Editing Pyroclastic flow (section)

==Origin of term==
[[File:BishopTuff.jpg|thumb|Pyroclastic rocks from the [[Bishop tuff]]; uncompressed with [[pumice]] (on left), compressed with [[fiamme]] (on right)]]

The word ''pyroclast'' is derived from the [[Greek language|Greek]] {{lang|grc|[[wikt:πῦρ|πῦρ]]}} (''pýr''), meaning "fire", and {{lang|grc|[[wikt:κλαστός|κλαστός]]}} (''klastós''), meaning "broken in pieces".<ref>See:
* {{cite book |last1=Jukes |first1=Joseph Beete |title=The Student's Manual of Geology |date=1862 |publisher=[[Adam and Charles Black]] |location=Edinburgh, Scotland, U.K. |page=[https://archive.org/details/bub_gb_xExMQScTSSoC/page/n90 68] |edition=2nd |url=https://archive.org/details/bub_gb_xExMQScTSSoC}} From p. 68: "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 or condition may be, when they are accumulated in such mass as to form beds of "rock." We might call them perhaps "pyroclastic materials," … "</ref><ref name="PerseusDict">{{cite web | url=https://www.perseus.tufts.edu/hopper/morph?l=klasto%2Fs&la=greek&can=klasto%2Fs0 | title=Definition of κλαστός | publisher=[[Tufts University]] | work=Perseus Greek Dictionary | access-date=8 October 2020}}</ref> A name for pyroclastic flows that glow red in the dark is '''{{lang|fr|nuée ardente}}''' (French, "burning cloud"); this was notably used to describe the disastrous 1902 eruption of [[Mount Pelée]] on [[Martinique]], a French island in the Caribbean.<ref>Lacroix, A. (1904) ''La Montagne Pelée et ses Eruptions'', Paris, Masson (in French)  [https://books.google.com/books?id=eh0hAQAAMAAJ&pg=PA38 From vol. 1, p. 38:]  After describing on p. 37 the eruption of a "dense, black cloud" (''nuée noire''), Lacroix coins the term {{lang|fr|nuée ardente}}:   "{{lang|fr|Peu après l'éruption de ce que j'appellerai désormais la {{strong|nuée ardente}}, un immense nuage de cendres couvrait l'ile tout entière, la saupoudrant d'une mince couche de débris volcaniques.}}"  (Shortly after the eruption of what I will call henceforth the ''dense, glowing cloud'' [''nuée ardente''], an immense cloud of cinders covered the entire island, sprinkling it with a thin layer of volcanic debris.)</ref><ref group=note>Although the coining of the term {{lang|fr|nuée ardente}} in 1904 is attributed to the French geologist [[Alfred Lacroix|Antoine Lacroix]], according to:
* {{cite journal |last1=Hooker |first1=Marjorie |title=The origin of the volcanological concept ''nuée ardente'' |journal=Isis |date=1965 |volume=56 |issue=4 |pages=401–407 |doi=10.1086/350041|s2cid=144772310 }}
the term was used in 1873 by Lacroix's father-in-law and former professor, French geologist [[Ferdinand André Fouqué]] in his description of the 1580 and 1808 eruptions of the volcano on the island of [[São Jorge Island|São Jorge]] in the [[Azores]].  
* {{cite journal |last1=Fouqué |first1=Ferdinand |title=San Jorge et ses éruptions |journal=Revue Scientifique de la France et de l'Étranger |date=1873 |volume=2 |issue=51 |pages=1198–1201 |url=https://books.google.com/books?id=wOmW-ihiBZAC&pg=PA1198 |series=2nd series |trans-title=São Jorge and its eruptions |language=fr }}
:* From p. 1199:  "{{lang|fr|Un des phénomènes les plus singuliers de cette grande éruption est la production de ce que les témoins contemporains ont appelé des {{strong|nuées ardentes}}.}}" (One of the strangest phenomena of this great eruption is the production of what contemporary witnesses called ''nuées ardentes''.)
:* From p. 1200:  "{{lang|fr|Les détonations cessent dans la journée du 17, mais alors apparaissent des {{strong|nuées ardents}} semblables à celles de l'éruption de 1580.}}" (The detonations cease on the day of the 17th, but then [there] appear burning clouds [''nuées ardents''] similar to those of the eruption of 1580.)
Marjorie Hooker – (Hooker, 1965), p. 405 – records that Father João Inácio da Silveira (1767–1852) from the village of Santo Amaro on São Jorge island wrote an account of the 1808 eruption in which he described an {{lang|pt|ardente nuven}} ("burning cloud" in Portuguese) that flowed down the slopes of the volcano. Silveira's account was published in 1871 and republished in 1883.
* {{cite book |last1=Silveira |first1=João Inácio da |editor1-last=Canto |editor1-first=Ernesto do |title=Archivo dos Açores [Archive of the Azores] |date=1883 |publisher=Archivo dos Açores |location=Ponta Delgada, São Miguel, Azores |pages=437–441 |url=https://books.google.com/books?id=WBMrAQAAMAAJ&pg=PA440 |language=pt |chapter=XXVIII. Anno de 1808. Erupção na ilha de S. Jorge [XXVIII. Year of 1808. Eruption on the island of São Jorge.]}} From pp. 439–440: "{{lang|pt|Em desassete do dito mês de Maio … de repente se levantou um tufão de fogo ou vulcão e introduzindo-se nas terras lavradias levantou todos aqueles campos até abaixo às vinhas com todas as árvores e bardos, fazendo-se uma medonha e {{strong|ardente nuvem}} e correndo até abaixo de igreja queimou trinta e tantas pessoas na igreja e nos campos…}}" (On the seventeenth of the said month of May … suddenly there arose a typhoon of fire out of the volcano and [it] entered the farm lands, heaved up all those fields down to the vineyards, with all the trees and hedges, forming a fearsome and burning cloud [''ardente nuvem''] and running down to the church, burned more than thirty people in the church and in the fields…)</ref>

Pyroclastic flows that contain a much higher proportion of gas to rock are known as "fully dilute pyroclastic density currents" or [[pyroclastic surge]]s. 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 {{convert|250|C|F|-1}}; 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, Martinique|Saint-Pierre]] and killed nearly 30,000 people.<ref>Arthur N. Strahler (1972), ''Planet Earth: its physical systems through geological time''</ref>

A pyroclastic flow is a type of [[gravity current]]; in scientific literature, it is sometimes abbreviated to PDC (pyroclastic density current).