Canyon

I

Introduction

Canyon, deep, cliff-sided chasm or gorge created by the erosive action of a river. Canyons most often occur in semiarid or arid regions where river erosion deepens the canyon faster than weathering (the breakdown of rocks) of the sides can widen it. The deepest and steepest canyons develop in regions with flat-lying, alternating beds of hard and soft rock. In such circumstances, the sides of the canyon resemble a staircase, with the layers of harder rocks forming cliffs and the layers of softer rock forming gentler slopes.

Canyons are particularly abundant across large areas of Arizona and Utah in the southwestern United States, where there is an optimal combination of climate and rock structure. The largest example in this region is the Grand Canyon of the Colorado River, which is 446 km (277 mi) long, more than 1,500 m (5,000 ft) deep, but only 29 km (18 mi) across from rim to rim at its widest. Canyons are also found in other dry regions around the world. One of the largest examples is the Fish River Canyon in Namibia, which extends for 160 km (100 mi). This canyon is 25 km (16 mi) wide and almost 500 m (1,600 ft) deep in places.

II

Box Canyons

Some canyons, called box canyons, start abruptly with a spring emanating from the base of a cliff at the canyon head. These canyons are called box canyons because, when traveling up the canyon floor to the canyon head, one becomes suddenly boxed in on all sides by vertical walls. Box canyons form where a flat-lying layer of permeable rock lies on top of an impermeable one. Permeable rocks allow rainwater to seep through them, while impermeable rocks do not. Rainfall seeps down through the permeable layer until it reaches the impermeable layer. The water then seeps horizontally through the bottom of the permeable layer until it emerges at the canyon head to form a spring. The flow of water enhances weathering and weakening of the rock, which leads to undermining and collapse of the rock wall at the head of the canyon, and the canyon is extended headward. Box canyons are particularly numerous across Arizona and Utah.

Features very similar to box canyons, deep valleys with steep walls and an abrupt start, have been discovered on Mars and appear to have formed when huge quantities of groundwater were released from springs at the bottom of cliffs at the heads of these features. However, water no longer flows along the bottom of these canyons.

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III

Submarine Canyons

The term canyon is also sometimes applied to steep-walled valleys that are formed by mechanisms other than river erosion. Submarine canyons are steep-walled valleys on the ocean floor. Scientists believe turbidity currents (flows of water mixed with sediment) scour out a canyon as they occasionally plunge down it.

IV

Valles Marineris

The largest known canyon in the solar system is the Valles Marineris on Mars. It is nine times longer and five times deeper than the Grand Canyon of the Colorado River. Although planetary scientists have found evidence that water once flowed along portions of the canyon, they believe that the canyon was formed primarily through movements of the Martian crust.

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