The lightning bolt is a fire symbol. At the American Panascope Corporation, Joe is literally surrounded by this symbol, which is the logo of the company, and he is constantly afraid. Since, according to J. P. Shanley, the factory represents the inside of Joe's head, this fire symbol indicates that his life is dominated by the fear which arose from his earlier experiences as a firefighter. (Dr. Ellison's analysis of Joe's problem is accurate.) After Dr. Ellison's 'death sentence', the lightning bolt is less prominent, although it is a 'fixture' in Joe's apartment (the crack in the wall). It reappears at times of fearful stress--when lightning hits the Tweedledee and in the shape of the procession up the volcano. As someone has pointed out at the alt.movies.joe-vs-volcano newsgroup, the symbol seems to prefigure the volcano. I think the volcano represents the ultimate fire. When Joe makes the jump, he is facing his worst fear. The ejection from the volcano would represent his victory over the fear.
The title 'Joe Versus the Volcano' says it all. The volcano is really in Joe's head. While at the factory, Joe was giving in to the fear, and the life-force (soul) was being drained away. After Dr. Ellison's diagnosis and the proposition by Graynamore, Joe becomes determined to overcome this fear, and he does so. In the battle of Joe versus the volcano, Joe emerges victorious.
By the way, the logo of the Southern Company (a power company based in Atlanta, Georgia, US) is very similar to the logo of American Panascope. (Thanks to Rob Lesley for this info.)

Sunday, June 22, 1997