Socompa as seen from the Salar de Atacama.
|Elevation||6,051 m (19,852 ft)|
|Prominence||2,015 m (6,611 ft)|
|Location||Argentina - Chile|
|Last eruption||5250 BC (?)|
Socompa is a large complex stratovolcano at the border of Argentina and Chile. It is best known for its large debris avalanche deposit, which is widely accepted as the best-preserved example of this type of deposit in the world, and also notable as the home of the world's most elevated known microbial ecosystems.
The western rim of the volcano borders the Monturaqui Basin, which is draped with the aforementioned deposit. Escondida Mining currently has a network of roads throughout this area, from beneath which they pump ground water for use at the nearby copper mine. The southern margin of the deposit is bordered by the Antofagasta to Salta trans-Andean railway, although this is rarely used.
The volcano is difficult to reach - either from the north along dirt tracks south of the Miscanti Pass, or from the west via the Escondida copper mine. Both routes require a full-day's driving and for any reasonable amount of time to be spent at Socompa would need significant planning.
Socompa debris avalanche deposit 
The Socompa deposit contains many features that are expected from a debris avalanche, including large-volume, rotated and slumped toreva blocks and hummocky topography. There is also evidence for a magmatic component (Bezymianny-type collapse) from the breadcrust texture of large dacitic blocks and a thin pyroclastic flow deposit. A large amphitheatre, open at 70° and 10 km wide at its mouth, marks the site of collapse on the remaining edifice. Since the failure, which occurred some 7000 years ago, this has been partially filled by subsequent lavas and pyroclastics.
The deposit's most striking aspects are its volume, deposition and composition. It has a volume of 25 km2 (10 sq mi)—around an order of magnitude greater than the Mount St. Helens collapse—in addition to 11 km2 (4 sq mi) of toreva blocks at the mouth of the amphitheatre. These dimensions set it apart from most other known terrestrial debris avalanches.
While a significant component of the deposit clearly originates from the ancestral Socompa edifice, there are also large amounts of ignimbrite and gravels which have been shown to have come from the substrata immediately below Socompa, and which make up the bulk (80%) of the deposit by volume. Despite originating at the lowest part of the failure zone, these units travelled the furthest distance and are found at the base of the deposit. The avalanche travelled down the regional slope for part of its course before mounting at least 250 m (820 ft) of topography near to its distal end, suggesting a high speed of emplacement, low friction and great mobility. There was also considerable remobilisation of the deposits and secondary flowage after the initial deposition, creating the lobe which was channeled northwards under gravity towards the Monturaqui Basin.
The large volume and stratification of the deposit suggests that the failure was not merely the result of slope failure of the volcanic cone. Structural evidence has been interpreted by van Wyk de Vries et al. (2001) to suggest that prior to the failure, the weak underlying substrata had been spreading under the load of the volcano. The remnants of thrust anticlines at La Flexura, west of the collapse amphitheatre, delineate the western edge of this spreading zone. The suggestion is that the deforming substrata suffered catastrophic failure as a result of gravitational spreading and was ejected to the northwest on the collapse of the basal anticlines. The substrata then formed the lower horizon of the debris avalanche, upon which the remainder of the edifice was carried and deposited. As a consequence, the large volume, high fluidity and stratification of the deposit can be explained.
Prior to the 1980 eruption of Mount St. Helens, many debris avalanches were misinterpreted, and the Socompa deposit was initially linked to pyroclastic flow products of a cataclysmic eruption. It was first recognised as resulting from volcano collapse by Peter Francis and others in 1985, when they described the major features and reclassified it as a debris avalanche deposit. Subsequent works studied the deposit itself in more detail.
Ecology of Socompa 
In the 1980s, Stephan Halloy discovered patches of plant communities (primarily mosses and liverworts) near Socompa's rim. In 2009 a research team from the University of Colorado at Boulder showed that Socompa is also home to bacterial ecosystems that are sustained by gasses from volcanic vents. With respect to elevation, these are the highest known microbial ecosystems in the world.
See also 
- List of volcanoes in Argentina
- List of volcanoes in Chile
- List of Ultras of South America
- Salta–Antofagasta railway (passing through Socompa Pass)
- "Socompa". Global Volcanism Program, Smithsonian Institution. http://www.volcano.si.edu/world/volcano.cfm?vnum=1505-109.
- Deruelle, B., 1978, The Negros de Aras nuée ardente deposits: a cataclysmic eruption of Socompa volcano (Andes of Atacama, north Chile), Bulletin of Volcanology, v. 41, p. 175-186
- Francis, P.W., Gardeweg, M., Ramirez, C.F., and Rothery, D.A., 1985, Catastrophic debris avalanche deposit of Socompa volcano, northern Chile, Geology, v. 13, p. 600-603
- Wadge, G., Francis, P.W., and Ramirez, C.F., 1995, The Socompa collapse and avalanche event, Journal of Volcanology and Geothermal Research, v. 66, p. 309-336
- van Wyk de Vries, B., Self, S., Francis, P.W., and Keszthelyi, L., 2001, A gravitational spreading origin for the Socompa debris avalanche, Journal of Volcanology and Geothermal Research, v. 105, p. 225-247
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