Pyroclastic flow

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

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 moves away from a volcano about 100 km/h (62 mph) on average but is capable of reaching speeds up to 700 km/h (430 mph). [2] The gases can reach temperatures of about 1,000 °C (1,830 °F).


Pyroclastic flows are a common and devastating 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.

Origin of term

Rocks from the Bishop tuff, uncompressed with pumice (on left); compressed with fiamme (on right). BishopTuff.jpg
Rocks from the Bishop tuff, uncompressed with pumice (on left); compressed with fiamme (on right).

The word pyroclast is derived from the Greek πῦρ , meaning "fire", and κλαστός , meaning "broken in pieces". [3] A name for pyroclastic flows which glow red in the dark is nuée ardente (French, "burning cloud"); this was first used to describe the disastrous 1902 eruption of Mount Pelée on Martinique. [4] [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 (482 °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. [5]

A pyroclastic flow is a type of gravity current; in scientific literature they are sometimes abbreviated to PDC (pyroclastic density current).


There are several mechanisms that can produce a pyroclastic flow:

Size and effects

Building remnant in Francisco Leon destroyed by pyroclastic surges and flows during eruption of El Chichon volcano in Mexico 1982. Reinforcement rods in concrete bent in the direction of the flow. PyroclasticFlow.jpg
Building remnant in Francisco Leon destroyed by pyroclastic surges and flows during eruption of El Chichon volcano in Mexico 1982. Reinforcement rods in concrete bent in the direction of the flow.
A scientist examines pumice blocks at the edge of a pyroclastic flow deposit from Mount St. Helens Pyroclastic Flow St. Helens.jpg
A scientist examines pumice blocks at the edge of a pyroclastic flow deposit from Mount St. Helens
The casts of some victims in the so-called "Garden of the Fugitives", Pompeii. Pompeii Garden of the Fugitives 02.jpg
The casts of some victims in the so-called "Garden of the Fugitives", Pompeii.

Flow volumes range from a few hundred cubic meters (yards) to more than 1,000 cubic kilometres (~240 cubic miles). Larger flows can travel for hundreds of kilometres (miles), although none on that scale has occurred for several hundred thousand years. Most pyroclastic flows are around 1 to 10 km3 (about ¼ to 2½ cubic miles) 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. [6]

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:

Interaction with water

Testimonial evidence from the 1883 eruption of Krakatoa, supported by experimental evidence, [9] 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. [9] One flow reached the Sumatran coast as far as 48 km (30 mi) away. [10]

A 2006 BBC documentary film, Ten Things You Didn't Know About Volcanoes, [11] demonstrated tests by a research team at Kiel University, Germany, of pyroclastic flows moving over water. [12] 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 1 km (0.6 mi) offshore. These show the water boiling as the flow passed over it. The flows eventually built a delta, which covered about 1 km2 (250 acres).

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 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. [13] [ non-primary source needed ]

See also

Related Research Articles

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Stratovolcano Tall, conical volcano built up by many layers of hardened lava and other ejecta

A stratovolcano, also known as a composite volcano, is a conical volcano built up by many layers (strata) of hardened lava, tephra, pumice and ash. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with a summit crater and periodic intervals of explosive eruptions and effusive eruptions, although some have collapsed summit craters called calderas. The lava flowing from stratovolcanoes typically cools and hardens before spreading far, due to high viscosity. The magma forming this lava is often felsic, having high-to-intermediate levels of silica, with lesser amounts of less-viscous mafic magma. Extensive felsic lava flows are uncommon, but have travelled as far as 15 km (9.3 mi).

Mount Pelée active volcano in the Lesser Antilles island

Mount Pelée is a volcano at the northern end of Martinique, an island and French overseas department in the Lesser Antilles Volcanic Arc of the Caribbean. Its volcanic cone is composed of stratified layers of hardened ash and solidified lava. The volcano has been dormant since its most recent eruption in 1932.

Lahar Pyroclastic mudflow

A lahar is a violent type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a river valley.

Plymouth, Montserrat Abandoned town in Montserrat

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Pyroclastic rock Clastic rocks composed solely or primarily of volcanic materials

Pyroclastic rocks or pyroclastics are sedimentary clastic rocks composed solely or primarily of volcanic materials. Where the volcanic material has been transported and reworked through mechanical action, such as by wind or water, these rocks are termed volcaniclastic. Commonly associated with unsieved volcanic activity—such as Plinian or krakatoan eruption styles, or phreatomagmatic eruptions—pyroclastic deposits are commonly formed from airborne ash, lapilli and bombs or blocks ejected from the volcano itself, mixed in with shattered country rock.

A pyroclastic surge, also referred as a dilute pyroclastic density current, is a flowing mixture of gas and rock fragments ejected during some volcanic eruptions. Pyroclastic surge refers to a specific type of pyroclastic current which moves on the ground as a turbulent flow with low particle concentration with support mainly from the gas phase. Pyroclastic surges are thus more mobile and less confined compared to dense pyroclastic flows, which allows them to override ridges and hills rather than always travel downhill.

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Santa María (volcano) mountain

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Soufrière Hills Volcano on Montserrat in the Caribbean

The Soufrière Hills are an active, complex stratovolcano with many lava domes forming its summit on the Caribbean island of Montserrat. Many volcanoes in the Caribbean are named Soufrière. These include La Soufrière or Soufrière Saint Vincent on the island of Saint Vincent, and La Grande Soufrière on Guadeloupe. After a long period of dormancy, the Soufrière Hills volcano became active in 1995 and has continued to erupt ever since. Its eruptions have rendered more than half of Montserrat uninhabitable, destroying the capital city, Plymouth, and causing widespread evacuations: about two thirds of the population have left the island.

Vulcanian eruption type of volcanic eruption

A Vulcanian eruption is a type of volcanic eruption characterized by a dense cloud of ash-laden gas exploding from the crater and rising high above the peak. They usually commence with phreatomagmatic eruptions which can be extremely noisy due the rising magma heating water in the ground. This is usually followed by the explosive clearing of the vent and the eruption column is dirty grey to black as old weathered rocks are blasted out of the vent. As the vent clears, further ash clouds become grey-white and creamy in colour, with convolutions of the ash similar to those of Plinian eruptions.

Explosive eruption Type of volcanic eruption in which lava is violently expelled

In volcanology, an explosive eruption is a volcanic eruption of the most violent type. A notable example is the 1980 eruption of Mount St. Helens. Such eruptions result when sufficient gas has dissolved under pressure within a viscous magma such that expelled lava violently froths into volcanic ash when pressure is suddenly lowered at the vent. Sometimes a lava plug will block the conduit to the summit, and when this occurs, eruptions are more violent. Explosive eruptions can send rocks, dust, gas and pyroclastic material up to 20 km (12 mi) into the atmosphere at a rate of up to 100,000 tonnes per second, traveling at several hundred meters per second. This cloud may then collapse, creating a fast-moving pyroclastic flow of hot volcanic matter.

Peléan eruption type of volcanic eruption

Peléan eruptions are a type of volcanic eruption. They can occur when viscous magma, typically of rhyolitic or andesitic type, is involved, and share some similarities with Vulcanian eruptions. The most important characteristic of a Peléan eruption is the presence of a glowing avalanche of hot volcanic ash, called a pyroclastic flow. Formation of lava domes is another characteristic. Short flows of ash or creation of pumice cones may be observed as well.

A pyroclastic fall is a uniform deposit of material which has been ejected from a volcanic eruption or plume such as an ash fall or tuff. Pyroclastic air fall deposits are a result of:

  1. Ballistic transport of ejecta such as volcanic blocks, volcanic bombs and lapilli from volcanic explosions
  2. Deposition of material from convective clouds associated with pyroclastic flows such as coignimbrite falls
  3. Ejecta carried in gas streaming from a vent. The material under the action of gravity will settle out from an eruption plume or eruption column
  4. Ejecta settling from an eruptive plume or eruption column that is displaced laterally by wind currents and is dispersed over great distances
Mount Hibok-Hibok stratovolcano on Camiguin Island in the Philippines

Mount Hibok-Hibok is a stratovolcano on Camiguin Island in the Philippines. It is one of the active volcanoes in the country and part of the Pacific ring of fire.

Types of volcanic eruptions Basic mechanisms of eruption and variations

Several types of volcanic eruptions—during which lava, tephra, and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are often named after famous volcanoes where that type of behavior has been observed. Some volcanoes may exhibit only one characteristic type of eruption during a period of activity, while others may display an entire sequence of types all in one eruptive series.

Bocas de Fogo Volcano in Portugal

Bocas de Fogo is a volcano near the community of Urzelina, Velas municipality, São Jorge Island, Azores. It erupted in May and June 1808, causing destruction and over 30 deaths in Urzelina and producing a basalt field of volcanic rock extending to the Ponta da Urzelina. The eruption was the last sub-aerial event observed in the Azores; most recent eruptions have occurred along submarine vents, with the Capelinhos eruption (1957–58) starting as a submarine eruption and the 1998–2001 Serreta eruption being exclusively submarine.

Volcanic ash and aviation safety

Plumes of volcanic ash near active volcanoes are a flight safety hazard, especially for night flights. Volcanic ash is hard and abrasive, and can quickly cause significant wear to propellers and turbocompressor blades, and scratch cockpit windows, impairing visibility. The ash contaminates fuel and water systems, can jam gears, and make engines flameout. Its particles have low melting point, so they melt in the engines' combustion chamber then the ceramic mass sticks to turbine blades, fuel nozzles, and combustors—which can lead to total engine failure. Ash can also contaminate the cabin and damage avionics.

A block and ash flow or block-and-ash flow is a flowing mixture of volcanic ash and large angular blocks commonly formed as a result of a gravitational collapse of a lava dome or lava flow. Block and ash flows are a type of pyroclastic flow and as such they form during volcanic eruptions. In contrast to other types of pyroclastic flows, block and ash flows do not contain pumice and the volume of block and ash flow deposits is usually small. Block and ash flow deposits have densities in the range of 1600 to 2000 kg/m3, two to five times greater than ash fall deposits. Some blocks in block and ash flow deposits may have thin and shiny coatings of carbon derived from charcoal formed from vegetation trapped by the flow.

1902 eruption of Mount Pelée

The 1902 eruption of Mount Pelée was a moderately large volcanic eruption on the island of Martinique in the Lesser Antilles Volcanic Arc of the eastern Caribbean. Eruptive activity began on 23 April as a series of phreatic explosions from the summit of Mount Pelée. Within days, the vigor of the explosions exceeded anything witnessed since the island was settled by Europeans. The intensity then subsided for a few days until early May when the explosions had increased again. Lightning laced the eruption clouds and trade winds dumped ash on villages to the west. Heavy ash fall at times caused total darkness. Some of the afflicted residents panicked and headed for the perceived safety of larger settlements, especially Saint-Pierre, about 10 km (6.2 mi) south of Pelée's summit. Saint-Pierre received its first ash fall on 3 May.


  1. Branney M.J. & Kokelaar, B.P. 2002, Pyroclastic Density Currents and the Sedimentation of Ignimbrites. Geological Society of London Memoir 27, 143pp.
  2. Pyroclastic flows USGS
  3. See:
    • Jukes, Joseph Beete (1862). The Student's Manual of Geology (2nd ed.). 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," … "
    • Wiktionary: pyroclastic (quotations)
  4. 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.)
  5. Arthur N. Strahler (1972), Planet Earth: its physical systems through geological time
  6. Myers and Brantley (1995). Volcano Hazards Fact Sheet: Hazardous Phenomena at Volcanoes, USGS Open File Report 95-231
  7. Weller, Roger (2005). Mount Vesuvius, Italy. Cochise College Department of Geology. Archived from the original on 23 October 2010. Retrieved 15 October 2010.
  8. Sutherland, Lin. Reader’s Digest Pathfinders Earthquakes and Volcanoes. New York: Weldon Owen Publishing, 2000.
  9. 1 2 Freundt, Armin (2003). "Entrance of hot pyroclastic flows into the sea: experimental observations". Bulletin of Volcanology . 65: 144–164. Bibcode:2002BVol...65..144F. doi:10.1007/s00445-002-0250-1.
  10. Camp, Vic. "KRAKATAU, INDONESIA (1883)." How Volcanoes Work. Department of Geological Sciences, San Diego State University, 31 Mar. 2006. Web. 15 Oct. 2010. .
  11. Ten Things You Didn't Know About Volcanoes (2006) on IMDb
  12. Entrance of hot pyroclastic flows into the sea: experimental observations, INIST.
  13. Cameron, W. S. (1964). "An Interpretation of Schröter's Valley and Other Lunar Sinuous Rills". J. Geophys. Res. 69 (12): 2423–2430. Bibcode:1964JGR....69.2423C. doi:10.1029/JZ069i012p02423.


  1. 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.
    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 … )