CHAPTER 1 An Introduction to Geology
34
century. Relative dates can be established by applying such
principles as the law of superposition and the principle of fossil
succession.
All science is based on the assumption that the natural world
behaves in a consistent and predictable manner. The process by
which scientists gather facts and formulate scientific hypotheses
and theories is called the scientific method. To determine what is
occurring in the natural world, scientists often (1) collect facts,
(2) ask questions and develop hypotheses that may answer these
questions, (3) develop observations and experiments to test the
hypotheses, and (4) accept, modify, or reject hypotheses on the
basis of extensive testing. Other discoveries represent purely
theoretical ideas that have stood up to extensive examination.
Still other scientific advancements have been made when a
totally unexpected happening occurred during an experiment.
Earth’ s physical environment is traditionally divided into three
major parts: the solid Earth, or geosphere; the water portion of
our planet, the hydrosphere; and Earth’ s gaseous envelope, the
atmosphere. In addition, the biosphere, the totality of life on Earth,
interacts with each of the three physical realms and is an equally
integral part of Earth.
Although each of Earth’ s four spheres can be studied separately, they are all related in a complex and continuously interacting whole that we call the Earth system. Earth system science
uses an interdisciplinary approach to integrate the knowledge of
several academic fields in the study of our planet and its global
environmental problems.
A system is a group of interacting parts that form a complex
whole. Closed systems are those in which energy moves freely in
and out, but matter does not enter or leave the system. In an open
system, both energy and matter flow into and out of the system.
Most natural systems have mechanisms that tend to enhance
change, called positive feedback mechanisms, and other mechanisms, called negative feedback mechanisms, that tend to resist
change and thus stabilize the system.
The two sources of energy that power the Earth system are
(1) the Sun, which drives the external processes that occur in the
atmosphere, hydrosphere, and at Earth’ s surface, and (2) heat
from Earth’ s interior that powers the internal processes that
produce volcanoes, earthquakes, and mountains.
The rock cycle is one of the many cycles or loops of the Earth
system in which matter is recycled. The rock cycle is a means of
viewing many of the interrelationships of geology. It illustrates
the origin of the three basic rock groups and the role of various
geologic processes in transforming one rock type into another.
The nebular theory describes the formation of the solar system. The planets and Sun began forming about 5 billion years
ago from a large cloud of dust and gases. As the cloud contracted, it began to rotate and assume a disk shape. Material that
was gravitationally pulled toward the center became the protosun.
Within the rotating disk, small centers, called planetesimals,
swept up more and more of the cloud’ s debris. Because of the
high temperatures near the Sun, the inner planets were unable to
accumulate many of the elements that vaporize at low temperatures. Because of the very cold temperatures existing far from the
Sun, the large outer planets consist of huge amounts of ices and
lighter materials. These substances account for the comparatively
large sizes and low densities of the outer planets.
Earth’ s internal structure is divided into layers based on differences in chemical composition and on the basis of changes in
physical properties. Compositionally, Earth is divided into a thin
outer crust, a solid rocky mantle, and a dense core. Other layers,
based on physical properties, include the lithosphere, asthenosphere, lower mantle, outer core, and inner core.
Two principal divisions of Earth’ s surface are the continents and
ocean basins. A significant difference is their relative levels. The
elevation differences between continents and ocean basins is
primarily the result of differences in their respective densities
and thicknesses.
The largest features of the continents can be divided into two
categories: mountain belts and the stable interior. The ocean floor
is divided into three major topographic units: continental margins,
deep-ocean basins, and oceanic (mid-ocean) ridges.
The theory of plate tectonics provides a comprehensive model of
Earth’ s internal workings. It holds that Earth’ s rigid outer lithosphere consists of several segments called lithospheric plates that
are slowly and continually in motion relative to one another.
Most earthquakes, volcanic activity, and mountain building are
associated with the movements of these plates.
The three distinct types of plate boundaries are (1) divergent
boundaries, where plates move apart; (2) convergent boundaries,
where plates move together, causing one to go beneath another,
or where plates collide, which occurs when the leading edges are
made of continental crust; and (3) transform fault boundaries,
where plates slide past one another.
Key Terms
abyssal plains (p. 26)
asthenosphere (p. 24)
atmosphere (p. 13)
biosphere (p. 14)
catastrophism (p. 4)
closed systems (p. 15)
continental margin (p. 25)
continental rise (p. 26)
continental shelf (p. 25)
continental slope (p. 25)
convergent boundaries (p. 29)
core (p. 24)
crust (p. 23)
deep-ocean basins (p. 26)
deep-ocean trenches (p. 26)
divergent boundaries (p. 29)
Earth system science (p. 15)
fossils (p. 7)
fossil succession,
principle of (p. 7)
geology (p. 2)
geosphere (p. 14)
historical geology (p. 2)
hydrosphere (p. 13)
hypothesis (p. 8)
igneous rocks (p. 20)
inner core (p. 24)
interface (p. 16)
lithosphere (p. 24)
lithospheric plates (p. 28)
lower mantle (p. 24)
magma (p. 20)
34
century. Relative dates can be established by applying such
principles as the law of superposition and the principle of fossil
succession.
All science is based on the assumption that the natural world
behaves in a consistent and predictable manner. The process by
which scientists gather facts and formulate scientific hypotheses
and theories is called the scientific method. To determine what is
occurring in the natural world, scientists often (1) collect facts,
(2) ask questions and develop hypotheses that may answer these
questions, (3) develop observations and experiments to test the
hypotheses, and (4) accept, modify, or reject hypotheses on the
basis of extensive testing. Other discoveries represent purely
theoretical ideas that have stood up to extensive examination.
Still other scientific advancements have been made when a
totally unexpected happening occurred during an experiment.
Earth’ s physical environment is traditionally divided into three
major parts: the solid Earth, or geosphere; the water portion of
our planet, the hydrosphere; and Earth’ s gaseous envelope, the
atmosphere. In addition, the biosphere, the totality of life on Earth,
interacts with each of the three physical realms and is an equally
integral part of Earth.
Although each of Earth’ s four spheres can be studied separately, they are all related in a complex and continuously interacting whole that we call the Earth system. Earth system science
uses an interdisciplinary approach to integrate the knowledge of
several academic fields in the study of our planet and its global
environmental problems.
A system is a group of interacting parts that form a complex
whole. Closed systems are those in which energy moves freely in
and out, but matter does not enter or leave the system. In an open
system, both energy and matter flow into and out of the system.
Most natural systems have mechanisms that tend to enhance
change, called positive feedback mechanisms, and other mechanisms, called negative feedback mechanisms, that tend to resist
change and thus stabilize the system.
The two sources of energy that power the Earth system are
(1) the Sun, which drives the external processes that occur in the
atmosphere, hydrosphere, and at Earth’ s surface, and (2) heat
from Earth’ s interior that powers the internal processes that
produce volcanoes, earthquakes, and mountains.
The rock cycle is one of the many cycles or loops of the Earth
system in which matter is recycled. The rock cycle is a means of
viewing many of the interrelationships of geology. It illustrates
the origin of the three basic rock groups and the role of various
geologic processes in transforming one rock type into another.
The nebular theory describes the formation of the solar system. The planets and Sun began forming about 5 billion years
ago from a large cloud of dust and gases. As the cloud contracted, it began to rotate and assume a disk shape. Material that
was gravitationally pulled toward the center became the protosun.
Within the rotating disk, small centers, called planetesimals,
swept up more and more of the cloud’ s debris. Because of the
high temperatures near the Sun, the inner planets were unable to
accumulate many of the elements that vaporize at low temperatures. Because of the very cold temperatures existing far from the
Sun, the large outer planets consist of huge amounts of ices and
lighter materials. These substances account for the comparatively
large sizes and low densities of the outer planets.
Earth’ s internal structure is divided into layers based on differences in chemical composition and on the basis of changes in
physical properties. Compositionally, Earth is divided into a thin
outer crust, a solid rocky mantle, and a dense core. Other layers,
based on physical properties, include the lithosphere, asthenosphere, lower mantle, outer core, and inner core.
Two principal divisions of Earth’ s surface are the continents and
ocean basins. A significant difference is their relative levels. The
elevation differences between continents and ocean basins is
primarily the result of differences in their respective densities
and thicknesses.
The largest features of the continents can be divided into two
categories: mountain belts and the stable interior. The ocean floor
is divided into three major topographic units: continental margins,
deep-ocean basins, and oceanic (mid-ocean) ridges.
The theory of plate tectonics provides a comprehensive model of
Earth’ s internal workings. It holds that Earth’ s rigid outer lithosphere consists of several segments called lithospheric plates that
are slowly and continually in motion relative to one another.
Most earthquakes, volcanic activity, and mountain building are
associated with the movements of these plates.
The three distinct types of plate boundaries are (1) divergent
boundaries, where plates move apart; (2) convergent boundaries,
where plates move together, causing one to go beneath another,
or where plates collide, which occurs when the leading edges are
made of continental crust; and (3) transform fault boundaries,
where plates slide past one another.
Key Terms
abyssal plains (p. 26)
asthenosphere (p. 24)
atmosphere (p. 13)
biosphere (p. 14)
catastrophism (p. 4)
closed systems (p. 15)
continental margin (p. 25)
continental rise (p. 26)
continental shelf (p. 25)
continental slope (p. 25)
convergent boundaries (p. 29)
core (p. 24)
crust (p. 23)
deep-ocean basins (p. 26)
deep-ocean trenches (p. 26)
divergent boundaries (p. 29)
Earth system science (p. 15)
fossils (p. 7)
fossil succession,
principle of (p. 7)
geology (p. 2)
geosphere (p. 14)
historical geology (p. 2)
hydrosphere (p. 13)
hypothesis (p. 8)
igneous rocks (p. 20)
inner core (p. 24)
interface (p. 16)
lithosphere (p. 24)
lithospheric plates (p. 28)
lower mantle (p. 24)
magma (p. 20)