In the Grand Canyon, rock layers preserve a relatively continuous record of Earth’s geologic history going back about 2 billion years. But between 520 million and 1.8 billion years, the record is noticeably empty. It’s as if someone broke into a museum and made off with almost an entire wing’s worth of rocks, and sorting out “whodunnit” is one of the coldest cases in all geology.
Now, scientists studying the Grand Canyon’s rocks say that a gigantic Earth-spanning cliff —with a global glaciation as its accomplice — could be to blame for the heist.
The giant cliff started forming about 800 million years ago as an ancient supercontinent called Rodinia split apart, Thomas Gernon, an earth scientist at the University of Southampton in England, and colleagues report August 17 in Geology. Forces like rain and ice weathered and eroded the rock and, like waves at the beach steadily erasing one layer of sand after another, deleted a billion years of Earth’s history.
The group used computer models to simulate how ancient Earth evolved over hundreds of millions of years. They watched as the cliff — known as an escarpment — formed around the time in Earth’s history when rocks in the region vanished.
The escarpment most likely extended across much of a continent called Laurentia, the ancient precursor to North America. It stretched from what was to become Alaska all the way to Arizona and would have been about one to two kilometers high.
“No study has previously suggested the existence of a continental-scale escarpment in the Grand Canyon region. Our study shows it’s a plausible, straightforward explanation for the immense erosion that took place in the Grand Canyon,” Gernon says. Such an immense cliff created steep slopes that were ideal places for the elements to loosen and make off with exposed rock.
And the escarpment had a lot of help when it came to its disappearing act: ice, lots and lots of ice. The theft of the rocks coincided with an extreme period of glaciation called Snowball Earth. Massive ice sheets formed and may have extended all the way from the poles to equatorial latitudes. As the ice expanded, it would have eroded great quantities of rock, helping create the gap in the record.
And the escarpment, due to its high elevation, would have created ideal conditions for Snowball Earth glaciers and ice sheets to form. In a separate study published in Science, Gernon and a team explained how an escarpment created by tectonic activity was responsible for the formation of the East Antarctic Ice Sheet starting some 34 million years ago. “The exposed rocky peaks of the escarpment would have acted as glacier factories,” Gernon says.
It remains unclear, however, whether the erosion that erased the rocks occurred before, during or after the Snowball Earth glaciations. “That’s the real hole that needs filling: pinning down the erosion history in space and time, continent by continent, precisely enough,” Gernon says.
Linking the disappearance of the rocks to verified events in Earth’s history such as the breakup of Rodinia bolsters the case, says Alan Collins, a geologist at Adelaide University in Australia who was not involved in the new study. Not only that, but Gernon’s team points to modern-day examples where similar erosive processes are at work, including a similar escarpment in Antarctica as well as one in South Africa.
Maybe not quite case closed yet, but “I find this very convincing,” Collins says.
