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Geomagnetism 3: The use of geomagnetism to infer past plate motion
Recommended reading:
The Solid Earth by C.M.R. Fowler
The use of geomagnetism to infer past plate motion: Polarity reversals and magnetization
The determination of past plate motion
is possible thanks to two facts:
• Magnetic pole reversals
• Rocks can record the Earth’s past
magnetic field
The use of geomagnetism to infer past plate motion: Palaeolatitude
The magnetic inclination and the magnetic
latitude are related each other through:
tan I  2tan l,
where I is the magnetic inclination and l is the
magnetic latitude, i.e. I=90-l.

Thus, measurement of the magnetic angle of
inclination of the remnant magnetization of a
suitable lava or the youngest sediment may be
used to infer the magnetic palaeolatitude of a
given piece of continent.
Unfortunately, it is not possible to infer palaeomagnetic data to infer palaeolongitude.
The use of geomagnetism to infer past plate motion: Palaeolatitude
• Isoclinal (lines of equal
inclination) chart of the Earth
magnetic field for 1945.
• The curviness of the
contours is due to the nondipole nature of the magnetic
field.
• Thus, the palaeolatitude
obtained from the equation
on the previous slide is
imprecise.
The use of geomagnetism to infer past plate motion: Magnetic stripes in the ocean
First detailed map of magnetic anomalies published in 1961:
• Over the ocean, magnetic field measurements
are made by towing a magnetometer behind
the ship.
• These instruments measure the magnitude of
the magnetic field, but not the direction.
• The magnetic anomaly is obtained by
subtracting the regional field from the
measured field.
• The magnetic stripes run parallel to the ridges
and are symmetric about their axes.
• The stripes are offset by fracture zones.
The use of geomagnetism to infer past plate motion: Magnetic stripes in the ocean
• The origin of the magnetic stripes was first
correctly understood in 1963.
• The width of the magnetic stripes is determined
by the half-spreading rate and the time intervals
between polarity reversals.
• Thus, to “decode” the magnetic pattern it is
necessary to know either when the earth’s field
reversed or else the half-spreading rate.
The use of geomagnetism to infer past plate motion: The Pacific Ocean
When combined with age data, the interpretation of magnetic anomalies becomes simple.
Note the 20 degrees difference between the present day trend of the JDF ridge and the
strike of older anomalies.
The use of geomagnetism to infer past plate motion: The Pacific Ocean
The 20 degrees difference between the present-day trend of the JDF ridge and the strike
of older anomalies is attributed to changes in ridge direction (i.e., changes in Euler pole).
Two possible models:
• The rotation model.
During the adjustment
period, the spreading is
asymmetric.
•The propagating rifting.
The rift jump, and then
lengthen. This model
seems suitable for the
JDF ridge.