Chapter 5 Notes - Chapter 5 Volcanoes and Other Igneous...

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Chapter 5: Volcanoes and Other Igneous Activity I) Volcanoes can either extrude magma violently or gently  A) depending on: 1) Composition 2) Temperature 3) Amount of dissolved gases B) All factors influence viscosity (magma’s mobility) 1) More viscous=greater resistance to flow=more explosive eruption 2) Strongly influenced by temperature--As lava cools its mobility decreases 3) Directly related to silica content a) More silica=greater viscosity b) Silica structures link together into long chains even before crystallization begins 4) Dissolved gases tend to increase fluidity of magma a) Escaping gases provide force to propel molten rock from volcanic vent II) During an eruption: A) Before eruption—summits begin to inflate 1) Magma is migrating into shallow reservoir inside cone 2) Volatiles migrate upward and accumulate near top of chamber 3) Upper portion of magma enriched in gases B) During eruption--Gas-charged magma moves from magma chamber and rises through conduit or  vent 1) Pressure drops—gases suddenly released and expand a) Fluid basaltic magmas allow gases to migrate and escape with ease—can create lava  fountains b) Viscous magmas explosively expel eruption columns (jets of ash-laden gases that evolve  into buoyant plumes) i) Magmatic differentiation occurs—iron rich minerals crystallize and settle out leaving  magma enriched in silica and dissolved gases Gases begin to collect as tiny bubbles Transformed into gas jet that is explosively ejected from volcano ii) Pressure of molten rock below drops—why there is often series of eruptions Explosive event usually followed by quiet emissions of “degassed” lavas 1. Soluble gases migrate slowly  2. Only uppermost portion of magma has enough gas to trigger explosive  eruptions iii) After eruptive phase, gas buildup starts anew—explains sporadic eruptive patterns of  viscous lavas III) Materials extruded during eruptions A) Lava flows 1) Over 90% of lava is basaltic a) Usually fluid
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b) Flow in thin, broad sheets or stream-like ribbons c) Can travel very far before congealing 2) Pahoehoe flows—relatively smooth skin that wrinkles as still-molten subsurface lava  continues to advance a) Fluid basaltic lavas of Hawaiian type b) Resembles twisting braids of ropes c) Lava tubes:  tunnels that once were horizontal conduits carrying lava from volcanic vent  to flow’s leading edge i) Develop in interior of flow when temperatures are high ii) Still-molten lava within conduits continues forward motion leaving behind lava tubes iii) Allow fluid lavas to advance great distances from source iv) Rare in andesitic/rhyolitic lavas d) Hotter, richer in gases, and faster than aa flows on same slope e)
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