Plate Tectonics

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Plate Tectonics
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Types of Plate Boundaries
Divergent
Convergent
Transform
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• Plates move apart, resulting in upwelling of
  material from the Mantle to create new sea floor.
• Plates move together, causing one of the slabs
  of lithosphere to be consumed into the Mantle as
  it decends beneath the overriding plate.
• Plates slide past each other, without creating
  or destroying lithosphere.

Divergent
Convergent
Transform Fault
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Fig.4.28

• W. W. Norton. Modified from Cox and Hardt, 1986.

Each plate is bounded by a combination of these
types of boundaries.
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New lithosphere is continually being created at
spreading centers Since the total surface area
of the Earth remains constant, lithosphere must
also be destroyed somewhere else. Zones of
Plate Convergence
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When two plates collide, the leading edge of one
is bent downward, allowing it to descend beneath
the other plate.
7
Upon entering the hot asthenosphere, the plunging
plate (which is relatively cold) begins to warm
and loses its rigidity.
8
The descending lithosphere reaches a depth of up
to 700 km (depending on its angle of descent)
before its leading edge becomes assimilated into
the material of the upper mantle.
W. W. Norton
9
Three types of convergent plate boundariesThe
nature of convergent boundaries is influenced by
the type of crustal material involved.
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Oceanic - Continental
Oceanic - Oceanic
Continental - Continental
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When the leading edge of a plate capped with
continental crust converges with oceanic
crust The less dense continental material
(granitic) remains floating, while the more
dense oceanic slab (basaltic) sinks into the
asthenosphere.
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A region where an oceanic plate descends into the
asthenosphere is called a subduction zone. As
the oceanic plate slides beneath the overriding
plate, the oceanic plate bends, producing a
deep-ocean trench.
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Oceanic-Continental Convergence
The oceanic crust is bent (at an angle of about
45º), permitting it to descend into the
lithosphere.
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Oceanic-Continental Convergence
Upon entering the hot asthenosphere, the downward
moving plate and the water soaked sediments
carried upon it begin to melt.
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This newly formed magma is less dense than the
surrounding mantle rocks, which causes the magma
to rise to the surface.
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Fig.4.14a
Most of this magma will be emplaced in the
continental crust and form intrusive igneous
rocks.
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Rock Cycle
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Fig.4.14a
The remaining magma will eventually migrate to
the surface as volcanic eruptions and will form
extrusive igneous rocks.
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Deep earthquakes occur at subduction zones.
20
The deepest earthquakes occur at depths as great
as 700 km below the surface.
W. W. Norton
21
The volcanic Andes Mountains were formed by
oceanic-continental convergence, when the Nazca
plate melted as it plunged beneath the continent
of South America.
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Plate Tectonics
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The largest earthquake ever recorded occurred
along the Nazca-South American plate
boundary. Chile May 22, 1960 Magnitude 9.5
Berkeley, CA Seismogram
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Fig. 6.18
Major Volcanoes of the World

• W. W. Norton

25
The Cascade Mountain Range, in western US, is a
volcanic mountain range that was formed as a
result of oceanic-continental convergence.
Eruption of Mount St. Helens in 1980
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The Cascade Mountain Range is a result of the
Juan de Fuca Plate subducting under the western
US.
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Oceanic - Continental
Oceanic - Oceanic
Continental - Continental
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Oceanic - Oceanic Convergence
When two oceanic plates converge, one descends
beneath the other initiating volcanic activity
(similar to the oceanic - continental case), but
the volcanoes form on the ocean floor rather than
on continents.
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Volcanic Island Arc
Dry land emerges from the ocean depths, forming a
chain of volcanic islands called a volcanic
island arc.
30
Examples of an island arcs are the island chains
of the western Pacific Ocean, such as Japan.
In the case of Japan, volcanic islands are
being created by the collision of the Pacific
plate with the Eurasian plate. The Pacific plate
being subducted beneath the Eurasian plate.
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Japan Subduction Zone and Island Arc
The Pacific plate subducting beneath the Eurasian
plate.
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The January 17, 1995 Kobe, Japan
Earthquake Magnitude 6.9
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Fig.4.28

• W. W. Norton. Modified from Cox and Hardt, 1986.

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Aleutian Islands Example of an island arc off the
coast of Alaska
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The second largest earthquake ever recorded
occurred along an oceanic - continental plate
boundary in Alaska. Great Alaska
Earthquake March 28, 1964 Magnitude 9.2
Elementary school in Anchorage, Alaska destroyed
by the 1964 Alaska earthquake.
Dallas, Texas Seismogram
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Oceanic - Continental
Oceanic - Oceanic
Continental - Continental