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Author Topic: How do mantle plumes work?  (Read 1440 times)

Offline Bill S

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How do mantle plumes work?
« on: 23/03/2014 14:19:56 »
Let us look first at temperature: We find that the temperature at the bottom of the mantle is about 3,740K, and the temperature at the top is about 930K.  Increased temperature leads to decreased density, so one might expect the mantle to be constantly convecting, like a pot of simmering soup.  However, gravity causes density to increase with depth, so we must look also at relative densities.  We find that the density of the material at the bottom of the mantle is about 5,560 kg/m3, while at the top it is about 3,370 kg/m3.  This seems to suggest that material at the bottom should stay at the bottom, and material at the top should stay at the top.  However, things are rarely that simple.

Of course, the first thing to recognise is that convection does not necessarily require that the rock at the bottom of the column be less dense than the rock at the top.  All that is needed to start convection is for the rock at any point in the column to become less dense than the rock directly above it; or to become more dense than the rock directly below.  The rate at which convection will then proceed depends to a great extent on the plasticity of the rock.  In general, the hotter the rock in any given environment, the more readily it will flow, and the more easily convection will progress.

The idea of mantle plumes seems to be gaining popularity in geological circles. 

My question is:   Does a plume necessarily involve a single-cell unit of Rayleigh-Benard type convection, or can it still be split into an ascending range of cells?



 

Offline evan_au

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Re: How do mantle plumes work?
« Reply #1 on: 24/03/2014 10:07:16 »
I suggest that a trigger for forming a plume is that one patch of mantle is hotter than the adjacent patch. The warmer patch will tend to move up, while the cooler patch will tend to move down, exposing it to the source of heat.
 
But the rising plume does not need to take the form of a geometrically regular convection cell.
 

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Re: How do mantle plumes work?
« Reply #1 on: 24/03/2014 10:07:16 »

 

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