CHAPTER INVESTIGATION
Geologic Time
OVERVIEW AND PURPOSE Geologists use information
from rocks, fossils, and other natural evidence to piece together
the history of Earth. The geologic time scale organizes Earth’s
history into intervals of time called eons, eras, periods, and
epochs. In this investigation you will
• construct a model of the geologic time scale
• place fossil organisms and geologic events in the correct
sequence on the timeline
Procedure
Complete the geologic time scale conversion chart. Use the conversion 1 mm = 1 million years to change the number of years
for each eon, era, period, and epoch on the chart into metric
measurements (millimeters, centimeters, and meters).
MATERIALS
• geologic time scale
conversion chart
• adding-machine paper
5 meters long
• scissors
• colored markers, pens,
or pencils
• metric tape measure or
meter stick
• sticky notes
Lay the adding-machine paper out in
front of you. At the far right end of the
strip write “TODAY” lengthwise along
the edge.
Starting from the TODAY mark, measure
back 4.6 meters, or 4600 million years.
Label this point “AGE OF EARTH.” Cut off
excess paper.
step 4
Fold the paper in half lengthwise and then
fold it in half lengthwise again. Unfold the
paper. The creases should divide your
paper into four rows.
At the far left end of the strip, label
each of the four rows as shown.
Using the numbers from your chart, measure
step 5
each eon. Start each measurement from the
TODAY line and measure back in time. For example,
the Archean eon started 3800 million years ago, so measure
back 3.8 meters from today. Mark that distance and write
“ARCHEAN EON.” Do the same for the other eons.
step 6
ARCHEAN EON
3800 million years ago (3.8 meters)
AGE OF EARTH
480 Unit 4: Life Over Time
TODAY
Repeat step 6 to measure and label the eras,
periods, and epochs.
After all the eons, eras, periods, and epochs are
measured and labeled, use the same measuring
technique to add the fossils and events from the
table below.
Table 1. Important Events in Earth’s History
Fossils and Events
First trilobite
First mammal
Greatest mass extinction
First green algae
Early humans
Extinction of dinosaurs
First life forms
Flowering plants
Time (millions of years ago)
554
210
248
1000
2
65
3800
130
Draw pictures of the fossils and events or
write the names of the fossils and events on
the timeline. If you do not have space to write
directly on the timeline, write on sticky notes
and then place the sticky notes at the correct
positions on the timeline.
Observe and Analyze
Write
It Up
1. COMPARE AND CONTRAST The
time from 4.6 billion years ago up until the
beginning of the Phanerozoic eon is called
Precambrian time. Find the part of your timeline that represents Precambrian time. How
does Precambrian time compare in length
with the rest of the geologic time scale?
2. COMPARE AND CONTRAST The
Cenozoic era is the most recent era, and it
includes the present. How does the Cenozoic
era compare in length with the other eras?
3. INTERPRET Where on the timeline are the
two major extinction events?
4. INFER What does the location of the
two major extinction events suggest about
how geologists divided the time scale into
smaller units?
Write
Conclude
It Up
1. INTERPRET Where are most of the life
forms that you placed on your time line
grouped?
2. INFER Judging by the locations of most of
the life forms on your timeline, why do you
think the shortest era on the timeline—the
Cenozoic era—has been divided into so many
smaller divisions?
3 EVALUATE What limitations or difficulties
did you experience in constructing or interpreting this model of the geologic time scale?
4. APPLY Think about the relationships
among fossils, rock layers, and the geologic
time scale. Why do you think the geologists
who first constructed the geologic time scale
found it difficult to divide the first three eons
into smaller time divisions?
INVESTIGATE Further
CHALLENGE Choose several more events or
life forms mentioned in the chapter. For each, find
either an absolute date or a relative date that will
allow you to place it in the correct position in the
geologic sequence. Draw or label these new
items on your timeline. What new patterns or
connections did adding these events or life forms
to the timeline reveal?
Chart
ale Conversion
Geologic Time Sc
Division of
Geologic Time
Millions of Years
Ago It Began
Eons
Hadean
4600
Archean
3800
Proterozoic
2500
Phanerozoic
Eras
Measurement
4.6 meters
544
544
Chapter 13: Views of Earth’s Past 481