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The Grand Canyon tells one of the world’s greatest geologic stories. Its distinctive features allow researchers to piece together the history of this unique location, one of America’s treasures and a UNESCO World Heritage Site. Thinking of the geologic record as a book is helpful to understand each page of Earth’s history. The beginning of the story starts at the bottom of the canyon and moves forward in time as you get closer to the rim.

Grand Canyon National Park from Powell Point on the South Rim. Photo taken during a ceremony commemorating the 150th anniversary of the John Wesley Powell expedition.
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(Credit: Annie Scott, USGS. Public domain.)

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Introduction to Grand Canyon Geologic Principles

Stratigraphy is the study of the rock layering, and reveals a wealth of information about what Earth was like when each layer formed. In the Grand Canyon, there are clear horizontal layers of different rocks that provide information about where, when, and how they were deposited, long before the canyon was even carved. The Law of Superposition states that sediment is deposited in layers in a sequence, the oldest rocks are on the bottom and the youngest rocks are on the top, similar to the way that sand piles up in an hour glass. This principle is a key part of determining the relative age of a rock layer. The three main rock layer sets in the Grand Canyon are grouped based on position and common composition and 1) Metamorphic basement rocks, 2) The Precambrian Grand Canyon Supergroup, and 3) Paleozoic strata. These three main sets of rocks were first described by the explorer and scientist John Wesley Powell during his expeditions of the Grand Canyon in the late 1860s and early 1870s. To learn more about the Powell expeditions, visit http://www.usgs.gov/Powell150. A USGS geologic field photograph map of the Grand Canyon can be viewed or downloaded here.

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The image on the right is a stratigraphic section of Grand Canyon by John Wesley Powell (1875). “A” is the metamorphic basement complex (Early Proterozoic Vishnu Group), with igneous intrusives labeled “a”; “B” is the Grand Canyon Supergroup (Middle and Late Proterozoic); “C” indicates the Paleozoic strata; “x” and “y” delineate the major unconformable contacts. The image on the left is a recent photograph of Grand Canyon from Walhalla Plateau, with the red line showing the Great Unconformity that was first noted by Powell (credit: Annie Scott/USGS).

Unconformities are gaps in the geologic record that occur when rocks or sediments are eroded away and time elapses before new deposition occurs. New sediment eventually forms new rock layers on top of the eroded surface, but there is a period of geologic time that is not represented. You can think of unconformities as missing “pages” in the book of the geologic record. Missing layers may seem like a problem, but the very fact that there is this gap in the record provides information to geologists, indicating changing ocean levels or changes in the Earth’s crust. In the Grand Canyon, unconformities are common in the Grand Canyon Supergroup and the Paleozoic Strata.

The three main types of rock are igneous, sedimentary and metamorphic. Igneous rocks are cooled magma (melted rock found underground) or lava (molten rock found above ground). Granite (cooled from magma, known as an intrusive igneous rock) and basalt (cooled from lava, known as an extrusive igneous rock) are two types of igneous rocks. Sedimentary rocks are formed by smaller pieces of sand and mud stick together in layers. Examples include: sandstone, mudstone ,shale , siltsone, chert, limestone, and more. Sedimentary rocks often contain fossils that can be used to help identify the age of the rock. Certain fossils, called index fossils, are particularly useful because they are abundant in a relatively narrow time range. Over time, pressure increases as sediment increases, and minerals help form these rock layers. Metamorphic rocks are formed when sedimentary or igneous rocks change due to exposure to heat and/or pressure. All three rock types can be found in the Grand Canyon, and each layer adds an important understanding to the geologic history of the region.

Metamorphic Basement Rocks

The oldest rocks in the Grand Canyon, found at the bottom of the canyon, areprimarily metamorphic, with igneous intrusions (the name given to when magma or lava enters or cools on top of previously formed rock). The intrusive igneous rocks here are called Zoroaster granite. The name given to this rock set (the combination of metamorphic and igneous rock of a certain age found at this location) is Vishnu Basement Rocks. Primarily schist (metamorphic) with granite (igneous), these rocks have visible crystals and are about 1.7 billion years old, from an era early in Earth history known as the Proterozoic. On Powell’s expedition to explore and map the Grand Canyon, he named this part of the exposed rock “The Granite Gorge.” This rock set tells the story of the creation of North America, when volcanic islands collided with the continental landmass, forming metamorphic rocks through the intense heat and pressure. Volcanism continued after the collision and igneous intrusions continued after metamorphosis. [1]

(Public domain.)

The Grand Canyon Supergroup

The middle rock set, the Grand Canyon Supergroup, is primarily sandstone and mudstone, both sedimentary rocks, with some areas of igneous rock. They are from the late Proterozoic, only slightly younger than the metamorphic basement rocks. These rocks do not contain many fossils, because they formed before complex life on Earth was common. The few fossils that are present include stromatolites, columns of sediment formed by cyanobacteria. The composition (sandstone) and presence of stromatolites indicate that this area was previously a very shallow sea. The rock layers in the Grand Canyon Supergroup have been tilted, whereas the other rocks above this set are horizontal. This is known as an angular unconformity. The top of these sediment layers was then eroded away, forming the Great Unconformity.

Paleozoic Strata

These layers are sedimentary, and primarily sandstone. This set makes up most of the typical reddish layers that you often see in images, and which come to mind when thinking of the Grand Canyon. Following the Great Unconformity, this set is much younger than the other rock layers, and fossils are prevalent. The most common fossils are small sea creatures, such as brachiopods, bryozoans, coral, and crinoids. Combined with the sandstone, this tells us that the region was a warm, shallow sea when these sediments were deposited.

Old Rocks, Young Canyon

A key feature of the Grand Canyon is, unsurprisingly, the canyon itself. The width and depth make it truly remarkable, and expose the rock layers that were discussed above. After all the rocks were deposited, there was a period of uplift (where plate tectonics literally force a section of the Earth upward), setting the stage for canyon formation. It provided a high enough elevation that water could flow downward, cutting through the rock as it went.

This incredible formation was carved over millions of years by the Colorado River. The canyon itself has formed much more recently than the deposition of rock layers, only about 5 million years ago (as opposed to the rocks, the youngest of which are a little less than300 million years old). The canyon has since been forming at varying rates, with periods of intense erosion carving the canyon. The river must have had periods of quick movement, carving deep, not only wide. To view or download a 2018 report about the Colorado River downstream of Grand Canyon, click here.

The confluence of the Colorado and Little Colorado rivers, showing their paths through the rock

(Public domain.)

The river continues to be an agent of change, reshaping the canyon over time. The canyon isn’t fully formed as long as there is water flowing. There is ongoing research about river flow, sediments, and geomorphology. The Glen Canyon Dam controls the Colorado River now, providing electricity to six states and changing the natural flow patterns. Since the construction of the dam in 1963, researchers have been studying how changes in river flow affect the erosion and deposition of sediment along the Colorado River and the changes to riparian vegetation and food webs.

More Information about Fossils

The Paleozoic Strata contain many fossils that help scientists learn about the geologic history of North America. Most of the fossils are ocean-dwelling creatures, telling us that the area now in the middle of Arizona was once a sea. Some of the most common fossils found in the Grand Canyon are listed below.

Trilobites were invertebrates that lived in shallow marine environments and varied widely in size. They are index fossils for the Paleozoic, and were particularly prominent during the Ordovician.

Tracks and burrows are known as trace fossils, because they are not preservations of the actual organism, but instead show where the organism moved and lived. They are commonly tunnels dug by trilobites and worms in muddy ocean sediment.

Brachiopods left shells behind that are quite common in Paleozoic rocks.

A photo of Grand Canyon rocks, displaying superposition, the Great Unconformity, and the angling of the Grand Canyon Supergroup rocks

(Public domain.) https://education.usgs.gov/lessons/schoolyard/superposition.html

The North Rim of the Grand Canyon, showing a tree growing on a ledge overlooking the canyon

(Public domain.)

Visualization of map of riverbed and canyon walls near Navajo Bridge, 4.5 miles downstream from Lees Ferry, Arizona. River bathymetry was measured with multibeam sonar and topography was measured with a boat-mounted laser scanner. The data from this survey collected in April 2016 will be used to measure changes in sand storage on the river bed and to model streamflow and sand transport.

(Public domain.)

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Learn More

Geologic Map of the Grand Canyon: https://pubs.usgs.gov/imap/i-2688

The Grand Canyon Monitoring and Research Center: https://www.gcmrc.gov/about/about_default.aspx

Geologic Maps of the Grand Canyon: https://geomaps.wr.usgs.gov/arizona/

Information about John Wesley Powell and his expeditions: https://pubs.er.usgs.gov/publication/pp669

Canyon News Dating Site

Grand Canyon Geology Training Manual: https://www.nps.gov/grca/learn/nature/geology_manual.htm

NPS Introduction to Grand Canyon Geology: https://www.nps.gov/grca/learn/nature/grca-geology.htm

USGS Educational Videos: https://education.usgs.gov/videos.html

Citizen Science: Find projects to get involved in with the USGS https://txpub.usgs.gov/myscience/

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The Trail of Time, an exhibit at the Grand Canyon showing different rocks and their ages: http://tot.unm.edu/w-elves.html

NPS Geologic Tours: https://www.nps.gov/subjects/geology/geologic-tour.htm

Grand Canyon Fact Sheets: https://www.nps.gov/grca/learn/education/learning/upload/GeoArticle-11-1-11-2017.pdf

Fossils in the Grand Canyon: https://www.nps.gov/grca/learn/nature/fossils.htm

Geologic Field Photograph Map of the Grand Canyon Region: https://pubs.usgs.gov/gip/0189/gip189.pdf

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References

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Timmons, Stacey. Grand Canyon Geology Training Manual. Dec 16, 2013.

Return to Grand Canyon National Park main page.

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Not all rift valleys are located underwater. Rifting can also happen on land, forming rift valleys such as The Great Rift Valley in Africa or the Rio Grande Rift here in North America.
The Rio Grande Rift runs south to north from the state of Chihuahua, Mexico, to Leadville, Colorado. It began forming between 30 to 35 million years ago and continues to widen very slowly today.
Rifts like the Rio Grande form basins, or valleys, that slowly fill with sediments. In Albuquerque, New Mexico, which is situated in the Rio Grande Rift, these basin sediments have reached a thickness of three miles.

How does a river like the Colorado carve such a big canyon?


The Colorado River has been carving away rock for the past five to six million years. Remember, the oldest rocks in Grand Canyon are 1.8 billion years old.
The canyon is much younger than the rocks through which it winds. Even the youngest rock layer, the Kaibab Formation, is 270 million years old, many years older than the canyon itself.
Geologists call the process of canyon formation downcutting. Downcutting occurs as a river carves out a canyon or valley, cutting down into the earth and eroding away rock.
Downcutting happens during flooding. When large amounts of water are moved through a river channel, large rocks and boulders are carried too. These rocks act like chisels, chipping off pieces of the riverbed as they bounce along.
Several factors increase the amount of downcutting that happens in Grand Canyon: the Colorado River has a steep slope, a large volume, and flows through an arid climate.
Gradient is a measure of slope. The higher the gradient, the steeper the slope. If you go sledding, a steeper slope means a faster ride down the hill. For riverbeds, a high, or steep, gradient means a faster current.
A river with a faster current has the power to carry larger rocks and boulders. While a steep gradient is not an uncommon characteristic of rivers, it is certainly an important one, adding to a river’s power.
On its 277 mile (446 km) journey through Grand Canyon, the Colorado River descends about 2,000 feet (610 m) in elevation. For a river this is a steep slope, which gives the river a tremendous amount of speed and the ability to transport big rocks during floods.
That is, there is a lot of water flowing in the Colorado River. A river with a large volume can carry a lot of sediment.
Compared with the volume of rivers like the Mississippi or the Nile, the volume of the Colorado is pretty small. But, during floods, its volume increases significantly.
The Colorado River through Grand Canyon averages 300 feet (91 m) across and about 40 feet (12 m) deep. The average flow is between 12,000 and 15,000 cubic feet per second (cfs). During a flood, the increased volume of water can flow at a rate of 300,000 cubic feet per second (cfs). Imagine 300,000 basketballs, each about the size of a cubic foot, barreling past you each second!
Some geologists suggest that when glaciers were melting in the Rocky Mountains at the close of the last ice age, there might have been as much as one million cfs moving through the Colorado River’s channel.
Mechanical weathering happens relatively quickly in arid regions. In more humid regions, plants cover the land and help slow erosion. In arid climates, there is often very little soil, leaving bedrock exposed.
The exposed rocks are more easily eroded by precipitation, daily fluctuations in surface temperature, and ice-wedging during winter months. Rivers flowing through arid landscapes can carve through rock more easily because the rock is exposed and weathered.

The Grand Canyon is a dynamic place


Weathering and erosion are ongoing processes. If we were to visit Grand Canyon in another couple million years, how might it look?
For one, it would be wider; we may not even be able to see across it anymore. Much of Grand Canyon’s width has been gained through the erosive action of water flowing down into the Colorado River via tributaries. As long as water from snow melt and rain continues to flow in these side drainages, erosion will continue.
In a few million years, Grand Canyon also may be a bit deeper, though the canyon isn’t getting deeper nearly as fast as it is getting wider. The rocks through which the river is currently downcutting are hard, crystalline igneous and metamorphic rocks, which are much stronger than the sedimentary rocks resting above them. More importantly, the river’s gradient has decreased, such that it has less power to battle with the hard rocks.
Finally, the river’s elevation near Phantom Ranch, a popular hiking destination in the canyon, is just 2,400 feet above sea level. Because sea level (0 ft.) is the ultimate base level for all rivers and streams, upon reaching sea level, the Colorado River will be done downcutting.
Currently controlled by Glen Canyon Dam, the Colorado River through Grand Canyon no longer experiences dramatic changes in water volume throughout the year.
Prior to the construction of Glen Canyon Dam, the volume of flow of the Colorado River would decrease to as low as 500 – 1000 cfs in late summer. Then, in late spring when snows in the Rocky Mountains were melting, the river would swell to 100,000 cfs or more.
In the past, downcutting occurred during these spring floods because the flood waters could carry tremendous amounts of sediment and had the power to sweep huge rocks downstream.
The highest recorded pre-dam flow was about 300,000 cfs in 1884. The highest recorded post-dam flow of just 92,400 cfs was in 1983.
Much of Northern Arizona is dotted with cinder cones, lava domes and stratovolcanoes. The San Francisco Volcanic Field, which contains at least 600 cinder cones, covers 1,800 square miles. Standing at about the center of this vast volcanic field are the prominent San Francisco Peaks of Flagstaff, Arizona, remnants of a geologically young stratovolcano.
Cinder cones are also found near the western end of Grand Canyon, between Toroweap Valley and Hurricane fault.
Beginning about 630,000 years ago, lava cascaded over the canyon’s North Rim from volcanoes such as Vulcan's Throne, damming the river at least 13 times. One such lava dam was over 2,000 feet high, reaching several hundred feet higher than Glen Canyon Dam does today.
Because these dams were not particularly strong, they were probably very short lived.
One thing we can be sure of is that the lava dams were removed by the erosive action of the Colorado River. Once water pooled enough to pour over the top of the dams, a phenomenon called “spillover” or “overtopping,” the river was able to quickly erode through the obstacle. Evidence of huge outburst floods has been well documented in western Grand Canyon.
Finally, about 400,000 years ago, volcanic eruptions came to an end, and the river was temporarily blocked for the last time.
Numerous normal faults cut across Grand Canyon. Normal faults form in response to extensional tectonics or in other words when a region is being slowly pulled apart, eventually resulting in a landscape such as Nevada’s basin and range.
The most active faults in northwestern Arizona are the Hurricane and Toroweap faults, which cut across Grand Canyon near river miles 179 and 191.
Significant offset can be observed along these faults, which have moved vertically between 520 and 720 feet in the past 2 million years. Ruptured rock layers are also common in this stretch, offering evidence for the occurrence of major earthquakes.
Learning more about these faults is exciting because they help scientists determine just how fast the Colorado River is downcutting.

How old are Grand Canyon and the Colorado River?


Landscapes are more difficult to date than rock formations. Still, by looking at relationships between rock formations, scientists are able to determine ages of landscapes with some precision. Scientists have used this type of relative dating technique to narrow the age of the Colorado River and Grand Canyon.
Scientists know that the Colorado River carved Grand Canyon. The river is thus slightly older than the canyon, though the two are certainly close in age.
Scientists have studied rock deposits along course of the present day Colorado River. By looking at the type of sediments the deposits contain, scientists determine whether or not the rocks were deposited by the river.
Rocks deposited by the river are younger than the river, as the river needed to be around to deposit them. Rocks not deposited by the river are older than the river because the river was not yet there to drop them.
When possible the scientists then date these rock deposits. The age of the river falls between the rocks determined to be older than the river and those determined to be younger. Through this method, scientists have estimated an age for the river, and thus the canyon through which it flows, of 5-6 million years.
The Hualapai Limestone was deposited in a warm, shallow freshwater lake just west of the Grand Wash Cliffs. It does not contain any river sediments, therefore, the Colorado River was not flowing yet when the Hualapai Limestone was deposited.
The estimated age of the Hualapai Limestone is 6 million years. It was obtained from the Fortification Basalt, a lava flow that is interbedded with the sediments. Because the Hualapai Limestone is older than the river, the river can’t be more than 6 million years old.
Directly below the Sandy Point Basalt rests a Colorado River gravel deposit. The river was therefore present and flowing before the basalt was deposited.
The Sandy Point Basalt is 4.4 million years old, which means that the river is at least that old.
The quartz-rich sands and silts of the Bouse Formation are found in just three basins along the modern Colorado River. Scientists believe that these sediments came from the Colorado River, though their precise depositional environment is debated. Some believe the Bouse Formation was deposited in an estuary, while others claim that deposition took place in a series of freshwater lakes fed by the Colorado.
The water in both of these possible environments would have been from the Colorado River. Therefore, the river was already flowing about 5 million years ago.
The Imperial Formation is composed of both marine and deltaic sediments, meaning that it was deposited at the mouth of a river on a delta that was sometimes covered by the sea. Colorado River sediments are found in this formation.
In particular, the Imperial Formation contains microfossils from rock layers on the Colorado Plateau. The Colorado River, then, must have been carrying eroded rock materials from the Plateau to its mouth by 5 million years ago.
The opening of the Colorado River’s outlet, the Gulf of California, was a significant event in the story of Grand Canyon. It was this event that allowed for the establishment of the modern, through-flowing Colorado River. Connecting the Colorado River of the Colorado Plateau with an outlet to the sea lowered the base level by about 5,000 feet (1,524 m). The base level is the lowest level to which a river can erode its bed. The ultimate base level is sea level (0 ft.).
The introduction of this lower base level created the conditions necessary for incision through the Colorado Plateau to occur.

How did the Colorado River and Grand Canyon come to be?


It is fairly easy to explain the formation of Grand Canyon through downcutting, weathering, and erosion. It is more difficult to explain just how the Colorado River came to be in its current location.
We can think of the development of the Colorado River as a history book with many chapters. The most recent chapter is familiar, because it is the chapter that is visible today. But, there are a number of chapters missing. The plotlines of some of those missing chapters are heavily debated.
There are a few chapters in the story that the majority of geologists agree upon. For one, evidence collected thus far suggests that the upper and lower reaches of the Colorado River are different ages.
The lower section in California, Nevada, and Arizona is younger than the upper portion in Utah and Colorado. The age of the younger portion of the river is estimated to be between 5 and 6 million years based on the various constraints listed in the Ages section.
The older portion, or “ancestral Colorado River,” is at least 7 million years old and may even be 10 million years old, based on the presence of river gravels found near Grand Junction, Colorado. When the upper and lower portions combined, an event called drainage integration, the Colorado River became what it is today.
In other words, the river that we know today as the Colorado River, was actually once either two different rivers that have been joined together or a much shorter river that was later lengthened.
Another widely accepted chapter in the Colorado River’s history book is that the landscape of the Colorado Plateau and surrounding areas looked very different at the time that the Colorado River developed. We know younger rocks once rested on top of the Kaibab Limestone because of land forms such as Red Butte south of Grand Canyon, which is composed of some of these younger rock layers. The majority of these rock layers, however, have been eroded away. We can only guess what the landscape might have looked like, but it is likely that this unknown topography influenced the present course of the river.
Today, the Kaibab Plateau, a high point through which the river gingerly slices, poses a problem for geologists. Water certainly doesn’t flow up hill, so geologists hypothesize that at the time the river started cutting through it, the Kaibab Plateau must not have been an obstacle.
It may have been smaller 10 to 30 million years ago, or perhaps it was buried beneath layers of flat-lying Mesozoic rocks, only to be revealed after the river had already established its course.
While keeping these few accepted chapters in the Colorado River’s history book in mind, geologists have proposed a number of different hypotheses to explain how the river became a through-flowing river with an outlet at the Gulf of California. As we learn more, some hypotheses may be confirmed or proven false.
Regardless, it is safe to say that geologists won’t ever know all of the answers! There is still much to be learned about the Colorado River and Grand Canyon. As more research is completed, we will continue to learn.
In attempting to address the age differences between the upper and lower reaches of the Colorado River, Edwin McKee(1967) suggested that the Colorado River in Grand Canyon formed when two different rivers met.
His research group suggested that the ancestral Colorado River followed its present day course until reaching the Kaibab Plateau, where it flowed on out to the Gulf of Mexico down the channel of today’s Little Colorado River.
The young Colorado River would have slowly worked its way back from the Gulf of California by headward erosion, to eventually capture the ancestral Colorado River and form the river as we know it today.
In order for this theory to work, the ancestral Colorado River would have needed to flow eastward over the continental divide. There is however no evidence in the Little Colorado River drainage system to support the idea of an eastward flowing river.
The young Colorado River would have slowly worked its way back from the Gulf of California by headward erosion, to eventually capture the ancestral Colorado River and form the river as we know it today.
In order for this theory to work, the ancestral Colorado River would have needed to flow eastward over the continental divide. There is however no evidence in the Little Colorado River drainage system to support the idea of an eastward flowing river.
Geologist Ivo Lucchitta and others believe that the ancestral Colorado River crossed the Kaibab Plateau as it does today and then veered off to the northwest alongside a ridge.
In order for this to work, the Kaibab Plateau couldn’t look quite like it does today- perhaps it was still buried by Mesozoic rocks or it may not have even existed yet.
Like McKee before him, Ivo Lucchitta then suggests that the young Colorado River captured the ancestral portion by headward erosion to create the modern through-flowing river. Unfortunately, no sedimentary deposits have been found to support this theory.
A few geologists have hypothesized that the ancestral Colorado River was temporarily dammed behind the Kaibab Plateau and other high points.
Just as water pools and collects behind Glen Canyon Dam to form Lake Powell today, the water from the ancestral Colorado River would have pooled and collected behind multiple high points, forming a chain of ancient lakes, one of which has been dubbed Lake Bidahochi.
Sediments believed to have been deposited by these ancient lakes have been found in Arizona. According to the hypothesis, the lakes would have remained until
a) the young Colorado River cut through the Kaibab Plateau and other high points by way of headward erosion, creating an outlet to the sea, or
b) the lake overflowed the plateaus, rapidly carving the canyon, and connected with the young Colorado River on the other side.
Carol Hill of the University of New Mexico recently proposed that the “ancestral” Colorado River integrated with the young Colorado through the collapse of a groundwater-karst system. This is a very new idea and it is still being developed.
Hill proposes that precipitation falling on the Colorado Plateau drained into the Redwall aquifer karst through features such as sinkholes, cracks and joints. The water in the Redwall aquifer then flowed through the Kaibab Uplift to connect with the headward-eroding young Colorado River in western Grand Canyon. Hill proposes that collapse and incision of the canyon then followed along this already weakened subterranean route.

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