Download Theory of Plate Tectonics

The Structure of
the Earth
&
Plate Tectonics
Restless planet
What is the theory of plate tectonics?
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The Theory of Plate Tectonics states that the
continents have moved and are still moving…about as
fast as your fingernails grow.
In 1912, Alfred Wegener introduced a hypothesis of
continental drift, but he did not fully understand what
caused the plates to move.
As scientists amassed more data, Wegener’s hypotheses
was amended to become
the Plate Tectonic Theory.
A theory is an explanation of a scientific process
that has been successfully tested.
What is the theory of plate tectonics
good for?
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The motion of Earth's plates help scientists to
understand why earthquakes, volcanoes, and
mountain building occur.
Scientists believe these plates have been moving for
millions of years.
Their research indicates that 250 millions years ago,
the Earth's continents were all grouped together
into one super-continent called Pangaea.
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About 200
million
years ago,
Pangaea
covered
approximate
ly 30% of
Earth’s
surface.
A large
ocean,
Panthalassa,
covered the
rest of the
planet.
Pangaea
Some clues leading scientists to accept the idea of
Pangaea were:
• Fossil clues
• Rock Clues
• Climate Clues
• Fossil clues: Fossils of the reptile Mesosaurus
have been found in South America and
Africa. Since the reptile couldn’t swim that
distance, Wegener hypothesized that this
reptile lived on both continents when they
were joined.
Additionally, plant fossils and matching rock
layers have been found on both continents.
• Rock Clues: Similar rocks were found in
United States, Western Europe, and
Greenland.
• Climate Clues: Glacial deposits found in
South America, Africa, India and Australia
provide additional evidence that continents
were connected.
Structure of the Earth
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The Earth is made
up of 3 main
layers:
• Crust
• Mantle
• Core
Crust
Mantle
Outer core
Inner core
The Crust
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This is where we live!
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The Earth’s crust is made of:
1. Continental Crust
- thick
- buoyant / kind of floats
-(less dense than
oceanic crust)
- mostly old
2. Oceanic Crust
- thin
- dense
- (sinks under
continental crust)
- young
Let’s model the Earth’s layers
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What are the layers
of the Earth?
How are the peach
layers similar to
Earth’s layers?
What are the
limitations of this
model?
The Earth’s layers
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The crust, mantle and core are labels for
describing the three different kinds of
materials the Earth is made of.
The lithosphere, asthenosphere, mesosphere,
outer core & inner core distinguish between
the 5 physical layers of the Earth.
Lick All My Orange Ice Cream
Question:
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Describe the difference between the crustmantle- core AND LAMOIC
How do we know what the Earth
is made of?
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Geophysical surveys: Mostly seismic wave experiments,
fieldwork, boreholes, mines
What is Plate Tectonics?
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If you look at a map of the world, you may
notice that some of the continents could fit
together like pieces of a puzzle.
Question
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Describe plate tectonics in your own words
Why are there tectonic plate
interactions?
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Earth is a warm planet sailing through cold space.
Most of the mantle of our planet is hot enough to
flow.
The surface of Earth, however, is chilled as it loses
heat into space. As a result, the rocks of Earth’s
surface are hard and brittle.
The cold outer layer of our planet, which holds
together as a rigid shell, is not made of one solid
piece.
Instead, this shell is broken into separate pieces, or
tectonic plates (lithosphere), that slide on top of
the mobile interior (asthenosphere).
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The tectonic plates are driven by the flowing
part of the mantle, it sets them in motion.
The movement of the plates causes a
complex puzzle of plate collisions around the
globe.
Plate Tectonics
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The Earth’s crust is divided into 12 major plates
which are moved in various directions.
(Plate examples: Eurasian, African, IndoAustrailian, Pacific, North American and South
American plates
This plate motion causes them to collide, pull
apart, or scrape against each other.
Each type of interaction causes a characteristic set
of Earth structures or “tectonic” features.
The word, tectonic, refers to the deformation of
the crust as a consequence of plate interaction.
What are tectonic plates made of?
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Plates are made
of rigid
lithosphere.
The lithosphere is
made up of the
crust and the upper
part of the mantle.
What lies beneath the tectonic plates?
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Below the
lithosphere
(which makes up
the tectonic
plates) is the
asthenosphere.
Plate Movement
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“Plates” of lithosphere are moved around by
the underlying hot mantle convection currents
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.
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There are three types of plate-plate
interactions based upon relative
motion:
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Convergent boundary - where plates collide
Divergent boundary - where plates separate
Transform boundary - where plates slide past
one another
What happens at a convergent boundary?
What forms around that boundary?
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Plates converge, which means two plates
move toward each other. Usually an oceanic
plate converges with a continental plate.
Since oceanic plates are more dense than
continental plates, ocean plates are
subducted or go down into the mantle in an
area called the subduction zone.
Convergent boundaries make
subduction zones
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A convergent
boundary can also
create a deep-sea
trench.
At the subduction
zone, the
temperature of the
rocks rises and causes
them to melt, which
forms magma that is
forced upwards to
create volcanoes.
FYI
Convergent Boundaries
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There are three styles of convergent plate
boundaries
• Continent-continent collision
• Continent-oceanic crust collision
• Ocean-ocean collision
Continent-Continent Collision
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Forms mountains, e.g. European Alps, Himalayas
Himalayas
Convergent boundaries making
mountains
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When two
continental plates
collide (same
densities) and push
up the crust, they
form mountain
ranges such as the
Himalayas. This is an
example of uplift.
No volcanoes form.
There is little or no subduction to produce the
high temperature required for melting the rocks.
Continent-Oceanic Crust Collision
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Called SUBDUCTION
Subduction
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Oceanic lithosphere subducts
underneath the continental
lithosphere
Oceanic lithosphere heats and
dehydrates as it subsides
The melt rises forming
volcanism
E.g. The Andes
Ocean-Ocean Plate Collision
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When two oceanic plates collide, one runs
over the other which causes it to sink into
the mantle forming a subduction zone.
The subducting plate is bent downward to
form a very deep depression in the ocean
floor called a trench.
The worlds deepest parts of the ocean are
found along trenches.
• Ex. The Mariana Trench is 11 km deep!
Divergent boundary
Plates separate, which means two plates
move away from each other.
 Usually an oceanic plate diverges from
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another oceanic plate.
Seafloor spreading occurs at divergent
oceanic plate boundaries and leads to
the creation of new ocean floor.
What happens at a divergent boundary?
What forms along that boundary?
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As two tectonic plates slowly separate,
molten material rises from within the
mantle to fill the opening. As the ocean
floor slowly separates, new rocks form
at the mid-ocean ridge that is
forming.
The Mid-Atlantic Ridge is getting wider
at a rate of 2.5 cm every year.
Mid-ocean
ridges
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Mid-ocean ridges are underwater
mountain chains.
Although named “mid-ocean”,
there are not necessarily at the
center of the ocean.
Divergent Boundaries
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Spreading ridges
• As plates move apart new material is erupted to fill
the gap
Sea floor spreading
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When the sea floor moves, magma
moves upward and flows from cracks at
the mid ocean ridge.
The magma cools and forms a new sea
floor.
This sea floor spreading idea is supported by the
following observations:
1. When scientists tested rocks at the mid-ocean ridge
and on both sides of the ridge, they found that the
youngest rocks were closer to the ridge and older
rocks were farther away from the ridge.
2. Earth has a magnetic field that from time to time
reverses magnetic north and south poles. Volcanic
rock provides a record of the magnetic poles at the
time the rock cooled, thus past reversals are
recorded in rocks forming along mid-ocean ridges.
Iceland: An example of continental rifting
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Iceland has a divergent plate
boundary running through
its middle
Transform Boundaries
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Where plates slide past each other
Above: View of the San Andreas
transform fault
What happens at the transform boundary?
What forms along that boundary?
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Plates slide past each other, and faults or breaks in
the Earth’s crust can form there from stress.
Stress- the amount of force put on a rock
When one plate suddenly slips past another plate
and releases the stress, earthquakes occur.
The San Andreas fault is part of a transform
boundary where the Pacific plate slides against the
North American plate.
Volcanoes and Plate
Tectonics…
…what’s the connection?
Pacific Ring of Fire
Hotspot volcano
chain- Hawaiian
Islands
Volcanos
are mostly
focused at
plate
boundaries
Pacific Ring of Fire
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Most of the active
volcanoes and
earthquakes are
located at the plate
boundaries.
The Pacific Ring of Fire
is an area of almost
constant earthquake
and volcanic activity.
It circles the Pacific
Ocean basin.
Volcanoes are formed by:
- Subduction - Rifting - Hotspots
What are Hotspot Volcanoes?
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Hot mantle plumes breaching the surface
in the middle of a tectonic plate
The Hawaiian island chain are
examples of hotspot volcanoes.
Photo: Tom Pfeiffer / www.volcanodiscovery.com
The tectonic plate moves over a fixed hotspot
forming a chain of volcanoes.
The volcanoes get younger from one end to the other.
Earthquakes and Plate
Tectonics…
…what’s the connection?
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As with volcanoes, earthquakes are not
randomly distributed over the globe
Figure showing
the distribution of
earthquakes
around the globe
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At the boundaries between plates, friction
causes plates to stick together. When built up
energy causes them to “unstick”, earthquakes
occur.
Where do earthquakes form?
Figure showing the tectonic setting of earthquakes
Plate Tectonics Summary
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The Earth is made up of 3 main layers (core,
mantle, crust) & 5 physical layers (LAMOIC)
On the surface of the Earth are tectonic plates
that slowly move around the globe
Plates are made of crust and upper mantle
(lithosphere) and move on the asthenosphere
There are 2 types of plates: oceanic and
continental
There are 3 types of plate boundaries:
convergent, divergent & transform
Volcanoes and Earthquakes are closely linked to
the borders of the tectonic plates
Convergent
Divergent
Transform
C-C form mountains
using uplift
Ex. Alps, Himalayas
O-O form mid-ocean
ridges (underwater
mountain chains) with
sea floor spreading
All will create fault lines
that build up pressure to
release earthquakes
Ex. San Andes fault CA
O-O form subduction
zones trenches
Ex. Mariana Trench
C-C breaks continental
crust apart and exposes
magma on Earth’s
surface
Ex. Iceland
C-O form volcanoes
from subduction zones
Ex. Andes Mts.
Age of Oceanic Crust
Courtesy of www.ngdc.noaa.gov
Questions...
What is the theory of plate
tectonics?
 What is the lithosphere?
 What is the asthenosphere?
 What is the connection between
the two?
 What are the two types of plates?
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Questions...
What are the three types of
boundaries?
 What direction do plates go for
each?
 Which boundary has a subduction
zone…what occurs at a subduction
zone?
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Questions...
What causes plates to move?
 How is a convection current
formed?
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