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A pyroclastic flow (also known scientifically as a pyroclastic density current) is a common and devastating result of certain explosive volcanic eruptions. The flows are fast-moving currents of hot gas and rock (collectively known as tephra), which travel away from the volcano at speeds generally as great as 700 km/hr (450 mi/h). The gas can reach temperatures of about . The flows normally hug the ground and travel downhill, or spread laterally under gravity. Their speed depends upon the density of the current, the volcanic output rate, and the gradient of the slope.

The word pyroclast is derived from the Greek πῦρ, meaning fire, and κλαστός, meaning broken. A name for some pyroclastic flows is nuée ardente (French for "glowing cloud"); this was first used to describe the disastrous 1902 eruption of Mount Peléemarker on Martiniquemarker. These pyroclastic flows glowed red in the dark.

Pyroclastic flows that contain a much higher proportion of gas to rock are known as "fully dilute pyroclastic density currents" or pyroclastic surges. The lower density sometimes allows them to flow over higher topographic features such as ridges and hills. They may also contain steam, water and rock at less than these are called "cold" compared with other flows, although the temperature is still lethally high. Cold pyroclastic surges can occur when the eruption is from a vent under a shallow lake or the sea. Fronts of some pyroclastic density currents are fully dilute, for example during the eruption of Montagne Peléemarker in 1902 a fully dilute current overwhelmed the city of Saint-Pierremarker and killed nearly 30,000 people.

A pyroclastic flow is a type of gravity current; in scientific literature they are sometimes abbreviated to PDC (pyroclastic density current).

It is common for pyroclastic flows to also generate pyroclastic surges.


There are several scenarios which can produce a pyroclastic flow:

  • Fountain collapse of an eruption column from a Plinian eruption (e.g., Mount Vesuviusmarker's destruction of Pompeiimarker, see Pliny the Younger). In such an eruption, the material ejected from the vent heats the surrounding air and the turbulent mixture rises, through convection, for many kilometres. If the erupted jet is unable to heat the surrounding air sufficiently, convection currents will not be strong enough to carry the plume upwards and it falls, flowing 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 Novaruptamarker in 1912 which produced the largest flows to be generated during recorded history.
  • Gravitational collapse of a lava dome or spine, with subsequent avalanches and flow down a steep slope e.g., Montserrat's Soufrière Hillsmarker volcano.
  • Fountain collapse of an eruption column associated with a vulcanian eruption e.g., Montserrat's Soufrière Hillsmarker volcano has generated many pyroclastic flows and surges.
  • The directional blast (or jet) when part of a volcano explodes or collapses (e.g. the May 18, 1980 eruption of Mount St. Helensmarker) As distance from the volcano increases, this rapidly transforms into a gravity-driven current.

Size and effects

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 Pompeiimarker and Herculaneummarker, Italymarker, 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 Unzenmarker, in Japanmarker, 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 Caribbeanmarker island of Montserratmarker. 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.

Crossing water

Testimonial evidence from the 1883 eruption of Krakatoamarker, supported by experimental evidence,that pyroclastic flows can cross significant bodies of water. One flow reached the Sumatranmarker coast as much as 48 km (30 mi) away.

A recent documentary film Ten Things You Didn't Know About Volcanoes demonstrated tests by a research team at Kiel Universitymarker, Germanymarker of pyroclastic flows moving over water. When the reconstructed pyroclastic flow (stream of mostly hot ash with varying densities) hit the water two things happened. The temperature of the ash caused the water to evaporate, propelling the pyroclastic flow now only consisting of the lighter material along at an even faster pace than before on a bed of superheated steam. The heavier material fell to the water, precipitating out from the pyroclastic flow and into the liquid. This large displacement of heavy ash into the water hence caused a similarly sized displacement of water. In reality, this would (and did with Krakatoamarker) cause a tsunami due to the displacement via precipitate mass.

In 1963, NASA astronomer Winifred Cameron proposed that sinuous rilles on the Moon may have been formed by the lunar equivalent of a terrestrial pyroclastic flows. In a lunar volcanic eruption, a pyroclastic cloud would follow local relief resulting in an often sinuous track. The Moon's Scroter's Valley is one example.


  • Sigurdson, Haraldur: Encyclopedia of volcanoes. Academic Press, 546-548. ISBN 0-12-643140-X.

See also

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