When we last checked in on the Old Canyon vs. Young Canyon debate, in late 2012, the Old Canyoneers had just put forth a new paper in the journal Science declaring the Grand Canyon to be roughly 70 million years old, having been excised by rivers other than the one that's currently at the bottom, the Colorado.
That figure, if accurate, would make the Grand Canyon so old that dinosaurs could have been among the first tourists.
But now comes another rebuttal, published Sunday online in the journal Nature Geoscience, stating that the canyon in Arizona as we see it today is 5 million to 6 million years old, with some older stretches in a couple of spots, including one patch that reaches almost back to the dinosaur era.
"The canyon we see today is young, but it made use of some old segments," said Karl Karlstrom, professor of geology at the University of New Mexico and lead author of the new paper.
He added, "The question is resolved that the canyon is not 70 million years old."
Not so fast. Consensus is hard to come by in a debate in which the opposing viewpoints have been separated by a yawning chasm. The Old Canyon advocates are sticking to their story.
The lead author of the 2012 paper, University of Colorado geologist Rebecca Flowers, said by email that "it will take a bit more time to understand fully why their interpretations are so different from ours."
Brian Wernicke, a California Institute of Technology geologist who co-authored that paper with Ms. Flowers, and who is considered a leader of the Old Canyon caucus, said he still believes that the westernmost stretch of the canyon is 70 million years old, when it was carved to within a thousand feet of its current depth.
"The idea that the westernmost Grand Canyon was filled up with rock [until 6 million years ago], that's just a nonstarter for us," Mr. Wernicke said.
Both camps rely on thermochronology, which measures the past temperatures of rocks along the canyon wall. Scientists take rock samples and study them by microscope, looking for crystals known as apatite. The crystals record the decay of radioactive elements into helium as hot, buried rocks are exposed to the surface by erosion and cool down.
Mr. Karlstrom's paper grants that a section of the eastern canyon is fairly old, having been excised 15 million to 25 million years ago. And there's an even older portion, known as the Hurricane fault segment, that the Karlstrom paper states was carved 50 million to 65 million years ago by a northwest-flowing river.
But in the Karlstrom formulation, that's not the Grand Canyon we see today. About 5 million to 6 million years ago, the Karlstrom paper says, the Colorado River integrated the two older canyon segments with two very young canyon segments and began flowing into the Gulf of California. The canyon then got wider and deeper. The canyon has since deepened at a fairly steady rate of roughly 328 to 656 feet every million years, the paper states.
Joel Pederson, a geologist at Utah State University, said of the new paper, "It certainly doesn't end the debate." He added, "If you want to call something 'Grand Canyon,' and you want to do it correctly, 'Grand Canyon' is less than 6 million years old and that's all there is to it."
Now comes a new geological wrinkle: James Sears, a professor of geology at the University of Montana, has published research arguing that the mysterious river that carved the eastern Grand Canyon about 25 million years ago could have flowed through Nevada, Idaho and Montana and all the way up through Canada to the Atlantic Ocean.
He bases this on gravel deposits found in Montana that don't match any of the local bedrock but do match the bedrock in Nevada. In that formulation, the ancestral Colorado River would have followed the base of a mountain range in what is now the western United States and flowed to the Atlantic in a huge watershed reminiscent of today's Amazon as it emerges from the Andes.
"The moral of the story is that the tectonics of western North America have been very active in the recent geologic past, and the way we see it today was not the way it was," Mr. Sears said. "It's very hard to read through the modern landscape to see this ancient landscape, because the tectonics have been so active."