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Chapter Eleven - Geophysical Properties of Planet Earth

Chapter Eleven - Geophysical Properties of Planet Earth. HOMOGENOUS EARTH. Exploring the Interior of the Earth. Geophysics- the study of the foundational properties of the Earth’s interior.

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Chapter Eleven - Geophysical Properties of Planet Earth

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  1. Chapter Eleven -Geophysical Properties of Planet Earth

  2. HOMOGENOUS EARTH

  3. Exploring the Interior of the Earth • Geophysics- the study of the foundational properties of the Earth’s interior. • Geophysicists- identify thickness, density, composition, structure and physical state of the layers of the Earth’s interior • Knowledge of Earth’s interior comes mainly from seismological station that records seismic body waves. • Analysis of waves arrival time recorded by seismographs • Seismic tomography- using same principles similar to CAT Scans to generate 3-D images of the Earth’s interior • P-wave travels through solid and liquid while S-wave travels only through solids • Refraction and reflection occur at contacts between different layers

  4. Earth’s Interior • Inaccessibility of Earth’s Interior • Deepest hole drilled ~ 13 km • General observations about Wave Propagation: • P waves compress mail material through which they travel; Medium returns to original volume; Travel through sold (Elastic) faster than Liquid or Gas (inelastic) • S waves travel as shear waves; admitted by elasticity of solids; omitted by inelasticity of liquid or gas; seismic wave velocity increases with depth

  5. Earth’s Layers • Earth is divided into continental and oceanic crust between different composition thickness & structure • Seismic discontinuity- MOHO- boundary between crust and mantle Transitional zone within the mantle (slowing) • Crust- silica rich igneous/metamorphic rocks- continental 20-70 km (12.5-45 mi) ~2.7-3g/cm3; oceanic- density 3.0 gm.cm3 • Mantle- upper 3.3 g/cm3 up to 400 km and more. Lower P velocity at boundary between mantle and core 700-2900 km (440-1800 mi) • Asthenosphere-region (100-350 km (62-217 mi)) where P & S slow down • Seismic discontinuity- mantle core

  6. Earth’s layers-contd. • Crust composed of Silicate-rich igneous rocks • Sampled directly by drilling • Studied extensively by seismic analysis • P-waves: ~6 km/s in continental crust; ~ 7 km/s in oceanic crust • Continental Crust: Thickness varies between 20-70 km; P-wave velocity varies between 6-7 km/s; density: 2.7-3.0 g cm-3 • Oceanic Crust: Studied by Deep-Sea Drilling; seismic analysis; 200-m deposit marine sed; 2-km layer of pillow sediment;6-km layer of Gabbro; aver. Den. ~ 3 g cm-3;

  7. Earth’s layers-contd. • Crust-Mantle Boundary: Moho discontinuity • Mantle: Density varies – 3.3-5.5 g cm-3; composed of elastic/plastic solids; Changes in P- & S-wave velocities reveal mantle layers; P-wave velocity from Moho to Asthenosphere: 8-8.3 km/s; P-wave velocity in Asthenosphere: < 8 km/s; Asthenosphere is partially molten because of unique temperature and pressure combination • Transition Zone: Below Asthenosphere; At 400-km, Mg olivine compresses to form spinel; At 700-km, spinel and other minerals change to metallic oxides

  8. Ultramafic mantle minerals collapsing

  9. Earth’s Mantle-Core Boundary

  10. Earth’s layers-contd. • Lower Mantle: 700-2,900 km deep; P-wave velocity from Asthenosphere to base of mantle: 8.3-13.6 km/s; composed of dense Mg silicates and oxides • Mantle-Core Boundary: P-wave velocity slows from 13.6 to 8.1 km/s; S-waves cease; Outer core: Liquid Iron-Nickel mix, density 10-13 g cm-3 • CORE: 1/6TH Earth’s volume, 1/3RD Of the Earth’s Mass; Pressure >3 Million atmosph.; Temp.~4,700ºC; Composition: IRON-NICKEL, Consistent with Seismic data, meteorite data, and mathematical model

  11. Velocity Change between layers

  12. Three major components of the Earth

  13. Seismic Wave

  14. The low-velocity zone

  15. Shadow Zones • Shadow zones- • segments of the earth opposite an Earthquake’s focus where no direct S & P waves can be received • S-Shadow zone- • produced because shearing S-wave cannot travel through liquid, hence S-Shadow zone occurs • P-Shadow zones- • are produced as P-waves are refracted when they enter a zone of lower rigidity • both zones help to confirm that earth’s outer core is liquid

  16. Shear Waves – Shadow zone

  17. Shear Waves – contd.

  18. P-Waves Globe

  19. P-waves Cut-away

  20. The Behavior of P- and S-waves

  21. Solid Inner Core

  22. Gravity Force of attraction that an object (A) exerts on another object (B), i.e. Force of gravity is proportional to mass of A x mass of B distance 2 • Gravimeter- measures variation in Earth’s gravity. • Gravity depends on the altitude of the land, latitude, and distance from the Earth’s center of gravity. • Gravity anomalies- • difference between actual gravimetric measurement to the expected theoretical values- positive attraction will be lower than expected and negative attraction higher than expected

  23. Gravitation attraction of the earth

  24. Gravitation-contd. • Isostacy- equilibrium between lithospheric segments and the asthenosphere beneath them • Magnetism- force associated with moving charged particles that enables certain substances to attract or repel similar materials- magnetic reversal, paleomagnetism

  25. Negative Gravity anomaly

  26. Positive Gravity anomaly

  27. Positive gravity anomaly over ore deposit

  28. Principle of isostacy - icebergs

  29. Principle of isostacy - mountains

  30. Isostatic adjustments

  31. Magnetic field of a bar magnet

  32. Prevailing Magnetic Field

  33. Electrically conductive field

  34. Magnetic field polarity within magnetite

  35. Terrestrial record of magnetic reversal

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