Incahuasi

Incahuasi is part of the Central Volcanic Zone of the Andes together with about 110 other Quaternary volcanoes, and lies in the southern sector of the volcanic zone;[8] other volcanic zones in the Andes are the Northern Volcanic Zone, the Southern Volcanic Zone and the Austral Volcanic Zone.[9] The history of volcanic activity is poorly known for most of these volcanoes owing to the lack of dating; only a few historical eruptions have been recorded, such as an eruption at Ojos del Salado in 1993.[8]

Incahuasi is located northeast of Ojos del Salado,[2] the highest volcano in the world.[10] Both volcanoes are found at the southern end of the Central Volcanic Zone.[11] They together with El Fraile, Cerro El Muerto, Nevado Tres Cruces and El Solo form a 50 kilometres (31 mi) long volcanic chain.[12]

The area is dominated by volcanoes that were active after 1.5 million years ago.[13] Also located close to Incahuasi are Falso Azufre and Nevado San Francisco,[6] as well as the Miocene Cerro Morocho and Cerro Ojo de Las Lozas volcanoes.[14] It has been suggested that a perpendicular chain of volcanoes including Ojos del Salado may be the consequence of the Juan Fernandez Ridge subducting in the Peru-Chile Trench.[15]

Volcanism in the area goes back to the Oligocene and Miocene, when the main volcanic arc was located 40 kilometres (25 mi) west in the Maricunga Belt. Between 9 and 6 million years ago volcanic activity in the Maricunga Belt decreased and eventually ceased. Simultaneously, the back-arc experienced increased volcanic activity.[11]

Incahuasi is formed by a caldera 3.5 kilometres (2.2 mi) wide. Two coalesced stratovolcanoes formed within the caldera[2] and have a diameter of 15 kilometres (9.3 mi).[16] A 6 by 4 kilometres (3.7 mi × 2.5 mi) wide lava dome is located on the eastern flank.[16] The volcano has a volume of about 231 cubic kilometres (55 cu mi)[17] and covers a surface area of about 207 square kilometres (80 sq mi).[18] With a height of 6,621 metres (21,722 ft) Incahuasi is the 12th highest mountain in South America[19] and one of the world's highest volcanoes.[18]

Incahuasi has two craters, a summit crater and an arcuate crater on the eastern slope that contains a lava dome.[2] The summit crater has dimensions of 750 by 900 metres (2,460 ft × 2,950 ft)[16] and is embedded within a 2 kilometres (1.2 mi) high summit plateau.[14] Subsidiary vents conversely are associated with fissure vents.[20]

The western and southwestern slopes of Incahuasi are dotted with lava domes,[2] which are more subdued than on other volcanoes in the region.[21] Less than 1 kilometre (0.62 mi) wide and 5 kilometres (3.1 mi) long[16] lava flows extend down the volcano.[2] They reach the Las Coladas salar east of Incahuasi.[22] Two 2 kilometres (1.2 mi) long coulees extend north and east of the main crater.[16]

7 kilometres (4.3 mi) northeast of Incahuasi four pyroclastic cones can be found. They have covered 10 square kilometres (3.9 sq mi) with lava[2] but they are probably an independent volcanic system, similar to other regional mafic volcanoes.[23] On Incahuasi's eastern flank lies a major lava dome and a field of lava flows.[14] Incahuasi volcano rises over a surface with elevations of 4,300–4,700 metres (14,100–15,400 ft).[24]

• 
  Incahuasi is the volcano at the left and El Fraile immediately right. The green lake is Laguna Verde
• 
  Incahuasi from Las Grutas. The eastern lava domes and the northeastern monogenetic volcanoes are clearly visible.

Like many Andean volcanoes, Incahuasi has erupted andesite containing hornblende and pyroxene,[6] but also trachyandesite and trachydacite.[25] Lava flows on the main stratovolcano are dacitic.[2]

The four cones northeast of the principal volcano have erupted basaltic andesite.[2] Likewise, parasitic cones have erupted magnesium-rich basaltic andesite.[26] Minerals contained in these rocks include clinopyroxene and olivine.[6]

The occurrence of such basic magmas in a volcanic setting dominated by dacites appears to be a consequence of local tectonics, which involve the extension of the crust compared to the compressional regime farther west.[13] Originating in the mantle, the magmas quickly ascended in faults and were contaminated by crustal material.[6] The mantle itself had been modified before by crustal material added by delamination of the lower crust and subduction erosion.[27]