Mount Doom, or Orodruin, was a volcano in Mordor.
Theories
According to Karen Fonstad, Mount Doom was obviously a stratovolcano, composited by alternating layers of ash and lava. Towering at only 4500 ft, it was relatively short.
Geography
It stood alone in an empty plain, the Plateau of Gorgoroth and was connected to the Dark Tower with Sauron's Road, rising about 4500 feet with its base about 3000 feet tall. The Road approached the east side of the base at a causeway and then wound up like a snake; at that point, the Road seemed damaged by the lava and re-repaired several times.
Inside its cone, were the Sammath Naur leading to the Crack of Doom, a fiery chasm where the One Ring was forged.
Melkor created Mount Doom in the First Age, and the name "Mordor" may have been given to the surrounding land before Sauron settled there because of its eruptions. When Sauron chose the land of Mordor as his dwelling-place in the Second Age, Orodruin was the reason for his choice. He "used the fire that welled there from the heart of the earth in his sorceries and his forging." The most famous result of his forging, and in fact the only one we know of for sure, was the One Ring.
The mountain erupted in S.A. 3429, signalling Sauron's attack on Gondor and it took the name Amon Amarth, "Mount Doom".
In T.A. 2954, Mount Doom reawakened and the last inhabitants of Ithilien terrified fled over Anduin. From then on it erupted sporadically until the end of the Age.
The Fellowship of the Ring's quest in the War of the Ring was to destroy the Ring at Mount Doom.
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