and mound-like topography (e.g., White et al., 2002).
At slower-spreading rates, tectonic forces (faulting and
plate separation) dominate over volcanic construction,
leading to the common axial valley morphology of the
ridge, with volcanic accretion commonly localized along
smaller ridges within this valley (e.g., Searle et al., 2010;
Yeo et al., 2012) and fault block-dominated rift valley
walls. At the very slowest-spreading rates, volcanic activity appears to be discontinuous along the highly tectonized
ridge axis, leading to exposures of volcanic, plutonic, and
even mantle rocks on the seafloor (e.g., Michael et al.,
2003; Cannat et al., 2008).
Eruption products
The different styles of submarine volcanic eruptions and
the deposits they produce are controlled by magma chemistry and physical properties, magma eruption rate, and
physical conditions at the volcanic vent (e.g., Gregg and
Fink, 1995; Perfit and Chadwick, 1998). The size, volume, thickness, and dispersal of both effusive and pyroclastic volcanic deposits provide key information about
the conditions of the eruption(s) that produced them. Both
effusive and explosive eruption styles are observed at
mid-ocean ridges, although effusive lava flows are by far
the dominant volcanic product of mid-ocean ridge eruptions. Lava flow thickness, run out, and surface morphology reflect variations in magma viscosity, effusion rate,
local slope, topographic obstructions, and the sequence
of emplacement events during individual eruptions (see
discussion in Rubin et al., 2012 and references therein).
Submarine lava flow morphologies are typically classified
by the length scale of the quenched-crust units that collectively make up a lava flow, which themselves form as a
function of flow rate and cooling rate. Common different
types of lava deposits are known as pillow, lobate, and
sheet lavas (see Figure 3). Common methods of detection
and mapping of these flow types involve direct or remote
visual observation from submersibles and towed camera
systems (see Rubin et al., 2012), although a recent development employs high-resolution sonar data from autonomous underwater vehicles that have been automatically
classified by surface geometry and texture to generate
detailed lava flow morphology maps over relatively large
areas of seabed (McClinton et al., 2013). Volumetrically
minor deposits of lapilli- to ash-sized explosive fragments
also occur throughout the depth range of observations in
the deep sea (e.g., Clague et al., 2009; Helo et al., 2011).
Eruption frequency and duration
Just like volcanoes on land, the periodicity of volcanism at
mid-ocean ridges reflects the rate of magma input to the
volcano, the buildup of tectonic stress, and conditions
within the crustal magma reservoir. Marine scientists currently lack the information to determine eruption frequency, size, and duration for all but a very small
number of submarine mid-ocean ridge volcanoes
(reviewed in Rubin et al., 2012). Individual eruptions
can last just days to perhaps a year. In terms of eruption
frequency, many mid-ocean ridge volcanoes likely operate
at a quasi-steady state (e.g., Wadge, 1982) in which eruption volume and repose (the time between eruptions) correlate positively as the result of a relatively constant
magma flux. Using such a steady-state assumption and
calculating magma supply rate from the spreading rate
and an assumption of constant crustal thickness, the
smallest average eruption repose interval (~10 year)
should occur at the fastest-spreading rates (for instance,
on the Southern East Pacific Rise; Sinton et al., 2002 or
at Axial Volcano, a ridge-centered hotspot on the Juan de
Fuca ridge with anomalously high magma supply). An
average repose interval of roughly 1,000 years should
occur on slower-spreading ridges like the Mid-Atlantic
Ridge (see Perfit and Chadwick, 1998; Sinton et al.,
2002 and references therein). The small number of direct
eruption observations (there are two submarine
mid-ocean ridge sites that have been observed to have
erupted twice) and studies utilizing volcano-scale geological mapping and deposit dating via high-precision radiometric or paleomagnetic chronological methods indicate
that volcanic eruption frequency and repose are likely
not steady state at the individual eruption scale, but that
averaged over several eruptions, they do occur at roughly
the predicted steady-state interval (e.g., Rubin et al., 2012;
Bowles et al., 2014).
Eruption dynamics
Our understanding of the dynamics and mechanisms of
submarine volcanic eruptions is informed by a relatively
recent but growing body of direct and remote sensing
observations. Intermediate- to fast-spreading mid-ocean
ridges (i.e., the only places we have such observations)
are dominated by short duration, high eruption rate events,
or clusters of events (see Rubin et al., 2012 and references
therein). Eruption rate variations produce a range of lava
morphologies. Lava flows will advance for hours or days
during each eruption pulse (e.g., Soule et al., 2012). Eruption pulses may continue if the pressure release was
incomplete and/or pressure continues to build in the
crustal melt lens by recharge. Widespread diffuse venting
of hydrothermal fluids continues through the carapace of a
new lava flow well after the eruption wanes. Focused flow
hydrothermal activity (i.e., from chimneys) can persist at a
site over multiple eruptions, although the character of
fluids and chimneys at any given vent site is dominated
by subterranean plumbing systems that are highly sensitive to changes that occur during eruptions (e.g., Von
Damm, 2004).
Decadal-scale studies at several mid-ocean ridge sites
with high melt supply reveal that magma chambers are
persistent features over eruptive cycles (e.g., Carbotte
et al., 2012). Magma chamber pressures increase over time
(e.g., Chadwick et al., 2006), triggering increased seismicity rates along the ridge over several years (e.g., Tolstoy
et al., 2006). When sufficient pressure has built in the
506
MID-OCEAN RIDGE MAGMATISM AND VOLCANISM
At slower-spreading rates, tectonic forces (faulting and
plate separation) dominate over volcanic construction,
leading to the common axial valley morphology of the
ridge, with volcanic accretion commonly localized along
smaller ridges within this valley (e.g., Searle et al., 2010;
Yeo et al., 2012) and fault block-dominated rift valley
walls. At the very slowest-spreading rates, volcanic activity appears to be discontinuous along the highly tectonized
ridge axis, leading to exposures of volcanic, plutonic, and
even mantle rocks on the seafloor (e.g., Michael et al.,
2003; Cannat et al., 2008).
Eruption products
The different styles of submarine volcanic eruptions and
the deposits they produce are controlled by magma chemistry and physical properties, magma eruption rate, and
physical conditions at the volcanic vent (e.g., Gregg and
Fink, 1995; Perfit and Chadwick, 1998). The size, volume, thickness, and dispersal of both effusive and pyroclastic volcanic deposits provide key information about
the conditions of the eruption(s) that produced them. Both
effusive and explosive eruption styles are observed at
mid-ocean ridges, although effusive lava flows are by far
the dominant volcanic product of mid-ocean ridge eruptions. Lava flow thickness, run out, and surface morphology reflect variations in magma viscosity, effusion rate,
local slope, topographic obstructions, and the sequence
of emplacement events during individual eruptions (see
discussion in Rubin et al., 2012 and references therein).
Submarine lava flow morphologies are typically classified
by the length scale of the quenched-crust units that collectively make up a lava flow, which themselves form as a
function of flow rate and cooling rate. Common different
types of lava deposits are known as pillow, lobate, and
sheet lavas (see Figure 3). Common methods of detection
and mapping of these flow types involve direct or remote
visual observation from submersibles and towed camera
systems (see Rubin et al., 2012), although a recent development employs high-resolution sonar data from autonomous underwater vehicles that have been automatically
classified by surface geometry and texture to generate
detailed lava flow morphology maps over relatively large
areas of seabed (McClinton et al., 2013). Volumetrically
minor deposits of lapilli- to ash-sized explosive fragments
also occur throughout the depth range of observations in
the deep sea (e.g., Clague et al., 2009; Helo et al., 2011).
Eruption frequency and duration
Just like volcanoes on land, the periodicity of volcanism at
mid-ocean ridges reflects the rate of magma input to the
volcano, the buildup of tectonic stress, and conditions
within the crustal magma reservoir. Marine scientists currently lack the information to determine eruption frequency, size, and duration for all but a very small
number of submarine mid-ocean ridge volcanoes
(reviewed in Rubin et al., 2012). Individual eruptions
can last just days to perhaps a year. In terms of eruption
frequency, many mid-ocean ridge volcanoes likely operate
at a quasi-steady state (e.g., Wadge, 1982) in which eruption volume and repose (the time between eruptions) correlate positively as the result of a relatively constant
magma flux. Using such a steady-state assumption and
calculating magma supply rate from the spreading rate
and an assumption of constant crustal thickness, the
smallest average eruption repose interval (~10 year)
should occur at the fastest-spreading rates (for instance,
on the Southern East Pacific Rise; Sinton et al., 2002 or
at Axial Volcano, a ridge-centered hotspot on the Juan de
Fuca ridge with anomalously high magma supply). An
average repose interval of roughly 1,000 years should
occur on slower-spreading ridges like the Mid-Atlantic
Ridge (see Perfit and Chadwick, 1998; Sinton et al.,
2002 and references therein). The small number of direct
eruption observations (there are two submarine
mid-ocean ridge sites that have been observed to have
erupted twice) and studies utilizing volcano-scale geological mapping and deposit dating via high-precision radiometric or paleomagnetic chronological methods indicate
that volcanic eruption frequency and repose are likely
not steady state at the individual eruption scale, but that
averaged over several eruptions, they do occur at roughly
the predicted steady-state interval (e.g., Rubin et al., 2012;
Bowles et al., 2014).
Eruption dynamics
Our understanding of the dynamics and mechanisms of
submarine volcanic eruptions is informed by a relatively
recent but growing body of direct and remote sensing
observations. Intermediate- to fast-spreading mid-ocean
ridges (i.e., the only places we have such observations)
are dominated by short duration, high eruption rate events,
or clusters of events (see Rubin et al., 2012 and references
therein). Eruption rate variations produce a range of lava
morphologies. Lava flows will advance for hours or days
during each eruption pulse (e.g., Soule et al., 2012). Eruption pulses may continue if the pressure release was
incomplete and/or pressure continues to build in the
crustal melt lens by recharge. Widespread diffuse venting
of hydrothermal fluids continues through the carapace of a
new lava flow well after the eruption wanes. Focused flow
hydrothermal activity (i.e., from chimneys) can persist at a
site over multiple eruptions, although the character of
fluids and chimneys at any given vent site is dominated
by subterranean plumbing systems that are highly sensitive to changes that occur during eruptions (e.g., Von
Damm, 2004).
Decadal-scale studies at several mid-ocean ridge sites
with high melt supply reveal that magma chambers are
persistent features over eruptive cycles (e.g., Carbotte
et al., 2012). Magma chamber pressures increase over time
(e.g., Chadwick et al., 2006), triggering increased seismicity rates along the ridge over several years (e.g., Tolstoy
et al., 2006). When sufficient pressure has built in the
506
MID-OCEAN RIDGE MAGMATISM AND VOLCANISM