MYSTERIES OF PLANET EARTH

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• Chapter Eleven -Geophysical Properties of Planet
  Earth

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HOMOGENOUS EARTH
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Exploring the Interior of the Earth

• Geophysics- the study of the foundational
  properties of the Earths interior.
• Geophysicists- identify thickness, density,
  composition, structure and physical state of the
  layers of the Earths interior
• Knowledge of Earths 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
  Earths interior
• P-wave travels through solid and liquid while
  S-wave travels only through solids
• Refraction and reflection occur at contacts
  between different layers

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Earths Interior

• Inaccessibility of Earths 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

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Earths 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

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Earths 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 sediment6-km layer of Gabbro
  aver. Den. 3 g cm-3

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Earths 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 lt 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

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Ultramafic mantle minerals collapsing
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Earths Mantle-Core Boundary
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Earths 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 Earths volume, 1/3RD Of the Earths
  Mass Pressure gt3 Million atmosph.
  Temp.4,700ºC Composition IRON-NICKEL,
  Consistent with Seismic data, meteorite data, and
  mathematical model

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Velocity Change between layers
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Three major components of the Earth
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Seismic Wave
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The low-velocity zone
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Shadow Zones

• Shadow zones-
• segments of the earth opposite an Earthquakes
  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 earths outer
  core is liquid

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Shear Waves Shadow zone
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Shear Waves contd.
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P-Waves Globe
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P-waves Cut-away
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The Behavior of P- and S-waves
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Solid Inner Core
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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 Earths
  gravity.
• Gravity depends on the altitude of the land,
  latitude, and distance from the Earths 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

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Gravitation attraction of the earth
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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

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Negative Gravity anomaly
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Positive Gravity anomaly
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Positive gravity anomaly over ore deposit
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Principle of isostacy - icebergs
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Principle of isostacy - mountains
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Isostatic adjustments
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Magnetic field of a bar magnet
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Prevailing Magnetic Field
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Electrically conductive field
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Magnetic field polarity within magnetite
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Terrestrial record of magnetic reversal