The early-June eruption at Guatemala’s Volcan de Fuego, whose pyroclastic flows killed at least 109 people, may have slipped away from the headlines, but that doesn’t mean the story’s over for those still living there. While volcanologists on the ground use drones to get a better sense of the violent changes that have occurred, NASA’s Landsat-8 satellite is peering at the devastation from up on high.
As reported by Earth Observatory’s Adam Voiland, these images reveal quite remarkably clearly where the pyroclastic flow deposits are, but that's not all. Somehow, three weeks after the eruption took place, you can still detect these flow deposits' incandescent thermal glow from space.
How, then, is this possible?
Pyroclastic flows are types of pyroclastic density currents (PDCs), turbulent streams of fresh volcaniclastic material that are produced by explosive volcanic activity. Flows are the most common – mixtures of hot ash, gas, entrained debris and flecks of lava – and they can form in a variety of ways.
In terms of Fuego, it appears many were generated as a result of parts of the sustained column of ash, shooting skyward from the vent, lost upward momentum and fell back down to Earth. This could have been through a lack of explosive thrust at the vent, or sufficient cooling – and subsequent lack of buoyancy – in the ash column.
Some, however, may have formed as the vent “boiled over”, and essentially spilled volcaniclastic material right over the peak and down the slopes. Either way, pyroclastic flows quickly formed and rushed down the valleys on Fuego’s flanks.
Forget that notion of cooling-and-collapse though: these flows are still astonishingly hot, with internal temperatures ranging from 200-700°C (390-1,300°F), moving at 80 kilometers (50 miles) per hour, roughly speaking. Anything they touched was blown away; people were instantly scorched, and suffered from extreme heat shock or asphyxiation.
Death is inevitable for anyone caught up in a pyroclastic flow, but what happens when they come to a stop?
Eventually, when these flows lose forward momentum, their particles begin to settle out.
There are plenty of varieties of pyroclastic flow deposits, each governed by the type of material produced, how chaotically it was propagating across the land, its original temperature, and so forth. Sometimes, flow deposits are so hot that the material in it welds together and forms a more singular deposit.
The point is that whatever flavor they come in, these beasts are frequently deposited very quickly, at very high temperatures, so cooling quickly isn't an option. When they do settle down, their exposed fridges cool quickly, but their bellies are often insulated (particularly in the thicker deposits).
Dr Janine Krippner, a volcanologist at Concord University, told IFLScience that rocks are poor conductors, particularly large masses of them. “The surface may be cooling, but the underlying rocks can stay hot for a long time.”
That’s what you can see in these Landsat-8 images, taken on June 24: the glow of the pyroclastic flow deposits indicate that they’re still 4 to 6°C (7.2 to 10.8°F) hotter than the surrounding landscape.
That’s not all these images show. If you look closely, you can see gray streams leading away from Fuego too. Several of these are the slurry-like deposits of lahars, which are fast-moving, concrete-like mudflows created by the saturation – i.e. by rain – of the pyroclastic flow deposits.
Lahars can sometimes be more dangerous than the flows themselves, especially as people’s guards tend to be down after the eruption has concluded. Pyroclastic flows may have killed 45,000 people in the 20th century alone, but in 1985, in Columbia, a lahar killed 23,000 people in one single night.