USGS: Pyroclastic flow at Mount Saint Helens on August 7, 1980. It hurtles out of the volcano and forms a pyroclastic flow.Of all volcanic hazards, pyroclastic flows are the most deadly. The only way to survive a pyroclastic flow is to avoid it. 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.