Andesite
sphene, and quartz |
Andesite (
Andesite is the extrusive equivalent of
The name andesite is derived from the Andes mountain range, where this rock type is found in abundance. It was first applied by Christian Leopold von Buch in 1826.[4]
Description
Andesite is an
Andesite is usually light to dark grey in colour, due to its content of hornblende or pyroxene minerals.[2] 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.[9]
The plagioclase in andesite varies widely in sodium content, from
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.[11] These minerals have the highest melting temperatures of the typical minerals that can crystallize from the melt[12] 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.
Andesitic volcanism
Andesite lava typically has a viscosity of 3.5 × 106
Block lava flows are typical of andesitic lavas from composite volcanoes. They behave in a similar manner to
Generation of melts in island arcs
Though andesite is common in other tectonic settings, it is particularly characteristic of
During subduction, the subducted
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.
Genesis
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
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.[20] These mafic minerals settle out of the magma, forming mafic cumulates.[21] There is geophysical evidence from several arcs that large layers of mafic cumulates lie at the base of the crust.[22][23] 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.[20]
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
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.[26] 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.[27] 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.[14]
Partial melting of metasomatized mantle
High-magnesium andesites (boninites) in island arcs may be primitive andesites, generated from metasomatized mantle.[28] [29] 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.[30][31][32]
Notable andesite structures
Notable stonemasonry structures built with andesite include:
- Borobudur in Java, Indonesia.[33]
- Sacsayhuamán citadel in Peru.[34][35]
- Gate of the Sun in Bolivia.[36]
- Templo Mayor ruins and other historic buildings in Mexico City, built from andesite and Tezontle basaltic andesite.[37]
Extraterrestrial samples
In 2009, researchers revealed that andesite was found in two meteorites (numbered GRA 06128 and GRA 06129) that were discovered in the
Along with basalts, andesites are a component of the
See also
- 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 – Type of igneous rock
- Porphyry – Textural form of igneous rock with large grained crystals in a fine matrix
References
- ^ "andesite". Merriam-Webster.com Dictionary.
- ^ ISBN 0824808320.
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- ^ "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.
- ^ ISBN 9780521880060.
- ^ Philpotts and Ague 2009, p. 139
- ISBN 0-7167-2438-3.
- ^ Blatt and Tracy 1996, p.57
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- ^ Philpotts & Ague 2009, pp. 23, 611.
- ^ a b Philpotts & Ague 2009, p. 377.
- ISBN 9783540436508.
- ^ Blatt & Tracy 1996, pp. 170–177.
- 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]
- ^ a b Blatt & Tracy 1996, pp. 172–177.
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- ^ Blatt & Tracy 1996, p. 176.
- ^ "Borobudur".
- ^ "Sacsayhuaman Ruins, Peru". 16 December 2021.
- ^ "Cusco - Sacsayhuaman".
- ^ "Gateway of the Sun, Tiwanaku".
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