Animalia Exotica
In order to be a lava bender you have to have a parent who is a firebender and a parent who is and earthbender.
This insinuates that you could be a fire or earthbender and still lavabend.
Problem with this is, Mako was never shown to lavabend. And no firebenders have ever been shown to bend lava at all, not the way Ghazan, the Avatars, or Bolin did. The Fire Nation Avatar before Yangchen doesn't count, since Avatars can bend all four elements, and Kiyoshi's bending of lava was probably earthbending.
In all likelihood the creators probably originally wanted it to be a mix of fire and earthbending, but then they couldn't think of something unique for Ghazan and thought, eh let's make lava an earthbending skill.