Llullaillaco is a stratovolcano at the border of Argentina and Chile. It lies in the Puna de Atacama, a region of very high volcanic peaks on a high plateau within the Atacama Desert, one of the driest places in the world. It is the fifth highest volcano in the world and it is also the seventh highest mountain of the Western Hemisphere.
Llullaillaco follows the typical Puna de Atacama volcano pattern: it is surrounded by large debris fields, and is perpetually capped by small snow patches, though there are no true glaciers due to the extreme aridity. The snow line in this region is the highest in the world at around 6,500 metres (21,300 ft), which is around 1,000 metres (3,300 ft) higher than in the Himalayas and 2,000 metres (6,600 ft) higher than in the Andes of Colombia and Ecuador.
It has been confirmed that Incas climbed Llullaillaco in the pre-Columbian period. Artifacts on the summit constitute the highest evidence of human presence worldwide before the late nineteenth century. Also, the huáqueros may have also reached its summit and those of other mountains in the region during their searches. The first recorded ascent was on December 1, 1952, by Bión González and Juan Harseim.
There are several climbing routes which do not require specialized climbing techniques, although the altitude imposes great difficulty and is by itself a very dangerous factor. Crampons and an ice axe are needed as most paths cross ice patches. However, the area on the Chilean side of the mountain is known to have some land mines that were installed during the Argentina-Chile conflict period of 1978–1982, thus rendering it extremely dangerous.
Two major geological stages can be highlighted in the history of the volcano: Llullaillaco I, the ancestral primary volcano, dates back to the Pleistocene. Two very eroded cones with associated lava flows, up to 20 km in length, distributed mainly to the West, evidence these stages.
Built upon it there is a well preserved secondary post-glacial edifice called Llullaillaco II, which has been active during human history. Many Holocene lava flows are associated with this latter phase; the two most notable are directed North and South of the volcano. These youthful-looking dacitic flows have been dated to be of late Pleistocene age. Moreover, hot avalanche deposits, extending up to 3 km, are associated with one of the southern lava flows.
There are still other very conspicuous flows remaining: one of the most striking, apparently caused by partial collapse of Lullaillaco I about 150,000 years ago, extends eastward into Argentina, diverging around Cerro Rosado stratovolcano 17 km to the East and terminating in the Salar del Llullaillaco. This deposit has not yet been thoroughly studied.
There are reports of eruptions in 1854, 1868, and 1877, possibly causing the youngest lava flows in the area, which are easily recognizable because of their very dark appearance.