A Appendix A: GLOBAL MUD DOME OCCURRENCES
A1. Aleutian/Alaskan Margin
 In the Copper River Basin in Alaska, mud volcanism has been reported to be associated with the occurrence of hot springs [Allen, 1887; Nichols and Yehle, 1961]. The MVs of the Klawasi group in the west and the Tolsona group in the east are triggered by different fluids. The Tolsona mud cones are made of mud with sand and pebbles of the underlying formations; the water has been related to connate waters expelled from Upper Cretaceous marine sediment [Grantz et al., 1962]. Gas analyses show methane and nitrogen as the predominant phases. In contrast, the Klawasi MVs emanate largely CO2 and also have authigenic carbonate crusts and cements at their surfaces. The fluids are proposed to be either volcanic or fossil interstitial waters of marine origin [Nichols and Yehle, 1961]. They are characterized by high salinities and enhanced contents of mobile elements and resemble oilfield brines. The clasts within the mud reflect unmetamorphosed source rocks of the Wrangell Mountains area [Grantz et al., 1962]. Despite a series of geophysical surveys and bottom sampling, no evidence for offshore MVism has been found to date in the Aleutian subduction zone of the Alaskan forearc [e.g., Wallmann et al., 1997]. This is particularly surprising because diffuse as well as conductive fluid expulsion with rapid flux rates have been demonstrated for the toe of the accretionary prism [von Huene et al., 1998]. However, deep ocean mud diapirs have been inferred from seaward dipping thrusts through overpressured shale [Seely, 1977].
A2. British Columbia
 The Mount Sullivan hydrothermal Cu/Zn/Pb deposit in southeastern British Columbia, Canada, has been proposed to have evolved from a mud volcano complex [Slack et al., 1998]. Both pebble-bearing mud volcano deposits and massive sulfide bodies are interbedded with Mesoproterozoic clastic rocks and seem connected with the Kimberley fault. The preserved MV deposits have been traced by drill holes along a 5-km-long transect, showing an average thickness of 300–400 m. In cross section [Slack et al., 1998, Figure 1] the conduits to the mud-clast breccias are often related to steep normal faults and contain sulfides, tourmalinites, pyrrothite, and sedex-type Pb-Zn ores. Such ancient seafloor tourmalinites resemble modern analogues such as the hydrothermal mud volcano fields in the Black Sea area [Shnyukov et al., 1986]. The model of MV evolution in a hydrothermal setting under anoxic conditions, as proposed by Slack et al. [1998, Figure 3], is discussed in section 3.3 (see also Figure 6d).
A3. Cascadia Margin/Oregon, Washington
 Along the Oregon Margin, there is only weak evidence for MVism. Although fluid venting and gas hydrate processes are well documented, the seismically opaque mounts and ridges on seismic profiles were only initially interpreted as mud volcanoes [MacKay et al., 1992]. A more detailed study, however, suggests that tectonic shortening causes the formation of plunging anticlines along which fluid venting and carbonate precipitation occurs [e.g., Suess et al., 1999]. Reevaluation of seismic lines together with new swath bathymetry lead Trehu et al.  to the hypothesis that three dome-shaped features on the flanks of Hydrate Ridge on the Cascadia accretionary prism, off Oregon, may be vent sites and MVs. Recent submersible evidence, however, identified at least one of the structures as a massive carbonate chemoherm [Brown et al., 1999]. Evidence from three-dimensional seismic data across southern Hydrate Ridge suggests that buried MVs exist (J. Chevallier, personal communication, 2002). The features reach up to 3 km in width, show incoherent seismic signatures (compared to clear reflectors in their surrounding), and probably relate to enhanced fluid flow in the past.
 Onshore in the Hoh accretionary complex on the Olympic Peninsula, diapiric mélanges have been mapped and described by Orange . They represent the onshore continuation of the Cascadia accretionary prism off Washington, exhibiting rocks of Miocene age. The mélanges result both from shear movements and from diapiric ascent of mud breccia due to buoyancy [Orange, 1990, Figure 16]. Similar mud ridges of diapiric origin have been inferred from seismic images across the offshore portion of the Washington margin [Silver, 1972; Barnard, 1978]. However, only one active mud volcano, the Garfield gas mound, has been mapped adjacent to the onshore outcrop of the Duck Creek mélange zone [Orange, 1990]. The feature is <20 m high, is several tens of meters across, and is presently belching methane. Fluid is expelled as a result of burial and tectonic stress.
 A different type of mud deposit has recently been described by Vallance and Scott . The occurrence of mudflows from hydrothermal activity and, specifically, phreatomagmatic eruptions was reported from the western flank of Mount Rainier, Washington. A total of 3.8 km3 of mud started as a water-saturated avalanche and covered some 200 km2 in the Osceola region. One peculiarity of the mudflow deposit is the downstream decrease in clay, most likely resulting from incipient incorporation of surrounding altered rocks of sand and gravel size [Vallance and Scott, 1997].
 Other examples of thermal activity triggering MVism are the Lake City hot springs, northeastern California [White, 1955], and Yellowstone, Wyoming [Pitt and Hutchinson, 1982; Sheppard et al., 1992]. The latter features emit gases of largely volcanic origin, which consist of CO2, N2, and generally <5% CH4 [Sheppard et al., 1992]. He isotopes indicate minor contributions of a mantle source. The mud volcano domain is a system of mud cauldrons, fumaroles, warm springs, and cold seeps and has previously been related to earthquake activity [Pitt and Hutchinson, 1982]. As a result, widened fracture networks at depth facilitate upward migration of hydrothermal fluids, which liquefy mudstones. Other small subaerial mud mounds (mudpots) in the Salton Sea area seem to be unrelated to the hot brines in the area [Sturz et al., 1992].
 In Monterey Bay, offshore of California, abundant cold fluid seeps and mud volcanoes are connected with the transform fault separating the Pacific and North America Plates [Orange et al., 1999]. Often, small (tens of meters across) mounds are covered with authigenic carbonate crusts, which originate from bacterial oxidation of the methane-rich fluids. Such mud volcanoes are circular to irregular surface expressions of diapiric mélanges, and the locally restricted fluid discharge and cementation of mud and talus allows formation of durable topographic highs [Orange et al., 1999, Figure 9]. Similar to accretionary wedges, fluid is augmented due to tectonic compaction between the San Gregorio fault zone, thus imparting overpressures and a buoyancy force in the sediment. A field of small MVs has also been observed in the nearby Santa Barbara Basin during acoustic imaging of the seafloor (P. Eichhubl, personal communication, 2001).
 The Diablo mountain range, Franciscan Belt, is one of the southern coast ranges in California where serpentine extrusions have been found [Oakeshott, 1968]. The serpentine represents an old marine layer, which got incorporated into subduction and orogenic processes. Low bulk densities made the serpentinized rocks a lubricant in thrusting and folding before some serpentine, which had been sheared into fold hinges, pierced the overlying strata. The extreme mobility of this mass allowed it to diapirically rise upward, with the erosion of some of its overburden aiding this movement. The upheaved domes reach diameters of ∼8 km and are currently mined for short-fiber chrysotile asbestos [Oakeshott, 1968].
 A different scenario in which mud extrusion has been observed is Pyramid Lake, Nevada [Miffin, 1970]. Here sediment mud lumps result from the rapid progradation of sediment over undercompacted offshore muds in the lake. The proposed mechanism of intrusion is excess pore pressure relative to the lithostatic load of the overburden [Miffin, 1970] (see also sections 3.3 and 3.4).
 In Mexico the active El Cocuite mud volcano, near Veracruz, has been described by Humphrey . It is 25 m wide, ∼6 m high, and of dome-shaped geometry. Its activity has been related to the viscosity of the mud that is mobilized from different stratigraphic levels within an anticlinal structure of kilometers-thick sediments. Its microfossil content and the presence of gas and heavier liquid hydrocarbons suggest an origin from Oligocene strata at several kilometers depth [Humphrey, 1963].
A6. Texas, Louisiana, and Mississippi/Gulf of Mexico
 A series of fossil mud volcanoes is preserved along an Oligocene-Miocene succession in south Texas [Freeman, 1968]. The circular hills are ∼500 m across and up to 40 m high and cover some 110 km along strike. The source is very likely related to Eocene clayey tuff deposits of the Frio formation, which diapirically extrude along fault planes into the younger Gueydan sedimentary rocks at the surface. Because of igneous boulders of inferred greater depth related to the mud extrusions, Freeman  inferred violent, gas-assisted eruptions and rise over enormous vertical distances. Also, individual blocks of up to 150 m3 among the ejecta support a forceful mechanism. Shale diapirism has thinned and penetrated several hundred meters to form the North LaWard diapir at the lower Texas Gulf Coast [Brooner, 1967]. No violent eruptions, venting, or tectonic relations are described, so that the phenomenon is attributed entirely to buoyancy of deep-water shales at depth.
 Offshore of Louisiana, oil slicks over the site of an active mud volcano were recorded from space [MacDonald et al., 1993]. The seepage generalized for the entire area is estimated to be on the order of 20,000 m3 yr−1. In the immediate vicinity, one active and two dormant MVs have been identified on the upper continental slope [Neurauter and Roberts, 1994]. Small intermittent eruptions of gas- and oil-rich muds cause spillover and growth of the feature in the crater area as well as on its flanks. Authigenic carbonate crusts were observed on the dormant MV [Neurauter and Roberts, 1994]. One of the mounds, Bush Hill MV, has been subject to an earlier study [Neurauter and Bryant, 1990], which revealed an irregular cone-shaped geometry of up to 1000 m diameter and 40> m height. From seismic images, mud extrusion seems to be related to steep normal faulting.
 Literature spanning several decades has been published on mud lumps in the Mississippi delta, Texas Gulf Coast. The features are described as small anticlines in fine-grained, rapidly deposited sediments in the distributories of the Mississippi River [e.g., Morgan, 1961; Morgan et al., 1968]. The island-type topographic highs have been related to diapiric ascent of overpressured clays, with vertical displacements on the order of 100–200 m. Cone-shaped mud vents are also known, some of which occur along fault lines [Morgan, 1961]. Prior and Coleman  have shown that mudflows in the area occur at slope angles of as little as 1°, suggesting a very low viscosity of the mud (see section 3.4).
A7. Lake Michigan
 When Colman et al.  studied the stratigraphy of deposits in southern Lake Michigan, a variety of features that disrupt the otherwise smooth, muddy surface of sediments were found. Next to circular depressions, which have been interpreted as pockmarks, a number of small slumps as well as low, irregular, subcircular mounds, possibly small mud volcanoes, were mapped. Their nature and origin remain unclear, although migrating gas may be the driving force for mud extrusion.
A8. Costa Rica
 MVs on the upper forearc off Costa Rica have been investigated during numerous geophysical surveys [Shipley et al., 1990; Stoffa et al., 1991; von Huene et al., 2000]. As has been shown recently by deep-sea drilling, the frontal wedge of the overriding plate is of nonaccretionary origin, so that the mud volcanoes on the sediment apron overlying the igneous forearc wedge either are dewatering products or relate to deep-seated faults [Kimura et al., 1997]. The mud domes are numerous, generally cone-shaped, of less than 1 km diameter, and do not contain clasts of underlying strata (on the basis of the evidence from low backscatter on side-scan sonar maps [von Huene et al., 2000]). Pore water studies show signatures close to seawater [Zuleger et al., 1996], so that mixing of deep fluids, gas hydrate water, and pore water is inferred [Kopf et al., 2000a]. Recent bottom sampling attests that the MVs are massive carbonate-cemented highs, similar to the chemoherms in Cascadia (see section A3).
A9. Northern Colombia and Panama
 In northern Colombia the Sinu and San Jacinto fold belts form an accretionary wedge of sediment from the Caribbean Plate, which was scraped off by a rigid buttress comprising South American basement. On the 12-km-thick imbricated sequence, folding, sediment overpressuring, and abundant mud volcanism have been reported [Toto and Kellogg, 1992]. Mud extrusion is related to undercompaction of the thick sedimentary pile as well as to tectonic shortening. The most detailed information exists for the area between the Gulf of Uraba and northeastward to Cartagena, where shield-shaped MVs have been described by Gansser . They occur in several groups within Tertiary clay-rich folded strata, emitting methane and, rarely, oil. The most prominent features, the Turbaco group, comprise various domes of several tens of meters in diameter, with soupy muds flowing into the surrounding jungle.
 It has been suggested that the small El Totuma mud volcano farther east is genetically connected to the marine features. It reaches 12 m height, is located onshore, and appears to be dormant at present [Humphrey, 1963]. Apart from El Totuma, a large number of similar features are located along the Cordillera Occidental of northern Colombia. Most likely, the onshore features are older remnants of convergent tectonics, accretion, and accumulation of thick sedimentary successions along this margin. However, some activity of mud and methane bubbling has been reported in places [Humphrey, 1963].
 More recently, Vernette et al.  reported mud diapirs off the Caribbean coast of Colombia in the Magdalena delta. The authors propose tectonic stress as well as undercompaction of the delta sediments to be the driving forces for extrusion. The features observed on seismic reflection profiles are up to 2.5 km in diameter and a few hundred meters high. Samples indicate the predominance of smectite and kaolinite. The 3.5-kHz profiling provided evidence for abundant gas seepage in the MV area. A total of 24 domes has been identified during this survey [Vernette et al., 1992, Figure 13].
 Offshore of north Panama, where the Caribbean Plate is subducted beneath Middle America, Reed et al.  have reported the occurrence of more than 40 MVs. The features are between 0.4 and 2 km wide, <100 m high, and usually occur at the lower slope of a seaward vergent thrust belt. They pierce the crestal part of folded ridges that have been accumulated from two depocenters with undercompacted sediments. Seismic evidence is provided for strong BSRs and thus for gas hydrate formation. A majority of the MVs are directly connected with fold and fault traces visible on the seafloor, both thrusts and strike-slip faults [Reed et al., 1990, Figure 3]. Sediment overpressure as the main driving force for mud extrusion is facilitated by high sedimentation rates, tectonic shortening, and gas hydrate processes (see section 3 and Figure 6a).
 A few occurrences of MVs have been reported by Sheppard  in southwestern Ecuador, where Tertiary sediments and olistostromes dominate the complex geology. Although the rocks (i.e., olistoliths) carried with the mudflows are described in detail [e.g., Marchant and Black, 1960], no information regarding the depth of mud mobilization or mechanism of extrusion exists.
 An extensive literature exists on MVs onshore and offshore of Barbados, Lesser Antilles. The island of Barbados represents the apex and backstop of an extremely broad accretionary prism, comparable to the Mediterranean Ridge accretionary complex in many respects (see sections 4.1 and A22). Here sediment is scraped off the Atlantic Plate and accumulated to the imbricate wedge during subduction beneath the Caribbean Plate. Senn  considered the olistostrome-like Joes River formation on the island as a product of sedimentary volcanism. While Kugler  argued that these deposits result from slumping or mudflows from Paleocene parent beds, Speed and Larue  more recently demonstrated that the Neogene uplift and Pleistocene arching of the island can be partly attributed to clay diapirism. The upwelled mass contains the mud debris flow suite, which is bounded by younger faulting in the north, and probably migrated upward as a rigid plug [Speed and Larue, 1982].
 Offshore of the island, MVs have been mapped on the toe of the accretionary prism during geophysical surveys [e.g., Stride et al., 1982]. A number of early studies have examined the occurrence of mud volcanoes seaward of the Barbados accretionary prism [Brown and Westbrook, 1988; Langseth et al., 1988; Henry et al., 1990, 1996; Martin et al., 1996], where their morphological variability is well exposed. More than 450 MVs have been observed on top of the southern part of the Barbados Ridge, where they cover an area of >700 km2 [Brown and Westbrook, 1988]. The features are circular to oval, with edifice diameters ranging from 200 m to 6 km and with a height of up to 200–300 m above the surrounding seafloor. Many, but not all of the MVs show a rough surface owing to boulders and seem to be aligned to fault lineaments. Although no seismic data were available, an origin from diapirism at depth has been inferred [Brown and Westbrook, 1988]. Regarding their size and distribution, the MVs are similar to a MV field farther south, that is, toward Trinidad [Mascle et al., 1979]. More recent studies in the latter area relate fluid venting and MVism to the complex pattern of the surface traces of faults, folds, and networks of conjugate fractures [e.g., Griboulard et al., 1998]. Both argillokinetic diapirism and mud volcanoes are found [Griboulard et al., 1998, Figure 6 and 10]. The MVs are up to >6 km wide, and their conduits are estimated to reach 2 km in diameter. The majority of the clayey diapiric material originates from Miocene beds [Faugéres et al., 1997]. The vertical migration of clays was partly initiated by tectonic deformation and was guided by major faults before it appears as ridges and cones on the seafloor [Faugéres et al., 1997].
 In MV fields off the deformation front (i.e., on the incoming Demerara abyssal plain), extensive geophysical and submersible studies revealed another mechanism for MVism [Henry et al., 1996; Sumner and Westbrook, 2001]. Between 13.5°N and 14.2°N along the Mercurus fracture zone, the fluids involved in mud extrusion originate from clay dehydration at depth as well as from lateral flux [Martin et al., 1996] and are transported beneath the décollement of the accretionary wedge by diffusive flow [Westbrook and Smith, 1983, Figure 5]. Recently, the reorganization of the fluid flow regime in the oceanic crust has been suggested to have allowed for outward migration of waters [Sumner and Westbrook, 2001]. Secondary processes triggering mud volcanism are gas hydrate dissociation or degradation of organic matter. In the small corridor mapped during the expedition, 23 mud features domes and diatremes) have been observed. In these MVs, free methane flux triggers mud convection and ascent due to lowered density and viscosity [Henry et al., 1996]. The amount of gas expelled from the biggest feature, Atalante diatreme, has been estimated from heat flow data. Gas flux ranges are on the order of 1800–11,000 m3 d−1 [Henry et al., 1996], although this value does not seem representative for the average Barbados MV. On these MVs, benthic communities are sustained by massive methane-rich fluid expulsion [Olu et al., 1997]. Chimneys and authigenic carbonate crusts are common, with salinity-driven fluid convection as main motor. On the basis of nannofossil occurrences, the oldest MVs were dated 750 ka or younger [Lance et al., 1998]. Different stages of activity have been deduced from the submersible observations, reaching from collapse to growth by mud extrusion. Mud temperatures may be as high as 21°C. The geometry of the feeder has been related to the material sampled and to the physical properties of the mud. Postcruise analog modeling confirmed that plastic mud allows the retention of conical mounds, while soupy mud and wide conduits lead to pie-like MVs [Lance et al., 1998]; see also section 5.
A12. Venezuela and Trinidad
 The southwestward continuation of the deformation front of the Southern Barbados Ridge terminates in a system of thrust and strike-slip faults and, most prominently, in the El Pilar fault zone, a right-lateral wrench fault. The earliest studies of MVism date back to Ferguson . Numerous small MVs have later been described by Gansser  and Arnold et al.  and have been related to Neogene sediments. The mud is mobilized from marine clays of the diapiric mass in the Pedernales anticline, which collected various rocks of overlying strata during ascent. Apart from many small cones, there are a few big domes (e.g., the flat-topped, 100 m-high Morne Diablo), which consist of Oligo/Miocene oil-bearing clays [Gansser, 1960].
 Farther east, on the island of Trinidad, a similar setting is met, where faults separate the anticlines. MVs also occur offshore, and their description dates back to Kugler . Other key references, from which the following summary is collated, include Kugler , Gansser ,Higgins and Saunders , Carr-Brown and Frampton , and Yassir . Among the 26 active and four fossil MVs described by Higgins and Saunders , the majority are found in the Southern Range deposits. They relate to thick mid-Tertiary clay sequences, specifically, the Navira-, Karamat-, and lower Cruse-Lengua Formations, which are characterized by low p wave velocities hinting toward sediment overpressuring [Yassir, 1989]. In these lithologies, sedimentary and tectonic mélanges have been described [Yassir, 1989]. An early study by Birchwood  estimated the depth of mud mobilization to be at least 3 km. The “mixed fauna clays” are of proposed diapiric origin and, according to wells onshore, originate from parent beds at depths between 1.2 and 1.8 km [Higgins and Saunders, 1974]. Onshore features can be divided into four types of MVs: (1) mud pools, ranging from a few tens of centimeters to hundreds of meters, forming shields or pies; (2) cones, reaching 60 m (Anglais Point) and more, with boulders of up to 42 m3 of Eocene or Cretaceous blocks and flows >250 m long [Higgins and Saunders, 1974]; (3) big tassiks, extending over up to 175 × 110 m (Lagon Bouffe), with several conduits; and (4) fossil MVs, like Moruga Bouffe, which may reach over 1200 m in diameter and include more than 600 extinct individual cones [Higgins and Saunders, 1974]. The subsurface structure of many of the MVs is well constrained by commercial well information and often resembles a diapiric main body with interfingering patterns from ancient mudflows (similar to the models in Figure 6b and 6c). For detailed descriptions of the MVs, reference is made to Higgins and Saunders , Yassir , and Dia et al. .
 Several historic reports of island formation due to mud volcanic extrusion exist from the Trinidad coast. Violent eruptions accompanied the rise of small islands off Chatham, Erin Bay, in 1911 [Arnold and MacReady, 1956], 1928 [Weeks, 1929], and 1964 [Higgins and Saunders, 1967]. Striated muds and absence of salt water imply that the source beds had undergone considerable tectonic loading prior to extrusion. Fresh fluid, probably from clay mineral dehydration within the lower Cruse-Lengua clays, caused overpressures and explosive eruption [Higgins and Saunders, 1967]. The size of such islands varies between tens and hundreds of meters in diameter; their geometry is usually a flat shield or cone that rarely reaches more than 10 m above sea level. Given the tidal variation in the area, they are short-lived structures and suffer rapid erosion (8 months in case of the 1964 Chatham Island [Higgins and Saunders, 1967]).
 Recently, extensive studies on the physical properties of the Trinidad muds [Yassir, 1989] and the fluid history and origin [Dia et al., 1999] were conducted. The muds generally have clay contents between 40 and 70%, corresponding to plasticity indices (i.e., range of Atterberg limits) of 15% and almost 80% [Yassir, 1989]. Shear strength is found to be a function of plasticity and clay content; weak, clay-rich muds also show the strongest pore pressure response [Yassir, 1989]. The fluid chemistry is indicative of two reservoirs separated by the major Los Bajos wrench fault. Chemical signatures inherited from high T (>150°C [Dia et al., 1999]) water-rock interaction suggest mixing of deep-generated fluids with meteoric water. This influx is believed to recharge the expulsion system with fluid. The gas phase expelled with the mud is mostly methane, with minor contributions of CO2 [Dia et al., 1999].
A13. Greenland and Northern Atlantic
 An arctic mud volcano, or pingo, situated in Svartenhuk, northwest Greenland, was subject to a phytoplankton study. The feature is apparently dormant, with a crater pond filled with meteoric water/ice [Kristiansen et al., 1995].
 In the Norwegian-Greenland Sea, side-scan sonar mapping discovered several fields of pockmarks within soft, stratified silty clays [Vogt et al., 1999]. Gas venting appears vigorous in places and has caused large-scale slope failure in the case of the Storegga slide. Tiny mounds (several to tens of meters across and only a few meters high) have also been observed [Vogt et al., 1999]. On acoustic profiles these features show an opaque, patchy signature, possibly illustrating the gas-charged conduit. Both bulk sediment density and heat flow are low [Vogt et al., 1999]. Some of the mounds can clearly be related to regional faulting.
 In the Porcupine Basin, northeastern Atlantic, along the eastern Irish continental margin, numerous MVs have been imaged during commercial [Croker and O'Loughlin, 1998] and scientific [Henriet et al., 1998] seismic surveys. The features, termed mud mounds, are of small to moderate size (200–600 m wide and 40–60 m high) and pierce the well-stratified sediments, as evidenced by their characteristic opaque signature. Four provinces, the Magellan, Hovland, Belgica, and Connemara fields, consist of pockmarks, shallow gas traps, and more than 120 mounds within the sedimentary succession. Some of them are buried (and overlain by contourites), while others are domes of ejecta surrounded by a moat, the latter possibly from collapse after gas venting ceased. Here onlapping sediments allow dating of the MVs. Preservation of the features relies on abundant microbial activity precipitating methane to authigenic carbonate cements.
 Complex interactions between the Iberian and African Plates since the Triassic create a complex structural pattern between the Gulf of Cadiz, Alboran Sea, and adjacent Moroccan continental margin. Gardner  described several seafloor structures to be gas hydrate related MVs. In the west of the study area, several sinuous ridges of proposed diapiric origin have been mapped and sampled, while farther east, at least six circular MVs and four more ridges have been sampled. Methane hydrates were recovered by gravity coring on one MV. The two biggest domes are 4 km across [Gardner, 1999].
 Onshore MVism in Spain is known in the Cantabrian Mountains, where diapiric clays deform the overlying carbonates [Stel, 1976]. The driving force for the ascent of the Lower Emsian La Vid Shales is believed to be undercompaction and enhanced pore fluid pressures from mineral dehydration reactions [Stel, 1976].
 MV formation was also observed in fluvial mid-Pleistocene deposits of the southwest Madrid Basin, Spain [Silva et al., 1997]. The features vary in size decimeters to tens of meters) and appearance from small domes and wavy ridges to hummocky surfaces of ancient flood plains. Although their origin was related to sediment loading, mud extrusion may have been triggered by earthquake activity.
 In the NE Gulf of Cadiz at the Iberian Margin, N-S oriented diapiric ridges have been described by Maldonado et al. . They comprise early-middle Miocene blue marls and mud breccia and seem related to slope instability and tectonic movements of the deformed sediment wedge. Recent oceanographic cruises discovered a total of eight circular to oval-shaped mud volcanoes, which consist largely of gas-saturated breccias with a strong H2S smell and tube worm fauna [Fernandez-Puga et al., 2002]. The domes show slope angles between 2° and 9° but locally reach up to 25°. The parent bed may be Eocene olistostrome deposits. Fluid and gas mobilization is related to convergent tectonics between Africa and Eurasia.
A16. North Sea, Barents Sea, and Baltic Sea
 In the North Sea several MV occurrences have been described in some detail. Among the most prominent features is Håkon Mosby mud volcano near the Norwegian-Barents-Svalbard continental margin [Vogt et al., 1997]. The dome is one of two 1-km-wide features next to each other, which supposedly results from fluid expulsion owing to gas hydrate processes and liquids trapped in mass wasting deposits. Heat flow data exceeding 1000 mW m−2 are among the highest in ocean basins (excluding hotspots and plate boundaries), and active venting creates warm water plumes above the MV [Eldholm et al., 1999]. A relatively limited biological community, dominated by tube worms and demersal fish, exists on Håkon Mosby [Milkov et al., 1999]. White mats on the seafloor are believed to represent both bacterial mats and massive gas hydrates, the latter being ∼1.8–2.5 × 106 m3 in volume [Ginsburg et al., 1999].
 In the area of the Sleipner gas reservoirs [Heggland, 1997], buried mounds are believed to be mud volcanoes generated during mid-Miocene time. Their locations seem to be associated with minor faults and fractures. Their conduits, seismic chimneys, indicate migration of gas from below this level through the faults and fractures and up to the seabed. In addition to the seepage, shallow gas accumulations found recently in late Pliocene sands above the MVs may be a result of such gas migration.
 Three fields of what has been termed “seafloor piercing diapiric structures,” the Vema, Vigrid, and Vivian fields, exist on the marginal Vøring Plateau [Hjelstuen et al., 1997]. These MVs rise 150 m above the surrounding seafloor and overlie well-stratified basin sequences. From seismic profiles and scientific drill holes, Eocene-Miocene biosiliceous oozes and muds have been identified as parent beds. The ooze mobilization started in late Pliocene and was induced by differential loading and bulk density contrasts within the succession [Hjelstuen et al., 1997]. Bouriak et al.  have also reported geophysical evidence for MVism on the Vøring Plateau. A seal of massive gas hydrates allows enhanced fluid pressures at depth, with mud extruding when sliding creates zones of weakness in this barrier.
 The Crater field, located several hundred kilometers north of Norway in the Barents Sea, shows an accumulation of ∼20 seafloor depressions and occasional mounds [Long et al., 1998]. The features reach only tens of meters in diameter and seem related to gas hydrate instability. In the subsurface the BSR is interrupted by a seismically opaque conduit to the feature. The proposed evolution relates to gas hydrate dissociation due to postglacial uplift. The pockmark-type depressions most likely reflect rapid gas dissipation, which is still ongoing, as evidenced by frequent dense colonization in the crater area (small hydrozoans, sponges, actinans, and soft cold water corals [Long et al., 1998]).
 In the northern Stockholm Archipelago, Baltic Sea, small domes associated with gas seepage have been reported [Söderberg and Flodén, 1992]. The features are only 30 cm high and a few meters in diameter, which is mostly a function of the thin sedimentary cover (∼10 m in thickness) on the shelf. The gas emanating along tectonic lineaments through the crystalline bedrock has a thermogenic origin, indicating a deep trigger of sediment disturbance and mud extrusion.
A17. Alboran Sea
 In the western Alboran Sea, Mediterranean, diapiric ridges and, to a lesser extent, subcircular MVs are abundant phenomena in geophysical records [Perez-Belzuz et al., 1997]. Although one would suspect that intrusion and extrusion result from extensional tectonics in the narrow rift basin, there is no straightforward pattern of the ridges relative to kinematic constraints. In fact, only a few ridges align with the southwest-northeast trend of the Alboran Ridge spreading axis, while many others are associated with north-south trending thrust or do not correspond to any known tectonic lineaments. The flat-topped nature of all features (sometimes the mud diapir does not pierce the seafloor sediments at all [Perez-Belzuz et al., 1997, Figures 4b–4e]) suggests that ascent of parts of the Miocene shales at depth (some diapirs are Aquitanian-Langhian and others are Seravallian-Tortonian) has occurred some time ago. The MVs form small cones (∼1 km across) and are fed by seismically well imaged, vertical conduits that root in upper Miocene sediments (upper Tortonian-Messinian [Perez-Belzuz et al., 1997]). The parent bed depth has been inferred to be between 10 and 12 km from industry seismic records (A. C. Weinzapfel, personal communication, 2001).
A18. Western Alps and Apennines/Italy
 In the Maritime Alps and Northern Apennines, reports of olistostromes, chaotic facies rocks, and diapiric mélanges of the argille scagliose date back as far as the late nineteenth century [Ferreti, 1878]. More recent studies imply that the strata involved in MVism dates back as far as Upper Cretaceous [Abbate et al., 1970]. These deposits are part of a prealpine accretionary wedge that was thrust beneath the Eurasian Plate during orogenesis [Di Giulio, 1992]. Because of undercompaction and flow of mud along detachment surfaces, mélanges and mud volcanoes formed [Di Giulio, 1992]. In the Apennine foredeep thrust belt, chemosynthetic communities of inferred Miocene age attest paleo-dewatering and seepage. In fact, detailed field analyses allow us to distinguish between periods of concentrated flow and diffuse background flux in the system (P. Vannucchi, personal communication, 2001).
 For most of the MVs in the western Alps, intense postdepositional deformation hinders the reliable reconstruction of the in situ geometry. However, in places, cemented conduits and gryphons were preserved. The authigenic carbonate cements together with analyses of boulders within the shales allow us to identify, for example, the Verrua MV near Monferrato, northwest Italy, as a former cold seep marine mud dome [Cavagna et al., 1998]. Carbon and oxygen isotope geochemistry are in favor of gas hydrate processes having been involved in carbonate formation [Cavagna et al., 1998]. The two best known areas of MVism in the Apennines are near the villages of Regnato and Maranello. Either MV field is characterized by an area of several hundred meters squared, each containing numerous domes and mud pools of several meters in diameter. Mudflows may stretch over tens of meters downhill, with mostly methane bubbling in the craters and pools [Martinelli, 1999]. The diameter of inactive central conduits varies from 10 to 30 cm (A. Kopf, unpublished data, 2002). Several generations of mudflows with variable contents of mostly angular claystone and carbonate clasts of variable abundance overlie each other. While the mud of the Regnato region originates from Cretaceous strata, the mud domes near Maranello are fed by Miocene deposits at depth of the faulted belt.
 Certain MVs in the Po plain, in the southeastward prolongation of the ancient MV occurrences, have recently emitted gas and liquids during local seismic activity [Martinelli and Ferrari, 1991; Martinelli et al., 1995]. More recently, Conti et al.  concluded from their chemical data on fluids that brine-type compositions indicate deep-seated tectonic features allowing upward migration. The ejected material typically shows two different compositions. Normally, the emissions show thermal reequilibration with the surrounding rock in the faulted Apennine mountains. However, during less frequent violent eruptions, when explosions of mud, brackish waters, petroleum, and gas occur, chemically anomalous waters and gases have been found [Martinelli and Ferrari, 1991]. In the eastern part of the Po plain, these researchers find active fluid and gas venting a useful precursor for earthquakes, with the water increasing in temperature relative to background values. In the less active western part, cemented chimneys document former MV activity [Martinelli and Ferrari, 1991].
 MVs in the vicinity of Paterno at the base of Mount Etna volcano, Sicily, were described already by von Gumbel  and Deeke . Recent investigations of three MVs at the southwest flank of Mount Etna, at the contact between the volcanics and onlapping sediments, have reported discharge of cold brines as well as large quantities of CO2 [Chiodini et al., 1996]. The composition of the gas and liquid phases hints toward a hydrothermal reservoir at 100°–150°C, which can be explained by groundwaters from the central part of Etna feeding the MVs through lateral flux. Farther west, three active MV areas have been recently investigated by Etiope et al. . The biggest field, Maccalube near Atagona, spreads over 1.4 km2 and has numerous small domes of less than 1 m height. Salty waters as well as gas are discharged at considerable rates (see section 5).
A20. Adriatic Sea and Greece
 Gas seepage and possible MV occurrences have been reported from the Adriatic Sea, east of Italy. Seismic reflection data are characterized by abundant highly reflective patches, which have been interpreted as gas-charged diapiric structures [Hovland and Curzi, 1989]. Often, they show updoming of the seafloor, forming small domes of several tens of meters up to ∼300 m in diameter [Hovland and Curzi, 1989, Figure 4]. The features are only a few meters higher than the surrounding seafloor and have not been sampled. However, “acoustic fountains” of gas seeping out of the seafloor indicate active fluid escape.
 One MV example is known from the Katakolo Peninsula, western Peloponnesus, Greece [Stamatakis et al., 1987]. The small dome has a conduit of 30 cm diameter that emits hydrogen sulfide and gaseous hydrocarbons. On the walls of the conduit as well as in the crestal area, native S, halotrichite, gypsum, and other sulfide minerals are found. While most of the peninsula's geology is dominated by clay stones, marlstones, and other sediments of Pliocene age, Triassic anhydrites from depth are believed to intrude as diapirs before piercing the Pliocene cover [Stamatakis et al., 1987]. Similar structures of updoming and MVism have been observed offshore (northwest of the peninsula) in the Ionian Sea. Papatheodorou et al.  mapped active gas seeps, MVs of a few meters in height, and pockmark-type depressions on the seafloor near the westernmost prolongation of the Hellenic Arc.
A21. Aegean Sea
 The Aegean Sea is surrounded by mainland Greece and is the fast-spreading back arc basin to the eastern Mediterranean Sea (see Figure 7, inset). Gas-charged sediments, pockmarks, and small MVs and ridges were identified during high-resolution geophysical profiling on the Sporades Shelf [Papatheodorou et al., 1993]. MVism is fed by Quaternary fine-grained deltaic deposits of two river systems. The sediment's overpressure triggers normal faulting and gas seepage. Mud volcanic ridges (100 m in length [Papatheodorou et al., 1993, Figure 8]) evolve along the seafloor outcrops of such faults.
A22. Eastern Mediterranean Sea
 As already mentioned in section 3 and 4, the eastern Mediterranean Sea (Figure 7) is arguably the region with the highest abundance of MVs and diapirs on Earth. Several hundred features of variable age, geometry, and origin have been discovered during the previous 2 decades of geophysical surveys. Thereafter, a wealth of data and samples have been collected by coring, deep drilling, and submersible studies, so that the region of convergence between Africa and Eurasia is well understood.
 The Mediterranean Ridge accretionary complex (MedRidge) is an arcuate structure of >1500 km length and up to 250 km width [Fusi and Kenyon, 1996], which is suffering incipient tectonic deformation and uplift. Accretion has started as a result of the exhumation of thrust nappes on Crete after subduction of the Pindos Ocean [Thomson et al., 1998]. Crete, as an outer arc high, acted as a backstop to offscraping Neotethyan sediments and presently represents the northern border at the apex of the accretionary prism (Figure 7 and 8). Although northeastward subduction of the African Plate is relatively slow (1–2 mm yr−1), half the Aegean spreading rate in the back arc sums up to a net rate of ∼5–6 mm yr−1 [Le Pichon et al., 1995]. Morphologically, there are two distinct areas forming the accretionary prism: the actual thrust wedge and the so-called Inner Ridge, which is believed to represent the paleo-MedRidge and which presently acts as a backstop to the modern wedge [Kopf et al., 2000b]. In the western part of the MedRidge, subduction of the Ionian abyssal sediment occurs orthogonally to the grain of the prism, while east of Crete the Herodotus abyssal plain is obliquely thrust beneath the accretionary complex. This bears consequences for the geometry of the prism's and forearc's morphological elements and has tectonic repercussions regarding the nature of mud extrusions on the wedge. While dome-shaped, small MVs dominate the western and central branch of the MedRidge, the prism southeast of Crete is covered by mud pies of up to several tens 2of kilometers in diameter [Mascle et al., 1999]. When following the MedRidge to its easternmost prolongation, right where the Hellenic Trench system joins the Cyprean Arc, cone-shaped MVs are found again in the Anaximander Mountains and Florence Rise areas [Woodside et al., 1998; Aloisi et al., 2000a, 2000b]. The first to discover MVism on the MedRidge were Cita et al.  in one of the densest MV fields south of Crete, the Olimpi region (Figure 7). Gravity coring of some of the domes recovered a potpourri of polymictic clasts in a clayey matrix, termed mud breccia [Cita et al., 1981]. Later on, the difference in geometry between the settings on the MedRidge was related to the nature of the tectonic features that they are associated with, which operate as conduits. While the domes in the western and central part of the prism relate to reverse and back thrust faulting (Figure 8b), the mud pies correspond with extensional or transtensional forces when oblique subduction takes place [Kopf et al., 2001]. Hence, in the first case, mud breccia domes result from mud and fault breccia having been mixed during upward migration and emplacement [Kopf et al., 1998]. By contrast, pull-apart forces allow overpressured mud to extrude, so that (according to low backscatter intensities [Volgin and Woodside, 1996]) clast-free pies form over wide areas (Figure 7). This is in good agreement with results from analogue modeling of MV geometry as a function of material properties and width of the conduit (see Lance et al.  and section 4). In the Anaximander Mountains in the northeast Mediterranean, MVism is related to strike-slip faulting and gas hydrate processes [Woodside et al., 1998]. Domes of generally hundreds of meters width carry rock fragments from the regional framework (mostly known from mainland Turkey) with them. Toward Cyprus on the Florence Rise, MVism is less pronounced and may result from thrusting and shortening that causes the offscraped Herodotus Basin sediments to expel their pore fluids.
 It has been widely accepted that the main driving force of MVism in the area is generally related to incipient compression [Camerlenghi et al., 1992, 1995]; however, a variety of hypotheses have been put forward concerning the nature and origin of the muds and fluids. As the variety of methods in MV research used to characterize the origin of the gaseous, liquid, and solid phases are described in section 4, only the key aspects are briefly stated here.
 One of the biggest steps forward in MV investigation was drilling two transects of deep holes into submarine MVs south of Crete [Robertson et al., 1996]. The main findings from the cores recovered included their long-lived, though episodic nature (>1-Myr-old mudflows intercalated with hemipelagics) and their eruptive (rather than diapiric) origin. The mud breccia shows variable lithologies and ages of clasts in a clayey matrix, some of which can be biostratigraphically dated back to the Cretaceous [Staffini et al., 1993]. However, some concerns have risen that reworking of accreted strata, or even accretion of reworked material, may have led to such estimates, because several arguments hint toward a Messinian age of the mud matrix (see review given by Kopf et al. [2000b]). Indirect evidence is provided by brine pools on the MedRidge [Westbrook et al., 1995], saline pore waters on some MVs [DeLange and Brumsack, 1998], characteristic vent organisms [Corselli and Basso, 1996], and physical properties of Messinian muds drilled elsewhere [Kastens et al., 1987]. Also, the difficulty of having to explain noncompaction of fine-grained sediments since Cretaceous time would not arise when the mud was deposited only 5 Myr ago. In fact, if evaporites were precipitated above the mud, this seal may have trapped pore fluids until recent extrusion [Zabanbark et al., 1998].
 Apart from brines, freshened pore waters as well as hydrogen sulfide, methane, and traces of higher hydrocarbon gases emanate from some of the MedRidge MVs [e.g., DeLange and Brumsack, 1998; Robertson et al., 1996]. Although it had been proposed that diluted pore waters relate to gas hydrate dissociation [DeLange and Brumsack, 1998], the lack of BSRs, the relatively high temperatures of Mediterranean bottom waters, and the amount of water discharge through MVism in the area (see section 4 and 5) make this hypothesis an unlikely one. Other sources of fresh water, like mineral dehydration reactions due to stress, seem a more likely explanation in a scenario with deep-seated faults near the apex of an intensely deformed prism [Kopf et al., 2001]. Massive gas hydrates, however, have been sampled in the Anaximander Mountains at deep-water MVs [Woodside et al., 1998]. Gas hydrate occurrence at the edge of its stability field may explain large slides (a single event of 550 km3 [Woodside et al., 1998]) and abundant gas vents and could act as trigger mechanism for MV evolution.
 Within this overview of worldwide MV distribution, no more detailed descriptions can be given about MVism in the eastern Mediterranean. However, because the MedRidge is an example of a large accretionary complex undergoing intense deformation and dewatering (and is arguably the region with the largest number of submarine MVs on Earth), certain aspects of mud extrusion have been highlighted in earlier sections. They include mechanism and rates of extrusion, physical properties of the phases involved (both covered in section 4), and quantitative estimates of mass transfer owing to MVism (section 5).
 Many dozens of MVs occur along the easternmost units of the Carpathians where the mountain chain abuts the thick Neogene sediments to the east. The description of the domes by an engineer in 1924 [Higgins and Saunders, 1974] mentions huge blocks weighing several tons scattered among the mudflows, the latter of which originate from Miocene parent beds. The fault trace along which the sediment is thrust over the Carpathian orogenic wedge is shown in Figure 10a (upper left).
A24. Tanzania/East Africa
 Mud volcanoes near the coast of Moa, south of Mombasa, have been reported by Richard . Mud extrusion and gas discharge seem to be connected to regional tectonics (i.e., along a fault separating the Mesozoic Karroo series from younger units), but are unrelated to the enhanced dewatering of deltaic deposits in the Pemba Channel.
A25. Black Sea/Crimea and Kerch Peninsulas (Ukraine)
 Extensive literature exists describing MVism on the Crimea Peninsula bordering the north Black Sea [Kulschin, 1845; Ansted, 1866; Borisyak, 1907]. At its southeastern tip, Crimea extends into the Kerch Peninsula between the Sea of Azov and the Black Sea (Figure 10a). Here MVs and their relationship to petroleum fields in the area were the subjects of early investigations [e.g., Ansted, 1866; Morosevitch, 1888]. Recent reconnaissance studies distinguished between episodically active, violent MVs (up to 400-m-long mudflows from 60-m-high crests) and continuously bubbling mud pools and pies [Akhmetjanov et al., 1996].
 Recently, numerous studies were directed to examine submarine mud volcanoes in the Black Sea [Limonov et al., 1994, 1995, 1997]. Nine large MVs are found adjacent to the west Crimea fault, which divides the western and eastern Black Sea basins (Figure 10a). While rifting in the western part lasted until the Miocene, the eastward part rotated, resulting in compression along the Greater Caucasus (see section A26). MVism at the junction of the two basins was facilitated by transtensional tectonic forces, while gas hydrate processes act as a trigger (although found in only some of the domes [Limonov et al., 1997]). Apart from massive gas hydrates, bacterial mats indicative of gas hydrates and 2authigenic carbonate crusts of oxidized methane from gas hydrate dissociation were sampled in the Sorokin Trough, south of Crimea [Ivanov et al., 1998]. The methane is of proposed thermogenic origin. The features can be divided into midsize (2.5 km across, 100–150 m height above the surrounding seafloor) domes with collapsed crater-type conduits and smaller, actively venting domes of presumed younger age. Multiple overlapping mudflows of >1 km in length as well as pockmarks have been identified on high-resolution sonographs. Seismic data suggest the conduits to be as wide as 1.5–3.5 km (at depth), extending to 7–9 km depth into the 13–14 km of Tertiary and Quaternary sediments [Limonov et al., 1997]. Apart from the features on the seafloor, seven buried MVs have been identified during seismic surveys [Meisner et al., 1996]. MVism is related to the 5-km-thick Maykopian shales of Miocene age.
 Geochemistry indicates high hydrocarbon and CO2 fluxes and odd fluid signatures. Both chlorine and boron contents in the pore fluids (the latter up to 915 ppm B [Lagunova, 1976]) give evidence of hydrothermal interaction at depth. Muds contain fragments of the overlying succession, but also contain authigenic sulfates such as barite and anhydrite [Shnyukov et al., 1986]. On the basis of the geochemical signatures, Slack et al.  proposed a model in which hydrothermal mud volcanism causes tourmaline alteration and “black-smoker”-type deposits when hot metal-bearing brines are expelled in an anoxic environment (Figure 6d). Such deposits would be strikingly similar to Pb-Zn tourmalinite deposits, like the Sullivan ore body [Slack et al., 1998] (see also section A2). In oxic environments, volatile-bearing geothermal waters would produce borate minerals due to lower temperature and alkaline pH [Palmer, 1991]. In other areas (i.e., the Fedosia region), mud volcanoes with active gas vents have been observed. Some features exceed 1 km in diameter and are related to gas hydrate processes [Soloviev and Ginsburg, 1994]. The released methane shows characteristic δ13C ratios of biogenic origin, which coincides with results from the Taman Peninsula [Soloviev and Ginsburg, 1994] (see section A26).
A26. Caucasus (Taman Peninsula, Georgia, and Azerbaijan)
 The Greater Caucasus is the eastward prolongation of the Taman Peninsula all the way through Georgia and Azerbaijan to the western border of the Caspian Sea (Figures 10a and 10b). MVism has long been recognized in the area, and detailed reports are given on violent eruptions accompanied by gas dissipation (e.g., Abriutski  during that very year). It was noted that earthquake activity preceded such eruptions [Abich, 1865, Abich, 1869] and that there is a clear relationship between petroleum and MVism [e.g., Redwood, 1913a; Jakubov et al., 1971]. The most comprehensive studies on MVs in the Caucasus area have been conducted by Jakubov et al.  and Lavrushin et al. , and a summary has been provided by Jevanshir .
 In a first systematic study on the Taman Peninsula, 35 prominent MVs have been located along the saddle of anticlines in the Greater Caucasus [Shardanov and Znamenskiy, 1965] (e.g., see Dashgil MV in Figure 10c). The Maykop Formation wildflysch deposits, possibly a chaotic reminiscent of submarine postdepositional slumping, are the presumed parent bed to the domes [Lavrushin et al., 1996]. However, some authors argue that the presence of Cretaceous boulders within the MV deposits indicate a Cretaceous origin of the parent bed [Jakubov et al., 1971]. These undercompacted sediments were squeezed out at the crests of the folds when the Neogene sequence suffered deformation, with variable intensity of activity of the more than 30 features [e.g., Basov and Meisner, 1996]. Previously, vigorous eruptions had caused circular rims and deep craters of blocky mud breccia [Basov and Meisner, 1996], while during the most recent past, gas bubbling of fluid-saturated muds and mud breccias has been reported. Chemical instability of the fluids suggests either mobilization from different reservoirs or in situ alteration owing to the inhomogeneous parent beds [Lavrushin et al., 1996; Rudakov et al., 1998].
 Farther southeast of the Taman Peninsula, MVism remains a common phenomenon in Georgia, with early descriptions dating back more than a century (e.g., Melikov  on Achtala MVs near Tiflis). Also, a dozen MVs have been reported farther east, near the border with Azerbaijan [Lavrushin et al., 1996]. Chemical analyses of gas, pore waters, and muds from the domes point to a mobilization from 3 to 5 km depth. Only occasionally, low δ13CCH4 and δ11B signatures favor fluid reservoirs as deep as 8–10 km [Lavrushin et al., 1996; Deyhle, 2000] (see also Figure 10). The mud and embedded rock fragments predominantly originate from the Maykop Formation, which is found at a depth of 2.5–6 km in the area.
 Still farther southeast, Azerbaijan is probably the area with the world's densest onshore MV population. The pioneers of MVism already noted activity over considerable periods of time and described some of the features in greater detail [e.g., Abich, 1863]. A major eruption occurred at Lok Botan, Aspheron Peninsula, in January 1887 [Sjogren, 1887], which was apparently related to an almost 2-year-long phase of violent eruptions from 1885 to 1887 [Sjogren, 1888]. In follow-on studies [e.g., Goubkin, 1934] the MVs were described as steep, up to 500-m-high cones made of mud with clasts (3–8 vol %). What was then termed “intensely crumpled plastic mud” most likely refers to scaly fabrics and mélange formation during intrusion and emplacement. In a major effort a total of more than 200 domes have been described in much detail by Jakubov et al. . The size of the MVs is variable (up to 600 m height and 10 km2 area; see Figure 10c) and so are the occurrence of gryphons (∼20), salses (30–75 m across), sinter mounds (aligned near the crest), and the ejection of oil or water or the emission of gas. Although most of the work is purely descriptive and lacks some comparison with similar phenomena in other areas on Earth, valuable information can be extracted. Many of the more impressive MVs have been proposed to be younger; conversely, areas with neighboring small domes are believed to be a product of incipient erosion. One of the most prominent features, Dashgil MV (Figure 10c), is known to have been active every 6 to 32 years in historic time. At present, it has been tentatively estimated to release ∼15 × 106 m3 yr−1 of radiative gases into the atmosphere [Jakubov et al., 1971]. A more recent evaluation of gas flux conservatively suggests an annual emission of 800 m3 (predominantly methane) during quiescent intervals [Hovland et al., 1997]. Similar to Taman and Georgia, the material having formed the bulk of the dome is assumed to be of the Oligocene/Miocene Maykop Formation (and, specifically, the wildflysch deposits therein [Higgins and Saunders, 1974]). The clayey matrix comprises polymictic clasts, from within both the Maykop Formation and the overlying country rock. From historical records and by studying 220 MVs, Bagirov et al. [1996a] compiled an impressive database. They show that over the past 100 years, four new MVs evolved, which are only small in size and are no hazard to mankind. Among these MVs, three to five eruptions per year occur on average [Jevanshir, 2002]. Next to the establishment of previously acquired data on gas composition, mud breccia, and geometry, estimates are given for gas emission during a (presently very unlikely) violent eruption. The amount of 6 × 108 m3 equals 750 kyr of quiescent background flux [Bagirov et al., 1996a]. During eruptions, self-ignition of the methane (CO2, H2S, and higher hydrocarbons contribute little to the overall gas discharge) can cause >100-m-high flames. The reader is referred to the wealth of statistical estimates that Bagirov et al. [1996a] put forward for likelihood of MV-related phenomena in the area.
A27. Caspian Sea
 The entire Caspian region has long been known to be rich in MVs (see section A26 on Azerbaijan). Marine MVs may have been an equally common phenomenon, but investigators had to await the development of geophysical acquisition techniques. On occasion, however, submarine MV eruptions were so powerful that such small islands appeared above sea level [Schweder, 1893]. As in Azerbaijan, flame eruptions are known offshore, which may cause heating hazards when occurring too close to shore (or near an oil exploration site [Bagirov and Lerche, 1998]). Offshore MVism, especially in the Chirag area, has been investigated by Bagirov et al. [1996a, 1996b], who suggest that MV activity is very low at present and may be connected to local earthquakes. On the other hand, gas hydrate processes are connected with MVism [Soloviev and Ginsburg, 1994], possibly acting as a separate trigger. More than 60 MVs floor the southwestern Caspian Sea [Soloviev and Ginsburg, 1994, Figure 4]. Mud breccias sampled at two of those MVs, Buzdag and Elm domes, contained up to 35 vol % massive gas hydrate. Given that massive gas hydrate is less dense than water, such large volumes have a profound impact on the buoyancy of the bulk mud and on its behavior during ascent [Soloviev and Ginsburg, 1994].
A28. Turkmenistan and Iran
 Active degassing and variable gas composition in MVs of southwest Turkmenistan have also been related to local tectonic activity [e.g., Voitov et al., 1991]. The ten features described border the western coast along the Caspian Sea and are generally domes of a few hundred meters width and tens of meters in height (for location, see Lavrushin et al. [1996, Figure 1d]). Gases emanating from these domes are reported to have distinct thermogenic δ13CCH4 signatures; their He isotope ratios suggest minor contribution of mantle gas, possibly owing to deep faulting during the Caucasian orogenesis [Lavrushin et al., 1996].
 Gansser  discusses the Gorgan Steppes, a series of wide MVs with caldera-type craters owing to subsidence and internal collapse after the main extrusion event, having created the 6-m high crater rim. The relationship to the regional geology is poorly known, but the fluidized, oil-bearing nature of the mud hints of a deep origin. Cone-shaped domes (like Napag MV, Belutchistan, Iran) of up to 20 m height and ∼80 m across have also been found [Gansser, 1960, Figure 11]. Abundant MVs have been reported in an early study by Stiffe  as being located along the Persian Gulf coast of Iran. Diapirism in the area has also been mentioned by Heim . Domes are usually cone shaped (40° flanks) and of heights of meter to decimeter scale. Formation of the MVs was apparently unrelated to the convergent tectonics having created the Makran Arc farther east (see section A29). Recent reevaluation of the colored mélanges shows that these chaotic rocks pierced through the gently folded Neogene pelagic and flysch cover, forming randomly distributed, isolated domes [Stöcklin, 1990]. The clasts and blocks within the shaly matrix are proposed to be derived entirely from deeper levels, with no gravitational gliding (olistostromes) involved.
A29. Makran and Pakistan
 An early survey following the entire coast from the Persian Gulf to Karachi, Pakistan, over hundreds of kilometers revealed abundant MVs [Stiffe, 1874]. On an expedition through Pakistan, Hart  described the Chandragup MV as a 100-m-high dome with two craters of >10 m diameter. Snead  compiled systematic differences between the MVs close to the coast compared with those farther inland in the Hala and Haro mountains. While the first are usually cones, the latter are ridges of up to 30 km along strike, with mudflows extending over several kilometers in length along their flanks. The gray-blue extrusive clays carry angular clasts and fragments (up to 1 m across) of the rocks overlying the Miocene parent beds. Both the sediment and tectonic loading were discussed as driving forces, with water and gas acting as a lubricant for upward migration [Snead, 1964]. More recently, some of the features were studied in greater detail, like the Chandragup or Jebel-u-Garab [Delisle et al., 2002]. Mud discharge varied between 0 and 1.4 m3 h−1, while gas emission ranged from negligible amounts to 1 m3 s−1 [Delisle et al., 2002]. The activity of some Makran MVs was correlated with earthquakes in the area [e.g., Skrine, 1936; Harrison, 1944]. In fact, after a major earthquake (Ms 8.25) on the Makran coast in November 1945, island formation resulting from mud ejection was reported some hundreds of meters off the coast [Sondhi, 1947]. Similarly, Malan Island formed in March 1999 as a result of a vigorous extrusion of 160,000 m3 of soft mud [Delisle et al., 2002]. Although not triggered by seismicity, overpressured mud and gas ascended some 2–3 km from folded Pliocene/Pleistocene sediments (Hjinglaj Formation). The island reached only 100 m2 in size and was destroyed by tidal activity during the monsoon season in November 1999 [Delisle et al., 2002].
 During recent cruises in the Arabian Sea along the Makran convergent margin, a number of small MVs (>1 km in diameter) were found during seismic profiling [White and Louden, 1982; Wiedicke et al., 2001] and bathymetric mapping [Flueh et al., 1997]. Some of these features are only tens of meters high and are characterized by gas plumes in the overlying water column. Their occurrence offshore as well as onshore is closely related to the location of thrust and strike-slip faults [Flueh et al., 1997]. Fluid venting and authigenic carbonate precipitation accompanies MVism [von Rad et al., 2000]. Apart from seven of the small domes, recent 4-kHz sediment echo sounding has identified two larger mud domes adjacent to the deformation front of the Makran accretionary complex [Wiedicke et al., 2001]. They reach 1.5–2 km in diameter and heights of 36–65 m. Sediments from the MVs are methane charged (∼40 ppm), which explains the seismically transparent root of the structures. Pore fluids from the features reveal enrichment of volatile elements (e.g., boron) relative to seawater (Figure 9). One buried mound, similar to the seafloor piercements, has also been identified [Wiedicke et al., 2001]. This led the researchers to suggest tectonically induced fluid expulsion as being the driving force for these features seaward of the décollement (in analogy to Barbados; see section A11 and Henry et al. ).
 Within the Indus fan sediments on Murray Ridge, Gulf of Oman, MVs occur as piercement features and as buried domes almost regularly spaced [Collier and White, 1990]. The features originate from a seismically opaque parent layer at ∼500 m depth below the seafloor and apparently rise at low to moderate rates. Although methane is known in the area (and could account for the bright patches on seismic profiles), no vigorous eruptions or disrupted chaotic strata have been observed [Collier and White, 1990]. Hence rapid sedimentation and fluid overpressure are believed to control diapirism.
 MVism is long known from the Arakan coast of India and Burma [e.g., Mallet, 1878, Mallet, 1879, 1880, 1881, 1885, 1907]. Frequent mud eruptions forming domes as well as shield-shaped islands have occurred in historic time, for example, in 1843, 1879, and 1907. Among the explanations of activity, both gas expansion [Mallet, 1880] and the influx of large amounts of meteoric water during the monsoon season [Mallet, 1885] have been suggested. On Baratang Island, Andaman, an area of ∼40 km2, is peppered with MVs in the south of the island around Wafter's Creek [Poddar, 1954]. MV ejecta contain microfossils of Cretaceous and mid-Eocene age that hint toward parent beds of considerable age and depth [e.g., Badve et al., 1984; Ling et al., 1995]. In the Jarawa Creek MV deposits on Baratang Island, Achyuthan and Eastoe  found volcanic glass, brines, and sulfide nodules in addition to the dominant illite and kaolinite. Methane is the dominant gas emitted. Sulfur isotope composition of the mud, brines, and nodules also indicates large influx of groundwater into the system [Achyuthan and Eastoe, 1999]. Tassik areas are up to 50 m wide, with pebbles of several centimeters floating in them [Poddar, 1954]. Domes are generally less than 10 m wide, have dried and fresh mudflows, and are aligned to fault traces [Poddar, 1954]. Eruptive activity is linked to local earthquakes [Jhingran, 1953].
 Banks of extruding terrigenous mud have been reported from the coast near Kerala in southwest India [Nair, 1976]. Their shallow water occurrence (only tens of meters) has been related to oxygen-deficient waters and gases from degradation of abundant organic matter in the fine silts. The banks are ∼4 km long, parallel the coastline, and consist of porous (60–80%) mud with clay contents between 45 and 90%. Active extrusion of soapy blue muds as well as gas bubbling has been reported from the nineteenth century and apparently caused fish to die in these coastal waters [Bristow, 1938].
 Apart from the descriptions in the nineteenth century (see section A30), Pascoe  describes two MV areas along the eastern and western borders of Yarakan Yoma. Along the Burmese coast the discovered features vary between a few to almost 50 m in height, while the eastern MVs are smaller (<30 m high). Both types are cone shaped, and in the west, violent eruptions with mud breccias containing angular clasts are more frequent. Over a distance of almost 1000 km, being sourced by a trough filled with thick Miocene/Pliocene sediments of the Pegu Series, occasional MVs occur. However, the parent bed may be located somewhere in the Oligocene deposits. Estimates of the pipes and veins acting as conduits range around 1.5 m [Pascoe, 1912], which Dudley Stamp  related to tectonic fracturing along a fault scarp.
A32. Java and Sumatra
 Goad  describes one flat dome on Java on the plains of Grobogan, where mud is ejected in the center of the crest. From the description, it appears as if both brines and gas were set free with the mud. The eruptive products of a MV near Poerwodadi have been studied by Ehrenberg . Hofer  revisited 2the same and some other MVs near Poerwodadi and Semarang, as well as on Madura island, and related ejection of fluidized mud to the occurrence of hot springs.
 In Sumatra, mud breccias with large boulders have been mentioned north of Langsa [Blumer, 1922]. Regarding both Java and Sumatra, the information given relies on the earlier MV compilation by Higgins and Saunders .
A33. Brunei/Borneo and Sabah
 On and around Borneo, shale diapirism and MVism have been known for many years from the Klias Peninsula [e.g., Blumer, 1922], where extrusion is restricted to anticlines. Resurgence of activity correlates with local earthquakes. Recently, two geophysical studies have shown MVism onshore [Morley et al., 1998] and offshore of Brunei [Van Rensbergen et al., 1999]. Regarding the first, multiphase intrusion of shale, indicated from dyke formation patterns, affected mid-Miocene anticlinal sediments [Morley et al., 1998]. Transpressive tectonics with small-scale normal faults allowed the shale to move, but then relatively high normal stresses exceeded fluid pressures. More recently, subsequent erosion and uplift remobilized the shale bodies and gave way to MV formation in the Jerudong anticline [Morley et al., 1998]. In the Ampa and Egret areas offshore in the Baram delta, the situation is more complex, with an intrusive phase followed by MVism, which in turn got buried, before migrating gas and water may have helped a renewal of ascent and extrusion [Van Rensbergen et al., 1999]. High-resolution seismic data across the structures suggest an interfingering of shales (showing a chaotic texture with incorporated country rock) and the wall rock (well stratified sediment). From the seismic signature, it was estimated that only 7% of the total 35 km3 of the Ampa feature represent parent bed shale, while the remainder is overburden having been incorporated during intrusion. It is hypothesized that during dyke formation along preexisting faults, the material reached the former seafloor and formed domes from multiphase mudflows (Figure 6c). Such MVs got either eroded or buried, leaving an irregularity in the cylindrical dyke, which in the future may act as a trap for fluids (the latter possibly triggering the next eruptive phase). Dating of the stratigraphic position of the Setap Shale parent bed (early to middle Miocene) and of the MV extrusion (latest Pliocene) allows the duration of inactivity of the undercompacted sediment to be dated as 9 Myr [Van Rensbergen et al., 1999].
 On Sabah, both west of Sandakan and on the Dent Peninsula at the junction of the Sulu and Celebes Seas, Reinhard and Wenk  mentioned MVs made of “giant breccias.” The lithic fragments are of pre-Tertiary and younger age, with the oldest rocks being metamorphosed. However, the interpretation of a deep-seated parent bed within such units [Reinhard and Wenk, 1951] supposedly lacks consideration that olistostromes of the Danau Formation of Upper Cretaceous age comprise those very components as well. The clay/shale component within these olistostromes may cause intrusion and MVism as a result of low bulk densities relative to their overburden. The first to suggest that parts of the mélanges of Sabah have a diapiric origin were Haile and Wong . Several hypotheses regarding the fractured, hardened clays have been put forward, the most convincing explanation being very local soft sediment deformation of the mud matrix during emplacement [Clennell, 1992]. There is no clear coevolution between the ∼24 flat mud domes (none of them exceeding 200 m width) and the massive outcrops of the East Sabah Mélange over several hundred square kilometers. In fact, evidence suggests that the mélanges rose only tens of meters while the MV ejecta entrained a thick cover of coherent rocks. Some of the MVs, for example, Pulau Batu Hairan MV in the north, near Banghi Island, source in the mélanges, hence ejecting enigmatic polymictic rocks [Clennell, 1992]. Other features, like Jeroco MV, are only a few meters in diameter and are free of clasts (Figure 4a). In general, both onshore and offshore of Sabah, a variety of MVs have been mapped. Commercial drill holes into the offshore area yielded anomalously high formation pressures associated with muddy sediment. Fluid comprises biogenic and thermocatalytic methane and water, sometimes accompanied by small amounts of CO2 and higher hydrocarbons [Wilford, 1967]. Water influx is most likely from near-surface groundwaters [Haile and Wong, 1965].
 Offshore of Nigeria in the Niger delta, numerous circular features have been reported from the upper continental slope [Graue, 2000]. The domes are both active and dormant, are usually 1–2 km in diameter, and have chaotic seismic signatures. Some features overlie a rolling anticline, whereas others juxtapose a diapir. Seabed coring revealed oil, gas, and clasts of shale and sandstone in the mud. Occasional carbonate nodules have also been found. The age of the material is Pliocene and Pleistocene, with the dormant features being older than the active ones [Graue, 2000]. Evidence from geophysical investigation and coring suggests that the parent bed to the MVs consists of an overpressured shale, which is overlain by low-integrity sediment. While some of the shale intrudes the overburden diapirically in a nonviolent manner (forming mud domes in the process), other features evolved from high fluid discharge from an underlying sediment wedge and appear as circular depressions in the surrounding seafloor. Buried MVs with ancient flows have been recently mapped using three-dimensional seismic acquisition [Heggland et al., 2001]. The features occur predominantly in large-scale anticlines (compressional folds) and are often underlain by strong BSRs.
A35. Timor-Ceram Arc
 Both in the east and west of Indonesian Timor, MVism and diapiric mélanges are abundant (Figure 4b). Despite some debate, the majority (or even all features) are fed by the Bobonaro clays, which cover some 4000 km2 of outcrop [Audley-Charles, 1968] and formed during the Permian-Triassic. The MVs are dome shaped with relatively steep flanks (of ∼50°) and show well-developed tassiks at their crest. The mud breccias show flow structures and carry clasts of >1 m and have been related to regional formations and explosive eruption ['t Hoen and van Es, 1928]. Mud volcanism is also known from islands like Samau (6 MVs), Roti (2 MVs), Tanimbar [cf. Heim, 1940], and Kambing, the last of which is a 2-km-wide MV representing the island itself [Higgins and Saunders, 1974]. On the island of Sumba, up to 18-km-long mud ridges pierce the overlying strata [Clennell, 1992]. Initially, Heim  related the diapiric muds to the overall orogenic movements in the southwest Moluccas. The Tanimbar mud breccia was studied in great detail by Yassir , revealing its very fine grain size (no particles >0.4 mm) and high kaolinite and illite contents (up to 99.4% of the entire sample). A detailed study of 21 MVs in western Timor revealed that the mud originates from diapiric rise of the Bobonaro clays [Tjokrosapoetro, 1978]. Ascent is allowed along a sinistral, southwest-northeast-striking wrench fault, with oil and gas acting as lubricants (see model given by Barber et al. [1986, Figure 5]). Also, the occurrence of 80% illite but only 20% smectite in the extruded clays suggests that mineral dehydration supplied water as an additional driving force for ascent [Barber et al., 1986]. In contrast to earlier interpretations, the scaly clays forming the Timor MVs have been interpreted as diapiric mélanges due to their shape and crosscutting relationship to the surrounding strata. Unlike olistostromes (where the blocks travel far and mostly horizontally), the mélanges of Timor show exotic, angular blocks of broken material [Barber et al., 1986, Figure 6].
 Offshore geophysical surveys revealed MVism near the island of Sumba [Breen et al., 1986] as well as along the Flores thrust zone [e.g., Silver et al., 1986] in the Sunda forearc and back arc, respectively. In the forearc wedge as well as on the abyssal plane off the Timor Trough, MVs and mud ridges are observed [Breen et al., 1986]. The domes in the forearc reach only a few hundred meters in diameter; however, the elongated ridges parallel to strike are several kilometers long. The outward migration of the long décollement with near-lithostatic fluid pressures probably causes mud extrusion in the protodeformation front region of the downgoing Australian Plate [Breen et al., 1986]. None of the material was sampled. Compared to the scale of deformation in the Timor Trough, the back arc thrusting is minor. However, along the Flores thrust zone the Flores and Bali Basins are overridden by the island arc, forming what has been termed a back arc accretionary wedge [Silver et al., 1986]. Scattered MVs and ridges occur over a length of 150–200 km along strike and correspond with the grain of the Flores thrust (i.e., the basal detachment to the back arc wedge). The biggest mud ridge reaches a length of 10 km and occurs some 5 km behind the “frontal” thrust (i.e., south of it). Fluid overpressuring has been the inferred driving force [Silver et al., 1986].
A36. Irian Raya/Indonesia and Papua New Guinea
 Shale diapirism and mélange formation has been reported for wide parts of Irian Raya and western Papua New Guinea [Williams et al., 1984]. MVism is concentrated in fields covering several tens to a few hundred square kilometers, which are bound to east-west-striking fault lineaments. The individual domes range from 3 m to 2.5 km width and reach maximum heights of 110 m [Williams et al., 1984]. The source rocks of the mud breccias are the Miocene Makats and Mamberano turbidites, into which limestones and serpentinites from deeper stratigraphic levels got incorporated during folding and faulting prior to ascent.
 Taiwan is located at the boundary between the Philippine Sea Plate and the Eurasian Plate, where the Luzon Arc collides with eastern Taiwan. In the old Taiwanese literature (e.g., Fukutome  as cited by Shih [1967, pp. 259–260]), there are records of mud volcanic eruptivity dating back as far as 1723, when “firelight lit up the sky. Two holes burst in the ground with black colored mud and water flowing out. The vegetation around was all baked into ashes. . dark mud gushes out night and day. The mud can be ignited. This is certainly a wonder.” On Taiwan mainland, eruptive MV centers are known from 17 areas in South Taiwan, with altogether 64 active and several extinct features [Shih, 1967]. The majority of the MVs are located in the southeast and southwest of the island, and incipient arc-continent collision and tectonic shortening are believed to account for extrusion. The ejecta have been associated with parent beds of the Lichi Formation (Pliocene/Pleistocene) and the mélange-bearing Gutingkeng Formation (Pliocene), respectively. All southwestern domes occur along fault traces and anticlines, which seem to be the onshore continuation of the trace of the marine deformation front [Lee et al., 1992]. Two diapiric domes are known from gravimetric mapping [Hsieh, 1972]. If the mud density is compared between the areas in the southwest and southeast, it is obvious that the southwestern domes are more consolidated and are presumably older [Hsieh, 1972]. The activity has somehow shifted to the east through time. The geometry of the MVs is highly variable, with domes of only several meters across and a few meters in height, but also with wider features with 150-m-wide tassiks. The conduit width, where visible, has been estimated to be only ∼10 cm [Yassir, 1989]. Two mud basins have also been found, which show continuous gas bubbling in the center of their 6-m-wide pond. Low-viscosity mudflows over their rims extend over tens of meters. A variety of physical properties have been measured [Yassir, 1989] and are compared to data from other areas of MVism in section 3.4.
 A comparative geochemical study on MV fluids from the southeast and southwest Taiwan MVs reveals similarities in the thermogenic δ13CCH4 gas composition, suggesting a deep source [Gieskes et al., 1992]. In contrast, the MV pore waters of the different areas differ considerably (Table 3). The southwest samples apparently source from a deeper reservoir, as indicated by much higher B, but lower Ca, Mg, and Na. Chlorinity is highly variable, but is always well below seawater (even for the southwest features adjacent to the deformation front), so that dilution from deep mineral dehydration within the large accretionary prism is suggested [Gieskes et al., 1992].
 Offshore of Taiwan, seismic surveys located numerous MVs connected with the occurrence of gas hydrates [Chi et al., 1998]. BSRs are located in the crest of anticlines and mud volcanoes, having been sourced from offscraped sediments derived from the Taiwan orogen and from the Chinese continental margin. Gas seepage (mostly methane) is a result of biogenic degradation of organic matter in the sediments accreted from the Manila Trench [Chi et al., 1998]. Farther to the north, Liu et al.  investigate a complex pattern of MVs, faults, and sedimentary features. The mud domes are only ∼100 m wide, have cone-shaped geometries, and show gas plumes in the overlying water column. The MVs lie structurally above subsurface shale diapirs of much larger extent. While some diapirs pierce the seafloor sediments, others have stopped ascending hundreds of meters below the seafloor. Diapiric movement is facilitated along steep normal faults in a scenario of abundant horst-and-graben structures along the upbreaking Chinese continental margin, but is also facilitated along thrusts in the Taiwan accretionary wedge [Liu et al., 1997].
 A special type of trigger mechanism has been inferred for a dozen MVs near Lichelieuyu Island, southwest of Taiwan. While most of the features are <200 m wide and are only a few meters high, Lichelieuyu MV extends over >1 km in diameter and is bound by fault scarps [Chow et al., 2001, Figure 3]. The diapiric root of the feature can be traced on seismic data, which also show abundant steep normal faults and gas craters. The source of MVism here is strongly connected to the sediment supply from the nearby Kaopingshi River in the north. The rapidly deposited material in the Kaoping submarine canyon is fine-grained, trapped gas, and it developed overpressures sufficient to allow the material to extrude [Chow et al., 2001]. Subsequent diapiric movement of the undercompacted material later caused the uppermost part of Lichelieuyu MV to rise above the seafloor, hence forming the island of the same name.
A38. Ryukyu Trench, Nankai Trough, Japan Trench, and Japan
 At the convergent margin off Japan, several MV occurrences are known both on the subducting and overriding plates. A small mud ridge of ∼1.2 km length and 130 m height is found on the incoming Philippine Sea plate at >5 km water depth in the Japan Trench area [Ogawa and Kobayashi, 1993]. Extrusion of fluidized sediment follows northeast-southwest-trending normal faults, which are believed to be caused by downward flexure of the oceanic plate. Frequent seismic activity of the area may have triggered MVism as well.
 In the Kumano Basin, offshore of Kii Peninsula (southern Honshu), several dozen MVs have been discovered during a recent geophysical survey (J.-O. Park, personal communication, 2001). The domes are ∼100 m high, are usually <1 km wide, and contain clasts of the older part of the accretionary prism (i.e., Shimanto Belt). Gas emanated is mostly methane of thermogenic origin, which indicates that the material may be rehydrated Shimanto clay stones. A few smaller features have been observed farther east (S. Lallemant, personal communication, 2001), but have not been sampled.
 To the south, in the eastern portion of the Nankai Trough accretionary prism, Kobayashi et al.  discovered three MVs with diameters of several hundred meters and heights of several tens of meters during bathymetric mapping. Submersible studies revealed cold seep vents and biological communities.
 Even farther south, in the Ryukyu forearc, side-scan sonar imaging and piston coring characterized MVism in accreted sediments [Ujiiè, 2000]. A total of >24 domes are observed. The example shown is related to a thrust along which overpressured material migrated upward. Microfossils of Eocene to present are found in mud-supported breccia, which is interlayered with background sediments and ash layers from the arc volcanoes. Piston core evidence suggests interfingering of mud breccia and hemipelagic sediments during the late Pliocene and Pleistocene [Ujiiè, 2000].
 Ancient chaotic breccias and mélanges were identified in the accretionary prism of the Shimanto belt and have been interpreted as extinct diapirs or MVs [Shimizu, 1985]. However, these accreted deposits from the pre-Eocene wedge now overlying the modern accretionary prism may equally originate from already disturbed parent beds with cataclastic material, like the former décollement zone. More recently, Lewis and Byrne  have described the ancient Tako mud diapir as a chaotic, pebble-bearing shale body within coherent rocks of the Kogawa Formation. The blocks are angular and range between centimeters and tens of meters, the smaller fragments being separated from larger units by hydrofracturing during ascent [Lewis and Byrne, 1996, Figure 3; Behrmann, 1991; Brown et al., 1994]. Diapiric rise, as well as hydrofracturing, is believed to be associated with underconsolidated shales.
 MVism on the island of Sakhalin, Sea of Ochotsk, has been known throughout historic time and seems to be related to the regional oil and gas fields [Siryk, 1962]. Eruptions seem to occur frequently (and sometimes explosively), for example, in March 1959 and September 1961 [Gorkun and Siryk, 1968]. In 1959 the Yuzhno-Sakhalinskiy MV ejected 150,000–200,000 m3 of mud, covering 60,000 m2, with the mud, rock fragments, and trees being propelled up to ∼100 m into the air. Similarly, the mud discharge for the Pugachevskiy MV in 1961 has been estimated to cover 8000 m2 of ground with some 7200 m3 of mud, which was thrown some 40–50 m into the air above the crater [Gorkun and Siryk, 1968]. The diameters of the conduits are reported to be 30–200 cm. A total of five active MVs are identified on Sakhalin, and the average emission of predominantly methane gas per vigorous eruption has been calculated to be ∼14,400 m3 [Gorkun and Siryk, 1968]. The equations used for the gas flux estimates are further used to estimate the mobilization depth of the ejecta, which again bears some errors (see discussion and equations of Kopf and Behrmann ). Although detailed fieldwork in the exhumed accretionary complex on the island of Sakhalin revealed shaly and olistostromal series [Kimura et al., 1992], no close relationship between these beds and the ejecta has been established. From what is known about the regional geology, however, it may be assumed that the parent bed of the mud breccias lies within argillites of the Bykovskaya Series at ∼2.3–3 km depth.
 A recent Ms 7.2 earthquake affecting northern Sakhalin in May 1995 was immediately followed by various secondary phenomena such as landslides, falls, soil liquefaction, and mud volcano eruptions [Ivashchenko et al., 1997]. However, this MV activity triggered by aftershocks and related to fault movement along the preexisting Upper Piltun strike-slip fault is an exception here. Usually, buoyancy (and gas-assisted lift) of shales and clay-rich sediments is the main driving force that mobilizes material from a few kilometers depth.
 A different form of diapirism and volcanism can be studied at some subduction zones where hydrothermally altered mantle wedge material (i.e., serpentinites) ascend from the deeper part of the plate interface. In the Mariana and Izu-Bonin forearc, two serpentinite seamounts were drilled during Ocean Drilling Program Legs 125 [Fryer and Mottl, 1992] and 195 ( M. Salisbury et al., Leg 195 scientific prospectus: Mariana convergent margin/West Philippine Sea seismic observatory, available at http://www-odp.tamu.edu/publications). These seamounts form as MVs, composed of undercompacted serpentine mudflows, or as horst blocks of serpentinized ultramafics that diapirically intruded the leading edge of the overlying plate [Maekawa et al., 1993]. As many as 100 up to 30-km-wide and 2-km-high features have been reported from the Mariana forearc as being 50–120 km behind the trench axis [Fryer et al., 1985]. Their size varies, but it is generally an order of magnitude larger than for sedimentary MVs. For instance, the Conical and Torishima seamounts drilled are >10 km across and have an elevation of ∼1 km relative to the surrounding seafloor [Fryer and Mottl, 1992]. Apart from flows, slumping and various soft sediment deformation can be found. The material recovered comprises serpentine with various amounts of clasts (both sedimentary and mafic rock types) underlain by serpentinized dunite, harzburgite, and metadiabase of various compositions at the base [Fryer and Mottl, 1992]. Hence an increase in alteration toward the seafloor can be inferred. However, fluid chemistry and authigenic phases are indicative of a fluid other than seawater having interacted with the mafic rock at depth (Table 3). Such fluids may be derived from mineral dehydration reactions. Although not sedimentary volcanism, the serpentine MVs play a similar role in mass transfer at active convergent margins as, for example, the MVs on the Mediterranean Ridge (see section A22). They occur in a similar location some tens of kilometers behind the deformation front and rheologically resemble their sedimentary counterparts [Phipps and Ballotti, 1992]. Also, there are numerous sedimentary serpentine deposits known from land in former convergent margin settings [e.g., Oakeshott, 1968; Lockwood, 1972].
A41. Australia (Gosses Bluff)
 In central Australia a circular feature known as Gosses Bluff rises from a plain of Paleozoic sedimentary rocks. It is ∼10 km in diameter (crater area ~3 km), with its core at the same topographic level as the surrounding rocks, but made of highly disturbed Cambrian and Ordovician rocks [Ranneft, 1970]. The elevated rim (180 m higher than the core area) of the feature comprises steep layers of Silurian to Carboniferous rocks, which may have been dragged upward if a diapiric mechanism of emplacement is inferred (either by sedimentary or salt intrusion and updoming and/or gas discharge from depth). However, nothing similar is known from elsewhere.
A42. New Zealand
 MVism on the northern island of New Zealand has been described for the Raukamura Peninsula by Ridd . Two of three domes are dormant at present; however, historic records exist regarding their episodic, earthquake-shock-generated nature and activity. The parent beds are bentonitic clays of either the Arnold or Dannevirke Series. These Paleocene/Eocene rocks form many diapirs, with abundant clasts of Cretaceous and younger age. High formation pressures in the ∼6-km-thick Tertiary sequence may play a crucial role in MVism [Ridd, 1970]. The MVs are small domes with crater mud lakes (in the case of the active Arakihi Road MV).
 Historic accounts of regional earthquakes mention bubbling of the shallow waters offshore of Raukamura Peninsula, which may have been submarine mud eruptions [Ridd, 1970]. Historic records link offshore MV eruptions at Open Bay and East Cape in 1877 with seismic tremor. It remains unclear, however, whether the mechanism is similar to the onshore features or whether processes along the active margin at the Australian-Pacific plate boundary caused submarine gas venting. More recently, Nelson and Healy  reported pockmarks that may result from MVism on the Poverty Bay seafloor. However, no subseafloor evidence has been provided.
 On the northern island of New Zealand, the Waimata MV erupted in 1908 [Adams, 1908]. The feature ejected debris with blocks of up to 27 kg to a height of 150 m, scattering them over tens of meters distance relative to the central conduit of the mud spring. Fluid overpressure hauled ∼100,000 tons of mud breccia within the first hour of the vigorous eruption [Adams, 1908]. Another 150,000 tons extruded during the similarly violent 1930 eruption [Strong, 1931]. Some 70% to almost pure methane escapes at high gas discharge rates with hissing sounds. The control of activity has been related to tectonic movements, with the rock fragments (up to 15 cm across) ejected being interpreted as removed fault breccia during upward migration of mud from a deeper reservoir (Cretaceous or younger [Strong, 1931]). The total volume of the underlying clay diapir, which rose vertically ∼3 km along the transcurrent fault system, has been estimated to range around 30–35 km3 [Stoneley, 1962]. Similarly, activity of Mangaehu Stsream MV farther west is controlled by tectonic movements along the complex Waimata-Mangaehu fault system [Ridd, 1970, Figure 5].
 In the Hikurangi subduction zone many small mud pools have recently been described by Lesedert et al. . Their activity is related to petroleum occurrence and gas seeps onshore. Fossil examples are also found. The conduits range from centimeter to decimeter size and often contain authigenic carbonates and zeolites. The parent bed to the MVs is proposed to represent an offshore diapiric wall south of the subducting margin [Lesedert et al., 2001].
A43. Egypt/Libyan Desert
 Hume  discovered “asymmetrical elongated domes of the diaper-type, with dips of 5° to 10° on the NE-flanks” east of Cairo, Egypt. One of these clay diapirs from the folded and faulted Paleozoic rocks in the Giran el Ful-Gebel el Hiqaf anticline has been described in more detail. The feature is ∼300 m in diameter and domes up near the intersection of a fault cutting the core of the anticline [Omara, 1964]. Also, clayey dykes have been observed in the area, possibly related to syntectonic injection of the plastic material into more indurated rock of Turonian age. Injection as well as diapirism have been interpreted to be tectonically rather than density driven [Omara, 1964], mostly because the less competent clays got accumulated locally during shear.
 One peculiarity of sedimentary volcanism closes the compilation of global MVism: peat diapirs in Pleistocene sediments in Netherlands [Paine, 1968]. The features are <2 m high, are <3 m wide, and occur in the ∼ 7-kyr-old Flevoland Lower Peat unit. Diapiric rise supposedly started 1–1.5 kyr ago and pierced the overlying clays and sands [Paine, 1968]. Some of the features have been related to normal faults, which allowed some features to crop out on the surface [Paine, 1968, Figure 2]. The peat is rich in plant debris, and these debris flow deposits on the Earth surface bear striking similarities to mud volcano flows elsewhere (compare to, e.g., Paine's Figure 4 and to Martinelli and Ferrari's Figure 1).
 In summary, mud volcanism is most abundant in compressional scenarios (see compilation of selected areas in Table 1) and, to a lesser extent, in deltas of great rivers. As for the first, tectonic activity is an additional trigger to the buoyant driving mechanism. Fluid for MVism is supplied from various sources (see section 4.2 and Figures 2 and 9), including meteoric and volcanic waters, pore water expulsion, hot springs, mineral dehydration reactions, and gas hydrate destabilization. Gas may originate from as deep as the upper mantle (see section 4.3). Among the MV occurrences known to date, the Barbados Ridge, Caucasus, and Mediterranean Ridge are the areas with the most abundant and best-studied features (see section 4 and 5).
 There are other areas of MVism known to the author; however, only sparse information exists. This is especially true for early publications in somewhat obscure journals (not all of which I was fortunate enough to get hold of). For an overview of several dozen such references, the reader is referred to volume III of Redwood's [1913b] bibliography. Another excellent collection of literature related to diapirism (not entirely of clays, shales, and muds, but also of salt, peat, and ice) has been compiled by Braunstein and O'Brien .