Download Oceans: The Last Frontier

Chapter 9 Lecture Outline
Oceans: The Last
Frontier
Focus Question 9.1
• How does the area of Earth’s surface
covered by oceans compare with the area
covered by continents?
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 – 81% ocean
The Vast World Ocean
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
The Vast World Ocean
Focus Question 9.2
• What are the two primary sources for the
elements that comprise the dissolved
components in seawater?
Composition of Seawater
• Salinity
– The total amount of solid material dissolved in
water
• Dissolved substances in seawater are in such
small quantities that expressed in parts per
thousand
• Most of the salt in seawater is sodium chloride
• Average salinity of seawater - 35 ‰
Dissolved Components in Seawater
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
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
– Surface varies 33‰ to 38‰
– Extreme ……. Red Sea- 42‰
Composition of Seawater
Focus Question 9.3
• What two factors influence seawater density?
Which one has the greater influence on
surface seawater density?
Variations in Temperature and Density
with Depth
• Temperature variations
– Thermocline
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Rapid change of temperature with depth
Between about 300-1000m
Thermo = heat
Cline = slope
Creates a vertical barrier in ocean column
Not present at high latitudes (isothermal)
Variations in Temperature and Density
with Depth
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)
Variations in Temperature and Density
with Depth
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
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.
An Emerging Picture of the Ocean Floor
• Bathymetry
– Mapping ocean depths and the shape, or
topography, of the ocean floor
– HMS Challenger – 1872-1876
• Traveled to every ocean except Arctic
• Measured all types of properties, depths
– Challenger Deep – deepest point –over 35,000ft
An Emerging Picture of the Ocean Floor
• Echo sounder
– sonar
– SOund Navigation And
Ranging
– Invented in the 1920s
– Primary instrument
for measuring depth
– Reflects sound from
ocean floor
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
An Emerging Picture of the Ocean Floor
Satellite altimeter
Satellites equipped
with radar altimeters
Measures variations of
the sea surface from
space
An Emerging Picture of the Ocean Floor
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
An Emerging Picture of the Ocean Floor
Focus Question 9.5
• Describe the differences between active and
passive continental margins. Where is each
type found?
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
Continental Margins
• Passive continental margin features
– Continental shelf
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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
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
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
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
Continental Margins
Continental Margins
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
Continental Margins
Continental Margins
Focus Question 9.6
• Explain how deep-ocean trenches are related
to plate boundaries.
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
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
Features of Deep-Ocean Basins
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.
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
The Oceanic Ridge
The Oceanic Ridge
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
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
Focus Question 9.8
• List and describe the three basic types of
seafloor sediments.
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
Seafloor Sediments
• Types of seafloor sediments
– Terrigenous sediment
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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
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
Seafloor Sediments
Seafloor Sediments
• Hydrogenous sediment
–Minerals that precipitate directly from
seawater
–Most common types include:
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Manganese nodules
Calcium carbonates
Metal sulfides
Evaporites
Seafloor Sediments
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)
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