Andesite porphyry or porphyritic andesite rocks are andesite rocks with large crystals set inside a finer-grained matrix. The larger, visible crystals are phenocrysts and the fine-grained matrix groundmass.
Porphyry (pronounced as POR-fə-ree) is an igneous textural term that only indicates the presence of consistently large crystals in a finer-grained groundmass. It doesn’t tell you about the absolute grain sizes or chemical composition, or mineralogy.
Andesite porphyry or porphyritic andesite have the same mineralogy and chemistry as ordinary andesite that don’t have this texture. So, this texture will only tell you more about formation.
Let us look more into the meaning of andesite porphyry or porphyritic andesite rocks. We will also talk about formation, uses, and occurrence.
What is an andesite rock?
Andesite is light to dark grey, bluish-grey, or sometimes a reddish extrusive igneous rock with a fine-grained (aphanitic) to porphyritic texture. It is an intermediate rock with a mineral composition between granite and basalt, i.e., it has 52%-63% silica.
Andesite rocks are high in feldspar, plagioclase, and amphibole and have small amounts of pyroxene and quartz minerals. Any mica present is a form of muscovite or biotite, and they don’t usually have olivine.
Lastly, andesite is a hard rock with a Mohs hardness of 7 and a moderate to high density of 2.6 to 2.8.
What is an andesite porphyry?
Andesite porphyry refers to andesite rock with porphyritic texture – larger, visible crystals or phenocrysts of plagioclase, amphibole, or pyroxene set in fine-grained greyish to purple matrix or groundmass.
However, the most common texture of andesite rock is aphanitic (fine-grained). But it may have an amygdaloidal or vesicular texture if there is gas-charged lava.
Also, some specimens may have a considerable amount of glass when rapidly cooling. When andesite porphyry rock has large crystals set in a glassy matrix, the texture is known as vitrophyric, a variant of porphyritic texture.
If you look at an andesite porphyry rock, you should see the phenocrysts. For instance, plagioclase appears whitish to bluish-white in a dark grey fine-grained groundmass.
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On the other hand, hornblende phenocrysts will appear as black, brownish, or dark green crystals, and they tend to be darker than the groundmass, and amphibole will have a range of colors, i.e., brown, green, white, lavender, or black.
Lastly, it is not only andesite that shows porphyritic texture. Almost all igneous rock, both intrusive and extrusive, can have it. Famous examples include granite, phonolite, diorite, and rhyolite porphyry.
Naming these rocks
Sometimes, the andesite porphyry rocks are named with the predominant large crystals or phenocrysts. For instance, a hornblende andesite porphyry indicates that hornblende is the dominant large crystal or phenocryst in that rock sample.
This way of naming will be the same if amphibole or plagioclase is the dominant large crystals. So, don’t be surprised if you get names like plagioclase or amphibole andesite porphyry.
How are andesite porphyry rocks formed?
Andesite porphyry rocks form like any other rocks with porphyry or porphyritic texture, i.e., through a two-stage cooling – a slow, followed by a faster cooling rate.
The larger crystals form during slow cooling deep within the Earth’s Crust by fractional crystallization, starting with those with the highest melting temperature. And since cooling is slow, these phenocrysts will have adequate time for crystal growth. Also, they will, in most instances, have well-formed, euhedral crystal structures.
On the other hand, the fine-grained groundmass forms from faster cooling that occurs when magma (containing already crystalized phenocrysts) moves and erupts on the Earth’s surface. There is no time for large crystals to form; sometimes, you may find a considerable amount of glass where cooling is too rapid.
If the phenocrysts have a different density from the residual melt, they form cumulates (float on magma is lighter or sink to the bottom if denser). However, the violet eruption contributes to mixing the phenocrysts inside the groundmass.
Therefore, porphyritic andesites formation indicates that 1) phenocrysts formed first while the magma was still deep inside the Earth’s crust, and 2) there was an eruption or magma movement to the Earth’s surface that resulted in faster final stage crystallization.
Occurrence
Andesite porphyry will occur on volcanoes above the ocean and continental convergent plate boundaries, i.e., above a subduction zone.
Going to specific locations in the USA, porphyritic andesite occurs at Cascades (Arizona), Newbury volcanic complex in Massachusetts, Powder Mill Group, Siemens Creek Volcanics in Michigan and Wisconsin, etc.
Places outside the US include the Andes in South America and Kamchatka in Japan. Also, they occur in Tuscany, Umbria, and Eolian in Italy.
Uses
The uses of andesite porphyry include as a fill, aggregate not intended for concrete (are high in silica), and making tiles that are slip resistant. Also, you can use these rocks in landscaping or making monuments and sculptures.
References
- Universität Salzburg. (n.d.). Andesit-porphyry. Retrieved August 28, 2022, from http://chc.sbg.ac.at/sri/thesaurus/node.php?id=30
- Department of Geological Sciences. (n.d.). Porphyritic andesite. East Carolina University. Retrieved August 28, 2022, from https://geology.ecu.edu/geol1501/igneous/porphyritic-andesite/
- Porphyry (geology). (2022, August 6). In Wikipedia. https://en.wikipedia.org/w/index.php?title=Porphyry_(geology)&oldid=1102772210