Grand Canyon

The Grand Canyon is a colorful, steep-sided gorge, carved by the Colorado River, in northern Arizona. The canyon is one of the seven natural wonders of the world and is largely contained in the Grand Canyon National Park — one of the first national parks in the United States. President Theodore Roosevelt was a major proponent of the Grand Canyon area, visiting on numerous occasions to hunt mountain lions and enjoy the breathtaking scenery.

The canyon, created by the Colorado River cutting a channel over millions of years, is about 277 miles (446 km) long, ranges in width from 4 to 18 miles (6 to 29 kilometers) and attains a depth of more than a mile (1,600 m). Nearly 2000 million years of the Earth's history has been exposed as the Colorado River and its tributaries cut through layer after layer of sediment as the Colorado Plateaus have uplifted.

The Grand Canyon was first seen by a European in 1540, García López de Cárdenas from Spain. The first scientific expedition to the canyon was led by U.S. Major John Wesely Powell in the late 1870s. Powell referred to the sedimentary rock units exposed in the canyon as "leaves in a great story book." However, long before that, the area was inhabited by Native Americans who built settlements in the canyon walls.

Geography

The Grand Canyon is a very deep - in places even a mile deep - 277 mile (446 km) long cut in the Colorado Plateaus that exposes uplifted Proteozoic and Paleozoic strata. The exposed strata are gradually revealed by the gentle incline beginning at Lee's Ferry and continuing to Phantom Ranch. At the point where the river crosses the Grand Wash Fault (near Lake Mead) the Canyon ends.

Uplift associated with plate tectonics-caused mountain building events later moved these sediments thousands of feet upward and created the Colorado Plateaus. The higher elevation has also resulted in greater precipitation in the Colorado River drainage area, but not enough to change the Grand Canyon area from being semi-arid. Landslides and other mass wasting events then caused headward erosion and stream capture - all of which tend to increase the depth and width of canyons in arid environments.

The uplift of the Colorado Plateaus is uneven, resulting in the North Rim of the Grand Canyon being over a thousand feet (about 300 meters) higher than the South Rim. The fact that the Colorado River flows closer to the South Rim is also explained by this asymmetrical uplift. Almost all runoff from the plateau behind the North Rim (which also gets more rain and snow) flows toward the Grand Canyon, while much of the runoff on the plateau behind the South Rim flows away from the canyon (following the general tilt). The result is much greater erosion and thus faster widening of the canyon and its tributary canyons north of the Colorado River.

Temperatures on the North Rim are generally lower than the South Rim because of the greater elevation (8000 feet/2438 m above sea level). Heavy snowfall is common during the winter months. Views from the North Rim tend to give a better impression of the expanse of the canyon rather than the views down which characterize the South Rim.

The Havasupai Indian Reservation is in a large tributary canyon on the south side of the Colorado River; it is administered by the Havasupai Indian Tribe

Geology

The major sedimentary rock units exposed in the Grand Canyon range in age from the 2000 million year old Vishnu Schist at the bottom of the Inner Gorge to the 230 million year old Kaibab Limestone on the Rim. Most of the formations were deposited in warm shallow seas, near-shore environments (such as beaches), and swamps as the seashore repeatedly advanced and retreated over the edge of a proto-North America. The major exception being the Cococino Sandstone which was laid down as sand dunes in a desert.

The great depth of the Grand Canyon and especially the height of its strata (most of which formed below sea level) can be attributed to 5000 to 10,000 feet (1500 to 3000 m) of uplift of the Colorado Plateaus starting about 65 million years ago (which occurred in a series of uplift events rather than a continuous rise). This uplift has steepened the stream gradient of the Colorado River and its tributaries, which in turn has increased their speed and thus their ability to cut through rock.

The Colorado River drainage (of which the Grand Canyon is a part) has developed in the past 40 million years and the Grand Canyon itself is probably less than five to six million years old (with most of the downcutting occurring in the last two million years). The result of all this erosion is one of the most complete geologic columns on the planet. The river is still actively cutting deeper and is thus exposing older and older rock.

Wetter conditions during ice ages also increased the amount of water in the Colorado River drainage system. The ancestral Colorado River responded by cutting its channel faster and deeper.

Then the base level and course of the Colorado River (or its ancestral equivalent) changed 5.3 million years ago when the Gulf of California opened and lowered the river's base level (its lowest point). This increased the rate of erosion and cut nearly all of the Grand Canyon's current depth by 1.2 million years ago. The terraced walls of the canyon were created by differential erosion.

A million years ago volcanic activity (mostly near the western canyon area) deposited ash and lava over the area which at times even dammed the Colorado. These are the youngest rocks in the park.

With its current stream gradient, the Colorado River should cut another 1200 to 2000 feet (370 to 600 meters) into the rock of the canyon before reaching its base level.

History

Little is known about the people who lived in western North America between 9,000 and 3,000 years ago. The earliest signs of human life in the Grand Canyon, radiocarbon dated to older than 3,000 years, belong to them - tiny willow twig representations of animals, a few of which were pierced with tiny twig spears. The desert culture were hunters and gatherers and made baskets and sandals, and hunted with stone-tipped spears. The first Europeans to find evidence of their activities were the 1934 Frazier, Eddy and Hatch expedition. From Wikipedia