extended. The rollback causes the island arc to migrate
towards the outside of the arc system which in turn results
in a strong extension of the backarc region. If the arc was
originally built on a continent, this process leads to the
separation of the island arc from the upper plate continent.
The resulting separation behind the island arc generates
a backarc basin and with sufficient extension, new oceanic
crust may form (e.g., Sea of Japan). The western Pacific is
characterized by numerous island arc–backarc systems
(Figure 1). The garland- or drape-like alignment of the
island arcs results from their separation from the continent. Typically, the garlands have lengths from 2,000 to
2,500 km. Because the Mariana Arc comprises an
intraoceanic subduction zone, the formation of the backarc
basin occurred solely within oceanic crust (Figure 2).
Subduction rollback that forms during Mariana-type
subduction has an effect on the topography of the plate
boundary system. The mean topographic elevation on
the edge of the upper plate is low because of the effect of
suction and extension. Like in the Marianas, the magmatic
arc may only form a chain of small islands representing the
peaks of the volcanoes. Also, the deep-sea trenches are
significantly deeper on average because the subducting
plate bends downward very steeply and the island arcs
do not deliver enough sedimentary material to even partially fill the trench. The world’s deepest trenches, all with
depths greater than 8,000 m, occur along westwarddirected subduction zones (including those of the Atlantic
Ocean) that involve old oceanic lithosphere (Figure 1).
The rollback of the westward-directed subduction zones
may be enhanced by an eastward-directed asthenospheric
current as calculated from global plate drift compared to
flow of the sublithospheric mantle.
A curious situation at subduction zones with backarc
basins (Mariana-type subduction) is the juxtaposition of
strong convergence and divergence. Although relative
plate movement velocities at convergent plate boundaries
in the western Pacific approach 9 cm/year, a wide area at
the edge of the upper plate is under extension. Compressive structures are restricted essentially to the tip of the
upper plate.
The subduction zones and convergent plate boundaries
of Chile-type subduction are fundamentally different in
many ways. The subducting plate is intermittently coupled
to the upper plate because of the buoyancy of the former;
this causes compression and thickening of the upper plate
(Figure 4). Thickening of continental crust leads to the
generation of a mountain range. Deep-sea trenches are
shallower and volcanic zones are characterized by significantly higher elevations than volcanoes associated with
Mariana-type subduction. The highly elevated hinterland
is more likely to deliver sedimentary material into the
trenches, and the sediment supply is generally not hampered by intervening ridges. Even more consequential
than the topographic height is the structural height, the
total amount of uplift within the volcanic zone, which
may exceed 20 km. Crustal structures originally formed
at depth are subsequently uncovered by erosion. In fact,
metamorphic and intrusive magmatic rocks at the surface
in the Andes can be used to estimate the amount of uplift
and subsequent amount of erosion.
Along Chile-type subduction zones, the compressional
forces are transferred far into the upper plate. Therefore,
earthquakes in this area are particularly strong and frequent. Regionally, compressional structures with
overthrusts evolve that are oceanward-directed in the
forearc and continentward-directed in the backarc region
(Figure 4). Consequently, the backarc area is also under
compressional stress. Crustal shortening occurs across
the entire magmatic zone, which in turn has an effect on
the magmatism itself. Melts rising from the asthenospheric wedge above the subduction zone typically
become trapped in the crust and crystallize as intrusions
or melt adjacent crustal rocks and feed highly explosive
acidic volcanoes. The collective processes of tectonic
stacking and magmatic accretion have formed a crustal
thickness of 70 km in the Andes, one of the areas of
thickest continental crust on Earth.
Bibliography
Bott, M. P. H., 1982. The Interior of the Earth: Its Structure, Constitution and Evolution, 2nd edn. London: Edward Arnold. 403pp.
Doglioni, C., Harabaglia, P., Merlini, S., Mongelli, F., Peccerillo,
A., and Piromallo, C., 1999. Orogens and slabs vs. their direction
of subduction. Earth Science Reviews, 45, 167–208.
Frisch, W., Meschede, M., and Blakey, R., 2011. Plate Tectonics.
Continental Drift and Mountain Building. Berlin/Heidelberg:
Springer. 212pp.
Nicolas, A., 1995. The Mid-Oceanic Ridges. Mountains Below Sea
Level. Berlin/Heidelberg: Springer. 217pp.
Cross-references
Active Continental Margins
Intraoceanic Subduction Zone
Island Arc Volcanism, Volcanic Arcs
Marginal Seas
Subduction
MUD VOLCANO
Achim Kopf
MARUM-Center for Marine Environmental Sciences,
University of Bremen, Bremen, Germany
Synonyms
Diatreme; Mud diapir; Pockmark
Definition
A type of sedimentary volcano (Kopf, 2002; van Loon,
2010) composed of fine-grained mud, in both subaqueous
and subaerial settings.
Landforms proposed to be mud volcanoes: Pitted cone,
Mound, Mud diapir, Diatreme.
MUD VOLCANO
527
towards the outside of the arc system which in turn results
in a strong extension of the backarc region. If the arc was
originally built on a continent, this process leads to the
separation of the island arc from the upper plate continent.
The resulting separation behind the island arc generates
a backarc basin and with sufficient extension, new oceanic
crust may form (e.g., Sea of Japan). The western Pacific is
characterized by numerous island arc–backarc systems
(Figure 1). The garland- or drape-like alignment of the
island arcs results from their separation from the continent. Typically, the garlands have lengths from 2,000 to
2,500 km. Because the Mariana Arc comprises an
intraoceanic subduction zone, the formation of the backarc
basin occurred solely within oceanic crust (Figure 2).
Subduction rollback that forms during Mariana-type
subduction has an effect on the topography of the plate
boundary system. The mean topographic elevation on
the edge of the upper plate is low because of the effect of
suction and extension. Like in the Marianas, the magmatic
arc may only form a chain of small islands representing the
peaks of the volcanoes. Also, the deep-sea trenches are
significantly deeper on average because the subducting
plate bends downward very steeply and the island arcs
do not deliver enough sedimentary material to even partially fill the trench. The world’s deepest trenches, all with
depths greater than 8,000 m, occur along westwarddirected subduction zones (including those of the Atlantic
Ocean) that involve old oceanic lithosphere (Figure 1).
The rollback of the westward-directed subduction zones
may be enhanced by an eastward-directed asthenospheric
current as calculated from global plate drift compared to
flow of the sublithospheric mantle.
A curious situation at subduction zones with backarc
basins (Mariana-type subduction) is the juxtaposition of
strong convergence and divergence. Although relative
plate movement velocities at convergent plate boundaries
in the western Pacific approach 9 cm/year, a wide area at
the edge of the upper plate is under extension. Compressive structures are restricted essentially to the tip of the
upper plate.
The subduction zones and convergent plate boundaries
of Chile-type subduction are fundamentally different in
many ways. The subducting plate is intermittently coupled
to the upper plate because of the buoyancy of the former;
this causes compression and thickening of the upper plate
(Figure 4). Thickening of continental crust leads to the
generation of a mountain range. Deep-sea trenches are
shallower and volcanic zones are characterized by significantly higher elevations than volcanoes associated with
Mariana-type subduction. The highly elevated hinterland
is more likely to deliver sedimentary material into the
trenches, and the sediment supply is generally not hampered by intervening ridges. Even more consequential
than the topographic height is the structural height, the
total amount of uplift within the volcanic zone, which
may exceed 20 km. Crustal structures originally formed
at depth are subsequently uncovered by erosion. In fact,
metamorphic and intrusive magmatic rocks at the surface
in the Andes can be used to estimate the amount of uplift
and subsequent amount of erosion.
Along Chile-type subduction zones, the compressional
forces are transferred far into the upper plate. Therefore,
earthquakes in this area are particularly strong and frequent. Regionally, compressional structures with
overthrusts evolve that are oceanward-directed in the
forearc and continentward-directed in the backarc region
(Figure 4). Consequently, the backarc area is also under
compressional stress. Crustal shortening occurs across
the entire magmatic zone, which in turn has an effect on
the magmatism itself. Melts rising from the asthenospheric wedge above the subduction zone typically
become trapped in the crust and crystallize as intrusions
or melt adjacent crustal rocks and feed highly explosive
acidic volcanoes. The collective processes of tectonic
stacking and magmatic accretion have formed a crustal
thickness of 70 km in the Andes, one of the areas of
thickest continental crust on Earth.
Bibliography
Bott, M. P. H., 1982. The Interior of the Earth: Its Structure, Constitution and Evolution, 2nd edn. London: Edward Arnold. 403pp.
Doglioni, C., Harabaglia, P., Merlini, S., Mongelli, F., Peccerillo,
A., and Piromallo, C., 1999. Orogens and slabs vs. their direction
of subduction. Earth Science Reviews, 45, 167–208.
Frisch, W., Meschede, M., and Blakey, R., 2011. Plate Tectonics.
Continental Drift and Mountain Building. Berlin/Heidelberg:
Springer. 212pp.
Nicolas, A., 1995. The Mid-Oceanic Ridges. Mountains Below Sea
Level. Berlin/Heidelberg: Springer. 217pp.
Cross-references
Active Continental Margins
Intraoceanic Subduction Zone
Island Arc Volcanism, Volcanic Arcs
Marginal Seas
Subduction
MUD VOLCANO
Achim Kopf
MARUM-Center for Marine Environmental Sciences,
University of Bremen, Bremen, Germany
Synonyms
Diatreme; Mud diapir; Pockmark
Definition
A type of sedimentary volcano (Kopf, 2002; van Loon,
2010) composed of fine-grained mud, in both subaqueous
and subaerial settings.
Landforms proposed to be mud volcanoes: Pitted cone,
Mound, Mud diapir, Diatreme.
MUD VOLCANO
527