I. Lecture Content
Origin
of Earth and our Solar System
What
Makes Earth a Dynamic Planet?
The Plate Tectonic Theory
Causes of Plate Motion
Origin of Magmas
Determining Plate Motions
Plate Tectonics as a
Unifying Principle
II.
Origin of Our Earth and Solar System
A. Solar System Formed from Interstellar Gas & Dust
1. Material came from two sources
· Original primordial gases
left over from Big Bang
ü Mostly Hydrogen & Helium
· Secondary material from
exploded star(s)
ü All sorts of elements from H
to Uranium
ü Both gases and solid matter
2. Coalescing of cold matter done by gravitational
fields
· Gravity waves through
interstellar space
·
Increasing
gravity force of condensing matter
B. Earth Accreted from the Rotating Solar
Nebula Disc
1. Condensing Solar nebula cloud began
rotating
2. Increasing angular momentum
caused nebula to flatten
from an irregular mass into a spinning disc-like form
3. Central mass condensed into
the "proto" Sun
4. Outer mass coalesced into
many planetisimals, which
eventually lumped
together to form "proto" planets
5. Over time, the
"proto" planets swept up the remaining
planetisimals to become
the nine known planets
6. Inner planets, including
Earth, formed mainly from the
cold accretion of solid materials
(heavier elements)
· Metals (mostly iron,
magnesium & nickel)
· Silicates
7. Outer planets, like Jupiter,
formed mainly from volatiles
and gases (lighter
elements)
· Hydrogen & Helium
· Water & Carbon Dioxide
· Ammonia & Methane
8. Accretion process probably
took 1 BY to 500 MY
C. Age of Earth & Solar System is
about 4.6 Billion Years
Ø Age come from the dating of
meteorite & Moon samples
D. Infant Earth Underwent Further
Differentiation
1. Segregation of Core and
Mantle (Earth entirely molten)
2. Theorized collision of a
Mars-sized planet with Earth to
form the Earth-Moon system - Extremely violent event
3. Intense period of
planetisimal bombardment
4. Sufficient cooling to
create a solid, thin, & very mobile
lithosphere made up of many fast-moving micro plates
5. Earth's atmosphere
underwent drastic changes
III.
What Forces Keep Earth Dynamic?
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. Evidence
of Continual Change Since Accretion
1. Historical differences in character of the Earth's rock
record for different periods of geologic time
Ø Unique rock types; their abundance & occurrence
Examples:
Komatiites
Sedimentary iron beds
Anorthosites
Greenbelts
Glacial tillites
Ø Unique fossils; their abundance & occurrence
Examples:
Trilobites
Ammonites
Dinosaurs
Ø Distinctive tectonic terranes and their locations
Examples:
Wrangalia
Ophiolites
Southern California & Baja
Mount Everest & the Himalayas
2. Present-day geologic activities and events
Ø Earthquakes; Volcanic
eruptions; Uplift & Erosion
C. Internal Forces Produce Heat and
Density Gradients
1. Gravity and radioactive decay
generate internal Earth heat
ü Gravitational heat left over
from accretionary event
ü Sustained nuclear
"barbeque" in mantle and crust
2. Internal Gravity and Thermal energy generate density
contrasts in the solid Earth = Internal mechanical
layering
ü
Core
ü
Mantle
ü
Lithosphere/Crust
3. Rotational momentum -
Coriolis effect; other effects
4. Organic
Life - Biochemical terra-forming
D. External Forces Produce Heat and Density Gradients
1. Solar Radiation generates thermal and
density contrasts in
the hydrosphere and
atmosphere
ü Dynamic Layering and
Circulation Patterns
2.
Gravity of the Moon and Sun generate tidal forces on Earth
ü Ocean tides
ü Earth tides
3.
Bolides (comet and meteor impacts)
IV. The Plate Tectonic Theory
A. Earth's Outermost Solid Layer is
called Lithosphere
1. Consists of two parts
(sub-layers)
· Crust (top part)
· Uppermost mantle (bottom
part)
2. Two major types of Lithosphere
· Continental lithosphere
· Oceanic lithosphere
3. The lithosphere "floats" on top of the partially melted
Athenosphere
4. The lithosphere is made-up of
a number of separate,
irregular segments called tectonic plates
· Six major plates
· Six or so minor plates
B.
The Lithospheric Plates are Mobile
1. The plates move over the
underlying mantle
2. Each plate has a separate and
unique plate motion
3. The tectonic plates jostle
with one another
4. Plates interact with one
another in three different ways
C. There are Three Types of Plate Boundaries
1. Divergent
ü tensional setting
ü plates move away from each
other
ü new plate material is
generated
ü spreading centers occur at
this boundary
2. Convergent
ü compressional setting
ü plates move toward each
other
ü old oceanic plate material
is destroyed
ü new continental material is
created
ü subduction zones occur at
this boundary
3. Transform
ü shearing setting
ü plates move laterally past
each other
ü plates are neither created
nor destroyed
ü great strike-slip faults
occur at this boundary
D. Plate Boundaries are the Primary Site of
Present-day
Mountain
Building Events (Orogenies)
1. Major Earthquakes
2. Magmatism and Volcanism
3. Crustal Uplift
4 Folding and Faulting
E. Seafloor Spreading Occurs at Divergent Boundaries
1. New oceanic lithosphere is
created where two plates are
actively pulling away from one another
2. Predominant regional
tensional forces at work
3. Site of the spectacular
mid-ocean ridge system
4. Examples: Mid Atlantic
Ridge & East Pacific Rise
F. Subduction Occurs at Convergent
Boundaries
1. Old, dense oceanic
lithosphere plunges back into the
underlying mantle
2. Predominant regional
compressional forces at work
3. The site of a paired oceanic
trench/ volcanic arc system
4. Examples: Cascades, the
Andes, the Alps, & Himalayas
V. 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"
VI. Plate Tectonics Causes Global-scale
Magmatism
A. Three Tectonics Settings Where Magmas Form
1. Spreading Centers
2.
Subduction Zones
3. Hot
Spots
B. Each
Tectonic Setting has a Unique Set of Conditions
That
Promotes Partial Melting (Magma)
1. Spreading Centers
ü
Lowered
pressure due to tensional forces
ü
Upwelling of hot ultramafic mantle rocks melt
ü
Typical
magmas are basaltic in composition
2. Subduction Zones
ü Lowering of melting
temperature due to release of
water from dehydrating down-going slab
ü Both the subducting basaltic
crust and overlying
ultramafic mantle rocks get melted
ü Rising magmas cause melting
at base of crust
ü Typical magmas are andesitic
in composition
3. Hot Spots
ü
Occurs
both within plates or at plate boundaries
ü
Strong upwelling of narrow plumes of very hot
and buoyant ultramafic mantle rocks
ü
Melting
of mantle rock occurs when pressure is sufficiently reduced near base of
lithosphere
ü
Magma
sources appear fixed deep in the mantle
as moving plates shift position over a hot spot
ü Typical magmas are basaltic
in composition
C. Each
Tectonic Setting has Characteristic Magmas
1.
Spreading centers & hot spots = Dry, hot, basaltic
2. Subduction
zones = Wet, cooler, andesitic
VII. 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 past times
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.
GPS-Satellite Technique
· Generate a record of highly accurate ground position measurements
over time
· GPS stations calculate their position relative to four
or more geostationary satellites, including
latitude, longitude, and elevation to within
millimeters
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
VIII. The Plate Tectonic Theory is a
Unifying Principle
1. Best explains the
relationships between many different
and seemingly unrelated geologic phenomena
2. Usefulness for understanding past, present, and
future
geologic
changes and events