Aracar is a large conical

stratovolcano A stratovolcano, also known as a composite volcano, is a typically conical volcano built up by many alternating layers (strata) of hardened lava and tephra. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with ...

in northwestern

Argentina Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...

, just east of the

Chile Chile, officially the Republic of Chile, is a country in western South America. It is the southernmost country in the world and the closest to Antarctica, stretching along a narrow strip of land between the Andes, Andes Mountains and the Paci ...

an border. It has a main summit crater about in diameter and sometimes contains

crater lake Crater Lake ( Klamath: ) is a volcanic crater lake in south-central Oregon in the Western United States. It is the main feature of Crater Lake National Park and is a tourist attraction for its deep blue color and water clarity. T ...

s and a secondary crater. The volcano has formed, starting during the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Tolar Grande about southeast of the volcano, but with no evidence of deformation of the volcano from satellite observations.

Inca The Inca Empire, officially known as the Realm of the Four Parts (, ), was the largest empire in pre-Columbian America. The administrative, political, and military center of the empire was in the city of Cusco. The History of the Incas, Inca ...

archeological sites are found on the volcano.

Geology

Aracar is located in the

Salta province Salta () is a Provinces of Argentina, province of Argentina, located in the northwest of the country. Neighboring provinces are from the east clockwise Formosa Province, Formosa, Chaco Province, Chaco, Santiago del Estero Province, Santiago del Es ...

, north of the Salar de Taca Taca and Arizaro and east of the Salar de Incahuasi and the Sierra de Taca Taca, close to the Chilean border. Volcanoes in the territory rise above the

endorheic An endorheic basin ( ; also endoreic basin and endorreic basin) is a drainage basin that normally retains water and allows no outflow to other external bodies of water (e.g. rivers and oceans); instead, the water drainage flows into permanent ...

sinks and landscape. Cerro Arizaro (9.0 ± 1.3 mya) is another volcano southeast of Aracar. The basement consists of

Paleozoic The Paleozoic ( , , ; or Palaeozoic) Era is the first of three Era (geology), geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma a ...

granite Granite ( ) is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly coo ...

s. The Laguna de Aracar Formation north of Aracar was formed by

Gondwana Gondwana ( ; ) was a large landmass, sometimes referred to as a supercontinent. The remnants of Gondwana make up around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia (continent), Australia, Zea ...

volcanism and has been dated by K-Ar methods to be 266 ± 28 Ma old; it is associated with the Llullaillaco Unit.

Tertiary Tertiary (from Latin, meaning 'third' or 'of the third degree/order..') may refer to: * Tertiary period, an obsolete geologic period spanning from 66 to 2.6 million years ago * Tertiary (chemistry), a term describing bonding patterns in organic ch ...

sedimentary rocks in the east and

arenite Arenite (from the Latin ''arena'', "sand") is a sedimentary clastic rock with sand grain size between 0.0625 mm (0.00245 in) and 2 mm (0.08 in) and containing less than 15% matrix. The related adjective is ''arenaceous''. The e ...

s in the south form the rest of the basement. The volcano's height over the surrounding terrain is between from north to south. Aracar is a polygenetic volcanic cone with a diameter of and a rectangular basis of , covering a surface area of . Four

lava domes In volcanology, a lava dome is a circular, mound-shaped protrusion resulting from the slow Extrusive rock, extrusion of viscous lava from a volcano. Dome-building eruptions are common, particularly in convergent plate boundary settings. Around 6 ...

extend southeast from the volcano. Grey

basaltic Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron ( mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% ...

lava flows Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or unde ...

descend from its summit, forming a gentle western flank and much steeper eastern, northern, and southern flanks. West of the main summit, a wide and several hundred meters deep crater forms Aracar's main crater. Snowmelt occasionally forms small ephemeral lakes in the main crater. A shallow deep secondary crater is surmounted by a flat semilunar wide surface. Small southbound andesitic lava flows are associated with the main crater. There are traces of a

sector collapse A sector collapse or lateral collapse is the structural failure and subsequent collapse of a minimum volume of of a volcano. Unlike smaller flank collapses, a sector collapse can involve the central volcanic pipe and historically this term had b ...

. Some deep gorges cut into the volcano, and erosion has removed of rock.

Moraine A moraine is any accumulation of unconsolidated debris (regolith and Rock (geology), rock), sometimes referred to as glacial till, that occurs in both currently and formerly glaciated regions, and that has been previously carried along by a gla ...

s, mainly occurring on the volcano's eastern side, descend to . A lava field is found beneath the Aracar volcano. It is constructed by lava flows that range in composition from

basaltic andesite Basaltic andesite is a volcanic rock that is intermediate in composition between basalt and andesite. It is composed predominantly of augite and plagioclase. Basaltic andesite can be found in volcanoes around the world, including in Central Ameri ...

bordering on

trachyandesite Trachyandesite is an extrusive igneous rock with a composition between trachyte and andesite. It has little or no free quartz, but is dominated by sodic plagioclase and alkali feldspar. It is formed from the cooling of lava enriched in alkal ...

over

dacite Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. ...

and smaller flows of partially silicic magma. Basal lava flows are heavily eroded and reach in length and width in the south, decreasing from to . They have cancelled out the prior landscape. These lower lava flows reach the Salar de Taca Taca and extend south-southeast. The main

andesitic Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomina ...

cone is high and wide, forming on top of older

dacitic Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. ...

lava flows. The dacite flows which form the edifice's bulk are covered with debris and have flow fronts high. The lava field formed over a north-south slope. Lavas have gray-black porphyric and in some places, vesicular textures.

Andesine Andesine is a silicate mineral, a member of the plagioclase feldspar solid solution series. Its chemical formula is ( Ca, Na)( Al, Si)4 O8, where Ca/(Ca + Na) (% anorthite) is between 30 and 50%. The formula may be written as Na0.7-0.5Ca0.3-0.5 ...

labradorite Labradorite (( Ca, Na)( Al, Si)4 O8) is a calcium-enriched feldspar mineral first identified in Labrador, Canada, which can display an iridescent effect ( schiller). Labradorite is an intermediate to calcic member of the plagioclase series. It ...

plagioclase Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...

and

pyroxene The pyroxenes (commonly abbreviated Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents ions of calcium (Ca), sodium (Na), iron ( ...

phenocryst image:montblanc granite phenocrysts.JPG, 300px, Granites often have large feldspar, feldspathic phenocrysts. This granite, from the Switzerland, Swiss side of the Mont Blanc massif, has large white phenocrysts of plagioclase (that have trapezoid sh ...

s are found in the lavas, which have a fine-grained groundmass.

Apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of Hydroxide, OH−, Fluoride, F− and Chloride, Cl− ion, respectively, in the crystal. The formula of the admixture of ...

augite Augite, also known as Augurite, is a common rock-forming pyroxene mineral with formula . The crystals are monoclinic and prismatic. Augite has two prominent cleavages, meeting at angles near 90 degrees. Characteristics Augite is a solid soluti ...

, and opaque mineral inclusions are also seen. Some lava flows display very small-scale flow bands with feldspat and plagioclase inclusions.

Xenolith A xenolith ("foreign rock") is a rock (geology), rock fragment (Country rock (geology), country rock) that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term ''xenolith'' is almost exclusi ...

s containing

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

and

gabbro Gabbro ( ) is a phaneritic (coarse-grained and magnesium- and iron-rich), mafic intrusive igneous rock formed from the slow cooling magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ch ...

nodules also play a part in the rock composition. The overall rock composition is

calc-alkaline The calc-alkaline magma series is one of two main subdivisions of the subalkaline magma series, the other subalkaline magma series being the tholeiitic series. A magma series is a series of compositions that describes the evolution of a mafic ...

, similar to other magmas in the

Central Volcanic Zone The Andean Volcanic Belt is a major volcanic belt along the Andean cordillera in Argentina, Bolivia, Chile, Colombia, Ecuador, and Peru. It is formed as a result of subduction of the Nazca plate and Antarctic plate underneath the South Americ ...

, with some intraplate and crustal components, with the magmas forming in an open

magma chamber A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it u ...

. Later magmas may have been influenced by the entry of basic magmas from the depth. The total volume of the edifice is about . The volcanic history of Aracar is poorly understood. The bottom lava flows have ages of 3.4 ± 1.2 to 2.6 ± 0.4 mya, but an age of 100,000 years has also been given. Presumably, at first, fluid basaltic lavas were erupted. Subsequently, dacite lavas erupted, accompanied by the formation of a strong slope in the edifice and

hydrothermal Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...

activity at a northwestern lava dome. Finally, the central crater and andesite lava flows were erupted. Beneath altitude, lava flows are well conserved. No historical activity is recorded but in March 1993, inhabitants of Tolar Grande southeast of Aracar observed a high ash or steam column rising from Aracar, which may be either an eruption or the result of

landslides Landslides, also known as landslips, rockslips or rockslides, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, mudflows, shallow or deep-seated slope failures and debris flows. Landslide ...

. Satellite images did not detect edifice deformation during this episode, probably due to

aliasing In signal processing and related disciplines, aliasing is a phenomenon that a reconstructed signal from samples of the original signal contains low frequency components that are not present in the original one. This is caused when, in the ori ...

. Aracar is considered Argentina's 17th most dangerous volcano out of 38.

History and human interaction

The mountain was climbed in 1958 by European climbers, including Mathias Rebitsch, who found an archeological site in the summit area.

Minefield A land mine, or landmine, is an explosive weapon often concealed under or camouflaged on the ground, and designed to destroy or disable enemy targets as they pass over or near it. Land mines are divided into two types: anti-tank mines, wh ...

are present on the northeastern flanks of Aracar, making exploration from that side impossible. A major

archeological site is found in Aracar. Two separate places exist on the summit and secondary crater. The summit place is formed by a

stone circle A stone circle is a ring of megalithic standing stones. Most are found in Northwestern Europe – especially Stone circles in the British Isles and Brittany – and typically date from the Late Neolithic and Early Bronze Age, with most being ...

and low stone walls which are placed just above the slopes down into the main crater. A terrace-shaped stone structure forms the secondary crater, which is placed on its slopes. A smaller terrace is located on its northeastern side.

Timber Lumber is wood that has been processed into uniform and useful sizes (dimensional lumber), including beams and planks or boards. Lumber is mainly used for construction framing, as well as finishing (floors, wall panels, window frames). ...

and wood are found around the stone structures. All these sites have no more than dimensions. These structures may be sanctuary sites that could be accessed by a bystanding public during ceremonies, with the sites themselves located within wind protected areas.

Notes

References

External links

"Cerro Aracar, Argentina" on Peakbagger
* (in Spanish; also includes volcanoes of Argentina) {{Andean volcanoes Stratovolcanoes of Argentina Subduction volcanoes Active volcanoes Mountains of Argentina Volcanic crater lakes Polygenetic volcanoes Six-thousanders of the Andes Puna de Atacama Pliocene stratovolcanoes Pleistocene stratovolcanoes

Geology

History and human interaction

Notes

See also

References

External links