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Lake Taupo, in the centre of New Zealand’s North Island, is the caldera of a large rhyolitic volcano called the Taupo Volcano. This huge volcano has produced two of the world’s most violent eruptions in geologically recent times. The Taupo Volcano forms part of the Taupo Volcanic Zone, a region of volcanic activity that extends from Ruapehu in the South, through the Taupo and Rotorua regions, to White Island, in the Bay of Plenty. Taupo began erupting about 300,000 years ago, but the main eruptions that still affect the surrounding landscape are the Oruanui eruption, about 26,500 years ago, which is responsible for the shape of the modern caldera, and the Hatepe eruption, about 1,800 years ago. However, there have been many more eruptions, with about one every thousand years or so.〔A continent on the move: New Zealand geoscience into the 21st century. Graham, Ian J. et al.; The Geological Society of New Zealand in association with GNS Science, 2008. ISBN 978-1-877480-00-3. page 66, 168.〕〔"Taupo the volcano" (a single sheet pamphlet), C.J.N. Wilson and B.F. Houghton, Institute of Geological & Nuclear Sciences, c2004.〕〔(Information from GNS Science on the Taupo Volcano )〕 ==Rhyolitic eruptions== The Taupo Volcano erupts rhyolite, a viscous magma, with a high silica content. If the magma does not contain much gas, rhyolite tends to just form a lava dome. However, when mixed with gas or steam, rhyolitic eruptions can be extremely violent. The magma froths to form pumice and ash, which is thrown out with great force. If the volcano creates a stable plume, high in the atmosphere, the pumice and ash is blown sideways, and eventually falls to the ground, draping the landscape like snow. If the material thrown out cools too rapidly, and becomes denser than the air, it cannot rise as high, and suddenly collapses back to the ground, forming a pyroclastic flow, hitting the surface like water from a waterfall, and spreading sideways across the land at enormous speed. When the pumice and ash settles, it is sufficiently hot to stick together as a rock called Ignimbrite. Pyroclastic flows can travel hundreds of kilometres an hour. 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「Taupo Volcano」の詳細全文を読む スポンサード リンク
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