Sabancaya

Sabancaya is an active stratovolcano in the Andes of southern Peru, about northwest of Arequipa. It is considered part of the Central Volcanic Zone of the Andes, one of the three distinct volcanic belts of the Andes. The Central Volcanic Zone includes a number of volcanoes, some of which like Huaynaputina have had large eruptions and others such as Sabancaya and Ubinas have been active in historical time. Sabancaya forms a volcanic complex together with Hualca Hualca to the north and Ampato to the south and has erupted andesite and dacite. It is covered by a small ice cap which leads to a risk of lahars during eruptions.

Sabancaya has generated numerous long lava flows especially during the early Holocene, while activity in the later Holocene has been more explosive. Historical reports indicate eruptions during the 18th century. The volcano returned to activity in 1986, culminating in a large eruption in 1990. Since then it has been continuously active with the emission of ash and gas.

Name

The name "Sabancaya" is Quechua and means tongue of fire or spitting volcano, likely a reference to the eruptive activity. The name is attested from 1595, implying that volcanic activity was observed since that date.

Geography and geomorphology

Sabancaya lies about northwest of Arequipa. The Rio Colca valley is located north of the Sabancaya-Hualca Hualca-Ampato volcano complex. The principal economic activities in the area are agriculture, animal husbandry, mining and tourism.

Regional

The subduction of the Nazca Plate beneath the South American Plate in the Peru-Chile Trench leads to volcanic activity in the Andes. This volcanic activity presently occurs in three segments, the Northern Volcanic Zone, the Central Volcanic Zone and the Southern Volcanic Zone. There is an additional volcanic belt south of the Southern Volcanic Zone, the Austral Volcanic Zone. Sabancaya is located in the Central Volcanic Zone of the Andes, which extends through southern Peru. Many volcanoes in the Central Volcanic Zone are poorly known, owing to their remote locations and adverse conditions such as high altitude.

Sabancaya is part of a series of volcanoes that line the southwestern coast of Peru at a distance of roughly from the shore. Of these volcanoes, Sabancaya, El Misti, Ubinas, Huaynaputina, Ticsani and Tutupaca have been active during historical time. Further volcanoes in the area with Pliocene-Quaternary activity are Sara Sara, Solimana, Coropuna, Ampato, Chachani, Yucamane, Casiri and Tacora. All these volcanoes are considered part of the Central Volcanic Zone of the Andes, and lie east of the Peru-Chile Trench. Notable among them are Ampato and Coropuna for exceeding a height of , Huaynaputina and El Misti for their large eruptions and Ubinas and Sabancaya for their recent activity.

These volcanoes are found in places where strike-slip faults which delimit the volcanic arc and strike along its length intersect additional faults formed by extensional tectonics. Such faults, mainly normal faults, occur around Sabancaya as well and include the Huambo-Cabanaconde, the Huanca, the Ichupampa, the Pampa Sepina, Sepina, Solarpampa and Trigal faults; the volcanoes Ampato and Sabancaya are aligned on this fault, which may thus be responsible for their existence. These fault systems are still active and experience occasional earthquakes and deformation, and their activity appears to be in part triggered by underground magma movements at Sabancaya.

Local

Sabancaya is high and rises above the surrounding terrain. It forms a group of volcanoes with the northern Hualca Hualca and the southern Ampato in the Cordillera Occidental, which tower above the Colca Canyon in the north and the Siguas Valley in the southwest. Ampato and the more heavily eroded Hualca Hualca are the dominant volcanoes of this group, with Sabancaya forming a northeastward extension of the former away from Ampato's summit. There is evidence of age progression from the oldest, Hualca Hualca, over Ampato, to the youngest volcano, Sabancaya.

Sabancaya consists of two separate centres that are formed by neighbouring domes, Sabancaya I North and Sabancaya II South. The summit crater of the volcano lies between these two domes, with traces of an additional crater just northeast. Despite the presence of an ice cap, lava flows are recognizable in the summit area. They have a total volume of .

A set of over 42 Holocene lava flows emanate from the volcano, and cover a surface area of about , with individual lava flows extending up to east and west from between its two neighbours. The lava flows at larger distances are older than the ones close to the vent. These flows are blocky, have lobe structures and reach thicknesses of ; the total thickness of this pile of lava flows is about . Pyroclastic flow deposits are also found, but they might originate from Ampato rather than Sabancaya.

Sabancaya, like its two neighbours, is covered by an ice cap which in 1988 extended to distances of from the summit. In 1997, a surface area of was reported. Between 1986 and 2016 the mountain lost over three quarters of its ice cap, and the remaining ice field broke up into several ice bodies. Moraines at elevations of above sea level testify to the occurrence of more extensive glaciation during the last ice age between 25,000 and 17,000 years before present; these moraines have diverted some lava flows. Younger moraines are found at higher altitudes, above sea level, and may have formed between 13,000 and 10,000 years ago, shortly after the beginning of the Holocene.

Earthquake activity has allowed the identification of a candidate magma reservoir beneath Pampa Sepina northeast of Sabancaya about away from the summit. Between 1992 and 1996 this area inflated at a depth of below sea level, indicating that the magma supply system of Sabancaya may not be centered directly below the volcano. Indeed, a phase of ground uplift at Hualca Hualca volcano and earthquake swarms in 1990 and later seismic activity under Hualca Hualca could indicate that the magma chamber of Sabancaya is actually under the neighbouring volcano, a not uncommon phenomenon at volcanoes.

Geology

The tectonic conditions in the region have not been constant over time; at various times the plates approached each other at higher speed, and this led to a compressional tectonic regimen. In the Western Cordillera however, tensional faulting facilitated the occurrence of voluminous volcanism. This faulting is still underway and produces earthquakes in the area.

The basement of the volcano is formed by Precambrian rocks, which are overlaid by various sediments and volcanic formations of Mesozoic and Cenozoic age. Especially during the Neogene, the supply of volcanic material was high and dominated the region, forming a volcanic "foot"; the present volcanoes are constructed on this volcanic "foot". This "foot" is made out of an ignimbrite plateau that drops down south. The "foot" beneath Ampato, Hualca Hualca and Sabancaya has been dated 2.2 ±0.15 million years ago, while a lava flow beneath the first and the last of these is about 0.8 ±0.04 million years old.

Composition

Fresh volcanites of Sabancaya consist of porphyritic andesite and dacite which form a potassium-rich calc-alkaline suite similar to other volcanoes in southern Peru; the andesites occasionally appear as fine-grained enclaves. The rocks are not very vesicular and contain a moderate amount of phenocrysts. Minerals encountered in both phenocrysts and groundmass are amphibole, biotite, hornblende, iron oxide, plagioclase, pyroxene and titanium oxide; degraded olivine is also found.

The magmas formed at temperatures of with uncertainties of ; the highest temperatures are associated with the 1992 eruption products. Isotope data indicate that the pre-magmas interacted with the crust at great depths before rising to shallower magma chambers. In these shallower magma chambers, magma genesis involved processes of magma mixing which formed at least part of the andesites and fractional crystallization which gave rise to the dacites. Partial crystallization and flow events within the magma chamber caused the formation of the andesite enclaves. The total magma production rate of Sabancaya without accounting for repose periods is about .

Sabancaya is a source of volcanic gases such as and . The amount of water emitted by Sabancaya is noticeably large for a volcano (about); the source of this water might be an evaporating hydrothermal system in the volcano. Together with Ubinas Sabancaya is among the main emitters of and in the Central Volcanic Zone of the Andes and among the top fifteen volcanic emitters on Earth.

Eruptive history

Initially, Ampato volcano was the active volcano before volcanism shifted to Sabancaya, after a period where both volcanoes erupted. Presently, most activity at Sabancaya occurs in the form of lava domes and lava flows, one of which is dated to 5,440 ±40 years before present on the eastern flank. Additional surface exposure dating has yielded ages of 6,650 ±320 and 6,300 ±310 on various lava flows, indicating that effusive activity started shortly after the beginning of the Holocene although the basal part of the edifice did not exist yet at that time. Pyroclastic eruptions are less common and have a low volume, with one such occurrence dated to 8,500 years before present. This tephra layer, along with layers dated 2500-2100 BC, 420–150 BC, 100 BC – 150 AD and between 1200 and 1400 AD, could have originated either on Sabancaya or Ampato, however. There is evidence that early and middle-Holocene Sabancaya mostly erupted lava, while the late-Holocene volcano was more explosive in its activity. It is possible that the Inca performed human sacrifices in response to eruptions of Sabancaya to calm down the mountain spirits.

Spanish chronicles mention probable eruptions in 1752 and 1784, which might have left layers of tephra. After the 18th century, the volcano we…