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Cerro Guacha is a Miocene caldera in southwestern Bolivia's Sur Lípez Province. Part of the volcanic system of the Andes, it is considered to be part of the Central Volcanic Zone (CVZ), one of the three volcanic arcs of the Andes, and its associated Altiplano-Puna volcanic complex (APVC). A number of volcanic calderas occur within the latter. Cerro Guacha and the other volcanoes of that region are formed from the subduction of the Nazca plate beneath the South America plate. Above the subduction zone, the crust is chemically modified and generates large volumes of melts that form the local caldera systems of the APVC. Guacha is constructed over a basement of sediments. Two major ignimbrites, the 5.6-5.8 mya Guacha ignimbrite with a volume of and the 3.5-3.6 mya Tara ignimbrite with a volume of were erupted from Cerro Guacha. More recent activity occurred 1.7 mya and formed a smaller ignimbrite with a volume of . The larger caldera has dimensions of with a rim altitude of . Extended volcanic activity has generated two nested calderas, a number of lava domes and lava flows and a central resurgent dome. == Geography and structure == The caldera was discovered in 1978 thanks to Landsat imagery. It lies in Bolivia next to the Chilean frontier. The terrain is difficult to access being located at altitudes between . The caldera is named after Cerro Guacha, a feature named as such by local topographic maps.〔 Later research by the Geological Service of Bolivia indicated the presence of three welded tuffs.〔 Paleogene red beds and Ordovician sediments form the basement of the caldera.〔 Cerro Guacha is part of the Altiplano-Puna volcanic complex, an area of extensive ignimbrite volcanism in the Central Andes between the Altiplano and the Atacama and associated with the Central Volcanic Zone of the Andes. Several large caldera complexes are found within this area, formed by crustal magma chambers generated by magmas derived from the melting of deep crustal layers. Present day activity is limited to geothermal phenomena in El Tatio, Sol de Manana and Guacha,〔 with recent activity encompassing the extrusion of Quaternary lava domes and flows. Deformation in the area occurs beneath Uturuncu volcano north of the Guacha centre.〔 A westward-facing semicircular scarp () contains subvertically banded Guacha ignimbrite layers rich in lithic clasts and is the presumable vent of the Guacha ignimbrite. The resulting caldera formed like a trapdoor and with a volume of is among the largest known. Volcanic structures are aligned along the eastern moat of this structure, which is filled by lacustrine deposits and welded ignimbrites. Another eastern collapse was generated by the Tara Ignimbrite eruption, with dimensions of .〔〔 The margins of the caldera-graben structure are about high while the caldera floors are about lower. Probably dacitic lava domes are found on the northern caldera rim, with the caldera floor possibly containing lava flows.〔 The caldera contains a resurgent dome, the western part of it is formed by the Tara ignimbrite while the eastern is part of the Guacha ignimbrite. This dome was cut by the Tara collapse, exposing of Guacha ignimbrites. The resurgent dome in the caldera rises about above the caldera floor.〔 A second resurgence episode occurred inside the Tara caldera.〔 The caldera is filled up to thick with ignimbrites. Three lava domes, roughly coeval with the Tara ignimbrite, are constructed on the northern side of the resurgent dome. The western dome is named Chajnantor and is the most silica-rich of the domes. Rio Guacha in the middle is more dacitic. The Puripica Chico lavas on the western side of the caldera are not associated with a collapse.〔 Dark coloured lava flows are found to the southwest of the caldera.〔 Some geothermal activity occurs within the caldera.〔 Laudrum ''et al.'' suggested that the heat from Guacha and Pastos Grandes may be transferred to the El Tatio geothermal system to the west.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cerro Guacha」の詳細全文を読む スポンサード リンク
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