Visitors to the Grand Canyon always want to know: How old is it?
Park rangers are instructed to tell them that the canyon has been carved by the Colorado River for 5-million to 6-million years. The National Park Service's Web site notes that "geologically speaking, the Grand Canyon is very young."
That might need revision. The canyon is more like 17-million years old, according to a new study published online Thursday by the journal Science.
And the Colorado River may not be the only river involved in its formation. The new study contends a smaller river cut the older, western part of the canyon. The canyon formed gradually, from west to east, on a westward-flowing river. Then something happened about 5-million to 6-million years ago — what, exactly, is unclear — to accelerate the rate of the canyon-carving.
"The canyon is older than we think," said Victor Polyak, a University of New Mexico geologist and the lead author on the Science paper. "And there's a two-step process, I guess you can say."
Not so fast, says Joel Pederson, a geomorphologist at Utah State University who has spent his career studying the Grand Canyon. He says the estimated age of 5-million to 6-million years is based on evidence amassed by scientists over many decades. The 17-million age is impossible, he says, because there's no evidence of a large quantity of sediment flowing out of a canyon before 6-million years ago.
"They clearly have not taken the time to be rigorous and actually understand the regional geography," Pederson said.
Polyak's research paired new lab techniques with field work. Researchers climbed canyon walls to reach caves containing crucial evidence. The scientists examined mammillaries, also known as cave clouds, which are rounded rock structures that tend to form underwater near the top of a water table.
In the canyon, these rocks also contain abundant amounts of uranium. In recent years, scientists have improved techniques for dating rocks based on the decay of uranium into lead.
The mammillaries offered strong evidence of where the water table had been in the past. But Pederson says Polyak went too far in assuming that the water table tracks the canyon-carving. Pederson notes that sometimes a spring will gush from the side of a canyon wall thousands of feet above the river.
Pederson also raises a where's-the-dirt question. "In order to cut a large canyon in the place of today's Grand Canyon, you have to remove that mass and put that detritus somewhere." There's no sign of that detritus, at least not from more than 6-million years ago, he says.
Polyak acknowledges that his study raises some difficult questions. "Where's this material go? Where is it? I think it's a good question," he said.