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Plate Tectonics
Earth Science
Mr. Ahearn
March 06
Continental Drift
Alfred Wegner
proposed in 1912 that
all continents were
once connected as
one large landmass
called Pangaea that
broke apart about 200
million yrs ago, this
was his theory of
continental drift.
Breakup of Pangaea
Evidence for Continental Drift
• Fossil Clues –
fossils from animals
such as the
Mesosaurus found in
S. America and Africa
Evidence for Continental Drift
Climate Cluesglacial deposits and grooved bedrock
indicate that ice once covered southern
areas of S. America, Africa, India, and
Australia. Northern continents have fossils
of tropical plants.
Glacial Evidence
Evidence for Continental Drift
Rock Cluesthere are similar rock structures found on
different continents.
ex. The Appalachian Mtns are similar to the
mtns. of Greenland and Western Europe.
Matching Mountain Ranges
Review Questions
1.) What is the theory of continental drift? Who
came up with it?
2.) What is the evidence that supports continental
drift?
3.) Why would you expect to see similar rocks and
rock structures on two landmasses that were
connected at one time?
Seafloor Spreading
Not until the invention of echo-sounding did
scientist discover the ocean is full of
mountains and valleys.
Mid-Ocean Ridgesform an underwater mtn. range that
extends through the center of much of
Earth’s oceans.
Seafloor Spreading
• Early 1960’s Princeton
University’s Harry Hess came
up with explanation for midocean ridges, known as:
Seafloor Spreading- hot, less
dense material in the mantle is
forced upward to the surface at
a mid-ocean ridge where the
material flows sideways,
carrying the seafloor away
from the ridge.
Seafloor Spreading
Evidence
Age- Glomar Challenger drilled into seafloor
rocks, discovering the rocks became older
as they moved away on each side of the
mid-ocean ridge.
Seafloor Spreading
Magnetic Clues
-Earth has reversed magnetic field in the
past.
-Fe aligns in ocean basalt according to
Earths Field.
- rocks show many field reversals moving
away from ridges.
Seafloor Spreading
• Deep sea sediments and rocks were collected
• Ocean rock  young 180 myo (compared to
continental 3.8 byo)
• Ocean sediment thin compared to continental
• Young rock and thinner sediments @ ridges
• Old rock and thicker sediment @ trenches
Isochron
• Lines connecting points of same age
The Missing Link
• Seafloor spreading explained the
mechanism that made continents drift,
completing Wegner’s model.
Review
• How does ocean ridges and trenches
support the theory of seafloor spreading?
• How is ocean rock & sediments evidence
for seafloor spreading?
• How does an isochron map of the ocean
floor support seafloor spreading?
Plate Tectonics
According to the plate tectonics theory, the
uppermost mantle, along with the
overlying crust, behaves as a strong, rigid
layer. This layer is known as the
lithosphere.
• A plate is one of numerous rigid sections
of the lithosphere that move as a unit over
the material of the asthenosphere.
Plate Boundaries
Divergent boundaries (also called
spreading centers) are the place where
two plates move apart.
Convergent boundaries form where two
plates move together.
Transform fault boundaries are margins
where two plates grind past each other
without the production or destruction of the
lithosphere.
Types of Plate Boundaries
Divergent Boundaries
Oceanic ridges are continuous elevated zones on
the floor of all major ocean basins. The rifts at
the crest of ridges represent divergent plate
boundaries.
Rift valleys are deep faulted structures found
along the axes of divergent plate boundaries.
They can develop on the seafloor or on land.
Seafloor spreading produces new oceanic
lithosphere.
Spreading Zones
Continental Rifts
When spreading centers develop within a
continent, the landmass may split into two
or more smaller segments, forming a rift.
East African Rift Valley
Convergent Boundaries
• A subduction zone occurs when one oceanic plate is
forced down into the mantle beneath a second plate.
Oceanic-Continental
• Denser oceanic slab sinks into the asthenosphere
• Pockets of magma develop and rise
• Continental volcanic arcs form in part by volcanic
activity caused by the subduction of oceanic lithosphere
beneath a continent
• Examples include the Andes, Cascades, and
the Sierra Nevadas
Ocean-Continent
Ocean-Ocean
Continent-Continent
Collision of India
Transform Boundary
• At a transform fault boundary, plates grind
past each other without destroying the
lithosphere
Evidence for Plate Tectonics
Paleomagnetism is the natural remnant
magnetism in rock bodies; this permanent
magnetization acquired by rock can be used to
determine the location of the magnetic poles at
the time the rock became magnetized.
• Normal polarity—when rocks show the same
magnetism as the present magnetism field
• Reverse polarity—when rocks show the
opposite magnetism as the present magnetism
field
Paleomagnetism in Lava Flows
Ocean Crust Polarity
Evidence for Plate Tectonics
• Earthquakes
• Ocean drilling
• Hot Spots
Hotspots
Complete #38- 50
Review Book p.69
Causes of Plate Tectonics
• Convection in the Mantle
Causes of Plate Tectonics
• Slab pull – ridge push
Complete Questions 51-57
Review Book p. 71
The End