MadSci Network: Earth Sciences

Re: Some rocks have alternating polarity layers. Did the earth flip over?

Date: Mon May 7 11:57:57 2001
Posted By: David Smith, Faculty Geology, Environmental Science
Area of science: Earth Sciences
ID: 988685501.Es

No, the earth did not flip over, only the polarity of its magnetic field 
changed.  The earth's magnetic field is internally generated, by 
circulation patterns in the liquid iron outer core.  The earth's magnetic 
field behaves as if the earth were a big bar magnet, but unlike bar 
magnets, the earth is not permanently magnetized.  The vast majority of the 
earth, including all of the metallic iron parts, are much too hot to be 
permanently magnetized (Metal magnets loose their permanent magnetism above 
a temperature called the Curie point - for Fe-oxides this is about 500-600 
degrees C).  

The polarity reversals on earth have something to do with changes in the 
core circulation, but to my knowledge, the details of the mechanisms are 
still unknown and are the subject of continuing research.  For example, see 
the recent paper:
Narteau, C. ; Blanter, E. ; Le Mouel, J. L. ; Shirnman, M. ; Allegre, C. J. 
, 2000, Reversal sequence in a multiple scale dynamo mechanism. Physics of 
the Earth and Planetary Interiors, v. 120, no. 4 p. 271-287.

Someone at the University of Richmond might be able to help you find this 
paper and others on the same topic.  This paper is written for 
professionals in the field to read and you may have a very hard time 
following the details, but you might be able to find useful information in 
the introduction and conclusion sections.  In this paper, the authors use a 
dynamo model (electro-magnetic fields generated by circulating fluids) to 
successfully reproduce  many of the features of earth's field, including 
reversals, which appear to be generated by feedbacks between the field and 
the motions of the fluids generating the field. For a scientifically 
rigorous explanation that is a little more accessible, see this web site:

What happens when the field reverses is well known from detailed studies of 
rock sequences at the times of reversals.  The strength of the magnetic 
field decreases  to zero and then increases again with the opposite 
polarity (compasses would point in the opposite direction).  Other than the 
malfunctioning of compasses, everything else would be perfectly normal and 
the effects on life would be negligible.

Dave Smith
La Salle University, Philadelphia, PA

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