avatars can bend lava. i'm guessing it's mostly/all firebending in the show, but logically it should mostly be earth since lava is hot earth.
waterbenders and earthbenders can both bend mud. mud = water+earth. when the water is bended, the earth dissolved in the water goes with it. and vice versa
and katara isn't a earthbender so of course toph is a better earthbender than she is