What is andesite?
Andesite is an extrusive igneous volcanic rock which is in between rhyolite and basalt. Texture of andesite is between aphanitic and porphyritic. Andesite is fine grained rock. Andesite lava is of moderate viscosity and forms thick lava flows and domes. Andesite is mainly composed of plagioclase with other minerals such as hornblende, pyroxene and biotite. It is volcanic equivalent of diorite.
Andesite is the name used for a family of fine-grained, extrusive igneous rocks that are usually light to dark gray in colour. They often weather to various shades of brown, and these specimens must be broken for proper examination. Andesite is rich in plagioclase feldspar minerals and may contain biotite, pyroxene, or amphibole. Andesite usually does not contain quartz or olivine.
Andesite is typically found in lava flows produced by stratovolcanoes. Because these lava cooled rapidly at the surface, they are generally composed of small crystals. The mineral grains are usually so small that they cannot be seen without the use of a magnifying device. Some specimens that cooled rapidly contain a significant amount of glass, while others that formed from gas-charged lava have a vesicular or amygdaloidal texture.
Where Does Andesite Form?
Andesite and diorite are common rocks of the continental crust above subduction zones. They generally form after an oceanic plate melts during its descent into the subduction zone to produce a source of magma. Diorite is a coarse-grained igneous rock that forms when the magma did not erupt, but instead slowly crystallised within Earth's crust. Andesite is a fine-grained rock that formed when the magma erupted onto the surface and crystallised quickly.
Andesite and diorite have a composition that is intermediate between basalt and granite. This is because their parent magmas formed from the partial melting of a basaltic oceanic plate. This magma may have received a granitic contribution by melting granitic rocks as it ascended or mixed with granitic magma.
Andesite derives its name from the Andes Mountains of South America. In the Andes it occurs as lava flows interbedded with ash and tuff deposits on the steep flanks of stratovolcanoes. Andesite stratovolcanoes are found above subduction zones in Central America, Mexico, Washington, Oregon, the Aleutian Arc, Japan, Indonesia, the Philippines, the Caribbean, and New Zealand, among other locations.
Andesite can also form away from the subduction zone environment. For example, it can form at ocean ridges and oceanic hot spots from partial melting of basaltic rocks. It can also form during eruptions at continental plate interiors where deep-source magma melts continental crust or mixes with continental magmas. There are many other environments where andesite might form.
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. 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.
Occasionally, andesites contain large, visible grains of plagioclase, amphibole, or pyroxene. These large crystals are known as "phenocrysts." They begin forming when a magma, which is cooling at depth, approaches the crystallisation temperature of some of its minerals. These high-crystallisation-temperature minerals begin forming below the surface and grow to visible sizes before the magma erupts.
When the magma erupts onto the Earth's surface, the rest of the melt crystallises quickly. This produces a rock with two different crystal sizes: large crystals that formed slowly at depth (known as "phenocrysts"), and small crystals that formed quickly at the surface (known as "groundmass"). "Andesite porphyry" is the name used for these rocks with two crystal sizes.
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 crystallisation of a mafic parent magma.
- Partial melting of crustal material.
- Magma mixing between felsic rhyolitic and mafic basaltic magma in a magma reservoir
To achieve andesitic composition via fractional crystallisation, a basaltic magma must crystallise 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 crystallise 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.
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 under-plate 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 under-plates 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 reaching 1100 - 1240 °C temperatures cannot provide enough heat to melt lower crustal amphibolite. Basalt can, however, melt pelitic upper crustal material. Andesitic magmas generated in island arcs, therefore, are probably the result of partial melting of the crust.
In continental arcs, such as the Andes, magma often pools in the shallow crust creating magma chambers. Magma in these reservoirs become evolved in composition (dacitic to rhyolitic) through both the process of fractional crystallisation and partial melting of the surrounding country rock. Over time as crystallisation 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.
Dissolved Gas and Explosive Eruptions
Some magmas that produce volcanic eruptions above subduction zones contain enormous amounts of dissolved gas. These magma can contain several percent dissolved gas by weight. This gas can have several origins, examples of which include the following:
- Water vapour produced when ocean-floor sediments on an oceanic plate are heated in a subduction zone.
- Water vapour produced when hydrous minerals dehydrate in the heat of a subduction zone.
- Carbon dioxide produced when rising magma encounters carbonate rocks, such as limestone, marble, or dolomite.
- Water vapour produced when a rising magma chamber encounters groundwater.
At depth, these gases can be dissolved in the magma like carbon dioxide dissolved in a can of cold beer. If that can of beer is shaken and suddenly depressurised by opening the can, the gas and the beer will erupt from the opening. A volcano behaves in a similar manner. A rising magma chamber instantly depressurised by a landslide, faulting, or other event can produce a similar but much larger explosive eruption.
Many volcanic plumes and ash eruptions occur when gas-charged andesitic magmas erupt. The gas pressure that causes the eruption blows large amounts of tiny rock and magma particles into the atmosphere. These particles can be blown high into the atmosphere and carried long distances by the wind. They often cause problems for aircraft operating downwind from the volcano.
Catastrophic eruptions like Mount St. Helens, Pinatubo, Redoubt, and Novarupta were produced by andesitic magma with enormous amounts of dissolved gas under high pressure. It is difficult to imagine how a magma can contain enough dissolved gas to produce one of these eruptions.
Uses of andesite
Andesite can be used as a crushed aggregate in construction but is not often choose to be ideal because of the high silica content.