Editing Basalt (section)

== Definition and characteristics ==
[[File:Basalt qapf.jpg|thumb|[[QAPF diagram]] with basalt/andesite field highlighted in yellow. Basalt is distinguished from andesite by SiO<sub>2</sub>&nbsp;<&nbsp;52%.]]
[[File:TAS-Diagramm-basalt.png|thumb|upright=1.35|Basalt is field B in the [[TAS classification]].]]
[[File:VesicularBasalt1.jpg|thumb|Vesicular basalt at [[Sunset Crater]], Arizona. [[US quarter]] (24mm) for scale.]]
[[File:Basalt columns in yellowstone 2.jpg|Columnar basalt flows in [[Yellowstone National Park]], US|thumb]]
Basalt is composed mostly of oxides of silicon, iron, magnesium, potassium, aluminum, titanium, and calcium. [[Geologists]] classify [[igneous rock]] by its mineral content whenever possible; the relative volume percentages of [[quartz]] (crystalline [[silica]] (SiO<sub>2</sub>)), [[alkali feldspar]], [[plagioclase]], and [[feldspathoid]] ([[QAPF diagram|QAPF]]) are particularly important. An [[aphanitic]] (fine-grained) igneous rock is classified as basalt when its QAPF fraction is composed of less than 10% feldspathoid and less than 20% quartz, and plagioclase makes up at least 65% of its feldspar content. This places basalt in the basalt/andesite field of the QAPF diagram. Basalt is further distinguished from andesite by its silica content of under 52%.<ref name="lebas-streckeisen-1991">{{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 name="bgs">{{Cite journal|date=1999|title=Rock Classification Scheme - Vol 1 - Igneous|url=http://nora.nerc.ac.uk/id/eprint/3223/1/RR99006.pdf |archive-url=https://web.archive.org/web/20180329022649/http://nora.nerc.ac.uk/id/eprint/3223/1/RR99006.pdf |archive-date=2018-03-29 |url-status=live|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>{{sfn|Philpotts|Ague|2009|pp=139–143}}

It is often not practical to determine the mineral composition of volcanic rocks, due to their very small grain size, in which case geologists instead classify the rocks chemically, with particular emphasis on the total content of alkali metal oxides and silica ([[TAS classification|TAS]]); in that context, basalt is defined as volcanic rock with a content of between 45% and 52% silica and no more than 5% alkali metal oxides. This places basalt in the B field of the TAS diagram.<ref name="lebas-streckeisen-1991"/><ref name="bgs"/>{{sfn|Philpotts|Ague|2009|pp=139–143}} Such a composition is described as [[mafic]].<ref>{{cite web |url=http://www.glossary.oilfield.slb.com/en/Terms/m/mafic.aspx |publisher=Schlumberger Ltd. |title=Oilfield Glossary |year=2021}}</ref>

Basalt is usually dark grey to black in colour, due to a high content of [[augite]] or other dark-coloured [[pyroxene]] minerals,{{sfn|Hyndman|1985|p={{pn|date=June 2021}}}}{{sfn|Blatt|Tracy|1996|p=57}}{{sfn|Levin|2010|p=63}} but can exhibit a wide range of shading. Some basalts are quite light-coloured due to a high content of plagioclase; these are sometimes described as ''leucobasalts''.<ref name="wilson-1985">{{cite journal |last1=Wilson |first1=F. H. |title=The Meshik Arc – an eocene to earliest miocene magmatic arc on the Alaska Peninsula |date=1985 |pages=PR 88 |doi=10.14509/2269|doi-access=free |journal=Alaska Division of Geological & Geophysical Surveys Professional Report |volume=88|bibcode=1985usgs.rept....1W }}</ref><ref name="nozhkin-etal-2016">{{cite journal |last1=Nozhkin |first1=A.D. |last2=Turkina |first2=O.M. |last3=Likhanov |first3=I.I. |last4=Dmitrieva |first4=N.V. |title=Late Paleoproterozoic volcanic associations in the southwestern Siberian craton (Angara-Kan block) |journal=Russian Geology and Geophysics |date=February 2016 |volume=57 |issue=2 |pages=247–264 |doi=10.1016/j.rgg.2016.02.003|bibcode=2016RuGG...57..247N }}</ref> It can be difficult to distinguish between lighter-colored basalt and [[andesite]], so [[field research]]ers commonly use a [[wiktionary:rule of thumb|rule of thumb]] for this purpose, classifying it as basalt if it has a [[color index (geology)|color index]] of 35 or greater.{{sfn|Philpotts|Ague|2009|p=139}}

The physical properties of basalt result from its relatively low silica content and typically high iron and magnesium content.<ref name="USGSGlossary">{{cite web | url=https://volcanoes.usgs.gov/vsc/glossary/basalt.html | title=Basalt | publisher=[[USGS]] | website=USGS Volcano Hazards program – Glossary | date=8 April 2015 | access-date=27 July 2018}}</ref> The average density of basalt is 2.9 g/cm<sup>3</sup>, compared, for example, to [[granite]]’s typical density of 2.7 g/cm<sup>3</sup>.{{sfn|Philpotts|Ague|2009|p=22}} The viscosity of basaltic magma is relatively low—around 10<sup>4</sup> to 10<sup>5</sup> [[Centipoise|cP]]—similar to the viscosity of [[ketchup]], but that is still several orders of magnitude higher than the viscosity of water, which is about 1 cP).{{sfn|Philpotts|Ague|2009|pp=23–25}}

Basalt is often [[porphyritic]], containing larger crystals ([[phenocryst]]s) that formed before the extrusion event that brought the magma to the surface, embedded in a finer-grained [[Matrix (geology)|matrix]]. These phenocrysts are usually made of augite, [[olivine]], or a calcium-rich plagioclase,{{sfn|Blatt|Tracy|1996|p=57}} which have [[Bowen's reaction series|the highest melting temperatures]] of any of the [[mineral]]s that can typically crystallize from the melt, and which are therefore the first to form solid crystals.{{sfn|Klein|Hurlbut|1993|pp=558–560}}<ref>{{cite web |last1=Nave |first1=R. |title=Bowen's Reaction Series |url=http://hyperphysics.phy-astr.gsu.edu/hbase/Geophys/Bowen.html |website=Hyperphysics |publisher=Georgia State University |access-date=24 March 2021}}</ref>

Basalt often contains [[vesicular texture|vesicles]]; they are formed when dissolved gases bubble out of the magma as it decompresses during its approach to the surface; the erupted lava then solidifies before the gases can escape. When vesicles make up a substantial fraction of the volume of the rock, the rock is described as [[scoria]].{{sfn|Blatt|Tracy|1996|pp=27, 42–44}}<ref>{{cite web |last1=Jones |first1=C.E. |title=Scoria and Pumice |url=https://www.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousTextures/8PumiceScoria.html |website=Department of Geology & Planetary Science |publisher=University of Pittsburgh |access-date=24 March 2021}}</ref>

The term ''basalt'' is at times applied to shallow [[intrusive rock]]s with a composition typical of basalt, but rocks of this composition with a [[phaneritic]] (coarser) groundmass are more properly referred to either as [[diabase]] (also called dolerite) or—when they are more coarse-grained (having crystals over 2&nbsp;mm across)—as [[gabbro]]. Diabase and gabbro are thus the [[hypabyssal]] and [[plutonic]] equivalents of basalt.<ref name="bgs"/>{{sfn|Levin|2010|pp=58–60}}
[[File:Szentgyörgyhegy03.jpg|thumb|upright|Columnar basalt at Szent György Hill, Hungary]]
During the [[Hadean]], [[Archean]], and early [[Proterozoic]] [[Eon (geology)|eon]]s of Earth's history, the chemistry of erupted magmas was significantly different from what it is today, due to immature crustal and [[asthenosphere]] differentiation. The resulting [[Ultramafic rock|ultramafic]] volcanic rocks, with silica (SiO<sub>2</sub>) contents below 45% and high magnesium oxide (MgO) content, are usually classified as [[komatiite]]s.{{sfn|Philpotts|Ague|2009|pp=399–400}}<ref>{{cite web |title=Komatiite |url=http://www.atlas-hornin.sk/en/record/54/komatiite |website=Atlas of Magmatic Rocks |publisher=Comenius University in Bratislava |access-date=24 March 2021}}</ref>

=== Etymology ===
The word "basalt" is ultimately derived from [[Late Latin]] {{Lang|la|basaltes}}, a misspelling of Latin {{Lang|la|basanites}} "very [[Hardness|hard]] stone", which was imported from [[Ancient Greek]] {{Lang|grc|βασανίτης}} ({{Transliteration|grc|basanites}}), from {{Lang|grc|βάσανος}} (''{{Transliteration|grc|basanos}}'', "[[touchstone (assaying tool)|touchstone]]").<ref>{{cite journal |last1=Tietz |first1=O. |last2=Büchner |first2=J. |title=The origin of the term 'basalt' |journal=Journal of Geosciences |date=29 December 2018 |pages=295–298 |doi=10.3190/jgeosci.273|doi-access=free }}</ref> The modern petrological term ''basalt'', describing a particular composition of [[lava]]-derived rock, became standard because of its use by [[Georgius Agricola]] in 1546, in his work ''[[De Natura Fossilium]]''. Agricola applied the term "basalt" to the volcanic black rock beneath the [[Bishop of Dresden-Meissen|Bishop of Meissen's]] [[Stolpen#Burg Stolpen|Stolpen castle]], believing it to be the same as the "basaniten" described by [[Pliny the Elder]] in AD&nbsp;77 in {{Lang|la|[[Natural History (Pliny)|Naturalis Historiae]]}}.<ref>{{cite journal |last1=Tietz |first1=Olaf |last2=Büchner |first2=Joerg |title=The origin of the term 'basalt' |journal=Journal of Geosciences |date=2018 |volume=63 |issue=4 |pages=295–298 |doi=10.3190/jgeosci.273 |url=http://www.jgeosci.org/content/jgeosci.273_tietz.pdf |archive-url=https://web.archive.org/web/20190428204828/http://www.jgeosci.org/content/jgeosci.273_tietz.pdf |archive-date=2019-04-28 |url-status=live |access-date=19 August 2020|doi-access=free }}</ref>

=== Types ===
[[File:Causeway23.jpg|thumb|Large masses must cool slowly to form a polygonal joint pattern, as here at the [[Giant's Causeway]] in Northern Ireland]]
[[File:Базальтове.jpg|thumb|right|Columns of basalt near [[Bazaltove]], Ukraine]]

On Earth, most basalt is formed by [[decompression melting]] of the [[mantle (geology)|mantle]].{{sfn|Philpotts|Ague|2009|pp=16–17}} The high pressure in the upper mantle (due to [[Overburden pressure|the weight of the overlying rock]]) raises the melting point of mantle rock, so that almost all of the upper mantle is solid. However, mantle rock is [[ductile]] (the solid rock slowly deforms under high stress). When [[Tectonics|tectonic forces]] cause hot mantle rock to creep upwards, pressure on the ascending rock decreases, and this can lower its melting point enough for the rock to [[Partial melting|partially melt]], producing basaltic magma.<ref name="green-ringwood-1969">{{cite book |doi=10.1029/GM013p0489 |chapter=The Origin of Basalt Magmas |title=The Earth's Crust and Upper Mantle |series=Geophysical Monograph Series |year=2013 |last1=Green |first1=D. H. |last2=Ringwood |first2=A. E. |volume=13 |pages=489–495 |isbn=978-1-118-66897-9 |bibcode=1969GMS....13..489G }}</ref>

Decompression melting can occur in a variety of tectonic settings, including in continental [[rift]] zones, at [[Mid-ocean ridge|mid-ocean ridges]], above [[Hotspot (geology)|geological hotspots]],{{sfn|Blatt|Tracy|1996|pp=151–156, 191–195, 162–163, 200}}{{sfn|Philpotts|Ague|2009|pp=236, 593–595}} and in [[back-arc basins]].<ref>{{cite journal |last1=Stern |first1=Robert J. |title=Subduction zones |journal=Reviews of Geophysics |date=2002 |volume=40 |issue=4 |pages=1012 |doi=10.1029/2001RG000108 |bibcode=2002RvGeo..40.1012S |s2cid=15347100 |doi-access=free }}</ref> Basalt also forms in [[subduction zones]], where mantle rock rises into a [[mantle wedge]] above the descending slab. The slab releases water vapor and other volatiles as it descends, which further lowers the melting point, further increasing the amount of decompression melting.{{sfn|Stern|2002|p=22–24}} Each tectonic setting produces basalt with its own distinctive characteristics.{{sfn|Philpotts|Ague|2009|pp=356–361}}  

* [[Tholeiitic basalt]], which is relatively rich in [[iron]] and poor in [[alkali metal]]s and [[aluminium]],{{sfn|Philpotts|Ague|2009|pp=143–146}} include most basalts of the [[ocean]] floor, most large [[oceanic island]]s,{{sfn|Philpotts|Ague|2009|pp=365–370}} and continental [[flood basalt]]s such as the [[Columbia River Basalt Group|Columbia River Plateau]].{{sfn|Philpotts|Ague|2009|pp=52–59}}
**High- and low-titanium basalt rocks, which are sometimes classified based on their [[titanium]] (Ti) content in High-Ti and Low-Ti varieties. High-Ti and Low-Ti basalt have been distinguished from each other in the [[Paraná and Etendeka traps]]<ref>{{cite journal |last1=Gibson |first1=S. A. |last2=Thompson |first2=R. N. |last3=Dickin |first3=A. P. |last4=Leonardos |first4=O. H. |title=High-Ti and low-Ti mafic potassic magmas: Key to plume-lithosphere interactions and continental flood-basalt genesis |journal=Earth and Planetary Science Letters |date=December 1995 |volume=136 |issue=3–4 |pages=149–165 |doi=10.1016/0012-821X(95)00179-G |bibcode=1995E&PSL.136..149G }}</ref> and the [[Emeishan Traps]].<ref name="cugb">{{cite journal |last1=Hou |first1=Tong |last2=Zhang |first2=Zhaochong |last3=Kusky |first3=Timothy |last4=Du |first4=Yangsong |last5=Liu |first5=Junlai |last6=Zhao |first6=Zhidan |title=A reappraisal of the high-Ti and low-Ti classification of basalts and petrogenetic linkage between basalts and mafic–ultramafic intrusions in the Emeishan Large Igneous Province, SW China |journal=Ore Geology Reviews |date=October 2011 |volume=41 |issue=1 |pages=133–143 |doi=10.1016/j.oregeorev.2011.07.005 |bibcode=2011OGRv...41..133H }}</ref>
** [[Mid-ocean ridge]] basalt (MORB) is a tholeiitic basalt that has almost exclusively erupted at ocean ridges; it is characteristically low in [[incompatible element]]s.{{sfn|Blatt|Tracy|1996|pp=156–158}}{{sfn|Hyndman|1985|p={{pn|date=June 2021}}}} Although all MORBs are chemically similar, geologists recognize that they vary significantly in how depleted they are in incompatible elements. When they are present in close proximity along mid-ocean ridges, that is seen as evidence for mantle inhomogeneity.<ref>{{cite journal |last1=Waters |first1=Christopher L. |last2=Sims |first2=Kenneth W. W. |last3=Perfit |first3=Michael R. |last4=Blichert-Toft |first4=Janne |author4-link=Janne Blichert-Toft |last5=Blusztajn |first5=Jurek |title=Perspective on the Genesis of E-MORB from Chemical and Isotopic Heterogeneity at 9–10°N East Pacific Rise |journal=Journal of Petrology |date=March 2011 |volume=52 |issue=3 |pages=565–602 |doi=10.1093/petrology/egq091 |doi-access=free }}</ref>
***Enriched MORB (E-MORB) is defined as MORB that is relatively undepleted in incompatible elements. It was once thought to be mostly located in hot spots along mid-ocean ridges, such as Iceland, but it is now known to be located in many other places along those ridges.<ref>{{cite journal |last1=Donnelly |first1=Kathleen E. |last2=Goldstein |first2=Steven L. |last3=Langmuir |first3=Charles H. |last4=Spiegelman |first4=Marc |title=Origin of enriched ocean ridge basalts and implications for mantle dynamics |journal=Earth and Planetary Science Letters |date=October 2004 |volume=226 |issue=3–4 |pages=347–366 |doi=10.1016/j.epsl.2004.07.019|bibcode=2004E&PSL.226..347D }}</ref>
***Normal MORB (N-MORB) is defined as MORB that has an average amount of incompatible elements.
***D-MORB, depleted MORB, is defined as MORB that is highly depleted in incompatible elements.
* [[Alkali basalt]] is relatively rich in alkali metals. It is [[normative mineralogy|silica-undersaturated]] and may contain [[feldspathoid]]s,{{sfn|Philpotts|Ague|2009|pp=143–146}} [[alkali feldspar]], [[phlogopite]], and [[kaersutite]]. Augite in alkali basalts is titanium-enriched augite; low-calcium pyroxenes are never present.{{sfn|Blatt|Tracy|1996|p=75}} They are characteristic of continental rifting and hotspot volcanism.{{sfn|Philpotts|Ague|2009|pp=368–370, 390–394}}
* High-alumina basalt has greater than 17% [[alumina]] (Al<sub>2</sub>O<sub>3</sub>) and is intermediate in composition between tholeiitic basalt and alkali basalt. Its relatively alumina-rich composition is based on rocks without phenocrysts of [[plagioclase]]. These represent the low-silica end of the [[calc-alkaline magma series]] and are characteristic of [[volcanic arc]]s above subduction zones.{{sfn|Philpotts|Ague|2009|pp=375–376}}
* [[Boninite]] is a high-[[magnesium]] form of basalt that is erupted generally in [[back-arc basin]]s; it is distinguished by its low titanium content and trace-element composition.{{sfn|Crawford|1989|p={{pn|date=June 2021}}}}
* [[Ocean island basalt]]s include both tholeiites and alkali basalts; the tholeiites predominate early in the eruptive history of the island. These basalts are characterized by elevated concentrations of incompatible elements, which suggests that their source mantle rock has produced little magma in the past (it is ''undepleted'').{{sfn|Philpotts|Ague|2009|pp=368–370}}