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No. 71: Sep-Oct 1990

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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