By Stephanie Pappas

Scientists are homing in on the mystery of the Great Unconformity.

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.

Even stranger, the Great Unconformity shows up in rocks worldwide, and always in rocks from the same era: about 550 million years ago and earlier.

"There are lots of unconformities that are observed locally in various places, but it's rare to have one that's observed that represents the same kind of gap in time from over a billion years ago to roughly 500 million years ago," said Barra Peak, a doctoral student in geology at the University of Colorado, Boulder.

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.

Missing layers

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