Mount Fuji will never erupt and I am almost sure that's where they made the poneglyphs which is why they are indestructible, they Kairouseki stone.
I'm not saying Poneglyphs aren't related or made in some part with Kairoseki, but they're not only kairoseki. There's something else as well.
And at some point Oda will show a character destroying a poneglyph (either by himself or with the help of an Ancient Weapon) to hype them up.