It's a similar principle to ice cubes exploding in a deep fryer. "Hot enough to melt rock" is enough to instantly melt and boil ice into steam, at which point it will expand explosively. It's so fast it wouldn't actually need to be contained.
Enjoyed the silly take on physics miced with magic! Thank you for the release.
By the way, how many credit/splash pages do you guys need? It's getting almost silly at this point. lol
I love how they toss the "use hot and cold in rapid succession!" shounen gimmick out there but skipped the obligatory (pseudo-)science explanation because Zero is an idiot ...not very studious and would have to go totally out of character to explain it.