Geomagnetism and the rate of Sea-floor Spreading

Hello there.This is a first post for me, although I've been having fun lurking for the last few months.I thought I'd just point out that the generation of the Earth's magnetic field has nothing to do with *mantle* convection, rather it is convection of molten iron in the outer core. This being a liquid, viscosity effects are much reduced, and more importantly, the process is much more dynamic. This is the reason why we see variation of the magnetic field, for example secular variation (the movement of magnetic north pole relative to the geographic pole) over our lifetimes.In contrast, in the mantle, convection occurs over *much* longer timescales (millions of years). This is due to high viscosity, but that in itself is no real barrier to convection, which only requires some sort of temperature variation within the system. It just means it occurs very slowly (from our perspective at least)If someone was proposing the magnetic field resulted from mantle convection, there would be a problem. But we don't, so there isn't.

No, in fact I struggle to see how you could model a fluid *without* considering viscosity. The scientist he quoted is not saying (I think) that viscosity is not considered, just that other factors (for example, the effect of the rotating earth) are more important controls on the behaviour of the geodynamo.I was giving Rod the benefit of the doubt - he seemed to have some idea that the magnetic field was something to do with the mantle. If that were the case, then his objections would have some validity, in that the behaviour of the field is inconsistent with what we know about how the mantle flows.

Well, as I've got involved I'll try to tackle this one too... Convection is one of the ways a system can even out temperature differences. Another way is by conductive heat flow. To put it simply, convection will occur if that is a faster method of heat transfer in that system than conduction.The relative amounts of heat exchange occurring by these processes depend on things like the magnitude of the temperature difference, the dimensions of the system, viscosity (see, it is in there) and thermal conductivity. These combine into a a parameter called the Rayleigh number, which is essentially a measure of the vigor of convection.Heres some references which may explain it better:http://engr.smu.edu/~arunn/html/convect/rbconvect/rbcon.html
Rayleigh Number -- from Eric Weisstein's World of Physics(sorry quick search, I could probably do better if you're really interested)Convection occurs when the Rayleigh number is above about 1000; the Rayleigh number of the mantle is thought to be around 1e6 (it depends on whether we have whole mantle or layered convection, so no-one knows for sure yet) either way it can be seen that convection is not really a problem: yes the viscosity is high but the high temperature differences inolved and the relatively low thermal conductivity of the mantle rocks mean the Rayliegh number is still very high.The 'pudding in a pan' example fails as a model of the mantle primarily because the Rayleigh number is wrong - mainly because you're scaling down the size of the system so much. A system with a more realistic Rayleigh number is actually a pan full of Golden syrup, which convects just fine. Don't believe me? A quick google reveals:http://www.geo.brown.edu/...ple/grads/weeraratne/convect.htmOk, if you look in figure one it's corn syrup, but the principle is the same.Sorry if this is a bit off topic, and thanks for the welcomes - I'm looking forward to contributing as well as reading!

As the convection is in liquid metal, the convection currents *are* the electric currents. They're not something separate.As for reversals, they definitely happen. The magnetic field has a direction as well as an intensity (it is a vector). When the poles switch the direction of the flux lines also switches. Here in the UK, the magnetic field currently points north and slightly down into the Earth. If the field was reversed it would point south and up. It is this change in direction which we look for. And measure all the time.The intensity of the field falls during transition, but then regains its strength once the reversal has occurred.But, as has already been pointed out, even if we are completely wrong about the geodynamo, it has nothing to do with PT. That is a manifestation of *mantle* convection.This message has been edited by gengar, 01-16-2005 18:33 AM

Hi Hippie.I'm glad you appreciate my posts. Apologies if I sounded a bit frustrated in the last one, but I think there is some serious talking at cross-purposes going on here, which is always frustrating. I will try to do better this time.The first question is, can convection occur in the outer core?There's all this smoke and noise about viscosity, and the wide range of estimates. This is because it's actually a very hard thing to measure. This paper does indeed give a wide range of estimates, covering a wide range of different methods for indirectly estimating outer core viscosity. This indirectness is why there's so much uncertainty. But does it matter? As previously stated, values of the individual properties by themselves are not diagnostic all that is required is that the Rayleigh number of the system is above the critical value of ~1000. So, lets do some calculations.The Rayleigh number Ra = g.a.p.T.d^3 / k.hHere's some estimates of all the parameters in the above equation other than viscosity (h):g = acceleration due to gravity = 10 ms-2
a = thermal expansion coefficient = 6e-6 K-1
k = thermal diffusivity = 1e-5 m2s-1
T = temperature difference = 1000 to 2000K
p = density = 1e7 to 1e9 kgm-3
d = thickness of layer = 2300 km (2,300,000 m)I doubt you'll dispute g. The next three parameters are derived from lab experiments which try to measure these properties at conditions similar to the core. There is some uncertainty here, particularly in T, because we don't know the exact composition of core material. The last two have been derived from seismological data, and are pretty reliable.Plug these into the equation and you get:Ra = 7.3e30 / hSo with a range of viscosity from 10e-3 to 10e11 Pas (coverted to SI from the figures in the paper above: 1 poise = 0.1 Pa S) we can estimate that the Rayleigh number for the outer core ranges from:7.3e19 < Ra < 7.3e33So, yes, there's a large range of possible values, but even the lower bound is still 16 orders of magnitude above the critical number, so convection is no only likely, but will be extremely vigorous. We'd have to be a long long way out on our numbers for this not to be the case.(I'll just add here that the viscosity of the mantle is approximately 10e19 Pa.s, and it still convects, because the Rayleigh number is high enough).The next issue is, can this convection generate a magnetic field? This seems to be where the real confusion is (on my side as well as yours, perhaps). I'll try and explain it as I understand it, perhaps you can show where you're unhappy.A conductor moving in a magnetic field generates a current; therefore a convection current involving liquid metal, moving in the present magnetic field, will generate a current.This current generates a magnetic field.In the outer core, these processes feed back into each other - the currents generate magnetic fields which maintain the currents. So all that was needed was a small 'seed' magnetic field to set the whole thing going, at some time in the past.As you correctly point out, there will be energy loss in this system due to resistive dissipation. But the system is constantly being supplied with heat to maintain convection. I suppose the term 'self-exciting dynamo' is potentially misleading. Perhaps 'self-organising' would be better.So on to the models. Running computer models of this process could have one of two aims:- proving the general valdity of this mechanism (i.e can this sort of process actually generate a stable field)?
- Trying to produce a field with behaviour that matches our observations of the Earth's.Only the second category strictly requires we exactly model based on the parameters of the Earth's outer core. And you're right in that we don't seem to do that: A brief trawl reveals that these models are generally run with much smaller Rayleigh number (of the order of 10 times the critical value). It seems that modelling a system with the ultra-large Rayleigh numbers I derived above is a bit of a computing challenge - I'm not sure why but I'd suspect its something to do with turbulent flow becoming a big issue.So it was a bit of a surprise when these models started producing fields which *did* look a lot like the Earth's, including the occurence of reversals. This is what is driving the current (ho ho) thinking about the primary importance of the Earth's rotation on the behaviour of the field.Just in closing, bear in mind that we have *observed* a dominantly dipolar field which reverses every few hundred thousands years. Our models try to explain this behaviour. Invalidating the model does not invalidate the observations. And, let me emphasise: REVERSALS HAPPEN. I STUDY THEM. I have gone to a sedimentary section. I have drilled and oriented samples from different levels. I have measured and analysed the samples in the lab, and I have found polarity reversals through the section. So, I'm wondering whether to be insulted that you don't rate my competence, or amused that you think I'm going to all this effort as part of the Grand Plate Tectonic Conspiracy. They don't pay me that well, you know!Please don't take that the wrong way - you're a very polite poster, and I appreciate that.This message has been edited by gengar, 01-21-2005 11:18 AM

Doh!You're right of course. i was so busy trying to keep my units right (hope I did) that I missed this.Yes, g will be reduced, but not to zero (to around 2.5ish I'd think)Still, this does not invalidate my arguement that the Rayleigh number is very high.This message has been edited by gengar, 01-21-2005 11:36 AM

Well, it depends on where you're sampling. If you're collecting samples on land, you drill around the bit of rock you want to collect, then put an orienting tool with attached compass into the hole to measure the direction and angle that you drilled at, in relation to a reference mark which is drawn/carved on the sample at the same time. Only then do you remove the core. These measurement are then used to correct the rock's measured magnetisation back into the correct coordinate system. If the beds you sample are dipping, you have to correct for them in the same way. You do have to be careful, but it's not rocket scienceOcean cores drill straight down, so orientation is pretty easy - you do have to be careful in sediment cores though because the drilling causes some disruption at the edge. There's also the possiblity of rotation of the core during drilling, but if you're just looking for reversals (for magnetostratigraphy) this is not an issue as you just need to establish if the field points up or down.The magnetic stripes we're mostly talking about here are somewhat different as most are remotely measured (ship-towed and aircraft magnetometers), although there has been some ground truthing by drilling to basement. I'm not sure how you could misorient yourself with those.

The clue to understanding this is in the more technical name for these features - marine magnetic anomalies. They are measured as deviations from a global reference field: the remanent magnetisation of the crust is being superimposed on Earth's present day field, leading to local variations in the magnetic intensity.Normal polarity stripes are have a magnetisation which points in the same direction as the present day field and therefore add to the measured intensity, resulting in a positive anomaly.Reversed polarity stripes have a magnetisation which points in the opposite direction, and subtract from the measured intensity, leading to a negative anomaly.A simple variation in intensity would not produce the negative anomalies. Also, we have the directly measured and dated reversal stratigraphy from sediment cores: the ages of normal and reversed polarity intervals correlate nicely with the ages of the positive and negative stripes, determined from radiometric dating of the crust itself. I'm not sure why, in standard oceanic crust you have around 2km of basaltic lavas and dykes which will be packed with magnetic minerals like magnetite. That said, it is still not entirely clear exactly where the magnetic anomalies are sourced - mainly because we have yet to drill completely through the crust. We're getting there though.

Sadly for HH, where drilling has reached sea floor basement, the rocks invariably have a normal polarity in the positive stripes and a reversed polarity in the negative. What we're not yet sure of is how much of the crust is contributing to the overall anomaly. The net magnetization of the upper portions we've sampled does not seem to be large enough to produce the anomaly we see, implying that the lower crust is also involved.From an Ocean Drilling Program report: So, whilst there are some unanswered questions about these anomalies, their existence is not in doubt. As your rather cool picture clearly demonstrates!

Hi Hippie,Thanks for your posts. You write a lot, and I think some of your points are possibly getting lost in the sheer volume, though. So I’m going to cut it down a bit, and try and concentrate on the most basic points first. I’ll address the geodynamo stuff in this post, and then I’ll do another about the sea floor stripes.Regarding my last post you write: I’m not sure why we should do that. You were arguing about uncertainty in the parameters. I agree. But my post demonstrates that even with this uncertainly, we can still be certain that the outer core convects. Or do you dispute that a system with a Rayleigh number of at least 1E16 will convect?The critical Rayleigh number is an experimental observation — systems where it is >1000-2000, convect. Always. So when you ask, My answer is again, ‘because the Rayleigh number of the system is much greater than 1000’. However, we can also map temperature anomalies in the mantle by looking at small variations in the speed of seismic waves (colder = faster, hotter = slower) which directly show us features indicating convection - particularly the cold downgoing slabs at subduction zones: Indeed, earlier in the LDEO webpage you quoted, it quite clearly states: But I’m glad that we’re finally clear that mantle and core convection are separate. Perhaps further discussion of your quote about plate tectonics and mantle convection should take place in another thread. I’d be happy to participate of course; it’s a very interesting topic.OK, back to the magnetic field. You gave a good explanation of lightning; however, as you yourself acknowledge it’s not really relevant in this case. In the inner core, we’re talking about a manifestation of the dynamo effect. The convection currents move a conductor, iron, in magnetic field, generating a current, according to the left hand rule we all learn in school: This effect will apply in the core just as much as it does in the generator at your local power station.At this stage, I’d like to stop and ask you some simple questions to see where we’re at. Do you agree or disagree with the following points?

• Convection will occur at Rayleigh numbers of greater than 1-2000.
• The outer core, even given our uncertainty over certain parameters, has a Rayleigh number which is much higher than 1-2000.
• A moving conductor in a magnetic field will generate a current.

I’ll wait for your answers on these before moving on.I’d also like to clarify that this desire to do away with reversals is to do with your clear preference that the magnetic field is a property of the inner core? You obviously couldn’t have reversals if it was. However, when you say: you’re forgetting that iron loses its ferromagnetic properties above about 700 degrees Celsius — the inner core is much hotter than this, so will not act as a permanent magnet. Thus your hypothesis suffers from the fate you ascribe to geodynamo theory — it violates known physical law.You also say Yes, but whilst Ptolemy was wrong in thinking the planets moved around the Earth, the planets still move. Geodynamo theory may be wrong, but REVERSALS STILL HAPPEN.Anyway, I’m glad you’re finding this discussion informative. As, you suspect, I’m a real ale man, although my friends in Phoenix will attest that I’ll drink anything at a pinchI’ll get to addressing the sea floor anomaly stuff now

So, on to the magnetic anomalies. This is as good a place to start as any: That depends on where you are on the Earth. Here’s a simple representation of the present day field: As you can see, the field does indeed point down in the northern hemisphere; but, in the southern hemisphere, it points up. However, we find a great many rocks in the northern hemisphere with a magnetization that points upwards, in the opposite direction that we’d expect; and crucially, rocks in the southern hemisphere of a corresponding age have magnetizations that point downwards. This is what tells us that at certain points in the past, the Earth’s magnetic field was reversed — the flux lines were all pointing in the opposite direction, as we see in the image below. There is actually significant variation in the reference field at all latitudes; it is not a simple beast. Did you go to the link I gave to the International Geomagnetic Reference Field?. As they say: The figures above aren’t a completely accurate representation of the field. Although the earth’s magnetic field is dominantly dipolar, there are also quadrapole and even octopole components (in other words, there do exist other, weaker, magnetic poles, it’s just that compasses respond most strongly to the largest one). Secular variation is caused by the change over time of the strength of these different components. The IGRF takes all our current measurements of field direction and intensity, all around the globe (satellite measurements are a major source nowadays) and then calculates the field which best fits all these measurements. Have a look here if you want to see maps of this field.This gives us an accurate (as it includes many measurements distributed over the whole planet) representation of the field being generated inside the Earth (however you want to say it is being generated). What it does not account for is for local sources of magnetism. If you are measuring the magnetic field and happen to be standing over, for example, a large mass of iron ore, the presence of a large mass of magnetic material is clearly going to have an effect on your reading — you will see a deviation away from the reference value. However, whilst it will have a very large influence on your reading when you are directly over it, the effect will reduce very quickly as you move away from it, becoming insignificant when you get more than a few km away.This is what it meant by a magnetic anomaly — an additional component in the locally measured field, which has a short wavelength — it can only be detected over a small area.When we measure the field over the ocean floor, we see such short wavelength anomalies — positive and negative deviations from the average field. These are the magnetic stripes. Such anomalies must have a shallow source (if they were deeper, they would be ‘smeared out’ over longer wavelengths), which is why we know they are due to the magnetization of the oceanic crust. And, just by considering the principles of superposition, we can work out the polarities of these magnetizations. In the northern hemisphere (field currently points down):

• A section of ocean floor with a normal (downward pointing) magnetization adds to, and will increase, the local magnetic field.
• A section of ocean floor with a reversed (upward pointing) magnetization subtracts from, and will decrease, the local magnetic field.

I think I’ll stop here. We can address any questions you have about the above before we move on.