The basement consists of Paleozoic granites. The Laguna de Aracar Formation north of Aracar was formed by Gondwana volcanism and has been dated by K-Ar methods to be 266 ± 28 Ma old, and it is associated with the Llullaillaco Unit. Tertiary sedimentary rocks in the east and arenites in the south form the rest of the basement. The height of the volcano over the surrounding terrain is between from north to south.