Antofagasta de la Sierra is a

volcanic field A volcanic field is an area of Earth's crust that is prone to localized volcanic activity. The type and number of volcanoes required to be called a "field" is not well-defined. Volcanic fields usually consist of clusters of up to 100 volcanoes ...

Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, th ...

. The main type of volcanic edifice in the area are

scoria cones A cinder cone (or scoria cone) is a steep conical hill of loose pyroclastic fragments, such as volcanic clinkers, volcanic ash, or scoria that has been built around a volcanic vent. The pyroclastic fragments are formed by explosive eruptions o ...

, it is formed by the La Laguna, Jote and Alumbrera volcanoes. The first and last of these form a sub-group which is better researched. Various dating methods have yielded ages from several million to several hundred thousand years ago, but some vents appear to be of

Holocene The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togethe ...

age.

Geology

Regional

Volcanic activity in the Andes occurs in four regions, the

Northern 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 Ame ...

, 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 ...

, the

Southern 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 American ...

and the

Austral Volcanic Zone The Andean Volcanic Belt is a major volcanic belt along the Andes, 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 Am ...

. The first three of these are found where the

Nazca Plate The Nazca Plate or Nasca Plate, named after the Nazca region of southern Peru, is an oceanic tectonic plate in the eastern Pacific Ocean basin off the west coast of South America. The ongoing subduction, along the Peru–Chile Trench, of the Na ...

subducts beneath the

South America Plate The South American Plate is a major tectonic plate which includes the continent of South America as well as a sizable region of the Atlantic Ocean seabed extending eastward to the African Plate, with which it forms the southern part of the Mid-A ...

, while the last zone occurs at the subduction zone of the

Antarctic Plate The Antarctic Plate is a tectonic plate containing the continent of Antarctica, the Kerguelen Plateau, and some remote islands in the Southern Ocean and other surrounding oceans. After breakup from Gondwana (the southern part of the superconti ...

.Báez ''et al.'' 2017, p.313 The Andean

runs along the Western Cordillera of the Andes and along the

Altiplano The Altiplano (Spanish for "high plain"), Collao (Quechua and Aymara: Qullaw, meaning "place of the Qulla") or Andean Plateau, in west-central South America, is the most extensive high plateau on Earth outside Tibet. The plateau is located at the ...

. During the

Neogene The Neogene ( ), informally Upper Tertiary or Late Tertiary, is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period million years ago ( Mya) to the beginning of the present Quaternary Period Mya. ...

, the position of this volcanic arc moved eastward and the arc became broader, probably due to a change in the tilt of the

slab Slab or SLAB may refer to: Physical materials * Concrete slab, a flat concrete plate used in construction * Stone slab, a flat stone used in construction * Slab (casting), a length of metal * Slab (geology), that portion of a tectonic plate that i ...

of the

. Volcanic activity during this time was heavily influenced by local

strike-slip fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

s which acted to channel the magma flows. About 7 million years ago, tectonics and volcanic activity changed in the region probably in response to the

delamination Delamination is a mode of failure where a material fractures into layers. A variety of materials including laminate composites and concrete can fail by delamination. Processing can create layers in materials such as steel formed by rolling and ...

of the crust beneath the region: Large scale

ignimbrite Ignimbrite is a type of volcanic rock, consisting of hardened tuff. Ignimbrites form from the deposits of pyroclastic flows, which are a hot suspension of particles and gases flowing rapidly from a volcano, driven by being denser than the surrou ...

s were erupted at

Galán 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 ...

and small

back-arc A back-arc basin is a type of geologic basin, found at some convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found in the western Pacific Ocean. Most of ...

volcanoes erupted

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...

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 ...

lavas. These small volcanic centres consist of

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

lava flow 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 und ...

maar A maar is a broad, low-relief volcanic crater caused by a phreatomagmatic eruption (an explosion which occurs when groundwater comes into contact with hot lava or magma). A maar characteristically fills with water to form a relatively shallow ...

scoria cone Scoria is a pyroclastic, highly vesicular, dark-colored volcanic rock that was ejected from a volcano as a molten blob and cooled in the air to form discrete grains or clasts.Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds. (2005) ''G ...

s and

tuff ring Phreatomagmatic eruptions are volcanic eruptions resulting from interaction between magma and water. They differ from exclusively magmatic eruptions and phreatic eruptions. Unlike phreatic eruptions, the products of phreatomagmatic eruptions conta ...

s.Báez ''et al.'' 2017, p.312

Local

The Antofagasta de la Sierra volcanic field lies within the north–south trending tectonic depression of the same name, in the

Catamarca Province Catamarca () is a province of Argentina, located in the northwest of the country. The province had a population of 334,568 as per the , and covers an area of 102,602 km2. Its literacy rate is 95.5%. Neighbouring provinces are (clockwise, fr ...

. Of the individual vents the La Laguna and Alumbrera cones are the best studied ones,Báez ''et al.'' 2017, p.313-314 they lie about south of

Antofagasta de la Sierra Antofagasta de la Sierra is a volcanic field in Argentina. The main type of volcanic edifice in the area are Cinder cone, scoria cones, it is formed by the La Laguna, Jote and Alumbrera volcanoes. The first and last of these form a sub-group wh ...

. These volcanoes are associated with black lava flow fields. The Jote volcanoes consist of a cluster of

cinder cone A cinder cone (or scoria cone) is a steep conical hill of loose pyroclastic fragments, such as volcanic clinkers, volcanic ash, or scoria that has been built around a volcanic vent. The pyroclastic fragments are formed by explosive eruptions o ...

s and lava flows. Alumbrera cone is high and has an elliptical shape with a maximum length of , probably due to wind influencing the growth of the cone. The cone shows signs of having been gravitationally unstable, including a

sector collapse A sector collapse is the collapse of a portion of a volcano due to a phreatic eruption, an earthquake, or the intervention of new magma. Occurring on many volcanoes, sector collapses are generally one of the most hazardous volcanic events, and will ...

on its northwestern flank. The cone has a wide

crater Crater may refer to: Landforms *Impact crater, a depression caused by two celestial bodies impacting each other, such as a meteorite hitting a planet *Explosion crater, a hole formed in the ground produced by an explosion near or below the surfac ...

and a few

fissure vent A fissure vent, also known as a volcanic fissure, eruption fissure or simply a fissure, is a linear volcanic vent through which lava erupts, usually without any explosive eruption, explosive activity. The vent is often a few metres wide an ...

s. Alumbrera has a volume of and is the source of a large

field which covers a surface of and surrounds the Alumbrera cone.

Lapilli Lapilli is a size classification of tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. ''Lapilli'' (singular: ''lapillus'') is Latin for "little stones". By definition lapilli range f ...

fill the crater of Alumbrera, which has alum,

ochre Ochre ( ; , ), or ocher in American English, is a natural clay earth pigment, a mixture of ferric oxide and varying amounts of clay and sand. It ranges in colour from yellow to deep orange or brown. It is also the name of the colours produced ...

and

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...

colours. La Laguna is smaller than Alumbrera: The cone is only high and wide, with a deep crater. As with Alumbrera La Laguna features one fissure vent and a lava flow field, with a smaller volume of and surface area of .Báez ''et al.'' 2017, p.314 Both La Laguna and Alumbrera were fed by northwest-striking fissures, which is unusual as most lineaments in the area strike northeastwards or north–south. The surfaces of erupted rocks have been subject to various degrees of wind-driven erosion which has left grooves in the most altered units. Most likely the magma supply at these two vents was so large that the rising dykes were able to force pre-existent lineaments open and to pass through them.Báez ''et al.'' 2017, p.328

Composition

Alumbrera and La Laguna has erupted alkaline to sub-alkaline

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% of a ...

trachyandesite Trachyandesite is an extrusive igneous Rock (geology), rock with a composition between trachyte and andesite. It has little or no free quartz, but is dominated by Sodium, sodic plagioclase and alkali feldspar. It is formed from the cooling of ...

trachybasalt Trachybasalt is a volcanic rock with a composition between trachyte and basalt. It resembles basalt but has a high content of alkali metal oxides. Minerals in trachybasalt include alkali feldspar, calcic plagioclase, olivine, clinopyroxene and li ...

with a

porphyritic Porphyritic is an adjective used in geology to describe igneous rocks with a distinct difference in the size of mineral crystals, with the larger crystals known as phenocrysts. Both extrusive and intrusive rocks can be porphyritic, meaning all ...

texture.

Phenocryst 300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter ...

s within the rock include

olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...

and

plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prope ...

clinopyroxene The pyroxenes (commonly abbreviated to ''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 calcium (Ca), sodium (Na), iron (Fe II) ...

, which form also the bulk of the

matrix Matrix most commonly refers to: * ''The Matrix'' (franchise), an American media franchise ** ''The Matrix'', a 1999 science-fiction action film ** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchis ...

.Báez ''et al.'' 2017, p.319-323 The Jote lavas have similar composition to the La Laguna rocks. The chemical composition of the old and the new La Laguna volcanoes is different, with the old lavas showing a more

volcanic arc A volcanic arc (also known as a magmatic arc) is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc lo ...

-like composition. "

Alum An alum () is a type of chemical compound, usually a hydrated double salt, double sulfate salt (chemistry), salt of aluminium with the general chemical formula, formula , where is a valence (chemistry), monovalent cation such as potassium or a ...

" (in reality a mixture of

aluminium sulfate Aluminium sulfate is a salt with the chemical formula, formula aluminium, Al2sulfate, (SO4)3. It is soluble in water and is mainly used as a Coagulation (water treatment), coagulating agent (promoting particle collision by neutralizing charge) in ...

and

magnesium sulfate Magnesium sulfate or magnesium sulphate (in English-speaking countries other than the US) is a chemical compound, a salt with the formula , consisting of magnesium cations (20.19% by mass) and sulfate anions . It is a white crystalline solid, ...

) found at Alumbrera was mined by the local population.Reichert 1907, p.80-81

Eruptive history

Eruptions at Alumbrera occurred in two phases, the first formed toothpaste lava and

aa lava 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 und ...

and the second in addition to lava small

pyroclastic flow A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of bu ...

s and a tephra blanket. A

scoria Scoria is a pyroclastic, highly vesicular, dark-colored volcanic rock that was ejected from a volcano as a molten blob and cooled in the air to form discrete grains or clasts.Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds. (2005) ''G ...

fallout is found as far as east of Alumbrera. During this second phase, the sector collapse at Alumbrera occurred and was followed by the emission of

lava bomb A volcanic bomb or lava bomb is a mass of partially molten rock (tephra) larger than 64 mm (2.5 inches) in diameter, formed when a volcano ejects viscous fragments of lava during an eruption. Because volcanic bombs cool after they l ...

s during Strombolian eruptions. La Laguna at first erupted several now heavily eroded lava flows and formed an old cone.Báez ''et al.'' 2017, p.325 Later during a second phase of activity

s and

formed a newer cone and two lava flow fields.Báez ''et al.'' 2017, p.326-327 At both volcanoes,

phreatomagmatic Phreatomagmatic eruptions are volcanic eruptions resulting from interaction between magma and water. They differ from exclusively magmatic eruptions and phreatic eruptions. Unlike phreatic eruptions, the products of phreatomagmatic eruptions cont ...

activity may have occurred after the magma pressure dropped so far that water could enter the conduit. The eruptions probably lasted several weeks to several years.Báez ''et al.'' 2017, p.332 A lava flow at La Laguna cone has been dated to 340,000 ± 60,000 years before present, consistent with its older appearance.Báez ''et al.'' 2017, p.327 A large-scale research project in the Puna yielded a number of Ar-Ar and K-Ar dates, including 3.2 and 4.5 million years ago for Jote and 0.3 and 7.3 million years ago for Alumbrera and La Laguna.Risse ''et al.'' 2008, p.4-5 The older age appears to refer to a

mesa A mesa is an isolated, flat-topped elevation, ridge or hill, which is bounded from all sides by steep escarpments and stands distinctly above a surrounding plain. Mesas characteristically consist of flat-lying soft sedimentary rocks capped by ...

just west of the Alumbrera lava field.Risse ''et al.'' 2008, p.10 The youngest age was obtained on Alumbrera cone by argon-argon dating: 37,000 ± 2,000. Historical activity is unknown, and in 1907 the volcanoes were considered to be extinct,Reichert 1907, p.79 but the field may have had eruptions during the

; the young appearance of Alumbrera suggests a Holocene age for this cone. Renewed activity in the field could cause ash to fall over hundreds of square kilometres of adjoining land and generate dangerous

pyroclastic density current A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of bu ...

s close to the vent.

Other

Between the Antofagasta and Alumbrera volcanoes lies the Antofagasta lake. The vegetation in the area is shrub steppe, and fairly sunny and dry. The Antofagasta River coming from the north forms a large

wetland A wetland is a distinct ecosystem that is flooded or saturated by water, either permanently (for years or decades) or seasonally (for weeks or months). Flooding results in oxygen-free (anoxic) processes prevailing, especially in the soils. The ...

before the relatively deep Antofagasta lake; the Alumbrera volcano borders the lake in the south. A major

archeological site An archaeological site is a place (or group of physical sites) in which evidence of past activity is preserved (either prehistoric or historic or contemporary), and which has been, or may be, investigated using the discipline of archaeology and ...

is emplaced on the Antofagasta volcano.

Gallery

File:Volcán Antofagasta visto desde el pueblo.jpg, Antofagasta cone from the town File:Volcan Alumbrera y Antofagasta.jpg, Alumbrera and Antofagasta cones File:Volcanic Cones in Antofagasta de la Sierra in Argentina.jpg, File:Pucará de la Alumbrera.jpg, A

pucara Pukara (Aymara and Quechuan "fortress", Hispanicized spellings ''pucara, pucará'') is a defensive hilltop site or fortification built by the prehispanic and historic inhabitants of the central Andean area (from Ecuador to central Chile and nort ...

close to the volcanoes

References

* * * {{Cite book , section=Yacimiento de Soda Natural, Azufre, Oro y Alumbre: IV. Yacimiento de Alumbre , section-url=https://archive.org/details/losyacimientosd00reicgoog/page/n85/mode/2up, title=Los yacimientos de boratos y otros porductos minerales explotables del territorio de los Andes: (Puna de Atacama) , last=Reichert , first=Fritz , date=1907 , location=Buenos Aires , publisher=Talleres de publicaciones de la oficina meteorológica Argentina, language=es, ref=none Volcanoes of Argentina Mountains of Argentina Landforms of Catamarca Province Volcanic fields Four-thousanders of the Andes Puna de Atacama