Something I was pondering briefly in bed last night as I mucked around with my old lava lamp. Can a mantle plume be small or low-energy enough so that when it rises from the core-mantle boundary, it doesn't actually reach the crust, but rather reaches a neutral bouyancy level somewhere in the upper or lower mantle?
If it does then reach this neutral bouyancy level, would it then cool to the same temperature as the surroundings? Since it formed in the deep lower mantle, would it then be denser due to different element compositions and then sink back down, or would it stay there?
Presumably it would also cause displacement of mantle above it leading to crustal doming, but without the attendant active volcanism associated with mantle plumes? And if it does cool and sink, would you then get basin formation on the surface where the doming collapses?
This stuff isn't something we've looked at in major detail at uni, so any older (or younger) and wiser opinions are welcome.
Geologists: Can mantle plumes collapse?
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Geologists: Can mantle plumes collapse?
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Mr Flibble
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Well this is what I remember from my tectonics course last semester and geophysics this semester, take it with a pinch of salt.
The current model for mantle convection is described as 'leaky' two layer convection. That is that the mantle has two layers undergoing separate convection, but occasionally they interact (i.e. leak), for example plumes coming from CMB (core-mantle boundary) to crust and the collapse of subducted slabs below the mantle transition zone (the mantle undergoes a change in mineral (same chemical, different structure) from olivine to spinel IV due to the pressure).
How does this relate to your question? I sort of interpret (that is code for I could be making this up) it this way: Well Plumes from the CMB which don't make it to the surface exist, and when the reach the density contrast of the mantle transition zone they stop rising cool and then sink back down, that is the lower layer convection cycle and is typical convection. Mantle plumes which go all the way to the surface are atypical; we just are able to see them. This occurs deep enough (IIRC ~600km? deep) that the effects aren?t realised at the surface, or at least we don't notice them. That and the cooling part of the convection cycle 'makes room' for the heating part.
Incidentally, seismic studies (tomography) seem to show that some of the mantle plumes at the surface originate at the transition zone, not the CMB, but data is limited and research ongoing into it.
Hope that was a little helpful, and that I'm not way off the mark. Captain Chewbacca would probably know better than me.
The current model for mantle convection is described as 'leaky' two layer convection. That is that the mantle has two layers undergoing separate convection, but occasionally they interact (i.e. leak), for example plumes coming from CMB (core-mantle boundary) to crust and the collapse of subducted slabs below the mantle transition zone (the mantle undergoes a change in mineral (same chemical, different structure) from olivine to spinel IV due to the pressure).
How does this relate to your question? I sort of interpret (that is code for I could be making this up) it this way: Well Plumes from the CMB which don't make it to the surface exist, and when the reach the density contrast of the mantle transition zone they stop rising cool and then sink back down, that is the lower layer convection cycle and is typical convection. Mantle plumes which go all the way to the surface are atypical; we just are able to see them. This occurs deep enough (IIRC ~600km? deep) that the effects aren?t realised at the surface, or at least we don't notice them. That and the cooling part of the convection cycle 'makes room' for the heating part.
Incidentally, seismic studies (tomography) seem to show that some of the mantle plumes at the surface originate at the transition zone, not the CMB, but data is limited and research ongoing into it.
Hope that was a little helpful, and that I'm not way off the mark. Captain Chewbacca would probably know better than me.