The Grand Canyon is one of the sites that preserves the Great Unconformity. Image: Saad Chaudhry on Unsplash
Scientists can reconstruct incredible details about bygone eras in Earth’s history using fossils, rocks, and other clues preserved in its crust. But sometimes, the absence of geological records is just as telling as their presence. The Great Unconformity, a missing chunk of time that appears in rocks across the world, is the ultimate example of this phenomenon. This giant lapse in Earth’s memory exceeds one billion years in some places, resulting in 550 million-year-old rocks sitting atop ancient layers that date back 1.7 billion years, with no trace of the many lost epochs in between.
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For over a century, scientists have debated the cause of this immense geological blackout, which erased a huge chunk of the Neoproterozoic era. Two main explanations have emerged, which are not mutually exclusive: One suggests that tectonic activity associated with the assembly and breakup of the supercontinent Rodinia created the Unconformity, while another points to erosion from widespread glaciation during our planet’s “Snowball Earth” phase some 700 million years ago. Now, a team led by Kalin McDannell, a postdoctoral researcher in earth sciences at Dartmouth College, has presented new evidence that glaciers were the main force that carved out this mysterious gap in time. The researchers discovered a strong “cooling signal” at four very different geological locations across North America, suggesting that continental-scale glaciation may be “the only foreseeable process that can account for both the formation and preservation of the Great Unconformity,” according to a study published Tuesday in The Proceedings of the National Academy of Sciences. “Something really unique was going on in terms of global geodynamics and surface processes that allowed the Great Unconformity to both form and then be preserved,” said McDannell in a joint call with C. Brenhin Keller, assistant professor of earth sciences at Dartmouth and a co-author of the study. “That’s my perspective on why this has captured people's imaginations.”
Previous studies on this massive gap in time have found evidence of tectonic erosion at sites such as Pikes Peak, Colorado, and the Ozark Plateau in Missouri, which were near Rodinia’s active fault lines 700 million years ago. In their study, McDannell’s team widened the scope from these regions to include rocks from East Lake Athabasca in Saskatchewan, Canada, and the Minnesota River Valley, both of which were far from any tectonic activity during this ancient era.
The team analyzed published thermochronology data from these diverse regions and discovered signs of synchronous rock cooling that is associated with glacial erosion. While tectonic activity could have contributed to the Great Unconformity in certain areas, McDannell and his colleagues concluded that glaciation is the only process that could have bulldozed miles of geological strata away in so many different regions.“Imagine taking the very middle of the U.S. today, and then just eroding kilometers of that in the span of a geologically short period of, let's say, 60 to 100 million years,” said Keller, who led a study published in 2019 that also supported a glacial origin for the Unconformity. “That's not normally what happens. If those kinds of erosion rates were normal, we would have no crust that was older than a few hundred million years.”
“It’s an exceptional cooling signal in the interior of the continent and it's hard to imagine many ways of doing that other than glaciation,” he added.While the new study bolsters the glaciation hypothesis for the Great Unconformity, the team plans to analyze samples from more locations around the world to get a broader sense of the forces that swallowed these missing years. In addition to solving one of geology’s most intriguing mysteries, these follow-up studies could have implications for understanding the emergence of complex life on Earth. This huge gap in the geological record ends roughly 550 million years ago, right before the explosion of complex life that has dominated the planet ever since. While there is no established link between the cause of the Great Unconformity and the emergence of modern species, the timing has intrigued scientists for years. “We can imagine at the very least, without becoming too speculative, that a period of widespread glacial erosion would provide a lot of finely comminuted igneous rock, and reworked crustal rock more broadly, with a lot of nutrients in it, which would certainly not hurt if you were an organism that was trying to trying to diversify and proliferate around that time period,” said Keller.“There's a huge missing chunk of rock area and right after it, we have this explosion of life,” added McDannell. “So you can, tenuously or tentatively, make this causal inference. I would not say it's well-known yet, but it's certainly an area that could be looked at.”