Transcript Section 18.1 The Geologic Time Scale

Section 18.1
The Geologic Time Scale
I Introduction
II The Geologic Column
A. Rock Layers
III Divisions of Geologic Times
A. Eras
1. Precambrian Time
2. The Paleozoic Era
3. The Mesozoic Era
4. The Cenozoic Era
B. Periods
C. Epochs
Introduction
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The surface of the earth is in a
constant state of change.
– Mountains form and erode
– oceans rise and fall
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As the surface of the earth changes:
– Various organism flourish.
– Then they become extinct.
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There is evidence of the changes
recorded in the rock layers of the
earth’s crust.
To explain the sequence and length
of the changes, scientists have
developed a geologic time scale.
– This scale is an outline of the
development of the earth and of life on
the earth.
The Geologic Column
• Using the law of superposition
and the study of fossils, scientists
were able to determine the
relative age of rock layers in areas
throughout the world.
• No single area on earth contained
a record of all geologic time.
• Scientists combined their
observations from around the
world to create a standard
arrangement of rock layers.
• The ordered arrangement of rock
layers, which is based upon the
ages of the rocks, is called the
geologic column.
– The geologic column represents a
time line of the earth’s history,
with the oldest rocks at the bottom
and the most recent rocks at the
top.
Rock Layers in a Geologic Column
• They are distinguished from
each other primarily by the
kinds of fossils they contain
and by the rock type.
• Fossils in the upper, morerecent layers resemble
modern plants and animals.
• Most fossils in the lower,
older layers are of plants
and animals that are very
different from those living
today and contain fossils of
many species that are
extinct today.
Questions
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What are some examples of how the earth is in a constant state of change?
What happens as the surface of the earth changes?
What is the geologic time scale used for?
What is the geologic time scale?
What did scientist use to find the relative ages of rock layers?
What is a geologic column?
What does a geologic column represent?
How can scientists tell the difference between two
different layers in a geologic column?
9. How are geologic times divided?
10. What is a unit of geologic time characterized by?
11. What is an era?
12. How many era are there?
13. What are the eras?
14. What is the oldest era?
15. Why is it hard to break Precambrian time into smaller units?
Divisions of Geologic Times
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Just like a calendar is divided into
months, weeks, and days, geologic
time is divided into units.
The geologic history of the earth is
scarred with major changes in the
earth’s surface or climate and by the
extinction of various species.
Geologists use these the scars as the
basis for dividing the geologic time
scale into smaller units.
Rocks grouped within each unit
contain a similar fossil record.
A unit of geologic time is often
characterized by fossils of a dominant
life-form (i.e. dinosaurs)
Eras
• A very large geologic time is an era.
• There are 4 geologic eras.
Precambrian Time
• This is the earliest era.
• It is referred to as the
Precambrian Time instead of
the Precambrian era because
it is much longer than the
other geologic eras.
• Very few fossils exist in early
precambrian rocks, making it
hard to divide the 4 billions
years of the Precambrian time
into smaller units.
• Precambrian rocks make up
the oldest layers of the
geologic column.
More Questions
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Which era following Precambrian time?
What is the meaning of the word Paleozoic?
How long did the Paleozoic Era last?
What do rocks from the Paleozoic era contain?
Which era follows the Paleozoic era?
About how long was the Mesozoic era?
What does the word Mesozoic mean?
What kind of fossils are found from the
Mesozoic era?
24. What is the present geologic era?
25. What does Cenozoic mean?
26. In terms of earth science what is a period? And how is it characterized?
27. What are periods broken down into?
28. How many epochs are there in the Cenozoic era?
Paleozoic Era
• The Paleozoic Era
followed Precambrian
time.
• The term Paleozoic comes
from the Greek words
meaning “ancient life.”
• The paleozoic Era lasted
292 million years.
• Rocks from this era
contain both marine and
land plants and animals.
Mesozoic Era
• The Mesozoic Era
follows the Paleozoic
era and contains a span
of about 183 millions
years.
• Mesozoic means
“middle life.”
• Mesozoic rocks hold
fossils of more-complex
organisms like reptiles
and birds.
Cenozoic Era
• The present geologic era
is the Cenozoic era.
• Cenozoic means “recent
life.”
• Fossils of mammals are
common in rocks of this
era.
• The fossil sequence in the
geologic column shows
that earlier forms of life
were generally followed
by new and morecomplex forms.
Periods
• The eras have been
divided into shorter time
spans called periods.
• Each period is
characterized by specific
fossils and is usually name
for the location in which
the rocks containing the
identifying fossils were
first found.
Epochs
• An Epoch is a subdivision of a geologic period.
• There are 7 epochs in the Cenozoic era.