Coropuna

Coropuna is a dormant compound volcano located in the Andes mountains of southeast-central Peru. The upper reaches of Coropuna consist of several perennially snowbound conical summits, lending it the name Nevado Coropuna in Spanish. The complex extends over an area of and its highest summit reaches an altitude of above sea level. This makes the Coropuna complex the third-highest of Peru. Its thick ice cap is the most extensive in Earth's tropical zone, with several outlet glaciers stretching out to lower altitudes. Below an elevation of , there are various vegetation belts which include trees, peat bogs, grasses and also agricultural areas and pastures.

The Coropuna complex consists of several stratovolcanos. These are composed chiefly of ignimbrites and lava flows on a basement formed by Middle Miocene ignimbrites and lava flows. The Coropuna complex has been active for at least five million years, with the bulk of the current cone having been formed during the Quaternary. Coropuna has had two or three Holocene eruptions and either or ago which generated lava flows, plus an additional eruption which may have taken place some 6,000 years ago. Current activity occurs exclusively in the form of hot springs.

Coropuna is located northwest of the city of Arequipa. People have lived on the slopes of Coropuna for millennia. The mountain was regarded as sacred by the Inca, and several archaeological sites have been discovered there, including the Inca sites of Maucallacta and Acchaymarca. The mountain was considered one of the most important Inca religious sites in their realm; human sacrifices were performed on its slopes, Coropuna forms part of many local legends and the mountain is worshiped to the present day.

The ice cap of Coropuna, which during the Last Glacial Maximum (LGM) had expanded to over , has been in retreat since at least 1850. Estimates published in 2018 imply that the ice cap will persist until about 2120. The retreat of the Coropuna glaciers threatens the water supply of tens of thousands of people relying upon its watershed, and interaction between volcanic activity and glacial effects has generated mudflows that could be hazardous to surrounding populations. Because of this, the Peruvian geological agency, INGEMMET, monitors Coropuna and has published a hazard map for the volcano.

Name and etymology

In Quechua, means "plateau", and is a common component of toponyms such as with Coro Coro, Bolivia, though its etymology is unclear. The name may mean , "Puna of Gold", "golden mountain", "cold, snowy" or "cut off at the top". The name is also spelled . The mountain is also called Nevado Coropuna; "Nevado" is the Spanish word for "snowy". There is another volcano in the Andahua volcanic field which has the same name, but is completely separate.

Geography and geomorphology

Coropuna lies in the Andes of Peru, on the border between the Castilla and Condesuyos Provinces of the Arequipa Department. Towns around the volcano belong to the Chuquibamba, Machaguay, Pampacolca and Viraco Districts. The volcano can be reached on paved roads through the town of Andahua, either from Arequipa or through Aplao from the Pan-American Highway. Roads also pass along the northern and western sides of the volcano.

Regional

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. More regionally, the volcano is in the , a mountain range which lies at an average of from the Pacific coastline, and contains nearly one hundred glaciers.

Coropuna is in the Central Volcanic Zone of the Andes, which contains 44 stratovolcanoesincluding many of the world's highestand several glaciated volcanoes. Besides Coropuna, some of the latter are Sara Sara, Solimana, Mismi, Ampato, Hualca Hualca, Sabancaya, Chachani, Misti, Ubinas, Huaynaputina, Tutupaca, Yucamane and Casiri. Also found nearby are Neogene-age calderas. Sixteen volcanoes in Peru are active or potentially active.

There is no habitation on Coropuna above , but numerous villages dot the lower slopes. Agriculture and animal husbandry are the most common economic activities; there are copper and gold mines as well. The city of Arequipa lies to the southeast.

Local

General outline

Seen from above, Coropuna has a pear-shaped outline and is a eastwest ridge that features four summits that are separated by broad saddles. In addition, there is another summit north of the eastwest trend. A high subsidiary peak named Cerro Cuncaicha lies east of Coropuna; it is a stratovolcano. Coropuna covers a surface area of and its various main summits rise about above the surrounding plateau.

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

The higher elevations of Coropuna consist of an ice cap and glaciated terrain but old lava flows with gentle slopes and blocky lava crop out from underneath the ice. Regions of hydrothermally altered rocks, lava flows, pyroclastic flows and areas covered by volcanic ash occur all around the mountain. Glacial activity has eroded these volcanic rocks, carving valleys into them or removing them altogether. 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. Glacial valleys of Coropuna are up to deep and long.

There are several collapse scarps on the mountain, especially around its central sector. 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. Also on the southern side, mud-water flow deposits have been found in the Capiza River valley and appear to relate to Coropuna; at least eight such debris flows have been identified. Lahars (mudflows) have reached the Colca River valley. Lahars are dangerous phenomena owing to their high speed and density, causing large scale destruction and fatalities, and can be generated both by volcanic and meteorological processes.

Lakes and rivers

Lakes lie on the flanks of the volcano. These include Lake Pallarcocha on the southwestern flank on formerly glaciated terrain, 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, Quebrada Sigue Chico and Quebrada Sepulturayoc on the western flank. The Rio Blanco and Rio Amayani eventually form the Rio Arma, while the Rio Capiza discharges water from Coropuna to the Colca River. During the winter dry season, most of these rivers do not carry substantial discharge.

The volcano is situated on a drainage divide. 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. An endorheic area that receives meltwater from the volcano also exists northeast from Coropuna, on Pampa Pucaylla where the lake of the same name lies.

Surrounding terrain

Coropuna rises above the surrounding terrain from a base elevation of , and about on the southern side where the Rio Llacllaja has incised the underlying basement almost to the foot of the volcano, forming sharp, amphitheatre-like valleys. In general, many deep valleys cut into the flanks of the volcano and give the mountain an "impressive topographic relief".

The region is characterised by high plateaus separated by deep canyons, including some of the world's deepest gorges that reach depths of . Apart from river erosion, giant landslides have affected the Altiplano below Coropuna, such as the Chuquibamba landslide, which took place over the last 120,000years in the form of multiple collapse events within a fault-controlled basin.

Geomorphologically, Coropuna lies at the edge of the Altiplano high plateau on the Western Cordillera mountain range; in the Central Andes this mountain chain is split into two rangesthe western and the eastern Cordilleraseparated by the Altiplano. The Pucuncho Basin and Firura volcano lie north of Coropuna, while Solimana volcano is northwest from Coropuna. Sara Sara is another volcano in the area. A large lava dome lies northwest of Coropuna while Cerro Pumaranra, a eroded volcano, is to the northeast. west-southwest from Coropuna lies the high Antapuna, while the Andahua " Valley of the Volcanoes" is east-northeast of Coropuna.

Elevation and size

Coropuna is the largest and highest volcano in Peru, the highest peak of the Cordillera Ampato and the third-highest mountain in Peru. The highest point of Coropuna is the northwestern dome named Coropuna Casulla, with elevation. Mountaineering sources also cite an elevation of for the El Toro summit, which would make Coropuna the 22nd highest mountain in the Andes.

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 ; other candidates were peaks in the Cordillera Blanca. In 1910 it was believed that the volcano was over high and thus the highest mountain in South America, ahead of Aconcagua, although a North American expedition during the preceding year had determined that Coropuna was not the highest, as they only found an elevation of , and Huascaran is higher than this. Varying snow elevations might also lead to varying height estimates.

Coropuna has several summits (up to ten overall according to one count) which exceed elevation, plus a northern summit. Those with individual names are the northwestern Coropuna Casulla at , El Toro, the western Nevado Pallacocha at , the central Coropuna Central II at , Escalera at in the western sector of the volcano, Paiche at in the central sector, and Coropuna Este and Yana Ranra at in the eastern sector.

Ice cap

Coropuna features the largest ice cap of the tropics. As of 2014 it was wide and long. It is larger than the Quelccaya Ice Cap farther northeast, which was considered to be the largest, but has since shrunk to a size less than Coropuna's. A subsidiary peak named Cerro Cuncaicha, east of Coropuna, has a small ice cap as well. In general, Peruvian glaciers form the bulk of the world's tropical glaciers. The ice cap consists of three ice domes and many glaciers. Perennial snow fields are present on Coropuna, sometimes making it hard to measure the true extent of glaciation or glacier retreat.

On average, the ice cap of Coropuna is about thick, with maximum thicknesses exceeding . In 20032004 the ice cap had a volume of about snow water equivalents. Ice cores have been taken from the Coropuna ice cap and from a summit crater; one of these ice cores covers a timespan beginning from 20,000years ago.

Penitentes reaching heights of and seracs (blocks of ice in glaciers delimited by cracks) occur on the glaciers, while debris cover is rare. Mudflows (lahars) originated from the ice cap and left deposits at the bottom of valleys.

Glaciers and periglacial phenomena

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

Glaciers descend to elevations of about on the southern flank, and to about on the northern flank. This is higher than the freezing level, owing to the dry climate; the freezing level at Coropuna lies at about elevation. In 2001, the ice limits were located at elevations of on the southern and at on the northern flank.

Moraines are mostly found on Coropuna's northern and southern side and reach lengths of , with longer moraines on the northern flank. In general, moraines on Coropuna are steep and have prominent crests as they are little eroded. Grey-coloured, fresh moraines up to 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 and small, discontinuous moraines.

Apart from normal glaciers, 78  rock glaciers have been counted on Coropuna, but only 11 of them are considered to be active. Permafrost occurs at elevations exceeding on the southern and on the northern flank. Cryoturbation, gelifluction, patterned grounds, solifluction and other periglacial landforms are noticeable at over elevation.

Recent area and retreat

Measuring the surface area of Coropuna's ice cap is difficult as seasonal snow can be mistaken for ice, 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. Retreat rates shortly before 2009 reached 13per cent in only 21years. Between 1980 and 2014 the ice cap shrank at a rate of with a 2015 estimate amounting to , and a brief slowdown observed during the late 1990s and early 2000s. Total shrinkage has been estimated to amount to 26per cent between 1962 and 2000, and by 18per cent between 1955 and 2007. Retreat is faster on the northern side of the mountain.

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. All of this retreat is attributed to global warming, and tends to increase during El Niño years owing to a drier climate. The glaciers lose mass through both sublimation and melting. Ablation occurs year-round and is diurnal. This meltwater rarely forms streams, though some do exist. The Quebrada Ullulo on the northern side is the largest such meltwater stream. Recently deglaciated terrain is covered by rock debris.

File:Coropuna glacier areas.png, alt=The ice area has been steadily decreasing since 1950 and may completely disappear in the future., Glacier trends and extrapolation
File:Coropuna ice profile.png, alt=Radar display of a transect through the ice cap, Ice profile
File:Ice loss Coropuna-2.png, alt=The thickest ice lies in the western portion, while marginal ice is thinner., Ice thickness
File:Radar profile and glacier evolution Coropuna.png, alt=A number of transects across the ice cap have been made, with the ice cap retreating at its margins., Ice schematics

Glacial history

Before the first human settlement of the area, the ice cap on Coropuna was much larger than today, with its surface exceeding and its glaciers descending to much lower elevations. Additionally, glaciers also expanded from the mountains Pumaranra, Pucaylla and Cuncaicha to the east of Coropuna. They covered the Pampa Pucaylla northeast from Coropuna and descended the Jellojello valley and other valleys to the east. Glacial valleys radiate from Coropuna, and glaciofluvial landforms are associated with moraines.

Regional climate oscillations are recorded in the ice masses of Coropuna. 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 and surface exposure dating using helium-3. Three separate moraine generations and about five separate glacial stages have been recorded on the volcano. Glacial advances on Coropuna appear to be synchronous to advances of the ice sheets in the Northern Hemisphere. Glaciers developed on other mountains in the region as well.

Last glacial maximum

During the Last Glacial Maximum (LGM) 25,00020,000years ago, valley glaciers on Coropuna were considerably longer than today. The longest glacier, at , was in the Quebrada Ullulo. 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 high, long, and wide. On the northern flank, moraine systems have been observed in the Santiago, Ullulo, Keaña, Queñua Ranra, Cuncaicha, Pommulca and Huajra Huire valleys, while the southeastern flank was covered by glaciers in the Yanaorco, Viques, Cospanja, Buena Vista Este, Buena Vista Oeste and Huasi valleys. Rock bars occur in some glacial valleys on the southern and southwestern side of the volcano. There are large cirques around Cerro Cuncaicha.

The LGM ice cap had an area of at least , with glaciers descending to elevation. Glacier ends were lower on the northern and western sides, probably due to airflow-mediated variations in sublimation. The growth of the ice cap has been explained by a decrease of the equilibrium line altitude of about . Assuming constant precipitation, temperatures may have decreased by . The glaciers began to retreat between 12,000 and 11,000years ago.

Other glacial periods

Ice has been present on Coropuna for at least 80,000years. At least two pre-LGM advances spread beyond the area that was covered with ice during the LGM, with an expansion occurring in particular in the eastern sector of the volcano. Moraines older than marine isotope stage 2 are widespread. Those close to the village of Viraco may date back 40,00045,000years and thus be part of an earlier glaciation, and old dates of 47,00031,000 and 61,00037,000years ago in the Huayllaura and Sigue Chico valleys could reflect even larger glacier expansions during marine isotope stage 3 or 4.

Glaciers retreated after the end of the last glacial maximum 20,00018,000years ago and then re-expanded. During the Lateglacial, a group of moraines formed between the position of the LGM moraines and the position of the recent moraines, with one lateglacial advance dated to either 13,40010,000 or 13,90011,900years ago. Full glacial conditions lasted until 10,0009,000years ago; minor advances took place about ago, and again some 6,000 years ago. The late glacial advances coincide with similar glacier expansions worldwide and some of them may correlate with the Younger Dryas cold period or the Antarctic Cold Reversal. 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 elevation.

Importance as a source of water

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. A 2003 study by Bryan G. Mark and Geoffrey O. Seltzer estimated that about 30per cent of the dry season runoff in the Cordillera Blanca comes from glaciers. Meltwater from the glaciers on Coropuna sustains the baseflow of the rivers during dry periods; Coropuna is an important source of water for the valleys of the surrounding areas and for the desert-like piedmont, with an estimated 38,000 people depending directly or indirectly on water originating from it. This water supply is threatened by the retreat of the glaciers 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. Glacial meltwater has a low content of regulated metals while springs sometimes have very high concentrations.

Geology

Regional setting

Off the coast of Peru, the Nazca Plate subducts beneath the South American Plate at a rate of or . 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. In the Cordillera Occidental (Western Cordillera) uplift commenced about 50million years ago in the Eocene, paused until 25 millionyears ago in the Oligocene, and increased substantially after about 10million years ago in the Miocene. Andean uplift in the area of Coropuna is ongoing.

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

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

Basement

Coropuna is constructed atop of 14million year old ignimbrites and lava flows of Neogene age. Individual ignimbrites crop out mainly in valleys; on the highlands they are buried beneath more recent volcanic products. 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.

Faults and lineaments

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

Composition

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

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

Eruption history

The beginning of Coropuna's growth has variously been placed over 5million years ago, during the Pliocene

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