Reverse Magnetization of Rocks and Its Connection with the Geomagnetic Field

From the present knowledge on the magnetism of rocks, palaeomagnetic study by means of the natural remanent magnetization of igneous rocks is reliable when and only when the natural remanent magnetization is ascertained to be the thermo-remanent magnetization acquired at the time of the rocks' formation. The authors propose, in this paper, several experimental methods for examining whether a given natural remanent magnetization can have definite palaeomagnetic significance or not, under the above mentioned criterion.In short, the proposed set of testing methods comprises: thermo-magnetic, crystallographic and chemical analyses of the ferromagnetic minerals contained in the rock, and the comparison of the stabilities of the natural remanent magnetization and of the thermo-remanent magnetization, produced in laboratory, against the demagnetizing processes by heating in non-magnetic space and by applying an alternating magnetic field.These proposed tests were actually carried out on several naturally adversely magnetized rocks found mainly in Japan. For three of them, including Dr. Hospers' Icelandic rock, it was concluded that their reverse natural remanence must have been brought about by the reversed earth's magnetic field at the times of their formation.As to the Haruna type of the self-reversal of the thermoremanent magnetization, the two-constituent mechanism was investigated in detail with the aid of electron-microscopic observation on the intergrowth of the two constituents.