Aucanquilcha is a large compound volcano in the Antofagasta Region of northern Chile, within the Central Volcanic Zone of the Andes. The volcano has the form of a ridge reaching an altitude of above sea level, and formed between 1.04 and 0.23 million years ago; there is ongoing fumarolic activity. The volcano is part of a larger cluster of volcanoes sometimes known as the "Aucanquilcha cluster" or "Aucanquilcha complex". This cluster includes lava domes and lava flows that have formed in several stages over 11 million years of activity with varying magma output. During the ice ages, Aucanquilcha was covered in an ice cap that left moraines and cirques.
The cluster has generated lava ranging in composition from andesite to dacite, with the main Aucanquilcha volcano having produced dacites exclusively. Systematic variations in temperature, crystal and biotite content have been recorded during the evolution of the cluster, reflecting changes in the crust from dispersed magma chambers to a centralized volcanic system.
The volcano has sulfur deposits in its summit area, which were mined in the past. One mine at an altitude of was opened in 1913 and remained in use from 1950 to 1992, constituting at that time the world's highest mine. As part of this mining activity, a road and an aerial cableway were built on Aucanquilcha, which still stand today although they are partially unusable. In 1986, four men were reported to be living at an altitude of , making them the highest permanent residents on Earth and drawing scientific attention.
Geography and geomorphology
Aucanquilcha is a compound volcano in the El Loa Province of Chile's Antofagasta Region. Roads and railways, including a branch line of the Antofagasta-Bolivia railway going to the Collahuasi mine, pass near the volcano. A road with numerous switchbacks leads up to the Aucanquilcha mine; it was built in 1972 and is now impassable because of rock falls, and might have made the mine on Aucanquilcha the highest point on Earth accessible by road. The mountain was evaluated as a potential site for the European Southern Observatory and its surroundings for the Submillimeter Array observatory.
The main volcano is composed of an east–west long chain of at least six stratovolcanoes, some with well-preserved volcanic craters, that rise above a platform of andesitic rocks. The mountains are formed by about of lava domes and pyroclastic rocks, making it one of the largest volcanoes of northern Chile. Four peaks exceed altitudes of ; at the end of a ridge is the highest peak, reaching above sea level. It is the summit of the Aucanquilcha volcano, and one can see the Salar de Uyuni and other Andean volcanoes from there. Lava flows extend mostly from the summit areas as far as from their vents; they are dark to grey in colour. 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 million-year-old andesite Cerro Tres Monos ridge; to the west lies the east-west Cerro Polan and La Luna ridge.
Aucanquilcha collapsed to the northwest, forming a debris avalanche that descended and ran for to the northwest and north, eventually covering an area of to form a tongue-shaped deposit. The flow was channeled between Miño Volcano and Cerro Cumbre Negra (an eroded lava massif) towards the dry Rio Loa valley, which helped to preserve the slide deposit. The slide lacks the hummocky topography usually found on debris avalanches but has radial ridges and grooves and lateral levees; its upper sector is buried by more recent volcanic rocks and moraines. 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 and a surface area of ; this is 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 is assumed based on a probable thickness of .
There are anecdotal reports of crater lakes on the mountain, and lake El Toro on its southern foot. Some of the source creeks and springs of the Rio Loa are on Aucanquilcha, forming its headwaters when they converge in Lago Seco. 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 only intermittently transport water, if at all.
The volcano is part of a quasi-circular cluster of about 19–20 volcanoes between the Rio Loa and the Chile–Bolivia border, covering a surface area of . Salt flats and alluvial deposits surround it to the east, north and south. On its southern side is the Cerro Chela volcano. The cluster lies on 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.
Geology
Off the western coast of South America, the Nazca Plate subducts beneath the South America Plate at a rate of . As it descends, it releases fluids into the mantle that induce its melting and the onset of volcanism at the surface. This volcanism of the Andean Volcanic Belt is distributed over four arcs, including the Northern Volcanic Zone (NVZ), the Central Volcanic Zone (CVZ) and Southern Volcanic Zone (SVZ), which are separated by gaps without volcanic activity. Aucanquilcha is part of the CVZ, which contains more than a thousand volcanoes (including ignimbrites and compound volcanoes). During the past 28 million years, it has produced more than of eruption products; the average rate of is one tenth of the global maximum rate of arc magma production. Rocks are typically silicic or have intermediate compositions between silicic and mafic. The arc has migrated eastward towards the high Andes from the Pacific Ocean coast since the Jurassic and has been producing ignimbrites and stratovolcanoes since the Miocene. The arc features an eastward offset at the latitude of Aucanquilcha. Lascar is the most active volcano of the CVZ, having erupted more than thirty times since 1800; other active volcanoes of the CVZ include Taapaca, Parinacota, Irruputuncu, Ollagüe, San Pedro, Socompa and Lastarria.
The crust under the CVZ reaches high thicknesses exceeding , 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 of the lower crust. The offset in the volcanic arc coincides with a boundary between two distinct crustal domains to the north and south in the Arequipa-Antofalla terrane. 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). Seismic and electric tomography indicate the presence of a hot zone in the crust, which may be a developing batholith.
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 volcano might be a surface manifestation of copper porphyry deposits and a batholith-plutonic complex resembling the Tuolumne Intrusive Suite of the Sierra Nevada.
Composition
The rocks from the main Aucanquilcha volcano are uniformly dacitic and show little evidence of composition changes over time, while the Aucanquilcha complex produced both andesite and dacite. Andesites appear as lava flows while dacites form lava domes and dome complexes. The rocks define a calc-alkaline suite rich in potassium (content ranges from 1.5 to 4%), typical for Central Andean volcanoes and subduction zone volcanoes in general. Plagioclase is the dominant component of the magma. Clinopyroxene + orthopyroxene + amphibole (hornblende and pargasite) or amphibole + biotite + minor amounts of pyroxene are subordinate components. Anhydrite, apatite, ilmenite, magnetite and zircon can also be found. Rocks have a porphyritic texture and the more recent (last 1 million years) rocks contain inclusions of mafic rocks. Basaltic andesites typically are less than 10% crystals while dacites generally are 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. There is evidence of magma mixing and mingling, and of multiple generations of crystals being extracted from the magma reservoir especially during inactive periods. The magmas form at depth.
Various parts of the main volcano have been subjected to hydrothermal and fumarolic alteration, especially the central parts of the complex and the Azufrera stage edifice. Hydrothermal alteration may be driven by the formation of a deep magma reservoir and resulting hydrothermal circulation in overlying rocks. The summit area and the area between the Azufrera and Angulo complex feature talus deposits containing sulfur, which formed at temperatures of in a now extinct fumarole. Some of the sulfur may have been emplaced in liquid form. The geologist Friedrich Ahlfeld in a 1947 publication reported ongoing formation of sulfur crystals in a cave on the summit. Pyrite and hypogene-derived copper sulfides such as covellite and idaite occur on Aucanquilcha in the sulfur deposits, cementing volcanic rocks. The copper sulfides found in the area appear to have formed through postvolcanic epithermal mineralization above deep porphyry copper mineralization, and before the sulfur deposits.
Magma output and evolution
The surrounding Aucanquilcha complex had a fairly stable magma output between 12 and 6 million years ago, followed by an increase that peaked between 4 and 1 million years ago (with a possible hiatus between 2 and 1 million years ago). The long-term magma output of Aucanquilcha itself 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. The low output of Aucanquilcha relative to other volcanoes in the region might reflect the fact that it is located at the edge of the Altiplano–Puna volcanic complex, and might explain why no major caldera-forming eruption ever took place.