Scientists report why a billion years of rock history is missing from Grand Canyon

• Computer models demonstrate that a continental-scale escarpment, stretching one to two kilometers high, formed about 800 million years ago during the breakup of the supercontinent Rodinia.
• The extreme ice age known as Snowball Earth acted as an accomplice, using the high-elevation cliff as a glacier factory to scour away massive quantities of rock.
• Alan Collins, a geologist at Adelaide University who was not involved in the new study, called the findings very convincing, though researchers note that the exact timing of the erosion remains to be pinned down.

The Breakup of Rodinia and the Formation of a Giant Escarpment

To solve how those rock layers vanished, Thomas Gernon and his colleagues at the University of Southampton used computer models to simulate how ancient Earth evolved over hundreds of millions of years. The team watched as a giant cliff, known as an escarpment, formed about 800 million years ago as the ancient supercontinent Rodinia split apart.

This escarpment most likely extended across much of a continent called Laurentia, the ancient precursor to North America. Reaching heights of one to two kilometers, the cliff stretched from the region that would eventually become Alaska all the way down to Arizona. No previous study had suggested the existence of a continental-scale escarpment in the Grand Canyon region, according to Gernon.

Glaciation and the Role of Snowball Earth

An immense cliff creates steep slopes that naturally weather and erode under rain and ice. However, the escarpment had major assistance from an extreme period of global glaciation called Snowball Earth. During this era, massive ice sheets formed and may have extended from the poles all as far as equatorial latitudes.

The high elevation of the escarpment created ideal conditions for these glaciers and ice sheets to form, acting essentially as glacier factories. As the ice expanded, it eroded great quantities of rock, helping create the vast gap in the geologic record. Gernon’s team previously published research in Science explaining how a similar tectonic escarpment was responsible for the formation of the East Antarctic Ice Sheet starting about 34 million years ago.

Researchers Seek Exact Sequence of Erosion Events

While the computer models and the scale of the escarpment provide a straightforward explanation for the immense erosion, critical questions remain regarding the exact sequence of events. It remains unclear whether the erosion that erased the rocks occurred before, during, or after the Snowball Earth glaciations. Pinning down that erosion history in space and time across continents is the real hole that needs filling, Gernon noted.

To support their conclusions, the researchers point to modern-day examples where similar erosive processes are active, including escarpments found in Antarctica and South Africa. Alan Collins, a geologist at Adelaide University who did not participate in the research, noted that linking the disappearance of the rocks to verified events like the breakup of Rodinia significantly bolsters the case.