An Investigation of Changes in the Magnetic Field of the Earth (Magnetic Anisothropy of Sedimentary Rocks)

Citation

Bhattacharya, Prabhat Kumar (1950) An Investigation of Changes in the Magnetic Field of the Earth (Magnetic Anisothropy of Sedimentary Rocks). Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/10jq-dy86. https://resolver.caltech.edu/CaltechTHESIS:12042020-181406559

Abstract

The paper gives the results of measurements of the magnetic anisotropy of 47 samples of sedimentary rocks ranging in age from Pliocene to Silurian time. The study was undertaken to test the validity of the hypothesis that fine grained sediments containing a large number of ferromagnetic particles should exhibit a magnetic anisotropy, the direction of the maximum susceptibility being in the direction of the Earth’s magnetic field existing at the time of deposition. The theory of the development of remnant moment and anisotropy in sedimentary formations has been discussed and it is claimed that the anisotropy measurements should give more reliable results than the remnant moment measurements regarding the magnetic history of the Earth. A highly sensitive Susceptibility Meter has been designed and constructed for the purpose, and the theory of measurements of the initial magnetic anisotropy in cubical rock samples has been developed. Preliminary measurements on 24 samples of Tertiary sediments show that for the past 60 million years or so, the average declination of the Earth’s magnetic fields has been within about 20° of the geographic north. Similar measurements on 23 samples of Silurian sediments indicate a significant difference between the Silurian and the present geomagnetic field. The effect of deformation of beds and of strong magnetic fields on anisotropy have also been studied and it is concluded that the direction of maximum susceptibility in sedimentary rocks is stable and is invariable with respect to any stray magnetic fields subsequent to deposition.

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