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Chapter 6
Plate Tectonics
Section 1: Inside the Earth
The Composition of the Earth
 There are two ways that scientists think about the
Earth’s layers:
 1) By composition
 2) By physical properties
 This includes temperature, density, and ability to flow
 The Earth is divided into three main layers:
 1) The Crust
 2) The Mantle
 3) The Core
 The lightest materials make up the outside of the Earth while
the heaviest make up the inside
The Crust
 Crust - is the outermost layer of the Earth
 It ranges from 5 to 100 km thick
 It is also the thinnest layer of the Earth
 Because we live on this layer we know more about this
layer than the other two.
 There are two types of crust:
 1) Continental - the crust above the ocean
 It is made up of mostly granite and has an average thickness of
about 30 km
 2) Oceanic - the crust under the ocean
 It is made up of mostly basalt (basalt is denser than granite)
and has an average thickness between 5 and 8 km thick
The Mantle
 Mantle - is the layer of the Earth between the crust
and the core
 The mantle is about 2,900 km thick and contains most
of the Earth’s mass
 No one has ever seen what the mantle really looks
like
 It’s just to deep to drill into
 In some cases mantle rock has been pushed to the
surface by tectonic forces allowing scientists to study
mantle rocks
 Scientists have determined that the composition of the
mantle is similar to the mineral olivine, which is a
mixture mostly of magnesium and iron
The Core
 Core - extends from the bottom of the
mantle to the center of the Earth.
 The diameter of the core is about 6,856 km,
which is slightly larger than the planet Mars
 The core contains 33% of the Earth’s mass
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The Structure of the Earth
The Structure of the Earth
 The Earth is divided into 5 main PHYSICAL
layers (outside / in):
 1) Lithosphere (“rock sphere”)- is the outermost, rigid
layer of the Earth.
 It is made up of the crust and the rigid part of the upper mantle
 Unlike the other physical layers of the Earth, the lithosphere is
not a single solid layer, it is made up of pieces called - tectonic
plates
 2) Asthenosphere (“weak sphere”) - is a soft layer of
the mantle on which the lithosphere moves
 It is made of a solid rock that moves very slowly like putty
 These low-strength rocks tend to lose their shape when stressed
 3) Mesosphere (“middle sphere”) - is the strong
lower part of the mantle beneath the
asthenosphere
 The mesosphere extends from the bottom of the
asthenosphere to the Earth’s core
 4) Outer Core - is the liquid layer of the Earth’s
core that lies beneath the mantle and surrounds
the inner core
 5) Inner Core - is the solid, dense center of our
planet that extends from the bottom of the outer
core to the center of the Earth
 The center of the Earth is 6,378 km beneath the
surface
Thicknesses of the Physical
Layers
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1) Lithosphere:
2) Asthenosphere:
3) Mesosphere:
4) Outer Core:
5) Inner Core:
15 - 300 km
250 km
2,550 km
2,200 km
1,228 km
Tectonic Plates
 Tectonic Plates - are pieces of the lithosphere that
move around on top of the asthenosphere
 Major Tectonic Plates:



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


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1) Pacific Plate
2) North American Plate
3) Cocos Plate
4) Nazca Plate
5) South American Plate
6) African Plate
7) Eurasian Plate
8) Indian Plate
9) Australian Plate
10) Antarctic Plate
A Giant Jigsaw Puzzle
 Looking at the previous slide, notice how
all of the major tectonic plates fit together
like a puzzle.
 Notice how all of the tectonic plates are not
the same
 For example the North American plate is much
larger than the Cocos plate
 Also the North American plate has an entire
continent on it, while the Cocos plate is
completely covered by ocean
 Like the North American plate, some plates
include both continental and oceanic crust
A Tectonic Plate Close Up
 What would a tectonic plate look like if you
could lift it out of place?
 It could consist of only oceanic crust, or only
continental crust, or both, like the North and
South American plates
 The thickest part would be where the mountain
ranges are, while the thinnest would be under the
ocean where ridges are located
 Remember oceanic crust is much thinner than
continental crust
 Why?????????????????????????????
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Mapping the Earth’s Interior
 How do we know so much about the Earth’s interior when
scientists have never even drilled through the Earth’s crust,
the thinnest part?
 The secret lies in Earthquakes
 When an earthquake occurs it produces vibrations called Seismic
Waves
 Depending on what the material is made of, seismic waves will
travel at different speeds
 For example, they travel faster through solid rock compared to liquids
 Scientists can measure these waves and their speeds with a
Seismograph which allows them to predict the consistency of the
Earth’s layers
Chapter 6 Quiz 1
 1) ___________ is the outer most layer of the
Earth based on composition.
 2) ___________ is the outermost rigid layer of the
Earth based on physical properties
 3) ___________ is the solid, dense portion of the
core
 4) ___________ are pieces of the lithosphere that
move around on the asthenosphere
 5) ___________ is beneath the asthenosphere and
when translated means “middle sphere”
 Bonus) _____________ is when two tectonic
plates slide past each other.
Section 2- Restless Continents
Wegener Theory of Continental Drift
 In the early 1900’s Alfred Wegener wrote a book
about his theory of continental drift
 Continental drift - is the theory that continents can drift
apart from one another and have done so in the past
 Does this help explain why the continents could fit
together like a puzzle?!?!?!?!?!?!?
 Does this explain why certain fossil types are found on
opposite sides of the Atlantic Ocean?!?!?!?!?!?
 Look at the following slide. Does continental drift
explain these findings?
 Does continental drift explain why glacier cut grooves
in the Earth match up if you put the continents back
together?!?!?!?!?!?
The Breakup of Pangaea
 Wegener studied many observations before
establishing his theory
 He thought that all the separate continents
of today were once joined as a single land
mass he called -Pangaea
 Pangaea is Greek for “All Earth”
 This occurred over 245 million years ago
Sea-Floor Spreading
 Sea-floor spreading - is the process by which new
oceanic lithosphere is created as older materials
are pulled away
 Mid-ocean Ridges are places where sea-floor spreading
takes place
 mid-ocean ridges are chains of underwater mountains that run
throughout the world
 As tectonic plates move away from each other, the seafloor spreads apart and magma rises up to fill the gap
 When observing sea-floor spreading, notice that the
newest part of the crust is where the spreading occurs
and gets older as it gets further away
Magnetic Reversals
 Some of the most important evidence of sea-floor
spreading comes from magnetic reversals recorded
in the ocean floor
 Magnetic Reversal - Is when the magnetic north and
south poles change place
 This has happened many times in Earth’s history
 Molten rock at mid-ocean ridges contain tiny
grains of magnetic minerals
 These grains act like compasses and align with the
magnetic field of the Earth
 Once cooled these minerals are set in place
 Scientists consider these as additional evidence
that sea-floor spreading occurs
Chapter 6 - Quiz 2
 1) ____________ is the theory that continents can drift
apart from one another and have done so in the past
 2) T/F There are identical fossil types on opposite sides of
the Atlantic Ocean
 3) ___________ is the name that Wegener came up with do
describe all the continents together as a single land mass
 4) ___________ are places where sea-floor spreading takes
place
 5) ___________ is the process by which oceanic
lithosphere is created as older materials are pulled away
 Bonus) __________ is the boundary where two plates
meet.
Section 3 - The Theory of Plate
Tectonics
 Plate Tectonics - is the theory that the Earth’s
lithosphere is divided into tectonic plates that
move around on top of the asthenosphere
 Because both oceanic and continental crust appear
to move, this NEW THEORY was devised to
explain both continental drift and sea-floor
spreading
 So what causes tectonic plates to move!?!?!?!?!?!?
Possible Causes of Tectonic Plate Motion
 It takes an incredible amount of energy to move
something as large as a tectonic plate
 Possible causes include;
 1) Slab Pull - is the process by which an oceanic plate
slides down and under a continental plate
 2) Ridge Push -is the process by which an oceanic plate
slides down the slope of the lithosphere/asthenosphere
boundary
 This happens because the ocean lithosphere is higher at mid
ocean ridges versus where it gets pushed down under the
continental lithosphere
 3) Convection - is the process by which hot material
from deep within the Earth rises, while cool material
near the surface sinks.
 This forms a circular motion that drags plates sideways away
from mid-ocean ridges
Pg. 147
Tectonic Plate Boundaries
 All tectonic plates have boundaries with other tectonic plates, and are
classified into three main types depending on how they move in
relation to one another
 1) Convergent Boundaries - are when two plates push into each
other
 There are three types of convergent boundaries
 A) Continental/Continental collisions - are when two plates with
continental crust collide
 This causes the plates to buckle and thicken pushing the
continental crust upward
 These collisions form some of the largest mountains in the
world
 B) Continental/oceanic collisions - is when a plate with
continental crust collides with a plate with oceanic crust
 This causes the oceanic plate (thinner) to slide under the
continental plate (thicker)
 The region where the ocean plate sinks down into the
asthenosphere is called a subduction zone
 C) Oceanic/Oceanic collisions - is when two oceanic plates
collide
 This causes one oceanic plate to slide under the other causing a
subduction zone
 2) Divergent Boundaries - are when two tectonic plates
move away from one another
 This type of boundary causes sea-floor spreading and midocean ridges
 These boundaries can also be found on continents
 Divergent boundaries are where new lithosphere is formed
 3) Transform Boundaries - are when two tectonic plates
slide past each other
 The San Andreas fault is an example of a transform boundary
 This fault marks the location where the Pacific Plate and the
North American plate slide past each other
 This type of boundary can cause EARTHQUAKES !!!!!!!!!!!!!
 This is because the plates do not slide smoothly past each other.
They grind and jerk as they slide past each other
Tracking Tectonic Plate Motion
 How fast a tectonic plate moves depends on many
factors including the type and shape of the plate
and how they are interacting with other plates
around them
 Normally plates move very slowly (a few cm per
year)
 Exceptions include transform boundaries like the
San Andreas fault, that can move several meters
during an Earthquake
 One way to measure plate movement is with a
Global Positioning System (GPS), which measures
how far ground stations placed on tectonic plates
move, by using a satellite in space
Ch. 6 Quiz 3
 1) ____________ is the theory that the Earth’s
lithosphere is divided into plates that move on top
of the asthenosphere
 2) LIST one of the three possible causes of plate
movement
 3) ___________ boundary is where two tectonic
plates push into each other
 4) ___________ boundary is where two tectonic
plates move away from each other
 5) ___________ boundary is where two tectonic
plates slide past each other
 Bonus) _____________ is the name given to the
rim of the pacific ocean, because of all the
volcanoes and volcanic activity that occurs there.
Section 4 - Deforming the Earth’s Crust
 Stress - is the amount of force per unit area that is
put on a given material
 The conditions under which a rock is stressed
determines its behavior
 When rock changes shape due to stress, this reaction is
called deformation
 Like a piece of spaghetti, rock can bend or break
depending on the type of stress that is placed on them
 Rocks can deform due to the forces of plate tectonics
 Compression - is the type of stress that occurs
when an object is squeezed
 This type of stress occurs when two plates collide
 Convergent boundary compression can cause large
mountain ranges to form (i.e. Rocky Mountains)
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Folding
 Folding - occurs when rock layers bend due
to stress in the Earths crust
 When you see a fold in the layers of the Earth’s
crust, you know deformation has taken place
 The two most common folds are anticlines and
synclines and form compression
 Monoclines are a third type of
fold that forms an upward or
downward force
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Faulting
 While some rock layers bend and fold under
stress, some rocks will break
 Fault - is the surface along which rocks
break and slide past each other
 The blocks of crust on each side of a fault are
called fault blocks
 If a fault is not vertical it is useful to
distinguish between the two sides - hanging
wall and foot wall
 Depending on how the hanging wall and
foot wall move relative to each other, two
main types of faults can form
 1) Normal Fault - causes the hanging wall to
move down relative to the footwall
 This fault normally occurs when rocks move away
from each other (divergent)
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 2) Reverse Fault - causes the hanging wall to
move up relative to the footwall
 Reverse faults usually occur when rocks push
together (convergent)
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 3) Strike-Slip Fault - occurs when opposite
forces cause rock to break and move
horizontally (transform)
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Plate Tectonics and Mountain Building
 When tectonic plates collide, land features
that start out as small folds and faults can
eventually become great mountain ranges
 The reason mountains exist is that tectonic
plates are continually moving and bumping
into each other
 Most major mountain ranges form at the edges
of tectonic plates
 Mountains can form in several different
ways
 1) Folded Mountains
 Form when rock layers are squeezed
together and pushed upward
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 2) Fault Block Mountains
 Tectonic forces can place so much force and tension on the
Earth’s crust that a large number of normal faults can form
in one area
• When large blocks of the earth’s crust drop down
relative to other blocks, fault block mountains are
formed
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 3) Volcanic Mountains
 Most of the world’s major volcanic mountains are located at
convergent boundaries
 These boundaries often include subduction zones where
one plate slips under another and is forced downward
 These rocks melt and become magma
 Volcanic mountains form when molten rock erupts onto the
Earth’s Surface
 Unlike folded and fault block mountains, volcanic mountains
form from new material being added to the Earth’s surface
 There are so many volcanic mountains around the rim of the
Pacific ocean that early explorers named it the RING OF FIRE
!!!!!!
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Folded Mountain
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Fault Block Mountain
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Volcanic Mountain
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Chapter 6 Quiz 4
 1) ____________ is the type of stress that is
formed when two plates collide
 2) ____________ occurs when rock layers bend
due to stress
 3) ____________ is the type of fault when the
hanging wall moves UP compared to the foot wall
 4) ____________ is the type of fault when the
hanging wall moves DOWN compared to the foot
wall
 5) ____________ is the type of fault when
opposite forces cause rock to break and move
horizontally
 Bonus) List 5 of the 10 major tectonic plates