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Lastarria is a high stratovolcano that lies on the border between Chile and Argentina. It is remote, and the surroundings are uninhabited but can be reached through an unpaved road. The volcano is part of the Central Volcanic Zone, one of the four segments of the volcanic arc of the Andes. Over a thousand volcanoes—of which about 50 are active—lie in this over long chain of volcanoes, which is generated by subduction of the Nazca Plate beneath the South American Plate.
The volcano is constituted by two volcanic edifices that form a ridge, and one subsidiary lava flow field southwest of the main volcanoes. The main edifice features several aligned craters that form a line. There is no recorded eruptive activity, but the volcano displays vigorous fumarolic activity on its northern side and within the craters. It is located on top of older volcanic rocks and features both andesite and dacite.
Lastarria produced a large landslide deposit when part of its southeastern flank collapsed. From a wide collapse scar, the landslide descended over a distance of . The intense fumarolic activity makes the volcano the largest source of volcanic gases in the region, and has produced fumarolic vent deposits as well as flows of molten sulfur. A progressive uplift of the terrain around Lastarria and farther south has been recognized; it appears to reflect a deep magma intrusion in the region.
Lastarria is part of the Andean Central Volcanic Zone, which extends over from Peru to Chile. Over 1,000 volcanic edifices have been identified in this zone, of which about 50 volcanoes are active or potentially active, with many exceeding in altitude. In addition, the zone features 18 monogenetic volcanoes and about 6 caldera/ignimbrite systems.
Five overlapping Volcanic crater are aligned in a north–south line on Lastarria's main cone, the craters 5-4 and 3-2 are nested within each other (counted from south to north). Volcanic activity has migrated north during the history of Lastarria, and the most recent eruption products are found on the northern and western slopes. A lava dome sits on the northernmost crater rim. The South Spur has two craters.
The volcano rises from a terrain of about altitude and has fairly steep slopes. Much of the surface, including the Southern Spur, is covered by deposits left by volcanic ash fall. Some parts of the Southern Spur display evidence of hydrothermal alteration. The total volume of the edifice is about .
The Negriales del Lastarria (also known as Big Joe) lava flow complex lies southwest of the Lastarria volcano and covers a large surface area. It is formed by several massive flows erupted from a single vent during three or eight pulses; the longest reaches a length of . These lavas are with flow ridges and levees. The total volume of the lava field is about and it is often grouped together with Lastarria and South Spur as the Lastarria Volcanic Complex.
Neighbouring mountains include Chili volcano northeast, the high Cerro Bayo northwest and the high Cerro Piramide close to Negriales de Lastarria in the southwest. Almost due north of Lastarria lies the Laguna de la Azufrera, a salt pan with a waterbody that is almost a lake, its name is a reference to the sulfur deposits of Lastarria. There are with temperatures exceeding on its eastern shore. The fumarolic system may drain into this waterbody, depositing sulfur at Azufrera's southern shore. Water levels in the lake were higher in the past, as evidenced by two recognizable shorelines, which owing to tectonics related to recent volcanism in the Corrida de Cori crop out mainly on the eastern side, and the lake's surface area reached . There is a valley and a lake also at the southeastern foot of Lastarria.
The debris avalanche deposit is long and well preserved with landforms such as lobes, levees and hummocks. After exiting the collapse scar over its northern opening, it overrode an older scoria cone before coming to rest. The slide, bordered by levee-like structures reaching a height of , formed wide and high lobes. Unlike many debris avalanche deposits, the Lastarria debris avalanche lacks large blocks and has only a few hummocks. The velocity of the avalanche has been estimated to have been over , a fairly high velocity for a volcanic debris avalanche, while later research proposed a maximum speed of . It is possible that air was entrained in the debris, which thus assumed properties similar to an ignimbrite. The collapse occurred without any preceding instability of the edifice.
The avalanche deposit consists mostly of loose material such as ash, lapilli, pumice, with only a few lithic blocks. This loose consistency may explain the lack of megablocks. Its total volume is about , less than the volume of the Mount St. Helens and Socompa deposits. It is comparable to the volume of the landslide that the Ancash earthquake triggered on Huascarán in Peru in 1970, resulting in more than 20,000 fatalities. There is evidence of previous flank collapses at Lastarria and ongoing flank instability.
The earliest volcanic activity on the west coast of South America goes back to the Jurassic, when the South Atlantic started to open. During the late Cenozoic, a volcanic chain was formed on top of Mesozoic and Paleozoic rocks and reached a width of in the area of Lastarria. This volcanism began 25million years ago, and the rocks are mostly .
Lastarria is located on a basement formed by andesite–dacite volcanic rocks in the form of ignimbrites, lava flows, and lava domes. They are of Miocene to Pleistocene age and are in turn underlaid by Paleozoic metamorphism volcanic and sedimentary rocks. The basement underneath Lastarria appears to have a different composition to that beneath Lascar. A major crustal lineament known as the Archibarca lineament intersects the main volcanic arc at Lastarria. Other volcanic centres such as Galán and ore deposits are also found on this lineament. The intersection between this lineament and the arc may act as a weakness zone that focuses the ascent of magma. Other lineaments in the region include the Imilac-Salina del Fraile and Pedernales-Arizaro faults of Miocene age.
A number of Weathering products are also present, some of which have been visualized by aerial imagery. Fumarole deposits contain encrustations and sublimates. They broadly consist of several components, mainly native sulfur, like anhydrite, baryte, gypsum and rhomboclase, like sassolite, like quartz and less commonly like galena, orpiment and pyrite. Cristobalite and magnetite are found in high-temperature vents. These form multicoloured deposits, ranging from yellow-white over grey to yellow-orange and red.
The petrogenesis of Lastarria rocks, like those of other volcanoes in the Central Volcanic Zone, involves the prolonged interaction with crustal rocks in magma chambers as well as the fractionation of certain minerals. Enriched lower crust and upper mantle might also contribute. Finally, the mixing of magma chamber contents with new and more mafic magma shortly before each eruption played an important role in rock genesis. In the case of Lastarria, this mixing occurs in a stratified magma chamber, with active convection occurring between lighter and colder upper contents and hotter and denser lower contents. Some rocks display "banding" features, implicating the mixing of different magmas during their formation. Some chemical differences exist among the Negriales rocks, the Lastarria lavas, and the Lastarria pyroclastics. The Negriales rocks are the richest in silicon dioxide, and their trace element composition sharply diverges as well. The Negriales rocks may originate from parental magmas that are different from the main Lastarria magmas.
Potassium–argon dating of Lastarria has yielded ages of 600,000 ± 300,000 and less than 300,000 years ago. The older date refers to the Negriales lava field, which has also been dated at 400,000116,000 ± 26,000 years ago. The South Spur is dated at 150,000 ± 50,000 years ago. The main edifice started forming 260,000 ± 20,000 years ago. One andesite lava flow was dated at 51,000 ± 13,000 years ago by argon–argon dating. There was then a lull in volcanic activity until the Holocene. Three were erupted during the Holocene between 4,850 ± 40 and 2,460 ± 40 years ago and were emplaced around the volcano, in particular north and west of the edifice.
Most of the volcano formed post-. Holocene explosive eruptions were separated by intervals of about 2,390 - 1,660 years. Three were erupted during the Holocene between 4,850 ± 40 and 2,460 ± 40 years ago and were emplaced around the volcano, in particular north and west of the edifice. Three were erupted during the Holocene between 4,850 ± 40 and 2,460 ± 40 years ago and were emplaced around the volcano, in particular north and west of the edifice. The landslide also occurred during the Holocene, 7430 (+136,−156) years before present. The lava dome on the northernmost crater rim is the youngest vent of Lastarria. The youngest dated deposit is 2,460 ± 50/60 years old, but at least one younger pyroclastic flow is present.
No historical eruptions are known but have been recorded at the volcano. Noticeable thermal hotspots are visible from ASTER imagery and are associated with the fumarolic areas. Temperatures observed at the hotspots are around . A potential of geothermal power at Lastarria was recognized already in 1974. It is considered Argentina's 9th most dangerous volcano out of 38.
The fumaroles release gases with temperatures ranging from . Carbon dioxide is the most important non-hydrous component of the gases; other components are hydrogen in variable amounts, hydrogen chloride, hydrogen fluoride, hydrogen sulfide, and variable amounts of nitrogen and sulfur dioxide. Additional components are , alkenes, argon, carbon monoxide especially in hotter fumaroles, helium, methane especially in colder fumaroles, and oxygen. The composition of the fumaroles indicates that most gases are of magmatic origin with little contribution from the atmosphere. Likewise, most water comes from the magma rather than from precipitation, as indicated by the oxygen isotope ratios. It is likely that the arid climate of the region reduces the input of meteoric water to the volcanic system.
Of five volcanoes analyzed in 2012 (Lascar, Lastarria, Ollague, Putana, and San Pedro), Lastarria had the highest flux rates. Individual gas fluxes in tons per day are registered as:
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The gases of Lastarria come from a geothermal system and, with temperatures ranging from and , supply the colder and hotter fumaroles, respectively. In turn, a magma system at depths of underpins and feeds this geothermal system. During their rise, the gases interact with the rocks of the surrounding country and with .
Lastarria's exhalations contain numerous volatile elements, like boron and chalcophiles. Arsenic is a harmful pollutant that occurs in above average concentrations in waters of northern Chile. Fumarolic exhalations at Lastarria can reach over of the fumarole sediments and the volcano is considered to be an important source of arsenic of the southern Central Volcanic Zone.
The conditions surrounding the emplacement of the sulfur have caused the sulfur to assume various colours, including black, brown-orange, orange, red, yellow, and yellow-orange. These colours vary along the length of the flows and between various flows, indicating that temperatures varied between one flow and another. Liquid sulfur has different viscosity and Rheology properties at different temperatures, and some variation occurred at Lastarria's flows as well.
Such sulfur flows are rare on Earth; they may be more common on the Jupiter moon Io. On Earth, they have been found at Kawah Ijen in Indonesia, Mount Iō (Shiretoko) in Japan, Mauna Loa on Hawaii, Momotombo in Nicaragua, and Sierra Negra on the Galapagos Islands. The sulfur flows on Lastarria may be threatened by future mining efforts in the region.
Ground uplift has been detected at Lastarria itself, amounting to . The uplifting region has either a surface area of or is wide, smaller than Lazufre. The Lastarria uplift started later than the Lazufre uplift and may be influenced by the latter. Possibly, magma injected into a Lazufre magma chamber is influencing the Lastarria hydrothermal system, with changes in fumarole output observed in 2006–2012. Modelling indicates that the source of this uplift lies at a depth of about and has the shape of a sphere. Another estimate places the source inside the volcanic edifice and assumes a size of , with the volume increasing by about .
Ground uplift is still underway but with a slowdown between 2006 and 2016. At other volcanoes, such uplift has been associated with changes in fumarolic activity or even the start of an eruption.
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