Geology Reference
In-Depth Information
In larger grains, the total magnetic energy is decreased
if the magnetization of each grain subdivides into indi-
vidual volume elements (magnetic domains) with diam-
eters of the order of a micrometre, within which there is
parallel coupling of dipoles. In the absence of any exter-
nal magnetic field the domains become oriented in such
a way as to reduce the magnetic forces between adjacent
domains.The boundary between two domains, the
Bloch
wall
, is a narrow zone in which the dipoles cant over from
one domain direction to the other.
When a multidomain grain is placed in a weak exter-
nal magnetic field, the Bloch wall unrolls and causes a
growth of those domains magnetized in the direction of
the field at the expense of domains magnetized in other
directions. This induced magnetization is lost when the
applied field is removed as the domain walls rotate back
to their original configuration. When stronger fields
are applied, domain walls unroll irreversibly across small
imperfections in the grain so that those domains
magnetized in the direction of the field are permanently
enlarged. The inherited magnetization remaining after
removal of the applied field is known as
remanent
, or
per-
manent
,
magnetization J
r
.The application of even stronger
magnetic fields causes all possible domain wall move-
ments to occur and the material is then said to be
magnetically saturated.
Primary remanent magnetization may be acquired
either as an igneous rock solidifies and cools through
the Curie temperature of its magnetic minerals
(thermoremanent magnetization, TRM) or as the
magnetic particles of a sediment align within the Earth's
field during sedimentation (detrital remanent magneti-
zation, DRM). Secondary remanent magnetizations
may be impressed later in the history of a rock as mag-
netic minerals recrystallize or grow during diagenesis
or metamorphism (chemical remanent magnetization,
CRM). Remanent magnetization may develop slowly
in a rock standing in an ambient magnetic field as the do-
main magnetizations relax into the direction of the field
(viscous remanent magnetization,VRM).
Any rock containing magnetic minerals may possess
both induced and remanent magnetizations
J
i
and
J
r
.The
relative intensities of induced and remanent magnetiza-
tions are commonly expressed in terms of the
Königs-
berger ratio
,
J
r
:
J
i
.These may be in different directions and
may differ significantly in magnitude. The magnetic ef-
fects of such a rock arise from the resultant
J
of the two
magnetization vectors (Fig. 7.4). The magnitude of
J
controls the amplitude of the magnetic anomaly and the
orientation of
J
influences its shape.
J
i
J
r
J
H
Fig. 7.4
Vector diagram illustrating the relationship between
induced (
J
i
), remanent (
J
r
) and total (
J
) magnetization
components.
7.3 Rock magnetism
Most common rock-forming minerals exhibit a very
low magnetic susceptibility and rocks owe their mag-
netic character to the generally small proportion of
magnetic minerals that they contain.There are only two
geochemical groups which provide such minerals. The
iron-titanium-oxygen group possesses a solid solution
series of magnetic minerals from magnetite (Fe
3
O
4
) to
ulvöspinel (Fe
2
TiO
4
). The other common iron oxide,
haematite (Fe
2
O
3
), is antiferromagnetic and thus does
not give rise to magnetic anomalies (see Section 7.12)
unless a parasitic antiferromagnetism is developed. The
iron-sulphur group provides the magnetic mineral
pyrrhotite (FeS
1+
x
,0<
x
< 0.15) whose magnetic sus-
ceptibility is dependent upon the actual composition.
By far the most common magnetic mineral is mag-
netite, which has a Curie temperature of 578°C. Al-
though the size, shape and dispersion of the magnetite
grains within a rock affect its magnetic character, it is rea-
sonable to classify the magnetic behaviour of rocks ac-
cording to their overall magnetite content. A histogram
illustrating the susceptibilities of common rock types is
presented in Fig. 7.5.
Basic igneous rocks are usually highly magnetic due to
their relatively high magnetite content. The proportion
of magnetite in igneous rocks tends to decrease with
increasing acidity so that acid igneous rocks, although
variable in their magnetic behaviour, are usually less
magnetic than basic rocks. Metamorphic rocks are also
variable in their magnetic character. If the partial pres-
sure of oxygen is relatively low, magnetite becomes re-
sorbed and the iron and oxygen are incorporated into
other mineral phases as the grade of metamorphism
increases. Relatively high oxygen partial pressure can,
however, result in the formation of magnetite as an
accessory mineral in metamorphic reactions.
