Essentials_Ch04

Essentials_Ch04 - Volcanoes and Other Igneous Activity Essentials of Geology 8e Stan Hatfield and Ken Pinzke Southwestern Illinois College The

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Unformatted text preview: Volcanoes and Other Igneous Activity Essentials of Geology, 8e Stan Hatfield and Ken Pinzke Southwestern Illinois College 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 actually control the viscosity of a given magma which in turn controls the nature of an eruption The Nature of Volcanic Eruptions ¡ Viscosity is a measure of a material’s resistance to flow (e.g., Higher viscosity materials flow with great 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) The Nature of Volcanic Eruptions ¡ Factors affecting viscosity continued – 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 The Nature of Volcanic Eruptions ¡ Factors affecting viscosity continued In Summary ¡ Fluid basaltic lavas generally produce quiet eruptions ¡ Highly viscous lavas (rhyolite or andesite) produce more explosive eruptions 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 A Pahoehoe lava flow A typical aa flow Materials extruded from a volcano ¡ Pyroclastic materials – “Fire fragments” Types of pyroclastic debris ¡ Ash and dust- fine, glassy fragments...
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This note was uploaded on 07/17/2008 for the course GS 100 taught by Professor Leavell during the Spring '06 term at Ohio State.

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Essentials_Ch04 - Volcanoes and Other Igneous Activity Essentials of Geology 8e Stan Hatfield and Ken Pinzke Southwestern Illinois College The

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