beneath the South America Plate in the Peru-Chile Trench. Volcanic activity is localized in four major volcanic belts, the Northern Volcanic Zone, the Central Volcanic Zone (CVZ), Southern Volcanic Zone and the Austral Volcanic Zone; these are separated by belts without volcanic activity. Where volcanic activity occurs, the subduction process releases fluids from the downgoing slab which trigger the formation of melts in the mantle that eventually ascend to the surface and give rise to volcanism. Ojos del Salado is part of the Pacific Ring of Fire. The CVZ spans Peru, Bolivia, Chile and Argentina and contains about 1,100 recognized volcanoes, many of which are extremely old and are still recognizable owing to the low erosion rates in the region. Apart from stratovolcanoes, the CVZ includes numerous calderas, isolated lava domes and lava flows, maars and pyroclastic cones. Most of the volcanoes are remote and thus constitute a low hazard. Ojos del Salado is part of the CVZ and constitutes its southern boundary. South of the volcano volcanism ceased during the last six million years and until 32° south, subduction takes place at a shallow angle and volcanism is absent in the "Pampean flat-slab". The shallow angle may be a consequence of the subduction of submarine topography, such as the Copiapo Ridge at the northern and of the Juan Fernández Ridge at the southern margin of the gap. Local The basement in the region crops out in the Cordillera Claudio Gay area, and consists of sedimentary rocks of Devonian-Carboniferous age. The rocks are intruded by and covered by granites and rhyolites associated with Permian volcanic rocks and the Choiyoi Group. Oligocene to recent volcanic rocks and volcano-sedimentary formations cover this basement. The topography at Ojos del Salado bears evidence of what may have been past magmatic uplift. Seismic tomography has yielded evidence of a low seismic velocity anomaly underneath the volcano that may constitute the pathway through which water emanating from the downgoing slab rises through the mantle and gives rise to melting. Volcanism in the region commenced 26 million years ago, when the Farallon Plate broke up and subduction speed increased. Initially between 26 and 11 million years ago activity was concentrated in the Maricunga region west of the Ojos del Salado region, where only small-volume volcanism took place and which constituted the back-arc to the Maricunga arc. Between 8-5 million years ago activity declined in the Maricunga region