Mud pots, also known as solfatares, are basically hot springs that are filled with hot, bubbling mud instead of water. The mud is made up of silica and various clay minerals and generally has a light gray color. Gasses such as hydrogen sulfide, carbon dioxide, and steam move from deep within the earth up through a hydrothermal vent and form large bubbles in the mud. When the bubbles pop bits of mud are hurled into the air. The mud often lands around the mud pot to form a cone several feet high similar to a miniature volcano. The best place to see mud pots is Yellowstone National Park in Wyoming. This park is home to several major mud pots, including Fountain Paint Pot, Mud Volcano, and Artist Paint Pot.
tWD disagrees:"I don't know that Yellowstone necessarily deserves the accolade of 'best' mud pots. New Zealand and South America certainly have some doozies, to my knowledge, and I seem to remember something about Scandinavia as well, or maybe Iceland." Yes, both Iceland and New Zealand have large geothermal fields with geysers, hot springs, and mud pots, and it's certainly possible that these regions have better mud pots than Yellowstone. Hopefully a direct comparision can be made someday :)
The consistency of the mud in the mud pot depends on the season. During the spring and early summer the mud is generally thin because of rain and snow runoff. At late summer most of this liquid has evaporated and the mud is extremely thick. This is the best time to view the mud pots because the thick mud produces especially large bubbles that can lob mud huge distances. Park officials at Yellowstone warn spectators that mud can even fly past guardrails and hit people.
Mud pots form only under certain conditions. The hydrothermal vent must contain a reasonable amount of hydrogen sulfide gas and only a small amount of water. Hydrogen sulfide gas is responsible for the characteristic “rotten egg” odor present at many hydrothermal features. Microorganisms that live in and around the vent are able to convert the gas into sulfuric acid. This makes the water in the mud pot very acidic, unlike most other hot springs and geysers that have alkaline water. The water slowly breaks down the rocks around the mud pot into mud.
Occasionally, certain compounds are present in the rock and resulting mud that turn the mud pot various colors. For example, sulfur compounds turn the mud yellow while certain iron compounds turn the mud red or pink. These mud pots are referred to as “paint pots.”
http://en.wikipedia.org/wiki/Mud_pot
www.sandiego.edu/~asturz/Mudpots.htm