By Joel Achenbach
Washington Post Staff Writer
Friday, March 7, 2008
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 the past 5 or 6 million years. The park's Web site, under Frequently Asked Questions, notes that the rocks exposed by the canyon are up to 2 billion years old, and then adds: "The Canyon itself -- an erosional feature -- has formed only in the past five or six million years. Geologically speaking, Grand Canyon is very young."
That might need revision. The canyon is more like 17 million years old, according to a study published today in the journal Science.
And the Colorado River may not be the only river involved in its formation. The study contends that a smaller river cut the older, western part of the canyon. Gradually the canyon formed from west to east on westward-flowing river. Then something happened about 5 or 6 million years ago -- what, exactly, is unclear -- to accelerate dramatically 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, said Joel Pederson, a geomorphologist at Utah State University who has spent his career studying the Grand Canyon. He said the estimated age of 5 million to 6 million years is based on abundant evidence amassed by scientists over many decades. Seventeen million is impossible, he said, because there is 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 intrepid fieldwork. Researchers had to climb canyon walls to reach caves containing crucial evidence of the canyon's history. The scientists examined mammillaries, also known as cave clouds, which are rounded mineral deposits that tend to form underwater near the top of a water table.
In the canyon, these deposits also contain a lot of uranium. In recent years, scientists have improved techniques for dating rocks based on the predictable decay of uranium into lead.
Polyak, working with geologist Carol Hill, suggested the research project to geochemist Yemane Asmerom, Polyak's boss at the University of New Mexico: Why not use the new lab techniques to measure the ages of the mammillaries? That ought to tell the story, Polyak reasoned, of how the river gradually cut through the plateau and lowered the water table.
Asmerom was skeptical. Scientists have long struggled to figure out the age of the 277-mile-long canyon, which is 18 miles across at its widest and reaches depths of 6,000 feet. Evidence of how and when the huge incision into Earth's crust took place tends to erode.
"Forget it. That was my reaction," Asmerom said. But he was persuaded to join the effort, which received funding from the National Science Foundation.
Not least among the challenges was simply reaching the caves, some of which are hundreds of feet high on canyon walls. Polyak and his team rafted the Colorado River, hiked side canyons and used climbing ropes. Expert climbers led the way. One cave was 400 feet up. "I always get nervous," Polyak said.
The mammillaries offered evidence of where the water table had been in the past. But Pederson, the critic of the study, said Polyak went too far in assuming that the water table tracks the canyon-carving. Pederson noted that sometimes a spring will gush from a canyon wall thousands of feet above the river.
Pederson also raises what might be called the where's-the-dirt question. "To cut a large canyon in the place of today's Grand Canyon," Pederson said, "you have to remove that mass and put that detritus somewhere."
But there's no sign of such canyon detritus in sediments older than 6 million years, Pederson said.
Hill, however, argued that there are multiple locations where material from the canyon may have been deposited. And keep in mind, she said, the older canyon wasn't that big.
"We're not talking about the canyon that's there now. We're talking about a small canyon that's slowly and steadily starting to down-cut," she said. "It's not the Colorado River; it's what we might call the proto-Colorado River."
So then: How did the proto-Colorado River turn into the much more dramatic Colorado River, which, in turn, transformed the not-so-big canyon into the Grand Canyon?
"That's the $64,000 question," Hill said.