Generation of plume (in the Earth's mantle)

[billiards]

I'm wondering if there is a generally accepted mechanism by which a plume is formed and how this then applies to the Earth's mantle, and so-called "mantle plumes".

Specifically I am a bit stuck on these questions:
Do plumes require a thermal boundary layer as their source?
What process operates to initiate the plume?
What exactly is a thermal boundary layer anyway?

 

[Darwin123]

I'm wondering if there is a generally accepted mechanism by which a plume is formed and how this then applies to the Earth's mantle, and so-called "mantle plumes".

Specifically I am a bit stuck on these questions:
Do plumes require a thermal boundary layer as their source?
What process operates to initiate the plume?
What exactly is a thermal boundary layer anyway?

It is like asking what forms a bubble when one heats a pot to boiling. Or how do plumes form in a lava lamp.
1) There doesn't have to be a boundary per se. There just has to be a convective instability. The temperature can be a smooth gradient. There may be discontinuous boundaries in this system, but they don't cause the mantle plumes to form. The discontinuities definitely affect the mantle plumes, but the boundaries aren't the fundamental cause.
2) The process of initiating a boundary plume involves nonlinear feedback mechanisms of the unstable system of a high weight density fluid on top of a low weight density fluid.
3) Are you sure that you mean "thermal boundary layer"? Maybe you mean "Layer with a thermal gradient". Or a "no gradient layer". Or "unstable stratified system."
The initiation of plumes is common in many systems which are intrinsically unstable. In the case of the earth, the instability is caused by the condition that the center of the Earth is hot, the surface of the Earth is cool and gravity points downward. On a geological time scale, the mantle and the core behave like fluids. So the mantle is unstable.
Small inhomogeneities are amplified by the buoyant and gravitational forces. The small inhomogeneities grow into mantle plumes. I believe this is also called convective inhomogeneities.
A mathematical description of the amplification process is called the “Rayleigh-Taylor instability. It is important in many systems other than the Earth's mantle. The general idea is a physics concept, but it is applicable all over.
The mantle forms due to an inhomogeneity in the surface of the mantle.
The process that amplifies this inhomogeneity has to do with the balance between buoyancy, gravity and viscosity. .

Here is a link to a reference on the general process.
http://en.wikipedia.org/wiki/Rayleigh–Taylor_instability
“The Rayleigh–Taylor instability, or RT instability (after Lord Rayleigh and G. I. Taylor), is an instability of an interface between two fluids of different densities, which occurs when the lighter fluid is pushing the heavier fluid.
…
The inviscid two-dimensional Rayleigh–Taylor (RT) instability provides an excellent springboard into the mathematical study of stability because of the exceptionally simple nature of the base state.
…
The analysis of the previous section breaks down when the amplitude of the perturbation is large. The growth then becomes non-linear as the spikes and bubbles of the instability tangle and roll up into vortices. Then, as in the figure, numerical simulation of the full problem is required to describe the system.”

http://scales.colorado.edu/reckinger/Pubs/p6_CFD.pdf
“Simulation of Classical
Rayleigh-Taylor Instability
SCOTT J. RECKINGER”

Here is a link to Earth related processes.
http://people.earth.yale.edu/sites/default/files/diapir-PEPI97.pdf
“The nonlinear initiation of diapers and plume heads
by Bercovici and Kelley (1997)
A simple theory is devised to describe the non-linear feedback mechanisms involved in the initial growth of a diaper or plume head from a low viscosity channel overlain by a much more viscous layer.”

Here is a link one type of inhomogeneity called delamination. http://en.wikipedia.org/wiki/Delamination_(geology)
“The second type, ductile delamination, is related to convective instabilities. The convection can simply peel away the lower crust. Or, in a different scenario, a Rayleigh-Taylor instability is created. Due to the instability in a local area, the base of the lithosphere breaks up into descending blobs fed by an enlarging region of thinning lithosphere. The space left by departing lithosphere is filled by an asthenosphere upwelling.”

 

[billiards]

Thanks for your input Darwin.

1) There doesn't have to be a boundary per se. There just has to be a convective instability. The temperature can be a smooth gradient. There may be discontinuous boundaries in this system, but they don't cause the mantle plumes to form. The discontinuities definitely affect the mantle plumes, but the boundaries aren't the fundamental cause.

You often hear the argument that plumes must come from the core-mantle boundary, because just above the core-mantle boundary there is apparently a thermal boundary layer. This argument was even cited in the Bercovici and Kelly paper you linked to. If plumes don't require a thermal boundary layer, then this argument does not stand up to scrutiny. I suspect that plumes do need a thermal boundary layer in the Earth, but that this is not generally true of all systems.

2) The process of initiating a boundary plume involves nonlinear feedback mechanisms of the unstable system of a high weight density fluid on top of a low weight density fluid.

You see perhaps this is why you do need the thermal boundary layer in the Earth's mantle. Because generally speaking, the (chemically) dense stuff in the Earth has already sunk down. The thermal boundary layer heats material up and encourages it to expand (and become less dense). Perhaps this is how it happens that a dense layer overlies a lighter density layer in the Earth.

3) Are you sure that you mean "thermal boundary layer"? Maybe you mean "Layer with a thermal gradient". Or a "no gradient layer". Or "unstable stratified system."

Yes I meant thermal boundary layer, it is a common thing in the Earth sciences. I believe a thermal boundary layer is a layer with a super-adiabatic thermal gradient, in which convection is the means by which heat is transferred. You get thermal boundary layers at the edges of convecting systems.

 

[Darwin123]

Thanks for your input Darwin.



You often hear the argument that plumes must come from the core-mantle boundary, because just above the core-mantle boundary there is apparently a thermal boundary layer. This argument was even cited in the Bercovici and Kelly paper you linked to. If plumes don't require a thermal boundary layer, then this argument does not stand up to scrutiny. I suspect that plumes do need a thermal boundary layer in the Earth, but that this is not generally true of all systems.



You see perhaps this is why you do need the thermal boundary layer in the Earth's mantle. Because generally speaking, the (chemically) dense stuff in the Earth has already sunk down. The thermal boundary layer heats material up and encourages it to expand (and become less dense). Perhaps this is how it happens that a dense layer overlies a lighter density layer in the Earth.


Yes I meant thermal boundary layer, it is a common thing in the Earth sciences. I believe a thermal boundary layer is a layer with a super-adiabatic thermal gradient, in which convection is the means by which heat is transferred. You get thermal boundary layers at the edges of convecting systems.

The core-mantle layer is not a boundary layer. It is a composition and phase layer. The core is an iron-nickel-cobalt liquid, and the mantle is made of solid-plastic insulating minerals (probably oxides of different types). The density of the material is discontinuous at this boundary. However, the temperature is probably continuous across this boundary. Because the temperature is continuous, the boundary isn't a thermal layer.
The discontinuity in density is probably the reason that mantle plumes start at this boundary. Convective plumes can't cross this discontinuity in density. A small change in temperture can't change the local density in the core enough to lift the material into the mantle.
There are probably chemical reactions and other things that generate the initial inhomogeneities that get amplified into mantle plumes. However, core plumes can't leave the core because there is a large discontinuity in density.

 

[billiards]

The core-mantle layer is not a boundary layer. It is a composition and phase layer. The core is an iron-nickel-cobalt liquid, and the mantle is made of solid-plastic insulating minerals (probably oxides of different types). The density of the material is discontinuous at this boundary. However, the temperature is probably continuous across this boundary. Because the temperature is continuous, the boundary isn't a thermal layer.

That said. Just above the core-mantle boundary there is a layer which does act as a thermal boundary layer, this layer is called D''.

The discontinuity in density is probably the reason that mantle plumes start at this boundary. Convective plumes can't cross this discontinuity in density. A small change in temperture can't change the local density in the core enough to lift the material into the mantle.
There are probably chemical reactions and other things that generate the initial inhomogeneities that get amplified into mantle plumes. However, core plumes can't leave the core because there is a large discontinuity in density.

No certainly not, no one is suggesting that plumes come from the core! Plumes are fed by material from the thermal boundary layer.