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Chachani is a volcanic group in southern Peru, northwest of the city of Arequipa. Part of the Central Volcanic Zone of the Andes, it is above sea level. It consists of several and individual volcanoes such as Nocarane, along with lava shields such as the Airport Domes. Underneath Chachani lies a caldera.
During the Pliocene and early Pleistocene, the volcanic group produced large such as the La Joya, Arequipa Airport, and Yura Tuff ignimbrites; afterwards, the volcanic group proper grew in the caldera until about 56,500 years ago. There have been no eruptions during historical times. Still, the volcano is considered to be only dormant, and due to its closeness to the city of Arequipa, it is considered high-risk.
Volcanoes in the southern part of Peru include from north to south Auquihuato, Firura, Coropuna, Andagua volcanic field, Sabancaya, Ampato, Chachani, El Misti, Pichu Pichu, Ubinas which has erupted intermittently since 1954, Huaynaputina where a large eruption occurred in 1600, Ticsani, Tutupaca, Yucamane, and Casiri. Some of these volcanoes are among the highest in the world,, and on average, there is one eruption of a Peruvian volcano every 13 years. The Peruvian volcanoes are part of the Central Volcanic Zone of the Andes, one of three separate volcanic belts in that mountain chain; the Central Volcanic Zone contains 44 named .
Chachani is an about and complex of , , and ; the highest summit is high, making Chachani the 84th-highest peak in the Andes. The Chachani complex has an arcuate shape encompassing both the main Chachani volcano and the Nocarane to the north of Chachani, while the La Horqueta together with El Rodado to its west and Chachani proper to its east forms an east–west-trending ridge. La Horqueta has a young appearance and resembles an ash cone with a Volcanic crater. Additional peaks are the northerly high los Ángeles and the southeasterly Trigo. In total, Chachani is made up of more than 12 edifices.
The Colorado which are also known as Cerro Penones in turn are located northwest from Nocarane. To the south of Chachani lie the Airport Domes, an lava shield with two discernible vents and a pristine appearance. The lava shield consists of overlapping lava flows with a wavy and rugose texture and a steep front which reaches heights of ; evidently these were formed by viscous lava flows. A volcanic caldera associated with widespread in the Arequipa area may be located underneath Chachani; to the north its outline is marked by a amphitheatre while its southern part is continuous with the Arequipa depression and is generally poorly recognizable. The El Misti volcano later arose on the margin of the Chachani caldera, where it intersects the border of the Altiplano.
The volcanic complex is formed mainly by aa lava and block lava that rarely reach lengths of about ; additionally, and tephra occur. The volcanic complex covers an area of about and has a present-day volume of about ; this makes Chachani one of the largest volcanoes of the Andes. Glacial erosion and landslides have affected the volcanic complex, forming and and removing much of the original shape of the individual volcanoes. Some moraines have been overrun by lava flows.
Chachani drains into the Rio Chili. The Quebrada Canchero, Quebrada Cabrería and Quebrada Traccra drain south to eastward into the river, which flows around the southeastern side of Chachani and has cut a canyon between Chachani and El Misti. The Rio Sumbay, one of its tributaries, runs along the eastern side of Chachani. The Rio Yura flows southward along the western side of Chachani, and upon reaching the then westward-flowing Rio Chili becomes the Rio Vitor, which eventually discharges into the Pacific Ocean together with the Rio Siguas.
Permafrost and however still exist at Chachani, especially close to Nocarane, and reach lengths of ; they are characterized by lobate appearances and their locations at the feet of tall . Some of them (about six in total) are still active and are located above elevation, while the lowest inactive ones end at elevation. Permafrost is expected to be continuous above with discontinuous occurrence above . Other periglacial phenomena such as needle ice are also encountered on the volcanic complex.
Volcanic activity in the Chachani region appears to have begun during the Cretaceous-Paleocene in the form of the "Toquepala" volcanics. The oldest volcanic rocks of the Western Cordillera are known under the name "Tacaza", and underwent erosion and folding before the next phase which is known as "Sillapaca". Finally, during the Miocene-Quaternary, the Sencca Formation and the "Barroso" volcanics developed; Chachani is classified as part of the Barroso volcanics although the oldest volcanism might belong to the Sillapaca unit.
The terrain of the volcano is formed by volcanic rocks of Eocene-to-recent age, which overlie a Precambrian basement and include widespread Neogene-Quaternary . Chachani straddles the margin between the higher-elevation Altiplano and the Arequipa basin at lower altitude.
The younger El Misti volcano lies southeast of Chachani, across the Rio Chili. Other volcanoes in the region are Ampato and Jollojello northwest, Baquetane, Hucullani and Nevado Calcha north, Yanarico east, and Pichu Pichu southeast from Chachani. Of these, Misti, Pichu Pichu, and Chachani are aligned on that trend northwest–southeast and have been active during the Holocene.
Chachani has erupted andesite and dacite, which define a potassium-rich calc-alkaline suite with unusual adakite characteristics; adakites are magmas that form when the downgoing plate in a subduction setting melts. include augite, biotite, hornblende, and hypersthene; the of the Arequipa ignimbrites additionally contain ilmenite, magnetite, plagioclase, quartz and sanidine. The composition of the volcanic rocks has varied over the lifespan of Chachani; sometimes rocks of basaltic andesite composition were erupted as well while the younger volcanoes are usually more homogeneous; this was paralleled by a decrease in the eruption rate.
From elevation, cacti, , Peruvian feather grass, yareta, , and grow on the slopes of Chachani and other regional volcanoes. Vegetation is scarce to absent above elevation, and much of its lower slopes were stripped of their vegetation during the colonial era. The dry soils are not suitable for agriculture. Part of the volcano is in the Salinas y Aguada Blanca National Reserve. Lizards on the mountain are the highest-altitude known reptile population in the world.
They occur in the wider region of Chachani and crop out in the Arequipa area and in river valleys. Their emplacement took place in the form of fast-moving hot streams of rock. An additional ignimbrite is the 13-million-year-old Miocene Rio Chili Ignimbrite; the vents of the Miocene units are unknown and the Rio Chili Ignimbrite appears to be related to the Huaylillas Ignimbrite of southern Peru.
Later, during the Quaternary, the Chachani volcanic complex grew in the caldera. The older activity formed the older volcanoes Chingana northeast, Estribo east, and Nocarane north of Chachani, as well as the Colorado north-northwest from Nocarane and the volcanoes Chachani Base and El Angel; argon-argon dating has yielded ages of 1,000,000–500,000 years ago for these volcanoes. Later, the volcanoes El Rodado, La Horqueta and Chachani ( 130,000–131,000 years ago) formed as a west–east alignment, along with the Uyupampa lava field (about 230,000–280,000 years ago) far west from Chachani, the Airport Domes south (between 290,000–400,000 years ago), the Cabrería lava domes south-southeast, and the Volcancillo dome northeast from Chachani.
The youngest reported date of 56,500 ± 31,600 years ago has been obtained on the Cabrería lava domes; formerly the Airport Domes/Los Angeles/Pampa de Palacio were considered to be the youngest and of Holocene age. However, Volcancillo may be even younger, and a post-glacial lava flow descends the western slope of Chachani between Chachani and Nocarane, and phreatic eruptions may have formed Holocene-age craters on Chachani. No historical eruptions are known at the volcanic complex, and the date of the last activity is likewise unclear.
The existence of in the summit region and of at Socosani and Yura has been reported, and frequent seismic activity occurs on Chachani's southwestern flank; this activity may relate to either geothermal or tectonic phenomena. Currently, the volcano is considered to be dormant volcano. Occasionally, Mudflow descend their slopes and cause damage at lower elevations.
Chachani is considered to be a high-risk volcano, and the Geophysical Institute of Peru in 2018 installed a monitoring station on Chachani. Several volcano hazard maps exist: One shows three levels of danger depending on the distance from the edifice, the other two show areas threatened by and pyroclastic fall. Chachani is monitored with and . Apart from eruptions, volcano-induced pollution of neighbouring water resources is a concern at Chachani.
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