Remember that when it's pitch dark a little light goes further than you think... The exception is that you need something powerful and with good throw to figure out the route over those lava rocks. When stationary you can get by with less light, prolonging run time.
Based on CPF reviews, I would think that the fenix HP10 would fit the bill nicely because you can easily adjust to your preferred level and it is quite powerful at the maximum setting. 3 minutes "turbo boost" should be quite helpful for you looking for a descent down those sharp lava rocks, I think. Apart from fenix HP10 I think it is hard to find powerful headlamps that won't break your bank. You can find several very good headlamps in the not so powerful cathegory (listed above), they do a good job but you will need to supplement with a thrower to find your way among those rocks.
You may also seriously consider a regular flashlight strapped to your head. Much easier to find something powerful at a good price, such as a quark and the nightcore headband. I also ordered
this diffuser and it looks promising (fits the quarks and also most fenix lights, including the HP10). Flipping from throwy light to a diffuse area lighting is very handy. I just received this diffuser and headband, so it is a little early to recommend it, but it certainly looks promising...
The quark prism setup also looks very tempting...
Also, you don't want to rely on one single light only. If you choose a nice work headlamp (perhaps not so powerful?) and a regular, powerful throwy flashlight with a headband you have backup and flexibility without breaking the bank (or your back, carrying all this stuff).
But first - you should consider battery options? You definitely want a rechargeable setup. My advise is that you choose between sanoy eneloop (low self discharge nimh at the standard AA size) or li-ion, and choose lights that run on your preferred battery. Search a little bit on CPF to find out which is best for your use.