Cerro Chao is a

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 un ...

complex associated with the

Cerro del León Cerro del León is a stratovolcano located in El Loa province, Antofagasta Region, Chile. It is part of the Chilean Central Volcanic Zone and forms a volcanic lineament with neighbouring Paniri and Toconce that was active into the Holocene. Cer ...

volcano in the

Andes The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S ...

. It is the largest known

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). It follows the Neogene Period and spans from 2.58 million year ...

silicic volcano body and part of the most recent phase of activity in the

Altiplano–Puna volcanic complex The Altiplano–Puna volcanic complex (), also known as APVC, is a complex of volcanic systems in the Puna of the Andes. It is located in the Altiplano area, a highland bounded by the Bolivian Cordillera Real in the east and by the main chain of ...

. Cerro Chao formed over the course of three eruptions preceded by a pyroclastic stage. Three large lobate lava flows erupted in the

col In geomorphology, a col is the lowest point on a mountain ridge between two peaks.Whittow, John (1984). ''Dictionary of Physical Geography''. London: Penguin, 1984, p. 103. . It may also be called a gap. Particularly rugged and forbidding co ...

between two volcanoes and advanced for a maximum length of . The eruption that originated the lava flows probably lasted more than one hundred years and occurred before the

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 togeth ...

Geology

Cerro Chao, also named Cerros de Chao, Chao lava or Chao volcano, is located in the volcanic front of 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 America ...

of the

, between the older

Paniri Paniri (Aymara for "he who comes, visits") is a stratovolcano located in El Loa Province, Antofagasta Region, Chile, and near the border with Bolivia. To its northwest lie the twin volcanoes San Pedro and San Pablo, and to its southeast lies Ce ...

and

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 predomin ...

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

es. The Central Volcanic Zone is one of the three volcanic belts in the Andes. The region is dominated by the

, where during the

Miocene The Miocene ( ) is the first 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 means "less recen ...

–

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

epoch large

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 surro ...

eruptions occurred. In the present day, volcanism is of andesitic composition forming volcanic cones. Activity is controlled by

fault zone 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 tectoni ...

s, some of these linked to the Pastos Grandes

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...

. Neighbouring volcanoes with similar characteristics to Cerro Chao include Cerro Chanca/Pabellon, Cerro Chascon–Runtu Jarita complex, Cerro Chillahuita and La Torta. Cerro Chao lies within a northwest-running belt of volcanoes known as the San Pedro-Linzor volcanic chain, some of them over high, of which San Pedro has historical activity. Cerro Chao is the largest such silicic lava flow known. The eruption is intriguing both in terms of the

viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the int ...

of the erupted lavas and its effusive nature. Conventional lava flows increase in viscosity with increasing crystal content; however Chao lava flows were erupted with similar viscosities and

yield strength In materials science and engineering, the yield point is the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning of plastic behavior. Below the yield point, a material will deform elastically and wi ...

s as

silicic Silicic is an adjective to describe magma or igneous rock rich in silica. The amount of silica that constitutes a silicic rock is usually defined as at least 63 percent. Granite and rhyolite are the most common silicic rocks. Silicic is the grou ...

domes. The formation of a lava flow instead of a lava dome may have been influenced by the formation of a carapace on the flow and the steep slopes the flows initially formed on; the late Chao III flow formed on the very gentle slope left by the previous Chao I and Chao II flows and shows some lava dome characteristics.

Origins

The

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 predo ...

inclusions contained in the lava are characteristic for magma mixing processes. The eruption forming the flow may have been caused by the injection of andesite in a pre-existent homogenous dacitic magma chamber. The injection did presumably modify the crystallization processes in the magma chamber and the volatiles in the magma to the point of forcing an eruption. The magmas that gave rise to Cerro Chao may either be the remnants of a previous magma body that gave rise to the neighbouring

s of the Altiplano-Puna volcanic complex, or may be the sign of a new injection of magmas in the crust. The significance of these theories is controversial.

Structure

Cerro Chao is a -long coulee. It has a volume of and its flow front is high. Based on volumetric considerations, the eruption lasted about 100-150 years with an average lava flux rate of . The volume of Chao is exceptional for a lava dome structure, although the lava flux rate generating it is low in comparison to a

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 ...

eruption like Laki in

Iceland Iceland ( is, Ísland; ) is a Nordic island country in the North Atlantic Ocean and in the Arctic Ocean. Iceland is the most sparsely populated country in Europe. Iceland's capital and largest city is Reykjavík, which (along with its ...

. This low flux rate is insufficient to cause

formation. Cerro Chao is the largest

silicic lava flow in the world. The vent location is related to an inferred fault zone emanating from one of the neighbouring volcanoes. The flow is underpinned by a

pyroclastic Pyroclastic rocks (derived from the el, πῦρ, links=no, meaning fire; and , meaning broken) are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroc ...

apron that extends from the flow front. Most of it is buried beneath the flow and only on the eastern side does some material emerge; its volume is estimated at . This deposit is formed from several layers of

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

s separated by erosion surfaces; at least one layer may be derived from

volcano. An overlapping pair of pyroclastic cones sits on top of the Chao flow and form its eruption vent. The cone has a dense rock equivalent volume of of

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 ...

and blocks. The northern side of the cone rises from terrain while the southern side is partially breached. The highest point of the cone lies at altitude. Its morphology suggests that it formed from a lava dome when it collapsed over the vent. The eruption of Cerro Chao occurred in several phases. In a first phase, Plinian–

Vulcanian A Vulcanian eruption is a type of volcanic eruption characterized by a dense cloud of ash-laden gas exploding from the crater and rising high above the peak. They usually commence with phreatomagmatic eruptions which can be extremely noisy due t ...

activity generated

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 ...

deposits to the south of the system. Most of the pyroclastics were formed during this phase, although some minor deposits formed from the collapse of the forming flow. A thin lapilli layer has been linked to San Pedro volcano. Explosive activity continued during the extrusion of the Chao flow, growing the pumice cone. The flow proper is subdivided in three subunits, the first two named Chao I and Chao II. Originally subdivided because of their morphology, they most likely represent various pulses of the same eruption. They have a combined volume exceeding and are formed from a long southbound flow with some lateral spillage. The flow itself is long and its flow front is high. Its structure is massive and lobate, with lobe diameters expanding downflow from . The flows are covered by

ogive An ogive ( ) is the roundly tapered end of a two-dimensional or three-dimensional object. Ogive curves and surfaces are used in engineering, architecture and woodworking. Etymology The earliest use of the word ''ogive'' is found in the 13th c ...

s (up to high and with spacing of ) and some structures interpreted as fossil

fumarole A fumarole (or fumerole) is a vent in the surface of the Earth or other rocky planet from which hot volcanic gases and vapors are emitted, without any accompanying liquids or solids. Fumaroles are characteristic of the late stages of volcani ...

s. The ridges are drawn out on the western flow margin. The folds in the surface layers may have been caused by the surface stiffening more quickly than the underlying flow due to cooling. The flow surface is blocky, with blocks occasionally displaying flow banding. The lowest Chao I flow covers an area of . The Chao III flow has a smaller volume of than Chao I and II. It has less ogives than Chao I and II and forms a single lobe, high. The flow overlies the pumice cone and parts of the Chao II on its eastern side. A

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 ...

formed above its vent and underwent several collapses, generating collapse scars. The flow is covered by

weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs '' in situ'' (on site, with little or no movement ...

-derived aeolian debris from the other flows. This flow has a surface area of .

Petrology

The Chao flow is of

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 rhyol ...

composition, with some non-vesicular small

inclusions that are more numerous in the Chao III and upper Chao II stages, up to 5% of the volume of some Chao III lavas and vesiculated there. The lava has a

porphyric Porphyry ( ) is a textural term for an igneous rock consisting of coarse-grained crystals such as feldspar or quartz dispersed in a fine-grained silicate-rich, generally aphanitic matrix or groundmass. The larger crystals are called phenoc ...

texture owing to its high crystal content of 45% and displays extensive flow banding. Chao III lavas have lower concentrations of crystals.

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 in the lava contain

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 ...

hornblende Hornblende is a complex inosilicate series of minerals. It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. Hornblende minerals are common in igneous and metamorphic rock ...

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 p ...

and

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical f ...

. Some hornblende crystals have diameters of up to .

Apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common ...

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 t ...

are accessory minerals. Based on geochemical considerations, the magmas equilibrated at depths of and temperatures of .

Geologic history

Potassium–argon dating and

argon–argon dating Argon–argon (or 40Ar/39Ar) dating is a radiometric dating method invented to supersede potassiumargon (K/Ar) dating in accuracy. The older method required splitting samples into two for separate potassium and argon measurements, while the newer ...

performed on rocks from the Chao I stage have indicated an average age of 423,000 ± 100,000 years. However, anomalous chemical compositions of the dated rocks suggest that they may overestimate the true age of the volcanics. Such alteration may be the result of the inclusion of

xenocryst A xenolith ("foreign rock") is a rock fragment (country rock) that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term ''xenolith'' is almost exclusively used to describe inclusions in ig ...

s or K leaching. A

glacial moraine A moraine is any accumulation of unconsolidated debris (regolith and 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 glacier or ice she ...

system lies on Cerro del León at altitude. One of these moraines abuts Cerro Chao, indicating that the dome must be older than the moraine and thus older than the last glaciation 11,000 years ago. An active magmatic body may still exist under Cerro Chao and Paniri.

References

External links

* * {{DEFAULTSORT:Chao, Cerro Volcanoes of Chile Pleistocene lava domes Lava fields Pre-Holocene volcanism