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Frank Press • Raymond Siever • John Grotzinger • Thomas H. Jordan Understanding Earth Fourth Edition Chapter 6: Volcanism Lecture Slides prepared by Bill Dupré • Peter Copeland Copyright © 2004 by W. H. Freeman & Company 4. …accumulating on the surface to form a volcano. 3. Lavas erupt from the magma chamber through central and side vents… 2. …rises through the lithosphere to form a magma chamber Fig. 6.1 1. Magma, which originates in the partially melted asthenosphere… Types of Lavas • Basaltic lavas: low-viscosity mafic lavas, typically erupted at 1000o to 1200o C; cool to form basalt. • Rhyolitic lavas: high-viscosity felsic lavas, typically erupted at 800o to 1200o C; cool to form rhyolite. • Andesitic lavas: intermediate in composition and viscosity between mafic and felsic magmas; cool to form andesite. Flood Basalts of the Columbia Plateau Types of Basalts • Flood Basalts: thick, widespread accumulations of basalt, typically fed by fissures • Pahoehoe: a very low viscosity basaltic lava characterized by a ropy texture • Aa: a relatively low viscosity basaltic lava characterized by a jagged, blocky texture • Pillow Basalts: a basaltic lava extruded beneath the water, characterized by glassy pillows filled with crystalline basalt Fig. 6.2 Vesicular Basalt: trapped gases form bubbles (vesicles) Fig. 6.3 Fig. 6.4 Volcanic Bomb Pyroclastic Material: Fragmentary volcanic rocks ejected into the air Fig. 6.5 Pyroclastic Flows: A density flow consisting of a hot (up to 800o C), poisonous mixture of gas and pyroclastic material moving downslope at speed in excess of 200 km/hr! Fig. 6.8 Fig. 6.6 Volcanic Features Formed by Central Eruptions • • • • • • Shield Volcanoes Volcanic domes Cinder-cones Volcanoes Composite Volcanoes Craters Calderas Eruptive Styles and Landforms Mauna Kea Shield Volcano Volcanic Dome Mauna Loa Cinder Cone Volcano Composite Volcano Crater Caldera Fig.6.9.a Fig. 6.9 Shield Volcano Lava Dome Fig. 6.9b Fig.6.9.a Lyn Topinka/USGS Volcanic Dome Cerro Negro Cinder Cone, near Managua, Nicaragua in 1968 Fig. 6.9b Mark Hurd Aerial Surveys Fig. 6.9c Mt Fujiyama, Japan Cinder Cone Fig. 6.9c Raga/The Stock Market Fig. 5.15 Caldera Stages in the Development of Crater Lake Fig. 6.10 Shiprock, Formation of a Diatreme New Mexico Fig. 6.12 Fig. 6.12 Fissure Eruptions A volcanic eruption originating along an elongate fissure rather than a central vent. Volcanoes along the Laki Fissure (Iceland) formed in 1783, resulting in the largest lava flow in recorded history Fig. 6.13 Mt. St. Helens: Before Hydrothermal Activity: • The circulation of water through hot volcanic rocks and magma • Forms fumaroles – volcanic vents emitting gases, some charged with dissolved minerals • Forms geysers – heated hydrothermal waters under pressure, that intermittently erupts at the surface • Provides the source for Geothermal Energy. Mt. St. Helens: During Sulfur-encrusted Fumarole Fig. 6.16 Volcanism Associated with Plate Tectonics Fig. 6.19 Fig. 6.20