Cerro Galán is a caldera in the Catamarca Province of Argentina. It is one of the largest exposed calderas in the world and forms part of the Central Volcanic Zone of the Andes, one of the three volcanic belts found in South America. One of several major caldera systems in the Central Volcanic Zone, the mountain is grouped into the Altiplano–Puna volcanic complex.
Volcanic activity at Galán is the indirect consequence of the subduction of the Nazca Plate beneath the South America Plate, and involves the infiltration of melts into the crust and the formation of secondary magmas which after storage in the crust give rise to the dacitic to rhyodacitic rocks erupted by the volcano.
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 largest eruption of Galán was 2.08 ± 0.02 million years ago and was the source of the Galán ignimbrite, which covered the surroundings of the caldera with volcanic material. The volume of this ignimbrite has been estimated to be about ; after this eruption much smaller ignimbrite eruptions took place and presently two hot springs are active in the caldera.
Geography and geomorphology The Galán caldera lies in the northwestern Catamarca Province of Argentina and was discovered in 1975 in a remote region of the Andes, using satellite images. The town of Antofagasta de la Sierra lies west-southwest of the Galán caldera, Tacuil is almost due northeast from the caldera and El Penon southwest of the volcano. The caldera is difficult to access. An Inka tambo was situated at Laguna Diamante, and an important prehistoric travel route passed through the caldera. Sacrificial offers were given on the summit of Galán.
Galán is part of the Central Volcanic Zone of the Andes, which lies on the western margin of South America, where the Nazca Plate subducts beneath the South America Plate. There are about 50 volcanoes with recent activity in the Central Volcanic Zone, and additional volcanoes exist in the Northern Volcanic Zone and the Southern Volcanic Zone, two other volcanic belts north and south.
The volcanic arc runs along the borders between Bolivia and Argentina with Chile, and behind the volcanic arc lies a chain of silicic volcanoes, of which Galán is a southern member. The whole region has been subject to substantial ignimbrite-forming volcanism with many eruptions producing volumes of rock larger than , although the actual vents often are only visible from space imagery. Many vents cluster in an area known as the Altiplano-Puna volcanic complex which occupies a surface of about approximately north of Galán, and which includes the large calderas of La Pacana, Cerro Guacha, Pastos Grandes and Cerro Panizos as well as more recent geothermal systems. This volcanism appears to be a surface expression of a pluton, and at depths of beneath the Altiplano-Puna volcanic complex electrical, gravity and seismic tomography data have localized a structure of partially molten rock called the "Altiplano Puna Magma Body". Volcanism in this "back" region may not be directly related to subduction processes despite the region itself being close to a subducting margin.
The Galán caldera lies on the eastern margin of the Andes, where the Sierras Pampeanas begin. The region is characterized by the Puna, a high plateau similar to Tibet in Asia. A magmatic body has been found under Galán, inside the crust.
Local
Galán is a caldera with topographic dimensions of , of which about are part of the caldera proper. Such dimensions make Galán one of the biggest calderas on Earth; it has been described as a supervolcano. The floor of the caldera reaches an elevation of or about , and the whole caldera has an elliptical shape extending in the north–south direction. Only the western margin of the caldera structure appears to be a true caldera margin, however, with different landforms forming the rest of the caldera walls and the actual collapse caldera covering only a portion of the topographic caldera expression; the latter has been defined to be a volcano-tectonic depression.
The caldera contains a resurgent dome, whose highest point in the frost-shattered Galán massif reaches an elevation of about -. Seismic tomography has identified a slow-speed anomaly beneath Galán, which has a volume of about and is considered to be a magma reservoir of the volcano.
Summits along the caldera margin include Cerro Aguas Calientes (a lava dome) to the north, Cerro Leon Muerto to the southeast, Cerro Pabellon to the southwest and Cerro Toconquis to the northwest. On the western rim, elevations of are reached. Younger volcanoes have developed on the western and northern rim of the Galán caldera.
Hydrology and hydrothermal system
The caldera contains a lake in its southwestern corner, which is known as Laguna Diamante and may formerly have occupied much of the caldera. Laguna Diamante has gained attention among scientists for the extreme environmental conditions that life within the lake has to withstand, including high arsenic contents of the waters and high insolation with ultraviolet radiation. 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. Tube-shaped microbialites have also been reported. A smaller lake known as Laguna Pabellon lies just south of Laguna Diamante. North of the resurgent dome, the Rio Aguas Calientes drains the caldera northward, while east of it the Rio Leon Muerto runs eastward out of the caldera. Another Leon Muerto river drains the southern side of the caldera into Laguna Pabellon.
Rivers in the caldera and neighbourhood display river terraces which may reflect pre-caldera formation uplift of the terrain and uplift associated with the resurgent dome. These drainages eventually converge in the Rio de Los Patos and end into the Salar del Hombre Muerto north of Galán. 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. The western flanks of the caldera drain into the Antofagasta de la Sierra valley through a number of drainages such as Rio Punilla, Rio Toconquis, Rio Miriguaca, Rio Las Pitas; the waters eventually end into the Laguna Antofagasta south of Antofagasta de la Sierra.
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, both emitting water with temperatures of about . The first one is known as the Aguas Calientes hydrothermal spring and features deposits of tufa and boiling water. Another geothermal system is known as La Colcha and includes fumaroles as well as boiling water; it drains into Laguna Diamante. Finally, on the northeastern side of the caldera is Piscinas Burbujeantes, with mud volcanoes and small fumaroles. These springs have emplaced clay, salt, sinter and travertine deposits and have been prospected for geothermal power development.
Geology The basement beneath the caldera consists of 600–365 million years old metamorphic and sedimentary rocks of Precambrian to Paleozoic age. These include intrusions of granitoid character and are overlain with Paleozoic marine sediments. Ordovician units are also present and form sediment layers up to thick. Basements outcrops occur in the northeastern margin of the caldera.
About 14.5 million years ago volcanic activity started in the region, first west of Galán but by 7 million years ago it shifted to the future caldera, forming the Cerro Colorado, Pabellon and Cerro Toconquis composite volcanoes on its future western rim. The more westerly centres are today represented by eroded volcanoes. Since about 6.6 million years ago the volcanic activity produced rocks of both mafic and silicic compositions. The increase of volcanic activity has been attributed to the steepening of the Nazca Plate slab which allowed mantle material to penetrate into the space between the lower crust and the slab. North of 21° degrees southern latitude ignimbritic volcanism started earlier, generating the Altos de Pica and Oxaya formations.
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. The positions of the exact vents are controlled by recent fault systems in the region.
Since about 10 million years ago, the area has been subject to reverse faulting which has disrupted the basement along north–south lines, forming a rift valley that also stretches from north to south. The magma erupted by the Galán system was likewise channelled along such fault systems, and neighbouring volcanoes were similarly influenced by them; the fault systems at Galán proper are known as the Diablillos-Galán faults. Another major lineament in the area is the Archibarca lineament, which is formed by a strike-slip fault that extends from the northwest to the southeast in the region and which intersects the Diablillos-Galán faults at the location of the caldera.
Composition Galán has erupted mainly potassium-rich dacitic to rhyolitic rocks that are often called rhyodacitic, and which reflect a calc-alkaline suite. Each ignimbrite has usually a uniform composition but there is some variation between individual ignimbrites; for example older rocks contain amphibole and younger rocks instead sanidine. Minerals contained in the eruption products include allanite, apatite, biotite, hornblende, ilmenite, magnetite, orthopyroxene, plagioclase, quartz, sanidine and zircon. Hydrothermal alteration has left calcite in some rocks. Trace element patterns are distinct in the Galán ignimbrite in comparison to the Toconquis Group rocks.
The formation of the Galán magma has been explained with melting of lower crustal rocks under the influence of rising basaltic magmas that supplied the heat needed for the melting processes, and which also directly contributed to magma formation through mixing events. Further metasomatism in the crust and fractional crystallization processes completed the magma genesis process. Probably under the influence of larger scale tectonics, magma that accumulated into a mid-crustal mush zone is eventually transferred into shallow magma chambers at depths of ; recharge events where deep magma entered the shallow magma bodies may have triggered eruptions at Galán. After eruption, a leftover pluton would have been generated inside the crust.
Based on the presence of two separate populations of pumice in the Galán ignimbrite it has been inferred that there were two types of magma in the magmatic system during the Galán eruption, a larger volume of so-called "white" magma and a "grey" magma which was injected into the "white" magma pool and eventually rose above the latter. More generally, it appears that before each eruption there were two batches of magma present beneath the volcano which however were very similar owing perhaps to a homogenization process that took place deep in the crust. Before the eruption, the magma is estimated to have been hot.
Climate and biology Galán lies in a region of arid climate, with annual precipitation amounting to about . Frosts occur year-round. Climate data are known for Salar de Hombre Muerto north of Galán; average temperatures there are in summer and winter, respectively. Precipitation occurs mostly during the summer months.
At high elevations there is no vegetation. Between elevation, vegetation consists of high altitude steppe dominated by Poaceae (grasses) such as Festuca (fescue) and Stipa (feather grass). At lower altitudes, wetlands have their own vegetation. In sheltered areas birds like ducks and flamingos can be observed.
Eruptive history