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Ch 21: Earth’s interior

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  1. Ch 21: Earth’s interior • Probing Earth’s interior • Seismic waves and Earth’s interior • Discovering Earth’s major boundaries • Geodynamo • Earth’s internal heat engine

  2. Ch 21: Earth’s interior - Study guide ·Know Earth’s major layers (and depths) based on compositional and mechanical differences. ·Properties of P-and S waves, and which ones move through solid, which through liquid (molten) rock? ·Difference between refraction and reflection ·What is the Moho and how was it discovered? ·How was the core-mantle boundary discovered? ·Variations of S-and P wave velocities with depth. How can you explain the changes in velocity inside the asthenosphere? What does it tell you about the composition of the mantle? ·Why does earth have a magnetic field? ·What is the source of heat inside earth and how it is transferred through earth (conduction and convection)?

  3. 1) Probing Earth’s interior • earthquake happens (slip on a fault) • seismic waves travel away from earthquake • they carry info about material they travel through

  4. earlierlecture today 1) Probing Earth’s interior • earthquake happens (slip on a fault) • seismic waves travel away from earthquake • they carry info about material they travel through • earthquakes make P-waves & S-waves • seismologists use them to: • locate earthquakes • determine what deep Earth is made of

  5. P waves: • compressional waves: are fastest • vibrate material back/forth in direction wave travels S waves: • shear waves: slower than P-waves • vibrate material side-to-side from direction wave travels • Don’t pass through liquids 1) Probing Earth’s interior • The nature of seismic waves Seismic wave speeds: • depend on material properties • are faster in more rigid materials • increase with increasing depth (higher pressure)

  6. “secondary” “primary” 1) Probing Earth’s interior • The nature of seismic waves • P waves always faster than S-waves • wave paths are “bent” when crossing from one material into another

  7. 1) Probing Earth’s interior • The nature of seismic waves • wave paths are “bent” when crossing from one material into another

  8. reflections refractions q4 q3 q q2 q2 q

  9. 1) Probing Earth’s interior • The nature of seismic waves • wave paths are “bend” when going deeper in Earth • higher pressure = higher wave speed

  10. 2) Seismic waves & Earth’s Interior • Compositional layers • 5 Physical/mechanical layers crust 3-70 km thick mantledown to 2900 km depth core2900-6370 km depth layer depth lithospherestiff/strong, 0-100 km asthenospheresoft/weak, 100-660 km

  11. boundaries between layers 3) Discovering Earth’s major boundaries • crust • mantle • core

  12. 3) Discovering Earth’s major boundaries • The Crust Thickness: ~ 30 km (continents, 70km under mountains) 3-15 km (oceanic) Composition: Continents: felsic (granite) and mafic (gabbro) rocks Oceani: Basalt, Gabbro

  13. 3) Discovering Earth’s major boundaries • The “Moho” Boundary between the crust and mantle Discovered in 1909 by Andrija Mohorovicic

  14. 3) Discovering Earth’s major boundaries • The Mantle upper mantle 400 Over 82% of Earth’s volume, mainly peridotite (minerals Olivine and pyroxene) 660 lower mantle mantle upper mantle 0 - 660 km lower mantle 660-2900 km D” region 2600-2900 km D” 400 & 660 km depth “phase transitions” Minerals suddenly compress to a more compact form (phase change) See Fig. 21.7

  15. Insert: Isostacy and crustal uplift/subsidence “isostacy” = balance between gravitational force and buoyancy force, see Figure Story 16.16

  16. Insert: Isostacy and crustal uplift/subsidence Less dense crust floats on top of the denser and deformable rocks of the mantle “isostacy” = balance between gravitational force and buoyancy force, see Figure Story 16.16

  17. Insert: Isostacy and crustal uplift/subsidence Isostatic rebound, adjustment: Readjustment of the isostatic equilibrium after the ice-shield is removed, as happens still in Scandinavia and Canada. See 21.1: Isostacy and postglacial uplift

  18. 3) Discovering Earth’s major boundaries • crust • mantle • core • about Mars sized • Nickel-iron alloy • Outer, liquid, spinning • Inner, solid • 4 mio times atm. pressure at center

  19. Core-mantle boundary mantle core 3) Discovering Earth’s major boundaries • The core-mantle boundary Boundary between the mantle and core Discovered in 1914 by Beno Gutenberg …how?

  20. P-wave shadow zone Fig. 21.2

  21. S-wave shadow zone Core was discovered …from a “shadow zone”

  22. Inner core mantle core 3) Discovering Earth’s major boundaries • The inner core Boundary between the outer liquid and solid inner core Discovered in 1936 by Inge Lehman …how?

  23. 3) Discovering Earth’s major boundaries • The inner core She discovered reflections (‘echoes’) of seismic waves ‘Lehman Discontinuity’

  24. 4) Earth’s magnetic field- Geodynamo • The Core Core behaves like a dynamo and thus sustains Earth’s magnetic field. Polarity of Earth’s magnetic field reverses about every million years

  25. 4) Earth’s magnetic field- Geodynamo Geographic and magnetic poles do not coincide! Fig. 21.11

  26. 4) Earth’s magnetic field See time-line of magnetic field reversals (paleomagnetic time-scale), Fig. 21.15 . Remember that magnetic reversals recorded in seafloor basalts were a major confirmation of seafloor spreading (Fig. Story 2.11).

  27. 5) Earth’s internal heat engine • 3 reasons for internal heat: 1) radioactive decay of uranium, thorium, potassium 2) heat released as inner core crystallized 3) from colliding particles during Earth formation • Ways to transfer heat: 1) conduction - molecular activity 2) convection - movement (circulation) of material TSP 17.13

  28. 5) Earth’s internal heat engine Fig. 17.14 Convective flow in the mantle

  29. 5) Earth’s internal heat engine The geotherm: Increase of temperature with depth

  30. Ch 21: Earth’s interior What is the approximate distance from the surface to the center of the Earth? A. 700 km B. 2900 km C. 6400 km D. 24,000 km

  31. Ch 21: Earth’s interior 4. The two kinks in the seismic wave path are examples of seismic __________. A. isostasy B. reflection C. refraction D. tomography Answer = C (page 485) What type of seismic wave is depicted by the ray path in the diagram? A. a P wave B. an S wave C. a surface wave D. all of the above

  32. Ch 21: Earth’s interior The two kinks in the seismic wave path are examples of seismic __________. A. isostasy B. reflection C. refraction D. tomography

  33. Ch 21: Earth’s interior Continental crust beneath mountains can be up to ___ kilometers thick A. 10 B. 40 C. 70 D. 100

  34. Ch 21: Earth’s interior Which of the following statements is false? A. P waves travel slower in the crust than in the mantle. B. The crust is denser than the mantle. C. The crust-mantle boundary is called the Mohorovicic discontinuity. D. The oceanic crust consists of basalt and gabbro. Answer = B (page 487)

  35. Ch 21: Earth’s interior Which of the following regions in the Earth consists primarily of olivine and pyroxene? A. the crust B. the upper mantle C. the lower mantle D. the inner core

  36. Ch 21: Earth’s interior Which of the following statements about the Earth’s core is true? A. The inner core and the outer core are both liquid. B. The inner core and the outer core are both solid. C. The inner core is liquid and the outer core is solid. D. The inner core is solid and the outer core is liquid.

  37. Ch 21: Earth’s interior What drives plate tectonics? A. erosion B. solar energy C. thermal conduction D. thermal convection