Legend of Korra

This is what I've been thinking this entire time. Lavabending is kind of a cop out as as a special Earthbending technique, in my opinion =/

I suppose a firebender could bend pre-existing lava (though prior to this the only people who have been seen bending lava were Avatars, I think, so they might have been using firebending and earthbending simultaneously), sorta like how Aang and Katara were able to bend clouds since they were made of both air and water; there are probably quite a few bending skills that overlap the basic styles.

It could be an ability that children of both earth and fire benders could use, I mean Mako and Balin have different bendings, and Among himself said how talented Mako was, so perhaps Balin (having an equal talent) could lava bend due to his heritage?

The only thing clear so far is that it's an earthbending technique, hence why the prision in the middle of the ocean.