March 6, 2008 -- Before the Colorado River had a chance to carve the Grand Canyon out of the Colorado Plateau, the waterway was swallowed up by the desert and disappeared underground.
A flurry of scientific papers this month have edged closer to providing a clear picture of the complicated series of events that led to the river's disappearance and the formation of what Utah State University geologist Joel Pederson calls "the greatest erosional feature in the solar system."
Pederson's latest contribution to the story appears in the March issue of GSA Today, in which he eliminates one popular theory about the location of the ancient Colorado River. For some time geologists had suspected that in times before the western half of today's Grand Canyon existed, the river there flowed northwest and ended at a basin in Nevada's southern tip.
But Pederson found the sediments that now exist along this supposed course, called the Muddy Creek Formation, have all the geological call signs not of the Colorado River but of a different river entirely.
"This (Muddy Creek) deposit had been the primary option," Pederson told Discovery News.
Without that option, the debate grows a bit louder. One possibility doesn't even look for deposits on the surface; it suggests that the river once flowed underground, diverted to several springs that leaked out of the sides of the plateau.
In the drier times prior to 6 million years ago, a lesser Colorado River might have gotten lost in the cavernous limestone under the Colorado Plateau.
"There are huge spring deposits right off the Colorado Plateau," said Pederson.
Bottoms-Up Canyon
Pederson's explanation jives, in a way, with what might be considered a radical new idea suggesting the river carved the Grand Canyon from the bottom up. The theory is presented in a paper in the March 15 issue of the journal Geomorphology.
Carol Hill of the University of New Mexico believes that the river sneaks through the region's subterranean plumbing -- the cave-riddled karst formations created by soluble limestone -- avoiding some otherwise very problematic high ground along the river's present course.
"If you don't understand the plumbing system, then you don't understand the system," Hill told Discovery News.
Her take on the Grand Canyon is that it's not so different from some other steep terrains that have been created by rivers flowing underground and eroding larger and larger caverns until their roofs eventually collapse. When the river carries away the debris, what's left are gigantic, steep-sided sinkholes, much like the bizarre "tiankeng" formations of southern China.
Connect these sinkholes and you have a deep, narrow canyon just like those seen in the Grand Canyon.
"Why couldn't the Grand Canyon form like this?" Hill asks. The main reason the idea hasn't been studied in the past, she said, is that few geologists are familiar with the details of karsts.
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
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