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Chapter 4: Igneous Rocks

Chapter 4: Igneous Rocks. Introduction. Igneous rocks = formed from “fire” Magma = completely or partially molten rock Lava = magma which reaches surface Melt = liquid portion of partially melted rock

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Chapter 4: Igneous Rocks

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  1. Chapter 4: Igneous Rocks

  2. Introduction • Igneous rocks = formed from “fire” • Magma = completely or partially molten rock • Lava = magma which reaches surface • Melt = liquid portion of partially melted rock • Crystallization = when magma cools, ions arrange themselves into orderly crystalline structures • Extrusive = erupts on surface • “Volcanic” from god Vulcan • Intrusive = cools under ground • “Plutonic” from god Pluto • Volatiles = gases & liquids dissolved into magma

  3. Crystal Size Effected By … • Cooling rate • Amount of silica • Amount of dissolved gases • Igneous rocks classified by texture & composition

  4. Aphanitic Vesicular Phaneritic Porphyritic Phenocrysts Groundmass Glassy Viscosity Pyroclastic AKA Fragmental Pegmatitic Igneous Textures

  5. Fine-grained “A” = not “Phaner” = visible Rapid cooling Characterized by color Light Intermediate dark Usually extrusive / volcanic Aphanitic Texture

  6. Type of aphanitic rock Vesicule = void left by gas bubble Spherical or elongate holes Usually form in upper layers of extrusive rocks Vesicular Texture

  7. Coarse-grained “Phaner” = visible Slow cooling Minerals can be identified Usually intrusive Exposed at surface after much erosion Phaneritic Texture

  8. Large crystals in matrix of smaller crystals Phenocryst = large crystals Groundmass = surrounding smaller matrix Due to minerals forming at different temps & rates Porphyritic Texture

  9. High SiO2 content forms chains which impede movement of ions High viscosity e.g., Obsidian Quick cooling also prevents crystallization e.g., Pele’s hair Glassy Texture

  10. AKA Fragmental Texture Forms from consolidation of rock fragments from violent eruption Large range in particle sizes Angular pieces Pyroclastic Texture Montserrat, 1996

  11. Pegmatitic Texture • Abnormally large crystals • > 1 cm • Late stages of crystallization • Rich in volatiles • Composition ~ granite

  12. Composed primarily of silicate minerals Dark (Ferromagnesian) minerals Examples: olivine, pyroxene, amphibole, biotite mica Light (nonferromagnesian) minerals Examples: quartz, feldspars, muscovite mica Igneous Compositions

  13. Granitic Composition Light-colored silicates Rich in SiO2 Up to 70% AKA Felsic From feldspar & silica AKA Silicic Major constituent of continental crust Basaltic Composition Dark silicates & Ca-rich feldspar Low in SiO2 ~45% AKA Mafic From magnesium & ferric (iron) Higher density than felsic rocks Major constituent of ocean floor & many islands Granitic vs. Basaltic Rocks

  14. Intermediate Contains ~25% dark silicates AKA Andesite & rhyotlie Associated with dome-building & explosive eruptions Associated with subduction zones Ultramafic Rare composition Rich in Mg & Fe Poor in SiO2 ~40% Composed entirely of ferromagnesian silicates e.g. olivine & pyroxene Peridotite & komatiite Other Compositions

  15. Exhibits considerable range in crustal rocks 40% - 73% SiO2 Influences magma behavior Granitic magmas More SiO2 = more viscous => more explosive Basaltic magmas Less SiO2 = more runny => more like fluid Silica Indicates Composition

  16. Igneous Rock Continuum

  17. Exhibits considerable range in crustal rocks 40% - 73% SiO2 Influences magma behavior Granitic magmas More SiO2 = more viscous => more explosive Basaltic magmas Less SiO2 = more runny => more like fluid Silica Indicates Composition

  18. Naming Igneous Rocks

  19. >25% Quartz ~40% Feldspar Intrusive = Granite Very abundant Associated w/ mountain building Extrusive = Rhyolite May contain glass & vesicles Less common than granite Silicic Composition

  20. Glassy Composition • Obsidian • Dark-colored glassy rock • Forms when SiO2-rich lava cools quickly • Pumice • Forms when large amounts of gas escape • Frothy texture • Usually found with obsidian

  21. 60%-67% SiO2 Intrusive = Diorite Phaneritic Extrusive = Andesite Named for Andes Mts. Aphanitic / Porphyritic Intermediate Composition

  22. 45%-50% SiO2 Composed of pyroxene & Ca-rich feldspar Intrusive = Gabbro Extrusive = Basalt Most common extrusive rock on Earth Basaltic Composition

  23. Composed of fragments ejucted during eruption Varieties: Ash Flow = loose agglomeration of ash-sized particles from fallout in ash cloud Tuff = lithified ash layer Welded tuff = hot enough to fuse Volcanic Breccia = particles larger than ash, usually angular & poorly sorted Porphyritic Rocks

  24. Origin of Magma • Highly debated topic • Formation of Magma from Solid Rock • Role of Heat • Role of Volatiles • Role of Pressure

  25. Fractional Crystallization Crystal Settling Magmatic differentiation Assimilation Magma Mixing Evolution of Magmas

  26. Bowen’s Reaction Series

  27. Assimilation & Magma Mixing

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