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A volcanic dam is a type of natural dam produced directly or indirectly by volcanism, which holds or temporarily restricts the flow of surface water in existing streams, like a man-made dam. There are two main types of volcanic dams, those created by the flow of molten lava, and those created by the primary or secondary deposition of pyroclastic material and debris. This classification generally excludes other, often larger and longer lived dam-type geologic features, separately termed crater lakes, although these volcanic centers may be associated with the source of material for volcanic dams, and the lowest portion of its confining rim may be considered as such a dam, especially if the lake level within the crater is relatively high. Volcanic dams generally occur world-wide, in association with former and active volcanic provinces, and are known to have existed in the geologic record, in historic times and occur in the present day. Their removal or failure is similarly recorded. The longevity, and extent varies widely, having periods ranging from a few days, weeks or years to several hundred thousand years or more, and dimensions ranging from a few meters to hundreds, to several thousand. The emplacement, internal structure, distribution and longevity of such dams can be related variously to the amount, rapidity and duration of (primary) geothermal energy released, and the rock material made available; other considerations include the rock types produced, their physical and toughness characteristics, and their various modes of deposition. Depositional modes include gravity flow of molten lava at the surface, gravity flow or fall of pyroclastics through the air, as well as the redistribution and transportation of those materials by gravity and water. ==Lava dam== Lava dams are formed by lava flowing or spilling into a river valley in sufficient quantity and height to temporarily overcome the explosive nature (steam) of its contact with water, and the erosive force of flowing water to remove it. The latter depends on the quantity of water flow and stream gradient. The lava may flow during numerous successive or repetitive eruptions and may emanate from single or numerous vents or fissures. Lava of this nature, like basalt, is usually associated with less explosive eruptions; more viscous lavas with lower mafic content, like dacites and rhyolites, can also flow, but tend to be more closely associated with eruptions of greater explosiveness and the formation of pyroclastics. Once initially established, continued lava flow creates a steeper upstream face as it battles the rising water, but with most lava flowing unimpeded downstream covering the now-dried river bed and its alluvial sediments, sometimes for miles. Thus emplaced, the shape of a lava dam resembles an elongated blob, wedged in the valley bottom. At the same time, the water continues to flow, the lake continues to rise and accumulate sediment, which previously had migrated unimpeded downstream. Sediment filling, over-topping, downward erosion, waterfalls and under-cutting inevitably follow,〔Jeremy Schmidt, (Grand Canyon National Park: A Natural History Guide ), p.34-37. Houghton Mifflin Harcourt, (1993)〕 unless an alternative outlet is established, for water and sediment elsewhere in the drainage. Large examples of lava dams from the geologic record include those repeatedly developed from the western side of the Grand Canyon, with the largest remnant now termed Prospect Dam,〔 and in several locations within the Snake River drainage. The former 'Lake Idaho', which existed for more than 6.5 million years, filled the western portion of the behind such a structure and created the western section of the Snake River Plain, and accumulated 4000 ft of lake sediments. Other locations include near American Falls, Idaho, and numerous others. Many of these were overtopped, washed out, or skirted by the outburst flood originating from ancestral Lake Bonneville.〔Harold E. Malde, (The Catastrophic Late Pleistocene Bonneville Flood in the Snake River Plain, Idaho )〕 Many other examples exist globally including, Caburgua Lake in Chile, Mývatn in Iceland and Lake Reporoa in New Zealand.〔V. Manville, ''Sedimentology and history of Lake Reporoa: an ephemeral supra-ignimbrite lake, Taupo Volcanic Zone, New Zealand''. In (Volcaniclastic sedimentation in lacustrine settings ), James D. L. White, Nancy R. Riggs, Eds., Wiley-Blackwell, (2001), p.194.〕 Examples in western Canada and others in northwestern United States include, Lava Lake and The Barrier, which still impounds Garibaldi Lake,〔(Catalogue of Canadian volcanoes:Garibaldi Lake volcanic field ) Retrieved on 2007-07-30〕 and Lava Butte. 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「Volcanic dam」の詳細全文を読む スポンサード リンク
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