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 kilometres although none on that scale 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, while an ash cloud rises above it because of the turbulence between the flow and the overlying air.
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 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 Pelée 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:
- Collapse of the ash column from a plinian eruption (e.g Mount Vesuvius, 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 will fall back down the flanks of the volcano. This is known as base surge. It was first documented from observations of underwater nuclear explosions, in which a cloud rolls outward from the bottom of the column as it rises through the air, and subsequently applied to volcanology.
- 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.
- Collapse of a volcanic dome and its subsequent flow down a steep slope (e.g. Montserrat).