The Movies Are Wrong About Lava

I’ll start with the answer: Everyone is wrong about how people die when they fall into lava.

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 … and here’s why.

Molten lava is nothing like water. Sure, everyone thinks that liquid rock (magma) is going to behave like any other liquid (e.g., water), but there are some key physical properties that tell us it just isn’t the case. Let’s compare!

  • Water has a density of 1000 kg/m3 and a viscosity of 0.00089 Pa·s.
  • Lava has a density of 3100 kg/m3 and a viscosity of 100-1000 Pa·s.

Pa·s is the SI unit for viscosity — some people might be familiar with other viscosity measures like poise. Viscosity is, more or less, the resistance to flow, so if you throw something in a liquid, a low viscosity liquid (like water) will “get out the way” and you’ll sink faster relative to a high viscosity liquid (like honey). The density of the liquid will also play a role in how quickly you might sink based on your own density. So, when we’re looking at water versus lava, lava is ~3.1 times the density and between ~100,000 to 1,100,000 times the viscosity. They are very different! As a result humans WOULD NOT sink into lava, they would just sit on top and burn, which would still be quite nasty!

So, there you have it. The next time you see someone in a movie fall into a cauldron of roiling lava and they sink into it, remember that it can never happen. Learn more here.

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One Response to “The Movies Are Wrong About Lava”

  1. Hyouten Says:

    Thanks for this post! My uneasy feelings about watching a movie character sink into the lava have finally been answered. I always thought there was something wrong with the thought of someone disappearing under lava.

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