# Plate tectonic mechanism ?

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The mechanism proposed in post#349 does not explain why the crust enters compression when the core shrinks, unless the crust started the cycle in compression, which was not stated.

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Hi arc, happy new year

Happy new year billiards! Hope you had a pleasant holiday.

I see there is no new material here (except the cartoons), and therefore there is nothing much left to say.

What! I do not recall ever discussing with you proto crust and the initiation of plate tectonics.

With regards to subduction, do you not agree that this is indeed mantle convection in action ...?

No, I have avoided falling into that trap that was reiterated in the link that Ophiolite offered in post #346 as a counter argument to my demand that anyone defending convection needs to furnish some predictions of observations;

http://people.earth.yale.edu/sites/default/files/files/Bercovici/51EPSL-Frontiers-PlateGen.pdf

"The purpose of this review is to first briefly explain why generating plates is such a problem, and then to survey some of the various directions and progress made in trying to generate them from mantle dynamics theory"
3.2. Subduction zones

"However, subduction zones are really only crudely predicted by simple convection theory, and there is much to them that is highly atypical of convection. First, if one were to only consider the strength of a cold super-viscous lithosphere, one would not expect to see subduction zones at all. Convection with purely temperature dependent viscosity typical of the Earth’s tends form a cold, hard, and immobile layer on the top, and all convective motion occurs beneath it, as if it were a rigid lid."

"Indeed, Earth seems particularly anomalous since it appears to be the only terrestrial planet with prominent subduction zones. Thus, subduction initiation presents formidable problem to convection models."

And I'm just following your lead on this;

To be fair, we can only theorise about what it is we see in the data. I'm saying that as a seismologist. Having said that, faults are particularly easy to see on good seismic data as they cut the stratigraphy often with dramatic effect. Further, countless "theorized" faults have been verified by subsequent drilling. As has already been pointed out, many faults are obvious from their surface expression.

Things get a lot murkier when we look at the deep earth, to be fair to the OP.

"For the last decade, the ‘plate-generation’ community has focused largely on explaining how and why convection looks plate-like at present. Yet little has been done to address another important feature of plate tectonics, i.e. plate evolution and rapid plate-motion changes. The plate tectonic record is filled with apparent plate-reorganization events"

"Both the initiation of subduction and the asymmetry or one-sidedness of subduction remain open first-order questions. Initiation might require some sort of mechanism to weaken thick cold lithosphere, such as a rifting event"

Hmm . . . Yes, . . . . someone does need to come up with a solution for that . . . . .

The mechanism proposed in post#349 does not explain why the crust enters compression when the core shrinks, unless the crust started the cycle in compression, which was not stated.

Hello studiot, welcome back to my thread. I will be a more gracious host this time. My apologies again for not responding properly in the past. There was and is no excuse. And I extend this acknowledgment to billiards as well and anyone else who feel I acted improper towards them. My apologies to them also.

'''why the crust enters compression when the core shrinks,"

Well, gravity. Just as the skin of an apple will wrinkle as the surface area of the hypanthium or flesh, to which it is attached, shrinks, the Earth's crust that is much more rigid will in turn begin to respond with similar compressive energies as the gravitational potential energy slowly increases from the inward displacement of the mantle. The only solution to this growing energy level is for the crust to go downward as subduction, or to go upward as mountain structures.

As the article mentioned;

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

During the early years of Earth's existence, when the planet was much hotter, greater degrees of melting at spreading oceanic ridges generated oceanic lithosphere with thick crust, much thicker than 20 km (12 mi)

I believe this would suggest for a proto crust that subduction is easier to accomplish after the dynamics of many cycles of inward and outward displacement had played out on a weaker and continually weakening boundary, as apposed to the lower vulnerabilities I believe a thicker crust would have to a potential vertical rupture during the same period. The current dynamics of greater continental masses can produce mountain structures when adequate crustal gravitational potential energies are present and supplied.

Edited by arc

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studiot, on 02 Jan 2015 - 12:40 PM, said:

The mechanism proposed in post#349 does not explain why the crust enters compression when the core shrinks, unless the crust started the cycle in compression, which was not stated.

Hello studiot, welcome back to my thread. I will be a more gracious host this time. My apologies again for not responding properly in the past. There was and is no excuse. And I extend this acknowledgment to billiards as well and anyone else who feel I acted improper towards them. My apologies to them also.

'''why the crust enters compression when the core shrinks,"

Well, gravity. Just as the skin of an apple will wrinkle as the surface area of the hypanthium or flesh, to which it is attached, shrinks, the Earth's crust that is much more rigid will in turn begin to respond with similar compressive energies as the gravitational potential energy slowly increases from the inward displacement of the mantle. The only solution to this growing energy level is for the crust to go downward as subduction, or to go upward as mountain structures.

Good morning arc, I nearly missed your reply in the flurry of activity after the disappointing closure of the matter continuum thread (not yours).

I don't think you quite understood my comment, so I will try to expand.

If you inflate a (semi)rigid skinned balloon (eg a metal foil skin) to put the skin into tension.

All that happens if you then remove the internal inflating force is that the balloon resumes its former diameter.

The skin does not enter compression.

At the beginning of the cycles the perimeter 'crust' is neither in compression nor tension.

If you stretch a body, and therefore put it into tension, and then release that tension, the bodydoes not automatically enter compression.

It simply returns to its unstressed state, unless it has something to convert the strain energy of tension to, in which case can oscillate between tension and compression using the something as an intermediary between tension strain energy and compression strain energy.

For example a mass on a spring converts the tension strain energy first to the kinetic energy of motion and then converts that to the strain energy of compression.

So it oscillates about its mean unstrained, unstressed, position.

To avoid the complication of gravity, that is how a loudspeaker works.

Now in your model earth, internal forces stretch the perimeter into tension and then the tension is released by cracking.

So there is no strain energy available to convert via an intermediary into compression.

You may wish to say that the perimeter is now longer than before, by virtue of the material that has welled up into the cracks and solidified.

This this longer perimeter would enter compression if it tried to shrink inwards.

If you wish to consider the Earth as a sphere, pulsing in and out of its unstrained radius the very simplest formula for the period of oscillation is

$T = 2\pi \sqrt {\frac{m}{k}}$

Where T is the period, m is the oscillating mass and k is the spring constant.

You can estimate the mass of the crust, but what you would do about the spring constant, I don't know.

Edited by studiot

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Arc, subduction is convection. It is not a "trap". It is a cold hard undisputed fact. It is the cold, dense, upper thermal boundary layer of a convecting system becoming unstable and embroiled into the convecting mantle. It is somewhat unusual as simple considerations tend to lead to "rigid" lid planets without plate tectonics (as exampled by other terrestrial planets).

I see your confusion. You see that it is difficult to explain subduction with "simple convection theory" and read "it is not convection". But that is your confusion, the Earth does not care whether you understand it or not. It is complex and cannot be easily explained by "simple convection theory" -- but it is still convection!

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Good morning arc, I nearly missed your reply in the flurry of activity after the disappointing closure of the matter continuum thread (not yours).

I don't think you quite understood my comment, so I will try to expand.

If you inflate a (semi)rigid skinned balloon (eg a metal foil skin) to put the skin into tension.

All that happens if you then remove the internal inflating force is that the balloon resumes its former diameter.

The skin does not enter compression.

At the beginning of the cycles the perimeter 'crust' is neither in compression nor tension.

If you stretch a body, and therefore put it into tension, and then release that tension, the bodydoes not automatically enter compression.

It simply returns to its unstressed state, unless it has something to convert the strain energy of tension to, in which case can oscillate between tension and compression using the something as an intermediary between tension strain energy and compression strain energy.

For example a mass on a spring converts the tension strain energy first to the kinetic energy of motion and then converts that to the strain energy of compression.

So it oscillates about its mean unstrained, unstressed, position.

To avoid the complication of gravity, that is how a loudspeaker works.

Now in your model earth, internal forces stretch the perimeter into tension and then the tension is released by cracking.

So there is no strain energy available to convert via an intermediary into compression.

You may wish to say that the perimeter is now longer than before, by virtue of the material that has welled up into the cracks and solidified.

This this longer perimeter would enter compression if it tried to shrink inwards.

If you wish to consider the Earth as a sphere, pulsing in and out of its unstrained radius the very simplest formula for the period of oscillation is

$T = 2\pi \sqrt {\frac{m}{k}}$

Where T is the period, m is the oscillating mass and k is the spring constant.

You can estimate the mass of the crust, but what you would do about the spring constant, I don't know.

Well, as I said in post #349;

This is a minimalists approach.

I have been chastised many times for posting to much at once. And it was probably justified, as it was difficult for anyone to commit to reading that much material for just a post response.

The balloon analogy doesn't work very well for this. We are dealing with pieces that are large and heavy yet deformable under extreme energies. I would think we could imagine, say, taking several large slabs of concrete and placing them over a support frame or form with a very subtle radius. The opposing ends of the slab assembly are buttressed in a manner that can support more weight or horizontal pressure than is needed for this experiment.

Next, with the slabs in place against each other and the buttresses on either end, the frame is fractionally lowered. I think everyone would agree the slabs would begin to experience compression. We could add more slabs, like say dozens of bricks, and make it resemble an almost flat arch but as experience shows these arrangements are unstable and the level of compression needed to accomplish self support will divert to a kinetic direction of force change, usually upwards in the center as the span fails and falls on either side of the vertical rupture. (A nice analogy to a mountain range in a continental interior BTW.)

This analogy when expanded to a global matrix of irregular shaped and sized tectonic plates works rather well.

The evidence of the outward displacement of the global plate matrix is not only seen in the simultaneous opening of divergent boundaries but in the observed evidence of the progression in movement of convergent boundaries in the opposite direction of the subducting plate. As if the convergent trench was being pulled as shear forces from below were extending not only the subducting plate but the entire plate matrix. This progression is currently observed at both convergent and divergent plate boundaries.

Seismology, thermodynamics and classical physics—the physics associated with the names of Fourier, Debye, Born, Gr€uneisen, Kelvin, Rayleigh, Rutherford, Ramberg and Birch—show that ambient shallow mantle under large long-lived plates is hundreds of degrees hotter than in the passive upwellings that fuel the global spreading ridge system, that potential temperatures in mantle below about 200 km generally decrease with depth and that deep mantle low shear wave-speed features are broad, sluggish and dome-like rather than narrow and mantle-plume-like. The surface boundary layer of the mantle is more voluminous and potentially hotter than regions usually considered as sources for intraplate volcanoes.

This would suggest there is evidence that the crust/mantle boundary contains large volumes of magma that would be at play in providing a hydraulic medium between the mantle and crust, extending the crust while simultaneously extruding into the divergent boundaries as is currently being observed. The model proposes the magma is dependent on the strain energy that is simultaneously released by the mantle during it's displacement.

In post #103 I wrote;

My model is predictive of the statement above;

The thermal expansion (of the core's liquid iron) will displace the mantle and release strain energy in the form of heat during its outward movement. The slow increase in the mantles circumference will require the crust to separate and adjust to release the continual tension. As the mantle is displaced outward the divergent plate boundaries are slowly separated, and as they do magma created from the strain energy at the crust/mantle boundary is forced under pressure into the slowly opening gap. The strain energy thermal content is produce as the mantle is forced to expand against gravity and its own viscosity, tearing its outer surface area and releasing the thermal energy.

​I apologize if this was too long or contained any unneeded verbiage.

Arc, subduction is convection. It is not a "trap". It is a cold hard undisputed fact. It is the cold, dense, upper thermal boundary layer of a convecting system becoming unstable and embroiled into the convecting mantle. It is somewhat unusual as simple considerations tend to lead to "rigid" lid planets without plate tectonics (as exampled by other terrestrial planets).

I see your confusion. You see that it is difficult to explain subduction with "simple convection theory" and read "it is not convection". But that is your confusion, the Earth does not care whether you understand it or not. It is complex and cannot be easily explained by "simple convection theory" -- but it is still convection!

I must disagree, as I just showed above, a very simple mechanical model that matches the field observations perfectly, is all that is needed. And as the link that Ophiolite furnished explains;

3.2. Subduction zones

"However, subduction zones are really only crudely predicted by simple convection theory, and there is much to them that is highly atypical of convection. First, if one were to only consider the strength of a cold super-viscous lithosphere, one would not expect to see subduction zones at all. Convection with purely temperature dependent viscosity typical of the Earth’s tends form a cold, hard, and immobile layer on the top, and all convective motion occurs beneath it, as if it were a rigid lid."

"Indeed, Earth seems particularly anomalous since it appears to be the only terrestrial planet with prominent subduction zones. Thus, subduction initiation presents formidable problem to convection models."

Convection has a formidable challenge to overcome.

Edited by arc

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No it was not too long, but I'm afraid it did not address my point.

I have raised and lowered several bridges in my time and can assure you that your slabs of concrete would not suffer additional compression, merely from raising and lowering.

That would require an external agent.

I do not know if the Earth pulsates over a long time period or not, so I am genuinely trying to help you make your model work.

This is a discussion not an attempt to prove you wrong or right.

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No it was not too long, but I'm afraid it did not address my point.

I have raised and lowered several bridges in my time and can assure you that your slabs of concrete would not suffer additional compression, merely from raising and lowering.

That would require an external agent.

I do not know if the Earth pulsates over a long time period or not, so I am genuinely trying to help you make your model work.

This is a discussion not an attempt to prove you wrong or right.

I understand, thank you.

That external agent is the mantle and the (hydraulic) magmatic fluid between the crust and mantle. They are always there supporting the crust throughout its various degrees of compressive energies.

"Next, with the slabs in place against each other and the buttresses on either end, the frame is fractionally lowered. I think everyone would agree the slabs would begin to experience compression. We could add more slabs, like say dozens of bricks, and make it resemble an almost flat arch but as experience shows these arrangements are unstable and the level of compression needed to accomplish self support will divert to a kinetic direction of force change, usually upwards in the center as the span fails and falls on either side of the vertical rupture." (A nice analogy to a mountain range in a continental interior BTW.)

In the model that I described above, I stated; the frame is fractionally lowered. This was to express that the slabs were never fulling unsupported, and actually the opposite is true. They are mostly supported and attain very little compression as compared to what there would be if they were mostly or completely unsupported.

The current observable evidence is that there is compression in the crust that is derived from and in the form of gravitational potential energy.

http://ceas.iisc.ernet.in/~aghosh/Ghosh_geology06.pdf

Gravitational potential energy of the Tibetan Plateau and the forces
driving the Indian plate

Attreyee Ghosh
William E. Holt
Department of Geosciences, State University of New York, Stony Brook, New York 11790, USA
Lucy M. Flesch*
Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA
A. John Haines†
Bullard Laboratories, University of Cambridge, Cambridge CB3 0EZ, UK
ABSTRACT
"We present a study of the vertically integrated deviatoric stress field for the Indian plate and the Tibetan Plateau associated with gravitational potential energy (GPE) differences. Although the driving forces for the Indian plate have been attributed solely to the mid-oceanic ridges that surround the entire southern boundary of the plate, previous estimates of vertically integrated stress magnitudes of 6–7 1012 N/m in Tibet far exceed those of 3 1012 N/m associated with GPE at mid-oceanic ridges, calling for an additional force to satisfy the stress magnitudes in Tibet. We use the Crust 2.0 data set to infer gravitational potential energy differences in the lithosphere. We then apply the thin sheet approach in order to obtain a global solution of vertically integrated deviatoric stresses associated only with GPE differences. Our results show large N-S extensional deviatoric stresses in Tibet that the ridge-push force fails to cancel."
. . . ."there is no complete dynamic explanation for this large GPE of the Tibetan Plateau and the relatively fast movement of the Indian plate. There is no apparent down going slab attached to the Indian plate that might assist in driving the plate into Eurasia through the slab pull mechanism" . . . . .
. . . . "However, the ridge push, or vertically integrated deviatoric stress magnitude, which is 3 1012 N/m (Richardson, 1992; Harper, 1975; Lister; 1975; Parsons and Richter, 1980), is not sufficient to satisfy inferred stress magnitudes of 6–7 1012 N/m that result from GPE differences between the Tibetan Plateau and the surrounding lowlands (Molnar and Lyon-Caen, 1988). An additional force is required to explain the disparity between the excess GPE of Tibet relative to that of the mid-oceanic ridges" . . . .
. . . ."Lithospheric density variations associated with the support of the high topography of the Tibetan Plateau give rise to lithospheric body forces and hence stresses. Although the sources of stress that drive plate motions have been ascribed to many parameters (Forsyth and Uyeda, 1975), from the point of view of stress continuity and force balance, the stresses that drive lithospheric motion arise from two sources: (1) gravity acting on density variations within the lithospheric shell on Earth, and (2) gravity acting on density variations deeper than the lithospheric shell. The latter gives rise to tractions (radial and tangential) that act on the base of the lithosphere, affecting the stress field of the lithosphere and producing dynamic topography. The former involves density variations associated with support of nondynamic components of topography".
Conclusions;
. . "It is clear that something is missing as a driving force that does not have its source within the lithospheric shell."
"calling for an additional force to satisfy the stress magnitudes in Tibet."
This would be the marginal but still present ~ compression that I expect to be present at all times within the entire plate matrix.
"(1) gravity acting on density variations within the lithospheric shell on Earth, and (2) gravity acting on density variations deeper than the lithospheric shell."
That describes exactly what I want my model to portray and to provide. The "density variations deeper than the lithospheric shell." providing that fractional change in support of the overlaying lithosphere or in my example above, the concrete slabs.
Edit: underline
Edited by arc

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Again good morning, arc.

You still have not understood my point so here are some facts and figures for you.

Please understand this is the very crudest 'ball park' analysis that would need some refining.

If you wish to start expounding on shell theory I suggest you study the classic book by Timoshenko

Timoshenko : Theory of Plates and Shells : McGraw- Hill

With reference to my sketches

1) As the ancient 'Earth' cooled the outer layer deformed continuously and plastically until it became solid enough to support direct loads. I will call this the base radius R0 and observe this is in the unstressed state.

2) Following your scenario the Earth expands to radius R1, straining the periphery as shown in the equations and inducing a peripheral hoop tensile stress. In my simple model there are no shear stresses at this point.

3) At some point in the expansion this peripheral stress is sufficient to fracture the crust by cracking. Since the crust is no longer continuous the hoop stress is relaxed. ( I have simplified this to zero). This relaxation induces shear stresses between the crust and the supporting interior. The shortening of the old crust leaves gaps in the periphery.

4) The Earth contracts back to its original radius R0 and the gaps disappear as the disconnected segments of crust rejoin. Note these are unstressed. The shear stresses are reversed but there can be no hoop stress since until the gaps close there is nothing to 'buttress' -your term is as good as any' the disconnected segments.

5) One alternative would be that the Earth contracted to a new radius, less than R0. This would indeed induce compression in the periphery as it subsided under gravity. This is the mechanism that could lead to oscillation.

6) Another alternative would be if the gaps were filled with new solid material, before the contraction began. In this case hoop compression would be induced in the periphery as the Earth contracted.

You should note carefully, however that the strain would be different for the contraction and expansion so the compression would not exactly mirror the tension, since the definition base length for strain is different in the two cases.

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I must disagree, as I just showed above, a very simple mechanical model that matches the field observations perfectly, is all that is needed. And as the link that Ophiolite furnished explains;

3.2. Subduction zones

"However, subduction zones are really only crudely predicted by simple convection theory, and there is much to them that is highly atypical of convection. First, if one were to only consider the strength of a cold super-viscous lithosphere, one would not expect to see subduction zones at all. Convection with purely temperature dependent viscosity typical of the Earth’s tends form a cold, hard, and immobile layer on the top, and all convective motion occurs beneath it, as if it were a rigid lid."

"Indeed, Earth seems particularly anomalous since it appears to be the only terrestrial planet with prominent subduction zones. Thus, subduction initiation presents formidable problem to convection models."

Convection has a formidable challenge to overcome.

I must disagree.

As I said in post #354:

You see that it is difficult to explain subduction with "simple convection theory" and read "it is not convection". But that is your confusion, the Earth does not care whether you understand it or not. It is complex and cannot be easily explained by "simple convection theory" -- but it is still convection!

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You still have not understood.

Hello again studiot,

Hopefully we are getting closer though.

1) As the ancient 'Earth' cooled the outer layer deformed continuously and plastically until it became solid enough to support direct loads. I will call this the base radius R0 and observe this is in the unstressed state.

Agreed.

2) Following your scenario the Earth expands to radius R1, straining the periphery as shown in the equations and inducing a peripheral hoop tensile stress. In my simple model there are no shear stresses at this point.

Agreed.

3) At some point in the expansion this peripheral stress is sufficient to fracture the crust by cracking. Since the crust is no longer continuous the hoop stress is relaxed. ( I have simplified this to zero). This relaxation induces shear stresses between the crust and the supporting interior. The shortening of the old crust leaves gaps in the periphery.

Agreed. With an addendum. Being this is the planet Earth, there would be a substantial amount of magma present in the crust/mantle boundary. This would be under hydrostatic pressure and would be extruded under pressure into any subsequent openings or "gaps" that "shortening of the old crust" would produce. This magma would take an active part in the expansion of the crust.

4) The Earth contracts back to its original radius R0 and the gaps disappear as the disconnected segments of crust rejoin. Note these are unstressed. The shear stresses are reversed but there can be no hoop stress since until the gaps close there is nothing to 'buttress' -your term is as good as any' the disconnected segments.

This is where our models part ways, in mine the Earth's crust will not be able to contract back to its original* radius without the processes of subduction or that of mountain building aka orogenic belts. The "gaps" cannot "disappear" because they are now full of solidified magma that is now considered an addition to the oceanic crust. The Earth's crustal shell will need to accommodate the slowly growing gravitational stresses in some other way.

* "Immediately prior radius" may be more accurate without knowing the actual limits that are possible.

5) One alternative would be that the Earth contracted to a new radius, less than R0. This would indeed induce compression in the periphery as it subsided under gravity. This is the mechanism that could lead to oscillation.

Well, now were back together. Just as this example would induce compression so would the crust in scenario #4 if the mid ocean ridge infill material (extruded magma) eliminated the gaps in the crust.

6) Another alternative would be if the gaps were filled with new solid material, before the contraction began. In this case hoop compression would be induced in the periphery as the Earth contracted.

And we are there! In my model the gaps now have the solidified magma in them, that "solid material" you refer to above. Once you have several operating subducting plates the crust can return to its smaller radius.

You should note carefully, however that the strain would be different for the contraction and expansion so the compression would not exactly mirror the tension, since the definition base length for strain is different in the two cases.

Agreed. In the model there is the idea that they are both present at some degree at all times. For example, in post #357 the paper "Gravitational potential energy of the Tibetan Plateau and the forces driving the Indian plate", concerns the time period beginning around 10 MA to the present. In the model the crust would be in a reduction of radius.

However, prior to this there was more of that Magma infill in the "gap" than usual, producing more crustal area, and by that, more GPE than could be processed by the convergent boundaries alone. This required another form of strain relief, a vertical movement of a very narrow convergent zone between the Indian continent and Asia. It started slowly with the raising of the Tibetan Plateau but increased dramatically with most of the uplift within the last several million years.

http://people.earth.yale.edu/sites/default/files/files/Bercovici/51EPSL-Frontiers-PlateGen.pdf

"Yet little has been done to address another important feature of plate tectonics, i.e. plate evolution and rapid plate-motion changes. The plate tectonic record is filled with apparent plate-reorganization events"

The simple mechanics we have discussed here in this post will provide these dynamics.

Currently the Himalayas are still rising, yet the divergent boundaries indicate we are once again in a very slow expansion mode. But this slow displacement is barely keeping pace with the tremendous GPE still present and dispersing in the largest plates. In configurations like these you would not see large scale extensional events such as the Basin and Range Province. All that GPE that has been in the crust for the last 10 MY would need to be gone for a Basin and Range to be possible.

So this model accommodates a very flexible and dynamic cycle that contains within it times of very low gravitational potential energy in the crust that may then allow large scale extensional events during expansion, and at other times may allow unusually large magma infill at mid-ocean ridges that will in time allow massive amounts of GPE to accumulate and in turn produce mountain ranges in just a few million years.

Both of these types of events undoubtedly on the extreme boundaries of this planet's range of geodynamic operation.

I must disagree.

As I said in post #354:

You see that it is difficult to explain subduction with "simple convection theory" and read "it is not convection". But that is your confusion, the Earth does not care whether you understand it or not. It is complex and cannot be easily explained by "simple convection theory" -- but it is still convection!

That paper in post #346 that Ophiolite put forth as a defense of convection was not the best choice. Yet what alternatives did he really have.

http://people.earth.yale.edu/sites/default/files/files/Bercovici/51EPSL-Frontiers-PlateGen.pdf

This is what I could easily extract from it;

A major outcome of work so far is that nearly all aspects of plate generation require lithosphericrheologies and shear-localizing feedback mechanisms that are considerably more exotic than rheologies typically used in simple fluid-dynamical models of mantle flow . . .

However, many other important problems remain unsolved, such as subduction initiation and asymmetry, temporal evolution of plate geometry, rapid changes in plate motion, and the Archaean initiation of the plate-tectonic mode of convection.

Plate Tectonics has been hailed as the grand unifying principle of geology, although its success at explaining at a multitude of geological and geophysical phenomena has overshadowed the effort to explain the existence of the plates themselves.

The plates are convection

A fair question to ask is, if plate tectonics is so difficult to generate from convection, then might it in fact have nothing to do with convection?

Plates generally subduct when they are heavy enough to completely founder, although initiation of subduction of a cold strong plate (even if heavy) is a serious problem that we will discuss later. . .

3. The plate-generation problem

The simple picture of slabs as convective currents works quite well at explaining the driving force for past and present plates. However, beyond that the picture of plates as convection gets complicated. In many ways, plates are not well described by simple fluid-dynamical convection theory. Indeed, the mathematical theory of plate tectonics works as well as it does because the plates appear to be fairly solid bodies and their motion can be treated with simple rigid-body dynamics. This assumes that all the deformation in the lithosphere primarily occurs between the plates at narrow boundaries.

3.2. Subduction zones

However, subduction zones are really only crudely predicted by simple convection theory, and there is much to them that is highly atypical of convection. First, if one were to only consider the strength of a cold super-viscous lithosphere, one would not expect to see subduction zones at all. Convection with purely temperature dependent viscosity typical of the Earth’s tends form a cold, hard, and immobile layer on the top, and all convective motion occurs beneath it, as if it were a rigid lid.

Indeed, Earth seems particularly anomalous since it appears to be the only terrestrial planet with prominent subduction zones. Thus, subduction initiation presents formidable problem to convection models.

Second, in most forms of thermal convection, both surface boundary layers converging on a sheet-like downwelling will descend, while this occurs nowhere at any terrestrial subduction zone; i.e. all subduction is one-sided, with only one plate descending into the mantle. This asymmetric downwelling is yet another major enigma not yet well explained in convection theory.

Finally, spreading boundaries or mid-ocean ridges and continental rift zones are in some ways not atypical of convection since they represent divergent flow above convective upwellings. However, it is very unlikely that deep mantle upwellings are driving this motion; it is far more probable that plate divergence is drawing up the mantle such that the upwelling is shallow and ‘passive’ (i.e. not moving under its own buoyancy). The clearest evidence for this is the weak gravity anomalies associated with ridges, which strongly indicates that ridge topography is isostatically supported by a shallow buoyant root, not a deep, hot upwelling root. Recent extensive seismic and marine-geophysical surveys of the East Pacific Rise confirm the idea that the ridges involve only shallow passive upwellings.

4. The physics of plate generation

To a large extent, the plateness of the lithosphere and the various plate-boundary enigmas mean that the lithosphere is not obeying simple fluid dynamics, which is indeed rather self-evident. However, the nature of the complicated physics that leads to plates and plate boundaries is both the key and the mystery of how plates are generated.

4.1. Does the lithosphere just shatter?

. . . does not simply break brittly under the various convective stresses . . . unfortunately, such simple brittle behavior only applies across the top few kilometers or so (up to about 10 km) of lithosphere . . .

. . . Although simple brittle behavior across the entire lithosphere is unlikely, pervasive cracking and ductile-cracking may leave faults and weak zones that are later reactivated to make ‘new’ plate boundaries . . .

One drawback of this approach is that it must prescribe a fault instead of letting it arise and evolve from the physics of the system; thus, for example, it cannot account for how a fault or weak zone is formed in lithosphere that is newly created at ridges. Nevertheless, these studies have clearly demonstrated that without low-friction, fault-like concentrations of deformation, called ‘shear localizations’, simple fluid behavior cannot generate plate-like motion.

Well, all I can say is that is a substantial amount of unknown information.

Edited by arc

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Good morning, arc.

This is becoming a habit, (though not necessary a bad one and certainly much more agreeable than discussing religion, polotics and such).

What I am trying to do is apply the scientific method (my version)

That take a premise and examine and compare the options and implications.

Given A then what about B compared to C. Would B lead to D whilst C leads to E and so on? What other results are required for A to be true?

So 4 and 5 are alternatives to be compared in this way.

Since we observe crustal compression now this leads to discarding 4 since it cannot produce contemporaneous observations.

5 leads to the conclusion that something must therefore fill the gaps.

Agreed. With an addendum. Being this is the planet Earth, there would be a substantial amount of magma present in the crust/mantle boundary. This would be under hydrostatic pressure and would be extruded under pressure into any subsequent openings or "gaps" that "shortening of the old crust" would produce. This magma would take an active part in the expansion of the crust.

No, the hydrostatic pressure at the free surface of any liquid, including magma, is zero.

Following the s. method above we need a mechanism for the extrusion of the magma that was extruded in the past (and we now observe as solid) and also that we observe being extruded today.

So you must also consider some form of convection, in following the s. method.

Subduction is also a stress release mechanism, albeit considerably more complicated, and we can observe it today.

However NASA has not reported any physical increase in the Earth's radius, as you state.

I did postgrad in geodesy and I am not aware of any measurements to support such an increase. I can assure you that we have the necessary technology to observe this.

I'm not saying that the modern view of techtonics is entirely correct, it is and has been evolving and has already been down blind alleys and will no doubt be subject to revision, perhaps major as well as minor in the future.

I also still think there is good mileage to be had from following my periodic radius oscillation suggestion, as it would provide pointers as to what to look for.

It may be the timescale would be significant. During an oscillation the rate of change of radius will vary and we may not be observing any at the moment because we are in a slow part of the cycle.

Edited by studiot

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Hello studiot, thanks for sticking with this.

5 leads to the conclusion that something must therefore fill the gaps.

No, the hydrostatic pressure at the free surface of any liquid, including magma, is zero.

OK, can we explore this a little? I have many questions about this.

studiot, on 05 Jan 2015 - 03:04 AM, said:

6) Another alternative would be if the gaps were filled with new solid material, before the contraction began. In this case hoop compression would be induced in the periphery as the Earth contracted.

I would have thought that the crustal shell that is compressing downward towards the Earth's center would contributed to the magma's pressure. I thought this was the source to the magma's movement towards the surface. Seeking a path to a lower pressure state.

I see my mistake was misidentifying the type of fluid pressure.

In the model, the outer core thermally expands and displaces the mantle. Magma, that is present in substantial quantities at the crust/mantle boundary is compressed between the outward displacing mantle and the crust that is displaced by it. As the crust is pushed outwards the divergent boundary opens and begins to leak magma like a hydraulic leak at a faulty seal.

For example, to model this, imagine a pressure vessel that has a flanged opening that is sealed with a several ton weight. The vessel is then pressurized with hydraulic fluid. The fluid pressure is raised until the the cap's weight is countered and the fluid begins to leak from between the flange and cap.

In the model, the cap represents the crust and the flange is the divergent boundary, in this example the working fluid or magma must attain enough pressure to displace the cap's weight (crustal mass) and leak under the higher pressure into the newly opening space (divergent boundary).

At the mid ocean ridge the magmatic working fluid will be forced into the gap that opens at a speed that is approximate to what a fingernail grows, continually sealing the gap as it widens.

Subduction is also a stress release mechanism, albeit considerably more complicated, and we can observe it today.

Yes, I agree, a stress relieving mechanism. But my model has a rather simple explanation for them.

However NASA has not reported any physical increase in the Earth's radius, as you state.

I did postgrad in geodesy and I am not aware of any measurements to support such an increase. I can assure you that we have the necessary technology to observe this.

I saw that about a year ago. I wouldn't think adding the few cm of the divergent boundaries infill per year to the 40075.16 kilometers of the Earth's circumference would be detectable. And considering that the convergent boundaries are active it would seem likely the two are in balance.

Everyone has the idea that by me saying the outer core's molten iron displaces the mantle and crust, that there should be a substantial change in the Earth's radius in time periods relative to several decades, but it doesn't likely achieve measurable metrics at these time scales and energy levels.

During the Miocene the Basin and Range Province was created, it was pulled up to 100% of its original size. The model would explain that this was a period of exceptional divergent boundary inventory that may have been detectable in decadal periodicities of the Earth's radius, but then again maybe not. These cycles are in time scales like 5-10 million years. But as this era ended around 10 million years ago a period of rapid mountain building occurred. The model would explain this as the outer core had cooled and by contracting converted that extra ordinary divergent boundary inventory into exceptional gravitational potential energy, more energy than the convergent boundaries could process. Thus resulting in the rapid rise of the Himalaya range.

I'm not saying that the modern view of techtonics is entirely correct, it is and has been evolving and has already been down blind alleys and will no doubt be subject to revision, perhaps major as well as minor in the future.

I also still think there is good mileage to be had from following my periodic radius oscillation suggestion, as it would provide pointers as to what to look for.

It may be the timescale would be significant. During an oscillation the rate of change of radius will vary and we may not be observing any at the moment because we are in a slow part of the cycle.

I think we are separated by much less than we think.

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Good morning

I think we are separated by much less than we think.

The difference, as I see it, is that you have produced an interesting (speculative) theory, whilst I can claim none at all.

If your theory can be made potentially viable it can proceed to the next stage of testing against observation.

OK, can we explore this a little? I have many questions about this.

Yes indeed, that is what we are doing.

I think it is important to be clear as to the distinction between Force, Stress and Pressure, which are different physical quantities, though very closely linked.

Other important mechanical considerations are the difference between a solid and a fluid and that of a confined v unconfined fluid or body.

Finally the important idea that an unconfined body suffers no stress on thermal expansion.

It is interesting that you think the radius of the Earth cannot be measured with sifficient accuracy, yet you think that the weight of a separated segment of crust can cause a measurable increase in magma pressure.

I would have said it is the other way round.

Particularly as this is your thesis.

That the magma pressure is the result of attempted thermal expansion against confinement.

Also technology achieved the ability to measure the 'radius' of the Earth to millimetric accuracy in about the 1880s, and we can do much better today.

However, something else to consider.

Geodesists do not talk of the 'radius' of the Earth as to a particular physical surface. Measurements are made in relation to gravitational equipotential surfaces.

So when you invoke gravity you need to consider movement measured against these surfaces, which may not be constant.

So when you talk about uplift ask uplift measured against what? Did the local gravity surfaces change as well as the topographical ones?

This is a huge task, but Everest 'discovered' the Himalayas because of gravitational anomalies affecting his results.

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The attempts to sweep mantle convection under the carpet are incredibly unscientific.

A true scientist will attempt to falsify their hypothesis. Not sweep inconvenient facts under the carpet.

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Good morning

The difference, as I see it, is that you have produced an interesting (speculative) theory, whilst I can claim none at all.

If your theory can be made potentially viable it can proceed to the next stage of testing against observation.

Well, as I see it, you early on introduced me to strain energy. Which I believe is the single most important mechanism involved with this idea. I was originally struggling to fit kinetic energy into that position. The rest as they . . . . .

I think it is important to be clear as to the distinction between Force, Stress and Pressure, which are different physical quantities, though very closely linked.

I'm sure I have misused those more times than I am aware.

Other important mechanical considerations are the difference between a solid and a fluid and that of a confined v unconfined fluid or body.

OK, but will need more clarification when the point in question arises.

Finally the important idea that an unconfined body suffers no stress on thermal expansion.

OK, noted but will need your clarification in regards to whether a continent with an attached ocean plate such as the N. American continent would attain any shear stress during the mantle's displacement. Or an extra large continent such as when N. America and Africa separated.

It is interesting that you think the radius of the Earth cannot be measured with sifficient accuracy, yet you think that the weight of a separated segment of crust can cause a measurable increase in magma pressure.

I would have said it is the other way round.

Particularly as this is your thesis.

Geodesists do not talk of the 'radius' of the Earth as to a particular physical surface. Measurements are made in relation to gravitational equipotential surfaces.

So when you invoke gravity you need to consider movement measured against these surfaces, which may not be constant.

So when you talk about uplift ask uplift measured against what? Did the local gravity surfaces change as well as the topographical ones?

This is a huge task, but Everest 'discovered' the Himalayas because of gravitational anomalies affecting his results.

Why studiot, I would almost think you were making fun of my ignorance. Well, in my defense if there is any, I am surprised as to why and how there can be such a large difference in total lengths between divergent and convergent boundaries;

Plate tectonicists insist that the volume of crust generated at mid ocean ridges is equaled by the volume subducted. But whereas 80,000 km of midocean ridges are supposedly producing new crust, only 30,500 km of trenches exist. Even if we add the 9000 km of "collision zones," the figure is still only half that of the "spreading centers" (Smoot, 1997a).

And to complicate it a little more consider that paper from post #357;

http://ceas.iisc.ernet.in/~aghosh/Ghosh_geology06.pdf

"Although the driving forces for the Indian plate have been attributed solely to the mid-oceanic ridges that surround the entire southern boundary of the plate, previous estimates of vertically integrated stress magnitudes of 6–7 1012 N/m in Tibet far exceed those of 3 1012 N/m associated with GPE at mid-oceanic ridges, calling for an additional force to satisfy the stress magnitudes in Tibet."

And add to that;

http://pubs.usgs.gov/gip/dynamic/himalaya.html

The Himalayas and the Tibetan Plateau to the north have risen very rapidly. In just 50 million years, peaks such as Mt. Everest have risen to heights of more than 9 km. The impinging of the two landmasses has yet to end. The Himalayas continue to rise more than 1 cm a year -- a growth rate of 10 km in a million years! If that is so, why aren't the Himalayas even higher?

So just looking at just these few examples I have shown out of probably many more if I continued to look, I am rather honestly surprised (not "I think the radius of the Earth cannot be measured with sufficient accuracy") but really surprised, that a variation of a few cm has not be seen in this metric during the period that the measurements were taken. 30,500 km of convergent trenches vs 80,000 km of mid ocean ridges and no sign of a ebb and flow of movement resulting in a few cm change in that measurement. It stands out to me in a rather anti intuitive way. I would think as the GPE in the crust distributes there would be a gain. But I don't doubt the results.

The attempts to sweep mantle convection under the carpet are incredibly unscientific.

A true scientist will attempt to falsify their hypothesis. Not sweep inconvenient facts under the carpet.

This is an experiment to build a mechanical model of geodynamics. Is there no merit in this? Nothing to be gained? Nothing to explore?

Have you never said once; Now let's try it without the _________.

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Good morning arc, gosh it's wet and windy here today.

No I am not making fun, just trying to help.

Your idea, right or wrong, should be properly considered because it is a good one.

If it eventually fails it will not be because of plausibility but because things just didn't happen that way.

I see you are now on line so am posting this encouragement quickly and will post something more considered later.

PS I have no idea what Billiards meant.

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I could be more clear.

A) Subduction is a mode of (mantle) convection. (Arc will disagree, but there is no science to debate here, this statement is completely uncontroversial fact.)

B) Arc's model incorporates subduction.

Combining (A) and (B): (1) Arc's model incorporates mantle convection.

However: (2) Arc claims his model has no mantle convection.

(1) and (2) contradict. Therefore we have a problem.

Either arc must soften his approach and admit that there is mantle convection (in his model), or he deny subduction (to his model, and therefore it loses on the spot).

Moving on ... one has to question the intentions of someone who has not changed their thesis at all in the last 19 pages. He's still spewing these dodgy numbers "80,000 km of midocean ridges are supposedly producing new crust, only 30,500 km of trenches exist" despite the fact they were shot down much earlier in this thread. Arc is basically soapboxing, not taking in any of the feedback, albeit in a friendly manner. He is clearly not interested in disproving his own theory. Has no concept of what a "prediction" is, and really hasn't the foggiest idea of how to test his theory. Nevertheless he would die for it. Passion yes, science no.

Edited by billiards

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I'm going to be taking a break from the forum to study for exams related to my job. Sorry about the timing but it can't be postponed. I will see everyone in a few weeks.

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arc,

Good luck.

Billiards,

Mantle convection is not contingent upon subduction and would occur without the presence of any plates.

arc has indeed listened to others in this thread and modified his hypotheses as a result.

For instance the original proposal was that the energy comes from a dynamo effeect of the Earth's core sweeping round in the Sun's magnetic field.

Addressing the source of energy has not been resolved since that was abandoned, but the new proposal seems to be gravitational.

Surely this process is exactly what we want to see at SF (and elsewhere in the scientific community)

The testing of ideas against observation and existing knowledge (theory).

Edited by studiot

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Mantle convection is not contingent upon subduction and would occur without the presence of any plates.

Not sure if this is a deliberate straw man of if you just haven't been following closely. You got my point backwards. Subduction is contingent on mantle convection. You can't have subduction without convection, because subduction is a type of convection!

arc has indeed listened to others in this thread and modified his hypotheses as a result.

For instance the original proposal was that the energy comes from a dynamo effeect of the Earth's core sweeping round in the Sun's magnetic field.

Addressing the source of energy has not been resolved since that was abandoned, but the new proposal seems to be gravitational.

Bad example. This has always been the theory and still is. The expansion of the core provides gravitational energy to explain surface dynamics. What, precisely, has changed?

Surely this process is exactly what we want to see at SF (and elsewhere in the scientific community)

The testing of ideas against observation and existing knowledge (theory).

Of course we would like to see good science here. The subject matter of plate tectonics is fascinating to me. My contribution here is to see the gaps and pick holes where the theory is weak. However, non of my points (or indeed the points of others) has been successfully rebutted. Every single one of my points has been avoided or slammed by ignorant arguments (the earthquake catalogue debate is a good example of my points being slammed by ignorance). The subduction point is not even an attack on the theory per se (if arc accepted his model has some convection I would be happy), but again it has been slammed by ignorance. If this is science then you can take my PhD away from me because I obviously haven't understood it.

Edited by billiards

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Billiards

of if you just haven't been following closely..............................

Who hasn't been following closely?

Page 1, post 1 lines 1-4.

arc

I think I hit onto a really nice fit for plate tectonics. The Earth's core is an electro - magnetic field generator. I could not imagine that it wouldn't have variability in it's cycling of current and field. Nothing is going to be constant, especially magnitohydrodynamic generation as it cycles current and field. I thought the Sun's magnetic field could be imposing variation over longer time periods giving the Earth a historic variable thermal cycle to move the tectonic plates.

Clearly I was correct.

studiot

For instance the original proposal was that the energy comes from a dynamo effeect of the Earth's core sweeping round in the Sun's magnetic field.

Addressing the source of energy has not been resolved since that was abandoned, but the new proposal seems to be gravitational.

As regards mantle convection

Billiards

Not sure if this is a deliberate straw man of if you just haven't been following closely. You got my point backwards. Subduction is contingent on mantle convection. You can't have subduction without convection, because subduction is a type of convection!

And I refuted your underlined statement.

Both subduction and mantle convection are possible without the other.

It is entirely possible for one of two impacting plates to ride over the other, which is a definition of subduction.

However without mantle convection another mechanism would be required to cause the horizontal advection of the plates in the first place.

We are examining arc's speculation that such a mechanism could be radial expansion.

It is worth noting that mantle convection does not require segments of colder more dense lithosphere to descend, in order to take place.

Mantle material falls within the Raleigh criterion for such convection to occur, but we cannot expect arc to solve as even the simplest equations to derive this lead to a sixth order differential equation.

Bad example. This has always been the theory and still is. The expansion of the core provides gravitational energy to explain surface dynamics. What, precisely, has changed?

Agreed arc's speculation requires core expansion, but that requires an energy source and we are setting aside the original proposal for the moment to examine if it is worth continuing since unless the speculation can be shown to produce the surface dynamics there is no point seeking an alternative energy source.

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Who hasn't been following closely?

Page 1, post 1 lines 1-4.

Clearly I was correct.

I must say I'm shocked. I expect better from a member of your standing, studiot. Arc's first post continues ...

So I started with a simple model, just a divergent plate boundary, a plate and a convergent boundary (trench). The cycle begins with a small thermal increase in the molten iron core from increased current due to induction from the strengthening of the Sun's magnetic field. As the molten core presses out from thermal expansion it expands the mantle ever so slightly which opens the divergent plate boundaries in the currently observed manner, filling with magma as they expand. After several million years of this solar increase induced cycle the Sun's magnetic field lowers and the Earth's field generator's core begins a cooler period of operation. As the core and mantle slowly recede the crust is put into compression against the newest divergent boundary deposits which leverages the crust towards the trench as the crust follows the mantle as it recedes from the cooling core. This is when the subduction takes place. The compression bleeds into the trenches until the next heat cycle increase.

You see that? The exact same theory written out in plain English in post #1. You just didn't quote the relevant part. Did you not read that far? Or were you deliberately being dishonest? CLEARLY you were wrong! Arc's thesis has not changed!

As regards mantle convection

1) And I refuted your underlined statement.

2) Both subduction and mantle convection are possible without the other.

It is entirely possible for one of two impacting plates to ride over the other, which is a definition of subduction.

However without mantle convection another mechanism would be required to cause the horizontal advection of the plates in the first place.

3) We are examining arc's speculation that such a mechanism could be radial expansion.

1) Where did you refute my statement? You must have forgot to post that bit.

2) You mean if one plate goes over another and the underlying plate does not sink, then that would be "subduction"* without convection? Perhaps, but then you would have to ignore the entire scientific literature on subduction zones, for example you would have to deny the existence of a "mantle wedge". And you would have a fun (and extremely difficult) time explaining the existence of "back-arc basins". Wadati-Benioff zones might cause a few headaches, not to mention reflectivity profiles, receiver functions, and tomography from seismology. The list could go on ...

* according to "a" definition

3) Good, that's one line that you have pursued, and I have been following it. (You might not have noticed but other lines of investigation have happened here too.)

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Billiards

Hmm let me see.

arc proposed an expanding Earth, powered by electrodynamic action, in order to explain observed effects. and you acknowledged this.

That is a chain of two separate sequential propositions and one set of results.

That entails two steps to heaven.

I stated that arc has suspended step 1 so that we can examine if step 2 would actually work as proposed.

And you have been berating me for that statement, in most intemperate language, ever since.

Would we be any more able to get to heaven if we followed steps 1 2 and 3?

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Studiot, sometimes I wonder whether we are reading the same thread ...

I thought we were talking about whether arc's thesis had changed at all.

Here is the whole exchange as I have read it:

I start with the initial criticism of arc:

one has to question the intentions of someone who has not changed their thesis at all in the last 19 pages... Arc is basically soapboxing, not taking in any of the feedback, albeit in a friendly manner.

You come to his defence:

arc has indeed listened to others in this thread and modified his hypotheses as a result.

For instance the original proposal was that the energy comes from a dynamo effeect of the Earth's core sweeping round in the Sun's magnetic field.

Addressing the source of energy has not been resolved since that was abandoned, but the new proposal seems to be gravitational.

According to that post you think arc has modified his original hypothesis. He hasn't, so I respond:

Bad example. This has always been the theory and still is. The expansion of the core provides gravitational energy to explain surface dynamics. What, precisely, has changed?

Now you quote half his theory (you don't respond to my request for pinpointing the bit that has changed):

Page 1, post 1 lines 1-4.

I think I hit onto a really nice fit for plate tectonics. The Earth's core is an electro - magnetic field generator. I could not imagine that it wouldn't have variability in it's cycling of current and field. Nothing is going to be constant, especially magnitohydrodynamic generation as it cycles current and field. I thought the Sun's magnetic field could be imposing variation over longer time periods giving the Earth a historic variable thermal cycle to move the tectonic plates.

Clearly I was correct.

Not clear at all, so I quote the other half to show that it is still the same (the same as the theory presented recently with all the diagrams):

Arc's first post continues ...

So I started with a simple model, just a divergent plate boundary, a plate and a convergent boundary (trench). The cycle begins with a small thermal increase in the molten iron core from increased current due to induction from the strengthening of the Sun's magnetic field. As the molten core presses out from thermal expansion it expands the mantle ever so slightly which opens the divergent plate boundaries in the currently observed manner, filling with magma as they expand. After several million years of this solar increase induced cycle the Sun's magnetic field lowers and the Earth's field generator's core begins a cooler period of operation. As the core and mantle slowly recede the crust is put into compression against the newest divergent boundary deposits which leverages the crust towards the trench as the crust follows the mantle as it recedes from the cooling core. This is when the subduction takes place. The compression bleeds into the trenches until the next heat cycle increase.

You see that? The exact same theory written out in plain English in post #1 ... CLEARLY you were wrong! Arc's thesis has not changed!

Which leads to your most recent post which now seems very out of context:

I stated that arc has suspended step 1 so that we can examine if step 2 would actually work as proposed.

And you have been berating me for that statement, in most intemperate language, ever since.

Not sure how we got here, but, I think breaking up the theory and examining it piece by piece is a good idea. Subduction zones seems an excellent place to start. There are many observations that severely test the theory.

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billiards

Not sure how we got here, but, I think breaking up the theory and examining it piece by piece is a good idea.

Here we have something we can agree on.

If you break arc's hypothesis into not just any old pieces but links in a chain then my position is that I have consistently (apart form post#21 which I will return to)

set aside the early link(s) and concentrated on purely mechanical link at the end, from my post 26 on.

Several others including Ophiolite and yourself have identified the same set of links as myself and asked some of the same questions, plus others I didn't ask.

I don't find that suprising.

In reviewing the thread I note several other instances of arc revising his notions in the light of comment, including one from Tim-the-plumber about relative densities.

In my case I queried arc's kinetic energy mechanism (post26) and was rewarded with post 27.

This should all be clear to anyone who has the stamina to read the thread.

Returning to post 21 I queried arc's thermodynamic statement.

I seem to remember (from elsewhere) that this is absolutely crucial to mantle convection since conventional mathematical models suggest that it is not temperature difference but heat flow difference that provides the energy for convection.

All this is why I recommended that arc catch up on some basic physics to inject into his hypothesis.

Edited by studiot

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