Okay, this is offically Armok's personal magama forge.
I mean, look at the shape of the thing! it's got two spires and a giant funnel! there's no way this is a regular volcano!
Well, they're a pretty regular occurrence now in DF.
It just means that there's a lot of magma forges, and demons down there are probably just fair laborers of the great world. Dwarves, being sadistic consumers as they are, do not understand what they break. They dig and mine out everything they find, not thinking that it could be someone's natural industry they break. Those adamantine spires are probably some natural equipment of the world's industry as well, like nerves for human body or something. That's why demons are pissed off at dwarves mining them out.
Actually, all this is just a speculation and random ideas, but... Why not? What do we know about the DF world? Nothing! Could it be that the world is itself an inorganic huge living (in some way) being (it even has a name!) with demons as its immune system guarding its blood (magma) and nevers (adamantine)? And those volcanoes are just natural furnaces which form obsidian and warm the air where needed?
Hehe, I love DF world.
P.S. The above ideas mean that "civilized" races are in fact some parasites on a great body of the world. Elves try to be fair with the world, but other races hate them for that and kill them o_O. It means that we (dorf-lovers and elf-haters) are just a part of a great infection.