From: cain@geomag.gly.fsu.edu (Joe Cain)
Subject: Re: Question about Earth's magnetic poles
Organization: Florida State University Geology Dept.
Date: Sat, 9 Jan 93 18:53:52 GMT

In article <145527@lll-winken.LLNL.GOV> hudek@polaris.llnl.gov (David Hudek) writes:

>I was reading a book the other day in which it was claimed there was
>(unspecified) geological evidence that the Earth's magnetic poles had
>shifted rather dramatically in the past. It was rather vague
>and offered no details/references so I tend to disbelieve it, but thought
>of what a wonderful resource the "net" is... can anyone confirm/deny
>the assertion? 

	The excellent responses already given should dispell your
concern as to the reality of past "pole shifts." To summarize some:

1. The original "polar wander" turned out to be mainly due to the
continents drifting around on plates. These curves of millions of
years are plotted assuming that all observations of the paleofield
result from a dipole, whose pole is located by the direction of the
field frozen into the rock at the time it cooled. This "virtual"
dipole can be different from what would be that which would be
determined for that epoch given enough data as it ignores the
non-dipole main field terms and crustal field which can sometimes be
overpowering. 

2. Reversals, or flips of the field have formed the basis of
magnetostratigraphy, which helps stratigraphers organize data given
some understanding of the changes in the rock's attitude since
cooling.

3. The geomagnetic field reverses on a tens of thousands year to
millions year time scale, or sometimes goes to a low level and recovers
to its original state without having changed direction. During the
times of low field, the main or dipole component, which is some 90% of
that observed at the surface today, may indeed zero, though recent
evidence indicates that the virtual dipole seems to have preferred
paths in going from one hemisphere to another. These analyses are
complicated by the fact that there is some tendency over short periods
of time for the non-dipole components to increase as the dipole
decreases. Such an increase has been shown to take place over the past
decades for example (Paper in the 1960's I think by Cox and Verosub)
and is continuing. 

4. The pole of a dipole is seldom where the actual field is vertical
due to the higher degree terms. This "dip" pole is often elsewhere by
thousands of kilometers. During a reversal there are conceivably more
than one pair of poles, especially if the quadrapole or higher terms
dominate. During such an interval the field is indeed lowered, but
likely never goes to zero even though it has lost its simple dipole
character. 

5. The "secular' change of the field has a time scale of like 10,000
years and is mostly east-west. These changes are those that mainly
result in the need to keep up with the field surveys for practical and
scientific reasons. Unfortunately, with its fascination for the Space
Station NASA has neglected its duty to monitor these changes by
accurate instruments since 1980 so that global maps are reverting to
the pre-satellite state. (The Navy is attempting to plug the gap with
"cheapsat," now making very approximate observations) 

Besides the Merrill and McIhinney book already recommended, there is a
new multivolume "Geomagnetism" edited by Jack Jacobs. M&M are getting
material together for a new edition.

Joseph Cain		cain@geomag.gly.fsu.edu   
cain@fsu.bitnet		scri::cain
(904) 644-4014		FAX (904) 644-4214 or -0098
Newsgroups: sci.geo.geology