A Trip Through Geologic Time
Table of Contents
Fossils
The Relative Age of Rocks
Radioactive Dating
The Geologic Time Scale
Early Earth
Eras of Earth’s History
A Trip Through Geologic Time - Fossils
How a Fossil Forms
Most fossils form when living things die and are buried by
sediment. The sediment slowly hardens into rock and
preserves the shapes of the organisms.
A Trip Through Geologic Time - Fossils
Changes Over Time
The fossils record provides
evidence about the history of
life and past environments on
Earth. The fossil record also
shows that different groups of
organisms have changed
over time. Fossils of many
different kinds of organisms
were formed in this ancient
lakeshore environment that
existed in Wyoming about 50
million years ago.
A Trip Through Geologic Time - Fossils
Changes Over Time
From fossils, scientists have reconstructed the
paleomastodon. This animal had a short trunk and short
tusks on both the upper and lower jaws. The paleomastodon
is an ancestor of the modern elephant.
A Trip Through Geologic Time - Fossils
Using Prior Knowledge
Before you read, look at the section headings and visuals to
see what this section is about. Then write what you know
about fossils in a graphic organizer like the one below. As
you read, write what you learn.
1.
2.
3.
What You Know
Fossils come from ancient organisms.
Fossils are found in hardened rock.
Fossils show us how some present-day organisms looked
different in the past.
1.
2.
3.
What You Learned
Molds and casts are types of fossils.
Organisms are also preserved in amber, tar, and ice.
Fossils tell us about past climates, changes in Earth’s surface,
and how organisms have changed over time.
A Trip Through Geologic Time - Fossils
Links on Fossils
Click the SciLinks button for links on fossils.
A Trip Through Geologic Time
End of Section:
Fossils
A Trip Through Geologic Time - The Relative Age of Rocks
The Position of Rock Layers
According to the law of superposition, in horizontal
sedimentary rock layers, the oldest layer is at the bottom.
Each higher layer is younger than the layers below it.
A Trip Through Geologic Time - The Relative Age of Rocks
Determining Relative Age
To determine relative age, geologists also study extrusions
and intrusions of igneous rock, faults, and gaps in the
geologic record.
A Trip Through Geologic Time - The Relative Age of Rocks
Determining Relative Age
An unconformity occurs where erosion wears away layers of
sedimentary rock. Other rock layers then form on top.
A Trip Through Geologic Time - The Relative Age of Rocks
Using Fossils to Date Rocks
Index fossils are useful because they tell the relative ages of
the rock layers in which they occur.
A Trip Through Geologic Time - The Relative Age of Rocks
Index Fossil Activity
Click the Active Art button to open a browser window and
access Active Art about index fossils.
A Trip Through Geologic Time - The Relative Age of Rocks
Asking Questions
Before you read, preview the red headings. In a graphic
organizer like the one below, ask a what or how question for
each heading. As you read, write answers to your questions.
Question
Answer
What does the position of rock
layer reveal?
The oldest layers—and the oldest
fossils—are at the bottom.
How do geologists determine
the relative age of a rock?
They examine the position of rock
layer, extrusions and intrusions of
igneous rock, faults, and gaps in
the geologic record.
The age of an index fossil tells the
age of the rock layer in which it
occurs.
How are fossils used to date
rocks?
A Trip Through Geologic Time - The Relative Age of Rocks
Rock Layers
Click the Video button to watch a movie about rock layers.
A Trip Through Geologic Time - The Relative Age of Rocks
Index Fossils
Click the Video button to watch a movie about index fossils.
A Trip Through Geologic Time
End of Section:
The Relative Age
of Rocks
A Trip Through Geologic Time - Radioactive Dating
Radioactive Decay
During radioactive decay, the
atoms of one element break
down to form atoms of another
element.
A Trip Through Geologic Time - Radioactive Dating
Radioactive Decay
The half-life of a radioactive element is the amount of time it
takes for half of the radioactive atoms to decay.
A Trip Through Geologic Time - Radioactive Dating
Determining Absolute Ages
Geologists use radioactive dating to determine the absolute
ages of rocks.
A Trip Through Geologic Time - Radioactive Dating
Percentages
What percentage of a radioactive element will be left after
three half-lives? First multiply 1/2 three times to determine
what fraction of the element will remain.
You can convert this fraction to a percentage by setting up a
proportion:
To find the value of d, begin by cross-multiplying, as for any
proportion:
1 X 100 = 8 X d
d=
d = 12.5%
A Trip Through Geologic Time - Radioactive Dating
Percentages
Practice Problem
What percent of a radioactive element will remain after five
half-lives?
3.125%
A Trip Through Geologic Time - Radioactive Dating
Determining Absolute Ages
The age of a sedimentary rock layer can be determined
relative to the absolute age of an igneous intrusion or
extrusion near the sedimentary rock.
A Trip Through Geologic Time - Radioactive Dating
Identifying Main Ideas
As you read the section “Determining Absolute Ages,” write
the main idea in a graphic organizer like the one below. Then
write three supporting details that further explain the main
idea.
Main Idea
Using radioactive dating, scientists can determine…
Detail
the absolute ages
of the most ancient
rocks using
potassium-40.
Detail
the absolute ages
of fossils up to
about 50,000
years ago using
carbon-14.
Detail
the ages of
sedimentary rocks
by dating the
igneous intrusions
and extrusions near
the sedimentary
rock.
A Trip Through Geologic Time - Radioactive Dating
More on Radioactive Dating
Click the PHSchool.com button for an activity about
radioactive dating.
A Trip Through Geologic Time
End of Section:
Radioactive
Dating
A Trip Through Geologic Time - The Geologic Time Scale
The Geologic Time Scale
Because the time
span of Earth’s past
is so great,
geologists use the
geologic time scale
to show Earth’s
history.
A Trip Through Geologic Time - The Geologic Time Scale
Sequencing
As you read, make a flowchart like the one below that shows
the eras and periods of geologic time. Write the name of each
era and period in the flowchart in the order in which it occurs.
Geologic Time Scale
Precambrian Time
Paleozoic Era: Permian
Paleozoic Era: Cambrian Period
Mesozoic Era: Triassic
Paleozoic Era: Ordovician Period
Mesozoic Era: Jurassic
Paleozoic Era: Silurian
Mesozoic Era: Cretaceous
Paleozoic Era: Devonian
Cenozoic Era: Tertiary
Paleozoic Era: Carboniferous
Cenozoic Era: Quaternary
A Trip Through Geologic Time - The Geologic Time Scale
More on the Geologic Time Scale
Click the PHSchool.com button for an activity about the
geologic time scale.
A Trip Through Geologic Time
End of Section:
The Geologic
Time Scale
A Trip Through Geologic Time - Early Earth
Earth’s Surface Forms
During the first several hundred million years of Precambrian
Time, an atmosphere, oceans, and continents began to form.
A Trip Through Geologic Time - Early Earth
Life Develops
Scientists have found fossils of single-celled organisms in
rocks that formed about 3.5 billion years ago. These earliest
life forms were probably similar to present-day bacteria.
A Trip Through Geologic Time - Early Earth
Comparing and Contrasting
As you read, compare and contrast the different types of
mass movement by completing a table like the one below.
Precambrian Earth
Later Precambrian
Earth
Feature
Early Earth
Atmosphere
Hydrogen and helium
Oceans
Earth’s surface is too hot.
All water evaporates into
water vapor.
Earth cools, water vapor
condenses, and rain falls.
Rain forms oceans.
Continents
Less dense rock at
surface forms continents.
Old continents break
apart, and new continents
form as a result of
continental drift.
Carbon dioxide, nitrogen,
and water vapor
A Trip Through Geologic Time - Early Earth
Links on Precambrian Earth
Click the SciLinks button for links on Precambrian Earth.
A Trip Through Geologic Time
End of Section:
Early Earth
A Trip Through Geologic Time - Eras of Earth’s History
Mass Extinctions
The graph shows how the
number of families of animals in
Earth’s oceans has changed.
A Trip Through Geologic Time - Eras of Earth’s History
Mass Extinctions
Reading Graphs:
What variable is shown on the
x-axis of the graph? On the yaxis?
The x-axis shows time in
millions of years before the
present; the y-axis shows the
number of families of ocean
animals.
A Trip Through Geologic Time - Eras of Earth’s History
Mass Extinctions
Interpreting Data:
How long ago did the most
recent mass extinction occur?
Slightly more than 50 million
years ago
A Trip Through Geologic Time - Eras of Earth’s History
Mass Extinctions
Interpreting Data:
Which mass extinction
produced the greatest drop in
the number of families of
ocean animals?
The one that occurred about
230 million years ago
A Trip Through Geologic Time - Eras of Earth’s History
Mass Extinctions
Relating Cause and Effect:
In general, how did the
number of families change
between mass extinctions?
The number of families of
ocean animals immediately
dropped but then increased.
A Trip Through Geologic Time - Eras of Earth’s History
Geologic History
A Trip Through Geologic Time - Eras of Earth’s History
Continental Drift Activity
Click the Active Art button to open a browser window and
access Active Art about continental drift.
A Trip Through Geologic Time - Eras of Earth’s History
Previewing Visuals
Before you read, preview Figure 22. Then write three
questions you have about Earth’s history in a graphic
organizer like the one below. As you read, answer your
questions.
Earth’s History
Q. What geologic events happened during Precambrian Time?
A. Earth, the oceans, and the first sedimentary rocks formed.
Q. When did the dinosaurs appear on Earth?
A. About 225 million years ago
Q. What caused the mass extinction at the end of the Cretaceous
Period?
A. An object from space struck Earth and blocked the sunlight.
A Trip Through Geologic Time
End of Section:
Eras of Earth’s
History
A Trip Through Geologic Time
Graphic Organizer
Fossils
include
Carbon
films
Rock
fossils
Preserved
fossils
include
include
Petrified
fossils
Molds
and casts
Trace
fossils
Amber
Tar
Ice
A Trip Through Geologic Time
End of Section:
Graphic Organizer