Try something along this route. I know its not the best paint over, but for 5 mins it demonstrates the point. You have the same saturation on your piece all around, keep in mind that the things in the foreground will have the most saturated/contrast in the piece,so to do this effectively, you have to knock down the saturation a lil bit somewhere in the middle
Also keep in mind that lava will have hot spots, and changes hue, so steam will also rise a lil. Lava also give light, so most likely the bridges will have some under light
THe last thing is that, in order to get a good effect for rocks (which i didn't really demonstrate) is that they are hard edged. You can see opacity flowing into the sky in some of your rocks, but try more of a 'hard edge' brush with not too much opacity jitter. The lava flowing is the same concept, where it isn't transparent, so you wont be able to see thru it, or have that 'mist' effect, and it needs to be solid and smooth
Where your lava is flowing on the rocks, the shadows have to stay consistent with the flow. You cant have a laving flowing in different paths when the shadows of the surrounding rocks aren't flowing in the same direction
I didn't really demonstrate everything I explained, but try it on your next piece, and don't be afraid to pull up some reference
Happy painting
CHeers
http://img232.imageshack.us/img232/530/tryxh.jpg
but how come i havent seen you around before, you obviously know what your takling about