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{{Short description|Rock formed from lava erupted from a volcano}}{{Redirect|Volcanics|Volcanic pistols|Volcanic Repeating Arms}}
{{For|the album|Volcanic Rock (album)}}
[[File:Ignimbrite.jpg|thumb|right|[[Ignimbrite]], a volcanic rock deposited by [[pyroclastic flow]]s]]

'''Volcanic rocks''' (often shortened to '''volcanics''' in scientific contexts) are [[Rock (geology)|rock]]s formed from [[lava]] erupted from a [[volcano]]. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into [[Subvolcanic rock|hypabyssal]] and [[metamorphic rock]]s and constitute an important element of some [[sediment]]s and [[sedimentary rock]]s. For these reasons, in geology, volcanics and shallow hypabyssal rocks are not always treated as distinct. In the context of [[Precambrian]] [[shield (geology)|shield]] geology, the term "volcanic" is often applied to what are strictly [[metavolcanic rock]]s. Volcanic rocks and sediment that form from [[magma]] erupted into the air are called "pyroclastics," and these are also technically sedimentary rocks.

Volcanic rocks are among the most common rock types on Earth's surface, particularly in the oceans. On land, they are very common at plate boundaries and in [[flood basalt|flood basalt province]]s. It has been estimated that volcanic rocks cover about 8% of the Earth's current land surface.<ref name="Wilkinson2008">{{cite journal | title=Global geologic maps are tectonic speedometers—Rates of rock cycling from area-age frequencies | first1=Bruce H | last1=Wilkinson | first2=Brandon J | last2=McElroy | first3=Stephen E | last3=Kesler | first4=Shanan E | last4=Peters | first5=Edward D | last5=Rothman | journal=Geological Society of America Bulletin | year=2008 | volume=121 | issue=5–6 | pages=760–79 | doi=10.1130/B26457.1 | bibcode=2009GSAB..121..760W }}</ref>

==Characteristics==

===Setting and size===
{{Expand section|date=May 2011}}
Volcanic rocks are classified based on their formation environment and particle size. They can originate from lava flows or be ejected explosively as fragmented material known as '''tephra'''.
*'''[[Lava]]''' – When molten rock erupts and solidifies on the Earth's surface, it forms coherent volcanic rocks such as basalt, andesite, and rhyolite. The size and structure of lava formations vary, with common types including pahoehoe (smooth, ropy lava) and ʻaʻā (rough, jagged lava).
*'''[[Tephra]]''' – Fragmented volcanic material ejected during eruptions, which varies in size and composition. Tephra includes:
**[[Volcanic bomb|'''Volcanic bomb''']] – Large, semi-molten fragments ejected from a volcano that solidify before reaching the ground. They often acquire aerodynamic shapes due to their flight through the air.
**'''[[Lapilli]]''' – Rock fragments between 2 and 64 mm in diameter, formed from lava droplets or broken volcanic material. Lapilli can accumulate to form volcanic breccia or tuff.
**[[Volcanic ash|'''Volcanic ash''']] – Fine particles (<2 mm) of pulverized rock, minerals, and glass created during explosive eruptions. Ash can travel long distances, affecting air quality and climate.
The size and setting of volcanic rock influence its distribution, physical properties, and impact on the environment.<br />
{| class="wikitable"
|+Classification of Volcaniclastic rocks and sediments<ref>{{Cite journal|last1=Le Bas|first1=M. J.|last2=Streckeisen |first2=AL |title=The IUGS systematics of igneous rocks|journal=Journal of the Geological Society|volume=148|issue=5|pages=825–33|doi=10.1144/gsjgs.148.5.0825|bibcode=1991JGSoc.148..825L|year=1991|s2cid=28548230}}</ref><ref>{{Cite journal|date=1999|title=Rock Classification Scheme - Vol 1 - Igneous|url=http://www.bgs.ac.uk/downloads/start.cfm?id=7|journal=British Geological Survey: Rock Classification Scheme|publisher=NERC|volume=1|pages=1–52|url-status=live|archive-url=https://web.archive.org/web/20161124091415/http://www.bgs.ac.uk/downloads/start.cfm?id=7|archive-date=24 November 2016}}</ref>
|-
|
|
! colspan="2" |Pyroclastic deposit
|-
!Clast size in mm
!Pyroclast
!Primarily unconsolidated: tephra
!Primarily consolidated: pyroclastic rock
|-
|> 64&nbsp;mm
|Bomb, block
|Agglomerate, bed of blocks or bomb, block tephra
|Agglomerate, pyroclastic breccia
|-
|64 to 2&nbsp;mm
|Lapillus
|Layer, bed of lapilli or lapilli tephra
|Lapilli tuff
|-
|2 to 1/16&nbsp;mm
|Coarse ash grain
|Coarse ash
|Coarse (ash tuff)
|-
| class="nowrap"|< 1/16 mm
|Fine ash grain (dust grain)
|Fine ash (dust)
|Fine (ash) tuff (dust tuff)
Type 3-Class Rogue
|}

===Texture===
[[File:LvMS-Lvm.jpg|thumb|Photomicrograph of a [[volcanic lithic fragment]] ([[sand grain]]); upper picture is plane-polarized light, bottom picture is cross-polarized light, scale box at left-center is 0.25 millimeter.]]
Volcanic rocks are usually fine-grained or [[aphanitic]] to glass in texture. They often contain [[xenolith|clasts]] of other [[Rock (geology)|rocks]] and [[phenocryst]]s. Phenocrysts are [[crystal]]s that are larger than the [[matrix (geology)|matrix]] and are identifiable with the unaided [[Human eye|eye]]. [[Rhomb porphyry]] is an example with large [[Rhombus|rhomb]] shaped [[phenocrysts]] embedded in a very fine grained matrix.<ref name=Oslo>{{cite journal |last1=Corfu |first1=Fernando |last2=Larsen |first2=Bjørn Tore |title=U-Pb systematics in volcanic and plutonic rocks of the Krokskogen area: Resolving a 40 million years long evolution in the Oslo Rift |journal=Lithos |date=December 2020 |volume=376-377 |article-number=105755 |doi=10.1016/j.lithos.2020.105755|bibcode=2020Litho.37605755C |hdl=10852/83877 |s2cid=225300187 |hdl-access=free }}</ref>

Volcanic rocks often have a [[vesicular texture]] caused by voids left by  [[Volatile (astrogeology)#Igneous petrology|volatiles]] trapped in the molten [[lava]]. [[Pumice]] is a highly vesicular rock produced in [[Explosive eruption|explosive volcanic eruptions]].{{citation needed|date=October 2021}}

===Chemistry===
Most modern petrologists classify igneous rocks, including volcanic rocks, 
by their chemistry when dealing with their origin. The fact that different mineralogies and textures may be developed from the same initial [[magma]]s has led petrologists to rely heavily on chemistry to look at a volcanic rock's origin.{{citation needed|date=October 2021}}

[[File:Classification extrusive rocks EN.svg|thumb|upright=1.3|[[IUGS]] 'total alkali silicates' classification of [[aphanitic]] volcanic rocks according to their relative alkali (Na<sub>2</sub>O + K<sub>2</sub>O) and silica (SiO<sub>2</sub>) weight contents. Blue area is roughly where alkaline rocks plot; yellow area where subalkaline rocks plot.

Original source:
*{{aut|Le Maitre, R.W.}} (''ed.''); 1989: ''A classification of igneous rocks and glossary of terms'', Blackwell Science, Oxford. ]]
The chemical classification of igneous rocks using the [[TAS classification]] is based first on the total content of [[silicon]] and alkali metals ([[sodium]] and [[potassium]]) expressed as weight fraction of silica and alkali oxides ([[Potassium oxide|K<sub>2</sub>O]] plus [[Sodium oxide|Na<sub>2</sub>O]]). These place the rock in one of the fields of the [[TAS classification|TAS diagram]]. [[Ultramafic rock]] and [[carbonatite]]s have their own specialized classification, but these rarely occur as volcanic rocks. Some fields of the TAS diagram are further subdivided by the ratio of potassium oxide to sodium oxide. Additional classifications may be made on the basis of other components, such as aluminum or iron content.<ref>{{Cite journal|last1=Le Bas|first1=M. J.|last2=Streckeisen|first2=A. L.|title=The IUGS systematics of igneous rocks|journal=Journal of the Geological Society|volume=148|issue=5|pages=825–833|doi=10.1144/gsjgs.148.5.0825|bibcode=1991JGSoc.148..825L|year=1991|citeseerx=10.1.1.692.4446|s2cid=28548230}}</ref><ref>{{Cite journal|date=1999|title=Rock Classification Scheme - Vol 1 - Igneous|url=http://nora.nerc.ac.uk/id/eprint/3223/1/RR99006.pdf|journal=British Geological Survey: Rock Classification Scheme|volume=1|pages=1–52}}</ref><ref>{{Cite web|url=http://geology.csupomona.edu/alert/igneous/igclass.htm|title=Classification of igneous rocks|archive-url=https://web.archive.org/web/20110930102012/http://geology.csupomona.edu/alert/igneous/igclass.htm|archive-date=30 September 2011|url-status=dead}}</ref><ref name="philpotts-ague-2009=139-148">{{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}}</ref>

Volcanic rocks are also broadly divided into subalkaline, alkaline, and peralkaline volcanic rocks. Subalkaline rocks are defined as rocks in which

SiO<sub>2</sub> < -3.3539 × 10<sup>−4</sup> × A<sup>6</sup> + 1.2030 × 10<sup>−2</sup> × A<sup>5</sup> - 1.5188 × 10<sup>−1</sup> × A<sup>4</sup> + 8.6096 × 10<sup>−1</sup> × A<sup>3</sup> - 2.1111 × A<sup>2</sup> + 3.9492 × A + 39.0

where both silica and total alkali oxide content (A) are expressed as [[molar fraction]]. Because the TAS diagram uses weight fraction and the boundary between alkaline and subalkaline rock is defined in terms of molar fraction, the position of this curve on the TAS diagram is only approximate. Peralkaline volcanic rocks are defined as rocks having Na<sub>2</sub>O + K<sub>2</sub>O > Al<sub>2</sub>O<sub>3</sub>, so that some of the alkali oxides must be present in sodic pyroxenes such as [[aegirine]] or sodic [[amphibole]] in addition to in [[feldspar]].<ref name="irvine-baragar-1971">{{cite journal |last1=Irvine |first1=T. N. |last2=Baragar |first2=W. R. A. |title=A Guide to the Chemical Classification of the Common Volcanic Rocks |journal=Canadian Journal of Earth Sciences |date=1 May 1971 |volume=8 |issue=5 |pages=523–548 |doi=10.1139/e71-055|bibcode=1971CaJES...8..523I }}</ref><ref name="philpotts-ague-2009=139-148"/>

The chemistry of volcanic rocks is dependent on two things: the initial composition of the primary magma and the subsequent differentiation. Differentiation of most magmas tends to increase the silica ([[Silicon dioxide|SiO<sub>2</sub>]]) content, mainly by [[Fractional crystallization (geology)|crystal fractionation]]. The initial composition of most magmas is [[basalt]]ic, albeit small differences in initial compositions may result in multiple differentiation series. The most common of these series are the subalkaline ([[Tholeiitic magma series|tholeiitic]], [[calc-alkaline]]) and [[Alkaline magma series|alkaline]].<ref name="irvine-baragar-1971"/><ref name="philpotts-ague-2009=139-148"/>

===Mineralogy===
Most volcanic rocks share a number of common [[mineral]]s. Differentiation of volcanic rocks tends to increase the silica (SiO<sub>2</sub>) content mainly by [[Fractional crystallization (geology)|fractional crystallization]]. Thus, more evolved volcanic rocks tend to be richer in minerals with a higher amount of silica such as [[phyllosilicate|phyllo]] and [[tectosilicate]]s including the feldspars, [[quartz]] [[Polymorphism (materials science)|polymorphs]] and [[muscovite]].  While still dominated by silicates, more primitive volcanic rocks have mineral assemblages with less silica, such as [[olivine]] and the [[pyroxene]]s. [[Bowen's reaction series]] correctly predicts the order of formation of the most common minerals in volcanic rocks.{{citation needed|date=October 2021}}

Occasionally, a magma may pick up crystals that crystallized from another magma; these crystals are called [[xenocryst]]s. [[Diamond]]s found in [[kimberlite]]s are rare but well-known xenocrysts; the kimberlites do not create the diamonds, but pick them up and transport them to the surface of the Earth.{{citation needed|date=October 2021}}

== Naming ==
[[Image:LvMS-Lvv.jpg|thumb|An aphanitic volcanic sand grain, with fine-grained groundmass, as seen under a [[petrographic microscope]]]]
[[File:Olivine basalt2.jpg|thumb|right|Vesicular olivine basalt from [[La Palma]] (green [[phenocrysts]] are [[olivine]]).]]
[[File:Pumice on 20 dollars.jpg|thumb|right|A {{convert|15|cm|adj=on|sp=us}} piece of [[pumice]] supported by a rolled U.S. $20 bill demonstrates its very low density.]]
Volcanic rocks are named according to both their [[chemical composition]] and texture. [[Basalt]] is a very common volcanic rock with low [[silica]] content. [[Rhyolite]] is a volcanic rock with high silica content. Rhyolite has silica content similar to that of [[granite]] while basalt is compositionally equal to [[gabbro]]. Intermediate volcanic rocks include [[andesite]], [[dacite]], [[trachyte]], and [[latite]].{{citation needed|date=October 2021}}

[[Pyroclastic rock]]s are the product of explosive volcanism. They are often [[felsic]] (high in silica). Pyroclastic rocks are often the result of volcanic debris, such as [[Volcanic ash|ash]], [[Volcanic bomb|bomb]]s and [[tephra]], and other [[volcanic]] [[ejecta]].  Examples of pyroclastic rocks are [[tuff]] and [[ignimbrite]].{{citation needed|date=October 2021}}

Shallow [[intrusion]]s, which possess structure similar to volcanic rather than [[pluton]]ic rocks, are also considered to be volcanic, shading into [[subvolcanic]].{{citation needed|date=October 2021}}

The terms lava stone and lava rock are more used by marketers than geologists, who would likely say "volcanic rock" (because [[lava]] is a molten liquid and [[Rock (geology)|rock]] is solid). "Lava stone" may describe anything from a friable [[silicic]] [[pumice]] to solid [[mafic]] flow basalt, and is sometimes used to describe rocks that were never [[lava]], but look as if they were (such as [[sedimentary]] [[Chinese scholar's rocks|limestone with dissolution pitting]]). To convey anything about the physical or chemical properties of the rock, a more specific term should be used; a good supplier will know what sort of volcanic rock they are selling.<ref name=what/>

== Composition of volcanic rocks ==
[[File:Aa next to pahoehoe lava at Craters of the Moon NM-750px.JPG|thumb|[[Lava#Volcanic morphologies|{{okina}}A{{okina}}ā next to pāhoehoe lava]] at the [[Craters of the Moon National Monument and Preserve]], Idaho, United States.]]
[[File:Latit - Boxberg, Hocheifel.jpg|thumb|A German example of [[latite]], a type of volcanic rock]]
The sub-family of rocks that form from volcanic lava are called [[igneous volcanic rock]]s (to differentiate them from igneous rocks that form from magma below the surface, called [[igneous plutonic rock]]s).

The lavas of different volcanoes, when cooled and hardened, differ much in their appearance and composition. If a [[rhyolite]] lava-stream cools quickly, it can quickly freeze into a black glassy substance called [[obsidian]]. When filled with bubbles of gas, the same lava may form the spongy appearing [[pumice]]. Allowed to cool slowly, it forms a light-colored, uniformly solid rock called rhyolite.{{citation needed|date=October 2021}}
[[File:Rhyolite 2013.jpg|thumb|A sample of rhyolite]]

[[File:Scoria AmsterdamIsland 2 edit.jpg|thumb|Basaltic [[scoria]] from [[Île Amsterdam|Amsterdam Island]] in the Indian Ocean]]

The lavas, having cooled rapidly in contact with the air or water, are mostly finely crystalline or have at least fine-grained ground-mass representing that part of the viscous semi-crystalline lava flow that was still liquid at the moment of eruption. At this time they were exposed only to atmospheric pressure, and the steam and other gases, which they contained in great quantity were free to escape; many important modifications arise from this, the most striking being the frequent presence of numerous steam cavities ([[Vesicular texture|vesicular]] structure) often drawn out to elongated shapes subsequently filled up with minerals by infiltration ([[Amygdule|amygdaloidal]] structure).<ref name=EB1911>{{EB1911|wstitle=Petrology |volume=21 |page=327 |inline=1 |first=John Smith |last=Flett}}</ref><ref>{{cite journal |title=Transient phenomena in vesicular lava flows based on laboratory experiments with analogue materials |journal=Journal of Volcanology and Geothermal Research|volume=132 |issue=2–3 |pages=115–36 |doi=10.1016/s0377-0273(03)00341-x  |last1=Pinkerton |first1=H |first2=N |last2=Bagdassarov |bibcode=2004JVGR..132..115B |year=2004 }}</ref><ref name=lavasteine/><ref>{{cite journal |last1=Pinkerton |first1=Harry |last2=Norton |first2=Gill |title=Rheological properties of basaltic lavas at sub-liquidus temperatures: laboratory and field measurements on lavas from Mount Etna |journal=Journal of Volcanology and Geothermal Research |date=1 November 1995 |volume=68 |issue=4 |pages=307–323 |doi=10.1016/0377-0273(95)00018-7 |bibcode=1995JVGR...68..307P }}</ref>

As crystallization was going on while the mass was still creeping forward under the surface of the Earth, the latest formed minerals (in the [[Matrix (geology)|ground-mass]]) are commonly arranged in subparallel winding lines that follow the direction of movement (fluxion or fluidal structure)—and larger early minerals that previously crystallized may show the same arrangement. Most lavas fall considerably below their original temperatures before emitted. In their behavior, they present a close analogy to hot solutions of salts in water, which, when they approach the saturation temperature, first deposit a crop of large, well-formed crystals (labile stage) and subsequently precipitate clouds of smaller less perfect crystalline particles (metastable stage).<ref name=EB1911/>

In igneous rocks the first generation of crystals generally forms before the lava has emerged to the surface, that is to say, during the ascent from the subterranean depths to the crater of the volcano. It has frequently been verified by observation that freshly emitted lavas contain large crystals borne along in a molten, liquid mass. The large, well-formed, early crystals ([[phenocryst]]s) are said to be [[porphyritic]]; the smaller crystals of the surrounding matrix or ground-mass belong to the post-effusion stage. More rarely lavas are completely fused at the moment of ejection; they may then cool to form a non-porphyritic, finely crystalline rock, or if more rapidly chilled may in large part be non-crystalline or glassy (vitreous rocks such as obsidian, [[tachylyte]], [[pitchstone]]).<ref name=EB1911/>

A common feature of glassy rocks is the presence of rounded bodies ([[spherulites]]), consisting of fine divergent fibres radiating from a center; they consist of imperfect crystals of feldspar, mixed with quartz or [[tridymite]]; similar bodies are often produced artificially in glasses that are allowed to cool slowly. Rarely these spherulites are hollow or consist of concentric shells with spaces between ([[lithophysae]]). [[Perlite|Perlitic]] structure, also common in glasses, consists of the presence of concentric rounded cracks owing to contraction on cooling.<ref name=EB1911/>

[[File:Volcanic rock.jpg|thumb|Volcanic rocks, [[Porto Moniz]], [[Madeira]]]]
The phenocrysts or porphyritic minerals are not only larger than those of the ground-mass; as the matrix was still liquid when they formed they were free to take perfect crystalline shapes, without interference by the pressure of adjacent crystals. They seem to have grown rapidly, as they are often filled with enclosures of glassy or finely crystalline material like that of the ground-mass . Microscopic examination of the phenocrysts often reveals that they have had a complex history. Very frequently they show layers of different composition, indicated by variations in color or other optical properties; thus augite may be green in the center surrounded by various shades of brown; or they may be pale green centrally and darker green with strong [[pleochroism]] (aegirine) at the periphery.<ref name=EB1911/>

In the feldspars the center is usually richer in calcium than the surrounding layers, and successive zones may often be noted, each less calcic than those within it. Phenocrysts of quartz (and of other minerals), instead of sharp, perfect crystalline faces, may show rounded corroded surfaces, with the points blunted and irregular tongue-like projections of the matrix into the substance of the crystal. It is clear that after the mineral had crystallized it was partly again dissolved or corroded at some period before the matrix solidified.<ref name=EB1911/>

Corroded phenocrysts of [[biotite]] and [[hornblende]] are very common in some lavas; they are surrounded by black rims of [[magnetite]] mixed with pale green augite. The hornblende or biotite substance has proved unstable at a certain stage of consolidation, and has been replaced by a paramorph of augite and magnetite, which may partially or completely substitute for the original crystal but still retains its characteristic outlines.<ref name=EB1911/>

==Mechanical behaviour of volcanic rocks==
The mechanical behaviour of volcanic rocks is complicated by their complex microstructure.<ref name="HeapViolay2021">{{cite journal | title=The mechanical behaviour and failure modes of volcanic rocks: a review | first1=Michael J | last1=Heap | first2=Marie | last2=Violay | journal=Bulletin of Volcanology | year=2021 | volume=83 | issue=33  | page=33 | doi=10.1007/s00445-021-01447-2 | bibcode=2021BVol...83...33H | s2cid=233217231 |issn=0258-8900 | doi-access=free }}</ref><ref name="Heap2015">{{cite journal | title=Fracture and compaction of andesite in a volcanic edifice | first1=Michael J | last1=Heap | first2=Jamie | last2=Farquharson | first3=Patrick | last3=Baud | first4=Yan | last4=Lavallée | first5=Thierry | last5=Reuschlé | journal=Bulletin of Volcanology | year=2015 | volume=77 | issue=55  | page=55 | doi=10.1007/s00445-015-0938-7 | pmid=26321781 | pmc=4551152 | bibcode=2015BVol...77...55H }}</ref> For example, attributes such as the partitioning of the void space (pores and microcracks), pore and crystal size and shape, and hydrothermal alteration can all vary widely in volcanic rocks and can all influence the resultant mechanical behaviour (e.g., Young's modulus, compressive and tensile strength, and the pressure at which they transition from brittle to ductile behaviour<ref name=HeapViolay2021/>). As for other crustal rocks, volcanic rocks are brittle and ductile at low and high effective confining pressures, respectively. Brittle behaviour is manifest as faults and fractures, and ductile behaviour can either be distributed (cataclastic pore collapse) or localised (compaction bands).<ref name=HeapViolay2021/> Understanding the mechanical behaviour of volcanic rocks can help us better understand volcanic hazards, such as flank collapse.{{citation needed|date=October 2021}}

==See also==
*[[Extrusive rock]]
*[[Intrusive rock]]

==References==
{{reflist|refs 
<ref name=what>{{cite web |url=http://reddome.com/lava-rock.html |title=What is Lava Rock |access-date=9 Sep 2017 |url-status=dead |archive-url=https://web.archive.org/web/20170910041116/http://reddome.com/lava-rock.html |archive-date=10 September 2017 |website= reddome.com |publisher= Red Dome Lava Rock}}</ref>
<ref name=lavasteine>{{cite web |url=https://lavasteine24.de/ |title=Der online Shop für Lavasteine |language=de |website=lavasteine24.de |access-date=27 Oct 2016 |archive-url=https://web.archive.org/web/20161027192833/https://lavasteine24.de/ |archive-date=27 October 2016 |url-status=live }}</ref>
}}

{{Igneous rocks}}
{{Volcanoes}}

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[[Category:Volcanic rocks| ]]
[[Category:Igneous petrology]]
[[Category:Volcanology]]