Download 403 Forbidden

Geomagnetism (I)
The Earth’s magnetic field

Magnetic field of the Earth measured at the surface
comes from three sources:
 97-99%
represents main field generated by dynamo
action in the outer core. Main field varies significantly
with time (secular variation means variations along
geological time)
 1-2% represents external field generated in space in
the magnetosphere. External field also varies on time
scales of seconds to days.
 1-2% represents crustal field from remnant
magnetization above the Curie depth.
The Earth’s magnetic field

The Earth’s magnetic field
would be:
 vertical


at the poles
horizontal at the equator
Today, the best-fit dipole is
currently oriented 11.5° from
the rotation axis of the
geographic north pole, but
this has varied with time.
Describing the Earth’s magnetic field







Declination (D)
Inclination (I)
Horizontal Intensity (H)
Vertical Intensity (Z)
North-South Intensity (X)
East-West Intensity (Y)
Total Intensity (B)
X
Y
Describing the Earth’s magnetic field

This first order simple model of the field allows to use
the paleomagnetic observations to determine past
plate motions

Magnetic potential is given by
The Earth’s best fit dipole moment (m) equals to 7.94x1022 Am2 in magnitude

Magnetic field is determined by the differentiating the
magnetic potential
given the magnetic permeability of free space, μ0 = 4x10-7 kg m A-2 s-2
Spherical polar coordinates


Conversion from/spherical into Cartesian
coordinates:
Gradient operator
f  (
f 1 f
1
f
,
,
)
R R  R sin  
Describing the Earth’s field


If the Earth’s magnetic dipole moment is aligned along
the z-axis:
At a latitude of θ and longitude , Magnetic field in
spherical polar coordinates can show three components:
Radial Component Br,
 Southerly Component B, and
 Easterly Components B

Describing the Earth’s field


For the best fit dipole, Three components are
given by
Total field is given by
Describing the Earth’s field



Then, the magmatic inclination (I) can be computed from
the following equation
At the North Pole, θ = 90° which gives I = 90°
At the Equator, θ = 0° which gives I = 0°
Describing the Earth’s field
Describing the Earth’s field
Describing the Earth’s field




The equation of the magnetic inclination is important
because it allows use to use a measurement of
inclination (I) to determine latitude (θ). This was once
used by mariners, but is most important in
paleomagnetism.
A rock can record the magnetic field present when it
crystallized (temperature fell below the Curie
temperature).
Thus we can find the latitude of a continent at some
time in the past.
This was the idea of Apparent Polar Wandering.
Diamagnetism and paramagnetism


The magnetic behaviour of minerals is due to atoms
behaving as small magnetic dipoles.
If a uniform magnetic field (H) is applied to a
mineral, there are two possible responses.
 Diamagnetic
behaviour
 Paramagnetic behaviour
Diamagnetic behaviour




This effect arises from the orbital motion of
electrons in atoms.
The atom develops a magnetic field that is
opposite direction to the applied magnetic field
Magnetic susceptibility is negative
All minerals diamagnetic but will be masked by
paramagnetism
Paramagnetic behaviour




This phenomena arises when the atoms have a net
magnetic dipole moment due to unpaired electrons.
The atoms align parallel to the applied magnetic
field H and increase the local magnetic field.
For paramagnetic materials Magnetic susceptibility
is positive.
Paramagnetic elements include iron, nickel and
cobalt.
Geomagnetism (II)
Rock magnetization and translation
Magnetizing Igneous Rocks


Curie Temperature

Temperature above which a mineral cannot be permanently magnetized

spontaneous magnetization when temperature drops below Curie temperature
Curie Depth


Is the depth at which magnetic behaviour ceases since temperature exceeds curie
temprature. Thermal vibrations of atoms prevents domain formation.
Blocking Temperature

Tens degrees less than the Curie point for most minerals

Temperature below which the orientation of the rock’s magnetization cannot change

magnetization cannot change once below blocking temperature

Both temperatures are much lower than that at which lavas crystallize.

The magnetization becomes permanent some time after lavas solidify.


This type of permanent residual magnetization is called thermoremanent magnetization
(TRM); atoms align when molten and freeze
The magnetism of TRM is larger in magnitude than that induced in the basalt by the earth’s
present field.
Magnetizing sedimentary rocks

Sedimentary rocks can acquire magnetization in
through:
 Depositional
or detrital remanent magnetization
(DRM); acquiring during the deposition of sedimentary
rocks.
 Chemical remanent magnetization (CRM); acquiring
after deposition during the chemical growth of iron
oxide grains as the case in sandstones.

Strength of DRM and CRM fields typically 1-2
orders of magnitude smaller than TRM
Detrital remnant magnetization

Detrital magnetization can
produce a weak remnant
magnetization in sedimentary
grains

Grains being deposited
contain some magnetite or
other magnetic minerals

Preferred orientation as they
are deposited
Chemical remnant magnetization


Can occur during alteration
Example from oil field in Gibson and Millegan
(1988)
Induced magnetic field
Calculating palaeomagnetic latitude (use of TMR)
Example

A rock sample was found at latitude of 34°N. Remnant
magnetization in the sample was found to have an inclination I
= 40 ° from the horizontal. Was the rock magnetized at the
location where it was sampled?
Locating the palaeomagnetic pole (use of TMR)
Locating the palaeomagnetic pole (use of TMR)
Polar wander paths
Magnetic stripes (Dating the oceans)




Using magnetometer with overseas vessel
Measure the total field intensity
Subtract regional value
Produce magnetic anomaly map
Magnetic stripes (Dating the oceans)


Raff and Mason, 1961
 First magnetic field map
 Off the western coast of North America
Magnetic anomaly map
 Take total magnetic field
intensity and subtract regional
average

Black Stripes: positive intensity

White Stripes: negative intensity

Which is normal/reversed polarity?

Coupled stripes with sea-floor spreading and magnetic pole
reversals
Origin of Seafloor magnetic anomalies
formed at mid-ocean ridges

Important evidence to support the hypothesis of continental
drift came from observations of magnetic fields measured by
survey ships on profiles that crossed the world’s oceans.

Basalt erupted and when cools it is permanently magnetized in
direction of Earth’s magnetic field at that time.

Sea floor spreading moves rocks away from ridge.

Magnetic field reverses direction
Magnetic stripes anomalies

Magnetic stripes anomalies are considered for two cases of
magnetic stripes anomalies:


High magnetic latitude
Low magnetic latitude

Magnetic stripes at High magnetic latitudes
Magnetic stripes anomalies of high
magnetic latitude are characterized
by:
 Earth’s magnetic field is close
to vertical.
 Remnant magnetization at the
ridge is in the same direction
as the Earth’s field.
 Positive magnetic anomaly at
the ridge crest
Magnetic stripes anomalies