I recently visited Mt. St Helens and at the visitors center they showed movies. Of course they couldn't show anything decent like Ant-man or Jurassic World. Oh no, they had to show a documentary about Mt. St. Helens. They didn't even have the courtesy to show any good trailers. Sigh the new generation doesn't have any good manners. Anyway the documentary gave me a weird idea. The Lonely Mountain from the hobbit must be volcanic. This is because if it was formed from tectonic plates brushing up against each other, it would be a whole mountain chain. It is called the Lonely Mountain for a reason. This means that it must be created from volcanic activity. Sure you might say "hey, random person who spends his time writing weird and disgusting things, the Lonely Mountain does not have any record of erupting." In which case I would say "Yay someone commented on my post!"
Just because it has not erupted doesn't mean it won't. Have you seen how far the dwarves have dug. Their tunnels are MASSIVE. Also note that they have dug straight down. All it would take is a little tectonic action and cablooey smug smaug would be smiling smackulously. Wait smackulously I don't think that smackulously is a word, but hey etymology isn't my thing. More like eatemology. Ha! take that viewer who has an English Major. Suffice to say that Thorin could have waited till the magma pilled up enough and despite Smaugs fire proof armor, everything has a melting point even Smaug. So Smaug the terrible would have a meltdown. And forget dwarven magic. Even they could not protect against magma.