San Pedro is a Holocene composite volcano in northern Chile and one of the tallest active volcanoes in the world. Part of the Chilean Andes' volcanic segment, it is part of the Central Volcanic Zone of the Andes, one of the four tracts of the Andean Volcanic Belt. This region of volcanism spans the borders of Peru, Bolivia, Chile and Argentina and contains a number of individual volcanic centres, including the world's two highest volcanoes Ojos del Salado and Llullaillaco.
San Pedro like other Andean volcanoes was formed by the subduction of the Nazca Plate beneath the South America Plate. It has a neighbouring volcano San Pablo and is itself formed by two separate edifices usually known as the Old Cone and the Young Cone. These edifices are formed by rocks ranging from basaltic andesite over andesite to dacite and are emplaced on a basement formed by Miocene volcanic rocks.
The Old Cone was active over one hundred thousand years ago and was eventually truncated by a giant landslide that removed its northwestern side. Within the landslide scar lava flows and pyroclastic flows constructed the Young Cone as well as the lateral centre La Poruña. This volcano was glaciated during the Pleistocene and a large Plinian eruption occurred at the beginning of the Holocene. Some eruptions occurred reportedly during historical time; presently the volcano is fumarolically active.