Geology Reference
In-Depth Information
Fig. 4.6
Historical spline regression curves of declination and inclination for London, England (Data from Malin and
Bullard (
1981
))
Fig. 4.7
Holocene secular
variation, recorded by
lacustrine sediments in
southeastern Oregon. Ages
come from radiocarbon
dating (Redrawn from
Butler (
1992
))
The geomagnetic poles' wandering has
been observed over a longer time interval
than that shown in Fig.
4.6
. For example, the
archeomagnetism of rapidly deposited lacustrine
and cave sediments has given further evidence
to the existence of an irregular precession of
the geomagnetic dipole axis about the spin
axis (e.g., Creer
1977
). Figure
4.7
shows the
Holocene record of variations of declination
and inclination at a site in NW United States.
Regarding the field magnitude, archeomagnetic
studies have shown that it decreased by 50 %
during the last 2,000 years, at a rate of
6.3 %
per century. In particular, the magnetic moment
was
9.4
10
22
Am
2
in 1600 ac and decreased
almost linearly to
7.9
10
22
If this rate were maintained, the magnetic
moment would go to zero within 1,600 years!
Finally, the westward migration of some
magnetic features (such as declination contour
lines) was observed for the first time by Halley
in 1683 around the Atlantic Ocean. However,
although the phenomenon seems to involve some
independent variables, it is
not
observed in the
Pacific region, thereby, it could be associated
with a process that operates at regional scale.
The discovery of the secular variation of the
geomagnetic field had an indirect but dramatic
consequence for the success of the plate tecton-
ics paradigm during the 1960s. The apparently
random wandering of the dipole axis about the
Earth's spin axis, along with the recognition that
Am
2
in 1980 ac.
