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Plate Tectonics
Chapter 9
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Plate Tectonic is a theory in science!
an ideal example of how a new theory
replaces a well-established older one
 the accepted paradigm in earth science
 merger of earlier ideas of continental drift
and sea-floor spreading
accepted only since the late 1960’s

THEORY
- well-substantiated explanation of
some aspect of the natural world
PARADIGM
- model used to explain a concept or theory.
- prevailing principle for a school of thought
Alfred Wegener
German
meteorologist
Proposed the
continental drift
hypothesis:
1915
1. The Shapes Match
Early ideas: Continents seem to be able to fit together like a
puzzle!! In 1858 Geographer Antonio Snider-Pellegrini
made these maps
Continents were once together
in a super continent that
Wegener called Pangea
(250 mya)
Broke up into two super
continents call Laurasia
and Gondwanaland (180
mya)
Continued to break apart
Continents move by plowing
through the crust like ships
in the sea
http://www.discovertsunamis.o
rg/multimedia/autoformat/g
et_swf.php?videoSite=hite
c&videoFile=tsunami_conti
2. Ancient
Climate
Clues indicate that the present
climate is not the same
throughout a continent’s
history.
Warm-weathered plants are found
in the arctic. (near the equator
at some point)
Glacial deposits in Africa, India,
and Australia (covered with
ice near the Earth’s South
Pole at some point)
3. Matching Fossils
Several fossil
organisms found
on different land
masses.
Considered:
Swimming and
Land Bridges
Determined:
Impossible
Considered Best
evidence
http://www.discovertsunamis.org/multimedia/autoformat/get_swf.php
?videoSite=hitec&videoFile=tsunami_plate_direction.swf%20&video
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Types &
Structures
Mountain belts that end at
one coastline, only to
reappear on a landmass
across the ocean.
Appalachians through
Newfoundland into Caledonian
Mts. of the British Isles
Cape Mounts in S. Africa:
Mountains in Argentina and
Australia
Hypotheses Rejected
 Centripetal Force and
tidal influence – not
accepted
 No evidence existed to
explain how continents
could move through the
ocean
 Idea – rejected because
he could not describe
mechanism
New Theory Emerges
 Technology from WWII-
mapping of the ocean
floor (HESS)
 Data on earthquake
activity
 Data on Magnetic Field
 1968 Plate tectonics
What is the Theory of Plate
Tectonics?
The theory is that the lithosphere acts
as a rigid layer sitting on a weaker
asthenosphere. The asthenosphere is
divided into plates which continually
change size and shape.
7 major plates
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What are Divergent
Boundaries?
when two plates move apart
Location: along the crests of oceanic ridges
New crust is created by magma pushing up from
mantle Volcanoes
2.5 cm/yr
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What are the results of
Divergent Boundaries?
Oceanic Ridges
– Occur along welldeveloped divergent
plate boundaries
seafloor is elevated
– Rift valleys
deep faulted structures
Seafloor Spreading
– the process of
producing new
oceanic lithosphere
Typical rates of
spreading average
around 5 cm per year
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http://www.discovertsunamis.org/multimed
ia/autoformat/get_swf.php?videoSite=hitec
&videoFile=tsunami_south_atlantic_sprea
ding.swf%20&videoTitle=
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Divergent Boundaries Cont.
Continental Rifts
– Magma rises and
squeezes through
the widening
cracks, sometimes
to erupt and form
volcanoes.
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East African Rift Valley
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http://www.discovertsunamis.org/multimed
ia/autoformat/get_swf.php?videoSite=hitec
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What are Convergent
Boundaries?
Convergent boundaries
– Occurs where two plates move together
Destructive plate margins - this is why the Earth
doesn’t get bigger (the crust is destroyed at the
same rate it is created)
Subduction zones
– As two plates slowly converge, the leading edge of
one is bent downward, allowing it to slide beneath the
other
ocean trench- surface feature produced by the descending
plate
3 Types
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What are Oceanic-Continental
Convergent Boundaries?
The leading edge of the
oceanic plate is
subducted into the
asthenosphere
– The continental plate is
less dense than the
oceanic plate
Continental volcanic
arc form
– Mountains produced
– May cause volcanic
eruptions.
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http://www.discovertsunamis.org/multimed
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What are Oceanic-Oceanic
Convergent Boundaries?
 Two oceanic slabs converge and one descends
beneath the other
 Volcanoes form on the ocean floor
 Recycling of crust takes place because one plate is
subducted under another.
 Where sinking of a plate occurs is called a subduction
zone.
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http://www.discovertsunamis.org/multimed
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What are Continental-Continental
Convergent Boundaries?
After all the oceanic crust is consumed, continents
collide.
Subduction is greatly reduced and massive fold
mountains are uplifted
Volcanic activity is minimal, but earthquakes are
common.
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http://www.discovertsunamis.org/multimed
ia/autoformat/get_swf.php?videoSite=hitec
&videoFile=tsunami_plate_tectonics.swf%
20&videoTitle=
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What are Transformation Fault
Boundaries?
Plates grind past each other without the
production or destruction of lithosphere
move in opposite directions
usually located in the ocaen
Continental transform faults
– San Andreas Fault
the Pacific plate is moving toward the NW, past
the North American plate
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http://www.discovertsunamis.org/multimed
ia/autoformat/get_swf.php?videoSite=hitec
&videoFile=tsunami_plate_tectonics.swf%
20&videoTitle=
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What are the Evidence for Plate
Tectonics? #1
Paleomagnetism
– Magnetic Field
– iron-rich mineral grains
cool down and become
magnetized in the
direction parallel to the
existing magnetic field
– Show magnetic pole
orientation at the time the
rock material is created
Strong evidence of seafloor
spreading see in text lab
Paleomanetism and Seafloor
spreading.
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http://www.discovertsunamis.org/multimed
ia/autoformat/get_swf.php?videoSite=hitec
&videoFile=tsunami_magnetic_stripe_reve
rsal.swf%20&videoTitle=
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Evidence for Plate Tectonics #2
Earthquake Patterns in & around ocean trenches
– Shallow-focus earthquakes occur within or adjacent to the
trench
– Intermediate- and deep-focus earthquakes occur toward
the mainland
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Evidence for Plate Tectonics #3 &
4
Ocean Drilling - data on seafloor sediment’s age
confirmed seafloor spreading;
- Youngest oceanic crust at the ridge
- Oldest at the continental margin
Hot Spots - enough heat in the mantle which
causes magma to rise
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What are the Causes of Plate
Movement?
Convection occurring in the mantle is the basic driving force for
plate movement.
–
–
–
–
convective flow generated by unequal heat - thermal convection
drives plate motion
causes mantle to rise and cool then to slide back dow
http://www.discovertsunamis.org/multimedia/autoformat/get_swf.php?vi
deoSite=hitec&videoFile=tsunami_convection_and_subduction.swf%20
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Convection Demo!
 http://www.youtube.com/watch?v=PdWYBAOqHrk&sa
fety_mode=true&persist_safety_mode=1&safe=active
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Summary of Types of Plate
Boundaries
Divergent boundaries (also called spreading
centers)
– Occurs when two plates move apart
– Create new seafloor
Upwelling of material from the mantle A relatively new
divergent boundary
Convergent boundaries
– Occurs where two plates move together
– oceanic lithosphere plunging beneath an overriding
plate
Transform fault boundaries
– where two plates grind past each other without the
production or destruction of lithosphere
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