Interpret Marine Magnetic Anomalies

The black line at the bottom shows the magnetic polarity scale, with the anomalies out to 20 Ma labeled above the line, and the ages below the line (Cande and Kent, 1995).  This shows the pattern that would be to the right of the ridge; the pattern would be symmetrical to the left.  Solid black is normal polarity, when the measured anomaly will be positive.

The three profiles ID the anomalies in red above each profile.

  1. The first step is to look for the symmetrical pattern about the ridge, which occurs here only on the lowest profile.  Depending on ridge orientation and latitude, the degree of symmetry varies.  The anomaly at the ridge should be among the largest positive values.  You are also looking for the other anomalies as mirror images about the ridge crest.  While not labeled here, anomalies 2 and 2A both appear on the left side of the profile, as does the small Jaramillo event with normal polarity about 1 Ma.
  2. The next step is to look for distinctive anomalies.  Note that Anomaly 3 has 4 closely spaced peaks, and 3A has two.
  3. The middle profile is from the left side of the ridge, which you must consider if you cannot get a good match from the pattern to the right.  If the profiles are in the correct spatial relationships, and do not match up, there are transform faults between profiles.
  4. You cannot always get a simple correlation between adjacent survey lines because:
    • Transform faults can occur between adjacent survey lines.
    • Spreading rates vary with distance from the Euler pole, so the distance between the same pair of anomalies also varies.
  5. You can get complexities within a single profile, if the survey line crossed a transform fault, or if there was a ridge jump.
  6. With slow spreading ridges, or noisy real world data, closely spaced reversals may be hard to pick out.

Profile Reversing

Last revision 2/26/2019