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Earth's Interior: Layers and Composition

Learn about the layers of the Earth's interior, their composition, and how they were determined through seismic waves, heat flow, gravity, and the magnetic field.

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Earth's Interior: Layers and Composition

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  1. Geology 12 Presents

  2. UNIT 3 • Chp 10 Earth’s Interior and Isostacy • Chp 11 Ocean Basin • Chp 12 Plate Tectonics • Chp 9 Seismology • Chp 13 Structure Handout WS 10.1 Note Helper

  3. Chapter 10Earth’s Interior and Isostacy atmosphere hydrosphere continental crust oceanic crust Lithosphere Moho Asthenosphere Upper mantle Outer core Lower mantle Inner core

  4. Earth’s Interior

  5. Layer Vol Density Composition % gms/cm3 Oceanic Crust 0.16 3.0 upper: basalt lower: gabbro Continental Crust 0.44 2.7 granodiorite Mantle 83 3.3 – 5.7 peridotite Outer Core 15.7 9.9 – 12.2 88%Fe, +S,O2,Ni, K Inner Core 0.7 12.6 – 13.0 80 - 90% Fe, +Ni

  6. How Layers of Earth was Determined • Seismic • Heat Flow • Gravity • Magnetic Field

  7. 1. Seismic • Seismic waves that travel through the Earth • Caused by earthquakes or underground nuclear tests • Two types • Surface waves • Body waves

  8. 1. Surface Waves • Travel on the surface • 2-3 km/sec • Rayleigh and Love waves (more on this in Chp 9) Surface waves Body waves EQ

  9. 1. Surface Waves • Travel on the surface • 2-3 km/sec • Rayleigh and Love waves (more on this in Chp 9)

  10. 2. Body Waves • Travel inside the Earth • a) P-waves/Primary waves • Fastest (7-13 km/sec) • A compressional/pull-push wave like sound • Travel through everything (rock, magma, water, air ( can sometimes hear EQ!)

  11. B) S-waves/Secondary waves • slower (3 - 7 km/sec) • Are shear waves because they move perpendicular to direction of travel • Travel only through solids (not fluids)

  12. Wave velocity affected by: i) rock density: higher density speeds waves (ex: Earth’s density increase towards the core: S-waves travel faster at the bottom of the mantle) ii) Rock elasticity = tendency to return to original shape higher elasticity increases wave speed (ex: P-waves travel slow through liquid outer core than through solid inner core) • Discontinuity = marked change in wave velocity • Basis for dividing Earth into its layers.

  13. Please now refer to WS 10.1 top of page 2 of note helper

  14. Gutenburg Discontinuity Low velocity zone = asthenophere crust

  15. Upon EQ/nuclear detonation, waves travel outward and inward in every direction (like ripples on a pond) • Waves’ direction of travel: • Refracted (bent) away from more dense/more elastic rocks back to surface • Most energy is refracted • Reflected (change direction) at major rock boundaries (discontinuities) • Reflected waves (wave velocity) + time to travel = depth to layer) gives us depth to layers (d = vt) • GEOPHYSICS

  16. EQ Wave Reflection Wave Refraction

  17. Layers of the Earth Found • 1. Core • a) Outer Core & S-waves Liquid Outer

  18. b) Inner Core & P-waves • Inner core is solid because higher pressure increases melting temperature • Inner core rotates 20 km/yr faster than outer core

  19. 2. Mantle • 1909 Andrija Mohorovicic detected a seismic disontinuity (Moho) at about 30 km.

  20. P-waves travel faster (8 km/sec) in mantle than in crust (6.75 km.sec) • Moho – bottom of crust • Below continents: 2- 90 km (aver’ 35 km) • Below seafloor: 5 -10 km. Please refer to WS 10.1, p.2 of notehelper, bottom diagram.

  21. Upper mantle M . Transition Zone Lower Mantle Low velocity zone = Asthenosphere C .

  22. Upper Mantle

  23. Seismic Tomography • Numerous seismic waves are analyzed to give a “CAT-scan” of the Earth • Hot and cold areas of the mantle have been detected by measuring wave velocities (hot rock is less elastic = lower velocity) • Depressions and rises have been detected where rising and sinking mantle deforms the mantle and core.

  24. Seismic Tomography

  25. 3. Crust • a) Continental Crust: • mainly granodiorite (P-waves 8 km/sec) • 35 km aver’ thickness • 20 km at rifts • 90 km at Himalayas

  26. b) Oceanic Crust • Top 1/2 : basalt • Bottom ½: gabbro • 5 km thick at spreading ridges • 10 km thick where it is oldest

  27. 3. Crust

  28. How Layers of Earth was Determined • Seismic • Heat Flow • Gravity • Magnetic Field Hand out WS 10.2 Note Helper

  29. 2: Heat Flow • Is very low (Earth is a good insulator/poor conductor) • Is measured with very sensitive instruments

  30. Highest at spreading centres and areas of recent volcanism; lowest at trenches 4 Oceanic ridge Island arc (volcanoes) 3 Heat flow Cal/cm2/sec 2 World average 1 new crust old crust trench 0

  31. 70% of heat is lost through oceans • Earth has been cooling off (less and less radioactive fuel) since its beginning. • Most heat created by radioactive decay of U, Th and K

  32. Remember Chp 7! • Geothermal Gradient = rate of increase in temperature with depth • Highest at spreading ridge; lowest in mtns’ Region Temperature Geothermal Gradient Crust base: 800- 1200 25 Mantle base: 3500-5000 1 Core base: 6500 <1

  33. How Layers of Earth was Determined • Seismic • Heat Flow • Gravity • Magnetic Field

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