A sample of andesite (dark groundmass) with amygdaloidal vesicules filled with zeolite. In Mineralogy, shape and size give rise to descriptive terms applied to the typical appearance or habit of Crystals The many terms used by mineralogists Zeolites (Greek zein, "to boil" lithos, "a stone" are hydrated Aluminosilicate Minerals and have a micro-porous structure Diameter of view is 8 cm.

Andesite (pronounced /ˈændəsaɪt/) is an igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. Igneous rocks (etymology from Latin ignis, fire are rocks formed by solidification of cooled Magma (molten rock Volcanic rock is an Igneous rock of volcanic origin Texture Volcanic rocks are usually fine-grained or Aphanitic to glassy in Igneous rocks (etymology from Latin ignis, fire are rocks formed by solidification of cooled Magma (molten rock Porphyry is a variety of Igneous rock consisting of large-grained Crystals such as Feldspar or Quartz, dispersed in a fine-grained The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. A mineral is a naturally occurring substance formed through geological processes that has a characteristic chemical composition a highly ordered atomic structure and specific Plagioclase is a very important series of tectosilicate Minerals within the Feldspar family The pyroxenes are a group of important rock-forming Silicate minerals found in many Igneous and metamorphic rocks. Hornblende is a complex inosilicate series of Minerals Hornblende is not a recognized mineral in its own right but the name is used as a general or field term to Biotite, quartz, magnetite, sphene are common accessory minerals. Biotite is a common phyllosilicate Mineral within the Mica group with the approximate chemical formula K(Mg Fe3AlSi3O10(F Quartz (from German) is the most abundant Mineral in the Earth 's Continental crust (although Feldspar is more common in Magnetite is not to be confused with Magnesite or Maghemite. Magnetite is a ferrimagnetic Mineral with chemical Titanite or sphene is a Calcium Titanium nesosilicate Mineral, Ca[[titanium Ti]] Si[[oxygen O]]5 Alkali feldspar may be present in minor amounts. The alkali feldspar group are those Feldspar Minerals rich in the Alkali elements Potassium and Sodium. The quartz-feldspar abundances in andesite and other volcanic rocks are illustrated in QAPF diagrams. A QAPF diagram is a double triangle Diagram which is used to classify Igneous rocks based on mineralogic composition Relative alkali and silica contents are illustrated in TAS diagrams. The TAS classification can be used to assign names to many common types of Volcanic rocks based upon the relationships between the combined alkali content and the silica content

Classification of andesites may be refined according to the most abundant phenocryst. A phenocryst is a relatively large and usually conspicuous Crystal distinctly larger than the grains of the rock groundmass of a Porphyritic Igneous Example: hornblende-phyric andesite, if hornblende is the principal accessory mineral. Hornblende is a complex inosilicate series of Minerals Hornblende is not a recognized mineral in its own right but the name is used as a general or field term to

Andesite can be considered as the extrusive equivalent of plutonic diorite. In Geology, an intrusion is a body of Igneous rock that has Crystallized from molten Magma below the surface of the Earth. Diorite (ˈdaɪəraɪt is a grey to dark grey intermediate intrusive Igneous rock composed principally of Plagioclase Feldspar (typically Andesites are characteristic of subduction zones, such as the western margin of South America. In Geology, a subduction zone is an area on Earth where two tectonic plates meet and move towards one another with one sliding underneath the other South America is a Continent of the Americas, situated entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a The name andesite is derived from the Andes mountain range. The Andes form the world's longest exposed Mountain range. They lie as a continuous chain of highland along the western coast of South America.

Genesis of andesite

Andesite is typically formed at convergent plate margins. Intermediate volcanic rocks are created via several processes:

1. Dehydration melting of peridotite and fractional crystallization
2. Melting of subducted slab containing sediments
3. Magma mixing between felsic rhyolitic and mafic basaltic magmas in an intermediate reservoir prior to emplacement or eruption. Fractional crystallization is one of the most important geochemical and physical processes operating within the Earth's crust and mantle. This page is about a volcanic rock For the ghost town see Rhyolite Nevada, and for the satellite system see Rhyolite/Aquacade. Basalt (bəˈsɔːlt ˈbeisɔːlt ˈbæsɔːlt is a common Extrusive Volcanic rock.

Via fractional crystallisation

Andesitic magma in island arc regions (i. Magma (Plurals magmas and magmata) is molten rock that sometimes forms beneath the surface of the Earth (or any other Terrestrial planet A volcanic arc is a chain of volcanic islands or Mountains formed by Plate tectonics as an oceanic Tectonic plate subducts under e. active oceanic margins) comes from the interplay of the subducting plate and the mantle wedge, the part of the overriding plate above the subducted plate.

Water in the subducted oceanic lithosphere 'boils off' from the slab by dehydration of hydrous minerals such as amphibole, zeolites, chlorite etc, which are formed in the oceanic lithosphere during hydrothermal circulation at the mid-ocean-ridge. Amphibole (pronounced amfi-bowl defines an important group of generally dark-colored rock-forming inosilicate Minerals composed of double chain SiO4 Zeolites (Greek zein, "to boil" lithos, "a stone" are hydrated Aluminosilicate Minerals and have a micro-porous structure The chlorites are a group of phyllosilicate Minerals Chlorites can be described by the following four endmembers based on their chemistry via substitution Hydrothermal circulation in its most general sense is the circulation of hot water 'hydros' in the Greek meaning water and 'thermos' meaning heat As these minerals are subjected to Barrovian Facies Sequence or Franciscan Facies Sequence metamorphism during subduction, they change to more stable, dehydrated forms, releasing water and soluble elements into the overlying wedge of mantle. Greenschist - also known as greenstone - is a general field petrologic term applied to metamorphic and/or altered Mafic Volcanic rock Blueschist (ˈbluːʃɪst is a rock that forms by the Metamorphism of Basalt and rocks with similar composition at high Pressures and low

The slab itself, or the overlying mantle wedge, may melt. If the slab melts, it may include subducted sediment as well. The water and initial slab melts rise into the mantle wedge, prompting melting of the peridotite to produce basaltic magma with a distinctive enrichment of soluble elements (e. A peridotite is a dense coarse-grained Igneous rock, consisting mostly of the minerals Olivine and Pyroxene. g. K, Ba, and Pb)compared to insoluble elements (e. g. Nb and Ti).

On its way to the surface, the melt stalls and cools, enabling the fractional crystallization of silica poor minerals, thus raising the silica content of the remaining melt and resulting in andesitic magma. The Chemical compound silicon dioxide, also known as silica or silox (from the Latin " Silex " is an Oxide

Via magma mixing

Basaltic magma may also mix with rhyolitic magma. This usually occurs in continental arc areas such as the Andes, where the high geothermal gradient above the subducted plate, and hydrothermal flows within the mantle wedge may create an underplate of softened, partially molten continental crust of intermediate or felsic composition. The Andes form the world's longest exposed Mountain range. They lie as a continuous chain of highland along the western coast of South America. The geothermal gradient is the rate of increase in temperature per unit depth in the Earth. Basaltic magmas intruded into this anomalously hot zone will prompt partial melting of the crust, and may mix with these melts to produce intermediate compositions, typically andesite to trachyte in composition.

Alternatively, the basaltic melt may heat up the overlying arc, prompting partial melting, and may even assimilate sediments, previous volcanic rocks, etcetera, whilst undergoing fractional crystallisation. These rocks are subordinate due to the difficulty in assimilating sufficient cold material by magmas without cooling to a degree that they become immobile.

Ultimately, the resultant composition of andesite and intermediate magmas is the result of fractional crystallisation, assimilation, partial melting and contamination by the subducted slab. These may take considerable effort to resolve the individual components.

See also

• List of rock types
• Continental crust and the oceanic crust
• Metamorphism
• Origins of granite
• Fractional crystallization (geology)
• Andesite line
• Porphyry

External links

• Igneous Rock Textures

References

• Origins of the Continental Crust
• Island arc magmatism
• Experimental and Theoretical Constraints on Peridotite Partial Melting in the Mantle Wedge