Four years ago, during my very first field season as a volcanologist, I was at the bottom of Barranca Ceniza (Ash Valley) in Guatemala. My colleagues and I had lowered ourselves into the valley with ropes and walked around looking for an ideal sampling site. It was muggy and overcast, and we couldn't see the summit of Volc-n de Fuego, the Guatemalan volcano that has been erupting since Sunday. But we could hear it rumbling above the clouds.
Earlier that morning, from the street in front of the local observatory in Panimache, we had seen some volcanic explosions -- small puffs of ash, spectacular to me as a newbie, yet not uncharacteristic of this volcano. But now, down on the floor of the "barranca," the surrounding scenery reminded us of how differently things could go here: we were walking amid layer upon layer of consolidated ash, which had made its way all the way down there, miles away from the summit, certainly during a much bigger eruption than those we had just witnessed from the observatory.
I worked nervously, knowing that if a big one came -- and it could have come at any time from this volcano, one of the most active in Latin America -- it would have found us on a preferential path, one that the ash had already taken many times before. A few hours later we made our way up safely and with plenty of samples. I felt very lucky that no major eruption had caught us there.
Right now, though, things are very different.
At least 75 people have died since Fuego began erupting four days ago, and that number may well grow as the eruption continues. Yet Kilauea -- the other volcano that has captured our attention with the slow, rolling devastation of its lava flow -- hasn't claimed a single life over the month since it started erupting on the big island of Hawaii.
And that's not likely to change. Why is that?
The two major eruptions we are witnessing, one in Guatemala and the other in Hawaii, are profoundly different. Unsurprisingly, not all volcanoes erupt in the same way. Some, like Kilauea, produce lava. Others, like Fuego, produce ash. What's the difference between lava and ash? Lava is, basically, a relatively tiny amount of gas in a lot of partially molten rock; ash is a relatively small amount of solid rock in a lot of hot gas, instead.
Whereas flowing lava moves slowly, destroying infrastructure but giving people the chance to escape, flowing ash moves very fast, making outrunning it impossible.
Pinning down one factor that makes a volcano erupt in an effusive (lava-making) or explosive (ash-making) way is tricky, and the same volcano can behave differently at different times. But for Kilauea and Fuego, the key is, once again, water: very little of it at Kilauea -- just enough to help its fluid magma rise to the surface -- and lots of it at Fuego, so much that its stickier magma can't keep up with it when it rises. I've written a little about how this works here.
Where does Fuego's water come from? Fuego is located on the Caribbean Plate, at what geologists call a subduction zone. At a subduction zone, one tectonic plate (in this case the Cocos Plate) goes beneath another (in this case the Caribbean Plate) -- we say it subducts. Now, the subducting plate has been sitting at the bottom of the ocean for millions of years, and it has soaked up a lot of water over that time. As it subducts beneath the Caribbean Plate, the Cocos Plate warms up, and that water gets released and prompts the upper plate to melt (much like salt prompts ice to melt on the roads), eventually ending up in the newly formed magma.
All is well as long as the magma sits deep in the magma chamber. But as it rises toward the surface, it is under less pressure, and the water contained within begins exsolving (or separating) from the magma, forming bubbles that grow bigger and bigger and move faster and faster, just like when you open a bottle of soda. The molten rock all around it can't quite keep up, and it breaks (or fragments, in volcanology jargon) into a billion pieces. So by the time magma reaches the vent, it isn't glowing, flowing lava coming out, but, rather, solid ash -- fine fragments of incandescent but solid rock.
That ash forms an eruptive column. Initially, the ash column rises because it leaves the vent at the top of the volcano really fast, propelled by that expanding water vapor. At that point, it rises because it is really hot, and hence not very dense compared with the surrounding atmosphere. But eventually, it becomes unstable and collapses under its own weight in a pyroclastic density current (PDC). And this is where it becomes lethal.
A PDC is a hot mixture of ash and gas traveling at speeds up to 450 miles/hour. Most PDCs separate into a pyroclastic flow and a surge soon after forming. The pyroclastic flow -- dense and rocky -- travels at the bottom. The surge -- gassy and buoyant -- travels above it. The pyroclastic flow follows the topography it encounters, quickly becoming channeled in pre-existing valleys and deepening them through erosion. Conversely, the surge can easily overcome valley sides, hills, and infrastructures; it travels in a straight line away from the volcano, and nothing can stop it, making it all the more dangerous.
Fuego's many barrancas, radiating from its vent outward, are the scars left by its own explosive activity, and serve us as a powerful reminder of its destructive potential. I thought about that potential very much while I was working at the bottom of a barranca in 2014, hoping I would never see it unleashed, the way we now have in this destructive eruption.