Nevado Tres Cruces

The volcano has an extended history of activity, going back at least 1.5 million years. A number of lava domes surround the complex and a number of craters lie on its summits. The main volcano is of rhyodacitic composition and has generated two major ignimbritic eruptions, one 1.5 million years ago and a second 67,000 years ago. The last eruption was 28,000 years ago, but the volcano is a candidate source for a Holocene eruption and could erupt again in the future.

Geography and geomorphology

Nevado Tres Cruces is located in the High Andes of Copiapo and straddles the boundary between Chile (Atacama Region) and Argentina (Catamarca Province). The Salar de Maricunga is located west of Nevado Tres Cruces, the Almagro valley north and its tributary the Barrancas Blancas valley northeast of it. The international road between Chile and Argentina from Paso de San Francisco passes north of Nevado Tres Cruces; an unpaved road runs through the Barrancas Blancas valley. The Rio Lomas and Rio Salado originate from its southwestern and southeastern flanks, respectively.

The volcano is massive, covering an area of about 1,000 square kilometres (390 sq mi), and consists of a 10 kilometres (6.2 mi) long and 5 kilometres (3.1 mi) wide north-south trending chain made up of at least three overlapping volcanoes. These volcanoes have diameters of 4–5 kilometres (2.5–3.1 mi) and rise about 800–1,600 metres (2,600–5,200 ft) above the surrounding terrain. The highest summit, and sixth-highest summit of South America, of Nevado Tres Cruces is the 6,748 metres (22,139 ft) high southern summit, which is also the least eroded of the three volcanoes that make up Nevado Tres Cruces. The southern summit consists of two overlapping cones, the western and older one of which has two explosion craters while the eastern one is capped by a summit lava dome. The central volcano reaches an elevation of 6,629 metres (21,749 ft), has the steepest slopes and is tilted to the west. The northern volcano has a summit elevation of 6,206 metres (20,361 ft) and is capped by a glacially eroded, 1 kilometre (0.62 mi) wide crater. There are two even more minor summits at the north end of the massif, Punta Torre 6,320 m (20,730 ft) and Punta Atacama 6,206 m (20,361 ft).

The volcanoes are formed by explosion craters, lava domes including couleés, lava flows, tephra, and base surge and pyroclastic flow deposits. Fallout of explosive eruptions cover the slopes of the southern summit and deposits of a large Plinian eruption and its eruption column cover much of Nevado Tres Cruces and its surroundings. Normal faults cut across the volcanic structures and a north-northwest trending fault system appears to have directed the development of the three volcanoes.

Domo del Indio on the southeastern flank is 3.2 by 1.8 kilometres (2.0 mi × 1.1 mi) wide and 235 metres (771 ft) high. Between it and Nevado Tres Cruces lies a 2 by 1.5 kilometres (1.24 mi × 0.93 mi) wide and 150 metres (490 ft) deep explosion crater that contains the La Espinilla dome, which is 45 metres (148 ft) high and 200–250 metres (660–820 ft) wide. Another lava dome is known as Domo las Vicuñas. The Tres Cruces Ignimbrite was erupted by Nevado Tres Cruces and lies between Nevado Tres Cruces and Ojos del Salado and reaches a thickness of 100 metres (330 ft). It covers a surface area of 81.31 square kilometres (31.39 sq mi). The ignimbrite consists of pumice and volcanic ash, is poorly welded and has a low crystal content.

These edifices rise over older volcanoes, which crop out north of Nevado Tres Cruces in the form of the volcanoes Cristi (5,900 metres (19,400 ft) high), Lemp and Rodrigo. The latter has a caldera at 5,950 metres (19,520 ft) elevation. Lemp is located just south of Rodrigo. Puntiagudo crops out south of Nevado Tres Cruces. Two other centres lie on the southwestern foot: 5,194 metres (17,041 ft) high Paitur and 5,361 metres (17,589 ft) Trioblite. The older structures are smoothened by erosion and lack primary features. A thick and large lava flow crops out west of the volcano; it has a surface area of 3.5 by 5.5 square kilometres (1.4 sq mi × 2.1 sq mi) and a thickness of 150–200 metres (490–660 ft). Three older lava domes, all heavily eroded, are found on the western flank.

Glaciation

Small glaciers occur on Nevado Tres Cruces on the eastern and southern sides above 5,500 metres (18,000 ft) elevation. They are best developed above 5,750 metres (18,860 ft) elevation and consist of small ice bodies (none exceeding 1 square kilometre (0.39 sq mi)) in glacial cirques and at the edges of lava flows. One of these is hosted in a cirque on the southeastern flank of the southern summit. Ice area was constant between 1937 and 1956 but declined by almost half between 1985 and 2016. Non-moving ice without crevasses has been found on the northern summit, and there are debris-covered glaciers on the volcanoes. Some sources however deny that any glacier occurs on Nevado Tres Cruces.

Moraines occur above 4,400 metres (14,400 ft) elevation and a well-developed terminal moraine at the foot of Nevado Tres Cruces, at 4,200 metres (13,800 ft) elevation, has been broken by the Lamas River. There are cirques at 5,500 metres (18,000 ft) on the eastern sides of Nevado Tres Cruces and traces of periglacial occur. Presently, the equilibrium line altitude lies at about 5,800 metres (19,000 ft); during the Last Glacial Maximum the equilibrium line altitude descended to 5,500 metres (18,000 ft).