We know that Ancalagon was killed by Earendil, but how exactly was he killed? Ancalagon was so incredibly massive and Earendil was just a dude in a ship with some Eagles.
This image ↓ isn't canon but it probably isn't far from Ancalagon's in-universe size description where he's said to have destroyed 3 giant volcanoes just by falling on them.
There is no canon answer to this. As you point out, literally the only description we have of this is that
Before the rising of the sun Eärendil slew Ancalagon the Black, the mightiest of the dragon-host, and cast him from the sky; and he fell upon the towers of Thangorodrim, and they were broken in his ruin.
(The Silmarillion, Chapter 24, "Of the Voyage of Eärendil and the War of Wrath")
However, consider the power of the Silmaril that Eärendil wore:
And the wise have said that it was by reason of the power of that holy jewel that they came in time to waters that no vessels save those of the Teleri had known; and they came to the Enchanted Isles and escaped their enchantment; and they came into the Shadowy Seas and passed their shadows, and they looked upon Tol Eressëa the Lonely Isle, but tarried not; and at the last they cast anchor in the Bay of Eldamar, and the Teleri saw the coming of that ship out of the East and they were amazed, gazing from afar upon the light of the Silmaril, and it was very great.
(The Silmarillion, Chapter 24, "Of the Voyage of Eärendil and the War of Wrath")
Further, this was the very Silmaril which Beren and Lúthien had taken from Morgoth, and which injured the wolf Carcharoth nearly to death:
Carcharoth looked upon that holy jewel and was not daunted, and the devouring spirit within him awoke to sudden fire; and gaping he took suddenly the hand within his jaws, and he bit it off at the wrist. Then swiftly all his inwards were filled with a flame of anguish, and the Silmaril seared his accursed flesh. ... Of all the terrors that came ever into Beleriand ere Angband's fall the madness of Carcharoth was the most dreadful; for the power of the Silmaril was hidden within him.
(The Silmarillion, Chapter 19, "Of Beren and Lúthien"; emphasis added)
Thus it is probably not beyond reason to believe that the power of the Silmaril aided Eärendil in some way, and perhaps even had some direct part in the death of Ancalagon. But we are never told in so many words.
To answer this question it's necessary to review the textual history of this passage, because what we have in the published Silmarillion is actually an editorial construct and implies a causal relationship between Ancalagon's fall and the Breaking of Thangorodrim that is weaker in JRRT's own writings.
The slaying of Ancalagon makes it's first entry in what CT labels the "Q II text" of the Quenta Noldorinwa, given in History of Middle-earth 4:
And Earendel slew Ancalagon the black and the mightiest of all the dragon-horde, and cast him from the sky, and in his fall the towers of Thangorodrim were thrown down.
This also appears in the Annals of Beleriand (text "AB I" - same source) as follows:
Earendel came in the sky and overthrew Ancalagon the Black Dragon, and in his fall Thangorodrim was broken.
In the Later Annals of Beleriand (moving on to History of Middle-earth 5) the text is virtually unchanged from AB I:
But Earendel came in the sky and overthrew Ancalagon the Black Dragon, and in his fall Thangorodrim was broken.
And in the 1937 Silmarillion (HoME 5 again) the text reads:
And ere the rising of the sun Earendel slew Ancalagon the Black, the mightiest of the dragon-host, and he cast him from the sky, and in his fall the towers of Thangorodrim were thrown down.
The final texts are given in History of Middle-earth 11, firstly from the revised ending of the Silmarillion:
And ere the rising of the sun Earendel slew Ancalagon the Black, the mightiest of the dragon-host, and he cast him from the sky, and he fell upon the towers of Thangorodrim and they were broken and thrown down.
And finally from the Tale of Years:
Ancalagon is cast down by Earendil and all save two of the Dragons are destroyed.
There is therefore no authorial warrant for the published text's statement that the towers of Thangorodrim "were broken in his ruin", and the constituent texts leave open the possibility that the slaying of Ancalagon and the Breaking of Thangorodrim were two separate events that just happened to occur at the same time.
This lack of a causal relationship is reinforced further by the opening text of Appendix B of Return of the King, which reads:
The First Age ended with the Great Battle, in which the Host of Valinor broke Thangorodrim and overthrew Morgoth.
Although evidently a summary, the statement that it was the Host of the West that broke Thangorodrim is notably unambiguous. This may be contrasted with Elrond's statement (in The Council of Elrond) which is also a summary but left vague:
And yet not so many, nor so fair, as when Thangorodrim was broken...
The summary of events is therefore:
Although a causal relationship between (5) and (6) is certainly implied, it is not required by JRRT's original texts; the editorial change makes it stronger than it originally was. A valid reading of the original texts is that "Earendil slew Ancalagon and then the Host of the West broke Thangorodrim".
All of which is a very long-winded way of coming to the conclusion: Ancalagon need not be as huge as he is commonly supposed to be.
This doesn't really answer the question, but as far as I have been able to measure things, in this picture, titled "Conversation With Smaug", by Tolkien himself, Smaug is about 11 times longer than the figure in the shadows and smoke on the right hand side of the image, who I assume is Bilbo.
Tolkien said hobbits rarely grew to be taller than about 3' 6" in the Third Age, and if we assume that Bilbo was roughly that height, then Smaug is around 38 feet long from the tip of his nose to the end of his tail.
In the image you posted, the visible portion of Ancalagon's body and head is at least 6 times longer than Smaug, and it is very likely that the portion of his body which isn't in the picture is at least 3 times that length. That makes him roughly 720 feet long, or about 2 1/2 football fields. By way of comparison, a Boeing 747 is only 250 feet. Ancalagon is almost as long as three 747's laid end to end.
This leads me to two possible conclusions:
The image you posted, and which you acknowledge is non-canonical, is wildly inaccurate, to the point of being useless.
The image you posted is roughly accurate, and Ancalagon is really that big, more or less.
If the latter is true, the only way any human/elf/half-elf could hope to kill such an enormous monster is with magic. Any blade short enough for a humanoid to wield would do little more than annoy the dragon, if it even noticed that it was stabbed at all. Its internal organs would probably be 15 to 20 feet inside its skin, behind several feet of scales, and a couple of yards worth of muscle and bone. To pierce its heart, you would need a spear 50 feet long. Basically, you would have to stand there with a spear too big for you to use it effectively, struggling to keep it upright, and hope that the dragon would be stupid enough to land directly on top of it, with enough force to drive the point deep into its abdomen.
Before the rising of the sun Eärendil slew Ancalagon the Black, the mightiest of the dragon-host, and cast him from the sky; and he fell upon the towers of Thangorodrim, and they were broken in his ruin.
Is there ever any conformation that he 100% destroyed Thangorodrim? I can't imagine Tolkien would have created a beast that was big enough to destroy things of that magnitude by falling on it...I mean think of how large an object would have to be to destroy Everest completely. It would probably have its own gravitational field...
If I had to choose between thinking that Tolkien meant this as a metaphor or that he literally destroyed Thangorodrim with his weight, I would go with the former. If that's the case, then Ancalagon is probably much smaller than all these pictures, and it doesn't become so unrealistic that he was killed by Eärendil + the eagles.
Eärendil's ship Vingilot could fly, and was used during the War of Wrath to fight Ancalagon. It was powered by the Silmaril which possessed great powers. I'm not sure how exactly he was killed, but the ship did play a big part of it. Perhaps they had some great weapon which wasn't written about or they were able to focus the power of the Silmaril against Ancalagon. I believe it was an air battle because when he fell, he crushed the mountains of Thangorodrim.
The Silmarillion is more mythological than The Hobbit and LOTR and there are just so many things which weren't written about.
I always believed that the fall of Analagon not only destroyed Thangorodrim but also started the series of gigantic earthquakes and volcanic eruptions which eventually destroyed Beleriand, so that everyone would have been periodically shaken off their feet by the recurring series of earthquakes, and sometimes dodging lava bombs, during the remainder of the battle and the evacuation of the Edain to Numenor.
Have you ever seen the coat of arms of Transylvania?
The horizontal red bar divides it into an upper and a lower image. The seven red objects in the lower image are not wheat sheaves as I assumed at first, but stylized fortresses, castles, or fortified towns, the seven cities of the German name of Transylvania.
If you look at it as a single image, you see a giant eagle rising from below the horizon above the tiny cities on the plain. And that is pretty much the way I have always pictured Ancalagon rising up from behind the mountains in an attempt to eat the sun and the moon.
I think that the image at the top shows Glaurung too large compared to Smaug. Remember that Glaurung's body was slender and thin enough that a sword probably less than six feet long could penetrate to a vital organ, thus making Glaurung probably much less than fifty or twenty five feet thick in the body. And after first reading The Hobbit I have always calculated that Smaug should have been at least ten feet thick in the body and two hundred feet long.
Note that Smaug could smash many tons of rock off the mountain with a swing of his tail. Then calculate the size of Ancalagon necessary to smash the towers of Thangorodrim.
Reading the passage where the towers of Thangorodrim were thrown down in his ruin as this being the act that killed him makes the most sense to me. I hadn't even considered this interpretation before but now it sounds to me like Tolkien gave us the answer but we were just not reading it. Earendil caused all three of the volcanic peaks to erupt at the same time and the force from this destruction is what killed Ancalagon. Now, if it took causing three simultaneous eruptions to kill Ancalagon I think it would be very hard to imagine him as bigger than the was intended to be. 3 explosions bigger than the most recent explosion of Mount St Hellens to destroy it? Ancalagon must have been huge and nearly as powerful as a Valar. Now that I see this interpretation I can't really read it any other way. This was a BIG dragon by any measure.
I agree with Jason, I believe that Ancalagon's fall started the sinking of Beleriand, but didn't start it by itself. What I think happened was that Earendil fought Ancalagon AT THE SAME TIME as the rest of the Valar wher battling Morgoth underground, and their battle weakened the earth underneath the peaks and destabilized the ground, so when Ancalagon fell he disturbed the earth, starting a cataclysm deep in the earth, so the Valar took down Morgoth as the disaster started, and then everyone either fled or drowned/was crushed.
Also, technically Thangoridrim themselves were towers. Yes, giant, mountain-shaped towers. I say this because in the Silmarillion it says that Thangorodrim was built out of the slag from Morgoth's forges, which tells me either:
Or
Eärendil (legendary hero blessed by the gods) had a flying ship (battle cruiser) with a Silmaril cannon attached to it (Yamato gun). Ancalagon was BTFO.
"Eärendil came, shining with white flame, and about Vingilótë were gathered all the great birds of heaven and Thorondor was their captain, and there was battle in the air all the day and through a dark night of doubt. Before the rising of the sun Eärendil slew Ancalagon the Black, the mightiest of the dragon-host."
My best guess was that Earendil (and the eagles he led) tore and cut at Ancalagon's wings, while bringing the battle ever higher, until the wings were too tattered and he fell, the fall killing him.
But while believe-able, my theory seems unlikely, given that Tolkien always writes that the killing came before the casting down
