Catastrophic floods built Grand Canyon

July 20, 2002 at 6:00 AM   |   0 comments

GRAND CANYON NATIONAL PARK, Ariz., July 20 (UPI) -- Dams of volcanic rock laid across the Grand Canyon have burst repeatedly and catastrophically over the past million years -- most recently about 165,000 years ago -- carrying enormous onrushing floods and carving out much of the great landmark in the blink of a geologic eye, new research by U.S. Geological Survey and University of Utah geologists suggests.

The findings tend to support other new data indicating the canyon's Inner Gorge may be no more than 700,000 years old, much younger than earlier estimates of 3 million to 5 million years, said Robert Webb, a research geologist with USGS.

"The newer interpretation is that there was a basalt dike that crossed the Grand Canyon that's been dated at 770,000 year ago," Webb told United Press International. "So the Inner Gorge wasn't there then. It's been downcut since then."

Downcutting refers to the phenomenon that occurs when enormous volumes of water are unleashed by sudden removal or failure of natural barriers such as lava dams. In the case of the Grand Canyon, downcutting means the Colorado River did not form the canyon through gradual erosion over millions of years. Instead, intermittent dam failures unleashed massive flash floods, in at least one case carrying many times more water than the largest Mississippi River overflow ever recorded.

"Large sustained floods can cause rapid downcutting in bedrock," Webb said. A similar event on a smaller scale occurred in 1976 when Idaho's Teton Dam failed. The water flow left a distinctive profile in soils and on canyon walls, he said. The water level dropped extremely rapidly in a phenomenon called a decay curve.

"We have that curve preserved from a lava dam that failed in the Grand Canyon 165,000 years ago," Webb said.

The lava was deposited by a chain of volcanoes that runs across the Grand Canyon. Periodic eruptions created the dams, which blocked the river's flow. Over time, enormous quantities of water backed up, eventually breaching the dams and continuing the downcutting process.

Webb, who collaborated with University of Utah researcher Cassandra Fenton, said, "There's a big volcanic field that straddles the Grand Canyon" and was active relatively recently. A minor eruption probably occurred there around 1,300 years ago, he said.

The most recent basalt dam probably occurred between 100,000 and 120,000 years ago, Webb said. However, "The one we've worked on most was there 165,000 years ago." When the natural lava dam failed, it unleashed 15 million cubic feet of water per second -- 37 times larger than the biggest Mississippi River flood -- helping to carve the lower canyon.

"These were some high dams," Webb said. "We estimate some were more than 1,500 feet tall."

The lava dams also were inherently unstable. The researchers explained when the molten basalt lava met cold river water, it cooled almost instantly, forming fragile walls of glass. "When basalt hits water, it shatters into glass, and there is just glass all over the place in these deposits," Webb said.

Another piece of evidence is the short life spans of the lakes building up behind the dams. Webb said the lakes filled quickly under pressure from large snowmelts in the Pleistocene era, which lasted from about 2 million years ago to the end of the last ice age -- around 9,000 B.C. The lakes did not have enough time to form deltas, he said.

USGS geologist George Billingsley, one of the leading experts on the age of the Grand Canyon, said although Webb and Fenton's work adds more data, he still regards the age issue as unresolved.

"We don't have enough hard-core evidence to prove it one way or the other," Billingsley told UPI. "All we're saying is that there has been a lot of Grand Canyon cutting in the last million years."

Other geologists share the skepticism, he said. "There are still a lot of holes to fill in. We need more data. That's why I'm hesitating to say 2,000 feet (of canyon depth) was cut in a million years." But some work, like the Webb-Fenton work, is supporting the hypothesis, he said.

One theory the catastrophic flood work does not support is the biblical flood story in Genesis. Some creationists have taken evidence of a "young Grand Canyon" as grounds for biblical literalism. One company, Another Viewpoint, offers trips through the canyon explaining its features as the result of Noah's flood.

"It isn't directly relevant to something like the Noachian flood because in this case we know what the source of the flood is," Webb said. "It is not like a rainfall flood that happened over an entire watershed. This is a river being blocked."

He added, "But it is a nice parallel to make. We do have in the geologic record where sometimes we can tell if they are rainfall related ... It's important to stress that a lot of the biggest floods worldwide have not been the result of unusual climatic events. They have been the result of the failure of a natural dam."

(Reported by Dan Whipple, UPI Science News, in Broomfield, Colo.)

© 2002 United Press International, Inc. All Rights Reserved. Any reproduction, republication, redistribution and/or modification of any UPI content is expressly prohibited without UPI's prior written consent.
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