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Divergent Boundaries: Origin and Evolution of the Ocean Floor
Divergent Boundaries: Origin and Evolution of the Ocean Floor begins with a brief overview of the ocean
floor and methods used for mapping the seafloor. Following an examination of the features associated with
passive and active continental margins, the chapter continues with investigations of trenches, abyssal plains,
and other features of deep-ocean floor. The origin and structure of oceanic crust is discussed followed by
continental rifting and the destruction of oceanic lithosphere. The chapter concludes with a discussion of the
opening and closing of ocean basins in the supercontinent cycle.
Learning Objectives
After reading, studying, and discussing the chapter, students should be able to:
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Briefly explain the technology utilized in mapping the ocean floor.
Discuss the geologic characteristics and features associated with passive and active continental
margins.
Briefly discuss features of the deep-ocean basins, including deep-ocean trenches, abyssal plains,
and seamounts.
Understand the origin of oceanic lithosphere including the role of oceanic ridges and the
mechanism of seafloor spreading.
Discuss the structure and geologic characteristics of oceanic crust.
Briefly discuss continental rifting and evolution of ocean basins.
Discuss subduction and the destruction of oceanic lithosphere.
Chapter Outline___________________________________________________________________
I.
Mapping the ocean floor
A. Depth was originally measured by
lowering weighted lines overboard
B. Echo sounder (also referred to as sonar)
1. Invented in the 1920s
2. Primary instrument for measuring
depth
3. Reflects sound from ocean floor
C. Multibeam sonar
1. Employs an array of sound sources
and listening devices
2.
Obtains a profile of a narrow strip of
seafloor
D. Seismic reflection profiles
1. Low-frequency sounds are produced
by explosions
2. Reflected sound waves reveal
contacts and fault zones on the
seafloor
E. Viewing the ocean floor from space
1. Satellites use radar altimeters to
measure subtle differences of the
ocean surface
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2. Small variations reflect the
gravitational pull of features on the
seafloor
F. Three major topographic units of the
ocean floor
1. Continental margins
2. Deep-ocean basins
3. Mid-ocean ridges
II.
Continental margins
A. Passive continental margins
1. Found along most coastal areas that
surround the Atlantic ocean
2. Not associated with plate boundaries
a. Experience little volcanism and
b. Few earthquakes
3. Features comprising a passive
continental margin
a. Continental shelf
1. Flooded extension of the
continent
2. Varies greatly in width
3. Gently sloping
4. Contains important mineral
deposits
a. Petroleum
b. Natural gas
c. Sand and gravel
5. Some areas are mantled by
extensive glacial deposits
6. Most consist of thick
accumulations of shallowwater sediments
b. Continental slope
1. Marks the seaward edge of the
continental shelf
2. Relatively steep structure
3. Boundary between continental
crust and oceanic crust
c. Continental rise
1. Found in regions where
trenches are absent
2. Continental slope merges into
a more gradual incline – the
continental rise
3. Thick accumulation of
sediment
4. At the base of the continental
slope turbidity currents that
follow submarine canyons
deposit sediment that forms
deep-sea fans
B. Active continental margins
1. Continental slope descends abruptly
into a deep-ocean trench
2. Located primarily around the Pacific
Ocean
3. Accumulations of deformed sediment
and scraps of ocean crust form
accretionary wedges
4. Some subduction zones have little or
no accumulation of sediments
III.
Features of the deep-ocean basin
A. Deep-ocean trench
1. Long, relatively narrow features
2. Deepest parts of ocean
3. Most are located in the Pacific Ocean
4. Sites where moving lithospheric
plates plunge into the mantle
5. Associated with volcanic activity
B. Abyssal plains
1. Likely the most level places on Earth
2. Sites of thick accumulations of
sediment
3. Found in all oceans
C. Seamounts, guyots and oceanic plateaus
1. Seamounts are isolated volcanic
peaks on the seafloor
2. Many seamounts form near oceanic
ridges
3. Seamounts sometimes emerge as an
island
4. May sink and form flat-topped
seamounts called guyots
5. Vast outpourings of basaltic lavas on
the ocean floor create extensive
volcanic structures called oceanic
plateaus
Divergent Boundaries: Origin and Evolution of the Ocean Floor
IV.
Anatomy of the oceanic ridge
A. Broad, linear swells along divergent
plate boundaries are called oceanic
ridges
1. Occupy elevated positions
2. Extensive faulting and earthquakes
3. High heat flow
4. Numerous volcanic structures
B. Oceanic ridge characteristics
1. Longest topographic feature on
Earth’s surface
a. Over 70,000 kilometers (43,000
miles) in length
b. Twenty percent of Earth’s surface
c. Winds through all major oceans
1. Term ridge is misleading – widths of
1000 to 4000 kilometers give the
appearance of broad swells
2. Axis of some ridge segments exhibit
deep down-faulted structures called
rift valleys
3. Portions of the mid-Atlantic ridge
have been studied in considerable
detail
V.
Origin of oceanic lithosphere
A. Seafloor spreading
1. Concept formulated in the early
1960s by Harry Hess
2. Seafloor spreading occurs along
relatively narrow zones, called rift
zones, located at the crests of ocean
ridges
3. As plates move apart, magma wells
up into the newly created fractures
and generates new slivers of oceanic
lithosphere
4. New lithosphere moves from the
ridge crest in a conveyor-belt
fashion
5. Zones of active rifting are 20 to 30
km wide
B. Why are oceanic ridges elevated
1. Primary reason is because newly
created oceanic lithosphere is hot and
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occupies more volume than cooler
rocks
2. As the basaltic crust travels away
from the ridge crest it is cooled by
seawater and because it is moving
away from the source of heat
3. As the lithosphere moves away it
thermally contracts and becomes
more dense
4. Thickness actually increases due to
mechanical properties of the mantle
C. Spreading rates and ridge topography
1. Ridge systems exhibit topographic
differences
2. Topographic differences are
controlled by spreading rates
a. At slow spreading rates (1–5
centimeters per year), a
prominent rift valley develops
along the ridge crest that is
usually
1. 30 to 50 kilometers across
2. 1500–3000 meters deep
3. Rugged
b. At intermediate spreading rates
(5–9 centimeters per year), rift
valleys that develop are
1. Shallow
2. Often less than 200 meters
deep
3. Topographically rather smooth
c. At spreading rates greater than 9
centimeters per year no median
rift valley develops and these
areas are
1. Usually narrow (roughly 10
kilometers wide) topographic
highs
2. Extensively faulted
3. Composed of numerous horsts
and grabens
VI. Structure of the oceanic crust
A. Four distinct layers
1. Layer 1 – sequence of unconsolidated
sediments
2. Layer 2– consisting of pillow lavas
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Lower layer – gabbro, in a
sequence of rocks called an
ophiolite complex
Formation of oceanic crust
1. Basaltic magma originates from
partially melted mantle peridotite
2. Molten rock injected into fractures
above the magma chambers creates
the sheeted dike complex
3. The submarine lava flows chill
quickly and the congealed margin is
forced forward by the accumulating
lava to produce large tube-shaped
protuberances known as pillow
basalts
4. Crystallization of the central magma
chamber forms the coarse-grained
gabbro
Interactions between seawater and
oceanic crust
1. Seawater circulates downward
through the highly fractured crust
2. Basaltic rock is altered by
hydrothermal metamorphism
3. Hydrothermal fluids dissolve ions of
various metals and precipitate them
on the seafloor as particle-filled
clouds called black smokers
4.
B.
C.
VII.
3. Layer 3 – numerous interconnected
dikes called sheet dikes
Continental rifting: The birth of a new
ocean basin
A. Evolution of an ocean basin
1. A new ocean basin begins with the
formation of a continental rift
a. Splits landmasses into two or
more smaller segments
b. Examples include the East
African Rift, Baikal rift, the
Rhine Valley, Rio Grand Rift,
and the Basin and Range
c. Produced by extensional forces
acting on the lithospheric plates
2. The Red Sea is an example of a rift
valley that has lengthened and
deepened in a narrow linear sea
3. If spreading continues the Red Sea
will grow wider and develop an
oceanic ridge similar to the Atlantic
Ocean
4. Not all rift valleys develop into fullfledged spreading centers (e.g., a
failed rift running through the
central United States from Lake
Superior to Kansas)
B. Mechanisms for continental rifting
1. Two mechanisms have been
proposed
2. Mantle plumes and hotspots
a. Regions of hotter than normal
mantle cause decompression
melting that results in a volcanic
region called a hotspot
b. Hot mantle plumes may cause
the overlying crust to dome and
weaken
c. Lifting and stretching of the crust
results in a continental rift
similar to the East African Rift
3. Slab pull and slab suction
a. Subduction of old oceanic
lithosphere may pull a continent
attached to a subducting slab and
create a rift
b. Another possible force might
result from sinking of a cold slab
causing the trench to retreat or
roll back due to flow in the
asthenosphere – this is known as
slab suction
VIII. Destruction of oceanic lithosphere
A. Why oceanic lithosphere subducts
1. Oceanic lithosphere subducts
because its overall density is
greater than the underlying mantle
2. Subduction of older, colder
lithosphere results in descending
angles of nearly 90 degrees
3. Younger, warmer oceanic
lithosphere is more buoyant and
angles of descent are small
Divergent Boundaries: Origin and Evolution of the Ocean Floor
B.
a. The lithospheric slab moves
horizontally beneath a block of
continental lithosphere
b. This phenomenon is called
buoyant subduction
4. Subduction may be prevented or
modified when oceanic crust is
unusually thick because of
seamounts
Subducting plates: The demise of an
ocean basin
1. Plate movements have been
reconstructed for the past 200
million years using magnetic
stripes on the ocean floor
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2. Research indicates that parts, or
even entire oceanic basins, have
been destroyed along subducted
zones
3. The Farallon plate once occupied
much of the eastern Pacific basin
a. Beginning 180 million years ago
the Farallon plate was subducting
beneath the Americas faster than
it was being generated
b. The plate got continually smaller
and now only fragments of the
original plate remain as the Juan
de Fuca, Cocos, and Nazca plates
Answers to the Review Questions
1. The echo sounder pulse travels a distance equal to twice the water depth. In 6 seconds the pulse travels
9000 meters (1500 m/sec X 6 sec = 9000 m); thus the depth (1/2 of 9000 m) is 4500 meters.
2. Satellites equipped with radar altimeters are able to determine features on the seafloor by measuring
subtle differences in elevation on the ocean surface. The satellites bounce microwaves off of the ocean
surface, which is affected by the gravitational pull of seafloor features. Therefore, mountains and ridges
produce elevated areas on the ocean surface while canyons and trenches cause slight depressions.
3. The three major topographic units of the ocean floor are the continental margins, deep-ocean basins, and
oceanic (mid-ocean) ridges.
4. The three major features listed in their order from the coastline seaward are the continental shelf,
continental slope, and continental rise. The shelf is a relatively flat, flooded extension of the continent.
Water depths increase very gradually seaward to the edge of the shelf. The continental slope extends
seaward and downward from the shelf edge and merges downward into the continental rise. It has the
steepest slopes of the three listed features. On trailing, passive continental margins, the shelf is part of the
continent. The slope marks the eroded scarp left from an original continental rift zone and the rise marks a
fan-shaped accumulation of clastic sediments carried down from the shelf and slope by turbidity currents.
5. An active continental margin exhibits a very narrow continental shelf and a relatively steep, narrow,
continental slope that merges into a deep ocean trench along most of the coastline. Along a passive
continental margin, the continental shelf is wide and the shelf is terminated by a well-defined, continental
slope with a wide, gently sloping continental rise at its base. There are no deep ocean trenches and the rise
merges oceanward into an abyssal plain. In terms of plate tectonics, the east coast of North America is a
good example of a passive continental margin and therefore is not very active tectonically. In contrast, the
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west coast of South America represents an active continental margin that is characterized by subduction,
volcanism, and earthquakes; all evidence of a tectonically active region.
6. To a great extent, the Pacific Ocean is rimmed by deep ocean trenches or deep near-shore basins that trap
sediment and prevent turbidity currents from moving farther offshore. The Atlantic Ocean is rimmed
mainly by passive, continental margins and turbidity currents with their sediment load can move far out
into the deeper parts of the ocean basin. The abyssal plain marks the smooth, upper surface of turbidity
current sediments deposited on the original, basaltic bedrock of the ocean floor. For the most part, the
Atlantic Ocean floor is not being subducted, so there has been plenty of geologic time for turbidite
sediments to accumulate.
7. Seamounts are isolated volcanic peaks that rise hundreds of meters above the ocean floor. Over time, if
the volcano may grow large enough to emerge as an island. During the time they exist as islands, some of
the volcanoes are eroded to near sea level by running water and wave action. Over millions of years, the
islands gradually sink below the surface of the water as a moving plate carries them away from their place
of origin. The submerged, flat-topped seamounts are called guyots.
8. Mid-ocean ridges are topographically elevated features that are located near the center of most ocean
basins. The ridges are broken into segments that are offset by large transform faults. Also, along the axis
of some of the segments are deep down-faulted structures called rift valleys. The rift valleys are
characterized by active volcanoes, recent underwater lava flows, and black smokers, which are a type of
hydrothermal vent that spews dark mineral-rich water.
9. Unlike continental mountains where compressional forces fold and metamorphose sedimentary rocks
along convergent boundaries, oceanic ridges form where tensional forces fracture and pull the oceanic
crust apart. As a result of the rifting, the oceanic ridges are composed of layers and piles of basaltic rocks
that are faulted into elongated blocks.
10. Along the oceanic ridges the lithospheric plates separate and hot mantle rocks rise upward to replace the
material that has shifted horizontally. The rising mantle rock experiences a decrease in confining pressure
and partially melts to create the basaltic magmas. This process is called decompression melting.
11. Oceanic ridges are thought to form over rising plumes of hot mantle rock. Partial melting occurs as the
plume approaches the top of the mantle. The basaltic magma rises and solidifies as lava flows and
intrusive gabbro forming new oceanic crust. Temperatures are higher at any given depth under the ridge
than under parts of the seafloor distant from the ridge. Magmas and high-temperature rocks occupy more
space than equivalent masses of cooler, solid rock; thus the seafloor is elevated along the mid-ocean
ridge. Also, the rising-mantle, convection currents may contribute to the “excess” elevation of the ridges.
12. Hydrothermal metamorphism of basalt on the seafloor is accomplished with heated seawater (from
circulating through the hot crust) reacting with the existing minerals, such as olivine and plagioclase
feldspar, to form more stable minerals such as chlorite and calcite. The seawater is heated during this
process and it becomes locally saturated with dissolved ions that are precipitated as metallic sulfide
minerals on the seafloor.
Divergent Boundaries: Origin and Evolution of the Ocean Floor
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13. A black smoker is a particle-filled cloud that results from the metallic-rich solutions along oceanic ridges
that emanate from hot springs on the seafloor. The black smokers mix with the cold seawater and metallic
sulfide minerals are precipitated.
14. Along slow spreading centers (1–5 centimeters per year) such as the Mid-Atlantic Ridge, a prominent rift
valley generally develops along the ridge crest. The rift valleys are 30 to 50 kilometers wide, 1500–2000
meters deep, and typically quite rugged. Along fast spreading centers (5–9 centimeters per year) such as
the East Pacific Rise, no median rift valleys develop. These areas are usually narrow (10 kilometers or
less) and extensively faulted.
15. The four layers of the ocean crust include: 1) a surface layer comprised of unconsolidated sediments; 2) a
second layer composed mainly of basaltic lavas that contain abundant pillow-like structures; 3) a rocky
layer below the lavas where numerous interconnected dikes have a nearly vertical orientation, which is
called the sheeted dike complex; and 4) a lower unit made up mainly of gabbro.
16. The sheeted dike complex forms from magma that is injected into fractures of the oceanic crust. The
lower unit represents crystallization of the central magma chamber as it cools slowly to form gabbro.
17. Several continental rifts exist on Earth, but the East African Rift is perhaps the best example of an active
continental rift zone.
18. Mantle plumes represent large, mushroom-shaped regions of heat that rise through the mantle and
generate voluminous outpourings of basaltic lava at the surface. In some regions, continental rifting is
apparently preceded by buoyant lifting of the crust causing it to dome and weaken. Uplift is followed by
basaltic volcanism both before and during rifting. Thus, mantle plumes are thought to initiate rifting
although other processes are necessary for the dispersal of crustal fragments.
19. Areas such as the Columbia River basalts in the Pacific Northwest or Russia’s Siberian Traps represent
regions dominated by basaltic volcanism associated with hotspots. However, neither area is characterized
by rifting or fragmentation of a continent. Thus, hotspot volcanism does not necessarily always lead to the
breakup of a continent.
20. When initially formed, oceanic crust is warm and buoyant, which explains the elevated position of
oceanic ridges above the deep-ocean basins. Over time the oceanic slab cools and thickens as it moves
away from the ridge. After about 15 million years the oceanic lithosphere is now more dense than the
underlying asthenosphere and subduction can occur.
21. Oceanic lithosphere thickens as it moves away from a ridge due to cooling and an increase in density. As
it continues to move further away, continued cooling thickens the lithosphere such that the thickest
oceanic lithosphere is found where the rocks are the oldest.
22. The Farallon was a large oceanic plate that once occupied much of the eastern Pacific basin. Around 180
million years ago the Americas were moving westward and the Farallon plate was being subducted along
the west coasts of North and South America. The Farallon plate was subducting beneath the Americas
faster than it was being generated so it got smaller and smaller. Today, the Juan de Fuca, Cocos, and
Nazca plates are the remaining fragments of the once much larger Farallon plate.
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Lecture outline, art-only, and animation PowerPoint presentations for each chapter of Earth,
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