Plate tectonics may control reversals in the Earth's magnetic field

Oct 24, 2011

The Earth's magnetic field has reversed many times at an irregular rate throughout its history. Long periods without reversal have been interspersed with eras of frequent reversals. What is the reason for these reversals and their irregularity? Researchers from CNRS and the Institut de Physique du Globe, France, have shed new light on the issue by demonstrating that, over the last 300 million years, reversal frequency has depended on the distribution of tectonic plates on the surface of the globe. This result does not imply that terrestrial plates themselves trigger the switch over of the magnetic field. Instead, it establishes that although the reversal phenomenon takes place, in fine, within the Earth's liquid core, it is nevertheless sensitive to what happens outside the core and more specifically in the Earth's mantle. This work is published on 16 October 2011 in Geophysical Research Letters.

The Earth's magnetic field is produced by the flow of liquid iron within its core, three thousand kilometers below our feet. What made researchers think of a link between plate tectonics and the ? The discovery that convective flows play a role in magnetic reversals: experiments and modeling work carried out over the last five years have in fact shown that a reversal occurs when the movements of are no longer symmetric with respect to the equatorial plane. This “symmetry breaking” could take place progressively, starting in an area located at the core-mantle boundary (the separates the Earth's liquid core from its crust), before spreading to the whole core (made of molten iron). 

Extending this research, the authors of the article asked themselves whether some trace of initial symmetry breakings behind the geomagnetic reversals that have marked the Earth's history, could be found in the only records of large-scale geological shifts in our possession, in other words the movements of continents (or ). Some 200 million years ago, Pangaea, the name given to the supercontinent that encompassed almost all of the Earth's land masses, began to break up into a multitude of smaller pieces that have shaped the Earth as we know it today. By assessing the surface area of continents situated in the Northern hemisphere and those in the Southern hemisphere, the researchers were able to calculate a degree of asymmetry (with respect to the equator) in the distribution of the continents during that period.

In conclusion, the degree of asymmetry has varied at the same rhythm as the magnetic reversal rate (number of reversals per million years). The two curves have evolved in parallel to such an extent that they can almost be superimposed. In other words, the further the centre of gravity of the continents moved away from the equator, the faster the rate of reversals (up to eight per million years for a maximum degree of asymmetry).

What does this suggest about the mechanism behind geomagnetic reversals? The scientists envisage two scenarios. In the first, terrestrial plates could be directly responsible for variations in the frequency of reversals: after plunging into the Earth's crust at subduction zones, the plates could descend until they reach the core, where they could modify the flow of iron. In the second, the movements of the plates may only reflect the mixing of the material taking place in the mantle and particularly at its base. In both cases, the movements of rocks outside the core would cause flow asymmetry in the liquid core and determine reversal frequency.

Explore further: Lava creeps toward lots in Big Island subdivision

More information: Plate Tectonics May Control Geomagnetic Reversal Frequency. F. Pétrélis, J. Besse, J.-P. Valet. Geophysical Research Letters, 16 October 2011.

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User comments : 19

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Megapixel
1 / 5 (9) Oct 24, 2011
The relationship between the position of Earth's continental masses and the core elements is the basis for the Gravity Theory of Mass Extinction. This article supports the GTME.

GTME posits that the continents, when moving laditudinally, would alter the Earth's angular momentum, an impossibility unless the Earth's angular velocity changed or the core elements moved. No substantial change in angular velocity has been observed, leaving the core elements movement the only option. Movement of the core elements would entail changes to surface gravity and an an explanation for dinosaur gigantism.

GTME has offerred an explanation of the two superchrons as being the result of the core element movement well before the CNRS study.
rubberman
1.8 / 5 (5) Oct 24, 2011
Interesting, I've never heard of the GTME....I am however reasonably sure that it is against the laws of planetary physics to propose that a spherical object of roughly the same symmetrical densities throughout could exhibit a different level of gravity large enough to cause gigantism in the sphere's suface dwelling inhabitants. Yes surface gravity fluctuates in a very small order depending on surface composition at a specific point........tell you what, let's just say it like this: earth floats in a vaccuum, continental drift is a very slow process....describe to me where the angular momentum comes from that could alter the gravity of the planet in this fashion......please.
Megapixel
1 / 5 (8) Oct 24, 2011
rubberman,
The Earths density becomes much greater near the center. It has been estimated that 80-85% of the Earths mass can be accounted for by the inner/outer cores and the densest part of the lower mantle surrounding the cores (i.e., the core elements).

Based on the above, it doesnt take a large movement of the core elements to produce a change in surface gravity on the Earths surface.

The shifting of the core elements is primarily a result of the movement of the continents (e.g., when Pangea consolidated) in a way that the center of mass (COM) of Pangea moved south of the equator. On a rotating body, in this case the Earth, in order to not violate the Law of Conservation of Angular Momentum, one of two things had to happen to compensate:
CONTINUED IN NEXT POST (seems like long posts not allowed)
Megapixel
1 / 5 (8) Oct 24, 2011
1. A change in the spin rate (i.e., angular velocity) of the Earth. This is usually explained in physics text with the spinning skaters changing their spinning rate by extending or retracting the arms.
or
2. Movement of the internal and most massive parts of the Earth away from rotation axis.

Since the southern movement of Pangeas COM would tend to lower Earths angular momentum, the Earth would have to have spun faster to compensate or the core elements would have to move off-center and away from Pangea. GTME posits that the latter event happened because there is no known evidence that the Earth spun faster during Pangeas existence.
barakn
2.5 / 5 (6) Oct 25, 2011
it doesnt take a large movement of the core elements to produce a change in surface gravity on the Earths surface.

That's actually quite wrong. Forget for the moment the huge question of whether you could disturb the radial symmetry of the Earth's center without it rapidly returning to equilibrium. Instead consider the implication of the inverse square law. When you are standing on the Earth's surface, the strongest pull is from the soil a few nanometers beneath your feet, not from the core thousands of km away.
rubberman
2 / 5 (4) Oct 25, 2011
Symmetrical densities throughout MP, this means that if you split the earth in two halves, all the layers would have the same densities from the core up to the crust. Nothing short of a very powerful Galactic anomaly could displace the core from the center of the earth and as barakn points out, it would return to equilibrium...provided the planet wasn't torn apart. Your core percentage of the earths mass is correct which is why I find it odd that you would support such an off the wall theory. The mantle makes up all of the rest of the earths mass except one percent which is in the outer crust and tectonic plates (lithosphere). There simply isn't enough planetary mass in the crust to alter the internal organizational structure of the planet with it's movement. The basic laws of physics make this theory completely implausible.
rawa1
1 / 5 (6) Oct 25, 2011
The plate tectonics is based on the motion of relatively thin crust at the surface of Earth. While I can imagine quite easily, the motion of such crust could correlate with changes in convection flux inside of Earth mantle or even Earth core, the reversed causality seems extremely improbable for me...
Megapixel
1 / 5 (8) Oct 25, 2011
barakn,
Your first post is incorrect, Newton proved that in a radially symmetric sphere, all of the mass can assumed to be at the center.

rubberman,
Most people believe what you wrote, i.e., that the tectonic plates are too small (mass-wise) to have an effect on the Earth.
Angular momentum of the plates depends on three things:
1. Their mass
2. Their distance from the axis of rotation
3. Their tangential velocity

Their mass may only be 1% of that of the Earth but their velocity is about 1670 km/hr (at the equator) and their axis displacement is about 6380 km.

Multiplying the above three variables gives a substantial angular momentum. If the Earth did not rotate, the plates A.M. would be insignificant. In other words, people view the tectonic plates/Earth as though the Earth was a static (i.e., a non-rotating entity) when they compare the two.
astro_optics
1 / 5 (4) Oct 25, 2011
a few bilion years ago the moon was much closer to the earth, not sure of the numbers, but it would have influenced the shape and gravitation on the surface especially on the equator.
Pirouette
1.1 / 5 (15) Oct 25, 2011
Please forgive me if I'm wrong on this, but I was under the impression that during subduction, the one plate that is sliding downward under another plate doesn't necessarily have to be plunging down toward the molten core, but instead could continue underneath the upper plate and add to its weight. . . .so that after a time, the weight of both plates may press down toward the core and cause it to, let's say, hiccup, producing a polar reversal. Is this incorrect? If you press upon an unbroken egg yolk, the contents move to the other side, for instance, due to the weight of your finger pressing in on it.
Pirouette
1 / 5 (14) Oct 25, 2011
Is this not what has happened to India and the Himalayas?
Megapixel
1 / 5 (7) Oct 26, 2011
Pirouette,
Don't confuse the two types of subduction. When a continental plate collides with another continental plate, as in the case of India, mountains are uplifted.

When an oceanic plate is subducted in a subduction zone it will, because it is much denser than the surrounding crust and upper mantle, plunge downward. How far down it descends before it is absorbed by the mantle I do not know.

The authors of the CNRS research suggest one possibility for the core reversals is the intrusion of these descending slabs into the outer core. My belief is that they are in error.

The article states that the further the center of gravity of the continents moves away from the equator, the faster the rate of reversals. So, there doesn't seem to be a clear connection. GTME posits that the above described movement would cause a shift of the core elements.....which has a more direct effect on the circulation of molten iron in the outer core and therefore the possibility of reversals.
barakn
1.8 / 5 (5) Oct 29, 2011
barakn,
Your first post is incorrect, Newton proved that in a radially symmetric sphere, all of the mass can assumed to be at the center.

No, my first (and 'til now only) post was spot on. Newton's shell theorem calculates the force of gravity at some arbitrary point by integrating the forces from an infinite number of infinitesimal volume elements in a shell of material of constant density. I'm not talking about the total integrated force due to the shell. I'm talking about the force due to just one of those volume elements from the part of a shell very nearby vs. that of one volume element (volume adjusted to contain the same amount of mass) from a shell much further away towards the interior.
Ojorf
1.6 / 5 (7) Oct 30, 2011
@ Megapixel. GMTE makes no sense for various reasons. One being that in order to experience 50% of g you have to be about 2600km above sea level, no amount of redistribution of the earths mass can possibly account for this. Please explain. Also can you point to a single published article in a pier reviewed journal dealing with GMTE.
Megapixel
1 / 5 (6) Oct 30, 2011
Ojorf,

A shift of the center of mass of the core elements by 2600km would be 2600km/12756km or .20 of the diameter of the Earth. This is quite possible.
You will not find any peer-reviewed articles on GTME primarily because the author is not a professional in any of the science fields.
What are the "various reasons" that you believe GTME makes "no sense", in your opinion?
Graeme
not rated yet Oct 30, 2011
The effect is more likely to be due to different temperatures at the base of the mantle. The cool spots due to subducting material from high up in the mantle and hot spots where it has been for longer. The core fluid would sink under the cooler parts of the mantle base.
Ojorf
1 / 5 (5) Oct 31, 2011
Well, that makes sense, I mean that the author is not a professional in the science fields. Articles do not get accepted on the credentials of the author though, but on merit.
If giantism in dinosaurs was due to a low gravity you would expect the same to have happened to plants and trees, not only to dinosaurs, this does not seem to have happened. Further biokinetic studies showed that even the largest sauropods did not need to wallow in water (as was first believed)to get by, but were quite capable of moving around on land. Also a lower gravity would have left its imprint in the sedimentary buildup of rock strata from the time.
Megapixel
1 / 5 (6) Oct 31, 2011
Ojorf,
Articles are not accepted from non-professionals.....there is a glass ceiling there. Does that mean a non-professional cannot write a valid theory?

Your conclusion that because giantism affected dinosaurs (i.e., sauropods) and therefore had to affect plants and trees is not valid. The growth of flora and fauna is determined by whether their chances of survival is enhanced by exceptional growth. Clearly, in the case of sauropods, this giantism was a response to make them less vulnerable to predators similar to extant elephants.

The realization that sauropods did not have to rely on bodies of water to support their large size is further support for reduced gravity. The original belief that they required swamp-like environments was based on the presumption that they couldn't exist with current surface gravity, which I obviously believe is true.

Linking sedimentary buildups and surface gravity is a tenuous concept...there are far more stronger points that support GTME.
Megapixel
1 / 5 (6) Oct 31, 2011
Ojorf,
Google "Plate tectonics may contol reversals in Earth's magnetic field", a recent study which supports GTME. The movement of the core elements would affect reversals due to the disturbance of the outer core. The authors of the paper, although they have come to an incorrect conclusion as to the cause of the reversals, have correctly linked the movement of the continents to the Earth's core. This is the very basis of GTME.