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#101
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![]() Robert Grumbine wrote: In article . com, don findlay wrote: Klaus wrote: [snip] Natural convection is simply heat transfer be circulation in a fluid, caused by gravity acting on fluids of differing density. There is a heat source at the bottom and a heat sink at the top. Hot fluid is generally less dense than cold fluid. The hot fluid rises, and the cold fluid sinks. These motions usually spontaneously set up currents. This can easily be oserved in a pot of water being heated on a stove, prior to boiling. The material of the mantle is hot enough to flow slowly as a fluid; it can undergo plastic deformation indefinitely. ('viscous', ..it's viscous) (not plastic) ... so continuing my sporadic comments about rheology, we're now to plastics, elastics, and fluids. To back up a second, rheology is the relationship between stress and the strain and/or rate of strain. The divergence of stress produces forces, which then can drive motion. Strain is the displacement of material from a reference position. Rate of strain is the speed at which this occurs. In a simple fluid, the stress is proportional to the rate of strain. Fluids have no 'memory' of where the particles started from, so only detect rates of change. In an elastic material, the particles do know where they started from, so stress is proportional to the strain itself. An ideal plastic (elastic-plastic) behaves like an elastic material as long as the stresses are small enough (stay below the yield stress of the material). Above that yield stress, they behave like viscous fluids -- stress becomes proportional to the rate of strain. In the case at hand, mantle material can be examined as a plastic medium. For low stresses, it is an elastic solid. For higher stresses, it is a viscous fluid. People concerned with portions of the earth which are always on one side of the yield stress talk about that behavior, hence a viscous lower mantle and elastic crust/lithosphere. Since stress, strain, and rate of strain are all tensors, the possible relations get quite involved. Gets even more involved when, as for glacial ice, stress is related to nonlinear functions of the rate of strain. Even worse when, as for sea ice, stress responds to both strain and rate of strain. What are you telling me this for all about stress and strain, when it's convection we're discussing? Klaus there was busily refuting what nasa was saying, and you feel a discourse on stress and strain is a necessary diversion? Why? I'm saying nasa's position is ridiculous, and earns me one strike for falsifying plate Tectonics. http://groups.google.com.au/group/sc...f6ec783?hl=en& Do you agree? |
#102
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In article om,
oriel36 wrote: Strip away all the polished prose and you are still working with a stationary Earth mechanism for crustal motion, Glad, I guess, you consider the prose polished. As to the papers, who said they involved crustal motion at all? -- Robert Grumbine http://www.radix.net/~bobg/ Science faqs and amateur activities notes and links. Sagredo (Galileo Galilei) "You present these recondite matters with too much evidence and ease; this great facility makes them less appreciated than they would be had they been presented in a more abstruse manner." Two New Sciences |
#103
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oriel36 wrote:
I will know when you reach my level of understanding on this matter .... you will enjoy the padded walls and the wraparound sleeves as much as oriel does. --D. |
#104
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don findlay wrote:
Robert Grumbine wrote: In article . com, don findlay wrote: Klaus wrote: [snip] Natural convection is simply heat transfer be circulation in a fluid, caused by gravity acting on fluids of differing density. There is a heat source at the bottom and a heat sink at the top. Hot fluid is generally less dense than cold fluid. The hot fluid rises, and the cold fluid sinks. These motions usually spontaneously set up currents. This can easily be oserved in a pot of water being heated on a stove, prior to boiling. The material of the mantle is hot enough to flow slowly as a fluid; it can undergo plastic deformation indefinitely. ('viscous', ..it's viscous) (not plastic) ... so continuing my sporadic comments about rheology, we're now to plastics, elastics, and fluids. To back up a second, rheology is the relationship between stress and the strain and/or rate of strain. The divergence of stress produces forces, which then can drive motion. Strain is the displacement of material from a reference position. Rate of strain is the speed at which this occurs. In a simple fluid, the stress is proportional to the rate of strain. Fluids have no 'memory' of where the particles started from, so only detect rates of change. In an elastic material, the particles do know where they started from, so stress is proportional to the strain itself. An ideal plastic (elastic-plastic) behaves like an elastic material as long as the stresses are small enough (stay below the yield stress of the material). Above that yield stress, they behave like viscous fluids -- stress becomes proportional to the rate of strain. In the case at hand, mantle material can be examined as a plastic medium. For low stresses, it is an elastic solid. For higher stresses, it is a viscous fluid. People concerned with portions of the earth which are always on one side of the yield stress talk about that behavior, hence a viscous lower mantle and elastic crust/lithosphere. Since stress, strain, and rate of strain are all tensors, the possible relations get quite involved. Gets even more involved when, as for glacial ice, stress is related to nonlinear functions of the rate of strain. Even worse when, as for sea ice, stress responds to both strain and rate of strain. What are you telling me this for all about stress and strain, when it's convection we're discussing? Wow, you really don't understand basic rheology at *all*. Have you *ever* studied transport phenomena (heat/mass/momentum transfer)? From your statements, the answer is 'no'. --D. |
#105
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David Iain Greig wrote:
What are you telling me this for all about stress and strain, when it's convection we're discussing? Wow, you really don't understand basic rheology at *all*. Have you *ever* studied transport phenomena (heat/mass/momentum transfer)? From your statements, the answer is 'no'. Have you ever studied geology? Could it be the answer is 'yes'? At school maybe, ..like me.. Then perhaps You can ('coz I can't) come up with an answer how the lithospheric plate with the floating crust on it, pushes the mantle plate down to make it subduct and drive convection - instead of the mantle plate just sliding under and not bothering about driving anything - just there at any rate... Do you think that could be the reason there are no subduction zones in the Atlantic? ..the mantle is just sliding under, ..say the Americas, say, ...South America, ..and going along until it meets the Nazca Plate and THEN bending down. Both of them. One of them bending sort of normally, and the other at an acute angle. And both of them bending creates this updraft that lifts the Andes? (There, do Plate Tectonics a favour and Falsify that one.) |
#106
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![]() David Iain Greig wrote: oriel36 wrote: I will know when you reach my level of understanding on this matter ... you will enjoy the padded walls and the wraparound sleeves as much as oriel does. --D. Suit yourselves,a response on a reasonable topic refering the dynamics of the Earth's shape and crustal motion should normally elicit interesting comments as there is nothing intrinsically difficult in the principles of differential rotation perpendicular to the Earth's rotational axis in generating the Earth's deviation from a perfect sphere and the motion of the fractured surface crust. I even made the level of understanding so loose in outlines that any further broadining of the rotational dynamics would be ineffective.The exposed plasma of the Sun and differential rotation is just a rough indication of the mechanism inherent in the Earth's interior,to make the Earth a celestial exception in favor of ugly convection cells with no link to the rotational dynamics generating the Earth's shape is simply not worth it.Let me show you a good animation of differential rotation and how elements of it can be applied to the Earth's interior dynamics - http://www.astronomynotes.com/starsun/sun-rotation.gif No doubt,leaving the outlines of this so loose can have the opposite affect with people who are hellbent in screwing things up.As long as the link between rotational dynamics as the common mechanism for the Earth's shape and crustal motion is grasped,it becomes harder to adhere to convection cells. If you find yourself trying to insult me,forget it,you insult yourselves quite nicely as far as I am concerned. |
#107
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In article , Robert Grumbine wrote:
*Extensive* parts of continents. Let me take you hill walking on the Scottish fringe of the Laurentian Shield one of these days G. Sure. Then come back here and go for a stroll around the north american Laurentians. No problem with that. Any particular part of Canada? Could you turn off the mosquitos for the day? I'll try and get you a Midge Exemption Certificate, if you can provide evidence of having been running around in a field screaming and rubbing your face in the past. -- Aidan Karley, FGS Aberdeen, Scotland Written at Wed, 21 Jun 2006 07:49 +0100, but posted later. |
#108
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In article , Robert Grumbine wrote:
I am not a geologist....however....at the margins, where subduction is occuring...would there not be shearing and scraping of sediments? Would there not be lower portions of CRUST being subducted as the mantle that it's ATTACHED to descends into the planet? Some portion of the crust/sediment gets scraped off, and some gets carried down in the subduction zone. Part of the subducted material is readily melted at the higher pressure and temperature and comes back up through volcanoes. For the scraped material, the term to look up is 'accretionary wedge'. iirc SE Asia has a good one. Worth looking up on "ophiolites" too, Ken. That's the inverse effect of bits of oceanic crust and upper mantle getting scraped off and mixed in with the material of the accretionary wedge, then pushed up onto the continent. The classical ophiolites are in Cyprus and Oman (good places for a holiday), but I have heard of nice ones in the Californian melange too, and I'm sure there are other examples closer to your home. There's a small ophiolite complex at Ballantrae, for example. -- Aidan Karley, FGS Aberdeen, Scotland Written at Wed, 21 Jun 2006 08:14 +0100, but posted later. |
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