Socompa is a large stratovolcano (composite volcano) on the border of Argentina and Chile. It has an elevation of 6,051 metres (19,852 ft) and is part of the Chilean and Argentine Andean Volcanic Belt (AVB). Socompa is within the Central Volcanic Zone, one of the segments of the AVB, which contains about 44 active volcanoes. It begins in Peru and runs first through Bolivia and Chile, and then Argentina and Chile. Socompa lies close to the pass of the same name where the Salta-Antofagasta railway crosses the Chilean border. Most of the northwestern slope of Socompa collapsed catastrophically 7,200 years ago to form an extensive debris avalanche deposit. The Socompa collapse is among the largest known on land with a volume of 19.2 cubic kilometres (4.6 cu mi) and a surface area of 490 square kilometres (190 sq mi); its features are well-preserved by the arid climate. The deposit was at first considered to be either a moraine or a pyroclastic flow deposit, until the 1980 eruption of Mount St. Helens prompted awareness of the instability of volcanic edifices and the existence of large-scale collapses. There are large toreva blocks, which were left behind within the collapse crater. After the landslide, the volcano was rebuilt by the effusion of lava flows and much of the scar is now filled in. Socompa is also noteworthy for the high-altitude biotic communities that are bound to fumaroles on the mountain. They are well above the sparse regular vegetation in the region, which does not extend up the mountains. The climate on the mountain is cold and dry.
Geography and geomorphology Socompa is on the border between Argentina and Chile, east-southeast of the Monturaqui railway station of the Salta–Antofagasta railway. The railway crosses the border between the two countries just below Socompa, making the volcano easily accessible despite its remote location. The same pass was an important route between the two countries and reportedly between 1940 and 1970 the Carabineros de Chile had a post there. Rails and roads at Socompa go up to an elevation of 3,860 metres (12,660 ft); from there the volcano can be climbed from its southern, eastern and northern flank. The mountain is considered to be an apu by the local population, and Inca constructions have been reported either from its slopes or from its summit. The name comes from the Kunza language and may be related to socke and sokor, which mean "spring" or "arm of water". Presently, the volcano is within two protected areas. The volcano is part of the Central Volcanic Zone, one of the four volcanic zones of the Andean Volcanic Belt. This volcanic zone spans Peru, Bolivia, Chile and Argentina and contains about 44 active volcanoes and several monogenetic volcanoes and silicic caldera volcanoes. Some older inactive volcanoes are well-preserved owing to the dry climate of the region. Many of these volcanoes are in remote regions and thus are poorly studied, but pose little threat to humans. The largest historical eruption in the Central Volcanic Zone occurred in 1600 at Huaynaputina in Peru, and the recently most active volcano is Lascar in Chile. Socompa is a 6,051-metre-high (19,852 ft) composite volcano consisting of a central cone and several lava domes; it is the most voluminous conical volcano of the Central Volcanic Zone and one of the highest edifices there, rising more than 2 kilometres (1.2 mi) above the surrounding terrain. Several dacitic lava flows form the summit area of the volcano, the youngest of which originates from a summit dome. This summit dome is capped off by a summit crater at an altitude of 5,850 metres (19,190 ft), and four other craters occur northeast of the summit at altitudes of 5,600 to 5,800 metres (18,400 to 19,000 ft). Northwest of the summit, a dacitic lava dome is the source of a 500-metre-high (1,600 ft) talus slope. The summit area is surrounded by an inwards-dropping scarp that opens to the northwest and whose southern margin is buried by lava flows. Pyroclastic flows crop out beneath lava flows in the northwestern segment of the volcano, within the scarp. On the southern and eastern side there are 5 kilometres (3.1 mi) long 200–400 metres (660–1,310 ft) high cliffs; the southern scarp is about 9 kilometres (5.6 mi) long in total. A large wedge-shaped scar is recognizable on the northwestern flank, delimited by prominent scarps running through the western and northern flanks of the edifice. The existence of a lake in the summit area within the scarps at an elevation of 5,300 metres (17,400 ft) has been reported. A pumice deposit is visible on the northeastern flank. Lava domes of various shapes are recognizable on the southern and western slopes; lava flows appear mainly on the eastern and northern slopes. The whole edifice has a diameter of 16 kilometres (9.9 mi) and, like many Central Andes volcanoes, is probably made up of lava domes, lava flows and pyroclastic formations. Its volume is about 102 cubic kilometres (24 cu mi), making Socompa one of the largest stratovolcanoes with Quaternary activity. The volcano apparently developed within a northwest-striking valley, the southern part of which now contains Laguna Socompa. This lake lies at an elevation of 3,400 metres (11,200 ft); to the north the volcano is bordered by the 3,200 metres (10,500 ft) high Monturaqui basin. The water table is at depths of 100–200 metres (330–660 ft), but surface runoff is only ephemeral. Magnetotelluric investigation has identified a structure at 2–7 kilometres (1.2–4.3 mi) depth which may be Socompa's magma chamber.
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