Download Structure of Earth notes part 2 [Compatibility Mode]

9/8/2015
Structure of Earth
Notes Part 2
https://www.khanacademy.org/science/cosmolog
y-and-astronomy/earth-history-topic/platetechtonics/v/plate-tectonics-difference-betweencrust-and-lithosphere
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Plate tectonics
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Lithosphere and Lithospheric plates,
pt. 1
• Lithosphere – rigid outer shell of the Earth
– Composed of crust and some of the upper mantle
– Composed of a series of moving plates
– Mature oceanic plate is approx. 80-100 km thick
• The upper 5-7 km is oceanic crust
– Continental plate is somewhat thicker, 150-200
km thicker
• Includes continental crust, 5-70 km thick
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Lithosphere and Lithospheric plates,
pt. 2
• A single plate may be part oceanic and part
continental. In this type of case, the boundary
between ocean crust and continental crust is
NOT the plate boundary.
• The mobile part of the mantle under the
lithosphere is called the asthenosphere.
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Boundaries between two Lithospheric
plates, pt 2
• Plate boundaries are most readily identified by
seismicity, the location of significant earthquake
activity.
• About 95% of the world’s Earthquakes occur
on plate boundaries and ALL earthquakes
occur in the Lithosphere.
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USGS.gov diagram
• The diagram on the next page shows 23 plate
boundaries and their direction of motion.
• Use this labeled diagram to help you label
your diagram.
• LEGEND
• Orange = divergent boundary
• Green = convergent boundary
• Blue = transform boundary
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Plate boundaries and direction of movement
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Boundaries between two
Lithospheric plates, pt 1
• Can be
–Divergent
–Convergent
–Or
–Transform
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Divergent Plate Boundaries
• At divergent plate boundaries, two
lithospheric plates move apart and new plate
(and crust) are created at the boundary.
• The new crust is created through volvanic
activity.
• This process ALWAYS PRODUCES OCEAN
CRUST, and thus oceanic plate.
• Thus divergent boundaries ultimately become
mid-ocean ridges.
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Divergent plate boundaries, pt 2
• As the lithosphere moves away from the ridge,
it thickens by a process of underplating, in
which asthenosphere solidifies onto the
underside of the plate as the plate cools.
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• Thickening lithosphere subsides as it ages.
Ridges are a result of this. Young lithosphere
near the ridge sits up high, while older
lithosphere sits lower.
• Ultimately, ocean lithosphere becomes more
dense than the asthenosphere, beneath it.
Thus old oceanic lithosphere will sink in to the
asthenosphere if it gets the chance.
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Convergent boundaries, pt 1
• Two lithospheric plates move together. The
denser plate sinks in a process called
subduction.
• The subduction process produces magmas
through other processes (that we will discuss
as needed later.)
• Only oceanic plate can subduct. Continental
crust is too thick and low density to sink. (acts
like a float).
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Convergent, pt 2
• Subducting plates (sinking into the mantle)
remain more rigid than the surrounding
mantle to depths of about 700km. This is the
maximum depth of earthquakes (which only
occur in rigid lithosphere).
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Convergent, pt 3
• At some convergent plates, oceanic plate
meets continental plate.
• An example: west coast of southern Mexico,
Central America and South America.
– An oceanic trench forms offshore.
– Chain of volcanic mountains forms just inland of
the coast.
– Called an active continental margin.
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Convergent, pt 4
• Other convergent plates: oceanic meets
oceanic.
• Results in oceanic trench and a chain of
volcanic islands known as an island arc.
• Japan, the Phillipines, and the Aleutions are
examples.
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Convergent plates, pt 5
• At other convergent plate boundaries, two
continents collide. Neither can subduct.
• Result: like a car crash.
• Both are crumpled. Large mountain belts are
formed, their insurance rates go up.
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Transform Boundaries, pt 1
• Occur when two plates move parallel to one
another.
• Neither divergence nor convergence
• Rare: occurs for short intervals along mid ocean
ridges.
• Along and perpendicular to mid ocean ridges.
• The San Andreas Fault in California is a transform
boundary. Another cuts across southern New
Zealand.
• No volcanism at transform boundaries
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What causes plate tectonics?, pt 1
• The ultimate cause of plate tectonics is the
cooling of the earth. The core and inner Earth
have retained enormous amounts of heat
from the initial formation of Earth. Since the
mantle is made of rock, it is a very good
insulator.
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What causes plate tectonics, pt 2
• The material of the mantle convects, enabling
the release of heat. Like a hot cup of coffee,
the quickest way to cool the Earth is to stir it.
The movement of plates is the surface
manifestation of that mantle convection.
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What causes plate tectonics, pt 3
• Oceanic plates are driven by their own sinking.
Thick, cold, dense oceanic lithosphere sinks at
subduction zones, pulling the rest of the
ocean plate. Since plates have limited
tensional strength, this "subduction pull"
cannot be the only force acting on an ocean
plate.
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What causes plate tectonics, pt 4
• "Ridge push" is the sliding of the young thin
part of the plate downhill away from the
ridge. A combination of "subduction pull" and
"ridge push" move the ocean plates along.
These forces are not separate from mantle
convection, but are both caused by and help
cause that convection. Continental plates are
pushed around by oceanic plates.
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