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:
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
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".
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
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
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:
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