Unlike hot springs and geysers, fumeroles do not have access to a huge supply of hot water within their plumbing system. Their vent system reaches far down into the earth where the rock masses are very hot, but very little water is found within these channels. What water does enter is instantly converted to steam and is expelled from the vent with carbon dioxide and a little hydrogen sulfide, accounting for the "rotten egg" odor associated with these features.
This material is abstracted from brochures distributed by The Yellowstone Association for Natural Science, History & Education, Inc.