Geology 12
Plate Boundaries
There are three types of plate boundaries
1. Divergent (or constructive) plate
boundary
2. Convergent (or destructive) plate
boundary
3. Transform (or slip & slide) plate
boundary
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Divergent Plate Boundary
Divergent Plate Boundary
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Plates move away from each other, often
causing mountain or ridge building
(constructive)
Result is to create new ocean basins (such as
the Atlantic) or huge valleys on land (called
rift valleys)
As the plates move away from each other,
magma flows from the mantle (same as water
between two blocks of wood)
Divergent Plate Boundary
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Mid-ocean ridges are formed when plates
move apart. They are giant undersea
mountain ranges that extend around the world.
One example is the Mid-Atlantic Ridge.
Earthquakes & eruptions of rock are frequent
along the ridges. Iceland formed atop a ridge.
Exotic creatures often live along ridges.
Convergent Plate Boundaries
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With convergent plate boundaries, plates move toward each
other
There are three possible collisions:
Oceanic crust colliding with oceanic crust causes one to dive
under the other (often forming deep trenches or island
chains)
Heavier, denser and thinner oceanic crust colliding with the
continental crust causes the oceanic crust to dive (often
forming deep trenches)
Two colliding continental crusts cause crumpling &
mountain building (ex. Himalayas & Mt. Everest;
Appalachians of Eastern North America, including the
mountains of Eskasoni)
Convergent Plate Boundaries
Transform Boundaries
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With transform boundaries, one plate slides
horizontally past another plate
The result is often a fault or massive “crack”
in the Earth’s crust (ex. San Andreas fault in
California; Aspy Fault in C.B. Highlands)
Earthquakes are very common (and often
violent) along transform boundaries
Transform Boundaries
Major Plates of the World
Model of Mantle Convection
What causes plate motions?
Scientists once thought that heat from the core
causes the plates to move. However, it is now
believed that three types of forces are at work:
1. Ridge Push: Plates near mid-ocean ridges are
pushed apart by the rising magma. The plates
then slide down the oceanic ridges.
2. Slab Pull: The cold, dense leading edge of the
sinking plate may pull the rest of the plate with
it.
3. Trench Suction: If a sinking plate falls at a
steep angle into a trench, the rest of the plate
is “sucked” forward.
What are mantle plumes?
Mantle plumes are narrow columns of hot,
rising magma. These plumes form “hot
spots” of active volcanism at the Earth’s
surface.
Mantle plumes have formed Iceland, the
Hawaiian Islands, and Ol’Faithful in
Yellowstone Park (USA)
Mantle Plume
Results of Magma Plumes
Formation of “hot spots” of active
volcanism around the world
 May help to drive plates apart (ex. MidAtlantic Ridge)
 Formation of domes, or bulges in the
crust, which may produce volcanic
eruptions
 Continent splitting (ex. Yellowstone
Park in U.S.A.)
 Formation of seamounts and guyots,
often in chains
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Importance of Understanding
Plate Tectonics
Explains many geological features, such as
volcanoes, earthquakes, sea-floor
spreading, seamounts, hot spots, and
general features of Earth
 Important to the prediction of volcanoes
and earthquakes to help protect people
 Explains much of the geology of Canada,
including the hills of Eskasoni and the
Canadian Shield
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Importance of Understanding
Plate Tectonics
Explains the breakup of the Pangea
Supercontinent
 Important mineral resources often formed
at plate margins
 BASICALLY, THE THEORY OF PLATE
TECTONICS CONNECTS TOGETHER MOST
TOPICS IN GEOLOGY, INCLUDING THIS
COURSE!!!
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Seamounts
Define the following:
Hot spot
 Volcanic dome
 Seamount
 Guyot
 Pangea
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