(Takahashi et al., 2007; Kodaira, 2007; Calvert, 2011).
In Izu, the crust varies sinusoidally from <25 to ~35 km
thick along the length of the arc with a wavelength of
80–100 km. Crust is thicker where the Quaternary arc
front volcanoes have erupted mostly basaltic magma and
thinner where the magmas are mostly rhyolitic in composition. The velocity structure is similar in both cases, with
a Vp ¼ 6.0–6.5 km/s middle crust at 5–10 km depth
that is characteristic of continents and consistent with
an andesitic composition and a lower crust that is consistent with plagioclase-bearing mafic cumulates
(Vp ¼ 6.8–7.2 km/s) overlying plagioclase-free mafic
rocks (olivine, pyroxene, amphibole: Vp ¼ 7.2–7.6 km/s).
This highest-velocity layer is sometimes assigned to the
lowermost crust (Kodaira, 2007) and sometimes to the
uppermost mantle (Takahashi et al., 2007; Tatsumi et al.,
2008). The Aleutian arc differs in that its Vp ¼ 6.0–6.5
layer is at a shallower depth (<5 km) and velocities generally are higher at all depths, indicating a more mafic composition. The overall velocity structure of arcs is
consistent with a two-stage process of arc crustal production and internal differentiation whereby juvenile magma
from the underlying mantle is basaltic and the crustal
structure reflects in situ magmatic differentiation and
anatexis (e.g., Tatsumi et al., 2008). The crust in all modern island arcs has higher velocity at depths >10 km than
old continental interiors.
Across-arc variations
All island arcs include volcanoes farther from the plate
boundary than the volcanic front in what is called the “rear
arc.” Rear arc volcanoes are usually smaller and more
likely to be submarine seamounts rather than subaerial
islands. Consequently, they are less well known. There
can be hundreds of them that extend up to 100 km behind
the volcanic front. The volcanic front and rear arc volcanoes together constitute the island arc or volcanic
arc. The thickness and velocity structure of the crust are
roughly the same for ~100 km across unrifted volcanic
arcs (Kodaira, 2007). Therefore, although the volcanoes
are smaller in modern rear arcs, the majority of arc crust
may be created in the rear arc because it is wider than
the volcanic front.
Volcanic rocks in the rear arc usually differ in chemical
composition from those at the front. They have higher
concentrations of K and most incompatible trace elements
(those favoring melt over crystals) but less enriched Sr,
Nd, Pb, and Hf isotope ratios. They are more likely to have
amphibole phenocrysts in mafic rocks. The differences
usually are attributed to smaller degrees of mantle melting
beneath the rear arc in response to less fluxing by liquids
from the subducting plate that are themselves less water
rich than beneath the volcanic front (e.g., Hochstaedter
et al., 2001).
Backarc magmas gradually lose their subductionrelated character altogether by several hundred kilometers
behind the volcanic front and become like those at
mid-ocean ridges (MORB) where there is active seafloor
spreading in the backarc (e.g., Central Lau, North Fiji,
and Manus Basins) or like those at ocean islands (OIB)
in broad extensional zones without spreading. The distinction between rear arc and backarc, and consequently the
width of volcanic arcs, is somewhat subjective depending
on what evidence of “subduction-related character” is
used. The following concentration ratios are indicative of
subduction-related magmatism: U/Nb > 0.03, Ba/La
> 20, Pb/Ce > 0.08, H 2 O/Ce > 250, and La/Nb >1. However, enrichment in these elements derived from the
subducting plate can postdate active subduction by as
much as a few million years.
Along-arc variations
As noted above, the volcanic fronts of island arcs are characterized by along-arc spacing of both volcanoes and the
thickness of the middle crust at 50–100 km intervals. In
addition, there are similarly spaced chains of volcanoes
across the width of many volcanic arcs (e.g., across the
Kermadec, Tonga, Mariana, Izu, Kurile, and Sangihe arcs;
e.g., Hochstaedter et al., 2001). In NE Honshu, Japan, the
along-arc spacing of these volcanic chains correlates with
the spacing of low-gravity anomalies at the surface and
low seismic velocities (especially Vs/Vp) in the upper
mantle (Tamura et al., 2002). This along-strike threedimensional structure of island arcs suggests spatial periodicity of magma production and mantle upwelling. This
spacing has been attributed directly to the rheology of melt
from the subducting plate (Marsh, 1974; Behn et al., 2011)
or indirectly to the rheology of flux-melted mantle in the
overlying wedge (Honda and Yoshida, 2005).
Relationship of island arcs to subduction
parameters
Island arcs differ most among themselves in the age and
velocity of the subducting plate and, therefore, slab surface temperatures and in the nature and thickness of
subducting sediment. The location of volcanic fronts is
thought to be controlled by the thermal structure of the
subducting plate or the overlying mantle or both.
A generic arc cross section is shown in Figure 2 of Stern
(Magmatism at Convergent Plate Boundaries). P-T paths
of the slab surface for different subducting plates, and
for different layers within the subducting crust of any
one arc, are shown in Figs. 1 and 2 (Syracuse et al.,
2010; van Keken et al., 2011). The location of the volcanic
front sometimes corresponds to the depth of maximum
dehydration or the onset of melting, in one or more layers
of subducted crust. Alternatively, the volcanic front may
overlie where the anhydrous solidus of the convecting
mantle comes closest to the plate boundary (England and
Katz, 2010). The across-arc width of island arcs may be
related to the loci of slab melting. Subducting sediment
is expected to melt beneath all volcanic fronts, and
subducting basalt beneath most volcanic fronts, if they
are water saturated (Figure 1). Because the P-T path of
380
ISLAND ARC VOLCANISM, VOLCANIC ARCS
In Izu, the crust varies sinusoidally from <25 to ~35 km
thick along the length of the arc with a wavelength of
80–100 km. Crust is thicker where the Quaternary arc
front volcanoes have erupted mostly basaltic magma and
thinner where the magmas are mostly rhyolitic in composition. The velocity structure is similar in both cases, with
a Vp ¼ 6.0–6.5 km/s middle crust at 5–10 km depth
that is characteristic of continents and consistent with
an andesitic composition and a lower crust that is consistent with plagioclase-bearing mafic cumulates
(Vp ¼ 6.8–7.2 km/s) overlying plagioclase-free mafic
rocks (olivine, pyroxene, amphibole: Vp ¼ 7.2–7.6 km/s).
This highest-velocity layer is sometimes assigned to the
lowermost crust (Kodaira, 2007) and sometimes to the
uppermost mantle (Takahashi et al., 2007; Tatsumi et al.,
2008). The Aleutian arc differs in that its Vp ¼ 6.0–6.5
layer is at a shallower depth (<5 km) and velocities generally are higher at all depths, indicating a more mafic composition. The overall velocity structure of arcs is
consistent with a two-stage process of arc crustal production and internal differentiation whereby juvenile magma
from the underlying mantle is basaltic and the crustal
structure reflects in situ magmatic differentiation and
anatexis (e.g., Tatsumi et al., 2008). The crust in all modern island arcs has higher velocity at depths >10 km than
old continental interiors.
Across-arc variations
All island arcs include volcanoes farther from the plate
boundary than the volcanic front in what is called the “rear
arc.” Rear arc volcanoes are usually smaller and more
likely to be submarine seamounts rather than subaerial
islands. Consequently, they are less well known. There
can be hundreds of them that extend up to 100 km behind
the volcanic front. The volcanic front and rear arc volcanoes together constitute the island arc or volcanic
arc. The thickness and velocity structure of the crust are
roughly the same for ~100 km across unrifted volcanic
arcs (Kodaira, 2007). Therefore, although the volcanoes
are smaller in modern rear arcs, the majority of arc crust
may be created in the rear arc because it is wider than
the volcanic front.
Volcanic rocks in the rear arc usually differ in chemical
composition from those at the front. They have higher
concentrations of K and most incompatible trace elements
(those favoring melt over crystals) but less enriched Sr,
Nd, Pb, and Hf isotope ratios. They are more likely to have
amphibole phenocrysts in mafic rocks. The differences
usually are attributed to smaller degrees of mantle melting
beneath the rear arc in response to less fluxing by liquids
from the subducting plate that are themselves less water
rich than beneath the volcanic front (e.g., Hochstaedter
et al., 2001).
Backarc magmas gradually lose their subductionrelated character altogether by several hundred kilometers
behind the volcanic front and become like those at
mid-ocean ridges (MORB) where there is active seafloor
spreading in the backarc (e.g., Central Lau, North Fiji,
and Manus Basins) or like those at ocean islands (OIB)
in broad extensional zones without spreading. The distinction between rear arc and backarc, and consequently the
width of volcanic arcs, is somewhat subjective depending
on what evidence of “subduction-related character” is
used. The following concentration ratios are indicative of
subduction-related magmatism: U/Nb > 0.03, Ba/La
> 20, Pb/Ce > 0.08, H 2 O/Ce > 250, and La/Nb >1. However, enrichment in these elements derived from the
subducting plate can postdate active subduction by as
much as a few million years.
Along-arc variations
As noted above, the volcanic fronts of island arcs are characterized by along-arc spacing of both volcanoes and the
thickness of the middle crust at 50–100 km intervals. In
addition, there are similarly spaced chains of volcanoes
across the width of many volcanic arcs (e.g., across the
Kermadec, Tonga, Mariana, Izu, Kurile, and Sangihe arcs;
e.g., Hochstaedter et al., 2001). In NE Honshu, Japan, the
along-arc spacing of these volcanic chains correlates with
the spacing of low-gravity anomalies at the surface and
low seismic velocities (especially Vs/Vp) in the upper
mantle (Tamura et al., 2002). This along-strike threedimensional structure of island arcs suggests spatial periodicity of magma production and mantle upwelling. This
spacing has been attributed directly to the rheology of melt
from the subducting plate (Marsh, 1974; Behn et al., 2011)
or indirectly to the rheology of flux-melted mantle in the
overlying wedge (Honda and Yoshida, 2005).
Relationship of island arcs to subduction
parameters
Island arcs differ most among themselves in the age and
velocity of the subducting plate and, therefore, slab surface temperatures and in the nature and thickness of
subducting sediment. The location of volcanic fronts is
thought to be controlled by the thermal structure of the
subducting plate or the overlying mantle or both.
A generic arc cross section is shown in Figure 2 of Stern
(Magmatism at Convergent Plate Boundaries). P-T paths
of the slab surface for different subducting plates, and
for different layers within the subducting crust of any
one arc, are shown in Figs. 1 and 2 (Syracuse et al.,
2010; van Keken et al., 2011). The location of the volcanic
front sometimes corresponds to the depth of maximum
dehydration or the onset of melting, in one or more layers
of subducted crust. Alternatively, the volcanic front may
overlie where the anhydrous solidus of the convecting
mantle comes closest to the plate boundary (England and
Katz, 2010). The across-arc width of island arcs may be
related to the loci of slab melting. Subducting sediment
is expected to melt beneath all volcanic fronts, and
subducting basalt beneath most volcanic fronts, if they
are water saturated (Figure 1). Because the P-T path of
380
ISLAND ARC VOLCANISM, VOLCANIC ARCS