A mud volcano, sometimes also called a gas-oil volcano, is geological formation created by the extrusion of pressurized gases and mud from below the Earth's crust. Just like igneous volcanoes, mud volcanoes can vary in size, and the strength of their eruptions is also quite diverse. The nation of Azerbaijan is famous for its mud volcanoes, most of which are located over petroleum deposits, but these volcanoes can be found all over the world.
An igneous or magma volcano erupts with lava, molten rock which pushes up through the surface of the Earth. Mud volcanoes, on the other hand, are essentially like steam vents for the planet: they erupt with pressurized gases, hot water, and various sediments. Many of them also contain petroleum products, which can cause the volcano to flare.
These volcanoes tend to start out as small bulges in the Earth which develop into cones. The bulge is created by a buildup of pressure underneath relatively plastic rock. Areas of increased tectonic activity are frequent sites for mud volcanoes, and these volcanoes can also appear over petroleum deposits, and in areas where volcanic activity is occurring. Geothermal springs are often associated with mud volcanoes as well, in which case they may erupt with superheated sprays of mineral water which can scald bystanders.
Some mud volcanoes erupt with relatively tame flows; a knee-high cone with a gentle trickle of mud is generally not terribly remarkable. Others erupt more explosively, shooting material into the air. This can be dangerous, as the material may be hot or actually on fire, causing damage wherever it lands. Many mud volcanoes are also very stinky, because they contain a variety of volatile gases.
A mud volcano is not the same thing as a mud pot, a pool of mud which bubbles as water and gases rise through it. Mud pots do not erupt, and they do not generally pose a safety hazard, although extremely hot or viscous mud pots can be dangerous for people who climb into them. While mud pots are sometimes used for recreation, this is not advisable with a mud volcano, due to the unpredictability of eruptions.
Sometimes, a mud volcano forces an evacuation. Extremely explosive eruptions or a heavy flow of mud can lead officials to decide that the neighboring population should be moved to a safer location. Residents of Porong, Indonesia, for example, were evacuated in 2008 after a mud volcano which had been erupting since 2006 became much more active, flooding roads and spewing black goo over the surrounding area.