Misti

The name "Misti" originates from either the Quechuan language or Spanish. It means 'mixed', 'mestizo' or 'white' and may refer to the volcano's snow cover. The indigenous names are Putina,^[2][3] which means 'mountain that growls'^[4] in the Puquina language, while the Aymara language terms for Misti are Anukara^[5] or Anuqara^[6] ('dog'). All three terms refer to the dog-like appearance of the volcano when viewed from the Andean Plateau, known as the Altiplano.^[4] The volcano was originally known as Putina and only became known as Misti beginning in the 1780s.^[7] Other names for the volcano are Guagua-Putina, El Volcán ('the volcano'), San Francisco and Volcán de Arequipa ('Arequipa volcano').^[8][9] Some Spanish chroniclers have confused it with other volcanoes like Ubinas and Huaynaputina.^[10]

Settlement of the region began more than 1,500 years ago.^[b] It is unclear whether the Inca were the first Altiplano political entities to influence the region or whether previous cultures played a role,^[11] but by the arrival of the Spanish, the area was densely populated,^[12] and there were canals, roads and buildings where Arequipa is today.^[13] The city itself was founded on 15 August 1540,^[14] and Misti is featured on its seal.^[15] The volcano is the house mountain of Arequipa,^[16] whose residents view themselves as the offspring of the mountain.^[15]

Misti lies north of Arequipa,^[17] the second-largest city in Peru,^[18] and is the best known volcano of Peru.^[19] The Inca empire's Condesuyos province included the volcano;^[20] presently Misti is in the Arequipa Department.^[21] The mountain is visible from the Pacific Ocean.^[22]

The volcano rises about 3.5 kilometres (2.2 mi) above Arequipa.^[17] Dirt roads heading from Arequipa to Chivay run along the northern/western foot of Misti, and those to Juliaca along the southern/eastern foot.^[23] Inca roads from the Arequipa area passed by the volcano.^[24] There are numerous dams on the Rio Chili, including the Aguada Blanca Dam and reservoir north of the volcano, El Frayle (both north of the volcano)^[25][26] and Hidroeléctrica Charcani I, II, III, IV, V and VI^[27] along the northwestern foot of Misti;^[25] their hydroelectric power plants provide electricity to Arequipa.^[28]

Italian geographer Gustavo Cumin [it] in 1925 stated that three small man-made structures in the crater had been known since 1677, but noted that their origin was unknown.^[29] Inca ceremonial platforms on the summit associated with human sacrifices were probably destroyed by human activity around 1900.^[30]

In 1893^[c],^[32] professor Solon Irving Bailey from the Harvard College Observatory installed what was then the world's highest weather station on Misti.^[33][34] The Misti observatory was in its time the highest permanently inhabited location on Earth.^[35] The selection of the volcano was motivated by the clear, calm atmosphere at Misti.^[36] The station was one of several stations built at the time to investigate the atmosphere at such high altitudes;^[37] it was also used for research on the response of the human body to high altitudes^[33] and on the solar eclipse of 16 April 1893.^[38] Another weather station, named "Mt. Blanc Station",^[39] was installed at the base of the volcano^[40][41] after 1888.^[42] Both were shut down in 1901 when Harvard College Observatory decided to only maintain a station in Arequipa;^[40][41] storms have since erased any trace of the summit observatory.^[43] Observation of physics phenomena, such as cosmic ray measurements,^[44] were sporadically carried out on Misti during the 20th century.^[43]

The volcanoes of Peru are part of the Andean Central Volcanic Zone (CVZ),^[45] 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.^[46] The CVZ extends for 1,000 kilometres (620 mi)^[47] from southern Peru through Bolivia to northern Argentina and Chile.^[48] Volcanoes are numerous in the CVZ, but most are poorly known due to the low population density of much of the Central Andes.^[49] Several Peruvian volcanoes have been active since the Spanish conquest: the Andagua volcanic field, Huaynaputina, Sabancaya and Ubinas, and possibly Ticsani, Tutupaca and Yucamane.^[50] Other Peruvian volcanoes in the CVZ are Ampato, Casiri, Coropuna, Huambo volcanic field, Purupuruni and Sara Sara;^[47] in total, there are more than 400 volcanoes in Peru but most are eroded to the point of being hard to recognize.^[51] Ubinas is the most active volcano in Peru, having erupted more than 23 times since 1550.^[52] The 1600 eruption of Huaynaputina claimed more than 1,000 casualties; recent eruptions of Sabancaya 1987–1998 and Ubinas 2006–2007 had severe economic and social impacts on the local populations.^[53]

The volcano is a young, symmetric^[d] cone with 30° degree steep slopes.^[50] The summit features nested summit craters: The outer crater is 835 to 950 metres (2,740 to 3,117 ft)^[55][56] wide and 120 metres (390 ft) deep.^[56] There is a gap in the southwestern rim, almost to the bottom of the crater;^[57] otherwise the inner crater walls are nearly vertical^[56] and consist of small sphere-shaped pieces of volcanic debris called lapilli,^[58] lava and volcanic ash.^[59] The western rim of the outer crater is about 150 metres (490 ft) higher than the southern.^[50] The 550-metre-wide (1,800 ft) and 200-metre-deep (660 ft) inner crater^[55] is in the southeastern part of the outer crater.^[60] The inner crater cuts through metre-thick ash, scoria^[e] deposits^[50] and historical lava domes; it is rimmed by scoria.^[55] In the crater is a 120-metre-wide (390 ft) and 15-metre-high (49 ft) volcanic plug^[f][63] or lava dome^[g].^[50] It is covered with cracks,^[29] boulders and fumarolic sulfur deposits^[60] and features active fumaroles^[h].^[65] The highest point of the volcano is at 5,822 metres (19,101 ft)^[i][67] on the northwestern outer crater rim; an iron cross marks the highest point.^[56] Other mountains of the Western Cordillera, including Ubinas and Pichu Pichu, can be seen from the summit.^[68]

The volcano is about 20 kilometres (12 mi) wide^[69] and rises abruptly from the surrounding terrain.^[70] Estimates of the mountain's volume range from 150 cubic kilometres (36 cu mi) to more likely values of 90 cubic kilometres (22 cu mi)^[71] or 40 cubic kilometres (9.6 cu mi).^[50] The stratovolcano^[j] is made up of pyroclastic rocks and stubby lava flows, which form a 2.2-kilometre-thick (1.4 mi) pile.^[17] On the northwestern foot, there is an outcrop of rhyolite named "Hijo de Misti" ("son of Misti"),^[73] while an older, eroded stratovolcano ("Misti 1"), lies underneath the Misti cone.^[17] Misti is surrounded by a fan of volcanic debris,^[l] which covers an area of 200 square kilometres (77 sq mi) on Misti and extends 25 kilometres (16 mi) from the volcano.^[17] On the southern side, the volcano is cut by 20-to-80-metre-deep (66 to 262 ft) ravines,^[75] while the northern side is flatter.^[50] Dune fields and volcanic ash deposits extend for 20 kilometres (12 mi) northeast of Misti; they are formed by wind-blown ash.^[19][67][76] The terrain between Arequipa and Misti is initially gently sloping, before reaching the steep flanks of the cone.^[77]

Volcanoes often experience the collapse of part of the cone, a so-called sector collapse, which form a landslide ("debris avalanche").^[78] There are no obvious traces of such on Misti,^[17] except on its western foot^[76] and a narrow chute on the northwestern flank of Misti that reaches its summit.^[79] 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 respectively. 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.^[17]

The Rio Chili^[m] rounds the northern and western sides of Misti,^[17] where it has cut the 20-kilometre-long (12 mi) and 150–2,600-metre-deep (490–8,530 ft)^[81] Charcani Gorge.^[71] 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 mountain. They eventually join to the Rio Chili west and Rio Andamayo south of Misti;^[82] the Andamayo joins the Chili south of Arequipa.^[83] Quebrada San Lazaro and Quebrada Huarangual have formed fan-like deposits of material carried by the streams at the foot of the volcano.^[84][17] The quebradas (dry valleys) carry water during the wet season in November–December and March–April.^[80]

The snowline lies above^[n] the summit.^[88] During December–August,^[89] snow can cover an area of 1–7 square kilometres (0.39–2.70 sq mi) on the upper cone^[90] and be mistaken for glaciers,^[91] but does not persist over time.^[92] Unlike neighbouring Chachani, Misti lacks any evidence of glacial or periglacial^[o] processes, probably due to its inner heat.^[94] Whether there was past glaciation is unclear;^[95][96] a thin ice cover may not have left traces on the volcano.^[95] Traces of glacial erosion^[96] like cirques,^[97] evidence of volcanic activity involving magma-water interaction and mudflows imply that Misti was glaciated during the first phase of the last glacial maximum of the Central Andes 43,000 years ago.^[76][98]

Off the western coast of Peru, the Nazca Plate subducts (descends) under South America^[50] at a rate of 5–6 centimetres per year (2.0–2.4 in/year).^[17] The subduction is responsible for the volcanism of the CVZ,^[47] as the downgoing slab releases fluids that chemically modify the mantle situated above the slab, causing it to produce melts.^[99] Most Peruvian volcanoes have produced potassium-rich andesitic magmas, derived from the mantle and further modified by fractional crystallization^[p] and the entry of material from the often thick crust into the magma.^[47]

Volcanic activity in southern Peru goes back to the Jurassic,^[q][102] but the currently recognizable volcanic arcs in Peru are more recent: the Tacaza Arc formed 30–15 million years ago, the Lower Barroso 9–4 million, the Upper Barroso 3–1 million 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^[r].^[104]

During the Cretaceous–Paleogene, the Toquepala Group of volcanics was emplaced. The Tacaza Arc is the source of the Huaylillas Formation and the Barroso arc of the Sencca Formation.^[105] The Nazca fracture zone on the Nazca Plate projects under Misti.^[106]

Misti is part of the Andean Western Cordillera.^[107] It is the youngest of a group of three Plio-Pleistocene volcanoes;^[75] the others are the dormant Chachani 15 kilometres (9.3 mi) northwest and extinct Pichu Pichu 20 kilometres (12 mi) southeast.^[18] This group lies at the margin of the Altiplano,^[50] next to the 600-square-kilometre (230 sq mi)^[108] tectonic depression of Arequipa where the city lies.^[109] The depression has dimensions of 30 by 15 kilometres (18.6 mi × 9.3 mi) and appears to be formed by fault activity.^[110] The terrain under Misti slopes south and this might make the mountain slip southward over time.^[111] Another group of volcanoes lies south of Chachani: The Yura volcanic group with Cerro Nicholson.^[112]

A northwest-southeast trending fault system includes the Huanca fault at Chachani and the Chili fault on Misti.^[113] The faults were active during the Holocene, offsetting tephra deposits,^[114] and may have provided a pathway for magma to ascend and form the volcanoes of Arequipa.^[56][115] One of these faults, the Incapuquio fault, produced two earthquakes that coincide with Misti's last eruptions.^[116] Other faults include north- and northeast-trending faults, which are inactive but could have influenced the formation of the Rio Chili canyon.^[18] The crust under the volcano is 55 kilometres (34 mi) thick.^[71]

The basement under Misti is exposed in the Rio Chili gorge. It consists of Proterozoic rocks of the Arequipa Terrane, which are more than a billion years old, Triassic-Jurassic sediments of the Chocolate Formation, Socosani Formation^[117][118] and Yura Group, and the Cretaceous–Paleogene La Caldera batholith.^[119] The batholith forms the hills south of Arequipa.^[120] These formations are covered by rhyodacitic ignimbrites^[17] known as "sillars".^[46] They are between 13.8 and 2.4 million years old;^[17] the older ignimbrites are part of the Huaylillas Formation and the younger of the Barroso Arc.^[121] Individual ignimbrites are exposed in the Rio Chili gorge^[122] and include the 300-metre (980 ft) thick Rio Chili ignimbrite from 13.19 ± 0.09 million years ago, the 4.89 ± 0.02 million-year-old La Joya ignimbrite or "sillar", the 1.65 ± 0.04 million-year-old Aeropuerto or Sencca ignimbrite,^[75] and the 1.02 million-year-old Yura Tuff and Capillune Formation.^[123] These ignimbrites were erupted from multiple calderas^[s], one of which is now buried under Chachani.^[125][66] The ignimbrites are covered by volcanic sedimentary rocks^[17] and debris from the sector collapse of Pichu Pichu.^[110]

Misti has erupted rocks mainly of andesitic composition, while dacitic^[126] and rhyolitic compositions are less common.^[127] Rhyolites and dacites are associated with explosive eruptions.^[128] The volcanic rocks are subdivided into several classes: Pyroxene-amphibole andesites, amphibole andesites, amphibole dacites and amphibole rhyolites.^[129] There are reports of trachyandesite erupted during the Holocene eruptions.^[130] Mica has also been reported.^[127] The rocks define a potassium-rich calc-alkaline suite^[127] typical for Peruvian volcanoes.^[131] Phenocrysts^[t] include amphibole, augite, biotite, enstatite, plagioclase and titanomagnetite.^[126] 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.^[128] The composition of Misti magmas and those of its neighbours Pichu Pichu and Chachani resemble adakite, an unusual kind of volcanic rock formed by the direct melting of a subducting plate.^[127] Some rocks erupted by the volcano show evidence of hydrothermal alteration, colouring them yellow.^[133]

The formation of the Misti magmas involves the arrival of new magma, assimilation of crustal material and fractional crystallization.^[126] 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,^[129] where they interact with Proterozoic gneisses.^[134] Assimilation of basement rocks gave rise to the rhyolitic magmas erupted 34,000–31,000 years ago.^[135] Crystal-poor magma can form in the magma plumbing system through numerous processes and gives rise to the rhyolites and the volcanic plug.^[136] The existence of a third magma storage zone hosting mafic magmas at the base of the crust has been proposed.^[137]

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.^[138] The reservoir appears to be 6–15 kilometres (3.7–9.3 mi) underground^[139] and has a volume of several cubic kilometres.^[126] Every few millennia, a secondary rhyolitic reservoir forms at about 3 kilometres (1.9 mi) depth;^[140] it was last reactivated during the eruption 2,000 years ago.^[102] The magma system is periodically recharged, but such an influx of new magma does not trigger eruptions;^[136] instead multiple recharges are necessary to cause activity.^[126][141] Numerous mixing and decompression events can happen to each magma batch before it is erupted,^[142] with mixing particularly important during the last 21,000 years.^[143] A recharge of the magma chamber may have occurred at some point before 2000 AD.^[144] The overall rate of magma supply is 0.63 cubic kilometres per millennium (0.15 cu mi/ka), comparable to other stratovolcanoes in volcanic arcs, but with brief surges reaching about 2.1 cubic kilometres per millennium (0.50 cu mi/ka)^[65] and an increased rate during the last 21,000 years.^[145]

Misti is a young volcano.^[28] It developed in four stages, numbered 1 through 4; a pre-Misti volcano may have formed the southwestern debris avalanche^[17] and the older volcanic structures lie mainly in the western sector of Misti.^[146] On average, sub-Plinian eruptions take place every 2,000–4,000 years, while ash fallout occurs every 500–1,500 years^[65] and large ignimbrite-producing eruptions every 20,000–10,000 years.^[147] Rock formations showing the stratigraphy of Misti are found mainly in the ravines on the southern side^[50] and the Rio Chili gorge;^[148] only a few eruptions have been thoroughly investigated.^[149] Seismic tomography has identified solidified buried magma bodies from the early stages of volcanism.^[150]

Long andesitic lava flows and ignimbrites, which reach a thickness of more than 400 metres (1,300 ft), form the oldest part of the volcano.^[17] They have an age of 833,000 years, but it is not clear if the ignimbrites and lava flows should be considered part of "Misti 1" or of a pre-Misti volcano.^[151] Sometimes, they are considered the first stage of Misti activity, with all the subsequent activity making up the second stage.^[96] 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 a mountain with an elevation of 4,000–4,500 metres (13,100–14,800 ft), in the southern and eastern sectors of present-day Misti.^[17] 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.^[152]

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,^[153] forming a 6-by-5-kilometre (3.7 mi × 3.1 mi) caldera.^[154] 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.^[153] This activity brought "Misti 2" to an end;^[155] subsequently lava domes built "Misti 3" to an elevation of 5,600 metres (18,400 ft), almost entirely erasing the caldera.^[156] 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.^[157] The activity between 50,000 and 20,000 years ago has been christened "Cayma stage",^[158] and several eruption deposits from this time have been named:^[159]

• The 44,900–38,700^[160] or 34,000–33,000 year old "Fibroso I",^[161] also known as "Cogollo".^[162]
• The 43,200–38,300 year old "Anchi".^[160]
• The 38,500–32,400 year old^[160] "Sacarosa", "Sacaroso" or "Sacaroide".^[162] This eruption produced two layers of pumice^[163] 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^[164] or 5. It was a two-stage event, with a change of magma dynamics or intensity occurring during the eruption.^[165]
• The 37,100–30,500 year old "Conchito"^[160] or "Fibroso II".^[162]
• The 30,300–28,800 year old "Chuma". Several additional eruptions took place between the "Conchito" and "Chuma" events.^[166]
• The 15,000 years old^[167] "Autopista"^[u].^[159] This eruption three layers of (mostly) pumice with smaller quantities of lithics.^[168] During its eruption about 0.16 cubic kilometres (0.038 cu mi) of volcanic ash fell west of the volcano.^[169] 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.^[170] The "Autopista" deposit is the best preserved of the late Pleistocene tephra layers.^[159]
• Deposits of eruptions after "Autopista" have been named according to two schemes: One spans the Pleistocene and Holocene^[159] and lists "Blanco", "La Zebra", "Espuma gris", "Espuma iridiscente" and "Rosado",^[171] the other includes tephra layers up to the eruption 2,000 years ago and lists "Ponche Iridescente", "Ponche Gris", "Sandwich Inferior", "Sandwich Superior", "Sancayo", "La Rosada", "Apo" and "Misquirichi".^[172]

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.^[173] 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.^[157] 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).^[174]

More than 10 eruptions took place during the last 11,000 years,^[55] with only brief pauses in activity.^[175] The activity between 21,000 and 2,000 years ago is known as the "Pacheco" stage.^[176] Holocene activity filled the younger caldera with scoria and lava flows, forming the "Misti 4" volcanic structure 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.^[55] The 9,000 and 8,500 years old eruptions produced the "Sándwich" deposits.^[177] They extend for more than 15 kilometres (9.3 mi) on the southwestern flank of Misti^[177] produced ash fall over the Pacific Ocean and Lake Titicaca.^[178] Radiocarbon dating has identified eruptions 8,140, 6,390, 5,200, 4,750, 3,800 and 2,050 years ago;^[179] the 3,800 eruption deposited fallout on Nevado Mismi^[180] more than 90 kilometres (56 mi) northwest of Misti.^[181] The Global Volcanism Program lists eruptions in 310 BCE ± 100 years, 2230 BCE ± 200 years, 3510 BCE ± 150 years, 4020 BCE ± 200 years, 5390 BCE ± 75 years and 7190 BCE ± 150 years.^[182]

The last major explosive eruption took place about 2,000 years ago in one or multiple events.^[175] The date is constrained to 2,060–1,920 years before present; ages of 2,300 BP are probably too old.^[130] It produced about 0.4 cubic kilometres (0.096 cu mi) dense rock equivalents of rock^[183] and probably lasted a few hours.^[184] The event had a volcanic explosivity index of 4 or 5.^[185]

The eruption was probably triggered when fresh andesitic magma entered a pre-existent rhyolitic body.^[186] Magma rose through the volcano and expelled part of the hydrothermal system,^[187] causing initial phreatic eruptions^[v].^[189] Tephra rained down around the mountain,^[190] with pumice falling 25 kilometres (16 mi) from the volcano.^[175] Owing to magma mixing, the pumice deposits have an appearance resembling chocolate and vanilla swirls.^[130] Eventually, the conduit fully cleared and a 29-kilometre (18 mi) high eruption column rose above the volcano.^[189] Pyroclastic flows^[w] emanated from the column and descended the southern flanks of the volcano, possibly through the gap in the crater rim.^[192] During the course of the eruption, collapses of the crater and conduit walls caused a temporary decline in the intensity of the column.^[193] The eruption column periodically collapsed and reformed, until the eruption ended with phreatomagmatic^[x] explosions.^[194]

Mudflows descended the mountain,^[189] although their importance relative to the pyroclastic flows is contentious.^[195] 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.^[95] Rainfall generated further mudflows after the eruption.^[196] The outer summit crater probably formed during this eruption.^[183] Tephra layers in the Sallalli and (in this case with less certainty) Mucurca peat bogs close to Sabancaya,^[197] and (tentatively) in an ice core in the Antarctic Plateau in Antarctica, are attributed to this eruption.^[198] The 2,000 years eruption is the only Plinian eruption during the Holocene at Misti.^[199]

After the eruption 2,000 years ago, 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.^[28][200] Pyroclastic flows and ash falls were emplaced 1,290 ± 100 and 620 ± 50 years ago.^[201] Mudflows–not all associated with eruptions^[28][200]–took place 1,035 ± 45, 520 ± 25, 340 ± 40 and 330 ± 60 years ago^[185] and left 5–15-metre (16–49 ft) thick deposits.^{[202] [201]}

The last eruption took place in AD 1440–1470^[y][65] and produced about 0.006 cubic kilometres (0.0014 cu mi) of ash.^[147] It was probably a prolonged eruption that lasted for months or years,^[204] depositing ash in the Peruvian Laguna Salinas^[199] and possibly as far as Siple Dome^[205] and Law Dome in Antarctica.^[206] It is the oldest eruption of a South American volcano for which historical records exist.^[207] The eruption was severe enough that Mama Ana Huarque Coya,^[208] the wife of the Inca emperor Pachacutec,^[z] came to Chiguata^[210] to provide assistance.^[210] There is no evidence that a supposed Inca settlement was destroyed by this eruption,^[199] but the local population fled and the Inca had to resettle the area.^[211] Along with other volcanic eruptions around that time and the beginning Spörer solar minimum, the AD 1440–1470 eruption of Misti may have affected global climate conditions.^[212] In 1600, the volcano was covered by ash from Huaynaputina.^[213]

Most sources state that there is no clear evidence of eruptions after the arrival of the Spaniards,^[126][210] while the Global Volcanism Program reports a last eruption in 1985.^[67] Mudflows descended the southern valleys until the 17th century.^[65] The mountain is sometimes reported to be "smoking" at its summit,^[214] including water vapour clouds.^[215] Phreatic eruptions may have taken place in 1577,^[216] 2 May 1677, 9 July 1784, 28 July 1787 and 10 October 1787. Questionable eruptions are recorded in 1542, 1599, 1826, 1830, 1831, 1869, 1870. They probably constitute fumarolic activity^[128] and often took place after heavy precipitation; the water would have infiltrated the mountain and evaporated from the volcanic heat.^[217] There is no record of the structure of the summit craters changing in historical records, implying that the craters and volcanic plug were emplaced in prehistoric times.^[199] Comparisons between 1967 photos of the volcanic plug and more recent images show no changes.^[218]

The volcano is seismically active, with long-period earthquakes, tremors, "tornillos"^[aa] and volcano tectonic earthquakes recorded.^[220] The hypocentres, the actual sites of the earthquakes, are found within the volcanic structure of Misti^[221] and cluster on the northwest flank of the volcano. The seismic activity appears to be linked to Misti's hydrothemal system.^[222] Seismic swarms were recorded in August 2012, May 2014 and June 2014.^[223] No deformation of the volcano is evident in satellite images.^[224][225] Clouds rising from the mountain are sometimes mistaken for renewed activity.^[226]

Misti is Peru's most dangerous volcano and one of the most dangerous in the world,^[227][228] owing to its location just 12 kilometres (7.5 mi) from Arequipa,^[28] a city of over one million residents.^[229] Over time the city has expanded and new^[28] and neighbouring towns such as Chiguata get within 11 kilometres (6.8 mi) of Misti.^[17] About 8.6% of Peru's GDP depends on Arequipa and would be impacted by a future eruption of Misti.^[230] The city is constructed on mudflow and pyroclastic flow deposits of the volcano^[231] and all the valleys that drain Misti pass directly or indirectly through Arequipa.^[71] At least 220,000 people south of Misti are threatened by floods, mudflows and pyroclastic flows^[17] channelled through the ravines.^[232]

Individual threats from Misti include:

• The eruptions 2,000 years ago and 1440–1470 AD eruptions deposited tephra over what today is Arequipa.^[26] Tephra fallout^[ab] can cause health problems, pollute water resources, cause roofs to collapse, bury fields,^[233] and cause road accidents and accidents during cleanup.^[234] Much closer to the volcano, large rocks can fall.^[235]
• Mudflows are mixtures of rocks and water. They are caused by rainfall or the melting of snow and ice and can happen without volcanic activity.^[236][237] At Misti, they occur on average every century or two.^[238] Small mudflows can reach the city^[239] and bury and destroy everything in their path.^[240] Eruptions of Misti could generate mudflows on Chachani, thus threatening settlements that are on the other side of the Rio Chili.^[241]
• 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.^[236] Pyroclastic flows and surges can extend 13 kilometres (8.1 mi) from the volcano,^[65] although denser flows are likely to stop before reaching the city.^[242]
• 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.^[242][243] Debris flows, like mudflows, can destroy everything in their path.^[236] Such collapses could also dam the Rio Chili, producing mudflows^[244] and threatening neighbourhoods like Vallecito, Av. La Marina and Club Internacional.^[27] Small landslides on the western side of the volcano could threaten the water supply of Arequipa.^[240]
• Toxic gases can accumulate in closed spaces to dangerous concentrations, or interact with precipitation to form acid rain. Lava flows are highly destructive, but their slow speed does not constitute a major threat to life.^[245]

Hazards at Misti not related to volcanic activity include flooding during the wet season in Arequipa.^[242] Heavy metals, presumably from Misti and Chachani, have been found in river water.^[246]

In 2001, there was neither emergency planning nor land-use planning around Misti;^[26] the 2002–2015 development plan mentioned volcanic hazards but did not envisage specific measures.^[247] The last eruption of Misti had taken place shortly before the foundation of Arequipa, and thus—unlike for earthquakes—there is no memory of the hazards of volcanic activity.^[248] Before the eruption of Ubinas in 2006–2007, volcanic hazards drew little attention from the Peruvian state and there was little awareness in Arequipa.^[53] The volcano is frequently considered a protective figure and not as a threat.^[249] A number of people associate volcanoes with lava flows and neglect other volcanic hazards.^[228]

Beginning in 2005, INGEMMET began monitoring volcanoes in Peru;^[250] the first monitoring equipment was at the Charcani V hot spring. Later the monitoring was extended to other hot springs, and the crater fumaroles were surveilled both from Arequipa and from the crater.^[251] Monitoring of seismic activity commenced in 2005.^[252] Beginning in 2008, geodesic measurement stations were installed on the northeastern and southern slopes of the volcano,^[251] and a new monitoring station for the volcano was inaugurated in 2012.^[250] In May 2009 and April 2010, two exercise evacuations of several suburbs of Arequipa were carried out.^[253] The Peruvian Volcano Observatory (OVI) was inaugurated in Arequipa in 2013; it monitors Misti, Ubinas, Ticsani and other Peruvian volcanoes.^[254] As of 2021^[update], 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 mountain.^[255] These efforts have yielded an increased awareness of the dangers posed by Misti, which is now being increasingly perceived as an active volcano.^[256] Efforts have been made to slow the growth of the northern suburbs of Arequipa, which are closest to Misti.^[257]

A volcano hazard map was developed in 2005 by numerous local and international organizations,^[244] and officially presented in early 2008.^[258] It defines three hazard categories: a red "high risk" zone, an orange "intermediate risk" zone and a yellow "low risk" zone.^[244] These are defined by the risk of debris flows, lava flows, mudflows, pyroclastic flows and tephra fallout.^[259] 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.^[260][261] Additional maps show areas at risk of tephra fallout^[262] and of being flooded by mudflows.^[263] The hazard map of Misti is the first hazard map of a Peruvian volcano.^[254] These maps serve to mitigate volcano hazards and to inform local development.^[264] A 3D map was published in 2018.^[265] In 2010, the municipality of Arequipa decreed that the hazard map would have to be considered in future city zoning decisions.^[248]

Three different scenarios of future eruptions have been evaluated.^[90] The first envisages a small eruption, similar to recent activity at Sabancaya,^[90] or the 1440–1470 AD eruption of Misti.^[129] 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 eruption 2,000 years ago. Thicker ash falls (exceeding 10 centimetres; 3.9 in) could cause building collapse and pyroclastic flows down the steep slopes south of Misti, reaching the suburbs of Arequipa and Chiguata.^[266][267] Most risk assessments are based on these two scenarios.^[240]

The third scenario is a Plinian eruption like the "Fibroso" and "Sacaroso" events or the 1600 Huaynaputina eruption;^[129] 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,^[268] including over the cities of El Alto, La Joya and agricultural areas.^[267] A Plinian eruption would require the evacuation of Arequipa.^[240] Other hazard scenarios are the emissions of short lava flows, the formation and collapse of lava domes and the collapse of part of the mountain.^[264]

Fumaroles exist in several places: the volcanic plug; the northern and northeastern walls of the volcano's inner crater; and the volc southeastern flank of the volcano.^[114] They make noises,^[269] produce visible clouds of water vapour and the smell of hydrogen sulfide. The smell reaches the crater rim,^[60] and, at times, the gas becomes so concentrated that it causes irritations to the eyes, nose and throat.^[269] Fumarolic activity has been reported since the 1440–1470 eruption.^[224] In 1948–1949 and 1984–1985, it was intense enough that it could be seen from Arequipa.^[128] The fumarolic activity is visible in satellite images as a temperature anomaly of about 6 K (11 °F).^[270]

Water is the most important component of the fumarole gases, followed by carbon dioxide, sulfur dioxide, hydrogen sulfide and hydrogen.^[271] The hydrogen chloride and hydrogen sulfide content makes them highly acidic.^[272] Fumarole temperatures have varied through the years, generally they are between 125–310 °C (257–590 °F)^[141] with peaks of 430 °C (806 °F).^[273] The present-day (21st century) fumarole gases appear to derive directly from magma, with no interaction with a hydrothermal system.^[141] The fumaroles outside of the summit crater are colder, with temperatures of 50–80 °C (122–176 °F),^[114] and do not smell of sulfur.^[274]

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.^[275] Elemental compositions and isotope ratios indicate that the fumarole deposits are derived from the leaching of volcanic rocks and the water from precipitation.^[276] The chemistry of the deposits changed between 1967 and 2018, with decreasing zinc and increasing lead concentrations, alongside with a warming of the fumarolic system^[277] that may have been due to the arrival of new magma in the volcano during the 20th century.^[278] Sometimes the temperature of the fumaroles is high enough to melt the sulfur^[279] and the fumarolic gases can ignite.^[269]

Hot springs occur at the foot of the volcano. To the north is the Humaluso (Umaluso) spring, while to the south and southwest lie 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^[280][281] south and southwest of Misti.^[274] The hottest of these is the Charcani V spring^[281] in the Rio Chili gorge;^[282] it is also the closest to the volcano, being only 6 kilometres (3.7 mi) from the crater.^[283] 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.^[281] Their waters appear to originate through the mixing of freshwater, magmatic water and chloride-rich deep water.^[284] Many of these springs form artificial pools or have water intakes,^[285] and several are monitored by INGEMMET for changes in activity.^[286]

High soil temperatures on the cone,^[287] hot springs and fumaroles indicate that Misti contains a hydrothermal system.^[283] Electric potential measurements indicate that the system appears to be confined between faults^[76] or to the older caldera.^[288] 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.^[282] Water temperatures decreased after the 2007 Peru earthquake.^[289] Over time old fumarolic vents shut down and new vents develop,^[269] but the configuration of the dome vents is stable over time.^[224] The fumarolic activity is correlated to earth tides,^[139] the deformation of the Earth caused by the Moon's and Sun's gravity.^[290]

The region has a semi-arid climate with mild temperatures.^[291] In Arequipa, temperatures are stable throughout the year, with minima of 6.9–11.2 °C (44.4–52.2 °F) and maxima of 23.2–22.1 °C (73.8–71.8 °F).^[292] Temperatures decrease with elevation:^[291] 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.^[293] In 1968 temperatures at the summit rose above freezing for a few days per year.^[68]

The summit is often covered in clouds.^[294] For most of the year, dry westerly winds blow over the Western Cordillera; during summer convection over the Amazon forces easterly flow that draws moisture to the Cordillera.^[295] Wind speeds at the summit can reach 5 metres per second (16 ft/s), with gusts to 16 metres per second (52 ft/s).^[296] Most precipitation falls during the austral summer (December to March); according to a 1974 publication, it reached 90 millimetres per year (3.5 in/year)^[21] and a 1910 study found most precipitation to be in the form of snow or hail.^[293] During the wet season, rainstorms and flash floods erode the volcanic debris deposits.^[175] The snow cover rapidly melts away during the dry season.^[297] The El Niño–Southern Oscillation and sea surface temperatures in the Atlantic and Pacific Oceans govern annual rainfall.^[298] 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. A subsequent dry period lasted until the 16th century AD, when the Little Ice Age began.^[181]

The region west of the Andes, including the terrain at the foot of Misti,^[297] is mostly desert with cacti and dwarf shrubs as the principal vegetation forms.^[299] 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)^[297] and by the bush^[300] Diplostephium tacorense above 3,000 metres (9,800 ft).^[301] Other bushes occur mainly in creeks and valleys.^[301] 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.^[302] 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).^[303][304] Polylepis species form woodlands.^[302] Vegetation cover decreases above 4,000 metres (13,000 ft) elevation.^[304]

Insects are the most important animals in the Peruvian mountains, and include beetles and hymenopterans (ants, bees, sawflies and wasps). Birds include the Andean condor.^[305] 358 plant, 37 mammal and 158 bird species have been recorded^[ac] in the region, including alpacas, guanacos, llamas and vicuñas.^[309] Most of the volcano is within the Salinas y Aguada Blanca National Reserve, which extends northwest of Misti^[310] and includes the volcano among its main attractions.^[309]

The mountain was considered the apu^[ad][312] and "volcano of the city".^[313] It was venerated by the inhabitants of Arequipa, a common practice for inhabitants of the Andes.^[9] The Aymara people viewed it as an abode of deceased souls,^[314] with different regions regarding it as either a friendly or hellish final destination.^[5] According to the late 16th-century chronicler Cristóbal de Albornoz,^[9][313] Misti was one of the important mountains (waqa, a kind of deity or idol^[315]) of the Arequipa area of the Inca Empire, along with Ampato, Coropuna, Sara Sara and Solimana.^[316] This tradition probably originated with the previous inhabitants of the area and was taken over by the Inca when they conquered the region.^[317] The Middle Horizon^[318] 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.^[319] Petroglyphs at Toro Muerto may represent astronomical alignments of Misti and Chachani.^[320]

The Inca gave the apus cups of gold and silver^[321] and settled people around Misti that would continue the mountain veneration.^[322] People used to alter the shape of the skulls of their infant children so that they resembled the volcano.^[323] Misti was considered to be an aggressive mountain that was always demanding sacrifices,^[324] and the mountain had to be exorcised in colonial times.^[325] After the Spanish conquest, the mountain was consecrated to St. Francis.^[326] According to the Jesuit College of Arequipa, "Indian sorcerers" thought that Huaynaputina volcano had asked Misti for assistance in expelling the Spaniards; Misti however had turned down, saying it was already Christianised, so Huaynaputina had proceeded alone.^[327] During episodes of increased activity, the inhabitants of Arequipa carried out religious ceremonies, including public penance and flagellations, to discourage the volcano.^[328] 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.^[329] 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^[330] as a celebration of the new century.^[331] The cross on the summit of Misti supposedly protects the city.^[332] To this day, religious ceremonies are carried out on the volcano.^[328] Peasants believe that after offering gifts to Misti will bear boys, while the same offers to Chachani will make them bear girls.^[333] Geologists are known to offer objects to the volcano before carrying out investigations.^[334]

Eight or nine mummies were found on Misti by the North American anthropologist Johan Reinhard^[325][335] and the Arequipan archaeologist José Antonio Chávez^[335] in 1998, inside the crater and below the summit.^[336] The mummies were of children, mostly boys around six years old^[337] and some infants, which were sometimes buried one on top of the other.^[338] Unusually, the mummies were buried in shared tombs.^[339] Along with the mummies were figurines, ceramics and other objects;^[325] the high number of figurines found on Misti (47) indicates that the site was important to the Incas.^[340][338] These mummies were Inca human sacrifices, called capacochas,^[336] and the Misti capacocha is the largest known.^[211][341] However, the hostile conditions within the crater had seriously damaged the mummies.^[340]

The sacrifices on Misti, and others on Chachani and Pichu Pichu, were probably motivated by the 1440–1470 eruption of Misti,^{[30][211][342]} which may explain the unusual location within the crater rather than on a summit.^[343] According to the 16th-century chronicler Martín de Murúa,^[313] the Inca emperor Thupa Yapanki sacrificed llamas to calm a volcano Putina close to Arequipa (probably Misti),^[344] going as closely as possible to the summit.^[345] According to stories, previous ceremonies had failed to calm the volcano and only the emperor's direct intervention quelled its anger.^[346] This description most likely refers to the 1600 eruption of Huaynaputina, rather than of eruptions at Misti.^[347]

Misti was first ascended by pre-Columbian people, who left archaeological evidence around the summit.^[348] The first documented ascent was by Álvaro Meléndez, a priest from Chiguata, on 1 May 1667.^[349] Numerous ascents of the volcano were made already during the 18th and 19th centuries.^[350] On 9 July 1988, U.S. cyclist Terry Powers went to the summit of Misti with a mountain bike from the southern slopes and rode down the northern slopes.^[351][352] 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.^[353] Ascent from Chiguata takes a few days.^[354] Climbers report difficulties due to the loose ground, noxious gases^[350] and altitude sickness.^[348] John Biggar cautioned that there is no source of potable water on the mountain.^[353]

The volcano is frequently visited by tourists,^[355] who come for the sight of the landscape surrounding Misti.^[356] Tourist activities at Misti include mountaineering,^[357] trekking^[334] and running down scree slopes.^[358] Ascents take place almost year-round.^[359] Misti and its neighbouring volcanoes have been investigated as potential geosites.^[360]

• List of volcanoes in Peru