Taapaca

Taapaca is a Holocene volcanic complex in northern Chile's Arica y Parinacota Region. Located in the Chilean Andes, it is part of the Central Volcanic Zone of the Andean Volcanic Belt, one of four distinct volcanic chains in South America. The town of Putre lies at the southwestern foot of the volcano.
Like other volcanoes of the Central Volcanic Zone, Taapaca formed from the subduction of the Nazca Plate beneath the South America Plate. It lies on the western margin of the Altiplano high plateau, on top of older volcanic and sedimentary units. Taapaca has mainly erupted dacite, in the form of numerous lava domes, although an andesitic stratovolcano is also present.
Volcanic activity at Taapaca occurred in several stages starting during the Plio-Pleistocene. The emplacement of lava domes was often followed by their collapse and the formation of block-and-ash avalanches, and parts of the volcano underwent sector collapses. It was at first assumed that activity ended during the Pleistocene, but late eruptions occurred until 2,300 years ago; the latest is dated to 320 BCE. The Chilean Geological Service monitors the volcano as it is a hazard to Putre, but eruptions could also impact local roads and areas as far east as Bolivia.

Name

The term tara paka is Aymara for "two-headed eagle" or "winter bird", and Quechua for Andean eagle. It is also known as Nevados de Putre; sometimes "Nevados de Putre" is used to refer to the volcanic complex and "Taapaca" to its highest summit. The term taapaca may also be the origin of the term tarapaca and could be an Aymara name for the founder deity Viracocha. Putre in turn appears to mean "sound of falling water" in Aymara.

Geography and geology

Taapaca lies in the Parinacota province of the Arica y Parinacota Region. Northern Chile has little documented volcanic activity during the last ten thousand years with the majority of eruptions documented at Guallatiri, Lascar and Parinacota. The first and the last of these three volcanoes as well as Taapaca itself are part of the Lauca National Park. Taapaca volcano can be reached from the Tambo Quemado-Arica international road.

Regional setting

Volcanism in the Andes is caused by the subduction of the Nazca Plate and the Antarctic Plate beneath the South America Plate in the Peru-Chile Trench, at rates of and, respectively. The process generates fluids that are ultimately responsible for the evolution of subduction-associated magmas when they interact with the mantle wedge above the downgoing plate.
This subduction does not result in volcanic activity everywhere; in places where the process is shallower there is no recent volcanism. Volcanism has been ongoing in the Andes since about 185 million years ago, with an increase about 27 million years ago when the Farallon Plate broke up. In 1994, the Andes were considered to contain about 178 volcanoes with Holocene activity, of which 60 were further assumed to have been active during historical time.

Local setting

Taapaca is part of the Central Volcanic Zone of the Andes, which along with the Northern Volcanic Zone, the Southern Volcanic Zone and the Austral Volcanic Zone is one of the four volcanic belts of the Andes; these volcanic belts are separated by areas where no recent volcanism has occurred.
The Central Volcanic Zone has about 44 active volcanoes and several other caldera/ignimbrite and volcanic field centres. Older volcanoes are often well preserved owing to the dry climate. This volcanic zone features the highest volcanoes in the world, which reach elevations of here. The largest historical eruption in the Central Volcanic Zone occurred at Huaynaputina in 1600 and Lascar is the most active volcano in the region; otherwise volcanic activity is poorly recorded as most edifices are remote from human habitation.
Taapaca is located on the western margin of the Altiplano, where the Western Cordillera has developed since the Oligocene. The basement beneath the volcano is formed by several principally volcanic formations, including the Lupica and sedimentary Huaylas formations and the Lauca ignimbrite ; this basement is of Oligocene to Pliocene age. In some places, a Proterozoic basement formed by amphibolites, gneisses and serpentinites crops out. The volcanoes of Condoriri, Pomerape, Larancagua and Parinacota lie to the east of Taapaca. The area of the volcano is subject to contractional tectonics, with a major thrust fault passing close by, but their relationship to Taapaca's volcanism is not clear.

Volcano

Taapaca reaches a height of above sea level and is a volcanic complex elongated from west to east; Larancagua lies just east of Taapaca. It consists primarily of many overlapping lava domes, with shapes ranging from almost elliptical to circular. Lava flows are uncommon. The volcano has generated an apron of block-and-ash flow deposits especially on the western, southwestern and eastern flanks, which has filled valleys. An andesitic stratovolcano is also present. The total volume of the edifice is about. Volcanic material covers a surface of.
Just north of the summit, a valley begins and curves clockwise until it opens southwestward onto the flanks of the volcano, and is bordered on the site opposed to the summit by an equally clockwise trending ridge. This valley is drained by the Quebrada Pacollo stream. The main summit is formed by a Holocene dome, with a late Pleistocene dome just west of the main summit.
Taapaca is usually covered by snow, but does not feature glaciers other than block glaciers. Moraines have been described as either poorly or well-developed with six separate stages, of which the lowest lies at elevation on the western slopes. Glacially eroded valleys have also been reported; in the past two glaciers descended Taapaca's western slopes while four glaciers developed between Taapaca and Larancagua, draining south. It is the origin of the Lluta River which has its headwaters on the mountain and subsequently flows through a north-south trending valley west of the volcano. The Quebrada Allane drains the northern flank westward into the Lluta River; parts of the mountain drain into the Rio Lauca watershed to the east. Southwest of Taapaca lies Putre, the main town of the northern Altiplano of Chile.

Composition

Taapaca is mostly formed by potassium-rich dacite, although andesite was erupted early during its activity, and one occurrence of rhyolite is reported. The composition of the rocks has been relatively uniform over the history of the volcano, and is characterized by a calc-alkaline suite of magmas.
Visible minerals found in rocks erupted at Taapaca include amphibole, apatite, biotite, clinopyroxene, orthopyroxene, hornblende, magnetite and hematite, plagioclase, quartz, sanidine which forms characteristic large crystals, and titanite. Dacitic rocks contain mafic inclusions, and such inclusions become increasingly common the younger the rocks they are embedded in are.
The presence of mafic inclusions indicates that magma mixing occurs at Taapaca, with renewed eruptive episodes having been triggered by the injection of new andesitic magma into preexisting dacitic magma chambers. These dacitic magma chambers appear to have relatively small volumes, with little movement of the magma within the chamber except for the episodes where the chambers were heated by new magma injection. Based on geothermometry, a temperature of has been inferred for the dacites. The formation of the magma has been hypothesized to occur in several steps. First, basaltic andesite, which is the typical calc-alkaline volcanic arc basalt, mixes with basalt derived from melting of sub-crustal basalt cumulates; then the resulting mixture interacts with rhyodacite melts derived from Proterozoic crustal material. The initial melt contributes most of the material in mafic inclusions and the rhyodacite contributes most of the dacite material.
The principal magma basin appears to be located at depth, although some petrological traits of the erupted rocks indicate a secondary area of petrogenesis at of depth. Fractional crystallization and partial melting are involved in the formation of Taapaca magmas.
Crustal assimilation at depths of more than was involved in the formation of the dacitic magma and contributes about 18% of the mass of the dacites. Taapaca is constructed on a fairly thick crust and rising magma, and thus undergoes substantial interaction with the crust, meaning that crustal contamination is important in the genesis of Taapaca's magmas. This magma is then transported to shallower levels, where it crystallizes.

Climate and biology

Taapaca lies in a region of tropical alpine climate, with large diurnal temperature fluctuations and frost a possibility during the night throughout the year; temperatures range between. Unlike most of Chile, precipitation occurs mainly during summer, with snowfall occasionally occurring during June and July; however the climate is largely arid, so vegetation is not widespread.
On the southern side of Taapaca, vegetation consists mostly of Puna shrub and steppe, which becomes shrubland farther west. Cushion plants such as the noticeable Azorella compacta and Polylepis woods also occur, along with wetlands known as bofedales. Woodlands were once more common in the region.
Fauna encountered in the region includes birds, flamingos, guanacos, huemuls, rheas, vicuñas and viscachas, along with pumas and rodents, which are fairly common. Much of the area is protected by the Lauca National Park, which includes Taapaca.

Eruptive activity

Taapaca was originally considered to have been active for the last 1.5 million years during three volcanic phases. Later a fourth stage was identified, and some rocks were interpreted as pre-dating 1.5 million years ago. Eruptions at Taapaca have consisted of lava dome-forming eruptions and explosive activity with avalanches of blocks and ash, which form when lava domes collapse, as has been observed in historical time at the Soufriere Hills and Unzen volcanoes, although these events were much smaller than reconstructed episodes at Taapaca. Only one subplinian eruption has occurred on Taapaca, and tephra fallout deposits are not widespread. Eruptive activity has moved south-southwestward over the history of Taapaca, with activity centred at the top of the edifice.
Late Pleistocene to Holocene activity consisted of discrete episodes lasting about 10,000 years and separated by tens of thousands of years with no recognized activity. Aside from actual eruptions, various types of edifice collapse are recorded at Taapaca including sector collapses of segments of the volcano and mass failure of individual domes, which generated block-and-ash flows.