Basalt occurs at all four volcanoes. Phonolite and trachyte are found at Mount Rees and Mount Steere, the former also at Mount Frakes; Mount Rees also features rhyolite. Phenocrysts include clinopyroxene, magnetite, olivine and plagioclase. The magma erupted in the Crary Mountains originated in the mantle and underwent fractional crystallization after formation.