New plate boundaries are created
in response to changes in the forces
acting on the lithosphere. For example, a relatively new divergent
boundary is located in Africa, in a
region known as the East African Rift
Valleys. If spreading continues in this
region, the African plate will split
into two plates, separated by a new ocean basin. At other locations plates
carrying continental crust are moving toward each other. Eventually these
continents may collide and be sutured together. Thus, the boundary that
once separated these plates disappears, and two plates become one.
As long as temperatures within the interior of our planet remain significantly higher than those at the surface, material within Earth will continue
to circulate. This internal flow, in turn, will keep the rigid outer shell of
Earth in motion. Thus, while Earth’ s internal heat engine is operating, the
positions and shapes of the continents and ocean basins will change and
Earth will remain a dynamic planet.
In the remaining chapters we will examine in more detail the workings
of our dynamic planet in light of the plate tectonics theory.
C O N C E P T C H E C K 1 . 1 2
What are the three basic types of plate boundaries and the relative movement associated with each?
Name the type of plate boundary with which each of the following is associated: deep-ocean trench, seafloor spreading, San Andreas Fault, subduction,
oceanic ridge, and Himalaya Mountains.
2
1
33
Chapter in Review
C H A P T E R
O N E
An Introduction to Geology
in Review
Geology means “the study of Earth.” The two broad areas of
the science of geology are (1) physical geology, which examines
the materials composing Earth and the processes that operate
beneath and upon its surface; and (2) historical geology, which
seeks to understand the origin of Earth and its development
through time.
The relationship between people and the natural environment
is an important focus of geology. This includes natural hazards,
resources, and human influences on geologic processes.
During the 17th and 18th centuries, catastrophism influenced
the formulation of explanations about Earth. Catastrophism
states that Earth’ s landscapes developed over short time spans
primarily as a result of great catastrophes. By contrast, uniformitarianism, one of the fundamental principles of modern geology advanced by James Hutton in the late 1700s, states that the
physical, chemical, and biological laws that operate today have
also operated in the geologic past. The idea is often summarized
as “The present is the key to the past.” Hutton argued that processes
that appear to be slow acting could, over long spans of time,
produce effects that are just as great as those resulting from
sudden catastrophic events.
Using the principles of relative dating, the placing of events in
their proper sequence or order without knowing their age in
years, scientists developed a geologic time scale during the 19th
San Andreas Fault
FIGURE 1.34 California’s San Andreas Fault is an example of
a transform fault boundary. (Photo by Michael Collier)
D I D Y O U K N O W ?
The average rate at which
plates move relative to each
other is roughly the same rate
at which human fingernails
grow.
in response to changes in the forces
acting on the lithosphere. For example, a relatively new divergent
boundary is located in Africa, in a
region known as the East African Rift
Valleys. If spreading continues in this
region, the African plate will split
into two plates, separated by a new ocean basin. At other locations plates
carrying continental crust are moving toward each other. Eventually these
continents may collide and be sutured together. Thus, the boundary that
once separated these plates disappears, and two plates become one.
As long as temperatures within the interior of our planet remain significantly higher than those at the surface, material within Earth will continue
to circulate. This internal flow, in turn, will keep the rigid outer shell of
Earth in motion. Thus, while Earth’ s internal heat engine is operating, the
positions and shapes of the continents and ocean basins will change and
Earth will remain a dynamic planet.
In the remaining chapters we will examine in more detail the workings
of our dynamic planet in light of the plate tectonics theory.
C O N C E P T C H E C K 1 . 1 2
What are the three basic types of plate boundaries and the relative movement associated with each?
Name the type of plate boundary with which each of the following is associated: deep-ocean trench, seafloor spreading, San Andreas Fault, subduction,
oceanic ridge, and Himalaya Mountains.
2
1
33
Chapter in Review
C H A P T E R
O N E
An Introduction to Geology
in Review
Geology means “the study of Earth.” The two broad areas of
the science of geology are (1) physical geology, which examines
the materials composing Earth and the processes that operate
beneath and upon its surface; and (2) historical geology, which
seeks to understand the origin of Earth and its development
through time.
The relationship between people and the natural environment
is an important focus of geology. This includes natural hazards,
resources, and human influences on geologic processes.
During the 17th and 18th centuries, catastrophism influenced
the formulation of explanations about Earth. Catastrophism
states that Earth’ s landscapes developed over short time spans
primarily as a result of great catastrophes. By contrast, uniformitarianism, one of the fundamental principles of modern geology advanced by James Hutton in the late 1700s, states that the
physical, chemical, and biological laws that operate today have
also operated in the geologic past. The idea is often summarized
as “The present is the key to the past.” Hutton argued that processes
that appear to be slow acting could, over long spans of time,
produce effects that are just as great as those resulting from
sudden catastrophic events.
Using the principles of relative dating, the placing of events in
their proper sequence or order without knowing their age in
years, scientists developed a geologic time scale during the 19th
San Andreas Fault
FIGURE 1.34 California’s San Andreas Fault is an example of
a transform fault boundary. (Photo by Michael Collier)
D I D Y O U K N O W ?
The average rate at which
plates move relative to each
other is roughly the same rate
at which human fingernails
grow.