IE was a part of a crew setting posts at right track, 24 wall in the mains. The crew set 2 posts and was in the process of setting a third when a large rock dislodged from the rib. IE dove out of the way of the incoming rock, EE's hard hat fell off and EE struck EE's head on a scoop parked in the crosscut. Injury was not the result of a training issue.