The following statements summarize and describe many of the key terms and concepts presented in this chapter.
Earth science is the name for
all the sciences that collectively seek to understand Earth and its neighbors
in space. It includes geology, oceanography, meteorology,
and astronomy. Geology is traditionally divided into two
broad areasphysical and historical.
to everything that surrounds and influences an organism. These influences
can be biological, social, or physical. When applied to Earth science today,
the term environmental is usually reserved for those aspects
that focus on the relationships between people and the natural environment.
Resources are an important environmental
concern. The two broad categories of resources are (1) renewable,
which means that they can be replenished over relatively short time spans,
and (2) nonrenewable. As population grows, the demand for
resources expands as well.
Environmental problems can be local, regional,
or global. Human-induced problems include urban air pollution, acid rain,
ozone depletion, and global warming. Natural hazards include earthquakes,
landslides, floods, and hurricanes. As world population grows, pressures
on the environment also increase.
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 through observation and careful measurement
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) develop a
scientific hypothesis, (3) construct experiments to validate the hypothesis,
and (4) accept, modify, or reject the hypothesis 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.
One of the challenges for those who study
Earth is the great variety of space and time scales. The geologic
time scale subdivides the 4.5 billion years of Earth history into
The nebular hypothesis 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 protoplanets,
swept up more and more of the cloud's debris. Because of their high temperatures
and weak gravitational fields, the inner planets were unable to accumulate
and retain many of the lighter components. Because of the very cold temperatures
existing far from the Sun, the large outer planets consist of huge amounts
of lighter materials. These gaseous substances account for the comparatively
large sizes and low densities of the outer planets.
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.
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. Based on physical properties, the layers of
Earth are (1) the lithospherethe cool, rigid outermost layer
that averages about 100 kilometers thick, (2) the asthenosphere,
a relatively weak layer located in the mantle beneath the lithosphere,
(3) the more rigid lower mantle, where rocks are very hot and capable of
very gradual flow, (4) the liquid outer core, where Earth's magnetic field
is generated, and (5) the solid 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
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.
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, which powers
the internal processes that produce volcanoes, earthquakes, and mountains.