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Lecture 8 Feb 6 2008 Volcanic disasters

Lecture 8 Feb 6 2008 Volcanic disasters - The Pacific...

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The Pacific Northwest is a region of historical volcanism. The Juan de Fuca plate is subducting beneath the North American plate. Relative to the melting basaltic ocean crust, the admixture of melted marine sediments and continental crust leads to an intermediate (andesitic) lava.
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Some Indians of the Pacific Northwest called it "Louwala-Clough" or "smoking mountain.“ Mount St. Helens is a relatively young volcano - 40,000 years old - that had been dormant since 1857. Mount St. Helens: 9,677 ft before May 18, 1980
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Mount St. Helens: 8,364 ft after May 18, 1980 The eruptions of May 1980 were the first volcanic eruptions in the conterminous United States since 1914 (Lassen Peak, CA).
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Seismic activity and venting of steam began in March 1980. These events were evidence of magma intruding at depth. By May 1, a bulge was growing at about 1.5 m/day.
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A magnitude 5.0 earthquake triggered a large landslide/debris avalanche. Debris traveled more than 20 km from the volcano. Much of the bulge was now gone.
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The initial failure of the bulge released internal pressure, and the confined gases blasted away a large portion of the mountain laterally.
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