The geology of the Grand Canyon is a story etched in stone, a nearly complete chronicle of Earth's history laid bare for anyone with the inclination to look. It's a testament to time, to immense forces, and to the persistent, relentless work of water. And honestly, it's a bit much, isn't it? All this ancient rock, all these layers. But if you insist on understanding it, fine.
Aspect of Geology
Look at the Grand Canyon from Navajo Point. You see the Colorado River snaking below, a mere thread from this height, and the distant, stark North Rim. What's truly remarkable, though, are the strata. Nearly every sedimentary layer worth mentioning is on display here, ranging from a relatively recent 200 million years old to a staggering 2 billion. These weren't formed in some dramatic cataclysm, but rather in the quiet, unremarkable environments of warm, shallow seas and ancient shorelines that once graced western North America. We're talking about marine and terrestrial sediments, even lithified sand dunes from a desert long since turned to dust. And the gaps – oh, the gaps. At least 14 known unconformities scar this geological record, like missing pages in a very long book.
Figure 1, a geologic cross section of the canyon, attempts to make some sense of it all. It's a visual representation of billions of years, compressed into a diagram.
The grand uplift of this region, a process that would eventually give birth to the Rocky Mountains to the east, began around 75 million years ago during the Laramide orogeny. The Colorado Plateau itself was heaved upwards by an estimated two miles. Meanwhile, to the west, the Basin and Range Province began its own dramatic transformation, stretching and thinning the Earth's crust starting about 18 million years ago. A drainage system that once flowed east across what is now the Grand Canyon found its path blocked, diverted into the sinking Basin and Range. Then, about 6 million years ago, the opening of the Gulf of California created a new, powerful force. A large river began to carve its way inland, capturing the older drainage and forming the ancestral Colorado River, the architect of this immense chasm.
The ice ages, beginning two million years ago, brought wetter climates that amplified the river's erosive power. By 1.2 million years ago, the canyon was already near its current depth. Then came the volcanoes. For over a million years, from 1.8 million to 500,000 years ago, lava flowed across the land, forming at least 13 lava dams that choked the Colorado River, creating vast lakes that reached depths of 2,000 feet. The end of the last ice age, coupled with the increasing presence of human activity, has significantly altered the river's ability to sculpt the canyon. Dams, in particular, have disrupted the natural flow of sediment and deposition. Even now, controlled floods from Glen Canyon Dam are an attempt to mimic the river's original power, a rather futile effort to restore what's been lost. And of course, the earth still shifts and groans with earthquakes, and mass-wasting events continue to reshape the canyon walls.
Vishnu Basement Rocks
These are the ancient foundations, the bedrock upon which everything else is built. The Vishnu Basement Rocks are a complex mix of volcanic debris and sediments, thoroughly transformed by heat and pressure.
Around 2.5 to 1.8 billion years ago, in the deep reaches of Precambrian time, the very building blocks of this region – sand, mud, silt, and volcanic ash – were laid down in a marine basin. This basin was nestled beside an orogenic belt, a zone of intense geological activity. Then, from 1.8 to 1.6 billion years ago, a dramatic collision occurred. At least two island arcs slammed into the proto-North American continent. This monumental act of plate tectonics compressed the marine sediments, fusing them to the mainland and thrusting them above the sea. Later, buried deep beneath the surface, perhaps 12 miles down, these rocks were subjected to immense heat and pressure, cooking them into metamorphic rock. The result is the Granite Gorge Metamorphic Suite, a part of the Vishnu Basement Rocks. It comprises the Vishnu Schist, a metasedimentary rock, and the Brahma and Rama Schists, which were once volcanic. These were formed between 1.75 and 1.73 billion years ago. This is the tough, resistant rock that forms the very bottom of the canyon, the Inner Gorge.
As these volcanic islands collided, around 1.7 billion years ago, molten magma from the subduction zone bled upwards, intruding the already formed Granite Gorge Metamorphic Suite. These molten intrusions, called plutons, cooled slowly to form the Zoroaster Granite. Some of this granite would later be transformed into gneiss. You can see this granite as lighter bands cutting through the darker, garnet-studded Vishnu Schist, a visual anomaly in the ancient darkness (look for it in figure 1b). The intrusion of this granite wasn't a single event; it happened in three phases, two during the initial Vishnu metamorphism and a third around 1.4 billion years ago. This third phase was accompanied by significant faulting, particularly along north-south lines, which began to tear the continent apart through rifting. This process thickened the continental crust by 5 to 6 miles, effectively expanding the continent from what is now the Wyoming–Colorado border all the way into Mexico. It created a colossal mountain range, the ancestral Mazatzal Mountains, some 5 to 6 miles high.
Then, for 300 million years, erosion took its toll. This relentless process stripped away much of the exposed sediment and the mighty mountains, reducing them to mere hills, mere tens of feet high. This vast gap in the geological record, this immense period of missing time, is known to geologists as the Great Unconformity. Other sediments may have been deposited, only to be completely erased by erosion. These gaps, these missing chapters in Earth's history, are called unconformities. The Great Unconformity is a prime example of a nonconformity, where layered rock sits directly atop older igneous or metamorphic rocks.
Grand Canyon Supergroup
In the late Precambrian, a significant geological event occurred: the thinning of Laurentia's continental crust due to the movement of large or small tectonic plates away from the continent. This thinning created extensive rift basins, depressions in the land that, ultimately, failed to split the continent apart. Eventually, this sunken region was inundated by a shallow sea, a vast body of water stretching from what is now Lake Superior to Glacier National Park in Montana, and extending all the way to the Grand Canyon and the Uinta Mountains.
This epic geological episode left behind the Grand Canyon Supergroup, a collection of nine distinct geologic formations. These sedimentary units were deposited between 1.2 billion and 740 million years ago. You can still see impressive exposures of the supergroup in the eastern Grand Canyon, particularly in the Inner Gorge and from viewpoints like Desert View, Lipan Point, and Moran Point.
The oldest part of this supergroup is the Unkar Group. It represents a diverse range of environments: fluvial (riverine), deltaic, tidal flats, nearshore marine, and offshore marine. The very first formation laid down within the Unkar Group was the Bass Formation. Initially, fluvial gravels accumulated in shallow river valleys, which later hardened into a basal conglomerate, known as the Hotauta Member of the Bass Formation. Following this, the Bass Formation itself was deposited in a shallow sea close to the coast, a mix of limestone, sandstone, and shale. Over time, diagenesis, the process of chemical and physical changes in sediment, transformed most of the limestone into dolomite. This formation, averaging 120 to 340 feet in thickness, is grayish in color. Dating back approximately 1250 million years, it holds the distinction of being the oldest layer in the Grand Canyon that contains fossils – specifically, stromatolites, ancient microbial mats.
Next comes the Hakatai Shale, a formation composed of thin beds of mudstones, sandstones, and shale, deposited in marginal-marine environments. These layers, collectively ranging from 445 to 985 feet thick, indicate a brief period when the seashore retreated, leaving behind mud flats. Today, this formation is a striking bright orange-red, giving the nearby Red Canyon its name. The Shinumo Quartzite is a remarkably resistant marine sedimentary quartzite. It was so durable that it eroded into isolated hills, or monadnocks, which stood as islands in the Cambrian sea, only to be re-buried by later sediments. The Dox Formation is a substantial unit, exceeding 3,000 feet in thickness. It's composed of sandstone, with interbedded shale and mudstone, deposited in fluvial and tidal settings. Features like ripple marks suggest its proximity to the ancient shoreline. This red to orange formation, visible in the eastern canyon, also yields fossils of stromatolites and algae. The youngest formation in the Unkar Group is the Cardenas Basalt, dated at 1070 ± 70 million years old. It consists of thick layers of dark brown basaltic rocks, evidence of massive lava flows that once extended up to 1,000 feet thick.
The Nankoweap Formation, around 1050 million years old, stands alone, not part of any group. This rock unit, composed of coarse-grained sandstone, was deposited in a shallow sea atop the eroded surface of the Cardenas Basalt. It's found only in the eastern part of the canyon. Following the Nankoweap, there's another gap in the geological record, an unconformity.
All the formations within the Chuar Group were laid down in coastal and shallow marine environments approximately 1000 to 700 million years ago. The Galeros Formation is predominantly greenish, a mix of interbedded sandstone, limestone, and shale, and it contains fossilized stromatolites. The Kwagunt Formation is characterized by black shale and red to purple mudstone, with some limestone. Isolated pockets of reddish sandstone can also be found around Carbon Butte, and stromatolites are present in this layer as well.
Around 800 million years ago, this entire supergroup was tilted by 15 degrees and fractured by block faulting during the Grand Canyon Orogeny. Some blocks were thrust upwards, others dropped down, and the faulting created mountain ranges aligned north-south. This was followed by approximately 100 million years of erosion, which stripped away much of the Chuar Group and parts of the Unkar Group, exposing the Shinumo Quartzite once more. The mountain ranges were reduced to hills, and in some places, the entire 12,000 feet of the supergroup were obliterated, revealing the basement rocks beneath. Any younger Precambrian rocks that might have been deposited on top were also completely removed, creating a significant unconformity that represents a staggering 460 million years of missing geological history.
Tonto Group
During the Paleozoic era, the western part of what would become North America lay near the equator, situated on a passive margin. This was the era of the Cambrian Explosion, a period of rapid diversification of life that unfolded over about 15 million years. The climate was warm, and invertebrates like the trilobites thrived. Around 550 million years ago, an ocean began to creep back into the Grand Canyon area from the west. As its shoreline advanced eastward, river systems deposited fluvial sediments, initially forming the Sixtymile Formation, a tan-colored sandstone with thin shale layers.
As the sea level continued to rise, it flooded the coastal plains, leading to the deposition of the Tapeats Sandstone, Bright Angel Shale, Muav Limestone, and Frenchman Mountain Dolostone. Finally, the Frenchman Mountain Dolostone accumulated in shallow seas. The Tonto Group, as a whole, is most easily recognized by the broad Tonto Platform that stretches just above the Colorado River.
The Tapeats Sandstone, averaging 525 million years old, is composed of medium to coarse-grained sand and conglomerate, deposited right at an ancient shoreline (see 3a in figure 1). You'll find common ripple marks in its upper layers, along with fossilized trace fossils of trilobites and brachiopods. It forms a cliff, typically 100 to 325 feet thick. The Bright Angel Shale, averaging 515 million years old, is primarily mudstone-derived shale, interspersed with sections of sandstone and shaly limestone, and a few thin beds of dolomite. Mostly deposited as mud just offshore, it contains fossils of brachiopods, trilobites, and worms (see 3b in figure 1). Its color varies through shades of green, with some brownish-tan and gray sections. It forms slopes and measures 270 to 450 feet thick. The green hue is due to glauconite. The Muav Limestone, averaging 505 million years old, is a gray, thin-bedded limestone deposited further offshore from calcium carbonate precipitates (see 3c in figure 1). The western canyon exhibits a much thicker sequence of Muav than the eastern part. This formation forms cliffs, ranging from 136 to 827 feet in thickness.
These three formations were laid down over a period of 30 million years, from the early to middle Cambrian. While fossils like trilobites and brachiopods are found, well-preserved specimens are relatively rare. The depositional pattern, with finer sediments layered over coarser ones, clearly indicates a transgressing shoreline – the sea advancing onto the land. Today, the Tonto Group defines the Tonto Platform; the Tapeats Sandstone and Muav Limestone create its cliffs, while the Bright Angel Shale forms its slopes. Unlike the older Proterozoic rocks below, the Tonto Group's beds are largely horizontal, maintaining their original orientation. The Bright Angel Shale acts as an aquiclude, a barrier to downward groundwater seepage, effectively collecting and directing water through the overlying Muav Limestone to feed springs in the Inner Gorge.
Temple Butte, Redwall, and Surprise Canyon
The Ordovician and Silurian periods, the next two chapters in Earth's geologic history, are conspicuously absent from the Grand Canyon sequence. Geologists are unsure if sediments were deposited and subsequently eroded, or if they were never laid down at all. Regardless, this break in the geological record spans about 65 million years, resulting in a disconformity – an unconformity characterized by erosional features like valleys and hills that were later buried by younger sediments.
Deep channels were carved into the top of the Muav Limestone during this period. Streams were likely responsible, though marine scour cannot be ruled out. These depressions were filled with freshwater limestone around 385 million years ago, in the Middle Devonian period, forming the Temple Butte Formation (see 4a in figure 1). These purplish-colored channels are particularly well-exposed in Marble Canyon in the eastern part of the park. The Temple Butte Formation acts as a cliff-former in the western part of the park, where it's a gray to cream-colored dolomite. Fossils of animals with backbones are found here: bony plates from freshwater fish in the east and numerous marine fish fossils in the west. The formation measures 100 to 450 feet thick, thinning near Grand Canyon Village and thickening in the western canyon. An unconformity, representing another 40 to 50 million years of lost history, marks the top of this formation.
The next prominent formation in the Grand Canyon's geological column is the cliff-forming Redwall Limestone, a formidable layer 400 to 800 feet thick (see 4b in figure 1). Composed of thick-bedded, dark brown to bluish-gray limestone and dolomite, it's studded with white chert nodules. It was deposited in a shallow, retreating tropical sea near the equator during the early to middle Mississippian, over a span of 40 million years. The Redwall is rich with fossils of marine organisms, including crinoids, brachiopods, bryozoans, horn corals, nautiloids, and sponges, along with large and complex trilobites. In the late Mississippian, the Grand Canyon region was slowly uplifted, and the Redwall was partially eroded, creating a Karst topography of caves, sinkholes, and subterranean river channels, which were later filled with more limestone. The characteristic red color of the exposed Redwall surface comes from rainwater dripping from the iron-rich redbeds of the Supai and Hermit shale formations above.
The Surprise Canyon Formation is a sedimentary layer of purplish-red shale, laid down in discontinuous beds of sand and lime above the Redwall (see 4c in figure 1). It formed in very late Mississippian or possibly earliest Pennsylvanian times as the land subsided and tidal estuaries filled river valleys with sediment. This formation exists only in isolated lenses, 50 to 400 feet thick. Discovered only in 1973, it's accessible solely by helicopter. Fossil logs, other plant material, and marine shells are found within this formation. An unconformity marks the top of the Surprise Canyon Formation, and in most places, this unconformity has completely removed it, exposing the underlying Redwall.
Supai Group
An unconformity spanning 15 to 20 million years separates the Supai Group from the Redwall Formation below. The Supai Group was deposited from the late Mississippian through the Pennsylvanian and into the Early Permian periods, roughly 320 to 270 million years ago. Both marine and non-marine deposits of mud, silt, sand, and calcareous sediments accumulated on a broad coastal plain, not unlike the Texas Gulf Coast today. Around this time, the Ancestral Rocky Mountains began to rise in Colorado and New Mexico, and streams carried eroded sediment from them to the Grand Canyon region.
Formations within the Supai Group in the western canyon contain limestone, indicating a warm, shallow sea, while the eastern part was likely a muddy river delta. This formation consists of red siltstones and shale capped by tan-colored sandstone beds, reaching a combined thickness of about 600 to 700 feet (around 200 m). The shale in the early Permian formations within this group was oxidized to a striking bright red color. Fossils of amphibian footprints, reptiles, and abundant plant material are found in the eastern part, with an increasing number of marine fossils appearing in the west.
The formations of the Supai Group, from oldest to youngest (with an unconformity at the top of each), are: The Watahomigi (see 5a in figure 1) is a slope-forming gray limestone with bands of red chert, sandstone, and purple siltstone, measuring 100 to 300 feet thick. The Manakacha (see 5b in figure 1) is a cliff- and slope-forming pale red sandstone and red shale, averaging 300 feet thick in the Grand Canyon. The Wescogame (see 5c in figure 1) is a ledge- and slope-forming pale red sandstone and siltstone, 100 to 200 feet thick. The Esplanade (see 5d in figure 1) is a ledge- and cliff-forming pale red sandstone and siltstone, ranging from 200 to 800 feet thick. An unconformity marks the top of the Supai Group.
Hermit, Coconino, Toroweap, and Kaibab
Similar to the Supai Group below it, the Permian-aged Hermit Formation was likely deposited on a vast coastal plain (see 6a in figure 1). The alternating thin beds of iron oxide, mud, and silt were deposited by freshwater streams in a semiarid environment around 280 million years ago. Fossils of winged insects, cone-bearing plants, and ferns are found in this formation, along with tracks of vertebrate animals. It's a soft, deep red shale and mudstone that forms slopes, approximately 100 to 900 feet thick. The development of these slopes periodically undermines the formations above, causing car- to house-sized blocks of rock to cascade down onto the Tonto Platform. An unconformity marks the top of this formation.
The Coconino Sandstone formed about 275 million years ago as the region dried out and sand dunes composed of quartz sand advanced across a growing desert (see 6b in figure 1). Some Coconino layers fill deep mudcracks in the underlying Hermit Shale, and the desert conditions that created the Coconino persisted for 5 to 10 million years. Today, the Coconino is a 57 to 600 feet thick golden white to cream-colored cliff-former found near the canyon's rim. The cross bedding patterns, visible in the frosted, fine-grained, well-sorted, and rounded quartz grains of its cliffs, are consistent with, though not definitive proof of, an eolian (wind-blown) environment. Fossilized tracks of lizard-like creatures, and what appear to be tracks of millipedes and scorpions, have also been discovered. An unconformity marks the top of this formation.
Next in the geological sequence is the 200-foot-thick Toroweap Formation (see 6c in figure 1). It's a mix of red and yellow sandstone and shaly gray limestone, interbedded with gypsum. The formation was deposited in a warm, shallow sea, marked by fluctuating shorelines that transgressed (advanced) and regressed (retreated) over the land. The average age of this rock is about 273 million years. Today, it forms ledges and slopes and contains fossils of brachiopods, corals, and mollusks, along with other marine animals and various terrestrial plants. The Toroweap is divided into three members: the Seligman, a slope-forming yellowish to reddish sandstone and siltstone; the Brady Canyon, a cliff-forming gray limestone with some chert; and the Wood Ranch, 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 visible in the Grand Canyon area is the Kaibab Limestone (see 6d in figure 1). It erodes into ledgy cliffs that are 300 to 400 feet thick. It was laid down in the latest early Permian, about 270 million years ago, by an advancing warm, shallow sea. The formation typically consists of sandy limestone resting on a layer of sandstone. This is the cream to grayish-white rock upon which park visitors stand when viewing the canyon from both rims. It also forms the surface rock covering much of the Kaibab Plateau north of the canyon and the Coconino Plateau immediately to the south. Shark teeth have been found in this formation, along with abundant fossils of marine invertebrates such as brachiopods, corals, mollusks, sea lilies, and worms. An unconformity marks the top of this formation.
Mesozoic Deposition
The dawn of the Mesozoic era was marked by uplift, and streams began to incise the newly exposed land. During Triassic time, streams flowing through broad, low valleys deposited sediment eroded from nearby uplands, creating the once 1,000-foot-thick Moenkopi Formation. This formation is composed of sandstone and shale, with layers of gypsum in between. Outcrops of Moenkopi are found along the Colorado River in Marble Canyon, on Cedar Mountain (a mesa near the southeastern park border), and at Red Butte (south of Grand Canyon Village). Remnants of the Shinarump Conglomerate, a member of the Chinle Formation, lie above the Moenkopi Formation near the top of Red Butte, though below a much younger lava flow.
Formations totaling over 4,000 to 5,000 feet in thickness were deposited in the region during the Mesozoic and Cenozoic eras, but they have been almost entirely stripped away from the Grand Canyon sequence by subsequent erosion. The geology of the Zion and Kolob canyons area and the geology of the Bryce Canyon area preserve some of these formations. Collectively, these rock units form a super sequence known as the Grand Staircase.
Cenozoic Regional Uplift and Erosion of the Canyon
Uplift and Nearby Extension
The Laramide orogeny left its mark across all of western North America, contributing significantly to the formation of the American cordillera. The Kaibab Uplift, Monument Upwarp, the Uinta Mountains, San Rafael Swell, and the Rocky Mountains were all, at least in part, uplifted by this colossal mountain-building event. This major orogeny began near the end of the Mesozoic, around 75 million years ago, and continued into the Eocene epoch of the Cenozoic. It was driven by the subduction of oceanic plates off the western coast of North America. Major north-south trending faults that traverse the canyon area were reactivated by this uplift, many of them originating in the Precambrian and remaining active to this day. In the early Miocene, streams draining the Rocky Mountains terminated in landlocked basins in Utah, Arizona, and Nevada; there's no evidence, however, of a single, major river system.
The uplift of the Colorado Plateaus forced rivers to accelerate their downward cutting.
Around 18 million years ago, tensional forces began to thin and drop the region to the west, giving rise to the Basin and Range Province. Basins, or grabens, subsided, while mountain ranges, or horsts, rose up between old and new north-south trending faults. Remarkably, for reasons not yet fully understood, the rock layers of the Colorado Plateaus remained largely horizontal throughout these events, even as they were uplifted by approximately two miles in two distinct pulses. The extreme western edge of the canyon terminates at one of these Basin and Range faults, the Grand Wash fault, which also serves as the boundary between the two provinces.
The combined forces of uplift from the Laramide orogeny and the formation of the Basin and Range province increased the gradient of streams flowing westward across the Colorado Plateau. These streams carved deep, eastward-extending channels into the western edge of the plateau, depositing their sediment loads into the widening Basin and Range region.
According to a study published in 2012, there is evidence suggesting that the western Grand Canyon might be as old as 70 million years.
Colorado River: Origin and Development
Rifting began to open up the Gulf of California to the south, starting between 6 and 10 million years ago. Around the same time, the western edge of the Colorado Plateau may have experienced a slight sag. Both of these events influenced the direction of many streams, redirecting them towards this subsiding region. The increased gradient of these streams led to a significant acceleration in downcutting. Between 5.5 and 5 million years ago, through a process of headward erosion to the north and east, these streams coalesced into a single major river and its associated tributary channels. This river, the ancestral Lower Colorado River, began to fill the northern arm of the gulf with estuary deposits, extending nearly to the location of Hoover Dam.
The Colorado River had already carved down to nearly the current depth of the Grand Canyon by 1.2 million years ago.
Concurrently, streams flowed from the highlands of central Arizona northward, crossing what is now the western Grand Canyon, possibly feeding a larger river system. The precise mechanism by which the ancestral Lower Colorado River captured this drainage, along with the drainage from much of the rest of the Colorado Plateau, remains unknown. Hypotheses include headward erosion or the breaching of a natural dam holding back a lake or river. Whatever the cause, the Lower Colorado River likely captured the landlocked Upper Colorado River somewhere west of the Kaibab Uplift. This significantly larger drainage area, coupled with an even steeper stream gradient, further accelerated the downcutting process.
The ice ages of the Pleistocene epoch brought a cooler, wetter pluvial climate to the region, beginning approximately 2 to 3 million years ago. The increased precipitation boosted runoff and the erosive power of the streams, particularly from spring meltwater and summer flash floods. With a greatly amplified flow volume, the Colorado River cut through the rock with unprecedented speed, rapidly excavating the Grand Canyon starting about 2 million years ago, and reaching nearly its modern depth by 1.2 million years ago.
The resulting Grand Canyon of the Colorado River stretches approximately 278 miles (447 km) from Lake Powell to Lake Mead. In this distance, the Colorado River drops 2,000 feet (610 m) and has excavated an estimated 1,000 cubic miles (4,200 km³) of sediment to form the canyon. This section of the river bisects the 9,000-foot-high (2,700 m) Kaibab Uplift and passes through seven plateaus: the Kaibab, Kanab, and Shivwits plateaus bound the northern part of the canyon, while the Coconino plateau bounds the southern part. Each of these plateaus is delineated by north-south trending faults and monoclines created or reactivated during the Laramide orogeny. Streams flowing into the Colorado River have subsequently exploited these faults to carve their own tributary canyons, such as Bright Angel Canyon.
Volcanic Activity in the Western Canyon
Volcanic activity began in the Uinkaret volcanic field, located in the western Grand Canyon, approximately 3 million years ago. Over 150 flows of basaltic lava dammed the Colorado River at least 13 times between 725,000 and 100,000 years ago. These dams typically formed within weeks, extending 12 to 86 miles (19 to 138 km) in length, standing 150 to 2,000 feet (46 to 610 m) high (thicker upstream, thinner downstream), and containing volumes ranging from 0.03 to 1.2 cubic miles (0.13 to 5.00 km³).
The longevity of these dams and their capacity to impound large lakes of Colorado River water has been a subject of debate. One hypothesis suggests that water from the Colorado River backed up behind these dams, forming extensive lakes that reached as far as Moab, Utah. Dams were overtopped relatively quickly; those 150 to 400 feet (46 to 122 m) high were breached by their lakes in just 2 to 17 days. Simultaneously, sediment rapidly filled the lakes behind the dams. Sediment would fill a lake behind a 150-foot (46 m) dam in less than a year, a lake behind an 1,150-foot (350 m) dam in about 345 years, and the lake behind the tallest dam in approximately 3,000 years. Water cascaded over the dams, forming waterfalls that migrated upstream. Most lava dams are estimated to have lasted for around 10,000 to 20,000 years. However, other researchers propose that these lava dams were far more ephemeral, failing catastrophically before overtopping. In this model, dams would fail due to fluid flow through fractures within the dams and around their abutments, as well as through permeable river deposits and alluvium.
Since the disappearance of these dams, the Colorado River has carved a maximum of about 160 feet (49 m) into the rocks of the Colorado Plateau.
Ongoing Geology and Human Impact
The end of the Pleistocene ice ages and the dawn of the Holocene marked a shift in the region's climate, moving from a cool, wet pluvial period to the drier, semi-arid conditions seen today. With less water available for erosion, the Colorado River's cutting power significantly diminished. Consequently, mass wasting processes became relatively more dominant than before. This led to the formation of steeper cliffs and further widening of the Grand Canyon and its tributary canyon system. On average, two debris flows per year reach the Colorado River from tributary canyons, contributing to the formation or expansion of rapids. This type of mass wasting is the primary mechanism for sediment transport in smaller, steeper side canyons, but it also plays a crucial role in excavating the larger canyons.
Glen Canyon Dam has drastically reduced the amount of sediment transported by the Colorado River through the Grand Canyon.
In 1963, Glen Canyon Dam and other upstream dams began regulating the flow of the Colorado River through the Grand Canyon. Prior to the dams, historical flows of the Colorado through the Grand Canyon ranged from 700 to 100,000 cubic feet (20 to 2,832 m³) per second, with at least one flood in the late 19th century reaching 300,000 cubic feet (8,500 m³) per second. Discharge from Glen Canyon Dam now exceeds 48,200 cubic feet (1,360 m³) per second only in situations where there is a risk of overtopping the dam or when the water level of Lake Powell needs to be lowered. An interim conservation measure, in place since 1991, has capped maximum flows at 20,000 cubic feet (570 m³) per second, even though the dam's power plant is capable of handling an additional 13,200 cubic feet (370 m³) per second.
The regulation of river flow by dams has significantly diminished the river's ability to scour rocks by substantially reducing the amount of sediment it carries. The dams on the Colorado River have also altered the character of the river water. Once both muddy and warm, the river is now clear and maintains an average temperature of 46°F (8°C) year-round. Experimental floods, approaching the 48,200 cubic feet (1,360 m³) per second level mentioned above, were conducted in 1996 and 2004 to study their effects on sediment erosion and deposition.
The Grand Canyon is situated at the southern end of the Intermountain West seismic belt. At least 35 earthquakes registering above 3.0 on the Richter Scale occurred in the Grand Canyon region during the 20th century. Of these, five registered above 5.0 on the Richter Scale, with the largest being a 6.2 magnitude quake in January 1906. Major, roughly north-south trending faults that cross the canyon include, from west to east, the Grand Wash, Hurricane, and Toroweap faults. Major northeast-trending fracture systems of normal faults intersecting the canyon include the West Kaibab and Bright Angel faults, while northwest-trending systems include the Grandview—Phantom faults. Most earthquakes in the region occur within a narrow northwest-trending band between the Mesa Butte and West Kaibab fracture systems. These seismic events are likely a consequence of eastward-migrating crustal stretching that may eventually move beyond the Grand Canyon area.
Trail of Time and Yavapai Geology Museum
The Trail of Time is an outdoor geological exhibit and nature trail located on the South Rim of Grand Canyon National Park. Each meter walked along the trail represents one million years of the Grand Canyon's geological history. Bronze markers are placed along the path, indicating your position in time. The trail begins at "Today," near the Yavapai Geology Museum, and concludes 2 billion years later at Verkamp's Visitor Center. Along the way, visitors encounter samples of the canyon's rocks, arranged as they would be encountered from the rim down to the river, with displays explaining the geological history of the canyon. The Trail of Time was opened in late 2010.
The Yavapai Geology Museum features three-dimensional models, photographs, and exhibits designed to help park visitors understand the complex geological story of the area. The museum building itself is the historic Yavapai Observation Station, constructed in 1928 and located one mile (1.6 km) east of Market Plaza. It offers expansive views of the canyon. A bookstore within the museum provides a variety of materials related to the area.