The Colorado River took the easy route when it carved the Grand Canyon through Arizona's ruddy sandstones and pastel limestones, a new study claims.
Instead of slicing through thousands of feet of unblemished rock, the Colorado River recycled ancient canyons, at least one of which was 70 million years old, researchers report Jan. 26 in the journal Nature Geoscience.
"I think the Colorado River found low places and paleocanyons and ancient topographies that led to the Grand Canyon," said Karl Karlstrom, lead study author and a geologist at the University of New Mexico in Albuquerque.
The new findings, which rely mainly on reinterpretations of other scientists' work, summarize decades of geologic sleuthing. But the study may do little to resolve the heated debate over the age of the Grand Canyon. For the past year, Karlstrom and others have stridently attacked work published Nov. 29, 2012, in the journal Science that suggested the westernmost Grand Canyon was 70 million years old.
But the debate over the Grand Canyon's age has raged for decades, in part because so much of the canyon's history is missing, carried away by the river. The little that's left means many things to many people. The argument also hinges on how one defines the Grand Canyon. Is there a Grand Canyon without the Colorado River running through it? [Video: Virtual Tour of Grand Canyon]
For Karlstrom, the answer is no. Even though his latest findings jibe with the 2012 Science paper, and he reuses that data, he asserts that the Grand Canyon is less than 6 million years old. He was also affronted by claims that dinosaurs walked on the Grand Canyon.
"The Colorado River found a path and carved the entire canyon 5 [million] to 6 million years ago," Karlstrom told LiveScience's OurAmazingPlanet. "I agree our data is not in disagreement, but I had the [National] Park Service call me up and say, 'Is it true that the park is 70 million years old?'"
Assembling the canyon
To prove the point, Karlstrom and his co-authors assembled published geologic evidence, along with four new "cooling ages" in the westernmost canyon. The cooling ages come from apatite crystals, which contain helium-producing uranium. When the apatite is hotter than about 122 degrees Fahrenheit (50 degrees Celsius), helium escapes. As the rocks cool — for instance, when a canyon-carving river removes their cover — the helium stays trapped in the apatite crystals. Measuring the helium is a gauge for how long the rock has been cool, and exposed near the surface.
Karlstrom's group snipped the Grand Canyon into pieces, and then calculated how long ago each segment was carved. Only two of the segments are less than 6 million years old, the age posted on the National Park Service's signage, they found.
Here's the breakdown, from east to west:
Marble Canyon — Less than 6 million years old
Eastern Grand Canyon — A 4,900-foot (1,500 meter) deep canyon carved 25 million years ago along the Kaibab Uplift.
Hurricane Canyon — Carved to half its current depth 70 million years ago, flowing north along the Hurricane Fault.
Westernmost Grand Canyon — Less than 6 million years old
The 2012 Science study also found a segment between the Hurricane Canyon and the westernmost Grand Canyon was cut to near-modern canyon depths about 70 million years ago.
Rebecca Flowers, lead author of the Science study, said she was interested to see the unusually young ages for the westernmost Grand Canyon, close to where both her group and Karlstrom's team had discovered 70-million-year-old cooling ages.
"Given the consistency of our combined helium data sets and the reproducibility of those results throughout this 35-mile [55 kilometers] section of the westernmost canyon, it will take a bit more time to understand fully why their interpretations are so different from ours and why they conclude that the erosion history varied so dramatically within this short reach of the canyon," Flowers, a geochemist at the University of Colorado, Boulder, said in an email interview.
Why is it there?
The Colorado River first emerged from the Rocky Mountains some 11 million years ago, according to old river gravels. So another huge puzzle remains: Where did the river flow before the Grand Canyon formed, and why did it finally end up in the Grand Canyon? [7 Amazing Grand Canyon Facts]
"To me, the greatest remaining mystery is how this got connected into a canyon," said Joel Pederson, a geomorphologist at the University of Utah, who was not involved in the study.
For now, here's Karlstrom's big picture: About 6 million years ago, something prompted the Colorado River to shift gears and head southwest. That event could have been a lake flood, climate change or subtle shift prompted by erosion. Whatever happened, the Colorado River grabbed its chance, cutting through the ancient rocks lining the gorge (up to 1.8 billion years old at the bottom) and bursting through to the Gulf of California.
Pederson agrees that Karlstrom's reanalysis won't resolve the Grand Canyon age debate, especially because geochemists can continue to debate how the cooling ages are interpreted.
"You have two groups of people who can take the same samples from the same results and come to really different conclusions," Pederson told LiveScience's OurAmazingPlanet. "That's the key battle."
But while the arguments will continue to play out in scientific journals and conferences, the big picture is getting clearer.
Long before anyone ever measured helium in apatite, geologists had discovered the Grand Canyon's ancestors, the paleocanyons that came before today's stunning vermillion walls were breached. First discovered in the 1950s, the history of these older canyons is now being refined and revised by scientists like Flowers and Karlstrom, with modern geochemical techniques.
"We now know there are parts of the Grand Canyon that are using ancient paleocanyons to a greater extent than we previously thought," Pederson said.
And geologists continue more old-fashioned detective work, tramping across the desert plateau in search of undiscovered clues about the Colorado River's history.
"There are a lot of ideas out there, but I don't think we're all in agreement yet," Karlstrom said.
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