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  • Dick writes about Earth and planetary science for Science magazine.
 

The Grand Canyon as Frankenstein

26 January 2014 1:00 pm
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From many, one. Here in the eastern Grand Canyon, where 5 million park visitors flock each year, the canyon was first carved 15 million to 25 million years ago and then deepened since 6 million years ago by the Colorado River.

Laura Crossey, UNM

From many, one. Here in the eastern Grand Canyon, where 5 million park visitors flock each year, the canyon was first carved 15 million to 25 million years ago and then deepened since 6 million years ago by the Colorado River.

It’s a debate that has vexed scientists for decades: Is the Grand Canyon young or old, geologically speaking? Both, a new study declares. A group of scientists reports that the famed formation is a hybrid of five different gorges of various ages that the Colorado River only tied into a single continuous canyon and deepened since 5 million or 6 million years ago.

The debate over the age of the Grand Canyon has been so drawn out largely because nature leaves so few clues as to the shape of the land tens of millions of years ago. Water must flow downhill to create canyons, but which way was down ages ago? Over the past 100 million years—since before the death of the dinosaurs—the incessant jostling of plate tectonics has repeatedly reshaped the landscape of the U.S. Southwest. The Rocky Mountains and the Colorado Plateau rose to the east of the present-day canyon, the Sierra Nevada grew to the west, the Basin and Range of Nevada and Utah stretched the crust to the north, and the Gulf of California split the crust to the south.

So, to figure out which way the land was sloping at a given time, geologist Karl Karlstrom of the University of New Mexico, Albuquerque, and 10 colleagues pulled together the relevant published data and added some of their own. Geologists have long recorded where sand, gravel, and cobbles have ended up as a sign of how, when, and where running water had been cutting into the rock. More recently, geochemists have been gauging when river erosion removed most of the crust overlying a particular rock now exposed at the surface. That is possible because the deeper a bit of rock is in the crust, the hotter it is. So as a river removes overlying rock to form a canyon, that once-deep bit of rock cools as it slowly nears the surface. Geochemists can now date when a rock cooled to a certain temperature—and therefore when the overlying rock was removed and a canyon formed—using three semi-independent techniques, each involving the decay of a different element.

As they report today in Nature Geoscience, Karlstrom and colleagues conclude that two of the present Grand Canyon’s five segments formed early on and two are relatively young. They found that two of the three middle segments formed long ago—between 70 million and 50 million years ago and between 25 million and 15 million years ago. (The third middle segment lacked sufficient data.) But the two end segments were carved in the past 5 million to 6 million years, since the Colorado River first began flowing down the length of the present canyon and into the Gulf of California. “There will be battles yet,” Karlstrom says, but “I think this really is the solution to the 140-year debate.”

The paper is getting a warm if not quite as categorical reception. “It doesn’t try to say the entire Grand Canyon is all very old or all very young, which is probably more realistic,” says geochemist Peter Reiners of the University of Arizona in Tucson. “Most in the geological community will consider this to be the more plausible scenario. The reception will be very positive.”

But battles there will be. They would be shorter and more productive, Reiners says, if the geochemical methods for determining the timing of ancient erosion and canyon formation were improved. Karlstrom would settle for researchers’ applying all three of the current methods to the same rocks so that interpretations would be more likely to converge. That will likely take more collegiality than seen to date.

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