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Volcanoes - VOLCANO Pressure Pressure drops Temperature...

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VOLCANO February 14, 2008
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Temperature Pressure Pressure drops
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Phase Diagram
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MAGMA Most magma originates in upper mantle (asthenosphere) High temperatures, relatively low pressure Plate tectonics settings Divergent plate boundaries – Subduction zones “hot spots” or mantle plumes
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MAGMA Magma composition controlled by: – Source material – Extent of melting Magma composition influences behavior – Determines type of eruption Major compositional variables – Silica – Iron – Magnesium
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Plate Distance from the plate VISCOSITY Water molecule τ
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Plate Distance from the plate VISCOSITY Velocity Velocity Gradient u Δ y = Δ u / Δ y τ =μ( Δ u / Δ y ) μ is referred to as (dynamic) viscosity. Δ u
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VISCOSITY Iron Ball Plastic Ball
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Plate Distance from the plate VISCOSITY Velocity Velocity
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Viscosity is also a function of density ν =μ/ ρ ρ is density; μ is referred to as (dynamic) viscosity; ν is referred to as kinematic viscosity.
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VISCOSITY Fluid with higher viscosity tends to flow slowly (other conditions being equal). (compare honey and water. Viscosity of honey is much greater than water).
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MAGMA More iron and magnesium More silicate Higher viscosity Mafic magma Felsic (silicic) magma The flow is slower
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MAGMA: COOLING Crystalline Plutonic (cools slower) Volcanic (cools faster)
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MAGMA • Mafic magmas – Divergent plate boundaries – Basalt formed on ocean floor (spreading ridges) – Low viscosity – Quiet eruptions
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MAGMA • Felsic magmas – Convergent plate boundaries – Granite formed above subduction zones – High viscosity – Explosive eruptions
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