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I read some of her book; having borrowed it but not owned it.

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. Originally Posted by drowsy turtle It’s possible I’m remembering wrong, but that’s how I remember it. Originally Posted by drowsy turtle Let’s be careful to distinguish the debate here. This is what I meant, yes.

Or it could be that she was being more open and honest in the lecture. Her book is fairly recent. economics essay writing service
I guess you read Gillian’s book? Even for Hawaii, she doesn’t really buy the plume model. 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. 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. So tectonics seem to be top down driven process.

It’s possible I’m remembering wrong, but that’s how I remember it. Plumes do occasionally hit the spreading ridge (e.g. Originally Posted by billiards The upwelling, spreading, and subduction of oceanic lithosphere looks seductively like the tracing out of classic convection cells. Originally Posted by drowsy turtle It’s not that the Hawaiian plume doesn’t affect the surface; it clearly does. This is repeatedly shown in introductory textbooks. Her book is fairly recent. 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. Perhaps in the comfort of her home institution she felt relaxed enough to speak her true mind? The midocean ridge basalts are pretty similar all over the world, whereas ocean island basalts are markedly different. Surely this would be, at best, a balanced view? 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 admit I could have been clearer. The Plate-driven view doesn’t completely rule out plumes at all. 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.

I doubt you could find any advocate of top-down tectonics who disagrees with this. The hot material still reaches the surface, but ceases to act as a ‘simple’ plume.That’s just my thought, anyway. This heat then forms a new convection cell in the upper mantle. Originally Posted by florian Originally Posted by drowsy turtle It’s possible I’m remembering wrong, but that’s how I remember it. Originally Posted by drowsy turtle Removing plumes from the equation completely doesn’t work, however. Originally Posted by florian Originally Posted by drowsy turtle Removing plumes from the equation completely doesn’t work, however. 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?

Originally Posted by billiards 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. It might be the case that she changed her mind, I misinterpreted what she said, or misheard altogether.

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. Originally Posted by billiards So tectonics seem to be top down driven process. This would be layered convection, which means that the Hawaiian magmas are sourced from the Upper mantle reservoir.

I guess you read Gillian’s book? Even for Hawaii, she doesn’t really buy the plume model. There’s no explanation for it’s occurence, save for the deep mantle plume directly beneath. 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. 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. May be she changed her mind between her lecture or the book writing?

Originally Posted by billiards 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. Originally Posted by drowsy turtle Also, note that the plume apparantly terminates in the asthenosphere, where the lower viscosity will probably make things more chaotic. I read some of her book; having borrowed it but not owned it. 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. Don’t get me wrong.

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. 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. 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. I doubt you could find any advocate of top-down tectonics who disagrees with this. Let’s be careful to distinguish the debate here. 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 is possible that her public stance is more aggressively anti-plume than her private mind. Originally Posted by billiards I think the idea is that no material penetrates through the transition zone into the Upper mantle, but the heat does, presumably by conduction.

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. Originally Posted by drowsy turtle I think you’re overstating it slightly. There’s no explanation for it’s occurence, save for the deep mantle plume directly beneath. 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. This heat then forms a new convection cell in the upper mantle.

Whereas, if plate tectonics is driven by the negative buoyancy of subducting oceanic lithosphere, then plumes are not necessary, yet may still exist. 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). 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. She argues for extensional stress in the Pacific lithosphere as the origin of the magmatic activity. 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? 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. 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. I certainly did not say that all phenomena are explained or must be explained in the context of plate tectonics.

Whereas, if plate tectonics is driven by the negative buoyancy of subducting oceanic lithosphere, then plumes are not necessary, yet may still exist. 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). 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. This would be layered convection, which means that the Hawaiian magmas are sourced from the Upper mantle reservoir. It doesn’t really matter. I think the idea is that no material penetrates through the transition zone into the Upper mantle, but the heat does, presumably by conduction. IIRC, the Hawaiian ‘hotspot’ is in an area of low tectonic stress, normal crustal/lithosphere thickness…

IIRC, the Hawaiian ‘hotspot’ is in an area of low tectonic stress, normal crustal/lithosphere thickness…

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