A new theory has emerged as to what caused the largest glaciation event in history that covered the Earth from pole-to-pole in ice 717 million years ago.

A research team from Harvard University believes that the event, known as ‘snowball Earth,’ occurred after a huge volcanic event devastated an area from present-day Alaska to Greenland.

“We know that volcanic activity can have a major effect on the environment, so the big question was, how are these two events related,” Francis Macdonald, the John L. Loeb Associate Professor of the Natural Sciences at Harvard, said in a statement.

Robin Wordsworth, assistant professor of Environmental Science and Engineering at the Harvard John A. Paulson School of Engineering and Applied Science, theorized that aerosols emitted from volcanoes could have rapidly cooled Earth under the right conditions.

“It is not unique to have large volcanic provinces erupting,” Wordsworth said in a statement. “These types of eruptions have happened over and over again throughout geological time but they’re not always associated with cooling events.

“So, the question is, what made this event different?”

Geological and chemical studies of the region—called the Franklin large igneous province—revealed that volcanic rocks erupted through sulfur-rich sediments, which would have been pushed into the atmosphere during eruption as sulfur dioxide.

When sulfur dioxide gets into the upper layers of the atmosphere, it’s very good at blocking solar radiation. For example, in 1991 the eruption of Mount Pinatubo in the Philippines shot about 10 million metric tons of sulfur into the air and reduced global temperatures by about one degree Fahrenheit for a year.

Sulfur dioxide is most effective at blocking solar radiation if it gets past the tropopause—the boundary separating the troposphere and the stratosphere. When it reaches this height it’s less likely to be brought back down to Earth in precipitation or mixed with other particles, extending its presence in the atmosphere from about a week to about a year.

The height of the tropopause barrier depends on the background climate of the planet where the cooler the planet the lower the tropopause.

The eruptions throwing sulfur into the air 717 million years ago weren’t considered one-off explosions of single volcanoes like Pinatubo but rather the volcanoes spanned almost 2,000 miles across Canada and Greenland.

Instead of singularly explosive eruptions, these volcanoes can erupt more continuously, similar to those in present-day Hawaii and Iceland.

The research team demonstrated that about 10 years of continual eruptions from these types of volcanoes could have poured enough aerosols into the atmosphere to rapidly destabilize the climate.

“Cooling from aerosols doesn’t have to freeze the whole planet; it just has to drive the ice to a critical latitude,” Macdonald said. “Then the ice does the rest.”

More sunlight is reflected the more ice is prevalent, which means the planet becomes cooler.

“It’s easy to think of climate as this immense system that is very difficult to change and in many ways that’s true. But there have been very dramatic changes in the past and there’s every possibility that as sudden of a change could happen in the future as well,” Wordsworth said.

Understanding how these dramatic changes occur could help researchers better understand how extinctions occurred. They may also help scientists better understand how proposed geoengineering approaches may impact climate and how climate changes on other planets.

“This research shows that we need to get away from a simple paradigm of exoplanets, just thinking about stable equilibrium conditions and habitable zones,” Wordsworth said. “We know that Earth is a dynamic and active place that has had sharp transitions.

“There is every reason to believe that rapid climate transitions of this type are the norm on planets, rather than the exception.”