Seafloor Spreading

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Drifting Continents
Objectives
• Describe one piece of early evidence that led
people to suggest that Earth’s continents may
have once been joined.
• Discuss evidence of continental drift.
• Explain why continental drift was not accepted
when it was first proposed.
Vocabulary
– continental drift
– Pangaea
Drifting Continents
Drifting Continents
• Earth’s surface appears to remain relatively
unchanged during the course of an average
human lifetime.
• On the geologic time scale, Earth’s surface is
changing at rates almost too great to imagine.
Drifting Continents
Early Observations
• In the late 1500s, Abraham Ortelius, a Dutch
mapmaker, noticed the apparent fit of continents
on either side of the Atlantic Ocean.
• In the late 1800s, Eduard Suess, an Austrian
geologist, hypothesized that the present southern
continents had once been joined as a single
landmass that he named Gondwanaland.
• The first time that the idea of moving continents
was proposed as a serious scientific hypothesis
was in 1912 by a German scientist named
Alfred Wegener.
Drifting Continents
Continental Drift
Wegener’s hypothesis, continental drift,
proposed that Earth’s continents had once been
joined as a single landmass.
• Wegener proposed that Pangaea began to break
apart about 200 million years ago and that the
continents had continued to slowly move to their
present positions.
Pangaea, a Greek word that means “all the
earth,” refers to the combined landmass.
Drifting Continents
Continental Drift
Drifting Continents
Continental Drift
Evidence from Rock Formations
– Wegener reasoned that large geologic structures,
such as mountain ranges, would have fractured as
the continents separated.
– Using this reasoning, Wegener hypothesized that
there should be areas of similar rock types on
opposite sides of the Atlantic Ocean.
– That similar groups of rocks were observed in the
United States, Greenland, and Europe supported
Wegener’s idea.
Drifting Continents
Continental Drift
Evidence from Rock Formations
Drifting Continents
Continental Drift
Evidence from Fossils
– Similar fossils of several different animals and
plants that once lived on land had been found on
widely separated continents.
– The ages of different fossils predated Wegener’s
time frame for the breakup of Pangaea.
– Fossils of Glossopteris, a seed fern that resembled
low shrubs, have been found on many continents,
indicating that the areas had a single climate that
was close to the equator.
Drifting Continents
Continental Drift
Evidence from Fossils
Drifting Continents
Continental Drift
Ancient Climatic Evidence
– Various sedimentary rocks offer evidence of vast
climatic changes on some continents.
– Coal deposits in Antarctica suggested that it must have
been closer to the equator.
– Glacial deposits found in Africa, India, Australia, and
South America suggested that these areas had once
been covered by thick ice caps.
– Wegener proposed that they once were located near
the south pole before Pangaea began to fracture.
Drifting Continents
Continental Drift
Ancient Climatic Evidence
Drifting Continents
A Rejected Hypothesis
• In the early 1900s, most scientists rejected
Wegener’s hypothesis of continental drift.
• Two unanswered questions—what forces could
move continents and how continents could move
without shattering—were the main reasons that
the hypothesis of continental drift was rejected.
Drifting Continents
Section Assessment
1. How did Pangaea differ from the present layout
of the continents?
During Pangaea, the continents were joined
and essentially located in one hemisphere
from the north to the south poles. Presently,
the continents have separated and are
distributed in both the eastern and western
hemisphere, with the majority of landmass in
the northern hemisphere.
Drifting Continents
Section Assessment
2. According to Wegener, what force caused the
continents to move?
Wegener suggested that the rotation of Earth
could be responsible for continental movement.
Drifting Continents
Section Assessment
3. Identify whether the following statements are
true or false.
______
true Similar rock types are located in South America
and Africa.
______
true Gondwanaland and Pangaea essentially refer to
the same thing.
______
false Coal located in India and Africa suggest that they
were once located in warmer climates.
______
true Geologic evidence suggests that North America,
Greenland, and Europe were once connected.
Seafloor Spreading
Objectives
• Summarize the evidence that led to the discovery of
seafloor spreading.
• Explain the significance of magnetic patterns on
the seafloor.
• Explain the process of seafloor spreading.
Vocabulary
– magnetometer
– isochron
– paleomagnetism
– seafloor spreading
– magnetic reversal
Seafloor Spreading
Seafloor Spreading
• Until the mid-1900s, most people, including
many scientists, thought that the ocean floor,
unlike the continents, was essentially flat.
• Many people also had the misconceptions that
oceanic crust was unchanging and was much
older than continental crust.
• Advances in technology during the 1940s and
1950s, however, proved all of these widely
accepted ideas to be wrong.
Seafloor Spreading
Help from Technology
• The development of echo-sounding methods
allowed scientists to study the ocean floor in
great detail.
• Sonar uses sound
waves to measure
water depth by
measuring the time it
takes for sound waves
to travel from the
device and back to
a receiver.
Seafloor Spreading
Help from Technology
A magnetometer is a device that can detect small
changes in magnetic fields, allowing scientists to
construct magnetic maps of the seafloor.
Seafloor Spreading
Ocean Floor Topography
• The maps made from the data collected by sonar
and magnetometers showed underwater mountain
chains called ocean ridges.
• The same data showed that these underwater
mountain chains have counterparts called
deep-sea trenches.
• These two topographic features of the ocean
floor puzzled geologists for over a decade after
their discovery.
Seafloor Spreading
Ocean Rocks and Sediments
• Analysis of deep-sea rocks and sediments
produced two important discoveries.
1. The ages of the rocks that make up the seafloor vary
in different places, and that the age of oceanic crust
consistently increases with distance from a ridge.
– The oldest part of the seafloor is geologically young
at about 180 million years old.
2. The thickness of ocean-floor sediment was, in general,
much less than expected and that the thickness of
the sediments increases with distance from an
ocean ridge.
Seafloor Spreading
Ocean Rocks and Sediments
Seafloor Spreading
Magnetism
• Rocks containing iron-bearing minerals provide
a record of Earth’s magnetic field.
Paleomagnetism is the study of Earth’s
magnetic record.
• Basalt, because it is rich in iron-bearing
minerals, provides an accurate record of
ancient magnetism.
Seafloor Spreading
Magnetism
The Geomagnetic Time Scale
– Studies of continental basalt flows
in the early 1960s revealed a pattern
of magnetic reversals over
geologic time.
– A magnetic reversal is a change in
Earth’s magnetic field.
• A magnetic field that is the same as
the present has normal polarity.
• A magnetic field that is opposite to
the present has reversed polarity.
Seafloor Spreading
Magnetism
The Geomagnetic Time Scale
– Towing magnetometers behind ships to measure the
magnetic field of the ocean floor revealed an
interesting pattern.
• In places where the magnetic readings of the ocean
floor matched Earth’s present field, a stronger-thannormal reading (+) was recorded.
• In places where the magnetic data were reversed in
relation to Earth’s present magnetic field, a lowerthan-normal reading (–) was recorded.
Seafloor Spreading
Magnetism
The Geomagnetic Time Scale
Seafloor Spreading
Magnetism
Magnetic Symmetry
– The positive and
negative areas of the
seafloor form a series
of stripes that were
parallel to ocean
ridges.
– The magnetic pattern
on one side of the
ridge is a mirror
image of the pattern
on the other side of
the ridge.
Seafloor Spreading
Magnetism
Magnetic Symmetry
– The magnetic data collected from the ocean floor
matched the pattern of magnetic reversals that had
been found in basalt flows on land.
– From this match, scientists were able to determine the
age of the ocean floor from a magnetic recording and
quickly create isochron maps of the ocean floor.
– An isochron is a line on a map that connects points
that have the same age.
Seafloor Spreading
Magnetism
Seafloor Spreading
Seafloor Spreading
• An American scientist named Harry Hess
proposed the theory of seafloor spreading.
Seafloor spreading states that new ocean crust
is formed at ocean ridges and destroyed at deepsea trenches.
– Magma is forced toward the
crust along an ocean ridge and
fills the gap that is created.
Seafloor Spreading
Seafloor Spreading
– When the magma hardens, a
small amount of new ocean floor
is added to Earth’s surface.
– Each cycle of spreading and
the intrusion of magma
results in the formation of
another small section of
ocean floor, which slowly
moves away from the ridge.
Seafloor Spreading
Seafloor Spreading
The Missing Link
– Seafloor spreading was the missing link needed by
Wegener to complete his model of continental drift.
– Continents are not pushing through ocean crust, as
Wegener proposed; they ride with ocean crust as it
slowly moves away from ocean ridges.
Seafloor Spreading
Section Assessment
1. Match the following terms with their definitions.
___
A magnetometer
___
C paleomagnetism
___
B isochron
___
D seafloor spreading
A. a device that can detect small
changes in magnetic fields
B. a line on a map that connects
points that have the same age
C. the study of Earth’s magnetic
record
D. a theory that states that new
ocean crust is formed at ocean
ridges and destroyed at deepsea trenches
Seafloor Spreading
Section Assessment
2. How does the distribution of ocean-floor
sediments support the theory of seafloor
spreading?
The thickness of ocean-floor sediments increases
with distance from an ocean ridge which indicates
that the seafloor is older with distance.
Seafloor Spreading
Section Assessment
3. Identify whether the following statements are
true or false.
______
true Earth’s magnetic field has reversed more than
20 times over the past five million years.
______
false The oldest part of the seafloor is over 500 million
years old.
______
false Alfred Wegener first proposed the theory of
seafloor spreading.
______
false Shale provides an accurate record of ancient
magnetism.
Theory of Plate Tectonics
Objectives
• Explain the theory of plate tectonics.
• Compare and contrast the three types of plate
boundaries and the features associated with each.
Vocabulary
– theory of plate tectonics
– subduction
– divergent boundary
– transform boundary
– rift valley
– convergent boundary
Theory of Plate Tectonics
Theory of Plate Tectonics
The theory of plate tectonics states that
Earth’s crust and rigid upper mantle are broken
into enormous moving slabs called plates.
• There are a dozen or so major plates and
several smaller ones.
• Tectonic plates move in different directions and
at different rates over Earth’s surface.
Theory of Plate Tectonics
Theory of Plate Tectonics
Theory of Plate Tectonics
Plate Boundaries
• Tectonic plates interact at places called plate
boundaries.
• At some plate boundaries:
– Plates come together, or converge
– Plates move away from one another, or diverge
– Plates move horizontally past one another
Theory of Plate Tectonics
Plate Boundaries
Divergent Boundaries
– Divergent boundaries are places where two tectonic
plates are moving apart.
– Most divergent boundaries are found in rifts, or faultbounded valleys, which form along the axis of an
ocean ridge.
– A rift valley, which is a
narrow depression, is
created when a
divergent boundary
forms on a continent.
Theory of Plate Tectonics
Plate Boundaries
Convergent Boundaries
– Convergent boundaries are places where two
tectonic plates are moving toward each other.
– There are three types of convergent boundaries:
1. Oceanic crust converging with oceanic crust
2. Oceanic crust converging with continental crust
3. Continental crust converging and colliding with
continental crust.
Theory of Plate Tectonics
Plate Boundaries
Convergent Boundaries
Theory of Plate Tectonics
Plate Boundaries
Convergent Boundaries
– Subduction occurs when one of the two converging
plates descends beneath the other.
– A subduction zone forms when one oceanic plate, which
has become denser as a result of cooling, descends
below another plate creating a deep-sea trench.
– The subducted plate descends into the mantle
and melts.
– Some of the magma forms new oceanic crust at the
ridge or is forced back to the surface, forming an arc of
volcanic islands that parallel the trench.
Theory of Plate Tectonics
Plate Boundaries
Convergent Boundaries
– When an oceanic plate converges with a continental
plate, the denser oceanic plate is subducted.
– Oceanic-continental convergence produces a trench
and a series of volcanoes along the edge of the
continental plate.
– Two continental plates collide when an ocean basin
between converging oceanic and continental plates is
entirely subducted.
– Because continental rocks are too buoyant to be forced
into the mantle, the colliding edges of the continents are
crumpled and uplifted to form a mountain range.
Theory of Plate Tectonics
Plate Boundaries
Transform Boundaries
– A transform boundary is a place
where two plates slide horizontally
past each other, deforming or
fracturing the crust.
– Transform boundaries are
characterized by long faults
and usually offset sections
of ocean ridges.
– The San Andreas Fault is an
exception to the fact that
transform boundaries rarely
occur on continents.
Theory of Plate Tectonics
Section Assessment
1. Match the following terms with their definitions.
___
B divergent
boundary
A. place where two tectonic plates
are moving toward each other
___
E rift valley
B. place where two tectonic
plates are moving apart
___
A convergent
boundary
___
D transform
boundary
___
C subduction
zone
C. when one of two converging
plates descends beneath
the other
D. place where two plates slide
horizontally past each other
E. result of a divergent boundary
forming on a continent
Theory of Plate Tectonics
Section Assessment
2. What happens to an oceanic plate once it
is subducted? What is created with the material?
The subducted plate melts in the mantle. Some
of the resulting magma is forced to the surface
creating a series of volcanoes that are parallel to
the subduction zone. Some of the magma is
recycled into new oceanic crust at the ridge.
Theory of Plate Tectonics
Section Assessment
3. Why does uplift occur when two continental plates
converge? Give an example of this process.
The rocks that make up a continental plate are
too buoyant to be forced into the mantle. As a
result, the colliding edges of the continents are
crumpled and uplifted to form a mountain range
such as the Himalayas.
Causes of Plate Motions
Objectives
• Explain the process of convection.
• Summarize how convection in the mantle is related to
the movements of tectonic plates.
• Compare and contrast the processes of ridge push
and slab pull.
Vocabulary
– ridge push
– slab pull
Causes of Plate Motions
Causes of Plate Motions
• The directions and rates of plate movements
have been measured.
• What actually causes the plates to move is not
well understood.
• One of the leading hypotheses proposes that
large-scale motion in the mantle is the mechanism
that drives the movement of tectonic plates.
Causes of Plate Motions
Mantle Convection
• Convection is the transfer of thermal energy by
the movement of heated matter.
• Convection currents in the mantle are thought to
be the driving mechanism of plate movements.
• Convection currents in this part of the mantle are
set in motion by the transfer of energy between
Earth’s hot interior and its cooler exterior.
• It is hypothesized that these convection currents
are probably set in motion by subducting slabs,
thus causing plates to move.
Causes of Plate Motions
Mantle Convection
• The rising part of a convection current spreads
out as it reaches the upper mantle and causes
both upward and lateral forces.
• These forces lift and split the lithosphere at
divergent plate boundaries.
• The downward part of a convection current occurs
where a sinking force pulls tectonic plates
downward at convergent boundaries.
Causes of Plate Motions
Mantle Convection
Push and Pull
– During the formation of an ocean ridge, forces in the
mantle cause the asthenosphere to rise.
– In a process called ridge push, the weight of the
uplifted ridge is thought to push an oceanic plate
toward the trench formed at the subduction zone.
– In addition to ridge push, the horizontal flow at the top
of a convection current could create drag on the
lithosphere and thereby contribute to plate motion.
Causes of Plate Motions
Mantle Convection
Push and Pull
– A sinking region of a mantle convection current
could suck an oceanic plate downward into a
subduction zone.
– In a process called slab pull, the weight of a
subducting plate helps pull the trailing lithosphere into
the subduction zone.
Causes of Plate Motions
Mantle Convection
Push and Pull
Causes of Plate Motions
Mantle Convection
Unanswered Questions
– There are unanswered questions about how these
convection currents originate and what their actual sizes
are.
– Some geologists have suggested that subducted slabs,
over time, might eventually reach Earth’s outer core.
– Other remaining questions concern relationships
between convection currents and the overlying plates.
– Most studies show that the process of slab pull is the
most important force driving tectonic plate motions.
Causes of Plate Motions
Mantle Convection
Unanswered Questions
– A similar set of questions surround the formation of
divergent continental plate boundaries.
– One hypothesis is that large continental masses
ultimately cause their own breakup by acting as
insulating blankets.
– The underlying mantle then becomes warmer and
causes the upward leg of a convection current to
develop, which eventually causes the continent to split.
Causes of Plate Motions
Section Assessment
1. How might a convection current cause a
divergent boundary?
An upward flow in the mantle causes the
asthenosphere to rise. This force causes the
lithosphere to rise and split. As the plates
separate, material rising from the mantle
supplies the magma that hardens to form
new ocean crust.
Causes of Plate Motions
Section Assessment
2. How might a convection current cause a
convergent boundary?
A sinking region of a mantle convection current
could suck an oceanic plate downward into a
subduction zone. The weight of a subducting
plate helps pull the trailing lithosphere into the
subduction zone in a process called slab pull.
Causes of Plate Motions
Section Assessment
3. How are slab push and slab pull related
processes?
Slab pull is thought to be the most important
process driving tectonic plate motions. The
material that is subducted through slab pull
enters the convection current that drives
slab push.
Chapter Resources Menu
Study Guide
Section 17.1
Section 17.2
Section 17.3
Section 17.4
Chapter Assessment
Image Bank
Section 17.1 Study Guide
Section 17.1 Main Ideas
• The matching coastlines of continents on opposite sides
of the Atlantic Ocean suggest that the continents were
once joined.
• Continental drift states that Earth’s continents were
joined as a single landmass that broke apart and sent
the continents adrift.
• Wegener supported his hypothesis of continental drift
with rock types, fossils, and ancient climatic data.
His hypothesis was not accepted at first because he
couldn’t explain how the continents moved or what
caused their motion.
Section 17.2 Study Guide
Section 17.2 Main Ideas
• Sonar and magnetic studies of ocean rocks and
sediments led to the proposal of the theory of
seafloor spreading.
• Magnetic patterns on the seafloor are symmetric in
relation to ocean ridges, indicating that ocean crust on
either side of the ridge is moving away from the ridge at
essentially the same rate.
• During seafloor spreading, magma rises and hardens to
form new crust, which becomes part of the ocean floor.
Each cycle of spreading and intrusion results in the
formation of another small section of ocean floor, which
slowly moves away from the ridge.
Section 17.3 Study Guide
Section 17.3 Main Ideas
• Plate tectonics states that Earth’s crust and rigid upper
mantle are broken into large slabs of rock called plates,
which move in different directions and at different rates
over Earth’s surface.
• At divergent plate boundaries, plates move apart. At
convergent boundaries, plates come together. At
transform boundaries, plates slide horizontally past
each other.
• High heat flow, volcanism, and earthquakes are
associated with divergent boundaries; trenches, island
arcs, and folded mountains with convergent boundaries;
and faults and earthquakes with transform boundaries.
Section 17.4 Study Guide
Section 17.4 Main Ideas
• Convection is the transfer of energy via the movement of
heated matter. Convection currents in the mantle are the
result of energy transfer between Earth’s hot interior and
cooler exterior.
• Ridge push occurs when the elevation of a ridge pushes
a plate toward a subduction zone. Slab pull occurs as
the weight of the subducting plate pulls a plate into a
subduction zone.
Chapter Assessment
Multiple Choice
1. Pangaea begin to break apart ____ years ago?
a. 65 million
c. 200 million
b. 135 million.
d. 400 million
Alfred Wegener hypothesized that Pangaea began to
break apart 200 million years ago based on geologic
evidence.
Chapter Assessment
Multiple Choice
2. The oldest oceanic crust is generally located near
what feature?
a. ridge
c. abyssal plain
b. trench
d. rift
Subduction occurs in a trench, meaning that oceanic
crust at a trench is older than any crust closer to a ridge.
A rift is a valley that forms along the axis of a ridge. It is in
the rift that new seafloor is created.
Chapter Assessment
Multiple Choice
3. The San Andreas Fault is an example of what
kind of boundary?
a. divergent
c. transform
b. convergent
d. rift
Two plates sliding past each other form the San Andreas
Fault. The San Andreas Fault is a rare example of a
transform boundary that occurs on a continent.
Chapter Assessment
Multiple Choice
4. Which device was instrumental in determining
the age of the ocean floor?
a. magnetometer
c. sonar
b. isochron
d. seismometer
A magnetometer allowed scientists to match patterns
magnetic patterns in the seafloor with known patterns and
ages of land-based material. From this they could
determine the age of the ocean floor.
Chapter Assessment
Multiple Choice
5. The oldest part of the seafloor is approximately
____ years old.
a. 70 million
c. 260 million
b. 180 million
d. 430 million
The discovery that the seafloor is geologically young was
one of the major factors in proposing the theory of
seafloor spreading. There are examples, for comparison,
of continental rocks that are 3.8 billion years old.
Chapter Assessment
Short Answer
6. Why are there more volcanoes around the rim of
the Pacific Ocean than the Atlantic Ocean?
There are subduction zones located around
the periphery of the Pacific Ocean. In the
Atlantic Ocean, the seafloor is spreading, but
with the exception of Caribbean, there are no
subduction zones.
Chapter Assessment
Short Answer
7. Why was Wegener’s hypothesis of continental
drift rejected when it was first proposed?
Two unanswered questions—what forces could
move continents and how continents could
move without shattering—were the main
reasons that the hypothesis of continental drift
was rejected when it was first proposed.
Chapter Assessment
True or False
8. Identify whether the following statements are
true or false.
______
false Wegener called his hypothesis Pangaea.
______
false Magnetic reversal events are longer in duration
than epochs.
______
true There are around a dozen major plates.
______
true Slab push is a process that is associated with
an ocean ridge.
______
false Folded mountain ranges are the result of a
convergent boundary involving oceanic crust
and continental crust.
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