A pyroclastic flow is a fast-moving current of hot gas and rock, which reaches speeds
moving away from a volcano of up to 700 km/h. The gas can reach temperatures of about 1,000 °C.
Pyroclastic 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. They are a common and devastating result of certain explosive
volcanic eruptions.
Origin of term The word pyroclast is derived from the Greek
πῦρ, meaning "fire", and κλαστός, meaning "broken in pieces". A name for some
pyroclastic flows is nuée ardente; this was first used to describe the disastrous 1902
eruption of Mount Pelée on Martinique. In the dark, these pyroclastic flows glowed red.
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 250 °C; 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 Mount Pelée in 1902
a fully dilute current 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. Causes
There are several factors which can produce a pyroclastic flow:
Fountain collapse of an eruption column from a Plinian eruption. 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.
Fountain collapse of an eruption column associated with a vulcanian eruption The gas and projectiles
create a cloud which is denser than the surrounding air and becomes a pyroclastic flow.
Frothing at the mouth of the vent during degassing of the erupted lava. This can lead to the
production of a rock called ignimbrite. This occurred during the eruption of Novarupta
in 1912. Gravitational collapse of a lava dome or spine,
with subsequent avalanches and flow down a steep slope.
The directional blast when part of a volcano explodes or collapses. As distance from the
volcano increases, this rapidly transforms into a gravity-driven current.
Size and effects
The 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, as they will incinerate
living organisms instantaneously: The towns of Pompeii and Herculaneum, Italy,
for example, were 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 Unzen, in Japan, 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.
The 1902 eruption of Mount Pelée destroyed the Martinique town of St. Pierre. Despite
signs of impending eruption, the government deemed St. Pierre safe due to hills and valleys
between it and the volcano, but the pyroclastic flow charred almost the entirety of the city,
killing all but two of its 30,000 residents. On 25 June 1997 a pyroclastic flow travelled
down Mosquito Ghaut on the Caribbean island of Montserrat. 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. Several others in the area suffered
severe burns. Interaction with water
Testimonial evidence from the 1883 eruption of Krakatoa, supported by experimental evidence,
shows that pyroclastic flows can cross significant bodies of water. One flow reached the Sumatran
coast as much as 48 km away. A 2006 documentary film, Ten Things You Didn't
Know About Volcanoes, demonstrated tests by a research team at Kiel University, Germany,
of pyroclastic flows moving over water. When the reconstructed pyroclastic flow hit the
water two things happened: The heavier material fell into the water, precipitating out from
the pyroclastic flow and into the liquid; The temperature of the ash caused the water
to evaporate, propelling the pyroclastic flow along at an even faster pace than before on
a bed of steam. During some phases of the Soufriere Hills
volcano on Montserrat, pyroclastic flows were filmed about 1 km offshore. These show the
water boiling as the flow passed over it. The flows eventually built a delta which covered
about 1 km2. A pyroclastic flow can interact with a body
of water to form a large amount of mud, which can then continue to flow downhill as a lahar.
This is one of several mechanisms that can create a lahar.
On the Moon In 1963, NASA astronomer Winifred Cameron
proposed that sinuous rilles on the Moon may have been formed by the lunar equivalent of
terrestrial pyroclastic flows. In a lunar volcanic eruption, a pyroclastic cloud would
follow local relief resulting in an often sinuous track. The Moon's Schroter's Valley
is one example. See also
Pyroclastic fall Pyroclastic rock
Pyroclastic surge Welded tuff
References
Sigurdson, Haraldur: Encyclopedia of volcanoes. Academic Press, 546-548. ISBN 0-12-643140-X.
External links Pyroclastic Flows video