The composition of the rocks ranges from basaltic andesite to andesite. Phenocrysts contained in Mentolat's rocks include: clinopyroxene, olivine, orthopyroxene and plagioclase. Mentolat tephras have noticeably lower potassium contents than the tephras of other volcanoes in the region and its magmas appear to originate from parental melts that contain more water and volatiles than the parental melts of the magmas of other volcanoes.