Grand Canyon

Grand Canyon, exceptionally deep, steep-walled canyon in north-western Arizona, United States. Formed by the eroding action of the Colorado River, the Grand Canyon is about 443 km (227 mi) long, 8 to 29 km (5 to 18 mi) wide, and more than 1.6 km (1 mi) deep. The entire canyon is extremely beautiful, containing towering buttes, mesas, and valleys within its main gorge. A spectacular section of the canyon, together with plateau areas on either side, are preserved as the Grand Canyon National Park, which receives about 5 million visitors a year.

The Grand Canyon cuts steeply through an arid plateau region that lies between about 1,525 and 2,745 m (5,000 and 9,000 ft) above sea level. This region, although lacking year-round streams in recent years, is sharply eroded, showing such characteristic forms as buttes; it is interspersed with old lava flows, hills composed of volcanic debris, and intrusions of igneous rock. The plateau area has a general downward slope to the south-west and in its upper reaches is sparsely covered with such evergreens as juniper and piñon. Parts of the northern rim of the canyon are forested. Vegetation in the depths of the valley consists principally of such desert plants as agave and Spanish bayonet. In general the entire canyon area has little soil. The climate of the plateau region above the canyon is severe, with extremes of both heat and cold. The canyon floor also becomes extremely hot in summer, sometimes reaching 46°C (115°), but seldom experiences frost in the winter.

The Grand Canyon has been sculpted in general by the downward cutting of the Colorado River, which flows through the canyon's lowest portions. Other factors have also played a part. The Kaibab Plateau, which forms the northern rim of the canyon, is about 365 m (1,200 ft) higher than the Coconino Plateau, which forms the southern rim. Water from the northern side has flowed into the canyon, forming tributary valleys, while the streams of the southern plateau flow away in a southerly direction without carving valleys in the canyon walls. The underlying rock beds also have a south-western slant, with the result that groundwater from the north finds its way into the canyon, but water from the south does not. In the entire canyon region, the rocks have been broken by jointing and faulting, and fractures in the rocks resulting from these processes have contributed to the rapid erosion of the gorge.

The Grand Canyon is relatively young in geological terms; the river began its work of erosion about 10 million years ago. Coupled with the downward cutting of the river has been a general rising or upwarping of the Colorado Plateau, which has added its effect to the action of the river.

Although the canyon itself is of comparatively recent origin, the rocks exposed in its walls are not. Most of the strata were originally deposited as marine sediment, indicating that for long periods of time the canyon area was the floor of a shallow sea.

In a typical section of the canyon, towards its eastern end, nine separate rock layers can be seen, stacked vertically. The topmost layer is a limestone, the Kaibab limestone. Below this layer is a thick deposit of sandstone, called the Coconino sandstone, and below that a layer of soft, shaly rock known as the Hermit shale. Still lower is a series of shales and sandstones interbedded with each other, collectively termed the Supai formation. The fossils found in the Supai and the rocks above it suggest that these rocks were all deposited in the Permian Period, at the end of the Palaeozoic Era, from 225 million to 280 million years ago. However, the Supai may be slightly older still. Next comes a deposit of light grey-blue limestone, the Redwall limestone, which in many places has been coloured red by seepage from the Supai beds above. The Redwall is 152 m (500 ft) thick and is easily identified because of the prominent sheer cliffs that it forms in the canyon walls. This layer has been identified as belonging to the Mississippian Period and was laid down more than 280 million years ago. A thin layer of sandstone, the Temple Butte, beneath the Redwall, gives evidence of having originated in the Devonian Period, about 345 million to 395 million years ago. The next three rock layers, consisting of the brown Muav limestone, the green Bright Angel shale, and the Tapeats sandstone, all belong to the Cambrian Period, at the dawn of the Palaeozoic Era, from 500 million to 570 million years ago. Beneath these layers, at the bottom of the canyon, are the most ancient rocks of all, Precambrian schists and gneisses, from half a billion to a billion years old.

The first Europeans to see the canyon were a group of soldiers led by Garcia Lopez de Cordeñas. They were members of a party headed by the Spanish explorer Francisco Vásquez de Coronado, which set out from New Spain (now Mexico) in February 1540. The sighting was made later that year. Because of the inaccessibility of the canyon, it was not until more than three centuries later that it was fully explored. Beginning about 1850, a series of expeditions commanded by officers of the United States Army surveyed the canyon and the surrounding area. The first passage of the canyon was accomplished in 1869 by the American geologist John Wesley Powell and ten companions, who made the difficult journey through the length of the gorge in four boats. The construction of the Glen Canyon Dam in Arizona in 1963 dramatically reduced the natural flow of sand and nutrients down the Colorado River and into the Grand Canyon. In March 1996 the federal government released more than 380 billion litres (100 billion gallons) of water from Glen Canyon Dam. This artificial flood added around 1 m (3 ft) to some beaches and cleared fish spawning grounds of debris and sediment. A similar process was carried out in November 2004, which was designed to re-establish beaches, backwaters, and sandbars along the canyon—the valuable habitats of many species of fish, birds, snails, and plants that are threatened with extinction.