Aucanquilcha

Aucanquilcha is part of the Central Volcanic Zone (CVZ) of the Andes, a highly silicic volcanic zone in South America. The CVZ generates magmas at a rate of 0.11 cubic kilometres per millennium (0.026 cubic miles per millennium), one tenth of the global average of arc magma production, and lies about 135–180 kilometres (84–112 mi) above the Wadati–Benioff zone. The arc has migrated eastward towards the high Andes from the Pacific Ocean coast since the Jurassic. The arc contains andesitic volcanoes, ignimbrites and compound volcanoes and has generated over 3,000 cubic kilometres (720 cu mi) of eruption products over 28 million years.[5]

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.[6]

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. 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.[5]

In this topographic map of the region, the white surface in the upper left is Aucanquilcha.

Aucanquilcha is part of a cluster of volcanoes located between the Rio Loa and the Chile–Bolivia border.[7] Aucanquilcha sits on top of a 2.7–3.3 mya andesite platform and rises more than 1,400 metres (4,600 ft) over it.[5] The main volcano is composed of an east-west 8-kilometre (5.0 mi) long chain of stratovolcanoes[3] and has an estimated volume of 37 cubic kilometres (8.9 cu mi).[5] The maximum slope of the summit area is 25°.[8] During the Cumbre Negra stage, a pyroclastic flow occurred on the northwestern side of the volcano. It covered 34 square kilometres (13 sq mi) on a run of 10 square kilometres (3.9 sq mi) and now has a volume of 0.3 cubic kilometres (0.072 cu mi). It was at first identified as a debris avalanche, but the lack of hummocky topography and the presence of large juvenile blocks identify it as a pyroclastic flow.[5] One block in the flow and the lava dome from which the flow originated have been dated at 0.6 mya.[7] 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.[5] 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.[5]

The volcanic cluster of which Aucanquilcha is a part contains about 19–20 volcanoes and has generated about 340 cubic kilometres (82 cu mi) of andesite and dacite over eleven million years. Covering a surface area of 700 square kilometres (270 sq mi), the cluster is surrounded on its northern, western and eastern sides by salt flats and alluvial deposits. On its southern side lies the Cerro Chela volcano.[6][7] The cluster lies on a 70-kilometre (43 mi) thick crust,[6] and arid conditions since the Miocene have preserved its structures. Its volcanoes are arranged in north-south and northwestern alignments, which may indicate a rupture of the crust above shallow magma reservoirs.[7]

Aucanquilcha underwent a northwestward flank collapse, generating a debris avalanche deposit. The debris avalanche descended 2,100 metres (6,900 ft) and ran for 17 kilometres (11 mi) with an azimuth of 211°, eventually covering an area of 59 square kilometres (23 sq mi). 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. The proximal parts of the slide are covered by younger eruptive products and also by moraines, indicating that the slide was followed by at least one glacial episode. The slide lacks the hummocky topography usually found on debris avalanches but has radial ridges and grooves.[8] 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).[5][7]

Petrographically, the composition of the cluster ranges from andesite to dacite with SiO₂ ranging from 62.8% to 65.7%. Andesites appear as lava flows while dacites form lava domes and dome complexes.[3] The rocks from the main Aucanquilcha volcano are uniformly dacitic and show little evidence of temporal variation in their composition.[5] Potassium content ranges from 1.5 to 4%.[9] 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. There is evidence of magma mixing and mingling.[5][6]

Rocks have a porphyritic texture.[5] Basaltic andesites typically contain less than 10% crystals while dacites generally have more than 20% crystals. The Alconcha group lavas of the first one million years are crystal-poor and lack biotite; later lavas contain biotite and more crystals. Based on geothermometric data, the highest temperatures occurred during phases of high activity and lower temperatures are associated with low output periods.[10] It is likely that crustal feedback and increased deep crustal influx of mantle-derived basalts drive increased magmatic flux. During the time of the Polan eruption on the west flank, magma output was spatially segregated with the peripheral Miño Volcano generating andesite lavas and the more central volcanos generating dacites.[7]

Various parts of the main volcano have been subjected to hydrothermal alteration. The Azufrera stage edifice was subject to the most alteration; especially in the summit area and between the Angulo and Azufrera summits lie sulfur rich talus deposits.[5] The central part of the complex is heavily altered by fumarole activity.[3] Hydrothermal alteration may be driven by the formation of a deep magma reservoir and resulting hydrothermal circulation in overlying rocks.[7]

Eruption rates increased 6 million years ago, coinciding with magmas becoming more uniform in composition and the onset of strong hydrothermal alteration. It is possible that solidification of magmas below the volcano insulated the feeding channels from heat loss and caused the temperatures in the system to increase. Activity waned again 2 million years ago, with magma and included crystals being drawn from depths of 3 to 23 kilometres (1.9 to 14.3 mi) and forming the Aucanquilcha volcano proper.[6][10] The 10-million-year duration of activity of the Altiplano–Puna volcanic complex systems is comparable to the duration of Aucanquilcha activity.[11] The 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.[3]

Aucanquilca

The Aucanquilcha cluster formed in four stages, each corresponding to a group. The Alconcha group, with seven volcanoes, is constructed from pyroxene, 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 collapse 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. 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).[7]

The Gordo group, which erupted 6–4 mya following a probable 2-million-year hiatus, 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. 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).[7]

The 3.6–2.3 mya Polan group, 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 cluster 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. 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).[7]

After the four main phases, some scoria cones of basaltic composition, including Poruñita and Luna de Tierra, formed between Aucanquilcha and Ollagüe.[10]

Landscape view

The main Aucanquilcha volcano formed in four stages. Between 1.04–0.92 mya the bulk of the volcano formed in the Azufrera stage. 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. 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). This volcano was probably an isolated cone, but the existence of a previous stage cannot be excluded.[5]

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. 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). A flank collapse, possibly triggered by a large earthquake, occurred during this stage.[5]

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 during a lava dome collapse as occurred on Merapi in Indonesia. 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).[5]

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. 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).[5]

There is no evidence of Holocene activity[12] but the volcano has active fumaroles though the low-temperature fumaroles are not visible through short wavelength infrared data from satellites.[13] Fumarole activity was observed in 1962[14] and is most conspicuous in the trenches dug during sulfur mining operations. Lava flow emissions and minor explosive activity, confined to the edifice of the volcano, are the most important risks from renewed activity.[15]

The sulfur mine is notable for being the highest mine in the world at 5,950 metres (19,520 ft)[27] and the highest permanently inhabited area.[8] An expedition in 1935, part of the International High Altitude Expedition, found that miners lived at an altitude of 5,300 metres (17,500 ft) in the town of Quilcha and reached the higher mine on foot. The expedition found that an even higher abandoned village at 5,639 metres (18,501 ft) existed, but miners refused to live there. The conclusion taken from the expedition was that 5,334 metres (17,500 ft) was the highest habitable altitude.[27]

West in 1986 reported that a few miners permanently lived in the mine area.[30] A small group of men, caretakers of the mine, has lived at an altitude of 5,900 metres (19,500 ft) in a galvanized iron hut. One of them reportedly had spent two years there.[31] These are considered to be the highest human inhabitants on Earth.[32] Research performed on Aymara miners of the Aucanquilcha mine indicates that they are fully acclimatized to the altitude, with less hyperventilation and higher hemoglobin than acclimatized people from lower areas.[33] Their families are born and raised at lower altitudes, however.[32]