TRoP – Orodruin, Mount Doom

“[There] was a fiery mountain in that land that the Elves named Orodruin. Indeed for that reason Sauron had set there his dwelling long before, for he used the fire that welled there from the heart of the earth in his sorceries and in his forging; and in the midst of the Land of Mordor he had fashioned the Ruling Ring.”

The Silmarillion, Of the Rings of Power and the Third Age

Morgoth created Mount Doom in the First Age. It was a tall mountain alone on an empty plateau. Orodruin was the primary reason Sauron chose the land of Mordor as his dwelling-place in the Second Age. Orodruin is more than just a "huge mass of ash and slag and burned stone, out of which a sheer-sided cone was raised into the clouds." It responded to Sauron's commands and his presence, lapsing into dormancy when he is away from Mordor and becoming active again when he returns. The Sammath Naur ("Chambers of Fire"), located high up in the mountain's cone, was made by the Dark Lord Sauron in the Second Age. It was in these fiery chambers that Sauron forged the One Ring.

The mountain erupted in Second Age 3429, signaling Sauron's resurgence at which time Orodruin was named Amon Amarth, "Mount Doom". In Third Age 2954, Mount Doom reawakened and it erupted sporadically until the end of that Age, providing the cover of darkness for Sauron’s Orcs.

The Fellowship of the Ring's quest in the War of the Ring was to destroy the One Ring at Mount Doom. When Sauron is defeated at the end of the Third Age, the "burning mountain" erupts violently. “Orodruin reeled”, “the side of the cone was riven open”, “and the Mountain was convulsed. Great rents opened in its side.”

The Rings of Power

Adar’s Orcs build trenches and tunnels leading into the magma chamber within Orodruin. The watch tower of Ostirith, used by the Elves to watch over the Southlanders, is also a rampart damming a lake above the valley. When breached this water was channeled directly into the fiery heart of Orodruin. The cool water pouring into the chamber of magma immediately causes the volcano to erupt. The pyroclastic explosion sends up an umbrella of grey clouds composed of ash and hot gasses shielding Adar’s Orcs from the sun. The surrounding lands are destroyed becoming “the great heaps and hills of slag and broken rock and blasted earth” that Frodo and Sam will eventual make their way through.

Orodruin – Sindarin "burning mountain" / "mountain of the red flame"

Ref:TolkienGateway

Artwork:J.R.R. Tolkien, NewLine Cinema/WingNut Films, PrimeVideo