[Galadriel55]

Ancalagon was "pretty darn big", whatever hyperbole Tolkien might have used. Big and powerful. I always thought that he only broke a part of Thangorodrim with his fall and thrashing, which was still a huge part. Perhaps he knocked off four mountains, maybe five, but definitely not the entire chain.
My answer as to why Orodruin errupted when the Ring was thrown in it: the Ring had so much power in it that when it was destroyed, the power caused the "natural disaster". It is also possible that the destruction is more near Mt Doom than any other place, because the Ring's power begun there; Sauron made it in Sammath Naur. I think that it is a metaphorical scene (about the Ring being thrown in, I mean), but exageration is also possible. I find a lot of Tolkien's exagerations to also be symbolic.