There is evidence that supports what I think, and one piece of evidence is the “Crystal Caves” below Canterlot. Crystals are often formed, and found, in volcanic tubes.
Not only that, but there was even a mine shaft that Princess Cadance and Twilight used to escape the Crystal Caves!
Thinking about how Canterlot might have been built on a volcano, I started thinking about the mountain named “Smoky Mountain” just outside of Ponyville that the mane six had to climb in the episode “Dragonshy.”
I believe that mountain is actually another volcano. There is also evidence to support this theory as well! What is this evidence of mine (besides the name) you ask? Three words: Sweet Apple Acres.
The Apple Family claims to have the best tasting apples in all of Equestria, and I wouldn’t doubt it! Volcanic soil is very rich in nutrients, and that is why their trees grow so well and the apples taste so good!
If you can recall in the episode “Family Appreciation Day,” we learn that Princess Celestia gives the Apple Family that land to build a farm.
Granny Smith: And as if the beauty of that city wasn’t enough, suddenly, she appeared. Princess Celestia, the most regal of all ponies. When lo and behold, she stopped to look at my pa’s seed collection. Then Princess Celestia saw that we were plumb-tuckered and hankerin’ to find our forever home. And bein’ a royal Princess and all, she knew exactly the place for us to lay down our stakes.