The basement is formed by metamorphic and sedimentary rocks of Paleozoic to Mesozoic age. The former are particularly represented east of Cerro Blanco and go back in part to the Precambrian, the latter occur mainly west and consist of Ordovician volcano-sedimentary units. Both are intruded by granitoids and mafic and ultramafic rocks. Permian sediments and Paleogene rocks complete the nonvolcanic geology. Local tectonic structures such as borders between crustal domains and northeast-southwest trending faults might control the position of volcanic vents. Tectonic processes may also be responsible for the elliptic shape of the Cerro Blanco caldera. There is evidence of intense earthquakes during the Quaternary and some faults such as the El Peñón Fault have been recently active.