Physics for Geologists, Second edition

Electricity and magnetism 83
is known as the declination in geophysics, variation2 in navigation and on
maps. This angle varies with position - and in any one position, it varies
with time. The compass needle does not usually point to the magnetic poles
but is aligned, as mentioned earlier, with a field line that eventually reaches
the poles.
Why does the Earth have a magnetic field? Assuming that we are correct
in saying that most of the interior of the Earth is above its Curie point, and
that it cannot therefore be permanently magnetized, the source of the field
must either lie in the shallower, low-temperature crust or be continuously
generated in the core. It is clearly something to do with the Earth's rotation
for the magnetic poles to be so close to the geographic poles. It is thought
to be caused by convection currents circulating conducting metallic fluids
in the outer core, a sort of self-exciting dynamo. The sudden changes of
polarity that have occurred in the past are not easily explained (sudden, that
is, in the context of geological time). There is a small component, as men-
tioned above, caused by external influences that give rise to small changes
on a short time-scale, such as the magnetic storms associated with sunspot
activity. These components are in the field caused by electric currents in the
ionosphere and solar wind.
Remanent magnetism
Remanent magnetism is the magnetism that remains after the magnetizing
force has been removed or changed. In rocks there are minerals that have
magnetic properties, such as magnetite (naturally!), that may have been mag-
netized in the past by a terrestrial magnetic field with its poles in a different
position or reversed. Today, such an object is in the present field, the strength
and direction of which is accurately known. Using a technique for removing
the magnetization due to the present terrestrial field, the remanent magnetism
can be determined.
When a ferrimagnetic material cools from above to below its Curie point,
as lava tends to do, it acquires its magnetism from the local field. This
is called thermoremanent magnetism. If new magnetic minerals grow dur-
ing diagenesis or metamorphism, they become permanently magnetized to
a measurable degree once they attain a certain size. If magnetized sedimen-
tary particles settle from suspension in still water, or are agitated gently
on the bottom before accumulating into the stratigraphic record, they may
orientate themselves in the geomagnetic field and so preserve a record of
it. All these are examples of remanent magnetism. Remanent magnetism is
the basis of palaeomagnetism, the elucidation of past magnetic fields and of
sea-floor spreading, continental drift and plate tectonics.
2 Deviation in ships is the difference between compass north and magnetic north due to the
ship itself.
Copyright 2002 by Richard E. Chapman