Pyroclastic Flow - Size and Effects

Size and Effects

The volumes range from a few hundred cubic meters to more than a thousand cubic kilometres. The larger ones can travel for hundreds of kilometres, although none on that scale have occurred for several hundred thousand years. Most pyroclastic flows are around one to ten cubic kilometres and travel for several kilometres. Flows usually consist of two parts: the basal flow hugs the ground and contains larger, coarse boulders and rock fragments, while an extremely hot ash plume lofts above it because of the turbulence between the flow and the overlying air, admixes and heats cold atmospheric air causing expansion and convection.

The kinetic energy of the moving boulders will flatten trees and buildings in their path. The hot gases and high speed make them particularly lethal:

• The towns of Pompeii and Herculaneum, Italy, for example, were famously engulfed by pyroclastic surges in 79 AD with many lives lost.
• A pyroclastic surge killed volcanologists Katia and Maurice Krafft and 41 other people on Mount Unzen, in Japan, on June 3, 1991. The surge started as a pyroclastic flow and the more energised surge climbed a spur on which the Kraffts and the others were standing; it engulfed them, and the corpses were covered with about 5 mm of ash.
• On 25 June 1997 a pyroclastic flow travelled down Mosquito Ghaut on the Caribbean island of Montserrat. A large energised pyroclastic surge developed. This surge could not be restrained by the Ghaut and spilled out of it, killing 19 people who were in the Streatham village area (which was officially evacuated). Several others in the area suffered severe burns.