Key Terms
abyssal plains (p. 400)
accretionary wedge (p. 398)
active continental
margins (p. 398)
bathymetry (p. 394)
black smokers (p. 407)
continental margins (p. 397)
continental rift (p. 407)
continental rise (p. 398)
continental shelf (p. 397)
continental slope (p. 398)
deep-ocean basin (p. 399)
deep-ocean trenches (p. 399)
deep-sea fan (p. 398)
echo sounders (p. 394)
guyots (p. 400)
mid-ocean ridge (p. 402)
oceanic plateaus (p. 400)
oceanic ridge (p. 402)
ophiolite complex (p. 405)
passive continental
margins (p. 397)
pillow basalts (p. 406)
rift valleys (p. 402)
seamounts (p. 400)
seismic reflection
profile (p. 395)
sheeted dike
complex (p. 405)
sonar (p. 394)
tablemounts (p. 400)
C H A P T E R
S I X T E E N
Origin and Evolution of the Ocean Floor
in Review
Ocean bathymetry is determined using echo sounders and multibeam
sonars, which bounce sonic signals off the ocean floor. Ship-based
receivers record the reflected echoes and accurately measure the
time interval of the signals. With this information, ocean depths
are calculated and plotted to produce maps of ocean floor topography. Recently, satellite measurements of the ocean surface have provided a new type of data that can be used to map the ocean floor.
Oceanographers studying the topography of ocean basins have
delineated three major units: continental margins, deep-ocean
basins, and oceanic (mid-ocean) ridges.
The zones that collectively make up a passive continental
margin include the continental shelf (a gently sloping, submerged
surface extending from the shoreline toward the deep-ocean
basin; the continental slope (the true edge of the continent, which
has a steep slope that leads from the continental shelf into deep
water); and the continental rise (a gradual incline composed of
sediments that have moved downslope from the continental shelf
to the deep-ocean floor).
Most active continental margins are located around the Pacific
Ocean in areas where the leading edge of a continent is overrunning oceanic lithosphere. At these sites, sediment scraped from
the descending oceanic plate is plastered against the continent to
form a collection of sediments called an accretionary wedge. An
active continental margin generally has a narrow continental
shelf, which grades into a deep-ocean trench.
The deep-ocean basin lies between the continental margin and
the oceanic ridge system. Its features include deep-ocean trenches
(long, narrow depressions that are the deepest parts of the ocean
and are located where moving crustal plates descend back into
the mantle); abyssal plains (among the most level places on Earth,
consisting of thick accumulations of sediments that were
deposited atop the low, rough portions of the ocean floor by
turbidity currents); seamounts (volcanic peaks on the ocean floor
that originate near oceanic ridges or in association with volcanic
hot spots); and oceanic plateaus (large, thick, flood basalt
provinces similar to those found on the continents).
Oceanic (mid-ocean) ridges, the sites of seafloor spreading,
are found in all major oceans and represent more than 20 percent of Earth’s surface. They are the most prominent features
in the oceans and form an almost continuous swell that rises 2
to 3 kilometers above the adjacent ocean basin floor. Ridges
are characterized by an elevated position, extensive faulting, and
volcanic structures that have developed on newly formed
oceanic crust. Most of the geologic activity associated with
ridges occurs along a narrow region on the ridge crest, called
the rift zone, where magma from the asthenosphere moves
upward to create new slivers of oceanic crust. The topography
of the oceanic ridge is controlled by the rate of seafloor
spreading.
New oceanic crust is formed in a continuous manner by the
process of seafloor spreading. The upper crust is composed of
pillow lavas of basaltic composition. Below this layer are numerous interconnected dikes (sheeted dike complex) that are underlain
by a thick layer of gabbro. This entire sequence is called an
ophiolite complex.
The development of a new ocean basin begins with the formation of a continental rift similar to the East African Rift. In
those settings where rifting continues, a narrow ocean basin
develops, exemplified by the Red Sea. Eventually, seafloor
spreading creates an ocean basin bordered by rifted continental
margins similar to the present-day Atlantic Ocean. Mechanisms
that drive continental rifting include: hot mantle plumes,
upwelling from shallow levels in the mantle, and forces that
arise from plate motions.
Oceanic lithosphere subducts because its overall density is
greater than the underlying asthenosphere. The subduction of
oceanic lithosphere may result in the destruction of sections of
or entire ocean basins. A classic example is the Farallon plate,
most of which subducted beneath the Americas as these
continents were displaced westward by seafloor spreading
in the Atlantic.
CHAPTER 16 Origin and Evolution of the Ocean Floor
412
abyssal plains (p. 400)
accretionary wedge (p. 398)
active continental
margins (p. 398)
bathymetry (p. 394)
black smokers (p. 407)
continental margins (p. 397)
continental rift (p. 407)
continental rise (p. 398)
continental shelf (p. 397)
continental slope (p. 398)
deep-ocean basin (p. 399)
deep-ocean trenches (p. 399)
deep-sea fan (p. 398)
echo sounders (p. 394)
guyots (p. 400)
mid-ocean ridge (p. 402)
oceanic plateaus (p. 400)
oceanic ridge (p. 402)
ophiolite complex (p. 405)
passive continental
margins (p. 397)
pillow basalts (p. 406)
rift valleys (p. 402)
seamounts (p. 400)
seismic reflection
profile (p. 395)
sheeted dike
complex (p. 405)
sonar (p. 394)
tablemounts (p. 400)
C H A P T E R
S I X T E E N
Origin and Evolution of the Ocean Floor
in Review
Ocean bathymetry is determined using echo sounders and multibeam
sonars, which bounce sonic signals off the ocean floor. Ship-based
receivers record the reflected echoes and accurately measure the
time interval of the signals. With this information, ocean depths
are calculated and plotted to produce maps of ocean floor topography. Recently, satellite measurements of the ocean surface have provided a new type of data that can be used to map the ocean floor.
Oceanographers studying the topography of ocean basins have
delineated three major units: continental margins, deep-ocean
basins, and oceanic (mid-ocean) ridges.
The zones that collectively make up a passive continental
margin include the continental shelf (a gently sloping, submerged
surface extending from the shoreline toward the deep-ocean
basin; the continental slope (the true edge of the continent, which
has a steep slope that leads from the continental shelf into deep
water); and the continental rise (a gradual incline composed of
sediments that have moved downslope from the continental shelf
to the deep-ocean floor).
Most active continental margins are located around the Pacific
Ocean in areas where the leading edge of a continent is overrunning oceanic lithosphere. At these sites, sediment scraped from
the descending oceanic plate is plastered against the continent to
form a collection of sediments called an accretionary wedge. An
active continental margin generally has a narrow continental
shelf, which grades into a deep-ocean trench.
The deep-ocean basin lies between the continental margin and
the oceanic ridge system. Its features include deep-ocean trenches
(long, narrow depressions that are the deepest parts of the ocean
and are located where moving crustal plates descend back into
the mantle); abyssal plains (among the most level places on Earth,
consisting of thick accumulations of sediments that were
deposited atop the low, rough portions of the ocean floor by
turbidity currents); seamounts (volcanic peaks on the ocean floor
that originate near oceanic ridges or in association with volcanic
hot spots); and oceanic plateaus (large, thick, flood basalt
provinces similar to those found on the continents).
Oceanic (mid-ocean) ridges, the sites of seafloor spreading,
are found in all major oceans and represent more than 20 percent of Earth’s surface. They are the most prominent features
in the oceans and form an almost continuous swell that rises 2
to 3 kilometers above the adjacent ocean basin floor. Ridges
are characterized by an elevated position, extensive faulting, and
volcanic structures that have developed on newly formed
oceanic crust. Most of the geologic activity associated with
ridges occurs along a narrow region on the ridge crest, called
the rift zone, where magma from the asthenosphere moves
upward to create new slivers of oceanic crust. The topography
of the oceanic ridge is controlled by the rate of seafloor
spreading.
New oceanic crust is formed in a continuous manner by the
process of seafloor spreading. The upper crust is composed of
pillow lavas of basaltic composition. Below this layer are numerous interconnected dikes (sheeted dike complex) that are underlain
by a thick layer of gabbro. This entire sequence is called an
ophiolite complex.
The development of a new ocean basin begins with the formation of a continental rift similar to the East African Rift. In
those settings where rifting continues, a narrow ocean basin
develops, exemplified by the Red Sea. Eventually, seafloor
spreading creates an ocean basin bordered by rifted continental
margins similar to the present-day Atlantic Ocean. Mechanisms
that drive continental rifting include: hot mantle plumes,
upwelling from shallow levels in the mantle, and forces that
arise from plate motions.
Oceanic lithosphere subducts because its overall density is
greater than the underlying asthenosphere. The subduction of
oceanic lithosphere may result in the destruction of sections of
or entire ocean basins. A classic example is the Farallon plate,
most of which subducted beneath the Americas as these
continents were displaced westward by seafloor spreading
in the Atlantic.
CHAPTER 16 Origin and Evolution of the Ocean Floor
412