Mud volcanoes (MVs) are the topographic expression of a system that transports fluids and sediment from depth to the surface. They are capable of providing information from depths unreachable by current drilling techniques. MVs are commonly observed on land and offshore along convergent plate margins in volcanically active areas, oil fields, and areas of high sediment accumulation. Although previous studies have revealed source depths and compositions, a comprehensive understanding of their structure and mechanics remains elusive. Understanding of MVs has progressed, although their definition and associated terminology have changed over time. Further confusion has arisen with the Japanese usage of either “doro-kazan” or “dei-kazan” as translations of “mud volcano”. We apply a global interdisciplinary database of MVs from a range of modern tectonic regimes to maximize information from active MVs observed at sedimentary ocean floor in Japan. An ongoing multi-year MV studies has now progressed to a point whereby newly compiled observations and use of ocean floor seismic observatories may yield new insights into submarine MV activity. In addition, these terrestrial MV studies should enable further insights to be gained into Martian MVs and their implications for planetary sciences.

Mud volcanism is tied closely to subsurface cycling of sediment, fluid and carbon, and thus plays an important role in the whole earth system. Yet, our understanding of mud volcanism falls far behind that of “magmatic” volcanism. This paper briefly overviews fundamental topics of submarine mud volcanoes, including their geological distribution and morphological features. The compilation of distribution represents that submarine mud volcanoes are found widely in most plate subduction margins, except for those in subpolar regions. The morphological study suggests that the shapes of submarine mud volcanoes are highly variable and likely related to geological properties of the proximal subduction margins. All the fundamental studies reviewed here lead to an increase in our understanding of mud eruption dynamics, as well as the dynamic interplay between mud volcanism and subsurface material cycling in plate subduction margins.

Mud volcanoes are formed by the upward intrusion of deformable, low-density material as mud diapirs. Most mud volcanoes are located near continental margins. Recent studies have revealed the biogeochemical and microbiological characteristics of the deep sediment found in mud volcanoes.

In the submarine mud volcanoes off Tanegashima island, sediment-inhabiting Atribacteria have been found in methane plumes in the overlying water column. This observation suggests that microbes are dispersed from the deep sedimentary biosphere to the overlying hydrosphere through submarine mud-volcanic activity. Scientific drilling conducted at the submarine mud volcano in the Kumano basin has revealed that the production and migration of fluids in an oceanic subduction zone is closely related to the production of natural gas by microorganisms living in deep sediments at mud volcanoes.

Inland mud volcanoes are distributed along anticline axes and faults. Based on studies of inland mud volcanoes in Japan and Taiwan over the last eight years, we discuss the relationship between mount shape and the geological structure that controls the distribution of mud volcanos and the ascent of erupting fluid composed of mud, groundwater, and gas. Mud volcanoes distributed along anticline axes (AMVs) are characterized by intense eruptions with topographic depressions several meters to one km in diameter and several tens of meters deep. These features arise from hydro-fracturing of the impermeable cap rock under high gas pressure, which is due to degassing in the saline mud chamber beneath the cap rock. The δ¹⁸O values of groundwater erupted from AMVs are 0‰-2‰. Thermogenic hydrocarbon gas from AMVs originates deep underground and ascends and mixes with biogenic gas under the cap rock before erupting at the surface. The mounts form low mud pools because of the high water content of the fluid. If the cap rock is not distributed at a crest of anticline, the ascending fluid erupts gently under the cap rock. Mud volcanoes distributed along faults (FMVs) are characterized by the gentle eruption of fluid that originates deep underground. The δ¹⁸O values of groundwater erupted from FMVs are 3-8‰. Fluid erupted from FMVs originates deeper compared with fluid at FMVs and ascends through faults at fracture zones. AMVs form high cones up to several tens of meters high as a result of the low water content of fluid from the clay-rich fault gauge. Thermogenic hydrocarbon gas at FMVs originates from microbial decomposition deep in the fracture zone. AMVs cause serious damage to infrastructure and are a threat to human life.