Chapter 6 - Click to edit Master title style 6 Volcanoes:...

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Unformatted text preview: Click to edit Master title style 6 Volcanoes: Tectonic Environments and Eruptions Hyndman/Hyndman Natural Hazards and Disasters , 3 rd Edition Cascade Range Volcanoes are Active Until 1970s, Cascade Range volcanoes thought to be extinct or dormant Rule of thumb: if volcano had significant glacial erosion, it had not erupted since end of last ice age and probably wouldnt erupt again 1914-1921 eruptions of Mount Lassen (northern California) seen as exception Cascade Range Volcanoes are Active With understanding of plate tectonics came realization that Cascade Range volcanoes sit over active subduction zone, therefore volcanoes are potentially active Active volcano is one that is likely to erupt again, ever Volcanologists dont consider volcanoes to be dormant only active or extinct , on geologic time scale Introduction to Volcanoes: Generation of Magma Volcano : cone-shaped hill or mountain formed at vent from which molten rock or gases reach Earths surface and erupt Magma : molten rock before it erupts Lava : magma after it reaches Earths surface Volcanoes only form at settings where magma is generated at depth and can rise to surface Introduction to Volcanoes: Generation of Magma Distinction between liquid , solid and gas Substances change from one to another with change in temperature and/or pressure Melting temperature depends on pressure and availability of water Fig. 6-1, p. 135 Stepped Art Solid Liquid Gas Fig. 6-1, p. 135 Introduction to Volcanoes: Generation of Magma Magma rises through crust because less dense than surrounding rocks Magma sometimes breaks off and incorporates pieces of adjacent rocks Magma chamber : mass of molten magma that rises through Earths crust, often erupting at surface to build volcano Magma Properties and Volcanic Behavior No two volcanoes exactly alike No two eruptions exactly alike (even at same volcano) Events of eruption depend on How fluid magma is ( viscosity ) Quantity of water vapor, other volcanic gases ( volatiles ) Type and amount of magma ( volume ) Melting Temperature of a Rock 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) Fig. 6-BTN1, p. 136 Magma Properties and Volcanic Behavior Viscosity: resistance to flow High viscosity magmas are thick and pasty Depends on chemical composition, internal arrangement of atoms and molecules Silica tetrahedra bonds are strong rigid silicate structures higher viscosity Differences in viscosity mainly due to differences in amount of silica Higher percentage of silica higher viscosity Fig. 6-2, p. 137 Between tetrahedra Stepped Art Break bonds Broken silicate structure Formerly shared oxygen atoms 1 h y d r o g e n a t t a c h e s t o a s h a r e d o x y g e n , b r e a k i n g s h a r e d b o n d Added water molecule: (1 oxygen + 2 hydrogens) Hydrogen Oxygen Fig. 6-2, p. 137 Magma Properties and Volcanic...
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This note was uploaded on 03/28/2011 for the course GEO 107 taught by Professor Stidham during the Spring '08 term at SUNY Stony Brook.

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Chapter 6 - Click to edit Master title style 6 Volcanoes:...

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