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III. Igneous Rocks Minerals Crystallized from Melts

III. Igneous Rocks Minerals Crystallized from Melts. 1. The Rock Cycle 2. Formation of Igneous Rocks 3. Classification of Igneous Rocks 4. Igneous Rocks and Plate Tectonics. Geological Materials Transformation Processes. The Rock Cycle. Igneous Rock Solidification Magma

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III. Igneous Rocks Minerals Crystallized from Melts

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  1. III. Igneous RocksMinerals Crystallized from Melts 1. The Rock Cycle 2. Formation of Igneous Rocks 3. Classification of Igneous Rocks 4. Igneous Rocks and Plate Tectonics

  2. Geological Materials Transformation Processes The Rock Cycle • Igneous Rock Solidification • Magma Partial Melting • Mantle Rock Fig 3.1 See Kehew, Fig. 1-6

  3. See Kehew, Fig 3-6 Partial Melting and Magma E.g., partial melting results in Silica enrichment Partial MeltingMagma Enriched: O, Si, Al, Na, K, Depleted: Ca, Fe, Mg Poor in: O, Si, Al, Na, K, (<50%) Rich in: Ca, Fe, Mg (>50% wt.)

  4. Formation ofMagma • How are rocks melted? • 1. Heating ■ 2. Depressurization • 3. Increase water content • 4. Increased silica content • Where do rocks melt? • Subduction zones (Silicic andIntermediate) • Mantle Plumes (“Hot Spots”) not only at Divergent Boundaries Mafic Hot and Low Pressure See Kehew, Fig 3-6 Hot and High Pressure See Kehew Fig. 3-8

  5. See Kehew, Fig 3-6 Solidification of Melts • Magma, Intruded or Extruded, • Solidifies (crystallizes) to form • Intrusive or Extrusive • Igneous Rocks Fig 3.12, 3.13

  6. Intrusive vs.Extrusive (Silicic Rocks) • Silicic (a.k.a, felsic) Magmas • Cool(<700oC) • Viscous(sticky, doesn’t flow easily) • Gaseous(steam of H2O and C02) • Silicic Rocks • Usuallyintrusive, course-grained, Silicic (Granite) to Intermediate (Diorite) rock forms plutons • If extrusive, fine-grained rocks formed by explosive volcanoes Rhyolite or Andesite Volcanoes • Also injects surrounding rocks with silica laden steam See Kehew, Fig 3-40

  7. E.g., Granite (Silicic, Phaneritic): Crystallized (Solidified) Silicic Magma Poor in: Fe, Mg, Ca, (<20%) Rich in: Silica (>70%) Biotite Quartz Na Plagioclase Intrusive Igneous Rock

  8. Composite Volcanoes • Silicic magma • High-viscosity • Gaseous • Explosive eruptions • Steep sloped, cone shaped volcanoes

  9. Extrusive vs. Intrusive(Mafic Rocks) • Mafic Magmas • Hot(>1000oC) • Non-Viscous(runny, flows easily) • “Dry”(no H2O or C02) • Mafic Rocks • UsuallyExtrusive, Fine-grained, Mafic (Basalt) rock forms oceanic crust, Shield Volcanoes and Basalt Floods • If Intrusive, course-grained mafic rocks are formed Gabbro. • If intrusive, Dikes and Sills more common See Kehew, Fig 3-40

  10. E.g., Mafic Volcanism, Hawaii • Mantle Hot Spot Volcano • See Kehew, 3-8

  11. HawaiiA long chain of inactive volcanoes • Island ages, millions of years Convergent Plate Boundary 59.6 56.2 48.1 Current Plate Motion 55.2 Midway 27.2 19.9 43.4 42.4 Hawaii 20.6 0-5 12.0 10.3

  12. Shield Volcanoes • Mafic Magma • Low-viscosity • Non-explosive eruptions • Gentle slopes • Covering large areas

  13. Types of Eruptions (Mafic Volc.) • Types of Eruptions • Lava floods • Lava fountains • Fissure eruptions • Rock Textures (Table 3-1) • Aphanitic • Porphyritic • Vesicular • Glassy (Obsidian)

  14. Surface Textures (Extrusive, Mafic Rocks) See Kehew, Fig 3-9, 3-10 • Ahah (Rubbley) • Mostly solid when flowing • Pahoehoe (Ropey) • mostly liquid when flowing)

  15. Volcanic Hazards • Relative Hazard (see Kehew, Fig. 3-30, 31) based on • Type of volcano • Active Vents • Earthquakes • Calderas • Historical Flows • Rift Zones • Slopes

  16. Mafic Sill: Intruded between layers • Mafic magma is less viscous and hotter so • Does not form plutons but • Cuts along layers (Sills) or even across layers (Dikes) • Also Baked Zones of adjacent country rock and Chill Zones within the intrusion

  17. Igneous Rock Classification Intrusive(Plutonic) Extrusive(Volcanic) See Kehew, Fig. 3-5 Mineral Percentage Continental CrustOceanicMantle Crust

  18. Frame- Double Single Iso. work Sheet Chain Chain Igneous Rocks and Silicates The Igneous Rock Classification parallels the systematic Silicate Mineral Classification Mineral Percentage Fig. 3-6

  19. Igneous Rock Classification SilicicIntermediateMafic Granite Diorite Gabbro Rhyolite Andesite Basalt (Porphyritic) Intrusive Extrusive

  20. Bowen’s Reaction Series Temperature of Crystallization Low Silica Magma Two series of minerals formed during crystallization of magma Intrus. Extrus. Gabbro Basalt Diorite Andesite Granite Rhyolite 1200oC 1000o Frame- Double Single Isolated work Sheet Chain Chain 750o High Silica Magma

  21. Crystallization ofGabbroandBasalt ~1,450oC ~1,400o ~1,350oThe remaining melt is enriched in silica (Int-Felsic) Ca-Feldspar Olivine Olivine converts Forms Forms to Pyroxene ~1,300o ~1,250o GabbroBasalt Remaining silica crystallizes into Ca-Feldspar. Intrusive Extrusive Cooling: Slowly Quickly Mafic Melt If melt is extracted Ultramafic Rock will be formed

  22. Crystallization ofDioriteandAndesite If cooled slowly in the magma chamber then extruded, Andesite Porphyry will be formed ~1,400oC ~1,300o ~1,200o Ca Feldspar Olivine converts Ca Feldspar Forms to Pyroxene absorbs Na ~1,100o ~1,000o DioriteAndesite Pyroxene converts Remaining silica Intrusive Extrusive to Amphibole forms Na-Ca Fldspr. Slowly Cooled Quickly Cooled Intermediate Melt

  23. Crystallization ofGraniteandRhyolite ~1,200oC ~1,100o ~1,000o Na-Feldspar Pyroxene Pyroxene dissolves forms and grows forms to form Amphibole ~900o ~750o GraniteRhyolite Amph. dissolves Remaining silica Intrusive Extrusive to form Biotite forms Quartz Slowly Cooled Quickly Cooled Silicic Melt

  24. Crystallization ofGraniteandRhyolite

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