Transcript Theory of Plate Tectonics

Continental Drift Theory
Continental Drift Theory
• Proposed by Alfred Wegener in 1912
• 250 million years ago, all of the continents
were combined into one super-continent
called “Pangaea”
• The continents gradually drifted apart to
where they are today
“Puzzle Pieces”
• Continents look like
they could be part
of a giant jigsaw
puzzle
Distribution of Fossils
• Plant and animal
fossils found on the
coastlines of different
continents
Ancient Climates
• Tropical plant remains
(coal deposits) found
in Antarctica
• Glaciation in Africa,
South America, India,
and Australia during
the same time
Sequence of Rocks
• Same rock patterns
found in South
America, India, Africa,
Antarctica and
Australia
Problems With The Theory
• Theory was not accepted by
scientists…why?
– Because Wegner could not identify the
cause of continental drift
Causes of Plate
Tectonics
Convection Currents
• Hot magma in the Earth
moves toward the surface,
cools, then sinks again.
• Creates convection currents
beneath the plates that cause
the plates to move.
Heat Transfer The Driving Force of
Plate Tectonics
Theory of Plate Tectonics
Plate Tectonics
Plate Boundaries
Causes of Plate Tectonics
Plate Tectonics
What is Plate Tectonics
• The Earth’s crust and upper
mantle are broken into
sections called plates
• Plates move around on top of
the mantle like rafts
What is the Lithosphere?
• The crust and part of the upper
mantle = lithosphere
–100 km thick
–Less dense than the material
below it so it “floats”
What is the Asthenoshere?
• The plastisity layer below the
lithosphere = asthenosphere
• The plates of the lithosphere
float on the asthenosphere
2 Types of Plates
• Ocean plates - plates below
the oceans
• Continental plates - plates
below the continents
Plate Boundaries
Divergent Boundaries
• Boundary between two plates
that are moving apart or rifting

• RIFTING causes SEAFLOOR
SPREADING
Divergent Boundaries
create…
• Mid-ocean ridges
• rift valleys
• fissure volcanoes
Convergent Boundaries
• Boundaries between two
plates that are colliding
 
• There are 3 types…
Type 1
• Ocean plate colliding with a
less dense continental plate
• Subduction Zone: where the
more dense plate slides under
the less dense plate
• VOLCANOES occur at
subduction zones
Andes Mountains,
South America
Type 2
• Ocean plate colliding with
another ocean plate
• The more dense plate slides
under the less dense plate
creating a subduction zone
called a TRENCH
Aleutian Islands, Alaska
Type 3
• A continental plate colliding
with another continental plate
• Have Collision Zones:
–a place where folded and thrust
faulted mountains form.
Transform Fault Boundaries
• Boundary between two plates that
are sliding past each other
• EARTHQUAKES along faults
• Another example of a transform
boundary is the San Andres Fault
San Andreas Fault, CA
Sea-Floor Spreading
Evidence
• Magnetic Stripes.
How does the
pattern of matching
stripes show
evidence of seafloor spreading?
Mapping the mid-ocean ridge
• The East Pacific Rise
has a mid-ocean
ridge.
• Curves around like a
baseball.
• Lies hidden under
hundreds of meters of
water.
Sonar
• A device that bounces
sound waves off
underwater objects
and then records the
echoes of these
sound waves.
• Sonar mapped midocean ridges.
Harry Hess
• An American
geologist who studied
mid-ocean ridges.
• He suggested that the
ocean floors move
like conveyor belts,
carrying the
continents along with
them.
• At the mid-ocean ridge, molten material
rises from the mantle and erupts. The
molten material then spreads out, pushing
older rock to both sides of the ridge. As
the molten material cools, it forms a strip
of solid rock in the center of the ridge.
Then more molten material flows into the
crack.
Sea-floor spreading
• The process that
continually adds new
material to the ocean
floor.
• Evidence molten
material, magnetic
stripes, and drilling
samples.
Evidence #1 - Molten Material
• The submersible,
Alvin, found strange
rocks shaped like
pillows or like
toothpaste squeezed
from a tube. Such
rocks can form only
when molten material
hardens quickly after
erupting under water.
Evidence #2 - Magnetic Stripes
• Scientists discovered
that the rock that
makes up the ocean
floor lies in a pattern
of magnetized
“stripes”. They hold a
record of reversals in
Earth’s magnetic field.
Evidence #3 - Drilling Samples
• The Glomar
Challenger did a
drilling sample and
found rocks that the
farther away from the
ridge the older the
rocks were. The
younger ones were in
the center of the
ridge.
Subduction at Deep-Ocean
Trenches
• Wider & wider? Deepocean trenches
• Ocean floor plunges into
deep underwater
canyons are deep-ocean
trenches.
• Subduction is the process
by which the ocean floor
sinks beneath a deepocean trench and back
into the mantle.
Subduction in the Pacific &
Atlantic
• Deep ocean trenches are swallowing more
oceanic crust than the mid-ocean ridge
can produce. Thus, the width of the
Pacific will shrink.
• The Atlantic is expanding. It has short
trenches. In some places, the oceanic
crust is attached to the continental crust
which moves the continents.
Subduction
• At deep-ocean trenches, subduction
allows part of the ocean floor to sink back
into mantle, over tens of millions of years.
• Subduction and Earth’s Oceans
• Earth’s ocean floor is renewed about every
200 million years.