Cerro Panizos
The lava domes in the centre of the image form the Panizos centre
Geography
Parent range	Cordillera de Lípez
Geology
Volcanic arc/belt	Altiplano-Puna volcanic complex
Last eruption	6.1 mya

22°15′S 66°45′W / 22.250°S 66.750°W / -22.250; -66.750^[1] Panizos is a Late Miocene caldera in the Potosí Department of Bolivia and the Jujuy Province of Argentina. It is part of the Altiplano-Puna volcanic complex of the Central Volcanic Zone in the Andes. 50 volcanoes active in recent times are found in the Central Volcanic Zone, and several major caldera complexes are situated in the area. The caldera is located in a difficult-to-access part of the Andes.

Panizos and the majority of these volcanoes are formed by the subduction of the oceanic Nazca Plate under the continental South American continental lithosphere. The caldera is located east of the main arc and is supplied chiefly by dacite magmas. Beneath Panizos lie Tertiary ignimbrites and a Paleozoic sedimentary basement.

The huge Panizos ignimbrite erupted by Cerro Panizos has a minimum volume of 950 cubic kilometres (230 cu mi). It erupted during an event 6.71 ± 0.04 mya and was preceded by another ignimbrite 7.9 mya ago. The last activity is a lava flow 6.1 mya ago.

The caldera is hidden beneath a shield with a diameter of 40 kilometres (25 mi) and some of its central summits are over 5,000 metres (16,000 ft) high. It has been referred to as an "ignimbrite shield".

Geography and geomorphology

Cerro Panizos^[a] lies in the Cordillera de Lipez^[3] of the Andean Altiplano-Puna high plateau.^[4] The volcano is a 40 kilometres (25 mi) wide, gently sloping ignimbrite shield surrounding a 10–15 kilometres (6.2–9.3 mi) wide lava dome^[5] semicircle.^[6] Of the domes, one forms the 5,228 metres (17,152 ft),^[7] 5,360 metres (17,590 ft) or 5,494 metres (18,025 ft) high^[8] Cerro Panizos proper^[b]; the others are the 5,480 metres (17,980 ft),^[10] 5,490 metres (18,010 ft)^[8] or 5,228 metres (17,152 ft) high Cerro Cuevas, 5,504 metres (18,058 ft) high Cerro Crucesnioc/Crucesnioj/^[11][1] El Volcán,^[12] 5,390 metres (17,680 ft) high Cerro Vicunahuasi to the west and 5,540 metres (18,180 ft) high Cerro La Ramada^[11][1]/Cerro Ramada^[12] to the north.^[11][1] The border between Argentina and Bolivia runs along the domes.^[13] There are two calderas, a larger one formed by downsag south of the lava dome semicircle, and a smaller collapse caldera outlined by the lava domes.^[14] Cerro Anta Cuevas, Cerro Chinchijaran, Cerro Limitayoc^[1]/Limitayo^[11] and Cerro Tucunquis are lava plateaus that rise from the ignimbrite shield.^[1] The 5,158 metres (16,923 ft) high^[11] Limitayoc formed along a north-south trending fault^[15] and has an elongated shape,^[16] with traces of hydrothermal alteration at its northern end.^[15] The whole region is raised above the surrounding terrain.^[17]

Owing to the arid climate, little erosion has taken place.^[1] Parts of the ignimbrite are covered with windblown sand. Landforms have conical, dome-like or table-like shapes.^[18] Erosion of the ignimbrite has formed cliffs and rows of pinnacles, the latter of which draw photographers owing to their exotic appearance.^[19] The ignimbrite shield of Panizos has been compared to paterae on Mars.^[20]

Hydrology and human geography & history

Several (often ephemeral^[10]) streams cut into the rocks^[5] and drain mostly to the east; from north to south they are Rio Khuchu Mayu^[1][21]/Khuchumayu,^[22] Quebrada Buenos Aires, Quebrada Cienago^[c], Quebrada Paicone, Quebrada Potrero, Quebrada Guanapata, Quebrada Pupusayo^[1]/Pupusayoc,^[23] Quebrada Cusi Cusi, Quebrada Cueva, Quebrada Garcia^[1]/Quebrada de Garcia^[21] and Quebrada Quenoal.^[1][21] Most of them eventually join the San Juan de Oro River,^[10] which flows into the Atlantic Ocean.^[3] Panizos can be accessed through these valleys.^[24] Small lakes dot the southwestern sector of the shield,^[1] and there are ephemeral lakes on its southeastern side.^[10] Neighbouring mountains are Cerro San Matias north, Cerro Lipez northwest^[25] and Corutu southwest of Panizos,^[1] while the San Juan de Oro depression is east of the volcano.^[26]

The region is remote and inhospitable.^[27] Most of the volcano is in Bolivia's Potosi Department (Sud Lipez Province), except for the southeastern quadrant which lies in Argentina's Jujuy Province (Rinconada and Santa Catalina departments).^[28][29][30] The towns of Cusi Cusi and San Antonio de Esmoruca^[11]/San Antonio de Esmoruco^[31] are southeast and north of the volcano, respectively.^[11] A branch of the Inka road system passed over the volcano, which features several archeological sites.^[32] Most of the central domes were first climbed in November 1939^[33] but the volcano itself was only identified as such in 1977, thanks to images from the Landsat satellite.^[34]

Climate, flora and fauna

The region is a desert with only sparse bushy vegetation. The climate is cold and dry, with maximum precipitation reaching 200–400 millimetres (7.9–15.7 in) per year and only sparse cloud cover. The day-night temperature contrast is high, and most days of the year have frosts. The region is a desert,^[3][18] with the only vegetation consisting of cushion plants, grasses and shrubs. Wetter areas feature wetlands (such as bofedales), and there are salt flats.^[29] Animals include the large guanacos, llamas, tarucas and vicuñas,^[32] the smaller chinchillas, vizcachas and numerous mice genera, and the carnivorous Andean mountain cats, cougars, culpeos, Pampas cats and South American gray foxes. Birds include the Andean, Chilean and James's flamingos, Andean geese, Darwin's rheas and ducks.^[32]

Geology

Off the western coast of South America, the Nazca and Antarctic Plates subduct underneath South America.^[35] The subduction is responsible for the volcanism of the Andean Volcanic Belt, which is subdivided into four volcanic segments;^[36] the Northern (NVZ), Central (CVZ), Southern (SVZ) and Austral (AVZ) volcanic zones.^[35] The CVZ consists of a volcanic arc with stratovolcanoes^[37] reaching 6,000 metres (20,000 ft) height^[d][38] and numerous large calderas, both in the main arc and east of it;^[39] they are the source of the largest pile of Neogene-Quaternary volcanic rocks in the Andes.^[36]

The largest assembly of volcanoes in the CVZ is the 70,000 square kilometres (27,000 sq mi) Altiplano-Puna volcanic complex (APVC),^[38][40] a system of calderas and ignimbrites that were active in the Altiplano-Puna high plateau^[e] during the Miocene.^[44] With a volume exceeding 12,800 cubic kilometres (3,100 cu mi),^[45] it is one of the largest ignimbrite provinces on Earth.^[46] Known calderas in the APVC are Cerro Guacha, Cerro Panizos, Coranzulí, Kapina, La Pacana, Pastos Grandes and Vilama, but other buried calderas may exist^[44] and only a few of these volcanoes have been studied in detail.^[47] In the crust under the APVC is the Altiplano-Puna Magmatic Body,^[40] a giant pile of rock-magma mush^[48] at 17–19 kilometres (11–12 mi) depth^[38] that extends under southern Bolivia, northeastern Chile and northwestern Argentina.^[49] The southern Bolivian tin belt overlaps with the APVC.^[35]

Volcanic activity began during the Jurassic in the Cordillera de la Costa and migrated eastward since then.^[43] During the late Miocene, subduction under the Puna became steeper, causing the mantle wedge to become thicker and part of the overlying crust to delaminate, increasing the production of melts.^[38] Volcanic activity shifted east into the Puna until the Pliocene, after which it returned to the main arc where it persists to this day.^[50] Numerous ignimbrites were emplaced between 25 and 1 million years ago, with the bulk dating to the late Miocene to Pliocene.^[37] Volcanic activity was episodic, with several recognizable "flare-ups" during which volcanic activity increased^[41] about 10, 8, 6 and 4 million years ago.^[51] Volcanism declined during the past 4 million years,^[52] with the last eruptions 271,000 and 85,000 years ago at Uturuncu and Cerro Chascon-Runtu Jarita complex, respectively.^[53] During the 21st century, ongoing uplift was discovered at Uturuncu.^[17] In Bolivia, about 8-5 million years ago Kari-Kari was active, 8.4-6.4 million years ago Morococala, 8-5 million years ago Los Frailes and during the last one million years Nuevo Mundo.^[54]

The basement is formed by volcanic, sedimentary and crystalline rocks, which have ages ranging from Paleozoic (Acoite Formation^[55]) to Cenozoic (Peñas Coloradas^[4]/Peña Colorada^[55] or Tiomayo Formations^[56]).^[57] Two older ignimbrites underlie the Panizos centre,^[35] one of which originated at Coruto.^[58] The crust is 70 kilometres (43 mi) thick^[57] and up to a billion years old,^[59] but it reached the present-day thickness only during the late Cenozoic.^[42] It is intersected by numerous lineaments, some formed during the uplift of the Andes and others are older structures that were reactivated. Most calderas of the CVZ lie on such lineaments;^[40] one northeast-southwest trending line may have influenced the formation of Panizos, Vilama and Cerro Guacha,^[40] and smaller scale structures at Panizos may reflect north-south and eastsoutheast-westnorthwest trending lineaments.^[22] There is evidence of faulting, both before^[22] and after the eruption of the Panizos ignimbrite.^[60]

Composition

The volcano has erupted dacite, which contains numerous crystals and has a homogeneous composition.^[35] Andesites are subordinate; both rocks define a peraluminous potassium-rich calc-alkaline suite.^[61][62] Phenocrysts include biotite and plagioclase, while apatite and zircon form accessory phases; orthopyroxene, quartz and sanidine are less common and clinopyroxene, hercynite, hornblende,^[63] hypersthene,^[16] ilmenite, magnetite^[63] and orthopyroxene rare.^[23] Many of the rarer minerals are xenoliths derived from the crust.^[63] Gold and silver deposits are found on the volcano,^[64] and an occurrence of antimony-copper-uranium has been described at Paicone.^[65]

The rocks derive from a magma chamber,^[66] where stored magmas crystallized and underwent some fractional crystallization without mixing completely.^[67] The magma chamber was fed by a combination of mantle-derived basalts and melts from the lower crust,^[68] which formed in a melt zone at the bottom of the crust^[69] that is percolated by ascending basalts.^[70]

Oval orbs formed by concentric layers of crystals around a core have been found at Panizos.^[71] They make up less than one percent of Panizos rocks and only occur in the ignimbrites^[72] east and southeast of the volcano.^[71] The core is typically formed by a non-volcanic rock fragment or a cluster of orthopyroxene crystals, while the millimetre-thick layers of crystals are biotite, bronzite, ilmenite, orthopyroxene and plagioclase.^[73] Large (up to 2 centimetres (0.79 in)) phenocrysts co-occur with orbs.^[74] The orbs probably formed when magma rapidly degassed during the eruption of the Panizos ignimbrite, prompting the formation of crystals around "seeds" like xenoliths or orthopyroxene crystals that eventually formed the orbs.^[75] Only a few volcanoes in the world have such orbs, probably because they require special conditions to form.^[76]

Eruption history

Panizos was active in the late Miocene,^[35] although early Miocene rocks north of Panizos^[30] and the 12.4 million years old Cusi Cusi ignimbrite may also be part of it.^[77][78] Panizos was active at the same time as Coranzulí and Vilama-Coruto.^[79]

Panizos is the source of two major ignimbrites: The first (Cienago^[35] or Panizos I^[52]) was erupted 7.9 million years ago and forms two flow units^[35] with a total volume >300 cubic kilometres (72 cu mi),^[52] each underlaid by pyroclastic fallout deposits^[15] several centimetres thick. The ignimbrite contains a high proportion of pumice.^[13] It might be the first eruption of Panizos.^[80] Afterwards, lava domes erupted on the southern side of the volcano,^[35] including Cerro Limitayoc^[81] where activity continued after the Panizos ignimbrite.^[82]

The second ignimbrite,^[83] the >650 cubic kilometres (160 cu mi) Panizos (or Panizos II^[52]) ignimbrite erupted from the volcano 6.7 million years ago. It was emplaced as two flow units,^[35] which are separated by multiple base surge, pyroclastic flow and volcanic ash deposits that reach thicknesses of several metres.^[63] The Panizos ignimbrite forms the shield around the central dome complex,^[13] reaching as far as the Rio Granada-San Juan de Oro valley east of the volcano.^[13] The ignimbrites reach maximum thicknesses of about a few hundred metres around the central dome complex^[84] and where they filled in the pre-existent topography, forming thick deposits within valleys.^[85] The ignimbrites were not very mobile.^[86] A 1 metre (3 ft 3 in) thick layer of lapilli underlies the ignimbrite.^[23]

The Panizos ignimbrite consists of crystal-rich,^[23] partially welded pumice deposits,^[84] with individual pumice fragments reaching sizes of 80 centimetres (31 in),^[85] and rare lithics. Some rocks have been altered by outgassing.^[23] Rocks in the lower flow unit contain fewer crystals and more vesicles than in the upper flow unit,^[63] and covers a much smaller area.^[13] The Panizos ignimbrite is one of several "super-eruptions" in the Central Andes; these are giant volcanic events^[87] that exceed the size of all known eruptions of the last 11,700 years.^[88] Ash layers possibly correlated to the Panizos ignimbrite have been found in the Cordillera de la Costa.^[89]

Both units of the Panizos ignimbrite were products of the same eruption.^[90] After an initial Plinian eruption produced an eruption column,^[91] a vent in the southeastern part of the dome complex^[92] produced the lower flow unit. Collapse of the first vent or the opening of a new one caused a break in the eruption; the layer between the units^[90] and the downsag caldera formed at this time.^[93] Activity continued from multiple vents, making the upper flow unit.^[90] The two units originated from different levels of the same magma chamber,^[59] with hotter magma yielding the upper flow unit.^[66] The upper flow unit ponded within the downsag caldera until the second caldera breached its margins, allowing ignimbrites to flow out on the eastern side.^[94]

The caldera was subsequently filled with dacitic lavas^[35] and is no longer a depression.^[37] The flows originated from ring vents in the caldera,^[72] and were later overlaid by the lava dome group.^[95] Collapses at the eastern end of the volcano exposed underlying country rocks,^[23] and hydrothermal activity took place in the central dome complex.^[60] The last volcanic activity was 6.1 million years ago.^[35] Aeolian and fluvial deposits are found in outcrops.^[58]

Notes

References

Sources

Additional sources

• Ort, Michael Harold (1991). Eruptive Dynamics and Magmatic Processes of Cerro Panizos, Central Andes. Santa Barbara: University of California. pp. 52–55. Retrieved 2 December 2015.