If you check out his bio you states he is an earth bender, and you can't be a fire bender first and move stones, plus you see the speed he generates with those stones making it more plausible he changes the minerals in the earth by heating it up plus it's in the bio, similar to metal bending I guess, and fire benders…

I really don't want to nitpick but Ghazan is not the fire bender of the group he's the earth bender the combustion girl is the fire bender of the group, the escapees have one of each element in their group
He just heat up and liquified the minerals in the earth to create lava. Gotta get them facts right.