Grand Canyon: A natural wonder

If you have heard about the seven natural wonders of the world, you would definitely know what the Grand Canyon is. The Grand Canyon is a very colourful gorge (narrow, steep-sided valley), carved by the Colorado River, in northern Arizona, USA.

It is largely contained within the Grand Canyon National Park - one of the first national parks in the United States. 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 15 miles (0.5 to 29 kilometres) 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, and as the Colorado Plateaus have uplifted (become raised).

The Grand Canyon is a very deep - in places, over a mile deep - cut in the Colorado Plateaus that exposes uplifted Proterozoic and Paleozoic (belonging to ancient eras) strata (layers). 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, the Canyon ends.

Uplift associated with mountain building caused by plate tectonics, later moved these sediments thousands of feet upward and created the Colorado Plateaus. The higher elevation has also resulted in greater precipitation (water) in the Colorado River drainage area, but not enough to change the Grand Canyon area from being semi-arid. Landslides and other such events then caused headward erosion, which tend to increase the depth and width of canyons in arid (harsh) 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 metres) higher than the South rim. The fact that the Colorado River flows closer to the South rim is also explained by this asymmetrical (two sides are different) 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 metres 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 (extent) of the Canyon, rather than the views from the South Rim.

The major geologic exposures in Grand Canyon range in age from the two billion year old Vishnu Schist, the oldest geological formation exposed at the bottom of the Inner Gorge, to the 230 million year old Kaibab Limestone (a layer composed primarily of a sandy limestone with a layer of sandstone below it) 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 early 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 (composed mainly of shale/a type of stone) that is intermixed with some small amounts of limestone and capped by sandstone.

The colour of this layer varies from red for the shale, to tan for the sandstone caps.

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 (slope) 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 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 original 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. The Grand Canyon was declared a national monument by the U.S. government in 1908.