Avatar Roku bends lava when he destroys the crescent island temple.
Avatar Kyoshi bends lava when she separates Kyoshi Island from the mainland.
Five lives back from Aang, which is the Fire Nation Avatar before Roku, lavabent when Roku was explaining the Avatar State in 201.
My thoughts are that lavabending can be done by firebenders, but it is an old and lost form of bending. Sozin absorbs steam and gasses from lava in the volcano, hardening the lava. Idk if this counts as lavabending or firebending, but my bet is a little of both. It seems like lavabending is only done by past avatars and Firelord Sozin. So it's obviously not practiced much in the current days.
Also, in the four cuts where Roku is explaining to Aang the Avatar State, it flashes to four different Avatar bending their natural element. And the firebender was lavabending. So that obviously leads to the notion that lavabending is part of firebending.
But what is lava? Lava's just molten earth.
So what do you think?













