Download Chapter 9.4 - Planet Earth

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Section 9.4
9.4 Testing Plate Tectonics
1 FOCUS
Section Objectives
Key Concepts
What evidence supports
the theory of plate
tectonics?
How does paleomagnetism
support the theory of plate
tectonics?
Vocabulary
◆
◆
◆
◆
paleomagnetism
normal polarity
reverse polarity
hot spot
Reading Strategy
9.11
Predicting Copy the table. Write a
prediction of where earthquakes will occur.
After you read, if your prediction was incorrect
or incomplete, write where earthquakes
actually occur.
9.12
Probable Locations
a.
?
Actual Locations
b.
?
Explain how paleomagnetism
and magnetic reversals provide
evidence that supports the
theory of plate tectonics.
Evaluate how earthquakes,
ocean drilling, and hot spots
provide evidence that supports
the theory of plate tectonics.
Reading Focus
Evidence for Plate Tectonics
Build Vocabulary
With the birth of the plate tectonics model, researchers from all of the
Earth sciences began testing it. You have already seen some of the evidence supporting continental drift and seafloor spreading. Additional
evidence for plate tectonics came as new technologies developed.
Paleomagnetism If you have ever used a compass to find direction, you know that the magnetic field has a north pole and a south
pole. These magnetic poles align closely, but not exactly, with the geoFigure 16 Paleomagnetism
Preserved in Lava Flows As the
graphic poles.
lava cools, it becomes magnetized
In many ways, Earth’s magnetic field is much like that produced
parallel to the magnetic field
present at that time. When the
by a simple bar magnet. Invisible lines of force pass through Earth and
polarity randomly reverses, a
extend from one pole to the other. A compass needle is a small magnet
record of the paleomagnetism is
that is free to move about. The needle aligns with these invisible lines
preserved in the sequence of
lava flows.
of force and points toward the magnetic poles.
Certain rocks contain iron-rich minerals, such as magnetite. When
heated above a certain temperature, these magnetic minerals lose their
Normal
magnetic
magnetism. However, when these iron-rich mineral grains cool down,
field
0.4 m.y.
they become magnetized in the direction parallel to the existing
ago
magnetic field. Once the minerals solidify, the magnetism they
0.8 m.y. (normal)
ago
possess stays frozen in this position. So magnetized rocks
(reversed)
1.2 m.y.
behave much like a compass needle because they point
ago
toward the existing magnetic poles. If the rock is moved
(normal)
or if the magnetic pole changes position, the rock’s magnetism retains its original alignment. Rocks formed
millions of years ago thus show the location of the magnetic poles at the time of their formation, as shown in
Figure 16. These rocks possess paleomagnetism.
Plate Tectonics
265
L2
Word Parts Have students break the
vocabulary term paleomagnetism into
roots, prefixes, or suffixes. Students may
need to use a dictionary to find the
meanings of some parts. (Paleo- is a
combination form of the Greek word
palaios meaning “ancient.” The word
magnetism comes from the Greek root
words Magnes (lithos), literally meaning
a stone of Magnesia, an ancient city in
Asia Minor.)
Reading Strategy
L2
a. at convergent plate boundaries
b. at all plate boundaries
2 INSTRUCT
Evidence for Plate
Tectonics
Integrate Biology
L2
Birds and Magnetism Tell students
that birds use Earth’s magnetic field to
locate places to stop and eat along their
migration route. In addition, the birds
use Earth’s magnetic field to navigate.
They read the angle at which magnetic
fields enter the ground and thus
determine their latitude relative to the
magnetic poles. Ask: Why is it so
important for birds to locate food
sources? (The location of these places is
critical because birds must have large
quantities of food to provide energy during
their long migrations.)
Verbal, Logical
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Section 9.4 (continued)
Polarity of Ocean Crust
A
Testing Minerals
for Magnetism
L1
Magma
Purpose Students test various minerals
with a magnet to determine whether
they have magnetic properties.
Period of normal magnetism
B
Materials magnet, minerals (include
at least one sample of a mineral that
contains iron or cobalt), compass
Procedure Have students test the
mineral samples with the magnet to see
if they are attracted by it. Have students
place the compass near each mineral
sample to see if the needle moves.
If it does, the material is magnetic.
Expected Outcomes Minerals that
contain iron or cobalt, such as
lodestone, have magnetic properties.
Meteorites also have magnetic
properties.
Kinesthetic, Visual
Use Visuals
L1
Figure 17 Have students study the
figure. Ask: Could the rocks in a strip
possessing reverse polarity ever possess
normal polarity? (No, once the rocks
solidify, their polarity is permanently set.)
How do you think the width of a strip
relates to the seafloor spreading rate?
(The faster the spreading rate is, the wider
the strip will be.)
Visual, Logical
Magma
Period of reverse magnetism
C
Magma
Period of normal magnetism
Figure 17 A As new material is
added to the ocean floor at the
oceanic ridges, it is magnetized
according to Earth’s existing
magnetic field. B This process
records each reversal of Earth’s
magnetic field. C Because new
rock is added in approximately
equal amounts to the trailing
edges of both plates, strips of
equal size and polarity parallel
both sides of the ocean ridges.
Applying Concepts Why are
the magnetized strips about
equal width on either side of
the ridge?
Geophysicists learned that
Earth’s magnetic field periodically
reverses polarity. The north magnetic pole becomes the south
magnetic pole, and vice versa. A rock
solidifying during one of the periods
of reverse polarity will be magnetized with the polarity opposite that
of rocks being formed today.
When rocks show the same
magnetism as the present magnetic
field, they are described as having
normal polarity. Rocks that show
the opposite magnetism are said to
have reverse polarity. A relationship was discovered between the
magnetic reversals and the seafloorspreading hypothesis. Ships towed
instruments called magnetometers
across segments of the ocean floor.
This research revealed alternating
strips of high- and low-intensity
magnetism that ran parallel to the ridges. The strips of high-intensity magnetism are regions where the paleomagnetism of the ocean
crust is of the normal type. These positively magnetized rocks
enhance the existing magnetic field. The low-intensity strips represent regions where the ocean crust is polarized in the reverse direction
and, therefore, weaken the existing magnetic field. As new basalt is
added to the ocean floor at the oceanic ridges, it becomes magnetized
according to the existing magnetic field, as shown in Figure 17.
The discovery of strips of alternating polarity, which lie as
mirror images across the ocean ridges, is among the strongest evidence of seafloor spreading.
Earthquake Patterns
Scientists found a close link
between deep-focus earthquakes and ocean trenches. Also, the
absence of deep-focus earthquakes along the oceanic ridge system
was shown to be consistent with the new theory.
Compare the distribution of earthquakes shown in Chapter 8 on
page 226 with the map of plate boundaries on pages 256–257. The
close link between plate boundaries and earthquakes is obvious. When
the depths of earthquake foci and their locations within the trench systems are plotted, a pattern emerges.
266 Chapter 9
Customize for English Language Learners
Explain to students that there are many uses of
the term polar, both in science and in everyday
usage. For example, in magnetism, polarity
refers to the magnetic poles. In chemistry,
266 Chapter 9
polar molecules have partial charges. Polar also
means diametrically opposite. Have students
look up the various meanings of the term polar
and use each meaning in a sentence.
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Volcanic island arc
Build Science Skills
Trench
Marginal sea
Oceanic
crust
China
lithosphere 100 km
anic
oce
g
Partial melting
n
f
cti
iof
du
en
ub
ti-B
S
200 km
a
d
Wa
e
n
zo
Japan
nch
Japan tre
Asthenosphere
300 km
Key
Shallow
Intermediate
Deep
Figure 18 Distribution of Earthquake Foci Note
that intermediate- and deep-focus earthquakes occur
only within the sinking slab of oceanic lithosphere.
Look at Figure 18. It shows the distribution of earthquakes near
the Japan trench. Here, most shallow-focus earthquakes occur within
or adjacent to the trench. Intermediate- and deep-focus earthquakes
occur toward the mainland.
In the plate tectonics model, deep-ocean trenches are produced
where cool, dense slabs of oceanic lithosphere plunge into the mantle.
Shallow-focus earthquakes are produced as the descending plate interacts with the lithosphere above it. As the slab descends farther into the
mantle, deeper-focus earthquakes are produced. No earthquakes have
been recorded below 700 kilometers. At this depth, the slab has been
heated enough to soften.
L2
Interpreting Diagrams Have students
study Figure 18. Ask:
• From the map, identify the direction
in which the sinking slab of oceanic
lithosphere is moving. (from right to
left)
• Locate Korea on the map. Why do
you think Korea has relatively few
earthquakes compared to Japan?
(Korea is located far from ocean
trenches; Japan is close to a trench.)
• What pattern does the map show?
(Deeper earthquakes occur farther from
the trench.) Be sure students can
distinguish the blue dots from the
green dots.
• What can geologists learn from this
pattern? (They can use the plotted foci
to track the plate’s descent into the
mantle.)
Visual, Logical
Ocean Drilling Some of the most convincing evidence confirming the plate tectonics theory has come from drilling directly into
ocean-floor sediment. The Deep Sea Drilling Project from 1968 to 1983
used the drilling ship Glomar Challenger to drill hundreds of meters
into the sediments and underlying crust.
When the oldest sediment from each drill site was plotted against
its distance from the ridge crest, it was revealed that the age of the sediment increased with increasing distance from the ridge.
The data
on the ages of seafloor sediment confirmed what the seafloorspreading hypothesis predicted. The youngest oceanic crust is at the
ridge crest and the oldest oceanic crust is at the continental margins.
The data also reinforced the idea that the ocean basins are geologically young. No sediment older than 180 million years was found. By
comparison, some continental crust has been dated at 4.0 billion years.
Plate Tectonics
267
Facts and Figures
During its 15 years of operation, the Glomar
Challenger drilled 1092 holes and obtained
more than 96 km of invaluable core samples.
The Ocean Drilling Program has succeeded
the Deep Sea Drilling Project and, like its
predecessor, is a major international program.
A more technologically advanced drilling ship,
the JOIDES Resolution, now continues the work
of the Glomar Challenger.
Answer to . . .
Figure 17 Both sides of the ocean
plate are moving away from the ridge
at equal rates, so the magnetized strips
will be about equal in width.
Plate Tectonics 267
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Page 268
Kauai
3.8–5.6
Section 9.4 (continued)
Build Reading Literacy
L1
Refer to p. 530D in Chapter 19, which
provides the guidelines for making
inferences.
Making Inferences Have students
read the section about hot spots on this
page. Ask: What can you infer about a
hot spot from the description of how
the islands in a volcanic island arc
form at different times by the hot
spot? (You can infer that the hot spot is
relatively stationary with respect to the
mantle, and so moves relative to the plate.
If it moved along with the plate, a line of
islands would not have formed.)
Verbal, Intrapersonal
Suiko
65 my
Emperor
Seamount chain
Reteach
L1
Have students explain in their own
words why data produced by drilling
into ocean-floor sediment supports the
tectonic plate theory.
Hawaii
Mantle
plume
Ages given
in millions of
years
Figure 19 Hot Spot The chain of
islands and seamounts that
extends from Hawaii to the
Aleutian trench results from the
movement of the Pacific plate
over a stationary hot spot.
Predicting Where will a new
Hawaiian island be located?
Hot Spots Mapping of seafloor volcanoes in the Pacific revealed a
chain of volcanic structures extending from the Hawaiian Islands to
Midway Island and then north to the Aleutian trench, as shown in
Figure 19. Dates of volcanoes in this chain showed that the volcanoes
increase in age with increasing distance from Hawaii. Suiko Seamount
is 65 million years old. Midway Island is 27 million years old. The
island of Hawaii formed less than a million years ago and is still
forming today.
A rising plume of mantle material is located below the island of
Hawaii. Melting of this hot rock as it nears the surface creates a volcanic area, or hot spot. As the Pacific plate moves over the hot spot,
successive volcanic mountains have been created. The age of each volcano indicates the time when it was situated over the hot spot. Kauai
is the oldest of the large islands in the Hawaiian chain. Its volcanoes are
extinct. The youthful island of Hawaii has two active volcanoes—
Mauna Loa and Kilauea.
Hot spot evidence supports the idea that
the plates move over Earth’s surface.
Section 9.4 Assessment
Reviewing Concepts
1.
List and describe the evidence for the
plate tectonics theory.
2.
Define the term paleomagnetism.
3. What is the age of the oldest ocean crust?
How do the ages of the ocean crust compare
to the age of continental rocks?
4. What is a hot spot?
Critical Thinking
The age of the seafloor increases with
increasing distance from the spreading
center at an ocean ridge. The theory of
seafloor spreading states that new ocean
lithosphere is created at ocean ridges,
so the ocean floor should be younger
closer to the ridges and older farther
from the ridge.
Answer to . . .
Figure 19 A new Hawaiian island
will form to the southeast of the island
of Hawaii.
268 Chapter 9
Hot
spot
Hawaii
0.7 to
present
Oceanic
lithosphere
L2
To assess students’ knowledge of section
content, have them make flashcards for
the vocabulary terms and the four lines
of evidence supporting the plate
tectonic theory. For each vocabulary
term, the card should include a
definition and an example, where
applicable. For each line of evidence, the
card should give an explanation of why
it supports the theory and include an
example. Students can use the cards to
quiz one another.
Direction of
plate motion
Maui
less
than 1.0
Hawaiian chain
Midway
Islands
27 my
3 ASSESS
Evaluate
Understanding
Oahu
2.2–3.3 Molokai
1.3–1.8
6. Predicting Would earthquakes occur at
depths of over 700 kilometers? Why or
why not?
Explanatory Paragraph Write a paragraph explaining why the age pattern of
the ocean floor supports seafloor spreading.
5. Applying Concepts How do hot spots and
the plate tectonics theory account for the
different ages of the Hawaiian Islands?
268 Chapter 9
Section 9.4
Assessment
1. paleomagnetism: iron-rich minerals in rocks
line up with the magnetic field at the time
they cool; earthquake patterns: earthquake
foci are concentrated at plate boundaries;
ocean drilling: the age of the ocean lithosphere was found from drilling; hot spots:
the location of hot mantle plumes shows
plate motion.
2. Paleomagnetism is the natural magnetism
in rocks, which was acquired from Earth’s
magnetic field at the time the rock formed.
3. The oldest ocean crust is about 180 million
years old. Some continental rocks are about
3.9 billion years old.
4. an area where a plume of hot mantle
material rises up and causes volcanic activity
5. Hot spots are relatively stationary plumes of
hot rock from the mantle. As a plate moves
over a hot spot, the hot material causes volcanic activity. The previously formed volcanoes become extinct and increase in age as
the distance from the hot spot (and the active
volcanic activity) increases.
6. No, below 700 km the plates are no longer
rigid enough.