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Oceanography
An Invitation to Marine Science
6th Edition
by
Tom Garrison
http://oceanography.brookscole.com/garrison6e
Chapter 4
Continental Margins and
Ocean Basins
Look For The Following Key Ideas
In Chapter 4

New bathymetric devices used to study seabed features include multibeam
echo sounder systems and satellites that use sensitive radar for altimetry.
Their use has revolutionized our understanding of ocean floor formation and
topography.

Seafloor features result from a combination of tectonic activity and the
processes of erosion and deposition.

Near shore, the features of the ocean floor are similar to those of the adjacent
continents because they share the same granitic basement. The transition to
basalt marks the true edge of the continent and divides ocean floors into two
major provinces. The submerged outer edge of a continent is called the
continental margin. The deep-sea floor beyond the continental margin is
properly called the ocean basin.

Features of the continental margins include continental shelves, continental
slopes, submarine canyons, and continental rises.

Features of the deep-ocean basins include oceanic ridges, hydrothermal vents,
abyssal plains and hills, seamounts, guyots, trenches, and island arcs.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
The Ocean Floor Is Mapped by
Bathymetry
The discovery and study of
ocean floor contours is
called Bathymetry.
(left) An illustration from the
Challenger Report (1880).
Seamen are handing the
steam winch used to lower a
weight on the end of a line
to the seabed to find ocean
depth.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Bathymetry:
The Study of Ocean Floor Contours
How did early scientists study the ocean floor?
• Early bathymetric studies were often performed using a
weighted line to measure the depth of the ocean floor.
Advances in Bathymetry
• Echo sounding
• Multibeam Systems
• Satellite Altimetry
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Echo Sounders Bounce Sound off
the Seabed
Echo sounding is a method of measuring seafloor depth using powerful sound pulses.
The accuracy of an echo sounder can be affected by water conditions and bottom
contours. The pulses of sound energy, or “pings,” from the sounder spread out in a
narrow cone as they travel from the ship. When depth is great, the sounds reflect from a
large area of seabed. Because the first sound of the returning echo is used to sense
depth, measurements over deep depressions are often inaccurate.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Multibeam Systems Combine Many
Echo Sounders
Multibeam systems can provide more accurate measurements than
echo sounders do. Multibeam systems collect data from as many as
121 beams to measure the contours of the ocean floor.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Satellites Can Be Used to Map
Seabed Contours
Satellite altimetry measures the sea surface height from orbit. Satellites can bounce
1,000 pulses of radar energy off the ocean surface every second.
(right) Geosat, a U.S. Navy satellite operated from 1985 through 1990,
provided measurements of sea surface height from orbit. Moving above the
ocean surface at 7 kilometers (4 miles) a second, Geosat bounced 1,000
pulses of radar energy off the ocean every second. Height accuracy was
within 0.03 meters (1 inch)!
(below) With the use of satellite altimetry, sea surface levels can be
measured more accurately, showing sea surface distortion. Distortion of
the sea surface above a seabed feature occurs when the extra gravitational
attraction of the feature “pulls” water toward it from the sides, forming a
mound of water over itself.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
The Topography of Ocean Floors
Cross section of the
Atlantic ocean basin
and the continental
United States,
showing the range of
elevations. The
vertical exaggeration
is 100:1.
Although ocean depth
is clearly greater than
the average height of
the continent, the
general range of
contours is similar.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Ocean-Floor Topography Varies
with Location
A graph showing the
distribution of elevations and
depths on Earth.
This graph is not a land-to-sea
profile of Earth, but rather a plot
of the area of Earth’s surface
above any given elevation or
depth below sea level.
Note that more than half of
Earth’s solid surface is at least
3,000 meters (10,000 feet)
below sea level.
The average depth of the
ocean (3,790 meters or 12,430
feet) is much greater than the
average elevation of the
continents (840 meters or
2,760 feet).
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Ocean-Floor Topography Varies
with Location
What are the two classifications of ocean floor?
• Continental Margins – the submerged outer edge of a
continent
• Ocean Basin – the deep seafloor beyond the continental
margin
What are the two types of continental margins?
• Passive margins, also called Atlantic-type margins, face the
edges of diverging tectonic plates. Very little volcanic or
earthquake activity is associated with passive margins.
• Active margins, known as Pacific-type margins, are located
near the edges of converging plates. Active margins are the
site of volcanic and earthquake activity.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Ocean-Floor Topography Varies
with Location
Continental margins have several distinct components.
(above) Cross section of a typical ocean basin flanked by passive continental margins.
The submerged outer edge of a continent is called the continental margin.
The deep-sea floor beyond the continental margin is properly called the ocean basin.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Ocean-Floor Topography Varies
with Location
Features of Earth’s
solid surface shown
as percentages of
the Planet’s total
surface.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Continental Margins May Be Active
or Passive
Continental margins have several components:
• Continental shelf – the shallow, submerged edge of the
continent.
• Continental slopes – the transition between the continental
shelf and the deep-ocean floor.
• Shelf break – the abrupt transition from continental shelf to
the continental slope.
• Continental rises – accumulated sediment found at the base
of the continental slope.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Continental Margins May Be Active
of Passive
Typical continental margins bordering the tectonically active (Pacific-type) and passive
(Atlantic-type) edges of a moving continent. The vertical scale has been exaggerated.
 Continental margins facing the edges of diverging plates are called passive margins.
 Continental margins near the edges of converging plates (or near places where plates
are slipping past each other) are called active margins.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Continental Shelves Are Seaward
Extensions of the Continents
The features of a
passive continental
margin:
(a) Vertical
exaggeration 50:1
(b) No vertical
exaggeration
margin.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Continental Shelves Are Seaward
Extensions of the Continents
Changes in sea level over the last
250,000 years, as traced by data
taken from ocean-floor cores. The
rise and fall of sea level is due
largely to the coming and going of
ice ages – periods of increased
and decreased glaciation,
respectively. Because water that
formed the ice-age glaciers came
from the ocean, sea level dropped.
Point a indicates a low stand of 125 meters (-410 feet) at the
climax of the last ice age some
18,000 years ago. Point b
indicates a high stand of +6 meters
(+19.7 feet) during the last
interglacial period about 120,000
years ago. Point c shows the
present sea level. Sea level
continues to rise as we emerge
from the last ice age and enter an
accelerating period of global
warming.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Submarine Canyons Form at the
Junction between Continental Shelf and
Continental Slope
Submarine canyons are a feature of some continental margins. They
cut into the continental shelf and slope, often terminating on the deepsea floor in a fan-shaped wedge of sediment.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Submarine Canyons Form at the
Junction between Continental Shelf and
Continental Slope
(right) A turbidity current
flowing down a submerged
slope off the island of
Jamaica. The propeller of a
submarine caused the
turbidity current by
disturbing sediment along
the slope
Avalanche-like sediment
movement caused when
turbulence mixes
sediments into water above
a sloping bottom are called
turbidity currents.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
The Topology of Deep-Ocean Basins
Differs from That of the Continental
Margin
What are some features of the deep-ocean floor?
•
Oceanic Ridges
•
Hydrothermal Vents
•
Abyssal Plains and Abyssal Hills
•
Seamounts and Guyots
•
Trenches and Island Arcs
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Oceanic Ridges Circle the World
An oceanic ridge is a mountainous chain of young,
basaltic rock at an active spreading center of an ocean.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Oceanic Ridges Circle the World
Transform faults and fracture zones along an oceanic ridge
Transform faults are fractures along which lithospheric plates slide horizontally past one
another. Transform faults are the active part of fracture zones.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Hydrothermal Vents Are Hot
Springs on Active Oceanic Ridges
Hydrothermal vents are sites where superheated water containing dissolved
minerals and gases escapes through fissures, or vents. Cool water (blue
arrows) is heated as it descends toward the hot magma chamber, leaching
sulfur, iron, copper, zinc, and other materials from the surrounding rocks. The
heated water (red arrows) returning to the surface carries these elements
upward, discharging them at hydrothermal springs on the seafloor.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Volcanic Seamounts and Guyots
Project above the Seabed
Seamounts are volcanic projections from the ocean floor that do not rise above sea level. Flat-topped
seamounts eroded by wave action are called guyots
Abyssal hills are flat areas of sediment-covered ocean floor found between the continental margins
and oceanic ridges. Abyssal hills are small, extinct volcanoes or rock intrusions near the oceanic
ridges.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Trenches and Island Arcs Form in
Subduction Zones
Trenches are arc-shaped depressions in the ocean floor caused by the subduction of a
converging ocean plate.
Most trenches are around the edges of the active Pacific. Trenches are the deepest
places in Earth’s crust, 3 to 6 kilometers (1.9 to 3.7 miles) deeper than the adjacent basin
floor. The ocean’s greatest depth is the Mariana Trench where the depth reaches 11,022
meters (36,163 miles) below sea level.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Trenches and Island Arcs Form in
Subduction Zones
The Mariana Trench
(a) Comparing the
Challenger Deep and
Mount Everest at the
same scale shows
that the deepest part
of the Mariana Trench
is about 20% deeper
than the mountain is
high.
(b) The Mariana
Trench shown without
vertical exaggeration.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Chapter 4 Summary
In this chapter you learned how difficult it has been to discover the shape of the
seabed. Even today, the surface contours of Mars are better known than those of our
ocean floor.
We now know that seafloor features result from a combination of tectonic activity and the
processes of erosion and deposition. The ocean floor can be divided into two regions:
continental margins and deep-ocean basins. The continental margin, the relatively
shallow ocean floor nearest the shore, consists of the continental shelf and the
continental slope. The continental margin shares the structure of the adjacent continents,
but the deep ocean floor away from land has a much different origin and history.
Prominent features of the deep ocean basins include rugged oceanic ridges, flat abyssal
plains, occasional deep trenches, and curving chains of volcanic islands. The processes
of plate tectonics, erosion, and sediment deposition have shaped the continental margins
and ocean basins.
In the next chapter you will learn that nearly all the ocean floor is blanketed with
sediment. With the exception of the spreading centers themselves, the broad shoulders
of the oceanic ridge systems are buried according to their age – the older the seabed, the
greater the sediment burden. Some oceanic crust near the trailing edges of plates may
be overlain by sediments more than 1,500 meters (5,000 feet) thick. Sediments have
been called the “memory of the ocean.” The memory, however, is not a long one. Before
continuing, can you imagine why that is so?
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
End of Chapter 4