The Grand Canyon: National Icon And Elusive Mystery

The Grand Canyon is a geologic and national icon with which even young school children are familiar and countless tourist visit each year. And yet the processes that went into the formation of this national symbol of awe, wonder and pride has remained an elusive mystery to geologists until rather recently. Only now, piece by piece, have scientists begun to unlock the secrets which this behemoth has dearly held on to for billions of years.

The Mountains That Once Existed Where The Grand Canyon Is Today

In order to understand the processes that led to the formation of the Grand Canyon we must first understand the geologic history going back a couple of billions years ago–before the formation of the Grand Canyon itself.

Major John Wesley Powell

The first to lead a scientific expedition down the Colorado River was the then thirty-five-year-old geologist and the second Director of the USGS from 1881-1894, Major John Wesley Powell (pictured above), who incidentally was the person to give the canyon its name. One of Powell’s discoveries was what he referred as “ugly black rocks”–so named because these extremely hard and resilient rocks were responsible for so many of the perilous rapids that Powell and his team had encountered during their long trek down the Colorado River through the entire length of the Grand Canyon. Unknown to Powell at the time, these “ugly black rocks” would prove to be one of the keys in unlocking the secrets of the Grand Canyon.

Karl E. Karlstrom, Ph.D.

As Karl E. Karlstrom, Ph.D., Professor of Structural Geology and Tectonics at the University of New Mexico (pictured above) realized, this black rock was a type of metamorphic rock known as Vishnu Schist, which is formed at great depths under extreme heat and pressure.

Karlstrom had the advantage of modern analytical technology that was not available at the time John Wesley Powell had done his scientific survey of the Grand Canyon. Karlstrom was able to ascertain with great accuracy two very important bits of information contained within the Vishnu Schist.

Through radiometric data, Karlstrom was able to determine that the age of the Vishnu Schist was around 1.7 billion years old. Another clue that Karlstrom had were the garnets contained within in the Schist, which only form under great pressure that can only be produced by millions of tons of overlying rock (i.e. mountains). By determining the amount of calcium contained within the garnets, it was possible for Karlstrom to determine precisely how deep this schist was buried when it formed–the higher the calcium content in the garnets, the deeper the schist in which the garnets were formed had been buried. What Karlstrom determined was that the schist had been buried at a depth of 6 miles beneath the mountain peeks that existed in this region 1.7 billion years ago. After a great period of time, these extremely tall mountains were eroded down to a low-lying flat plain, just as the Appalachian and Rocky Mountains are being eroded today.

Marine Fossil In The Upper Layers Of Rock

The next clue as to the processes that went into the formation of what is today the Grand Canyon is the marine fossils that have been found in the layers of limestone, sandstone and shale towards the top of the Grand Canyon, almost a mile above the Vishnu Schist. This begs the question: How in the world did the fossils of marine organisms end up in rock around one mile above sea level?

About a half a billion years ago the Grand Canyon region was a low-lying plain that for over hundred of millions of years had been covered by inland seas–at least eight times. After one sea retreated having deposited sediments with the remains of dead marine life (i.e. hard tissue such as shells, exoskeletons, etc.) contained within it, another sea would cover the landscape depositing different kinds of sediments also with the remains of dead marine life from another time contained within it. This process continued over and over again until about 80 million years ago, with each inland sea depositing either mud or calcium carbonate from living organism or sand, which all in time would become buried. And by being subjected to the weight of overlying rock and sediment, each layer of sediment would undergo a process known as lithification, in which the mud would be transformed into shale, the calcium carbonate into limestone and the sand into sandstone.

But that still doesn’t explain how these layers of sedimentary rock rose to a mile above sea level. So, exactly how did they get up that high?

The Laramide Orogeny

This is where plate tectonics and convergent plate boundaries come in. The Laramide orogeny is something that is not well understood, and quite honestly, there is a lot of controversy surrounding exactly when it occurred and the precise processes that took place. It is, however, believed to have started during the Late Cretaceous towards the end of the Mesozoic Era some 70 to 80 million years ago.

It is believed that during the Laramide orogeny the angle at which the Kula and the Farallon Plates were being subducted under the west coast of the North American plate shallowed-out and ran along the bottom of the North American Plate rather than dropping down steeply into the mantle as the Pacific Plate does today. It is believed that as a result of this shallowing-out of the subducted plates that it created a kind of drag underneath the continental crust and in this manner, not only uplifted the region where the Grand Canyon exists today as a plateau instead of folding and tilting the land into mountains like the Rockies, but also created the unusual structures known as the flatirons in Colorado.

This is the current theory and is perhaps the most controversial part of the story as to how the Grand Canyon came into existence.

It is also believed that when the Colorado Plateau, that would later become the Grand Canyon, was uplifted, the water from the last inland sea covering the plateau ran off toward the northeast, leaving a dry and very high plateau behind.

Okay, so now we have a very high plateau, but still no canyon. So where did the water come from that incised the canyon which later became what we know as the Grand Canyon?

The Spill-Over Theory

During the sixties, it was generally believed by geologists that Hindu Canyon was the original source for the Colorado River. But in 1969 Dr. Richard Young, then a 24-year-old graduate student at the University of Washington, was sent to Hindu Canyon to investigate. What Young discovered is that the alignment of rim pebbles in Hindu Canyon indicated that the river that once flowed there ran in the opposite direction that the Colorado River does and could have never possibly have been the source of the Colorado River. Furthermore, Young found at Muddy Creek, located a relatively short distance from where the Colorado exits the Grand Canyon, that up until 5.5 million years ago a fresh-water lake once existed there with marine organism that had produced layers of limestone. Then 5.5 million years ago the limestone was no longer being produced. From this one can infer that the organisms that once lived there all of a sudden became extinct. On top of the limestone are large deposits of mud and silt which could only have been deposited by a powerful, eroding river, i.e., the Colorado River. Marine organisms that create limestone cannot survive in silty and muddy water. So when the silty and muddy waters that created the Colorado River spilled over the escarpment of the western edge of the Colorado Plateau into the existing fresh-water lake, it killed the organisms from which the limestone had been created. And this is how the age (i.e. 5.5 million years old) of the Colorado River was determined.

With these two new revelations in hand, Young conclusively proved that the Colorado River 1) always flowed toward the Pacific and 2) was 5.5 million years old—much younger than anyone had previously thought. So the question still remained, what was the source of the water that carved out the Grand Canyon?

Enter John Douglass, Ph.D. with a radical idea known as the spill-over theory. What this theory entails is that rivers from mountains to the east of the Grand Canyon poured water into a giant basin, larger than Lake Michigan today. Once the water reached the top of the western rim of the lake, it spilled over onto the plateau where the Grand Canyon currently is.

But, Douglass needed evidence for his radical theory. So, the first thing he set out to do is prove using a scale model of the area that it was possible for something like this to happen. In geology we often use scaled-down models (e.g. a flow tank) to test theories about phenomena that we observed in nature. This is possible because many geological processes are scale independent. So, that’s exactly what John Douglass did which you can see in the short video clip below.

But, contrary to what is stated in the video, we now know that this is what happened with a pretty high degree of certainty. How do we know that this is what actually occurred? Well let’s put the pieces together:

1. We know from Young’s work that the Colorado River always flowed toward the Pacific and never could have flowed toward an easterly direction.

2. Further investigation done by John Douglass in analyzing the sediment of Lake Bidahochi revealed green clay deposits that can only form in extremely deep lakes where oxidation is very low.

3. Douglass also found within the sediment the fossil shells of fresh-water mollusks as young as 6 million years old, making Lake Bidahochi a half a million years older than the Grand Canyon, thus putting Lake Bidahochi at the right place at the right time.

4. Water travels naturally from higher elevations to lower elevations and not the other way around. That would make the mountains to the East of the Colorado Plateau the only logical source for the water.

5. A subsequent and extensive survey done by Douglass of the terrain of ancient Lake Bidahochi and the watermarks left behind on the landscape, proved that at one time Lake Bidahochi held more water than Lake Michigan–more than enough water to have flooded the Colorado plateau at the time.

6. A competing theory of merging rivers from the East creating the Colorado River has never panned-out. The evidence is simply not there.

7. The great altitude of the Colorado Plateau at the time, as a result of the Laramide orogeny, explains how the water flowing over the Colorado Plateau over the escarpment at the western edge of the plateau could easily have cut out the deep gash in the plateau within a little over 5 million years by means of a great waterfall that cut its way back to the present-day source of the Colorado River.

8. New analytical techniques of changes in the atomic structure of sand near the source of the Colorado River resulting from exposure to sunlight at various times in the evolution of the Colorado River as well as the Grand Canyon shows that the Colorado River cut through the Colorado Plateau by means of a retreating waterfall about 1 foot per century or a little over one inch per year.

So, even though we can never be 100% certain that this was the way that the Grand Canyon was formed, nevertheless, the evidence is pretty conclusive.

Today, John’s spill-over theory has gained wide acceptance in the geological community and is considered the only reasonable explanation for the origin of the Colorado River.

So, with this tremendous source of water from Lake Bidahochi, the matter of incising a long gash into the Colorado Plateau becomes a relatively simple thing to explain. With the high altitude of the plateau you would have had a very high escarpment at the western edge of the plateau. This basically would result in a very tall waterfall that over time would have cut a narrow gash through the bedrock of the plateau—working its way back just as Niagara Falls has been doing and is still doing today. That would explain the depth of the incision into the plateau, but it would not explain how the Grand Canyon became so wide.

Remember earlier when I was talking about how layers of sedimentary rock were laid down over 100 of millions of years? Well, in between harder rock layers of limestone and sandstone you have much softer shale, which when it gets wet from rain, swells-up and crumbles. After enough of the softer shale has been eroded away, the weight of the overlying limestone and sandstone exceeds the tensile strength of these rock layers which in time leads to structural failure and landslides of catastrophic proportions. The debris from these landslides is washed downstream over time as rock debris and sediment, and the cycle repeats itself over and over again. The canyon continues to widen and will do so for many millions of years. And that is how we have ended up with beautiful vistas such as the image below of the south rim of the canyon, which is the widest part of the Grand Canyon.

South Rim of the Grand Canyon

The research is far from finished. And I don’t pretend that we have all of the answers yet regarding the formation of the Grand Canyon. But we have learned a heck of a lot in the last few decades, and we’re continuing to learn.

(All images used in this article are in the public domain and were obtained from copyright free sources.)

Recommended reading:

The Exploration of the Colorado River and Its Canyons by John Wesley Powell (also available at Amazon.com)

Copyright © 2010 Eric F. Diaz

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