[quote=Gowi]
The problem I had with Ghazan's lavabending (which was more magmabending if you ask me) was how does an Earthbender heat up Earth to turn it into that? What are the fundamental mechanics behind it? Earthbenders don't deal with temperature but mineral content. 
[/quote]

It's likely that Ghazan was using friction to melt the rocks into lava, I imagine it would work like rubbing you legs to warm them up or rubbing two sticks together to produce a fire. Except instead of two sticks you are rubbing every individual ''rock particle'' in the targeted area together in order to generate massive amounts of heat; the result of which melts the rock into lava.