Editing Pyroclastic rock (section)

==Classification==
Pyroclasts include juvenile pyroclasts derived from chilled magma, mixed with accidental pyroclasts, which are fragments of [[Country rock (geology)|country rock]]. Pyroclasts of different sizes are classified (from smallest to largest) as [[volcanic ash]], [[lapilli]], or [[volcanic block]]s (or, if they exhibit evidence of having been hot and molten during emplacement, [[volcanic bomb]]s). All are considered to be pyroclastic because they were formed (fragmented) by volcanic explosivity, for example during explosive decompression, shear, thermal [[decrepitation]], or by attrition and abrasion in a volcanic conduit, volcanic jet, or pyroclastic density current.<ref>Heiken, G. and Wohletz, K., 1985 ''Volcanic Ash'', University of California Press;, pp. 246.</ref>

{| class="wikitable"
|-
! Clast size !! Pyroclast !! Mainly unconsolidated (tephra) !! Mainly consolidated: pyroclastic rock
|-
| > 64&nbsp;mm || block (angular)<br />bomb (if fluidal-shaped) || blocks; agglomerate || pyroclastic breccia; agglomerate
|-
| < 64&nbsp;mm || lapillus || lapilli || lapillistone ([[lapilli tuff]] is where lapilli are supported within a matrix of tuff)
|-
| < 2&nbsp;mm || coarse ash || coarse ash || coarse [[tuff]]
|-
| < 0.063&nbsp;mm || fine ash || fine ash || fine tuff
|}

Pyroclasts are transported in two main ways: in atmospheric eruption plumes, from which pyroclasts settle to form topography-draping [[pyroclastic fall]] layers, and by pyroclastic density currents (PDCs) (including [[pyroclastic flow]]s and [[pyroclastic surge]]s),<ref>{{cite book |last1=Philpotts |first1=Anthony R. |last2=Ague |first2=Jay J. |title=Principles of igneous and metamorphic petrology |date=2009 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=9780521880060 |edition=2nd |page=73}}</ref> from which pyroclasts are deposited as pyroclastic density current deposits, which tend to thicken and coarsen in valleys, and thin and fine over topographic highs.

During [[Plinian eruption]]s, [[pumice]] and [[volcanic ash|ash]] are formed when foaming [[silicic]] [[magma]] is fragmented in the volcanic conduit, because of rapid shear driven by decompression and the growth of microscopic bubbles. The pyroclasts are then entrained with hot gases to form a supersonic jet that exits the volcano, admixes and heats cold atmospheric air to form a vigorously buoyant [[eruption column]] that rises several kilometers into the stratosphere and cause [[Volcanic ash and aviation safety|aviation hazards]].{{sfn|Schmincke|2003|pp=155-176}} Particles fall from atmospheric eruption plumes and accumulate as layers on the ground, which are described as fallout deposits.{{sfn|Fisher|Schmincke|1984|p=8}}

Pyroclastic density currents arise when the mixture of hot pyroclasts and gases is denser than the atmosphere and so, instead of rising buoyantly, it spreads out across the landscape. They are one of the greatest hazards at a volcano, and may be either 'fully dilute' (dilute, turbulent ash clouds, right down to their lower levels) or 'granular fluid based' (the lower levels of which comprise a concentrated dispersion of interacting pyroclasts and partly trapped gas).<ref>{{cite journal |last1=Breard |first1=Eric C.P. |last2=Lube |first2=Gert |title=Inside pyroclastic density currents – uncovering the enigmatic flow structure and transport behaviour in large-scale experiments |journal=Earth and Planetary Science Letters |date=January 2017 |volume=458 |pages=22–36 |doi=10.1016/j.epsl.2016.10.016|bibcode=2017E&PSL.458...22B }}</ref> The former type are sometimes called ''pyroclastic surges'' (even though they may be sustained rather than "surging") and lower parts of the latter are sometimes termed ''pyroclastic flows'' (these, also, can be sustained and quasi steady or surging). As they travel, pyroclastic density currents deposit particles on the ground, and they entrain cold atmospheric air, which is then heated and thermally expands.{{sfn|Schmincke|2003|pp=177-208}} Where the density current becomes sufficiently dilute to loft, it rises into the atmosphere as a 'phoenix plume'<ref>{{cite journal |last1=Sulpizio |first1=Roberto |last2=Dellino |first2=Pierfrancesco |title=Chapter 2 Sedimentology, Depositional Mechanisms and Pulsating Behaviour of Pyroclastic Density Currents |journal=Developments in Volcanology |date=2008 |volume=10 |pages=57–96 |doi=10.1016/S1871-644X(07)00002-2|isbn=9780444531650 }}</ref> (or 'co-PDC plume').<ref>{{cite journal |last1=Engwell |first1=S. |last2=Eychenne |first2=J. |title=Contribution of Fine Ash to the Atmosphere From Plumes Associated With Pyroclastic Density Currents |journal=Volcanic Ash |date=2016 |pages=67–85 |doi=10.1016/B978-0-08-100405-0.00007-0|isbn=9780081004050 |url=http://nora.nerc.ac.uk/id/eprint/516054/1/Engwell%26Eychenne.pdf }}</ref> These phoenix plumes typically deposit thin ashfall layers that may contain little pellets of aggregated fine ash.<ref>{{cite journal |last1=Colombier |first1=Mathieu |last2=Mueller |first2=Sebastian B. |last3=Kueppers |first3=Ulrich |last4=Scheu |first4=Bettina |last5=Delmelle |first5=Pierre |last6=Cimarelli |first6=Corrado |last7=Cronin |first7=Shane J. |last8=Brown |first8=Richard J. |last9=Tost |first9=Manuela |last10=Dingwell |first10=Donald B. |title=Diversity of soluble salt concentrations on volcanic ash aggregates from a variety of eruption types and deposits |journal=Bulletin of Volcanology |date=July 2019 |volume=81 |issue=7 |pages=39 |doi=10.1007/s00445-019-1302-0|bibcode=2019BVol...81...39C |s2cid=195240304 |url=http://dro.dur.ac.uk/29046/1/29046.pdf }}</ref>

Hawaiian eruptions such as those at [[Kīlauea]] produce an upward-directed jet of hot droplets and clots of magma suspended in gas; this is called a [[lava fountain]]<ref name="macdonald-etal-2983">{{cite book |last1=Macdonald |first1=Gordon A. |last2=Abbott |first2=Agatin T. |last3=Peterson |first3=Frank L. |title=Volcanoes in the sea : the geology of Hawaii |date=1983 |publisher=University of Hawaii Press |location=Honolulu |isbn=0824808320 |pages=6, 9, 96–97 |edition=2nd}}</ref> or 'fire-fountain'.<ref name="allaby-2013">{{cite book |editor1-last=Allaby |editor1-first=Michael |title=A dictionary of geology and earth sciences |date=2013 |publisher=Oxford University Press |isbn=9780199653065 |edition=Fourth |chapter=Fire-fountain}}</ref> If sufficiently hot and liquid when they land, the hot droplets and clots of magma may agglutinate to form 'spatter' ('agglutinate'), or fully coalesce to form a clastogenic [[lava flow]].<ref name="macdonald-etal-2983"/><ref name="allaby-2013"/>