You will want to make a spinning lamp. Your outer volcano shell will have transparent areas where you want the lava flow to show through. Inside suspended over the light you will need a translucent cylinder that can spin on a needle over the bulb. This cylinder will have your lava pastern on it. Much like the picture below. As it spins it will give the effect of flowing lava.

In the image the piece in the center sits over the light and is balanced on a needle over the light. The top of the cylinder is vented so the heat from the bulb will keep that cylinder spinning. As the light shines through it the pattern is seen on the outer cylinder. In this case it creates a swirling effect and makes the bear logo flash go from swirling to fully light and back again as it spins.

This is the same method used in fake fire places to give the effect of a real fire on the back wall of the unit. The hard part for you will be getting a translucent lava like pattern made.