The Grand Canyon & the Colorado River, Arizona
June 21, 2003: Take Out, Las Vegas, Sprawl
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Rafting the Colorado is a fun and exciting way to see one the world's most spectacular natural formations, the Grand Canyon.
Grand Canyon National Park, a World Heritage Site, encompasses 1,218,375 acres and lies on the Colorado Plateau in northwestern Arizona. The land is semi-arid and consists of raised plateaus and structural basins typical of the southwestern United States. Drainage systems have cut deeply through the rock, forming numerous steep-walled canyons. Forests are found at higher elevations while the lower elevations are comprised of a series of desert basins. [National Park Service Excerpt]
Recommended travel guides on the Grand Canyon:
Grand Canyon Geology [Wikitravel]:
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 .