake, became entrenched as the land slowly rose around them, locking them into their courses. In limestone country, collapsing cave systems can leave canyons behind, as in England's Mendip Hills and Yorkshire Dales.
The forms vary. Box canyons are short and narrow, with steep walls on three sides, which made them handy natural corrals in the American West once the entrance was fenced. Slot canyons are extremely narrow and often smooth-walled. Submarine canyons cut into the continental slope are thought to be carved by turbidity currents and landslides rather than rivers.
The name is Spanish, from cañón. In the United States, place names lean towards canyon in the southwest near Mexico and gorge in the northeast near French Canada, shading between the two elsewhere. South Africa uses kloof alongside both, and Britain occasionally says defile. Size rankings stay fuzzy because largest could mean depth, length or area, and Himalayan canyons are so remote they are often left out. In the Americas, Peru's Colca and Cotahuasi canyons both exceed 3,500 metres.
Source: Canyon
