Acidic gases and steam rising from hot magma turns the overlying rock into soupy clay, which can bubble or spurt out of fissures on occasion. This mud volcano is in the Norris Geyser Basin in the Yellowstone region.
This small mud volcano (about 40 cm high) is of hydrothermal origin. (more below)
S.R. Brantley, U.S. Geological Survey photo
©2009 About.com, a part of The New York Times Company.
All rights reserved.