Apacheta-Aguilucho volcanic complex

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Apacheta-Aguilucho volcanic complex (also known as Cerro Pabellón) 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. 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, with another unit coming into service in 2022. It is the first geothermal power plant in Chile.

The Apacheta-Aguilucho volcanic complex lies in northern Chile, close to the border with Bolivia. The city of Calama lies 105 kilometres (65 mi)-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 and mining-associated infrastructure the area is remote and uninhabited. Presently, the region has an arid climate. Vegetation, if present, consists mainly of grasses and shrubs. 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 that produced over 15,000 cubic kilometres (3,600 cu mi) of rock; after that it produced lava flows and lava domes like Cerro Chao and geothermal fields such as El Tatio and Sol de Mañana, accompanied by a decreased production of volcanic material. The Altiplano-Puna volcanic complex is underpinned by a magmatic body that appears to be a batholith, the Altiplano-Puna Magmatic Body. Volcanic activity is the consequence of the subduction of the Nazca Plate beneath the South America Plate. Aside from volcanic activity, crustal shortening has resulted in a thickened crust since about 35 million years ago.

Apacheta-Aguilucho is a volcanic complex formed by two volcanoes, Cerro Apacheta and Cerro Aguilucho, both composite volcanoes; with the highest summit of the complex reaching a height of 5,557 metres (18,232 ft). 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, which as the oldest part of the edifice is heavily eroded. North and east of the complex, the two lava domes Chac-Inca and Cerro Pabellón (also known as Apacheta or Pabellóncito) form the youngest part of the volcanic complex. Moraines are encountered both west-southwest of Apacheta and east of the Chac-Inca dome, and traces of glacial erosion are observed on the Aguilucho crater region. The moraines developed during the Last Glacial Maximum.

Apacheta volcano is fumarolically active on its northern and eastern slopes as well as on the summit. Two fumaroles on the summit of Apacheta produce 108–118 °C (226–244 °F) superheated steam at a high output; the gas has a composition typical of geothermal system fumaroles, with some differences. They are located within a wider c. 0.03 square kilometres (0.012 sq mi) area that features diffuse degassing, gryphons, mud pools and vents. 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. This fumarolic activity has resulted in hydrothermal alteration of many parts of the volcanic complex; this activity has generated a sulfur deposit on the west flank of Aguilucho. The mine Mina Aguilucho is located on Aguilucho's western flank. The Pabellón geothermal system was discovered in 1999, extends over an area of 25 square kilometres (9.7 sq mi) and is classified as a "blind" geothermal system owing to its scarce activity at the surface. The only exceptions are the fumaroles of Aguilucho and a warm spring 17 kilometres (11 mi) from the power plant site; the ascent of gases to these fumaroles is presumably facilitated by the presence of intersecting faults. Thick clay layers encountered between about 165 metres (541 ft) and 490 metres (1,610 ft) depth in drill holes presumably seal the geothermal system and block ascent to the surface. The water remains in the system for a long time and undergoes prolonged interaction with the country rock. 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. Another geothermal area is found in the Cordón de Inacaliri range southeast from Apacheta-Aguilucho.

Adapted from Wikipedia (CC BY-SA). Photography via Wikimedia Commons.