MID-OCEAN RIDGE GEOCHEMISTRY AND
PETROLOGY
M. R. Perfit, Department of Geological Sciences,
University of Florida, Gainsville, FL, USA
Copyright & 2001 Elsevier Ltd.
Introduction
The most volcanically active regions of our planet
are concentrated along the axes of the globe, encircling mid-ocean ridges. These undersea mountain
ranges, and most of the oceanic crust, result from the
complex interplay between magmatic (i.e., eruptions
of lavas on the surface and intrusion of magma at
depth) and tectonic (i.e., faulting, thrusting, and
rifting of the solid portions of the outer layer of the
earth) processes. Magmatic and tectonic processes
are directly related to the driving forces that cause
plate tectonics and seafloor spreading. Exploration
of mid-ocean ridges by submersible, remotely operated vehicles (ROV), deep-sea cameras, and other
remote sensing devices has provided clear evidence of
the effects of recent magmatic activity (e.g., young
lavas, hot springs, hydrothermal vents and plumes)
along these divergent plate boundaries. Eruptions are
rarely observed because of their great depths and
remote locations. However, over 60% of Earth’s
magma flux (approximately 21 km
3 year
À1
) currently
occurs along divergent plate margins. Geophysical
imaging, detailed mapping, and sampling of midocean ridges and fracture zones between ridge segments followed by laboratory petrologic and
geochemical analyses of recovered rocks provide us
with a great deal of information about the composition and evolution of the oceanic crust and the
processes that generate mid-ocean ridge basalts
(MORB).
Mid-ocean ridges are not continuous but rather
broken up into various scale segments reflecting
breaks in the volcanic plumbing systems that feed the
axial zone of magmatism. Recent hypotheses suggest
that the shallowest and widest portions of ridge
segments correspond to robust areas of magmatism,
whereas deep, narrow zones are relatively magmastarved. The unusually elevated segments of some
ridges (e.g., south of Iceland, central portion of the
Galapagos Rift, Mid-Atlantic Ridge near the Azores)
are directly related to the influence of nearby mantle
plumes or hot spots that result in voluminous
magmatism.
Major differences in the morphology, structure,
and scales of magmatism along mid-ocean ridges
vary with the rate of spreading. Slowly diverging
plate boundaries, which have low volcanic output,
are dominated by faulting and tectonism whereas
fast-spreading boundaries are controlled more by
volcanism. The region along the plate boundary
within which volcanic eruptions and high-temperature hydrothermal activity are concentrated is called
the neovolcanic zone. The width of the neovolcanic
zone, its structure, and the style of volcanism within
it, vary considerably with spreading rate. In all cases,
the neovolcanic zone on mid-ocean ridges is marked
by a roughly linear depression or trough (axial
summit collapse trough, ASCT), similar to rift zones
in some subaerial volcanoes, but quite different from
the circular craters and calderas associated with
typical central-vent volcanoes. Not all mid-ocean
ridge volcanism occurs along the neovolcanic zone.
Relatively small (o1 km high), near-axis seamounts
are common within a few tens of kilometers of fast
and intermediate spreading ridges. Recent evidence
also suggests that significant amounts of volcanism
may occur up to 4 km from the axis as off-axis
mounds and ridges, or associated with faulting and
the formation of abyssal hills.
Lava morphology on slow spreading ridges is
dominantly bulbous, pillow lava (Figure 1A), which
tends to construct hummocks (o50 m high, o500 m
diameter), hummocky ridges (1–2 km long), or small
circular seamounts (10s–100s of meters high and
100s–1000s of meters in diameter) that commonly
coalesce to form axial volcanic ridges (AVR) along
the valley floor of the axial rift zone. On fast
spreading ridges, lavas are dominantly oblong, lobate flows and fluid sheet flows that vary from remarkably flat and thin (o4 cm) to ropy and jumbled
varieties (Figure 1). Although the data are somewhat
limited, calculated volumes of individual flow units
that have been documented on mid-ocean ridges
show an inverse exponential relationship to spreading rate, contrary to what might be expected. The
largest eruptive units are mounds and cones in the
axis of the northern Mid-Atlantic Ridge whereas the
smallest units are thin sheet/lobate flows on the East
Pacific Rise. Morphologic, petrologic, and structural
studies of many ridge segments suggest they evolve
355
PETROLOGY
M. R. Perfit, Department of Geological Sciences,
University of Florida, Gainsville, FL, USA
Copyright & 2001 Elsevier Ltd.
Introduction
The most volcanically active regions of our planet
are concentrated along the axes of the globe, encircling mid-ocean ridges. These undersea mountain
ranges, and most of the oceanic crust, result from the
complex interplay between magmatic (i.e., eruptions
of lavas on the surface and intrusion of magma at
depth) and tectonic (i.e., faulting, thrusting, and
rifting of the solid portions of the outer layer of the
earth) processes. Magmatic and tectonic processes
are directly related to the driving forces that cause
plate tectonics and seafloor spreading. Exploration
of mid-ocean ridges by submersible, remotely operated vehicles (ROV), deep-sea cameras, and other
remote sensing devices has provided clear evidence of
the effects of recent magmatic activity (e.g., young
lavas, hot springs, hydrothermal vents and plumes)
along these divergent plate boundaries. Eruptions are
rarely observed because of their great depths and
remote locations. However, over 60% of Earth’s
magma flux (approximately 21 km
3 year
À1
) currently
occurs along divergent plate margins. Geophysical
imaging, detailed mapping, and sampling of midocean ridges and fracture zones between ridge segments followed by laboratory petrologic and
geochemical analyses of recovered rocks provide us
with a great deal of information about the composition and evolution of the oceanic crust and the
processes that generate mid-ocean ridge basalts
(MORB).
Mid-ocean ridges are not continuous but rather
broken up into various scale segments reflecting
breaks in the volcanic plumbing systems that feed the
axial zone of magmatism. Recent hypotheses suggest
that the shallowest and widest portions of ridge
segments correspond to robust areas of magmatism,
whereas deep, narrow zones are relatively magmastarved. The unusually elevated segments of some
ridges (e.g., south of Iceland, central portion of the
Galapagos Rift, Mid-Atlantic Ridge near the Azores)
are directly related to the influence of nearby mantle
plumes or hot spots that result in voluminous
magmatism.
Major differences in the morphology, structure,
and scales of magmatism along mid-ocean ridges
vary with the rate of spreading. Slowly diverging
plate boundaries, which have low volcanic output,
are dominated by faulting and tectonism whereas
fast-spreading boundaries are controlled more by
volcanism. The region along the plate boundary
within which volcanic eruptions and high-temperature hydrothermal activity are concentrated is called
the neovolcanic zone. The width of the neovolcanic
zone, its structure, and the style of volcanism within
it, vary considerably with spreading rate. In all cases,
the neovolcanic zone on mid-ocean ridges is marked
by a roughly linear depression or trough (axial
summit collapse trough, ASCT), similar to rift zones
in some subaerial volcanoes, but quite different from
the circular craters and calderas associated with
typical central-vent volcanoes. Not all mid-ocean
ridge volcanism occurs along the neovolcanic zone.
Relatively small (o1 km high), near-axis seamounts
are common within a few tens of kilometers of fast
and intermediate spreading ridges. Recent evidence
also suggests that significant amounts of volcanism
may occur up to 4 km from the axis as off-axis
mounds and ridges, or associated with faulting and
the formation of abyssal hills.
Lava morphology on slow spreading ridges is
dominantly bulbous, pillow lava (Figure 1A), which
tends to construct hummocks (o50 m high, o500 m
diameter), hummocky ridges (1–2 km long), or small
circular seamounts (10s–100s of meters high and
100s–1000s of meters in diameter) that commonly
coalesce to form axial volcanic ridges (AVR) along
the valley floor of the axial rift zone. On fast
spreading ridges, lavas are dominantly oblong, lobate flows and fluid sheet flows that vary from remarkably flat and thin (o4 cm) to ropy and jumbled
varieties (Figure 1). Although the data are somewhat
limited, calculated volumes of individual flow units
that have been documented on mid-ocean ridges
show an inverse exponential relationship to spreading rate, contrary to what might be expected. The
largest eruptive units are mounds and cones in the
axis of the northern Mid-Atlantic Ridge whereas the
smallest units are thin sheet/lobate flows on the East
Pacific Rise. Morphologic, petrologic, and structural
studies of many ridge segments suggest they evolve
355