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Chaitén is a volcanic caldera in diameter, west of the elongated ice-capped Michinmahuida volcano and northeast of the town of Chaitén, near the Gulf of Corcovado in southern Chile. The most recent eruptive phase of the volcano erupted on 2008. Originally, radiocarbon dating of older tephra from the volcano suggested that its last previous eruption was in 7420 BC ± 75 years. However, recent studies have found that the volcano is more active than thought. According to the Global Volcanism Program, its last eruption is 1640 ± 18 years. The caldera rim reaches above sea level. Before the current eruption, it was mostly filled by a rhyolite obsidian lava dome that reached a height of , partly devoid of vegetation. Two small lakes occupied the caldera floor on the west and north sides of the lava dome.〔 The translucent grey obsidian which had erupted from the volcano was used by pre-Columbian cultures as a raw material for artifacts and has been found as far away as to the south and north, for example in Chan-Chan.〔 Pino, Mario and Navarro, Rayen. (Geoarqueología del sitio arcaico Chan-Chan 18 ). Revista Geológica de Chile, 2005.〕 ==2008 eruption (May 2, 2008 – November 2008)== The Chaitén volcano entered a new eruptive phase for the first time in about 9,500 years on the morning of May 2, 2008. The Chilean government began an evacuation of the nearby town of Chaitén (population 4,200) and the surrounding area the same day, the main phase of which was completed by May 3, 2008. One elderly person died while at sea en route to Puerto Montt.〔〔 By the afternoon of May 3, the plume of ash from the eruption had spread across Chile and Argentina to the Atlantic Ocean, contaminating water supplies, and reportedly coating the town of Futaleufú located southeast to a depth of .〔 Ash thickness estimates are often exaggerated during volcanic crises; later field investigations suggest that the average ash thickness deposited across Futaleufú was less than 5–10 cm (2–4 in).〔 A team of scientists from the US was dispatched to the area to assess the air quality and the risks from chemicals in the falling ash.〔 The initial phase of the actual eruption in 2008 was characterised by ash emissions and seismic activity; local seismic measurements in 2005 registered earthquakes up to magnitude 3.6 MW below the Chaitén volcano.〔 On May 6, 2008 the force of eruption increased significantly, producing pyroclastic flows and possibly some lava explosions, and raising the eruption column to a height of perhaps . The remaining personnel and almost all inhabitants of Chaitén and nearby villages were evacuated, as was Futaleufú. In the early phase of the eruption (May 2, 2008), two separate vents had developed in the old lava dome. An overflight on 6 May 2008 found that these had fused into one vent roughly across. OVDAS warned of possible major pyroclastic incidents, and the likelihood of prolonged activity. On May 8, 2008 the government said it would force the last residents from the danger area, but this was later legally challenged by some residents and left to no effect by the Supreme Court.〔 Government personnel later returned to attend to livestock and rescue dogs and other animals.〔 Through the remainder of May and June 2008 the eruption continued as a variable but gradually decreasing emission of ash, with intermittent seismic activity and pyroclastic flows. On May 21, a new lava dome was observed to be forming in the crater, which by May 24 exceeded the height of the old dome. Initially, the dome extended towards the north side of the caldera, but following the emergence of two new vents in the south of the old dome around June 11 and a later one to the west, the expansion moved to the south, eventually blocking the drainage from the caldera floor.〔 As of July 3, 2008, Chaitén continued to erupt, with associated seismic activity, an eruptive column of ash up to , and a growing lava dome.〔 Whether the dome will be stable remains uncertain, and there is an ongoing risk of collapse and explosive pyroclastic eruption. In August 2008, an expedition reached the summit of Chaiten volcano. The summit crater contained a high lava dome. Earthquakes were felt at the summit. The lava dome was loudly degassing, and avalanches of lava boulders fell from the dome side to the crater floor. On February 19, 2009, a partial dome collapse caused pyroclastic flows to descend through the Chaitén river valley reaching down to approximately from the town of Chaitén. The ash once again reached Futaleufú and parts of Chubut province in neighboring Argentina. The approximately 160 people that were in Chaitén were strongly urged to leave, and all but 25 people who refused to leave were evacuated that day. 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「Chaitén (volcano)」の詳細全文を読む スポンサード リンク
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