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Pyroclastic flows are a common and devastating result of some volcanic eruptions. They are fast moving fluidized bodies of hot gas, ash and rock (collectively known as tephra) which can travel away from the vent at up to 150 km/h. The gas is usually at a temperature of 100-800 degrees Celsius. The flows normally hug the ground and travel downhill under gravity, their speed depending upon the gradient of the slope and the size of the flow.

Volumes range from a few hundred cubic metres 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. Most 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 ash cloud rises above it because of the turbulence between the flow and the overlying air.

While moving, the kinetic energy of the boulders will flatten trees and buildings in their path. The hot gases and high speed make them particularly lethal. For example, the towns of Pompeii and Herculaneum in Italy were famously engulfed by them in 79 with heavy loss of life, and in June 1997 flows killed 20 people on the Caribbean island of Montserrat.

Another name for a pyroclastic flow is a nuée ardente ( French for "fiery cloud"), which was first used to describe the 1902 Martinique eruption. A pyroclastic flow has a red glow in the dark.

Flows containing a high proportion of gas to rock are known as pyroclastic surges. The lower density sometimes allows them to flow over higher topographic features such as ridges and hills. They may also be "cold," containing steam, water and rock at less than 100 degrees Celsius. Cold surges can occur when the eruption is from a vent under a lake or the sea.

Hot pyroclastic surges may form ahead of flows, for example during the eruption of Mount PeléeMount Pelee ( French: Montagne Pelee is an active volcano on the northern tip of the French departement of Martinique in the Caribbean. Though it was previously considered extinct, Mount Pelee erupted on May 8, 1902, destroying the town of Saint Pierre, a in 1902 a surge overwhelmed the city of Saint-PierreSaint-Pierre is the name of several communes of France: Saint-Pierre, in the Alpes-de-Haute-Provence departement Saint-Pierre, in the Cantal departement Saint-Pierre, in the Haute-Garonne departement Saint-Pierre, in the Jura departement Saint-Pierre, in and killed nearly 30,000 people.

A pyroclastic flow is a type of gravity currentIn fluid dynamics, a gravity current is a primarily horizontal flow in a gravitational field that is driven by a density difference. Typically, the density difference is small enough for the Boussinesq approximation to be valid. Gravity currents are typic; in scientific literature they are sometimes abbreviated to PDC - pyroclastic density current.

There are several scenarios which can produce a pyroclastic flow:

Collapse of the ash column from a plinian eruption (e.g Mount VesuviusMount Vesuvius (Monte Vesuvio) is a volcano east of Naples, Italy, located at 40°49′N 14°26′ E. It is the only active volcano on the European mainland, although it is not currently in eruption. It is one of four active volcanos in Italy, situa, see Pliny the YoungerCaius Plinius Caecilius Secundus ( 63 ca. 113), better known as Pliny the Younger was a lawyer, an author and a scientist of Ancient Rome. Born in Como, Italy, Pliny the Younger was the nephew of Pliny the Elder, who is considered by many to be the greate). In such an eruption, the material ejected from the vent heats the surrounding air and the turbulent mixture rises for many kilometres through convection. 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 will fall back down the flanks of the volcano. This is known as base surge. It was first documented from observations of underwater nuclear explosionA nuclear explosion nuclear detonation has occurred: twice using a nuclear weapon many times testing a nuclear weapon Potential other applications: nuclear pulse propulsion, including using a nuclear explosion as asteroid deflection strategy. Nuclear expls, in which a cloud rolls outward from the bottom of the column as it rises through the air, and subsequently applied to volcanologyVolcanology (also spelled vulcanology is the study of volcanoes, lava, magma and related geological phenomena. A volcanologist (also spelled vulcanologist is a person who studies in this field. Volcanologists frequently visit volcanoes, especially active.

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 rockIgneous rocks are formed when molten rock ( magma) cools and solidifies, with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive ( volcanic) rocks. This magma can be derived from either the Ea called ignimbrite. This occurred during the eruption of Mount Katmai in 1912 which produced the largest flows to be generated during recorded history.

Collapse of a volcanic dome and its subsequent flow down a steep slope (e.g. Montserrat).

The directional blast when part of a volcano explodes or collapses (e.g. the May 18, 1980 eruption of Mount St. Helens)

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Pyroclastic flowPyroclastic flows are a common and devastating result of some volcanic eruptions. They are fast moving fluidized bodies of hot gas, ash and rock (collectively known as tephra) which can travel away from the vent at up to 150 km/h. The gas is usually at a	PyroclasticPyroclastic is a term used in volcanology to describe air fall material ejected during a volcanic eruption. This material, known as pyroclast may include pumice, volcanic bombs, ignimbrite and ash. See also Pyroclastic flow Pyroclastic rock Pyroclastic su	Pyroclastic rockPyroclastic rocks are formed from lavas which are ejected into the air, as occur in pyroclastic flows or Plinian eruptions. They include pumice, volcanic bombs, ignimbrite and ash. The airborne fragments themselves are called pyroclasts (often abbreviated
Pyroclastic 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 contains a much higher of proportion of gas to rock, which makes it more turbulent and all

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