A fumarole, also known as a steam vent, is a geothermal feature best described as a dry geyser. Both fumaroles and geysers are powered by an underground, volcanic heat source and both have vents that span from the heat source to the surface. However, fumarole vents contain very little water compared to geyser vents and mainly emit gas instead of water. Any groundwater that does enter the vent is immediately converted into steam that is also emitted from the opening. Fumaroles were named from the Latin word "fumus" (smoke) because this escaping steam has the appearance of smoke.
The type and amount of gas emitted from a fumarole varies greatly. The major gases in a fumarole are carbon dioxide, hydrogen sulfide and sulfur dioxide (responsible for the "rotten egg" smell characteristic of many geothermal features), chlorine gases, and ammonia gas. Fumaroles that emit sulfurous gases are also called solfataras, from the Italian word "solfo" (sulfur). The opening of solfataras tends to be yellow because of sulfur residue deposited by the gases. Fumaroles with a high output of gas (over 20 meters/second) are also called "roaring fumaroles" because they tend to make a loud roaring or hissing sound that can be heard hundreds of feet away.
Fumaroles can be found in geothermal areas around the world, especially in major geyser fields in Iceland, New Zealand, and Yellowstone National Park in Wyoming. Fumaroles are also a major feature in the Valley of Ten Thousand Smokes located in Katmai National Park in Alaska. Like other geothermal features, the life span of fumaroles is very unpredictable and they can be active from weeks to centuries. If the heat source shifts position away from the vent the fumarole quickly becomes inactive and if the heat source moves to a different area new fumaroles may form there. Additionally, if there is an increase in underground water levels the fumarole can fill with water and turn into a geyser, hot spring, or mud pot. Some features are geysers during the spring and early summer when there is enough water and then turn into fumaroles in the late summer when water levels are low.
http://en.wikipedia.org/wiki/Fumarole
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