Pyroclastic flow
Encyclopedia
Volcanic gas
|250px|thumb|right|Image of the [[rhyolitic]] [[lava dome]] of [[Chaitén Volcano]] during its 2008-2010 eruption.Volcanic gases include a variety of substances given off by active volcanoes...
(which can reach temperatures of about 1000 °C (1,832 °F)) and rock (collectively known as tephra
Tephra
200px|thumb|right|Tephra horizons in south-central [[Iceland]]. The thick and light coloured layer at center of the photo is [[rhyolitic]] tephra from [[Hekla]]....
), which reaches speeds moving away from a volcano of up to 700 km/h (450 mph). 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. They are a common and devastating result of certain explosive volcanic eruptions
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
.
The word pyroclast is derived from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
, meaning "fire", and , meaning "broken in pieces". A name for some pyroclastic flows is nuée ardente (French
French language
French is a Romance language spoken as a first language in France, the Romandy region in Switzerland, Wallonia and Brussels in Belgium, Monaco, the regions of Quebec and Acadia in Canada, and by various communities elsewhere. Second-language speakers of French are distributed throughout many parts...
for "glowing cloud"); this was first used to describe the disastrous 1902 eruption of Mount Pelée
Mount Pelée
Mount Pelée is an active volcano at the northern end of the island and French overseas department of Martinique in the Lesser Antilles island arc of the Caribbean. Its volcanic cone is composed of layers of volcanic ash and hardened lava....
on Martinique
Martinique
Martinique is an island in the eastern Caribbean Sea, with a land area of . Like Guadeloupe, it is an overseas region of France, consisting of a single overseas department. To the northwest lies Dominica, to the south St Lucia, and to the southeast Barbados...
. 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 surge
Pyroclastic surge
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 it has a lower density or contains a much higher proportion of gas to rock ratio, which makes it more turbulent and allows it to rise...
s. 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 °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
Mount Pelée
Mount Pelée is an active volcano at the northern end of the island and French overseas department of Martinique in the Lesser Antilles island arc of the Caribbean. Its volcanic cone is composed of layers of volcanic ash and hardened lava....
in 1902 a fully dilute current overwhelmed the city of Saint-Pierre
Saint-Pierre, Martinique
Saint-Pierre is a town and commune of France's Caribbean overseas department of Martinique, founded in 1635 by Pierre Belain d'Esnambuc. Before the total destruction of Saint-Pierre in 1902 by a volcanic eruption, it was the most important city of Martinique culturally and economically, being known...
and killed nearly 30,000 people.
A pyroclastic flow is a type of gravity current
Gravity current
In fluid dynamics, a gravity current is a primarily horizontal flow in a gravitational field that is driven by a density difference, hence gravity currents also sometimes being referred to as "density currents"...
; in scientific literature they are sometimes abbreviated to PDC (pyroclastic density current).
Causes
There are several scenarios which can produce a pyroclastic flow:- Fountain collapse of an eruption columnEruption columnAn eruption column consists of hot volcanic ash emitted during an explosive volcanic eruption. The ash forms a column rising many kilometres into the air above the peak of the volcano. In the most explosive eruptions, the eruption column may rise over 40 km, penetrating the stratosphere...
from a Plinian eruptionPlinian eruptionPlinian eruptions, also known as 'Vesuvian eruptions', are volcanic eruptions marked by their similarity to the eruption of Mount Vesuvius in AD 79 ....
(e.g., Mount VesuviusMount VesuviusMount Vesuvius is a stratovolcano in the Gulf of Naples, Italy, about east of Naples and a short distance from the shore. It is the only volcano on the European mainland to have erupted within the last hundred years, although it is not currently erupting...
's destruction of PompeiiPompeiiThe city of Pompeii is a partially buried Roman town-city near modern Naples in the Italian region of Campania, in the territory of the comune of Pompei. Along with Herculaneum, Pompeii was destroyed and completely buried during a long catastrophic eruption of the volcano Mount Vesuvius spanning...
, see Pliny the YoungerPliny the YoungerGaius Plinius Caecilius Secundus, born Gaius Caecilius or Gaius Caecilius Cilo , better known as Pliny the Younger, was a lawyer, author, and magistrate of Ancient Rome. Pliny's uncle, Pliny the Elder, helped raise and educate him...
). 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, 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 columnEruption columnAn eruption column consists of hot volcanic ash emitted during an explosive volcanic eruption. The ash forms a column rising many kilometres into the air above the peak of the volcano. In the most explosive eruptions, the eruption column may rise over 40 km, penetrating the stratosphere...
associated with a vulcanian eruption (e.g., Montserrat's Soufrière HillsSoufrière HillsThe Soufrière Hills volcano is an active complex stratovolcano with many lava domes forming its summit on the Caribbean island of Montserrat. After a long period of dormancy, it became active in 1995, and has continued to erupt ever since...
volcano has generated many of these deadly pyroclastic flows and surges.) In this style of eruption, the gas and projectiles create a cloud which is denser than the surrounding air and become 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 ignimbriteIgnimbriteAn ignimbrite is the deposit of a pyroclastic density current, or pyroclastic flow, a hot suspension of particles and gases that flows rapidly from a volcano, driven by a greater density than the surrounding atmosphere....
. This occurred during the eruption of NovaruptaNovaruptaNovarupta, meaning "new eruption", is a volcano located on the Alaska Peninsula in Katmai National Park and Preserve, about southwest of Anchorage. Formed in 1912 during the largest volcanic eruption of the 20th century, Novarupta released 30 times the volume of magma as the 1980 eruption of...
in 1912 which produced the largest flows to be generated during recorded history. - Gravitational collapse of a lava domeLava dome|250px|thumb|right|Image of the [[rhyolitic]] lava dome of [[Chaitén Volcano]] during its 2008–2009 eruption.In volcanology, a lava dome is a roughly circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano...
or spine, with subsequent avalanches and flow down a steep slope e.g., Montserrat's Soufrière HillsSoufrière HillsThe Soufrière Hills volcano is an active complex stratovolcano with many lava domes forming its summit on the Caribbean island of Montserrat. After a long period of dormancy, it became active in 1995, and has continued to erupt ever since...
volcano which caused nineteen deaths in 1997. - The directional blast (or jet) when part of a volcano explodes or collapses (e.g., the May 18, 1980, eruption of Mount St. Helens1980 eruption of Mount St. HelensThe 1980 eruption of Mount St. Helens, a stratovolcano located in Washington state, in the United States, was a major volcanic eruption. The eruption was the only significant one to occur in the contiguous 48 U.S. states since the 1915 eruption of Lassen Peak in California...
) As distance from the volcano increases, this rapidly transforms into a gravity-driven current.
Size and effects
The kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
of the moving boulders will flatten trees and buildings in their path. The hot gases and high speed make them particularly lethal:
- The towns of PompeiiPompeiiThe city of Pompeii is a partially buried Roman town-city near modern Naples in the Italian region of Campania, in the territory of the comune of Pompei. Along with Herculaneum, Pompeii was destroyed and completely buried during a long catastrophic eruption of the volcano Mount Vesuvius spanning...
and HerculaneumHerculaneumHerculaneum was an ancient Roman town destroyed by volcanic pyroclastic flows in AD 79, located in the territory of the current commune of Ercolano, in the Italian region of Campania in the shadow of Mt...
, ItalyItalyItaly , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...
, for example, were famously engulfed by pyroclastic surges in 79 AD with many lives lost. - A pyroclastic surge killed volcanologistsVolcanologyVolcanology is the study of volcanoes, lava, magma, and related geological, geophysical and geochemical phenomena. The term volcanology is derived from the Latin word vulcan. Vulcan was the ancient Roman god of fire....
Katia and Maurice KrafftKatia and Maurice KrafftKatia Krafft and her husband, Maurice Krafft 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...
and 41 other people on Mount UnzenMount Unzenis an active volcanic group of several overlapping stratovolcanoes, near the city of Shimabara, Nagasaki Prefecture, on the island of Kyūshū, Japan’s southernmost main island....
, in JapanJapanJapan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
, 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 of ash. - On 25 June 1997 a pyroclastic flow travelled down Mosquito Ghaut on the CaribbeanCaribbeanThe Caribbean is a crescent-shaped group of islands more than 2,000 miles long separating the Gulf of Mexico and the Caribbean Sea, to the west and south, from the Atlantic Ocean, to the east and north...
island of MontserratMontserratMontserrat is a British overseas territory located in the Leeward Islands, part of the chain of islands called the Lesser Antilles in the West Indies. This island measures approximately long and wide, giving of coastline...
. A large energised pyroclastic surge developed. This surge 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 Krakatoa1883 eruption of Krakatoa
The 1883 eruption of Krakatoa began in May 1883 and culminated with the destruction of Krakatoa on 27 August 1883. Minor seismic activity continued to be reported until February 1884, though reports after October 1883 were later dismissed by Rogier Verbeek's investigation.-Early phase:In the years...
, supported by experimental evidence,shows that pyroclastic flows can cross significant bodies of water. One flow reached the Sumatra
Sumatra
Sumatra is an island in western Indonesia, westernmost of the Sunda Islands. It is the largest island entirely in Indonesia , and the sixth largest island in the world at 473,481 km2 with a population of 50,365,538...
n coast as much as 48 km (30 mi) away.
A 2006 documentary film, Ten Things You Didn't Know About Volcanoes, demonstrated tests by a research team at Kiel University, Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
, of pyroclastic flows moving over water. 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. This large displacement of heavy ash into the water caused a displacement of water; in reality, this would cause a tsunami
Tsunami
A tsunami is a series of water waves caused by the displacement of a large volume of a body of water, typically an ocean or a large lake...
due to the displacement via precipitate mass, as it did with the Krakatoa eruption. The temperature of the ash caused the water to evaporate, propelling the pyroclastic flow (now only consisting of the lighter material) along at an even faster pace than before on a bed of steam.
On the Moon
In 1963, NASA astronomer Winifred Cameron proposed that sinuous rilleRille
Rille is typically used to describe any of the long, narrow depressions in the lunar surface that resemble channels. Typically a rille can be up to several kilometers wide and hundreds of kilometers in length...
s on the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
may have been formed by the lunar equivalent of terrestrial pyroclastic flows. In a lunar volcanic eruption, a pyroclastic cloud would follow local relief resulting in an often sinuous track. The Moon's Schroter's Valley is one example.
See also
- Pyroclastic fallPyroclastic fallA 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:...
- Pyroclastic rockPyroclastic rockPyroclastic rocks or pyroclastics are 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...
- Pyroclastic surgePyroclastic surgeA 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 it has a lower density or contains a much higher proportion of gas to rock ratio, which makes it more turbulent and allows it to rise...
- LaharLaharA lahar is a 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. The term is a shortened version of "berlahar" which originated in the Javanese language of...
- Welded tuff
External links
- Pyroclastic Flows video