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Apacheta-Aguilucho volcanic complex (also known as Cerro Pabellón[2]) is a volcanic complex in Chile. It consists of two volcanoes Cerro Apacheta and Cerro Aguilucho, which are constructed mainly by lava flows and surrounded by outcrops of lava. A sector collapse and its landslide deposit are located on Apacheta's eastern flank. Two lava domes are associated with the volcanic complex, Chac-Inca and Pabellón.

Apacheta-Aguilucho volcanic complex
Highest point
Elevation	5,557 m (18,232 ft)[1]
Coordinates	[1]
Geography
Apacheta-Aguilucho volcanic complex

The volcanic complex was active from the Miocene to the Pleistocene, but ongoing fumarolic activity has been observed and a geothermal system is present. A geothermal power plant was being built in 2015 and became active in late 2017. It is the first geothermal power plant in Chile.

Geography and geomorphology

The Apacheta-Aguilucho volcanic complex lies in northern Chile, close to the border with Bolivia.[3] The city of Calama lies 105 kilometres (65 mi)[4]-120 kilometres (75 mi) southwest of Apacheta-Aguilucho and El Tatio is about 60 kilometres (37 mi) south-southwest, but with the exception of geothermal power[2] and mining-associated infrastructure the area is remote and uninhabited.[3] Presently, the region has an arid climate.[5] Vegetation, if present, consists mainly of grasses and shrubs.[6]

The volcanic complex is part of the Central Volcanic Zone of the Andes. The Central Volcanic Zone further includes the Altiplano-Puna volcanic complex, which between 10 and 1 million years ago was the source of large ignimbrite eruptions[4] that produced over 15,000 cubic kilometres (3,600 cu mi) of rock;[7] after that it produced lava flows and lava domes like Cerro Chao and geothermal fields such as El Tatio and Sol de Mañana,[4] accompanied by a decreased production of volcanic material.[7] The Altiplano-Puna volcanic complex is underpinned by a magmatic body that appears to be a batholith.[8] Volcanic activity is the consequence of the subduction of the Nazca Plate beneath the South America Plate.[9] Aside from volcanic activity, crustal shortening has resulted in a thickened crust since about 35 million years ago.[8]

The volcanic complex

Apacheta-Aguilucho is a volcanic complex formed by two volcanoes,[1] Cerro Apacheta and Cerro Aguilucho, both composite volcanoes;[10] with the highest summit of the complex reaching a height of 5,557 metres (18,232 ft).[1] The northern Aguilucho volcano and the southern Apacheta volcano are in their central portions formed by rhyolitic lava flows, with the surrounding edifice formed by andesitic-dacitic lavas. An andesitic lahar and pyroclastic flow crops out south and east of Apacheta,[11] which as the oldest part of the edifice is heavily eroded.[3] North and east of the complex, the two lava domes Chac-Inca and Cerro Pabellón[11] (also known as Apacheta or Pabellóncito[12]) form the youngest part of the volcanic complex.[3]

Moraines are encountered both west-southwest of Apacheta and east of the Chac-Inca dome,[11] and traces of glacial erosion are observed on the Aguilucho crater region.[13] The moraines developed during the last glacial maximum.[14]

Geothermal manifestations

Apacheta volcano is fumarolically active on its northern[15] and eastern slopes[16] as well as on the summit.[17] Two fumaroles on the summit of Apacheta produce 108–118 °C (226–244 °F)[18] superheated steam at a high output; the gas has a composition typical of geothermal system fumaroles, with some differences.[15] They are located within a wider c. 0.03 square kilometres (0.012 sq mi) area that features diffuse degassing, gryphons,[19] mud pools and vents.[18]

Apacheta-Aguilucho has a geothermal system and the chemistry of fumarole gases suggests that Apacheta-Aguilucho features an active magma system at depth. Drilling has found temperatures of over 200 °C (392 °F) at 500 metres (1,600 ft) depth.[4] This fumarolic activity has resulted in hydrothermal alteration of many parts of the volcanic complex;[1] this activity has generated a sulfur deposit on the west flank of Aguilucho.[10] The mine Mina Aguilucho is located on Aguilucho's western flank.[15]

The Pabellón geothermal system was discovered in 1999,[9] extends over an area of 25 square kilometres (9.7 sq mi)[20] and is classified as a "blind" geothermal system owing to its scarce activity at the surface,[21] despite the fumaroles existing at the top of Aguilucho;[22] the ascent of gases to these fumaroles is presumably facilitated by the presence of intersecting faults.[23] Thick clay layers encountered between about 165 metres (541 ft) and 490 metres (1,610 ft) depth[24] in drill holes presumably seal the geothermal system.[25] The water remains in the system for a long time and undergoes prolonged interaction with the country rock.[20] The source of heat is unclear; the last eruptions of Apacheta-Aguilucho are too long ago and the more recent lava domes are unlikely to be associated with significant heat reserves.[26] Another geothermal area is found in the Cordón de Inacaliri range southeast from Apacheta-Aguilucho.[27]

Sector collapse

A landslide deposit extends 4.5 kilometres (2.8 mi) away from the volcanic complex on its eastern flank. The landslide deposit covers a surface of about 3 square kilometres (1.2 sq mi),[1] and a large collapse scar with signs of hydrothermal alteration faces the western end of the landslide.[13] In its upper part, the deposit is bordered to the north and south by levees.[28] Downslope, the deposit consists at first of a lobate structure that east of the Pabellón dome grades into a more hummocky deposit. The deposit is increasingly dominated by hydrothermally altered material farther east, while the hummocks consist of lava blocks.[29] The material of the landslide deposit itself features both solid blocks and a finer matrix.[30] Rocks involved in the landslide include both andesitic and dacitic lavas and hydrothermally altered material. The landslide deposit is confined between two older lava flows.[1]

Sector collapses like the one at the Apacheta-Aguilucho volcanic complex have been observed on other volcanoes (such as Mount St Helens during its 1980 eruption), and they result in characteristic landslide deposits known as debris avalanche deposits; these feature hummock-like hills and often levees. Such collapses occur for different reasons for each event and are fairly common; in the Central Andes alone 14 volcanoes feature deposits of such collapses.[31] Such deposits commonly feature a stratigraphy that resembles that of the source edifice, as well as jigsaw-like cracks in rocks that form when rocks disintegrate during the collapse and sliding.[32]

In the case of Apacheta-Aguilucho, it is likely that the sector collapse was triggered by hydrothermal alteration of the edifice that weakened its structure until it failed, and its path was likely influenced by regional tectonics.[33] After the edifice had failed, the landslide moved eastward until it was deflected by older topography; the material became increasingly fragmented.[34]

Geology

Off the west coast of South America, the Nazca Plate subducts beneath the South America Plate in the Peru-Chile Trench. This subduction process is responsible for volcanism in the Central Andes, which occurs at distances of 250–300 kilometres (160–190 mi) from the trench.[1]

Two major fault systems are found close to Apacheta-Aguilucho. The first is the major regional Calama-Olacapato-El Toro fault, the second fault system runs from neighbouring Inacaliri volcano northwest of Apacheta-Aguilucho towards the southeast, where it forms the Pabellóncito graben;[1] the volcanic complex is built within that graben[4] and the eruption of Pabellón was influenced by the graben faults, which allowed magma to reach the surface. The Pabellóncito fault system was active during the Pliocene[22] and Pleistocene; the activity of these regional fault systems appears to be a consequence of tectonic regime changes during the early Pleistocene, which drove extensional tectonics in the area,[14] or of gravitational spreading of the crust under the weight of volcanoes in the area.[35] Northwest of Apacheta-Aguilucho, the axis of the graben is buried beneath other volcanoes starting with Cerro del Azufre[36] which form a northwest-southeast alignment[21] known as Inacaliri lineament.[37]

The basement beneath Apacheta-Aguilucho is of Eocene to Miocene age. It consists of various volcanic and sedimentary formations[36][1] including eroded volcanoes,[4] and it is in part covered by 7.5 million years old ignimbrites.[1]

Composition

Apacheta-Aguilucho has principally erupted andesite, dacite and rhyolite[1] which define a potassium-rich calc-alkaline suite.[12] These rocks define a potassium-rich calc-alkaline suite which contains amphibole, biotite, clinopyroxene, iron-titanium oxides, orthopyroxene, olivine, plagioclase, quartz, sanidine and titanite. Olivine is often altered to clay, goethite and hematite[38] and there are areas of hydrothermally altered rocks on the eastern flank[22] and in the summit fumarole field. At the fumarole field, minerals like chlorite, gypsum, halloysite, hematite, mica and quartz formed through supergene and acid leaching processes.[18] The magmas appear to derive from the Altiplano-Puna magmatic complex volcanic body and underwent fractional crystallization and other geochemical processes before erupting.[39]

Eruption history

The volcanic complex is of Pliocene to Pleistocene age[10] and developed over several different stages. In the first stage, the so-called Aguilucho ignimbrite consisting of moderately welded, white-to-pink pyroclastic flows was emplaced 7.5 ± 0.6 million years ago and andesitic lava flows 6.7 ± 0.3 million years ago. Subsequently, more lava flows were emplaced, consisting of blocky dacite.[16]

The Apacheta and Aguilucho volcanoes were constructed consecutively. Apacheta consists of lava flows and pyroclastic material made out of andesite, and its crater is covered by pyroclastic flows and a 2.5 kilometres (1.6 mi) long rhyolitic lava flow. Aguilucho is constructed by lava flows. Finally, several lava flow fields were emplaced together with the lava domes Pabellón and Chac-Inca.[16] 1,204,000 ± 33,000 years ago an ignimbrite named Aguilucho ignimbrite was erupted[40] from the Apacheta volcano.[8]

Apacheta grew between about 1.024 and 0.9 million years ago and Aguilucho between 0.7 and 0.6 million years ago.[5] Lava flows on Apacheta-Aguilucho have been dated to 910,000 ± 140,000 and 700,000 ± 200,000 years before present,[1] with one lava flow being dated to 652,000 ± 12,000 years ago.[40] The date of the sector collapse is not known with certainty but was probably after the last lava flow was emplaced on Apacheta-Aguilucho.[33]

Dating of Chac-Inca has produced an age of 140,000 ± 80,000 years before present. Different dates have been obtained on Cerro Pabellón: Potassium-argon dating yields an age of 130,000 - 80,000 years before present, while argon-argon dating has yielded an age of 50,000 ± 10,000 years before present.[16] After the cessation of volcanic activity at Apacheta-Aguilucho, it appears to have migrated to the neighbouring Azufre volcano.[41] Future eruptions may consist of small explosions with local impact, of perhaps phreatic nature.[37]

Geothermal power production

The discovery of steam during the drilling of a freshwater well in the 1990s led to investigations by ENAP and UNOCAL in the area and the discovery of the fumaroles. In 2002, a joint enterprise by ENAP and CODELCO obtained the permission to explore the area, but while a geothermal system was discovered political questions about ENAP's involvement derailed the project.[3]

Another joint enterprise this time including the Italian ENEL performed further research in the area between 2006-2007, and obtained a permit to exploit the system in 2009.[3] The joint enterprise started the geothermal power project Cerro Pabellón, and in 2015 the construction of two power plants with a projected output of 24 MW each started, to be completed in 2017-2018.[42] The plant on Pampa Apacheta[22] 3.5 kilometres (2.2 mi) east of Apacheta-Aguilucho[5] was inaugurated by the Chilean president Michelle Bachelet on 12 September 2017, becoming the first[lower-alpha 1] geothermal power station in South America[44] and as of 2021^[update] the only operating one.[45] Situated at an elevation of over 4,500 metres (14,800 ft), it is the highest large-scale geothermal plant in the world.[46]

The Cerro Pabellón project is expected to produce 340 Gigawatt-hours per year, enough to cover the electricity consumption of 165,000 families.[47] The plant is expected to reduce Chile's carbon dioxide emissions by about 166,000,000 kilograms per year (166,000 t/a)[2] and is owned by the companies Geotérmica del Norte S.A.(GDN) and Empresa Nacional de Geotermia; the latter is a joint venture between ENAP and ENEL.[5] A further expansion began in 2019,[46] aiming at increasing output by about 33 MW[48] to 81 MW.[46] The plant uses hydrogen for energy storage.[49]

Notes

See also

• El Tatio
• Geothermal power in Chile

References

Sources

External link

• Cerro Pabellon Geothermal Project (Apacheta). UNFCC (Report).
• Cristian Andrés Gutiérrez Villalobos. Ficha del Proyecto: Central Geotérmica Cerro Pabellón (Report). Forma de Presentación: Estudio de Impacto Ambiental (in Spanish). SGA.

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