Andesites

Andesites are a fine-grained, extrusive igneous rock of intermediate composition. Mineralogically, they are composed primarily of plagioclase feldspar (typically andesine or oligoclase, hence the name) and one or more mafic minerals such as pyroxene (augite and/or orthopyroxene), hornblende, and biotite. Quartz may be present in small amounts, but typically makes up less than 20% of the rock. Olivine is generally absent.

Andesites are the extrusive equivalent of diorite. The fine-grained texture indicates relatively rapid cooling at or near the Earth's surface. Porphyritic textures, where larger crystals (phenocrysts) are embedded in a fine-grained matrix (groundmass), are common. These phenocrysts represent minerals that crystallized slowly at depth before the magma erupted. Vesicular textures, resulting from the expansion of gases during eruption, are also frequently observed.

Andesites are commonly found in volcanic arcs above subduction zones, where oceanic crust is subducted beneath continental crust or another oceanic plate. They are particularly abundant along the Andes Mountains of South America, from which the rock derives its name. Other significant occurrences include island arcs in the Pacific Ocean, such as the Cascade Range in North America, and parts of Indonesia.

The chemical composition of andesites typically falls between that of basalt (mafic) and dacite (felsic). They are characterized by a silica (SiO2) content between approximately 52% and 63% by weight. The relatively high silica content contributes to their higher viscosity compared to basaltic magmas, which can lead to more explosive eruptions.

The color of andesites can vary, ranging from light gray to dark gray or even reddish-brown, depending on the relative proportions and alteration of the constituent minerals. Alteration processes, such as hydrothermal alteration, can significantly modify the original mineral assemblage, leading to the formation of secondary minerals like clay minerals, chlorite, and epidote.

Andesites play a significant role in the formation of stratovolcanoes, also known as composite volcanoes, which are characterized by their steep slopes and alternating layers of lava flows, ash, and pyroclastic debris. The explosive eruptions associated with andesitic magmas can pose significant hazards to human populations and infrastructure.