Ollagüe

Ollagüe () or Ullawi () is a massive andesite stratovolcano in the Andes on the Bolivia–Chile border, within the Antofagasta Region of Chile and the Potosi Department of Bolivia. Part of the Central Volcanic Zone of the Andes, its highest summit is above sea level and features a summit crater that opens to the south. The western rim of the summit crater is formed by a compound of lava domes, the youngest of which features a vigorous fumarole that is visible from afar.

Ollagüe is mostly of Pleistocene age. It started developing more than one million years ago, forming the so-called Vinta Loma and Santa Rosa series mostly of andesitic lava flows. A fault bisects the edifice and two large landslides occurred in relation to it. Later two groups of dacitic lava domes formed, Ch'aska Urqu on the southeastern slope and La Celosa on the northwestern. Another centre named La Poruñita formed at that time on the western foot of the volcano, but it is not clear whether it is part of the main Ollagüe system. Activity at the summit continued during this time, forming the El Azufre sequence.

This phase of edifice growth was interrupted by a major collapse of the western flank of Ollagüe. Debris from the collapse spread in the form of hummocks down the western slope and into an adjacent salt pan, splitting it in two. The occurrence of this collapse was perhaps facilitated by a major crustal lineament that crosses Ollagüe from southeast to northwest. Later volcanic activity filled up the collapse scar, forming the Santa Cecilia series. This series includes lava flows as well as a compound lava dome on the western rim of the summit crater, which represent the youngest volcanic activity of Ollagüe. While there is no clear evidence of historical eruptions at Ollagüe, the volcano is considered to be potentially active and is monitored by the National Geology and Mining Service (SERNAGEOMIN) of Chile. Hydrothermal alteration has formed sulfur deposits on the volcano, which is the site of several sulfur mines. Later glaciations have formed moraines on the volcano.

Name

The original Aymara name of the volcano was Ullawi. It is derived from Aymara ullaña to see, to look at, to watch, and wi which is a nominalizing suffix to indicate a place, thus "viewpoint".

The common name is Ollagüe.Template:Efn Other alternate names are Oyague, Ollagua and Oyahué.

Geography and geomorphology

Ollagüe straddles the border between Chile and Bolivia, with most of the edifice lying on the Bolivian side.Template:Sfn The Chilean portion lies in the commune of Ollague, in the El Loa province of the Antofagasta Region, while the Bolivian segment lies in the Potosi department. Towns and human sites close to Ollagüe are Amincha, Buenaventura,Template:Sfn Cosca, El Chaco, Ollague and Santa Rosa,Template:Sfn and the main road of Ollagüe runs along the western foot of the volcano.Template:Sfn The first documented ascent was in 1888. The mountain reportedly can be climbed from the eastern side. The occurrence of warning signs about minefields has been reported.

Regional

Ollagüe is part of the Central Volcanic Zone (CVZ),Template:Sfn one of the volcanic arcs that exist in the Andes. The Andes have segments with volcanic activity and segments without; volcanic activity occurs only where the angle of subduction is relatively steep. There are four such segments, the Northern Volcanic Zone, the CVZ, the Southern Volcanic Zone and the Austral Volcanic Zone. The subducted part of the plate (slab) loses water as it sinks into the mantle, and this water and other components migrate into the mantle that lies between the subducted plate and the overlying crust (mantle wedge) and cause the formation of melts in the wedge.Template:Sfn

The CVZ is located between 16° and 28° southern latitude, on the western margin of South America.Template:Sfn At this latitude, west of the CVZ,Template:Sfn the oceanic Nazca Plate subducts steeply beneath the continental South America Plate in the Peru–Chile Trench.Template:Sfn East of the CVZ lies the Altiplano, a plateau with average elevations of .Template:Sfn The CVZ contains about 1,100 volcanoes of Cenozoic age, including Parinacota, San Pedro and Tata Sabaya. Many volcanoes in the CVZ have summit heights exceeding ,Template:Sfn forming the Occidental Cordillera of the Andes at these latitudes.Template:Sfn About 34 of these volcanoes are considered to be active;Template:Sfn most of the volcanoes have not received detailed scientific reconnaissance.Template:Sfn A notable feature of the volcanoes of the CVZ is that they formed over a fairly thick crust, which reaches a thickness of ;Template:Sfn as a consequence contamination with crustal material has heavily affected the magmas that formed the volcanoes. The crust is not uniform along the length of the south-central CVZ because the northern segment is of Proterozoic and the southern of Paleozoic age.Template:Sfn

The Central Andes formed first during the Paleozoic–Eocene and were worn down by erosion during the Oligocene. The recent volcanic activity started during the Miocene and includes major ignimbrite eruptions of dacitic to rhyolitic composition; such large eruptions began 23 million years ago and caused the formation of calderas like Galán. The total volume of this formation exceeds . Stratovolcanoes also began to form 23 million years ago, although most were constructed in the last 6 million years. They are volumetrically much smaller and were formed by magmas whose composition ranges from basaltic andesite to dacite. Finally, small alkaline volcanic centres are found primarily in the back-arc region and appear to be young.Template:Sfn A notable trait of the Central Andes are the long strike-slip faults that extend from the Eastern Cordillera northwest through the Altiplano into the volcanic arc. These include from north to south the Pastos Grandes–Lipez–Coranzuli, Calama–Olacapato–El Toro, Archibarca–Cerro Galan and Chulumpaja–Cerro Negro lineaments. MonogeneticTemplate:Efn centres are aligned on these faults.Template:Sfn

Local

Ollagüe is a stratovolcano and lies isolated slightly east of the main volcanic arc.Template:Sfn The volcano is usually covered with snow, which together with yellow and red colours gives Ollagüe a "beautiful" appearance. Other than some past glacial activity, the arid climate of the Altiplano region has kept erosion rates low, meaning that the volcanic edifice is well preserved.Template:Sfn On the other hand, lack of erosion also means that relatively little of its internal structure is exposed.Template:Sfn

Ollagüe has two summits, Ollagüe South is high and Ollagüe North .Template:Sfn Southwest of the summit is theTemplate:Sfn summit crater below the summit with a narrow opening towards the south, which forms the Quebrada El Azufre. The rim of the crater culminates into high Ollagüe South. The western rim is formed by several lava domes.Template:Sfn These lava domes feature landslide deposits and lava flows that emanate from the foot of the dome. Originally they were considered to be a single lava dome,Template:Sfn before it was found that the dome is formed by four individual domes.Template:Sfn Just north of the summit crater lies another semicircular crater rim which encircles the summit crater on its northern side and whose high point is high Ollagüe North.Template:Sfn The northeastern part of the edifice is old and affected by glaciation and the development of gullies, while the southwestern part has experienced younger activity and flank collapses.Template:Sfn The volume of the well exposed edifice is about Template:Sfn covering a surface area of . Ollagüe rises about above the surrounding terrain.Template:Sfn

• Ollagüe summit from Bolivia
  Ollagüe summit from Bolivia
• View from the summit of Ollagüe to southwest
  View from the summit of Ollagüe to southwest

File:Txu-oclc-224571178-sf19-07.jpg

Map of the region

The volcano has a number of adventive vents on its slopes, especially the northwestern and southeastern slope. These include Ch'aska Urqu on the southeastern slope and La Celosa (; also known as El Ingenio) on the northwestern.Template:Sfn They lie at distances of and from the summit vent, respectively.Template:Sfn The alignment of these subsidiary vents with the summit vents suggests that a N55°W striking lineament influenced their eruption; such channelling of magma along radial fractures has also been observed on other volcanoes such as Medicine Lake volcano, Mount Mazama and South Sister.Template:Sfn A normal fault runs across the main edifice but is not aligned with these adventive vents,Template:Sfn and the Pastos Grandes-Lipez-Coranzuli lineament intersects with the volcanic arc at Ollagüe.Template:Sfn Fault scarps are found on the northwestern and southeastern side of the edifice.Template:Sfn Overall, northwest trending lineaments exercised a strong influence on the tectonic development of Ollagüe,Template:Sfn and may be the path that feeder dykes of the more recent eruptions followed.Template:Sfn The basement undergoes extension perpendicularly to the lineament.Template:Sfn

A wide phreatomagmatic vent named La Poruñita lies on the western slope, on the deposit formed by the sector collapse.Template:Sfn It lies at an elevation of , is constructed out of tephra and formed on the sector collapse deposit.Template:Sfn Farther up on the edifice, two cinder cones are found just north and west of the highest summit of Ollagüe.Template:Sfn

Older volcanic centres around Ollagüe are Cerro Chijliapichina southwest (also known as Cerro PeinetaTemplate:Sfn), Cerro Canchajapichina south and Wanaku east of the volcano. These centres are unrelated to Ollagüe and were deeply affected by glaciation.Template:Sfn On the eastern foot the Carcote ignimbrite crops out,Template:Sfn a 5.9–5.5 million years old ignimbrite that is part of the Altiplano–Puna volcanic complex. These ignimbrites form the basement in much of the region.Template:Sfn The Carcote ignimbrite originally formed a plateau that extended around the volcano.Template:Sfn Off the western foot of Ollagüe lies a smaller volcanic centre that forms an effusive shield.Template:Sfn

File:Laguna verde Salar de carcote.JPG

Salar de Carcote. Ollagüe is the white mountain on the right.

The Salar de Ollague is located due north, while the Salar de San Martin lies southwest and Salar de Chiguana northeast of Ollagüe.Template:Sfn They are situated at elevations of .Template:Sfn The Salar de San Martin and the Salar de Ascotán farther south form a northwest–southeast trending graben delimited by the same normal fault that crosses the edifice of Ollagüe.Template:Sfn A ring plain formed by debris shed from Ollagüe surrounds the volcano.

Glaciation

Presently, high insolation and evaporation as well as the dry climate prevent the formation of glaciers or the existence of a snow cover.Template:SfnTemplate:Sfn Ollagüe lies in one of the driest regions of South America.Template:Sfn Thus, the present-day snowline is higher than the volcano. Underground ice deposits have been found on Ollagüe; presumably they form through evaporation cooling.Template:Sfn

Ollagüe has experienced glacial activity. Moraines are found on top of young lava flows and glacial valleys cut into the slopes.Template:Sfn On the western side, there are remnants of a moraine girdle,Template:Sfn which reaches an elevation of on the southwestern foot of the volcano.Template:Sfn Another possibly separate moraine girdle has been reported in the summit region, at elevations of about . This moraine is thought to have been formed during the Little Ice Age.Template:Sfn The Pleistocene snowline may have occurred at elevations of . Template:Sfn

Debris avalanche

A major sector collapse occurred on the western flank of the edifice, with the deposit formed by the collapse extending west from it.Template:Sfn Debris from the collapse flowed for into the Salar de San MartinTemplate:Sfn/Salar de Carcote, which slowed down the landslide.Template:Sfn Only the distal sector of the collapse deposit is still visible; the parts higher up on the edifice have been buried by more recent lava domes and lava flows.Template:Sfn The distal segment is also slightly raised compared to the more proximal parts.Template:Sfn The collapse deposit covers a surface area of and has a hummocky appearance, similar to the collapse deposit formed by the 1980 eruption of Mount St. Helens.Template:Sfn The avalanche deposit separates the Salar de San Martin from the Salar de Ollague.

The younger debris avalanche deposit has a volume of about . It was believed that it occurred about 600,000–400,000 yearsTemplate:Sfn or 800,000 ± 100,000 years ago, but dating of the andesites cut by the collapse yielded a maximum age of 292,000 ± 25,000 years ago.Template:Sfn Later the deposit was covered by lake deposits and debris from the piedmont,Template:Sfn and evaporites accumulated in depressions within the deposit.Template:Sfn Several lake terraces are set into the avalanche deposit,Template:Sfn with the traces of the highstand of Lake Tauca being recognizable; thus the sector collapse predates the highstand.Template:Sfn

Andesitic lava bombs on top of the deposit may indicate that an eruption occurred during the collapse.Template:Sfn Indeed, pyroclastic materials have been found at the foot of the volcano within the collapse deposit, where they fill small depressions. These materials are formed by several units of pumice and ash, generated by fallout and lava dome collapses.Template:Sfn

The sector collapse was probably caused by the edifice oversteepening as it grew,Template:Sfn with Ollagüe reaching a critical height before the collapse.Template:Sfn Magma pressurization probably triggered the failure, as the remnants of a lava lake in its summit indicate that magma pressure in the edifice was high at the time of the collapse. Conversely, hydrothermal alteration – which tends to weaken the stability of a volcanic edifice – was not involved in the onset of instability.Template:Sfn The northwest–southeast cutting fault probably additionally destabilized the edifice, allowing it to fail into a southwestern direction.Template:Sfn A previous southwesterly tilt of the basement also assisted in focusing the failure into that direction.Template:Sfn

The sector collapse formed a wide collapse scar on the upper western flank,Template:Sfn although the summit itself was probably unaffected.Template:Sfn This scar however was later filled by subsequent volcanic activity and modified by glaciation and is thus not conclusively identifiable.Template:Sfn

Two old sector collapses occurred during the older stages of volcanic activity. Their collapse scars are noticeable on the southeastern-southern and northwestern areas of the summit. The first is high and long, the second long and many high. Hydrothermally altered breccia with block sizes of several from the first collapse fills a valley on the western slope of the volcano.Template:Sfn Compared to the younger collapse, they are much narrower and have a highly unusual rectilinear form.Template:Sfn These collapses occurred about 450,000 years ago along the strike of a normal fault that cuts across Ollagüe.Template:Sfn Like in the young collapse, the summit was unaffected.Template:Sfn The lava domes that form the western rim of the summit crater have been subject to smaller sector collapses as well.Template:Sfn

• File:Volcán de Ollagüe, Bolivia, 2016-02-03, DD 90.JPG
• Fumarole clearly visible
  Fumarole clearly visible
• File:Volcán de Ollagüe, Chile, 2016-02-09, DD 72.JPG
• File:Volcán Ollague.jpg
• File:Volcán Ollagüe - panoramio.jpg
• Panoramic image, with Ollagüe in the centre
  Panoramic image, with Ollagüe in the centre

Composition

Ollagüe has erupted rocks ranging from basaltic andesite to dacite.Template:Sfn Blobs of basaltic andesite are found in all rocks from the volcano;Template:Sfn they probably formed when mafic magma was quenched by colder felsic magma.Template:Sfn The andesites and dacites are relatively rich in crystals.Template:Sfn Phenocrysts in the main andesite-dacite series include amphibole, apatite, biotite, clinopyroxene, ilmenite, magnetite, orthopyroxene, plagioclase and rarely olivine, quartz and zircon. The more acidic rocks also contain rare sphene. Some of the phenocrysts are surrounded by reaction rims, Template:Efn suggesting that they were not in chemical equilibrium with surrounding magma. Cumulates of phenocrysts indicate their formation during the magma differentiation process.Template:Sfn

Overall, the composition of Ollagüe's rocks fits into a high-potassium calc-alkaline series.Template:Sfn Gabbroic clots embedded in the lavas probably formed from cumulates.Template:Sfn Xenocrysts with large reaction rimsTemplate:Efn testify to a strong crustal contamination of the forming magma.Template:Sfn

File:Ollagüe from Bolivia(Jan2012).jpg

Ollagüe, with the road to the sulfur mines

Areas of hydrothermal alteration are found on Ollagüe, including in the summit crater, on its northeastern and northwestern rim and low on the northwestern slope.Template:Sfn Alunite, gypsum and sulfur were formed by the alteration on the summit and the northwestern slope,Template:Sfn and chalcedony, clay, kaolinite and opal are found as well.

The overall magma temperatures ranged for the andesitic and dacitic magmas and in the basaltic andesite.Template:Sfn The magmas became cooler over time, with the post-collapse magmas being colder than the pre-collapse eruptive products.Template:Sfn Variations in temperature between the outside and the inside of phenocrysts suggest that the magma chamber of Ollagüe was occasionally reheated by fresh magmas.Template:Sfn Water contents of the main edifice magmas range 3-5% by weight; in the Ch'aska Urqu and La Celosa magmas the water content is less well determined,Template:Sfn but is comparable to that of the main edifice magmas.Template:Sfn Later research, however, has raised questions about the reliability of the method used to determine water content in magma, which may have been lower than 3–5%.

Element compositions match those of other volcanoes in the CVZ.Template:Sfn Ollagüe magmas did not exclusively form from fractional crystallization; magma mixing and crustal contamination contributed to the formation of the magmas although it is not easy to determine what the composition of contaminants was.Template:Sfn Probably, it was in part hydrothermally altered upper crustal rock,Template:Sfn and in part Miocene age ignimbrites that crop out close to the volcano in Bolivia.Template:Sfn Crystal fractionation with some minor contamination by crustal components is probably the most satisfactory explanation for the magma chemistry of Ollagüe.Template:Sfn It is however difficult to tell the relative importance of contamination vs. assimilation.Template:Sfn

The composition data indicate that Ollagüe was underpinned by a large magma chamber that was the source of the main edifice building andesite magmas.Template:Sfn In this main magma chamber, differentiation processes generated the andesitic and dacitic magmas from basaltic andesite. The chamber itself was chemically zoned.Template:Sfn Episodically, new mafic magmas were injected into the magma chamber from below.Template:Sfn Subsidiary magma chambers which developed beneath the northwestern and southeastern flank gave rise to the La Celosa and Ch'aska Urqu volcanic centres, respectively. These subsidiary pathways also allowed basaltic andesite magmas to ascend to the surface; the main magma chamber would have intercepted any mafic magmas ascending into the central vent as such mafic magmas are denser.Template:Sfn The walls of the magma chamber were also affected by strong hydrothermal alteration processes, with weaker alteration also occurring in the walls of the subsidiary magma chambers.Template:Sfn La Poruñita was probably formed by magmas from the floor of the main magma chamber, or from the magma that enters the magma chamber from below; it had already undergone some crustal contamination in the depths of the crust when it erupted.Template:Sfn

Fumarolic activity

A major fumarole is active on the summit of the volcano, its plume reaching heights of .Template:Sfn It is strong enough that it can be seen on the ground from over away. The vent of the fumarole lies in the summit lava domes,Template:Sfn more specifically in a high and wide collapse scar in the southeasternmost lava dome of the compound summit lava dome.Template:Sfn Other volcanoes in the area with fumarolic activity include San Pedro and Putana.Template:Sfn

Fumarole temperatures appear to be so low (less than ) that in 1989 the exhalations could not be detected in the Thematic Mapper infrared band of the Landsat satellite even during night.Template:Sfn More recent satellite observations have shown the existence of hotspots with temperature anomalies of about ;Template:Sfn the relatively poor visibility of the hotspots in satellite images contrasts with the good visibility of the fumarole from the ground and may reflect the relatively small surface area of the hotspots, which makes them difficult to isolate in satellite images.

Fumarolic gases are made up primarily by Template:Chem/link and Template:Chem/link; Template:Chem/link is a subordinate component. The amounts of Template:Chem/link released have been measured; quantities vary but in December 2013 appeared to be about .Template:Sfn

Eruption history

Not many radiometric dates have been obtained on Ollagüe. Most dates are younger than one million years.Template:Sfn One proposed timeline subdivides the volcano into three stages: Ollagüe I between 1.2 million and 900,000 years ago, Ollagüe II 900,000–600,000 years ago and Ollagüe III 400,000 years ago to present.Template:Sfn La Poruñita, once considered of Holocene age, has been dated at 680,000 ± 200,000 to 420,000 ± 200,000 years ago; it is also not clear if it belongs to the Ollagüe volcanic system.Template:Sfn Magma output during the history of the volcano is about .

Vinta Loma and Santa Rosa

The oldest stage of activity is known as Vinta Loma and formed the bulk of the volcanic edifice, especially on the eastern side and in the summit area.Template:Sfn During this stage, lava flows and some pyroclastic flows were erupted from a central vent.Template:Sfn The pyroclastic flows are exposed as a thick sequence in a cirque close to the summit and reflect the occurrence of Plinian eruptions during this stage of volcanic activity.Template:Sfn The Vinta Loma series is subdivided into two groups separated by an unconformity, which are dated to 870,000 ± 80,000–641,000 ± 9,000 and 910,000 ± 170,000–1,230,000 ± 80,000 years ago respectively.Template:Sfn The Vinta Loma series more recently was partitioned into two series, Vinta Loma proper and the younger Santa Rosa.Template:Sfn Two summit crater rims and sector collapses formed during these stages.Template:Sfn The northern summit cinder/scoria cone and some lateral lava flows have been assigned to the Santa Rosa series.Template:Sfn

Lava flows from these stages have gray colours and rocky appearance which sometimes appears like it is covered by plates, with flow folds and some breccia. Their thicknesses and widths range , increasing on gentler slopes. Especially on the upper slopes, old colluvium conceals the surface of Vinta Loma lava flows.Template:Sfn The texture of the lavas ranges from porphyritic to seriate.Template:Sfn Two-pyroxene andesite is the dominant component but dacite has been found as well.Template:Sfn

The Vinta Loma edifice developed on top of an older fault. During the progression of volcanism the fault itself progressively propagated up and across the edifice and caused the southwest sector of the volcano to subside, without changes in volcanic activity. Eventually, the subsidence prevented lava flows of the Santa Rosa series from flowing northeast across the fault trace.Template:Sfn Then, the two older sector collapses occurred on the southwestern sides of the fault.Template:Sfn

Ch'aska Urqu, El Azufre and La Celosa series

Later the Ch'aska Urqu stage was erupted on top of Vinta Loma depositsTemplate:Sfn through radial vents on the southeastern flank. This stage is named after the high Ch'aska Urqu lava dome on the southeastern flank.Template:Sfn The stage generated lava flows, lava domes and couleesTemplate:Efn with compositions ranging from basaltic andesite to dacite, the former forming the base of the stage and the andesites and dacites being deposited above it.Template:Sfn These basaltic andesites form thick grey coloured lava flows and a thick plate-covered flow on top of the smaller ones.Template:Sfn

About 10 lava andesitic-dacitic domes and coulees were erupted on top of the basaltic andesite lava flows. They are short and have steep slopes, often ending with scree at the front. On the foot of the volcano they sometimes developed pressure ridges, and an deep cleft in Ch'aska Urqu may have formed when the dome spread laterally during its formation.Template:Sfn As with Vinta Loma lavas,Template:Sfn the upper parts of the coulees are covered with thin colluvium.Template:Sfn

Simultaneously, another dacitic lava dome stage occurred on the northwestern flank, forming the La Celosa lava dome-coulee complex.Template:Sfn Its age has been controversial,Template:Sfn with it being first associated with the youngest post collapse stages through argon–argon dating;Template:Sfn then with the oldest stages of volcanic activity. Eventually potassium-argon dating yielded an age of 507,000 ± 14,000 years ago.Template:Sfn Two other dates obtained from northern lava domes are 450,000 ± 100,000 and 340,000 ± 150,000 years ago. It has a lobate appearance, and similar to the Ch'aska Urqu dome a wide rift cuts through the dome. The La Celosa complex was erupted from two separate vents, and owing to its low altitude it has not been affected by glaciation.Template:Sfn

The andesites and dacites are of grey to light grey colour respectively, with porphyritic to vitrophyric textures.Template:Sfn In this stage, dacites are more common than in the Vinta Loma deposits. Basaltic andesite contains olivine, while the dacites tend to contain more amphibole and biotite.Template:Sfn There is a tendency of silicic acid contents to increase in the upper parts of the exposure.Template:Sfn

Later evidence has indicated that some lava flows were erupted from the summit during the Ch'aska Urqu stage. Also, a structure interpreted as a former lava lake formed close to the summit during this time. The lava lake-like structure itself is undated; one of the lava flows was dated 410,000 ± 80,000 years ago and the southern summit cinder cone is 292,000 ± 25,000 years old. This series is known as El Azufre.Template:Sfn The El Azufre series was emplaced within a sector collapse, a collapse which generated pyroclastic deposits in the Poroto section of the southwestern flank.Template:Sfn

• Ollagüe from the town of the same name, the La Celosa lava domes are in the foreground
• File:Vista del volcán de Ollagüe desde Ollagüe, Chile, 2016-02-09, DD 79.JPG
• File:Ollagüe Volcano.jpg

Post-collapse and Santa Cecilia series

The principal sector collapse occurred after the Ch'aska Urqu stage. It was followed by the eruption of andesitic lava flows and the compound lava dome in the summit region,Template:Sfn all focused into the collapse scar; this focusing is a phenomenon noted at other volcanoes which underwent flank collapses such as Planchón-Peteroa.Template:Sfn This formation has been named the Santa Cecilia series.Template:Sfn The compound summit lava dome probably fills the collapse scar but young lavas and glacial erosion make this assessment difficult.Template:Sfn Dates obtained on the summit lava domes range from 220,000 ± 50,000 years ago to 130,000 ± 40,000 years ago.Template:Sfn The youngest date was obtained on the youngest dome and shows an age of 65,000 years ago. Tephras identified in the Salar Grande close to the Pacific coast and dated to be less than 330,000 years old may come from Ollagüe or Irruputuncu.

The lava flows are best exposed on the western flank and have a grey colour. They display levees and pressure ridges and appear to be younger than the Ch'aska Urqu flows.Template:Sfn They originate at elevations of and extend over distances of .Template:Sfn The summit lava dome has a volume of ;Template:Sfn blocks with sizes of up to were formed by landslides during its growth.Template:Sfn Later research has shown that the summit lava dome is actually formed by several separate lava domes that extend southeast along a feeder fissure and become younger to the southeast. The foot of the compound dome is formed by scree-like breccia deposits.Template:Sfn

Compositionally, the post-collapse magmas appear to fit into two distinct groups. Older flows are dominated by pyroxene with only small quantities of amphibole and biotite. Younger shorter flows farther up on the edifice and the summit lava dome conversely contain relatively large quantities of amphibole and biotite.Template:Sfn

Recent activity and hazards

The post-collapse lava flows have been affected by glacial activity, indicating that eruptive activity ceased before the end of the last glacial stage;Template:Sfn thus the volcano was largely constructed in pre-Holocene times.Template:Sfn However, a long and wide lava flow extending from the youngest summit lava dome appears to post-date glaciation, and the dome itself is also unmodified.Template:Sfn

An uncertain report of an eruption on 3 December 1903 exists,Template:Sfn as well as on 8 October 1927 when the population of Ollagüe fled for several days. Increased fumarolic activity was observed in 1854, 1888, 1889, and 1960. SubstantialTemplate:Sfn earthquake activity occurs at Ollagüe in a diffuse pattern around the volcano, sometimes in the form of seismic swarms.Template:Sfn

The volcano is considered to be potentially active because of the fumarolic activity,Template:Sfn and SERNAGEOMIN publishes a volcano hazard index for Ollagüe. A seismometer array was deployed in 2010–2011.Template:Sfn Future eruptions of Ollagüe may threaten the town of Ollague away and the highway Template:Interlanguage link.

Sulfur mining and processing

Sulfur deposits on Ollague and neighbouring AucanquilchaTemplate:Sfn and OlcaTemplate:Sfn have been mined. On Ollague, the Santa Cecilia mine is on the northwestern rim of the crater and Santa Rosa mine in its centre.Template:Sfn In 1990, it was estimated that of sulfur can be mined at the Santa Rosa mine. According to a report in 1894, fumes released from sulfur beds on the volcano can incapacitate a man in seconds, making ascents difficult.

Large-scale exploitation of natural resources in the area commenced in the late 19th century, when after the Saltpeter War Chile acquired the territories, began to exploit themTemplate:Sfn and capitalism and industrialization came to the region.Template:Sfn After initial efforts in 1899, 1902 and 1916, in 1932-1936 Luis Borlando set up a companyTemplate:Sfn and began to mine sulfur on Ollagüe in response to demand by the saltpeter and copper industries.Template:Sfn The Chilean government had been encouraging mining development during the 1930s-1970s,Template:Sfn but only after the cessation of mining did it set up the infrastructure of the town of Ollague.Template:Sfn Mining was still underway in 1988Template:Sfn but ceased in the 1990sTemplate:Sfn as fluctuations in the global markets and the inability of the Chilean sulfur industry to compete on global markets forced its decline.Template:Sfn

A road reaching up to an altitude of leads to the western and southern mines. Sulfur was transported through an aerial tramway, which had replaced llamas.Template:Sfn A reduction plant with autoclaves is also found at Ollagüe, it was the first such plant in Chile,Template:Sfn while south of the town a mining camp was set up at Buenaventura.Template:Sfn Worker camps with factory and residential buildings,Template:Sfn and railway stations of the Ferrocarril de Antofagasta a Bolivia railway between Bolivia and Chile, completed the infrastructure.Template:SfnTemplate:Sfn

Mining activity at Ollagüe is mostly documented by many technical reports and by local oral tradition.Template:Sfn Presently, much of the infrastructure is in ruins and is the backdrop of a past interplay between migration, modernization and economic activity.Template:Sfn Some of the sites were dismantled, others were left with virtually all their equipment.Template:Sfn Since 2015, an investigation project has been running in the town of Ollagüe to record and preserve the history of sulfur mining and industrialization in the region.Template:Sfn

Sulfur mining was mostly carried out by an indigenous workforce,Template:Sfn as other people are not adapted to the extreme conditions at high altitudes (cold, hypoxia, intense winds) and thus unable to perform the work.Template:Sfn One of the worker camps had a population of 85, according to a 1930 census.Template:Sfn The harsh climate and precarious social status of this workforce conditioned work at Ollagüe, where sulfur mining and processing occurred under unique conditions.Template:Sfn Contemporary references to working conditions are ambiguous, as there were both concerns about the working conditions in newspapers of the 1930s and the impact that working conditions could have on economic productivity.Template:Sfn There was a high turnover in the workforce, which came to a large degree from BoliviaTemplate:Sfn to the point that the Bolivian government curtailed it in 1925, triggering a decline in the Chilean sulfur industry.Template:Sfn

See also

• Aucanquilcha
• Olca
• List of volcanoes in Bolivia
• List of volcanoes in Chile

External links

• Population data and map of San Pedro de Quemes Municipality
• Template:Cite peakware
• AVA