A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) 100 km/h (62 mph) on average but is capable of reaching speeds up to 700 km/h (430 mph). The gases can reach temperatures of about 1,000 °C (1,830 °F).is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that moves away from a volcano about
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.
The word pyroclast is derived from the Greek πῦρ , meaning "fire", and κλαστός , meaning "broken in pieces". 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.
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.
A pyroclastic flow is a type of gravity current; in scientific literature they are sometimes abbreviated to PDC (pyroclastic density current).
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There are several mechanisms that can produce a pyroclastic flow:
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.
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:
Testimonial evidence from the 1883 eruption of Krakatoa, supported by experimental evidence, km (30 mi) away.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. One flow reached the Sumatran coast as far as 48
A 2006 BBC documentary film, Ten Things You Didn't Know About Volcanoes,demonstrated tests by a research team at Kiel University, Germany, 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; 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.
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. [ non-primary source needed ]
Mount Vesuvius is a somma-stratovolcano located on the Gulf of Naples in Campania, Italy, about 9 km (5.6 mi) east of Naples and a short distance from the shore. It is one of several volcanoes which form the Campanian volcanic arc. Vesuvius consists of a large cone partially encircled by the steep rim of a summit caldera caused by the collapse of an earlier and originally much higher structure.
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 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.
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 is a ghost town on the island of Montserrat, an overseas territory of the United Kingdom located in the Leeward Island chain of the Lesser Antilles, West Indies.
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.
Mount Unzen 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.
Santa María Volcano is a large active volcano in the western highlands of Guatemala, in the Quetzaltenango Department near the city of Quetzaltenango.
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.
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.
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 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:
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.
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 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.
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.
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.
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