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subglacial volcano
A subglacial volcano, also known as a glaciovolcano, is a volcanic form produced by subglacial eruptions or eruptions beneath the surface of a glacier or ice sheet which is then melted into a lake by the rising lava. Today they are most common in Iceland and Antarctica; older formations of this type are found also in British Columbia and Yukon Territory, Canada.
During the eruption, the heat of the lava from the subglacial volcano melts the overlying ice. The water quickly cools the lava, resulting in pillow lava shapes similar to those of underwater volcanoes. When the pillow lavas break off and roll down the volcano slopes, pillow breccia, tuff breccia, and hyaloclastite form. The meltwater may be released from below the ice as happened in Iceland in 1996 when the Grímsvötn caldera erupted, melting 3 km3 of ice and giving rise to a large glacial lake outburst flood.
The shape of subglacial volcanoes tends to be quite characteristic and unusual, with a flattened top and steep sides supported against collapse by the pressure of the surrounding ice and meltwater. If the volcano eventually melts completely through the ice layer, then horizontal lava flows are deposited, and the top of the volcano assumes a nearly level form. However, if significant amounts of lava are later erupted subaerially, then the volcano may assume a more conventional shape. In Canada the volcanos have been known to form both conical and nearly level shapes.〔http://gsc.nrcan.gc.ca/volcanoes/type_e.php〕 The more distinctly flat-topped, steep-sided subglacial volcanoes are called tuyas, named after Tuya Butte in northern British Columbia by Canadian geologist Bill Mathews in 1947. In Iceland, such volcanoes are also known as table mountains.
==Jökulhlaups==
Subglacial eruptions often cause jökulhlaups or great floods of water. In November 1996 the Grímsvötn Volcano beneath the Vatnajökull ice sheet erupted and caused a Jökulhlaup that affected more than and destroyed or severely damaged several bridges.〔http://www.accessscience.com.ezproxy.spl.org:2048/content.aspx?searchStr=subglacial+volcano&id=735200〕 During the ice ages, such floods from Lake Missoula were estimated to have discharges exceeding 17 × 106 m³/s (4.5 × 109 gal/s) and covered a third of eastern Washington state.〔http://www.accessscience.com.ezproxy.spl.org:2048/content.aspx?id=290400〕 Sonia Esperanca, program director in the National Science Foundation commented on the danger of subglacial volcanos: "When an ice-covered volcano erupts, the interplay among molten magma, ice and meltwater can have catastrophic results."〔http://geology.com/press-release/glaciovolcanoes/〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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