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Constructive Force 1
A Plate Tectonics
1)Alfred Wegener hypothesized
that all of the continents were
once connected to form one
large land mass called Pangaea.
Evidence of Continental drift
 a)As
a student, and scholar
he saw the globe as a
broken jigsaw puzzle.
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Evidence for Continental Drift
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b)as a scholar, he collected samples of
fossils from all over the world. What he
found was that a fossil found on one
continent could also be found on
another continent usually near the
shore .
Evidence for Continental Drift 4
c)
he discovered deep
scratches in rock on the
shores of South Africa. He
identified these as caused by
glaciers.
This shows Climate change
More evidence
 d)
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Alfred also discovered coal
deposits at exactly the same
depth and type located near the
same latitude but across the
ocean from each other.
What Wagoner couldn’t
answer
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What force is large enough
to split and move
continents apart from each
other?
Advances in Technology Allow 7
More Accurate Observations
Sonar developed in WWII is used in
1950 to continue mapping the ocean
floor.
 An underwater ridge is discovered that
that encompasses the Earth.
A new question is asked.

What is this underwater ridge? 8
 More
observations were
needed before a hypothesis
could be formed.
Drilling begins

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Scientists begin to drill into the ocean
floor near the ridge to study rock
samples to see if they could learn
anything about the ocean floors and the
ridge that ran through them.
What is learned through 10
drilling and rock samples
The ocean floor is made of basalt!
 The farther away from ridge the older
the rock!
 What does this tell you ?
 What is this underwater ridge?
 Hypothesize
 Model

Three types of heat transfer 11
Which of these three types of heat transfer
can cause the plates to move?
Explain
 Radiation
 Conduction
 Convection
However this can’t be the only force. Can you find
other forces that would cause the plates to
move?
2) Plate movements
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 a)Translation-
 When
two plates slide past each
each-other in opposite directions.
 1) The place where these plates
meet are called a transform
boundary
 IE. The San Andreas Fault in CA.
b) Converging plates 13
Where
the two plates are
colliding
The boundary where the two
plates meet is called a
convergent boundary
IE. The island of Japan and the
country of Iceland
c) Subduction
When
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one plate moves
underneath another plate. This
happens because one plate is
denser and therefore heavier
than the other. This is most
likely to happen when an
oceanic plate collides with a
continental plate.
d)Spreading15
Spreading is when two
plates move apart from
each-other
1)divergent
boundaries is
the point at which the two
plates are moving away
from each -other.
Example of spreading 16
Africa’s
Great Rift
Valley (on land)
Mid-Oceanic Ridge
(under the ocean)
Plate movements below 17
create landforms above
Diverging plates
Continental crust away from continental
crust = Rift valley
Oceanic crust away from oceanic crust =
Mid ocean ridge
Plate movements below 18
create landforms above
Converging plates
Continental crust into continental crust =
Fault block mountains, folded mountains
Continental crust into oceanic crust =
Coastal and Volcanic mountains, ocean trench,
Plate movements below
create landforms above
 Translating
plates
Folded hills, riverbed offsets.
No new crust is created or
destroyed.
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Final slide

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