A Volcano is a rift or an opening in the upper surface of the world that expels molten lava when it erupts.
The origin of volcanoes in the world of Arda has not been made clear. They may be naturally occurring phenomenon or some unnatural corruption of something else. It is most likely that they are a little bit of both.
Significance in the StoryEdit
The main volcano shown in Middle-earth is Mount Doom in Mordor which was heavily tied into the power of Sauron. The Silmarillion suggests that Sauron used Mount Doom as a direct conduit to access the power of the Secret Fire that Eru placed at the heart of Arda.
Sauron created the One Ring by the heat of the volcano. Elrond and Gandalf state that the volcano's heat is the only thing that can destroy the One Ring, hence the Quest, in which the Ring is ultimately cast back into the very fire that forged it, thus destroying it forever.
The lava lake within Mount Doom was the cause of the death of Gollum, who fell in while in the midst of an ecstatic dance after recovering the Ring from Frodo by biting off his finger. (In the films, Gollum fell into the lava after a protracted fight with Frodo, who was attempting to retrieve the Ring from Gollum.)
The Thangorodrim were a group of three volcanoes in the Iron Mountains in the north of Beleriand that were destroyed in the War of Wrath. Udûn in Mordor may be the remnant of an early volcano. It is possible that the Echoriath may also be the remnant of an earlier gigantic volcano's caldera with Amon Gwareth as its secondary cone.