Warm Up 11/33
What hypothesis states that the continents were once joined
to form a single supercontinent?
a. continental drift
c. plate tectonics
b. seafloor spreading d. paleomagnetism
2. What kind of plate boundary occurs where two plates grind
past each other without destroying or producing lithosphere?
a. convergent boundary
c. divergent boundary
b. transitional boundary
d. transform fault boundary
3. The lithospheric plates move an average of ____.
a. 50 inches per year c. 5 centimeters per year
b. 5 inches per year d. 50 centimeters per year
Answers: 1) a. 2) d. 3) c.
1.
Actions at Plate
Boundaries
Chapter 9, Section 3
Divergent Boundaries
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Most divergent boundaries are located
along the crests of oceanic ridges
These plate boundaries can be
thought of as constructive plate
boundaries, because it is where new
oceanic lithosphere is created
Location of Most
Divergent Boundaries
Oceanic Ridges and Seafloor
Spreading
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Oceanic Ridge – along well-developed divergent
plate boundaries, area where seafloor is elevated
The system of ridges is the longest physical feature on
Earth’s surface (70,000 km long)
These features are 1000 to 4000 km wide, not narrow
at all
Rift Valley – Deep faulted structures found along the
ridge system
Seafloor Spreading – process by which plate
tectonics produces new lithosphere
Typically the rates of spreading average about 5
cm/year, fast enough that all of the current oceans
have been produced in the last 200 million years
Seafloor Spreading
Continental Rifts
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When spreading centers develop within a
continent, the landmass may split into two
or more smaller segments
The most widely accepted view is that
areas where plumes of hot rock rise from
the mantle are weaker then other areas,
stretching the areas above it causing
faulting and volcanism
In the East African rift valley, the
beginning stages of continental breakup
may be occurring creating the Red Sea
East African Rift Valleys
Concept Check
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How do rifts begin to form?
The lithosphere is stretched and a
plume of hot rock from the mantle
weakens and then splits the
lithosphere
Convergent Boundaries
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Because lithosphere is being “destroyed” at
convergent boundaries, they are also called
destructive plate margins
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Subduction Zones – destructive plate margins
where oceanic crust is being pushed down into
the mantle
Trench – Surface feature produced by
descending slab, some of the deepest spots on
Earth
Convergent boundaries are controlled by the
type of crust involved in the convergence
Oceanic-Continental
Convergence
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When a continental plate meets an oceanic
plate, the less dense continental plate remains
floating as the oceanic plate is pushed
underneath
When the descending plate reaches a depth of
~100-150 km, some of the surrounding material
will begin to melt, forming magma
Some of this magma may reach the surface and
cause volcanic eruptions
Continental Volcanic Arc – mountains
produced by the volcanic activity that is caused
by the subduction of oceanic lithosphere under
continental (Andes Mountains)
Oceanic-Continental
Convergence
Oceanic-Oceanic
Convergence
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When two oceanic slabs converge, one
descends below the other
This causes volcanic activity similar to the
oceanic-continental boundary, with the
volcanoes forming on the ocean floor instead
It will eventually form a chain of volcanic
structures that will become islands
Volcanic Island Arc – newly formed land
consisting of an arc-shaped chain of small
volcanic islands (Aleutian Islands, Alaska)
Oceanic-Oceanic
Convergence
Continental-Continental
Convergence
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Eventually, subduction may bring two continents
together and a collision occurs
The result of this collision is usually seen in the form
of complex mountains such as the Himalayas in South
Asia
When the continents collide, after all of an ocean
basin between them has been subducted, the collision
folds and deforms the sediments along the margin
A mountain chain forms that is composed of deformed
and metamorphosed sedimentary rocks
The Himalayas, Alps, Appalachians, and Urals were all
formed by this process
Continental-Continental
Convergence
Collision of India and Asia
Concept Check
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What happens when continents
collide?
Mountains form as the lithosphere is
forced upward
Transform Fault
Boundaries
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At a transform fault boundary, plates grind past each
other without destroying lithosphere
Most transform faults join two segments of mid-ocean
ridges, present about every 100 km along the ridge
axes
The seafloor produced at one ridge axis moves in the
opposite direction as the seafloor produced at an
opposing ridge segment, and these segments grind
past each other on their ways
Most transform faults are found within the ocean
basins, a few cut through the continental crust (San
Andreas Fault)
Transform Faults
The Role of Transform
Faults
The World’s Oceanic Trenches,
Ridge System, and Transform
Faults
Assignment
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Chapter 9 Assessment
P. 275 – 276 # 1 – 31
P. 277 # 1 – 9