Incapillo

Incapillo is a Pleistocene-age caldera (a depression formed by the collapse of a volcano) in the La Rioja Province of Argentina. It is the southernmost volcanic centre in the Andean Central Volcanic Zone (CVZ) that erupted during the Pleistocene. Incapillo is one of several ignimbrite or caldera systems that, along with 44 active stratovolcanoes, comprise the CVZ. Subduction of the Nazca Plate beneath the South American Plate is responsible for most of the volcanism in the CVZ. After activity in the volcanic arc of the western Maricunga Belt ceased six million years ago, volcanism commenced in the Incapillo region, forming the high volcanic edifices Monte Pissis, Cerro Bonete Chico and Sierra de Veladero. Later, a number of lava domes were emplaced between these volcanoes. Incapillo is the source of the Incapillo ignimbrite, a medium-sized deposit comparable to the Katmai ignimbrite. The Incapillo ignimbrite was erupted 0.52 ± 0.03 and 0.51 ± 0.04 million years ago and has a volume of about 20.4 cubic kilometres (4.9 cu mi). A caldera with dimensions of 5 by 6 kilometres (3.1 mi × 3.7 mi) formed during the eruption. Later volcanism generated more lava domes within the caldera and a debris flow in the Sierra de Veladero. The lake within the caldera may overlie an area of ongoing hydrothermal activity.

Geography and structure Incapillo, located in Argentina's La Rioja province, is the highest caldera stemming from explosive volcanism in the world. The name Incapillo means 'Crown of the Inca' in Quechua; it is also known as Bonete caldera, Corona del Inca or Inca Pillo. The surrounding mountain peaks were visited by pre-Hispanic people. The crater is marketed as a tourist destination, with visits possible between December and April. Incapillo is part of the Andean Central Volcanic Zone (CVZ), which extends through the countries of Chile, Bolivia, and Argentina and includes six or more Quaternary caldera or ignimbrite systems, about 44 stratovolcanoes, and over 18 smaller centres. One of these stratovolcanoes, Ojos del Salado, is the world's highest volcano. This zone also includes the Altiplano–Puna volcanic complex and the Galán caldera south of it. Incapillo is the southernmost volcano of the CVZ that erupted during the Pleistocene; the next volcano to the south to have erupted during the Pleistocene epoch is Tupungato in the Southern Volcanic Zone at 33° southern latitude.

Incapillo is a caldera with a diameter of 5 by 6 kilometres (3.1 mi × 3.7 mi) and lies at an elevation of 5,750 metres (18,860 ft) or 5,386 metres (17,671 ft). The three adjacent volcanic centres of Monte Pissis (6,882 metres (22,579 ft)), Cerro Bonete Grande (6,436 metres (21,115 ft)), and Cerro Bonete Chico (6,759 metres (22,175 ft)) are also considered part of the Incapillo volcanic complex and are among the highest on Earth. These centres surround the ignimbrite and lava domes. The caldera is about 400 metres (1,300 ft) deep and its walls reach heights of 250 metres (820 ft). The pumice-containing Incapillo ignimbrite forms the bulk of the caldera walls. Forty lava domes surround the caldera, distributed in a northwest–southeast pattern. There is an eastern group of lava domes between Monte Pissis and Cerro Bonete Chico, and a western one on the Sierra de Veladero. The domes have heights of 100–600 metres (300–2,000 ft), and eroded dome material forms an apron about 1 kilometre (0.6 mi) in width around many of them. The aprons consist of erosional material. Some domes have water-filled craters with widths of 20 metres (66 ft) at their top. Domes on the caldera's northern side are dacitic and show signs of hydrothermal alteration. Some domes are probably part of the pre-caldera volcanic complex, and several rhyodacitic domes were eroded after caldera formation; these were formerly considered erosional remnants. Older domes have reddish oxidised colours in satellite images. The total combined volume of the domes is about 16 cubic kilometres (3.8 cu mi).

The Laguna Corona del Inca, considered to be the highest navigable lake in the world, lies next to a heavily altered lava dome in the centre of the caldera. Various dimensions have been reported: The lake may be either 350-metre (1,150 ft) or 13-metre (43 ft) deep, it might lie at 5,300 metres (17,400 ft) or 5,495 metres (18,028 ft) altitude, and its surface area has been variously given as 2 by 1 kilometre (1.24 mi × 0.62 mi), 3.34 square kilometres (334 ha) between 1986 and 2017 or 1.8 square kilometres (0.69 sq mi). The lake has probably generated the evaporite and lacustrine deposits that lie on the caldera floor. Water temperatures of 13 °C (55 °F) obtained by satellite measurements suggest that hydrothermal activity persists in the lake. The lake is fed by meltwater; its surface area declined between 1986 and 2017. Other lakes occur in topographic depressions.

Geology The Nazca plate is subducting beneath the South American plate in the area of the CVZ at a speed of 7–9 centimetres (2.8–3.5 in) per year. The subduction results in volcanism along the Occidental Cordillera 240–300 kilometres (150–190 mi) east of the trench formed by the subduction. Incapillo is one of at least six different ignimbrite or caldera volcanoes that are part of the CVZ in Chile, Bolivia and Argentina. The CVZ is one of four different volcanic arcs in the Andes. The Maricunga Belt, about 50 kilometres (31 mi) west of Incapillo, is where volcanism started 27 million years ago. Phases of ignimbritic and stratovolcanic activity occurred, including Copiapo volcano, until activity ceased with the last eruption of Nevado de Jotabeche 6 million years ago. South of Incapillo, the Pampean flat slab region is associated with tectonic deformation and lack of volcanic activity until Tupungatito volcano farther south. S. L. de Silva and P. Francis suggested in their 1991 book Volcanoes of the Central Andes that the CVZ should be subdivided into two systems of volcanoes: one in Peru and another in Chile, on the basis of the orientation (northwest–southeast versus north–south). Charles R. Stern notes that C.A. Wood, G. McLaughlin and P. Francis in a 1987 paper at the American Geophysical Union instead suggested a subdivision into nine different groups.

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