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That the exotic bending we see if due to mixing races.
Lava bending is earth bending and fire bending. That's why Bolin can lava bend, but his earth bending isn't pure enough for him to metal bend.
Explosion bending is air and fire. Sand bending is earth and air.
I'm pretty sure this is just canon...
What? No, it's never mentioned in the series at all.
Edit: after some reading, it seems that lava bending was originally only possible for avatars to do, as it required both fire and earth bending. It was later ret-conned into an earth sub-bending. One reason proposed for why Bolin can lava bend is that he is a very laid-back person compared to most earth benders. This would explain why he could never master the rigidity of metal bending and why flowing lava came more naturally to him.
I mean we see sozin pull the heat directly out of lava. It stands to reason he could put the heat back into rock. And fire benders seem to excells at controlling heat sources.
That's another possible explanation. I still like mine more, but still cool.
The retcon makes it seem like lava bending should be more common though. Like, you would think King Boomy would be able to lava bend, with his chill demeanor and philosophy.
There have been 3 non-avatars to do it, if you include the comics. Maybe, due to how dangerous it is, lava benders have a tendency to... remove themselves from history... the first time that they lava bend.
It also takes a bending style more like waterbending. Bumi probably had the personality for it, but Bolin was also already trained for fluid motions and a looser stance.
My theory is that avatars can't lava-bend. They can use earth and fire bending to move already existing lava or use air bending to cool lava, but they can't create lava from cold stone like lavabenders can or freeze lava back into stone