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Chapter 6
Plate Tectonics
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Section 1 Earth's Structure
Section 2 The Theory of Plate Tectonics
Section 3 Deforming Earth's Crust
Section 4 California Geology
Concept Map
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Chapter 6
Section 1 Earth's Structure
Bellringer
Many fossils of the same ancient plants and animals are
found on different continents separated by oceans.
Write a few sentences to explain how this could happen
and what it suggests about the continents.
Write your answers in your science journal.
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Chapter 6
Section 1 Earth's Structure
What You Will Learn
• Earth’s interior can be divided into layers based on
chemical composition and physical properties.
• Scientists use seismic waves to study Earth’s interior.
• Continents are drifting apart from each other now and
have done so in the past.
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Chapter 6
Section 1 Earth's Structure
The Layers of the Earth
• Earth is made of several layers.
• The materials in each layer have distinct properties.
• Earth’s layers can be described in terms of their
chemical composition or physical properties.
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Chapter 6
Section 1 Earth's Structure
The Layers of the Earth, continued
The Compositional Layers of Earth
•
Earth is divided into three compositional layers.
1. At Earth’s center, the dense metallic core is made
mainly of the metal iron.
2. The dense, thick middle layer is the mantle.
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Chapter 6
Section 1 Earth's Structure
The Layers of the Earth, continued
• The mantle is made up largely of silicon, oxygen, and
magnesium.
3. The surface layer, or crust, is composed mostly of
silicon, oxygen, and aluminum.
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Chapter 6
Section 1 Earth's Structure
The Layers of the Earth, continued
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Chapter 6
Section 1 Earth's Structure
The Layers of the Earth, continued
Continental and Oceanic Crust
• There are two types of crust.
• Continental crust is thicker than oceanic crust.
• Both types are made mainly of the elements oxygen,
silicon, and aluminum.
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Chapter 6
Section 1 Earth's Structure
The Layers of the Earth, continued
• But oceanic crust has almost twice as much iron,
calcium, and magnesium as continental crust does.
• These three elements form minerals that are denser
than the minerals in continental crust.
• These dense minerals make the thin oceanic crust
heavier than the thicker continental crust.
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Chapter 6
Section 1 Earth's Structure
The Layers of the Earth, continued
The Physical Structure of Earth
•
Earth is divided into five layers based on physical
properties.
1. Earth’s outer layer is the lithosphere, which is a
cool, rigid layer that includes the crust and the upper
part of the mantle.
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Chapter 6
Section 1 Earth's Structure
The Layers of the Earth, continued
•
The lithosphere is divided into pieces called tectonic
plates.
2. Below the lithosphere is the asthenosphere, which
is a layer of the mantle that is made of very slowflowing solid rock.
•
Tectonic plates move on top of the asthenosphere.
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Chapter 6
Section 1 Earth's Structure
The Layers of the Earth, continued
3. Below the asthenosphere is the mesosphere, which
is the lower part of the mantle.
• The mesosphere flows even more slowly than the
asthenosphere.
• The Earth’s core has two layers.
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Chapter 6
Section 1 Earth's Structure
The Layers of the Earth, continued
4. The outer core is a layer of liquid iron and nickel.
5. At Earth’s center is the solid inner core. This layer is
made mostly of nickel and iron.
• The inner core is very hot, but it is solid because it is
under enormous pressure.
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Chapter 6
Section 1 Earth's Structure
Mapping Earth’s Interior
• Scientists have learned about Earth’s interior by
studying earthquakes.
• An earthquake produces vibrations called seismic
waves.
• Seismic waves travel through Earth at various
speeds.
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Chapter 6
Section 1 Earth's Structure
Mapping Earth’s Interior, continued
• Machines called seismometers measure the time
seismic waves take to travel various distances from
an earthquake’s center.
• Scientists use these distances and times to calculate
the density and thickness of Earth’s layers.
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Chapter 6
Section 1 Earth's Structure
Mapping Earth’s Interior, continued
• The speed of seismic waves is affected by the type of
material that the waves are traveling through.
• For example, some types of waves can travel through
rock but not through liquids.
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Chapter 6
Section 1 Earth's Structure
Mapping Earth’s Interior, continued
• These waves never reach the seismometers on the
side of Earth opposite the earthquake.
• Therefore, part of Earth’s interior must be liquid.
• This liquid layer is the outer core.
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Chapter 6
Section 1 Earth's Structure
Mapping Earth’s Interior, continued
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Chapter 6
Section 1 Earth's Structure
Continental Drift
• Continental drift is the idea that a single large
landmass broke up into smaller landmasses to form
the continents, which then drifted to their present
locations. (OLD Theory, no longer relevant)
• However, some parts of this theory helped to create
the current theory of plate tectonics.
• First, it explains how the continents seem to fit
together like puzzle pieces.
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Chapter 6
Section 1 Earth's Structure
Continental Drift, continued
• Second, Continental Drift explained why fossils of the
same plant and animal species were found on
continents that are far away from each other.
• Many of these ancient species could not have
crossed an ocean, so, the continents must have been
connected at one point.
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Chapter 6
Section 1 Earth's Structure
Continental Drift, continued
• The locations of mountain ranges and similar types of
rock also support continental drift.
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Chapter 6
Section 1 Earth's Structure
Continental Drift, continued
• Scientists have used rock and fossil evidence to
reconstruct past patterns of climate regions.
• The distribution of these ancient climatic zones
supports the idea of continental drift, too.
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Chapter 6
Section 1 Earth's Structure
The Breakup of Pangaea
• Alfred Wegener, the scientist who proposed the
theory of continental drift, proposed that the large
continent of Pangaea gave rise to today’s continents.
• Scientists have determined that Pangaea existed
about 245 million years ago.
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Chapter 6
Section 1 Earth's Structure
The Breakup of Pangaea, continued
• Pangaea split into two continents—Laurasia and
Gondwana—about 135 million years ago.
• These two continents then split into the continents we
know today.
• These continents slowly drifted to their present
positions. (centimeters per year)
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Chapter 6
Section 1 Earth's Structure
The Breakup of Pangaea, continued
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Chapter 6
Section 1 Earth's Structure
Sea-Floor Spreading
• Evidence for continental drift lies on the sea floor.
• A chain of submerged mountains runs through the
center of the Atlantic Ocean (Mid-Atlantic Ridge).
• This mountain chain is part of a worldwide system of
mid-ocean ridges.
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Chapter 6
Section 1 Earth's Structure
Sea-Floor Spreading, continued
• Mid-ocean ridges show patterns of magnetism.
• The pattern on one side of a ridge is the mirror image
of the pattern on the other.
• The magnetism of rocks aligns with Earth’s magnetic
field as it was when the rocks formed.
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Chapter 6
Section 1 Earth's Structure
Sea-Floor Spreading, continued
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Chapter 6
Section 1 Earth's Structure
Sea-Floor Spreading, continued
• Throughout Earth’s history, the north and south
magnetic poles have changed place many times.
• This is called magnetic reversal.
• As rock forms from magma, minerals that contain iron
form.
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Chapter 6
Section 1 Earth's Structure
Sea-Floor Spreading, continued
• Some of these minerals are magnetic and act like
compasses.
• They form so that their magnetic fields align with the
magnetic fields on Earth.
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Chapter 6
Section 1 Earth's Structure
Sea-Floor Spreading, continued
• When the molten rock cools, these tiny compasses
are locked into position in the rock.
• After Earth’s magnetic field reverses, new magnetic
minerals that align in the opposite direction form.
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Chapter 6
Plate Tectonics
Magnetic Reversals and Sea-Floor Spreading
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Chapter 6
Section 1 Earth's Structure
Sea-Floor Spreading, continued
• At a mid-ocean ridge, magma rises through fractures
in the sea floor.
• As magma cools, it forms new rock.
• As the new rock forms, the older rock gets pulled
away from the mid-ocean ridge.
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Chapter 6
Section 1 Earth's Structure
Sea-Floor Spreading, continued
• The process by which new sea floor forms as old sea
floor is pulled away is called sea-floor spreading.
• The record of magnetic reversals on the sea floor
provides evidence that the continents are moving.
• Sea-floor spreading is one process that moves
continents.
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Chapter 6
Section 1 Earth's Structure
Sea-Floor Spreading, continued
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Chapter 6
Section 2 The Theory of Plate
Tectonics
Bellringer
When water is heated in a pot over a flame, the flame
touches only the bottom of the pot. How does the
water become heated?
Why does all of the air in a room become warm even if
heat enters the room only through one furnace vent?
Write your answers in your science journal.
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Chapter 6
Section 2 The Theory of Plate
Tectonics
What You Will Learn
• Earth’s lithosphere is broken into pieces called
tectonic plates.
• Heat from Earth’s interior causes convection in the
mantle.
• Tectonic plates move at an average rate of a few
centimeters per year.
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Chapter 6
Section 2 The Theory of Plate
Tectonics
Tectonic Plates
• Plate tectonics is the theory that Earth’s lithosphere
is divided into tectonic plates that move around on
top of the asthenosphere.
• Pieces of the lithosphere that move around on top of
the asthenosphere are called tectonic plates.
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Chapter 6
Section 2 The Theory of Plate
Tectonics
Tectonic Plates, continued
• Earth’s tectonic plates differ in size.
• Some plates contain both continental and oceanic
crust.
• Others contain mostly oceanic crust, or mostly
continental crust.
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Chapter 6
Section 2 The Theory of Plate
Tectonics
Tectonic Plates, continued
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Chapter 6
Section 2 The Theory of Plate
Tectonics
Tectonic Plates, continued
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Chapter 6
Section 2 The Theory of Plate
Tectonics
Tectonic Plates, continued
• Tectonic plates float on the asthenosphere.
• The plates cover the surface of the asthenosphere,
and they touch one another and move around.
• Thick plates made of continental lithosphere displace
more asthenosphere than do thin plates made of
oceanic lithosphere.
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Chapter 6
Section 2 The Theory of Plate
Tectonics
Tectonic Plate Boundaries
• A boundary is a place where tectonic plates meet.
• Tectonic plate boundaries are located by studying the
locations of earthquakes, volcanoes, and landforms
such as mid-ocean ridges and ocean trenches.
• A plate boundary can be a convergent, divergent, or
transform boundary.
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Chapter 6
Section 2 The Theory of Plate
Tectonics
Tectonic Plate Boundaries, continued
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Chapter 6
Section 2 The Theory of Plate
Tectonics
Tectonic Plate Boundaries, continued
Convergent Boundaries
• The boundary at which two tectonic plates collide is
a convergent boundary.
•
At a convergent boundary, three types of collisions
may happen.
1. Continental/Continental: Two plates made of
continental lithosphere collide, forming a high
mountain range.
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Chapter 6
Section 2 The Theory of Plate
Tectonics
Tectonic Plate Boundaries, continued
2. Oceanic/Continental: A plate of oceanic lithosphere
collides with a plate of continental lithosphere.
•
The denser oceanic lithosphere will sink beneath
the less-dense continental crust, in a process
called subduction.
•
Subduction can cause a chain of volcanoes to
form parallel to the plate boundary.
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Chapter 6
Section 2 The Theory of Plate
Tectonics
Tectonic Plate Boundaries, continued
3. Oceanic/Oceanic: Two plates of oceanic lithosphere
may collide.
•
The denser of the two plates will subduct.
•
A series of volcanic islands, called an island arc,
may form parallel to the plate boundary.
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Chapter 6
Section 2 The Theory of Plate
Tectonics
Tectonic Plate Boundaries, continued
Divergent Boundaries
• The boundary at which two tectonic plates separate
is a divergent boundary.
•
Most divergent boundaries happen on the sea floor.
•
These boundaries are characterized by mid-ocean
ridges.
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Chapter 6
Section 2 The Theory of Plate
Tectonics
Tectonic Plate Boundaries, continued
•
As the plates pull away from each other, fractures
form in the oceanic lithosphere.
•
Magma rises through these fractures to the ocean
floor.
•
There, the magma solidifies to form new
lithosphere.
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Chapter 6
Section 2 The Theory of Plate
Tectonics
Tectonic Plate Boundaries, continued
Transform Boundaries
•
The boundary at which two tectonic plates slide past
each other is a transform boundary.
•
Most transform boundaries occur in the sea floor at
mid-ocean ridges.
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Chapter 6
Section 2 The Theory of Plate
Tectonics
Tectonic Plate Boundaries, continued
•
At these locations, transform boundaries run
perpendicular to the ridge where plates are pulling
apart.
•
The transform boundaries cause offsets between
shorter segments of the ridge.
•
These offsets give mid-ocean ridges a zigzag
pattern.
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Chapter 6
Section 2 The Theory of Plate
Tectonics
Tectonic Plate Boundaries, continued
•
The San Andreas fault system is a well-known
example of a transform boundary.
•
This system occurs both on the sea floor and on
land.
•
The fault system is located where the Pacific and
North American plates are sliding past each other.
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Chapter 6
Section 2 The Theory of Plate
Tectonics
Causes of Tectonic Plate Motion
•
Tectonic plate motion is the result of density
differences that are caused by the flow of heat
within Earth.
•
Earth’s core and mantle are very hot, due to
minerals that have radioactive atoms.
•
These atoms release heat as they decay.
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Chapter 6
Section 2 The Theory of Plate
Tectonics
Causes of Tectonic Plate Motion, continued
•
Heat from Earth’s center flows toward the surface.
•
However, rock is a poor conductor of heat.
•
Therefore, most heat transfer happens through
convection.
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Chapter 6
Section 2 The Theory of Plate
Tectonics
Causes of Tectonic Plate Motion, continued
•
When rock is heated, it expands, becomes less
dense, and rises toward Earth’s surface.
•
At the surface, cold, dense rock tends to sink during
subduction.
•
This process causes convection currents in the
mantle.
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Chapter 6
Plate Tectonics
Causes of Tectonic Plate Motion
•There are Three Forces that drive
Tectonic Plate Motion:
1.Convection
2. Ridge Push
3.Slab Pull
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Chapter 6
Section 2 The Theory of Plate
Tectonics
Tracking Tectonic Plate Motion
•
Tectonic plate motion is so gradual that it is
measured in centimeters per year.
•
The average rate of movement for different plates
ranges between 2.5 cm/year and 15 cm/year.
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Chapter 6
Section 2 The Theory of Plate
Tectonics
Tracking Tectonic Plate Motion, continued
•
Scientists use GPS to measure the movement of
tectonic plates.
•
GPS continuously records the exact distance
between satellites and ground stations.
•
Scientists use changes in distances to calculate
rates of tectonic plate motion.
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Chapter 6
Section 3 Deforming Earth's Crust
Bellringer
Look at the photographs of the mountains displayed by
your teacher. Explain how each mountain might have
formed. (Pg. 208-209 Figures 4,5, and 6)
Write your answers in your science journal.
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Chapter 6
Section 3 Deforming Earth's Crust
What You Will Learn
• Stress is placed on rock as plates move. The stress
causes rocks to fold and break.
• The formation of mountains results from the motion of
tectonic plates.
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Chapter 6
Section 3 Deforming Earth's Crust
Deformation
• Stress is the amount of force per unit area on a given
material.
• Rock may bend or break when different amounts of
stress are applied.
• The process by which the shape of a rock changes in
response to stress is called deformation.
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Chapter 6
Section 3 Deforming Earth's Crust
Folding
• The bending of rock layers in response to stress is
called folding.
• Scientists assume that all rock layers start as
horizontal layers.
• When scientists see a fold, they know that
deformation has taken place.
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Chapter 6
Section 3 Deforming Earth's Crust
Folding, continued
• All folds have a hinge and two limbs.
• Limbs are the sloping sides of a fold.
• A hinge is the bend where the two limbs meet.
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Chapter 6
Section 3 Deforming Earth's Crust
Folding, continued
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Chapter 6
Section 3 Deforming Earth's Crust
Folding, continued
• Anticlines and synclines are the two most common
types of folds.
• An anticline is a fold in which the oldest rock layers
are in the center of the fold, like an arch.
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Chapter 6
Section 3 Deforming Earth's Crust
Folding, continued
• A syncline is a fold in which the youngest rock layers
are in the center of the fold, like a trough.
• Folds can range from centimeters wide to hundreds
of kilometers wide.
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Chapter 6
Section 3 Deforming Earth's Crust
Folding, continued
• Rock layers can bend into symmetrical or
asymmetrical folds.
• In a symmetrical fold, each limb dips in the same
way.
• In an asymmetrical fold, one limb may dip more
steeply than the other does.
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Chapter 6
Section 3 Deforming Earth's Crust
Folding, continued
• An overturned fold is a fold in which one limb is tilted
beyond 90°.
• Rock layers may also be bent so much that a rock
appears to be lying on its side.
• Geologists call this lying-down fold a recumbent fold.
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Chapter 6
Section 3 Deforming Earth's Crust
Faulting
• Rock may break if placed under too much stress.
• The surface along which rocks break and slide past
each other is called a fault.
• The blocks of crust on either side of the fault are
called fault blocks.
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Chapter 6
Section 3 Deforming Earth's Crust
Faulting, continued
• When a fault is not vertical, it forms two types of fault
blocks.
• The footwall is the block of rock that lies below the
plane of the fault.
• The hanging wall is the block that lies above the
plane of the fault.
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Chapter 6
Section 3 Deforming Earth's Crust
Faulting, continued
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Chapter 6
Section 3 Deforming Earth's Crust
Faulting, continued
• Faults are classified into three categories according
to how the fault blocks move relative to each other.
• The type of fault that formed can be used to
determine the type of stress that caused the fault.
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Chapter 6
Section 3 Deforming Earth's Crust
Faulting, continued
Normal Faults
• Along a normal fault, the hanging wall moves down
relative to the footwall.
• Normal faults usually form where tectonic plate
motions cause tension. ->
• Tension is stress that pulls rocks apart.
• Therefore, normal faults are common at mid-ocean
ridges.
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Chapter 6
Section 3 Deforming Earth's Crust
Faulting, continued
Reverse Faults
• Along a reverse fault, the hanging wall moves up
relative to the footwall.
• Reverse faults usually form where tectonic plate
motions cause compression. -> <• Compression is stress that causes rocks to push
together.
• Therefore, reverse faults are common in subduction
zones.
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Chapter 6
Section 3 Deforming Earth's Crust
Faulting, continued
Strike-Slip Faults
• Along a strike-slip fault, the two fault blocks move
past eachother horizontally.
• Strike-slip faults usually form where tectonic plate
motions cause shear stress parallel to Earth’s
surface.
• Therefore, strike-slip faults are common along
transform boundaries.
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Chapter 6
Section 3 Deforming Earth's Crust
Faulting, continued
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Chapter 6
Section 3 Deforming Earth's Crust
Faulting, continued
Recognizing Faults
• The position of rock layers can help scientists to
recognize a fault and determine offset.
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Chapter 6
Section 3 Deforming Earth's Crust
Faulting, continued
• Features such as grooves, striations, or polished
surfaces called slickensides also show where rocks
have moved.
• Fault offset is often obvious for faults that have
lengths of many kilometers.
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Chapter 6
Section 3 Deforming Earth's Crust
Faulting, continued
• Streams commonly change their direction of flow at a
fault.
• A scarp, or row of cliffs formed by faulting, can also
identify a fault.
• Scarps form when rock on one side is raised
vertically relative to rock on the other side of the fault.
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Chapter 6
Section 3 Deforming Earth's Crust
Plate Tectonics and Mountain Building
• As tectonic plates move around Earth’s surface, their
edges grind against each other.
• Over time, this process may crumple and push up the
margins of the plates.
• When this happens, mountain-building may occur.
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Chapter 6
Section 3 Deforming Earth's Crust
Plate Tectonics and Mountain Building,
continued
• When rock layers are squeezed together and pushed
upward, folded mountains form.
• These mountain ranges form at convergent
boundaries where continents have collided.
• When continents collide, compression folds and
uplifts the rock.
• Examples include the Appalachians and the
Himalayas
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Chapter 6
Section 3 Deforming Earth's Crust
Plate Tectonics and Mountain Building,
continued
• When tension in Earth’s crust causes the crust to
break into a large number of normal faults, fault-block
mountains form.
• These mountains form when tension causes large
blocks of Earth’s crust to drop down relative to other
blocks.
• The Tetons in Idaho and Wyoming are an example of
Fault-Block Mountains
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Chapter 6
Section 3 Deforming Earth's Crust
Plate Tectonics and Mountain Building,
continued
• When molten rock erupts onto Earth’s surface,
volcanic mountains form.
• Most of the world’s major volcanic mountains are
located at convergent boundaries.
• At these boundaries, the motion of the plates causes
hot mantle rocks to rise beneath the plate.
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Chapter 6
Section 3 Deforming Earth's Crust
Plate Tectonics and Mountain Building,
continued
• The molten rock rises to the surface and erupts.
• Volcanic mountains form both on land and on the
ocean floor.
• Sometimes, these mountains can rise above the
ocean surface to become islands (like Hawaii).
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Chapter 6
Section 3 Deforming Earth's Crust
Plate Tectonics and Mountain Building,
continued
• Most of the active volcanic mountains on Earth have
formed around the tectonically active rim of the
Pacific Ocean, this area is known as the Ring of Fire.
• Subduction zones are what allow volcanoes to form
at these convergent boundaries.
• Mount Shasta in northern California is in the Ring of
Fire.
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Chapter 6
Plate Tectonics
Types of Mountains
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Chapter 6
Section 4 California Geology
Bellringer
How many geologic features from California can you
name? Brainstorm a list of specific features of
California geology, such as faults and mountains.
Write your list in your science journal.
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Chapter 6
Section 4 California Geology
What You Will Learn
• Plate tectonics has been the most important force in
shaping California’s geologic history.
• The San Andreas fault marks a transform boundary
between the North American plate and the Pacific
plate.
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Chapter 6
Section 4 California Geology
Building California by Plate Tectonics
• The region that we know as California has been at an
active plate boundary for the past 225 million years.
• As a result, plate tectonics has been the most
important force shaping California’s geologic history.
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Chapter 6
Section 4 California Geology
Building California by Plate Tectonics,
continued
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Chapter 6
Section 4 California Geology
Building California by Plate Tectonics,
continued
• Before about 225 million years ago, North America’s
western edge was much farther east than it is now.
• The area where Nevada and the eastern deserts of
California are today was the west coast of North
America.
• Most of what is now California was either part of a
distant oceanic plate or did not exist.
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Chapter 6
Section 4 California Geology
Building California by Plate Tectonics,
continued
• When Pangaea began to break up, the North
American plate moved west.
• The continent’s western edge became an active
convergent plate boundary.
• A long period of subduction began, which was an
important period of geologic “building” in California.
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Chapter 6
Section 4 California Geology
Building California by Plate Tectonics,
continued
• Three major tectonic plates influenced California’s
geologic history: the North American plate, the
Farallon plate, and the Pacific plate.
• A convergent boundary existed between the North
American and Farallon plates.
• The Farallon plate subducted beneath the North
American plate.
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Chapter 6
Section 4 California Geology
Building California by Plate Tectonics,
continued
• The ancient Farallon plate lay between the North
American and Pacific plates.
• About 25 million years ago, the entire Farallon plate
was subducted at one part of the boundary.
• The Pacific Plate touched North America for the first
time, forming a transform boundary.
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Chapter 6
Section 4 California Geology
Building California by Plate Tectonics,
continued
• As the Farallon plate continues to subduct, the
transform boundary continues to grow longer.
• Today, it is about 2,600 km long.
• The Juan de Fuca plate off northern California is part
of what remains of the ancient Farallon plate.
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Chapter 6
Section 4 California Geology
Building California by Plate Tectonics,
continued
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Chapter 6
Section 4 California Geology
Subduction and Volcanism
• The subduction of the Farallon plate caused rocks to
melt and caused chunks of rock to collide with the
North American continent.
• Subduction caused a great deal of magma, or molten
rock, to form in the lithosphere.
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Chapter 6
Section 4 California Geology
Subduction and Volcanism, continued
• This magma solidified to form a huge mass of granite
called the Sierra Nevada batholith.
• A batholith is a large mass of igneous rock that
forms deep below the surface.
• Batholiths are the “roots” of subduction zone
volcanoes.
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Chapter 6
Section 4 California Geology
Subduction and Volcanism, continued
• A chain of huge volcanoes must have formed above
the giant magma chamber.
• These volcanoes probably stood twice as tall as
today’s Sierra Nevadas.
• The granite batholith is exposed in the Sierra Nevada
mountain range.
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Chapter 6
Section 4 California Geology
Subduction and Volcanism, continued
• Today, the Juan de Fuca plate is subducting beneath
the North American plate.
• This area is known as the Cascadia subduction zone.
• A chain of active volcanoes is present in this zone, in
the Cascade Mountains of California, Oregon, and
Washington.
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Chapter 6
Section 4 California Geology
Subduction and Accretion
• During subduction, pieces of the plate that subducts
may be scraped off and attached to the overriding
plate.
• This process, called accretion, forms mountain
chains.
• These mountain chains are parallel to the plate
boundary.
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Chapter 6
Section 4 California Geology
Subduction and Accretion, continued
• The rocks in California’s Coast Ranges and
Transverse Ranges are thought to have been formed
by accretion.
• The Central Valley, Los Angeles Basin, and Ventura
Basin separate some of these mountain ranges.
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Chapter 6
Section 4 California Geology
Subduction and Accretion, continued
Accreted Terranes
• The chunks of lithosphere that are scraped off of
subduction plates and added to the edge of a
continent are called accreted terranes.
• The rocks of a terrane differ from the surrounding
rocks by age or composition.
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Chapter 6
Section 4 California Geology
Subduction and Accretion, continued
California Gold
• The foothills along the western side of the Sierra
Nevadas contain rocks filled with gold.
• These rocks are thought to be accreted terranes.
• This rock formed near submarine volcanic vents.
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Chapter 6
Section 4 California Geology
Subduction and Accretion, continued
• After the terranes were accreted, the gold became
concentrated in the quartz veins of the Mother Lode.
• Gold is an important part of California’s history.
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Chapter 6
Section 4 California Geology
The San Andreas Fault System
• California is home to the most famous transform plate
boundary in the world, the San Andreas fault system.
• The San Andreas fault system extends for about
1,000 km.
• The San Andreas fault forms the boundary between
the Pacific and North American plates.
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Chapter 6
Section 4 California Geology
The San Andreas Fault System, continued
• Most of California is on the North American plate.
• A small part of California, west of the San Andreas
fault, lies on the Pacific plate.
• The Pacific plate is moving to the northwest relative
to the North American plate.
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Chapter 6
Section 4 California Geology
The San Andreas Fault System, continued
• Not all plate movement takes place on the San
Andreas fault itself.
• In the San Francisco area and southern California,
motion takes place on other faults of the system.
• These faults lie west and east of the San Andreas
fault.
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Chapter 6
Section 4 California Geology
The San Andreas Fault System, continued
• In these areas, it is best to think of the boundary
between the North American and Pacific plates as a
zone, not a line.
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Chapter 6
Section 4 California Geology
The San Andreas Fault System, continued
• The Pacific and North American plates have been
moving along the San Andreas fault system for about
25 million years.
• During the last 16 million years, the separation, or
offset, along the fault has been 315 km.
• Geologists use rocks to estimate the amount and rate
of movement along the fault.
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Chapter 6
Section 4 California Geology
The San Andreas Fault System, continued
• Geologists determine offset by matching unusual
rocks that have been separated by the fault.
• They date these rocks to determine when they
formed.
• Then geologists use that date to determine when the
areas were not separated.
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Chapter 6
Section 4 California Geology
The San Andreas Fault System, continued
• In southern California, the San Andreas fault makes a
huge bend as it passes east of Los Angeles.
• Because of this bend, the Pacific and North American
plates collide as they move past each other.
• As a result, the motion along this boundary is partly
convergent.
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Chapter 6
Section 4 California Geology
The San Andreas Fault System, continued
• Because southern California is being compressed,
areas near the bend are being uplifted or dropped
down by active faults.
• The San Bernardino Mountains and the San Gabriel
Mountains are tectonically created mountain ranges.
• The Los Angeles basin is a large depression
bordered by active faults.
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Chapter 6
Section 4 California Geology
Plate Tectonics and the California Landscape
• Much of California’s landscape has been formed by
plate tectonics.
• Compression has recently uplifted California’s rugged
mountains.
• The steep, rocky coastlines have been formed by
uplift along the plate boundary.
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Chapter 6
Section 4 California Geology
Plate Tectonics and the California Landscape,
continued
• Major river valleys, mountain ranges, and the
coastline are oriented in a northwesterly direction.
• A northwesterly orientation is parallel to the faults of
the plate boundary.
• The Transverse Ranges are oriented east-west, due
to motion from the San Andreas fault system.
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Chapter 6
Section 4 California Geology
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Chapter 6
Plate Tectonics
Concept Map
Use the terms below to complete the concept map
on the next slide.
folds
synclines
strike-slip
faults
reverse
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Chapter 6
Plate Tectonics
Concept Map
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Chapter 6
Plate Tectonics
Concept Map
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