Hydrate-bearing sediments, which often are associated with mud volcano activity. Source: USGS, 1996.
The geothermal phenomena known as "mud volcanoes" are often not true mud volcanoes (pelovolcano). See mudpot for further information.
A mud volcano or mud dome is a landform created by the eruption of mud or slurries, water and gases. Several geological processes may cause the formation of mud volcanoes. Mud volcanoes are not true igneousvolcanoes as they do not produce lava and are not necessarily driven by magmatic activity. The Earth continuously exudes a mud-like substance, which may sometimes be referred to as a "mud volcano". Mud volcanoes may range in size from merely 1 or 2 meters high and 1 or 2 meters wide, to 700 meters high and 10 kilometers wide. Smaller mud exudations are sometimes referred to as mud-pots.
The mud produced by mud volcanoes is mostly formed as hot water, which has been heated deep below the Earth's surface, begins to mix and blend with subterranean mineral deposits, thus creating the mud slurry exudate. This material is then forced upwards through a geological fault or fissure due to local subterranean pressure imbalances. Mud volcanoes are associated with subduction zones and about 1100 have been identified on or near land. The temperature of any given active mud volcano generally remains fairly steady and is much lower than the typical temperatures found in igneous volcanoes. Mud volcano temperatures can range from near 100 °C (212 °F) to occasionally 2 °C (36 °F), some being used as popular "mud baths."
About 86% of the gas released from these structures is methane, with much less carbon dioxide and nitrogen emitted. Ejected materials are most often a slurry of fine solids suspended in water that may contain a mixture of salts, acids and various hydrocarbons.
Possible mud volcanoes have been identified on Mars.
A mud volcano may be the result of a piercement structure created by a pressurized muddiapir that breaches the Earth's surface or ocean bottom. Their temperatures may be as low as the freezing point of the ejected materials, particularly when venting is associated with the creation of hydrocarbonclathrate hydrate deposits. Mud volcanoes are often associated with petroleum deposits and tectonic subduction zones and orogenic belts; hydrocarbon gases are often erupted. They are also often associated with lava volcanoes; in the case of such close proximity, mud volcanoes emit incombustible gases including helium, whereas lone mud volcanoes are more likely to emit methane.
Approximately 1,100 mud volcanoes have been identified on land and in shallow water. It has been estimated that well over 10,000 may exist on continental slopes and abyssal plains.
Gryphon: steep-sided cone shorter than 3 meters that extrudes mud
Mud cone: high cone shorter than 10 meters that extrudes mud and rock fragments
Scoria cone: cone formed by heating of mud deposits during fires
Salse: water-dominated pools with gas seeps
Spring: water-dominated outlets smaller than 0.5 metres
Most liquid and solid material is released during eruptions, but seeps occur during dormant periods.
First-order estimates of mud volcano emissions have been made (1 Tg = 1 million metric tonnes).
2002: L. I. Dimitrov estimated that 10.2–12.6 Tg/yr of methane is released from onshore and shallow offshore mud volcanoes.
2002: Etiope and Klusman estimated at least 1–2 and as much as 10–20 Tg/yr of methane may be emitted from onshore mud volcanoes.
2003: Etiope, in an estimate based on 120 mud volcanoes: "The emission results to be conservatively between 5 and 9 Tg/yr, that is 3–6% of the natural methane sources officially considered in the atmospheric methane budget. The total geologic source, including MVs (this work), seepage from seafloor (Kvenvolden et al., 2001), microseepage in hydrocarbon-prone areas and geothermal sources (Etiope and Klusman, 2002), would amount to 35–45 Tg/yr."
2003: analysis by Milkov et al. suggests that the global gas flux may be as high as 33 Tg/yr (15.9 Tg/yr during quiescent periods plus 17.1 Tg/yr during eruptions). Six teragrams per year of greenhouse gases are from onshore and shallow offshore mud volcanoes. Deep-water sources may emit 27 Tg/yr. Total may be 9% of fossil CH4 missing in the modern atmospheric CH4 budget, and 12% in the preindustrial budget.
2003: Alexei Milkov estimated approximately 30.5 Tg/yr of gases (mainly methane and CO2) may escape from mud volcanoes to the atmosphere and the ocean.
2003: Achim J. Kopf estimated 1.97×1011 to 1.23×1014 m³ of methane is released by all mud volcanoes per year, of which 4.66×107 to 3.28×1011 m³ is from surface volcanoes. That converts to 141–88,000 Tg/yr from all mud volcanoes, of which 0.033–235 Tg is from surface volcanoes.
Mud volcanism is a common phenomena in Indonesia with dozens of structures present onshore and offshore.
The Indonesian Lusi mud eruption is often (and wrongly) described as a mud volcano. Geochemical, pertrography and geophysical results reveal instead that Lusi is a sediment-hosted hydrothermal system connected at depth with the neighboring Arjuno Welirang volcanic complex.
Drilling or an earthquake in the Porong subdistrict of East Java province, Indonesia, may have resulted in the Sidoarjo mud flow on May 29, 2006,. The mud covered about 440 hectares, 1,087 acres (4.40 km2) (2.73 mi2), and inundated four villages, homes, roads, rice fields, and factories, displacing about 24,000 people and killing 14. The gas exploration company involved was operated by PT Lapindo Brantas and the earthquake that may have triggered the mud volcano was the 6.3 magnitudeYogyakarta earthquake of May 27, 2006. According to geologists who have been monitoring Lusi and the surrounding area, the system is beginning to show signs of catastrophic collapse. It was forecasted that the region could sag the vent and surrounding area by up to 150 metres (490 ft) in the next decade. In March 2008, the scientists observed drops of up to 3 metres (9.8 ft) in one night. Most of the subsidence in the area around the volcano is more gradual, at around 1 millimetre (0.039 in) per day. A study by a group of Indonesian geo-scientists led by Bambang Istadi predicted the area affected by the mudflow over a ten-year period. More recent studies carried out in 2011 predict that the mud will flow for another 20 years, or even longer. Now named Lusi – a contraction of Lumpur Sidoarjo, where lumpur is the Indonesian word for "mud" – the eruption represent an active hybrid system.
In the Suoh caldera in Lampung, dozens of mud cones and mudpots varying in temperature are found.
Wushanding mud volcano is a well-known attraction.
Many mud volcanoes exist on the shores of the Black Sea and Caspian Sea. Tectonic forces and large sedimentary deposits around the latter have created several fields of mud volcanoes, many of them emitting methane and other hydrocarbons. Features over 200 metres (656 ft) high occur in Azerbaijan, with large eruptions sometimes producing flames of similar scale. There are mud volcanoes in Georgia, such as the one at Akhtala.
Iran and Pakistan possess mud volcanoes in the Makran range of mountains in the south of the two countries. A large mud volcano is located in Balochistan, Pakistan. It is known as Baba Chandrakup (literally Father Moonwell) on the way to Hinglaj and is a Hindupilgrim site.
Azerbaijan and its Caspian coastline are home to nearly 400 mud volcanoes, more than half the total throughout the continents. In 2001, one mud volcano 15 kilometres (9 mi) from Baku made world headlines when it started ejecting flames 15 metres (49 ft) high.
In Azerbaijan, eruptions are driven from a deep mud reservoir which is connected to the surface even during dormant periods, when seeping water shows a deep origin. Seeps have temperatures that are generally above ambient ground temperature by 2 °C (3.6 °F) – 3 °C (5.4 °F).Lökbatan Mud Volcano since 1998 submitted to UNESCO's World Heritage Site tentative list.
In Pakistan there are more than 80 active mud volcanoes in Balochistan province; there are about 10 locations with clusters of mud volcanoes. In the west, in Gwadar District, the mud volcanoes are very small and mostly sit in the south of Jabal-e-Mehdi toward Sur Bandar. Many more are in the northeast of Ormara. The remainder are in Lasbela District and are scattered between south of Gorangatti on Koh Hinglaj to Koh Kuk in the North of Miani Hor in the Hangol Valley. In this region, the heights of mud volcanoes range between 800 to 1,550 feet (243.8 to 472.4 m). The most famous is Chandragup. The biggest crater found at 25°33'13.63"N. 65°44'09.66"E is about 450 feet (137.16 m) in diameter. Most mud volcanoes in this region are in out-of-reach areas having very difficult terrain. Dormant mud volcanoes stand like columns of mud in many other areas.
In the Turtle Islands, in the province of Tawi-Tawi, the southwestern edge of the Philippines bordering Malaysia, presence of mud volcanoes are evident on three of the islands – Lihiman, Great Bakkungan and Boan Islands. The northeastern part of Lihiman Island is distinguished for having a more violent kind of mud extrusions mixed with large pieces of rocks, creating a 20-m (66-ft) wide crater on that hilly part of the island. Such extrusions are reported to be accompanied by mild earthquakes and evidence of extruded materials can be found high in the surrounding trees. Submarine mud extrusions off the island have been observed by local residents.
Other Asian locations
Mud volcano landscape at Oesilo, Oecusse District, Timor-Leste
There are mud volcanoes at the Minn Buu Township, Magway division in Myanmar (Burma).
There are two active mud volcanoes in South Taiwan and several inactive ones. The Wushan Mud Volcanoes are in the Yanchao District of Kaohsiung City. There are active mud volcanoes in Wandan township of Pingtung County.
There are mud volcanoes on the island of Pulau Tiga, off the western coast of the Malaysian state of Sabah on Borneo.
The Meritam Volcanic Mud, locally called the 'lumpur bebuak', located about 35 km from Limbang, Sarawak, Malaysia is a tourist attraction.
A drilling accident offshore of Brunei on Borneo in 1979 caused a mud volcano which took 20 relief wells and nearly 30 years to halt.
Mud volcanoes of the North American continent include:
A field of small (<2 metres (6.6 ft) high) fault-controlled, cold mud volcanoes is on California's Mendocino Coast, near Glenblair and Fort Bragg, California. The fine-grained clay is occasionally harvested by local potters.
An unnamed mud volcano 30 metres (98 ft) high and with a top about 100 metres (328 ft) wide, 24 kilometres (15 mi) off Redondo Beach, California, and 800 metres (2,620 ft) under the surface of the Pacific Ocean.
A field of small (<3 metres (9.8 ft)) mud volcanoes in the Salton Sea geothermal area near the town of Niland, California. Emissions are mostly CO2. One, known as the Niland Geyser, continues to move erratically.
The name of Yellowstone National Park's "Mud Volcano" feature and the surrounding area is misleading; it consists of hot springs, mud pots and fumaroles, rather than a true mud volcano. Depending upon the precise definition of the term mud volcano, the Yellowstone formation could be considered a hydrothermal mud volcano cluster. The feature is much less active than in its first recorded description, although the area is quite dynamic. Yellowstone is an active geothermal area with a magma chamber near the surface, and active gases are chiefly steam, carbon dioxide, and hydrogen sulfide.
However, there are mud volcanoes and mud geysers elsewhere in Yellowstone. One, the "Vertically Gifted Cyclic Mud Pot" sometimes acts as a geyser, throwing mud up to 30 feet high.
The mud volcano[which?] in Yellowstone was previously a mound until a thermal explosion in the 1800's ripped it apart. 
One of the Devil's Woodyard Volcano (Trinidad & Tobago)
There are many mud volcanoes in Trinidad and Tobago in the Caribbean, near oil reserves in southern parts of the island of Trinidad. As of August 15, 2007, the mud volcano titled the Moruga Bouffle was said to being spitting up methane gas which shows that it is active. There are several other mud volcanoes in the tropical island which include:
the Chatham mud volcano underwater in the Columbus Channel; this mud volcano periodically produces a short-lived island.
the Erin Bouffe mud volcano near Los Iros beach
L'eau Michel mud volcano in Bunsee Trace, Penal
A number of large mud volcanoes have been identified on the Barbados accretionary complex, offshore Barbados.
Yagrumito mud volcano in Monagas, Venezuela (6 km from Maturín)
The eastern part of Venezuela contains several mud volcanoes (or mud domes), all of them having an origin related to oil deposits. The mud of 6 kilometres (3.7 mi) from Maturín, contains water, biogenic gas, hydrocarbons and an important quantity of salt. Cattle from the savanna often gather around to lick the dried mud for its salt content.
Volcan El Totumo, which marks the division between Bolívar and Atlantico in Colombia. This volcano is approximately 50 feet (15 m) high and can accommodate 10 to 15 people in its crater; many tourists and locals visit this volcano due to the alleged medicinal benefits of the mud; it is next to a cienaga, or lake. This volcano is under legal dispute between the Bolívar and Atlántico Departamentos because of its tourist value.
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^Achim J. Kopf (2003). "Global methane emission through mud volcanoes and its past and present impact on the Earth's climate". International Journal of Earth Sciences. 92 (5): 806–816. Bibcode:2003IJEaS..92..806K. doi:10.1007/s00531-003-0341-z. ISSN 1437-3254 (Paper) ISSN 1437-3262 (Online)
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^Barnard, A.; Sager, W. W.; Snow, J. E.; Max, M. D. (2015-06-01). "Subsea gas emissions from the Barbados Accretionary Complex". Marine and Petroleum Geology. 64: 31–42. doi:10.1016/j.marpetgeo.2015.02.008.