Paleomagnetism and Earth History - Quantitative Plate Tectonics - page 174

6

Paleomagnetism and Earth History

Abstract
Even in times preceding the worldwide acceptation of the plate tectonics
paradigm, paleomagnetic data have furnished the most striking evidence
of Wegener's continental drift theory. In this chapter, I present selected
topics from paleomagnetism, which are essential for using paleomagnetic
data in plate kinematics modelling. First, I introduce Neel's theory of
thermo-remanent magnetization, which represents the form of primary
remanent magnetism acquired by most igneous rocks. Then, the basics of
paleomagnetic sampling and statistics are discussed. Finally, the chapter
presents some advanced applications, such as the construction of apparent
polar wander paths and true polar wander, as well as a global plate motions
model since the Triassic.

heating, and other secondary processes. However,
such a long-term stability does not imply an exact
conservation of the total magnetization vector,
because for H D 0 the final state of equilibrium
of a rock containing a large ensemble of SD
grains requires in any case that M r D 0. This
demagnetized limit equilibrium state is not a
consequence of the fact that the magnetization
vectors of the individual grains have decreased to
zero, because an SD grain is always magnetized
to saturation (see Sect. 3.5 ) . The total magneti-
zation of an ensemble of SD grains decays to
zero as a consequence of the tendency for the
magnetization vectors of the individual grains to
acquire a random alignment. At any temperature,
the thermal energy is responsible for this relax-
ation towards a demagnetized equilibrium state.
Now we are going to introduce the classic theory

6.1 Néel's Theory

of Single-Domain TRM

In Chap. 3 , we have introduced the physics of
ferromagnetism in its generality, without consid-
ering the different kinds of NRM that are carried
by Earth's rocks. Here we are going to describe
TRM, which is the form of primary remanent
magnetism acquired by most igneous rocks and
some high-grade metamorphic rocks when they
are cooled in a weak field H below Curie's
temperature. This kind of NRM is considered the
most reliable record of the past directions and
intensities of the geomagnetic field, especially
when it is carried by SD grains. In general, the
TRM of SD grains is very stable over geological
times, despite geomagnetic polarity reversals, re-