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Geologists May Have Solved a Grand Canyon Mystery

The 'Great Unconformity' of rocks has been a puzzle for 150 years
By John Johnson,  Newser Staff
Posted Aug 20, 2021 9:46 AM CDT
Updated Aug 22, 2021 8:00 AM CDT
Geologists May Have Solved a Grand Canyon Mystery
One aspect of the Grand Canyon has long puzzled geologists: Where did a billion years of rocks go?   (Getty/ASTROBOY)

(Newser) – The Grand Canyon may be a geologic marvel, but it's also a geological puzzle—one that may have finally been cracked. The puzzle is known as the Great Unconformity, explains a release at Phys.org. In parts of the canyon, layers of rock about 520 million years old sit directly atop layers roughly 1 billion years older. The cutoff, visible to the naked eye, was first pointed out by geologist and explorer John Wesley Powell in the 1860s. Since then, scientists have been trying to answer a basic question: Where did a billion years of rocks go?

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Now, a study out of the University of Colorado published in the journal Geology suggests an answer. We can blame Rodinia, an ancient supercontinent that researchers say wreaked havoc on the landscape when it broke up and washed those "missing" rocks into the ocean, per Forbes. "The Great Unconformity is one of the first well-documented geologic features in North America," says lead author Barra Peak. Above layers of rocks that have been pushed together vertically at the canyon's bottom, "you have these beautiful horizontal layers that form the buttes and peaks that you associate with the Grand Canyon."

To figure out how this came to be, Peak's team used a method called "thermochronology," which detects ancient heat signatures in rock formations caused by enormous pressure. They concluded that when Rodinia broke up, it "may have torn at the eastern and western halves of the Grand Canyon in different ways and at slightly different times—producing the Great Unconformity in the process," per the release. The Great Unconformity is not unique to the Grand Canyon, but the canyon makes the phenomenon easy to see. Peak's team is now looking at other Great Unconformity sites in North America to see if their findings apply there, too. (Read more Grand Canyon stories.)

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