Re: heat corpuscles in the mantle
- From: James McNangle <mcnangle@xxxxxxxxxxxxxx>
- Date: Sun, 11 Dec 2005 15:11:17 +1100
"don findlay" <don@xxxxxxxxxxxx> wrote:
>The question was:- In a mantle that is convecting because of the heat
>produced by radioactive elements which might be considered little
>corpuscles of heat tied up in minerals which are locked in the solid
>rock, ..if these 'corpuscles' rise with the heated rock as convection
>says they should, how do other corpuscles manage to stay behind in
>order to keep the thing going next year, or in a million years? I
>mean, ..don't they all rise at once - whether they want to or not?
>Wouldn't the ones that do, carry with them the ones that don't, so that
>they'd all end up pretty quickly at the top of the mantle? A once-off
>rise? And heat things from there?
>
>I'm sure this is a question that gets asked many times and there must
>be a pat answer, but what is it? If the heat loss is so low it can't
>drive convection in water, why would it drive it in solid rock? (And
>that bit about corpuscles.)
There doesn't seem to be any particular problem to me. Charts of continental
drift, such as http://sideshow.jpl.nasa.gov/mbh/series.html, indicate that the
convection cells are 5,000 to 15,000 km across, and the continents are racing
around at speeds of less than 0.1 m per year. It seems probable that the flow
velocity in the convection cells would not be much higher, and these figures
indicate that it would take something like 200 million years for one complete
circuit of a medium-sized cell.
The material in the magma contains a certain amount of radioactive matter, and
this will be decaying steadily, and releasing heat as it does so. As each
individual atom decomposes, it will release a minute pulse of heat (your heat
corpuscles?), which will warm up its surroundings. As atoms steadily decompose,
they will cause the temperature of the magma to rise steadily. When the magma
rises to the top of the convection cell, it will be cooled by conduction through
the earth's crust, and the heat flowing out through the crust will give rise to
the well-known increase in temperature with depth in the crust.
As well as the steady release of energy by radioactive decay, the mantle
material will pick up heat by convection from the core when it is at the bottom
of the cell, and shed this heat when it is at the top.
The amount of energy to drive the convection cell does not the need to be high.
If energy were continually being released in the mantle material at a rate of
one microwatt per tonne, or 30 joules per year, it would cause the temperature
of the material to rise at a rate of roughly 25° C. per per million years. If
we ignore conduction, this would be sufficient to heat the rock by 2,500° C.
during its passage around the bottom half of our medium-sized convection cell.
Knowing the thermal conductivity of the rock in the crust, and the rate at which
temperature rises with depth, we can readily calculate the amount of heat
released into the atmosphere, or the ocean, from the mantle. This energy is
certainly a significant factor in helping to keep the globe warm, but it is far
smaller than the energy from the sun which drives the oceanic and atmospheric
convection cells.
So the answer to your final question is that because of the exceedingly low
mobility of the mantle material the amount of heat actually carried by the
convection cells is very small in comparison with vastly greater amounts of heat
transported by the convection cells in the far more mobile ocean and atmosphere.
And your "heat corpuscles" are just a red herring.
James McNangle
.
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