Aucanquilcha

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 . 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 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 , making them the highest permanent residents on Earth.

Geography and geology

Regional setting

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 , one tenth of the global average of arc magma production, and lies about 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 of eruption products over 28 million years.

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 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.

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 (, followed by ). 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.

Local setting

Aucanquilcha is part of a cluster of volcanoes located between the Rio Loa and the Chile–Bolivia border. Aucanquilcha sits on top of a 2.7–3.3 mya andesite platform and rises more than over it. The main volcano is composed of an east-west long chain of stratovolcanoes and has an estimated volume of . The maximum slope of the summit area is 25°. During the Cumbre Negra stage, a pyroclastic flow occurred on the northwestern side of the volcano. It covered on a run of and now has a volume of . 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. One block in the flow and the lava dome from which the flow originated have been dated at 0.6 mya. Lava flows, mostly from the summit areas, are dark to grey in colour and extend as far as from their vents. It is likely that two small lava domes (Cerro Cumbre Negra and Summit 5867) on the northwest flank occupy flank vents. 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.

The volcanic cluster of which Aucanquilcha is a part contains about 19–20 volcanoes and has generated about of andesite and dacite over eleven million years. Covering a surface area of , 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. The cluster lies on a thick crust, 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.

Aucanquilcha underwent a northwestward flank collapse, generating a debris avalanche deposit. The debris avalanche descended and ran for with an azimuth of 211°, eventually covering an area of . 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. Another landslide occurred during the Redondo stage on the eastern side of the main volcano into the Salar de Carcote avalanche of 1980 and one-third of the surface area of the Ollagüe avalanche. A volume of is assumed based on a probable thickness of .

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. The rocks from the main Aucanquilcha volcano are uniformly dacitic and show little evidence of temporal variation in their composition. Potassium content ranges from 1.5 to 4%. 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.

Rocks have a porphyritic texture. 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. 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.

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. The central part of the complex is heavily altered by fumarole activity. Hydrothermal alteration may be driven by the formation of a deep magma reservoir and resulting hydrothermal circulation in overlying rocks.

Eruptive history

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 and forming the Aucanquilcha volcano proper. The 10-million-year duration of activity of the Altiplano–Puna volcanic complex systems is comparable to the duration of Aucanquilcha activity. 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.

Aucanquilcha cluster

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 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 , indicating a flux rate of .

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 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 , indicating a flux rate of .

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 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 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 , indicating a flux rate of .

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

Aucanquilcha proper

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 . A vent at altitude fed most of this edifice; a second vent on the northern flank at the 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 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 , indicating a flux rate of . This volcano was probably an isolated cone, but the existence of a previous stage cannot be excluded.

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 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.  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 , indicating a flux rate of . A flank collapse, possibly triggered by a large earthquake, occurred during this stage.

The third stage is the ''Cumbre Negra'' stage, named after the westernmost summit and principal vent of this stage, Cerro Cumbre Negra (). 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 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 , indicating a flux rate of .

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 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 from the vent, and with the exception of a due south flow thick they are thin, with thicknesses of . 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 , indicating a flux rate of .

There is no evidence of Holocene activity but the volcano has active fumaroles though the low-temperature fumaroles are not visible through short wavelength infrared data from satellites. Fumarole activity was observed in 1962 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.

Glaciation and hydrology

The volcano is currently not glaciated despite its height, due to the aridity of the climate. The Quebrada de Chaigüire valley originates at the foot of Aucanquilcha. Some of the source springs of the Rio Loa are on Aucanquilcha and it 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