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A Grand Old Canyon
29 November 2012 3:35 pm
That giant hole in the ground cutting across Arizona is still sparking debate—and now, new analyses hint that parts of Arizona's Grand Canyon may be millions of years older than previously thought. The key evidence, scientists say, comes from the concentration and distribution of helium atoms found in nearby mineral samples. But many researchers are skeptical, noting that it's not clear whether these findings radically change current scenarios of how and when the iconic gorge was carved.
Debate about the age and history of the Grand Canyon has raged for more than 150 years, says Rebecca Flowers, a geologist at University of Colorado, Boulder, and the lead author of the new study. "If history were as simple as the popular view, the canyon's origins wouldn't continue to be a topic of hot debate," Flowers notes.
Over the years, scientists have taken a variety of approaches to determine the canyon's age, such as estimating how quickly sediment traveled from one end of the canyon to the other and studying the age of formations within the canyon. Some produced ages as young as 5 million to 6 million years for portions of the canyon, and others ages as old as 17 million years.
Flowers and Kenneth Farley, a geologist at the California Institute of Technology in Pasadena, are employing yet another approach: using concentrations of the noble gas helium in calcium phosphate minerals called apatites found in the canyon's rocks. Helium in these minerals can fluctuate in several ways over time: For instance, concentrations increase due to the radioactive decay of uranium and thorium in the apatites. As long as the apatites remain deep within Earth—at a temperature above 70°C—the helium can escape from the minerals through diffusion, Flowers explains. But as the minerals rise toward the Earth's surface—or as erosion carves a canyon downward—the rocks cool, trapping the helium within the apatites so that it begins to accumulate. So, helium concentrations in apatite can help scientists estimate when the rocks cooled.
In the new study, published online today in Science, Flowers and Farley analyzed four rock samples from the western portions of the Grand Canyon and four from the eastern reaches of the gorge. The pattern of helium concentrations in the samples suggests that substantial parts of the western portion of the Grand Canyon were already carved to within a few hundred meters of their current depth by about 70 million years ago and that erosion hasn't increased dramatically in recent eras, the researchers report. That's a far cry from the 5-million-to 6-million-year-old age suggested by previous research, and is about quadruple the oldest previous estimate from other teams for the canyon's age.
But that hardly closes the debate on the canyon's age, other researchers say. "I like the work [this team is] doing, and a lot of the stuff they've done is really interesting," says Richard Young, a geologist at the State University of New York, Geneseo. "But there's a lot of evidence for a young Grand Canyon," he adds.
Indeed, says William Dickinson, a professor emeritus of geosciences at the University of Arizona in Tucson, "this [notion of an old Grand Canyon] isn't what most people are thinking, but it's not totally new either. The Grand Canyon is a very young-looking feature to this geologist's eye."
Some estimates of the younger age are based on mid-20th century field studies that described the ancient flow of sediment through the canyon, revealing that sediments from the eastern Grand Canyon didn't show up at the western end of the gorge until between 5 million and 6 million years ago. Researchers often cite those field data—including in research published as recently as 2008 in journals such as Geology—to bolster the idea that the canyon wasn't present before that time. But those 5-million-to 6-million-year-old sediments don't constrain the age of the canyon, only the direction of the river's flow within it, Flowers says—and, for example, changes in the slope of the terrain over geological time could have altered the flow of the ancient river. Or the eastern and western portions of the canyon may have formed separately, joining only recently.
Other previous research, meanwhile, supports the notion that parts of the canyon are old—for instance, one study that includes analyses of cave formations in the region but not directly along the Grand Canyon suggests that erosion may have begun as much as 17 million years ago. But those results are controversial. Because the caves aren't located right on the river, the water table at those sites may not accurately reflect the level of the water table at the canyon edge.
Flowers and Farley's approach is particularly compelling, however, because looks can be deceiving, and evidence is everything, says Brian Wernicke, a geologist at California Institute of Technology who has collaborated with Flowers in the past but wasn't involved in the current study. "This thermal fingerprint is all you've got in terms of hard data."
"It's hard to look at a landscape and discern its erosional history," he notes. "This study hammers a hard nail into the idea that there has been significant erosion since 70 million years ago."
Nevertheless, not everyone is convinced by the team's evidence. Karl Karlstrom, a structural geologist at the University of New Mexico, Albuquerque, describes the findings as "out in left field." His team has also analyzed helium concentrations in apatites that were collected just a couple of kilometers downstream from where Flowers and Farley collected their samples in the western Grand Canyon. And their preliminary results, Karlstrom says, bolster the notion of a young gorge. Those soon-to-be-published results suggest that those rocks were still between 50° and 60°C—implying that they were well over 1 kilometer below the surface of Earth's crust—between 15 million and 20 million years ago.
Despite the contentiousness of the dispute about the age and erosion history of the Grand Canyon, "it's a legitimate debate," Karlstrom says. "It takes a community to come together to evaluate the evidence."