Earth Science, 6e
The Ocean FloorChapter - 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
HemisphereFigure 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. Continental shelf
b. Continental slope
c. Continental rise
d. Oceanic trench
Which one is NOT true of deep ocean trenches?
a. They are long and narrow depressions at subduction zones
b. They are sites where plates plunge back into the mantle
c. They are associated with mid ocean ridges
d. They are part of active continental margins
Calcareous ooze and siliceous ooze are an example of
a. Terrigenous sediment
b. Biogenous sediment
c. Hydrogenous sediment
d. Both terrigenous sediment and biogenous sediment
The _?_ represents a rapid __?_change with depth
a. thermocline; temperature
b. pycnocline; temperature
c. halocline; pressure
d. thermocline; salinity
Deep-ocean circulation is referred to as __?_circulation and
is driven by __?_differences.
a. Thermocline, temperature
b.Pycnocline, density
c. Thermohaline, density
d.Coriolis, latitude
This instrument greatly enhanced our knowledge of the ocean floor.
a. Pulsar
b. Echo sounder
c. Submarine transit
d. Tuzometer
The oldest ocean floor rock is about _?_ years of age
a. 2 billion
b. 200 million
c. 4.5 billion
d. 15 billion
The eastern coast of the US is a (n)
a. Active continental margin
b. Passive continental margin
c. None of the above
Which is the correct ordering of the passive margin's
subdivisions?a. Continental shelf-Continental slope-Continental rise-Abyssal plain
b. Continental shelf-Abyssal Plain-Continental rise-Continental slope
c. Abyssal plain- Continental rise-Continental slope-Continental shelf
Where would you expect to find the thickest accumulation of
seafloor sediment?
a. Abyssal plain
b. Continental slope
c. Atop seamounts
d. Deep-ocean trench
e. Mid-ocean ridge
Which one is NOT one of the three broad subdivisions of the
ocean floor?
a. coastal plain
b. oceanic ridge
c. deep ocean basin
d. continental margin