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Chapter 6 – IGNEOUS ROCKS

Chapter 6 – IGNEOUS ROCKS. How, Why & Where Rocks Melt. Begins as solid Molecules warm & begin vibrating = softening Molecules may vibrate violently enough to break bonds. How, Why & Where Rocks Melt. Heat & pressure inside Earth Magma chamber Geothermal gradient

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Chapter 6 – IGNEOUS ROCKS

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  1. Chapter 6 – IGNEOUS ROCKS

  2. How, Why & Where Rocks Melt Begins as solid Molecules warm & begin vibrating = softening Molecules may vibrate violently enough to break bonds

  3. How, Why & Where Rocks Melt Heat & pressure inside Earth Magma chamber Geothermal gradient Continental vs. ocean crust Composition varies melting point Pressure increases melting point Water decreases melting point

  4. Heat and pressure inside Earth Fractional melt (partial melting) Mix of molten & solid rock 1 magma body mayproduce several different igneous compositions How, Why & Where Rocks Melt

  5. How, Why &Where Rocks Melt Magma (Intrusive) Molten rock below Earth’s surface Lava (Extrusive) Molten rock when it reaches Earth’s surface

  6. Magma & Lava Composition Silica content varies (~ 45-75%) Water vapor & carbon dioxide Temperature Temperature varies (~ 750°C – 1200°C) Viscosity (resistance to flow) Varies in ability to flow Influenced by silica content & temperature How, Why & Where Rocks Melt

  7. Tectonic setting Characteristics influenced by location Oceanic, divergent margins Hot, low viscosity basaltic lava Subduction (convergent) zones Cooler, viscous lavas with more silica Ocean hot spots Hot & basaltic; build giant shield volcanoes Continental hot spots Cooler & granitic; high silica lava How, Why & Where Rocks Melt

  8. Cooling and Crystallization Crystallization Process where mineral grains form & grow in cooling magma (or lava) Classified based on: Texture (size of mineral crystals) Volcanic (extrusive) = small grains due to rapid cooling Plutonic (intrusive) = large grains due to slow cooling Composition (silica content)

  9. Rate of Cooling Extrusive Textures Glassy Cools too rapidly to form crystals Example: obsidian

  10. Rate of Cooling Extrusive Textures Aphanitic Fine grained (small crystals) Examples: basalt, andesite, rhyolite

  11. Rate of Cooling Extrusive Textures Vesicular Form from trapped gas bubbles Examples: pumice, scoria

  12. Rate of Cooling Extrusive Textures Pyroclastic or fragmental Includes rock fragments Example: volcanic tuff

  13. Rate of Cooling Intrusive Texture Phaneritic Course grained (large crystals); slow cooling inside Earth Examples: granite, syenite, diorite, gabbro, peridotite

  14. Chemical composition Igneous rocks subdivided into 4 categories based on silica content Felsic Intermediate Mafic Ultramafic

  15. Igneous Rock Classification

  16. Igneous Rock Classification

  17. Fractional Crystallization Crystals separate from liquids during crystallization Bowens reaction series Predictable melting & cooling of minerals

  18. Plutons and Plutonism Plutons Any body of intrusive igneous rock, regardless of size or shape Massive vs. Tabular Concordant vs. Discordant

  19. Plutons& Plutonism Batholith Large, irregular shaped pluton Massive & Discordant Laccolith Mushroom-shaped pluton Massive & Concordant

  20. Plutons and Plutonism Dikes Magma squeezes into cross cutting fracture & solidifies Tabular & Discordant Sills Magma intrudes between 2 layers; parallel to layers Tabular & Concordant

  21. Plutons& Plutonism Volcanic pipe = remnant Volcanic neck = remnant exposed via erosion

  22. Economics of Igneous Rocks • Uncommon uses: • Mining • Pumice stone • Lava soap • Fingernail files • Surgical tools

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