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 Earths 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 Earths 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

     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

 

       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

       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

 

       Pelagic clays

 

       Silica and carbonates Oozes

 

            Oceanic igneous crustal column is also layered

 

       Pillow lava basalt

 

       Sheeted gabbroic dikes

 

       Massive gabbro (intrusions)

 

       Layered gabbro (intrusions)

 

       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:

       Mid-ocean rift valleys and ridge flanks

       Vast expanses of ocean crust (abyssal plains)

       Chains of volcanoes (seamounts and islands)

       Transform fracture systems

       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

       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

 

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

 

       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

 

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

 

       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

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

 

       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.

                                          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