Láscar

Lascar is a stratovolcano in Chile within the Central Volcanic Zone of the Andes, a volcanic arc that spans Peru, Bolivia, Argentina and Chile. It is the most active volcano in the region, with records of eruptions going back to 1848. It is composed of two separate cones with several summit craters. The westernmost crater of the eastern cone is presently active. Volcanic activity is characterized by constant release of volcanic gas and occasional vulcanian eruptions.

Lascar has been active since at least 56,000 years ago, though some argue for activity beginning 220,000 years ago. The first known activity occurred at the eastern cone and was characterized by lava flows, before shifting to the western cone where lava domes were emplaced. An eruption event known as Piedras Grandes was followed by the large Soncor eruption. A new western edifice was constructed on top of the Soncor vent, during the Holocene activity then shifted again to the eastern edifice and continues there to this day. The magma supplied to the volcano ultimately comes from the subduction of the Nazca Plate beneath the South America Plate. A number of other volcanoes are found in the region, such as Aguas Calientes, Cordon de Puntas Negras and the giant La Pacana caldera.

The volcano experienced at least three major eruptions throughout its history: One is the Soncor eruption about 26,450 ± 500 years ago, another in 7,250 BCE and the third in 1993. The first of these eruptions released of material and is known as the Soncor eruption. The largest eruption of Lascar known to recorded history occurred in April 1993 and caused ash fall as far away as Buenos Aires. Because Lascar is located in a remote area, it is monitored primarily by remote sensing. Explosive eruptions are the greatest hazard at Lascar.

Etymology

The name originates from the Atacameño word ''láskar'' or ''lassi'' ( en, tongue), thought to refer to the shape of the volcano. Other names for the volcano are Hlàscar, Hlascar, Ilascar, Kar Las, Laskar, Toconado and Toconao.

Human use

The new town of Talabre is west of Lascar. , it had a population of 50 inhabitants. Toconao and San Pedro de Atacama lie and from the volcano, respectively. , stockbreeding and farming were the principal economic activities in Talabre. Chile Route 23 passes about west of Lascar.

Lascar, like El Tatio, is a destination for volcano tourism. Unlike the neighbouring volcanoes Acamarachi, Licancabur and Quimal, there is no evidence of archeological sites on Lascar, possibly because of the volcanic activity. However, the inhabitants of the town of Camar consider Lascar a protective mountain spirit and in Susques (Argentina) it is believed that snow will fall if Lascar is steaming strongly.

Geography and geological context

Regional setting

Volcanoes in the Andes occur in four separate regions: the Northern Volcanic Zone between 2°N and 5°S, the Central Volcanic Zone between 16°S and 28°S, the Southern Volcanic Zone between 33°S and 46°S, and the Austral Volcanic Zone, south of the Southern Volcanic Zone. These volcanic zones are separated by areas where recent volcanism is absent; one common theory is that the subduction processes responsible for volcanism form a subducting plate that is too shallow to trigger the formation of magma. This shallow subduction appears to be triggered by the Nazca Ridge and the Juan Fernandez Ridge; the areas where they subduct beneath the Peru-Chile Trench coincide with the limits of the Central Volcanic Zone. It is possible that when these ridges are subducted, the buoyancy they carry disrupts the subduction process and reduces the supply of water, which is important for the formation of melts.

Of these volcanic zones, the Central Volcanic Zone of which Lascar is a member of is the largest, covering parts of Peru, Bolivia, Argentina and Chile. The Central Volcanic Zone is located between two areas where subduction is shallower and volcanic activity is absent. In the Central Volcanic Zone, volcanism has been active for 120 million years, although it has undergone eastward migration during this time. Water released from the subducting plate triggers the formation of basaltic magmas that are then injected into the crust.

About 122 volcanoes with Holocene eruptions exist in the Andean Volcanic Belt, including Ojos del Salado which with a height of is the highest volcano in the world. Many of these volcanoes are covered by snow and ice. A number of supervolcanoes exist in the Central Volcanic Zone, they are part of the Altiplano-Puna volcanic complex.

Local setting

The volcanism of Lascar relates to the subduction of the Nazca Plate beneath the South America Plate. The Central Andes contain many hundreds of volcanoes, extending over the countries of Argentina, Bolivia, Chile and Peru. In this remote territory, where eruptions are poorly recorded, many volcanoes are higher than . They are constructed on a crust that is between thick. Volcanic centres include calderas and associated large ignimbrites, lava domes and stratovolcanoes; among the better-researched volcanoes are Galan, Nevados de Payachata, Ollague, Purico Complex, San Pedro– San Pablo, La Pacana, Tata Sabaya and Tumisa. Over 44 volcanoes in the region are considered potentially active, with a number of young volcanoes featuring fumarolic or hydrothermal activity. Guallatiri, for example, features fumarolic activity that is visible in satellite images. Also fumarolically active are: Sabancaya, El Misti, Ubinas, Tacora, Isluga, Irruputuncu, Olca, Ollague, San Pedro, Putana and Lastarria. The largest historical eruption occurred at Huaynaputina in 1600. Given the low population density around many of these volcanoes, there is often little information on their activity.

Lascar is located in the Antofagasta Region of Chile, and is , , or high, according to different sources. With a surface area of , the volcano has a volume of . Geographically, the area of Lascar is located between the Altiplano and the Salar de Atacama farther west; the terrain at Lascar dips in the direction of the Salar.

Lascar is located in the main volcanic arc, on the western margin of the Altiplano. The andesitic-dacitic Aguas Calientes is located east of Lascar; it may have formed a lava flow close to the summit during the Holocene. Aguas Calientes is older than Lascar, and it might share a magma chamber. Miocene– Quaternary volcanic centres in the neighbourhood include Cerro Negro in the north, Acamarachi northeast, Tumisa southwest, and the Cordon de Puntas Negras in the south, which Lascar is sometimes considered to be part of. Tumisa, to the south of Lascar, was active between 2.5 and 0.4 million years ago, is composed of dacite and surrounded by pyroclastic flow deposits. East of Lascar lies the La Pacana caldera.

Cerro Opla, west of Lascar, is a hill formed by Permian–Triassic granite. An area of increased electrical conductivity has been identified beneath Lascar and extends to some neighbouring volcanoes, reaching a depth of over south of Lascar.

The Quebrada de Chaile, the Quebrada de Soncor and the Quebrada de Talabre canyons run towards Salar de Atacama; they are deep and wide. These valleys were probably formed by erosion during glacial periods. The valleys drain the western, northern and southwestern slopes of Lascar. The southeastern slopes drain into Laguna Lejía which is close to the volcano, and the northwestern slope drains through the Quebrada de Morro Blanco.

Lascar is located atop of a ridge formed by the Cerro Corona and Cerro de Saltar lava domes, south and north of Lascar, respectively. Cerro Corona gets its name from a crown-shaped structure at its top. These domes cover a surface area of about . These lava domes are about 5 million years old, and are composed of dacite and smaller amounts of pyroxene andesite, along with rhyolite and visible minerals including biotite and hornblende. An eruption 16,700 years ago from Corona deposited tephra containing biotite and quartz in Laguna Lejía and generated a rhyodacitic lava flow. Another debris flow from Corona spread towards Salar de Atacama.

Geology

Lascar is a steep volcano formed by two irregularly shaped truncated cones that extend east–west, on a trend that includes Aguas Calientes. Six craters are located on the volcano, but sometimes only five craters are counted, in which case the central crater is considered to be the active one. The extinct western cone (also known as Apagado) is composed of layers of lava and pyroclastics. Its large crater is filled by another cone, which forms the highest summit of the Lascar volcano. Immediately east of it lies the eastern cone, which is contiguous with the western cone. The eastern cone (also known as Activo) is capped off with three distinct craters which are delimited by arcuate fractures. Measurements made from 1961 to 1997 determined that the eastern crater is wide and deep and thus the largest, the central crater is wide and deep, and the western crater is wide and deep, increasing to depth in 2005–2006. The craters show evidence that activity has migrated westward. The westernmost of these three eastern craters is the currently active one, surrounded by rims that reach heights of . In 1985, a hot spot in this crater was observed in satellite images. In the centre of the westernmost crater lies a smaller crater, deep and wide. There are many fumaroles along the rim of the inner crater.

Layers of lava and pyroclastics are discernible in the craters. These craters are not collapsed calderas, and there is no evidence of the deposits a large explosion would produce. Remnants of a previous edifice are visible in the craters; this older edifice constitutes the bulk of the eastern cone. There are traces of a collapse of the volcano towards the northeast, with an associated horseshoe-shaped scar.

Large lava flows are noticeable on the flanks of the volcano, with a total of eight lava flows recognized. They extend from the summit craters, although none of them appear to be associated with the currently active crater. Flows from the first stage of Lascar's activity are exposed at its western foot, while lava flows are buried beneath pyroclastic material on the eastern flank. A lava flow on the northern flank reaches almost to the village of Talabre. This lava flow is known as the Tumbres–Talabre lava flow; its margins are high, and it features a central channel. The flow advanced just north of the head of Quebrada Talabre before passing over cliffs and entering it. Another lava flow on the southwest flank is known as the Capricorn Lava. This dacitic lava was erupted on Lascar at high altitude and has a blocky surface. It features well developed levees and a flow front. Its rocks have a pale gray-blue colour, and their composition resembles the Soncor flow, despite more mafic lavas and pyroclastics being erupted in the time period between the emplacement of the Soncor flow and the Capricorn Lava.

An early pyroclastic flow, the Saltar Flow, is exposed on the eastern flank. It was emplaced after the collapse of the oldest edifice, covering Aguas Calientes' western slopes. The flow deposit was later modified by glacial activity. The Soncor flow is found primarily on the western side of Lascar, with part of it also southeast of Lascar. On the western slope, it buries the even older Piedras Grandes flow, which crops out only at the margins of the Soncor flow. While the Piedras Grandes flow was formed by a glacier run that transported blocks with sizes of up to , Soncor was formed by a large eruption. The large eruption gave rise to a pyroclastic flow that extended westward and contained breccia and various magmas. It was accompanied by a Plinian fall deposit. Finally, the andesitic pumice Tumbres flow is found on the northwest–west–southwestern slopes of Lascar.

The Quebrada Talabre cuts into the upper flanks of Lascar and eventually joins the Quebrada Soncor. Lahar deposits are found in adjacent valleys, suggesting that wetter periods had occurred during Lascar's activity. The Quebrada Talabre was scoured by pyroclastic flows during the 1993 eruption, exposing bedrock and Tertiary ignimbrites. Traces of glacial action are found on the older parts of Lascar at altitudes above and include meltwater gorges, striated rock surfaces, and U-shaped valleys. Moraines are found at Tumisa down to an altitude of .

The volcano sits above a major local geological trend, the north–south Miscanti Line. Other volcanic centres are also located on this line, including the Corona and Saltar lava domes, and the Miscanti and Lejia volcanoes. The Miscanti Line dissects the Quaternary basement beneath Lascar, and it may be a hinge of a fold that is being propagated by faults. The formation of the first cone at Lascar may have been facilitated by the intersection between the Miscanti Line and another east–west lineament formed by Pliocene tectonic compression of the region, and the lineament would have worked as an ascent path for magma. At least four alignments of volcanoes are recognized in the region.

Composition

Lascar rocks consist of andesite and dacite. These rocks have a composition mainly characterized as "two-pyroxene", but the old Piedras Grandes and Soncor rocks contain hornblende. Other minerals include anhydrite, augite, plagioclase which is also the dominant phenocryst phase in Lascar rocks, apatite, ilmenite, magnetite, olivine, orthopyroxene, phyrrotite, quartz, rhyolite in the groundmass, and spinel in inclusions. Dacite has more plagioclase and rhyolite. Additional component minerals found at Lascar include anorthite, augite bordering on diopside, bronzite, fassaite, forsterite, hypersthene, pigeonite and more.

The rocks of Lascar belong to the calc-alkaline series. concentrations range from 55.5 to 67.8% by weight, and the rocks have medium to large concentrations of potassium. The magmas are contaminated by the local crust, but not to the extent found in the Galan or Purico complex eruption products. The magma interacts with former salar deposits before ascending. The chemistry of Lascar's rocks is fairly similar to those of neighbouring Tumisa volcano.

Magma erupted by Lascar appears to form from the mixing of mafic and more evolved magmas; the 1993 eruption deposits contain bands of different rocks. Specifically, basaltic andesite magma is periodically injected into a magma chamber, where crystal fractionation and mixing processes take place. The process happens frequently, thus the magmas are relatively unevolved; presumably, if the supply of mafic magma is steady, the products are andesitic, otherwise dacite forms. This origin of Lascar magmas is reflected in the textures of rocks. Petrologic investigations indicate that at least three components give rise to Lascar's magmas, an upper crustal one, a mantle component and an enriched component that may come either from the lower crust or the downgoing slab. The overall magma supply rate of Lascar is .

The magma chamber of Lascar appears to lie at depths of , although the lack of deformation of the edifice during the 1993 eruption indicates that it may be deeper, over or even over deep. Magma petrology implies that there is another reservoir at depth. There appear to be two distinct chamber systems, an andesitic one that is responsible for the frequent andesite lava and pyroclastic flow activity, and a dacitic one that was involved in the Piedras Grandes and Soncor activities.

Temperatures of the magma chamber range from ; the mafic magmas that are injected in the chamber are about hotter than the extant andesite and dacite. The chamber may be surrounded by skarnic alteration. This alteration gives rise to wollastonite and pyroxene-containing skarn, depending on the distance from the magma chamber walls. Metasomatism does further affect rocks derived from magma chamber walls. The conditions at the magma chamber may be comparable to these under which epithermal mineral deposits form. The oxidation conditions in the magma chamber are favourable for the formation of sulfate, but unfavourable for the deposition of sulfide minerals.

A number of xenoliths occur in Lascar's rocks; a large amount of the phenocrysts are ultimately derived from them. Hornfels, skarn, and rocks that are part of Lascar's lava dome ridge are the source of these xenoliths. Minerals encountered in xenoliths include andradite