What makes Mt. Merapi different from other volcanoes?

Reuters photo.jpg

This is what happens when your kitchen meets a nuée ardente.

Literally a "burning cloud", the name is French for pyroclastic flow—a mass of hot gas, ash and rock released in some volcanic eruptions. Basically, it's an avalanche that happens to be hot enough to sear flesh. The danger of these things is that they move fast—hundreds of miles per hour, in some cases—and that they hug the ground, burning and suffocating everything in their path. Almost all the big, famous eruptions—from Mt. Vesuvius to Mt. St. Helens—included pyroclastic flows. And so have the recent eruptions at Mt. Merapi in Indonesia.

In fact, these things happen so often at Mt. Merapi that the Mountain has become the namesake of a specific type of pyroclastic eruption, different from the ones that buried Pompeii and the Toutle River. You're probably most familiar with Pelean-type eruptions. Named after the volcano that flattened the Caribbean town of Saint-Pierre in 1902, these eruptions are often phenomenally destructive. Pressure builds up for decades under a dried lava "cork". When the lava dome collapses and the cork finally pops, the pyroclastic flows spill out—fast and furious.

Merapi, in contrast, erupts a lot more frequently—on the order of every 4 or 5 years. Between eruptions, it builds up a lava cork of its own. In this case, though, the cork isn't very well supported by the underlying structure of the mountain, so it collapses any time it gets a bit too big. Collapse triggers pyroclastic flows, but, because so little time has passed, there isn't nearly as much pressure behind them. At Mt. Merapi, pyroclastic flows happen more frequently, but they're considered to be less dangerous.

That doesn't mean they can't kill you, however. Most of the people in the way of these recent eruptions were evacuated ahead of time. But not all. As of yesterday, 153 people were reported dead. The Boston Globe's Big Picture blog has some truly devastating photos of the human toll. They are, in many cases, more explicit than you're used to seeing. But I think it's important to not get so caught up in the the zoomed-out, awe-inspiring perspective that we lose track of the impact these eruptions have had on individual people. The pyroclastic flow went through somebody's kitchen.

Image: Dwi Oblo / Reuters

(Thanks to Howard Koerth—Happy Mutant, raconteur, Dad—for bringing the Boston Globe photos to my attention.)