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Guide
to Reading 
This chapter deals with geologic time, which stretches
to such staggering length as the billions of years in an eon, and
examines the means by which geologists puzzle out Earth’s history.
You learn to decipher the clues Earth offers, to date Earth materials
and events, and to match this to an appropriate time scale.
When human society advanced enough to “have
time on its hands,” it used some of it to speculate about our
planet. Many cultures asked “How old is Earth?” and “What’s
been happening to Earth throughout all of its existence?” Some
persons earned their places in history by trying to answer these
questions. You read about James Hutton, Charles Lyell, William Smith,
John Wesley Powell, and Lord William Kelvin and their contributions
to establishing the time frame of Earth’s history.
Telling when something happened is an important
part of any historical narrative. Scientists had to figure out not
only what came first, last, and in between in Earth’s history
(relative dating), but they had to apply real numbers (numerical
or absolute dating) to Earth’s materials and events. Relative
dating is based on the application of several commonsense principles;
numerical dating requires more science. Therefore Earth happenings
were put in proper order before they were dated. You’ll read
about the common-sense principles of relative dating and work with
them in this study guide—principles of uniformitarianism, superposition,
original horizontality, continuity, baked zones, cross-cutting relations,
inclusions, and fossil succession.
Geologists were quite confident they were getting
the events of Earth’s history in proper sequence long before
they felt much confidence in the numbers they assigned to the events.
There were several creative lines of logic applied to the problem
including Lord Kelvin’s analysis of the temperature of Earth.
Unfortunately, new data and newer and better interpretation of old
data always showed fatal flaws in these schemes. Finally, during
the mid 1900s, observations of the statistical regularity of radioactive
decay allowed geologists to assign dates to ancient geologic materials
and events that are firmly believed to this day. The dating method
is termed radiometric dating. Your author discusses it thoroughly:
the actual procedures used, what the special case of carbon 14 dating
is all about, the accuracy of the method and the uncertainty of measurement,
and the mechanics of radioactive decay (half-lives, parent and daughter
isotopes, and isotope ratios).
Several other nonradioactive procedures have played
their parts in dating Earth events. Rock layers, some with fossils
in them, read like pages in a book to reveal Earth’s history.
Sometimes there are breaks in the rock record—pages missing—called
unconformities, which often can be accounted for by finding the missing
pages (rock layers) elsewhere in the world (a procedure called correlation).
As time passed and communications got better, correlations
worldwide became complete enough to compile a geologic column showing
all (or almost all) of Earth’s history as written in the rocks.
Improved communications also resulted in the development of a dated
geologic time scale. Its organization is a bit lacking because it
grew by bits and pieces over more than a century, but its terminology
is essential to any discussion of Earth’s history. This chapter
presents the largest, most basic divisions of the scale (Precambrian,
Hadean, Archean, Proterozoic, Phanerozoic, Paleozoic, Mesozoic, and
Cenozoic). Chapter 11 will go into greater detail.
In a study of geology you get used to hearing about
millions and billions of years. You may be very comfortable with
the words and know how many zeros go with each, but as humans we
all lead lives that revolve around smaller figures and much less
time. Therefore your author concludes the chapter with an analogy
that tries to fit these immense numbers onto a time frame we can
feel. He equates all of Earth history to one calendar year. It is
a humbling paragraph.
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