Geology Lecture Outline -

  Divergent Plate Tectonism and Ocean Floor Geology – (Ch 13)

 

I. Lecture Content

       Introduction – Earth's Hidden Surface - The Seafloor

       Ocean Exploration - Earth's Last Frontier

       Seafloor Topography - Undersea Bottom Relief

       Seafloor Physiology - Major Features of the Sea bottom

Continental Margins - Where Ocean Meets Continent

        Deep-Sea Floors - Basaltic Crust Conveyor Systems

Deep Sea Sedimentation - Mucky Clays and Oozes

Islands, Reefs and Atolls - Interesting Places

Life and Death of an Ocean Basin - The Wilson Cycle

        Sea Bottom Resources - Natural Treasure

 

II. Introduction

A. The Earth’s Seafloor is Topographically Rugged and Symmetrical

                1. Seafloors cover nearly 3/4's of Earth's surface.

 

  2. A highly variable topography, which includes, rugged mountain ranges,

       very deep trenches, amazing island chains, ocean plateaus, seamounts,

      and deep canyons that rival anything seen on land.

3.      Symmetrical cross-section – very rugged basaltic mid ocean ridge mountain

     chain near center; 4-km deep hilly to flat abyssal plains on each side of MOR;

     either continental rise or deep sea trench between abyssal plains and

     continental slope and shelf.

4. The seafloor is the final receptacle for vast amounts of terriginous and organic sediment.

 

 

       B. The Earth’s Seafloor is Geologically Young and Made of Basaltic Material

              1. Generated at Oceanic Divergent Plate Boundaries by Seafloor Spreading.

 

·        Seafloor Spreading Process creates new seafloor

 

·         Mid-Ocean Ridge systems are the site of seafloor spreading

 

·      Basaltic magmas generated by decompression melting of rising asthenospheric

      mantle beneath MOR

 

 

III. Seafloor Topography - Underwater Bottom Relief

A. Earth has Two Distinctive Topographic Regions

·        ·      Continental Highlands - Continents

·       ·      Oceanic Lowlands - Ocean basins

               

       B. The Earth's Seafloors are Rugged in Appearance and

            Have Considerable Topographic Relief.

·       ·     See Figures

·       ·      Much more topographic relief than the continents  

·       ·      Seafloors have distinctive topographic features

·       ·     Seafloors look much different than dry continents  

 

C. Earth's Seafloor is Divided into Two Major Provinces

1. Continental Margins

·        ·      Submerged shallow platforms

·        ·      Floored mostly by granitic rock

·        ·      Varies greatly width, depth, and topographic relief

·        ·      Vast majority of marine life concentrated there

 

2. Deep-ocean basins are rugged with variable relief, and

                        have a wide variety of distinctive physiological features

Ø   Ø  See Figure  - Seafloor Topographic Map

 

                   1. Mid-ocean ridges

 

                                      2. Mid-ocean ridge fractures

 

                   3. Hydrothermal vents

 

                   4. Abyssal plains and Abyssal hills

 

5. Seamounts and Guyouts

 

6. Oceanic island chains

 

7. Oceanic plateaus

 

8. Trenches and Island arcs

 

                3.  The two-province division is based upon the major

                       inherent differences between continental and oceanic

                       crust.

·       ·     Composition (density)

·       ·     Thickness

·       ·     Isostatic equilibrium

 

IV. Continental Margins - Shallow Marine

       A. Shallow Seafloor Rims of Ocean Basins

      1. Continental margins - the submerged edges of continents

 

  2. Continental margins are underlain by faulted blocks of

         granitic crust , overlying sediment piles, and possible

         accreted subduction zone material

 

B. Continental Margins are Classified into Two Types

      

      1. Passive Margins = Atlantic Ocean style

 

§       §      Situated within a plate

 

§       §      Develops after continental rifting and opening of a new

     ocean basin opening

 

§       §      Typically broad (avg. 100's km) with a very thick pile of

         accumulating sediments

 

§       §      Lacks much seismic or volcanic activity

 

      2. Active Margins = Pacific Ocean style

 

·       ·     Situated at the leading edge of a continental plate

 

·       ·     Develops after initiation of subduction

 

·       ·     Typically narrow with rugged topography

 

·       ·     Outer edge typically forms inner wall of ocean trench

 

·       ·     Regionally unstable with much seismicity & volcanism

 

     3. See Figures )

 

C. Physiological Features of a Continental Margin

                  

·        ·     Continental Shelf

 

·        ·     Continental Slope

 

·        ·     Submarine Canyons

 

·          ·       Continental Rise

D. The Continental Shelf   (See Figs )

 

·       ·     Shallow, submerged edge of continent between the

                   shoreline and continental slope (shelf-slope break)

 

·       ·     Has a very low sloping angle (<< 1degree)

 

·       ·     Typically shallow water depths (avg. = 75 m = 250 ft)

 

·       ·     Greatly influenced by fluctuations in sea level

 

·       ·     Shelf sediments are mainly influenced by waves and

        tidal currents

 

·       ·     Site of abundant mineral resources and sea life

 

E. Continental Slope and Rise  (See Figs )

 

·       ·     Deeper, steeper, outermost edge of continent between the

    continental shelf and the deep ocean floor

 

·       ·     A continental rise may separate the continental slope

            from the deep ocean basin along passive margins

 

ü    ü    A continental rise forms a thick pile of sediments

                                     that have accumulated at the base of the

     continental slope

 

·       ·     The shelf-slope break marks the abrupt transition between

      the slope and the shelf

 

·       ·     Location of Earth's greatest depository of sediments

ü    ü   Roughly 70% of Earth's sediments

 

·       ·     Slope and Rise sediments are mainly influenced by

       gravity, and are transported down-slope via strong

       turbidity currents and deposit as submarine fans.

 

·        ·     Submarine canyons and fan deposits are present on all

      continental slopes and rises, and on some continental

                           shelves

 

·        ·      Submarine fan deposits grade into deep-ocean deposits

 

V. Deep-Ocean Basins - True Oceanic Seafloor

A.  Ocean Basins are Classified by Size and Extent

 

·        ·      Oceans - broad, large, and globally extensive

Examples: Pacific, Atlantic and Indian

 

·        ·      Seas - narrow, smaller, and regionally limited

Example: Mediterranean, South China, & Red

 

B. Deep-ocean basins are underlain by basaltic crust

       1. Ocean Crust - A typical cross section (See Figs.)

 

·       ·     Layered basaltic crust covered by sediments

 

·       ·     Rugged volcanic surface covered by layers upon

        layers of very fine pelagic sediment

 

ü    ü   Pelagic clays

 

ü    ü   Silica and carbonates Oozes

 

·       ·     Oceanic igneous crustal column is also layered

 

Ø    Ø   Pillow lava basalt

 

Ø    Ø   Sheeted gabbroic dikes

 

Ø    Ø   Massive gabbro (intrusions)

 

Ø    Ø   Layered gabbro (intrusions)

 

Ø    Ø   Layered Peridotite

 

·       ·     Oceanic crustal sections found on land are

        termed an ophiolite suite

 

C. Ocean Basins are Relatively Young Earth Features

·        ·      Oldest part of ocean basins is 180 million years old

 

·        ·      Average age of deep ocean seafloor is 60 million y.o.

 

·        ·      Age distribution pattern of deep-ocean crust is striking

 

·          ·        See Figure - Ocean crust age map

 

 

 

E. Most Deep Ocean Features are the Result of Seafloor

             Spreading Processes Occurring at Mid Ocean Ridges

      

1. Seafloor spreading processes create:

       

ü    ü   Mid-ocean rift valleys and ridge flanks

ü    ü   Vast expanses of ocean crust (abyssal plains)

ü    ü   Chains of volcanoes (seamounts and islands)

ü    ü   Transform fracture systems

ü    ü   Hydrothermal systems (black smokers)

 

2. See Figures

 

VI. Deep Sea Sedimentation - Pelagic Clays and Oozes

A. Nearly All Deep Ocean Sediment is Very Fine Grained

·        ·      No mechanisms to transport coarse-grained material

ü    ü   Exception is icebergs

 

·        ·      Vast majority of deep seafloor sediment is deposited via

           vertical settling of suspended material

 

·        ·      Sediment deposited from suspension is termed pelagic

 

     B. Deep Ocean Sediment Comes From Two Major Sources

 

        1. Windblown dust and volcanic ash

·        ·      Continentally derived

·        ·      Deep-sea clays are termed pelagic clays

 

       2. Microscopic marine organism skeletons

 

·        ·      Carbonate hard parts = calcareous ooze

 

·        ·      Silica hard parts = siliceous ooze

 

    C. Distribution of Deep-Sea Sediments - A Global Pattern

 

        1. Pelagic clays carpet the deepest parts of the oceans

 

ü    ü   Cover about 38% of world's deep-sea bottoms

 

ü    ü   Deposition rate is roughly 2 mm/1000 years

 

        2. Pelagic calcareous oozes cover a good portion of the

 Atlantic, Indian and Southern Pacific Ocean seafloors

 

ü    ü   Cover about 48% of world's deep-sea bottoms

 

ü    ü   Accumulation rate is 1 to 6 cm per 1000 years

 

3. Pelagic siliceous oozes cover equatorial Pacific and

        Indian Ocean and high latitude ocean seafloors

 

ü    ü    Cover about 14% of world's deep-sea bottoms

 

ü    ü   Accumulation rate is 1 to 6 cm per 1000 years

 

4. See Figure - Global deep-sea sediment map

 

VII. Origin of Islands, Atolls, Guyouts and Reefs

A. Islands and Seamounts are formed by volcanism

 

        1. Formed on or near mid-ocean ridges

 

        2. Basaltic shield volcanoes

 

        3. Migrate away from mid-ocean ridges over time

 

B. Atolls and Guyouts are Modified Oceanic Islands

 

        1. Circular coral reef systems develop around islands.

 

2. Oceanic crust cools and subsides with increasing age,

       causing the attached islands to also subside over time.

 

3. Islands slowly wear down to sea level by wave erosion.

 

        4. Upwards reef growth keep ups with sinking island.

 

        5. Island eventually worn down to below sea level, with

               only the growing reef able to maintain at sea level.

ü                        ü                    This stage of an island is termed an atoll.

 

6.  Eventually reef growth lags behind rate of atoll

       subsidence, and entire atoll structure becomes

permanently submerged - this is termed a guyout.

 

VIII. Birth, Growth and Death of an Ocean Basin

       ---- The Wilson Cycle ---- 

 

      A.  Initiation of New Ocean Basin via Continental Rifting

   1. Initial stages of plate divergence

 

   2. Rift valley floored by new basaltic (oceanic) crust.

 

   3. Further widening of rift, marine waters begin filling valley

 

      B. Young Ocean Basin is Born - A True Sea

 

         1. Continued plate divergence now in full swing

 

   2.True seafloor spreading in operation =  Mini ocean basin

 

           3. Matching set of opposing coastlines frame the sea

 

     C. Full Maturation of Ocean Basin -

 

        1. Divergence begins to stall - spreading rate slows

 

 2. Continental margins, abyssal seafloors, and mid-ocean ridge

 

         3. Fully-developed ocean has emerged with an age 200-400 Ma

 

    D. Mature Ocean Basin Starts to Collapse near Its Margins

 

1. Old, dense ocean lithosphere becomes isostatically unstable

 

2. Subduction initiated; ocean basin lithosphere dives into

       upper mantle forming ocean trenches and island arcs.

 

3. Beginning of plate convergence of sides of ocean basin

 

    E. Collapsing Ocean Basin Becomes Narrow and Irregular

 

         1.  Plate convergence in full swing

 

         2. Subduction zones established along continental margins

 

 3. Extensive volcanic and uplifted mountain chains result

                 from continued subduction and intense collision forces

 

   F. Total Collapse of Ocean Basin - Suturing of Continents

 

         1.  Plate convergence reaches an apex -  subduction wanes

 

         2. Last of oceanic lithosphere subducted - Ocean basin gone

 

         3. Massive thrusted and uplifted mountain ranges form a

                complex continental suture zone marking the site of the

now totally collapsed ocean basin

 

IX. Sea Bottom Resources - Natural Treasure

       A. Continental Margins

 

                1. Oil and Gas

 

                2. Sand and Gravel

 

                3. Plankton, Fish and Shellfish

 

       B. Deep-Sea Bottom

 

               1. Manganese nodules

 

                2. Massive sulphide deposits

 

                3. Migratory fish

 

X. Ocean floor Vocabulary - Chapter 13

 

Abyssal plain

Active continental margin

Atoll

Black Smoker

Continental margin

Continental rise

Continental slope

Guyout

Isostatic equilibrium

Hydrothermal Vent

Mid-oceanic ridge

Oceanic trench

Ooze

Ophiolite

Passive continental margin

Pelagic clay

Reef

Ridge fracture zones

Seafloor Spreading

Seamount

Submarine canyon

Submarine hydrothermal vent

Submarine fan

Turbidity current

Wilson cycle