Mud volcanoes, as the name suggests, are volcanoes made of mud. To say it's mud is not hard to understand, because it is indeed composed of clay, rock debris, salt powder and other mud. However, it's not a normal volcano. The basic characteristics of a normal volcano are formed by magma and have magma channels, while mud volcanoes are formed by mud and do not have magma channels. Nevertheless, mud volcanoes not only look like volcanoes and have eruption outlets, but also have eruption phenomena. In this regard, they are not volcanoes, but they are just like volcanoes.
The author has investigated mud volcanoes in Usur, Dushanzi in Xinjiang and Kaohsiung in Taiwan. The mud volcanoes in Usur, Xinjiang are presented as a series of mud springs and mud ponds. Most of them are round, not large in size, with diameters ranging from a few centimeters to 1-2 meters, scattered across the valleys. The mud in the springs and ponds often bubbles from time to time, just like the earth is boiling, emitting odorous gases such as methane and hydrogen sulfide, some of which can be ignited. People may not think that the rolling mud has a very low temperature, so cold that they can put their hands in and feel it's cold. Therefore, some people call mud volcanoes "cold volcanoes". The eruption outlets of mud volcanoes are often distributed along faults or fissures, some in the form of deep ditches, others like deep wells. The rolling mud constantly overflows from the eruption outlets and dries up into mud hills over time, similar to ordinary volcanic cones, hence the name mud volcanoes. The scale of mud volcanoes is much smaller than that of real volcanoes. The diameter of the base of the mud volcano cone is usually only a few meters to dozens of meters, and the height is generally no more than 10 meters. Those dried-up mud volcanoes often form various landforms: hilly, ridged, funnel-shaped... patchy and layered, even showing red, yellow, orange, ochre, and green, with colorful hues.
In China, in addition to Xinjiang, active mud volcanoes have been found in Kaohsiung and Hengchun in Taiwan. There, mud volcanoes not only have typical landforms, but also have natural eruption phenomena. There are also many mud volcanoes in other parts of the world. The more famous ones include Makran in Iran, Buzau in Romania, and the largest mud volcanoes in Baku, Azerbaijan, as well as many mud volcanoes in Yellowstone Park in the United States.
How are mud volcanoes formed? At present, researchers generally divide mud volcanoes into two categories. One is related to sedimentation. That is, if there are thick layers of organic-rich sediments underground, there will be a large amount of water and hydrocarbon gases. Once the crustal cracks are encountered, the high-pressure water and gases will expand and rise, bringing with them the surrounding mud and debris, erupting onto the surface with gas and mud, and sometimes erupting fire (methane and other gases self-ignite at high temperatures), forming mud volcanoes. These mud volcanoes are often formed in areas with developed oil and gas deposits, so they have also been called "gas-oil volcanoes". In a sense, mud volcanoes are a sign of oil and gas deposits.
The other type of mud volcanoes is related to volcanic activity. The most representative one is Yellowstone Park in the United States. There are not only mud volcanoes, but also many intermittent springs related to mud volcanoes, with a large number and large scale. Many mud volcanoes and intermittent springs are developed on the basis of some cracks and fracture zones in the original volcanic crater, and some gases and water are also residues of the original volcanoes.