Andesites

Andesite is an igneous rock of intermediate composition that forms as lava erupts at convergent plate margins. It is the volcanic equivalent of diorite and is named after the Andes mountain range.

Andesite typically contains plagioclase feldspar as the dominant mineral, with pyroxene or amphibole. Quartz is uncommon in most varieties, and minor minerals such as biotite or magnetite may be present. Its silica content is about 57–63 weight percent, placing it between basalt and rhyolite in the silica continuum. The rock is commonly subalkaline.

Textures range from fine-grained (aphanitic) to porphyritic, with early-formed phenocrysts of plagioclase and mafic minerals in a finer groundmass. Vesicular textures are common in surface flows and volcanic ejecta. Some varieties are glassy or have a mixed glassy-porphyritic appearance.

Andesite forms primarily in subduction-related environments, where fluids released from a sinking oceanic plate lower the melting point of mantle and crust, generating intermediate magmas. These magmas often evolve through fractional crystallization and interaction with crustal rocks, yielding calc-alkaline suites typical of volcanic arcs.

Distribution and significance: andesite is widespread in continental volcanic arcs, notably the Andes, the Cascades, Japan, the Philippines, and Central America. It also occurs in other arc settings. The plutonic counterpart is diorite.

Economic and practical aspects: andesitic magmatism is closely linked to hydrothermal ore deposits, including porphyry copper systems. Andesite rocks have been used locally as construction material and for decorative stone where suitable.