By Allison Kubo Hutchison

USGS: Pyroclastic Current at Mount Saint Helens on August 7, 1980.

The volcano erupts. The immense pressure inside the volcano from the accumulation of gases causes fragmentation. The thicker and more viscous the magma, the more fragmentation takes place (read more about it here). The fragmented magma cools down to sharp, glassy ash and larger blocks. It rushes out of the volcano and forms a pyroclastic flow.

Of all volcanic hazards, pyroclastic currents are the deadliest. They’re extremely fast-paced, deceptively enough. The thick opaque waves that accompany a current hide the fast-moving avalanche that forms the core of the current. They are also very hot, between 100 ° C and 500 ° C. Because of the high steam content in them and their high speed, even “low” temperatures can be fatal to humans. During some eruptions, the currents could penetrate houses, move under doors and through ventilation, and kill people inside and outside due to the immense heat and asphyxiating gas. The only way to survive a pyroclastic flow is to avoid it. Avoiding them is not easy as they can reach speeds of up to 100 km / h. Their high speed combined with driving over obstacles such as valleys or hills was a mystery.

Recent research has shown that these currents can flow on a layer of heated gas that rises from the floor like a magic carpet. Researchers call it “air lubrication,” which is used to reduce the friction these currents feel. When the current is racing downhill, the shear reduces the pressure at the base of the flow height. This pressure drop, in turn, causes gases to enter this area, creating a local area of ​​low ash concentration that reduces friction. However, as the friction decreases the velocity and hence the shear, it increases and results in a positive feedback mechanism that further increases the velocity of the pyroclastic flow.

This phenomenon could explain the extreme distance observed during ancient eruptions. Looking at the pyroclastic current deposits, some super-eruptions in the past have seen currents that traveled hundreds of kilometers and left behind meters of ash. A better understanding of the complex physics within these flows will lead to better prediction and hazard mitigation


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