Pyroclastic flow

A pyroclastic flow (also known as a pyroclastic density current) is a common and devastating result of some volcanic eruptions. The flows are fast-moving currents of hot gas, and rock (collectively known as tefra), which can travel away from the volcano at up to 700 km/h. The gas can reach temperatures of up to 1,000 degrees Celsius. 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.

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Naming issues

The word pyroclast is derived from the Greek πυρος, meaning fire, and κλαστός, meaning broken. An old name for small pyroclastic flows was nuée ardente (French for "burning cloud"); this was first used to describe the disastrous 1902 eruption of Mount Pelée on Martinique.^[1] 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 be "cold," containing steam, water and rock at less than 250 degrees Celsius. 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 Mount Pelée in 1902 a fully dilute current overwhelmed the city of Saint-Pierre and killed nearly 30,000 people.^[2]

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

Causes

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.

Size and effects

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.

References

See also

• Pyroclastic surge
• Pyroclastic rock
• Lahar