Exploring Earths Interior

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Exploring Earths Interior
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Magnetic Field of the Earth
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Earth's magnetic field

• The Earth behaves as a magnet whose poles are
  nearly coincident with the spin axis (i.e., the
  geographic poles).
• Magnetic lines of force emanate from the
  magnetic poles such that a freely suspended
  magnet is inclined upward in the southern
  hemisphere, horizontal at the equator, and
  downward in the northern hemisphere

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Earth's magnetic field

• declination horizontal angle between
  magnetic N and true N
• inclination angle made with horizontal

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Earth's magnetic field

• It was first thought that the Earth's magnetic
  field was caused by a large, permanently
  magnetized material deep in the Earth's interior.
  
• In 1900, Pierre Currie recognized that permanent
  magnetism is lost from magnetizable materials at
  temperatures from 500 to 700 C (Currie point).

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Self-exciting dynamo

• A dynamo produces electric current by moving a
  conductor in a magnetic field and vise versa.
  (i.e., an electric current in a conductor
  produces a magnetic field.

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Self-exciting dynamo

• It is believed that the outer core is in
  convective motion (because it is liquid and in a
  temperature gradient).
• A "stray" magnetic field (probably from the Sun)
  interacts with the moving iron in the core to
  produce an electric current that is moving about
  the Earth's spin axis yielding a magnetic fielda
  self-exciting dynamo!

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Self-exciting dynamo

• The theory has this going for it
• It is plausible.
• It predicts that the magnetic and geographic
  poles should be nearly coincident.
• The polarity is arbitrary.
• The magnetic poles move slowly.

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Magnetic reversals

• The polarity of the Earth's magnetic field has
  changed thousands of times in the Phanerozoic
  (the last reversal was about 700,000 years ago).
• These reversals appear to be abrupt (probably
  last 1000 years or so).

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Recording the Magnetic Field in Newly Deposited
Sediment
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Lavas Recording Reversals in Earths Magnetic
Field
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The GeomagneticTime Scale
Based on determining the magnetic characteristics
of rocks of known age (from both the oceans and
the continents). We have a good record of
geomagnetic reversals back to about 60 Ma.
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Formation of Magnetic Anomalies
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Magnetic Anomalies
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Age of Seafloor Crust
R. Dietmar Muller, 1997
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Structure of the Earth

• Seismic velocity depends on the composition of
  material and pressure.
• We can use the behavior of seismic waves to tell
  us about the interior of the Earth.
• When waves move from one material to another they
  change speed and direction.

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World Seismicity, 19632000
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Earthquakes Associated with Divergent and
Transform Margins
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Earthquakes Associated with Convergent Plate
Margins
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Comparison of P-wave and S-wave Motion
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Two Types of Surface Waves
Rayleigh waves
Love waves
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Seismograph Record of P, PP, S, and Surface Waves
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Refraction and Reflection of a Beam of Light
Refraction
Reflection
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P-wave Shadow Zone
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S-wave Shadow Zone
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Seismic Velocities

• Vp (K4/3m) / r
• Vs m / r

Where K bulk modulus mshear modulus
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P-and S-wave Pathways Through Earth
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Changes in P-and S- wave Velocity Reveal Earths
Internal Layers
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Structure of the Earth

• Study of the behavior of seismic waves tells
• us about the shape and composition of the
• interior of the Earth
• Crust 1070 km, intermediate composition
• Mantle 2800 km, mafic composition
• Outer core 2200 km, liquid iron
• Inner core 1500 km, solid iron

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Composition of the Earth

• Seismology tells us about the density
• of rocks
• Continental crust 2.8 g/cm3
• Oceanic crust 3.2 g/cm3
• Asthenosphere 3.3 g/cm3

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Structure of the Crust and Upper Mantle
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Earths internal heat

• Original heat
• Subsequent radioactive decay
• Conduction
• Convection

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Upper Mantle Convection as a Possible Mechanism
for Plate Tectonics
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Fig. 20.25d
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Seismic Tomography Scan of a Section of the Mantle
Subducted slab
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Temperature vs. Depth
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Examples of Plate Boundaries
O-C convergent
O-O divergent
C-C divergent
O-O divergent
O-O convergent
O-O divergent
O-C convergent
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Driving mechanism of plate tectonics

• Thought to be convection of the mantle
• Friction at base of the lithosphere transfers
  energy from the asthenosphere to the lithosphere.
• Convection may have overturned asthenosphere 46
  times.

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Examples of Plate Boundaries
O-C convergent
O-O divergent
C-C divergent
O-O divergent
O-O convergent
O-O divergent
O-C convergent
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Rifting and Seafloor Spreading
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Spreading Centers Offset by Transform Boundary
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OceanOcean Convergent Plate Boundary
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Parts of an OceanContinentConvergent Plate
Boundary
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Mantle subdivision(b) How do we know what
mantle rocks looks like?(c) What is the mantle
made out of?Evidence?(c) Why does the mantle
melt?