Socompa

Socompa is a large stratovolcano at the border of Argentina and Chile with an elevation of 6051 m metres. Part of the Chilean and Argentine Andean Volcanic Belt (AVB), it is within the Central Volcanic Zone, one of the various segments of the AVB. This part of the Andean volcanic belt begins in Peru and runs first through Bolivia and Chile, and then through Argentina and Chile, and contains about 44 active volcanoes. Socompa lies close to the pass of the same name, where the Salta-Antofagasta railway crosses the border.

Socompa is known for its large debris avalanche, which was formed 7,200 years ago when most of the northwestern slope collapsed and slid down, forming an extensive deposit. It was at first considered to be either a moraine or a nuee ardende deposit, until the 1980 eruption of Mount St. Helens prompted awareness of the instability of volcanic edifices and the existence of large scale collapses on them. The Socompa collapse is among the largest known with a volume of 19.2 km3 and covers a surface area of 490 km2, and its features are well preserved by the arid climate. Notable are the 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 and form well above the regular vegetation in the region. The climate on the mountain is cold and dry.

Geography and geomorphology
Socompa is situated on the border between Argentina and Chile, due east of Monturaqui. The Salta–Antofagasta 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 3860 m; 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".

The volcano is part of the Central Volcanic Zone, one of the four volcanic zones of the Andean Volcanic Belt. The Central Volcanic Zone spans Peru, Bolivia, Chile and Argentina and contains about 44 active volcanoes, plus a number of monogenetic volcanoes and silicic caldera volcanoes. A number of older inactive volcanoes are well preserved owing to the dry climate of the region. Many of these systems 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 6051 m high 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 km above the surrounding terrain. Several dacitic lava flows form the summit area of the volcano, the youngest of which originates in a summit dome. This summit dome is capped off by a summit crater at an altitude of 5850 m, and four additional craters occur northeast of the summit at altitudes of 5600 to(-). Northwest of the summit, a dacitic lava dome is the source of a 500 m high 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 the scarp is 5 km long and 200 - 400 m high, while the southern side is about 9 km long. 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 5300 m has been reported.

On the northeastern flank a pumice deposit is clearly visible. Lava domes have various shapes and are recognizable in the southern and western slopes, while lava flows appear mainly on the eastern and northern slopes. The whole edifice has a diameter of 16 km and, like many Central Andes volcanoes, is probably made up of lava domes, lava flows and various pyroclastic formations. Its volume is about 102 km3, 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 3400 m; to the north the volcano is bordered by the 3200 m high Monturaqui basin. The water table is at depths of 100 - 200 m, but surface runoff is only ephemeral. Magnetotelluric investigation has identified a structure at 2 - 7 km depth that may be Socompa's magma chamber.

Sector collapse
Socompa suffered a major sector collapse during the Holocene, forming one of the largest terrestrial collapse deposits. The deposit left by the collapse was first discovered on aerial photography in 1978 but it was correctly identified as a landslide in 1985; at first, it was interpreted as a form of moraine, then as a large pyroclastic flow and the collapse scar as a caldera.

The collapse removed about 70° (about 12 km) of Socompa's circumference on its northwestern side, descended over a vertical distance of about 3000 m and spread over distances of over 40 km, at a modelled speed of c. 100 m/s. As it descended, the landslide had sufficient energy that it was able to override topographic obstacles and climb over an elevation of about 250 m; secondary landslides occurred on the principal deposit and there is evidence that the landslide was reflected back from its margins. The collapse occurred in several steps, with 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 19.2 km3, which was dilated as it flowed and eventually ended up as a deposit with a volume of 25.7 km3; 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 6300 m high.

The collapse left a triangle-shaped collapse scar, which was partly filled by blocks left over by the collapse. The walls of the amphitheatre were about 2000 m high, so high that secondary landslides occurred. The largest of these detached from a dome northwest of the summit and descended a horizontal distance of 6 km, forming a landslide structure notable in its own right and covering about 12 km2. 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 300 m. 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, may either be a remnant of the pre-collapse volcano, or collapse debris.

A similar collapse took place in the 1980 eruption of Mount St. Helens. The occurrence of the large landslide at Mount St. Helens probably aided in the subsequent identification of the Socompa deposit as a landslide remnant. 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 collapse happened about $6,180$ years ago, it was not witnessed in historical records. This event probably lasted only 12 minutes, based on simulations. The growth rate of the volcano increased after the collapse, probably due to the mass removal unloading the magmatic system.

There is evidence in the collapse deposit that a lava flow was being erupted on the volcano when the landslide occurred, which together with the presence of pyroclastic fallout on the southwestern side of Socompa implies the collapse 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 thrusts at its foot. Evidence of such spreading of the basement under Socompa has been found.

The collapse generated a large amount of energy, about 380 PJ. 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 sector collapse events are 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.

Landslide deposit


The collapse deposit covers a surface area of 490 km2, and is thus not as large as the deposit left by the Mount Shasta collapse or by the Nevado de Colima collapse. It forms the Negros de Aras surface northwest of the volcano and the El Cenizal surface due north, where it has a hook-like surface distribution; the name "Negros de Aras" was given to the deposit before it was known that it had been formed by a landslide. The thickness of the deposit varies, with thin segments in the extreme southeastern and southwestern parts being less than 10 m thick and the central parts reaching 90 m.

The deposit spreads to a maximum width of 20 km and is bounded by levees higher than 40 m, 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 the surface morphology of the collapse. 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 itself of several subunits, one of which includes basement rocks that were integrated into the collapse as it occurred. Likewise, the El Cenizal unit entrained basement rocks such as playa 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 toreva blocks, which were torn from the mountain and came to a standstill unmodified, forming ridges up to several 100 m high; the largest such blocks are 2.5 km long and 1 km wide, and their total volume is about 11 km3. 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 25 m 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.

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 7 - 9 cm/year. Volcanism does not occur 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 6000 m; 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 unlike the eruption products of Socompa itself.

A 200 km 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 6000 m high Salín volcano neighbours Socompa; other volcanoes in the area are the 5340 m high Cerro Bayo and the 5200 m 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 250 m 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 Purilactis 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 30 m thick while the Tucucaro reaches a thickness of 5 m.

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 has erupted andesite and dacite, with dacite dominating. Phenocrysts found in the rocks of the avalanche include the minerals augite, hornblende, hypersthene, magnetite and plagioclase; dacites also contain biotite while andesites also contain olivine. In the summit area, hydrothermal alteration took place, and clay, silt and sulfur bearing rocks are also found.

Climate and ecology
There are few data on climate at Socompa. The area is windy and dry given that the volcano lies in the Desert Puna, with frequent snow cover, there are penitentes but no glaciers. The low cloud cover means that insolation is high. Weather data collected in 1991 found an average temperature of -5.5 C, a large diurnal air temperature cycle (and a larger soil temperature cycle of c. 60 - -10 C ) and low evaporation. The present-day precipitation has been estimated to be 400 mm/year, with other estimates assuming less than 200 mm/year. Periglacial landforms indicate that in the past the area was wetter, possibly thanks to the Little Ice Age. There is, however, no evidence for Pleistocene glaciation including no cirques, which may be due to the volcano's young age.

Socompa features autotrophic communities associated with fumaroles and thermal anomalies at high altitude, between 5750 - 6050 m of elevation. The autotrophic communities on Socompa are the highest known in the world, and they occur both on the actual fumaroles and on "cold fumaroles". The various species are often extremophiles since the environment on Socompa is harsh, and the communities also include heterotrophic species. Such heterotrophs include ascomycota and basidiomycota, the latter of which have noticeable similarity to Antarctic basidiomycota.

The fumaroles on Socompa also feature stands of bryophytes such as liverworts and mosses as well as lichens and algae, 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 cloud forests 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.

Eruptive history
Activity at Socompa commenced with the extrusion of andesites, which were followed later by dacites. Several Plinian eruptions have occurred on Socompa. Several dates have been obtained on rocks, including 2,000,000 ± 1,000,000, 1,300,000 ± 500,000, 800,000 ± 300,000 and less than 500,000 years ago. An age of 3,340,000 ± 600,000 years may be of an older volcano, now buried beneath the Socompa edifice. Lava domes and lava flows on the southern side of the volcano have yielded ages of 69,200 ± 6,000, 31,400 ± 3,200, 29,800 ± 3,300 and 22,100 ± 1,900 years ago. After the sector collapse 7,200 years ago, activity continued filling the collapse scar. The explosion craters on the summit are the youngest volcanic landforms on Socompa, one dome in the scar has been dated to 5,910 ± 430 years ago. An eruption 7,220 ± 100 years before present produced the El Túnel pyroclastic deposit on the western side of Socompa. The youngest eruption was dated to have occurred 5,250 years before present.

The absence of moraines 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 have been observed. The fumarolic activity occurs at at least six sites and is relatively weak; anecdotal reports indicate a smell of sulfur on the summit. Ongoing uplift of the edifice began in November 2019 and is 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 Argentina's 13th most dangerous volcano out of 38. 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 pyroclastic fallout west of the volcano, while during the other seasons fallout would be concentrated east of it.

Groundwater is warmer and richer in 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. Hot springs are found at Laguna Socompa as well. In 2011, the Chilean mining company Escondida Mining 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.