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LECTURE_09_02-23-10_VOLCANOES_Chap_6___7

LECTURE_09_02-23-10_VOLCANOES_Chap_6___7 - Tectonic Hazards...

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Tectonic Hazards – Volcanoes NEW STUFF (Chap. 6 and 7) Volcanoes – Tectonic setting Volcanoes – Products/hazards Volcanoes – Types of volcanoes Volcanoes – Examples/case studies
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Fig. 2-8, p. 17 Outer edge of the Earth is a dynamic system….all parts/plates are in motion Plate Tectonics - Volcanoes
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Volcanoes – Tectonic Setting Would you expect faults and earthquakes in these settings? (1) (1) Convergent Margins – Subduction zones (2) (2) (2) Divergent Margins – Spreading ridges and rifts (3) (3) Hot Spot Volcanoes
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Volcanoes Earthquake distribution Volcano distribution
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Volcanoes Why no volcanoes here? Convergent and Transform plate boundaries along the North American plate
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Volcanoes – Tectonic Setting (1) (1) Convergent Margins – Subduction zones (2) (2) (2) Divergent Margins – Spreading ridges and rifts (3) (3) Hot Spot Volcanoes
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Magma is molten rock may contain mineral grains and dissolved gas Volcano - where magma reaches the surface lava is magma that flows on the surface Magma characteristics depend on: Composition Temperature: 800-1200ºC Water & other gases Mantle and Magma
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Magma and Volcano types Magma Composition Viscosity Volatiles Volume Product Basalt 50 % silica low low large lava flows Andesite 60 % silica med med medium lava & ash Rhyolite 70 % silica high high large ash Magma type controlled by : (1) Composition and the three VVVs (2) V iscosity (resistance to flow) (3) V olatiles (gases) (4) V olume Remember this chart!!!
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Volcanoes – Tectonic Setting – Subduction Zones (1) (1) Convergent Margins – Subduction zones (2) Divergent Margins – Spreading ridges and rifts (3) Hot Spot Volcanoes
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Environments of magma formation Subduction Zone Subduction zones What causes the melting? 1. Increase temperature downward 2. Boil off water from subduction zone: add water to hot rocks above subduction zone : causes melting to form basalt magma 3. Silica-poor magma rises and heats the continental crust causing melting to form silica-rich magma Continental crust Mantl e
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10,000 melt solid Temperature ( o C) Pressure 0 1000 wet melting dry melting Melting by addition of water At subduction zones –  the addition of water  decreases the melting  temperature of rocks = Less heat required for   melt magma   volcanoes
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Volcanoes – Tectonic Setting – Spreading Ridges (1) Convergent Margins – Subduction zones (2) (2) (2) Divergent Margins – Spreading ridges and rifts (3) Hot Spot Volcanoes
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Spreading Centers/Ocean Ridges 1) Convection causes asthenosphere to rise Most spreading centers are at oceanic ridges (e.g. Mid-Atlantic Ridge. Nearly all of the oceanic crust is produced this way . 2) Lithospheric plates rift apart (diverge) 3) As asthenosphere rises , pressure drops and it melts to form silica-poor basalt magma .
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