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.