The helicopter was equipped with 2 cameras and was doing filming runs of a volcano vent crater and its associated smoke plume for a motion picture. The pilot said that on the third pass over the crater he noticed the main rotor RPM decreasing and saw the rotor caution light. He said he lowered the collective and inadvertently entered the volcano smoke and steam cloud. After turning he exited the cloud and autorotated to the crater bottom. The main rotor struck the shear rock wall during the flare and separated from the helicopter. Two camera operators onboard said the pilot announced 'we've got a problem' as they entered the smoke cloud. Both witnesses said they did not hear the low rotor warning tone and perceived no change in engine sound during the event. One said he felt 'sideward yawing motions' during the descent. The camera operators said that after the helicopter emerged from the cloud near the bottom of the crater the rotor struck the rock wall.