The term "mud-volcano" generally is applied to a more or less violent eruption or surfaces extrusion of watery mud or clay which almost invariably is accompanied by methane gas, and which commonly tends to build up a solid mud or clay deposit around its orifice which may have a conical or volcano-like shape. The source of a mud volcano commonly may be traced to a substantial subsurface layer or diapir of highly plastic, and probably undercompacted, mud or shale. Mud volcanoes also commonly appear to be related to lines of fracture, faulting, or sharp folding. There appears to be a close interrelation between undercompacted (overpressured) muds or shale bodies, mud or shale diapirs, mud lumps, and mud volcanoes; and all degrees of gradation from one to another. Mud volcanoes are one of the most useful surface sources of information on the nature of materials in mud diapirs and undercompacted shale bodies.
The motivating force responsible for a mud volcano is, in part, simply the weight of rock overburden borne by the fluid content of undecompacted shales. However, mud volcanoes all over the world are associated so invariably with quietly or explosively escaping methane gas that it is reasonable to conclude that the presence of methane gas in the subsurface is also an essential feature of the phenomenon. The mud of the volcanoes is a mixture of clay and salt water which is kept in the state of a slurry by the boiling or churning activity of escaping methane gas. Probably the methane gas was derived either directly from organic matter in muds or shales or from secondary accumulations in sand stringers within the source-rock shale or from larger reservoirs just above or just below such shales. Some liquid oil often, but not always, is associated with the hydrocarbon gases of mud volcanoes.
Commonly the activity of a mud volcano is simply a mild surface upwelling of muddy and usually saline water accompanied by gas bubbles. However, many ,instances are known of highly explosive eruptions where large masses of rock have been violently blown out hundreds of feet into the air and scattered widely over the countryside. These intermittent violent eruptions strongly suggest that motive force is not merely weight of gradually increasing overburden but is due to periodic buildup and release of internal pressure from the generation of methane gas within the shale body or diapir.

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