Transcript Plate Tectonics

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By Alex McCracken & Nick Froemming
With help from Alexandra H, Brooke T,
Tom L and Tim D.
and Mr. Brown
Plate Tectonics
• Theory that the Earth’s
crust is made of rigid
plates that float on the
molten layer of the
mantle.
• Comes from the Greek
word meaning “builder”.
• Explains nearly all of
Earth’s major surface
features and activities:
faults and earthquakes,
volcanoes and volcanism,
mountains and mountain
building, and the origin
of the continents.
Continental Drift
• About 50 years earlier,
German geophysicist
Alfred Wegener
developed a related
theory known as
continental drift.
• His theory is supported
by the fact that the
world’s continents fit
together like pieces of a
puzzle.
• This super continent is
called PANGAEA.
Rodinia
History of Plate Tectonics
• Wegener’s idea was supported
by:
• the fit of the continents
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the distribution of fossils
a similar sequence of rocks at
numerous locations
ancient climates
• the apparent wandering of the
Earth's polar regions
• Wegener used his observations
to hypothesize that all of the
present-day continents were
once part of a single super
continent called Pangaea.
Fossil Record
Ancient Climates
Problems with Wegener’s
Theory
• Wegener’s Theory was
not accepted by most
geologists.
• They said that ocean
currents or winds could
scatter the ancient
plants and animals.
• They also thought that
maybe it was possible
for the poles to wander.
The Big Problem
• Wegener could not
provide an
explanation for how
the continents could
move.
• He couldn’t explain
what forces could
actually cause the
continents to move.
The Big Solution
• Arthur Holmes, (Scottish geologist) made this proposal in 1928.
• He believed heat trapped in the Earth caused convection
currents, areas where fluids beneath the Earth's crust rise,
flow laterally, and then fall. The currents would rise beneath
continents, pushing them apart, and then plunge beneath the
oceans.
• Unfortunately, Wegener died in 1930 while exploring the
Greenland ice cap. He never had the opportunity to adapt
Holmes' ideas to his views of continental drift.
Theory of Plate Tectonics
• Totally
revolutionized our
understanding of
the Earth, past and
present.
• Theory ranks with
the theories on
evolution, relativity,
the Big Bang, and
Newton’s Laws.
Chapter 7
Section 1 Inside the Earth
Tectonic Plates
Click below to watch the Visual Concept.
Visual Concept
Plate Structure
• The lithosphere is
broken into rigid
slabs of rock called
tectonic plates.
• These plates are
moving about on top
of the
asthenosphere.
• Resembles a giant
jig saw puzzle.
Plate Movement
• The plates are
pushed along by
convection currents in
the mantle.
• Plates move at rates
of about 1 inch per
year.
• The fastest move
more than 4 inches a
year.
Chapter 7
Section 3 The Theory of Plate Tectonics
Causes of Tectonic Plate Motion
Click below to watch the Visual Concept.
Visual Concept
Convection Currents Inside the
Earth
The tectonic plates float on the magma that lies in
the core of the earth
This magma is just one big convection current
These currents push the continental plates around.
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Plate Boundaries
This is where all the action is:
Volcanoes
Earthquakes
New crust being created, old crust being destroyed
Plate Interaction
• Tectonic plates can
interact in three ways
• Converge (→←),
diverge (←,→) and
transform(↑↓).
• Boundaries where
plates meet are
known as plate
margins.
Chapter 7
Section 2 Restless Continents
Continental Drift
Click below to watch the Visual Concept.
Visual Concept
Chapter 7
Section 3 The Theory of Plate Tectonics
Tectonic Plate Boundaries
Click below to watch the Visual Concept.
Visual Concept
Converging Boundary O-O
• When two plates
converge, one of the
plates subducts or sinks
underneath the other
forming a deep
depression called an
ocean or deep sea
trench. Here crust is
destroyed.
• The subducted plate
sinks downward into the
mantle and begins to
melt.
• The molten rock rises
towards the surface and
forms a chain of volcanic
islands.
Converging Boundary O-O
Converging Boundary O-C
• Instead of a volcanic
arc, you get a
volcanic mountain
range.
•Subduction
zone
Converging Boundary O-C
Converging Boundary C-C
• Continental crust is
too light to be carried
downward into a
trench.
• Two continental
plates converge to
form folded mountain
ranges.
• Andes and
Himalayans are
examples.
Converging Boundary C-C
Transform Boundary
• A transform boundary is
when two plates slip
past each other, moving
in opposite directions.
• Earthquakes often
happen in areas with
transform boundaries.
• The San Andreas fault
in California is a good
example of this type of
fault
• Tend to have the most
earthquakes.
• Crust is not created or
destroyed
Transform Boundary
Divergent Boundaries
• These are places where the plates
are moving away from each other.
• Magma from deep in the mantle is
pushing the plates apart. This
creates new crust.
• They cause the mid-ocean ridges.
They are the longest mountain
ranges on the planet.
• Mid-ocean ridges on land are called
rift valleys.
• The rift valley in east Africa is
about 3,000 km long.
• Could be the beginning of a new
ocean.
• This is called sea floor spreading.
• The mid-ocean ridges wrap almost
completely around the Earth!
Chapter 7
Section 2 Restless Continents
Magnetic Reversals and Sea-Floor Spreading
Click below to watch the Visual Concept.
Visual Concept
Rifting
Hot Spots
• Some volcanoes can
occur in the middle of
plates.
• Sometimes the plates
move over an area where
the magma is close to
the surface.
• Volcanoes occur at the
same place, but because
the plate is moving, a
chain of volcanoes can
occur.
• Hawaiian Islands were
formed this way.
Future Earth
• What do you think the world will look
like in a million years???
• It is predicted that the continents will
eventually crash together on the other
side of the world, creating a new
Pangaea.
Future World
150 million years in the future
Return to Pangaea