Grand Canyon

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categorie: Engleza

nota: 9.54

nivel: Liceu

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 o[...]
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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.

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III.1 RELIEF

The Colorado Plateau is a flat, dry, semidesert region that covers the northern two-fifths of the state. Many rivers have carved deep canyons into this region, with the most famous example being the Grand Canyon of the Colorado River. The river in the Grand Canyon is as much as 1,500 m (5,000 ft) below the level of the surrounding plateau. Canyon de Chelly and Oak Creek Canyon are also beautiful, but lesser known, canyons carved into the plateau by tributaries of the Colorado River.

About 2 billion years ago this area, now mostly 1,500 to 2,400 m (5,000 to 8,000 ft) above sea level, lay under a vast sea. Through the ages the land emerged and resubmerged repeatedly, and many different rock types, including igneous, sedimentary, and metamorphic, formed. Rivers cut through layers of soil and rock to reach the ancient granites, quartzites, and rocks now revealed. Water and wind have eroded the edges of canyons and the surface of the plateaus, carving isolated, steep-sided, flat-topped hills called mesas. Underlying rocks such as sandstones, shales, and limestones have also been exposed by erosion, creating a kaleidoscope of brilliant colors on canyon and mesa walls.

North of the Grand Canyon is the Kaibab Plateau, which is an area extending from Utah into Arizona. The Kaibab Plateau resembles a peninsula.
The section of the Colorado Plateau located in the northern and northeastern part of the state is a maze of valleys and mesas. Carved knobs, rounded domes, and tall rock spires that pierce broad valleys earned the area its name of Monument Valley. The Painted Desert, where red, yellow, purple, blue, brown, and gray rocks alternate in a vivid display of colors, extends south from the Grand Canyon to the Mogollon Rim. Within the Painted Desert is the Petrified Forest National Park, an area of giant, ancient fallen trees that slowly petrified over thousands of years.

South of the Grand Canyon lies the San Francisco Plateau, which is covered by ancient lava flows and dotted with extinct volcanic cones such as Humphreys Peak, 3,851 m (12,633 ft) tall, the highest point in Arizona. The southeastern part of the Colorado Plateau in Arizona is part of the Datil section, noted for its solidified lava flows and other volcanic features.

The southern border of the Colorado Plateau is distinguished by an extensive volcanic escarpment known as the Mogollon Rim. The Rim, which extends from central Arizona toward the southeast and terminates in the Mogollon Mountains, was originally created by tectonic pressure, uplift of the plateau, and, most important, erosion of the Transition Zone. The steep rock wall reaches about 600 m high in some places. To the south of the Mogollon Rim is a narrow strip of land known as the Transition Zone.

The Transition Zone is characterized by mountain ranges so close together that the area appears as a cluster of rugged peaks separated by steep, narrow valleys. The Mazatzal, Santa Maria, Sierra Ancha, and White mountain ranges are found in this zone, which occupies part of the area once known as the Central Highlands. So uninviting was the landscape that prospectors did not explore the region until the late 19th century. Since then, more than 90 percent of Arizona’s mining activity has taken place in this area.

The Basin and Range region, known to Arizona residents as the Sonoran Desert, occupies most of the southern part of Arizona. It is composed of a series of smooth-floored desert basins separated by mountain ranges that extend from northwest to southeast. Mountains in this region include the Chiricahua, Gila, Pinaleno, Huachuca, Hualapai, Santa Catalina, Santa Rita and Superstition ranges. The portion of the Basin and Range region that lies to the south and west is a low, dry landscape.

Elevations in this area range from as low as 43 m (141 ft) at Yuma to 3,267 m (10,717 ft) atop Mount Graham. While the land has little rain, along the western border of Arizona farms irrigated with waters from the Colorado River produce abundant crops. Most of the loose material on the mountains in this region has been carried down by infrequent but violent cloudbursts to form thick fans of sand and gravel where the steep slopes meet the basin floor. When irrigated, this area produces excellent crops.

Also, the state’s largest cities are located in this region. Most elevations in the Basin and Range region are from 150 to 1,500 m (500 to 5,000 ft), but some mountains rise to more than 3,400 m (11,000 ft). The region is generally higher in the east than in the west. Its desert plains are drained by the lower courses of the Colorado and Gila rivers.

III.2 Rivers and Lakes

The most important river in Arizona is the Colorado, which enters the state in the north, flows southwestward to the state’ western boundary, and then follows the boundary south into Mexico. In the Colorado Plateau, its main course is joined by the Little Colorado, which runs from south to north. Because of the rain-shadow effect of the Mogollon Rim, the Little Colorado draws very little water from a relatively large watershed, usually containing a mere trickle of water in its riverbed.

The Colorado River’s principal tributary is the Gila River, which flows all the way across the southern part of the state from New Mexico to the California border. From the mountains and plateaus of central Arizona, the Gila River receives the Salt, Agua Fria, and Hassayampa rivers. The Salt River is itself fed by the Verde River. The Gila River also is joined by rivers draining the Mogollon Rim and other mountains in the Central Highlands, as well as mountain ranges such as the Sierriata and Santa Catalinas in the southern part of the state. Heavy rainfalls typical in the summer months over the Mogollon Rim drain into the Black, White, and Verde rivers.

In February, March, and April melting snow from the same regions occasionally creates flood conditions along the Salt River and around Phoenix. These three rivers are important tributaries to the Salt River, which is a primary source of water for the highly-populated Phoenix metropolitan and surrounding agrarian areas. The Santa Cruz and San Pedro rivers flow northward out of southern mountain ranges near the Mexican border and feed into the Gila River as well.
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