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GEOL: CHAPTER 4. Igneous Rocks and Intrusive Igneous Activity. Learning Outcomes. LO1: Describe the properties and behavior of magma and lava LO2: Explain how magma originates and changes LO3: Identify and classify igneous rocks by their characteristics

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  1. GEOL: CHAPTER 4 Igneous Rocks and Intrusive Igneous Activity

  2. Learning Outcomes • LO1: Describe the properties and behavior of magma and lava • LO2: Explain how magma originates and changes • LO3: Identify and classify igneous rocks by their characteristics • LO4: Recognize intrusive igneous bodies, or plutons • LO5: Explain how batholiths intrude into Earth’s crust

  3. Igneous Rocks • Molten rock (magma or lava) that cools and crystallizes to form minerals • Intrusive: underground, magma, plutons • Extrusive: above ground, lava, volcanic eruptions • Large parts of continents • All of oceanic crust

  4. Magma and Lava • Magma: molten rock below surface • Less dense than surroundings and wants to rise • Most solidifies underground: plutons • Lava flows: when magma reaches the surface • Volcanic rocks = extrusive igneous rocks • Lava flows • Pyroclastic materials

  5. Composition of Magma • Silicate rocks usually the source • Silica is primary constituent • Other constituents: • Aluminum • Calcium • Sodium • Iron • Magnesium • Potassium

  6. Three Types of Magma • Felsic magma • >65% silica • Considerable sodium, potassium, aluminum • Little calcium, iron, magnesium • Mafic magma • <52% silica • Silica poor • Proportionally more calcium, iron, magnesium • Intermediate magma • Composition between felsic and mafic magma

  7. Magma/Lava Temperatures • Lava usually 700ºC to 1,200ºC • Magma hotter, but can’t measure reliably • Mafic lava nonexplosive, easier to measure • Felsic more explosive, harder to measure • New igneous rocks take years or millennia to cool

  8. Viscosity • Resistance to flow • Higher temperatures reduce viscosity • Hotter magma/lava moves more readily • Increased silica content increases viscosity • Mafic lavas flow far • Felsic lavas don’t flow far • Higher amounts of dissolved gases reduce viscosity

  9. Origination of Magma • Can be 100-300 km deep • Usually shallower: upper mantle and lower crust • Accumulates in magma chambers • Some magma cools: plutons • Some rises through surface: volcanic

  10. Bowen’s Reaction Series • Minerals crystallize from cooling magma in a predictable sequence • Discontinuous branch • Continuous branch • Crystallization occurs on both branches simultaneously • Continued crystallization changes the composition of the melt

  11. Bowen’s Reaction Series, cont. Discontinuous branch • Ferromagnesian silicates only • One mineral changes to another over specific temperature ranges • Olivine to pyroxene to amphibole to biotite • Reactions often incomplete, so can have all ferromagnesian silicates in one rock

  12. Bowen’s Reaction Series, cont. Continuous branch • Plagioclase feldspar silicates only • Calcium-rich plagioclase crystallizes first • Then increasing amounts of sodium are incorporated until all sodium and calcium are gone • Rapid cooling gives calcium-rich core surrounded by zones of increasingly rich sodium

  13. Types of magma Calcium-rich plagioclase Olivine Reaction Mafic (45–52% silica) Pyroxene (augite) Plagioclase feldspars Continuous branch Continuous reaction series Reaction Amphibole (hornblende) Intermediate (53–65% silica) Decreasing temperature Reaction Sodium-rich plagioclase Biotite mica Potassium feldspar Felsic (>65% silica) Muscovite mica Quartz Discontinuous branch Stepped Art Fig. 4-3, p. 69

  14. Magma at Spreading Ridges • Geothermal gradient: 25ºC/km • Lower pressure at ridges allows melting • Ultramafic rocks undergo partial melting • Release more silica-rich minerals (Bowen’s reaction series) • Create mafic magma

  15. Magma at Subduction Zones • Volcanoes and plutons near leading edge of overriding plate • Partial melting at depth • Releases water from hydrous minerals • Water rises and enhances melting • Mafic rocks melt, creating intermediate and felsic magma

  16. Hot-Spot Magma • Interior portions of plates • Mantle plumes: rising magma from the core-mantle boundary • Creates volcanoes • Hawaiian Islands

  17. Changing Magma Composition: Crystal Settling • Physical separation of minerals by crystallization and settling • Olivine, first formed, denser than magma, so it sinks • Makes remaining magma less mafic, more felsic

  18. Changing Magma Composition: Assimilation • Magma reacts with country rock • Country rock melts and changes composition of magma • Inclusions of incompletely melted country rock

  19. Changing Magma Composition: Magma Mixing • A volcano can erupt lavas of different composition • Some of these magmas mix, which changes composition

  20. Igneous Rock Textures • Mineral appearance • Size most important • Cooling rate of magma or lava • Shape • Arrangement

  21. Aphanitic Texture • Rapid cooling • Mineral nuclei form faster than mineral growth • Fine-grained • Lava flows: extrusive

  22. Phaneritic Texture • Slow cooling • Magma underground • Mineral growth faster than nuclei formation • Coarse-grained • Plutons: intrusive

  23. Porphyritic Texture • Minerals of markedly different sizes • Phenocrysts = large minerals • Groundmass = small minerals • Complex cooling history • Porphyry

  24. Glassy Texture • Lava • Very rapid cooling • No ordered 3-D framework of minerals • Natural glass

  25. Vesicles • Magma can contain water vapor and other gases • Gasses trapped in cooling lava • Vesicular: many small holes from gases

  26. Pyroclastic Texture • Also called fragmental texture • Explosive volcanic activity • Consolidated ash from eruptions

  27. Phenocrysts Stepped Art Fig. 4-7, p. 73

  28. Classifying Igneous Rocks • Texture • Aphanitic to Phaneritic • Composition • Ultramafic <45% silica • Mafic 45% to 52% silica • Intermediate 53%-65% silica • Felsic >65% silica

  29. Ultramafic Rocks • <45% silica • Mostly ferromagnesian silicates • Darker minerals: dark rocks • Peridotite: mostly olivine • Pyroxenite: mostly pyroxene • Komatiites: very old lava flows

  30. Basalt-Gabbro • Mafic magma: 45% to 52% silica • Basalt: aphanitic, lava flows • Gabbro: phaneritic, lower part oceanic crust • Large proportion ferromagnesian silicates • Dark color

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