Pre‐history of the Earth's magnetic field

Abstract

SummaryIn order to determine the origin and nature of the Earth's magnetic field and to test the various hypotheses which have been advanced to explain the field, it is desirable to determine the history of this field throughout geologic time and to investigate more carefully its spatial variations, both inside and outside the Earth's surface. This research is concerned with the determination of the history of the Earth's field as it can be deduced from the present polarization of crustal material. Unconsolidated fresh‐ and salt‐water sediments have been investigated. These sediments are in the form of clays and offer one of the simplest types of polarization, since the clays can be redeposited under laboratory conditions. A particularly lengthy investigation of the polarization of glacial varves has been made, together with measurements on core samples of sediments from the Pacific. From a study of anomalous deposits in the glacial clays, the geologic stability of the polarization of these clays has been established over geologic time. From the measurements of the glacial clays, it is concluded that the Earth's field has not changed substantially in direction or intensity during the last 15,000 years. From measurements of the Pacific cores, it is tentatively concluded that the direction and intensity of the Earth's magnetic field has probably remained substantially constant during the last million years. A much more complete investigation is necessary to verify these tentative conclusions. It would be desirable to extend the measurements to periods of the order of one billion years. These results are consistent with the “fundamental” theory proposed by Schuster, Babcock, and Blackett, but do not provide positive evidence to support this theory.