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Theory of Plate Tectonics
Terri Henry
2nd lesson
Evidence of Continental Drift
• Continents fir together like
puzzle pieces.
• Same fossils of animals and
plants found on different
continents.
• Similar rock layers on different
continents.
• Striation left from glacier
movement.
• Tropical plant fossils in
Antarctica.
• Satellite technology collects data
about plate movement.
Theory of Plate Tectonics
• All of the evidence supports scientist belief that
the continents move.
• Why?
• Through a collection of data scientist have
pieced together the Theory of Plate Tectonics.
• Scientist believe that the earth’s surface is
broken in to many different sized plates that
move.
What is a Tectonic Plate?
• A massive, irregularly
shaped slab of solid
rock, usually
composed of both
continental and
oceanic crust.
Why do Plates move?
• Must investigate the interior of the earth.
• 4.6 billion years ago, scientist believe that a huge
explosion of gas and matter occurred in space.
• Over time the gas and matter cooled to formed planets.
• A layer of solid matter or crust formed around the outside
of a a ball.
• The lighter or less dense matter moved toward the
center.
• Four main layers formed as the Earth cooled and its
matter separated.
Crust
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The outermost layer.
Includes soil, sand and rocks, deep rocks.
Very thin and brittle.
Contains elements Oxygen and Silicon.
Less dense than the mantle, this is why it floats.
Contains continental crust 20km – 70km and
oceanic crust 5km – 10km.
• Also known as the lithosphere.
Mantle /Asthenosphere
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Mantle is a solid rocky layer below the crust.
It is made up of magnesium, silicon, and oxygen.
The mantle makes up 80% of the Earth’s volume.
The mantle makes up 68% of the Earth’s mass.
It begins 200km below the surface and extends down to
3,000km.
There is a big temperature difference that causes the
convection currents.
The convection currents are what move the crust.
Asthenosphere is the upper mantle to about 700km deep.
The Asthenosphere is a hot semi-solid material (plasticity).
The Asthenosphere glides over the lower mantle. This is what
causes the crust to move.
Outer Core
• Made of iron and nickel.
• The temperature (3,800°C) is so great that
these metals form a moving liquid layer.
This liquid layer causes the magnetic field
of earth.
• The outer core is 2,200km thick.
Inner Core
• A solid Sphere.
• Made of iron and nickel.
• Temperature is 5,000°C and pressure is
so great (from the weight of all the other
layers) the iron is squeezed into a solid
form.
• About 1,300km thick.
Why does the crust move?
• Inside the Earth, the source of heat is extreme.
• Heat form the interior rises toward the surface of
the Earth.
• This causes the liquid rock in the mantle to move
with it.
• Hot or less dense liquid moves upwards.
• Cool heavy rock sinks downwards.
• The heated magma rises to the surface, spreads
and begins to cool, then sinks to the bottom of
the mantle where it is reheated and rises again.
Convection currents continued
• The plates ride on the convection currents.
• Plate motion results in volcanoes,
earthquakes, and mountain building.
• Convection currents caused Pangaea to
break up.
• Convection currents have pushed the
continents to where they are today.
• Convection currents also causes the crust
to be destroyed or recycled
Why Does the Plate Float?
• Continental crust is
composed of rocks
that are made of
relatively lightweight Quartz
minerals such as
quartz and feldspar.
• All of which are found
in granite.
• Oceanic crust is
composed of basaltic
rock which is heavier.
Feldspar
Basaltic
Types of Plate Movement
• There are three main types of plate
movement.
Divergent boundaries
Convergent boundaries
Transform boundaries
Divergent Boundaries
• Occur along spreading centers
where plates are moving apart.
• New crust is created by
magma pushing up from the
mantle.
• Can be any combination of
plates (oceanic and
continental)
• Best known divergent
boundary is the Mid-Atlantic
Ridge or sea floor spreading.
Divergent Boundaries
• Causes underwater volcanoes and islands.
• Creates rift valleys.
• The stress at this boundary is tension. A stress
that pulls on the crust, stretching rock so that it
becomes thinner.
• Type of fault associated with divergent
boundaries is a Normal fault.
Iceland
• Iceland lies across the Mid-Atlantic
Ridge.
• It becomes wider every year as the
North American and Eurasian Plates
drift apart.
• Parts of Iceland are spreading 9cm
per year.
Sea Floor Spreading
Transform Boundaries
• The zone between two plates
sliding horizontally past one
another.
• Do not easily slide past each other.
There is a lot of pressure and
stress that builds up between the
plates as they move.
• The place where plates move and
cause a break in the crust is called
a fault. Transform boundaries
create earthquakes when they slide
past each other.
• Most transform boundaries exist on
the ocean floor.
Transform Boundary
• Type of stress: shearing a stress
that pushes a mass of rock in two
opposite directions.
• Type of fault: Strike –Slip
• Motion at the boundary: creates
faults
• Crust action: there is no new
crust made at this boundary.
• Land features: Faults are
created
San Andreas Fault
The San Andreas Fault slices through 2/3 or the length of
California. The Pacific Plate is moving horizontally past
the North American Plate at an average rate of 5 cm/yr.
Convergent Boundaries
• Recycling of crust occurs along
these boundaries.
• Plates are moving toward each
other or colliding.
• Usually occurs where one
plate is sinking under another
plate (subduction zone).
• Convergence may occur
between an oceanic and
continental plate, or between
two oceanic plates, or between
two continental plates.
Convergent Boundary
Continental to Continental
• Occurs when two continental
plates collide with each other
and push upwards.
• Tall folding Mountains form
The land has no where to go
but up.
• This is pushing up is known as
uplifting.
• This type of convergent
boundary does not cause
subduction.
Convergent
Continental to Continental
• Type of stress:
Compression: a stress that
squeezes rock until it folds
or breaks.
• Type of Fault: Reverse
• Motion at this boundary:
Folding and bending
• Crust Action: no new crust
is created and crust is not
recycled, it is reformed.
• Features: Mountain building
Creation of the Himalayan
Mountain Range
The Himalayas continue to rise
more than 1 cm a year—a growth
rate of 10km in a million years.
Convergent Boundary
Oceanic to Oceanic
• When two ocean plates
collide, one plate moves
under the other or subducts.
• Usually the plate with older
rock moves under the
younger rock because it is
more dense.
• The old crust is forced into the
Earth and melts to become
liquid rock.
Convergent
Oceanic to Oceanic
• Type of stress: compression: stress that
squeezes rock until it folds or breaks.
• Motion at the boundary: subduction
• Type of fault: Reverse
• Crust is recycled into the earth.
• Land feature: trench is formed.
Marianas Trench
• Located between the
Phillippine and Pacific
plates.
• Deepest trench on Earth.
• Reaches almost 11,000m
below sea level.
• It is over 2,000m deeper
than the tallest mountain
on Earth.
continental
ocean
Convergent Boundary
Continental to Oceanic
• When oceanic and continental
crust collide, the oceanic crust
pushes under the continental
crust.
• Oceanic crust slides under the
continental crust because it is
more dense.
• Eat and pressure below the
earth’s surface get very intense.
Earthquake and volcanoes occur
near the subduction zones to
release the pressure.
Convergent
Continental to Oceanic
• Type of stress: compression: a
stress that squeezes rock until
it folds or breaks.
• Motion at the boundary:
subduction
• Type of fault: Reverse
• Crust Action: recycling of old
crust to form new crust
• Land Form: Mountain ranges
and volcanoes
Cascade Mountain Range
• Occurs as the pacific plate moves under
the Juan de Fuca plate.
• The heat and pressure cause the oceanic
crust to melt.
• The crust is broken down when it forms
liquid rock or magma.
• The magma rises to the surface of the
earth and the result is a volcano.
Convergent Boundaries
India and Asia to create the
Himalayas
Oceanic Nazca Plate is subducting
under the South American Plate to
create the Andes mountains and Peru
Chili Trench
Two oceanic plates colliding, one must
subduct under the other creating a
trench (Marianas Trench)