Geology Reference
In-Depth Information
Figure 2.2
(a) geometry for
calculating the dipole field for
the dipole moment shown by the
arrow using Equations (2.1) and
(2.2) in the text. P is the point
where the field is calculated.
(b) Interference pattern for the
radial fields for two monopoles
paired together to create a dipole.
The dipole field is the result of
the interference between two
radial fields for two sources close
together, viewed from a distance
(the far field).
(a)
(b)
S
r
θ
P
N
where θ is the angle between the dipole moment and the radial vector out
to the point in space where the field is calculated, r is the distance from the
dipole moment out to the point in space, and M is the dipole moment's
strength (Figure 2.2). The equations above are written for cgs (centimeter-
gram-second) units; for SI (System International) units, the denominators
would also include a 4π factor multiplying the cubed distance term. One way
of conceptualizing the dipole moment is that it is the result of the interference
between the radial field lines of separate (and imagined) magnetic monopoles
when observed in the far field, i.e., the monopoles are very close together com-
pared to the distance of the observation point from the dipole (Figure 2.2).
The magnetic measurements made in a paleomagnetism laboratory or rock
magnetism laboratory are in reality the measurements of the dipole fields gen-
erated by the samples. Typically for rock magnetic cyclostratigraphy measure-
ments, the material's magnetic dipole moment is normalized by the mass of
the material. Converting from cgs magnetics units to SI units is tricky and
often involves a 4π factor, but the conversion from mass normalized cgs units
(emu/g) to mass normalized SI units (Am
2
/kg) is easy; the conversion factor is
simply unity. It is very important to keep track of units in any rock magnetic
study, since proper units are critical to quantitative comparison of results bet-
ween different environmental magnetic studies. Tauxe (2010) and Butler
(1992) both provide good discussions of magnetic units and the conversion
between cgs units, which are typically used in most paleomagnetic labora-
tories, and SI units, which must be used for publication. However, for
convenience, Table 2.1 gives the critical conversions between cgs and SI
magnetic units for magnetization, field, and susceptibility.
All materials, either natural or human-made, are magnetic in that either
they carry a permanent magnetization (remanent magnetization) or they
become magnetized during exposure to a magnetic field (induced magne-
tization). Basically, there are three different types of material magnetism:
diamagnetism, paramagnetism, and ferromagnetism.
Diamagnetism is a weak induced magnetization in which a material
acquires a magnetization with a magnetic moment opposite in direction to
