|USGS: Pyroclastic flow at Mount Saint Helens on August 7, 1980.|
The volcano erupts. The immense pressure within the volcano due to the build-up of gases causes fragmentation. The thicker and more viscous the magma the more fragmentation occurs (Read more about that here). The fragmented magma cools into sharp, glasslike ash and larger blocks. It hurtles out of the volcano and forms a pyroclastic flow.
Of all volcanic hazards, pyroclastic flows are the most deadly. They are extremely fast-moving, deceptively so. The thick opaque billows that accompany a flow hide the fast-moving avalanche that makes up the core of the flow. They are also very hot, ranging between 100 C to 500 C. Due to the high vapor content within them and their high velocity even “low” temperatures can be fatal for humans. In some eruptions, the flows have been able to penetrate homes, moving under doors and through ventilation, killing those inside and outside due to the immense heat and choking gas. The only way to survive a pyroclastic flow is to avoid it. Avoiding it isn’t easy since they can travel at speeds of up to 100 km/hour. Their high speed coupled with passing over obstacles such as valleys or hills has been a mystery.
Recent research highlights that these flows are able to flow on top of a layer of heated gas, elevated from the ground just like a magic carpet. Researchers call it “air lubrication” which acts to reduce the friction that these flows feel. As the flow races down a hill, the shear decreases the pressure at the base of the head of the flow. This decrease in pressure in turn causes gases to enter that area creating a local area of low ash concentration which lowers the friction. However, as the friction lowers the speed and thus shear increases and leads to a positive feedback mechanism which further increases the pyroclastic flow’s speed.
This phenomenon might explain the extreme distance that has been observed in ancient eruptions. Looking at the deposits of pyroclastic flows some super-eruptions in the past have had flows that traveled 100s of kilometers and deposited meters of ash in their wake. A better understanding of the complex physics within these flows will lead to increased hazard prediction and mitigation