How the Grand Canyon could have lost 1 billion years of geologic history

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. Some 65 million years ago, the Great Escarpment of Laurentia would have stretched across much of North America, including the spot that would one day become the Grand Canyon and the site of an enduring geologic mystery. The escarpment itself is thought to have formed around 800 million years ago.

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.