One of Geology's Great Mysteries May Actually Be Many Smaller Mysteries

One of Geology’s Great Mysteries May Actually Be Many Smaller Mysteries

The “Great Unconformity” is a big chunk of missing deep time.

by Isaac Schultz April 28, 2020

The discrepancy pops up in the Grand Canyon, where ancient rock abuts, well, less ancient rock.

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It’s called the “Great Unconformity,” and it has long vexed geologists from Nevada to Scotland. Geology is often the study of layers, set one on top of each other for billions of years and compressed into sequences that provide geologists insight into how the Earth has evolved through the eons. Under the best circumstances, that sequence is more or less uninterrupted, but there can be gaps—sometimes big ones, like the Great Unconformity, which can be seen all over, from the Rockies to southern Africa to northern China. This gap spans one of the murkier periods in Earth’s history, before the Cambrian explosion, around 540 million years ago, when the diversity of life on Earth went wild.

Erosion is one natural process that wears layers away from the stack of geological deposits, but how so much was wiped out across such a wide range of places in one go has remained unknown. Even the unconformity isn’t uniform, ranging in scale from over a billion years of missing time to a mere couple hundred million. In the Grand Canyon, the timeframe of the unconformity jumps multiple times along its length.

According to a new paper published in the journal Proceedings of the National Academy of Sciences, there’s evidence that this erosion didn’t all happen at once. There might have been many events involved—lots of little unconformities—the origins of which can be traced to about a billion years ago on the supercontinent Rodinia, a landmass three times as old as the supercontinent Pangea that broke up to form the world as we know it.

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At Manitou Springs, Colorado, one can clearly see a line between layers of rock that differ in age by hundreds of millions of years. Courtesy Christine Siddoway

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The unconformity has recently been associated with the theory of “Snowball Earth,” which posits that the planet froze over entirely around 700 million years ago, following Rodinia’s formation. The thought is that glaciation during the snowball years ground away rock all over the world, creating the gap in geological time. The Cambrian explosion followed, and can be seen clearly in the layers that eventually went down over the gap. Flowers’s recent work suggests that the unconformity her team analyzed, near Pike’s Peak in Colorado, was caused by erosion related to tectonic movement that preceded Snowball Earth.

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