Transcript Chapter 6

Chapter 8
The Theory of Plate
Tectonics
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Objectives
Given information about Plate Tectonics you
will be able to describe:
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The history & development of the theory
Supporting evidence
Plate tectonic processes
Phenomena explained by tectonic theory
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Guiding Questions
• What is the evidence for Gondwanaland?
• How does paleomagnetism demonstrate that continents
have moved in the course of time?
• How was paleomagnetism used to show that
lithosphere forms at mid-ocean ridges and then
migrates away?
• How did features of the seafloor engender the concept
of plate tectonics?
• Why do faulting and volcanism occur along oceanic
plate margins?
• What causes lithospheric plates to move?
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• How can geologists measure rates of plate movement?
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Plate Tectonics
• Tectonics
– Movement of
Earth’s crust
• Plate tectonics
– Movement of
discrete segments
of Earth’s crust in
relation to one
another
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Continental Drift
• Idea that continents
move horizontally
over Earth’s surface
– Alfred Wegener
– Alexander Logie du
Toit
Wegener
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du Toit
Continental Drift
• Early scientists
recognized
relationship
between fossils on
continents
separated by sea
– Proposed land
bridges
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Continental Drift
• Glossopteris flora
present only in
southern
hemisphere
continents
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Continental Drift
• Early recognition
that Mid-Atlantic
Ridge was site of
landmass rupture
and Atlantic Ocean
formation
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Continental Drift
• Wegener’s Evidence
– Continents fit together
• Pangaea
– Geologic similarities
– Floral and faunal similarities
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Continental Drift
• du Toit’s evidence
– Expanded Wegener’s
ideas
– Proposed Pangaea split
into Laurasia &
Gondwanaland
– Mesosaurus fossils
• Found on Gondwana
continents
• Freshwater
– Could not swim
across Atlantic
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Continental Drift
• Geologic
Similarities
– Brazil and South
Africa have nearly
identical geologic
sequences
• Similar in
Antarctica and
India
– Glacial sediments
– Coal
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Continental Drift
• Glaciers
– Orientation of
glacial markings
on all continents
suggests they were
linked
– Must reconfigure
continents back to
Pangaea for this to
make sense
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Additional Evidence
• Lystrosaurus
– Heavyset herbivore
– Pig-like tetrapod
– Found in Antarctica
• 1969
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Paleomagnetism
• Magnetization of
ancient rocks at the
time of their formation
• Declination
– Angle that a compass
needle makes with the
line running to the
geographic north pole
• Rocks lock in this
orientation at formation
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Paleomagnetism
• Apparent Polar
Wander
– First studies
indicated poles had
moved
– Instead, plates had
moved
– North American
and European paths
met
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Rise of Plate Tectonics
• Harry Hess, 1962
– Geopoetry
• Continents didn’t plow
through seafloor
• Entire crust moved
– Crust must be created
and destroyed
• Sedimentary cover too
thin for four billion
years of accumulation
– Driven by convective
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cells
Rise of Plate Tectonics
• Ridges
– Site of crustal
formation
– Hot rising mantle
material rises to top
of lithosphere,
cools
– Ocean crust is
formed
– Bends away from
center to form ridge
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Rise of Plate Tectonics
• Guyots
– Had identified flattopped seamounts
in Pacific
– Realized they were
volcanoes that had
been eroded by
waves at sea level
– Postulated as crust
moved away from
ridge it cools and
sinks
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Rise of Plate Tectonics
• Crust is destroyed at subduction zones
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Test of Plate Tectonics
• Paleomagnetism
– Vine and Matthews,
1963
– Measured
magnetization of
rocks across the
Indian Ocean
central ridge
• Found normal and
reversed “stripes”
• Mirror image
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Processes at Plate Boundaries
• Normal faults
– Extensional motion
• Thrust faults
– Compressional motion
• Strike-slip faults
– San Andreas fault
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Processes at Plate Boundaries
• Mid-Ocean Ridges
(MOR)
– Graben
• Valley bounded by
normal faults along
which a central
block has slipped
downward
– Pillow basalt
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Processes at Plate Boundaries
• Transform faults
– Offset MORs
– Enormous strikeslip faults
– Seismically active
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Processes at Plate Boundaries
• Subduction
– Descending slab undergoes
dehydration which causes
partial melting of the
overlying mantle
– Molten material is less
dense, rises
• Common around
Pacific
– Ring of Fire
• Location of most of the
world’s trenches
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Processes at Plate Boundaries
• Associated with:
– Volcanoes
• Island arc
– Deep-focus
earthquakes
• > 300 km depth
– Forearc basin
• Zone of intensely
deformed rocks in
belt between island
arc and deep-sea
trench
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Processes at Plate Boundaries
• Forearc Basin
– Deep-ocean dark muds
and graywackes with
ocean crust mixed in
• Mélange
– Chaotic deformed
mixture of rocks
• Accretionary Wedge
– Body of rock that
accumulates as plate is
subducted
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Plate Motion
• Why plates move:
– Drag on the base of the plate – “Slab Pull”
– Elevation at ridge pushes plate ahead of it – “Ridge Push”,
“Gravity Sliding”
– Plate is pulled into subduction zone by preceding parent plate –
“Slab Pull”
– Broken plate segments create additional forces
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Plate Motion
• Eight large plates
• Several small plates
• Not all plates move
at the same rate
– Relative motion
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Plate Motion
• Absolute plate
motion
– Establish using
fixed point
– Hot spot
• Small geographic
area where heating
and igneous activity
occur within the
crust
• Yellowstone
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• Hawaii
Plate Motion
• Hawaiian hot spot
– Thermal plume
creates volcano
– Plate moves away
from plume
– Stranded volcano
cools, leaves a
chain
– Chain indicates
direction and rate
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Plate Motion
• GPS
– Global Positioning
System
– Earth-orbiting
satellites identify
motion
• Transmitter on
satellite
• Ground-based
receiver
• Average rate
– 5 cm/year
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Summary
• Continental drift –proposed by Wegener, added to
by du Toit
• Plate Tectonic theory developed by late 1960’s
• Evidence = fossils, rock types, shapes of
continents, paleomagnetic data, gps
• Driven by convection – heat in earth’s interior
• 3 plate motions – convergent, divergent, transform
• Phenomena explained on earth = mountains,
trenches, ore deposits, volcanoes, earthquakes,
tsunamis, etc.
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