Grand Canyon

Geology

2006-05-07T00:02:00.000-07:00

The details of the canyon's formation are still highly controversial. Geologists continue to debate ideas about the formation of Grand Canyon. According to Geologist Wayne Ranney: "To date, geologists have been unable to determine the canyon's precise age and what specific processes were at work in carving it". There is no authoritative theory on the formation of Grand Canyon.
The major geologic exposures in 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. Many 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. Major exceptions include the Cococino Sandstone which was laid down as sand dunes in a desert and several parts of the Supai formation.
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 (during the Laramide Orogeny). 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 (see the elevation summary of the Colorado River for present conditions).
The Colorado River basin (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.
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 completely obstructed the river. These volcanic rocks are the youngest in the canyon.

Geography

2006-05-06T23:54:00.000-07:00

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

Grand Canyon

2006-05-06T23:46:00.000-07:00

The Grand Canyon is a very colorful, steep-sided gorge, carved by the Colorado River, in northern Arizona, USA. The canyon appears on many versions of the Seven Natural Wonders of the World list, although none of these lists are by any means authoritative. It is largely contained within 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 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 0.25 to 18 miles (0.5 to 29 kilometers) and attains a depth of more than a mile (1,600 m). Nearly two billion 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 first recorded sighting of the Grand Canyon by a European was in 1540, García López de Cárdenas from Spain. The first scientific expedition to the canyon was led by U.S. Major John Wesley 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 within the canyon walls.