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 = 4x10-7 kg m A-2 s-2

Spherical polar coordinates Conversion from/spherical into

Cartesian coordinates:

Gradient operator

)sin

1,

1,(

f

R

f

RR

ff

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