Transcript Theory of Plate Tectonics III

Plate Tectonics
Types of
Plate
Boundaries
Divergent
Convergent
Transform
 Plates move apart, resulting
in upwelling of material from
the Mantle to create new sea
floor.
Divergent
 Plates move together,
causing one of the slabs of
lithosphere to be consumed
into the Mantle as it decends
beneath the overriding plate.
Convergent
 Plates slide past each other,
without creating or destroying
lithosphere.
Transform Fault
W. W. Norton. Modified from Cox and Hardt, 1986.
Each plate is bounded by a combination of these
types of boundaries.
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
When two plates collide, the leading edge
of one is bent downward, allowing it to
descend beneath the other plate.
Upon entering
the hot
asthenosphere,
the plunging
plate (which is
relatively cold)
begins to warm
and loses its
rigidity.
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
Three types of
convergent plate
boundaries:
The nature of
convergent
boundaries is
influenced by the
type of crustal
material involved.
Oceanic - Continental
Oceanic - Oceanic
Continental - Continental
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.
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.
Oceanic-Continental Convergence
The oceanic crust is bent (at an angle of about 45º),
permitting it to descend into the lithosphere.
Oceanic-Continental Convergence
Upon entering the hot asthenosphere, the
downward moving plate and the water soaked
sediments carried upon it begin to melt.
This newly formed magma is less dense than
the surrounding mantle rocks, which causes
the magma to rise to the surface.
Most of this magma will be emplaced in the
continental crust and form intrusive igneous
rocks.
Rock
Cycle
The remaining magma will eventually migrate
to the surface as volcanic eruptions and will
form extrusive igneous rocks.
Deep earthquakes occur at subduction zones.
The deepest
earthquakes occur
at depths as great
as 700 km below
the surface.
W. W. Norton
The volcanic Andes Mountains were formed by
oceanic-continental convergence, when the
Nazca plate melted as it plunged beneath the
continent of South America.
Plate Tectonics
The largest earthquake ever
recorded occurred along the
Nazca-South American plate
boundary.
Chile
May 22, 1960
Magnitude 9.5
Berkeley, CA Seismogram
Major Volcanoes of the World
Fig. 6.18
W. W. Norton
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
The Cascade Mountain Range
is a result of the “Juan de
Fuca Plate” subducting
under the western US.
Oceanic - Continental
Oceanic - Oceanic
Continental - Continental
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.
Volcanic Island Arc
Dry land emerges from the ocean depths, forming a chain
of volcanic islands called a volcanic island arc.
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.
Japan Subduction Zone and Island Arc
The Pacific plate subducting beneath the Eurasian plate.
The January 17, 1995 Kobe, Japan Earthquake
Magnitude 6.9
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W. W. Norton. Modified from Cox and Hardt, 1986.
Aleutian Islands
Example of an island arc off the coast of Alaska
The second largest
earthquake ever recorded
occurred along an oceanic continental plate boundary in
Alaska.
Great Alaska Earthquake
March 28, 1964
Magnitude 9.2
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Elementary school in Anchorage,
Alaska destroyed by the 1964
Alaska earthquake.
Dallas, Texas Seismogram
Oceanic - Continental
Oceanic - Oceanic
Continental - Continental