Andesite () is a
volcanic rock of
intermediate composition In igneous petrology, an intermediate composition refers to the chemical composition of a rock that has 5263 wt% SiO2 being an intermediate between felsic and mafic compositions. Typical intermediate rocks include andesite, dacite, and trachyan ...
. In a general sense, it is the intermediate type between
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
-poor
basalt and silica-rich
rhyolite. It is fine-grained (
aphanitic
Aphanites (adj. ''aphanitic''; ) are igneous rocks that are so fine-grained that their component mineral crystals are not visible to the naked eye (in contrast to phanerites, in which the crystals are visible to the unaided eye). This ge ...
) to
porphyritic in texture, and is composed predominantly of sodium-rich
plagioclase
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more p ...
plus
pyroxene or
hornblende.
Andesite is the extrusive equivalent of
plutonic diorite
Diorite ( ) is an intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metals. It is intermediate in composition between low-sil ...
. Characteristic of
subduction
Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, ...
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.
The name ''andesite'' is derived from the
Andes
The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S ...
mountain range, where this rock type is found in abundance. It was first applied by
Christian Leopold von Buch in 1826.
Description
Andesite is an
aphanitic
Aphanites (adj. ''aphanitic''; ) are igneous rocks that are so fine-grained that their component mineral crystals are not visible to the naked eye (in contrast to phanerites, in which the crystals are visible to the unaided eye). This ge ...
(fine-grained) igneous rock that is intermediate in its content of
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
and low in
alkali metals. It has less than 20% quartz and 10%
feldspathoid
The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks, and are usually not found in rocks containing ...
by volume, with at least 65% of the
feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) felds ...
in the rock consisting of
plagioclase
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more p ...
. This places andesite in the
basalt/andesite field of the
QAPF diagram
A QAPF diagram is a double ternary diagram which is used to classify igneous rocks based on mineralogic composition. The acronym QAPF stands for " Quartz, Alkali feldspar, Plagioclase, Feldspathoid (Foid)". These are the mineral groups us ...
. 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
Basaltic andesite is a volcanic rock that is intermediate in composition between basalt and andesite. It is composed predominantly of augite and plagioclase. Basaltic andesite can be found in volcanoes around the world, including in Central Am ...
, 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.
[Philpotts and Ague 2009, p. 139]
The plagioclase in andesite varies widely in sodium content, from
anorthite to
oligoclase
Oligoclase is a rock-forming mineral belonging to the plagioclase feldspars. In chemical composition and in its crystallographic and physical characters it is intermediate between albite ( Na Al Si3 O8) and anorthite ( CaAl2Si2O8). The albite: ...
, but is typically
andesine, in which
anorthite makes up about 40
mol%
In chemistry, the mole fraction or molar fraction (''xi'' or ) is defined as unit of the amount of a constituent (expressed in moles), ''ni'', divided by the total amount of all constituents in a mixture (also expressed in moles), ''n''tot. This ...
of the plagioclase. The pyroxene minerals that may be present include
augite,
pigeonite, or
orthopyroxene.
Magnetite,
zircon,
apatite,
ilmenite,
biotite, and
garnet
Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.
All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different ...
are common accessory minerals.
Alkali feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldspa ...
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
Matrix most commonly refers to:
* ''The Matrix'' (franchise), an American media franchise
** '' The Matrix'', a 1999 science-fiction action film
** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchi ...
. Phenocrysts of pyroxene or hornblende are also common.
[Blatt and Tracy 1996, p.57] These minerals have
the highest melting temperatures of the typical
mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
s 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''.
Andesitic volcanism
Andesite lava typically has a viscosity of 3.5 × 10
6 cP at . 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
A mantle wedge is a triangular shaped piece of mantle that lies above a subducting tectonic plate and below the overriding plate. This piece of mantle can be identified using seismic velocity imaging as well as earthquake maps. Subducting oceanic ...
'', 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 tectonic
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
s that the Earth has been described as an "andesite planet".
During subduction, the subducted
oceanic crust
Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic ...
is subjected to increasing pressure and temperature, leading to
metamorphism.
Hydrous
In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was underst ...
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
A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that.
Mantle may refer to:
*Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear
**Mantle (vesture), an Eastern Orthodox ve ...
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
Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmos ...
(K),
barium (Ba), and
lead
Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, ...
(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.
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
Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals ( phenocrysts) in an otherwise fine-grained groundmass. The ...
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
olivine
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers qui ...
s 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
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
and
magnesium
Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ...
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
Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals ( phenocrysts) in an otherwise fine-grained groundmass. The ...
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
Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flak ...
. Basalt can, however, melt
pelitic
A pelite ( Greek: ''pelos'', "clay") or metapelite is a metamorphosed fine-grained sedimentary rock, i.e. mudstone or siltstone. The term was earlier used by geologists to describe a clay-rich, fine-grained clastic sediment or sedimentary roc ...
upper crustal material.
Magma mixing
In continental arcs, such as the
Andes
The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S ...
, 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 (''
boninite
Boninite is an extrusive rock high in both magnesium and silica, thought to be usually formed in fore-arc environments, typically during the early stages of subduction. The rock is named for its occurrence in the Izu-Bonin arc south of Japan ...
s'') 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.
Extraterrestrial samples
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.
See also
*
*
*
*
*
References
External links
Origins of the Continental Crust, AbstractOrigins of the Continental Crust, Full paperIsland arc magmatism Experimental and Theoretical Constraints on Peridotite Partial Melting in the Mantle Wedge
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Aphanitic rocks
Porphyritic rocks
Intermediate rocks
Volcanic rocks
Subvolcanic rocks