Dating of rock erosion pegs the ancient chasm as 70 million years old

By Alexandra Witze

Web edition: November 29, 2012
Print edition: January 12, 2013; Vol.183 #1 (p. 15)

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Ages of Rock

An ancient river had carved the Grand Canyon to nearly its current depths by 70 million years ago, a new study suggests.

Credit: Realbrvhrt/Wikimedia Commons

Rock of ages it may be, but the Grand Canyon’s age itself is under fire. New work suggests the iconic chasm was already in place 70 million years ago — making it far older than commonly believed.

By most geologists’ definition, the Grand Canyon proper emerged between 5 million and 6 million years ago, as the Colorado River flowed across and eroded its way through layer after layer of rock. Evidence for this age comes from, among other things, great piles of washed-out gravel at the canyon’s western end that appeared around that time.

But the new study, reported online November 29 in Science, looks instead at the chemistry of rocks exposed throughout the canyon. Rocks get cooler as erosion strips away the material above them. That cooling is chemically preserved in several ways, including in helium within the mineral apatite.

“When the apatite is hot, the helium simply diffuses out of the crystal; when the apatite is cold, helium is completely retained,” says study leader Rebecca Flowers of the University of Colorado Boulder. “So by measuring the helium we are constraining when the rock went from hot to cold as it moved closer to the Earth’s surface, or as the Earth’s surface moved closer to the rock as the canyon was carved.”

Flowers and Kenneth Farley of Caltech looked at helium in apatite crystals throughout the canyon, including how the element was distributed within the crystals — which gives more clues as to whether the rock cooled slowly or quickly. The scientists concluded that some ancient river must have carved out a chasm roughly the shape and size of the Grand Canyon by around 70 million years ago.

Not everyone is convinced by the new findings, which build on earlier work from the same research team. Karl Karlstrom, a geologist at the University of New Mexico, says that there must have been canyons throughout the area 70 million years ago, but most geological evidence supports the modern canyon — what visitors see today when standing at the rim — appearing only in the last 5 million to 6 million years.

“The question is, how come we don’t know the shape of all the landscapes that eroded away to make this spectacular thing we see today?” Karlstrom says. “That’s a hard thing to tell.”