Editing Andesite (section)

=== Fractional crystallization ===
{{anchor|Basalt-andesite-rhyolite association}}
To achieve andesitic composition via [[Fractional crystallization (geology)|fractional crystallization]], a basaltic magma must crystallize specific minerals that are then removed from the melt. This removal can take place in a variety of ways, but most commonly this occurs by crystal settling. The first minerals to crystallize and be removed from a basaltic parent are [[olivine]]s and [[amphibole]]s.{{sfn|Blatt|Tracy|1996|pp=172-177}} These mafic minerals settle out of the magma, forming mafic cumulates.<ref>{{cite journal |last1=Béziat |first1=Didier |last2=Bourges |first2=François |last3=Debat |first3=Pierre |last4=Lompo |first4=Martin |last5=Martin |first5=François |last6=Tollon |first6=Francis |title=A Paleoproterozoic ultramafic-mafic assemblage and associated volcanic rocks of the Boromo greenstone belt: fractionates originating from island-arc volcanic activity in the West African craton |journal=Precambrian Research |date=May 2000 |volume=101 |issue=1 |pages=25–47 |doi=10.1016/S0301-9268(99)00085-6|bibcode=2000PreR..101...25B }}</ref> There is geophysical evidence from several arcs that large layers of mafic cumulates lie at the base of the crust.<ref>{{cite journal |last1=Hayes |first1=Jorden L. |last2=Holbrook |first2=W. Steven |last3=Lizarralde |first3=Dan |last4=van Avendonk |first4=Harm J. A. |last5=Bullock |first5=Andrew D. |last6=Mora |first6=Mauricio |last7=Harder |first7=Steven |last8=Alvarado |first8=Guillermo E. |last9=Ramírez |first9=Carlos |title=Crustal structure across the Costa Rican Volcanic Arc: CRUSTAL STRUCTURE OF THE COSTA RICAN ARC |journal=Geochemistry, Geophysics, Geosystems |date=April 2013 |volume=14 |issue=4 |pages=1087–1103 |doi=10.1002/ggge.20079|hdl=1912/6029 |s2cid=21897249 |hdl-access=free }}</ref><ref>{{cite journal |last1=DeBari |first1=Susan M. |last2=Coleman |first2=R. G. |title=Examination of the deep levels of an island arc: Evidence from the Tonsina Ultramafic-Mafic Assemblage, Tonsina, Alaska |journal=Journal of Geophysical Research: Solid Earth |date=10 April 1989 |volume=94 |issue=B4 |pages=4373–4391 |doi=10.1029/JB094iB04p04373|bibcode=1989JGR....94.4373D }}</ref> Once these mafic minerals have been removed, the melt no longer has a basaltic composition. The silica content of the residual melt is enriched relative to the starting composition. The [[iron]] and [[magnesium]] contents are depleted. As this process continues, the melt becomes more and more evolved eventually becoming andesitic. Without continued addition of mafic material, however, the melt will eventually reach a [[rhyolite|rhyolitic]] composition. This produces the characteristic basalt-andesite-rhyolite association of island arcs, with andesite the most distinctive rock type.{{sfn|Blatt|Tracy|1996|pp=172-177}}