[yamo (OP)]

If geothermal energy is implemented on a large scale, might we cool the core of the earth?  Would this change our magnetic fields?

[Geezer]

I suppose removing thermal energy from the core would have to accelerate the rate of cooling. I'm assuming it is actually cooling already, although that may be a bad assumtion. (I think a fair amount of the energy is the result of ongoing nuclear reactions within the core.)

However, unless you were to do this on a truly massive scale, I doubt that it would have a noticeable effect.

[litespeed]

I agree with you geezer. The earths core is continually warmed by radioactive decay, however, we do not know by how much. Further, we have no good enough information to compare this with cooling caused by out migration of magma, hot water, and various gases.

Thermal energy capture seems inconsequential for a couple of reasons. First, anything we can capture is a pitance of what comes out all along the "Pacific Ring Of Fire" let alone all the continuous and intermitent outflows in other places. Second, anything we capture would be near the surface to begin with, and would likely out migrate soon anyway.