The terrain beneath the volcano is formed in part by the crystalline basement of Precambrian-Paleozoic (Antofalla Metamorphites) age mainly north of the volcano and often interpreted as ophiolite, and by sedimentary units of Eocene-Miocene age that crop out on its southern side and by a conglomerate unit known as the Potrero Grande Formation. Parts of the basement crop out where it have been exposed by erosion, such as in the Rio Antofalla and the Quebrada de las Minas, and more generally in two sectors north and south of the volcano.