Transcript mag_reversals302

Reversals of the Geomagnetic
Field
• Secular variations- historic to modern changes in
the field
• Archaeomagnetism: changes during the Holocene
• Reversals of the dipole polarity
• Reversal chronology for past 5 million years: the
terrestrial record
• reversal chronology for past 200 million years: the
seafloor spreading “tape recorder
Locations of the
north pole of the
dipole component of
the geomagnetic
field from 19452000.
positions of the north magnetic pole during the past 3700 years.
-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)
units: nT/yr
contour interval: 5 nT/yr
Main field: 30,000 to 60,000 nT
units: minutes/yr
contour interval: 2 min/yr
units: minutes/yr
contour interval: 1 min/yr
Dipole moment determined from the strength of magnetization of
archaeological material (archaeomagnetic results from TRM in
ancient hearths and pottery)
Years before present (BP)
Schematic plot of magnetic field variations in time
dipole component
non-dipole component
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.
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. Our solution shows how convection in the fluid outer core
is continually trying to reverse the field but that the solid inner core inhibits magnetic
reversals because the field in the inner core can only change on the much longer time
scale of diffusion [2]. Only once in many attempts is a reversal successful, which is
probably the reason why the times between reversals of the Earth's field are long and
randomly distributed.
The key to determining the chronology of the geomagnetic
field reversals is to be able to date the time during which
robust magnetizations were attained in a given rock sample.
The classical work was done in the latter half of the last
century on basaltic rocks, which cool rapidly and acquire a
strong thermo-remanent magnitization (TRM). These rocks
can be dated effectively with the Potassium-Argon
method, which uses the decay of K-40 into the chemically
inert Ar-40.
Ar-40 is trapped and accumulates in the rock only since the
last time the rock was melted – the time when the basalt
was extruded and solidified. While liquid, the prior Ar-40, a
gas, leaves the magma. Since the basalt is extruded on the
surface, cooling is rapid and the acquisition of TRM occurs
soon after the trap is set for accumulation of Ar-40.
Reversal captured in Columbia River basalt
flows ( Steens Mtn., Oregon: Miocene, 15.5 Ma)
Steens Mtn: Kiger Gorge from the Steens Mountain Loop Road
Steens Mtn: View northwest from the short trail/road to the summit.
High resolution
record of
geomagnetic field
reversal
3500 yrs
3600 yrs
Reversal captured in
Columbia River basalt
flows ( Steens Mtn.,
Oregon: Miocene, 15.5
Ma)
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
Magnetizations (DRM) recovered from deep ocean sediments
Magnetizations (DRM) recovered from deep ocean sediments
Note minimum intensities
during reversals
Geomagnetic field
reversal chronology for
past 5 million years
based mainly on K-Ar
dating of terrestrial
volcanic rocks
Why only to 5 Ma?
Geomagnetic field
reversal chronology for
past 5 million years
based mainly on K-Ar
dating of terrestrial
volcanic rocks
Why only to 5 Ma?
Errors in K-Ar dates
become too large
compared to reversal
periods
The chronology of geomagnetic field reversals earlier
than 5 Ma is well preserved in the magnetization of
basalts extruded on the ocean floor in the process of
sea-floor spreading.
The chronology of geomagnetic field reversals earlier than 5
Ma is well preserved in the magnetization of basalts
extruded on the ocean floor in the process of sea-floor
spreading.
How does this work? Need to understand 1. Magnetic anomalies observed by ships at sea
2. The sea-floor spreading “tape recorder”
Marine Magnetic anomalies
• Ships tow magnetometers which measure the “total intensity” of the
geomagnetic field, the magnitude of the geomagnetic field vector, often
symbolized by F, or Fobs , to denote that it is the observed total intensity. These
measurements lead to a plot of Fobs versus distance along the track.
magnetic field intensity,Fobs
Smoothly varying global field plus small, short
wavelength effects due to crustal magnitizations
0
distance along
ship track
Marine Magnetic anomalies
• Ships tow magnetometers which measure the “total intensity” of the
geomagnetic field, the magnitude of the geomagnetic field vector, often
symbolized by F, or Fobs , to denote that it is the observed total intensity. These
measurements lead to a plot of Fobs versus distance along the track.
• The main internal geomagnetic field (produced in the outer core), Fg, is
determined for the earth as a function of time as the International Geomagnetic
Reference Field (IGRF).
• The IGRF field can then be subtracted from the observed value to produce a
total intensity anomaly, DF = Fobs - Fg
• DF results only from effects of rocks magnetized near the surface, and can thus
be compared with models of the magnetization of the ocean bottom rocks.
magnetic field intensity,Fobs
Smoothly varying global field plus small, short
wavelength effects of crustal magnitizations
0
distance along
ship track
intensiy anomaly, DF
Total intensity anomaly, DF
0
distance along
ship track
Marine Magnetic anomalies
The rocks with the strongest magnetizations by far are the basalts
extruded and rapidly cooled, acquiring thermo-remanent
magnetization (TRM) via the process of seafloor spreading.
The following slides give an example of the large anomalies
associated with seafloor spreading
Global bathymetry, showing ocean ridge system
Global bathymetry, showing ocean ridge system
Global bathymetry, showing ocean ridge system
Map shown
in next slide
Ship tracks across the East Pacific Rise which obtained the magnetic anomalies
shown in the next slide. The measurements were made in the 1960’s by the Columbia
University research vessel Eltanin.
21
20
19
Eltanin profiles of
magnetic anomalies
The four profiles show total
intensity anomalies and
bathymetry (ocean depth in km)
along the four tracks shown on
the previous map. Note that
track 20 crosses the ridge
system twice.
Eltanin profiles of
magnetic anomalies
Magnetic anomaly,
gamma
Ocean depth, km
The vertical scale for total intensity anomaly, DF,
is shown in “gammas”. This is the same as
nanoTeslas or nT. The horizontal lines are at zero
anomaly; the scale is thus minus 500 to plus 500
nT.
Note that the total intensity
anomaly fluctuates from large
positive to large negative values.
This can only be explained by
systematic changes in the
directions of magnetizations of
ocean floor rocks; merely
changing the induced
magnetizations won’t work.
Also note that peaks and
troughs in the curves can be
correlated from track to track,
indicating that the magnetized
material on the ocean floor with
a positive or negative
magnetization can be traced
along the strike of the ocean
ridge system. These correlations
are shown by the numbers,
which identify correlatable
features in the wiggly lines.
The incredible symmetry of the Eltanin 19 profile
ESE
WNW
WNW
ESE
total intensity anomaly calculated from model
mirror image of measured
profile to show symmetry
measured profile of
total intensity anomalies
Modeling the magnetic anomaly pattern
mirror image of measured profile to show symmetry
ESE
WNW
Observed profile of total
intensity anomalies
WNW
ESE
total intensity anomaly
calculated from model
cross section through model of
normal (black) and reversed
(white) magnetized upper crust
reversal chronology from
paleomagnetic studies on land
Magnetic field lines for vertically
downwards magnetization in
cross-sectional view
---------------J
+ + + + + + + + + + + + +++
Magnetic field lines for vertically
upwards magnetization
+ ++ + + + + + + + + + + + +
- - - - - - - - - -J- - - - - -
Magnetic field due to
magnetized prism taken
along the surface above
the prism (directions
only)
ocean surface
---------------J
+ + + + + + + + + + + + +++
Earth’s field, He
Vertically downwards
magnetization parallel to
vertical earth’s field
Magnetized prism
field adds to Earth’s
field, DF positive
---------------J
+ + + + + + + + + + + + +++
Earth’s field, He
Magnetic field due to
magnetized prism taken
along the surface above
the prism (directions
only)
Magnetized prism
field perpendicular
to He, DF = 0
---------------J
+ + + + + + + + + + + + +++
Earth’s field, He
Magnetic field due to
magnetized prism taken
along the surface above
the prism (directions
only)
Magnetized prism
field subtracts from
He, DF negative
---------------J
+ + + + + + + + + + + + +++
Earth’s field, He
Magnetic field due to
magnetized prism taken
along the surface above
the prism (directions
only)
distance
along track
reversal
sea surface
reversal
axis of seafloor spreading
Direction of
modern
geomagnetic
field
reversal
-
reversal
Intensity anomaly, DF
+
ocean bottom
Basalt magnetized upon solidification along axis of spreading ridge
Intensity anomaly, DF
distance
along track
reversal
Magnetization
decreases main
field
sea surface
reversal
reversal
Direction of
modern
geomagnetic
field
reversal
Magnetization
decreases main
field
axis of seafloor spreading
Magnetization
increases main
field
ocean bottom
Basalt magnetized upon solidification along axis of spreading ridge
The interpretation of the ocean floor magnetization is provided by the seafloor spreading
model, shown schematically below.
Schematic representation of
upper crustal magnetized layer
Age, Ma
lithosphere
crust
moho
upper mantle
convecting mantle
Seafloor spreading is a tape recorder of the geomagnetic field!
Age, Ma
The recorded
reversal chronology
crust
moho
upper mantle
The tape drive
The recording head
of the tape recorder
The seafloor spreading tape recorder extends the
record of geomagnetic field reversals out as far as
we have ocean basins- this turns out to be about 200
million years worth of recording.
The seafloor spreading tape recorder extends the record
of geomagnetic field reversals out as far as we have
ocean basins- this turns out to be about 200 million
years worth of recording.
All that is needed is to determine the timing of the
recording system back beyond 5 million years.
The seafloor spreading tape recorder extends the record
of geomagnetic field reversals out as far as we have
ocean basins- this turns out to be about 200 million
years worth of recording.
All that is needed is to determine the timing of the
recording system back beyond 5 million years.
How? Drilling to the bottom of the sediments that
cover the basalts, and use the
biostratigraphic/geochemical time scale to date
when sedimentation started. The Ocean Drilling
Program, which started in 1968, did just this
throughout the world’s oceans.
paleontological age
magnetic anomaly number
Seafloor ages from
deep sea drilling
versus
geomagnetic reversal
chronology
data for Atlantic ocean;
similar data from older
oceans permit reversal
chronology to be
calibrated back to 180
Ma
age (Ma) from geomagnetic reversal chronology extrapolated
in South Atlantic assuming constant rate of spreading
Chronology of
geomagnetic field
reversals recorded
on ocean floor
magnetic anomaly
“number” is a
convenient identifier of
specific features of the
magnetic anomaly
profiles that have
proven useful for
correlation between
different profiles.
Ocean floor age,
millions of years
(Ma), determined
largely from deep
sea drilling (ODP
program)