Geology Lecture Outline

Plate Tectonics (Chapters 2, 13, 14)

 

I. Lecture Content

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"

Geology of the Seafloor Nothing Like the Continents

Major Discoveries about the Seafloor - Let the Spreading Begin

Seafloor Spreading and Subduction - Creation and Destruction

Theory of Plate Tectonics - Unifying, Earth-Shaking Paradigm

Plates and Plate Boundaries - Inter-Plate Relationships

Determining Plate Movement & Motion - Past, Present & Future

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

Pangea and The Wilson Cycle - 500 Ma Supercontinent cycles

Paleogeographic Reconstruction - Modeling Earth's Past

Plate Tectonics and Natural Resources - Rhyme and Reason

 

II. 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 Supports Plate Tectonics

1) Seafloor and Continent Anatomy

2) Seismic Data

3) Fossil Record

4) Rock Dates

5) Magnetic Patterns

6) Satellite Geodesy

7) Volcanic Chemistry

8) Exotic Terranes

 

III. Early Ideas About 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 were 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 Pangea, 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

 

      Climatology 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?

IV. Overview of Earths Seafloor

 

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

       Starts at base of continental margins

 

       Deep seafloor consists primarily of:

1. High-standing mid-ocean ridge systems and

 

2. Low-standing sediment-covered abyssal plains

 

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

inherent differences between continental and oceanic

crust.

      Composition (density)

      Thickness

      Isostatic equilibrium

 

V. 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

 

VI. 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

D. 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

 

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

 

VII. 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

o     See Figure

       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.

      See Figure

C. Deep-sea Drilling Projects Confirm Seafloor Spreading

1. Analysis of ocean crust seafloor core samples

      Sediments and Basalts = ophiolite sequence

      See Fig.

 

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.

 

VIII. 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.

 

IX. 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 (see Fig. 12.14)

      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 athenosphere 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

 

X. Three Types of Plate Boundaries (See Table

 

A. Divergent Plate Boundaries - Two Styles

 

    A line along which two plates move apart

 

    Tensional tectonic forces dominate

 

    Oceanic crust forms along divergent boundaries

 

    See Figures

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

 

    See Figures

 

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

 

    See Figures

 

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

 

XI. 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

between anomalies by the amount of time that has

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

 

XII. Causes of Plate Motion - Plate Driving Mechanisms

A. Presently there are three proposed mechanisms for

driving the movement of the 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"

 

       See Figure

 

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"

 

       See Figure

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"

 

       See Figure

 

xIII. 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

 

XIV. 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

 

XV. 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

o     See Figure

 

2. Gem vein deposits

o  Plutonic fluid activity

o  Example: Pala District, San Diego County

o  See Figure

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

 

XVI. Seafloor and Plate Tectonics Vocabulary

 

Abyssal plain

Active continental margin

Atoll

Continental margin

Continental rise

Continental slope

Guyout

Isostatic equilibrium

Mid-oceanic ridge

Oceanic trench

Ooze

Ophiolite

Passive continental margin

Pelagic clay

Reef

Ridge fracture zones

Seamount

Submarine canyon

Submarine hydrothermal vent

Submarine fan

Turbidity current

Wilson cycle

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

 

Abyssal plain

Active continental margin

Atoll

Continental margin

Continental rise

Continental slope

Guyout

Isostatic equilibrium

Mid-oceanic ridge

Oceanic trench

Ooze

Ophiolite

Passive continental margin

Pelagic clay

Reef

Ridge fracture zones

Seamount

Submarine canyon

Submarine hydrothermal vent

Submarine fan

Turbidity current

Wilson cycle