Ubinas

Ubinas is an active stratovolcano in the Moquegua Region of southern Peru, approximately east of the city of Arequipa. Part of the Central Volcanic Zone of the Andes, it rises above sea level. The volcano's summit is cut by a and caldera, which itself contains a smaller crater. Below the summit, Ubinas has the shape of an upwards-steepening cone with a prominent notch on the southern side. The gently sloping lower part of the volcano is also known as Ubinas I and the steeper upper part as Ubinas II; they represent different stages in the volcano's geological history.

The most active volcano in Peru, Ubinas has a history of small to moderate explosive eruptions as well as a few larger eruptions, such as in 1667, along with persistent degassing and ash emissions. Activity at the volcano began in the Pleistocene epoch, and led to the growth of the current mountain in two phases. Among the recent eruptions was the 2006–2007 event, which produced eruption columns and led to ash fall in the region, resulting in health issues and evacuations. During the most recent activity, from 2013 to 2017, a lava flow formed inside the crater, and further ash falls led to renewed evacuations of surrounding towns. Ubinas is monitored by the Peruvian geological service INGEMMET, which has published a volcano hazard map for Ubinas and regular volcanic activity reports.

Name and mythology

The historian and geographer Mariano Felipe Paz Soldán relates the name Ubinas to two terms in two different languages. In the indigenous language Quechua, means "to stuff" or "to fill", and is translated as "to grow" or "to increase". In Aymara, means "weep" or "murmur"; is the genitive of . Local inhabitants believed that Ubinas was infested by demons and the souls of people who had fallen from God. The volcano is also known as Uvillas or Uvinas.

Geography and structure

Ubinas lies in the Ubinas District of the General Sánchez Cerro Province, Moquegua Region of Peru, east of Arequipa in the Peruvian Western Cordillera.

Like other Peruvian volcanoes, Ubinas belongs to the Central Volcanic Zone of the Andes. The Central Volcanic Zone is one of four volcanic belts in the Andes; the others are the Northern Volcanic Zone, the Southern Volcanic Zone, and the Austral Volcanic Zone. The Central Volcanic Zone is long, and 69 of its volcanoes have been active in the Holocene epoch.

Peruvian volcanoes include stratovolcanoes, which are typically active for less than 500,000 years, long-lived clusters of lava domes, and monogenetic volcanic fields. Historical eruptions have been recorded at seven Peruvian volcanoes: El Misti, Huaynaputina, Sabancaya, Ticsani, Tutupaca, Ubinas, and Yucamane. The volcanoes Ampato, Casiri, Chachani, Coropuna, and Sara Sara are dormant.

Reaching an elevation of , Ubinas is a conical, truncated stratovolcano with upper slopes that reach angles of up to 45 degrees, and more gently sloping lower flanks. The more gently sloping lower part of the volcano is also known as Ubinas I and the steeper upper part as Ubinas II. The southern flank is cut by a noticeable notch, which is probably not an eruption vent and may have been formed by mudflows and rockslides. Due to weathering, the upper sector of the volcano has a worn appearance. Glacial valleys such as the Ubinas and Para valleys, as well as cirques and moraines down to and at the foot of the volcano, indicate that glaciers developed on Ubinas during the last glacial maximum. Other volcanic cones in the region all show heavy erosion by glaciation.

The volcano rises from a circular surface at the margin of a high plateau. Volcanic ash and some lava flows cover the terrain north and east of Ubinas. Four lava domes crop out around the volcano and may be related to it. The Ubinas and Para valleys border the volcano in its southeastern sector; the difference in elevation between the floor of the Ubinas valley and the plateau is about . The total volume of the mountain is estimated to be about .

The summit of the volcano is an elliptical caldera wide and deep, formed by collapses of the summit and explosive eruptions. The caldera walls are made of lava flows bearing traces of hydrothermal alteration; the caldera floor is covered by lava flows and pyroclastic debris from explosive eruptions. It contains one or two ash cones with a triangle-shaped crater wide and deep; its walls are fractured and hydrothermally altered. Geophysical surveys have indicated the presence of an even larger buried caldera in Ubinas.

A debris avalanche on the southeastern flank reached a distance of from the volcano, and left a collapse scar that is drained by the Volcanmayo River. This collapse took place early in the history of the volcano and removed a volume of about of rock from the mountain and underlying basement. Further collapses have occurred throughout the history of the volcano and into the Holocene epoch, including one collapse that left a hummocky deposit on the southern flank. The sloping terrain that Ubinas is built upon predisposes the mountain to south-directed landslides; future collapses in that direction are possible, with the heavily fractured southern flank of the caldera particularly at risk.

Hydrology and human geography

In the 1970s, an ephemeral crater lake appeared in the crater after wet seasons; another lake formed in 2016 after the crater floor was covered by the ongoing eruptions with impermeable material. Acid springs occur in the crater, and their water is capable of corroding silicon after a few hours' exposure. Lake Piscococha is located on the volcano's western foot and during December–April receives meltwater from its slopes, while the Para River and Sacuaya River flow past its eastern and southern slopes, respectively. Other rivers on the slopes of Ubinas are the Quebrada Infiernillo on the southeastern, Volcanmayo River on the southern and Quebrada Postcone on the southwestern flank. The Sacuaya River becomes the Ubinas River and after confluence with the Para ends in the Tambo River which eventually flows into the Pacific Ocean; the Ubinas River valley is densely inhabited.

Ubinas lies in the Salinas y Aguada Blanca National Reserve of Peru, which was founded in 1979. The town of Ubinas and the villages of Querapi, Tonohaya, Ubinas and Viscachani lie southeast, south, southeast and northwest of the volcano, respectively, and other villages in the area include Anascapa, Escacha, Huarina, Huatahua, Sacuaya, San Miguel and Tonohaya. In total about 5,000 people live within from the volcano, with Querapi being only away from Ubinas and thus the closest town to it. Agriculture and animal husbandry are the most important economic activities in these towns, agriculture prevailing at lower elevations. Water reservoirs and mining projects also exist in the wider region. Paved roads run along the northern and southern-southwestern foot of Ubinas, connecting towns close to the volcano to Arequipa and allowing access to the volcano over its western flank.

Geology

Off the western coast of South America, the Nazca Plate subducts beneath the South American Plate at a rate of in the Peru-Chile Trench. This subduction process is responsible for the formation of the Andes and the Altiplano- Puna plateau within the last 25 million years, as well as for volcanism and earthquakes. The magma erupted by the volcanoes is formed by the partial melting of the mantle after fluids originating in the downgoing slab have altered the mantle; the magmas often undergo fractional crystallization and absorb crustal material.

Southern Peru has been affected by volcanic activity since the Ordovician and the Permian-Jurassic period, subduction-related volcanism becoming important from the Cretaceous onwards. Beginning 91 million years ago, several volcanic arcs have been active in southern Peru: from the Toquepala arc 91 – 45 million years ago over the Andahuaylas-Anta 45–30 million years ago, the Huaylillas 24–10 million years ago, the two Barroso arcs 10–1 million years ago, to the recent arc in the last million years. The switching between the volcanic arcs was accompanied by northeastward or southwestward shifts of the zone of main volcanic activity. Furthermore, there was little relief in the region before about 45 million years ago when major uplift commenced.

Local setting

Ubinas, Ticsani and Huaynaputina form a group of volcanoes that extend in north-south direction north of the chain of volcanoes that make up the rest of the Central Volcanic Zone. These volcanoes have erupted rocks with similar geochemical traits and they are located around a graben occupied by the Rio Tambo; the marginal faults of this graben are the sites of the volcanic vents and probably acted as magma conduits. The magmas erupted by all three volcanoes appear to originate in a common magma chamber at depth, with seismic activity localized along the margins of the chamber. Aside from this deep reservoir, Ubinas also has a shallower magma chamber at depth which appears to have a small size. An underground magmatic connection between Ubinas and Huaynaputina was postulated already by Antonio Vázquez de Espinosa after the 1600 eruption of the latter volcano, which was the largest historical eruption in the Andes and had an enormous impact, including causing a cold summer in the northern hemisphere.

The basement of Ubinas consists of volcanic and sedimentary rocks. The sedimentary rocks include the Jurassic Chocholate Formation, the Socosani Formation and the Yura Group. The oldest volcanic rocks of the Matalaque Volcanics date to the Late Cretaceous and crop out east and southeast of Ubinas, far away from the volcano. Most of the volcanics in proximity to Ubinas are the younger, Eocene-to- Oligocene Tacaza Group and the more restricted Miocene-Pliocene

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