Archeobacteria (thermophile anerobic protists) live near thermal vents on the ocean floor, in gysers, and underground down to the beadrock. No deeper. Thats far from the mantle.
I looked up the archaea and found that they live near volcanic vents, not in molten lava. They use the heat to synthesize food. So we are assuming the existance of life forms that can exist below the earths crust. "It's not the heat of the molten material itself, but the instability of such regions, the turbulence and such." I would suppose that the areas near a volcano are a bit MORE turbulent, but lava isn't exactly stationary. As for the lava thing, where exactly are you hypothesizing these subterranean creatures would live? There are no caves in lava. If you are thinking about caves, they would be in the crust, not under it.
Unfortunately science is against you in finding anything bigger than microbes anywhere below the crust, and sadly I learned that apparently silicon isn't nearly as good as carbon in bonding complex molecules, so silica lifeforms are less likely than I had thought, here or elsewhere. But, given that you're writing a story, you could go past current science, and speculate on possibilities. There's one book about life found on the surface of a neutron star (I can't recall the name), your setting isn't any more extreme than that. I would suggest go past the crust, and use the fluid mantle as a setting for the creature's world, and the creatures themselves be a collection of solidified, organized minerals (crystalline?) Have the reason they don't come up where we are be two fold...it's much too cold for them to survive normally, and the pressure lessens, making them lose cohesion (opposite of us, too hot, and crushing pressures). Just some ideas...I haven't found any good science that discusses anything larger than bacteria.
What about bacteria that, individually, are just bacteria, but together in large bunches can form a kind of hive-minded colony?
Why would bacteria need a hive-mind? They are allready the most successful kingdom on earth and NOTHING eats Archeobacteria. Basically the whole of the abyssal plain is a giant mass of these things. Evolution dosnt happen without an incentive.
