Fast-moving current of hot gas and volcanic matter that moves away from a volcano From Wikipedia, the free encyclopedia
A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud)[1] is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of 100km/h (30m/s; 60mph) but is capable of reaching speeds up to 700km/h (190m/s; 430mph).[2] The gases and tephra can reach temperatures of about 1,000°C (1,800°F).
Pyroclastic flows are the most deadly of all volcanic hazards[3] and are produced as a result of certain explosive eruptions; they normally touch the ground and hurtle 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.
Pyroclastic rocks from the Bishop tuff; uncompressed with pumice (on left), compressed with fiamme (on right)
The word pyroclast is derived from the Greekπῦρ (pýr), meaning "fire", and κλαστός (klastós), meaning "broken in pieces".[4][5] A name for pyroclastic flows that glow red in the dark is nuée ardente (French, "burning cloud"); this was notably used to describe the disastrous 1902 eruption of Mount Pelée on Martinique, a French island in the Caribbean.[6][note 1]
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 or water such as ridges, hills, rivers, and seas. They may also contain steam, water, and rock at less than 250°C (480°F); 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.[7]
A pyroclastic flow is a type of gravity current; in scientific literature, it is sometimes abbreviated to PDC (pyroclastic density current).
This section does not cite any sources. (March 2018)
Several mechanisms can produce a pyroclastic flow:
Fountain collapse of an eruption column from a Plinian eruption (e.g. Mount Vesuvius' destruction of Herculaneum and Pompeii in 79 AD). In such an eruption, the material forcefully ejected from the vent heats the surrounding air and the turbulent mixture rises, through convection, for many kilometers. 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.[8]
Fountain collapse of an eruption column associated with a Vulcanian eruption (e.g., Montserrat's Soufrière Hills volcano has generated many of these deadly pyroclastic flows and surges). The gas and projectiles create a cloud that 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 flows down a steep slope (e.g., Montserrat's Soufrière Hills volcano, which caused nineteen deaths in 1997).
Building remnant in Francisco Leon destroyed by pyroclastic surges and flows during eruption of El Chichon volcano in Mexico in 1982. Reinforcement rods in the concrete were bent in the direction of the flow.A scientist examines pumice blocks at the edge of a pyroclastic flow deposit from Mount St. HelensThe casts of some victims in the so-called "Garden of the Fugitives", Pompeii
Flow volumes range from a few hundred cubic meters to more than 1,000 cubic kilometres (240cumi). Larger flows can travel for hundreds of kilometres, although none on that scale has occurred for several hundred thousand years. Most pyroclastic flows are around oneto ten cubic kilometres (1⁄4–2+1⁄2cumi) 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, admixing and heating cold atmospheric air causing expansion and convection.[9] Flows can deposit less than 1 meter to 200 meters in depth of loose rock fragment.[10]
The kinetic energy of the moving cloud will flatten trees and buildings in its path. The hot gases and high speed make them particularly lethal, as they will incinerate living organisms instantaneously or turn them into carbonized fossils:
The cities of Pompeii and Herculaneum, Italy, for example, were engulfed by pyroclastic surges in 79 AD with many lives lost.[11]
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 three of its 30,000 residents.[citation needed]
A pyroclastic surge killed volcanologistsHarry Glicken and Katia and Maurice Krafft and 40 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 (1⁄4in) of ash.[12]
On June 25, 1997, a pyroclastic flow travelled down Mosquito Ghaut on the Caribbean island of Montserrat. A large, highly energized pyroclastic surge developed. This flow could not be restrained by the Ghaut and spilled out of it, killing 19 people who were in the Streatham village area (which was officially evacuated). Several others in the area suffered severe burns.[citation needed]
Interaction with water
Testimonial evidence from the 1883 eruption of Krakatoa, supported by experimental evidence,[13] shows that pyroclastic flows can cross significant bodies of water. However, that might be a pyroclastic surge, not flow, because the density of a gravity current means it cannot move across the surface of water.[13] One flow reached the Sumatran coast as far as 48 kilometres (26 nautical miles) away.[14]
A 2006 BBC documentary film, Ten Things You Didn't Know About Volcanoes,[15] demonstrated tests by a research team at Kiel University, Germany, of pyroclastic flows moving over the water.[16] When the reconstructed pyroclastic flow (stream of mostly hot ash with varying densities) 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 (now only consisting of the lighter material) along on a bed of steam at an even faster pace than before.
During some phases of the Soufriere Hills volcano on Montserrat, pyroclastic flows were filmed about 1km (1⁄2nmi) offshore. These show the water boiling as the flow passes over it. The flows eventually built a delta, which covered about 1km2 (250 acres). Another example was observed in 2019 at Stromboli when a pyroclastic flow traveled for several hundreds of meters above the sea.[17]
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.[citation needed]
On other celestial bodies
In 1963, NASA astronomer Winifred Cameron proposed that the lunar equivalent of terrestrial pyroclastic flows may have formed sinuous rilles on the Moon. In a lunar volcanic eruption, a pyroclastic cloud would follow local relief, resulting in an often sinuous track. The Moon's Schröter's Valley offers one example.[18][non-primary source needed]
Some volcanoes on Mars, such as Tyrrhenus Mons and Hadriacus Mons, have produced layered deposits that appear to be more easily eroded than lava flows, suggesting that they were emplaced by pyroclastic flows.[19]
Jukes, Joseph Beete (1862). The Student's Manual of Geology (2nded.). Edinburgh, Scotland, U.K.: Adam and Charles Black. p.68. From p. 68: "The word "ash" is not a very good one to include all the mechanical accompaniments of a subaerial or subaqueous eruption, since ash seems to be restricted to a fine powder, the residuum of combustion. A word is wanting to express all such accompaniments, no matter what their size or condition may be, when they are accumulated in such mass as to form beds of "rock." We might call them perhaps "pyroclastic materials," … "
Lacroix, A. (1904) La Montagne Pelée et ses Eruptions, Paris, Masson (in French) From vol. 1, p. 38: After describing on p. 37 the eruption of a "dense, black cloud" (nuée noire), Lacroix coins the term nuée ardente: "Peu après l'éruption de ce que j'appellerai désormais la nuée ardente, un immense nuage de cendres couvrait l'ile tout entière, la saupoudrant d'une mince couche de débris volcaniques." (Shortly after the eruption of what I will call henceforth the dense, glowing cloud [nuée ardente], an immense cloud of cinders covered the entire island, sprinkling it with a thin layer of volcanic debris.)
Weller, Roger (2005). Mount Vesuvius, Italy. Cochise College Department of Geology. Archived from the original on 23 October 2010. Retrieved 15 October 2010.
Camp, Vic. "KRAKATAU, INDONESIA (1883)". How Volcanoes Work. Department of Geological Sciences, San Diego State University, 31 Mar. 2006. Web. 15 Oct. 2010. Archived 2014-12-16 at the Wayback Machine.
Zimbelman, James R.; Garry, William Brent; Bleacher, Jacob Elvin; Crown, David A. (2015). "Volcanism on Mars". In Sigurdsson, Haraldur; Houghton, Bruce; McNutt, Steve; Rymer, Hazel; Stix, John (eds.). The Encyclopedia of Volcanoes (Seconded.). Amsterdam: Zimbelman. pp.717–728. ISBN978-0-12-385938-9.
Sigurdson, Haraldur: Encyclopedia of volcanoes. Academic Press, 546–548. ISBN0-12-643140-X.
Although the coining of the term nuée ardente in 1904 is attributed to the French geologist Antoine Lacroix, according to:
Hooker, Marjorie (1965). "The origin of the volcanological concept nuée ardente". Isis. 56 (4): 401–407. doi:10.1086/350041. S2CID144772310.
the term was used in 1873 by Lacroix's father-in-law and former professor, French geologist Ferdinand André Fouqué in his description of the 1580 and 1808 eruptions of the volcano on the island of São Jorge in the Azores.
Fouqué, Ferdinand (1873). "San Jorge et ses éruptions"[São Jorge and its eruptions]. Revue Scientifique de la France et de l'Étranger. 2nd series (in French). 2 (51): 1198–1201.
From p. 1199: "Un des phénomènes les plus singuliers de cette grande éruption est la production de ce que les témoins contemporains ont appelé des nuées ardentes." (One of the strangest phenomena of this great eruption is the production of what contemporary witnesses called nuées ardentes.)
From p. 1200: "Les détonations cessent dans la journée du 17, mais alors apparaissent des nuées ardents semblables à celles de l'éruption de 1580." (The detonations cease on the day of the 17th, but then [there] appear burning clouds [nuées ardents] similar to those of the eruption of 1580.)
Marjorie Hooker – (Hooker, 1965), p. 405 – records that Father João Inácio da Silveira (1767–1852) from the village of Santo Amaro on São Jorge island wrote an account of the 1808 eruption in which he described an ardente nuven ("burning cloud" in Portuguese) that flowed down the slopes of the volcano. Silveira's account was published in 1871 and republished in 1883.
Silveira, João Inácio da (1883). "XXVIII. Anno de 1808. Erupção na ilha de S. Jorge [XXVIII. Year of 1808. Eruption on the island of São Jorge.]". In Canto, Ernesto do (ed.). Archivo dos Açores [Archive of the Azores] (in Portuguese). Ponta Delgada, São Miguel, Azores: Archivo dos Açores. pp.437–441. From pp. 439–440: "Em desassete do dito mês de Maio … de repente se levantou um tufão de fogo ou vulcão e introduzindo-se nas terras lavradias levantou todos aqueles campos até abaixo às vinhas com todas as árvores e bardos, fazendo-se uma medonha e ardente nuvem e correndo até abaixo de igreja queimou trinta e tantas pessoas na igreja e nos campos…" (On the seventeenth of the said month of May … suddenly there arose a typhoon of fire out of the volcano and [it] entered the farm lands, heaved up all those fields down to the vineyards, with all the trees and hedges, forming a fearsome and burning cloud [ardente nuvem] and running down to the church, burned more than thirty people in the church and in the fields…)
