How was the Grand Canyon formed? The short answer
The Grand Canyon was carved by the Colorado River, working for about 5 to 6 million years through a high plateau of layered rock.17 Geologists at Grand Canyon National Park sum up the story with one easy word, DUDE:1
- Deposition: over hundreds of millions of years, sand, mud and shells piled up in layers that hardened into rock.
- Uplift: between about 70 and 30 million years ago, the slow push of Earth's plates lifted the whole region into the high, fairly flat Colorado Plateau.
- Downcutting: starting 5 to 6 million years ago, the river began sawing down through the plateau, mostly during floods.
- Erosion: rain, side streams, ice and rockfalls widened the walls into the canyon we see today.
The river made the canyon deep. Rain, ice and gravity made it wide.7
Step 1: Deposition, a stack of lost worlds
Before there could be a canyon, there had to be rock. The walls are built from layer upon layer of rock. The National Park Service counts nearly 40 of them.2 Most are sedimentary: loose sand, mud or bits of shell that settled, got buried and slowly turned to stone, often trapping fossils as they did.5 (Want the details of how that works? See how sedimentary rocks are formed.)
Layers pile up from the bottom, so the oldest rocks are at the bottom and the youngest are on top. Geologists call this the principle of superposition. They also know layers are first laid down flat, so tilted layers mean something moved them later.1
Each layer records a different world. Here are the main ones, from the rim down. Ages are the Park Service's best estimates.3
The oldest dated rock in the canyon is the Elves Chasm granite, at about 1,840 million years old.3 Even the youngest layer on the rim, at about 270 million years, is older than the oldest known dinosaurs. That's why you won't find dinosaur fossils in the Grand Canyon.5
The Great Unconformity: pages torn out of the book
The record isn't complete. In places, flat layers about 500 million years old sit directly on top of the ancient Vishnu rocks, with the rocks from the time in between missing. Some were never laid down, and some were worn away. Geologists call gaps like this unconformities, and the biggest one here is famous as the Great Unconformity, where more than a billion years of history is missing.74 The rocks from the "Age of Dinosaurs" that once lay on top of the rim have mostly been eroded away too. You can still see them in nearby Red Butte and in the cliffs of southern Utah.27
Step 2: Uplift, how sea-floor rock ended up 9,000 feet high
The Kaibab limestone on the rim formed at the bottom of a sea. Yet today it sits as high as 9,000 feet above sea level.1 Between about 70 and 30 million years ago, plate tectonics pushed the whole region upward, creating the Colorado Plateau.1
What's strange is how it rose. The rocks of the Rocky Mountains were crunched and folded as they were pushed up. The plateau's layers were lifted high and nearly flat, like a table being raised.1 Scientists are still investigating why. A leading idea is that an ocean plate, the Farallon Plate, slid under North America at an unusually shallow angle. Another is that the crust kept rising as erosion took weight off it, a process called isostatic rebound.1
Uplift mattered because it gave the river somewhere to fall. A high plateau means a steep, fast river, and a steep, fast river can cut deep.7
Step 3: Downcutting, how the Colorado River carved the canyon
Water on its own is soft, so how does a river cut through rock? It uses tools. During floods, the Colorado River sweeps along huge amounts of sand, gravel and boulders, and these rocks act like chisels, chipping away at the riverbed as they bounce along.1 Before Glen Canyon Dam was built, the river carried an average of about 500,000 tons of sediment a day.7
Three things made the Colorado an unusually good canyon-cutter:1
- A steep slope. Through the canyon, the river drops about 2,000 feet over 277 miles. For a river, that's steep, so it flows fast and can move big rocks.
- Big floods. The river averages 12,000 to 15,000 cubic feet per second, but before the dams spring floods reached 100,000 cfs or more, and the biggest recorded, in 1884, hit about 300,000 cfs, roughly 20 times the average.
- A dry climate. With little soil or plant cover, bare rock is exposed and weathers quickly.
steep slope + big floods + rock "tools" = fast downcutting
Another key event was the opening of the Gulf of California. Once the river had a path to the sea, the lowest level it could cut down to dropped by about 5,000 feet, which set the stage for carving through the plateau.1
How do we know it took 5 to 6 million years?
Landscapes are harder to date than rocks, so geologists date the deposits around the river. The Hualapai Limestone, about 6 million years old, formed in a lake and has no river sediment in it, so the Colorado wasn't flowing there yet. River gravels lie under the Sandy Point Basalt, a lava flow 4.4 million years old, so the river was flowing by then. Other deposits downstream show the river was carrying plateau rock to the sea by about 5 million years ago.1 Put together, the canyon's age comes out at about 5 to 6 million years.18
Step 4: Erosion, why the Grand Canyon is so wide
A river on its own cuts a narrow slot. The canyon is up to 18 miles wide because other forces work on the walls:7
- Side streams. Rain and snowmelt rush down tributary canyons. Much of the canyon's width comes from these side drainages.1
- Ice wedging. In winter, water freezes in cracks, expands and pries rocks loose.1
- Temperature swings and rain break down the bare rock.17
- Gravity. Loosened rock falls, and the river carries the pieces away.
Not every layer wears down at the same speed. Hard rocks like the Coconino Sandstone and Redwall Limestone stand as steep cliffs. Softer shales like the Bright Angel and Hermit crumble into slopes. That's what gives the canyon its stair-step look.2
The dry climate is the secret ingredient. Rain is rare, so the walls wear back slowly while the river cuts down fast. In a wetter place, rain would wash the walls into a broad, gentle valley.27 Britannica goes as far as saying that without the semiarid climate, there would be no Grand Canyon.7
How old is the Grand Canyon compared with its rocks?
This is the big surprise. The rocks are old, up to about 1.8 billion years, but the canyon cut into them is young, only about 6 million years.16 Earth is about 4,560 million years old.3 If you squeezed all of Earth's history into one 24-hour day, the Grand Canyon would appear in roughly the last two minutes.
The Park Service's Trail of Time on the South Rim lets you walk this: every meter is a million years, so the canyon's entire carving fits in your last six steps, while the rocks at the bottom are nearly 2,000 steps back.63
What scientists still debate
The basic story, DUDE, is well supported. But some chapters are still argued about, and the Park Service says so openly.12
- How the river found its path. The Colorado slices right through the Kaibab Plateau, a high point, and water doesn't flow uphill. The upper river seems to be older (at least 7 million years) than the lower river (5 to 6 million years), so it may once have been two rivers that joined. Several ideas compete.1
- Whether parts of the canyon are much older. A 2012 study argued that the western Grand Canyon could be about 70 million years old.9 A 2014 study answered that older canyon segments existed but were only linked into today's through-flowing canyon 5 to 6 million years ago.8
- Exactly how the plateau rose so flat (see Uplift above).1
That's normal science: the evidence points clearly to a river-carved canyon, and new data keep refining the details.
Is the Grand Canyon still changing?
Yes. The same forces are still at work, slowly deepening and widening the canyon.1 Today it's widening faster than it's deepening: the river is now cutting through very hard ancient rock, its slope has eased, and Glen Canyon Dam has tamed its floods.1 The volcanoes were busy too. Starting about 630,000 years ago, lava poured over the North Rim and dammed the river at least 13 times. The river cut through every one.1
Common misconceptions
"The canyon is billions of years old." The
rocks are up to 1.8 billion years old. The canyon carved into them is only about 5 to 6 million years old.
1
"The river made it wide." The river made it deep. Side streams, rain, ice and rockfalls made it wide.
17
"A glacier carved it." Glaciers never existed in or near the Grand Canyon.
1
"It was made in one giant flood." Dated lava flows and river deposits show carving over millions of years, during many floods.
1
"The river cut down while the land stayed put." Uplift raised the land high first, which made the river steep and powerful enough to cut a mile deep.
17
FAQ
How was the Grand Canyon formed, in simple words?
First, layers of sand, mud and shells piled up over hundreds of millions of years and turned into rock. Then the land was pushed up into a high plateau. About 5 to 6 million years ago the Colorado River started cutting down through it, and rain, ice and side streams widened the walls. Geologists remember it as DUDE: deposition, uplift, downcutting, erosion.
How old is the Grand Canyon?
The canyon itself is about 5 to 6 million years old, based on dated rocks and river gravels along the Colorado River. The rocks in its walls are much older: from about 270 million years at the rim to about 1.8 billion years at the bottom. A few scientists argue that parts of the canyon are much older, so this is still being studied.
Was the Grand Canyon once a sea?
The canyon wasn't, but many of its rock layers formed under shallow seas. The Kaibab Limestone on the rim, the Redwall Limestone and the Tonto Group all hold fossils of sea animals such as brachiopods, crinoids and trilobites. Other layers formed in deserts of sand dunes or on muddy river plains.
Where did all the dirt from the Grand Canyon go?
The Colorado River carried it away downstream, eventually toward the Gulf of California. Before Glen Canyon Dam was built, the river moved an average of about 500,000 tons of sediment every day. The sand and gravel it carried also acted like sandpaper that helped cut the canyon.
Did a glacier or a single flood make the Grand Canyon?
No. The National Park Service notes that glaciers never existed in or near the Grand Canyon. Dated lava flows and river deposits show the canyon was carved over millions of years, mostly during repeated floods of the Colorado River.
Why are the Grand Canyon's walls striped and stepped?
Each stripe is a different rock layer formed in a different environment, so each has its own colour. Hard layers such as the Coconino Sandstone and Redwall Limestone form steep cliffs, while softer shales crumble into slopes. That mix makes the canyon's famous staircase shape.
Is the Grand Canyon still getting bigger?
Yes, slowly. Side streams, rain and ice keep widening it, and the river keeps cutting, although it is now working through very hard ancient rock and its flow is controlled by Glen Canyon Dam, so the canyon is widening faster than it is deepening.
Sources
- National Park Service, Grand Canyon: Geology (Deposition, Uplift, Downcutting, Erosion)
- National Park Service, Grand Canyon: Geologic Formations
- National Park Service, Numeric Ages of Grand Canyon Rocks (Telling Time at Grand Canyon)
- National Park Service, Grand Canyon's Three Sets of Rocks
- National Park Service, Grand Canyon: Fossils
- National Park Service, The Trail of Time
- Encyclopaedia Britannica, Grand Canyon
- Karlstrom, K. E. et al. (2014), Formation of the Grand Canyon 5 to 6 million years ago through integration of older palaeocanyons, Nature Geoscience 7: 239–244
- Flowers, R. M. and Farley, K. A. (2012), Apatite 4He/3He and (U-Th)/He evidence for an ancient Grand Canyon, Science 338: 1616–1619