Sairecabur

Geological context

West of South America, the Nazca Plate subducts beneath the South America Plate. This process has formed the Andean Volcanic Belt, which is subdivided into the Northern Volcanic Zone, the Central Volcanic Zone and the Southern Volcanic Zone. These belts have different underlying crusts and have thus different typical magma compositions. These volcanic zones are separated from each other by zones where there is no volcanism, associated with a shallow dip of the seismic zone of the subducting plate.

Sairecabur is part of the Andean Central Volcanic Zone (CVZ). A number of stratovolcanoes can be found in the area, many of them are now ruins. Effusion of large amounts of lava took place from several cones. Many volcanoes are extinct and only a few have documented activity. A work in 1950 estimated that there were about 800 volcanoes in northern Chile, about 37 of them east of the Salar de Atacama. Also part of the volcanism are large ignimbrites, which are usually thought to be of Miocene age. These in the area of Sairecabur often originated in the neighbourhood to the conical volcanoes. The volcanoes formed over crust with thicknesses reaching 70 kilometres (43 mi), thus the erupted magmas are heavily influenced by the crust. A number of the highest stratovolcanoes in the world are in the CVZ. Historical activity has been low in comparison to the volcanic zones of southern Chile and Colombia/Ecuador. In Chile, much of the area of the CVZ is desertic and thus difficult to research.

Sairecabur is constructed on the two Pliocene-Pleistocene Puripicar and Chaxas formations, some lavas have overrun the Purico formation, which is of Pleistocene age and includes ignimbrites from the Purico Complex. The basement beneath Sairecabur and Licancabur contains a large number of faults.

Geography

Sairecabur is located at a distance of 25 kilometres (16 mi) from San Pedro de Atacama. The volcano saddles the frontier between Bolivia and Chile, where it lies in the Antofagasta Region. Laguna Verde lake, Licancabur volcano and Portezuelo de Chaxas pass lie south of Sairecabur. East of the Sairecabur range lie Mount Nelly and Cerro Laguna Verde.

The Treaty of Peace and Friendship (1904) traced the frontier between Bolivia and Chile along the Sairecabur chain. Disagreements between the topographic maps in the two countries mean that the naming of the mountains is often confusing. Escalante is also known as Apagado.

The Sairecabur range forms a drainage divide between the Salar de Atacama on the western side and a number of small endorheic basins in Bolivia on the eastern side. Licancabur and Juriques farther south and Tocorpuri farther north are part of the same divide. Dry valleys on the eastern side of the Sairecabur range ultimately drain into Laguna Verde. The Rio Purifica originates on Sairecabur's slope at an altitude of 3,950 metres (12,960 ft), it later converges with the Rio Puritama to form the San Pedro de Atacama River.

Geology

The Sairecabur range is a 22 kilometres (14 mi) long chain of volcanoes going from Escalante volcano south of Putana in the north to Sairecabur proper in the south, including at least ten centres which have been active in postglacial time. Two additional centres exist northeast of Escalante.

5,971 metres (19,590 ft) high Sairecabur is the highest volcano in the range. Lava flows extend from Sairecabur as far as 2.5 kilometres (1.6 mi) to the northwest. A 4.5 kilometres (2.8 mi) wide caldera exist south of Sairecabur and formed on an older volcano, several young lava flows formed south of this caldera. 5,819 metres (19,091 ft) high Escalante (22°37′00″S 67°33′00″W) has a crater lake. Puritama volcano west of Sairecabur has generated 16 kilometres (9.9 mi) lava flows that extend along tectonic canyons. 5,722 metres (18,773 ft) high Curiquinca (22°36′00″S 67°52′00″W) and 5,748 metres (18,858 ft) high Cerro Colorado (22°35′00″S 67°55′00″W) are found northwest and northeast, respectively, of the range.

The caldera is bounded by cliffs reaching a height of 400 metres (1,300 ft), which are buried by lava flows from Sairecabur on the northern rim; one of these reaches the caldera bottom. The pre-collapse volcano was about 7,000 metres (23,000 ft) high and would have been one of the highest volcanoes in the world; Ojos del Salado reaches an altitude of 6,885 metres (22,589 ft). Sairecabur proper has three cones. Thick (10 metres (33 ft)) and short lava flows emanated from the northern cone. Glacial erosion has affected Sairecabur and moraines cover some lava flows south of Sairecabur. The total volume of the Sairecabur proper is about 35 cubic kilometres (8.4 cu mi).

Lava flows with a young appearance extend from each of these ten centres. An older centre has generated a 30 kilometres (19 mi) long lava flow that flowed southwestwards. Some older lava flows from Sairecabur were later buried by lava flows from Licancabur.

Cerro Colorado and Curiquinca are aligned in an west-east pattern. Other volcanoes in the area such as Lascar-Aguas Calientes, Licancabur-Juriques and La Torta-Tocorpuri also form such alignments which seem to be controlled by north-south tension in the crust. Sairecabur is located on faults which are also apparent at Laguna Verde.

Petrology

Escalante and Sairecabur have erupted dark andesites, later also dacites. Mafic enclaves are found in the post-caldera lavas. The colour of the rocks is black, brown or gray. Minerals include amphibole, biotite, bronzite, Ca-containing augite, clinopyroxene, hornblende, magnetite, orthopyroxene, plagioclase, pyroxene and quartz. In addition, apatite, ilmenite, iron oxides and zirconia are found. At least one lava erupted after the caldera-forming eruption contains olivine. Lavas erupted before the caldera-forming eruption of Sairecabur contain glass and have a microlithic texture. The magmas are calc-alkaline with medium-high K content.

Fumarolically altered rocks are found on the eastern flanks of the chain. Desert patine covers post-caldera lavas.

Based on crystal composition, the magmas of Sairecabur formed at temperatures of 850–950 °C (1,560–1,740 °F). The process started by partial melting of the mantle involving peridotite and subsequent interaction with the crust and fractional crystallization. Andesites erupted before the caldera formation formed at temperatures 90 °C (160 °F) higher than dacites erupted after the formation of the caldera. O, Pb and Sr isotope ratios are typical for magmas in CVZ. The Pb isotope ratios are consistent with these found in the crust, specifically of the so-called "Antofalla" domain of Andean crust, the remnant of a terrane of Gondwana.

Geological history

The 7 million years old Chaxas ignimbrite massif has been related to the caldera-forming eruption at Sairecabur. These dacitic ignimbrites spread southwest towards the Salar de Atacama. Such old age for the Chaxas ignimbrite however has been questioned, considering that it is inconsistent with stratigraphic relationships of this ignimbrite to older ignimbrite. The lava formations named Post-Caldera Lavas I at Sairecabur are of Pleistocene and Post-Caldera Lavas II of Holocene age. A fresh flow that extends northwest from Sairecabur appears to be the most recent flow.

The formation of the caldera preceded the formation of the other cones in the range. The edifice this caldera formed on formed during the Pliocene-Quaternary considering the morphology of its deposits.

Sairecabur volcano is the youngest volcano in the chain; Escalante is also young but not as young as Sairecabur. There are no reports of contemporaneous activity nor has fumarolic activity been reported. Future activity at Sairecabur may disturb activity at Atacama Large Millimeter Array.

Climate and biology

The climate at Sairecabur is dry and cloudless, leading to the absence of glaciers and permanent snowcover at Sairecabur. Overall the Atacama Altiplano has a desert climate with precipitation below 200 millimetres per year (0.25 in/Ms), but during the late Pleistocene and early Holocene the climate was wetter. T isotope ratios of present-day snowfall are consistent with the isotope ratios determined for precipitation that arises in the continent, rather than from the Pacific Ocean. The rainshadow effect exercised by the Andes and the stability of the South Pacific High are responsible for this dryness.

The dry and cloudless climate together with the low latitude and high altitude gives the region some of the highest insolations on Earth; at Sairecabur it amounts to 98% of the solar constant. The coincidence between the southern hemisphere summer solstice on 21 December and the perihelion, the point of lowest Earth-Sun distance, on 3 January contribute to the high insolation. Ultraviolet radiation is also high in the area, with values 15.6–36.4 watts per square metre (0.00194–0.00453 hp/sq ft) reported for ultraviolet radiation B and ultraviolet radiation A respectively.

Temperatures at 5,820 metres (19,090 ft) ranged −8.7 – −16.3 °C (16.3–2.7 °F) in 1991–1994. A series of measurements in 1995 indicated that surface temperatures at an altitude of 5,820 metres (19,090 ft) range from −20–20 °C (−4–68 °F) in winter, and soil temperatures at depths of 5 centimetres (2.0 in) also in winter between almost 10 °C (50 °F) and less than −10 °C (14 °F). There are large differences between daytime and night temperatures. Between 1991–1994 the average speed of wind amounted to 5–11 metres per second (16–36 ft/s).

About 250 species have been found in the valley west of Sairecabur. Extremophilic yeast species have been found at Sairecabur. In 1955, penitentes, a form of ice, was reported to be widespread at Saciel.

Pre-modern times

Andesite found at some archeological sites in the Atacama may come from Sairecabur.

Sairecabur and Curiquinca both have mountain sanctuaries made by the Inca. Licancabur and Juriques farther south were also sites of such sanctuaries. Sairecabur was considered to be a sacred mountain.

Recent times

A sulfur mine is active at Saciel, north of Sairecabur. Sulfur mining there, at Cerro Colorado and Putana in the 1950s contributed to the growth of San Pedro de Atacama, where the mined sulfur was transported to. A report in 1955 indicated the presence of about 600,000 tonnes (590,000 long tons; 660,000 short tons) ore containing 55–60% sulfur. A 21 kilometres (13 mi) long mining dirt road with a single lane leads from the El Tatio highway to Sairecabur.

Sairecabur is since 2003 the site of the 0.8 metres (2 ft 7 in) diametre Receiver Lab Telescope, a telescope which operates in the terahertz range of the electromagnetic spectrum; ground-based astronomy in this range was long considered to be impossible since the atmosphere absorbs radiation in that frequency range heavily. With an altitude of 5,525 metres (18,127 ft) the telescope is the highest submillimeter telescope in the world.