Lastarria

Lastarria is a 5697 m 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 1500 km 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 1 km wide collapse scar, the landslide descended over a distance of 8 km. 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.

Geography and structure
Lastarria is situated in the Central Andes, in the Antofagasta Region of Chile, and straddles the border with Argentina's Salta Province. The city of Antofagasta lies 250 km northwest of Lastarria. The Central Andes are difficult to access and its volcanoes are usually poorly monitored. There are no human populations within 150 km of Lastarria. From the former Catalina railway station 120 km west, an unpaved road leads to Lastarria. Unlike most other volcanoes in the region, no Inka archeological site has been found on Lastarria. Either the volcano was too active or too unimportant in pre-Hispanic times.

Lastarria is part of the Andean Central Volcanic Zone, which extends over 1500 km 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 6000 m in altitude. In addition, the zone features 18 monogenetic volcanoes and about 6 caldera/ignimbrite systems.

Edifice proper
Lastarria is formed by two coalesced edifices, the main cone and the older South Spur (Espolón Sur), which are joined at an altitude of about 5500 m and form a 10 km long ridge. The main cone comprises lava domes, lava flows, pyroclastic flows, and scoria, and reaches a summit height of 5697 m. Most of the volcano is covered by pyroclastic material, some of which extends to the southeastern margin of the Salar de Aguas Calientes. Lava flows are exposed mainly on the northwestern slope, where they reach thicknesses of 40 m. The South Spur has also generated lava flows. The volcano covers a surface area of about 156 km2.

Five overlapping craters 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 4200 m 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 10.1 km3.

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 10 km. These lavas are block lavas with flow ridges and levees. The total volume of the lava field is about 5.4 km3 and it is often grouped together with Lastarria and South Spur as the Lastarria Volcanic Complex.

Neighbouring mountains include Chili volcano northeast, the 4709 m high Cerro Bayo northwest and the 5214 m 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. 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 7 mi2. There is a valley and a lake also at the southeastern foot of Lastarria.

Landslide scar
A major sector collapse occurred on Lastarria's southeastern flank, leaving a clearly defined north–south scarp in the volcano that opens to the east-southeast. The scarp reaches a maximum height of 120 m and forms a semicircle just under 1 km wide; the northern part is longer than the southern and the western part directly abuts the margin of the southernmost crater of Lastarria. The highest point of the scarp lies at an altitude of 5575 m.

The debris avalanche deposit is 8 km 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 20 m, formed 500 m wide and 40 m 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 84 m/s, a fairly high velocity for a volcanic debris avalanche, while later research proposed a maximum speed of 208 - 270 km/h. 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 0.091 km3, 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.

Internal structure
The internal structure of a volcano has been visualized with a technique known as seismic wave tomography. An inverted funnel-shaped low velocity anomaly with a width of 4 by extends to a depth of 1 km beneath the volcano and appears to be associated with areas of high fumarolic activity; it may be the hydrothermal system. An even stronger anomaly at depths of 3 to 6 km may be the magma chamber of the volcano and an associated fluid-filled system. Magnetotelluric imaging showed structures similar to those revealed using seismic imaging.

Geology
Off the western coast of South America, the Nazca Plate subducts beneath the South America Plate at a rate of 7 - 9 cm/year. Volcanism in the Andes occurs in four distinct regions: the Northern Volcanic Zone, the Central Volcanic Zone, the Southern Volcanic Zone, and the Austral Volcanic Zone. All but the last are geographically associated with the subduction of the Nazca Plate beneath the South American Plate; the Austral Volcanic Zone involves the subduction of the Antarctic Plate beneath the South American Plate. Magmatic processes important in the Central Andes include the partial melting of the subducting plate and its sediments and of mantle peridotite, and fractional crystallization of ascending magma in the crust.

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 100 to 150 km in the area of Lastarria. This volcanism began 25million years ago, and the rocks are mostly acidic rocks.

Local
Lastarria and Cordón del Azufre form a group of volcanoes on the Altiplano, on the border between Chile and Argentina. They were active during the Quaternary. The Bayo volcano is sometimes considered part of this complex as well. Lastarria and Cordón del Azufre, together with some more local volcanic centres, may be part of a larger silicic volcanic complex that has not yet formed a caldera. This complex is characterized by a 500 m high doming with a central depression. The large Los Colorados caldera lies south-southeast of Lastarria. Farther south lie volcanoes such as Wheelwright Caldera and Cerro Blanco, the last of which shows evidence of recent unrest.

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 metamorphized 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.

Geologic record
The Altiplano started to form during the Eocene, when subduction of the Nazca Plate beneath the South American Plate caused compression along the plate margin. Strong volcanism and tectonic uplift occurred between 15and 20million years ago.

Composition
Lastarria is composed of rocks ranging from basalt over andesite to dacite, and the rocks define a potassium-rich suite characteristic of calc-alkaline magmas of the Central Volcanic Zone. The appearance of Lastarria lavas is porphyritic. Phenocrysts include plagioclase in andesite with smaller amounts of amphibole, biotite, clinopyroxene, and orthopyroxene. Apatite and zircon form accessory minerals. Dacites have similar composition but also contain hornblende. Olivine is found in the andesites and quartz in the dacites.

A number of alteration 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, sulfates like anhydrite, baryte, gypsum and rhomboclase, borates like sassolite, oxides like quartz and less commonly sulfides 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.

Climate and vegetation
Lastarria has a montane climate characterized by extreme aridity as it is located at the intersection between the summer rain region of the Altiplano and the Atacama Desert. Temperatures of -11 F and precipitation of 20 - 50 mm/year have been recorded on Lastarria, although the precipitation may be underestimated. Low bush vegetation exists in the area.

Eruptive history
The South Spur edifice is the oldest structure found at Lastarria. The Negriales lava field formed later. The five craters of Lastarria proper formed in five different stages. An alternative view holds that Negriales formed before the South Spur, and that the main edifice formed in ten different stages. Block-and-ash flows, hot avalanches, lava domes, lava flows, and pyroclastic flows have all been involved in the activity of Lastarria. Most deposits on the northern slopes were erupted during the last two stages, with the exception of several exposures of the older stages on the northwestern flank and the western "pink pyroclastic flow" deposit. Overall, later and Holocene activity at Lastarria was highly explosive, unlike the more effusive earlier eruptions including the Negriales eruptions.

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 400,000 – 116,000 ± 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 ignimbrites 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-glacially. Holocene explosive eruptions were separated by intervals of about 2,390 - 1,660 years. Three ignimbrites 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 ignimbrites 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 earthquakes 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 279 K. A potential of geothermal power at Lastarria was recognized already in 1974. It is considered Argentina's 9th most dangerous volcano out of 38.

Fumarolic activity
Lastarria displays vigorous fumarolic activity on the summit and down the northwestern slopes. Such activity has been observed since the European discovery of Lastarria, in the late 19thcentury. Lastarria is the only volcano in the area with ongoing fumarolic activity. It manifests in fumaroles forming 15 cm high chimneys, small cones reaching heights of 2 m, 2.5 m wide craters, 100 by fractures, in a diffuse fashion through pyroclastic deposits, as well as degassing through cracks and fissures on surfaces. Individual fumaroles have been found in the craters, on the crater edges, and on the slopes. A northwest–southeast striking fracture is associated with some of the fumaroles. Four different fumarole fields have been found, one along this fissure on the northwestern slope at altitudes of about 4950 to 5140 m, two on the rims of the fourth crater, and one in the fifth crater. The fissure field is the largest, covering a surface area of 0.023 km2, while the other fields can be as small as 0.001 km2. The alignment of fumarolic vents suggest that their position is controlled by the structure of the volcano. There is evidence of changes in fumarolic gas compositions, with an increased influence of magma beginning 2009-2012.

The fumaroles release gases with temperatures ranging from 80 to 408 C. 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 alkanes, alkenes, argon, carbon monoxide especially in hotter fumaroles, helium, methane, 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: The composition of Lastarria's gases has changed over time, with an increase in the magmatic component between 2009 and 2012, which may be due either to different measurement methods or to changes in the volcanic activity at Lastarria. Decreased temperatures have been observed after rainfall.

The gases of Lastarria come from a geothermal system and, with temperatures ranging from 280 to 370 C and 560 to 680 C, supply the colder and hotter fumaroles, respectively. In turn, a magma system at depths of 7 to 15 km underpins and feeds this geothermal system. During their rise, the gases interact with the rocks of the surrounding country and with aquifers.

Arsenic is a harmful pollutant that occurs in above average concentrations in waters of northern Chile. Fumarolic exhalations at Lastarria can reach over 1 g/kg of the fumarole sediments and the volcano is considered to be an important source of arsenic of the southern Central Volcanic Zone.

Sulfur
The fumaroles on Lastarria have created widespread deposits of sulfur. The sulfur has also formed flows, of which the two largest are 350 m and 250 m long. The longer flow is partly buried by the shorter one and has already lost part of its surface structure. No vents have been found; sulfur flows appear to emerge from the fumarolic terrain. One andesite lava flow has generated several subsidiary sulfur flows that resemble pahoehoe flows and have widths of 1 to 2.5 m. It is likely that sulfur deposited by fumaroles formed such flows. Some fumaroles currently release centimetre-long sulfur flows. Flows of sulfur are highly fragile constructs that can easily be destroyed.

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 rheomorphic 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
InSAR observations performed in the years 1998 to 2000 have yielded evidence of a pattern of ground uplift centered between Lastarria and Cordón del Azufre. This pattern, also known as "Lazufre", covers a surface area of 45 by. This uplift appears to be caused by the injection of magma at depth, with a pattern of progressively increasing flux between 2003 and 2006. The source of this uplift appears to lie at depths of 9 to 17 km, later recalculated at 2 to 14 km. This uplift may have been ongoing for about 400,000 years and has influenced the final position of lava flows of Lastarria and other volcanoes in the area.

Ground uplift has been detected at Lastarria itself, amounting to 9 mm/yr. The uplifting region has either a surface area of 6 km2 or is 6 km 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 1000 m and has the shape of a sphere. Another estimate places the source inside the volcanic edifice and assumes a size of 230 to 360 m, with the volume increasing by about 8000 to 18000 m3/yr.

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.

Threats
The volcano is in a remote area and thus constitutes little risk to human settlements. The closest populations are at Mina Vaquillas, Mina El Guanaco, and Campamento Pajonales. The Chilean SERNAGEOMIN published a volcano alert rating for Lastarria. A permanent seismometer was installed on the volcano in late 2013. It is considered the 45th most dangerous volcano in Chile.