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No. 41: Sep-Oct 1985

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Anatomy Of A Magnetic Field Reversal

"A highly detailed record of both the direction and intensity of the Earth's magnetic field as it reverses has been obtained from a Miocene volcanic sequence. The transitional field is low in intensity and is typically non-axisymmetric. Geomagnetic impulses corresponding to astonishingly high rates of change of the field sometimes occur, suggesting that liquid velocity within the Earth's core increases during geomagnetic reversals."

The time period required for the field to reverse was about 4500 years, as measured at Steens Mountain, Oregon. There were three periods of very rapid change (impulses), which hint at radical changes in the core. The average magnetic field at the earth's surface decreased to 20% of normal during the reversal.

(Prevot, Michel, et al; "How the Geomagnetic Field Vector Reverses Polarity," Nature, 316:230, 1985.)

Comment. The illustration reveals that the reversal was far from a clean 180 flip; there was much meandering. Just what was happening in the core during the reversal is a mystery. When the magnetic field dropped to low levels, flux of cosmic rays and other radiation at the earth's surface probably increased drastically. Terrestrial life might have been adversely affected.

Steene Mountains directional record The Steene Mountains directional record. The numbers refer to the samples used from the volcanic sequence, in order of increasing age. Dotted lines represent field directions in the opposite hemisphere.

From Science Frontiers #41, SEP-OCT 1985. 1985-2000 William R. Corliss

Science Frontiers Sourcebook Project Reviewed in:


  • "A sourcebook of unexplained phenomena is therefore a valuable addition to a collection of scientific literature. William R. Corliss has provided this in the past with his source books of scientific anomalies in several subjects, and now he has provided it for astronomy. He has done an excellent job of collecting and editing a large amount of material, taken in part from scientific journals and in part from scientific reporting in the popular or semi-scientific press." -- "The Mysterious Universe: A Handbook of Astronomical Anomalies", reviwed by Thomas Gold, Cornell University, in Icarus, vol.41, 1980

  • "An interesting, systematic presentation of unusual weather [..] This book is recommended for a general audience" --"Corliss, William R., Tornados, Dark Days, Anomalous Precipitation, and Related Weather Phenomena, Sourcebook Project, 1983.", revieweed in Choice, September 1983
  • "..the science is necessarily somewhat speculative, but Corliss's symthesis is based on reputable sources." -- "Corliss, William R. (Compiler). Lightning, Auroras, Nocturnal Lights, and Related Luminous Phenomena" reviwed by Joseph M. Moran, Univ. of Wisconsin in Science Books and Films, Sep/Oct 1983

  • "Before opening the book, I set certain standards that a volume which treads into dangerous grounds grounds like this must meet. The author scrupulously met, or even exceeded those standards. Each phenomenon is exhaustively documented, with references to scientific journals [..] and extensive quotations" -- "Book Review: The moon and planets: a catalog of astronomical anomalies", The Sourcebook Project, 1985., Corliss, W. R., Journal of the Royal Astronomical Society of Canada, Vol. 81, no. 1 (1987), p. 24., 02/1987