Parinacota (in Hispanicized spelling), Parina Quta or Parinaquta is a dormant stratovolcano on the border of Bolivia and Chile. Together with Pomerape it forms the Nevados de Payachata volcanic chain. Part of the Central Volcanic Zone of the Andes, its summit reaches an elevation of above sea level. The symmetrical cone is capped by a summit crater with widths of or . Farther down on the southern slopes lie three parasitic centres known as the Ajata cones. These cones have generated lava flows. The volcano overlies a platform formed by lava domes and andesitic lava flows.
The volcano started growing during the Pleistocene and formed a large cone. At some point between the Pleistocene and the Holocene, the western flank of the volcano collapsed, generating a giant landslide that spread west and formed a large, hummocky landslide deposit. The avalanche crossed and dammed a previously existing drainage, impounding or enlarging Lake Chungará; numerous other lakes now forming the headwaters of the Rio Lauca sprang up within the deposit. Volcanic activity rebuilt the cone after the collapse, cancelling out the collapse scar.
Parinacota had numerous effusive and explosive eruptions during the Holocene, the latest about 200 years ago. While there are no recorded eruptions, legends of the local Aymara people imply that they may have witnessed one eruption. Renewed activity at Parinacota is possible in the future, although the relatively low population density in the region would limit potential damage. Some towns and a regional highway between Bolivia and Chile are potentially exposed to the effects of a new eruption.
Name
The name "Parinacota" is Aymara. Parina means flamingo and quta lake. Parinacota and its neighbour Pomerape are also known as the Nevados de Payachata, "twins". This refers to the fact that the volcanoes resemble each other.
Geomorphology and geology
Parinacota lies in the Altiplano, a high plateau in the Central Andes. The border between Bolivia and Chile bisects the volcano and runs along the rim of the crater, which lies in Bolivia. In Chile, where most of the edifice is located, Parinacota lies in the commune of Putre, Arica y Parinacota Region, and in Bolivia in the Oruro Department of the Sajama Province. The towns of Ajata and Parinacota lie southwest and west of the volcano, respectively. The region lies at high altitude and access is difficult, hampering research on the volcanoes of the Central Andes.
Regional
The Nazca Plate and Antarctic Plate subduct beneath the South America Plate in the Peru-Chile Trench at a pace of and , respectively, resulting in volcanic activity in the Andes. Present-day volcanism occurs within four discrete belts: The Northern Volcanic Zone (NVZ), the Central Volcanic Zone (CVZ), the Southern Volcanic Zone (SVZ) and the Austral Volcanic Zone (AVZ). These extend between 2°N-5°S, 16°S-28°S, 33°S-46°S and 49°S-55°S, respectively. Between them they contain about 60 active volcanoes and 118 volcanoes which appear to have been active during the Holocene, not including potentially active very large silicic volcanic systems or very small monogenetic ones. These belts of active volcanism occur where the Nazca Plate subducts beneath the South America Plate at a steep angle, while in the volcanically inactive gaps between them the subduction is much shallower; thus there is no asthenosphere between the slab of the subducting plate and the overriding plate in the gaps.
Parinacota is part of the CVZ, which contains about 44 active volcanoes. Most volcanoes of the CVZ are relatively poorly researched and many exceed of elevation. Some of these edifices were active during historical time; these include El Misti, Lascar, San Pedro and Ubinas; the largest historical eruption of the CVZ occurred in 1600 at Huaynaputina. Other volcanoes in the CVZ that have been the subject of research are Galan and Purico complex. The CVZ has a characteristically thick crust () and the volcanic rocks have peculiar oxygen and strontium isotope ratios in comparison to the SVZ and NVZ. Parinacota lies in a segment of the CVZ where the Peru-Chile Trench undergoes a 45° curvature, and where the direction of subduction changes from diagonal to perpendicular. The crust is especially thick there, the reasons for this are not agreed upon yet and may vary between the western and eastern sides of the CVZ.
Subduction-related volcanism in the region has been ongoing since 200 million years ago, burying most of the Precambrian basement. Various units of sedimentary and volcanic origin form most of the outcropping basement in the region. A dramatic increment of volcanic activity occurred approximately 27 million years ago, when the Farallon Plate broke apart and subduction increased substantially. On the Chilean side, the basement is formed by the Oligocene-Miocene Lupica formation, the Miocene Ajoya volcanics, the Lauca formation and the Lauca Ignimbrite. On the Bolivian side the oldest volcanites are the Oligocene Kollukollu formation 34 million years ago and the 23 million years old Rondal Lavas. Miocene volcanic activity generated the Berenguela, Carangas and Mauri formations, followed by the Perez formation during the Pliocene and Pleistocene. These formations were all affected by terrain uplift and folding, probably linked to changes in the subduction regime. Volcanism continued into the late Pleistocene and Holocene (Condoriri 650,000±70,000 and Pomerape between 300,000-100,000 years ago), and was accompanied by glacial activity during the Pleistocene. During this whole time period, volcanic activity progressively migrated westward; presently, it is located on the Bolivia-Chile border.
Local
Parinacota is a highly symmetric volcanic cone, having the classical "regular cone" shape of a stratovolcano. The volcano is or high and features both blocky lava flows and scoria flows. Lava flows are fresh with levees, lobes and flow ridges, and reach lengths of on the slopes of the cone. The lava flows are between thick and can spread to widths of at the foot of the volcano. Pyroclastic flows are also found, reaching lengths of and are usually poorly consolidated, containing breadcrust bombs and breccia.
The volcano is capped by a wide and deep summit crater, which has a pristine appearance. Other data imply a width of and a depth of . The crater is the source of pumice flows, which have well conserved surface features such as levees and lobes especially down on the eastern slope. These pumice flows extend as far as away from the crater. An ashfall deposit spreads east from Parinacota to a distance of in Bolivia. Ash and lapilli deposits have been found at the shores of Lake Chungará as well.
The cone sits atop a thick multilobed andesitic platform known as the "Chungará Andesites" which crop out on the north shore of Lake Chungará in the form of a shelf. Overlying this shelf is a system of lava domes, which reach thicknesses of . The lava domes are accompanied by block and ash flow deposits that reach lengths of . A steep descent leads to Lake Chungará.South of the main edifice lie the parasitic vents known as the Ajata cones, which formed along a fissure that emanates from the main cone and is aligned with the regional Condoriri-Parinacota lineament. The dimensions of the cones reach width and height. The High Ajata flow emanates from a single cone and spreads southwest as a lobated lava flow. The middle Ajata flow is much smaller and is sourced to three different cones below the source of the High Ajata, each cone having its own small flow field. The upper and lower Ajata flows are only slightly smaller than the High Ajata flow and form superposed lava flows lower on the edifice. These lava flows are gray-black aa lava flows, commonly up to thick; the longest of these flows reaches a length of .
Older are the large dacitic lava flows known as the "Border Dacites" on the southeastern side of Parinacota, which are over horizontal distance. A similar but smaller lava flow lies west of the Border Dacites, entirely within Chile. These three lava flows have a total volume of about . Overall, Parinacota rises from a surface of ; the resulting edifice has a volume of
On the northern side Parinacota partly overlaps with Pomerape, which in turn overlies the rocks of Condoriri farther north; together the volcanoes form a north-northeast trending volcano chain. Parinacota, Pomerape, and volcanoes farther south like Quisiquisini, Guallatiri and Poquentica constitute the eastern margin of the Lauca basin. This is a relatively gentle plain drained by the Rio Lauca. A chain of dormant or extinct volcanoes farther west like Taapaca forms the western margin of the basin and separates the Altiplano from the steep dropoff to the Atacama west of the Lauca basin.
Glaciers
The old cone was subject to glaciation, and traces of glacial erosion are preserved on its lava flows. A system of moraines can be seen at an elevation of on the southeastern foot of the volcano, where they partly cross the shores of Lake Chungará. Six such high moraines have been identified there, they were formed during the regional Last Glacial Maximum (which did not coincide with the global last glacial maximum) although a pre-last glacial maximum origin has been proposed. Other, unspecified glacial deposits have also been observed in this area.
Presently, a or large ice cap covers the upper parts of the volcano and drops down to an elevation of about . There is also a large glacier on its southern flank. Some reports disagree with calling any part of Parinacota's ice cap a "glacier", however. Between 1987 and 2016, ice area at Parinacota and Pomerape declined by 1.94% every year. A retreat of was noted between 2002 and 2003, and most of the ice lies on the western slope of the mountain.
Sector collapse
Parinacota shows evidence of a major sector collapse (a giant landslide), whose deposit was originally interpreted to be a lava flow. The collapse removed a volume of about from the cone, plunged over vertical distance and flowed west, covering a surface area of or with debris; the volume is not very well established.
As the volcano grew, it put more and more load on relatively weak sedimentary material that the volcano had developed on, deforming it, until these sedimentary rocks gave way. The western slope might have been weakened by glacial action, further facilitating the onset of the collapse. The collapse was probably sequential from the lower part of the edifice to the summit, and it formed an avalanche of rocks that flowed down the volcano. This flow was probably laminar and extremely fast ( ), judging from the morphologies of the avalanche deposit, and it incorporated substantial pre-collapse sediments from the Lauca basin. As the avalanche descended the slopes of the volcano, it picked up enough speed to run up on some topographical obstacles. Such collapses have occurred on other volcanoes in the CVZ such as Llullaillaco, Ollagüe, Socompa and Tata Sabaya; the most recent event occurred between 1787 and 1802 at Tutupaca in Peru and was much smaller than the Parinacota sector collapse.
The collapse event resembled the one that occurred on Mount St. Helens during the latter's eruption in 1980, although the Parinacota collapse was three times larger. A separate, minor sector collapse occurred on a lava dome on the southwestern foot of the volcano at an unknown time. Such sector collapses are a common phenomenon on volcanoes.