Cerro Panizos

Cerro Panizos is a late Miocene-age shield-shaped volcano spanning parts of Bolivia and Argentina. It features two calderas (a depression formed by the collapse of a volcano) and a group of lava domes in the Potosi Department of Bolivia and the Jujuy Province of Argentina. It is part of the Andean Central Volcanic Zone (CVZ) and the Altiplano-Puna volcanic complex (APVC), a group of calderas and associated ignimbrites (a form of volcanic rock) that erupted during the past ten million years. Incapillo is one of several ignimbrite or caldera systems that, along with 44 active stratovolcanoes, are part of the CVZ.

Subduction of the Nazca Plate beneath South America is responsible for most of the volcanism in the CVZ. After activity commenced in the APVC about ten million years ago, it produced the large volcanic calderas Panizos, Vilama, Cerro Guacha and the volcano Uturuncu; the last shows evidence of ongoing activity. The formation of the APVC has been linked to the existence of a giant magmatic body in the crust of the Central Andes.

Panizos is the source of two major ignimbrites, the older Cienago Ignimbrite and the more recent Panizos Ignimbrite. The former was erupted about 7.9 million years ago and the second 6.7 million years ago. The Panizos Ignimbrite has a total volume exceeding 650 km3, and has been noted for volcanic rocks containing orbs. Several volcanic cones such as Limitayoc were active between the ignimbrite eruptions, and a plateau of lava flows and lava domes formed in the central area of the Panizos Ignimbrite after the last eruptions.

Geography and geomorphology
Cerro Panizos lies in the Cordillera de Lipez mountain range of the Andean Altiplano-Puna high plateau. The volcano is a 40 km, gently sloping ignimbrite shield surrounding a 10 - 15 km lava dome semicircle. Cerro Panizos proper is a 5228 m, 5360 m or 5494 m lava dome in the southeastern semicircle. The other domes are Cerro Cuevas, Cerro Crucesnioc/Crucesnioj/El Volcán, Cerro Vicunahuasi west and 5540 m Cerro La Ramada/Cerro Ramada north of Cerro Panizos.

Two calderas have been identified at Cerro Panizos. The larger one was formed by downward sagging of the ground and its centre is south of the lava dome semicircle. A smaller collapse caldera – outlined by the lava domes – is nested within the larger caldera. Cerro Anta Cuevas, Cerro Chinchijaran, Cerro Limitayoc/Limitayo and Cerro Tucunquis are lava plateaus that rise from the ignimbrite shield. The 5158 m Limitayoc formed along a north-south trending fault and has an elongated shape, with traces of hydrothermal alteration at its northern end.

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

Hydrology and human geography & history
Several (often ephemeral) streams cut into the rocks of the shield to form a radial drainage system. The streams run mostly to the east; from north to south they are Rio Khuchu Mayu/Khuchumayu, Quebrada Buenos Aires, Quebrada Cienago, Quebrada Paicone, Quebrada Potrero, Quebrada Guanapata, Quebrada Pupusayo/Pupusayoc, Quebrada Cusi Cusi, Quebrada Cueva, Quebrada Garcia/Quebrada de Garcia and Quebrada Quenoal. Most of them eventually join the San Juan de Oro River, which flows into the Atlantic Ocean. Panizos can be accessed through the valleys of these streams. Small lakes dot the southwestern sector of the shield, and there are ephemeral lakes on its southeastern side. The mountain Cerro San Matias borders Panizos to the north, Cerro Lipez northwest and Corutu southwest of Panizos, while the San Juan de Oro depression is east of the volcano.

The region is remote and inhospitable. 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). The border between Argentina and Bolivia runs along the domes. The towns of Cusi Cusi and San Antonio de Esmoruca/San Antonio de Esmoruco are southeast and north of the volcano, respectively. There are several archeological sites on Panizos, and a branch of the Inca road system passed over the volcano. Most of the central domes were first climbed in November 1939 but the volcano itself was only identified as such in 1977, thanks to images from the Landsat satellite.

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 mm per year and only sparse cloud cover. The day-night temperature contrast is high, and most days of the year have frosts. The only vegetation consists of cushion plants, grasses and shrubs. Wetter areas feature wetlands (such as bofedales), and there are salt flats. Animals include the large guanacos, llamas, tarucas and vicuñas, 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.

Geology
Off the western coast of South America, the Nazca and Antarctic Plates subduct underneath South America. The subduction is responsible for the volcanism of the Andean Volcanic Belt, which is subdivided into four volcanic segments; the Northern (NVZ), Central (CVZ), Southern (SVZ) and Austral (AVZ) volcanic zones. The CVZ consists of two parts: a volcanic arc with stratovolcanoes reaching 6000 m elevation, including the highest volcano in the world, Ojos del Salado; and numerous large calderas in the main arc and farther east, which produced the largest volume of Neogene-Quaternary volcanic rocks in the Andes. About 44 volcanoes in the CVZ were active in historical time, Lascar being the most active of them.

The largest assembly of volcanoes in the CVZ is the 70000 km2 Altiplano-Puna volcanic complex (APVC), a system of calderas and ignimbrites that were active in the Altiplano-Puna high plateau during the Miocene. With a volume exceeding 15000 km3, it is one of the largest ignimbrite provinces on Earth. Known calderas in the APVC are Cerro Guacha, Cerro Panizos, Coranzulí, Kapina, La Pacana, Pastos Grandes and Vilama, but other buried calderas may exist and only a few of these volcanoes have been studied in detail. Within the crust under the APVC is the Altiplano-Puna Magma Body, a giant pile of rock-magma mush at 9 - 31 km depth that extends under southern Bolivia, northeastern Chile and northwestern Argentina. It has a volume of 500000 km3. The southern Bolivian tin belt overlaps with the APVC, and Panizos is the easternmost APVC volcano.

The basement is formed by volcanic, sedimentary and crystalline rocks, which have ages ranging from Paleozoic (Acoite Formation) to Cenozoic (Peñas Coloradas/Peña Colorada or Tiomayo Formations). The San Juan de Oro erosion surface forms the surface on which later volcanic rocks were emplaced. Two older ignimbrites underlie the Panizos centre, one of which originated at Corutu. The crust is 70 km thick and up to a billion years old, but it reached its present-day thickness only during the late Cenozoic. 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; one northeast-–southwest trending line may have influenced the formation of Panizos, Vilama and Cerro Guacha, and smaller scale structures at Panizos may reflect north-south and east-southeast–west-northwest trending lineaments. There is evidence of faulting, both before and after the eruption of the Panizos ignimbrite.

Geochronology
Volcanic activity in the region began during the Jurassic in the Cordillera de la Costa and has migrated eastward since then. 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. Volcanic activity shifted east into the Puna until the Pliocene, after which it returned to the main arc where it persists to this day. Numerous ignimbrites were emplaced between 25 and 1 million years ago, with the bulk dating from the late Miocene to Pliocene. Volcanic activity was episodic, with several recognizable "flare-ups" during which volcanic activity increased about 10, 8, 6 and 4 million years ago. Each of these flare-ups is associated with numerous ignimbrites: The first with the Artola, San Antonio, Lower Rio San Pedro, Divisoco, Granada, Pairique and Coyaguayma; the second with the Sifon and Vilama; the third with the Panizos, Coranzuli, Toconce, Pujsa, Guacha, Chuhuilla, Carcote and Alota; and the fourth with the Atana-Toconao, Tara and Puripicar Ignimbrites. Sometimes the first and the second stages are considered together. 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.

Volcanism declined during the past 4 million years, yielding smaller ignimbrites such as the Patao, Talabre-Pampa Chamaca, Laguna Colorada, Puripica Chico, Purico, Tatio, Filo Delgado and Tuyajto. The last eruptions took place 271,000 and 85,000 years ago at Uturuncu and the Cerro Chascon-Runtu Jarita complex, respectively. During the 21st century, ongoing uplift was discovered at Uturuncu.

Composition
The volcano has erupted dacite, which contains numerous crystals and has a homogeneous composition; andesites are subordinate. Both rocks constitute a peraluminous potassium-rich calc-alkaline suite. Phenocrysts include biotite and plagioclase, while apatite and zircon form accessory phases; orthopyroxene, quartz and sanidine are less common and clinopyroxene, hercynite, hornblende, hypersthene, ilmenite and magnetite are rare. Many of the rarer minerals are xenoliths derived from the crust. Gold and silver deposits are found on the volcano, and an occurrence of antimony-copper-uranium has been described at Paicone.

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

Oval orbs formed by concentric layers of crystals around a core have been found at Panizos. They make up less than one percent of Panizos rocks and only occur in the ignimbrites east and southeast of the volcano. 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. Large (up to 2 cm) phenocrysts co-occur with orbs. 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. Only a few volcanoes in the world have such orbs, probably because they require special conditions to form.

Eruption history
Panizos was active in the late Miocene, although early Miocene rocks north of Panizos and the 12.4-million-years-old Cusi Cusi ignimbrite may also be part of it. Panizos was active at the same time as Coranzulí and Vilama-Coruto. It is the source of two major ignimbrites: The first (Cienago or Panizos I) was erupted 7.9 million years ago and forms two flow units with a total volume exceeding 300 km3, each underlaid by pyroclastic fallout deposits several centimetres thick. The ignimbrite contains a high proportion of pumice. It might be the first eruption of Panizos. Afterwards, lava domes erupted on the southern side of the volcano, including Cerro Limitayoc, where activity continued after the Panizos ignimbrite.

The second ignimbrite, the more than 650 km2 Panizos (or Panizos II) Ignimbrite erupted from the volcano 6.7 million years ago. It was emplaced as two flow units, which are separated by multiple base surge, pyroclastic flow and volcanic ash deposits that reach thicknesses of several metres. A 1 m thick layer of lapilli underlies the ignimbrite. The Panizos ignimbrite forms the shield around the central dome complex, reaching as far as the Rio Granada-San Juan de Oro valley east of the volcano. The ignimbrite has a maximum thickness of about a few hundred metres, mostly around the central dome complex and where it filled in the pre-existent topography, forming thick deposits within valleys. The ignimbrite was not very mobile.

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

Both units of the Panizos Ignimbrite were products of the same eruption. After an initial Plinian eruption produced an eruption column, a vent in the southeastern part of the dome complex 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 and the downsag caldera formed at this time. Activity continued from multiple vents, making the upper flow unit. The two units originated from different levels of the same magma chamber, with hotter magma yielding the upper flow unit. 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.

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