The Incapillo ignimbrite is formed by potassium-rich and magnesium-poor rhyodacite, forming glassy and porous pumice with individual clasts of in diameter. Typical pumice contains crystals of biotite, hornblende, plagioclase, quartz, and sanidine, with minor amounts of apatite, iron oxides, and titanite. The lava domes have uniform crystalline compositions that are richer in magnesium than the ignimbrite. Within the lava dome rocks lie phenocrysts of amphibole, biotite, plagioclase, quartz, and titanite with some alkali feldspar in some domes. Older domes have higher amphibole and lower quartz content than younger domes. Post-caldera domes are strongly hydrothermally altered.