How big was Ancalagon?

[Puddleglum]

Originally Posted by Ibrîniðilpathânezel

On the other hand, there is some precedent for Tolkien's fiery critters breaking mountains when they fall and die.

Originally Posted by Ibrîniðilpathânezel

Then again, maybe when these fire-critters fall and die, they explode in a nuclear fashion and flatten the landscape.

It's the "Dark Side" energy being released when they die <g>

Seriously, if you look at the other dragon deaths which Tolkien described, I think we have to allow for more than just a dead weight falling.

• When Glaurung was mortally stabbed, he started thrashing around breaking things.
• When Smaug received his mortal wound he shot spouting into the air, crashed down on Esgaroth, and completed the total destruction of the town by his death-flailings.

Also, if the "three peaks of Thangorodrim" were more in the style of mighty towers of slag and ash - then think how it only took a single Boeing 767 to bring down each of the WTC Towers on 9/11.
All it takes is for a thrashing Ancalagon (mightiest of the winged dragons) to weaken the supportting infrastructure of the towers enough (in his death throes) that they collapse under their weight - not unlike the WTC towers.

[Fordim Hedgethistle]

Pretty darn big.

[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.