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Continental Drift
Chapter 10
Wegener’s Hypothesis
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Once a single supercontinent
Started breaking up about 200 mya
Continents drifted to current location
Evidence, but no mechanism
Pangaea
Pangaea
Wegener’s Evidence
1. Fossils
–
Similar ones found in areas thought to have
been connected
2. Rock formations
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Rock ages same on African & S. American coast
Continuation of mountain ranges across
continents
3. Climatic evidence
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Glaciers in Africa and S. America
Tropical fossils in colder climates
Rock Evidence
Fossil Evidence
Climate evidence
Glacial
evidence in
south Africa,
Australia, South
America, and
India. Occurred
at same time.
Pangaea Breakup
Breakup of Pangaea (cont)
Breakup of Pangaea (cont)
Breakup of Pangaea (cont)
Mid – Ocean Ridges
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Under sea mountain ranges
Sediment thicker farther away from ridges
Sediment closer to ridges is younger
Ocean floor is young compared to Earth
Mid - ocean Ridges
Mid – Ocean Ridges
Sea Floor Spreading
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Suggested by Harry Hess
Ocean ridge is a crack (Rift) in Earth’s crust
Magma fills the crack
As ocean floor spreads, continents also
move
Sea Floor Spreading
Sea Floor Spreading
Paleomagnetism
 Study of magnetic properties of rocks
 Iron rich minerals in magma align with
Earth’s magnetic field
 As magma solidifies, magnetic orientation
becomes permanent
 Residual magnetism is paleomagnetism
Earth as a Magnet
Polar Wandering
Makes
more sense
for the
continents
to move
than for the
North Pole
to move
Magnetic Properties from Pangaea
Paths
coincide if
continents
started as
Pangaea
Magnetic Reversals
 Earth’s magnetic field does not always point
north
 Magnetic reversals – orientation is opposite
of normal
 “Normal” rocks and “Reversal” rocks line up
by time period
 Alternating normal and reversed polarity
over time
 Geomagnetic time scale
Magnetic Reversals
Magnetic Reversals
Plate Tectonics
 Theory that explains how large pieces of
lithosphere, called plates, move and change
shape
 Study of formation of features in Earth’s
crust
Lithosphere
 Solid outer layer of Earth
 Consists of the crust and rigid upper part of
the mantle
 Broken into blocks called plates
 Plates float on the asthenosphere
Asthenosphere
 Solid plastic layer of mantle below the
lithosphere
 Made of mantle rock that flows very slowly
 Allows tectonic plates to move
Lithosphere & Asthenosphere
Crust types
1.Oceanic crust
– Dense
– Rich in magnesium and iron
2.Continental crust
– Low density
– Rich in silica (SiO2)
Continental & Oceanic Crust
Tectonic plates
 Can be either or both oceanic and
continental crusts
 15 major plates
 Earthquakes – sudden shifts along plate
boundaries
 Volcanoes – plate motions generate magma
that erupts
Tectonic Plates
Types of Plate Boundaries
1. Divergent boundaries
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Plates move away from each other
Most on the ocean floor
Form ocean ridges (rifts)
Plates separate at rift valley
Divergent Boundary
Divergent Boundary (on land)
Types of Plate Boundaries
2. Convergent boundaries
– Plates move toward each other
– Plates eventually collide
– 3 types of collisions
Convergent Boundary
 Continental – Oceanic Crust Collisions
– Subduction zone – oceanic sinks under
continental
– Magma rises to surface and forms volcanic
mountains
Continental – Oceanic
Collision
Convergent Boundary
 Oceanic – Oceanic Crust Collision
– Forms ocean trench (subduction)
– Magma forms island arc (Example: Japan)
Convergent Boundary
 Continental – Continental Collision
– Forms mountains
– Edges crumple and thicken
– Creates uplift
Continent Collisions
The Himalayas
Types of Boundaries
3. Transform boundaries
– Plates slide past each other
– Sudden motion produces Earthquakes
– No magma involved
Transform boundary
San Andreas
Fault
Earthquakes – 1 Year
North America – Earthquakes
10 Year Period
Causes of Plate Motion
1. Mantle convection
– Convection currents drag plates
2. Ridge Push
– Newly formed rock slides downhill at mid
ocean ridge
3. Slab pull
– Subducting plate pulls more crust with it
Plate Motion
Rifting
 Breaking apart of Earth’s crust
Terrane
 Piece of
lithosphere
with a unique
geologic
history
incorporated
into another
piece of
lithosphere
Western U.S. Terrane
Alaska Accretions
Continental Growth
Effects of Continental Change
 Climate changes
 Life
– As populations
separate, new species
develop