Download Oceanic Crust

Plate Tectonics
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Plate Tectonics
• Earth’s crust is cracked and
fragmented
– Fragments called plates
• The Earth’s mantle is very hot and
fluid, so the plates move atop the
mantle
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Earth’s Crust
• Two kinds of crust exist
–Granitic (felsic) and basaltic (mafic)
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Earth’s Crust
• Think of the Earth as
completely covered with a
layer of thin, dense basalt rock
with chunks of thicker, less
dense granite here and there
–The granite chunks are the
continents riding on the basalt
like wood floats on water
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Plate Boundaries--where tectonic
plates meet and interact
 Where
can you
find this
image?
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Plate Boundaries
• Three types
–Convergent
–Divergent
–Transform
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Divergent Boundaries
• Where plates are moving apart
–Cracks in the crust are filled by
upwelling molten rock
 Give one example of a
divergent boundary:
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Divergent Boundaries
• Divergent boundaries on continents
are called rifts
–Example: the African Rift Valley
• Splitting Africa in two
• Divergent boundaries under oceans
form mid-ocean ridges
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Divergent Boundaries
• Several plates are pulling apart
beneath the Atlantic ocean forming
the Mid-Atlantic Ridge
–Diverging ocean plates cause sea
floor spreading
• Where new sea floor is constantly being
added
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EAST AFRICAN RIFT VALLEY – CONTINENTAL RIFT
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In Afar, Ethiopia, a 40-mile magmatic
rift that opened up 1 year ago
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Spreading Center
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Mid-ocean ridges (rifts) normally form where tectonic plates are
(1) converging
(3) stationary
(2) diverging
(4) sliding past each other
The motion of the convection currents in the mantle beneath the Atlantic Ocean
appears to be mainly making this ocean basin
(1) deeper
(3) wider
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(2) shallower
(4) narrower
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Convergent Boundaries
• Where plates are coming together
Here for
example
 What
is the name
of this
boundary?
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Convergent Boundaries
• Where mountains form
• Where earthquakes occur (and
sometimes volcanoes)
–Several plates are colliding with the
Pacific Plate creating the Pacific Ring
of Fire
• An area that experiences lots of
earthquake and volcanic activity
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Convergent Boundary Types
• Ocean-Ocean
• Continental-Continental
• Ocean-Continental
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Convergent Boundaries
CONVERGENT BOUNDARIES
 Oceanic crust - Oceanic Crust
older, denser crust subducts forming ocean
trench
 Oceanic Crust – Continental Crust
denser oceanic crust subducts forming ocean
trench
 Continental Crust – Continental Crust
resists subduction - folding, faulting, and 20
mountain building
Ocean-Ocean
Convergence
• Forms ocean trenches
– Deepest places on Earth
 Name one ocean-ocean
boundary
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Oceanic Crust – Oceanic Crust
Older, denser oceanic crust is SUBDUCTED
VOLCANIC ISLAND ARC FORMED – ALEUTIAN ISLANDS
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Ocean-Continental Convergence
Site of subduction zones
• Denser ocean plate slides beneath the less
dense continental plate
–Example: the Pacific Northwestern U.S.
• Creates many of the Earth’s volcanic
mountain chains as the ocean plate dives
down, melts and is forced back to the
surface

Name one ocean-continental
boundary
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Oceanic-Continental
Convergent Boundary - SUBDUCTION
DENSER OCEANIC CRUST
GOES DOWN INTO MANTLE
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Continental-Continental
Convergence
• How non-volcanic mountains are formed
– Example: the Appalachian chain and the
Himalayas
– Rising of land due to plate collisions is called
uplift
 Name one continental-continental
boundary
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Continental Crust – Continental Crust
Convergent Boundary
NO SUBDUCTION - COLLISION ZONE
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Collision of India and Asia
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TRANSFORM PLATE BOUNDARIES
Transform Fault Boundaries
 At a transform fault boundary, plates grind
past each other without creating or
destroying the lithosphere.
 SHALLOW EARTHQUAKES
 Transform faults
• Most join two segments of a mid-ocean ridge.
• At the time of formation, they roughly parallel
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the direction of plate movement.
Transform Fault Boundary
AT MID-OCEAN RIDGES
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So, what causes plates to move?
• Convection currents: drag and move the
lithospheric plates above the asthenosphere
(three sources of heat produce the convection currents):
(1)Leftover heat from earth’s formation
(2)Decay of Radioactive elements
(3)Plate friction
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Convection Cells
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Convection Currents in the mantle
drag and pull the lithospheric plates
above them
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Mid-Atlantic Ridge
• As the plates
pull apart new
ocean floor is
added
 What 2
continents are
moving away
from each
other as a
result of the
mid-Atlantic
Ridge?
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Convection Currents
• It is theorized
convection currents
within the Mantle
cause plates to
move around
• There are other
theories about
what causes plates
to move!
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Evidence for Plate Tectonics
 Ocean Drilling
• The data on the ages of seafloor sediment
confirmed what the seafloor spreading
hypothesis predicted.
• The youngest oceanic crust is at the ridge crest,
and the oldest oceanic crust is at the continental
margins.
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Which graph best represents the geologic age of the surface bedrock on the ocean bottom?
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More Evidence for Plate
Tectonics
 Earthquake Patterns
• Scientists found a close link between
deep-focus earthquakes and ocean
trenches.
• The absence of deep-focus
earthquakes along the oceanic ridge
system was shown to be consistent
with the new theory.
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Convergent plate (subduction) boundaries
have deeper earthquakes
Divergent plate boundaries and collision
zones have shallow earthquakes
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SUMMARY
3 PLATE BOUNDARIES
 DIVERGING
 CONVERGING (SUBDUCTION ZONE)
Oceanic Crust – Continental Crust
Oceanic Crust - Oceanic Crust
 CONVERGING (COLLISION ZONE)
Continental Crust – Continental Crust
 TRANSFORM
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Boundaries: Diverging
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Boundaries: Converging
SUBDUCTION
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CONVERGING –- COLLISION ZONE
No subduction – continental crust less dense - buoyant
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Boundaries: Transform
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Due to plate tectonics the world is
constantly changing
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