Oceanogrpahy Lecture Notes OutlinE  

Plate Tectonics

 

I. Lecture Content-  Topics Covered

Introduction Earth's Ever-Changing Surface - How so?

Early Ideas - Pondering Earth's Continental Jigsaw Puzzle

Wegener and the Continental Drift Hypothesis - Eye-Opener

Evidence for Continental Drift - Many Continent "Connections"

Major Discoveries about the Seafloor - Let the Spreading Begin

Seafloor Spreading and Subduction - Creation and Destruction

Theory of Plate Tectonics - Unifying, Earth-Shaking Paradigm

Pangea and The Wilson Cycle - 500 Ma Supercontinent cycles

Plates and Plate Boundaries - Inter-Plate Relationships

Determining Plate Movement & Motion - Past, Present & Future

Driving Forces of Plate Tectonics - Where's the Big Ponies?

Paleogeographic Reconstruction - Modeling Earth's Past

The Rock and Water Cycles Interplay with Plate Tectonics

Plate Tectonics and Natural Resources - Rhyme and Reason

 

I. Introduction

A. How and Why Does the Earth Continue Changing?

                1. Earth's surface has never stopped changing since it first formed nearly 4.6

billion years ago.

 

                2. There must be very energetic, long-lived forces within the Earth to maintain the

global-scale earthquake, volcanic, and mountain-building activities we

observe.

 

                3. Earth scientists have been studying the Earth for several 100 years in hopes of

answering this question.

 

                4. Numerous ideas or theories have been proposed to explain Earth's long and

eventful geologic history, and its amazing variety of features and phenomena.

 

                5. The unifying Theory of Plate Tectonics is, by far, the best and most accepted

theory for explaining all of Earth's geologic and some biological phenomena.

 

        B. Ever-Mounting Evidence Supporting Plate Tectonics

 

II. Early Ideas About Movements of Continents (Drift)

A.  Concept of "Drift" First Derived From Continent Fit

1. Fit of Africa and South America gave several people the idea that they

once a single landmass and that they eventually "drifted" apart.

        Leonardo de Vinci - 1500's

        Francis Bacon - 1620

        Edward Suess - 1885

       Alfred Wegener - 1912

 

        B. Alfred Wegener and His Continental Drift Theory

              1. German meteorologist and polar explorer

 

                2. Credited with the Theory of Continental Drift (1912)

 

                3. Outline of Continental Drift Theory

    All of Earth's landmasses had once been joined into a single supercontinent (named Pangaea) surrounded by a single superocean (named Panthalassa).

 

     Pangea broke into smaller pieces (today's continents) around 200 Ma.  (Ma = million years ago)

 

      After the breakup of Pangaea, the pieces (continents) started moving

away from one another; have been moving ever since; and are still moving.

                               

                4. Wegener amassed numerous lines of evidence from all corners of the world to

support his 'outrageous' theory   

       Geological data:

    Continental margin fits

    Match-up of truncated mountain ranges and faults

    Match-up of 'stratigraphic' sequences and mineral deposits

 

       Paleontological data:

    Match-up of extinct plant species fossil localities

    Match-up of extinct animal species fossil localities

 

       Climatologic data:

    Match-up of temporally-equivalent glacial deposits

    Discovery of coal deposits in Antarctica

 

             5. Wegener proposed a mechanism for continental drift.

 

       Heavy continents were slung toward the equator by centrifugal forces generated by the spinning Earth.

 

       The slinging force coupled with tidal drag created by sun's and moon's gravity caused continents to drift.

 

             6. Wegener had many harsh critics who cut him down over both the (unselected)

evidence, and his drift mechanism.

 

        How do continents move through solid ocean crust?

 

        If so, then where is the "wake" left behind on seafloor?

 

        Drift mechanism deemed geophysically impossible.

    Ideas about the mantle were different than today

 

        No known power source to cause drift.

 

             7. Wegener's Continental Drift Theory nearly dies with him

 

   C.  Post-Wagener Research Keeps the "Drift" Idea Alive

               1. Several scientists transform the "drift" idea with new research on Pacific

volcanism and earthquake.

 

        Kiyoo Wadati - 1935 - Connected EQ's with "drift"

 

        Hugo Benioff - 1940 - Mapped "Pacific Ring of Fire"

 

               2. Radiometric age-dating of world's seafloors reveal that the oldest rocks and

overlying sediments < 200 Ma.

 

        Why was oceanic crust so young?

 

               3. Oceanographers in the Atlantic map the Mid-Atlantic Ridge

 

        The Mid-Atlantic Ridge mimics the continent outlines

 

        Seafloor sediments thin at ridge; thickens landward

 

             4. Oceanographer maps flat-topped seamounts (guyouts)

 

              5. Mantle studies reveal a partially-melted layer in the upper mantle, termed the

asthenosphere.

 

              6.  Upper mantle shown to act like a very viscous plastic

        The idea of crust isostically "floating" in mantle

 

              7. Paleomagnetic studies of numerous lava flows of all ages from each of the

continents revealed "wandering" of the Earth's magnetic pole (mapped polar

wander paths).

 

       Each continent has a unique "wander path".

 

    Question: Can magnetic poles wander from the equator to the geographic pole?

 

       For a given geologic age, there appears to be two or more North magnetic poles.

 

    Question: Can there be more than one North magnetic pole at any one given time?

                                            

III. Modern Revelations about the Seafloor

        A. Navy Oceanographer Proposes Seafloor Spreading

 

               1. Harry Hess formally proposes the theory of seafloor spreading in 1962 to explain

movement of continents.

 

               2. Hess's theory of seafloor spreading in a "nutshell".

        Ocean and continental crust move together over convecting cells of viscous upper mantle material

 

        New ocean crust is created at mid-ocean ridges by upwelling mantle from the mantle.

 

        Newly formed crust is then split apart by divergent forces and rafted laterally off the ridge and down the flanks of the ocean ridge, and eventually

 

                                3. Evidence sited: guyouts, seafloor topography, crustal age profiles, and

mantle characteristics.

 

             - Further Support of Seafloor Spreading Theory -

        B. Scripps Oceanographers Discover Magnetic Stripes

                                1. Paleomagnetic polarity-reversal stripe patterns, termed magnetic

anomalies, are discovered on Pacific seafloor.

 

                2. Group of scientists from East Coast schools propose a model to explain the

magnetic anomaly stripes (1963)

o      Vine and Mathews - Cambridge

o      L. W. Morley - Canada

        Magma intruded at the crest of the ocean ridge records the polarity at the time it cooled

        Newly formed crust splits and moves away to make room for new magma

        Over time the repeated intrusion events would form a symmetrical set of magnetic stripes

 

        4. Similar magnetic anomalies found across the mid-ocean ridge off of Iceland

and other ocean ridges world-wide.

 

        5. Paleomagnetic data-generated age profile of seafloors confirmed Vine,

Mathews and Morley's proposal, and greatly support Hess's theory of

seafloor spreading.

       

      C. Deep-sea Drilling Projects Confirm Seafloor Spreading

      

               1. Analysis of ocean crust seafloor core samples

       Sediments and Basalts = ophiolite sequence

 

                 2. Seismic profiles of seafloors (oceanic crust x-sections)

 

     D. Researchers Discover that Crust Plunges into Mantle

 

                1. Close correspondence between ocean trenches, active island arcs, and

earthquake-packed Benioff zones

 

                   2. Seismic profiles of trench-arc complexes

 

                   3. The term subduction is used to describe the process.

 

IV. The Unifying Theory of Plate Tectonics - A Paradigm

    A. Theory of Plate Tectonics Proposed (1965)

              1.  Conceptualized by geophysicist  J.T. Wilson

        Also proposed the "Wilson Cycle"

        Pangea and the 500 My Supercontinent cycle

 

                 2. Combined ideas of continental drift, seafloor spreading, subduction, and

mantle convection into a single concept.

 

                 3. The theory of plate tectonics is termed a unifying theory because it is able to

explain a great many geological (and some biological) phenomenon.

 

     B. The Basic Components of the Plate Tectonics Theory

                      1. The Earth's rigid outer layer is broken up into a dozen or so separate

lithospheric plates

       Lithosphere = crust + uppermost mantle

       Large plates = continental and oceanic crust

 

2. The lithospheric plates are floating on the hot and plastically mobile athenosphere

 

3. Heat convection cells in the athenosphere causes it to expand & rise up beneath the lithospheric plates

 

4. The rising athenosphere laterally diverges beneath the lithosphere, causing a tensional drag effect at the base of the lithosphere plate.

 

                                 5. The asthenosphere drags the lithospheric plate with it laterally until it

turns downward with the descending portion of the mantle

thermal convection cell.

 

                 6. The lithosphere plates jostle with each other as they move independently about

under the influence of the underlying athenosphere.

                

                 7. Three types of plate interactions = 3 types of boundaries

       Divergent boundaries

       Convergent boundaries

       Transform boundaries 

 

V. Three Types of Plate Boundaries

        A.  Divergent Plate Boundaries - Two Styles

 

    A line along which two plates move apart

 

    Tensional tectonic forces dominate

 

    Oceanic crust forms along divergent boundaries

                        1. Continental (rifting)

        Spreading center

    Rift valley (pull-apart basin)

 

        Examples: East Africa Rift Valley

 

                        2. Oceanic (basin extension)

        Spreading center

 

    Mid-ocean ridge system

   Transform fracture system

 

       Examples: Mid Atlantic Ridge

 

        B.  Convergent Plate Boundaries - Three Styles

    A line along which two plates move towards each other

 

    Compressional tectonic forces usually dominate

 

    Ocean crust is consumed at convergent boundaries

 

                        1. Oceanic-Oceanic Plate Convergence

        Subduction zone complex

    Oceanic trench

    Volcanic island arc

 

        Examples: Aleutian Island trench/arc belt

 

                  2. Oceanic-Continental Plate Convergence

        Subduction zone complex

    Oceanic trench

    Volcanic continental margin arc

 

        Examples: Andes trench/arc belt

 

                  3. Continental-Continental Plate Convergence

        Continental collision complex

    Uplifted fold/thrust mountain belt

    Collapsed ocean basin suture zone

 

        Examples: Himalayas

        

C.  Transform Plate Boundaries - Three Styles

    Line along which two plates slide laterally past the other

 

    Shearing tectonic forces usually dominate

 

    Crust is neither created of destroyed at this boundary

 

                        1. Oceanic-Oceanic Plate Transform

       Transform fault

    Ridge-ridge fracture zone

    Ridge-trench fracture zone

    Trench-trench fracture zone

       Examples: Mendocino fracture zone

 

                        2. Oceanic-Continental Plate Transform

       Transform fault

    Great strike-slip fault zone

 

       Examples: Queen Charlotte Fault

 

                        3. Continental-Continental Plate Transform

       Transform fault

    Great strike-slip fault zone

 

       Examples: San Andreas Fault

        

VI. Determining Plate Motion - Past, Present and Future 

      A.  Several Aspects of Determining Tectonic Plate Motion

             1. Determine present rate (speed) of motion of each plate

 

             2. Determine present direction of motion for each plate

        Relative motion - in relation to other plates

        Absolute motion - in relation to fixed point in mantle

 

            3. Determine past rates and directions of motion of each plate

 

            4. Reconstruct ancient plate configurations for various past  time periods

 

            5. Predict future plate configurations

 

            B. Methods Used for Determining Plate Motions

 

           1.  Magnetic Anomaly Dating of the seafloor crust

          Distance from the ridge axis to the any specific  magnetic anomaly indicates the width of new oceanic seafloor crust that formed since the magnetic anomaly was recorded (a time interval)

 

        For a given interval of time, the wider the (magnetic anomaly) strip of seafloor, the faster the plate moved.

 

        Both present average rate of movement and relative direction of motion can be determined with this method

 

 

        Both past average rates of movement and relative directions of motion

can also be calculated with this method for various past time periods.

 

        Past rates are calculated by dividing the distance elapsed between the anomalies

 

      Past plate positions can also be calculated, because magnetic anomalies are parallel and symmetrical with respect to the ocean spreading ridge

    Determine continent position by move the anomaly stripes back to the spreading ridge

 

       2. Laser-Satellite Ranging Technique

 

        Shooting a laser beam pulse from one tectonic plate to another by bouncing it off a geo-stationary satellite

 

        As the plates move relative to one another, the sending and receiving laser stations will also move

 

        The rate of movement and direction of relative motion of the two plates can be calculated from differences in the recorded elapsed times of the laser pulses taken over a given period of time

 

        Only useful for present plate motion rates and direction

 

        The results of this method correlate with those made with the magnetic anomaly dating method

 

       3. Quasar Radio Signal Ranging Technique

        Virtually identical to the above laser-satellite method, except in a sort of reverse fashion

 

        Only difference is that the time-elapsed signal is not ground based, rather its from fixed object in space

                

      4. The Hot Spot Technique

        Only method that may provide absolute rates of movement and direction of motion of plates

 

        Absolute determinations possible because active hot spots mark the sites of fixed mantle plumes that appear to originate from deep within the mantle

 

         Hot spots are independent of lithospheric plates and fixed with respect to Earth's rotational axis

 

 Useful as reference points for determining paleolatitude

 

VII. Causes of Plate Motion - Plate Driving Mechanisms

       A. Presently there are three proposed mechanisms for

             driving the movement of tectonic plates

 

    Mantle Convection

 

    Ridge Push

 

   Slab Pull

 

              1.  Friction of mantle (athenosphere) convection currents against bottom of plates

 

        Plates dragged by coupled traction forces

 

        Like a raft carried by a river current

 

        Termed "plate drag"

 

              2. Lateral outward push of new, high-standing mid- ocean ridge lithosphere

 

        Plate slides off raised ridge, due to force of gravity; raised end

exerts a pushing effect on low end

 

        Like a sliding cookies off a tipped baking sheet

 

        Termed "ridge push   

                

              3. Downward pull of a descending plate's cold, dense leading edge.

 

     Extra-dense plate edge isostatically sinks down into the mantle under its own weight; the rest (of the plate) gets pulled along with it.

 

        Like a table cloth slipping off the end of a table

 

        Termed "slab pull"

 

 

VIII. Paleogeographic Reconstruction

     A. Term used to describe the technique of model-mapping ancient geographic settings

on Earth, using numerous geologic and biologic criteria recorded in rock record:

o      Paleomagnetism

o      Paleontology

o      Stratigraphy

o      Paleotectonics

o      Paleoclimatology

 

IX. The Rock and Hydrologic Cycles

       A. The Rock Cycle

              1. A multi-process recycling (creation & destruction) of one rock type into another

 

                2. Three major rock types (material reservoirs)

       Igneous

       Sedimentary

       Metamorphic

 

3.  Several major multi-step rock-forming processes

       Partial melting (magma), cooling & crystallization

 

       Weathering, erosion, deposition, compaction, & cementation and/or crystallization

 

       Recrystallization & neocystallization of solid rock under elevated temperature and/or pressure

 

                4. The various rock reservoirs and related processes are all interconnected under

the title: Rock Cycle

 

       B. The Hydrologic Cycle

            1. A multi-process (re)cycling of water between the hydrosphere, atmosphere, and

lithosphere

 

             2. Phase changes of water during cycling

 

              3. Several major water reservoirs

       Ocean THE Biggie

       Atmosphere

      Glaciers

       Lakes and Rivers

       Groundwater

       Plants

 

4.  Several water-transforming & moving processes

       Evaporation

       Transpiration

       Precipitation

       Runoff

       Ocean Currents

       Wind and Cyclones

 

     5. Ocean Circulation Modified by Moving Continental Masses

       Long-term ocean basin shape modification

       Affects on long-term climate

 

X. Tectonically-Controlled Mineral Resources

       A. Divergent Seafloor Spreading Processes

                1. Massive metal sulphides deposits

o      Hydrothermal vent activity

o      Example: Cyprus, Mediterranean Sea

 

        B. Convergent Subduction Zone Processes

                1. Porphyry metal lead/sulphide deposits

o      Hydrothermal plutonic activity

o      Example: Bingham, Utah

 

                   2. Gem vein deposits

o   Plutonic fluid activity

o   Example: Pala District, San Diego County

                    

        C. Continental Collision Zone Processes

                1. Petroleum development and concentration

o   Ocean basin collapse

o   Example: Mid-East

 

                   2.  Various mineral and gem deposits

o   Mountain-building processes

o   Example: Himalayas

 

XI. Plate Tectonics Vocabulary

Athenosphere

Benioff zone

Continental-continental boundary

Continental drift

Continental margin arc

Convergent plate boundary

Divergent plate boundary

Hot spot

Lithosphere (plate)

Magnetic anomalies

Mantle thermal convection cell

Paleogeographic reconstruction

Pangaea

Plate tectonic theory

Polar wander paths

Oceanic-continental boundary

Oceanic-oceanic boundary

Oceanic ridge

Oceanic trench

Ophiolite

Seafloor spreading

Slab pull

Slab push

Subduction

Transform fault

Transform plate boundary

Volcanic island arc