Remember the scene inside Mount Doom where Frodo and Gollum wrestle for the ring? In the movie, they are in a mostly enclosed space, on a bridge high over the lava. They were sweating but the bridge is high and the cave is large. Shouldn’t really be too hot, should it? Alas, Sam, Frodo and Gollum would have been cooked the moment they stepped into that cave, unless there was some magical protection that was unmentioned in book or movie.
As a thought experiment, imagine a magma chamber underground. Everything in that pool of molten rock will be pretty much at the same temperature (about 700 to 1200 degrees C, per Wikipedia.) This is because it’s a very hot body enclosed in a very good insulator (lots of rock). In fact, it will start altering and melting the adjacent rock, an effect you can see at the edges of granitic intrusions (plutons) that have weathered to the surface. Now, imagine a small gas cavity at the top of the magma. How hot is that pocket of gas? Why, just as hot as the magma because it will be heated by the magma and can’t dissipate much heat into the insulating rock.
Let’s expand that pocket of gas to the size of, say, the Mount Doom cave. How hot is the air above the magma? It is still the same temperature as the magma: there’s no where for the heat to go (technically, there is heat flow through the rock but it is very low.) Now, let’s cut a tiny window into this cave. In the real world, what you get is a volcanic eruption as the pressure on the magma is released and it boils away exactly as what happens in a shaken soda-can when suddenly opened. But let’s ignore that and pretend there is no pressure differential and, thus, no eruption. How hot is it in the cave? Well, it’s still the temperature of the magma, throughout the entire cave, including all rock surfaces and the air. A small hole will provide minimal heat loss. The cave isn’t going to cool.
Okay, let’s try a bigger hole, about the size of the opening to the Mount Doom chamber in the movie. Now is it cooler? Not really. Perhaps near the doorway it will be not quite so balmy but that opening still is fairly insignificant. The magma will have no trouble heating the walls, air, bridge, etc., to about its temperature. This means, enter the cave and, poof, you are suddenly exposed to 1200C (judging by the fluidity of the lava in the movie, let’s go with the higher temperature). That’s pretty much an instant burst into flames. But wait, what if cool air was being drawn into the cave? That is possible. Maybe old lava tubes provide conduits into the cave. A chimney effect might draw quite a lot of air through the cave and out the door. I’m just going to wave my hands here but at the very least the air flow would likely be immense, making it probably impossible to walk into the cave or a real danger of getting sucked into the lava tube. But regardless, it still wouldn’t solve the problem you would have in this cave because aside from convective heating of the air, the magma is radiantly heating everything in line of sight of it.
Anyone who has cooked knows a heating element can burn without making contact (think toasters, broilers, etc.): infra-red (light) can burn. Huge pools of molten rock create a lot of radiant heat.
So, Sam, Frodo and Gollum need something magical to protect them in this cave. Certainly something that could have been present, though, again, there was no mention of it.
We are used to this sort of movie physics from many action-adventure shows. How many times does the hero outrun a fireball in a tunnel, for instance? Aside from the fact that few explosions would move slow enough to be outrun, if there was a flaming ball of gas in the tunnel, all that radiant heat from the fireball would be bouncing down the tunnel at you like a big death ray. You would be badly burned long before the actual flames touched you. If you have spent any time around a fireplace or firepit, this, too, is no surprise. It’s something we all know. For me, it was really driven home when I watched a show about a fire in a large fuel tank. The tank caught fire, the fire department poured water onto it. This contained it until the water that pooled atop the oil got hot enough to boil at which point the water turned to steam which ejected the oil in a fine mist which promptly exploded like a huge fuel-air bomb. It was all rather sudden (no time to out run the fireball) and interestingly, people standing over 800 feet away were not touched by flames, were not affected much by blast (because it mostly went up) but they did suffer 3rd degree burns on exposed flesh from radiant heat. Ouch.
As always with a well established trope, this can be ignored. Only we engineers seem to groan at such things (probably fire fighters, too, but I don’t know any). But, again, you might enrich your setting or story by doing something about it. It might help the verisimilitude if your smith has to figure out how to tolerate the heat of lava as part of forging the ring of doom. Because we all know, if we stop to think about it, you can get badly burned by really hot things without actually touching or falling into them.
So let’s talk Mustafar of Star Wars fame. Here, there do appear to be shields protecting the installation from the radiant heat (good). In fact, when disabled, it appears the radiant heat can quickly melt thick metal, as when the collector bends and drops into the lava stream. Strangely, the dueling Jedi don’t seem to suffer the same effect. There must be some Jedi trick to reflect heat (not entirely implausible in that universe, I think the old KOTOR games may have had a similar affect at one point). But even so, we have radiant heat that can quickly melt metal at 100 meters or so and can cause Anakin to burst into flame while lying on the shore but don’t affect Obi-wan or Anakin while riding those little ore carts. Must be more Jedi magic that stops working once you get your legs and an arm lopped off.
But that’s the least of the implausibilities with Mustafar. Where does the breathable air come from on a lava planet? How would endemic, complex creatures evolve there? And why are they mining lava? That’s undifferentiated material and economically of little value: the important stuff is mixed in with too much unimportant stuff. On Earth, while you might quarry igneous rock as a building material, you don’t mine it until other processes have concentrated the valuable minerals and elements. Lava is almost by definition maximally undifferentiated, i.e., of the least possible mineral value. The only exception would be if you could tap a magma chamber that was mostly solidified. The remaining melt will have lots of odd elements in it that don’t play well with others and are generally of value. But in that case, you are taking advantage of differentiation in a solidifying magma body, not free running lava.
You could also ask why does Star Wars make so much use of single biome planets (Ice Planet Hoth, Lava Planet Mustafar, Forest Moon Endor, etc.)? For biomes, it doesn’t really make a lot of sense from all we know about Earth and suspect about other planets. But, while it would not have a breathable atmosphere, you could have a lava planet. We already know about planets orbiting so close to their sun they are probably molten. But in an upcoming post, I’d like to explore young planets because they will have a lot more radioactive elements and consequently be a lot hotter, which could make for interesting science fiction settings.
- Cave Of Crystals: Where To Find These 55 Ton Giants… (fox41blogs.typepad.com) Not molten but really, really hot.
- Microscope: Fire vs lava (csirohelixblog.com)
- Bizarre! Supervolcano’s Ash So Hot It Turned to Lava (livescience.com)
- Magma Can ‘Fester’ Beneath Earth’s Upper Crust For Thousands Of Years Without Triggering Eruption; When Will Yellowstone’s Volcano Go Off? (hngn.com)
- Molten magma can survive in upper crust for hundreds of millennia (sciencedaily.com)
- BLEVEs: what happens when a contained liquid heats up beyond what the containment vessel can hold.