Mountain in Argentina
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 pass of the same name where the Salta-Antofagasta railway crosses the Chilian 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 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 ; 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 and , 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 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 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 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 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 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 ; 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. 
Sector collapse.
Socompa suffered a major sector collapse during the Holocene, forming one of the largest terrestrial 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 scar as a caldera. Traces of such events are widespread on Central Andean volcanoes; Socompa's is the largest in the region and one of the better studied. 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 c. . 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 landslides 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 simulations. 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 pyroclastic fallout 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 thrusts at its foot. Evidence of such spreading of the basement under Socompa has been found. Other potential causes are earthquakes 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 generated 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.
Landslide deposit.
The collapse deposit covers a surface area of , and is thus not as large as the deposit left by the Mount Shasta or Nevado de Colima collapses. The deposit forms the Negros de Aras (also a name for the deposit) surface northwest of the volcano and the El Cenizal surface due north, where it has a hook-like surface distribution. The thickness of the deposit varies, with thin segments in the extreme southeastern and southwestern parts being less than thick and the central parts reaching .
The deposit spreads to a maximum width of and is bounded by levees 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 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 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