Bison, Fumaroles & Geysers by Gregory “Slobirdr” Smith
Via Flickr:
Bison grazing as they pass the the confluence of the Little Firehole and Firehole rivers. Fumaroles emit mixtures of steam and gases (why you might experience that rotten egg smell…) and usually have brightly colored rocks associated with the steam vent. They are caused by channels from below passing through water tables…

Magma, degassing…

Under the high pressures and temperatures down in a magma chamber or deeper, molten rock carries within a number of gases in solution. Common ones include carbon dioxide, sulphur dioxide and water vapour. When it reaches the surface through a fissure or vent the gases dissipate into the atmosphere, but the drop in temperature forces some sulphur to precipitate out around the edges of the fumarole (aka solfatara). This photo was taken on the big island of Hawaii at the at Puʻu ʻŌʻō Crater.

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