Geologic Time
Chapter 17 section 1
(This is transcribed from a Prentice Hall
read only slide show)
Geologic Time Scale:
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Information
To understand Earth, it is important to
realize that our planet is very old.
Modern methods of radiometric dating put
Earth’s age at 4.5 billion years.
This immense time span is hard to
comprehend. Guess about how long it would
take just to count to 4.5 billion if you could
count one number per second without
stopping. 150 years
An appreciation for the magnitude of geologic
time is important because many of Earth’s
processes are so gradual that vast spans of
time are needed for significant changes to
occur.
It took millions of years to create these
mountains and it will take millions more to
wear them away.
Geologists have divided all of Earth history
into units of varying magnitude. Together
they comprise the geologic time scale.
The time scale provides a framework for
arranging events of the geologic past.
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Eon represents the greatest expanses of
time.
The eon that began about 540 million years
ago is called the Phanerozoic, Greek for
“visible life”.
The sedimentary rocks and deposits of
Phanerozoic eon contain an abundance of
fossils.
The Phanerozoic eon is divided into units
called eras.
The era names refer to import differences in
dominant life-forms.
 Paleozoic refers to ancient life.
Example Trilobite fossils.
 Mesozoic refers to middle life.
 Cenozoic refers to recent life.
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Each era is divided into smaller units called
periods.
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The changing fossil record also played an
important part in establishing each of the
periods.
Periods may be divided into epochs. Seven
epochs are names for the periods of the
Cenozoic era.
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The epochs of other periods usually are just
termed early, middle, and late.
The Pleistocene epoch, which ended just
10,000 years ago, coincides with a time span
commonly known as the ice age.
The detail on the time scale does not begin
until about 540 million years ago, the date for
the start of the Cambrian Period.
The vast expanse of time prior to the
Cambrian Period is divided into three eonsthe Hadean, the Archean, and the
Proterozoic.
This greater than 4 billion-year span is also
commonly called the Precambrian.
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About what percent of Earth’s 4.5 billion-year
history is represented by Precambrian time?
88%
Why are the eons of Precambrian time not
divided into numerous eras and periods?
One reason is that Precambrian life was softbodied and therefore the fossil record is
meager compared to the Phanerozoic eon.
In addition, much of the Precambrian rock
record is buried beneath younger rocks
and/or altered by metamorphism.
The Grand Canyon’s ancient Vishnu schist is
buried beneath a huge pile of Paleozoic rocks.
Which is the greatest expanse of time? eon
Which is a subdivision of an epoch? None of
these
Mesozoic is an example of era.
Eons are divided into these era.
Which is the smallest time span on the
geologic time scale? epoch
Cambrian and Jurassic are examples of
period.
The age of the Earth is about: 4.5 billion
years
The Phanerozoic eon began about years ago.
We are presently living in the Cenozoic era.
The Hadean, Archean, and Proterozoic are:
eons that collectively comprise the
Precambrian.
Which era of the Phanerozoic eon has the
most periods? Paleozoic
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Eocene, Pleistocene, and Holocene are
examples of: epochs