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PLATE TECTONICS
Continental Drift
E.B. Taylor (1910) and Alfred Wegener (1915) published on Continental Drift. Later Alexander du Toit (1937)gave more supporting evidence.
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Continental Drift
Wegener and duToit’s evidence
1. Fit of the Continents 2. Fossil Evidence 3. Rock Type and Structural Similarities 4. Paleoclimatic Evidence
Continental Drift
Du Toit’s evidence
Fossils - Mesosaurs
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Continental Drift – Wegener’s Evidence,
fossils
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Continental Drift – Wegener’s Evidence
Rocks and Geologic Structures
Same rock types
Same order
Same faults and folds
Same age
300my old orogeny
Continental Drift- Wegener’s Evidence
Climates
Evidence of glaciation
Unlikely places
Better explained by fit
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Continental Drift – Du Toit’s Evidence
Climate
Coal Deposits
Should be in warm temperate areas or equatorial areas
Better explained by fit
Continental Drift
Wegener lacked a mechanism for continental movement through oceanic crust.
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Evidence: Polar Wandering – Keith Runcorn
Plate Tectonics
Evidence from exploration of the ocean floor Global oceanic ridge system and guyots
Paleomagnetic stripes
Deep ocean trenches associated with earthquakes – Benioff zones
All seafloor less than 180my old
Sediments thin where expected to be thick
Ocean deepest away from center, near edges
Rocks youngest near center, ageing toward edges
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Plate Tectonics
Harry Hess (1906-1969)
Surveyed ocean floor
Discovered guyots
Hypothesized sea-floor spreading
Plate Tectonics – the ocean floor
The mid-ocean ridge system is a nearly continuous volcanic ridge found in all oceans.
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Sea Floor Spreading
A process known as seafloor spreading occurs where
magma from the mantle wells up into the divergent
boundary - forming new basaltic seafloor.
Spreading rates average ~5 cm/year.
Sea Floor Spreading
The mid-ocean ridge has an elevated position on the seafloor because it is formed from relatively hot igneous rocks.
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Plate Tectonics- Sea Floor Spreading
As the seafloor moves
away from the ridge, it
cools and contracts — thus
the seafloor generally is at
a greater depth as you
move away from the mid-
ocean ridge.
Sea Floor Spreading
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Plate Tectonics – Sea Floor Spreading
Fred Vine and D.H. Mathews
Interpreted “magnetic stripes”
Earth has experienced periods of reverse polarity
As magma solidifies it is magnetized according to the polarity at the time
Stripes of magnetic polarity are formed at mid-oceanic ridges
Magnetic Stripes
In the figure, regions of
normal polarity are indicated
in white where magnetic
north is coincident with the
geographic north.
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Magnetic Stripes
Stripes are parallel to mid-oceanic ridge
Stripes are mirror images across ridge
Dates of stripes indicate sea floor spreading
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PLATE TECTONICS
Sea-floor spreading provided the missing mechanism for Continental Drift
Plate Tectonics
If the sea floor is spreading, and the earth is not expanding, where does the excess crust go?
Subduction zones
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Subduction Zones
Studies of crust around deep sea trenches showed zones of earthquakes beneath the crust called “Wadati-Benioff Zones”
Heat and heat flow studies around trenches revealed a “cold slab”
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Plate Tectonics – Subduction Zones
Earthquake epicenters revealed plate boundaries.
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Plate tectonics is a theory about how
the surface of the Earth evolves due to
strong internal forces. The surface of
the Earth is composed of rigid plates
that are mobile and move relative to one
another.
Plate tectonics is a unifying theory in Geology — different geologic
phenomena such as mountain building, earthquakes, volcanoes, and the
distribution of fossils and organisms can be explained through plate
tectonics.
Earth’s Major Plates
The Earth’s surface is composed of a strong, rigid layer known as the
lithosphere. The lithosphere is broken into pieces known as tectonic
plates.
Lithospheric plates are thinnest in the oceans (<100 km thick) and may
be more than 250 km thick on the continents.
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The lithospheric plates
overlie a weaker region
of the mantle known as
the asthenosphere.
The rocks in the
asthenosphere are near
their melting point and
are relatively weak and
ductile.
The asthenosphere
allows the plates to
move above it. Plates
move slowly but
continuously - generally
on the order of a ~5
cm/year.
Note that the plates generally include a continent or a portion of a
continent AND a portion of the ocean floor.
Each plate moves as a coherent unit relative to others
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Plate Tectonics- Plate Boundaries
Plate Boundaries are where earthquakes, volcanoes, and crustal deformation take place.
There are three general types of boundaries:
Divergent –Plates move away from each other
Convergent- Plates move toward each other
Transform – Plates move past each other
Divergent Boundaries
Most divergent boundaries are located along mid-ocean ridges.
Divergent plate boundaries are known as constructive margins because
they are the site where new oceanic crust (lithosphere) is generated.
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Some divergent boundaries occur under continental crust
The East African Rift represents a modern example of a continental rift. If this rift is successful, eastern Africa may split off from the rest of the continent and a new ocean basin may form between the two “Africas.”
Convergent margins are also known as destructive margins since oceanic crust is destroyed or consumed.
Most convergent margins are associated with a subduction zone
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The map shows the world’s oceanic trenches. Note that the Pacific is
Nearly encircled in deep-ocean trenches.
Convergent Boundaries: Oceanic-Continental Convergence
Oceanic-continental
convergence occurs when
leading edge of one plate is
composed of continental
rocks (granitic) and the other
is oceanic (basaltic).
The denser oceanic plate dives beneath (subducts) the lower-density
continental plate. Lower density granitic rocks tend to float in the
asthenosphere, i.e., via isostacy
Dewatering of the subducted slab causes melting in the wedge of the
asthenosphere above it. The magma that is produced is buoyant and
rises through the mantle toward the Earth’s surface.
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The magma results in volcanic activity along a line parallel to the
subduction zone known as a continental volcanic arc.
Examples of continental volcanic arcs include
the Cascade volcanoes such as Mt. Rainier and
Mt. St. Helens and the volcanoes of the Andes
mountains along the west coast of South
America.
Mt. St. Helens
Convergent Boundaries: Oceanic-Oceanic Convergence
Oceanic-oceanic convergence
occurs when the leading edge
of both plates consists of
oceanic crust. These plate
boundaries have many of the
same features as in oceanic-
continental convergence.
In oceanic-oceanic convergence,
the line of volcanoes forms a
string of islands parallel to the
subduction zone known as a
volcanic island arc.
Examples of island arc systems
include the Aleutian Islands,
Tonga, Indonesia, and Japan.
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Ocean to Ocean Convergence NOTE: Oceanic crust subducts other oceanic crust Volcanoes form islands
Continent-continent convergence usually begins as oceanic-
continental convergence (ex. Andes). As the oceanic crust is
subducted, a continental block on the subducting plate may approach
the continent.
The Himalayan mountains were formed by the collision of the Indian
subcontinent into the Asian mainland.
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These figures show the convergence of
India into Asia over the last 71 million
years.
Continent to Continent Collision
Uplifted continental crust No volcanoes Folding and faulting
NOTE:
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Transform Fault Boundaries
Transform plate boundaries
are where plates slide past one
another.
Most transform boundaries are
associated with mid-ocean
ridges where they form linear
breaks in the ridge system.
The active transform boundary
exists between the two offset
ridge segments
Transform Faulting
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The Breakup of
Pangaea
Now that we
understand plate
tectonics, we can use
geologic data to
reconstruct Pangaea
and model the
movement of the
continents during the
last 200 million
years.
Continental Drift animation
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What Drives Plate Motions?
This is an active area of research
and there is a diversity of opinions.
Most geologists agree on the
following points about the driving
forces for plate motion:
1. The Earth’s mantle is convecting - hotter rocks rise buoyantly and
cooler denser rocks sink. This motion helps drive plate motion.
2. Mantle convection and plate tectonics are part of the same system.
3. Density differences due to the unequal distribution of heat within
the Earth’s mantle ultimately drive the mantle convection cells and
plate motion.