By ALICIA CHANG
AP Science Writer
LOS ANGELES (AP) - The
awe-inspiring Grand Canyon was probably carved about 70 million years
ago, much earlier than thought, a provocative new study suggests - so
early that dinosaurs might have roamed near this natural wonder.
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.
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.
___
Online:
Science: http://www.sciencemag.org
___
Follow Alicia Chang at http://twitter.com/SciWriAlicia
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2012 The Associated Press. All rights reserved. This material may not
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