Pyroclastic flow - pyroclastic flow

Japanese: 火山砕屑流 - かざんさいせつりゅう（英語表記）pyroclastic flow

During a volcanic eruption, volcanic debris such as lava fragments and a mixture of volcanic ash and gas flow down at high speeds. This phenomenon is called a pyroclastic flow, and can be classified into a variety of types, including dry avalanches, base surges, debris flows, mud flows, and subaqueous pyroclastic flows. Aramaki has reorganized the traditional classification of pyroclastic flows, stating that it is best to limit the term to turbulent flows of a mixture of high-temperature gas and powder [Aramaki: 1957, 1979]. Pyroclastic flows are somewhere between lava and pyroclastic rock, and when volcanic debris is released from the crater as a collection of fragments and powder, it flows rapidly down the slopes of the volcano in a turbulent state. This is a fluidized liquid phase dispersed in a gas medium, and such a mass is called a thermal cloud (nuee ardante). Products of various eruption origins include pyroclastic flows, which include welded tuff and ignimbrite. Some say that the term "pyroclastic flows" should be broadened to include volcaniclastic deposits that have been moved by fluidization mechanisms, such as base surge deposits associated with phreatomagmatic eruptions [Waters & Fisher: 1971]. Since pyroclastic flows are rapidly deposited in a turbulent state, sorting by air does not work, and cooling is often insufficient. In addition, since they flow down in low areas, their distribution shows completely different characteristics from fall deposits that uniformly cover the earth's surface. The deposits do not show a stratified structure at the outcrop level [Aramaki: 1979]. However, it is unclear whether all the turbulent masses are deposited in a turbulent state, and it is possible that they may partially form a stratified structure due to fall deposition. Furthermore, these do not necessarily maintain sufficiently high temperatures, and it is expected that the depositional environment and cooling conditions will vary in some parts of the turbulent flow, so it is unlikely that the conditions for welding would have been maintained uniformly in all parts [Suzuki: 1994].

Pyroclastic flow

Pyroclastic flows are formed when pyroclastic material flows from the crater. Pyroclastic flows are intermediate between lava and pyroclastic rock. When pyroclastic material is released from the crater as a collection of fragments and powder, it flows rapidly down the slopes of the volcano in a turbulent state as a hot cloud (nuee ardante). As a result, there is no separation by air, and cooling is often insufficient, so the flow tends to flow over low areas of the terrain.

Source: Asakura Publishing Dictionary of Petrology Information

Japanese:

火山の噴火に際して,熔岩の破片や火山灰とガスの混合物などの火山砕屑物が高速で流下する現象を火山砕屑流（火砕流）とよぶが，その内容にはドライ・アバランシュ（dry avalanche），ベース・サージ（base surge），土石流（debris flow），泥流（mud flow），水中火砕流（subaqueous pyroclastic flow）など様々なものが含まれる．荒牧は従来の火砕流の分類を整理して，高温のガスと粉体の混合物の乱流に限って使用するのが望ましいとしている［荒牧 : 1957, 1979］．火山砕屑流は熔岩と火山砕屑岩との中間に相当するもので，火口から破片や粉体の集合として火山砕屑物が放出されると，乱流状態で火山の斜面を急速に流下する．これはガスの媒体中に液体相が分散して流動化したもので，このような一団を熱雲（nuee ardante）という．色々な噴出起源の生成物に火山砕屑流があり，これには熔結凝灰岩やイグニンブライトが含まれている．火山砕屑流は水蒸気マグマ噴火（phreatomagmatic eruption）に伴われたベース・サージ堆積物などのように流動機構によって移動した火山砕屑堆積物を含めて拡大解釈すべきという意見がある［Waters & Fisher : 1971］．火山砕屑流は乱流の状態で急速に堆積するので，空気による分級作用は働かず，冷却作用も不十分な場合が多い．また地形の低い所を選んで流下するので，その分布は一様に地表を覆う降下堆積物とは全く異なる特徴を示している．堆積物には露頭単位では成層構造がみられない［荒牧 : 1979］．しかし乱流の塊がすべて乱流の状態で堆積するかどうかは不明で，部分的には降下堆積による成層構造をなす可能性がある．またこれらが十分に高温を保つとは限らず，乱流中では部分的に堆積環境や冷却状況が異なることが予想され，熔結するような条件がすべての部分で一様に保たれたとは考えられない［鈴木 : 1994］．

火山砕屑流

火山砕屑物が火口から流れて溜まったもの．火山砕屑流は熔岩と火山砕屑岩の中間的なもので，火口から破片や粉体の集合として火山砕屑物が放出されると，乱流状態で火山の斜面を熱雲（nuee ardante）の状態で急速に流下する．この結果空気による分級作用が行われず，冷却作用も不十分な場合が多く，地形は低いところを選んで流下する特徴がある．

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