Editing Basalt (section)

== Alteration of basalt ==

=== Weathering ===
{{See also|Weathering}}
[[File:Absolute iron accumulation in kaolinized basalt. C 015.jpg|thumb|alt=This rock wall shows dark veins of mobilized and precipitated iron within kaolinized basalt in Hungen, Vogelsberg area, Germany.|Kaolinized basalt near Hungen, Vogelsberg, Germany]]

Compared to granitic rocks exposed at the Earth's surface, basalt [[outcrop]]s weather relatively rapidly. This reflects their content of minerals that crystallized at higher temperatures and in an environment poorer in water vapor than granite. These minerals are less stable in the colder, wetter environment at the Earth's surface. The finer grain size of basalt and the [[volcanic glass]] sometimes found between the grains also hasten weathering. The high iron content of basalt causes weathered surfaces in humid climates to accumulate a thick crust of [[hematite]] or other iron oxides and hydroxides, staining the rock a brown to rust-red colour.{{sfn|Blatt|Middleton|Murray|1980|pp=254–257}}<ref name="mackin-1961">{{cite journal |last1=Mackin |first1=J.H. |year=1961 |title=A stratigraphic section in the Yakima Basalt and the Ellensburg Formation in south-central Washington |journal=Washington Division of Mines and Geology Report of Investigations |volume=19 |url=https://www.dnr.wa.gov/Publications/ger_ri19_strat_yakima_basalt_ellensburg_form.pdf |archive-url=https://web.archive.org/web/20100124083749/http://www.dnr.wa.gov/Publications/ger_ri19_strat_yakima_basalt_ellensburg_form.pdf |archive-date=2010-01-24 |url-status=live }}</ref><ref name="usgs-holyoke">{{cite web |title=Holyoke Basalt |url=https://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=CTJho%3B0 |website=USGS Mineral Resources Program |publisher=United States Geological Survey |access-date=13 August 2020}}</ref><ref name="anderson-1987">{{cite journal |last1=Anderson |first1=J. L. |title=Geologic map of the Goldendale 15' quadrangle, Washington |journal=Washington Division of Geology and Earth Resources Open File Report |date=1987 |volume=87-15 |url=https://www.dnr.wa.gov/Publications/ger_ofr87-15_goldendale_39k.pdf |archive-url=https://web.archive.org/web/20091220013300/http://www.dnr.wa.gov/Publications/ger_ofr87-15_goldendale_39k.pdf |archive-date=2009-12-20 |url-status=live |access-date=13 August 2020}}</ref> Because of the low potassium content of most basalts, weathering converts the basalt to calcium-rich [[clay]] ([[montmorillonite]]) rather than potassium-rich clay ([[illite]]). Further weathering, particularly in tropical climates, converts the montmorillonite to [[kaolinite]] or [[gibbsite]]. This produces the distinctive tropical [[soil]] known as [[laterite]].{{sfn|Blatt|Middleton|Murray|1980|pp=254–257}} The ultimate weathering product is [[bauxite]], the principal ore of aluminium.{{sfn|Blatt|Middleton|Murray|1980|pp=263–264}}

Chemical weathering also releases readily water-soluble cations such as [[calcium]], [[sodium]] and [[magnesium]], which give basaltic areas a strong [[buffer capacity]] against [[Soil acidification|acidification]].<ref name="gillman-etal-2002">{{cite journal |last1=Gillman |first1=G. P. |last2=Burkett |first2=D. C. |last3=Coventry |first3=R. J. |title=Amending highly weathered soils with finely ground basalt rock |journal=Applied Geochemistry |date=August 2002 |volume=17 |issue=8 |pages=987–1001 |doi=10.1016/S0883-2927(02)00078-1|bibcode=2002ApGC...17..987G }}</ref> Calcium released by basalts binds [[carbon dioxide|CO<sub>2</sub>]] from the atmosphere forming [[Calcium carbonate|CaCO<sub>3</sub>]] acting thus as a CO<sub>2</sub> trap.<ref name="mcgrail-etal-2006">{{cite journal |last1=McGrail |first1=B. Peter |last2=Schaef |first2=H. Todd |last3=Ho |first3=Anita M. |last4=Chien |first4=Yi-Ju |last5=Dooley |first5=James J. |last6=Davidson |first6=Casie L. |title=Potential for carbon dioxide sequestration in flood basalts: Sequestration in flood basalts |journal=Journal of Geophysical Research: Solid Earth |date=December 2006 |volume=111 |issue=B12 |pages=n/a |doi=10.1029/2005JB004169|doi-access=free }}</ref>

=== Metamorphism ===
[[File:Archean Greenstone Pillow Lava in Michigan USA 3.jpg|thumb|Metamorphosed basalt from an [[Archean]] [[greenstone belt]] in Michigan, US. The minerals that gave the original basalt its black colour have been metamorphosed into green minerals.]]

Intense heat or great pressure transforms basalt into its [[metamorphic rock]] equivalents. Depending on the temperature and pressure of metamorphism, these may include [[greenschist]], [[amphibolite]], or [[eclogite]]. Basalts are important rocks within metamorphic regions because they can provide vital information on the conditions of [[metamorphism]] that have affected the region.{{sfn|Blatt|Tracy|1996|loc=chapter 22}}

Metamorphosed basalts are important hosts for a variety of [[hydrothermal]] [[ore]]s, including deposits of gold, copper and [[volcanogenic massive sulfide ore deposit|volcanogenic massive sulfide]]s.<ref>{{Cite journal|last1=Yardley|first1=Bruce W. D.|last2=Cleverley|first2=James S.|date=2015|title=The role of metamorphic fluids in the formation of ore deposits|journal=Geological Society, London, Special Publications|language=en|volume=393|issue=1|pages=117–134|doi=10.1144/SP393.5|bibcode=2015GSLSP.393..117Y|s2cid=130626915|issn=0305-8719|doi-access=free}}</ref>