Fine-grained volcanic rock (named after the Andes mountains) characterized by the presence of plagioclase feldspars of the oligoclase–andesine end of the range, and of some combination of augite, orthopyroxene, and hornblende. In chemical and mineralogical terms andesites are similar to diorites. The feldspars frequently occur as strongly zoned phenocrysts. Glassy andesites are rare. An increase in the quartz percentage marks the gradation of andesites into dacites. An increase in the percentages of alkali feldspars marks the change from andesites into trachyandesites, some of which (e.g. mugearite and hawaiite) exhibit basaltic features, including a higher olivine content. See basalt.