What Are Lahars? Guatemala's Mt Fuego May Cause Volcanic Mudflows With Deadly Speeds

Sunday's eruption of the Fuego volcano in Guatemala has ended, at least for now, after killing at least 25 people, according to the country's disaster response agency. But even if the volcano remains quiet, residents of the area aren't necessarily safe quite yet.

Guatemala's national agency monitoring earthquakes, volcanos and the weather, has warned that in the wake of the eruption, areas around the volcano may be at risk of what scientists call lahars, which are sort of like very fast mudflows, but filled with rock particles released by the eruption.

That risk reflects how the eruption played out: there was no lava involved, only pyroclastic flows. Despite the name, pyroclastic flows aren't liquids, they're a dense mixture of super-hot rocks—from boulders all the way down to the tiny particles of rock scientists refer to as volcanic ash. Pyroclastic flow can reach temperatures as high as 1500 degrees Fahrenheit and speeds of 50 miles per hour or more, barreling through pretty much whatever is in its path.

As if the initial destruction weren't bad enough, pyroclastic flows bring a second, longer-lasting threat to areas surrounding a volcano. That's because all that rock and ash stays on the ground. And if heavy rains come along, they can send that mass flowing again, since it's loosely packed particles the consistency of sand and rock. (In colder regions, lahars can also be triggered by melting glaciers.)

"If you think of trying to build a sand castle on a beach, the water just carries it away," Janine Krippner, a volcanologist at Concord University in West Virginia, told Newsweek.

The result is what scientists call a lahar, essentially an incredibly fast-moving volcanic mudflow that can reach speeds up to 120 miles per hour. As a lahar travels, it can pick up boulders, debris, cars and other objects in its way.

Pyroclastic flow curls down the side of Guatemala’s Volcan de Fuego (“Volcano of Fire”), on June 4. A group of survivors of the volcanic eruption has been given special dispensation to get treatment in the United States. Johan Ordonez/AFP/Getty Images

Lahars are a known volcanic threat. "They really are just part of living next to an explosive eruption where you have rainfall," Krippner said. And unlike eruptions themselves, which are unpredictable, scientists can give some warning about rain-triggered lahars based on local weather forecasts.

That's particularly helpful because lahars can reach areas that weren't originally affected by the eruption itself. And the risk of lahars lingers after an eruption—in some regions, like Indonesia, where lahars are particularly common, they can occur years after the original eruption.

As for the eruptive future at Fuego itself, that's more difficult to predict. "This volcano has been erupting for a long time, it's quite an active volcano," Krippner said, adding that that means it's likely to remain fairly active. Sunday's eruption wasn't unusual for taking place at all, it was unusual for its scale. "The size of this eruption is what's different, it's much bigger than what usually occurs at this volcano."