05 Volcanoes

05 Volcanoes - Volcanoes and Other Igneous Activity...

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Unformatted text preview: Volcanoes and Other Igneous Activity Volcanic eruptions The Nature of Volcanic Eruptions What Comes out of Volcanoes? Three Types of Volcanoes Other Volcanic Landforms Living in the Shadow of Volcanoes Intrusive Igneous Formations Plate Tectonics and Volcanoes Volcanic eruptions Factors that determine the violence of an eruption Viscosity of magma Composition of the magma Temperature of the magma Dissolved gases in the magma Viscosity is a measure of a material's resistance to flow Volcanic eruptions Viscosity of magma Factors affecting viscosity Temperature (hotter magmas are less viscous) Composition (silica content) High silica high viscosity (e.g., rhyolitic lava) Low silica more fluid (e.g., basaltic lava) Dissolved gases (volatiles) Mainly water vapor and carbon dioxide Gases expand near the surface Volcanic eruptions Viscosity of magma Factors affecting viscosity Dissolved gases (volatiles) Provide the force to extrude lava Violence of an eruption is related to how easily gases escape from magma Easy escape from fluid magma Viscous magma produces a more violent eruption Volcanic eruptions The Nature of Volcanic Eruptions What Comes out of Volcanoes? Three Types of Volcanoes Other Volcanic Landforms Living in the Shadow of Volcanoes Intrusive Igneous Formations Plate Tectonics and Volcanoes Materials associated with volcanic eruptions Types of lava Gases Pahoehoe lava (resembles braids in ropes) Aa lava (rough, jagged blocks) One to 5 percent of magma by weight Mainly water vapor and carbon dioxide A Pahoehoe lava flow A typical aa flow Materials associated with volcanic eruptions Pyroclastic materials "Fire fragments" Types of pyroclastic material Ash and dust fine, glassy fragments Pumice from "frothy" lava Cinders "peasized" Lapilli "walnut" size Blocks hardened lava Bombs ejected as hot lava A volcanic bomb Bomb is approximately 4 inches long Volcanoes General features Conduit, or pipe caries gasrich magma to the surface Vent, the surface opening (connected to the magma chamber via a pipe) Crater Steepwalled depression at the summit Caldera (a summit depression greater than 1 km diameter) General Features Volcanic eruptions The Nature of Volcanic Eruptions What Comes out of Volcanoes? Three Types of Volcanoes Other Volcanic Landforms Living in the Shadow of Volcanoes Intrusive Igneous Formations Plate Tectonics and Volcanoes Volcanoes Types of volcanoes Shield volcano Broad, slightly domed Primarily made of basaltic (fluid) lava Generally large size e.g., Mauna Loa in Hawaii A shield volcano Volcanoes Types of volcanoes Cinder cone Built from ejected lava fragments Steep slope angle Rather small size Frequently occur in groups Sunset Crater a cinder cone near Flagstaff, Arizona Volcanoes Types of volcanoes Composite cone (or stratovolcano) Most are adjacent to the Pacific Ocean (e.g., Mt. Rainier) Large size Interbedded lavas and pyroclastics Most violent type of activity A composite volcano Mt. St. Helens a typical composite volcano Mt. St. Helens following the 1980 eruption A size comparison of the three types of volcanoes A size comparison of the three types of volcanoes Volcanic eruptions The Nature of Volcanic Eruptions What Comes out of Volcanoes? Three Types of Volcanoes Other Volcanic Landforms Living in the Shadow of Volcanoes Intrusive Igneous Formations Plate Tectonics and Volcanoes Stratospheric dust plume 8 hours after eruption Volcano Hazards May produce a lahar volcanic mudflow Often produce nue ardente Fiery pyroclastic flow made of hot gases infused with ash Flows down sides of a volcano at speeds up to 200 km (125 miles) per hour A lahar along the Toutle River near Mt. St. Helens A nue ardente on Mt. St. Helens Martinique 1902 Pompeii August 24, 79 A.D. Volcanic eruptions The Nature of Volcanic Eruptions What Comes out of Volcanoes? Three Types of Volcanoes Other Volcanic Landforms Living in the Shadow of Volcanoes Intrusive Igneous Formations Plate Tectonics and Volcanoes Other volcanic landforms Calderas Fissure eruptions and lava plateaus Steep walled depression at the summit Formed by collapse Nearly circular Size exceeds one kilometer in diameter Fluid basaltic lava extruded from crustal fractures called fissures e.g., Columbia Plateau Crater Lake, Oregon is a good example of a caldera Crater Lake in Oregon Fissure eruptions Hawaii What will happen when North America overrides the Hawaiian Island hot spot? The Columbia River lava flows Other volcanic landforms Volcanic pipes and necks Pipes are short conduits that connect a magma chamber to the surface Volcanic necks (e.g., Ship Rock, New Mexico) are resistant vents left standing after erosion has removed the volcanic cone Formation of a volcanic neck An eroding landscape Volcanic necks are the result of erosion long after a volcano is dormant This is a reminder that igneous activity occurs mostly underground. Volcanic eruptions The Nature of Volcanic Eruptions What Comes out of Volcanoes? Three Types of Volcanoes Other Volcanic Landforms Living in the Shadow of Volcanoes Intrusive Igneous Formations Plate Tectonics and Volcanoes Intrusive igneous activity Most magma is emplaced at depth An underground igneous body is called a pluton Plutons are classified according to shape and orientation to surrounding rocks. Intrusive igneous activity Plutons are classified according to Shape Orientation with respect to the surrounding rock Tabular thin in one of its dimensions Massive large in three dimensions Discordant cuts across existing structures Concordant parallel to features such as sedimentary strata Intrusive igneous activity Examples of igneous intrusive features Dike, a tabular, discordant pluton Sill, a tabular, concordant pluton Laccolith, a massive, concordant pluton Batholith, a massive, discordant pluton A sill in the Salt River Canyon, Arizona Intrusive igneous activity Types of igneous intrusive features Laccolith Batholith Lens shaped mass Arches overlying strata upward Largest intrusive body Often occur in groups Surface exposure 100+ square kilometers (smaller bodies are termed stocks) Frequently form the cores of mountains Intrusive and extrusive igneous structures Intrusive igneous structures exposed by erosion A batholith exposed by erosion Volcanic eruptions The Nature of Volcanic Eruptions What Comes out of Volcanoes? Three Types of Volcanoes Other Volcanic Landforms Living in the Shadow of Volcanoes Intrusive Igneous Formations Plate Tectonics and Volcanoes Plate tectonics and igneous activity Global distribution of igneous activity is not random Most volcanoes are located on the margins of the ocean basins (Composite Volcanoes) Second group is confined to the deep ocean basins (Shield Volcanoes) Third group includes those found in the interiors of continents (Cinder Cones) Locations of some of Earth's major volcanoes Plate tectonics and igneous activity Plate motions provide the mechanism by which mantle rocks melt to form magma Convergent plate boundaries Deepocean trenches are generated Descending plate partially melts Magma slowly rises upward Rising magma can form Volcanic island arcs in an ocean (Aleutian Islands) Continental volcanic arcs (Andes Mountains) Plate tectonics and igneous activity Plate motions provide the mechanism by which mantle rocks melt to form magma Divergent plate boundaries The greatest volume of volcanic rock is produced along the oceanic ridge system Lithosphere pulls apart Less pressure on underlying rocks Partial melting occurs Large quantities of fluid basaltic magma are produced Plate tectonics and igneous activity Plate motions provide the mechanism by which mantle rocks melt to form magma Intraplate igneous activity Activity within a rigid plate Plumes of hot mantle material rise Form localized volcanic regions called hot spots Examples include the Hawaiian Islands and the Columbia River Plateau in the northwestern United States Hawaiian Hot Spot End of Chapter 8 ...
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This note was uploaded on 08/07/2008 for the course GEO 1408 taught by Professor Greene during the Summer '07 term at Baylor.

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