Details

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Details

• Problem set 2, questions 1-3 due today. No late
  homework accepted.
• Problem set 2, question 4 due next Thursday.
  THIS IS A DIFFERENT PROBLEM THAN I PASSED OUT
  LAST WEEK!
• Syllabus .

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Details

• March 1 (Monday) Maps available for 2-credit
  option (see me in my office).
• March 4 Deep seismics (P waves, S waves, the
  core, and more).
• March 11 No class (spring break).
• March 18 Shallow seismics (the structure of the
  mantle) Really shallow seismics (crustal
  structure and faulting). Problem set 3.
• March 25 Earthquakes, tsunamis, and other
  geophysical catastrophes. Problem set 3 due.
• April 1 Ocean drilling and seismic
  stratigraphy. Papers due (2-credit option).
  Problem set 4.
• April 8 GPS Measuring how plates move
  Satellite interferometry. Problem set 4 due.
• April 15 Mantle plumes, plume-ridge
  interactions, and mass extinctions.
• April 22 Heat flow.

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Magnetic reversals

• 1993 ODP study in Pacific by Institut de
  Physique du Globe de Paris
• Plots strength of the dipole field as a
  function of time
• Shows quick increase in dipole field strength
  after reversals (or failed reversals), followed
  by gradual weakening
• Idea Reversals occur when the dipole field
  decays enough to be influenced by the much weaker
  non-dipole field

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Calibrating the stripes

• Two approaches terrestrial and marine.
• Terrestrial
• Go to the field and measure the magnetic
  orientation of a rock sample in situ.
• Then measure its age in the lab.
• Completed in 1966 went back 4 Ma.
• Marine
• Collect magnetic anomaly profiles perpendicular
  to the axis of the Mid-Atlantic Ridge in the
  South Atlantic.
• Assume a constant spreading rate for the last 80
  Ma.
• Calibrate using seafloor samples collected by
  DSDP (precursor to ODP), dated using fossils.
• Completed in 1968.

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Calibrating the stripes

• With additional marine data, the geomagnetic
  reversal time scale has undergone continual
  refinement.

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Geomagnetic Time Scale

• Specific reversal anomalies have been numbered
  for reference.
• 0 to 80 Ma Anomaly 0 to Anomaly 33
• 125 to 157 Ma M0 to M27 (M Mesozoic)
• Magnetic Quiet Zone Cretaceous, 83-124 Ma

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Using the stripes

1. Go out on a cruise
2. Collect marine gravity data.
3. Reduce to magnetic anomalies.
4. Make a magnetic anomaly profile.
5. Wiggle-match.

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Lines of constant age isochrons
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Crustal Age Map

• Compile magnetic anomaly picks from many cruises.
• Plot locations of specific reversals on map.
• Contour ages (plot isochrons).
• Result Crustal anomaly age map.
• IMPORTANT NOTE Oldest seafloor is 180 Ma.
  Everything else has been subducted.

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Crustal Age Map
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