Tucson geologists dispute study dating Grand Canyon to 17M years ago
by B. William Poole on Sep. 20, 2008, under Education, Local, Special
The beginnings of the Grand Canyon.
Scientists from the Arizona Geological Survey are disputing a finding by University of New Mexico scientists that the Grand Canyon is older than previously thought.
In an article published March 7 in the journal Science, three University of New Mexico geologists claimed new evidence shows the canyon began forming about 17 million years ago, not 5 million as many scientists had thought.
The study received wide media attention.
But the data does not take into account alternative – and according to the Arizona geologists, more likely – interpretations, said Tucson-based geologists Jon Spencer and Phil Pearthree, who Friday published a short rebuttal to the Science article.
“The data is probably fine. It’s the interpretation that was flawed. They should have considered alternative interpretations,” Stewart said.
Victor Polyak, the geologist from University of New Mexico who led the study, agreed that the data can be interpreted in more than one way. He stands by his finding and hopes to bolster it with further study.
“We’ll just continue on, and hopefully we’ll find some more data points in the western canyon that support our interpretation,” Polyak said.
Radioactive dating of mineral deposits shows caves in and around the canyon dried out as long ago as 17 million years, when canyon cutting pulled down the water table, Polyak, Carol Hill and Yemane Asmerom wrote in Science.
But only two of the 10 caves analyzed by the New Mexico scientists showed ages beyond the widely accepted 5 million to 6 million years, and both are miles from the canyon proper. Though the caves were likely dried out at the times the study showed, it was not because the Grand Canyon was emerging, the Arizona scientists said in a technical comment published Friday in Science.
One site about 24 miles north of the Colorado River that showed water loss about 7.5 million years ago could have dried because of cliff erosion or drainage of a lake believed to have emptied about the same time, Pearthree and Stewart wrote.
And the other old site dried out about 17 million years ago. That one, about 18 miles south of the river, more likely dried when shifting of the Earth’s crust caused water table changes, they wrote.
“The two samples indicating middle to late Miocene water-table decline probably have no direct bearing on Grand Canyon incision,” they wrote in the technical comment.
Polyak does not intend to defend his position at all costs. He has two years of funding left to continue studying caves.
“We hope to obtain more samples,” he said. “And if those support their interpretation, then we’ll accept that.”
