Why Guatemala’s Volcano Has Been More Deadly Than Hawaii’s

The recent volcanic eruptions in Hawaii and Guatemala are a reminder that not all volcanoes are created equal: Some types pose a far greater hazard to life than others.

In the most recent eruption of Kilauea on the Big Island of Hawaii that began a month ago, most of the lava has oozed from fissures, spreading slowly enough that it is no threat to people — although hundreds of houses and other buildings in the lava’s path have been destroyed.

But the two eruptions that have occurred so far at the Fuego volcano near Guatemala City have been explosive, creating pyroclastic flows — fast-moving, searingly hot clouds of volcanic ash and gases — that have killed more than 100 people so far.

“Lava flows rarely kill people,” said Paul Segall, a professor of geophysics at Stanford University. “They typically move slow enough that you can walk out of the way.”

By contrast, pyroclastic flows are extremely dangerous, he said, with temperatures as high as 1,000 degrees Fahrenheit. Because the clouds are denser than air, they can rush down valleys or gullies at speeds of 100 mph or more and overcome people trying to run away.

“You will not survive a pyroclastic flow,” Segall said.

Kilauea and Fuego are very different volcanoes. Kilauea is a shield type — a broad, gently sloping lump of old lava. Fuego is a stratovolcano, with a conical shape and steep sides.

The difference in shape, and in eruptions, is related to the viscosity of the molten rock, or magma, within each volcano.

At Kilauea the magma is relatively thin, so that each time it erupts the lava quickly spreads out, leaving a broad low mound. At Fuego the magma is much thicker, so it spreads less rapidly, leaving a higher mound.

Kilauea’s thinner magma also explains why it is less explosive. As magma comes to the surface, a lot of gases — carbon dioxide, sulfur dioxide and others — come with it. Kilauea’s magma is thin enough that most of the gas bubbles out.

At Fuego and other stratovolcanoes, the thicker magma traps much of the gas. At some point, the mass of magma and gas explodes. Some stratovolcano eruptions are so big they eject huge amounts of gas and ash high into the atmosphere, where the gas can reflect sunlight and lower global temperatures for several years. The eruption of Krakatau in 1883 in what is now Indonesia, for example, caused temperatures to temporarily drop about 2 degrees Fahrenheit.

Neither of Fuego’s eruptions were anywhere near that large. And a Krakatau-scale explosion of a shield volcano like Kilauea is never going to happen, Segall said.

Not that there haven’t been a few small explosions on Kilauea in the past month. These may have been the result of hot magma contacting groundwater, which then flashes into steam. But the explosions were predictable and relatively small. Only one injury occurred.

Kilauea has experienced “fountaining” of lava, where the molten rock is thrust a hundred feet or more into the air as gas bubbles out of it. Segall said that this suggests that the lava during this eruption contains more gas than usual.

Fuego has also produced lahars, essentially mudslides where the mud is actually volcanic ash and other debris. Lahars are usually cooler than pyroclastic flows — they occur sometime after the eruption, when the ash has settled on the ground — but they can be extremely destructive and deadly, burying houses and people in their path.