By measuring the amounts of uranium and helium contained in samples collected from the Upper Granite Gorge, the researchers could determine the time at which rocks there cooled, indicating when they were unearthed by erosion.
Because temperatures are hotter below Earth's surface, these cooling rates helps scientists determine when rocks were buried deep underground, and when the forces of erosion exposed them to cooler temperatures closer to the surface.
The results showed that the rocks in Upper Granite Gorge were exposed to cooler temperatures near Earth's surface around 55 million years ago.
It also showed that both the rim and the bottom of the gorge had cooled at the same time, suggesting that the gorge formed from previously existing canyons that eventually connected, rather than a plateau.
"If there had not been a canyon, the gorge and rim samples would have been different," noted Brian Wernicke, a geology professor at Caltech.
Karl Karlstrom, a geologist at the University of New Mexico, pointed out that precursor canyons to the Grand Canyon have been proposed before and that the Caltech research supports those theories.
"Every time we get a new tool we learn something new," Karlstrom said.
"This is a strong addition of data that helps to document what has been proposed—that ancestral canyons existed and became linked together."
Other recent research, including a study by geologists at the University of New Mexico published recently in the journal Science, further challenges the idea that the Grand Canyon was created in one fell swoop by the Colorado.
That research found that parts of the canyon were likely formed about 17 million years ago, adding perhaps more credibility to the notion that the parts of the Grand Canyon were created at different times.
More to Learn
Though there is mounting evidence for the existence of precursor canyons around the Grand Canyon, scientists are still a long way from understanding the full history of how the natural landmark was created.
The new research does not call into question, however, that the Colorado River did, on its present course, start spilling sediment into Lake Mead six million years ago.
"That six-million-year figure is solid, and they are not disputing that," said Joel Pederson, a geologist at Utah State University who researches the Grand Canyon.
What is not known is how the precursor canyons were linked together and what the exact course of the Colorado River was before it started unloading sediment around Lake Mead, he said.
"There have been so many hypotheses how the Colorado got pieced together and when," Pederson said.
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