andesite
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an·de·site
(ăn'dĭ-zīt') 
n.
A gray, fine-grained volcanic rock, chiefly plagioclase and feldspar.
n.
A gray, fine-grained volcanic rock, chiefly plagioclase and feldspar.
[After the ANDES.]
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andesite
Britannica Concise Encyclopedia:
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andesite
Any member of a large family of rocks that occur in most of the world's volcanic areas, mainly as surface deposits and to a lesser extent as dikes and small plugs. The Andes, where the name was first applied, and most of the cordillera (parallel mountain chains) of Central and North America consist largely of andesites. They also occur in abundance in volcanoes along practically the entire margin of the Pacific basin. Andesites are most often porphyritic (having distinct crystals in a fine-grained base) rocks.
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A typical volcanic rock erupted from a volcano associated with convergent plate boundaries. The process of subduction, which defines convergent plate boundaries, pushes oceanic lithosphere beneath either oceanic lithosphere or continental lithosphere. Andesites are the principal rocks forming the volcanoes of the “ring of fire,” the arcuate chains of volcanoes which rim the Pacific Ocean basin. The Marianas and Izu-Bonin islands, the islands of Japan, the Aleutian Islands, the Cascades Range of the northwest United States, the Andes mountain chain of South America, and the Taupo Volcanic Zone of New Zealand are andesitic. See also Lithosphere; Plate tectonics; Volcano.
Andesites are mostly dark-colored vesicular volcanic rocks which are typically porphyritic (containing larger crystals set in a fine groundmass). Phenocrysts (the larger crystals) comprise plagioclase; calcium-rich, calcium-poor pyroxene; and iron-titanium oxides set in a fine-grained, frequently glassy, groundmass. Some andesites contain phenocrysts of olivine, and some contain amphibole and biotite; these latter rocks generally contain more potassium. The porphyritic nature of andesites is derived from a complicated history of magmatic crystallization and evolution as the melts rise toward the surface from deep in the Earth. Phenocryst minerals commonly are strongly zoned and show evidence for disequilibrium during growth, consistent with an origin involving crystal fractionation and mixing processes. Andesites are readily classified in terms of their silicon dioxide (SiO2) content, between 53 and 63 wt %, and potassium oxide (K2O) content at a given SiO2 content. They can also be readily discriminated on a total alkali versus SiO2 diagram. Most andesite volcanoes erupt lavas and tephras (volcanic ash) which range in composition from basaltic andesite to dacite. Eruptions are often explosive, reflecting the relatively high water and gas content of the magmas. Pyroclastic flows are a particular feature of andesite-type volcanism and are among the most dangerous of volcanic hazards. See also Basalt; Lava; Pyroclastic rocks.
Oxford Dictionary of Geography:
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andesite
A fine-grained volcanic rock, taking its name from the Andes Mountains. The andesite line is, essentially, the boundary between the basic rocks of the oceanic crust and islands and the acid rocks of the continental crust in a belt surrounding the Pacific. It is the boundary between oceanic sima and continental sial.
Cosmic Lexicon:
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Andesite
dark-colored, fine-grained extrusive igneous rock with about 52 to 63 weight percent silica (SiO2). Andesite consists mainly of plagioclase and one or more mafic minerals. The word andesite is derived from the Andes Mountains, located along the western edge of South America, where andesite rock is common.
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Very fine crystalline extrusive rock of volcanic origin composed largely of plagioclase feldspar (oligoclase or andesine) with smaller amounts of dark-colored mineral (hornblende, biotite, or pyroxene). The extrusive equivalent of diorite.
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Andesite
- For the extinct cephalopod genus, see Andesites.
| Igneous Rock | |
 Photomicrograph of andesite in thin section (between crossed polars) |
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Intermediate |
Andesite (
/ˈændəsaɪt/) is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and dacite, and ranges from 57 to 63% SiO2 as illustrated in TAS diagrams. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite, zircon, apatite, ilmenite, biotite, and garnet are common accessory minerals.[1] Alkali feldspar may be present in minor amounts. The quartz-feldspar abundances in andesite and other volcanic rocks are illustrated in QAPF diagrams.
Classification of andesites may be refined according to the most abundant phenocryst. Example: hornblende-phyric andesite, if hornblende is the principal accessory mineral.
Andesite can be considered as the extrusive equivalent of plutonic diorite. Characteristic of subduction zones, andesite represents the dominant rock type in island arcs, such as the Aleutian arc in Alaska. The average composition of the continental crust is andesitic.[2] Along with basalts they are a major component of the martian crust.[3] The name andesite is derived from the Andes mountain range.
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Contents
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Generation of melts in island arcs
Magmatism in island arc regions (i.e., active oceanic margins) comes from the interplay of the subducting plate and the mantle wedge, the wedge-shaped region between the subducting and overriding plates.
During subduction, the subducted oceanic crust is submitted to increasing pressure and temperature, leading to metamorphism. Hydrous minerals such as amphibole, zeolites, chlorite etc. (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.[4] Due to the lower density of the partially molten material, it rises through the wedge until it reach 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. K, Ba, and 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 3 components (crust, sediment, and wedge) to the generated basalts is still a matter of debate.[5]
Genesis of andesite
Andesite is typically formed at convergent plate margins but may occur in other tectonic settings. 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
Fractional crystallization
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 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.
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.[6] Basalt can, however, melt pelitic upper crustal material.[7] Andesitic magmas generated in island arcs, therefore, are probably the result of fractional crystallization rather than partial melting of the crust.
Magma mixing
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.
Andesite in space
In 2009, researchers revealed that andesite was found in two meteorites (numbered GRA 06128 and GRA 06129) that were discovered in the Graves Nunatak Icefield during the US Antarctic Search for Meteorites 2006/2007 field season. This possibly points to a new mechanism to generate andesite crust.[8]
See also
- List of rock types
- Continental crust and the oceanic crust
- Metamorphism
- Origins of granite
- Fractional crystallization (geology)
- Andesite line
- Porphyry
References
- ^ Blatt, Harvey and Robert J. Tracy, 1996, Petrology, Freeman, ISBN 0-7167-2438-3
- ^ Rudnick,R.I. and Fountain, D.M., 1995, "Nature and Composition of the Continental Crust: a lower crustal perspective." Reviews of Geophysics 33 (267-309)
- ^ Cousins, C. R. and Crawford, I. A., 2011, "Volcano-Ice Interaction as a Microbial Habitat on Earth and Mars." Astrobiology, Vol.11 No.7.
- ^ Tatsumi, Y. (1995). Subduction Zone Magmatism. Oxford: Blackwell Scientific.
- ^ Eiler, J.M. (2003). Inside the Subduction Factory. San Fransisco: AGU Geophysical Monograph 138.
- ^ Petford, N.; Gallagher, K. (2001). "Partial melting of mafic (amphibolitic) lower crust by periodic influx of basaltic magma". Earth and Planetary Science Letters.
- ^ 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: 937-955. Bibcode 2002E&PSL.203..937A. doi:10.1016/S0012-821X(02)00929-9.
- ^ Scientists Find Evidence of Asteroids with Earth-Like Crust Newswise, Retrieved on January 19, 2008.
- Origins of the Continental Crust
- Island arc magmatism
- Experimental and Theoretical Constraints on Peridotite Partial Melting in the Mantle Wedge
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