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Andesite

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Andesite is volcanic rock named after the Andes Mountains. Intermediate in silica content, it is usually gray in color and may be fine-grained or porphyritic. Andesite is the volcanic equivalent of diorite. It consists of the plagioclase feldspar minerals andesine and oligoclase, together with one or more dark, ferromagnesian minerals such as pyroxene and biotite. Amygdaloidal andesite occurs when the voids left by gas bubbles in the solidifying magma are later filled in, often with zeolite minerals. Andesite erupts from volcanoes and is commonly found interbedded with volcanic ash and tuff. Ancient andesites are used to map ancient subduction zones because andesitic volcanoes form on continental or ocean crust above these zones.

Name origin: Rock name is after Andes – the mountain chain extending along the western coast of the southern America.

Colour: Variable, but typically bluish-grey or grey (lighter coloured than basalt).

Structure: Compact

Group – volcanic.

Texture: Aphanitic to porphyric with redish phenocrysts of garnet and plagioclase.

Alterations: Plagioclases are in places transformed to clay minerals.

Major minerals of Andesite: Plagioclase, hornblende, almandine.

Accessory minerals of Andesite:Ilmenite, apatite and orthopyroxene.

Classification

According to modal composition projected within the QAPF discrimination diagram for volcanic rock (Streckeisen, 1978), the andesite project within basalt field. However, the andezit has higher SiO2 content (> 52 wt. %) compared to that in basalt with less than 52 wt. % SiO2.

Modal composition projected within the QAPF discrimination diagram for volcanic rock (Streckeisen, 1978)

Chemical Composition of Andesite

Andesite is an intermediate sub-alkalic rock with SiO2 contents ranging between 57 and 63 wt. %, and Na2O + K2O contents around 5 wt. %. Intermediate rocks are also characterized by an increased CaO content compared to that in acidic rocks. Similar CaO contents (6 – 7 wt. %) are also typical for diorite – the plutonic equivalent of andesite. The andesite from Šiatorska Bukovinka is metaluminous, medium-potassic rock with A/CNK = 0.95 and A/NK = 2.38. The Mg/(Mg + Fe2+) ratio was recalculated after the conversion of all Fe2O3 to FeO. Trace element contents in andesites with garnets are similar to those without garnets. They only show a moderate enrichment in large lithophile elements (LILE – K, Rb, Cs, Sr, Ba), negative Nb anomaly and positive Pb anomaly pronounced in normalized records of trace elements. Such trends are typical for the magmas originating in subduction zones. Contents of rare earth elements La-Eu in garnet-bearing andesites are similar to those in garnet-free andesites. However, the garnet-bearing andesites are little depleted in heavy rare earth elements compared to the garnet-free andesites what probably reflects the garnet fractionation (Harangi et al., 2001).

Andesite Formation

Andesite generally occurs in convergent plate cages. Contains some processes in its formation.

  • Fractional crystallization of a mafic parent magma.
  • Partial melting of crustal material.
  • Magma mixing between the magmas in a magma reservoir

For the formation of andesite, a basaltic magma must then crystallize certain minerals removed from the melt. The first minerals that crystallize and emerge from a basaltic base material are olivine and amphiboles. These mafic minerals are separated from the magma and form mafic cumulates. Once these mafic minerals have been removed, the melt has no residual basaltic composition. The silica content of the melt is now enriched with respect to the starting composition. As this process continues, the melt gradually develops and eventually becomes andesitic.

In the mantle wedge section, the molten basalt moves upwards until it reaches the base of the dominant shell. Once there, the basaltic melt can underline in its shell, there may be a layer of molten material, or it may go into the top plate in the form of dams. Together, the basalt melts the material of the pelitic upper crust. It is the result of melting in the crust of island arches and andesitic magmas.

In the continental springs such as the Andes, magma is pooled in the shallow shell and forms magma chambers. As cristalization continues and the system loses heat, these reservoirs cool down in time. In order to remain active, magma chambers should have continued to reload the hot basaltic solution into the system. When this basaltic material is mixed with advanced riolitic magma, the composition is returned to the intermediate phase andesite.

Where Andesite is Located

The Andesite derives its name from Andes mountain ranges, which possess a large number of andesite rocks. Besides this mountain range, the Cordillera mountain range of North and Central America houses these rocks too.

In New Zealand, it is found in Raglan-Kawhia area, Coromandel Peninsula, Great and Little Barrier Islands, West Auckland, Western Bay of Plenty, Central North Island, and Taranaki. A few active volcanoes include Popocatépetl (Mexico), Mount Ngauruhoe (New Zealand), Mount Shasta (U.S.), Mount Adams (U.S.), Soufrière Saint Vincent (Caribbean), Krakatoa (Indonesia), Mount Fuji (Japan), Montagne Pelée (Caribbean), Bandai-san (Japan), and Mount Hood (U.S.).

Characteristics and Properties of Andesite Rock

  • Andesite, together with pyroxene, consists of plagioclase feldspar. In addition, it may contain hornblende.
  • The minerals that this rock can contain are apatite, garbet, ilmenite, biotite, magnetite, zircon. It may also contain trace amounts of alkali feldspar.
  • Silica content is moderate. In other words, this mineral is neither rich nor deficient. The silica content is 50-65%.
  • The density of such rocks is 2.11 – 2.36 g / cm3.
  • It has a porphyritic structure. The term ‘porphyric’ refers to the incorporation of large crystals into a fine-grained rock.
  • The specific gravity of this rock is 2,5 – 2,8.
  • It usually occurs in shades of gray. However, it is lighter in color than basalt.
  • It is said that thicker or dome-shaped structures are formed.
  • The hardness of andesite rocks on the Moh scale is 7.

Uses

  • It is used for making tiles because it is resistant to slipping.
  • It is used as filling material in construction and road construction.
  • Used in landscaping and garden designs.
  • Used in the construction of sculptures and monuments.

Facts

They are formed by cooling lava and are therefore classified as volcanic.

There are too many of these rocks in the continental crust.

The dark color is mistaken by basalt rock.

This rock is regarded as the equivalent of volcanic diorite, a granular crystal rock.

References

  • Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
  • Harangi, S. (2001). Neogene to Quaternary volcanism of the Carpathian-Pannonian region; a review. Acta Geologica Hungarica, 44(2), 223-258.
  • Atlas-hornin.sk. (2019). Atlas of magmatic rocks. [online] Available at: http://www.atlas-hornin.sk/en/home [Accessed 13 Mar. 2019].
Cite this article as: Geology Science. (2019). Andesite. [online] Available at: http://geologyscience.com/rocks/igneous-rocks/andesite/ [21st October 2019 ]
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