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Misti

Coordinates: 16°17′47″S 71°24′38″W / 16.29639°S 71.41056°W / -16.29639; -71.41056
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Misti
Misti as viewed from Arequipa
Highest point
Elevation5,822 m (19,101 ft)
Prominence1,785 m (5,856 ft)[1]
ListingUltra
Coordinates16°17′47″S 71°24′38″W / 16.29639°S 71.41056°W / -16.29639; -71.41056
Geography
Misti is located in Peru
Misti
Misti
Peru
CountryPeru
RegionArequipa
Parent rangeAndes
Geology
Mountain typeStratovolcano
Volcanic arc/beltCentral Volcanic Zone

Misti is a dormant volcano located in the Andes mountains in southern Peru, rising above Peru's second-largest city, Arequipa. It is a conical volcano with two nested summit craters, the inner one of which contains a volcanic plug or lava dome with active fumaroles. The summit of the volcano lies on the margin of the outer crater and is 5,822 metres (19,101 ft) above sea level. Snow falls on the summit during the wet season, but does not persist; there are no glaciers. The upper slopes of the volcano are barren, while the lower slopes are covered by bushland.

The volcano developed over four different stages. During each stage, lava flows and lava domes built up a mountain, whose summit then collapsed to form a caldera. The volcano is part of a volcano group with Chachani to the northwest and Pichu Pichu to the southeast, and developed on top of a basement formed by numerous Miocene-Pliocene ignimbrites and volcano-derived debris. Numerous intense explosive eruptions took place during the last 50,000 years and covered the surrounding terrain with tephra. The last two significant eruptions were 2,000 years ago and in 1440–1470 AD; since then, phases of increased fumarolic activity have sometimes been mistaken for eruptions.

Misti is one of the most dangerous volcanoes in the world, as it lies less than 20 kilometres (12 mi) from Arequipa. The city's population exceeds one million people and its northeastern suburbs have expanded on to the slopes of the volcano. The narrow valleys on western and southern flanks are a particular threat, as mudflows and pyroclastic flows can be channelled into the urban area and into important infrastructure, like hydropower plants. Even moderate eruptions can deposit volcanic ash and tephra over most of the city. Until 2005, there was little awareness or monitoring of the volcano. Since then, the Peruvian INGEMMET has set up a volcano observatory in Arequipa, run public awareness campaigns on the dangers of renewed eruptions and published a hazard map. The Inca viewed the volcano as a threat and during the 1440–1470 eruption offered human sacrifices (capacocha) on its summit and that of its neighbours to calm the volcano down; the mummies on Misti are the largest Inca sacrifice known.

Name and settlement history

[edit]

The name is either Quechua or Spanish and means "mixed", "mestizo" or "white"; it may refer to snow cover. The indigenous names are Putina[2][3] ("mountain that growls"[4]) and in Aymara Anuqara[5] ("dog"); they both refer to the dog-like appearance of the volcano when viewed from the Altiplano.[4] The original name of the volcano was Putina; it only became known as "Misti" beginning in the 1780s.[6] Other names for the volcano are Guagua-Putina, El Volcán ("the volcano"), San Francisco, and Volcán de Arequipa ("the Arequipa volcano").[7][8] Sometimes chroniclers confused it with other volcanoes like Ubinas and Huaynaputina.[9]

Settlement of the region began about 1,500 years ago. It is unclear whether the Inka were the first Altiplano polities to influence the region or whether previous cultures played a role,[10] but by the arrival of the Spanish the area was densely populated.[11] The pre-Hispanic people built canals, roads and buildings in the area where Arequipa is today.[12] The city itself was founded on 15 August 1540.[13] Misti is the house mountain of Arequipa,[14] appearing on the seal of the city for example.[15]

Human geography

[edit]

The old roads heading from Arequipa to Chivay and Juliaca run along the northern/western and southern/eastern foot of Misti, respectively.[16] Inca roads from the Arequipa area passed by the volcano.[17] There are numerous dams on the Rio Chili, including the Aguada Blanca Dam and reservoir north of the volcano,[18] El Fraile, and Hidroeléctrica Charcani I, II, III, IV, V and VI.[19] These dams have hydroelectric power plants which supply electricity to Arequipa. The river is also the principal water resource for the city. Roads leaving the city cross the river on bridges.[20]

According to Italian geographer Cumin 1925, there were three small man-made structures of unknown origin in the crater. He noted that they were known since 1677.[21] Inka ceremonial platforms on the summit associated with human sacrifices were probably destroyed by human activities around 1900 AD.[22] Professor S. I. Bailey from the Harvard College Observatory in 1893 installed[23] the world's highest[a] weather station on Misti.[26][27] The station was one of several high-altitude stations built at the time, which aimed to investigate the atmosphere at such high altitudes;[28] additionally, the Observatory performed research on the response of the human body to high altitudes[26] and on the solar eclipse of April 16, 1893.[29] Misti was in its time the highest permanently inhabited location on Earth.[30] Another weather station, named "Mt. Blanc Station",[31] was installed at the base of the volcano[32][33] after 1888.[34] Both were shut down in 1901 when the Observatory decided to only maintain a station in Arequipa;[32][33] subsequently storms have erased any trace of the summit observatory.[35] Observation of physics phenomena, such as cosmic ray measurements,[36] were sporadically carried out on Misti during the 20th century.[35]

Geography and geomorphology

[edit]

Misti rises about 3.5 kilometres (2.2 mi) above Arequipa,[37] the second-largest city in Peru,[38] and is the best known volcano of Peru.[39] The Inka empire's Condesuyos province included the volcano;[40] presently it is in the Arequipa Department.[41] The mountain is visible from the sea.[42]

Regional

[edit]

The volcanoes of Peru are part of the Andean Central Volcanic Zone (CVZ),[43] one of the four volcanic belts of the Andes; the others are the Northern Volcanic Zone, the Southern Volcanic Zone and the Austral Volcanic Zone.[44] The CVZ extends for 1,000 kilometres (620 mi)[45] from southern Peru through Bolivia to northern Argentina and Chile.[46] Volcanoes are numerous in the CVZ, but most are poorly known due to the low population density of much of the Central Andes.[47] Several Peruvian volcanoes have been active since the Spanish conquest: Andagua volcanic field, Huaynaputina, Sabancaya and Ubinas, and possibly Ticsani, Tutupaca and Yucamane.[48] Other Peruvian volcanoes in the CVZ are Ampato, Casiri, Coropuna, Huambo volcanic field, Purupuruni and Sara Sara.[45] Ubinas is the most active volcano in Peru, having erupted 24 times since 1550.[49] The 1600 eruption of Huaynaputina claimed more than 1,000 casualties; recent eruptions of Sabancaya 1987–1998 and Ubinas 2006–2007 had severe impacts on the local populations.[50]

Local

[edit]

General outline

[edit]

The volcano is a young, symmetric[b] cone with 30° degree slopes[48] and a nested summit crater. The outer crater has a diameter of 950 metres (3,120 ft)[52] or 835 metres (2,740 ft) and is 120 metres (390 ft) deep.[53] There is a gap in the southwestern rim, almost to the bottom of the crater;[54] otherwise the inner crater walls are nearly vertical[53] and consist of lapilli, lava and volcanic ash.[55] The 550-metre (1,800 ft) wide and 200-metre (660 ft) deep inner crater[52] is in the southeastern part of the outer crater.[56] The inner crater cuts across metre-thick ash and scoria deposits[48] and historical lava domes and is rimmed by scoria.[52] In the crater is a 120-metre (390 ft) wide and 15-metre (49 ft) high volcanic plug[57]/lava dome,[48] covered with cracks,[21] boulders and fumarolic sulfur deposits;[56] it is fumarolically active.[58] The highest point of the volcano is at 5,822 metres (19,101 ft)[c][60] on the northwestern outer crater rim; an iron cross marks the highest point.[53] Other mountains of the Western Cordillera, including Ubinas and Pichu Pichu, can be seen from the summit.[61]

The crater of El Misti (2005).

The volcano is about 20 kilometres (12 mi) wide[62] and rises abruptly from the surrounding terrain.[63] Estimates of the edifice's volume range reach 150 cubic kilometres (36 cu mi), but more likely its volume is only 90 cubic kilometres (22 cu mi)[64] or 40 cubic kilometres (9.6 cu mi). It is notably asymmetric, with the western side more heavily eroded and featuring older rocks than the eastern side. The western rim of the outer crater is about 150 metres (490 ft) higher than the southern.[48] Underneath the Misti cone is an older, eroded stratovolcano ("Misti 1"). The stratovolcano is made up of pyroclastic rocks and stubby lava flows, which form a 2.2-kilometre (1.4 mi) thick pile.[37] On the northwestern foot, there is a rhyolitic landform named "Hijo de Misti" ("son of Misti").[65] Misti is surrounded by a fan of volcanic debris,[d] which covers an area of 200 square kilometres (77 sq mi) on Misti and extends 25 kilometres (16 mi) from the volcano.[37] On the southern side, the volcano is cut by 20-to-80-metre (66 to 262 ft) deep ravines,[66] while the northern side is flatter.[48] Dune fields and volcanic ash deposits extend for 20 kilometres (12 mi) northeast of Misti; they are formed by wind-blown ash.[67][60][39] The terrain between Arequipa and Misti is initially gently sloping, before reaching the steep flanks of the cone.[68]

There are no obvious traces of a sector collapse on the volcano,[37] except on its western foot[67] and a narrow chute on the northwestern flank of Misti that reaches its summit.[69] Two debris avalanche deposits lie on the southeastern and southwestern-southern side of Misti, extending 25 kilometres (16 mi) and 12 kilometres (7.5 mi) from the volcano. The first is made up by hummock-shaped hills of mixed debris and covers an area of 100 square kilometres (39 sq mi); the second forms a flat-topped terrain with an area of about 40 square kilometres (15 sq mi) on both sides of the Rio Chili.[37]

Hydrology and glaciology

[edit]

The perennial[70] Rio Chili rounds the northern and western sides of Misti,[37] where it has cut the 20-kilometre (12 mi) long and 150–2,600-metre (490–8,530 ft) deep[71] Charcani Gorge.[64] From southeast to southwest the Quebrada Carabaya, Quebrada Honda, Quebrada Grande, Quebrada Agua Salada, Quebrada Huarangual, Quebrada Chilca, Quebrada San Lazaro and Quebrada Pastores drain the edifice. They eventually join to the Rio Chili west and Rio Andamayo south of Misti;[67] the Andamayo joins the Chili south of Arequipa.[72] Quebrada San Lazaro and Quebrada Huarangual have formed alluvial fans at the foot of the volcano.[37] The quebradas (dry valleys) carry water during the wet season in November–December and March–April.[70]

During the wet season,[18] snow can cover an area of 1–7 square kilometres (0.39–2.70 sq mi) on the upper cone.[73] Unlike neighbouring Chachani, Misti lacks any evidence of glacial[e] or periglacial[f] processes, probably due to its inner heat.[77] There is no clear indication of past glaciation, either, except possibly on the western flank. A thin ice cover may not have left traces on the volcano, however.[78] The present-day snowline lies above 5,800 metres (19,000 ft) elevation.[61]

Geology

[edit]

Regional setting

[edit]

Off the western coast of Peru, the Nazca Plate subducts under South America[48] at a rate of 5–6 centimetres per year (2.0–2.4 in/year).[37] The subduction is responsible for the volcanism of the CVZ,[45] as the downgoing slab releases fluids that chemically modify the overlying mantle, causing it to produce melts.[79] Most Peruvian volcanoes have produced potassium-rich andesitic magmas, derived from the mantle and further modified by fractional crystallization and assimilation of material from the often thick crust.[45]

Volcanic activity in southern Peru goes back to the Jurassic.[80] Various volcanic arcs formed in Peru during the past 30 million years: The Tacaza Arc 30–15 million years ago, the Lower Barroso 9–4 million years ago, the Upper Barroso 3–1 million years ago and the Pleistocene-Holocene Frontal Arc during the past one million years. Two distinct episodes of uplift took place 24–13 and 9–4 million years ago, and were accompanied by the emplacement of numerous large ignimbrites.[81]

During the Cretaceous-Paleogene, the Toquepala Group of volcanics was emplaced. The Tacaza Arc is the source of the Huaylillas Formation and the Barroso Group of the Sencca Formation.[82] The Nazca fracture zone on the Nazca Plate projects under Misti.[83]

Local setting

[edit]
Aerial picture of Ubinas with Misti in the back (2015).

Misti is part of the Western Cordillera of the Andes.[84] It is the youngest of a group of three Plio-Pleistocene volcanoes;[66] the others are the dormant Chachani 15 kilometres (9.3 mi) northwest and extinct Pichu Pichu 20 kilometres (12 mi) southeast.[38] This group lies at the margin of the Altiplano,[48] next to the 600-square-kilometre (230 sq mi)[85] tectonic depression of Arequipa where the city lies.[86] The depression has dimensions of 30 by 15 kilometres (18.6 mi × 9.3 mi) and appears to be formed by fault activity.[87] The terrain under Misti slopes south and this might make the edifice slip southward over time.[88]

A northwest-southeast trending fault system includes the Huanca fault at Chachani and the Chili fault on Misti.[89] The faults were active during the Holocene, offsetting tephra deposits,[90] and may have provided a pathway for magma to ascend and form the volcanoes of Arequipa.[53][91] Other faults include north- and northeast-trending faults, which are inactive but could have influenced the formation of the Rio Chili canyon.[38] The crust under the volcano is 55 kilometres (34 mi) thick.[64]

Basement

[edit]

The basement under Misti crops out in the Rio Chili gorge. It consists of Proterozoic rocks of the Arequipa Terrane, which are more than a billion years old, Jurassic sediments of the Socosani Formation[92] and Yura Group, and the Cretaceous-Paleogene La Caldera batholith.[93] The batholith forms the hills south of Arequipa.[94] These formations are covered by rhyodacitic ignimbrites[37] known as "sillars".[44] They are between 13.8 and 2.4 million years old;[37] the older are part of the Huaylillas Formation and the younger of the Barroso Arc.[95] Individual ignimbrites crop out in the Rio Chili gorge[96] and include the 300-metre (980 ft) thick Río Chili ignimbrite from 13.19 ± 0.09 million years ago, the 4.89 ± 0.02 million years old La Joya ignimbrite or "sillar", the 1.65 ± 0.04 million years old Aeropuerto or Sencca ignimbrite,[66] and the 1.02 million years old Yura Tuff and Capillune Formation.[97] They were erupted from multiple calderas, one of which is now buried under Chachani.[98][59] The ignimbrites are covered by volcanic sedimentary rocks[37] and debris from the sector collapse of Pichu Pichu.[87]

Composition

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Misti has erupted mainly andesite, while dacite[99] and rhyolite are less common.[100] There are reports of trachyandesite erupted during the Holocene eruptions.[101] Rhyolites and dacites are associated with explosive eruptions.[102] The volcanic rocks are subdivided into several classes: Pyroxene-amphibole andesites, amphibole andesites, amphibole dacites and amphibole rhyolites;[103] mica has also been reported.[100] The rocks define a potassium-rich calc-alkaline suite[100] typical for Peruvian volcanoes.[104] Phenocrysts include amphibole, augite, biotite, enstatite, plagioclase and titanomagnetite.[99] Magma composition has varied over time and the most recent volcanic stage has produced slightly different magmas, but overall the composition of Misti magmas is highly homogeneous.[102] The composition of Misti magmas and these of its neighbours Pichu Pichu and Chachani resembles adakite, an unusual kind of volcanic rock formed by the direct melting of a subducting plate.[100] Some rocks erupted by the volcano show evidence of hydrothermal alteration.[105]

Magma genesis and storage

[edit]

The formation of the magmas of Misti is a complicated process, involving the arrival of new magma, assimilation of crustal material, and fractional crystallization.[99] Initially mantle-derived melts pool in a reservoir at the base of the crust, where they assimilate crustal material and undergo fractional crystallization. Afterwards they ascend to a shallower reservoir,[103] where they interact with Proterozoic gneisses.[106] Assimilation of basement rocks gave rise to the rhyolitic magmas erupted 34,000–31,000 years ago.[107] Crystal-poor magma can form in the magmatic system through numerous processes and gives rise to the rhyolites and the volcanic plug.[108]

It is not clear whether Misti has a single magma chamber or multiple magma reservoirs at depth, although the rock composition implies that only one large magma system is present.[109] The reservoir appears to be located at 6–15 kilometres (3.7–9.3 mi) depth[110] and has a volume of several cubic kilometres.[99] Every few millennia, a secondary rhyolitic reservoir forms at about 3 kilometres (1.9 mi) depth;[111] it was last reactivated during the eruption 2 ka ago.[80] The magma system is periodically recharged, but such an influx of new magma does not trigger eruptions;[108] instead multiple recharges are necessary to cause activity.[99][112] Numerous mixing and decompression events can happen to each magma batch before it is erupted.[113] A recharge of the magma chamber may have occurred at some point before 2000 AD.[114] The overall rate of magma supply is 0.63 cubic kilometres per kiloare (0.15 cu mi/ka), comparable to other stratovolcanoes in volcanic arcs, but with brief surges reaching about 2.1 cubic kilometres per kiloare (0.50 cu mi/ka).[58]

Eruption history

[edit]

Misti is a young volcano.[20] It developed in four stages, numbered 1 through 4; a pre-Misti volcano may have formed the southwestern debris avalanche.[37] On average, sub-Plinian eruptions take place every 2,000–4,000 years, while ash fallout occurs every 500–1,500 years[58] and large ignimbrite-producing eruptions every 20,000–10,000 years.[115] Outcrops showing the stratigraphy of Misti are found mainly in the ravines on the southern side[48] and the Rio Chili gorge;[116] the older volcanic structures lie mainly in the western sector of Misti.[117] Seismic tomography has identified solidified buried magma bodies from the early stages of volcanism.[118]

Long andesitic lava flows and ignimbrites, which reach a thickness of more than 400 metres (1,300 ft), form the oldest edifice.[37] They have an age of 833,000 years, but it is not clear if they should be considered part of "Misti 1" or of a pre-Misti volcano.[119] Sometimes, they are considered the first stage of Misti activity, with all the subsequent activity making up the second stage.[120] After the south-southwestern collapse, the present stratovolcano began to grow 112,000 years ago. During the following 42,000 years lava flows and lava domes built an edifice with an elevation of 4,000–4,500 metres (13,100–14,800 ft), in the southern and eastern sectors of present-day Misti.[37] During the subsequent 20,000 years, repeated collapses of lava domes deposited blocks, fallout deposits and scoria on the southern side of Misti and on Chachani to the northwest.[121] Traces of glacial erosion[120] like cirques,[122] evidence of hydromagmatic activity and mudflows imply that Misti was glaciated during the first last glacial maximum of the Central Andes 43,000 years ago.[67][123]

Between 50,000 and 40,000 years ago, the summit of Misti collapsed one or more times above 4,400 metres (14,400 ft) elevation,[124] forming a 6-by-5-kilometre (3.7 mi × 3.1 mi) caldera.[125] Intense pyroclastic eruptions yielded ignimbrites with volumes of 3–5 cubic kilometres (0.72–1.20 cu mi), which cover an area of 100 square kilometres (39 sq mi) on the southern side of Misti.[124] This activity brought "Misti 2" to an end;[126] subsequently lava domes built "Misti 3" to an elevation of 5,600 metres (18,400 ft), almost entirely erasing the caldera.[127] Between 36,000 and 20,000 years ago collapses of lava domes produced numerous block-and-ash flows of dacitic to andesitic composition, which reach thicknesses of several tens of metres on the southern side of Misti.[128] Several eruption deposits from this time have been named:[129]

  • 34,000–33,000 years old "Fibroso I".[130]
  • 21,000[131][129] or 37,000 years old "Sacarosa",[132] "Sacaroso" or "Sacaroide"[131][129] produced two layers of pumice[133] from a 22-kilometre (14 mi) high eruption column. The total volume of tephra is about 0.5–1.5 cubic kilometres (0.12–0.36 cu mi), equivalent to a volcanic explosivity index of 4.[132]
  • 15,000 years old[134] "Autopista"[g][129] produced three layers of (mostly) pumice with smaller quantities of lithics.[135] During its eruption about 0.16 cubic kilometres (0.038 cu mi) of volcanic ash fell west of the volcano.[136] The "Autopista" eruption with a volcanic explosivity index of 4 produced about 0.6 cubic kilometres (0.14 cu mi) of tephra; a similar eruption today would cover parts of Arequipa with 10 centimetres (3.9 in) of pumice.[137] The "Autopista" deposit is the best preserved of the late Pleistocene tephra layers.[129]
  • Other deposits with ages spanning the Pleistocene and Holocene[129] are the "Fibroso II", "Blanco", "La Zebra", "Espuma gris", "Espuma iridiscente" and "Rosado".[138]

Eruptions 43,000 and 14,000 years ago dammed the Rio Socabaya and Rio Chili, forming temporary lakes south and north of the volcano that were later affected by earthquakes.[139] Between 24,000 and 12,000 years ago ice fields formed on Chachani and Misti during the last glacial maximum; tephra fell on ice and was reworked by meltwater.[128] Two eruptions 13,700 and 11,300 years ago produced pyroclastic surges that extended 12 kilometres (7.5 mi) away from the volcano; a 2-kilometre (1.2 mi) wide caldera formed at an elevation of 5,400 metres (17,700 ft).[140]

Holocene

[edit]

More than ten eruptions took place during the last 11,000 years,[52] with only brief pauses in activity.[141] They filled the younger caldera with scoria and lava flows, forming the "Misti 4" edifice with the nested summit craters. Tephra forms 5–6-metre (16–20 ft) thick deposits around the volcano, and pyroclastic surges reached distances of many kilometres more than 6,400 and 5,200 years ago.[52] The 9,000 and 8,500 years old eruptions produced the "Sándwich" deposits.[142] They extend for more than 15 kilometres (9.3 mi) on the southwestern flank of Misti,[142] but they also resulted in ash fall over the Pacific Ocean and Lake Titicaca.[143] Radiocarbon dating has identified eruptions 8,140, 6,390, 5,200, 4,750, 3,800 and 2,050 years ago;[144] the 3,800 eruption deposited fallout on Nevado Mismi[145] more than 90 kilometres (56 mi) northwest of Misti.[146] The Global Volcanism Program lists eruptions in 310 BCE ± 100, 2230 BCE ± 200, 3510 BCE ± 150, 4020 BCE ± 200, 5390 BCE ± 75 and 7190 BCE ± 150.[147]

2 ka eruption and later activity

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The last major explosive eruption – one or several events – took place about 2,000 years ago.[141] The date is constrained to 2,060–1,920 years before present; ages of 2,300 BP are probably too old.[101] The eruption produced about 0.4 cubic kilometres (0.096 cu mi) dense rock equivalents of rock[148] and probably lasted a few hours.[149] The eruption had a volcanic explosivity index of 4 or 5.[150]

The eruption was probably triggered when fresh andesitic magma entered a pre-existent rhyolitic body.[151] Magma rose through the edifice and expelled part of the hydrothermal system,[152] causing initial phreatic eruptions.[153] Tephra rained down around the edifice,[154] with pumice falling 25 kilometres (16 mi) from the volcano.[141] Owing to magma mixing, the pumice deposits have an appearance resembling chocolate and vanilla swirls.[101] Eventually, the conduit fully cleared and a 29-kilometre (18 mi) high eruption column rose above the volcano.[153] Pyroclastic flows emanated from the column and descended the southern flanks of the volcano, possibly through the gap in the crater rim.[155] During the course of the eruption, collapses of the crater and conduit walls caused a temporary decline in the intensity of the column.[156] The eruption column periodically collapsed and reformed, until the eruption ended with phreatomagmatic explosions.[157]

Mudflows descended the mountain.[153] The water source for the mudflows is unclear, but the eruption took place during the neoglacial between 2,500 and 1,000 years ago. Thus Misti may have featured a snow or ice cap at the time of the eruption; its melting would have given rise to mudflows.[78] Rainfall generated further mudflows after the eruption.[158] The relative importance of pyroclastic flows and mudflows during the 2 ka eruption is contentious.[159] The outer summit crater probably formed during this eruption.[148] Tephra layers in the Sallalli and (in this case with less certainty) Mucurca peat bogs close to Sabancaya,[160] and (tentatively) for an ice core in the Antarctic Plateau in Antarctica, are attributed to this eruption.[161] This is the only Plinian eruption during the Holocene.[162]

After the 2 ka eruption, activity was limited to small Vulcanian eruptions, mudflows and tephra fallout, including scoria and volcanic ash. Dating has yielded ages of 330, 340, 520, 620, 1035 and 1,300 years before present for several such events.[20][163] Mudflows took place 1,035 ± 45, 520 ± 25, 340 ± 40 and 330 ± 60 years ago[150] and left 5–15-metre (16–49 ft) thick deposits.[164] Not all of these mudflows are associated with eruptions.[20][163] Pyroclastic flows and ash falls were emplaced 1,290 ± 100 and 620 ± 50 years ago.[165]

Historical activity and seismicity

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The last eruption took place in AD 1440–1470[h][58] and produced about 0.006 cubic kilometres (0.0014 cu mi) of ash.[115] It was probably a prolonged eruption that lasted for months or years,[167] depositing ash in the Peruvian Laguna Salinas[162] and possibly as far as Siple Dome[168] and Law Dome in Antarctica.[169] It is the oldest eruption of a South American volcano for which historical records exist.[170] The eruption was severe enough that Mama Ana Huarque Coya,[171] the wife of the Inka emperor Pachacutec,[i] came to Chiguata,[173] where black ash had fallen,[174] to provide assistance.[173] There is no evidence that a supposed Inka settlement was destroyed by this eruption,[162] but the local population fled and the Inka had to resettle the area.[175] Along with other volcanic eruptions around that time and the beginning Spörer Minimum, the AD 1440–1470 eruption of Misti may have affected global climate conditions.[176] In 1600, the volcano was covered by ash from the Huaynaputina.[177]

There is no clear evidence of historical eruptions,[j][99] while the Global Volcanism Program reports a last eruption in 1985.[60] Mudflows descended the southern valleys until the 17th century.[58] Phreatic eruptions may have taken place in 1577,[178] on the 2 May 1677, 9 July 1784, 28 July 1787 and 10 October 1787. Questionable eruptions are recorded in 1542, 1599, August 1826, August 1830, 1831, September 1869, March 1870. They probably constitute fumarolic activity[102] and often took place after heavy precipitation; the water would have infiltrated the edifice and evaporated from the volcanic heat.[179] Comparisons between 1967 photos of the volcanic plug and more recent images show no changes.[180]

The volcano is seismically active, with long-period earthquakes, tremors, "tornillos"[k] and volcano-tectonic earthquakes recorded.[182] The hypocentres, the actual sites of the earthquakes, are found within the edifice of Misti[183] and cluster on the northwest flank of the volcano. The seismic activity appears to be linked to Misti's hydrothemal system.[184] Seismic swarms were recorded in August 2012, May 2014 and June 2014.[185] No deformation of the volcanic edifice is evident in satellite images.[186][187] Clouds rising from the mountain are sometimes mistaken for renewed activity.[188]

Hazards

[edit]
This mosaic of two astronaut photographs taken from the ISS illustrates the proximity of Arequipa to Misti, just 17 km away (2009).

Misti is Peru's most dangerous volcano and one of the most dangerous in the world,[189][190] owing to its proximity (17 kilometres (11 mi)) to Arequipa,[191] where more than a million inhabitants live.[192] The city has expanded to within 12 kilometres (7.5 mi) of the volcano, with new towns like Alto Selva Alegre, Mariano Melgar, Miraflores and Paucarpata[20] and towns such as Chiguata getting within 11 kilometres (6.8 mi).[37] About 8.6% of Peru's GDP depends on Arequipa and would be impacted by future eruption of Misti.[193] The city is constructed on mudflow and pyroclastic flow deposits of the volcano[194] and all the valleys that drain Misti pass directly or indirectly through Arequipa.[64] At least 220,000 people live on the alluvial fans and in the ravines on the southern side of Misti, and are threatened by floods, mudflows and pyroclastic flows emanating from the volcano[37] that can be channelled through the ravines.[191]

Individual threats from Misti include:

  • There are few outcrops of tephra within Arequipa; however, this probably reflects erosion and the dense urban environment.[191] The 2 ka and 1440–1470 AD eruptions deposited tephra over what today is Arequipa.[18] Tephra fallout can cause health problems, pollute water resources, cause roofs to collapse, bury fields,[195] and cause road accidents and accidents during cleanup.[196] Much closer to the volcano, volcanic bombs can fall.[197]
  • Mudflows are mixtures of rocks and water. They are caused by rainfall or the melting of snow and ice; thus they can occur in the absence of volcanic activity.[198][199] At Misti, they occur on average every century or two.[200] Even small mudflows can reach the city[201] and they bury and destroy everything in their path.[202] Eruptions of Misti could generate mudflows on Chachani, thus threatening settlements that are on the other side of the Rio Chili.[203]
  • Pyroclastic flows are hot (300–800 °C (600–1,000 °F)) masses of gas and rocks that can descend the slopes at speeds of 200–400 kilometres per hour (60–100 m/s); they can flow over topographic obstacles and reach large distances from the volcanic vent.[198] Pyroclastic flows and surges can reach 13 kilometres (8.1 mi) from the volcano,[58] although denser flows are likely to stop before reaching the city.[204]
  • The steep slopes put Misti at risk of sector collapses. Debris avalanches from the collapse of volcanoes can reach large distances, larger than that between Arequipa and Misti.[204][205] Debris flows, like mudflows, can destroy everything in their path.[198] Such collapses could also dam the Rio Chili, producing mudflows[206] and threaten neighbourhoods like Vallecito, Av. La Marina and Club Internacional.[19] Even small landslides on the western side of the volcano could threaten the water supply of Arequipa.[202]
  • Other threats are: Toxic gases can accumulate in closed spaces to dangerous concentrations, or interact with precipitation to form acid rains. Lava flows are highly destructive, but their slow speed means that they do not constitute a major threat to life.[207]

Hazards at Misti are not limited to volcanism. During the wet season, Arequipa is frequently flooded.[204] Heavy metals, presumably from Misti and Chachani, have been found in river water.[208]

Monitoring and hazard management

[edit]

In 2001, there was neither emergency planning nor land use planning around Misti;[18] the 2002–2015 development plan mentioned volcanic hazards but did not envisage any specific measures.[209] The last eruption of Misti had taken place shortly before the foundation of Arequipa, and thus there is no memory of the hazards of volcanic activity, unlike the hazards of earthquake.[210] Before the eruption of Ubinas in 2006–2007, volcanic hazards drew little attention from the Peruvian state and there was little awareness in Arequipa.[50] The volcano is frequently considered a protective figure and not as a threat.[211] A number of people associate volcanoes with lava flows and neglect other volcanic hazards.[190]

Beginning in 2005, INGEMMET began monitoring volcanoes in Peru;[212] the first monitoring equipment was targeted at the Charcani V hot spring. Later the monitoring was extended to other hot springs and to fumaroles in the crater; the latter both visually from Arequipa and in the crater.[213] Monitoring of seismic activity commenced in 2005.[214] Beginning in 2008 geodesic measurement stations were installed on the northeastern and southern slopes of the volcano.[213] In 2012, a new monitoring station for the volcano was inaugurated.[212] In May 2009[215] and April 2010, two exercise evacuations of several suburbs of Arequipa were carried out.[216] In 2013, the Peruvian Volcano Observatory (OVI) was inaugurated in Arequipa; it monitors Misti, Ubinas, Ticsani and other Peruvian volcanoes.[217] As of 2021, the monitoring network on Misti includes seismometers, equipment that measures the composition and temperature of hot springs and fumaroles, and sensors for movements or deformations of the edifice.[218] These efforts have yielded an increased awareness of the dangers posed by Misti, which is now being increasingly perceived as an active volcano.[219] Efforts have been made to slow the growth of the northern suburbs of Arequipa, which are closest to Misti.[220]

A volcano hazard map was developed in 2005 by numerous local and international organizations,[206] and officially presented on the 17 January 2008.[221] It defines three hazard categories: A red "high risk" zone, an orange "intermediate risk" zone and a yellow "low risk" zone.[206] These are defined by the risk of debris flows, lava flows, mudflows, pyroclastic flows, and tephra fallout.[222] The "high risk" zone encompasses the entire volcanic cone, its immediate surroundings and the valleys that emanate from it. Parts of Arequipa lie in the "high risk" zone. The "intermediate risk" zone surrounds the "high risk" zone, including the lower slopes of neighbouring mountains and most of the northeastern parts of Arequipa. The "low risk" zone in turn surrounds the "intermediate risk" zone and includes the rest of the city.[223][224] Additional maps show areas at risk of tephra fallout[225] and of being flooded by mudflows.[226] The hazard map of Misti is the first hazard map of a Peruvian volcano.[217] These maps serve to mitigate volcano hazards and to inform local development.[227] A 3D map was published in 2018.[228] In November 2010, the municipality of Arequipa decreed that the hazard map would have to be considered in future city zoning decisions.[210]

Scenarios

[edit]

Three different scenarios of future eruptions have been evaluated.[73] The first envisages a small eruption, similar to recent activity at Sabancaya[73] or the 1440–1470 AD eruption of Misti.[103] Ash fall would occur around the volcano, reaching 5 centimetres (2.0 in) in the urban area and shutting down the Arequipa Airport, landslides could damage the dams on the Rio Chili, and mudflows would descend the southern slopes. The second scenario involves an eruption like the 2 ka eruption. Thicker ash falls (exceeding 10 centimetres (3.9 in)) could cause buildings to collapse, and pyroclastic flows down the steep slopes south of Misti would reach the suburbs of Arequipa and Chiguata.[229][230] Most risk assessments are based on these two scenarios.[202]

The third scenario is a Plinian eruption like the "Fibroso" and "Sacaroso" events or the 1600 Huaynaputina eruption;[103] pyroclastic flows would sweep all the flanks of Misti and past Arequipa, blocking the Rio Chili. Thick ash fall would occur over the entire region,[231] including over the cities of El Alto, La Joya and agricultural areas.[230] A Plinian eruption would require the evacuation of Arequipa.[202] Other hazard scenarios are the emissions of short lava flows, the formation and collapse of lava domes and the collapse of part of the volcanic edifice.[227]

Fumarolic and geothermal system

[edit]

Fumaroles on Misti occur in three locations: on the volcanic plug, the northern/northeastern walls of the inner crater, and on the southeastern flank of the volcano.[90] They emit noises,[72] visible clouds of water vapour and the smell of hydrogen sulfide. The smell reaches the crater rim,[56] and, at times, the gas becomes so concentrated that it causes irritations to the eyes, nose and throat.[72] Fumarolic activity has been reported since the 1440–1470 eruption.[186] In 1948–1949 and 1984–1985 it was intense enough to be seen from Arequipa.[102] The fumarolic activity is visible in satellite images as a temperature anomaly of about 6 K (11 °F).[232]

Water is the most important component of the fumarole gases, followed by carbon dioxide, sulfur dioxide, hydrogen sulfide and hydrogen.[233] The gases are highly acidic, containing hydrogen chloride and hydrogen sulfide.[234] Fumarole temperatures have varied through the years, generally they are between 125–310 °C (257–590 °F)[112] with peaks of 430 °C (806 °F).[235] The present-day (21st century) fumarole gases appear to derive directly from magma, with no interaction with a hydrothermal system.[112] The fumaroles outside of the summit crater are colder, with temperatures of 50–80 °C (122–176 °F),[90] and do not smell of sulfur.[236]

Fumarolic vents are surrounded by concentric deposits of anhydrite close to the vent, gypsum at some distance, and sulfur in the colder vents. Other minerals are ammonium sulfate, hematite, ralstonite, soda alum and sodium chloride.[237] Elemental compositions and isotope ratios indicate that the fumarole deposits are derived from the leaching of volcanic rocks and the water from precipitation.[238] The chemistry of the deposits changed between 1967 and 2018, with decreasing zinc and increasing lead concentrations, concomitant with a warming of the fumarolic system[239] that may be due to the arrival of new magma in the volcano during the 20th century.[240] Sometimes the temperature of the fumaroles is high enough to melt the sulfur[241] and the fumarolic gases can ignite.[72]

Hot springs occur at the foot of the volcano. These include the Humaluso/Umaluso spring north and the Agua Salada, Bedoya/La Bedoya, Calle Cuzco, Charcani V, Chilina Norte, Chilina Sur, Jésus, Ojo de Milagro, Puente de Fierro, Sabandia, Tingo, Yumina and Zemanat[242][243] south and southwest of Misti.[236] The hottest of these is[243] the Charcani V spring in the Rio Chili gorge;[244] it is also the closest to the volcano, being only 6 kilometres (3.7 mi) from the crater.[245] The Jésus and Umaluso springs produce gas bubbles. The springs are fed by a low-temperature geothermal system that mostly produces alkaline waters containing bicarbonate, chloride and sulfate.[243] Their waters appear to originate through the mixing of freshwater, magmatic water and chloride-rich deep water.[246] Many of these springs form artificial pools or have water intakes,[247] and several are monitored by INGEMMET for changes in activity.[248]

High soil temperatures on the cone,[249] hot springs and fumaroles indicate that Misti contains a hydrothermal system.[245] Electric potential measurements indicate that the system appears to be confined between faults[67] or to the older caldera.[250] The activity has not been stable over time; after the 2001 southern Peru earthquake flow at the Charcani V spring and the temperature of the crater emissions increased noticeably.[244] Water temperatures decreased after the 2007 Peru earthquake.[251] Over time old fumarolic vents shut down and new vents develop,[72] but the configuration of the dome vents is stable over time.[186] The fumarolic activity is correlated to earth tides.[110]

Climate and vegetation

[edit]

The region has a semi-arid climate with temperate temperatures;[252] the annual mean temperature in Arequipa is 13.7 °C (56.7 °F).[41] Temperatures decrease with elevation;[252] in 1910 monthly mean temperatures at the summit ranged from −6 °C (21 °F) in January to −9.7 °C (14.5 °F) in May, June and August[253] but in 1968 temperatures at the summit rose above freezing for a few days per year.[61] During most of the year, dry westerly winds blow over the Western Cordillera except during summer months, when convection over the Amazon forces easterly flow that draws moisture to the Cordillera.[254] Most precipitation falls during the austral summer (December to March) and amounts to 89.1 millimetres per year (3.51 in/year),[41] a 1910 study found most precipitation to be in the form of snow or hail.[253] During the wet season, rainstorms and flash floods erode the volcanic debris deposits.[141] The snow cover rapidly melts away during the dry season.[255] The El Niño-Southern Oscillation and sea surface temperatures in the Atlantic and Pacific Oceans govern annual rainfall.[256] After a wet and cold start to the Holocene, the climate in the Western Cordillera may have been moist until 5,200–5,000 years ago, followed by a dry period that lasted until the 16th century AD when the Little Ice Age began.[146]

The region west of the Andes, including the terrain at the foot of Misti,[255] is mostly desert with cacti and dwarf shrubs as the principal vegetation forms.[257] The vegetation belt is known as the "Misti zone". There is an altitudinal gradation: vegetation is dominated by Franseria bushes between 2,200–2,900 metres (7,200–9,500 ft)[255] and by Diplostephium tacorense above 3,000 metres (9,800 ft).[258] Other bushes occur mainly in creeks and valleys.[258] At higher elevations, other genera such as Adesmia and Senecio idiopappus become more frequent, and at an elevation of about 3,900 metres (12,800 ft) Lepidophyllum quadrangulare becomes the dominant plant.[259] Cacti, herbs, yareta cushion plants, ichu (Jarava ichu), as well as pioneer species like lichens and mosses, are important above 3,500 metres (11,500 ft).[260][261] Polylepis species form woodlands.[259] Vegetation cover decreases above 4,000 metres (13,000 ft) elevation.[261]

Insects are the most important animals in the Peruvian mountains, and include beetles and hymenopterans. Birds include the Andean condor.[262] 358 plant, 37 mammal and 158 bird species have been recorded[l] in the region, including alpacas, guanacos, llamas and vicuñas.[266] Most of the volcano is within the Salinas y Aguada Blanca National Reserve, which extends northwest of Misti[267] and includes the volcano among its main attractions.[266]

Religious importance

[edit]
Misti, as seen from Arequipa (2015).

The mountain was considered the apu[268] and "volcano of the city".[269] It was venerated by the inhabitants of Arequipa, a common practice for inhabitants of the Andes.[8] The Aymara people viewed it as an abode of deceased souls.[270] According to the late 16th-century chronist Cristóbal de Albornoz,[8][269] Misti was one of the important mountains (waqa, a kind of deity or idol[271]) of the Arequipa area of the Inka Empire, along with Ampato, Coropuna, Sara Sara and Solimana.[272] This tradition probably originated with the previous inhabitants of the area and was taken over by the Inka when they conquered the region.[273] The Middle Horizon[274] Millo archeological site in the Rio Vitor valley was constructed in a manner that allowed a good view of Misti, which was probably the apu of this place.[275] The Inka gave them cups of gold and silver[276] and settled people around Misti that would continue the mountain veneration.[277] People used to alter the shape of the skulls of their infant children so that they resembled the volcano.[278] Misti was considered to be an aggressive mountain that was always demanding sacrifices,[279] and the mountain had to be exorcised in colonial times.[280] After the Spanish conquest, the mountain was consecrated to St. Francis.[281] According to the Jesuit College of Arequipa, "Indian sorcerers" thought that Huaynaputina had asked Misti for assistance in expelling the Spaniards; Misti however had turned down, saying it was already Christianized, so Huaynaputina had proceeded alone.[282] A group of converts and Franciscans in 1600 climbed on Misti and threw saints' relics and a cross into its crater to discourage the volcano.[283] Another expedition was launched in 1784, after an earthquake had destroyed Arequipa, and planted a cross on the summit. This cross was replaced twice: first a decade later and then in 1900.[284] The cross on the summit of Misti supposedly protects the city.[285] To this day, peasants believe that after offering gifts to Misti women will bear boys, while the same offers to Chachani will make them bear girls.[286]

Mummies

[edit]

Eight or nine mummies were found on Misti by Johan Reinhard[280] and José Antonio Chávez in 1998, inside the crater and below the summit.[287] The mummies were of children, mostly boys around six years old.[288] Unusually, the mummies were buried in shared tombs.[289] Along with the mummies were figurines, ceramics and other objects;[280] the high number of figurines found on Misti (47) indicates that the site was important to the Inkas.[290][291] These mummies were Inka human sacrifices, so-called capacochas,[287] and the Misti capacocha is the largest known.[175][292] However, the hostile conditions within the crater had seriously damaged the mummies.[290] They included infants and children, which were sometimes buried one on top of the other.[291]

The sacrifices on Misti, and others on Chachani and Pichu Pichu, were probably motivated by the 1440–1470 eruption of Misti,[22][175] which may explain the unusual location within the crater rather than on a summit.[293] According to the 16th century chronicler Martín de Murúa,[269] the Inka Thupa Yapanki sacrificed llamas to calm a volcano Putina close to Arequipa (probably Misti),[294] going as closely as possible to the summit.[295] Previous ceremonies had failed to calm the volcano and only the emperor's direct intervention quelled its anger.[296] This description however most likely refers to the 1600 eruption of Huaynaputina, rather than of eruptions at Misti.[297]

Climbing and recreation

[edit]

Misti was first ascended by pre-Columbian people, who left archaeological evidence around the summit.[298] Numerous ascents of the volcano were made already during the 18th and 19th centuries.[299] On July 9, 1988, U.S. cyclist Terry Powers went to the summit of El Misti with a mountain bike from the southern slopes and rode down the northern slopes.[300][301] The iron cross on the summit was placed in 1784 and was still there a century later.[298] The main ascent route according to mountaineer John Biggar is from the Aguada Blanca dam; a permit is needed to cross the dam. There are campsites at around 4,600 metres (15,100 ft) elevation, accessible from the Aguada Blanca dam and the town of Chihuata south of Misti. The ascent to the summit takes about one long day. A less common route starts at Apurimac San Luis on the southern flank, through Tres Cruces and Los Pastores.[302] The climbers reported difficulties due to the loose ground, noxious gases[299] and altitude sickness,[298] and John Biggar cautioned that there is no source of potable water on the mountain.[302] The volcano is frequently visited by tourists.[303] Tourist activities at Misti include running down scree slopes.[304]

See also

[edit]

Notes

[edit]
  1. ^ The selection of the volcano was motivated by the clear, calm atmosphere at Misti.[24] The volcano was also evaluated as a potential site for an astronomical observatory.[25]
  2. ^ It has been christened the Fuji of Peru.[51]
  3. ^ An altitude of 5,850 metres (19,190 ft) has also been proposed.[59]
  4. ^ This fan consists of mudflow, pyroclastic flow and tephra deposits.[64]
  5. ^ Misti has been cited as an example of a volcano where glaciers are retreating due to global warming,[74] but the source does not mention this mountain.[75]
  6. ^ Although patterned ground and solifluction lobes were observed in the crater.[76]
  7. ^ "Highway", referring to the appearance of the deposits in a stratigraphic section.[129]
  8. ^ The exact date is uncertain due to possible inaccuracies in the Inka chronologies.[166]
  9. ^ After who the deposit of the eruption was named.[172]
  10. ^ Historical records begin in 1540 AD when the Spaniards arrived,[173] and there is no record of the structure of the summit craters changing since then, implying that the craters and volcanic plug were emplaced in prehistoric times.[162]
  11. ^ Tornillos are a type of earthquake with long period and long coda; their waveforms have shapes resembling screws, which in Spanish translates to "tornillo".[181]
  12. ^ The Bolivian grass mouse[263] and two plant species, the stonecrop Sedum ignescens[264] and Cantua volcanica, were discovered at Misti; the latter was named after where it was found.[265]

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[edit]
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Bibliography

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