I must say, first off, that I am a little jealous you got to shoot this. :)
I have to agreee with the previous commenters to certain extent. It would have have been nice to have them the people in better focus. Its understandable why this wasn't so. It looks like your focus was on the lava and not the people. Unless you were to back up a lot more they would have been out of focus no matter what. And you were shooting at f/4 which didn't give you a lot of depth of field to play with. True you could have tried to stop down but I don't think you would have gotten the depth of field anyways and you would have introduced more noise using a higher ISO or messed up the perfect capture of the lava which you got with your shutter speed. I think given all that, having the people sharp AND the lava sharp might not have been an option.
The composition works well for me. It is a little central but I think it works in this case. The people could have been separated a bit more but again, that probably wasn't an option and I think there's sufficient separation to pull it off.
In regards to the challenge? Yup...that's definitely orange. :)
One of the more interesting volcano shots. I love the colors and the millions of little dots of lava.
But I really really feel like the people should have been in sharper focus. Especially the two blurry ones on the left are very distracting. It is the people's silhouettes that make this photograph interesting and sets it apart from the other volcano shots, so the fact that they are not all in focus is a downer. For me, it makes the difference between giving you a 6 and an 8.