magma body, diking initiates (e.g., Dziak et al., 2007) and
may reach the surface given sufficient overpressure. Event
plumes, which are thermally buoyant volumes of seawater
carrying chemical signatures of volcanic and hydrothermal inputs, likely form at this time by advection of magmatic heat to the seafloor and/or discharge of
hydrothermal fluids stored in the crust (e.g., Baker et al.,
2011, 2012). The amount of magmatic gas within the system and the eruption depth will determine the extent of
explosive activity that might accompany the effusive component of the eruption (e.g., Clague et al., 2009; Helo
et al., 2011).
Eruption detection
This discipline of submarine eruption detection and study
is quite a bit younger than complementary studies of eruptions on land. The first evidence for an eruption at a
mid-ocean ridge came in the form of a hydrothermal event
plume detected along the Cleft segment of the Juan de
Fuca Ridge (JdFR) in 1986 (Baker et al., 1987) – a
follow-up study of the seabed led to the discovery of a
series of pillow mounds up to 75 m high in the same area,
making them the first young deep-sea lava flows of known
age (Chadwick et al., 1991; Embley et al., 1991). In 1991,
the Alvin submersible happened upon the aftermath of a
volcanic eruption at 9
50
0 N on the East Pacific Rise, finding dead and charred tube worms strewn among and under
fresh lava and “snowblower” hydrothermal vents diffusely
spewing white sulfur-rich microbial floc (Haymon et al.,
1993). Radiometric dating subsequently demonstrated
that this discovery occurred just 2–4 weeks after the eruption had stopped (Rubin et al., 1994). The first remotely
detected mid-ocean ridge eruption was in 1993, when scientists at NOAA/PMEL detected an earthquake swarm on
the CoAxial segment of the Juan de Fuca Ridge within the
first month of real-time acoustic monitoring using the US
Navy Sound Surveillance System (SOSUS) (Fox et al.,
1994). Since then, multiple other eruptions have been
detected and described by both remote and direct methods
(see Rubin et al., 2012 for a full listing). For instance, the
real-time hydroacoustic monitoring capability has led to
multiple eruption discoveries and responses in the NE
Pacific (e.g., Perfit and Chadwick, 1998; Dziak et al.,
2007; Dziak et al., 2011; Baker et al., 2012).
Current investigations and controversies
The scientific community has learned a great deal about
mid-ocean ridges over the half century, since the realization that they were centers of volcanism and seafloor
spreading. Yet there is a great deal that is not well known
about both volcanism and magmatism that occur in the
Earth’s most active and prolific volcanic province, so
research continues on multiple fronts. These include the
timescales of magmatic processes, magmatic processes
leading to intrusions, and the architecture of the ridge at
depth, eruption characteristics, and what are the spatial
scales of mantle heterogeneity and how is this
heterogeneity sampled and modified by magmatic activity. This section highlights some of these current topics
of investigation.
Magmatic timescales
Researchers are actively developing and employing novel
methods to unravel the timescales of magmatic processes
at mid-ocean ridges, from the duration of melting to the
rate of magma transport from mantle depths to the crust
and the duration of magma residence in the crust before
eruption. The nuclides of the naturally occurring U and
Th decay series provide the best tracers for these applications because collectively they have a range of half-lives
and chemical characteristics, hence varying recovery rates
to secular equilibrium following chemical perturbations
(where secular equilibrium is the condition of all isotopes
in a chain decaying at the same activity, measured as decay
constant multiplied by the number of atoms present).
Recent U-series radioactive disequilibria studies have
demonstrated that melting, melt extraction, and melt accumulation rates can range from millennial (Elliott and
Spiegelman, 2007; Stracke et al., 2006; Sinton et al.,
2002) to decadal timescales (Rubin et al., 2001;
Bergmanis et al., 2007). For instance, covariation of
short-lived
210
Pb–
226
Ra
disequilibria
(half-life¼ 22 years.) and other compositional attributes of historically erupted MORB at intermediate to fast-spreading
ridges indicate that magmatic conditions can fluctuate rapidly, even between successive eruptions separated by just
a decade or two (Rubin et al, 2005; Bergmanis et al.,
2007). And eruption of lavas with
226
Ra–
230
Th
disequilibria (half-life ¼ 1,600 year.) far away from the
axis of spreading implies eruption ages of one to eight
thousand years even when located on crust of 75–100
kyr spreading age (i.e., assuming steady-state volcanism
and post-eruption movement of lava flows by plate
spreading; Waters et al., 2013 Standish and Sims, 2010).
Despite these successes, it must be noted that the
U-series constraints in the aforementioned studies and
others like them are semi-quantitative and highly dependent upon model parameterizations of melting using
imprecisely known melting rates, melt fractions, melt
porosities, melt flow conditions, lithological diversity,
and the spatial scales of mantle compositional and mineralogical variations, all of which are targets for future
research.
At even finer temporal scales, the
210 Po–
210
Pb dating
method provides ultrahigh-resolution lava eruption ages
(Æ1–3 weeks) by charting the post-eruption in-growth of
initially degassed, volatile
210
Po from erupting magma.
The method has been used on several submarine eruptions
since its first use on the East Pacific Rise in 1994 (Rubin
et al., 1994) to provide temporal constraint on posteruption recovery of hydrothermal systems and biological
colonization and ecological succession on newly
emplaced seafloor (summarized in Rubin et al, 2012).
Recent applications using large sample numbers along
508
MID-OCEAN RIDGE MAGMATISM AND VOLCANISM
may reach the surface given sufficient overpressure. Event
plumes, which are thermally buoyant volumes of seawater
carrying chemical signatures of volcanic and hydrothermal inputs, likely form at this time by advection of magmatic heat to the seafloor and/or discharge of
hydrothermal fluids stored in the crust (e.g., Baker et al.,
2011, 2012). The amount of magmatic gas within the system and the eruption depth will determine the extent of
explosive activity that might accompany the effusive component of the eruption (e.g., Clague et al., 2009; Helo
et al., 2011).
Eruption detection
This discipline of submarine eruption detection and study
is quite a bit younger than complementary studies of eruptions on land. The first evidence for an eruption at a
mid-ocean ridge came in the form of a hydrothermal event
plume detected along the Cleft segment of the Juan de
Fuca Ridge (JdFR) in 1986 (Baker et al., 1987) – a
follow-up study of the seabed led to the discovery of a
series of pillow mounds up to 75 m high in the same area,
making them the first young deep-sea lava flows of known
age (Chadwick et al., 1991; Embley et al., 1991). In 1991,
the Alvin submersible happened upon the aftermath of a
volcanic eruption at 9
50
0 N on the East Pacific Rise, finding dead and charred tube worms strewn among and under
fresh lava and “snowblower” hydrothermal vents diffusely
spewing white sulfur-rich microbial floc (Haymon et al.,
1993). Radiometric dating subsequently demonstrated
that this discovery occurred just 2–4 weeks after the eruption had stopped (Rubin et al., 1994). The first remotely
detected mid-ocean ridge eruption was in 1993, when scientists at NOAA/PMEL detected an earthquake swarm on
the CoAxial segment of the Juan de Fuca Ridge within the
first month of real-time acoustic monitoring using the US
Navy Sound Surveillance System (SOSUS) (Fox et al.,
1994). Since then, multiple other eruptions have been
detected and described by both remote and direct methods
(see Rubin et al., 2012 for a full listing). For instance, the
real-time hydroacoustic monitoring capability has led to
multiple eruption discoveries and responses in the NE
Pacific (e.g., Perfit and Chadwick, 1998; Dziak et al.,
2007; Dziak et al., 2011; Baker et al., 2012).
Current investigations and controversies
The scientific community has learned a great deal about
mid-ocean ridges over the half century, since the realization that they were centers of volcanism and seafloor
spreading. Yet there is a great deal that is not well known
about both volcanism and magmatism that occur in the
Earth’s most active and prolific volcanic province, so
research continues on multiple fronts. These include the
timescales of magmatic processes, magmatic processes
leading to intrusions, and the architecture of the ridge at
depth, eruption characteristics, and what are the spatial
scales of mantle heterogeneity and how is this
heterogeneity sampled and modified by magmatic activity. This section highlights some of these current topics
of investigation.
Magmatic timescales
Researchers are actively developing and employing novel
methods to unravel the timescales of magmatic processes
at mid-ocean ridges, from the duration of melting to the
rate of magma transport from mantle depths to the crust
and the duration of magma residence in the crust before
eruption. The nuclides of the naturally occurring U and
Th decay series provide the best tracers for these applications because collectively they have a range of half-lives
and chemical characteristics, hence varying recovery rates
to secular equilibrium following chemical perturbations
(where secular equilibrium is the condition of all isotopes
in a chain decaying at the same activity, measured as decay
constant multiplied by the number of atoms present).
Recent U-series radioactive disequilibria studies have
demonstrated that melting, melt extraction, and melt accumulation rates can range from millennial (Elliott and
Spiegelman, 2007; Stracke et al., 2006; Sinton et al.,
2002) to decadal timescales (Rubin et al., 2001;
Bergmanis et al., 2007). For instance, covariation of
short-lived
210
Pb–
226
Ra
disequilibria
(half-life¼ 22 years.) and other compositional attributes of historically erupted MORB at intermediate to fast-spreading
ridges indicate that magmatic conditions can fluctuate rapidly, even between successive eruptions separated by just
a decade or two (Rubin et al, 2005; Bergmanis et al.,
2007). And eruption of lavas with
226
Ra–
230
Th
disequilibria (half-life ¼ 1,600 year.) far away from the
axis of spreading implies eruption ages of one to eight
thousand years even when located on crust of 75–100
kyr spreading age (i.e., assuming steady-state volcanism
and post-eruption movement of lava flows by plate
spreading; Waters et al., 2013 Standish and Sims, 2010).
Despite these successes, it must be noted that the
U-series constraints in the aforementioned studies and
others like them are semi-quantitative and highly dependent upon model parameterizations of melting using
imprecisely known melting rates, melt fractions, melt
porosities, melt flow conditions, lithological diversity,
and the spatial scales of mantle compositional and mineralogical variations, all of which are targets for future
research.
At even finer temporal scales, the
210 Po–
210
Pb dating
method provides ultrahigh-resolution lava eruption ages
(Æ1–3 weeks) by charting the post-eruption in-growth of
initially degassed, volatile
210
Po from erupting magma.
The method has been used on several submarine eruptions
since its first use on the East Pacific Rise in 1994 (Rubin
et al., 1994) to provide temporal constraint on posteruption recovery of hydrothermal systems and biological
colonization and ecological succession on newly
emplaced seafloor (summarized in Rubin et al, 2012).
Recent applications using large sample numbers along
508
MID-OCEAN RIDGE MAGMATISM AND VOLCANISM