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River Vanished to Build Grand Canyon

Larry O'Hanlon, Discovery News

Thanks for the Mammillaries

Hill says her scenario is in agreement with new information on the history of changing water tables in the Grand Canyon over millions of years. That new groundwater data appears in a paper in the March 7 issue of Science. Hill is a coauthor.

Using uranium-lead radioisotopes to date Grand Canyon cave formations called mammillaries, the University of New Mexico's Victor Polyak, Hill, and Yemane Asmerom analyzed the bulbous calcite formations at nine Grand Canyon cave sites to see how the groundwater levels have changed over millions of years.

Mammillaries are known to form near the water surfaces of groundwater lakes and ponds found in caves.

"We interpret that the groundwater level is at the river level," explained Polyak.

So the timing and rate that the river carved out the canyon ought to be reflected in the dates of the cave formations as they formed at the top of the water table in caves along different parts of the canyon.

What Polyak and his team found were declines in the water level -- i.e. dropping of the river as it ate its way down into the Colorado Plateau -- which suggests the western Grand Canyon was being carved deeper at a rate of 55 to 123 meters per million years, over the last 17 million years.

This 17-million-year-old western Grand Canyon, says Pederson, is too old to fit easily into the picture of a surface river cutting the chasm.

According to his understanding of more than a century of geological research, the western Grand Canyon must be far younger for one simple reason: "The river didn't exist until 6 million years ago," he said.

So whatever Polyak and his colleagues are seeing in the falling groundwater, says Pederson -- and he doesn't dispute their data -- it's not just a canyon being cut into the Colorado Plateau, he says.

Missing Mountain

That matter and other details about the Grand Canyon's history will likely get a lot more attention in coming months and years, if the present and ongoing scientific hubbub is any indication, said Hill.

It's big, famous and there's a history of research going all the way back to John Wesley Powell, who led the first expedition down the Grand Canyon in 1869.

"There's so much motivation to speculate about it," agreed Pederson, who also expressed concern that sometimes the science can suffer from too much speculation.

The Grand Canyon offers is a profound challenge to any geologist -- a geological feature that is made not of rock, but of missing rock.

Hill poses the central question facing any researcher venturing near: "How do you date something that has been washed away?"

Related Links:

Larry O'Hanlon's blog: Earth Impacts

How Stuff Works: Grand Canyon Travel Guide

USGS: Grand Canyon Monitoring and Research Center