Naked Science Forum
Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: billyaimee on 14/06/2018 14:30:27
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So the general idea of plate tectonics is that the hardened crust is subducting into the hot mantle, where the material is then 'recycled' by magma convection currents, eventually forming into new crust at seafloor spreading zones.
(https://upload.wikimedia.org/wikipedia/commons/thumb/2/27/Oceanic_spreading.svg/450px-Oceanic_spreading.svg.png)
Notice in the above image that the magma convection currents appear to be operating at depths of 700 km.
Yet recent studies appear to show that magma will nolonger rise upwards after it has reached a depth of roughly 200 km in the mantle. From what I gather, this is because there is so much downward pressure from above at this depth, that the magma will begin to be forced downwards deeper into the earth's core. Here's an excerpt from the study:
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Anomalous Compression of Basaltic Magma: Implications to Pressure-induced Structural Change in Silicate Melt
Satoru Urakawa, Tatsuya Sakamaki, Eiji Ohtani
The Graduate School of Natural Science and Technology,
Okayama University
The transportation of magmas, which are generated by the partial melting of deep-seated mantle rocks, in the planetary interior is driven by buoyant force. The relative density between magmas and surrounding rocks controls whether magma ascends to the surface or remains in the deep interior... A linear extrapolation of our data, if correct, may shift the pressure of density crossover toward a pressure lower than that of the previous estimation. Compared with the density of olivine, which is the crystalline phase coexisting with basaltic magma in the mantle, the density of the basaltic magma would exceed that of olivine at approximately 7 GPa, which is lower than the pressure of density crossover predicted by Agee by 1 GPa. The basaltic magma could not ascend from a position deeper than 200 km in the Earth's interior.
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So is the whole idea of these mantle convection currents flawed, or would they still work at more shallow depths under 200 km? I haven't seen any discussion about this so I'm curious what geologists have to say about it.
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the hardened crust is subducting into the hot mantle, where the material is then 'recycled' by magma convection currents, eventually forming into new crust at seafloor spreading zones
There are two main types of rock we see:
- Lighter Aluminium-rich silicates (SIAL), mostly seen in the continental plates
- Denser Magnesium-rich silicates (SIMA), mostly seen in deep ocean seafloor (and, mostly out of sight, also present in the upper mantle).
At a trench at the continental edge, the lighter SIAL rocks float over the denser SIMA seafloor, pushing the SIMA rocks into the upper mantle.
At the mid-ocean ridge, new SIMA seafloor is formed from the upper mantle.
AFAIK, there is no need for the subducted SIMA material from the trench to be recycled via convection currents that take it down near the outer core and back up to the mid-ocean ridge.
So the 200 km depth limit is feasible - the crust is quite thin at the mid-ocean ridges.
See: https://en.wikipedia.org/wiki/Sial
https://en.wikipedia.org/wiki/Sima_(geology)
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I think those huge convection cells are generally regarded a over simplified, these days. One thing to remember, though, is that if something is being forces down, something is, generally, going to come up, whatever the depth, in order to restore isostatic equilibrium.
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You appear to be confusing two related, but quite distinct processes.
Mantle convection currents involve movement of solid rock. Convection is possible in solids via sub-microscopic mineral realignments and recrystallisations. (This is a gross simplification, but sufficient summary for our purposes.)
Magma generation through partial melting of the mantle occurs when temperature and pressure are such as to permit a liquid phase. Then, as noted in the article, bouyant magma will rise through the upper layers of the mantle, likely reposing for a time in magma chambers. (These may be made up of a crystal mush, with less than 20% molten rock, contrary to popular images.)
This partial melting is most likely to occur where rising convection currents of high temperature solid rock are present. What the research has shown is that only above a depth of 200 km is the magma sufficiently bouyant to rise. If the research findings are correct all this means is that magma, as magma, cannot have originated from a greater depth.
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I not contradict anyting here . I do , however , augment sumpin' . All this talk of relative density & boyancy skirts the issue that makes folks exclaim " whoa ! " and " wow ! " . I'm talkin' 'bout 2000 ft. tall lava geysers , and exploding mountains ! Most folks just go WTF , figure there's some pressure somewhere , and let it go . Little do they know , they're holding an Earth in their hand everytime they're about to open a soda container . Shake that puppy first , and you've got your eruption !
The Earth is pressurized by volitile compounds ( water , hydrocarbons , etc. ) trying to separate out from the magma it is dissolved in ( from crust subduction & melting ) . This gas pressure can be incredibly high , ergo 2000 ft. liquid rock geysers . The soda pressure is far lower . It is just CO2 pressure-dissolved into water . MUCH smaller geyser !
OK , pressure rules world ! P.M.
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I can't reply to your question, but I want to say that then and now I am still afraid of cooling the earth's core. Scientists would better try to find a solution for this case rather than understand the pressure issue...
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I can't reply to your question, but I want to say that then and now I am still afraid of cooling the earth's core. Scientists would better try to find a solution for this case rather than understand the pressure issue...
I wouldn't sweat the cooling core, it is going to stay warm for quite a while.
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Gentlemen ,
What might be especially interesting here would be a comparison of the cores of Earth and Venus , in light of the above analyses .
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Gentlemen ,
What might be especially interesting here would be a comparison of the cores of Earth and Venus , in light of the above analyses .
Feel free to make the comparison.
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For some reason , NSF won't let me make the necessary links to expert planetary geologists .
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For some reason , NSF won't let me make the necessary links to expert planetary geologists .
Then do it yourself.