Geology of the Grand Canyon area

thumb|300px|alt=Wide canyon with exposed red- and tan-colored rock|The [[Grand Canyon from Navajo Point. The Colorado River is to the right and the North Rim is visible at all in the distance. The view shows nearly every sedimentary layer described in this article.]]

The geology of the Grand Canyon area includes one of the most complete and studied sequences of rock on Earth. The nearly 40 major sedimentary rock layers exposed in the Grand Canyon and in the Grand Canyon National Park area range in age from about 200 million to nearly 2 billion years old. Most were deposited in warm, shallow seas and near ancient, long-gone sea shores in western North America. Both marine and terrestrial sediments are represented, including lithified sand dunes from an extinct desert. There are at least 14 known unconformities in the geologic record found in the Grand Canyon.

thumb|upright=1.25|Figure 1. A geologic [[cross section (geology)|cross section of the Grand Canyon.]]

Uplift of the region started about 75 million years ago during the Laramide orogeny; a mountain-building event that is largely responsible for creating the Rocky Mountains to the east. In total, the Colorado Plateau was uplifted an estimated . The adjacent Basin and Range Province to the west started to form about 18 million years ago as the result of crustal stretching. A drainage system that flowed through what is today the eastern Grand Canyon emptied into the now lower Basin and Range province. The opening of the Gulf of California around 6 million years ago enabled a large river to cut its way northeast from the gulf. The new river captured the older drainage to form the ancestral Colorado River, which in turn started to form the Grand Canyon.

Wetter climates brought upon by ice ages starting 2 million years ago greatly increased excavation of the Grand Canyon, which was nearly as deep as it is now, 1.2 million years ago. Volcanic activity deposited lava over the area 1.8 million to 500,000 years ago. At least 13 lava dams blocked the Colorado River, forming lakes that were up to deep. The end of the last ice age and subsequent human activity has greatly reduced the ability of the Colorado River to excavate the canyon. Dams in particular have upset patterns of sediment transport and deposition. Controlled floods from Glen Canyon Dam upstream have been conducted to see if they have a restorative effect. Earthquakes and mass-wasting erosive events still affect the region.

Vishnu Basement Rocks

thumb|left|alt=Gray and reddish rock face with rough surface adjacent to a river.|The [[Vishnu Basement Rocks were deposited as volcanic rocks and sediments but were later metamorphosed and intruded by igneous rock.]]

At about 2.5 and 1.8 billion years ago in Precambrian time, sand, mud, silt, and ash were laid down in a marine basin adjacent to an orogenic belt. From 1.8 to 1.6 billion years ago at least two island arcs collided with the proto-North American continent. This process of plate tectonics compressed and grafted the marine sediments in the basin onto the mainland and uplifted them out of the sea. Later, these rocks were buried under the surface and pressure-cooked into metamorphic rock. The resulting Granite Gorge Metamorphic Suite, which is part of the Vishnu Basement Rocks, consists of the metasedimentary Vishnu Schist and the metavolcanic Brahma and Rama Schists that were formed 1.75 billion to 1.73 billion years ago. This is the resistant rock now exposed at the bottom of the canyon in the Inner Gorge.

As the volcanic islands collided with the mainland around 1.7 billion years ago, blobs of magma rose from the subduction zone and intruded the Granite Gorge Metamorphic Suite. These plutons slowly cooled to form the Zoroaster Granite; part of which would later be metamorphosed into gneiss. This rock unit can be seen as light-colored bands in the darker garnet-studded Vishnu Schist (see 1b in figure 1). The intrusion of the granite occurred in three phases: two during the initial Vishnu metamorphism period, and a third around 1.4 billion years ago. The third phase was accompanied by large-scale faulting, particularly along north–south faults, leading to a partial rifting of the continent.

Subsequent erosion lasting 300 million years stripped much of the exposed sediments and the mountains away. This reduced the very high mountains to small hills a few tens to hundreds of feet (tens of meters) high.

alt=Layered dark brown rock in stairstep pattern in ledges above a river in a canyon with exposed reddish and tan rock|thumb|left|The [[Cardenas Basalt was laid on top of the rest of the Grand Canyon Supergroup]]

The oldest section of the supergroup is the Unkar Group. It accumulated in a variety of fluvial, deltaic, tidal, nearshore marine, and offshore marine environments. The first formation to be laid down in the Unkar Group was the Bass Formation. Fluvial gravels initially accumulated in shallow river valleys. They later lithified into a basal conglomerate that is known as the Hotauta Member of the Bass Formation. The Bass Formation was deposited in a shallow sea near the coast as a mix of limestone, sandstone, and shale. Diagenesis later altered the bulk of the limestone into dolomite. It is thick and grayish in color. Averaging 1250 million years old, this is the oldest layer exposed in the Grand Canyon that contains fossils—stromatolites. This formation indicates a short-lived regression (retreat) of the seashore in the area that left mud flats. Ripple marks and other features indicate it was close to the shore. Outcrops of this red to orange formation can be seen in the eastern parts of the canyon. Fossils of stromatolites and algae are found in this layer. At 1070 ± 70 million years old, the Cardenas Basalt is the youngest formation in the Unkar Group. It is made of layers of dark brown basaltic rocks that flowed as lava up to thick. This rock unit is made of coarse-grained sandstone, and was deposited in a shallow sea on top of the eroded surface of the Cardenas Basalt. The Galeros Formation is a mainly greenish formation composed of interbedded sandstone, limestone, and shale. Fossilized stromatolites are found in the Galeros. The Kwagunt Formation consists of black shale and red to purple mudstone with some limestone. Isolated pockets of reddish sandstone are also found around Carbon Butte. Stromatolites are found in this layer.

About 800 million years ago the supergroup was tilted 15° and block faulted in the Grand Canyon Orogeny. Some of the block units moved down and others moved up while fault movement created north–south-trending fault-block mountain ranges.

Tonto Group

upright|thumb|alt=A ledge made of pebbly rock with lichen on it.|Sixtymile Formation is the basal unit, where present, of the Tonto Group

During the Paleozoic era, the western part of what would become North America was near the equator and on a passive margin. Climate was warm and invertebrates, such as the trilobites, were abundant. An ocean started to return to the Grand Canyon area from the west about 550 million years ago.

thumb|left|alt=Wide canyon with steep tan colored walls. A river inside a valley is below a broad gently sloping surface.|Tonto Group is most easily seen as the broad Tonto Platform just above the Colorado River

Tapeats Sandstone averages 525 million years old and is made of medium- to coarse-grained sand and conglomerate that was deposited on an ancient shore (see 3a in figure 1). Ripple marks are common in the upper members of this dark brown thin-bedded layer. Fossils and imprint trails of trilobites and brachiopods have also been found in the Tapeats. Today it is a cliff-former that is thick. Bright Angel Shale averages 515 million years old and is made of mudstone-derived shale that is interbedded with small sections of sandstone and shaly limestone with a few thin beds of dolomite. Glauconite is responsible for the green coloration of the Bright Angel. Muav Limestone averages 505 million years old and is made of gray, thin-bedded limestone that was deposited farther offshore from calcium carbonate precipitates (see 3c in figure 1). The Muav is a cliff-former, thick.

These three formations were laid down over a period of 30 million years from early-to-middle Cambrian time. Trilobites followed by brachiopods are the most commonly reported fossils in this group but well-preserved fossils are relatively rare. An unconformity representing 40 to 50 million years of lost geologic history marks the top of this formation.

The next formation in the Grand Canyon geologic column is the cliff-forming Redwall Limestone, which is thick (see 4b in figure 1). Redwall is composed of thick-bedded, dark brown to bluish gray limestone and dolomite with white chert nodules mixed in. Many fossilized crinoids, brachiopods, bryozoans, horn corals, nautiloids, and sponges, along with other marine organisms such as large and complex trilobites have been found in the Redwall. Surprise Canyon was unknown to science until 1973 and can be reached only by helicopter. Both marine and non-marine deposits of mud, silt, sand and calcareous sediments were laid down on a broad coastal plain similar to the Texas Gulf Coast of today.

Supai Group formations in the western part of the canyon contain limestone, indicative of a warm, shallow sea, while the eastern part was probably a muddy river delta. This formation consists of red siltstones and shale capped by tan-colored sandstone beds that together reach a thickness of 600 to 700 ft (around 200 m).

Formations of the Supai Group are from oldest to youngest (an unconformity is present at the top of each): Watahomigi (see 5a in figure 1) is a slope-forming gray limestone with some red chert bands, sandstone, and purple siltstone that is thick. Manakacha (see 5b in figure 1) is a cliff- and slope-forming pale red sandstone and red shale that averages thick in Grand Canyon. Wescogame (see 5c in figure 1) is a ledge- and slope-forming pale red sandstone and siltstone that is thick. Esplanade (see 5d in figure 1) is a ledge- and cliff-forming pale red sandstone and siltstone that is thick. An unconformity marks the top of the Supai Group.

Hermit, Coconino, Toroweap, and Kaibab

Like the Supai Group below it, the Permian-aged Hermit Formation was probably deposited on a broad coastal plain (see 6a in figure 1). Slope development will periodically undermine the formations above and car- to house-sized blocks of that rock will cascade down onto the Tonto Platform. An unconformity marks the top of this formation .

thumb|alt=Indentations of roundish footprints with claw or toe marks in tan-colored rock|Lizard-like animals left their footprints in Coconino Sandstone

Coconino Sandstone formed about 275 million years ago as the area dried out and sand dunes made of quartz sand invaded a growing desert (see 6b in figure 1). Today, the Coconino is a thick golden white to cream-colored cliff-former near the canyon's rim. Cross bedding patterns of the frosted, fine-grained, well-sorted and rounded quartz grains seen in its cliffs is compatible with but does not substantiate conclusively an eolian environment. Also fossilized are tracks from lizard-like creatures and what look like tracks from millipedes and scorpions. An unconformity marks the top of this formation.

thumb|left|alt=Dark mass in bluish gray rock with shells in it.|Fossils, such as this brachiopod and fragments of crinoids, are common in the Toroweap and Kaibab formations

Next in the geologic column is the -thick Toroweap Formation (see 6c in figure 1). Seligman is a slope-forming yellowish to reddish sandstone and siltstone. Brady Canyon is a cliff-forming gray limestone with some chert. Wood Ranch is a slope-forming pale red and gray siltstone and dolomitic sandstone. An unconformity marks the top of this formation.

One of the highest, and therefore youngest, formations seen in the Grand Canyon area is the Kaibab Limestone (see 6d in figure 1). It erodes into ledgy cliffs that are thick and was laid down in latest early Permian time, about 270 million years ago, This is the cream to grayish-white rock that park visitors stand on while viewing the canyon from both rims. It is also the surface rock covering much of the Kaibab Plateau just north of the canyon and the Coconino Plateau immediately south. Shark teeth have been found in this formation as well as abundant fossils of marine invertebrates such as brachiopods, corals, mollusks, sea lilies, and worms. An unconformity marks the top of this formation.

Mesozoic deposition

thumb| alt=A large mound of rock and dirt with reddish and grayish soil and mostly covered with vegetation. |Reddish Moenkopi outcrop below volcanic rubble on [[Red Butte]]

Uplift marked the start of the Mesozoic and streams started to incise the newly dry land. Streams flowing through broad low valleys in Triassic time deposited sediment eroded from nearby uplands, creating the once -thick Moenkopi Formation. The formation is made from sandstone and shale with gypsum layers in between. Moenkopi outcrops are found along the Colorado River in Marble Canyon, on Cedar Mountain (a mesa near the southeastern park border), and in Red Butte (located south of Grand Canyon Village). The geology of the Zion and Kolob canyons area and the geology of the Bryce Canyon area records some of these formations. All these rock units together form a super sequence of rock known as the Grand Staircase.

Cenozoic regional uplift and erosion of the canyon

Uplift and nearby extension

The Laramide orogeny affected all of western North America by helping to build the American cordillera. The Kaibab Uplift, Monument Upwarp, the Uinta Mountains, San Rafael Swell, and the Rocky Mountains were uplifted, at least in part, by the Laramide orogeny. This major mountain-building event started near the end of the Mesozoic, around 75 million years ago, Streams draining the Rocky Mountains in early Miocene time terminated in landlocked basins in Utah, Arizona and Nevada but there is no evidence for a major river.

frame|left|Uplift of the Colorado Plateaus forced rivers to cut down faster.| alt=Relief map of the roughly oval shape of the Colorado Plateau surrounding the point where the U.S. States of Utah, Colorado, New Mexico and Arizona meet.

Around 18 million years ago, tensional forces started to thin and drop the region to the west, creating the Basin and Range Province. The extreme western part of the canyon ends at one of the Basin and Range faults, the Grand Wash, which also marks the boundary between the two provinces.

Colorado River: origin and development

Rifting started to create the Gulf of California far to the south 6 to 10 million years ago. This river, the ancestral Lower Colorado River, started to fill the northern arm of the gulf, which extended nearly to the site of Hoover Dam, with estuary deposits. The mechanism by which the ancestral Lower Colorado River captured this drainage and the drainage from much of the rest of the Colorado Plateau is not known. Possible explanations include headward erosion or a broken natural dam of a lake or river. The added precipitation increased runoff and the erosive ability of streams (especially from spring melt water and flash floods in summer). With a greatly increased flow volume the Colorado cut faster than ever before and started to quickly excavate the Grand Canyon 2 million years before present, almost reaching the modern depth by 1.2 million years ago.

The resulting Grand Canyon of the Colorado River trends roughly east to west for between Lake Powell and Lake Mead. In that distance, the Colorado River drops and has excavated an estimated of sediment to form the canyon. This part of the river bisects the -high Kaibab Uplift and passes seven plateaus (the Kaibab, Kanab, and Shivwits plateaus bound the northern part of the canyon and the Coconino bounds the southern part).

Volcanic activity in the western canyon

thumb|[[Vulcan's Throne volcano above Lava Falls. Lava flows, such as this heavily eroded remnant, once dammed the Colorado River.|alt=Dark-colored mass of rock draped over the side of a canyon]]

Volcanic activity started in Uinkaret volcanic field (in the western Grand Canyon) about 3 million years ago. Over 150 flows of basaltic lava dammed the Colorado River at least 13 times from 725,000 to 100,000 years ago. The dams typically formed in weeks, were long, high (thicker upstream and thinner downstream) and had volumes of .

The longevity of the dams and their ability to hold Colorado River water in large lakes has been debated. In one hypothesis water from the Colorado River backed up behind the dams in large lakes that extended as far as Moab, Utah. Dams were overtopped in short time; those that were high were overtopped by their lakes in 2 to 17 days. At the same time, sediment filled the lakes behind the dams. Sediment would fill a lake behind a -high dam in 10.33 months, filled a lake behind an -high dam in 345 years, and filled the lake behind the tallest dam in 3000 years. However others have proposed that the lava dams were much more ephemeral and failed catastrophically before overtopping. In this model dams would fail due to fluid flow through fractures in the dams and around dam abutments, through permeable river deposits and alluvium.

Since the demise of these dams the Colorado River has carved a maximum of about into the rocks of the Colorado Plateau This type of mass wasting is the main way the smaller and steeper side canyons transport sediment but it also plays a major role in excavating the larger canyons. An interim conservation measure since 1991 has held maximum flows at per second even though the dam's power plant can handle per second more flow.

Controlling river flow by use of dams has diminished the river's ability to scour rocks by substantially reducing the amount of sediment it carries.

Grand Canyon lies on the southern end of the Intermountain West seismic belt. At least 35 earthquakes larger than 3.0 on the Richter Scale occurred in the Grand Canyon region in the 20th century. Of these, five registered over 5.0 on the Richter Scale and the largest was a 6.2 quake that occurred in January 1906. Major northeast-trending fracture systems of normal faults that intersect the canyon include the West Kaibab and Bright Angel while northwest-trending systems include the Grandview—Phantom. Most earthquakes in the region occur in a narrow northwest-trending band between the Mesa Butte and West Kaibab fracture systems. These events are probably the result of eastward-migrating crustal stretching that may eventually move past the Grand Canyon area.

The Yavapai Geology Museum include three-dimensional models, photographs, and exhibits which allow park visitors to see and understand the complicated geologic story of the area. The museum building, the historic Yavapai Observation Station (built 1928), located one mile (1.6 km) east of Market Plaza, features expansive canyon views. A bookstore offers a variety of materials about the area.