The basement at Socompa is formed by Paleozoic and Mesozoic formations and by Quaternary sedimentary and volcanic rocks. The former crop out in the Sierra de Alameida and Alto del Inca west of Socompa and the latter as the thick Quebrada Salin Beds east of the volcano. Part of these beds were taken up into the avalanche as it collapsed and form the Flexura inliner, others appear in the Loma del Inca area north and the Monturaqui area due west of Socompa. The basement rocks are subdivided into three named formations, the Purilactus Formation of Paleozoic-Mesozoic age, the San Pedro and Tambores formations of Oligocene-Miocene age and the Miocene-Pliocene Salin formation; part of the latter formation may have been erupted by Socompa itself. The volcano is situated at the point where the Sierra de Alameida meets the Puna block.