Grand Canyon 'formed recently'

By Jonathan Amos
Science correspondent, BBC News

image copyrightLaura Crossey/UNM
image captionThe eastern Marble Canyon segment was only cut in the last six million years, says the team

The world famous Grand Canyon, which snakes through the American state of Arizona, only took its present form relatively recently.

New research suggests that most of it was put in place just five to six million years ago.

Earlier studies had claimed the canyon was perhaps 70 million years old.

The latest investigation, published in the journal Nature Geoscience, agrees that some segments are very ancient, but says the full system is young.

"The 'old canyon model' has argued that the Grand Canyon was carved 70 million years ago in the same place and to nearly the same depth as the modern canyon. We are refuting that," said Prof Karl Karlstrom from the University of New Mexico, Albuquerque.

"We are also refuting the 'young canyon model', which claims the canyon was cut entirely in the last six million years. Instead, we show that the Colorado River used some old segments as it found its path from the Rockies to the Gulf of California in the past six million years.

media captionProf Karl Karlstrom: "It's a resolution of the old and young canyon hypotheses"

"What's different here I think is that we finally have a description of the Grand Canyon that honours all the hard-won data," he told BBC News.

Whether Karlstrom's and colleagues' paper will actually end the debate that has raged for 140 years remains to be seen. What is in little doubt is the great splendour of the canyon.

Running for almost 450km and to a depth of 1,800m, it is simply too vast for the five million tourists who visit the National Park each year to take it in. Many try their best by taking a plane or helicopter ride through the deep incision, which records nearly two billion years of Earth history.

That huge scale has also been problematic for scientists who have had to gather data from many different locations through the canyon in an attempt to gauge its true age.

The latest study used a couple of techniques that go under the term thermochronology.

This measures changes in the structure of rocks' mineral crystals as they get cooler through time. This transition occurs as deeply buried rocks come closer to the surface as erosion removes overlying layers of material.

Karlstrom's team used thermochronology to constrain the timing of the formation of four of the Grand Canyon's five segments.

They found that two of the three central segments - known as the "Hurricane" segment and the "Eastern Grand Canyon" - were indeed ancient palaeocanyons. The former was cut between 50 and 70 million years ago; the latter was incised some 15 to 25 million years ago.

But they determined that the two end segments of the canyon - known as the "Marble Canyon" and the "Westernmost Grand Canyon" - had to have been carved in the last five to six million years, when the Colorado River managed to link up the full system that everyone recognises today.

"If you were to add up the 280-mile length and ask, 'how much is young? More than half of it is young; a quarter of it is middle-aged - 15-25 million years old; and the rest of it is 70 million years old," said Prof Karlstrom.

"It continues to deepen today, of course. Right now, over the course of the last half-a-million-years or so, it's been deepening by about the thickness of a piece of paper every year.

"It's a beautiful place to work and a wonderful laboratory where the geology is laid bare. It's great for research, and for the many students we take there to teach."

image copyrightLaura Crossey/UNM
image captionThe Eastern Grand Canyon segment was half-carved between 15-25 million years ago and follow me on Twitter: @BBCAmos

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