Igneous rocks


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Nature of Igneous Rocks, Magma, Lava, Textures, Types classification,compositions,Bowen’s Reaction Series, characteristics of magma, Origin of Magmas, Evolution of Magma, Magma Differentiation,Partial Melting,Fractional Crystallization, Plate Tectonic Setting of Igneous Rocks

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Igneous rocks

  1. 1. Igneous Rocks
  2. 2. Sawtooth Mtns. near Stanley, ID -example of a intrusive igneous rocks….not to mention a very cool place to do fieldwork……
  3. 3. Lava flow in Hawaii [magma @ the Earth’s surface] Pyroclastic Flow Montserrat Volcano
  4. 4. What is a rock? [Yes, you really do care.]
  5. 5. The Nature of Igneous Rocks • Form from Magma [Greek=“paste”] – Hot, partially molten mixture of solid liquid and gas – Gases: H2O, CO2, etc. – less dense than solid rock – solidify upon cooling
  6. 6. “The beer I had for breakfast wasn’t bad so I had one more for desert.” -Sunday Morning Coming Down by Kris Kristofferson
  7. 7. • Magma vs. Lava – Magma: molten rock beneath the surface – Lava: molten rock that has reached the surface – Magma: form intrusive igneous rocks – Lava: form extrusive igneous rocks
  8. 8. • Composition varies widely – Oxygen plus major elements – Generally a silica (SiO2) melt – Silica and water content control viscosity – Silica content used in classification
  9. 9. Mafic Magmas • Silica content ~ 50% • High Fe, Mg and Ca • High temperature molten magma – 1000o to 1200oC • Major minerals: – Olivine - Ca Plagioclase – Pyroxene
  10. 10. Silicic Magma • Silica content: 65-77% • High Al, Na and K • Lower temperature magmas – Less than 850oC • Major minerals: – Feldspars - Micas – Quartz
  11. 11. Magma Viscosity -resistance to flow
  12. 12. • Controlled by silica and water content, and temperature • As magma cools-silica tetrahedron form links • Linkages control viscosity: •High Silica=high viscosity •Low Silica=low viscosity
  13. 13. Magma Viscosity • High Silica=high viscosity/Low Silica=low viscosity • Cooler Temperatures=higher viscosity/Higher Temperatures=lower viscosity • Water and volatiles break linkages=lower viscosity
  14. 14. Occurrence of Igneous Rocks • Found globally • Formed in discrete geologic settings – Convergent plate margins – Divergent plate margins – Mantle plumes
  15. 15. Igneous Textures • Texture - the size, shape and relationship of minerals in the rock • Cooling history of the magma or lava • Crystal size increases as rate of cooling slows • Texture - the size, shape, and arrangement of interlocking minerals • Factors affecting crystal size • Rate of cooling Slow rate = fewer but larger crystals Fast rate = many small crystals Very fast rate forms glass
  16. 16. IGNEOUS TEXTURES  Types of igneous textures Aphanitic (fine-grained) texture  Rapid rate of cooling  Microscopic crystals  May contain vesicles (holes from gas bubbles) Phaneritic (coarse-grained) texture  Slow cooling  Large, visible crystals  Kinds of igneous textures  Porphyritic texture  Glassy texture  Pyroclastic texture  Pegmatitic texture  Exceptionally coarse grained  Form in late stages of crystallization of granitic magmas
  19. 19. Glassy Texture • Very rapid cooling - quenched – Volcanic glass – Conchoidal fracture – Rock is called obsidian • No apparent crystals Lava flow quenching in ocean water.
  21. 21. Glassy texture in obsidian
  22. 22. Crystalline Textures • Crystal growth requires time for ions to migrate - form minerals • Slow rate of cooling=time for crystal growth • Crystals grow until melt is quenched or completely solidified
  23. 23. Aphanitic Texture • Fine grained texture • Few crystals visible in hand specimen • Relatively rapid rate of cooling
  24. 24. Aphanitic texture in rhyolite
  25. 25. Phaneritic Texture • Coarse grained texture • Relatively slow rate of cooling • Equigranular, interlocking crystals • Slow cooling = crystallization at depth
  26. 26. Phaneritic texture in granite
  27. 27. Pegmatites from the Sawtooth Mtns. -unusually large crystals
  28. 28. Porphyritic Texture  Minerals form at different temperatures  Large crystals (phenocrysts) are embedded in a matrix of smaller crystals (groundmass) • Well formed crystals (phenocrysts) • Fine grained matrix (groundmass) • Complex cooling history – Initial stage of slow cooling – Later stage of rapid cooling
  30. 30. Pyroclastic Texture • Fragmental appearance produced by violent volcanic eruptions • Explosive volcanic eruptions • Appear porphyritic with visible crystals – Crystals show breakage or distortion • Matrix dominated by glassy fragments – Hot fragments may “weld” together
  31. 31. IGNEOUS COMPOSITIONS  Granitic versus basaltic compositions Granitic [felsic] composition  Light-colored silicates  Termed felsic (feldspar and silica) in composition  High silica (SiO2) content  Major constituent of continental crust Basaltic [mafic] composition  Dark silicates and Ca-rich feldspar  Termed mafic (magnesium and ferrum, for iron) in composition  Higher density than granitic rocks  Comprise the ocean floor and many volcanic islands
  32. 32. GRANITE
  33. 33. RHYOLITE
  34. 34. BASALT
  35. 35. GABBRO
  36. 36. IGNEOUS COMPOSITIONS  Other compositional groups Intermediate (or andesitic) composition  Contain 25% or more dark silicate minerals  Associated with explosive volcanic activity Ultramafic composition  Rare composition that is high in magnesium and iron  Composed entirely of ferromagnesian silicates
  37. 37. ANDESITE
  38. 38. DIORITE
  39. 39. IGNEOUS COMPOSITIONS  Silica content as an indicator of composition Crustal rocks exhibit a considerable range  45% to 70%  Silica content influences magma behavior Granitic magmas = high silica content and viscous Basaltic magmas = much lower silica content and more fluid-like behavior
  40. 40. GENERAL CHARACTERISTICS OF MAGMA  Igneous rocks form as molten rock cools and solidifies  General characteristics of magma  Parent material of igneous rocks  Forms from partial melting of rocks  Magma at surface is called lava  Rocks formed from lava = extrusive, or volcanic rocks  Rocks formed from magma at depth = intrusive, or plutonic rocks  The nature of magma  Consists of three components:  Liquid portion = melt  Solids, if any, are silicate minerals  Volatiles = dissolved gases in the melt, including water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2)
  41. 41. Classification of Igneous Rocks –Texture • Aphanitic • Phanaritic –Composition • Silicic • Intermediate • Mafic • Ultramafic Combination of Texture and Composition produces rock name
  42. 42. Origin of Magmas Generating magma from solid rock • Role of heat  Temperature increases in the upper crust (geothermal gradient) average between 20oC to 30oC per kilometer  Rocks in the lower crust and upper mantle are near their melting points  Additional heat may induce melting • Role of pressure  Increases in confining pressure increases a rock’s melting temperature  When confining pressures drop, decompression melting occurs • Role of volatiles  Volatiles (primarily water) cause melting at lower temperatures  Important factor where oceanic lithosphere descends into the mantle • Solid rock is at equilibrium with its surrounding • Changes in the surroundings cause solid rock magma [melting] • Lowering P  Mantle convection moves deep mantle rocks upwards • Raising T  Hot mafic magma intrudes into the crust • Changing composition  Adding small amounts of water
  43. 43. How do rocks melt? Heat: Geothermal Gradient Temperature of most Silicic magmas [600-800oC] -about the base of crust Temperature of Mafic Magmas [1100-1200oC] -upper mantle conditions
  44. 44. EVOLUTION OF MAGMAS  A single volcano may extrude lavas exhibiting very different compositions  Bowen’s reaction series Minerals crystallize in a systematic fashion based on their melting points During crystallization, the composition of the liquid portion of the magma continually changes
  46. 46. EVOLUTION OF MAGMAS  Processes responsible for changing a magma’s composition Magmatic differentiation  Separation of a melt from earlier formed crystals Assimilation  Changing a magma’s composition by incorporating surrounding rock bodies into a magma Magma mixing  Two chemically distinct magmas may produce a composition quite different from either original magma
  47. 47. Magma Differentiation • Magmas, and the resulting igneous rocks, show a wide range of compositions • Source Rock – variations cause major and minor variations in the magma • Magma Mixing • Assimilation
  48. 48. • Partial Melting – Individual minerals in source rock melt at different T – Magma is enriched in many elements, especially SiO2 How do we know this? Norman Bowen [1887-1956] -Experimental Petrologist -developed Bowen’s Reaction Series
  49. 49. Bowen’s reaction series
  50. 50. • Fractional Crystallization – Individual minerals precipitate from magma at different T – Magma is enriched in many elements, especially SiO2 – Magma may migrate [buoyancy]
  51. 51. Bowen’s reaction series
  52. 52. Plate Tectonic Setting of Igneous Rocks • Divergent Plate Boundaries – Partial melting of mantle produces basaltic magma • Convergent Plate Boundaries – Subduction produces partial melting of basalt, sediments, parts of mantle – Andesitic and rhyolitic magma – Ascending magma assimilates lower crustal material • Mantle Plumes – Partial melting of plumes of mantle material – Basaltic magma is produced – Rising magma produce • Intraplate island chains • Flood basalt [Columbia River Basalts]
  54. 54. FIGURE 4.21