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Igneous Rocks I

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Igneous Rocks I

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    3. The Nature of Igneous Rocks Form from Magma [Greek=“paste”] Hot, partially molten mixture of solid liquid and gas Gases: H2O, CO2, etc. less dense than solid rock solidify upon cooling

    4. Magma vs. Lava Magma: molten rock beneath the surface Lava: molten rock that has reached the surface Magma: form intrusive igneous rocks Lava: form extrusive igneous rocks

    8. Composition varies widely Oxygen plus major elements Generally a silica (SiO2) melt Silica and water content control viscosity Silica content used in classification

    9. Mafic Magmas Silica content ~ 50% High Fe, Mg and Ca High temperature molten magma 1000o to 1200oC Major minerals: Olivine - Ca Plagioclase Pyroxene

    12. Silicic Magma Silica content: 65-77% High Al, Na and K Lower temperature magmas Less than 850oC Major minerals: Feldspars - Micas Quartz

    14. Magma Viscosity -resistance to flow

    16. Magma Viscosity High Silica=high viscosity/Low Silica=low viscosity Cooler Temperatures=higher viscosity/Higher Temperatures=lower viscosity Water and volatiles break linkages=lower viscosity

    17. Occurrence of Igneous Rocks Found globally Formed in discrete geologic settings Convergent plate margins Divergent plate margins Mantle plumes

    19. Igneous Textures Texture - the size, shape and relationship of minerals in the rock Cooling history of the magma or lava Crystal size increases as rate of cooling slows

    20. Glassy Texture Very rapid cooling - quenched Volcanic glass Conchoidal fracture No apparent crystals

    22. Crystalline Textures Crystal growth requires time for ions to migrate - form minerals Slow rate of cooling=time for crystal growth Crystals grow until melt is quenched or completely solidified

    24. Aphanitic Texture Fine grained texture Few crystals visible in hand specimen Relatively rapid rate of cooling

    26. Phaneritic Texture Coarse grained texture Relatively slow rate of cooling Equigranular, interlocking crystals Slow cooling = crystallization at depth

    29. Porphyritic Texture Well formed crystals (phenocrysts) Fine grained matrix (groundmass) Complex cooling history Initial stage of slow cooling Later stage of rapid cooling

    31. Pyroclastic Texture Explosive volcanic eruptions Appear porphyritic with visible crystals Crystals show breakage or distortion Matrix dominated by glassy fragments Hot fragments may “weld” together

    33. Classification of Igneous Rocks Texture Aphanitic Phanaritic Composition Silicic Intermediate Mafic Ultramafic

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