Trimis la data: 2007-05-19
Nota: 7.37 / 10
Autor: Moise Daniel Adrian
Tipul fisierelor: doc
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Grand Canyon is an exceptionally deep, steep-walled canyon in northwestern Arizona, excavated by the Colorado River. The Grand Canyon is 446 km (277 mi) long, up to 29 km (18 mi) wide, and more than 1500 m (5000 ft) 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 of it, are preserved as the Grand Canyon National Park, which receives about four million visitors a year.
The plateau area has a general downward slope to the southwest 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, but seldom experiences frost in the wintertime.
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 (about 1200 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 southwestern 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 of relatively recent origin; apparently the river began its work of erosion about six 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, toward its eastern end, nine separate rock layers can be seen, piled vertically like a stack of pancakes. 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 (290 million to 240 million years ago) at the end of the Paleozoic Era. However, the Supai may be slightly older still. Next comes a deposit of light gray-blue limestone, the Redwall limestone, which in many places has been colored 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 (360 million to 330 million years ago). A thin layer of sandstone, the Temple Butte, beneath the Redwall, gives evidence of having originated in the Devonian Period (410 million to 360 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 (570 million to 500 million years ago), at the dawn of the Paleozoic Era.
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 members of a group 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 rowboats.
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 liters (100 billion gallons) of water from Glen Canyon Dam. This artificial flood added more than three feet to some beaches and cleared fish spawning grounds of debris and sediment.
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