Magma is a mixture of molten rock, volatiles and solids. As such, waterbenders would not be able to manipulate any of its components, as none are solutions. Bending of lava is an Avatar-exclusive technique, as its manipulation requires a complex, sophisticated and simultaneous changes between earth and fire in several states. This implies that firebenders and earthbenders are incapable of performing the technique and that it can only be done by an Avatar.