Study Links Glaciers to Earth’s ‘Great Unconformity’

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Ancient ice action seems responsible for massive erosion of rock across the planet.

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A photo of the Grand Canyon.
The massive loss of rock known as the Great Unconformity was first named in the Grand Canyon. (Photo by Saad Chaudhry on Unsplash)
1/26/2022
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The planet was different 700 million years ago. In the Cryogenian period, Earth featured a single supercontinent known as Rodinia, complex organisms had yet to explode on to the scene, and the globe was an ice-covered snowball.

But at some point during “Snowball Earth,” rock as much as 3 miles deep was carved away by geological forces. The result is up to a billion years of missing geological time known as the “Great Unconformity.”

“The fact that so many places are missing the sedimentary rocks from this time period has been one of the most puzzling features of the rock record,” says C. Brenhin Keller, assistant professor of earth sciences.

For years, researchers have debated the cause of the massive erosion of rock. Some believe that the phenomenon was the result of ice age glacial activity, while others point to plate tectonics from the assembly and breakup of the Rodinia supercontinent.

“The underlying concept is pretty simple: Something removed a whole lot of rock, resulting in a whole lot of missing time,” says Keller of the phenomenon that was first named in the Grand Canyon in the late 1800s.

Research led by Keller in 2019 first proposed that erosion by continental ice sheets was responsible for the lost rock formations hundreds of millions of years ago.

A new study by Keller and Kalin McDannell, a postdoctoral researcher in earth sciences, provides further evidence that the rocks were carved away by ancient glaciers.

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C. Brenhin Keller and Kalin McDannell
C. Brenhin Keller, assistant professor of earth sciences, left, and Kalin McDannell, a postdoctoral researcher in earth sciences. (Photo by Eli Burakian ’00)

“Glaciation is the simplest explanation for erosion across a vast area during the Snowball Earth period,” says McDannell, the first author of the study that was published in Proceedings of the National Academy of Sciences.

To study the rock record, the team used thermochronometry – a research method that analyzes time and temperature information to identify when missing rock was removed and when current exposed rocks may have been exhumed.

They found a widespread cooling signal in the data that indicated the erosion of rocks 2 to 3 miles deep during Snowball Earth glaciations.

“This work provides strong support for the controversial 2019 hypothesis,” says Keller.

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Ladder Canyon, Colorado
In Colorado’s Ladder Canyon, rocks that differ in age by about a billion years sit together across the Great Unconformity . (Photo by C. Brenhin Keller)

According to the team, the findings also help explain how and when nutrient-rich sediment was transported to the ocean about 530 million years ago, leading to the emergence of complex organisms.

“This was a fascinating time in Earth’s history,” says McDannell, “The Great Unconformity sets the stage for the Cambrian explosion of life in the fossil record, which has always been puzzling since it is so abrupt.”

The Dartmouth team says that both glaciation and tectonics could have contributed to the loss of rock in different parts of the planet. However, their study argues that only glaciation can explain erosion far from the tectonic margins in the center of North America.

The team will repeat their work on other continents, where they hope to further test their hypotheses about how the Great Unconformity was created and preserved.

Written by
David Hirsch