so I'm guessing melting rock + ice made for steam explosion (if water/ice turns to gas fast enough the pressure can be explosive, it's stupid hard to make happen IRL but add magic...)
as for the barrier, well, it lets spirits in so presumably after reincarnating he didn't come back as a human, not that that's really surprising at this point considering most humans can't see normal spirits but can see greater spirits if they meet (I'm guessing this either means he's some new greater spirit or maybe just something outright new that's a similar tier of existence)