Aucanquilcha

Aucanquilcha
Aucaquilcha and Auncanquilcha1
View of Aucanquilcha volcano from the northwest
Highest point
Elevation	6,176 m (20,262 ft)2
Prominence	2,145 m (7,037 ft)3
Parent peak	Huascaran
Coordinates	21°11′S 68°35′W / 21.183°S 68.583°W / -21.183; -68.5834
Naming
Etymology	Quechua: Auqa/aukka; "cruel"/"enemy", kichka/khiska; "spine"; "cruel spine"5
Pronunciation	OW-kahn-KEEL-chuh
Geography
Aucanquilcha Antofagasta
Parent range	Andes
Geology
Rock age	Up to 11 Ma (Late Miocene)
Mountain type	Stratovolcano
Rock type	Dacite
Volcanic zone	Central Volcanic Zone
Last eruption	240,000 ± 50,000 years ago (Pleistocene)6

Aucanquilcha is a massive stratovolcano located in the Antofagasta Region of northern Chile, just west of the border with Bolivia and within the Alto Loa National Reserve. Part of the Central Volcanic Zone of the Andes, the stratovolcano has the form of a ridge with a maximum height of 6,176 metres (20,262 ft). The volcano is embedded in a larger cluster of volcanoes known as the Aucanquilcha cluster. This cluster of volcanoes was formed in stages over eleven million years of activity with varying magma output, including lava domes and lava flows. Aucanquilcha volcano proper is formed from four units that erupted between 1.04–0.23 million years ago. During the ice ages, both the principal Aucanquilcha complex and the other volcanoes of the cluster were subject to glaciation, resulting in the formation of moraines and cirques.

The cluster has generated lava ranging in composition from andesite to dacite, with the main volcano being exclusively of dacitic composition. Systematic variations in temperature, crystal and biotite content have been recorded during the evolution of the cluster.

At Aucanquilcha volcano there is some fumarolic activity, and sulfur deposits are found at the summit. Several sulfur mines lie in the complex. One mine at an altitude of 5,950 metres (19,520 ft) was opened in 1913 and remained in use from 1950 to 1992. It was the world's highest mine during that period. Originally, sulfur obtained at the mine was transported down with llamas. Subsequently, an aerial cableway was employed to transport the sulfur to the town of Amincha. To bring the sulfur down, a road network to the summit was constructed in 1972, although it is now impassable.

In 1986, four men were reported to be living at an altitude of 5,900 metres (19,400 ft), making them the highest permanent residents on Earth.

Aucanquilcha is a compound volcano⁷ that rises 1.4 kilometres (0.87 mi) above an andesite platform⁸ in the El Loa Province⁹ of Chile's Antofagasta Region.¹⁰ Roads and railways pass around the volcano,⁹ a branch line of the Antofagasta-Bolivia railway going to the Collahuasi mine¹¹ passes by Aucanquilcha.¹² A road with numerous switchbacks¹³ leads up to the Aucanquilcha mine; it was built in 1972 and is now impassable because of rock falls,¹⁴ and might have made the mine on Aucanquilcha the highest point on Earth accessible by road.¹² The mountain was evaluated as a potential site for the European Southern Observatory.¹⁵

The main volcano is composed of an east-west 8-kilometre (5.0 mi) long chain of stratovolcanoes⁴ and has an estimated volume of 37 cubic kilometres (8.9 cu mi)^a,⁸ making it one of the largest volcanoes of northern Chile.¹⁷ Four peaks^b exceed altitudes of 6,000 metres (20,000 ft);¹⁹ the highest peak and summit of the Aucanquilcha volcano reaches 6,176 metres (20,262 ft)^c17 and one can see the Salar de Uyuni and other Andean volcanoes from there.²¹ Lava flows, mostly from the summit areas, are dark to grey in colour and extend as far as 2–3 kilometres (1.2–1.9 mi) from their vents. It is likely that two small lava domes (Cerro Cumbre Negra and Summit 5867) on the northwest flank occupy flank vents.²² To the north lies the 3.3 mya old andesite Cerro Tres Monos ridge; to the west lies the east-west Cerro Polan and La Luna ridge.⁸

Aucanquilcha collapsed to the northwest,²³ forming a 1 cubic kilometre (0.24 cu mi) debris avalance²⁴ that descended 2,100 metres (6,900 ft) and ran for 17 kilometres (11 mi) with an azimuth of 311°, eventually covering an area of 59 square kilometres (23 sq mi)¹⁸ to form a tongue-shaped deposit.²⁵ The flow was channeled between Miño Volcano and Cerro Cumbre Negra (an eroded lava massif) towards the dry Rio Loa valley, favouring the preservation of the slide deposit.²⁶ The slide lacks the hummocky topography usually found on debris avalanches but has radial ridges and grooves and lateral levees;²⁴ its upper sector is buried by more recent volcanic rocks and moraines.²⁷ Another landslide occurred during the Redondo stage on the eastern side of the main volcano into the Salar de Carcote. It has the classic hummocky topography of landslides and covers a length of 17 kilometres (11 mi) and a surface area of 35 square kilometres (14 sq mi), roughly half of the surface area of the Mount St Helens avalanche of 1980 and one-third of the surface area of the Ollagüe avalanche. A volume of 0.35 cubic kilometres (0.084 cu mi) is assumed based on a probable thickness of 10 metres (33 ft).²⁸²⁹

The volcano is part of a quasi-circular cluster of about 19-20 volcanoes³⁰¹⁹ between the Rio Loa and the Chile–Bolivia border,³¹ covering a surface area of 700 square kilometres (270 sq mi). salt flats and alluvial deposits surround it to the east, north and south. On its southern side lies the Cerro Chela volcano.³⁰¹⁹ The cluster lies on a 70-kilometre (43 mi) thick crust,³⁰ and arid conditions since the Miocene have preserved its structures.³¹ Its volcanoes are arranged in north-south and northwestern alignments, which may indicate a rupture of the crust above shallow magma reservoirs.³²

There are anecdotal reports of crater lakes on the mountain,³³ and a map shows a lake (El Toro) on its southern foot.³⁴ Some of the source creeks and springs of the Rio Loa are on Aucanquilcha,³⁵³⁶ forming its headwaters when they converge in Lago Seco.³⁵³⁷ The Loa drains the western and northwestern sides of the volcano; the eastern side drains into the Salar de Ollagüe salt pan, the northeastern into the Salar de Laguani, and the southeastern into the Salar de Carcote. Most valleys^d only intermittently transport water, if at all.⁴⁰

Off the western coast of South America, the Nazca Plate subducts beneath the South America Plate at a rate of 7 centimetres per year (2.8 in/year).⁴¹ As it descends, it releases fluids into the mantle that induce its melting and the onset of volcanism at the surface.⁴² This volcanism of the Andean Volcanic Belt is distributed over four arcs,⁴³ including the Northern Volcanic Zone (NVZ), the Central Volcanic Zone (CVZ) and Southern Volcanic Zone (SVZ),⁴⁴ which are separated by gaps without volcanic activity.⁴⁵ Aucanquilcha is part of the CVZ,⁸ which contains more than a thousand⁴⁵ andesitic volcanoes, ignimbrites and compound volcanoes. During the past 28 million years, it has produced more than 3,000 cubic kilometres (720 cu mi) of eruption products;⁸ the average rate of 0.11 cubic kilometres per millennium (0.026 cubic miles per millennium) is one tenth of the global maximum rate of arc magma production.⁸ Rocks are typically silicic or have intermediate compositions between silicic and mafic.⁷ The arc has migrated eastward towards the high Andes from the Pacific Ocean coast since the Jurassic⁸ and has been producing ignimbrites and stratovolcanoes since the Miocene.¹⁹ The arc features an eastward offset at the latitude of Aucanquilcha.²² Lascar is the most active volcano of the CVZ, having erupted more than thirty times since 1800;⁴³ other active volcanoes of the CVZ include Taapaca, Parinacota, Irruputuncu, Ollagüe, San Pedro, Socompa and Lastarria.⁴⁶

The crust under the CVZ reaches high thicknesses exceeding 60–70 kilometres (37–43 mi), leading to widespread contamination of ascending magmas. The basement of the Central Andes consists of volcanic rocks and marine sediments from the Mesozoic, and volcanic and volcano-sedimentary rocks from the Cenozoic; they overlie metamorphic rocks and amphibolite in the lower crust.³⁰⁸ The offset in the volcanic arc coincides with a boundary between two distinct crustal domains to the north and south⁴⁷ in the Arequipa-Antofalla terrane.⁴⁸ A fracture in the crust under Aucanquilcha might be responsible both for its elongated shape and the dominance of effusive eruptions (as it would facilitate degassing).⁴⁹ Seismic and electric tomography indicate the presence of a hot zone in the crust, which may be a developing batholith.¹⁹

The Aucanquilcha complex lies northwest of the Altiplano–Puna volcanic complex (APVC), a local large igneous province. The APVC is underpinned below at a depth of 20 kilometres (12 mi) by a slow seismic velocity zone that has been linked to the presence of 15–25% of partial melts in the zone. The Aucanquilcha complex is much smaller volumetrically than the APVC ignimbrites, but the duration of activity and the location indicate that Aucanquilcha is a subcomponent of the APVC complex.³⁰ The volcano might be a surface manifestation of copper porphyry deposits and a batholith³²-plutonic complex resembling the Tuolumne Intrusive Suite of the Sierra Nevada.⁵⁰

The rocks from the main Aucanquilcha volcano are uniformly dacitic and show little evidence of temporal variation in their composition,⁵¹ while the Aucanquilcha complex produced both andesite and dacite. Andesites appear as lava flows while dacites form lava domes and dome complexes.⁴ The rocks define a calc-alkaline suite⁵² rich in potassium (content ranges from 1.5 to 4%⁵³), typical for Central Andean volcanoes and subduction zone volcanoes in general.³¹⁸ Plagioclase is the dominant component of the magma. Clinopyroxene+orthopyroxene+amphibole (hornblende and pargasite) or amphibole+biotite+minor amounts of pyroxene are subordinate components. Apatite, ilmenite, magnetite and zircon can also be found.⁵¹⁵⁴ Rocks have a porphyritic texture and the more recent (last 1 million years) rocks contain inclusions of mafic rocks.⁵⁵ Basaltic andesites typically contain less than 10% crystals while dacites generally have more than 20% crystals. The Alconcha group lavas form two distinct families: Early lavas are crystal-poor and lack biotite, later rocks contain biotite and more crystals.⁵⁶ There is evidence of magma mixing and mingling,⁵¹³⁰ and of multiple generations of crystals being sampled from the magma reservoir⁵⁷ especially during inactive periods.⁵⁸ The magmas form at 5–20 kilometres (3.1–12.4 mi) depth.⁵⁹

Various parts of the main volcano have been subjected to hydrothermal alteration²⁸ and alteration by fumarolic activity, especially the central parts of the complex⁴ and the Azufrera stage edifice.²⁸ The central part of the complex is heavily altered by fumarole activity.⁴ Hydrothermal alteration may be driven by the formation of a deep magma reservoir and resulting hydrothermal circulation in overlying rocks.⁶⁰ The summit area and the area between the Azufrera and Angulo complex feature sulfur rich talus deposits,²⁸ including pyrite⁶¹ and hypogene-derived copper sulfides such as covellite⁶² that were emplaced before the sulfur deposits.⁶³ The copper sulfides found in the area appear to have formed through postvolcanic epithermal mineralization above deep porphyry copper mineralization.⁶⁴ The sulfur itself formed at temperatures of 450 °C (842 °F)⁶⁵ in a now extinct fumarole,⁶⁶ although Ahlfeld 1947 reported ongoing formation of sulfur crystals in a cave on the summit.⁶⁷

The Aucanquilcha complex had a fairly stable magma output between 12 and 6 million years ago, followed by an acceleration that resulted in a peak between 4 and 1 million years ago (with a possible hiatus between 2 and 1 million years ago).⁶⁸ The long-term magma output of Aucanquilcha is comparable to the magma output of other long-term active volcanoes in the central Andes such as Ollagüe and Llullaillaco. In all such cases, an early peak in magma output is followed by later lower-volume activity (0.1 to 0.2 cubic kilometres per millennium (0.024 to 0.048 cubic miles per millennium), followed by 0.01 to 0.02 cubic kilometres per millennium (0.0024 to 0.0048 cubic miles per millennium)). Unzen in Japan and Mount Duff and Lassen Peak in California have similar eruption histories.⁶⁹ Such decreases may occur because of the lithostatic load imposed by the edifices on the magma chambers and the increased travel distance of the magma through the edifice.⁷⁰ The comparatively low output of Aucanquilcha might reflect the fact that it is located at the edge of the Altiplano-Puna volcanic complex,⁷¹ and might explain why no major caldera-forming eruption ever took place.⁷²

The repeated formation of magma chambers heated the crust under the volcano, warming the magmatic system.⁷³ This crustal feedback and increased deep crustal influx of mantle-derived basalts drove increased magma production,³² and ephemeral shallow magma chambers formed which were supplied from the deeper reservoir.⁷⁴ Eruption rates and hydrothermal activity increased,⁷⁵ and crystal populations in the magmatic system shifted.⁷⁶ 4 million years ago the magmatic system became centralized⁷⁷ and stable, allowing the continuous production of dacitic magmas.⁷⁸ Dacite built up under the volcano and formed a barrier to the ascent of denser magmas,⁷¹ so that andesite was spatially segregated with the peripheral Miño Volcano while the more central volcanoes erupted dacites.⁷⁹ The Aucanquilcha volcanic complex has been interpreted as the surface component of a developing pluton.⁵⁵

Crustal rocks were absorbed in the magma, especially the more evolved (dacitic) ones⁸⁰ and during declining stages.⁸¹ Magma and included crystals being drawn from depths of 3 to 23 kilometres (1.9 to 14.3 mi) during the main Aucanquilcha stage of volcanism,³⁰⁸² with evidence of stronger crustal contributions or of a change of their composition.⁸⁰ Based on geothermometric data, the highest temperatures occurred during phases of high activity and lower temperatures are associated with low output periods.⁷³

Volcanic activity of the Aucanquilcha cluster commenced 11 million years ago,⁷ increasing 6-5 million years ago at the same time as magmas becoming more uniform in composition and strong hydrothermal alteration commenced. The Aucanquilcha cluster generated about 340 ± 20 cubic kilometres (81.6 ± 4.8 cu mi) of volcanic rock,³⁰ with each centre being active for a few hundred thousand years.⁷¹ Over time, centres became less distributed and their magmas more uniform.⁸³ Activity waned 2-1 million years ago,³⁰⁸² the Aucanquilcha volcano proper may be a waning stage of the overall cluster.³² Both the timing⁸⁰ and the duration of volcanic activity of the Aucanquilcha cluster are comparable to that of the Altiplano–Puna volcanic complex.⁸⁴

Aucanquilcha itself began erupting about 1 million years ago, producing mostly lava flows with little variation in eruption style⁸⁵ and with a prolonged decline after an initial phase of high output.⁸⁶ Eruptions occurred every 50,000-100,000 years, with breaks reaching 250,000 years.⁸⁷The age of the lava flows ranges from heavily glaciated andesite flows overlying less-eroded ones to possibly postglacial lava flows that may be tens of thousands of years old.⁴ The site of volcanic activity migrated over time.⁸⁸

Both the Aucanquilcha cluster and Aucanquilcha proper formed in four stages each:

• The Alconcha group, with seven volcanoes, is constructed from andesite and dacite and formed 11–8 mya. It is constructed from two northern composite cones, Alconcha and Volcan Tuco (also known as Cerro Garage, dated 10.96–10.51 mya), and five lava domes on the northeastern side of the cluster. Alconcha has a large breach on the southern side of its crater that is likely the product of a flank collapse⁷⁹ but the avalanche deposit may be buried beneath younger material. Lavas within the breach are dated 10.78–10.43 mya. The centres of Volcan Tuco and Alconcha are heavily eroded, and Alconcha's lavas and scoria lie on top of Tuco. The Ujina ignimbrite was erupted 9.4 mya from an unknown vent and has a volume of 2 cubic kilometres (0.48 cu mi) of dacite. While the vent location is unknown, the composition of the ignimbrite, and its dating and distribution, indicate an association with this group. The domes are poorly researched, with the Coscalito dome dated 8.9–8.7 mya and Cerro Amincha 8.01.⁸⁹ The total volume of this group is 46 cubic kilometres (11 cu mi), indicating a flux rate of 0.013 cubic kilometres per millennium (0.0031 cubic miles per millennium).⁹⁰
• The Gordo group, which erupted 6–4 mya following a probable 2-million-year hiatus,⁷⁹ is located in the southern and western parts of the cluster. Cerro Gordo itself (5.49 mya) has a crater that is breached westwards, exposing about twelve radial dykes but with no trace of a debris avalanche. One of the larger centres of the cluster, Gordo is associated with a lava field on its western side that is dated 4.9 mya. Cerro Puquíos and Cerro Negro (5.81–5.28 mya) lie on the southern side of the cluster, and glacial cirques cutting their northeastern flanks reveal layers of scoria and lavas. Puquíos has an amphitheatre structure on its western side. Paco Paco (4.41–4.27 mya) is located north of most Gordo group volcanoes. It forms a 4 kilometres (2.5 mi) wide stratocone with a lava-filled crater, and layers of scoria and agglutinated lavas dip from it. Volcan Pabellón (4.14–4.12 mya) sits southwest of the Puquíos-Negro ridge. The Las Bolitas lava field (5.23–5.13 mya) is associated with the Gordo group but the vent locations are unknown.⁶⁸ The total volume of this group is 55 cubic kilometres (13 cu mi), indicating a flux rate of 0.027 cubic kilometres per millennium (0.0065 cubic miles per millennium).⁹⁰
• The 3.6–2.3 mya Polan group,⁶⁸ with ten dispersed volcanoes including Miño Volcano, is the largest group in the cluster and includes Tres Monos, La Luna, Cerro Polan, Chaihuiri, Miño Volcano and the lavas of the Aucanquilcha platform. Cerro Polan's (3.5–3 mya) eastern side is deeply dissected, and the exposed materials are heavily altered in the deeper sections. Lava fields to Polan's west and southwest (2.6 mya on one western field) are associated with it. La Luna (2.97–2.57) lies just east of Polan; these two volcanoes were probably one volcano in the past. La Luna has a lava dome surrounded by a glaciated but unaltered lava table. Cerro Tres Monos (3.4–2.78 mya) forms a northbound 14-kilometre (8.7 mi) long ridge with at least six vents. Hydrothermal alteration has affected some lavas and pyroclastics from Tres Monos, and the western side has lateral and terminal moraines. The Aucanquilcha platform (3.6–2.7 mya) sits underneath the main Aucanquilcha volcano, and its lava mostly flowed north. Its southern side is a 4,500-metre (14,800 ft) table with one hill, Cerro Campana, dated to 3.3 mya. The platform presumably forms one third of the total volume of the Aucanquilcha cluster⁹¹ and may have originated from a part of the ridge of the La Luna-Polan trend, now buried beneath Aucanquilcha. Chaihuiri (2.39 mya) is a lava dome with moraines and two short lava flows; it is the youngest Polan group volcano.²⁹ The total volume of this group is 154 cubic kilometres (37 cu mi), indicating a flux rate of 0.077 cubic kilometres per millennium (0.018 cubic miles per millennium).⁹⁰
• The main Aucanquilcha volcano is the youngest volcano in the cluster⁷ and formed in four stages:
  • Between 1.04–0.92 mya the bulk of the volcano formed in the Azufrera stage.²² One lava flow toward the southwest is unusually long at 6 kilometres (3.7 mi). A vent at 6,116 metres (20,066 ft) altitude fed most of this edifice; a second vent on the northern flank at the 5,887 metres (19,314 ft) summit generated three lava flows, two shorter ones and a long one to the northwest. The Azufrera stage lavas are blocky dacites with large clasts and flow fronts up to 20 metres (66 ft) high. These flows are moderately altered and have oxidation rinds. There is little evidence of explosive activity, but it may have been obscured by glacial erosion.²⁸ The total volume is 21.1 cubic kilometres (5.1 cu mi), indicating a flux rate of 0.16 cubic kilometres per millennium (0.038 cubic miles per millennium).⁵¹ This volcano was probably an isolated cone, but the existence of a previous stage cannot be excluded.⁹²
  • The second stage, named Rodado, lasted from 0.95 to 0.85 mya. It formed on the eastern slopes of the Azufrera volcano, with one vent at the 6,073 metres (19,925 ft) summit. Rodado stage lavas are blocky and platy and usually thicker than Azufrera stage lavas. Some of the summit vent lavas are among the most vesicular of this stage. They are also less weathered (oxidation rinds are c. 1 centimetre (0.39 in) thick) and less subject to solfataric alteration. The Cerro Chinchillas lavas are the oldest of this stage; erupted from an unknown vent, they lack amphiboles.²⁸ The total volume is 9.1 cubic kilometres (2.2 cu mi), indicating a flux rate of 0.09 cubic kilometres per millennium (0.022 cubic miles per millennium).⁵¹ A flank collapse, possibly triggered by a large earthquake, occurred during this stage.⁹²
  • The third stage is the Cumbre Negra stage, named after the westernmost summit and principal vent of this stage, Cerro Cumbre Negra (5,670 metres (18,600 ft)). The time course of its activity is less defined than the previous two stages; it may have occurred between 1–0.47 mya, but most likely 0.6–0.5 mya based on potassium–argon dating. Four lava flows derived from the main vent, all less than one kilometre long and 40–60 metres (130–200 ft) thicker than previous stages. They all have hydration rinds but no native sulfur deposits. This stage generated Aucanquilcha's only pyroclastic flow^e during a lava dome collapse as occurred on Merapi in Indonesia.²⁸ The total volume of this stage is 0.7 cubic kilometres (0.17 cu mi), indicating a flux rate of 0.005 cubic kilometres per millennium (0.0012 cubic miles per millennium).⁵¹
  • The youngest stage, known as Angulo, lasted from 0.66 to 0.24 mya. It was centered between the Azufrera and Rodado stage edifices 0.35–0.23 mya.²⁸ Most lava flows from this stage originate on a 1-kilometre (0.62 mi) long ridge that includes Aucanquilcha's highest summit. One crater on the northeast side of the ridge fed lavas to the north. Other than that, most flows extend southwards 4 to 9 kilometres (2.5 to 5.6 mi) from the vent, and with the exception of a due south flow 50 metres (160 ft) thick they are thin, with thicknesses of 15–20 metres (49–66 ft). One of the oldest flows has been compared in length to the 50% longer Chao Dacite flow but is much thinner. The flows from this stage are weakly weathered and partially overlie glacial deposits. The total volume is 5.8 cubic kilometres (1.4 cu mi), indicating a flux rate of 0.015 cubic kilometres per millennium (0.0036 cubic miles per millennium).⁵¹

The last dated eruption occurred 450,000±70,000 years ago.⁹³ Some scoria cones of basaltic composition, including Poruñita and Luna de Tierra, formed between Aucanquilcha and Ollagüe after Aucanquilcha's last activity.⁸³ There is no clear evidence of Holocene activity,⁹⁴ except possibly for lava flows overlying moraines,⁹²¹⁷ and no known historical eruptions⁹⁵ or ongoing ground deformation.⁹⁶ There are active fumaroles¹⁷ or hot ground,⁶¹ most conspicuous in the trenches dug during sulfur mining operations.^87f The emitted gases have temperatures of 60–85 °C (140–185 °F);⁹⁹ alternatively they have been described as "low temperature".⁹⁸

The Chilean geology agency SERNAGEOMIN in 2019 considered Aucanquilcha a low hazard volcano, ranking it 70th out of 92 Chilean volcanoes¹⁰⁰ and 73th out of 91 in 2014.¹⁰¹ As of 2021 Aucanquilcha was not monitored.¹⁰² Lava flow emissions and minor explosive activity, confined to the edifice of the volcano, are the most important risks from renewed activity,⁸⁷ a hazard map has been published.³⁴ There are signs that warn of volcanic danger in the area.¹⁰³

Temperatures at the summit are usually below freezing,^104g but insolation is among the highest on Earth's surface¹⁵ and can cause sunburns.¹⁰⁶ Mean annual precipitation at Aucanquilcha is unknown, but at Ollagüe in 1963 it did not exceed 100 millimetres (3.9 in) per year¹⁰⁷ and occurs mostly during summer as the so-called Bolivian winter.¹⁰⁸ Aridity has prevailed here since the Miocene³¹ but with short-term fluctuations, a more humid period between 14,000-9,500 years ago and a drier period between 9,000 and 4,000 years ago for example.⁴⁵ The summit region is windy¹⁰⁹ and can experience snowstorms,¹¹⁰ and there is a seasonal snow cover that can reach a substantial thickness.¹¹¹

Three distinct vegetation belts occur in the region. While the aridity prevents plant growth below 3,100 metres (10,200 ft) altitude, a sparse cover of Acantholippia punensis exists above this altitude. Between 3,750–3,300 metres (12,300–10,830 ft) vegetation is denser, featuring Baccharis boliviensi and Fabiana densa. A more diverse plant community is found between 4,100 metres (13,500 ft) and 3,750 metres (12,300 ft), consisting of bushes and grasses. Species there include Azorella compacta, Festuca chrysophylla, Pycnophyllum and Stipa venusta. The occurrence of Polylepis has been reported. The exact vegetation structure depends on the availability of water.¹¹²¹⁰⁸ Vegetation cover ends at about 4,900 metres (16,100 ft) altitude.¹¹³ Mice have been found around 4,500 metres (14,800 ft) altitude;¹¹⁴ regional mice populations are known to reach extreme altitudes.¹¹⁵

There is some snow and ice on the mountains,¹¹⁶ as well as rock glaciers between 5,200–5,500 metres (17,100–18,000 ft)¹¹⁷ and permafrost down to 4,385 metres (14,386 ft) altitude,¹¹⁸ but with the exception of one 1999 report¹¹⁹ sources agree that there are no active glaciers on Aucanquilcha¹²⁰¹²¹ as the climate is too dry for glacier development despite the low temperature.¹²²

During glacials a 45 square kilometres (17 sq mi) ice cap covered the volcano,¹¹⁶ leaving cirques,¹²³ glacial striations¹⁹ and extensive moraines¹⁷ on both on the main volcano^h and its subsidiaries.³⁷ The exact altitude of the ice edge ranged between 4,500–5,200 metres (14,800–17,100 ft)ⁱ,¹⁹¹²⁴ with lower altitudes reached on the eastern and southeastern side.¹²⁵ At least two distinct glacier stages took place.³⁷ The development of glaciers in the region was probably a consequence of a wetter climate.¹²⁶

Aucanquilcha features a mine at 5,950 metres (19,520 ft) altitude,¹²⁷ famous for being the highest mine in the world.¹²⁸ It produced sulfur ore with a grade of 30%¹²⁹ emplaced by fumaroles¹³⁰ in the area of hydrothermally altered rocks around the summit.⁹⁵ The mine was opened by Julian B. Carrasco in 1913, who subsequently established the Compañía Minera y Azufrera Carrasco S.A in 1933,¹³¹ first at Ollagüe and after 1950 in Amincha.¹³² In 1942, the mine produced about 180 tons of ore per day,¹³³ in 1953 annual production was 12,000 tons.¹³⁴ The sulfur was transported down first with llamas, later by trucks¹³⁵ and eventually through an aerial cableway system from Quilca/El Ángulo to Amincha station¹³⁶¹³⁷ (3,900 metres (12,800 ft))¹⁴ where the sulfur was refined.¹³⁸ The sulfur was then transported to Chuquicamata to be converted into sulfuric acid¹³⁹ or to Antofagasta to be shipped;¹³⁷ the Antofagasta-Bolivia railway was essential for the sulfur mining in the Ollagüe area.¹⁴⁰ In 1977 other sulfur mines were present to the west between Cerro Polan and Cerro Gordo and south of the main Aucanquilcha massif,⁴⁰ and another camp (Polan) is marked on modern maps west-southwest of Aucanquilcha.³⁴ Neighbouring volcanoes were also mined.¹¹

In 1935, mining took place throughout the year¹⁴¹ but typical workhours were only about 8 hours per day owing to the conditions.¹⁴² Owing to the extreme conditions, sulfur from the mines in the Ollagüe area was only rarely competitive on global markets.¹⁴⁰ After an initial crisis in the 1950s,¹⁴³ sulfur mining began to decline in the Ollagüe region during the 1970s;¹⁴⁴ during the 1980s¹⁴⁵–1990s the worker camps were abandoned.¹³² Amincha was closed in 1992¹⁴⁵ or 1993,¹⁴⁶ and the last reported mining activity on the mountain was in 1993¹⁴⁷ or 1994.⁸ The mines and adjacent infrastructure were investigated beginning in 2015 as part of the Alto Cielo Archaeological Project¹⁴⁸ and there has been research on the living conditions¹⁴⁹ and ethnic relations between Bolivian and Chilean workers in the region.¹⁵⁰

There are still ruins of the cableway,¹⁵¹ a relic network of roads leading up to 5,900 metres (19,400 ft)⁸ as well as ruins of the Amincha and El Ángulo stations.¹⁵² Apart from mining activity, an Inca mountain sanctuary with offerings of llamas has been reported to exist on Aucanquilcha.¹⁵³ Such Inca findings are common on mountaintops of the region.¹¹¹ Other archaeological sites are found around the volcano,¹⁵⁴ including the Inca road Qhapaq Ñan,¹⁵⁵ and there might have been copper mining at Miño.¹⁵⁶

The inhabitants of Ollagüe viewed Aucanquilcha as the devil's mountain¹⁵⁷ and nicknamed it "El Rebelde".¹²⁸ Its masculine nature (reflecting the hard work of mining) was contrasted to the female, more ambiguous, interpretation of Ollagüe volcano.¹⁵⁸ Anecdotal reports indicate that mine workers presented offerings to the mountain to secure a high mine yield.¹⁵⁹ According to a local folktale, when the Spaniards attempted to rename the mountain to San José, it resisted as it had been named by the goddess Pachamama and occasionally throws sulfur and kills miners on the mountain.¹⁶⁰

The sulfur mine is notable for being the highest mine in the world at 5,950 metres (19,520 ft)¹⁶¹ and just below it⁹⁵ is (formerly) the highest permanently inhabited place on Earth²³ in the form of a galvanized iron hut.¹⁶² In 1986, several caretakers lived there, including Justo Copa who had been living there for two years;¹⁶³¹⁶² they were the highest^j human inhabitants on Earth.¹⁶⁵

With increasing altitude, atmospheric pressure decreases and with it the availability of oxygen for breathing, eventually leading to altitude sickness.¹⁶⁶ Aucanquilcha has been used for studies on the effects of high altitude on human behaviour¹⁶⁷ and medical parameters. They¹⁶⁸ indicated that the Aymara miners are fully acclimatized to the altitude, with less hyperventilation and higher hemoglobin than acclimatized people from lower areas,¹⁶⁹ and are more tolerant of the high altitude than the researchers who studied them in 1942,¹⁷⁰ although some European workers managed to tolerate the environment too.¹¹¹ Their families are born and raised at lower altitudes^k,¹⁶⁵ although they could handle the high altitude conditions as well.¹⁷¹ Only a small fraction of the mineworkers could live on Aucanquilcha for longer timespans,¹⁷² leading to a frequent turnover in personnel¹⁷³ and high sickness and mortality.¹⁷⁴