Geoscience Reference
In-Depth Information
Chapter 3
Past plate motions
3.1 The role of the Earth's magnetic field
3.1.1 Introduction
It should be clear from the preceding chapter that it is possible, without too much
difficulty, to calculate the relative motions of pairs of plates at any location along
their common boundary and to see what may occur in the future. This chapter
deals with the past motions of the plates and shows how to reconstruct their
previous interactions from evidence they have left.
Two important facts together make it possible to determine past plate motions.
The first is that the Earth's magnetic field has not always had its present (normal)
polarity with the 'north' magnetic pole close to the north geographic pole
1
and
the 'south' magnetic pole close to the south geographic pole. Over geological
history the magnetic field has intermittently reversed. Thus, there have been times
in the past when the north magnetic pole has been located close to the present-
day south geographic pole and the south magnetic pole has been located close to
the present-day north geographic pole; then the field is said to be reversed. The
second fact is that, under certain circumstances (discussed in Section 3.2) rocks
can record the Earth's past (
palaeo
) magnetic field. Together, these facts enable us
to estimate dates and past positions of the plates from magnetic measurements.
3.1.2 The Earth's magnetic field
To specify the geomagnetic field at any point on the Earth's surface both a mag-
nitude and a direction are required: the geomagnetic field is a vector quantity.
It is far from being constant either in magnitude or in direction and varies spa-
tially over the surface of the Earth as well as in time. Systematic mapping of
the magnetic field began some five hundred years ago with the magnetic com-
pass measurements carried out by the early mariners. The internationally agreed
values of the geomagnetic field are updated and published every few years as
the
International Geomagnetic Reference Field
(IGRF). Figure 3.1(a) shows the
1
The geomagnetic pole at present situated in the northern hemisphere is in fact a south pole since it
attracts the north poles of magnets (compass needles)! See Fig. 3.2.
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