Ampato

About

Ampato (possibly from Quechua hamp'atu or from Aymara jamp'atu, both meaning "frog") is a dormant 6,288-metre (20,630 ft) stratovolcano in the Andes of southern Peru. It lies about 70–75 kilometres (43–47 mi) northwest of Arequipa and is part of a north-south chain that includes the volcanoes Hualca Hualca and Sabancaya, the last of which has been historically active. Ampato consists of three volcanic cones, which lie on top of an older eroded volcanic edifice. They were formed sequentially by extrusion of lava flows, but Ampato has also had explosive eruptions which have deposited ash, lapilli and pumice in the surrounding landscape. One young lava flow has been dated to 17,000 ± 6,000 years before present, but a summit lava dome is even younger, and Holocene ash layers in surrounding peat bogs may testify to the occurrence of recent eruptions. The present-day volcano is covered by an ice cap, and during the Last Glacial Maximum glaciers advanced to low altitudes. In 1995, an Inca mummy known as Mummy Juanita was discovered on Ampato by Johan Reinhard; it had been offered as a human sacrifice around five hundred years earlier on the mountain.

Ampato lies south of the Colca Canyon and at the southern end of a chain of volcanoes formed by Hualca Hualca and Sabancaya, the last of which has been historically active. Clockwise from northeast the towns of Colihuiri, Cajamarcana, Sallalli, Japo, Baylillas, Corinta and Collpa surround the volcano; the city of Arequipa lies 70–75 kilometres (43–47 mi) to the southeast. Ampato is part of the Central Volcanic Zone of the Andes, which in Peru manifests itself as several dozen Pleistocene volcanoes, some of which erupted in historical time including El Misti, Huaynaputina, Sabancaya and Ubinas. The largest historical eruption of the Andes took place at Huaynaputina. Other volcanoes in the Peruvian Central Volcanic Zone are Sara Sara, Solimana, Coropuna, Andagua volcanic field-Huambo volcanic field, Chachani, Ticsani, Tutupaca, Yucamane, Purupuruni and Casiri.

The volcano Ampato consists of three individual steep-sided cones which rise from a gentle glacially eroded foot. These three cones are lined up in southwest-northeast direction and the highest one reaches an elevation of 6,280 metres (20,600 ft) or 6,288 metres (20,630 ft). Ampato is one of the highest volcanoes in the Central Volcanic Zone and the 35th highest summit in the Andes. The volume of the edifice is about 38–42 cubic kilometres (9.1–10.1 cu mi), it covers an area of about 90–100 square kilometres (35–39 sq mi). The summit of the volcano is covered with an ice cap, and the edifice is incised by cirques and glacial valleys. The volcano is surrounded by three sets of moraines, the lowermost one at 4,250–4,450 metres (13,940–14,600 ft) elevation has been attributed to the Last Glacial Maximum between 25,000-17,000 years ago, the middle one between 4,400–4,650 metres (14,440–15,260 ft) to a late readvance at the Pleistocene-Holocene epoch boundary and the higher ones above 4,800 metres (15,700 ft) to Holocene advances.

Off the western coast of South America, the Nazca Plate subducts beneath the South America Plate at a rate of 5–6 centimetres per year (2.0–2.4 in/year). This subduction process is responsible for volcanism of the Central Volcanic Zone (CVZ) in Peru, Bolivia and Chile. The CVZ is one of the four volcanic belts in the Andean Volcanic Belt; the others are the Northern Volcanic Zone, the Southern Volcanic Zone and the Austral Volcanic Zone. The subduction started during the Jurassic period after the opening of the southern Atlantic Ocean, which triggered the onset of subduction of the Nazca Plate. Volcanic arc-associated volcanism originally occurred within the Cordillera de la Costa in the Jurassic, but later it migrated resulting in the emplacement of the Tacaza and Toquepala groups and finally the Neogene Barroso group. The present-day volcanic arc is situated in the area of the Barroso group but has a narrower extent. The pre-volcanic basement consists of 1.9-1.0-billion-year-old rocks of the "Arequipa Massif", which consists of gneiss and granulite. They are covered by the Yura Group of the Mesozoic and the Mesozoic-Paleogene Tiabaya unit; the former consists of marine sediments and the latter of volcanic sediments intruded by plutons. The Tacaza and Barroso groups are emplaced on the sediments. The basement beneath Ampato is formed by sedimentary and volcanic rocks of the Western Cordillera of Peru, and the rocks are of Mesozoic to Cenozoic age. A high plateau formed by ignimbrites and lavas of Pliocene to Miocene age rises above this basement. The terrain is cut by several different fault systems; one of these, the northeastward striking Sepina fault has been seismically active in the 20th and 21st centuries and seems to have controlled the development of the Ampato and Sabancaya volcanoes.

Adapted from Wikipedia (CC BY-SA). Photography via Wikimedia Commons.