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A tuya is a type of distinctive, flat-topped, steep-sided volcano formed when lava erupts through a thick glacier or ice sheet. They are somewhat rare worldwide, being confined to regions which were covered by glaciers and had active volcanism during the same period. Lava that erupts under a glacier cools very quickly and cannot travel far, so it piles up into a steep-sided hill. If the eruption continues long enough, it either melts all the ice or emerges through the top of the ice and then creates normal-looking basalt flows that make a flat cap on top of the hill. Discovering and dating the lava flows in a tuya has proven useful in reconstructing past glacial ice extents and thicknesses. ==Formation== Tuyas are a type of subglacial volcano that consists of nearly horizontal beds of basaltic lava capping outward-dipping beds of fragmental volcanic rocks, and they often rise in isolation above a surrounding plateau. Tuyas are found throughout Iceland, British Columbia, the Santiam Pass region in Oregon, the Antarctic Peninsula and beneath the West Antarctic Ice Sheet in Antarctica. Tuyas in Iceland are sometimes called table mountains because of their flat tops. S. Holland, a geographer for the British Columbia government, described tuyas in the following way:〔 *''Landforms of British Columbia: A Physiographic Outline'', S. Holland, Govt of BC, 1976, pp. 51–52.〕 :"They have a most interesting origin ... (were ) formed by volcanic eruptions which had been thawed through the Pleistocene ice-sheet by underlying volcanic heat. The lavas capping the mountains were extruded after the volcanoes were built above lake-level, and the outward-dipping beds were formed by the chilling of the lava when it reached the water's edge." Because they erupt under ice and water, tuyas have phreatomagmatic eruptions create layers of breccia and hyaloclastite above of pillow lavas. If the volcano breaches the surface of the glacier it will be topped by a subaerially erupted lava plateau. 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「Tuya」の詳細全文を読む スポンサード リンク
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