A look at how the Grand Canyon in Arizona was believed to have been formed by the slow draining of an ancient lake which formed the Colorado River and carved the canyon out over millions of years. The Grand Canyon (Hopi: Ongtupqa; Yavapai: Wi:kaʼi:la) is a steep-sided canyon carved by the Colorado River in the United States in the state of Arizona. It is contained within and managed by Grand Canyon National Park, the Hualapai Tribal Nation, and the Havasupai Tribe. President Theodore Roosevelt was a major proponent of preservation of the Grand Canyon area, and visited it on numerous occasions to hunt and enjoy the scenery. It is considered one of the Seven Natural Wonders of the World. The Grand Canyon is 277 miles (446 km) long, up to 18 miles (29 km) wide and attains a depth of over a mile (6,000 feet or 1,800 meters). Nearly two billion years of the Earth's geological history has been exposed as the Colorado River and its tributaries cut their channels through layer after layer of rock while the Colorado Plateau was uplifted. While the specific geologic processes and timing that formed the Grand Canyon are the subject of debate by geologists, recent evidence suggests the Colorado River established its course through the canyon at least 17 million years ago. Since that time, the Colorado River continued to erode and form the canyon to its present-day configuration. For thousands of years, the area has been continuously inhabited by Native Americans who built settlements within the canyon and its many caves. The Pueblo people considered the Grand Canyon ("Ongtupqa" in Hopi language) a holy site and made pilgrimages to it. The first European known to have viewed the Grand Canyon was García López de Cárdenas from Spain, who arrived in 1540. The Grand Canyon is part of the Colorado River basin which has developed over the past 40 million years. A recent study places the origins of the canyon beginning about 17M years ago. Previous estimates had placed the age of the canyon at 5--6 million years. The study, which was published in the journal Science in 2008, used uranium-lead dating to analyze calcite deposits found on the walls of nine caves throughout the canyon. There is a substantial amount of controversy because this research suggests such a substantial departure from prior widely supported scientific consensus. In December 2012, a study published in the journal Science claimed new tests had suggested the Grand Canyon could be as old as 70M years. However, this study has been criticized as "[an] attempt to push the interpretation of their new data to their limits without consideration of the whole range of other geologic data sets." The result of all this erosion is one of the most complete geologic columns on the planet. The major geologic exposures in the Grand Canyon range in age from the 2-billion-year-old Vishnu Schist at the bottom of the Inner Gorge to the 230M-year-old Kaibab Limestone on the Rim. There is a gap of about a billion years between the 500M-year-old stratum and the level below it, which dates to about 1.5 billion years ago. This large unconformity indicates a period of erosion between two periods of deposition. 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 Permian Coconino Sandstone, which contains abundant geological evidence of aeolian sand dune deposition. Several parts of the Supai Group also were deposited in non--marine environments. 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 5--10 thousand feet (1,500 to 3,000 m) of uplift of the Colorado Plateau, starting about 65M 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). Weather conditions during the 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.

• published: 12 Dec 2013
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