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Grand old age for Canyon

Around five million tourists flock to Arizona each year to marvel at the 446 kilometre long chasm, which plunges 1800 metres deep in some places.

Now a new study indicates that it was probably carved much earlier than thought, about 70 million years ago ? so early in fact 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 five million to six million years.

Not everyone is convinced with the latest viewpoint published 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.

One expert described the Grand Canyon as a ?geologic layer cake?, with the most recent rock formations near the rim stacked on top of older rocks that date back two 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 argues that canyon cutting occurred long before that.

The team crushed rocks collected from the bottom of the canyon to analyse 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 is that 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 conceded not everyone will accept this alternative view, which minimises the role of the Colorado River.

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, was 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.

Many scientists find it hard to imagine an ancient Grand Canyon since the oldest gravel and sediment that washed downstream dates to about six million years ago, and there are no signs of older deposits.

And while they welcome advanced dating methods to decipher the canyon?s age, Karl Karlstrom of the University of New Mexico does not think the latest effort is very accurate. He said it defied 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. He said Flowers ?wants to have a canyon there, I want to have a cliff there?.

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 smaller rivers chiseled a good portion of the Grand Canyon long ago and then the Colorado came along and finished the job.

Sources: OptusZoo News, Los Angles Times, Durango Herald

 

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