Page 79 - Global Tectonics

66 CHAPTER 3

Figure 3.12 Development of detrital remanent magnetization.

tometer. The latter instrument is extremely sensitive The geomagnetic field undergoes progressive
and capable of measuring NRM orientations of rocks changes with time, resulting from variations in the con-
with a very low concentration of ferromagnetic vective circulation pattern in the core, known as secular
minerals. variation. One manifestation of this phenomenon is that

the direction of the magnetic field at a particular geo-
graphic location rotates irregularly about the direction
3.6.4 The past and present implied by an axial dipole model with a periodicity of
a few thousand years. In a paleomagnetic study the

geomagnetic field effects of secular variation can be removed by collecting
samples from a site which span a stratigraphic interval
The magnetic field of the Earth approximates the fi eld of many thousands of years. Averaging the data from

that would be expected from a large bar magnet embed- these specimens should then remove secular variation
ded within it inclined at an angle of about 11° to the so that for the purposes of paleomagnetic analysis the

spin axis. The actual cause of the geomagnetic fi eld geomagnetic field in the past may be considered to
is certainly not by such a magnetostatic process, as originate from a dipole aligned along the Earth’s axis of
the magnet would have to possess an unrealistically rotation.
large magnetization and would lie in a region where the Paleomagnetic measurements provide the intensity,
temperatures would be greatly in excess of the Curie azimuth and inclination of the primary remanent

temperature. magnetization, which reflect the geomagnetic para-

The geomagnetic field is believed to originate from meters at the time and place at which the rock was
a dynamic process, involving the convective circulation formed. By assuming the axial geocentric dipole model

of electrical charge in the fluid outer core, known as for the geomagnetic field, discussed above, the inclina-

magnetohydrodynamics (Section 4.1.3). However, it is tion I can be used to determine the paleolatitude φ at
convenient to retain the dipole model as simple calcula- which the rock formed according to the relation-
tions can then be made to predict the geomagnetic fi eld ship 2 tan φ = tan I. With a knowledge of the paleo-
at any point on the Earth. latitude and the azimuth of the primary remanent