visible geologic noxious gas

[David Krentz <ddkrentz@charter.net>]

  First, let me preface this email with the fact that I failed in  
chemistry and many other science based subjects-heck, I failed art  
once.  Please be gentle with me.

The scene I've created for the show I'm on involves dinosaurs being  
choked to death in a low altitude by some kind of geologic event.

What kinds of VISIBLE noxious gases do volcanoes emit before  
eruptions?  From what I understand carbon dioxide is odorless and  
invisible. Unfortunately its something that does not play well  
visually.  Is there some way it could become visible?  Mixed with  
water vapour?  Do sulfer gases have the same effect on wildlife that  
carbon monoxide does?  Is sulfer as heavy as CO2?  Are there any  
modern locations that I can site?

  The scene I've created for the show I'm on involves dinosaurs being  
choked to death in a low altitude by some kind of geologic event.

D  (thats my initial, not my grade)