Geology Reference
In-Depth Information
Chapter Summary
Earth can be viewed as a system of interconnected com-
ponents that interact and affect one another. The princi-
pal subsystems of Earth are the atmosphere, hydrosphere,
biosphere, lithosphere, mantle, and core. Earth is consid-
ered a dynamic planet that continually changes because of
the interactions among its various subsystems and cycles.
Geology, the study of Earth, is divided into two broad
areas: Physical geology is the study of Earth materials, as
well as the processes that operate within Earth and on its
surface; historical geology examines the origin and evolu-
tion of Earth, its continents, oceans, atmosphere, and life.
The scientifi c method is an orderly, logical approach that
involves gathering and analyzing facts about a particular
phenomenon, formulating hypotheses to explain the phe-
nomenon, testing the hypotheses, and fi nally proposing a
theory. A theory is a testable explanation for some natural
phenomenon that has a large body of supporting evidence.
Geology is part of the human experience. We can fi nd
examples of it in art, music, and literature. A basic
understanding of geology is also important for dealing
with the many environmental problems and issues
facing society.
Geologists engage in a variety of occupations, the main
one being exploration for mineral and energy resources.
They are also becoming increasingly involved in environ-
mental issues and making short- and long-range predic-
tions of the potential dangers from such natural disasters
as volcanic eruptions, tsunami, and earthquakes.
The universe began with a big bang approximately
14 billion years ago. Astronomers have deduced this age
by observing that celestial objects are moving away from
each other in an ever-expanding universe. Furthermore,
the universe has a background radiation of 2.7 K above
absolute zero (2.7 K = -270.3°C) which is thought to be
the faint afterglow of the Big Bang.
About 4.6 billion years ago, our solar system formed from
a rotating cloud of interstellar matter. As this cloud con-
densed, it eventually collapsed under the infl uence of gravity
and fl attened into a counterclockwise-rotating disk. Within
this rotating disk, the Sun, planets, and moons formed from
the turbulent eddies of nebular gases and solids.
Earth formed from a swirling eddy of nebular material
4.6 billion years ago, accreting as a solid body, and soon
thereafter differentiated into a layered planet during a
period of internal heating.
Earth's outermost layer is the crust, which is divided
into continental and oceanic portions. The crust and
underlying solid part of the upper mantle, also known
as the lithosphere, overlie the asthenosphere, a zone that
behaves plastically and fl ows slowly. The asthenosphere is
underlain by the solid lower mantle. Earth's core consists
of an outer liquid portion and an inner solid portion.
The lithosphere is broken into a series of plates that
diverge, converge, and slide sideways past one another.
Plate tectonic theory provides a unifying explanation
for many geologic features and events. The interaction
between plates is responsible for volcanic
eruptions, earthquakes, the formation of mountain
ranges and ocean basins, and the recycling of rock
materials.
The three major rock groups are igneous, sedimentary,
and metamorphic. Igneous rocks result from the
crystallization of magma or the consolidation of
volcanic ejecta. Sedimentary rocks are typically
formed by the consolidation of rock fragments,
precipitation of mineral matter from solution, or
compaction of plant or animal remains. Metamorphic
rocks result from the alteration of other rocks, usually
beneath Earth's surface, by heat, pressure, and
chemically active fl uids.
The rock cycle illustrates the interactions between Earth's
internal and external processes and how the three rock
groups are interrelated.
The central thesis of the theory of organic evolution is
that all living organisms evolved (descended with modifi -
cations) from organisms that existed in the past.
Time sets geology apart from the other sciences except
astronomy, and an appreciation of the immensity of geo-
logic time is central to understanding Earth's evolution.
The geologic time scale is the calendar geologists use to
date past events.
The principle of uniformitarianism is basic to the inter-
pretation of Earth history. This principle holds that the
laws of nature have been constant through time and that
the same processes operating today have operated in the
past, although not necessarily at the same rates.
Geology is an integral part of our lives. Our standard of
living depends directly on our consumption of natural
resources, resources that formed millions and billions of
years ago.
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