Pyroclastic flow

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Pyroclastic flows sweep down the flanks of Mayon Volcano, Philippines, in 1984

A pyroclastic flow (also known as a pyroclastic density current)^[1] is a common and devastating result of some 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 greater than 80 km/h (50mph).^[2] The gas can reach temperatures of about 1,000 degrees Celsius (1,800 F). 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 πυρος, 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 Pelée on Martinique.^[3] 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 250 degrees Celsius; 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 Pelée in 1902 a fully dilute current overwhelmed the city of Saint-Pierre and killed nearly 30,000 people.^[4]

A pyroclastic flow is a type of gravity current; in scientific literature they are sometimes abbreviated to PDC (pyroclastic density current). Snow avalanches are the cold equivalent and can be just as deadly.

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

1 Pyroclastic Surges
2 Causes
3 Size and effects
- 3.1 Crossing water
4 References
5 See also
6 External links

[edit] Pyroclastic Surges

These are often associated with pyroclastic flows. They are highly energised and very fluid, travel at high velocities and are known to be able to climb obstructions - valley spurs, cliffs; flow up valleys. They deposit thin - mm thick, layers, often exhibiting similar depositional features of water borne sediments. Surges can occur during a pyroclastic flow and may even develop as a result of processes within a pyroclastic flow.

[edit] Causes

There are several scenarios which can produce a pyroclastic flow:

Fountain collapse 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, through convection, for many kilometres. 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 falls, then to flow 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 avalanches and flow down a steep slope e.g., Montserrat's Soufrière Hills volcano.

Fountain collapse of an eruption column associated with a vulcanian eruption e.g., Montserrat's Soufrière Hills 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. Helens) As distance from the volcano increases, this rapidly transforms into a gravity-driven current.

[edit] Size and effects

Scientist examines pumice blocks at the edge of a pyroclastic flow from Mount St. Helens

Volumes range from a few hundred cubic meters 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 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. For example, the towns of Pompeii and Herculaneum, Italy were famously engulfed by pyroclastic surges in 79 AD with heavy loss of life. 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 5mm of ash. On 19th June 1997 a pyroclastic flow travelled down the Tar River valley on the Caribbean island of Montserrat. A large energised pyroclastic surge developed. This surge climbed a spur and killed 25 people who were in a prohibited zone. Several others in the area suffered severe burns.

[edit] Crossing water

There is testimonial evidence from the 1883 eruption of Krakatoa, supported by experimental evidence, that pyroclastic flows can cross significant bodies of water; one flow reached the Sumatran coast as much as 25 miles (40 km) away, having apparently moved across the water on a "cushion" of superheated steam.

A recent documentary film showed tests by a research team at Kiel University, Germany of pyroclastic flows moving over water[1]. The tests revealed that hot ash traveled over the water on a cloud of superheated steam, continuing to be a pyroclastic flow after crossing water; the heavy matter precipitated out of the flow shortly after initial contact with the water, creating a tsunami due to the precipitate mass.

[edit] References

^ Branney M.J. & Kokelaar, B.P. 2002, Pyroclastic Density Currents and the Sedimentation of Ignimbrites. Geological Society London Memoir 27, 143pp.
^ Volcanic Hazards: Pyroclastic flows and surges
^ Lacroix, A. (1904) La Montagne Pelée et ses Eruptions, Paris, Masson (in French)
^ Arthur N. Strahler (1972), Planet Earth: its physical systems through geological time