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Lecture%20#12%20Geol%203950%20notes%202010%20CR%20Stern -...

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Lecture #12 notes; Geology 3950 2010; CR Stern Global effects of volcanism Sector collapse – when Mt St Helens erupted in 1980, the eruption was triggered by the collapse of the northern part of the volcano which had been over steepened by magma below the surface causing the bulging up of the north side of the volcano. The collapse of a sector, or part, of a large volcano is called “sector collapse” and is a common process in the evolution of a large volcano, which can generate very large debris flows and/or a super-tsunamis if the volcano is located in the ocean. Sector collapse can occur in association with an eruption or simply as a result of mechanical failure. The potential for the mechanical failure of a large volcano is enhanced because of circulation of hot water inside the volcanic edifice, which causes some part of the edifice to weather into very some rocks full of clay minerals. These rocks will not support the weight of the rest of the volcano and it collapses. Mt Rainer, one of the largest volcanoes in the Cascade range, and the one with the most snow and ice cover (see figure 11 in lecture #10), underwent a sector collapse 5000 years ago which generated the Osceola mudflow that reached all the way to Puget sound, covering an area now occupied by approximately 1,000,000 people. The potential for sector collapse of oceanic island volcanoes is also enhanced by the fact that the base of these volcanoes is built onto a submarine apron of soft marine sediments, so that the flanks of these volcanoes tend to slide out to sea.
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The Hawaiian islands (figure 2) have undergone a history of sector collapse events which has reduced the size of the islands and created large debris fans on the ocean floor, and must have generated large tsunami waves as well, but all these events where pre-historic.
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