Chapter Introduction
Lesson 1
The Continental
Drift Hypothesis
Lesson 2
Development
of a Theory
Lesson 3
The Theory of
Plate Tectonics
Chapter Wrap-Up
Arctic_Images/Getty Images
What is the theory
of plate tectonics?
What do you think?
Before you begin, decide if you agree or
disagree with each of these statements.
As you view this presentation, see if you
change your mind about any of the
statements.
Do you agree or disagree?
1. India has always been north of the
equator.
2. All the continents once formed one
supercontinent.
3. The seafloor is flat.
Do you agree or disagree?
4. Volcanic activity occurs only on the
seafloor.
5. Continents drift across a molten mantle.
6. Mountain ranges can form when
continents collide.
The Continental Drift Hypothesis
• What evidence supports continental
drift?
• Why did scientists question the
continental drift hypothesis?
The Continental Drift Hypothesis
• Pangaea
• continental drift
Pangaea
• Nearly 100 years ago, Alfred Wegener
proposed that all the continents were
once part of a supercontinent called
Pangaea.
• Over time, Pangaea began breaking
apart and the continents slowly moved
to their present position.
Pangaea (cont.)
• Wegener proposed the hypothesis of
continental drift, which suggested
that continents are in constant motion
on the surface of Earth.
• Wegener observed the similarities of
continental coastlines now separated
by oceans and how they could fit
together like pieces of a puzzle.
The eastern coast of
South America
mirrors the shape of
the west coast of
Africa.
Evidence That Continents Move
• Evidence to support Wegener’s
hypothesis is found in
• climate clues;
• fossil clues;
• rock clues.
• When Wegener pieced Pangaea
together, he proposed that the
continents were located closer to the
South Pole 250 million years ago.
Evidence That Continents Move (cont.)
• Wegener suggested that a large sheet
of ice covered the continents.
• Wegener studied the sediments left
behind and the glacial grooves that
formed when the ice sheets melted and
Pangaea spread apart.
• This provided climate evidence for
continental drift.
The presence of an ice sheet covering
Pangea could explain glacial features
found on some continents today.
Evidence That Continents Move (cont.)
• Animals and plants that live on
separate continents can be unique to
that continent alone.
• Fossils of similar organisms have been
found on several continents separated
by oceans.
• Fossils of a plant called Glossopteris
have been found on continents that are
now separated by oceans.
• The orange
area shows
where fossils of
Glossopteris
have been
found.
• Fossils provide
evidence for
continental drift.
Evidence That Continents Move (cont.)
• Wegener observed that mountain ranges
and rock formations on different
continents had common origins,
providing rock evidence for continental
drift.
• Volcanic rock that is identical in
chemistry and age has been found on
both the western coast of Africa and the
eastern coast of South America.
The Caledonia mountain range in northern
Europe and the Appalachian Mountains in
eastern North America are similar in age,
structure, and rock type.
Evidence That Continents Move (cont.)
How were similar rock types
used to support the
continental drift hypothesis?
What was missing?
• Wegener’s ideas were not widely
accepted until nearly four decades later.
• Scientists questioned continental drift
because it was a slow process and
Wegener could not measure how fast
continents moved or how they moved.
• Scientists could not understand how
continents could push their way through
the solid rock of the mantle and the
seafloor.
What was missing? (cont.)
mantle
Science Use the middle layer of
Earth, situated between the crust
above and the core below
Common Use a loose,
sleeveless garment worn over
other clothes
What was missing? (cont.)
Why did scientists argue
against Wegener’s
continental drift hypothesis?
• All continents were once part of a
supercontinent called Pangaea.
• Alfred Wegener proposed that
continents move around on Earth’s
surface.
What term did Wegener use to
describe the constant motion of
continents on the surface of Earth?
A. Pangaea
B. continental drag
C. continental movement
D. continental drift
In which of these did Wegener
observe similarities that suggested
continents might fit together like the
pieces of a puzzle?
A. fossils
B. ice sheets
C. plates
D. continental coastlines
Wegener proposed that 250 million
years ago, South America, Africa,
India, and Australia were located
closer to what?
A. equator
B. South Pole
C. North Pole
D. Pacific ocean
Do you agree or disagree?
1. India has always been north of the
equator.
2. All the continents once formed one
supercontinent.
Development of a Theory
• What is seafloor spreading?
• What evidence is used to support
seafloor spreading?
Development of a Theory
• mid-ocean ridge
• seafloor spreading
• normal polarity
• magnetic reversal
• reversed polarity
Mapping the Ocean Floor
• During the late 1940s scientists were
able to determine the depth of the ocean
using a device called an echo sounder.
• Once ocean depths were determined,
scientists used these data to create a
topographic map of the sea floor that
revealed vast mountain ranges, called
mid-ocean ridges, that stretch for many
miles deep below the ocean’s surface.
Seafloor Topography
Seafloor Spreading
• By the 1960s, scientists discovered the
process of seafloor spreading.
• Seafloor spreading is the process by
which new oceanic crust forms along a
mid-ocean ridge and older oceanic
crust moves away from the ridge.
• When the seafloor spreads, the mantle
below melts and forms magma.
Seafloor Spreading (cont.)
• Magma erupts on Earth’s surface as
lava, which cools and crystallizes on
the seafloor, forming rock.
• Because the lava erupts into water, it
cools rapidly and forms rounded
structures called pillow lavas.
• As the seafloor continues to spread
apart, the older oceanic crust moves
away from the mid-ocean ridge.
Seafloor Spreading (cont.)
Seafloor Spreading (cont.)
Scientists argued that if the seafloor
spreads, the continents must also be
moving.
What is seafloor spreading?
Seafloor Spreading (cont.)
• The rugged mountains that make up
the mid-ocean ridge system can form
in two different ways.
• Large amounts of lava can erupt from
the center of the ridge, cool, and build
up around the ridge.
Seafloor Spreading (cont.)
• Or, as the lava cools and forms new
crust, it cracks and the rocks move up or
down along these cracks in the seafloor,
forming jagged mountain ranges.
• The abyssal plain, the smooth part of the
seafloor, is made when the layer of
sediment that accumulates far from the
mid-ocean ridge becomes thick enough.
Continents move as the seafloor spreads
along a mid-ocean ridge.
Development of a Theory
• The first evidence used to support
seafloor spreading was discovered in
rocks on the seafloor.
• Scientists studied the magnetic signature
of minerals in these rocks.
• Earth’s magnetic field today is described
as having normal polarity—a state in
which magnetized objects, such as
compass needles, will orient themselves
to point north.
Development of a Theory (cont.)
• Sometimes a magnetic reversal
occurs and the magnetic field reverses
direction.
• The opposite of normal polarity is
reversed polarity: a state in which
magnetized objects reverse direction
and orient themselves to point south.
Development of a Theory (cont.)
• Volcanic rock on the seafloor contains
iron-rich minerals that are magnetic.
• Magnetic minerals in cooling lava from
the mid-ocean ridge record the direction
of Earth’s magnetic field.
• Scientists have discovered parallel
patterns in the magnetic signature of
rocks on either side of a mid-ocean
ridge.
Minerals in fresh lava record Earth’s
magnetic signature.
Development of a Theory (cont.)
• Scientists studied magnetic minerals in
rocks from the seafloor using a
magnetometer to measure and record
the magnetic signature.
• They discovered parallel magnetic
stripes on either side of the mid-ocean
ridge.
Development of a Theory (cont.)
• Each pair of stripes has a similar
composition, age, and magnetic
character.
• The pairs of magnetic stripes confirm
that the ocean crust formed at midocean ridges is carried away from the
center of the ridges in opposite
directions.
Seafloor Spreading Theory
Development of a Theory (cont.)
• Other measurements made on the
seafloor confirm seafloor spreading.
• Measuring the amount of thermal energy
leaving the Earth shows that more
thermal energy leaves Earth near midocean ridges than is released from
beneath the abyssal plains.
Development of a Theory (cont.)
Sediment collected from the seafloor can
be dated to show that the sediment closest
to the mid-ocean ridge is younger than the
sediment farther away from the ridge.
• New ocean crust forms along midocean ridges.
• Mid-ocean ridges are large mountain
ranges that
extend throughout
Earth’s oceans.
• A magnetic reversal occurs when
Earth’s magnetic field changes
direction.
As the seafloor continues to spread
apart, the older oceanic crust
moves in which direction with
respect to the mid-ocean ridge?
A. toward it
B. above it
C. away from it
D. under it
Today’s magnetic field is
described as having which of
these?
A. reversed polarity
B. normal polarity
C. magnetic reversal
D. no polarity
In which state do magnetized
objects reverse themselves to
point south?
A. reversed polarity
B. normal polarity
C. seafloor spreading
D. magnetic polarity
Do you agree or disagree?
3. The seafloor is flat.
4. Volcanic activity occurs only on the
seafloor.
The Theory of Plate Tectonics
• What is the theory of plate tectonics?
• What are the three types of plate
boundaries?
• Why do tectonic plates move?
The Theory of Plate Tectonics
• plate tectonics
• lithosphere
• divergent plate boundary
• transform plate boundary
• convergent plate boundary
The Theory of Plate Tectonics
• subduction
• convection
• ridge push
• slab pull
The Plate Tectonics Theory
• The theory of plate tectonics,
proposed in the late 1960s, states that
Earth’s surface is made of rigid slabs of
rock, or plates, that move with respect
to each other.
• Plate tectonics suggests that Earth’s
surface is divided into large plates of
rigid rock and each plate moves over
Earth’s hot and semiplastic mantle.
The Plate Tectonics Theory (cont.)
What is plate tectonics?
The Plate Tectonics Theory (cont.)
• Geologists use the word tectonic to
describe the forces that shape Earth’s
surface and the rock structures that
form as a result.
• The cold and rigid outermost rock layer
of the Earth is called the lithosphere.
• The lithosphere is made up of the crust
and the solid, uppermost mantle.
Earth’s Tectonic Plates
The Plate Tectonics Theory (cont.)
• The lithosphere is thin below mid-ocean
ridges and thick below continents.
• Earth’s tectonic plates are large pieces
of the lithosphere that fit together like the
pieces of a giant jigsaw puzzle.
• The layer of Earth below the lithosphere,
called the asthenosphere, is so hot that it
behaves like a plastic material and
enables Earth’s plates to move.
The Plate Tectonics Theory (cont.)
plastic
Science Use capable of being
molded or changing shape
without breaking
Common Use any of numerous
organic, synthetic, or processed
materials made into objects
Plate Boundaries
• A divergent plate boundary forms
where two plates separate.
• When the seafloor spreads at a midocean ridge, lava erupts, cools, and
forms new oceanic crust.
• Divergent plate boundaries can also
exist in the middle of a continent,
where they pull continents apart and
form rift valleys.
Plate Boundaries (cont.)
• A transform plate boundary forms
where two plates slide past each other.
• As they move past one another, the
plates can get stuck and stop moving.
• Stress builds up where the plates are
stuck until they eventually break and
suddenly move apart, resulting in a
rapid release of energy as
earthquakes.
Plate Boundaries (cont.)
• Convergent plate boundaries form
where two plates collide.
• The denser plate sinks below the more
buoyant plate in a process called
subduction.
• The area where a denser plate
descends into Earth along a
convergent plate boundary is called a
subduction zone.
Plate Boundaries (cont.)
subduction
from Latin subductus, means “to
lead under, removal”
Plate Boundaries (cont.)
• When an oceanic plate and a continental
plate collide, the denser oceanic plate
subducts under the edge of the
continent, creating a deep ocean trench
and a line of volcanoes above the
subducting plate on the edge of the
continent.
• When two continental plates collide,
neither plate is subducted, and
mountains form from uplifted rock.
Plate Boundaries (cont.)
What are the three types of
plate boundaries?
Evidence for Plate Tectonics
• Scientists can measure how fast
continents move using a network of
satellites called the Global Positioning
System.
• Because plates are rigid, tectonic
activity occurs where plates meet.
Evidence for Plate Tectonics (cont.)
• Volcanoes form where plates separate
along a mid-ocean ridge or continental
rift or collide along a subduction zone.
• Mountains can form where two
continents collide.
Notice the relationship between
earthquake epicenters, volcanoes, and
plate boundaries.
Evidence for Plate Tectonics (cont.)
How are earthquakes and
volcanoes related to the
theory of plate tectonics?
Plate Motion
• Earth’s plates move because the
asthenosphere moves underneath the
lithosphere.
• Convection is the circulation of material
caused by differences in density.
• Hot mantle material rises upward and
comes in contact with Earth’s crust.
Plate Motion (cont.)
• As the mantle cools, it becomes denser
and then sinks, forming a convection
current.
• Convection currents in the
asthenosphere act like a conveyor belt
moving the lithosphere above it.
• There are three forces that interact to
cause plate motion: basal drag, ridge
push, and slab pull.
Plate Motion (cont.)
What causes convection?
Plate Motion (cont.)
• Basal drag refers to how convection
currents in the asthenosphere circulate
and drag the lithosphere like a
conveyor belt.
• Rising mantle material at mid-ocean
ridges creates the potential for plates
to move away from the ridge with a
force called ridge push.
As a slab, or dense plate, sinks, it pulls
on the rest of the plate with a force called
slab pull.
A Theory in Progress
• Plate tectonics has become the
unifying theory of geology, but several
unanswered questions remain.
• Why is Earth the only planet in the
solar system that has plate tectonic
activity?
• Why do some earthquakes and
volcanoes occur far away from plate
boundaries?
• What forces dominate plate motion?
• What will scientists study next? This is
a 3-D image
of seismic
wave
velocities
from
a new
technique
called
anisotropy.
• Tectonic plates are
made of cold and
rigid slabs of rock.
• Mantle convection—
the circulation of
mantle material due to density
differences—drives plate motion.
• The three types of plate boundaries
are divergent, convergent, and
transform boundaries.
What word do geologists use to
describe the forces that shape
Earth’s surface?
A. semiplastic
B. lithosphere
C. tectonic
D. mantle
Which of these is made up of the
crust and the solid, uppermost
mantle?
A. seafloor
B. lithosphere
C. biosphere
D. hydrosphere
What type of boundary forms
where two plates separate?
A. convergent plate boundary
B. transform plate boundary
C. new plate boundary
D. divergent plate boundary
Do you agree or disagree?
5. Continents drift across a molten
mantle.
6. Mountain ranges can form when
continents collide.
Key Concept Summary
Interactive Concept Map
Chapter Review
Standardized Test Practice
The theory of plate
tectonics states that
Earth’s lithosphere is
broken up into rigid
plates that move over
Earth’s surface.
Lesson 1: The Continental Drift
Hypothesis
• The puzzle piece fit of
continents, fossil evidence,
climate, rocks, and mountain
ranges supports the
hypothesis of continental
drift.
• Scientists were skeptical of
continental drift because
Wegener could not explain
the mechanism for
movement.
Lesson 2: Development of a Theory
• Seafloor spreading provides a mechanism for
continental drift.
• Seafloor spreading occurs at mid-ocean ridges.
• Evidence of magnetic reversal in rock, thermal
energy trends, and
the discovery of
seafloor spreading
all contributed to
the development
of the theory of
plate tectonics.
Lesson 3: The Theory of Plate
Tectonics
• Types of plate boundaries, the location of
earthquakes, volcanoes, and mountain ranges, and
satellite measurement of plate motion support the
theory of plate tectonics.
• Mantle convection, ridge
push, and slab pull are
the forces that cause
plate motion. Radioactivity
in the mantle and thermal
energy from the core
produce the energy for
convection.
Which of these have been found on
several continents separated by
oceans, suggesting that they were
once geographically closer together?
A. animals
B. ice sheets
C. plant fossils
D. plate boundaries
Which of these refers to the process
by which new oceanic crust forms
along a mid-ocean ridge as older
oceanic crust moves away from the
ridge?
A. continental drift
B. plate tectonics
C. seafloor spreading
D. magnetic reversal
What is the rigid outermost layer
of Earth called?
A. mantle
B. lithosphere
C. plate
D. transform plate boundary
What forms where two plates
slide past each other?
A. transform plate boundary
B. divergent plate boundary
C. convergent plate boundary
D. semiplastic mantle
What is the process in which a
denser plate sinks below a more
buoyant plate?
A. seafloor spreading
B. convergence
C. convection
D. subduction
What is the name of the
supercontinent that all continents
were once part of?
A. Wegener
B. Pangaea
C. mantle
D. seafloor
Where did scientists find glacial
features that are usually
associated with cooler climates?
A. Antarctica
B. plate boundaries
C. mountain ranges
D. Africa
Which term refers to vast
mountain ranges on the seafloor?
A. volcanoes
B. oceanic crust
C. pillow lava
D. mid-ocean ridges
Which of these states that Earth’s
surface is made of rigid slabs of rock
that move with respect to one another?
A. the theory of plate tectonics
B. the theory of mid-ocean ridges
C. the theory of divergent plate
boundaries
D. the theory of transform plate
boundaries
What is the name for the force
that pulls on the rest of a plate as
part of a dense plate sinks?
A. ridge push
B. basal drag
C. slab pull
D. plate tectonics