Earth Science, 10^th edition

Chapter 12: The Ocean Floor

 

 

I.     The vast world ocean

Seventy-one percent of Earth’s surface is represented by oceans and marginal seas

Continents and islands comprise the remaining 29%

Northern Hemisphere is called the land hemisphere, and the Southern Hemisphere the

              water hemisphere

Four main ocean basins

1.    Pacific Ocean - the largest and has the greatest depth

2.    Atlantic Ocean – about half the size of the Pacific and not quite as deep

3.    Indian Ocean – slightly smaller than the Atlantic, largely a southern Hemisphere body

4.    Arctic Ocean – about 7 percent the size of the Pacific

 

II.    Mapping the ocean floor

A.        Bathymetry – measurement of ocean depths and the charting of the shape

        or topography of the ocean floor

B.        Echo sounder (also referred to as sonar)

1.       Reflects sound from ocean floor

C.        Measuring the shape of the ocean surface from space

D.       Three major topographic units of the ocean floor

1.        Continental margins

2.        Ocean basin floor

3.        Mid-ocean ridge

 

III. Continental margins

A.        Passive continental margins

1.        Found along most coastal area that surround the Atlantic ocean

2.        Not associated with plate boundaries

             3.    Features comprising a passive continental margin

                  a.    Continental shelf

                      1.    Flooded extension of the continent

                      2.    Varies greatly in width

          3.    Gently sloping

                  b.    Continental slope

                      1.    Marks the seaward edge of the continental shelf

                      2.    Relatively steep structure

                      3.    Boundary between continental crust and oceanic crust

                  c.    Submarine canyons and turbidity currents

                       1.    Submarine canyons

                           a.    Deep, steep-sided valleys cut into the continental slope

                           b.    Some are seaward extensions of river valleys

                           c.    Most appear to have been eroded by turbidity currents

                      2.    Turbidity currents

                           a.    Downslope movements of dense, sediment-laden water

                  d. Continental rise

1.        Found in regions where trenches are absent

2.        Continental slope merges into a more gradual incline – the continental rise

                      3.    Thick accumulation of sediment

                       4.    At the base of the continental slope turbidity currents that follow

       submarine canyons deposit sediment that forms deep-sea fans

B.         Active continental margins

1.        Continental slope descends abruptly into a deep-ocean trench

2.        Located primarily around the Pacific Ocean

3.        Some subduction zones have little or no accumulation of sediments

 

IV.   Ocean basin floor

A.        Deep-ocean trenches

             1.    Sites where moving lithospheric plates plunge into the mantle

       B.    Abyssal plains

             1.    Likely the most level places on Earth

             2.    Sites of thick accumulations of sediment

             3.    Found in all oceans

       C.    Seamounts and guyots

1.        Isolated volcanic peaks

2.        Many form near oceanic ridges

3.        May emerge as an island

4.        May sink and form flat-topped seamounts called guyots or tablemounts

       D.    Mid-ocean ridge

             1.   Characterized by

a.       An elevated position

b.       Extensive faulting

c.       Numerous volcanic structures that have developed on newly formed crust

             2.    Interconnected ridge system is the longest topographic feature on Earth’s surface

 

 

V.       Seafloor sediments

A.        Ocean floor is mantled with sediment

B.         Sources

1.        Turbidity currents

2.        Sediment that slowly settles to the bottom from above

       C.    Mud is the most common sediment on the deep-ocean floor

       D.    Types of seafloor sediments

1.        Terrigenous sediment

a.        Material weathered from continental rocks

             2.    Biogenous sediment

                  a.    Shells and skeletons of marine animals and plants

3.        Hydrogenous sediment

a.  Minerals that crystallize directly from seawater

E.        Distribution

1.    Coarse terrigenous deposits dominate continental margin areas

2.    Fine-grained terrigenous material is common in deeper areas of the ocean basin

3.    Hydrogenous sediment comprises only a small portion of deposits in the ocean

F.         Seafloor sediments and climate change

1.    Seafloor sediments provide clues to Earth’s climate history

2.    Numbers and types of organisms living near the sea surface change

     with the climate