Page 78 - Global Tectonics

CONTINENTAL DRIFT 65

trons. Magnetic fields are generated by the spin and Curie temperature, which is realized after solidifi ca-
orbital motions of the electrons. In shells with paired tion. At this stage its ferromagnetic minerals pick up a

electrons, their magnetic fields essentially cancel each magnetism in the same sense as the geomagnetic fi eld
other. The unpaired electrons present in paramagnetic at that time, which is retained during its subsequent
substances cause the atoms to act as small magnets or history.
dipoles. The primary remanence in clastic sedimentary rocks
When a paramagnetic substance is placed in a is known as detrital remanent magnetization (DRM). As
weak external magnetic field, such as the Earth’s the sedimentary particles settle through the water

field, the atomic dipoles rotate so as to become par- column, any ferromagnetic minerals present align in

allel to the external field direction. This induced the direction of the geomagnetic field. On reaching

magnetization is lost when the substance is removed bottom the particles flatten out, and if of elongate form

from the field as the dipoles return to their original preserve the azimuth of the geomagnetic field but not

orientations. its inclination (Fig. 3.12). After burial, when the sedi-
Certain paramagnetic substances which contain a ment is in a wet slurry state, the magnetic particles
large number of unpaired electrons are termed ferro- realign with the geomagnetic field as a result of micro-

magnetic. The magnetic structure of these substances seismic activity, and this orientation is retained as the
tends to devolve into a number of magnetic domains, rock consolidates.
within which the atoms are coupled by the interaction Secondary NRM is acquired during the subse-

of the magnetic fields of the unpaired electrons. This quent history of the rock according to various pos-
interaction is only possible at temperatures below the sible mechanisms. Chemical remanent magnetization
Curie temperature, as above this temperature the (CRM) is acquired when ferromagnetic minerals are
energy level is such as to prohibit interatomic magnetic formed as a result of a chemical reaction, such as
bonding and the substance then behaves in an ordinary oxidation. When of a sufficient size for the forma-

paramagnetic manner. tion of one or more domains, the grains become
Within each domain the internal alignment of magnetized in the direction of the geomagnetic fi eld
linked atomic dipoles causes the domain to possess at the time of reaction. Isothermal remanent magneti-
a net magnetic direction. When placed in a magnetic zation (IRM) occurs in rocks which have been sub-
field the domains whose magnetic directions are in jected to strong magnetic fields, as in the case of a

the same sense as the external field grow in size at lightning strike. Viscous remanent magnetization (VRM)

the expense of domains aligned in other directions. may arise when a rock remains in a relatively weak

After removal from the external field a preferred magnetic field over a long period of time as the
direction resulting from the growth and shrinkage of magnetic domains relax and acquire the external
the domains is retained so that the substance exhib- fi eld direction.
its an overall magnetic directionality. This retained Some CRM may be acquired soon after formation,
magnetization is known as permanent or remanent for example during diagenesis, or during a metamor-
magnetism. phic event of known age, and hence preserve useful
paleomagnetic information.
CRM, TRM, and DRM tend to be “hard,” and
3.6.3 Natural remanent remain stable over long periods of time, whereas
certain secondary components of NRM, notably
magnetization VRMs, tend to be “soft” and lost relatively easily. It is
thus possible to destroy the “soft” components and
Rocks can acquire a natural remanent magnetization isolate the “hard” components by the technique of
(NRM) in several ways. If the NRM forms at the same magnetic cleaning. This involves monitoring the orien-
time as the rock it is referred to as primary; if acquired tation and strength of the magnetization of a rock
during the subsequent history of the rock it is termed sample as it is subjected either to an alternating fi eld
secondary. of increasing intensity or to increasing temperature.
The primary remanence of igneous rocks is known Having isolated the primary remanent magnetization,
as thermoremanent magnetization (TRM). It is acquired its strength and direction are measured with either a
as the rock cools from its molten state to below the spinner magnetometer or superconducting magne-