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Rangers and docents at Grand Canyon National Park hear the question a lot: “So, just how old is it?”

The prevailing theory has been that the Grand Canyon is about 5 million to 6 million years old and was carved by the west-flowing Colorado River.

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The West Rim of the Grand Canyon

But researchers from the University of Colorado in Boulder and the California Institute of Technology now say the natural wonder could be as much as 70 million years old.

That would date its formation to the era of the dinosaur.

The study was published online recently in the journal Science.

Lead researcher Rebecca Flowers is an assistant professor in the University of Colorado Department of Geosciences. She says her team used a new technique to date rocks found on the canyon's floor in the western part of the canyon, near the Hualapai Reservation.

“We used a radiometric dating method that involved determining the amounts of uranium, thorium and helium inside individual crystals of a phosphate mineral called apatite,” says Flowers. “And so essentially we used the radioactive decay ... to date these minerals.”

According to Flowers, the amounts of those minerals left behind in the rock are a function of temperature, which is cooler near the surface of the earth. This knowledge allowed them to calculate the time since the canyon was formed.

“In this case, our data say that those rocks were very cold for a very long time,” Flowers says. “And really the only way to do that in this setting is to have the rock close to the surface, due to there being a big canyon present there by 70 million years.”

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Photo: National Park Service

Poster for Grand Canyon National Park, 1938

Not all geologists agree. William Dickinson is an emeritus professor of geosciences at the University of Arizona who says there's a lot of geologic evidence that the western Grand Canyon is, at the most, 19 million years old.

“Well, their data is very interesting and undoubtedly will mean something,” says Dickinson. “But I think frankly that their conclusion is implausible ... they have the data, but they have to do some pretty fancy rocket science to figure out what it means.”

Over the years, many studies have been conducted to determine the canyon's age. A common approach was to measure how quickly sediment traveled the canyon's length. Those studies have put the canyon's age at anywhere between 5 million and 19 million years.

It's not just the canyon's age that's up for debate in the world of geology. It's also how the canyon was formed.

Many scientists have long believed that the chasm was cut into the Colorado Plateau by the Colorado River, flowing westward from the snows of the Rockies toward the Sea of Cortez. But the new study from Rebecca Flowers proposes that an ancient river that flowed from west to east carved the Grand Canyon.

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Sunset at Yavapai Point, along the Grand Canyon's South Rim.