Chapter 4 igneous rocks


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Chapter 4 igneous rocks

  1. 1. Essentials of Geology 3 rd Edition Chapter 4 Norton Media Library
  2. 2. Up from the Inferno: Magma and Igneous Rocks Prepared by: Ronald Parker , Senior Geologist Fronterra Geosciences Houston, Oklahoma City, Denver, Anchorage, Dallas, Midland, Aberdeen, Vienna, Buenos Aires, Neuquén
  3. 3. Igneous Rocks <ul><li>Volcano – A vent where molten rock comes out of Earth. </li></ul><ul><ul><li>Example: Kilauea Volcano, Hawaii. </li></ul></ul><ul><ul><ul><li>Hot (~1,200 o C) lava pools around the volcanic vent. </li></ul></ul></ul><ul><ul><ul><li>Hot, syrupy lava runs downhill as a lava flow. </li></ul></ul></ul><ul><ul><ul><li>The lava flow slows, loses heat and crusts over. </li></ul></ul></ul><ul><ul><ul><li>Finally, the flow stops and cools, forming an igneous rock. </li></ul></ul></ul>
  4. 4. Igneous Rocks <ul><li>Solidified molten rock that freezes at high temp. </li></ul><ul><ul><li>1,100°C to 650°C. </li></ul></ul><ul><ul><li>Temp depends on composition. </li></ul></ul><ul><li>Earth is mostly igneous rock. </li></ul><ul><ul><li>Magma – Subsurface melt. </li></ul></ul><ul><ul><li>Lava – Melt at the surface. </li></ul></ul><ul><li>Volcanoes erupt magma. </li></ul>
  5. 5. Igneous Rocks <ul><li>Melted rock can cool above or below ground. </li></ul><ul><ul><li>Intrusive igneous rocks – Cool slowly underground. </li></ul></ul><ul><ul><li>Extrusive igneous rocks – Cool quickly at the surface. </li></ul></ul><ul><ul><ul><li>Lava – Cooled liquid. </li></ul></ul></ul><ul><ul><ul><li>Pyroclastic debris – Cooled fragments. </li></ul></ul></ul><ul><ul><ul><ul><li>Volcanic ash. </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Fragmented lava. </li></ul></ul></ul></ul><ul><li>Many types of igneous rocks. </li></ul><ul><ul><li>All oceanic crust. </li></ul></ul><ul><ul><li>Most continental crust. </li></ul></ul>
  6. 6. Sources of Heat <ul><li>Sources of heat to the early Earth: </li></ul><ul><ul><li>Planetesimal and meteorite accretion. </li></ul></ul><ul><ul><li>Gravitational compression. </li></ul></ul><ul><ul><li>Differentiation. </li></ul></ul><ul><ul><li>Decay of radioactive minerals. </li></ul></ul><ul><ul><li>Tidal pull of the Sun and Moon. </li></ul></ul><ul><li>The crust does NOT float on a </li></ul><ul><li>sea of molten rock. </li></ul>
  7. 7. Magma Formation <ul><li>Magma forms in special settings that melt existing rocks. </li></ul><ul><li>Partial melting occurs in the crust and upper mantle. </li></ul><ul><li>Melting is caused by… </li></ul><ul><ul><li>Pressure release. </li></ul></ul><ul><ul><li>Volatile addition. </li></ul></ul><ul><ul><li>Heat transfer. </li></ul></ul>
  8. 8. Magma Formation <ul><li>Geothermal gradient – The Earth is hot inside. </li></ul><ul><ul><li>Crustal temperature (T) averages 25°C / km of depth. </li></ul></ul><ul><ul><li>At the base of the lithosphere T ~ 1280°C. </li></ul></ul><ul><li>The geothermal gradient varies from place to place. </li></ul>
  9. 9. Magma Formation <ul><li>Pressure release. </li></ul><ul><ul><li>Base of the crust is hot enough to melt mantle rock. </li></ul></ul><ul><ul><li>Due to high pressure, the rock does not melt. </li></ul></ul><ul><ul><li>A drop in pressure initiates “decompressional melting.” </li></ul></ul><ul><ul><ul><li>Pressure drops when hot rock </li></ul></ul></ul><ul><ul><ul><li>rises to shallower depths. </li></ul></ul></ul>
  10. 10. Addition of Volatiles <ul><li>Volatiles lower the melting T of a hot rock. </li></ul><ul><li>Common volatiles include water and carbon dioxide. </li></ul><ul><li>Subduction introduces water to the mantle, melting rock. </li></ul>
  11. 11. Magma Formation <ul><li>Heat transfer. </li></ul><ul><ul><li>Rising magma carries mantle heat with it. </li></ul></ul><ul><ul><li>This raises the T in nearby crustal rock, which then melts. </li></ul></ul>
  12. 12. What is Magma Made of? <ul><li>Magmas have three components (solid, liquid and gas). </li></ul><ul><ul><li>Solid – Solidified mineral crystals are borne by the melt. </li></ul></ul><ul><ul><li>Liquid – The melt itself is comprised of mobile ions. </li></ul></ul><ul><ul><ul><li>Dominantly Si and O; lesser Al, Ca, Fe, Mg, Na, and K. </li></ul></ul></ul><ul><ul><ul><li>Other ions present to a lesser extent. </li></ul></ul></ul><ul><ul><li>Different mixes of elements yield different magmas. </li></ul></ul>
  13. 13. What is Magma Made of? <ul><ul><li>Gas – Magmas contain abundant dissolved volatile gas. </li></ul></ul><ul><ul><ul><li>Dry magma – Scarce volatiles. </li></ul></ul></ul><ul><ul><ul><li>Wet magma – To 15% volatiles. </li></ul></ul></ul><ul><ul><ul><ul><li>Water vapor (H 2 O) </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Carbon dioxide (CO 2 ) </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Sulfur dioxide (SO 2 ) </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Nitrogen (N 2 ) </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Hydrogen (H 2 ) </li></ul></ul></ul></ul>
  14. 14. Magma Compositions <ul><li>There are four major magma types based on silica (SiO 2 ) percentage. </li></ul><ul><ul><li>Felsic (feldspar and silica) 66 to 76% SiO 2 . </li></ul></ul><ul><ul><li>Intermediate 52 to 66% SiO 2 . </li></ul></ul><ul><ul><li>Mafic (Mg and Fe-rich) 45 to 52% SiO 2 . </li></ul></ul><ul><ul><li>Ultramafic 38 to 45% SiO 2 . </li></ul></ul>
  15. 15. Magma Compositions <ul><li>Composition controls magma density, T, and viscosity. </li></ul><ul><ul><li>The most important factor is silica (SiO 2 ) content. </li></ul></ul><ul><ul><ul><li>Silica-rich magmas are thick and viscous. </li></ul></ul></ul><ul><ul><ul><li>Silica-poor magmas are thin and “runny.” </li></ul></ul></ul><ul><ul><li>These characteristics govern eruptive style. </li></ul></ul>Type Density Temperature Viscosity Felsic Very low Very low (600 to 850 ° C) Very High: Explosive eruptions. Intermediate Low Low High: Explosive eruptions. Mafic High High Low: thin, hot runny eruptions. Ultramafic Very high Very high (up to 1300 ° C) Very low.
  16. 16. Magma Variation <ul><li>Why are there different magma compositions? </li></ul><ul><li>Magmas vary chemically due to… </li></ul><ul><ul><li>Initial source rock compositions. </li></ul></ul><ul><ul><li>Partial melting. </li></ul></ul><ul><ul><li>Assimilation. </li></ul></ul><ul><ul><li>Fractional crystallization. </li></ul></ul>
  17. 17. Magma Variation <ul><li>The source of the melt dictates the initial composition. </li></ul><ul><ul><li>Mantle source – Ultramafic and mafic magmas. </li></ul></ul><ul><ul><li>Crustal source – Mafic, intermediate, and felsic magmas. </li></ul></ul>
  18. 18. Partial Melting <ul><li>Upon melting, rocks rarely dissolve completely. </li></ul><ul><li>Instead, only a portion of the rock melts. </li></ul><ul><ul><li>Silica-rich minerals melt first. </li></ul></ul><ul><ul><li>Silica-poor minerals melt last. </li></ul></ul><ul><li>Partial melting, then, yields a silica-rich magma. </li></ul><ul><li>Removing a partial melt from its source creates… </li></ul><ul><ul><li>Felsic magma. </li></ul></ul><ul><ul><li>Mafic residue. </li></ul></ul>
  19. 19. <ul><li>Magma melts the country rock it passes through. </li></ul><ul><li>Blocks of country rock (xenoliths) fall into magma. </li></ul><ul><li>Assimilation of these rocks alters magma composition. </li></ul>Assimilation
  20. 20. Magma Mixing <ul><li>Different magmas may blend in a magma chamber. </li></ul><ul><li>The result combines the characteristics of the two magmas. </li></ul><ul><li>Often magma mixing is incomplete, resulting in blobs of one rock type suspended within the other. </li></ul>
  21. 21. Magma Migration <ul><li>Magma doesn’t stay put; it tends to rise upward. </li></ul><ul><ul><li>Magma may move upward in the crust. </li></ul></ul><ul><ul><li>Magma may breach the surface – a volcano. </li></ul></ul><ul><li>This transfers mass from deep to shallow parts of Earth. </li></ul><ul><ul><li>A crucial process in the Earth System. </li></ul></ul><ul><ul><li>Provides the raw material for soil, atmosphere, and ocean. </li></ul></ul>
  22. 22. Magma Migration <ul><li>Magma moves by… </li></ul><ul><ul><li>Injection into cracks. </li></ul></ul><ul><ul><li>Melting overlying rocks. </li></ul></ul><ul><ul><li>Squeezed by overburden. </li></ul></ul><ul><li>Low viscosity eases movement. Lower viscosity from… </li></ul><ul><ul><li>Higher T. </li></ul></ul><ul><ul><li>Lower Silica content. </li></ul></ul><ul><ul><li>Higher volatile content. </li></ul></ul>
  23. 23. Magma Migration <ul><li>Viscosity depends on T, volatiles, and silica. </li></ul><ul><ul><li>Temperature: </li></ul></ul><ul><ul><ul><li>Hotter – Lower viscosity </li></ul></ul></ul><ul><ul><ul><li>Cooler – Higher viscosity. </li></ul></ul></ul><ul><ul><li>Volatile content: </li></ul></ul><ul><ul><ul><li>More volatiles – Lower viscosity. </li></ul></ul></ul><ul><ul><ul><li>Less volatiles – Higher viscosity. </li></ul></ul></ul><ul><ul><li>Silica (SiO 2 ) content: </li></ul></ul><ul><ul><ul><li>Less SiO 2 (Mafic) – Lower viscosity. </li></ul></ul></ul><ul><ul><ul><li>More SiO 2 (Felsic) – Higher viscosity. </li></ul></ul></ul>
  24. 24. Cooling Rates <ul><li>Cooling rate – How fast does magma cool? </li></ul><ul><ul><li>Depth: Deep is hot; shallow is cool. </li></ul></ul><ul><ul><ul><li>Deep plutons lose heat very slowly and cool slowly. </li></ul></ul></ul><ul><ul><ul><li>Shallow flows lose heat rapidly and cool quickly. </li></ul></ul></ul><ul><ul><li>Shape: Spherical bodies cool slowly; tabular bodies cool faster. </li></ul></ul><ul><ul><li>Ground water: Ground water removes heat. </li></ul></ul>
  25. 25. Fractional Crystallization <ul><li>As magma cools, early crystals settle by gravity. </li></ul><ul><li>Melt composition changes as a result. </li></ul><ul><ul><li>Fe, Mg, and Ca are removed in early settled solids. </li></ul></ul><ul><ul><li>Si, Al, Na, and K remain in melt and increase in abundance. </li></ul></ul>The original melt is mafic. As early-formed minerals settle, the melt becomes more felsic.
  26. 26. Fractional Crystallization <ul><li>Felsic magma can evolve from mafic magma. </li></ul><ul><ul><li>Progressive removal of mafics depletes Fe, Mg, and Ca. </li></ul></ul><ul><ul><li>Remaining melt becomes enriched in Na, K, Al, and Si. </li></ul></ul>
  27. 27. Bowen’s Reaction Series <ul><li>N. L. Bowen, in the 1920s, ran experiments with melts. </li></ul><ul><li>He found with cooling, early-forming crystals settled out. </li></ul><ul><li>Settling removed elements from the remaining melt. </li></ul><ul><li>He discovered that minerals solidify in a specific series. </li></ul><ul><ul><li>Continuous – Plagioclase changed from Ca-rich to Na-rich. </li></ul></ul><ul><ul><li>Discontinuous – Minerals that solidify in a narrow T range. </li></ul></ul><ul><ul><ul><li>Olivine </li></ul></ul></ul><ul><ul><ul><li>Pyroxene </li></ul></ul></ul><ul><ul><ul><li>Amphibole </li></ul></ul></ul><ul><ul><ul><li>Biotite </li></ul></ul></ul>
  28. 28. Igneous Environments <ul><li>Two major categories – Based on cooling site. </li></ul><ul><ul><li>Extrusive settings – Cool at or near the surface. </li></ul></ul><ul><ul><ul><li>Cool rapidly. </li></ul></ul></ul><ul><ul><ul><li>Chill too fast to grow big crystals. </li></ul></ul></ul><ul><ul><li>Intrusive settings – Cool at depth. </li></ul></ul><ul><ul><ul><li>Lose heat slowly. </li></ul></ul></ul><ul><ul><ul><li>Crystals grow large. </li></ul></ul></ul><ul><li>Most mafic magmas extrude. </li></ul><ul><li>Most felsic magmas don’t. </li></ul>
  29. 29. Extrusive Settings <ul><li>Lava flows – Cool as blankets that often stack vertically. </li></ul><ul><li>Lava flows exit volcanic vents and spread outward. </li></ul><ul><li>Low-viscosity lava (basalt) can flow long distances. </li></ul><ul><li>Detail on extrusive igneous rocks appears in Chapter 5. </li></ul>
  30. 30. Extrusive Settings <ul><li>Explosive ash eruptions. </li></ul><ul><ul><li>High-viscosity felsic magma builds volcanic pressure. </li></ul></ul><ul><ul><li>Violent eruptions yield huge volumes of volcanic ash. </li></ul></ul><ul><ul><li>Ash can cover large regions. </li></ul></ul>
  31. 31. Intrusive Settings <ul><li>Intrusive rocks cool at depth; they don’t surface. </li></ul><ul><li>Magma invading colder country rock initiates… </li></ul><ul><ul><li>Thermal (heat) metamorphism and melting. </li></ul></ul><ul><ul><li>Inflation of fractures which wedges the country rock apart. </li></ul></ul><ul><ul><li>Incorporation of country rock fragments (xenoliths). </li></ul></ul><ul><ul><li>Hydrothermal (hot water) alteration. </li></ul></ul>
  32. 32. Intrusive Settings <ul><li>Intrusive contacts preserve evidence of high heat. </li></ul><ul><ul><li>Baked zone – Rim of heat-altered country rock. </li></ul></ul><ul><ul><li>Chill margin – Quenched magma at contact = tiny crystals. </li></ul></ul><ul><li>Xenolith – Altered country rock fragment in magma. </li></ul><ul><ul><li>Magma cooled before xenolith could be assimilated. </li></ul></ul>
  33. 33. Intrusive Activity <ul><li>Magma intrudes into other rocks in two ways. </li></ul><ul><ul><li>As planar, tabular bodies (dikes and sills) </li></ul></ul><ul><ul><li>As balloon-shaped blobs (plutons). </li></ul></ul><ul><li>Size varies widely. </li></ul><ul><ul><li>Plutons can be massive. </li></ul></ul><ul><ul><li>Dikes and sills tend to </li></ul></ul><ul><ul><li>be smaller. </li></ul></ul>
  34. 34. Tabular Intrusions <ul><li>Tend to have a uniform thickness. </li></ul><ul><li>Can be traced laterally. </li></ul><ul><li>Two major subdivisions. </li></ul><ul><ul><li>Sill – Parallels rock fabric. </li></ul></ul><ul><ul><li>Dike – Crosscuts rock fabric. </li></ul></ul>
  35. 35. Tabular Intrusions <ul><li>Dikes and sills modify invaded country rock. </li></ul><ul><ul><li>They cause the rock to expand and inflate. </li></ul></ul><ul><ul><li>They thermally alter the country rock. </li></ul></ul><ul><li>Dikes… </li></ul><ul><ul><li>Cut across preexisting layering (bedding or foliation). </li></ul></ul><ul><ul><li>Spread rocks sideways. </li></ul></ul><ul><ul><li>Dominate in extensional settings. </li></ul></ul>
  36. 36. Tabular Intrusions <ul><li>Sills… </li></ul><ul><ul><li>Are injected parallel to preexisting layering. </li></ul></ul><ul><ul><li>Are usually intruded close to the surface. </li></ul></ul><ul><li>Both dikes and sills exhibit wide variability in... </li></ul><ul><ul><li>Size. </li></ul></ul><ul><ul><li>Thickness (or width). </li></ul></ul><ul><ul><li>Lateral continuity. </li></ul></ul>
  37. 37. Tabular Intrusions <ul><li>Sills </li></ul><ul><ul><li>This basalt sill (dark band) was intruded into sandstones (light colored rock) in Antarctica. </li></ul></ul><ul><ul><li>When intruded, it </li></ul></ul><ul><ul><li>lifted the entire </li></ul></ul><ul><ul><li>landscape above it. </li></ul></ul>
  38. 38. Plutonic Activity <ul><li>Most magma is emplaced at depth in the Earth. </li></ul><ul><ul><li>A large, deep, igneous body is called a pluton. </li></ul></ul><ul><li>Plutonic intrusions modify the crust. </li></ul><ul><ul><li>Push aside preexisting rock. </li></ul></ul><ul><ul><li>Add new material. </li></ul></ul><ul><ul><li>Add heat. </li></ul></ul>
  39. 39. Plutonic Activity <ul><li>Plutons sometimes coalesce to form a larger batholith. </li></ul><ul><ul><li>Plutons are created above subduction zones. </li></ul></ul><ul><ul><li>Magma generation may occur over tens of millions of years. </li></ul></ul><ul><ul><li>A long subduction history is linked </li></ul></ul><ul><ul><li>to the genesis of large batholiths. </li></ul></ul>
  40. 40. Intrusive and Extrusive <ul><li>Intrusive and extrusive rocks commonly co-occur. </li></ul><ul><li>Magma chambers feed overlying volcanoes. </li></ul><ul><li>Magma chambers may cool to become plutons. </li></ul><ul><li>Many igneous geometries are possible. </li></ul>
  41. 41. Intrusive and Extrusive <ul><li>With erosion, progressively deeper features are exposed. </li></ul><ul><ul><li>Dikes. </li></ul></ul><ul><ul><li>Sills. </li></ul></ul><ul><ul><li>Laccoliths – Mushroom-shaped intrusions. </li></ul></ul>
  42. 42. Influence on Landscape <ul><li>Continued uplift and erosion exposes the pluton. </li></ul><ul><ul><li>Intrusive rocks are commonly more resistant to erosion. </li></ul></ul><ul><ul><li>Thus, intrusive rocks often stand high on the landscape. </li></ul></ul><ul><li>“ Unroofing” takes long periods of geologic time. </li></ul>
  43. 43. Igneous Textures <ul><li>The size, shape, and arrangement of the minerals. </li></ul><ul><ul><li>Interlocking – Mineral crystals fit like jigsaw puzzle pieces. </li></ul></ul><ul><ul><li>Fragmental – Pieces of preexisting rocks, often shattered. </li></ul></ul><ul><ul><li>Glassy – Made of solid glass or glass shards. </li></ul></ul><ul><li>Texture directly reflects magma history. </li></ul>Fragmental texture Interlocking or crystalline texture Glassy texture
  44. 44. Crystalline Igneous Textures <ul><li>Texture reveals cooling history. </li></ul><ul><ul><li>Aphanitic (finely crystalline). </li></ul></ul><ul><ul><ul><li>Rapid cooling. </li></ul></ul></ul><ul><ul><ul><li>Crystals do not have time to grow. </li></ul></ul></ul><ul><ul><ul><li>Extrusive. </li></ul></ul></ul><ul><ul><li>Phaneritic – (coarsely crystalline). </li></ul></ul><ul><ul><ul><li>Slow cooling. </li></ul></ul></ul><ul><ul><ul><li>Crystals have a long time to grow. </li></ul></ul></ul><ul><ul><ul><li>Intrusive. </li></ul></ul></ul>
  45. 45. Crystalline Textures <ul><li>Texture reveals cooling history. </li></ul><ul><ul><li>Porphyritic texture – A mixture of coarse and fine crystals. </li></ul></ul><ul><ul><ul><li>Indicates a two-stage cooling history. </li></ul></ul></ul><ul><ul><ul><ul><li>Initial slow cooling creates large phenocrysts. </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Subsequent eruption cools remaining magma more rapidly. </li></ul></ul></ul></ul>
  46. 46. Igneous Classification <ul><li>Classification is based upon composition and texture. </li></ul><ul><ul><li>Composition – Felsic, intermediate, mafic, and ultramafic. </li></ul></ul><ul><ul><li>Texture – Fine (aphanitic); coarse (phaneritic). </li></ul></ul>Type Aphanitic (fine) Phaneritic (coarse) Felsic Rhyolite Granite Intermediate Andesite Diorite Mafic Basalt Gabbro Ultramafic Very high Very high (up to 1300 ° C) A B A B C2 C1 C2 C1
  47. 47. <ul><li>Composition. </li></ul><ul><li>Texture. </li></ul><ul><li>A specific composition may </li></ul><ul><li>occur as different textures. </li></ul><ul><li>Example: The finely crystalline equivalent </li></ul><ul><li>of a coarse granite is a rhyolite. </li></ul><ul><li>A specific texture may be </li></ul><ul><li>found in varying compositions. </li></ul><ul><li>Example: Finely crystalline mafics are basalts; </li></ul><ul><li>finely crystalline felsics are rhyolites. </li></ul>Crystalline Classification
  48. 48. Pegmatites <ul><li>Coarse mineral crystals found in dikes. </li></ul><ul><li>Large crystals are not due to slow cooling. </li></ul><ul><li>Instead, pegmatites form from water-rich melts. </li></ul><ul><ul><li>Many unusual minerals are found in pegmatites. </li></ul></ul><ul><ul><li>Some pegmatites are rich in prized minerals. </li></ul></ul>
  49. 49. <ul><li>Form by very rapid cooling of lava in water or air. </li></ul><ul><ul><li>Glassy textures are more common in felsic magmas. </li></ul></ul><ul><ul><li>They often preserve gas bubbles (vesicles). </li></ul></ul><ul><ul><li>Underwater, basalt lava quenches into “pillows.” </li></ul></ul>Glassy Textures
  50. 50. Glassy Classification <ul><li>Glassy Igneous Rocks. </li></ul><ul><ul><li>Obsidian – Volcanic glass from rapidly cooled lava. </li></ul></ul><ul><ul><ul><li>Quenching – Lava flowing into water. </li></ul></ul></ul><ul><ul><ul><li>High silica lavas – These can make glass without quenching. </li></ul></ul></ul><ul><ul><li>Pumice – Frothy felsic rock full of vesicles; it floats. </li></ul></ul><ul><ul><li>Scoria – Glassy, vesicular, mafic rock. </li></ul></ul>
  51. 51. Fragmental Textures <ul><li>Preexisting rocks that were shattered by eruption. </li></ul><ul><li>After fragmentation, the pieces fall and are cemented. </li></ul>
  52. 52. Fragmental Classification <ul><li>aka Pyroclastic – Fragments of violent eruptions. </li></ul><ul><ul><li>Tuff – Volcanic ash that has fallen on land and solidified. </li></ul></ul><ul><ul><li>Volcanic breccia – Made of larger volcanic fragments. </li></ul></ul>
  53. 53. Igneous Activity Distribution <ul><li>Igneous activity tracks tectonic plate boundaries. </li></ul>
  54. 54. W. W. Norton & Company Independent and Employee-Owned <ul><li>This concludes the Norton Media Library PowerPoint Slide Set for Chapter 4 </li></ul><ul><li>Essentials of Geology </li></ul><ul><li>3 rd Edition (2009) </li></ul><ul><li>by Stephen Marshak </li></ul>