Fossils

1
Table of Contents

• Fossils
• The Relative Age of Rocks
• Radioactive Dating
• The Geologic Time Scale
• Early Earth
• Eras of Earths History

2
How a Fossil Forms
- Fossils

• 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.

3
Changes Over Time
- Fossils

• 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.

4
Changes Over Time
- Fossils

• 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.

5
Using Prior Knowledge
- Fossils

• 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.

What You Know

1. Fossils come from ancient organisms.
2. Fossils are found in hardened rock.
3. Fossils show us how some present-day organisms
   looked different in the past.

What You Learned

1. Molds and casts are types of fossils.
2. Organisms are also preserved in amber, tar, and
   ice.
3. Fossils tell us about past climates, changes in
   Earths surface, and how organisms have changed
   over time.

6
Links on Fossils
- Fossils

• Click the SciLinks button for links on fossils.

7
End of SectionFossils
8
The Position of Rock Layers
- The Relative Age of Rocks

• 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.

9
Determining Relative Age
- The Relative Age of Rocks

• To determine relative age, geologists also study
  extrusions and intrusions of igneous rock,
  faults, and gaps in the geologic record.

10
Determining Relative Age
- The Relative Age of Rocks

• An unconformity occurs where erosion wears away
  layers of sedimentary rock. Other rock layers
  then form on top.

11
Using Fossils to Date Rocks
- The Relative Age of Rocks

• Index fossils are useful because they tell the
  relative ages of the rock layers in which they
  occur.

12
Index Fossil Activity
- The Relative Age of Rocks

• Click the Active Art button to open a browser
  window and access Active Art about index fossils.

13
Asking Questions
- The Relative Age of Rocks

• 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.

What does the position of rock layer reveal?
The oldest layersand the oldest fossilsare 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.
How are fossils used to date rocks?
The age of an index fossil tells the age of the
rock layer in which it occurs.
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Rock Layers
- The Relative Age of Rocks

• Click the Video button to watch a movie about
  rock layers.

15
Index Fossils
- The Relative Age of Rocks

• Click the Video button to watch a movie about
  index fossils.

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End of SectionThe Relative Age of Rocks
17
Radioactive Decay
- Radioactive Dating

• During radioactive decay, the atoms of one
  element break down to form atoms of another
  element.

18
Radioactive Decay
- Radioactive Dating

• The half-life of a radioactive element is the
  amount of time it takes for half of the
  radioactive atoms to decay.

19
Determining Absolute Ages
- Radioactive Dating

• Geologists use radioactive dating to determine
  the absolute ages of rocks.

20
Percentages
- Radioactive Dating

• 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

21
Percentages
- Radioactive Dating

• Practice Problem
• What percent of a radioactive element will remain
  after five half-lives?

• 3.125

22
Determining Absolute Ages
- Radioactive Dating

• 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.

23
Identifying Main Ideas
- Radioactive Dating

• 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
Detail
Detail
the absolute ages of the most ancient rocks
usingpotassium-40.
the absolute ages of fossils up to about 50,000
years ago using carbon-14.
the ages of sedimentary rocks by dating the
igneous intrusions and extrusions near the
sedimentary rock.
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More on Radioactive Dating
- Radioactive Dating

• Click the PHSchool.com button for an activity
  about radioactive dating.

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End of SectionRadioactive Dating
26
The Geologic Time Scale
- The Geologic Time Scale

• Because the time span of Earths past is so
  great, geologists use the geologic time scale to
  show Earths history.

27
Sequencing
- The Geologic Time Scale

• 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
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More on the Geologic Time Scale
- The Geologic Time Scale

• Click the PHSchool.com button for an activity
  about the geologic time scale.

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End of SectionThe Geologic Time Scale
30
Earths Surface Forms
- Early Earth

• During the first several hundred million years of
  Precambrian Time, an atmosphere, oceans, and
  continents began to form.

31
Life Develops
- Early Earth

• 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.

32
Comparing and Contrasting
- Early Earth

• As you read, compare and contrast the different
  types of mass movement by completing a table like
  the one below.

Precambrian Earth
Feature
Later Precambrian Earth
Early Earth
Atmosphere
Hydrogen and helium
Carbon dioxide, nitrogen, and water vapor
Oceans
Earth cools, water vapor condenses, and rain
falls. Rain forms oceans.
Earths surface is too hot. All water evaporates
into water vapor.
Continents
Old continents break apart, and new continents
form as a result of continental drift.
Less dense rock at surface forms continents.
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Links on Precambrian Earth
- Early Earth

• Click the SciLinks button for links on
  Precambrian Earth.

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End of SectionEarly Earth
35
Mass Extinctions
- Eras of Earths History

• The graph shows how the number of families of
  animals in Earths oceans has changed.

36
Mass Extinctions
- Eras of Earths History

• Reading Graphs
• What variable is shown on the x-axis of the
  graph? On the y-axis?

• The x-axis shows time in millions of years before
  the present the y-axis shows the number of
  families of ocean animals.

37
Mass Extinctions
- Eras of Earths History

• Interpreting Data
• How long ago did the most recent mass extinction
  occur?

• Slightly more than 50 million years ago

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Mass Extinctions
- Eras of Earths History

• 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

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Mass Extinctions
- Eras of Earths History

• 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.

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Geologic History
- Eras of Earths History
41
Continental Drift Activity
- Eras of Earths History

• Click the Active Art button to open a browser
  window and access Active Art about continental
  drift.

42
Previewing Visuals
- Eras of Earths History

• Before you read, preview Figure 22. Then write
  three questions you have about Earths history in
  a graphic organizer like the one below. As you
  read, answer your questions.

Earths 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.
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End of SectionEras of Earths History
44
Graphic Organizer

Fossils
include
Rock fossils
Preserved fossils
include
include
Molds and casts
Carbon films
Petrified fossils
Trace fossils
Amber
Ice
Tar
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End of SectionGraphic Organizer