8. Igneous Rocks and Processes -know about the three compositional types of magma/lava (properties: silica vs Fe-Mg mineralogy, viscosity, explosive of eruption, landform type. 1) Basaltic Magma Comes from partial melt of asthenosphere. Can form at hotspots, seafloor spreading, and basaltic magma at subduction zone. Composed of dry melting, melting temp decreases with decrease in pressure as magma rises As basaltic magma melts its density decrease= rise until reach the surface Low viscosity/ flow great distance from vent Basalt when erupted along seafloor spreading/Hawaii= liquid Ex) pahoehoe (ropey texture) & aa flow (block texture) Mineralogy (lava type) = Mafic Composition/ Basaltic (dark color/pyroxene & ca-rich plagioclase) Explosive of Eruption= Quiet and effusive Landform type= Tectonic setting magma is found at= Divergent plate boundaries, oceanic intraplate hot spots, intraplate rifts 2) Granitic/ Rhyolite magma Forms from partial melt of continental crust contains dissolved water Dissolved water in magma decrease melting temp with increasing pressure Solidifies as it rises, melting temp increase = geothermal gradient decrease Rarely reach surface-granitic magma is a wet melt & the melting temp increase with decreasing pressure so it solidifies as it rises to the surface. Opposite of basaltic magma Produce at continental collision margins, as continental crust thickens it begins to partially melt Mineralogy= Felsic Composition Explosive of Eruption= very explosive, pyroclastic Landform type= Tectonic setting magma is found at= Subduction zones, especially at continental margins, intracontinental rifts, intracontinental hot spots 3) Andesitic magma Produced from partial melt of oceanic crust along subduction zones. Introduction of water forced out of subducting plate lowers the melting temp. of the upper emanttle which rises and may mix with the overlying continental crust. Example= Mt. St Helens= intermediate compostion Mineralogy= Intermediate Composition Explosive of Eruption= fairly explosive, pyroclastic Landform type= Tectonic setting magma is found at= subduction zones Dry Melt Vs. Wet Melt Why does granitic composition magma rarely make it to the surface? granitic magma is a wet melt & the melting temp increase with decreasing pressure so it solidifies as it rises to the surface. Opposite of basaltic magma Bowen?s Reaction Series (magmatic fractionation by partial melting and fractional crystallization). -Be familiar with the igneous rock classification diagram in Chapter 3 of the text. Can you identify and igneous rock if I give you mineral composition and texture information (the diagram will be provided). Show range of most common igneous rock compostsion could be produced by the same basaltic magma. Illustrates different minerals have different crystallization temp.
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