Mid-Atlantic Ridge
Seibold, Eugen, and Wolfgang H. Berger. The Sea Floor:
An Introduction to Marine Geology. Berlin and
New York: Springer-Verlag, 1982; 3rd rev. and
updated ed., 1996.
Useful Web Sites
United States Geological Survey. “Exploring the Deep
Ocean Floor: Hot Springs and Strange Creatures.”
<http://pubs.usgs.gov/publications/text/exploring.html>.
Related Articles
Atlantic Ocean; Chemosynthesis; Convergent Plate
Boundary; Divergent Plate Boundary; Hydrothermal
Vent; Mid-Ocean Ridge; Pangea; Plate Tectonics; Seafloor
Spreading; Seamount; Transform Fault
Midlittoral Zone
The intertidal, or littoral, zone is the shallow area
of seafloor closest to land that lies between the
highest high and lowest low tides. In areas of the
world where the shoreline is rocky, ecologists
divide the intertidal zone into three bands: the
uppermost supralittoral fringe, the midlittoral
zone, and the lowermost infralittoral fringe. Of
these zones, the midlittoral is the broadest in
extent: It lies between mean high and low tide
and is covered by seawater at least once a day.
Researchers sometimes subdivide the midlittoral
zone into upper, middle, and lower bands
because the zone can be quite broad; sometimes
its upper and lower reaches can experience different physical and environmental conditions.
Because rocky intertidal zones throughout the
world exhibit similar patterns of zonation
(prominent horizontal bands composed of similar types of organisms), the distribution of
organisms can also be used to define the boundaries of these zones. Under this zonation scheme,
the midlittoral zone lies between the highest
point inhabited by barnacles and the highest
point at which large laminarian kelps grow.
The midlittoral zone and its tide pools (areas of
rocky shore that retain seawater when the tide goes
out) can host a great variety of organisms, including anemones, chitons, hermit crabs, limpets,
mussels, snails, and many types of algae. These
organisms spend time both under water and in air
and are exposed to drastic environmental and
physical stresses that accompany the ebb and flow
of the tide. However, the inhabitants of the midlittoral zone are well adapted to face the fluctuations
in temperature and salinity and the threat of desiccation (drying out) and dislodgment by wave
action that they experience on a daily basis. For
example, to prevent desiccation during low tide,
anemones retract their tentacles and barnacles and
mussels close their shells. To avoid dislodgment by
wave action, snails, limpets, and chitons use their
muscular foot to clamp themselves firmly to the
rock surface. Mussels use byssal threads to anchor
themselves into rock crevices.
Lynn L. Lauerman
Further Reading
Nybakken, James W. Marine Biology: An Ecological
Approach, 5th ed. San Francisco: Benjamin
Cummings, 2001.
Ricketts, Edward F., and Jack Calvin. Between Pacific
Tides, 5th ed. rev. by Joel W. Hedgpeth. Stanford,
Calif.: Stanford University Press, 1985.
Related Articles
Infralittoral Fringe; Intertidal; Littoral Zone;
Supralittoral Fringe
Mid-Ocean Ridge
The largest and most continuous mountain
ranges on Earth are the extensive systems of
ridges, rifts, fault zones, and volcanic constructs
that make up the globe-encircling mid-ocean
ridge (MOR). This approximately 60,000-kilometer (37,280-mile)-long submarine chain is
formed as a consequence of seafloor spreading
and marks the location where new oceanic crust
is created and Earth’s tectonic plates diverge at
rates of 1 to 16 centimeters (about 0.4 to 6
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Mid-Ocean Ridge
inches) per year. The undersea mountains and
rift zones that comprise the MOR form as a result
of the complex interactions between magmatic
(i.e., eruptions of lava on the surface of the
seafloor and intrusion of magma at depth within
the oceanic crust and upper mantle) and tectonic
(i.e., faulting, thrusting, and rifting of the solid
portions of the outer layer of Earth) processes.
Although rarely explosive or exposed above sea
level (Iceland is an exception), they are the most
volcanically active regions of our planet; 60 percent of Earth’s magma production occurs at the
MOR. This magmatism is believed to be a consequence of the upwelling of hot mantle material
that partially melts to form basaltic magma as it
rises from depths around 100 kilometers (about
62 miles) or less. Exploration of mid-ocean ridges
by human-operated submersibles (HOVs),
remote-operated vehicles (ROVs), deep-sea cameras, and other remote sensing devices has provided clear evidence that MORs are associated
with volcanic eruptions, earthquakes (seismic
activity), hot springs (hydrothermal vents), and
unique biological communities.
Ridge Segmentation
Although the ridge system is a globe-encircling
feature, it is broken up into segments that vary
from long, first-order segments [about 400 to 600
kilometers (249 to 373 miles)] bounded by major
transform faults or fracture zones, to short,
fourth-order segments [about 10 kilometers (6.2
miles)] defined by subtle bends or offsets in the
linearity of the ridge crest. Ridge axis discontinuities that occur at locally deep areas along ridges
can be long-lived, substantial tectonic features
such as transform faults (first order), smaller [0.5
to 30 kilometers (0.3 to 18.6 miles)] overlapping
spreading centers (second and third order), or
subtle [< 1 kilometer (3280 feet)], short-lived and
mobile deviations from axial linearity called
devals (fourth order). All of these discontinuities
appear to reflect breaks in the volcanic plumbing
systems that feed the axial zone of magmatism
where volcanism is concentrated.
Transform faults are tectonically active zones
that offset major MOR segments at roughly right
angles and are characterized by narrow, deep
troughs where faulting is common, lower crustal
rocks are often exposed, and volcanism and
hydrothermal activities are very rare. The traces
of these fault zones extend as prominent fracture
zones for hundreds to a few thousands of kilometers (about 100 to 2000 miles) beyond the axis of
the MOR. Recent studies suggest that the shallowest and widest portions of ridge segments correspond to areas of robust magmatism, whereas
deep, narrow zones are relatively magma-starved.
Elevated and wide segments of some ridges (e.g.,
south of Iceland, central portion of the Galápagos
Rift, Mid-Atlantic Ridge near the Azores) are
influenced by nearby island formation related to
mantle plumes or hotspots that generate voluminous magma deep within Earth.
Major differences in the morphology, structure, and scale of magmatism along the MOR vary
with the rate of spreading (see figure). Slowly
spreading or diverging plate boundaries [1 to 4
centimeters (< 2 inches) per year total divergence
from the axis], which have low volcanic output,
are dominated by faulting and the brittle rupture
of the crust, whereas fast-spreading boundaries [8
to 16 centimeters (about 3 to 6 inches) per year]
are controlled more by volcanic processes.
Intermediate-spreading-rate [4 to 8 centimeters
(about 1.6 to 3 inches) per year] ridges are transitional in that they may be dominated by volcanism during some time periods and then by
tectonism as magmatic activity wanes. Whereas
slow-spreading ridges typically have deep and
wide fault-bounded valleys (graben) with rugged
flanking morphology, fast-spreading MORs have
elevated, dome-shaped cross sections (rises) with
smoother flanking morphology.
The region along the MOR within which
volcanic eruptions and high-temperature
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Mid-Ocean Ridge
hydrothermal activity (vents, black smokers, white
smokers, sulfide chimneys) are concentrated is
called the neovolcanic zone. The part of the ridge
crest that encompasses the neovolcanic zone has
variously been described as the axial valley, rift
valley, inner valley floor, median valley, elongate
summit depression, axial summit graben, axial
summit caldera, and axial summit collapse
trough. The width of the neovolcanic zone, its
structure, and the style of volcanism within it vary
considerably with spreading rate. The continuous
spectrum of MOR axial morphologies that are
observed is a function of two competing processes
that vary in time and space: volcanic activity,
which results in the construction of relatively
smooth features on the seafloor, and faulting/rifting, which results in the creation of rough, linear
features that range from small fractures to larger
fissures and steep escarpments that can have hundreds of meters (>300 feet) of relief. In all cases,
the neovolcanic zone on the MOR is marked by a
roughly linear depression or trough, similar to
volcanic rift zones on land (e.g., East African Rift
Valley, East Rift Zone on Hawaii), but quite different from the circular craters and calderas associated with typical terrestrial volcanoes (e.g., Crater
Lake). These fault-bounded valleys or troughs are
called graben. Not all MOR volcanism occurs
along the neovolcanic zone. Relatively small [<1
kilometer (<3280 feet) high] near-axis seamounts
are common within approximately 10 to 40 kilometers (6 to 25 miles) of fast and intermediate
spreading ridges, and some off-axis volcanism
may occur up to a few kilometers from the axis as
pillow mounds and ridges, or associated with
faulting and the formation of linear ridges called
abyssal hills that parallel the MOR axis.
Many features on the MOR vary with spreading rate—the width of the neovolcanic zone, the
overall morphology of the ridge crest, the size
and frequency of eruptions, and the morphology
of lava flows—all of which appear to be fundamentally related to whether or not a steady-state
magma reservoir or chamber is maintained.
Where a steady-state magma reservoir exists, volcanism can keep pace with or dominate over tectonism; where magma reservoirs are intermittent,
tectonism tends to dominate.
Slow-Spreading Ridges
Slow-spreading ridges such as the Mid-Atlantic
Ridge (see figure), which extends down the center
of the Atlantic Ocean, have large, discontinuous
axial valleys [8 to 20 kilometers (5 to 12 miles)
wide and 1 to 2 kilometers (3280 to 6560 feet)
deep], and the neovolcanic zone can extend
across the entire axial valley floor [5 to 12 kilometers (3 to 7.5 miles) wide] because magmatism
there is relatively unfocused, both across and
along the axis. Lava morphology on slow ridges is
dominantly pillow lava, which tends to construct
hummocks, hummocky ridges, or small circular
seamounts that often coalesce or overlap one
another to form axial volcanic ridges along the
inner valley floor. The prevalence of small
seamounts in the neovolcanic zone of slowspreading ridges is in marked contrast with fastspreading ridges, where virtually no seamounts
or large constructional forms are found in the
neovolcanic zone, and at intermediate ridges,
where seamounts are only rarely associated with
the neovolcanic zone.
The inner valley floor of slow-spreading MORs
is more faulted and fissured and large earthquakes
(indicating major faulting events) occur more
commonly than on fast-spreading ridges. These
features, rough topography and large axial valleys,
reflect the dominance of extensional faulting and
tectonism over volcanism at slow ridges. Lower
magma supply, thicker crust, and greater cooling
of the crust due to deeper circulation of seawater
all lead to volcanic events being relatively infrequent. In addition, slow-spreading ridges probably
do not maintain long-lived magma reservoirs or
chambers in the shallow crust, so that volcanic
episodes tend to be limited in extent and longevity.
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Mid-Ocean Ridge
Cross-axis bathymetric profiles of selected mid-ocean ridges with different spreading rates. Profiles across fast-spreading
(Southern East Pacific Rise) and slow-spreading (Northern Mid-Atlantic Ridge) ridges show the morphologic contrast between
a narrow axial high and a wide rift valley, whereas intermediate-spreading ridges (Juan de Fuca Ridge) have transitional
features. Location of the axis where volcanism and spreading are commonly focused is shown as an arrow on each profile.
On the other hand, tectonic faulting events are
more frequent than at fast-spreading ridges,
because there is more opportunity for tensional
stresses (i.e., extensional pull) perpendicular to the
ridge to build up over long periods of gradual plate
divergence without interruption from volcanic
intrusions. This faulting leads to the exposure of
plutonic rocks (magmas that have been intruded
into the crust and cooled slowly to form large
crystals) and peridotites (ultramafic rocks rich in
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Mid-Ocean Ridge
magnesium and iron that are residues of mantle
melting) that make up the deep crust and upper
mantle, and the development of strong geologic
and structural asymmetry in the axes at some
slow-spreading ridges. During nonmagmatic periods, tectonic stretching and thinning of the crust
creates numerous low-angle listric or detachment
faults that extend down to a ductile (plastic) zone
in the oceanic crust. This faulting results in the formation of megamullions—broad exposures of
deep-level crustal rocks similar to core complexes
in highly extended continental rift zones.
and thin [<4 centimeters (1.5 inches)] to ropy
and jumbled varieties with chaotically folded and
deformed surfaces that can be tens of centimeters
thick. The textures of some sheet flows are similar to the fluid pahoehoe flows erupted on land
from basaltic volcanoes such as in Hawaii.
Smoother, more bulbous flows known as lobate
lavas are also common at fast-spreading MORs
and typically develop an extensive collapse zone
surrounding the axis where lavas have flooded
and overflowed the axial trough.
Intermediate-Spreading Ridges
Fast-Spreading Ridges
There is no rift valley or prominent axial valley or
graben on fast-spreading MORs, but along most
of the axial crest, there is a narrow, linear depression or trough, which is typically 5 to 40 meters
(16.4 to 130 feet) deep and 40 to 250 meters (130
to 820 feet) wide, that marks the locus of the neovolcanic zone. The most striking characteristic of
the neovolcanic zone at fast-spreading ridges
such as the Southern East Pacific Rise is that it is
very narrow [generally, <2 kilometers (<1.24
miles)], indicating that narrow, elongate igneous
intrusions (dikes) that feed magma to the
seafloor are focused beneath the ridge in a linear
fashion. This focusing of magmatism along the
axis is apparently the direct consequence of the
fast rate of plate spreading, greater magma supply, a shallower and more steady-state magma
reservoir or chamber, and more frequent intrusive events than at slow-spreading centers.
Observations indicate that the axial summit
trough forms as a series of elongate and irregularly shaped collapse features forming over
drained lava ponds and channels over zones of
primary fissuring and diking. There are essentially no seamounts within the neovolcanic zone
at fast-spreading ridges, although lava domes are
known to form at the ends of volcanic segments.
Lavas erupted within the trough are dominantly
fluid sheet flows that vary from remarkably flat
The ridge morphology and neovolcanic zone at
intermediate-spreading ridges have morphological characteristics that are transitional between
those at fast and slow ridges (see figure). However,
individual ridge segments on an intermediate
ridge can be significantly different in character
from one another—some closer to one end member or the other. The morphology of intermediate
ridges is variable, but generally consists of a small
axial valley 1 to 5 kilometers (about 0.6 to 3 miles)
wide, with bounding faults 50 to 1000 meters (164
to 3280 feet) high. Well-studied intermediate
ridges include the Juan de Fuca and Gorda Ridges
in the Northeast Pacific, the Galápagos Ridge, the
northernmost East Pacific Rise at 21°N, and the
Southeast Indian Ridge.
Future Research
MOR studies are trending toward multidisciplinary, multiscale research and long-term monitoring of specific segments. Future focus will be
on determining the relationships between volcanic and tectonic processes and the origin and
development of hydrothermal systems, as well
as on life in the deep sea. This research will rely
on systematic studies of various MORs that
utilize mapping, dating, sampling, geophysics,
and arrays of crustal drill holes. Deep-ocean
oceanographic research in the coming decades
will require the development of new sensors,
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Midwater Trawl
deep-submergence vehicle systems, and seafloor
observatory complexes.
Michael Perfit
Further Reading
Carbotte, Suzanne, and Ken Macdonald. “Comparison
of Seafloor Tectonic Fabric at Intermediate, Fast, and
Superfast Spreading Ridges: Influence of Spreading
Rate, Plate Motions, and Ridge Segmentation on
Fault Patterns.” Journal of Geophysical Research, Vol.
99 (1994), pp. 13,609–13,631.
Fornari, Daniel, and Robert Embley. “Tectonic and
Volcanic Controls on Hydrothermal Processes at the
Mid-ocean Ridge: An Overview Based on NearBottom and Submersible Studies.” In Susan E.
Humphris, ed., Seafloor Hydrothermal Systems:
Physical, Chemical, Biological, and Geological
Interactions. Geophysical Monograph 91. Washington,
D.C.: American Geophysical Union, 1995; pp. 1–46
Fox, Paul J., Nancy Grindlay, and Ken Macdonald. “The
Mid-Atlantic Ridge (31°S–34°30'S): Temporal and
Spatial Variations of Accretionary Processes.” Marine
Geophysical Researches, Vol. 13 (1991), pp. 1–20.
Karson, Jeffrey, and Peter Rona. “Block-Tilting, Transfer
Faults, and Structural Control of Magmatic and
Hydrothermal Processes in the TAG Area, MidAtlantic Ridge 26°N.” Geological Society of America
Bulletin, Vol. 102 (1990), pp. 1635–1645.
Macdonald, Ken. “The Crest of the Mid-Atlantic Ridge:
Models for Crustal Generation Processes and
Tectonics.” In P. R. Vogt and B. E. Tucholke, eds., The
Geology of North America: The Western North Atlantic
Region. Boulder, Colo.: Geological Society of
America, 1986; pp. 51–68
———. “Linkages between Faulting, Volcanism,
Hydrothermal Activity and Segmentation on Fast
Spreading Centers.” In W. R. Buck, P. T. Delaney, J. A.
Karson, and Y. Lagabrielle, eds., Faulting and
Magmatism at Mid-Ocean Ridges. American
Geophysical Monograph 106. Washington, D.C.:
American Geophysical Union, 1998, pp. 27–58.
Mutter, John, and Jeffrey Karson. “Structural Processes
at Slow-Spreading Ridges.” Science, Vol. 257 (1992),
pp. 627–634.
Nicolas, Adolphe. The Mid-Oceanic Ridges: Mountains
below Sea Level. Translated by Thomas Reimer. Berlin
and New York: Springer-Verlag, 1995.
Perfit, Michael, and William Chadwick, Jr. “Magmatism at
Mid-ocean Ridges: Constraints from Volcanological
and Geochemical Investigations.” In W. R. Buck, P. T.
Delaney, J. A. Karson, and Y. Lagabrielle, eds., Faulting
and Magmatism at Mid-ocean Ridges. American
Geophysical Monograph 106. Washington, D.C.:
American Geophysical Union, 1998; pp. 59–115.
Scheirer, Daniel, and Ken Macdonald. “Variation in
Cross-Sectional Area of the Axial Ridge Along the
East Pacific Rise: Evidence for the Magmatic Budget
of a Fast Spreading Center.” Journal of Geophysical
Research, Vol. 98 (1993), pp. 7871–7885.
Smith, Deborah, and Johnson Cann. “Mid-Atlantic Ridge
Volcanic Processes: How Erupting Lava Forms the
Earth’s Anatomy.” Oceanus, Vol. 41 (1998), pp. 11–14.
Related Articles
Abyssal Hill; Gorda Ridge; Hydrothermal Vent;
Juan de Fuca Ridge; Mid-Atlantic Ridge; Plate Tectonics;
Seafloor Spreading
Midwater Trawl
Midwater trawls are used to catch pelagic animals
living in the water column between the upper
ocean and the seabed. Commercial midwater
trawls are used to catch fish such as herring and
mackerel and invertebrates such as krill. They are
nets with large trawl doors rigged to act as
hydroplanes—both keeping the net open and
dragging it down. The trawlers use sonars to locate
the shoals and target them with the net. When successful, such trawls can catch tens of tons of fish on
each tow, although in recent years overfishing and
climate change have reduced the catchability of
many stocks of fish.
When used for scientific purposes, midwater
trawls ideally provide both qualitative and quantitative information about the depth distributions and abundances of invertebrate and
vertebrate species. The aim is to catch a representative sample of the animals from a known depth
range. However, there are problems with this
approach. Large animals tend to be less frequent
than smaller ones and can usually swim faster.
As the depth of fishing increases, the species composition changes and their abundances decline.
289