在地質學中，“由火山作用形成”，特別是指火山爆發中快速移動、密集、超熱的火山灰、氣體和岩石； 到1862年，指的是由此產生的岩石； 見 pyro- “火”+ clastic，表示“破碎成碎片”。
The word "ash" is not a very good one to include all the mechanical accompaniments of a subaerial or subaqueous eruption, since ash seems to be restricted to a fine powder, the residuum of combustion. A word is wanting to express all such accompaniments, no matter what their size and condition may be, when they are accumulated in such mass as to form beds of "rock." We might call them perhaps "pyroclastic materials," but I have endeavoured in vain to think of an English word which should express this meaning, and believe, therefore, that the only plan will be to retain the word "ash," giving it an enlarged technical meaning, so as to include all the fragments accumulated during an igneous eruption, no matter what size or what shape they may be. [J. Beete Jukes, "The Student's Manual of Geology," Edinburgh, 1862]
“灰”這個詞並不是一個很好的詞來包括所有的亞空氣或水下爆發的機械伴隨物，因爲灰似乎僅限於細粉末，即燃燒的殘渣。需要一個詞來表達所有這些伴隨物，無論它們的大小和狀態如何，當它們積聚成爲“岩石”層時。也許我們可以稱它們爲“火山碎屑材料”，但我一直徒勞地想着一個能表達這個意思的英語單詞，因此相信唯一的計劃將是保留“灰”這個詞，賦予它一個擴大的技術含義，以包括在火山爆發期間積累的所有碎片，無論它們的大小或形狀如何。[J. Beete Jukes，“地質學學生手冊”，愛丁堡，1862年]
