Earth Science, 6e
The Ocean Floor
Chapter - 9
A-The vast world ocean (page 260)
Earth is often referred to as the blue planet
• 71% of Earth’s surface is represented by oceans
and marginal seas
• Continents and islands comprise the remaining
29
Oceans represent 97% of the Hydrosphere
Land
Hemisphere
&
Water
Hemisphere
Figure 9.1
The vast world ocean
Four main ocean basins
• Pacific Ocean - the largest and deepest
• Atlantic Ocean – about half the size of the
Pacific Ocean
• Indian Ocean – slightly smaller than the
Atlantic
• Arctic Ocean – about 7 percent the size of the
Pacific
The oceans of Earth
Figure 13.2 B
Mapping the ocean floor
(page 265)
Bathymetry – measurement of ocean depths
and the shape or topography of the ocean
floor
Echo sounder (also referred to as sonar)
• Invented in the 1920s
• Primary instrument for measuring depth
• Reflects sound from ocean floor
Echo sounder and
multibeam sonar
Figure 9.11
Three major topographic units
of the Ocean floor
1. Continental margins (Active and Passive)
• Active margins
• Passive margins
2. Ocean basin floor
3. Mid-ocean ridge
Major topographic divisions of
the North Atlantic Ocean
Figure 9.14
I - Continental margins
Passive continental margins
• Found along most coastal areas that surround
the Atlantic Ocean
• Not associated with plate boundaries
• Experience little volcanism and
• Few earthquakes
• Wider sandy beaches
Features of a passive
continental margin
Figure 13.9
Passive continental margins
• Comprises three features:
– Continental Shelf
– Continental Slope
– Continental Rise
Passive continental margins
• Features comprising a passive continental
margin
1. Continental shelf
• Flooded extension of the continent
• Contains oil and important mineral deposits
Passive continental margins
• Features comprising a passive continental
margin
2. Continental slope
• Marks the seaward edge of the continental shelf
• Relatively steep structure
• Submarine canyons and turbidity currents
Turbidity currents
Figure 13.10
Continental margins
Passive continental margins
• Features comprising a passive continental
margin
3. Continental rise
• Found in regions where trenches are absent
• Continental slope merges into a more gradual
incline – the continental rise
• Thick accumulation of sediment
Active continental margins
• Continental slope descends abruptly into a
deep-ocean trench
• Located primarily around the Pacific Ocean
• Accumulations of deformed sediment and
scraps of ocean crust form accretionary wedges
• Some subduction zones have little or no
accumulation of sediments (narrow beaches)
An active continental margin
Figure 9.18
Active continental margins
Deep-ocean trenches
•
•
•
•
Long, relatively narrow features
Deepest parts of ocean
Most are located in the Pacific Ocean
Sites where moving lithospheric plates plunge
into the mantle
• Associated with volcanic activity
• Volcanic islands arcs (Japan)
• Continental volcanic arcs (Andes, Cascades mts)
II - Ocean basin floor
 Abyssal plains
• Likely the most level places on Earth
• Sites of thick accumulations of sediment
• Found in all oceans
 Seamounts and guyots
• Isolated volcanic peaks
• Many form near oceanic ridges
Ocean basin floor
Seamounts and guyots
• May emerge as an island
• May sink and form flat-topped seamounts
called guyots or tablemounts
III - Mid-ocean ridge
•
•
•
•
•
Mid-ocean ridge
Characterized by
An elevated position
Extensive faulting
Numerous volcanic structures that have
developed on newly formed crust
Mid-ocean ridge
• Interconnected ridge system is the longest
topographic feature on Earth’s surface
• Over 70,000 kilometers (43,000 miles) in length
• Twenty-three percent of Earth’s surface
• Winds through all major oceans
• Along the axis of some segments are deep
down faulted structures called rift valleys
Mid-ocean ridge
• Consist of layer upon layer of basaltic rocks
that have been faulted and uplifted
• Mid-Atlantic Ridge has been studied more
thoroughly than any other ridge system
Seafloor sediments
Ocean floor is mantled with sediment
Sources
• Turbidity currents
• Sediment that slowly settles to the bottom from
above
Thickness varies
• Thickest in trenches – accumulations may
approach 10 kilometers
Seafloor sediments
Thickness varies
• Pacific Ocean – about 600 meters or less
• Atlantic Ocean – from 500 to 1000 meters thick
Mud is the most common sediment on the
deep-ocean floor
Seafloor sediments
Types of seafloor sediments
1. Terrigenous sediment
•
•
•
•
Material weathered from continental rocks
Virtually every part of the ocean receives some
Fine particles remain suspended for a long time
Oxidation often produces red and brown colored
sediments
Seafloor sediments
Types of seafloor sediments
2. Biogenous sediments
• Shells and skeletons of marine animals and plants
• Most common are calcareous oozes produced from
microscopic organisms that inhabit warm surface
waters
• Siliceous oozes composed of skeletons of diatoms
and radiolarians
• Phosphate rich materials derived from the bones,
teeth, and scales of fish and other marine organisms
Seafloor sediments
Types of seafloor sediments
3. Hydrogenous sediment
• Minerals that crystallize directly from seawater
• Most common types include
• Manganese nodules
• Calcium carbonates
• Metal sulfides
• Evaporites
Distribution of marine sediments
Figure 13.17
Seafloor sediments
Distribution
• Coarse terrigenous deposits dominate
continental margin areas
• Fine-grained terrigenous material is common in
deeper areas of the ocean basin
• Hydrogenous sediment comprises only a small
portion of deposits in the ocean
• There are a few places where very little
sediment accumulates (Mid-ocean ridges)
Resources from the seafloor
Energy resources
• Oil and gas
• Gas hydrates
Other resources
• Sand and gravel
• Evaporative salts
• Manganese nodules
End of Section 1
B - Ocean Chemistry (Page 261)
• Salinity is the total amount of material
dissolved in water. It is the ration of the
mass of dissolved substances to the mass of
the water. It is expressed in per cent or part
per thousand.
Origin of Sea salt
Sources of sea salts
• Chemical weathering of rocks
• Outgassing – gases from volcanic eruptions
Processes affecting seawater salinity
• Variations in salinity are a consequence of
changes in the water content of the solution
Composition of seawater
 80% of the ocean depth is salty, cold and
dark
 Seawater consists of about 33‰ to 38‰
(by weight) dissolved minerals
 Salinity is the total amount of solid
material dissolved in water
• Typically expressed in parts-per-thousand (‰)
• Average salinity is 35‰
• Major constituent is sodium chloride
Relative proportions of
water and dissolved
components in seawater
Figure 9.3
Composition of seawater
Processes affecting seawater salinity
• Processes that decrease salinity (add water)
•
•
•
•
Precipitation
Runoff from land
Icebergs melting
Sea ice melting
• Processes that increase salinity (remove water)
• Evaporation
• Formation of sea ice
Ocean temperature
Surface water temperature varies with the
amount of solar radiation received
• Lower surface temperatures are found in
high-latitude regions (Polar regions)
• Higher temperatures found in low-latitude
regions (Equatorial regions)
Flashback: Latitude and Longitude
Ocean temperature
Temperature variation with depth
• Low-latitudes (Equatorial regions)
• High temperature at the surface
• Rapid decrease in temperature with depth
(thermocline)
• High-latitudes (Polar regions)
• Cooler surface temperatures
• No rapid change in temperature with depth;
(thermocline absent)
Variations in ocean water
temperature with depth
Figure 14.4
Ocean density
Density is mass per unit volume - how
heavy something is for its size
Determines the water’s vertical position in
the ocean
Factors affecting seawater density
• Salinity (function of temperature)
• Temperature - the greatest influence
Variations in the ocean’s
surface temperature and
salinity with latitude
Figure 14.3
Ocean density
Ocean Density Variations with depth
• Low-latitudes (Equatorial regions)
• Low density at the surface
• Density increases rapidly with depth (pycnocline)
because of colder water
• High-latitudes (Polar regions)
• High-density (cold) water at the surface
• Little change in density with depth (pycnocline
absent)
Variations in ocean water
density with depth
Figure 14.5
Ocean density
Ocean layering
• Layered according to density
• Three-layered structure
1. Surface mixed zone
• Sun-warmed zone
• Zone of mixing
• Shallow (300 meters)
Ocean density
Ocean layering
• Three-layered structure
2. Transition zone
• Between surface layer and deep zone
• Zone of Thermocline and pycnocline
3. Deep zone
• Sunlight never reaches this zone
• Temperatures are just a few degrees above
freezing
• Constant high-density water
Layering in the ocean
Figure 14.6
PRS Review Test
No talking please.
Which one is NOT part of a
passive continental margin?
a.
b.
c.
d.
Continental shelf
Continental slope
Continental rise
Continental trench
Which one is NOT true of deep
ocean trenches?
a.
b.
c.
d.
They are long and narrow depressions at
subduction zones
They are sites where plates plunge back
into the mantle
They are associated with mid ocean
ridges
They are part of active continental
margins
Calcareous ooze and siliceous
ooze are an example of
a.
b.
c.
d.
Terrigenous sediment
Biogenous sediment
Hydrogenous sediment
Both terrigenous sediment and biogenous
sediment
The _?_ represents a rapid
__?_change with depth
a.
b.
c.
d.
thermocline; temperature
pycnocline; temperature
halocline; pressure
thermocline; salinity
Deep-ocean circulation is
referred to as __?_circulation and
is driven by __?_differences.
a.
b.
c.
d.
Thermocline, temperature
Pycnocline, density
Thermohaline, density
Coriolis, latitude
This instrument greatly enhanced
our knowledge of the ocean floor.
a.
b.
c.
d.
Pulsar
Echo sounder
Submarine transit
Tuzometer
The oldest ocean floor rock is
about _?_ years of age
a.
b.
c.
d.
2 billion
200 million
4.5 billion
15 billion
The eastern coast of the US
is a (n)
a.
b.
c.
Active continental margin
Passive continental margin
None of the above
Which is the correct ordering of
the passive margin's
subdivisions?
a.
b.
c.
d.
Continental shelf-Continental slopeContinental rise-Abyssal plain
Continental shelf-Abyssal PlainContinental rise-Continental slope
Abyssal plain- Continental riseContinental slope-Continental shelf
Continental rise-Continental shelfContinental slope-Abyssal plain
Where would you expect to find
the thickest accumulation of
seafloor sediment?
a.
b.
c.
d.
e.
Answer abyssal plain
Continental slope
Atop seamounts
Deep-ocean trench
Mid-ocean ridge
Which one is NOT one of the
three broad subdivisions of the
ocean floor?
a.
b.
c.
d.
coastal plain
oceanic ridge
deep ocean basin
continental margin