Geology Reference
In-Depth Information
The most accurate radiometric dates are obtained
from igneous rocks.
The geologic time scale evolved primarily during the
19th century through the efforts of many people.
Stratigraphic terminology includes units based on
content and units related to geologic time.
Geologic time is an important element in the study of
climate change.
In some respects, time is defi ned by the methods used to mea-
sure it. Geologists use two different frames of reference when
discussing geologic time.
Relative dating
is placing geologic
events in a sequential order as determined from their posi-
tion in the geologic record. Relative dating will not tell us how
long ago a particular event took place, only that one event
preceded another. A useful analogy for relative dating is a
television guide that does not list the times that programs are
shown. You cannot tell what time a particular program will be
shown, but by watching a few shows and checking the guide,
you can determine whether you have missed the show or how
many shows are scheduled before the one you want to see.
The various principles used to determine relative dating
were discovered hundreds of years ago, and since then, they
have been used to construct the
relative geologic time scale
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In 1869, Major John Wesley Powell, a Civil War veteran who
lost his right arm in the battle of Shiloh, led a group of hardy
explorers down the uncharted Colorado River through the
Grand Canyon. With no maps or other information, Powell
and his group ran the many rapids of the Colorado River in
fragile wooden boats, hastily recording what they saw. Powell
wrote in his diary that “all about me are interesting geologic
records. The topic is open and I read as I run.”
From this initial reconnaissance, Powell led a second ex-
pedition down the Colorado River in 1871. This second trip
included a photographer, a surveyor, and three topographers.
Members of the expedition made detailed topographic and
geologic maps of the Grand Canyon area, as well as the fi rst
photographic record of the region.
Probably no one has contributed as much to the under-
standing of the Grand Canyon as Major Powell. In recogni-
tion of his contributions, the Powell Memorial was erected
on the South Rim of the Grand Canyon in 1969 to com-
memorate the 100th anniversary of this history-making fi rst
expedition.
Most tourists today, like Powell and his fellow explor-
ers in 1869, are astonished by the seemingly limitless time
represented by the rocks exposed in the walls of the Grand
Canyon. For most visitors, viewing a 1.5-km-deep cut into
Earth's crust is the only encounter they'll ever have with the
magnitude of geologic time. When standing on the rim and
looking down into the Grand Canyon, we are really looking
far back in time, all the way back to the early history of our
planet. In fact, more than 1 billion years of history are pre-
served in the rocks of the Grand Canyon.
Vast periods of time set geology apart from most of the
other sciences, and an appreciation of the immensity of geo-
logic time is fundamental to understanding the physical and
biological history of our planet. In fact, understanding and
accepting the magnitude of geologic time are major contri-
butions that geology has made to the sciences.
Besides providing an appreciation for the immensity
of geologic time, why is the study of geologic time impor-
tant? One of the most valuable lessons you will learn in
this chapter is how to reason and apply the fundamental
geologic principles in solving geologic problems. The logic
used in applying these principles to interpret the geologic
history of an area involves basic reasoning skills that can
be transferred to and used in almost any profession or
discipline.
Figure 17.1). Furthermore, these principles are still widely
used by geologists today.
Absolute dating
provides specifi c dates for rock units or
events expressed in years before the present. In our analogy
of the television guide, the time when the programs are actu-
ally shown would be the absolute dates. In this way, you not
only can determine whether you have missed a show (rela-
tive dating), but also know how long it will be until a show
you want to see will be shown (absolute dating).
Radiometric dating is the most common method of
obtaining absolute ages. Dates are calculated from the natural
decay rates of various radioactive elements present in trace
amounts in some rocks. It was not until the discovery of
radioactivity near the end of the 19th century that absolute
ages could be accurately applied to the relative geologic time
scale. Today, the geologic time scale is really a dual scale: a
relative scale based on rock sequences with radiometric dates
expressed as years before the present (Figure 17.1).
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AND THE AGE OF EARTH
The concept of geologic time and its measurement have
changed throughout human history. Many early Christian
scholars and clerics tried to establish the date of creation by an-
alyzing historical records and the genealogies found in Scrip-
ture. One of the most infl uential scholars was James Ussher
(1581-1656), Archbishop of Armagh, Ireland, who, based upon
Old Testament genealogy, asserted that God created Earth on
Sunday, October 23, 4004
B
.
C
. In 1701, an authorized version of
the Bible made this date accepted Church doctrine. For nearly a
century thereafter, it was considered heresy to assume that Earth
and all of its features were more than about 6000 years old.
Thus, the idea of a very young Earth provided the basis for most
Western chronologies of Earth history prior to the 18th century.
During the 18th and 19th centuries, several attempts
were made to determine Earth's age on the basis of scientifi c
evidence rather than revelation. The French zoologist Georges
Louis de Buffon (1707-1788) assumed that Earth gradu-
ally cooled to its present condition from a molten beginning.
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