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Socompa is a large stratovolcano (composite volcano) on the border of Argentina and Chile. It has an elevation of 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 Mountain 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 and a surface area of ; 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 Sector collapse. There are large , 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 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.
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 composite volcano consisting of a central cone and several ; it is the most voluminous conical volcano of the Central Volcanic Zone and one of the highest edifices there, rising more than 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 , and four other craters occur northeast of the summit at altitudes of . Northwest of the summit, a dacitic lava dome is the source of a Scree slope. The summit area is surrounded by an inwards-dropping Escarpment that opens to the northwest and whose southern margin is buried by lava flows. crop out beneath lava flows in the northwestern segment of the volcano, within the scarp. On the southern and eastern side there are long high cliffs; the southern scarp is about 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 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 and, like many Central Andes volcanoes, is probably made up of lava domes, lava flows and pyroclastic formations. Its volume is about , making Socompa one of the largest 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 ; to the north the volcano is bordered by the high Monturaqui basin. The water table is at depths of , but surface runoff is only ephemeral. Magnetotelluric investigation has identified a structure at depth which may be Socompa's magma chamber.
The event removed a 70° sector (about of circumference and of radius) on Socompa's northwestern side. The landslide descended over a vertical distance of about and spread over distances of over , at a modelled speed of . As it descended, the landslide had sufficient energy that it was able to override topographic obstacles and climb over an elevation of about ; secondary landslides occurred on the principal deposit and there is evidence that the landslide was reflected back from its margins. The event occurred in several steps, the first parts to fail ending up at the largest distances from the volcano; it is not established whether the collapse happened in a single event or as several separate failures.
The total volume of material removed was about , which was dilated as it flowed and eventually ended up as a deposit with a volume of ; thorough mixing of the avalanche material occurred as the landslide progressed. The summit of the volcano was cut by the collapse and some lava domes embedded within the volcano were exposed in the rim of the collapse amphitheatre; before the collapse the volcano was about high.
The collapse left a triangle-shaped collapse scar partly filled by leftover blocks. The walls of the amphitheatre were about high, so high that secondary occurred. The largest of these detached from a dome northwest of the summit and descended a horizontal distance of , forming a landslide structure notable in its own right and covering about . The central section of the collapse amphitheatre was not a simple collapse structure, but instead contained a secondary scarp. At the mouth of the collapse scar, the walls were lower, about . After the principal collapse, lava flows and pyroclastic flows – some of which emerge from the western rim of the collapse scar – filled up the scar left by the collapse. A structure in the scar, named Domo del Núcleo, might either be a remnant of the pre-collapse volcano, or collapse debris.
The collapse happened about years ago and is estimated to have lasted around 12 minutes, based on . The growth rate of the volcano increased, in the aftermath probably due to the mass removal unloading the magmatic system. A similar collapse took place in the 1980 eruption of Mount St. Helens. Identification of the Socompa deposit as a landslide remnant was made after the occurrence of the large landslide at Mount St. Helens drew more attention to such events. Other volcanoes have suffered from large-scale collapses as well; this includes Aucanquilcha, Lastarria and Llullaillaco. In the case of Socompa, the occurrence of the collapse was probably influenced by a northwest tilt of the basement the volcano was constructed on; it caused the volcano to slide downward in its northwestern sector and made it prone to a collapse in that direction.
The precise circumstances leading to the collapse are unknown, although there are several hypotheses. There is evidence in the deposit that a lava flow was being erupted on the volcano when the landslide occurred, which together with the presence of on the southwestern side of Socompa implies the event may have been started by volcanic activity. The quantity of water in the edifice rocks was probably minor. Another theory assumes that the volcanic edifice was destabilized by ductile and mechanically weak layers beneath Socompa; under the weight of the volcano these layers can deform and "flow" outward from the edifice, causing the formation of Thrust fault at its foot. Evidence of such spreading of the basement under Socompa has been found. Other potential causes are and the intrusion of new magma. Climatic factors for the Socompa collapse, which have been proposed as triggers for other volcanoes, are speculative.
The event released a large amount of energy, about . Some evidence in the form of tephra suggests that the collapse was accompanied by a lateral blast, but other research found no such evidence. Such events are classified as catastrophic phenomena, and the debris avalanches associated with them can reach large distances from the original volcano. The fragmentation of rocks during the landslide and the fine material generated during this process might enhance the fluidity of the avalanche, allowing it to spread far away from the source.
The deposit spreads to a maximum width of and is bounded by higher than , which are less prominent on the eastern side. As later parts of the collapse overrode the earlier segments, they formed a northeast-trending scarp in the deposit, across which there is a striking difference in its surface morphology. The landslide deposit has been stratigraphically subdivided into two units, the Monturaqui unit and the El Cenizal unit. The first unit forms most of the surface and consists of several subunits, one of which includes basement rocks that were integrated as it occurred. Likewise, the El Cenizal unit entrained basement rocks such as Dry lake deposits. The amount of basement material is noticeably large and might form as much as 80% of the landslide volume; the topography of the northwestern side of the volcano may have prevented the mass failure from being localized along the basement-edifice surface area, explaining the large volume of basement involved. Further, the basement-derived material was probably mechanically weak and thus allowed the landslide to move over shallow slopes. This basement material forms part of the white surfaces in the landslide deposit; other bright areas are formed by fumarolically altered material. The basement material was originally considered to be pumice.
The landslide deposit contains large blocks, so called , which were torn from the mountain and came to a standstill unmodified, forming ridges up to several hundred metres high; the largest such blocks are long and wide, and their total volume is about . These blocks form an almost closed semicircle at the mouth of the collapse amphitheatre and in part retain the previous stratigraphy of the volcano. Such toreva blocks are far more frequent in submarine landslides than subaerial ones and their occurrence at Socompa may reflect the relatively non-explosive nature of the collapse and material properties of the collapsed mass. Aside from the toreva blocks, individual blocks with sizes of up to occur in the deposit and form large boulder fields. As well as 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. Owing to 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.
The style of subduction has changed over time. About 27 million years ago, the Farallon Plate had been subducting beneath South America but broke up and the pace of subduction increased, leading to greater levels of volcanism. Around the same epoch, 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.
A elongated geologic structure (a 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 Cerro Bayo and the Socompa Cairis, all of which show evidence of glacial activity unlike the younger Socompa.
During the Pliocene this basement was covered by the Arenosa and Tucucaro (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, and the Tucucaro reaches a thickness of .
Some appear in the area north of Socompa and appear to run through the edifice. Although 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. Directly north-northwest of Socompa lie three probably formed under the influence of the mass of both Socompa and Pajonales: The Loma del Inca, Loma Alta and La Flexura.
Socompa features autotrophic communities associated with and thermal anomalies at high altitude, between of elevation. The autotrophic communities on Socompa are the highest known in the world, and they occur both on the actual fumaroles, on "cold fumaroles" and at a few metres from the vents. The different species are often since the environment on Socompa is harsh, and the communities also include heterotrophic species. Such heterotrophs include ascomycota and basidiomycota, the latter of which are similar to Antarctic basidiomycota.
The fumaroles on Socompa also feature stands of such as and as well as and , and animals have been found in the stands. These stands are among the highest in the world and cover noticeably large surface areas despite their elevation, and are fairly remote from other plant life in the region. There is a noticeable diversity between separate stands, and the vegetation is quite dissimilar to the vegetation in the surroundings but resembles that found in the paramo and in South America and the subantarctic islands. A sparse vegetation cover is also found on the lower slopes of Socompa. The black-headed lizard and its relative Liolaemus porosus live on its slopes, and mice have been observed in the summit area.
The absence of on Socompa suggests that volcanic activity occurred during post-glacial time. The volcano also has a young appearance, similar to historically active Andean volcanoes such as San Pedro, implying recent volcanic activity.
There is no evidence for historical activity at Socompa and the volcano is not considered an active volcano, but both fumarolic activity and the emission of carbon dioxide have been observed. The fumarolic activity occurs at at least six sites and is relatively weak; anecdotal reports indicate a smell of sulphur on the summit. Uplift of the edifice began in November 2019 and was ongoing , and could be caused by the arrival of new magma. there is no ground-based monitoring of the volcano.
Socompa is considered to be a high-risk volcano; a 2021 survey labelled it Argentina's 13th most dangerous volcano out of 38. The area is only thinly populated, and apart from the Socompa railway station and mining camps west of the volcano, there is little infrastructure that could be impacted by future eruptions. Large explosive eruptions during summer may result in west of the volcano; during the other seasons fallout would be concentrated east of it.
Groundwater is warmer and richer in carbon dioxide the closer to Socompa it is pumped, also suggesting that volcanic gas fluxes still occur at the volcano and that the volcano influences groundwater systems. are found at Laguna Socompa as well. In 2011, the copper mining company Minera Escondida was considering building a geothermal power plant on Socompa to supply energy; the Argentine Servicio Geológico Minero agency started exploration work in January 2018 for geothermal power production.
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