Wired had this interesting article which states:
Gollum, if you remember, dove into the lava of Mount Doom after his precious ring was thrown in — he proceeds to sink into the lava (see below) and leaves the ring floating on the lava until it melts away. Guess what? Sinking into lava just will not happen if you’re a human (or remotely human). You’d need to be a Terminator to sink into molten rock/metal …
On a discussion of the article on some other site, author John Scalzi wrote:
In a film with spiders of physically impossible size, talking trees, ugly warriors birthed out of mud and a disembodied malevolence causing a ring to corrupt the mind of anyone who wears it (and also turn them invisible), we’re going to complain that the lava is not viscous enough?
I can understand Scalzi’s point, but I disagree with his argument that one shouldn’t complain about the physics of Middle Earth lava just because one has accepted the existence of fantastical Middle Earth creatures.
I agree with Scalzi when he writes:
… you should consider the work in its totality and ask whether in the context of the work, this specific thing is inconsistent with the worldbuilding.
I’d also add (though it should be obvious) that this will be a subjective issue. Some people can more easily suspend their disbelief about certain things than others. If your area of expertise includes lava, lava falsehoods will stand out to you more than talking trees (and Ents are not trees, by the way).
Personally, the lava issue doesn’t bother me, but I’ve never seen a living creature fall into lava before. And it’s not something I ever really want to see. But if I did see such a thing, aside from being scarred and depressed for the rest of my sad sorry life, I can understand why Gollum’s death goop might stop working for me, even while I accept all the other magic of Middle Earth. There’s nothing in the story that signifies that the lava should behave in any other way than it does here on earth. Similarly, gravity behaves the same way, temperature behaves the same way, elf and hobbit and wizard emotions behave the same way. So it’s not like we assume that everything is so different that we have to just accept everything that comes our way.
Imagine if Gollum had bounced on the lava as if it were a trampoline. Who would accept that? Would me saying “hey, you accepted talking trees!” make you change your mind? I doubt it. You expect the lava to behave a certain way in the context of the story.
I would say that most audiences accept the physics of Gollum’s death because that is exactly how most of us imagine falling into lava should look, because most of us haven’t witnessed creatures falling into lava before. When we watch videos of rivers of lava pouring down the side of a volcano, it looks as viscous as it does in Gollum’s death scene. So our acceptance of the physics of Gollum’s death is based on our own lava-physics ignorance, not on our consideration of our own acceptance of the wizards and talking trees and giant spiders that preceded it. This lava-physics ignorance is also what makes the Wired article interesting at all in the first place (at least to me). It’s fun and educational!
Also, I think we could argue that as the lava liquefies Gollum’s innards, because the ring of power has turned him into the ugly gross unnatural goblin-like creature he is, his unnatural innards would liquefy in such a way that they mix in the lava in such a way that what we see in the movie makes perfect sense. That is, the Wired article may be right about the physics of the lava, but it hath no knowledge about the physics of melting Gollum guts, which might become extremely dense at high temperatures. (Sure, why not?) Or perhaps his skin vaporizes easily at lava temperatures, and lava pours into muscles and bones. He’s not really sinking; he’s being pulled down by the flow of the lava. Why didn’t Tolkien specify these sorts of things? He could’ve had an entire section of the appendix for this!