up to occur in the deposit and form large boulder fields. In addition to the blocks, the surface of the landslide deposit contains hummock-like hills and small topographic depressions. Part of the landslide deposit was later covered by pyroclastic flows, and this covered area is known as the Campo Amarillo. As it descended, the landslide deposit filled a shallow valley that previously existed northwest of the volcano, as well as a larger northeast-striking depression. A lava flow was rafted on the avalanche to the El Cenizal area and ended up there almost unmodified. The collapse deposit is well preserved by the arid climate, among the best preserved such deposits in the world. However, because of its sheer size its structure and stratigraphy were only appreciated with the help of remote sensing. Pleistocene lava flows and a northwest-striking drainage were buried by the landslide but can still be discerned from aerial imagery; apart from these and some hills most of the area covered by the landslide was relatively flat. At La Flexura, part of the basement beneath the avalanche crops out from the ground. Geology Regional Volcanism in the Central Volcanic Zone of the Andes is caused by the subduction of the Nazca Plate beneath the South America Plate in the Peru-Chile Trench at a rate of . It does not cause volcanism among the entire length of the trench; where the slab is subducting beneath the South America Plate at a shallow angle there is no recent volcanic activity. The style of subduction has changed over time. About 27 million years ago, the Farallon Plate which hitherto had been subducting beneath South America broke up and the pace of subduction increased, causing increased volcanism. Around the same time, after the Eocene, the subduction angle increased beneath the Altiplano and caused the development of this plateau either from magmatic underplating and/or from crustal shortening; eventually the crust there became much thicker. Local Socompa forms a northeast-trending alignment with neighbouring volcanoes such as Pular and Pajonales, which reach elevations of about ; Socompa is their youngest member. The presence of two calderas southeast and east of Socompa has been inferred. Monogenetic volcanoes were active in the area as well during the Pliocene and Quaternary and generated lava flows. One of these centres is El Negrillar just north of the collapse deposit, which was active during the Pleistocene and formed andesite-basaltic andesite lavas