How do oceans and continents differ Chapter 4 IGNEOUS ROCKS 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 1 Polymerization 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 Polymerization of Silica Olivine has none isolated SiO4 groups Pyroxene has 50 shared SiO3 chains Mica has has 75 shared Si4O10 sheets Quartz and feldspar have 100 Si Al O2 framework Polymerization of Silica Pyroxene Tetrahedron Tetrahedron Polymerization of Silica Mica Sheet Quartz Framework Olivine Mg2SiO4 MgSiO3 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 2 Igneous Rocks Learning Goals Intrusive Igneous Rocks Composition Mineralogy Geologic Setting What does igneous mean Composition Mineralogy Mantle Oceanic Crust Continental Crust Mantle Oceanic Crust Continental Crust Igneous Fractionation Igneous Fractionation How the chemistry evolves How the chemistry evolves Igneous 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 Names and Compositions Volcanic Composition Intrusive Name Rock Name Rock Name Ultramafic Mafic Intermediate Silicic felsic Peridotite Gabbro Diorite Granite Komatiite Basalt Andesite Rhyolite Igneous Rock Compositions Igneous rocks vary in composition SiO2 content 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 Mineralogy Peridotite Ultramafic Gabbro Mafic Olivine Mg Fe 2SiO4 Pyroxene Mg Fe Ca SiO3 Garnet Mg3Al2Si3O12 or Spinel MgAl2O4 Feldspar CaAl2Si2O8 Pyroxene Mg Fe Ca SiO3 Olivine Mg Fe 2SiO4 3 Igneous Rock Names and Compositions Igneous Rock Mineralogy Diorite Intermediate Granite Silicic Feldspar CaAl2Si2O8 Feldspar NaAlSi3O8 Pyroxene Mg Fe Ca SiO3 Mica K Mg Fe 3 Si Al 2O10 OH 2 Quartz SiO2 Feldspar NaAlSi3O8 Feldspar KAlSi3O8 Mica biotite K Mg Fe 3 Si Al 4O10 OH 2 Mica muscovite KAl2 Si Al 4O10 OH 2 Geologic Settings Compositions Where Ultramafic Mafic Intermediate Silicic felsic Volcanic Composition Intrusive Name Rock Name Rock Name Ultramafic Mafic Intermediate Silicic felsic Peridotite Gabbro Diorite Granite Komatiite Basalt Andesite Rhyolite Geologic Settings Mantle Mid ocean ridges Oceanic Islands Island arcs Continental Mantle Oceanic Subduction Zones Continental Mass 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 4 Peridotite Peridotite Partial Melting Peridotite Partial Melting First melt at 3 mineral junctions Garnet Orthopyroxene Olivine Clinopyroxene Melt composition is different from host Melt is less dense 5 Melt flows when pockets touch Peridotite Basalt Melt Igneous Fractionation 6 Layered Mafic Intrusion Granite Outcrop Intrusive forms Dikes Tabular discordant Pluton Any large discordant intrusion at depth Batholith large 100 km2 Stock small 100 km2 Dike Tabular non conformable Sill Tabular conformable Dikes Tabular discordant Sills Tabular conformable 7 Sills Tabular conformable How 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 Sill 8 Assignment Grotzinger Chapter 5 Sedimentary Rocks Clicker 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 Reefs Crust and Mantle Lithosphere and Asthenosphere Clicker 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 Reefs Clicker 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 Clicker 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 Lava 9 Clicker 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 Lava Clicker Question 2 The most abundant element in the Earth is A Hydrogen B Oxygen C Magnesium D Silicon E Iron Clicker Question 2 The most abundant element in the Earth is Clicker Question 3 The composition of the mantle is said to be Clicker Question 3 A Ultramafic B Mafic C Intermediate D Silicic E Sedimentary A Ultramafic B Mafic C Intermediate D Silicic E Sedimentary Clicker Question The composition of the mantle is said to be A Hydrogen B Oxygen C Magnesium D Silicon E Iron Any small or large discordant intrusion at depth is known as a A Pluton B Batholith C Stock D Dike E Sill 10 Clicker Question Any small or large discordant
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