Most of the volcanic rocks found at Cerro Blanco are rhyolites. Minerals encountered in the volcanic rocks include biotite, feldspar, ilmenite, magnetite quartz, less commonly amphibole, clinopyroxene, orthopyroxene, and rarely apatite, allanite-epidote, muscovite, titanite and zircon. Fumarolic alteration on the caldera ground has produced alunite, boehmite and kaolinite and deposited opal, quartz and silica.