Nov. 3, 2008 -- The Grand Canyon's age, the source of over a century of scientific controversy, may finally get a definitive number.
In recent years scientists have generally come to agree that the mile-deep gash in northern Arizona was probably carved by the Colorado River starting around six million years ago. But earlier this year researchers claimed in the journal Science to have found rocks in a cave near the western section of the canyon that proved the huge chasm was at least 17 million years old.
Now, in a rebuttal paper published this month in the journal Geology, Karl Karlstrom of the University of New Mexico and a team of researchers seek to end the debate once and for all. They argue that a combination of three faults in the area and upwelling hot mantle material pushed the region's rocks upward, causing the canyon to form in segments from east to west over the last six million years.
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"I think the other group really missed the boat on interpreting the two dates from the caves, one at 17 million years ago and one at seven million," Karlstrom said.
But the dates only record the last time water was in the caves. The assumption is that the river cut down into the canyon, bringing the water table with it, while the cave stayed at the same elevation. Today the caves are 4,600 feet (1,400 meters) above the present-day Colorado River.
This is a big assumption; water tables are rarely flat features. When Karlstrom went back and examined the area, he found the water table was just 1,300 feet (400 meters) below the cave, and perched more than half a mile higher than the Colorado.
"Perched water tables are very common, and we certainly see that here," Phillip Pearthree of the Arizona Geological Survey said. Because the river and water table are at different levels, the cave dates are in no way related to the formation of the canyon, he added.
Karlstrom's team also believes that hot, buoyant mantle material is flowing out from beneath the thick crust of the Colorado Plateau and pushing up underneath the canyon.
Over the past six million years, this uplift has been most pronounced in the east, accounting for 1,300 feet (400 meters) of its 5,200-foot (1,600-meter) depth near the famous South Rim. The uplift becomes less dramatic heading west, where the canyon is cut by a series of major faults.
"Everyone talks about canyons being passively incised by a river," Karlstrom said. "But no, it's actively uplifting. It's like cutting a layer cake with a knife. The cake is cutting the knife, but there's also a component of the cake coming up through the knife."
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