Sillajhuay (also known as Sillajguay or Alto Toroni) is a volcano on the border between

Bolivia , image_flag = Bandera de Bolivia (Estado).svg , flag_alt = Horizontal tricolor (red, yellow, and green from top to bottom) with the coat of arms of Bolivia in the center , flag_alt2 = 7 × 7 square p ...

and Chile. It is part of a volcanic chain that stretches across the border between Bolivia and Chile and forms a

mountain massif In geology, a massif ( or ) is a section of a planet's crust that is demarcated by faults or flexures. In the movement of the crust, a massif tends to retain its internal structure while being displaced as a whole. The term also refers to a ...

that is in part covered by ice; whether this ice should be considered a glacier is debatable but it has been retreating in recent decades. The volcano has developed on top of older ignimbrites. The volcano was active within the last one million years, but not within recent times considering the heavy glacial erosion of the mountain and the widespread periglacial modifications. Non-eruptive activity however occurs in the form of surface deformation and earthquake activity.

Geography and geomorphology

Sillajhuay is located in the Andes on the border between

and Chile ( commune of Pica- Colchane, Tarapaca Region) although only a small easterly sector of the mountain is located in Bolivia. The volcano lies in a thinly inhabited region; the towns of

Cancosa Cancosa is a Chilean commune. It is a highland in Aymara town in the Tarapacá Region. Cancosa is slightly on the slopes of the Sillajhuay volcano, near the border by Bolivia. According to the Chile National Archive the town was founded by the M ...

and

Villa Blanca A villa is a type of house that was originally an ancient Roman upper class country house. Since its origins in the Roman villa, the idea and function of a villa have evolved considerably. After the fall of the Roman Republic, villas became ...

lie southeast and northeast of Sillajhuay, respectively, and a road runs west of the volcano. The volcano is also known as Alto Toroni, Sillajguay, or sometimes Candelaria. The name "Sillajhuay" means "devil's chair" in Aymara but the part ''silla'' may also refer to which means llama. About 50 different volcanoes and geothermal features have been active in the Central Andes during the Holocene, with earthquakes observed at Guallatiri, Irruputuncu, Isluga, Lascar, Olca, Parinacota and Putana. Most volcanoes 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 Americ ...

(CVZ) are relatively poorly researched and many exceed of elevation. Some of these volcanoes were active during historical time; these include El Misti, Lascar, San Pedro and Ubinas; the largest historical eruption of the CVZ occurred in 1600 at Huaynaputina. Other volcanoes in the CVZ that have been the subject of research are Galan and the Purico complex. The CVZ has a characteristically thick crust () and the volcanic rocks have peculiar oxygen and

strontium Strontium is the chemical element with the symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is ex ...

isotope ratio The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundanc ...

s in comparison to 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 ...

(SVZ) and 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 ...

(NVZ). The mountain is most commonly stated to be maximally high, but a higher heights or a lower height of are also possible. It is the highest peak in the region. Sillajhuay is part of a larger massif that rises above the surrounding flat pedimented terrain to a median elevation of elevation. Subsidiary summits in the massif include the high Cerro Carvinto southwest of Sillajhuay, the Cerro Picavilque west-northwest of Sillajhuay, high Cerro Irpa southeast,

Cerro Candelaria Candelaria is a mountain in the Andes of Bolivia. It has a height of . It is situated in the Potosí Department, Daniel Campos Province, Llica Municipality, Chacoma Canton, north east of Alto Toroni and south of Alto Totoni. Generally, the massif is elongated northeastwards and its summit region is poorly accessible. Above elevation, glaciation has eroded the massif and thus the volcano is heavily degraded with no recognizable

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 ...

s; below that elevation volcanic landforms are more clearly expressed. The volcano was the source of thick

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 ...

s that reached lengths of about and valleys occur all around it. Farther west lie the Cerros de Quimsachata which form a volcanic chain with Sillajhuay.

Glaciation

Firn including penitentes occurs on the mountain at elevations of over and is visible over large distances but there are no presently active, moving glaciers unless they are buried beneath a snow cover. Some sources consider Sillajhuay's firn a glacier however, in which case it would be considered to be the southernmost glacier north of the Arid Diagonal of the Andes. Between 1989 and 2011 the firn lost over half of its surface, interrupted by some small advances, and further retreat is likely. Ice loss between 2000 and 2003 amounted to about . In the past during the Late Quaternary, the mountain was more extensively glaciated, with about nine glaciers surrounding it including a subsidiary summit to the south. Former glaciers did reach lengths of and their tongues descended to elevations of on the northern, eastern and southern flanks; they have left well developed

glacial striae Glacial striations or striae are scratches or gouges cut into bedrock by glacial Abrasion (geology), abrasion. These scratches and gouges were first recognized as the result of a moving glacier in the late 18th century when Swiss alpinists firs ...

, glacial valleys and various types of

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 shee ...

s. The lowest moraines are found on the eastern flank, with the northern flanks having the highest moraines and the southern flank moraines reaching intermediate elevations. Some ancient tills have been overrun by

porphyries 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 phenocr ...

. The extent of glacial erosion suggests that at least two stages of glaciation occurred at Sillajhuay. Some

rock glacier Rock glaciers are distinctive geomorphological landforms, consisting either of angular rock debris frozen in interstitial ice, former "true" glaciers overlain by a layer of talus, or something in-between. Rock glaciers are normally found at high ...

s are found on the southern side of Sillajhuay and mostly in the Rincon Tucuruma valley; they occur at elevations of and the longest reaches a length of . Solifluction landforms and other surfaces generated by periglacial processes are common on the southern and northern-northwestern flanks of the massif.

Hydrography

Erosion has cut steep valleys into the massif; these include clockwise the Rio Blanco southeast, Ricon Tacurma south, Quebrada Mina Chucha southwest, Quebrada Seca northwest and Quebrada Quisimachiri north-northwest of the volcano. These valleys reach up to the summit plateau and contain perennial rivers; additional valleys contain ephemeral streams, and they are often linked to alluvial fans down where eroded material has been deposited.

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 ...

ous springs are active on the massif. The valleys descending the volcano have steep slopes, the Rio Blanco valley for example has a drop over . All the drainages from Sillajhuay eventually flow eastward towards the

Salar de Coipasa __NOTOC__ Lago Coipasa or Salar de Coipasa is a lake in Sabaya Province, Oruro Department, Bolivia. At an elevation of 3657 m, its surface area is 806 km². It is on the western part of Altiplano, 20 km north of Salar de Uyuni and south ...

. On the southern flank of Sillajhuay, the Rio Blanco and the Ricon Tacurma drain into the Rio Ocacucho; there was formerly a lake known as the

Cancosa paleolake The paleolake of Cancosa is a former lake at Cancosa, Chile. The existence of the lake was inferred on the basis of limnic sediments in the area, which lies in the Tarapacá Region of Chile. Presently, the area is occupied by a basin at about ...

south of Sillajhuay. There is evidence that during the middle Pleistocene, a landslide from Sillajhuay dammed the

Rio Cancosa Rio or Río is the Portuguese, Spanish, Italian, and Maltese word for " river". When spoken on its own, the word often means Rio de Janeiro, a major city in Brazil. Rio or Río may also refer to: Geography Brazil * Rio de Janeiro * Rio do Sul, ...

and generated a body of water, into which the Cancosa Strata formation was deposited. Farther west, away from Sillajhuay, drainages conversely descend to the Pampa del Tamarugal.

Geology

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 ...

and Antarctic Plate subduct beneath the South America Plate in the Peru-Chile Trench at a pace of and , respectively, resulting in volcanic activity and geothermal manifestations in the Andes. Present-day volcanism occurs within four discrete belts: the NVZ (between 2°N–5°S), the CVZ (16°S–28°S), the SVZ (33°S–46°S) 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 ...

(AVZ) (49°S-55°S). Between them they contain about 60 active volcanoes and 118 volcanoes which appear to have been active during the Holocene, not including potentially active very large silicic volcanic systems or very small monogenetic ones. These belts of active volcanism occur where the Nazca Plate subducts beneath the South America Plate at a steep angle, while in the volcanically inactive gaps between them the subduction is much shallower; thus there is no

asthenosphere The asthenosphere () is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between ~ below the surface, and extends as deep as . However, the lower boundary of the asthenosphere is not ...

between the slab of the subducting plate and the overriding plate in the gaps. Among the oldest volcanics in the region are Eocene effusive andesitic volcanics known as the Icanche Formation and associated subvolcanic bodies, such as the Alantaya intrusive complex. These also include

granodioritic Granodiorite () is a coarse-grained ( phaneritic) intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar. The term banatite is sometimes used informally for various rocks ranging from ...

to tonalitic plutons. During the Eocene-

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...

Incaic deformation phase this basement was uplifted and eroded and subsequently covered by rhyolitic ignimbrites called the Utayane Formation. Along with the Utayane, andesitic volcanism led to the emplacement of additional andesitic lava formations such as the Puchuldiza and Chojña Chaya Formations. Rhyolitic ignimbritic volcanism however continued and was accompanied during the Miocene by the uplift of mountain ranges. Eventually large central volcanoes developed during the Miocene and Pliocene and are mostly uneroded and not affected by tectonic deformation. Sillajhuay developed during this time period. Finally, the mountains were altered by at least two cycles of glaciation.

Local

The regional geography is characterized by north–south trending mountain chains which are separated by relatively flat plains covered by

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 years ...

sediments. Sillajhuay lies on top of older ignimbrites, which in turn were emplaced on top of granitic, sedimentary and volcanic rocks of Paleozoic to Mesozoic age. Some of these ignimbrites have been identified as the 19.38 million years old Oxaya Ignimbrite, the much younger Ujina Tsu ignimbrite and finally the Pastillos Ignimbrite. Tectonic stress during the subduction process has led to the development of a horst that Sillajhuay is part of, perpendicular to the main strike of the Andes where magma formation was increased. The mountain is also part of the Serranía Intersalar mountain chain which separates the

from the

Salar de Uyuni Salar de Uyuni (or "Salar de Tunupa") is the world's largest salt flat, or playa, at over in area. It is in the Daniel Campos Province in Potosí in southwest Bolivia, near the crest of the Andes at an elevation of above sea level. The Sal ...

and lacks recent volcanic activity. Another isolated volcano

Cerro Cariquima Cerro Cariquima is a high volcano in the Andes. It is a sacred mountain of the territory, featuring a mountain sanctuary. The edifice presently has a volume of and based on erosion an age of 4.6 million years has been inferred. The true age o ...

rises north of Sillajhuay, the volcanic centres of Churullo northwest and the volcanic chain Pumiri northeast of Sillajhuay form the rest of the neighbouring centres. The volcano is formed by dacite and porphyry, including

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 ...

-containing porphyry which has a yellow colour and solfataric deposits; the volcanic rocks define a potassium-rich calc alkaline suite. Phenocrysts include plagioclase, with less common

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 ...

, hornblende and quartz.

Isotope ratio The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundanc ...

s of volcanic rocks indicate a strong crustal influence on the magmas that were erupted at Sillajhuay.

Climate and vegetation

The mountain lies in an arid region and features a montane climate; estimated precipitation rises from at to at elevation mostly during summer, although rainfall might exceed . During the night the temperatures can fall to under . Grasses and

shrub A shrub (often also called a bush) is a small-to-medium-sized perennial woody plant. Unlike herbaceous plants, shrubs have persistent woody stems above the ground. Shrubs can be either deciduous or evergreen. They are distinguished from trees ...

s with rare trees form the vegetation, mostly on the eastern flank and sometimes reaching high elevations. Among the plant species that grow in the area are yareta plants. The dry climate is caused by the South East Pacific High and compounded by the

Humboldt Current The Humboldt Current, also called the Peru Current, is a cold, low- salinity ocean current that flows north along the western coast of South America.Montecino, Vivian, and Carina B. Lange. "The Humboldt Current System: Ecosystem components and pr ...

off the coast, which cools the atmosphere and reduces evaporation. Only during the summer months does convection on the Bolivian Altiplano lead to the arrival of moisture, leading to a predominant summer precipitation. The climate becomes even drier farther south. Cut-off lows can sometimes reach Sillajhuay in winter but are uncommon. In the past, such as 28,000, 8,000 and 3,700 - 1,500 years ago the climate was more humid and this led frequently to glacier advances when it was also cold enough. In return, glaciers on Sillajhuay may have enhanced the moisture supply to other mountains in the area such as

Chuquiananta Chuquiananta (possibly from Aymara ''chuqi'' gold, Quechua ''chuqi'' metal, gold ( is a mountain in the

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 (W/m ...

leads to a strongly diurnal temperature cycle on the mountain with a day-night temperature gradient of about that in some environments can increase to over ; there are thus active freeze-thaw cycles. The warming also leads to the development of mountain breeze and valley breeze, convective clouds as well as occasional landspouts.

Human activity

The summit of Sillajhuay can be climbed and features Inca ruins on the summit; there are a number of such high altitude ruins in the Andes such as at Llullaillaco. This site was discovered in 2013 by the Scottish mountaineer John Biggar. Mining takes place east of Sillajhuay, including

mines; estimated deposits are of ore with 47% sulfur. The area has also been prospected for the possibility of obtaining

geothermal power Geothermal power is 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 is currently used in 2 ...

.The first recorded climb was by Friedrich Adolf Ernest Ahlfeld (Germany) in 1926.

Eruptive history

The whole volcano is considered to be of Pliocene- Pleistocene age, although the lack of detailed study precludes a precise dating of volcanic activity. The strong glacial modification implies that volcanism at Sillajhuay took place during the older Pleistocene. The maximum age of 730,000 ± 160,000 years is set by the age of the underlying ignimbrites although dates obtained directly on the volcanic rocks from Sillajhuay imply ages of as much as 2.47 ± 0.06 million years ago. Most volcanic activity probably took place around 600,000 - 400,000 years ago with potassium-argon dating yielding an age of 890,000 ± 500,000 years ago. Very young activity may have formed some gravel plains in the river valleys, when the heat from the eruption melted the permafrost of the summit region. However, between 2007 and 2010 a ground uplift of about was observed as Sillajhuay over an area wide. In addition seismic activity was recorded at the volcano, and hot springs can be observed close to Sillajhuay, including the Pampa Lirima field southwest of Sillajhuay. These patterns indicate that magma may still exist below the volcano and that it should be classified as a potentially active volcano.

Notes

References

Sources

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