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Structure of Earth

Structure of Earth. Chapter 2. Most simply. Crust – cold, rigid, thin. Mantle – warmer, more dense; outer part rigid and inner part plastic (deformable). Outer core – transition zone then thick liquid zone. 4. Inner core – solid but warm, very dense, rich in magnetic

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Structure of Earth

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  1. Structure of Earth Chapter 2

  2. Most simply • Crust – • cold, rigid, thin • Mantle – • warmer, more • dense; outer part • rigid and inner • part plastic • (deformable) • Outer core – • transition zone • then thick liquid • zone 4. Inner core – solid but warm, very dense, rich in magnetic materials (Ni, Fe) The earth is layered & density stratified

  3. How do we know this? • All we see is the crust! • Deepest drill-hole – 12,063 m (7.5 miles) • Still crustal • Deepest ocean drilling – 2 km (1.2 miles) • Still crustal • Studies of the earth’s orbit – gave an idea of mass • Surface rocks predicted lower total mass if the earth were homogeneous

  4. Mohorovicic “Moho” discontinuity • Density discontinuity – P waves arrived at seismic station before they should have in an homogeneous earth • Boundary between the crust and mantle • Discovered by Croatian geophysicist based on observations of seismic waves generated by earthquakes. • Fun fact – there was an effort to drill a “Mohole” but failed due to lack of $$ and technology

  5. Evidence for layering • Mainly we know depend on seismology • Seismic waves generated from earthquakes • “Primary” P-waves (compression waves; longitudnally propagated waves; oscillate in same direction as movement like sound waves) • “Secondary” S-waves (transverse waves; horizontally propagated; oscillate perpendicular to movement like water waves) • 1900 – identified P & S waves on a seismograph (Oldham) • Waves were passing through the earth faster than predicted • Wave speed increases with increasing density! • Waves were being refracted (bent so they changed direction) • Hypothesized that there were areas of Earth with different densities • 1906 – no S-waves passed through the earth • Shadow zone – no S-waves • P-waves took longer than expected

  6. Why are these waves important? • We can detect • these waves • independently • They behave • differently passing • through different • media

  7. Point of origin of seismic source. Prediction of earthquake waves passing through a planet of regularly changing density. Prediction of earthquake waves passing through a homogeneous planet.

  8. What P waves do in & around liquid outer core (bend) – see book. What S waves do around liquid outer core (do not penetrate). P-wave shadow zone 142o P-wave shadow zone 142o

  9. Seismology • Changes in travel time and path tell us about the earth’s structure • Refraction of waves led to discovery of earth’s core and Moho • Travel time of waves led to discovery of layers • Now we use changes in travel time and path tell us about location of disturbances (earthquakes or bombs)

  10. Earth’s functional layers • Crust – we know most about it; continental crust is less dense • Moho – a density discontinuity that separates crust from the mantle • Depth varies under continents and oceans • First thought that this was layer where crust moved relative to earth’s interior BUT, outer layer of mantle moves with crust! • Lithosphere – crust plus rigid mantle (not totally rigid but, movements cause things like earthquakes and volcanoes • Asthenosphere – plastic layer of mantle; lithosphere floats on asthenosphere • Mantle includes part of lithosphere, asthenosphere and solid mesosphere

  11. Chemical composition • of layers: • Crust – lightweight (0.4% mass/1% volume of earth) – ocean crust (basalt – O, Si, Mg & Fe) is denser than continental crust (granite – O, Si, Al) • Mantle – denser (68% mass/83% volume of earth) - Si, O, Fe & Mg • Core – densest (31.5% mass/16% volume of earth) - mainly Fe & Ni with some Si, S and heavy elements

  12. Physical responses Lower mantle ~3400 Core 2900 – 6370 km Dense, viscous liquid Solid inner core

  13. Classifying layers By composition

  14. Isostatic equilibrium and rebound • This concept helps us understand the “floating” of lithosphere on asthenosphere

  15. Isostacy • Ocean basins and continents “float” on asthenosphere at equilibrium so that total pressure at depth in mantle is everywhere the same. • Depending on density, things will float at a certain height and displace a different amount of water • Most mass is below the surface, what sticks out of the fluid is supported by bouyancy of displaced fluid below the surface • Examples – icebergs, ships, blocks of wood of different densities in water

  16. What does this mean? • Mountains have roots that are deeper than surface expression • As erosion removes mass from the top of a mountain, the roots shrink upward or the asthenosphere “rebounds” • Example: younger (higher) Rockies have deeper roots than older Appalacians • Example: continental rebound from glaciers (Great Lakes & Long Island Sound examples); sea level decreases even though more water!

  17. Take home points • Layers of the earth – density stratification • How do we know earth’s structure – seismology and the role of S and P waves • Moho, lithosphere and asthenosphere • Isostacy; Isostatic equilibrium

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