Abstract :
The Earth’s main magnetic field can be approximated by an axial, geocentric dipole. The
remaining non-dipole field is much smaller and is a regional rather than a global feature – quite
large changes can occur in a few ka. This review is concerned with changes in the dipole component
of the geomagnetic field, and one of the problems is in separating the non-dipole from the dipole
contributions to the field. Unlike the many determinations of the direction of the Earth’s magnetic
field in the past (which have led to fundamental contributions to our understanding of plate tectonics
and shown that the field can on occasion reverse its polarity), estimates of the intensity of the field
are comparatively few, especially before the Holocene. This is mainly the result of experimental
difficulties in obtaining reliable measurements of the field. These problems are discussed in some
detail and are followed by a short account of archaeomagnetic intensities and results from Hawaii
where many of the first determinations were obtained. Measurements for the last 100 ka from both
lavas and lacustrine and oceanic sediments are reviewed and results from different areas compared.
An asymmetric saw-tooth pattern has been observed in some of the records over the last few Ma,
and this rather controversial question is discussed. Finally an account is given of the far more limited
data on palaeointensities in earlier times.
A short discussion is given of the interpretation of coherent short wavelength variations which are
observed in many marine magnetic profiles. Although short reversals of the field may be responsible
for some of these “tiny wiggles”, it is more likely that in general they are the result of changes in the
strength of the Earth’s magnetic field.
