Page 88 - Global Tectonics

SEA FLOOR SPREADING AND TRANSFORM FAULTS 75

continues for some time afterwards, with a total dura-
tion of about 10,000 years.
There is no general theory for the origin of the
geomagnetic field. However, it is recognized that the

main part originates within the Earth, and must be
caused by dynamic processes. A magnetostatic origin
appears impossible as no known material is suffi ciently
magnetic to give rise to the magnitude of the fi eld
observed at the surface, and subsurface temperatures
would be well in excess of the Curie point, even given
that its dependence upon pressure is largely unknown.
The temporal variation of the internally generated
field would also be inexplicable with such a model.

The geomagnetic field is believed to originate by

magnetohydrodynamic processes within the fl uid
(outer) part of the Earth’s core, magnetohydrodynam-
ics being that branch of physics concerned with the
interaction of fl uid motions, electric currents, and

magnetic fields. Indeed, this process is also believed to
be responsible for the magnetic fields of other planets

and certain stars. The process requires the celestial
body to be rotating and to be partly or completely
composed of a mobile fluid which is a good electrical

conductor. The turbulent or convecting fl uid consti-
tutes a dynamo, because if it moves in a pre-existing
magnetic field it generates an electric current which

Figure 4.2 Magnetic lineations either side of the mid- has a magnetic field associated with it. When the mag-

Atlantic ridge south of Iceland. Positive anomalies in netic field is supplied solely by the electric currents, the
black (after Heirtzler et al., 1966, in Deep Sea Research dynamo is said to be “self-excited.” Once “excited,” the
13, 428, with permission from Pergamon Press. dynamo becomes self-perpetuating as long as there is
Copyright Elsevier 1966). a primary energy source to maintain the convection
currents. The process is complex, and analytic solu-
certain volcanic sequences were explicable in terms of tions are only available for the very simplest confi gura-
a reversed magnetization vector. tions, which cannot approach the true confi guration in
By the early 1960s the concept of geomagnetic fi eld the core. The field is thought to be maintained by

reversals was being revived, both because of the large convection in the outer core, which is thermally or
number of paleomagnetic measurements revealing gravitationally driven, either by heat sources in the
40
reversed magnetization, and the demonstration that core, such as potassium ( K), or the latent heat and
self-reversal, whereby a reversed magnetization can light constituents released during solidification of the

originate from interaction with normally magnetized inner core due to the slow cooling of the Earth (Section
material, was a very rare phenomenon. By the mid 2.9) (Merrill et al., 1996).
1960s, following the work of Cox et al. (1964, 1967) on A mathematical formulation of the geodynamo has
lava flows erupted within the past few million years, the not been possible because of the complexity of the

concept was widely accepted. More recently, paleomag- physical processes occurring in the Earth’s fl uid core.
netic studies of rapidly deposited sediments, lava fl ow Consequently theoreticians have had to resort to
sequences, and slowly cooled igneous intrusions have numerical modeling. Initially these simulations were
shown that a magnetic reversal occurs over a time inter- severely limited by the computing power available for
val of about 5000 years. It is accompanied by a reduc- the very large number of numerical integrations
tion in field intensity to about 25% of its normal value involved. Large-scale, realistic simulations of the

which commences some time before the reversal and dynamo model had to await the advent of the so-called