Lava vent system of MountDoom, here in the SammathNaur, the Chambers of Fire, the OneRing was forged by Sauron and in the ThirdAge destroyed (involuntarily) by Gollum.
also called Cracks of Doom.
The LotR video game - oops I mean the movie - is going to get all this totally wrong, so hopefully the following extrapolates the existing canon in a more Tolkienesque, and geologically accurate, direction...
MountDoom comes in two parts; a wide ash cone, and a steep lava cone, shaped like an oast.
Into the bottom of the oast Sauron's servants have tunnelled a small hole; probably only 20 feet high by 25 wide. Just big enough for Sauron in his "Dark Lord" form to enter and work. The gate of the hole should be visualized with simple stone posts and lintels holding it open; thinner posts and lintels, set into the walls, can be dimly seen within.
The hole should be visualized as bare, dry, unadorned, and coated with black soot and dark grey ash. Beyond the actual crack there may be dim shapes; possibly stone supports for the ceiling, possibly some collapsed, possibly part of the ceiling hanging down, and possibly remnants of an ancient and very large forge, with anvils, metal ingots, tongs, etc. All are over four thousand years old, and - though Orodruin did not erupt all that time - they should all have a thick shaggy coat of grey ash. Orodruin has been erupting since 10 years after the Quest of Erebor.
Halfway down the tunnel, as if there by accident, a rift cuts through all the rock of the volcano. So the floor, ceiling, and walls show a gap of maybe just 2 yards or less. The crack shouldn't form a right angle with the floor; it should lean out from the center of the volcano by, say, 20 degrees, following a volcano's natural subsidence. The mountain's rock, along the walls, may split around the rift a little, and in parallel to it. The near and far rims of the crack should match in shape.
The crack vents a magma chamber, but lets its heat and gas escape up, not into the tunnel. Let's say Orodruin erupts constantly, like Mt. Etna or Mauna Loa, and that Sauron has contrived a lava tube below this chamber to carry the main torrent away. Such volcanos have nearly permanent lava tube systems.
The SammathNaur is a "skylight" above this tube. The crack interrupts the lava tube just as it does the corridor above. As the tube flows across the fissure, the walls and ceiling that it constantly renews fail, and the magma encounters air.
Because magma contains dissolved gas, and because the skylight is probably the first place the magma has seen atmosphere, the reduced pressure decompresses the dissolved gas explosively. The magma bubbles, and throws up sprays and fireworks, with an irregular but frequent rhythm; the analog of a geyser.
Mt. Stromboli, the most active volcano in the world, shows this effect:
'The lava from Stromboli is typically less fluid than those of say, Hawaii. The result is thicker, shorter lava that is a tough substance for gas to escape from. The gas bubbles become pressurized and burst at the top of the magma column, producing small explosions and throwing clumps of molten lava into the air. When this type of explosion occurs at other volcanoes, it is referred to as "Strombolian" in nature.'
So, seen from the entry of the tunnel, we see a drab, bare corridor leading in - no forge equipment remaining on this side of the Crack. But only a short distance in a black crack cuts thru the corridor. The walls and ceiling around this crack glow with orange light from the skylight. The lava tube rumbles like a huge underground machine laboring. Hobbits unfamiliar with lava would have no other reference for sounds from a lava tube escaping thru a skylight. And every now and then, a great leap of "flame" shoots up thru the crack. This "Strombolian" incandescent explosion rears up and flops back down like a fountain turning on and off.
With each leap, some of the magma spatters the near rim of the crack. Then most of this material drips back down, but some cools in place and stays, like candle wax accumulating around the rim of a candle holder. This edge of the crack, downhill from the vector of the crack's plane, has an irregular "curb"; a broken ridge of accreted rock. This curb is cool, dark, hard to notice, and easy to trip over.
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