Start by making a box like this. You will need to make it large enough to contain all the lava for the entire animation:
Add a turbosmooth and some noise, for geometrical variation:
Create an FFD box (as a seperate object, NOT as a modifier!) around part of the lava, like this, and bind the lava to the FFD with a spacewarp bind:
Bend the lava into shape by moving the FFD box control points:
Go to the final frame in the timeline, turn on ANIMATE, and move the lava through the FFD box:
Voila, you're done! Play back the animation, and the lava is flowing.
Note that you will need to change the keyframe types to linear, if you want the lava's speed to be constant.
Also, as it appears in the picture above, as soon as the lava exits the FFD box, it will pop back to its unmodified position. You will need to take this into account. ;)