Geology Lecture Outline
Plate Tectonics
– (Chapters 2, 13, 14)
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:
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
· Climatology 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?
IV. Overview
of Earth’s 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
ü 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
ü 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
ü Pelagic clays
ü Silica and carbonates Oozes
· Oceanic igneous
crustal column
is also layered
Ø Pillow lava basalt
Ø Sheeted gabbroic dikes
Ø Massive gabbro (intrusions)
Ø Layered gabbro (intrusions)
Ø 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
Ø 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:
ü Mid-ocean rift valleys and
ridge flanks
ü Vast expanses of ocean crust
(abyssal plains)
ü Chains of volcanoes
(seamounts and islands)
ü Transform fracture systems
ü 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.
ü
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
Ø
A
line along which two plates move apart
Ø
Tensional
tectonic forces dominate
Ø
Oceanic
crust forms along divergent boundaries
Ø
See
Figures
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
Ø
See
Figures
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
Ø
See
Figures
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
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
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
XII. Causes of Plate Motion - Plate Driving Mechanisms
A. Presently there are three
proposed mechanisms for
driving the movement of the 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"
·
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