Editing Basalt (section)

=== 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}}