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From: erkyrath@netcom.com (Andrew Plotkin)
Subject: Re: Magnetic field reversals
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Date: Wed, 18 Dec 1996 05:24:15 GMT
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Judson Lester (lester@whiskey.ee.tulane.edu) wrote:
> >The reversals can actually be timed quite accurately, by measuring the
> >thickness of the sediment layers with different magnetic polarity, by
> >measuring isotope distributions, etc. I don't remember any exact
> >figures, but I think the flip takes on the order of several thousands
> >or tens of thousands of years.

> This isn't what the orginal poster wanted to know.  (At least, it wasn't 
> what I wanted to know and my guess is that he had the same question in 
> mind.)  Yes, several thousand years pass _between_ flips, but how long 
> does a flip take? 

No, he was answering the question. :-) That's how long a flip takes.

Ok, I have here "American Scientist" magazine, Nov-Dec 1996, page 552:
"The Reversal of the Earth's Magnetic Field". 

Well, I'll just summarize:

The last field reversal was 780,000 years ago. The present rate is between
one or two reversals per million years, but the rate has varied.  (For
example, there was a period of no reversals which started 120 megayears
ago and ended 80 megayears ago.)

(So, yes, by the numbers, we could have one any tenth-of-a-megayear now.)

The reversal itself takes a few thousand years.

During a reversal, the field strength of the Earth decreases by about a
factor of ten, and the field direction fluctuates anomalously (at any 
given measuring site.) It's not a simple case of a dipole field rotating 
180 degrees. The whole thing gets messy, and sort of bubbly -- there's a 
neat (entirely theoretical, computer simulated) picture of a sphere 
halfway through (somebody's model of a) reversal, and it's got big 
patches and bands of "north pole" and "south pole" wandering around the 
planet.

(This is all background material in the article -- the article itself is
about some possible statistical results coming from our many samples of
the "messy" in-reversal field. I won't try to explain that. The real short
form is "It kinda looks like something which is directly influencing the
reversal is affected by fluid motion at the top of the Earth's core, or
possibly the other way around, and also by seismic wave velocity in the
lower mantle (which we think is affected by temperature), or possibly the
other way around, and the current field direction at the core surface ties
in there too. But this could all be a statistical artifact. We'll get back
to you.")

So, yes, a reversal could happen "soon", but it would probably take a
human lifetime to notice one starting up. Don't expect compasses to just
stop working some Tuesday afternoon. 

And, to answer the *original* original question, yes, the magnetic field
is caused by the rotation of the Earth. When the planet is not in the
process of a reversal, the magnetic poles are always near the rotational
poles. 

--Z


-- 

"And Aholibamah bare Jeush, and Jaalam, and Korah: these were the
borogoves..."
