GLY 155 Introduction to Physical Geology, W. Altermann. Grotzinger Jordan. Understanding Earth. Sixth Edition

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1 Grotzinger Jordan Understanding Earth Sixth Edition Chapter 4: IGNEOUS ROCKS Solids from Melts 2011 by W. H. Freeman and Company Chapter 4: Igneous Rocks: Solids from Melts 1

2 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. 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

3 1. How Do Igneous Rocks Differ from One Another? Texture size of crystals Coarse-grained rocks Fine-grained rocks Mixed texture rocks 1. How Do Igneous Rocks Differ from One Another? 3

4 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 1. How Do Igneous Rocks Differ from One Another? Texture is related to rate of cooling. Intrusive igneous rocks Extrusive igneous rocks 4

5 1. How Do Igneous Rocks Differ from One Another? Pyroclasts Volcanic ash Bomb Pumice Extrusive pyroclasts form in violent eruptions from lava in the air. Porphyry Intrusive rocks Extrusive rocks Mafic Basalt Felsic Rhyolite Extrusive igneous rocks cool rapidly and are finegrained. Gabbro Granite Intrusive igneous rocks cool slowly, allowing large, coarse crystals to form. Phenocrysts Some phenocrysts grow large, but the remaining melt cools faster, forming smaller Porphyry crystals during an eruption. Chemical and Mineral Composition of Igneous Rocks Two basic compositional groups: Felsic igneous rocks Mafic igneous rocks 5

6 Chemical and Mineral Composition of Igneous Rocks Four compositional groups: Felsic igneous rocks Intermediate igneous rocks Mafic igneous rocks Ultramafic igneous rocks 6

7 2. How Do Magmas Form? Why do rocks melt? 7

8 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. 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 C to occur. 8

9 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. 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

10 3. Magmatic Differentiation THE PALISADES INTRUSION An excellent example from South Africa is the Bushveld Intrusion 3. Magmatic Differentiation BOWEN S S REACTION SERIES Temperature ~600 C ~1200 C Olivine Orthoclase feldspar Muscovite mica Quartz Biotite mica Amphibole Pyroxene Plagioclase feldspar Simultaneous crystallization Sodiumrich Calciumrich Magma composition Felsic, Rhyolitic (high silica) Intermediate, andesitic Mafic, basaltic Ultramafic (low silica) 10

11 BOWEN S S REACTION SERIES As magma temperature decreases Temperature ~600 C ~1200 C Olivine Orthoclase feldspar Muscovite mica Quartz Biotite mica Amphibole Pyroxene Plagioclase feldspar Simultaneous crystallization Magma composition Felsic, Rhyolitic (high silica) Intermediate, andesitic Mafic, basaltic Ultramafic (low silica) BOWEN S S REACTION SERIES As magma temperature decreases Temperature ~600 C ~1200 C materials crystallize in an ordered series Olivine Orthoclase feldspar Muscovite mica Quartz Biotite mica Amphibole Pyroxene Plagioclase feldspar Simultaneous crystallization Sodiumrich Calciumrich Sodiumrich Calciumrich Magma composition Felsic, Rhyolitic (high silica) Intermediate, andesitic Mafic, basaltic Ultramafic (low silica) 11

12 BOWEN S S REACTION SERIES Temperature ~600 C As magma temperature decreases materials crystallize in an ordered series Orthoclase feldspar Muscovite mica Quartz while plagioclase feldspar crystallizes, from calcium-rich to sodium-rich form Magma composition Felsic, Rhyolitic (high silica) ~1200 C Olivine Biotite mica Amphibole Pyroxene Plagioclase feldspar Simultaneous crystallization Intermediate, andesitic Mafic, basaltic Ultramafic (low silica) BOWEN S S REACTION SERIES Temperature ~600 C As magma temperature decreases materials crystallize in an ordered series Orthoclase feldspar Muscovite mica Quartz while plagioclase feldspar crystallizes, from calcium-rich sodium-rich form Magma composition Felsic, Rhyolitic (high silica) ~1200 C Olivine Biotite mica Amphibole Pyroxene Plagioclase feldspar Simultaneous crystallization Sodiumrich Calciumrich Sodiumrich Calciumrich Intermediate, andesitic Mafic, basaltic Ultramafic (low silica) and the composition of magma changes from ultramafic to andesitic. 12

13 3. Magmatic Differentiation GRANITE AND BASALT Crystallizing minerals Magma chamber A Magma chamber B Partial melting of country rock Basaltic magma Partial melting creates a magma of a particular composition. Cooling causes minerals to crystallize and settle. Crystallizing minerals Magma chamber A Magma chamber B Magma chamber A 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. Partial melting of country rock Basaltic magma 13

14 4. Forms of Igneous Intrusions Plutons Discordant intrusions Batholiths Stocks Dikes Concordant intrusions Sills Veins 4. Forms of Igneous Intrusions 14

15 Country rock Volcano Lava flow (extrusive) Ash falls and pyroclasts (effusive) Volcanic neck with radiating dikes Stock Dike Sill Sill Sill Dike Dike Pluton Pluton Batholith Dikes cut across layers of country rock but sills run parallel to them. Batholiths are the largest forms of plutons, covering at least 100 km 2. 15

16 5. Igneous Processes and Plate Tectonics Magma factories: Spreading centers Subduction zones Mantle plumes 16

17 5. Igneous Processes and Plate Tectonics 17

18 5. Igneous Processes and Plate Tectonics Origin of magma in magma factories: Decompression melting in spreading centers Fluid-induced induced melting in subduction zones 18

19 Decompression Melting: Spreading Centers Pillow lava Newer, thinner sediments Older, thicker sediments Oceanic crust Moho Mantle Sheeted dikes in basalt Gabbro Peridotite layer Spreading center Hot mantle rises, decompresses, and melts. Pillow lava Newer, thinner sediments Older, thicker sediments Oceanic crust Moho Mantle Sheeted dikes in basalt Gabbro Peridotite layer Spreading center 19

20 Hot mantle rises, decompresses, and melts. A thin dike erupts, spilling lava in pillows. Dikes Dikes intruding dikes Pillow lava Newer, thinner sediments Older, thicker sediments Oceanic crust Moho Mantle Sheeted dikes in basalt Gabbro Peridotite layer Spreading center Hot mantle rises, decompresses, and melts. Oceanic crust Moho Mantle Cold seawater Sheeted dikes A thin dike erupts, spilling lava in pillows. Pillow lava Spreading center Heated seawater carrying dissolved minerals Mantle Sheeted dikes in basalt Gabbro Peridotite layer Dikes Newer, thinner sediments Older, thicker sediments Magma chamber Peridotite layer Dikes intruding dikes Dikes intrude dikes to form sheeted dikes. Sediments are deposited on the spreading seafloor. The gabbro layer metamorphoses by contact with the magma. 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

21 Fluid-Induced Melting Subduction Zones Trench Oceanic sediments Oceanic crust basalt Oceanic mantle lithosphere Magma chamber Asthenosphere Trench Oceanic sediments Oceanic crust basalt Oceanic mantle lithosphere Magma chamber Asthenosphere Subducting oceanic crust carries sediments with it. Sediment grains Water 21

22 Trench Oceanic sediments Oceanic crust basalt Oceanic mantle lithosphere Magma chamber 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 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 chamber H 2 O H 2 O H 2 O The trapped water is released as the temperature increases, The water and molten sediments melt parts of the overlying plate. causing the sedimentary rocks to melt at lower temperatures. 22

23 Thought questions for this chapter How would you classify a coarse-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 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? 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

24 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 mid-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? Key terms and concepts Andesite Basalt Batholith Bomb Concordant intrusion Country rock Dacite Decompression melting Dike Discordant intrusion Extrusive igneous rock Felsic rock Fluid-induced induced melting Fractional crystallization Gabbro 24

25 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 Key terms and concepts Pyroclast Rhyolite Sill Stock Tuff Ultramafic rock Vein Viscosity Volcanic ash 25