On Sat, Jun 20, 2009 at 9:57 AM, <[email protected]> wrote: > In reply to David Jonsson's message of Wed, 17 Jun 2009 14:48:21 +0200: > Hi David, > [snip] > >> > The magma is hot becasue it is pressurised. > > This is not the only reason it is hot. There is also *at least* radioactive > decay. (And perhaps also some CF considering the small amount of Tritium > that is > occasionally also detected - though this could also be a byproduct of > spontaneous fission). > > >>> When you pick > >> it > >> >up to earth it will expand and cool. > >> [snip] > >> Volcano. > >> > > > >OK, I have to admit I haven't studied the magma but only the crust. And it > >surprises me much that the magma has an adiabatic gradient of only 0.3 > K/km. > >How was that calculated? > > You did the calculation. AFAIK others just measure it.
No this calculation was made by someone else. I just referred to it from http://en.wikipedia.org/wiki/Geothermal#Variations It would be very interesting to know how they calculated it. From solid state physics, experimentally or something else. Measuring an adiabatic gradient is not easy and doing it on location below the crust seems impossible. > > > > >Admit that the crust will cool if picked up. > > Certainly something will cool (the energy to fight gravity has to come from > somewhere), but what comes to the surface may not be the same thing that > cools. The calculation I presented earlier, which I now removed, showed that the heat gradient balances compression in the crust. Thermal expansion compensates for elastic compression. Why? > I'm not sure how relevant it is, but I did the following simple > calculation, > which assumes that the gravitational energy of a falling body is all > converted > into heat. For stone, the specific heat is about 2 cal/gm*K. If we divide g > (gravitational acceleration at the Earth's surface) by this, we get 11.7 > K/km. Corect, I also did that initially, and that is the method used for gases where the constituent moleculeas mostly are in free fall. In solid and liquid matter this is no longer the case where molecules and atomes are tightly bound to each other. But exactly how tightly bound are they? I suggest someone with a centrifuge to make an experiment. Desktop centrifuges can now produce one million g. Maybe solid state physics can determine how tightly bond atoms are in a crystal? Regards David
