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 Using a new dating tool, a team of scientists came up with a different age for the gorge's western section, challenging conventional wisdom that much of the canyon was scoured by the mighty Colorado River in the last 5 million to 6 million years.
Not everyone is convinced with the latest viewpoint published online Thursday in the journal Science. Critics contend the study ignores a mountain of evidence pointing to a geologically young landscape and they have doubts about the technique used to date it.
The notion that the Grand Canyon existed during the dinosaur era is "ludicrous," said geologist Karl Karlstrom of the University of New Mexico in Albuquerque.
How the Grand Canyon became grand -- with its vertical cliffs and flat plateaus -- has been debated since John Wesley Powell navigated the whitewater rapids and scouted the sheer walls during his famous 1869 expedition.
Some 5 million tourists flock to Arizona each year to marvel at the 277-mile-long chasm, which plunges a mile deep in some places. It's a geologic layer cake with the most recent rock formations near the rim stacked on top of older rocks that date back 2 billion years.
Though the exposed rocks are ancient, most scientists believe the Grand Canyon itself was forged in the recent geologic past, created when tectonic forces uplifted the land that the Colorado River later carved through.
The new work by researchers at the University of Colorado Boulder and California Institute of Technology argued that canyon-cutting occurred long before that. They focused on the western end of the Grand Canyon occupied today by the Hualapai Reservation, which owns the Skywalk attraction, a horseshoe-shaped glass bridge that extends from the canyon's edge.
To come up with the age, the team crushed rocks collected from the bottom of the canyon to analyze a rare type of mineral called apatite. The mineral contains traces of radioactive elements that release helium during decay, allowing researchers to calculate the passage of time since the canyon eroded.
Their interpretation: The western Grand Canyon is 70 million years old and was likely shaped by an ancient river that coursed in the opposite direction of the west-flowing Colorado.
Lead researcher Rebecca Flowers of the University of Colorado Boulder realizes not everyone will accept this alternative view, which minimizes the role of the Colorado River.
"Arguments will continue over the age of Grand Canyon, and I hope our study will stimulate more work to decipher the mysteries," Flowers said in an email.
It's not the first time that Flowers has dug up evidence for an older Grand Canyon. In 2008, she authored a study that suggested part of the eastern Grand Canyon, where most tourists go, formed 55 million years ago. Another study published that same year by a different group of researchers put the age of the western section at 17 million years old.
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 If the Grand Canyon truly existed before dinosaurs became extinct, it would have looked vastly different because the climate back then was more tropical. Dinosaurs that patrolled the American West then included smaller tyrannosaurs, horned and dome-headed dinosaurs and duckbills.
If they peered over the rim, it would not look like "the starkly beautiful desert of today, but an environment with more lush vegetation," said University of Maryland paleontologist Thomas Holtz.
Many scientists find it hard to imagine an ancient Grand Canyon since the oldest gravel and sediment that washed downstream date to about 6 million years ago and there are no signs of older deposits. And while they welcome advanced dating methods to decipher the canyon's age, Karlstrom of the University of New Mexico does not think the latest effort is very accurate.
Karlstrom said it also defies logic that a fully formed canyon would sit unchanged for tens of millions of years without undergoing further erosion.
Geologist Richard Young of the State University of New York at Geneseo said his own work suggests there was a cliff in the place of the ancient Grand Canyon.
Flowers "wants to have a canyon there. I want to have a cliff there. Obviously, one of us can't be right," he said.
Whatever the age, there may be a middle ground, said Utah State University geologist Joel Pederson.
Researchers have long known about older canyons in the region cut by rivers that flow in a different direction than the Colorado River. It's possible that a good portion of the Grand Canyon was chiseled long ago by these smaller rivers and then the Colorado came along and finished the job, he said.
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Online:
Science:
http://www.sciencemag.org/
[Associated
Press; By ALICIA CHANG]
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