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La mont saint helens est en éruption, une fumée épaisse s'échappe de la cheminée du volcan. Dans cette épaisse fumée grise, des tonnes de cailloux dévalent la pente à plus de 700 km/h et les gaz composant la fumée sont à 1000°C de température. Courir devient alors la seule solution.
There are several scenarios which can produce a pyroclastic flow:
Fountaining of an eruption column from a plinian eruption (e.g., Mount Vesuvius's destruction of Pompeii, see Pliny the Younger). In such an eruption, the material ejected from the vent heats the surrounding air and the turbulent mixture rises for many kilometres through convection. If the erupted jet is unable to heat the surrounding air sufficiently, there will not be enough convection to carry the plume upwards and it fountains back down the flanks of the volcano.
Frothing at the mouth of the vent during degassing of the erupted lava at the mouth. This can lead to the production of a type of igneous rock called ignimbrite. This occurred during the eruption of Mount Katmai in 1912 which produced the largest flows to be generated during recorded history.
Gravitational collapse of a lava dome or spine, with subsequent avalanching and flow down a steep slope (e.g. Montserrat's Soufriere Hills volcano).
The directional blast (or jet) when part of a volcano explodes or collapses (e.g. the May 18, 1980 eruption of Mount St. Helens) This rapidly transforms into a gravity driven current with distance from the volcano.
Volumes range from a few hundred cubic meters to more than a thousand cubic kilometers, and 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 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.
While moving, the kinetic energy of the boulders will flatten trees and buildings in their path. The hot gases and high speed make them particularly lethal. For example, the towns of Pompeii and Herculaneum in Italy were famously engulfed by pyroclastic flows in 79 AD with heavy loss of life. A pyroclastic flow killed couple Katia and Maurice Krafft, French volcanologists on Mount Unzen, in Japan, on June 3, 1991, and in June 1997 flows killed twenty people on the Caribbean island of Montserrat.
This is the 1991 Mount Unzen eruption which erupted violently, possibly as a result of depressurization of the magma column after a landslide in the crater. This pyroclastic flow reached 4.5 km from the crater and claimed the lives of 43 scientists and journalists, including volcanologists Katia and Maurice Krafft and Harry Glicken.
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