Eons ago Earth experienced a wild transformation: it turned into a giant snowball. These massive glaciation events, where ice encased the planet from pole-to-pole, are fittingly named “snowball Earth.” There were at least two occurrences: one around 717 million and another some 645 million years ago.

Although geologists have good evidence Earth experienced these snowball events, they still cannot figure out howthey happened. Scientists have debated for decades over what set off the most profound climatic changes in the planet’s geologic record. Now researchers at Harvard University have a new idea that may finally provide an answer: They say volcanic regions, located in the right place at the right time, may have triggered at least the one of these giant glaciation events.

If you traveled back in time to Earth about 700 million years ago, you would have found ice hundreds of meters thick covering the oceans and continents, although the land masses may have also had some bare, dry areas dotted with ice-covered hypersaline lakes. The average global temperature fell around negative 37 degrees Fahrenheit. The snowball-like Earth was largely uninhabitable. Thankfully, these apocalyptic glacial periods happen rarely—but that fact also makes it hard for scientists to determine how such an extreme climate formed. “The further we go back in time, the more Earth resembles a world very different from the one we live on today,” explains Linda Sohl, a paleoclimatologist at Columbia University's Center for Climate Systems Research and NASA's Goddard Institute for Space Studies “So we can’t readily interpret the past based on our knowledge of the present.”

Researchers have proposed a host of ideas about what sparked snowball Earths. The cause—whatever it was—had to cool the planet so that enough ice formed to reflect much of the sun’s incoming energy, creating a runaway cooling effect. One hypothesis suggests a large meteorite hit the planet and threw up so much dust and ash into the air it reduced the incoming solar radiation for a couple years and chilled the planet. Other ideas involve similar types of brief but catastrophic events, such as a gigantic volcanic eruption. Yet another hypothesis proposes some kind of organism evolved that could remove a large amount of carbon from the surface of the ocean and bury it in deep sediments after they died and settled on the ocean floor; that mechanism would theoretically have kept enough carbon out of the atmosphere to cause runaway cooling. None of these ideas have much—if any—physical evidence to back them up, however.

One of the most popular ideas focuses on weathering, a natural process that captures and stores carbon via the chemical breakdown of rocks. When the supercontinent Rodinia broke up around 750 million years ago, the new, smaller continents scattered to locations around the equator where it was warm and wet—prime conditions for weathering. In addition, large volcanic regions would have emerged as the giant land mass fragmented, which would have been extremely vulnerable to weathering.

The problem: weathering works incredibly slowly—the process is constantly happening but it affects the global climate on a million-year time scale. Earth’s climate system usually self-corrects in that amount of time. Plus, the greater volcanic activity would have released carbon dioxide, making it even harder to push Earth into a snowball state. This supercontinent breakup scenario could have caused a runaway cooling effect only if weathering outpaced other feedbacks in the climate system, explains Francis Macdonald, an associate professor of geology at Harvard.

Because none of the ideas is completely satisfactory, Macdonald and colleague Robin Wordsworth, an assistant professor of environmental science and engineering, set out to find another explanation. In 2010 Macdonald published a paper that, for the first time, pinned down the precise date when the Sturtian glaciation—the first of the two snowball Earths—began. “We could suddenly say within a few hundred thousand years when this event actually occurred,” Macdonald explains. “Before, it had only been known within tens of millions of years.” He discovered Sturtian glaciation started around 717 million years ago.

Around the same time, Macdonald dated a volcanic region, called the Franklin Large Igneous Province (LIP). He discovered the Franklin LIP became active close to when the first snowball Earth event began. “I started thinking: How could these be so coincident? How might they be related?” he says.

Armed with this new information, Macdonald and Wordsworth used a combination of geologic evidence and modeling to test whether the Franklin LIP could be the culprit. In a new study, published in February in Geophysical Research Letters, they show the Franklin LIP’s volcanic activity could have caused extreme climate cooling. That is because of a unique combination of factors: First, the Franklin LIP formed in an area rich in sulfur; as it erupted, large plumes of hot gas and dust would have lofted sulfur particles kilometers into the air. Sulfur particles block the incoming sun and also keep heat from escaping Earth, which can create either a warming or cooling effect, depending on the location. That’s why the next piece of physical evidence is key—geologic records show the Franklin LIP sat at the equator where Earth receives more solar energy than the amount of heat it radiates back out to space. According to the researchers’ model, if enough sulfur particles reached high enough into the atmosphere at this equatorial location, it would block enough of the sun’s incoming energy to trigger runaway cooling. The sulfur aerosols would have spread over the planet as well via mixing that occurs in the stratosphere, but the equatorial region would have the greatest density of sulfur particles, severely blocking the sun. The eruptions would have needed to blast sulfur into the atmosphere for about five years to push Earth into a snowball state.

Such a scenario would also require a relatively cool Earth ahead of time. Macdonald says that is because sulfur particles need to reach the altitude of the stratosphere to have maximum cooling effect. In a colder climate the stratosphere settles a little closer to Earth’s surface, making it possible for the sulfur-rich hot air plumes to reach. Although scientists have not determined exactly what the climate was like prior to snowball Earth, this new hypothesis is appealing, Macdonald says. “It provides a positive feedback mechanism. As you start cooling, then it gets easier and easier to put more sulfur aerosols up there, then Earth cools more, and so on,” he explains. This process would happen potentially so fast that it would overwhelm other climate feedbacks that might make the planet warmer.”

Other experts find Macdonald’s and Wordsworth idea compelling. “I would say it’s probably the best idea we have, because it’s actually based on observations,” says Joseph Kirschvink, a geobiologist at California Institute of Technology, who coined the term “snowball Earth.” Paul Hoffman, an emeritus professor of geology at Harvard, says the timing between the sulfur-rich Franklin volcanism and snowball Earth makes it an attractive explanation. But “it could just be a coincidence with no relation,” he explains. Linda Sohl says the pair have come up with an intriguing hypothesis, although she also says, “Does it explain all snowball events in Earth’s history? Almost certainly not.”

Hoffman also points out the researcher’s idea does not explain the second snowball event that came soon after the first, called the Marinoan glaciation. “I think that’s the weakest point in the idea,” he says. “So far as we know, there’s no large [volcanic regions] associated with the onset of the second.” Macdonald says there could have been one but that geologic evidence becomes patchy that far back in time. Macdonald himself is not convinced his and Wordsworth’s version of events is what actually occurred 717 million years ago. “We’re not saying this had to happen, just that it’s feasible and it’s a pretty impressive coincidence,” he explains.

Along with this new idea, Macdonald expresses a note of caution to people who have proposed geoengineering projects using sulfur aerosols to combat global warming. “It’s a little frightening if we want to play with these particles, to know they may have caused major climate change in the past,” he says. “On the other hand, we’re already geoengineering with carbon dioxide. The cat’s already out of the bag.”