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pyroclastic flow
A pyroclastic flow (also known scientifically as a pyroclastic density current〔Branney M.J. & Kokelaar, B.P. 2002, Pyroclastic Density Currents and the Sedimentation of Ignimbrites. Geological Society London Memoir 27, 143pp.〕) is a fast-moving current of hot gas and rock (collectively known as tephra), which reaches speeds moving away from a volcano of up to 700 km/h (450 mph).〔(Pyroclastic flows USGS )〕 The gas can reach temperatures of about 1,000 °C (1,830 °F). Pyroclastic 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.
==Origin of term==
The word ''pyroclast'' is derived from the Greek , meaning "fire", and , meaning "broken in pieces". 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.〔Lacroix, A. (1904) ''La Montagne Pelée et ses Eruptions'', Paris, Masson (in French)〕 In the dark, these pyroclastic flows glowed red.
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, and sea. They may also contain steam, water and rock at less than ; 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.〔Arthur N. Strahler (1972), ''Planet Earth: its physical systems through geological time''〕
A pyroclastic flow is a type of gravity current; in scientific literature they are sometimes abbreviated to PDC (pyroclastic density current).
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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