Cerro Chascon-Runtu Jarita is a complex 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 ...

s located inside, but probably unrelated to, the Pastos Grandes caldera. It is part of the more recent phase of activity of the Altiplano-Puna volcanic complex. Accompanied with little explosive activity on the main dome Cerro Chascon, it contains ten lava domes arranged in a chain. Located in the floor of the Pastos Grandes caldera, these domes were erupted after injection of

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 ...

magmas in the deep less than 100,000 years ago. The largest dome has a volume of .

Geology

The complex is a chain of seven

s named Runtu Jarita whose largest member is named Cerro Chascon. This chain is comparable with the Mono-Inyo craters in the United States. Together with other lava domes like Cerro Chao this complex is part of the young surface expression of the APVC and may indicate the future location of a caldera.

Origins

The lava domes are contained within the moat and floor of the

Pastos Grandes Pastos Grandes is the name of a caldera and its crater lake in Bolivia. The caldera is part of the Altiplano-Puna volcanic complex, a large ignimbrite province that is part of the Central Volcanic Zone of the Andes. Pastos Grandes has erupted a ...

caldera, but it may not be part of that caldera complex proper. The lava dome complex is part of the Altiplano-Puna volcanic complex, an igneous province located in the Central Andes. During the Late

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 ...

, large ignimbrite eruptions took place covering an area of triggered by the formation of melts deep in the crust and their subsequent rise to the upper crust. Geochemical analysis led de Silva (1994) to suggest that they may be the latest eruptive cycle of the APVC, either as part of a reactivation or waning of that system. Pastos Grandes is the source of two major ignimbrites, the 8.1

mya Mya may refer to: Brands and product names * Mya (program), an intelligent personal assistant created by Motorola * Mya (TV channel), an Italian Television channel * Midwest Young Artists, a comprehensive youth music program Codes * Burmese ...

Sifon and the 5.3 mya Chuhuilla ignimbrite, as well as the 3.1 mya Cerro Juvina ignimbrite shield on the northern caldera flank. The eruption of this complex may have been formed by a dyke.

Structure

The volcanic complex extends from to with base altitudes . The complex is split into a northern group encompassing Cerro Chascon and three other domes (named Cerro Guichi, Morro Chascoso and Pabelloncita Loma), and a southern group of six domes. Their orientation in northwestward direction is consistent with the regional trends but may also be influenced by the Pastos Grandes caldera structure. The arid climate of the area has impeded erosion in the area, leaving the volcanic complex relatively unaltered. Cerro Chascon is formed from a cicle of lava flow lobes with a central plug with a diameter of . Explosive activity occurred before the dome formation and generated a thick

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 vol ...

deposit composed from

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 mineral ...

obsidian Obsidian () is a naturally occurring volcanic glass formed when lava extrusive rock, extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock. Obsidian is produced from felsic lava, rich in the lighter elements s ...

beneath the dome. The lava dome has a volume of with a maximum altitude of . Its surface is covered by mounds ( high) of presumably diapiric origin, with sand trails eroded from the mounds separating them. Morro Chascoso and Pabelloncita Loma are cone shaped domes with blocky slopes with talus and central depressions. The much smaller Cerro Guichi ( high and long) may be an exposed

intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

, considering that it lacks any indication of surface flow. The two southernmost domes of the southern group have similar shapes to Chascoso and Pabelloncita. However the second southernmost dome is accompanied with an andesitic

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 ...

. The rest of the domes contain a silicic core with surrounding andesitic flows. The domes become less andesitic towards the north.

Petrology

The lavas of this complex are of

potassium Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosphe ...

-rich calc-alkaline origin, and are highly viscous with large difference between two magma types. The lava domes are formed 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 predomi ...

in the south and

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

in the north. The dacites are crystal rich and some lavas show evidence of magma mixing. Phenocryst contents ranges from 35% by volume in dacite and 48% by volume of rhyolite. In the northern group these are primarily composed from

feldspar Feldspars are 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 ''plagioclase'' (sodium-calcium) feldsp ...

with minor components of

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 A ...

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 alumino ...

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 form ...

. The southern group rocks have similar petrologies, but are of

composition and contain

components. They contain a rhyolitic core surrounded with andesitic lavas that contains primarily

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 ...

phenocrysts. Temperatures of for dacitic lava and for

rhyolite 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 mineral ...

lava have been estimated. Petrology indicates that the andesitic lavas of the southern domes are derived from the more silicic magmas by addition of more mafic andeistes. Conversely, the northern dome magmas formed by

fractional crystallization Fractional crystallization may refer to: * Fractional crystallization (chemistry), a process to separate different solutes from a solution * Fractional crystallization (geology) Fractional crystallization, or crystal fractionation, is one of the ...

with the most evolved components being erupted explosively.

Geologic history

The lava domes were erupted between 89,000-94,000 years ago. In Runtu Jarita, Argon-argon dating on

sanidine Sanidine is the high temperature form of potassium feldspar with a general formula K(AlSi3O8). Sanidine is found most typically in felsic volcanic rocks such as obsidian, rhyolite and trachyte. Sanidine crystallizes in the monoclinic crystal system ...

has yielded ages of 88,000±4,000 to 97,000±2,000 BP. The eruption may have been caused by the injection of

magmas in the magmatic system. Presumably, the magmas were in the process of forming a

pluton In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

when the injection of new magmas led to eruption. The eruptions of Chascon initially were

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 ...

and influenced by local lakewaters. In the southern domes, some time occurred between the formation of the andesitic magmas by mixing and the eruption. Both andesitic and rhyolitic magmas were simultaneously extruded.

Geology

Origins

Structure

Petrology

Geologic history

See also

References

Further reading