unlike the eruption products of Socompa itself. A long lineament known as the Socompa Lineament is associated with the volcano. Other volcanoes such as Cordon de Puntas Negras and the rim of the large La Pacana caldera farther north are also influenced by this lineament. A north-south trending lineament called the Llullaillaco Lineament is also linked to Socompa and to the Mellado volcano farther south. To the west Socompa is bordered by the Sierra de Alameida (or Almeida), which farther north merges into the Cordon de Lila. To the east the high Salín volcano neighbours Socompa; other volcanoes in the area are the high Cerro Bayo and the high Socompa Cairis, all of which show evidence of glacial activity unlike the younger Socompa. Basement The basement at Socompa is formed by Paleozoic and Mesozoic formations and by Quaternary sedimentary and volcanic rocks. The former crop out in the Sierra de Alameida and Alto del Inca west of Socompa and the latter as the thick Quebrada Salin Beds east of the volcano. Part of these beds were taken up into the avalanche as it collapsed and form the Flexura inliner, others appear in the Loma del Inca area north and the Monturaqui area due west of Socompa. The basement rocks are subdivided into three named formations, the Purilactus Formation of Paleozoic-Mesozoic age, the San Pedro and Tambores formations of Oligocene-Miocene age and the Miocene-Pliocene Salin formation; part of the latter formation may have been erupted by Socompa itself. The volcano is situated at the point where the Sierra de Alameida meets the Puna block. During the Pliocene this basement was covered by the Arenosa and Tucucaro ignimbrites (2.5 and 3.2 million years ago by potassium-argon dating, respectively) which also crop out west of Socompa; Socompa is probably constructed on top of these ignimbrites. The Arenosa ignimbrite is about thick while the Tucucaro reaches a thickness of . Some normal faults appear in the area north of Socompa and appear to run through the edifice. While they are not visible in the edifice itself, Socompa was uplifted on its southeastern side by the fault motion. This might have aided in the onset of edifice instability and the collapse event. In addition, directly north-northwest of Socompa lie three anticlines probably formed under the influence of the mass of both Socompa and Pajonales: The Loma del Inca, Loma Alta and La Flexura. Composition Socompa