Coropuna

The Andes stretch along the western coast of South America from Tierra del Fuego northwards to Venezuela, forming the longest mountain chain in the world.[18] More regionally, the volcano is in the Cordillera Ampato, a mountain range which lies at an average of 100 kilometres (62 mi) from the Pacific coastline,[19] and contains nearly one hundred glaciers.[20]

Coropuna is in the Central Volcanic Zone of the Andes,[12][21] which contains 44 stratovolcanoes[22] – including many of the world's highest[21] – and several glaciated volcanoes.[23] Besides Coropuna, some of the latter are Sara Sara, Solimana, Mismi, Ampato, Hualca Hualca, Sabancaya, Chachani, Misti, Ubinas, Huaynaputina, Tutupaca, Yucamane and Casiri.[24][25] Also found nearby are Neogene-age calderas.[24] Sixteen volcanoes in Peru are active or potentially active.[26]

There is no habitation on Coropuna above 5,200 metres (17,100 ft),[27] but numerous villages dot the lower slopes.[lower-alpha 3] Agriculture and animal husbandry are the most common economic activities;[29] there are copper and gold mines as well.[30] The city of Arequipa lies 150 km (93 mi) to the southeast.[12]

Coropuna seen from the south in 1988

Seen from above, Coropuna has a pear-shaped outline[31] and is a 20 km (12 mi) east–west ridge[14] that features four summits that are separated by broad saddles.[12][32] In addition, there is another summit north of the east–west trend.[1] A 5,558 m (18,235 ft) high subsidiary peak named Cerro Cuncaicha lies east of Coropuna;[33] it is a stratovolcano.[34] Coropuna covers a surface area of 240 square kilometres (93 sq mi)[35] and its various main summits rise about three kilometres (1.9 mi) above the surrounding plateau.[23]

The volcano is formed from alternating layers of ignimbrite and lava,[31] and consists of coalesced stratovolcanoes[36] and seven separate coulees.[37] Ice cover makes discerning its structure difficult,[38] but about[38] six separate peaks[35][39][40] as well as six not readily recognisable summit craters have been counted.[28][31] Additional lava domes form a southeastward trending line on the southwestern side of the volcano[28] and dikes crop out close to Lake Pallarcocha.[28] Coropuna overlies the margin of a buried caldera.[41]

The higher elevations of Coropuna consist of an ice cap and glaciated terrain[35] but old lava flows with gentle slopes[42] and blocky lava crop out from underneath the ice.[22] Regions of hydrothermally altered rocks, lava flows, pyroclastic flows and areas covered by volcanic ash occur all around the mountain.[28] Glacial activity has eroded these volcanic rocks, carving valleys into them or removing them altogether.[43] This process created U-shaped valleys such as Buenavista, Cospanja and Tuilaqui on the southern flank, and glacial valleys such as Chaque, Mapa Mayo, Río Blanco, Torcom and Ullulo on the northern slopes.[44] Glacial valleys of Coropuna are up to 300 m (980 ft) deep and seven km (4.3 mi) long.[45]

There are several collapse scarps on the mountain, especially around its central sector.[34] A sector collapse took place on the southwestern flank and formed a landslide deposit as well as a horseshoe-shaped valley that was later filled by glaciers.[45] Also on the southern side, mud-water flow deposits have been found in the Capiza River valley and appear to relate to Coropuna;[46] at least eight such debris flows have been identified.[47] Lahars (mudflows) have reached the Colca River valley.[48] Lahars are dangerous phenomena owing to their high speed and density, causing large scale destruction and fatalities,[46] and can be generated both by volcanic and meteorological processes.[49]

Coropuna seen from Lake Pallacocha

Lakes lie on the flanks of the volcano.[50] These include Lake Pallarcocha on the southwestern flank on formerly glaciated terrain,[51] Laguna Pucaylla on Coropuna's northeastern side and Laguna Caracara on the southeastern side. A number of streams and rivers originate on the mountain. Clockwise around Coropuna, these include Quebrada Chauqui-Huayco, Rio Amayani on the northern side, Quebrada Chinchina/Infernillo, Quebrada Jollpa, Quebrada Caspanja with the lake Laguna Caracara, Quebrada Buena Vista, Quebrada Tuallqui, Rio Testane on the southern flank, Rio de Huayllaura on the southwestern flank, Quebrada del Apacheta,[17] Quebrada Sigue Chico[52] and Quebrada Sepulturayoc on the western flank.[17] The Rio Blanco and Rio Amayani eventually form the Rio Arma,[53] while the Rio Capiza discharges water from Coropuna to the Colca River.[54] During the winter dry season,[2] most of these rivers do not carry substantial discharge.[55]

The volcano is situated on a drainage divide.[56] To the west, the Rio Arma is a tributary of the Ocoña River, while to the east, the Colca River is part of the Majes River watershed.[43] An endorheic area that receives meltwater from the volcano also exists northeast from Coropuna, on Pampa Pucaylla[57] where the lake of the same name lies.[17]

Coropuna rises two km (1.2 mi) above the surrounding terrain[2][32] from a base elevation of 4,500 m (14,800 ft),[14] and about 3.5 km (2.2 mi) on the southern side where the Rio Llacllaja has incised the underlying basement[2][32] almost to the foot of the volcano, forming sharp, amphitheatre-like valleys.[55] In general, many deep valleys cut into the flanks of the volcano[58] and give the mountain an "impressive topographic relief".[1]

The region is characterised by high plateaus separated by deep canyons, including some of the world's deepest gorges[52] that reach depths of 600–3,000 m (2,000–9,800 ft).[59] Apart from river erosion, giant landslides have affected the Altiplano below Coropuna,[60] such as the Chuquibamba landslide, which took place over the last 120,000 years in the form of multiple collapse events within a fault-controlled basin.[61]

Geomorphologically, Coropuna lies at the edge of the Altiplano high plateau on the Western Cordillera mountain range;[62] in the Central Andes this mountain chain is split into two ranges – the western and the eastern Cordillera – separated by the Altiplano.[63] The Pucuncho Basin[10] and Firura volcano lie north of Coropuna, while Solimana volcano is northwest from Coropuna.[12] Sara Sara is another volcano in the area.[35] A large lava dome lies northwest of Coropuna[17] while Cerro Pumaranra, a 5,089 m (16,696 ft)[33] eroded volcano, is to the northeast.[28] 19 km (12 mi) west-southwest from Coropuna lies the 4,855 m (15,928 ft) high Antapuna,[64] while the Andahua "Valley of the Volcanoes" is 20 km (12 mi) east-northeast of Coropuna.[65]

Coropuna Este

Coropuna is the largest[66] and highest volcano in Peru, the highest peak of the Cordillera Ampato[2] and the third-highest mountain in Peru.[6][7] The highest point of Coropuna is the northwestern dome[1][35] named Coropuna Casulla,[15] with 6,377 metres (20,922 ft) elevation.[1][67][40] Mountaineering sources also cite an elevation of 6,425 m (21,079 ft) for the El Toro summit,[68][69] which would make Coropuna the 22nd highest mountain in the Andes.[22][lower-alpha 4]

Estimates on the height of Coropuna have changed over time. In the 19th century, it was one of the candidates for "highest mountain in Peru", with the Mapa del Perú (Map of Peru) of Antonio Raimondi giving an estimated height of 6,949 m (22,799 ft); other candidates were peaks in the Cordillera Blanca.[73] In 1910 it was believed that the volcano was over 7,000 m (23,000 ft) high and thus the highest mountain in South America, ahead of Aconcagua,[74][75] although a North American expedition during the preceding year had determined that Coropuna was not the highest, as they only found an elevation of 6,615 m (21,703 ft), and Huascaran is higher than this.[76] Varying snow elevations might also lead to varying height estimates.[68]

Coropuna has several summits (up to ten overall according to one count)[31] which exceed 6,000 m (20,000 ft) elevation,[56] plus a 5,623 m (18,448 ft) northern summit.[15] Those with individual names are the northwestern Coropuna Casulla at 6,377 m (20,922 ft),[35] El Toro,[68][69] the western Nevado Pallacocha at 6,171 m (20,246 ft), the central Coropuna Central II at 6,161 m (20,213 ft),[77] Escalera at 6,171 m (20,246 ft) in the western sector of the volcano, Paiche at 6,330 m (20,770 ft) in the central sector,[78][34] and Coropuna Este[79] and Yana Ranra at 6,305 m (20,686 ft) in the eastern sector.[34][78]

A number of glaciers flow down from the ice cap,[28] their number variously estimated to be 15,[32] 17[93][56] and 23.[80] Some glaciers have been named; on the southwestern flank two glaciers are known as Azufrioc 1 and 2, three Rio Blanco 1 through 3 and six Tuialqui 1 through 6.[94] Eighteen separate accumulation areas have been found as well.[95] There are no substantive valley glaciers presently on Coropuna[38] and some glaciers, especially in the eastern side, emanate from cirques.[28] The ongoing downward movement of the ice on Coropuna produces icequakes.[40][96]

Glaciers descend to elevations of about 5,100 to 5,300 m (16,700 to 17,400 ft) on the southern flank, and to about 5,500 to 5,800 m (18,000 to 19,000 ft) on the northern flank.[12][37][80] This is higher than the freezing level, owing to the dry climate;[2] the freezing level at Coropuna lies at about 4,900 m (16,100 ft) elevation.[32] In 2001, the ice limits were located at elevations of 5,300 m (17,400 ft) on the southern and at 5,600 m (18,400 ft) on the northern flank.[97]

Moraines are mostly found on Coropuna's northern and southern side[17] and reach lengths of three to eight km (1.9 to 5.0 mi), with longer moraines on the northern flank.[44] In general, moraines on Coropuna are steep and have prominent crests as they are little eroded.[83] Grey-coloured, fresh moraines up to 500 m (1,600 ft) from the ice cap may reflect the position of the glaciers before the onset of glacier retreat which has left small mounds that often contain ice between these moraines and the ice cap[83] and small, discontinuous moraines.[98]

Apart from normal glaciers, 78 rock glaciers have been counted on Coropuna, but only 11 of them are considered to be active.[99] Permafrost occurs at elevations exceeding 5,100 metres (16,700 ft) on the southern and 5,750 metres (18,860 ft) on the northern flank.[100] Cryoturbation,[101] gelifluction, patterned grounds,[31] solifluction[102] and other periglacial landforms are noticeable[31] at over 4,500 m (14,800 ft) elevation.[31]

Extent of the ice cap over the years, from various sources:[lower-alpha 5]

Measuring the surface area of Coropuna's ice cap is difficult as seasonal snow can be mistaken for ice,[105] and different studies come to various conclusions about the retreat rate, due to the use of different time periods and methodological practices. However, all studies conclude that the net retreat trend is obvious and that the ice cap is diminishing.[106] Retreat rates shortly before 2009 reached 13 per cent in only 21 years.[107] Between 1980 and 2014 the ice cap shrank at a rate of 0.409 km²/a (0.158 sq mi/a)[80] with a 2015 estimate amounting to 0.5 km²/a (0.19 sq mi/a),[108] and a brief slowdown observed during the late 1990s and early 2000s.[109] Total shrinkage has been estimated to amount to 26 per cent between 1962 and 2000, and by 18 per cent between 1955 and 2007.[2] Retreat is faster on the northern side of the mountain.[110]

The Coropuna ice cap retreat follows the pattern recorded elsewhere in Peru such as in the Cordillera Blanca, Cordillera Vilcanota and the mountains Ampato, Quelccaya and Sabancaya.[111] All of this retreat is attributed to global warming,[97] and tends to increase during El Niño years owing to a drier climate. The glaciers lose mass through both sublimation and melting.[32] Ablation occurs year-round and is diurnal.[112] This meltwater rarely forms streams, though some do exist. The Quebrada Ullulo on the northern side is the largest such meltwater stream.[32] Recently deglaciated terrain is covered by rock debris.[113]

• 
  Glacier trends and extrapolation
• 
  Ice profile
• 
  Ice thickness
• 
  Ice schematics

Before the first human settlement of the area,[114][115] the ice cap on Coropuna was much larger than today, with its surface exceeding 500 km² (190 sq mi)[116] and its glaciers descending to much lower elevations.[52] Additionally, glaciers also expanded from the mountains Pumaranra, Pucaylla and Cuncaicha to the east of Coropuna.[117] They covered the Pampa Pucaylla northeast from Coropuna and descended the Jellojello valley and other valleys to the east.[118] Glacial valleys radiate from Coropuna,[38] and glaciofluvial landforms are associated with moraines.[32]

Regional climate oscillations are recorded in the ice masses of Coropuna.[119] The glacial history of the volcano has been reconstructed with tephrochronology (using dated tephra layers such as those from the 1600 Huaynaputina eruption), radiocarbon dating[37] and surface exposure dating using helium-3.[32] Three separate moraine generations[34] and about five separate glacial stages have been recorded on the volcano.[120] Glacial advances on Coropuna appear to be synchronous to advances of the ice sheets in the Northern Hemisphere.[121] Glaciers developed on other mountains in the region as well.[122]

During the Last Glacial Maximum (LGM) 25,000–20,000 years ago,[79] valley glaciers on Coropuna were considerably longer than today.[37] The longest glacier, at 12 km (7.5 mi), was in the Quebrada Ullulo.[79] The glaciers had a cover of boulders and gravel and formed tall moraines, and both lateral and terminal moraines where outlet glaciers ended. At the crest, these moraines were as much as 100 m (330 ft) high, eight km (5.0 mi) long, and five–ten m (16–33 ft) wide.[123] On the northern flank, moraine systems have been observed in the Santiago, Ullulo,[124] Keaña, Queñua Ranra, Cuncaicha, Pommulca and Huajra Huire valleys,[57] while the southeastern flank was covered by glaciers in the Yanaorco, Viques, Cospanja, Buena Vista Este, Buena Vista Oeste and Huasi valleys.[118] Rock bars occur in some glacial valleys on the southern and southwestern side of the volcano.[28] There are large cirques around Cerro Cuncaicha.[37][83]

The LGM ice cap had an area of at least 365 km² (141 sq mi), with glaciers descending to 3,780–4,540 m (12,400–14,900 ft) elevation. Glacier ends were lower on the northern[79] and western sides, probably due to airflow-mediated variations in sublimation.[125] The growth of the ice cap has been explained by a decrease of the equilibrium line altitude of about 750 m (2,460 ft). Assuming constant precipitation, temperatures may have decreased by 4.5–5.5 °C (8.1–9.9 °F).[126] The glaciers began to retreat between 12,000 and 11,000 years ago.[127]

Ice has been present on Coropuna for at least 80,000 years.[128] At least two pre-LGM advances spread beyond the area that was covered with ice during the LGM,[32] with an expansion occurring in particular in the eastern sector of the volcano.[129] Moraines older than marine isotope stage 2 are widespread.[130] Those close to the village of Viraco may date back 40,000–45,000 years and thus be part of an earlier glaciation,[131] and old dates of 47,000–31,000 and 61,000–37,000 years ago in the Huayllaura and Sigue Chico valleys could reflect even larger glacier expansions during marine isotope stage 3 or 4.[132]

Glaciers retreated after the end of the last glacial maximum 20,000–18,000 years ago and then re-expanded.[120] During the Lateglacial, a group of moraines formed between the position of the LGM moraines and the position of the recent moraines,[83] with one lateglacial advance dated to either 13,400–10,000 or 13,900–11,900 years ago.[133] Full glacial conditions lasted until 10,000–9,000 years ago;[128] minor advances took place about 13,000–9,000 years ago, and again some 6,000 years ago.[134] The late glacial advances coincide with similar glacier expansions worldwide[135] and some of them may correlate with the Younger Dryas cold period or the Antarctic Cold Reversal.[136] During the Little Ice Age, glaciers on Coropuna did not expand much, although some rock glaciers might have formed during that time. The glaciers descended to 4,900 m (16,100 ft) elevation.[121]

Glaciers in Peru are important sources of water for local communities and for hydropower generation, especially during the dry season; their shrinkage is thus of concern.[137] A 2003 study by Bryan G. Mark and Geoffrey O. Seltzer estimated that about 30 per cent of the dry season runoff in the Cordillera Blanca comes from glaciers.[138] Meltwater from the glaciers on Coropuna sustains the baseflow of the rivers[139] during dry periods;[93] Coropuna is an important source of water for the valleys of the surrounding areas and for the desert-like piedmont,[111] with an estimated 38,000 people depending directly or indirectly on water originating from it.[72] This water supply is threatened by the retreat of the glaciers[111] and would require costly mitigation measures to compensate for its reduction. The Peruvian government is making preparations for Coropuna ceasing to be a contributor to the local water supply by 2025; a 2018 study and re-evaluation of past data concluded that the ice cap should persist until about 2120, and recommends that greater in situ monitoring of Coropuna's glaciers is needed to aid future planning and mitigation.[140] Glacial meltwater has a low content of regulated metals[141] while springs sometimes have very high concentrations.[142]

The larger tectonic plates around South America

Off the coast of Peru, the Nazca Plate subducts beneath the South American Plate at a rate of five–seven centimetres per year (2.0–2.8 in/year)[143] or nine centimetres per year (3.5 in/year).[144] This subduction process, along with the subduction of the Antarctic Plate also underneath the South American Plate, is responsible for the volcanism in the Andes and the uplift of the mountain chain.[145] In the Cordillera Occidental (Western Cordillera) uplift commenced about 50 million years ago in the Eocene, paused until 25 million years ago in the Oligocene, and increased substantially after about 10 million years ago in the Miocene.[146] Andean uplift in the area of Coropuna is ongoing.[31]

Coropuna is part of the volcanic arc of southern Peru[46] and is considered to be a member of the Barroso volcanic arc.[97] There are over six hundred volcanoes in southern Peru,[147] and the entire Cordillera Occidental from southern Peru to northern Chile is covered with volcanic rocks, although present-day volcanic activity is scarce.[55] Many of the older volcanoes are deeply eroded by glaciation, while younger volcanoes often still resemble cones.[63]

Volcanic activity in the Andes occurred during three eras. The first was between 195 and 190 million years ago in the Early Jurassic, and generated the Chocolate Formation. The second between 78 and 50 million years ago (Late Cretaceous to Early Eocene) generated the Toquepala Formation and the Andean batholiths.[146] Volcanic activity in southern Peru commenced about 13 million years ago in the Miocene.[148] One volcanic unit – after being folded and eroded – was covered by a second lava and tuff unit, which in turn was followed by the emplacement of large volcanoes.[63] Ignimbrites and stratovolcano activity, at times subdivided into a "rhyolitic" and an "andesitic" formation, alternated.[55]

Coropuna is constructed atop of 14 million year old ignimbrites[66] and lava flows of Neogene age.[14] Individual ignimbrites crop out mainly in valleys; on the highlands they are buried beneath more recent volcanic products.[31] The volcanic basement includes the Miocene to Plio-Pleistocene Tacaza, Huaylillas, Sencca and Barroso Formations; the latter formation includes Coropuna itself. Below these formations lie the sedimentary Murco and Socosani formations and the Yura Group, which are sediments of Jurassic-Cretaceous age with intruded plutons of the same age; finally there is a Basal Complex of Precambrian age.[149]

The basement is cut by faults and lineaments such as the Viraco-San Antonio Fault that crosses the edifice,[150] Pampacolca Fault on the southern side of the volcano and the Pumaranra and Cerro Casulla lineaments,[148] which trend southeast–northwest and northeast–southwest, respectively. One east–west lineament may have influenced the recent volcanism;[151] the alignment of Coropuna with Sara Sara, Solimana and El Misti may indicate a tectonic control on the volcano in general.[152] On the southern flank, Holocene normal faults have offset lava flows and streams.[38]

The rocks released by Coropuna are dark brown to black and porphyritic.[153] They consist of andesite,[2] dacite,[43] rhyodacite,[154] rhyolite,[155] trachy-basaltic andesite, trachyandesite and trachydacite.[156] The more recent lava flows have been dacitic[157] or trachydacitic.[15] Phenocryst phases include amphibole, biotite, plagioclase, pyroxene and titanomagnetite.[66] Aside from the volcanic rocks, deposits of salts, sulfur and travertine produced by hot springs are found on the southern flank.[158]

The volcanic rocks define a calc-alkaline[155] potassium-rich suite which resembles that of[156] Chilean[159] and Peruvian volcanoes such as Tutupaca.[156] They contain large amounts of rubidium, strontium and barium.[156] Complicated processes[160] of crystallisation and interaction with Earth's crust appear to have produced the magma.[161]

No eruptions of Coropuna during historical[175] or modern times are known,[137] and the volcano was considered to be long-extinct.[39] However, young-looking[38] ʻaʻā lava[176] or block lava[23] flows erupted during the Holocene and in part overlie late-glacial moraines.[12][157][176] Their vents are now hidden beneath glacier ice,[22] and the flows have been affected by later glacial advances.[177] These lava flows are found on the west–northwest, south–southeast and northeast side of the mountain:[83]

• A northwesterly lava flow – Coropuna's longest[157] at 8.5 km (5.3 mi) – occupies the Cerro Sepulturayoc valley.[118] It has been dated to about 6,000 years ago,[118] but research published in 2019 has suggested it may have erupted somewhat earlier, during the Late Glacial period.[178]
• A southeasterly flow lies in the Cospanja valley and is either 1,100 ± 100[179] or 700 ± 200 years old,[34] the latter age being derived from cosmogenic isotope dating.[46] It was probably formed during a single eruption and is four kilometres (2.5 mi) long.[180]
• A dark, young-looking lava[181] flow runs northeasterly[33] in the Queñua Ranra valley[57] and is five kilometres (3.1 mi) long.[182] The eruption took place about 2,100 ± 200 years ago[183] according to cosmogenic isotope dating.[46] Its deposition was preceded by the eruption of lava bombs that cover the valley and by the production of a lahar that advanced 14 km (8.7 mi) from its source. Whether a secondary lava flow in the same valley occurred at the same time or later is not clear, as that flow has not yet been dated.[183]

The ages of the flows indicate an eastward shift in activity.[128] The southeasterly and northeasterly flows may have been erupted within 500 years from the same fissure,[178] while the eruption of the northwesterly flow might be a consequence of the retreat of the ice cap.[184] These lava flows are the most recent manifestation of volcanic activity[15] and they imply that Coropuna is still active;[137] it is thus considered to be a dormant volcano, rather than an extinct one.[185] There is no evidence of Holocene tephras in peat bog drill cores[71] and volcanism at Coropuna since the last ice age has been primarily effusive.[178]

Steam rising on Coropuna Este

The volcano is still hydrothermally active.[15] Six hot springs are found on Coropuna, mostly on the southeastern foot,[186] such as at Acopallpa, Antaura/Antauro, Viques, Ccollpa/Collpa, Buena Vista and Aguas Calientes and, on its northern flank, at Huamaní Loma. Their water temperatures range between 18 and 51 °C (64 and 124 °F).[187][188] With the exception of the last two, which are situated in glacial terrain, these hot springs rise within valleys via rock fractures.[158] Geochemical analyses of the water from these springs published in 2015 show no major variations in composition, implying a stable volcanic system.[189] Whether solfataric or fumarolic activity occurs at Coropuna is unclear,[1][190][188] and the thick glaciation indicates that the summit craters have no thermal activity.[38] A lahar took place on the southeastern flank on 22 December 2016, causing damage to water infrastructure[191] and pastures below the volcano.[192]

Some of the hot springs on Coropuna are used for bathing.[158] The volcano had been considered a potential site for geothermal power production,[193] but research published in 1998 concluded that the available energy of the Coropuna area was insufficient.[194]

The first volcano activity report published in 2018 noted ongoing seismic activity involving volcano-tectonic earthquakes.[40] Seismic swarms were observed at Coropuna after the 2001 southern Peru earthquake[195] and were possibly triggered by that earthquake.[196] Observations of deformation of the volcanic edifice have shown that gravitational instability and soil water absorption result in movements of part of the volcano but, as a whole, Coropuna shows no evidence of volcanic deformation.[197]

The Peruvian Instituto Geológico Minero y Metalúrgico (INGEMMET) monitors Coropuna for activity. It uses data such as the composition of hot spring waters[198] and the shape of the volcano as estimated by satellite images,[199] GPS and geodesy,[200] as well as information from five seismic stations.[67] Seismic monitoring of the volcano began in 2008-2010 and was supplemented with geophysical monitoring in 2018.[201] A volcanic hazard map[202] and scenarios of lahar generation have been published,[49] the Peruvian government publishes regular status reports.[203] The Peruvian Geophysical Institute considers Coropuna a "high risk" volcano;[204] about 90,000 people live in risk areas,[67] and the sites most in danger are towns in the steep southern valleys.[156]

Together with El Misti, Sabancaya and Ubinas, Coropuna is considered to be one of Peru's most dangerous volcanoes.[205] The presence of a large ice cap,[206] and therefore the risk of incandescent volcanic rocks melting that ice, creates a hazard of lahars, or mudflows, such as those that in 1985 killed over 23,000 people at Nevado del Ruiz volcano in Colombia.[29][137] The risk to life is further increased by Coropuna's steep slopes and by the concentration of people in nearby valleys.[207] The terrain around the volcano has one of the greatest topographic reliefs in the world and a number of towns lie on the floor of the Majes valley, right down to the Pacific Ocean where the district capital Camaná[38] with 20,000 inhabitants is situated.[175] Although there is no evidence of past mudflows of such size, lahars could reach as far as the coast,[208] affecting a number of towns[209] and infrastructure such as roads, antennas and small hydropower plants[29] in the provinces Condesuyos, Castilla and Camaná.[190] According to the 2007 census, 110,481 people lived in the provinces that span Coropuna and lie downstream of it.[137]

Lava flows are also a potential danger at Coropuna.[156] Other hazards with lesser probabilities are directed volcanic blasts, lava dome collapses,[156] fast-moving massive pyroclastic flows[210] and flows of pumice and volcanic ash,[156] lava bombs[211] and shock waves from volcanic explosions.[212]

Coropuna lies between the semi-humid Altiplano and the arid western slope of the Andes.[213] Its climate is semi-arid, with precipitation at 6,080 m (19,950 ft) elevation reaching 390 millimetres per year (15 in/year).[12] Other reported precipitation values range between 700 mm/a (28 in/year)[214] and 1,000 mm/a (39 in/year).[52] Lower down the mountain, at altitudes between at 3,000 and 4,000 m (9,800 and 13,100 ft), annual precipitation levels increase to between 226 and 560 mm/a (8.9 and 22.0 in/year) (semi-humid). Even further down, at altitudes around 2,000–3,000 m (6,600–9,800 ft), they decrease again to 98–227 mm/a (3.9–8.9 in/year) (desert).[27] Cold water brought from Antarctica along the Pacific Ocean by the Humboldt Current,[215] the presence of a stable anticyclone [216] and of a temperature inversion over the Pacific, and the Andean rainshadow are all responsible for this dryness.[12]

Most precipitation falls as hail or snow.[27] This happens mostly during the summer[12] wet season, between December and March,[52] when the ITCZ moves south[217] and a summer monsoon is active over South America.[215] Most precipitation is brought by easterly winds coming from the Amazon and the Atlantic Ocean, whereas the westerly winds that dominate during the dry season do not carry much moisture.[2] Thus, humidity generally decreases in a westward direction.[216]

The amount of precipitation is modulated by the El Niño Southern Oscillation. During phases of El Niño, the weather is drier, snow cover smaller and glacier retreat increases.[111][218] Over longer timespans, precipitation in the region increases whenever iceberg discharge and cooling occur in the North Atlantic. This was the case during the Heinrich events and the Younger Dryas when lakes formed on the Bolivian Altiplano: The Sajsi formed about 25,000–19,000 years ago, Tauca about 18,000–14,000 and Coipasa 13,000–11,000 years ago.[215] Cold periods in the Southern Hemisphere such as the Antarctic Cold Reversal between 14,500 and 12,900 years ago may have pushed the polar front north and increased precipitation as well.[216] That increased precipitation may have delayed the retreat of Coropuna's glaciers after the end of the last glacial maximum.[219] Coropuna experienced moist conditions during the early Holocene, whereas the late Holocene beginning 5,200 years ago was drier there,[220] with a pronounced dry period lasting from 5,200 to 3,000 years ago.[221]

Temperatures decrease with altitude gain, and at lower elevations around 2,000–3,000 m (6,600–9,800 ft) they average 12–17 °C (54–63 °F). Between 3,000 and 4,000 m (9,800 and 13,100 ft) they average 7.8 °C (46.0 °F) and at 4,000–5,200 m (13,100–17,100 ft) elevation they average 0–6 °C (32–43 °F). At altitudes above 5,200 m (17,100 ft) they remain below freezing.[27] Temperatures fluctuate more over daily timescales than over seasonal ones when measured close to the glaciers.[56] Southerly cold waves can sometimes reach Coropuna, leaving traces in ice cores in the form of southern pollen.[222] During the Little Ice Age, at 5,000–5,200 m (16,400–17,100 ft) elevation temperatures decreased to −5 to −7 °C (23 to 19 °F).[121] Warm fluctuations between about 2,200 and 900 years ago, plus a cold fluctuation between around 970 to 1010 AD, are also recorded.[223]

A larger number of archaeological sites go back to the 2nd Intermediate Period[238] and during the Inca era. The Inca expanded preexisting irrigation and terrace systems which are in part still existing today.[239] These include the highest irrigation system in the world,[240] which was possibly constructed on Coropuna to allow the cultivation of bitter potatoes.[241] Inca sites are often found at higher elevations than the sites left by preceding civilisations; the highest one is located at 5,700 m (18,700 ft) elevation,[242] and there is evidence of Inca presence to 6,200 m (20,300 ft) elevation.[240] In addition, an important branch of the Inca road system passes by the western foot of Coropuna.[240] The region was densely populated; the close location of the mountains and favourable climatic conditions facilitated its settlement.[243]

As noted by Spanish chroniclers[244] such as Pedro Cieza de León,[245] Coropuna played an important role in Inca religion, and an important temple was situated there,[246] possibly at Maucallacta.[247] Pedro Cieza de León considered Coropuna to be the fifth most important holy site of the Inca empire.[245] One archaeological site on the volcano may have been a stopover for religious ceremonies to its summit.[248] Capacocha, a form of human sacrifice, were offered to the mountain;[244] reportedly, in 1965, a mummy was found there.[249]

Among the archaeological sites at Coropuna is the important Inca site of Maucallacta, on the southwestern flank of the mountain.[250] Some of the structures there were built to evoke the appearance of the mountain.[251] A royal residence, an oracle and a political unit were associated with Maucallacta,[252] and the oracle of Coropuna would have answered the rulers' queries all year round.[253] The Maucallacta site was probably the most important one at Coropuna; the western summit today known as "La Niña" was apparently also significant.[254]

Another important site associated with Coropuna is Acchaymarca, to the west of the mountain,[255] where about 280 Inca stone structures have been found.[243] It is likely that many pilgrims came there for ceremonies honouring the apus of Coropuna and Solimana.[256]