Ollagüe (Spanish pronunciation: [oˈʎaɣwe]) or Ullawi (Aymara: [uˈʎawi]) is a massive andesite stratovolcano in the Andes on the Bolivia–Chile border, within the Antofagasta Region of Chile and the Potosi Department of Bolivia.

The western rim of the summit crater is formed by a compound of lava domes, the youngest of which features a vigorous fumarole that is visible from afar.

It started developing more than one million years ago, forming the so-called Vinta Loma and Santa Rosa series mostly of andesitic lava flows.

Later two groups of dacitic lava domes formed, Ch'aska Urqu on the southeastern slope and La Celosa on the northwestern.

Another centre named La Poruñita formed at that time on the western foot of the volcano, but it is not clear whether it is part of the main Ollagüe system.

While there is no clear evidence of historical eruptions at Ollagüe, the volcano is considered to be potentially active and is monitored by the National Geology and Mining Service (SERNAGEOMIN) of Chile.

[24] A notable trait of the Central Andes are the long strike-slip faults that extend from the Eastern Cordillera northwest through the Altiplano into the volcanic arc.

[16] Other than some past glacial activity, the arid climate of the Altiplano region has kept erosion rates low, meaning that the volcanic edifice is well preserved.

These include Ch'aska Urqu on the southeastern slope and La Celosa (4,320 metres (14,170 ft); also known as El Ingenio[1]) on the northwestern.

[37] A normal fault runs across the main edifice but is not aligned with these adventive vents,[21] and the Pastos Grandes-Lipez-Coranzuli lineament intersects with the volcanic arc at Ollagüe.

[39] Overall, northwest trending lineaments exercised a strong influence on the tectonic development of Ollagüe,[40] and may be the path that feeder dykes of the more recent eruptions followed.

[42] A 700 metres (2,300 ft) wide[43] phreatomagmatic vent named La Poruñita lies on the western slope, on the deposit formed by the sector collapse.

[45] Presently, high insolation and evaporation as well as the dry climate prevent the formation of glaciers or the existence of a snow cover.

Hydrothermally altered breccia with block sizes of several 0.1–1 metre (3.9 in – 3 ft 3.4 in) from the first collapse fills a valley on the western slope of the volcano.

[50] Phenocrysts in the main andesite-dacite series include amphibole, apatite, biotite, clinopyroxene, ilmenite, magnetite, orthopyroxene, plagioclase and rarely olivine, quartz and zircon.

[14] Alunite, gypsum and sulfur were formed by the alteration on the summit and the northwestern slope,[39][11] and chalcedony, clay, kaolinite and opal are found as well.

[83] Subsidiary magma chambers which developed beneath the northwestern and southeastern flank gave rise to the La Celosa and Ch'aska Urqu volcanic centres, respectively.

[87] Fumarole temperatures appear to be so low (less than 100 °C (212 °F)) that in 1989 the exhalations could not be detected in the Thematic Mapper infrared band of the Landsat satellite even during night.

[38] La Poruñita, once considered of Holocene age,[1] has been dated at 680,000 ± 200,000 to 420,000 ± 200,000 years ago;[43] it is also not clear if it belongs to the Ollagüe volcanic system.

[91] The oldest stage of activity is known as Vinta Loma and formed the bulk of the volcanic edifice, especially on the eastern side and in the summit area.

[2] The pyroclastic flows are exposed as a 60 metres (200 ft) thick sequence in a cirque close to the summit and reflect the occurrence of Plinian eruptions during this stage of volcanic activity.

[96] Later the Ch'aska Urqu stage was erupted on top of Vinta Loma deposits[2] through radial vents on the southeastern flank.

On the foot of the volcano they sometimes developed pressure ridges, and an 80 metres (260 ft) deep cleft in Ch'aska Urqu may have formed when the dome spread laterally during its formation.

[35] The summit lava dome has a volume of 0.35 cubic kilometres (0.084 cu mi);[57] blocks with sizes of up to 10 metres (33 ft) were formed by landslides during its growth.

Younger shorter flows farther up on the edifice and the summit lava dome conversely contain relatively large quantities of amphibole and biotite.

[11] According to a report in 1894, fumes released from sulfur beds on the volcano can incapacitate a man in seconds, making ascents difficult.

[109] After initial efforts in 1899, 1902 and 1916, in 1932-1936 Luis Borlando set up a company[110] and began to mine sulfur on Ollagüe in response to demand by the saltpeter and copper industries.

[120] Since 2015, an investigation project has been running in the town of Ollagüe to record and preserve the history of sulfur mining and industrialization in the region.

[113] Sulfur mining was mostly carried out by an indigenous workforce,[121] as other people are not adapted to the extreme conditions at high altitudes (cold, hypoxia, intense winds) and thus unable to perform the work.

[124] There was a high turnover in the workforce, which came to a large degree from Bolivia[125] to the point that the Bolivian government curtailed it in 1925, triggering a decline in the Chilean sulfur industry.