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PALEOMAGNETIC PITFALLS

"Magnetism in rocks has provided a traditional tool for studies of the Earth's geomagnetic field. These studies have tended to rely on the assumption that the direction of magnetization was 'frozen in' during formation of the rock. But many sedimentary rocks formed during the Palaeozoic acquired their remanent magnetization through alteration processes that occurred after deposition of the sediment. The causes and geological significance of this phenomenon have been much debated."

The foregoing paragraph is enough to send shivers throughout the geological world. Does this undermine paleomagnetism and generalizations flowing from it, such as plate tectonics?

The "alteration processes" mentioned in the above quotation include: (1) The chemical conversion of pyrite into magnetite in ancient rocks after they were deposited; and (2) The reorientation of remanent magnetization following exposure to moderately high temperatures. That these processes can be important is evident in a second quotation:

"During the past eight years, however, evidence has accumulated that the remanent magnetization of many carbonate sediments was not acquired at the time of deposition, thereby invalidating some previous interpretations of the palaeomagnetic data. Instead, magnetization seems to have been acquired over a limited time span during the late Palaeozoic, from about 310 to 250 million years ago."

(Reynolds, Richard L.; "A Polished View of Remagnetization," Nature, 345: 570, 1990.)

Reference. Many other problems afflict paleomagnetism. See: EZP in our catalog: Inner Earth. More information on the book here.

From Science Frontiers #71, SEP-OCT 1990. 1990-2000 William R. Corliss

Science Frontiers Sourcebook Project Reviewed in:

Quotes

  • "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