Frosted it with chocolate frosting, roughly applied.
As you can see, I experimented with two types of lava. I should have left out the lighter one; it was orange buttercream frosting that I made runnier, then pooled it in the crater at the top of the cake, letting it run down the side.
The explosive lava is orange hard candy, melted, then poured onto wax paper in lava flow shapes, and left to dry. Then I stuck it over the cake.