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GLY 155 Introduction to Physical Geology, W. Altermann © Grotzinger • Jordan Understanding Earth Sixth Edition Chapter 4: IGNEOUS ROCKS Solids from Melts © 2011 by W. H. Freeman and Company GLY 155 Introduction to Physical Geology, W. Altermann © Chapter 4: Igneous Rocks: Solids from Melts 1 GLY 155 Introduction to Physical Geology, W. Altermann © About Igneous Rocks • Igneous rocks form from liquid rock (magma) in several different ways. • Igneous processes within the Earth produce intrusive igneous rocks. • Igneous processes on or near Earth’s surface produce extrusive igneous rocks. GLY 155 Introduction to Physical Geology, W. Altermann © Lecture Outline 1. How do igneous rocks differ from one another? 2. How do magmas form? 3. Magmatic differentiation 4. Forms of igneous intrusions 5. Igneous processes and plate tectonics 2 GLY 155 Introduction to Physical Geology, W. Altermann © 1. How Do Igneous Rocks Differ from One Another? Texture – size of crystals Coarse-grained rocks Fine-grained rocks Mixed texture rocks GLY 155 Introduction to Physical Geology, W. Altermann © 1. How Do Igneous Rocks Differ from One Another? 3 GLY 155 Introduction to Physical Geology, W. Altermann © 1. How Do Igneous Rocks Differ from One Another? Clues about significance of texture Early studies of volcanic rocks Laboratory crystallization studies Studies of slow cooling in granite GLY 155 Introduction to Physical Geology, W. Altermann © 1. How Do Igneous Rocks Differ from One Another? Texture is related to rate of cooling. Intrusive igneous rocks Extrusive igneous rocks 4 GLY 155 Introduction to Physical Geology, W. Altermann © 1. How Do Igneous Rocks Differ from One Another? Pyroclasts Volcanic ash Extrusive rocks Bomb Pumice Mafic Basalt Felsic Rhyolite Gabbro Granite Porphyry Intrusive rocks Extrusive pyroclasts form in violent eruptions from lava in the air. Extrusive igneous rocks cool rapidly and are finegrained. Intrusive igneous rocks cool slowly, allowing large, coarse crystals to form. Phenocrysts Porphyry Some phenocrysts grow large, but the remaining melt cools faster, forming smaller crystals during an eruption. GLY 155 Introduction to Physical Geology, W. Altermann © Chemical and Mineral Composition of Igneous Rocks Two basic compositional groups: Felsic igneous rocks Mafic igneous rocks 5 GLY 155 Introduction to Physical Geology, W. Altermann © GLY 155 Introduction to Physical Geology, W. Altermann © Chemical and Mineral Composition of Igneous Rocks Four compositional groups: Felsic igneous rocks Intermediate igneous rocks Mafic igneous rocks Ultramafic igneous rocks 6 GLY 155 Introduction to Physical Geology, W. Altermann © GLY 155 Introduction to Physical Geology, W. Altermann © 2. How Do Magmas Form? Why do rocks melt? 7 GLY 155 Introduction to Physical Geology, W. Altermann © 2. How Do Magmas Form? What is a magma chamber? A rising mass of magma that pushes aside crustal rocks as it rises through the crust. GLY 155 Introduction to Physical Geology, W. Altermann © 2. How Do Magmas Form? A temperature of about 1000°C is required for partial melting of crustal rocks. A depth of at least 40 km is required for temperatures of 1000°C to occur. 8 GLY 155 Introduction to Physical Geology, W. Altermann © 3. Magmatic Differentiation A process by which rocks of varying composition can arise from a uniform parent magma. The first minerals to crystallize from a cooling magma are the ones that are the last to melt. GLY 155 Introduction to Physical Geology, W. Altermann © 3. Magmatic Differentiation Fractional crystallization is the process by which the crystals are formed in a cooling magma and are segregated from the remaining liquid. Example: basaltic intrusion like Palisades, New Jersey 9 GLY 155 Introduction to Physical Geology, W. Altermann © 3. Magmatic Differentiation THE PALISADES INTRUSION An excellent example from South Africa is the Bushveld Intrusion GLY 155 Introduction to Physical Geology, W. Altermann © 3. Magmatic Differentiation BOWEN’S REACTION SERIES Magma composition Temperature ~600°C Orthoclase feldspar Quartz Biotite mica Pyroxene Olivine Sodiumrich e l as ioc ar a g sp Pl eld f Amphibole ~1200°C Felsic, Rhyolitic (high silica) Muscovite mica Simultaneous crystallization Calciumrich Intermediate, andesitic Mafic, basaltic Ultramafic (low silica) 10 GLY 155 Introduction to Physical Geology, W. Altermann © BOWEN’S REACTION SERIES As magma temperature decreases… Magma composition Temperature ~600°C Orthoclase feldspar Quartz Sodiumrich Biotite mica e l as ioc ar a g sp Pl eld f Amphibole Pyroxene ~1200°C Felsic, Rhyolitic (high silica) Muscovite mica Olivine Simultaneous crystallization Calciumrich Intermediate, andesitic Mafic, basaltic Ultramafic (low silica) GLY 155 Introduction to Physical Geology, W. Altermann © BOWEN’S REACTION SERIES As magma temperature decreases… Temperature ~600°C …materials crystallize in an ordered series… Magma composition Orthoclase feldspar Quartz Biotite mica Pyroxene Olivine Sodiumrich e l as ioc ar a g sp Pl eld f Amphibole ~1200°C Felsic, Rhyolitic (high silica) Muscovite mica Simultaneous crystallization Calciumrich Intermediate, andesitic Mafic, basaltic Ultramafic (low silica) 11 GLY 155 Introduction to Physical Geology, W. Altermann © BOWEN’S REACTION SERIES …while plagioclase feldspar crystallizes, from calcium-rich to sodium-rich form… As magma temperature decreases… Temperature ~600°C …materials crystallize in an ordered series… Magma composition Orthoclase feldspar Quartz Sodiumrich Biotite mica e l as ioc ar a g sp Pl eld f Amphibole Pyroxene ~1200°C Felsic, Rhyolitic (high silica) Muscovite mica Olivine Simultaneous crystallization Intermediate, andesitic Mafic, basaltic Ultramafic (low silica) Calciumrich GLY 155 Introduction to Physical Geology, W. Altermann © BOWEN’S REACTION SERIES …while plagioclase feldspar crystallizes, from calcium-rich sodium-rich form… As magma temperature decreases… Temperature ~600°C …materials crystallize in an ordered series… Magma composition Orthoclase feldspar Quartz Biotite mica Pyroxene Olivine Sodiumrich e l as ioc ar a g sp Pl eld f Amphibole ~1200°C Felsic, Rhyolitic (high silica) Muscovite mica Simultaneous crystallization Calciumrich Intermediate, andesitic Mafic, basaltic Ultramafic (low silica) …and the composition of magma changes from ultramafic to andesitic. 12 GLY 155 Introduction to Physical Geology, W. Altermann © 3. Magmatic Differentiation GRANITE AND BASALT Crystallizing minerals Magma chamber A Magma chamber B Partial melting of country rock Basaltic magma GLY 155 Introduction to Physical Geology, W. Altermann © Partial melting creates a magma of a particular composition. Cooling causes minerals to crystallize and settle. Crystallizing minerals Magma chamber A Magma chamber A Magma chamber B Partial melting of country rock A basaltic magma chamber breaks through. Mixing results in andesitic magma. Magma chamber B Crystals may accumulate on the sides and roof of the chamber due to turbulence. Basaltic magma 13 GLY 155 Introduction to Physical Geology, W. Altermann © 4. Forms of Igneous Intrusions Plutons Discordant intrusions Batholiths Stocks Dikes Concordant intrusions Sills Veins GLY 155 Introduction to Physical Geology, W. Altermann © 4. Forms of Igneous Intrusions 14 GLY 155 Introduction to Physical Geology, W. Altermann © Lava flow (extrusive) Ash falls and pyroclasts (effusive) Country rock Volcano Volcanic neck with radiating dikes Dik e Stock Dikes cut across layers of country rock… Sill Sill Dik e Dike Sill ton Plu lith tho Ba …but sills run parallel to them. Batholiths are the largest forms of plutons, covering at least 100 km2. GLY 155 Introduction to Physical Geology, W. Altermann © 15 GLY 155 Introduction to Physical Geology, W. Altermann © GLY 155 Introduction to Physical Geology, W. Altermann © 5. Igneous Processes and Plate Tectonics Magma factories: Spreading centers Subduction zones Mantle plumes 16 GLY 155 Introduction to Physical Geology, W. Altermann © 5. Igneous Processes and Plate Tectonics GLY 155 Introduction to Physical Geology, W. Altermann © 17 GLY 155 Introduction to Physical Geology, W. Altermann © GLY 155 Introduction to Physical Geology, W. Altermann © 5. Igneous Processes and Plate Tectonics Origin of magma in magma factories: Decompression melting in spreading centers Fluid-induced melting in subduction zones 18 GLY 155 Introduction to Physical Geology, W. Altermann © Decompression Melting: Spreading Centers Pillow lava Newer, thinner sediments Older, thicker sediments Sheeted dikes in basalt Oceanic crust Gabbro Moho Peridotite layer Mantle Spreading center GLY 155 Introduction to Physical Geology, W. Altermann © Hot mantle rises, decompresses, and melts. Pillow lava Newer, thinner sediments Older, thicker sediments Sheeted dikes in basalt Oceanic crust Gabbro Moho Peridotite layer Mantle Spreading center 19 GLY 155 Introduction to Physical Geology, W. Altermann © Dikes A thin dike erupts, spilling lava in “pillows.” Dikes intruding dikes Hot mantle rises, decompresses, and melts. Pillow lava Newer, thinner sediments Older, thicker sediments Sheeted dikes in basalt Oceanic crust Gabbro Moho Peridotite layer Mantle Spreading center Dikes A thin dike erupts, spilling lava in “pillows.” Dikes intruding dikes Hot mantle rises, decompresses, and melts. Pillow lava Newer, thinner sediments Older, thicker sediments Sheeted dikes in basalt Oceanic crust Gabbro Cold seawater Sheeted dikes Sediments are deposited on the spreading seafloor. The gabbro layer metamorphoses by contact with the magma. Moho Peridotite layer Mantle Dikes intrude dikes to form sheeted dikes. Spreading center Heated seawater carrying dissolved minerals Magma chamber Peridotite layer Mantle Seawater filters through the basalt layer, where it is heated. The heated seawater then rises. Dissolved minerals precipitate in the ocean. Crystals settle out of the magma, forming the peridotite layer. 20 GLY 155 Introduction to Physical Geology, W. Altermann © Fluid-Induced Melting – Subduction Zones Trench Oceanic sediments Magma chamber Oceanic crust basalt Oceanic mantle lithosphere Asthenosphere GLY 155 Introduction to Physical Geology, W. Altermann © Trench Oceanic sediments Magma chamber Oceanic crust basalt Oceanic mantle lithosphere Asthenosphere Subducting oceanic crust carries sediments with it. Sediment grains Water 21 GLY 155 Introduction to Physical Geology, W. Altermann © Trench Oceanic sediments Magma chamber Oceanic crust basalt Oceanic mantle lithosphere Asthenosphere Subducting oceanic crust carries sediments with it. Water remains trapped as the pressure and temperature increase. Sediment grains Water Magma of intermediate composition is erupted to form arc volcanoes. Molten sediments combine with lithospheric magma. Trench Oceanic sediments Magma chamber Oceanic crust basalt Oceanic mantle lithosphere The water and molten sediments melt parts of the overlying plate. Asthenosphere Subducting oceanic crust carries sediments with it. H2O H2O H2O Water remains trapped as the pressure and temperature increase. Sediment grains Water …causing the sedimentary rocks to melt at lower temperatures. The trapped water is released as the temperature increases,… 22 GLY 155 Introduction to Physical Geology, W. Altermann © Thought questions for this chapter How would you classify a coarsecoarse-grained igneous rock that contains about 50 percent pyroxene and 50 percent olivine? What kind of rock would contain some plagioclase feldspar crystals about 5 mm long “floating” floating” in a dark gray matrix of crystals less than 1 mm? What differences in crystal size might you expect to find between two sills, one intruded at a depth of about 12 km, where country rock was very hot, and the other at a depth of 0.5 km, where the country rock was moderately warm? GLY 155 Introduction to Physical Geology, W. Altermann © Thought questions for this chapter Assume that a magma with a certain ratio of calcium to sodium starts to crystallize. If fractional crystallization occurs during the solidification process, will the plagioclase feldspars formed after complete crystallization have the same ratio of sodium that characterized the magma? Why are plutons more likely than dikes to show the effects of fractional crystallization? What might be the origin of a rock composed almost entirely of olivine? What processes create the unequal sizes of crystals in porphyries? 23 GLY 155 Introduction to Physical Geology, W. Altermann © Thought questions for this chapter What observations would show that a pluton solidified during fractional crystallization? If you were to drill a hole through the crust of a midmid-ocean ridge, what intrusive or extrusive igneous rocks might you expect to encounter at or near the surface? What intrusive or extrusive igneous rocks might you expect at the base of the crust? Water is abundant in the sedimentary rocks and oceanic crust of subduction zones. How would the water affect melting in these zones? Why are granitic and andesitic rocks so plentiful? GLY 155 Introduction to Physical Geology, W. Altermann © Key terms and concepts Andesite Basalt Batholith Bomb Concordant intrusion Country rock Dacite Decompression melting Dike Discordant intrusion Extrusive igneous rock Felsic rock FluidFluid-induced melting Fractional crystallization Gabbro 24 GLY 155 Introduction to Physical Geology, W. Altermann © Key terms and concepts Granodiorite Intermediate igneous rock Intrusive igneous rock Lava Mafic rock Magma chamber Magmatic differentiation Obsidian Ophiolite suite Partial melting Pegmatite Peridotite Pluton Porphyry Pumice GLY 155 Introduction to Physical Geology, W. Altermann © Key terms and concepts Pyroclast Rhyolite Sill Stock Tuff Ultramafic rock Vein Viscosity Volcanic ash 25