Ch07_Outline - Chapter7 FiresWithin:Igneous Activity...

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 Chapter 7  Fires Within: Igneous  Activity
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    The Nature of  Volcanic Eruptions Characteristics of a magma  determine the “violence” or  explosiveness of an eruption Composition  Temperature Dissolved gases   The above three factors actually  control the viscosity of a magma
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    The Nature of  Volcanic Eruptions Viscosity   is a measure of a material’s  resistance to flow Factors affecting viscosity Temperature—Hotter magmas are less  viscous Composition—Silica (SiO 2 ) content Higher silica content = higher viscosity  Lower silica content  = lower viscosity
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    The Nature of  Volcanic Eruptions Dissolved gases Gases expand within a magma as it  nears Earth’s surface due to decreasing  pressure The violence of an eruption is related to  how easily gases escape  In summary Basaltic lavas = mild eruptions Rhyolitic or andesitic lavas =  explosive eruptions
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    Materials Extruded  from a Volcano Lava flows Basaltic lavas exhibit fluid   behavior Types of basaltic flows Pahoehoe  lava (resembles a twisted or  ropey texture) Aa  lava (rough, jagged blocky texture) Dissolved gases 1%–6% by weight Mainly H 2 O and CO 2
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    Aa Lava Flow Figure 7.5 A
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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 Cinders Pea-sized material Lapilli Walnut-sized material  Particles larger than lapilli Blocks Hardened or cooled lava Bombs Ejected as hot lava
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  A Volcanic Bomb Figure 7.6 (top)
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Ch07_Outline - Chapter7 FiresWithin:Igneous Activity...

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