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CU-Boulder GEOL 1010 - Igneous Rocks

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1IGNEOUS ROCKSChapter 4How do oceans and continents differ?The average ocean depth is 3000m.If the Earth’s surface is 30% land and 70% ocean, why is there any land at all?Why are there not just a few little islands?How do oceans and continents differ?How do oceans and continents differ?How do oceans and continents differ?How do oceans and continents differ?Ocean crust is thin, dense, and young.Continent crust is thick, light, and old.2Polymerization of Silica•Each Si atom is surrounded by 4 oxygens.•Polymerization is the degree to which the oxygens are shared with other Si atoms.•Polymerization is the number of Si-O-Si bonds per tetrahedron.–Olivine has none: isolated SiO4groups–Pyroxene has 50% shared: SiO3chains–Mica has has 75% shared: Si4O10sheets–Quartz and feldspar have 100% (Si,Al)O2framework.Polymerization of SilicaTetrahedronPolymerization of SilicaTetrahedronOlivine (Mg2SiO4)PyroxeneMgSiO3Polymerization of SilicaMica (Sheet)Quartz (Framework)MAGMA•Magma is the term for any molten silicate material, whether below the surface or on top.•Volcanic rocks are erupted on the surface.–Volcanic rocks are fine-grained (<1 mm)•Intrusive igneous rocks crystallize from magma below the surface.–Intrusive igneous rocks are coarse-grained (> 1mm)–Pegmatites are very coarse-grained (> 1cm)3Igneous Rocks:Learning Goals•What does ‘igneous’ mean?•Composition•Mineralogy–Mantle–Oceanic Crust–Continental Crust•Igneous Fractionation–How the chemistry evolvesIntrusive Igneous Rocks•Composition•Mineralogy•Geologic Setting–Mantle–Oceanic Crust–Continental Crust•Igneous Fractionation–How the chemistry evolvesIgneous Rock Compositions•Rock compositions are described in weight percents of oxides:•SiO2, MgO, FeO, Al2O3, etc•The principal variation in igneous rock compositions is silica (SiO2) content.•The degree of polymerization of silica increases with silica content. (in both crystals and melt).Igneous Rock Compositions•Igneous rocks vary in composition (SiO2content)–ultramafic (~40 wt%) (peridotite)–mafic (45-55%) (gabbro / basalt) –intermediate (55-65%) (diorite/andesite)–silicic (65-75 wt %) (rhyolite/granite)•The mantle is peridotite (ultramafic).•The ocean crust is gabbro (mafic). •The continents are granite (silicic)Igneous Rock Names and Compositions• Intrusive Rock Name• Peridotite• Gabbro• Diorite• Granite• Composition Name• Ultramafic•Mafic• Intermediate• Silicic (felsic)• Volcanic Rock Name• (Komatiite)• Basalt• Andesite• RhyoliteIgneous Rock Mineralogy• Peridotite (Ultramafic)– Olivine ((Mg,Fe)2SiO4)– Pyroxene ((Mg,Fe,Ca)SiO3)– Garnet (Mg3Al2Si3O12) or– Spinel (MgAl2O4)• Gabbro (Mafic)– Feldspar (CaAl2Si2O8)– Pyroxene ((Mg,Fe,Ca)SiO3)– Olivine ((Mg,Fe)2SiO4)4• Diorite (Intermediate)– Feldspar (CaAl2Si2O8) – Feldspar (NaAlSi3O8)– Pyroxene ((Mg,Fe,Ca)SiO3)– Mica K(Mg,Fe,)3(Si,Al)2O10(OH)2Igneous Rock Mineralogy• Granite (Silicic)– Quartz (SiO2)– Feldspar (NaAlSi3O8) – Feldspar (KAlSi3O8)– Mica (biotite) (K(Mg,Fe,)3(Si,Al)4O10(OH)2– Mica (muscovite)– (KAl2(Si,Al)4O10(OH)2Igneous Rock Names and Compositions• Intrusive Rock Name• Peridotite• Gabbro• Diorite• Granite• Composition Name• Ultramafic•Mafic• Intermediate• Silicic (felsic)• Volcanic Rock Name• (Komatiite)• Basalt• Andesite• RhyoliteGeologic Settings•Where?•Mantle•Oceanic•Subduction Zones•Continental Mass•Compositions•Ultramafic•Mafic•Intermediate•Silicic (felsic)Geologic Settings:MantleMid-ocean ridges Oceanic IslandsIsland arcs Continental There appear to be 13 major plates that cover the globe. Igneous Fractionation:Peridotite-basalt-granite •Partial Melting–Rocks have a melting interval of several hundred degrees C.–First melt is rich in silica, water and trace elements.•Fractional Crystallization–First-formed crystals low in silica settle to the bottom.5PeridotitePeridotite Partial MeltingPeridotite Partial MeltingGarnetOlivineOrthopyroxeneClinopyroxeneFirst melt at 3-mineral junctionsMelt composition is different from hostMelt is less dense6Melt flows when pockets touchPeridotite + Basalt MeltIgneous Fractionation:7Layered Mafic Intrusion Granite OutcropIntrusive forms•Pluton: Any large discordant intrusion at depth–Batholith: large > 100 km2–Stock: small < 100 km2•Dike: Tabular non-conformable•Sill: Tabular conformableDikes:Tabular discordantDikes:Tabular discordantSills:Tabular conformable8Sills: Tabular conformableHow do oceans and continents differ?How do oceans and continents differ?•Oceanic crust is:–Thin (~7 km), –Dense (~3.1g/cm3) –Mafic (45-50% SiO2)–Young (< 200 my)•Continental Crust is:–Thick (>30 km),–Light (~2.7 g/cm3)–Silicic (> 60% SiO2)–Old (> 1000 my)How do oceans and continents differ?•Difference is result of partial melting and fractional crystallization.–Mid ocean ridge makes basalt from peridotite.–Partial melting at subduction zones makes andesite from basalt.Terms•Polymerization•Magma•Pegmatitie•Igneous Fractionation•Partial melting•Fractional Crystallization•Ultramafic•Mafic•Intermediate•Silicic•Peridotite•Gabbro•Diorite•Granite•Pluton•Stock•Batholith•Dike•Sill9Assignment•Grotzinger Chapter 5•Sedimentary RocksClicker Question•What is the most likely geologic setting in which conglomerate was deposited?–A. Continental alluvial fans–B. Continental deserts and beaches –C. Shallow-water marine–D. Deep-water marine –E. ReefsClicker Question•What is the most likely geologic setting in which sandstone was deposited?–A. Continental alluvial fans–B. Continental deserts and beaches –C. Shallow-water marine–D. Deep-water marine –E. ReefsClicker Question•What is the most likely geologic setting in which shale was deposited?–A. Continental alluvial fans–B. Continental deserts and beaches –C. Shallow-water marine–D. Deep-water marine –E. Reefs(Tropical Shallow Marine)Crust and MantleLithosphere and AsthenosphereClicker Question 1•The term for any molten silicate material on or below the Earth’s surface is:–A. Granite–B. Basalt –C. Magma–D. Pegmatite–E. Lava10Clicker Question 1•The term for any molten silicate material on or below the Earth’s surface is:–A. Granite–B. Basalt –C. Magma–D.


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CU-Boulder GEOL 1010 - Igneous Rocks

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