Ch07_Outline - Chapter 7 Fires Within: Igneous Activity The...

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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 Fow ±actors a²ecting 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 fows Basaltic lavas exhibit fuid behavior Types oF basaltic fows 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 - Chapter 7 Fires Within: Igneous Activity The...

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