Chapter 4 igneous rocks

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