reflects cumulative heating by innumerable pulses of
~1,300
C magma moving from the upper mantle through
the crust towards the surface. The crust is further heated
when basaltic magma stalls in it without erupting. Crustal
melts and products of fractional crystallization are both
felsic combine to generate andesitic and felsic melts. The
more felsic the magma, the more polymerized it is,
because Si and O link up to form long chains of silicon
and oxygen that resist flow. The Si-rich, viscous nature
of magmas are half of the reason that convergent margin
eruptions are the most violent of among the three great tectonic settings.
The volatile-rich nature (especially H 2 O, CO 2 , and S)
of convergent margin magmas is the other half of the reason that convergent margin magmas can erupt so explosively – and why most ore deposits are associated with
ancient arcs. Of the three great magmatotectonic settings,
convergent margin magmas contain by far the greatest
concentration of magmatic gases and volatiles
(Roggensack et al., 1997; Plank et al., 2013). Convergent
margin high-Si melts inherit the water- and sulfur-rich
nature of the mantle melt, which ultimately result from
the fluids released by the subducted slab. The volatiles
are dissolved in the magma when it forms in the upper
mantle but are released as magma approaches the surface
and pressure is greatly reduced. The exsolved volatiles
turn into gas bubbles with a tremendous increase in volume. The gas bubbles can escape quietly through
low-viscosity mafic magma but must break out violently
if they are in viscous felsic magma.
A significant proportion of convergent margin melts
crystallize several kilometers deep within the crust, where
they form large bodies of crystalline rocks known as plutons (Figure 3). Plutons are similar in scale to the overlying volcano, typically several to a few tens of kilometers
in diameter. Although lavas and other products of volcanoes characterize an active island arc, once a subduction
zone stops operating (perhaps due to plate collision), the
associated magmatic arc shuts down; erosion remove
the volcanic rocks and exposes the plutonic roots of the
arc. Because island arcs are elongate magmatic systems,
an ancient arc can be identified by similarly elongate
aggregations of plutons called batholiths. Batholiths, like
island arcs, are typically about 100 km wide and thousands of kilometers long. Examples of batholiths include
the Sierra Nevada of California, which is spectacularly
exposed at Yosemite National Park, USA.
Volcanism
Eruptions from island arc volcanoes can be especially violent and dangerous. This is partly due to high volatile contents (especially water) in both primitive (basaltic)
convergent margin magmas and more evolved felsic
magmas, and this powers the violence of eruptions from
volcanoes at convergent margins. Because the solubility
of volatiles in magma decreases as pressure falls during
magma ascent, volatiles are progressively released as
magma rises towards the surface prior to eruption. This
involves a tremendous increase in volume when pressure
is low enough for magmatic volatiles to flash into gas.
Depending on the viscosity of the magma, the rate of volatile diffusion from the melt into the gas phase varies,
which – depending on magma ascent rate – can lead to
volatile supersaturation in magmas and dramatic explosive bubble nucleation and greater magma acceleration.
Because magmatic volatiles readily exsolve out of low
viscosity mafic magmas and the bubbles can rise faster
than the melt phase (i.e., are decoupled), basaltic eruptions
tend to be less dangerous (but note that there are examples
of more violent “Plinian” basaltic eruptions, such as 1886
Mt. Tarawera, New Zealand eruption). In contrast, in high
viscosity felsic melt bubbles often remain coupled, driving
high magma buoyancy and acceleration, making subaerial
felsic eruptions the most violent on Earth.
Today, eruption violence is measured using the Volcanic Explosivity Index (VEI; Newhall and Self, 1982) a
logarithmic scale (base 10) for erupted volume. The largest Cenozoic eruptions have VEI ¼ 8: Yellowstone
630 ka; Toba 74 ka; Taupo 26.5 ka. These entail violent
and catastrophic eruptions where >1,000 km
3 of magma
is erupted over short durations of likely weeks to months.
Submarine arc volcanism is not yet well understood
compared to subaerial volcanism but stunning discoveries
(Embley et al., 2007) encourage further exploration. In
some ways submarine volcanism is very different than
subaerial counterparts. In contrast to eruptions into the
atmosphere, eruption into water strongly modifies eruption and pyroclast transport processes due to the
contrasting properties of water compared to air. Increasing
hydrostatic pressure associated with increasing eruption
depth influences volatile exsolution and particularly
expansion in the shallow conduit during magma ascent.
Magma fragmentation and pyroclast transport processes
are also strongly modified in the submarine setting due
to the greater heat capacity, density and viscosity of water
compared with air.
Submarine arc volcanism is not yet well understood,
however it is very likely that this mode of eruption shows
as much variability in style as subaerial volcanism. Submarine basaltic eruptions range from quiet effusion
(including pillow basalts), and low to moderate intensity
explosive eruptions. Felsic eruptions are also common
along submarine volcanic arcs, from effusive dome
growth and spalling of the pumiceous carapace to low –
moderate explosive cone-forming eruptions and likely
catastrophic caldera-forming eruptions (Fiske et al.,
2001; Smith et al., 2003). Subaqueous eruptions are
poorly understood for several reasons, most notably the
lack of witnessed eruptions, but it is clear from theory
and seafloor exploration with submergence vehicles that
increasing hydrostatic pressure has a profound effect on
limiting explosivity (e.g., Allen and McPhie 2009). In
the last decade remote sensing methods together with
some good luck has led to the observations of both basaltic
and felsic submarine eruptions (e.g., West Mata volcano in
MAGMATISM AT CONVERGENT PLATE BOUNDARIES
403
~1,300
C magma moving from the upper mantle through
the crust towards the surface. The crust is further heated
when basaltic magma stalls in it without erupting. Crustal
melts and products of fractional crystallization are both
felsic combine to generate andesitic and felsic melts. The
more felsic the magma, the more polymerized it is,
because Si and O link up to form long chains of silicon
and oxygen that resist flow. The Si-rich, viscous nature
of magmas are half of the reason that convergent margin
eruptions are the most violent of among the three great tectonic settings.
The volatile-rich nature (especially H 2 O, CO 2 , and S)
of convergent margin magmas is the other half of the reason that convergent margin magmas can erupt so explosively – and why most ore deposits are associated with
ancient arcs. Of the three great magmatotectonic settings,
convergent margin magmas contain by far the greatest
concentration of magmatic gases and volatiles
(Roggensack et al., 1997; Plank et al., 2013). Convergent
margin high-Si melts inherit the water- and sulfur-rich
nature of the mantle melt, which ultimately result from
the fluids released by the subducted slab. The volatiles
are dissolved in the magma when it forms in the upper
mantle but are released as magma approaches the surface
and pressure is greatly reduced. The exsolved volatiles
turn into gas bubbles with a tremendous increase in volume. The gas bubbles can escape quietly through
low-viscosity mafic magma but must break out violently
if they are in viscous felsic magma.
A significant proportion of convergent margin melts
crystallize several kilometers deep within the crust, where
they form large bodies of crystalline rocks known as plutons (Figure 3). Plutons are similar in scale to the overlying volcano, typically several to a few tens of kilometers
in diameter. Although lavas and other products of volcanoes characterize an active island arc, once a subduction
zone stops operating (perhaps due to plate collision), the
associated magmatic arc shuts down; erosion remove
the volcanic rocks and exposes the plutonic roots of the
arc. Because island arcs are elongate magmatic systems,
an ancient arc can be identified by similarly elongate
aggregations of plutons called batholiths. Batholiths, like
island arcs, are typically about 100 km wide and thousands of kilometers long. Examples of batholiths include
the Sierra Nevada of California, which is spectacularly
exposed at Yosemite National Park, USA.
Volcanism
Eruptions from island arc volcanoes can be especially violent and dangerous. This is partly due to high volatile contents (especially water) in both primitive (basaltic)
convergent margin magmas and more evolved felsic
magmas, and this powers the violence of eruptions from
volcanoes at convergent margins. Because the solubility
of volatiles in magma decreases as pressure falls during
magma ascent, volatiles are progressively released as
magma rises towards the surface prior to eruption. This
involves a tremendous increase in volume when pressure
is low enough for magmatic volatiles to flash into gas.
Depending on the viscosity of the magma, the rate of volatile diffusion from the melt into the gas phase varies,
which – depending on magma ascent rate – can lead to
volatile supersaturation in magmas and dramatic explosive bubble nucleation and greater magma acceleration.
Because magmatic volatiles readily exsolve out of low
viscosity mafic magmas and the bubbles can rise faster
than the melt phase (i.e., are decoupled), basaltic eruptions
tend to be less dangerous (but note that there are examples
of more violent “Plinian” basaltic eruptions, such as 1886
Mt. Tarawera, New Zealand eruption). In contrast, in high
viscosity felsic melt bubbles often remain coupled, driving
high magma buoyancy and acceleration, making subaerial
felsic eruptions the most violent on Earth.
Today, eruption violence is measured using the Volcanic Explosivity Index (VEI; Newhall and Self, 1982) a
logarithmic scale (base 10) for erupted volume. The largest Cenozoic eruptions have VEI ¼ 8: Yellowstone
630 ka; Toba 74 ka; Taupo 26.5 ka. These entail violent
and catastrophic eruptions where >1,000 km
3 of magma
is erupted over short durations of likely weeks to months.
Submarine arc volcanism is not yet well understood
compared to subaerial volcanism but stunning discoveries
(Embley et al., 2007) encourage further exploration. In
some ways submarine volcanism is very different than
subaerial counterparts. In contrast to eruptions into the
atmosphere, eruption into water strongly modifies eruption and pyroclast transport processes due to the
contrasting properties of water compared to air. Increasing
hydrostatic pressure associated with increasing eruption
depth influences volatile exsolution and particularly
expansion in the shallow conduit during magma ascent.
Magma fragmentation and pyroclast transport processes
are also strongly modified in the submarine setting due
to the greater heat capacity, density and viscosity of water
compared with air.
Submarine arc volcanism is not yet well understood,
however it is very likely that this mode of eruption shows
as much variability in style as subaerial volcanism. Submarine basaltic eruptions range from quiet effusion
(including pillow basalts), and low to moderate intensity
explosive eruptions. Felsic eruptions are also common
along submarine volcanic arcs, from effusive dome
growth and spalling of the pumiceous carapace to low –
moderate explosive cone-forming eruptions and likely
catastrophic caldera-forming eruptions (Fiske et al.,
2001; Smith et al., 2003). Subaqueous eruptions are
poorly understood for several reasons, most notably the
lack of witnessed eruptions, but it is clear from theory
and seafloor exploration with submergence vehicles that
increasing hydrostatic pressure has a profound effect on
limiting explosivity (e.g., Allen and McPhie 2009). In
the last decade remote sensing methods together with
some good luck has led to the observations of both basaltic
and felsic submarine eruptions (e.g., West Mata volcano in
MAGMATISM AT CONVERGENT PLATE BOUNDARIES
403