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 Mayon Volcano is an active Stratovolcano in the Philippines on the Island of Luzon, in the province of Albay The Philippines ( Filipino: Pilipinas, officially known as the Republic of the Philippines (fil ''Republika ng Pilipinas'' RP Plate tectonics and hotspots Divergent plate boundaries At the 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). Tephra is air-fall material produced by a volcanic eruption regardless of composition or fragment size ^[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 πῦρ, meaning fire, and κλαστός, meaning broken. Greek (el ελληνική γλώσσα or simply el ελληνικά — "Hellenic" is an Indo-European language, spoken today by 15-22 million people mainly 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. French ( français,) is a Romance language spoken around the world by 118 million people as a native language and by about 180 to 260 million people Mount Pelée ( French: Montagne Pelée, or "Bald Mountain" is a dormant Volcano on the northern tip of the French Overseas department Martinique is an Island in the eastern Caribbean Sea, having a land area of 1128 km² ^[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. A pyroclastic surge is a fluidized mass of turbulent gas and rock fragments which is ejected during some Volcanic eruptions It is similar to a Pyroclastic flow but 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. Mount Pelée ( French: Montagne Pelée, or "Bald Mountain" is a dormant Volcano on the northern tip of the French Overseas department Saint-Pierre was the former primary city of France 's Caribbean Département d'outre-mer of Martinique, founded in 1635 by ^[4]

A pyroclastic flow is a type of gravity current; in scientific literature they are sometimes abbreviated to PDC (pyroclastic density current). In Fluid dynamics, a gravity current is a primarily horizontal flow in a gravitational field that is driven by a Density difference Snow avalanches are the cold equivalent and can be just as deadly.

It is common for pyroclastic flows to also generate pyroclastic surges. A pyroclastic surge is a fluidized mass of turbulent gas and rock fragments which is ejected during some Volcanic eruptions It is similar to a Pyroclastic flow but

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.

Causes

There are several scenarios which can produce a pyroclastic flow:

• Fountain collapse of an eruption column from a plinian eruption (e. An eruption column consists of hot Volcanic ash emitted during an explosive Volcanic eruption. Plinian eruptions are Volcanic eruptions marked by their similarity to the eruption of Mt g. , Mount Vesuvius's destruction of Pompeii, see Pliny the Younger). Mount Vesuvius (in Italian Monte Vesuvio and in Latin Mons Vesuvius) is an active Stratovolcano east of Naples Pompeii is a ruined and partially buried Roman town-city near modern Naples and Caserta in the Italian region of Campania, in Gaius Plinius Caecilius Secundus, born Gaius Caecilius or Gaius Caecilius Cilo (61/63 - ca 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. Igneous rocks (etymology from Latin ignis, fire are rocks formed by solidification of cooled Magma (molten rock Ignimbrite is a Volcanic Pyroclastic rock often of Dacitic or Rhyolitic composition This occurred during the eruption of Mount Katmai in 1912 which produced the largest flows to be generated during recorded history. Mount Katmai is a large Stratovolcano (composite volcano on the Alaska Peninsula in southern Alaska, located within Katmai National Park and

• Gravitational collapse of a lava dome or spine, with subsequent avalanches and flow down a steep slope e. In Volcanology, a lava dome or plug dome is a roughly circular Mound -shaped protrusion resulting from the slow eruption of Felsic Lava g. , Montserrat's Soufrière Hills volcano. The Soufrière Hills volcano ( French "Sulphur" Hills is an active complex Stratovolcano with many lava domes forming its summit on

• Fountain collapse of an eruption column associated with a vulcanian eruption e. An eruption column consists of hot Volcanic ash emitted during an explosive Volcanic eruption. Vulcanian eruptions usually commence with Phreatomagmatic eruptions which can be extremely noisy due the rising magma heating water in the ground g. , Montserrat's Soufrière Hills volcano has generated many pyroclastic flows and surges. The Soufrière Hills volcano ( French "Sulphur" Hills is an active complex Stratovolcano with many lava domes forming its summit on

• 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. Events 1152 - Henry II of England marries Eleanor of Aquitaine. The 1980 eruption of Mount St Helens, a volcano located in Washington state in the United States was a major volcanic eruption.

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. Mount St Helens is an active Stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United States 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 kinetic energy of an object is the extra Energy which it possesses due to its motion 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. Pompeii is a ruined and partially buried Roman town-city near modern Naples and Caserta in the Italian region of Campania, in Herculaneum (in modern Italian Ercolano) is an ancient Roman town located in the territory of the current commune of Ercolano. Italy (Italia officially the Italian Republic, (Repubblica Italiana is located on the Italian Peninsula in Southern Europe, and on the two largest A pyroclastic surge killed volcanologists Katia and Maurice Krafft and 41 other people on Mount Unzen, in Japan, on June 3, 1991. Volcanology (also spelled vulcanology) is the study of Volcanoes, Lava, Magma, and related geological and Geophysical phenomena Katia Krafft ( Mulhouse, 17 April 1942 – 3 June 1991) and her husband Maurice Krafft ( Guebwiller, 25 Mount Unzen ( jp 雲仙岳 Unzendake) is an active volcanic complex of several overlapping Stratovolcanoes also known as a Composite volcano For a topic outline on this subject see List of basic Japan topics. Events 350 - Roman usurper Nepotianus, of the Constantinian dynasty, proclaims himself Roman Emperor, entering Year 1991 ( MCMXCI) was a Common year starting on Tuesday (link will display full calendar of the Gregorian Calendar. 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. The Caribbean (ˌkærəˡbiən kæ'rəbiən Cariben|Caraïben or Caraïben; Caraïbe or more commonly Antilles; Caribe is a Region consisting Montserrat (ˌmɒntsəˈræt is British overseas territory located in the Leeward Islands, part of the chain of islands called the Lesser Antilles 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.

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. Krakatoa ( Indonesian: Krakatau) also spelled Krakatao or Krakatowa, is a volcanic island in the Sunda Strait Sumatra (also spelled Sumatera) is the sixth largest island in the world (approximately 470000 km² and is the largest island entirely in Indonesia (two

A recent documentary film showed tests by a research team at Kiel University, Germany of pyroclastic flows moving over water[1]. The University of Kiel ( German Christian-Albrechts-Universität zu Kiel, CAU) is a University in the city of Kiel, Germany Germany, officially the Federal Republic of Germany ( ˈbʊndəsʁepuˌbliːk ˈdɔʏtʃlant is a Country in Central Europe. 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.

References

• Sigurdson, Haraldur: Encyclopedia of volcanoes. Academic Press, 546-548. ISBN 0-12-643140-X.

See also

• Pyroclastic surge
• Pyroclastic rock
• Lahar

External links

• Pyroclastic Flows video