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Thread: Mantle convection -- top down or bottom up?

  1. #1 Mantle convection -- top down or bottom up? 
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    What's more important cooling at the top or heating at the bottom? Plates and/or plumes? Single or multi-layered mantle convection?

    I confess to having a special interest in the deep Earth.

    Let's fire up the old plume debate!


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    In my opinion, the data is still best explained by the plume paradigm. While the plate theory works in theory, it seems a little contrived, and plumes offer a much simpler solution to the problem.


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    Ay yes I agree. Plumes seem to really exist, for sure.

    The story is complex though. What is the nature of plumes? Do they originate at the core mantle boundary? Do they reach the surface? To what extent are they responsible for Earth's heat loss and how do they influence the Earth's dynamics?

    These questions are largely unknown, but there has been some interesting recent work that has made it into the "news".

    e.g.

    Hawaii plume originates at CMB but does not penetrate all the way up into the upper mantle. Discontinuity under the Aloha State | Singularities - Physics Today
    (as a Hawaiian side note: why is there that kink in the Hawaii Emperor volcanic chain?)

    Whereas in another study the authors discuss a plume that did seemingly reach the surface (and gave us the Deccan Traps), and may have even played an active role in the tectonics, pushing the Indian plate towards Asia and slowing down Africa.
    Mantle plume propelled India towards Asia : Nature News

    In contrast Don Anderson is a great proponent of the "top-down tectonics", which deems that plate tectonics is driven by causes at the surface, and doesn't care about the deep Earth. http://www.mantleplumes.org/WebDocuments/TopDown.pdf
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    Quote Originally Posted by billiards View Post
    Hawaii plume originates at CMB but does not penetrate all the way up into the upper mantle. Discontinuity under the Aloha State | Singularities - Physics Today
    The Hawaiian plume is a unique phenomenon. Other intraplate 'hotspots' are often not particularly hot, and have distinctly different characteristics, whereas the Hawaiian volcanism is certainly correlatable to the deep-mantle plume it sits above. While the plume doesn't penetrate into the upper mantle, it's termination will inevitably lead to convection cells above it, leading to the uplift and volcanism we see. Also, note that the plume apparantly terminates in the asthenosphere, where the lower viscosity will probably make things more chaotic.

    Quote Originally Posted by billiards
    (as a Hawaiian side note: why is there that kink in the Hawaii Emperor volcanic chain?)
    It's just due to a change in plate motion.

    Quote Originally Posted by billiards
    Whereas in another study the authors discuss a plume that did seemingly reach the surface (and gave us the Deccan Traps), and may have even played an active role in the tectonics, pushing the Indian plate towards Asia and slowing down Africa.
    Mantle plume propelled India towards Asia : Nature News
    It's not that the Hawaiian plume doesn't affect the surface; it clearly does. I think it's probably more that where it reaches the upper mantle, it behaves less like a plume due to increased variation in temperature and pressure, and instead becomes a number of seperate convection cells. The hot material still reaches the surface, but ceases to act as a 'simple' plume.

    That's just my thought, anyway.

    Quote Originally Posted by billiards
    In contrast Don Anderson is a great proponent of the "top-down tectonics", which deems that plate tectonics is driven by causes at the surface, and doesn't care about the deep Earth. http://www.mantleplumes.org/WebDocuments/TopDown.pdf
    I think you're overstating it slightly. The Plate-driven view doesn't completely rule out plumes at all. For instance, I think I recall Gillian Foulger (whose website you referenced) stating that a top-down approach simply can't explain the Hawaiian hotspot, and possibly certain other examples in the stratigraphic record.

    Since you mentioned the Deccan Traps, it's worth noting that it is hypothesised that the associated volcanism was triggered by an asteroid impact - probably larger than the one creating the crater at chicxulub at around the same time. Study suggests dinosaurs killed off by more than one asteroid
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    Quote Originally Posted by billiards View Post
    In contrast Don Anderson is a great proponent of the "top-down tectonics", which deems that plate tectonics is driven by causes at the surface, and doesn't care about the deep Earth. http://www.mantleplumes.org/WebDocuments/TopDown.pdf
    He's not the only one. The orthodox via is that slab pull drives plate tectonics, i.e, top-down tectonics. Plumes are truly alien to plate tectonics, this explain why this plate vs plume debate exist. And Foulger argues that the plume model is flawed because there are as many type of plumes as plumes so that the plume model is not falsifiable.
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    Quote Originally Posted by florian View Post
    He's not the only one. The orthodox via is that slab pull drives plate tectonics, i.e, top-down tectonics. Plumes are truly alien to plate tectonics, this explain why this plate vs plume debate exist. And Foulger argues that the plume model is flawed because there are as many type of plumes as plumes so that the plume model is not falsifiable.
    Bull. Slab pull drives much of plate tectonics, but many other phenomena are at work, including plumes. Only a fool would believe that only one process describes everything.

    Oops, sorry, I forgot who I was speaking to ...
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    Quote Originally Posted by florian View Post
    Quote Originally Posted by billiards View Post
    In contrast Don Anderson is a great proponent of the "top-down tectonics", which deems that plate tectonics is driven by causes at the surface, and doesn't care about the deep Earth. http://www.mantleplumes.org/WebDocuments/TopDown.pdf
    He's not the only one. The orthodox via is that slab pull drives plate tectonics, i.e, top-down tectonics. Plumes are truly alien to plate tectonics, this explain why this plate vs plume debate exist. And Foulger argues that the plume model is flawed because there are as many type of plumes as plumes so that the plume model is not falsifiable.
    Removing plumes from the equation completely doesn't work, however. IIRC, the Hawaiian 'hotspot' is in an area of low tectonic stress, normal crustal/lithosphere thickness... There's no explanation for it's occurence, save for the deep mantle plume directly beneath. I doubt you could find any advocate of top-down tectonics who disagrees with this.
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    Quote Originally Posted by MeteorWayne View Post
    Bull. Slab pull drives much of plate tectonics, but many other phenomena are at work, including plumes. Only a fool would believe that only one process describes everything.

    Don't get me wrong. I certainly did not say that all phenomena are explained or must be explained in the context of plate tectonics. I remind the reader that the current orthodox view is that without subduction as a driving force, there is no plate tectonics.
    Here is a few quotes from Robert J Stern in a fairly recent review (Subduction initiation: spontaneous and induced Earth and Planetary Science Letters 226 (2004) 275–292 ):

    "The most important misconception is that mantle convection moves the lithosphere (see dJargon BoxT), dragging the plates as it moves. This is repeatedly shown in introductory textbooks. In fact, Earth’s mantle convects mostly because cold lithosphere sinks at subduction zones [3] with mantle plumes representing a d. . .clearly resolved but secondary mode of mantle convection"

    "There is a consensus among geodynamicists that the sinking of cold, gravitationally unstable litho- sphere drives the plates and indirectly causes mantle to well up beneath mid-ocean ridges. Some estimate that 90% of the force needed to drive the plates comes from the sinking of lithosphere in subduction zones, with another 10% coming from ridge push [11]. "


    I think it is very clear.


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    Quote Originally Posted by drowsy turtle View Post
    Removing plumes from the equation completely doesn't work, however. IIRC, the Hawaiian 'hotspot' is in an area of low tectonic stress, normal crustal/lithosphere thickness... There's no explanation for it's occurence, save for the deep mantle plume directly beneath. I doubt you could find any advocate of top-down tectonics who disagrees with this.
    I guess you read Gillian's book? Even for Hawaii, she doesn't really buy the plume model. She argues for extensional stress in the Pacific lithosphere as the origin of the magmatic activity.

    BTW, I evidently disagree with her view, because in alternative theories, diapirs and plumes are thermal or compositional or a mix, which explains the large variety of effects.
    Last edited by florian; July 25th, 2011 at 04:19 AM.
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    Quote Originally Posted by drowsy turtle View Post
    Also, note that the plume apparantly terminates in the asthenosphere, where the lower viscosity will probably make things more chaotic.
    I think you are saying that the plume kind of chaotically breaks up when it reaches the asthenosphere?
    In fact the plume seems to terminates at the base of the transition zone (some way away from the low velocity zone that defines the asthenosphere).

    Quote Originally Posted by drowsy turtle View Post
    It's not that the Hawaiian plume doesn't affect the surface; it clearly does. I think it's probably more that where it reaches the upper mantle, it behaves less like a plume due to increased variation in temperature and pressure, and instead becomes a number of seperate convection cells. The hot material still reaches the surface, but ceases to act as a 'simple' plume.

    That's just my thought, anyway.
    I think the idea is that no material penetrates through the transition zone into the Upper mantle, but the heat does, presumably by conduction. This heat then forms a new convection cell in the upper mantle. This would be layered convection, which means that the Hawaiian magmas are sourced from the Upper mantle reservoir.

    Quote Originally Posted by drowsy turtle View Post
    I think you're overstating it slightly. The Plate-driven view doesn't completely rule out plumes at all. For instance, I think I recall Gillian Foulger (whose website you referenced) stating that a top-down approach simply can't explain the Hawaiian hotspot, and possibly certain other examples in the stratigraphic record.
    Let's be careful to distinguish the debate here. I know it's quite convoluted and I probably haven't helped in my discussion so far, but there is the issue of whether plumes exist, and there is the issue of what drives mantle convection and plate tectonics. They are convoluted because if you are to say that mantle convection is driven by thermal upwellings in the form of plumes derived from the core-mantle boundary then you inherently assume that plumes DO exist. Whereas, if plate tectonics is driven by the negative buoyancy of subducting oceanic lithosphere, then plumes are not necessary, yet may still exist. Now, I don't believe I have ever stated that plumes are incompatible with the top-down view, although it just so happens that there has been a public debate as to the existence of plumes wherein some people have gone as far as to say that plumes do not exist, and those people have naturally been proponents of top-down tectonics.
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    Quote Originally Posted by billiards View Post
    I think the idea is that no material penetrates through the transition zone into the Upper mantle, but the heat does, presumably by conduction. This heat then forms a new convection cell in the upper mantle. This would be layered convection, which means that the Hawaiian magmas are sourced from the Upper mantle reservoir.
    This is what I meant, yes. I admit I could have been clearer.

    Quote Originally Posted by drowsy turtle View Post
    Let's be careful to distinguish the debate here. I know it's quite convoluted and I probably haven't helped in my discussion so far, but there is the issue of whether plumes exist, and there is the issue of what drives mantle convection and plate tectonics. They are convoluted because if you are to say that mantle convection is driven by thermal upwellings in the form of plumes derived from the core-mantle boundary then you inherently assume that plumes DO exist. Whereas, if plate tectonics is driven by the negative buoyancy of subducting oceanic lithosphere, then plumes are not necessary, yet may still exist. Now, I don't believe I have ever stated that plumes are incompatible with the top-down view, although it just so happens that there has been a public debate as to the existence of plumes wherein some people have gone as far as to say that plumes do not exist, and those people have naturally been proponents of top-down tectonics.
    I do find it difficult to see how one could advocate a top-down approach and at the same time aknowledge the existance of plumes in locations that can be matched to volcanism and intrusive bodies. Surely this would be, at best, a balanced view?
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    Quote Originally Posted by florian View Post
    Quote Originally Posted by drowsy turtle View Post
    Removing plumes from the equation completely doesn't work, however. IIRC, the Hawaiian 'hotspot' is in an area of low tectonic stress, normal crustal/lithosphere thickness... There's no explanation for it's occurence, save for the deep mantle plume directly beneath. I doubt you could find any advocate of top-down tectonics who disagrees with this.
    I guess you read Gillian's book? Even for Hawaii, she doesn't really buy the plume model. She argues for extensional stress in the Pacific lithosphere as the origin of the magmatic activity.
    I read some of her book; having borrowed it but not owned it. I was thinking of a lecture of hers I went to rather than her book, where she pretty much conceded that the plume model more easily explains the Hawaiian 'hotspot', but with the proviso that even in the plume model it's unique [which I suppose it is - although as you went on to say, every plume is really]. It's possible I'm remembering wrong, but that's how I remember it.
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    Quote Originally Posted by drowsy turtle View Post
    It's possible I'm remembering wrong, but that's how I remember it.
    Her book is fairly recent. May be she changed her mind between her lecture or the book writing?
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    Quote Originally Posted by florian View Post
    Quote Originally Posted by drowsy turtle View Post
    It's possible I'm remembering wrong, but that's how I remember it.
    Her book is fairly recent. May be she changed her mind between her lecture or the book writing?
    The lecture was more recent than the book, since she was basically advertising it. It might be the case that she changed her mind, I misinterpreted what she said, or misheard altogether. It doesn't really matter.
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    Or it could be that she was being more open and honest in the lecture. It is possible that her public stance is more aggressively anti-plume than her private mind. Perhaps in the comfort of her home institution she felt relaxed enough to speak her true mind?
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    So tectonics seem to be top down driven process. This is primarily because the force of slab pull is an order of magnitude greater than ridge push, although this force is combatted by friction in the descending slab which may work to over power slab pull to the point at which slab pull roughly balances ride push.

    But where does ridge push come from? Is it the ambient upwelling of mantle, forced up due to the downgoing slabs? Or is it rather pushing its way out with gusto? Which ever way it seems that mid ocean ridges are not plume driven. The midocean ridge basalts are pretty similar all over the world, whereas ocean island basalts are markedly different. This hints that midocean ridge basalts are derived from a well mixed upper mantle, ocean island basalts probably come from somewhere different -- these are the plumes that may come from as deep as the core mantle boundary!

    The upwelling, spreading, and subduction of oceanic lithosphere looks seductively like the tracing out of classic convection cells. Plumes do occasionally hit the spreading ridge (e.g. Iceland), but to what extent are they important? Plumes are caused by instabilities in a basal thermal boundary layer, therefore an excellent candidate for the origin of plumes would be the thermal boundary layer at the base of the mantle -- it's called D" (pronounced D-double prime). Could it be that most of the heat lost from the core comes in the form of mantle plumes? The core contributes less than 40% of the total amount of heat that is lost to space (should probably find a reference for that one!), the mantle contributes greater than 50 % by the decay of radiogenic isotopes. This internal heating of the mantle provides total buoyancy that seems to contribute to ridge push, but it is not so focussed as plumes, being spread out over the whole mantle.

    This has been a complete brain dump, for which I apologise. I am wondering though, would we have plate tectonics at all if we did not have mantle plumes? How important is D" in the system?
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    Quote Originally Posted by billiards View Post
    So tectonics seem to be top down driven process. This is primarily because the force of slab pull is an order of magnitude greater than ridge push, although this force is combatted by friction in the descending slab which may work to over power slab pull to the point at which slab pull roughly balances ride push.
    No, friction does apply to both slab pull and ridge push.

    Question: what is the tensile strength of basalt and how does it compare to slab pull?

    Quote Originally Posted by billiards View Post
    But where does ridge push come from? Is it the ambient upwelling of mantle, forced up due to the downgoing slabs?
    The later is the orthodox view.
    Quote Originally Posted by billiards View Post
    This hints that midocean ridge basalts are derived from a well mixed upper mantle
    Yes, the melts come from the upper mantle, just below the ridge.
    Quote Originally Posted by billiards View Post
    The upwelling, spreading, and subduction of oceanic lithosphere looks seductively like the tracing out of classic convection cells.
    Not really, mantle convection is very chaotic without clearly defined convection cells. The term "avalanche" is also use to describe slabs sinking thru the lower mantle.

    Quote Originally Posted by billiards View Post
    Plumes are caused by instabilities in a basal thermal boundary layer.
    That's only one type of plume. Plume can also be compositional rather than thermal (think to salt diapirs).

    Quote Originally Posted by billiards View Post
    Could it be that most of the heat lost from the core comes in the form of mantle plumes? The core contributes less than 40% of the total amount of heat that is lost to space (should probably find a reference for that one!), the mantle contributes greater than 50 % by the decay of radiogenic isotopes. This internal heating of the mantle provides total buoyancy that seems to contribute to ridge push, but it is not so focussed as plumes, being spread out over the whole mantle.
    According to geoneutrinos counting, about half the heat of Earth comes from radionucleotide decay (see this paper), so half of the heat is primordial, but this heat is not concentrated in the core. Though, the heat released from the crystallization of the inner core could also contribute significantly to the geothermal flux.
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    Hello at all, I hope I can enrich (... how adequate to our issue) (y)our discussion

    Quote Originally Posted by florian View Post
    "There is a consensus among geodynamicists that the sinking of cold, gravitationally unstable litho- sphere drives the plates and indirectly causes mantle to well up beneath mid-ocean ridges. Some estimate that 90% of the force needed to drive the plates comes from the sinking of lithosphere in subduction zones, with another 10% coming from ridge push [11]. "


    I think it is very clear.


    Really? So if this is so, how do you argue about the moving of the mid atlantic Ridge?

    Quote Originally Posted by billiards View Post
    I think the idea is that no material penetrates through the transition zone into the Upper mantle, but the heat does, presumably by conduction. This heat then forms a new convection cell in the upper mantle. This would be layered convection, which means that the Hawaiian magmas are sourced from the Upper mantle reservoir.
    well thats really unlikely:
    1. There are really many fluids in the hawaiian magmas where should they come from if there is only a heat transfer
    2. The isotopegeochemistry of those Islands is very homogenous AND is evidently contaminated with enriched components I only say HIMU. You can't declare both points without any deep mantle source.

    Quote Originally Posted by drowsy turtle View Post
    Quote Originally Posted by florian View Post
    Quote Originally Posted by drowsy turtle View Post
    It's possible I'm remembering wrong, but that's how I remember it.
    Her book is fairly recent. May be she changed her mind between her lecture or the book writing?
    The lecture was more recent than the book, since she was basically advertising it. It might be the case that she changed her mind, I misinterpreted what she said, or misheard altogether. It doesn't really matter.
    I borrowed it, too. I think I remember a picture about the discussion of two mantle reservoirs for Hawaii (Kea and Loa Trend) so it didn't seemed so plume-unfriendly to me.

    Quote Originally Posted by drowsy turtle View Post
    I do find it difficult to see how one could advocate a top-down approach and at the same time aknowledge the existance of plumes in locations that can be matched to volcanism and intrusive bodies. Surely this would be, at best, a balanced view?
    Well I think this position should be defendable, because if you know earths physics you know that there MUST BE plumes, because the conduction of stones (the mantle is not fluid) is very low you need convection from the bottom to the top but for this point it doesn't mind if there are one two or three convection layers.

    So tectonics seem to be top down driven process.


    I think thats very complicated to discuss and even a matter of time cause plate tectonics (exatly I should say subduction like we know it today) evidently started at 3.000 Ma before today and I think its still gaining on influence, what there was before? Honestly, thats speculation!

    According to geoneutrinos counting, about half the heat of Earth comes from radionucleotide decay (see this paper), so half of the heat is primordial, but this heat is not concentrated in the core.


    we should mind that this number is an estimation which could vary

    If you like to know the origin of my knowledge I will try to find the paper I read again, but I can't guarantee for that.
    Last edited by The Geographer; December 18th, 2011 at 10:05 PM.
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    Quote Originally Posted by The Geographer View Post
    I borrowed it, too. I think I remember a picture about the discussion of two mantle reservoirs for Hawaii (Kea and Loa Trend) so it didn't seemed so plume-unfriendly to me.
    Indeed, although IIRC, how the reservoir melt would be generated was not discussed (and so is assumed to be from a plume?). The Hawaiian volcanism is an irritating anomaly for a top-down approach.

    Quote Originally Posted by The Geographer View Post
    Well I think this position should be defendable, because if you know earths physics you know that there MUST BE plumes, because the conduction of stones (the mantle is not fluid) is very low you need convection from the bottom to the top but for this point it doesn't mind if there are one two or three convection layers.
    True enough, but this isn't realy in debate. The question is whether or not this convection leads to tectonic movement.

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    Quote Originally Posted by drowsy turtle View Post
    Indeed, although IIRC, how the reservoir melt would be generated was not discussed (and so is assumed to be from a plume?). The Hawaiian volcanism is an irritating anomaly for a top-down approach.
    continous isotop anomalies for certain lines of volcanism... hardly to be declared seriously by platetectonics. I knew the paper where those pictures from those were written by pro plume guys thats because I didn't read those lines.

    True enough, but this isn't realy in debate. The question is whether or not this convection leads to tectonic movement.
    also read this?
    I think thats very complicated to discuss and even a matter of time cause plate tectonics (exatly I should say subduction like we know it today) evidently started at 3.000 Ma before today and I think its still gaining on influence, what there was before? Honestly, thats speculation!
    I admit that this is rather opinion than knowledge
    Last edited by The Geographer; December 18th, 2011 at 10:53 PM. Reason: bad english
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    Quote Originally Posted by The Geographer View Post
    I think thats very complicated to discuss and even a matter of time cause plate tectonics (exatly I should say subduction like we know it today) evidently started at 3.000 Ma before today and I think its still gaining on influence, what there was before? Honestly, thats speculation!

    I admit that this is rather opinion than knowledge
    The oldest rocks on Earth are ~3.5 Myrs old, but this probably relates to the first occurance of large bodies of granitic rocks, rather than the beginning of plate tectonics. At this time the lithosphere would have been thinner and hotter; the upper mantle less highly evolved in composition. Perhaps granites only began to occur in large amounts and form enduring continents once the upper mantle had become more evolved, and the lithosphere cooled and became more brittle?

    Also just a speculation.
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    Quote Originally Posted by drowsy turtle View Post
    The oldest rocks on Earth are ~3.5 Myrs old, but this probably relates to the first occurance of large bodies of granitic rocks, rather than the beginning of plate tectonics.
    Unless you are using very large error bars this turns out to be incorrect.

    The Acasta Gneiss is 4.05 Ga Source.

    Then there is is this little beauty, the Nuvvuagittuq greenstone belt. From the abstract of this paper: A line fit only to the faux-amphibolite compositional group that shows the widest range in LREE enrichment, including corresponding co-genetic ultramafic sills gives a 146Sm-142Nd age of 4381 +67-123 Ma (n=21). The Hadean age for the faux-amphibolite is supported by a 4079 ± 110 Ma (n=15) 147Sm-143Nd age for intruding gabbro sills.

    And of course we have crustal materials, namely the Jack Hills zircons, with ages as great as 4.4 Ga ~. Here is a fascinating paper on deductions about the early crust based on detailed isotope analyses of these zircons.
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    Welcome to the discussion The Geographer. This discussion kind of came in the context of earlier threads (in particular the infamous plate tectonics thread) -- in which the participant florian denies the existence of plate tectonics! This thread was trying to push the discussion away from that and back into talking about the current scientific state of knowledge with regards to plate tectonics.

    Quote Originally Posted by The Geographer View Post
    Hello at all, I hope I can enrich (... how adequate to our issue) (y)our discussion

    Quote Originally Posted by florian View Post
    "There is a consensus among geodynamicists that the sinking of cold, gravitationally unstable litho- sphere drives the plates and indirectly causes mantle to well up beneath mid-ocean ridges. Some estimate that 90% of the force needed to drive the plates comes from the sinking of lithosphere in subduction zones, with another 10% coming from ridge push [11]. "


    I think it is very clear.


    Really? So if this is so, how do you argue about the moving of the mid atlantic Ridge?
    I think this question is central and we kind of let Florian get away with that statement. In fact there are other opinions out there (e.g. Dziewonski, and Romanowicz et al.) and this is a question I am keen to explore in more detail. In particular I think the Large Low Shear Velocity Provinces (LLSVPs) at the core mantle boundary aka "super plumes" or "thermochemical piles" (depending on your preference), are interesting features worthy of discussion from the "bottom up" perspective.

    Quote Originally Posted by The Geographer View Post
    Quote Originally Posted by billiards View Post
    I think the idea is that no material penetrates through the transition zone into the Upper mantle, but the heat does, presumably by conduction. This heat then forms a new convection cell in the upper mantle. This would be layered convection, which means that the Hawaiian magmas are sourced from the Upper mantle reservoir.
    well thats really unlikely:
    1. There are really many fluids in the hawaiian magmas where should they come from if there is only a heat transfer
    2. The isotopegeochemistry of those Islands is very homogenous AND is evidently contaminated with enriched components I only say HIMU. You can't declare both points without any deep mantle source.
    Well, perhaps. I'm not taking any sides on that debate, only sharing my interpretation of the Cao and van der Hilst paper. There is definitely something very odd showing up in the transition zone 1000 km offset from the Hawaiian volcanism. It is odd in that it doesn't fit neatly into the thermal anomaly interpretation of the olivine phase transitions which are used so readily everywhere else. We expect to see the transition zone thin if it is particularly hot, or thicken if it is cold. What is observed is the transition zone bulges down, so it is thinner at the top (hot) and thicker at the bottom (cold). To me I think it is a stretch to interpret this structure in terms of the temperature, it suggests there must be some chemical heterogeneity in that region and we cannot assume the structure is related to phase transitions in olivine.

    As for the isotope geochemistry, I would appreciate some kind of primer on that. My understanding was that there were two volcanic trends that are quite distinct yet remarkably persistent over the length of the chain. I was also under the impression that the geochemical signatures showed signs of recycled sediments. I do not know how anybody can infer where these melts came from without knowledge of the available reservoirs. We do not know what reservoirs are down there so it all seems like speculation as to whether it's CMB material or whatever.
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    Quote Originally Posted by billiards View Post
    Well, perhaps. I'm not taking any sides on that debate, only sharing my interpretation of the Cao and van der Hilst paper. There is definitely something very odd showing up in the transition zone 1000 km offset from the Hawaiian volcanism. It is odd in that it doesn't fit neatly into the thermal anomaly interpretation of the olivine phase transitions which are used so readily everywhere else. We expect to see the transition zone thin if it is particularly hot, or thicken if it is cold. What is observed is the transition zone bulges down, so it is thinner at the top (hot) and thicker at the bottom (cold). To me I think it is a stretch to interpret this structure in terms of the temperature, it suggests there must be some chemical heterogeneity in that region and we cannot assume the structure is related to phase transitions in olivine.

    As for the isotope geochemistry, I would appreciate some kind of primer on that. My understanding was that there were two volcanic trends that are quite distinct yet remarkably persistent over the length of the chain. I was also under the impression that the geochemical signatures showed signs of recycled sediments. I do not know how anybody can infer where these melts came from without knowledge of the available reservoirs. We do not know what reservoirs are down there so it all seems like speculation as to whether it's CMB material or whatever.
    Wow! I'm impressed! That's the most clever text I ever read from you
    And no, I do not -deny- plate tectonics. It is just another obsolete, refuted theory.
    Last edited by florian; December 20th, 2011 at 03:21 PM.
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    Quote Originally Posted by billiards View Post
    [...]how do you argue about the moving of the mid atlantic Ridge?
    I think this question is central and we kind of let Florian get away with that statement. In fact there are other opinions out there (e.g. Dziewonski, and Romanowicz et al.) and this is a question I am keen to explore in more detail. In particular I think the Large Low Shear Velocity Provinces (LLSVPs) at the core mantle boundary aka "super plumes" or "thermochemical piles" (depending on your preference), are interesting features worthy of discussion from the "bottom up" perspective.
    I would say, "my preference" is hard to describe, when I look at the empereor chain or at Tristan da Cunha my point of view is clearly ok, but I wouldn't deny in general that there could have been a continent what was seduced by top-down elements. Last week I talked to one of my professors (about adakitic melts) and he said (I try to replay how it sounded to me though its translated very free) "well, there is - EVERYTHING".

    Well, perhaps. I'm not taking any sides on that debate, only sharing my interpretation of the Cao and van der Hilst paper. There is definitely something very odd showing up in the transition zone 1000 km offset from the Hawaiian volcanism. It is odd in that it doesn't fit neatly into the thermal anomaly interpretation of the olivine phase transitions which are used so readily everywhere else. We expect to see the transition zone thin if it is particularly hot, or thicken if it is cold. What is observed is the transition zone bulges down, so it is thinner at the top (hot) and thicker at the bottom (cold). To me I think it is a stretch to interpret this structure in terms of the temperature, it suggests there must be some chemical heterogeneity in that region and we cannot assume the structure is related to phase transitions in olivine.
    I don't say we know all about those islands that would be out of the range I think this paper opens a door to understand shallow mantle convection perhaps we even can declare by this "odd" issue those odd suboceanic magmatic "provinces" in the south of the hawaiian change which are a little bit younger (up to ca. 60 Ma IIRC) and look at the canary chain there you have those edge driven convection which is disturbing a plume (by the way the canary islands are one of the most interesting examples for hotspots there is a very good site unfortunatelly for you in german but the pictures of the modells are worth a look Der Kanarische Hotspot - Edge Driven Convection ). Also I'm not fully convinced about those LLSVP's like Burke argues, those provinces are existing nowadays ok, but that doesn't have to be related to plate tectonics today. Nobody can say how the CMB or the D'' looked like 500 Ma before today so we should not forget other mechanisms which can drive bottom-up tectonics. (Main point that is disturbing me on burkes paper is his speculation on longterm stability)


    As for the isotope geochemistry, I would appreciate some kind of primer on that. My understanding was that there were two volcanic trends that are quite distinct yet remarkably persistent over the length of the chain. I was also under the impression that the geochemical signatures showed signs of recycled sediments. I do not know how anybody can infer where these melts came from without knowledge of the available reservoirs. We do not know what reservoirs are down there so it all seems like speculation as to whether it's CMB material or whatever.
    Well one of the main points that this material came from somewhere... let's say near to the D'' layer is look at the length of this chain in central europe we have some things like Eifel, Rhön, Vogelsberg they didn't last very long and those things seem to me to be from the transition zone.

    Another detail I have to mention are the magmatic series which are interpreted to be the hot upper zones of a blob of a plume and the colder bottom of those blobs, a plate tectonic model could not declare this so easily.

    Another word to the evidence for the start of plate tectonics in diamonds we can find little enclaves of eclogite or peridotit BUT the eclogite is everytime younger than 3.0 Ga and thats going to be interpreteted as the start of subduction like we know it and plate tectonics

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    Quote Originally Posted by florian View Post
    And no, I do not -deny- plate tectonics. It is just another obsolete, refuted theory.
    how has it been refuted, in your eyes ?

    + as an aside, if this thread becomes a carbon copy of the "plate tectonics" thread, we may have to merge them, or otherwise split off the "expanding earth - plate tectonics" posts
    "Reality is that which, when you stop believing in it, doesn't go away." (Philip K. Dick)
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    Quote Originally Posted by marnixR View Post
    Quote Originally Posted by florian View Post
    And no, I do not -deny- plate tectonics. It is just another obsolete, refuted theory.
    how has it been refuted, in your eyes ?

    + as an aside, if this thread becomes a carbon copy of the "plate tectonics" thread, we may have to merge them, or otherwise split off the "expanding earth - plate tectonics" posts
    So why ask him in this thread then?
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    Quote Originally Posted by billiards View Post
    So why ask him in this thread then?
    Yes, why not following the other thread?
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    ok, if that's your reaction, then i close this thread + leave the "plate tectonics" thread as the only vehicle for the "florian vs. most other people" debate
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    re-opened at request - but let's try and stay clear from making this a "plate tectonics v.2" thread
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    I think I'm tending to agree that mantle convection is "bottom up".

    Dziewonski et al. Mantle Anchor Structure: An argument for bottom up tectonics. Earth Planet Sc Lett (2010) vol. 299 (1-2) pp. 69-79

    Discuss.
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    Quote Originally Posted by billiards View Post
    I think I'm tending to agree that mantle convection is "bottom up".

    Dziewonski et al. Mantle Anchor Structure: An argument for bottom up tectonics. Earth Planet Sc Lett (2010) vol. 299 (1-2) pp. 69-79

    Discuss.
    its not just bottom up or top down its more complicated. If you have a look here http://user.uni-frankfurt.de/~schmelin/convection1.mpg you can see that the convection is said not to be stable if you just mind the mantle but with the outter core and the crust being involved in the convection you get a stable convection
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    Quote Originally Posted by The Geographer View Post
    Quote Originally Posted by billiards View Post
    I think I'm tending to agree that mantle convection is "bottom up".

    Dziewonski et al. Mantle Anchor Structure: An argument for bottom up tectonics. Earth Planet Sc Lett (2010) vol. 299 (1-2) pp. 69-79

    Discuss.
    its not just bottom up or top down its more complicated. If you have a look here http://user.uni-frankfurt.de/~schmelin/convection1.mpg you can see that the convection is said not to be stable if you just mind the mantle but with the outter core and the crust being involved in the convection you get a stable convection
    Naturally a cold top is necessary in any convecting system. The real point is that it is the structures at the bottom of the Earth's mantle that controls the pattern of mantle convection. That is what is meant by "bottom up" convection.
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    Quote Originally Posted by billiards View Post
    Quote Originally Posted by The Geographer View Post
    Quote Originally Posted by billiards View Post
    I think I'm tending to agree that mantle convection is "bottom up".

    Dziewonski et al. Mantle Anchor Structure: An argument for bottom up tectonics. Earth Planet Sc Lett (2010) vol. 299 (1-2) pp. 69-79

    Discuss.
    its not just bottom up or top down its more complicated. If you have a look here http://user.uni-frankfurt.de/~schmelin/convection1.mpg you can see that the convection is said not to be stable if you just mind the mantle but with the outter core and the crust being involved in the convection you get a stable convection
    Naturally a cold top is necessary in any convecting system. The real point is that it is the structures at the bottom of the Earth's mantle that controls the pattern of mantle convection. That is what is meant by "bottom up" convection.
    You missed my point I will try to declare it on another way. You need differences in the convecting system (not just vertical even horizontally) to get such big convection cells, if you have got a homogenous mantle there are no differences and you've got no convection. So how can we get temperature differences into the mantle? ...just by subduction of crust, so the first subducted cold plates sinking down to the mantle bottom could have created the upwelling zone at the bottom of the mantle which (maybe) had been not stable (just speculation). Later the core was created and stabilized the convection.

    PS: please mind at the beginning there had been no core, elements like uranium had been in the mantle and heated it up so at first there had been at least two mantle layers, so we should be talking about the point of time where the mantle had been convecting in one cell maybe at 3 Ga
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    Quote Originally Posted by The Geographer View Post

    You missed my point I will try to declare it on another way. You need differences in the convecting system (not just vertical even horizontally) to get such big convection cells, if you have got a homogenous mantle there are no differences and you've got no convection.
    In fact the horizontal differences are the important ones. But yes I agree with this statement.

    Quote Originally Posted by The Geographer View Post
    So how can we get temperature differences into the mantle? ...just by subduction of crust, so the first subducted cold plates sinking down to the mantle bottom could have created the upwelling zone at the bottom of the mantle which (maybe) had been not stable (just speculation). Later the core was created and stabilized the convection.
    So you're saying we had subduction before any convection? Or at least, subduction from the surface was the first bit of convection. Also we had a crust with basic plate tectonics before the core formed?

    Far more likely the Earth has always had heterogeneities and so convection has always been active. The Earth would not have accreted homogeneously at the fine length scales that would be important for convection.


    Quote Originally Posted by The Geographer View Post
    PS: please mind at the beginning there had been no core, elements like uranium had been in the mantle and heated it up so at first there had been at least two mantle layers, so we should be talking about the point of time where the mantle had been convecting in one cell maybe at 3 Ga
    OK not sure I am missing something in your English, but I think you are saying we should be talking about an Earth at least 3 Ga old? In which case I agree, in fact, we are talking about an Earth that is 4.5 Ga old, i.e. the Earth of today. Not to say you can't talk about an earlier Earth if you want, but I think that that is hard given the lack of constraints. At least we have some constraints on mantle convection today.
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    Quote Originally Posted by billiards View Post
    So you're saying we had subduction before any convection? Or at least, subduction from the surface was the first bit of convection.
    Before this there had been also convection but totally different mainly separated in the upper mantle and the lower mantle maybe there had been little plumes out of the lower mantle but no big whole mantle convection like we know it today

    Quote Originally Posted by billiards View Post
    Also we had a crust with basic plate tectonics before the core formed?
    yes but no subduction. The inner core seems to me to be about 1 Ga old there are hints that could be interpreted to say that the convection of the outer core started at 2.7 Ga before today (I always mean before today when I'm talking about those numbers)

    this is by the way referring to this "At the broadest level our compilation confirms an important deep-Earth event at c. 2.7 Ga that is manifested in an abrupt increase in geodynamo palaeointensity"
    Palaeoproterozoic supercontinents and global evolution: correlations from core to atmosphere
    Palaeoproterozoic supercontinents and global evolution: correlations from core to atmosphere
    S. M. Reddy1,* and
    D. A. D. Evans2



    Quote Originally Posted by billiards View Post
    Far more likely the Earth has always had heterogeneities and so convection has always been active. The Earth would not have accreted homogeneously at the fine length scales that would be important for convection.
    yes but not such big ones, and we convection had been all time active but not like today what we wanted to know

    OK not sure I am missing something in your English, but I think you are saying we should be talking about an Earth at least 3 Ga old? In which case I agree, in fact, we are talking about an Earth that is 4.5 Ga old, i.e. the Earth of today. Not to say you can't talk about an earlier Earth if you want, but I think that that is hard given the lack of constraints. At least we have some constraints on mantle convection today.
    3 Ga before today.
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