Characteristics

An extrusive rock forms when magma is exposed to the air which starts rapid cooling of the magma/lava. Due to this rapid cooling, minerals don't have time to develop and form so this rock is composed of small crystals. The mineral grains can't be spotted with the naked eye.

Andesite can have a various amount of minerals present where plagioclase feldspar and amphibole minerals are abundant. There may also be some quartz, pyroxene minerals, biotite or muscovite present. A more detailed description can be found in the picture on the right. Andesite is an intrusive rock which means phenocrysts can be present. This are large crystals suspended in a matrix of fine crystals. These are present due to mixing of other rocks into the cooling parent magma.

Environment

As mentioned before is diorite the intrusive variant of andesite. Both rocks form above subduction zones of the continental crust. They often form after an oceanic plate melts during its decent into the subduction zone to produce a source of magma.

Diorite is a coarse-grained igneous rock that forms when the magma remains below Earth's surface and cools slowly. Slow cooling facilitates the growth of large mineral crystals in the rock. Andesite is a fine-grained rock that forms when the magma erupts onto the surface and crystallizes 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.[1]
References

• https://geology.com/