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Monday, February 26, 2007

Rapid reversals of the magnetic field



A few years ago, I began to look at the claims of young earth creationists for rapid reversals of the earth's magnetic field. Here is the introduction from that site:

One of the more intriguing arguments put forth by young earth creationists for the age of the earth has is related to the strength of the magnetic field. The notion that the earth could not be older than ~10,000 years on the basis of magnetic field strength was forwarded by Thomas Barnes (1971, 1973). Barnes, using the known decay of the dipole moment over this century, argued that the magnetic field has decayed via free decay since creation no more than 10,000 years ago. Barnes' analysis relied, at least in part, on fitting an exponential curve to the observed decay whereas a linear decay model would give a maximum age on the order of 100 million years (Brush, 1983). Barnes rejected the idea that the earth's magnetic field had reversed polarity. Reversals were originally suggested in the early 1900's (David, 1904; Bruhnes, 1906) although it was unclear if this was a real feature of the magnetic field or a rock magnetic artifact. Subsequent work has demonstrated that reversals of the main field have taken place (Jacobs, 1994).

A magnetic reversal, in simple terms, occurs when the force between the two poles reverses direction. In a reversed magnetic field (by definition, today's field is 'normal') a compass will point to the south geomagnetic pole and the inclination (dip of the magnetic needle) will also invert itself at all locations on the globe with the exception of the magnetic equator (Figure 1). However, even this is a simplistic definition of a reversal. A more precise definition requires that the geocentric axial dipole term changes sign on a global basis, the change in sign must have some stability (typically unchanged for a few thousand years), Thus, strictly speaking there can be at most only a few reversals if the time scale favored by young earth creationists is correct. Shorter term variations in directions are described as 'excursions' or 'events' (Jacobs, 1994).

As noted, Barnes (1973) did not accept that magnetic field reversals have taken place in the past. One of the reasons for rejecting field reversals was that reversals provided some evidence that the main field could be generated by a dynamo process. It should be noted that a dynamo does not require a reversal as there are reversing and non-reversing dynamos. However, if reversals do occur, then the decay of the dipole field cannot, in and of itself, be used to measure the age of the earth. A few decades later, it became apparent that reversals of the magnetic field have taken place and creationists had to move quickly to accomodate these new observations. Humphreys (1988,1993), a self-professed 'fan' of Barnes took up the gauntlet of trying to rescue the young magnetic field argument once reversals became undeniable. Whenever rates are discussed amongst ye-creationists, there is a nearly universal decree that says "the rates implied by conventional geology are wrong" and the real rates are faster. Humphreys quickly adopted the 'rapid reversal' explanation, but made several crucial mistakes along the way.

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I am in the midst of planning a symposium later this year and one of the invitees is Profesor Rob Coe (UC Santa Cruz). Rob is one of the world's best paleomagnetists who also has an interest in the history of field intensity and reversal. In 1989, Coe and Prevot published an article entitled "Evidence suggesting extremely rapid field variation during a magnetic reversal" which Humphreys seized on as evidence for rapid reversal. However, it is important to actually look at what Coe and Prevot (1989) were discussing. First, Coe and Prevot (1989) made no argument that the reversal process itself was rapid, only that the reversal period included evidence of rapid fluctuation in field intensity and direction. A subsequent publication in 1995 by Coe and co-authors was titled "New evidence for extremely rapid change of the geomagnetic field during a reversal". Humphreys used these papers to make the following claim:

"In 1988, startling new evidence was found for the most essential prediction of my theory--very rapid reversals". and "I cited newly discovered evidence for rapid reversals (Coe and Prevot, 1989), evidence in thin lava flows confirming my 1986 prediction. Since then, even more such evidence has become known (Coe, Prevot, and Camps, 1995)"


This is a false representation of Coe et al.'s work and I describe the error in some detail on the weblink posted at the beginning of this article. When Rob replied to me with the title of his talk for the upcoming meeting, I asked him if he knew that creationists were misusing his work. He replied:

"Yes, that is unfortunate because the rapid-field-change hypothesis offers no basis for any of their stuff. Insofar as there is any evidence for the duration of the Steens polarity transition, from estimates of rates of SV, it is several thousand years. The question we examine is whether the systematic streaking of directions of samples as a function of height in a single flow could have been caused by an EPISODE of rapid movement of field direction while that flow was cooling through its blocking temperatures. We state as much in everything we have written, but they ignore that."


Indeed, ignoring data or inventing data seems to be part and parcel of the creationism/id movement.

Cheers

Joe Meert

4 Comments:

At 5:09 AM, Anonymous SteveF said...

I'm not entirely sure, but I think I can claim credit for informing Rob about creationist misuse of his work. I was doing the third year of my undergraduate degree at UCSC, on exchange from the UK. I'd just started reading about creationism at much the same time as I took Rob's class. I let him know about this and seem to recall that he wasn't overly impressed. Rob's a very cool guy.

 
At 6:33 AM, Anonymous Anonymous said...

Yes indeed. It will be interesting to see what new work he has done on the section.

Cheers

Joe Meert

 
At 10:42 AM, Anonymous Anonymous said...

SteveF,

Maybe you could get Humphries along to present his latest research.

On the subject of Humphries, I think I can fairly confidently predict his response to your article. It will go something like this:

"Coe and Prevot provide evidence that rapid activity within the magnetic field is possible. The other evidence for reversals taking longer is based on the acceptance of an old earth chronology which I reject."

This approach would be fairly typical of the have your cake and eat it methodology of the creationists. Take the timescales from conventional geology that vaguely support them, whilst ignoring the timescales from conventional geology that do not.

 
At 11:26 AM, Blogger Joe Meert said...

He can state that, but he is still faced with a number of conundrums. (1) The volcanic rocks in question were erupted on land (not during a flood). If he says they are 'pre-flood', then all flood rocks must be younger than 15.5 Ma (ie Cenozoic). That's kind of troubling, isn't it--even for young earth creationists.
(2) The archeomagnetic data he uses show no reversals.
(3) Coe et al. show a rapid pulse of motion of the pole not a full reversal. He needs rapid full reversals.


Cheers

Joe Meert

 

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