Editing Basalt (section)

== Petrology ==
[[File:Мікрофотографія шліфа базальту із заповідника Базальтові стовпи в поляризованому світлі.jpg|thumb|[[Photomicrograph]] of a [[thin section]] of basalt from [[Bazaltove]], Ukraine]]
The mineralogy of basalt is characterized by a preponderance of calcic plagioclase [[feldspar]] and [[pyroxene]]. [[Olivine]] can also be a significant constituent.{{sfn|Levin|2010|p=62}} Accessory [[mineral]]s present in relatively minor amounts include [[iron oxide]]s and iron-titanium oxides, such as [[magnetite]], [[ulvöspinel]], and [[ilmenite]].{{sfn|Blatt|Tracy|1996|p=75}} Because of the presence of such [[oxide]] minerals, basalt can acquire strong [[magnetism|magnetic]] signatures as it cools, and [[paleomagnetism|paleomagnetic]] studies have made extensive use of basalt.{{sfn|Levin|2010|p=185}}

In [[tholeiitic basalt]], pyroxene ([[augite]] and [[orthopyroxene]] or [[pigeonite]]) and [[calcium]]-rich plagioclase are common phenocryst minerals. Olivine may also be a phenocryst, and when present, may have rims of pigeonite. The [[groundmass]] contains interstitial quartz or [[tridymite]] or [[cristobalite]]. ''Olivine tholeiitic basalt'' has augite and orthopyroxene or pigeonite with abundant olivine, but olivine may have rims of pyroxene and is unlikely to be present in the [[matrix (geology)|groundmass]].{{sfn|Blatt|Tracy|1996|p=75}}

[[Alkali basalt]]s typically have mineral assemblages that lack orthopyroxene but contain olivine. Feldspar phenocrysts typically are [[labradorite]] to [[andesine]] in composition. Augite is rich in titanium compared to augite in tholeiitic basalt. Minerals such as [[feldspar|alkali feldspar]], [[leucite]], [[nepheline]], [[sodalite]], [[phlogopite]] mica, and [[apatite]] may be present in the groundmass.{{sfn|Blatt|Tracy|1996|p=75}}

Basalt has high [[liquidus]] and [[Solidus (chemistry)|solidus]] temperatures—values at the Earth's surface are near or above 1200&nbsp;°C (liquidus){{sfn|McBirney|1984|pp=366–367}} and near or below 1000&nbsp;°C (solidus); these values are higher than those of other common igneous rocks.{{sfn|Philpotts|Ague|2009|p=252}}

The majority of tholeiitic basalts are formed at approximately 50–100&nbsp;km depth within the mantle. Many alkali basalts may be formed at greater depths, perhaps as deep as 150–200&nbsp;km.<ref>{{cite book |doi=10.1016/B978-075063386-4/50003-3 |chapter=Tectonic settings |title=Plate Tectonics and Crustal Evolution |year=1997 |last1=Condie |first1=Kent C. |pages=69–109 |isbn=978-0-7506-3386-4 }}</ref><ref>{{cite journal |last1=Kushiro |first1=Ikuo |title=Origin of magmas in subduction zones: a review of experimental studies |journal=Proceedings of the Japan Academy, Series B |date=2007 |volume=83 |issue=1 |pages=1–15 |doi=10.2183/pjab.83.1 |pmid=24019580 |pmc=3756732 |bibcode=2007PJAB...83....1K }}</ref> The origin of high-alumina basalt continues to be controversial, with disagreement over whether it is a [[magma|primary melt]] or derived from other basalt types by fractionation.<ref>{{cite journal|last1=Ozerov|first1=Alexei Y|title=The evolution of high-alumina basalts of the Klyuchevskoy volcano, Kamchatka, Russia, based on microprobe analyses of mineral inclusions|journal=Journal of Volcanology and Geothermal Research|date=January 2000|volume=95|issue=1–4|pages=65–79|doi=10.1016/S0377-0273(99)00118-3|bibcode=2000JVGR...95...65O|url=http://repo.kscnet.ru/2569/1/my_2000_f_en_JVGR.pdf |archive-url=https://web.archive.org/web/20200306105257/http://repo.kscnet.ru/2569/1/my_2000_f_en_JVGR.pdf |archive-date=2020-03-06 |url-status=live}}</ref>{{rp|65}}

=== Geochemistry ===
Relative to most common igneous rocks, basalt compositions are rich in [[magnesium oxide|MgO]] and [[calcium oxide|CaO]] and low in [[silicon dioxide|SiO<sub>2</sub>]] and the alkali oxides, i.e., [[sodium oxide|Na<sub>2</sub>O]] + [[potassium oxide|K<sub>2</sub>O]], consistent with their [[TAS classification]]. Basalt contains more silica than [[picrobasalt]] and most [[basanite]]s and [[tephrite]]s but less than [[basaltic andesite]]. Basalt has a lower total content of alkali oxides than [[trachybasalt]] and most basanites and tephrites.{{sfn|Philpotts|Ague|2009|pp=139–143}}

Basalt generally has a composition of 45–52 [[wt%]] SiO<sub>2</sub>, 2–5 wt% total alkalis,{{sfn|Philpotts|Ague|2009|pp=139–143}} 0.5–2.0 wt% [[titanium dioxide|TiO<sub>2</sub>]], 5–14 wt% [[iron(II) oxide|FeO]] and 14 wt% or more [[alumina|Al<sub>2</sub>O<sub>3</sub>]]. Contents of CaO are commonly near 10 wt%, those of MgO commonly in the range 5 to 12 wt%.<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>

High-alumina basalts have aluminium contents of 17–19 wt% Al<sub>2</sub>O<sub>3</sub>; [[boninite]]s have [[magnesium]] (MgO) contents of up to 15 percent. Rare [[feldspathoid]]-rich [[mafic]] rocks, akin to alkali basalts, may have Na<sub>2</sub>O + K<sub>2</sub>O contents of 12% or more.{{sfn|Irvine|Baragar|1971}}

The abundances of the [[lanthanide]] or [[rare-earth element]]s (REE) can be a useful diagnostic tool to help explain the history of mineral crystallisation as the melt cooled. In particular, the relative abundance of europium compared to the other REE is often markedly higher or lower, and called the [[europium anomaly]]. It arises because Eu<sup>2+</sup> can substitute for Ca<sup>2+</sup> in plagioclase feldspar, unlike any of the other lanthanides, which tend to only form <sup>3+</sup> [[cation]]s.{{sfn|Philpotts|Ague|2009|p=359}}

Mid-ocean ridge basalts (MORB) and their intrusive equivalents, gabbros, are the characteristic igneous rocks formed at mid-ocean ridges. They are tholeiitic basalts particularly low in total alkalis and in [[Compatibility (geochemistry)|incompatible]] trace elements, and they have relatively flat REE patterns normalized to mantle or [[chondrite]] values. In contrast, alkali basalts have normalized patterns highly enriched in the light REE, and with greater abundances of the REE and of other incompatible elements. Because MORB basalt is considered a key to understanding [[plate tectonics]], its compositions have been much studied. Although MORB compositions are distinctive relative to average compositions of basalts erupted in other environments, they are not uniform. For instance, compositions change with position along the [[Mid-Atlantic Ridge]], and the compositions also define different ranges in different ocean basins.<ref>{{cite book |doi=10.1016/B978-0-08-095975-7.00203-5 |chapter=Sampling Mantle Heterogeneity through Oceanic Basalts: Isotopes and Trace Elements |title=Treatise on Geochemistry |year=2014 |last1=Hofmann |first1=A.W. |pages=67–101 |isbn=978-0-08-098300-4 }}</ref> Mid-ocean ridge basalts have been subdivided into varieties such as normal (NMORB) and those slightly more enriched in incompatible elements (EMORB).{{sfn|Philpotts|Ague|2009|p=312}}

[[Isotope]] ratios of [[chemical element|elements]] such as [[strontium]], [[neodymium]], [[lead]], [[hafnium]], and [[osmium]] in basalts have been much studied to learn about the evolution of the [[Earth's mantle]].{{sfn|Philpotts|Ague|2009|loc=Chapter 13}} Isotopic ratios of [[noble gas]]es, such as <sup>3</sup>[[Helium|He]]/<sup>4</sup>He, are also of great value: for instance, ratios for basalts range from 6 to 10 for mid-ocean ridge tholeiitic basalt (normalized to atmospheric values), but to 15–24 and more for ocean-island basalts thought to be derived from [[mantle plume]]s.<ref name="class-goldstein-2005">{{cite journal |last1=Class |first1=Cornelia |last2=Goldstein |first2=Steven L. |title=Evolution of helium isotopes in the Earth's mantle |journal=Nature |date=August 2005 |volume=436 |issue=7054 |pages=1107–1112 |doi=10.1038/nature03930|pmid=16121171 |bibcode=2005Natur.436.1107C |s2cid=4396462 }}</ref>

Source rocks for the partial melts that produce basaltic magma probably include both [[peridotite]] and [[pyroxenite]].<ref name="sobolev-etal-2007">{{cite journal|author=Alexander V. Sobolev|author2=Albrecht W. Hofmann|author3=Dmitry V. Kuzmin|author4=Gregory M. Yaxley|author5=Nicholas T. Arndt|author6-link=Sun-Lin Chung|author6=Sun-Lin Chung|author7=Leonid V. Danyushevsky|author8=Tim Elliott|author9=Frederick A. Frey|author10=Michael O. Garcia|author11=Andrey A. Gurenko|author12=Vadim S. Kamenetsky|author13=Andrew C. Kerr|author14=Nadezhda A. Krivolutskaya|author15=Vladimir V. Matvienkov|author16=Igor K. Nikogosian|author17=Alexander Rocholl|author18=Ingvar A. Sigurdsson|author19=Nadezhda M. Sushchevskaya|author20=Mengist Teklay|name-list-style=amp |title=The Amount of Recycled Crust in Sources of Mantle-Derived Melts|journal=Science|date=20 April 2007|volume=316|issue=5823|pages=412–417|bibcode=2007Sci...316..412S|doi=10.1126/science.x|pmid=17395795|url=http://eprints.utas.edu.au/2614/1/Science2007.pdf}}
</ref>

=== Morphology and textures ===
[[File:20011005-0039 DAS large.jpg|thumb|An active basalt lava flow]]
The shape, structure and [[rock microstructure|texture]] of a basalt is diagnostic of how and where it erupted—for example, whether into the sea, in an explosive [[Scoria|cinder]] eruption or as creeping [[pāhoehoe]] lava flows, the classic image of [[Hawaii]]an basalt eruptions.{{sfn|Schmincke|2003|p={{pn|date=June 2021}}}}

==== Subaerial eruptions ====
{{Main|Subaerial eruption}}
Basalt that erupts under open air (that is, [[subaerial]]ly) forms three distinct types of lava or volcanic deposits: [[scoria]]; [[volcanic ash|ash]] or cinder ([[breccia]]);{{sfn|Blatt|Tracy|1996|pp=27–28}} and lava flows.{{sfn|Blatt|Tracy|1996|pp=22–23}}

Basalt in the tops of subaerial lava flows and [[cinder cone]]s will often be highly [[Vesicular texture|vesiculated]], imparting a lightweight "frothy" texture to the rock.{{sfn|Blatt|Tracy|1996|pp=43–44}} Basaltic cinders are often red, coloured by oxidized [[iron]] from weathered iron-rich minerals such as [[pyroxene]].{{sfn|Lillie|2005|p=41}}

[[Lava#{{okina}}A{{okina}}ā|{{okina}}A{{okina}}ā]] types of blocky cinder and breccia flows of thick, viscous basaltic [[lava]] are common in Hawai{{okina}}i. Pāhoehoe is a highly fluid, hot form of basalt which tends to form thin aprons of molten lava which fill up hollows and sometimes forms [[lava lake]]s. [[Lava tube]]s are common features of pāhoehoe eruptions.{{sfn|Blatt|Tracy|1996|pp=22–23}}

Basaltic [[tuff]] or [[Pyroclastic rock|pyroclastic]] rocks are less common than basaltic lava flows. Usually basalt is too hot and fluid to build up sufficient pressure to form explosive lava eruptions but occasionally this will happen by trapping of the lava within the volcanic throat and buildup of [[volcanic gas]]es. Hawai{{okina}}i's [[Mauna Loa]] volcano erupted in this way in the 19th century, as did [[Mount Tarawera]], New Zealand in its violent 1886 eruption. [[Maar]] volcanoes are typical of small basalt tuffs, formed by explosive eruption of basalt through the crust, forming an apron of mixed basalt and wall rock breccia and a fan of basalt tuff further out from the volcano.{{sfn|Schmincke|2003|loc=Chapter 12}}

Amygdaloidal structure is common in relict [[vesicle (geology)|vesicles]] and beautifully [[crystal]]lized species of [[zeolite]]s, [[quartz]] or [[calcite]] are frequently found.{{sfn|Philpotts|Ague|2009|p=64}}

===== Columnar basalt =====
{{Main|Columnar jointing}}
{{See also|List of places with columnar basalt}}
[[File:Causeway-code poet-4.jpg|thumb|The [[Giant's Causeway]] in Northern Ireland]]
[[File:Boyabat.jpg|thumb|Columnar [[Joint (geology)|jointed]] basalt in [[Turkey]]]]
[[File:Мыс Столбчатый. После заката.jpg|thumb|Columnar basalt at [[Cape Stolbchaty]], Russia]]
During the cooling of a thick lava flow, contractional [[Joint (geology)|joints]] or fractures form.<ref>{{cite journal |last1=Smalley |first1=I. J. |title=Contraction Crack Networks in Basalt Flows |journal=Geological Magazine |date=April 1966 |volume=103 |issue=2 |pages=110–114 |doi=10.1017/S0016756800050482 |bibcode=1966GeoM..103..110S |s2cid=131237003 }}</ref> If a flow cools relatively rapidly, significant [[Thermal expansion#Contraction effects (negative expansion)|contraction]] forces build up. While a flow can shrink in the vertical dimension without fracturing, it cannot easily accommodate shrinking in the horizontal direction unless cracks form; the extensive fracture network that develops results in the formation of [[Columnar jointing|column]]s. These structures, or [[basalt prism]]s, are predominantly hexagonal in cross-section, but polygons with three to twelve or more sides can be observed.<ref>{{cite journal |last1=Weaire |first1=D. |last2=Rivier |first2=N. |title=Soap, cells and statistics—random patterns in two dimensions |journal=Contemporary Physics |date=January 1984 |volume=25 |issue=1 |pages=59–99 |doi=10.1080/00107518408210979 |bibcode=1984ConPh..25...59W }}</ref> The size of the columns depends loosely on the rate of cooling; very rapid cooling may result in very small (<1&nbsp;cm diameter) columns, while slow cooling is more likely to produce large columns.<ref name="spry-1962">{{cite journal |last1=Spry |first1=Alan |title=The origin of columnar jointing, particularly in basalt flows |journal=Journal of the Geological Society of Australia |date=January 1962 |volume=8 |issue=2 |pages=191–216 |doi=10.1080/14400956208527873 |bibcode=1962AuJES...8..191S }}</ref>

==== Submarine eruptions ====
{{Main|Submarine eruption}}
[[File:Pillow basalt crop l.jpg|thumb|Pillow basalts on the Pacific seafloor]]

The character of submarine basalt eruptions is largely determined by depth of water, since increased pressure restricts the release of volatile gases and results in effusive eruptions.<ref name="francis">Francis, P. (1993) ''Volcanoes: A Planetary Perspective'', Oxford University Press.</ref> It has been estimated that at depths greater than {{convert|500|m||}}, explosive activity associated with basaltic magma is suppressed.{{sfn|Parfitt|Parfitt|Wilson|2008|p={{pn|date=June 2021}}}} Above this depth, submarine eruptions are often explosive, tending to produce [[pyroclastic rock]] rather than basalt flows.<ref name="head and wilson">{{cite journal |last1=Head |first1=James W. |last2=Wilson |first2=Lionel |title=Deep submarine pyroclastic eruptions: theory and predicted landforms and deposits |journal=Journal of Volcanology and Geothermal Research |date=2003 |volume=121 |issue=3–4 |pages=155–193 |doi=10.1016/S0377-0273(02)00425-0 |bibcode=2003JVGR..121..155H }}</ref> These eruptions, described as Surtseyan, are characterised by large quantities of steam and gas and the creation of large amounts of [[pumice]].<ref name="Smithson">[http://www.volcano.si.edu/galleries.cfm?p=11], Smithsonian Institution National Museum of Natural History Global Volcanism Program (2013).</ref> 

===== Pillow basalts =====
{{Main|Pillow lava}}
When basalt erupts underwater or flows into the sea, contact with the water quenches the surface and the lava forms a distinctive ''pillow'' shape, through which the hot lava breaks to form another pillow. This "pillow" texture is very common in underwater basaltic flows and is diagnostic of an underwater eruption environment when found in ancient rocks. Pillows typically consist of a fine-grained core with a glassy crust and have radial jointing. The size of individual pillows varies from 10&nbsp;cm up to several metres.{{sfn|Schmincke|2003|p=64}}

When ''[[pahoehoe|pāhoehoe]]'' lava enters the sea it usually forms pillow basalts. However, when ''{{okina}}a{{okina}}ā'' enters the ocean it forms a [[littoral cone]], a small cone-shaped accumulation of tuffaceous debris formed when the blocky ''{{okina}}a{{okina}}ā'' lava enters the water and explodes from built-up steam.{{sfn|Macdonald|Abbott|Peterson|1983|p={{pn|date=June 2021}}}}

The island of [[Surtsey]] in the [[Atlantic Ocean]] is a basalt volcano which breached the ocean surface in 1963. The initial phase of Surtsey's eruption was highly explosive, as the magma was quite fluid, causing the rock to be blown apart by the boiling steam to form a tuff and cinder cone. This has subsequently moved to a typical pāhoehoe-type behaviour.<ref name="kikelaar-durant-1983">{{cite journal |last1=Kokelaar |first1=B.Peter |last2=Durant |first2=Graham P. |title=The submarine eruption and erosion of Surtla (Surtsey), Iceland |journal=Journal of Volcanology and Geothermal Research |date=December 1983 |volume=19 |issue=3–4 |pages=239–246 |doi=10.1016/0377-0273(83)90112-9|bibcode=1983JVGR...19..239K }}</ref><ref name="moore-1985">{{cite journal |last1=Moore |first1=James G. |title=Structure and eruptive mechanisms at Surtsey Volcano, Iceland |journal=Geological Magazine |date=November 1985 |volume=122 |issue=6 |pages=649–661 |doi=10.1017/S0016756800032052 |bibcode=1985GeoM..122..649M |s2cid=129242411 }}</ref>

[[Volcanic glass]] may be present, particularly as rinds on rapidly chilled surfaces of lava flows, and is commonly (but not exclusively) associated with underwater eruptions.{{sfn|Blatt|Tracy|1996|pp=24–25}}

Pillow basalt is also produced by some [[Subglacial eruption|subglacial]] volcanic eruptions.{{sfn|Blatt|Tracy|1996|pp=24–25}}