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From when it all began 2 billion years ago up to the canyon we see today, many different geologic processes have acted on the Grand Canyon. The four most major of these processes can be easily remembered with a simple acronym: D.U.D.E.
From when it all began 2 billion years ago up to the canyon we see today, many different geologic processes have acted on the Grand Canyon. The four most major of these processes can be easily remembered with a simple acronym: D.U.D.E.
D
D
Deposition - After the formation of the igneous and metamorphic layers at the bottom of the Grand Canyon, layer upon layer of sediment deposited. Over millions of years, this sediment built up to form the countless layers we see today.
U
U
Uplifting - Plate tectonics then pushed all these layers upward to form the Colorado Plateau.
D
D
Downcutting - The Colorado River flowed quickly and cut down through the Colorado Plateau like sandpaper. The high volume of water was able to pull large rocks and even boulders through the channel.
E
E
Erosion - The Colorado River continued to erode and downcut for millions of years. The arid climate, coupled with the steep slope and large volume of water in this river meant that the Grand Canyon saw erosion faster than typical conditions would allow for. Tributaries running into the Colorado River worked to slowly deepen and widen the canyon as well.
Scientists are confident about all of these processes, but exactly how the Colorado Plateau uplifted has left them puzzled for years...
Why is the uplifting of the Colorado Plateau so special?
The rock that makes up the Colorado Plateau didn't deform! Usually when an area uplifts, something big and rocky like a mountain range is formed, but this wasn't the case at all for this plateau. Scientists currently have two main theories as to the processes responsible for this smooth uplifting:
1.) Subduction would have occurred during the Laramide Orogeny, when the Farallon Plate went underneath the North American Plate. This reverse fault would have seen movement at a relatively fast pace and a shallow angle. This theory would also explain how the plateau wound up so far from the plate margin today. The figure at the left shows how this subduction would have occurred, as well as how it would also explain the formation of the Sierra Nevada Mountain Range.
2.) Continued uplift due to isostatic rebound would mean that the Colorado Plateau uplifted well into the middle/late Cenozoic Period. Large amounts of erosion during this period would have released a lot of pressure off of Earth's crust, causing it to rise upward. Scientists have seen isostatic rebound in action from glaciers around the world. The figure at the right shows how this glacio-static rebound occurs. It would be the same in the case of the uplift of the Grand Canyon, except instead of ice being removed it would have been a lot of sediment.
This image shows how Spillover Theory works. As you can see, local base-level (the lake bed) will fill up until it cannot anymore, then water will spillover the edge and cause rapid downcutting and erosion as it flows toward the next base-level. The process will repeat until the water reaches ultimate base-elevel (sea level).
But what gave the Colorado River its exceptional power?
But what gave the Colorado River its exceptional power?
A drastic change in base-level millions of years ago!
Base-Level Change in the Spillover Theory
Base-Level Change in the Spillover Theory
To begin, what is a river's base-level?
The base-level of a river is the lowest elevation to which a river can flow. Gravity, coupled with water always wanting to reach equilibrium, means that rivers will always flow toward their base-level. The ultimate base-level is sea level, but rivers can have local base-levels when there are large land forms in their way.
The Colorado River, for example, used to be two separate rivers: the Upper Colorado River and the Lower Colorado River. The Upper Colorado River's local base-level was Lake Bidahochi, and the Lower Colorado River's base level was the Gulf of California. As Lake Bidahochi filled up, eventually it spilled over, linking the two rivers into one. This brand new base level for the Upper Colorado River caused it to gain a ton of energy and cause rapid downcutting and erosion right where the two rivers now became one. This incredible downcutting and erosion is exactly what formed the Grand Canyon over millions of years to come.
Volcanism
Volcanism
The Western end of the Grand Canyon used to be dotted with cinder cones. These cinder cones caused lava to cascade over the North Rim about 630,000 years ago which dammed the Colorado River at least 13 times. Each time, water would accumulate behind the dams until it overflowed, and then the flow of the water over the dam would quickly erode the dam the lava flow caused. This would lead to huge outburst floods. The last time the river was blocked was about 400,000 years ago.
Present Day
Present Day
Although much slower than millions of years ago, today we still see the last two letters of D.U.D.E. acting on the canyon. Downcutting and eroding is still occurring from both The Colorado River and the tributaries that flow into it. The Colorado River is now controlled by the Glen Canyon Dam, so there are no longer huge changes in the volume of water flowing in the canyon. Major volcanic activity in the area has also stopped, which means huge outburst floods are no more. On top of this, the rock that the river now cuts through is igneous and metamorphic so it takes a lot longer to erode through. Tributaries, however, are still cutting through sedimentary rock, so they erode the land the run over more quickly. This means the canyon is currently getting wider much more quickly than it is getting deeper.
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