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Nevado Sajama (; ) is an extinct volcano and the highest peak in Bolivia. The mountain is located in Sajama Province, in Oruro Department. It is situated in Sajama National Park and is a composite volcano consisting of a stratovolcano on top of several . It is not clear when it erupted last but it may have been during the Pleistocene or Holocene.
The mountain is covered by an ice cap, and Polylepis tarapacana trees occur up to elevation.
In Bolivia, the Andes mountain chain splits into two branches separated by a high plateau, the Altiplano. Nevado Sajama lies in the Western Andes of Bolivia and in the western side of the Altiplano; more specifically the mountain is located before the Western Cordillera.
Nevado Sajama rises about from the surrounding terrain to a height of (earlier estimates of its height are ), making it the highest mountain of Bolivia. Below the mountain is characterized by parasitic vents and a cover of lava fragments and volcanic ash. Two secondary summits and occur west and east-northeast from Sajama respectively; the former is named Cerro Huisalla and the second is Huayna Potosi. The mountain has a conical shape and is capped by a summit crater that, owing to its ice fill, appears to be linked to the flat summit plateau of Sajama but other records do not indicate the presence of a crater. The Patokho, Huaqui Jihuata and Phajokhoni valleys are located on the eastern flank; at lower elevations the whole volcano features glacially deepened valleys.
The terrain is characterized by a continuous ice cover in the central sector of the mountain, exposures of bedrock, deposits and in some sites, and scree in the periphery of Sajama and forming a girdle around the upper sector of Sajama. The ground moraines are the most prominent moraines on Sajama, and have varying colours depending on the source of their component rocks. Vegetation and small lakes occur in their proximity, while other parts are unvegetated. They mostly occur within glacial valleys, but some appear to have formed underneath small plateau ice caps on flatter terrain. At lower elevations the glaciers lead to lava flows and matorral and tolar vegetation, and finally to grassland at lower altitudes.
A number of called occur on the mountain. Starting in the lake Waña Quta on the northwestern foot of Sajama, the Tomarapi River flows first northeastward, then east, south and southeast around the northern and eastern flanks of the volcano; the Sicuyani River, which originates on Sajama, joins it there. The southern flanks give rise to the Huaythana River, which flows directly south and then makes a sharp turn to the east. The Sajama River originates on the western side of the volcano, flowing due south and increasingly turning southeast before joining the Lauca River. The other rivers draining Sajama and its ice cap also eventually join the Lauca River and end in the Salar de Coipasa.
The mountain is a stratovolcano located atop several . The stratovolcano consists of and Pyroclastic rock material that radiate away from the centre of the volcano. Some parasitic vents occur southeast of Sajama and have produced lava domes and lava flows. The parasitic vents away from the volcano are older and their location appears to be controlled by radial dikes; the whole complex is a compound volcano. Two later volcanic units are known as the Colquen Wilqui lavas and the Jacha Khala tuff. The Sajama volcano rises within a caldera that has been buried by later volcanic activity so that it is only recognizable on its eastern-northeastern side. A circular structure around Sajama may be the origin of the 2.7 million years old Lauca-Perez Ignimbrite.
Argon-argon dating has yielded ages of 679,000 years ago from Sajama and of 80,900 to 25,000 years ago for the Kkota Kkotani lavas, which are unrelated to the main Sajama volcano. The date of the last eruption is not known, it may have occurred in the Pleistocene or Holocene. occur on the Junthuma River and reflect the presence of geothermal heat with temperatures of about on the western foot of Sajama, and volcanic rocks of Sajama bear traces of fumarolic activity.
Three major geologic occur in the region, the north-northwesterly trending Sajama lineament, a west-southwesterly one aligned with high topographical features and a west-northwesterly one. The west-southwesterly one played an important role in the development of Sajama volcano.
Sajama is located between two climate regimes, a westerly one characterized by a dry climate and the Southeast Pacific High and an easterly one with a moister atmosphere. During the southern hemisphere summer, easterly winds carry moist air towards Sajama where solar insolation then triggers showers and ; the moisture ultimately originates in the Atlantic Ocean. During winter, dry westerly winds prevail although cold air outbreaks from the westerlies belt sometimes trigger intense snowfall which is often underestimated by precipitation data. Overall, on the Altiplano precipitation diminishes from the northeast to the southwest.
Summer precipitation is typically reduced during El Nino years, but on Nevado Sajama there is little correlation.
Nevado Sajama features the highest treeline on Earth at elevation, as Polylepis tarapacana forms that have both a sharp upper and a sharp lower limit on the mountain. The trees are usually no higher than and are separated by large distances from each other and appear to localize to spots where water is available. The current woods are remnants; whether the decrease is caused by human impact or climate change is not clear. Protecting these woodlands was the impetus for the 1939 creation of the Sajama National Park.
Sajama and neighbouring mountains featured much larger glaciers in the past. The history of glaciation on Sajama in general is poorly known, but it appears that the outermost glacial features originated during the late last glacial maximum and the intermediary features during the Middle Holocene which is usually considered to be a warm and dry period in the region.
There are a number of archaeological sites on the mountain, including chullpa burials and pukara fortifications, distributed over various altitudes and connected through paths. About 40 such sites have been found, most of them at low elevations on the western and northern foot of Sajama. Some of these sites have yielded . The archaeological sites demonstrate a direct link between the mountain and their builders and with the resolution of local conflicts.
During the second undisputed ascent on the mountain in 1946, one mountaineer disappeared and his body was never found. In August 2001, two teams of Sajama villagers and Bolivian mountain guides played a football match on top of Mount Sajama in an effort to show that altitude itself is not a limitation to physical strain. In 2015, a challenge to hold a political debate on the summit of Sajama was made by a candidate to an election. The 50-boliviano Bolivian banknote launched in October 2018 shows Sajama on its reverse.
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