I would replace the vertical spikes by some (raised) billowed fractal, so that the lava streams (?) are passing through them. The shrub are hardly discernible, so maybe some blackened tree stumps would draw more attention. For blackened grass you could use the internal grass, and give it some black/ash color, blended by a larger scale fractal (passed through transform shader, world position) to give the fractal(s) variation.
I put something together, but haven't got time to render it out. Maybe it's of use.