The Great Unconformity shows up in rocks worldwide, and always in rocks from the same era: about 550 million years ago and earlier.
The Grand Canyon is a layer cake of geological history, with rocks stacked neatly upon one another as they were laid down millions of years ago. That is, until you get deep into the canyon and find the Great Unconformity, a gap between rock layers representing a billion years in some places.
Now, Peak and her colleagues have found that in the Grand Canyon, at least, these rock layers were lost during a tectonic upheaval caused by the breakup of a supercontinent. The findings suggest that although the Great Unconformity is found in rocks from around the world, the reason for its presence may be different in every place.
To do that, the researchers looked at helium trapped within the mineral zircon in the rock. Helium is a byproduct of the radioactive decay of uranium into lead. Under high heat, helium can escape from the mineral matrix. But in cooler rocks, the helium stays trapped. Thus, measuring helium levels in a rock of a certain age can tell you when that rock reached the surface and cooled.
The overall picture suggests that the western half of what is now the canyon rose to the surface about 700 million years ago; the eastern half rose closer to 500 million years ago. But even within this broad picture, there are differences of tens or hundreds of millions of years in spots just a few dozen miles from each other.
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