Since you're metering a luminous subject (flowing lava), I'd probably try to get a spot meter reading and then overexpose by 1 or 2 stops. You probably want to portray the flow as accurately as possible, regardless of the ambient light or surrounding conditions, which will probably be very dark. This is where an incident meter or a wider reflective meter probably won't be of much use, as there won't be much ambient light and any cooled lava surrounding the flow will be almost black. There may be some online resources for subjects like this, but it sounds like it could be a challenging situation. Good Luck!