What Are Pyroclastic Flows? Why Guatemala's Fuego Volcano Eruption Was Far Deadlier Than Hawaii's Kilauea

The eruption of the Fuego volcano—or volcano of fire—in Guatemala on Sunday and the flows that followed have killed more than 60 people so far, according to authorities. The volcano is one of the most active in Central America and has erupted more than 60 times since 1524, according to Oregon State University.

Sunday's eruption was one of the most violent in decades and has destroyed entire towns in its vicinity. The volcano sits at an elevation of more than 12,000 feet and is one of three in the area near the city of Antigua. It's a stratovolcano, meaning it's built up over time to have steep slopes, especially near the cone. They're some of the most common—and most deadly—volcanoes on Earth, and they emit a combination of lava and pyroclastic flows.

But Kilauea, the Hawaiian volcano that has been erupting for weeks, spews a different material than the Fuego volcano. In Hawaii, there have been slow flows of liquid rock coming from the volcano for weeks. "On Hawaii, you have something that is mainly basalt," Tamsin Mather, professor of earth sciences at the University of Oxford, told Newsweek.

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U.S. Army National Guard First Lt. Aaron Hew Len takes measurements for dangerous levels of sulfur dioxide gas in front of a lava flow and downed power lines on a residential street in the Leilani Estates neighborhood in the aftermath of eruptions from the Kilauea volcano on Hawaii's Big Island on May 8, 2018 in Pahoa, Hawaii. Mario Tama/Getty Images

That liquid rock on Hawaii moves slowly enough that people can evacuate if the lava begins to head in their direction. In Guatemala, that's not the case. "In Fuego, you've got lots of gas in the magma, and it's much more explosive," Mather said. The energy release from Kilauea and that from Fuego are so different that they're difficult to compare.

"In Fuego, the main devastation was due to the pyroclastic flows," Mather told Newsweek. "This is very different to a lava flow." Pyroclastic flows are a mixture of hot gas along with volcanic matter, like ash and rock fragments, she said. They are incredibly hot and can travel faster than a moving vehicle in some cases.

Photos taken during and following the eruption show this material and the devastation it's caused in the areas around the Fuego volcano. What came down the side of the Fuego volcano on Sunday, while still the product of a volcanic eruption, is not the same as what's been coming out of the Kilauea volcano on Hawaii.

The pyroclastic flows are "like a hot cloud traveling at speed down the side of a volcano, and it devastated everything in its path," Mather said. These flows cause death from the heat, or from being overwhelmed by the hot gases or even by being hit by chunks of rock. In Guatemala, "people just didn't have time to get out of the path," according to Mather.

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A man stands on the roof of a house in the ash-covered village of San Miguel Los Lotes, in Escuintla Department, about 21 miles southwest of Guatemala City. The photo was taken on June 4, after the eruption of the Fuego Volcano. Johan Ordonez/Getty Images