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Op ch06 lecture_earth3, igneous rocks

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Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava. Igneous rock may ...

Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava. Igneous rock may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can be derived from partial melts of pre-existing rocks in either a planet's mantle or crust. Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Over 700 types of igneous rocks have been described, most of them having formed beneath the surface of Earth's crust

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Op ch06 lecture_earth3, igneous rocks Presentation Transcript

  • 1. Chapter 6 Up from the Inferno: Magma and Igneous Rocks LECTURE OUTLINE earth Portrait of a Planet Third Edition ©2008 W. W. Norton & Company, Inc. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 2. Up from the Inferno: Magma and Igneous Rocks Prepared by Ron Parker Earlham College Department of Geosciences Richmond, Indiana
  • 3. Igneous Rocks  Solidified molten rock (which freezes at high temp). 1,100°C to 650°C.  Depends on composition.   Earth is mostly igneous rock. Magma – Subsurface melt.  Lava – Melt at the surface.   Magma erupts via volcanoes. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 4. 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. Ash. Fragmented lava.  There are many varieties of igneous rocks. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 5. Magma Formation  Why does magma form?  Earth is hot inside. Why? Planetesimal and meteorite accretion. Gravitational compression. Differentiation. Radioactive mineral decay. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 6. Magma Formation Partial melting in crust / upper mantle.  Melting is from…  Pressure release.  Volatile addition.  Heat transfer.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 7. Magma Formation  Geothermal gradient – Earth is hot inside. Crustal temperature (T) increases 25°C / km with depth.  At the base of the lithosphere T ~ 1,280 °C.   Geothermal gradient varies from place to place. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 8. 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.”  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 9. Magma Formation  Heat transfer. Rising magma carries mantle heat.  This raises T in crustal rock.  Crustal rock melts at lower T.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 10. Addition of Volatiles  Volatiles cause rocks to melt at much lower T. Water.  Carbon dioxide.  Adding volatiles to hot, dry rocks initiates melting.  Important subduction process.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 11. What Is Magma Made of?  Magmas have 3 components (solid, liquid, and gas). Solid – Solidified minerals are carried by the liquid.  Liquid – The melt itself is comprised of mobile ions.  Dominantly Si and O; lesser Ca, Fe, Mg, Al, Na, and K. Other ions to a lesser extent.  Different mixes of elements yield different magmas. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 12. What Is Magma Made of?  Gas – Volatiles dissolved in the melt. Dry magma – No volatiles. Wet magma – To 15% volatiles. Water vapor (H2O) Carbon dioxide (CO2) Sulfur dioxide (SO2) Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 13. Magma Compositions  4 major magma types based on % silica (SiO2). Felsic (Feldspar and silica)  Intermediate  Mafic (Mg and Fe-rich)  Ultramafic  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak 66 to 76% silica. 52 to 66% silica. 45 to 52% silica. 38 to 45% silica. Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 14. Magma Compositions  Composition controls density, T, and viscosity.  Most important is the content of silica (SiO2). Silica-rich magmas are thick and viscous. Silica-poor magmas and 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 1,300°C) Very low Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 15. Magma Variation Why are there different magma compositions?  Magmas vary chemically due to…  Initial source rock compositions.  Partial melting.  Assimilation.  Fractional crystallization.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 16. Magma Variation  Source rock dictates initial magma composition. Mantle source – Ultramafic and mafic magmas.  Crustal source – Mafic, intermediate, and felsic magmas.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 17. Partial Melting Upon heating, silica-rich minerals melt first.  Partial melting, then, yields a silica-rich magma.  Removing a partial melt from its source creates:  Felsic magma.  Mafic residue.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 18. Assimilation Magma melts the country rock it passes through.  Assimilated materials change magma composition.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 19. Magma Mixing Different magmas may blend in a magma chamber.  The result combines the characteristics of the two.  Often magma mixing is incomplete, resulting in blobs of one rock type suspended within the other.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 20. Fractional Crystallization As magma cools, early crystals settle by gravity.  Melt composition changes as a result.  Fe, Mg, and Ca is removed in earlsettled solids.  Si, Al, Na, and K remain in melt and increase.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 21. Fractional Crystallization Felsic magma can evolve from mafic magma.  Progressive removal of mafic minerals.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 22. Magma Migration Magma is less dense than rock; it moves upward.  Magma moves by…  Injection into cracks.  Melting overlying rocks.  Squeezed by overburden.  Pressure decrease with upward migration releases volatiles (bubbles), thereby decreasing density.  Viscosity controls migration ease.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 23. Magma Migration  Viscosity depends on temp, volatiles, and silica.  Temperature: Hotter - Lower viscosity Cooler – Higher viscosity.  Volatile content: More volatiles – Lower viscosity. Less volatiles – Higher viscosity.  Silica (SiO2) content: Less SiO2 (Mafic) – Lower viscosity. More SiO2 (Felsic) – Higher viscosity. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 24. 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 do not.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 25. Extrusive Characteristics Lava flows – Sheets of cooled lava.  Lava flows exit volcanic vents and flow outward.  Lava cools as it flows, eventually solidifying.  Low-viscosity lava (basalt) can flow long distances.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 26. Extrusive Characteristics  Explosive ash eruptions. High-viscosity felsic magma builds volcanic pressure.  Violent eruptions yield huge volumes of volcanic ash.  Ash can cover large regions.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 27. Intrusive Characteristics Intrusive rocks cool at depth, they don’t surface.  Magma invading colder country rock initiates…  Thermal (heat) metamorphism and melting.  Inflation of fractures pushing the rock aside.  Incorporation of country rock fragments (xenoliths).  Hydrothermal (hot water) alteration.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 28. Intrusive Characteristics  Intrusive contacts preserve evidence of high heat. Baked zone – Rim of heat altered country rock.  Chill margin – Magma at contact that cooled rapidly.   Xenolith - Country rock fragment in magma. Thermally altered.  Magma cooled before zenolith could be assimilated.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 29. Intrusive Activity  Magma intrudes into other rocks in 2 ways. As planar, tabular bodies (dikes and sills), and  As balloon-shaped blobs (plutons).   Size varies widely; plutons can be massive. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 30. Tabular Intrusions Tend to have a uniform thickness.  Can be traced laterally.  Two major subdivisions.  Sill – Parallels rock fabric.  Dike – Crosscuts rock fabric.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 31. Tabular Intrusions  Dikes and sills modify invaded country rock. They cause the rock to expand and inflate.  They thermally alter the country rock.   Dikes… Spread rocks sideways.  Dominate in extensional settings.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 32. Tabular Intrusions  Sills Lift entire landscapes skyward.  Usually intruded near the surface.   Both dikes and sills exhibit wide variability. Size.  Thickness (or width).  Lateral continuity.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 33. Large Sill Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 34. 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. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 35. Plutonic Activity  Plutons may coalesce to form a larger batholith. Plutons are created at subduction zones.  Magma generation may occur of over 10s of Ma.  Long subduction history linked to large batholiths.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 36. 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.  Dikes.  Sills.  Laccoliths.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 37. Influence on Landscape  Deeper features are exposed by uplift and erosion. Intrusive rocks are more resistant to erosion.  Intrusive rocks often stand above the landscape.   “Unroofing” takes long periods of geologic time. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 38. Cooling Rates  Cooling rate – How fast is heat lost?  Depth: Deep is hot, shallow is cool. Deep plutons cool slowly. Shallow flows cool rapidly.  Shape: Surface to volume ratio. Spherical bodies cool slowly. Tabular bodies cool faster.  Ground water. Ground water removes heat. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 39. Igneous Textures  The size, shape, and arrangement of the minerals. Glassy – Made of solid glass or glass shards.  Interlocking crystals – Minerals that fit like jigsaw pieces.  Fragmental – Pieces of pre-existing rocks.   Texture directly reflects magma history. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 40. Glassy Textures  Form by very rapid cooling of lava in water or air. Quenching forms obsidian (volcanic glass).  Basalts may quench into blobs of lava called pillows.  Glasses may fragment explosively.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 41. Crystalline Textures  Texture immediately reveals cooling history.  Aphanitic (finely crystalline). Rapid cooling – extrusive. Crystals do not have time to grow.  Phaneritic – (coarsely crystalline). Slow cooling – Intrusive. Crystals have a long time to grow. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 42. Crystalline Textures  Texture immediately reveals cooling history.  Porphyritic – A mixture of coarse and fine crystals. Indicates a 2-stage history. Initial slow cooling creates large phenocrysts. Subsequent eruption cools remaining magma quickly. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 43. Other Textures  Pegmatitic - Exceptionally coarse mineral crystals. From late stage crystallization of granitic magmas  Many unusual minerals are found in pegmatites.  Made from ions that don’t easily fit into crystals.  Some pegmatites are rich in prized minerals. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 44. Igneous Classification  Based upon composition and texture. Composition – Felsic, intermediate, mafic, ultramafic.  Texture - Fine (aphanitic), coarse (phaneritic).  Type Aphanitic (fine) Phaneritic (coarse) Felsic Rhyolite Granite Intermediate Andesite Diorite Basalt Gabbro Very high Very high (up to 1300°C) Mafic Ultramafic Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 45. 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.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 46. Crystalline Classification Composition.  Texture.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 47. Pyroclastic Classification  Pyroclastic – Fragments of violent eruptions. Tuff – Volcanic ash that has fallen on land.  Volcanic breccia – Made of larger volcanic fragments.  Hyaloclasite – Fragments of lava exploded into water.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 48. Igneous Activity Distribution  Igneous activity tracks tectonic plate boundaries. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 49. Igneous Activity Distribution  Igneous activity tracks tectonic plate boundaries. Hot spots – Mafic volcanic activity.  Convergent boundaries – Felsic igneous activity.  Divergent boundaries – Mafic igneous activity.  Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks
  • 50. This concludes the Chapter 6 Up from the Inferno: Magma and Igneous Rocks LECTURE OUTLINE earth Portrait of a Planet Third Edition ©2008 W. W. Norton & Company, Inc. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks