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12-VOLCANISM-S08 - Volcanism Chap 12 Mt Rainier Outline...

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Volcanism – Chap. 12 Mt Rainier
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Outline Lavas and Volcanic Deposits Eruptive styles and Landforms Where are the volcanoes Hazards and Benefits 1. What lava composition forms each type of volcanic deposits? 1. What lava composition is associated with each eruptive style (or land form)? 2. Where do volcanoes form?
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Extrusive (Volcanic) Rocks
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Bowen’s Reaction Series
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Rock Names 100% 65% 65% 48% 40% 36% 45% 30% % = Silica content
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Rock Names vs. Viscosity High Viscosity Low Viscosity High Silica Low Silica
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Volcanic Styles Main Control on style of Volcanism: Silica content !!! Where ….. - Low silica, low viscosity = liquid flows along ground - High silica, high viscosity = violent airborne eruptions Volatile content will also affect style of volcanism
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Low SiO 2 Lavas: Basalts Very fluid – erupts at 1000 to 1200°C Low silica and high temperature means low viscosity Flows very quickly and covers large areas
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Pahoehoe : Fluid basalt with high gas content Aa : more viscous basalt that has degassed. Textures Pahoehoe Aa Pahoehoe Pahoehoe Aa
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Flood Basalts Flood Basalts : Fluid basaltic lava erupting onto flat terrain to produce multiple layers of thin flows Columbia Plateau: Multi-layered Flood basalt covering a wide area in Washington, Oregon, and Idaho
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Columnar fracture Devils Postpile National Monument Sierra Nevada Columnar jointing (slow cooling) Avg. columns = 2 ft dia. 60 ft long basalt http://volcanoes.usgs.gov/yvo/gallery_volcanic.html
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Lava Tubes
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Pillow Lava When basaltic lavas erupt underwater, rapid cooling creates an outer skin/scum “Pillow lavas” are indicators that ancient volcanism occurred under water
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Vesicular basalt Vesicle : A small cavity in a glassy igneous rock, formed by the expansion of a bubble of gas or steam during solidification.
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Rock Names vs. Viscosity High Viscosity Low Viscosity High Silica Low Silica
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