Lastarria is a 5,697 metres (18,691 ft) high stratovolcano that lies on the border between Chile and Argentina. It is remote, and the surroundings are uninhabited but can be reached through an unpaved road. The volcano is part of the Central Volcanic Zone, one of the four segments of the volcanic arc of the Andes. Over a thousand volcanoes—of which about 50 are active—lie in this over 1,500 kilometres (930 mi) long chain of volcanoes, which is generated by subduction of the Nazca Plate beneath the South American Plate. The volcano is constituted by two volcanic edifices that form a ridge, and one subsidiary lava flow field southwest of the main volcanoes. The main edifice features several aligned craters that form a line. There is no recorded eruptive activity, but the volcano displays vigorous fumarolic activity on its northern side and within the craters. It is located on top of older volcanic rocks and features both andesite and dacite. Lastarria produced a large landslide deposit when part of its southeastern flank collapsed. From a 1 kilometre (0.62 mi) wide collapse scar, the landslide descended over a distance of 8 kilometres (5.0 mi). The intense fumarolic activity makes the volcano the largest source of volcanic gases in the region, and has produced fumarolic vent deposits as well as flows of molten sulfur. A progressive uplift of the terrain around Lastarria and farther south has been recognized; it appears to reflect a deep magma intrusion in the region.
Geography and structure Lastarria is situated in the Central Andes, in the Antofagasta Region of Chile, and straddles the border with Argentina's Salta Province. The city of Antofagasta lies 250 kilometres (160 mi) northwest of Lastarria. The Central Andes are difficult to access and its volcanoes are usually poorly monitored. There are no human populations within 150 kilometres (93 mi) of Lastarria. From the former Catalina railway station 120 kilometres (75 mi) west, an unpaved road leads to Lastarria. Unlike most other volcanoes in the region, no Inka archeological site has been found on Lastarria. Either the volcano was too active or too unimportant in pre-Hispanic times. Lastarria is part of the Andean Central Volcanic Zone, which extends over 1,500 kilometres (930 mi) from Peru to Chile. Over 1,000 volcanic edifices have been identified in this zone, of which about 50 volcanoes are active or potentially active, with many exceeding 6,000 metres (20,000 ft) in altitude. In addition, the zone features 18 monogenetic volcanoes and about 6 caldera/ignimbrite systems.
Edifice proper Lastarria is formed by two coalesced edifices, the main cone and the older South Spur (Espolón Sur), which are joined at an altitude of about 5,500 metres (18,000 ft) and form a 10-kilometre (6.2 mi) long ridge. The main cone comprises lava domes, lava flows, pyroclastic flows, and scoria, and reaches a summit height of 5,697 metres (18,691 ft). Most of the volcano is covered by pyroclastic material, some of which extends to the southeastern margin of the Salar de Aguas Calientes. Lava flows are exposed mainly on the northwestern slope, where they reach thicknesses of 40 metres (130 ft). The South Spur has also generated lava flows. The volcano covers a surface area of about 156 square kilometres (60 sq mi). Five overlapping craters are aligned in a north–south line on Lastarria's main cone, the craters 5-4 and 3-2 are nested within each other (counted from south to north). Volcanic activity has migrated north during the history of Lastarria, and the most recent eruption products are found on the northern and western slopes. A lava dome sits on the northernmost crater rim. The South Spur has two craters. The volcano rises from a terrain of about 4,200 metres (13,800 ft) altitude and has fairly steep slopes. Much of the surface, including the Southern Spur, is covered by deposits left by volcanic ash fall. Some parts of the Southern Spur display evidence of hydrothermal alteration. The total volume of the edifice is about 10.1 cubic kilometres (2.4 cu mi). The Negriales del Lastarria (also known as Big Joe) lava flow complex lies southwest of the Lastarria volcano and covers a large surface area. It is formed by several massive flows erupted from a single vent during three or eight pulses; the longest reaches a length of 10 kilometres (6.2 mi). These lavas are block lavas with flow ridges and levees. The total volume of the lava field is about 5.4 cubic kilometres (1.3 cu mi) and it is often grouped together with Lastarria and South Spur as the Lastarria Volcanic Complex. Neighbouring mountains include Chili volcano northeast, the 4,709-metre (15,449 ft) high Cerro Bayo northwest and the 5,214-metre (17,106 ft) high Cerro Piramide close to Negriales de Lastarria in the southwest. Almost due north of Lastarria lies the Laguna de la Azufrera, a salt pan with a waterbody that is almost a lake, its name is a reference to the sulfur deposits of Lastarria. There are hot springs with temperatures exceeding 40 °C (104 °F) on its eastern shore. The fumarolic system may drain into this waterbody, depositing sulfur at Azufrera's southern shore. Water levels in the lake were higher in the past, as evidenced by two recognizable shorelines, which owing to tectonics related to recent volcanism in the Corrida de Cori crop out mainly on the eastern side, and the lake's surface area reached 18 square kilometres (7 mi2). There is a valley and a lake also at the southeastern foot of Lastarria.
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