PLATE TECTONICS

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PLATE TECTONICS
Hypothesis Earths Lithosphere moves on top of
Aesthenosphere due to Mantle
Convection Evidence of Lithospheric (Plate)
motions (Tectonics) Driving Mechanism of Plate
Tectonics
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EVIDENCE OF LARGE-SCALE HORIZONTAL MOTIONS OF THE
CRUST AND UPPER MANTLE Contributions of
Land-based Geologists (observed from 1600s to
present)

• Fit of coastlines, especially South America and
  Africa
• Identical species of land-dwelling fossils on
  different continents
• Alignments of rare minerals and fossils with
  other
• continents
• Linear patterns of mountain ranges, volcanoes,
  and earthquakes

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Reconstructions of Continents at Various Times in
the Past by Alfred Wegener
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Fossil Relationships of Rare or Limited Species
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EVIDENCE OF A DYNAMIC EARTH
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Evidence from Ocean-based Data(1950-present)

• Young Age of Seafloor (Continents (up to 4.5 b.y.)
• Discovery of 40,000 km long Mid Ocean Ridges
  (MOR)
• Age of seafloor increases symmetrically away from
  MOR crest

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Age of Seafloor
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New seafloor comes to the Surface at MOR At
MIMORs
Topography of the Mid Ocean Ridge N. Atlantic
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MAGNETIC ANOMALIES
Earths magnetic field periodically reverses its
polarity on average about every 400,000 years,
but range is 10,000 yrs to millions of
yrs Fe-rich minerals crystallizing in molten
rock align with Earths magnetic field and point
to N magnetic pole Magnetic signature is locked
in as rock solidifies Subsequent pole reversals
do not change the mineral alignment in solid
rock
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Seafloor-Spreading at Mid-Ocean Ridges a
Divergent Plate Boundary
5 4 3 2 1 0 0 1 2 3
4 5
Age of seafloor in m.y.
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Earthquake Evidence

• Quakes form linear zones
• Usually associated with volcanoes
• Deepest quakes reach 700km implies brittle
  failure occurring
• In some areas, there is an inclined plane of
  seismicity, marking the contact between a
  subducting plate and the overriding plate

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Earthquakes define Plate Boundaries
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Andean-type Convergent Plate Boundary
Subduction Zone where 2 plates collide, one
sinks, or subducts. Evidence for subduction comes
from inclined plane of seismicity, Called a
Wadati-Benioff Zone
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Earthquake Epicenters for central South America
Color change from yellow to dark blue represents
increasing depth
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Driving Mechanism of thePlatesConvection in
the Mantle
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How Convection Works
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Large convection currents in the aesthenosphere
transfer heat to the surface, where plumes of
less dense magma break apart the plates at the
spreading centers, creating divergent plate
boundaries. As the plates move away from the
spreading centers, they cool, and the higher
density basalt rocks that make up ocean crust get
consumed at subduction zones. The crust is
recycled back into the aesthenosphere.
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CONVECTION CELLS in the EARTH
MOR
MANTLE
OC
SUBDUCTION ZONE
IC
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THREE TYPES OF PLATE BOUNDARIES
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Transform Plate Boundary San Andreas Fault
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Japan-Sea Type Convergent Plate Boundary
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Andean-type Convergent Plate Boundary
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Himalayan-type Convergent Plate Boundary
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Indian Plate Collides with Asian Plate
S
N
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OBSERVATION Because ocean plates are denser
than continental plates, when these two types of
plates converge, the ocean plates are subducted
beneath the continental plates. The collision
of plates is accompanied by earthquakes and
volcanoes.
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Hot Spot Beneath Pacific Plate Creates
the Hawaiian Islands
Not all volcanic activity occurs at plate
boundaries
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SUMMARY TABLEProcess---Feature---Plate Boundary
Type

• Seafloor Spreading---Mid Ocean Ridges and
  Continental Rifts---Divergent Plate
• Boundaries
• Subduction---Subduction Zones---Convergent Plate
  Boundaries
• Shear---Strike-slip Faults---Transform Plate
  Boundaries