Pyroclastic flow

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.

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

Origin of term

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:

  • Fountain collapse of an eruption column from a Plinian eruption (e.g. Mount Vesuvius' destruction of Herculaneum and Pompeii). 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.
  • 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).
  • The directional blast (or jet) when part of a volcano collapses or explodes (e.g., the eruption of Mount St. Helens in May 18, 1980). As distance from the volcano increases, this rapidly transforms into a gravity-driven current.

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.
Pyroclastic Flow St. Helens
A scientist examines pumice blocks at the edge of a pyroclastic flow deposit from Mount St. Helens
Pompeii Garden of the Fugitives 02
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:

  • The cities of Pompeii and Herculaneum, Italy, for example, were engulfed by pyroclastic surges on August 24, 79 AD with many lives lost.[7]
  • 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.
  • A pyroclastic surge killed volcanologists Harry 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 5 mm (0.2 in) of ash.[8]
  • On 25 June, 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.

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 much 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 sinews 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]

See also


  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. ^ a b 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. [1].
  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.
  • Sigurdson, Haraldur: Encyclopedia of volcanoes. Academic Press, 546–548. ISBN 0-12-643140-X.


  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 … )

External links

Bishop Tuff

The Bishop Tuff is a welded tuff that formed 767,100 ± 900 years ago as a rhyolitic pyroclastic flow during the eruption that created the Long Valley Caldera. Large outcrops of the tuff are located in Inyo and Mono Counties, California, United States.

Block and ash flow

A block and ash flow or block-and-ash flow is a flowing mixture of volcanic ash and large (>26 cm) angular blocks commonly formed as 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 difference to other types of pyroclastic flows block and ash fows do no contain pumice and the volume of block and ash flow deposits is usually small. Block and ash flows deposits have densities in the range of 1600 to 2000 kg/m3, two to five time 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.Volcanoes known for their production of block and ash flows since the 1990s include Mount Unzen in Japan, Mount Merapi in Java and Soufrière Hills in the Lesser Antilles.

Bridge River Vent

The Bridge River Vent is a volcanic crater in the Pacific Ranges of the Coast Mountains in southwestern British Columbia, Canada. It is located 51 km (32 mi) west of Bralorne on the northeastern flank of the Mount Meager massif. With an elevation of 1,524 m (5,000 ft), it lies on the steep northern face of Plinth Peak, a 2,677 m (8,783 ft) high volcanic peak comprising the northern portion of Meager. The vent rises above the western shoulder of the Pemberton Valley and represents the northernmost volcanic feature of the Mount Meager massif.

At least eight volcanic vents compose the Meager massif, with the Bridge River Vent being the most recent to form. It is the only vent of the massif to exhibit volcanic activity in the past 10,000 years and one of the several vents in the Garibaldi Volcanic Belt to erupt since the end of the last glacial period. The crater constitutes a bowl-shaped depression overlain by glacial ice and volcanic debris that were deposited during volcanic activity. Its breached northern rim has been a pathway for lava and ash flows that have traveled throughout the nearby Pemberton Valley.

Descabezado Grande

Descabezado Grande (also Cerro Azul or Quizapu) is a stratovolcano located in the Maule Region of central Chile. It is capped by a 1.4-kilometre-wide (0.9 mi) ice-filled caldera and named for its flat-topped form, as descabezado means "headless" in Spanish. A smaller crater about 500 metres (1,600 ft) wide is found in the northeast part of the caldera, and it has active fumaroles.

The volcano is composed of andesite and rhyodacite lava flows along with pyroclastic flow deposits. It has a basal diameter of about 10 by 12 kilometres (6 mi × 8 mi) and a total volume of about 30 cubic kilometres (7.2 cu mi). Along with Cerro Azul, 7 kilometres (4.3 mi) to the south, it lies at the center of a 20-by-30-kilometre (12 mi × 20 mi) volcanic field.

Explosive eruption

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.

Gangsta Rap Made Me Do It

"Gangsta Rap Made Me Do It" is the first single from Ice Cube's studio album, Raw Footage. It was released with a music video on his MySpace page on January 3, 2008. The song contains a "chopped and screwed" line from Cube's previous single, Child Support. (" niggas know my pyroclastic flow...") Several members of Westside Connection make cameo appearances in the video. In the song Ice Cube comments on the exploitation of gangsta rap as a scapegoat for society's problems.

A remix to the song was made featuring Nas and Scarface. It is credited to being featured on the video game Midnight Club: Los Angeles.

Geology of Jan Mayen

The geology of Jan Mayen is part of the larger Jan Mayen Ridge, an undersea volcanic ridge that forms the boundary of the Iceland Plateau to the northeast. North of the island, the sea floor slopes steeply, plunging a depth of greater than two kilometers in the vicinity of Jan Mayen Rift Zone. The region is highly tectonically active, at the junction of the European and American plates. This activity produces volcanism and earthquakes on the island itself. Beerenberg, a 2.27 kilometer tall volcano rises on the north end of the island, covered in more than 20 glaciers.

Jan Mayen is made up of basalt flows and pyroclastic flow related mafic deposits. The two most recent eruptions were in 1970 and 1985, although the Central Crater and Egg Island craters on the southern side of the mountain have continuous fumarole activity. Tectonic geologists have identified Jan Mayen as a microcontinent, which has experienced significant deformation around its boundaries due to sea-floor spreading and the formation of new plate boundaries since the Paleocene, with full separation as a microcontinent in the Oligocene. A model published in 2009 suggests a failed ridge offshore oriented toward the Faroe Islands and the Aegir Ridge extending northeast of Jan Mayen.

Harry R. Truman

Harry Randall Truman (October 30, 1896 – May 18, 1980) was a resident of the U.S. state of Washington who lived near Mount St. Helens. He was the owner and caretaker of Mount St. Helens Lodge at Spirit Lake near the foot of the mountain, and he came to fame as a folk hero in the months preceding the volcano's 1980 eruption after he refused to leave his home despite evacuation orders. Truman is presumed to have been killed by a pyroclastic flow that overtook his lodge and buried the site under 150 ft (46 m) of volcanic debris.

After Truman's death, his family and friends reflected on his love for the mountain. In 1981, Art Carney portrayed Truman in the docudrama film St. Helens. He was commemorated in a book by his niece and a number of musical pieces, including songs by Headgear and Billy Jonas.


Karangetang (also known as Api Siau) is a volcano on the north side of Siau Island off the coast of Sulawesi, Indonesia. The island is inhabited by 22,000 people. It is one of the most active volcanoes in Indonesia having erupted 41 times since 1675. A pyroclastic flow in 1997 killed three people.

Katia and Maurice Krafft

Catherine Joséphine "Katia" Krafft (née Conrad; 17 April 1942 – 3 June 1991) and her husband, Maurice Paul Krafft (25 March 1946 – 3 June 1991), were French volcanologists who died in a pyroclastic flow on Mount Unzen, in Japan, on June 3, 1991. The Kraffts were known for being pioneers in filming, photographing and recording volcanoes, often getting within feet of lava flows. Their obituary appeared in the Bulletin of Volcanology. Werner Herzog's documentary Into the Inferno mentions them.

Mount Rausu

Mount Rausu (羅臼岳, Rausu-dake) is a stratovolcano on the Shiretoko Peninsula in Hokkaidō, Japan. It sits on the border between the towns of Shari and Rausu. Mount Rausu is the northeastern most Holocene volcano on Hokkaidō. It is one of the 100 famous mountains in Japan.

Mount Rausu's opening festival is held annually on July 3. This day officially opens the climbing season.In the past 2200 years it is believed that Mount Rausu erupted thrice, with a Plinian Eruption roughly 1400 years ago and a pyroclastic flow about 500 years ago.

Mount Sinabung

Mount Sinabung (Indonesian: Gunung Sinabung, also Dolok Sinabung, Deleng Sinabung, Dolok Sinaboen, Dolok Sinaboeng and Sinabuna) is a Pleistocene-to-Holocene stratovolcano of andesite and dacite in the Karo plateau of Karo Regency, North Sumatra, Indonesia, 40 kilometres (25 mi) from the Lake Toba supervolcano. Many old lava flows are on its flanks and the last known eruption, before recent times, occurred 1200 years before present, between 740 - 880 CE. Solfataric activities (cracks where steam, gas, and lava are emitted) were last observed at the summit in 1912; recent documented events include an eruption in the early hours of 29 August 2010 and eruptions in September and November 2013, January, February and October 2014. A pyroclastic flow in May 2016 killed seven people. Between 2013 and 2014, the alert for a major event was increased with no significant activity. On 2 June 2015, the alert was again increased, and on 26 June 2015, at least 10,000 people were evacuated, fearing a major eruption. The long eruption of Mount Sinabung is similar to that of Mount Unzen in Japan, which erupted for five years after lying dormant for 200 years.

Mount Unzen

Mount Unzen (雲仙岳, Unzen-dake) is an active volcanic group of several overlapping stratovolcanoes, near the city of Shimabara, Nagasaki on the island of Kyushu, Japan's southernmost main island.

In 1792, the collapse of one of its several lava domes triggered a megatsunami that killed 14,524 people in Japan's worst volcanic-related disaster. The volcano was most recently active from 1990 to 1995, and a large eruption in 1991 generated a pyroclastic flow that killed 43 people, including three volcanologists.

Its highest peaks are Fugen-dake (普賢岳) at 1,359 metres (4,459 ft) and Heisei-shinzan (平成新山) at 1,486 metres (4,875 ft). The latter emerged during the eruptions of the early, eponymous Heisei era (1989–2019).

Osorno (volcano)

Osorno Volcano is a 2,652-metre (8,701 ft) tall conical stratovolcano lying between Osorno Province and Llanquihue Province, in Los Lagos Region of Chile. It stands on the southeastern shore of Llanquihue Lake, and also towers over Todos los Santos Lake. Osorno is known worldwide as a symbol of the local landscape, and is noted for its similar appearance to Mount Fuji.

Osorno is one of the most active volcanoes of the southern Chilean Andes, with 11 historical eruptions recorded between 1575 and 1869. The basalt and andesite lava flows generated during these eruptions reached both Llanquihue and Todos los Santos Lakes. The upper slopes of the volcano are almost entirely covered in glaciers despite its very modest altitude and latitude, sustained by the substantial snowfall in the very moist maritime climate of the region. This mountain also produces pyroclastic flow, since it is a composite volcano.

Osorno sits on top of a 250,000-year-old eroded stratovolcano, La Picada, with a 6-km-wide caldera.

Peléan 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.

The initial phases of eruption are characterized by pyroclastic flows. The tephra deposits have lower volume and range than the corresponding Plinian and Vulcanian eruptions. The viscous magma then forms a steep-sided dome or volcanic spine in the volcano's vent. The dome may later collapse, resulting in flows of ash and hot blocks. The eruption cycle is usually completed in a few years, but in some cases may continue for decades, like in the case of Santiaguito.The 1902 eruption of Mount Pelée is the first described case of a Peléan eruption; the term is derived from the name of the volcano.

Other examples of Peléan eruptions include:

the 1948–1951 eruption of Hibok-Hibok;

the 1951 eruption of Mount Lamington, which remains the most detailed observation of this kind;

the 1968 eruption of Mayon Volcano

Pyroclastic rock

Pyroclastic rocks or pyroclastics (derived from the Greek: πῦρ, meaning fire; and κλαστός, meaning broken) 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.

Pyroclastic rocks may be a range of clast sizes, from the largest agglomerates, to very fine ashes and tuffs. Pyroclasts of different sizes are classified as volcanic bombs, lapilli, and volcanic ash. Ash is considered to be pyroclastic because it is a fine dust made up of volcanic rock. One of the most spectacular forms of pyroclastic deposit are the ignimbrites, deposits formed by the high-temperature gas-and-ash mix of a pyroclastic flow event.


Sillar is a variety of rhyolite, which is a type of volcanic rock. Although sillar is of rhyolitic composition, it has been erupted from volcanoes which mostly erupt andesite lava, and sillar contains small fragments of andesite. A pink variety of sillar owes its colour to crystals of hematite within the rock. A white variety lacks these hematite crystals. Sillar is found as pyroclastic flow deposits of tuff near volcanoes in southern Peru, for example the now-extinct Chachani volcano which erupted flows of sillar during the Pleistocene epoch.

Sillar facies Orvieto-Bagnoregio Ignimbrite (black blocks of scoria in red tuff) occurs at Civita di Bagnoregio in the Vulsini volcanic district of central Italy.


Soche is a 3,955-metre-high (12,976 ft) dacitic volcano in Ecuador and is located on the northern end of a secondary volcanic chain. Constructed on a Paleozoic substratum, it contains an eastwards-opening caldera in the summit region. A large eruption in 6650 BCE generated ashfall into Colombia and two lava domes in the caldera. The ash- and lapilli-fall is about a metre thick in the Interandean valley and the neighbouring cordilleras and most likely represented a long-lasting obstacle for human population. Earlier eruptive events involving a lava flow that was subsequently offset by a fault zone named the Cayambe-Chingual fault by 110m occurred 9.67 ka BP, and another involving a pyroclastic flow was dated at 37.22 ± 0.63 ka BP.


Tutupaca is a volcano in the region of Tacna in Peru. It is part of the Peruvian segment of the Central Volcanic Zone, one of several volcanic belts in the Andes. Tutupaca consists of three overlapping volcanoes formed by lava flows and lava domes made out of andesite and dacite, which grew on top of older volcanic rocks. The highest of these is usually reported to be 5,815 metres (19,078 ft) tall and was glaciated in the past.

Several volcanoes in Peru have been active in recent times, including Tutupaca. Their volcanism is caused by the subduction of the Nazca Plate beneath the South America Plate. One of these volcanoes collapsed in historical time, probably in 1802, generating a large debris avalanche with a volume likely exceeding 0.6–0.8 cubic kilometres (0.14–0.19 cu mi) and a pyroclastic flow. The associated eruption was among the largest in Peru for which there are historical records. The volcano became active about 700,000 years ago, and activity continued into the Holocene, but whether there were historical eruptions was initially unclear; some eruptions were instead attributed to the less eroded Yucamane volcano. The Peruvian government plans to monitor the volcano for future activity. Tutupaca features geothermal manifestations with fumaroles and hot springs.

Volcanic rocks
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Сomposition and structure
Historical geology


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