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chapter7 - Click to edit MasterHazards Volcanoes title...

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Click to edit Master title style 7 Volcanoes: Hazards and Mitigation Hyndman/Hyndman Natural Hazards and Disasters , 3 rd Edition
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Mount St. Helens Erupts May 18, 1980 : few people camping or logging in restricted zone north and west of Mount St. Helens 8:32 am : Dr. David Johnston was lone volcanologist stationed on high ridge facing volcano (last minute replacement at post for someone else) when he radioed Cascades Volcano Observatory Vancouver, Vancouver – this is it! ”, before dying in blast
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Mount St. Helens Erupts Survivors’ eyewitness accounts describe black, boiling clouds of pyroclastic flows Some people trapped and burned, many did not survive Geologist and husband detoured on way home from conference for quick flight around volcano, just as eruption began – captured impressive photos of landslide and lateral blast
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Volcanic Hazards Millions of people around world live in risk of volcanic hazard Volcanic materials give rich, fertile soil for agriculture Majestic volcanic peaks give beautiful vistas Hawaiian climate appeals to retirees Most volcanic eruptions kill no more than few hundred people Few volcanic eruptions have potential to kill many more, depending on how many people live in close proximity
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Fig. 7-1, p. 164
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Lava Flows Flowing basalt lava may cool, blacken on outside, but still be hot enough to ignite wood structures Objects that aren’t burned may be surrounded or buried by flow Engulfed trees rot away and leave behind cast Although incredibly destructive, lava flows move so slowly and cover small enough area that they pose little threat to human life Rare, fluid, alkali-rich lava flows faster – from Vesuvius, killed 3000 people in 1631 Basaltic magma very rarely contains enough gas to blast out in Plinian eruption
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Pyroclastic Flows and Surges Mixture of hot volcanic ash and steam that flows downhill because too dense to rise Also called ash flow, nuee ardente, glowing avalanche, ignimbrite deposit Race down side of volcano at 50-200 km/hr Form as part of eruption column collapse Ash-rich, shock wave surge may race ahead of pyroclastic flow, up to 600 km/hr Pyroclastic flow means certain death unless near edges, in building or vehicle o
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Pyroclastic Flows and Surges Most dangerous place: bottom of valley Can approximate travel distance of pyroclastic flow with energy line sloping downward from top of ash-rich part of eruption column Actual hazard distance can be much greater if eruption is fueled by lateral blast Pyroclastic flows can travel over surface of water
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Fig. 7-3, p. 164
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Fig. 7-7, p. 167
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Fig. 7-7, p. 167 Stepped Art Heavier mass of ash and water continues across sea bottom Sea Accompanying ash cloud Ash plume from secondary steam explosions Fine ash and steam Dilute ash cloud surge continues over surface of sea Ash takes in water Pumice flow Violent explosions as ash mixes with seawater
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Ash and Pumice Falls Volcanic ash is composed of bits of pumice
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