Real Science Radio: Earth & Mercury's Decaying Magnetic Fields
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How does a solid convect?
gcthomas
New member
Slowly. The crystals slide past each other. The whole convection cycle takes 200 million years, so there's plenty of time.
Crystals don't just "slide past each other" for no reason.
Why does one crystal prefer to go down and another upward?
gcthomas
New member
It is convection ,so it is powered by density differences which cause the buoyancy force to be bigger than the weight.
You are happy for the rocks to experience pressure at depth even though pressure is usually only a property of fluids: the rocks are only solid at the elevated temperatures because of the pressure. Materials act in rather unfamiliar ways under such conditions.
"Unfamiliar" does not mean against the rules of physics.
What causes the density differences? Why would a deeper crystal, under greater pressure, become less dense than an overlying crystal under less pressure?
Sorry.
As we have seen, sufficiently higher pressures mean rocks contract upon heating.
Perhaps you'd like to explain what I have gotten wrong instead of only chiming in to lend moral support to those who adhere to your religion.
| At low pressure, olivine fractionation lowers the density of basic magmas, but above 13-14 GPa this trend is reversed. All of these basic to ultrabasic liquids are predicted to have similar densities at 13-14 GPa, and this density is approximately equal to the density of the bulk mantle in this pressure range. This suggests that melts derived from a peridotitic mantle may be inhibited from ascending from depths greater than 400 km. |
What about this is not supportive of what I said?
Is that the situation you spoke of at 200 km depth where you say the rocks will sink?
gcthomas
New member
Try this Nature paper summarising mantle studies.
http://www.geo.uu.nl/~jeannot/geo4-1401/2001HW.pdf
The abstracts is here:
| Seismological images of the Earth's mantle reveal three distinct changes in velocity structure, at depths of 410, 660 and 2,700 km. The ®rst two are best explained by mineral phase transformations, whereas the thirdÐthe D0 layerÐprobably re¯ects a change in chemical composition and thermal structure. Tomographic images of cold slabs in the lower mantle, the displacements of the 410-km and 660-km discontinuities around subduction zones, and the occurrence of small-scale heterogeneities in the lower mantle all indicate that subducted material penetrates the deep mantle, implying whole-mantle convection. In contrast, geochemical analyses of the basaltic products of mantle melting are frequently used to infer that mantle convection is layered, with the deeper mantle largely isolated from the upper mantle. We show that geochemical, seismological and heat-flow data are all consistent with whole-mantle convection provided that the observed heterogeneities are remnants of recycled oceanic and continental crust that make up about 16 and 0.3 per cent, respectively, of mantle volume. |
And here in this intro to the topic:
Why does melting occur? Melting temperatures of most silicate minerals increase with increasing pressures. So temperatures of solid mantle material at depth may be higher than the melting point of mantle near the earth's surface. As hot deep mantle rises beneath spreading ridges it will, as pressure falls, rise above its solidus, and begin melting. The simplified situation is as follows: 
Mantle melting phase diagram As the uprising mantle crosses the geotherm it begins to melt, and as the solidus temperature of mantle falls with decreasing pressure, the temperature of the melt increases relative to this solidus, thus effectively giving higher degress of melting with decompression, as shown. The amount of melt generated will be limited by the latent heat of fusion (which is high for silicates), and as the melting range of mantle peridotite lies between ca. 1100°C and ca. 1700°C, it is likely that most ridge basalts are partial (rather than complete) melts of mantle. The magma may enter a chamber in the ocean crust and begin crystallising, giving the following P-T path: 
Magma ascent and crystallisation |
Note the "melting with decompression" bit? That means that the liquid has a larger volume or lower density than the solid. And the motion of the bouyant material is shown, going (surprise, surprise!) UPWARDS!
Why don't you do your own research? There is more to science that Walt's references and misinterpretations.
Reading - it would be good for you Stripe! :thumb:
At 200km the pressure is not high enough to cause the reversal spoken of in the report.
This is all theory.
Theory has to fit facts.
Fact is, at great depths, magma is more compact than the rock it is derived from.
gcthomas
New member
Not really - it is based on diamond anvil and shock wave compression of samples to see how they behave.
Yup.
How was this 'fact' arrived at? Your referenced paper was the same mix of theory and lab work as my Nature one - will you reject your own reference as well as mine? :think:
IN any case, the bulk convection in the mantle is OF ROCK NOT MAGMA! Magma forms above descending slabs, since the addition of water and other materials to the mantle lowers the melting point, which then convect upwards (although there are, as you notes, specific conditions that MAY prevent convetion there in some cases).
This doesn't apply to the bulk of the mantle is which the solid convection currents occur.
"At low pressure, olivine fractionation lowers the density of basic magmas, but above 13-14 GPa this trend is reversed. All of these basic to ultrabasic liquids are predicted to have similar densities at 13-14 GPa, and this density is approximately equal to the density of the bulk mantle in this pressure range. This suggests that melts derived from a peridotitic mantle may be inhibited from ascending from depths greater than 400 km."
But the technical article that Walt Brown references (Satoru Urakawa et al) specifically concludes: “The basaltic magma could not ascend from a position deeper than 200 km in the Earth's interior.” The study Walt Brown linked to is 15 years more current than the one you are quoting from.
Like I say, the creationists need to decide on what depth magma can’t ascend from instead of playing whack-a-mole.
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So you've accepted the facts then? At great pressures, rock becomes denser upon melting.
400km.
Not that the depth being touted makes any significant difference to the discussion. Not like when evolutionists wave about three different mechanisms by which plates move and cannot describe an energy source for any of them.
And as you well know, the different studies will yield different depths because they focus on different kinds of rocks, which have different melting points and compressibility. We are interested in an overall picture, which can be easily seen by looking at the issue from a different angle -- seismicity.
You should be quickly able to understand all this far better than I could, but instead you play this silly game of testing me, waiting for me to say something wrong. Why don't you just have the conversation and we can deal with the mistakes I make as I make them? Why the childish games?
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I suspect what you would present as facts are not the things I would accept. If your “facts” are those in the article you reference, are you willing to accept all of what it says?
Are you basing this conclusion on the study you reference? It looks to me like the study was specifically focused on komattiite.
I will take this as a point you disagree with Walt Brown on.
Slab pull - I take it you must not accept gravitational potential energy as a source of energy.Not that the depth being touted makes any significant difference to the discussion. Not like when evolutionists wave about three different mechanisms by which plates move and cannot describe an energy source for any of them.
You have something relevant to offer dealing with seismicity?
I have asked you relevant questions, even numbering some of them (see post 109). Your either ignored them, or gave answers that avoided answering what I asked.
gcthomas
New member
The basic magma you mention is derived from rising mantle rock. As the mantle rock rises, it expands due to the lower pressure. The expansion causes cooling just like gases cool if expands (think CO2 fire extinguisher). For mantle rock the cooling is at a rate of 0.3 deg C per km of ascent. But, the lower pressures reduce the melting temperature of the rock by several degrees per km. (Water penetration in the shallow parts of the mantle could also reduce the melting temp massively.)
So, the mantle rock melts to make the magma not by being heated up, but by decompression melting (melting by expansion). The expansion makes it easier for the atoms to move about like a liquid - at higher pressures they can't move, so it is a solid. Liguids are in general less dense than their originating solids for this reason.
So your "rock becomes denser upon melting" should be "rocks melt by becoming less dense".
200km/400km doesn't make much of a difference in prnciple. Seismological studies suggest the 400km figure is more likely, but not proven yet.
BUT the solid mantle rocks can come up from much deeper and rising to shallower than that depth before generating the magma by melting. The MAGMA convection limits don't apply to the solid MANTLE convection.
I don't accept facts based on where they appear. I accept facts based on evidence. :thumb:
You should have finished reading my post before responding to it. :up:
Get your sources right, mate. :thumb:
And I am not disagreeing with any particular figure.
You promise?
Of course. However, if you were as up to date with Walt Brown's book as you like to portray, you'd already know what I was referring to. :up:
It's all rock. If you have got some reason why heated rock at greater depth becomes less dense than magma does below the crossover depth, we'd sure be glad to hear it. :up: