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Thread: Expanding earth video

  1. #511
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    Quote Originally Posted by hhEb09'1
    Ah. But there are many tectonic ( (mountain) building ) theories, most of them supplanted by Plate Tectonics.
    I think most of them are incorporated into plate tectonics, not supplanted by the theory. Semantics maybe, so to elaborate, I consider Tectonic Theory the theory which describes all manner of tectonic actitivity arising from the movement of solid plates over the molton core.

    Quote Originally Posted by hhEb09'1
    I guess I'm interested--ABD in geophysics. Brad Hager was on my committee (gave me an Excellent on his section of my writtens, didn't make it to my orals).
    ABD? Not sure what you mean by this. I don't know of Brad, but his credentials seems beyond reproach. Just curious--what was your topic?

    Quote Originally Posted by hhEb09'1
    The idea is that once it occurs, it would modify the flow, as I said, so that the boundary would then become convergent. It would seem to make reasonable sense that if the oceanic part of an Atlantic boundary started to fall, there would almost certainly be a reaction of the crust on the other side. In everyday convective flow, a region of descending material is naturally convergent.
    Except that no such area of spontanious subduction is seen that I am aware of. Furthermore, if an area begins to subduct because of convergence, it means that convectional forces have changed their flow to make an area where there was not convergence begin to converge. I think you are confusing cause with result.

  2. #512
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    Quote Originally Posted by Duane
    I think most of them are incorporated into plate tectonics, not supplanted by the theory. Semantics maybe, so to elaborate,
    No, there were many precursors to plate tectonics. Even the expanding earth model qualifies.
    ABD? Not sure what you mean by this.
    All But Done, or All But Dissertation.
    Except that no such area of spontanious subduction is seen that I am aware of.
    Subduction doesn't occur overnight!
    Furthermore, if an area begins to subduct because of convergence, it means that convectional forces have changed their flow to make an area where there was not convergence begin to converge. I think you are confusing cause with result.
    I'm not. I'm not even advocating that particular theory. It's a perfectly reasonable one though.

    Are you familiar with the old "slab pull" versus "ridge push" debate?

    [fixed quote passages, or tried to ]
    Last edited by hhEb09'1; 2006-Feb-08 at 09:19 PM.

  3. #513
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    Quote Originally Posted by hhEb09'1
    No, there were many precursors to plate tectonics. Even the expanding earth model qualifies.
    Well, ok (hesitantly). My point was (and is) Tectonic Theory is a disipline in geoology/geophysics now, and has successfully incorporated most (all?) precursor theories under the broad umbrella. The theory, as a whole, quite successfully explains the geologic features we see all around us.

    Quote Originally Posted by hhEb09'1
    All But Done, or All But Dissertation.
    Nope. still lost on this. It's not important to the jist of our discussions but I just hate misunderstanding.

    So, IOW, you said: I guess I'm interested--ABD in geophysics meaning, I am interested all but {done}{dissertation} in geophysics. Maybe I'm losing it, but I don't understand quite what you intend here.

    Quote Originally Posted by hhEb09'1
    (Subduction doesn't occur overnight!
    Certainly not, which lends more credibility to my argument. If it is the case that older, hydrated oceanic lithosphere can sink without being subjected to a convergent plate zone, why is it that we do not see any partially sunk, relatively newly formed subduction zones?

    The closest parallel I can see is the drawn down area of the Australian plate, where a recently cut-off subduction slab is exerting gravitational pull on the plate. This is causing it to sink and be overrun by seawater, much as the NA plate was during the bulk of the Mesozoic era. Even that immense pull is not enough to cause the plate to start subducting in the middle, rather it simply over-rides the subduction zone and rebounds once it is past the influence of that gravity well.

    Quote Originally Posted by hhEb09'1
    I'm not. I'm not even advocating that particular theory. It's a perfectly reasonable one though.
    Lost me again (perhaps I'm a bit obtuse!) (shuddup Josh!!)

    Quote Originally Posted by hhEb09'1
    Are you familiar with the old "slab pull" versus "ridge push" debate?
    Absolutely. Both were wrong. The plates are not dependant on pull (as in lithoshere being "pulled" into the mantle) nor push (as in the plates being "pushed" apart by upwelling mantle.) These are terms used in the very early studies of plate tectonics (well, very early is a relative term--the mid to late 70's). P&S wave studies have dispelled alot of the misconceptions surrounding the cause of plate movement, especially as it relates to overlying plates riding on convection cells within the mantle.

    I am going to try to draw a simple mind picture. As one plate rides over another, the deflection angle of the subducting edge of the over-ridden plate causes it to be pushed into the deep mantle. The slab will continue to follow the deflection angle until it melts or is cut off by the plate over-riding the subduction trench. This is a process which takes hundreds of millions of years, because plate movement occurs at a rate of only a few centimetres per year. It is so slow, it is essentially imperceptable to us humans. Why would there be need for an expanding mass in this scenerio?

    Now, hh, I know that you are not the main (or frankly even a clear)proponant of this EE idea, but help me out a bit. I have asked a number of clear, direct questions relating to subducted paleoslabs, continental uplift, gravity field measurements etc, none of which have been addressed by the main proponants of this "theory". Dennis has run away, and even EEmann is now lurking a bit. Can you postulate an answer?

    I will give the specific questions:

    1.) The seismic waves studies undertaken of the mantle under the NA plate have identified a series of paleoslabs that have been (apparently) cutoff as the NA plate over-ran them. How can EE theory explain the existance of these paleoslabs?

    2).) Evidence exists which clearly indicates there was a substancial inland sea covering the NA continent during the Mesozoic era (and somewhat beyond) which seems to have clearly arisen from the gravitational influence of the overrun subducted paleoslab now seen to exist in seismic studies approximately 450 KM east of the eastern NA plate. How does EE explain this?

    3.) The Australian plate is currently under the gravitational influence of a similar paleoslab to that in 2, resulting on the area now seen in the indochina region to be slumped, and thus under water. Precise gravitation field measurements undertaken by the GRACE spacecraft confirm the field strength is higher in this region, as would be expected by the presence of an over-riden subduction zone. How does EE account for this?

    4.) Seismic wave studies have identified structure to a few metres down to the D" (pronounced dee double prime) zone at the core/mantle boundary some 2800km below the lithoshere. There is no evidence of "tubes" or "vents" or anything else that would allow some imaginary "eother" or whatever to flow to the surface. What then powers the "expansion"?

    I invite EEMann, John Kierien, and especially Dennis McCarthy to provide specific answers. En garde!

  4. #514
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    Quote Originally Posted by Duane
    Well, ok (hesitantly). My point was (and is) Tectonic Theory is a disipline in geoology/geophysics now, and has successfully incorporated most (all?) precursor theories under the broad umbrella.
    I think tectonic theory always has been a discipline in geology/geophysics, even before the advent of plate tectonics. I just looked at a couple online university catalogs, and their courses on tectonic theory include a review of previous theories, as well as current plate tectonic theory.
    Certainly not, which lends more credibility to my argument. If it is the case that older, hydrated oceanic lithosphere can sink without being subjected to a convergent plate zone, why is it that we do not see any partially sunk, relatively newly formed subduction zones?
    The current general plate orientation has been in place for quite a while, the opening of the Atlantic etc. As I said, some people expect it at any moment (meaning, within the next ten million years )
    The closest parallel I can see is the drawn down area of the Australian plate, where a recently cut-off subduction slab is exerting gravitational pull on the plate. This is causing it to sink and be overrun by seawater, much as the NA plate was during the bulk of the Mesozoic era. Even that immense pull is not enough to cause the plate to start subducting in the middle, rather it simply over-rides the subduction zone and rebounds once it is past the influence of that gravity well.
    I don't think it works that way. First, the material below the plate is "pulled down" as well, or even more. Second, the middle of the plate is stronger than the edge.
    Now, hh, I know that you are not the main (or frankly even a clear)proponant of this EE idea, but help me out a bit.
    I am not a proponent at all. Just ask ExpErdMann
    I have asked a number of clear, direct questions relating to subducted paleoslabs, continental uplift, gravity field measurements etc, none of which have been addressed by the main proponants of this "theory". Dennis has run away, and even EEmann is now lurking a bit.

    :: snip ::

    I invite EEMann, John Kierien, and especially Dennis McCarthy to provide specific answers. En garde!
    respondez! or be banned

  5. #515
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    Quote Originally Posted by Duane
    Certainly not, which lends more credibility to my argument. If it is the case that older, hydrated oceanic lithosphere can sink without being subjected to a convergent plate zone, why is it that we do not see any partially sunk, relatively newly formed subduction zones?
    I'm inclined to side with you on this. Why should a section of oceanic crust sink on its own any more than a piece of continental crust? But if you deny hhEb09'1's point on this, then you still need to account for every single bit of older (eg, Precambrian) oceanic crust having run up against a plate boundary and been subducted. Looking at the southern oceans, this appears most unlikely.

    Absolutely. Both were wrong. The plates are not dependant on pull (as in lithoshere being "pulled" into the mantle) nor push (as in the plates being "pushed" apart by upwelling mantle.) These are terms used in the very early studies of plate tectonics (well, very early is a relative term--the mid to late 70's). P&S wave studies have dispelled alot of the misconceptions surrounding the cause of plate movement, especially as it relates to overlying plates riding on convection cells within the mantle.
    This doesn't jive with my reading on the topic. I think ridge push has been discounted, so they're all going for slab pull.

    I am going to try to draw a simple mind picture. As one plate rides over another, the deflection angle of the subducting edge of the over-ridden plate causes it to be pushed into the deep mantle. The slab will continue to follow the deflection angle until it melts or is cut off by the plate over-riding the subduction trench. This is a process which takes hundreds of millions of years, because plate movement occurs at a rate of only a few centimetres per year. It is so slow, it is essentially imperceptable to us humans. Why would there be need for an expanding mass in this scenerio?
    Well, this sounds a lot like slab pull.

    Now, hh, I know that you are not the main (or frankly even a clear)proponant of this EE idea, but help me out a bit. I have asked a number of clear, direct questions relating to subducted paleoslabs, continental uplift, gravity field measurements etc, none of which have been addressed by the main proponants of this "theory". Dennis has run away, and even EEmann is now lurking a bit. Can you postulate an answer?
    Sorry, can't always participate as much as I would like.

    I will give the specific questions:

    1.) The seismic waves studies undertaken of the mantle under the NA plate have identified a series of paleoslabs that have been (apparently) cutoff as the NA plate over-ran them. How can EE theory explain the existance of these paleoslabs?
    Not all EE theories deny subduction. Slow EE actually requires subduction. Even fast EE can be compatible with subduction, but most fast EEers would say no, because the strongest argument for fast EE is that all the ocean crust we see is all the ocean crust there ever was.

    2).) Evidence exists which clearly indicates there was a substancial inland sea covering the NA continent during the Mesozoic era (and somewhat beyond) which seems to have clearly arisen from the gravitational influence of the overrun subducted paleoslab now seen to exist in seismic studies approximately 450 KM east of the eastern NA plate. How does EE explain this?
    I think you (or someone) has gone way too far in this idea. Think of the subduction below Chile. Is that descending slab pulling Chile down (by gravity) or pushing it up. (Hope everyone voted for the latter).

    3.) The Australian plate is currently under the gravitational influence of a similar paleoslab to that in 2, resulting on the area now seen in the indochina region to be slumped, and thus under water. Precise gravitation field measurements undertaken by the GRACE spacecraft confirm the field strength is higher in this region, as would be expected by the presence of an over-riden subduction zone. How does EE account for this?
    Same answer.

    4.) Seismic wave studies have identified structure to a few metres down to the D" (pronounced dee double prime) zone at the core/mantle boundary some 2800km below the lithoshere. There is no evidence of "tubes" or "vents" or anything else that would allow some imaginary "eother" or whatever to flow to the surface. What then powers the "expansion"?
    This is a mishmash of ideas. My suggestion about a network of magma tubes going down to the core was just an extension of Meyerhoff's idea in Surge Tectonics. How this would appear on a seismogram is beyond me to say. Nothing to do with ether though. As for what powers expansion, that's of course the $1 million question. I think I have a part of the answer and may start a thread on it some time. (And again, I point out that the power source for PT is unknown, but generally assumed to be residual heat or radioactivity).

    I invite EEMann, John Kierien, and especially Dennis McCarthy to provide specific answers. En garde!
    Acec plaisir! I wonder if I can make a few comments though. There is a history to this whole business as hhEb09'1 points out. EE was a part of it. Are you aware that Bruce Heezen, Samual Carey, Tuzo Wilson and others were all to various degrees advocates at one time or another for expansion? I'm reading an interesting book at the moment Ocean of Truth by one of the PT pioneers Menard. You can see that EE was taken seriously. The mechanism is still to be defined and there are other problems (such as the slowdown of Earth's rotation), but there is still a lot to like in EE. And now we're seeing signs of expansion on Enceladus and other moons.

  6. #516
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    Quote Originally Posted by hhEb09'1
    I am not a proponent at all. Just ask ExpErdMann
    But you once said you had nothing against EE. (I should have bookmarked that one!)

  7. #517
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    Quote Originally Posted by ExpErdMann
    I'm inclined to side with you on this. Why should a section of oceanic crust sink on its own any more than a piece of continental crust?
    Because continental crust is lighter than the material beneath it, whereas oceanic crust is heavier (after a short ten million years according to that link I provided earlier)?

    Obviously the situation is complicated, since ocean crust greater than ten million years old is still around, but such crust has a certain amount of strength and connection to other crust.
    Quote Originally Posted by ExpErdMann
    But you once said you had nothing against EE. (I should have bookmarked that one!)
    And context is key.

    But I repeat, I am not a proponent of expanding earth theories.

  8. #518
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    Quote Originally Posted by hhEb09'1
    Because continental crust is lighter than the material beneath it, whereas oceanic crust is heavier (after a short ten million years according to that link I provided earlier)?
    The link from here?: "It's not unreasonable--I googled and this isn't the best site but it mentions that oceanic crust 10 million years old has cooled to the point where it is denser than the material beneath it." It didn't work.

    Obviously the situation is complicated, since ocean crust greater than ten million years old is still around, but such crust has a certain amount of strength and connection to other crust.
    Well, if the main heat loss from the slab is in the first 10 million years, say, then do you expect the additional loss after that to do the trick? I suppose you must, or else lots of Pacific crust not encountering a plate boundary would have sunk by now.

    And context is key.
    Maybe it was "be kind to EE day".

    But I repeat, I am not a proponent of expanding earth theories.
    Why not?

    (hey just kidding - I know your position on this from about page 1! )
    Last edited by ExpErdMann; 2006-Feb-04 at 03:17 PM.

  9. #519
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    Quote Originally Posted by ExpErdMann
    *snip* But if you deny hhEb09'1's point on this, then you still need to account for every single bit of older (eg, Precambrian) oceanic crust having run up against a plate boundary and been subducted. Looking at the southern oceans, this appears most unlikely.
    I'm sorry, I don't understand what you are trying to say here. (I've read it a half dozen times over the last few days, and it still doesn't make sense to me). Could you reword the question please?


    Quote Originally Posted by ExpErdMann
    This doesn't jive with my reading on the topic. I think ridge push has been discounted, so they're all going for slab pull.
    Well, ok, to a point. It is true that the higher density oceanic subducted slabs are going to be affected more by gravity, thereby creating some amount of pull on the slabs. This also seems to correlate with areas where the longest subduction zones are also the areas where the lithosphere is moving the fastest. (See here--very technical 16 page pdf) The point is, however, that slab pull occurs after subduction has begun, and is therefore not the originating cause of subduction. Rather, boundary deflection begins the process, and once enough lithosphere has subducted, gravitational influence on the subducted slab might cause the slab to subduct faster, thereby pulling the attached non-subducted lithosphere along with it.

    It might also be that the softened, plasticized lithosphere merely stretches under the gravitational force. This is an issue that will be resolved, with time. One of the problems with the study of the subducted plates is that the process is almost imperceptively slow.

    Quote Originally Posted by ExpErdMann
    Well, this sounds a lot like slab pull.
    Then you are not reading it very carefully. While the similarities are many, the process I refer to is boundary defection--that is, the over-riding plate defelects the over-ridden plate into the mantle. No need for a gravity assist!


    Quote Originally Posted by ExpErdMann
    Sorry, can't always participate as much as I would like.
    No issue! I can't always either.


    Quote Originally Posted by ExpErdMann
    Not all EE theories deny subduction. Slow EE actually requires subduction. Even fast EE can be compatible with subduction, but most fast EEers would say no, because the strongest argument for fast EE is that all the ocean crust we see is all the ocean crust there ever was.
    The problem with subduction in the slow EE models is that the subducted slabs seen in seismic wave studies extend all the way down to the core/mantle boundary. (caveat--not all of them!) Remembering that the mantle is some 2800km thick, and that the movement of the slabs is a few centimetres per year, a simple math calculation shows that the slabs have been descending for hundreds of millions or even billions of years. Further, EE cannot (or maybe has not) accounted for the cut-off paleo slabs seen in several areas around the planet.

    The simple answer to the paleoslabs is that subduction zones were cut off when an overriding plate overrode the subduction zone. This is seen today, where the NA plate is moving to cut off the Cascadian subduction zone, an event that will occur is a few million years. (A blink in geologic time)

    Quote Originally Posted by ExpErdMann
    I think you (or someone) has gone way too far in this idea. Think of the subduction below Chile. Is that descending slab pulling Chile down (by gravity) or pushing it up. (Hope everyone voted for the latter).
    This is a different beast. The SA plate is rising because it is an area of convergence between an oceanic and continental plate. The continental plate is being pushed up by slab-defection. (well, this is very simplified, there are a number of processes at work) Once the SA plate over-rides the Nazca plate and cuts off the subduction zone, the over-ridden slab will exert a gravitation pull on the SA plate. (the Australian plate is already under the influence of such a pull--source.)

    That the NA continent was once split by an inland sea is uncontroversial. (See here, for eg) The cause of this sea is downwelling resultant from the gravitational pull of over-ridden lithospheric slabs. (A good review is here.)

    The fact that paleoslabs are identified in seismic wave studies is a given. What I have not seen from you (or any other EE proponant) is an explanation for how these slabs came to be, using the "theory" of EE.

    Quote Originally Posted by ExpErdMann
    Same answer.
    Same question.


    Quote Originally Posted by ExpErdMann
    This is a mishmash of ideas. My suggestion about a network of magma tubes going down to the core was just an extension of Meyerhoff's idea in Surge Tectonics. How this would appear on a seismogram is beyond me to say. Nothing to do with ether though. As for what powers expansion, that's of course the $1 million question. I think I have a part of the answer and may start a thread on it some time. (And again, I point out that the power source for PT is unknown, but generally assumed to be residual heat or radioactivity).
    Generally accepted. It is not one or the other either, it is a combination of residual heat and ongoing radioactivity.


    Quote Originally Posted by ExpErdMann
    Acec plaisir! I wonder if I can make a few comments though. There is a history to this whole business as hhEb09'1 points out. EE was a part of it. Are you aware that Bruce Heezen, Samual Carey, Tuzo Wilson and others were all to various degrees advocates at one time or another for expansion? I'm reading an interesting book at the moment Ocean of Truth by one of the PT pioneers Menard. You can see that EE was taken seriously. The mechanism is still to be defined and there are other problems (such as the slowdown of Earth's rotation), but there is still a lot to like in EE. And now we're seeing signs of expansion on Enceladus and other moons.
    Avec plaisir I think you mean! Yes, in the past EE theory was proposed as a possible mechanism. On the whole, it was abandoned because it does not:
    1.) match observation
    2.) meet predictions
    3.) have an explainable mechanism

  10. #520
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    Quote Originally Posted by ExpErdMann
    The link from here?: "It's not unreasonable--I googled and this isn't the best site but it mentions that oceanic crust 10 million years old has cooled to the point where it is denser than the material beneath it." It didn't work.
    Yes, that was it. And it did seem to not work for a while, maybe we hammered it
    Well, if the main heat loss from the slab is in the first 10 million years, say, then do you expect the additional loss after that to do the trick? I suppose you must, or else lots of Pacific crust not encountering a plate boundary would have sunk by now.
    Plates are defined by their edges. If the Atlantic oceanic crust were to sink, then that oceanic crust would be on a separate plate from the continental crust next to it.

    But I'm not sure I understand your comment about the additional heat loss? Could you expand (heh) on it?
    Quote Originally Posted by Duane
    This is a different beast. The SA plate is rising because it is an area of convergence between an oceanic and continental plate. The continental plate is being pushed up by slab-defection. (well, this is very simplified, there are a number of processes at work) Once the SA plate over-rides the Nazca plate and cuts off the subduction zone, the over-ridden slab will exert a gravitation pull on the SA plate. (the Australian plate is already under the influence of such a pull--source.)

    That the NA continent was once split by an inland sea is uncontroversial. (See here, for eg) The cause of this sea is downwelling resultant from the gravitational pull of over-ridden lithospheric slabs. (A good review is here.)
    I'm interested in this, but I'm a little dubious. The wikipedia article you cite says it was a result of high eustatic sea levels. The last link is a series of abstracts, only one of which seems to be concerned with downwelling (Downwelling Along the Western Edges of the Great Plains: Implication for Small-scale Convection) but it doesn't say much about over-ridden slabs. The other link, the Sci.Am. article by Michael Gurnis, I'll have to spend a little more time on. Which part of it are you referring to?

  11. #521
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    Quote Originally Posted by hhEb09'1
    I'm interested in this, but I'm a little dubious. The wikipedia article you cite says it was a result of high eustatic sea levels.
    I was only making the point of the inland sea. High water levels may certainly have contributed to the sea, but it is now relatively certain that downwelling was a major factor. (PS the article also says there is no modern equivalent, ignoring the Indonesian downwelling zone. It is only Wikipedia after all.... )

    Quote Originally Posted by hhEb09'1
    The last link is a series of abstracts, only one of which seems to be concerned with downwelling (Downwelling Along the Western Edges of the Great Plains: Implication for Small-scale Convection) but it doesn't say much about over-ridden slabs. The other link, the Sci.Am. article by Michael Gurnis, I'll have to spend a little more time on. Which part of it are you referring to?
    The first link is a whole series of discussions regarding tectonic activity throughout the western NA plate. It is an example of the type of measurements and findings that are coming out of seismic and other geologic studies. Most of the articles reference subducted and/or melted oceanic slabs. Long reading, but well worth it.

    Gurnis's article discusses a whole range of upwelling and downwelling events around the globe. It is an excellent review of our current understanding, so the whole article is what I refer to.

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    Now, after all my yapping about slab pull, I come across a paper that uses it in combination with shear traction to cause the plate movements.

    How Mantle Slabs Drive Plate Tectonics Clinton P. Conrad* and Carolina Lithgow-Bertelloni (Science Magazine Oct 4, 2002).

    This does seem to help explain the movement of smaller plates (Caribbean for eg) and provides the right velocities to non-subducting plates, something slab pull could not do on it's own.

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    Quote Originally Posted by Duane
    I'm sorry, I don't understand what you are trying to say here. (I've read it a half dozen times over the last few days, and it still doesn't make sense to me). Could you reword the question please?
    PT requires that the lithospheric ocean plates move around at high velocities and are eventually subducted. Given that the stresses of plates act in various directions corresponding to the plate motions, it just seems odd that somewhere out in the Pacific or Indian Oceans some pieces of old ocean crust didn't become detached and left unsubducted. PT requires a perfect sweep of the ocean floor every 200 Ma. Is nature that perfect?

    Now, having said that, I should add that the same problem affects slow EE, or even Owen's intermediate model (Earth was 80 per cent of present size when Pangea breakup began), since considerable erasure of ocean crust also occurs in these cases. On this point fast EE has the advantage.

    Well, ok, to a point. It is true that the higher density oceanic subducted slabs are going to be affected more by gravity, thereby creating some amount of pull on the slabs. This also seems to correlate with areas where the longest subduction zones are also the areas where the lithosphere is moving the fastest. (See here--very technical 16 page pdf) The point is, however, that slab pull occurs after subduction has begun, and is therefore not the originating cause of subduction. Rather, boundary deflection begins the process, and once enough lithosphere has subducted, gravitational influence on the subducted slab might cause the slab to subduct faster, thereby pulling the attached non-subducted lithosphere along with it.

    It might also be that the softened, plasticized lithosphere merely stretches under the gravitational force. This is an issue that will be resolved, with time. One of the problems with the study of the subducted plates is that the process is almost imperceptively slow.
    If you think of plates many miles thick bumping into each other edge-on, it seems unlikely that deflection alone could do the trick. However, if you suppose that the oceanic crust starts to 'pile up' at the continental margin, since it has nowhere to go, the weight of that piled up crust will eventually cause the crust to subside along that margin. This would match with the general subsidence in the Western Pacific (guyots, etc). I'll check your link though.


    Then you are not reading it very carefully. While the similarities are many, the process I refer to is boundary defection--that is, the over-riding plate defelects the over-ridden plate into the mantle. No need for a gravity assist!
    With the type of subsidence I described above, it's perhaps more explicable why a surface oceanic crust could be dragged down gravitationally. It's just isostasy at work.

    The problem with subduction in the slow EE models is that the subducted slabs seen in seismic wave studies extend all the way down to the core/mantle boundary. (caveat--not all of them!) Remembering that the mantle is some 2800km thick, and that the movement of the slabs is a few centimetres per year, a simple math calculation shows that the slabs have been descending for hundreds of millions or even billions of years. Further, EE cannot (or maybe has not) accounted for the cut-off paleo slabs seen in several areas around the planet.

    The simple answer to the paleoslabs is that subduction zones were cut off when an overriding plate overrode the subduction zone. This is seen today, where the NA plate is moving to cut off the Cascadian subduction zone, an event that will occur is a few million years. (A blink in geologic time)
    Keep in mind that, just because expansion is slow (.5 mm/yr or so) in slow EE, that doesn't mean that the crustal motions can't be as large as in PT. The expansion is just a minor background feature in slow EE. Except that it would be that expansion which is ultimately driving the plate motions. (see for example Weijermars' model)

    This is a different beast. The SA plate is rising because it is an area of convergence between an oceanic and continental plate. The continental plate is being pushed up by slab-defection. (well, this is very simplified, there are a number of processes at work) Once the SA plate over-rides the Nazca plate and cuts off the subduction zone, the over-ridden slab will exert a gravitation pull on the SA plate. (the Australian plate is already under the influence of such a pull--source.)

    That the NA continent was once split by an inland sea is uncontroversial. (See here, for eg) The cause of this sea is downwelling resultant from the gravitational pull of over-ridden lithospheric slabs. (A good review is here.)
    Well, I'll have a look, but it's not clear why the slabs should exert a greater downward pull than the mantle that was there before. It all has to be isostatic.

    The fact that paleoslabs are identified in seismic wave studies is a given. What I have not seen from you (or any other EE proponant) is an explanation for how these slabs came to be, using the "theory" of EE.
    I've mentioned that slow EE can accommodate this. But I do have some reservations about this data. As mentioned in earlier posts, expansion in the mantle below oceans would cause a surface swell that would spread out laterally (but slowly). When it encounters the continents, subduction could arise, but it would really be a large section of the mantle being subducted as well, not just the crust. I wonder if this tomographic data is so precise that a 'slab' can be truly discerned, or is the slab-like nature merely presumed to fit theory?

    Avec plaisir I think you mean!
    Yes, my only attempt at French and a typo!

    Yes, in the past EE theory was proposed as a possible mechanism. On the whole, it was abandoned because it does not:
    1.) match observation
    2.) meet predictions
    3.) have an explainable mechanism
    In a separate post I'll try to assemble a new proposal which addresses all this.

  14. #524
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    Quote Originally Posted by ExpErdMann
    PT requires that the lithospheric ocean plates move around at high velocities and are eventually subducted. Given that the stresses of plates act in various directions corresponding to the plate motions, it just seems odd that somewhere out in the Pacific or Indian Oceans some pieces of old ocean crust didn't become detached and left unsubducted. PT requires a perfect sweep of the ocean floor every 200 Ma. Is nature that perfect?
    Not true! There are many areas where island arcs have been swept up and incorporated into the continents (source) and other areas where ancient seafloor (ie oceanic lithosphere) has been raised and incorporated as well. (source) Further, the subducting slabs have built up sediment scraped off during the subduction process, leading to accretionary wedges, which also add to the boundaries of the continents. (source) The NA continent show significant accretion wedge sedimental material in the eastern area of the mountains, for eg. (source, for eg)

    You have also noted the 200 million year age of the oldest seafloor, ignoring the fact (as agreed to by both PT and EE proponants) that most of the ocean floor except the Pacific basin has arisen since the breakup of the last super-continent.

    Furthermore, it is always oceanic crust that decouples, then descends into the mantle. (source)

    Quote Originally Posted by ExpErdMann
    Now, having said that, I should add that the same problem affects slow EE, or even Owen's intermediate model (Earth was 80 per cent of present size when Pangea breakup began), since considerable erasure of ocean crust also occurs in these cases. On this point fast EE has the advantage.
    Au contraire mon amie! Fast EE, especially, has a very tough row to hoe in attemting to explain the various processes I noted above, and most especially the sedimentary wedge accumulation.



    Quote Originally Posted by ExpErdMann
    If you think of plates many miles thick bumping into each other edge-on, it seems unlikely that deflection alone could do the trick. However, if you suppose that the oceanic crust starts to 'pile up' at the continental margin, since it has nowhere to go, the weight of that piled up crust will eventually cause the crust to subside along that margin. This would match with the general subsidence in the Western Pacific (guyots, etc). I'll check your link though.
    Well, you have a couple of errors here. First "many miles" might work for the continental crust, at least at the roots, but it does not work for oceanic crust. On average, oceanic crust is only 7 or so miles thick (source) much thinner than the continental crust. It doesn't "build up" it strikes or is struck by other crust and deflects downwards.

    Guyots are associated with hotspots and ridge activity. I don't understand how you associate these with "piled up crust" causing subsidence.


    Quote Originally Posted by ExpErdMann
    With the type of subsidence I described above, it's perhaps more explicable why a surface oceanic crust could be dragged down gravitationally. It's just isostasy at work.
    Hmm....

    Quote Originally Posted by ExpErdMann
    Keep in mind that, just because expansion is slow (.5 mm/yr or so) in slow EE, that doesn't mean that the crustal motions can't be as large as in PT. The expansion is just a minor background feature in slow EE. Except that it would be that expansion which is ultimately driving the plate motions. (see for example Weijermars' model)
    How? If slow exapnsion is occurring (a BIG if IMHO) how does it fit against the known movements of all of the plates, and especially slip-strike and microplate zones? (PS--this is a direct question!)

    Quote Originally Posted by ExpErdMann
    Well, I'll have a look, but it's not clear why the slabs should exert a greater downward pull than the mantle that was there before. It all has to be isostatic.
    Because the subducted plates are quite dense, partially hydrated (to ~600 miles or so) and significantly heavier per m^3 than the material making up the mantle they are passing through.

    Quote Originally Posted by ExpErdMann
    I've mentioned that slow EE can accommodate this. But I do have some reservations about this data. As mentioned in earlier posts, expansion in the mantle below oceans would cause a surface swell that would spread out laterally (but slowly). When it encounters the continents, subduction could arise, but it would really be a large section of the mantle being subducted as well, not just the crust. I wonder if this tomographic data is so precise that a 'slab' can be truly discerned, or is the slab-like nature merely presumed to fit theory?
    Well, a simple google search will give you alot of data to answer this question. The short answer is absolutely, well discerned subducted slabs are seen, extending from the surface right through to the C/M boundary.

    Quote Originally Posted by ExpErdMann
    Yes, my only attempt at French and a typo!
    Heh!


    Quote Originally Posted by ExpErdMann
    In a separate post I'll try to assemble a new proposal which addresses all this.
    I look forward to it.

  15. #525
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    Quote Originally Posted by hhEb09'1
    Plates are defined by their edges. If the Atlantic oceanic crust were to sink, then that oceanic crust would be on a separate plate from the continental crust next to it.

    But I'm not sure I understand your comment about the additional heat loss? Could you expand (heh) on it?
    It's just that the link between cooling and subsiding is so ad hoc. In general, the heat emission is quite similar between oceanic and continental crust anyway, and also between different oceanic plates. True there is some extra heat over the spreading ridges, but it seems insufficient to be a driving force. Also, in the paper Duane cited "How Mantle Slabs Drive Plate Tectonics", note that cooling of slabs doesn't seem to enter the picture. Oddly, one reads there of yet another slab force "slab suction", i.e., detached slabs can cause a sort of viscous traction that pulls plates along. Imaginative but real?

  16. #526
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    Quote Originally Posted by Duane
    Not true! There are many areas where island arcs have been swept up and incorporated into the continents (source) and other areas where ancient seafloor (ie oceanic lithosphere) has been raised and incorporated as well. (source) Further, the subducting slabs have built up sediment scraped off during the subduction process, leading to accretionary wedges, which also add to the boundaries of the continents. (source) The NA continent show significant accretion wedge sedimental material in the eastern area of the mountains, for eg. (source, for eg)
    True, some oceanic crust was added onto the continental margins. But this is a very tiny fraction of all the ocean floor that has been recycled every 200 Ma or so according to PT. The growth of continents since the early Proterozoic is thought to be less than 20 per cent of the present continental area. That's oceans of crust that have been subducted without a trace left in the basins themselves. According to Glikson (Tectonophysics 63, 193-234, 1980), this is in part because during the whole period of 2.5 - 1.0 Ga there was no appreciable seafloor spreading. I'll return to this below.

    <>

    Well, you have a couple of errors here. First "many miles" might work for the continental crust, at least at the roots, but it does not work for oceanic crust. On average, oceanic crust is only 7 or so miles thick (source) much thinner than the continental crust. It doesn't "build up" it strikes or is struck by other crust and deflects downwards.

    Guyots are associated with hotspots and ridge activity. I don't understand how you associate these with "piled up crust" causing subsidence.
    I left out something important. The eruption of flood basalts on the ocean floor over time can gradually cause subsidence. As one moves away from the mid-ocean ridges, the combined weight of the volcanic eruptions increases. This at least what some people thought when trying to explain the apparent subsidence in the western Pacific.

    How? If slow exapnsion is occurring (a BIG if IMHO) how does it fit against the known movements of all of the plates, and especially slip-strike and microplate zones? (PS--this is a direct question!)
    You're still not getting the main point that the expansion effect can be small compared to the plate motions (eg, Weijermars' model). In other words, in slow EE you could, if you wanted, just tack on the expansion on the typical PT framework.


    Well, a simple google search will give you alot of data to answer this question. The short answer is absolutely, well discerned subducted slabs are seen, extending from the surface right through to the C/M boundary.
    Google always cuts both ways!


    I look forward to it.
    Here it is in a nutshell. I'm always improvising, so pay no mind if I back off of this later. Let's take Glikson as being correct that there was no significant seafloor spreading in the period between 2.5 - 1.0 Ga. For simplicity let's also assume that not much expansion happened in the Archaean. That means the expansion has happened since 1.0 Ga. This is the time of Rodinia. Now note that in Rodinia configurations, Australia and Antarctica are placed right up against North America. In EE we could take this as evidence that the Pacific was not there yet. This could also partly connect to Dennis's model. The Pacific then opened up due to expansion in the late Proterozoic. Let's suppose that expansion was evenly spaced from 1.0 Ga to present. That would correspond to a radial growth of 200 km/100 Ma or 2 mm/yr. That's about 4 times faster than Weijermars' slow EE model. At the same time this rate is only 1/5 or so of Carey's fast expansion rate. Now Weijermars criticized fast EE because it conflicts with Earth's rotational history. Owen's model was also in conflict (Owen proposed that Pangea broke up when the globe was 80 per cent of present). Slow EE however is not in conflict. I've looked at the numbers a bit and my intermediate model looks like it can squeak through. We could incorporate various PT aspects in it, but the main driving force is expansion. I haven't included an energy source, but it would be similar in magnitude to what is driving PT. I'll expand later, but this may do for a start.

  17. #527
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    Quote Originally Posted by Duane
    The first link is a whole series of discussions regarding tectonic activity throughout the western NA plate. It is an example of the type of measurements and findings that are coming out of seismic and other geologic studies. Most of the articles reference subducted and/or melted oceanic slabs. Long reading, but well worth it.
    Still, none of them address the issue, so I'll put them aside for when I need a late night soporific.
    Gurnis's article discusses a whole range of upwelling and downwelling events around the globe. It is an excellent review of our current understanding, so the whole article is what I refer to.
    Gurnis's article does not describe an increased gravitational pull of the cold descending slabs--it's more of a viscous pull.

    Brad Hager's work is featured prominently in the article.
    Quote Originally Posted by ExpErdMann
    It's just that the link between cooling and subsiding is so ad hoc.
    Why ad hoc? Things get denser as they cool, dense things sink. Doesn't get more basic than that--that pretty much describes convection in a nutshell.

  18. #528
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    Cooling slabs

    Well, maybe I was too hasty. I kind of like the description in this abstract by Hamilton. He also mentions that the angles of slabs can't be taken as trajectories and that they don't descend below 660 km. I think that cooling can't be the whole story though. What about the addition of volcanic material as in flood basalts? That would also increase the weight of slabs over time.

    In the EE picture, of course, we're talking about more than just the lithospheric slabs. A better picture is the upwelling of mantle leads to a bulge in the oceans which spreads laterally under gravity and pushes up the continents at their edges.

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