Grand Canyon Geology - mark pettersen€¦ · Early Study of Grand Canyon Geology To early geologists the Grand Canyon appeared to be “an open book” to the geology of the Earth’s

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The Grand Canyon and the Colorado River from Lipan PointPhoto Paul Fretheim

Geology of the Grand Canyonby Paul Fretheim

When Garcia Lopez de Cardenas, the first European explorer tosee the Grand Canyon, found himself at the canyon rim in Septem-ber of 1540 he exclaimed, “¿Que pasa aqui?” (“What happenedhere?”)

Over the intervening centuries millions of visitors have wonderedthe same thing as they have gazed upon the magnificent spectacleof the Grand Canyon of the Colorado.

The Story Has Two PartsTwo things impress the eye upon seeing the canyon for the first

time: A huge gash has been carved into the Earth’s surface; The

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canyon walls have multi-colored horizontal bands of rock that arestrikingly uniform and symmetrical.

These two aforementioned phenomena are representative ofthe two threads in the story of the geology of the Grand Canyon.

The first part of the story is that of the recent events, geologi-cally speaking, of the last few million years that carved the canyon.

The second part of the story is that of the much longer periodof time over which the layers of rock into which the canyon wascarved were formed.

Uplift and Erosion Result in Canyon TopographyThe Colorado River has flowed westward from the Rocky Moun-

tains to the Pacific Ocean for at least the past 80 million years.

The part of the North American continent centered on the FourCorners, east of the Great Basin and west of the Rocky Mountains isknown as the Colorado Plateau. Click on the link below to bring upthe USGS web page on the Colorado Plateau.

http://geology.wr.usgs.gov/docs/parks/province/coloplat.html

This part of North America began to be uplifted by tectonic forcessome 5 - 10 million years ago. The Colorado River at this time wasalready flowing westward across the Colorado Plateau. For the past60 million years, at least, the gradient of the river from the high-lands of the Rockies to the Pacific has always been quite steep. Be-cause of this the water in the Colorado River has always flowed swiftly.Fast flowing streams can carry exponentially more sediment thanslower flowing streams and have tremendous power to erode awaythe rock of the riverbed. Because of these factors, the Coloradohas always been capable of cutting its way down to “base level.”

The Colorado Plateau has been being uplifted for the past fewmillion years. Some geologist place the date for the beginning ofthe uplift as recently as three million years ago, others date the startof the uplift as long as ten million years ago. As the Colorado Pla-teau has been rising, the Colorado River has been able to cut its waydown through the bedrock of the plateau as fast as it has been ris-ing up in the river’s path.

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So, expressed most accurately, the Colorado River did not cutdown into the Earth’s surface to create the Grand Canyon, but, rather,the rock of the Colorado Plateau rose beneath the river and the rivercut its way through the rock to maintain its level nearly constantdespite the nearly 2000 meters of uplift which the Colorado Plateauhas undergone over the past few million years.

The First GeologistsGeology, the study of the structure and prehistoric past of the

Earth, had begun a generation or two before the exploration of theGrand Canyon region. The first scientists to make an organized studyof the Earth’s past began their work in England. Charles Lyell andCharles Darwin are two of the most famous of those early geolo-gists.

Lyell, who was a generation older than Darwin and a mentor ofhis, was among the first to systematically study fossilized life forms.He noticed that the rocks of the Earth’s surface seemed to occur indistinct layers. After a period of time studying the fossilized lifeforms, he noticed that below a certain rock formation the numberof fossils dropped off dramatically. Lyell understood that the layerof rock where a large number of fossils first appeared marked thetime when complex life first appeared on the Earth.

Since Lyell found this layer of rock near Cambridge he named it“Cambrian.”

Many of the names we still use today when describing rock for-mations which correspond to a specific time in Earth’s geologicalpast come from this era of early study in 19th century England. Asanother distinct rock formation was identified it was given a namecorresponding to the area where that layer was found. For example,the Devonian was first identified in Devonshire County, England.

Early Study of Grand Canyon GeologyTo early geologists the Grand Canyon appeared to be “an open

book” to the geology of the Earth’s past. They believed that surelysuch a deep canyon exposed rocks from the very beginning of time.Actually, the oldest rocks of the Inner Gorge are about 1.7 billionyears old, which is only about one third as old as the Earth itself,

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which has been datedby measuring the ratiosof isotopes of uraniumas being about 4.6 bil-lion years old.

After his widelyhailed trips of explora-tion down the previ-ously unknown Colo-rado River, John WesleyPowell parleyed his wellearned fame to becomedirector of the UnitedStates Geological Survey.As director he contin-ued in his efforts to un-cover the scientific se-crets of the ColoradoPlateau, and in 1880 hesent a party which in-cluded geologistClarence E. Dutton tomap and study the ge-ology of the Grand Can-yon district.

Upon his return tocivilization in 1881,

Dutton published the results of his findings under the title “The Physi-cal Geology of the Grand Canyon District.” The completeness andaccuracy of this geological treatise are really quite astounding con-sidering that it was written in the latter part of the 19th century.This work is considered by many to contain, even to this day, themost colorful and best descriptive prose of the Grand Canyon everwritten.

At the close of his monograph on the Geology of the Grand Can-yon District Dutton wrote:

John Wesley Powell and Tau-Gu, Chief of thePaiutes overlooking the Virgin River. 1873.

GRCA 13806

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No doubt the question will often be asked, how long hasbeen the time occupied in the excavation of the Grand Cañon. Un-fortunately there is no mystery more inscrutable than the durationof geological time. On this point geologists have obtained no satis-factory results in any part of the world. Whatever periods may havebeen assigned to the antiquity of past events have been assignedprovisionally only, and the inferences are almost purely hypotheti-cal. In the Plateau country, Nature has, in some respects, been farmore communicative than in other regions, and has answered manyquestions far more fully and graciously. But here, as elsewhere,whenever we interrogate her about time other than relative, herlips are sternly closed, and her face becomes as the face of theSphinx.

The most significant addition to the understanding of the Geol-ogy of the Grand Canyon District since Dutton’s time has been theability of geologists to add absolute dating to the relative datingthat was largely worked out by Dutton in 1880 - 1881. Absolutedates, that is, when, in years an event or series of events took placein the past, has been worked out in the last 50 years. This achievmenthas been accomplished by geologists utilizing dating techniquesbased upon the ratios of different radioactive isotopes of a variouselements found in rock formations.

Tectonic Forces and Periods of Erosion and DepositionExcept for the basement metamorphic and igneous rocks of the

Inner Gorge, which are as much as 1.7 billion years old, nearly all therock formations of the Grand Canyon are from the period of timegeologists call the Paleozoic. This name means “old life” in English.

There have been at least two major events in Earth’s past thatresulted in extinctions of a large percentage of the species whichexisted at the time of the extinctions. That is to say, that at twopoints in the fossil record, one 250 million years ago and one 65million years ago, the number of different species drops off sud-denly and many of the species whose fossils are found in rocks olderthan those dates do not appear after those times.

The life forms that lived after the first flowering of species dur-ing the Cambrian up until the first great extinction are the life formsthat geologists are referring to when they use the term Paleozoic.

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This time period, the Paleozoic, stretches for about 230 million years,from the emergence of complex multi-cellular life on the planetduring the Cambrian, some 580 million years ago, until about 250million years ago, the time of the first great extinction.

It was during this time, the Paleozoic, that the landmass thattoday embodies the Colorado Plateau was intermittently submergedunder seas of varying depth. The sediments that comprise today’scolored layer rock formations of the Grand Canyon collected at thebottom of these seas that covered the Colorado Plateau during Pa-leozoic times. ( See: Colorado Plateau Through Geologic Time)

Most areas of the Earth’s surface have been subjected to foldingand other distortions of the original horizontal orientation of theirsedimentary layers of rock. One of the unique features of GrandCanyon geology and of the Colorado Plateau in general is the lack ofdeformation of the rock formations over an extensive area. This re-tention of the formations organization gives the Grand Canyon itsstrikingly uniform multi-hued layered character.

In the time since the Colorado Plateau rose above the sea some250 million years ago, the forces of erosion have been at work andhave removed over 2000 meters of rock from the surface over thegeneral area of the Colorado Plateau. Dutton referred to this as“The Great Denudation.” We know now that it was not the result ofany single cataclysmic event, but was theproduct of the accumulated effects oferosional forces over a very long periodof time.

This article was meant to give thereader only the most brief overview ofGrand Canyon geology. For furtherreading see An Introduction to GrandCanyon Geology by L. Greer Price, whichis great little book and is available fromthe Grand Canyon Association at a veryreasonable price. You can contact theGrand Canyon Association via their on-line bookstore at:

http://www.grandcanyon.org/bookstore