[CHAPTER 16: ORIGINAND EVOLUTION OF THE OCEAN FLOOR] EXAM 3 MATERIAL
TERMS:
Abyssal plain: deep, incredibly flat features; relatively featureless topography
due to thick accumulations of sediment that have buried the rugged ocean
floor. Most abundant in Atlantic Ocean because it has few trenches to trap
sediment.
Accretionary wedge: Sediments from the ocean floor and pieces of oceanic
crust are scraped from the descending oceanic plate and plastered against the
edge of the overriding continent; leads to accumulation of sediments along a
continental margin
Active continental margin: located mainly around Pacific Ocean where
oceanic lithosphere is being subducted beneath the leading edge of a
continent; narrow margin consisting of highly deformed sediments that were
scraped from the descending lithospheric slab
Passive Continental margin: include continental shelf, continental slope,
continental rise; found along most of the costal areas surrounding Atlantic and
Indian oceans; NOT situated along active plate boundary so have very little
volcanism/earthquake activity.
Continental rift: an elongated depression in which the entire thickness of the
lithosphere has been deformed; beginning of new ocean basin.
Continental rise: In regions where trenches do not exist, the steep continental
slope merges into a more gradual incline known as continental rise; thick
accumulation of sediment that moved downslope from the continental shelf to
the deep-ocean floor
Continental shelf: a gently sloping submerged surface extending from the
shoreline toward the deep-ocean basin; flooded extension of the continents
Continental slope: marks seaward edge of continental shelf; relatively steep
structure that marks the boundary between a continental crust and oceanic
crust
Deep-ocean basin: between the continental margin and the oceanic ridge;
includes abyssal plains, seamounts, guyots, deep ocean trenches, and oceanic
plateaus
[CHAPTER 16: ORIGINAND EVOLUTION OF THE OCEAN FLOOR] EXAM 3 MATERIAL
Deep-ocean trench: long, relatively narrow creases in the seafloor that form the
deepest parts of the ocean; mainly along margins of Pacific Ocean
Mid-ocean ridge: most prominent features in the ocean; site of seafloor
spreading; well-developed divergent plate boundaries where the seafloor is
elevated forming a broad linear swell
Ophiolite complex: The sequence of rocks composing the oceanic crust; Layers
1-4: Peridotite (upper mantle  layered gabbro  gabbro  sheeted dike
complex  basaltic pillow lavas  deep sea sediment
Rift valley: deep down-faulted structures along the axis of some segments of
the oceanic ridge system
Seamount: submarine volcanoes; form linear chains
REVIEW QUESTIONS
What are the three major topographic provinces of the ocean floor?
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Continental margins
Deep-ocean basins
Oceanic (mid-ocean) ridges
List the three major features that comprise a passive continental margin. Which
of these features is considered a flooded extension of the continent? Which one
has the steepest slope?
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Three major features of passive continental margin:
o Continental shelf
 Avg inclination 1/10 degree
o Continental slope
 5-25 degree inclination
o Continental rise
 1/3 degree inclination
Continental Slope has steepest slope
Describe the differences between active and passive continental margin.
Which of these features is considered a flooded extension of the continent?
Which one has the steepest slope?
[CHAPTER 16: ORIGINAND EVOLUTION OF THE OCEAN FLOOR] EXAM 3 MATERIAL
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Passive margins are NOT situated along an active plate boundary and
experience little volcanism and few earthquakes
Active continental margins occur where oceanic lithosphere is being
subducted beneath the edge of a continent
o Continental slope descends abruptly into a deep-ocean trench
Continental shelf is a flooded extension of the continent
Continental Slope has steepest slope
Describe the differences between active and passive continental margins. Be
sure to include how various features relate to plate tectonics and give a
geographic example of each type of margin.
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Passive Continental Margins
o Continental Shelf: gently sloping submerged surface extending
from shoreline toward deep-ocean basin
 Mostly along East Coast of U.S.
o Continental Slope: Marks seaward edge of continental shelf; steep
structure that marks boundary between continental crust and
oceanic crust
o Continental Rise: regions where trenches do not exist; steep
continental slope merges into gradual incline or continental rise.
 Thick accumulation of sediment moved downward from
continental shelf to ocean floor by turbidity currents that flow
down submarine canyons
Active Continental Margins
o Areas (mostly around Pacific Ocean) where oceanic lithosphere is
being subducted beneath leading edge of continent
 Acretionary Wedge: pieces of oceanic crust scraped from
descending oceanic plate and plastered against edge of
overriding continent
 Large accretionary wedge found along northern coast
of Japan’s Honshu Island
Why area abyssal plains more extensive on the floor of the Atlantic than on the
floor of the pacific?
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The Atlantic Ocean has few trenches to act as traps for sediment
carried down the continental slope
How does a flat-topped Seamount or guyot form?
[CHAPTER 16: ORIGINAND EVOLUTION OF THE OCEAN FLOOR] EXAM 3 MATERIAL
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Some volcanic structures are lowered near sea level by weathering
and erosion; islands will gradually sink and disappear below the
water surface as the moving plate slowly carries them away from
the elevated oceanic ridge or hot spot where they originated
Briefly describe the oceanic ridge system.
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Oceanic ridges are broad linear swells of elevated sea floor along
well-developed divergent boundaries.
Characterized by an elevated position, extensive faulting,
earthquakes, high heat flow, numerous volcanic structures
Form where tensional forces fracture and pull ocean crust apart
Consists of layers and piles of newly formed basaltic rocks that have
been faulted into elongated blocks that are buoyantly uplifted.
What is the source of magma for seafloor spreading?
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Below the ridge axis, the lithospheric plates separate; solid hot mantle
rocks rise upward to replace the material that has shifted horizontally
Decompression melting – as rock rises it experiences a decrease in
confining pressure and may undergo melting without the addition of heat
o This is how magma is generated along ridge axis
o Partial melting  basaltic magma
What is the primary reason for the elevated position of the oceanic ridge
system?
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Newly created oceanic lithosphere is hot, occupies more volume, and is
therefore less desne than cooler rocks of the deep-ocean basin. As newly
formed basaltic crust travels away from the ridge crest, it is cooled from
above as seawater circulates through the pore spaces and fractures in
the rock (also cools b/c moving away from zone of upwelling/main
source of heat). Lithosphere gradually cools, contracts, becomes more
dense
Briefly describe the four layers of the ocean crust.
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Layer 1: Upper layer comprised of unconsolidated sediments
o Thin near axes of oceanic ridges but thick near continents
Layer 2: Rock unit composed mainly of basaltic lavas that contain
abundant pillowlike structures called pillow basalts
[CHAPTER 16: ORIGINAND EVOLUTION OF THE OCEAN FLOOR] EXAM 3 MATERIAL
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Layer 3: Middle rocky layer made of numerous interconnected dikes
having nearly vertical orientation – sheeted dike complex
o Dikes former pathways where magma rose to feed lava flows on
ocean floor.
Layer 4: made up mainly of gabbro, the coarse-grained equivalent of
basalt, which crystallized in a magma chamber below the ridge axis
LAYERS MAKE UP OPHIOLITE COMPLEX
Name a place that exemplifies a continental rift.
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East African Rift
o Consists of several interconnected rift valleys
o Exemplifies initial stage in break up of a continent
Red Sea
o Continental rift becomes a narrow linear sea with an outlet to the
ocean
Atlantic Ocean
o Passive continental margins
o Rifted continental margins move slowly away from one another
cooling, contracting, subsiding,
What happened to the farallon plate? Name the remnants of this plate.
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The farallon plate was sbducting beneath the Americas faster than it was
being generated so it got smaller and smaller
Surface area decreased, it broke into smaller pieces some of which
subducted entirely.
Remaining fragments are Juan de Fuca, Cocos, and Nazca plates
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