The composite volcano of San Pedro in the arid Atacama desert of northern Chile is one of the world's highest historically active volcanoes. The basaltic andesite-to-dacitic edifice is 6 km W of the older San Pablo. The youngest cone was constructed within a large horseshoe-shaped escarpment left by the collapse of an older edifice, which produced a large debris avalanche to the west, perhaps accompanied by a major pumice eruption. Thick dacitic lava flows with steep-sided fronts mantle the upper portion of the younger cone. The youthful-looking La Poruña scoria cone on the W flank produced an 8-km-long lava flow, but Worner et al. (2000) obtained a surprisingly old Helium surface-exposure age of 103,000 years from a juvenile block of the lava flow. Reports of variable reliability mention historical eruptions in the 19th and 20th centuries.