A mud volcano is a small volcano-shaped cone of fine sediment, usually less than 1-2 m high. Mud volcanoes are built by a mixture of hot water and sediment that either (1) pours gently from a vent in the ground like a fluid flow; or (2) is ejected into the air like a fountain by escaping volcanic gas and boiling water.
Mud volcano in Yellowstone National Park, Wyoming; about 40 cm tall. Photo: © S.R. BrantleyLots of mud (New Zealand)