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Oceanography
An Invitation to Marine Science, 7th
Tom Garrison
Chapter 4
Continental Margins and Ocean
Basins
Chapter 4 Study Plan
• The Ocean Floor Is Mapped by Bathymetry
• Ocean-Floor Topography Varies with
Location
• Continental Margins May Be Active or
Passive
• The Topology of Deep-Ocean Basins Differs
from That of the Continental Margin
• The Grand Tour
Chapter 4 Main Concepts
• Tectonics forces shape the seabed.
• The ocean floor is divided into continental margins and deepocean basins. The continental margins are seaward
extensions of the adjacent continents and are usually
underlain by granite; the deep seabeds have different
features and are usually underlain by basalt.
• Continental margins may be active (earthquakes, volcanoes)
or passive, depending on the local sense of plate movement.
• The mid-ocean ridge system is perhaps Earth’s most
prominent feature. Most of the water of the world ocean
circulates through hot oceanic crust in the ridges about every
10 million years.
• Using remote sensing methods, oceanographers have
mapped the world ocean floor in surprising detail.
The Ocean Floor Is Mapped by
Bathymetry
• The discovery and study of
ocean floor contours is called
Bathymetry.
• 85 B.C.- PosidoniusMediterranean (≈2 km/ 1.25 mi)
– Used a stone on a rope
• 1818- Sir James Clark RossSouth Atlantic (≈16054 ft/
4893m)
• 1870s- HMS Challenger
– Used the same techniques but used
a steam powered winch to raise and
lower the line and weight.
– Confirmed Matthew Maury’s
discovery of the Mid- Atlantic Ridge.
The Ocean Floor Is Mapped by
Bathymetry
Echo sounding
– June 1922- US Navy vessel, USS Stewart, made
the first continuous profile across an ocean basin.
– 1925 to 1927- German ship, Meteor, made 14
profiles of the Atlantic. The winding path of the MidAtlantic Ridge was revealed.
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 (temperature,
pressure, salinity) 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. Because the first
sound of the returning echo is used to sense depth, measurements over deep
depressions are often inaccurate.
Multi-beam Systems Combine
Many Echo Sounders
Multi-beam systems provide more accurate measurements than
echo sounders. Multi-beam systems collect data from up to 121
beams to measure the contours of the ocean floor.
Satellites Can Be Used to Map
Seabed Contours
Satellite altimetry is an indirect way of measuring ocean depth, by measuring 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.
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.
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).
Ocean-Floor Topography Varies
with Location
• What are the two classifications of ocean floor?
Continental Margins
– The submerged outer edge of a continent
– Marked by thicker, less dense granitic crust
Ocean Basin
– The deep seafloor beyond the continental margin
– Marked by the transition to thinner, denser basaltic crust
Ocean-Floor Topography Varies
with Location
Continental margins have several distinct components.
(above) Cross section of a typical ocean basin
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.
Ocean-Floor Topography Varies
with Location
• 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.
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.
Passive margins = continental margins facing the edges of diverging plates
Active margins = continental margins near the edges of converging plates (or near places
where plates are slipping past each other)
Ocean-Floor Topography Varies
with Location
Features of
Earth’s solid
surface shown as
percentages of
the Planet’s total
surface.
Continental Margins May Be Active
or Passive
• Continental margins have several components
• Continental shelf – the shallow, submerged
edge of the continent.
• Continental slope – the transition between the
continental shelf and the deep-ocean floor.
• Shelf break – the abrupt transition from
continental shelf to the continental slope.
• Continental rise – accumulated sediment found
at the base of the continental slope.
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.
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.
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.
Submarine Canyons
(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.
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
Oceanic Ridges Circle the World
An oceanic ridge is a mountainous chain of young,
basaltic rock at an active spreading center of an ocean.
Oceanic Ridges Circle the World
• Stetches across 65,000 km (40,000 miles)
• Can rise 2 km above the seafloor
• Can lead to the formation of islands
– Iceland
– The Azores
– Easter Island
• Slowly spreading centers have a steeper
rise than rapidly cooling ones
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. Shallow earthquakes are common at these faults.
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.
Volcanic Seamounts and Guyots
Project above the Seabed
Seamounts are volcanic projections from the ocean floor that do not rise above sea level. Some form at hotspots,
but most are believed to be submerged in active volcanoes that formed at spreading centers. ≈ 10,000 (1/2 total)
are found in the Pacific.
Guyots are flat-topped seamounts eroded by wave action. They once were tall enough to penetrate the sea
surface, but were eroded by waves.
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. Earth’s most
common landform.
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.
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.
Chapter 4 in Perspective
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?