The erupting volcano may be terrifying in terms of explosive force, but the aftermath of fast-moving ash flows is far more deadly and destructive.
A pyroclastic flow is a fluidized mixture of solid to semi-solid fragments and hot, expanding gases that flows down the flank of a volcanic edifice. These awesome features are heavier-than-air emulsions that move much like a snow avalanche, except that they are fiercely hot, contain toxic gases, and move at phenomenal, hurricane-force speeds, often over 100 km/hour. They are the most deadly of all volcanic phenomena.
Pyroclastic flows can form in several different ways. The flow is fluidized because it contains water and gas from the eruption, water vapor from melted snow and ice, and air from the flow overriding air as it moves downslope. Ignimbrites and nuees ardentes are two types of pyroclastic flows. An ignimbite contains mostly vesiculated material whereas a nuee ardente contains denser material.
Nuee ardente means glowing cloud and was named for the pyroclastic flows seen at Mount Pelee. These flows were often accompanied by a cloud of ash elutriated from the flow. When the incandescent ash particles are observed at night, the flow looks like a glowing cloud moving away from the volcano. Pyroclastic flows can move very fast. Small pyroclastic flows can move as fast as 10 to 30 m/s while larger flows can move at rates of 200 m/s.
At Mount Pinatubo in the Philippines, pyroclastic flow deposits were 220 m thick in some valleys but averaged 30 to 50 m thick in others . Pyroclastic flows can be very hot. In fact, pyroclastic flows from Mount Pelee had temperatures as high as 1075 degrees C (Bryant, 1991)! Some Pyroclastic flows from Pinatubo had temperatures of 750 degrees C and pyroclastic flows from Mount St. Helens had temperatures of 350 degrees C. Such high temperature flows can burn man made structures, vegetation, and, for those unlikely enough to be caught by then, human skin.
Scientists recognize the hazards of pyroclastic flows, and so there is currently a lot of research going on in this area. Important research with regard to hazards prevention is the study of past pyroclastic flow deposits. Areas that have old pyroclastic flow deposits are likely to receive new pyroclastic flow deposits if the volcano erupts again.
People living near the summit of an active volcano, especially those in valley areas, are most likely to be in danger from a pyroclastic flow. The best course of action for these people to take when a volcano erupts is to evacuate valley areas and head for higher ground away from the volcano. Of course, if the volcano gives ample warning that it is going to erupt, then the best thing to do is evacuate the area and get as far away from the volcano as possible.