review: earth magnetic field

Review: Earth Magnetic Field

GEOMAGNETIC FIELD = DIPOLE +

NONDIPOLE

Dipole field

A comment on branton adjacment...

The origin of the dipole field is in the liquidcore. This field and its reversal have beensimulated numerically by Glatzmaire and

Roberts [1995]. Movies of these simulations maybe viewed on:http://www.psc.edu/research/graphics/gallery/geodynamo.html

Non-dipole field

Question: What gives rise to the nondipole

component?

Two effects act to produce a nondipole field:

1) Solar wind

2) Screening by the mantle and lithosphere

The following chapter is based on the book of

Butler [1992]. An electronic edition may be

downloaded from:

http://www.geo.arizona.edu/Paleomag/book/

Magnetization

Induced magnetization, Ji. When a material is

exposed to a magnetic field H, it acquires an

induced magnetization. These are related

through the magnetic susceptibility, χ.

Ji = χH

If the material magnetic properties are

anisotropic, χ is a tensor, and Ji need not be

parallel to H. (talk about units)

Remanent magnetization, Jr. This remanent

magnetization is a recording of past magnetic

field that have acted on the material.

There types of magnetic properties:

(a) Diamagnetic substance.

The diamagnetic response to application of

magnetic field is acquisition of small induced

magnetization opposite to the applied field. The

magnetization depends linearly on the applied

field and reduces to zero on removal of the field.

(b) Paramagnetic substance.

Paramagnetic These solids contain atoms with

atomic magnetic moment and acquire induced

magnetization parallel to the applied field. For

any geologically relevant conditions Ji is

linearly dependent on H. As with diamagnetic

materials, magnetization reduces to zero when

the magnetizing field is removed.

(c) Ferromagnetic substance.

Ferromagnetic solids have atoms with magnetic

moments, but unlike the paramagnetic case,

adjacent atomic moments interact strongly.

The effect of interaction is to produce

magnetization that can be orders of magnitude

larger than for the paramagnetic solids. Upon

removal of the magnetizing field, magnetization

does not return to zero but retains a record of

the applied field.

Natural Remanent Magnetization, J (NRM). In

situ magnetization of rocks is the vector sum

of two components:

J = Ji + Jr

REMANENTINDUCED

NRM

NRM is the remanent magnetization present in

a rock sample prior to laboratory treatment. It

depends on the geomagnetic field and geological

processes during rock formation and during the

history of the rock.

Question: For a rock to show NRM, what type

of materials must be present?

NRM = primary NRM + secondary NRM

Three forms of primary NRM:

Thermoremanent magnetization, acquired

during cooling from high temperature.

Chemical remanent magnetization, formed by

growth of ferromagnetic grains below the Curie

temperature.

Detrital remanent magnetization, acquired

during accumulation of sedimentary rocks

containing detrital ferromagnetic minerals (see

cartoon on next page).

Secondary NRM:

Results from chemical changes affecting

ferromagnetic minerals, exposure to nearby

lighting strikes, or long-term exposure to the

geomagnetic field subsequent to rock formation.

Sampling

The first step of paleomagnetic survey is to

collect oriented cores. The information of each

sample includes coordinates, azimuth and dip

(or hade).

Measurement of NRM

NRM is measured with a special devise called

magnetometer.

Display of Data

Vector directions are described in terms of

inclination and declination. This

information is then projected onto a

stereographic plot.

Rotation of the sample coordinates to

geographic direction

Bedding-tilt correction

Geological applications

Fold Test

Synfolding

Conglomerate Test

Flow direction in volcanic dikes

While the magma is flowing in the dike,

elongate particles become imbricated against

the chilled margins. In the ideal case, the flow

directions from the two margins are distinct

and fall on either side of the dike plane. The

fact that the western margin data plot on the

western side and the eastern margin data plot

in the eastern side suggests that the flow was

upward.