The Grand Canyon is a geological wonder, a vast chasm stretching 277 miles west to east through northern Arizona. The canyon offers one of the best geological cross sections in the world, with nearly 30 distinct layers found from the bottom to the top; its mesas, buttes, colorful slopes and spires illustrate the geological story of the great American southwest. School children are fed a simple tale about how the canyon was formed, but as any honest geologist will admit, nobody knows exactly how it got there.
The true cause of the Grand Canyon is still hotly debated among geologists, and all recognize there's no solid answer. There are too many missing pieces. The basic park ranger explanation is that the Colorado Plateau – 130,000 square miles covering northern Arizona, southern and eastern Utah, western Colorado and northwestern New Mexico – began to rise up 50-70 million years ago, causing the existing Colorado River to downcut. After millions of years of steady uplift, the Colorado River carved out the Grand Canyon. The higher the plateau rose, pushed upward by magma from deep in the earth, the more powerful the erosional forces of the river proved to be.
The Colorado River was tamed a great deal when the Glen Canyon Dam was finished in 1966. The dam controls the flow of the river now. The Colorado's flashfloods once carried boulders the size of VW Buses, and it transported an estimated 160 million tons of sediment every year, scouring the canyon bottom. Still, many people consider the vast expanse of the Grand Canyon and despise the Colorado as an underfit river, one that could not have possibly hauled out all the necessary sediment.
A Few Puzzles:
The simple explanation does have serious geological issues. The Colorado River flows into the Gulf of California – the Sea of Cortez – which contains sediments from geologically young Pliocene layers - not older layers. It appears that the eastern part of the Grand Canyon is much older than the western part of the Grand Canyon, but nobody knows exactly what happened.
The western end of the canyon is fairly young. Local sediments come from the Basin and Range area to the west of the Canyon and are from Miocene layers. No river could have carved through there until after the Miocene. There's also no evidence that an older Colorado River ran through the Grand Wash Cliffs at the western end of the canyon. There is therefore an upper Colorado River system to the east that did not originally continue west of the Colorado Plateau.
The first thought, of course, is that the deposits were carried away by a different route. Perhaps the canyon was carved by a river that flowed down through Marble Canyon through what is now the Little Colorado River, draining into the Rio Grande. According to the theory, another small but steep and vigorous river rushed westward across the Basin and Range. It moved eastward by headward erosion until it ran into the upper Colorado. It "captured" the Colorado , caveman style, and the two married and carved the Grand Canyon in 4-6 million years, dumping into the Sea of Cortez like today.
There's a problem there, though. The Continental Divide would have prevented the Little Colorado River from reaching the Rio Grande, and the sediment evidence is not there.
Some argue that the old river flowed south at Peach Springs Canyon, until the river burst through whatever blocked the way to the western part of the Grand Canyon. Some argue that the Colorado Plateau tipped one way and then the other as it rose so that the Colorado River flowed in the opposite direction. Some argue the river flowed underground, or flowed northwest, draining into lakes in Nevada and Utah.
The deposits to prove these drainage systems haven't been found. That's part of the problem; there is a lot of data that is just plain missing.
A Breached Dam:
According to one theory, two major lakes formed in the so-called Bidahochi Formation to the east of the current day Grand Canyon. The Colorado River overflowed the basins of the Colorado Rockies, filling the vast Hopi Lake and Grand Lake to the east. When the natural dam holding back the lakes broke, they gushed down, tearing out the soft limestone and sandstone layers of the Grand Canyon. With a rupture in the Kaibab Upwarp, the Colorado River changed course and followed the fissure down through what is now the western end of the canyon and south into the Gulf of California. In other words, the Colorado did not really cause the Grand Canyon at all, it merely followed the easiest path down to the Sea of Cortez after the Canyon had been formed.;
The breached dam theory has an interesting bit of support from the legends of the local tribes. More than 500 Native American sites in the Grand Canyon park indicate that significant populations lived in the vicinity throughout history. In his video Grand Canyon, part of the Great National Parks series, Dan Goldblatt refers to a Navajo legend about the formation of Grand Canyon. In the full story, it rained in the land for many days. It rained so much that waters rose high over the tops of the mountains. After the rain stopped, whatever was holding back the waters broke, and the waters rushed down and carved out the canyon.
Young Earth geologists like Steve Austin believe that these great lakes were leftover from the Flood of Noah. When the natural dam that held them broke, the waters ripped through the Kaibab Plateau with a fury. If the lakes were quite large – three times the size of Lake Michigan, by some estimates - the erosion would not have had to take the millions of years that geologists would have expected.
In the end, the scientists will keep battling it out. Only God knows exactly how that fantastic crevasse in the earth was formed, but every bit of information adds to the tremendous fun of figuring out the mystery.
Source: http://www.khouse.org/enews/2011-06-14/#1Continue reading