Essentials of Geology (10th Edition)

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Essentials of Geology Volcanoes and Other Igneous  Activity Chapter 4
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The nature of volcanic eruptions Factors determining the “violence” or  explosiveness of a volcanic eruption Composition  of the magma Temperature  of the magma Dissolved gases  in the magma The above three factors control the  viscosity of a given magma, which in  turn controls the nature of an eruption
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The nature of volcanic  eruptions Viscosity  is a measure of a material’s  resistance to flow (e.g., higher viscosity  materials flow with greater difficulty) Factors affecting viscosity Temperature – hotter magmas are less  viscous Composition –  silica  (SiO 2 ) content Higher silica content = higher  viscosity  (e.g., felsic lava such as rhyolite)
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The nature of volcanic  eruptions Factors affecting viscosity Lower silica content = lower viscosity or  more fluid-like behavior (e.g., mafic lava  such as basalt) Dissolved gases Gas content affects magma mobility Gases expand within a magma as it nears  the Earth’s surface due to decreasing  pressure The violence of an eruption is related to  how easily gases escape from magma
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The nature of volcanic  eruptions Factors affecting viscosity In summary Fluid basaltic lavas generally produce  quiet eruptions Highly viscous lavas (rhyolite or  andesite) produce more explosive  eruptions
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 Materials extruded from a  volcano Lava flows Basaltic lavas are much more fluid   Types of basaltic flows Pahoehoe  lava (resembles a twisted or  ropey texture) Aa  lava (rough, jagged blocky texture) Dissolved gases One to six percent of a magma by  weight Mainly water vapor and carbon dioxide
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A typical aa flow
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Pahoehoe vs. aa basalt lava flows Pahoehoe vs. aa basalt lava flows
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Materials extruded from a  volcano Pyroclastic  materials – “Fire fragments” Types of pyroclastic debris Ash  and  dust    fine, glassy fragments Pumice    porous rock from “frothy” lava Lapilli    walnut-sized material  Cinders    pea-sized material  Particles larger than lapilli Blocks    hardened or cooled lava Bombs    ejected as hot lava
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  A volcanic bomb
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Volcanoes General features
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Ch04 - EssentialsofGeology VolcanoesandOtherIgneous...

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