What is the evidence for geomagnetic reversal ?

[rogerharris]

Just to clarify (I hadn't finished my morning coffee when I wrote this).

I say that hcp Fe loses magnetic ordering as it due to the structural change "in addition to" it's low Tc. This is should really be that as Fe transforms to hcp Fe loses magnetic ordering due to the fact that hcp Fe has a lower Tc (these experiments were at room temperature).

im a believer ! not seeing how there could be a permanent magnet anyway.

here comes the columbo bit tho

whats the core mechanism for reversal ? if these inner cores are massive upward pointing ultra compressed paramagnetic crystals, and the dipole field is created by the liquid sloshing around it, then the crystals inner core still has the dominant paramagnetic order by comparison to the outer core.

how do these highly pressurized crystals get flipped around by outer core fluids which are going to be kind of random, inconsistent and paramagnetically weaker ?

 

[geo101]

ere comes the columbo bit tho

Do that mean I am the killer?

Now we are getting into the territory of another thread...
https://www.physicsforums.com/showthread.php?t=518080"

For the core you can completely ignore any ferro/ferri/antiferro/para/dia magnetic effects. The geomagnetic field is generated by the geodynamo. Because I am lazy, I'll copy paste from the other thread

The geomagnetic field is generated by the fluid motion of the electrically conductive fluid outer core around the solid inner core. The fluid motion of the outer core is driven by both thermal and compositional convection (as Fe freezes out of the liquid lighter elements remain creating compositional buoyancy) and is dominated by large scale flow. The Earth’s rotation also plays a big role; it produces convection columns within the outer core that align along the rotation axis. So to answer your question, thermal and compositional convection along with the Earth’s rotation control fluid flow in the outer core... The flow of fluid is approximately axial-symmetric. The thermal and compositional convection is radial and the Earth’s rotation adds a helical twist to the fluid motion. [So the geomagnetic field aligns with the geographic poles, and not, for example the equator]

I guess what you are really asking is why does the geomagnetic field point north or south? First some background. As I mentioned the outer core is conductive and, in the presence of a magnetic field, electric currents will be produce inducing new magnetic. This is the basic premise of the self-sustaining dynamo, which is a big feedback system of convection to electric currents to magnetic fields, which then modify convection currents (through magnetohydrodynamics; http://en.wikipedia.org/wiki/Magnetohydrodynamics" ), and so on.

Now, imagine you have a convecting Earth-like core system in the absence of any magnetic field, i.e., convection without magnetic induction. Then a seed magnetic field was instantaneous “switched” on; the final stable configuration would depend on the interplay of the strength and direction of the seed field, the configuration of convection before the seed field (and how it changed through time, i.e., magnetohydrodynamics), magnetic diffusion through the solid inner core and its relative scale to that in the outer core (the inner core can act as a breaking system to changes in the magnetic field induced by convection in the outer core), controls on the heat flux through the core mantle boundary, and a host of other details. So those are (some of) the factors that control the orientation of the magnetic field.

So what causes the field to reverse? The short answer is that we don't really know, but basically major changes in the convective regime are likely to cause reversals of the geomagnetic field. The heat flux across the core-mantle boundary, which the main driving force behind the thermal convection, will play a key role in this.

 

[billiards]

So what causes the field to reverse? The short answer is that we don't really know, but basically major changes in the convective regime are likely to cause reversals of the geomagnetic field. The heat flux across the core-mantle boundary, which the main driving force behind the thermal convection, will play a key role in this.

How important is lateral heterogeneity in the mantle above the core mantle boundary to the geomagnetic field?

To me it seems "obvious" that the temperature gradients in the rocks in the lowermost mantle will control the flow of heat by conduction out of the core. One can envisage more heat flowing towards colder regions of the lowermost mantle (which are colder in the first place because they are the sites of downwelling in mantle convection). How important is this effect in controlling outer core convection -- given that this is to first order controlled by the Earth's rotation axis -- would the thermal gradients exert second order eddies? How important are eddy currents to the geomagnetic field?

Also hot and cold regions in the lowermost mantle seem to be quite permanent features, which to me suggests that heat flux across the core mantle boundary should not vary much in a short span of time. Unless a very slight perturbation in the heat flux can cause the field to reverse I would be surprised that this influence alone could be responsible for the flipping.

I seem to remember Glatzmeier modelling reversal as happening spontaneously. The geodynamo is inherently chaotic, I guess the flipping is an emergent property of the system, there is not a simple north/south switch that is being flicked. It is something less tangible to us.

 

[Dotini]

billiards, do you accept that sea-floor striping and the other evidences/anomalies constitute proof that the whole Earth magnetic field has flipped in the past?

Respectfully submitted,
Steve

 

[billiards]

billiards, do you accept that sea-floor striping and the other evidences/anomalies constitute proof that the whole Earth magnetic field has flipped in the past?

Respectfully submitted,
Steve

Yes I do.

Why do you ask?

 

[rogerharris]

Do that mean I am the killer?

Now we are getting into the territory of another thread...
https://www.physicsforums.com/showthread.php?t=518080"

For the core you can completely ignore any ferro/ferri/antiferro/para/dia magnetic effects. The geomagnetic field is generated by the geodynamo. Because I am lazy, I'll copy paste from the other thread



So what causes the field to reverse? The short answer is that we don't really know, but basically major changes in the convective regime are likely to cause reversals of the geomagnetic field. The heat flux across the core-mantle boundary, which the main driving force behind the thermal convection, will play a key role in this.

thanks geo. good clear explanation again. My inner Colombo feels at rest (for a while). I am guessing he will start coming out to play when I get into magnetohydrodynamics properly. I know from biophysics there is a lot of reaction diffusion going on in MHD.

Ok I am not getting into that today

 

[rogerharris]

Yes I do.

Why do you ask?

well actually I am interested in this as well. I mean i speed read and downloaded a lot of the papers and tomes i could get which are inked here, and was still trying to find that one conclusive ocean striping study or meta analysis.

I expected it would be something like this.

Geologists dig down and take a dozen deep crust samples from south to north pole with the sample rod marked with its compass points. These are all time synchronized and labeled, so a clear picture emerges with no doubt that the field has flipped 180 degrees across the polar axis on all the samples.

I still can't find this. Any pointers as to the seminal work which makes it undoubtedly clear ?

 

[Dotini]

Yes I do.

Why do you ask?

Well, I was trying to avoid the necessity of explaining global geomagnetic reversal, hoping to associate sea-floor striping with local tectonic anomalies, similar to Etruscan vases and Israel copper slag. But you are definitive, and I appreciate that very much.

Respectfully,
Steve

 

[Evo]

well actually I am interested in this as well. I mean i speed read and downloaded a lot of the papers and tomes i could get which are inked here, and was still trying to find that one conclusive ocean striping study or meta analysis.

I expected it would be something like this.

Geologists dig down and take a dozen deep crust samples from south to north pole with the sample rod marked with its compass points. These are all time synchronized and labeled, so a clear picture emerges with no doubt that the field has flipped 180 degrees across the polar axis on all the samples.

I still can't find this. Any pointers as to the seminal work which makes it undoubtedly clear ?

It's an established science, I'm not sure exactly what you're looking for.

Anyway for those with an interest

http://en.wikipedia.org/wiki/Paleomagnetism

http://en.wikipedia.org/wiki/Magnetostratigraphy

And a discussion of where paleomagnetism is used for dating.

http://archserve.id.ucsb.edu/courses/anth/fagan/anth3/Courseware/Chronology/11_Paleomag_Archaeomag.html

 

[geo101]

Any pointers as to the seminal work which makes it undoubtedly clear ?

Much of the seminal work is old, some going back over 100 hundred years. I would recommend reading through the book chapters that I suggested in post #23. First, they will give you a good overview of the weight of evidence. Second they will have most of the references that you are looking for. Essentially you are looking for papers like Matuyama (1929), who developed the first (albeit crude) global polarity timescale (GPTS, wiki/google search this). Or others like http://gsabulletin.gsapubs.org/content/96/11/1419.abstract"


Matuyama, M. (1929), On the direction of magnetisation of basalts in Japan, Tyosen and Manchuria, Proc. Imp. Acad. Jap., 5, 203-205.

 

[Evo]

Much of the seminal work is old, some going back over 100 hundred years. I would recommend reading through the book chapters that I suggested in post #23. First, they will give you a good overview of the weight of evidence. Second they will have most of the references that you are looking for. Essentially you are looking for papers like Matuyama (1929), who developed the first (albeit crude) global polarity timescale (GPTS, wiki/google search this). Or others like http://gsabulletin.gsapubs.org/content/96/11/1419.abstract"


Matuyama, M. (1929), On the direction of magnetisation of basalts in Japan, Tyosen and Manchuria, Proc. Imp. Acad. Jap., 5, 203-205.

BTW, welcome to the forum geo101!

 

[geo101]

BTW, welcome to the forum geo101!

Cheers

 

[rogerharris]

Much of the seminal work is old, some going back over 100 hundred years. I would recommend reading through the book chapters that I suggested in post #23. First, they will give you a good overview of the weight of evidence. Second they will have most of the references that you are looking for. Essentially you are looking for papers like Matuyama (1929), who developed the first (albeit crude) global polarity timescale (GPTS, wiki/google search this). Or others like http://gsabulletin.gsapubs.org/content/96/11/1419.abstract"


Matuyama, M. (1929), On the direction of magnetisation of basalts in Japan, Tyosen and Manchuria, Proc. Imp. Acad. Jap., 5, 203-205.

Ok thank. ill try and dig that up. The uni didnt have that book and google omits chapter 4. I am wary of forking out my grant if its not going to provide a specifically overwhelming case. I reckon i might know the seminal paper actually. if its overwhelmingly good i won't be back !

 

[rogerharris]

It's an established science, I'm not sure exactly what you're looking for.

At last one paper where they take a load of samples across the entire earth, say what, 60 samples would be good i suppose to eliminate the errors but 20 would stll be something.

They then aggregate the samples and overwhelmingly find at least one reversal happening across the planet from north to south pole at the same time.

I haven't found such a paper yet, but i think it does exist and know somebody who can tell me where to find it :)

I have been vey surprised that others do not demand to see at least this baseline of evidence to consider reversal a credible theory.

 

[geo101]

http://www.nature.com/nature/journal/v435/n7043/full/nature03674.html"

I'll write a better description in the morning. This is as "overwhelming" as you will get. What you have to remember is that establishment of geomagnetic reversals did not happen with one paper, but with hundreds of global records over a long period of time. The discovery of seafloor spreading and associated magnetic anomalies was the final nail in the coffin as it were.

 

[rogerharris]

http://www.nature.com/nature/journal/v435/n7043/full/nature03674.html"

I'll write a better description in the morning. This is as "overwhelming" as you will get. What you have to remember is that establishment of geomagnetic reversals did not happen with one paper, but with hundreds of global records over a long period of time. The discovery of seafloor spreading and associated magnetic anomalies was the final nail in the coffin as it were.

thanks ill see if i can get that. Nature articles are hard to get even at british universities.

 

[billiards]

I've been away the past few days.

Geo101 seems to have the references more or less covered.

I would like to add this paper which charts a whole reversal over a time period constrained to be 4,500 to 11,200 years.


http://www.agu.org/pubs/crossref/1984/JB089iB02p01049.shtml"

 

[billiards]

Well, I was trying to avoid the necessity of explaining global geomagnetic reversal, hoping to associate sea-floor striping with local tectonic anomalies, similar to Etruscan vases and Israel copper slag. But you are definitive, and I appreciate that very much.

Respectfully,
Steve

So because I believe the evidence you stop thinking?

I might be a "crank" or a "fool" or perhaps neither but still simply "wrong". Follow your own mind, convince yourself.

Now you have got me interested in the Etruscan vases and the Israel copper slag. I do not know anything about those.

 

[Evo]

Roger, this is a brief history with tons of references.

This article is a condensed history of the early evidence
of geomagnetic field reversals, showing some of the
achievements up to the end of the 1950s that led the way
for the acceptance of field reversals during the 1960s. It is
based upon a number of the original texts and the following
detailed sources: Bullard [1]; Glen [2]; Kristjánsson
[3, 4]; Didier & Roche [5]; Laj et al. [6]; Courtillot &
Le Mouël [7]; Kono [8]; Irving [9]. I would also like to
draw attention to Our Magnetic Earth by Merrill [10].

http://www.irm.umn.edu/quarterly/irmq20-3.pdf

 

[geo101]

I'll write a better description in the morning

Errr... is it morning??
Better late than never.

So one thing that we didn't talk about so far is how the strength of the field changes as the field reverses. All of the available data tell us that the main dipole field drops to about 20% of it's pre-reversal strength. Again this is seen at various times and across the globe.

The Valet paper stacked together relative paleointensity (geomagnetic field strength) records over the past 2 million years from 10 globally distributed deep sea sediment core, essentially confirming that the lows associated with reversal are a global feature.


Evo - good catch on the IRM Quarterly, I miss that one