Basanite, basalt, benmoreite, hawaiite, phonolite, trachyandesite, trachyte and tristanite have been recovered from The Pleiades. These volcanic rocks define two separate sodium and potassium-rich magma suites and may originate from separate levels of the same magma chamber or through fractional crystallization. Ultimately, these magmas originate from a metasomatized mantle and were altered through assimilation of crustal material as they ascended. Overall, these volcanic rocks define one of the most complete magmatic series of the McMurdo Volcanic Group. It is possible that the volcanoes first erupted trachyte and later basalts, but later findings indicate that the two suites were erupted simultaneously. Phenocrysts include anorthoclase, apatite, augite, biotite, kaersutite, magnetite, oligoclase and olivine. Essexite, granodiorite, granite and syenite xenoliths also occur. Hydrothermal alteration at Taygete Cone has produced hematite and sulfur which coat and stain bleached trachyte.