Origin of term
The Greek “diapeirein” means “to pierce,” and the first
mention of diapiric structures dates back to Leymerie
(1881), who suggested the word “tiphon.” This nomenclature was adapted by Choffat (1882) when describing sedimentary diapiric manifestations in Portugal, calling these
small domes “tiphonique.” Mrazec’s (1927) definition of
diapirism as the process of forceful movement of a more
or less plastic body from areas of greater pressure to areas
of less pressure has since then widely been accepted. It has
often been restricted to salt structures instead of having
been applied on mechanical grounds, although some
workers used it for effusive or intrusive magmatic processes (e.g., Wegmann, 1930). The term “sedimentary volcanism” supposedly (Saunders, personal communication,
2000) goes back to Kugler (1933).
Introduction and historical background
Mud volcanoes on Earth extrude relatively low temperature slurries of gas, liquid, and rock to the surface from
depths of meters to kilometers (Kopf, 2002). Slurries build
circular to subcircular deposits of mud and rock breccia
that range in diameter from a few meters up to tens of kilometers. They can produce a variety of morphologies,
domes, cones, caldera-like forms, or relatively flat structures, reflecting physical properties of the rising material.
The depth of origin of the different phases may be variable
within the same mud volcano and ranges from a few
meters to several kilometers sub-bottom.
Mud intrusion and extrusion are well-known phenomena whereby fluid-rich, fine-grained sediments ascend
within a lithologic succession due to their buoyancy.
These processes have long been recognized as related to
the occurrence of petroleum, regional volcanic and earthquake activity, and orogenic belts (see the enlightening
presentation by Ansted (1866)). The abundance of mud
volcanoes was first examined systematically on a broad
scale by Higgins and Saunders (1974). These authors
mostly focused on mud volcanoes on land and used industry drillhole data to establish relationships between mud
volcanism, hydrocarbons, and regional tectonics. The
onshore mud volcanoes and their important role in
predicting petroleum reservoirs have been summarized
by Rhakmanov (1987) and were heavily used at times
when drilling the subsurface had not been invented yet.
Recent improvements in seafloor imagery and seismic
exploration led to the discovery of countless mud extrusive provinces all over the world (Figure 1). In fact, compared to the compilation by Higgins and Saunders (1974),
more than twice as many occurrences are presently
known, with the number gradually increasing through
time. Most importantly, mud volcanoes occur along convergent plate margins where fluid-rich sediment is accumulated in deep-sea trenches at high rates. Such deposits
then enter the subduction factory, where liquids and volatiles are released due to incipient compactional stress and
temperature. Studies of geophysical data and samples of
mud volcanoes have considerably improved the understanding of the mechanics, driving forces, and evolution
of the features through the most recent Earth history
(e.g., Barber et al., 1986; Brown, 1990). In addition, deep
ocean drilling and submersible studies shed crucial light
on eruptivity, emission of volatiles, and potential hazard
originating from violent mud extrusion (e.g., Bagirov
et al., 1996; Robertson et al., 1996; Kopf, 1999). The
wealth of results attests that mud extrusion predominantly
occurs in collisional settings (i.e., zones of convergent
plate motion), with mostly pore fluids during early (often
marine) stages and hydrocarbons at later stages (often on
land; e.g., Jakubov et al., 1971; Tamrazyan, 1972; Speed
and Larue, 1992). Although quantification of fluid and
mud discharge in mud volcanoes is not easy due to their
short-lived nature and inaccessibility on the seafloor,
first-order estimates regarding flux rates have recently
been attempted for various features and regions (e.g.,
Henry et al., 1996; Kopf and Behrmann, 2000). When
put into a broader context, such estimates indicate that
mud extrusion contributes significantly to fluid backflux
from the lithosphere to the hydrosphere. Along wide parts
of large accretionary prisms (like the Barbados or Mediterranean Ridges), hundreds of features can cause fluid
expulsion at rates exceeding those at the frontal part of
the prism (see discussion in Kopf et al., 2001). Provided
that the process of mud volcanism has been equally common in the past, these features have been major players in
fluid and gas budgets and geochemical cycling in collision
zones.
Formation, occurrence, and habitus
Mud volcanoes (MVs) are the surface expressions of
deeply rooted vertical structures, and they are composed
of coarse, poorly sorted “mud breccia” made up of materials which migrated upward from zones of overpressure.
Sediment extrusion is driven by a mobile fluid phase, most
often a mixture of aqueous and gaseous fluids (on Earth,
water, hydrocarbons, and CO 2 ; on Mars: it is proposed
to be partly methane). Mud volcanoe (MV) occurrences
have the following aspects in common: (1) an origin from
thick, rapidly deposited sequences of marine clays, (2) a
tertiary age, (3) a structural association due to tectonic
shortening and/or earthquake activity, (4) sediment
overpressuring and accompanied fluid emission (free
gas, brines, gas hydrate water, rarely oil), and
(5) polymictic assemblages of the surrounding rock present in the ejected argillaceous matrix (e.g., Ansted,
1866; Higgins and Saunders, 1974; Fertl, 1976; Yassir,
1989; Kopf, 2002). Less commonly, volcanic and geothermal activity, often associated with ascent of hydrothermal
fluids, may form mud domes and mud pools (e.g., at the
basal flanks of the Costa Rica volcanic arc onshore). See
Figure 2.
The possible causes of undercompaction are numerous,
and sediment loading is one mechanism which may apply
in any setting. It can be enhanced by rapid plate motion in
528
MUD VOLCANO
The Greek “diapeirein” means “to pierce,” and the first
mention of diapiric structures dates back to Leymerie
(1881), who suggested the word “tiphon.” This nomenclature was adapted by Choffat (1882) when describing sedimentary diapiric manifestations in Portugal, calling these
small domes “tiphonique.” Mrazec’s (1927) definition of
diapirism as the process of forceful movement of a more
or less plastic body from areas of greater pressure to areas
of less pressure has since then widely been accepted. It has
often been restricted to salt structures instead of having
been applied on mechanical grounds, although some
workers used it for effusive or intrusive magmatic processes (e.g., Wegmann, 1930). The term “sedimentary volcanism” supposedly (Saunders, personal communication,
2000) goes back to Kugler (1933).
Introduction and historical background
Mud volcanoes on Earth extrude relatively low temperature slurries of gas, liquid, and rock to the surface from
depths of meters to kilometers (Kopf, 2002). Slurries build
circular to subcircular deposits of mud and rock breccia
that range in diameter from a few meters up to tens of kilometers. They can produce a variety of morphologies,
domes, cones, caldera-like forms, or relatively flat structures, reflecting physical properties of the rising material.
The depth of origin of the different phases may be variable
within the same mud volcano and ranges from a few
meters to several kilometers sub-bottom.
Mud intrusion and extrusion are well-known phenomena whereby fluid-rich, fine-grained sediments ascend
within a lithologic succession due to their buoyancy.
These processes have long been recognized as related to
the occurrence of petroleum, regional volcanic and earthquake activity, and orogenic belts (see the enlightening
presentation by Ansted (1866)). The abundance of mud
volcanoes was first examined systematically on a broad
scale by Higgins and Saunders (1974). These authors
mostly focused on mud volcanoes on land and used industry drillhole data to establish relationships between mud
volcanism, hydrocarbons, and regional tectonics. The
onshore mud volcanoes and their important role in
predicting petroleum reservoirs have been summarized
by Rhakmanov (1987) and were heavily used at times
when drilling the subsurface had not been invented yet.
Recent improvements in seafloor imagery and seismic
exploration led to the discovery of countless mud extrusive provinces all over the world (Figure 1). In fact, compared to the compilation by Higgins and Saunders (1974),
more than twice as many occurrences are presently
known, with the number gradually increasing through
time. Most importantly, mud volcanoes occur along convergent plate margins where fluid-rich sediment is accumulated in deep-sea trenches at high rates. Such deposits
then enter the subduction factory, where liquids and volatiles are released due to incipient compactional stress and
temperature. Studies of geophysical data and samples of
mud volcanoes have considerably improved the understanding of the mechanics, driving forces, and evolution
of the features through the most recent Earth history
(e.g., Barber et al., 1986; Brown, 1990). In addition, deep
ocean drilling and submersible studies shed crucial light
on eruptivity, emission of volatiles, and potential hazard
originating from violent mud extrusion (e.g., Bagirov
et al., 1996; Robertson et al., 1996; Kopf, 1999). The
wealth of results attests that mud extrusion predominantly
occurs in collisional settings (i.e., zones of convergent
plate motion), with mostly pore fluids during early (often
marine) stages and hydrocarbons at later stages (often on
land; e.g., Jakubov et al., 1971; Tamrazyan, 1972; Speed
and Larue, 1992). Although quantification of fluid and
mud discharge in mud volcanoes is not easy due to their
short-lived nature and inaccessibility on the seafloor,
first-order estimates regarding flux rates have recently
been attempted for various features and regions (e.g.,
Henry et al., 1996; Kopf and Behrmann, 2000). When
put into a broader context, such estimates indicate that
mud extrusion contributes significantly to fluid backflux
from the lithosphere to the hydrosphere. Along wide parts
of large accretionary prisms (like the Barbados or Mediterranean Ridges), hundreds of features can cause fluid
expulsion at rates exceeding those at the frontal part of
the prism (see discussion in Kopf et al., 2001). Provided
that the process of mud volcanism has been equally common in the past, these features have been major players in
fluid and gas budgets and geochemical cycling in collision
zones.
Formation, occurrence, and habitus
Mud volcanoes (MVs) are the surface expressions of
deeply rooted vertical structures, and they are composed
of coarse, poorly sorted “mud breccia” made up of materials which migrated upward from zones of overpressure.
Sediment extrusion is driven by a mobile fluid phase, most
often a mixture of aqueous and gaseous fluids (on Earth,
water, hydrocarbons, and CO 2 ; on Mars: it is proposed
to be partly methane). Mud volcanoe (MV) occurrences
have the following aspects in common: (1) an origin from
thick, rapidly deposited sequences of marine clays, (2) a
tertiary age, (3) a structural association due to tectonic
shortening and/or earthquake activity, (4) sediment
overpressuring and accompanied fluid emission (free
gas, brines, gas hydrate water, rarely oil), and
(5) polymictic assemblages of the surrounding rock present in the ejected argillaceous matrix (e.g., Ansted,
1866; Higgins and Saunders, 1974; Fertl, 1976; Yassir,
1989; Kopf, 2002). Less commonly, volcanic and geothermal activity, often associated with ascent of hydrothermal
fluids, may form mud domes and mud pools (e.g., at the
basal flanks of the Costa Rica volcanic arc onshore). See
Figure 2.
The possible causes of undercompaction are numerous,
and sediment loading is one mechanism which may apply
in any setting. It can be enhanced by rapid plate motion in
528
MUD VOLCANO