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2008

How to Date the Grand Canyon: Go With the Flow
Dissection column, Wired.com, March 6, 2008
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The Grand Canyon is a victim of terrible press.

Its banded walls make up one of the most magnificent landscapes on Earth. And yet it seems the only time reporters bother to mention its geology is when they are writing about creationists and their bogus claims that the Grand Canyon formed a few thousand years ago. It's a shame, because the real story of the Grand Canyon is a riveting epic. Even its scientific history is fascinating: Figuring out just how old the Grand Canyon is has challenged geologists for 150 years. And just this week, the mystery may be solved.

Creationists would have you believe that the Grand Canyon formed in Noah's flood. They offer raft trips down the Colorado River where they explain how this supposedly happened. Once you're done with the trip, you can stop by one of the park’s bookstores and pick up a copy of The Grand Canyon: A Different View, written by tour leader Tom Vail.

The book appeared in the park stores in 2003, whereupon National Park Service geologists went ballistic. They demanded that it be pulled. Vail's lawyers threatened to sue. As the national media's attention turned to a juicy fight, the National Park Service hemmed and hawed, saying that they would review the matter. They never did. As I worked on this story I checked with the National Park Service, five years after the book appeared in their stores, to see if it was still for sale. It is.

I can only imagine how galling that must be to geologists. After all, when a geologist first set eyes on the Grand Canyon 150 years ago, he immediately recognized that it was immensely old. John Strong Newberry also speculated that water was responsible for carving down through a mile of rock, but he did not envision a biblical flood. Instead, he believed the Colorado River had gradually eroded it.

A century and a half of research has backed Newberry up, but geologists have not had an easy time determining just how long it took for the canyon to form. Their struggle is all the more remarkable when you consider that it took far less time to figure out the age of the Earth. In 1956 geologist Clair Patterson dated our planet at 4.5 billion years old, and 50 years of subsequent research has barely budged that figure.

Scientists know the Earth is 4.5 billion years old because planets and asteroids swept up uranium in the early days of the solar system. Ever since, the uranium has been decaying into stable lead isotopes at a predictable rate. Meteorites and Earth rocks all have proportions of uranium and lead pointing to the same age. The trouble with the Grand Canyon is that geologists couldn’t find a good clock to measure its growth. Part of the problem was that it was hard to find anything to date. The rocks themselves are ancient; the water that cut through them did not seem to leave behind a time stamp. As a result, estimates of the age of the canyon, and the speed at which it formed, have remained rough.

It turns out that the time stamps were there all along. They were just hidden away inside the hundreds of caves inside the Grand Canyon's walls. Strange formations known as mammilary coatings -- named for their vague resemblance to breasts -- line some of the cave walls. Mammilary coatings form on the walls of caves that are submerged just below the water table. As the Colorado River sliced deeper down into the Colorado Plateau, the water table gradually dropped. Mammilary coatings marked the river's fall. And as mammilary coatings form, they also happen to trap a lot of uranium. By measuring their age, scientists can measure how long ago they were near the water table.

Three geologists from the University of New Mexico have explored caves along the Grand Canyon, ranging from the very bottom to the rim. In this week's issue of Science, they report that the highest caves have mammilary coatings dating back about 17 million years, and the lowest ones date to about 800,000 years. And all the caves between the top and bottom have the intermediate ages you’d expect. By measuring the distance from the rim to the caves, the geologists were then able to estimate how fast the Colorado River carved the canyon. The downstream end of the canyon formed first, and only later did the upstream end catch up. These new measurements show that even as the river sank down into the earth, the earth itself rose, lifted by hot rock welling up through the crust.

The Grand Canyon is far older than Noah's flood, but at just 17 million years or so, it’s geologically infantile. For 99.99 percent of Earth's history, the Grand Canyon as we know it did not exist. Other canyons formed and vanished time and again, and only recently -- even as our own ancestors came out of the trees and stood upright -- a conspiracy of knife-sharp water and restless crust swiftly brought the Grand Canyon into being.

Now that's a story worth reporting.

Copyright 2008 Carl Zimmer
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