Transcript lecture36

A snapshot of the 3D
magnetic field structure
simulated with the
Glatzmaier-Roberts
geodynamo model.
Magnetic field lines are
blue where the field is
directed inward and yellow
where directed outward.
The rotation axis of the
model Earth is vertical and
through the center. A
transition occurs at the
core-mantle boundary from
the intense, complicated
field structure in the fluid
core, where the field is
generated, to the smooth,
potential field structure
outside the core. The field
lines are drawn out to two
Earth radii. Magnetic field
is wrapped around the
"tangent cylinder" due to
the shear of the zonal fluid
flow.
View online magnetograms
from USGS geomagnetic
observatories
Local view of the relationships of the
geomagnetic field vector seen at the
site of measurement. Shown is the
total field H, the declination angle, D,
measured in the horizontal plane, and
the inclination angle, I, measured in
the vertical plane. D is measured
clockwise with respect to geographic
north.
SITE
North Pole (NP)
This is the
longitudinal
great circle that
passes through
the North Pole
and the
observation site
Red grid: geomagnetic
latitudes and longitudes
about geomagnetic north
pole (shown as VGP here)
This is the great
circle through
the observation
site and the
VGP; it is the
same great
circle shown in
sections in the
preceding
figures.
SITE
Black grid: geographic
latitudes and longitudes
Pointing in the direction of
geographic North, along
the great circle joining site
and North Pole
Pointing in the direction of
magnetic North, along the
great circle joining site and
geomagnetic North Pole
(VGP in diagrams)
SITE
North Pole (NP)
Thus from measurements of D and I at
the site, one can
1) determine the great circle through
the site which makes an angle, D,
with the longitudinal great circle
through the site;
2) measure the angular distance q,
calculated from I, along the segment
of the great circle (shown in red)
connecting the SITE and the VGP.
This determines the location of the
“Virtual Geomagnetic Pole” or VGP
which is the location of north
geomagnetic pole assuming that the
declination and inclination at the site
result from a simple earth centered
dipole.
Locations of the
north pole of the
dipole component of
the geomagnetic
field from 1600 2001.
Canadian geomagnetic program:
http://www.geolab.nrcan.gc.ca/geomag/northpole_e.shtml
Locations of the north pole of the
dipole component of the
geomagnetic field from 1904 -2001.
Magnetic field “jerks”
Canadian geomagnetic program:
http://www.geolab.nrcan.gc.ca/geomag/northpole_e.shtml
Paleosecular poles from
western US volcanic rocks
Hagstrum and Champion, 2002, A Holocene
paleosecular variation record from 14C-dated
volcanic rocks in western North America,
J.Geophys. Res.,v.107, B1.
Summary of “continuous” data
-30 to 3690 BP
Average pole position
for all data
(94 poles):
88.4 N
23.8 W
1.6 degrees from
geographic North Pole
-30 to 800 BP
800 to 1940 BP
1940 to 3690 BP
Calibrated radiocarbon years before
present, (B.P, AD1950=0)
VGP’s determined
for magnetized
rocks less than 5 Ma
modern
geomagnetic
pole
90 E
180
VGP’s average Earth’s rotation
axis!
Polar projection showing VGP’s
derived from magnetization of
igneous rocks at many sites, all
dated at less than 20 million years
old (too young to be significantly
affected by plate motions).
geographic North Pole
0
90 W
500yrs before
middle of reversal
500yrs after
“About 36,000 years into the simulation the magnetic field underwent a reversal of its
dipole moment (Figure 3), over a period of a little more than a thousand years. The
intensity of the magnetic dipole moment decreased by about a factor of ten during the
reversal and recovered immediately after, similar to what is seen in the Earth's
paleomagnetic reversal record.”
(See mag_field.ppt for the source of the models shown above)
Excursions and transitions
Excursions
Reversal rate, per Ma:
Superchrons
Magnetizations (DRM) recovered from deep ocean sediments
Magnetizations (DRM) recovered from deep ocean sediments
Note minimum
intensities during
reversals
Reversal captured in Columbia River basalt
flows ( Steens Mtn., Oregon: Miocene, 15.5 Ma)
Steens Mtn: Kiger Gorge from the Steens Mountain Loop Road
High resolution
record of
geomagnetic field
reversal
3500 yrs
3600 yrs
Extrusions at rate of
about 43 m/1000 yrs
5000 yrs
Mankinen, et al., 1985, J.
Geophys. Res., v. 90, p, 10400
Steens Mtn
results: VGP’s in
time
Figure 5 Spatial patterns of
large-scale mantle and core
dynamical structures.
a, Map showing spatial
distribution of subducted slabs
that are expected to have penetrated
to the base of the mantle (blue lines)
and buoyancy flux weighted
hotspots (red dots).
b, Map showing virtual
geomagnetic pole (VGP) reversal
paths (green lines). To first order,
reversal paths and hotspots appear
to be anti-correlated, with reversal
paths generally following
approximately longitudinal bands
in which the ULVZ has not been
observed to be present