WASHINGTON - THE Grand Canyon, carved out over the eons by rushing river water, began to form 17 million years ago, making it nearly three times older than previously thought, scientists said on Thursday.

The general consensus among geologists had been that the famed natural landmark in Arizona was about 6 million years old.

But now University of New Mexico scientists say it is far older based on their findings using a technique called uranium-lead isotope to date mineral deposits in caves from nine sites in the canyon's walls.

The gorge is 446km long, 1.8km deep at its deepest point and up to 29km wide. The Colorado River that runs through it arises in the Rocky Mountains of Colorado and flows south-west about 2,250km to the Gulf of California.

The scientists found that there is a kind of dual history to the canyon, with the western portion starting to form about 17 million years ago when water from a predecessor to the Colorado River began the slow, inexorable process of eroding the rock, they said.

The eastern portion began forming closer to the 6 million-year-old date that had been the estimate for the whole canyon, they said.

'Essentially, it formed in two parts,' said Professor Carol Hill, a University of New Mexico geologist. 'The older Grand Canyon formed and then hooked up once the Colorado River became through-flowing (all the way to the Gulf of California), then it hooked up with the eastern Grand Canyon.'

The findings were published in the journal Science.

'Not only were we able to figure out the time scale information but also the way that the canyon may have formed. It was one of those beautiful complete stories,' University of New Mexico scientist Yemane Asmerom added.

Untold tonnes of rocks were eroded away by the river.

'One of the problems with trying to determine the age of the canyon is that the canyon-forming process itself removes all the material that would otherwise hint to its history and how old it is,' University of New Mexico geologist Victor Polyak added.

Even with the latest findings, the canyon remains a relatively recent geological formation, considering the 4.5 billion-year history of the planet. For example, the Rocky Mountains began to rise about 70 million years ago. -- REUTERS

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