A mud volcano in the Norris Geyser Basin, Yellowstone National Park, Wyoming. It is about 40 cm tall. Credit: S. R. Brantley / U.S. Geological Survey.
A mud volcano is a small volcano-shaped cone of mud and clay, usually less than 1–2 meters tall. These small mud volcanoes are built by a mixture of hot water and fine sediment (mud and clay) that either (1) pours gently from a vent in the ground like a fluid lava flow; or (2) is ejected into the air like a lava fountain by escaping volcanic gas and boiling water. The fine mud and clay typically originates from solid rock – volcanic gases and heat escaping from magma deep below turn groundwater into a hot acidic mixture that chemically changes the rock into mud- and clay-sized fragments.