Pyroclastic flow

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

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.

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).

Other Languages
eesti: Lõõmpilv
Esperanto: Piroklasta fluo
français: Nuée ardente
Gaeilge: Nuée ardente
한국어: 화산쇄설류
Bahasa Indonesia: Aliran piroklastik
íslenska: Gusthlaup
Kreyòl ayisyen: Ekoulman piwoklastik
Bahasa Melayu: Aliran piroklastik
日本語: 火砕流
norsk nynorsk: Pyroklastisk straum
Simple English: Pyroclastic flow
slovenščina: Piroklastični tok
српски / srpski: Пирокластични ток