Andesite

Andesite is an extrusive igneous rock of intermediate composition, with silica content typically around 57–63 percent by weight. It is dominated by plagioclase feldspar, usually in the range from andesine to labradorite, and one or more ferromagnesian minerals such as pyroxene or amphibole; quartz is rare in most varieties. Andesite is the volcanic counterpart of the plutonic rock diorite.

Andesite most commonly forms in subduction-related volcanic arcs, where mantle-derived magmas evolve by fractional crystallization or mixing with crustal material. This process produces andesitic magmas that feed many stratovolcanoes. It is widespread in convergent-margin settings, including the Andes, the Cascades, the Japan–Kurile arc, and Indonesia.

Texture and appearance vary from fine-grained, aphanitic varieties to porphyritic ones with larger phenocrysts of plagioclase and hornblende or pyroxene. Color is typically gray to greenish-gray, sometimes with a rusty weathering rind. Because of its intermediate viscosity, andesitic lava tends to produce lava flows that form lava domes and steep-sided volcanoes, and its eruptions can be explosive, generating pyroclastic flows and volcanic bombs.

In classification, andesite sits between basalt and rhyolite/dacite in silica content. The plutonic equivalent is diorite. Economic use of andesite is limited, but it can be quarried for crushed stone and construction materials in some regions.