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Plate Tectonics
• The Earth’s surface consists of
rigid moving pieces called plates.
• Each plate is made up of crust
and upper mantle collectively
known as lithosphere.
• The plates move as a result of
convection currents within the
asthenosphere.
• There are 8 large plates and
many smaller plates.
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Mantle
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Plate Tectonics Combines:
• Continental Drift – the continents are
slowly moving apart; and,
• Sea-floor spreading – the ocean floor
is forming at the mid-oceanic ridge
and spreading away from it.
Convection Patterns
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Australia-Pangaea
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Plate Movement
Converge
Diverge
Slide past each other
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Alfred Wegener in
the early 1900’s
proposed that the
continents were
once joined
together in a single
large land mass he
called Pangea.
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He believed that around 200-250 million years
ago the supercontinent, Pangea, began
splitting apart.
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250 million years ago: Wegener named the southern
portion of Pangaea Gondwana, and the northern
portion Laurasia.
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70 million years ago: The Atlantic Ocean is forming.
Eventually India would collide with Eurasia and form
the Himalayan Mountains.
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Today the continents are still slowly moving.
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Evidence for Plate Tectonics
1. Jigsaw puzzle connections of
continents (noticed in 1620 by
Francis Bacon)
2. Fossils on different continents were
similar
3. Similar rock structures on different
continents for older rock yet
younger rock is very different
• Using the middle of
the continental
shelf (the
continental edges
underwater) as the
edge of a continent
provides a better fit
than does the
shoreline.
Glossopteris, a fern that requires warm
climates, was found on Antarctica,
Southern South America, Australia,
Southern Africa and India.
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Evidence for Plate Tectonics
4. Apparent movement of the north
magnetic pole (continents moving)
5. Magnetic polarity reversals in
igneous rock on ocean floor
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Evidence for Plate Tectonics
6. Similar freshwater fossils are
separated by ocean
7. Inconsistencies with glaciation
8. Pattern of volcanoes and
earthquakes
Mesosaurus, a
freshwater reptile,
has had fossils
found in Africa and
South America.
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•Glacial sediment has
been found in places
where glaciers do not
exist today.
•When the continents
are reassembled,
striations line up.
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Evidence for Plate Tectonics
9. Variation in heat flow. It is higher at
actively spreading sites.
Earthquake Locations and Plate Boundaries
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Pattern of Volcanic Activity
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Convection
• Warmer regions are
less dense and rise.
• Cooler region are more
dense and sink.
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• Warmer, rising currents cause
plates to move apart.
• Cooler, sinking currents cause
plates to pull together.
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Types of Plate Boundaries
1. Diverging boundaries occur where
two lithospheric plates are moving
apart.
– These regions are marked by:
• Magnetic reversals
• Heat flow variations
• Mid-ocean ridges
• Mid-ocean ridges have rift valleys.
• Rift valleys are:
– The boundary between the lithospheric
plates
– The place where new rocks form
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Rift Valley Formation – on land
• Rising magma upwarps the crust
causing cracks.
• As the crust pulls apart, large slabs of
rock sink generating a rift zone.
• Further spreading generates a
narrow sea.
• Eventually an expansive ocean basin
and ridge system are created.
Rift Valley
Formation -land
Red Sea – looking SE
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2. Sliding Boundaries
• a. k. a.
Transform
Faults –
plates
slide past
each other
San Andreas
fault
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There is no production/destruction of
material.
They connect mid-ocean ridge segments
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Converging Plates
• Three types:
– Ocean and continent
– Ocean and ocean
– Continent and continent
• Oceanic crust is more dense and will
sink beneath continental crust.
Ocean and Continent Convergence:
• Results in the formation of an ocean
trench.
• Results in the formation of a
mountain range near the trench.
Ocean and Continent
• As the ocean slab descends, some
sediment is scraped off the
continental plate and carried with it.
• The sediment is less dense than the
oceanic crust. Upon melting it will
rise. Volcanic activity will occur on
land. Mountains – like the Cascades
(Washington) will form.
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Ocean and Continent Collision
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Ocean and Ocean Convergence:
• Results in the formation of an ocean
•
•
trench and islands nearby.
One slab will descend beneath the other.
The Aleutian Islands (off Alaska) are an
example.
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Continent and Continent Convergence:
• Neither plate will subduct. Instead,
the continental crusts will buckle
forming mountains.
• The Himalayas are an example.
Nepal
Continent and Continent Convergence
Continent and Continent Collision
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What About the Hawaiian Islands?
• Formed as a
result of ocean
plate moving
slowly across a
stationary hot
spot.
Convergence – Revisited