Pyroclastic material

Japanese: 火山砕屑物 - かざんさいせつぶつ（英語表記）pyroclastic material

A general term for solid volcanic material that is ejected from a volcano onto the earth's surface during a volcanic eruption. It is also called pyroclastic material or tephra. It is classified by size and shape (structure), with particles with a diameter of 2 mm or less called volcanic ash, particles with a diameter of 2 to 64 mm called lapilli, and particles with a diameter of 64 mm or more called volcanic blocks. Those with distinctive shapes are classified as volcanic bombs, pumice (floating stones), scoria (rock scavenging), etc. Tephra, a synonym for pyroclastic material, refers to materials that fall on the wind, mostly volcanic ash. However, since volcanic ash is defined as being 2 mm or less in size, the term tephra came to refer to ash-like particles that fall from high eruption clouds and form layers over a wide area. Pyroclastic rocks include essential materials (→ essential minerals) derived from the magma directly involved in the eruption, as well as other materials derived from old ejecta that are closely related to the magma that caused the eruption and that already constitute the volcanic edifice, and other materials derived from rocks that are volcanic but unrelated to the magma. Pyroclastic rocks are also called pyroclastic rocks.

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

火山の噴火によって地表に噴出した火山噴出物のうち，固体をなす物質の総称。火砕物，テフラともいう。大きさや外形（構造）によって分類され，直径 2mm以下の粒子状のものを火山灰，直径 2～64mmのものを火山礫（ラピリ），直径 64mm以上のものを火山岩塊といい，特徴的な外形をもつものは，火山弾，軽石（浮石），スコリア（岩滓）などとして分類される。火山砕屑物と同義語のテフラ tephraは，風に乗って降下する物質，多くは火山灰をさす。しかし火山灰は 2mm以下と定義されているため，高い噴火雲から落下して広範囲に層を形成する灰状粒子をテフラと呼ぶようになった。火山砕屑物には，噴火に直接関係するマグマから導かれた本質物質（→本質鉱物）のほかに，噴火を引き起こしたマグマと近縁関係にあり，すでに火山体を構成している古い噴出物である類質物質，火山物質であるが同マグマとは成因が無関係な岩石に由来する異質物質が含まれる。また，火山砕屑物が再び膠結して生じたものを火山砕屑岩（火砕岩）という。

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