Apparent Polar Wandering?

Discussion in 'Earth Science' started by kingwinner, Sep 12, 2005.

  1. kingwinner Registered Senior Member

    Messages:
    796
    The correspondence of Apparent Polar Wandering Paths?

    I am kind of confused with the kind of poles that apparent polar wandering is saying...

    Are the Apparent Polar Wandering Paths saying that the MAGNETIC pole (not the geographic pole) is apparently moving as time goes on? (I understand that the poles weren't really moving, the continents did.) What kind of pole is seemingly moving?
    What and where is the geographic pole and is it always stationary all the time throughout the earth's history?

    I would need some clarification on these to get out of this mess!

    Edit: Please see the 11th reply (figure B) for the correspondence of Apparent Polar Wandering Paths and I am now puzzled with its meanings...
     
    Last edited: Sep 18, 2005
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  3. invert_nexus Ze do caixao Valued Senior Member

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    9,686
    Yeah. The magnetic pole moves around quite a bit. Here's a picture from APOD a few months back. The pole must have shifted quite a bit since then (the pole in the picture is from 94.)

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    Explanation: A magnetic compass does not point toward the true North Pole of the Earth. Rather, it more closely points toward the North Magnetic Pole of the Earth. The North Magnetic Pole is currently located in northern Canada. It wanders in an elliptical path each day, and moves, on the average, more than forty meters northward each day. Evidence indicates that the North Magnetic Pole has wandered over much of the Earth's surface in the 4.5 billion years since the Earth formed. The Earth's magnetic field is created by Earth's partially ionized outer core, which rotates more rapidly than the Earth's surface. Indicated in the above picture is Ellef Ringnes Island, the location of Earth's North Magnetic Pole in 1994.​

    http://antwrp.gsfc.nasa.gov/apod/ap040919.html

    Edit: Hmm. The picture says 99. The text says 94. Oh well.

    And, by the way, there is a recent article in Science about the inner core rotating faster than the planet.

    Earth's Inner Core Is Running a Tad Faster Than the Rest of the Planet. Science 26 August 2005; Vol. 309; Issue. 5739; p. 1313
    (You need a subscription to view it online. I know. A bitch. Damn Science and other journals for being so damned spendy...)
     
    Last edited: Sep 12, 2005
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  5. kingwinner Registered Senior Member

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    796
    So what actually is the apparent polar wander? I don't understand now...
     
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  7. spidergoat Valued Senior Member

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    There is the apparent pole, which is magnetic, and the true pole, which is the axis the Earth spins around. The apparent polar wander is the phenomenon of the magnetic north pole seeming to drift around relative to the actual pole.

    Yes, it has shifted in the past as well, sometimes alarmingly suddenly (for geologists). I'm not sure why.
     
  8. kingwinner Registered Senior Member

    Messages:
    796
    So is the apparent polar wander the "fake" magnetic polar wander at times where people thought that the continents are stationary? (which led them to draw the wrong conclusions with wrong evidences)

    Is the geogrpahic pole usually interpreted as always staying the same? (I found out definitions of this word and they say so)
     
  9. kingwinner Registered Senior Member

    Messages:
    796
    Thanks invert_nexus for your detailed explanation, but I think you are talking about the TRUE polar wandering, instead of the APPARENT polar wandering which I am concerned of...
     
  10. invert_nexus Ze do caixao Valued Senior Member

    Messages:
    9,686
    Ah. Yeah. You're right. I've never heard of this apparent and true polar wandering.

    Here's what I've found by googling.

    What it seems to me is that true polar wandering is the actual measurement of the polar wandering. Something that can only be done in real time and thus, before these measurements were taken, are impossible.

    However, that's where apparent polar wandering comes in. Apparent polar wandering takes advantage of the direction of iron crystals in the rock that was formed at any particular time in from md-oceanic ridges (possible from ordinary volcanoes as well, for all I know. Bear in mind this is merely the result of a few minutes of web searching.)

    So, one can examine the rocks and determine from them where the poles were at the time that the rock was formed. However, this is sure to be inaccurate as plate-tectonics has shifted the rocks around this way and that. And thus its only apparent rather than true. An approximation.

    That's what I've found anyway. I could be way off-base.

    Here's the pages that I've found.

    http://www.uh.edu/~jbutler/physical/chapter20notes.html

    http://www.geocities.com/CapitolHill/Lobby/1777/papers/spaleo.html

    That first link has animations of both true and apparent polar wanderings, but they're in a restricted area of the site requiring user name and passwords... A shame. A dedicated internet search might find the animations (or similar animations) available elsewhere.

    Edit: Ah that was easy:
    www.hisf.no/anf/rmg/geogrk/pWander.swf

    Follow the instructions on the first link to view the various aspects of the animation.
     
  11. kingwinner Registered Senior Member

    Messages:
    796
    Thank you for your explanation!

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  12. kingwinner Registered Senior Member

    Messages:
    796
    I found out from the web that there are 3 types of polar wander: apparent polar wander, magnetic polar wander, true polar wander. And the true polar wander means the migration of the geographic poles.
    Magnetic polar wander is the accurate movement of the magnetic poles with respect to the geographic poles (when the geographic poles are considered stationary)
     
    Last edited: Sep 17, 2005
  13. kingwinner Registered Senior Member

    Messages:
    796
    "In the past, the geologists examining ancient rocks (paleomagnetism) assumed the continents did not move so the magnetic north pole APPEARED to be moved, hence this is called apparent polar wander.
    When the continents are considered to be moving (RATHER THAN THE POLE), the PATHWAYS correspond."

    When they considered the continents move instead of the pole moving, the pathways correspond, ie Fitting the continents back together results in a single path............but the last sentence in the quote confuses me. It says that scientists consider that the continents are moving instead of the magnetic pole (ie consider the change in position and orientation of the continents), and this gives a perfect match of the apparent polar wander paths for different continents (North America and Europe, for example), but as they consider that the magnetic pole is not moving, why are there still pathways of polar wander? Is this the real MAGNETIC polar wander--the movement of the magnetic pole with respect to the geographic pole, which is considered stationary at most times?)

    Edit: Ha...I have found a nice figure that claries what I mean...the following is the kind of thing I am saying, the paths of the bottom figure, are they the true magnetic polar wander throughout the earth's history?

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    Last edited: Sep 17, 2005
  14. DwayneD.L.Rabon Registered Senior Member

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    locked
     
    Last edited: Oct 12, 2005
  15. kingwinner Registered Senior Member

    Messages:
    796
    But what type of polar wandering is the single pathway in figure B of the picture that I posted?
    If is the real magnetic polar wander throughout these 500 million of years, it looks strange to me as the path is moving very slow (it is a 500 million years path) comparing to today's magnetic pole moving rate. And also, for a long time the position of the magnetic pole should be averaged near the geogrpahic pole...the path in figure B just looks really strange to me...
     
    Last edited: Sep 18, 2005
  16. Andre Registered Senior Member

    Messages:
    889
    It appear that you guys are mixing things up a little.

    APW, TPW and RTPW (rapid)TPW are all related to assumed geographic polar wandering, not magnetic polar wandering. The magnetic pole movements are hopelessly erratic anyway. A deliciously delicate subject.

    What do you want to know about it?
     
  17. kingwinner Registered Senior Member

    Messages:
    796
    Andre, I searched a lot of web sites and I am pretty sure that APW is dealing with the MAGNETIC POLE because it is recorded by paleomagnetism on rocks (scientists measure their magnetic orientation).

    The thing that I don't get is the type of polar wander of the single path (APW paths for N. America and Europe correspond) in figure B...does anyone understand the meaning of figure B and can explain to me? I would really appreciate!
     
  18. Andre Registered Senior Member

    Messages:
    889
    Sorry Kingw. You deserve more attention than I can give right now. You're most certainly right that paleo magnetic oriention is the sole and only detector for whatever kind of polar wander.

    But consider this, the last million years the Earth has seen about 10 paleo magnetic excursions (PME) with large abberations of the north pole. How would rocks look like that are formed this way. We would assume that the average would be about the geographic north pole. So how would we judge rocks of 500 million years old? The resolution is far too low to discriminate any PME so we are left with the presumption that the average magnetic orientation equals the geographic orientation.

    AGW is defined as far as I've told to plate tectonics, moving the continents around along north and south pole but without changing anything in the Earth spin axis orientation with regards to the spinning axis of Earts mantle.

    TPW does indeed move the Earth mantle with regards to the spin axis. Google search term: TPW + Vermeersen (he lives two streets away from me)

    RTPW is the same but with a breathtaking rate, google search tpw + Kirschvink

    Good discussion.
     
  19. kingwinner Registered Senior Member

    Messages:
    796
    I think you are talking things that are beyond the scope of my course (I am a high school student), please forgive me that I don't quite understand your meaning...

    "However, the magnetism in old rocks was generally found to be inconsistent with the present field. The magnetism in these older rocks pointed in directions other than towards the pole and the recorded latitudes often did not coincide with the rocks' present latitudes. The magnetic direction and inclination (or latitude) recorded in the rocks allows an apparent pole to be calculate for rocks of that age. The apparent magnetic poles for old rocks are often far removed from the geographic pole. Moreover, Runcorn showed that these apparent poles were closest to the geographic pole for younger rocks and farther from the geographic pole for older rocks. "

    Quote from
    http://myweb.cwpost.liu.edu/vdivener/notes/cont_drift_to_tectonics.htm

    This is just one of the sites that say that APW is concerning the magnetic pole...and this saying is even consistent with my text book...
     
  20. Andre Registered Senior Member

    Messages:
    889
    Well, as the magnetic poles travel around with several miles per year, 12 miles I think is the average current rate, it's rather impossible to find a magnetic north pole back at say 25,687,688 years ago. Because say 25,687,419 years ago the pole was perhaps 1257 miles more to the West, that's the accuracy you need to follow those pole movements. But the rocks tell us that they are 25,5 +/- 0.25 million years old, an error large enough for the poles to travel around the world (which actually happened BTW). So we assume -and that is not in the text- that the magnetic poles have been hovering around balance or pivot points. And we also assume that these balance point happens to be the geographic poles. So if we can get a magnetic direction of that rock of 25,5 +/- 0.25 million years, we assume that this is the average of all magnetic directions between 25.25 and 25.75 million years and we hope that average being the geographic pole. Does it help?
     

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