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
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
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:
ü Continental margin fits
ü Match-up of truncated mountain ranges and faults
ü Match-up of 'stratigraphic' sequences and mineral deposits
· Paleontological data:
ü Match-up of extinct plant species fossil localities
ü Match-up of extinct animal species fossil localities
· Climatologic data:
ü Match-up of temporally-equivalent glacial deposits
ü 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.
ü 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
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
· Each continent has a unique "wander path".
Ø 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.
Ø 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
- 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
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
· 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
Ø A line along which two plates move apart
Ø Tensional tectonic forces dominate
Ø Oceanic crust forms along divergent boundaries
1. Continental (rifting)
· Spreading center
ü Rift valley (pull-apart basin)
· Examples: East Africa Rift Valley
2. Oceanic (basin extension)
· Spreading center
ü Mid-ocean ridge system
ü Transform fracture system
· Examples: Mid Atlantic Ridge
B. Convergent Plate Boundaries - Three Styles
Ø A line along which two plates move towards each other
Ø Compressional tectonic forces usually dominate
Ø Ocean crust is consumed at convergent boundaries
1. Oceanic-Oceanic Plate Convergence
· Subduction zone complex
ü Oceanic trench
ü Volcanic island arc
· Examples: Aleutian Island trench/arc belt
2. Oceanic-Continental Plate Convergence
· Subduction zone complex
ü Oceanic trench
ü Volcanic continental margin arc
· Examples: Andes trench/arc belt
3. Continental-Continental Plate Convergence
· Continental collision complex
ü Uplifted fold/thrust mountain belt
ü Collapsed ocean basin suture zone
· Examples: Himalayas
C. Transform Plate Boundaries - Three Styles
Ø Line along which two plates slide laterally past the other
Ø Shearing tectonic forces usually dominate
Ø Crust is neither created of destroyed at this boundary
1. Oceanic-Oceanic Plate Transform
· Transform fault
ü Ridge-ridge fracture zone
ü Ridge-trench fracture zone
ü Trench-trench fracture zone
· Examples: Mendocino fracture zone
2. Oceanic-Continental Plate Transform
· Transform fault
ü Great strike-slip fault zone
· Examples: Queen Charlotte Fault
3. Continental-Continental Plate Transform
· Transform fault
ü 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
ü 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
Ø Mantle Convection
Ø Ridge Push
Ø 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:
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)
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
2. Phase changes of water during cycling
3. Several major water reservoirs
· Ocean – THE “Biggie”
· Lakes and Rivers
4. Several water-transforming & moving processes
· 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
Continental margin arc
Convergent plate boundary
Divergent plate boundary
Mantle thermal convection cell
Plate tectonic theory
Polar wander paths
Transform plate boundary
Volcanic island arc