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Oceans: The Last Frontier

Chapter 9 Lecture

Natalie Bursztyn

Utah State University

Foundations of Earth Science

Eighth Edition

Discuss the extent and distribution of oceans and continents on Earth.

Identify Earth’s four main ocean basins.

Focus Questions 9.1

Oceanography is the study of all aspects of the 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

The Vast World Ocean

Geography of the Oceans

Four main ocean basins

Pacific Ocean

The largest with the greatest depth

Atlantic Ocean

About half the size of Pacific and not as deep

Indian Ocean

Slightly smaller than Atlantic, largely in Southern Hemisphere

Arctic Ocean

About 7% the size of the Pacific

Geography of the Oceans

Define salinity.

List the main elements that contribute to the ocean’s salinity.

Describe the sources of dissolved substances in seawater and causes of variations in salinity.

Focus Questions 9.2

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

Salinity

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

Sources of Sea Salts

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

Processes Affecting Seawater Salinity

Discuss temperature, salinity, and density changes with depth on the open ocean.

Focus Question 9.3

Thermocline

Rapid change of temperature with depth

Thermo = heat

Cline = slope

Creates a vertical barrier in ocean column

Not present at high latitudes (isothermal)

Temperature 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)

Density Variations

Layered ocean

Layered according to density

Three-layer structure

Shallow surface mixed zone

Transition zone

Deep zone

Does not exist in high latitudes

Ocean Layering

Define bathymetry.

Summarize the various techniques used to map the ocean floor.

Focus Questions 9.4

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

Mapping the Seafloor

Sidescan sonar

Provides view of the ocean floor

No bathymetry data

High-resolution multibeam sonar

Array of sound sources and listening devices

Obtains a profile of a narrow strip of seafloor

Mapping the Seafloor

An Emerging Picture of the Ocean Floor

Satellite altimeter

Satellites equipped with radar altimeters

Measures variations of the sea surface from space

Mapping the Ocean Floor from Space

An Emerging Picture of the Ocean Floor

[insert Figure 9.14 here]

Three major topographic units

Continental margins

Passive

Active

Ocean basin floor

Seamount

Abyssal plain

Mid-ocean ridge

Rift valley

Provinces of the Ocean Floor

[insert Figure 9.16 here]

Provinces of the Ocean Floor

Compare a passive continental margin with an active continental margin.

List the major features of each.

Focus Questions 9.5

Continental margins

Outer margins where continental crust transitions to oceanic crust

Two types:

Passive

Active

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

Passive Continental Margins

Passive continental margin features

Continental shelf

Submerged extension of the continent

Varies greatly in width

Gently sloping

Relatively featureless

Some dissected by extensions of river valleys (shelf valleys)

Eroded during last Ice Age

Contains oil and important mineral deposits

Passive Continental Margins

Passive continental margin features

Continental slope

Marks the seaward edge of the continental shelf

Relatively steep feature

Boundary between continental crust and oceanic crust

Passive Continental Margins

Passive continental margin features

Continental rise

Continental slope merges into a more gradual incline

Thick accumulations of sediment that moved down the continental slope

At base of the continental slope, turbidity currents deposit deep-sea fans

Passive 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

Passive Continental Margins

Active continental margins

Oceanic lithosphere is subducted beneath leading edge of a continent

Expressed as deep-ocean trenches

Located primarily around the Pacific Ocean

Accumulations of deformed sediment and ocean crust form accretionary wedges

Subduction erosion causes sediment and rock from the overriding plate to be transported into the mantle

Active Continental Margins

Continental Margins

List and describe the major features associated with deep-ocean basins.

Focus Question 9.6

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

Oceanic plateaus

Generated from vast outpouring of fluid basaltic lavas

Features of Deep-Ocean Basins

Summarize the basic characteristics of oceanic ridges.

Focus Question 9.7

Mid-ocean ridge (oceanic ridge or rise)

Found along well-developed oceanic divergent plate boundaries

Elevated seafloor forms a broad linear swell

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

The Oceanic Ridge System

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

Anatomy of The Oceanic Ridge System

Along axis of some segments are deep downfaulted structures called rift valleys

Anatomy of The Oceanic Ridge System

Elevated position of ridge

Due to hot (less dense) newly formed crust

Cools, contracts and becomes denser as it moves away from mantle upwelling

Why Is the Oceanic Ridge Elevated?

Distinguish among three categories of seafloor sediment.

Explain why some of these sediments can be used to study climate change.

Focus Questions 9.8

Ocean floor is covered 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 deep-ocean floor

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

Accumulates rapidly and forms thick deposits on continental margins

Types of Seafloor Sediments

Biogenous sediment

Shells and skeletons of marine organisms

Mostly produced by organisms living in sunlit surface waters

Most common are calcareous oozes

Siliceous oozes composed of skeletons of diatoms and radiolarians

Phosphate-rich materials derived from bones, teeth, and fish scales

Types of Seafloor Sediments

Hydrogenous sediment

Minerals that crystallize directly from seawater

Most common types include:

Manganese nodules

Calcium carbonates

Metal sulfides

Evaporites

Types of Seafloor Sediments

Organisms recording climate data become part of the sedimentary record

Records of temperature changes revealed in sediment cores from ocean floor

Seafloor Sediment—A Storehouse of Climate Data