Ch05_Outline

Ch05_Outline - Chapter 5 Volcanoes and Other Igneous...

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1 Chapter 5 Volcanoes and Other Igneous Activity The nature of volcanic eruptions ± Characteristics of a magma determine the “violence” or explosiveness of a volcanic eruption Composition Temperature Dissolved gases ± The above three factors actually control the viscosity of a given magma 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 (e.g., felsic lava such as rhyolite) Lower silica content = lower viscosity (e.g., mafic lava such as basalt)
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2 The nature of volcanic eruptions 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 In summary Basaltic lavas = mild eruptions Rhyolitic or andesitic lavas = explosive eruptions 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 A pahoehoe lava flow Figure 5.5 A
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3 Aa lava flow Figure 5.5 B 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 A volcanic bomb Bomb is approximately 10 cm long Figure 5.7
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This note was uploaded on 07/17/2008 for the course GS 121 taught by Professor Leavell during the Fall '07 term at Ohio State.

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Ch05_Outline - Chapter 5 Volcanoes and Other Igneous...

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