Legend of Korra

How the heck does Lavabending work though? what's the cutoff point of the earthbending turning into firebending?

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.