|Primary||Intermediate: plagioclase (often andesine) and pyroxene or hornblende|
|Secondary||Magnetites, biotite, sphene, and quartz|
Andesite (//) is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predominantly of sodium-rich plagioclase plus pyroxene or hornblende.
Andesite is the extrusive equivalent of plutonic diorite. Characteristic of subduction zones, andesite represents the dominant rock type in island arcs. The average composition of the continental crust is andesitic. Along with basalts, andesites are a component of the Martian crust.
Andesite is an aphanitic (fine-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%. 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 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 gray in colour, due to its content of hornblende or pyroxene minerals. but can exhibit a wide range of shading. Darker andesite can be difficult 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.
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. 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. These minerals have the highest melting temperatures of the typical minerals that can crystallize from the melt 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 × 106 cP at 1,200 °C (2,190 °F). This is slightly greater than the viscosity of smooth peanut butter. 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.
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.
Generation of melts in island arcs
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. 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".
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. 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.
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
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. These mafic minerals settle out of the magma, forming mafic cumulates. There is geophysical evidence from several arcs that large layers of mafic cumulates lie at the base of the crust. 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.
Partial melting of the crust
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. Basalt can, however, melt pelitic upper crustal material.
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. 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. 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.
Partial melting of metasomatized mantle
High-magnesium andesites (boninites) in island arcs may be primitive andesites, generated from metasomatized mantle.  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.
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.
Along with basalts, andesites are a component of the Martian crust. 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.
- List of rock types – List of rock types recognized by geologists
- Metamorphism – Change of minerals in pre-existing rocks without melting into liquid magma
- Oceanic crust – Uppermost layer of the oceanic portion of a tectonic plate
- Origins of granite – Common type of intrusive, felsic, igneous rock with granular structure
- Porphyry – Textural form of igneous rock with large grained crystals in a fine matrix
- "andesite". Merriam-Webster Dictionary.
- Macdonald, Gordon A.; Abbott, Agatin T.; Peterson, frank L. (1983). Volcanoes in the sea : the geology of Hawaii (2nd ed.). Honolulu: University of Hawaii Press. p. 127. ISBN 0824808320.
- Rudnick, Roberta L.; Fountain, David M. (1995). "Nature and composition of the continental crust: A lower crustal perspective". Reviews of Geophysics. 33 (3): 267–309. Bibcode:1995RvGeo..33..267R. doi:10.1029/95RG01302.
- Jackson, Julia A., ed. (1997). "andesite". Glossary of geology (Fourth ed.). Alexandria, Virginia: American Geological Institute. ISBN 0922152349.
- Le Bas, M. J.; Streckeisen, A. L. (1991). "The IUGS systematics of igneous rocks". Journal of the Geological Society. 148 (5): 825–833. Bibcode:1991JGSoc.148..825L. CiteSeerX 10.1.1.692.4446. doi:10.1144/gsjgs.148.5.0825. S2CID 28548230.
- "Rock Classification Scheme - Vol 1 - Igneous" (PDF). British Geological Survey: Rock Classification Scheme. 1: 1–52. 1999.
- "CLASSIFICATION OF IGNEOUS ROCKS". Archived from the original on 30 September 2011.
- Philpotts, Anthony R.; Ague, Jay J. (2009). Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. pp. 139–143. ISBN 9780521880060.
- Philpotts and Ague 2009, p. 139
- Blatt, Harvey; Tracy, Robert J. (1996). Petrology : igneous, sedimentary, and metamorphic (2nd ed.). New York: W.H. Freeman. p. 57. ISBN 0-7167-2438-3.
- Blatt and Tracy 1996, p.57
- Tilley, C. E. (1957). "Norman Levi Bowen 1887-1956". Biographical Memoirs of Fellows of the Royal Society. 3: 6–26. doi:10.1098/rsbm.1957.0002. JSTOR 769349. S2CID 73262622.
- Philpotts & Ague 2009, pp. 23, 611.
- Philpotts & Ague 2009, p. 377.
- Schmincke, Hans-Ulrich (2003). Volcanism. Berlin: Springer. p. 132. ISBN 9783540436508.
- Blatt & Tracy 1996, pp. 170–177.
- Tatsumi, Yoshiyuki; Sato, Takeshi; Kodaira, Shuichi (December 2015). "Evolution of the Earth as an andesite planet: water, plate tectonics, and delamination of anti-continent". Earth, Planets and Space. 67 (1): 91. Bibcode:2015EP&S...67...91T. doi:10.1186/s40623-015-0267-2. S2CID 59357096.
- Tatsumi, Y. (1995). Subduction Zone Magmatism. Oxford: Blackwell Scientific.[page needed]
- Eiler, J.M. (2003). Inside the Subduction Factory. San Francisco: AGU Geophysical Monograph 138.[page needed]
- Blatt & Tracy 1996, pp. 172–177.
- Béziat, Didier; Bourges, François; Debat, Pierre; Lompo, Martin; Martin, François; Tollon, Francis (May 2000). "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". Precambrian Research. 101 (1): 25–47. Bibcode:2000PreR..101...25B. doi:10.1016/S0301-9268(99)00085-6.
- Hayes, Jorden L.; Holbrook, W. Steven; Lizarralde, Dan; van Avendonk, Harm J. A.; Bullock, Andrew D.; Mora, Mauricio; Harder, Steven; Alvarado, Guillermo E.; Ramírez, Carlos (April 2013). "Crustal structure across the Costa Rican Volcanic Arc: CRUSTAL STRUCTURE OF THE COSTA RICAN ARC". Geochemistry, Geophysics, Geosystems. 14 (4): 1087–1103. doi:10.1002/ggge.20079. hdl:1912/6029. S2CID 21897249.
- DeBari, Susan M.; Coleman, R. G. (10 April 1989). "Examination of the deep levels of an island arc: Evidence from the Tonsina Ultramafic-Mafic Assemblage, Tonsina, Alaska". Journal of Geophysical Research: Solid Earth. 94 (B4): 4373–4391. Bibcode:1989JGR....94.4373D. doi:10.1029/JB094iB04p04373.
- Petford, Nick; Gallagher, Kerry (2001). "Partial melting of mafic (amphibolitic) lower crust by periodic influx of basaltic magma". Earth and Planetary Science Letters. 193 (3–4): 483–99. Bibcode:2001E&PSL.193..483P. doi:10.1016/S0012-821X(01)00481-2.
- Annen, C.; Sparks, R.S.J. (2002). "Effects of repetitive emplacement of basaltic intrusions on thermal evolution and melt generation in the crust". Earth and Planetary Science Letters. 203 (3–4): 937–55. Bibcode:2002E&PSL.203..937A. doi:10.1016/S0012-821X(02)00929-9.
- Troll, Valentin R.; Deegan, Frances M.; Jolis, Ester M.; Harris, Chris; Chadwick, Jane P.; Gertisser, Ralf; Schwarzkopf, Lothar M.; Borisova, Anastassia Y.; Bindeman, Ilya N.; Sumarti, Sri; Preece, Katie (2013-07-01). "Magmatic differentiation processes at Merapi Volcano: inclusion petrology and oxygen isotopes". Journal of Volcanology and Geothermal Research. Merapi eruption. 261: 38–49. Bibcode:2013JVGR..261...38T. doi:10.1016/j.jvolgeores.2012.11.001. ISSN 0377-0273.
- Reubi, Olivier; Blundy, Jon (2009). "A dearth of intermediate melts at subduction zone volcanoes and the petrogenesis of arc andesites". Nature. 461 (7268): 1269–1273. Bibcode:2009Natur.461.1269R. doi:10.1038/nature08510. PMID 19865169. S2CID 4417505.
- 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
- Beier, Christoph; Haase, Karsten M.; Brandl, Philipp A.; Krumm, Stefan H. (11 April 2017). "Primitive andesites from the Taupo Volcanic Zone formed by magma mixing". Contributions to Mineralogy and Petrology. 172 (5): 33. Bibcode:2017CoMP..172...33B. doi:10.1007/s00410-017-1354-0. S2CID 133574938.
- Wood, Bernard J.; Turner, Simon P. (June 2009). "Origin of primitive high-Mg andesite: Constraints from natural examples and experiments". Earth and Planetary Science Letters. 283 (1–4): 59–66. Bibcode:2009E&PSL.283...59W. doi:10.1016/j.epsl.2009.03.032.
- Mitchell, Alexandra L.; Grove, Timothy L. (23 November 2015). "Erratum to: Melting the hydrous, subarc mantle: the origin of primitive andesites". Contributions to Mineralogy and Petrology. 170 (5–6). doi:10.1007/s00410-015-1204-x.
- Blatt & Tracy 1996, p. 176.
- Day, James M. D.; Ash, Richard D.; Liu, Yang; Bellucci, Jeremy J.; Rumble, Douglas; McDonough, William F.; Walker, Richard J.; Taylor, Lawrence A. (2009). "Early formation of evolved asteroidal crust". Nature. 457 (7226): 179–82. Bibcode:2009Natur.457..179D. doi:10.1038/nature07651. PMID 19129845. S2CID 4364956.
- Cousins, Claire R.; Crawford, Ian A. (2011). "Volcano–Ice Interaction as a Microbial Habitat on Earth and Mars" (PDF). Astrobiology. 11 (7): 695–710. Bibcode:2011AsBio..11..695C. doi:10.1089/ast.2010.0550. hdl:10023/8744. PMID 21877914.
- Pavri, Betina; Head, James W.; Klose, K. Brennan; Wilson, Lionel (1992). "Steep-sided domes on Venus: Characteristics, geologic setting, and eruption conditions from Magellan data". Journal of Geophysical Research. 97 (E8): 13445. Bibcode:1992JGR....9713445P. doi:10.1029/92JE01162.