Download Oceans: The Last Frontier

Chapter 9 Lecture Outline
Foundations of
Earth Science
Seventh Edition
Oceans: The Last
Frontier
Natalie Bursztyn
Utah State University
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Focus Question 9.1
• How does the area of Earth’s surface
covered by oceans compare with the area
covered by continents?
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The Vast World Ocean
• Earth is referred to as the blue planet
– 71% of Earth’s surface is oceans and
marginal seas
– Continents and islands comprise the
remaining 29%
• The Northern Hemisphere is the land
hemisphere
• The Southern Hemisphere is the water
hemisphere
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The Vast World Ocean
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The Vast World Ocean
• Four main ocean basins
1. Pacific Ocean
• The largest with the greatest depth
2. Atlantic Ocean
• About half the size of Pacific and not as deep
3. Indian Ocean
• Slightly smaller than Atlantic, largely in Southern
Hemisphere
4. Arctic Ocean
• About 7% the size of the Pacific
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The Vast World Ocean
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Focus Question 9.1
• How does the area of Earth’s surface
covered by oceans compare with the area
covered by continents?
– 71% covered by oceans; 29% by land
– Most land in northern hemisphere
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Focus Question 9.2
• What are the two primary sources for the
elements that comprise the dissolved
components in seawater?
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Composition of Seawater
• Salinity
– The total amount of solid material dissolved in
water
• Typically expressed as a percent
• Dissolved substances in seawater are in such
small quantities that expressed in parts per
thousand
• Most of the salt in seawater is sodium chloride
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Dissolved Components in Seawater
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Composition of Seawater
• Sources of sea salts
– Chemical weathering of continental rocks
– Earth’s interior through volcanic eruptions
• Process called outgassing
• Composition of seawater has been relatively
stable for millions of years
– Material removed as rapidly as added
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Composition of Seawater
• Processes affecting seawater salinity
– Changes in water content of the solution
• Addition of fresh water
– Precipitation
– Runoff
– Icebergs and sea-ice melting
• Removal of fresh water
– Evaporation
– Formation of sea ice
– Latitude and seasons
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Composition of Seawater
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Focus Question 9.2
• What are the two primary sources for the
elements that comprise the dissolved
components in seawater?
– Chemical weathering of continental rocks
– Earth’s interior through volcanic eruptions
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Focus Question 9.3
• What two factors influence seawater density?
Which one has the greater influence on
surface seawater density?
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Variations in Temperature and Density
with Depth
• Temperature variations
– Thermocline
• Rapid change of temperature with depth
• Thermo = heat
• Cline = slope
• Creates a vertical barrier in ocean column
• Not present at high latitudes (isothermal)
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Variations in Temperature and Density
with Depth
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Variations in Temperature and Density
with Depth
• Density variations
– Influenced by salinity
• Increased salinity = increased density
– Influenced by temperature
• Increased temperature = decreased density
• Greater variations in temperature result in a
greater impact of this factor on density
– Influenced by depth
• Increased depth = increased density
• Pycnocline = rapid change of density with depth
• Not present at high latitudes (isopycnal)
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Variations in Temperature and Density
with Depth
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Variations in Temperature and Density
with Depth
• Layered ocean
– Layered according to density
– 3-layer structure
• Shallow surface mixed zone
• Transition zone
• Deep zone
– Does not exist in high latitudes
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Focus Question 9.3
• What two factors influence seawater density?
Which one has the greater influence on
surface seawater density?
– Salinity and temperature
– Temperature has the greater influence
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Focus Question 9.4
• Describe how satellites orbiting Earth can
determine features on the seafloor without
being able to directly observe them beneath
several kilometers of seawater.
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An Emerging Picture of the Ocean Floor
• Bathymetry
– Mapping ocean depths and the shape, or
topography, of the ocean floor
• Echo sounder (sonar)
– Invented in the 1920s
– Primary instrument for measuring depth
– Reflects sound from ocean floor
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An Emerging Picture of the Ocean Floor
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An Emerging Picture of the Ocean Floor
• Multibeam sonar
– Array of sound sources and listening
devices
– Obtains a profile of a narrow strip of
seafloor
• Satellite altimeter
– Satellites equipped with radar altimeters
– Measures variations of the sea surface
from space
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An Emerging Picture of the Ocean Floor
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An Emerging Picture of the Ocean Floor
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An Emerging Picture of the Ocean Floor
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An Emerging Picture of the Ocean Floor
• Provinces of the Ocean Floor
– Three major topographic units
1. Continental margins
• Passive
• Active
2. Ocean basin floor
• Seamount
• Abyssal plain
3. Mid-ocean ridge
• Rift valley
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An Emerging Picture of the Ocean Floor
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Focus Question 9.4
• Describe how satellites orbiting Earth can
determine features on the seafloor without
being able to directly observe them beneath
several kilometers of seawater.
– Satellites measure differences across the
ocean surface. These differences are due to
water displacement caused by variations of
the ocean floor.
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Focus Question 9.5
• Describe the differences between active and
passive continental margins. Where is each
type found?
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Continental Margins
• Passive continental margins
– Found along most coastal areas that
surround the Atlantic Ocean
– Not associated with plate boundaries
• Experience little volcanism
• Few earthquakes
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Continental Margins
• Passive continental margin features
– Continental shelf
• Flooded extension of the continent
• Varies greatly in width
• Gently sloping
• Contains oil and important mineral deposits
• Some areas mantled by extensive glacial
deposits
• Most consist of thick accumulations of
shallow-water sediments
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Continental Margins
• Passive continental margin features
– Continental slope
• Marks the seaward edge of the
continental shelf
• Relatively steep structure
• Boundary between continental crust and
oceanic crust
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Continental Margins
• Passive continental margin features
– Submarine canyons
• Deep, steep-sided valleys cut into
continental slope
• Some are seaward extensions of river
valleys
• Most appear to have been eroded by
turbidity currents
– Turbidity currents
• Downslope movements of dense,
sediment-laden water
• Deposits are turbidites
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Continental Margins
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Continental Margins
• Passive continental margin features
– Continental rise
• Found in regions where trenches are absent
• Continental slope merges into a more
gradual incline
• At base of the continental slope, turbidity
currents deposit deep-sea fans
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Continental Margins
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Continental Margins
• Active continental margins
– Continental slope descends abruptly into a
deep-ocean trench
– Located primarily around the Pacific Ocean
– Accumulations of deformed sediment and
ocean crust form accretionary wedges
– Some subduction zones have little or no
accumulation of sediments
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Continental Margins
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Continental Margins
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Focus Question 9.5
• Describe the differences between active and
passive continental margins. Where is each
type found?
– Passive margins mark the boundary between
continental and oceanic crust on a single
tectonic plate
– Passive margins accumulate sediment
– Active margins mark the boundary between
tectonic plates
– Active margins are found along all subduction
zones
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Focus Question 9.6
• Explain how deep-ocean trenches are related
to plate boundaries.
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Features of Deep-Ocean Basins
• Deep-ocean trenches
–
–
–
–
–
Long, relatively narrow features
Deepest parts of ocean
Most are located in the Pacific Ocean
Located at subduction zones
Associated with volcanic activity
• Volcanic island arcs
• Continental volcanic arcs
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Features of Deep-Ocean Basins
• 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
– May emerge as islands
– May sink and form flat-topped seamounts called
guyots or tablemounts
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Features of Deep-Ocean Basins
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Focus Question 9.6
• Explain how deep-ocean trenches are related
to plate boundaries.
– Deep ocean trenches form at subduction
zones, which are a form of convergent plate
boundary.
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Focus Question 9.7
• Although oceanic ridges can be as tall as
some mountains on the continents, list some
ways that oceanic ridges are different.
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The Oceanic Ridge
• Mid-ocean ridge
– Interconnected ridge system is the longest
topographic feature on Earth’s surface
• Over 70,000 km (43,000 mi) in length
• 23% of Earth’s surface
• Winds through all major oceans
– Along axis of some segments are deep
downfaulted structures called rift valleys
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The Oceanic Ridge
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The Oceanic Ridge
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The Oceanic Ridge
• Oceanic ridges are characterized by:
– An elevated position
– Extensive faulting
– Numerous volcanic structures that have
developed on newly formed crust
• Consist of layer upon layer of faulted and
uplifted basaltic rocks
• Mid-Atlantic ridge has been studied more
thoroughly than any other ridge system
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The Oceanic Ridge
• Elevated position of ridge
– Due to hot (less dense) newly formed crust
– Cools, contracts and becomes denser as it
moves away from mantle upwelling
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Focus Question 9.7
• Although oceanic ridges can be as tall as
some mountains on the continents, list some
ways that oceanic ridges are different.
– Type of plate boundary
– Type of volcanism (and composition of lava)
– Type of tectonic stress and resulting features
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Focus Question 9.8
• List and describe the three basic types of
seafloor sediments.
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Seafloor Sediments
• Ocean floor is mantled with sediment
• Sources
– Turbidity currents
– Sediment slowly settles to bottom from
above
• Thickness varies
– Thickest in trenches: up to 10 km
– Pacific Ocean: 600 m or less
– Atlantic Ocean: 500 to 1000 m
– Mud is most common sediment on deepocean floor
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Seafloor Sediments
• Types of seafloor sediments
– Terrigenous sediment
• Material weathered from continental rocks
• Virtually every part of the ocean receives it
• Fine particles remain suspended for a long time
• Oxidation often produces red and brown
colored sediments
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Seafloor Sediments
• Biogenous sediment
– Shells and skeletons of marine
organisms
– Most common are calcareous oozes
– Siliceous oozes composed of skeletons
of diatoms and radiolarians
– Phosphate rich materials derived from
bones, teeth, and fish scales
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Seafloor Sediments
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Seafloor Sediments
• Hydrogenous sediment
– Minerals that precipitate directly from
seawater
– Most common types include:
• Manganese nodules
• Calcium carbonates
• Metal sulfides
• Evaporites
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Seafloor Sediments
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Seafloor Sediments
• Distribution
– Coarse terrigenous deposits dominate
continental margin areas
– Fine terrigenous material common in
deeper areas of the ocean basin
– Hydrogenous sediment comprises only a
small portion of deposits in the ocean
– Few places where very little sediment
accumulates (mid-ocean ridges)
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Seafloor Sediments
• A Storehouse of Climate Data
– Organisms recording climate data become
part of the sedimentary record
– Records of temperature changes revealed
in sediment cores from ocean floor
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Focus Question 9.8
• List and describe the three basic types of
seafloor sediments.
1. Terrigenous — weathered from continental rocks
2. Hydrogenous — precipitated directly from seawater
3. Biogenous — produced by marine organisms
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