Jurassic World: Fallen Kingdom is a movie full of numerous complex practical and digital effects. However, the film came up with a pretty simple way to create a stand-in for lava when it was needed on set. While all of the lava that the audience sees in FallenKingdom is a digital creation, director J.A. Bayona needed something to play the part of the lava on set that the actors could react to. The solution, set cat litter on fire, of course. According to visual effects supervisor David Vickery...
We had this crack in the ceiling filled with cat litter that had been soaked in some sort of flammable liquid. There were 12 different nozzles that could drop this cat litter at different points, and that gave us these huge beautiful streams of liquid fire that dropped down from the ceiling. It also gave us this wonderful interactive lighting and a really interesting billowing smoky atmosphere.
It makes perfect sense why Jurassic World: Fallen Kingdom would need to come up with something to stand in for the lava while the actors are on set. During an early scene in the film that sees the character of Claire and Franklin cornered by a dinosaur in old, overgrown, portion of the old theme park, lava is flowing everywhere around them as the volcano begins to erupt and slowly (at first) destroy the island. The actors need to know where the digital lava is going to be placed when it's created in post-production so they don't accidentally stand in the exact wrong spot that has it pouring on their heads.
What's a little less clear is why or how somebody got the idea that the thing they needed to play the part of lava was flaming kitty litter. That's not explained to Thrillist. It's not exactly the first thing that comes to mind. Kitty litter does pour fairly smoothly, which may be part of the reason. You should get the constant stream you need with little or no gaps in the flow. Of course, you also need something that will generate light, which means that simply using cat litter won't do, you also need to set it on fire.
On the plus side, since one has to assume that burning cat litter pouring down around you is a fairly unpleasant and mildly scary idea, director J.A. Bayona almost certainly got some very realistic performances from Justice Smith and Bryce Dallas Howard. Getting burning by flaming cat litter probably hurts a lot less than lava, but that doesn't exactly mean it's fun.
It's not clear if using kitty litter in place of fire has been a regular occurrence, but now I have to wonder if this is a special effects move that has been used before. How much kitty litter does the average film production actually go through?