Yes rocks can explode, depends what type of rocks they are. Don't ask me which ones explode, I am not that much of an expert! I have boiled and scrubbed and soaked all my rocks that I have put in my hamsters cages, no explosions.
Personally I wouldn't stack rocks up in a cage, as I would be concerned that they got knocked over and landed on a hamster. I know lots of people use bendy bridges on their sides, but the problem with that is sometimes they have big gaps in between and it has been known for hamsters paws to get stuck in these and hurt themselves, badly. A wooden barrier or cardboard may be the best way to go,oh I just had an idea, what about a chew mat or 2 as a barrier.. i will post a link
https://smile.amazon.co.uk/Rosewood-...s%2C289&sr=8-5
I use megazorb and my hamsters like to tunnel in that, you may want to mix it with some other substrate though.