On the other hand, there is some precedent for Tolkien's fiery critters breaking mountains when they fall and die. The Balrog of Moria "fell from that high place and broke the mountain-side where he smote it in his ruin." Not quite as spectacular as Ancalagon breaking the peaks of Thangorodrim, but the Balrog was smaller, and I doubt that Tolkien ever meant that the dragon took out the mountains (ala the Bakshi interpretation of the end of the Balrog). There is also a possibility that the peaks of the reference are not those of a naturally occurring mountain range, but rather something that Morgoth deliberately fashioned to look more menacing, like spires of rock much closer together than, say, the peaks of the Himilayas.
Then again, maybe when these fire-critters fall and die, they explode in a nuclear fashion and flatten the landscape. Perhaps the Yellowstone caldera isn't a leftover from an old volcano, after all....
