True Polar Wander: Shifting of the Earth’s Surface Relative to its Spin Axis

Posted: November 16, 2012 by Rog Tallbloke in Cycles, Geomagnetism, Gravity, Natural Variation, solar system dynamics, Tides

Here’s an interesting article from the IOP website; How Earth’s wandering poles return home. It takes the Solar System about 225–250 million years to complete one orbit around the Galaxy (a Galactic year), so there may be a relationship between this and the cycle of  the ‘tipping’ of the surface relative to the spin axis.

True polar wander (TPW) can be defined as the relative movement between the mantle (and so the surface of the Earth) and the Earth’s spin axis or its rotational axis. Incredibly, researchers believe that over the past one billion years, the Earth’s surface has “tipped over” and then returned to its original location six times along the same axis – this is the process of “oscillatory true polar wander”.

What has eluded researchers is a theory that clearly explains how and why the pole returns to its original location, or the “oscillatory true polar wander”. In the new work, graduate student Jessica Creveling, also of the Earth and Planetary Science Department at Harvard, along with Mitrovica and colleagues, provides an explanation.

The first mechanism relates to the Earth’s equatorial bulge. The Earth is not a perfect sphere – rather it is an oblate spheroid, as it is flattened at the poles and bulges at the equator….  if the Earth, with its bulging equator, tips over, it prefers to right itself again. “So, this girdle of excess mass actually has a very stabilizing effect, acting as a self-righting mechanism for the Earth’s rotation,”

The second mechanism relates to the strength of the tectonic plates. If the Earth’s surface tips over relative to the rotational axis, the 12 larger tectonic plates all get deformed to a small extent, like elastic bands. In a similar way to a stretched elastic band, the plates want to go back to their original size, and these stabilizing elastic stresses also play a role in the oscillatory return of the pole.

Read the whole article.

The paper is published by Nature here:
http://www.nature.com/nature/journal/v491/n7423/full/nature11571.html

Comments
  1. Project722 says:

    Hasn’t NASA even confirmed the NMP is racing towards Siberia at a rate of 40 miles per year? That does not seem to me like “wandering”.

  2. Project722 says:

    I suppose this is the “move along, nothing to see here folks” follow up to that NASA article.

  3. vukcevic says:

    Project722 says:
    November 16, 2012 at 3:29 pm
    Hasn’t NASA even confirmed the NMP is racing towards Siberia at a rate of 40 miles per year? That does not seem to me like “wandering”.

    That is the consensus science, the reality as we know often is different.
    NMP simply jumped from Hudson Bay (Canada) to the central Siberia in mid 1990s.
    http://www.vukcevic.talktalk.net/MFTI.htm
    See for more details the NOAA’s magnetic maps (1900-2010)
    http://www.ngdc.noaa.gov/wist/magfield.jsp

  4. Project722 says:

    vukcevic says:
    November 16, 2012 at 7:48 pm

    Project722 says:
    November 16, 2012 at 3:29 pm
    Hasn’t NASA even confirmed the NMP is racing towards Siberia at a rate of 40 miles per year? That does not seem to me like “wandering”.

    That is the consensus science, the reality as we know often is different.
    NMP simply jumped from Hudson Bay (Canada) to the central Siberia in mid 1990s.
    http://www.vukcevic.talktalk.net/MFTI.htm
    See for more details the NOAA’s magnetic maps (1900-2010)
    http://www.ngdc.noaa.gov/wist/magfield.jsp

    Thanks for the links although I can’t make out a distinct movement in the NMP during the mid 1990′s like you say. Are you saying it moved back?

  5. Project722 says:

    Vukcevic you have some nice data on your website. I was looking at the solar polar field strength on this one below.

    http://www.vukcevic.talktalk.net/LFC6.htm

    It looks like we are at max now and magnetics will be changing very soon. Curious though, how come the sunspots aren’t more equatorial if we are right at solar max/flip time?

  6. Climate Change and the Earth’s Magnetic Poles,
    A Possible Connection

    See my website

  7. oldbrew says:

    Another curiosity is that the North Pole is currently shifting faster than the South Pole.

    http://www.ngdc.noaa.gov/geomag/GeomagneticPoles.shtml

  8. tallbloke says:

    Feel free to continue the discussion on geomagnetics, but I’ll just point out that the paper this post is highlighting is talking about movement of the Earth’s crust, not the movement of the magnetic poles.

  9. Project722 says:

    vukcevic says:
    November 16, 2012 at 7:48 pm

    “NMP simply jumped from Hudson Bay (Canada) to the central Siberia in mid 1990s.”

    I don’t see a jump of this magnitude during this time frame. Check this site and download the animation for the last few hundred years.

    http://geomag.org/info/declination.html

  10. Project722 says:

    oldbrew says:
    November 17, 2012 at 11:24 am

    “Another curiosity is that the North Pole is currently shifting faster than the South Pole.”

    Follow the link in my last post. Download and watch the animation since the 1600′s. Notice the slow gradual wander of the NMP over the course of the last few hundred years. Then notice the very rapid straight line movement towards Siberia in the last few seconds of the video. Am I the only one who finds this odd?

  11. oldbrew says:

    Project722 – Re movement towards Siberia, it might have something to do with the Siberian traps?

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

    Imagine what was coming up from the core to generate a lava field the size of western Europe, or perhaps even bigger.

    This link says ‘The frequent polarity reversals of Earth’s magnetic field can also be connected with processes in the mantle.’

    http://www.sciencedaily.com/releases/2012/07/120729142156.htm

  12. Coldish says:

    Thanks, Tallbloke, for introducing the topic of TPW. I’d be cautious about placing much confidence in any article on a ‘new’ topic in a generalist journal like Nature or Science without first looking at the specialist scientific literature. Nature has been known to publish junk science – think of MBH98.
    I haven’t had time to look up the lit, but a few points occurred to me.
    In order to establish the existence of TPW one needs at least one fixed reference point on the earth. The whole of the lithosphere (the solid outer layer of the earth, averaging 100km thick) is and likely always has been in constant motion, with parts (‘plates’) moving in different directions relative to the other parts and relative to the rotation axis. This process is called plate tectonics and is as far as I know not a matter of dispute. Plate motion is of the order of 5cm per year. There are however ‘hotspots’ which do not move with the plates; these typically form chains of volcanic islands as the plates move over them. The Hawaiian chain is the best-known example. How deep the source of hotspots is not clear, but a likely depth is the core-mantle boundary. They seem also to be in motion relative to each other (and so relative to the rotation axis) although this must be at a rate at least an order of magnitude slower than plate movements.
    So is anything fixed?
    The key to all this is of course paleomagnetism. Many rocks contain minerals which record the inclination and declination of the earth’s magnetic field at the time the rock formed or cooled. If we also know the age of the rock, and that the rock has not been rotated by folding or tilting, we can determine the approx. position of the magnetic poles at that date (in other words the paleomagnetic latitude of the sample location). With lots of samples of the same age from different locations we can be more precise and with sets of samples of different ages we can construct what are called ‘apparent polar wander’ (APW) paths. If the samples are taken from different plates, this can enable us to plot past movements of the plates relative to each other as well as to the magnetic poles.
    All this of course assumes that the magnetic poles have not in the past strayed far from their present position relative to the pole of rotation. That is a big assumption. We can observe that the magnetic poles do move rather rapidly (about 6 orders of magnitude faster than plate movements). At the moment the north magnetic pole is about 10º away from the pole of rotation. It does indeed seem to be the case that it has generally remained within about 20º from the pole of rotation – otherwise it would not be possible to construct meaningful APW paths.
    Does this bring us any nearer to being able to recognise TPW (if it has occurred)? I think it does. Suppose we found an episode of the earth’s history when all the APWs shifted by, say, 90º in the same sense, so that the magnetic poles appeared to be near the equator. If the pole of rotation was still within 20º of the magnetic poles, then bingo, we have TPW.
    However, I don’t really like the idea of a rotating lopsided earth, with one equatorial diameter 20km shorter than another.
    Time to get back to work.

  13. tallbloke says:

    Thanks Coldish.

    I think you are strongly overestimating the rigidity of the crust. Plates have boundaries, and the oblateness due to the rate of spin and the Moon’s gravitational pull aren’t going to be overcome so easily.

    Plenty of volcanic activity would surely result from the buckling of the plate boundaries. I wonder if the record of volcanism can be tied into this study.

  14. Brian H says:

    I also wonder if it had any thing to do with the transition into the Ice Age, and the end of Hothouse Earth.