Galán was active between 5.6 and 4.51 million years ago, when it generated a number of ignimbrites known as the Toconquis group which crop out mainly west of the caldera.

The Galán caldera lies in the northwestern Catamarca Province of Argentina and was discovered in 1975 in a remote region of the Andes,[2] using satellite images.

[16] The whole region has been subject to substantial ignimbrite-forming volcanism with many eruptions producing volumes of rock larger than 100 cubic kilometres (24 cu mi), although the actual vents often are only visible from space imagery.

[17] Many vents cluster in an area known as the Altiplano-Puna volcanic complex which occupies a surface of about 70,000 square kilometres (27,000 sq mi)[13] approximately 200 kilometres (120 mi) north of Galán,[18] and which includes the large calderas of La Pacana, Cerro Guacha, Pastos Grandes and Cerro Panizos as well as more recent geothermal systems.

[28] Seismic tomography has identified a slow-speed anomaly beneath Galán, which has a volume of about 22,000 cubic kilometres (5,300 cu mi) and is considered to be a magma reservoir of the volcano.

[43][44] The water is hyper-alkaline and five times as salty as the sea but supports microorganisms which form microbial mats and provide food for a colony of flamingos.

[46] These drainages eventually converge in the Rio de Los Patos and end into the Salar del Hombre Muerto north of Galán.

[47][48] The high Cerro Galán intercepts moisture transported from east, thus nourishing the Rio de Los Patos in a region where long permanent watercourses are unusual.

[50] Two hot springs are found within the caldera, the first close to its northern end and the second on the southwestern foot of the resurgent dome,[51] both emitting water with temperatures of about 56–85 °C (133–185 °F).

[57] The basement beneath the caldera consists of 600–365 million years old metamorphic[58] and sedimentary rocks of Precambrian to Paleozoic age.

[65] North of 21° degrees southern latitude ignimbritic volcanism started earlier, generating the Altos de Pica and Oxaya formations.

[66] Mafic volcanism occurred south and west of Galán both before its large eruption and afterwards, in the valley of Antofagasta de la Sierra and may have continued to less than ten thousand years ago.

[67] Since about 10 million years ago, the area has been subject to reverse faulting which has disrupted the basement along north–south lines,[61] forming a rift valley that also stretches from north to south.

[70] Galán has erupted mainly potassium-rich dacitic to rhyolitic rocks that are often called rhyodacitic,[71] and which reflect a calc-alkaline suite.

[73] Minerals contained in the eruption products include allanite, apatite, biotite, hornblende, ilmenite, magnetite, orthopyroxene, plagioclase, quartz, sanidine and zircon.

[80] More generally, it appears that before each eruption there were two batches of magma present beneath the volcano[78] which however were very similar owing perhaps to a homogenization process that took place deep in the crust.

[83] Climate data are known for Salar de Hombre Muerto north of Galán; average temperatures there are 8–23 °C (46–73 °F) in summer and winter, respectively.

[86][87] These stages have left an ignimbrite plateau that surrounds the caldera[4] except on its southern side, and which is noticeable on satellite images.

[28] It covers a surface area of about 3,500 square kilometres (1,400 sq mi)[17] and is the largest ignimbrite system in the Puna plateau.

[94] On the northern side of the Galán complex, ignimbrites extend up to 80 kilometres (50 mi) away from the caldera and may have reached even larger distances prior to erosion,[92] and they have thicknesses of 300 metres (980 ft).

[98] Emission of lava flows occurred during the Toconquis phase as well,[99] in general there was vigorous volcanic activity between the eruptions that formed the main ignimbrites.

[107] A contrary view is that the Galán ignimbrite was largely eroded only on its northern side by wind action, forming yardangs.

[31] The "Toba Dacitica" 270 kilometres (170 mi) outcrop away from the volcano was once considered part of the Galán eruption but later compositional differences were found.

The most recent activity was of tectonic nature and consists of movements along the faults and mafic volcanism ("Incahuasi Formation"[123]) farther west.

[57] A phreatic explosion took place at La Colcha at some point in the past, and stronger hydrothermal activity was necessary to emplace the travertines.