The Grand Canyon is a massive rift in the Colorado Plateau that exposes uplifted Proterozoic and Paleozoic strata and is also one of the six distinct physiographic sections of the Colorado Plateau province. The Grand Canyon is unmatched throughout the world for the vistas it offers to visitors on the rim. It is not the deepest canyon in the world – both the Cotahuasi Canyon (11598 feet or 3535 m), Colca Canyon (10499 feet or 3200 m), both in Arequipa, Peru and Hell’s Canyon on the Oregon-Idaho border are deeper – but Grand Canyon is known for its overwhelming size and its intricate and colorful landscape.
Geologically it is significant because of the thick sequence of ancient rocks that are beautifully preserved and exposed in the walls of the canyon. These rock layers record much of the early geologic history of the North American continent.Uplift associated with mountain building events later moved these sediments thousands of feet upward and created the Colorado Plateau. 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.
The uplift of the Colorado Plateau is uneven, and the north-south trending Kaibab Plateau that Grand Canyon bisects is over a thousand feet higher at the North Rim (about 1,000 ft/300 m) than at the South Rim. The fact that the Colorado River flows in a curve around the higher North Rim part of the Kaibab Plateau and closer to the South Rim part of the plateau is also explained by this asymmetry.
Ivo Lucchitta of the U.S. Geological Survey first suggested that, as the Colorado River developed before significant erosion of the region, it naturally found its way across or around the Kaibab Uplift by following a “racetrack” path to the south of the highest part of the plateau. Almost all runoff from 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 deeper and longer tributary washes and canyons on the north side and shorter and steeper side canyons on the south side.
Temperatures on the North Rim are generally lower than the South Rim because of the greater elevation (averaging 8,000 ft/2,438 m above sea level). Heavy rains are common on both rims during the summer months. Access to the North Rim via the primary route leading to the canyon (Arizona State Route 67) is limited during the winter season due to road closures. Views from the North Rim tend to give a better impression of the expanse of the canyon than those from the South Rim.
The principal consensus among geologists is that the Colorado River basin (of which the Grand Canyon is a part) has developed in the past 40 million years and that 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 major geologic exposures in Grand Canyon range in age from the 2 billion year old Vishnu Schist at the bottom of the Inner Gorge to the 230 million year old Kaibab Limestone on the Rim. Interestingly, there is a gap of about one billion years between the stratum that is about 500 million years old and the lower level, which is about 1.5 billion years old. That 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 most (though not all) geologists claim was laid down as sand dunes in a desert, and several parts of the Supai Group.
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 5,000 to 10,000 feet (1500 to 3000 m) of uplift of the Colorado Plateau, 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).
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.
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.
About one 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.