Andesite#Andesitic volcanism

File:Basalt qapf.jpg with basalt/andesite field highlighted in yellow. Andesite is distinguished from basalt by SiO₂ > 52%.]]

File:TAS-Diagramm-andesite.png.]]

Andesite is an aphanitic (fine-grained) to porphyritic (coarse-grained) igneous rock that is intermediate in its content of silica and low in alkali metals. It has less than 20% quartz and 10% feldspathoid by volume, with at least 65% of the feldspar in the rock consisting of plagioclase. This places andesite in the basalt/andesite field of the QAPF diagram. Andesite is further distinguished from basalt by its silica content of over 52%.{{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}}{{Cite journal|date=1999|title=Rock Classification Scheme - Vol 1 - Igneous|url=http://nora.nerc.ac.uk/id/eprint/3223/1/RR99006.pdf|journal=British Geological Survey: Rock Classification Scheme|volume=1|pages=1–52}}{{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}}{{cite book |last1=Philpotts |first1=Anthony R. |last2=Ague |first2=Jay J. |title=Principles of igneous and metamorphic petrology |date=2009 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=9780521880060 |edition=2nd |pages=139–143}} However, it is often not possible to determine the mineral composition of volcanic rocks, due to their very fine grain size, and andesite is then defined chemically as volcanic rock with a content of 57% to 63% silica and not more than about 6% alkali metal oxides. This places the andesite in the O2 field of the TAS classification. Basaltic andesite, with a content of 52% to 57% silica, is represented by the O1 field of the TAS classification but is not a distinct rock type in the QAPF classification.

Andesite is usually light to dark grey in colour, due to its content of hornblende or pyroxene minerals. but can exhibit a wide range of shading. Darker andesite can be challenging to distinguish from basalt, but a common rule of thumb, used away from the laboratory, is that andesite has a color index less than 35.Philpotts and Ague 2009, p. 139

The plagioclase in andesite varies widely in sodium content, from anorthite to oligoclase, but is typically andesine, in which anorthite makes up about 40 mol% of the plagioclase. The pyroxene minerals that may be present include augite, pigeonite, or orthopyroxene. Magnetite, zircon, apatite, ilmenite, biotite, and garnet are common accessory minerals.{{cite book |last1=Blatt |first1=Harvey |last2=Tracy |first2=Robert J. |title=Petrology : igneous, sedimentary, and metamorphic. |date=1996 |publisher=W.H. Freeman |location=New York |isbn=0-7167-2438-3 |page=57 |edition=2nd}} Alkali feldspar may be present in minor amounts.

Andesite is usually porphyritic, containing larger crystals (phenocrysts) of plagioclase formed prior to the extrusion that brought the magma to the surface, embedded in a finer-grained matrix. Phenocrysts of pyroxene or hornblende are also common.Blatt and Tracy 1996, p.57 These minerals have the highest melting temperatures of the typical minerals that can crystallize from the melt{{Cite journal | last1 = Tilley | first1 = C. E. | author-link1 = Cecil Edgar Tilley| doi = 10.1098/rsbm.1957.0002 | title = Norman Levi Bowen 1887-1956 | journal = Biographical Memoirs of Fellows of the Royal Society | volume = 3 | pages = 6–26 | year = 1957 | jstor = 769349| s2cid = 73262622 }} and are therefore the first to form solid crystals. Classification of andesites may be refined according to the most abundant phenocryst. For example, if hornblende is the principal phenocryst mineral, the andesite will be described as a hornblende andesite.

Andesite lava typically has a viscosity of 3.5 million cP (3500 Pa⋅s) at {{cvt|1200|C||}}. This is slightly greater than the viscosity of smooth peanut butter.{{sfn|Philpotts|Ague|2009|pp=23, 611}} As a result, andesitic volcanism is often explosive, forming tuffs and agglomerates. Andesite vents tend to build up composite volcanoes rather than the shield volcanoes characteristic of basalt, with its much lower viscosity resulting from its lower silica content and higher eruption temperature.{{sfn|Philpotts|Ague|2009|p=377}}

File:Block lava in Lassen Volcanic National Park.jpg in Lassen Volcanic National Park]]

Block lava flows are typical of andesitic lavas from composite volcanoes. They behave in a similar manner to ʻaʻā flows but their more viscous nature causes the surface to be covered in smooth-sided angular fragments (blocks) of solidified lava instead of clinkers. As with ʻaʻā flows, the molten interior of the flow, which is kept insulated by the solidified blocky surface, advances over the rubble that falls off the flow front. They also move much more slowly downhill and are thicker in depth than ʻaʻā flows.{{cite book |last1=Schmincke |first1=Hans-Ulrich |title=Volcanism |date=2003 |publisher=Springer |location=Berlin |isbn=9783540436508|page=132}}

File:Andesite pmg ss 2006.jpg (between crossed polarizers)]]

File:Zarnov.jpg), Slovakia]]

File:Andesite pillar.jpg

Though andesite is common in other tectonic settings, it is particularly characteristic of convergent plate margins. Even before the Plate Tectonics Revolution, geologists had defined an andesite line in the western Pacific that separated basalt of the central Pacific from andesite further west. This coincides with the subduction zones at the western boundary of the Pacific Plate. Magmatism in island arc regions comes from the interplay of the subducting plate and the mantle wedge, the wedge-shaped region between the subducting and overriding plates.{{sfn|Blatt|Tracy|1996|pp=170-177}} The presence of convergent margins dominated by andesite is so characteristic of the Earth's unique plate tectonics that the Earth has been described as an "andesite planet".{{cite journal |last1=Tatsumi |first1=Yoshiyuki |last2=Sato |first2=Takeshi |last3=Kodaira |first3=Shuichi |title=Evolution of the Earth as an andesite planet: water, plate tectonics, and delamination of anti-continent |journal=Earth, Planets and Space |date=December 2015 |volume=67 |issue=1 |pages=91 |doi=10.1186/s40623-015-0267-2|bibcode=2015EP&S...67...91T |s2cid=59357096 |doi-access=free |hdl=20.500.14094/90002866 |hdl-access=free }}

During subduction, the subducted oceanic crust is subjected to increasing pressure and temperature, leading to metamorphism. Hydrous minerals such as amphibole, zeolites, or chlorite (which are present in the oceanic lithosphere) dehydrate as they change to more stable, anhydrous forms, releasing water and soluble elements into the overlying wedge of mantle. Fluxing water into the wedge lowers the solidus of the mantle material and causes partial melting.{{cite book|last1=Tatsumi|first1=Yashiyuki|last2=Eggins|first2=Steve|title=Subduction Zone Magmatism|year=1995|publisher=Blackwell Scientific|location=Oxford|isbn=086542361X}}{{page needed|date=August 2013}} Due to the lower density of the partially molten material, it rises through the wedge until it reaches the lower boundary of the overriding plate. Melts generated in the mantle wedge are of basaltic composition, but they have a distinctive enrichment of soluble elements (e.g. potassium (K), barium (Ba), and lead (Pb)) which are contributed from sediment that lies at the top of the subducting plate. Although there is evidence to suggest that the subducting oceanic crust may also melt during this process, the relative contribution of the three components (crust, sediment, and wedge) to the generated basalts is still a matter of debate.{{cite book|last=Eiler|first=J.M.|title=Inside the Subduction Factory|year=2003|publisher=AGU Geophysical Monograph 138|location=San Francisco}}{{page needed|date=August 2013}}

Basalt thus formed can contribute to the formation of andesite through fractional crystallization, partial melting of crust, or magma mixing, all of which are discussed next.

Intermediate volcanic rocks are created via several processes:

• Fractional crystallization of a mafic parent magma.
• Partial melting of crustal material.
• Magma mixing between felsic rhyolitic and mafic basaltic magmas in a magma reservoir
• Partial melting of metasomatized mantle

{{anchor|Basalt-andesite-rhyolite association}}

To achieve andesitic composition via 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 olivines and amphiboles.{{sfn|Blatt|Tracy|1996|pp=172-177}} These mafic minerals settle out of the magma, forming mafic cumulates.{{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 }} There is geophysical evidence from several arcs that large layers of mafic cumulates lie at the base of the crust.{{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 }}{{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 }} 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 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}}

Partially molten basalt in the mantle wedge moves upwards until it reaches the base of the overriding crust. Once there, the basaltic melt can either underplate the crust, creating a layer of molten material at its base, or it can move into the overriding plate in the form of dykes. If it underplates the crust, the basalt can (in theory) cause partial melting of the lower crust due to the transfer of heat and volatiles. Models of heat transfer, however, show that arc basalts emplaced at temperatures 1100–1240 °C cannot provide enough heat to melt lower crustal amphibolite.{{cite journal |doi=10.1016/S0012-821X(01)00481-2 |title=Partial melting of mafic (amphibolitic) lower crust by periodic influx of basaltic magma |year=2001 |last1=Petford |first1=Nick |last2=Gallagher |first2=Kerry |journal=Earth and Planetary Science Letters |volume=193 |issue=3–4 |pages=483–99|bibcode = 2001E&PSL.193..483P }} Basalt can, however, melt pelitic upper crustal material.{{cite journal |bibcode=2002E&PSL.203..937A |title=Effects of repetitive emplacement of basaltic intrusions on thermal evolution and melt generation in the crust |last1=Annen |first1=C. |last2=Sparks |first2=R.S.J. |volume=203 |year=2002 |pages=937–55 |journal=Earth and Planetary Science Letters |doi=10.1016/S0012-821X(02)00929-9 |issue=3–4}}

In continental arcs, such as the Andes, magma often pools in the shallow crust creating magma chambers. Magmas in these reservoirs become evolved in composition (dacitic to rhyolitic) through both the process of fractional crystallization and partial melting of the surrounding country rock.{{Cite journal|last1=Troll|first1=Valentin R.|last2=Deegan|first2=Frances M.|last3=Jolis|first3=Ester M.|last4=Harris|first4=Chris|last5=Chadwick|first5=Jane P.|last6=Gertisser|first6=Ralf|last7=Schwarzkopf|first7=Lothar M.|last8=Borisova|first8=Anastassia Y.|last9=Bindeman|first9=Ilya N.|last10=Sumarti|first10=Sri|last11=Preece|first11=Katie|date=2013-07-01|title=Magmatic differentiation processes at Merapi Volcano: inclusion petrology and oxygen isotopes|url=http://www.sciencedirect.com/science/article/pii/S0377027312003253|journal=Journal of Volcanology and Geothermal Research|series=Merapi eruption|language=en|volume=261|pages=38–49|doi=10.1016/j.jvolgeores.2012.11.001|bibcode=2013JVGR..261...38T|issn=0377-0273|url-access=subscription}} Over time as crystallization continues and the system loses heat, these reservoirs cool. In order to remain active, magma chambers must have continued recharge of hot basaltic melt into the system. When this basaltic material mixes with the evolved rhyolitic magma, the composition is returned to andesite, its intermediate phase.{{cite journal|doi=10.1038/nature08510 |title=A dearth of intermediate melts at subduction zone volcanoes and the petrogenesis of arc andesites |year=2009 |last1=Reubi |first1=Olivier| last2=Blundy |first2=Jon |journal=Nature |volume=461 |issue=7268 | pages=1269–1273|bibcode = 2009Natur.461.1269R |pmid=19865169|s2cid=4417505 }} Evidence of magma mixing is provided by the presence of phenocrysts in some andesites that are not in chemical equilibrium with the melt in which they are found.{{sfn|Philpotts|Ague|2009|p=377}}

High-magnesium andesites (boninites) in island arcs may be primitive andesites, generated from metasomatized mantle.Kelemen, P.B., Hanghøj, K., and Greene, A.R. "One View of the Geochemistry of Subduction-Related Magmatic Arcs, with an Emphasis on Primitive Andesite and Lower Crust." In Treatise on Geochemistry, Volume 3. Editor: Roberta L. Rudnick. Executive Editors: Heinrich D. Holland and Karl K. Turekian. pp. 659. {{ISBN|0-08-043751-6}}. Elsevier, 2003., p.593-659

{{cite journal |last1=Beier |first1=Christoph |last2=Haase |first2=Karsten M. |last3=Brandl |first3=Philipp A. |last4=Krumm |first4=Stefan H. |title=Primitive andesites from the Taupo Volcanic Zone formed by magma mixing |journal=Contributions to Mineralogy and Petrology |date=11 April 2017 |volume=172 |issue=5 |page=33 |doi=10.1007/s00410-017-1354-0|bibcode=2017CoMP..172...33B |s2cid=133574938 }} Experimental evidence shows that depleted mantle rock exposed to alkali fluids such as might be given off by a subducting slab generates magma resembling high-magnesium andesites.{{cite journal |last1=Wood |first1=Bernard J. |last2=Turner |first2=Simon P. |title=Origin of primitive high-Mg andesite: Constraints from natural examples and experiments |journal=Earth and Planetary Science Letters |date=June 2009 |volume=283 |issue=1–4 |pages=59–66 |doi=10.1016/j.epsl.2009.03.032|bibcode=2009E&PSL.283...59W }}{{cite journal |last1=Mitchell |first1=Alexandra L. |last2=Grove |first2=Timothy L. |title=Erratum to: Melting the hydrous, subarc mantle: the origin of primitive andesites |journal=Contributions to Mineralogy and Petrology |date=23 November 2015 |volume=170 |issue=5–6 |doi=10.1007/s00410-015-1204-x|doi-access=free |hdl=1721.1/103410 |hdl-access=free }}{{sfn|Blatt|Tracy|1996|p=176}}

File:Borobudur-Temple-Park Indonesia Stupas-of-Borobudur-01.jpg temple, Indonesia]]

File:Sacsayhuamán-21.jpg citadel, Peru]]

Notable stonemasonry structures built with andesite include:

• Borobudur in Java, Indonesia{{cite web | url=https://www.worldhistory.org/Borobudur/ | title=Borobudur }}
• Sacsayhuamán citadel in Peru{{cite web | url=https://www.journeymachupicchu.com/sacsayhuaman-ruins-peru/ | title=Sacsayhuaman Ruins, Peru | date=16 December 2021 }}{{cite web | url=https://www.megalithicbuilders.com/south-america/peru/cusco-sacsayhuaman | title=Cusco - Sacsayhuaman }}
• Gate of the Sun in Bolivia{{cite web | url=https://www.worldhistory.org/image/2374/gateway-of-the-sun-tiwanaku/ | title=Gateway of the Sun, Tiwanaku }}
• Templo Mayor ruins and other historic buildings in Mexico City, built from andesite and Tezontle basaltic andesite{{cite journal | doi=10.1007/s12665-011-1075-z | title=Natural building stones of Mexico–Tenochtitlán: Their use, weathering and rock properties at the Templo Mayor, Palace Heras Soto and the Metropolitan Cathedral | year=2011 | last1=Wedekind | first1=Wanja | last2=Ruedrich | first2=Joerg | last3=Siegesmund | first3=Siegfried | journal=Environmental Earth Sciences | volume=63 | issue=7–8 | pages=1787–1798 | bibcode=2011EES....63.1787W | s2cid=130452483 | doi-access=free }}

In 2009, researchers revealed that andesite was found in two meteorites (numbered GRA 06128 and GRA 06129) that were discovered in the Graves Nunataks icefield during the US Antarctic Search for Meteorites 2006/2007 field season. This possibly points to a new mechanism to generate andesite crust.{{cite journal |doi=10.1038/nature07651 |url=http://newswise.com/articles/view/547877/ |title=Early formation of evolved asteroidal crust |year=2009 |last1=Day |first1=James M. D. |last2=Ash |first2=Richard D. |last3=Liu |first3=Yang |last4=Bellucci |first4=Jeremy J. |last5=Rumble |first5=Douglas |last6=McDonough |first6=William F. |last7=Walker |first7=Richard J. |last8=Taylor |first8=Lawrence A. |journal=Nature |volume=457 |issue=7226 |pages=179–82 |pmid=19129845|bibcode = 2009Natur.457..179D |s2cid=4364956 |url-access=subscription }}

Along with basalts, andesites are a component of the Martian crust.{{cite journal |doi=10.1089/ast.2010.0550 |title=Volcano–Ice Interaction as a Microbial Habitat on Earth and Mars |year=2011 |last1=Cousins |first1=Claire R. |last2=Crawford |first2=Ian A. |journal=Astrobiology |volume=11 |issue=7 |pages=695–710 |pmid=21877914|bibcode = 2011AsBio..11..695C |hdl=10023/8744 |url=https://research-repository.st-andrews.ac.uk/bitstream/10023/8744/1/AST_2010_0550R2Cousins_forCE.pdf |hdl-access=free }} The presence of distinctive steep-sided domes on Venus suggests that andesite may have been erupted from large magma chambers where crystal settling could take place.{{cite journal |last1=Pavri |first1=Betina |last2=Head |first2=James W. |last3=Klose |first3=K. Brennan |last4=Wilson |first4=Lionel |title=Steep-sided domes on Venus: Characteristics, geologic setting, and eruption conditions from Magellan data |journal=Journal of Geophysical Research |date=1992 |volume=97 |issue=E8 |pages=13445 |doi=10.1029/92JE01162|bibcode=1992JGR....9713445P }}

• {{annotated link|List of rock types}}
• {{annotated link|Metamorphism}}
• {{annotated link|Oceanic crust}}
• {{annotated link|Granite#Origin|Origins of granite}}
• {{annotated link|Porphyry (geology)|Porphyry}}

{{Reflist}}

{{commons category}}

• [https://web.archive.org/web/20071104205758/http://nsw.royalsoc.org.au/journal_archive/132_34.html Origins of the Continental Crust, Abstract]
• [https://archive.org/stream/journal13113319982000roya#page/n229/mode/2up Origins of the Continental Crust, Full paper]
• [http://www.nsm.buffalo.edu/courses/gly206/SubductionMagmas.pdf Island arc magmatism]
• [http://www.nsf-margins.org/Eugene_PDF/SubFac_abstract_Gaetani.pdf Experimental and Theoretical Constraints on Peridotite Partial Melting in the Mantle Wedge] {{Webarchive|url=https://web.archive.org/web/20160305084623/http://www.nsf-margins.org/Eugene_PDF/SubFac_abstract_Gaetani.pdf |date=2016-03-05 }}
• [https://web.archive.org/web/20071208203514/http://geology.csupomona.edu/alert/igneous/texture.htm Igneous Rock Textures]

{{Igneous rocks}}

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