Quite impressive. I agree about the smoke, the volcano should be smoking if it's erupting.
I think you should add a bit more depth to it. Make sure that it's easy to see that the mountains are in the background, maybe get some atmospheric perspective. It's confusing to see where everything fits together. Is the lava flowing into the river? If it is, then there should be some major steam rising from that area. Also, the black on the right bothers me. It's clear that you need to have another darker color if you want to put in some shading that dark. I'm not sure why that side of the waterfall would be that dark, there's no real indication of anything that's casting a shadow on it.