the ridge accumulate and focus so that they feed a
relatively narrow region (a few kilometers) along the
axis of the ridge (Figure 1).
During ascent from the mantle and cooling in the
crust, primary mantle melts are subjected to a variety
of physical and chemical processes such as fractional
crystallization, magma mixing, crustal assimilation,
and thermogravitational diffusion that modify and
differentiate the original melt composition. Consequently, primary melts are unlikely to erupt on the
seafloor without undergoing some modification.
Picritic lavas and magnesian glasses thought to represent likely primary basalts have been recovered
from a few ocean floor localities; commonly in
transform faults (Table 1). MgO contents in these
basalts range from B10 wt% to over 15 wt% and the
lavas typically contain significant amounts of olivine
crystals. Based on comparisons with high-pressure
melting experiments of likely mantle peridotites, the
observed range of compositions may reflect variations in source composition and mineralogy (in part
controlled by pressure), depth and percentage melting (largely due to temperature differences), and/or
types of melting (e.g., batch vs. fractional).
Ocean Floor Volcanism and
Construction of the Crust
Oceanic crust formed at spreading ridges is relatively
homogeneous in thickness and composition compared to continental crust. On average, oceanic crust
is 6–7 km thick and basaltic in composition as
compared to the continental crust which averages
35–40 km thick and has a roughly andesitic composition. The entire thickness of the oceanic crust has
not been sampled in situ and therefore the bulk
composition has been estimated based on investigations of ophiolites (fragments of oceanic and
back-arc crust that have been thrust up on to the
continents), comparisons of the seismic structure of
the oceanic crust with laboratory determinations of
seismic velocities in known rock types, and samples
recovered from the ocean floor by dredging, drilling,
submersibles, and remotely operated vehicles.
Rapid cooling of MORB magmas when they come
into contact with cold sea water results in the formation of glassy to finely crystalline pillows, lobate
flows, or sheet flows (Figure 1). These lava flows
typically have an B0.5–1 cm-thick outer rind of
glass and a fine-grained, crystalline interior containing only a few percent of millimeter-sized crystals
of olivine, plagioclase, and more rarely clinopyroxene in a microscopic matrix of the same minerals.
MORB lavas erupt, flow, and accumulate to form the
uppermost volcanic layer (Seismic Layer 2A) of
ocean crust (Figure 2). Magmas that do not reach the
seafloor cool more slowly with increasing depth
forming intrusive dikes at shallow levels (0.5–3 km)
in the crust (layer 2B) and thick bodies of coarsely
crystalline gabbros and cumulate ultramafic rocks at
the lowest levels (3–7 km) of the crust (layer 3)
(Figure 2).
Although most magma delivered to a MOR is focused within the neovolcanic zone, defined by the
axial summit collapse trough or axial valley, off-axis
volcanism and near-axis seamount formation appear
to add significant volumes of material to the uppermost crust formed along ridge crests. In some portions of the fast spreading East Pacific Rise, off-axis
eruptions appear to be related to syntectonic volcanism and the formation of abyssal hills. Near-axis
seamount formation is common along both the East
Pacific Rise and medium spreading rate Juan de Fuca
Ridge. Even in areas where there are abundant offaxis seamounts they may add only a few percent to
the volume of the extrusive crust. More detailed
studies of off-axis sections of ridges are needed before accurate estimates of their contribution to the
total volume of the oceanic crust can be made.
Oceanic transform faults are supposed to be plate
boundaries where crust is neither created nor destroyed, but recent mapping and sampling indicate
that magmatism occurs in some transform domains.
Volcanism occurs in these locales either at short,
intratransform spreading centers or at localized
eruptive centers within shear zones or relay zones
between the small spreading centers.
Mid-ocean Ridge Basalt Composition
Ocean floor lavas erupted along mid-ocean ridges are
low-potassium tholeiites that can range in composition from picrites with high MgO contents to
ferrobasalts and FeTi basalts containing lower MgO
and high concentrations of FeO and TiO 2 , and even
to rare, silica-enriched lavas known as icelandites,
ferroandesites and rhyodacites (Table 1). In most
areas, the range of lava compositions, from MgO-rich
basalt to FeTi basalt and ultimately to rhyodacite,
is generally ascribed to the effects of shallow-level
(low-pressure) fractional crystallization in a subaxial
magma chamber or lens (Figure 2). A pronounced
iron-enrichment trend with decreasing magnesium
contents (related to decreasing temperature) in suites
of genetically related lavas is, in part, what classifies
MORB as tholeiitic or part of the tholeiitic magmatic
suite (Figure 3).
MID-OCEAN RIDGE GEOCHEMISTRY AND PETROLOGY 357
relatively narrow region (a few kilometers) along the
axis of the ridge (Figure 1).
During ascent from the mantle and cooling in the
crust, primary mantle melts are subjected to a variety
of physical and chemical processes such as fractional
crystallization, magma mixing, crustal assimilation,
and thermogravitational diffusion that modify and
differentiate the original melt composition. Consequently, primary melts are unlikely to erupt on the
seafloor without undergoing some modification.
Picritic lavas and magnesian glasses thought to represent likely primary basalts have been recovered
from a few ocean floor localities; commonly in
transform faults (Table 1). MgO contents in these
basalts range from B10 wt% to over 15 wt% and the
lavas typically contain significant amounts of olivine
crystals. Based on comparisons with high-pressure
melting experiments of likely mantle peridotites, the
observed range of compositions may reflect variations in source composition and mineralogy (in part
controlled by pressure), depth and percentage melting (largely due to temperature differences), and/or
types of melting (e.g., batch vs. fractional).
Ocean Floor Volcanism and
Construction of the Crust
Oceanic crust formed at spreading ridges is relatively
homogeneous in thickness and composition compared to continental crust. On average, oceanic crust
is 6–7 km thick and basaltic in composition as
compared to the continental crust which averages
35–40 km thick and has a roughly andesitic composition. The entire thickness of the oceanic crust has
not been sampled in situ and therefore the bulk
composition has been estimated based on investigations of ophiolites (fragments of oceanic and
back-arc crust that have been thrust up on to the
continents), comparisons of the seismic structure of
the oceanic crust with laboratory determinations of
seismic velocities in known rock types, and samples
recovered from the ocean floor by dredging, drilling,
submersibles, and remotely operated vehicles.
Rapid cooling of MORB magmas when they come
into contact with cold sea water results in the formation of glassy to finely crystalline pillows, lobate
flows, or sheet flows (Figure 1). These lava flows
typically have an B0.5–1 cm-thick outer rind of
glass and a fine-grained, crystalline interior containing only a few percent of millimeter-sized crystals
of olivine, plagioclase, and more rarely clinopyroxene in a microscopic matrix of the same minerals.
MORB lavas erupt, flow, and accumulate to form the
uppermost volcanic layer (Seismic Layer 2A) of
ocean crust (Figure 2). Magmas that do not reach the
seafloor cool more slowly with increasing depth
forming intrusive dikes at shallow levels (0.5–3 km)
in the crust (layer 2B) and thick bodies of coarsely
crystalline gabbros and cumulate ultramafic rocks at
the lowest levels (3–7 km) of the crust (layer 3)
(Figure 2).
Although most magma delivered to a MOR is focused within the neovolcanic zone, defined by the
axial summit collapse trough or axial valley, off-axis
volcanism and near-axis seamount formation appear
to add significant volumes of material to the uppermost crust formed along ridge crests. In some portions of the fast spreading East Pacific Rise, off-axis
eruptions appear to be related to syntectonic volcanism and the formation of abyssal hills. Near-axis
seamount formation is common along both the East
Pacific Rise and medium spreading rate Juan de Fuca
Ridge. Even in areas where there are abundant offaxis seamounts they may add only a few percent to
the volume of the extrusive crust. More detailed
studies of off-axis sections of ridges are needed before accurate estimates of their contribution to the
total volume of the oceanic crust can be made.
Oceanic transform faults are supposed to be plate
boundaries where crust is neither created nor destroyed, but recent mapping and sampling indicate
that magmatism occurs in some transform domains.
Volcanism occurs in these locales either at short,
intratransform spreading centers or at localized
eruptive centers within shear zones or relay zones
between the small spreading centers.
Mid-ocean Ridge Basalt Composition
Ocean floor lavas erupted along mid-ocean ridges are
low-potassium tholeiites that can range in composition from picrites with high MgO contents to
ferrobasalts and FeTi basalts containing lower MgO
and high concentrations of FeO and TiO 2 , and even
to rare, silica-enriched lavas known as icelandites,
ferroandesites and rhyodacites (Table 1). In most
areas, the range of lava compositions, from MgO-rich
basalt to FeTi basalt and ultimately to rhyodacite,
is generally ascribed to the effects of shallow-level
(low-pressure) fractional crystallization in a subaxial
magma chamber or lens (Figure 2). A pronounced
iron-enrichment trend with decreasing magnesium
contents (related to decreasing temperature) in suites
of genetically related lavas is, in part, what classifies
MORB as tholeiitic or part of the tholeiitic magmatic
suite (Figure 3).
MID-OCEAN RIDGE GEOCHEMISTRY AND PETROLOGY 357