Chapter
16
Origin and Evolution of
The Ocean Floor
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
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
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. Contain 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 shallow- water 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.
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
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
2. Term ridge is misleading - widths of 1000 to 4000 km give the appearance of broad, elongated
swells
3. Axis of some ridge segments exhibit deep down-faulted structures called rift valleys
4. 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 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
3. Layer 3 - numerous interconnected dikes called sheet dikes
4. Layer 4 - gabbro, in a sequence of rocks called an ophiolite
B. 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
C. 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 dissolves ions of various metals and precipitates them on the seafloor as particlefilled clouds called black smokers
VII. 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 full- fledged 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 result 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
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
B. 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
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