Guatemala Volcano Fuego: What Makes Pyroclastic Lava So Fast-Moving

Guatemala’s Volcán de Fuego explosively erupted on Sunday for 16 and a half hours. The devastating event left approximately 25 people dead and nearly 300 injured, a number that is expected to rise. The stratovolcano is one of the most active in Central America and its ability to destroy is directly linked to the lava mixture it emits: Unlike the slow-moving flows recently seen gurgling from Hawaii’s Kilauea volcano, Fuego spits out immensely dangerous pyroclastic flows.

Pyroclastic flows are why this eruption has engulfed entire villages and forced approximately 3,000 residents to rapidly evacuate. The United States Geological Survey describes pyroclastic flows as a “chaotic mixture” of hot lava blocks, pumice, ash, and volcanic gas which “destroys nearly everything in their path.” The speed of the flow underlies its danger: Rock fragments within the flow can travel up to 50 miles per hour, and the mixture itself usually moves at tens of meters per second.

Meanwhile, the extreme temperature within a pyroclastic flow can ignite fires as it gushes through a landscape: In the Guatemalan towns of El Rodeo, Alotenango, and San Miguel los Lotes, this has turned property to ash. Eddy Sanchez, Ph.D., the director of Guatemala’s seismological, volcanic and meteorological institute told The Guardian that, “Temperatures in the pyroclastic flow can exceed 700 degrees [Celsius] and volcanic ash can rain down on a 15 kilometer [9.32-mile] radius. That could cause more mudflows and nearby rivers to burst their banks.”

The BBC reports that Guatemala’s national institute of volcanology, Insivumeh, said that the last pyroclastic flow was recorded at 18:45 local time on Sunday. But the institute warns that people should stay away from the affected ravines, in case there is a “reactivation.”

The institute has also warned of the presence of lahar, which may further explain why these flows — which would already be moving fast — have moved at an incredible speed. Also known as volcanic mudflow, lahar is a mixture of water and volcanic debris that emerges as a secondary hazard to pyroclastic flows. Sometimes the consistency is like muddy dishwater, and other times it’s more like wet cement — it all depends on the ratio of water to debris. There’s a currently a warning about the possibility of lahar in place for villages south, southwest, and southeast of the volcano.

As a stratovolcano, Fuego is a steep, conical volcano subject to explosive and effusive eruptions. Hawaii’s Kilauea, which has steadily erupted for the past four weeks, is a basaltic shield volcano, which gurgles out a type of basalt lava called tholeiite. It’s a very different type of volcano: Shield volcanos typically only have explosive eruptions if water gets into the vent and 90 percent of the volcano is lava rather than pyroclastic material.

For the last month, Hawaiian officials have been trying to persuade those in Kilauea’s path to evacuate. The Guatemalans affected by Fuego tragically did not have that option. According to the USGS, the best advice it can offer for a volcano like Fuego is also the most instinctual: “If you witness a pyroclastic flow, run in the opposite direction as quickly as possible.”