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Pyroclastic flow

Pyroclastic flows are a common and devastating result of some volcanic eruptions. They are fast moving fluidized bodies of hot gas, ash and rock (collectively known as tephra) which can travel away from the vent at up to 150 km/h. The gas is usually at a temperature of 100-800 degrees Celsius. The flows normally hug the ground and travel downhill under gravity, their speed depending upon the gradient of the slope and the size of the flow.

Volumes range from a few hundred cubic metres to more than a thousand cubic kilometres, and the larger ones can travel for hundreds of kilometre although none have occurred for several hundred thousand years. Most 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, whilst an ash cloud rises above it because of the turbulence between the flow and the overlying air.

Whilst 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 them in 79 with heavy loss of life; and in June 1997 flows killed 20 people on the Caribbean island of Montserrat.

Another name for a pyroclastic flow is a nuée ardente (French for "fiery cloud"), which was first used to describe the 1902 Martinique eruption. A pyroclastic flow has a red glow in the dark.

Flows containing a high proportion of gas to rock are known as 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 100 degrees Celsius. Cold surges can occur when the eruption is from a vent under a lake or the sea.

Hot pyroclastic surges may form ahead of flows, for example during the eruption of Mount Pelee in 1902 a surge overwhelmed the city of Saint-Pierre 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.

There are several scenarios which can produce a pyroclastic flow:

See also Pyroclastic rock