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Guatemala volcano killed dozens with its pyroclastic flow. Here’s what that is.

A police officer stumbles while running from a new pyroclastic flow spewed by the Fuego volcano in the community of San Miguel Los Lotes in Escuintla, Guatemala, on June 4, 2018. (Luis Echeverria/Reuters)

Guatemala’s Volcan de Fuego — Volcano of Fire — erupted spectacularly Sunday, shooting a plume of ash and gas nearly six miles into the sky and spreading ash and debris across towns and farms more than 10 miles away from the volcano.

A river of lava, rocks and ash poured down the mountain, burying homes and people. This river, or pyroclastic flow, may not be glowing orange like molten earth, but it is undeniably the most deadly part of the volcano.

At least 33 dead in Guatemala as volcano erupts, burying an entire village

Do you recall that scene in “Independence Day,” the one where the aliens detonate their weapon of mass destruction over New York City, Washington and Los Angeles? A wall of fire tears through the cities, incinerating everything in its path. Jasmine, played by Vivica A. Fox, leaps into a tunnel service closet with her son and dog Boomer, narrowly escaping the fiery wave. It was a close call, but Boomer does not end up on the very bad list at, and Jasmine and her son are safe.

Pyroclastic flow is kind of like the alien wall of fire in Independence Day. It’s full of rocks, ash and hot gas. It blasts across the ground at speeds over 50 mph and is incredibly hot — sometimes over 1,000 degrees Fahrenheit.

“It is universally deadly,” the U.S. Geological Survey says.

There’s no service closet in the world that could save you when pyroclastic flow is shooting down the mountain.

Pyroclastic flow is how 30,000 people died in the eruption of Mount Pelee in 1902. When Vesuvius erupted in 79 A.D., more than 13,000 people died in pyroclastic flow, although the exact death toll is unknown. And it’s also being blamed for dozens of deaths in the Fuego eruption Sunday.

“Hundreds of police officers, soldiers and emergency workers have been sent to affected areas on the slopes of the volcano,” the BBC reports. “They found charred bodies resting on steaming remnants of pyroclastic flow.”

Guatemala's Volcan de Fuego, or “Volcano of Fire,” erupted for the second time this year, killing dozens of people on June 3. (Video: Amber Ferguson/The Washington Post)

There are several ways pyroclastic flow can develop during an eruption.

  • The plume of ash and gas above the volcano cools and falls to the ground. Amplified by gravity, the flow pulls hot debris down with it.
  • Instead of erupting into a big plume, the volcano can “boil over” and its contents flow rapidly down the mountain.
  • Like an avalanche, lava that piles up can become unstable, break off and shoot down the mountain.

Even though the pyroclastic flow just looks like ash, it’s actually a flash flood of rocks and boulders. Just like a flash flood, it will naturally flow into dips and valleys, but if the valley narrows or the volume of flow becomes too large, it will spill out and flood the surrounding area.

A boulder crushed a man fleeing the Vesuvius eruption. Archaeologists just found his remains.

That’s exactly what happened in this video, which shows people watching the pyroclastic flow from Fuego shoot down a ravine. When it reaches the bridge, it explodes into the air. Right at the end of the clip, you can see the most dangerous part of the pyroclastic flow — hot rocks and boulders — shoot across the road.

“The clouds you see mask a dangerous interior to these currents,” Rebecca Williams, a volcanologist at the University of Hull, said on Twitter. “Often a dense avalanche is at the base, capable of carrying huge blocks of rock. This has great force and will destroy buildings and flatten forests.”

The USGS says that pyroclastic flow — made up of pieces of rock, ash and hot gas — can vary in speed and is universally deadly and destructive. “A pyroclastic flow will destroy nearly everything in its path,” the USGS says. “With rock fragments ranging in size from ash to boulders traveling across the ground at speeds typically greater than [50 mph], pyroclastic flows knock down, shatter, bury or carry away nearly all objects and structures in their way.

“The extreme temperatures of rocks and gas inside pyroclastic flows, generally between [400 and 1,300 degrees Fahrenheit], can cause combustible material to burn, especially petroleum products, wood, vegetation and houses.”

Correction: A previous version of this story said that the majority of deaths in the Krakatoa eruption were due to pyroclastic flow. The pyroclastic flow in the 1883 Krakatoa eruption caused a tsunami which is thought to be responsible for the vast majority of the 36,000 deaths — not the pyroclastic flow itself.