GG304 Lecture 10
2/15/11
1
LECTURE 10: GEOMAGNETISM
The Earth’s magnetic field is produced by the combination of Earth’s rotation
and convection in the Earth’s metallic core. The geomagnetic potential is:
n+1
+ n "
R%
where R is Earth’s radius
W = R( ( $ ' gnm cos m) + hnm sinm) Pnm cos *
n=1 m=0 # r &
(
) (
)
This is a multipole expansion. The Gauss coefficients gnm and hnm get smaller with
! increasing order n. The most important component is the dipole field.
!
Dipole Field (note reversal of N and S)
!
Quadrupole field
The dipole potential can be written as:
4" 3 0
µ m cos "
m=
R g1
W= 0 2
where
µ0
r
This is the field due to the geocentric axial
!
magnetic dipole m, which
! is the strongest
component. The dipole magnetic field can be
broken into radial and tangential components:
#W µ 0 2m cos %
1 $W µ 0 m sin"
Br = "
=
B" = #
=
3
#r
4$
r $" 4% r 3
r
B
tanI = r = 2cot " = 2 tan #
The inclination I is:
B"
!
!
Clint Conrad
!
10-1
University of Hawaii
GG304 Lecture 10
2/15/11
2
Earth’s field is measured in nT
(nanoTeslas), and can be separated
into Inclination (I) and Declination (D).
Deviations from a dipole field are
caused by non-axial dipole
components and higher-order
components.
Intensity (nT)
Inclination (degrees)
Declination (degrees)
The Earth’s field has several non-dipole complexities, and changes with time:
-- The magnetic poles are not exactly opposite each other.
-- The magnetic field is stronger near high latitudues (weakest over S. Atlantic).
Clint Conrad
10-2
University of Hawaii
GG304 Lecture 10
2/15/11
3
-- Strength of the dipole field is decreasing at 3.2%/century from 1550 to
1900 and then accelerated to 5.8%/century for last 80 years (would be zero in
year 4000 – the beginning of a geomagnetic field reversal?)
-- The location of geomagnetic pole is changing with time.
-- The dipole field drifts westward at 0.044-0.14°/yr (2500-8000 yr period).
-- Non-dipole surface features drift westward 0.2-0.7°/yr (550-1650 yr period).
-- The magnetic field polarity reverses periodically.
Paleomagnetism is the study of geomagnetic
field recorded in rock magnetizations.
Remenant magnetism of rock samples is
measured using an: astatic magnetometer
(rarely), a spinner magnetometer, or a
cryogenic magnetometer (most sensitive).
Dipole and non-dipole components of the
magnetic field average to zero over timescales of 104 yr, thus rocks that
average the field over longer timescales longer
than ~104 yr, and only the average dipole field
remains (axial geocentric dipole hypothesis).
Using the inclination (I), the paleomagnetic pole
can be computed – many observations yield
an average pole near the geographic north pole.
To determine if a set of paleomagnetic poles
is representative of the field at the time of
formation, several tests have been developed:
Clint Conrad
10-3
University of Hawaii
GG304 Lecture 10
2/15/11
4
Fold test: “undo” the fold
to see if orientations align.
Baked contact test: If country
Rock orientations are the same
as those within an intrusion, then field has not changed since intrusion.
Reversals test: If a reversal happens between samples, the paleomagnetic poles
should be exactly opposite each other.
Tectonics can be inferred from
Paleomagnetic data. If a tectonic plate has
moved, since the remanent field was
recorded, then the inclination (I) gives the
distance to the virtual geomagnetic pole
and the declination (D) can be used to
position it. Poles for different ages trace
an apparent polar wander path that can be
linked to plate motions. Each continent has its own polar wander path.
Geomagnetic polarity reversals
(switching of the orientation of
the geomagnetic field) occur
with varying frequencies, take
place over ~3000-5000 yr, and
can be used to calibrate
seafloor age. Plate tectonic
reconstructions can be deduced
from seafloor age maps.
Clint Conrad
10-4
University of Hawaii