The Intensity of the Ancient Geomagnetic Field: A Review and Analysis

Summary In this review all palaeomagnetic determinations of the intensity of the ancient geomagnetic field known to the author at 1966 June are brought together and analysed. The lists of data cover determinations from both historic (archaeomagnetic) and geological specimens. The problems encountered in the determination of ancient field intensities from rocks, especially that of rock alteration during the formation of artificial reman‐ence and that of the presence of secondary components of magnetization, are discussed, and methods used to overcome them are described. Experimental techniques used hitherto are briefly summarized. Methods of treating data are reviewed; and suggestions for comparing data from different latitudes are made. The main conclusions drawn from an analysis of both the field intensity data and the problems involved in obtaining this data are: (i) that the problem of rock alteration during the heating necessary to induce an artificial TRM may be overcome by heating selected naturally highly‐oxidized specimens in air; (ii) that the effects of secondary components of magnetization may be avoided by utilizing that part of the primary moment which remains after the removal of secondary moments by thermal or a. c. demagnetization; (iii) that the geomagnetic dipole is not constant within any given polarity but fluctuates in strength, possibly with a period of the order of 10 4 years; (iv) that because of these fluctuations and non‐dipole field variations no conclusions regarding the mean strength of the geomagnetic dipole at any given time may be made if only a few rock samples are measured, and hence that future workers should use sufficient samples to average out these effects; (v) that the limited data from transition zones suggest that during a field reversal the dipole moment reduces to zero but the non‐dipole field remains.