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 Types of Lavas volcano. • Basaltic lavas: low-viscosity mafic lavas, 3. Lavas erupt from typically erupted at 1000o to 1200o C; cool to the magma chamber form basalt. through central and side vents… • Rhyolitic lavas: high-viscosity felsic lavas, typically erupted at 800o to 1200o C; cool to 2. …rises through the lithosphere to form a form rhyolite. magma chamber • Andesitic lavas: intermediate in composition 1. Magma, which and viscosity between mafic and felsic originates in the magmas; cool to form andesite. partially melted Fig. 6.1 asthenosphere… 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 Pyroclastic Volcanic Bomb Material: Fragmentary volcanic rocks ejected into the air Fig. 6.5 Fig. 6.6 Volcanic Features Formed by Pyroclastic Flows: Central Eruptions A density flow consisting • Shield Volcanoes of a hot (up to 800o C), • Volcanic domes poisonous mixture of gas and pyroclastic material • Cinder-cones Volcanoes moving downslope at • Composite Volcanoes speed in excess of 200 • Craters km/hr! • Calderas Fig. 6.8 Eruptive Styles and Landforms Mauna Kea Shield Volcano Volcanic Dome Mauna Loa Cinder Cone Volcano Composite Volcano Crater Caldera Fig. 6.9 Fig.6.9.a 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 Cinder Cone Mt Fujiyama, Japan 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 Volcanoes along the Laki Fissure A volcanic eruption originating (Iceland) formed in 1783, resulting along an elongate fissure rather in the largest lava than a central vent. flow in recorded history Fig. 6.13 Mt. St. Helens: Before Mt. St. Helens: During Hydrothermal Activity: • The circulation of water through hot Sulfur-encrusted Fumarole 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. Fig. 6.16 Volcanism Associated with Plate Tectonics Fig. 6.19 Fig. 6.20.