I've come up with a way to approach my concept pieces. As Phil rightly said, the text excerpts included a lot more than just tunnels and rocks. I think I got far too hung up on the generic volcano, and not enough on the  fantasty. I am going to create one painting based around the inside of the volcano, the design is below:


This scene, shows the inside of a huge cavern, with rocky walkways heading away from the viewer. I wanted to try and give a sense of scale in this piece, which is why I'm showing the paths leading into the distant rock-wall. I felt that the scenes the text were describing seemed quite calm.. just caverns and tunnels. I wanted to put a bit of a twist on that and create a really hostile looking enviroment.  The way I'm going to do this is by creating huge pools of lava below the walkways, with a atmosphere within the cavern showing steam and flames. I will do this all with colour once I create the above piece digitally. I want there to be a contrast between the rock (Dark and granite) and the bright lava and flame encompassing it, making the whole scene very dangerous.