Grand Canyon

The Colorado River's Grand Canyon is considered the Earth's most extensive erosional canyon. Besides its awe-inspiring appearance, the Grand Canyon allows geologists to investigate the uppermost portions of the Earth's crust and examine the interrelationship of igneous, metamorphic, and sedimentary rocks exposed there.

Principal Terms

diagenesis: the alteration of sediment at its initial deposition, which takes place during and after its transformation into sedimentary rock

dip: the angle between a structural feature (for example, a joint, fault, or bedding plane) and the horizontal, which is usually the Earth's surface

dolomite: a mineral consisting of calcium and magnesium carbonate compounds that often form from precipitation from seawater

eolian: related to wind deposits or environments

fault: a fracture in rock along which an appreciable amount of displacement has occurred

stratigraphy: the study of sedimentary strata, which includes the concept of time, possible correlation of the rock units, and characteristics of the rocks themselves

unconformity: a significant break in a stratigraphic sequence because of nondeposition or erosion of the missing rock layers; much younger rocks are positioned above older rocks

Discovery and Physical Characteristics

The Grand Canyon of the Colorado River (henceforth referred to as the Grand Canyon) is considered the most extensive and impressive erosional canyon on Earth. With an average depth of over 1,675 meters below the North Rim (1,375 meters below the South Rim) and 350 kilometers long, the Grand Canyon is readily identified even from space. Images from the Landsat Earth resource satellites clearly show the enormous extent of the canyon as it winds south and then west across northern Arizona. This view is very different from that experienced in 1540 by a lieutenant of the famous Spanish explorer Francisco Vásquez de Coronado. Don Garcia Lopez de Cardenas, with the help of Hopi guides, was the first non-American Indian to see the canyon as he led a group of Spanish explorers through what is now the southwestern United States. Before that expedition, the surrounding area and parts of the canyon itself had been home to various American Indian peoples for thousands of years. Eleven federally recognized Nations are associated with the area, and parts of the canyon remain within the Navajo Nation, the Havasupai Indian Reservation, and the Hualapai Indian Reservation.

The South Rim is the best-known portion of the more than 486,000 hectares surrounding the canyon, included in Grand Canyon National Park. The central area of the Grand Canyon was initially designated as a national monument in 1908 by President Theodore Roosevelt to protect it from mining claims. In 1919, Congress proclaimed the area a national park. Several other regions in northern Arizona near the original park boundaries were named as national monuments in 1932 and 1969. Finally, in 1975, the National Park Service reorganized and consolidated these monuments to increase the total area of Grand Canyon National Park. The northernmost extent of the park is at the confluence of the Paria River and the Colorado River near Lees Ferry, Arizona. The Colorado River flows south and then west for 459 kilometers through the park until it empties into Lake Mead. From the confluence of the Little Colorado River, the distance of the Colorado River in the park is 350 kilometers. This is the segment of the park that bears the name Grand Canyon. The park's western edge is near the Grand Wash Cliffs near the Arizona-Nevada border. In this area, Grand Canyon National Park is contiguous with Lake Mead National Recreational Area, which extends from southern Nevada into northwestern Arizona.

Until 1966, the Colorado flowed unimpeded through the canyon. Once Glen Canyon Dam was completed, the natural balance of river flow and seasonal floods was upset. The river's flow became a function of the water supply and electrical power needs of the Southwest. The transformation of the Colorado River for large-scale human use has prevented the river from reaching the sea for many years, though restoration efforts have been implemented and environmental groups have blocked the construction of further dams. Despite protection as a national park, the Grand Canyon continues to face threats. Development projects threaten to impinge upon the rim of the canyon itself; such construction would also exacerbate existing pollution issues in the Grand Canyon. Experts have also voiced concerns over the effects of global warming and other forms of climate change on the canyon environment.

The distance separating the rims of the Grand Canyon on either side of the Colorado River varies from approximately 150 meters in Marble Canyon south of Lees Ferry to 29 kilometers at the downstream portion near Lake Mead. The South Rim is separated from the North Rim by 15 kilometers. It is a drive of 344 kilometers, however, to travel from one rim to the other.

As one looks from the South Rim toward the North Rim, it is evident that the North Rim is higher. The average elevation of the North Rim is more than 2,440 meters, approximately 300 meters above that of the South Rim. A series of uplifted plateaus accounts for this higher elevation. The largest plateau is the Kaibab Plateau, a north-south trending uplifted block that extends westward along the Colorado River from the confluence of the Little Colorado River to Kanab Creek, a major tributary draining the smaller Kanab Plateau to the west. Although the North and South Rims both lie on the Colorado Plateau, the fauna and flora of each area are quite different because of the elevation differences. Numerous hiking trails descend from either rim into the inner canyon. Four biological life zones are traversed in making the rim-to-rim hike.

The Grand Canyon is situated near the southwestern edge of the Colorado Plateau, a major physiographic province in the western United States, which is characterized by vast amounts of nearly horizontal sedimentary rocks. Although sedimentary rocks are usually deposited in a horizontal manner, it is puzzling to find massive amounts of horizontal strata in this major uplifted area. Geologists expect sedimentary layers to display some significant amount of dip when they are associated with major uplift. Sedimentary units that exist on both the North and South Rims display very little dip. Dip values average about 1 or 2 degrees toward the southwest. As a result, only streams located on the North Rim flow directly into the Colorado River as it passes through the canyon. Drainage on the Coconino Plateau on the South Rim is to the south into the Salt River system of southern Arizona. In the eastern section of the park, the Colorado River drops from an elevation of 852 meters to 544 meters. This vertical distance of 308 meters occurs over a horizontal distance of 169 kilometers. Thus, the gradient or downstream slope of the Colorado River in this area is 1.8 meters per kilometer.

Faults and Monoclines

There are many faults present in the Grand Canyon. The amount of vertical displacement for these faults ranges up to 300 meters. The majority of these faults have either a northeast or a northwest trend. Perhaps the most visibly evident fault is the Bright Angel fault, which cuts through the Kaibab Plateau. This fault has produced a major zone of weakness that serves as the channel for Bright Angel Creek, a tributary of the Colorado River draining the North Rim. The Kaibab Trail parallels the creek, giving hikers a way of getting from the Colorado River up to the North Rim. Bright Angel fault extends to the South Rim, where it plunges under the surface. Sufficient faulting and mass movement have occurred to produce broken and slumped material. The Bright Angel Trail descends from the South Rim over this broken, slumped material and heads out across the Tonto Platform down to the river.

Numerous structural features called monoclines pass through this part of northern Arizona. These rather simple flexures bend a series of originally horizontal rock layers. Several more prominent monoclines were likely formed by the reactivation of Precambrian faults later buried by sedimentary strata. Movement along the faults propagated upward and produced a bend in the sedimentary strata overlying the fault. Among the more prominent monoclines in the Grand Canyon are the East Kaibab along the park's eastern border, Grandview-Phantom in the middle portion, and the West Kaibab farther to the west. These monoclines trend basically north-south or northwest-southeast. Because of the flexural nature of these large-scale folds, they are sometimes subtler and, hence, more difficult to recognize than some of the major faults in the area.

Geological Formations

The superb exposures of rocks and geomorphic features in the Grand Canyon invite all types of geologists to examine them to further their understanding of the Earth's history. Over the past century, geologists have conducted countless hours of fieldwork, research, and interpretation in their quest to understand what the Grand Canyon has to offer both scientists and casual viewers.

The exposed geology within the canyon changes from place to place. In the easternmost part, the rocks are generally younger. Downriver from Lees Ferry, the sheer cliffs of the Redwall limestone (formed during the Mississippian 330 million years ago) and the overlying Supai group (formed 300 million years ago) are exposed along the Colorado. Farther downstream, the river cuts deeper into the rocks, exposing progressively older rocks. In the central portion of the canyon, rock members of the Vishnu group begin to crop out along the riverbanks. These metavolcanics and metasediments (igneous and sedimentary rocks that underwent metamorphism after being formed) are about 1.7 billion years old. They represent the oldest rocks exposed in the Grand Canyon. Rocks found in the Grand Canyon represent a period of the Earth's history ranging from approximately 1.7 billion years ago to about 200 million years ago. Within this time span are several important missing periods. These unconformities represent either periods of nondeposition of the sediments or periods when rocks that were deposited were later eroded, thus removing their evidence from the geologic record.

Older Precambrian rocks compose the portion along the riverbanks in the central part of the Grand Canyon. Farther west, these rocks form the Inner Gorge. The Vishnu group consists of volcanic debris and sediments that were deposited and later metamorphosed to form the Vishnu schist. These rocks are injected with quartz veins and dikes, along with younger granitic bodies, and are the erosional remnants of a Proterozoic mountain range. The Vishnu group forms the foundation upon which all the other rocks in the Grand Canyon rest.

Overlying these rocks is the Grand Canyon supergroup, a very thick sequence of sedimentary rocks and interbedded lava flows. Sedimentary rock types are varied, ranging from siltstone and shales to sandstones. Some portions of the supergroup include metamorphic and igneous rocks. Members of this supergroup include the Bass limestone, Hakatai shale, Shinumo quartzite, Dox sandstone, and Cardenas lavas. The Nankoweap formation and the Chuar group are found in portions of the eastern Grand Canyon, which are generally inaccessible to most people.

Three distinct geological formations are representative of the Cambrian period (542 to 488 million years ago): the Tapeats sandstone, the Bright Angel shale, and the Muav limestone. These three formations constitute the Tonto group. During the early Paleozoic, which began 542 million years ago, northern Arizona was covered by a shallow sea lying to the west. The onset or transgression of this sea, along with migrating coastal sand dunes, produced the first relatively horizontal layer in the lowermost reaches of the canyon—the Tapeats sandstone. This unit ranges in thickness from 30 to 90 meters and consists of medium-to coarse-grained sandstone ranging from gray to chocolate brown. The Tapeats sandstone unconformably overlies the Grand Canyon supergroup and is readily recognized from a distance as the first prominent cliff former at the base of the canyon. The Tapeats essentially forms the boundary between the Inner Gorge and the Outer Canyon, two major topographic divisions within the Grand Canyon. The Inner Gorge, about 365 meters deep, is a narrow, V-shaped channel cut into the older Proterozoic rocks (primarily the Vishnu schist and Zoroaster granite). The lower reaches of the Outer Canyon are delineated by the Tonto Platform, an extensive bench that is held up by the Tapeats sandstone. The Outer Canyon is roughly 1,220 meters deep and contains all the Paleozoic rocks exposed in the Grand Canyon.

Conformably overlying the Tapeats is the Bright Angel shale, the second formation associated with the Paleozoic era. This formation is a distinct slope former, ranging in thickness from 60 to 135 meters. The Bright Angel shale, a mixture of mudstones and fine-grained sandstones, is dominated by a shaly, green mudstone. There are occasional breaks in the slope as low cliffs of more resistant sandstone and siltstones crop out. The rocks have a variegated color in different outcrops within the same canyon. The Bright Angel shale is one of the most fossil-rich units in the canyon, containing numerous traces left by soft-bodied organisms that were not themselves fossilized. The finer-grained rocks of this unit were deposited in quiet, deeper water as the sea that produced the Tapeats sandstone transgressed or moved across the area.

The Muav limestone, a ledge and cliff former composed primarily of mottled gray limestone, overlies the Bright Angel shale. The Muav forms a series of resistant cliffs throughout its 45-to 240-meter thickness. It should be noted that limestones tend to form cliffs and ledges in arid climates. This condition results from the lack of chemical weathering associated with copious rainfall, which is slightly acidic and attacks the calcite present in the limestone. Intermittent recesses on the surface result from relatively thin layers of mudstones and siltstones that have differentially weathered out. The Muav becomes progressively thicker to the west because of the deeper water of the transgressive sea. The occurrence of dolomite in the Muav points to stages of the sea's regression.

Above the Muav limestone is an erosional surface representing the loss of about 160 million years of geologic history (spanning the Late Cambrian to the Early or Middle Devonian). This major unconformity produces no discernible interruption in the sequence of horizontally layered strata. Some deposition probably took place, but the rocks that formed were later eroded. Above this unconformity lies the Temple Butte limestone, a prominent cliff former of the Late Devonian age (385 million years ago). In the eastern Grand Canyon, this unit consists of intermittent outcrops of channel fill. Two types of dolomite are found in the Temple Butte: The first is a light-colored, thin-bedded dolomite having a porcelain-like texture; the second is sandier and finer-grained. The unit was deposited as a limestone but later underwent diagenesis and was converted into dolomite. The Temple Butte thickens considerably to the west, with outcrops ranging up to 125 meters in thickness. Very few invertebrate fossils are found in the Temple Butte.

Widespread erosion of the Temple Butte limestone took place throughout the region. Above the Temple Butte is an erosional surface that is difficult to recognize in most localities within the canyon. Overlying this erosional surface is the Redwall limestone, certainly one of the most conspicuous layers of rock within the Grand Canyon. It forms a sheer cliff more than 150 meters in height. Four members constitute the Redwall limestone: the lowermost Whitmore Wash, the Thunder Springs, the Mooney Falls (thickest of the four), and the uppermost Horseshoe Mesa. All four members are present throughout the canyon. The Redwall limestone is so named because its outer surface is stained by iron oxides that have washed down from the overlying Supai group rocks. For hikers who traverse the many trails in the canyon, their confrontation with the Redwall produces the most difficult portion of their hike. Because of the sheerness of the Redwall, trails often must follow slopes produced by the slumping and sliding of the overlying Supai group or Hermit shale. The Redwall limestone is the product of precipitation of limestone and other carbonates from a warm, shallow sea that transgressed and regressed three times during the Early to Middle Mississippian (330 million years ago). The unit is rich in fossils, such as bryozoans, brachiopods, and crinoids, all indicative of a sea that did not receive much sediment from adjacent continental areas.

The Surprise Canyon formation rests unconformably on the Horseshoe Mesa member of the Redwall. The formation crops out in isolated patches throughout most of the central portion of the Grand Canyon. Thicker deposits of channel fill of Surprise Canyon material occur in isolated areas in the western Grand Canyon; deposits are much less evident toward the east. Thicknesses can reach 90 meters.

About 25 to 30 million years elapsed before the deposition of the Supai group over the Redwall and intermittent channels of the Surprise Canyon formation. The Supai, which is up to 310 meters thick, is readily evident in the canyon as it is an extensive ledge and cliff former with some slopes separating them. The Supai group is the source of the iron oxide that stained the underlying Redwall limestone. The Supai has been divided into four formations: the basal Watahomigi, Manakacha, Wescogame, and the Esplanade, a distinct sandstone ledge-forming unit. In the western Grand Canyon, the percentage of resistant rock increases, resulting in more cliffs being produced in the process of erosion. The formations of the Supai do contain fossils, but they are scarcer to the east. The Supai group, which spans the boundary of the Pennsylvanian and Permian periods (300 million years ago), formed in a combination of coastal plain, eolian, and shallow river environments. The 90-meter-thick, red siltstones of the overlying Hermit shale produce a sloped surface above the Supai. The environment in which it formed was similar to that of the Supai—shallow swamps and lagoons. Cross-bedding present in portions of the unit indicates a sediment source from the north.

Near the top of the canyon, sheer cliffs of the Coconino sandstone overlie the Hermit shale. The Coconino is a very distinctive layer within the stratigraphy of the Grand Canyon, as it forms a wide, white band near the upper edge. The Coconino sandstone is a relatively pure, well-sorted quartz sandstone that formed when northern Arizona was covered by a vast sand sea that was driven by the prevailing south and southeast winds. Large cross-bedded dunes are preserved throughout northern Arizona and are exposed within the canyon. The thickness of the unit ranges from 18 to 30 meters in outcrops on the North Rim to more than 90 meters on the South Rim. The unit extends into central Arizona, where thicknesses of more than 245 meters have been recorded. The only fossil record preserved in the sandstone consists of tracks and trails produced by invertebrate and vertebrate animals. Surprisingly, no fossilized remains of the animals have been discovered.

Above the precipitous cliffs of the Coconino sandstone is the Toroweap formation, a ledge and cliff former made of limestone and sandstone layers containing occasional deposits of gypsum. The Toroweap formation interfingers with the Coconino Sandstone, which implies that the transgressive and regressive seas that formed the Toroweap inundated parts of the sand sea associated with the Coconino. Because the sea came from the west, layers thicken in that direction within this unit. The Toroweap is moderately fossiliferous, containing corals, bryozoans, brachiopods, and mollusks.

The Kaibab limestone, the uppermost surface of the Grand Canyon, is approximately 90 meters thick and is composed of a sandy limestone. The same sea that formed the Toroweap formation returned to deposit the layers of the Kaibab limestone. It was a shallow, warm-water marine environment from which the sediments were deposited. The Kaibab is an extensive deposit ranging from southern Utah to central Arizona. It is fairly rich in invertebrate marine fossils, including mollusks, brachiopods, and crinoids.

Evidence exists at the eastern edge of the Grand Canyon that Triassic deposits were laid down in the area. Cedar Mountain, situated about five kilometers east of the eastern entrance to Grand Canyon National Park, consists of more than 185 meters of Triassic deposits. This is the only sizable outcrop of Triassic or younger sediments in the area. These sediments were deposited but later eroded from the area, exposing the present-day surface of Kaibab limestone.

Colorado River

The Colorado River has had a complex history in the area. Several hypotheses have been proposed to explain its present course and how the river cut the Grand Canyon. Although these hypotheses differ in their details, they all point to some common points in the evolution of the present river course. Ancestral drainage existed in what is now the Upper Colorado River system, which includes the Green and San Juan Rivers, two major tributaries. The Kaibab-Coconino uplift diverted the southward course of the ancestral Colorado River.

One theory has the river flowing to the northwest; another has it flowing to the southeast, where extensive lake deposits are found. When the Gulf of California opened about five million years ago, headward erosion of the lower Colorado River system began. As the river eroded upstream, it managed to capture small streams situated to the west of what is now the Grand Canyon. With increased headward erosion, the lower Colorado River cut its way back through the Grand Wash Cliffs and started to carve the western Grand Canyon. Eventually, the lower portion captured the drainage of the upper portion, leading to the lake's drainage formed by the upper system. This increased water flow enhanced the downcutting of the Grand Canyon to its present form.

The vast majority of Colorado River water is now derived from snowmelt in Colorado and Wyoming. This water, along with that flowing in the San Juan River, passes through the Green and upper Colorado rivers before being impounded behind Glen Canyon Dam. The Little Colorado River drains the arid northeastern Arizona landscape and, thus, produces a small contribution to the overall flow of the Colorado River as it moves through the Grand Canyon.

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