Cerro Blanco (, "White Hill") is a

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcanic eruption. An eruption that ejects large volumes of magma over a short period of time can cause significant detriment to the str ...

in the

Andes The Andes ( ), Andes Mountains or Andean Mountain Range (; ) are the List of longest mountain chains on Earth, longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range ...

of the Catamarca Province in Argentina. Part of the Central Volcanic Zone of the Andes, it is a volcano collapse structure located at an altitude of in a depression. The caldera is associated with a less well-defined caldera to the south and several

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 ...

s. The caldera has been active for the last eight million years, and eruptions have created several

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. An eruption occurred 73,000 years ago and formed the Campo de la Piedra Pómez ignimbrite layer. About 2,300 ± 160 BCE, the largest known volcanic eruption of the Central Andes, with a VEI-7, occurred at Cerro Blanco, forming the most recent caldera as well as thick ignimbrite layers. About of

tephra Tephra is fragmental material produced by a Volcano, volcanic eruption regardless of composition, fragment size, or emplacement mechanism. Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground, ...

were erupted then. The volcano has been dormant since then with some deformation and geothermal activity. A major future eruption would put nearby communities to the south at risk. The volcano is also known for giant

ripple marks In geology, ripple marks are sedimentary structures (i.e., bedforms of the lower flow regime) and indicate agitation by water ( current or waves) or directly by wind. Defining ripple cross-laminae and asymmetric ripples * ''Current ripple ...

that have formed on its ignimbrite fields. Persistent wind action on the ground has shifted gravel and sand, forming wave-like structures. These ripple marks have heights up to and are separated by distances up to . These ripple marks are among the largest on Earth and have been compared to

Martian Mars, the fourth planet from the Sun, has appeared as a setting in works of fiction since at least the mid-1600s. Trends in the planet's portrayal have largely been influenced by advances in planetary science. It became the most popular celes ...

ripple marks by geologists.

Geography and geomorphology

The volcano lies at the southern margin of the Argentine Puna, on the border between the Antofagasta de la Sierra Department and the Tinogasta Department in the Catamarca Province of Argentina.

Trail A trail, also known as a path or track, is an unpaved lane or a small paved road (though it can also be a route along a navigable waterways) generally not intended for usage by motorized vehicles, usually passing through a natural area. Ho ...

s run through the area, and there are abandoned mining operations. Provincial Route 34 (Catamarca) between Fiambalá and Antofagasta de la Sierra runs past Cerro Blanco. The volcano is sometimes known as Cerro Blanco, meaning "white hill" in Spanish, and sometimes as Robledo; the

Smithsonian Institution The Smithsonian Institution ( ), or simply the Smithsonian, is a group of museums, Education center, education and Research institute, research centers, created by the Federal government of the United States, U.S. government "for the increase a ...

uses the latter name.

Calderas and lava domes

Cerro Blanco lies at an elevation of and consists of four nested calderas with discontinuous borders, fallout deposits,

s and pyroclastic deposits. The two inconspicuous El Niño and Pie de San Buenaventura calderas are nested in the northern part of the complex and form a wide depression; El Niño is sometimes referred to as a scarp. Only their northern margins are recognisable in satellite images; their southern parts are filled with block-and-ash flows from the southern calderas. The southern calderas are the Robledo and Cerro Blanco calderas, which form a southeast-northwest trending pair. Alternative interpretations consider the Pie de San Buenaventura, Robledo and Cerro Blanco calderas as one caldera, that the Robledo and Cerro Blanco calderas are one system or envisage the existence of only three calderas. The Cerro Blanco caldera is about wide and its walls are up to high. They are formed by

breccia Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...

, ignimbrites and lava domes cut by the caldera margins. The caldera floor is almost entirely covered by block-and-ash flows, apart from an area where

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 has left white sinter deposits. A slight circular uplift on the caldera floor may be a cryptodome. The caldera has an almost perfectly circular outline with the exception of the southwestern margin which is cut by a wide lava dome. This dome is also known as Cerro Blanco or Cerro Blanco del Robledo and reaches a height of above sea level. Three additional lava domes surround this dome, and an explosion crater lies to its southwest. West of this crater there are three pinkish lava domes lined up in west-southwest direction away from the main dome; these are surrounded by pyroclastic cones and depressions. Owing to erosion, the Robledo caldera is less well defined than the Cerro Blanco caldera. A site southeast of the Robledo caldera is known as Robledo. South of the Robledo caldera lies the Portezuelo de Robledo

mountain pass A mountain pass is a navigable route through a mountain range or over a ridge. Since mountain ranges can present formidable barriers to travel, passes have played a key role in trade, war, and both Human migration, human and animal migration t ...

, the south-eastward trending El Médano plain and the Robledo valley. About northeast of Cerro Blanco lies a wide and deep vent known as El Escondido or El Oculto. It does not have a strong topographic expression but is conspicuous on satellite images as a semi-circular patch of darker material. Gravimetric analysis has found a number of gravity anomalies around the caldera.

Surrounding terrain

The terrain northeast-east from Cerro Blanco is covered by its ignimbrites and by

Plinian Plinian eruptions or Vesuvian eruptions are volcanic eruptions characterized by their similarity to the eruption of Mount Vesuvius in 79 AD, which destroyed the ancient Roman cities of Herculaneum and Pompeii. The eruption was described in a le ...

fallout deposits which radiate away from the calderas. Cerro Blanco lies at the southwestern end of the Carachipampa valley, a volcano-

tectonic Tectonics ( via Latin ) are the processes that result in the structure and properties of the Earth's crust and its evolution through time. The field of ''planetary tectonics'' extends the concept to other planets and moons. These processes ...

depression flanked by

normal 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 extends to Carachipampa. This depression appears to have formed in response to north-south tectonic extension of the Puna and is covered by volcanic deposits from Cerro Blanco. These volcanic deposits form the "Campo de Pedra Pomez" and extend away from the volcano. To the north, the El Niño scarp of the El Niño caldera separates the Cerro Blanco caldera from the Purulla valley. Other valleys are the Purulla valley northwest from Cerro Blanco and Incahuasi due north; all three contain both volcanic deposits from Cerro Blanco and salt flats or lakes. In the Incahuasi valley an ignimbrite also known as the "white ignimbrite" reaches a distance of over . Wind has carved deep channels into the ignimbrites.

Aeolian landscapes

One of the most spectacular aeolian landscapes is found at Cerro Blanco, where large wind-formed

ripple mark In geology, ripple marks are sedimentary structures (i.e., bedforms of the lower flow regime) and indicate agitation by water (Ocean current, current or wind wave, waves) or directly by wind. Defining ripple cross-laminae and asymmetric ripples ...

s occur. These ripples cover Cerro Blanco ignimbrites and reach heights of and wavelengths of , making them the largest ripples known on Earth and comparable to similar ripple fields on

Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...

. Wind-driven erosion of ignimbrites has generated the ripples, which consist of gravel, pebbles and sand and are covered with gravel. Smaller

gravel Gravel () is a loose aggregation of rock fragments. Gravel occurs naturally on Earth as a result of sedimentation, sedimentary and erosion, erosive geological processes; it is also produced in large quantities commercially as crushed stone. Gr ...

ly ripples lie atop the larger ripples and troughs and there are intermediate sized forms ( high); they may be precursors to the large ripples and make up most of the ripples in the fields. Their wind-driven movement is fast enough that trails abandoned four years before are already partly covered with them. The ripple marks cover areas of about or in the Carachipampa and or in the Purulla valley. A field of large ripples covers an area of in the Purulla valley and is accompanied by yardangs; this field is also the place where the largest ripples occur. Various wind-dependent mechanisms have been proposed to explain their large size, including the presence of roll vortexes, Helmholtz instability-like phenomena, atmospheric

gravity wave In fluid dynamics, gravity waves are waves in a fluid medium or at the interface between two media when the force of gravity or buoyancy tries to restore equilibrium. An example of such an interface is that between the atmosphere and the oc ...

s or creep-like movement when pumice fragments and sand are lifted from the ground by wind and fall back. The latter view envisages that undulating terrain triggers the development of ripples through the accumulation of gravel and sand at such undulations. Their formation appears to be influenced by whether the rock material available can be moved by wind while a role of the bedrock structure or the size of the material is controversial. Wind has also formed demoiselles and yardangs in the ignimbrites. These are particularly well expressed in the Campo de Piedra Pomez area southeast of the Carachipampa valley, a area where yardangs, hoodoos and wind-exposed cliffs create a majestic landscape. The structures reach widths of and heights of and form an array-like assembly. They have fluted surfaces. The yardangs appear to form beginning from either a pre-existing topographic elevation or a fumarolic vent where the rock has been hardened, and eventually develop through a series of early, intermediate and late yardang forms as wind and wind-transported particles erode the rocks. Their layout may be influenced by regional tectonics, pre-existent topography and the patterns formed by the ignimbrite deposits. Exposed rocks are often covered with brown, orange or beige desert varnish and sometimes are oversteepened and collapse. Bedrock ridges are cut into ignimbrites of the Incahuasi valley. This terrain gradually leads over into the megaripple-covered surface through an increased gravel cover. The development of these megaripples appears to have been influenced by the underlying bedrock ridges which move along with the overlying ripples. These bedrock ridges are formed through erosion by wind and by wind-transported particles, it is not clear how they are then exposed from the ripples. Additional aeolian landforms in the region are known and include

ventifact A ventifact (also wind-faceted stone, windkanter) is a rock (geology), rock that has been abraded, pitted, etched, grooved, or polished by wind-driven sand or ice crystals. The word "Ventifact" is derived from the Latin word "Ventus" meaning 'wind ...

s and so-called "aeolian rat tails"; these are small structures which form when erosion-resistant rock fragments slow wind erosion in their lee, thus leaving a tail-like area where less rock is eroded. Wind streaks occur in groups. The Campo de Piedra Pómez makes up the , a

protected area Protected areas or conservation areas are locations which receive protection because of their recognized natural or cultural values. Protected areas are those areas in which human presence or the exploitation of natural resources (e.g. firewood ...

of Catamarca Province. It was among the finalists in the "Seven Wonders of Argentina" contest but was not selected when the results were announced in 2019. The area is also part of the

Ramsar site A Ramsar site is a wetland site designated to be of international importance under the Ramsar Convention,8 ha (O) *** Permanent 8 ha (P) *** Seasonal Intermittent < 8 ha(Ts) **

San Francisco San Francisco, officially the City and County of San Francisco, is a commercial, Financial District, San Francisco, financial, and Culture of San Francisco, cultural center of Northern California. With a population of 827,526 residents as of ...

and Falso Azufre and the Paso de San Francisco. It marks the boundary between the steep

subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...

to the north from the shallower subduction to the south. A series of

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 ...

to dacitic

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 ...

es ranging in age from 1 to 6 million years old make up the Cordillera de San Buenaventura, and

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS), as well as the current and most recent of the twelve periods of the ...

basalt Basalt (; ) is an aphanite, 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 planetary surface, surface of a terrestrial ...

ic volcanoes are dispersed over the wider region. In the surroundings of Cerro Blanco lies the Cueros de Purulla volcano north and the Nevado Tres Cruces- El Solo-

Ojos del Salado Nevado Ojos del Salado is a Dormant volcano, dormant complex volcano in the Andes on the Argentina–Chile border. It is the highest volcano on Earth and the highest peak in Chile. The upper reaches of Ojos del Salado consist of several overlapp ...

complex farther west.

Geology

Subduction of the

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

beneath the South America Plate occurs in the Peru-Chile Trench at a rate of . It is responsible for the volcanism in the Andes, which is localised in three volcanic zones known as the Northern Volcanic Zone, Central Volcanic Zone and Southern Volcanic Zone. Cerro Blanco is part of the Andean Central Volcanic Zone (CVZ), and one of its southernmost volcanoes. The CVZ is sparsely inhabited and recent volcanic activity is only poorly recorded; Lascar is the only regularly active volcano there. The CVZ extends over the Altiplano-Puna where calc-alkaline volcanism has been ongoing since the

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

. Characteristic for the CVZ are the large fields of ignimbritic volcanism and associated

s, chiefly in the Altiplano-Puna volcanic complex. In the southern part of the CVZ such volcanic systems are usually small and are poorly studied. During the

Neogene The Neogene ( ,) 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 million years ago. It is the second period of th ...

, volcanism commenced in the Maricunga belt and eventually shifted to its present-day location in the Western Cordillera. Volcanic activity occurred in bouts, named "flare-ups". The subduction of submarine ridges influences volcanism in the overlying crust; Cerro Blanco appears to overlie the subducted portion of the Copiapo Ridge. Tectonic processes also took place, such as two phases of east-west compression; the first was in the middle

and the second began 7 million years ago. Volcanism in the southern Puna region initiated about 8 million years ago and took place in several stages, which were characterised by the emplacement of lava domes and of ignimbrites such as the 4.0–3.7 million year old Laguna Amarga-Laguna Verde ignimbrites. Some of the domes are located close to the border with Chile in the Ojos del Salado and Nevado Tres Cruces area. Later there also were

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 ...

eruptions, which generated lava flows in the Carachipampa and Laguna de Purulla area. The late mafic eruption products and the Cerro Blanco volcanics are geologically classified as making up the "Purulla Supersynthem". From the Miocene to the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58La Hoyada volcanic complex was active southwest of Cerro Blanco in the form of several stratovolcanoes that produced the Cordillera de San Buenaventura; afterwards came a two-million year long hiatus. Cerro Blanco overlies this volcanic complex and outcrops of La Hoyada are found inside and around the calderas; sometimes it is considered part of La Hoyada or of a wider Cordillera de San Buenaventura volcanic system. The

basement A basement is any Storey, floor of a building that is not above the grade plane. Especially in residential buildings, it often is used as a utility space for a building, where such items as the Furnace (house heating), furnace, water heating, ...

is formed by

metamorphic Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...

, sedimentary and volcanic rocks of

Neoproterozoic The Neoproterozoic Era is the last of the three geologic eras of the Proterozoic geologic eon, eon, spanning from 1 billion to 538.8 million years ago, and is the last era of the Precambrian "supereon". It is preceded by the Mesoproterozoic era an ...

Paleogene The Paleogene Period ( ; also spelled Palaeogene or Palæogene) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period Ma (million years ago) to the beginning of the Neogene Period Ma. It is the fir ...

age. The former are particularly represented east of Cerro Blanco and go back in part to the

Precambrian The Precambrian ( ; or pre-Cambrian, sometimes abbreviated pC, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of t ...

, the latter occur mainly west and consist of

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

volcano-sedimentary units. Both are intruded by granitoids and mafic and

ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are usua ...

rocks.

Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...

sediments and Paleogene rocks complete the nonvolcanic geology. The crust is thick. Local tectonic structures such as borders between crustal domains and northeast-southwest trending faults might control the position of volcanic vents. Tectonic processes may also be responsible for the elliptic shape of the Cerro Blanco caldera. There is evidence of intense

earthquakes An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they c ...

during the Quaternary and some faults - such as these north of the volcano and the El Peñón Fault - have been recently active.

Composition

Most of the volcanic rocks found at Cerro Blanco are

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture (geology), texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained matri ...

s and define two suites of calc-alkaline rocks. Minerals encountered in the volcanic rocks include

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron- endmember annite, and the magnesium-endmember phlogopite; more al ...

feldspar Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagiocl ...

ilmenite Ilmenite is a titanium-iron oxide mineral with the idealized formula . It is a weakly magnetic black or steel-gray solid. Ilmenite is the most important ore of titanium and the main source of titanium dioxide, which is used in paints, printi ...

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula . It is one of the iron oxide, oxides of iron, and is ferrimagnetism, ferrimagnetic; it is attracted to a magnet and can be magnetization, magnetized to become a ...

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 ...

, less commonly

amphibole Amphibole ( ) is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is ...

, clinopyroxene, orthopyroxene, and rarely

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 ...

, allanite-

epidote Epidote is a calcium aluminium iron sorosilicate mineral. Description Well developed crystals of epidote, Ca2Al2(Fe3+;Al)(SiO4)(Si2O7)O(OH), crystallizing in the monoclinic system, are of frequent occurrence: they are commonly prismatic in ha ...

muscovite Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula KAl2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavage y ...

, titanite and

zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of th ...

. Fumarolic alteration on the caldera ground has produced

alunite Alunite is a hydroxylated aluminium potassium sulfate mineral, formula potassium, Kaluminium, Al3(sulfur, Soxygen, O4)2(Ohydrogen, H)6. It was first observed in the 15th century at Tolfa, near Rome, where it was mined for the manufacture of alum ...

boehmite Boehmite or böhmite is an aluminium oxide hydroxide (γ-AlO(OH)) mineral, a component of the aluminium ore bauxite. It is dimorphous with diaspore. It crystallizes in the orthorhombic dipyramidal system and is typically massive in habit. It is ...

and

kaolinite Kaolinite ( ; also called kaolin) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedral sheet of alumina () ...

and deposited

opal Opal is a hydrated amorphous form of silicon dioxide, silica (SiO2·''n''H2O); its water content may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to the amorphous (chemical) physical structure, it is classified as a ...

, quartz and

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

. Magma temperatures have been estimated to range between . The rhyolites erupted at Cerro Blanco appear to form from

andesite 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 ...

magmas, through processes such as fractional crystallisation, the absorption of crustal materials and the entry and mixing of new magmas. The magmatic system spans the entire thickness of the crust; the rhyolites are stored in a magma chamber at about depth. Magma composition has changed over time, initially becoming more

before returning to

felsic In geology, felsic is a grammatical modifier, modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted ...

magmas.

Climate and vegetation

Mean temperatures in the region are below but daily temperature fluctuations can reach and

insolation Solar irradiance is the power per unit area ( surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre ...

is intense. Vegetation in the region is classified as a high desert vegetation. It is bushy and relatively sparse, with thicker plant growth found at hot springs and in the craters where humid soils occur, perhaps wetted by ascending vapour. Annual precipitation is less than and moisture in the region comes from the

Amazon Amazon most often refers to: * Amazon River, in South America * Amazon rainforest, a rainforest covering most of the Amazon basin * Amazon (company), an American multinational technology company * Amazons, a tribe of female warriors in Greek myth ...

in the east. This aridity is a consequence of the region being within the Andean Arid Diagonal, which separates the northern

monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in Atmosphere of Earth, atmospheric circulation and precipitation associated with annu ...

precipitation regime from the southern

westerlies The westerlies, anti-trades, or prevailing westerlies, are prevailing winds from the west toward the east in the middle latitudes between 30 and 60 degrees latitude. They originate from the high-pressure areas in the horse latitudes (about ...

precipitation regime, and the

rain shadow A rain shadow is an area of significantly reduced rainfall behind a mountainous region, on the side facing away from prevailing winds, known as its leeward side. Evaporated moisture from body of water, bodies of water (such as oceans and larg ...

of the Andes, which prevents eastern moisture from reaching the area. The climate of the region has been

arid Aridity is the condition of geographical regions which make up approximately 43% of total global available land area, characterized by low annual precipitation, increased temperatures, and limited water availability.Perez-Aguilar, L. Y., Plata ...

since the Miocene but fluctuations in humidity occurred especially during the last glacial and between 9,000–5,000 years ago when climate was wetter. The aridity results in a good preservation of volcanic products. Strong winds blow at Cerro Blanco. Average windspeeds are unknown owing to the lack of measurements in the thinly populated region and there are contrasting reports on wind speed extremes but gusts of have been recorded in July and wind speeds in early December 2010 regularly exceeded . Winds blow mainly from the northwest, and have been stable in that orientation for the past 2 million years. This favoured the development of extensive aeolian landforms although winds coming from other directions also play a role. Thermal winds are generated by differential heating of surfaces in the region, and diurnal winds are controlled by the day-night cycle. Winds kick up pyroclastic material, generating

dust storm A dust storm, also called a sandstorm, is a meteorological phenomenon common in arid and semi-arid regions. Dust storms arise when a gust front or other strong wind blows loose sand and dirt from a dry surface. Fine particles are transpo ...

s which remove dust and sand from the area. Some of the dust is carried out into the

Pampa The Pampas (; from Quechuan languages, Quechua 'plain'), also known as the Pampas Plain, are fertile South American low grasslands that cover more than and include the Argentina, Argentine Provinces of Argentina, provinces of Buenos Aires Pro ...

, where it forms

loess A loess (, ; from ) is a clastic rock, clastic, predominantly silt-sized sediment that is formed by the accumulation of wind-blown dust. Ten percent of Earth's land area is covered by loesses or similar deposition (geology), deposits. A loess ...

deposits, and dust deposition at Cerro Blanco can quickly obscure vehicle tracks.

Dust devil A dust devil (also known regionally as a dirt devil) is a strong, well-formed, and relatively short-lived whirlwind. Its size ranges from small (18 in/half a metre wide and a few yards/metres tall) to large (more than 30 ft/10 m ...

s have been observed.

Eruption history

The Cerro Blanco volcanic system has been active during the

Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

and

Holocene The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...

. The oldest volcanic rock formation related to Cerro Blanco is the over 750,000 years old so-called "Cortaderas Synthem". Its outcrops are limited to the Laguna Carachipampa area. It consists of two ignimbrites, the Barranca Blanca Ignimbrite and the Carachi Ignimbrite, which erupted a long time apart. The former is a massive, white, unwelded ignimbrite, the latter is massive, rose-coloured and weakly welded. They contain

pumice Pumice (), called pumicite in its powdered or dust form, is a volcanic rock that consists of extremely vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicula ...

and fragments of extraneous rock and consist of

rhyodacite Rhyodacite is a volcanic rock intermediate in composition between dacite and rhyolite. It is the extrusive equivalent of those plutonic rocks that are intermediate in composition between monzogranite and granodiorite. Rhyodacites form from rapid ...

unlike later units. These ignimbrites, whose chronological relation to each other is unknown, were probably produced by "boil-over" of a volcanic vent rather than by an eruption column. Their exact source vent is unknown. The Campo de la Piedra Pómez Ignimbrite covers an area of about north of Cerro Blanco and has a volume of about . It was emplaced in two units a short time from each other. They both contain pumice and fragments of country rock, similar to the Cortaderas Synthem. The most reliable radiometrically obtained dates for this ignimbrite indicate an age of 73,000 years; previous estimates of their age were 560,000 ± 110,000 and 440,000 ± 10,000 years before present. The 73,000 age is considered to be more reliable but in 2022 an age of 54,600 ± 600 years was proposed for this eruption. The eruption reached level 6 on the

Volcanic Explosivity Index The volcanic explosivity index (VEI) is a scale used to measure the size of explosive volcanic eruptions. It was devised by Christopher G. Newhall of the United States Geological Survey and Stephen Self in 1982. Volume of products, eruption c ...

and is also known as the first cycle ignimbrite. The eruption has been described as the largest caldera collapse at Cerro Blanco but the source vent for this eruption has not been found, and there is no agreement whether the Robledo Caldera is the source. The volcano-tectonic depression northeast of Cerro Blanco or the Pie de San Buenaventura and El Niño scarps have been proposed as a source. As with the Cortaderas Synthem, this ignimbrite was produced by a boiling-over vent and the

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 b ...

s lacked the intensity to override local topography. It is possible that the eruption proceeded in two phases, with a magmatic reinvigoration of the system between the two. After the ignimbrite cooled and solidified, cracks formed in the rocks and were later eroded by wind. The Campo de la Piedra Pómez Ignimbrite crops out mainly on the southeastern and northwestern sides of the Carachipampa valley, as between these two outcrops it was buried by the later Cerro Blanco ignimbrite; other outcrops lie in the Incahuasi and Purulla valleys. The Robledo and Pie de San Buenaventura calderas were formed during the early activity. Several

layers identified in various sites of northwestern Argentina may come from Cerro Blanco: The 54,000 years old "Tuff B"/"Cerro Paranilla"/VP ash (which may be a product of the Campo de la Piedra Pómez eruption), a 22,700–20,900 years old tephra deposit in a lake of northwestern Argentina and the 10,000 years old "El Paso"/V0 ash. The volcano appears to have erupted repeatedly during the Holocene.

Explosive eruption In volcanology, an explosive eruption is a volcanic eruption of the most violent type. A notable example is the 1980 eruption of Mount St. Helens. Such eruptions result when sufficient gas has dissolved under pressure within a Viscosity, viscous ...

s took place between 8,830 ± 60 and 5,480 ± 40 years

before present Before Present (BP) or "years before present (YBP)" is a time scale used mainly in archaeology, geology, and other scientific disciplines to specify when events occurred relative to the origin of practical radiocarbon dating in the 1950s. Because ...

and deposited tephra and ignimbrites south of Cerro Blanco. Two tephra deposits in the Calchaquí valley have been attributed to Cerro Blanco; one of these is probably linked to the 4.2 ka eruption.

Sulfur oxide Sulfur oxide Sulfur oxides (SOx) are a group of chemical compounds formed by the combination of sulfur and oxygen. The most common SOx are sulfur dioxide (SO2) and sulfur trioxide (SO3). SOx are produced naturally through volcanic activity and are ...

gases from recent activity at Cerro Blanco may have degraded rock paintings in the Salamanca cave, south of the volcano.

4.2 ka eruption

A large eruption occurred approximately 4,200 years ago. Block-and-ash flow deposits (classified as "CB") found around the caldera have been interpreted as indicating that a lava dome was erupted prior to the caldera collapse at Cerro Blanco, although it is not clear by how much this eruption predates the main eruption. Deposits from this lava dome-forming episode consist of blocks which sometimes exceed sizes of embedded within ash and lapilli. A vent opened up, presumably on the southwestern side of the future caldera, and generated a 27 km (17 mi)-high

eruption column An eruption column or eruption plume is a cloud of super-heated Volcanic ash, ash and tephra suspended in volcanic gas, gases emitted during an explosive eruption, explosive volcanic eruption. The volcanic materials form a vertical column or Plu ...

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 activity. The vent is often a few metres wide and may be many kilo ...

s may have opened as well. After an initial, unstable phase during which alternating layers of lapilli and

volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, produced during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to r ...

(unit "CB1") fell out and covered the previous topography, a more steady column deposited thicker

rhyolitic Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals ( phenocrysts) in an otherwise fine-grained groundmass. The miner ...

tephra layers (unit "CB2"). At this time, a change in rock composition occurred, perhaps due to new magma entering the

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 ...

. Windy conditions dispersed most of the tephra to the east-southeast, covering a surface of about with about of tephra. The thickness of the tephra decreases eastwards away from Cerro Blanco and reaches a thickness of about away from Cerro Blanco in

Santiago del Estero Santiago del Estero (, Spanish for ''Saint-James-Upon-The-Lagoon'') is the capital of Santiago del Estero Province in northern Argentina. It has a population of 252,192 inhabitants, () making it the twelfth largest city in the country, with a sur ...

. The tephra deposits in the Valles Calchaquies and Tafi del Valle area are known as mid-Holocene ash, Ash C, Buey Muerto ash, and V1 ash layer, and it has been found northeast of Antofagasta de la Sierra. The tephra from the 4.2 ka eruption has been used as a chronological marker in the region. Modelling suggests the tephra might have reached Brazil and Paraguay farther east. Close to the vent, tephra fallout was emplaced on the Cordillera de San Buenaventura. Some of the tephra deposits close to the caldera have been buried by sediments, or

soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...

development has set in. Wind removed the volcanic ash, leaving block and lapilli sized pebbles that cover most of the deposits; in some places dunes have formed from pebbles. Pyroclastic flows also formed, perhaps through instability of the eruption column (unit "CB3"), and spread away from the volcano through surrounding valleys. They reached distances of from Cerro Blanco and while many of their up to thick deposits are heavily eroded well-exposed outcrops occur south of the volcano at Las Papas. They consist of

fragments of varying sizes embedded within ash, as well as country rock that was torn up and embedded in the flows. In the south, pyroclastic flows descending valleys partially overflowed their margins to flood adjacent valleys and reached the . North-westward and north-eastward flowing ignimbrites generated ignimbrite fans in the Purulla and Carachipampa valleys, respectively. The deposits from this event are also known as Cerro Blanco Ignimbrite, as Ignimbrite of the second cycle or El Médano or Purulla Ignimbrite. Formerly these were dated to be 12,000 and 22,000 years old, respectively, and related to the Cerro Blanco and (potentially) Robledo calderas. Cerro Blanco is considered to be the youngest caldera of the Central Andes. With a volume of of tephra, the 4.2 ka eruption has been tentatively classified as 7 on the Volcanic Explosivity Index, making it comparable to the largest known Holocene volcanic eruptions. It is the largest known Holocene eruption in the Central Andes and of the Central Volcanic Zone, larger than the 1600 Huaynaputina eruption, the largest historical eruption of the Central Volcanic Zone. Most of the erupted volume was ejected by the eruption column, while only about ended up in pyroclastic flows. Caldera collapse occurred during the course of the eruption, generating the unusually small (for the size of the eruption) Cerro Blanco caldera through a probably irregular collapse. Some authors have postulated that mid-Holocene eruptions of Cerro Blanco impacted human communities in the region. Tephra deposits in the Formative Period archaeological site of Palo Blanco in the Bolsón de Fimabalá have been attributed to Cerro Blanco, as is a tephra layer in an archaeological site close to Antofagasta de la Sierra. At Cueva Abra del Toro in northeastern Catamarca Province, rodents disappeared after the eruption and there was a change in human activity. The eruptions of Cerro Blanco may – together with more local

seismic Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes (or generally, quakes) and the generation and propagation of elastic ...

activity – be responsible for the low population density of the Fiambalá region, Chaschuil valley and western Tinogasta Department during the Archaic period between 10,000 and 3,000 years ago. The eruption is not recorded in the known

oral tradition Oral tradition, or oral lore, is a form of human communication in which knowledge, art, ideas and culture are received, preserved, and transmitted orally from one generation to another.Jan Vansina, Vansina, Jan: ''Oral Tradition as History'' (19 ...

of the region. The 4.2 kiloyear climatic event occurred at the same time; it may be in some way related to the Cerro Blanco eruption.

Post–4.2 ka activity

After the caldera-forming eruption, renewed

effusive eruption An effusive eruption is a type of volcanic eruption in which lava steadily flows out of a volcano onto the ground. Overview There are two major groupings of eruptions: effusive and explosive. Effusive eruption differs from explosive eruption ...

s generated the lava domes southwest of and on the margin of the Cerro Blanco caldera and

phreatic ''Phreatic'' is a term used in hydrology to refer to aquifers, in speleology to refer to cave passages, and in volcanology to refer to a type of volcanic eruption. Hydrology The term phreatic (the word originates from the Greek , meaning "well" ...

/ phreatomagmatic activity occurred. The current topography of Cerro Blanco is formed by the deposits from this stage, whose activity was influenced by intersecting fault systems including a northeast-southwest trending fault that controls the position of lava domes outside and fumarolic vents within the caldera. It's not clear how long after the 4.2 ka eruption this activity occurred, but it has been grouped as the "CB" unit (the domes are classified as "CB1"). This activity also generated block-and-ash deposits (unit "CB2") on the caldera floor. The domes are of rhyolitic composition, the block-and-ash deposits consist of ash and lapilli and appear to have formed when domes collapsed. As lava domes grow, they tend to become unstable as their vertical extent increases until they collapse. Additionally, internally generated explosions appear to have occurred at Cerro Blanco as lava domes grew and sometimes completely destroyed the domes. The 3.5 ka "Alemanía" ash in northwestern Argentina may be a product of post-4.2 ka eruptions of Cerro Blanco.

Present-day status

No historical eruptions have been observed or recorded at Cerro Blanco, but various indicators imply that it is still active. In 2007–2009, seismic swarms were recorded at less than depth. Geothermal activity occurs at Cerro Blanco, and manifests itself on the caldera floor through hot ground, fumaroles, diffuse degassing of at a rate of , and reportedly

hot spring A hot spring, hydrothermal spring, or geothermal spring is a Spring (hydrology), spring produced by the emergence of Geothermal activity, geothermally heated groundwater onto the surface of the Earth. The groundwater is heated either by shallow ...

s and

mud volcano A mud volcano or mud dome is a landform created by the eruption of mud or Slurry, slurries, water and gases. Several geological processes may cause the formation of mud volcanoes. Mud volcanoes are not true Igneous rock, igneous volcanoes as th ...

es; phreatic eruptions may have occurred in the past. Fumaroles release mainly carbon dioxide and water vapour with smaller amounts of

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...

and

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...

; they reach temperatures of while temperatures of have been reported for the hot ground. Past intense hydrothermal activity appears to have emplaced silicic material up to thick, and

steam explosion A steam explosion is an explosion caused by violent boiling or flashing of water or ice into steam, occurring when water or ice is either superheated, rapidly heated by fine hot debris produced within it, or heated by the interaction of molten ...

s took place within the caldera. Active fumaroles and

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...

cones formed by fumarolic activity are also found in the phreatic crater. The geothermal system appears to consist of an

aquifer An aquifer is an underground layer of water-bearing material, consisting of permeability (Earth sciences), permeable or fractured rock, or of unconsolidated materials (gravel, sand, or silt). Aquifers vary greatly in their characteristics. The s ...

hosted within pre-volcano rocks and heated by a magma chamber from below, with the Cerro Blanco ignimbrites acting as an effective seal. Supporting the effectiveness of the seal, total emissions of carbon dioxide exceed but are considerably lower than at other active geothermal systems of the Andes. It has been prospected for possible

geothermal power Geothermal power is electricity generation, electrical power generated from geothermal energy. Technologies in use include dry steam power stations, flash steam power stations and binary cycle power stations. Geothermal electricity generation i ...

generation. A second geothermal field related to Cerro Blanco is located south of the volcano and is known as Los Hornitos or Terma Los Hornos, in the area of the Los Hornos and Las Vizcachas creeks. It is located in a

ravine A ravine is a landform that is narrower than a canyon and is often the product of streambank erosion. Ravines are typically classified as larger in scale than gullies, although smaller than valleys. Ravines may also be called a cleuch, dell, ...

and consists of three clusters of bubbling pools, hot springs, up to high

travertine Travertine ( ) is a form of terrestrial limestone deposited around mineral springs, especially hot springs. It often has a fibrous or concentric appearance and exists in white, tan, cream-colored, and rusty varieties. It is formed by a process ...

domes that discharge water and extinct

geyser A geyser (, ) is a spring with an intermittent water discharge ejected turbulently and accompanied by steam. The formation of geysers is fairly rare and is caused by particular hydrogeological conditions that exist only in a few places on Ea ...

cones; these cones give the field its name and some of them were active until 2000. Water temperatures range between , the vents are settled by

extremophilic An extremophile () is an organism that is able to live (or in some cases thrive) in extreme environments, i.e., environments with conditions approaching or stretching the limits of what known life can adapt to, such as extreme temperature, pres ...

organisms. The springs deposit travertine, forming cascades, dams, pools and terraces of varying size, as well as pebbles. Fossil travertine deposits are also found and form a

carbonate A carbonate is a salt of carbonic acid, (), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate group ...

rock plateau generated by waters rising from a fissure. The Los Hornos system has been interpreted as a leak from the Cerro Blanco geothermal system, and south-westward trending fault systems might connect it to the Cerro Blanco magmatic system.

Deformation and hazards

Subsidence Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope mov ...

at a rate of has been noted at the caldera since 1992 in InSAR images. The rate of subsidence was originally believed to have decreased from over between 1992 and 1997 to less than between 1996 and 2000 and ceased after 2000. Later measurements found that the subsidence rate instead had been steady between 1992 and 2011 with , but with a faster phase between 1992 and 1997 and a slower phase between 2014 and 2020 of , and the location the subsidence is centred on has changed over time. The subsidence occurs at depth and has been related to either a cooling magmatic system, changes in the hydrothermal system or to subsidence that followed the 4.2 ka eruption and is still ongoing. Uplift in the area surrounding the caldera has also been identified. The Argentinian Mining and Geological Service has ranked Cerro Blanco eight in its scale of hazardous volcanoes in Argentina. Rhyolitic caldera systems like Cerro Blanco can produce large eruptions separated by short time intervals. Future activity might involve either a "boiling-over" of pyroclastic flows or Plinian eruptions. Given that the region is sparsely inhabited, the primary effects of a new eruption at Cerro Blanco would come from the eruption column, which could spread tephra eastwards and impact air traffic there. Also, pyroclastic flows could through narrow valleys reach the Bolsón de Fiambalá valley south of Cerro Blanco, where many people live. , the volcano is unmonitored.

Research history

Research in the region commenced in the 19th century and was mainly concentrated on mining. Cerro Blanco received attention from scientists after satellite images in the early 21st century observed deflation of the caldera. A number of Holocene tephra layers have been identified in the region, but linking these to specific eruptions has been difficult until 2008–2010 when some of these were linked to the Cerro Blanco vent. Scientific interest rose in the 2010s due to the discovery of the large 4.2 ka eruption.

Notes

References

Sources

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External links

Estilo eruptivo y dinámica de flujo de las corrientes de densidad piroclásticas asociadas a la gran erupción del Cerro Blanco (4200 AP), Puna Austral

Informe Geológico Correspondiente a la Mina La Hoyada, Departamento Tinogasta, Provincia de Catamarca

Travel information of the Provincial government of Catamarca (in Spanish)
* {{cite web, title=Robledo ASTER Imagery, url=http://ava.jpl.nasa.gov/ASTER_data.php?id=1505-21-, website=Aster Volcano Archive, publisher=

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

, access-date=15 September 2015, url-status=dead, archive-url=https://web.archive.org/web/20150910092945/http://ava.jpl.nasa.gov/volcano.php?id=1505-21-, archive-date=10 September 2015 VEI-7 volcanoes Volcanoes of Catamarca Province Pleistocene calderas Holocene calderas Calderas of Argentina