Geology and geomorphology
Ojos del Salado along with several other high volcanoes such as El Muerto, El Solo, Nevado Incahuasi and Nevado Tres Cruces lies at the southern end of the Central Volcanic Zone of the Andes.
Volcanic activity in the region commenced 26 million years ago in the
Cordillera Claudio Gay, at the same time as the neighbouring
Maricunga Belt volcanoes were active. Starting 18 million years ago, local subduction of the Nazca Plate beneath the South America Plate became increasingly shallower and this caused volcanism to shift from the Maricunga Belt to the Ojos del Salado region.
The Paleozoic basement crops out only northwest of Nevado Tres Cruces. Other geologic units in the region are Oligocene sedimentary units and volcanic rocks ranging from Oligocene age to less than 1.5 million years old. The area is part of a tectonic boundary between a volcanically active region north of the boundary and a less volcanically active region south of the boundary, which is also characterized by geographical differences, e.g. the presence of transverse valleys.
Ojos del Salado like other major neighbouring mountains is a complex of lava domes and lava flows, with the main summit surrounded by peripheral domes such as El Solo and El Fraile and many other lateral centres. Fumaroles are encountered at the summit of Ojos del Salado within a depression. Potassium-argon dating has yielded ages of 1.2 ± 0.3 million years ago and less than 1 million years ago from rocks north of Ojos del Salado.
There are glaciers on the mountain, in the crater and in the form of penitentes which reach heights of 8–5 metres (26–16 ft). Other reports indicate the absence of glaciers on Ojos del Salado. Permafrost exists on the mountain and its melting nourishes several lakes; one lake fed by a creek lies at 6,500 metres (21,300 ft) altitude.
Ojos del Salado is an active volcano, but the question of whether it should be considered currently (or "historically") active is arguable. According to the Smithsonian Institution's Global Volcanism Program, the most recent known eruption was about 1,300 years ago, with a large margin of uncertainty. However, there is also some evidence for a minor ash emission in 1993, which would definitely qualify the volcano as historically active. The presence of fumaroles high on the mountain and recent-looking lava flows, albeit of uncertain age, also argues in favor of a categorization as "active." By these definitions, Ojos del Salado is the highest historically active volcano on Earth. If the older date is accepted, the title of "highest historically active volcano" might reside instead with the somewhat lower Llullaillaco volcano, which certainly has erupted in historic times (most recently in 1877) and is considered active.
Salado's rock is predominantly potassium-rich dacite and rhyodacite. Its lavas are high in biotite, hornblende, plagioclase, and opaques, with lower levels of augite, quartz, and hypersthene.