Fig. 1.13. Locations of the North Geomagnetic Pole (dipole axis) over the past 10,000 years at 100-year intervals estimated from archeomagnetic measurements. Locations are given for each 2000-yr interval as well as for the entire 10,000 yr interval. After Ohno and Hamano (1992).
can be regarded as a recurring feature, or that the average over the preceding 10,000 yr would also coincide with the geographic pole.
Studies of the intensity of the geomagnetic field over archeological times can be undertaken with a much wider variety of materials, such as pottery fragments, because the orientation of the samples need not be known. The method used is that developed by Thellier (see review by Thellier and Thellier, 1959a,b). Under the GAD hypothesis, measurements of ancient geomagnetic intensity are a function only of latitude and the magnitude of the Earth's dipole moment (1.2.2). Thus, paleointensity measurements from all over the world can be normalized by calculating an equivalent dipole moment, referred to as the virtual dipole moment (VDM). This is the intensity analog of the VGP. For a more detailed discussion see Merrill et al. (1996).
To smooth rapid variations of the nondipole field at any one locality, the virtual dipole moments must be averaged not only from different parts of the world but also in class intervals of a few hundred years, as has been done in the analysis of archeomagnetic VGPs. McElhinny and Senanayake (1982) calculated the variation in the Earth's dipole moment over the past 10,000 yr by averaging over 500-yr intervals back to 4000 yr B.P. and over 1000-yr intervals prior to that. The results are shown in Fig. 1.14 together with 95% confidence bars.
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