Geology And Geophysics

Geological History of the Grand Canyon

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"Geological History of the Grand Canyon"
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Of all the beauty of North America, there is one place that consistently turns up on lists of the "seven natural wonders of the world." That's the Grand Canyon of the Colorado River, the enormous gorge that knifes across northern Arizona. This geographic feature is so large and imposing that squirrels evolved into different species on opposite sides! Many tourists visiting the USA don't count their visit complete unless they've seen the Canyon. It is one site of which almost everyone who sees it says, "Photographs don't do it justice!"

More than four million visitors gaze into the Canyon every year. Far fewer descend almost a mile to reach the river crossing at Phantom Ranch. Even visitors who hike to the bottom rarely notice that the journey mimics the formation of the canyon; except it's much faster! It is almost incomprehensible to many that such an enormous space was created by the "tiny" river at its bottom, but that's just what happened. The gorge - almost 300 miles long, up to 6000 feet deep, and as much as eighteen miles wide - was cut by a single river. It was a river with a mission!

The Grand Canyon of the Colorado River doesn't stand alone. It's part of a vast region of canyons, mesas, gorges, and plateaus that cover parts of four states: Arizona, Colorado, New Mexico, and Utah. The region, locally known as canyon country, is a geographic province scientists call the Colorado Plateau. The Plateau rises from the deserts on its east and south sides like a mighty stone ship; soaring as much as 8000 feet above them. To the east and north lie mountain chains drained by the Colorado and its tributaries. Large, rapid changes in elevation like this gives moving water enormous energy. This is because, more than anything else, water wants to get to the lowest possible point as fast as it can. This gives moving water great strength. Before the Colorado was tamed by the Glen Canyon Dam at Page, Arizona, spring floods roared down the river with enough power to send house-sized boulders tumbling along its bed. Even with a river working that hard, though, it still took millions of years to move all that rock!

The immense size of the Canyon is clear testament to the power of erosion. While the waters of the Colorado and its many tributary streams moved the rock and sediment downstream out of the Canyon, other forces were always at work. Those other agents of erosion actually do the heavy work. Ice, for instance, acts as "slow dynamite." Freeze-thaw cycles crack large rocks into small ones; and once in a great while loosen huge slabs of rock so that gravity can send them tumbling. This form of erosion, called mass wasting, created the Canyon's steep cliffs and the massive piles of rock at their base. Mass wasting in dry climates like northern Arizona is highly effective. Erosion by water may have opened the canyon and moved away the debris, but almost the only places where visitors can see rock sculpted by water is in the lowest canyon walls.

Another factor that helped shape the Canyon is what geologists call differential erosion. This means that rocks break apart at different rates because some are harder than others. Places where you see broad, flat areas down in the canyon are places where layers of soft rock were exposed to erosion. One such surface is the Tonto Platform, a flat space just above the inner gorge of the Canyon near the South Rim Visitor's Center. Soft shale at this level of the Canyon was rapidly eroded by the river, undercutting harder rock above it and allowing for rapid sideways expansion. The Colorado now, however, is grinding down into hard granite and metamorphic rocks in the inner gorge. When eroding into hard rocks like these, the river uses all of its energy to cut downward. When that happens, the canyon becomes very narrow.

All of this erosion takes time: lots of time. Geologists continue to debate how long the Colorado has been carving its way down. Up until a recently, the prevailing opinion was that the Canyon has been forming for about 5-6,000,000 years. This estimate was indirect, and was based in part on studies of how fast river downcutting has taken place in the region. A more recent study placed the age at about 9,000,000 years by using direct examination of minerals found in caves in the Canyon itself. A single measurement is never good enough, though, so the jury still remains "out."

Until the great western explorer John Wesley Powell rafted the length of the Canyon in 1869, mapmakers were unable to connect the Colorado River along the Arizona-California border to the river that drains the western slope of the Rocky Mountains. Until Powell established that they are one and the same, the upstream section was called the Grand River. That name is preserved in the Colorado towns of Granby and Grand Junction. In fact, it's preserved in the name of the Canyon itself: the Grand Canyon.

That historical difference in names may be more significant than a blank area on old maps. Geologists studying the history of the region believe that the upper Colorado River once flowed northwest of the modern-day Canyon. That makes sense, because the modern Canyon cuts across the southern part of a topographic bulge called the Kaibab Plateau. The scientists think that an early version of the Canyon was cut by what is now the Little Colorado River, which joins the Colorado at the east end of Grand Canyon National Park. This theory proposes that the Little Colorado River captured the drainage of the Grand River between 1-2,000,000 years ago. Evidence for this theory is visible in the drainage pattern of the Dolores River upstream from the confluence of the Colorado and Little Colorado. This drainage pattern seems to indicate that at one time the Dolores flowed north into Colorado, but it now flows south. Other streams in the area of the Uncompahgre Plateau in western Colorado show a similar pattern. Why these drainage patterns could have changed is a mystery.

Another prehistoric difference in the river running through the Canyon is a difference in climate. During the Ice Ages, the area was wetter and colder than it is now. This more rugged climate would have increased rock breakup caused by ice, and increased rain and snow means that the river ran even deeper than it does today.

The history of the Grand Canyon has been carefully studied because it is a national treasure. We know the basics of erosion and the power of water, both liquid and ice. We know that it took millions of years for this wonder to form. Even so, geologists still do not know all of its secrets. Some spend weeks and months every year investigating the Canyon to help us understand this great feature even better. All I can say about that is what a gorgeous place to work!

More information: Grand Canyon National Park website

U. S. Geological Survey "Tapestry" site

More about this author: Hai Shunxi

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