Download Plate Tectonics

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J. Tuzo Wilson was a Canadian scientist.
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He observed that there were cracks in
the continental crust similar to those on
the ocean floor.
Wilson proposed
that the lithosphere
is broken into
separate sections
called plates and
that they fit together
closely along cracks
in the crust.
Plate Tectonics
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Wilson’s theory came to be known as the theory of Plate
Tectonics.
This theory explains the formation, movement and
subduction of the Earth’s crust.
No plate can move without affecting the other plates
surrounding it.
As the plates move, they collide, pull apart, or grind past
each other, causing changes in the Earth’s surface.
Plate Boundaries
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A boundary is a place where tectonic plates
touch.
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There are three
types of plate
boundaries:
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Divergent
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Convergent
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Transform
 The
type of boundary depends on how
the plates move in relation to one
another.
 They
can collide,
pull apart, or slide
past each other.
 Earthquakes
can occur at all three types
of plate boundaries.
Convergent Boundaries
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Plates collide at a convergent boundary.
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What happens next depends on what type of
crust is involved in the collision.
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There are three types of
collisions:
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Continental and Oceanic crust
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Oceanic and Oceanic crust
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Continental and Continental crust
Oceanic – Oceanic Collision
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When two pieces of oceanic
crust meet at a deep-ocean
trench, the one that is farthest
away from the mid-ocean
ridge, and therefore denser,
dives, or subducts under the
less dense plate and returns
to the mantle.
 This can trigger volcanic
activity. If the activity
continues the volcanoes will
emerge as islands in the
ocean.
 This newly formed land consists of an arc-shaped chain of
small volcanic islands is called a volcanic island arc.
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The Aleutian Islands are examples of volcanic island
arcs.
Oceanic – Continental Collision
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When there is a collision
between continental and
oceanic crust, the less dense
continental crust cannot sink
under the denser oceanic
crust.
Instead, the denser oceanic
crust begins to sink and
plunges underneath the
continental plate, back into the
asthenosphere where it is
recycled.
This area is called a subduction zone.
An example of this is the western part of South America
where the Andes Mountains are.
Continental – Continental Collision
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When two pieces of
continental crust
collide, subduction
does not take place.
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Since both plates
are mostly lowdensity granite, they
squeeze together
forming mountain
ranges, such as the
Himalayas.
Divergent Boundaries
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The place where two plates move apart, or diverge, is
called a divergent boundary.
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Most divergent boundaries occur at the Mid-Ocean
Ridge, but they also can occur on land.
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A deep valley, called
a rift valley forms
along the divergent
boundary as the land
moves apart.

The Great Rift Valley
in East Africa is an
example of a
divergent plate
boundary.
Transform Boundaries
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Along transform boundaries, crust is neither
created, nor destroyed.
 A transform boundary is a place where two
plates slip past each other, moving in opposite
directions.
 Earthquakes
frequently occur
along these
boundaries.
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The San Andreas Fault in California is an
example of a transform boundary.
Remember!
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Faults are breaks in the Earth’s crust where
rocks have slipped past each other.