Editing Pyroclastic rock (section)

{{short description|Clastic rocks composed solely or primarily of volcanic materials}}{{For other uses|Pyroclastic (disambiguation)}}[[Image:Pyroclastic Flow St. Helens.jpg|thumb|right|300px|[[United States Geological Survey|USGS]] scientist examines [[pumice]] blocks at the edge of a pyroclastic flow from [[Mount St. Helens]]]]
[[File:BishopTuff.jpg|thumb|right|Rocks from the [[Bishop Tuff]], uncompressed with [[pumice]] on left; compressed with [[fiamme]] on right.]]
[[File:Volcanic Stone 2D.ogv|thumb|right|Flight through a [[computed tomography|μCT]]-image stack of a [[lapilli|lapillus]] of the volcano [[Katla volcano|Katla]] in [[Iceland]]. Find spot: Beach near [[Vik, Sogn|Vik]] at the end of road 215. Acquisition done using "CT Alpha" by "Procon X-Ray GmbH", Garbsen, Germany. Resolution 11,2μm/[[Voxel]], width approx. 24&nbsp;mm.]]
[[File:Volcanic Stone 3D.ogg|thumb|right|3D-Rendering of the above image stack, in parts transparent. Heavy particles in red.]]
'''Pyroclastic rocks''' are [[clastic rock]]s composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as '''pyroclasts'''. Pyroclastic rocks are a type of [[volcaniclastic]] deposit, which are deposits made predominantly of volcanic particles.<ref name="fisher-1961">{{cite journal |last1=Fisher |first1=Richard V. |title=Proposed classification of volcaniclastic sediments and rocks |journal=Geological Society of America Bulletin |date=1961 |volume=72 |issue=9 |pages=1409 |doi=10.1130/0016-7606(1961)72[1409:PCOVSA]2.0.CO;2|bibcode=1961GSAB...72.1409F }}</ref><ref name="fisher-schminke-1984-89">{{cite book |last1=Fisher |first1=Richard V. |last2=Schmincke |first2=H.-U. |title=Pyroclastic rocks |year=1984 |publisher=Springer-Verlag |location=Berlin |isbn=3540127569}}</ref> 'Phreatic' pyroclastic deposits are a variety of pyroclastic rock that forms from volcanic steam explosions and they are entirely made of accidental clasts. 'Phreatomagmatic' pyroclastic deposits are formed from explosive interaction of [[magma]] with [[groundwater]].{{sfn|Fisher|1961|p=1409}} The word ''pyroclastic'' is derived from the Greek {{lang|el|πῦρ}}, meaning fire; and {{lang|el|κλαστός}}, meaning broken.{{cn|date=November 2024}}

Unconsolidated accumulations of pyroclasts are described as [[tephra]]. Tephra may become [[lithified]] to a pyroclastic rock by cementation or chemical reactions as the result of the passage of hot gases ([[fumarolic]] alteration) or groundwater (e.g. [[hydrothermal alteration]] and [[diagenesis]]) and burial, or, if it is emplaced at temperatures so hot that the soft glassy pyroclasts stick together at point contacts, and deform: this is known as [[Welding (geology)|welding]].<ref name="schmincke-2003-138">{{cite book |last1=Schmincke |first1=Hans-Ulrich |title=Volcanism |date=2003 |publisher=Springer |location=Berlin |isbn=9783540436508 |page=138}}</ref>

One of the most spectacular types of pyroclastic deposit is an [[ignimbrite]], which is the deposit of a ground-hugging pumiceous pyroclastic density current (a rapidly flowing hot suspension of pyroclasts in gas). Ignimbrites may be loose deposits or solid rock, and they can bury entire landscapes. An individual ignimbrite can exceed 1000&nbsp;km<sup>3</sup> in volume, can cover 20,000&nbsp;km<sup>2</sup> of land, and may exceed 1&nbsp;km in thickness, for example where it is ponded within a volcanic caldera.