volcanoes - 9/23/09 Volcanoes: Tectonic Environments and...

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9/23/09 Volcanoes: Tectonic Environments and Eruptions Volcano Terminology o Volcano: hill or mountain formed at vent from which molten rock or gases reach Earth’s surface and erupt o Magma: molten rock before it erupts (liquid rock) o Lava: magma after it reaches Earth’s surface o Volcanoes form only where magma is generated at depth and can rise to surface Generation of Magma o Distinction between liquid , solid and gas o Substances change from one to another with change in temperature and/or pressure o Melting temperature depends on pressure and availability of water o Magma rises through crust because it is less dense than surrounding rocks o Magma can break off and incorporates pieces of adjacent rocks o Magma chamber : mass of molten magma that rises through Earth’s crust, often erupting at surface to build volcano Magma Properties o Eruptions depend on How fluid magma is ( viscosity ) - runny or thick Quantity of water vapor, other volcanic gases ( volatiles ) Type and amount of magma ( volume ) o Melting temperature Depends on depth and amount of water Rock may melt by: Increase in temperature Decrease in pressure Addition of water (shifts melting curve to lower temperatures)
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o Viscosity: resistance to flow o High viscosity magmas are thick and pasty o Depends on chemical composition, internal arrangement of atoms and molecules o Differences in viscosity mainly due to amount of silica Higher percentage of silica = higher viscosity o Low viscosity basalt magma: around 50% silica- runny like in Hawaii- dark colored o Intermediate andesite magma: around 60% silica- Andes Mountains in South America o High viscosity rhyolite magma: around 70% silica- Mount St. Helens- light colored o Temperature affects viscosity o As magma cools, more bonds form between atoms and molecules magma becomes more viscous o Temperature of 1100-1200 o C: basalt lava pours downhill, spreads across flat ground o Temperature of 800-900 o C: rhyolite magma erupts with dull red heat o Volatiles: dissolved gases in magma o Water vapor, carbon dioxide, other gases o Water in magma expands violently to steam when it reaches low pressure near surface o At 20 km depth, rhyolite magma can hold 2% water vapor o At Earth’s surface, magma can hold almost no gas – comes out of solution and forms bubbles o Gas bubbles can escape easily from low- viscosity basalt magma o Gas bubbles remain trapped in high viscosity rhyolite magma o Rhyolite magma explodes into clouds of steam, foamy pumice and white rhyolite ash Tectonic Environments o Plate boundaries: subduction zones or spreading zones Spreading centers typically have peaceful eruptions (basalt) Subduction zones typically have violent eruptions (andesite) o Hotspots
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This note was uploaded on 02/21/2010 for the course GEOL 116 taught by Professor Kruger during the Spring '09 term at Binghamton University.

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volcanoes - 9/23/09 Volcanoes: Tectonic Environments and...

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