Llullaillaco

Llullaillaco is part of the Central Volcanic Zone.[9] At least 44 volcanic centres with historical activity and 18 large caldera-forming volcanoes have been identified in the Central Volcanic Zone.[8] Volcanism in the Central Volcanic Zone mostly occurs on the Altiplano and the Cordillera Occidental. A number of volcanoes there reach heights of over 6,000 metres (20,000 ft) above sea level. Large Miocene ignimbrites that cover large surfaces are also part of the regional geology.[10] Llullaillaco is located about 300 kilometres (190 mi) east of the Peru-Chile Trench.[11] The Wadati-Benioff zone lies at 180 kilometres (110 mi) depth.[12]

Llullaillaco is located in the northwestern Argentine Andes,[13] towards the southern end of the Puna.[14] It lies in the Puna de Atacama, a region of very high volcanic peaks on a high plateau[15] close to the Atacama Desert,[16] one of the driest places in the world.[17] The border between Argentina (Salta Province) and Chile crosses the volcano.[2]

Llullaillaco seen from Cerro Paranal, 189 km away.

The volcano is an imposing mountain, rising by 3,800 metres (12,500 ft) above the Salar de Punta Negra 36 kilometres (22 mi) away.[18] With a summit height of 6,723 metres (22,057 ft),[19][1] or alternatively 6,739 metres (22,110 ft)[2] or 6,752 metres (22,152 ft),[20] it is the second highest active volcano in the world after Ojos del Salado (6,887 metres (22,595 ft), 27°07′S 68°33′W).[19][1] The height of the mountain and the clear air in the region make Llullaillaco visible from Cerro Paranal, 189 kilometres (117 mi) away as measured through Google Maps.[21] The Nevados de Cachi mountains and Ojos del Salado 250 kilometres (160 mi) farther south are visible from Lllullaillaco.[22] The region is dry and located at high altitudes, making work in the area difficult.[18]

17 kilometres (11 mi) farther east[2] of Llullaillaco lies the Miocene Cerro Rosado volcano (5,383 metres (17,661 ft)). It erupted dacitic lava flows on its northeastern and southern flanks during the Pliocene-Quaternary.[23] 20 kilometres (12 mi) east of Llullaillaco,[18] and behind Cerro Rosado is the Salar de Llullaillaco (3,750 metres (12,300 ft) [24]), a salt pan with warm springs at its western and southwestern shores. There are three abandoned borate mines Mina Amalia and the salt mines Mina Luisa and Mina Maria[23] and the recent lithium prospect "Proyecto Mariana" at Salar de Llullaillaco.[25] Mitral mountain (5,015 metres (16,453 ft)) lies southwest of Llullaillaco and is of Miocene age. It features an eroded crater that opens to the northwest.[26][27] Iris mountain (5,461 metres (17,917 ft)) north of Llullaillaco is constructed of Pliocene rocks.[26] Other volcanoes in the neighbourhood are Dos Naciones, Cerro Silla, and Cerro 5074.[28] Llullaillaco is associated with a local crustal upwarp.[29]

Llullaillaco is a stratovolcano which rises 2,200 metres (7,200 ft) above the surrounding terrain.[1][19] The edifice has an elliptical shape with dimensions of 23 by 17 kilometres (14 mi × 11 mi) and has a volume of about 50 cubic kilometres (12 cu mi)[19][30]-37 cubic kilometres (8.9 cu mi).[31] It is formed by thick dacitic lava flows erupted during the Quaternary.[32] These flows extend away from the volcano and form its summit. An older unit is formed by ignimbrites and pyroclastic flows.[12] Older lava flows extend west from the volcano and are partly buried by sediments closer to the edifice. These flows reach lengths of 20 kilometres (12 mi) and form about 70% of the surface of the volcano.[33]

The slopes of the volcano are fairly steep, with an altitude drop of 1,800 metres (5,900 ft) over only 3 kilometres (1.9 mi) horizontal distance.[18][34] The slopes high up are steeper than those at lower altitudes.[35] A crater at 6,100 metres (20,000 ft) altitude was formed early in the development of the Llullaillaco volcano.[36] Erosion has reduced it to a plateau.[33] This crater is filled with snow and ice. The ice shows evidence of recent geothermal heating.[37]

Llullaillaco seen from space, with lava flows clearly visible.

The summit of Llullaillaco is formed by a small cone with about four associated lava domes,[33] which reach lengths of 1–3 kilometres (0.62–1.86 mi) and have abrupt walls.[38] Large dacitic coulees emanate from the summit of the volcano and are young in appearance.[39] One extends north of the volcano and the other one south.[2] Their overall length is 4.5–8 kilometres (2.8–5.0 mi) and their flow fronts are up to 15 metres (49 ft) thick. Morphologically, these flows are reddish-black aa lava flows and feature blocks with sizes of 5 metres (16 ft).[33] The southern lava flow is 6 kilometres (3.7 mi) long and was fairly viscous when it was erupted.[39] It almost reaches a road southwest of the volcano.[26] These flows feature levees and ridges. At first they were considered to be Holocene, but argon-argon dating indicates they are of late Pleistocene age.[2] Some traces of glacier activity are found in the summit area.[36]

At least two stages of construction are recognized, Llullaillaco I and Llullaillaco II. The first stage originated from two centres and is now extensively degraded by glaciation and hydrothermal alteration.[40] This applies especially to the 5,561 metres (18,245 ft) high Azufrera Esperanto mountain 5 kilometres (3.1 mi) north of Llullaillaco, where little original volcanic substance is preserved and where erosion has exposed deeply altered white rock.[33] Llullaillaco II on the southern and northeastern flank is better preserved; the toes of the lava flows reach thicknesses of 500 metres (1,600 ft).[39] Its lava flows are less extensive than the ones of Llullaillaco I.[12] Pyroclastic flow deposits with a composition similar to Llullaillaco II are found on the southern slope of the volcano and may have formed before the lava eruptions began.[38]

Volcanism in the Central Volcanic Zone may be affected by deep-seated lineaments, which control where volcanoes and geothermal systems form.[41] Such lineaments extend diagonally across the volcanic arc and are accompanied by volcanic manifestations at substantial distances from the arc.[42] One of these lineaments, the Archibarca, runs beneath Llullaillaco and is also associated with the Escondida copper deposit, [43] Corrida de Cori,[41] Archibarca, Antofalla,[44] and the Galán caldera.[43] Other lineaments include the Calama-El Toro.[42]

Several dry valleys originate on Llullaillaco, including Quebrada de las Zorritas on the north-northeastern slope, Quebrada El Salado and Quebrada Llullaillaco on the northwestern slope, and Quebrada La Barda on the southwestern slope.[45] The Quebrada Llullaillaco drains into the Salar de Punta Negra northwest of the volcano.[46] The volcano is relatively unaffected by water erosion,[35] water is only episodically present on the mountain.[47] On the northwestern flank there is a 0.9 hectares (2.2 acres) lake called the "Lago Llullaillaco" on the mountain; with an elevation of 6,170 metres (20,240 ft)[48] it is one of the highest lakes in the world.[49][50]

Two abandoned sulfur mines can be found north and south of Llullaillaco.[23] The northern mine is known as Azufrera Esperanto and associated with an area of hydrothermal alteration. A path or road leads up to that mine from northwest.[27]

• 
  Llullaillaco during sunset
• 
  Llullailaco with a road leading to the mountain.
• 
  Llullaillaco from the base camp.
• 
  Lava flow from Llullaillaco
• 
  Llullaillaco in Salta

A major landslide occurred in the volcano's history during the late Pleistocene, probably triggered by volcanic activity.[32] This landslide descended the eastern-southeastern flanks of the volcano into Argentina,[39][2] first over a steep slope of 20° on the volcano,[51] split around Cerro Rosado and entered the Salina de Llullaillaco 25 kilometres (16 mi) east of the summit,[39] extending up to 5 kilometres (3.1 mi) into the salar.[52] The toe of the avalanche reaches a thickness of 10 metres (33 ft) above the salar in the avalanche's southern lobe. Part of the avalanche was channelled in a valley between Cerro Rosado and an unnamed volcano farther south.[53] When it reached Cerro Rosado, the avalanche climbed about 400 metres (1,300 ft) and mostly continued to flow southeast into the main avalanche path, with only a small flow continuing northeastwards. Part of the run-up avalanche later collapsed backwards over the main avalanche deposit.[54] Unlike Socompa farther north, a landslide scar is only poorly developed at Llullaillaco despite the large size of the collapse;[55] it was largely filled in later by lava flows and volcaniclastic debris.[56]

This landslide has been subdivided into four facies and features landforms like levees up to 50 metres (160 ft) high,[57] longitudinal ridges and a run-up mark on Cerro Rosado.[52] Such ridges may be associated with uneven underlying terrain.[58] The surface of the slide is covered by lava bombs less than 1 metre (3 ft 3 in) long, blocks exceeding 2 metres (6 ft 7 in) width, cobbles, and gravel-like rocks.[59] Close to the collapse scarp on Llullaillaco the largest blocks with sizes of up to 20 metres (66 ft) are found.[54] Overall, the margins of the landslide are very crisp and the surface covered by hummocks.[60]

The landslide deposit covers a surface of about 165 square kilometres (64 sq mi).[39] Its volume has been estimated at 1–2 cubic kilometres (0.24–0.48 cu mi) and the speed at 45–90 metres per second (150–300 ft/s). This speed range is comparable to that of the Colima, Lastarria, and Mount St. Helens debris avalanches.[61] The landslide occurred no later than 156,000 – 148,000 ± 5,000 years ago,[39] it might coincide with the 48,000 year old lava flow.[51] Some volcanic rocks were still hot at the time of the collapse, indicating that volcanic activity occurred immediately before the collapse.[55] A smaller undated avalanche occurred on the northeastern flank.[39]

Such landslides are common at volcanoes, where they are favoured by the steep edifices that form from lava flows being stacked on top of each other. It is not usually known what triggers the collapse, although eruption-associated earthquakes are suspected to play a role. Other volcanoes in the region with sector collapses are: Lastarria, Ollague, San Pedro, Socompa, and Tata Sabaya.[62] The Mellado and Cerro Rosado volcanoes close to Llullaillaco also display evidence of sector collapses.[56]

Snow patches on Llullaillaco

In 2006 the General Water Directorate of Chile stated that there were seven separate ice bodies on the mountain.[63] Small glaciers are found above 6,000 metres (20,000 ft) altitude.[32] Other sources contend there are no glaciers on Llullaillaco, which would make it the highest mountain in the world without one;[64] although the existence of an ice slab on the western slope was reported in 1958[65] between 5,600–6,500 metres (18,400–21,300 ft) elevation[66] and in 1992 there was an ice body which however lacked all evidence of glacial motion.[67] However, Llullaillaco has snow fields[34] tied to permafrost in the ground.[68] 1–1.5 metres (3 ft 3 in – 4 ft 11 in) high penitentes occur above 5,000 metres (16,404 ft) altitude,[69] especially in more sheltered areas.[70] 1 metre (3 ft 3 in) high penitentes were observed in 1954.[71] Snow covers the terrain above 5,400–5,800 metres (17,700–19,000 ft) altitude.[3] Gullies that are found in the area probably result from meltwater runoff.[28] The snowfields on Llullaillaco supply water to the Salar de Punta Negra.[72] The lack of glaciers is a consequence of the dry climate,[73] as the high insolation and dry air allow all snow to evaporate before it can form glaciers.[74]

The existence of a cirque on the northwestern slope has been postulated.[75] It was once believed that Llullaillaco had experienced three large glaciations,[76] but the "moraines" are actually mudflow deposits.[77] However, during the Pleistocene glaciers reached down to 5,100 metres (16,700 ft) altitude on the northwestern slope. Lava flows have overrun some of the glaciated surfaces and it is possible that lava flows induced the melting of glaciers.[77] A number of glacial landforms elsewhere were destroyed by mudflows during the time of Llullaillaco's last activity.[78] Llullaillaco may not have been glaciated at all during the Pleistocene.[79]

Periglacial phenomena are observed on Llullaillaco, commencing at 4,300 metres (14,100 ft) altitude and reaching their maximum around 5,100–5,400 metres (16,700–17,700 ft) on the Chilean and 5,350–5,700 metres (17,550–18,700 ft) on the Argentine side. There, solifluction and cryoplanation surfaces are developed,[80] including lobe-shaped ground and block ramparts.[81] These landforms have been mapped on the northwestern side of the volcano.[82] Patterned ground is also common.[83] Permafrost is found at higher altitudes.[80] Cryoplanation and solifluction landforms are also observed on Iris and Mitral.[84] Other than on periglacially influenced terrain, the ground at Llullaillaco is formed mostly by lavic rocks and block debris, which are frequently buried by tephra.[85]

Llullaillaco has mostly erupted dacites with medium potassium content, with rocks becoming more felsic the younger they are.[86] Rock samples taken from Llullaillaco are mostly porphyritic or vitrophyric, with a glassy or microcrystalline matrix. Phenocrysts are mostly plagioclase, with mafic phenocrysts being dominated by orthopyroxene and smaller amounts of biotite, clinopyroxene, and hornblende. Ilmenite, magnetite, and sulfide minerals are also present, magnetite especially in the more oxidized older lavas.[39] Some phenocrysts show evidence of a complex history.[87] Quartz is rare. Older lavas have developed a red colour and contain oxidized iron in form of hematite.[88] Lavas from Llullaillaco I are grey.[12] The content of SiO
₂ is 65–67%.[30]

The rocks belong to the calc-alkaline series.[12] Trace element data are typical for Central Volcanic Zone rocks.[86] High potassium content is typical for shoshonite-like lavas erupted at large distance from the trenches.[11]

The composition may reflect magma differentiation in a solitary magma chamber, but with occasional replenishment with more primitive magma.[86] Magma mixing and plagioclase crystallization occurred in it.[89] A lithospheric structure probably directed magma flows over long time periods along the same pathway.[61] The depth of the magma source probably varied over the history of the volcano.[90] The total magma output at Llullaillaco is about 0.05 cubic kilometres per millennium (0.012 cu mi/ka)[30]-0.02–0.04 cubic kilometres per millennium (0.0048–0.0096 cu mi/ka).[31]

The terrain around Llullaillaco consists of andesite and dacite lavas and pyroclastics of Miocene to Pliocene age.[23] Some Oligocene-Miocene layers are exposed in the Quebrada de las Zorritas.[84] Dates obtained by argon-argon dating range between 11.94 ± 0.13 and 5.48 ± 0.07 million years ago. 15 kilometres (9.3 mi) west of Llullaillaco do Paleozoic granites and volcanite rocks crop out. Elsewhere these layers are buried by Cenozoic rocks.[40] There are several faults in the region, such as the Imilac-Salina del Fraile lineament whose movement has influenced the activity of Llullaillaco.[91]

Research by J.P. Richards and M. Villeneuve has allowed the determination of the geological history of the region. Had volcanism during the Eocene and Oligocene been centered on the Precordillera, increased subduction of the Nazca Plate during the late Oligocene caused the volcanic arc to broaden to about 250 kilometres (160 mi). 25 million years ago, the "Quechua event" triggered the uplift of the Puna-Altiplano, a highland covering a surface area of 500,000 square kilometres (190,000 sq mi) and reaching an altitude of 3,700 metres (12,100 ft). In the late Miocene-Pliocene a phase of strong ignimbrite volcanism occurred. About 2 million years ago, the "Diaguita deformation" was characterized by a change in the deformation regimen from crustal shortening to strike-slip faulting and of volcanism from voluminous felsic eruptions to isolated stratovolcanoes and back-arc mafic volcanism. A slowdown in the subduction may have caused this change. Nowadays most volcanism occurs at the western edge of the Puna, where volcanoes such as Lascar and Llullaillaco formed.[9][42]