Public message 3283 SCIENCE Area 19:51 Wednesday 4-Sep-91 To: MIKE DAVEY Re: EARTH'S M

Public message 3283  SCIENCE Area 19:51  Wednesday   4-Sep-91
From: HENRY SHAW
  To: MIKE DAVEY
  Re: EARTH'S MAGNETIC FIELD 1

 MB> I understand that 'magnetic mapping' (for lack of the real term)
 MB> has produced evidence that the magnetic 'north pole' has shifted
 MB> many times in the earths past leaving some forms of rocks with
 MB> magnetic 'map' pointing to where the magnetic pole was at the
 MB> time the rock was formed.

 MB> Just out of curiousity, do you happen to know how many times this
 MB> shift is evident and the average time interval between them, and
 MB> when the last one was?

Many times. Dozens.  But I don't know an exact number.  The last major
reorganization of the magnetic field was a reversal at about 700,000 years
ago.  There does not appear to be any regularity to the pattern of
reversals.  That is, they are not periodic.  Recently, people have postulated
that they are a "chaotic" (in the strict sense) phenomenon.

Our knowledge of the history of the earth's magnetic field, naturally
enough, gets poorer as one goes back in time.  This is due to several
things: (1)  older rocks are relatively rarer than younger rocks;  (2) older
rocks are more likely to have undergone more complex histories (e.g.
undergone multiple heating events, which act to "reset" the remanent
magnetism); (3)  absolute errors (not percentage errors) on the age of
samples increases with the age of the sample, making it difficult to do the
correlations required for high resolution magnetostratigraphy.

The basis for paleomagnetic studies is the fact that many igneous rocks
contain minerals that are naturally magnetic (magnetite, or "lodestone" is
an obvious example of a common rock forming mineral like this, but there
are others).  These minerals are only magnetic below a certain
temperature known as the "Curie point".  Above this temperature, the
minerals have no net magnetism.  When a rock cools below this
temperature, the minerals acquire a magnetic moment (i.e. become like
little compass needles) aligned with the earth's field.  This direction is
"locked in" over a relatively small temperature range, and persists even if
the the relative orientation of the rock and the earth's field change at some
later time (either by movement of the field, or by movement of the rock).

[The following stuff is taken from a message I wrote to Pat Goltz in this
echo several years ago.]

The magnetic field of the earth is not a static thing.  It undergoes
fluctuations on timescales from milliseconds to hundreds of millions of
years. The high frequency fluctuations are due to external (to the earth)
causes, mainly variations in the solar wind.  The longer-term fluctuations
are due to variations in the motion of the fluid outer core of the earth at
a depth of ~2900km.  (For a review of the subject, see:  Courtillot, V. and
LeMouel, J.-L., 1988, Geomagnetic Time Variations, Annual Review of
Earth and Planetary Science, v16, pp389-476.)

continued next message....
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