Public message 3285 SCIENCE Area 19:52 Wednesday 4Sep91 To: MIKE DAVEY Re: EARTH'S MAGNETI

Public message 3285  SCIENCE Area 19:52  Wednesday   4-Sep-91
From: HENRY SHAW
  To: MIKE DAVEY
  Re: EARTH'S MAGNETIC FIELD 3

....continued from previous

Over longer periods of time (hundreds of thousands to millions of years),
there is evidence that the dipole field dissapeared completely and then
reappeared with the opposite polarity numerous times over geologic
history. Such events are called magnetic reversals.  Perhaps the most
compelling evidence for the reversals comes from the ocean floor, where
the record of reversals is is clearly laid out as a pattern of symmetric
magnetic "stripes" flanking both sides of the mid-oceanic spreading centers.
This is not meant to neglect the very large body of data from land that also
contains the record of reversals.

Detailed studies of the period of transition from one polarity to another
suggest that the transition takes 1-2 thousand years, once initiated.  There
is also evidence that prior to the reversals, there are larger-than-average
fluctuations in the partitioning of energy between the various harmonic
components of the field.  There are also instances of similar fluctuations
(called excursions) that are not associated with an actual reversal.  These
may be instances of "failed reversals", in which the convective motions of
the outer core underwent a reorganization that did not result in a change
in the net polarity of the field.

--- TBBS v2.1/NM
 * Origin: Diablo Valley PCUG-BBS, Walnut Creek, CA 510/943-6238 (1:161/55)