6–7 km plus or minus a few kilometers and composed of
basalt (pillow basalt, sheeted feeder dikes), gabbro
(lower crust), and the mantle rock (variations of peridotite). The Moho discontinuity is seismologically detectable almost ubiquitously in oceans, but its petrologic
characteristics dependent on the spreading rate and postulated to be the gabbro to peridotite boundary or an alteration front of serpentinized peridotite remain to be
verified.
Since the seafloor spreading hypothesis was verified on
Leg 3 of DSDP, the physics and chemistry of how the oceanic crusts are formed at ocean ridges and age and cool
away from the ridge axes forming the uppermost layer of
the lithosphere have been an important focus of ocean drilling. The difference in the seafloor spreading rate is currently considered a major factor in determining the
petrologic structure of oceanic crusts. Where magma supply is sufficient to balance the spreading rate, the crustal
layering is expected to be vertically stratified. Crusts
formed at fast spreading rate (>~100 mm/year) are in this
category. Crusts formed at slow spreading rate
(<~50 mm/year) accompany tectonic adjustments such
as exhumation of deeper crustal or mantle rocks, as the
magma supply seems insufficient to accommodate the
spreading rate and to cause larger lateral heterogeneity
than crusts from fast spreading ridges.
Deep penetration coring has been made since DSDP at
Sites 332 (Leg 37, mid-Atlantic), 395 and 920–922 (Legs
45, 153, mid-Atlantic), 418 (Legs 52–53, Bermuda Rise),
504 (Legs 69, 70, 83, 111, 137, 140, 148, E. Pacific, Nazca
Plate), 735 (Legs 118, 176, SW Indian Ridge), 801 (Legs
129, 185, W. Pacific), 1256 (Leg 206, Exps. 309, 312,
335, E. Pacific, Cocos Plate), and 1309 (Exps. 304, 305,
mid-Atlantic, Atlantis Massif). Hole 332B was the first
multiple reentry hole to deepen the hole with new bits.
Holes 735B and 1309D recovered gabbroic rocks
representing the lower crust. Hole 1256D, although not
the deepest, reached gabbros at 1,407 m below seafloor
(Wilson et al., 2006). These with drilling attempts to
shallower penetrations into the igneous crusts contributed
in observing the lateral change away from and in parallel
to the ridge segmentation (e.g., Leg 209, mid-Atlantic
Ridge) or test hypotheses of finding exhumed lower crust
or mantle rocks (Leg 147, Exp. 345, Hess Deep). Other
attempts include drilling into continental rifted margins
(Leg 103, Galicia; Legs 149, 173, Iberia; Leg 210, Newfoundland; Leg 104, Norway; Leg 152, Greenland) to
investigate the continental breakup process from
continent-ocean transition structure and their relation to
magma feed at the time of rifting.
Through ocean drilling combined with surface geophysical surveys, the understanding of petrologic evolution of oceanic crust has progressed to infer its relation
to spreading rates, seismic models (e.g., Layer 2/Layer
3 transition; Detrick et al., 1994; Swift et al., 2009), interactions with fluids, and inclusions of microbes (Mason
et al., 2010). To further the understanding of oceanic
crusts in relation to mantle convection and plate tectonics,
the current state of sampling is insufficient depth-wise and
also plate age-wise. The feasibility of deep penetration
into the lower crust and reaching the mantle across the
Moho boundary is being seriously studied with the advent
of the riser drillship Chikyu.
Fluids
The notion of the dark and motionless world of the seafloor and below is completely revised by numerous seafloor investigations and drilling expeditions. Fluids are
seeping up and down not only at hydrothermal vents but
also at plate subduction zones and possibly elsewhere providing energy for microbes to thrive and massive sulfide
minerals to deposit or serpentinizing mantle peridotites.
The physical behavior of the fluid fluxes is being studied
at and around ridges by borehole observatories including
chemical and biological parameters (Davis et al., 2000,
Davis et al., 2000; IODP Exps. 336, 337).
Marine gas hydrates have become important scientific
targets in order to understand their formation, distribution,
and role in affecting the environment from the viewpoints
of natural resource potential, methane storage, and control
in relation to sub-seafloor ecosystem and geohazard risk.
Observatories
Ocean-drilled boreholes have long been perceived to provide a stable environment for global seismic observatories
(e.g., Byrne et al., 1987; Stephen et al., 2003). The global
seismic network has progressively advanced the understanding of earthquakes and the entire internal structure
of the Earth as the frequency bandwidth and dynamic
range widened. 3-D tomography of the Earth and complex
fault motion description are direct products of the network
data. The weakness of the global network has been the
inhomogeneous distribution of the seismic stations
because of the lack of stations in the oceans (71 % of
Earth’s surface). There have been several attempts to overcome this (Suyehiro et al., 2006) utilizing ocean drilling
holes. Technically, a broadband digital seismic observatory with equal fidelity as on-land observatories can be
established in the boreholes on solid basement rock that
provide more quiet observational environment than on
seafloor or in soft sediments. Feeding power for long term
and getting data back in real time are the remaining
challenges.
While complementing the global resolution of the network, ocean sites uniquely contribute in obtaining new
data at upper mantle depths where on-land stations do
not have the resolving power. For example, a new implication is made on the thickness of the ocean lithosphere
suggesting that the lithosphere-asthenosphere boundary
is not a simple rheological change produced by the cooling
of the lithosphere (Kawakatsu et al., 2009) or a detailed
anisotropic structure of the ocean upper mantle
(Shinohara et al., 2008). Regional-scale seafloor cable networks are being established to cover tectonically active
areas such as off Japan or off North America to obtain
550
OCEAN DRILLING
basalt (pillow basalt, sheeted feeder dikes), gabbro
(lower crust), and the mantle rock (variations of peridotite). The Moho discontinuity is seismologically detectable almost ubiquitously in oceans, but its petrologic
characteristics dependent on the spreading rate and postulated to be the gabbro to peridotite boundary or an alteration front of serpentinized peridotite remain to be
verified.
Since the seafloor spreading hypothesis was verified on
Leg 3 of DSDP, the physics and chemistry of how the oceanic crusts are formed at ocean ridges and age and cool
away from the ridge axes forming the uppermost layer of
the lithosphere have been an important focus of ocean drilling. The difference in the seafloor spreading rate is currently considered a major factor in determining the
petrologic structure of oceanic crusts. Where magma supply is sufficient to balance the spreading rate, the crustal
layering is expected to be vertically stratified. Crusts
formed at fast spreading rate (>~100 mm/year) are in this
category. Crusts formed at slow spreading rate
(<~50 mm/year) accompany tectonic adjustments such
as exhumation of deeper crustal or mantle rocks, as the
magma supply seems insufficient to accommodate the
spreading rate and to cause larger lateral heterogeneity
than crusts from fast spreading ridges.
Deep penetration coring has been made since DSDP at
Sites 332 (Leg 37, mid-Atlantic), 395 and 920–922 (Legs
45, 153, mid-Atlantic), 418 (Legs 52–53, Bermuda Rise),
504 (Legs 69, 70, 83, 111, 137, 140, 148, E. Pacific, Nazca
Plate), 735 (Legs 118, 176, SW Indian Ridge), 801 (Legs
129, 185, W. Pacific), 1256 (Leg 206, Exps. 309, 312,
335, E. Pacific, Cocos Plate), and 1309 (Exps. 304, 305,
mid-Atlantic, Atlantis Massif). Hole 332B was the first
multiple reentry hole to deepen the hole with new bits.
Holes 735B and 1309D recovered gabbroic rocks
representing the lower crust. Hole 1256D, although not
the deepest, reached gabbros at 1,407 m below seafloor
(Wilson et al., 2006). These with drilling attempts to
shallower penetrations into the igneous crusts contributed
in observing the lateral change away from and in parallel
to the ridge segmentation (e.g., Leg 209, mid-Atlantic
Ridge) or test hypotheses of finding exhumed lower crust
or mantle rocks (Leg 147, Exp. 345, Hess Deep). Other
attempts include drilling into continental rifted margins
(Leg 103, Galicia; Legs 149, 173, Iberia; Leg 210, Newfoundland; Leg 104, Norway; Leg 152, Greenland) to
investigate the continental breakup process from
continent-ocean transition structure and their relation to
magma feed at the time of rifting.
Through ocean drilling combined with surface geophysical surveys, the understanding of petrologic evolution of oceanic crust has progressed to infer its relation
to spreading rates, seismic models (e.g., Layer 2/Layer
3 transition; Detrick et al., 1994; Swift et al., 2009), interactions with fluids, and inclusions of microbes (Mason
et al., 2010). To further the understanding of oceanic
crusts in relation to mantle convection and plate tectonics,
the current state of sampling is insufficient depth-wise and
also plate age-wise. The feasibility of deep penetration
into the lower crust and reaching the mantle across the
Moho boundary is being seriously studied with the advent
of the riser drillship Chikyu.
Fluids
The notion of the dark and motionless world of the seafloor and below is completely revised by numerous seafloor investigations and drilling expeditions. Fluids are
seeping up and down not only at hydrothermal vents but
also at plate subduction zones and possibly elsewhere providing energy for microbes to thrive and massive sulfide
minerals to deposit or serpentinizing mantle peridotites.
The physical behavior of the fluid fluxes is being studied
at and around ridges by borehole observatories including
chemical and biological parameters (Davis et al., 2000,
Davis et al., 2000; IODP Exps. 336, 337).
Marine gas hydrates have become important scientific
targets in order to understand their formation, distribution,
and role in affecting the environment from the viewpoints
of natural resource potential, methane storage, and control
in relation to sub-seafloor ecosystem and geohazard risk.
Observatories
Ocean-drilled boreholes have long been perceived to provide a stable environment for global seismic observatories
(e.g., Byrne et al., 1987; Stephen et al., 2003). The global
seismic network has progressively advanced the understanding of earthquakes and the entire internal structure
of the Earth as the frequency bandwidth and dynamic
range widened. 3-D tomography of the Earth and complex
fault motion description are direct products of the network
data. The weakness of the global network has been the
inhomogeneous distribution of the seismic stations
because of the lack of stations in the oceans (71 % of
Earth’s surface). There have been several attempts to overcome this (Suyehiro et al., 2006) utilizing ocean drilling
holes. Technically, a broadband digital seismic observatory with equal fidelity as on-land observatories can be
established in the boreholes on solid basement rock that
provide more quiet observational environment than on
seafloor or in soft sediments. Feeding power for long term
and getting data back in real time are the remaining
challenges.
While complementing the global resolution of the network, ocean sites uniquely contribute in obtaining new
data at upper mantle depths where on-land stations do
not have the resolving power. For example, a new implication is made on the thickness of the ocean lithosphere
suggesting that the lithosphere-asthenosphere boundary
is not a simple rheological change produced by the cooling
of the lithosphere (Kawakatsu et al., 2009) or a detailed
anisotropic structure of the ocean upper mantle
(Shinohara et al., 2008). Regional-scale seafloor cable networks are being established to cover tectonically active
areas such as off Japan or off North America to obtain
550
OCEAN DRILLING