Sillajhuay

Sillajhuay (also known as Sillajguay or Alto Toroni) is a volcano on the border between Bolivia and Chile. It is part of a volcanic chain that stretches across the border between Bolivia and Chile and forms a mountain massif 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 Bolivia 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 and Villa Blanca 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 (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 isotope ratios in comparison to the Southern Volcanic Zone (SVZ) and the Northern Volcanic Zone (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 due east and Morro Chuncaron and Alto Totoni northeast. 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 craters; below that elevation volcanic landforms are more clearly expressed. The volcano was the source of thick lava flows 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 valleys and various types of moraines. 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. The extent of glacial erosion suggests that at least two stages of glaciation occurred at Sillajhuay.

Some rock glaciers 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. Sulfurous 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. 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 south of Sillajhuay. There is evidence that during the middle Pleistocene, a landslide from Sillajhuay dammed the Rio Cancosa 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 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 (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 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 to tonalitic plutons. During the Eocene-Oligocene 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.

Local

The regional geography is characterized by north–south trending mountain chains which are separated by relatively flat plains covered by Quaternary 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 Salar de Coipasa from the Salar de Uyuni and lacks recent volcanic activity. Another isolated volcano Cerro Cariquima 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-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, hornblende and quartz. Isotope ratios 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 shrubs 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 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

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