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“I CAN” Statements Theory of Plate Tectonics
•Explain each type of plates interactions, and give example of the end
result(s) i.e. geological features [L2]
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A scientific revolution begins
– PALEOMAGNETISM
(evidence of past magnetism preserved in the rocks)
was the most convincing evidence set forth
to support the concepts of continental drift
[polar wandering] and seafloor spreading
[magnetic reversals].
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• Basalt is rich in iron-bearing materials, like
magnetite.
• As the basalt lava cools, the Fe-bearing
minerals become oriented parallel to the
Earth’s magnetic field.
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• As the lava hardens, the magnetic
orientation is preserved and provides a
record of the current Earth’s magnetic field.
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Theory of Plate Tectonics
From the hypotheses of Continental Drift and
Seafloor Spreading was formulated the
Theory of Plate Tectonics
TECTONICS is the study of the features of
the Earth’s crust [tektonikos = construction]
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Theory of Plate Tectonics
 The Earth’s crust and rigid upper mantle
[LITHOSPHERE] are broken into huge slabs
or plates.
 Those plates are riding/floating onto the
asthenosphere [like wood blocks on water],
at different rates and different directions.
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Directions and rates of plate motions
Subduction
Spreading
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The oceans and the
continents are carried
as passengers onto the
moving plates.
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The constant movement of those
plates is responsible for the
changes to the Earth’s surface
by the way of:
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EARTHQUAKES
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V
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MOUNTAIN RANGES
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8 major plates:
1)
2)
3)
4)
5)
6)
7)
8)
North American
South American
Eurasian
African
Pacific [largest]
Indian/ Australian
Antarctica
Nazca
The minor ones:
Juan de Fuca, Cocos, Caribbean, Scotia,
Arabian, Philippine
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BOUNDARIES
•Tectonic plates interact at places called
BOUNDARIES.
•The continents / oceans do not
necessarily resemble the outline of the
plate boundaries.
•There are three types of boundaries
•Divergent
•Convergent
•Transform
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Divergent Boundaries
Two plates moving AWAY from each other
and forming a gap or RIFT.
• Mostly associated with OCEANIC crust
[seafloor spreading = Mid-Atlantic Ridge]
– as molten rock [MAGMA] from the
asthenosphere rises the plates move apart,
and fills the space between the plates.
– as it cools, it hardens onto the edges of the
plates and creates new crust.
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DIVERGENT PLATES
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Divergent Boundaries
• Some involve the CONTINENTAL crust
– when it begins to separate, the stretched crust
forms a long, narrow, depression called a
RIFT VALLEY
• it is currently happening between the African and
Arabian plates  Red Sea is therefore widening
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DIVERGENT
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RIFT VALLEYS
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Mediterranean Sea
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Divergent Boundaries
• Some involve the CONTINENTAL crust
– the crust can also stretch to the point of forming
FISSURE VOLCANOES
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Convergent Boundaries
Two plates moving TOWARDS each other.
• Direct COLLISION of one plate with
another one.
• There are three POSSIBILITIES:
– CONTINENTAL – OCEANIC
– OCEANIC – OCEANIC
– CONTINENTAL – CONTINENTAL
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CONTINENTAL - OCEANIC
• As the plates are colliding, the denser plate will be
forced downwards
• Since oceanic crust is denser than the continental
crust [3.0 vs. 2.7g/cm3], the ocean plate is pushed
under continental plate. [crust destruction]
The area is called a SUBDUCTION ZONE, and
forms a deep-ocean trench
• As the plate sinks into the asthenosphere, it melts
and becomes magma. The magma rises through the
continental plate [WHY?] and forms a volcanic chain
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OCEANIC   CONTINENTAL
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Mount Hood near Portland, Oregon
Cascades range
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Mount St. Helens, Washington
Cascades range
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Volcanic mountains of the Andes,
(Nazca and South American Plates)
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TRENCH
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OCEANIC – OCEANIC
• The more dense of the 2 will go under and form
a subduction zone / OCEAN TRENCH
• The new mantle material produced from the
melting of the subducted plate will eventually
resurface to produce chain of volcanic islands
on the ocean floor called ISLAND ARCS
• As magma accumulates over time, the
volcanoes may rise above sea level to form
volcanic islands:
Mariana trench and
the Mariana Islands in the west Pacific ocean
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OCEANIC-OCEANIC CONVERGENCE
“DESTRUCTIVE”
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CONVERGENT PLATES
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CONTINENTAL - CONTINENTAL
• As the 2 continental plates are colliding,
neither plate is subducted i.e. neither plate
goes beneath the other  WHY?
– because the Continental crust is too buoyant to
sink into the asthenosphere
[2.7 vs. 3.3 to 5.5g/cm3]
• The colliding edges buckle [crumple] and are
pushed upward [uplift] to form
mountain ranges.
Indian/Australian plate collide with the
Eurasian plate to form the HIMALAYAS.
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CONTINENTAL – CONTINENTAL CONVERGENCE
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The collision of India and Asia
produced the Himalayas (before)
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The collision of India and Asia
produced the Himalayas (after)
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Mountain Building
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