xviii
Concept Checks
Within the chapter each major section concludes with “Concept Checks”
that allow students to monitor their understanding and comprehension of
significant facts and ideas.
367
The Great Debate
The movement of Earth’s tectonic plates is the cause of our most destructive earthquakes. Pisco, Peru,
following a powerful earthquake on August 16, 2007. (Sergio Erday/epa/Corbis)
to generate extensive expanses of glacial ice over much of these landmasses. At the same
time, this geography would place today’ s northern continents nearer the equator and
account for the tropical swamps that generated the vast coal deposits. Wegener was so convinced that his explanation was correct that he wrote, “This evidence is so compelling that
by comparison all other criteria must take a back seat.”
How does a glacier develop in hot, arid central Australia? How do land animals migrate
across wide expanses of the ocean? As compelling as this evidence may have been, 50 years
passed before most of the scientific community accepted the concept of continental drift
and the logical conclusions to which it led.
C O N C E P T C H E C K 1 5 . 2
What was the first line of evidence that led early investigators to suspect the continents were
once connected?
Describe the four kinds of evidence that Wegener and his supporters gathered to substantiate
the continental drift hypothesis.
Explain why the discovery of the fossil remains of Mesosaurus in both South America and
Africa, but nowhere else, supports the continental drift hypothesis.
Early in the 20th century, what was the prevailing view of how land animals migrated across
vast expanses of open ocean?
How did Wegener account for the existence of glaciers in the southern landmasses at a time
when areas in North America, Europe, and Asia supported lush tropical swamps?
5
4
3
2
1
The Great Debate
Wegener’ s proposal did not attract much
open criticism until 1924, when his book
was translated into English, French, Spanish, and Russian. From that point until his
death in 1930, the drift hypothesis encountered a great deal of hostile criticism. The
respected American geologist R.T. Chamberlain stated, “Wegener’ s hypothesis in
general is of the foot-loose type, in that it
takes considerable liberty with our globe,
and is less bound by restrictions or tied
down by awkward, ugly facts than most of
its rival theories. Its appeal seems to lie in
the fact that it plays a game in which there
are few restrictive rules and no sharply
drawn code of conduct.”
One of the main objections to
Wegener’ s hypothesis stemmed from his
inability to identify a credible mechanism
for continental drift. Wegener proposed
Give It Some Thought
Chapters conclude with a new section called “Give It Some Thought.” It
includes six to ten questions and problems that challenge learners by
involving them in activities requiring higher-order thinking skills that
include the synthesis, analysis, and application of material in the chapter.
G I V E I T S O M E T H O U G H T
After referring to the section in Chapter 1 entitled “The Nature of
Scientific Inquiry” answer the following:
a. What observation led Alfred Wegener to develop his continental
drift hypothesis?
b. What evidence did he gather to support his proposal?
c. Why was the continental drift hypothesis rejected by the majority
of the scientific community?
d. Do you think Wegener followed the basic principles of scientific
inquiry? Support your answer.
1
Referring to the accompanying diagrams that illustrate the three
types of convergent plate boundaries, complete the following:
a. Identify each type of convergent boundary.
b. Volcanic island arcs develop on what type of crust?
c. Why are volcanoes largely absent where two continental blocks collide?
d. Describe two ways that oceanic–oceanic convergent boundaries
are different from oceanic–continental boundaries? How are they
similar?
2
C O N C E P T C H E C K 1 5 . 2
What was the first line of evidence that led early investigators to suspect the continents were
once connected?
Describe the four kinds of evidence that Wegener and his supporters gathered to substantiate
the continental drift hypothesis.
Explain why the discovery of the fossil remains of Mesosaurus in both South America and
Africa, but nowhere else, supports the continental drift hypothesis.
Early in the 20th century, what was the prevailing view of how land animals migrated across
vast expanses of open ocean?
How did Wegener account for the existence of glaciers in the southern landmasses at a time
when areas in North America, Europe, and Asia supported lush tropical swamps?
5
4
3
2
1
CHAPTER 15 Plate Tectonics: A Scientific Revolution Unfolds
390
Convergent plate boundaries occur where plates move together,
resulting in the subduction of oceanic lithosphere into the mantle along a deep-ocean trench. Convergence of an oceanic and
continental block results in subduction of the oceanic slab and
the formation of a continental volcanic arc such as the Andes of
South America. Oceanic–oceanic convergence results in an arcshaped chain of volcanic islands called a volcanic island arc.
When two plates carrying continental crust converge, the
buoyant continental blocks collide, resulting in the formation
of a mountain belt as exemplified by the Himalayas.
Transform fault boundaries occur where plates grind past each
other without the production or destruction of lithosphere. Most
transform faults join two segments of a mid-ocean ridge where
they provide the means by which oceanic crust created at a ridge
crest can be transported to its site of destruction—a deep-ocean
trench. Still others, like the San Andreas Fault, cut through
continental crust.
The theory of plate tectonics is supported by (1) the ages of
sediments from the floors of the deep-ocean basins; (2) the existence of island groups that formed over hot spots and that provide
a frame of reference for tracing the direction of plate motion; and
(3) Paleomagnetism, the direction and intensity of Earth’ s
magnetism in the geologic past.
Two basic models for mantle convection are currently being
evaluated. Mechanisms that contribute to this convective flow are
slab pull and ridge push. Slab pull occurs where cold, dense
oceanic lithosphere is subducted and pulls the trailing
lithosphere along. Ridge push results when gravity sets the
elevated slabs astride oceanic ridges in motion. Hot, buoyant
mantle plumes are considered the upward flowing arms of mantle
convection. One model suggests that mantle convection occurs
in two layers separated at a depth of 660 kilometers (410 miles).
Another model proposes whole-mantle convection that stirs the
entire 2900-kilometer-thick (1800-mile-thick) rocky mantle.
Key Terms
asthenosphere (p. 369)
continental drift (p. 363)
continental rift (p. 371)
continental volcanic
arcs (p. 374)
convergent boundaries (p. 369)
Curie point (p. 381)
deep-ocean trenches (p. 372)
divergent boundaries (p. 369)
fossil magnetism (p. 382)
fracture zones (p. 376)
hot-spot (p. 380)
hot-spot track (p. 380)
island arc (p. 374)
lithosphere (p. 368)
lithospheric plates (p. 369)
magnetometers (p. 382)
magnetic reversal (p. 382)
magnetic time scale (p. 382)
mantle plume (p. 380)
normal polarity (p. 382)
oceanic ridge
system (p. 368)
paleomagnetism (p. 382)
Pangaea (p. 363)
partial melting (p. 374)
plate tectonics (p. 368)
reverse polarity (p. 382)
ridge push (p. 387)
rift valley (p. 370)
seafloor spreading (p. 370)
slab pull (p. 387)
spreading centers (p. 369)
subduction zones (p. 372)
supercontinent (p. 363)
tectonic plates (p. 369)
transform fault
boundaries (p. 369)
volcanic island arc (p. 374)
G I V E I T S O M E T H O U G H T
After referring to the section in Chapter 1 entitled “The Nature of
Scientific Inquiry” answer the following:
a. What observation led Alfred Wegener to develop his continental
drift hypothesis?
b. What evidence did he gather to support his proposal?
c. Why was the continental drift hypothesis rejected by the majority
of the scientific community?
d. Do you think Wegener followed the basic principles of scientific
inquiry? Support your answer.
1
Referring to the accompanying diagrams that illustrate the three
types of convergent plate boundaries, complete the following:
a. Identify each type of convergent boundary.
b. Volcanic island arcs develop on what type of crust?
c. Why are volcanoes largely absent where two continental blocks collide?
d. Describe two ways that oceanic–oceanic convergent boundaries
are different from oceanic–continental boundaries? How are they
similar?
2
Concept Checks
Within the chapter each major section concludes with “Concept Checks”
that allow students to monitor their understanding and comprehension of
significant facts and ideas.
367
The Great Debate
The movement of Earth’s tectonic plates is the cause of our most destructive earthquakes. Pisco, Peru,
following a powerful earthquake on August 16, 2007. (Sergio Erday/epa/Corbis)
to generate extensive expanses of glacial ice over much of these landmasses. At the same
time, this geography would place today’ s northern continents nearer the equator and
account for the tropical swamps that generated the vast coal deposits. Wegener was so convinced that his explanation was correct that he wrote, “This evidence is so compelling that
by comparison all other criteria must take a back seat.”
How does a glacier develop in hot, arid central Australia? How do land animals migrate
across wide expanses of the ocean? As compelling as this evidence may have been, 50 years
passed before most of the scientific community accepted the concept of continental drift
and the logical conclusions to which it led.
C O N C E P T C H E C K 1 5 . 2
What was the first line of evidence that led early investigators to suspect the continents were
once connected?
Describe the four kinds of evidence that Wegener and his supporters gathered to substantiate
the continental drift hypothesis.
Explain why the discovery of the fossil remains of Mesosaurus in both South America and
Africa, but nowhere else, supports the continental drift hypothesis.
Early in the 20th century, what was the prevailing view of how land animals migrated across
vast expanses of open ocean?
How did Wegener account for the existence of glaciers in the southern landmasses at a time
when areas in North America, Europe, and Asia supported lush tropical swamps?
5
4
3
2
1
The Great Debate
Wegener’ s proposal did not attract much
open criticism until 1924, when his book
was translated into English, French, Spanish, and Russian. From that point until his
death in 1930, the drift hypothesis encountered a great deal of hostile criticism. The
respected American geologist R.T. Chamberlain stated, “Wegener’ s hypothesis in
general is of the foot-loose type, in that it
takes considerable liberty with our globe,
and is less bound by restrictions or tied
down by awkward, ugly facts than most of
its rival theories. Its appeal seems to lie in
the fact that it plays a game in which there
are few restrictive rules and no sharply
drawn code of conduct.”
One of the main objections to
Wegener’ s hypothesis stemmed from his
inability to identify a credible mechanism
for continental drift. Wegener proposed
Give It Some Thought
Chapters conclude with a new section called “Give It Some Thought.” It
includes six to ten questions and problems that challenge learners by
involving them in activities requiring higher-order thinking skills that
include the synthesis, analysis, and application of material in the chapter.
G I V E I T S O M E T H O U G H T
After referring to the section in Chapter 1 entitled “The Nature of
Scientific Inquiry” answer the following:
a. What observation led Alfred Wegener to develop his continental
drift hypothesis?
b. What evidence did he gather to support his proposal?
c. Why was the continental drift hypothesis rejected by the majority
of the scientific community?
d. Do you think Wegener followed the basic principles of scientific
inquiry? Support your answer.
1
Referring to the accompanying diagrams that illustrate the three
types of convergent plate boundaries, complete the following:
a. Identify each type of convergent boundary.
b. Volcanic island arcs develop on what type of crust?
c. Why are volcanoes largely absent where two continental blocks collide?
d. Describe two ways that oceanic–oceanic convergent boundaries
are different from oceanic–continental boundaries? How are they
similar?
2
C O N C E P T C H E C K 1 5 . 2
What was the first line of evidence that led early investigators to suspect the continents were
once connected?
Describe the four kinds of evidence that Wegener and his supporters gathered to substantiate
the continental drift hypothesis.
Explain why the discovery of the fossil remains of Mesosaurus in both South America and
Africa, but nowhere else, supports the continental drift hypothesis.
Early in the 20th century, what was the prevailing view of how land animals migrated across
vast expanses of open ocean?
How did Wegener account for the existence of glaciers in the southern landmasses at a time
when areas in North America, Europe, and Asia supported lush tropical swamps?
5
4
3
2
1
CHAPTER 15 Plate Tectonics: A Scientific Revolution Unfolds
390
Convergent plate boundaries occur where plates move together,
resulting in the subduction of oceanic lithosphere into the mantle along a deep-ocean trench. Convergence of an oceanic and
continental block results in subduction of the oceanic slab and
the formation of a continental volcanic arc such as the Andes of
South America. Oceanic–oceanic convergence results in an arcshaped chain of volcanic islands called a volcanic island arc.
When two plates carrying continental crust converge, the
buoyant continental blocks collide, resulting in the formation
of a mountain belt as exemplified by the Himalayas.
Transform fault boundaries occur where plates grind past each
other without the production or destruction of lithosphere. Most
transform faults join two segments of a mid-ocean ridge where
they provide the means by which oceanic crust created at a ridge
crest can be transported to its site of destruction—a deep-ocean
trench. Still others, like the San Andreas Fault, cut through
continental crust.
The theory of plate tectonics is supported by (1) the ages of
sediments from the floors of the deep-ocean basins; (2) the existence of island groups that formed over hot spots and that provide
a frame of reference for tracing the direction of plate motion; and
(3) Paleomagnetism, the direction and intensity of Earth’ s
magnetism in the geologic past.
Two basic models for mantle convection are currently being
evaluated. Mechanisms that contribute to this convective flow are
slab pull and ridge push. Slab pull occurs where cold, dense
oceanic lithosphere is subducted and pulls the trailing
lithosphere along. Ridge push results when gravity sets the
elevated slabs astride oceanic ridges in motion. Hot, buoyant
mantle plumes are considered the upward flowing arms of mantle
convection. One model suggests that mantle convection occurs
in two layers separated at a depth of 660 kilometers (410 miles).
Another model proposes whole-mantle convection that stirs the
entire 2900-kilometer-thick (1800-mile-thick) rocky mantle.
Key Terms
asthenosphere (p. 369)
continental drift (p. 363)
continental rift (p. 371)
continental volcanic
arcs (p. 374)
convergent boundaries (p. 369)
Curie point (p. 381)
deep-ocean trenches (p. 372)
divergent boundaries (p. 369)
fossil magnetism (p. 382)
fracture zones (p. 376)
hot-spot (p. 380)
hot-spot track (p. 380)
island arc (p. 374)
lithosphere (p. 368)
lithospheric plates (p. 369)
magnetometers (p. 382)
magnetic reversal (p. 382)
magnetic time scale (p. 382)
mantle plume (p. 380)
normal polarity (p. 382)
oceanic ridge
system (p. 368)
paleomagnetism (p. 382)
Pangaea (p. 363)
partial melting (p. 374)
plate tectonics (p. 368)
reverse polarity (p. 382)
ridge push (p. 387)
rift valley (p. 370)
seafloor spreading (p. 370)
slab pull (p. 387)
spreading centers (p. 369)
subduction zones (p. 372)
supercontinent (p. 363)
tectonic plates (p. 369)
transform fault
boundaries (p. 369)
volcanic island arc (p. 374)
G I V E I T S O M E T H O U G H T
After referring to the section in Chapter 1 entitled “The Nature of
Scientific Inquiry” answer the following:
a. What observation led Alfred Wegener to develop his continental
drift hypothesis?
b. What evidence did he gather to support his proposal?
c. Why was the continental drift hypothesis rejected by the majority
of the scientific community?
d. Do you think Wegener followed the basic principles of scientific
inquiry? Support your answer.
1
Referring to the accompanying diagrams that illustrate the three
types of convergent plate boundaries, complete the following:
a. Identify each type of convergent boundary.
b. Volcanic island arcs develop on what type of crust?
c. Why are volcanoes largely absent where two continental blocks collide?
d. Describe two ways that oceanic–oceanic convergent boundaries
are different from oceanic–continental boundaries? How are they
similar?
2