This is a video of a "professional" demonstrating what happens when you step on a lava flow. You don't actually sink, it just gives a little before trying to set your shoe on fire. But what about the even hotter, redder lava in the crater of a volcano? Would a really light person sink into that right away? I only ask because whether or not I write Peter Jackson an angry letter depends on it.
Keep going for the short video.
Thanks to Mack, who tried to tell me the lava in Mount Doom is magical and that's why that little ring-greedy Gollum sank so fast.