Earth’s ice is melting at a rapid clip today. But some scientists think that during several ancient episodes, the planet plunged into a deep freeze known as “Snowball Earth,” when ice sheets grew to cover almost the entire planet. However, the number of these episodes, their extent, and just how fast Earth turned into an ice cube have long been a mystery. Now, analysis of a newly discovered rock sequence in Ethiopia supports a Snowball Earth event some 717 million years ago and suggests it took place in mere thousands of years—the geologic equivalent of a cold snap.
The new work, grounded in Earth’s rock record, means the Snowball Earth hypothesis is “hanging in there, big time,” says Carol Dehler, a geologist at Utah State University in Logan, who was not involved in the research.
Geologists have suspected these rapid planetary freezes since at least the 1990s. But even though computer models supported the idea, there was little actual evidence from the planet’s rock record. That’s why Scott Maclennan, a Ph.D. student studying geology at Princeton University, and his advisers were excited when they got a tipoff from Robert Bussert, a geologist at the Technical University of Berlin, about rocks in northern Ethiopia that supposedly formed around the same time as one of the suspected Snowball Earth episodes, known as the Sturtian glaciation.
Maclennan and colleagues ventured to the small town of Samre, Ethiopia, where they came across a type of rock—which they later dated to roughly 717 million years old—that could have only formed through glacial activity. These rocks, called diamictites, are made of huge boulders transported great distances by glaciers. Buried just below those glacial rocks were older layers of carbonate rocks. As the ancient supercontinent Rodinia was breaking apart, these rocks formed in shallow waters with the aid of microbial marine organisms—a sign that the same location was warm just a bit earlier in time, Maclennan explains.
Together, these layers suggest the ancient climate quickly shifted from tropical paradise to frozen wasteland, Maclennan and colleagues report this month in Geology. Because the carbonate rock layers transition into the glacial rock layers in a smooth fashion—without any significant breaks—there was likely little to no missing time between the warm and frozen episodes, Maclennan says. But exactly how long it took for the freeze to finish is more difficult to discern. Maclennan explains that it could have been, from beginning to end, anywhere from 1000 to 100,000 years.
Many scientists think that such a rapid expansion of ice happened thanks to a so-called ice albedo feedback loop. Within such a loop, ice sheets reflect incoming sunlight back out into space, driving down atmospheric temperatures. These low temperatures, in turn, drive even more ice growth, and as more ice forms, more solar energy escapes out into space. This process snowballed until, in this case, most of the planet froze. “Earth can do things that you could never imagine,” Dehler says.
The find supports theoretical models of snowball glaciation, which suggest that once ice extends down to 30 degrees of latitude, rapid planetwide glaciation follows. It also supports the only other find from the Sturtian glaciation that has been dated with high precision—rocks from northwest Canada that put the onset at about 717 million years ago. “Given how crazy the planet’s climate must have been to bring about a snowball Earth, this positive test for the hypothesis is surprising,” Maclennan says.